- Email: [email protected]
The cadherin superfamily and dendrite development
Update
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signalling pathway for different subclasses of semaphorins. This idea is supported by the observation that mutations in the GAP homology domains of plexin-A1 and the expression of constitutively active Ras prevent Sema3A-mediated growth-cone collapse [6,14]. Remarkably little is known about the nature and extent of molecular cross-talk between semaphorin and other signalling cascades. Intriguingly, the work of Oinuma et al. suggests that plexins, through the regulation of small GTPases, can suppress signalling pathways that normally counter semaphorin-induced collapse. Future research will, undoubtedly, refine the role of GTPases in plexin signalling and will provide molecular insight into the signalling cascades downstream of plexin–GTPase interactions. Acknowledgements I thank Alex Kolodkin for critical reading of the manuscript. Research in my laboratory is supported by the Human Frontier Science Program Organization and the Netherlands Organization for Scientific Research.
References 1 Fiore, R. and Pu¨schel, A.W. (2003) The function of semaphorins during nervous system development. Front. Biosci. 8, s484–s499 2 Fujisawa, H. (2004) Discovery of semaphorin receptors, neuropilin and plexin, and their functions in neural development. J. Neurobiol. 59, 24–33 3 Tamagnone, L. and Comoglio, P.M. (2000) Signalling by semaphorin receptors: cell guidance and beyond. Trends Cell Biol. 10, 377–383 4 Castellani, V. and Rougon, G. (2002) Control of semaphorin signaling. Curr. Opin. Neurobiol. 12, 532–541 5 Ohta, K. et al. (1995) Plexin: a novel neuronal cell surface molecule that mediates cell adhesion via a homophilic binding mechanism in the presence of calcium. Neuron 14, 1189–1199 6 Oinuma, I. et al. (2004) The Semaphorin 4D receptor Plexin-B1 is a GTPase activating protein for R-Ras. Science 305, 862–865 7 Symons, M. and Settleman, J. (2000) Rho family GTPases: more than simple switches. Trends Cell Biol. 10, 415–419 8 Pasterkamp, R.J. and Kolodkin, A.L. (2003) Semaphorin junction: making tracks toward neural connectivity. Curr. Opin. Neurobiol. 13, 79–89 9 Aurandt, J. et al. (2002) The semaphorin receptor plexin-B1 signals through a direct interaction with the Rho-specific nucleotide exchange factor, LARG. Proc. Natl. Acad. Sci. U. S. A. 99, 12085–12090
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10 Hirotani, M. et al. (2002) Interaction of plexin-B1 with PDZ domain containing Rho guanine nucleotide exchange factors. Biochem. Biophys. Res. Commun. 297, 32–37 11 Perrot, V. et al. (2002) Plexin B regulates Rho through the guanine nucleotide exchange factors leukemia-associated RhoGEF (LARG) and PDZ-RhoGEF. J. Biol. Chem. 277, 43115–43120 12 Swiercz, J.M. et al. (2002) Plexin-B1 directly interacts with PDZRhoGEF/LARG to regulate RhoA and growth cone morphology. Neuron 35, 51–63 13 Driessens, M.H.E. et al. (2002) B plexins activate Rho through PDZRhoGEF. FEBS Lett. 529, 168–172 14 Rohm, B. et al. (2000) The semaphorin 3A receptor may directly regulate the activity of small GTPases. FEBS Lett. 486, 68–72 15 Scheffzek, K. et al. (1998) GTPase-activating proteins: helping hands to complement an active site. Trends Biochem. Sci. 23, 257–262 16 Zanata, S.M. et al. (2002) Antagonistic effects of Rnd1 and RhoD GTPases regulate receptor activity in Semaphorin 3A-induced cytoskeletal collapse. J. Neurosci. 22, 471–477 17 Oinuma, I. et al. (2003) Direct interaction of Rnd1 with Plexin-B1 regulates PDZ-RhoGEF-mediated Rho activation by Plexin-B1 and induces cell contraction in COS-7 cells. J. Biol. Chem. 278, 25671–25677 18 Driessens, M.H.E. et al. (2001) Plexin-B semaphorin receptors interact directly with active Rac and regulate the actin cytoskeleton by activating Rho. Curr. Biol. 11, 339–344 19 Zhang, Z. et al. (1996) Integrin activation by R-ras. Cell 85, 61–69 20 Barberis, D. et al. (2004) Plexin signaling hampers integrin-based adhesion, leading to Rho-kinase independent cell rounding, and inhibiting lamellipodia extension and cell motility. FASEB J. 18, 592–594 21 Vikis, H.G. et al. (2000) The semaphorin receptor plexin-B1 specifically interacts with active Rac in a ligand-dependent manner. Proc. Natl. Acad. Sci. U. S. A. 97, 12457–12462 22 Vikis, H.G. et al. (2002) The plexin-B1/Rac interaction inhibits PAK activation and enhances Sema4D ligand binding. Genes Dev. 16, 836–845 23 Turner, L.J. et al. (2004) The activity of the plexin-A1 receptor is regulated by Rac. J. Biol. Chem. 279, 33199–33205 24 Swiercz, J.M. et al. (2004) Plexin-B1/RhoGEF-mediated RhoA activation involves the receptor tyrosine kinase ErbB-2. J. Cell Biol. 165, 869–880 25 Wennerberg, K. et al. (2003) Rnd proteins function as RhoA antagonists by activating p190 RhoGAP. Curr. Biol. 13, 1106–1115
0962-8924/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2004.12.005
The cadherin superfamily and dendrite development Bing Ye and Yuh Nung Jan Howard Hughes Medical Institute, Departments of Physiology, Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0725, USA
Dendrite morphogenesis is a highly dynamic process that involves constant extension and retraction of branches, and stabilization of these dynamics has pivotal roles in determining the number and length of branches. The classic cadherin N-cadherin is involved in stabilization of dendritic spines and branches. Recent Corresponding author: Jan, Y.N. ([email protected]). Available online 5 January 2005 www.sciencedirect.com
work in mammals by Shima et al., and earlier work in Drosophila by Gao et al. and Sweeney et al. have determined the presence of a non-classic, seven-pass transmembrane cadherin in this process.
