Regulation of contraction in striated muscle

Differential cross-bridge kinetics of FHC myosin mutations R403Q and R453C in heterozygous mouse myocardium

The kinetic effects of the cardiac myosin point mutations R403Q and R453C, which underlie lethal forms of familial hypertrophic cardiomyopathy (FHC), were assessed using isolated myosin and skinned strips taken from heterozygous (R403Q/+ and R453C/+) male mouse hearts. Compared with wild-type (WT) mice, actin-activated ATPase was increased by 38% in R403Q/+ and reduced by 45% in R453C/+, maximal velocity of regulated thin filament ( VRTF) in the in vitro motility assay was increased by 8% in R403Q/+ and was not different in R453C/+, myosin concentration at half-maximal VRTF was reduced by 30% in R403Q/+ and not different in R453C/+, and the characteristic frequency for oscillatory work production ( b frequency), determined by sinusoidal analysis in the skinned strip at maximal calcium activation, was 27% lower in R403Q/+ and 18% higher in R453C/+. The calcium sensitivity for isometric tension in the skinned strip was not different in R403Q/+ (pCa50 5.64 ± 0.02) and significantly enhanced in R453C/+ (5.82 ± 0.03) compared with WT (5.58 ± 0.02). We conclude that isolated myosin and skinned strips of R403Q/+ and R453C/+ myocardium show marked differences in cross-bridge kinetic parameters and in calcium sensitivity of force production that indicate different functional roles associated with the location of each point mutation at the molecular level.

Switching of Troponin I: Ca2+ and Myosin-induced Activation of Heart Muscle

The principal task of the Ca(2+) activation of striated muscle is the release of the troponin I (TnI) inhibitory region (TnI-I) from actin. TnI-I release facilitates the repositioning of tropomyosin across the actin surface and the formation of strong, force generating, actin-myosin cross-bridges. Full activation of the Ca(2+) regulatory switch (CRS) requires two switching steps in cTnI: binding of the TnI regulatory region to hydrophobic sites in the N-domain of Ca(2+)-bound troponin C and release of the adjacent TnI-I from actin. Using Förster resonance energy transfer, we have examined the requirements for full activation of the cardiac CRS. In the presence of actin, both Ca(2+) and strong cross-bridges are required for full activation. Actin desensitizes the CRS to Ca(2+) and produces cooperativity in the Ca(2+) activation of the CRS. Strong cross-bridges eliminate cooperativity and re-sensitize the CRS to Ca(2+). We propose a kinetic scheme and a structural model to account for these findings.

Troponin C isoform composition determines differences in Sr2+-activation characteristics between rat diaphragm fibers

Single fibers of rat diaphragm containing different naturally occurring combinations of myofibrillar protein isoforms were used to evaluate the contribution of troponin C (TnC) isoforms to fiber type-related differences with respect to sensitivity to Sr2+of the contractile system. Mechanically skinned fibers were studied for their isometric force vs. Sr2+concentration ([Sr2+]) relationships and then analyzed electrophoretically for myofibrillar protein isoform composition. Our data demonstrate that fiber-type differences in Sr2+dependence of contractile activation processes are primarily determined by the TnC isoform composition, with the slow isoform conferring on average a sevenfold greater sensitivity to Sr2+than the fast isoform. Moreover, the ratio of TnC isoforms determined functionally from the force-pSr (−log10[Sr2+]) curves is tightly ( r2= 0.97) positively correlated with that estimated electrophoretically. Together, these results validate the use of Sr2+activation characteristics to distinguish fibers containing different proportions of fast and slow TnC isoforms and to study the mechanisms by which divalent cations activate the contractile apparatus. We also found that the functionally and electrophoretically determined ratios of TnC isoforms present in a fiber display similar sigmoidal relationships with the ratio of myosin heavy chain (MHC) isoform types expressed. These relationships 1) offer further insight in the functional and molecular expression of TnC in relation to the molecular expression of MHC isoform types and 2) may provide the basis for predicting sensitivity to Sr2+, TnC, and MHC isoforms in pure and hybrid skeletal muscle fibers.

Molecular and integrated biology of thin filament protein phosphorylation in heart muscle

An increasing body of evidence points to posttranslational modifications of the thin filament regulatory proteins, cardiac troponin T (cTnT) and cardiac troponin I (cTnI) by protein kinase C (PKC) phosphorylation as important in both long- and short-term regulation of cardiac function and potentially implicated in the transition between compensated hypertrophy and decompensation. The main sites for PKC-dependent phosphorylation on cTnI are Ser43, Ser45, and Thr144 and on cTnT are Thr197, Ser201, Thr206, and Thr287 (mouse sequence). We analyzed the function of each phosphorylation residue using a phosphorylation mimic approach introducing glutamates (E) at PKC phosphorylation sites and then measuring the isometric tension of fiber bundles exchanged with these mutants. We also directly phosphorylated cTnI and cTnT by PKC, incorporated the phosphorylated troponins in the myofilament lattice, and determined the isometric tension at varying Ca(2+) concentrations. We followed the experimental data with computational analysis prediction of helical content of cTnI and cTnT peptides that undergo phosphorylation. Here we summarize our recent data on the specific functional role of PKC phosphorylation sites of cTnI and cTnT.

Designing calcium-sensitizing mutations in the regulatory domain of cardiac troponin C

Cardiac troponin C belongs to the EF-hand superfamily of calcium-binding proteins and plays an essential role in the regulation of muscle contraction and relaxation. To follow calcium binding and exchange with the regulatory N-terminal domain (N-domain) of human cardiac troponin C, we substituted Phe at position 27 with Trp, making a fluorescent cardiac troponin C(F27W). Trp(27) accurately reported the kinetics of calcium association and dissociation of the N-domain of cardiac troponin C(F27W). To sensitize the N-domain of cardiac troponin C(F27W) to calcium, we individually substituted the hydrophobic residues Phe(20), Val(44), Met(45), Leu(48), and Met(81) with polar Gln. These mutations were designed to increase the calcium affinity of the N-domain of cardiac troponin C by facilitating the movement of helices B and C (BC unit) away from helices N, A, and D (NAD unit). As anticipated, these selected hydrophobic residue substitutions increased the calcium affinity of the regulatory domain of cardiac troponin C(F27W) approximately 2.1-15.2-fold. Surprisingly, the increased calcium affinity caused by the hydrophobic residue substitutions was largely due to faster calcium association rates (2.6-8.7-fold faster) rather than to slower calcium dissociation rates (1.2-2.9-fold slower). The regulatory N-domains of cardiac troponin C(F27W) and its mutants were also able to bind magnesium competitively and with physiologically relevant affinities (1.2-2.7 mm). The design of calcium-sensitizing cardiac troponin C mutants presented in this work enhances the understanding of how to control cation binding properties of EF-hand proteins and ultimately their structure and physiological function.

Cardiac-like behavior of an insect flight muscle

The synchronous wing depressor muscles of the hawkmoth Manduca sexta undergo large amplitude motions at lengths that lie entirely on the ascending region of their twitch length–tension curve. Moreover, these muscles bear a striking functional resemblance to mammalian cardiac muscle in both the shape of their length–tension curve and in their working length range. Although operation on the ascending region of the twitch length–tension curve sacrifices maximal force, it does permit the generation of larger forces at greater strains. In the case of cardiac muscle,this mechanical behavior is critical for accommodating the increasing stresses associated with greater ventricular filling. Similar characteristics in moth flight muscle suggest an analogous regulatory mechanism for skeletal muscles performing repetitive oscillatory work; the strong length dependence of force over their working length range should give the wing depressors the capacity to generate larger forces as wing stroke amplitude increases. These results support the notion that the length–tension relationship of muscle can be tuned to function in locomotor muscles.

Activation of the calcium-regulated thin filament by myosin strong binding

The current study was undertaken to investigate the relative contribution of calcium and myosin binding to thin filament activation. Using the in vitro motility assay, myosin strong binding to the thin filament was controlled by three mechanisms: 1), varying the myosin concentration of the motility surface, and adding either 2), inorganic phosphate (Pi) or 3), adenosine diphosphate (ADP) to the motility solutions. At saturating myosin conditions, Pi had no effect on thin filament motility. However, at subsaturating myosin concentrations, velocity was reduced at maximal and submaximal calcium in the presence of Pi. Adding ADP to the motility buffers reduced thin filament sliding velocity but increased the pCa(50) of the thin filament. Thus by limiting or increasing myosin strong binding (with the addition of Pi and ADP, respectively), the calcium concentration at which half maximal activation of the thin filament is achieved can be modulated. In experiments without ADP or Pi, the myosin concentration on the motility surface required to reach maximal velocity inversely correlated with the level of calcium activation. Through this approach, we demonstrate that myosin strong binding is essential for thin filament activation at both maximal and submaximal calcium levels, with the relative contribution of myosin strong binding being greatest at submaximal calcium. Furthermore, under conditions in which myosin strong binding is not rate limiting (i.e., saturating myosin conditions), our data suggest that a modulation of myosin cross-bridge kinetics is likely responsible for the graded response to calcium observed in the in vitro motility assay.

