Receptor complexes for each of the Class 3 Semaphorins

NRP1 function and targeting in neurovascular development and eye disease

Neuropilin 1 (NRP1) is expressed by neurons, blood vessels, immune cells and many other cell types in the mammalian body and binds a range of structurally and functionally diverse extracellular ligands to modulate organ development and function. In recent years, several types of mouse knockout models have been developed that have provided useful tools for experimental investigation of NRP1 function, and a multitude of therapeutics targeting NRP1 have been designed, mostly with the view to explore them for cancer treatment. This review provides a general overview of current knowledge of the signalling pathways that are modulated by NRP1, with particular focus on neuronal and vascular roles in the brain and retina. This review will also discuss the potential of NRP1 inhibitors for the treatment for neovascular eye diseases.

Comprehensive behavioral phenotyping of a new Semaphorin 3 F mutant mouse

BACKGROUND

Semaphorin 3 F (Sema3F) is a secreted type of the Semaphorin family of axon guidance molecules. Sema3F and its receptor neuropilin-2 (Npn-2) are expressed in a mutually exclusive manner in the embryonic mouse brain regions including olfactory bulb, hippocampus, and cerebral cortex. Sema3F is thought to have physiological functions in the formation of neuronal circuitry and its refinement. However, functional roles of Sema3F in the brain remain to be clarified. Here, we examined behavioral effects of Sema3F deficiency through a comprehensive behavioral test battery in Sema3F knockout (KO) male mice to understand the possible functions of Sema3F in the brain.

RESULTS

Male Sema3F KO and wild-type (WT) control mice were subjected to a battery of behavioral tests, including neurological screen, rotarod, hot plate, prepulse inhibition, light/dark transition, open field, elevated plus maze, social interaction, Porsolt forced swim, tail suspension, Barnes maze, and fear conditioning tests. In the open field test, Sema3F KO mice traveled shorter distance and spent less time in the center of the field than WT controls during the early testing period. In the light/dark transition test, Sema3F KO mice also exhibited decreased distance traveled, fewer number of transitions, and longer latency to enter the light chamber compared with WT mice. In addition, Sema3F KO mice traveled shorter distance than WT mice in the elevated plus maze test, although there were no differences between genotypes in open arm entries and time spent in open arms. Similarly, Sema3F KO mice showed decreased distance traveled in the social interaction test. Sema3F KO mice displayed reduced immobility in the Porsolt forced swim test whereas there was no difference in immobility between genotypes in the tail suspension test. In the fear conditioning test, Sema3F KO mice exhibited increased freezing behavior when exposed to a conditioning context and an altered context in absence of a conditioned stimulus. In the tests for assessing motor function, pain sensitivity, startle response to an acoustic stimulus, sensorimotor gating, or spatial reference memory, there were no significant behavioral differences between Sema3F KO and WT mice.

CONCLUSIONS

These results suggest that Sema3F deficiency induces decreased locomotor activity and possibly abnormal anxiety-related behaviors and also enhances contextual memory and generalized fear in mice. Thus, our findings suggest that Sema3F plays important roles in the development of neuronal circuitry underlying the regulation of some aspects of anxiety and fear responses.

Split Hand/Foot Malformation Associated with 7q21.3 Microdeletion: A Case Report

Split hand/foot malformation (SHFM) or ectrodactyly is a rare genetic condition affecting limb development. SHFM shows clinical and genetic heterogeneity. It can present as an isolated form or in combination with additional anomalies affecting the long bones (nonsyndromic form) or other organ systems including the craniofacial, genitourinary and ectodermal structures (syndromic ectrodactyly). This study reports a girl with SHFM who also exhibited developmental delay, mild dysmorphic facial features and sensorineural hearing loss. High-resolution banding analysis indicated an interstitial deletion within the 7q21 band. FISH using locus-specific BAC probes confirmed the microdeletion of 7q21.3. Chromosomal microarray analysis also revealed a microdeletion of 1.856 Mb in 7q21.3. However, a larger 8.44-Mb deletion involving bands 7q21.11q21.2 was observed, and the breakpoints were refined. The phenotype and the candidate genes underlying the pathogenesis of this disorder are discussed.

The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity

During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.