Introduction Dendrites and axons are the major input and output apparatus of a neuron. How the tremendous diversity of
Update
TRENDS in Cell Biology
Vol.15 No.2 February 2005
dendritic morphology is generated is one of the most challenging problems in cell biology. Among the unsolved problems involved in dendrite morphogenesis [1–3], such as establishing axon–dendrite polarity and guiding dendritic branches to their proper destination, a fundamental issue is how a dendritic branch grows and stops properly. Thanks to recent advances in live imaging, it is now known that the formation of a dendritic tree is highly dynamic and is characterized by constant extension and retraction of individual filopodia and dendritic branches [4,5]. The identification of molecules that control the dynamics of dendritic branch formation has become the subject of much focus in the field of dendrite development. Recent studies suggest that members of the cadherin superfamily are involved in this process. In this article, we discuss these findings and focus on a more recent study of the roles of the non-classic, seven-pass transmembrane cadherin Flamingo in dendrite morphogenesis [6]. The formation of a dendritic branch For dendrites that receive synaptic input, the growth of a branch can be divided approximately into an early and a late phase, depending on whether synaptic contact has been formed between the branch and afferent axonal terminals (Figure 1). The early phase of branch growth happens before the formation of synaptic contact and is
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initiated by the appearance of a filopodium. The filopodium protrudes to form a new branch segment, the stabilization of which probably involves the invasion of microtubules [7]. As the neuron matures, branch growth enters the late phase. Synapses start to form along the dendrite and provide a new mechanism for stabilization of the dynamic branches [1,8]. This synaptic-contact-dependent stabilization mechanism is functionally selective because only dendritic branches that are contacted by the appropriate inputs are stabilized and, therefore, maintained [9]. For dendrites of sensory neurons, which do not receive synaptic input, little is known about how the dynamics are stabilized in the late stage, although it is conceivable that the target tissues of these neurons, in addition to the homotypic dendritic exclusion that exists in some types of sensory neuron [10], might provide a stabilization signal. Understanding the cellular and molecular mechanisms underlying the extension and retraction of dendritic branches has been a focus of the field of dendrite morphogenesis in recent years and it has been fueled by the advances in live-imaging techniques. Several molecules have been found to be required for extension (such as motor molecules involved in microtubule-dependent transport [11,12]), retraction (such as the actin-cytoskeleton regulator RhoA [2,13]) and dynamics (such as CaMKII [5] and Rac1 [14]) of dendritic branches. Recently,
Early phase
Late phase
Dynamic
Retraction Extension force force Synapses formed Retraction Extension force force
Stabilized
(Dendrites with synapses)
Stabilized Dynamic Dendritic exclusion established
Retraction Extension force force Stabilized
(Sensory dendrites)
Homotypic branch
Presynaptic terminal Synapses fail to form
Retraction Extension force force Stabilized
Retraction force Destabilized
Extension force Microtubules Depolymerized microtubules
TRENDS in Cell Biology
Figure 1. A model of dendritic branch growth. For dendrites that receive synaptic input, the growth of a branch can be divided into an early phase (before synapses are formed along the branch) and a late phase (after synapses are formed). In the early phase, branch growth is initiated by the appearance of a highly dynamic filopodium [4]. The filopodium protrudes to form a new branch segment, the stabilization of which probably involves the invasion of microtubules [7]. In the late phase, synapses form along the dendrite and provide stabilization signals to the branch [1,8]. If synapses fail to form, stabilization of the dendritic branch is affected and, consequently, the branch either retracts or overextends. For dendrites that do not receive synaptic input, such as those of sensory neurons, the stabilization signal in the late phase comes from other sources. For example, dendritic exclusion between homotypic branches might provide such a signal in some neurons [10]. www.sciencedirect.com
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several studies have suggested that the adhesion molecules of the cadherin superfamily are involved in the stabilization of dendrites. N-cadherin in dendrite morphogenesis The cadherin superfamily can be divided into classic and non-classic subfamilies [15]. The intracellular signaling mechanism of the classic cadherins involves a- and b-catenins, whereas the mechanism of the non-classic cadherins remains elusive. Among the classic cadherins, N-cadherin has been reported to be involved in the morphogenesis of both the dendritic spine [16] and the dendritic branch [17]. Interfering with N-cadherin or aN-catenin (a neuron-specific a-catenin) function in cultured hippocampal neurons leads to elongation of dendritic spines [16]. Live imaging reveals that the defects are due to a failure in the stabilization of dendritic filopodia and spines, which seems to be an activitydependent event [18]. Because cadherin-mediated