The Myofibrillar Complex and Fatigue: A Review

The basis for all biological movement is the conversion of chemical energy to mechanical energy by different classes of motor proteins. In skeletal muscle this motor protein is myosin II, a thick filament-based molecule that harnesses the free energy furnished by ATP hydrolysis to perform mechanical work against actin proteins of the thin filament. The cyclic attachment and detachment of myosin with actin that generates muscle force and shortening is Ca2+ regulated. Intense muscle activity may lead to metabolically induced inhibitions to the function of these myofibrillar proteins when Ca2+ regulation is normal, a phenomenon referred to as myofibrillar fatigue. Studies using single muscle fibers at room temperature or lower have shown that myosin motor function is inhibited by the accumulation of the ATP-hydrolysis products ADP, Pi, and H+ as well as by excess generation of reactive oxygen species (ROS). These metabolically induced impairments to myosin motor function reduce muscle work and power output by impairing maximal Ca2+ activated force, the Ca2+ sensitivity of force, and/or unloaded shortening velocity. Based on uncertainties about their inhibitory effect on muscle function at more physiological temperatures, the influence of ATP-hydrolysis product and ROS accumulation on myofibrillar protein function of human skeletal muscle remains to be clarified. Key words: actin, myosin, muscle contraction

Myosin Crossbridge Activation of Cardiac Thin Filaments

At the level of the myofibrillar proteins, activation of myocardial contraction is thought to involve switch-like regulation of crossbridge binding to the thin filaments. A central feature of this view of regulation is that Ca2+ binding to the low-affinity (approximately 3 micromol/L) site on troponin C alters the interactions of proteins in the thin filament regulatory strand, which leads to movement of tropomyosin from its blocking position on the thin filament and binding of crossbridges to actin. Although Ca2+ binding is a critical step in initiating contraction, this event alone does not account for the activation dependence of contractile properties of myocardium. Instead, activation is a highly cooperative process in which initial crossbridge binding to the thin filaments recruits additional crossbridge binding to actin as well as increased Ca2+ binding to troponin C. This review addresses possible roles of thin filament cooperativity in myocardium as a process that modulates the activation dependence of force and the rate of force development and also possible mechanisms by which cooperative signals are transmitted along the thick filament. Emerging evidence suggests that such mechanisms could contribute to the regulation of fundamental mechanical properties of myocardium and alterations in regulation that underlie contractile disorders in diseases such as cardiomyopathies.

[Postnatal maturation of the diaphragm muscle: ultrastructural and functional aspects]

OBJECTIVE

In the diaphragm muscle, postnatal maturation is associated with major histological and biochemical modifications, as well as a progressive development of the sarcoplasmic reticulum (SR), which in turn are responsible for the progressive postnatal improvement in diaphragmatic contractility. However, the mechanisms by which postnatal maturation induces this improvement in diaphragmatic contractility remain poorly understood and controversial. The aim of this review is to analyze the data from the literature regarding the process involved in the postnatal improvement in diaphragmatic contractility.

DATA SOURCES

References obtained from Pubmed((R)) databank using keywords (diaphragm muscle, postnatal maturation, contractility, muscular fatigue, cross-bridge).

DATA SYNTHESIS

From a cytological point of view, the postnatal development of the diaphragm muscle is processed in two successive generations of fiber types, corresponding to the progressive adaptation of the diaphragm muscle to its physiological function. Indeed, the proportion in type I (slow, aerobic) and type IIB fibers (fast, anaerobic) progressively increases with postnatal maturation, while the proportion in type IIA fibers (fast, intermediate) progressively decreases. The histochemical classification of the type of fiber corresponds to the expression of the different isoforms of myosin heavy chains (MHC). Two types of MHC: MHC embryologic (MCH-emb) and MHC neonatal (MCH-neo), and one type of myosin light chains (MLC) are expressed in the foetal skeletal muscles, then are progressively eliminated during postnatal maturation. For many authors, this progressive transition from immature MHC (MCH-emb and neo) to adult MHC (by chronological order of appearance: MHC-2A, MHC-lente, MHC-2X, MHC-2B) could be responsible for the progressive improvement in postnatal diaphragmatic contractility. This transition could be modulated by external factors, mainly including neural and hormonal stimuli. For others, this transition in MHC expression do not play a major role, and other factors, including the postnatal maturation of the ryanodine receptor (RyR) or developmental changes in cross-bridges (CB) properties should play a central role. The most recent hypotheses proposed included the possibility of a postnatal transition in the expression of structural proteins, which are playing a major role in the maintenance of the stability of the sarcomer, and therefore in force generation.

Gene transfer of troponin I isoforms, mutants, and chimeras

Thin filament proteins play an essential role in the regulation of myocardial pressure development. Within the thin filament of the sarcomere, troponin I (TnI) plays a key role in regulating the Ca(2+) sensitivity of force. During myocardial development, there is a transition in TnI isoform expression from the slow skeletal isoform (ssTnI) in embryonic/fetal myocardium to the cardiac isoform (cTnI) expressed in adult hearts. Over a similar developmental time window, the calcium sensitivity of force development also decreases. Gene transfer of ssTnI, and chimeras derived from ssTnI and cTnI, into adult ventricular myocytes have provided insights into the isoform-specific domains of TnI responsible for differentially influencing myofilament Ca(2+) sensitivity. Two separate isoform-specific regions, located in the carboxyl- and amino-portions of the protein, have been identified by comparing Ca(2+)-activated isometric tension in myocytes expressing the TnI isoforms or chimeras. The carboxyl-portion of TnI also contributes to isoform-dependent differences in myofilament sensitivity to acidic pH, which ensues during several myocardial disease states. In contrast, the diminished Ca(2+) sensitivity observed in response to beta-adrenergic-mediated phosphorylation of cardiac TnI requires the amino-portion of the cardiac TnI isoform yet, does not depend on the presence of a specific isoform in the carboxyl-region of TnI. Recent studies with a mutation linked to hypertrophic cardiomyopathy have demonstrated that changes in protein charge also influence the ability of TnI isoforms to regulate myofilament Ca(2+) sensitivity. Information gained from these, and future studies on more localized and specific changes in the amino acid sequence, may one day lead to the use of genetically engineered TnI for therapeutic manipulation of contractile function.

Cooperative attachment of cross bridges predicts regulation of smooth muscle force by myosin phosphorylation

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of approximately 0.15 mol P(i)/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser(19)-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser(19)-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

The computational integrated myocyte: a view into the virtual heart

The presentation outlines an integrative approach for developing a computational model of cardiomyocytes. A modular approach is proposed, and strategies of linking the modules (intermediary metabolism, electrophysiology, and mechanics) of the model are presented. A strong recommendation is given toward an integrated system approach backed by experimental validation.

Effects of Protein Kinase C Dependent Phosphorylation and a Familial Hypertrophic Cardiomyopathy-Related Mutation of Cardiac Troponin I on Structural Transition of Troponin C and Myofilament Activation

In experiments reported here, we compared tension and thin filament Ca(2+) signaling in preparations containing either wild-type cardiac troponin I (cTnI) or a mutant cTnI with an R146G mutation [cTnI(146G)] linked to familial hypertrophic cardiomyopathy. Myofilament function is altered in association with cTnI phosphorylation by protein kinase C (PKC), which is activated in hypertrophy. Whether there are differential effects of PKC phosphorylation on cTnI compared to cTnI(146G) remains unknown. We therefore also studied cTnI and cTnI(146G) with PKC sites mutated to Glu, which mimics phosphorylation. Compared to cTnI controls, binary complexes with either cTnI(146G) or cTnI(43E/45E/144E) had a small effect on Ca(2+)-dependent structural opening of the N-terminal regulatory domain of cTnC as measured using Förster resonance energy transfer. However, this structural change was significantly reduced in the cTnC-cTnI(43E/45E/144E/146G) complex. Exchange of cTnI in skinned fiber bundles with cTnI(146G) induced enhanced Ca(2+) sensitivity and an elevated resting tension. Exchange of cTnI with cTnI(43E/45E/144E) induced a depression in Ca(2+) sensitivity and maximum tension. However, compared to cTnI(146G), cTnI(43E/45E/144E/146G) had little additional effects on myofilament response to Ca(2+). By comparing activation of tension to the open state of the N-domain of cTnC with variations in the state of cTnI, we were able to provide data supporting the hypothesis that activation of cardiac myofilaments is tightly coupled to the open state of the N-domain of cTnC. Our data also support the hypothesis that pathological effects of phosphorylation are influenced by mutations in cTnI.

