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Abnormal Purkinje cell dendritogenesis in AFG3L2-deficient mice. Calbindin staining of the developing dendritic tree of Purkinje cells in WT animals at P7 (A) and P14 (C). In Afg3l2 Emv66/Emv66 mice, dendritogenesis is strongly impaired (B and D). At P7, the Purkinje cells have migrated to their position, but still retain bipolar morphology and fail to develop dendrites (B). Dendrites are still largely underdeveloped at P14 (D). Bar: 50 mm. Figure 5. Degeneration of hippocampal CA3 pyramidal neurons. (A and B) Reactive astrogliosis is detected in the hippocampus with an anti-GFAP antibody in the lacunosum-moleculare and radial layers of the CA3 field, deep into the hippocampal fissure (hf), in Spg7 2/2 Afg3l2 Emv66/þ mice at 13 weeks of age (B). (C and D) Semithin sections across the pyramidal layer of the CA3 field reveal loss of neuron and degenerative features in the Spg7 2/2 Afg3l2 Emv66/þ mice (D, arrow). (E and F) Granule neurons of the dentate gyrus appear normal in both genotypes. Bars: 200 mm in A–B; 20 mm in C– F. hf, hippocampal fissure; dg, dentate gyrus; CA1, CA1 field; CA3, CA3 field.
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The mitochondrial m-AAA protease has a crucial role in axonal development and maintenance. Human mitochondria possess two m-AAA protease isoenzymes: a hetero-oligomeric complex, composed of paraplegin and AFG3L2 (Afg3 like 2), and a homo-oligomeric
AFG3L2 complex. Loss of function of paraplegin (encoded by the SPG7 gene) causes hereditary spastic p...
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The m-AAA (ATPases Associated with a variety of cellular Activities) is an evolutionary conserved metalloprotease complex located in the internal mitochondrial membrane. In the mouse, it is a hetero-oligomer variably formed by the Spg7, Afg3l1, and Afg3l2 encoded proteins, or a homo-oligomer formed by either Afg3l1 or Afg3l2. In humans, AFG3L2 and...
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... A novel G671R [c.2011G > C] mutation that was located in a highly conserved region of AFG3L2 gene in five patients of a Hungarian family showed similar characteristics to previously identified cases with no cognitive impairment [106] and [99] have identified two mutations-homozygous missense and homozygous null-that caused lethality in mice models due to impaired axon development in both CNS and PNS delayed myelination, and weak axonal radial growth [56]. In Spg7 deficient mice, Emv66 mutants of AFG3L2 show exacerbating axonopathy and severe neuromuscular defects in hetero-and homozygous conditions respectively with loss of PCs and parallel fibers [109]. Heterozygous P688T [c.2062C > A] mutation at highly conserved site in exon 16 in three patients of a family for the first time reported non-neuronal skeletal muscle fiber atrophy type I along with SCA28 phenotypes [110]. ...
AFG3L2 is a zinc metalloprotease and an ATPase localized in an inner mitochondrial membrane involved in mitochondrial quality control of several nuclear- and mitochondrial-encoded proteins. Mutations in AFG3L2 lead to diseases like slow progressive ataxia, which is a neurological disorder. This review delineates the cellular functions of AFG3L2 and its dysfunction that leads to major clinical outcomes, which include spinocerebellar ataxia type 28, spastic ataxia type 5, and optic atrophy type 12. It summarizes all relevant AFG3L2 mutations associated with the clinical outcomes to understand the detailed mechanisms attributable to its structure-related multifaceted roles in proteostasis and quality control. We face early diagnostic challenges of ataxia and optic neuropathy due to asymptomatic parents and variable clinical manifestations due to heterozygosity/homozygosity of AFG3L2 mutations. This review intends to promote AFG3L2 as a putative prognostic or diagnostic marker.
Functions, mutations, and clinical manifestations in AFG3L2, a mitochondrial AAA + ATPases.
... 啮齿动物中m-AAA 27,28] . HSP截瘫患者的SPG缺 失不会导致脑线粒体中m-AAA蛋白酶活性的丧失, 可 能是由于形成具有改变底物特异性的m-AAA蛋白酶 导致 [83] . ...
