Control of retinal ganglion cell axon growth: a new role for Sonic hedgehog

CXCL12 targets the primary cilium cAMP/cGMP ratio to regulate cell polarity during migration

Directed cell migration requires sustained cell polarisation. In migrating cortical interneurons, nuclear movements are directed towards the centrosome that organises the primary cilium signalling hub. Primary cilium-elicited signalling, and how it affects migration, remain however ill characterised. Here, we show that altering cAMP/cGMP levels in the primary cilium by buffering cAMP, cGMP or by locally increasing cAMP, influences the polarity and directionality of migrating interneurons, whereas buffering cAMP or cGMP in the apposed centrosome compartment alters their motility. Remarkably, we identify CXCL12 as a trigger that targets the ciliary cAMP/cGMP ratio to promote sustained polarity and directed migration. We thereby uncover cAMP/cGMP levels in the primary cilium as a major target of extrinsic cues and as the steering wheel of neuronal migration.

Hedgehog signaling guides migration of primordial germ cells to the Drosophila somatic gonad

In addition to inducing nonautonomous specification of cell fate in both Drosophila and vertebrates, the Hedgehog pathway guides cell migration in a variety of different tissues. Although its role in axon guidance in the vertebrate nervous system is widely recognized, its role in guiding the migratory path of primordial germ cells (PGCs) from the outside surface of the Drosophila embryo through the midgut and mesoderm to the SGPs (somatic gonadal precursors) has been controversial. Here we present new experiments demonstrating (1) that Hh produced by mesodermal cells guides PGC migration, (2) that HMG CoenzymeA reductase (Hmgcr) potentiates guidance signals emanating from the SGPs, functioning upstream of hh and of 2 Hh pathway genes important for Hh-containing cytonemes, and (3) that factors required in Hh receiving cells in other contexts function in PGCs to help direct migration toward the SGPs. We also compare the data reported by 4 different laboratories that have studied the role of the Hh pathway in guiding PGC migration.

The activation of gastric inhibitory peptide/gastric inhibitory peptide receptor axis via sonic hedgehog signaling promotes the bridging of gapped nerves in sciatic nerve injury

Schwann cells play an essential role in peripheral nerve regeneration by generating a favorable microenvironment. Gastric inhibitory peptide/gastric inhibitory peptide receptor (GIP/GIPR) axis deficiency leads to failure of sciatic nerve repair. However, the underlying mechanism remains elusive. In this study, we surprisingly found that GIP treatment significantly enhances the migration of Schwann cells and the formation of Schwann cell cords during recovery from sciatic nerve injury in rats. We further revealed that GIP and GIPR levels in Schwann cells were low under normal conditions, and significantly increased after injury demonstrated by real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Wound healing and Transwell assays showed that GIP stimulation and GIPR silencing could affect Schwann cell migration. In vitro and in vivo mechanistic studies based on interference experiment revealed that GIP/GIPR might promote mechanistic target of rapamycin complex 2 (mTORC2) activity, thus facilitating cell migration; Rap1 activation might be involved in this process. Finally, we retrieved the stimulatory factors responsible for GIPR induction after injury. The results indicate that sonic hedgehog (SHH) is a potential candidate whose expression increased upon injury. Luciferase and chromatin immunoprecipitation (ChIP) assays showed that Gli3, the target transcription factor of the SHH pathway, dramatically augmented GIPR expression. Additionally, in vivo inhibition of SHH could effectively reduce GIPR expression after sciatic nerve injury. Collectively, our study reveals the importance of GIP/GIPR signaling in Schwann cell migration, providing a therapeutic avenue toward peripheral nerve injury.

The Role of Small Molecules and Their Effect on the Molecular Mechanisms of Early Retinal Organoid Development

Early in vivo embryonic retinal development is a well-documented and evolutionary conserved process. The specification towards eye development is temporally controlled by consecutive activation or inhibition of multiple key signaling pathways, such as the Wnt and hedgehog signaling pathways. Recently, with the use of retinal organoids, researchers aim to manipulate these pathways to achieve better human representative models for retinal development and disease. To achieve this, a plethora of different small molecules and signaling factors have been used at various time points and concentrations in retinal organoid differentiations, with varying success. Additions differ from protocol to protocol, but their usefulness or efficiency has not yet been systematically reviewed. Interestingly, many of these small molecules affect the same and/or multiple pathways, leading to reduced reproducibility and high variability between studies. In this review, we make an inventory of the key signaling pathways involved in early retinogenesis and their effect on the development of the early retina in vitro. Further, we provide a comprehensive overview of the small molecules and signaling factors that are added to retinal organoid differentiation protocols, documenting the molecular and functional effects of these additions. Lastly, we comparatively evaluate several of these factors using our established retinal organoid methodology.

New Tricks for an Old (Hedge)Hog: Sonic Hedgehog Regulation of Astrocyte Function

The Sonic hedgehog (Shh) molecular signaling pathway is well established as a key regulator of neurodevelopment. It regulates diverse cellular behaviors, and its functions vary with respect to cell type, region, and developmental stage, reflecting the incredible pleiotropy of this molecular signaling pathway. Although it is best understood for its roles in development, Shh signaling persists into adulthood and is emerging as an important regulator of astrocyte function. Astrocytes play central roles in a broad array of nervous system functions, including synapse formation and function as well as coordination and orchestration of CNS inflammatory responses in pathological states. Neurons are the source of Shh in the adult, suggesting that Shh signaling mediates neuron-astrocyte communication, a novel role for this multifaceted pathway. Multiple roles for Shh signaling in astrocytes are increasingly being identified, including regulation of astrocyte identity, modulation of synaptic organization, and limitation of inflammation. This review discusses these novel roles for Shh signaling in regulating diverse astrocyte functions in the healthy brain and in pathology.

