Semaphorin 3A is a chemoattractant for cortical apical dendrites

Distinct cytoplasmic domains in Plexin-A4 mediate diverse responses to semaphorin 3A in developing mammalian neurons

Guidance receptor signaling is crucial for neural circuit formation and elicits diverse cellular events in specific neurons. We found that signaling from the guidance cue semaphorin 3A diverged through distinct cytoplasmic domains in its receptor Plexin-A4 to promote disparate cellular behavior in different neuronal cell types. Plexin-A4 has three main cytoplasmic domains--C1, Hinge/RBD, and C2--and interacts with family members of the Rho guanine nucleotide exchange factor FARP proteins. We show that growth cone collapse occurred in Plexin-A4-deficient dorsal root ganglion sensory neurons reconstituted with Plexin-A4 containing either the Hinge/RBD or C2 domain, whereas both of the Hinge/RBD and C1 domains were required for dendritic arborization in cortical neurons. Although knockdown studies indicated that both the collapse and arborization responses involved FARP2, mutations in the cytoplasmic region of Plexin-A4 that reduced its interaction with FARP2 strongly inhibited semaphorin 3A-induced dendritic branching but not growth cone collapse, suggesting that different degrees of interaction are required for the two responses or that developing axons have an indirect path to FARP2 activation. Thus, our study provided insights into the multifunctionality of guidance receptors, in particular showing that the semaphorin 3A signal diverges through specific functions of the modular domains of Plexin-A4.

Wnt5a induces Ryk-dependent and -independent effects on callosal axon and dendrite growth

The non-canonical Wnt receptor, Ryk, promotes chemorepulsive axon guidance in the developing mouse brain and spinal cord in response to Wnt5a. Ryk has also been identified as a major suppressor of axonal regrowth after spinal cord injury. Thus, a comprehensive understanding of how growing axons and dendrites respond to Wnt5a-mediated Ryk activation is required if we are to overcome this detrimental activity. Here we undertook a detailed analysis of the effect of Wnt5a/Ryk interactions on axonal and dendritic growth in dissociated embryonic mouse cortical neuron cultures, focusing on callosal neurons known to be responsive to Ryk-induced chemorepulsion. We show that Ryk inhibits axonal growth in response to Wnt5a. We also show that Wnt5a inhibits dendrite growth independently of Ryk. However, this inhibition is relieved when Ryk is present. Therefore, Wnt5a-mediated Ryk activation triggers divergent responses in callosal axons and dendrites in the in vitro context.

Function of the Drosophila receptor guanylyl cyclase Gyc76C in PlexA-mediated motor axon guidance

The second messengers cAMP and cGMP modulate attraction and repulsion mediated by neuronal guidance cues. We find that the Drosophila receptor guanylyl cyclase Gyc76C genetically interacts with Semaphorin 1a (Sema-1a) and physically associates with the Sema-1a receptor plexin A (PlexA). PlexA regulates Gyc76C catalytic activity in vitro, and each distinct Gyc76C protein domain is crucial for regulating Gyc76C activity in vitro and motor axon guidance in vivo. The cytosolic protein dGIPC interacts with Gyc76C and facilitates Sema-1a-PlexA/Gyc76C-mediated motor axon guidance. These findings provide an in vivo link between semaphorin-mediated repulsive axon guidance and alteration of intracellular neuronal cGMP levels.

Light-reactive dextran gels with immobilized guidance cues for directed neurite growth in 3D models

Immobilized NT-3 enhanced DRG neurite growth while Sema3A strongly repelled it, versus neutravidin controls, in a hydrogel choice point model.

Distinct temporal hierarchies in membrane and cytoskeleton dynamics precede the morphological polarization of developing neurons

Final morphological polarization of neurons, with the development of a distinct axon and of several dendrites, is preceded by phases of non-polarized architecture. The earliest of these phases is that of the round neuron arising from the last mitosis. A second non polarized stage corresponds to the bipolar neuron, with two morphologically identical neurites. Both phases have their distinctive relevance in the establishment of neuronal polarity. During the round cell stage a decision is made as to where from the cell periphery a first neurite will form, thus creating the first sign of asymmetry. At the bipolar stage a decision is made as to which of the two neurites becomes the axon in neurons polarizing in vitro and the leading edge in neurons in situ. In this study we analysed cytoskeletal and membrane dynamics in cells at these two “pre-polarity” stages. By mean of time lapse imaging in dissociated hippocampal neurons and ex vivo cortical slices we show that both stages are characterized by polarized intracellular arrangements, however with distinct temporal hierarchies: polarized actin dynamics marks the site of first polarization in round cells, whereas polarized membrane dynamics precedes asymmetric growth in the bipolar stage.

Soluble guanylate cyclase generation of cGMP regulates migration of MGE neurons

Here we have provided evidence that nitric oxide-cyclic GMP (NO-cGMP) signaling regulates neurite length and migration of immature neurons derived from the medial ganglionic eminence (MGE). Dlx1/2(-/-) and Lhx6(-/-) mouse mutants, which exhibit MGE interneuron migration defects, have reduced expression of the gene encoding the α subunit of a soluble guanylate cyclase (Gucy1A3). Furthermore, Dlx1/2(-/-) mouse mutants have reduced expression of NO synthase 1 (NOS1). Gucy1A3(-/-) mice have a transient reduction in cortical interneuron number. Pharmacological inhibition of soluble guanylate cyclase and NOS activity rapidly induces neurite retraction of MGE cells in vitro and in slice culture and robustly inhibits cell migration from the MGE and caudal ganglionic eminence. We provide evidence that these cellular phenotypes are mediated by activation of the Rho signaling pathway and inhibition of myosin light chain phosphatase activity.

