Mitochondrial respiration in B lymphocytes is essential for humoral immunity by controlling the flux of the TCA cycle.

To elucidate the function of oxidative phosphorylation (OxPhos) during B cell differentiation, we employ CD23Cre-driven expression of the dominant-negative K320E mutant of the mitochondrial helicase Twinkle (DNT). DNT-expression depletes mitochondrial DNA during B cell maturation, reduces the abundance of respiratory chain protein subunits encoded by mitochondrial DNA, and, consequently, respiratory chain super-complexes in activated B cells. Whereas B cell development in DNT mice is normal, B cell proliferation, germinal centers, class switch to IgG, plasma cell maturation, and T cell-dependent as well as T cell-independent humoral immunity are diminished. DNT expression dampens OxPhos but increases glycolysis in lipopolysaccharide and B cell receptor-activated cells. Lipopolysaccharide-activated DNT-B cells exhibit altered metabolites of glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle and a lower amount of phosphatidic acid. Consequently, mTORC1 activity and BLIMP1 induction are curtailed, whereas HIF1α is stabilized. Hence, mitochondrial DNA controls the metabolism of activated B cells via OxPhos to foster humoral immunity.

Mutation in NDUFA13/GRIM19 leads to early onset hypotonia, dyskinesia and sensorial deficiencies, and mitochondrial complex I instability.

Mitochondrial complex I (CI) deficiencies are causing debilitating neurological diseases, among which, the Leber Hereditary Optic Neuropathy and Leigh Syndrome are the most frequent. Here, we describe the first germinal pathogenic mutation in the NDUFA13/GRIM19 gene encoding a CI subunit, in two sisters with early onset hypotonia, dyskinesia and sensorial deficiencies, including a severe optic neuropathy. Biochemical analysis revealed a drastic decrease in CI enzymatic activity in patient muscle biopsies, and reduction of CI-driven respiration in fibroblasts, while the activities of complex II, III and IV were hardly affected. Western blots disclosed that the abundances of NDUFA13 protein, CI holoenzyme and super complexes were drastically reduced in mitochondrial fractions, a situation that was reproduced by silencing NDUFA13 in control cells. Thus, we established here a correlation between the first mutation yet identified in the NDUFA13 gene, which induces CI instability and a severe but slowly evolving clinical presentation affecting the central nervous system.

Hereditary spastic paraplegia-like disorder due to a mitochondrial ATP6 gene point mutation.

Hereditary spastic paraplegia refers to a genetically heterogeneous syndrome. We identified five members of a family suffering from a late-onset spastic paraplegia-like disorder, carrying the homoplasmic m.9176 T>C mutation in the mitochondrial ATP6 gene. The clinical severity of the disease observed in the family was correlated with the biochemical and assembly defects of the ATP synthase. The m.9176 T>C mutation has been previously associated to Leigh syndrome or familial bilateral striatal necrosis. Other factors such as modifying genes may be involved in the phenotypic expression of the disease. The family belongs to the mitochondrial haplogroup J, previously shown to play a role in modulating the phenotype of mitochondrial diseases and be associated with longevity. Moreover other nuclear modifying genes or environmental factors may contribute to the disease phenotype. This finding extends the genetic heterogeneity of the hereditary spastic paraplegia together with the clinical spectrum of mutations of the ATP6 gene.