AFG3L2 supports mitochondrial protein synthesis and Purkinje cell survival

Abstract

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...

Ebselen Alters Mitochondrial Physiology and Reduces Viability of Rat Hippocampal Astrocytes

“…Additionally, redistribution of mitochondria within the cell can be a component of regulatory pathways (Murphy, 2012b). It has been proposed that fragmentation of the mitochondrial network is related neurodegeneration (Almajan et al, 2012). In agreement to these observations, the effect of ebselen on mitochondrial network could be deleterious to astrocyte physiology.…”

Control of mitochondrial integrity in ageing and disease

“…This molecular defect manifests itself as early onset spastic ataxia-neuropathy (SPG5, MIM:614487) with features characteristic for mitochondrial disorders [136]. Yet another molecular mechanism that underlies neurodegeneration caused by AFG3L2 mutations is the fragmentation of the mitochondrial network [176] and abnormal distribution of mitochondria in neurons [177]. Defective mitochondrial protein synthesis has been proposed as the molecular mechanism of fragmentation in Purkinje neurons [177].…”

“…Yet another molecular mechanism that underlies neurodegeneration caused by AFG3L2 mutations is the fragmentation of the mitochondrial network [176] and abnormal distribution of mitochondria in neurons [177]. Defective mitochondrial protein synthesis has been proposed as the molecular mechanism of fragmentation in Purkinje neurons [177]. The fragmentation leaves many mitochondria without ER connections limiting the Ca 2þ distribution along the mitochondrial network [176].…”

Assignment of 2′-O-Methyltransferases to Modification Sites on the Mammalian Mitochondrial Large Subunit 16 S Ribosomal RNA (rRNA)

“…Many of the disease mutations are found in tRNA or mRNA sequences; however, some have been found in the 12 S and 16 S rRNAs (3-6) and nuclear-encoded mitochondrial ribosomal proteins (MRPs) 2 (7)(8)(9). Mutations in AFG3L2 or paraplegin, subunits of an AAA protease that processes mitochondrial ribosomal protein MRPL32 and functions in the assembly of the mitochondrial ribosome (mitoribosome), are associated with hereditary spastic paraplegia, spinocerebellar ataxia type 28, and spastic ataxia-neuropathy syndrome, highlighting the importance of proper mitochondrial protein synthesis in mammals (10,11).…”