Targeting of retinal axons requires the metalloproteinase ADAM10

The mechanics of the retina: Müller glia role on retinal extracellular matrix and modelling

The retina is a highly heterogeneous tissue, both cell-wise but also regarding its extracellular matrix (ECM). The stiffness of the ECM is pivotal in retinal development and maturation and has also been associated with the onset and/or progression of numerous retinal pathologies, such as glaucoma, proliferative vitreoretinopathy (PVR), age-related macular degeneration (AMD), epiretinal membrane (ERM) formation or uveitis. Nonetheless, much remains unknown about the biomechanical milieu of the retina, and specifically the role that Müller glia play as principal mechanosensors and major producers of ECM constituents. So far, new approaches need to be developed to further the knowledge in the field of retinal mechanobiology for ECM-target applications to arise. In this review, we focus on the involvement of Müller glia in shaping and altering the retinal ECM under both physiological and pathological conditions and look into various biomaterial options to more accurately replicate the impact of matrix stiffness in vitro.

ADAM10 facilitates rapid neural stem cell cycling and proper positioning within the subventricular zone niche via JAMC/RAP1Gap signaling

The mechanisms that regulate neural stem cell (NSC) lineage progression and maintain NSCs within different domains of the adult neural stem cell niche, the subventricular zone are not well defined. Quiescent NSCs are arranged at the apical ventricular wall, while mitotically activated NSCs are found in the basal, vascular region of the subventricular zone. Here, we found that ADAM10 (a disintegrin and metalloproteinase 10) is essential in NSC association with the ventricular wall, and via this adhesion to the apical domain, ADAM10 regulates the switch from quiescent and undifferentiated NSC to an actively proliferative and differentiating cell state. Processing of JAMC (junctional adhesion molecule C) by ADAM10 increases Rap1GAP activity. This molecular machinery promotes NSC transit from the apical to the basal compartment and subsequent lineage progression. Understanding the molecular mechanisms responsible for regulating the proper positioning of NSCs within the subventricular zone niche and lineage progression of NSCs could provide new targets for drug development to enhance the regenerative properties of neural tissue.

The regulatory mechanism and therapeutic potential of transcription factor EB in neurodegenerative diseases

The autophagy-lysosomal pathway (ALP) is involved in the degradation of protein aggregates and damaged organelles. Transcription factor EB (TFEB), a major regulator of ALP, has emerged as a leading factor in addressing neurodegenerative disease pathology, including Alzheimer's disease (AD), Parkinson's disease (PD), PolyQ diseases, and Amyotrophic lateral sclerosis (ALS). In this review, we delineate the regulation of TFEB expression and its functions in ALP. Dysfunctions of TFEB and its role in the pathogenesis of several neurodegenerative diseases are reviewed. We summarize the protective effects and molecular mechanisms of some TFEB-targeted agonists in neurodegenerative diseases. We also offer our perspective on analyzing the pros and cons of these agonists in the treatment of neurodegenerative diseases from the perspective of drug development. More studies on the regulatory mechanisms of TFEB in other biological processes will aid our understanding of the application of TFEB-targeted therapy in neurodegeneration.

Identification of quantitative trait loci associated with calmness and gentleness in honey bees using whole-genome sequences

The identification of quantitative trait loci (QTL) through genome-wide association studies (GWAS) is a powerful method for unravelling the genetic background of selected traits and improving early-stage predictions. In honey bees (Apis mellifera), past genetic analyses have particularly focused on individual queens and workers. In this study, we used pooled whole-genome sequences to ascertain the genetic variation of the entire colony. In total, we sampled 216 Apis mellifera mellifera and 28 Apis mellifera carnica colonies. Different experts subjectively assessed the gentleness and calmness of the colonies using a standardised protocol. Conducting a GWAS for calmness on 211 purebred A. m. mellifera colonies, we identified three QTL, on chromosomes 8, 6, and 12. The two first QTL correspond to LOC409692 gene, coding for a disintegrin and metalloproteinase domain-containing protein 10, and to Abscam gene, coding for a Dscam family member Abscam protein, respectively. The last gene has been reported to be involved in the domestication of A. mellifera. The third QTL is located 13 kb upstream of LOC102655631, coding for a trehalose transporter. For gentleness, two QTL were identified on chromosomes 4 and 3. They are located within gene LOC413669, coding for a lap4 protein, and gene LOC413416, coding for a bicaudal C homolog 1-B protein, respectively. The identified positional candidate genes of both traits mainly affect the olfaction and nervous system of honey bees. Further research is needed to confirm the results and to better understand the genetic and phenotypic basis of calmness and gentleness.

