SPG7

SPG7 (paraplegin) is a subunit of the mitochondrial inner membrane m-AAA protease complex, where it co-assembles with AFG3L2 to perform ATP-dependent proteolysis essential for mitochondrial proteostasis, respiratory chain function, and mitochondrial DNA maintenance (PMID:14623864, PMID:24727571). The m-AAA protease degrades non-assembled EMRE, thereby preventing constitutively active MCU channels from causing mitochondrial Ca²⁺ overload and neuronal death (PMID:27642048). SPG7 is also required for transient mPTP flickering through a SIRT3–cyclophilin D acetylation axis; loss of SPG7 impairs mPTP opening, disrupts presynaptic neurotransmitter release, and causes corticospinal neurodegeneration underlying autosomal recessive hereditary spastic paraplegia type 7 and progressive external ophthalmoplegia with multiple mtDNA deletions (PMID:33045469, PMID:9635427, PMID:24727571).

1. 1998 High

   Identification of SPG7 as a nuclear-encoded mitochondrial metalloprotease whose mutations cause hereditary spastic paraplegia established the first link between mitochondrial proteolysis and corticospinal neurodegeneration.

   Evidence Immunofluorescence, mitochondrial import assays, patient mutation analysis, and muscle biopsy OXPHOS studies in human cells and patient tissue

   PMID:9635427

   Open questions at the time

   • Protease substrates unknown
   • Assembly state of paraplegin in the inner membrane not defined
   • Mechanism linking OXPHOS defect to selective neurodegeneration unresolved
2. 2003 High

   Demonstration that paraplegin co-assembles with AFG3L2 into the m-AAA protease complex resolved the quaternary structure of the active enzyme and linked complex I deficiency and oxidative stress sensitivity to loss of this complex.

   Evidence Co-immunoprecipitation, Blue Native PAGE, enzymatic activity assays, yeast complementation, and patient fibroblast studies

   PMID:14623864

   Open questions at the time

   • Direct substrates of the human m-AAA protease not yet identified
   • Relative contribution of homo- vs hetero-oligomeric complexes in different tissues unclear
3. 2012 Medium

   Extension of the SPG7 disease spectrum to optic neuropathy — including a dominant missense variant — broadened understanding of paraplegin's role beyond spastic paraplegia to visual pathway neurodegeneration.

   Evidence MLPA, Sanger sequencing, OCT imaging, and segregation analysis in large pedigrees

   PMID:23065789

   Open questions at the time

   • Biochemical consequence of the Asp411Ala variant not characterized
   • Mechanism of dominance not established
   • Limited to genetic and clinical evidence without reconstitution
4. 2014 High

   Discovery that SPG7 mutations cause progressive external ophthalmoplegia with multiple mtDNA deletions revealed a direct role for the m-AAA protease in mitochondrial DNA maintenance.

   Evidence Whole exome sequencing, mitochondrial network analysis, single-fibre mtDNA deletion mapping, and deep mtDNA resequencing in patient muscle and fibroblasts

   PMID:24727571

   Open questions at the time

   • Molecular mechanism by which paraplegin loss leads to mtDNA deletions undefined
   • Whether mtDNA instability is a primary or secondary consequence of proteostasis failure unclear
5. 2015 High

   Identification of SPG7 as a core component of the mitochondrial permeability transition pore, interacting with VDAC and cyclophilin D, established an unexpected structural role beyond canonical proteolysis.

   Evidence RNAi screen, co-immunoprecipitation of the PTP complex, mitochondrial Ca²⁺ retention capacity assays, ΔΨm measurements, and cell death assays

   PMID:26387735

   Open questions at the time

   • Whether the mPTP role is separable from the protease catalytic function not determined
   • Stoichiometry and architecture of the SPG7-VDAC-CypD complex unresolved
6. 2016 High

   Identification of EMRE as a direct m-AAA protease substrate explained how paraplegin loss causes mitochondrial Ca²⁺ overload: unprocessed EMRE generates constitutively active MCU channels lacking MICU1/MICU2 gatekeeper subunits, linking protease activity to Ca²⁺ signaling and neuronal death.

   Evidence Neuronal interactome mapping by co-IP/mass spectrometry, EMRE turnover assays, BN-PAGE MCU complex analysis, mitochondrial Ca²⁺ uptake assays, and genetic mouse models

   PMID:27642048

   Open questions at the time

   • Full substrate repertoire of the SPG7-containing m-AAA protease beyond EMRE remains unknown
   • Contribution of MAIP1 (C2ORF47) cofactor to substrate specificity not fully defined
7. 2020 High

   Dissection of the SIRT3–cyclophilin D axis showed that paraplegin loss upregulates SIRT3, hyperdeacetylates CypD, and thereby impairs transient mPTP flickering — a mechanism directly responsible for defective presynaptic neurotransmitter release; pharmacological bypass with Bz-423 rescued motor deficits in vivo.

   Evidence Live-cell mPTP flickering assays, SIRT3 activity and CypD acetylation analysis, electrophysiology of synaptic transmission, and Bz-423 pharmacological rescue in Spg7⁻/⁻ mice

   PMID:33045469

   Open questions at the time

   • How SPG7 loss leads to SIRT3 upregulation is mechanistically unclear
   • Whether mPTP flickering defect contributes to all SPG7-associated phenotypes (e.g., optic neuropathy, PEO) untested

• A unified structural model explaining how SPG7 participates simultaneously in m-AAA protease catalysis and mPTP regulation — and whether these are separable or interdependent functions — remains unresolved.
• No high-resolution structure of the human SPG7-AFG3L2 m-AAA complex
• Complete substrate catalog of the m-AAA protease not established
• Mechanism linking paraplegin loss to mtDNA instability remains undefined