Sonic hedgehog is indirectly required for intraretinal axon pathfinding by regulating chemokine expression in the optic stalk

Successful axon pathfinding requires both correct patterning of tissues, which will later harbor axonal tracts, and precise localization of axon guidance cues along these tracts at the time of axon outgrowth. Retinal ganglion cell (RGC) axons grow towards the optic disc in the central retina, where they turn to exit the eye through the optic nerve. Normal patterning of the optic disc and stalk and the expression of guidance cues at this choice point are necessary for the exit of RGC axons out of the eye. Sonic hedgehog (Shh) has been implicated in both patterning of ocular tissue and direct guidance of RGC axons. Here, we examine the precise spatial and temporal requirement for Hedgehog (Hh) signaling for intraretinal axon pathfinding and show that Shh acts to pattern the optic stalk in zebrafish but does not guide RGC axons inside the eye directly. We further reveal an interaction between the Hh and chemokine pathways for axon guidance and show that cxcl12a functions downstream of Shh and depends on Shh for its expression at the optic disc. Together, our results support a model in which Shh acts in RGC axon pathfinding indirectly by regulating axon guidance cues at the optic disc through patterning of the optic stalk.     

Distinct roles for Robo2 in the regulation of axon and dendrite growth by retinal ganglion cells

Guidance factors act on the tip of a growing axon to direct it to its target. What role these molecules play, however, in the control of the dendrites that extend from that axon’s cell body is poorly understood. Slits, through their Robo receptors, guide many types of axons, including those of retinal ganglion cells (RGCs). Here we assess and contrast the role of Slit/Robo signalling in the growth and guidance of the axon and dendrites extended by RGCs in Xenopus laevis. As Xenopus RGCs extend dendrites, they express robo2 and robo3, while slit1 and slit2 are expressed in RGCs and in the adjacent inner nuclear layer. Interestingly, our functional data with antisense knockdown and dominant negative forms of Robo2 (dnRobo2) and Robo3 (dnRobo3) indicate that Slit/Robo signalling has no role in RGC dendrite guidance, and instead is necessary to stimulate dendrite branching, primarily via Robo2. Our in vitro culture data argue that Slits are the ligands involved. In contrast, both dnRobo2 and dnRobo3 inhibited the extension of axons and caused the misrouting of some axons. Based on these data, we propose that Robo signalling can have distinct functions in the axon and dendrites of the same cell, and that the specific combinations of Robo receptors could underlie these differences. Slit acts via Robo2 in dendrites as a branching/growth factor but not in guidance, while Robo2 and Robo3 function in concert in axons to mediate axonal interactions and respond to Slits as guidance factors. These data underscore the likelihood that a limited number of extrinsic factors regulate the distinct morphologies of axons and dendrites.     

Patterning of the basal telencephalon and hypothalamus is essential for guidance of cortical projections

We have investigated the mechanisms that control the guidance of corticofugal projections as they extend along different subdivisions of the forebrain. To this aim, we analyzed the development of cortical projections in mice that lack Nkx2-1, a homeobox gene whose expression is restricted to two domains within the forebrain: the basal telencephalon and the hypothalamus. Molecular respecification of the basal telencephalon and hypothalamus in Nkx2-1-deficient mice causes a severe defect in the guidance of layer 5 cortical projections and ascending fibers of the cerebral peduncle. These axon tracts take an abnormal path when coursing through both the basal telencephalon and hypothalamus. By contrast, loss of Nkx2-1 function does not impair guidance of corticothalamic or thalamocortical axons. In vitro experiments demonstrate that the basal telencephalon and the hypothalamus contain an activity that repels the growth of cortical axons, suggesting that loss of this activity is the cause of the defects observed in Nkx2-1 mutants. Furthermore, analysis of the expression of candidate molecules in the basal telencephalon and hypothalamus of Nkx2-1 mutants suggests that Slit2 contributes to this activity.     

