Investigating the brain in mouse models of Duchenne muscular dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is an X-linked recessive muscle wasting disease caused by mutations in the DMD gene, which encodes the large cytoskeletal protein dystrophin. Alongside severe muscle pathology, one-third of DMD patients exhibit cognitive problems ranging from reduced verbal intelligence to severe autism. There is conclusive evidence that the muscle pathology exhibited by DMD patients is progressive, yet it remains unknown whether the cognitive impairments in DMD are also progressive. Previous studies have highlighted a cognitive impairment in the mdx mouse model of DMD, but no studies have investigated if this cognitive impairment worsens with age. We assessed the consequences of dystrophin deficiency on brain morphology and cognitive function in two dystrophin-deficient mouse models (mdx and Cmah-/-mdx mice). The overall project aim was to identify outcome measures to monitor central nervous system (CNS) pathology non-invasively in DMD mice. Magnetic resonance imaging (MRI) identified a total brain volume increase in DMD mice, alongside morphological changes in brain ventricles. Behavioural testing revealed a deficit in hippocampal spatial learning and memory, particularly long-term memory, in mdx mice, which appears to progressively worsen with age. Immunoblotting identified a progressive reduction of aquaporin-4 (AQP4) expression, the major water channel of the CNS, in DMD mice. Moreover, contrast enhancing MRI and Evans blue extravasation demonstrated a progressive impairment in blood-brain barrier (BBB) integrity in mdx mice. Proteomic profiling of the mdx cerebellum identified changes in expression of mitochondrial subunit complexes, suggestive of changes in mitochondrial function. Additionally, elevated levels of inflammatory markers were identified and confirmed in the mdx cerebellum. Our studies suggest that dystrophin deficiency causes a progressive cognitive impairment in mdx mice. We also present evidence showing that changes in osmotic equilibrium may be involved in the pathogenesis of DMD, with reductions in AQP4 expression and BBB disruptions. We speculate that some of the changes in the mdx cerebellar proteome, in comparison to wild type mice, iii serve as compensatory mechanisms whilst others may contribute directly to cognitive dysfunction in DMD. These results support a role for dystrophin in normal brain morphology and cognitive function.