Covalent and noncovalent modification of thin filament action: the essential role of troponin in cardiac muscle regulation

Troponin is essential for the regulation of cardiac contraction. Troponin is a sarcomeric molecular switch, directly regulating the contractile event in concert with intracellular calcium signals. Troponin isoform switching, missense mutations, proteolytic cleavage, and posttranslational modifications are known to directly affect sarcomeric regulation. This review focuses on physiologically relevant covalent and noncovalent modifications in troponin as part of a thematic series on cardiac thin filament function in health and disease.

A single-fibre study of the relationship between MHC and TnC isoform composition in rat skeletal muscle

In the present study, we investigated the possibility that MHC (myosin heavy chain) and TnC (troponin C) isoforms exist in specific combinations in rat-skeletal-muscle fibres. Single fibres (numbering 245) from soleus (predominantly slow-twitch) and sternomastoid (predominantly fast-twitch) muscles of adult rats were analysed for MHC and TnC isoform composition, using alanine-SDS/PAGE for separating MHC isoforms, and a novel method (based on the previously reported influence of Ca2+ on the mobility of Ca2+-binding proteins in SDS gels) for unequivocal identification of TnC isoforms in single-fibre segments. In this study, all fibres that contained only one MHC isoform (slow or fast) contained only the matching TnC isoform and all fibres that contained multiple fast MHC isoforms contained only the fast TnC isoform. Fibres expressing both slow and fast MHC isoforms displayed either both TnC isoforms or only one TnC isoform of a type depending on the relative proportion of fast/slow MHC present. Our results suggest a close relationship between MHC and TnC isoform composition in non-transforming skeletal muscles of adult rat.

Modulation of thin filament activation by breakdown or isoform switching of thin filament proteins: physiological and pathological implications

In the heart, the contractile apparatus is adapted to the specific demands of the organ for continuous rhythmic contraction. The specialized contractile properties of heart muscle are attributable to the expression of cardiac-specific isoforms of contractile proteins. This review describes the isoforms of the thin filament proteins actin and tropomyosin and the three troponin subunits found in human heart muscle, how the isoform profiles of these proteins change during development and disease, and the possible functional consequences of these changes. During development of the heart, there is a distinctive switch of isoform expression at or shortly after birth; however, during adult life, thin filament protein isoform composition seems to be stable despite protein turnover rates of 3 to 10 days. The pattern of isoforms of actin, tropomyosin, troponin I, troponin C, and troponin T is not affected by aging or heart disease (ischemia and dilated cardiomyopathy). The evidence for proteolysis of thin filament proteins in situ during ischemia and stunning is evaluated, and it is concluded that C-terminal cleavage of troponin I is a feature of irreversibly injured myocardium but may not play a role in reversible stunning.

Inorganic phosphate speeds loaded shortening in rat skinned cardiac myocytes

Force generation in striated muscle is coupled with inorganic phosphate (P(i)) release from myosin, because force falls with increasing P(i) concentration ([P(i)]). However, it is unclear which steps in the cross-bridge cycle limit loaded shortening and power output. We examined the role of P(i) in determining force, unloaded and loaded shortening, power output, and rate of force development in rat skinned cardiac myocytes to discern which step in the cross-bridge cycle limits loaded shortening. Myocytes (n = 6) were attached between a force transducer and position motor, and contractile properties were measured over a range of loads during maximal Ca(2+) activation. Addition of 5 mM P(i) had no effect on maximal unloaded shortening velocity (V(o)) (control 1.83 +/- 0.75, 5 mM added P(i) 1.75 +/- 0.58 muscle lengths/s; n = 6). Conversely, addition of 2.5, 5, and 10 mM P(i) progressively decreased force but resulted in faster loaded shortening and greater power output (when normalized for the decrease in force) at all loads greater than approximately 10% isometric force. Peak normalized power output increased 16% with 2.5 mM added P(i) and further increased to a plateau of approximately 35% with 5 and 10 mM added P(i). Interestingly, the rate constant of force redevelopment (k(tr)) progressively increased from 0 to 10 mM added P(i), with k(tr) approximately 360% greater at 10 mM than at 0 mM added P(i). Overall, these results suggest that the P(i) release step in the cross-bridge cycle is rate limiting for determining shortening velocity and power output at intermediate and high relative loads in cardiac myocytes.

Phosphorylation and mutation of human cardiac troponin I deferentially destabilize the interaction of the functional regions of troponin I with troponin C

We have utilized 2D [(1)H,(15)N]HSQC NMR spectroscopy to elucidate the binding of three segments of cTnI in native, phosphorylated, and mutated states to cTnC. The near N-terminal region (cRp; residues 34-71) contains the protein kinase C (PKC) phosphorylation sites S41 and S43, the inhibitory region (cIp; residues 128-147) contains another PKC site T142 and a familial hypertrophic cardiomyopathy (FHC) mutation R144G, and the switch region (cSp; residues 147-163) contains the novel p21-activated kinase (PAK) site S149 and another FHC mutation R161W. While S41/S43 phosphorylation of cRp had minimal disruption in the interaction of cRp and cTnC.3Ca(2+), T142 phosphorylation reduced the affinity of cIp for cCTnC.2Ca(2+) by approximately 14-fold and S149 phosphorylation reduced the affinity of cSp for cNTnC.Ca(2+) by approximately 10-fold. The mutation R144G caused an approximately 6-fold affinity decrease of cIp for cCTnC.2Ca(2+) and mutation R161W destabilized the interaction of cSp and cNTnC.Ca(2+) by approximately 1.4-fold. When cIp was both T142 phosphorylated and R144G mutated, its affinity for cCTnC.2Ca(2+) was reduced approximately 19-fold, and when cSp was both S149 phosphorylated and R161W mutated, its affinity for cNTnC.Ca(2+) was reduced approximately 4-fold. Thus, while the FHC mutation R144G enhances the effect of T142 phosphorylation on the interaction of cIp and cCTnC.2Ca(2+), the FHC mutation R161W suppresses the effect of S149 phosphorylation on the interaction of cSp and cNTnC.Ca(2+), demonstrating linkages between the FHC mutation and phosphorylation of cTnI. The observed alterations corroborate well with structural data. These results suggest that while the modifications in the cRp region have minimal influence, those in the key functional cIp-cSp region have a pronounced effect on the interaction of cTnI and cTnC, which may correlate with the altered myofilament function and cardiac muscle contraction under pathophysiological conditions.

Tropomyosin and troponin are required for ovarian contraction in the Caenorhabditis elegans reproductive system

Ovulation in the nematode Caenorhabditis elegans is coordinated by interactions between the somatic gonad and germ cells. Myoepithelial sheath cells of the proximal ovary are smooth muscle-like cells, but the regulatory mechanism of their contraction is unknown. We show that contraction of the ovarian muscle requires tropomyosin and troponin, which are generally major actin-linked regulators of contraction of striated muscle. RNA interference of tropomyosin or troponin C caused sterility by inhibiting ovarian contraction that is required for expelling mature oocytes into the spermatheca where fertilization takes place, thus causing accumulation of endomitotic oocytes in the ovary. Tropomyosin and troponin C were associated with actin filaments in the myoepithelial sheath, and the association of troponin C with actin was dependent on tropomyosin. A mutation in the actin depolymerizing factor/cofilin gene suppressed the ovulation defects by RNA interference of tropomyosin or troponin C. These results strongly suggest that tropomyosin and troponin are the actin-linked regulators for contraction of ovarian muscle in the C. elegans reproductive system.

