Roles of Id1 in Aβ-induced cell cycle re-entry in postmitotic neurons

Abstract

Senile plaque containing aggregation of amyloid-beta peptide (Aß) is one of the major hallmarks of Alzheimer's disease (AD). Several lines of evidence suggest the correlation between Aß-induced neuronal death and cell cycle re-entry, but the underlying mechanisms remain unclear. Inhibitor of differentiation 1 (Id1) is involved in the regulation of cellular differentiation and cell cycle progression. Doublecortin (DCX) is a neuronal lineage marker that expresses at proliferative stage and early differentiating stage. Subsequent to exit from cell cycle, NeuN is expressed at early and late postmitotic maturation stage. Glial fibrillary acidic protein (GFAP) is a specific protein marker for astrocyte. Bromodeoxyuridine (BrdU) is incorporated during DNA synthesis in proliferating cells. Thus, colocalization of BrdU with DCX or NeuN in fully differentiated neurons may serve as a marker for cell cycle re-entry. We investigate whether Id1 induction by Aß causes cell cycle re-entry in postmitotic neurons leading to cell death. The expression profiles of Id1 and cyclins upon exposure to Aß25-35 were determined by real-time RT-PCR and Western blotting in primary cortical neurons. Immunocytochemistry was utilized to quantitatively determine the numbers of DCX+/BrdU+, NeuN+/BrdU+, GFAP+/BrdU+, and caspase 3+/BrdU+ cells after Aß25-35 treatment. Finally, siRNA was employed to suppress Id1 expression in cortical neurons. Aß25-35 induced Id1, cyclin D1, E1 and A2 mRNA in primary cortical neurons. Aß25-35 and Aß1-42 induced expression of Id1 protein. Aß25-35 increased the numbers of DCX+/BrdU+, NeuN+/BrdU+, GFAP+/BrdU+, and caspase 3+/BrdU+ cells, indicating Aß25-35-mediated interference of neuronal differentiation, increases in astrocytosis, and a heightened extent of apoptosis in cultured cortical neurons. Suppression of Id1 expression by siRNA in part recovered cell survival after Aß25-35 and Aß1-42 treatments in primary cortical neurons. Id1 induction by Aß may play a significant role in Aß-induced cell cycle re-entry and neuronal death.