epithelial junction formation activates Rac1 [19], which is known to regulate the dynamics of dendritic growth itself [14], it is conceivable that signaling downstream of N-cadherin in dendrites might involve Rac1. N-cadherin also regulates dendrite development in Drosophila olfactory projection neurons (PNs) [20]. Normally, the dendrites of a PN are confined to a particular glomerulus following an initial overgrowth into adjacent glomeruli. In N-cadherin-deficient PNs, the dendrites overextend in both stages and, consequently, innervate multiple glomeruli. This defect seems to be caused by a compromised dendrodendritic interaction that is mediated by N-cadherin because dendrites of wild-type neurons also exhibit the defects when they are adjacent to N-cadherindeficient neurons. Flamingo in dendrite morphogenesis Flamingo, also known as Starry night, is a member of the non-classic cadherin subfamily. In addition to multiple cadherin domains that are common to all cadherins, Flamingo contains epidermal growth factor (EGF)-like domains, Laminin G domains, hormone receptor domains, a seven-pass transmembrane domain and an intracellular domain [6,21]. In Drosophila, it has roles in planar polarity through the seven-pass transmembrane protein Frizzled and the adaptor protein Disheveled [21,22], and is also involved in axon growth and target selection [23,24]. In rodents, there are three flamingo homologs – Celsr1, Celsr2 and Celsr3 – that are differentially expressed in the nervous system and other tissues [25]. Notable among them, Celsr2 protein is expressed in postmitotic neurons in addition to in differentiating cells and it is present in dendrites and/or axons [26]. Recently, using a vector-based RNA interference (RNAi) approach, Shima et al. provided compelling evidence that Celsr2 is involved in dendrite morphogenesis [6]. In cortical pyramidal cells and cerebellar Purkinje cells, impairment of Celsr2 expression reduced branch number and length, resulting in a simplified dendritic tree. The authors followed the dendritic arbor growth of control small hairpin RNA (shRNA)- and Celsr2 small interference RNA (siRNA)-transfected Purkinje neurons at www.sciencedirect.com
Vol.15 No.2 February 2005
four time points and found that, in the first 24 h after transfection, the control and experimental groups were indistinguishable. After that, whereas the dendrites of the control neurons grew further, those of the siRNA-transfected neurons started to retract. The authors suggest that Celsr2 is required for the maintenance of dendrites and that defects in it lead to retraction of branches. The term ‘maintenance’ is often used to describe the persistence of existing dendritic spines and branches over a certain period of time. It is noteworthy that this is more of a descriptive term than a mechanistic one. In other words, whether a ‘maintenance activity’ is required for proper dendrite morphogenesis and whether specific molecules are required for such activity have yet to be proven experimentally. By contrast, the fact that molecules such as Rac1 can regulate branch dynamics without affecting the final branch length or numbers [14] indicates that there are specific activities that regulate dendritic branch dynamics, rather than just extension or retraction, and therefore the stabilization of developing branches, which can be assessed by time-lapse experiments [4,5]. The defects observed in Celsr2-knockdown neurons could also be due to a defect in dendrite stabilization in the late phase of dendritic branch morphogenesis. It would be desirable to perform time-lapse experiments to find out whether and how Celsr2 regulates branch dynamics. Shima et al. also explored the domain requirement of the role of Celsr2 in dendrite morphogenesis by examining the effects of expressing deletion mutants of Celsr2 in Celsr2-defective neurons. They suggest that Celsr2– Celsr2 homophilic interaction and intracellular signaling transduced by Celsr2 are important for dendrite development [6]. The Drosophila ortholog of Celsr, flamingo, also controls dendrite morphogenesis. In multidendritic (md) neurons, loss of flamingo function results in overextension of the dorsal dendrites as a result of premature initiation and failure to stop at the appropriate location [27,28]. It seems that the dendritic localization of Flamingo protein is essential for proper dendrite morphogenesis because a missense mutation that causes the Flamingo protein to fail to localize to the dendrites and axons leads to overextension of dorsal dendrites. Similar to md neurons, mushroom-body neurons defective in Flamingo function exhibit overextension of dendrites [29]. How does Flamingo regulate dendrite morphology? The apparently different consequences of Flamingo and Celsr2 deficiency in Drosophila and mammals, namely overextension and retraction of dendritic branches, respectively, can be reconciled by assuming that members of the cadherin superfamily provide a signal for dendrite stabilization. The loss of stabilization activity could lead to either retraction or overextension of dendrites. The final outcome of such a loss depends on the prevailing mechanism that regulates the balance between retraction and extension (Figure 1). Time-lapse analysis is likely to be instrumental in testing this hypothesis. It is also possible that the signals downstream of Flamingo are different in mammalian and fly neurons. Currently, the nature of these signals remains elusive in both cases.