GAP-43 mediates retinal axon interaction with lateral diencephalon cells during optic tract formation

GAP-43 is an abundant intracellular growth cone protein that can serve as a PKC substrate and regulate calmodulin availability. In mice with targeted disruption of the GAP-43 gene, retinal ganglion cell (RGC) axons fail to progress normally from the optic chiasm into the optic tracts. The underlying cause is unknown but, in principle, can result from either the disruption of guidance mechanisms that mediate axon exit from the midline chiasm region or defects in growth cone signaling required for entry into the lateral diencephalic wall to form the optic tracts. Results here show that, compared to wild-type RGC axons, GAP-43-deficient axons exhibit reduced growth in the presence of lateral diencephalon cell membranes. Reduced growth is not observed when GAP-43-deficient axons are cultured with optic chiasm, cortical, or dorsal midbrain cells. Lateral diencephalon cell conditioned medium inhibits growth of both wild-type and GAP-43-deficient axons to a similar extent and does not affect GAP-43-deficient axons more so. Removal or transplant replacement of the lateral diencephalon optic tract entry zone in GAP-43-deficient embryo preparations results in robust RGC axon exit from the chiasm. Together these data show that RGC axon exit from the midline region does not require GAP-43 function. Instead, GAP-43 appears to mediate RGC axon interaction with guidance cues in the lateral diencephalic wall, suggesting possible involvement of PKC and calmodulin signaling during optic tract formation.     

Kinase independent function of EphB receptors in retinal axon pathfinding to the optic disc from dorsal but not ventral retina

Optic nerve formation requires precise retinal ganglion cell (RGC) axon pathfinding within the retina to the optic disc, the molecular basis of which is not well understood. At CNS targets, interactions between Eph receptor tyrosine kinases on RGC axons and ephrin ligands on target cells have been implicated in formation of topographic maps. However, studies in chick and mouse have shown that both Eph receptors and ephrins are also expressed within the retina itself, raising the possibility that this receptor-ligand family mediates aspects of retinal development. Here, we more fully document the presence of specific EphB receptors and B-ephrins in embryonic mouse retina and provide evidence that EphB receptors are involved in RGC axon pathfinding to the optic disc. We find that as RGC axons begin this pathfinding process, EphB receptors are uniformly expressed along the dorsal-ventral retinal axis. This is in contrast to the previously reported high ventral-low dorsal gradient of EphB receptors later in development when RGC axons map to CNS targets. We show that mice lacking both EphB2 and EphB3 receptor tyrosine kinases, but not each alone, exhibit increased frequency of RGC axon guidance errors to the optic disc. In these animals, major aspects of retinal development and cellular organization appear normal, as do the expression of other RGC guidance cues netrin, DCC, and L1. Unexpectedly, errors occur in dorsal but not ventral retina despite early uniform or later high ventral expression of EphB2 and EphB3. Furthermore, embryos lacking EphB3 and the kinase domain of EphB2 do not show increased errors, consistent with a guidance role for the EphB2 extracellular domain. Thus, while Eph kinase function is involved in RGC axon mapping in the brain, RGC axon pathfinding within the retina is partially mediated by EphB receptors acting in a kinase-independent manner.     

Robo2 is required for Slit-mediated intraretinal axon guidance

The developing optic pathway has proven one of the most informative model systems for studying mechanisms of axon guidance. The first step in this process is the directed extension of retinal ganglion cell (RGC) axons within the optic fibre layer (OFL) of the retina towards their exit point from the eye, the optic disc. Previously, we have shown that the inhibitory guidance molecules, Slit1 and Slit2, regulate two distinct aspects of intraretinal axon guidance in a region-specific manner. Using knockout mice, we have found that both of these guidance activities are mediated via Robo2. Of the four vertebrate Robos, only Robo1 and Robo2 are expressed by RGCs. In mice lacking robo1 intraretinal axon guidance occurs normally. However, in mice lacking robo2 RGC axons make qualitatively and quantitatively identical intraretinal pathfinding errors to those reported previously in Slit mutants. This demonstrates clearly that, as in other regions of the optic pathway, Robo2 is the major receptor required for intraretinal axon guidance. Furthermore, the results suggest strongly that redundancy with other guidance signals rather than different receptor utilisation is the most likely explanation for the regional specificity of Slit function during intraretinal axon pathfinding.     

