- Email: [email protected]
Dorsal-ventral patterning in the mammalian telencephalon
50
Dorsal-ventral patterning in the mammalian telencephalon Kenneth Campbell The telencephalon is the most diverse region of the brain with respect to both morphology and neuronal subtypes. This fact makes the task of unraveling the mechanisms underlying the development of this brain region rather daunting. Recent attempts to subdivide the embryonic telencephalon into distinct progenitor domains along the dorsal–ventral axis have provided an important framework on which to begin this process. These progenitor domains are defined by the restricted expression of transcriptional regulators and are proposed to give rise to specific subtypes of neurons. Work over recent years has provided important insights into the establishment and maintenance of these progenitor domains in the developing telencephalon. Addresses Division of Developmental Biology, Children’s Hospital Research Foundation, 3333 Burnet Avenue, Cincinnati, OH 45229, USA e-mail: [email protected]
Current Opinion in Neurobiology 2003, 13:50–56 This review comes from a themed issue on Development Edited by Magdalena Go¨tz and Samuel L Pfaff 0959-4388/03/$ – see front matter ß 2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S0959-4388(03)00009-6
Abbreviations BMP bone morphogenetic protein CGE caudal ganglionic eminence DV dorsal–ventral LGE lateral ganglionic eminence MGE medial ganglionic eminence RA retinoic acid SHH sonic hedgehog SVZ subventricular zone VZ ventricular zone
Introduction The vertebrate telencephalon derives from the most anterior portion of the developing CNS. This brain region is instrumental in organizing intellectual functions and voluntary movements. Two of the principal telencephalic structures that regulate these processes are the cerebral cortex and the basal ganglia (including both the striatum and the globus pallidus). Although the cortex is located in the dorsal telencephalon, its neuronal constituents derive from both the dorsal and ventral halves of the embryonic telencephalon: whereas cortical projection neurons arise in the dorsal telencephalon from a thin neuroepithelium [1], called the pallium (Figure 1), most of the interneurons that populate cortical structures originate in the Current Opinion in Neurobiology 2003, 13:50–56
ventral telencephalon (i.e. subpallium) and subsequently migrate tangentially into the cortex [2,3,4]. Neurons of the basal ganglia, by contrast, derive exclusively from the ganglionic eminences of the ventral telencephalon (Figure 1). At rostral levels, the ganglionic eminences form separate medial (MGE) and lateral (LGE) elevations, whereas at caudal levels there is a single eminence, known as the caudal ganglionic eminence (CGE). The MGE is a very heterogeneous structure, giving rise to projection neurons of the pallidum and basal forebrain [5,6], as well as to interneurons that populate the ventral telencephalon and cortical structures [3,4,6,7]. The LGE produces the projection neurons of the striatum [4,5,6,8] and interneurons that migrate rostrally to the olfactory bulb [4,9]. Although the LGE and CGE share many similarities, a recent study has shown that the CGE gives rise to both projection neurons and interneurons that are distinct from those produced in either the LGE or the MGE [10]. Thus, the neuronal diversity present in many telencephalic structures arises through the generation of neuronal subtypes from localized subregions of the embryonic telencephalon and the subsequent distribution of these neurons through radial or tangential migration. An important issue in CNS development is what regulates the regional production of specific neuronal subtypes. Work on the developing spinal cord has shown that distinct neuronal subtypes are generated from discrete progenitor domains located along the dorsal–ventral (DV) axis [11,12]. These progenitor domains are characterized by the restricted expression of transcription factor genes and seem to be established by the coordinated signaling of bone morphogenetic proteins (BMPs) and sonic hedgehog (SHH) from the dorsal midline and ventral midline, respectively [11,12]. Gene expression studies have established that similar progenitor domains can be identified along the DV axis of the developing telencephalon. This review will cover recent work on the molecular mechanisms underlying the establishment and maintenance of these telencephalic progenitor domains.
Regional subdivisions along the dorsal– ventral axis of the telencephalon The morphological boundaries in the embryonic telencephalon do not always correlate with the progenitor domains defined by gene expression. Notably, the boundary between the dorsal and ventral halves of the telencephalon (i.e. the pallio-subpallial boundary) does not lie at the angle of the pallium and LGE but is slightly more ventral in the dorsal-most portion of the LGE www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 51
Figure 1
Dorsal
pallium
MP
DP
Ventral dLGE
VZ SVZ
LP VP
LGE
vLGE MGE
MGE Palliosubpallial boundary Current Opinion in Neurobiology
Coronal hemisections of the mouse telencephalon at embryonic day 12.5, showing morphologically defined structures and the progenitor subdomains identified by restricted expression of transcription factor genes. The ventricular zone (VZ) extends along the whole DV axis and contains precursor cells. The subventricular zone (SVZ, indicated by broken blue lines) also contains precursor cells, which is a unique feature of the telencephalon. Note the placement of the DV boundary (i.e. pallio-subpallial boundary) in the dorsal portion of the LGE. Broken red lines indicate the approximate boundaries between distinct telencephalic progenitor domains (see text for details). dLGE, dorsal LGE; DP, dorsal pallium; LP, lateral pallium; MP, medial pallium; vLGE, ventral LGE; VP, ventral pallium.
(Figure 1). This is evident from the restricted expression of numerous transcription factors, including Pax6 on the pallial side and Gsh2 on the subpallial side [13,14]. In the subventricular zone (SVZ) and the mantle regions, the pallio-subpallial boundary is marked by a stream of Pax6positive cells originating from the region where Pax6 and Gsh2 meet in the ventricular zone (VZ) and continuing down to the pial surface (Figure 1; [13–15]). Recent studies [13,15–17] have suggested that the pallium can been subdivided into separate ventral, lateral, dorsal and medial compartments (Figure 1) that give rise to projection neurons of the clastroamygdaloid complex, lateral cortex, neocortex and hippocampus, respectively. The ventral pallium was initially defined both by its expression of Pax6 in the VZ and Tbr1 in the mantle region and by its lack of Emx1 expression [15,16]. Recent work has shown that the VZ of the ventral pallium uniquely expresses the homeobox gene Dbx [13] and the putative Wnt antagonist SFRP2 [18]. Unlike the ventral pallium, the specific boundaries of the other pallial domains are not as easily marked by discrete gene expression. In these regions, graded gene expression is more common. For example, Emx1, Emx2 [19,20] and Lhx2 [21] all show high expression in the medial pallium, with a progressive reduction of expression in more www.current-opinion.com
ventral regions. Conversely, Pax6 and Tbr2 show the opposite profile with their highest expression in the VZ of the ventral and lateral pallium [13,14,17]. In the subpallium, many genes are expressed in both the LGE and MGE, including Gsh2 in the VZ [22] and members of the Dlx gene family (i.e. Dlx1, Dlx2, Dlx5 and Dlx6) in the VZ and the SVZ [23]. The ventromedial telencephalon, including the MGE, steptum and the preoptic area, expresses Nkx2.1 [24,25]. Given the considerable heterogeneity of the ventromedial telencephalon with respect to the oligodendrocytes [26,27] and the different interneurons that it generates [3,7], it seems likely that future studies will identify separate progenitor subdomains in this telencephalic region as well. Recently, the LGE has been divided into a small dorsal domain (dorsal LGE) and a larger ventral domain (ventral LGE) (Figure 1; [13]). The dorsal LGE expresses the Ets gene Er81 [13]. Although this gene is expressed in the VZ, protein expression is observed only in the SVZ of the dorsal LGE. This Er81-positive SVZ domain has been suggested to give rise to interneurons that migrate in the rostral migratory stream to populate the olfactory bulb [28]. Progenitors in the SVZ of the ventral LGE uniquely express the LIM homeobox protein Islet-1 and generate striatal projection neurons [14,28]. Current Opinion in Neurobiology 2003, 13:50–56
52 Development
