Endosomes and lysosomes in cardiomyocytes : a study on morphology and function

Abstract

This study focused on the characterization of the lysosomal and endosomal structures and their function in rat heart muscle cells. Since controversial data has been published on the involvement of lysosomes during myocardial ischemia and necrosis, special emphasis was put on the function of lysosomal and endosomal structures after isoproterenol-induced infarct-like necrosis both in vivo and in vitro. High-uptake lysosomal enzymes were shown to bind to membrane fractions from rat heart muscle by mannose 6-phosphate recognition. Various parts of the heart muscle showed no apparent differences in their binding efficiency. However, in contrast to the results from other tissues after subcellular fractionation, myocardial sarcolemmal fraction showed a relatively high specific binding of the ligands, suggesting that the endocytic route is important in the delivery of the lysosomal enzymes to lysosomes. The morphology study showed that, in fact, the bulk of the cation independent mannose 6-phosphate receptor (CI-MPR) is located in large membraneous structures very similar to the prelysosomes recently characterized by Griffiths et al. (1988). Only low amounts of CI-MPR were found in derivatives of the sarcolemma and in the trans-Golgi network (TGN). Cathepsin L and lgp120 were found in CI-MPR-negative structures designated as lysosomes as well as in prelysosomes together with CI-MPR. Prelysosomes were often associated with autophagy. Infarct-like myocardial necrosis induced by a single injection of a large dose of isoproterenol caused a dramatic change in CI-MPR distribution. During the early course of myocardial cell injury, well before irreversible damage and the inflammatory reaction, the bulk of the CI-MPR, i.e. the late endosomal structures, fragmented and migrated towards the cellular periphery, whereas the distribution of lysosomes was unchanged. Lysosomes seemed, however, to aggregate in larger structures but showed no change in structural latency. Late endosomal fragmentation and migration towards the periphery was also reproduced in vitro by isoproterenol treatment and shown to be microtubule-dependent. These phenomena are most probably due to the lowered cytoplasmic pH during infarct-like myocardial necrosis since after isoproterenol-treatment the myocyte cytoplasm was acidified and similar results were produced by mere acidification of the cytoplasm.