Protein kinase LKB1 regulates polarized dendrite formation of adult hippocampal newborn neurons

Adult-born granule cells in the dentate gyrus of the rodent hippocampus are important for memory formation and mood regulation, but the cellular mechanism underlying their polarized development, a process critical for their incorporation into functional circuits, remains unknown. We found that deletion of the serine-threonine protein kinase LKB1 or overexpression of dominant-negative LKB1 reduced the polarized initiation of the primary dendrite from the soma and disrupted its oriented growth toward the molecular layer. This abnormality correlated with the dispersion of Golgi apparatus that normally accumulated at the base and within the initial segment of the primary dendrite, and was mimicked by disrupting Golgi organization via altering the expression of Golgi structural proteins GM130 or GRASP65. Thus, besides its known function in axon formation in embryonic pyramidal neurons, LKB1 plays an additional role in regulating polarized dendrite morphogenesis in adult-born granule cells in the hippocampus.

An extracellular adhesion molecule complex patterns dendritic branching and morphogenesis

Robust dendrite morphogenesis is a critical step in the development of reproducible neural circuits. However, little is known about the extracellular cues that pattern complex dendrite morphologies. In the model nematode Caenorhabditis elegans, the sensory neuron PVD establishes stereotypical, highly branched dendrite morphology. Here, we report the identification of a tripartite ligand-receptor complex of membrane adhesion molecules that is both necessary and sufficient to instruct spatially restricted growth and branching of PVD dendrites. The ligand complex SAX-7/L1CAM and MNR-1 function at defined locations in the surrounding hypodermal tissue, whereas DMA-1 acts as the cognate receptor on PVD. Mutations in this complex lead to dramatic defects in the formation, stabilization, and organization of the dendritic arbor. Ectopic expression of SAX-7 and MNR-1 generates a predictable, unnaturally patterned dendritic tree in a DMA-1-dependent manner. Both in vivo and in vitro experiments indicate that all three molecules are needed for interaction.

Biallelic SEMA3A defects cause a novel type of syndromic short stature

Chromosomal microarray testing is commonly used to identify disease causing de novo copy number variants in patients with developmental delay and multiple congenital anomalies. In such a patient we now observed an 150 kb deletion on chromosome 7q21.11 affecting the first exon of the axon guidance molecule gene SEMA3A (sema domain, immunoglobulin domain (Ig), short basic domain, secreted, (semaphorin) 3A). This deletion was inherited from the healthy father, but considering the function of SEMA3A and phenotypic similarity to the knock-out mice, we still assumed a pathogenic relevance and tested for a recessive second defect. Sequencing of SEMA3A in the patient indeed revealed the de novo in-frame mutation p.Phe316_Lys317delinsThrSerSerAsnGlu. Cloning of the mutated allele in combination with two informative SNPs confirmed compound heterozygosity in the patient. While the altered protein structure was predicted to be benign, aberrant splicing resulting in a premature stop codon was proven by RT-PCR to occur in about half of the transcripts from this allele. Expression profiling in human fetal and adult cDNA panels, confirmed a high expression of SEMA3A in all brain regions as well as in adult and fetal heart and fetal skeletal muscle. Normal intellectual development in the patient was surprising but may be explained by the remaining 20% of SEMA3A expression level demonstrated by quantitative RT-PCR. We therefore report a novel autosomal recessive syndrome characterized by postnatal short stature with relative macrocephaly, camptodactyly, septal heart defect and several minor anomalies caused by biallelic mutations in SEMA3A.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

Simulating cortical development as a self constructing process: a novel multi-scale approach combining molecular and physical aspects

Current models of embryological development focus on intracellular processes such as gene expression and protein networks, rather than on the complex relationship between subcellular processes and the collective cellular organization these processes support. We have explored this collective behavior in the context of neocortical development, by modeling the expansion of a small number of progenitor cells into a laminated cortex with layer and cell type specific projections. The developmental process is steered by a formal language analogous to genomic instructions, and takes place in a physically realistic three-dimensional environment. A common genome inserted into individual cells control their individual behaviors, and thereby gives rise to collective developmental sequences in a biologically plausible manner. The simulation begins with a single progenitor cell containing the artificial genome. This progenitor then gives rise through a lineage of offspring to distinct populations of neuronal precursors that migrate to form the cortical laminae. The precursors differentiate by extending dendrites and axons, which reproduce the experimentally determined branching patterns of a number of different neuronal cell types observed in the cat visual cortex. This result is the first comprehensive demonstration of the principles of self-construction whereby the cortical architecture develops. In addition, our model makes several testable predictions concerning cell migration and branching mechanisms.

The proper operation of the brain depends on the correct developmental wiring of billions of neurons. Understanding this process of living self-construction is crucial not only for biological explanation and medical therapy, but could also provide an entirely new approach to industrial fabrication. We are approaching this problem through detailed simulation of cortical development. We have previously presented a software package that allows for simulation of cellular growth in a 3D space that respects physical forces and diffusion of substances, as well as an instruction language for specifying biologically plausible ‘genetic codes’. Here we apply this novel formalism to understanding the principles of cortical development in the context of multiple, spatially distributed agents that communicate only by local metabolic messages.

Two specific populations of GABAergic neurons originating from the medial and the caudal ganglionic eminences aid in proper navigation of callosal axons

The corpus callosum (CC) plays a crucial role in interhemispheric communication. It has been shown that CC formation relies on the guidepost cells located in the midline region that include glutamatergic and GABAergic neurons as well as glial cells. However, the origin of these guidepost GABAergic neurons and their precise function in callosal axon pathfinding remain to be investigated. Here, we show that two distinct GABAergic neuronal subpopulations converge toward the midline prior to the arrival of callosal axons. Using in vivo and ex vivo fate mapping we show that CC GABAergic neurons originate in the caudal and medial ganglionic eminences (CGE and MGE) but not in the lateral ganglionic eminence (LGE). Time lapse imaging on organotypic slices and in vivo analyses further revealed that CC GABAergic neurons contribute to the normal navigation of callosal axons. The use of Nkx2.1 knockout (KO) mice confirmed a role of these neurons in the maintenance of proper behavior of callosal axons while growing through the CC. Indeed, using in vitro transplantation assays, we demonstrated that both MGE- and CGE-derived GABAergic neurons exert an attractive activity on callosal axons. Furthermore, by combining a sensitive RT-PCR technique with in situ hybridization, we demonstrate that CC neurons express multiple short and long range guidance cues. This study strongly suggests that MGE- and CGE-derived interneurons may guide CC axons by multiple guidance mechanisms and signaling pathways.

