Molecular Pathogenesis of Genetic and Inherited Diseases Differential Effects of Myopathy-Associated Caveolin-3 Mutants on Growth Factor Signaling

Caveolin-3 is an important scaffold protein of cholesterol-rich caveolae. Mutations of caveolin-3 cause hereditary myopathies that comprise remarkably different pathologies. Growth factor signaling plays an important role in muscle physiology; it is influenced by caveolins and cholesterol-rich rafts and might thus be affected by caveolin-3 dysfunction. Prompted by the observation of a marked chronic peripheral neuropathy in a patient suffering from rippling muscle disease due to the R26Q caveolin-3 mutation and because TrkA is expressed by neuronal cells and skeletal muscle fibers, we performed a detailed comparative study on the effect of pathogenic caveolin-3 mutants on the signaling and trafficking of the TrkA nerve growth factor receptor and, for comparison, of the epidermal growth factor receptor. We found that the R26Q mutant slightly and the P28L strongly reduced nerve growth factor signaling in TrkA-transfected cells. Surface biotinylation experiments revealed that the R26Q caveolin-3 mutation markedly reduced the internalization of TrkA, whereas the P28L did not. Moreover, P28L expression led to increased, whereas R26Q expression decreased, epidermal growth factor signaling. Taken together, we found differential effects of the R26Q and P28L caveolin-3 mutants on growth factor signaling. Our findings are of clinical interest because they might help explain the remarkable differences in the degree of muscle lesions caused by caveolin-3 mutations and also the co-occurrence of peripheral neuropathy in the R26Q caveolinopathy case presented. (Am J Pathol 2010, 177:261–270; DOI: 10.2353/ajpath.2010.090741) Caveolae are flask-shaped invaginations of the plasma membrane implicated in membrane-trafficking and signal transduction. Caveolins are major scaffolding proteins of caveolae and lipid rafts. In these structures, cholesterol and lipid-modified signaling molecules such as Src-like kinases, H-Ras, and G-proteins accumulate. Caveolin-3 (M-Caveolin) is one of three members of the caveolin protein family. In contrast to caveolin-1 and caveolin-2, which are ubiquitously expressed, caveolin-3 is predominantly expressed in muscle fibers. The caveolin-3 proteins form homomeric complexes that are transported to the plasma membrane where they are associated with the dystrophin glycoprotein complex. Point mutations in caveolin-3 lead to a dissociation of the assembly process and a decrease of sarcolemmal caveolin-3 levels. The caveolin-3 mutant R26Q has been shown to accumulate in the Golgi complex, which decreases the halflife of both the mutated and the wild-type-caveolin-3 protein. Interestingly, the different caveolin-3 mutations lead to a wide spectrum of phenotypes ranging from clinically asymptomatic hyperCKemia to rippling muscle disease, distal myopathy, and limb girdle muscular dystrophy type 1C. For instance, the point mutation R26Q can cause heterogeneous disease patterns encompassing this whole spectrum of disorders. In contrast, the mutation P28L, which is located in close proximity to mutation R26Q, causes persistent elevated serum levels of creatine kinase only, without overt clinical symptoms. The caveolin-3 mutation A45T leads to the same heterogeneous phenotype as R26Q except for distal myopathy, whereas the G55S mutation is associated with the limb girdle muscular dystrophy type 1C phenotype.