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Articles by G Mearini
Total Records ( 4 ) for G Mearini
  N Vignier , S Schlossarek , B Fraysse , G Mearini , E Kramer , H Pointu , N Mougenot , J Guiard , R Reimer , H Hohenberg , K Schwartz , M Vernet , T Eschenhagen and L. Carrier

Rationale: Mutations in the MYBPC3 gene encoding cardiac myosin-binding protein (cMyBP)-C are frequent causes of hypertrophic cardiomyopathy, but the mechanisms leading from mutations to disease remain elusive.

Objective: The goal of the present study was therefore to gain insights into the mechanisms controlling the expression of MYBPC3 mutations.

Methods and Results: We developed a cMyBP-C knock-in mouse carrying a point mutation. The level of total cMyBP-C mRNAs was 50% and 80% lower in heterozygotes and homozygotes, respectively. Surprisingly, the single G>A transition on the last nucleotide of exon 6 resulted in 3 different mutant mRNAs: missense (exchange of G for A), nonsense (exon skipping, frameshift, and premature stop codon) and deletion/insertion (as nonsense but with additional partial retention of downstream intron, restoring of the reading frame, and almost full-length protein). Inhibition of nonsense-mediated mRNA decay in cultured cardiac myocytes or in vivo with emetine or cycloheximide increased the level of nonsense mRNAs severalfold but not of the other mRNAs. By using sequential protein fractionation and a new antibody directed against novel amino acids produced by the frameshift, we showed that inhibition of the proteasome with epoxomicin via osmotic minipumps increased the level of (near) full-length mutants but not of truncated proteins. Homozygotes exhibited myocyte and left ventricular hypertrophy, reduced fractional shortening, and interstitial fibrosis; heterozygotes had no major phenotype.

Conclusions: These data reveal (1) an unanticipated complexity of the expression of a single point mutation in the whole animal and (2) the involvement of both nonsense-mediated mRNA decay and the ubiquitin–proteasome system in lowering the level of mutant proteins.

  A Belus , N Piroddi , C Ferrantini , C Tesi , O Cazorla , L Toniolo , M Drost , G Mearini , L Carrier , A Rossi , A Mugelli , E Cerbai , J van der Velden and C. Poggesi

Rationale: Chronic atrial fibrillation (cAF) is associated with atrial contractile dysfunction. Sarcomere remodeling may contribute to this contractile disorder.

Objective: Here, we use single atrial myofibrils and fast solution switching techniques to directly investigate the impact of cAF on myofilament mechanical function eliminating changes induced by the arrhythmia in atrial myocytes membranes and extracellular components. Remodeling of sarcomere proteins potentially related to the observed mechanical changes is also investigated.

Methods and Results: Myofibrils were isolated from atrial samples of 15 patients in sinus rhythm and 16 patients with cAF. Active tension changes following fast increase and decrease in [Ca2+] and the sarcomere length–passive tension relation were determined in the 2 groups of myofibrils. Compared to sinus rhythm myofibrils, cAF myofibrils showed (1) a reduction in maximum tension and in the rates of tension activation and relaxation; (2) an increase in myofilament Ca2+ sensitivity; (3) a reduction in myofibril passive tension. The slow β-myosin heavy chain isoform and the more compliant titin isoform N2BA were up regulated in cAF myofibrils. Phosphorylation of multiple myofilament proteins was increased in cAF as compared to sinus rhythm atrial myocardium.

Conclusions: Alterations in active and passive tension generation at the sarcomere level, explained by translational and post-translational changes of multiple myofilament proteins, are part of the contractile dysfunction of human cAF and may contribute to the self-perpetuation of the arrhythmia and the development of atrial dilatation.

  D. J Duncker , N. M Boontje , D Merkus , A Versteilen , J Krysiak , G Mearini , A El Armouche , V. J de Beer , J. M.J Lamers , L Carrier , L. A Walker , W. A Linke , G. J.M Stienen and J. van der Velden

Background— Myofilament contractility of individual cardiomyocytes is depressed in remote noninfarcted myocardium and contributes to global left ventricular pump dysfunction after myocardial infarction (MI). Here, we investigated whether β-blocker therapy could restore myofilament contractility.

Methods and Results— In pigs with a MI induced by ligation of the left circumflex coronary artery, β-blocker therapy (bisoprolol, MI+β) was initiated on the first day after MI. Remote left ventricular subendocardial biopsies were taken 3 weeks after sham or MI surgery. Isometric force was measured in single permeabilized cardiomyocytes. Maximal force (Fmax) was lower, whereas Ca2+ sensitivity was higher in untreated MI compared with sham (both P<0.05). The difference in Ca2+ sensitivity was abolished by treatment of cells with the β-adrenergic kinase, protein kinase A. β-blocker therapy partially reversed Fmax and Ca2+ sensitivity to sham values and significantly reduced passive force. Despite the lower myofilament Ca2+ sensitivity in MI+β compared with untreated myocardium, the protein kinase A induced reduction in Ca2+ sensitivity was largest in cardiomyocytes from myocardium treated with β-blockers. Phosphorylation of β-adrenergic target proteins (myosin binding protein C and troponin I) did not differ among groups, whereas myosin light chain 2 phosphorylation was reduced in MI, which coincided with increased expression of protein phosphatase 1. β-blockade fully restored the latter alterations and significantly reduced expression of protein phosphatase 2a.

Conclusions— β-blockade reversed myofilament dysfunction and enhanced myofilament responsiveness to protein kinase A in remote myocardium after MI. These effects likely contribute to the beneficial effects of β-blockade on global left ventricular function after MI.

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