Navigating the Anterior-Posterior Axis with Wnts
2006; Cell Press; Volume: 49; Issue: 6 Linguagem: Inglês
10.1016/j.neuron.2006.03.004
ISSN1097-4199
Autores Tópico(s)Nerve injury and regeneration
ResumoRecent studies have begun to shed light on the molecular guidance cues controlling anterior-posterior axon guidance. Two recent studies in the current issue of Developmental Cell show that Wnts play critical roles in patterning processes and directing neuronal migration in C. elegans. Together with previous findings in vertebrates and flies, these new results establish conserved function of Wnts in A-P guidance. Recent studies have begun to shed light on the molecular guidance cues controlling anterior-posterior axon guidance. Two recent studies in the current issue of Developmental Cell show that Wnts play critical roles in patterning processes and directing neuronal migration in C. elegans. Together with previous findings in vertebrates and flies, these new results establish conserved function of Wnts in A-P guidance. Despite their enormous numbers and complexity, axonal networks are extremely carefully organized. Much of the scaffold of the network is established during earlier developmental stages, when growth cones navigate in embryonic structures. Remarkably, early growth cone navigation largely follows the logic of embryonic patterning along the same major body axes: anterior-posterior (A-P) and dorsal-ventral. Axon guidance cues along the dorsal-ventral axis are relatively well understood. For example, Netrins and Slits are conserved cues that play roles in directing growth cones along the dorsal-ventral axis as they grow toward or away from the midline (Dickson, 2002Dickson B.J. Science. 2002; 298: 1959-1964Crossref PubMed Scopus (1170) Google Scholar). Little has been known about the identity of the cues axons recognize along the A-P axis. This gap in understanding has begun to be filled with the realization that Wnt family proteins act as directional guidance cues in the A-P axis of the vertebrate spinal cord (Lyuksyutova et al., 2003Lyuksyutova A.I. Lu C.C. Milanesio N. King L.A. Guo N. Wang Y. Nathans J. Tessier-Lavigne M. Zou Y. Science. 2003; 302: 1984-1988Crossref PubMed Scopus (464) Google Scholar, Imondi and Thomas, 2003Imondi R. Thomas J.B. Science. 2003; 302: 1903-1904Crossref PubMed Scopus (8) Google Scholar, Liu et al., 2005Liu Y. Shi J. Lu C.C. Wang Z.B. Lyuksyutova A.I. Song X. Zou Y. Nat. Neurosci. 2005; 8: 1151-1159Crossref PubMed Scopus (224) Google Scholar, Dickson, 2005Dickson B.J. Nat. Neurosci. 2005; 8: 1130-1132Crossref PubMed Scopus (12) Google Scholar). Ascending sensory axons are attracted to higher concentration of Wnts anteriorly via Frizzled 3, a seven transmembrane domain receptor, and conversely, descending corticospinal tract axons are repelled from higher levels of Wnts via Ryk (Derailed), a Wnt receptor first found to mediate Wnt repulsion in Drosophila. A guidance role for Wnts is conserved in the Drosophila ventral nerve cord, where Wnt5 determines the pathway choice during midline crossing, allowing a subset of commissural axons to cross the midline only through the anterior commissure in each segment because they avoid Wnt5 in the posterior commissure via the Derailed receptor (Yoshikawa et al., 2003Yoshikawa S. McKinnon R.D. Kokel M. Thomas J.B. Nature. 2003; 422: 583-588Crossref PubMed Scopus (343) Google Scholar). Whether Wnts have a global A-P guidance role in Drosophila in addition to this intrasegmental A-P role is unknown. In the current issue of Developmental Cell, two elegant papers provide compelling evidence that multiple Wnts act as directional cues to control the A-P migration of growth cones and neuronal cell bodies as well as the initial polarity of neuronal processes in C. elegans (Hilliard and Bargmann, 2006Hilliard M.A. Bargmann C.I. Dev. Cell. 2006; 10: 379-390Abstract Full Text Full Text PDF PubMed Scopus (155) Google Scholar, Pan et al., 2006Pan C.