External Member: LARSSON Adjunct Group

Mitochondrial Biology

The adjunct group of Nils-Göran Larsson focuses on mitochondrial genetics and the impact of mitochondrial dysfunction on disease and ageing.

Mitochondrial dysfunction is heavily implicated in the ageing process. Ageing humans have increased levels of somatic mtDNA mutations that typically undergo clonal expansion to cause mosaic patterns of respiratory chain deficiency in affected organs. The oxidative phosphorylation (OXPHOS) system produces adenosine triphosphate (ATP), the universal energy currency in mammalian tissues. The OXPHOS system is composed of two functional entities, i.e. the respiratory chain (complexes I-IV) and the phosphorylation system, which includes the ATP synthase (complex V). Transporting electrons through complex I-IV is coupled to the creation of a proton gradient, which drives the ATP synthesis by complex V. Respiratory chain dysfunction, and thus insufficient supply of ATP, can cause a variety of phenotypes associated with ageing and age-related mitochondrial diseases.

Mitochondria harbor their own genome, which is ~16 kb in mammals. Animal mitochondrial DNA (mtDNA) typically encodes 2 ribosomal RNAs (rRNAs) and 22 transfer RNAs (tRNAs). Furthermore, it encodes 13 proteins, which are all components of the oxidative phosphorylation system. The remaining proteins of the OXPHOS system are nuclear encoded. Respiratory chain proteins form complexes that are located in the inner membrane of mitochondria in a dynamic system in which the individual complexes coexist with superassembled structures.

In the group of Nils-Göran Larsson, we use the house mouse (Mus musculus) as a model organism to investigate mechanisms controlling mtDNA maintenance and the functional role of superassemblies of respiratory chain complexes.

Selected projects

Regulation of mtDNA maintenance.

Replication of mammalian mitochondrial DNA (mtDNA) is an essential process that requires high fidelity and control at multiple levels to ensure proper mitochondrial function. The machinery required for mtDNA replication consists of the heterotrimeric mitochondrial DNA polymerase (POLG), the hexameric helicase TWINKLE, the tetrameric mitochondrial single-stranded binding protein (mtSSB) and the mitochondrial RNA polymerase (POLRMT). Besides these basic replisome components, many additional proteins with nuclease, helicase and topoisomerase activities are necessary for mtDNA maintenance. The mitochondrial genome maintenance exonuclease 1 (MGME1; also known as DDK1) is involved in the final steps of mtDNA synthesis and has a role in processing DNA flap structures to allow ligation of the nascent DNA once replication is completed. Mutations in the mitochondrial genome maintenance exonuclease 1 (MGME1) gene were recently reported in patients with mitochondrial disease. To study MGME1 in vivo function, we generated Mgme1 knockout mice and reported that in addition to its role in 5´flap removal this mitochondrial nuclease may have an Important regulatory role at the end of the mtDNA control region (Matic et al., 2018). As a consequence of defective mtDNA replication, the MGME1 homozygous knockouts mice develop mtDNA depletion and accumulate substantial amounts of long linear subgenomic mtDNA molecules in a variety of mouse tissues. Moreover, MGME1 knockout mice display tissue specific replication stalling patterns as revealed by neutral-neutral two-dimensional agarose gel electrophoresis (2DNAGE) and mtDNA sequence coverage patterns obtained by next generation sequencing. These molecular phenotypes emphasize the importance of the MGME1 knockout mouse line as a valuable tool to investigate tissue-specificity of mtDNA maintenance disorders.
In this project we are using MGME1 knockout mice to understand regulatory processes controlling mtDNA manintenance and the tissue-specific manifestations of mtDNA maintenance disorders.

Superassembly of respiratory chain complexes.

Respiratory chain dysfunction plays an important role in human disease and aging. It is now well established that the respiratory complexes are highly enriched in the tubular inner mitochondrial membrane invaginations called cristae membranes. The respiratory chain complexes are frequently organized into different types of supermolecular assemblies, denoted supercomplexes. In mitochondria from mammalian tissues, BN-PAGE has revealed supercomplexes of varying stoichiometry including: CI/CIII2/CIV1-4 (a.k.a. respirasomes), CI/CIII2, and CIII2/CIV1-2. Structures for these macromolecular assemblies were recently determined by electron cryo-microscopy, but the reason why supercomplexes exist remains an enigma (Milenkovic et al, 2017). We have shown that C57BL/6 mice, that are widely used in metabolism research, lack a COX7a2l protein and therefore cannot form supercomplex III2IVn. However, this mouse strain nevertheless contains respirasomes and have normal respiratory chain function (Mourier et al, 2014; Pérez-Pérez et al., 2016).

In this project we are investigating the function of respiratory chain supercomplexes in normal physiology and ageing.