Determination of depolarisation- and agonist-evoked calcium fluxes on skeletal muscle cells in primary culture

Changes in intracellular calcium concentration ([Ca2+]i) evoked by prolonged depolarisation (120 mM KCl) or by the application of 15 mM caffeine were measured on skeletal muscle cells in primary culture. The extrusion rate (PVmax) of calcium from the myoplasm was determined, which in turn enabled the calculation of the calcium flux (Fl) underlying the measured calcium transients. PVmax was found to increase during differentiation, from 107 +/- 10 microM/s at the early myotube stage to 596 +/- 36 microM/s in secondary myotubes. This was paralleled by a decrease in resting [Ca2+]i from 99 +/- 4 to 51 +/- 2 nM. The depolarisation-evoked Fl rose to peak and then ceased despite the continuous presence of KCl. In contrast, the caffeine-induced Fl showed a peak and a clear steady-level with a peak-to-steady ratio of 5.6 +/- 1.2. Removal of external calcium suppressed the depolarisation--induced flux by 88 +/- 5% indicating that both an influx and a release from the SR underlie the K(+)-evoked calcium transients. Subsequent applications of caffeine resulted in essentially identical fluxes indicating an efficient refilling of the internal stores. Moreover, if a depolarisation-induced calcium transient preceded the second caffeine-evoked release, the latter was significantly larger than the first suggesting that much of the calcium that entered was stored in the SR rather than extruded.

Ca2+-induced Rolling of Tropomyosin in Muscle Thin Filaments

Tropomyosin is a filamentous coiled-coil protein directly involved in the regulation of the actomyosin interaction responsible for muscle contraction: it transmits the local calcium-induced conformational change in troponin to the helical array of myosin-binding sites on the surface of the actin filament. McLachlan and Stewart (McLachlan, A. D., and Stewart, M. (1976) J. Mol. Biol. 103, 271-298) proposed that the tropomyosin coiled-coil structure can be divided into 14 alternating 19- to 20-residue "alpha- and beta-bands," which could act as alternate 7-fold sets of sites for specific binding to actin in the different conformational states of the regulated thin filament. Here we present the first direct experimental evidence in support of the alpha- and beta-band hypothesis: we analyze the acrylamide quenching of the fluorescence of mutant tropomyosins containing 5-hydroxytryptophan residues at different positions along the coiled-coil structure under a variety of conditions (alone, complexed with actin, and complexed with actin and troponin with or without Ca(2+)). We show that fluorescent probes placed in the alpha-bands become less solvent-exposed in the absence of calcium, whereas those in the beta-bands become less solvent-exposed in the presence of calcium. A model in which the tropomyosin coiled-coil rolls across the actin surface in response to Ca(2+)-binding to troponin most easily explains these observations.

Genetically Encoded Indicators of Cellular Calcium Dynamics Based on Troponin C and Green Fluorescent Protein

Genetic calcium probes offer tremendous potential in the fields of neuroscience, cell biology, and pharmaceutical screening. Previously, ratiometric and non-ratiometric indicators of cellular calcium dynamics have been described that consist of mutants of the green fluorescent protein (GFP) as fluorophores and calmodulin as calcium-binding moiety in several configurations. However, these calmodulin-based types of probes have a series of deficiencies, such as reduced dynamic ranges, when expressed within transgenic organisms and lack of calcium sensitivity in certain targetings. We developed novel types of calcium probes based on troponin C variants from skeletal and cardiac muscle. These indicators have ratio changes up to 140%, K(d)s ranging from 470 nm to 29 microm, and improved subcellular targeting properties. We targeted the indicators to the plasma membrane of HEK293 cells and primary hippocampal neurons. Upon long lasting depolarization, submembrane calcium levels in hippocampal neurons were found to be in equilibrium with bulk cytosolic calcium levels, suggesting no standing gradient persists from the membrane toward the cytosol. We expect that such novel indicators using specialized calcium sensing proteins will be minimally interacting with the cellular biochemical machinery.

Troponin I is required for myofibrillogenesis and sarcomere formation inDrosophilaflight muscle

Myofibrillar proteins assemble to form the highly ordered repetitive contractile structural unit known as a sarcomere. Studies of myogenesis in vertebrate cell culture and embryonic developmental systems have identified some of the processes involved during sarcomere formation. However, isoform changes during vertebrate muscle development and a lack of mutants have made it difficult to determine how these proteins assemble to form sarcomeres. The indirect flight muscles (IFMs) of Drosophila provide a unique genetic system with which to study myofibrillogenesis in vivo. We show in this paper that neither sarcomeric myosin nor actin are required for myoblast fusion or the subsequent morphogenesis of muscle fibres, i.e. fibre morphogenesis does not depend on myofibrillogenesis. However, fibre formation and myofibrillogenesis are very sensitive to the interactions between the sarcomeric proteins. A troponin I (TnI) mutation, hdp3, leads to an absence of TnI in the IFMs and tergal depressor of trochanter (TDT) muscles due to a transcript-splicing defect. Sarcomeres do not form and the muscles degenerate. TnI is part of the thin filament troponin complex which regulates muscle contraction. The effects of the hdp3 mutation are probably caused by unregulated acto-myosin interactions between the thin and thick filaments as they assemble. We have tested this proposal by using a transgenic myosin construct to remove the force-producing myosin heads. The defects in sarcomeric organisation and fibre degeneration in hdp3 IFMs are suppressed, although not completely, indicating the need for inhibition of muscle contraction during muscle development. We show that mRNA and translated protein products of all the major thin filament proteins are reduced in hdp3 muscles and discuss how this and previous studies of thin filament protein mutants indicate a common co-ordinated control mechanism that may be the primary cause of the muscle defects.

A specific C-terminal deletion in tropomyosin results in a stronger head-to-tail interaction and increased polymerization

Tropomyosin is a 284 residue dimeric coiled-coil protein that interacts in a head-to-tail manner to form linear filaments at low ionic strengths. Polymerization is related to tropomyosin's ability to bind actin, and both properties depend on intact N- and C-termini as well as alpha-amino acetylation of the N-terminus of the muscle protein. Nalpha-acetylation can be mimicked by an N-terminal Ala-Ser fusion in recombinant tropomyosin (ASTm) produced in Escherichia coli. Here we show that a recombinant tropomyosin fragment, corresponding to the protein's first 260 residues plus an Ala-Ser fusion [ASTm(1-260)], polymerizes to a much greater extent than the corresponding full-length recombinant protein, despite the absence of the C-terminal 24 amino acids. This polymerization is sensitive to ionic strength and is greatly reduced by the removal of the N-terminal Ala-Ser fusion [nfTm(1-260)]. CD studies show that nonpolymerizable tropomyosin fragments, which terminate at position 260 [Tm(167-260) and Tm(143-260)], as well as Tm(220-284), are able to interact with ASTm(1-142), a nonpolymerizable N-terminal fragment, and that the head-to-tail interactions observed for these fragment pairs are accompanied by a significant degree of folding of the C-terminal tropomyosin fragment. These results suggest that the new C-terminus, created by the deletion, polymerizes in a manner similar to the full-length protein. Head-to-tail binding for fragments terminating at position 260 may be explained by the presence of a greater concentration of negatively charged residues, while, at the same time, maintaining a conserved pattern of charged and hydrophobic residues found in polymerizable tropomyosins from a variety of sources.

Extracellular Ca2+-sensing receptors—an overview

Extracellular Ca2+-sensing receptors (CaRs) are the molecular basis by which specialized cells detect and respond to changes in the extracellular [Ca2+] ([Ca2+]o). CaRs belong to the family C of G-protein coupled receptors (GPCRs). Activation of CaRs triggers signaling pathways that modify numerous cell functions. Multiple ligands regulate the activation of CaRs including multivalent cations, L-amino acids, and changes in ionic strength and pH. CaRs in parathyroid cells play a central role in systemic Ca2+ homeostasis in terrestrial tetrapods. Mutations of the CaR gene in humans cause diseases in which serum and urine [Ca2+] and parathyroid hormone (PTH) levels are altered. CaR homologues are also expressed in organs critical to Ca2+ transport in ancient and modern fish, suggesting that similar receptors may have long been involved in Ca2+ homeostasis in lower vertebrates before parathyroid glands developed in terrestrial vertebrates. CaR mRNA and protein are also expressed in tissues not directly involved in Ca2+ homeostasis. This implies that there may be other biological roles for CaRs. Studies of CaR-knockout mice confirm the importance of CaRs in the parathyroid gland and kidney. The functions of CaRs in tissues other than kidney and parathyroid gland, however, remain to be elucidated.

