Morphogens as growth cone signalling molecules

Development of neuroendocrine neurons in the mammalian hypothalamus

The neuroendocrine system consists of a heterogeneous collection of (mostly) neuropeptidergic neurons found in four hypothalamic nuclei and sharing the ability to secrete neurohormones (all of them neuropeptides except dopamine) into the bloodstream. There are, however, abundant hypothalamic non-neuroendocrine neuropeptidergic neurons developing in parallel with the neuroendocrine system, so that both cannot be entirely disentangled. This heterogeneity results from the workings of a network of transcription factors many of which are already known. Olig2 and Fezf2 expressed in the progenitors, acting through mantle-expressed Otp and Sim1, Sim2 and Pou3f2 (Brn2), regulate production of magnocellular and anterior parvocellular neurons. Nkx2-1, Rax, Ascl1, Neurog3 and Dbx1 expressed in the progenitors, acting through mantle-expressed Isl1, Dlx1, Gsx1, Bsx, Hmx2/3, Ikzf1, Nr5a2 (LH-1) and Nr5a1 (SF-1) are responsible for tuberal parvocellular (arcuate nucleus) and other neuropeptidergic neurons. The existence of multiple progenitor domains whose progeny undergoes intricate tangential migrations as one source of complexity in the neuropeptidergic hypothalamus is the focus of much attention. How neurosecretory cells target axons to the medial eminence and posterior hypophysis is gradually becoming clear and exciting progress has been made on the mechanisms underlying neurovascular interface formation. While rat neuroanatomy and targeted mutations in mice have yielded fundamental knowledge about the neuroendocrine system in mammals, experiments on chick and zebrafish are providing key information about cellular and molecular mechanisms. Looking forward, data from every source will be necessary to unravel the ways in which the environment affects neuroendocrine development with consequences for adult health and disease.

Prostaglandin E2 alters Wnt-dependent migration and proliferation in neuroectodermal stem cells: implications for autism spectrum disorders

Prostaglandin E2 (PGE₂) is a natural lipid-derived molecule that is involved in important physiological functions. Abnormal PGE₂ signalling has been associated with pathologies of the nervous system. Previous studies provide evidence for the interaction of PGE₂ and canonical Wnt signalling pathways in non-neuronal cells. Since the Wnt pathway is crucial in the development and organization of the brain, the main goal of this study is to determine whether collaboration between these pathways exists in neuronal cell types. We report that PGE₂ interacts with canonical Wnt signalling through PKA and PI-3K in neuroectodermal (NE-4C) stem cells. We used time-lapse microscopy to determine that PGE₂ increases the final distance from origin, path length travelled, and the average speed of migration in Wnt-activated cells. Furthermore, PGE₂ alters distinct cellular phenotypes that are characteristic of Wnt-induced NE-4C cells, which corresponds to the modified splitting behaviour of the cells. We also found that in Wnt-induced cells the level of β-catenin protein was increased and the expression levels of Wnt-target genes (Ctnnb1, Ptgs2, Ccnd1, Mmp9) was significantly upregulated in response to PGE₂ treatment. This confirms that PGE₂ activated the canonical Wnt signalling pathway. Furthermore, the upregulated genes have been previously associated with ASD. Our findings show, for the first time, evidence for cross-talk between PGE₂ and Wnt signalling in neuronal cells, where PKA and PI-3K might act as mediators between the two pathways. Given the importance of PGE₂ and Wnt signalling in prenatal development of the nervous system, our study provides insight into how interaction between these two pathways may influence neurodevelopment.

Structure and function of the ependymal barrier and diseases associated with ependyma disruption

The neuroepithelium is a germinal epithelium containing progenitor cells that produce almost all of the central nervous system cells, including the ependyma. The neuroepithelium and ependyma constitute barriers containing polarized cells covering the embryonic or mature brain ventricles, respectively; therefore, they separate the cerebrospinal fluid that fills cavities from the developing or mature brain parenchyma. As barriers, the neuroepithelium and ependyma play key roles in the central nervous system development processes and physiology. These roles depend on mechanisms related to cell polarity, sensory primary cilia, motile cilia, tight junctions, adherens junctions and gap junctions, machinery for endocytosis and molecule secretion, and water channels. Here, the role of both barriers related to the development of diseases, such as neural tube defects, ciliary dyskinesia, and hydrocephalus, is reviewed.

