The speed of swelling kinetics modulates cell volume regulation and calcium signaling in astrocytes: A different point of view on the role of aquaporins

Abstract

Regulatory volume decrease (RVD) is a process by which cells restore their original volume in response to swelling. In this study, we have focused on the role played by two different Aquaporins (AQPs), Aquaporin-4 (AQP4), and Aquaporin-1 (AQP1), in triggering RVD and in mediating calcium signaling in astrocytes under hypotonic stimulus. Using biophysical techniques to measure water flux through the plasma membrane of wild-type (WT) and AQP4 knockout (KO) astrocytes and of an astrocyte cell line (DI TNC1) transfected with AQP4 or AQP1, we here show that AQP-mediated fast swelling kinetics play a key role in triggering and accelerating RVD. Using calcium imaging, we show that AQP-mediated fast swelling kinetics also significantly increases the amplitude of calcium transients inhibited by Gadolinium and Ruthenium Red, two inhibitors of the transient receptor potential vanilloid 4 (TRPV4) channels, and prevented by removing extracellular calcium. Finally, inhibition of TRPV4 or removal of extracellular calcium does not affect RVD. All together our study provides evidence that (1) AQP influenced swelling kinetics is the main trigger for RVD and in mediating calcium signaling after hypotonic stimulus together with TRPV4, and (2) calcium influx from the extracellular space and/or TRPV4 are not essential for RVD to occur in astrocytes. Main Points: (1) The speed of swelling kinetics is the main trigger for Regulatory Volume Decrease (RVD) and for calcium response in astrocytes; (2) Calcium influx from the extracellular space and TRPV4 are not essential for RVD.

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

“…In Müller glia (an “astrocyte-like” glial cell type in the retina), rapid water influx through AQP4 produces membrane stretch, which then activates nearby stretch-activated TRPV4 channels to permit Ca 2+ entry to promote RVD (Iuso and Krizaj 2016; Jo et al 2015). While the requirement of TRPV4 opening and Ca 2+ influx in astrocytic RVD has recently been questioned (Mola et al 2016), these findings suggest that rapid water influx is an important trigger for subsequent astrocyte volume regulation (Benfenati et al 2011; Mola et al 2016). …”

TRPV4 Contributes to Resting Membrane Potential in Retinal Müller Cells: Implications in Cell Volume Regulation

“…In astrocyte glial cells, the Transient Receptor Potential Vanilloid type 4 (TRPV4), a calcium‐permeable nonselective cation channel, was proposed as a mediator in the swelling‐induced elevation of intracellular Ca 2+ related to cell volume regulation [Benfenati et al, ; Butenko et al, ]. Nevertheless, a recent study suggests that TRPV4 and Ca 2+ do not seem to be essential for RVD to occur [Mola et al, ]. Until recently, the participation of TRPV4 in the RVD of Müller glial cells was less explored.…”

TRPV4-AQP4 interactions ‘turbocharge’ astroglial sensitivity to small osmotic gradients

“…5 A similar conclusion was concurrently reached for cortical astroglia. 7 The study also addressed the potential contribution of TRPV4-AQP4 interactions to regulatory volume decrease (RVD), a key process that protects cells against pathological swelling. Its precise mechanism is cell type-dependent but was suggested to require TRPV4 and AQP4 in cortical astrocytes.…”