Dorsal telencephalic patterning The roof plate is thought to have an essential role in patterning the dorsal spinal cord, primarily through the expression of BMPs [11,12]. A recent study has assessed the role of the roof plate in dorsal telencephalic patterning [21]. In this study, genetic ablation of the telencephalic roof plate resulted in a severely reduced expression of Lhx2 and a severe reduction in cortical size. A requirement for BMPs was implied from these results because the dorsal midline of the telencephalon expresses numerous BMPs [29]. Indeed, explant studies support a role for BMPs in dorsal telencephalic development [29] and, in particular, in the expression of Lhx2 [21]. The requirement for BMP signaling in dorsal telencephalic patterning has recently been assessed by conditionally inactivating the BMP receptor 1a specifically in the mouse telencephalon [30]. Although the choroid plexus (a dorsal midline derivative) does not form properly in these conditional mutants, the telencephalon seems to be patterned correctly along the DV axis. Compensation by the BMP receptor 1b in the dorsalmost telencephalon (i.e. medial and dorsal pallium) was not observed, indicating that BMP signaling is required absolutely for midline development but other factors can organize dorsal patterning in the telencephalon. The dorsal midline of the telencephalon is also known to express Wnt molecules [31], and Wnt signaling is required for hippocampal development (i.e. medial pallium) [32,33]. Furthermore, certain Wnts are also expressed throughout the pallial VZ at early stages of telencephalic development [31,34] and thus may have a broader role in patterning the dorsal telencephalon. Interestingly, ablation of the roof plate in the telencephalon also resulted in the loss of the Wnt2b expression from the dorsal midline [21], suggesting that the observed phenotype may result from the combined loss of BMP and Wnt signaling. A recent study has identified a telencephalic enhancer of the Emx2 gene that is expressed at its highest levels in the medial and dorsal pallium (similar to Lhx2) [35]. This enhancer contains requisite binding sites for both Smad and Tcf proteins, which are transcriptional mediators of BMP and Wnt signaling, respectively. In addition, both BMPs and Wnts can activate this DNA enhancer in midbrain explants. Thus, it may be that BMP signaling from the dorsal midline is functionally redundant with Wnt signaling in specifying certain aspects of dorsal telencephalic fates. The downstream effects of BMP and/or Wnt signaling in the dorsal telencephalon ultimately involves the combinatorial actions of transcription factors such as Emx1, Emx2 and Lhx2, which function to specify and to expand the medial and dorsal pallium. This is most evident in the Current Opinion in Neurobiology 2003, 13:50–56
Lhx2 mutants, which lack most of the hippocampus and neocortex [21,36,37]. The Emx mutants do not show as marked phenotypes in the dorsal telencephalon as do the Lhx2 mutants, although hippocampal development is impaired in Emx2 mutants [38,39]. This is not thought to result from altered specification, however, but rather from reduced growth and maturation [40]. Another transcription factor that is crucially involved in dorsal patterning is the zinc-finger gene Gli3 [41,42,43]. Loss of Gli3 function results in a loss of Emx gene expression as well as the ectopic expression of certain genes characteristic of ventral telencephalic progenitors, such as Gsh2. With respect to the latter phenotype, a recent study has shown that Gli3 is required to antagonize the ventralizing signal SHH in the dorsal telencephalon [43]. It is currently unclear whether Gli3 lies upstream, downstream or in a parallel pathway with BMP and Wnt signaling.
Ventral telencephalic patterning The secreted glycoprotein SHH is required for ventral development at caudal levels of the developing CNS (reviewed in [11,12]), and many studies have shown that SHH is also involved in patterning the ventral telencephalon [44–48]. Results obtained from the analysis of Shh-null mice [49] also support a role for SHH in telencephalic patterning, but it does not seem to be required for all aspects of ventral telencephalic development. Although these mutants lack any sign of MGE development, such as expression of Nkx2.1 [43,50], many of them express genes normally found in both the MGE and LGE, such as Gsh2 and Dlx2, as well as the LGE-specific cellular retinol-binding protein 1 ([43]; and H Toresson, K Campbell, unpublished data). Thus, SHH is required for ventromedial (i.e. MGE) telencephalic development, but aspects of ventrolateral (i.e. LGE) telencephalic patterning can occur in its absence. Although it is currently unclear whether dorsal LGE characteristics are present in the Shh-null mice, Islet-1 cells (i.e. ventral LGE) can be found (H Toresson, K Campbell, unpublished data). As mentioned above, a role of Gli3 is to antagonize SHH signaling. Recent work has shown that this is a mutually antagonistic interaction with SHH repressing Gli3 function both in the ventral spinal cord [51] and in the telencephalon [43]. In the spinal cord of Shh/;Gli3/ mutants, ventral patterning is greatly improved over that observed in single Shh mutants, however, the ventralmost cell types (i.e. the floor plate and V3 neurons) do not form [51]. This may be different in the telencephalon, because Shh/;Gli3/ and even Shh/;Gli3þ/ mutants show Nkx2.1 expression and a morphologically distinct MGE [43]. It remains unclear, however, whether the ventral-most telencephalic regions (e.g. preoptic area) are rescued in the Shh/;Gli3/ mutants. Notably, www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 53
patterning in the dorsal telencephalon of Shh;Gli3 double mutants is improved over that in Gli3 single mutants, indicating that excessive SHH signaling is involved in the manifestation of the Gli3 mutant phenotype. It is currently unknown what patterns the telencephalon in the absence of SHH and Gli3. Other hedgehog proteins are not likely candidates because double mutants of Smoothened (an essential component of SHH signaling [52]) and Gli3 also show improved DV patterning similar to that observed in the Shh/;Gli3/ embryos [43]. As suggested by Rallu et al. [43] other (potentially novel) signaling pathways may direct DV patterning in the absence of SHH and Gli3. If this is true, it will be interesting to determine whether these pathways normally have a role in DV patterning or if they do so only as a compensatory mechanism. The source of SHH for telencephalic DV patterning remains unclear. Unlike the developing spinal cord, the telencephalon lacks both a floor plate and the underlying mesoderm expressing SHH [44]. In addition, the expression of Shh in the ventral telencephalon is an unlikely source for patterning because it coincides with, rather than precedes, ventral specification (e.g. see [24]). Previously, the ventral midline of the diencephalon was suggested to be the source of SHH for telencephalic ventralization [44]; however, a more recent study has proposed that SHH from the node controls this process [46]. If this is true, then the initiation of ventral patterning in the telencephalon occurs much earlier than was previously thought.
Patterning the intermediate telencephalon The intermediate region of the telencephalon includes the LGE, as well as the ventral and lateral pallium, and is divided by the pallio-subpallial boundary. Although signals from both the dorsal and the ventral aspects of the telencephalon probably have an impact on the patterning of this region, it is also likely that distinct lateral signals are involved (Figure 2). Indeed, these putative lateral signals may contribute to the expression of ventrolateral genes in the ventral midline of the Shh mutant (see above). The expression profiles of certain genes in the intermediate telencephalon also support the notion of distinct lateral signals. For example, Pax6 and Gsh2 show graded expression patterns in the dorsal and ventral portions, respectively, with their highest expression occurring at the pallio-subpallial boundary [13,14]. Previous work has shown that a SHH-independent retinoic acid (RA) pathway can pattern the intermediate region of the developing spinal cord [53]. Interestingly, at early stages of telencephalic patterning, mesenchymal cells in the olfactory placodes that produce RA are in close proximity to the ventral telencephalon and induce the expression of retinoid reporter genes in the ventrolateral region [54]. Thus, RA may participate in patterning the telencephalon. www.current-opinion.com
Figure 2
BMPs Wnts
rp
MP
DP LP VP dLGE
RA? ?
vLGE MGE
SHH ? Current Opinion in Neurobiology
The mouse telencephalon at embryonic day 10, showing the extrinsic signals that establish different progenitor domains along the DV axis. BMPs are most highly expressed in dorsal–medial portions of the telencephalon, where they are required to act locally for midline development. Wnts are expressed throughout the pallium and may function more broadly in dorsal specification than BMPs. Although SHH is required for ventromedial specification, some aspects of ventrolateral patterning can occur in its absence. DV patterning can occur in the absence of both Shh and Gli3, suggesting the existence of alternative signals and/or pathways in ventralizing the telencephalon. Lateral signals are proposed to participate in patterning the intermediate telencephalon. RA may be involved in this process. dLGE, dorsal LGE; DP, dorsal pallium; LP, lateral pallium; MP, medial pallium; rp, roof plate; vLGE, ventral LGE; VP, ventral pallium.