The multiple activities of BMPs during spinal cord development

Bone morphogenetic proteins (BMPs) are one of the main classes of multi-faceted secreted factors that drive vertebrate development. A growing body of evidence indicates that BMPs contribute to the formation of the central nervous system throughout its development, from the initial shaping of the neural primordium to the generation and maturation of the different cell types that form the functional adult nervous tissue. In this review, we focus on the multiple activities of BMPs during spinal cord development, paying particular attention to recent results that highlight the complexity of BMP signaling during this process. These findings emphasize the unique capacity of these signals to mediate various functions in the same tissue throughout development, recruiting diverse effectors and strategies to instruct their target cells.

Calcium signals and FGF-2 induced neurite growth in cultured parasympathetic neurons: spatial localization and mechanisms of activation

The growth of neuritic processes in developing neurons is tightly controlled by a wide set of extracellular cues that act by initiating downstream signaling cascades, where calcium signals play a major role. Here we analyze the calcium dependence of the neurite growth promoted by basic fibroblast growth factor (bFGF or FGF-2) in chick embryonic ciliary ganglion neurons, taking advantage of dissociated, organotypic, and compartmentalized cultures. We report that signals at both the growth cone and the soma are involved in the promotion of neurite growth by the factor. Blocking calcium influx through L- and N-type voltage-dependent calcium channels and transient receptor potential canonical (TRPC) channels reduces, while release from intracellular stores does not significantly affect, the growth of neuritic processes. Simultaneous recordings of calcium signals elicited by FGF-2 at the soma and at the growth cone show that the factor activates different patterns of responses in the two compartments: steady and sustained responses at the former, oscillations at the latter. At the soma, both voltage-dependent channel and TRPC blockers strongly affect steady-state levels. At the growth cone, the changes in the oscillatory pattern are more complex; therefore, we used a tool based on wavelet analysis to obtain a quantitative evaluation of the effects of the two classes of blockers. We report that the oscillatory behavior at the growth cone is dramatically affected by all the blockers, pointing to a role for calcium influx through the two classes of channels in the generation of signals at the leading edge of the elongating neurites.

The neurobiology of lipid metabolism in autism spectrum disorders

Autism is a neurodevelopmental disorder characterized by impairments in communication and reciprocal social interaction, coupled with repetitive behavior, which typically manifests by 3 years of age. Multiple genes and early exposure to environmental factors are the etiological determinants of the disorder that contribute to variable expression of autism-related traits. Increasing evidence indicates that altered fatty acid metabolic pathways may affect proper function of the nervous system and contribute to autism spectrum disorders. This review provides an overview of the reported abnormalities associated with the synthesis of membrane fatty acids in individuals with autism as a result of insufficient dietary supplementation or genetic defects. Moreover, we discuss deficits associated with the release of arachidonic acid from the membrane phospholipids and its subsequent metabolism to bioactive prostaglandins via phospholipase A(2)-cyclooxygenase biosynthetic pathway in autism spectrum disorders. The existing evidence for the involvement of lipid neurobiology in the pathology of neurodevelopmental disorders such as autism is compelling and opens up an interesting possibility for further investigation of this metabolic pathway.

