Protective mitochondrial fission induced by stress-responsive protein GJA1-20k

Abstract

The Connexin43 gap junction gene GJA1 has one coding exon, but its mRNA undergoes internal translation to generate N-terminal truncated isoforms of Connexin43 with the predominant isoform being only 20 kDa in size (GJA1-20k). Endogenous GJA1-20k protein is not membrane bound and has been found to increase in response to ischemic stress, localize to mitochondria, and mimic ischemic preconditioning protection in the heart. However, it is not known how GJA1-20k benefits mitochondria to provide this protection. Here, using human cells and mice, we identify that GJA1-20k polymerizes actin around mitochondria which induces focal constriction sites. Mitochondrial fission events occur within about 45 s of GJA1-20k recruitment of actin. Interestingly, GJA1-20k mediated fission is independent of canonical Dynamin-Related Protein 1 (DRP1). We find that GJA1-20k-induced smaller mitochondria have decreased reactive oxygen species (ROS) generation and, in hearts, provide potent protection against ischemia-reperfusion injury. The results indicate that stress responsive internally translated GJA1-20k stabilizes polymerized actin filaments to stimulate non-canonical mitochondrial fission which limits ischemic-reperfusion induced myocardial infarction.

GJA1-20k and Mitochondrial Dynamics

“…Canonical mitochondrial fission is associated with DRP1 and the degradation (breakdown) of dysfunctional and damaged mitochondria ( Jheng et al, 2012 ; Friedman and Nunnari, 2014 ). In contrast, we have found that GJA1-20k induces a protective form of mitochondrial fission ( Shimura et al, 2021a ). Interestingly, GJA1-20k also stabilizes actin cytoskeleton.…”

“…Endogenous GJA1-20k expression has been reported in not just cardiomyocytes, but also in various cell types such as C33A, NF-1, MEF, A549, HEK293 ( Salat-Canela et al, 2014 ; Basheer et al, 2018 ; Shimura et al, 2021a ). In all these cell types, GJA1-20k expression is the most abundant among the N-terminus truncated small isoforms of Cx43.…”

“…Interestingly, GJA1-20k also stabilizes actin cytoskeleton. GJA1-20k stabilization of actin is likely the common denominator of GJA1-20k divergent roles from trafficking to mitochondrial regulation ( Basheer et al, 2017 ; Shimura et al, 2021a ).…”

Gap Junction-Dependent and -Independent Functions of Connexin43 in Biology

“…Cx43-20k also regulates the transport of mitochondria to mediate cellular response to oxidative stress, preserving mitochondrial localization and function [ 153 ]. Under cellular stress, Cx43-20k facilitates polymerization of actin around mitochondria to form focal constriction sites to initiate mitochondrial fission [ 154 ].…”

“…Cx43-20K possesses a transmembrane domain and functions in cytosol [ 150 , 151 , 152 , 153 , 154 ]. Smaller fragments without any transmembrane domain, collectively called Cx43 C-terminal fragments (Cx43-CT), have been suspected to exist, and some of them have a unique nuclear localization [ 155 , 156 ].…”