Mic60/Mitofilin Homeostasis Determines MICOS Assembly Essential For Mitochondrial Dynamics And MtDNA Nucleoid Organization

The MICOS complex (mitochondrial contact site and cristae organizing system) is essential for mitochondrial inner membrane organization and mitochondrial membrane contacts. Mic60 and Mic19 are the key components of MICOS complex in yeast, but how MICOS is assembled and what are physiological functions of MICOS in mammals are still obscure. Here, we report that the downregulation of MICOS subunit such as Mic60/Mitofilin, Mic19/CHCHD3 or Mic10/MICOS1, leads to disorganized mitochondrial cristae in MEFs. The downregulation of Mic60/Mitofilin or Mic19/CHCHD3 results in instability of other MICOS components and disassembly of MICOS complex, and direct interaction between Mic60/Mitofilin and Mic19/CHCHD3 is curial for their stabilization in mammals. In addition, we identified that the mitochondrial i-AAA protease Yme1L regulates Mic60/Mitofilin homeostasis. Furthermore, impaired MICOS induced by Mic60/Mitofilin or Mic19/CHCHD3 knockdown (shMic60 or shMicl9) causes a large number of enlarged spherical mitochondria which are termed "giant mitochondria"; however, the giant mitochondria induced by shMic60 have attenuated mitochondrial fusion and fission due to the reduced levels of mitochondrial fission and fusion factors such as Drp1, Mff, Mid49, Mfn2 or OPA1. Finally, in shMic60 MEFs where MICOS complex is impaired, mtDNA nucleoids are disorganized and clustered severely into giant mitochondria which are independent of Drpl loss, and mtDNA transcription in shMic60 mitochondria is attenuated due to remarkable downregulation of some key mtDNA nucleoids associated proteins. Thus, these findings demonstrate that Mic60/Mitofilin homeostasis regulated by Yme1L is central to the MICOS assembly which is required for maintenance of mitochondrial morphology and organization of mtDNA nucleoids.