Xenopus sonic hedgehog guides retinal axons along the optic tract

The role of classic morphogens such as Sonic hedgehog (Shh) as axon guidance cues has been reported in a variety of vertebrate organisms (Charron and Tessier-Lavigne [2005] Development 132:2251-2262). In this work, we provide the first evidence that Xenopus sonic hedgehog (Xshh) signaling is involved in guiding retinal ganglion cell (RGC) axons along the optic tract. Xshh is expressed in the brain during retinal axon extension, adjacent to these axons in the ventral diencephalon. Retinal axons themselves express Patched 1 and Smoothened co-receptors during RGC axon growth. Blocking Shh signaling causes abnormal ventral pathfinding, and targeting errors at the optic tectum. Misexpression of exogenous N-Shh peptide in vivo also causes pathfinding errors. Retinal axons grown in culture respond to N-Shh in a dose-dependent manner, either by decreasing extension at lower concentrations, or retracting axons in the presence of higher doses. These data suggest that Shh signaling is required for normal RGC axon pathfinding and tectal targeting in the developing visual system of Xenopus. We propose that Shh serves as a ventral optic tract repellent that helps to define the caudal boundary for retinal axons in the diencephalon, and that this signaling is also required for initial target recognition at the optic tectum.

Hedgehog antagonist GDC-0449 is effective in the treatment of advanced basal cell carcinoma

OBJECTIVES/HYPOTHESIS

To demonstrate the efficacy of the hedgehog pathway inhibitor GDC-0449 in the treatment of advanced basal cell carcinoma.

DESIGN STUDY

Case series.

METHODS

Three patients treated in a referral center for locally advanced basal cell carcinoma, one with metastases, were referred for treatment in a GDC-0449 phase I clinical trial. The treatment was once per day continuous therapy with oral GDC-0449.

RESULTS

Two patients showed complete clinical and radiologic resolution of disease, whereas one patient had significant reduction in tumor burden with radiologic evidence of slowly progressive local disease. Side effects were taste changes, mild to moderate hair loss, and muscle cramps in one patient.

CONCLUSIONS

GDC-0449 showed significant inhibitory activity in the treatment of advanced basal cell carcinoma.

Gene expression of axon growth promoting factors in the deer antler

The annual regeneration cycle of deer (Cervidae, Artiodactyla) antlers represents a unique model of epimorphic regeneration and rapid growth in adult mammals. Regenerating antlers are innervated by trigeminal sensory axons growing through the velvet, the modified form of skin that envelopes the antler, at elongation velocities that reach one centimetre per day in the common deer (Cervus elaphus). Several axon growth promoters like NT-3, NGF or IGF-1 have been described in the antler. To increase the knowledge on the axon growth environment, we have combined different gene-expression techniques to identify and characterize the expression of promoting molecules not previously described in the antler velvet. Cross-species microarray analyses of deer samples on human arrays allowed us to build up a list of 90 extracellular or membrane molecules involved in axon growth that were potentially being expressed in the antler. Fifteen of these genes were analysed using PCR and sequencing techniques to confirm their expression in the velvet and to compare it with the expression in other antler and skin samples. Expression of 8 axon growth promoters was confirmed in the velvet, 5 of them not previously described in the antler. In conclusion, our work shows that antler velvet provides growing axons with a variety of promoters of axon growth, sharing many of them with deer's normal and pedicle skin.

Progranulin modulates zebrafish motoneuron development in vivo and rescues truncation defects associated with knockdown of Survival motor neuron 1

BACKGROUND

Progranulin (PGRN) encoded by the GRN gene, is a secreted glycoprotein growth factor that has been implicated in many physiological and pathophysiological processes. PGRN haploinsufficiency caused by autosomal dominant mutations within the GRN gene leads to progressive neuronal atrophy in the form of frontotemporal lobar degeneration (FTLD). This form of the disease is associated with neuronal inclusions that bear the ubiquitinated TAR DNA Binding Protein-43 (TDP-43) molecular signature (FTLD-U). The neurotrophic properties of PGRN in vitro have recently been reported but the role of PGRN in neurons is not well understood. Here we document the neuronal expression and functions of PGRN in spinal cord motoneuron (MN) maturation and branching in vivo using zebrafish, a well established model of vertebrate embryonic development.