Slit1 and Slit2 cooperate to prevent premature midline crossing of retinal axons in the mouse visual system.

During development, retinal ganglion cell (RGC) axons either cross or avoid the midline at the optic chiasm. In Drosophila, the Slit protein regulates midline axon crossing through repulsion. To determine the role of Slit proteins in RGC axon guidance, we disrupted Slit1 and Slit2, two of three known mouse Slit genes. Mice defective in either gene alone exhibited few RGC axon guidance defects, but in double mutant mice a large additional chiasm developed anterior to the true chiasm, many retinal axons projected into the contralateral optic nerve, and some extended ectopically-dorsal and lateral to the chiasm. Our results indicate that Slit proteins repel retinal axons in vivo and cooperate to establish a corridor through which the axons are channeled, thereby helping define the site in the ventral diencephalon where the optic chiasm forms.     

NF-Protocadherin Regulates Retinal Ganglion Cell Axon Behaviour in the Developing Visual System

Cell adhesion molecules play a central role in mediating axonal tract development within the nascent nervous system. NF-protocadherin (NFPC), a member of the non-clustered protocadherin family, has been shown to regulate retinal ganglion cell (RGC) axon and dendrite initiation, as well as influencing axonal navigation within the mid-optic tract. However, whether NFPC mediates RGC axonal behaviour at other positions within the optic pathway remains unclear. Here we report that NFPC plays an important role in RGC axonogenesis, but not in intraretinal guidance. Moreover, axons with reduced NFPC levels exhibit insensitivity to Netrin-1, an attractive guidance cue expressed at the optic nerve head. Netrin-1 induces rapid turnover of NFPC localized to RGC growth cones, suggesting that the regulation of NFPC protein levels may underlie Netrin-1-mediated entry of RGC axons into the optic nerve head. At the tectum, we further reveal a function for NFPC in controlling RGC axonal entry into the final target area. Collectively, our results expand our understanding of the role of NFPC in RGC guidance and illustrate that this adhesion molecule contributes to axon behaviour at multiple points in the optic pathway.     

GABA and development of the Xenopus optic projection

In the developing visual system of Xenopus laevis retinal ganglion cell (RGC) axons extend through the brain towards their major target in the midbrain, the optic tectum. Enroute, the axons are guided along their pathway by cues in the environment. In vitro, neurotransmitters have been shown to act chemotropically to influence the trajectory of extending axons and regulate the outgrowth of developing neurites, suggesting that they may act to guide or modulate the growth of axons in vivo. Previous work by Roberts and colleagues (1987) showed that populations of cells within the developing Xenopus diencephalon and mid-brain express the neurotransmitter gamma amino butyric acid (GABA). Here we show that Xenopus RGC axons in the midoptic tract grow alongside the GABAergic cells and cross their GABA immunopositive nerve processes. Moreover, RGC axons and growth cones express GABA-A and GABA-B receptors, and GABA and the GABA-B receptor agonist baclofen both stimulate RGC neurite outgrowth in culture. Finally, the GABA-B receptor antagonist CGP54626 applied to the developing optic projection in vivo causes a dose-dependent shortening of the optic projection. These data indicate that GABA may act in vivo to stimulate the outgrowth of Xenopus RGC axons along the optic tract.     

