Bridging Integrator 1 (BIN1) Initiates T-Tubule Growth during Cardiac Development and Disease

The cardiomyocyte t-tubule network is malleable; t-tubules develop after birth and are disrupted during diseases such as heart failure (HF). In failing cells, loss of the primarily transverse orientation of tubules is accompanied by growth of longitudinal t-tubules (LTTs), which we have shown are compensatory. We presently hypothesized that bridging integrator 1 (BIN1), a membrane tubulating protein, triggers t-tubule growth during both development and disease. Confocal microscopy and di-8-ANEPPS staining revealed cardiomyocytes isolated from 10 day-old mice predominantly contained LTTs, whereas the mature transverse network began developing 10-15 days following birth. Additionally, we observed that cardiac BIN1 protein levels in mice were highest during periods of t-tubule biogenesis, suggesting that BIN1 may be involved during early stages of development. In rats examined 6 weeks following myocardial infarction, appearance of LTTs was accompanied by a 2-fold increase in BIN1 transcript levels, despite marked loss of transverse elements. Immunostaining of failing cardiomyocytes fixed in 4% paraformaldehyde confirmed the presence of BIN1 in newly formed LTTs. To directly examine whether BIN1 upregulation could underlie the observed appearance of LTTs in developing and diseased cardiomyocytes, we expressed BIN1 in cultured cardiomyocytes lacking t-tubules (HL-1 cells). Following transfection with exogenous BIN1, a dense network of BIN1-positive sarcolemmal membrane invaginations was formed in HL-1 cells as early as 12 hours after transfection. Importantly, these tubules were chaotically organized and resembled the LTTs grown prior to t-tubule maturation. Labeling of intracellular Ca2+ with fluo-4 revealed that these tubules have t-tubule-like functionality, as Ca2+ release events occurred adjacent to BIN1-positive tubules. We propose that BIN1 initiates LTT growth during development and disease, and that upregulation of BIN1 in the diseased heart may represent a return to an immature gene program.