- Email: [email protected]
Control of photoreceptor development
Control of photoreceptor David Harvard
Recent
Altshuler
and Laura Lillien
Medical School, Boston, Massachusetts,
studies of cell type determination
that rod photoreceptor are mediated, this review
are compared
development
in the vertebrate
involves interactions
at least in part, by temporally
the
development
strategies
used to generate
with those described
Introduction The retina is composed of cells which detect, process, and transmit visual information to higher brain centers. In the Drosopbdu compound eye, a single class of cells, termed photoreceptors, serve these functions, while in the vertebrate retina, the functions are divided among several types of cell: rod and cone photoreceptor cells (detection), amacrine, horizontal, and bipolar cells (processing), and ganglion cells (transmission). In both Drosophila and vertebrate retinas, different cell types develop from multipotent progenitor cells [l-7]. This has made the retina a useful model system for studying mechanisms that determine cell identity. In Drosophila, the study of mutations that affect retinal development has led to rapid advances in our understanding of the molecular bases of cell type determination [ 81. Molecular characterization of vertebrate retinal development has lagged behind. Because of difficulties in attempting genetic analysis in vertebrates, studies of vertebrate retinal development have instead relied on direct manipulation of cells. Many authors have suggested that results obtained from the analysis of Drosophika retinal development might fa cilitate the study of the vertebrate retina, particularly in the identification of genes controlling development. There are similarities as well as differences in the structure of tly and vertebrate retinas, and in the ways that these structures arise. Both develop from a seemingly unpatterned monolayer epithelium (Fig. 1). Mitosis occurs at the apical (ventricular) boundary of the retina, and inductive signaling is thought to occur in this region as well. Their development may also differ, however. In the Drosophila retina, cell type determination and pattern formation occur simultaneously, and position within the developing ommatidium is predictive of the type of cell that will be specified. In the vertebrate retina, the relationship between cell type determination and position is not so clear. The mature retina is a complex multilayered structure (Fig. l), and cell type is thought to be
diffusible
suggest ceils that
signals. In
development
retina in the
retina.
in Neurobiology
Opinion
retina among
rods in the vertebrate
for photoreceptor
Drosophila
Current
regulated
USA
1992,
2:1&22
determined before the cells assume their final position. AS the relative positions of cells at the time they become determined have not been identified, the role of positiondependent mechanisms is not known. It is therefore possible that the developmental strategies used to generate the fly and vertebrate retinas may differ, This review will focus on the development of rod photoreceptors, one of the most abundant and best characterized cell types in the vertebrate retina, comparing the strategies used in their development to the mechanisms controlling determination of photoreceptors in the Drosophila compound eye.
Sequential
development
of cell types
In both fly and vertebrate retinas, the generation of different cell types appears to occur in a defined order. In Drosophila, ommatidial precursor cells stop dividing in two waves [9]. The post-mitotic cells differentiate (as monitored using a number of criteria, including expression of neuron-specific antigens [9]) in a characteristic temporal sequence which is related to each cell’s position within the developing ommatidia. This sequence starts with R8 and ends with R7 (Fig. la), and is thought to reflect the order of their induction [9]. The time lag between induction and expression of these markers is not clear, however. There is also a lag between the expression of these early markers and the manifestation of a more fully differentiated phenotype, as measured, for example, by expression of opsin [ 101. In vertebrates, cells also appear to be generated in an ordered sequence, although it is difficult to estimate the time of cellular determination. Three lines of evidence suggest that commitment might occur around the time of a cell’s final mitosis: first, cell fate is correlated with the time during development that a progenitor cell stops dividing (known as its ‘birthday’) [11,12*,13]; second,
Abbreviations boss-bride
of sevenless;
EGkpidermal
growth
factor;
FGF-fibroblast
SOS-son of sevenless; svpseven-up; @
Current
Biology
growth
factor;
TGFu-transforming
Ltd ISSN 09594388
sev-sevenless;
growth
factor
CL.
ha-seven
in absentia;
Control of photoreceptor development
(a)
Drosophila
(b)
Altshuler
and
Lillien
Vertebrate
1
Retina
Photoreceptors% Retina
Outer nuclear layer
Inner nuclear layer
~
Gang2 layer
Fig. 1. A diagram illustrating the cells of the Drosophila and vertebrate retina. (a) The Drosophila eye imaginal disc is a monolayer epithelium which gives rise to the repeated ommatidial units that make up the adult compound eye. Each ommatidium consists of a stereotyped arrangement of eight photoreceptor cells (termed RI-R8), four cone cells (which secrete lens), and eight other accessory cells (not shown). Ommatidia arise in a posterior to anterior gradient across the imaginal disc after passage of the morphogenetic furrow, and appear to develop as independent units. R8 cells differentiate first, followed by R2 and R5 (the cells that lie adjacent to R8), R3 and R4 (the cells adjacent to R2 and R5), RI and R6 (the cells adjacent to R2 and R5), and finally R7. Cone and other accessory cells are generated after photoreceptors. A cell that joins a developing ‘precluster’ at any given stage experiences a characteristic pattern of interactions with the cells already in the precluster, and position within the precluster is predictive of the type of photoreceptor a cell will become. Within the precluster cells are in close contact with each other, particularly at their apical microvilli. Adapted from [9,101. (b) The vertebrate retina arises from a seemingly unpatterned monolayer epithelium into a multi-layered structure with different cell types taking up characteristic positions within the laminae. Development proceeds in a gradient from center to periphery. Photoreceptors occupy the outermost layer of the retina (outer nuclear layer), next to the pigment epithelium. Horizontal, bipolar, Muller glia, and amacrine cells are found in the inner nuclear layer. The innermost layer of the retina, the ganglion cell layer, contains ganglion cells and displaced amacrine cells.
lineage analyses suggest that choice of cell type occurs during or after a cell’s last mitosis [3]; and third, some differentiation markers are expressed on newly-born cells in the apical (ventricular) region of the retina, where mitosis occurs [14,15]. Birthdays can thus be used to estimate the timing of commitment. The birthdays of different retinal cell types occur in sequence, such that some cell types are produced early in retinal development (for example, ganglion cells), and others late (for example, rod photoreceptors [11,12*,13]). The order of birthdays
is generally conserved across widely divergent species (reviewed in [ 16]), although in some organisms, such as the frog, retinal development is so rapid that the birthdays of different cell types almost entirely overlap [ 71. Differentiation markers have been identified for many retinal cell types (for a review, see [17]). Some markers are expressed around the time of cell birth [ 14,151, while in other cases, a variable lag exists between birthdate and expression. For example, more than 2 days pass before opsin can be detected in newly generated rods [l&19].