Ephrin‐B2 elicits differential growth cone collapse and axon retraction in retinal ganglion cells from distinct retinal regions

The circuit for binocular vision and stereopsis is established at the optic chiasm, where retinal ganglion cell (RGC) axons diverge into the ipsilateral and contralateral optic tracts. In the mouse retina, ventrotemporal (VT) RGCs express the guidance receptor EphB1, which interacts with the repulsive guidance cue ephrin‐B2 on radial glia at the optic chiasm to direct VT RGC axons ipsilaterally. RGCs in the ventral retina also express EphB2, which interacts with ephrin‐B2, whereas dorsal RGCs express low levels of EphB receptors. To investigate how growth cones of RGCs from different retinal regions respond upon initial contact with ephrin‐B2, we utilized time‐lapse imaging to characterize the effects of ephrin‐B2 on growth cone collapse and axon retraction in real time. We demonstrate that bath application of ephrin‐B2 induces rapid and sustained growth cone collapse and axon retraction in VT RGC axons, whereas contralaterally‐projecting dorsotemporal RGCs display moderate growth cone collapse and little axon retraction. Dose response curves reveal that contralaterally‐projecting ventronasal axons are less sensitive to ephrin‐B2 treatment compared to VT axons. Additionally, we uncovered a specific role for Rho kinase signaling in the retraction of VT RGC axons but not in growth cone collapse. The detailed characterization of growth cone behavior in this study comprises an assay for the study of Eph signaling in RGCs, and provides insight into the phenomena of growth cone collapse and axon retraction in general. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 781–794, 2010     

Perturbations of MicroRNA Function in Mouse Dicer Mutants Produce Retinal Defects and Lead to Aberrant Axon Pathfinding at the Optic Chiasm

Background

During development axons encounter a variety of choice points where they have to make appropriate pathfinding decisions. The optic chiasm is a major decision point for retinal ganglion cell (RGC) axons en route to their target in order to ensure the correct wiring of the visual system. MicroRNAs (miRNAs) belong to the class of small non-coding RNA molecules and have been identified as important regulators of a variety of processes during embryonic development. However, their involvement in axon guidance decisions is less clear.

Methodology/Principal Findings

We report here that the early loss of Dicer, an essential protein for the maturation of miRNAs, in all cells of the forming retina and optic chiasm leads to severe phenotypes of RGC axon pathfinding at the midline. Using a conditional deletion approach in mice, we find in homozygous Dicer mutants a marked increase of ipsilateral projections, RGC axons extending outside the optic chiasm, the formation of a secondary optic tract and a substantial number of RGC axons projecting aberrantly into the contralateral eye. In addition, the mutant mice display a microphthalmia phenotype.

Conclusions

Our work demonstrates an important role of Dicer controlling the extension of RGC axons to the brain proper. It indicates that miRNAs are essential regulatory elements for mechanisms that ensure correct axon guidance decisions at the midline and thus have a central function in the establishment of circuitry during the development of the nervous system.     

The development of a simple scaffold of axon tracts in the brain of the embryonic zebrafish, Brachydanio rerio

We have examined neuronal differentiation and the formation of axon tracts in the embryonic forebrain and midbrain of the zebrafish, between 1 and 2 days postfertilisation. Axons were visualised with three techniques; immunocytochemistry (using HNK-1 and antiacetylated tubulin antibodies) and horseradish peroxidase (HRP) labelling in whole-mounted brains, and transmission electron microscopy. Differentiation was monitored by histochemical staining for acetylcholinesterase (AChE). These independent methods demonstrated that a simple grid of tracts and commissures forms the initial axon scaffold of the brain. At 1 day, the olfactory nerve, four commissures, their associated tracts and three other non-commissural tracts are present. By 2 days, these tracts and commissures have all greatly enlarged and, in addition, the optic nerve and tract, and three new commissures and their associated tracts have been added. Small applications of HRP at various sites revealed the origins and projections of some of these earliest axons. Retrogradely labelled cell bodies originated from regions that were also positive for AChE activity. At 1 day, HRP-labelled axons were traced: (1) from the olfactory placode through the olfactory nerve to the dorsal telencephalon; (2) from the telencephalon into the tract of the anterior commissure and also to the postoptic region of the diencephalon; (3) from the hindbrain through the ventral midbrain and diencephalon to the postoptic commissure; (4) from the dorsal diencephalon (in or near the epiphysis) to the tract of the postoptic commissure; (5) from ventral and rostral midbrain through the posterior commissure. Three new projections were demonstrated at 2 days: (1) from the retina through the tract of the postoptic commissure to the tectum; (2) from the telencephalon to the contralateral diencephalon; and (3) from the telencephalon to the ventral flexure. These results show that at 1 day, the zebrafish brain is impressively simple, with a few small, well-separated tracts but by 2 days the brain is already considerably more complex. Most of the additional axons added onto pre-existent tracts rather than pioneered new ones supporting the notion that other axons play a crucial role in the guidance of early central nervous system (CNS) axons.     