BMP Signaling Interferes with Optic Chiasm Formation and Retinal Ganglion Cell Pathfinding in Zebrafish

Decussation of axonal tracts is an important hallmark of vertebrate neuroanatomy resulting in one brain hemisphere controlling the contralateral side of the body and also computing the sensory information originating from that respective side. Here, we show that BMP interferes with optic chiasm formation and RGC pathfinding in zebrafish. Experimental induction of BMP4 at 15 hpf results in a complete ipsilateral projection of RGC axons and failure of commissural connections of the forebrain, in part as the result of an interaction with shh signaling, transcriptional regulation of midline guidance cues and an affected optic stalk morphogenesis. Experimental induction of BMP4 at 24 hpf, resulting in only a mild repression of forebrain shh ligand expression but in a broad expression of pax2a in the diencephalon, does not per se prevent RGC axons from crossing the midline. It nevertheless shows severe pathologies of RGC projections e.g., the fasciculation of RGC axons with the ipsilateral optic tract resulting in the innervation of one tectum by two eyes or the projection of RGC axons in the direction of the contralateral eye.

Sonic hedgehog signaling in astrocytes

Astrocytes are complex cells that perform a broad array of essential functions in the healthy and injured nervous system. The recognition that these cells are integral components of various processes, including synapse formation, modulation of synaptic activity, and response to injury, underscores the need to identify the molecular signaling programs orchestrating these diverse functional properties. Emerging studies have identified the Sonic hedgehog (Shh) signaling pathway as an essential regulator of the molecular identity and functional properties of astrocytes. Well established as a powerful regulator of diverse neurodevelopmental processes in the embryonic nervous system, its functional significance in astrocytes is only beginning to be revealed. Notably, Shh signaling is active only in discrete subpopulations of astrocytes distributed throughout the brain, a feature that has potential to yield novel insights into functional specialization of astrocytes. Here, we discuss Shh signaling and emerging data that point to essential roles for this pleiotropic signaling pathway in regulating various functional properties of astrocytes in the healthy and injured brain.

Ocular findings of albinism in DYRK1A-related intellectual disability syndrome

BACKGROUND

Pathogenic variants in DYRK1A are associated with DYRK1A-related intellectual disability syndrome (DIDS). Common features of this diagnosis include microcephaly, intellectual disability, speech impairment, and distinct facial features. Reported ocular features include deep-set eyes, myopia, and strabismus. We present a case of DYRK1A-related intellectual disability syndrome with ocular findings of albinism and explore the possible pathogenesis of this previously unreported manifestation.

MATERIALS AND METHODS

This is a single, retrospective case report of a child with DIDS who underwent an ophthalmic exam including detailed visual electrophysiology. Results: A 21-month-old female with microcephaly, failure to thrive, language delay, cleft palate, and cardiac defects had an ophthalmic exam showing myopia, strabismus, a hypopigmented fundus and crossed asymmetry on visual evoked potential (VEP), consistent with ocular findings of albinism. Whole exome sequencing identified a pathogenic DYRK1A variant; no albinism gene variants were reported. Her constellation of features is consistent with a diagnosis of DYRK1A-related intellectual disability syndrome; however, ocular features of albinism have not previously been reported in this condition.

CONCLUSIONS

This is, to the best of our knowledge, the first report of ocular findings of albinism in a case of DYRK1A-related intellectual disability syndrome. We propose that ocular albinism is a novel ocular phenotype of DYRK1A-related disease. Ophthalmic exams in patients with this diagnosis should include thorough evaluation for ocular albinism, including VEPs.

Hedgehog signaling promotes endoneurial fibroblast migration and Vegf-A expression following facial nerve injury

BACKGROUND

Peripheral nerve injuries are a common clinical problem which may result in permanent loss of motor or sensory function. A better understanding of the signaling pathways that lead to successful nerve regeneration may help in discovering new therapeutic targets. The Hedgehog (Hh) signaling pathway plays significant roles in nerve development and regeneration. In a mouse model of facial nerve injury, Hedgehog-responsive fibroblasts increase in number both at the site of injury and within the distal nerve. However, the role of these cells in facial nerve regeneration is not fully understood. We hypothesize that the Hh pathway plays an angiogenic and pro-migratory role following facial nerve injury.

METHODS

Hedgehog pathway modulators were applied to murine endoneurial fibroblasts isolated from the murine facial nerve. The impact of pathway modulation on endoneurial fibroblast migration and cell proliferation was assessed. Gene expression changes of known Hedgehog target genes and the key angiogenic factor Vegf-A were determined by qPCR. In vivo, mice were treated with pathway agonist (SAG21k) and injured facial nerve specimens were analyzed via immunofluorescence and in situ hybridization.

RESULTS

Hedgehog pathway activation in facial nerve fibroblasts via SAG21k treatment increases Gli1 and Ptch1 expression, the rate of cellular migration, and Vegf-A expression in vitro. In vivo, expression of Gli1 and Vegf-A expression appears to increase after injury, particularly at the site of nerve injury and the distal nerve, as detected by immunofluorescence and in situ hybridization. Additionally, Gli1 transcripts co-localize with Vegf-A following transection injury to the facial nerve.

DISCUSSION

These findings describe an angiogenic and pro-migratory role for the Hedgehog pathway mediated through effects on nerve fibroblasts. Given the critical role of Vegf-A in nerve regeneration, modulation of this pathway may represent a potential therapeutic target to improve facial nerve regeneration following injury.

