Type 2 transglutaminase is involved in the autophagydependent clearance of ubiquitinated proteins and damaged mitochondria

Eukaryotic cells are equipped with an efficient quality control system to selectively eliminate misfolded, damaged proteins and organelles. Autophagy plays a critical role in the selective elimination of aggregated proteins such as mutated and misfolded proteins with an increased aggregation tendency, as oligomeric and higher-order structures that become inaccessible to the narrow proteasome barrel. The autophagic process is also required for the degradation of damaged and dysfunctional mitochondria (mitophagy). Type 2 Transglutaminase (TG2), the most ubiquitous member of the TG family, is a peculiar multifunctional enzyme involved in a variety of cellular processes such as differentiation, cell death, autophagy, inflammation, cell migration, and wound healing. Under stressful conditions, Ca2+ acts as a switch activating TG2 transamidating activity. Here, we show that Type 2 transglutaminase (TG2) knockout mice display impaired autophagy and accumulate ubiquitinated protein aggregates upon starvation. Furthermore, we also demonstrate that, under cellular stressful conditions, TG2 physically interacts with p62 and they are localized in cytosolic protein aggregates, which are then recruited into autophagosomes, where TG2 is degraded. Interestingly, the enzyme's crosslinking activity is activated during autophagy and its inhibition leads to the accumulation of ubiquitinated proteins. Taken together, these data indicate that the TG2 transamidating activity has an important role in the assembly of protein aggregates, as well as in their clearance. Moreover, we show that cells lacking TG2 display fragmented, damage and depolarized mitochondria. By measuring the extracellular acidification rate (ECAR), we observed that these cells also display high rates of aerobic glycolysis in comparison to the wild type. Furthermore, we demonstrate that the defective mitochondria observed in absence of TG2 are not removed by autophagy and their accumulation determines caspase-3 dependent cell death. In keeping with these results, TG2 transamidating activity is induced during mitophagic process. These findings suggest a key role for TG2 also in the regulation of energetic metabolism and mitochondrial homeostasis.