Mutations of hydrophobic residues in the N-terminal domain of troponin C affect calcium binding and exchange with the troponin C-troponin I96-148 complex and muscle force production

Interactions between troponin C and troponin I play a critical role in the regulation of skeletal muscle contraction and relaxation. We individually substituted 27 hydrophobic Phe, Ile, Leu, Val, and Met residues in the regulatory domain of the fluorescent troponin C(F29W) with polar Gln to examine the effects of these mutations on: (a) the calcium binding and dynamics of troponin C(F29W) complexed with the regulatory fragment of troponin I (troponin I(96-148)) and (b) the calcium sensitivity of force production. Troponin I(96-148) was an accurate mimic of intact troponin I for measuring the calcium dynamics of the troponin C(F29W)-troponin I complexes. The calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes varied approximately 243-fold, whereas the calcium association and dissociation rates varied approximately 38- and approximately 33-fold, respectively. Interestingly, the effect of the mutations on the calcium sensitivity of force development could be better predicted from the calcium affinities of the troponin C(F29W)-troponin I(96-148) complexes than from that of the isolated troponin C(F29W) mutants. Most of the mutations did not dramatically affect the affinity of calcium-saturated troponin C(F29W) for troponin I(96-148). However, the Phe(26) to Gln and Ile(62) to Gln mutations led to >10-fold lower affinity of calcium-saturated troponin C(F29W) for troponin I(96-148), causing a drastic reduction in force recovery, even though these troponin C(F29W) mutants still bound to the thin filaments. In conclusion, elucidating the determinants of calcium binding and exchange with troponin C in the presence of troponin I provides a deeper understanding of how troponin C controls signal transduction.

A troponin switch that regulates muscle contraction by stretch instead of calcium

The flight muscles of many insects have a form of regulation enabling them to contract at high frequencies. The muscles are activated by periodic stretches at low Ca2+ levels. The same muscles also give isometric contractions in response to higher Ca2+. We show that the two activities are controlled by different isoforms of TnC (F1 and F2) within single myofibrils. F1 binds one Ca2+ with high affinity in the C-terminal domain and F2 binds one Ca2+ in the C-terminal domain and one exchangeable Ca2+ in the N-terminal domain. We have characterised the isoforms and determined their effect on the development of stretch-activated and Ca2+-activated tension by replacing endogenous TnC in Lethocerus flight muscle fibres with recombinant isoforms. Fibres with F1 gave stretch-activated tension and minimal isometric tension; those with F2 gave Ca2+-dependent isometric tension and minimal stretch-activated tension. Regulation by a TnC responding to stretch rather than Ca2+ is unprecedented and has resulted in the ability of insect flight muscle to perform oscillatory work at low Ca2+ concentrations, a property to which a large number of flying insects owe their evolutionary success.

Ca transients from Ca channel activity in rat cardiac myocytes reveal dynamics of dyad cleft and troponin C Ca binding

The properties of the dyad cleft can in principle significantly impact excitation-contraction coupling, but these properties are not easily amenable to experimental investigation. We simultaneously measured the time course of the rise in integrated Ca current ( ICa) and the rise in concentration of fura 2 with Ca bound ([Ca-fura 2]) with high time resolution in rat myocytes for conditions under which Ca entry is only via L-type Ca channels and sarcoplasmic reticulum (SR) Ca release is blocked, and compared these measurements with predictions from a finite-element model of cellular Ca diffusion. We found that 1) the time course of the rise of [Ca-fura 2] follows the time course of integrated ICaplus a brief delay (1.36 ± 0.43 ms, n = 6 cells); 2) from the model, high-affinity Ca binding sites in the dyad cleft at the level previously envisioned would result in a much greater delay (≥3 ms) and are therefore unlikely to be present at that level; 3) including ATP in the model promoted Ca efflux from the dyad cleft by a factor of 1.57 when low-affinity cleft Ca binding sites were present; 4) the data could only be fit to the model if myofibrillar troponin C (TnC) Ca binding were low affinity (4.56 μM), like that of soluble troponin C, instead of the high-affinity value usually used (0.38 μM). In a “good model,” the rate constants for Ca binding and dissociation were 0.375 times the values for soluble TnC; and 5) consequently, intracellular Ca buffering at the rise of the Ca transient is inferred to be low.

Tropomyosin exon 6b is troponin-specific and required for correct acto-myosin regulation

The specificity of tropomyosin (Tm) exon 6b for interaction with and functioning of troponin (Tn) has been studied using recombinant fibroblast Tm isoforms 5a and 5b. These isoforms differ internally by exons 6a/6b and possess non-muscle exons 1b/9d at the termini, hence they lack the primary TnT(1)-tropomyosin interaction, allowing study of exon 6 exchange in isolation from this. Using kinetic techniques to measure regulation of myosin S1 binding to actin and fluorescently labeled Tm to directly measure Tn binding, we show that binding of Tn to both isoforms is similar (0.1-0.5 microm) and both produce well regulated systems. Calcium has little effect on Tn binding to the actin.Tm complex and both exons produce a 3-fold reduction in the S1 binding rate to actin.Tm.Tn in its absence. This confirms previous results that show exon 6 has little influence on Tn affinity to actin.Tm or its ability to fully inhibit the acto-myosin interaction. Thin filaments reconstituted with Tn and Tm5a or skeletal Tm (containing exon 6b) show nearly identical calcium dependence of acto-myosin regulation. However, Tm5b produces a dramatic increase in calcium sensitivity, shifting the activation mid-point by almost an order of magnitude. This shows that exon 6 sequence and, hence, Tm structure in this region have a significant effect upon the calcium regulation of Tn. This finding supports evidence that familial hypertrophic cardiomyopathy mutations occurring adjacent to this region can effect calcium regulation.

Red muscle recruitment during steady swimming correlates with rostral–caudal patterns of power production in trout

Rainbow trout (Oncorhynchus mykiss) and brook trout (or charr, Salvelinus fontinalis) display different rostral-caudal patterns of power production by the red or aerobic muscle during steady swimming. The anterior muscle of rainbow trout produces much less power for swimming than the posterior, while in brook trout there is no variation in power output. To determine if red muscle recruitment is associated with anterior-posterior patterns of power production, electromyography (EMG) was used to record red muscle activity at three body positions across a range of swimming speeds in fish of each species. The initial recruitment of the anterior red muscle in swimming rainbow trout was predicted to lag behind, i.e. occur at higher speeds, that of the posterior due to the variation in power production, but no variation in recruitment was expected for brook trout. Burst of red muscle EMG activity occurring with each tailbeat was analyzed for frequency (tailbeat frequency), duty cycle (DC) (duration of burst relative to the period of the tailbeat) and burst intensity (BI) (magnitude of the measured EMG activity). Brook trout swam with higher tailbeat frequencies and longer values of DC than rainbow trout. Both species showed a pattern of longitudinal variation in DC, with longer DC values in the anterior red muscle. BI also differed significantly along the length of rainbow trout but not brook trout. In the former, BI of anterior muscle was significantly less than the posterior at lower steady swimming speeds. The EMG data suggest that power production and muscle recruitment are related. In rainbow trout, where there is longitudinal variation in muscle power output, there are also significant rostral-caudal differences in red muscle recruitment.

Two fast-type fibers in claw closer and abdominal deep muscles of the Australian freshwater crustacean,Cherax destructor, differ in Ca2+ sensitivity and troponin-I isoforms

One type of fast fiber and two types of slow (slow-twitch, S1 and slow-tonic, S2) fibers are found in decapod crustacean skeletal muscles that differ in contractile properties and myofibrillar protein isoform compositions. In this study the structural characteristics, protein isoform compositions, and Ca2+-activation properties of fast fibers in the claw closer (F1) and abdominal deep flexor (F2) muscles of Cherax destructor were analyzed. For comparison, myofibrillar protein isoform compositions of slow (long-sarcomere) fibers from claw and abdomen were also determined; our results indicate that the slow fibers in the claw closer were the slow-twitch (S1) type and those in the abdominal superficial flexor were primarily slow-tonic (S2) type. F1 fibers had shorter resting sarcomere lengths (2.93 microm in unstretched fibers and 3.06 microm in stretched fibers) and smaller fiber diameter (256 microm) than F2 fibers (sarcomere lengths 3.48 microm in unstretched and 3.46 microm in stretched; 747 microm diameter). Moreover, F1 fibers showed a narrower range in sarcomere lengths than F2 fibers (2.81 to 3.28 microm vs. 2.47 to 4.05 micro m in unstretched fibers). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting showed that the fast fibers from claw and abdomen differed in troponin-I composition; F1 fibers expressed two isoforms of troponin-I (TnI1 and TnI2) in approximately equal amounts, whereas F2 fibers expressed primarily TnI3 and lower levels of TnI1. F1 fibers were more sensitive to Ca2+, as shown by higher pCa values at threshold activation (pCa(10)=6.50+/-0.07) and at 50% maximum force (pCa(50)=6.43+/-0.07) than F2 fibers (pCa(10)=6.12+/-0.04 and pCa(50)=5.88+/-0.03, respectively). F1 fibers also had a greater degree of co-operativity in Ca2+ activation, as shown by a higher maximum slope of the force-pCa curve (n(Ca)=12.98+/-2.27 vs. 4.34+/-0.64). These data indicate that there is a greater fast fiber-type diversity in crustacean muscles than was previously supposed. Moreover, the differences in activation properties suggest that the TnI isoform composition influences the Ca2+ sensitivity of the contractile mechanism.

