Insights into autophagic flux across three eukaryotic kingdoms

Abstract

Macroautophagy (herein autophagy) is a major intracellular pathway that is indispensable for maintaining cellular homeostasis. Autophagy has been reported in various organisms ranging from yeast to humans suggesting that it is an evolutionarily conserved process. This process was first reported by Christian de Duve (Deter et al., 1967). He identified this process of self-cannibalization which involves sequestration of cellular components within a double membranous structure called the autophagosome, which then fuses with the lysosomes and releases the cargo into it for degradation. The degradation products, like amino acids are recycled back to the cytoplasm and are used up by the cell (Klionsky, 2007; Rabinowitz & White, 2010). This entire phenomenon of cargo capture and ultimately its degradation in the lysosomes is called ‘autophagic flux’. Basal levels of autophagy occur in all cells during nutrient rich conditions and help in housekeeping functions (Mizushima et al., 2004; Musiwaro et al., 2013). However, levels of autophagy dramatically increase via extracellular or intracellular stimulation such as starvation, pathogen invasion, organelle damage and protein aggregation in cytoplasm. Thus autophagy can serve as an adaptive response to provide energy and nutrients to the cell during starvation conditions (Takeshige et al., 1992; Tsukada & Ohsumi, 1993) and also get rid of aggregated proteins, unnecessary or damaged organelles and intracellular pathogens (xenophagy) to act as a cellular rubbish disposal mechanism. Because autophagy is central to maintaining cellular homeostasis, dysfunctional autophagy has been attributed to a variety of disease conditions such as cardiovascular diseases, atherosclerosis, certain myopathies, innate and adaptive immune responses and cancer (Kroemer, 2015). Autophagy also serves a neuroprotective role, as it clears large aggregates of proteins that are resistant to proteasomal degradation. Aim and scope of the study Although the core autophagy machinery and the proteins involved in disease conditions might be known, but the exact mechanism of action and how the autophagic flux is regulated is not completely understood which leads to many unanswered questions. Understanding and controlling the autophagic flux either through a genetic or pharmacological approach is a highly promising approach and of great scientific interest. Studies with genetic modulations of autophagic flux have been carried out in the past with immense success. Yoshinori Ohsumi was awarded the Nobel Prize in 2016 as a pioneer in the field and his contribution to the study of autophagic flux. In the present study however, we used the pharmacological approach as a means to study autophagy. Chemical modulation has an advantage over genetic manipulations that the phenotype could be observed just on the addition of the compound and the action could be reversed on its withdrawal. The method is less laborious and the putative modulators could be used as leads for pharmacological purposes in certain disease conditions.