Astrocytes become active form to proliferate and migrate around the lesion site induced by the injury to the central nervous system (CNS). We have already reported that expression of auaporin-4 (AQP4), one of the water channels exclusively detected in the brain, up-regulated in reactive astrocytes after the brain injury. AQP4 seemed to have an important role in astroglial activation, however its molecular mechanism is largely unknown.
In the present study, we examined AQP4 function in reactive astrocytes compared between wild-type (WT) and AQP4-deficient (AQP4/KO) mice after a stab wound to the cerebral cortex. To examine activity of astrocytes, proliferating cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) incorporated in the drinking water provided to mice. By the immunofluorescent analysis using anti-BrdU antibody, astrocyte reactivity was at high level around the lesion site for WT mice 3 days after the stab wound to the brain, while it was much less for AQP4/KO mice. To identify the molecules related in injured mouse brain, we performed microarray analysis and found that more than 400 genes around the lesion site were upregulated 3 days after the wounding in WT mice.
Surprisingly, most of these up-regulations were significantly attenuated in AQP4/KO mice. We focused on to osteopontin (OPN) because real-time RT-PCR and immunofluorescence showed that extremely high for WT mice, however its expression was much lower in AQP4/KO mice. Since OPN is one of a pro-inflammatory cytokine inducer, we examined some of these gene expressions around lesion site. As expected, the up-regulation of pro-inflammatory cytokines was intensely high for WT mice, while it was significantly attenuated in AQP4/KO mice.
Taken together, these results suggested that AQP4 plays an important role in neuroimmunological function in collaboration with OPN under pathological conditions in the CNS.