During neuronal migration in the developing brain, it is thought that the centrosome precedes the nucleus and provides a cue for nuclear migration along the microtubules. In time-lapse imaging studies of radially migrating granule cells in mouse cerebellar slices, we observed that the movements of the nucleus and centrosome appeared to occur independently of each other. The nucleus often migrated ahead of the centrosome during its saltatory movement, negating the supposed role of the centrosome in pulling the nucleus. The nucleus was associated with dynamic microtubules enveloping the entire nucleus and stable microtubules extending from the leading process to the anterior part of the nucleus. Neither of these perinuclear microtubules converged at the centrosome. Disruption or excess formation of stable microtubules attenuated nuclear migration, indicating that the configuration of stable microtubules is crucial for nuclear migration. The inhibition of LIS1 function, a regulator of a microtubule motor dynein, specifically blocks nuclear migration without affecting the coupling of the centrosome and microtubules in the leading process, suggesting that movements of the nucleus and centrosome are differentially regulated by dynein motor function. Thus, the nucleus moves along the microtubules independently of the position of the centrosome in migrating neurons.neuronal migration ͉ nucleus ͉ LIS1 ͉ acetylated tubulin ͉ tyrosinated tubulin

Neural activity plays roles in the later stages of development of cortical excitatory neurons, including dendritic and axonal arborization, remodeling, and synaptogenesis. However, its role in earlier stages, such as migration and dendritogenesis, is less clear. Here we investigated roles of neural activity in the maturation of cortical neurons, using calcium imaging and expression of prokaryotic voltage-gated sodium channel, NaChBac. Calcium imaging experiments showed that postmigratory neurons in layer II/III exhibited more frequent spontaneous calcium transients than migrating neurons. To test whether such an increase of neural activity may promote neuronal maturation, we elevated the activity of migrating neurons by NaChBac expression. Elevation of neural activity impeded migration, and induced premature branching of the leading process before neurons arrived at layer II/III. Many NaChBac-expressing neurons in deep cortical layers were not attached to radial glial fibers, suggesting that these neurons had stopped migration. Morphological and immunohistochemical analyses suggested that branched leading processes of NaChBac-expressing neurons differentiated into dendrites. Our results suggest that developmental control of spontaneous calcium transients is critical for maturation of cortical excitatory neurons in vivo: keeping cellular excitability low is important for migration, and increasing spontaneous neural activity may stop migration and promote dendrite formation.

Lissencephaly is a devastating neurological disorder due to defective neuronal migration. LIS1 (or PAFAH1B1) was identified as the gene mutated in lissencephaly patients, and was found to regulate cytoplasmic dynein function and localization. Here, we show that more than half of LIS1 is degraded via calpain-dependent proteolysis, and that inhibition or knockdown of calpains protects LIS1 from proteolysis, resulting in the augmentation of LIS1 levels in Lis1+/− mouse embryonic fibroblast (MEF) cells, which leads to rescue of the aberrant distribution of cytoplasmic dynein, mitochondria and β-COP positive vesicles. We also show that calpain inhibitors improve neuronal migration of Lis1+/− cerebellar granular neurons. Intra-peritoneal injection of ALLN to pregnant Lis1+/− dams rescued apoptotic neuronal cell death and neuronal migration defects in Lis1+/− offspring. Furthermore, in utero knockdown of calpain by shRNA rescued defective cortical layering in Lis1+/− mice. Thus, the inhibition of calpain is a potential therapeutic intervention for lissencephaly.

Nuclear migration of newly born neurons is essential for cortex formation in the brain. The nucleus is translocated by actin and microtubules, yet the actual force generated by the interplay of these cytoskeletons remains elusive. High-resolution time-lapse observation of migrating murine cerebellar granule cells revealed that the nucleus actively rotates along the direction of its translocation, independently of centrosome motion. Pharmacological and molecular perturbation indicated that spin torque is primarily generated by microtubule motors through the LINC complex in the absence of actomyosin contractility. In contrast to the prevailing view that microtubules are uniformly oriented around the nucleus, we observed that the perinuclear microtubule arrays are of mixed polarity and both cytoplasmic dynein complex and kinesin-1 are required for nuclear rotation. Kinesin-1 can exert a point force on the nuclear envelope via association with nesprins, and loss of kinesin-1 causes failure in neuronal migration Thus, microtubules steer the nucleus and drive its rotation and translocation via a dynamic, focal interaction of nesprins with kinesin-1 and dynein, and this is necessary for neuronal migration during brain development.

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