... The human genome contains only two functional genes encoding for subunits of the m-AAA protease, however in the mouse a third homologue, Afg3l1, encodes for an additional component of homo-oligomeric and hetero-oligomeric m-AAA complexes. 10,49 The levels of Afg3l1 in the nervous system are low 50 and Afg3l1 −/− mice do not show a neuropathological phenotype 39 ; however, the expression of this subunit may partially compensate for the loss of Spg7. Indeed, the previously developed model for Spg7 deficiency displayed mild and late onset axonal degeneration in the spinal cord, but did not show cerebellar abnormalities. ...
Hereditary spastic paraplegia is a neurological condition characterized by predominant axonal degeneration in long spinal tracts, leading to weakness and spasticity in the lower limbs. The NAD + -consuming enzyme SARM1 has emerged as a key executioner of axonal degeneration upon nerve transection and in some neuropathies. An increase in the nicotinamide mononucleotide/NAD+ ratio activates SARM1, causing catastrophic NAD+ depletion and axonal degeneration. However, the role of SARM1 in the pathogenesis of hereditary spastic paraplegia has not been investigated.
Here, we report an enhanced mouse model for hereditary spastic paraplegia caused by mutations in SPG7. eSpg7 knock-out mice carry a deletion in both Spg7 and Afg3l1, a redundant homologue expressed in mice but not in humans. eSpg7 knock-out mice recapitulate the phenotypic features of human patients, showing progressive symptoms of spastic-ataxia and degeneration of axons in the spinal cord as well as the cerebellum. We show that the lack of SPG7 rewires the mitochondrial proteome in both tissues, leading to an early onset decrease in mitoribosomal subunits and a remodelling of mitochondrial solute carriers and transporters.
To interrogate mechanisms leading to axonal degeneration in this mouse model, we explored the involvement of SARM1. Deletion of SARM1 delays the appearance of ataxic signs, rescues mitochondrial swelling and axonal degeneration of cerebellar granule cells and dampens neuroinflammation in the cerebellum. The loss of SARM1 also prevents endoplasmic reticulum abnormalities in long spinal cord axons, but does not halt the degeneration of these axons. Our data thus reveal a neuron-specific interplay between SARM1 and mitochondrial dysfunction caused by lack of SPG7 in hereditary spastic paraplegia.
... These homologous proteins compose the m-AAA protease, a hexameric proteolytic complex embedded into sever mitochondrial membrane (IMM), which is conserved from yeast to human (Patron et al., 2018). In most cells, homo-oligomeric AFG3L2 complexes are more abundant than hetero-oligomeric complexes of SPG7 (also called paraplegin) and AFG3L2, although neurons contain increased levels of SPG7 (Koppen et al., 2007;Martinelli et al., 2009). So far, there is no evidence for substrate specificities for the different assemblies, and it is thus possible that the manifestation of HSP or spinocerebellar ataxia, upon mutations of m-AAA protease subunits, derives from different expression levels of functionally redundant subunits in individual neurons. ...
Degenerative ataxias and hereditary spastic paraplegias (HSPs) form a continuous, often overlapping disease spectrum sharing not only phenotypic features and underlying genes, but also cellular pathways and disease mechanisms. Mitochondrial metabolism presents a major molecular theme underlying both multiple ataxias and HSPs, thus indicating a heightened vulnerability of Purkinje cells, spinocerebellar tracts, and motor neurons to mitochondrial dysfunction, which is of particular interest for translational approaches. Mitochondrial dysfunction might be the primary (upstream) or secondary (downstream) result of a genetic defect, with underlying genetic defects in nuclear-encoded genes being much more frequent than in mtDNA genes in both, ataxias and HSPs. Here, we outline the substantial number of ataxias, spastic ataxias and HSPs caused by mutated genes implicated in (primary or secondary) mitochondrial dysfunction, highlighting several key "mitochondrial" ataxias and HSPs which are of particular interest for their frequency, pathogenesis and translational opportunities. We then showcase prototypic mitochondrial mechanisms by which disruption of these ataxia and HSP genes contributes to Purkinje cells or corticospinal neuron dysfunction, thus elucidating hypotheses on Purkinje cells and corticospinal neuron vulnerability to mitochondrial dysfunction.
