Mechanisms underlying postnatal development of primary somatosensory cortex

Abstract

Layer IV of the mouse somatosensory cortex contains discrete cytoarchitectonic units called 'barrels', formed in response to thalamocortical axon invasion by two processes: translocation of cortical cells to form a cell-dense barrel wall and cell-sparse barrel hollow, and selective dendritic elaboration toward thalamocortical afferents to form oriented dendritic branch patterns. Interestingly gene knockout of several members of the N-Methyl-D-Aspartate (NMDA) receptor -complex (NRC) disrupts barrel formation, indicating that synaptic activity is critical for barrel formation. Little is known of the cellular processes initiated by glutamate receptor activation; however, recent evidence suggests an interaction between neuronal activity and Wnt signalling. Wnts are secreted glycoproteins, are powerful regulators of cell proliferation and differentiation, and their signalling pathway involves proteins that directly participate in both gene transcription and cell adhesion. Wnt7a and Dv11 knockout mice exhibit delays in glomerular rosette formation; a cerebellar structure similar to barrels whose development involves granule cell migration and dendritic rearrangement. Furthermore activity dependent Wnt release can regulate the enhancement of dendrite arborisation, raising the possibility that NRC components and Wnts may interact to regulate barrel development.Recent findings suggest that membrane association guanylyl kinases (MAGUKs) may be the key scaffolding molecules that mediate the interaction between glutamate receptor and Wnt signalling pathways. The MAGUK family includes Postsynaptic Density (PSD)-95 and Synapse Associated Protein (SAP)-102, two key molecules of the NRC during barrel formation. These MAGUKs also bind the Wnt receptor family Frizzled and SAP-102 binds to APC, a key Frizzled-signalling protein. As a first step in examining a role for Wnts in barrel formation, the gene expression patterns of members of the Wnt, Frizzleds and secreted Frizzled related protein (sFRPs) families during barrel cortex development were measured using degenerate primer RT-PCR, quantitative real-time PCR and in-situ hybridization. Wnts 2b, 3, 4, 5a, la, 7b, 9a, 11, 16 were found in the barrel cortex, suggesting that these members of the Wnt family may play a role in barrel development, and Wnt7b, Frizzled 4, Frizzled 9, and Frizzled 3 were conspicuously downregulated in mutant mice that lack barrels, namely Plc-ß1⁻/⁻, Pkar2ß⁻/⁻ and Mglur5⁻/⁻ mice.In order to determine whether Wnts, members of the Wnt signaling machinery and MAGUKs associated with Wnt signaling are essential for barrel formation, the barrel phenotype of all available postnatally viable Wnt gene knockouts, Wnt2b⁻/⁻, Wnt7a⁻/⁻, Wnt8b⁻/⁻, Wnt signaling component knockouts Dvl⁻/⁻, MAGUK knockouts Sap-102'1', Psd-95'1' and double knockouts of Wnt7a"Dvr'~ and of Sap-102⁻/⁻Psd-95⁻/⁻ were examined. Barrels appeared normal in all mutants, apart from compound Sap-102⁻/⁻Psd-95⁻/⁻ and Sap-102⁻/⁻Psd-95⁻/⁻ mice, which exhibited poorer barrel segregation compared to wild type.In order to achieve a detailed understanding of the mechanisms by which neuronal activity regulates barrel development, we need a detailed understanding of the intracellular pathways activated by NMDA receptors during development. In searching the literature, data concerning the developmental expression patterns of NRC components can be difficult to locate, as the prevailing database tools used either search only title, author and keyword abstract text (NCBI PubMed) potentially missing desired information, or as in the unique case of Google Scholar, search the full text of electronically published papers, but yield overwhelming numbers of results in the process.The Jackson Laboratories MGI suite offers an impressive way of mining the literature for such data, but the content is sparse, relying on author submission and attempting to map expression throughout the whole mouse. For example, the only gene present in MGI for postnatal layer IV cerebral cortex is Apc2.In order to bring together the data in the literature and from the mouse genome projects into a usable and accessible way, we decided to create a web-based centralised resource for the developmental neuroscience community, containing expression profiles of NRC components within mouse somatosensory cortex. By performing exhaustive literature searches utilising Google Scholar and PubMed, and linking to sequence and mutant mouse availability information elsewhere, BGI offers a portal for such information and also offers a forum for the notification of unpublished observations of transgenic animals displaying normal barrel formation, preventing duplication of experiments. Barrelgene.info should provide a key resource for any researcher interested in the molecular basis of cortical development.