Promotion of Axon Growth by the Secreted End of a Transcription Factor

Axon growth is regulated externally by attractive and repulsive cues generated in the environment. In addition, intrinsic pathways govern axon development, although the extent to which axons themselves can influence their own growth is unknown. We find that dorsal root ganglion (DRG) axons secrete a factor supporting axon growth and identify it as the C terminus of the ER stress-induced transcription factor CREB3L2, which is generated by site 2 protease (S2P) cleavage in sensory neurons. S2P and CREB3L2 knockdown or inhibition of axonal S2P interfere with the growth of axons, and C-terminal CREB3L2 is sufficient to rescue these effects. C-terminal CREB3L2 forms a complex with Shh and stabilizes its association with the Patched-1 receptor on developing axons. Our results reveal a neuron-intrinsic pathway downstream of S2P that promotes axon growth.

Retinal Ganglion Cell Axon Wiring Establishing the Binocular Circuit

Binocular vision depends on retinal ganglion cell (RGC) axon projection either to the same side or to the opposite side of the brain. In this article, we review the molecular mechanisms for decussation of RGC axons, with a focus on axon guidance signaling at the optic chiasm and ipsi- and contralateral axon organization in the optic tract prior to and during targeting. The spatial and temporal features of RGC neurogenesis that give rise to ipsilateral and contralateral identity are described. The albino visual system is highlighted as an apt comparative model for understanding RGC decussation, as albinos have a reduced ipsilateral projection and altered RGC neurogenesis associated with perturbed melanogenesis in the retinal pigment epithelium. Understanding the steps for RGC specification into ipsi- and contralateral subtypes will facilitate differentiation of stem cells into RGCs with proper navigational abilities for effective axon regeneration and correct targeting of higher-order visual centers.

Shh-ushing Midline Crossing through Remote Protein Transport

Shh contributes to neural circuit formation with different mechanisms. In this issue, Peng and colleagues (2018) identify a novel trans-axonal mechanism by which Shh derived from contralateral projecting retinal ganglion cells prevents midline crossing of Boc-expressing ipsilateral axons at the optic chiasm.

Dorsoventral differences in cAMP levels and correlated changes in the subcellular distribution of the PKA catalytic domain, provide further evidence that PKA signaling coordinates dorsoventral patterning of the otocyst

Dorsoventral (DV) patterning of the otocyst gives rise to formation of the morphologically and functionally complex membranous labyrinth composed of unique dorsal and ventral sensory organs. DV patterning results from extracellular signaling by secreted growth factors, which presumably form reciprocal concentration gradients across the DV axis of the otocyst. Previous work suggested a model in which two important growth factors, bone morphogenetic protein (BMP) and SHH, undergo crosstalk through an intersecting pathway to coordinate DV patterning. cAMP-dependent protein kinase A (PKA) lies at the heart of this pathway. Here, we provide further evidence that PKA signaling coordinates DV patterning, showing that both BMPs and SHH regulate cAMP levels, with BMPs increasing levels in the dorsal otocyst and SHH decreasing levels in the ventral otocyst. This, in turn, results in regional changes in the subcellular distribution of the catalytic domain of PKA, as well as DV regulation of PKA activity, increasing it dorsally and decreasing it ventrally. These new results fill an important gap in our previous understanding of how ligand signaling acts intracellularly during otocyst DV patterning and early morphogenesis, thereby initiating the series of events leading to formation of the inner ear sensory organs that function in balance and hearing.

Wdpcp promotes epicardial EMT and epicardium-derived cell migration to facilitate coronary artery remodeling

During coronary vasculature development, endothelial cells enclose the embryonic heart to form the primitive coronary plexus. This structure is remodeled upon recruitment of epicardial cells that may undergo epithelial-mesenchymal transition (EMT) to enable migration and that give rise to smooth muscle cells. In mice expressing a loss-of-function mutant form of Wdpcp, a gene involved in ciliogenesis, the enclosure of the surface of the heart by the subepicardial coronary plexus was accelerated because of enhanced chemotactic responses to Shh. Coronary arteries, but not coronary veins in Wdpcp mutant mice, showed reduced smooth muscle cell coverage. In addition, Wdpcp mutant hearts had reduced expression of EMT and mesenchymal markers and had fewer epicardium-derived cells (EPDCs) that showed impaired migration. Epicardium-specific deletion of Wdpcp recapitulated the coronary artery defect of the Wdpcp mutant. Thus, Wdpcp promotes epithelial EMT and EPDC migration, processes that are required for remodeling of the coronary primitive plexus. The Wdpcp mutant mice will be a useful tool to dissect the molecular mechanisms that govern the remodeling of the primitive plexus during coronary development.

Sonic Hedgehog Regulation of the Neural Precursor Cell Fate During Chicken Optic Tectum Development

During nervous system development, neurons project axons over long distances to reach the appropriate targets for correct neural circuit formation. Sonic hedgehog (Shh) is a secreted protein and plays a key role in regulating vertebrate embryogenesis, especially in central nervous system (CNS) patterning, including neuronal migration and axonal projection in the brain and spinal cord. In the developing ventral midbrain, Shh is sufficient to specify a striped pattern of cell fates. Little is known about the molecular mechanisms underlying the Shh regulation of the neural precursor cell fate during the optic tectum development. Here, we aimed at studying how Shh might regulate chicken optic tectum patterning. In the present study, in ovo electroporation methods were employed to achieve the overexpression of Shh in the optic tectum during chicken embryo development. Besides, the study combined in ovo electroporation and neuron isolation culturing to study the function of Shh in vivo and in vitro. The fluorescent immunohistochemistry methods were used to check the related indicators. The results showed that Shh overexpression caused 87.8% of cells to be distributed to the stratum griseum central (SGC) layer, while only 39.3% of the GFP-transfected cells resided in the SGC layer in the control group. Shh overexpression also reduced the axon length in vivo and in vitro. In conclusion, we provide evidence that Shh regulates the neural precursor cell fate during chicken optic tectum development. Shh overexpression impairs neuronal migration and may affect the fate determination of transfected neurons.

