Autophagy is constitutively active in normal mouse sino-atrial nodal cells

This study was designed to examine the autophagy in sino-atrial (SA) nodal cells from the normal adult mouse heart. Autophagy is the cellular process responsible for the degradation and recycling of long-lived and/or damaged cytoplasmic components by lysosomal digestion. In the heart, autophagy is known to occur at a low level under physiological conditions, but to become upregulated when cells are exposed to certain stresses, such as ischemia. We examined whether the basal level of autophagy in SA nodal cells was different from that in ventricular or atrial myocytes. An ultrastructural analysis revealed that the SA nodal cells contained a number of autophagic vacuoles (autophagosomes) with various stages of degradation by lysosomal digestion, whereas the number of those in ventricular or atrial myocytes was either negligible or very small. The immunostaining of autophagosome marker microtubule-associated protein 1 light chain 3 (LC3) and lysosome marker lysosome-associated membrane protein 1 (LAMP1) indicated that the content of both autophagosomes and lysosomes were much greater in SA nodal cells than in ordinary cardiomyocytes. Our results provide evidence that the autophagy is active in normal SA nodal cells, which is not a stress-activated process but a constitutive event in the mouse heart.