17
18
Development
Control
of proliferation
The correlation of birthdate with cell type suggests that mechanisms that control proliferation may directly or indirectly influence the determination of cell type. The patterns of cell division in fly and vertebrate retinas appear different, however. In Drosophila, the passage of the morphogenetic furrow (Fig. 1) determines when the cells stop dividing. The progenitors that give rise to photoreceptors R8, R2, R5, R3 and R4 stop dividing at approximately the time that the furrow passes, while presumptive R6, Rl and R7 cells go through an additional round of cell division and differentiate after the first group [9]. Little is known about the mechanisms that control the proliferation of these progenitor cells, though a role for the Drosophila homolog of the epidermal growth factor (EGF) receptor [20] has been suggested [lo]. In addition, it has been proposed that dividing cells may be refractory to inducing signals [lo]. In vertebrates, progenitor cells proliferate extensively early in development, with mitosis decreasing as development proceeds. In lineage mapping experiments performed in the mouse retina, average clone size decreases from 3&52 cells early in development to l-2 cells around the time of birth [3,4]. In principle, the decline in proliferation could be due to changes in the cell’s environment, intrinsic changes in progenitor cells, or both. Heterochronic transplantation studies in the chick suggest that the environment of the early chick retina supports more extensive proliferation than does the environment of older chick retina (D Fekete, personal communication). Experiments in vitro in which old and young rat retinal cells were co-cultured, however, indicate that changes in the environment alone do not control proliferation. These studies have shown that young and old progenitor cells differ in their proliferative responses to the same environment, suggesting that progenitor cells change during development [ 193. Clues about the molecular nature of mitotic signals in the rat retina have come from studies showing that proliferation of rat retinal cells in vitro can be stimulated by sev eral peptide growth factors normally found in the developing retina, including fibroblast growth factors (FGFs), transforming growth factor alpha (TGFa) and EGF (L Lillien and C Cepko, unpublished data) [21]. Consistent with the suggestion that progenitor cells change, the responsiveness of progenitor cells to FGFs and TGFa has also been observed to change during development (L Mien and C Cepko, unpublished data). The changes in progenitor cells, along with changes in environmental signals, may both contribute to the control of proliferation in the retina. Although the correlation between the time at which a cell becomes post-mitotic and its ultimate cell type raises the possibility that the mechanisms controlling these processes may be linked, a recent study suggests that such a linkage may not be obligatory, as some retinal cells have been observed to develop in the apparent absence of cell division [22].
Determination
of cell type by cell-cell
interactions Studies of development in Drosophila and vertebrates indicate that in both retinas cell type is not restricted by cell lineage, implying that it may be determined by interactions among cells. Evidence for such interactions in Drosophila has come from analysis of mutations in which the development of a genetically normal cell is perturbed by neighboring ceils expressing a mutant gene. Two well characterized examples are mutations in the genes bride of sevenless (boss) and rough. Bossfunction is required in R8, but not in any other cell, in order for R7 determination to take place [ 231. Similarly, when R2 and R5 are mutant for the rough gene, normal development of adjacent cells into R3 and R4 cannot take place [24,25*]. These mutations are thought to influence commitment, rather than differentiation or survival, as affected cells become another cell type, rather than remaining undifferentiated or dying. In the case of boss,for example, cells that would become R7 cells, instead become cone cells [26*=]. Arother class of mutants, which include notch, disrupt development in a less specific manner. The perturbation seen in notch mutants suggests that notch function plays a permissive role in many types of cell-cell interactions 1271. Vertebrate homologues of notch are expressed in the developing retina [28*,29=], and it will be interesting to see what role they play in the vertebrate retina.
The importance of cell-cell interactions in the determination of cell fate in the vertebrate retina has been suggested by experiments in which the normal interactions between retinal cells have been disrupted by physically separating cells from each other in vitro. In one such study, this was accomplished by interspersing dissociated neonatal rat retinal cells with either younger retinal cells or other types of cells, in aggregate cultures [ 191.In this situation, differentiation of the postnatal cells into rod photoreceptors was reduced by the presence of the other cell populations. Conversely, the proportion of embryonic retinal cells that differentiated into rods was increased by co-culture with postnatal retinal cells [ 191 (see below). In another study, cell-cell interactions were disrupted by suspending neonatal rat retinal cells in collagen gels at various densities [30**]. Above a threshold cell density, normal levels of rod development occurred. A small reduction (to 25% of original) in cell density, however, completely abolished rod genera tion, without changing cell survival or proliferation. Rod development could be restored by co-culture with gels containing a higher density of cells. In the lower density cultures, which lacked rod development, progenitor cells appeared to adopt an alternative fate, as demonstrated by an increased number of cells expressing a marker characteristic of bipolar cells. An over-representation of bipolar cells is also observed when retinal cells are grown in monolayer cultures that do not support rod development (L Lillien and C Cepko, unpublished data). These results suggest that signals produced by neonatal retinal cells elicit a choice between the rod and bipolar cell fates. In
Control
the chick, the normal determination of cell type appears to be disrupted in sparse monolayer cultures as well, in this case leading to an over-representation of cone photoreceptor cells [31]. The cell types over-represented In these experiments might be induced by o;her emronmental signals, or represent intrinsically determined default pathways.
Nature of the signals Given that cell type determination depends upon ce&cell interactions, the sequential development of different types of retinal cells might reflect temporal changes in either environmental signals, the responsiveness of progenitor cells to these signals, or both. The molecular identification of signals underlying specific cell-cell interactions in retinal development has just begun. In Drosophila, the best characterized ligand-receptor pair is that encoded by the genes bossand sezjenless (sev). Sev is required for R7 development in a ceR-autonomous manner [32], and encodes a tyrosine-kinase receptor [ 33,341. A number of lines of evidence suggest that the ligand for this receptor is the boss gene product. First, bos is required in R8 for an adjacent ommatidial precursor to develop into R7 [23]. Second, the sequence of the bossgene predicts a transmembrane protein with a large extracellular domain, consistent with a role in signaling adjacent cells [26**]. Third, the bossprotein product is localized to R8 cells, and appears significantly before R7 induction takes place [35**]. Fourth, bossencoded protein is internalized into presumptive R7 cells; this internalization is dependent upon sev encoded protein but not sev activity [35-l. Fifth, cells expressing bossaggregate in vitro with those expressing sev, and the adhesion is blocked by antibodies to either bossor set/[35-l. Two observations suggest that the strict localization of sev activation limits where R7 cells form. Ectopic expression of boss (cited in [35-l), or auto-activation of the sez, receptor [36*], both lead to ectopic development of R7 cells. The molecular characterization of bossprovides strong evidence for the model that, in the Drosophila retina, highly localized, temporally regulated inducing signals control the determination of cell type. Not all of the signals that regulate retinal cell type require direct cell contact, however. Scabrous, for example, encodes a secreted protein that is expressed by R8 cells, the first cells to differentiate [ 37**,38]. Scabrous is thought to act by inhibiting other ommatidial precursor cells from developing into R8 cells, thereby helping to establish the spacing pattern of ommatidia [37-e]. In the veiebrate retina, assays have recently become available that allow the analysis of signals that mediate the cell-cell interactions involved in the development of rod photoreceptors. The experiments have shown that production of rod-promoting activity by rat retinal cells in vitro is temporally regulated in a manner that parallels the generation of rods in vivo [19,30*-l. This suggests that control of access to ligand may also play a role in the vertebrate retina in temporally regulating rod production. In addition, the rod-promoting activity appears to be
of photoreceptor
development
Altshuler
and Lillien
diffusible [30**,39-j. It has not been further characterized, nor have its cellular sources been identified, though the activity does not appear to be produced by several non-retinal cell types [19,30**,3P*]. Thus, unlike boss, which is membrane bound, the vertebrate rod-promoting activity can act in vitro without direct cell contact. It is possible that in vivo, rod promoting signals are localized by other mechanisms, such as association with the extracellular matrix. Given the high proportion of rods in the rodent retina ( -70% of retinal cells), however, it may be advantageous to have a mechanism whereby a single cell can induce multiple rods without having to contact all of ., them directly.
