H₂O₂ Induces Calcium and ERMES Complex-Dependent Mitochondrial Constriction and Division as Well as Mitochondrial...

The dynamin-like protein DnmA and its receptor FisA are essential for H₂O₂-induced mitochondrial division in Aspergillus nidulans. Here, we show that in the absence of DnmA or FisA, mitochondria show few spontaneous transient constrictions, the frequency of which is extensively increased by H₂O₂ or the carbonyl cyanide m-chlorophenyl hydrazone (CCCP). While H₂O₂-induced constrictions are transient, CCCP induces a drastic and irreversible alteration of mitochondrial filaments. H₂O₂ induces a gradual mitochondrial depolarization, while CCCP-induced depolarization is abrupt. The calcium chelator BAPTA-AM prevents the formation of mitochondrial constrictions induced by either H₂O₂ or CCCP. H₂O₂ also induces major rearrangements of the mitochondrial outer membrane, which remain after constrictions dissipate, as well as changes in endoplasmic reticulum (ER) and nuclear morphology. Similar mitochondrial constriction, ER and nuclear morphology changes are detected during the early stages of asexual development. ER and ER-Mitochondria encounter structure (ERMES) complex-composed of proteins Mdm10, Mmm1, Mdm43 and Mdm12-are important for mitochondrial division in Saccharomyces cerevisiae. As the Mdm10 ortholog MdmB was found to be essential in A. nidulans, we evaluated its functions in ΔmdmB terminal mutants and ΔmdmB heterokaryons. ΔmdmB conidia produce a short germ tube that fails to grow further, in which inherited mitochondria become gigantic and round shaped, lacking clear contacts with the ER. In slow-growing ΔmdmB heterokaryotic mycelia, multiple hyphae contain very long mitochondria with high ROS levels, as occur in ΔdnmA and ΔfisA mutants. In this hyphae, H₂O₂ fails to induce mitochondrial constrictions but not outer mitochondrial membrane reshaping, indicating that these are two separate effects of H₂O₂. Our results indicate that H₂O₂ induces a generalized mitochondrial constriction response, prior to actual division, involving gradual depolarization; they also indicate that Ca²⁺ and the ERMES complex are critical for both mitochondrial constriction and division. This supports a view of mitochondrial dynamics as the result of a cascade of signaling events that can be initiated in vivo by H₂O₂.