SUR1‐TRPM4 and AQP4 form a heteromultimeric complex that amplifies ion/water osmotic coupling and drives astrocyte swelling

Abstract

Astrocyte swelling occurs after central nervous system injury and contributes to brain swelling, which can increase mortality. Mechanisms proffered to explain astrocyte swelling emphasize the importance of either aquaporin-4 (AQP4), an astrocyte water channel, or of Na 1 -permeable channels, which mediate cellular osmolyte influx. However, the spatio-temporal functional interactions between AQP4 and Na 1 -permeable channels that drive swelling are poorly understood. We hypothesized that astrocyte swelling after injury is linked to an interaction between AQP4 and Na 1 -permeable channels that are newly upregulated. Here, using co-immunoprecipitation and F€ orster resonance energy transfer, we report that AQP4 physically co-assembles with the sulfonylurea receptor 1-transient receptor potential melastatin 4 (SUR1-TRPM4) monovalent cation channel to form a novel heteromultimeric water/ion channel complex. In vitro cell-swelling studies using calcein fluorescence imaging of COS-7 cells expressing various combinations of AQP4, SUR1, and TRPM4 showed that the full tripartite complex, comprised of SUR1-TRPM4-AQP4, was required for fast, high-capacity transmembrane water transport that drives cell swelling, with these findings corroborated in cultured primary astrocytes. In a murine model of brain edema involving cold-injury to the cerebellum, we found that astrocytes newly upregulate SUR1-TRPM4, that AQP4 co-associates with SUR1-TRPM4, and that genetic inactivation of the solute pore of the SUR1-TRPM4-AQP4 complex blocked in vivo astrocyte swelling measured by diolistic labeling, thereby corroborating our in vitro functional studies. Together, these findings demonstrate a novel molecular mechanism involving the SUR1-TRPM4-AQP4 complex to account for bulk water influx during astrocyte swelling. These findings have broad implications for the understanding and treatment of AQP4-mediated pathological conditions. K E Y W O R D Sastrocyte, ion channels, ions, osmosis, water

Cell Volume Control in Healthy Brain and Neuropathologies

“…In an animal stroke model, SUR1 expression steadily increased up to 6 hours after the initiation of ischemia, and treatment with the sulfonylurea inhibitor glibenclamide reduced brain edema and tissue damage (Simard et al, 2006). Their most recent work demonstrated that SUR1-TRPM4 can form a tripartite complex with AQP4, and such association mediates the pathological, high-capacity influx of water that is ~7.6-fold higher than that stimulated by TRPM4 alone, and ~3.2-fold higher than that induced by the SUR1-TRPM4 assembly (Stokum et al, 2018) (Fig. 12.3E).…”

“…Perhaps, AQP4 plays more complex roles in regulating water movement and osmotic gradients, via modulating activities of ion channels and transporters. E.g., AQP4 can form functional complexes with the non-selective cation channel TRPV4 (Benfenati et al, 2011), the SUR1-TRPM4 channel complex (Stokum et al, 2018), and also regulates the activity of VRAC (Benfenati et al, 2007). …”

“…12.3E). TRPM4 knockout mice displayed reduced astrocytic cell swelling following cold-injury (Stokum et al, 2018), but additional studies are needed to determine if this heteromeric complex is critical for cell swelling in other pathologies.…”

“…Besides direct effect on water fluxes, the observed changes in AQP4-null animals can be explained by other factors, such as adaptive changes in the expression of other genes or the indirect effects of AQP4 on activities of ion transporters and channels. This protein has been shown to physically associate with or functionally regulate activities of the K + channel Kir4.1, volume-sensitive VRAC, and nonselective cation channels TRPV4 and TRPM4/SUR1 [(Nagelhus, Mathiisen, & Ottersen, 2004; Benfenati et al, 2007; Benfenati et al, 2011; Stokum et al, 2018), but see an alternative opinion for Kir4.1 (Zhang & Verkman, 2008)]. Therefore, effects of AQP4 deletion may develop independent of or be upstream of cell volume changes [see (Assentoft, Larsen, & MacAulay, 2015) for detailed discussion].…”

A Precision Medicine Approach to Cerebral Edema and Intracranial Hypertension after Severe Traumatic Brain Injury: Quo Vadis?

“…Supramolecular assemblies of the M23 AQP4 isoform tetramers have been reported to aggregate into orthogonal arrays and facilitate water movement [80, 92]. This pathway has reported connections with other molecular contributors to cerebral edema, including a tripartite-complex association between Sur1-Trpm4-AQP4, a glutamate uptake channel EAAT1/2, and inflammatory cytokines [79, 93, 94]. Increased intracellular water movement mediated by AQP4 channels on perivascular astrocytic foot processes contributes to cytotoxic edema.…”