Haploinsufficiency ofAFG3L2, the Gene Responsible for Spinocerebellar Ataxia Type 28, Causes Mitochondria-Mediated Purkinje Cell Dark Degeneration

Abstract

Paraplegin and AFG3L2 are ubiquitous nuclear-encoded mitochondrial proteins that form hetero-oligomeric paraplegin-AFG3L2 and homo-oligomeric AFG3L2 complexes in the inner mitochondrial membrane, named m-AAA proteases. These complexes ensure protein quality control in the inner membrane, jointly with a chaperone-like activity on the respiratory chain complexes. Despite coassembling in the same complex, mutations of either paraplegin or AFG3L2 cause two different neurodegenerative disorders. Indeed, mutations of paraplegin are responsible for a recessive form of hereditary spastic paraplegia, whereas mutations of AFG3L2 have been recently associated to a dominant form of spinocerebellar ataxia (SCA28). In this work, we report that the mouse model haploinsufficient for Afg3l2 recapitulates important pathophysiological features of the human disease, thus representing the first SCA28 model. Furthermore, we propose a pathogenetic mechanism in which respiratory chain dysfunction and increased reactive oxygen species production caused by Afg3l2 haploinsufficiency lead to dark degeneration of Purkinje cells and cerebellar dysfunction.

Purkinje neuron Ca2+ influx reduction rescues ataxia in SCA28 model

“…However, the coexpression of WT and mutant AFG3L2 is able to correct the respiratory defect for most of the mutations, indicating that haploinsufficiency is the disease-causing mechanism (2). Remarkably, a partial AFG3L2 deletion (exons [14][15][16] has been recently identified as the cause of SCA28 in 2 large Belgian families, and functional studies revealed a reduced number of m-AAA complexes in these patients, demonstrating that SCA28 is due to haploinsufficiency (15). Indeed, the Afg3l2-haploinsufficient mouse resembles the phenotype of SCA28 patients, showing a progressive decline in motor performance caused by dark degeneration of Purkinje cells (PC-DCD) (16).…”

“…Remarkably, a partial AFG3L2 deletion (exons [14][15][16] has been recently identified as the cause of SCA28 in 2 large Belgian families, and functional studies revealed a reduced number of m-AAA complexes in these patients, demonstrating that SCA28 is due to haploinsufficiency (15). Indeed, the Afg3l2-haploinsufficient mouse resembles the phenotype of SCA28 patients, showing a progressive decline in motor performance caused by dark degeneration of Purkinje cells (PC-DCD) (16). The latter is a specific form of a cell degeneration intermediate between necrosis and apoptosis and is characterized by cell shrinkage, cytoplasm darkening, and chromatin condensation (17).…”

“…The latter is a specific form of a cell degeneration intermediate between necrosis and apoptosis and is characterized by cell shrinkage, cytoplasm darkening, and chromatin condensation (17). In the SCA28 mouse, this phenomenon is unique, since it originates from mitochondrial dysfunction (16), while in other forms of SCA, it is associated with excitotoxicity and high levels of intracellular Ca 2+ (18)(19)(20).…”

Early onset and slow progression of SCA28, a rare dominant ataxia in a large four-generation family with a novel AFG3L2 mutation

“…Hence, this mouse model seems to be adequate to unravel the pathological cascade leading to this form of SCA. 8 Notably, the six AFG3L2 mutations known so far occur in exons 15 and 16, both of which contribute to the peptidase M41 domain.…”

Lgr4 Protein Deficiency Induces Ataxia-like Phenotype in Mice and Impairs Long Term Depression at Cerebellar Parallel Fiber-Purkinje Cell Synapses

“…In the balance beam test, mice were trained to walk from a start platform along a 28-cm wide, 1-m long square beam suspended 50 cm above bedding (30,31). Mice were allowed to stay on the beam for a maximum of 60 s. The latency to traverse each beam and the number of times the hind feet slipped off each beam were recorded for each trial.…”