Hypoxia/ischemia up-regulates Id2 expression in neuronal cells in vivo and in vitro

• Id2 mRNA and protein are up-regulated in neuronal cells in vivo and in vitro.
• The increased expression of Id2 might be associated with neuronal apoptosis.
• The up-regulation of Id2 is consistent with the enhanced expression of Bax.
• Co-localization of Id2, TUNEL and NeuN was observed in ischemic neurons.

Inhibitor of DNA binding/differentiation 2 (Id2) belongs to a family of transcriptional modulators characterized by a helix-loop-helix (HLH) motif that lacks the basic amino acid domain necessary to bind DNA. The aim of this study was to obtain a better understanding of the role of Id2 in hypoxia/ischemia (H/I)-induced neuronal apoptosis. Following H/I induction in a rat model of middle cerebral artery occlusion (MCAO)/reperfusion, the number of TUNEL-positive cells in cerebral cortices of the penumbra area increased gradually, while the Id2 mRNA and protein expression were also significantly up-regulated. The hypoxia-mimetic, cobalt chloride (CoCl2)-treated rat neuroblastoma B35 cell line also demonstrated enhanced Id2 mRNA and protein expression as well as increased number of cells in the sub-G1 populations after H/I exposure. Consistently, the expression of Bax, a proapoptotic protein, was also up-regulated in vivo and in vitro. Moreover, triple immunofluorescence demonstrated the obvious co-localization of Id2, TUNEL and NeuN in neurons of the penumbra area. These data suggest that H/I up-regulates Id2 expression in neuronal cells, and Id2 might play an important role in H/I-induced neuronal apoptosis.