Neuropilin-2/Semaphorin-3F-mediated repulsion promotes inner hair cell innervation by spiral ganglion neurons

Auditory function is dependent on the formation of specific innervation patterns between mechanosensory hair cells (HCs) and afferent spiral ganglion neurons (SGNs). In particular, type I SGNs must precisely connect with inner HCs (IHCs) while avoiding connections with nearby outer HCs (OHCs). The factors that mediate these patterning events are largely unknown. Using sparse-labeling and time-lapse imaging, we visualized for the first time the behaviors of developing SGNs including active retraction of processes from OHCs, suggesting that some type I SGNs contact OHCs before forming synapses with IHCs. In addition, we demonstrate that expression of Semaphorin-3F in the OHC region inhibits type I SGN process extension by activating Neuropilin-2 receptors expressed on SGNs. These results suggest a model in which cochlear innervation patterns by type I SGNs are determined, at least in part, through a Semaphorin-3F-mediated inhibitory signal that impedes processes from extending beyond the IHC region.

DOI:http://dx.doi.org/10.7554/eLife.07830.001

The process of hearing begins when sound waves enter the outer ear, causing the eardrum to vibrate. The three small bones of the middle ear pass these vibrations on to the cochlea, a fluid-filled structure shaped like a spiral. Tiny hair cells inside the cochlea move in response to the vibrations and convert them into electrical signals, which are transmitted by cells called spiral ganglion neurons (SGNs) to the brain.

Hair cells can be divided into ‘inner’ and ‘outer’ hair cells. Inner hair cells transmit most of the information about a sound to the brain, via connections with type I SGNs. Outer hair cells are thought to amplify sound and connect to type II SGNs. How the type I and II SGNs connect to the correct type of hair cell as the ear develops is not well understood, despite these connections being essential for hearing.

Coate et al. have now used time-lapse imaging and fixed specimens to follow individually labeled SGNs as they establish these connections within the cochlea of a mouse embryo. Although the type I SGNs ultimately formed connections with inner hair cells, many of them made contact with outer hair cells first. These contacts were short-lived thanks to a protein found near the outer hair cells, named Semaphorin-3F. This protein repels the type I SGNs by activating a receptor on their surface called Neuropilin-2, and so directs the type I SGNs towards the inner hair cells.

One of the mysteries that remains to be solved is how type II SGNs are ‘permitted’ to extend into the outer hair cell region, even though they are also confronted by Semaphorin-3F. In addition, it will also be important to determine how SGNs adapt to cues from different Semaphorins from different parts of the cochlea as they navigate into different hair cell regions.

DOI:http://dx.doi.org/10.7554/eLife.07830.002

Prostate cancer cells induce osteoblastic differentiation via semaphorin 3A

BACKGROUND

Prostate cancer metastasis to bone is the second most commonly diagnosed malignant disease among men worldwide. Such metastatic disease is characterized by the presence of osteoblastic bone lesions, and is associated with high rates of mortality. However, the various mechanisms involved in prostate cancer-induced osteoblastic differentiation have not been fully explored. Semaphorin 3A (Sema 3A) is a newly identified regulator of bone metabolism which stimulates differentiation of pre-osteoblastic cells under physiological conditions. We investigated in this study whether prostate cancer cells can mediate osteoblastic activity through Sema 3A.

METHODS

We cultured osteoprogenitor MC3T3-E1 cells in prostate cancer-conditioned medium, and analyzed levels of Sema 3A protein in diverse prostate cancer cell lines to identify cell lines in which Sema 3A production showed a positive correlation with osteo-stimulation. C4-2 cells were stably transfected with Sema 3A short hairpin RNA to further determine whether Sema 3A contributes to the ability of C4-2 cells to induce osteoblastic differentiation.

RESULTS

Down-regulation of Sema 3A expression decreased indicators of C4-2 CM-induced osteoblastic differentiation, including alkaline phosphatase production and mineralization. Additionally, silencing or neutralizing Sema 3A in C4-2 cells resulted in diminished β-catenin expression in osteogenitor MC3T3-E1 cells.

CONCLUSIONS

Our results suggest that prostate cancer-induced osteoblastic differentiation is at least partially mediated by Sema 3A, and may be regulated by the β-catenin signalling pathway. Sema 3A may represent a novel target for treatment of prostate cancer-induced osteoblastic lesions.