As mentioned above, the pallio-subpallial boundary divides the telencephalon into a dorsal and ventral portion. Pax6 and Gsh2 are crucial for the positioning of this boundary through the regulation of mutually repressive genetic programs [13,14]. The telencephalic subdomains lying directly on each side of the palliosubpallial boundary (i.e. the dorsal LGE and the ventral pallium) are most dependent on correct positioning of this boundary. In the absence of Pax6, genes characteristic of the dorsal LGE become ectopically expressed early in the ventral pallium and subsequently spread as far as the dorsal pallium [13,28]. The opposite is true in Gsh2 mutants, in which ventral pallial markers are ectopically expressed in the LGE [13]. These alterations in gene expression do not represent a transformation of the dorsal LGE into the ventral pallium in Gsh2 mutants or vice versa in Pax6 mutants, because other markers of DV identity are expressed normally in these mutants [14]. Current Opinion in Neurobiology 2003, 13:50–56
54 Development
The fact that Pax6 and Gsh2 do not control all aspects of the DV identity in the intermediate telencephalon indicates that parallel and overlapping genetic pathways are also involved. Indeed, Gli3 mutants show ectopic ventral gene expression in the pallium similar to that seen in Pax6 mutants, despite the fact that Pax6 seems to be normally expressed [41,43]. In addition, a recent study [55] has shown that in Emx2/;Pax6/ mutants dorsal patterning is considerably more perturbed than in single Pax6 mutants. Although Emx2 mutants do not show overt patterning defects in the intermediate telencephalon, removal of Pax6 gene function from the Emx2 mutant background causes the whole pallium to lose its dorsal identity [55]. In the absence of pallial gene expression, markers of the ventral LGE (e.g. Islet-1) are expressed in the dorsal telencephalon of these double mutants, suggesting that there is a transformation of the pallium into subpallium. Thus, correct DV patterning in the telencephalon relies on the superimposition of several genetic pathways.
Conclusions Work carried out to date has provided a substantial framework on which to uncover the mechanisms that control DV identity in the developing telencephalon. The identification and refinement of progenitor domains along the DV axis has been and will continue to be central to this endeavor. Signals from each of the dorsal, ventral and possibly lateral aspects of the developing telencephalon participate in the establishment and maintenance of these progenitor domains, by regulating both separate and overlapping genetic pathways (Figure 2). In future studies it will be important to define further the roles of BMPs, RA, SHH and Wnts, as well as other as yet unknown signals, in DV patterning of the telencephalon. Many of the mechanisms involved in patterning the DV axis of the spinal cord are also at work in the telencephalon. What is it, then, that makes the mature telencephalon so much more complex than the spinal cord? A recent study has provided evidence that, in addition to spatial (e.g. DV) patterning, temporal patterning events contribute extensively to generating neuronal diversity in the Drosophila CNS [56]. Because the telencephalon shows a very protracted period of neurogenesis as compared with the spinal cord, it may be that different neuronal subtypes are generated from each telencephalic progenitor domain at specific time points during development. Thus, it will be important in future work to establish a connection between DV patterning and temporal patterning to account for the generation of all of the subtypes of neurons present in the telencephalon.
identity [57]. Interestingly, a genetic interaction between TLX and Pax6 was found to be crucial for this process.
Acknowledgements I thank Doug Epstein, Magdalena Go¨ tz, Mike Matise and Jan Stenman for critically reading the manuscript, and Gord Fishell for providing preprints of papers discussed in the review.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Gorski JA, Talley T, Qiu M, Puelles L, Rubenstein JL, Jones KR: Cortical excitatory neurons and glia, but not GABAergic neurons, are produced in the Emx1-expressing lineage. J Neurosci 2002, 22:6309-6314.
2.
Stuhmer T, Puelles L, Ekker M, Rubenstein JL: Expression from a Dlx gene enhancer marks adult mouse cortical GABAergic neurons. Cereb Cortex 2002, 12:75-85.
3.
Marin O, Rubenstein JL: A long, remarkable journey: tangential migration in the telencephalon. Nat Rev Neurosci 2001, 2:780-790.
4.
Wichterle H, Turnbull DH, Nery S, Fishell G, Alvarez-Buylla A: In utero fate mapping reveals distinct migratory pathways and fates of neurons born in the mammalian basal forebrain. Development 2001, 128:3759-3771. Although many slice culture studies have shown tangential migration of neurons from the medial ganglionic eminence to the dorsal telencephalon, this study was the first in vivo demonstration of this important phenomenon. 5.
Olsson M, Campbell K, Turnbull DH: Specification of mouse telencephalic and mid-hindbrain progenitors following heterotopic ultrasound-guided embryonic transplantation. Neuron 1997, 19:761-772.
6.
Olsson M, Bjorklund A, Campbell K: Early specification of striatal projection neurons and interneuronal subtypes in the lateral and medial ganglionic eminence. Neuroscience 1998, 84:867-876.
7.
Marin O, Anderson SA, Rubenstein JL: Origin and molecular specification of striatal interneurons. J Neurosci 2000, 20:6063-6076.
8.
Deacon TW, Pakzaban P, Isacson O: The lateral ganglionic eminence is the origin of cells committed to striatal phenotypes: neural transplantation and developmental evidence. Brain Res 1994, 668:211-219.
9.
Wichterle H, Garcia-Verdugo JM, Herrera DG, Alvarez-Buylla A: Young neurons from medial ganglionic eminence disperse in adult and embryonic brain. Nat Neurosci 1999, 2:461-466.
10. Nery S, Fishell G, Corbin JG: The caudal ganglionic eminence is a novel source of distinct cortical and subcortical cell populations. Nat Neurosci 2002, 5:1279-1282. The CGE has been proposed to represent a distinct structure in the developing telencephalon. This study provides the first functional evidence of this by using ultrasound-guided embryonic transplantation to map the fates of cells derived from this region. 11. Jessell TM: Neuronal specification in the spinal cord: inductive signals and transcriptional codes. Nat Rev Genet 2000, 1:20-29. 12. Briscoe J, Ericson J: Specification of neuronal fates in the ventral neural tube. Curr Opin Neurobiol 2001, 11:43-49. 13. Yun K, Potter S, Rubenstein JL: Gsh2 and Pax6 play complementary roles in dorsoventral patterning of the mammalian telencephalon. Development 2001, 128:193-205.
Update
14. Toresson H, Potter SS, Campbell K: Genetic control of dorsal– ventral identity in the telencephalon: opposing roles for Pax6 and Gsh2. Development 2000, 127:4361-4371.
A recent study has demonstrated a role for the orphan nuclear receptor TLX in the establishment of the palliosubpallial boundary and maintenance of ventral pallial
15. Puelles L, Kuwana E, Puelles E, Rubenstein JL: Comparison of the mammalian and avian telencephalon from the perspective of gene expression data. Eur J Morphol 1999, 37:139-150.