Mapping CRMP3 domains involved in dendrite morphogenesis and voltage-gated calcium channel regulation

Although hippocampal neurons are well-distinguished by the morphological characteristics of their dendrites and their structural plasticity, the mechanisms involved in regulating their neurite initiation, dendrite growth, network formation and remodeling are still largely unknown, in part because the key molecules involved remain elusive. Identifying new dendrite-active cues could uncover unknown molecular mechanisms that would add significant understanding to the field and possibly lead to the development of novel neuroprotective therapy because these neurons are impaired in many neuropsychiatric disorders. In our previous studies, we deleted the gene encoding CRMP3 in mice and identified the protein as a new endogenous signaling molecule that shapes diverse features of the hippocampal pyramidal dendrites without affecting axon morphology. We also found that CRMP3 protects dendrites against dystrophy induced by prion peptide PrP(106-126). Here, we report that CRMP3 has a profound influence on neurite initiation and dendrite growth of hippocampal neurons in vitro. Our deletional mapping revealed that the C-terminus of CRMP3 probably harbors its dendritogenic capacity and supports an active transport mechanism. By contrast, overexpression of the C-terminal truncated CRMP3 phenocopied the effect of CRMP3 gene deletion with inhibition of neurite initiation or decrease in dendrite complexity, depending on the stage of cell development. In addition, this mutant inhibited the activity of CRMP3, in a similar manner to siRNA. Voltage-gated calcium channel inhibitors prevented CRMP3-induced dendritic growth and somatic Ca(2+) influx in CRMP3-overexpressing neurons was augmented largely via L-type channels. These results support a link between CRMP3-mediated Ca(2+) influx and CRMP3-mediated dendritic growth in hippocampal neurons.

WNT signaling in neuronal maturation and synaptogenesis

The Wnt signaling pathway plays a role in the development of the central nervous system and growing evidence indicates that Wnts also regulates the structure and function of the adult nervous system. Wnt components are key regulators of a variety of developmental processes, including embryonic patterning, cell specification, and cell polarity. In the nervous system, Wnt signaling also regulates the formation and function of neuronal circuits by controlling neuronal differentiation, axon outgrowth and guidance, dendrite development, synaptic function, and neuronal plasticity. Wnt factors can signal through three very well characterized cascades: canonical or β-catenin pathway, planar cell polarity pathway and calcium pathway that control different processes. However, divergent downstream cascades have been identified to control neuronal morphogenesis. In the nervous system, the expression of Wnt proteins is a highly controlled process. In addition, deregulation of Wnt signaling has been associated with neurodegenerative diseases. Here, we will review different aspects of neuronal and dendrite maturation, including spinogenesis and synaptogenesis. Finally, the role of Wnt pathway components on Alzheimer’s disease will be revised.

Neuropilin-1 biases dendrite polarization in the retina

The majority of neurons in the nervous system exhibit a polarized morphology, with multiple short dendrites and a single long axon. It is clear that multiple factors govern polarization in developing neurons, and the biased accumulation of intrinsic determinants to one side of the cell, coupled with responses to asymmetrically localized extrinsic factors, appears to be crucial. A number of intrinsic factors have been identified, but surprisingly little is known about the identity of the extrinsic signals. Here, we show in vivo that neuropilin-1 (Nrp1) and its co-receptor plexinA1 (Plxna1) are necessary to bias the extension of the dendrites of retinal ganglion cells to the apical side of the cell, and ectopically expressed class III semaphorins (Sema3s) disrupt this process. Importantly, the requirement for Nrp1 and Plxna1 in dendrite polarization occurs at a developmental time point after the cells have already extended their basally directed axon. Thus, we propose a novel mechanism whereby an extrinsic factor, probably a Sema3, acts through Nrp1 and Plxna1 to promote the asymmetric outgrowth of dendrites independently of axon polarization.

Bimodal control of dendritic and axonal growth by the dual leucine zipper kinase pathway

Knowledge of the molecular and genetic mechanisms underlying the separation of dendritic and axonal compartments is not only crucial for understanding the assembly of neural circuits, but also for developing strategies to correct defective dendrites or axons in diseases with subcellular precision. Previous studies have uncovered regulators dedicated to either dendritic or axonal growth. Here we investigate a novel regulatory mechanism that differentially directs dendritic and axonal growth within the same neuron in vivo. We find that the dual leucine zipper kinase (DLK) signaling pathway in Drosophila, which consists of Highwire and Wallenda and controls axonal growth, regeneration, and degeneration, is also involved in dendritic growth in vivo. Highwire, an evolutionarily conserved E3 ubiquitin ligase, restrains axonal growth but acts as a positive regulator for dendritic growth in class IV dendritic arborization neurons in the larva. While both the axonal and dendritic functions of highwire require the DLK kinase Wallenda, these two functions diverge through two downstream transcription factors, Fos and Knot, which mediate the axonal and dendritic regulation, respectively. This study not only reveals a previously unknown function of the conserved DLK pathway in controlling dendrite development, but also provides a novel paradigm for understanding how neuronal compartmentalization and the diversity of neuronal morphology are achieved.