Regulation
of responsivenes$,.
-‘,
The studies discussed above suggest that the development of particular cell types is regulated by the availability of inductive or inhibitoty signals. In addition, the expression of either receptors for these signals, or components of the signal transduction pathway, could also be regu lated. Several components required for transduction of R7-inducing signals have been identified. Seven in absentia (sinu) [40-l and son of sevenless (sos) [41**] are both required for R7 development in a cell-autonomous manner, and are thought to act downstream of sev. Sev and sina are known to be expressed in many ommatidial precursor cells, and not only in presumptive R7 cells [40-,42,43]. Moreover, ectopic expression of sev does not result in the production of additional R7 cells [ 44,451. The expression of sev and sina in multiple ommatidial precursor cells may be a reflection of their initial multipotency [ 101. Given that multiple cells both express sev and contact boss-expressing R8 cells, why does only one cell become R7? One reason appears to be that some of the other cells express a gene product that blocks their development into R7. This gene, seven-up (sup), encodes a steroid receptor-like protein [46]. In the ‘sup loss of function’ mutants, R3, R4, Rl and R6 can be transformed into additional R7 cells [46]. One interpretation that has been suggested is that sup is required for determination of R3 and R4, and that when this pathway is defective, the cells can respond to R7 induction [46]. In any event, it appears that a combination of inducing signals (boss) and inhibitory influences (sup) uniquely specify R7. The receptors and signal transduction machinery that mediate rod-promoting signals in vertebrates have not yet been identified. There is evidence, however, that the timing of responsiveness to rod-promoting signals may be regulated in the vertebrate retina. It has not been possible in vitro to induce embryonic rat retinal cells to develop into rods prematurely, despite co-culture with older retinal cells, which both produced and responded to rod promoting signals [ 191. This result could reflect a lack of responsiveness to rod-promoting signals in the younger cells, but other, more indirect, mechanisms could also be responsible. For example, it is possible that cells must become postmitotic in order to respond to inducing sig-
19
20
Development
nals; the younger (unresponsive) ceils in these cultures divide extensively, while the older (responsive) cells divide very little. Lack of rod development could therefore also be explained as an indirect consequence of continued proliferation. Regardless of the cause, however, these in vitro studies suggest that both the timed production of an inductive signal, and temporal changes in responsiveness to that signal, may contribute to the temporal regulation of rod development.
Regulation
of differentiation
and pattern
formation We have focused on mechanisms that control photoreceptor determination. In Drosophila, mutations have been identified that alfect other steps in photoreceptor development, such as the expression of photoreceptor specific genes and cell survival (glass> [47,48*,49], formation of appropriate morphology (chuoptic) [50,51 I, and synaptic connections to other cells (disco) [52]. As described above, pattern formation depends upon scabrous function for the proper spacing of ommatidia [ 37**,38]. In vertebrates, it has been proposed that spacing of cells in the tangential plane (parallel to the layers) may involve functions like those of scubrous. Recent studies in the fish [ 531 and monkey [54], for example, indicate that subpopulations of cone photoreceptors are regularly spaced across the retina before most other cells are generated, leading the authors to speculate that this early pattern may be established by lateral inhibitory mechanisms, and may influence subsequent patterning or cell type of neighboring retinal cells. Development of the retina’s laminar organization may reflect an interaction with the pigment epithelium, which apposes the outer surface of the retina in vivo [ 551. The pigment epithelium may also play a role in supporting the survival of photoreceptors, as shown by studies of mutant rats in which photoreceptors degenerate several weeks after birth [56]. Photoreceptor survival can be restored in these animals by treatment with basic FGF [ 57.1, though a role for FGF as a survival factor for photoreceptors in the normal retina has not yet been established.
have been implicated in the generation of subpopulations of amacrine cells [ 58,591. As the genes involved in vertebrate retinal development become available, it will be possible to directly compare the molecular mechanisms controlling cell type in these two types of retina.
Acknowledgements We would like to thank C Cepko, D Fekete, M Raff, L Ryder and C Tabin for helpful comments on the manuscript, and D Fekete, T Watanabe and M RaE for sharing data prior to publication. We are especially grateful to C Cepko for encouragement and many useful discussions.
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KRAMERH, CAGANRL, ZIPLJR~KV SL: Interaction of bride
of sew enless Membrane-Bound Ligand and the sevenless TyrosineKInase Receptor. Nature 1991, 352:207-212. Antibodies to harr encoded protein are used to show that if is localized to R8 and expressed prior to R7 induction. Bare- encoded protein is demonstrated to be internalized into a large vesicle in R7, a process dependent on seu. Cells expressing boss are shown to heterotypically aggregate with cells expressing set,. This is blocked by antibodies to either protein. These data provide strong aidence that boss encodes the ligand for sev, and thereby induces the production of R7 cells. 35. ..
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36. .
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Developing Rat Retina. Developmenf 1992, 114: in press. This group had previously shown that neonatal retinal cells could stimulate the development of rod photoreceptors in vitro. In this report they ..
21
22
DeveioDment demonstrate that the rod-promoting activity can operate without direct cell-cell contact, and is not produced by a number of other cell types. In addition, the activity is shown fo affect rod development somewhat specifically, in that it does nor increase the expression of a marker for one other type of retinal cell. 40. ..
CAI~THEW RW, RUBY GM: Seven in absentia, a Gene Required for Specification of R7 CeU Fate in the Drosophila Eye. Cell 1990, 63~571-577. An extremely complete paper that identifies mutations in, and cloning of, seven in absentia &ka), a gene required ceU autonomously for R7 development. Sina encoded protein is found in many cells in each ommatidia, as is sev. Sinu is localized to the nucleus, suggesting a role in gene regulation. 41. ..
ROCCE RD, KARU&XH CA, BANERJEEU: Genetic Dissection of a Neurodevelopmental Pathway: son of sevenless Functions Downstream of the sevenless and EGF Receptor Tyrosine Kinases. Cell 1991, 64:3+48. Describes a genetic locus, termed son of sevenless (sos), identified as an allele-specific suppressor of a sev mutation. Sos is required in a cell-autonomous manner for suppression of sev, and is also shown to interact with boss and the Drosophila EGF receptor. Loss of function alleles of square recessive lethals. These data suggest that sosis required for signal transduction through multiple pathways. 42.
BANERJEEU, RENFRANZ PJ, HINTONDR, RABINBA, BENZER S: The Sevenless+ Protein is Expressed ApicaIIy in CeII Membranes of Developing Drosophila Retina; it is not Restricted to CeU’R7. Cell 1987, 51:151-158.
43.
TOMLINSON A, Bownzu DDL, HAFEN E, RUBINGM: Localization of the sevenless Protein, a Putative Receptor for Positional Information, in the Eye ImaginaI Disk of Drosophila. Cell 1987, 51:143-150.
