Memo1-Mediated Tiling of Radial Glial Cells Facilitates Cerebral Cortical Development.

Polarized, non-overlapping, regularly spaced, tiled organization of radial glial cells (RGCs) serves as a framework to generate and organize cortical neuronal columns, layers, and circuitry. Here, we show that mediator of cell motility 1 (Memo1) is a critical determinant of radial glial tiling during neocortical development. Memo1 deletion or knockdown leads to hyperbranching of RGC basal processes and disrupted RGC tiling, resulting in aberrant radial unit assembly and neuronal layering. Memo1 regulates microtubule (MT) stability necessary for RGC tiling. Memo1 deficiency leads to disrupted MT minus-end CAMSAP2 distribution, initiation of aberrant MT branching, and altered polarized trafficking of key basal domain proteins such as GPR56, and thus aberrant RGC tiling. These findings identify Memo1 as a mediator of RGC scaffold tiling, necessary to generate and organize neurons into functional ensembles in the developing cerebral cortex.

Site-specific HNK-1 epitope on alternatively spliced fibronectin type-III repeats in tenascin-C promotes neurite outgrowth of hippocampal neurons through contactin-1.

The human natural killer-1 (HNK-1) carbohydrate epitope, composed of a unique sulfated trisaccharide (HSO3-3GlcAß1-3Galß1-4GlcNAc-R), is highly expressed during brain development and regulates higher brain function. However, it remains unclear which glycoprotein carries the HNK-1 epitope in the embryonic brain and the functional role it plays. Here, we showed that one of the major HNK-1 carrier proteins in the embryonic brain is tenascin-C (TNC), an extracellular matrix protein that regulates neurite outgrowth by interacting with the GPI-anchored protein contactin-1 (CNTN). Because the alternatively spliced fibronectin type-III (FNIII) repeats in TNC give rise to many isoforms and affect neuronal function, we evaluated neurite outgrowth of primary hippocampal neurons on purified recombinant FNIII repeats with or without the HNK-1 epitope as a substrate. We found that the presence of the HNK-1 epitope on the C domain of TNC promoted neurite outgrowth, and that this signal was mediated by CNTN, which is an HNK-1-expressing neuronal receptor. The neurite-promoting activity of the HNK-1 epitope on TNC required neuronal HNK-1 expression, which was defective in neurons lacking the glucuronyltransferases GlcAT-P and GlcAT-S. These results suggest that the HNK-1 epitope is a key modifier of TNC and CNTN in the regulation of embryonic brain development.

APC sets the Wnt tone necessary for cerebral cortical progenitor development.

Adenomatous polyposis coli (APC) regulates the activity of ß-catenin, an integral component of Wnt signaling. However, the selective role of the APC-ß-catenin pathway in cerebral cortical development is unknown. Here we genetically dissected the relative contributions of APC-regulated ß-catenin signaling in cortical progenitor development, a necessary early step in cerebral cortical formation. Radial progenitor-specific inactivation of the APC-ß-catenin pathway indicates that the maintenance of appropriate ß-catenin-mediated Wnt tone is necessary for the orderly differentiation of cortical progenitors and the resultant formation of the cerebral cortex. APC deletion deregulates ß-catenin, leads to high Wnt tone, and disrupts Notch1 signaling and primary cilium maintenance necessary for radial progenitor functions. ß-Catenin deregulation directly disrupts cilium maintenance and signaling via Tulp3, essential for intraflagellar transport of ciliary signaling receptors. Surprisingly, deletion of ß-catenin or inhibition of ß-catenin activity in APC-null progenitors rescues the APC-null phenotype. These results reveal that APC-regulated ß-catenin activity in cortical progenitors sets the appropriate Wnt tone necessary for normal cerebral cortical development.