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TRENDS in Cell Biology
Identification of such mechanisms will improve the understanding of why the consequences of impairment of Flamingo function are different between mammals and Drosophila. Despite circumstantial evidence suggesting that homophilic cadherin adhesion is required for dendrite stabilization, including homophilic adhesion in heterologous cells [6] and the synaptic localization of some cadherins and catenins [16,30,31], there is still a lack of definitive proof. One finding suggests that the scenario is probably more complicated. Although flamingo is expressed in both md neurons and their neighboring non-neuronal cells in Drosophila, driving flamingo expression in md neurons is sufficient to rescue the dendrite defects in flamingo-mutant animals [27]. These data raise the possibility that heterophilic adhesion or, perhaps, a receptor function of Flamingo is responsible for proper dendrite stopping. Therefore, it is important to determine whether the members of the cadherin superfamily regulate dendrite morphogenesis through homophilic or heterophilic adhesion or, alternatively, through binding to unknown ligands. It will also be important to determine whether the functions of Flamingo in dendrite morphogenesis are synaptic-contact dependent. If the late onset of the dendrite defects seen in Celsr2-defective neurons [6] faithfully reflects the timing of Celsr2 function in vivo, rather than being a consequence of a delayed RNAi effect, it is conceivable that Celsr2 might be involved in synapticcontact-dependent stabilization of dendrites. Concluding remarks In summary, several studies suggest that cadherins contribute to dendrite development by regulating the stabilization of dendritic branches or spines. In both Drosophila and mammals, the non-classic cadherin Flamingo has been found to control dendrite morphogenesis, probably by regulating dendrite stabilization. Whether homophilic cadherin adhesion is responsible for dendrite stabilization remains to be determined. Identification of the adhesion partners and downstream signaling mechanisms will provide better understanding of how cadherins, especially Flamingo, regulate dendrite morphogenesis. Acknowledgements We thank Wesley Grueber, Michael Kim, Chay Kuo, Jay Parrish, Fabrice Roegiers, Tadashi Uemura and Rebecca Yang for helpful comments and suggestions. B.Y. is a recipient of the National Research Service Award (1 F32 NS46847–01) and Y.N.J. is an investigator of the Howard Hughes Medical Institute.
References 1 Cline, H.T. (2001) Dendritic arbor development and synaptogenesis. Curr. Opin. Neurobiol. 11, 118–126 2 Scott, E.K. and Luo, L. (2001) How do dendrites take their shape? Nat. Neurosci. 4, 359–365 3 Jan, Y.N. and Jan, L.Y. (2003) The control of dendrite development. Neuron 40, 229–242 4 Dailey, M.E. and Smith, S.J. (1996) The dynamics of dendritic structure in developing hippocampal slices. J. Neurosci. 16, 2983–2994 5 Wu, G.Y. and Cline, H.T. (1998) Stabilization of dendritic arbor structure in vivo by CaMKII. Science 279, 222–226 www.sciencedirect.com
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6 Shima, Y. et al. (2004) Regulation of dendritic maintenance and growth by a mammalian 7-pass transmembrane cadherin. Dev. Cell 7, 205–216 7 Baas, P.W. (1999) Microtubules and neuronal polarity: lessons from mitosis. Neuron 22, 23–31 8 Wong, R.O. and Ghosh, A. (2002) Activity-dependent regulation of dendritic growth and patterning. Nat. Rev. Neurosci. 3, 803–812 9 Vaughn, J.E. (1989) Fine structure of synaptogenesis in the vertebrate central nervous system. Synapse 3, 255–285 10 Grueber, W.B. et al. (2003) Dendrites of distinct classes of Drosophila sensory neurons show different capacities for homotypic repulsion. Curr. Biol. 13, 618–626 11 Liu, Z. et al. (2000) Drosophila Lis1 is required for neuroblast proliferation, dendritic elaboration and axonal transport. Nat. Cell Biol. 2, 776–783 12 Sharp, D.J. et al. (1997) Identification of a microtubule-associated motor protein essential for dendritic differentiation. J. Cell Biol. 138, 833–843 13 Van Aelst, L. and Cline, H.T. (2004) Rho GTPases and activitydependent dendrite development. Curr. Opin. Neurobiol. 