Recovery from experimental parkinsonism by semaphorin-guided axonal growth of grafted dopamine neurons

Cell therapy in animal models of Parkinson's disease (PD) is effective after intrastriatal grafting of dopamine (DA) neurons, whereas intranigral transplantation of dopaminergic cells does not cause consistent behavioral recovery. One strategy to promote axonal growth of dopaminergic neurons from the substantia nigra (SN) to the striatum is degradation of inhibitory components such as chondroitin sulphate proteoglycans (CSPG). An alternative is the guidance of DA axons by chemotropic agents. Semaphorins 3A and 3C enhance axonal growth of embryonic stem (ES) cell-derived dopaminergic neurons in vitro, while Semaphorin 3C also attracts them. We asked whether intranigral transplantation of DA neurons, combined with either degradation of CSPG or with grafts of Semaphorin 3-expressing cells, towards the striatum, is effective in establishing a new nigrostriatal dopaminergic pathway in rats with unilateral depletion of DA neurons. We found depolarization-induced DA release in dorsal striatum, DA axonal projections from SN to striatum, and concomitant behavioral improvement in Semaphorin 3-treated animals. These effects were absent in animals that received intranigral transplants combined with Chondroitinase ABC treatment, although partial degradation of CSPG was observed. These results are evidence that Semaphorin 3-directed long-distance axonal growth of dopaminergic neurons, resulting in behavioral improvement, is possible in adult diseased brains.

Development of the principal nucleus trigeminal lemniscal projections in the mouse

The principal sensory (PrV) nucleus-based trigeminal lemniscus conveys whisker-specific neural patterns to the ventroposteromedial (VPM) nucleus of the thalamus and subsequently to the primary somatosensory cortex. Here we examined the perinatal development of this pathway with carbocyanine dye labeling in embryonic and early postnatal mouse brains. We developed a novel preparation in which the embryonic hindbrain and the diencephalon are flattened out, allowing a birds-eye view of the PrV lemniscus in its entirety. For postnatal brains we used another novel approach by sectioning the brain along an empirically determined oblique horizontal angle, again preserving the trigeminothalamic pathway. PrV neurons are born along the hindbrain ventricular zone and migrate radially for a short distance to coalesce into a nucleus adjacent to the ascending trigeminal tract. During migration of the spindle-shaped cell bodies, slender axonal processes grow along the opposite direction towards the floor plate. As early as embryonic day (E) 11, pioneering axons tipped with large growth cones cross the ventral midline and immediately make a right angle turn. By E13 many PrV axons form fascicles crossing the midline and follow a rostral course. PrV axons reach the midbrain by E15 and the thalamus by E17. While the target recognition and invasion occurs prenatally, organization of PrV axon terminals into whisker-specific rows and patches takes place during the first 4 postnatal (P) days. Initially diffuse and exuberant projections in the VPM at P1 coalesce into row and whisker specific terminal zones by P4.

SEMA3A signaling controls layer-specific interneuron branching in the cerebellum

BACKGROUND

GABAergic interneurons regulate the balance and dynamics of neural circuits, in part, by elaborating their strategically placed axon branches that innervate specific cellular and subcellular targets. However, the molecular mechanisms that regulate target-directed GABAergic axon branching are not well understood.

RESULTS

Here we show that the secreted axon guidance molecule, SEMA3A, expressed locally by Purkinje cells, regulates cerebellar basket cell axon branching through its cognate receptor Neuropilin-1 (NRP1). SEMA3A was specifically localized and enriched in the Purkinje cell layer (PCL). In sema3A(-/-) and nrp1(sema-/sema-) mice lacking SEMA3A-binding domains, basket axon branching in PCL was reduced. We demonstrate that SEMA3A-induced axon branching was dependent on local recruitment of soluble guanylyl cyclase (sGC) to the plasma membrane of basket cells, and sGC subcellular trafficking was regulated by the Src kinase FYN. In fyn-deficient mice, basket axon terminal branching was reduced in PCL, but not in the molecular layer.

CONCLUSIONS

These results demonstrate a critical role of local SEMA3A signaling in layer-specific axonal branching, which contributes to target innervation.

Phosphorylation of CRMP2 is involved in proper bifurcation of the apical dendrite of hippocampal CA1 pyramidal neurons

The neural circuit in the hippocampus is important for higher brain functions. Dendrites of CA1 pyramidal neurons mainly receive input from the axons of CA3 pyramidal neurons in this neural circuit. A CA1 pyramidal neuron has a single apical dendrite and multiple basal dendrites. In wild-type mice, most of CA1 pyramidal neurons extend a single trunk, or alternatively, the apical dendrite bifurcates into two daughter trunks at the stratum radiatum layer. We previously reported the proximal bifurcation phenotype in Sema3A-/-, p35-/-, and CRMP4-/- mice. Cdk5/p35 phosphorylates CRMP2 at Ser522, and inhibition of this phosphorylation suppressed Sema3A-induced growth cone collapse. In this study, we analyzed the bifurcation points of the apical dendrites of hippocampal CA1 pyramidal neurons in CRMP2KI/KI mice in which the Cdk5/p35-phosphorylation site Ser522 was mutated into an Ala residue. The proximal bifurcation phenotype was not observed in CRMP2KI/KI mice; however, severe proximal bifurcation of apical dendrites was found in CRMP2KI/KI;CRMP4-/- mice. Cultured hippocampal neurons from CRMP2KI/KI and CRMP2KI/KI;CRMP4-/- embryos showed an increased number of dendritic branching points compared to those from wild-type embryos. Sema3A increased the number of branching points and the total length of dendrites in wild-type hippocampal neurons, but these effects of Sema3A for dendrites were not observed in CRMP2KI/KI and CRMP2KI/KI;CRMP4-/-hippocampal neurons. Binding of CRMP2 to tubulin increased in both CRMP2KI/KI and CRMP2KI/KI:CRMP4-/- brain lysates. These results suggest that CRMP2 and CRMP4 synergistically regulate dendritic development, and CRMP2 phosphorylation is critical for proper bifurcation of apical dendrite of CA1 pyramidal neurons.