-L. Howell J.E. Clark S.G. Hilliard M. Cordes S. Bargmann C.I. Garriga G. Dev. Cell. 2006; 10: 367-377Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar). A third independent study also led to the finding that Wnt signaling controls neuronal polarity in the A-P axis in C. elegans (Prasad and Clark, 2006Prasad, B.C., and Clark, S.G. (2006). Development, in press.Google Scholar). These new exciting findings not only establish a conserved role of Wnt family signaling proteins in axon patterning along the long axis in animals but also provide intriguing new insights into the diverse mechanisms neurons adopt to utilize the directional information provided by Wnts. Given the number of Wnt proteins and receptors and their multitude of mechanisms, this family of guidance cues may play a major role in circuit assembly along the A-P axis. The search for A-P guidance mechanisms in C. elegans was first conducted by investigating neuronal cell migration along the A-P axis. Cynthia Kenyon's laboratory identified wnt/egl-20 as a gene required for normal A-P migration of the QL and QR neuroblasts, which give rise to sensory neurons on the left and right sides of the worm. frizzled/lin-17 and frizzled/mig-1 were also shown to be part of this regulatory system (Harris et al., 1996Harris J. Honigberg L. Robinson N. Kenyon C. Development. 1996; 122: 3117-3131PubMed Google Scholar). Subsequent analyses established that QL and QR cells respond differently to EGL-20 in a dose-dependent manner, such that QL is more sensitive to EGL-20 than QR. High levels of EGL-20 promote posterior migration by activating the canonical Wnt gene expression pathway in QL, inducing expression of the Hox gene mab-5 and a change in cell identity and A-P position (Maloof et al., 1999Maloof J.N. Whangbo J. Harris J.M. Jongeward G.D. Kenyon C. Development. 1999; 126: 37-49PubMed Google Scholar). Low levels of EGL-20 promote anterior migration of QR through a different and unknown pathway (Whangbo and Kenyon, 1999Whangbo J. Kenyon C. Mol. Cell. 1999; 4: 851-858Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar). Because the QR descendents do not require a localized source of EGL-20 to migrate anteriorly, EGL-20 was thought to be a permissive cue rather than a directional guidance cue for A-P cell migration (Whangbo and Kenyon, 1999Whangbo J. Kenyon C. Mol. Cell. 1999; 4: 851-858Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar). Taking advantage of the wealth of mutant wnt and frizzled strains available in C. elegans, Gian Garriga's group from the University of California, Berkeley systematically analyzed the function of Wnts and Frizzled receptors in neuronal migration and axon guidance along the A-P axis in C. elegans. Remarkably, they found that all five Wnts and four Frizzleds in C. elegans function in neuronal migration and a subset of Wnts control anterior axon guidance, and at least Wnt/EGL-20 can function as a repellent that is sensed by Frizzled proteins. The hermaphrodite-specific neurons (HSNs), a pair of motor neurons that control egg laying, are born at the posterior end of the worm body and migrate anteriorly. Wnt/EGL-20 and Frizzled/MIG-1 are required for this anterior migration: mutations in either gene cause HSNs to terminate their migration posterior to their normal positions. Although single mutants of four other wnt genes, cwn-1, cwn-2, lin-44, and mom-2, showed few or no posteriorly displaced HSNs, double mutants significantly enhanced egl-20 mutant phenotypes. Through an impressive series of genetic analyses, involving a sensitized background that eliminated another signaling cell, the Garriga lab convincingly showed that EGL-20 expressed in the tail of embryos and larvae serves as a repellent for the HSN cell bodies. Similarly, single mutations of the frizzled genes, mom-5 and cfz-2, and the Derailed/Ryk gene lin-18, had no effect on HSN on their own but enhanced the HSN migration defects of mig-1/frizzled mutants. Interestingly, another Frizzled gene, lin-17, appeared to antagonize mig-1, suggesting that Frizzled signaling has unexpected and unexplored complications. Building on this observation, the Garriga lab found that redundant Wnts and Frizzleds direct axon pathfinding along the A-P axis. The AVM and PVM mechanosensory neurons each extend a single process that initially projects ventrally and enters the ventral nerve cord then turn anteriorly and grow along the A-P axis. None of the Wnt, Frizzled, or lin-18/Ryk single mutants showed significant defects in AVM and PVM axon guidance, but double Wnt and Frizzled mutants displayed several A-P guidance defects, including premature stopping, abnormal branching at the