44.
BILLER K, HAFIXN E: Ubiquitous Position-Dependent Specification 243:931-933.
45.
BOWIFU DDL, SIMON MA, RUIN GM: Ommatidia in the Developing Drosophila Eye Require and can Respond to seu enless for Only a Restricted Period. Cell 1989, 56931-936.
46.
MLODZIKM, HIROMI Y, WEBER U, G~~D~IAN CS, RUBIN GM: The Drosophila seven-up Gene, a Member of the Steroid Receptor Gene Superfamily, Fates. CelI 1990, 60:211-224.
47.
Expression of sevenless of CeU Fate. Science 1989,
Controls
Photoreceptor
49.
MOSES K, RIJBIN GM: Glass Encodes a Site-Spectic DNABinding Protein that is Regulated in Response to Positional Signals in the Developing Drosophila Eye. Genes Dev 1991, 5:583-593.
50.
REINKER, KRANTZDE, YEN D, ZI~UR~KVSL: Chaoptin, a CeU Surface Glycoprotein Required for Drosophila Photoreceptor CeU Morphogenesis, Contains a Repeat Motif Found in Yeast and Human. Cell 1988, 52:291-301.
51.
VAN VACTOR D, KRANTz DE, REINKE R, ZIPURSKV SL: Analy sis of Mutants in Chaoptin, a Photoreceptor Cell-Specific Glycoprotein in Drosophila, Reveals its Role in CeUuIar Morphogenesis. Cell 1988, 52:281-230.
52.
SELLER H, FISCHBACH K-F, RUBINGM: Disconnected a Locus Required for Neuronal Pathway Formation in the Visual System of Drosophila. Cell 1987, 50:113F1153.
53.
IAIUSONKD, BREM~LLER R: Early Onset of Phenotype and CeU Patterning in the Embryonic Zebra&h Retina Devefqtmzent 1990, 109~567-576.
54.
WIKLERKC, RAKlc P: Relation of an Array of Early-Differentiating Cones to the Photoreceptor Mosaic in the Primate Retina. Nature 1991, 351:397400.
55.
VOLLMER G, IAYERPG: An In Vitro Model of Proliferation and Differentiation of the Chick Retina: Coaggregates of Retinal and Pigment EpitheIiaI Cells. J Neurasci 1986, 6:188>1896.
56.
SHEEDLOWHJ, GAUR V, Lr L, SEATONAD, TURNERJE: Transplantation to the Diseased and Damaged Retina. Trends Neurosci 1991, 14:347-350.
FAKTOROVICH E, STEINBERGR, Y~~UM~JRA D, MATTESMT, ~AVA~L MM: Photoreceptor Degeneration in Inherited Retinal Dystrophy Delayed by Basic Fibroblast Growth Factor. Nature 1990, 34783-86. This study describes the ability of basic FGF fo restore photoreceptor survival in the RCS rat, a mutant in which photoreceptors degenerate due to a defect in the pigment epithelium. The authors discuss possible mechanisms whereby FGF treatment could compensate for this mutation. 57. .
58.
NEGISHIK, TERANISHIT, KATO S, NAKAMURA Y: Paradoxical Induction of Dopaminergic Cells Following Intravitreal Injection of High Doses of 6-Hydroxydopamine in Juvenile Carp Retina. Dev Brain Res 1987, 33~67-79.
59.
REH TA, TUUY T: Regulation of Tyrosine Hydroxylase-Conmining Amacrine CeU Number in LarvaI Frog Retina. Dev Biol 1986, 114:46%469.
CeU
MOSES K, ELUS MC, RUBIN GM: The Glass Gene Encodes a Zinc-Finger Protein Required by Drosophila Photoreceptor CeUs. Nature 1989, 340:531-536.
48. MOSES K: The Role of Transcription Factors in ,the Devel. oping Drosophila Eye. Trend Neurosci 1991, 7:25G255. An up-to-date review of four genes (rough glass, sev and sinu) that are thought to regulate Dmqbila retina development as transcription factors.
D Al&huler and L Lillien, Department of Genetics, Harvard Medical School, 25 Shattuck Street, Boston, Massachusetts 02115, USA
Recent
Altshuler
and Laura Lillien
Medical School, Boston, Massachusetts,
studies of cell type determination
that rod photoreceptor are mediated, this review
are compared
development
in the vertebrate
involves interactions
at least in part, by temporally
the
development
strategies
used to generate
with those described
Introduction The retina is composed of cells which detect, process, and transmit visual information to higher brain centers. In the Drosopbdu compound eye, a single class of cells, termed photoreceptors, serve these functions, while in the vertebrate retina, the functions are divided among several types of cell: rod and cone photoreceptor cells (detection), amacrine, horizontal, and bipolar cells (processing), and ganglion cells (transmission). In both Drosophila and vertebrate retinas, different cell types develop from multipotent progenitor cells [l-7]. This has made the retina a useful model system for studying mechanisms that determine cell identity. In Drosophila, the study of mutations that affect retinal development has led to rapid advances in our understanding of the molecular bases of cell type determination [ 81. Molecular characterization of vertebrate retinal development has lagged behind. Because of difficulties in attempting genetic analysis in vertebrates, studies of vertebrate retinal development have instead relied on direct manipulation of cells. Many authors have suggested that results obtained from the analysis of Drosophika retinal development might fa cilitate the study of the vertebrate retina, particularly in the identification of genes controlling development. There are similarities as well as differences in the structure of tly and vertebrate retinas, and in the ways that these structures arise. Both develop from a seemingly unpatterned monolayer epithelium (Fig. 1). Mitosis occurs at the apical (ventricular) boundary of the retina, and inductive signaling is thought to occur in this region as well. Their development may also differ, however. In the Drosophila retina, cell type determination and pattern formation occur simultaneously, and position within the developing ommatidium is predictive of the type of cell that will be specified. In the vertebrate retina, the relationship between cell type determination and position is not so clear. The mature retina is a complex multilayered structure (Fig. l), and cell type is thought to be
diffusible
suggest ceils that
signals. In
development
retina in the
retina.
in Neurobiology
Opinion
retina among
rods in the vertebrate
for photoreceptor
Drosophila
Current
regulated
USA
1992,
2:1&22
determined before the cells assume their final position. AS the relative positions of cells at the time they become determined have not been identified, the role of positiondependent mechanisms is not known. It is therefore possible that the developmental strategies used to generate the fly and vertebrate retinas may differ, This review will focus on the development of rod photoreceptors, one of the most abundant and best characterized cell types in the vertebrate retina, comparing the strategies used in their development to the mechanisms controlling determination of photoreceptors in the Drosophila compound eye.
