Carbonic anhydrase XIV in the normal and hypertrophic myocardium

Abstract

Two AE3 transcripts, full-length (AE3fl) and cardiac (AE3c) are expressed in the heart. AE3 catalyzes electroneutral Cl−/HCO3− exchange across cardiomyocyte sarcolemma. AE proteins associate with carbonic anhydrases (CA), including CAII and CAIV, forming a HCO3− transport metabolon (BTM), increasing HCO3− fluxes and regulating cardiomyocytes pH. CAXIV, which is also expressed in the heart's sarcolemma, is a transmembrane enzyme with an extracellular catalytic domain. Herein, AE3/CAXIV physical association was examined by coimmunoprecipitation using rodent heart lysates. CAXIV immunoprecipitated with anti-AE3 antibody and both AE3fl and AE3c were reciprocally immunoprecipitated using anti-CAXIV antibody, indicating AE3fl–AE3c/CAXIV interaction in the myocardium. Coimmunoprecipitation experiments on heart lysates from a mouse with targeted disruption of the ae3 gene, failed to pull down AE3 with the CAXIV antibody. Confocal images demonstrated colocalization of CAXIV and AE3 in mouse ventricular myocytes. Functional association of AE3fl and CAXIV was examined in isolated hypertrophic rat cardiomyocytes, using fluorescence measurements of BCECF to monitor cytosolic pH. Hypertrophic cardiomyocytes of spontaneously hypertensive rats (SHR) presented elevated myocardial AE-mediated Cl−/HCO3− exchange activity (JHCO3− mM.min−1) compared to normal (Wistar) rats (7.5±1.3, n=4 versus 2.9±0.1, n=6, respectively). AE3fl, AE3c, CAII, CAIV, and CAIX protein expressions were similar in SHR and Wistar rat hearts. However, immunoblots revealed a twofold increase of CAXIV protein expression in the SHR myocardium compared to normal hearts (n=11). Furthermore, the CA-inhibitor, benzolamide, neutralized the stimulatory effect of extracellular CA on AE3 transport activity (3.7±1.5, n=3), normalizing AE3-dependent HCO3− fluxes in SHR. CAXIV/AE3 interaction constitutes an extracellular component of a BTM which potentiates AE3-mediated HCO3− transport in the heart. Increased CAXIV expression and consequent AE3/CAXIV complex formation would render AE3 hyperactive in the SHR heart.