Current Opinion in Neurobiology 2003, 13:50–56
www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 55
16. Puelles L, Kuwana E, Puelles E, Bulfone A, Shimamura K, Keleher J, Smiga S, Rubenstein JL: Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx-2, Emx-1, Nkx- 2.1, Pax-6, and Tbr-1. J Comp Neurol 2000, 424:409-438. 17. Stoykova A, Treichel D, Hallonet M, Gruss P: Pax6 modulates the dorsoventral patterning of the mammalian telencephalon. J Neurosci 2000, 20:8042-8050. 18. Kim AS, Anderson SA, Rubenstein JL, Lowenstein DH, Pleasure SJ: Pax-6 regulates expression of SFRP-2 and Wnt-7b in the developing CNS. J Neurosci 2001, 21:RC132(1-5). 19. Simeone A, Gulisano M, Acampora D, Stornaiuolo A, Rambaldi M, Boncinelli E: Two vertebrate homeobox genes related to the Drosophila empty spiracles gene are expressed in the embryonic cerebral cortex. EMBO J 1992, 11:2541-2550. 20. Gulisano M, Broccoli V, Pardini C, Boncinelli E: Emx1 and Emx2 show different patterns of expression during proliferation and differentiation of the developing cerebral cortex in the mouse. Eur J Neurosci 1996, 8:1037-1050. 21. Monuki ES, Porter FD, Walsh CA: Patterning of the dorsal telencephalon and cerebral cortex by a roof plate–Lhx2 pathway. Neuron 2001, 32:591-604. This study provides evidence that the roof plate is an important signaling center for dorsal telencephalic development. The results from this study implicate BMPs and possibly also Wnts in patterning the dorsal telencephalon. 22. Hsieh-Li HM, Witte DP, Szucsik JC, Weinstein M, Li H, Potter SS: Gsh-2, a murine homeobox gene expressed in the developing brain. Mech Dev 1995, 50:177-186. 23. Eisenstat DD, Liu JK, Mione M, Zhong W, Yu G, Anderson SA, Ghattas I, Puelles L, Rubenstein JL: DLX-1, DLX-2, and DLX-5 expression define distinct stages of basal forebrain differentiation. J Comp Neurol 1999, 414:217-237. 24. Shimamura K, Hartigan DJ, Martinez S, Puelles L, Rubenstein JL: Longitudinal organization of the anterior neural plate and neural tube. Development 1995, 121:3923-3933. 25. Sussel L, Marin O, Kimura S, Rubenstein JL: Loss of Nkx2.1 homeobox gene function results in a ventral to dorsal molecular respecification within the basal telencephalon: evidence for a transformation of the pallidum into the striatum. Development 1999, 126:3359-3370. 26. Nery S, Wichterle H, Fishell G: Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain. Development 2001, 128:527-540. See annotation [27]. 27. Tekki-Kessaris N, Woodruff R, Hall AC, Gaffield W, Kimura S, Stiles CD, Rowitch DH, Richardson WD: Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon. Development 2001, 128:2545-2554. Together with [26], this study shows that the ventromedial telencephalon is a principal source of oligodendrocytes for this brain region and that SHH is involved in generating this cell type. 28. Stenman J, Toresson H, Campbell K: Identification of two distinct progenitor populations in the lateral ganglionic eminence: implications for striatal and olfactory bulb neurogenesis. J Neurosci 2003, 23:167-174. This study shows that the Dlx-expressing subventricular zone of the LGE can be divided into at least two distinct progenitor domains: a dorsal domain marked by Er81, which is proposed to generate interneurons that migrate rostrally to the olfactory bulb; and a more ventral domain expressing Islet-1, which gives rise to the striatal projection neurons. 29. Furuta Y, Piston DW, Hogan BL: Bone morphogenetic proteins (BMPs) as regulators of dorsal forebrain development. Development 1997, 124:2203-2212. 30. Hebert J, Mishina Y, McConnell S: BMP signaling is required locally to pattern the dorsal telencephalic midline. Neuron 2002, 35:1029. This study addresses the role of BMPs in patterning the dorsal telencephalon by conditional inactivation of the BMP receptor 1a. The results indicate that loss of BMP signaling in the dorsal telencephalon results in a normal dorsal–ventral pattern but a lack of normal development of the www.current-opinion.com
choroid plexus, suggesting a local role for BMPs only in the dorsal telencephalic midline. 31. Grove EA, Tole S, Limon J, Yip L, Ragsdale CW: The hem of the embryonic cerebral cortex is defined by the expression of multiple Wnt genes and is compromised in Gli3-deficient mice. Development 1998, 125:2315-2325. 32. Lee SM, Tole S, Grove E, McMahon AP: A local Wnt-3a signal is required for development of the mammalian hippocampus. Development 2000, 127:457-467. 33. Galceran J, Miyashita-Lin EM, Devaney E, Rubenstein JL, Grosschedl R: Hippocampus development and generation of dentate gyrus granule cells is regulated by LEF1. Development 2000, 127:469-482. 34. Parr BA, Shea MJ, Vassileva G, McMahon AP: Mouse Wnt genes exhibit discrete domains of expression in the early embryonic CNS and limb buds. Development 1993, 119:247-261. 35. Theil T, Aydin S, Koch S, Grotewold L, Ruther U: Wnt and Bmp signalling cooperatively regulate graded Emx2 expression in the dorsal telencephalon. Development 2002, 129:3045-3054. In this paper, a telencephalic enhancer for the Emx2 gene is identified and shown to contain requisite Smad- and Tcf-binding sites, implicating both BMPs and Wnts in the regulation of this dorsal telencephalic gene. 36. Porter FD, Drago J, Xu Y, Cheema SS, Wassif C, Huang SP, Lee E, Grinberg A, Massalas JS, Bodine D et al.: Lhx2, a LIM homeobox gene, is required for eye, forebrain, and definitive erythrocyte development. Development 1997, 124:2935-2944. 37. Bulchand S, Grove EA, Porter FD, Tole S: LIM-homeodomain gene Lhx2 regulates the formation of the cortical hem. Mech Dev 2001, 100:165-175. 38. Pellegrini M, Mansouri A, Simeone A, Boncinelli E, Gruss P: Dentate gyrus formation requires Emx2. Development 1996, 122:3893-3898. 39. Yoshida M, Suda Y, Matsuo I, Miyamoto N, Takeda N, Kuratani S, Aizawa S: Emx1 and Emx2 functions in development of dorsal telencephalon. Development 1997, 124:101-111. 40. Tole S, Goudreau G, Assimacopoulos S: Grove EA: Emx2 is required for growth of the hippocampus but not for hippocampal field specification. J Neurosci 2000, 20:2618-2625. 41. Theil T, Alvarez-Bolado G, Walter A, Ruther U: Gli3 is required for Emx gene expression during dorsal telencephalon development. Development 1999, 126:3561-3571. 42. Tole S, Ragsdale CW, Grove EA: Dorsoventral patterning of the telencephalon is disrupted in the mouse mutant extra-toes(J). Dev Biol 2000, 217:254-265. 43. Rallu M, Machold RP, Gaiano N, Corbin JG, McMahon AP, Fishell G: Dorsoventral patterning is established in the telencephalon of mutants lacking both Gli3 and Hedgehog signaling. Development 2002, 129:4963-4974. The results of this paper provide evidence for the cross-repressive activities of SHH and Gli3 in the telencephalon. In the absence of both Shh and Gli3 gene function, dorsal–ventral patterning is markedly better than in either single mutant. These findings suggests that another signal or signals can pattern the telencephalon in the absence of SHH and Gli3. 44. Ericson J, Muhr J, Placzek M, Lints T, Jessell TM, Edlund T: Sonic hedgehog induces the differentiation of ventral forebrain neurons: a common signal for ventral patterning within the neural tube. Cell 1995, 81:747-756. 45. Kohtz JD, Baker DP, Corte G, Fishell G: Regionalization within the mammalian telencephalon is mediated by changes in responsiveness to Sonic Hedgehog. Development 1998, 125:5079-5089. 46. Gunhaga L, Jessell TM, Edlund T: Sonic hedgehog signaling at gastrula stages specifies ventral telencephalic cells in the chick embryo. Development 2000, 127:3283-3293. 47. Gaiano N, Kohtz JD, Turnbull DH, Fishell G: A method for rapid gain-of-function studies in the mouse embryonic nervous system. Nat Neurosci 1999, 2:812-819. Current Opinion in Neurobiology 2003, 13:50–56
56 Development
48. Corbin JG, Gaiano N, Machold RP, Langston A, Fishell G: The Gsh2 homeodomain gene controls multiple aspects of telencephalic development. Development 2000, 127:5007-5020.
54. LaMantia AS, Colbert MC, Linney E: Retinoic acid induction and regional differentiation prefigure olfactory pathway formation in the mammalian forebrain. Neuron 1993, 10:1035-1048.
49. Chiang C, Litingtung Y, Lee E, Young KE, Corden JL, Westphal H, Beachy PA: Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function. Nature 1996, 383:407-413.
55. Muzio L, DiBenedetto B, Stoykova A, Boncinelli E, Gruss P, Mallamaci A: Conversion of cerebral cortex into basal ganglia in Emx2/ Pax6Sey/Sey double-mutant mice. Nat Neurosci 2002, 15:15. In this study, loss of function for both Pax6 and Emx2 is shown to result in a respecification of the pallium towards subpallial fates typical of the ventral LGE (i.e. striatal specification). Because Emx2 mutants do not show overt patterning defects, these findings reveal a role for Emx2 in the absence of Pax6 gene function.
50. Pabst O, Herbrand H, Takuma N, Arnold HH: NKX2 gene expression in neuroectoderm but not in mesendodermally derived structures depends on sonic hedgehog in mouse embryos. Dev Genes E 2000, 210:47-50. 51. Litingtung Y, Chiang C: Specification of ventral neuron types is mediated by an antagonistic interaction between Shh and Gli3. Nat Neurosci 2000, 3:979-985. 52. Ingham PW, McMahon AP: Hedgehog signaling in animal development: paradigms and principles. Genes Dev 2001, 15:3059-3087. 53. Pierani A, Brenner-Morton S, Chiang C, Jessell TM: A sonic hedgehog-independent, retinoid-activated pathway of neurogenesis in the ventral spinal cord. Cell 1999, 97:903-915.