DSCAM contributes to dendrite arborization and spine formation in the developing cerebral cortex

Down syndrome cell adhesion molecule, or DSCAM, has been implicated in many neurodevelopmental processes including axon guidance, dendrite arborization, and synapse formation. Here we show that DSCAM plays an important role in regulating the morphogenesis of cortical pyramidal neurons in the mouse. We report that DSCAM expression is developmentally regulated and localizes to synaptic plasma membranes during a time of robust cortical dendrite arborization and spine formation. Analysis of mice that carry a spontaneous mutation in DSCAM (DSCAM(del17)) revealed gross morphological changes in brain size and shape in addition to subtle changes in cortical organization, volume, and lamination. Early postnatal mutant mice displayed a transient decrease in cortical thickness, but these reductions could not be attributed to changes in neuron production or cell death. DSCAM(del17) mutants showed temporary impairments in the branching of layer V pyramidal neuron dendrites at P10 and P17 that recovered to normal by adulthood. Defects in DSCAM(del17) dendrite branching correlated with a temporal increase in apical branch spine density and lasting changes in spine morphology. At P15 and P42, mutant mice displayed a decrease in the percentage of large, stable spines and an increase in the percentage of small, immature spines. Together, our findings suggest that DSCAM contributes to pyramidal neuron morphogenesis by regulating dendrite arborization and spine formation during cortical circuit development.

CRMP4 suppresses apical dendrite bifurcation of CA1 pyramidal neurons in the mouse hippocampus

Collapsin response mediator proteins (CRMPs) are a family of cytosolic phosphoproteins that consist of 5 members (CRMP 1-5). CRMP2 and CRMP4 regulate neurite outgrowth by binding to tubulin heterodimers, resulting in the assembly of microtubules. CRMP2 also mediates the growth cone collapse response to the repulsive guidance molecule semaphorin-3A (Sema3A). However, the role of CRMP4 in Sema3A signaling and its function in the developing mouse brain remain unclear. We generated CRMP4-/- mice in order to study the in vivo function of CRMP4 and identified a phenotype of proximal bifurcation of apical dendrites in the CA1 pyramidal neurons of CRMP4-/- mice. We also observed increased dendritic branching in cultured CRMP4-/- hippocampal neurons as well as in cultured cortical neurons treated with CRMP4 shRNA. Sema3A induces extension and branching of the dendrites of hippocampal neurons; however, these inductions were compromised in the CRMP4-/- hippocampal neurons. These results suggest that CRMP4 suppresses apical dendrite bifurcation of CA1 pyramidal neurons in the mouse hippocampus and that this is partly dependent on Sema3A signaling.

TAG1 regulates the endocytic trafficking and signaling of the semaphorin3A receptor complex

Endocytic trafficking of membrane proteins is essential for neuronal structure and function. We show that Transient Axonal Glycoprotein 1 (TAG1 or CNTN2), a contactin-related adhesion molecule, plays a central role in the differential trafficking of components of the semaphorin3A (Sema3A) receptor complex into distinct endosomal compartments in murine spinal sensory neuron growth cones. The semaphorin3A receptor is composed of Neuropilin1 (NRP1), PlexinA4, and L1, with NRP1 being the ligand-binding component. TAG1 interacts with NRP1, causing a change in its association with L1 in the Sema3A response such that L1 is lost from the complex following Sema3A binding. Initially, however, L1 and NRP1 endocytose together and only become separated intracellularly, with NRP1 becoming associated with endosomes enriched in lipid rafts and colocalizing with TAG1 and PlexinA4. When TAG1 is missing, NRP1 and L1 fail to separate and NRP1 does not become raft associated; colocalization with PlexinA4 is reduced and Plexin signaling is not initiated. These observations identify a novel role for TAG1 in modulating the intracellular sorting of signaling receptor complexes.

Regulation of dendritic branching by Cdc42 GAPs

Nerve cells form elaborate, highly branched dendritic trees that are optimized for the receipt of synaptic signals. Recent work published in this issue of Genes & Development by Rosario and colleagues (pp. 1743-1757) shows that a Cdc42-specific GTPase-activating protein (NOMA-GAP) regulates the branching of dendrites by neurons in the top layers of the mouse cortex. The results raise interesting questions regarding the specification of arbors in different cortical layers and the mechanisms of dendrite branching.

Comparative study of human and mouse postsynaptic proteomes finds high compositional conservation and abundance differences for key synaptic proteins

Direct comparison of protein components from human and mouse excitatory synapses is important for determining the suitability of mice as models of human brain disease and to understand the evolution of the mammalian brain. The postsynaptic density is a highly complex set of proteins organized into molecular networks that play a central role in behavior and disease. We report the first direct comparison of the proteome of triplicate isolates of mouse and human cortical postsynaptic densities. The mouse postsynaptic density comprised 1556 proteins and the human one 1461. A large compositional overlap was observed; more than 70% of human postsynaptic density proteins were also observed in the mouse postsynaptic density. Quantitative analysis of postsynaptic density components in both species indicates a broadly similar profile of abundance but also shows that there is higher abundance variation between species than within species. Well known components of this synaptic structure are generally more abundant in the mouse postsynaptic density. Significant inter-species abundance differences exist in some families of key postsynaptic density proteins including glutamatergic neurotransmitter receptors and adaptor proteins. Furthermore, we have identified a closely interacting set of molecules enriched in the human postsynaptic density that could be involved in dendrite and spine structural plasticity. Understanding synapse proteome diversity within and between species will be important to further our understanding of brain complexity and disease.

Gli3 controls subplate formation and growth of cortical axons

The formation of a functional cortical circuitry requires the coordinated growth of cortical axons to their target areas. While the mechanisms guiding cortical axons to their targets have extensively been studied, very little is known about the processes which promote their growth in vivo. Gli3 encodes a zinc finger transcription factor which is expressed in cortical progenitor cells and has crucial roles in cortical development. Here, we characterize the Gli3 compound mutant Gli3(Xt/Pdn), which largely lacks Neurofilament(+) fibers in the rostral and intermediate neocortex. DiI labeling and Golli-τGFP immunofluorescence indicate that Gli3(Xt/Pdn) cortical neurons form short and stunted axons. Using transplantation experiments we demonstrate that this axon growth defect is primarily caused by a nonpermissive cortical environment. Furthermore, in Emx1Cre;Gli3(Pdn/fl) conditional mutants, which mimic the reduction of Gli3 expression in the dorsal telencephalon of Gli3(Xt/Pdn) embryos, the growth of cortical axons is not impaired, suggesting that Gli3 controls this process early in telencephalic development. In contrast to cortical plate neurons, Gli3(Xt/Pdn) embryos largely lack subplate (SP) neurons which normally pioneer cortical projections. Collectively, these findings show that Gli3 specifies a cortical environment permissive to the growth of cortical axons at the progenitor level by controlling the formation of SP neurons.

