Journal Article

Genetic interaction between the <i>m</i>-AAA protease isoenzymes reveals novel roles in cerebellar degeneration

Paola Martinelli, Veronica La Mattina, Andrea Bernacchia, Raffaella Magnoni, Federica Cerri, Gregory Cox, Angelo Quattrini, Giorgio Casari and Elena I. Rugarli

in Human Molecular Genetics

Volume 18, issue 11, pages 2001-2013
Published in print June 2009 | ISSN: 0964-6906
Published online March 2009 | e-ISSN: 1460-2083 | DOI: http://dx.doi.org/10.1093/hmg/ddp124
Genetic interaction between the m-AAA protease isoenzymes reveals novel roles in cerebellar degeneration

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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 paraplegia, a disease characterized by retrograde degeneration of cortical motor axons. Spg7−/− mice show a late-onset degeneration of long spinal and peripheral axons with accumulation of abnormal mitochondria. In contrast, Afg3l2Emv66/Emv66 mutant mice, lacking the AFG3L2 protein, are affected by a severe neuromuscular phenotype, due to defects in motor axon development. The role of the homo-oligomeric m-AAA protease and the extent of cooperation and redundancy between the two isoenzymes in adult neurons are still unclear. Here we report an early-onset severe neurological phenotype in Spg7−/− Afg3l2Emv66/+ mice, characterized by loss of balance, tremor and ataxia. Spg7−/− Afg3l2Emv66/+ mice display acceleration and worsening of the axonopathy observed in paraplegin-deficient mice. In addition, they show prominent cerebellar degeneration with loss of Purkinje cells and parallel fibers, and reactive astrogliosis. Mitochondria from affected tissues are prone to lose mt-DNA and have unstable respiratory complexes. At late stages, neurons contain structural abnormal mitochondria defective in COX-SDH reaction. Our data demonstrate genetic interaction between the m-AAA isoenzymes and suggest that different neuronal populations have variable thresholds of susceptibility to reduced levels of the m-AAA protease. Moreover, they implicate impaired mitochondrial proteolysis as a novel pathway in cerebellar degeneration.

Journal Article.  7516 words.  Illustrated.

Subjects: Genetics and Genomics

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