Contributions of Noncanonical Smoothened Signaling During Embryonic Development

The Sonic Hedgehog (Shh) signaling pathway is active during embryonic development in metazoans, and provides instructional cues necessary for proper tissue patterning. The pathway signal transducing component, Smoothened (Smo), is a G protein-coupled receptor (GPCR) that has been demonstrated to signal through at least two effector routes. The first is a G protein–independent canonical route that signals to Gli transcriptional effectors to establish transcriptional programs specifying cell fate during early embryonic development. The second, commonly referred to as the noncanonical Smo signal, induces rapid, transcription-independent responses that are essential for establishing and maintaining distinct cell behaviors during development. Herein, we discuss contributions of this noncanonical route during embryonic development. We also highlight important open questions regarding noncanonical Smo signal route selection during development, and consider implications of noncanonical signal corruption in disease.

Sonic -'Jack-of-All-Trades' in Neural Circuit Formation

As reflected by the term morphogen, molecules such as Shh and Wnts were identified based on their role in early development when they instruct precursor cells to adopt a specific cell fate. Only much later were they implicated in neural circuit formation. Both in vitro and in vivo studies indicated that morphogens direct axons during their navigation through the developing nervous system. Today, the best understood role of Shh and Wnt in axon guidance is their effect on commissural axons in the spinal cord. Shh was shown to affect commissural axons both directly and indirectly via its effect on Wnt signaling. In fact, throughout neural circuit formation there is cross-talk and collaboration of Shh and Wnt signaling. Thus, although the focus of this review is on the role of Shh in neural circuit formation, a separation from Wnt signaling is not possible.

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action.

SHH ventralizes the otocyst by maintaining basal PKA activity and regulating GLI3 signaling

During development of the inner ear, secreted morphogens act coordinately to establish otocyst dorsoventral polarity. Among these, Sonic hedgehog (SHH) plays a critical role in determining ventral polarity. However, how this extracellular signal is transduced intracellularly to establish ventral polarity is unknown. In this study, we show that cAMP dependent protein kinase A (PKA) is a key intracellular factor mediating SHH signaling through regulation of GLI3 processing. Gain-of-function experiments using targeted gene transfection by sonoporation or electroporation revealed that SHH signaling inactivates PKA, maintaining a basal level of PKA activity in the ventral otocyst. This, in turn, suppresses partial proteolytic processing of GLI3FL, resulting in a low GLI3R/GLI3FL ratio in the ventral otocyst and the expression of ventral-specific genes required for ventral otocyst morphogenesis. Thus, we identify a molecular mechanism that links extracellular and intracellular signaling, determines early ventral polarity of the inner ear, and has implications for understanding the integration of polarity signals in multiple organ rudiments regulated by gradients of signaling molecules.

Gas1 is a receptor for sonic hedgehog to repel enteric axons

The myenteric plexus of the enteric nervous system controls the movement of smooth muscles in the gastrointestinal system. They extend their axons between two peripheral smooth muscle layers to form a tubular meshwork arborizing the gut wall. How a tubular axonal meshwork becomes established without invading centrally toward the gut epithelium has not been addressed. We provide evidence here that sonic hedgehog (Shh) secreted from the gut epithelium prevents central projections of enteric axons, thereby forcing their peripheral tubular distribution. Exclusion of enteric central projections by Shh requires its binding partner growth arrest specific gene 1 (Gas1) and its signaling component smoothened (Smo) in enteric neurons. Using enteric neurons differentiated from neurospheres in vitro, we show that enteric axon growth is not inhibited by Shh. Rather, when Shh is presented as a point source, enteric axons turn away from it in a Gas1-dependent manner. Of the Gαi proteins that can couple with Smo, G protein α Z (Gnaz) is found in enteric axons. Knockdown and dominant negative inhibition of Gnaz dampen the axon-repulsive response to Shh, and Gnaz mutant intestines contain centrally projected enteric axons. Together, our data uncover a previously unsuspected mechanism underlying development of centrifugal tubular organization and identify a previously unidentified effector of Shh in axon guidance.

Connecting the retina to the brain

The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed.

Cilia: traffic directors along the road of cortical development

While the presence of a primary cilium on neural progenitors and on post-mitotic neurons was noted long ago, a primary cilium has been observed on migrating cortical interneurons only recently. As in fibroblasts, the cilium of interneurons controls the directionality of migration. It plays an important role in the reorientation of cortical interneurons toward the cortical plate. The morphogen Shh, which is expressed in the migratory pathway of interneurons, is one of the signals that control this reorientation. After a short description of the migratory pathways of cortical interneurons, we focus on cellular mechanisms that allow interneurons to reorient their trajectory during their long-distance migration. Then we examine the role of the primary cilium in cell migration and how ciliogenesis might be related to the migration cycle in interneurons. Finally, we review the molecular mechanisms at the basis of the sensory function of the primary cilium and examine how Shh signals could influence the migratory behavior of cortical interneurons. These novel data provide a cellular basis to further understanding cognitive deficits associated with human ciliopathies.

Transcriptome of Atoh7 retinal progenitor cells identifies new Atoh7-dependent regulatory genes for retinal ganglion cell formation

