Samuel J. Pleasure

Slit Proteins Prevent Midline Crossing and Determine the Dorsoventral Position of Major Axonal Pathways in the Mammalian Forebrain

We report that Slit proteins, a family of secreted chemorepellents, are crucial for the proper development of several major forebrain tracts. Mice deficient in Slit2 and, even more so, mice deficient in both Slit1 and Slit2 show significant axon guidance errors in a variety of pathways, including corticofugal, callosal, and thalamocortical tracts. Analysis of multiple pathways suggests several generalizations regarding the functions of Slit proteins in the brain, which appear to contribute to (1) the maintenance of dorsal position by prevention of axonal growth into ventral regions, (2) the prevention of axonal extension toward and across the midline, and (3) the channeling of axons toward particular regions.

Neuropilin-2 Regulates the Development of Select Cranial and Sensory Nerves and Hippocampal Mossy Fiber Projections

Neuropilin-1 and neuropilin-2 bind differentially to different class 3 semaphorins and are thought to provide the ligand-binding moieties in receptor complexes mediating repulsive responses to these semaphorins. Here, we have studied the function of neuropilin-2 through analysis of a neuropilin-2 mutant mouse, which is viable and fertile. Repulsive responses of sympathetic and hippocampal neurons to Sema3F but not to Sema3A are abolished in the mutant. Marked defects are observed in the development of several cranial nerves, in the initial central projections of spinal sensory axons, and in the anterior commissure, habenulo-interpeduncular tract, and the projections of hippocampal mossyfiber axons in the infrapyramidal bundle. Our results show that neuropilin-2 is an essential component of the Sema3F receptor and identify key roles for neuropilin-2 in axon guidance in the PNS and CNS.

Cell migration from the ganglionic eminences is required for the development of hippocampal GABAergic interneurons.

GABAergic interneurons have major roles in hippocampal function and dysfunction. Here we provide evidence that, in mice, virtually all of these cells originate from progenitors in the basal telencephalon. Immature interneurons tangentially migrate from the basal telencephalon through the neocortex to take up their final positions in the hippocampus. Disrupting differentiation in the embryonic basal telencephalon (lateral and medial ganglionic eminences) through loss of Dlx1/2 homeobox function blocks the migration of virtually all GABAergic interneurons to the hippocampus. On the other hand, disrupting specification of the medial ganglionic eminence through loss of Nkx2.1 homeobox function depletes the hippocampus of a distinct subset of hippocampal interneurons. Loss of hippocampal interneurons does not appear to have major effects on the early development of hippocampal projection neurons nor on the pathfinding of afferrent tracts.

Aberrant seizure-induced neurogenesis in experimental temporal lobe epilepsy.

Neurogenesis in the hippocampal dentate gyrus persists throughout life and is increased by seizures. The dentate granule cell (DGC) layer is often abnormal in human and experimental temporal lobe epilepsy, with dispersion of the layer and the appearance of ectopic granule neurons in the hilus. We tested the hypothesis that these abnormalities result from aberrant DGC neurogenesis after seizure‐induced injury. Bromodeoxyuridine labeling, in situ hybridization, and immunohistochemistry were used to identify proliferating progenitors and mature DGCs in the adult rat pilocarpine temporal lobe epilepsy model. We also examined dentate gyri from epileptic human hippocampal surgical specimens. Prox‐1 immunohistochemistry and pulse‐chase bromodeoxyuridine labeling showed that progenitors migrate aberrantly to the hilus and molecular layer after prolonged seizures and differentiate into ectopic DGCs in rat. Neuroblast marker expression indicated the delayed appearance of chainlike progenitor cell formations extending into the hilus and molecular layer, suggesting that seizures alter migratory behavior of DGC precursors. Ectopic putative DGCs also were found in the hilus and molecular layer of epileptic human dentate gyrus. These findings indicate that seizure‐induced abnormalities of neuroblast migration lead to abnormal integration of newborn DGCs in the epileptic adult hippocampus, and implicate aberrant neurogenesis in the development or progression of recurrent seizures. Ann Neurol 2005

Retinoic acid from the meninges regulates cortical neuron generation

Extrinsic signals controlling generation of neocortical neurons during embryonic life have been difficult to identify. In this study we demonstrate that the dorsal forebrain meninges communicate with the adjacent radial glial endfeet and influence cortical development. We took advantage of Foxc1 mutant mice with defects in forebrain meningeal formation. Foxc1 dosage and loss of meninges correlated with a dramatic reduction in both neuron and intermediate progenitor production and elongation of the neuroepithelium. Several types of experiments demonstrate that retinoic acid (RA) is the key component of this secreted activity. In addition, Rdh10- and Raldh2-expressing cells in the dorsal meninges were either reduced or absent in the Foxc1 mutants, and Rdh10 mutants had a cortical phenotype similar to the Foxc1 null mutants. Lastly, in utero RA treatment rescued the cortical phenotype in Foxc1 mutants. These results establish RA as a potent, meningeal-derived cue required for successful corticogenesis.Extrinsic signals controlling generation of neocortical neurons during embryonic life have been difficult to identify. In this study we demonstrate that the dorsal forebrain meninges communicate with the adjacent radial glial endfeet and influence cortical development. We took advantage of Foxc1 mutant mice with defects in forebrain meningeal formation. Foxc1 dosage and loss of meninges correlated with a dramatic reduction in both neuron and intermediate progenitor production and elongation of the neuroepithelium. Several types of experiments demonstrate that retinoic acid (RA) is the key component of this secreted activity. In addition, Rdh10- and Raldh2-expressing cells in the dorsal meninges were either reduced or absent in the Foxc1 mutants, and Rdh10 mutants had a cortical phenotype similar to the Foxc1 null mutants. Lastly, in utero RA treatment rescued the cortical phenotype in Foxc1 mutants. These results establish RA as a potent, meningeal-derived cue required for successful corticogenesis.

