Mutations in the AFG3L2 gene have been linked to spinocerebellar ataxia type 28 and spastic ataxia-neuropathy syndrome in humans; however, the pathogenic mechanism is still unclear. AFG3L2 encodes a subunit of the mitochondrial m-AAA protease, previously implicated in quality control of misfolded inner mitochondrial membrane proteins and in regulatory functions via processing of specific substrates. Here, we used a conditional Afg3l2 mouse model that allows restricted deletion of the gene in Purkinje cells (PCs) to shed light on the pathogenic cascade in the neurons mainly affected in the human diseases. We demonstrate a cell-autonomous requirement of AFG3L2 for survival of PCs. Examination of PCs prior to neurodegeneration revealed fragmentation and altered distribution of mitochondria in the dendritic tree, indicating that abnormal mitochondrial dynamics is an early event in the pathogenic process. Moreover, PCs displayed features pointing to defects in mitochondrially encoded respiratory chain subunits at early stages. To unravel the underlying mechanism, we examined a constitutive knockout of Afg3l2, which revealed a decreased rate of mitochondrial protein synthesis associated with impaired mitochondrial ribosome assembly. We therefore propose that defective mitochondrial protein synthesis, leading to early-onset fragmentation of the mitochondrial network, is a central causative factor in AFG3L2-related neurodegeneration. IntroductionEukaryotic cells monitor the quality of their mitochondria using mechanisms acting at the interorganelle or intraorganelle level. Mitochondrial fusion and fission ensure maintenance of a functional network, whereas intramitochondrial proteases remove misfolded and aggregated proteins and also have a role in regulating mitochondrial function by processing specific substrates (1). Both levels of quality control are crucial in neurons, as emphasized by the increasing number of neurodegenerative diseases linked to impaired mitochondrial dynamics or mutations in mitochondrial proteases (1).Subunits of the matrix ATPase associated with various cellular activities (m-AAA) protease have emerged as crucial players in defending neurons against neurodegeneration. The m-AAA protease is an evolutionary conserved hexameric complex in the inner mitochondrial membrane, exposing the catalytic domains to the matrix (2, 3). Studies in yeast and mammals have shown that the m-AAA protease degrades misfolded polypeptides and excess protein components lacking assembly partners in the inner mitochondrial membrane (2,4,5). Moreover, the yeast m-AAA protease mediates proteolytic maturation of the mitochondrial ribosomal component MRPL32 (6). MRPL32 maturation is a prerequisite for mitochondrial translation and the synthesis of mitochondrially encoded (mt-encoded) respiratory chain subunits, explaining respiratory deficiencies of yeast cells lacking the m-AAA protease (6). In a reconstituted yeast system, the mammalian m-AAA protease was able to process MRPL32, suggesting evolutionary conservation of this...

Mitochondrial dysfunction causes neurodegeneration but whether impairment of mitochondrial homeostasis in astrocytes contributes to this pathological process remains largely unknown. The m ‐AAA protease exerts quality control and regulatory functions crucial for mitochondrial homeostasis. AFG3L2 , which encodes one of the subunits of the m ‐AAA protease, is mutated in spinocerebellar ataxia SCA28 and in infantile syndromes characterized by spastic‐ataxia, epilepsy and premature death. Here, we investigate the role of Afg3l2 and its redundant homologue Afg3l1 in the Bergmann glia (BG), radial astrocytes of the cerebellum that have functional connections with Purkinje cells (PC) and regulate glutamate homeostasis. We show that astrocyte‐specific deletion of Afg3l2 in the mouse leads to late‐onset motor impairment and to degeneration of BG, which display aberrant morphology, altered expression of the glutamate transporter EAAT2, and a reactive inflammatory signature. The neurological and glial phenotypes are drastically exacerbated when astrocytes lack both Afg31l and Afg3l2 , and therefore, are totally depleted of the m ‐AAA protease. Moreover, mitochondrial stress responses and necroptotic markers are induced in the cerebellum. In both mouse models, targeted BG show a fragmented mitochondrial network and loss of mitochondrial cristae, but no signs of respiratory dysfunction. Importantly, astrocyte‐specific deficiency of Afg3l1 and Afg3l2 triggers secondary morphological degeneration and electrophysiological changes in PCs, thus demonstrating a non‐cell‐autonomous role of glia in neurodegeneration. We propose that astrocyte dysfunction amplifies both neuroinflammation and glutamate excitotoxicity in patients carrying mutations in AFG3L2 , leading to a vicious circle that contributes to neuronal death.