Role of cell adhesion molecule DM-GRASP in growth and orientation of retinal ganglion cell axons

The cell adhesion molecule (CAM) DM-GRASP was investigated with respect to a role for axonal growth and navigation in the developing visual system. Expression analysis reveals that DM-GRASP's presence is highly spatiotemporally regulated in the chick embryo retina. It is restricted to the optic fiber layer (OFL) and shows an expression maximum in a phase when the highest number of retinal ganglion cell (RGC) axons extend. In the developing retina, axons grow between the DM-GRASP-displaying OFL and the Laminin-rich basal lamina. We show that DM-GRASP enhances RGC axon extension and growth cone size on Laminin substrate in vitro. Preference assays reveal that DM-GRASP-containing lanes guide RGC axons, partially depending on NgCAM in the axonal membrane. Inhibition of DM-GRASP in organ-cultured eyes perturbs orientation of RGC axons at the optic fissure. Instead of leaving the retina, RGC axons cross the optic fissure and grow onto the opposite side of the retina. RGC axon extension per se and navigation from the peripheral retina towards the optic fissure, however, is not affected. Our results demonstrate a role of DM-GRASP for axonal pathfinding in an early phase of the formation of the higher vertebrate central nervous system.     

Role of Slit proteins in the vertebrate brain.

Diffusible chemorepellents play a major role in guiding developing axons towards their correct targets by preventing them from entering or steering them away from certain regions. Genetic studies in Drosophila revealed a novel repulsive guidance system that prevents inappropriate axons from crossing the CNS midline; this repulsive system is mediated by the Roundabout (Robo) receptors and their secreted ligand Slits. Three distinct slit genes (slit1, slit2 and slit3) and three distinct robo genes (robo1, robo2 and rig-1) have been cloned in mammals. In collagen gel co-cultures, Slit1 and Slit2 can repel and collapse olfactory axons. However, there is also some positive effect associated with Slits, as Slit2 stimulates the formation of axon collateral branches by NGF-responsive neurons of the dorsal root ganglia (DRG). Slit2 is a large ECM glycoproteins of about 200 kD, which is proteolytically processed into 140 kD N-terminal and 55-60 kD C-terminal fragments. Slit2 cleavage fragments appear to have different cell association characteristics, with the smaller C-terminal fragment being more diffusible and the larger N-terminal and uncleaved fragments being more tightly cell associated. This suggested that the different fragments might have different functional activities in vivo. We have begun to explore these questions by engineering mutant and truncated versions of hSlit2 representing the two cleavage fragments, N- and C-, and the uncleavable molecule and examining the activities of these mutants in binding and functional assays. We found that an axon's response to Slit2 is not absolute, but rather is reflective of the context in which the protein is encountered.     

Slit1a inhibits retinal ganglion cell arborization and synaptogenesis via Robo2-dependent and -independent pathways

Upon arriving at their targets, developing axons cease pathfinding and begin instead to arborize and form synapses. To test whether CNS arborization and synaptogenesis are controlled by Slit-Robo signaling, we followed single retinal ganglion cell (RGC) arbors over time. ast (robo2) mutant and slit1a morphant arbors had more branch tips and greater arbor area and complexity compared to wild-type and concomitantly more presumptive presynaptic sites labeled with YFP-Rab3. Increased arborization in ast was phenocopied by dominant-negative Robo2 expressed in single RGCs and rescued by full-length Robo2, indicating that Robo2 acts cell-autonomously. Time-lapse imaging revealed that ast and slit1a morphant arbors stabilized earlier than wild-type, suggesting a role for Slit-Robo signaling in preventing arbor maturation. Genetic analysis showed that Slit1a acts both through Robo2 and Robo2-independent mechanisms. Unlike previous PNS studies showing that Slits promote branching, our results show that Slits inhibit arborization and synaptogenesis in the CNS.     