Autophagy-Dependent Increased ADAM10 Mature Protein Induced by TFEB Overexpression Is Mediated Through PPARα

Nonamyloidogenic processing of amyloid precursor protein (APP) by augmenting ADAM10 is a promising therapeutic strategy for Alzheimer's disease (AD). Therefore identification of molecular pathways that regulate ADAM10 expression is crucial. Autophagy is strongly dysregulated in AD, and TFEB was recently shown to be a master regulator of autophagy-lysosome pathway (ALP). Here, we report that TFEB expression in HeLa cells increased ADAM10 mature form by 72% (p < 0.01, n = 4), while TFEB knockdown by CRISPR strategy reduced ADAM10 mature form by 36% (p < 0.05, n = 4). Autophagy inhibition by 3-methyladenine (3-MA), but not bafilomycin A1 (BAF1), reduced ADAM10 mature form by 49% (p < 0.05, n = 4) in the TFEB expressing HeLa cells. Autophagy activation by 3 h of starvation increased ADAM10 to 91% (p < 0.001, n = 6) relative to 51% (p < 0.01, n = 6) in the nutrient-fed cells. Further, siRNAs targeted against PPARα in HeLa cells decreased ADAM10 levels by 28% (p < 0.05, n = 6) relative to the cells treated with scrambled siRNAs. Further, incubation of EGFP-TFEB expressing HeLa cells with PPARα antagonist, but not PPARβ or PPARγ antagonists, prevented TFEB-induced increase in ADAM10 levels. Importantly, flag-TFEB expression in the brain also increased ADAM10 by 60% (p < 0.05, n = 3) in the cortical and 34% (p < 0.001, n = 3) in the hippocampal homogenates. ADAM10 activity also increased by 57% (p < 0.01, n = 3) in the HeLa cells. Finally, TFEB-induced ADAM10 potentiation led to increased secretion of sAPPα by 154% (p < 0.001, n = 3) in the cortex and 62% (p < 0.001, n = 3) in the hippocampus. Thus, TFEB expression enhances nonamyloidogenic processing of APP. In conclusion, TFEB expression induces ADAM10 in an autophagy-dependent manner through PPARα.

The Expression of Key Guidance Genes at a Forebrain Axon Turning Point Is Maintained by Distinct Fgfr Isoforms but a Common Downstream Signal Transduction Mechanism

During development the axons of neurons grow toward and locate their synaptic partners to form functional neural circuits. Axons do so by reading a map of guidance cues expressed by surrounding tissues. Guidance cues are expressed at a precise space and time, but how guidance cue expression is regulated, and in a coordinated manner, is poorly understood. Semaphorins (Semas) and Slits are families of molecular ligands that guide axons. We showed previously that fibroblast growth factor (Fgf) signaling maintains sema3a and slit1 forebrain expression in Xenopus laevis, and these two repellents cooperate to guide retinal ganglion cell (RGC) axons away from the mid-diencephalon and on towards the optic tectum. Here, we investigate whether there are common features of the regulatory pathways that control the expression of these two guidance cues at this single axon guidance decision point. We isolated the sema3a proximal promoter and confirmed its responsiveness to Fgf signaling. Through misexpression of truncated Fgf receptors (Fgfrs), we found that sema3a forebrain expression is dependent on Fgfr2-4 but not Fgfr1. This is in contrast to slit1, whose expression we showed previously depends on Fgfr1 but not Fgfr2-4. Using pharmacological inhibitors and misexpression of constitutively active (CA) and dominant negative (DN) signaling intermediates, we find that while distinct Fgfrs regulate these two guidance genes, intracellular signaling downstream of Fgfrs appears to converge along the phosphoinositol 3-kinase (PI3K)-Akt signaling pathway. A common PI3K-Akt signaling pathway may allow for the coordinated expression of guidance cues that cooperate to direct axons at a guidance choice point.

Fibroblast growth factor receptor 1 signaling transcriptionally regulates the axon guidance cue slit1

Axons sense molecular cues in their environment to arrive at their post-synaptic targets. While many of the molecular cues have been identified, the mechanisms that regulate their spatiotemporal expression remain elusive. We examined here the transcriptional regulation of the guidance gene slit1 both in vitro and in vivo by specific fibroblast growth factor receptors (Fgfrs). We identified an Fgf-responsive 2.3 kb slit1 promoter sequence that recapitulates spatiotemporal endogenous expression in the neural tube and eye of Xenopus embryos. We found that signaling through Fgfr1 is the main regulator of slit1 expression both in vitro in A6 kidney epithelial cells, and in the Xenopus forebrain, even when other Fgfr subtypes are present in cells. These data argue that a specific signaling pathway downstream of Fgfr1 controls in a cell-autonomous manner slit1 forebrain expression and are novel in identifying a specific growth factor receptor for in vivo control of the expression of a key embryonic axon guidance cue.

ADAM23 promotes neuronal differentiation of human neural progenitor cells

Background

ADAM23 is widely expressed in the embryonic central nervous system and plays an important role in tissue formation.