Breast cancer cells promote osteoblastic differentiation via Sema 3A signaling pathway in vitro

Breast cancer bone metastases are attributed to multiple cellular and molecular interactions between the cancer cells and the bone microenvironment. Some breast cancers (about 10%) manifest predominant osteoblastic bone metastases. However, the effects of cancer cell-produced factors on osteoblastic differentiation are not fully understood. Semaphorin 3A (Sema 3A) is a newly identified regulatory factor of bone rebuilding. In the present study, we demonstrated that human breast cancer MCF-7 cells, which preferentially form osteoblastic bone metastases, exhibited increased Sema 3A expression levels. We also found that MCF-7 cell-derived Sema 3A stimulated osteoblastic differentiation and nuclear β-catenin accumulation, and these effects could be blocked by shRNA Sema 3A or a Sema 3A-neutralizing antibody. In conclusion, our data suggest that MCF-7 cell-derived Sema 3A plays a causative role in osteoblastic bone metastases progression by stimulating osteoblastic differentiation.

The potential of class 3 semaphorins as both targets and therapeutics in cancer

INTRODUCTION

Semaphorins have been originally identified as a family of evolutionary conserved soluble or membrane-associated proteins involved in diverse developmental phenomena. This family of proteins profoundly influences numerous pathophysiological processes, including organogenesis, cardiovascular development and immune response. Apart from steering the neural networking process, these are implicated in a broad range of biological operations including regulation of tumor progression and angiogenesis.

AREAS COVERED

Members of class 3 semaphorin family are known to modulate various cellular processes involved in malignant transformation. Some of the family members trigger diverse signaling processes involved in tumor progression and angiogenesis by binding with plexin and neuropilin. A better understanding of the various signaling mechanisms by which semaphorins modulate tumor progression and angiogenesis may serve as crucial tool in crafting new semaphorin-based anticancer therapy. These include treatment with recombinant tumor suppressive semaphorins or inhibition of tumor-promoting semaphorins by their specific siRNAs, small-molecule inhibitors or specific receptors using neutralizing antibodies or blocking peptides that might serve as novel strategies for effective management of cancers.

EXPERT OPINION

This review focuses on all the possible avenues to explore various members of class 3 semaphorin family to serve as therapeutics for combating cancer.

A perspective on the role of class III semaphorin signaling in central nervous system trauma

Traumatic injury of the central nervous system (CNS) has severe impact on the patients’ quality of life and initiates many molecular and cellular changes at the site of insult. Traumatic CNS injury results in direct damage of the axons of CNS neurons, loss of myelin sheaths, destruction of the surrounding vascular architecture and initiation of an immune response. Class III semaphorins (SEMA3s) are present in the neural scar and influence a wide range of molecules and cell types in and surrounding the injured tissue. SEMA3s and their receptors, neuropilins (NRPs) and plexins (PLXNs) were initially studied because of their involvement in repulsive axon guidance. To date, SEMA3 signaling is recognized to be of crucial importance for re-vascularization, the immune response and remyelination. The purpose of this review is to summarize and discuss how SEMA3s modulate these processes that are all crucial components of the tissue response to injury. Most of the functions for SEMA3s are achieved through their binding partners NRPs, which are also co-receptors for a variety of other molecules implicated in the above processes. The most notable ligands are members of the vascular endothelial growth factor (VEGF) family and the transforming growth factor family. Therefore, a second aim is to highlight the overlapping or competing signaling pathways that are mediated through NRPs in the same processes. In conclusion, we show that the role of SEMA3s goes beyond inhibiting axonal regeneration, since they are also critical modulators of re-vascularization, the immune response and re-myelination.

Quantitative temporal viromics: an approach to investigate host-pathogen interaction

A systematic quantitative analysis of temporal changes in host and viral proteins throughout the course of a productive infection could provide dynamic insights into virus-host interaction. We developed a proteomic technique called “quantitative temporal viromics” (QTV), which employs multiplexed tandem-mass-tag-based mass spectrometry. Human cytomegalovirus (HCMV) is not only an important pathogen but a paradigm of viral immune evasion. QTV detailed how HCMV orchestrates the expression of >8,000 cellular proteins, including 1,200 cell-surface proteins to manipulate signaling pathways and counterintrinsic, innate, and adaptive immune defenses. QTV predicted natural killer and T cell ligands, as well as 29 viral proteins present at the cell surface, potential therapeutic targets. Temporal profiles of >80% of HCMV canonical genes and 14 noncanonical HCMV open reading frames were defined. QTV is a powerful method that can yield important insights into viral infection and is applicable to any virus with a robust in vitro model.