Stereotyped Pruning of Long Hippocampal Axon Branches Triggered by Retraction Inducers of the Semaphorin Family

Like naturally occurring neuronal cell death, stereotyped pruning of long axon branches to temporary targets is a widespread regressive phenomenon in the developing mammalian brain that helps sculpt the pattern of neuronal connections. The mechanisms controlling stereotyped pruning are, however, poorly understood. Here, we provide evidence that semaphorins, activating the Plexin-A3 receptor, function as retraction inducers to trigger-stereotyped pruning of specific hippocampal mossy fiber and pyramidal axon branches. Both pruning events are defective in Plexin-A3 mutants, reflecting a cell-autonomous requirement for Plexin-A3. The distribution of mRNAs for Sema3F and Sema3A makes them candidates for triggering the pruning. In vitro, hippocampal neurons respond to semaphorins by retracting axon branches. These results implicate semaphorins as retraction inducers controlling stereotyped pruning in the mammalian brain.

Plexin-A3 Mediates Semaphorin Signaling and Regulates the Development of Hippocampal Axonal Projections

Plexins are receptors implicated in mediating signaling by semaphorins, a family of axonal chemorepellents. The role of specific plexins in mediating semaphorin function in vivo has not, however, yet been examined in vertebrates. Here, we show that plexin-A3 is the most ubiquitously expressed plexin family member within regions of the developing mammalian nervous system known to contain semaphorin-responsive neurons. Using a chimeric receptor construct, we provide evidence that plexin-A3 can transduce a repulsive signal in growth cones in vitro. Analysis of plexin-A3 knockout mice shows that plexin-A3 contributes to Sema3F and Sema3A signaling and that plexin-A3 regulates the development of hippocampal axonal projections in vivo.

Wnt Signaling Mutants Have Decreased Dentate Granule Cell Production and Radial Glial Scaffolding Abnormalities

LRP6 mutant mice have generalized defects in the Wnt/β-catenin signaling pathway because of the crucial function of LRP6 as a Wnt signaling co-receptor (Pinson et al., 2000). We examined the hippocampal phenotype of single LRP6 mutant mice as well as LRP6/Lef1 double mutant mice. LRP6 mutants had reduced production of dentate granule neurons and abnormalities of the radial glial scaffolding in the forming dentate gyrus. These defects were more severe with the addition of a single Lef1 null allele to an LRP6 null background. Pyramidal cell fields were unaffected in the LRP6, Lef1, or double mutants. The dentate defects were accompanied by decreased numbers of mitotic precursors in the migratory pathway to the dentate and in the displaced proliferative zone in the dentate itself. At earlier gestational ages, there was a reduction in the number of dentate granule cell progenitors in the dentate ventricular zone before the emigration of the earliest differentiated granule neurons and precursors to form the dentate anlage.

Wnt Signaling Regulates Neuronal Differentiation of Cortical Intermediate Progenitors

Cortical intermediate progenitors (IPs) comprise a secondary neuronal progenitor pool that arises from radial glia (RG). IPs are essential for generating the correct number of cortical neurons, but the factors that regulate the expansion and differentiation of IPs in the embryonic cortex are essentially unknown. In this study, we show that the Wnt–β-catenin pathway (canonical Wnt pathway) regulates IP differentiation into neurons. Upregulation of Wnt–β-catenin signaling by overexpression of Wnt3a in the neocortex induced early differentiation of IPs into neurons and the accumulation of these newly born neurons at the subventricular zone/intermediate zone border. Long-term overexpression of Wnt3a led to cortical dysplasia associated with the formation of large neuronal heterotopias. Conversely, downregulation of Wnt–β-catenin signaling with Dkk1 during mid and late stages of neurogenesis inhibited neuronal production. Consistent with previous reports, we show that Wnt–β-catenin signaling also promotes RG self-renewal. Thus, our findings show differential effects of the Wnt–β-catenin pathway on distinct groups of cortical neuronal progenitors: RG self-renewal and IP differentiation. Moreover, our findings suggest that dysregulation of Wnt signaling can lead to developmental defects similar to human cortical malformation disorders.

Wnt signaling in development and disease

The Wnt signaling pathway is one of the central morphogenic signaling pathways regulating early vertebrate development. In recent years, it has become clear that the Wnt pathway also regulates many aspects of nervous system development from the patterning stage through the regulation of neural plasticity. In this review, we first present an overview of the components of the Wnt signaling pathway and then go on to discuss the literature describing the multitude of roles of Wnts in nervous system. In the latter portion of the review, we turn to the ways that defects in Wnt signaling lead to neurologic disease.