Temporal-spatial expressions of p27kip1 and its phosphorylation on Serine-10 after acute spinal cord injury in adult rat:: Implications for post-traumatic glial proliferation

p27(kip1), as a member of Cip/Kip family of cyclin-dependent kinase inhibitors, plays important roles in cell cycle regulation and neurogenesis in the developing central nervous system. Serine-10 is the major phosphorylation site of p27(kP1), and post-translational regulation of p27(kip1) by different phosphorylation events is critical for its function. To elucidate the expressions and possible functions of p27(kip1) and its phosphorylation in central nervous system lesion and repair, we performed an acute spinal cord contusion injury model in adult rats. Our work studied the temporal-spatial expression patterns of p27(kip1) and Serine-10 phosphorylated p27(kip1) (p-p27s 10). Western blot analysis showed p27(kip1) level significantly decreased at day 3 after damage, while p-p27s10 was detected at a high-level at the same time reaching the uninjured level. Moreover, immunofluorescence double labeling suggested these changes were striking in microglia and astrocytes, which were largely proliferated. Immunohistochemical analysis revealed subcellular localization changes of p27(kip1) and p-p27s10 staining between nucleus and cytoplasm after injury in about 20% of total positive cells including neurons and glial cells. We also investigated the increased interactions of p27(kip1) and p-p27s 10 with CRM1 3 days after injury by co-immunoprecipitation studies. Taken together, we hypothesized spinal cord injury stimulated mitogenic signals to induce a serine-threonine kinase KIS (kinase interacting stathmin) to phosphorylate p27(kip1) on Serine-10, so that p27(kip1) could bind to CRM1 and be exported from nuclei for degradation. Such an event facilitated cell cycle progression of glial cells, especially microglia and astrocytes which had a prevalent proliferation. (C) 2008 Elsevier Ltd. All rights reserved.