Current Opinion in Neurobiology 2003, 13:50–56
56. Isshiki T, Pearson B, Holbrook S, Doe CQ: Drosophila neuroblasts sequentially express transcription factors which specify the temporal identity of their neuronal progeny. Cell 2001, 106:511-521. 57. Stenman J, Yu RT, Evans RM, Campbell K: TLX and Pax6 cooperate genetically to establish the pallio-subpallial boundary in the embryonic mouse telencephalon. Development 2003, in press. See update.
www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Kenneth Campbell The telencephalon is the most diverse region of the brain with respect to both morphology and neuronal subtypes. This fact makes the task of unraveling the mechanisms underlying the development of this brain region rather daunting. Recent attempts to subdivide the embryonic telencephalon into distinct progenitor domains along the dorsal–ventral axis have provided an important framework on which to begin this process. These progenitor domains are defined by the restricted expression of transcriptional regulators and are proposed to give rise to specific subtypes of neurons. Work over recent years has provided important insights into the establishment and maintenance of these progenitor domains in the developing telencephalon. Addresses Division of Developmental Biology, Children’s Hospital Research Foundation, 3333 Burnet Avenue, Cincinnati, OH 45229, USA e-mail: [email protected]
Current Opinion in Neurobiology 2003, 13:50–56 This review comes from a themed issue on Development Edited by Magdalena Go¨tz and Samuel L Pfaff 0959-4388/03/$ – see front matter ß 2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S0959-4388(03)00009-6
Abbreviations BMP bone morphogenetic protein CGE caudal ganglionic eminence DV dorsal–ventral LGE lateral ganglionic eminence MGE medial ganglionic eminence RA retinoic acid SHH sonic hedgehog SVZ subventricular zone VZ ventricular zone
Introduction The vertebrate telencephalon derives from the most anterior portion of the developing CNS. This brain region is instrumental in organizing intellectual functions and voluntary movements. Two of the principal telencephalic structures that regulate these processes are the cerebral cortex and the basal ganglia (including both the striatum and the globus pallidus). Although the cortex is located in the dorsal telencephalon, its neuronal constituents derive from both the dorsal and ventral halves of the embryonic telencephalon: whereas cortical projection neurons arise in the dorsal telencephalon from a thin neuroepithelium [1], called the pallium (Figure 1), most of the interneurons that populate cortical structures originate in the Current Opinion in Neurobiology 2003, 13:50–56
ventral telencephalon (i.e. subpallium) and subsequently migrate tangentially into the cortex [2,3,4]. Neurons of the basal ganglia, by contrast, derive exclusively from the ganglionic eminences of the ventral telencephalon (Figure 1). At rostral levels, the ganglionic eminences form separate medial (MGE) and lateral (LGE) elevations, whereas at caudal levels there is a single eminence, known as the caudal ganglionic eminence (CGE). The MGE is a very heterogeneous structure, giving rise to projection neurons of the pallidum and basal forebrain [5,6], as well as to interneurons that populate the ventral telencephalon and cortical structures [3,4,6,7]. The LGE produces the projection neurons of the striatum [4,5,6,8] and interneurons that migrate rostrally to the olfactory bulb [4,9]. Although the LGE and CGE share many similarities, a recent study has shown that the CGE gives rise to both projection neurons and interneurons that are distinct from those produced in either the LGE or the MGE [10]. Thus, the neuronal diversity present in many telencephalic structures arises through the generation of neuronal subtypes from localized subregions of the embryonic telencephalon and the subsequent distribution of these neurons through radial or tangential migration. An important issue in CNS development is what regulates the regional production of specific neuronal subtypes. Work on the developing spinal cord has shown that distinct neuronal subtypes are generated from discrete progenitor domains located along the dorsal–ventral (DV) axis [11,12]. These progenitor domains are characterized by the restricted expression of transcription factor genes and seem to be established by the coordinated signaling of bone morphogenetic proteins (BMPs) and sonic hedgehog (SHH) from the dorsal midline and ventral midline, respectively [11,12]. Gene expression studies have established that similar progenitor domains can be identified along the DV axis of the developing telencephalon. This review will cover recent work on the molecular mechanisms underlying the establishment and maintenance of these telencephalic progenitor domains.
Regional subdivisions along the dorsal– ventral axis of the telencephalon The morphological boundaries in the embryonic telencephalon do not always correlate with the progenitor domains defined by gene expression. Notably, the boundary between the dorsal and ventral halves of the telencephalon (i.e. the pallio-subpallial boundary) does not lie at the angle of the pallium and LGE but is slightly more ventral in the dorsal-most portion of the LGE www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 51
Figure 1
Dorsal
pallium
MP
DP
Ventral dLGE
VZ SVZ
LP VP
LGE
vLGE MGE
MGE Palliosubpallial boundary Current Opinion in Neurobiology
Coronal hemisections of the mouse telencephalon at embryonic day 12.5, showing morphologically defined structures and the progenitor subdomains identified by restricted expression of transcription factor genes. The ventricular zone (VZ) extends along the whole DV axis and contains precursor cells. The subventricular zone (SVZ, indicated by broken blue lines) also contains precursor cells, which is a unique feature of the telencephalon. Note the placement of the DV boundary (i.e. pallio-subpallial boundary) in the dorsal portion of the LGE. Broken red lines indicate the approximate boundaries between distinct telencephalic progenitor domains (see text for details). dLGE, dorsal LGE; DP, dorsal pallium; LP, lateral pallium; MP, medial pallium; vLGE, ventral LGE; VP, ventral pallium.
(Figure 1). This is evident from the restricted expression of numerous transcription factors, including Pax6 on the pallial side and Gsh2 on the subpallial side [13,14]. In the subventricular zone (SVZ) and the mantle regions, the pallio-subpallial boundary is marked by a stream of Pax6positive cells originating from the region where Pax6 and Gsh2 meet in the ventricular zone (VZ) and continuing down to the pial surface (Figure 1; [13–15]). Recent studies [13,15–17] have suggested that the pallium can been subdivided into separate ventral, lateral, dorsal and medial compartments (Figure 1) that give rise to projection neurons of the clastroamygdaloid complex, lateral cortex, neocortex and hippocampus, respectively. The ventral pallium was initially defined both by its expression of Pax6 in the VZ and Tbr1 in the mantle region and by its lack of Emx1 expression [15,16]. Recent work has shown that the VZ of the ventral pallium uniquely expresses the homeobox gene Dbx [13] and the putative Wnt antagonist SFRP2 [18]. Unlike the ventral pallium, the specific boundaries of the other pallial domains are not as easily marked by discrete gene expression. In these regions, graded gene expression is more common. For example, Emx1, Emx2 [19,20] and Lhx2 [21] all show high expression in the medial pallium, with a progressive reduction of expression in more www.current-opinion.com
ventral regions. Conversely, Pax6 and Tbr2 show the opposite profile with their highest expression in the VZ of the ventral and lateral pallium [13,14,17]. In the subpallium, many genes are expressed in both the LGE and MGE, including Gsh2 in the VZ [22] and members of the Dlx gene family (i.e. Dlx1, Dlx2, Dlx5 and Dlx6) in the VZ and the SVZ [23]. The ventromedial telencephalon, including the MGE, steptum and the preoptic area, expresses Nkx2.1 [24,25]. Given the considerable heterogeneity of the ventromedial telencephalon with respect to the oligodendrocytes [26,27] and the different interneurons that it generates [3,7], it seems likely that future studies will identify separate progenitor subdomains in this telencephalic region as well. Recently, the LGE has been divided into a small dorsal domain (dorsal LGE) and a larger ventral domain (ventral LGE) (Figure 1; [13]). The dorsal LGE expresses the Ets gene Er81 [13]. Although this gene is expressed in the VZ, protein expression is observed only in the SVZ of the dorsal LGE. This Er81-positive SVZ domain has been suggested to give rise to interneurons that migrate in the rostral migratory stream to populate the olfactory bulb [28]. Progenitors in the SVZ of the ventral LGE uniquely express the LIM homeobox protein Islet-1 and generate striatal projection neurons [14,28]. Current Opinion in Neurobiology 2003, 13:50–56
52 Development
Dorsal telencephalic patterning The roof plate is thought to have an essential role in patterning the dorsal spinal cord, primarily through the expression of BMPs [11,12]. A recent study has assessed the role of the roof plate in dorsal telencephalic patterning [21]. In this study, genetic ablation of the telencephalic roof plate resulted in a severely reduced expression of Lhx2 and a severe reduction in cortical size. A requirement for BMPs was implied from these results because the dorsal midline of the telencephalon expresses numerous BMPs [29]. Indeed, explant studies support a role for BMPs in dorsal telencephalic development [29] and, in particular, in the expression of Lhx2 [21]. The requirement for BMP signaling in dorsal telencephalic patterning has recently been assessed by conditionally inactivating the BMP receptor 1a specifically in the mouse telencephalon [30]. Although the choroid plexus (a dorsal midline derivative) does not form properly in these conditional mutants, the telencephalon seems to be patterned correctly along the DV axis. Compensation by the BMP receptor 1b in the dorsalmost telencephalon (i.e. medial and dorsal pallium) was not observed, indicating that BMP signaling is required absolutely for midline development but other factors can organize dorsal patterning in the telencephalon. The dorsal midline of the telencephalon is also known to express Wnt molecules [31], and Wnt signaling is required for hippocampal development (i.e. medial pallium) [32,33]. Furthermore, certain Wnts are also expressed throughout the pallial VZ at early stages of telencephalic development [31,34] and thus may have a broader role in patterning the dorsal telencephalon. Interestingly, ablation of the roof plate in the telencephalon also resulted in the loss of the Wnt2b expression from the dorsal midline [21], suggesting that the observed phenotype may result from the combined loss of BMP and Wnt signaling. A recent study has identified a telencephalic enhancer of the Emx2 gene that is expressed at its highest levels in the medial and dorsal pallium (similar to Lhx2) [35]. This enhancer contains requisite binding sites for both Smad and Tcf proteins, which are transcriptional mediators of BMP and Wnt signaling, respectively. In addition, both BMPs and Wnts can activate this DNA enhancer in midbrain explants. Thus, it may be that BMP signaling from the dorsal midline is functionally redundant with Wnt signaling in specifying certain aspects of dorsal telencephalic fates. The downstream effects of BMP and/or Wnt signaling in the dorsal telencephalon ultimately involves the combinatorial actions of transcription factors such as Emx1, Emx2 and Lhx2, which function to specify and to expand the medial and dorsal pallium. This is most evident in the Current Opinion in Neurobiology 2003, 13:50–56
Lhx2 mutants, which lack most of the hippocampus and neocortex [21,36,37]. The Emx mutants do not show as marked phenotypes in the dorsal telencephalon as do the Lhx2 mutants, although hippocampal development is impaired in Emx2 mutants [38,39]. This is not thought to result from altered specification, however, but rather from reduced growth and maturation [40]. Another transcription factor that is crucially involved in dorsal patterning is the zinc-finger gene Gli3 [41,42,43]. Loss of Gli3 function results in a loss of Emx gene expression as well as the ectopic expression of certain genes characteristic of ventral telencephalic progenitors, such as Gsh2. With respect to the latter phenotype, a recent study has shown that Gli3 is required to antagonize the ventralizing signal SHH in the dorsal telencephalon [43]. It is currently unclear whether Gli3 lies upstream, downstream or in a parallel pathway with BMP and Wnt signaling.
