Ca²⁺ Nanodomain-Mediated Component of Swelling-Induced Volume-Sensitive Outwardly Rectifying Anion Current Triggered by Autocrine Action of ATP in Mouse Astrocytes

Abstract

The volume-sensitive outwardly rectifying (VSOR) anion channel provides a major pathway for anion transport during cell volume regulation. It is typically activated in response to cell swelling, but how the channel senses the swelling remains unclear. Meanwhile, we recently found that in mouse astrocytes the channel is activated by an inflammatory chemical mediator, bradykinin, without cell swelling and that the activation is regulated via high concentration regions of intracellular Ca²⁺ ([Ca²⁺]_i) in the immediate vicinity of open Ca²⁺-permeable channels, so-called Ca²⁺ nanodomains. Here we investigated whether a similar mechanism is involved in the swelling-induced VSOR channel activation in the astrocytes. A hypotonic stimulus (25% reduction in osmolality) caused the [Ca²⁺]_i rises in the astrocytes, and the rises were abolished in the presence of an ATP-degrading enzyme, apyrase (10 U/ml). Application of ATP (100 µM) under isotonic conditions generated the current through VSOR channels via Ca²⁺ nanodomains, as bradykinin does. The current induced by the hypotonic stimulus was suppressed by ∼40% in the Ca²⁺-depleted condition where the ATP-induced VSOR current was totally prevented. Thus the swelling-induced VSOR channel activation in mouse astrocytes is partly regulated via Ca²⁺ nanodomains, whose generation is triggered by an autocrine action of ATP.

DCPIB, the Proposed Selective Blocker of Volume-Regulated Anion Channels, Inhibits Several Glutamate Transport Pathways in Glial Cells

“…7A, ATP caused transient stimulation of D-[ 3 H]aspartate release that was strongly inhibited by DCPIB, consistent with the previous findings from this and other laboratories (see for example Abdullaev et al, 2006;Heacock et al, 2006;Liu et al, 2009). These data do not necessarily point to activation of Best1 channels because ATP was previously shown to activate the DCPIB-sensitive VRAC channels (Mongin and Kimelberg, 2002;Liu et al, 2009;Akita et al, 2011). VRAC can be regulated by various G-protein-coupled receptors even in the absence of cell swelling (reviewed in Fisher et al, 2010).…”

“…The niflumic acid sensitivity clearly discriminated this release pathway from VRAC channels. VRAC can be also activated in a Ca 21 -sensitive manner by ATP and other G-protein-coupled receptors but they are largely insensitive to niflumic acid (Pedersen et al, 1998;Mongin and Kimelberg, 2002;Fisher et al, 2010;Akita et al, 2011). The niflumic acid sensitivity, however, is not sufficient to infer contribution of Best1 because members of a different family of the Ca 21 -activated Cl 2 channels, anoctamins or TMEM16, are also potently blocked by this compound (Verkman and Galietta, 2009).…”

Volume-regulated anion channel—a frenemy within the brain

“…The impetus for these studies was created by an early report in hepatoma cells, which proposed that release of ATP and stimulation of purinergic P2Y receptors serve as an obligatory autocrine signal linking cell swelling to VRAC opening [207]. Although the obligatory nature of ATP signaling had not been confirmed for brain cells, or any other cell types, we and others found that activation of P2Y receptors in astrocyte cultures by ATP and other purinergic agonists produces limited, Ca 2+ -dependent stimulation of VRAC [7, 42, 131, 134, 199]. The ATP-induced increases in VRAC activity and osmolyte release were smaller than that induced by cellular swelling and involved distinct signaling mechanisms.…”

Expression and Functional Significance of the Ca2+-Activated Cl-Channel ANO6 in Dendritic Cells

“…A possible mechanism of ANO6-regulated DC migration may include ANO6-mediated local volume changes required for lamellipodium growth and/ or cell rear end retraction by uptake and/or efflux of osmotically active Cl -. nanodomains induces ORCC channel activation [70]. Moreover in astrocytes, inflammatory chemical mediator, bradykinin [71] and extracellular ATP [70] lead to ORCC activation by [Ca 2+ ] i rise in Ca 2+ nanodomains without cell swelling.…”

“…nanodomains induces ORCC channel activation [70]. Moreover in astrocytes, inflammatory chemical mediator, bradykinin [71] and extracellular ATP [70] lead to ORCC activation by [Ca 2+ ] i rise in Ca 2+ nanodomains without cell swelling. ANO6 may further provide a positive feedback to store-operated Ca 2+ influx, which is required for DC migration [7].…”