Results

In this study, we showed that ADAM23 contributes to cell survival and is involved in neuronal differentiation during the differentiation of human neural progenitor cells (hNPCs). Upregulation of ADAM23 in hNPCs was found to increase the number of neurons and the length of neurite, while its downregulation decreases them and triggers cell apoptosis. RNA microarray analysis revealed mechanistic insights into genes and pathways that may become involved in multiple cellular processes upon up- or downregulation of ADAM23.

Conclusions

Our results suggest that ADAM23 regulates neuronal differentiation by triggering specific signaling pathways during hNPC differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s11658-017-0045-1) contains supplementary material, which is available to authorized users.

The expression pattern of Adam10 in the central nervous system of adult mice: Detection by in situ hybridization combined with immunohistochemistry staining

ADAM10 (a disintegrin and metalloprotease 10) is a member of the ADAMs family, which is key in the development of the nervous system, by regulating proliferation, migration, differentiation and survival of various cells, including axonal growth and myelination. Previous studies have investigated the embryonic or postnatal expression of ADAM10, however, detailed information regarding its cellular distribution in the adult stage, to the best of our knowledge, is not available. The present study investigated the expression pattern of the ADAM10 gene in the adult mouse central nervous system (CNS) using an ADAM10 complementary RNA probe for in situ hybridization (ISH). Immunohistochemical staining was used to identify the type of the ISH staining-positive cells with neuron- or astrocyte-specific antibodies. The results of the current study demonstrated that the ADAM10 gene was predominantly expressed in the neurons of the cerebral cortex, hippocampus, thalamus and cerebellar granular cells in adult mouse CNS.

Cell adhesion and invasion mechanisms that guide developing axons

Axon extension, guidance and tissue invasion share many similarities to normal cell migration and cancer cell metastasis. Proper cell and growth cone migration requires tightly regulated adhesion complex assembly and detachment from the extracellular matrix (ECM). In addition, many cell types actively remodel the ECM using matrix metalloproteases (MMPs) to control tissue invasion and cell dispersal. Targeting and activating MMPs is a tightly regulated process, that when dysregulated, can lead to cancer cell metastasis. Interestingly, new evidence suggests that growth cones express similar cellular and molecular machinery as migrating cells to clutch retrograde actin flow on ECM proteins and target matrix degradation, which may be used to facilitate axon pathfinding through the basal lamina and across tissues.

The alpha secretase ADAM10: A metalloprotease with multiple functions in the brain

Proteins belonging to the 'A Disintegrin And Metalloproteinase' (ADAM) family are membrane-anchored proteases that are able to cleave the extracellular domains of several membrane-bound proteins in a process known as 'ectodomain shedding'. In the central nervous system, ADAM10 has attracted the most attention, since it was described as the amyloid precursor protein α-secretase over ten years ago. Despite the excitement over the potential of ADAM10 as a novel drug target in Alzheimer disease, the physiological functions of ADAM10 in the brain are not yet well understood. This is largely because of the embryonic lethality of ADAM10-deficient mice, which results from the loss of cleavage and signaling of the Notch receptor, another ADAM10 substrate. However, the recent generation of conditional ADAM10-deficient mice and the identification of further ADAM10 substrates in the brain has revealed surprisingly numerous and fundamental functions of ADAM10 in the development of the embryonic brain and also in the homeostasis of adult neuronal networks. Mechanistically, ADAM10 controls these functions by utilizing unique postsynaptic substrates in the central nervous system, in particular synaptic cell adhesion molecules, such as neuroligin-1, N-cadherin, NCAM, Ephrin A2 and A5. Consequently, a dysregulation of ADAM10 activity is linked to psychiatric and neurological diseases, such as epilepsy, fragile X syndrome and Huntington disease. This review highlights the recent progress in understanding the substrates and function as well as the regulation and cell biology of ADAM10 in the central nervous system and discusses the value of ADAM10 as a drug target in brain diseases.

Rho kinase is required to prevent retinal axons from entering the contralateral optic nerve

To grow out to contact target neurons an axon uses its distal tip, the growth cone, as a sensor of molecular cues that help the axon make appropriate guidance decisions at a series of choice points along the journey. In the developing visual system, the axons of the output cells of the retina, the retinal ganglion cells (RGCs), cross the brain midline at the optic chiasm. Shortly after, they grow past the brain entry point of the optic nerve arising from the contralateral eye, and extend dorso-caudally through the diencephalon towards their optic tectum target. Using the developing visual system of the experimentally amenable model Xenopus laevis, we find that RGC axons are normally prevented from entering the contralateral optic nerve. This mechanism requires the activity of a Rho-associated kinase, Rock, known to function downstream of a number of receptors that recognize cues that guide axons. Pharmacological inhibition of Rock in an in vivo brain preparation causes mis-entry of many RGC axons into the contralateral optic nerve, and this defect is partially phenocopied by selective disruption of Rock signaling in RGC axons. These data implicate Rock downstream of a molecular mechanism that is critical for RGC axons to be able to ignore a domain, the optic nerve, which they previously found attractive.