The bHLH transcription factor ATOH7 (Math5) is essential for establishing retinal ganglion cell (RGC) fate. However, Atoh7-expressing retinal progenitor cells (RPCs) can give rise to all retinal cell types, suggesting that other factors are involved in specifying RGCs. The basis by which a subpopulation of Atoh7-expressing RPCs commits to an RGC fate remains uncertain but is of critical importance to retinal development since RGCs are the earliest cell type to differentiate. To better understand the regulatory mechanisms leading to cell-fate specification, a binary genetic system was generated to specifically label Atoh7-expressing cells with green fluorescent protein (GFP). Fluorescence-activated cell sorting (FACS)-purified GFP(+) and GFP(-) cells were profiled by RNA-seq. Here, we identify 1497 transcripts that were differentially expressed between the two RPC populations. Pathway analysis revealed diminished growth factor signaling in Atoh7-expressing RPCs, indicating that these cells had exited the cell cycle. In contrast, axon guidance signals were enriched, suggesting that axons of Atoh7-expressing RPCs were already making synaptic connections. Notably, many genes enriched in Atoh7-expressing RPCs encoded transcriptional regulators, and several were direct targets of ATOH7, including, and unexpectedly, Ebf3 and Eya2. We present evidence for a Pax6-Atoh7-Eya2 pathway that acts downstream of Atoh7 but upstream of differentiation factor Pou4f2. EYA2 is a protein phosphatase involved in protein-protein interactions and posttranslational regulation. These properties, along with Eya2 as an early target gene of ATOH7, suggest that EYA2 functions in RGC specification. Our results expand current knowledge of the regulatory networks operating in Atoh7-expressing RPCs and offer new directions for exploring the earliest aspects of retinogenesis.

Hedgehog: Multiple Paths for Multiple Roles in Shaping the Brain and Spinal Cord

Since the discovery of the segment polarity gene Hedgehog in Drosophila three decades ago, our knowledge of Hedgehog signaling pathway has considerably improved and paved the way to a wide field of investigations in the developing and adult central nervous system. Its peculiar transduction mechanism together with its implication in tissue patterning, neural stem cell biology, and neural tissue homeostasis make Hedgehog pathway of interest in a high number of normal or pathological contexts. Consistent with its role during brain development, misregulation of Hedgehog signaling is associated with congenital diseases and tumorigenic processes while its recruitment in damaged neural tissue may be part of the repairing process. This review focuses on the most recent data regarding the Hedgehog pathway in the developing and adult central nervous system and also its relevance as a therapeutic target in brain and spinal cord diseases.

Retinal stem cells and regeneration of vision system

The vertebrate retina is a well-characterized model for studying neurogenesis. Retinal neurons and glia are generated in a conserved order from a pool of mutlipotent progenitor cells. During retinal development, retinal stem/progenitor cells (RPC) change their competency over time under the influence of intrinsic (such as transcriptional factors) and extrinsic factors (such as growth factors). In this review, we summarize the roles of these factors, together with the understanding of the signaling pathways that regulate eye development. The information about the interactions between intrinsic and extrinsic factors for retinal cell fate specification is useful to regenerate specific retinal neurons from RPCs. Recent studies have identified RPCs in the retina, which may have important implications in health and disease. Despite the recent advances in stem cell biology, our understanding of many aspects of RPCs in the eye remains limited. PRCs are present in the developing eye of all vertebrates and remain active in lower vertebrates throughout life. In mammals, however, PRCs are quiescent and exhibit very little activity and thus have low capacity for retinal regeneration. A number of different cellular sources of RPCs have been identified in the vertebrate retina. These include PRCs at the retinal margin, pigmented cells in the ciliary body, iris, and retinal pigment epithelium, and Müller cells within the retina. Because PRCs can be isolated and expanded from immature and mature eyes, it is possible now to study these cells in culture and after transplantation in the degenerated retinal tissue. We also examine current knowledge of intrinsic RPCs, and human embryonic stems and induced pluripotent stem cells as potential sources for cell transplant therapy to regenerate the diseased retina.

Cell biology in neuroscience: RNA-based mechanisms underlying axon guidance

Axon guidance plays a key role in establishing neuronal circuitry. The motile tips of growing axons, the growth cones, navigate by responding directionally to guidance cues that pattern the embryonic neural pathways via receptor-mediated signaling. Evidence in vitro in the last decade supports the notion that RNA-based mechanisms contribute to cue-directed steering during axon guidance. Different cues trigger translation of distinct subsets of mRNAs and localized translation provides precise spatiotemporal control over the growth cone proteome in response to localized receptor activation. Recent evidence has now demonstrated a role for localized translational control in axon guidance decisions in vivo.

Axon guidance effect of classical morphogens Shh and BMP7 in the hypothalamo-pituitary system

Hypothalamus plays a key role in homeostasis, and functions of the hypothalamus depend on the accurate trajectory of hypothalamic neuroendocrine axons. Thus, understanding the guidance of hypothalamic neuroendocrine axons is crucial for knowing how hypothalamus works. Previous studies suggest FGF10 deriving from the medial ventral midline of the hypothalamus plays an important role in axon guidance of the developing hypothalamus. Here we show that Shh and BMP7, which are from the anterior and posterior hypothalamic ventral midline respectively, together repel hypothalamic axons towards the medial ventral midline.

Functional organisation of visual pathways in a patient with no optic chiasm

Congenital achiasma offers a rare opportunity to study reorganization and inter-hemispheric communication in the face of anomalous inputs to striate cortex. We report neuroimaging studies of a patient with seesaw nystagmus, achiasma, and full visual fields. The subject underwent structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) studies, and functional MRI (fMRI) using monocular stimulation with checkerboards, motion, objects and faces, as well as retinotopic quadrantic mapping. Structural MRI confirmed the absence of an optic chiasm, which was corroborated by DTI tractography. Lack of a functioning decussation was confirmed by fMRI that showed activation of only ipsilateral medial occipital cortex by monocular stimulation. The corpus callosum was normal in size and anterior and posterior commissures were identifiable. In terms of the hierarchy of visual areas, V5 was the lowest level region to be activated binocularly, as were regions in the fusiform gyri responding to faces and objects. The retinotopic organization of striate cortex was studied with quadrantic stimulation. This showed that, in support of recent findings, rather than projecting to an ectopic location contiguous with the normal retinotopic map of the ipsilateral temporal hemi-retina, the nasal hemi-retina's representation overlapped that of the temporal hemi-retina. These findings show that congenital achiasma can be an isolated midline crossing defect, that information transfer does not occur in early occipital cortex but at intermediate and higher levels of the visual hierarchy, and that the functional reorganisation of striate cortex in this condition is consistent with normal axon guidance by a chemoaffinity gradient.

