Intercommunications between brain capillary endothelial cells and glial cells increase the transcellular permeability of the blood–brain barrier during ischaemia

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Abstract
Increased cerebrovascular permeability is an important factor in the development of cerebral oedema after stroke, implicating the blood–brain barrier (BBB). To investigate the effect of hypoxia on the permeability changes, we used a cell culture model of the BBB consisting of a co‐culture of brain capillary endothelial cells and glial cells. When endothelial cells from this co‐culture model were submitted alone to hypoxic conditions, long exposures (48 h) were necessary to result in an increase in endothelial cell monolayer permeability to [3H]inulin. When endothelial cells were incubated in presence of glial cells, a huge increase in permeability occurred after 9 h of hypoxic conditions. Oxygen glucose deprivation (OGD) resulted in a much shorter time (i.e. 2 h) required for an increase in permeability. We have demonstrated that this OGD‐induced permeability increase involves a transcellular rather than a paracellular pathway. Conditioned medium experiments showed that glial cells secrete soluble permeability factors during OGD. However, endothelial cells have to be made sensitive by OGD in order to respond to these glial soluble factors. This work shows that an early cross‐talk between glial and endothelial cells occurs during ischaemic stroke and alters BBB transcellular transport by means of glial factor secretions.