Asymmetric cell division plays an indispensable role during corticogenesis for producing new neurons while maintaining a self-renewing pool of apical progenitors. The cellular and molecular determinants favouring asymmetric division are not completely understood. Here, we identify a novel mechanism for generating cellular asymmetry through the active transportation and local translation of Cyclin D2 mRNA in the basal process. This process is regulated by a unique cis-regulatory sequence found in the 3 0 untranslated region (3 0 UTR) of the mRNA. Unequal inheritance of Cyclin D2 protein to the basally positioned daughter cell with the basal process confers renewal of the apical progenitor after asymmetric division. Conversely, depletion of Cyclin D2 in the apically positioned daughter cell results in terminal neuronal differentiation. We demonstrate that Cyclin D2 is also expressed in the developing human cortex within similar domains, thus indicating that its role as a fate determinant is ancient and conserved.

Development of appropriate dendritic arbors is crucial for neuronal information transfer. We show, using seizure-related gene 6 (sez-6) null mutant mice, that Sez-6 is required for normal dendritic arborization of cortical neurons. Deep-layer pyramidal neurons in the somatosensory cortex of sez-6 null mice exhibit an excess of short dendrites, and cultured cortical neurons lacking Sez-6 display excessive neurite branching. Overexpression of individual Sez-6 isoforms in knockout neurons reveals opposing actions of membrane-bound and secreted Sez-6 proteins, with membrane-bound Sez-6 exerting an antibranching effect under both basal and depolarizing conditions. Layer V pyramidal neurons in knockout brain slices show reduced excitatory postsynaptic responses and a reduced dendritic spine density, reflected by diminished punctate staining for postsynaptic density 95 (PSD-95). In behavioral tests, the sez-6 null mice display specific exploratory, motor, and cognitive deficits. In conclusion, cell-surface protein complexes involving Sez-6 help to sculpt the dendritic arbor, in turn enhancing synaptic connectivity.

The mechanisms that coordinate the three-dimensional shape of the vertebrate brain during development are largely unknown. We have found that sonic hedgehog (Shh) is crucial in driving the rapid, extensive expansion of the early vesicles of the developing midbrain and forebrain. Transient displacement of the notochord from the midbrain floor plate resulted in abnormal folding and overall collapse of the vesicles, accompanied by reduced cell proliferation and increased cell death in the midbrain. Simultaneously, expression of Shh decreased locally in the notochord and floor plate, whereas overt patterning and differentiation proceeded normally. Normal midbrain expansion was restored by implantation of Shh-secreting cells in a dose-dependent manner; conversely, expansion was retarded following antagonism of the Shh signaling pathway by cyclopamine. Our results indicate that Shh signaling from the ventral midline is essential for regulating brain morphogenesis during early development.

Neuronal migration is integral to the development of the cerebral cortex and higher brain function. Cortical neuron migration defects lead to mental disorders such as lissencephaly and epilepsy. Interaction of neurons with their extracellular environment regulates cortical neuron migration through cell surface receptors. However, it is unclear how the signals from extracellular matrix proteins are transduced intracellularly. We report here that mouse embryos lacking the Ras family guanine nucleotide exchange factor, C3G (Rapgef1, Grf2), exhibit a cortical neuron migration defect resulting in a failure to split the preplate into marginal zone and subplate and a failure to form a cortical plate. C3G-deficient cortical neurons fail to migrate. Instead, they arrest in a multipolar state and accumulate below the preplate. The basement membrane is disrupted and radial glial processes are disorganised and lack attachment in C3G-deficient brains. C3G is activated in response to reelin in cortical neurons, which, in turn, leads to activation of the small GTPase Rap1. In C3G-deficient cells, Rap1 GTP loading in response to reelin stimulation is reduced. In conclusion, the Ras family regulator C3G is essential for two aspects of cortex development, namely radial glial attachment and neuronal migration.

The modulation of cortical activity by GABAergic interneurons is required for normal brain function and is achieved through the immense level of heterogeneity within this neuronal population. Cortical interneurons share a common origin in the ventral telencephalon, yet during the maturation process diverse subtypes are generated that form the characteristic laminar arrangement observed in the adult brain. The long distance tangential and short-range radial migration into the cortical plate is regulated by a combination of intrinsic and extrinsic signalling mechanisms, and a great deal of progress has been made to understand these developmental events. In this review, we will summarize current findings regarding the molecular control of subtype specification and provide a detailed account of the migratory cues influencing interneuron migration and lamination. Furthermore, a dysfunctional GABAergic system is associated with a number of neurological and psychiatric conditions, and some of these may have a developmental aetiology with alterations in interneuron generation and migration. We will discuss the notion of additional sources of interneuron progenitors found in human and non-human primates and illustrate how the disruption of early developmental events can instigate a loss in GABAergic function.

