Complex I Function and Supercomplex Formation Are Preserved in Liver Mitochondria Despite Progressive Complex III Deficiency

Mina Davoudi,Eva Hansson,Vineta Fellman,Heike Kotarsky

doi:10.1371/journal.pone.0086767

Abstract

Functional oxidative phosphorylation requires appropriately assembled mitochondrial respiratory complexes and their supercomplexes formed mainly of complexes I, III and IV. BCS1L is the chaperone needed to incorporate the catalytic subunit, Rieske iron-sulfur protein, into complex III at the final stage of its assembly. In cell culture studies, this subunit has been considered necessary for supercomplex formation and for maintaining the stability of complex I. Our aim was to assess the importance of fully assembled complex III for supercomplex formation in intact liver tissue. We used our transgenic mouse model with a homozygous c.232A>G mutation in Bcs1l leading to decreased expression of BCS1L and progressive decrease of Rieske iron-sulfur protein in complex III, resulting in hepatopathy. We studied supercomplex formation at different ages using blue native gel electrophoresis and complex activity using high-resolution respirometry. In isolated liver mitochondria of young and healthy homozygous mutant mice, we found similar supercomplexes as in wild type. In homozygotes aged 27–29 days with liver disorder, complex III was predominantly a pre-complex lacking Rieske iron-sulfur protein. However, the main supercomplex was clearly detected and contained complex III mainly in the pre-complex form. Oxygen consumption of complex IV was similar and that of complex I was twofold compared with controls. These complexes in free form were more abundant in homozygotes than in controls, and the mRNA of complex I subunits were upregulated. In conclusion, when complex III assembly is deficient, the pre-complex without Rieske iron-sulfur protein can participate with available fully assembled complex III in supercomplex formation, complex I function is preserved, and respiratory chain stability is maintained.

Highlights

In intact mitochondria, the respiratory chain consists of appropriately assembled complexes, which are further arranged in supercomplexes, called respirasomes or supramolecular formations [1]
We addressed the problem of supercomplex assembly in vivo using a transgenic mouse model, in which a homozygous mutation (c.232A.G) in the CIII chaperone gene Bcs1l causes progressive CIII deficiency due to decreasing incorporation of the Rieske ironsulfur protein (RISP) subunit into CIII [15]
The bands representing CIII subunit Core1, CI subunit NDUFA9, and CIV Subunit Va in isolated mitochondria and mitochondrial membranes were similar to controls (Figure 1)

Summary

Introduction

The respiratory chain consists of appropriately assembled complexes, which are further arranged in supercomplexes, called respirasomes or supramolecular formations [1]. Supercomplexes contain mainly complexes I, III, and IV (CI, CIII and CIV) in different stoichiometric combinations [2,3]. Supercomplex formation is dependent on the presence of phospholipids [8] and is facilitated by recently identified supercomplex assembly factors in yeast [9,10,11] and mammals [12,13]. A recent human cancer cell study assessing respirasome assembly after reversible inhibition of mitochondrial translation suggested that CI plays a central role in the formation of supercomplexes [14]. A CI assembly intermediate would serve as a scaffold for incorporating CIII and CIV before addition of the NADH dehydrogenase module

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