Ventral telencephalic patterning The secreted glycoprotein SHH is required for ventral development at caudal levels of the developing CNS (reviewed in [11,12]), and many studies have shown that SHH is also involved in patterning the ventral telencephalon [44–48]. Results obtained from the analysis of Shh-null mice [49] also support a role for SHH in telencephalic patterning, but it does not seem to be required for all aspects of ventral telencephalic development. Although these mutants lack any sign of MGE development, such as expression of Nkx2.1 [43,50], many of them express genes normally found in both the MGE and LGE, such as Gsh2 and Dlx2, as well as the LGE-specific cellular retinol-binding protein 1 ([43]; and H Toresson, K Campbell, unpublished data). Thus, SHH is required for ventromedial (i.e. MGE) telencephalic development, but aspects of ventrolateral (i.e. LGE) telencephalic patterning can occur in its absence. Although it is currently unclear whether dorsal LGE characteristics are present in the Shh-null mice, Islet-1 cells (i.e. ventral LGE) can be found (H Toresson, K Campbell, unpublished data). As mentioned above, a role of Gli3 is to antagonize SHH signaling. Recent work has shown that this is a mutually antagonistic interaction with SHH repressing Gli3 function both in the ventral spinal cord [51] and in the telencephalon [43]. In the spinal cord of Shh/;Gli3/ mutants, ventral patterning is greatly improved over that observed in single Shh mutants, however, the ventralmost cell types (i.e. the floor plate and V3 neurons) do not form [51]. This may be different in the telencephalon, because Shh/;Gli3/ and even Shh/;Gli3þ/ mutants show Nkx2.1 expression and a morphologically distinct MGE [43]. It remains unclear, however, whether the ventral-most telencephalic regions (e.g. preoptic area) are rescued in the Shh/;Gli3/ mutants. Notably, www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 53
patterning in the dorsal telencephalon of Shh;Gli3 double mutants is improved over that in Gli3 single mutants, indicating that excessive SHH signaling is involved in the manifestation of the Gli3 mutant phenotype. It is currently unknown what patterns the telencephalon in the absence of SHH and Gli3. Other hedgehog proteins are not likely candidates because double mutants of Smoothened (an essential component of SHH signaling [52]) and Gli3 also show improved DV patterning similar to that observed in the Shh/;Gli3/ embryos [43]. As suggested by Rallu et al. [43] other (potentially novel) signaling pathways may direct DV patterning in the absence of SHH and Gli3. If this is true, it will be interesting to determine whether these pathways normally have a role in DV patterning or if they do so only as a compensatory mechanism. The source of SHH for telencephalic DV patterning remains unclear. Unlike the developing spinal cord, the telencephalon lacks both a floor plate and the underlying mesoderm expressing SHH [44]. In addition, the expression of Shh in the ventral telencephalon is an unlikely source for patterning because it coincides with, rather than precedes, ventral specification (e.g. see [24]). Previously, the ventral midline of the diencephalon was suggested to be the source of SHH for telencephalic ventralization [44]; however, a more recent study has proposed that SHH from the node controls this process [46]. If this is true, then the initiation of ventral patterning in the telencephalon occurs much earlier than was previously thought.
Patterning the intermediate telencephalon The intermediate region of the telencephalon includes the LGE, as well as the ventral and lateral pallium, and is divided by the pallio-subpallial boundary. Although signals from both the dorsal and the ventral aspects of the telencephalon probably have an impact on the patterning of this region, it is also likely that distinct lateral signals are involved (Figure 2). Indeed, these putative lateral signals may contribute to the expression of ventrolateral genes in the ventral midline of the Shh mutant (see above). The expression profiles of certain genes in the intermediate telencephalon also support the notion of distinct lateral signals. For example, Pax6 and Gsh2 show graded expression patterns in the dorsal and ventral portions, respectively, with their highest expression occurring at the pallio-subpallial boundary [13,14]. Previous work has shown that a SHH-independent retinoic acid (RA) pathway can pattern the intermediate region of the developing spinal cord [53]. Interestingly, at early stages of telencephalic patterning, mesenchymal cells in the olfactory placodes that produce RA are in close proximity to the ventral telencephalon and induce the expression of retinoid reporter genes in the ventrolateral region [54]. Thus, RA may participate in patterning the telencephalon. www.current-opinion.com
Figure 2
BMPs Wnts
rp
MP
DP LP VP dLGE
RA? ?
vLGE MGE
SHH ? Current Opinion in Neurobiology
The mouse telencephalon at embryonic day 10, showing the extrinsic signals that establish different progenitor domains along the DV axis. BMPs are most highly expressed in dorsal–medial portions of the telencephalon, where they are required to act locally for midline development. Wnts are expressed throughout the pallium and may function more broadly in dorsal specification than BMPs. Although SHH is required for ventromedial specification, some aspects of ventrolateral patterning can occur in its absence. DV patterning can occur in the absence of both Shh and Gli3, suggesting the existence of alternative signals and/or pathways in ventralizing the telencephalon. Lateral signals are proposed to participate in patterning the intermediate telencephalon. RA may be involved in this process. dLGE, dorsal LGE; DP, dorsal pallium; LP, lateral pallium; MP, medial pallium; rp, roof plate; vLGE, ventral LGE; VP, ventral pallium.