Regulation of ECM degradation and axon guidance by growth cone invadosomes

Invadopodia and podosomes, collectively referred to as invadosomes, are F-actin-rich basal protrusions of cells that provide sites of attachment to and degradation of the extracellular matrix. Invadosomes promote the invasion of cells, ranging from metastatic cancer cells to immune cells, into tissue. Here, we show that neuronal growth cones form protrusions that share molecular, structural and functional characteristics of invadosomes. Growth cones from all neuron types and species examined, including a variety of human neurons, form invadosomes both in vitro and in vivo. Growth cone invadosomes contain dynamic F-actin and several actin regulatory proteins, as well as Tks5 and matrix metalloproteinases, which locally degrade the matrix. When viewed using three-dimensional super-resolution microscopy, F-actin foci often extended together with microtubules within orthogonal protrusions emanating from the growth cone central domain. Finally, inhibiting the function of Tks5 both reduced matrix degradation in vitro and disrupted motoneuron axons from exiting the spinal cord and extending into the periphery. Taken together, our results suggest that growth cones use invadosomes to target protease activity during axon guidance through tissues.

ADAM metalloproteases promote a developmental switch in responsiveness to the axonal repellant Sema3A

During embryonic development, axons can gain and lose sensitivity to guidance cues, and this flexibility is essential for the correct wiring of the nervous system. Yet, the underlying molecular mechanisms are largely unknown. Here we show that receptor cleavage by ADAM (A Disintegrin And Metalloprotease) metalloproteases promotes murine sensory axons loss of responsiveness to the chemorepellant Sema3A. Genetic ablation of ADAM10 and ADAM17 disrupts the developmental downregulation of Neuropilin-1 (Nrp1), the receptor for Sema3A, in sensory axons. Moreover, this is correlated with gain of repulsive response to Sema3A. Overexpression of Nrp1 in neurons reverses axonal desensitization to Sema3A, but this is hampered in a mutant Nrp1 with high susceptibility to cleavage. Lastly, we detect guidance errors of proprioceptive axons in ADAM knockouts that are consistent with enhanced response to Sema3A. Our results provide the first evidence for involvement of ADAMs in regulating developmental switch in responsiveness to axonal guidance cues.

ADAM10 negatively regulates neuronal differentiation during spinal cord development

Members of the ADAM (a disintegrin and metalloprotease) family are involved in embryogenesis and tissue formation via their proteolytic function, cell-cell and cell-matrix interactions. ADAM10 is expressed temporally and spatially in the developing chicken spinal cord, but its function remains elusive. In the present study, we address this question by electroporating ADAM10 specific morpholino antisense oligonucleotides (ADAM10-mo) or dominant-negative ADAM10 (dn-ADAM10) plasmid into the developing chicken spinal cord as well as by in vitro cell culture investigation. Our results show that downregulation of ADAM10 drives precocious differentiation of neural progenitor cells and radial glial cells, resulting in an increase of neurons in the developing spinal cord, even in the prospective ventricular zone. Remarkably, overexpression of the dn-ADAM10 plasmid mutated in the metalloprotease domain (dn-ADAM10-me) mimics the phenotype as found by the ADAM10-mo transfection. Furthermore, in vitro experiments on cultured cells demonstrate that downregulation of ADAM10 decreases the amount of the cleaved intracellular part of Notch1 receptor and its target, and increases the number of βIII-tubulin-positive cells during neural progenitor cell differentiation. Taken together, our data suggest that ADAM10 negatively regulates neuronal differentiation, possibly via its proteolytic effect on the Notch signaling during development of the spinal cord.

ADAM10 mediates N-cadherin ectodomain shedding during retinal ganglion cell differentiation in primary cultured retinal cells from the developing chick retina

Here, we examined the role of ADAM10 during retinal cell differentiation in retinal sections and in vitro cultures of developing chick retinal cells from embryonic day 6 (ED6). Immunohistochemistry showed that ADAM10 is abundantly expressed in the inner zone of neuroblastic layer at ED5, and it becomes more highly expressed in the ganglion cell layer at ED7 and ED9. Western blotting confirmed that ADAM10 was expressed as an inactive pro-form that was processed to a shorter, active form in control cultured cells, but in cultures treated with an ADAM10 inhibitor (GI254023X) and ADAM10-specific siRNA, the level of mature ADAM10 decreased. Phase-contrast microscopy showed that long neurite extensions were present in untreated cultures 24 h after plating, whereas cultures treated with GI254023X showed significant decreases in neurite extension. Immunofluorescence staining revealed that there were far fewer differentiated ganglion cells in ADAM10 siRNA and GI254023X-treated cultures compared to controls, whereas the photoreceptor cells were unaltered. The Pax6 protein was more strongly detected in the differentiated ganglion cells of control cultures compared to ADAM10 siRNA and GI254023X-treated cultures. N-cadherin ectodomain shedding was apparent in control cultures after 24 h, when ganglion cell differentiation was observed, but ADAM10 siRNA and GI254023X treatment inhibited these processes. In contrast, N-cadherin staining was strongly detected in photoreceptor cells regardless of ADAM10 siRNA and GI254023X treatment. Taken together, these data indicate that the inhibition of ADAM10 can inhibit Pax6 expression and N-cadherin ectodomain shedding in retinal cells, possibly affecting neurite outgrowth and ganglion cell differentiation.