14, 297–304 14 Li, Z. et al. (2000) Rho GTPases regulate distinct aspects of dendritic arbor growth in Xenopus central neurons in vivo. Nat. Neurosci. 3, 217–225 15 Tepass, U. et al. (2000) Cadherins in embryonic and neural morphogenesis. Nat. Rev. Mol. Cell Biol. 1, 91–100 16 Togashi, H. et al. (2002) Cadherin regulates dendritic spine morphogenesis. Neuron 35, 77–89 17 Yu, X. and Malenka, R.C. (2003) b-Catenin is critical for dendritic morphogenesis. Nat. Neurosci. 6, 1169–1177 18 Abe, K. et al. (2004) Stability of dendritic spines and synaptic contacts is controlled by aN-catenin. Nat. Neurosci. 7, 357–363 19 Kaibuchi, K. et al. (1999) Regulation of cadherin-mediated cell–cell adhesion by the Rho family GTPases. Curr. Opin. Cell Biol. 11, 591–596 20 Zhu, H. and Luo, L. (2004) Diverse functions of N-cadherin in dendritic and axonal terminal arborization of olfactory projection neurons. Neuron 42, 63–75 21 Usui, T. et al. (1999) Flamingo, a seven-pass transmembrane cadherin, regulates planar cell polarity under the control of Frizzled. Cell 98, 585–595 22 Lu, B. et al. (1999) Flamingo controls the planar polarity of sensory bristles and asymmetric division of sensory organ precursors in Drosophila. Curr. Biol. 9, 1247–1250 23 Lee, R.C. et al. (2003) The protocadherin Flamingo is required for axon target selection in the Drosophila visual system. Nat. Neurosci. 6, 557–563 24 Senti, K.A. et al. (2003) Flamingo regulates R8 axon–axon and axon– target interactions in the Drosophila visual system. Curr. Biol. 13, 828–832 25 Tissir, F. et al. (2002) Developmental expression profiles of Celsr (Flamingo) genes in the mouse. Mech. Dev. 112, 157–160 26 Shima, Y. et al. (2002) Differential expression of the seven-pass transmembrane cadherin genes Celsr1-3 and distribution of the Celsr2 protein during mouse development. Dev. Dyn. 223, 321–332 27 Gao, F.B. et al. (2000) Control of dendritic field formation in Drosophila: the roles of flamingo and competition between homologous neurons. Neuron 28, 91–101 28 Sweeney, N.T. et al. (2002) Genetic manipulation of single neurons in vivo reveals specific roles of flamingo in neuronal morphogenesis. Dev. Biol. 247, 76–88 29 Reuter, J.E. et al. (2003) A mosaic genetic screen for genes necessary for Drosophila mushroom body neuronal morphogenesis. Development 130, 1203–1213 30 Benson, D.L. and Tanaka, H. (1998) N-cadherin redistribution during synaptogenesis in hippocampal neurons. J. Neurosci. 18, 6892–6904 31 Uchida, N. et al. (1996) The catenin/cadherin adhesion system is localized in synaptic junctions bordering transmitter release zones. J. Cell Biol. 135, 767–779
0962-8924/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2004.12.003
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signalling pathway for different subclasses of semaphorins. This idea is supported by the observation that mutations in the GAP homology domains of plexin-A1 and the expression of constitutively active Ras prevent Sema3A-mediated growth-cone collapse [6,14]. Remarkably little is known about the nature and extent of molecular cross-talk between semaphorin and other signalling cascades. Intriguingly, the work of Oinuma et al. suggests that plexins, through the regulation of small GTPases, can suppress signalling pathways that normally counter semaphorin-induced collapse. Future research will, undoubtedly, refine the role of GTPases in plexin signalling and will provide molecular insight into the signalling cascades downstream of plexin–GTPase interactions. Acknowledgements I thank Alex Kolodkin for critical reading of the manuscript. Research in my laboratory is supported by the Human Frontier Science Program Organization and the Netherlands Organization for Scientific Research.