PaperClip

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>8,000 proteins quantified over eight time points, including 1,200 cell-surface proteins

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Temporal profiles of 139/171 canonical HCMV proteins and 14 noncanonical HCMV ORFs

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Multiple families of cell-surface receptors selectively modulated by HCMV

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Multiple signaling pathways modulated during HCMV infection

Changes in expression of Class 3 Semaphorins and their receptors during development of the rat retina and superior colliculus

Background

Members of the Semaphorin 3 family (Sema3s) influence the development of the central nervous system, and some are implicated in regulating aspects of visual system development. However, we lack information about the timing of expression of the Sema3s with respect to different developmental epochs in the mammalian visual system. In this time-course study in the rat, we document for the first time changes in the expression of RNAs for the majority of Class 3 Semaphorins (Sema3s) and their receptor components during the development of the rat retina and superior colliculus (SC).

Results

During retinal development, transcript levels changed for all of the Sema3s examined, as well as Nrp2, Plxna2, Plxna3, and Plxna4a. In the SC there were also changes in transcript levels for all Sema3s tested, as well as Nrp1, Nrp2, Plxna1, Plxna2, Plxna3, and Plxna4a. These changes correlate with well-established epochs, and our data suggest that the Sema3s could influence retinal ganglion cell (RGC) apoptosis, patterning and connectivity in the maturing retina and SC, and perhaps guidance of RGC and cortical axons in the SC. Functionally we found that SEMA3A, SEMA3C, SEMA3E, and SEMA3F proteins collapsed purified postnatal day 1 RGC growth cones in vitro. Significantly this is a developmental stage when RGCs are growing into and within the SC and are exposed to Sema3 ligands.

Conclusion

These new data describing the overall temporal regulation of Sema3 expression in the rat retina and SC provide a platform for further work characterising the functional impact of these proteins on the development and maturation of mammalian visual pathways.

Assay development for the discovery of semaphorin 3B inducing agents from natural product sources

Semaphorins are a class of membrane-bound and secreted proteins. They have been found to regulate basic cell functions such as axonal growth cone guidance and recent studies have focused on their effect on tumor progression. Semaphorin 3B (Sema3B) particularly is a secreted protein that has been known to modulate proliferation and apoptosis, processes that are critical for tumor progression and development. In spite of its importance, there is yet no high-throughput screening assay available to detect or quantify the expression of Sema3B for natural product anticancer drug discovery purposes. Therefore, the development of a new high-throughput bioassay for the discovery of Sema3B inducing agents from natural product sources is described herein. A wide variety of pure compounds and extracts from plants and microorganisms has been found suitable for screening using this Sema3B assay to detect and quantify the effect of Sema3B inducing agents and thereby identify new selective bioactive Sema3B lead compounds for anticancer drug discovery and development. Also, this new bioassay procedure is based on a high-throughput platform using an enzyme-linked immunosorbent assay that involves the optimization of sensitivity and selectivity levels as well as accuracy, reproducibility, robustness, and cost effectiveness.

Transmembrane recognition of the semaphorin co-receptors neuropilin 1 and plexin A1: coarse-grained simulations

The cancer associated class 3 semaphorins require direct binding to neuropilins and association to plexins to trigger cell signaling. Here, we address the role of the transmembrane domains of neuropilin 1 and plexin A1 for the dimerization of the two receptors by characterizing the assembly in lipid bilayers using coarse-grained molecular dynamics simulations. From experimental evidence using a two-hybrid system showing the biochemical association of the two receptors transmembrane domains, we performed molecular simulations in DOPC and POPC demonstrating spontaneously assembly to form homodimers and heterodimers with a very high propensity for right-handed packing of the helices. Inversely, left-handed packing was observed with a very low propensity. This mode of packing was observed uniquely when the plexin A1 transmembrane domain was involved in association. Potential of mean force calculations were used to predict a hierarchy of self-association for the monomers: the two neuropilin 1 transmembrane domains strongly associated, neuropilin 1 and plexin A1 transmembrane domains associated less and the two plexin A1 transmembrane domains weakly but significantly associated. We demonstrated that homodimerization and heterodimerization are driven by GxxxG motifs, and that the sequence context modulates the packing mode of the plexin A1 transmembrane domains. This work presents major advances towards our understanding of membrane signaling platforms assembly through membrane domains and provides exquisite information for the design of antagonist drugs defining a novel class of therapeutic agents.

