Association of annexin A5 with Na+/Ca2+ exchanger and caveolin-3 in non-failing and failing human heart.

Annexin A5 is a Ca2+ dependent phosphatidylserine binding protein mainly located in the T-tubules and sarcolemma of cardiomyocytes. Our objectives were to determine whether annexin A5 was associated with various protein(s) and whether such an association was modified in failing (F) hearts. The association between annexin A5 and the cardiac Na+/Ca2+ exchanger (NCX) was demonstrated by immunohistofluorescence, annexin A5-biotin overlay and co-immunoprecipitations (IPs) performed with microsomal preparations (MPs) from non-failing (NF) (n = 8) and F (dilated cardiomyopathy, n = 7) human hearts. We moreover found caveolin-3 in the immunoprecipitates, indicating the presence of multimolecular subsarcolemmal complexes. Surface plasmon resonance assays in NF MPs allowed us to demonstrate direct interaction between the NCX and caveolin-3 and immobilized annexin A5. Interaction was Ca2+-dependent and inhibited by the specific antibody. In addition, dissociation by zwittergent 3-14 (ZW 3-14) of the complexes from MPs increased specific interactions. In F hearts, specific interactions were blunted in native MPs but were fully recovered after treatment with ZW 3-14. In conclusion, we demonstrated that a direct interaction between annexin A5 and the cardiac NCX occurs in complexes including caveolin-3. In F hearts, despite the increase in the exchanger level, almost all of the NCX was involved in complexes. These interactions probably occurred in the intracytoplasmic regulatory loop of the exchanger, suggesting a different regulation of the exchanger in heart failure, consistent with a role in altered Ca2+ handling.