m-AAA proteases, mitochondrial calcium homeostasis and neurodegeneration

Area of Science:

• Mitochondrial biology
• Neurogenetics
• Molecular cell biology

Background:

• Mitochondrial inner membrane m-AAA proteases are essential ATP-dependent peptidases involved in maintaining mitochondrial function.
• These proteases, including AFG3L2 and SPG7, form hexameric complexes and are implicated in neurodegenerative disorders like spinocerebellar ataxia (SCA28) and hereditary spastic paraplegia (HSP).
• m-AAA proteases play pleiotropic roles in mitochondrial quality control, protein synthesis, respiration, and axonal transport.

Purpose of the Study:

• To explore the intricate relationship between m-AAA proteases and mitochondrial calcium homeostasis.
• To elucidate the role of m-AAA proteases in the pathogenesis of neurodegenerative diseases.
• To present findings from a novel mouse model lacking the mitochondrial calcium uniporter (MCU) specifically in Purkinje cells.

Main Methods:

• Analysis of AFG3L2 and SPG7 subunit assembly into homo- and hetero-oligomeric complexes.
• Investigation of mitochondrial dysfunction, including protein synthesis, respiration, and axonal transport, in protease-deficient models.
• Examination of the impact of m-AAA proteases on the mitochondrial calcium uniporter (MCU) complex assembly and function, particularly the degradation of the EMRE subunit.
• Utilizing a novel mouse model with Purkinje cell-specific MCU deletion to assess the role of calcium influx in ataxia.

Main Results:

• Mutations in AFG3L2 cause SCA28, affecting Purkinje cells, while SPG7 mutations lead to HSP, affecting motor neurons.
• Loss of m-AAA protease function results in mitochondrial fragmentation, impaired protein synthesis, and disrupted mitochondrial axonal transport.
• AFG3L2 deficiency leads to impaired EMRE degradation, deregulated MCU complex formation, increased mitochondrial calcium uptake, and heightened neuronal vulnerability to calcium-induced cell death.
• Reduced cytosolic calcium influx in Purkinje cells ameliorates ataxia in an AFG3L2-deficient mouse model.

Conclusions:

• m-AAA proteases are critical regulators of mitochondrial calcium homeostasis, linking protease function to neuronal vulnerability.
• Dysregulation of mitochondrial calcium handling by m-AAA proteases is a key mechanism in the pathogenesis of associated neurodegenerative disorders.
• Targeting mitochondrial calcium regulation presents a potential therapeutic strategy for neurodegeneration linked to m-AAA protease dysfunction.