Kurt W. Prins

Dystrophin is a microtubule-associated protein

Cytolinkers are giant proteins that can stabilize cells by linking actin filaments, intermediate filaments, and microtubules (MTs) to transmembrane complexes. Dystrophin is functionally similar to cytolinkers, as it links the multiple components of the cellular cytoskeleton to the transmembrane dystroglycan complex. Although no direct link between dystrophin and MTs has been documented, costamere-associated MTs are disrupted when dystrophin is absent. Using tissue-based cosedimentation assays on mice expressing endogenous dystrophin or truncated transgene products, we find that constructs harboring spectrinlike repeat 24 through the first third of the WW domain cosediment with MTs. Purified Dp260, a truncated isoform of dystrophin, bound MTs with a Kd of 0.66 µM, a stoichiometry of 1 Dp260/1.4 tubulin heterodimer at saturation, and stabilizes MTs from cold-induced depolymerization. Finally, α- and β-tubulin expression is increased ∼2.5-fold in mdx skeletal muscle without altering the tubulin–MT equilibrium. Collectively, these data suggest dystrophin directly organizes and/or stabilizes costameric MTs and classifies dystrophin as a cytolinker in skeletal muscle.

Effects of Beta-Blocker Withdrawal in Acute Decompensated Heart Failure: A Systematic Review and Meta-Analysis

OBJECTIVES This study sought to evaluate the effects of beta-blocker withdrawal in acute decompensated heart failure (ADHF). BACKGROUND Published reports showed trends for either no harm or increased risk of in-hospital mortality, short-term mortality, and rehospitalization rates in patients admitted for ADHF that discontinued beta-blockers; however, a comprehensive analysis has not been conducted. METHODS Relevant studies from January 2000 through January 2015 were identified in the PubMed, EMBASE, and COCHRANE electronic databases. Where appropriate data were available, weighted relative risks were estimated using random-effects meta-analysis techniques. RESULTS Five observational studies and 1 randomized clinical trial (n = 2,704 patients who continued beta-blocker therapy and n = 439 patients who discontinued beta-blocker therapy) that reported the short-term effects of beta-blocker withdrawal in ADHF were included in the analyses. In 2 studies, beta-blocker withdrawal significantly increased risk of in-hospital mortality (risk ratio: 3.72; 95% confidence interval [CI]: 1.51 to 9.14). Short-term mortality (relative risk: 1.61; 95% CI: 1.04 to 2.49; 4 studies) and combined short-term rehospitalization or death (relative risk: 1.59; 95% CI: 1.03 to 2.45; 4 studies) were also significantly increased. CONCLUSIONS Discontinuation of beta-blockers in patients admitted with ADHF was associated with significantly increased in-hospital mortality, short-term mortality, and the combined endpoint of short-term rehospitalization or mortality. These data suggest beta-blockers should be continued in ADHF patients if their clinical picture allows.

Context-dependent functional substitution of α-skeletal actin by γ-cytoplasmic actin

We generated transgenic mice that overexpressed γ‐cyto actin 2000‐fold above wild‐type levels in skeletal muscle. 7‐cyto actin comprised 40% of total actin in transgenic skeletal muscle, with a concomitant 40% decrease in α‐actin. Surprisingly, transgenic muscle was histologically and ultrastructurally identical to wild‐type muscle despite near‐stoichiometric incorporation of γ–cyto actin into sarcomeric thin filaments. Furthermore, several parameters of muscle physiological performance in the transgenic animals were not different from wild type. Given these surprising results, we tested whether overexpression of γ‐cyto actin could rescue the early postnatal lethality in α‐sk actin‐null mice (Acta1–/–). By quantitative Western blot analysis, we found total actin levels were decreased by 35% in Actal–/– muscle. Although transgenic overexpression of γ‐cyto actin on the Actal /_ background restored total actin levels to wild type, resulting in thin filaments composed of 60% γ‐cyto actin and a 40% mixture of cardiac and vascular actin, the life span of transgenic Actal–/– mice was not extended. These results indicate that sarcomeric thin filaments can accommodate substantial incorporation of γ‐cyto actin without functional consequences, yet γ‐cyto actin cannot fully substitute for α‐sk actin.—Jaeger, M. A., Sonnemann, K. J., Fitzsimons, D. P., Prins, K. W., Ervasti, J. M. Contextdependent functional substitution of a‐skeletal actin by y‐cytoplasmic actin. FASEB J. 23, 2205–2214 (2009)