References 1 Fiore, R. and Pu¨schel, A.W. (2003) The function of semaphorins during nervous system development. Front. Biosci. 8, s484–s499 2 Fujisawa, H. (2004) Discovery of semaphorin receptors, neuropilin and plexin, and their functions in neural development. J. Neurobiol. 59, 24–33 3 Tamagnone, L. and Comoglio, P.M. (2000) Signalling by semaphorin receptors: cell guidance and beyond. Trends Cell Biol. 10, 377–383 4 Castellani, V. and Rougon, G. (2002) Control of semaphorin signaling. Curr. Opin. Neurobiol. 12, 532–541 5 Ohta, K. et al. (1995) Plexin: a novel neuronal cell surface molecule that mediates cell adhesion via a homophilic binding mechanism in the presence of calcium. Neuron 14, 1189–1199 6 Oinuma, I. et al. (2004) The Semaphorin 4D receptor Plexin-B1 is a GTPase activating protein for R-Ras. Science 305, 862–865 7 Symons, M. and Settleman, J. (2000) Rho family GTPases: more than simple switches. Trends Cell Biol. 10, 415–419 8 Pasterkamp, R.J. and Kolodkin, A.L. (2003) Semaphorin junction: making tracks toward neural connectivity. Curr. Opin. Neurobiol. 13, 79–89 9 Aurandt, J. et al. (2002) The semaphorin receptor plexin-B1 signals through a direct interaction with the Rho-specific nucleotide exchange factor, LARG. Proc. Natl. Acad. Sci. U. S. A. 99, 12085–12090
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10 Hirotani, M. et al. (2002) Interaction of plexin-B1 with PDZ domain containing Rho guanine nucleotide exchange factors. Biochem. Biophys. Res. Commun. 297, 32–37 11 Perrot, V. et al. (2002) Plexin B regulates Rho through the guanine nucleotide exchange factors leukemia-associated RhoGEF (LARG) and PDZ-RhoGEF. J. Biol. Chem. 277, 43115–43120 12 Swiercz, J.M. et al. (2002) Plexin-B1 directly interacts with PDZRhoGEF/LARG to regulate RhoA and growth cone morphology. Neuron 35, 51–63 13 Driessens, M.H.E. et al. (2002) B plexins activate Rho through PDZRhoGEF. FEBS Lett. 529, 168–172 14 Rohm, B. et al. (2000) The semaphorin 3A receptor may directly regulate the activity of small GTPases. FEBS Lett. 486, 68–72 15 Scheffzek, K. et al. (1998) GTPase-activating proteins: helping hands to complement an active site. Trends Biochem. Sci. 23, 257–262 16 Zanata, S.M. et al. (2002) Antagonistic effects of Rnd1 and RhoD GTPases regulate receptor activity in Semaphorin 3A-induced cytoskeletal collapse. J. Neurosci. 22, 471–477 17 Oinuma, I. et al. (2003) Direct interaction of Rnd1 with Plexin-B1 regulates PDZ-RhoGEF-mediated Rho activation by Plexin-B1 and induces cell contraction in COS-7 cells. J. Biol. Chem. 278, 25671–25677 18 Driessens, M.H.E. et al. (2001) Plexin-B semaphorin receptors interact directly with active Rac and regulate the actin cytoskeleton by activating Rho. Curr. Biol. 11, 339–344 19 Zhang, Z. et al. (1996) Integrin activation by R-ras. Cell 85, 61–69 20 Barberis, D. et al. (2004) Plexin signaling hampers integrin-based adhesion, leading to Rho-kinase independent cell rounding, and inhibiting lamellipodia extension and cell motility. FASEB J. 18, 592–594 21 Vikis, H.G. et al. (2000) The semaphorin receptor plexin-B1 specifically interacts with active Rac in a ligand-dependent manner. Proc. Natl. Acad. Sci. U. S. A. 97, 12457–12462 22 Vikis, H.G. et al. (2002) The plexin-B1/Rac interaction inhibits PAK activation and enhances Sema4D ligand binding. Genes Dev. 16, 836–845 23 Turner, L.J. et al. (2004) The activity of the plexin-A1 receptor is regulated by Rac. J. Biol. Chem. 279, 33199–33205 24 Swiercz, J.M. et al. (2004) Plexin-B1/RhoGEF-mediated RhoA activation involves the receptor tyrosine kinase ErbB-2. J. Cell Biol. 165, 869–880 25 Wennerberg, K. et al. (2003) Rnd proteins function as RhoA antagonists by activating p190 RhoGAP. Curr. Biol. 13, 1106–1115
0962-8924/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2004.12.005
The cadherin superfamily and dendrite development Bing Ye and Yuh Nung Jan Howard Hughes Medical Institute, Departments of Physiology, Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0725, USA
Dendrite morphogenesis is a highly dynamic process that involves constant extension and retraction of branches, and stabilization of these dynamics has pivotal roles in determining the number and length of branches. The classic cadherin N-cadherin is involved in stabilization of dendritic spines and branches. Recent Corresponding author: Jan, Y.N. ([email protected]). Available online 5 January 2005 www.sciencedirect.com
work in mammals by Shima et al., and earlier work in Drosophila by Gao et al. and Sweeney et al. have determined the presence of a non-classic, seven-pass transmembrane cadherin in this process.
