Astrocyte-endothelial cell calcium signals conveyed by two signalling pathways

Abstract

Astrocytes and endothelial cells are in close contact with each other at the blood-brain barrier, where important molecular transports take place. Despite these key morphological and functional properties, little is known regarding the dynamic signalling processes that occur between these two cell types. We investigated astrocyte-endothelial cell calcium signalling mechanisms in a coculture model prepared from primary rat cortical astrocytes and ECV304 cells. We used flash photolysis of caged inositol-trisphosphate (IP3) and gentle mechanical stimulation to trigger astrocyte-endothelial cell calcium signals and to investigate the underlying propagation mechanisms. Photolytically releasing IP3 in a single cell triggered increases in cytoplasmic calcium concentration that propagated between astrocytes and endothelial cells in either direction. These propagating calcium signals did not cross cell-free zones and were not affected by fast superfusion or by the purinergic inhibitors apyrase and suramin, indicating that they are communicated through an intracellular pathway in conjunction with gap junctions. Electrophysiological experiments confirmed a low degree of astrocyte-endothelial cell electrical cell-to-cell coupling. Mechanical stimulation of a single cell also triggered astrocyte-endothelial cell calcium signals but, in contrast to the former triggering mode, these signals crossed cell-free zones and were significantly inhibited by apyrase, thus indicating the involvement of an extracellular and purinergic messenger. Astrocyte-endothelial cell calcium signalling also occurred in cocultures prepared with astrocytes and primary rat brain capillary endothelial cells. We conclude that astrocytes and endothelial cells can exchange fast-acting calcium signals (time scale of seconds) that can be communicated through an intracellular/gap junctional pathway and an extracellular purinergic pathway.

Pancreatic Islet Production of Vascular Endothelial Growth Factor-A Is Essential for Islet Vascularization, Revascularization, and Function

“…For example, there is growing evidence that in the brain, vascular endothelial cells actively participate in communication of calcium signals or calcium signal propagation, a feature that until recently was attributed only to glial cells, neurons, or astrocytes (57). Astrocyte-endothelial cell cocultures demonstrated that calcium signals could be communicated bidirectionally utilizing both gap junctions and ATPmediated paracrine signaling through purinergic receptors (58,59). The components of both these signaling pathways are present in the ␤-cells and intra-islet endothelial cells (60,61).…”

Quinolinic acid induces NMDA receptor-mediated lipid peroxidation in rat brain microvessels

“…This would not invalidate the central hypothesis since the NMDA-R could exist within the membranes of the astrocytic processes 24,25 or, perhaps, on the autonomic nerve terminals 3,26 adjacent to the brain capillary barrier. Such NMDA-R could be involved in a Ca 2+ signalling cascade capable of activating glucose transporter (GLUT-1) 27 and/or producing CO 28 in brain endothelial cells. Thus NMDA-R and the associated Ca 2+ signalling in brain capillary system may be instrumental in the coupling of neuronal activity to the extraction of glucose from the blood stream.…”

The Blood-Brain Barrier/Neurovascular Unit in Health and Disease

“…Subsequent in vivo studies have indicated that cerebral microvessels can survive and maintain barrier integrity in areas of the brain that have experienced extensive astrocyte loss (Krum et al, 1997), although these results are in conflict with a more recent study that showed loss and restoration of barrier integrity in vivo following a temporary focal loss of astrocytes (Willis et al, 2004). It has also been speculated that astrocytes may act as intermediaries to or in conjunction with neurons in the moment-to-moment regulation of cerebral microvascular permeability (Ballabh et al, 2004), in particular via dynamic Ca 2ϩ signaling between astrocytes and the endothelium via gap junctions and purinergic transmission (Braet et al, 2001;Zonta et al, 2003).…”