The Critical Role of Pulmonary Arterial Compliance in Pulmonary Hypertension

The normal pulmonary circulation is a low-pressure, high-compliance system. Pulmonary arterial compliance decreases in the presence of pulmonary hypertension because of increased extracellular matrix/collagen deposition in the pulmonary arteries. Loss of pulmonary arterial compliance has been consistently shown to be a predictor of increased mortality in patients with pulmonary hypertension, even more so than pulmonary vascular resistance in some studies. Decreased pulmonary arterial compliance causes premature reflection of waves from the distal pulmonary vasculature, leading to increased pulsatile right ventricular afterload and eventually right ventricular failure. Evidence suggests that decreased pulmonary arterial compliance is a cause rather than a consequence of distal small vessel proliferative vasculopathy. Pulmonary arterial compliance decreases early in the disease process even when pulmonary artery pressure and pulmonary vascular resistance are normal, potentially enabling early diagnosis of pulmonary vascular disease, especially in high-risk populations. With the recognition of the prognostic importance of pulmonary arterial compliance, its impact on right ventricular function, and its contributory role in the development and progression of distal small-vessel proliferative vasculopathy, pulmonary arterial compliance is an attractive target for the treatment of pulmonary hypertension.

Skeletal Muscle-Specific Ablation of γcyto-Actin Does Not Exacerbate the mdx Phenotype

We previously documented a ten-fold increase in γcyto-actin expression in dystrophin-deficient skeletal muscle and hypothesized that increased γcyto-actin expression may participate in an adaptive cytoskeletal remodeling response. To explore whether increased γcyto-actin fortifies the cortical cytoskeleton in dystrophic skeletal muscle, we generated double knockout mice lacking both dystrophin and γcyto-actin specifically in skeletal muscle (ms-DKO). Surprisingly, dystrophin-deficient mdx and ms-DKO mice presented with comparable levels of myofiber necrosis, membrane instability, and deficits in muscle function. The lack of an exacerbated phenotype in ms-DKO mice suggests γcyto-actin and dystrophin function in a common pathway. Finally, because both mdx and ms-DKO skeletal muscle showed similar levels of utrophin expression and presented with identical dystrophies, we conclude utrophin can partially compensate for the loss of dystrophin independent of a γcyto-actin-utrophin interaction.

Quadriceps myopathy caused by skeletal muscle-specific ablation of βcyto-actin

Quadriceps myopathy (QM) is a rare form of muscle disease characterized by pathological changes predominately localized to the quadriceps. Although numerous inheritance patterns have been implicated in QM, several QM patients harbor deletions in dystrophin. Two defined deletions predicted loss of functional spectrin-like repeats 17 and 18. Spectrin-like repeat 17 participates in actin-filament binding, and thus we hypothesized that disruption of a dystrophin–cytoplasmic actin interaction might be one of the mechanisms underlying QM. To test this hypothesis, we generated mice deficient for βcyto-actin in skeletal muscles (Actb-msKO). Actb-msKO mice presented with a progressive increase in the proportion of centrally nucleated fibers in the quadriceps, an approximately 50% decrease in dystrophin protein expression without alteration in transcript levels, deficits in repeated maximal treadmill tests, and heightened sensitivity to eccentric contractions. Collectively, these results suggest that perturbing a dystrophin–βcyto-actin linkage decreases dystrophin stability, which results in a QM, and implicates βcyto-actin as a possible candidate gene in QM pathology.

Pulmonary Hypertension Secondary to Heart Failure With Preserved Ejection Fraction

Pulmonary hypertension (PH) secondary to heart failure with preserved ejection fraction (HFpEF) is an increasingly recognized cause of PH due to an emerging epidemic of HFpEF. The mechanisms underlying the pathogenesis of PH in HFpEF are not well established, but the presence of PH and right ventricular dysfunction in HFpEF is associated with worse prognosis. Currently, it is unclear whether PH is just a marker of underlying disease severity or whether it could be a target of treatment in HFpEF. Although PH-HFpEF and pulmonary arterial hypertension share several clinical characteristics, the evidence supporting the use of pulmonary arterial hypertension-specific therapies in PH-HFpEF is limited. Here, we review the disease classification, epidemiology, proposed pathophysiology, and treatments for PH-HFpEF. Our limited understanding highlights an urgent need for more research to elucidate the pathogenesis of PH in HFpEF and to develop novel therapies for this challenging syndrome.

