On starvation-induced autophagy in Teraahymena

Abstract

Starvation-induced autophagy in Tetrahymena was studied over a 48-hour period during which the cells decreased in size to a quarter of their original volume. The autophagosomes were divided into 4 types: I) Clearly identifiable cytoplasmic content delimited by 2 membranes. II) Clearly identifiable content delimited by one membrane. III) Content of remnants permitting identification of origin. And, IV) Completely disintegrated content of unknown origin. All types were present in 30-minute starved cells and Type I organelles were seen throughout starvation, thus indicating that autophagy occurred continuously. The number of autophagosomes per unit cytoplasmic area remained constant but with a decrease of the first 3 types and an accumu-lation of Type IV. Acid phosphatase was found in some Type II organelles and in most Types III and IV; these organelles are autolysosomes but Type I is not. Disintegration of the inner autophagosomal membrane (passage from Type I to Type II) thus occurs in the absence of lysosomal enzymes but could be ascribed to an assumed acidification of the organelles. In addition to autophagosomes, profiles were observed of double membrane systems, isolation membranes, enveloping cell constituents as an indication of autophagy in progress. These isolation membranes seemed to derive from Golgi complexes as tubula, or sac-like structures; moreover, such structures were seen in association with microtubules which indicates a possible transport mechanism to the site of autophagy. An analysis of the origin of the autophagosomal contents, revealed that cytoplasm was enclosed throughout starvation, whereas enclosure of mitochondria, peroxisomes, and membrane whorls occurred in a time-dependent manner. Hence in this ciliate, autophagy occurs in a well-defined and controlled manner.