Semaphorin 3d and semaphorin 3e direct endothelial motility through distinct molecular signaling pathways

Class 3 semaphorins were initially described as axonal growth cone guidance molecules that signal through plexin and neuropilin coreceptors and since then have been established to be regulators of vascular development. Semaphorin 3e (Sema3e) has been shown previously to repel endothelial cells and is the only class 3 semaphorin known to be capable of signaling via a plexin receptor without a neuropilin coreceptor. Sema3e signals through plexin D1 (Plxnd1) to regulate vascular patterning by modulating the cytoskeleton and focal adhesion structures. We showed recently that semaphorin 3d (Sema3d) mediates endothelial cell repulsion and pulmonary vein patterning during embryogenesis. Here we show that Sema3d and Sema3e affect human umbilical vein endothelial cells similarly but through distinct molecular signaling pathways. Time-lapse imaging studies show that both Sema3d and Sema3e can inhibit cell motility and migration, and tube formation assays indicate that both can impede tubulogenesis. Endothelial cells incubated with either Sema3d or Sema3e demonstrate a loss of actin stress fibers and focal adhesions. However, the addition of neuropilin 1 (Nrp1)-blocking antibody or siRNA knockdown of Nrp1 inhibits Sema3d-mediated, but not Sema3e-mediated, cytoskeletal reorganization, and siRNA knockdown of Nrp1 abrogates Sema3d-mediated, but not Sema3e-mediated, inhibition of tubulogenesis. On the other hand, endothelial cells deficient in Plxnd1 are resistant to endothelial repulsion mediated by Sema3e but not Sema3d. Unlike Sema3e, Sema3d incubation results in phosphorylation of Akt in human umbilical vein endothelial cells, and inhibition of the PI3K/Akt pathway blocks the endothelial guidance and cytoskeletal reorganization functions of Sema3d but not Sema3e.

cAMP-induced expression of neuropilin1 promotes retinal axon crossing in the zebrafish optic chiasm

Growing axons navigate a complex environment as they respond to attractive and repellent guidance cues. Axons can modulate their responses to cues through a G-protein-coupled, cAMP-dependent signaling pathway. To examine the role of G-protein signaling in axon guidance in vivo, we used the GAL4/UAS system to drive expression of dominant-negative heterotrimeric G-proteins (DNG) in retinal ganglion cells (RGCs) of embryonic zebrafish. Retinal axons normally cross at the ventral midline and project to the contralateral tectum. Expression of DNGα(S) in RGCs causes retinal axons to misproject to the ipsilateral tectum. These errors resemble misprojections in adcy1, adcy8, nrp1a, sema3D, or sema3E morphant embryos, as well as in sema3D mutant embryos. nrp1a is expressed in RGCs as their axons extend toward and across the midline. sema3D and sema3E are expressed adjacent to the chiasm, suggesting that they facilitate retinal midline crossing. We demonstrate synergistic induction of ipsilateral misprojections between adcy8 knockdown and transgenic DNGα(S) expression, adcy8 and nrp1a morphants, or nrp1a morphants and transgenic DNGα(S) expression. Using qPCR analysis, we show that either transgenic DNGα(S)-expressing embryos or adcy8 morphant embryos have decreased levels of nrp1a and nrp1b mRNA. Ipsilateral misprojections in adcy8 morphants are corrected by the expression of an nrp1a rescue construct expressed in RGCs. These findings are consistent with the idea that elevated cAMP levels promote Neuropilin1a expression in RGCs, increasing the sensitivity of retinal axons to Sema3D, Sema3E, or other neuropilin ligands at the midline, and consequently facilitate retinal axon crossing in the chiasm.

