Autophagy is a catabolic pathway that delivers cytoplasmic material to lysosomes for degradation. Under vegetative conditions, the pathway serves as quality control system, specifically targeting damaged or superfluous organelles and protein-aggregates. Selective autophagy is also involved in regulating cellular responses related to immunity, inflammation, and antimicrobial defense. Central to all these responses is the autophagic core machinery that comprises two interconnected Ubiquitin (Ub)-like conjugation systems. The conjugation machinery targets cellular components to the lysosome for degradation by inducing complex membrane remodeling events. The underlying molecular mechanism is, however, poorly understood. Obtaining insights into the function of the conjugation machinery is needed to unravel how impaired autophagy contributes to human diseases including cancer, neurodegeneration, and autoimmune diseases.
We investigated the molecular mechanism of autophagy by applying an interdisciplinary and highly innovative approach that combined in vitro reconstitutions using purified components with cell biological and biochemical methods. We first reconstituted the human Ub-like conjugation systems on model membranes in vitro in order to uncover how this machinery regulates autophagy. We found that LC3C specifically interacts with the lysosomal protein TECPR1 in vitro. Furthermore, this interaction was found to be critical to target autophagosomes that contain protein aggregates to lysosomes for degradation. LC3C thus functions as ‘molecular label’ and facilitates the turnover of protein aggregates by targeting autophagosomes to lysosomes. In cells that lack LC3C, protein aggregates accumulates and provokes cytotoxic secondary effects. By contrast, an augmented activity of TECPR1 promoted autophagy in vivo. The function of LC3C and TECPR1 was particularly important in neural cells. A stimulation of TECPR1 protected neural stem cells from accumulating toxic protein aggregates even under unfavourable conditions. This observation is of particular interest since many neurodegenerative diseases are characterized by a steady increase in protein aggregates. We hope that our results will lead to new therapeutic approaches to treat neurodegenerative diseases such as Alzheimer’s or Parkinson’s disease.