Expression patterns of the ADAMs in early developing chicken cochlea

Members of the ADAM (a disintegrin and metalloprotease) family are type I transmembrane proteins involved in biological processes of proteolysis, cell adhesion, cell-matrix interaction, as well as in the intracellular signaling transduction. In the present study, expression patterns of seven members of the ADAM family were investigated at the early stages of the developing cochlea by in situ hybridization. The results show that each individual ADAM is expressed and regulated in the early developing cochlea. ADAM9, ADAM10, ADAM17, and ADAM23 are initially and widely expressed in the otic vesicle at embryonic day 2.5 (E2.5) and in the differential elements of the cochlear duct at E9, while ADAM12 is expressed in acoustic ganglion cells at E7. ADAM22 is detectable in cochlear ganglion cells as early as from E4 and in the basilar papilla from E7. Therefore, the present study extends our previous results and suggests that ADAMs also play a role in the early cochlear development.

Axonal growth towards Xenopus skin in vitro is mediated by matrix metalloproteinase activity

We have previously demonstrated that the growth of peripheral nervous system axons is strongly attracted towards limb buds and skin explants in vitro. Here, we show that directed axonal growth towards skin explants of Xenopus laevis in matrigel is associated with expression of matrix metalloproteinase (MMP)-18 and also other MMPs, and that this long-range neurotropic activity is inhibited by the broad-spectrum MMP inhibitors BB-94 and GM6001. We also show that forced expression of MMP-18 in COS-7 cell aggregates enhances axonal growth from Xenopus dorsal root ganglia explants. Nidogen is the target of MMPs released by cultured skin in matrigel, whereas other components remain intact. Our results suggest a novel link between MMP activity and extracellular matrix breakdown in the control of axonal growth.

Dynamic responses of Xenopus retinal ganglion cell axon growth cones to netrin-1 as they innervate their in vivo target

Netrin-1 influences retinal ganglion cell (RGC) axon pathfinding and also participates in the branching and synaptic differentiation of mature RGC axons at their target. To investigate whether netrin also serves as an early target recognition signal in the brain, we examined the dynamic behavior of Xenopus RGC axons soon after they innervate the optic tectum. Time-lapse confocal microscopy imaging of RGC axons expressing enhanced yellow fluorescent protein demonstrated that netrin-1 is involved in early axon branching, as recombinant netrin-1 halted further advancement of growth cones into the tectum and induced back branching. RGC growth cones exhibited differential responses to netrin-1 that depended on the degree of differentiation of the axon and the developmental stage of the tadpole. Netrin-1 decreased the total number of branches on newly arrived RGC growth cones at the target, but increased the dynamic branching of more mature arbors at the later developmental stage. To further explore the response of axonal growth cones to netrin, Xenopus RGC axons were followed in culture by time-lapse imaging. Exposure to netrin-1 rapidly increased the forward advancement of the axon and decreased the size and expanse of the growth cone, while also inducing back branching. Taken together, the differential in vivo and in vitro responses to netrin-1 suggest that netrin alone is not sufficient to induce the cessation of growth cone advancement in the absence of a target but can independently modulate axon branching. Collectively, our findings reveal a novel role for netrin on RGC axon branch initiation as growth cones innervate their target.

The Xenopus retinal ganglion cell as a model neuron to study the establishment of neuronal connectivity

Neurons receive inputs through their multiple branched dendrites and pass this information on to the next neuron via long axons, which branch within the target. The shape the neuron acquires is thus the key to its proper functioning in the neural circuit in which it participates. Both axons and dendrites grow in a directed fashion to their target partner neurons by responding to a large number of molecular cues in the milieu through which they extend. They then go through the process of synaptogenesis, first choosing a neuron on which to synapse, and then the appropriate subcellular location. How a neuron acquires its unique shape, establishes and modifies appropriate synaptic connectivity, and the molecular signals involved, are key questions in developmental neurobiology. Such questions of nervous system wiring are being pursued actively with a variety of different animal models and neuron types, each with its own unique advantages. Among these, the developing retinal ganglion cell (RGC) of the South African clawed frog, Xenopus laevis, has proven particularly fruitful for revealing the secrets of how axons and dendrites acquire their final morphology and connectivity. In this review, we describe how this system can be used to understand the multiple molecular events that instruct the incorporation of RGCs into the neural circuit that controls vision.