As mentioned above, the pallio-subpallial boundary divides the telencephalon into a dorsal and ventral portion. Pax6 and Gsh2 are crucial for the positioning of this boundary through the regulation of mutually repressive genetic programs [13,14]. The telencephalic subdomains lying directly on each side of the palliosubpallial boundary (i.e. the dorsal LGE and the ventral pallium) are most dependent on correct positioning of this boundary. In the absence of Pax6, genes characteristic of the dorsal LGE become ectopically expressed early in the ventral pallium and subsequently spread as far as the dorsal pallium [13,28]. The opposite is true in Gsh2 mutants, in which ventral pallial markers are ectopically expressed in the LGE [13]. These alterations in gene expression do not represent a transformation of the dorsal LGE into the ventral pallium in Gsh2 mutants or vice versa in Pax6 mutants, because other markers of DV identity are expressed normally in these mutants [14]. Current Opinion in Neurobiology 2003, 13:50–56
54 Development
The fact that Pax6 and Gsh2 do not control all aspects of the DV identity in the intermediate telencephalon indicates that parallel and overlapping genetic pathways are also involved. Indeed, Gli3 mutants show ectopic ventral gene expression in the pallium similar to that seen in Pax6 mutants, despite the fact that Pax6 seems to be normally expressed [41,43]. In addition, a recent study [55] has shown that in Emx2/;Pax6/ mutants dorsal patterning is considerably more perturbed than in single Pax6 mutants. Although Emx2 mutants do not show overt patterning defects in the intermediate telencephalon, removal of Pax6 gene function from the Emx2 mutant background causes the whole pallium to lose its dorsal identity [55]. In the absence of pallial gene expression, markers of the ventral LGE (e.g. Islet-1) are expressed in the dorsal telencephalon of these double mutants, suggesting that there is a transformation of the pallium into subpallium. Thus, correct DV patterning in the telencephalon relies on the superimposition of several genetic pathways.
Conclusions Work carried out to date has provided a substantial framework on which to uncover the mechanisms that control DV identity in the developing telencephalon. The identification and refinement of progenitor domains along the DV axis has been and will continue to be central to this endeavor. Signals from each of the dorsal, ventral and possibly lateral aspects of the developing telencephalon participate in the establishment and maintenance of these progenitor domains, by regulating both separate and overlapping genetic pathways (Figure 2). In future studies it will be important to define further the roles of BMPs, RA, SHH and Wnts, as well as other as yet unknown signals, in DV patterning of the telencephalon. Many of the mechanisms involved in patterning the DV axis of the spinal cord are also at work in the telencephalon. What is it, then, that makes the mature telencephalon so much more complex than the spinal cord? A recent study has provided evidence that, in addition to spatial (e.g. DV) patterning, temporal patterning events contribute extensively to generating neuronal diversity in the Drosophila CNS [56]. Because the telencephalon shows a very protracted period of neurogenesis as compared with the spinal cord, it may be that different neuronal subtypes are generated from each telencephalic progenitor domain at specific time points during development. Thus, it will be important in future work to establish a connection between DV patterning and temporal patterning to account for the generation of all of the subtypes of neurons present in the telencephalon.
identity [57]. Interestingly, a genetic interaction between TLX and Pax6 was found to be crucial for this process.
Acknowledgements I thank Doug Epstein, Magdalena Go¨ tz, Mike Matise and Jan Stenman for critically reading the manuscript, and Gord Fishell for providing preprints of papers discussed in the review.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Gorski JA, Talley T, Qiu M, Puelles L, Rubenstein JL, Jones KR: Cortical excitatory neurons and glia, but not GABAergic neurons, are produced in the Emx1-expressing lineage. J Neurosci 2002, 22:6309-6314.
2.
Stuhmer T, Puelles L, Ekker M, Rubenstein JL: Expression from a Dlx gene enhancer marks adult mouse cortical GABAergic neurons. Cereb Cortex 2002, 12:75-85.
3.
Marin O, Rubenstein JL: A long, remarkable journey: tangential migration in the telencephalon. Nat Rev Neurosci 2001, 2:780-790.
4.
Wichterle H, Turnbull DH, Nery S, Fishell G, Alvarez-Buylla A: In utero fate mapping reveals distinct migratory pathways and fates of neurons born in the mammalian basal forebrain. Development 2001, 128:3759-3771. Although many slice culture studies have shown tangential migration of neurons from the medial ganglionic eminence to the dorsal telencephalon, this study was the first in vivo demonstration of this important phenomenon. 5.
Olsson M, Campbell K, Turnbull DH: Specification of mouse telencephalic and mid-hindbrain progenitors following heterotopic ultrasound-guided embryonic transplantation. Neuron 1997, 19:761-772.
6.
Olsson M, Bjorklund A, Campbell K: Early specification of striatal projection neurons and interneuronal subtypes in the lateral and medial ganglionic eminence. Neuroscience 1998, 84:867-876.
7.
Marin O, Anderson SA, Rubenstein JL: Origin and molecular specification of striatal interneurons. J Neurosci 2000, 20:6063-6076.
8.
Deacon TW, Pakzaban P, Isacson O: The lateral ganglionic eminence is the origin of cells committed to striatal phenotypes: neural transplantation and developmental evidence. Brain Res 1994, 668:211-219.
9.
Wichterle H, Garcia-Verdugo JM, Herrera DG, Alvarez-Buylla A: Young neurons from medial ganglionic eminence disperse in adult and embryonic brain. Nat Neurosci 1999, 2:461-466.
10. Nery S, Fishell G, Corbin JG: The caudal ganglionic eminence is a novel source of distinct cortical and subcortical cell populations. Nat Neurosci 2002, 5:1279-1282. The CGE has been proposed to represent a distinct structure in the developing telencephalon. This study provides the first functional evidence of this by using ultrasound-guided embryonic transplantation to map the fates of cells derived from this region. 11. Jessell TM: Neuronal specification in the spinal cord: inductive signals and transcriptional codes. Nat Rev Genet 2000, 1:20-29. 12. Briscoe J, Ericson J: Specification of neuronal fates in the ventral neural tube. Curr Opin Neurobiol 2001, 11:43-49. 13. Yun K, Potter S, Rubenstein JL: Gsh2 and Pax6 play complementary roles in dorsoventral patterning of the mammalian telencephalon. Development 2001, 128:193-205.
Update
14. Toresson H, Potter SS, Campbell K: Genetic control of dorsal– ventral identity in the telencephalon: opposing roles for Pax6 and Gsh2. Development 2000, 127:4361-4371.
A recent study has demonstrated a role for the orphan nuclear receptor TLX in the establishment of the palliosubpallial boundary and maintenance of ventral pallial
15. Puelles L, Kuwana E, Puelles E, Rubenstein JL: Comparison of the mammalian and avian telencephalon from the perspective of gene expression data. Eur J Morphol 1999, 37:139-150.
