IgG from patients with liver diseases inhibit mitochondrial respiration in permeabilized oxidative muscle cells: impaired function of intracellular energetic units?

The effect of IgG purified from the sera of healthy persons and patients with primary biliary cirrhosis (PBC) and chronic hepatitis (CH) on ADP dependent respiration (oxidative phosphorylation) in skinned muscle fibers from rat oxidative muscles (heart and M. soleus) and glycolytic skeletal muscle (M. gastrocnemius) was studied. The results show that IgG from three different sources inhibited the rate of respiration by 13, 44 and 42%, respectively, these effects being equally expressed in both types of oxidative muscles, whereas no inhibition was observed in glycolytic muscle. The following washout of unbound IgG did not abolish the inhibition of respiration suggesting that the specific interaction of IgG with antigens had taken place. Laser confocal analysis revealed binding of IgG predominantly to the sarcomeric structures such as Z-disk and M-lines in the cardiomyocytes. The staining of IgG within Z-disks and intermitochondrial space coincided throughout the muscle cells so that transversally serial spaces, each containing mitochondria and adjacent sarcomere, became clearly visible. When the IgG from a CH patient was incubated with the skinned myocardial fibers of the desmin knockout mice, its binding to Z-disks and the sarcomeric area was found to be similar to that in normal cardiac muscle. However, the transversal staining pattern was disintegrated, because of the slippage of the myofibrils in relation to each other and accumulation of mitochondria between them. These observations support the recent hypothesis that in oxidative muscles the mitochondria and adjacent sarcomeres form complexes, termed as the intracellular energetic units, ICEUs. Moreover, they indicate that human autoantibodies can be useful tools for localizing the proteins responsible for formation of ICEUs and modulation of their function. Thus, it appears that the proteins associated with the Z-disks and M-lines may participate in formation of ICEUs and that binding of IgG to these proteins decreases the access of exogenous adenine nucleotides to mitochondria, which manifests as decreased rate of ADP-dependent respiration.