Inward Operation of Sodium-Bicarbonate Cotransporter 1 Promotes Astrocytic Na+ Loading and Loss of ATP in Mouse Neocortex during Brief Chemical Ischemia

Abstract

Ischemic conditions cause an increase in the sodium concentration of astrocytes, driving the breakdown of ionic homeostasis and exacerbating cellular damage. Astrocytes express high levels of the electrogenic sodium-bicarbonate cotransporter1 (NBCe1), which couples intracellular Na+ homeostasis to regulation of pH and operates close to its reversal potential under physiological conditions. Here, we analyzed its mode of operation during transient energy deprivation via imaging astrocytic pH, Na+, and ATP in organotypic slice cultures of the mouse neocortex, complemented with patch-clamp and ion-selective microelectrode recordings and computational modeling. We found that a 2 min period of metabolic failure resulted in a transient acidosis accompanied by a Na+ increase in astrocytes. Inhibition of NBCe1 increased the acidosis while decreasing the Na+ load. Similar results were obtained when comparing ion changes in wild-type and Nbce1-deficient mice. Mathematical modeling replicated these findings and further predicted that NBCe1 activation contributes to the loss of cellular ATP under ischemic conditions, a result confirmed experimentally using FRET-based imaging of ATP. Altogether, our data demonstrate that transient energy failure stimulates the inward operation of NBCe1 in astrocytes. This causes a significant amelioration of ischemia-induced astrocytic acidification, albeit at the expense of increased Na+ influx and a decline in cellular ATP.

Deletion of Slc9a1 in Cx3cr1+ cells stimulated microglial subcluster CREB1 signaling and microglia-oligodendrocyte crosstalk

“…While Hvcn1 (encoding Hv1) gene expression in MG3 subpopulation is relatively low compared to other microglia/macrophages (MG1, 2, 4, and 5; 2.13-fold decrease, p < 0.0001), we found a small but significant elevation of Hvcn1 gene expression in the MG3 of cKO brains post-stroke (1.32-fold increase, p < 0.01; Additional file 1 : Figure S7), which shall alkalinize pH i . Other potential pH i regulating mechanism includes the Na + /HCO3 − cotransporter (NBCe1), which facilitates HCO3 − outward transport (coupled with Na + efflux) to buffer extracellular H + loads in response to neuronal activity [ 64 ], or stimulates inwardly directed HCO3 − transport (coupled with Na + influx) in ischemic stroke conditions [ 65 ]. We detected similarly low levels of Slc4a4 (encoding NBCe1) gene expression in MG3 compared to other microglia/macrophage subpopulations, but stroke triggered a small but significant increase of Slc4a4 gene expression in the MG3 of cKO brains (1.36-fold increase, p < 0.01; Additional file 1 : Figure S7).…”

The deletion of AQP4 and TRPV4 affects astrocyte swelling/volume recovery in response to ischemia-mimicking pathologies

“…Neuronal activity and local increase of [K + ] O causes astrocyte membrane depolarization during OGD, which activates astrocytic sodium-bicarbonate cotransporters (NBCs) ( Köhler et al, 2018 ). Their activation then increases intracellular Na + and simultaneously reduces the level of intracellular ATP in astrocytes ( Everaerts et al, 2023 ), and therefore contributes to their swelling ( Florence et al, 2012 ; Larsen and MacAulay, 2017 ). Interestingly, based on previous research Walch and Fiacco suggested in their review that the NBCs-dependent swelling occurs after approximately 10 min of the exposure to pathological conditions, and the first 10 min are NBCs-independent ( Walch and Fiacco, 2022 ).…”