The subcommissural organ and the development of the posterior commissure

Growing axons navigate through the developing brain by means of axon guidance molecules. Intermediate targets producing such signal molecules are used as guideposts to find distal targets. Glial, and sometimes neuronal, midline structures represent intermediate targets when axons cross the midline to reach the contralateral hemisphere. The subcommissural organ (SCO), a specialized neuroepithelium located at the dorsal midline underneath the posterior commissure, releases SCO-spondin, a large glycoprotein belonging to the thrombospondin superfamily that shares molecular domains with axonal pathfinding molecules. Several evidences suggest that the SCO could be involved in the development of the PC. First, both structures display a close spatiotemporal relationship. Second, certain mutants lacking an SCO present an abnormal PC. Third, some axonal guidance molecules are expressed by SCO cells. Finally, SCO cells, the Reissner's fiber (the aggregated form of SCO-spondin), or synthetic peptides from SCO-spondin affect the neurite outgrowth or neuronal aggregation in vitro.

An evolutionary recent neuroepithelial cell adhesion function of huntingtin implicates ADAM10-Ncadherin

The Huntington's disease gene product, huntingtin, is indispensable for neural tube formation, but its role is obscure. We studied neurulation in htt-null embryonic stem cells and htt-morpholino zebrafish embryos and found a previously unknown, evolutionarily recent function for this ancient protein. We found that htt was essential for homotypic interactions between neuroepithelial cells; it permitted neurulation and rosette formation by regulating metalloprotease ADAM10 activity and Ncadherin cleavage. This function was embedded in the N terminus of htt and was phenocopied by treatment of htt knockdown zebrafish with an ADAM10 inhibitor. Notably, in htt-null cells, reversion of the rosetteless phenotype occurred only with expression of evolutionarily recent htt heterologues from deuterostome organisms. Conversely, all of the heterologues that we tested, including htt from Drosophila melanogaster and Dictyostelium discoideum, exhibited anti-apoptotic activity. Thus, anti-apoptosis may have been one of htt’s ancestral function(s), but, in deuterostomes, htt evolved to acquire a unique regulatory activity for controlling neural adhesion via ADAM10-Ncadherin, with implications for brain evolution and development.

Expression patterns of ADAMs in the developing chicken lens

In the present study the expression patterns of ADAM (a disintegrin and metalloprotease) genes in the chicken developing lens were analyzed. Using in situ hybridization, we found that seven members of the ADAM family including ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 are expressed in the developing embryonic lens. From embryonic incubation day (E) 2 to E3, most of the ADAMs investigated here are expressed in the lens placode and lens vesicle. From E5 to E7, all seven ADAMs, but predominantly ADAM9 and ADAM10, are throughly expressed in the central epithelium, as well as in the proliferating lens epithelium and the equatorial lens epithelium. From E9 to E14, expression of ADAM9, ADAM10, and ADAM17 decreases moderately in these regions. ADAM12 and ADAM13 are weakly expressed in the central epithelium and the lens epithelium, and are not detectable from E14 onward. ADAM22 and ADAM23 are expressed in the central epithelium, the lens epithelium and the equatorial lens epithelium at E5 and decrease gradually afterwards in the same regions. At E16, only weak ADAM9, ADAM10 and ADAM17 signals are found in the anterior lens epithelium. The changing spatiotemporal expression of the seven ADAMs suggests a regulatory role for these molecules during chicken lens development.

Regulation of axonal outgrowth and pathfinding by integrin-ECM interactions

Developing neurons use a combination of guidance cues to assemble a functional neural network. A variety of proteins immobilized within the extracellular matrix (ECM) provide specific binding sites for integrin receptors on neurons. Integrin receptors on growth cones associate with a number of cytosolic adaptor and signaling proteins that regulate cytoskeletal dynamics and cell adhesion. Recent evidence suggests that soluble growth factors and classic axon guidance cues may direct axon pathfinding by controlling integrin-based adhesion. Moreover, because classic axon guidance cues themselves are immobilized within the ECM and integrins modulate cellular responses to many axon guidance cues, interactions between activated receptors modulate cell signals and adhesion. Ultimately, growth cones control axon outgrowth and pathfinding behaviors by integrating distinct biochemical signals to promote the proper assembly of the nervous system. In this review, we discuss our current understanding how ECM proteins and their associated integrin receptors control neural network formation.

Differential expression of the ADAMs in developing chicken retina

The expression patterns of the seven members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 were analyzed in the developing chicken retina by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the developing retinal layers. ADAM9, ADAM10 and ADAM17 are widely expressed in the differential layers of the retina throughout the whole embryonic period, while ADAM12 and ADAM13 are mainly expressed in the ganglion cell layer at a later stage. ADAM22 and ADAM23 are restricted to the inner nuclear layer and the ganglion cell layer at a later stage. Furthermore, ADAM10 protein is co-expressed with the four members of the classic cadherins, N-cadherin, R-cadherin, cadherin-6B and cadherin-7 in distinct retinal layers. Therefore, the differential expression of the investigated ADAMs in the developing retina suggests the contribution of them to the retina development.

