Mice harbouring a SCA28 patient mutation in AFG3L2 develop late-onset ataxia associated with enhanced mitochondrial proteotoxicity

Abstract

Spinocerebellar ataxia 28 is an autosomal dominant neurodegenerative disorder caused by missense mutations affecting the proteolytic domain of AFG3L2, a major component of the mitochondrial m-AAA protease. However, little is known of the underlying pathogenetic mechanisms or how to treat patients with SCA28. Currently available Afg3l2 mutant mice harbour deletions that lead to severe, early-onset neurological phenotypes that do not faithfully reproduce the late-onset and slowly progressing SCA28 phenotype. Here we describe production and detailed analysis of a new knock-in murine model harbouring an Afg3l2 allele carrying the p.Met665Arg patient-derived mutation. Heterozygous mutant mice developed normally but adult mice showed signs of cerebellar ataxia detectable by beam test. Although cerebellar pathology was negative, electrophysiological analysis showed a trend towards increased spontaneous firing in Purkinje cells from heterozygous mutants with respect to wild-type controls. As homozygous mutants died perinatally with evidence of cardiac atrophy, for each genotype we generated mouse embryonic fibroblasts (MEFs) to investigate mitochondrial function. MEFs from mutant mice showed altered mitochondrial bioenergetics, with decreased basal oxygen consumption rate, ATP synthesis and mitochondrial membrane potential.Mitochondrial network formation and morphology was altered, with greatly reduced expression of fusogenic Opa1 isoforms. Mitochondrial alterations were also detected in cerebella of 18-month-old heterozygous mutants and may be a hallmark of disease. Pharmacological inhibition of de novo mitochondrial protein translation with chloramphenicol caused reversal of mitochondrial morphology in homozygous mutant MEFs, supporting the relevance of mitochondrial proteotoxicity for SCA28 pathogenesis and therapy development.

ATPase Domain AFG3L2 Mutations Alter OPA1 Processing and Cause Optic Neuropathy

“…Overall, our results conclusively demonstrate that AFG3L2 mutations with clearly distinct biochemical impact account for the different patient phenotypes (ie, SCA28 vs DOA). Further evidence indicating that variants associated with SCA28 behave differently from those reported in the present article comes from a recently described knock‐in SCA28 mouse model carrying a patient‐derived missense mutation (p.M665R) in the AFG3L2 proteolytic domain, which shows mitochondrial fragmentation and increased OPA1 processing from long to short isoforms in cerebellum and brain, but not in optic nerves …”

“…Further evidence indicating that variants associated with SCA28 behave differently from those reported in the present article comes from a recently described knock-in SCA28 mouse model carrying a patient-derived missense mutation (p.M665R) in the AFG3L2 proteolytic domain, which shows mitochondrial fragmentation and increased OPA1 processing from long to short isoforms in cerebellum and brain, but not in optic nerves. 49 Interestingly, 3 families (F6, F11, and F12) harbored AFG3L2 biallelic mutations. In Families 11 and 12, the 2 sporadic patients were compound heterozygous for a missense change located outside the ATPase domain (p.K306E or p.Y605C) and a frameshift mutation (p.S634* or p.E793*).…”

Unique structural features of the mitochondrial AAA+ protease AFG3L2 reveal the molecular basis for activity in health and disease

“…AFG3L2-containing complexes regulate diverse biological functions, including mitochondrial ribosome assembly, the expression, maturation, and degradation of electron transport chain complexes, supervision of mitochondrial dynamics, and the regulation of calcium homeostasis [14][15][16][17] . Furthermore, m-AAA proteases are essential for axonal development in mammals, and loss or reduction of wild-type AFG3L2 lead to pleiotropic phenotypes, such as mitochondrial transport defects, mitochondrial fragmentation, and reductions in both mitochondrial membrane potential and ATP-linked respiration [18][19][20][21][22][23] . In fact, single point mutations localized throughout the catalytic core of AFG3L2 are linked to multiple neurodegenerative disorders in humans that present with diverse pathologies and severity.…”

Investigating the influence of mtDNA and nuclear encoded mitochondrial variants on high intensity interval training outcomes

“…An intronic variant within AFG3L2 was also shown to be associated with the composite exercise response phenotype (rs7231304), but this gene has not previously been associated with exercise response. However, mutations in AFG3L2 have been shown to cause spinocerebellar ataxia through the development of mitochondrial proteotoxicity 27,28 . As such, the intronic variation within this gene might inhibit exercise response through dysregulation of mitochondrial structure and function.…”