Nrp2 deficiency leads to trabecular bone loss and is accompanied by enhanced osteoclast and reduced osteoblast numbers

Neuropilin 1 (Nrp1) and Nrp2 are transmembrane receptors that can bind class 3 semaphorins (Sema3A-G) in addition to VEGF family members to play important roles in axonal guidance, vascularization and angiogenesis, as well as immune responses. Moreover, recent evidence implicates Sema3A/Nrp-mediated signaling in bone regulation. However, to date the expression of Nrp2 in bone has not been investigated and a possible role for Nrp2 in the maintenance of bone homeostasis in vivo remains unexplored. Here we show that Nrp2, together with its possible coreceptors (Plexin A family members and Plexin D1) and class 3 semaphorin ligands, were expressed during in vitro osteogenic differentiation of bone marrow stromal cells. Moreover, Nrp2 transcript and protein levels were highly induced in hematopoietic bone marrow cell-derived osteoclast cultures. Osteoblastic as well as osteoclastic Nrp2 expression was confirmed by immunohistochemistry of the long bones of mice. Interestingly, Nrp2 knockout mice were characterized by a low bone mass phenotype which was accompanied by an increased number of osteoclasts and a decreased osteoblast count. Collectively, these data point to a physiological role for Nrp2 in bone homeostasis.

Semaphorin 3C is a novel adipokine linked to extracellular matrix composition

AIMS/HYPOTHESIS

Alterations in white adipose tissue (WAT) function, including changes in protein (adipokine) secretion and extracellular matrix (ECM) composition, promote an insulin-resistant state. We set out to identify novel adipokines regulated by body fat mass in human subcutaneous WAT with potential roles in adipose function.

METHODS

Adipose transcriptome data and secretome profiles from conditions with increased/decreased WAT mass were combined. WAT donors were predominantly women. In vitro effects were assessed using recombinant protein. Results were confirmed by quantitative PCR/ELISA, metabolic assays and immunochemistry in human WAT and adipocytes.

RESULTS

We identified a hitherto uncharacterised adipokine, semaphorin 3C (SEMA3C), the expression of which correlated significantly with body weight, insulin resistance (HOMA of insulin resistance [HOMAIR], and the rate constant for the insulin tolerance test [KITT]) and adipose tissue morphology (hypertrophy vs hyperplasia). SEMA3C was primarily found in mature adipocytes and had no direct effect on human adipocyte differentiation, lipolysis, glucose transport or the expression of β-oxidation genes. This could in part be explained by the significant downregulation of its cognate receptors during adipogenesis. In contrast, in pre-adipocytes, SEMA3C increased the production/secretion of several ECM components (fibronectin, elastin and collagen I) and matricellular factors (connective tissue growth factor, IL6 and transforming growth factor-β1). Furthermore, the expression of SEMA3C in human WAT correlated positively with the degree of fibrosis in WAT.

CONCLUSIONS/INTERPRETATION

SEMA3C is a novel adipokine regulated by weight changes. The correlation with WAT hypertrophy and fibrosis in vivo, as well as its effects on ECM production in human pre-adipocytes in vitro, together suggest that SEMA3C constitutes an adipocyte-derived paracrine signal that influences ECM composition and may play a pathophysiological role in human WAT.

Elimination of aberrant DRG circuitries in Sema3A mutant mice leads to extensive neuronal deficits

Axon guidance molecules determine the pattern of neuronal circuits. Accuracy of the process is ensured by unknown mechanisms that correct early guidance errors. Since the time frame of error correction in Sema3A null mice partly overlaps with the period of naturally occurring cell death in dorsal root ganglia (DRG) development, we tested the hypothesis that apoptosis of misguided neurons enables error correction. We crossed BAX null mice, in which DRG apoptosis is blocked, with Sema3A null mice to induce errors. Analyses of these double-null mouse embryos showed that the elimination of abnormal projections is not blocked in the absence of BAX. Surprisingly however, there are fewer surviving neurons in Sema3A null or Sema3A/BAX double-null newborn mice than in wild-type mice. These results suggest that guidance errors are corrected by a BAX-independent cell death mechanism. Thus, aberrant axonal guidance may lead to reductions in neuronal numbers to suboptimal levels, perhaps increasing the likelihood of neuropathological consequences later in life.

Structural insights into semaphorins and their receptors

Ten years ago nothing was known of the three-dimensional structure of members of the semaphorin family of cell guidance cues, nor of their major receptors, the plexins. The structural biology of this cell surface ligand-receptor system has now come of age. Detailed atomic level information is available on the architecture of semaphorin and plexin ectodomains and their recognition complexes. Similarly the structure of the plexin cytoplasmic region, and its interactions with members of the Rho family of small GTPases have been unveiled. These structural analyses, in combination with biochemical, biophysical and cellular studies, have progressed our understanding of this signalling system into the realm of molecular mechanism.