Primary neuron culture for nerve growth and axon guidance studies in zebrafish (Danio rerio)

Zebrafish (Danio rerio) is a widely used model organism in genetics and developmental biology research. Genetic screens have proven useful for studying embryonic development of the nervous system in vivo, but in vitro studies utilizing zebrafish have been limited. Here, we introduce a robust zebrafish primary neuron culture system for functional nerve growth and guidance assays. Distinct classes of central nervous system neurons from the spinal cord, hindbrain, forebrain, and retina from wild type zebrafish, and fluorescent motor neurons from transgenic reporter zebrafish lines, were dissociated and plated onto various biological and synthetic substrates to optimize conditions for axon outgrowth. Time-lapse microscopy revealed dynamically moving growth cones at the tips of extending axons. The mean rate of axon extension in vitro was 21.4±1.2 µm hr⁻¹ s.e.m. for spinal cord neurons, which corresponds to the typical ∼0.5 mm day⁻¹ growth rate of nerves in vivo. Fluorescence labeling and confocal microscopy demonstrated that bundled microtubules project along axons to the growth cone central domain, with filamentous actin enriched in the growth cone peripheral domain. Importantly, the growth cone surface membrane expresses receptors for chemotropic factors, as detected by immunofluorescence microscopy. Live-cell functional assays of axon extension and directional guidance demonstrated mammalian brain-derived neurotrophic factor (BDNF)-dependent stimulation of outgrowth and growth cone chemoattraction, whereas mammalian myelin-associated glycoprotein inhibited outgrowth. High-resolution live-cell Ca²⁺-imaging revealed local elevation of cytoplasmic Ca²⁺ concentration in the growth cone induced by BDNF application. Moreover, BDNF-induced axon outgrowth, but not basal outgrowth, was blocked by treatments to suppress cytoplasmic Ca²⁺ signals. Thus, this primary neuron culture model system may be useful for studies of neuronal development, chemotropic axon guidance, and mechanisms underlying inhibition of neural regeneration in vitro, and complement observations made in vivo.

Integration of guidance cues: parallel signaling and crosstalk

Growing axons are exposed to various guidance cues en route to their targets. Although many guidance molecules have been identified and their effects on axon behavior extensively studied, how axons react to combinations of signals remains largely unexplored. We review recent studies investigating the combined actions of guidance cues present at the same choice points. Two main scenarios are emerging from these studies: parallel signaling and crosstalk between guidance systems. In the first case, cues act in an additive manner, whereas in the second case the outcome is non-additive and differs from the sum of individual effects, suggesting more complex signal integration in the growth cone. Some of the molecular mechanisms underlying these interactions are beginning to be unraveled.

Steerable-filter based quantification of axonal populations at the developing optic chiasm reveal significant defects in Slit2(-/-) as well as Slit1(-/-)Slit2(-/-) embryos

Background

Previous studies have suggested that the axon guidance proteins Slit1 and Slit2 co-operate to establish the optic chiasm in its correct position at the ventral diencephalic midline. This is based on the observation that, although both Slit1 and Slit2 are expressed around the ventral midline, mice defective in either gene alone exhibit few or no axon guidance defects at the optic chiasm whereas embryos lacking both Slit1 and Slit2 develop a large additional chiasm anterior to the chiasm’s normal position. Here we used steerable-filters to quantify key properties of the population of axons at the chiasm in wild-type, Slit1^−/−, Slit2^−/− and Slit1^−/−Slit2^−/− embryos.

Results

We applied the steerable-filter algorithm successfully to images of embryonic retinal axons labelled from a single eye shortly after they have crossed the midline. We combined data from multiple embryos of the same genotype and made statistical comparisons of axonal distributions, orientations and curvatures between genotype groups. We compared data from the analysis of axons with data on the expression of Slit1 and Slit2. The results showed a misorientation and a corresponding anterior shift in the position of many axons at the chiasm of both Slit2^−/− and Slit1^−/−Slit2^−/− mutants. There were very few axon defects at the chiasm of Slit1^−/− mutants.

Conclusions

We found defects of the chiasms of Slit1^−/−Slit2^−/− and Slit1^−/− mutants similar to those reported previously. In addition, we discovered previously unreported defects resulting from loss of Slit2 alone. This indicates the value of a quantitative approach to complex pathway analysis and shows that Slit2 can act alone to control aspects of retinal axon routing across the ventral diencephalic midline.

The Hedgehog signal transduction network

Hedgehog (Hh) proteins regulate the development of a wide range of metazoan embryonic and adult structures, and disruption of Hh signaling pathways results in various human diseases. Here, we provide a comprehensive review of the signaling pathways regulated by Hh, consolidating data from a diverse array of organisms in a variety of scientific disciplines. Similar to the elucidation of many other signaling pathways, our knowledge of Hh signaling developed in a sequential manner centered on its earliest discoveries. Thus, our knowledge of Hh signaling has for the most part focused on elucidating the mechanism by which Hh regulates the Gli family of transcription factors, the so-called "canonical" Hh signaling pathway. However, in the past few years, numerous studies have shown that Hh proteins can also signal through Gli-independent mechanisms collectively referred to as "noncanonical" signaling pathways. Noncanonical Hh signaling is itself subdivided into two distinct signaling modules: (i) those not requiring Smoothened (Smo) and (ii) those downstream of Smo that do not require Gli transcription factors. Thus, Hh signaling is now proposed to occur through a variety of distinct context-dependent signaling modules that have the ability to crosstalk with one another to form an interacting, dynamic Hh signaling network.

