Cyclin D2 is critical for intermediate progenitor cell proliferation in the embryonic cortex.

Expression of cyclins D1 (cD1) and D2 (cD2) in ventricular zone and subventricular zone (SVZ), respectively, suggests that a switch to cD2 could be a requisite step in the generation of cortical intermediate progenitor cells (IPCs). However, direct evidence is lacking. Here, cD1 or cD2 was seen to colabel subsets of Pax6-expressing radial glial cells (RGCs), whereas only cD2 colabeled with Tbr2. Loss of IPCs in cD2(-/-) embryonic cortex and analysis of expression patterns in mutant embryos lacking cD2 or Tbr2 indicate that cD2 is used as progenitors transition from RGCs to IPCs and is important for the expansion of the IPC pool. This was further supported by the laminar thinning, microcephaly, and selective reduction in the cortical SVZ population in the cD2(-/-)cortex. Cell cycle dynamics between embryonic day 14-16 in knock-out lines showed preserved parameters in cD1 mutants that induced cD2 expression, but absence of cD2 was not compensated by cD1. Loss of cD2 was associated with reduced proliferation and enhanced cell cycle exit in embryonic cortical progenitors, indicating a crucial role of cD2 for the support of cortical IPC divisions. In addition, knock-out of cD2, but not cD1, affected both G(1)-phase and also S-phase duration, implicating the importance of these phases for division cycles that expand the progenitor pool. That cD2 was the predominant D-cyclin expressed in the human SVZ at 19-20 weeks gestation indicated the evolutionary importance of cD2 in larger mammals for whom expansive intermediate progenitor divisions are thought to enable generation of larger, convoluted, cerebral cortices.

Cyclin D1 in excitatory neurons of the adult brain enhances kainate-induced neurotoxicity.

G1-phase cyclin D1 (cD1) expression has been documented in post-mitotic neurons undergoing apoptosis, leading others to propose that attempted cell cycle re-entry may induce cell death. Here, cD1 immunoreactivity was found in a subpopulation of healthy excitatory neurons throughout the brain. Most striking was the selective cD1 expression in hippocampal pyramidal neurons, an especially vulnerable cell group. Seizure threshold, cD1 induction and CA1 neuron death were examined following application of kainate (KA) or pentylenetetrazole (PTZ) in cD1 heterozygous (+/-) and wildtype mice to determine whether baseline cD1 correlates with pathology. cD1+/- mice displayed resistance to KA, but not PTZ-induced seizures and had reduced or equivalent cytotoxicity respectively, compared with wildtype. KA administration, but not PTZ, induced cD1 expression. These findings suggest that basal cD1 expression may render hippocampal circuits more susceptible to particular epileptogenic agents and excitotoxic cell death, though cD1 is not a direct precipitant in apoptosis.

Identification of a novel recessive RELN mutation using a homozygous balanced reciprocal translocation.

Two siblings from a consanguineous Egyptian marriage showed an identical phenotype of cortical lissencephaly with cerebellar hypoplasia, severe epilepsy, and mental retardation. Examination of karyotype revealed 46, t(7;12)(q22;p13)mat (7;12)(q22;p13)pat in both affected children, suggesting a homozygous reciprocal balanced translocation. Each healthy parent was a carrier of the balanced translocation in the heterozygous state, suggesting homozygous disruption of a gene involved in brain development. There were early spontaneous abortions in this family, as would be expected from transmission of an unbalanced chromosome. A disruption of RELN at 7q22.1 with absence of encoded protein was identified. This is the first demonstration that such rare homozygous translocations can be used to identify recessive disease gene mutations.

Calcium-dependent interaction of Lis1 with IQGAP1 and Cdc42 promotes neuronal motility.

Lis1 gene defects impair neuronal migration, causing the severe human brain malformation lissencephaly. Although much is known about its interactions with microtubules, microtubule-binding proteins such as CLIP-170, and with the dynein motor complex, the response of Lis1 to neuronal motility signals has not been elucidated. Lis1 deficiency is associated with deregulation of the Rho-family GTPases Cdc42, Rac1 and RhoA, and ensuing actin cytoskeletal defects, but the link between Lis1 and Rho GTPases remains unclear. We report here that calcium influx enhances neuronal motility through Lis1-dependent regulation of Rho GTPases. Lis1 promotes Cdc42 activation through interaction with the calcium sensitive GTPase scaffolding protein IQGAP1, maintaining the perimembrane localization of IQGAP1 and CLIP170 and thereby tethering microtubule ends to the cortical actin cytoskeleton. Lis1 thus is a key component of neuronal motility signal transduction that regulates the cytoskeleton by complexing with IQGAP1, active Cdc42 and CLIP-170 upon calcium influx.