David D. Ginty

Bone morphogenetic protein (BMP) signaling has emerged as an important regulator of sensory neuron development. Using a three-generation forward genetic screen in mice we have identified Megf8 as a novel modifier of BMP4 signaling in trigeminal ganglion (TG) neurons. Loss of Megf8 disrupts axon guidance in the peripheral nervous system and leads to defects in development of the limb, heart, and left-right patterning, defects that resemble those observed in Bmp4 loss-of-function mice. Bmp4 is expressed in a pattern that defines the permissive field for the peripheral projections of TG axons and mice lacking BMP signaling in sensory neurons exhibit TG axon defects that resemble those observed in Megf8^−/− embryos. Furthermore, TG axon growth is robustly inhibited by BMP4 and this inhibition is dependent on Megf8. Thus, our data suggest that Megf8 is involved in mediating BMP4 signaling and guidance of developing TG axons.

Androgen-Driven Sequestration: Male and female mice differ in the neuronal patterns that serve the mammary glands. Yin Liu et al. (p. 1357 ) now describe how gonadal hormones drive development of distinct male and female sensory innervations. Although both male and female mammary glands develop their sensory innervation similarly in early embryogenesis, once the androgens take effect, the developmental trajectories diverge. By birth, the rich network of sensory neurons present in the female is absent in the male. Androgens cause a switch from expression of the full-length neurotrophin receptor TrkB to its truncated form, TrkB.T1, both of which are expressed on the neurons. In males, truncated TrkB.T1 sequesters brain-derived neurotrophic factor (BDNF) from further activity, whereas in females, full-length TrkB binds BDNF and supports neuronal development.

The patterns of nociceptive innervation in skin and spinal cord differ in mammals and lower vertebrates. Guo et al. report that nerve growth factor induces the transcription factor Hoxd1 in mouse, but not chick, sensory neurons, and that expression of Hoxd1 is necessary and sufficient for a mammalian-like pattern of nociceptive projections in vivo. Species are endowed with unique sensory capabilities that are encoded by divergent neural circuits. One potential explanation for how divergent circuits have evolved is that conserved extrinsic signals are differentially interpreted by developing neurons of different species to yield unique patterns of axonal connections. Although nerve growth factor (NGF) controls survival, maturation and axonal projections of nociceptors of different vertebrates, whether the NGF signal is differentially transduced in different species to yield unique features of nociceptor circuits is unclear. We identified a species-specific signaling module induced by NGF and mediated by a rapidly evolving Hox transcription factor, Hoxd1. NGF promoted robust expression of Hoxd1 in mice, but not chickens, both in vivo and in vitro. Mice lacking Hoxd1 displayed altered nociceptor circuitry that resembles that normally found in chicks. Conversely, ectopic expression of Hoxd1 in developing chick nociceptors promoted a pattern of axonal projections reminiscent of the mouse. Thus, conserved growth factors control divergent neuronal transcriptional events that mediate interspecies differences in neural circuits and the behaviors that they control.

We have performed a three-generation, forward genetic screen to identify recessive mutations that affect the patterning of the peripheral nervous system. Using this assay, we identifiedSema3A^K108N, a novel loss-of-function allele ofSema3A. Class 3 semaphorins, which include Sema3A, are structurally conserved secreted proteins that play critical roles in the development and function of the nervous system.Sema3A^K108Nmutant mice phenocopySema3A-null mice, and Sema3A^K108Nprotein fails to repel or collapse DRG axonsin vitro. K108 is conserved among semaphorins, yet the loss-of-function effects associated with K108N are not the result of impaired expression, secretion, or binding of Sema3A to its high-affinity receptor Neuropilin-1 (Npn-1). Usingin silicomodeling and mutagenesis of other semaphorin family members, we predict that Sema3A^K108Ninteracts poorly with the Npn-1/PlexA holoreceptor and, thus, interferes with its ability to signal at the growth cone. Therefore, through the use of a forward-genetic screen we have identified a novel allele of Sema3A that provides structural insight into the mechanism of Sema3A/Npn-1/PlexinA signaling.

Although it is known that different dendritic domains on the same neuron can receive distinct input, the molecular codes underlying these differences have been unknown. Here it is shown that a specific semaphorin signalling pathway, the Sema3F–Npn-2/PlexA4 cascade, that was previously associated with axonal guidance, is involved in the control of dendritic spine development and synapse formation in different dendritic arbours. Dendritic spines carry the majority of excitatory synapses, and spine morphology and distribution are critical for synaptic transmission, synaptic integration and plasticity. Here, signalling by the secreted semaphorin Sema3F is shown to control spine distribution along select dendritic processes, and distinct secreted semaphorin signalling events orchestrate CNS connectivity through the differential control of a variety of processes. The majority of excitatory synapses in the mammalian CNS (central nervous system) are formed on dendritic spines1, and spine morphology and distribution are critical for synaptic transmission2,3,4,5,6, synaptic integration and plasticity7. Here, we show that a secreted semaphorin, Sema3F, is a negative regulator of spine development and synaptic structure. Mice with null mutations in genes encoding Sema3F, and its holoreceptor components neuropilin-2 (Npn-2, also known as Nrp2) and plexin A3 (PlexA3, also known as Plxna3), exhibit increased dentate gyrus (DG) granule cell (GC) and cortical layer V pyramidal neuron spine number and size, and also aberrant spine distribution. Moreover, Sema3F promotes loss of spines and excitatory synapses in dissociated neurons in vitro, and in Npn-2-/- brain slices cortical layer V and DG GCs exhibit increased mEPSC (miniature excitatory postsynaptic current) frequency. In contrast, a distinct Sema3A–Npn-1/PlexA4 signalling cascade controls basal dendritic arborization in layer V cortical neurons, but does not influence spine morphogenesis or distribution. These disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of Npn-2 to apical dendrites and of Npn-1 (also known as Nrp1) to all dendrites of cortical pyramidal neurons. Therefore, Sema3F signalling controls spine distribution along select dendritic processes, and distinct secreted semaphorin signalling events orchestrate CNS connectivity through the differential control of spine morphogenesis, synapse formation, and the elaboration of dendritic morphology.

Mouse mutants for Sec24b, a component of COPII-coated ER-to-Golgi vesicles, have defects in convergent extension, neural tube closure and other phenotypes related to planar cell polarity (PCP). The PCP component Vangl2 is sorted by Sec24b, and Vangl2 mutants defective in convergent extension do not exit the ER. Craniorachischisis is a rare but severe birth defect that results in a completely open neural tube. Mouse mutants in planar cell polarity (PCP) signalling components have deficits in the morphological movements of convergent extension that result in craniorachischisis. Using a forward genetic screen in mice, we identified Sec24b, a cargo-sorting member of the core complex of the endoplasmic reticulum (ER)-to-Golgi transport vesicle COPII, as critical for neural tube closure. Sec24bY613 mutant mice exhibit craniorachischisis, deficiencies in convergent extension and other PCP-related phenotypes. Vangl2, a key component of the PCP-signalling pathway critical for convergent extension, is selectively sorted into COPII vesicles by Sec24b. Moreover, Sec24bY613 genetically interacts with a loss-of-function Vangl2 allele (Vangl2LP), causing a marked increase in the prevalence of spina bifida. Interestingly, the Vangl2 looptail point mutants Vangl2D255E and Vangl2S464N, known to cause defects in convergent extension, fail to sort into COPII vesicles and are trapped in the ER. Thus, during COPII vesicle formation, Sec24b shows cargo specificity for a core PCP component, Vangl2, of which proper ER-to-Golgi transport is essential for the establishment of PCP, convergent extension and closure of the neural tube.