Sequential
development
of cell types
In both fly and vertebrate retinas, the generation of different cell types appears to occur in a defined order. In Drosophila, ommatidial precursor cells stop dividing in two waves [9]. The post-mitotic cells differentiate (as monitored using a number of criteria, including expression of neuron-specific antigens [9]) in a characteristic temporal sequence which is related to each cell’s position within the developing ommatidia. This sequence starts with R8 and ends with R7 (Fig. la), and is thought to reflect the order of their induction [9]. The time lag between induction and expression of these markers is not clear, however. There is also a lag between the expression of these early markers and the manifestation of a more fully differentiated phenotype, as measured, for example, by expression of opsin [ 101. In vertebrates, cells also appear to be generated in an ordered sequence, although it is difficult to estimate the time of cellular determination. Three lines of evidence suggest that commitment might occur around the time of a cell’s final mitosis: first, cell fate is correlated with the time during development that a progenitor cell stops dividing (known as its ‘birthday’) [11,12*,13]; second,
Abbreviations boss-bride
of sevenless;
EGkpidermal
growth
factor;
FGF-fibroblast
SOS-son of sevenless; svpseven-up; @
Current
Biology
growth
factor;
TGFu-transforming
Ltd ISSN 09594388
sev-sevenless;
growth
factor
CL.
ha-seven
in absentia;
Control of photoreceptor development
(a)
Drosophila
(b)
Altshuler
and
Lillien
Vertebrate
1
Retina
Photoreceptors% Retina
Outer nuclear layer
Inner nuclear layer
~
Gang2 layer
Fig. 1. A diagram illustrating the cells of the Drosophila and vertebrate retina. (a) The Drosophila eye imaginal disc is a monolayer epithelium which gives rise to the repeated ommatidial units that make up the adult compound eye. Each ommatidium consists of a stereotyped arrangement of eight photoreceptor cells (termed RI-R8), four cone cells (which secrete lens), and eight other accessory cells (not shown). Ommatidia arise in a posterior to anterior gradient across the imaginal disc after passage of the morphogenetic furrow, and appear to develop as independent units. R8 cells differentiate first, followed by R2 and R5 (the cells that lie adjacent to R8), R3 and R4 (the cells adjacent to R2 and R5), RI and R6 (the cells adjacent to R2 and R5), and finally R7. Cone and other accessory cells are generated after photoreceptors. A cell that joins a developing ‘precluster’ at any given stage experiences a characteristic pattern of interactions with the cells already in the precluster, and position within the precluster is predictive of the type of photoreceptor a cell will become. Within the precluster cells are in close contact with each other, particularly at their apical microvilli. Adapted from [9,101. (b) The vertebrate retina arises from a seemingly unpatterned monolayer epithelium into a multi-layered structure with different cell types taking up characteristic positions within the laminae. Development proceeds in a gradient from center to periphery. Photoreceptors occupy the outermost layer of the retina (outer nuclear layer), next to the pigment epithelium. Horizontal, bipolar, Muller glia, and amacrine cells are found in the inner nuclear layer. The innermost layer of the retina, the ganglion cell layer, contains ganglion cells and displaced amacrine cells.
lineage analyses suggest that choice of cell type occurs during or after a cell’s last mitosis [3]; and third, some differentiation markers are expressed on newly-born cells in the apical (ventricular) region of the retina, where mitosis occurs [14,15]. Birthdays can thus be used to estimate the timing of commitment. The birthdays of different retinal cell types occur in sequence, such that some cell types are produced early in retinal development (for example, ganglion cells), and others late (for example, rod photoreceptors [11,12*,13]). The order of birthdays
is generally conserved across widely divergent species (reviewed in [ 16]), although in some organisms, such as the frog, retinal development is so rapid that the birthdays of different cell types almost entirely overlap [ 71. Differentiation markers have been identified for many retinal cell types (for a review, see [17]). Some markers are expressed around the time of cell birth [ 14,151, while in other cases, a variable lag exists between birthdate and expression. For example, more than 2 days pass before opsin can be detected in newly generated rods [l&19].
17
18
Development
Control
of proliferation
The correlation of birthdate with cell type suggests that mechanisms that control proliferation may directly or indirectly influence the determination of cell type. The patterns of cell division in fly and vertebrate retinas appear different, however. In Drosophila, the passage of the morphogenetic furrow (Fig. 1) determines when the cells stop dividing. The progenitors that give rise to photoreceptors R8, R2, R5, R3 and R4 stop dividing at approximately the time that the furrow passes, while presumptive R6, Rl and R7 cells go through an additional round of cell division and differentiate after the first group [9]. Little is known about the mechanisms that control the proliferation of these progenitor cells, though a role for the Drosophila homolog of the epidermal growth factor (EGF) receptor [20] has been suggested [lo]. In addition, it has been proposed that dividing cells may be refractory to inducing signals [lo]. In vertebrates, progenitor cells proliferate extensively early in development, with mitosis decreasing as development proceeds. In lineage mapping experiments performed in the mouse retina, average clone size decreases from 3&52 cells early in development to l-2 cells around the time of birth [3,4]. In principle, the decline in proliferation could be due to changes in the cell’s environment, intrinsic changes in progenitor cells, or both. Heterochronic transplantation studies in the chick suggest that the environment of the early chick retina supports more extensive proliferation than does the environment of older chick retina (D Fekete, personal communication). Experiments in vitro in which old and young rat retinal cells were co-cultured, however, indicate that changes in the environment alone do not control proliferation. These studies have shown that young and old progenitor cells differ in their proliferative responses to the same environment, suggesting that progenitor cells change during development [ 193. Clues about the molecular nature of mitotic signals in the rat retina have come from studies showing that proliferation of rat retinal cells in vitro can be stimulated by sev eral peptide growth factors normally found in the developing retina, including fibroblast growth factors (FGFs), transforming growth factor alpha (TGFa) and EGF (L Lillien and C Cepko, unpublished data) [21]. Consistent with the suggestion that progenitor cells change, the responsiveness of progenitor cells to FGFs and TGFa has also been observed to change during development (L Mien and C Cepko, unpublished data). The changes in progenitor cells, along with changes in environmental signals, may both contribute to the control of proliferation in the retina. Although the correlation between the time at which a cell becomes post-mitotic and its ultimate cell type raises the possibility that the mechanisms controlling these processes may be linked, a recent study suggests that such a linkage may not be obligatory, as some retinal cells have been observed to develop in the apparent absence of cell division [22].
Determination
of cell type by cell-cell
interactions Studies of development in Drosophila and vertebrates indicate that in both retinas cell type is not restricted by cell lineage, implying that it may be determined by interactions among cells. Evidence for such interactions in Drosophila has come from analysis of mutations in which the development of a genetically normal cell is perturbed by neighboring ceils expressing a mutant gene. Two well characterized examples are mutations in the genes bride of sevenless (boss) and rough. Bossfunction is required in R8, but not in any other cell, in order for R7 determination to take place [ 231. Similarly, when R2 and R5 are mutant for the rough gene, normal development of adjacent cells into R3 and R4 cannot take place [24,25*]. These mutations are thought to influence commitment, rather than differentiation or survival, as affected cells become another cell type, rather than remaining undifferentiated or dying. In the case of boss,for example, cells that would become R7 cells, instead become cone cells [26*=]. Arother class of mutants, which include notch, disrupt development in a less specific manner. The perturbation seen in notch mutants suggests that notch function plays a permissive role in many types of cell-cell interactions 1271. Vertebrate homologues of notch are expressed in the developing retina [28*,29=], and it will be interesting to see what role they play in the vertebrate retina.
