Respiratory dysfunction by AFG3L2 deficiency causes decreased mitochondrial calcium uptake via organellar network
Francesca Maltecca1, Diego De Stefani, Laura Cassina
1San Raffaele Scientific Institute, Vita-Salute San Raffaele University and Center for Translational Genomics and Bioinformatics, Milan-I, Italy.
Abstract:
The mitochondrial protein AFG3L2 forms homo-oligomeric and hetero-oligomeric complexes with paraplegin in the inner mitochondrial membrane, named m-AAA proteases. These complexes are in charge of quality control of misfolded proteins and participate in the regulation of OPA1 proteolytic cleavage, required for mitochondrial fusion. Mutations in AFG3L2 cause spinocerebellar ataxia type 28 and a complex neurodegenerative syndrome of childhood. In this study, we demonstrated that the loss of AFG3L2 in mouse embryonic fibroblasts (MEFs) reduces mitochondrial Ca(2+) uptake capacity. This defect is neither a consequence of global alteration in cellular Ca(2+) homeostasis nor of the reduced driving force for Ca(2+) internalization within mitochondria, since cytosolic Ca(2+) transients and mitochondrial membrane potential remain unaffected. Moreover, experiments in permeabilized cells revealed unaltered mitochondrial Ca(2+) uptake speed in Afg3l2(-/-) cells, indicating the presence of functional Ca(2+) uptake machinery. Our results show that the defective Ca(2+) handling in Afg3l2(-/-) cells is caused by fragmentation of the mitochondrial network, secondary to respiratory dysfunction and the consequent processing of OPA1. This leaves a number of mitochondria devoid of connections to the ER and thus without Ca(2+) elevations, hampering the proper Ca(2+) diffusion along the mitochondrial network. The recovery of mitochondrial fragmentation in Afg3l2(-/-) MEFs by overexpression of OPA1 rescues the impaired mitochondrial Ca(2+) buffering, but fails to restore respiration. By linking mitochondrial morphology and Ca(2+) homeostasis, these findings shed new light in the molecular mechanisms underlining neurodegeneration caused by AFG3L2 mutations.
Insights
Loss of AFG3L2 protein impairs mitochondrial calcium uptake due to mitochondrial fragmentation, not direct defects in calcium handling machinery. This fragmentation disrupts calcium diffusion and is linked to neurodegeneration.
Area of Science:
- Mitochondrial biology
- Neurogenetics
- Cellular physiology
Background:
- AFG3L2 forms m-AAA proteases in the inner mitochondrial membrane, crucial for protein quality control and OPA1 processing.
- Mutations in AFG3L2 are linked to spinocerebellar ataxia type 28 and childhood neurodegenerative syndromes.
- Mitochondrial calcium (Ca2+) handling is vital for cellular function and neuronal health.
Purpose of the Study:
- To investigate the impact of AFG3L2 loss on mitochondrial calcium uptake capacity.
- To elucidate the mechanisms underlying defective mitochondrial calcium handling in AFG3L2-deficient cells.
- To explore the relationship between mitochondrial morphology, calcium homeostasis, and neurodegeneration.
Main Methods:
- Utilized mouse embryonic fibroblasts (MEFs) lacking AFG3L2 (Afg3l2-/-).
- Assessed mitochondrial calcium uptake using various techniques, including in permeabilized cells.
- Analyzed mitochondrial network morphology, respiratory function, and OPA1 processing.
- Investigated the effect of OPA1 overexpression on mitochondrial calcium buffering and respiration.
Main Results:
- Loss of AFG3L2 reduces mitochondrial Ca2+ uptake capacity without affecting global cellular Ca2+ homeostasis or mitochondrial membrane potential.
- Mitochondrial Ca2+ uptake machinery remains functional, but defective handling arises from mitochondrial network fragmentation.
- Fragmentation is secondary to respiratory dysfunction and OPA1 processing, disrupting ER-mitochondria Ca2+ transfer.
- OPA1 overexpression rescues mitochondrial Ca2+ buffering but not respiration in Afg3l2-/- cells.
Conclusions:
- Mitochondrial fragmentation, not a direct defect in Ca2+ uptake, causes impaired mitochondrial Ca2+ handling in AFG3L2-deficient cells.
- This study links mitochondrial morphology and Ca2+ homeostasis to the pathogenesis of AFG3L2-associated neurodegeneration.
- Findings provide new insights into the molecular mechanisms of neurodegenerative diseases caused by AFG3L2 mutations.
Related Concept Videos
01:30Acute Respiratory Failure-III
01:21Acute Respiratory Failure-II
The underlying physiological abnormalities that contribute to hypoxemic respiratory failure include:
01:30The Electron Transport Chain
Inhibitors of the electron transport chain
Rotenone, a widely used pesticide, prevents electron transfer from Fe-S cluster to ubiquinone or Q in...
01:41The Supercomplexes in the Crista Membrane
01:48ATP Synthase: Mechanism
01:10Atelectasis II: Pathophysiology