Tumour necrosis factor alpha inhibits purinergic calcium signalling in blood-brain barrier endothelial cells

The breaching of the blood-brain barrier is an essential aspect in the pathogenesis of neuroinflammatory diseases, in which tumour necrosis factor alpha (TNF-alpha) as well as endothelial calcium ions play a key role. We investigated whether TNF-alpha could influence the communication of calcium signals between brain endothelial cells (GP8 and RBE4). Intercellular calcium waves triggered by mechanical stimulation or photoliberation of InsP(3) in single cells were significantly reduced in size after TNF-alpha exposure (1000 U/mL, 2 and 24 h). Calcium signals are communicated between cells by means of gap junctional and paracrine purinergic signalling. TNF-alpha significantly inhibited gap junctional coupling, stimulated the basal release of ATP, and dose-dependently blocked the triggered component of ATP release. The cytokine displayed similar effects on the uptake of a fluorescent reporter dye into the cells. Previous work with connexin mimetic peptides demonstrated that the triggered ATP release in these cells is connexin-related; these peptides did, however, not influence the elevated basal ATP release caused by TNF-alpha. We conclude that TNF-alpha depresses calcium signal communication in blood-brain barrier endothelial cells, by reducing gap junctional coupling and by inhibiting triggered ATP release. The cytokine thus inhibits connexin-related communication pathways like gap junctions and connexin hemichannels.