In this study we investigated whether the pattern of expression of

In this study we investigated whether the pattern of expression of the cyclin-dependent kinase inhibitor p19INK4d by the unique progenitor cells of the neonatal anterior subventricular zone (SVZa) can account for their ability to divide even though they express phenotypic characteristics of differentiated neurons. an anteriorhighCposteriorlow gradient of p19INK4d expression. On the basis of the p19INK4d immunoreactivity and BrdU incorporation, SVZa-derived cells appear to exit and reenter the cell cycle successively. Thus, in contrast to telencephalic VZ cells, SVZa cells continue to undergo multiple rounds of division and differentiation before becoming postmitotic. hybridization, have analyzed the expression pattern of the CDKIs in the developing mouse brain (Zindy et al., 1997b), to better NPI-2358 understand the spatiotemporal distribution of p19INK4d in the developing fore-brain, immunocytochemical analysis is also required. hybridization studies showed that the p19INK4d mRNA was expressed throughout all laminae of the developing neocortex. From this finding it was concluded that it is present in both proliferating and differentiating cell populations of the cerebral cortex. In contrast, our immunocytochemical results showed a differential expression pattern of p19INK4d among the different layers of the developing cerebral cortex. Specifically, VZu progenitors that are NPI-2358 in the S phase of the cell cycle do not express p19INK4d. However, many cells in the VZl do express p19INK4d, and most likely these are immature neurons that have withdrawn recently from the cell cycle. Furthermore, reports that regulation of CDKIs occurs at the post-translational level (Pagano et al., 1995) underscore the need for immunocytochemical analyses of their expression. Although our data demonstrate that p19INK4d appears to be differentially regulated by the cells of the VZl and VZu, these data do not exclude the possibility that VZl progenitor cells temporarily express p19INK4d before they initiate a new round of interkinetic nuclear migration. A few studies have suggested that under specific circumstances, p19INK4d is expressed by cells undergoing division, disputing the idea that p19INK4d is only expressed by postmitotic cells. In particular, Hirai et al. (1995) and Thullberg et al. (2000) demonstrated the oscillation of p19INK4d expression by cultured macrophages and fibroblasts, after they were induced to reenter the cell cycle from a quiescent serum-deprived state. In these cells, the p19INK4d mRNA levels were low during G1, NPI-2358 highest in S, and low again as they approached the subsequent G1. When retroviral-mediated gene transfer was used to express p19INK4d constitutively in cultures of cycling fibroblasts, however, the cells were arrested at the G1 phase. This indicates that in fibroblasts the induction of p19INK4d during the G1 phase is sufficient to block the G1CS progression. Our data argue that the temporal sequence of expression of p19INK4d, described for macrophages and fibroblasts, is not what occurs when the progenitor NPI-2358 cells of the VZ undergo division in vivo. Instead, our analysis of the temporal pattern of p19INK4d expression during interkinetic nuclear migration demonstrates that there is a negligible level of p19INK4d expression when the VZ progenitor cells are in the S phase and indicates that p19INK4d is expressed at high levels in the VZl by newly generated postmitotic neurons. An alternative explanation to the scenario NPI-2358 that only postmitotic cells of the VZl express p19INK4d is that some actively dividing VZ Rabbit Polyclonal to GRK5 progenitor cells temporarily express p19INK4d before they undergo another round of interkinetic nuclear migration. Although our studies cannot exclude this as a formal possibility for VZ cells, it is likely to be the case for the cells of the RMS. However, the cells situated within the SVZa show the highest levels of BrdU incorporation and the lowest levels of p19INK4d expression. Therefore, we have proposed that cells traversing the RMS may undergo dedifferentiation before successive rounds of cell division. The different patterns of p19INK4deborah reflection displayed by the.