Getting neural circuits into shape with semaphorins

Semaphorins are key players in the control of neural circuit development. Recent studies have uncovered several exciting and novel aspects of neuronal semaphorin signalling in various cellular processes--including neuronal polarization, topographical mapping and axon sorting--that are crucial for the assembly of functional neuronal connections. This progress is important for further understanding the many neuronal and non-neuronal functions of semaphorins and for gaining insight into their emerging roles in the perturbed neural connectivity that is observed in some diseases. This Review discusses recent advances in semaphorin research, focusing on novel aspects of neuronal semaphorin receptor regulation and previously unexplored cellular functions of semaphorins in the nervous system.

Developmental origins and architecture of Drosophila leg motoneurons

Motoneurons are key points of convergence within motor networks, acting as the "output channels" that directly control sets of muscles to maintain posture and generate movement. Here we use genetic mosaic techniques to reveal the origins and architecture of the leg motoneurons of Drosophila. We show that a small number of leg motoneurons are born in the embryo but most are generated during larval life. These postembryonic leg motoneurons are produced by five neuroblasts per hemineuromere, and each lineage generates stereotyped lineage-specific projection patterns. Two of these postembryonic neuroblasts generate solely motoneurons that are the bulk of the leg motoneurons. Within the largest lineage, lineage 15, we see distinct birth-order differences in projection patterns. A comparison of the central projections of leg motoneurons and the muscles they innervate reveals a stereotyped architecture and the existence of a myotopic map. Timeline analysis of axonal outgrowth reveals that leg motoneurons reach their sites of terminal arborization in the leg at the time when their dendrites are elaborating their subtype-specific shapes. Our findings provide a comprehensive description of the origin, development, and architecture of leg motoneurons that will aid future studies exploring the link between the assembly and organization of connectivity within the leg motor system of Drosophila.

Hierarchical clustering of gene expression patterns in the Eomes + lineage of excitatory neurons during early neocortical development

BACKGROUND

Cortical neurons display dynamic patterns of gene expression during the coincident processes of differentiation and migration through the developing cerebrum. To identify genes selectively expressed by the Eomes + (Tbr2) lineage of excitatory cortical neurons, GFP-expressing cells from Tg(Eomes::eGFP) Gsat embryos were isolated to > 99% purity and profiled.

RESULTS

We report the identification, validation and spatial grouping of genes selectively expressed within the Eomes + cortical excitatory neuron lineage during early cortical development. In these neurons 475 genes were expressed ≥ 3-fold, and 534 genes ≤ 3-fold, compared to the reference population of neuronal precursors. Of the up-regulated genes, 328 were represented at the Genepaint in situ hybridization database and 317 (97%) were validated as having spatial expression patterns consistent with the lineage of differentiating excitatory neurons. A novel approach for quantifying in situ hybridization patterns (QISP) across the cerebral wall was developed that allowed the hierarchical clustering of genes into putative co-regulated groups. Forty four candidate genes were identified that show spatial expression with Intermediate Precursor Cells, 49 candidate genes show spatial expression with Multipolar Neurons, while the remaining 224 genes achieved peak expression in the developing cortical plate.

CONCLUSIONS

This analysis of differentiating excitatory neurons revealed the expression patterns of 37 transcription factors, many chemotropic signaling molecules (including the Semaphorin, Netrin and Slit signaling pathways), and unexpected evidence for non-canonical neurotransmitter signaling and changes in mechanisms of glucose metabolism. Over half of the 317 identified genes are associated with neuronal disease making these findings a valuable resource for studies of neurological development and disease.

'Til Eph do us part': intercellular signaling via Eph receptors and ephrin ligands guides cerebral cortical development from birth through maturation

Eph receptors, the largest family of surface-bound receptor tyrosine kinases and their ligands, the ephrins, mediate a wide variety of cellular interactions in most organ systems throughout both development and maturity. In the forming cerebral cortex, Eph family members are broadly and dynamically expressed in particular sets of cortical cells at discrete times. Here, we review the known functions of Eph-mediated intercellular signaling in the generation of progenitors, the migration of maturing cells, the differentiation of neurons, the formation of functional connections, and the choice between life and death during corticogenesis. In synthesizing these results, we posit a signaling paradigm in which cortical cells maintain a life history of Eph-mediated intercellular interactions that guides subsequent cellular decision-making.

N-cadherin: a new player in neuronal polarity

Comment on: Gärtner A, et al. EMBO J 2012; <span class="b">31</span>:1893-903

Autism spectrum disorder susceptibility gene TAOK2 affects basal dendrite formation in the neocortex

How neurons develop their morphology is an important question in neurobiology. Here we describe a new pathway that specifically affects the formation of basal dendrites and axonal projections in cortical pyramidal neurons. We report that thousand-and-one-amino acid 2 kinase (TAOK2), also known as TAO2, is essential for dendrite morphogenesis. TAOK2 downregulation impairs basal dendrite formation in vivo without affecting apical dendrites. Moreover, TAOK2 interacts with Neuropilin 1 (Nrp1), a receptor protein that binds the secreted guidance cue Semaphorin 3A (Sema3A). TAOK2 overexpression restores dendrite formation in cultured cortical neurons from Nrp1(Sema-) mice, which express Nrp1 receptors incapable of binding Sema3A. TAOK2 overexpression also ameliorates the basal dendrite impairment resulting from Nrp1 downregulation in vivo. Finally, Sema3A and TAOK2 modulate the formation of basal dendrites through the activation of the c-Jun N-terminal kinase (JNK). These results delineate a pathway whereby Sema3A and Nrp1 transduce signals through TAOK2 and JNK to regulate basal dendrite development in cortical neurons.