RESULTS

Whole-mount in situ hybridization and immunohistochemical analyses of zebrafish embryos revealed that zfPGRN-A is expressed within the peripheral and central nervous systems including the caudal primary (CaP) MNs within the spinal cord. Knockdown of zfPGRN-A mRNA translation mediated by antisense morpholino oligonucleotides disrupted normal CaP MN development resulting in both truncated MNs and inappropriate early branching. Ectopic over-expression of zfPGRN-A mRNA resulted in increased MN branching and rescued the truncation defects brought about by knockdown of zfPGRN-A expression. The ability of PGRN to interact with established MN developmental pathways was tested. PGRN over-expression was found to reverse the truncation defect resulting from knockdown of Survival of motor neuron 1 (smn1). This is involved in small ribonucleoprotein biogenesis RNA processing, mutations of which cause Spinal Muscular Atrophy (SMA) in humans. It did not reverse the MN defects caused by interfering with the neuronal guidance pathway by knockdown of expression of NRP-1, a semaphorin co-receptor.

CONCLUSIONS

Expression of PGRN within MNs and the observed phenotypes resulting from mRNA knockdown and over-expression are consistent with a role in the regulation of spinal cord MN development and branching. This study presents the first in vivo demonstration of the neurotrophic properties of PGRN and suggests possible future therapeutic applications in the treatment of neurodegenerative diseases.

Sonic hedgehog as a chemoattractant for adult NPCs

The developmental morphogen Sonic hedgehog (Shh) is well known for its role in modulating the proliferation and survival of neural progenitor cells in the developing mouse brain. A recent report now showed that Shh could regulate the migration of neuroblast in the adult subventricular zone (SVZ) along the rostral migratory stream (RMS) to the olfactory bulb, by functioning as a chemoattractant. Functions of Shh in regulating the migration and survival of neural progenitor cells in the adult central nervous system are suggestive of its potential roles in neural regeneration and CNS oncogenesis.

ActRIIA and BMPRII Type II BMP receptor subunits selectively required for Smad4-independent BMP7-evoked chemotaxis

Bone morphogenetic protein (BMP)-evoked reorientation and chemotaxis of cells occurs with rapid onset and involves events local to the cell membrane. The signaling pathways underlying these rapid processes likely diverge from those mediating classical transcriptional responses to BMPs but it remains unclear how BMP receptors are utilized to generate distinct intracellular mechanisms. We show that BMP7-evoked chemotaxis of monocytic cells depends on the activity of canonical type II BMP receptors. Although the three canonical type II BMP receptors are expressed in monocytic cells, inhibition of receptor subunit expression by RNAi reveals that ActRIIA and BMPRII, but not ActRIIB, are each essential for BMP7-evoked chemotaxis but not required individually for BMP-mediated induction. Furthermore, the chemotactic response to BMP7 does not involve canonical Smad4-dependent signaling but acts through PI3K-dependent signaling, illustrating selective activation of distinct intracellular events through differential engagement of receptors. We suggest a model of a BMP receptor complex in which the coordinated activity of ActRIIA and BMPRII receptor subunits selectively mediates the chemotactic response to BMP7.

Autonomous and non-autonomous Shh signalling mediate the in vivo growth and guidance of mouse retinal ganglion cell axons

In non-mammalian vertebrates, the relatively homogeneous population of retinal ganglion cells (RGCs) differentiates and projects entirely to the contralateral side of the brain under the influence of sonic hedgehog (Shh). In mammals, by contrast, there are two different RGC types: the Zic2-positive ipsilateral projecting and the Isl2-positive contralateral projecting. We asked whether the axons of these two populations respond to Shh and if their response differs. We have also analysed whether midline- and RGC-derived Shh contributes to the growth of the axons in the proximal visual pathway. We show that these two RGC types are characterised by a differential expression of Shh signalling components and that they respond differently to Shh when challenged in vitro. In vivo blockade of Shh activity, however, alters the path and distribution mostly of the contralateral projecting RGC axons at the chiasm, indicating that midline-derived Shh participates in funnelling contralateral visual fibres in this region. Furthermore, interference with Shh signalling in the RGCs themselves causes abnormal growth and navigation of contralateral projecting axons in the proximal portion of the pathway, highlighting a novel cell-autonomous mechanism by which Shh can influence growth cone behaviour.

