p15(INK4b) regulates cell cycle signaling in hippocampal astrocytes of aged rats

Cyclin-dependent kinase inhibitor p15(INK4b) is thought to be an important player in regulating astrocytic cell cycle. However, little is known with regard to the expression of p15(INK4b) and its function in hippocampal astrocytes. This study evaluated the expression of p15(INK4b) and its function during different development stages in hippocampal astrocytes.

In this study, we cultured hippocampal astrocytes from neonatal adult and aged rats. The expression of p15(INK4b) in neonatal, adult and aged astrocytes was examined. Short interfering RNA (siRNA) was then used to study the functional effects of p15(INK4b) down-regulation during cell cycle regulation.

We found the expression of p15(INK4b) in hippocampal astrocytes was detectable on postnatal day 7, was expressed at moderate levels in adult mice (9 months old) astrocytes and peaked in aged rat (24 months old) astrocytes. Incubation with siRNA significantly suppressed p15(INK4b) expression at the mRNA and protein levels in astrocytes. Down-regulation of p15(INK4b) increased [(3)H]-thymidine incorporation into DNA and allowed cells to pass the G0/G1-S checkpoint in aged but not in neonatal or adult astrocytes.

These observations suggest p15(INK4b) is expressed at a steady level in neonatal and adult rat hippocampal astrocytes with no effect on cell cycle regulation. Importantly, aged astrocyte cell cycle regulation was significantly affected by high expression levels of p15(INK4b) suggesting a role for p15(INK4b) in cell cycle regulation when it is expressed at high but not moderate or low levels in hippocampal astrocytes.