Long OPA1 isoforms mediate a membrane potential‐dependent threshold for mitochondrial fusion in AC16 cardiomyocytes

Abstract

The bioenergetic function of mitochondria is linked to its organellar structural dynamics by optic atrophy‐1 (OPA1), a nuclear‐encoded protein that mediates fusion of the mitochondrial inner membrane. OPA1‐mediated inner membrane fusion is dependent on the transmembrane potential across the mitochondrial inner membrane (Δψm): long (L‐OPA1) isoforms accomplish fusion for an interconnected reticular morphology when Δψm is intact. Upon loss of Δψm, L‐OPA1 is cleaved by the OMA1 metalloprotease, causing accumulation of short S‐OPA1 and collapse of mitochondrial fusion to a fragmented morphology. We examined Δψm‐sensitive mitochondrial fusion in human AC16 cardiomyocytes treated with carbonyl cyanide m‐chlorophenyl hydrazone (CCCP), an uncoupler of Δψm. Upon increasing titration with CCCP, mitochondrial fusion collapses at [CCCP] > 2 μM, as determined using blinded ImageJ quantification of mitochondrial morphology. Similarly, OPA1 Western blotting revealed that AC16s show a significant loss of fusion‐active L‐OPA1 at [CCCP] > 2 μM. Taken together, these findings indicate that L‐OPA1 maintains mitochondrial fusion, but that OMA1 is activated at [CCCP] > 2 μM, causing L‐OPA1 cleavage and collapse of mitochondrial fusion. Experiments in progress are examining OMA1 activation at this Δψm breakpoint, as well as impact of altered OPA1 and OMA1 expression.Support or Funding InformationNIGMS 5SC3GM116669 (to R.G.)NIGMS 1SC3GM132053 (to M.K.)UTRGV Presidential Graduate Research Assistantship (to P. D.)