Quantitative and dynamic expression profile of premature and active forms of the regional ADAM proteins during chicken brain development

The ADAM (A Disintegrin and Metalloprotease) family of transmembrane proteins plays important roles in embryogenesis and tissue formation based on their multiple functional domains. In the present study, for the first time, the expression patterns of the premature and the active forms of six members of the ADAM proteins — ADAM9, ADAM10, ADAM12, ADAM17, ADAM22 and ADAM23 — in distinct parts of the developing chicken brain were investigated by quantitative Western blot analysis from embryonic incubation day (E) 10 to E20. The results show that the premature and the active forms of various ADAM proteins are spatiotemporally regulated in different parts of the brain during development, suggesting that the ADAMs play a very important role during embryonic development.

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.

Regional expression of the ADAMs in developing chicken cochlea

The expression patterns of five members of the ADAM (a disintegrin and metalloprotease) family including ADAM9, ADAM10, ADAM17, ADAM22, and ADAM23 were analyzed in different anatomical structures of the developing chicken cochlea by in situ hybridization and immunohistochemistry. Results show that ADAM9, ADAM10, and ADAM17 are widely expressed in the sensory epithelium of the basilar papilla, by homogene cells, spindle-shaped cells, and acoustic ganglion cells, and in the tegmentum vasculosum, each with a different pattern. ADAM22 expression is restricted to spindle-shaped cells and acoustic ganglion cells, while ADAM23 is prominently expressed by hair cells and acoustic ganglion cells. Furthermore, ADAM10 protein is coexpressed with several members of the classic cadherins, including cadherin-7, N-cadherin, and R-cadherin in distinct anatomical regions of the cochlea except for acoustic ganglion cells. The expression of the ADAMs in the developing cochlea suggests a contribution of the ADAMs to the development of distinct cochlear structures.

Expression of seven members of the ADAM family in developing chicken spinal cord

The expression patterns of seven members of the ADAM (a disintegrin and metalloprotease) family, including ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23, were analyzed in the developing chicken lumbar spinal cord by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the lumbar cord and its surrounding tissues. ADAM9, ADAM10, ADAM22, and ADAM23 are expressed predominantly by motoneurons in the motor column and by sensory neurons in the dorsal root ganglia, each with a different expression pattern. ADAM12 and ADAM13 are mainly expressed in the meninges around the lumbar cord and in the condensed sheets of chondroblasts around the vertebrae. ADAM17 expression is strong in the ventricular layer and limited to early stages. The differential expression of the ADAMs in the lumbar cord suggests that the ADAMs play a regulatory role in development of the spinal cord.

Upregulation of the alpha-secretase ADAM10--risk or reason for hope?

A decade ago, a disintegrin and metalloproteinase 10 (ADAM10) was identified as an alpha-secretase and as a key proteinase in the processing of the amyloid precursor protein. Accordingly, the important role that it plays in Alzheimer's disease was manifested. Animal models with an overexpression of ADAM10 revealed a beneficial profile of the metalloproteinase with respect to learning and memory, plaque load and synaptogenesis. Therefore, ADAM10 presents a worthwhile target with respect to the treatment of a neurodegenerative disease such as Morbus Alzheimer. Initially, ADAM10 was suggested to be an enzyme, shaping the extracellular matrix by cleavage of collagen type IV, or to be a tumour necrosis factor alpha convertase. In a relatively short time, a wide variety of additional substrates (with amyloid precursor protein probably being the most prominent) has been identified and the search is still ongoing. Hence, any side effects concerning the therapeutic enhancement of ADAM10 alpha-secretase activity have to be considered. The present review summarizes our knowledge about the structure and function of ADAM10 and highlights the opportunities for enhancing the expression and/or activity of the alpha-secretase as a therapeutic target.

Dynamic expression of axon guidance cues required for optic tract development is controlled by fibroblast growth factor signaling

Axons are guided to their targets by molecular cues expressed in their environment. How is the presence of these cues regulated? Although some evidence indicates that morphogens establish guidance cue expression as part of their role in patterning tissues, an important question is whether morphogens are then required to maintain guidance signals. We found that fibroblast growth factor (FGF) signaling sustains the expression of two guidance cues, semaphorin3A (xsema3A) and slit1 (xslit1), throughout the period of Xenopus optic tract development. With FGF receptor inhibition, xsema3A and xslit1 levels were rapidly diminished, and retinal ganglion cell axons arrested in the mid-diencephalon, before reaching their target. Importantly, direct downregulation of XSema3A and XSlit1 mostly phenocopied this axon guidance defect. Thus, FGFs promote continued presence of specific guidance cues critical for normal optic tract development, suggesting a second later role for morphogens, independent of tissue patterning, in maintaining select cues by acting to regulate their transcription.

