Carlo Minetti

Phenotypic Behavior of Caveolin-3 Mutations That Cause Autosomal Dominant Limb Girdle Muscular Dystrophy (LGMD-1C) RETENTION OF LGMD-1C CAVEOLIN-3 MUTANTS WITHIN THE GOLGI COMPLEX

Caveolin-3, a muscle-specific caveolin-related protein, is the principal structural protein of caveolae membrane domains in striated muscle cell types (cardiac and skeletal). Autosomal dominant limb girdle muscular dystrophy (LGMD-1C) in humans is due to mutations within the caveolin-3 gene: (i) a 9-base pair microdeletion that removes three amino acids within the caveolin scaffolding domain (ΔTFT) or (ii) a missense mutation within the membrane spanning domain (P → L). The molecular mechanisms by which these two mutations cause muscular dystrophy remain unknown. Here, we investigate the phenotypic behavior of these caveolin-3 mutations using heterologous expression. Wild type caveolin-3 or caveolin-3 mutants were transiently expressed in NIH 3T3 cells. LGMD-1C mutants of caveolin-3 (ΔTFT or P → L) were primarily retained at the level of a perinuclear compartment that we identified as the Golgi complex in double-labeling experiments, while wild type caveolin-3 was efficiently targeted to the plasma membrane. In accordance with these observations, caveolin-3 mutants formed oligomers of a much larger size than wild type caveolin-3 and were excluded from caveolae-enriched membrane fractions as seen by sucrose density gradient centrifugation. In addition, these caveolin-3 mutants were expressed at significantly lower levels and had a dramatically shortened half-life of ∼45–60 min. However, caveolin-3 mutants were palmitoylated to the same extent as wild type caveolin-3, indicating that targeting to the plasma membrane is not required for palmitoylation of caveolin-3. In conclusion, we show that LGMD-1C mutations lead to formation of unstable high molecular mass aggregates of caveolin-3 that are retained within the Golgi complex and are not targeted to the plasma membrane. Consistent with its autosomal dominant form of genetic transmission, we demonstrate that LGMD-1C mutants of caveolin-3 behave in a dominant-negative fashion, causing the retention of wild type caveolin-3 at the level of the Golgi. These data provide a molecular explanation for why caveolin-3 levels are down-regulated in patients with this form of limb girdle muscular dystrophy (LGMD-1C).

Mutation in the CAV3 gene causes partial caveolin-3 deficiency and persistent elevated levels of serum creatine kinase

Article abstract Mutations in the caveolin-3 (CAV3) gene are associated with autosomal dominant limb–girdle muscular dystrophy (LGMD1C). The authors report a novel sporadic mutation in the CAV3 gene in two unrelated children with persistent elevated levels of serum creatine kinase (hyperCKemia) without muscle weakness. Immunohistochemistry and quantitative immunoblot analysis of caveolin-3 showed reduced expression of the protein in muscle fibers. Our data indicate that a partial caveolin-3 deficiency should be considered in the differential diagnosis of idiopathic hyperCKemia.