The corpus callosum is the largest commissural system in the mammalian brain, but the mechanisms underlying its development are not well understood. Here we report that neuronal activity is necessary for the normal development and maintenance of callosal projections in the mouse somatosensory cortex. We labeled a subpopulation of layer II/III callosal neurons via in utero electroporation and traced their axons in the contralateral cortex at different postnatal stages. Callosal axons displayed region-and layer-specific projection patterns within the first 2 weeks postnatally. Prenatal suppression of neuronal excitation was achieved via electroporation-induced overexpression of the inward rectifying potassium channel Kir2.1 in layer II/III cortical neurons. This resulted in abnormal callosal projections with many axons extending beyond layers II-III to terminate in layer I. Others failed to terminate at the border between the primary and secondary somatosensory cortices. Blocking synaptic transmission via expression of the tetanus toxin light chain (TeNT-LC) in these axons produced a more pronounced reduction in the projections to the border region, and the eventual disappearance of callosal projections over the entire somatosensory cortex. When Kir2.1 and TeNT-LC were coexpressed, callosal axon targeting exhibited a more severe phenotype that appeared to represent the addition of the effects produced by individual expression of Kir2.1 and TeNT-LC. These results underscore the importance of activity in regulating the developing neural connections and suggest that neuronal and synaptic activities are involved in regulating different aspects of the development of callosal projection.

SUMMARY Abnormalities in dendritic spines are manifestations of several neurodevelopmental and psychiatric diseases. TAOK2 is one of the genes in the 16p11.2 locus, copy number variations of which are associated with autism and schizophrenia. Here, we show that the kinase activity of the serine/threonine kinase encoded by TAOK2 is required for spine maturation. TAOK2 depletion results in unstable dendritic protrusions, mislocalized shaft-synapses, and loss of compartmentalization of NMDA receptor-mediated calcium influx. Using chemical-genetics and mass spectrometry, we identified several TAOK2 phosphorylation targets. We show that TAOK2 directly phosphorylates the cytoskeletal GTPase Septin7, at an evolutionary conserved residue. This phosphorylation induces translocation of Septin7 to the spine, where it associates with and stabilizes the scaffolding protein PSD95, promoting dendritic spine maturation. This study provides a mechanistic basis for postsynaptic stability and compartmentalization via TAOK2-Sept7 signaling, with implications toward understanding the potential role of TAOK2 in neurological deficits associated with the 16p11.2 region.

How developing axons in the corpus callosum (CC) achieve their homotopic projection to the contralateral cortex remains unclear. We found that axonal position within the CC plays a critical role in this projection. Labeling of nearby callosal axons in mice showed that callosal axons were segregated in an orderly fashion, with those from more medial cerebral cortex located more dorsally and subsequently projecting to more medial contralateral cortical regions. The normal axonal order within the CC was grossly disturbed when semaphorin3A/neuropilin-1 signaling was disrupted. However, the order in which axons were positioned within the CC still determined their contralateral projection, causing a severe disruption of the homotopic contralateral projection that persisted at postnatal day 30, when the normal developmental refinement of contralateral projections is completed in wild-type (WT) mice. Thus, the orderly positioning of axons within the CC is a primary determinant of how homotopic interhemispheric projections form in the contralateral cortex.axon development | axon fiber order | cortical axon guidance | cortical development T he largest commissural tract in the human brain is the corpus callosum (CC), with more than 200 million axons connecting the two cerebral hemispheres. Callosal axons originate primarily from neurons of layer II/III and layer V of the neocortex (1) and project homotopically to the contralateral cortex. For example, callosal axons of the primary motor cortex (M1) and primary somatosensory cortex (S1) project to topographically equivalent locations in the contralateral M1 and S1, respectively. This pattern of homotopic projection is essential for coordinated motor and somatosensory functions as well as for higher associative and cognitive processes (2-4). Abnormal CC development has been noted in psychiatric and developmental disorders (5, 6), and deviant asymmetry of cortical areas found in patients with developmental dyslexia also may be attributed to callosal abnormalities (7,8). However, the mechanism by which normal homotopic projection pattern is achieved during development remains largely unknown.The majority of axonal projections in the nervous system are organized topographically. To facilitate the formation of orderly projections over long distances, axons originating from adjacent areas may preserve their topographic order within the nerve tract along their entire path toward the target region (9, 10). This preservation of topographic order has been shown in the thalamocortical tract (11)(12)(13)(14) and in the optic and olfactory nerves (15-17). By performing a series of random microinjections of biotinylated dextran amine in the dorsal thalamus and reconstructing the labeled fibers, Powell et al. (14) showed that labeled axons within the thalamocortical tract preserve a topography similar to that in the ventral telencephalon before they reach the cortex. In the developing olfactory system, axons within the olfactory nerve from three different regions of the olfactory epitheli...