Loss of Sam50 in hepatocytes induces cardiolipin‐dependent mitochondrial membrane remodeling to trigger mtDNA release and liver injury

Abstract

Background and Aims Sam50, a key component of the sorting and assembly machinery (SAM) complex, is also involved in bridging mitochondrial outer‐membrane and inner‐membrane contacts. However, the physiological and pathological functions of Sam50 remain largely unknown. Approach and Results Here we show that Sam50 interacts with MICOS (mitochondrial contact site and cristae organizing system) and ATAD3 (ATPase family AAA domain‐containing protein 3) to form the Sam50‐MICOS‐ATAD3‐mtDNA axis, which maintains mtDNA stability. Loss of Sam50 causes mitochondrial DNA (mtDNA) aggregation. Furthermore, Sam50 cooperates with Mic60 to bind to cardiolipin, maintaining the integrity of mitochondrial membranes. Sam50 depletion leads to cardiolipin externalization, which causes mitochondrial outer‐membrane and inner‐membrane (including crista membrane) remodeling, triggering Bax mitochondrial recruitment, mtDNA aggregation, and release. Physiologically, acetaminophen (an effective antipyretic and analgesic)–caused Sam50 reduction or Sam50 liver‐specific knockout induces mtDNA release, leading to activation of the cGAS‐STING pathway and liver inflammation in mice. Moreover, exogenous expression of Sam50 remarkably attenuates APAP‐induced liver hepatoxicity. Conclusions Our findings uncover the critical role of Sam50 in maintaining mitochondrial membrane integrity and mtDNA stability in hepatocytes and reveal that Sam50 depletion–induced cardiolipin externalization is a signal of mtDNA release and controls mtDNA‐dependent innate immunity.

Mitochondrial membrane proteins and VPS35 orchestrate selective removal of mtDNA

“…Disruption of mitochondrial integrity triggers mtDNA release into the cytosol, where it can elicit the innate immune response either through the expression of type I interferons or via the inflammasome, leading to the activation of genes for pro-inflammatory cytokines. Here, and in agreement with recent research 37 , we demonstrate that SAMM50 controls BAK distribution in the mitochondrial outer membrane and Samm50 KD leads to mtDNA release and activation of the innate immune response. Remarkably, BAK/BAX macropores control mitochondrial herniation and inner membrane permeabilization 39 , which might facilitate mitochondrial nucleoids to trespass the mitochondrial inner barrier.…”

“…5i). In agreement with an inhibitory role of SAMM50 towards BAK 37 , we found elevated levels of transcription factors related to the innate immune response in Samm50 KD cells, which are activated upon increased levels of mtDNA in the cytoplasm 42 (Fig. 5j).…”

“…This process is linked to the permeabilization of the mitochondrial membranes allowing release of mitochondrial components 40 . Interestingly, SAMM50 has been found to interact with BAK 37 , a feature which we could also replicate (Supplementary Fig. 8k).…”

“…ATAD3 and SAMM50 have been found to be part of a big mitochondrial protein complex containing several inner and outer membrane proteins and regulating mitochondrial inner and outer contact sites 37 (Supplementary Fig. 6f).…”

Reply

“…We have read the comments by Rao et al on our recent published article [ 1 ] and thank them for their interest.…”