Impaired flickering of the permeability transition pore causes SPG7 spastic paraplegia

Abstract

Background Mutations of the mitochondrial protein paraplegin cause hereditary spastic paraplegia type 7 (SPG7), a so-far untreatable degenerative disease of the upper motoneuron with still undefined pathomechanism. The intermittent mitochondrial permeability transition pore (mPTP) opening, called flickering, is an essential process that operates to maintain mitochondrial homeostasis by reducing intra-matrix Ca 2+ and reactive oxygen species (ROS) concentration, and is critical for efficient synaptic function. Methods We use a fluorescence-based approach to measure mPTP flickering in living cells and biochemical and molecular biology techniques to dissect the pathogenic mechanism of SPG7. In the SPG7 animal model we evaluate the potential improvement of the motor defect, neuroinflammation and neurodegeneration by means of an mPTP inducer, the benzodiazepine Bz-423. Findings We demonstrate that paraplegin is required for efficient transient opening of the mPTP, that is impaired in both SPG7 patients-derived fibroblasts and primary neurons from Spg7 −/− mice. We show that dysregulation of mPTP opening at the pre-synaptic terminal impairs neurotransmitter release leading to ineffective synaptic transmission. Lack of paraplegin impairs mPTP flickering by a mechanism involving increased expression and activity of sirtuin3, which promotes deacetylation of cyclophilin D, thus hampering mPTP opening. Pharmacological treatment with Bz-423, which bypasses the activity of CypD, normalizes synaptic transmission and rescues the motor impairment of the SPG7 mouse model. Interpretation mPTP targeting opens a new avenue for the potential therapy of this form of spastic paraplegia. Funding Telethon Foundation grant (TGMGCSBX16TT); Dept. of Defense, US Army, grant W81XWH-18–1–0001

MICS1 is the Ca2+/H+ antiporter of mammalian mitochondria

“…The lack of a functional CHE also has severe implications on the permeability transition when Na + dependent Ca 2+ efflux is concomitantly blocked, as revealed by thapsigargin-induced hypersensitization of PTP opening. One reason that may explain why MICS1KO mitochondria are so sensitive to the PTP opening is reduced levels of Sirt3, which is responsible for deacetylation of CypD, a key PTP sensitizer (Sambri et al, 2020). The result is in accordance with the modulatory effect of thapsigargin on shifting the ratio between bound and free Ca 2+ towards free Ca 2+ (Korge and Weiss, 1999).…”

Calpain-mediated protein targets in cardiac mitochondria following ischemia–reperfusion

“…A total of 7 proteins were identified to be more abundant in the deletion ischemia–reperfusion hearts (> two-fold, p value < 0.05) including α-crystallin B chain, paraplegin, and sarcalumenin (Table 4 ). Among these up-regulated proteins, paraplegin, an ATP-dependent zinc metalloprotease, plays a role in assembling and regulating the mitochondrial permeability transition pore (MPTP) 35 .…”

Mitochondrial membrane proteins and VPS35 orchestrate selective removal of mtDNA

“…Mitochondrial matrix proteases (mAAAs) represent a group of enzymes related to mitochondrial quality control and mitochondrial membrane remodeling upon proteolytic cleavage of Opa1 and Oma1. Spg7 has been found to copurify with Prohibitin participating in the formation of the permeability transition pore 58,59 . Mutations in the SPG7 gene are the cause for Hereditary Spastic Paraplegia Type 7 but also a Progressive External Optalmoplegia-like syndrome with accumulation of mtDNA deletions 60 .…”