The importance of cell-cell interactions in the determination of cell fate in the vertebrate retina has been suggested by experiments in which the normal interactions between retinal cells have been disrupted by physically separating cells from each other in vitro. In one such study, this was accomplished by interspersing dissociated neonatal rat retinal cells with either younger retinal cells or other types of cells, in aggregate cultures [ 191.In this situation, differentiation of the postnatal cells into rod photoreceptors was reduced by the presence of the other cell populations. Conversely, the proportion of embryonic retinal cells that differentiated into rods was increased by co-culture with postnatal retinal cells [ 191 (see below). In another study, cell-cell interactions were disrupted by suspending neonatal rat retinal cells in collagen gels at various densities [30**]. Above a threshold cell density, normal levels of rod development occurred. A small reduction (to 25% of original) in cell density, however, completely abolished rod genera tion, without changing cell survival or proliferation. Rod development could be restored by co-culture with gels containing a higher density of cells. In the lower density cultures, which lacked rod development, progenitor cells appeared to adopt an alternative fate, as demonstrated by an increased number of cells expressing a marker characteristic of bipolar cells. An over-representation of bipolar cells is also observed when retinal cells are grown in monolayer cultures that do not support rod development (L Lillien and C Cepko, unpublished data). These results suggest that signals produced by neonatal retinal cells elicit a choice between the rod and bipolar cell fates. In
Control
the chick, the normal determination of cell type appears to be disrupted in sparse monolayer cultures as well, in this case leading to an over-representation of cone photoreceptor cells [31]. The cell types over-represented In these experiments might be induced by o;her emronmental signals, or represent intrinsically determined default pathways.
Nature of the signals Given that cell type determination depends upon ce&cell interactions, the sequential development of different types of retinal cells might reflect temporal changes in either environmental signals, the responsiveness of progenitor cells to these signals, or both. The molecular identification of signals underlying specific cell-cell interactions in retinal development has just begun. In Drosophila, the best characterized ligand-receptor pair is that encoded by the genes bossand sezjenless (sev). Sev is required for R7 development in a ceR-autonomous manner [32], and encodes a tyrosine-kinase receptor [ 33,341. A number of lines of evidence suggest that the ligand for this receptor is the boss gene product. First, bos is required in R8 for an adjacent ommatidial precursor to develop into R7 [23]. Second, the sequence of the bossgene predicts a transmembrane protein with a large extracellular domain, consistent with a role in signaling adjacent cells [26**]. Third, the bossprotein product is localized to R8 cells, and appears significantly before R7 induction takes place [35**]. Fourth, bossencoded protein is internalized into presumptive R7 cells; this internalization is dependent upon sev encoded protein but not sev activity [35-l. Fifth, cells expressing bossaggregate in vitro with those expressing sev, and the adhesion is blocked by antibodies to either bossor set/[35-l. Two observations suggest that the strict localization of sev activation limits where R7 cells form. Ectopic expression of boss (cited in [35-l), or auto-activation of the sez, receptor [36*], both lead to ectopic development of R7 cells. The molecular characterization of bossprovides strong evidence for the model that, in the Drosophila retina, highly localized, temporally regulated inducing signals control the determination of cell type. Not all of the signals that regulate retinal cell type require direct cell contact, however. Scabrous, for example, encodes a secreted protein that is expressed by R8 cells, the first cells to differentiate [ 37**,38]. Scabrous is thought to act by inhibiting other ommatidial precursor cells from developing into R8 cells, thereby helping to establish the spacing pattern of ommatidia [37-e]. In the veiebrate retina, assays have recently become available that allow the analysis of signals that mediate the cell-cell interactions involved in the development of rod photoreceptors. The experiments have shown that production of rod-promoting activity by rat retinal cells in vitro is temporally regulated in a manner that parallels the generation of rods in vivo [19,30*-l. This suggests that control of access to ligand may also play a role in the vertebrate retina in temporally regulating rod production. In addition, the rod-promoting activity appears to be
of photoreceptor
development
Altshuler
and Lillien
diffusible [30**,39-j. It has not been further characterized, nor have its cellular sources been identified, though the activity does not appear to be produced by several non-retinal cell types [19,30**,3P*]. Thus, unlike boss, which is membrane bound, the vertebrate rod-promoting activity can act in vitro without direct cell contact. It is possible that in vivo, rod promoting signals are localized by other mechanisms, such as association with the extracellular matrix. Given the high proportion of rods in the rodent retina ( -70% of retinal cells), however, it may be advantageous to have a mechanism whereby a single cell can induce multiple rods without having to contact all of ., them directly.
Regulation
of responsivenes$,.
-‘,
The studies discussed above suggest that the development of particular cell types is regulated by the availability of inductive or inhibitoty signals. In addition, the expression of either receptors for these signals, or components of the signal transduction pathway, could also be regu lated. Several components required for transduction of R7-inducing signals have been identified. Seven in absentia (sinu) [40-l and son of sevenless (sos) [41**] are both required for R7 development in a cell-autonomous manner, and are thought to act downstream of sev. Sev and sina are known to be expressed in many ommatidial precursor cells, and not only in presumptive R7 cells [40-,42,43]. Moreover, ectopic expression of sev does not result in the production of additional R7 cells [ 44,451. The expression of sev and sina in multiple ommatidial precursor cells may be a reflection of their initial multipotency [ 101. Given that multiple cells both express sev and contact boss-expressing R8 cells, why does only one cell become R7? One reason appears to be that some of the other cells express a gene product that blocks their development into R7. This gene, seven-up (sup), encodes a steroid receptor-like protein [46]. In the ‘sup loss of function’ mutants, R3, R4, Rl and R6 can be transformed into additional R7 cells [46]. One interpretation that has been suggested is that sup is required for determination of R3 and R4, and that when this pathway is defective, the cells can respond to R7 induction [46]. In any event, it appears that a combination of inducing signals (boss) and inhibitory influences (sup) uniquely specify R7. The receptors and signal transduction machinery that mediate rod-promoting signals in vertebrates have not yet been identified. There is evidence, however, that the timing of responsiveness to rod-promoting signals may be regulated in the vertebrate retina. It has not been possible in vitro to induce embryonic rat retinal cells to develop into rods prematurely, despite co-culture with older retinal cells, which both produced and responded to rod promoting signals [ 191. This result could reflect a lack of responsiveness to rod-promoting signals in the younger cells, but other, more indirect, mechanisms could also be responsible. For example, it is possible that cells must become postmitotic in order to respond to inducing sig-
19
20
Development
nals; the younger (unresponsive) ceils in these cultures divide extensively, while the older (responsive) cells divide very little. Lack of rod development could therefore also be explained as an indirect consequence of continued proliferation. Regardless of the cause, however, these in vitro studies suggest that both the timed production of an inductive signal, and temporal changes in responsiveness to that signal, may contribute to the temporal regulation of rod development.
