Myopalladin promotes muscle growth through modulation of the serum response factor pathway

Myopalladin (MYPN) is a striated muscle-specific, immunoglobulin-containing protein located in the Z-line and I-band of the sarcomere as well as the nucleus. Heterozygous MYPN gene mutations are associated with hypertrophic, dilated, and restrictive cardiomyopathy, and homozygous loss-of-function truncating mutations have recently been identified in patients with cap myopathy, nemaline myopathy, and congenital myopathy with hanging big toe

The power output of a myosin II-based nanomachine mimicking the striated muscle

This thesis reports the realization and first application of a synthetic nanomachine, able to reproduce in vitro the performance emerging from the array arrangement of myosin II motors in the sarcomere of the striated muscle. The nanomachine consists of an ensemble of less than ten myosin dimers from fast skeletal muscle disposed on a functionalized support carried by a piezoelectric nanopositioner and brought to interact with an actin filament attached with the correct polarity via gelsolin to a bead (Bead Tailed Actin, BTA) trapped into the focus of a Dual Laser Optical Tweezers (DLOT). In solution with [ATP] = 2 mM the nanomachine is able to produce steady force and shortening, delivering a maximum power of 5 aW. The nanomachine performances are interpreted with a kinetic model based on mechanics and energetics of fast skeletal muscle. In this way it is possible to define the minimal conditions that allow an actomyosin system in vitro to produce force and power with the efficiency of the striated muscle, in the absence of the confusing contribution of the other sarcomeric proteins. In turn, since the system is assembled one piece at a time, it allows different degrees of reconstitution of the sarcomeric assembly. Therefore it will be possible to characterize the function of native and engineered contractile, regulatory and accessory proteins. For future investigations on the Ca2+-dependent thin filament activation, the preparation of BTA has been implemented using a Ca2+-independent gelsolin fragment and the procedure for thin filament reconstitution has been established during my visit to the Institute for Biophysical Chemistry, MHH, Germany

Orthophosphate increases the efficiency of slow muscle-myosin isoform in the presence of omecamtiv mecarbil

Omecamtiv mecarbil is a small molecule effector under clinical trial for the treatment of systolic heart failure. Here the authors define the molecular mechanisms of its inotropic action and find it can increase the efficiency of contraction in muscle fibres when the orthophosphate concentration rises with the beat frequency

Orthophosphate increases the efficiency of slow muscle-myosin isoform in the presence of omecamtiv mecarbil

Omecamtiv mecarbil (OM) is a putative positive inotropic tool for treatment of systolic heart dysfunction, based on the finding that in vivo it increases the ejection fraction and in vitro it prolongs the actin-bond life time of the cardiac and slow-skeletal muscle isoforms of myosin. OM action in situ, however, is still poorly understood as the enhanced Ca2+-sensitivity of the myofilaments is at odds with the reduction of force and rate of force development observed at saturating Ca2+. Here we show, by combining fast sarcomere-level mechanics and ATPase measurements in single slow demembranated fibres from rabbit soleus, that the depressant effect of OM on the force per attached motor is reversed, without effect on the ATPase rate, by physiological concentrations of inorganic phosphate (Pi) (1-10 mM). This mechanism could underpin an energetically efficient reduction of systolic tension cost in OM-treated patients, whenever [Pi] increases with heart-beat frequency

The force and stiffness of myosin motors in the isometric twitch of a cardiac trabecula and the effect of the extracellular calcium concentration

Key points: Fast sarcomere-level mechanics in intact trabeculae, which allows the definition of the mechano-kinetic properties of cardiac myosin in situ, is a fundamental tool not only for understanding the molecular mechanisms of heart performance and regulation, but also for investigating the mechanisms of the cardiomyopathy-causing mutations in the myosin and testing small molecules for therapeutic interventions. The approach has been applied to measure the stiffness and force of the myosin motor and the fraction of motors attached during isometric twitches of electrically paced trabeculae under different extracellular Ca2+ concentrations. Although the average force of the cardiac myosin motor (∼6 pN) is similar to that of the fast myosin isoform of skeletal muscle, the stiffness (1.07 pN nm–1) is 2- to 3-fold smaller. The increase in the twitch force developed in the presence of larger extracellular Ca2+ concentrations is fully accounted for by a proportional increase in the number of attached motors. Abstract: The mechano-kinetic properties of the cardiac myosin were studied in situ, in trabeculae dissected from the right ventricle of the rat heart, by measuring the stiffness of the half-sarcomere both at the twitch force peak (Tp) of an electrically paced intact trabecula at different extracellular Ca2+ concentrations ([Ca2+]o), and in the same trabecula after skinning and induction of rigor. Taking into account the contribution of filament compliance to half-sarcomere compliance and the lattice geometry, we found that the stiffness of the cardiac myosin motor is 1.07 ± 0.09 pN nm–1, which is slightly larger than that of the slow myosin isoform of skeletal muscle (0.6-0.8 pN nm–1) and 2- to 3-fold smaller than that of the fast skeletal muscle isoform. The increase in Tp from 61 ± 4 kPa to 93 ± 9 kPa, induced by raising [Ca2+]o from 1 to 2.5 mm at sarcomere length ∼2.2 μm, is accompanied by an increase of the half-sarcomere stiffness that is explained by an increase of the fraction of actin-attached motors from 0.08 ± 0.01 to 0.12 ± 0.02, proportional to Tp. Consequently, each myosin motor bears an average force of 6.14 ± 0.52 pN independently of Tp and [Ca2+]o. The application of fast sarcomere-level mechanics to intact trabeculae to define the mechano-kinetic properties of the cardiac myosin in situ represents a powerful tool for investigating cardiomyopathy-causing mutations in the myosin motor and testing specific therapeutic interventions