Semaphorin signaling in vertebrate neural circuit assembly

Neural circuit formation requires the coordination of many complex developmental processes. First, neurons project axons over long distances to find their final targets and then establish appropriate connectivity essential for the formation of neuronal circuitry. Growth cones, the leading edges of axons, navigate by interacting with a variety of attractive and repulsive axon guidance cues along their trajectories and at final target regions. In addition to guidance of axons, neuronal polarization, neuronal migration, and dendrite development must be precisely regulated during development to establish proper neural circuitry. Semaphorins consist of a large protein family, which includes secreted and cell surface proteins, and they play important roles in many steps of neural circuit formation. The major semaphorin receptors are plexins and neuropilins, however other receptors and co-receptors also mediate signaling by semaphorins. Upon semaphorin binding to their receptors, downstream signaling molecules transduce this event within cells to mediate further events, including alteration of microtubule and actin cytoskeletal dynamics. Here, I review recent studies on semaphorin signaling in vertebrate neural circuit assembly, with the goal of highlighting how this diverse family of cues and receptors imparts exquisite specificity to neural complex connectivity.

Assembly of synaptic laminae by axon guidance molecules

A prominent feature of nervous systems is the organization of synapses into discrete layers, or laminae. Such laminae are essential for the spatial segregation of synaptic connections conveying different types of information. A prime example of this is the inner plexiform layer (IPL) of the vertebrate retina, which is subdivided into at least ten sublaminae. Another example gaining prominence is the layered neuropil of the zebrafish optic tectum. Three recent papers have shed light on the extracellular signals that control the precise stratification of pre- and postsynaptic neuronal processes in these two areas. The new studies implicate well-known axon guidance cues, including class 5 and 6 semaphorins in the retina, as well as Slit in the optic tectum. Remarkably, the short-range action of Slit, which is required for neurite stratification, appears to be achieved by anchoring the secreted guidance factor to the basement membrane at the surface of the tectum.

Semaphorin3A facilitates axonal transport through a local calcium signaling and tetrodotoxin-sensitive voltage-gated sodium channels

Semaphorin3A (Sema3A), a secreted factor that navigates axons and dendrites of developing neurons, facilitates axonal transport. However, little is known about the mechanism underlying Sema3A-induced facilitation and its functional implications. Here we show that Sema3A induces facilitation of axonal transport via local calcium signaling in growth cone. The facilitation of axonal transport was blocked by inhibitors of voltage-gated sodium channels (tetrodotoxin, TTX), L-type voltage-gated calcium channel, and ryanodine receptor (RyR). Sema3A evoked intracellular Ca(2+) elevation in growth cone by local application of Sema3A to growth cone. Sema3A also activated RyR in growth cone as well as cell body. Notably, TTX suppressed Sema3A-induced RyR activation in cell body but not in growth cone. Our results identify a novel mechanism of Sema3A-induced axonal transport, and further suggest that Sema3A-induced local calcium signaling in growth cone is propagated to cell body in a TTX-sensitive manner.

Development of the corticothalamic projections

In this review we discuss recent advances in the understanding of corticothalamic axon guidance; patterning of the early telencephalon, the sequence and choreography of the development of projections from subplate, layers 5 and 6. These cortical subpopulations display different axonal outgrowth kinetics and innervate distinct thalamic nuclei in a temporal pattern determined by cortical layer identity and subclass specificity. Guidance by molecular cues, structural cues, and activity-dependent mechanisms contribute to this development. There is a substantial rearrangement of the corticofugal connectivity outside the thalamus at the border of and within the reticular thalamic nucleus, a region that shares some of the characteristics of the cortical subplate during development. The early transient circuits are not well understood, nor the extent to which this developmental pattern may be driven by peripheral sensory activity. We hypothesize that transient circuits during embryonic and early postnatal development are critical in the matching of the cortical and thalamic representations and forming the cortical circuits in the mature brain.

Dendritic structural plasticity

Dendrites represent the compartment of neurons primarily devoted to collecting and computating input. Far from being static structures, dendrites are highly dynamic during development and appear to be capable of plastic changes during the adult life of animals. During development, it is a combination of intrinsic programs and external signals that shapes dendrite morphology; input activity is a conserved extrinsic factor involved in this process. In adult life, dendrites respond with more modest modifications of their structure to various types of extrinsic information, including alterations of input activity. Here, the author reviews classical and recent evidence of dendrite plasticity in invertebrates and vertebrates and current progress in the understanding of the molecular mechanisms that underlie this plasticity. Importantly, some fundamental questions such as the functional role of dendrite remodeling and the causal link between structural modifications of neurons and plastic processes, including learning, are still open.

Semaphorin 4A exerts a proangiogenic effect by enhancing vascular endothelial growth factor-A expression in macrophages

The axon guidance cues semaphorins (Semas) and their receptors plexins have been shown to regulate both physiological and pathological angiogenesis. Sema4A plays an important role in the immune system by inducing T cell activation, but to date, the role of Sema4A in regulating the function of macrophages during the angiogenic and inflammatory processes remains unclear. In this study, we show that macrophage activation by TLR ligands LPS and polyinosinic-polycytidylic acid induced a time-dependent increase of Sema4A and its receptors PlexinB2 and PlexinD1. Moreover, in a thioglycollate-induced peritonitis mouse model, Sema4A was detected in circulating Ly6C(high) inflammatory monocytes and peritoneal macrophages. Acting via PlexinD1, exogenous Sema4A strongly increased macrophage migration. Of note, Sema4A-activated PlexinD1 enhanced the expression of vascular endothelial growth factor-A, but not of inflammatory chemokines. Sema4A-stimulated macrophages were able to activate vascular endothelial growth factor receptor-2 and the PI3K/serine/threonine kinase Akt pathway in endothelial cells and to sustain their migration and in vivo angiogenesis. Remarkably, in an in vivo cardiac ischemia/reperfusion mouse model, Sema4A was highly expressed in macrophages recruited at the injured area. We conclude that Sema4A activates a specialized and restricted genetic program in macrophages able to sustain angiogenesis and participates in their recruitment and activation in inflammatory injuries.

