Autophagy Is Constitutively Active in Normal Mouse Sino-Atrial Nodal Cells

Abstract

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.

The Presence of LC3 and LAMP1 Is Greater in Normal Sino-Atrial Nodal Cells Than in Ordinary Cardiomyocytes, Indicating a Constitutive Event

“…In the normal mouse heart, only faint immunofluorescent signals of LC3 are seen in the cardiac ventricle (Figure 14.1A), consistent with the fact that the level of autophagy is very low in the cardiac ventricles under physiological conditions (Decker and Wildenthal, 1980;Matsui et al, 2007;Omatsu-Kanbe et al, 2011). The atrium also shows very weak signals of LC3.…”

“…In contrast to the ordinary cardiac myocytes, central SA nodal cells contain vacuoles of various sizes in the cytoplasm, including the perinuclear area, interspaces between myofibers and the peripheral area (Omatsu-Kanbe et al, 2011). The presence of a number of vacuoles in the cytoplasmic space is not observed in ventricular or atrial myocytes.…”

“…Evidence that the normal SA nodal cells contain a number of autophagosomes in various stages of degradation, express membrane-bound LC3, and display lysosomal accumulation, but not ventricular or atrial myocytes (Figures 14.1 and 14.2), indicates that the autophagy in the SA node is not a stress-activated process, but a constitutive event, at least in the mouse heart. Phosphorylation of mTOR at Ser 2448, active mTOR, can be detected in the normal SA node, as well as in the ventricle and atrium, suggesting that autophagy in the normal SA nodal cells is not likely due to the inactivation of mTOR (Omatsu-Kanbe et al, 2011). Phosphorylation of mTOR at Ser 2448, active mTOR, can be detected in the normal SA node, as well as in the ventricle and atrium, suggesting that autophagy in the normal SA nodal cells is not likely due to the inactivation of mTOR (Omatsu-Kanbe et al, 2011).…”

“…Under physiological conditions, autophagy in those cardiac myocytes stays at a low level to play a role in the maintenance of the cells, involving the degradation of long-lived proteins and removing damaged organelles, so that the number of autophagosomes in those cells is very small or negligible (Decker and Wildenthal, 1980;Matsui et al, 2007;Omatsu-Kanbe et al, 2011). The evidence that disruption of this pathway causes ventricular dilation and severe contractile dysfunction (Nakai et al, 2007) indicates that autophagy is necessary for normal cardiac myocytes even with the low levels of the basal state.…”

“…Central nodal cells, primary pacemaker cells, located in the center of the node contain a low amount of myofibers running without order. One of the morphological characteristics specifically observed in the central SA nodal cells is the presence of numerous pinocytotic vesicles invaginated from the plasma membrane (James et al, 1966;Masson-Pevet et al, 1979;Omatsu-Kanbe et al, 2011), which indicates that pinocytosis is highly activated in the central nodal cells but not so much in other cardiac myocytes. Atrial cells, peripheral transitional cells, located at the peripheral zone and going over into the atrium exhibit similar morphological characteristics to those in atrial myocytes, and possess ANPcontaining atrial granules in the cytoplasmic space.…”