Neogenin Recruitment of the WAVE Regulatory Complex to Ependymal and Radial Progenitor Adherens Junctions Prevents Hydrocephalus.

Denudation of the ependyma due to loss of cell adhesion mediated by cadherin-based adherens junctions is a common feature of perinatal hydrocephalus. Junctional stability depends on the interaction between cadherins and the actin cytoskeleton. However, the molecular mechanism responsible for recruiting the actin nucleation machinery to the ependymal junction is unknown. Here, we reveal that loss of the netrin/RGM receptor, Neogenin, leads to severe hydrocephalus. We show that Neogenin plays a critical role in actin nucleation in the ependyma by anchoring the WAVE regulatory complex (WRC) and Arp2/3 to the cadherin complex. Blocking Neogenin binding to the Cyfip1/Abi WRC subunit results in actin depolymerization, junctional collapse, and denudation of the postnatal ventricular zone. In the embryonic cortex, this leads to loss of radial progenitor adhesion, aberrant neuronal migration, and neuronal heterotopias. Therefore, Neogenin-WRC interactions play a fundamental role in ensuring the fidelity of the embryonic ventricular zone and maturing ependyma.

RGMa regulates cortical interneuron migration and differentiation.

The etiology of neuropsychiatric disorders, including schizophrenia and autism, has been linked to a failure to establish the intricate neural network comprising excitatory pyramidal and inhibitory interneurons during neocortex development. A large proportion of cortical inhibitory interneurons originate in the medial ganglionic eminence (MGE) of the ventral telencephalon and then migrate through the ventral subventricular zone, across the corticostriatal junction, into the embryonic cortex. Successful navigation of newborn interneurons through the complex environment of the ventral telencephalon is governed by spatiotemporally restricted deployment of both chemorepulsive and chemoattractive guidance cues which work in concert to create a migratory corridor. Despite the expanding list of interneuron guidance cues, cues responsible for preventing interneurons from re-entering the ventricular zone of the ganglionic eminences have not been well characterized. Here we provide evidence that the chemorepulsive axon guidance cue, RGMa (Repulsive Guidance Molecule a), may fulfill this function. The ventricular zone restricted expression of RGMa in the ganglionic eminences and the presence of its receptor, Neogenin, in the ventricular zone and on newborn and maturing MGE-derived interneurons implicates RGMa-Neogenin interactions in interneuron differentiation and migration. Using an in vitro approach, we show that RGMa promotes interneuron differentiation by potentiating neurite outgrowth. In addition, using in vitro explant and migration assays, we provide evidence that RGMa is a repulsive guidance cue for newborn interneurons migrating out of the ganglionic eminence ventricular zone. Intriguingly, the alternative Neogenin ligand, Netrin-1, had no effect on migration. However, we observed complete abrogation of RGMa-induced chemorepulsion when newborn interneurons were simultaneously exposed to RGMa and Netrin-1 gradients, suggesting a novel mechanism for the tight regulation of RGMa-guided interneuron migration. We propose that during peak neurogenesis, repulsive RGMa-Neogenin interactions drive interneurons into the migratory corridor and prevent re-entry into the ventricular zone of the ganglionic eminences.

Netrin-1: diversity in development.

In 1990, the discovery of three Caenorhabditis elegans genes (unc5, unc6, unc40) involved in pioneer axon guidance and cell migration marked a significant advancement in neuroscience research [Hedgecock EM, Culotti JG, Hall DH. The unc-5, unc-6, and unc-40 genes guide circumferential migrations of pioneer axons and mesodermal cells on the epidermis in C. elegans. Neuron 1990;4:61-85]. The importance of this molecular guidance system was exemplified in 1994, when the vertebrate orthologue of Unc6, Netrin-1, was discovered to be a key guidance cue for commissural axons projecting toward the ventral midline in the rodent embryonic spinal cord [Serafini T, Kennedy TE, Galko MJ, Mirzayan C, Jessell TM, Tessier-Lavigne M. The netrins define a family of axon outgrowth-promoting proteins homologous to C. elegans UNC-6. Cell 1994;78:409-424]. Since then, Netrin-1 has been found to be a critical component of embryonic development with functions in axon guidance, cell migration, morphogenesis and angiogenesis. Netrin-1 also plays a role in the adult brain, suggesting that manipulating netrin signals may have novel therapeutic applications.

Neogenin: A multi-functional receptor regulating diverse developmental processes.

Neogenin, a close relative of the axon guidance receptor Deleted in Colorectal Cancer (DCC), has been shown to be a receptor for members of the Netrin and Repulsive Guidance Molecule (RGM) families. While Netrin-1-Neogenin interactions result in a chemoattractive axon guidance response, the interaction between Neogenin and RGMa induces a chemorepulsive response. Evidence is now accumulating that Neogenin is a multi-functional receptor regulating many diverse developmental processes, including neural tube and mammary gland formation, myogenesis and angiogenesis. Little is known of the function of Neogenin in the adult, however, a novel role in the regulation of iron homeostasis is now emerging. While the signal transduction pathways activated by Neogenin are poorly understood, it is clear that the functional outcome of Neogenin activation, at least in the embryo, depends on both the developmental context as well as the nature of the ligand.