ventral nerve cord, and posterior turning at the ventral nerve cord. The repulsion of AVM and PVM growth cones by these Wnts is mediated by two Frizzled receptors, MIG-1 and MOM-5. Importantly, overexpression of EGL-20 showed that a localized source of EGL-20 is important for AVM/PVM process guidance. Ectopic anterior expression of EGL-20 from a lim4 promoter caused AVM and PVM axon stopping and turning away from the EGL-20-expressing cells. Therefore, multiple Wnts may act as directional cues for axon and cell migrations along the A-P axis of the C. elegans. Looking into the A-P patterning of two other mechanosensory neurons, PLM and ALM, Cori Bargmann's group at the Rockefeller University found a distinct and novel function of Wnt signaling in regulating neuronal polarity. The PLM neuron has a longer anterior process and shorter posterior process that extend directly from the cell body. The ALM neuron has one single anterior process that extends from the cell body. Wnt signaling is again important for the A-P disposition of these processes. However, rather than acting as either attractive or repulsive directional cues, Wnts appear to control neuronal polarity along the A-P axis and thus affect the A-P orientation of the processes. In both lin-44 and lin-17 mutants, the PLM anterior process was severely shortened, whereas the posterior process became much lengthened. Analyses of the synaptic vesicle protein SNB-1 indicated that the entire PLM neuron was reversed along the body axis, sending its synapses into the wrong process. In addition, lin-44 and lin-17 double mutants showed similar defects to single mutants, suggesting that lin-44 and lin-17 are in the same genetic pathway. The gene lin-17 appears to function autonomously in the PLM, an important control for a frizzled gene that has many effects on cell fate patterning. What is interesting is that, in contrast to EGL-20 in HSN migration and AVM/PVM migration, the localization of LIN-44 appears to be relatively unimportant to the A-P orientation of the processes, for which LIN-44 might therefore act as a permissive cue. Alternatively, other Wns may also contribute to polarizing neuronal cell bodies along the A-P axis, because Wnt signaling was suggested to play an instructive role in polarizing nonneuronal cells along the A-P axis in C. elegans (Goldstein et al., 2006Goldtein B. Takeshita H. Mizumoto K. Sawa H. Dev. Cell. 2006; 10: 391-396Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar). An elegant protein localization experiment from the Bargmann lab revealed a promising clue as to how Wnt signaling regulates neuronal polarity: a LIN-17-mRFP translational fusion protein was enriched in the PLM posterior process, suggesting that the Frizzled protein is distributed asymmetrically along the A-P axis. The asymmetric distribution of LIN-17 is dependent on LIN-44, although LIN-44 overexpression and misexpression had little effect on PLM neurons. For the ALM, located on the anterior side of the worm body, LIN-17 and LIN-44 are not important, but the redundant wnts, cwn-1 and egl-20, have an analogous polarity-reversing effect on ALM development. It is currently unknown which Frizzled is involved in ALM, because lin-17 mutants showed normal A-P polarity. However, overexpression of LIN-17 caused reversed ALM morphology. It is interesting that the A-P polarity phenotype appears reversed rather than randomized, suggesting that Wnt signaling may act as a balancing factor against another polarizing activity along the A-P axis. These new studies help to establish that Wnt signaling is an ancient strategy in patterning axonal connections along the A-P axis, from mammals to nematodes. Meanwhile, several new insights have emerged from these results. First, Frizzleds are diverse in function. So far, vertebrate studies have only identified attractive function of Frizzleds: Frizzled 3 mediates the attraction of commissural axons to Wnt4 and, probably, Wnt7b, during the anterior turning after midline crossing (Lyuksyutova et al., 2003Lyuksyutova A.I. Lu C.C. Milanesio N. King L.A. Guo N. Wang Y. Nathans J. Tessier-Lavigne M. Zou Y. Science. 2003; 302: 1984-1988Crossref PubMed Scopus (464) Google Scholar), and several Frizzleds mediate the attractive response of dorsal retinal ganglion cell axons to low concentrations of Wnt3 in retinotopic mapping (Schmitt et al., 2006Schmitt A.M. Shi J. Wolf A.M. Lu C.C. King L.A. Zou Y. Nature. 