Previous work demonstrated that members of the semaphorin family, Sema3A and Sema3C, act as repulsive and attractive guidance signals, respectively, for cortical axons. During the development of ...corticofugal projections, these semaphorins are expressed in adjacent cortical zones, but there is a considerable overlap between Sema3A and Sema3C expression in the subventricular zone. We used different in vitro assays to examine the response of cortical axons exposed to defined mixtures of these opposing guidance cues. Results showed that even at very low concentrations, Sema3A overrides the effects of Sema3C. Moreover, experiments with function-blocking antibodies directed against neuropilin provided insights into how cortical axons integrate disparate guidance signals at the receptor level. These in vitro data suggest that the pathway of corticofugal axons is defined by an attractive cue in the intermediate zone, where Sema3C is expressed alone. To directly test this hypothesis in vivo, we performed axon-tracing experiments in Sema3C-deficient mice. Compared with wild-type animals, corticofugal axons take a more superficial route in Sema3C(-/-) mice, and the corticofugal pathway is more compacted. This phenotype is expected when an attractive cue for cortical axons, Sema3C, is eliminated and a repulsive cue, Sema3A, becomes predominant.
The spatial orientation of cell divisions is fundamental for tissue architecture and homeostasis. Here we analysed neuroepithelial progenitors in the developing mouse spinal cord to determine whether ...extracellular signals orient the mitotic spindle. We report that Semaphorin3B (Sema3B) released from the floor plate and the nascent choroid plexus in the cerebrospinal fluid (CSF) controls progenitor division orientation. Delivery of exogenous Sema3B to neural progenitors after neural tube opening in living embryos promotes planar orientation of their division. Preventing progenitor access to cues present in the CSF by genetically engineered canal obstruction affects the proportion of planar and oblique divisions. Sema3B knockout phenocopies the loss of progenitor access to the CSF. Sema3B binds to the apical surface of mitotic progenitors and exerts its effect via Neuropilin receptors, GSK3 activation and subsequent inhibition of the microtubule stabilizer CRMP2. Thus, extrinsic control mediated by the Semaphorin signalling orients progenitor divisions in neurogenic zones.
The guidance of axons within the developing nervous system is orchestrated by a variety of cues that successively and complementary attract or repel axons to achieve a stereotyped wiring of neural ...circuits. Here we present a version of a method that has been widely used to identify and characterize the effect of guidance cues on the orientation of axons. We describe the coculture, within a three-dimensional environment, of dorsal spinal cord explants together with a cell aggregate secreting a candidate cue and the method to quantify the effect of this cue on axon orientation.
Axonal receptors for class 3 semaphorins (Sema3s) are heterocomplexes of neuropilins (Nrps) and Plexin‐As signalling coreceptors. In the developing cerebral cortex, the Ig superfamily cell adhesion ...molecule L1 associates with Nrp1. Intriguingly, the genetic removal of L1 blocks axon responses of cortical neurons to Sema3A in vitro despite the expression of Plexin‐As in the cortex, suggesting either that L1 substitutes for Plexin‐As or that L1 and Plexin‐A are both required and mediate distinct roles. We report that association of Nrp1 with L1 but not Plexin‐As mediates the recruitment and activation of a Sema3A‐induced focal adhesion kinase–mitogen‐activated protein kinase cascade. This signalling downstream of L1 is needed for the disassembly of adherent points formed in growth cones and subsequently their collapse response to Sema3A. Plexin‐As and L1 are coexpressed and present in common complexes in cortical neurons and both dominant‐negative forms of Plexin‐A and L1 impair their response to Sema3A. Consistently, Nrp1‐expressing cortical projections are defective in mice lacking Plexin‐A3, Plexin‐A4 or L1. This reveals that specific signalling activities downstream of L1 and Plexin‐As cooperate for mediating the axon guidance effects of Sema3A.
We previously observed that cadherin-11, a type II cadherin, is expressed in growing motor and sensory axons in the mouse embryo. Here, we assessed its functional involvement in the regulation of ...axon elongation and fasciculation by evaluating the activity of a specific cadherin-11 homophilic ligand, cad11-Fc (cadherin-11 extracellular region fused to Fc fragment of IgG), on the length and organization of motor axons outgrowing from embryonic ventral spinal cord explants. Cad11-Fc substrate enhanced axon growth and prevented interactions occurring between growing axons, providing evidences for a role of cadherin-11 in the control of growth cone progression. Comparison of cadherin-11 with N-cadherin, a type I cadherin concomitantly expressed by motor axons, revealed similarities in their functional properties, including the ability to reorganize the actin cytoskeleton through interactions with catenins, but differences in their axon growth-promoting activity, arguing for subtle differences in their contributions to peripheral nerve elongation.
During axon navigation, growth cones continuously interact with molecular cues in their environment, some of which control adherence and bundle assembly, others axon elongation and direction. Growth ...cone responses to these different environmental cues are tightly coordinated during the development of neuronal projections. Several recent studies show that axon sensitivity to guidance cues is modulated by extracellular and intracellular signals. This regulation may enable different classes of cues to combine their effects and may also represent important means for diversifying pathway choices and for compensating for the limited number of guidance cues. This chapter focuses on the modulation exerted by Ig Superfamily cell adhesion molecules (IgSFCAMs) on guidance cues of the class III secreted semaphorins
Semaphorins are major chemorepellents for developing neuronal projections. Their persistent expression at adult stages suggests that they may contribute to the functioning of neuronal circuits. We ...investigated the functional properties of semaphorin3A (Sema3A) in adult hippocampal neurons, and report that exogenous application of this cue decreases the efficacy of synaptic transmission evoked in the CA1 region of hippocampal slices. In situ hybridization, imaging and biochemical techniques showed that the Sema3A receptor component neuropilin‐1 is present at hippocampal synapses and localizes in the presynaptic membrane. In differentiated cultured hippocampal neurons, Sema3A elicited Erk1/2 phosphorylation in somata and neuritic compartments. Furthermore, Sema3A application resulted in a striking reduction of synaptophysin and postsynaptic density 95 puncta without affecting the axon diameter. These observations reveal novel functional potentialities for secreted semaphorins, which suggest that these cues could modulate the morphology and function of synapses in the adult brain.
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