Thyroid hormones increase bone turnover in vivo and stimulate bone resorption in vitro. Clinical states associated with excess circulating thyroid hormone levels are known to produce osteoporosis. To determine the effect of T3 on bone resorption, we used an isolated rat osteoclast bone resorption assay in the absence or presence of added osteoblasts. This makes it possible to distinguish between direct and indirect effects of thyroid hormones on osteoclasts. In short settlement osteoclast cultures, which contain relatively few osteoblasts, 24-h treatment with T3 (10(-10)-10(-8) M) produced no stimulation of bone resorption. However, after 48-h incubation in the presence of T3, an increase in resorption was observed (2.3-fold at 10(-9) M). In cocultures of osteoclasts and osteoblasts (UMR 106-01 osteoblast-like cells or long settlement cultures), T3 stimulated resorption at 24 h. Furthermore, stimulation of resorption occurred when osteoblasts (UMR 106-01 or rat calvarial cells) were pretreated with T3 and the subsequent osteoblast-osteoclast cocultures conducted for 24 h in the absence of T3. Thus, direct exposure of osteoclasts to T3 was not required for the stimulatory effect. Treatment for 48 h with T3 (10(-9) M) or PTH (10(-8) M) had no effect on bone resorption in osteoblast-free cultures derived from human osteoclastoma tumours. T4 was 100-fold less potent than T3 as a stimulator of osteoclast activity, and rT3 had no effect. T3-induced stimulation was inhibited by salmon calcitonin (10(-10) M). These findings indicate that thyroid hormone can act on osteoblasts to indirectly stimulate osteoclastic bone resorption.

PTH-related protein (PTHrP) interacts, via its amino-terminal 34 residues, with PTH receptors on osteoblasts to stimulate osteoclastic bone resorption indirectly. We now report that mature hPTHrP-(1-141) (EC50, approximately 10(-11) M) and a carboxyl-terminal fragment, PTHrP-(107-139) (EC50, approximately 10(-15) M), are potent inhibitors of resorption in an isolated rat osteoclast bone resorption assay, whereas hPTHrP-(1-108) and hPTHrP-(1-34) are inactive in this respect. PTHrP-(107-139) also inhibits resorption in a rat long bone organ culture system and reduces osteoclast spreading. PTHrP-(107-139) does not act on osteoclasts via a cAMP signal transduction mechanism, but its effects may be mediated by protein kinase-C. This previously unrecognized action of PTHrP, to inhibit osteoclastic bone resorption directly, indicates that PTHrP may be a precursor of multiple biologically active peptides with differing physiological functions.

Thyroid hormones increase bone turnover in vivo and stimulate bone resorption in vitro. Clinical states associated with excess circulating thyroid hormone levels are known to produce osteoporosis. To determine the effect of T3 on bone resorption, we used an isolated rat osteoclast bone resorption assay in the absence or presence of added osteoblasts. This makes it possible to distinguish between direct and indirect effects of thyroid hormones on osteoclasts. In short settlement osteoclast cultures, which contain relatively few osteoblasts, 24-h treatment with T3 (10(-10)-10(-8) M) produced no stimulation of bone resorption. However, after 48-h incubation in the presence of T3, an increase in resorption was observed (2.3-fold at 10(-9) M). In cocultures of osteoclasts and osteoblasts (UMR 106-01 osteoblast-like cells or long settlement cultures), T3 stimulated resorption at 24 h. Furthermore, stimulation of resorption occurred when osteoblasts (UMR 106-01 or rat calvarial cells) were pretreated with T3 and the subsequent osteoblast-osteoclast cocultures conducted for 24 h in the absence of T3. Thus, direct exposure of osteoclasts to T3 was not required for the stimulatory effect. Treatment for 48 h with T3 (10(-9) M) or PTH (10(-8) M) had no effect on bone resorption in osteoblast-free cultures derived from human osteoclastoma tumours. T4 was 100-fold less potent than T3 as a stimulator of osteoclast activity, and rT3 had no effect. T3-induced stimulation was inhibited by salmon calcitonin (10(-10) M). These findings indicate that thyroid hormone can act on osteoblasts to indirectly stimulate osteoclastic bone resorption.