2006; 439: 31-37Crossref PubMed Scopus (233) Google Scholar); similarly, in Drosophila, DFrizzled 2 mediates an attractive response of photoreceptor axons to DWnt4 in ventral lamina (Sato et al., 2006Sato M. Umetsu D. Murakami S. Yasugi T. Tabata T. Nat. Neurosci. 2006; 9: 67-75Crossref PubMed Scopus (70) Google Scholar). Conversely, in Drosophila, Derailed mediates Wnt5 repulsion during midline pathway selection (Yoshikawa et al., 2003Yoshikawa S. McKinnon R.D. Kokel M. Thomas J.B. Nature. 2003; 422: 583-588Crossref PubMed Scopus (343) Google Scholar), and vertebrate Ryk mediates repulsion of corticospinal tract axons from Wnt1 and Wnt5a and the repulsion of retinal ganglion cell axons from Wnt3 (Liu et al., 2005Liu Y. Shi J. Lu C.C. Wang Z.B. Lyuksyutova A.I. Song X. Zou Y. Nat. Neurosci. 2005; 8: 1151-1159Crossref PubMed Scopus (224) Google Scholar, Schmitt et al., 2006Schmitt A.M. Shi J. Wolf A.M. Lu C.C. King L.A. Zou Y. Nature. 2006; 439: 31-37Crossref PubMed Scopus (233) Google Scholar). In C. elegans, it appears that two Frizzled receptors, MIG-1 and MOM-5, actually mediate repulsion by EGL-20. Therefore, Frizzleds can mediate repulsion as well as attraction. Furthermore, another Frizzled receptor, LIN-17, antagonizes the function of MIG-1. These interactions may add to the repertoire of Frizzled receptor functions in nervous system wiring. It will be informative to compare the sequence differences of these different Frizzled receptors in both C. elegans and in vertebrates to discern the molecular mechanisms of this diversity. In a new piece of cell biology reminiscent of planar cell polarity, Frizzleds control neuronal cell polarity along the A-P axis, which has major implication in the studies of neuronal polarity. The diversity of Frizzled function suggests that interesting results will come from analysis of intracellular signaling cascades downstream of Frizzled in the nervous system. Second, Frizzled proteins can be asymmetrically localized in neurons. The asymmetric localization of the Frizzled proteins may determine the initial A-P neuronal polarity and potentially the direction of further growth along the A-P axis. Although guidance cues are known to cause directional turning of growth cones, it is still unknown what intracellular asymmetry allows growth cones to turn along a certain direction in response to a guidance cue. In the case of cell polarity, a mechanism involving Frizzled protein localization provides a possible insight. However, this is the first example where a guidance receptor is differentially distributed in the direction of polarity and growth. It will be of great interest to test how other Frizzled proteins, such as MIG-1 and MOM-5, are distributed during the guidance of the AVM and PVM processes along the A-P axis. Third, the direction of the Wnt gradients can be opposite in different animals. In mammals, Wnt-Frizzled signaling appears to respond to an anterior high, posterior low Wnt gradient (Lyuksyutova et al., 2003Lyuksyutova A.I. Lu C.C. Milanesio N. King L.A. Guo N. Wang Y. Nathans J. Tessier-Lavigne M. Zou Y. Science. 2003; 302: 1984-1988Crossref PubMed Scopus (464) Google Scholar). In C. elegans, high posterior Wnt expression was observed. In the fly midline, Wnt5 is enriched in the posterior commissural in each embryonic segment, in a posterior high gradient, the same direction as in C. elegans. Therefore, although Wnt signaling has a conserved role in controlling A-P directionality, the direction of gradients can be distinct. In retinotectal map formation, vertebrate Wnt3 is expressed in a dorsal high, ventral low gradient and repels most of the retinal ganglion cell axons via Ryk (Schmitt et al., 2006Schmitt A.M. Shi J. Wolf A.M. Lu C.C. King L.A. Zou Y. Nature. 2006; 439: 31-37Crossref PubMed Scopus (233) Google Scholar). The Drosophila visual system also appears to use Wnt signaling for retinotopic mapping along the dorsal-ventral axis, except that DWnt4 is enriched in the ventral target and attracts photoreceptor axons via DFrizzled 2 (Sato et al., 2006Sato M. Umetsu D. Murakami S. Yasugi T. Tabata T. Nat. Neurosci. 