Matrix metalloproteinases are required for retinal ganglion cell axon guidance at select decision points

Axons receive guidance information from extrinsic cues in their environment in order to reach their targets. In the frog Xenopus laevis, retinal ganglion cell (RGC) axons make three key guidance decisions en route through the brain. First, they cross to the contralateral side of the brain at the optic chiasm. Second, they turn caudally in the mid-diencephalon. Finally, they must recognize the optic tectum as their target. The matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase (ADAM) families are zinc (Zn)-dependent proteolytic enzymes. The latter functions in axon guidance, but a similar role has not yet been identified for the MMP family. Our previous work implicated metalloproteinases in the guidance decisions made by Xenopus RGC axons. To test specifically the importance of MMPs, we used two different in vivo exposed brain preparations in which RGC axons were exposed to an MMP-specific pharmacological inhibitor (SB-3CT), either as they reached the optic chiasm or as they extended through the diencephalon en route to the optic tectum. Interestingly, SB-3CT affected only two of the guidance decisions, with misrouting defects at the optic chiasm and tectum. Only at higher concentrations was RGC axon extension also impaired. These data implicate MMPs in the guidance of vertebrate axons, and suggest that different metalloproteinases function to regulate axon behaviour at distinct choice points: an MMP is important in guidance at the optic chiasm and the target, while either a different MMP or an ADAM is required for axons to make the turn in the mid-diencephalon.     

Leaving the midline

In the developing nervous system, pathfinding axons navigate through a series of intermediate targets in order to form synaptic connections. Vertebrate spinal commissural axons extend toward and across the floor plate (FP), a key intermediate target located at the ventral midline (VM). Subsequently, post-crossing commissural axons grow either alongside or significant distances away from the floor plate (FP), but never re-cross the VM. Consistent with this behavior, post-crossing commissural axons lose responsiveness to the FP-associated chemoattractants, Netrin-1 and SHH, and gain responsiveness to Slits, which are potent midline repellents, in vitro. In addition, the results of several in vivo studies suggest that the upregulation of Slit-binding repulsive Robo receptors, Robo1/2, alters the responsiveness of decussated commissural axons to midline guidance cues. Nevertheless, in vertebrates, it is unclear whether Robo1/2 are the sole or major repellent receptors responsible for driving these commissural axons away from the VM and preventing their re-entry into the FP. We recently re-visited these issues in the chick spinal cord by assessing the consequences of manipulating Robo expression on commissural axons in ovo. Our findings suggest that, at least in chick embryos, the upregulation of repulsive Robos on post-crossing axons alters the responsiveness of these axons to midline repellents and facilitates their expulsion from, but is not likely to have a significant role in preventing their re-entry into the VM.     

Chondroitin sulfate disrupts axon pathfinding in the optic tract and alters growth cone dynamics

Little is known about the cues that guide retinal axons across the diencephalon en route to their midbrain target, the optic tectum. Here we show that chondroitin sulfate proteoglycans are differentially expressed within the diencephalon at a time when retinal axons are growing within the optic tract. Using exposed brain preparations, we show that the addition of exogenous chondroitin sulfate results in retinal pathfinding errors. Retinal axons disperse widely from their normal trajectory within the optic tract and extend aberrantly into inappropriate regions of the forebrain. Time-lapse analysis of retinal growth cone dynamics in vivo shows that addition of exogenous chondroitin sulfate causes intermittent stalling and increases growth cone complexity. These results suggest that chondroitin sulfate may modulate the guidance of retinal axons as they grow through the diencephalon towards the optic tectum.     

Migration and function of glia in the developing Drosophila eye.

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.