Hiroki Hagiwara

Muscular atrophy of caveolin-3–deficient mice is rescued by myostatin inhibition

Caveolin-3, the muscle-specific isoform of caveolins, plays important roles in signal transduction. Dominant-negative mutations of the caveolin-3 gene cause autosomal dominant limb-girdle muscular dystrophy 1C (LGMD1C) with loss of caveolin-3. However, identification of the precise molecular mechanism leading to muscular atrophy in caveolin-3-deficient muscle has remained elusive. Myostatin, a member of the muscle-specific TGF-beta superfamily, negatively regulates skeletal muscle volume. Here we report that caveolin-3 inhibited myostatin signaling by suppressing activation of its type I receptor; this was followed by hypophosphorylation of an intracellular effector, Mad homolog 2 (Smad2), and decreased downstream transcriptional activity. Loss of caveolin-3 in P104L mutant caveolin-3 transgenic mice caused muscular atrophy with increase in phosphorylated Smad2 (p-Smad2) as well as p21 (also known as Cdkn1a), a myostatin target gene. Introduction of the myostatin prodomain, an inhibitor of myostatin, by genetic crossing or intraperitoneal administration of the soluble type II myostatin receptor, another inhibitor, ameliorated muscular atrophy of the mutant caveolin-3 transgenic mice with suppression of p-Smad2 and p21 levels. These findings suggest that caveolin-3 normally suppresses the myostatin-mediated signal, thereby preventing muscular atrophy, and that hyperactivation of myostatin signaling participates in the pathogenesis of muscular atrophy in a mouse model of LGMD1C. Myostatin inhibition may be a promising therapy for LGMD1C patients.

Mechanism of taxane neurotoxicity.

The two taxanes (paclitaxel and docetaxel) are widely employed in standard antineoplastic practice. Although these agents are now well established, some toxic side effects have been reported. Toxicity of these agents includes bone marrow suppression (principally neutropenia), hypersensitivity reactions, cutaneus reactions, edema and neurotoxicity. The most prominent neurotoxicity is a sensory neuropathy. Controlling neuropathy is crucial for maintaining the quality of life of patients because it is usually persistent and hard to manage. The precise mechanism for taxane-induced neuropathy is still unknown. The taxanes are known to promote aggregation of intracellular microtubules. Abnormal aggregation of micro-tubules in the neuronal cells may cause this neuropathy. In addition, the taxanes have been suggested to have intrinsic toxicity and directly injure the cells. A better understanding of the mechanism for this neuropathy may improve the quality of life of patients who undergo taxane antineoplastic therapy.

Biglycan regulates the expression and sarcolemmal localization of dystrobrevin, syntrophin, and nNOS

The dystrophin‐associated protein complex (DAPC) provides a linkage between the cytoskeleton and the extracellular matrix (ECM) and is also a scaffold for a host of signaling molecules. The constituents of the DAPC must be targeted to the sarcolemma in order to properly function. Biglycan is an ECM molecule that associates with the DAPC. Here, we show that biglycan null mice exhibit a mild dystrophic phenotype and display a selective reduction in the localization of α‐dystrobrevin‐1 and ‐2, α‐ and βl‐syntrophin, and nNOS at the sarcolemma. Purified biglycan induces nNOS redistribution to the plasma membrane in cultured muscle cells. Biglycan protein injected into muscle becomes stably associated with the sarcolemma and ECM for at least 2 wk. This injected biglycan restores the sarcolemmal expression of α‐dystrobrevin‐1 and ‐2, and β1‐ and β2‐syntrophin in biglycan null mice. We conclude that biglycan is important for the maintenance of muscle cell integrity and plays a direct role in regulating the expression and sarcolemmal localization of the intracellular signaling proteins dystrobrevin‐1 and ‐2, α‐ and β1‐syntrophin and nNOS.—Mercado, M. L., Amenta, A. R., Hagiwara, H., Rafii, M. S., Lechner, B., Owens, R. T., McQuillan, D. J., Froehner, S. C., Fallon, J. R. Biglycan regulates the expression and sarcolemmal localization of dystrobrevin, syntrophin, and nNOS. FASEB J. 20, E1075–E1085 (2006)