... Human i-AAA mutations that destabilize complex formation lead to neuromuscular disorders including intellectual disability, motor developmental delay, optic atrophy, ataxia and movement deficiencies (Hartmann et al., 2016;Sprenger et al., 2019). Two types of m-AAA proteases are present in human: the AFG3L2 homocomplex and the AFG3L2/SPG7 heterocomplex (Martinelli et al., 2009;Tatsuta and Langer, 2009;Pierson et al., 2011). Recessive mutations in SPG7 (paraplegin) cause hereditary spastic paraplegia (HSP7) (Casari et al., 1998). ...
The FtsH family of proteases are membrane-anchored, ATP-dependent, zinc metalloproteases. They are universally present in prokaryotes and the mitochondria and chloroplasts of eukaryotic cells. Most bacteria bear a single ftsH gene that produces hexameric homocomplexes with diverse house-keeping roles. However, in mitochondria, chloroplasts and cyanobacteria, multiple FtsH homologs form homo- and heterocomplexes with specialized functions in maintaining photosynthesis and respiration. The diversification of FtsH homologs combined with selective pairing of FtsH isomers is a versatile strategy to enable functional adaptation. In this article we summarize recent progress in understanding the evolution, structure and function of FtsH proteases with a focus on the role of FtsH in photosynthesis and respiration.
... Paraplegin, encoded by SPG7, and AFG3L2, are components of the m-AAA protease complex, located in the inner mitochondrial membrane. This proteolytic complex degrades misfolded proteins and regulates ribosome assembly [32,103]. Recently, it has been described that mutations in AFG3L2 destabilize OPA1 leading to mitochondrial fragmentation [104]. ...
Mitochondrial DNA depletion and multiple deletions syndromes (MDDS) constitute a group of mitochondrial diseases defined by dysfunctional mitochondrial DNA (mtDNA) replication and maintenance. As is the case for many other mitochondrial diseases, the options for the treatment of these disorders are rather limited today. Some aggressive treatments such as liver transplantation or allogeneic stem cell transplantation are among the few available options for patients with some forms of MDDS. However, in recent years, significant advances in our knowledge of the biochemical pathomechanisms accounting for dysfunctional mtDNA replication have been achieved, which has opened new prospects for the treatment of these often fatal diseases. Current strategies under investigation to treat MDDS range from small molecule substrate enhancement approaches to more complex treatments, such as lentiviral or adenoassociated vector-mediated gene therapy. Some of these experimental therapies have already reached the clinical phase with very promising results, however, they are hampered by the fact that these are all rare disorders and so the patient recruitment potential for clinical trials is very limited.
... Vertebrates produce two distinct m-AAA proteases, one composed entirely of Afg3l2 and a second composed of Afg3l2 and Paraplegin, which is encoded by the SPG7 gene in humans [17]. It has also been shown that haploinsufficiency of AFG3L2 exacerbates the axonopathy observed in Paraplegin-deficient mice, thus supporting the functional relationship between these two proteases [18]. To test whether Afg3l2 and Paraplegin also assemble into a functional heteromultimeric complex in Drosophila, we performed two experiments. ...
The m-AAA proteases play a critical role in the proteostasis of inner mitochondrial membrane proteins, and mutations in the genes encoding these proteases cause severe incurable neurological diseases. To further explore the biological role of the m-AAA proteases and the pathological consequences of their deficiency, we used a genetic approach in the fruit fly Drosophila melanogaster to inactivate the ATPase family gene 3-like 2 (AFG3L2) gene, which encodes a critical component of the m-AAA proteases. We found that null alleles of Drosophila AFG3L2 die early in development, but partial inactivation of AFG3L2 using RNAi allowed survival to the late pupal and adult stages of development. Flies with partial inactivation of AFG3L2 exhibited behavioral defects, neurodegeneration, accumulation of unfolded mitochondrial proteins, and diminished respiratory chain (RC) activity. Further work revealed that the reduced RC activity was primarily a consequence of severely diminished mitochondrial transcription and translation. These defects were accompanied by activation of the mitochondrial unfolded protein response (mito-UPR) and autophagy. Overexpression of mito-UPR components partially rescued the AFG3L2-deficient phenotypes, indicating that protein aggregation partly accounts for the defects of AFG3L2-deficient animals. Our work suggests that strategies designed to activate mitochondrial stress pathways and mitochondrial gene expression could be therapeutic in the diseases caused by mutations in AFG3L2.