2006; 9: 67-75Crossref PubMed Scopus (70) Google Scholar). In recently years, the question of axon guidance along the A-P axis has received increasing attention. With the identification of these molecular cues, many new questions can be addressed. Wnt signaling has been suggested as a conserved mechanism to convey A-P directionality. In addition, Shh was proposed to regulate A-P guidance in the chick (Bourikas et al., 2005Bourikas D. Pekarik V. Baeriswyl T. Grunditz A. Sadhu R. Nardo M. Stoeckli E.T. Nat. Neurosci. 2005; 8: 297-304Crossref PubMed Scopus (199) Google Scholar, Stoeckli, 2006Stoeckli E.T. Curr. Opin. Neurobiol. 2006; 16: 35-39Crossref PubMed Scopus (29) Google Scholar). It is interesting to ask whether all axons are guided by Wnts and Shh or whether there are additional guidance cues. Along the vertebrate dorsal-ventral axis, Netrin and Shh pull axons ventrally, and BMPs push axons from the dorsal side (Schnorrer and Dickson, 2004Schnorrer F. Dickson B.J. Curr. Biol. 2004; 14: R19-R21Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). Along the A-P axis, Wnts appear to be one major force to direct A-P growth. Are there any other forces that push and pull axons together with Wnts along this axis? Do Wnts and Shh act together on the same axons, or on different axons? In the case of cell polarity control in C. elegans, loss of Wnt signaling appears to reverse the anterior and posterior processes, suggesting that there is another opposing force and that, once the Wnt activity is eliminated, the other force, which is likely weaker, takes over. What is the other competing force? This unidentified force could be another A-P guidance mechanism. Yet another important question is how these axons stop in the right A-P position. Certainly, understanding these guidance systems along the long axis will significantly advance our knowledge of how the nervous system is wired into highly organized networks. In human spinal cord, there are over 40 different longitudinal tracts. Knowing how they are guided along the A-P axis will likely benefit the efforts of therapeutic design for spinal cord injury and regeneration. Converging studies in mammals, flies, and C. elegans point to a common mechanism whereby directionality along the A-P axis may be conveyed by Wnt concentration gradients. Much remains to be done to explore the mechanisms of gradients in axonal guidance. Questions like how steep these gradients are in vivo, how they are detected to allow directional response of the growth cone, and how they are established will continue to stimulate investigation in years to come. Wnt Signals and Frizzled Activity Orient Anterior-Posterior Axon Outgrowth in C. elegansHilliard et al.Developmental CellMarch, 2006In BriefSecreted proteins of the Wnt family affect axon guidance, asymmetric cell division, and cell fate. We show here that C. elegans Wnts acting through Frizzled receptors can shape axon and dendrite trajectories by reversing the anterior-posterior polarity of neurons. In lin-44/Wnt and lin-17/Frizzled mutants, the polarity of the PLM mechanosensory neuron is reversed along the body axis: the long PLM process, PLM growth cone, and synapses are posterior to its cell body instead of anterior. Similarly, the polarity of the ALM mechanosensory neuron is reversed in cwn-1 egl-20 Wnt double mutants, suggesting that different Wnt signals regulate neuronal polarity at different anterior-posterior positions. Full-Text PDF Open ArchiveMultiple Wnts and Frizzled Receptors Regulate Anteriorly Directed Cell and Growth Cone Migrations in Caenorhabditis elegansPan et al.Developmental CellMarch, 2006In BriefA set of conserved molecules guides axons along the metazoan dorsal-ventral axis. Recently, Wnt glycoproteins have been shown to guide axons along the anterior-posterior (A/P) axis of the mammalian spinal cord. Here, we show that, in the nematode Caenorhabditis elegans, multiple Wnts and Frizzled receptors regulate the anterior migrations of neurons and growth cones. Three Wnts are expressed in the tail, and at least one of these, EGL-20, functions as a repellent. We show that the MIG-1 Frizzled receptor acts in the neurons and growth cones to promote their migrations and provide genetic evidence that the Frizzleds MIG-1 and MOM-5 mediate the repulsive effects of EGL-20. Full-Text PDF Open Archive
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