Mutations that affect flightin expression in Drosophila alter the viscoelastic properties of flight muscle fibers

Striated muscles across phyla share a highly conserved sarcomere design yet exhibit broad diversity in contractile velocity, force, power output, and efficiency. Insect asynchronous flight muscles are characterized by high-frequency contraction, endurance, and high-power output. These muscles have evolved an enhanced delayed force response to stretch that is largely responsible for their enhanced oscillatory work and power production. In this study we investigated the contribution of flightin to oscillatory work using sinusoidal analysis of fibers from three flightless mutants affecting flightin expression: 1) fln0, a flightin null mutant, 2) Mhc13, a myosin rod point mutant with reduced levels of flightin, and 3) Mhc6, a second myosin rod point mutant with reduced levels of phosphorylated flightin. Fibers from the three mutants show deficits in their passive and dynamic viscoelastic properties that are commensurate with their effect on flightin expression and result in a significant loss of oscillatory work and power. Passive tension and passive stiffness were significantly reduced in fln0 and Mhc13 but not in Mhc6. The dynamic viscous modulus was significantly reduced in the three mutants, whereas the dynamic elastic modulus was reduced in fln0 and Mhc13 but not in Mhc6. Tension generation under isometric conditions was not impaired in fln0. However, when subjected to sinusoidal length perturbations, work-absorbing processes dominated over work-producing processes, resulting in no net positive work output. We propose that flightin is a major contributor to myofilament stiffness and a key determinant of the enhanced delayed force response to stretch in Drosophila flight muscles.

Local Destabilization of the Tropomyosin Coiled Coil Gives the Molecular Flexibility Required for Actin Binding†

Tropomyosin, a coiled coil protein that binds along the length of actin filaments, contains 40 uninterrupted heptapeptide repeats characteristic of coiled coils. Yet, it is flexible. Regions of tropomyosin that may be important for binding to the filament and for interacting with troponin deviate from canonical coiled coil structure in subtle ways, altering the local conformation or energetics without interrupting the coiled coil. In a region rich in interface alanines (an Ala cluster), the chains pack closer than in canonical coiled coils, and are staggered, resulting in a bend [Brown et al. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 8496-8501]. Brown et al. suggested that bends at alanine clusters allow tropomyosin to wind on the actin filament helix. Another explanation is that local destabilization of the coiled coil, rather than close packing of the chains at Ala clusters per se, allows flexibility. Changing three Ala residues to canonical interface residues, A74L-A78V-A81L, greatly stabilized tropomyosin, measured using circular dichroism and differential scanning calorimetry, and reduced actin affinity >10-fold. Normal actin affinity and stability were restored in a mutant A74Q-A78N-A81Q that mimicked the stability of the Ala cluster but not the close packing of the chains. Analysis and modeling of comparable mutations introduced closer to the N-terminus show that the effects on stability and function depend on context. Models based on tropomyosin crystal structures give insight into possible effects of the mutations on the structure. We conclude that the significance of the Ala clusters in allowing flexibility of tropomyosin is stability-driven.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

Quantitative Reconstruction of Cardiac Electromechanics in Human Myocardium:

INTRODUCTION

Myocytes from normal and failing myocardium show significant differences in electromechanical behavior. Mathematical modeling of the behavior provides insights into the underlying physiologic and pathophysiologic mechanisms. Electromechanical models of cardiomyocytes exist for various species, but models of human myocytes are lacking.

METHODS AND RESULTS

A mathematical model of electromechanics in normal and failing cardiac myocytes in humans was created by assembly and adaptation of parameters of an electrophysiologic model at the level of single cells and a force development model at the level of the sarcomere. The adaptation was performed using data from recent studies of ventricular myocytes and myocardium. The model was applied to quantitatively reconstruct measurement data from different experimental studies of normal and failing myocardium. Several simulations were performed to quantify the transmembrane voltage Vm, intracellular concentration of calcium[Ca2+]i, the [Ca2+]i-force relationship, and force transients. Furthermore, frequency dependencies and restitution of action voltage duration to 90% recovery APD90, peak [Ca2+]i, duration to 50% force recovery FD50, and peak force were determined.

CONCLUSION

The presented mathematical model was capable of quantitatively reconstructing data obtained from different studies of electrophysiology and force development in normal and failing myocardium of humans. In future work, the model can serve as a component for studying macroscopic mechanisms of excitation propagation, metabolism, and electromechanics in human myocardium.

Damped elastic recoil of the titin spring in myofibrils of human myocardium

The giant protein titin functions as a molecular spring in muscle and is responsible for most of the passive tension of myocardium. Because the titin spring is extended during diastolic stretch, it will recoil elastically during systole and potentially may influence the overall shortening behavior of cardiac muscle. Here, titin elastic recoil was quantified in single human heart myofibrils by using a high-speed charge-coupled device-line camera and a nanonewtonrange force sensor. Application of a slack-test protocol revealed that the passive shortening velocity (Vp) of nonactivated cardiomyofibrils depends on: (i) initial sarcomere length, (ii) release-step amplitude, and (iii) temperature. Selective digestion of titin, with low doses of trypsin, decelerated myofibrillar passive recoil and eventually stopped it. Selective extraction of actin filaments with a Ca2+-independent gelsolin fragment greatly reduced the dependency of Vp on release-step size and temperature. These results are explained by the presence of viscous forces opposing myofibrillar passive recoil that are caused mainly by weak actin-titin interactions. Thus, Vp is determined by two distinct factors: titin elastic recoil and internal viscous drag forces. The recoil could be modeled as that of a damped entropic spring consisting of independent worm-like chains. The functional importance of myofibrillar elastic recoil was addressed by comparing instantaneous Vp to unloaded shortening velocity, which was measured in demembranated, fully Ca2+-activated, human cardiac fibers. Titin-driven passive recoil was much faster than active unloaded shortening velocity in early phases of isotonic contraction. Damped myofibrillar elastic recoil could help accelerate active contraction speed of human myocardium during early systolic shortening.

Human diaphragm remodeling associated with chronic obstructive pulmonary disease: clinical implications

Diaphragm remodeling associated with chronic obstructive pulmonary disease (COPD) consists of a fast-to-slow fiber type transformation as well as adaptations within each fiber type. To try to explain disparate findings in the literature regarding the relationship between fiber type proportions and FEV1, we obtained costal diaphragm biopsies on 40 subjects whose FEV1 ranged from 118 to 16% of the predicted normal value. First, we noted that our exponential regression model indicated that changes in FEV1 can account for 72% of the variation in the proportion of Type I fibers. Second, to assess the impact of COPD on diaphragm force generation, we measured maximal specific force generated by single permeabilized fibers prepared from the diaphragms of two patients with normal pulmonary function tests and two patients with severe COPD. We noted that fibers prepared from the diaphragms of severe COPD patients generated a lower specific force than control fibers (p < 0.001) and Type I fibers generated a lower specific force than Type II fibers (p < 0.001). Our finding of an exponential relationship between the proportion of Type I fibers and FEV1 accounts for discrepancies in the literature. Moreover, our single-fiber results suggest that COPD-associated diaphragm remodeling decreases diaphragmatic force generation by adaptations within each fiber type as well as by fiber type transformations.

