We have used time-lapse multiphoton microscopy to map the migration and settling pattern of GABAergic interneurons that originate in the ganglionic eminence of the ventral forebrain and incorporate into the neocortex of the cerebral hemispheres. Imaging of the surface of the cerebral hemispheres in both explant cultures and brains of living mouse embryos revealed that GABAergic interneurons migrating within the marginal zone originate from three different sources and migrate via distinct and independent streams. After reaching their areal destination, interneurons descend into the underlying cortex to assume positions with isochronically generated, radially derived neurons. The dynamics and pattern of cell migration in the marginal zone (see movies, available at www.jneurosci.org) suggest that the three populations of interneurons respond selectively to distinct local cues for directing their migration to the appropriate areas and layers of the neocortex. This approach opens a new avenue for study of normal and abnormal neuronal migration in their native environment and indicate that interneurons have specific programs for their areal and laminar deployment.

The mode of neural stem cell division in the forebrain proliferative zones profoundly influences neocortical growth by regulating the number and diversity of neurons and glia. Long-term time-lapse multiphoton microscopy of embryonic mouse cortex reveals new details of the complex three-dimensional rotation and oscillation of the mitotic spindle before stem cell division. Importantly, the duration and amplitude of spindle movement predicts and specifies the eventual mode of mitotic division. These technological advances have provided dramatic data and insights into the kinetics of neural stem cell division by elucidating the involvement of spindle rotation in selection of the cleavage plane and the mode of neural stem cell division that together determine the size of the mammalian neocortex.T he number and diversity of cells in the cerebral cortex is due, in great measure, to the mode of progenitor cell division in the proliferative ventricular zone (VZ) before birth (1-4). At each mitotic division, the proliferative fate of each daughter cell, either reentering or exiting the cell cycle, dramatically influences neocortical growth (5). Symmetrical founder cell divisions, which predominate early, yield more progenitors and lead to an exponential expansion of the VZ population. In contrast, asymmetrical divisions, which prevail during later stages of neurogenesis, lead to cell commitment and diversity of the cortical architecture (1). Critical parameters of cortical growth are, therefore, the duration of the early symmetrical division phase and the time and extent of the transition to the asymmetrical mode of division (1-3). Despite the importance of this developmental strategy for brain growth, it is unknown how conversion between these major types of mitotic divisions is achieved in the VZ of the mammalian forebrain.The current model for how different daughter cell fates result from asymmetric stem cell divisions assumes that cytoplasmic fate-determining molecules are unequally distributed during mother cell mitosis (6-9). This asymmetrical segregation can initiate diverse cellular programs during development of the daughter cells, including restriction of potential and differentiation (10-13). A prerequisite for producing asymmetric allocation of intracellular contents, observed in rodent telencephalon, is rotation and alignment of the mitotic spindle before division (14). Supporting such a causative role in cell fate determination, spindle movement before anaphase and the choice of the final cleavage plane has been correlated with daughter cell behavior in both invertebrates (6, 15-19) and vertebrates (refs. 10, 13, 20, and 21; see Fig. 1). In mammalian studies, however, determination of the molecular mechanisms linking spindle orientation, parcellation of intracellular factors, and fate determination has suffered from the lack of an intact model system. We sought to develop the capability for real-time analysis of these issues in living tissue that preserves the three-dimensional relationship between stem and ...

Neurons of the cerebral neocortex in mammals, including humans, are generated during fetal life in the proliferative zones and then migrate to their final destinations by following an inside-tooutside sequence. The present study examined the effect of ultrasound waves (USW) on neuronal position within the embryonic cerebral cortex in mice. We used a single BrdU injection to label neurons generated at embryonic day 16 and destined for the superficial cortical layers. Our analysis of over 335 animals reveals that, when exposed to USW for a total of 30 min or longer during the period of their migration, a small but statistically significant number of neurons fail to acquire their proper position and remain scattered within inappropriate cortical layers and͞or in the subjacent white matter. The magnitude of dispersion of labeled neurons was variable but systematically increased with duration of exposure to USW. These results call for a further investigation in larger and slower-developing brains of non-human primates and continued scrutiny of unnecessarily long prenatal ultrasound exposure.brain malformations ͉ cerebral cortex ͉ embryonic development A fundamental feature of cerebral cortical organization is that positions of its neuronal constituents into horizontal (laminar) and vertical (radial) arrays ultimately define their connectivity and function (1). Cortical neurons acquire appropriate positions by migration from the site of their origin in the proliferative zones lining the cerebral ventricle, according to a precise schedule (2, 3) and along well defined pathways (4-6). When the rate of neuronal migration and the sequence of arrival are altered because of genetic and͞or environmental factors, various consequences, including abnormal behavior, have been observed (7-13). In terms of orientation and directionality of movement, neuronal migration to the cerebral cortex can be classified into radial (proceeding radially from the ventricular to the pial surface) (5, 14) and tangential (running parallel to the brain surface) (15)(16)(17). Contact interaction between migrating neurons and the surfaces of neighboring cells plays a decisive role in selecting the migratory pathway and determining their final position (18,19). Neuronal migration involves translocation of the nucleus and the surrounding cytoplasm with the leading process, which requires rearrangement of the cytoskeleton (20, 21). As a consequence of these complex cellular and molecular interactions, the process of neuronal migration is highly sensitive to a variety of biological, physical, and chemical agents, as well as to specific genetic mutations (7-13). For example, repeated exposure of the rodent and primate fetal brain to environmental agents, such as alcohol (9), drugs (22), neurotrophic viruses (23), and ionizing irradiation (24, 25), causes misplacement of neurons and behavioral deficits.To our knowledge, the effect of ultrasound waves (USW) on the rate of migration in the cerebral cortex has never been tested, although it has been reported ...

The transcription factor DeltaFosB is induced in the hippocampus and other brain regions by repeated electroconvulsive seizures (ECS), an effective antidepressant treatment. The unusually high stability of this protein makes it an attractive candidate to mediate some of the long-lasting changes in the brain caused by ECS treatment. To understand how DeltaFosB might alter brain function, we examined the gene expression profiles in the hippocampus of inducible transgenic mice that express DeltaFosB in this brain region by the use of cDNA expression arrays that contain 588 genes. Of the 430 genes detected, 20 genes were consistently upregulated, and 14 genes were downregulated, by >50%. One of the upregulated genes is cyclin-dependent kinase 5 (cdk5). On the basis of its purported role in regulating neuronal structure, we studied directly whether cdk5 is a true target for DeltaFosB. Upregulation of cdk5 immunoreactivity in the hippocampus was confirmed by Western blotting in the DeltaFosB-expressing transgenic mice as well as in rats treated chronically with ECS. Chronic ECS treatment also increased, in the hippocampus, the phosphorylation state of tau, a microtubule-associated protein that is a known substrate for cdk5. A 1.6 kb fragment of the cdk5 promoter was cloned, and activity of the promoter was found to be increased after overexpression of DeltaFosB in cell culture. Moreover, mutation of the single consensus activator protein-1 site contained within the cdk5 promoter fragment completely abolished activation of the promoter by DeltaFosB. Together, these results suggest that cdk5 is one target by which DeltaFosB produces some of its physiological effects in the hippocampus and thereby mediates certain long-term consequences of chronic ECS treatment.