Mitochondria: one of the origins for autophagosomal membranes?

Macroautophagy is a transport pathway to the lysosome/vacuole that contributes to the degradation of numerous intracellular components. Despite the recent advances achieved in the understanding of the molecular mechanism underlying macroautophagy, the membrane origin of autophagosomes, the hallmark of this process is still a mystery. It has been suggested that mitochondria may be one of the lipid sources for autophagosome formation and that possibly this organelle provides the phosphatidylethanolamine (PE) that covalently links to the members of the ubiquitin-like Atg8/microtubule-associated protein 1 light chain 3 (LC3) protein family. These lipidated proteins are inserted into the outer and inner surface of autophagosomes and are essential for the biogenesis of these large double-membrane vesicles. However, because PE is an integral component of all cellular membranes, designing appropriate experiments to determine the origin of the autophagosomal PE is not easy. In this review, we discuss the idea that mitochondria provide the pool of PE necessary for the autophagosome biogenesis and we propose some possible experimental approaches aimed to explore this possibility.