PTEN contributes to profound PI3K/Akt signaling pathway deregulation in dystrophin-deficient dog muscle

Duchenne muscular dystrophy is the most common and severe form of muscular dystrophy, and although the genetic basis of this disease is well defined, the overall mechanisms that define its pathogenesis remain obscure. Alterations in individual signaling pathways have been described, but little information is available regarding their putative implications in Duchenne muscular dystrophy pathogenesis. Here, we studied the status of various major signaling pathways in the Golden Retriever muscular dystrophy dog that specifically reproduces the full spectrum of human pathology. Using antibody arrays, we found that Akt1, glycogen synthase kinase-3beta (GSK3beta), 70-kDa ribosomal protein S6 kinase (p70S6K), extracellular signal-regulated kinases 1/2, and p38delta and p38gamma kinases all exhibited decreased phosphorylation in muscle from a 4-month-old animal with Golden Retriever muscular dystrophy, revealing a deep alteration of the phosphatidylinositol 3-kinase (PI3K)/Akt and mitogen-activated protein kinase pathways. Immunohistochemistry analysis revealed the presence of muscle fibers exhibiting a cytosolic accumulation of Akt1, GSK3beta, and phosphatidylinositol-3,4,5-trisphosphate 3-phosphatase (PTEN), an enzyme counteracting PI3K-mediated Akt activation. Enzymatic assays established that these alterations in phosphorylation and expression levels were associated with decreased Akt and increased GSK3beta and PTEN activities. PTEN/GSK3beta-positive fibers were also observed in muscle sections from 3- and 36-month-old animals, indicating long-term PI3K/Akt pathway alteration. Collectively, our data suggest that increased PTEN expression and activity play a central role in PI3K/Akt/GSK3beta and p70S6K pathway modulation, which could exacerbate the consequences of dystrophin deficiency.