Sonic hedgehog distribution within mature hippocampal neurons

Sonic hedgehog (Shh) regulates neural progenitor cells in the adult brain but its role in postmitotic mature neurons is not well understood. Using immunoelectron microscopy, we have recently demonstrated the postsynaptic distribution of Patched (Ptch) and Smoothened (Smo), the receptors for Shh, in hippocampal neurons of the adult rat brain. In this study, we describe the distribution of Shh protein in these adult hippocampal neurons. We find that Shh is present in both presynaptic and postsynaptic terminals. In presynaptic terminals, Shh is located either at the center or on the side of the synaptic junction. In postsynaptic terminals, Shh is mostly located on the side of the synaptic junction. We also find Shh in dendrites. Synaptic and dendritic Shh often reside in or are associated with vesicular structures that include dense-cored vesicles, synaptic vesicles, and endosomes. Thus, our subcellular map of Shh and its receptors provides a foundation for elucidating the functional significance of Shh signaling in mature neurons.

Hedgehog-stimulated chemotaxis is mediated by smoothened located outside the primary cilium

Regulation of the Hedgehog (Hh) pathway relies on an interaction of two receptors. In the absence of Hh, Patched1 (Ptch1) inhibits the pathway. Binding of the ligand Hh to Ptch1 stimulates the localization of the activating receptor Smoothened (Smo) to the primary cilium, which is required for the transcriptional Hh response. Hh can also induce chemotaxis through a nontranscriptional pathway. We assessed the effects of defective ciliary localization of Smo on its subcellular trafficking and ability to mediate chemotactic signaling. Cells expressing mutants of Smo that could not localize to the primary cilium or cells lacking the primary cilium showed altered intracellular trafficking of Smo and, in response to Hh or Smo agonists, decreased transcriptional signaling and enhanced chemotactic responsiveness. Thus, the ciliary localization machinery appears to transport Smo to subcellular sites where it can mediate transcriptional signaling and away from locations where it can mediate chemotactic signaling. The subcellular localization of Smo is thus a crucial determinant of its signaling characteristics and implies the existence of a pool of Smo dedicated to chemotaxis.

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.

Sonic hedgehog expression in corticofugal projection neurons directs cortical microcircuit formation

VIDEO ABSTRACT

The precise connectivity of inputs and outputs is critical for cerebral cortex function; however, the cellular mechanisms that establish these connections are poorly understood. Here, we show that the secreted molecule Sonic Hedgehog (Shh) is involved in synapse formation of a specific cortical circuit. Shh is expressed in layer V corticofugal projection neurons and the Shh receptor, Brother of CDO (Boc), is expressed in local and callosal projection neurons of layer II/III that synapse onto the subcortical projection neurons. Layer V neurons of mice lacking functional Shh exhibit decreased synapses. Conversely, the loss of functional Boc leads to a reduction in the strength of synaptic connections onto layer Vb, but not layer II/III, pyramidal neurons. These results demonstrate that Shh is expressed in postsynaptic target cells while Boc is expressed in a complementary population of presynaptic input neurons, and they function to guide the formation of cortical microcircuitry.

Subcellular localization of Patched and Smoothened, the receptors for Sonic hedgehog signaling, in the hippocampal neuron

Cumulative evidence suggests that, aside from patterning the embryonic neural tube, Sonic hedgehog (Shh) signaling plays important roles in the mature nervous system. In this study, we investigate the expression and localization of the Shh signaling receptors, Patched (Ptch) and Smoothened (Smo), in the hippocampal neurons of young and mature rats. Reverse transcriptase-polymerase chain reaction and immunoblotting analyses show that the expression of Ptch and Smo remains at a moderate level in young postnatal and adult brains. By using immunofluorescence light microscopy and immunoelectron microscopy, we examine the spatial distribution of Ptch and Smo within the hippocampal neurons. In young developing neurons, Ptch and Smo are present in the processes and are clustered at their growth cones. In mature neurons, Ptch and Smo are concentrated in dendrites, spines, and postsynaptic sites. Synaptic Ptch and Smo often co-exist with unusual structures-synaptic spinules and autophagosomes. Our results reveal the anatomical organization of the Shh receptors within both the young and the mature hippocampal neurons.

Megalin mediates the influence of sonic hedgehog on oligodendrocyte precursor cell migration and proliferation during development

Oligodendrocyte precursor cells (OPCs) of the optic nerve are generated in the preoptic area, from where they migrate to colonize it entirely. Sonic hedgehog (Shh) induces the proliferation of these cells as well as influencing their migration, acting through its canonical receptor (Ptc-1). However, the multiligand receptor megalin (or LRP-2) is also involved in Shh-induced OPC proliferation and migration, and thus, we have evaluated the relevance of this interaction. During the stages at which Shh influences OPC development, we found megalin to be selectively expressed by optic nerve astrocytes, whereas Ptc-1 and Gli1 were found in OPCs. Indeed, this pattern of expression paralleled the rostral-caudal expression of the three Shh-related molecules during the time course of plp-dm20(+) -OPC colonization. The blockage of megalin partially abolished OPC chemoattraction and fully impaired Shh-induced proliferation. Using in vitro co-cultures of dissociated optic nerve cells, we demonstrated that Shh was internalized by astrocytes via megalin, and sufficient Shh was subsequently released to produce the biological effects on OPCs observed in the nerve. Together, these data indicate that at least part of the influence of Shh on OPCs is mediated by megalin during optic nerve development, and that astrocytes expressing megalin transiently capture Shh to present it to OPCs and/or to control the gradient of this molecule during development.

