The mammalian neocortex contains many cell types, but whether they organize into repeated structures has been unclear. We discovered that major cell types in neocortical layer 5 form a lattice structure in many brain areas. Large-scale three-dimensional imaging revealed that distinct types of excitatory and inhibitory neurons form cell type-specific radial clusters termed microcolumns. Thousands of microcolumns, in turn, are patterned into a hexagonal mosaic tessellating diverse regions of the neocortex. Microcolumn neurons demonstrate synchronized in vivo activity and visual responses with similar orientation preference and ocular dominance. In early postnatal development, microcolumns are coupled by cell type-specific gap junctions and later serve as hubs for convergent synaptic inputs. Thus, layer 5 neurons organize into a brainwide modular system, providing a template for cortical processing.

During cortical development, newly generated neurons migrate radially toward their final positions. Although several candidate genes essential for this radial migration have been reported, the signaling pathways regulating it are largely unclear. Here we studied the role of phosphatidylinositol (PI) 3-kinase and its downstream signaling molecules in the radial migration of cortical neurons in vivo and in vitro. The expression of constitutively active and dominant-negative PI 3-kinases markedly inhibited radial migration. In the neocortical slice culture, a PI 3-kinase inhibitor suppressed the formation of GTP-bound Rac1 and Cdc42 and radial migration. Constitutively active and dominant-negative forms of Rac1 and Cdc42 but not Akt also significantly inhibited radial migration. In migrating neurons, wild-type Rac1 and Cdc42 showed different localizations; Rac1 localized to the plasma membrane and Cdc42 to the perinuclear region on the side of the leading processes. These results suggest that both the PI 3-kinase/Rac1 and Cdc42 pathways are involved in the radial migration of cortical neurons and that they have different roles.

A major question in neocortical research is the extent to which neuronal organization is stereotyped. Previous studies have revealed functional clustering and neuronal interactions among cortical neurons located within tens of micrometers in the tangential orientation (orientation parallel to the pial surface). In the tangential orientation at this scale, however, it is unknown whether the distribution of neuronal subtypes is random or has any stereotypy. We found that the tangential arrangement of subcerebral projection neurons, which are a major pyramidal neuron subtype in mouse layer V, was not random but significantly periodic. This periodicity, which was observed in multiple cortical areas, had a typical wavelength of 30 m. Under specific visual stimulation, neurons in single repeating units exhibited strongly correlated c-Fos expression. Therefore, subcerebral projection neurons have a periodic arrangement, and neuronal activity leading to c-Fos expression is similar among neurons in the same repeating units. These results suggest that the neocortex has a periodic functional micro-organization composed of a major neuronal subtype in layer V.

The activity-dependent remodeling of postsynaptic structure is a fundamental process underlying learning and memory. Insulin receptor substrate p53 (IRSp53), a key player in cytoskeletal dynamics, is enriched in the postsynaptic density (PSD) fraction, but its significance in synaptic functions remains unclear. We report here that IRSp53 is accumulated rapidly at the postsynaptic sites of cultured hippocampal neurons after glutamate or NMDA stimulation in an actin cytoskeleton-dependent manner. Pharmacological profiles showed that a PKC inhibitor, but not other kinase inhibitors, specifically suppressed the synaptic translocation of IRSp53 in response to NMDA, and the selective activation of PKC with phorbol ester markedly induced the synaptic translocation. Reverse transcriptase-PCR and Western blotting showed that IRSp53-S is the major isoform expressed in cultured hippocampal neurons. The synaptic targeting of IRSp53-S was found to be mediated through N-terminal coiled-coil domain and the PDZ (PSD-95/Discs large/zona occludens-1)-binding sequence at its C-terminal end and regulated by the PKC phosphorylation of its N terminus. In electrophysiological experiments, overexpression of IRSp53-S wild type and IRSp53-S mutant that is spontaneously accumulated at the postsynaptic sites enhanced the postsynaptic function as detected by an increased miniature EPSC amplitude. These data suggest that IRSp53 is involved in NMDA receptor-linked synaptic plasticity via PKC signaling.

the synaptic plasma membrane demonstrated that PSDZip70 was localized to the PSD and the dendritic raft. In Madin-Darby canine kidney (MDCK) cells, exogenous PSD-Zip70 was targeted to the apical plasma membrane of microvilli, and its N-myristoylation was necessary for this targeting. In hippocampal neurons, N-myristoylation was also required for the membrane localization and the Cterminal region was critically involved in the synaptic targeting. These results suggest that PSD-Zip70 may be involved in the dynamic properties of the structure and function of the postsynaptic site.