Molecular Mechanisms Governing Aquaporin-4 Mediated Cerebral Edema

Abstract

Cerebral edema, which accompanies all forms of CNS injury, is a pressing clinical problem. Aquaporin-4 (Aqp4), a passive astrocytic transmembrane water channel, has a central role in cerebral edema formation. However, the molecular mechanisms that govern Aqp4 activity after CNS injury are incompletely understood. Here, I sought to characterize the role of two Aqp4-governing mechanisms in Aqp4-mediated cerebral edema: (i) Aqp4 expression and subcellular localization, and (ii) transmembrane osmotic gradients. I determined that, following ischemic stroke, Aqp4 is upregulated to a greater extent in white matter astrocytes versus grey matter astrocytes. I hypothesized that regional heterogeneity in Aqp4 expression generates regional differences in tissue swelling. I report that white matter exhibits greater swelling than grey matter after cerebral ischemia. These results demonstrate a direct correlation between the pattern of Aqp4 expression and the differential propensity of white matter versus grey matter to swell after ischemic insult. Astrocyte swelling occurs after CNS injury and manifests as brain swelling. Mechanisms proffered to explain astrocyte swelling emphasize the importance of either aquaporin-4 (Aqp4), an astrocyte water channel, or of Na+ channels and pumps, which mediate cellular osmolyte influx. However, it is unclear how Na+ and water interact to drive swelling. I hypothesized that, after injury, Aqp4 physically co-associates with newly-expressed Na+ channels, accounting for astrocyte swelling. I report that the Aqp4 water channel physically co-assembles with the Sur1-Trpm4 monovalent cation channel to form a novel heteromultimeric water/ion channel. In vitro functional studies showed that ion and water flux through the Sur1-Trpm4-Aqp4 complex synergize to mediate fast, high-capacity water transport that generates cell swelling. In a murine model of brain edema, astrocytes de novo expressed Sur1-Trpm4, which co-associated with Aqp4. Genetic inactivation of the Sur1-Trpm4-Aqp4 complex via knockout of Trpm4 blocked astrocyte swelling, corroborating in vitro functional studies of cell swelling. Together, these findings demonstrate a novel molecular mechanism involving the Sur1-Trpm4-Aqp4 complex to account for bulk water influx during astrocyte swelling. Furthermore, these data indicate that ion channel antagonists could be used to indirectly modulate Aqp4.