Sonic Hedgehog regulates Wnt activity during neural circuit formation

Gradients of secreted morphogens, such as Sonic hedgehog (Shh), Wnt, and TGFβ/Bmp, have classically been shown to control many aspects of early development by regulating cell proliferation and determining cell fate. However, recent studies demonstrate that these molecules also play important and evolutionarily conserved roles in later aspects of neural development. Depending on the context, these molecules can elicit gene transcription in the nucleus, or alternatively can provide instructive signals at the growth cone that induce local and rapid changes in cytoskeletal organization. Shh can activate different cellular transduction pathways via its binding to alternative coreceptor complexes or simply by adaptation of its "classical" signaling pathway. However, in most of its activities during neural development, Shh does not act alone but rather in concert with other morphogens, particularly the Wnts. This review provides an overview of the mechanisms by which Shh signaling acts in concert with Wnts to mediate a myriad of cellular processes that are required for neural circuit formation.

Shh signaling guides spatial pathfinding of raphespinal tract axons by multidirectional repulsion

Relatively little is known about the molecular mechanisms underlying spatial pathfinding in the descending serotonergic raphespinal tract (RST) in the developing spinal cord, one of the most important nerve pathways for pain, sensory and motor functions. We provide evidence that ventral floor plate-secreted Sonic hedgehog (Shh) is responsible for the establishment of decreasing gradients in both the anterior-to-posterior (A-P) and the medial-to-lateral (M-L) directions in the ventral spinal cord during serotonergic RST axon projection. Downstream components of the Shh pathway, Patched 1 (Ptch1) and Smoothened (Smo), were expressed in the serotonergic caudal raphe nuclei and enriched in the descending serotonergic RST axons. Diffusible Shh repulsion of serotonergic RST axons was shown to be mediated by Shh-Ptch1 interactions and derepression of Smo. Using a co-culture assay, we showed that A-P graded repulsion mediated by Shh signaling pushed the serotonergic axons caudally through the ventral spinal cord and M-L graded repulsion mediated by Shh signaling simultaneously restricted the serotonergic axons to the ventral and ventral-lateral funiculus. Prominent pathfinding errors of serotonergic RST axons were observed in various Shh, Ptch1 and Smo mutants. We conclude that Shh signaling-mediated multidirectional repulsion is required to push descending serotonergic RST axons in the A-P direction, and to restrict these axons to the ventral and ventral-lateral funiculus in the M-L direction. This is the first demonstration that Shh signaling-mediated multidirectional repulsion of serotonergic RST axons maintains spatial axon pathfinding in the developing spinal cord.

VEGF signaling through neuropilin 1 guides commissural axon crossing at the optic chiasm

During development, the axons of retinal ganglion cell (RGC) neurons must decide whether to cross or avoid the midline at the optic chiasm to project to targets on both sides of the brain. By combining genetic analyses with in vitro assays, we show that neuropilin 1 (NRP1) promotes contralateral RGC projection in mammals. Unexpectedly, the NRP1 ligand involved is not an axon guidance cue of the class 3 semaphorin family, but VEGF164, the neuropilin-binding isoform of the classical vascular growth factor VEGF-A. VEGF164 is expressed at the chiasm midline and is required for normal contralateral growth in vivo. In outgrowth and growth cone turning assays, VEGF164 acts directly on NRP1-expressing contralateral RGCs to provide growth-promoting and chemoattractive signals. These findings have identified a permissive midline signal for axons at the chiasm midline and provide in vivo evidence that VEGF-A is an essential axon guidance cue.

Appropriate Bmp7 levels are required for the differentiation of midline guidepost cells involved in corpus callosum formation

Guidepost cells are essential structures for the establishment of major axonal tracts. How these structures are specified and acquire their axon guidance properties is still poorly understood. Here, we show that in mouse embryos appropriate levels of Bone Morphogenetic Protein 7 (Bmp7), a member of the TGF-β superfamily of secreted proteins, are required for the correct development of the glial wedge, the indusium griseum, and the subcallosal sling, three groups of cells that act as guidepost cells for growing callosal axons. Bmp7 is expressed in the region occupied by these structures and its genetic inactivation in mouse embryos caused a marked reduction and disorganization of these cell populations. On the contrary, infusion of recombinant Bmp7 in the developing forebrain induced their premature differentiation. In both cases, changes were associated with the disruption of callosal axon growth and, in most animals fibers did not cross the midline forming typical Probst bundles. Addition of Bmp7 to cortical explants did not modify the extent of their outgrowth nor their directionality, when explants were exposed to a focalized source of the protein. Together, these results indicate that Bmp7 is indirectly required for corpus callosum formation by controlling the timely differentiation of its guidepost cells.

The alpha subunit of the G protein G13 regulates activity of one or more Gli transcription factors independently of smoothened

Smoothened (Smo) is a seven-transmembrane (7-TM) receptor that is essential to most actions of the Hedgehog family of morphogens. We found previously that Smo couples to members of the G(i) family of heterotrimeric G proteins, which in some cases are integral although alone insufficient in the activation of Gli transcription factors through Hedgehog signaling. In response to a report that the G(12/13) family is relevant to Hedgehog signaling as well, we re-evaluated the coupling of Smo to one member of this family, G(13), and investigated the capacity of this and other G proteins to activate one or more of forms of Gli. We found no evidence that Smo couples directly to G(13). We found nonetheless that Gα(13) and to some extent Gα(q) and Gα(12) are able to effect activation of Gli(s). This capacity is realized in some cells, e.g. C3H10T1/2, MC3T3, and pancreatic cancer cells, but not all cells. The mechanism employed is distinct from that achieved through canonical Hedgehog signaling, as the activation does not involve autocrine signaling or in any other way require active Smo and does not necessarily involve enhanced transcription of Gli1. The activation by Gα(13) can be replicated through a G(q)/G(12/13)-coupled receptor, CCK(A), and is attenuated by inhibitors of p38 mitogen-activated protein kinase and Tec tyrosine kinases. We posit that G proteins, and perhaps G(13) in particular, provide access to Gli that is independent of Smo and that they thus establish a basis for control of at least some forms of Gli-mediated transcription apart from Hedgehogs.