What’s the role of autophagy in trypanosomes?

Anatomical Institute, Central University of Venezuela. Caracas, Venezuela. * Corresponding Authors: Katherine Figarella, Carretera Nacional de Hoyo de la Puerta, Valle de Sartenejas; Baruta 1080, Venezuela; Tel: 58 (0) 2129035163; E-mail: kfigarella@idea.gob. ve; and Nestor L. Uzcategui, Paseo Los Ilustres, Ciudad Universitaria de Caracas; Caracas 1040, Venezuela; Tel: 58 (0) 4120101485; E-mail: nestor.uzcategui@ucv.ve Trypanosomes are protozoa that diverged early in the eukaryotic lineage exhibiting unusual molecular and bio-chemical features. For example, trypanosomes use a gluta-thione-spermidine conjugate named trypanothione to re-generate thiols [1], decode their mitochondrial transcripts through extensive RNA editing [2], and possess a com-partmentalized glycolysis in unusual peroxisomes called glycosomes [3]. Trypanosoma brucei, the causative agent of sleeping sickness in humans, experiences complex envi-ronmental changes during its complicated life cycle, in which it undergoes differentiation processes in both a ver-tebrate and an invertebrate host. In the vertebrate host, the medically relevant bloodstream forms (BSF) exist as two distinct stages that are morphologically and metaboli-cally different: (i) Long slender forms, which rapidly prolif-erate and (ii) short stumpy forms that are cell cycle-arrested but adapted to survive in the tsetse fly. When taken up by the invertebrate host (the tsetse fly), the para-sites transform into procyclic forms (PCF). The molecular mechanisms regulating the conversion of long slender forms to short stumpy forms are not clear. It is known that proliferating long slender forms live in a glucose-rich milieu and consequently possess a simple metabolism based on obtaining ATP from the breakdown of glucose [4]. In con-trast, PCF have a functional mitochondrion, glycolysis is coupled to oxidative phosphorylation, and their metabo-lism depends largely on amino acids [5]. Short stumpy forms contain a better developed mitochondrion than long slender parasites, which allows them to survive upon up-take by the fly [6]. Under these circumstances parasite organelle turnover during its life cycle is crucial. The main cellular pathway involved in the degradation of cytoplasmic organelles and long-lived proteins is au-tophagy [7]. Autophagy may occur as a non-selective or a selective process. Depending on the nature of the material to be surrounded, it can be defined as macroautophagy, when fractions of the cytoplasm, containing bulk cytosol and organelles, become surrounded by a double-membrane structure to form the phagophore; or microau-tophagy, when part of cytoplasmic components are direct-ly taken up by lysosomal membrane engulfment. Sorting of redundant or damaged organelles for their degradation occur throughout various autophagy-related processes such as pexophagy, mitophagy, ER-phagy, ribophagy, and micronucleophagy (reviewed in 8). In this scenario, T. brucei must also possess mechanisms for the breakdown and recycling of obsolete cellular components in order to guarantee homeostasis during its differentiation and/or development. Indeed, orthologues of yeast autophagy-related (Atg) proteins have been identified in T. brucei by bioinformatic analyses [9]. Most of the understanding on the autophagy process is due in large part to investigations using Saccharomyces cerevisiae . The high tractability of these cells’ genetics, in combination with intensive cellular and physiological re-search, has established S. cerevisiae as a model of autoph-agy. Hence, research groups have taken advantage of the knowledge on yeast autophagy and have extrapolated this information to other single-cell organisms by comparing and analyzing orthologous genes. In T. brucei, this can be performed by knocking down genes using RNAi [10]. The paper by Proto et al. in this issue of Microbial Cell [11], and that of Li et al. (2012) [12], take advantage of the RNAi method to develop reporter cell lines for the autophagy pathway in T. brucei. In the future, this method will allow a detailed and systematic evaluation of all orthologous au-tophagy-related proteins and help piecing together the puzzle of the T. brucei autophagy pathway. Proto et al. genetically engineered PCF and BSF of a T. brucei cell line well suited for RNAi, to express fluorescent-ly-tagged chimeric proteins such as YFP-Atg8.1 and YFP-Atg8.2 serving as autophagy-specific reporters. The system was validated by subjecting the parasite to starvation and following the re-localization of the fusion proteins from the