Regulation
of differentiation
and pattern
formation We have focused on mechanisms that control photoreceptor determination. In Drosophila, mutations have been identified that alfect other steps in photoreceptor development, such as the expression of photoreceptor specific genes and cell survival (glass> [47,48*,49], formation of appropriate morphology (chuoptic) [50,51 I, and synaptic connections to other cells (disco) [52]. As described above, pattern formation depends upon scabrous function for the proper spacing of ommatidia [ 37**,38]. In vertebrates, it has been proposed that spacing of cells in the tangential plane (parallel to the layers) may involve functions like those of scubrous. Recent studies in the fish [ 531 and monkey [54], for example, indicate that subpopulations of cone photoreceptors are regularly spaced across the retina before most other cells are generated, leading the authors to speculate that this early pattern may be established by lateral inhibitory mechanisms, and may influence subsequent patterning or cell type of neighboring retinal cells. Development of the retina’s laminar organization may reflect an interaction with the pigment epithelium, which apposes the outer surface of the retina in vivo [ 551. The pigment epithelium may also play a role in supporting the survival of photoreceptors, as shown by studies of mutant rats in which photoreceptors degenerate several weeks after birth [56]. Photoreceptor survival can be restored in these animals by treatment with basic FGF [ 57.1, though a role for FGF as a survival factor for photoreceptors in the normal retina has not yet been established.
have been implicated in the generation of subpopulations of amacrine cells [ 58,591. As the genes involved in vertebrate retinal development become available, it will be possible to directly compare the molecular mechanisms controlling cell type in these two types of retina.
Acknowledgements We would like to thank C Cepko, D Fekete, M Raff, L Ryder and C Tabin for helpful comments on the manuscript, and D Fekete, T Watanabe and M RaE for sharing data prior to publication. We are especially grateful to C Cepko for encouragement and many useful discussions.
References
and recommended
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TOMLINSONA, READY DF: Neuronal
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Conclusion Genetic and molecular studies of Drosophila retinal development have shown that a cell’s position within the developing ommatidium specifies its cell type through the action of highly localized, contact-mediated interactions. In vertebrates, cell-type specification is thought to occur before the final pattern is established, and in the case of rods, to involve inducing signals that can act without direct cell contact. It is not unlikely that strategies analogous to those employed in Drosophila will be found in the vertebrate retina. Little is known at present about the role of cell-cell interactions in the specification of other vertebrate retinal cell types, though cellular interactions
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BANERJEE U, ZIP~JRSKY SL: The Role of CeE-CeE Interaction in
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REINKE R, ZIPURSKY SL: CeU-CeU Interaction in the Drosophila Retina: the bride of sevenless Gene is Required in Photoreceptor CeU R8 for R7 Cell Development. Cell 1988, 55:321330.
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a Drosophila Homeobox Gene Required in Photoreceptors R2 and R5 for Inductive Interactions iu the Developing Eye. Cell 1988, 55:771-784
TOMUN~~N A, KIMMEL BE, RUBIN GM: Rough,
HEBERLEINU, MLODZIK M, RLIBIN GM: Cell-Fate Determination in the Developing Drosophila Eye: Role of the Rough Gene. Development 1991, 112:70%712. Provides evidence that R2 and R5 cells fail to develop normally in rough mutants and are instead transformed into cells of the R3, R4, Rl or R6 type. This is demonstrated by expression of a svplacZinsertion, and by their transformation into R7-like cells in svp-/roughdouble mutant ommatidia. It is suggested that because of this transformation, R2 and R5 fail to signal R3 and R4 to develop. The study also provides evidence that rough mutant ommatidia develop extra R8 and R7 cells, suggesting a ‘complex network of cellular interactions’.
25. .
26.
HART AC, KRAMERH, VANVACT’OR DLJ, PAIDHLINGAT M, ZIP~JK~KV
SL: Induction of CeU Fate In the Drosophila Retina: the Bride of Sevenless Protein is Predicted to Contain a Large Extracellular Domain and Seven Transmembrane Segments. Genes Dev 1990, 4:1835-1847. Describes the cloning of the bm gene and a molecular analysis of mutant alleles. Boss encoded protein is thought to be localized to the cell membrane, with a large extracellular domain and seven putative transmembrane regions. Two models are presented, one in which boss encodes the ligand for sev, the other in which boss acts as a receptor for a signal that is indirectly required for sev activation. ..
27.
CAGAN RL, READY DF: No&h is Required for Successive CeU Decisions in the Developing Drosophila Retina. Gerzes Dev 1989, 3:109+1112.
COFFMANC, HARRISW, KINTER C: Xotch, the Xenopus Homo28. log of Drosophila notch. Science 1990, 249:143%1441. . First report of a vertebrate homolog to notch, cloned from Xenopus Like not&, it contains EGF-like repeats and a sequence homologous to cdcl0. It is shown to be expressed in the marginal zone of the retina, the region in which new cells are being generated, suggesting a possible role in proliferation and/or ceU type decisions.
of photoreceptor
development
Altshuler
and Mien
WEINMASTER G, ROBERTS VJ, LEMKE G: A Homolog of Drosophila notch Expressed During Mammalian Development. DeuelqOment 1991, 113:199-205. Reports the cloning of a rat homolog to the no&b gene and describes its distribution during development. In particular, it is shown fo be expressed throughout the embryonic retina, consistent with a role in proliferation, determination or differentiation.
29. .
AUSHULER D, CEPKO C: A Temporally Regulated, Diffusible Activity is Required for Rod Photoreceptor Development In Vitro. Development 1992, 114: in press. Rod photoreceptor development is shown fo be density dependent in vitro, indicating that this process requires the action of rod-promoting signals. Evidence is presented that the signal(s) influences the choice of progenitor cells between the rod and bipolar ceU types, rather than affecting differentiation or survival of photoreceptors. The signal(s) is shown to be diffusible in vitro, and to be temporally regulated in parallel to rod production in viva.
30. ..
31.
32.
ADLER R, HATLEE M: Plasticity and Differentiation of Embryonic Retinal Cells After Terminal Mitosis. Science 1989, 243:391-393. _,. . .
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33.
a CeUSpecific Homeotic Gene of Drosophila, Encodes a Putative Transmembrane Receptor with a Tyrosine Kinase Domain. Science 1987, 236:5563.
34.
BANERJEEU, RENFRANZPJ, POLL~CK JA, BENZER S: Molecular Characterization and Expression of sevenless, a Gene InvoIved in Neural Pattern Formation in the Drosophila Eye. Cell 1987, 49:281-291.
HAFEN E, BASLERK, EDSTROMJE, RUBIN GM: Sevenless,
KRAMERH, CAGANRL, ZIPLJR~KV SL: Interaction of bride
of sew enless Membrane-Bound Ligand and the sevenless TyrosineKInase Receptor. Nature 1991, 352:207-212. Antibodies to harr encoded protein are used to show that if is localized to R8 and expressed prior to R7 induction. Bare- encoded protein is demonstrated to be internalized into a large vesicle in R7, a process dependent on seu. Cells expressing boss are shown to heterotypically aggregate with cells expressing set,. This is blocked by antibodies to either protein. These data provide strong aidence that boss encodes the ligand for sev, and thereby induces the production of R7 cells. 35. ..
BA.SLERK, CHRISTENB, HAFEN E: Ligand-Independent Activation of the sevenless Receptor Tyrosine KInase Changes the Fate of Cells in the Developing Drosophila Eye. Cell 1991, 64106~1081. Demonstrates that over-expression of a truncated, constitutively active setrenless receptor leads to formation of exfra R7 cells. The development of supernumerary R7 cells is shown to be independent of boss function. All cells expressing this sev encoded receptor initiate neural development, transforming mystery cells and presumptive cone cells into neurons. The authors argue that R7 determination depends upon the localized activation of sev, without the need for additional signals from Rl and R6.
