Annexin VI is a member of a family of Ca(2+)-dependent
phospholipid-
binding proteins that is expressed in many tissues, including the heart. It is a regulator of membrane-associated events, including the skeletal muscle
ryanodine-sensitive Ca2+ release channel and the cardiac Na+/Ca2+ exchanger. The potential roles of
annexin VI in Ca2+ signaling in cardiac myocytes were evaluated by targeting its overexpression to the hearts of transgenic mice. Expression of full-length human
annexin VI cDNA was targeted to the heart using the
alpha-myosin heavy chain gene promoter (Subramaniam, A., W. K. Jones, J. Gulick, S. Wert, J. Neumann, and J. Robbins. J. Biol. Chem. 266: 24613-24620, 1991). Five transgenic lines exhibited at least 10-fold overexpression of
annexin VI protein in both atria and ventricles. Pathological evaluation indicated mice overexpressing
annexin VI had enlarged dilated hearts, acute diffuse
myocarditis, lymphocytic infiltration, moderate to severe
fibrosis throughout the heart, and mild
fibrosis around the pulmonary veins of the lungs. Contractile mechanics of cardiomyocytes isolated from hearts of transgenic animals showed frequency-dependent reduced percent shortening and decreased rates of contraction and relaxation compared with control animals. Cardiomyocytes isolated from transgenic animals had lower basal levels of intracellular free Ca2+ and a reduced rise in free Ca2+ following depolarization. After stimulation, intracellular free Ca2+ returned to basal levels faster in transgenic cells than in cells from control animals. These data demonstrate that the overexpression of
annexin VI in the heart disrupts normal Ca2+ homeostasis and suggests that this dysfunction may be due to
annexin VI regulation of pumps and/or exchangers in the membranes of cardiomyocytes.