Spectrofluorometric analysis of length-dependent conformational changes in cardiac troponin C

Length modulation of cardiac muscle is manifested in the Frank-Starling relation of the heart. Recently, it has been shown that length-dependent changes in SH reactivity of cardiac troponin C (cTnC) occurred in association with cross-bridge attachment and Ca2+. However, the presence of two SH groups (Cys-35 and Cys-84) in the regulatory region of cTnC complicates efforts to detect conformational changes. In this study skinned porcine cardiac fibers were reacted with 7-diethylamino-3-[4'maleimidylphenyl]-4-methylcoumarin (CPM). Alkaline urea gel electrophoresis, along with protein elution, was used to isolate filament bound cTnC. Analysis of fluorescence measurement showed that there is a Ca(2+)-increased fluorescence for CPM-labeled cTnC in long fibers (sarcomere length = 2.2 approximately 2.5 microm) but not in short fibers (sarcomere length = 1.6 approximately 1.8 microm). In addition, the labeled cTnC was measured for the fluorescence decrease over time by adding a non-fluorescence energy acceptor, 4-dimethylaminophenylazophenyl-4'maleimide (DABMI), in the presence and absence of Ca2+. Fluorescence quenching by DABMI is not affected by Ca2+ in long fibers but it is significantly increased in short fibers. However, the fibers maintained in the relaxed state with 5 mM MgATP and 1 mM Vanadate showed no length effect on the CPM-labeled cTnC in terms of the Ca(2+)-mediated changes in fluorescence spectrum and in fluorescence quenching by DABMI. All together, our results suggest that the relative reactivities of Cys-35 and Cys-84 vary with sarcomere length.

Identification of a functionally critical protein kinase C phosphorylation residue of cardiac troponin T

Cardiac Troponin T (cTnT) is one prominent substrate through which protein kinase C (PKC) exerts its effect on cardiomyocyte function. To determine the specific functional effects of the cTnT PKC-dependent phosphorylation sites (Thr197, Ser201, Thr206, and Thr287) we first mutated these residues to glutamate (E) or alanine (A). cTnT was selectively mutated to generate single, double, triple, and quadruple mutants. Bacterially expressed mutants were evaluated in detergent-treated mouse left ventricular papillary muscle fiber bundles where the endogenous troponin was replaced with a recombinant troponin complex containing either cTnT phosphorylated by PKC-alpha or a mutant cTnT. We simultaneously determined isometric tension development and actomyosin Mg-ATPase activity of the exchanged fiber bundles as a function of Ca2+ concentration. Our systematic analysis of the functional role of the multiple PKC phosphorylation sites on cTnT identified a localized region that controls maximum tension, ATPase activity, and Ca2+ sensitivity of the myofilaments. An important and novel finding of our study was that Thr206 is a functionally critical cTnT PKC phosphorylation residue. Its exclusive phosphorylation by PKC-alpha or replacement by Glu (mimicking phosphorylation) significantly decreased maximum tension, actomyosin Mg-ATPase activity, myofilament Ca2+ sensitivity, and cooperativity. On the other hand the charge modification of the other three residues together (T197/S201/T287-E) had no functional effect. Fibers bundles containing phosphorylated cTnT-wt (but not the T197/S201/T206/T287-E) exhibited a significant decrease of tension cost as compared with cTnT-wt.

Temperature and the expression of seven muscle-specific protein genes during embryogenesis in the Atlantic cod Gadus morhua L

Seven cDNA clones coding for different muscle-specific proteins (MSPs) were isolated from the fast muscle tissue of Atlantic cod Gadus morhua L. In situ hybridization using cRNA probes was used to characterize the temporal and spatial patterns of gene expression with respect to somite stage in embryos incubated at 4 degrees C, 7 degrees C and 10 degrees C. MyoD transcripts were first observed in the presomitic mesoderm prior to somite formation, and in the lateral compartment of the forming somites. MyoD expression was not observed in the adaxial cells that give rise to the slow muscle layer, and expression was undetectable by in situ hybridization in the lateral somitic mesoderm after the 35-somite stage, during development of the final approximately 15 somites. RT-PCR analysis, however, confirmed the presence of low levels of the transcript during these later stages. A phylogenetic comparison of the deduced aminoacid sequences of the full-length MyoD cDNA clone and those from other teleosts, and inference from the in situ expression pattern suggested homology with a second paralogue (MyoD2) recently isolated from the gilthead seabream Sparus aurata. Following MyoD expression, alpha-actin was the first structural gene to be switched on at the 16-somite stage, followed by myosin heavy chain, troponin T, troponin I and muscle creatine kinase. The final mRNA in the series to be expressed was troponin C. All genes were switched on prior to myofibril assembly. The troponin C sequence was unusual in that it showed the greatest sequence identity with the rainbow trout Oncorhynchus mykiss cardiac/slow form, but was expressed in the fast myotomal muscle and not in the heart. In addition, the third TnC calcium binding site showed a lower level of sequence conservation than the rest of the sequence. No differences were seen in the timing of appearance or rate of posterior progression (relative to somite stage) of any MSP transcripts between embryos raised at the different temperatures. It was concluded that myofibrillar genes are activated asynchronously in a distinct temporal order prior to myofibrillar assembly and that this process was highly canalized over the temperature range studied.

Expression of muscle-related genes and two MyoD genes during amphioxus notochord development

The notochord is one of the diagnostic features of the phylum Chordata. Despite the similarities in the early morphogenetic patterns of the notochords of various chordates, they are strikingly distinct from one another at the histological level. The amphioxus notochord is one example of an evolutionary novelty because it is made up of muscle cells. Our previous expressed sequence tag analysis, targeting messenger RNAs expressed in the adult amphioxus notochord, demonstrated that many muscle-related genes are expressed there. To characterize amphioxus notochord cells and to gain insights into the myogenic program in the notochord, we determined the spatial and temporal expre-ssion patterns of these muscle-related genes during amphioxus development. We found that BbNA1 (notochord actin), Amphi-Trop I (troponin I), Amphi-TPmyosin (tropo-myosin), Amphi-MHC2 (myosin heavy chain), Amphi-nMRLC (notochord-specific myosin regulatory light chain), Amphi-nTitin/MLCK (notochord-specific titin/myosin light chain kinase), Amphi-MLP/CRP3 (muscle LIM protein), and Amphi-nCalponin (notochord-specific calponin) are expres-sed with characteristic patterns in notochord cells, including the central cells, dorsally located cells, and ventrally located cells, suggesting that each notochord cell has a unique molecular architecture that may reflect its function. In addition, we characterized two MyoD genes (Amphi-MyoD1 and Amphi-MyoD2) to gain insight into the genetic circuitry governing the formation of the notochord muscle. One of the MyoD genes (Amphi-MyoD2) is expressed in the central notochord cells, and the coexistence of Amphi-MyoD2 transcripts along with the Amphi-MLP/CRP3 transcripts implies the participation of Amphi-MyoD2 in the myogenic program in the notochord muscle.

Muscle disease caused by mutations in the skeletal muscle alpha-actin gene (ACTA1)

Mutations in the skeletal muscle alpha-actin gene (ACTA1) associated with congenital myopathy with excess of thin myofilaments, nemaline myopathy and intranuclear rod myopathy were first described in 1999. At that time, only 15 different missense mutations were known in ACTA1. More than 60 mutations have now been identified. This review analyses this larger spectrum of mutations in ACTA1. It investigates the molecular consequences of the mutations found to date, provides a framework for genotype-phenotype correlation and suggests future studies in light of results of investigation of normal and mutant actin in other systems, notably the actin specific to the indirect flight muscles of Drosophila. The larger series confirms that the majority of ACTA1 mutations are dominant, a small number are recessive and most isolated cases with no previous family history have de novo dominant mutations. The severity of the disease caused ranges from lack of spontaneous movements at birth requiring immediate mechanical ventilation, to mild disease compatible with life to adulthood. Overall, the mutations within ACTA1 are randomly distributed throughout the protein. However, the larger series of mutations now available indicates that there may be clustering of mutations associated with some phenotypes, e.g. actin myopathy. This would suggest that interference with certain actin functions may be more associated with certain phenotypes, though the exact pathophysiology of the actin mutations remains unknown.