... In addition, an early-onset severe neurologic phenotype in Spg7-null/Afg3l2 +/− mice characterized by loss of balance, tremor, and ataxia were detected. These mice displayed acceleration and worsening of the axonopathy as observed in Spg7-null mice [45]. Seipin KO mice displayed anxiety and depression-like symptoms. ...
Motor neuron circuitry is one of the most elaborate circuitries in our body, which ensures voluntary and skilled movement that requires cognitive input. Therefore, both the cortex and the spinal cord are involved. The cortex has special importance for motor neuron diseases, in which initiation and modulation of voluntary movement is affected. Amyotrophic lateral sclerosis (ALS) is defined by the progressive degeneration of both the upper and lower motor neurons, whereas hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS) are characterized mainly by the loss of upper motor neurons. In an effort to reveal the cellular and molecular basis of neuronal degeneration, numerous model systems are generated, and mouse models are no exception. However, there are many different levels of complexities that need to be considered when developing mouse models. Here, we focus our attention to the upper motor neurons, which are one of the most challenging neuron populations to study. Since mice and human differ greatly at a species level, but the cells/neurons in mice and human share many common aspects of cell biology, we offer a solution by focusing our attention to the affected neurons to reveal the complexities of diseases at a cellular level and to improve translational efforts.
... To explore the function of the m-AAA protease in astrocytes, we developed two novel mouse models in which the Afg3l2 gene, alone or together with the redundant Afg3l1 paralogue, was specifically deleted in astrocytes postnatally. Afg3l1 is highly homologous to Afg3l2 in the mouse genome and has similar functional properties but a different expression pattern (Koppen et al., 2007;Martinelli et al., 2009), while it is a pseudogene in humans (Kremmidiotis et al., 2001). Afg3l1 knockout mice have no detectable phenotype up to 1 year of age . ...
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.
... Correspondingly, genetic loss of any of AAA proteases is associated with severe pleiotropic phenotypes, including defects in respiratory function and in mitochondrial morphology as well as oxidative stress [10,13,[23][24][25]65,66]. Depending on species/tissue context, the absence of the AAA proteases-dependent surveillance may have diverse consequences, including cell death in the worst-case scenario. For instance, m-AAA malfunction in mammals is linked to substantial neuronal loss [62,[67][68][69][70][71][72]. Recent studies indicated that neuronal death triggered by m-AAA protease deficiency is mainly a consequence of the deregulation of mitochondrial Ca 2+ homeostasis [44,67,68]. ...
... Respectively, disturbances in the performance of these ATP-dependent enzymes are associated with decline in mitochondrial health leading to development of many pathological conditions [10,23,67]. Studies using mouse models indicated that depletion in activities of AAA proteases might result in malfunction of critical cellular processes, including decline in activity of OXPHOS complexes, impairment in mitochondrial translation, disturbances in mitochondrial morphology, calcium deregulation as well as dysfunction of mitochondrial anterograde transport [62,[66][67][68][69][70][71][72]. Mitochondrial abnormalities predominantly influence organs and tissues with the high energetic requirements contributing to onset of cardiovascular, neurodegenerative, or complex metabolic diseases such as type 2 diabetes mellitus [10,23,67]. ...
Mitochondria are dynamic, semi-autonomous organelles that execute numerous life-sustaining tasks in eukaryotic cells. Functioning of mitochondria depends on the adequate action of versatile proteinaceous machineries. Fine-tuning of mitochondrial activity in response to cellular needs involves continuous remodeling of organellar proteome. This process not only includes modulation of various biogenetic pathways, but also the removal of superfluous proteins by adenosine triphosphate (ATP)-driven proteolytic machineries. Accordingly, all mitochondrial sub-compartments are under persistent surveillance of ATP-dependent proteases. Particularly important are highly conserved two inner mitochondrial membrane-bound metalloproteases known as m-AAA and i-AAA (ATPases associated with diverse cellular activities), whose mis-functioning may lead to impaired organellar function and consequently to development of severe diseases. Herein, we discuss the current knowledge of yeast, mammalian, and plant AAA proteases and their implications in mitochondrial function and homeostasis maintenance.
