Cullin-3 regulates vascular smooth muscle function and arterial blood pressure via PPARγ and RhoA/Rho-kinase.

Dominant-negative (DN) mutations in the nuclear hormone receptor peroxisome proliferator-activated receptor-γ (PPARγ) cause hypertension by an unknown mechanism. Hypertension and vascular dysfunction are recapitulated by expression of DN PPARγ specifically in vascular smooth muscle of transgenic mice. DN PPARγ increases RhoA and Rho-kinase activity, and inhibition of Rho-kinase restores normal reactivity and reduces arterial pressure. RhoBTB1, a component of the Cullin-3 RING E3 ubiquitin ligase complex, is a PPARγ target gene. Decreased RhoBTB1, Cullin-3, and neddylated Cullin-3 correlated with increased levels of the Cullin-3 substrate RhoA. Knockdown of Cullin-3 or inhibition of cullin-RING ligase activity in aortic smooth muscle cells increased RhoA. Cullin-RING ligase inhibition enhanced agonist-mediated contraction in aortic rings from normal mice by a Rho-kinase-dependent mechanism, and it increased arterial pressure in vivo. We conclude that Cullin-3 regulates vascular function and arterial pressure, thus providing a mechanistic link between mutations in Cullin-3 and hypertension in humans.

Caveolin 3 is associated with the calcium release complex and is modified via in vivo triadin modification.

The triadin isoforms Trisk 95 and Trisk 51 are both components of the skeletal muscle calcium release complex. To investigate the specific role of Trisk 95 and Trisk 51 isoforms in muscle physiology, we overexpressed Trisk 95 or Trisk 51 using adenovirus-mediated gene transfer in skeletal muscle of newborn mice. Overexpression of either Trisk 95 or Trisk 51 alters the muscle fiber morphology, while leaving unchanged the expression of the ryanodine receptor, the dihydropyridine receptor, and calsequestrin. We also observe an aberrant expression of caveolin 3 in both Trisk 95- and Trisk 51-overexpressing skeletal muscles. Using a biochemical approach, we demonstrate that caveolin 3 is associated with the calcium release complex in skeletal muscle. Taking advantage of muscle and non-muscle cell culture models and triadin null mouse skeletal muscle, we further dissect the molecular organization of the caveolin 3-containing calcium release complex. Our data demonstrate that the association of caveolin 3 with the calcium release complex occurs via a direct interaction with the transmembrane domain of the ryanodine receptor. Taken together, these data suggest that caveolin 3-containing membrane domains and the calcium release complex are functionally linked and that Trisk 95 and Trisk 51 are instrumental to the regulation of this interaction, the integrity of which may be crucial for muscle physiology.

Does peroxisome proliferator-activated receptor-gamma (PPAR gamma) protect from hypertension directly through effects in the vasculature?

Peroxisome proliferator-activated receptor-gamma (PPAR gamma) is a ligand-activated transcription factor of the nuclear hormone receptor superfamily. Increasing evidence suggests that PPAR gamma is involved in the regulation of vascular function and blood pressure in addition to its well recognized role in metabolism. Thiazolidinediones, PPAR gamma agonists, lower blood pressure and have protective vascular effects through largely unknown mechanisms. In contrast, loss-of-function dominant-negative mutations in human PPAR gamma cause insulin resistance and severe early onset hypertension. Recent studies using genetically manipulated mouse models have begun to specifically address the importance of PPAR gamma in the vasculature. In this minireview, evidence for a protective role of PPAR gamma in the endothelium and vascular smooth muscle, derived largely from studies of genetically manipulated mice, will be discussed.

Sarcoglycan complex: implications for metabolic defects in muscular dystrophies.

The sarcoglycans are known as an integral subcomplex of the dystrophin glycoprotein complex, the function of which is best characterized in skeletal muscle in relation to muscular dystrophies. Here we demonstrate that the white adipocytes, which share a common precursor with the myocytes, express a cell-specific sarcoglycan complex containing beta-, delta-, and epsilon-sarcoglycan. In addition, the adipose sarcoglycan complex associates with sarcospan and laminin binding dystroglycan. Using multiple sarcoglycan null mouse models, we show that loss of alpha-sarcoglycan has no consequence on the expression of the adipocyte sarcoglycan complex. However, loss of beta- or delta-sarcoglycan leads to a concomitant loss of the sarcoglycan complex as well as sarcospan and a dramatic reduction in dystroglycan in adipocytes. We further demonstrate that beta-sarcoglycan null mice, which lack the sarcoglycan complex in adipose tissue and skeletal muscle, are glucose-intolerant and exhibit whole body insulin resistance specifically due to impaired insulin-stimulated glucose uptake in skeletal muscles. Thus, our data demonstrate a novel function of the sarcoglycan complex in whole body glucose homeostasis and skeletal muscle metabolism, suggesting that the impairment of the skeletal muscle metabolism influences the pathogenesis of muscular dystrophy.

Defective membrane repair in dysferlin-deficient muscular dystrophy.

Muscular dystrophy includes a diverse group of inherited muscle diseases characterized by wasting and weakness of skeletal muscle. Mutations in dysferlin are linked to two clinically distinct muscle diseases, limb-girdle muscular dystrophy type 2B and Miyoshi myopathy, but the mechanism that leads to muscle degeneration is unknown. Dysferlin is a homologue of the Caenorhabditis elegans fer-1 gene, which mediates vesicle fusion to the plasma membrane in spermatids. Here we show that dysferlin-null mice maintain a functional dystrophin-glycoprotein complex but nevertheless develop a progressive muscular dystrophy. In normal muscle, membrane patches enriched in dysferlin can be detected in response to sarcolemma injuries. In contrast, there are sub-sarcolemmal accumulations of vesicles in dysferlin-null muscle. Membrane repair assays with a two-photon laser-scanning microscope demonstrated that wild-type muscle fibres efficiently reseal their sarcolemma in the presence of Ca2+. Interestingly, dysferlin-deficient muscle fibres are defective in Ca2+-dependent sarcolemma resealing. Membrane repair is therefore an active process in skeletal muscle fibres, and dysferlin has an essential role in this process. Our findings show that disruption of the muscle membrane repair machinery is responsible for dysferlin-deficient muscle degeneration, and highlight the importance of this basic cellular mechanism of membrane resealing in human disease.

Human skeletal muscle triadin: gene organization and cloning of the major isoform, Trisk 51.

We obtained the gene organization of human triadin gene by aligning the DNA coding sequence of human 95-kDa triadin (Trisk 95) with human genomic database. We identified a novel human triadin isoform, a potential human homologue of rat Trisk 51. We show that both isoforms of triadin, Trisk 51 and Trisk 95, are alternative splice variants of the same gene. We demonstrated experimentally the existence of this Trisk 51 transcript in human skeletal muscle and cloned its full length cDNA. We further demonstrated that the protein encoded by this transcript is expressed in the human skeletal muscle. In addition, unlike other species, Trisk 51 is the major triadin isoform expressed in human skeletal muscle, whereas Trisk 95 is below the detection level in the two types of muscles tested.