Ca2+ regulation of rabbit skeletal muscle thin filament sliding: role of cross-bridge number

We investigated how strong cross-bridge number affects sliding speed of regulated Ca(2+)-activated, thin filaments. First, using in vitro motility assays, sliding speed decreased nonlinearly with reduced density of heavy meromyosin (HMM) for regulated (and unregulated) F-actin at maximal Ca(2+). Second, we varied the number of Ca(2+)-activatable troponin complexes at maximal Ca(2+) using mixtures of recombinant rabbit skeletal troponin (WT sTn) and sTn containing sTnC(D27A,D63A), a mutant deficient in Ca(2+) binding at both N-terminal, low affinity Ca(2+)-binding sites (xxsTnC-sTn). Sliding speed decreased nonlinearly as the proportion of WT sTn decreased. Speed of regulated thin filaments varied with pCa when filaments contained WT sTn but filaments containing only xxsTnC-sTn did not move. pCa(50) decreased by 0.12-0.18 when either heavy meromyosin density was reduced to approximately 60% or the fraction of Ca(2+)-activatable regulatory units was reduced to approximately 33%. Third, we exchanged mixtures of sTnC and xxsTnC into single, permeabilized fibers from rabbit psoas. As the proportion of xxsTnC increased, unloaded shortening velocity decreased nonlinearly at maximal Ca(2+). These data are consistent with unloaded filament sliding speed being limited by the number of cycling cross-bridges so that maximal speed is attained with a critical, low level of actomyosin interactions.

Soluble Fas (FasB) regulates luteal cell apoptosis during luteolysis in murine ovaries

During luteolysis, luteal cell apoptosis is induced by the Fas ligand (FasL)/Fas system. In murine luteal bodies, we demonstrated the expression of mRNA of soluble form of Fas (FasB), which binds to FasL and prevents apoptosis induction. By in situ hybridization, strong expression of FasB mRNA was observed in normal luteal bodies, in which no apoptotic cells were detected, but negative/trace expression in regressing luteal bodies, in which many apoptotic cells were observed. Immunohistochemical staining revealed that Fas and TNF-alpha were localized in both normal and regressing luteal bodies, but IFN-gamma was localized only in regressing luteal bodies. Apoptosis was induced in primary cultured luteal cells, when they were pretreated with TNF-alpha and IFN-gamma and then incubated with TNF-alpha, IFN-gamma, and mouse recombinant FasL (rFasL). However, no apoptosis was detected in the cells, when they were treated with rFasL alone, TNF-alpha alone, IFN-gamma alone, TNF-alpha and rFasL, IFN-gamma and rFasL, or TNF-alpha and IFN-gamma. Fas mRNA expression in cultured luteal cells was up-regulated by the treatment of TNF-alpha, IFN-gamma, or TNF-alpha and IFN-gamma. The expression of FasB mRNA was down-regulated, when the cells were treated with TNF-alpha and IFN-gamma, but its expression was not changed by the treatment of TNF-alpha alone or IFN-gamma alone. We conclude that FasB inhibits the apoptosis induction in luteal cells of normal luteal bodies, and that decreased FasB production induced by TNF-alpha and IFN-gamma made possible the apoptosis induction in the luteal cells of regressing luteal bodies.

Unloaded shortening increases peak of Ca2+ transients but accelerates their decay in rat single cardiac myocytes

It is of paramount importance to investigate the relation between the time-dependent change in intracellular Ca2+ concentration ([Ca2+]i) (Ca2+ transients) and the mechanical activity of isolated single myocytes to understand the regulatory mechanisms of heart function. However, because of technical difficulties in performing mechanical measurements with single myocytes, the simultaneous recording of Ca2+ transients and mechanical activity has mainly been performed with multicellular cardiac preparations that give conflicting results concerning Ca2+ transients during isometric twitches and during twitches with unloaded shortening. In the present study, we coupled intracellular Ca2+ measurement optics with a force measurement system using carbon fibers to examine the relation between Ca2+ transients and the mechanical activity of rat single ventricular myocytes over a wide range of load. To minimize the possible load dependence of sarcoplasmic reticulum Ca2+ loading, contraction mode was switched at every twitch from unloaded shortening to isometric contraction. During a twitch with unloaded shortening, the Ca2+ transients exhibited a higher peak and a higher rate of decay than transients during an isometric twitch. Similarly, when we changed the contraction mode in every pair of twitches, Ca2+ transients were dependent only on the mode of contraction. Mechanical uncoupling with 2,3-butanedione monoxime abolished this dependence on the mode of contraction. Our results suggest that Ca2+ transients reflect the affinity of troponin C for Ca2+, which is influenced by the change in strain on the thin filament but not by the length change per se.

The C terminus (amino acids 75-94) and the linker region (amino acids 42-54) of the Ca2+-binding protein S100A1 differentially enhance sarcoplasmic Ca2+ release in murine skinned skeletal muscle fibers

S100A1, a Ca2+-binding protein of the EF-hand type, is most highly expressed in striated muscle and has previously been shown to interact with the skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel/ryanodine receptor (RyR1) isoform. However, it was unclear whether S100A1/RyR1 interaction could modulate SR Ca2+ handling and contractile properties in skeletal muscle fibers. Since S100A1 protein is differentially expressed in fast- and slow-twitch skeletal muscle, we used saponin-skinned murine Musculus extensor digitorum longus (EDL) and Musculus soleus (Soleus) fibers to assess the impact of S100A1 protein on SR Ca2+ release and isometric twitch force in functionally intact permeabilized muscle fibers. S100A1 equally enhanced caffeine-induced SR Ca2+ release and Ca2+-induced isometric force transients in both muscle preparations in a dose-dependent manner. Introducing a synthetic S100A1 peptide model (devoid of EF-hand Ca2+-binding sites) allowed identification of the S100A1 C terminus (amino acids 75-94) and hinge region (amino acids 42-54) to differentially enhance SR Ca2+ release with a nearly 3-fold higher activity of the C terminus. These effects were exclusively based on enhanced SR Ca2+ release as S100A1 influenced neither SR Ca2+ uptake nor myofilament Ca2+ sensitivity/cooperativity in our experimental setting. In conclusion, our study shows for the first time that S100A1 augments contractile performance both of fast- and slow-twitch skeletal muscle fibers based on enhanced SR Ca2+ efflux at least mediated by the C terminus of S100A1 protein. Thus, our data suggest that S100A1 may serve as an endogenous enhancer of SR Ca2+ release and might therefore be of physiological relevance in the process of excitation-contraction coupling in skeletal muscle.

Truncation by Glu180 nonsense mutation results in complete loss of slow skeletal muscle troponin T in a lethal nemaline myopathy

A lethal form of nemaline myopathy, named "Amish Nemaline Myopathy" (ANM), is linked to a nonsense mutation at codon Glu180 in the slow skeletal muscle troponin T (TnT) gene. We found that neither the intact nor the truncated slow TnT protein was present in the muscle of patients with ANM. The complete loss of slow TnT is consistent with the observed recessive pattern of inheritance of the disease and indicates a critical role of the COOH-terminal T2 domain in the integration of TnT into myofibrils. Expression of slow and fast isoforms of TnT is fiber-type specific. The lack of slow TnT results in selective atrophy of type 1 fibers. Slow TnT confers a higher Ca2+ sensitivity than does fast TnT in single fiber contractility assays. Despite the lack of slow TnT, individuals with ANM have normal muscle power at birth. The postnatal onset and infantile progression of ANM correspond to a down-regulation of cardiac and embryonic splice variants of fast TnT in normal developing human skeletal muscle, suggesting that the fetal TnT isoforms complement slow TnT. These results lay the foundation for understanding the molecular pathophysiology and the potential targeted therapy of ANM.

Familial hypertrophic cardiomyopathy mutations in troponin I (K183D, G203S, K206Q) enhance filament sliding

A major cause of familial hypertrophic cardiomyopathy (FHC) is dominant mutations in cardiac sarcomeric genes. Linkage studies identified FHC-related mutations in the COOH terminus of cardiac troponin I (cTnI), a region with unknown function in Ca(2+) regulation of the heart. Using in vitro assays with recombinant rat troponin subunits, we tested the hypothesis that mutations K183Delta, G203S, and K206Q in cTnI affect Ca(2+) regulation. All three mutants enhanced Ca(2+) sensitivity and maximum speed (s(max)) of filament sliding of in vitro motility assays. Enhanced s(max) (pCa 5) was observed with rabbit skeletal and rat cardiac (alpha-MHC or beta-MHC) heavy meromyosin (HMM). We developed a passive exchange method for replacing endogenous cTn in permeabilized rat cardiac trabeculae. Ca(2+) sensitivity and maximum isometric force did not differ between preparations exchanged with cTn(cTnI,K206Q) or wild-type cTn. In both trabeculae and motility assays, there was no loss of inhibition at pCa 9. These results are consistent with COOH terminus of TnI modulating actomyosin kinetics during unloaded sliding, but not during isometric force generation, and implicate enhanced cross-bridge cycling in the cTnI-related pathway(s) to hypertrophy.