Biglycan binds to α- and γ-sarcoglycan and regulates their expression during development†

The dystrophin‐associated protein complex (DAPC), which links the cytoskeleton to the extracellular matrix, is essential for muscle cell survival, and is defective in a wide range of muscular dystrophies. The DAPC contains two transmembrane subcomplexes—the dystroglycans and the sarcoglycans. Although several extracellular binding partners have been identified for the dystroglycans, none have been described for the sarcoglycan subcomplex. Here we show that the small leucine‐rich repeat (LRR) proteoglycan biglycan binds to α‐ and γ‐sarcoglycan as judged by ligand blot overlay and co‐immunoprecipitation assays. Our studies with biglycan‐decorin chimeras show that α‐ and γ‐sarcoglycan bind to distinct sites on the polypeptide core of biglycan. Both biglycan proteoglycan as well as biglycan polypeptide lacking glycosaminoglycan (GAG) side chains are components of the dystrophin glycoprotein complex isolated from adult skeletal muscle membranes. Finally, our immunohistochemical and biochemical studies with biglycan null mice show that the expression of α‐ and γ‐sarcoglycan is selectively reduced in muscle from young (P14‐P21) animals, while levels in adult muscle (≥P35) are unchanged. We conclude that biglycan is a ligand for two members of the sarcoglycan complex and regulates their expression at discrete developmental ages. J. Cell. Physiol. 209: 439–447, 2006.

Biglycan Is an Extracellular MuSK Binding Protein Important for Synapse Stability

The receptor tyrosine kinase MuSK is indispensable for nerve-muscle synapse formation and maintenance. MuSK is necessary for prepatterning of the endplate zone anlage and as a signaling receptor for agrin-mediated postsynaptic differentiation. MuSK-associated proteins such as Dok7, LRP4, and Wnt11r are involved in these early events in neuromuscular junction formation. However, the mechanisms regulating synapse stability are poorly understood. Here we examine a novel role for the extracellular matrix protein biglycan in synapse stability. Synaptic development in fetal and early postnatal biglycan null (bgn−/o) muscle is indistinguishable from wild-type controls. However, by 5 weeks after birth, nerve-muscle synapses in bgn−/o mice are abnormal as judged by the presence of perijunctional folds, increased segmentation, and focal misalignment of acetylcholinesterase and AChRs. These observations indicate that previously occupied presynaptic and postsynaptic territory has been vacated. Biglycan binds MuSK and the levels of this receptor tyrosine kinase are selectively reduced at bgn−/o synapses. In bgn−/o myotubes, the initial stages of agrin-induced MuSK phosphorylation and AChR clustering are normal, but the AChR clusters are unstable. This stability defect can be substantially rescued by the addition of purified biglycan. Together, these results indicate that biglycan is an extracellular ligand for MuSK that is important for synapse stability.

Bone marrow transplantation improves outcome in a mouse model of congenital muscular dystrophy

We examined whether pathogenesis in dystrophin‐deficient (mdx) mice and laminin‐α2‐deficient (dy) mice is ameliorated by bone marrow transplantation (BMT). Green fluorescent protein (GFP) mice were used as donors. In mdx mice, BMT failed to produce any significant differences in muscle pathology, although some GFP‐positive fibers with restored dystrophin expression were observed. In contrast, in the dy mice, BMT led to a significant increase in lifespan and an increase in growth rate, muscle strength, and respiratory function. We conclude that BMT improved outcome in dy mice but not mdx mice.

Reduced amplitude of the sural nerve sensory action potential in PARK2 patients

The authors performed nerve conduction studies in nine PARK2 and eight idiopathic Parkinson disease patients and found a significant reduction of sural sensory nerve action potential (SNAP) amplitude in eight PARK2 patients who mostly remained asymptomatic. These data suggest that sensory axonal neuropathy may be a common clinical feature of PARK2 and a reduced amplitude of sural SNAP could be a diagnostic indicator of PARK2.