Ca2+ transfer via the ER-mitochondria tethering complex in neuronal cells contribute to cadmium-induced autophagy

Mitochondrial-associated endoplasmic reticulum (ER) membranes (MAMs) play a key role in several physiological functions, including calcium ion (Ca2+) transfer and autophagy; however, the molecular mechanism controlling this interaction in cadmium (Cd)-induced neurotoxicity is unknown. This study shows that Cd induces alterations in MAMs and mitochondrial Ca2+ levels in PC12 cells and primary neurons. Ablation or silencing of mitofusin 2 (Mfn2) in PC12 cells or primary neurons blocks the colocalization of ER and mitochondria while reducing the efficiency of mitochondrial Ca2+ uptake. Moreover, Mfn2 defects reduce interactions or colocalization between GRP75 and VDAC1. Interestingly, the enhancement of autophagic protein levels, colocalization of LC3 and Lamp2, and GFP-LC3 puncta induced by Cd decreased in Mfn2-/- or Grp75-/- PC12 cells and Mfn2- or Grp75-silenced primary neurons. Notably, the specific Ca2+ uniporter inhibitor RuR blocked both mitochondrial Ca2+ uptake and autophagy induced by Cd. Finally, this study proves that the mechanism by which IP3R-Grp75-VDAC1 tethers in MAMs is associated with the regulation of autophagy by Mfn2 and involves their role in mediating mitochondrial Ca2+ uptake from ER stores. These results give new evidence into the organelle metabolic process by demonstrating that Ca2+ transport between ER-mitochondria is important in autophagosome formation in Cd-induced neurodegeneration.