Destabilization of the Dystrophin-Glycoprotein Complex without Functional Deficits in α-Dystrobrevin Null Muscle

α-Dystrobrevin is a component of the dystrophin-glycoprotein complex (DGC) and is thought to have both structural and signaling roles in skeletal muscle. Mice deficient for α-dystrobrevin (adbn−/−) exhibit extensive myofiber degeneration and neuromuscular junction abnormalities. However, the biochemical stability of the DGC and the functional performance of adbn−/− muscle have not been characterized. Here we show that the biochemical association between dystrophin and β-dystroglycan is compromised in adbn−/− skeletal muscle, suggesting that α-dystrobrevin plays a structural role in stabilizing the DGC. However, despite muscle cell death and DGC destabilization, costamere organization and physiological performance is normal in adbn−/− skeletal muscle. Our results demonstrate that myofiber degeneration alone does not cause functional deficits and suggests that more complex pathological factors contribute to the development of muscle weakness in muscular dystrophy.

Microtubule-Mediated Misregulation of Junctophilin-2 Underlies T-Tubule Disruptions and Calcium Mishandling in mdx Mice

Summary Cardiac myocytes from the mdx mouse, the mouse model of Duchenne muscular dystrophy, exhibit t-tubule disarray and increased calcium sparks, but a unifying molecular mechanism has not been elucidated. Recently, improper trafficking of junctophilin (JPH)-2 on an altered microtubule network caused t-tubule derangements and calcium mishandling in a pressure-overload heart failure model. Mdx cardiac myocytes have microtubule abnormalities, but how this may affect JPH-2, t-tubules, and calcium handling has not been established. Here, we investigated the hypothesis that an inverse relationship between microtubules and JPH-2 underlies t-tubule disruptions and calcium mishandling in mdx cardiac myocytes. Confocal microscopy revealed t-tubule disorganization in mdx cardiac myocytes. Quantitative Western blot analysis demonstrated JPH-2 was decreased by 75% and showed an inverse hyperbolic relationship with α- and β-tubulin, the individual components of microtubules, in mdx hearts. Colchicine-induced microtubule depolymerization normalized JPH-2 protein levels and localization, corrected t-tubule architecture, and reduced calcium sparks. In summary, these results suggest microtubule-mediated misregulation of JPH-2 causes t-tubule derangements and altered calcium handling in mdx cardiac myocytes.

Assessing continuous renal replacement therapy as a rescue strategy in cardiorenal syndrome 1

Background Patients with acute decompensated heart failure (ADHF) and cardiorenal syndrome (CRS) 1 have poor outcomes. Ultrafiltration (UF) is used to mechanically remove salt and water in ADHF patients with diuretic resistance. However, little is known about the outcomes of ADHF patients on inotropes and/or vasopressors who require continuous renal replacement therapy (CRRT) for both UF and solute clearance in severe acute kidney injury. Methods We retrospectively analyzed 37 consecutive critically ill patients who were admitted for ADHF from 2005–13 and were on inotropes and/or vasopressors at the time of CRRT initiation. The primary outcome was in-hospital mortality. Results In-hospital mortality rate was 62%. Median survival was 15.5 days after CRRT initiation, and 10 months following hospital discharge. When comparing renal and cardiovascular variables for survivors and non-survivors at baseline, admission and CRRT initiation, survivors were less likely to need vasopressors. After controlling for multiple predictors, vasopressor use remained associated with time to death (HR 9.9; 95% CI 2.3–43.3; P = 0.002). Patients with isolated right ventricular dysfunction had an in-hospital mortality of 45% compared with 69% in those with left ventricular dysfunction (P = 0.27). Age of >70 years was associated with 100% in-hospital mortality. Conclusions Rescue therapy using CRRT in refractory CRS1 was associated with high in-hospital mortality, especially when vasopressors were used and when patient age exceeded 70 years. Additionally, survivors had a poor long-term prognosis.