Retinal axon growth at the optic chiasm: to cross or not to cross

At the optic chiasm, retinal ganglion cell axons from each eye converge and segregate into crossed and uncrossed projections, a pattern critical for binocular vision. Here, we review recent findings on optic chiasm development, highlighting the specific transcription factors and guidance cues that implement retinal axon divergence into crossed and uncrossed pathways. Although mechanisms underlying the formation of the uncrossed projection have been identified, the means by which retinal axons are guided across the midline are still unclear. In addition to directives provided by transcription factors and receptors in the retina, gene expression in the ventral diencephalon influences chiasm formation. Throughout this review, we compare guidance mechanisms at the optic chiasm with those in other midline models and highlight unanswered questions both for retinal axon growth and axon guidance in general.

Graded hedgehog and fibroblast growth factor signaling independently regulate pituitary cell fates and help establish the pars distalis and pars intermedia of the zebrafish adenohypophysis

The vertebrate adenohypophysis forms as a placode at the anterior margin of the neural plate, requiring both hedgehog (Hh) and fibroblast growth factor (Fgf) mediated cell-cell signaling for induction and survival of endocrine cell types. Using small molecule inhibitors to modulate signaling levels during zebrafish development we show that graded Hh and Fgf signaling independently help establish the two subdomains of the adenohypophysis, the anteriorly located pars distalis (PD) and the posterior pars intermedia (PI). High levels of Hh signaling are required for formation of the PD and differentiation of anterior endocrine cell types, whereas lower levels of Hh signaling are required for formation of the PI and differentiation of posterior endocrine cell types. In contrast, high Fgf signaling levels are required for formation of the PI and posterior endocrine cell differentiation, whereas anterior regions require lower levels of Fgf signaling. Based on live observations and marker analyses, we show that the PD forms first at the midline closest to the central nervous system source of Sonic hedgehog. In contrast the PI appears to form from more lateral/posterior cells close to a central nervous system source of Fgf3. Together our data show that graded Hh and Fgf signaling independently direct induction of the PD and PI and help establish endocrine cell fates along the anterior/posterior axis of the zebrafish adenohypophysis. These data suggest that there are distinct origins and signaling requirements for the PD and PI.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Molecular guidance cues necessary for axon pathfinding from the ventral cochlear nucleus

During development, multiple guidance cues direct the formation of appropriate synaptic connections. Factors that guide developing axons are known for various pathways throughout the mammalian brain; however, signals necessary to establish auditory connections are largely unknown. In the auditory brainstem the neurons whose axons traverse the midline in the ventral acoustic stria (VAS) are primarily located in the ventral cochlear nucleus (VCN) and project bilaterally to the superior olivary complex (SOC). The circumferential trajectory taken by developing VCN axons is similar to that of growing axons of spinal commissural neurons. Therefore, we reasoned that netrin-DCC and slit-robo signaling systems function in the guidance of VCN axons. VCN neurons express the transcription factor, mafB, as early as embryonic day (E) 13.5, thereby identifying the embryonic VCN for these studies. VCN axons extend toward the midline as early as E13, with many axons crossing by E14.5. During this time, netrin-1 and slit-1 RNAs are expressed at the brainstem midline. Additionally, neurons within the VCN express RNA for DCC, robo-1, and robo-2, and axons in the VAS are immunoreactive for DCC. VCN axons do not reach the midline of the brainstem in mice mutant for either the netrin-1 or DCC gene. VCN axons extend in pups lacking netrin-1, but most DCC-mutant samples lack VCN axonal outgrowth. Stereological cell estimates indicate only a modest reduction of VCN neurons in DCC-mutant mice. Taken together, these data show that a functional netrin-DCC signaling system is required for establishing proper VCN axonal projections in the auditory brainstem.