Current Opinion in Neurobiology 2003, 13:50–56
www.current-opinion.com
Dorsal-ventral patterning in the mammalian telencephalon Campbell 55
16. Puelles L, Kuwana E, Puelles E, Bulfone A, Shimamura K, Keleher J, Smiga S, Rubenstein JL: Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx-2, Emx-1, Nkx- 2.1, Pax-6, and Tbr-1. J Comp Neurol 2000, 424:409-438. 17. Stoykova A, Treichel D, Hallonet M, Gruss P: Pax6 modulates the dorsoventral patterning of the mammalian telencephalon. J Neurosci 2000, 20:8042-8050. 18. Kim AS, Anderson SA, Rubenstein JL, Lowenstein DH, Pleasure SJ: Pax-6 regulates expression of SFRP-2 and Wnt-7b in the developing CNS. J Neurosci 2001, 21:RC132(1-5). 19. Simeone A, Gulisano M, Acampora D, Stornaiuolo A, Rambaldi M, Boncinelli E: Two vertebrate homeobox genes related to the Drosophila empty spiracles gene are expressed in the embryonic cerebral cortex. EMBO J 1992, 11:2541-2550. 20. Gulisano M, Broccoli V, Pardini C, Boncinelli E: Emx1 and Emx2 show different patterns of expression during proliferation and differentiation of the developing cerebral cortex in the mouse. Eur J Neurosci 1996, 8:1037-1050. 21. Monuki ES, Porter FD, Walsh CA: Patterning of the dorsal telencephalon and cerebral cortex by a roof plate–Lhx2 pathway. Neuron 2001, 32:591-604. This study provides evidence that the roof plate is an important signaling center for dorsal telencephalic development. The results from this study implicate BMPs and possibly also Wnts in patterning the dorsal telencephalon. 22. Hsieh-Li HM, Witte DP, Szucsik JC, Weinstein M, Li H, Potter SS: Gsh-2, a murine homeobox gene expressed in the developing brain. Mech Dev 1995, 50:177-186. 23. Eisenstat DD, Liu JK, Mione M, Zhong W, Yu G, Anderson SA, Ghattas I, Puelles L, Rubenstein JL: DLX-1, DLX-2, and DLX-5 expression define distinct stages of basal forebrain differentiation. J Comp Neurol 1999, 414:217-237. 24. Shimamura K, Hartigan DJ, Martinez S, Puelles L, Rubenstein JL: Longitudinal organization of the anterior neural plate and neural tube. Development 1995, 121:3923-3933. 25. Sussel L, Marin O, Kimura S, Rubenstein JL: Loss of Nkx2.1 homeobox gene function results in a ventral to dorsal molecular respecification within the basal telencephalon: evidence for a transformation of the pallidum into the striatum. Development 1999, 126:3359-3370. 26. Nery S, Wichterle H, Fishell G: Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain. Development 2001, 128:527-540. See annotation [27]. 27. Tekki-Kessaris N, Woodruff R, Hall AC, Gaffield W, Kimura S, Stiles CD, Rowitch DH, Richardson WD: Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon. Development 2001, 128:2545-2554. Together with [26], this study shows that the ventromedial telencephalon is a principal source of oligodendrocytes for this brain region and that SHH is involved in generating this cell type. 28. Stenman J, Toresson H, Campbell K: Identification of two distinct progenitor populations in the lateral ganglionic eminence: implications for striatal and olfactory bulb neurogenesis. J Neurosci 2003, 23:167-174. This study shows that the Dlx-expressing subventricular zone of the LGE can be divided into at least two distinct progenitor domains: a dorsal domain marked by Er81, which is proposed to generate interneurons that migrate rostrally to the olfactory bulb; and a more ventral domain expressing Islet-1, which gives rise to the striatal projection neurons. 29. Furuta Y, Piston DW, Hogan BL: Bone morphogenetic proteins (BMPs) as regulators of dorsal forebrain development. Development 1997, 124:2203-2212. 30. Hebert J, Mishina Y, McConnell S: BMP signaling is required locally to pattern the dorsal telencephalic midline. Neuron 2002, 35:1029. This study addresses the role of BMPs in patterning the dorsal telencephalon by conditional inactivation of the BMP receptor 1a. The results indicate that loss of BMP signaling in the dorsal telencephalon results in a normal dorsal–ventral pattern but a lack of normal development of the www.current-opinion.com
choroid plexus, suggesting a local role for BMPs only in the dorsal telencephalic midline. 31. Grove EA, Tole S, Limon J, Yip L, Ragsdale CW: The hem of the embryonic cerebral cortex is defined by the expression of multiple Wnt genes and is compromised in Gli3-deficient mice. Development 1998, 125:2315-2325. 32. Lee SM, Tole S, Grove E, McMahon AP: A local Wnt-3a signal is required for development of the mammalian hippocampus. Development 2000, 127:457-467. 33. Galceran J, Miyashita-Lin EM, Devaney E, Rubenstein JL, Grosschedl R: Hippocampus development and generation of dentate gyrus granule cells is regulated by LEF1. Development 2000, 127:469-482. 34. Parr BA, Shea MJ, Vassileva G, McMahon AP: Mouse Wnt genes exhibit discrete domains of expression in the early embryonic CNS and limb buds. Development 1993, 119:247-261. 35. Theil T, Aydin S, Koch S, Grotewold L, Ruther U: Wnt and Bmp signalling cooperatively regulate graded Emx2 expression in the dorsal telencephalon. Development 2002, 129:3045-3054. In this paper, a telencephalic enhancer for the Emx2 gene is identified and shown to contain requisite Smad- and Tcf-binding sites, implicating both BMPs and Wnts in the regulation of this dorsal telencephalic gene. 36. Porter FD, Drago J, Xu Y, Cheema SS, Wassif C, Huang SP, Lee E, Grinberg A, Massalas JS, Bodine D et al.: Lhx2, a LIM homeobox gene, is required for eye, forebrain, and definitive erythrocyte development. Development 1997, 124:2935-2944. 37. Bulchand S, Grove EA, Porter FD, Tole S: LIM-homeodomain gene Lhx2 regulates the formation of the cortical hem. Mech Dev 2001, 100:165-175. 38. Pellegrini M, Mansouri A, Simeone A, Boncinelli E, Gruss P: Dentate gyrus formation requires Emx2. Development 1996, 122:3893-3898. 39. Yoshida M, Suda Y, Matsuo I, Miyamoto N, Takeda N, Kuratani S, Aizawa S: Emx1 and Emx2 functions in development of dorsal telencephalon. Development 1997, 124:101-111. 40. Tole S, Goudreau G, Assimacopoulos S: Grove EA: Emx2 is required for growth of the hippocampus but not for hippocampal field specification. J Neurosci 2000, 20:2618-2625. 41. Theil T, Alvarez-Bolado G, Walter A, Ruther U: Gli3 is required for Emx gene expression during dorsal telencephalon development. Development 1999, 126:3561-3571. 42. Tole S, Ragsdale CW, Grove EA: Dorsoventral patterning of the telencephalon is disrupted in the mouse mutant extra-toes(J). Dev Biol 2000, 217:254-265. 43. Rallu M, Machold RP, Gaiano N, Corbin JG, McMahon AP, Fishell G: Dorsoventral patterning is established in the telencephalon of mutants lacking both Gli3 and Hedgehog signaling. Development 2002, 129:4963-4974. The results of this paper provide evidence for the cross-repressive activities of SHH and Gli3 in the telencephalon. In the absence of both Shh and Gli3 gene function, dorsal–ventral patterning is markedly better than in either single mutant. These findings suggests that another signal or signals can pattern the telencephalon in the absence of SHH and Gli3. 44. Ericson J, Muhr J, Placzek M, Lints T, Jessell TM, Edlund T: Sonic hedgehog induces the differentiation of ventral forebrain neurons: a common signal for ventral patterning within the neural tube. Cell 1995, 81:747-756. 45. Kohtz JD, Baker DP, Corte G, Fishell G: Regionalization within the mammalian telencephalon is mediated by changes in responsiveness to Sonic Hedgehog. Development 1998, 125:5079-5089. 46. Gunhaga L, Jessell TM, Edlund T: Sonic hedgehog signaling at gastrula stages specifies ventral telencephalic cells in the chick embryo. Development 2000, 127:3283-3293. 47. Gaiano N, Kohtz JD, Turnbull DH, Fishell G: A method for rapid gain-of-function studies in the mouse embryonic nervous system. Nat Neurosci 1999, 2:812-819. Current Opinion in Neurobiology 2003, 13:50–56
56 Development
48. Corbin JG, Gaiano N, Machold RP, Langston A, Fishell G: The Gsh2 homeodomain gene controls multiple aspects of telencephalic development. Development 2000, 127:5007-5020.
54. LaMantia AS, Colbert MC, Linney E: Retinoic acid induction and regional differentiation prefigure olfactory pathway formation in the mammalian forebrain. Neuron 1993, 10:1035-1048.
49. Chiang C, Litingtung Y, Lee E, Young KE, Corden JL, Westphal H, Beachy PA: Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function. Nature 1996, 383:407-413.
55. Muzio L, DiBenedetto B, Stoykova A, Boncinelli E, Gruss P, Mallamaci A: Conversion of cerebral cortex into basal ganglia in Emx2/ Pax6Sey/Sey double-mutant mice. Nat Neurosci 2002, 15:15. In this study, loss of function for both Pax6 and Emx2 is shown to result in a respecification of the pallium towards subpallial fates typical of the ventral LGE (i.e. striatal specification). Because Emx2 mutants do not show overt patterning defects, these findings reveal a role for Emx2 in the absence of Pax6 gene function.
50. Pabst O, Herbrand H, Takuma N, Arnold HH: NKX2 gene expression in neuroectoderm but not in mesendodermally derived structures depends on sonic hedgehog in mouse embryos. Dev Genes E 2000, 210:47-50. 51. Litingtung Y, Chiang C: Specification of ventral neuron types is mediated by an antagonistic interaction between Shh and Gli3. Nat Neurosci 2000, 3:979-985. 52. Ingham PW, McMahon AP: Hedgehog signaling in animal development: paradigms and principles. Genes Dev 2001, 15:3059-3087. 53. Pierani A, Brenner-Morton S, Chiang C, Jessell TM: A sonic hedgehog-independent, retinoid-activated pathway of neurogenesis in the ventral spinal cord. Cell 1999, 97:903-915.
Current Opinion in Neurobiology 2003, 13:50–56
56. Isshiki T, Pearson B, Holbrook S, Doe CQ: Drosophila neuroblasts sequentially express transcription factors which specify the temporal identity of their neuronal progeny. Cell 2001, 106:511-521. 57. Stenman J, Yu RT, Evans RM, Campbell K: TLX and Pax6 cooperate genetically to establish the pallio-subpallial boundary in the embryonic mouse telencephalon. Development 2003, in press. See update.
www.current-opinion.com