Cystatin C Rescues Degenerating Neurons in a Cystatin B-Knockout Mouse Model of Progressive Myoclonus Epilepsy

Abstract

In vitro studies have shown that cystatin C (CysC) is neuroprotective. Here we demonstrate that CysC is neuroprotective in vivo, in a mouse model of the inherited neurodegenerative disorder, progressive myoclonic epilepsy type 1 (EPM1). Loss-of-function mutations in the cystatin B (CysB) gene, an intracellular cysteine protease inhibitor, lead to this human disease. A CysB-knockout (CysBKO) mouse model develops symptoms that mimic EPM1. CysB deficiency in these mice results in enhanced cathepsin B and D activities, indicating lysosomal dysfunction. We show that expression of CysC is enhanced in the brains of CysBKO mice. Crossbreeding of CysBKO mice with either CysC-overexpressing transgenic mice or CysCknockout mice demonstrates that clinical symptoms and neuropathologies, including motor coordination disorder, cerebellar atrophy, neuronal loss in the cerebellum and cerebral cortex, and gliosis caused by CysB deficiency, are rescued by CysC overexpression and exacerbated by CysC deficiency. Thus, CysC effectively rescues the CysB loss-of-function mutations, facilitating the reversal of pathophysiological changes and suggesting a novel therapeutic intervention for patients with EPM1 and other neurodegenerative disorders.

Cystatin C in aging and in Alzheimer’s disease

“…Enhanced CysC expression in specific cell populations is also observed in other neurodegenerative conditions, such as epilepsy, ischemia, and progressive myoclonus epilepsy (Aronica et al, 2001; Hendriksen et al, 2001; Ishimaru et al, 1996; Kaur et al, 2010; Lukasiuk et al, 2002; Palm et al, 1995). Discussions of the role that enhanced CysC expression plays in these disorders has lead to contradictory interpretations, with arguments being made that increased CysC cellular expression in the brain is associated with neurodegenerative and pathological processes, or alternatively, is part of a neuroprotective response aimed at preventing or minimizing neurodegeneration.…”

“…The progressive cerebellar atrophy caused by CysB deficiency implicates a required role for CysB expression in the development of the cerebellum and in normal neuronal survival. Increased mRNA, protein, and enzymatic activity levels of the two lysosomal enzymes Cat B and Cat D were also demonstrated in the brains of CysBKO mice (Kaur et al, 2010), and multiple findings suggest that increased proteolysis by lysosomal cathepsins is responsible for the phenotypic characteristics of EPM1 (Houseweart et al, 2003; Lieuallen et al, 2001; Rinne et al, 2002). Consistent with excess cathepsin activity driving the disease, deletion of Cat B in CysBKO mice resulted in a reduction in the amount of cerebellar granule cell apoptosis (Houseweart et al, 2003).…”

“…Further evidence that CysC expression in vivo can reduce neurodegeneration by inhibition of cysteine protease has come from studies of a mouse model of an inherited neurodegenerative disorder, progressive myoclonic epilepsy (Kaur et al, 2010). In the rare autosomal disorder Unverricht-Lundborg disease (EPM1) (Houseweart et al, 2003; Lalioti et al, 1997; Pennacchio et al, 1996), the most common form of progressive myoclonus epilepsies (Lafreniere et al, 1997), loss-of-function mutations in the CysB gene lead to the disorder (Houseweart et al, 2003; Lalioti et al, 1997; Pennacchio et al, 1996).…”

“…While CysB has overlapping inhibitory activity with CysC, it is a member of the cystatin family 1 of cysteine protease inhibitors and evolutionarily distinct (Barrett et al, 1986). CysB is mainly localized in lysosomes (Kaur et al, 2010) and also diffusely distributed in the cytoplasm (Houseweart et al, 2003; Kaur et al, 2010). EPM1 has an onset of symptoms at 6–15 years of age and the disease progresses with myoclonic and tonic-clonic seizures (Koskiniemi et al, 1974; Norio and Koskiniemi, 1979), neurological decline, and severe ataxia (Eldridge et al, 1983; Koskiniemi et al, 1974).…”

Role of Cystatin C in Neuroprotection and Its Therapeutic Implications

“…However, the in vivo relevance of these findings remained unclear. In this issue of the journal, the article by Kaur et al 2 presents for the first time results supporting the in vivo efficacy of CysC as a neuroprotective protein.…”