Sonic hedgehog promotes the migration and proliferation of optic nerve oligodendrocyte precursors

Optic nerve (ON) oligodendrocyte precursors (OPCs) are generated under the influence of the Sonic hedgehog (Shh) in the preoptic area from where they migrate to colonise the entire nerve. The molecular events that control this migration are still poorly understood. Recent studies suggested that Shh is often used by the same cell population to control different processes, including cell proliferation and migration, raising the possibility that Shh could contribute to these aspects of OPC development. In support of this idea, we show here that Shh induces the proliferation of OPCs derived from embryonic mouse ON explants and acts as a chemoattractant for their migration. In ovo injections of hybridomas secreting Shh-specific blocking antibody decreases the number of OPCs present in chick ONs, particularly in the retinal portion of the nerve. Altogether these data indicate that Shh contributes to OPC proliferation and distribution along the ON, in addition to their specification.

Hedgehog signalling in skin development and cancer

Basal cell carcinoma (BCC) is the most common human malignancy, affecting 750,000 Americans each year. The understanding of mutations that are known to activate hedgehog (Hh) signalling pathway genes, including PATCHED (PTCH), sonic hedgehog (Shh) and smoothened (Smo), has substantially expanded our current understanding of the genetic basis of BCC development. The Hh signalling pathway is one of the most fundamental signal transduction pathways in embryonic development. In skin, the Shh pathway is crucial for maintaining stem cell population, and for regulating hair follicle and sebaceous gland development. This pathway plays a minimal role in adult tissues, but is known to be activated in many neoplasms, including those arising in the skin. In this review, we attempt to summarize the results of published studies on some important aspects of the Shh pathway and its involvement in skin development and carcinogenesis. We also provide a description of various animal models that have been developed, based on our knowledge of the Shh pathway in human skin cancers. Additionally, we include a brief description of studies conducted in our laboratory and by others on the chemoprevention of BCCs. This review therefore provides a current understanding of the role of the Shh pathway in skin development and neoplasia. It also provides a basis for the molecular target-based chemoprevention and therapeutic management of skin cancer.

Axon guidance cues in auditory development

The innervation of the cochlear sensory epithelium is intricately organized, allowing the tonotopy established by the auditory hair cells to be maintained along the ascending auditory pathways. These auditory projections are patterned by several gene families that regulate neurite attraction and repulsion, known as axon guidance cues. In this review, the roles of various axon guidance molecules, including fibroblast growth factor, ephs, semaphorins, netrins and slits, are examined in light of their known contribution to auditory development. Additionally, morphogens are discussed in the context of their recently described influence on axonal pathfinding in other sensory systems. The elucidation of these various mechanisms may guide the development of therapies aimed at maximizing the connectivity of auditory neurons in the context of congenital or acquired sensorineural hearing loss, especially as pertains to cochlear implants. Further afield, improved understanding of the molecular processes which regulate innervation of the organ of Corti during normal development may prove useful in connecting regenerated hair cells to the central nervous system.

SFRP1 regulates the growth of retinal ganglion cell axons through the Fz2 receptor

Axon growth is governed by the ability of growth cones to interpret attractive and repulsive guidance cues. Recent studies have shown that secreted signaling molecules known as morphogens can also act as axon guidance cues. Of the large family of Wnt signaling components, only Wnt4 and Wnt5 seem to participate directly in axon guidance. Here we show that secreted Frizzled-related protein 1 (SFRP1), a proposed Wnt signaling inhibitor, can directly modify and reorient the growth of chick and Xenopus laevis retinal ganglion cell axons. This activity does not require Wnt inhibition and is modulated by extracellular matrix molecules. Intracellularly, SFRP1 function requires G(alpha) protein activation, protein synthesis and degradation, and it is modulated by cyclic nucleotide levels. Because SFRP1 interacts with Frizzled-2 (Fz2) and interference with Fz2 expression abolishes growth cone responses to SFRP1, we propose a previously unknown function for this molecule: the ability to guide growth cone movement via the Fz2 receptor.