36. .
BAKER NE, MLO~ZIK M, KUBIN GM: Spacing DitTerentiation in the Developing Drosophila Eye: a Fibrinogen-Related Lateral Inhibitor Encoded by Scabrous. Science 1990, 250:137&1377. Describes the cloning of scabrous which encodes a protein homologous to fibrinogen that is thought to be secreted. Scubrotls is required in R8, and is thought to act as a lateral inhibitor that regulates the spacing and determination of R8 cells, and thereby the spacing of ommatidia. The roles of notch and the Drosophila EGF receptor in these processes are also described. 37. ..
38.
MLODZIKM, BAKERNE, R~JBINGM: Isolation and Expression of scabrous, a Gene Regulating Neurogenesis in Drosophila Genes Deu 1990, 4:184%1861.
39.
WATAN~E T, RAFFM: Diffusible Rod-Promoting Signals in the
Developing Rat Retina. Developmenf 1992, 114: in press. This group had previously shown that neonatal retinal cells could stimulate the development of rod photoreceptors in vitro. In this report they ..
21
22
DeveioDment demonstrate that the rod-promoting activity can operate without direct cell-cell contact, and is not produced by a number of other cell types. In addition, the activity is shown fo affect rod development somewhat specifically, in that it does nor increase the expression of a marker for one other type of retinal cell. 40. ..
CAI~THEW RW, RUBY GM: Seven in absentia, a Gene Required for Specification of R7 CeU Fate in the Drosophila Eye. Cell 1990, 63~571-577. An extremely complete paper that identifies mutations in, and cloning of, seven in absentia &ka), a gene required ceU autonomously for R7 development. Sina encoded protein is found in many cells in each ommatidia, as is sev. Sinu is localized to the nucleus, suggesting a role in gene regulation. 41. ..
ROCCE RD, KARU&XH CA, BANERJEEU: Genetic Dissection of a Neurodevelopmental Pathway: son of sevenless Functions Downstream of the sevenless and EGF Receptor Tyrosine Kinases. Cell 1991, 64:3+48. Describes a genetic locus, termed son of sevenless (sos), identified as an allele-specific suppressor of a sev mutation. Sos is required in a cell-autonomous manner for suppression of sev, and is also shown to interact with boss and the Drosophila EGF receptor. Loss of function alleles of square recessive lethals. These data suggest that sosis required for signal transduction through multiple pathways. 42.
BANERJEEU, RENFRANZ PJ, HINTONDR, RABINBA, BENZER S: The Sevenless+ Protein is Expressed ApicaIIy in CeII Membranes of Developing Drosophila Retina; it is not Restricted to CeU’R7. Cell 1987, 51:151-158.
43.
TOMLINSON A, Bownzu DDL, HAFEN E, RUBINGM: Localization of the sevenless Protein, a Putative Receptor for Positional Information, in the Eye ImaginaI Disk of Drosophila. Cell 1987, 51:143-150.
44.
BILLER K, HAFIXN E: Ubiquitous Position-Dependent Specification 243:931-933.
45.
BOWIFU DDL, SIMON MA, RUIN GM: Ommatidia in the Developing Drosophila Eye Require and can Respond to seu enless for Only a Restricted Period. Cell 1989, 56931-936.
46.
MLODZIKM, HIROMI Y, WEBER U, G~~D~IAN CS, RUBIN GM: The Drosophila seven-up Gene, a Member of the Steroid Receptor Gene Superfamily, Fates. CelI 1990, 60:211-224.
47.
Expression of sevenless of CeU Fate. Science 1989,
Controls
Photoreceptor
49.
MOSES K, RIJBIN GM: Glass Encodes a Site-Spectic DNABinding Protein that is Regulated in Response to Positional Signals in the Developing Drosophila Eye. Genes Dev 1991, 5:583-593.
50.
REINKER, KRANTZDE, YEN D, ZI~UR~KVSL: Chaoptin, a CeU Surface Glycoprotein Required for Drosophila Photoreceptor CeU Morphogenesis, Contains a Repeat Motif Found in Yeast and Human. Cell 1988, 52:291-301.
51.
VAN VACTOR D, KRANTz DE, REINKE R, ZIPURSKV SL: Analy sis of Mutants in Chaoptin, a Photoreceptor Cell-Specific Glycoprotein in Drosophila, Reveals its Role in CeUuIar Morphogenesis. Cell 1988, 52:281-230.
52.
SELLER H, FISCHBACH K-F, RUBINGM: Disconnected a Locus Required for Neuronal Pathway Formation in the Visual System of Drosophila. Cell 1987, 50:113F1153.
53.
IAIUSONKD, BREM~LLER R: Early Onset of Phenotype and CeU Patterning in the Embryonic Zebra&h Retina Devefqtmzent 1990, 109~567-576.
54.
WIKLERKC, RAKlc P: Relation of an Array of Early-Differentiating Cones to the Photoreceptor Mosaic in the Primate Retina. Nature 1991, 351:397400.
55.
VOLLMER G, IAYERPG: An In Vitro Model of Proliferation and Differentiation of the Chick Retina: Coaggregates of Retinal and Pigment EpitheIiaI Cells. J Neurasci 1986, 6:188>1896.
56.
SHEEDLOWHJ, GAUR V, Lr L, SEATONAD, TURNERJE: Transplantation to the Diseased and Damaged Retina. Trends Neurosci 1991, 14:347-350.
FAKTOROVICH E, STEINBERGR, Y~~UM~JRA D, MATTESMT, ~AVA~L MM: Photoreceptor Degeneration in Inherited Retinal Dystrophy Delayed by Basic Fibroblast Growth Factor. Nature 1990, 34783-86. This study describes the ability of basic FGF fo restore photoreceptor survival in the RCS rat, a mutant in which photoreceptors degenerate due to a defect in the pigment epithelium. The authors discuss possible mechanisms whereby FGF treatment could compensate for this mutation. 57. .
58.
NEGISHIK, TERANISHIT, KATO S, NAKAMURA Y: Paradoxical Induction of Dopaminergic Cells Following Intravitreal Injection of High Doses of 6-Hydroxydopamine in Juvenile Carp Retina. Dev Brain Res 1987, 33~67-79.
59.
REH TA, TUUY T: Regulation of Tyrosine Hydroxylase-Conmining Amacrine CeU Number in LarvaI Frog Retina. Dev Biol 1986, 114:46%469.
CeU
MOSES K, ELUS MC, RUBIN GM: The Glass Gene Encodes a Zinc-Finger Protein Required by Drosophila Photoreceptor CeUs. Nature 1989, 340:531-536.
48. MOSES K: The Role of Transcription Factors in ,the Devel. oping Drosophila Eye. Trend Neurosci 1991, 7:25G255. An up-to-date review of four genes (rough glass, sev and sinu) that are thought to regulate Dmqbila retina development as transcription factors.
D Al&huler and L Lillien, Department of Genetics, Harvard Medical School, 25 Shattuck Street, Boston, Massachusetts 02115, USA