Involvement of ADAM10 in axonal outgrowth and myelination of the peripheral nerve

The disintegrin and metalloproteinase 10 (ADAM10) is a membrane-anchored metalloproteinase with both proteolytic and disintegrin characteristics. Here, we investigate the expression, regulation, and functional role of ADAM10 in axonal outgrowth and myelination of the peripheral nerve. Expression pattern analysis of 11 ADAM family members in co-cultures of rat dorsal root ganglia (DRG) neurons and Schwann cells (SCs) demonstrated the most pronounced mRNA expression for ADAM10. In further studies, ADAM10 was found to be consistently upregulated in DRG-SC co-cultures before the induction of myelination. Neurons as well as SCs widely expressed ADAM10 at the protein level. In neurons, the expression of ADAM10 was exclusively limited to the axons before the induction of myelination. Inhibition of ADAM10 activity by the hydroxamate-based inhibitors GI254023X and GW280264X resulted in a significant decrease in the mean axonal length. These data suggest that ADAM10 represents a prerequisite for myelination, although its activity is not required during the process of myelination itself as demonstrated by expression analysis of myelin protein zero (P0) and Sudan black staining. Hence, during the process of myelin formation, ADAM10 is highly upregulated and appears to be critically involved in axonal outgrowth that is a requirement for myelination in the peripheral nerve.

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.

Axon growth and guidance: receptor regulation and signal transduction

The development of precise connectivity patterns during the establishment of the nervous system depends on the regulated action of diverse, conserved families of guidance cues and their neuronal receptors. Determining how these signaling pathways function to regulate axon growth and guidance is fundamentally important to understanding wiring specificity in the nervous system and will undoubtedly shed light on many neural developmental disorders. Considerable progress has been made in defining the mechanisms that regulate the correct spatial and temporal distribution of guidance receptors and how these receptors in turn signal to the growth cone cytoskeleton to control steering decisions. This review focuses on recent advances in our understanding of the mechanisms mediating growth cone guidance with a particular emphasis on the control of guidance receptor regulation and signaling.

LIMK1 acts downstream of BMP signaling in developing retinal ganglion cell axons but not dendrites

The actin cytoskeleton inside extending axonal and dendritic processes must undergo continuous assembly and disassembly. Some extrinsic factors modulate actin turnover through controlling the activity of LIM kinase 1 (LIMK1), which phosphorylates and inactivates the actin depolymerizing factor cofilin. Here, we for the first time examine the function and regulation of LIMK1 in vivo in the vertebrate nervous system. Upon expression of wildtype or kinase-dead forms of the protein, dendrite growth by Xenopus retinal ganglion cells (RGCs) was unchanged. In contrast, maintaining a low, but significant level, of LIMK1 function in the RGC axon is critical for proper extension. Interestingly, bone morphogenetic protein receptor II (BMPRII) is a major regulator of LIMK1 in extending RGC axons, as expression of a BMPRII lacking the LIMK1 binding region caused a dramatic shortening of the axons. Previously, we found that BMPRIIs stimulate dendrite initiation in vivo. Thus, the fact that manipulation of LIMK1 activity failed to alter dendrite growth suggests that BMPs may activate distinct signalling pathways in axons and dendrites.

Pursuing a 'turning point' in growth cone research

Growth cones are highly motile structures found at the leading edge of developing and regenerating nerve processes. Their role in axonal pathfinding has been well established and many guidance cues that influence growth cone behavior have now been identified. Many studies are now providing insights into the transduction and integration of signals in the growth cone, though a full understanding of growth cone behavior still eludes us. This review focuses on recent studies adding to the growing body of literature on growth cone behavior, focusing particularly on the level of autonomy the growth cone possesses and the role of local protein synthesis.

TGFbeta ligands promote the initiation of retinal ganglion cell dendrites in vitro and in vivo

Each type of neuron develops a unique morphology critical to its function, but almost all start with the basic plan of one long axon and multiple short, branched dendrites. Though extrinsic signals are known to direct many steps in the development of neuronal structure, little is understood about the initiation of processes, particularly dendrites. We find that Xenopus retinal ganglion cells (RGCs) explanted early will extend axons and not dendrites in dissociated cultures. If RGCs develop longer in vivo prior to culturing, many now extend dendrite-like processes in vitro, suggesting that an extrinsic factor is required to stimulate dendrite initiation. Members of the transforming growth factor beta (TGFbeta) superfamily, bone morphogenetic protein 2 (BMP2), and growth and differentiation factor 11 (GDF11), can signal cultured RGCs to form dendrites. Furthermore, TGFbeta ligands have an endogenous role: blocking BMP/GDF signaling with a secreted antagonist or inhibitory receptors reduces the number of primary dendrites extended in vivo.