The Effect of Metallothionein Overexpression on Inflammation Associated with Mitochondrial Complex I Deficiency

Abstract

Mitochondria form an integral part of the immune system because complex I (CI) of the mitochondrial oxidative phosphorylation system has been identified as a key regulator of the inflammatory response. CI activity suppresses inflammation under normal physiological conditions. However, CI deficiency, due to loss of one of its most important structural subunits NADH dehydrogenase (ubiquinone) iron-sulfur protein 4 (NDUFS4), promotes non-resolving inflammation by inducing persistent pro-inflammatory activity in macrophages. An abnormal increase in intracellular reactive oxygen species (ROS) is a well-known hallmark of CI deficiency and suspected to be directly involved in the underlying mechanism. Metallothioneins (MTs) are small intracellular proteins that play a vital role in cellular detoxification and also demonstrate effective ROS scavenging ability. Therefore, upregulation of MTs might serve as a potential endogenous treatment strategy to treat many of the symptoms associated with CI deficiency by reducing excess ROS. In effect, this might lead to the resolution of inflammation associated with CI deficiency. A well-suited experimental animal model to investigate this has recently been developed. In this study, NDUFS4 knockout mice were crossbred with MT overexpressing mice to obtain the four genotypes: CI deficient-, MT overexpressing-, CI deficient MT overexpressing-, and wild-type mice. Bone marrow-derived macrophages (BMDMs) were generated from all four mouse genotypes and the intracellular ROS levels, as well as the inflammatory activity of these cells, were evaluated. In addition, the overall inflammatory status of the mice was also evaluated in the serum. Although the results indicated that MT exhibits significant ROS scavenging ability, it did not reduce excessive ROS levels in CI deficient BMDMs and only led to minimal inhibition of pro-inflammatory activity in these cells. Thus, MT overexpression does not have a strong therapeutic effect on BMDMs. However, further investigation from serum analyses revealed that even though MT overexpression does not completely resolve inflammation, it does have the potential to attenuate it.