Semaphorin 3A suppresses tumor growth and metastasis in mice melanoma model

BACKGROUND

Recent understanding on cancer therapy indicated that targeting metastatic signature or angiogenic switch could be a promising and rational approach to combat cancer. Advancement in cancer research has demonstrated the potential role of various tumor suppressor proteins in inhibition of cancer progression. Current studies have shown that axonal sprouting inhibitor, semaphorin 3A (Sema 3A) acts as a potent suppressor of tumor angiogenesis in various cancer models. However, the function of Sema 3A in regulation of melanoma progression is not well studied, and yet to be the subject of intense investigation.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, using multiple in vitro and in vivo approaches we have demonstrated that Sema 3A acts as a potent tumor suppressor in vitro and in vivo mice (C57BL/6) models. Mouse melanoma (B16F10) cells overexpressed with Sema 3A resulted in significant inhibition of cell motility, invasiveness and proliferation as well as suppression of in vivo tumor growth, angiogenesis and metastasis in mice models. Moreover, we have observed that Sema 3A overexpressed melanoma clone showed increased sensitivity towards curcumin and Dacarbazine, anti-cancer agents.

CONCLUSIONS

Our results demonstrate, at least in part, the functional approach underlying Sema 3A mediated inhibition of tumorigenesis and angiogenesis and a clear understanding of such a process may facilitate the development of novel therapeutic strategy for the treatment of cancer.

Phosphorylation of CRMP2 (collapsin response mediator protein 2) is involved in proper dendritic field organization

Collapsin response mediator proteins (CRMPs) are intracellular proteins that mediate signals for several extracellular molecules, such as Semaphorin3A and neurotrophins. The phosphorylation of CRMP1 and CRMP2 by Cdk5 at Ser522 is involved in axonal guidance and spine development. Here, we found that the Ser522-phosphorylated CRMP1 and/or CRMP2 are enriched in the dendrites of cultured cortical neurons and P7 cortical section. To determine the physiological role of CRMPs in dendritic development, we generated CRMP2 knock-in mutant mice (crmp2ki/ki) in which the Ser residue at 522 was replaced with Ala. Strikingly, the cortical basal dendrites of double mutant crmp2ki/ki and crmp1-/- mice exhibited severe abnormal dendritic patterning, which we defined as "curling phenotype." These findings demonstrate that the function of CRMP1 and CRMP2 synergistically control dendritic projection, and the phosphorylation of CRMP2 at Ser522 is essential for proper dendritic field organization in vivo.

N-cadherin specifies first asymmetry in developing neurons

The precise polarization and orientation of developing neurons is essential for the correct wiring of the brain. In pyramidal excitatory neurons, polarization begins with the sprouting of opposite neurites, which later define directed migration and axo-dendritic domains. We here show that endogenous N-cadherin concentrates at one pole of the newborn neuron, from where the first neurite subsequently emerges. Ectopic N-cadherin is sufficient to favour the place of appearance of the first neurite. The Golgi and centrosome move towards this newly formed morphological pole in a second step, which is regulated by PI3K and the actin/microtubule cytoskeleton. Moreover, loss of function experiments in vivo showed that developing neurons with a non-functional N-cadherin misorient their cell axis. These results show that polarization of N-cadherin in the immediate post-mitotic stage is an early and crucial mechanism in neuronal polarity.

Semaphorin 7A promotes angiogenesis in an experimental corneal neovascularization model

PURPOSE

To characterize the involvement of Semaphorin 7A (Sema7a) in corneal neovascularization (NV).

METHODS

We generated anti-Sema7A antibodies to detect protein expression in corneal fibroblasts. Corneal fibroblast cells were cultured, stimulated with basic fibroblast growth factor (bFGF or FGF-2), immunostained with anti-Sema7A antibodies, and visualized by confocal microscopy. bFGF pellets were implanted in mouse corneal micropockets for 3-10 days, and corneal sections were immunostained with anti-Sema7A antibodies. Mouse corneas were injected with a Sema7A expression vector or a control vector for 3, 7, and 10 days. Mouse corneas were imaged by slit lamp microscopy, and areas of corneal NV were calculated using the ImageJ program. Mouse corneal sections were also immunostained with anti-macrophage marker (F4/80) and anti-vascular endothelial growth factor (VEGF)-A antibodies.

RESULTS

Our data showed enhanced Sema7A expression levels in bFGF-stimulated cultured corneal fibroblasts. bFGF corneal implantation also demonstrated enhanced Sema7A expression. Corneas injected with a Sema7A expression vector showed evidence of significant corneal NV compared to controls on day 10 (1.8 mm(2) vs. 0.11 mm(2); p < 0.02). Additionally, immunolocalization of Sema7A expression vector-injected corneas (at day 7) revealed macrophage recruitment and enhanced VEGF-A levels.

CONCLUSIONS

We demonstrated that Sema7A was expressed in vascularized corneas and showed pro-angiogenic properties in our corneal model. Understanding the mechanism of Sema7A in angiogenesis may provide a therapeutic target for the treatment of corneal angiogenesis-related disorders.

Motor neuron position and topographic order imposed by β- and γ-catenin activities

Neurons typically settle at positions that match the location of their synaptic targets, creating topographic maps. In the spinal cord, the organization of motor neurons into discrete clusters is linked to the location of their muscle targets, establishing a topographic map of punctate design. To define the significance of motor pool organization for neuromuscular map formation, we assessed the role of cadherin-catenin signaling in motor neuron positioning and limb muscle innervation. We find that joint inactivation of β- and γ-catenin scrambles motor neuron settling position in the spinal cord but fails to erode the predictive link between motor neuron transcriptional identity and muscle target. Inactivation of N-cadherin perturbs pool positioning in similar ways, albeit with reduced penetrance. These findings reveal that cadherin-catenin signaling directs motor pool patterning and imposes topographic order on an underlying identity-based neural map.