Introduction Dendrites and axons are the major input and output apparatus of a neuron. How the tremendous diversity of
Update
TRENDS in Cell Biology
Vol.15 No.2 February 2005
dendritic morphology is generated is one of the most challenging problems in cell biology. Among the unsolved problems involved in dendrite morphogenesis [1–3], such as establishing axon–dendrite polarity and guiding dendritic branches to their proper destination, a fundamental issue is how a dendritic branch grows and stops properly. Thanks to recent advances in live imaging, it is now known that the formation of a dendritic tree is highly dynamic and is characterized by constant extension and retraction of individual filopodia and dendritic branches [4,5]. The identification of molecules that control the dynamics of dendritic branch formation has become the subject of much focus in the field of dendrite development. Recent studies suggest that members of the cadherin superfamily are involved in this process. In this article, we discuss these findings and focus on a more recent study of the roles of the non-classic, seven-pass transmembrane cadherin Flamingo in dendrite morphogenesis [6]. The formation of a dendritic branch For dendrites that receive synaptic input, the growth of a branch can be divided approximately into an early and a late phase, depending on whether synaptic contact has been formed between the branch and afferent axonal terminals (Figure 1). The early phase of branch growth happens before the formation of synaptic contact and is
65
initiated by the appearance of a filopodium. The filopodium protrudes to form a new branch segment, the stabilization of which probably involves the invasion of microtubules [7]. As the neuron matures, branch growth enters the late phase. Synapses start to form along the dendrite and provide a new mechanism for stabilization of the dynamic branches [1,8]. This synaptic-contact-dependent stabilization mechanism is functionally selective because only dendritic branches that are contacted by the appropriate inputs are stabilized and, therefore, maintained [9]. For dendrites of sensory neurons, which do not receive synaptic input, little is known about how the dynamics are stabilized in the late stage, although it is conceivable that the target tissues of these neurons, in addition to the homotypic dendritic exclusion that exists in some types of sensory neuron [10], might provide a stabilization signal. Understanding the cellular and molecular mechanisms underlying the extension and retraction of dendritic branches has been a focus of the field of dendrite morphogenesis in recent years and it has been fueled by the advances in live-imaging techniques. Several molecules have been found to be required for extension (such as motor molecules involved in microtubule-dependent transport [11,12]), retraction (such as the actin-cytoskeleton regulator RhoA [2,13]) and dynamics (such as CaMKII [5] and Rac1 [14]) of dendritic branches. Recently,
Early phase
Late phase
Dynamic
Retraction Extension force force Synapses formed Retraction Extension force force
Stabilized
(Dendrites with synapses)
Stabilized Dynamic Dendritic exclusion established
Retraction Extension force force Stabilized
(Sensory dendrites)
Homotypic branch
Presynaptic terminal Synapses fail to form
Retraction Extension force force Stabilized
Retraction force Destabilized
Extension force Microtubules Depolymerized microtubules
TRENDS in Cell Biology
Figure 1. A model of dendritic branch growth. For dendrites that receive synaptic input, the growth of a branch can be divided into an early phase (before synapses are formed along the branch) and a late phase (after synapses are formed). In the early phase, branch growth is initiated by the appearance of a highly dynamic filopodium [4]. The filopodium protrudes to form a new branch segment, the stabilization of which probably involves the invasion of microtubules [7]. In the late phase, synapses form along the dendrite and provide stabilization signals to the branch [1,8]. If synapses fail to form, stabilization of the dendritic branch is affected and, consequently, the branch either retracts or overextends. For dendrites that do not receive synaptic input, such as those of sensory neurons, the stabilization signal in the late phase comes from other sources. For example, dendritic exclusion between homotypic branches might provide such a signal in some neurons [10]. www.sciencedirect.com
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several studies have suggested that the adhesion molecules of the cadherin superfamily are involved in the stabilization of dendrites. N-cadherin in dendrite morphogenesis The cadherin superfamily can be divided into classic and non-classic subfamilies [15]. The intracellular signaling mechanism of the classic cadherins involves a- and b-catenins, whereas the mechanism of the non-classic cadherins remains elusive. Among the classic cadherins, N-cadherin has been reported to be involved in the morphogenesis of both the dendritic spine [16] and the dendritic branch [17]. Interfering with N-cadherin or aN-catenin (a neuron-specific a-catenin) function in cultured hippocampal neurons leads to elongation of dendritic spines [16]. Live imaging reveals that the defects are due to a failure in the stabilization of dendritic filopodia and spines, which seems to be an activitydependent event [18]. Because cadherin-mediated epithelial junction formation activates Rac1 [19], which is known to regulate the dynamics of dendritic growth itself [14], it is conceivable that signaling downstream of N-cadherin in dendrites might involve Rac1. N-cadherin also regulates dendrite development in Drosophila olfactory projection neurons (PNs) [20]. Normally, the dendrites of a PN are confined to a particular glomerulus following an initial overgrowth into adjacent glomeruli. In N-cadherin-deficient PNs, the dendrites overextend in both stages and, consequently, innervate multiple glomeruli. This defect seems to be caused by a compromised dendrodendritic interaction that is mediated by N-cadherin because dendrites of wild-type neurons also exhibit the defects when they are adjacent to N-cadherindeficient neurons. Flamingo in dendrite morphogenesis Flamingo, also known as Starry night, is a member of the non-classic cadherin subfamily. In addition to multiple cadherin domains that are common to all cadherins, Flamingo contains epidermal growth factor (EGF)-like domains, Laminin G domains, hormone receptor domains, a seven-pass transmembrane domain and an intracellular domain [6,21]. In Drosophila, it has roles in planar polarity through the seven-pass transmembrane protein Frizzled and the adaptor protein Disheveled [21,22], and is also involved in axon growth and target selection [23,24]. In rodents, there are three flamingo homologs – Celsr1, Celsr2 and Celsr3 – that are differentially expressed in the nervous system and other tissues [25]. Notable among them, Celsr2 protein is expressed in postmitotic neurons in addition to in differentiating cells and it is present in dendrites and/or axons [26]. Recently, using a vector-based RNA interference (RNAi) approach, Shima et al. provided compelling evidence that Celsr2 is involved in dendrite morphogenesis [6]. In cortical pyramidal cells and cerebellar Purkinje cells, impairment of Celsr2 expression reduced branch number and length, resulting in a simplified dendritic tree. The authors followed the dendritic arbor growth of control small hairpin RNA (shRNA)- and Celsr2 small interference RNA (siRNA)-transfected Purkinje neurons at www.sciencedirect.com
