m6A-methylated Lonp1 drives mitochondrial proteostasis stress to induce testicular pyroptosis upon environmental cadmium exposure

Cadmium (Cd) is a widely distributed typical environmental pollutant and one of the most toxic heavy metals. It is well-known that environmental cadmium causes testicular damage by inducing classic types of cell death such as cell apoptosis and necrosis. However, as a new type of cell death, the role and mechanism of pyroptosis in cadmium-induced testicular injury remain unclear. In the current study, we used environmental cadmium to generate a murine model with testicular injury and AIM2-dependent pyroptosis. Based on the model, we found that increased cytoplasmic mitochondrial DNA (mtDNA), activated mitochondrial proteostasis stress occurred in cadmium-exposed testes. We used ethidium bromide to generate mtDNA-deficient testicular germ cells and further confirmed that increased cytoplasmic mtDNA promoted AIM2-dependent pyroptosis in cadmium-exposed cells. Uracil-DNA glycosylase UNG1 overexpression indicated that environmental cadmium blocked UNG-dependent repairment of damaged mtDNA to drive the process in which mtDNA releases to cytoplasm in the cells. Interestingly, we found that environmental cadmium activated mitochondrial proteostasis stress by up-regulating protein expression of LONP1 in testes. Testicular specific LONP1-knockdown significantly reversed cadmium-induced UNG1 protein degradation and AIM2-dependent pyroptosis in mouse testes. In addition, environmental cadmium significantly enhanced the m6A modification of Lonp1 mRNA and its stability in testicular germ cells. Knockdown of IGF2BP1, a reader of m6A modification, reversed cadmium-induced upregulation of LONP1 protein expression and pyroptosis activation in testicular germ cells. Collectively, environmental cadmium induces m6A modification of Lonp1 mRNA to activate mitochondrial proteostasis stress, increasing cytoplasmic mtDNA content, and triggering AIM2-dependent pyroptosis in mouse testes. These findings suggest that mitochondrial proteasome stress is a potential target for the prevention of testicular injury.