Density dependent modulation of cell cycle protein expression in astrocytes

Abstract

The proliferation of type-1 astrocytes is strongly inhibited by homotypic cell-contact. To examine the mechanisms mediating this inhibition of proliferation, the expression of cell cycle related proteins was compared between exponential growth-phase and contact-inhibited astrocytes. Expression of the cyclin-dependent kinase (Cdk) inhibitor p27Kip1 was upregulated 10-fold in confluent compared with growth-phase cultures. Density-induced expression of p27Kip1 was reversible. When confluent cultures of astrocytes expressing high levels of p27Kip1 were replated at low density, the expression of p27Kip1 decreased rapidly. In contrast to p27Kip1, the expression levels of the cell cycle protein, cyclin A was decreased ten-fold in confluent cultures compared with those in growth phase. In addition, the ratio of hyperphosphorylated to hypophosphorylated retinoblastoma protein (pRb) decreased concomitantly with the increase of p27Kip1 and the decrease of cyclin A levels. These results suggest that increased expression of p27Kip1 and decreased expression of cyclin A underlie the reduction in proliferation of contact inhibited astrocytes. High levels of mitogenic stimulation could transiently override contact-dependent inhibition of astrocyte proliferation. Addition of exogenous epidermal growth factor (EGF) resulted in elevated proliferation at high density and formation of multiple cell layers. Addition of EGF did not substantially alter levels of p27Kip1 or cyclin A, but did elevate the levels of cyclin D1. Such changes in cell cycle protein expression may contribute to elevated cell proliferation seen in reactive gliosis after injury to the adult CNS.

Olig2-dependent developmental fate switch of NG2 cells

“…Furthermore, the normal increase in the number of EGFP-negative existing (normal) astrocytes seen in the Ctr neocortex between P4 and P21 did not occur in the Cko neocortex. As astrocytes are known to occupy non-overlapping territories (Bushong et al, 2002), there may be a contact-dependent mechanism that regulates their density (Nakatsuji and Miller, 2001).…”

Ganglioside GM3 promotes cell migration by regulating MAPK and c-Fos/AP-1

“…Especially, the accumulation of cyclin D1 results in the cyclin D-cdk4/6 complex formation, leading to a redistribution of p21 and p27 (Danen and Yamada, 2001). In addition, Nakatsuji and Miller (2001) have reported that GM3 showed no cytotoxic effects on the fibroblast, however, decreased expression of p27 was detected. In our study, WT MEFs showed the typical expression pattern of cyclin D1, p21 and p27 during their process of confluency, although GM3 KO MEFs did not show any features of cell cycle arrest (Figure 5c).…”

Retinoic Acid Signaling Identifies a Distinct Precursor Population in the Developing and Adult Forebrain

“…These cells also express p27Kip1 (Fig. 2 I), an RAregulated modulator of cell proliferation associated with neural precursors as well as astrocytes in a variety of mitotic states (Dirks et al, 1997;Legrier et al, 2001;Nakatsuji and Miller, 2001;Doetsch et al, 2002). The coincident expression of three characteristic markers (Nestin, Sox1, and Sox2) as well as an established RA-sensitive regulator of cell proliferation (p27Kip1) suggests that RA-activated cells in the SVZ are mitotically active precursors rather than recently postmitotic or differentiated neurons.…”