No abstract

Collagen VI is an extracellular matrix protein that forms a microfilamentous network in skeletal muscles and other organs. Inherited mutations in genes encoding collagen VI in humans cause two muscle diseases, Bethlem myopathy and Ullrich congenital muscular dystrophy. We previously generated collagen VI-deficient (Col6a1-/-) mice and showed that they have a muscle phenotype that strongly resembles Bethlem myopathy. The pathophysiological defects and mechanisms leading to the myopathic disorder were not known. Here we show that Col6a1-/- muscles have a loss of contractile strength associated with ultrastructural alterations of sarcoplasmic reticulum (SR) and mitochondria and spontaneous apoptosis. We found a latent mitochondrial dysfunction in myofibers of Col6a1-/- mice on incubation with the selective F1F(O)-ATPase inhibitor oligomycin, which caused mitochondrial depolarization, Ca2+ deregulation and increased apoptosis. These defects were reversible, as they could be normalized by plating Col6a1-/- myofibers on collagen VI or by addition of cyclosporin A (CsA), the inhibitor of mitochondrial permeability transition pore (PTP). Treatment of Col6a1-/- mice with CsA rescued the muscle ultrastructural defects and markedly decreased the number of apoptotic nuclei in vivo. These findings indicate that collagen VI myopathies have an unexpected mitochondrial pathogenesis that could be exploited for therapeutic intervention.

Organelle-specific integral membrane proteins were identified by a novel strategy which gives rise to monospecific antibodies to these proteins as well as to the cDNA clones encoding them. A cDNA expression library was screened with a polyclonal antiserum raised against Triton X-114-extracted organelle proteins and clones were then grouped using antibodies affinity-purified on individual fusion proteins. The identification, molecular cloning and sequencing are described of a type 1 membrane protein (TGN38) which is located specifically in the trans-Golgi network.

TGF-beta proteins are main regulators of blood vessel development and maintenance. Here, we report an unprecedented link between TGF-beta signaling and arterial hypertension based on the analysis of mice mutant for Emilin1, a cysteine-rich secreted glycoprotein expressed in the vascular tree. Emilin1 knockout animals display increased blood pressure, increased peripheral vascular resistance, and reduced vessel size. Mechanistically, we found that Emilin1 inhibits TGF-beta signaling by binding specifically to the proTGF-beta precursor and preventing its maturation by furin convertases in the extracellular space. In support of these findings, genetic inactivation of Emilin1 causes increased TGF-beta signaling in the vascular wall. Strikingly, high blood pressure observed in Emilin1 mutants is rescued to normal levels upon inactivation of a single TGF-beta1 allele. This study highlights the importance of modulation of TGF-beta availability in the pathogenesis of hypertension.

To gain insight into the function of type VI collagen, the col6a1 gene was inactivated by targeted gene disruption in the mouse. The homozygous mutants lacked collagen VI in the tissues and showed histological features of myopathy such as fiber necrosis and phagocytosis and a pronounced variation in the fiber diameter. Muscles also showed signs of stimulated regeneration of fibers. Necrotic fibers were particularly frequent in the diaphragm at all ages examined. Similar, although milder, alterations were detected in heterozygous mutant mice, indicating haploinsufficiency of the col6a1 gene function. The data led us to conclude that collagen VI is necessary for maintenance of the integrity of muscle fibers and that the col6a1 -deficient mouse can be considered an animal model of Bethlem myopathy.