Calcium signaling in and between brain astrocytes and endothelial cells

Two in vitro co-culture models of the blood-brain barrier (BBB) were developed using primary rat cortical astrocytes and ECV304 endothelial cells or primary rat brain capillary endothelial cells. We showed that intercellular calcium (Ca2+) waves can mediate bidirectional astrocyte-endothelial Ca2+ signal communication in both co-culture models. It appears that two signaling pathways are involved. A first mechanism is related to intercellular diffusion of the Ca2+ mobilizing messenger inositoltrisphosphate (IP3) through gap junctions as studied with flash photolysis of caged-IP3. A second pathway involves extracellular diffusion of a purinergic messenger as studied with the purinergic inhibitors apyrase and suramin. Using gap junction-deficient HeLa cells and connexin-GFP transfected HeLa cells, the existence of the two Ca2+ signaling pathways mentioned was confirmed and their subcellular characteristics and differences were further studied. We also suggest that astrocytic intercellular Ca2+ waves may be accompanied by and contribute to some of the ionic shifts observed in the brain upon traumatic brain injury. Further work will be needed to confirm our data in brain slices or even in the brain in vivo, and to establish the role and function of intercellular Ca2+ signals at the BBB.