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four time points and found that, in the first 24 h after transfection, the control and experimental groups were indistinguishable. After that, whereas the dendrites of the control neurons grew further, those of the siRNA-transfected neurons started to retract. The authors suggest that Celsr2 is required for the maintenance of dendrites and that defects in it lead to retraction of branches. The term ‘maintenance’ is often used to describe the persistence of existing dendritic spines and branches over a certain period of time. It is noteworthy that this is more of a descriptive term than a mechanistic one. In other words, whether a ‘maintenance activity’ is required for proper dendrite morphogenesis and whether specific molecules are required for such activity have yet to be proven experimentally. By contrast, the fact that molecules such as Rac1 can regulate branch dynamics without affecting the final branch length or numbers [14] indicates that there are specific activities that regulate dendritic branch dynamics, rather than just extension or retraction, and therefore the stabilization of developing branches, which can be assessed by time-lapse experiments [4,5]. The defects observed in Celsr2-knockdown neurons could also be due to a defect in dendrite stabilization in the late phase of dendritic branch morphogenesis. It would be desirable to perform time-lapse experiments to find out whether and how Celsr2 regulates branch dynamics. Shima et al. also explored the domain requirement of the role of Celsr2 in dendrite morphogenesis by examining the effects of expressing deletion mutants of Celsr2 in Celsr2-defective neurons. They suggest that Celsr2– Celsr2 homophilic interaction and intracellular signaling transduced by Celsr2 are important for dendrite development [6]. The Drosophila ortholog of Celsr, flamingo, also controls dendrite morphogenesis. In multidendritic (md) neurons, loss of flamingo function results in overextension of the dorsal dendrites as a result of premature initiation and failure to stop at the appropriate location [27,28]. It seems that the dendritic localization of Flamingo protein is essential for proper dendrite morphogenesis because a missense mutation that causes the Flamingo protein to fail to localize to the dendrites and axons leads to overextension of dorsal dendrites. Similar to md neurons, mushroom-body neurons defective in Flamingo function exhibit overextension of dendrites [29]. How does Flamingo regulate dendrite morphology? The apparently different consequences of Flamingo and Celsr2 deficiency in Drosophila and mammals, namely overextension and retraction of dendritic branches, respectively, can be reconciled by assuming that members of the cadherin superfamily provide a signal for dendrite stabilization. The loss of stabilization activity could lead to either retraction or overextension of dendrites. The final outcome of such a loss depends on the prevailing mechanism that regulates the balance between retraction and extension (Figure 1). Time-lapse analysis is likely to be instrumental in testing this hypothesis. It is also possible that the signals downstream of Flamingo are different in mammalian and fly neurons. Currently, the nature of these signals remains elusive in both cases.
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Identification of such mechanisms will improve the understanding of why the consequences of impairment of Flamingo function are different between mammals and Drosophila. Despite circumstantial evidence suggesting that homophilic cadherin adhesion is required for dendrite stabilization, including homophilic adhesion in heterologous cells [6] and the synaptic localization of some cadherins and catenins [16,30,31], there is still a lack of definitive proof. One finding suggests that the scenario is probably more complicated. Although flamingo is expressed in both md neurons and their neighboring non-neuronal cells in Drosophila, driving flamingo expression in md neurons is sufficient to rescue the dendrite defects in flamingo-mutant animals [27]. These data raise the possibility that heterophilic adhesion or, perhaps, a receptor function of Flamingo is responsible for proper dendrite stopping. Therefore, it is important to determine whether the members of the cadherin superfamily regulate dendrite morphogenesis through homophilic or heterophilic adhesion or, alternatively, through binding to unknown ligands. It will also be important to determine whether the functions of Flamingo in dendrite morphogenesis are synaptic-contact dependent. If the late onset of the dendrite defects seen in Celsr2-defective neurons [6] faithfully reflects the timing of Celsr2 function in vivo, rather than being a consequence of a delayed RNAi effect, it is conceivable that Celsr2 might be involved in synapticcontact-dependent stabilization of dendrites. Concluding remarks In summary, several studies suggest that cadherins contribute to dendrite development by regulating the stabilization of dendritic branches or spines. In both Drosophila and mammals, the non-classic cadherin Flamingo has been found to control dendrite morphogenesis, probably by regulating dendrite stabilization. Whether homophilic cadherin adhesion is responsible for dendrite stabilization remains to be determined. Identification of the adhesion partners and downstream signaling mechanisms will provide better understanding of how cadherins, especially Flamingo, regulate dendrite morphogenesis. Acknowledgements We thank Wesley Grueber, Michael Kim, Chay Kuo, Jay Parrish, Fabrice Roegiers, Tadashi Uemura and Rebecca Yang for helpful comments and suggestions. B.Y. is a recipient of the National Research Service Award (1 F32 NS46847–01) and Y.N.J. is an investigator of the Howard Hughes Medical Institute.
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