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.

Blood Brain Barrier in Hypoxic-Ischemic Conditions

“…Some of the processes of astrocytes surrounding the blood capillaries reach neurons, transporting ions and other substances to sustain neurons and to regulate the extracellular environment (Fields and Stevens-Graham, 2002). Astrocytes have also been reported to act in conjunction with neurons in the regulation of cerebral microvascular permeability (Ballabh et al, 2004), particularly via dynamic Ca 2+ signaling between astrocytes and the endothelium (Braet et al, 2001;Zonta et al, 2003). However, in the developing brain, some neurons have been observed to make contact with the endothelial cells which has been thought to influence the induction of BBB properties in the endothelial cells .…”

Both sides now: multiple interactions of ATP with pannexin-1 hemichannels. Focus on “A permeant regulating its permeation pore: inhibition of pannexin 1 channels by ATP”

“…"Ca 2ϩ wave propagation" is the process whereby acute changes in cytosolic Ca 2ϩ levels within one cell can rapidly be propagated to nearby cells. Connexins and gap junctions play critical roles in Ca 2ϩ wave propagation in astrocytes and other cell types (9,26,27,45,49). One mechanism is that gap junction channels between adjacent cells directly facilitate the movement of Ca 2ϩ per se and Ca 2ϩ -mobilizing inositol 1,4,5-trisphosphate (IP 3 ) from one cell to another (9).…”

“…Connexins and gap junctions play critical roles in Ca 2ϩ wave propagation in astrocytes and other cell types (9,26,27,45,49). One mechanism is that gap junction channels between adjacent cells directly facilitate the movement of Ca 2ϩ per se and Ca 2ϩ -mobilizing inositol 1,4,5-trisphosphate (IP 3 ) from one cell to another (9). The alternative mechanism for wave propagation, as demonstrated by the Nedergaard group, is that one cell releases an extracellular signaling molecule, such as ATP, which then activates Ca 2ϩ -mobilizing receptors in nearby cells (Fig.…”

Modulation of Brain Hemichannels and Gap Junction Channels by Pro-Inflammatory Agents and Their Possible Role in Neurodegeneration

“…Cx37 and Cx43 also form myoendothelial gap junctions (137) that enable electrical coupling between muscle and endothelial layers and facilitate the synchronization of calcium waves and vasomotor activity along arterioles (137). Although, there is no in vivo evidence of direct intercellular coupling between astrocytes and endothelial cells at the capillary level, which might be prevented by the basal lamina between the two cell types, in vitro studies demonstrate the presence of weak electrical coupling between astrocytes and BBB endothelial cells, which is associated with propagation of both gap junction dependent and independent Ca 2+ waves (43). …”

“…In agreement with a role of hemichannels in ATP release during inflammatory conditions triggered by bacterial infections, Shigella infection of epithelial cells promote ATP release through Cx26 hemichannels, resulting in the activation of purinergic receptors on neighboring cells and bacterial dissemination (367). On the other hand, a reduction in gap junction communication between brain endothelial cells may impair astrocyte-endothelial cell calcium signaling (43) and increase BBB permeability by reducing tight junction formation. The latter is in agreement with the observation that block of GJCs increases transendothelial electrical resistance and enhanced paracellular flux of permeability tracers (250).…”