Regulation of functional expression of the electrogenic sodium bicarbonate cotransporter 1, NBCe1 (SLC4A4), in mouse astrocytes

Abstract

The electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4) is expressed in many cell types and is a major regulator of intracellular, and extracellular pH. In astrocytes, membrane depolarization leads to intracellular alkalinization through the activation of NBCe1. However, the molecular mechanisms regulating functional expression of NBCe1 in astrocytes are largely unknown. Astrocytes also express voltage-dependent K+ channels that are activated after depolarization and are sensitive to the K+ blocker 4-aminopyridine (4AP). Using acute hippocampal slices and primary hippocampal and cortical astrocyte cultures, we have investigated the role of 4AP for the regulation of NBCe1 and elucidated the underlying signaling pathways by quantitative RT-PCR, immunoblotting, biotinylation of surface proteins, immunofluorescence, and intracellular H+ recording using the H+-sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein. The results show significant upregulation of NBCe1 transcript, protein, and surface expression after the application of 4AP in both hippocampal slices and astrocyte cultures, effects that were suppressed after the inhibition of c-jun N-terminal kinase (JNK), proto-oncogene tyrosine-protein kinase Src, and Src/extracellular-signal-regulated kinases signaling. In the presence of 4AP, the rate and amplitude of intracellular H+ changes upon challenging NBCe1 increased in wild-type astrocytes but not in cortical astrocytes from NBCe1-deficient mice. 4AP-dependent effects were suppressed after the inhibition of JNK and Src signaling. Our results demonstrate that transcriptional regulation and targeting of NBCe1, as well as functional operation of NBCe1, may occur through multiple signaling pathways. GLIA 2015;63:1226-1239. Main Points: 4AP up-regulates transcript, protein and functional expression of NBCe1 in astrocytes. Transcriptional expression and targeting of NBCe1 occurs through multiple signaling pathways. 4AP-dependent regulation of NBCe1 in cortical astrocytes is not related to membrane depolarization.