The Croonian Lecture, 1977. Stretch activation of muscle: function and mechanism

Sensorimotor delays constrain robust locomotion in a 3D kinematic model of fly walking

Walking animals must maintain stability in the presence of external perturbations, despite significant temporal delays in neural signaling and muscle actuation. Here, we develop a 3D kinematic model with a layered control architecture to investigate how sensorimotor delays constrain robustness of walking behavior in the fruit fly, Drosophila. Motivated by the anatomical architecture of insect locomotor control circuits, our model consists of three component layers: a neural network that generates realistic 3D joint kinematics for each leg, an optimal controller that executes the joint kinematics while accounting for delays, and an inter-leg coordinator. The model generates realistic simulated walking that matches real fly walking kinematics and sustains walking even when subjected to unexpected perturbations, generalizing beyond its training data. However, we found that the model's robustness to perturbations deteriorates when sensorimotor delay parameters exceed the physiological range. These results suggest that fly sensorimotor control circuits operate close to the temporal limit at which they can detect and respond to external perturbations. More broadly, we show how a modular, layered model architecture can be used to investigate physiological constraints on animal behavior.

Structure of mavacamten-free human cardiac thick filaments within the sarcomere by cryoelectron tomography

Heart muscle has the unique property that it can never rest; all cardiomyocytes contract with each heartbeat which requires a complex control mechanism to regulate cardiac output to physiological requirements. Changes in calcium concentration regulate the thin filament activation. A separate but linked mechanism regulates the thick filament activation, which frees sufficient myosin heads to bind the thin filament, thereby producing the required force. Thick filaments contain additional nonmyosin proteins, myosin-binding protein C and titin, the latter being the protein that transmits applied tension to the thick filament. How these three proteins interact to control thick filament activation is poorly understood. Here, we show using 3-D image reconstruction of frozen-hydrated human cardiac muscle myofibrils lacking exogenous drugs that the thick filament is structured to provide three levels of myosin activation corresponding to the three crowns of myosin heads in each 429Å repeat. In one crown, the myosin heads are almost completely activated and disordered. In another crown, many myosin heads are inactive, ordered into a structure called the interacting heads motif. At the third crown, the myosin heads are ordered into the interacting heads motif, but the stability of that motif is affected by myosin-binding protein C. We think that this hierarchy of control explains many of the effects of length-dependent activation as well as stretch activation in cardiac muscle control.

Structure of the Flight Muscle Thick Filament from the Bumble Bee, Bombus ignitus, at 6 Å Resolution

Four insect orders have flight muscles that are both asynchronous and indirect; they are asynchronous in that the wingbeat frequency is decoupled from the frequency of nervous stimulation and indirect in that the muscles attach to the thoracic exoskeleton instead of directly to the wing. Flight muscle thick filaments from two orders, Hemiptera and Diptera, have been imaged at a subnanometer resolution, both of which revealed a myosin tail arrangement referred to as “curved molecular crystalline layers”. Here, we report a thick filament structure from the indirect flight muscles of a third insect order, Hymenoptera, the Asian bumble bee Bombus ignitus. The myosin tails are in general agreement with previous determinations from Lethocerus indicus and Drosophila melanogaster. The Skip 2 region has the same unusual structure as found in Lethocerus indicus thick filaments, an α-helix discontinuity is also seen at Skip 4, but the orientation of the Skip 1 region on the surface of the backbone is less angled with respect to the filament axis than in the other two species. The heads are disordered as in Drosophila, but we observe no non-myosin proteins on the backbone surface that might prohibit the ordering of myosin heads onto the thick filament backbone. There are strong structural similarities among the three species in their non-myosin proteins within the backbone that suggest how one previously unassigned density in Lethocerus might be assigned. Overall, the structure conforms to the previously observed pattern of high similarity in the myosin tail arrangement, but differences in the non-myosin proteins.

Musculoskeletal wing-actuation model of hummingbirds predicts diverse effects of primary flight muscles in hovering flight

Hummingbirds have evolved to hover and manoeuvre with exceptional flight control. This is enabled by their musculoskeletal system that successfully exploits the agile motion of flapping wings. Here, we synthesize existing empirical and modelling data to generate novel hypotheses for principles of hummingbird wing actuation. These may help guide future experimental work and provide insights into the evolution and robotic emulation of hummingbird flight. We develop a functional model of the hummingbird musculoskeletal system, which predicts instantaneous, three-dimensional torque produced by primary (pectoralis and supracoracoideus) and combined secondary muscles. The model also predicts primary muscle contractile behaviour, including stress, strain, elasticity and work. Results suggest that the primary muscles (i.e. the flight 'engine') function as diverse effectors, as they do not simply power the stroke, but also actively deviate and pitch the wing with comparable actuation torque. The results also suggest that the secondary muscles produce controlled-tightening effects by acting against primary muscles in deviation and pitching. The diverse effects of the pectoralis are associated with the evolution of a comparatively enormous bicipital crest on the humerus.

Molecular regulation of stretch activation

Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac myofibrillar preparations, few studies have measured mechanisms of stretch activation in mammalian skeletal muscle fibers. We measured stretch activation following rapid step stretches [∼1%-4% sarcomere length (SL)] during submaximal Ca2+ activations of rat permeabilized slow-twitch skeletal muscle fibers before and after protein kinase A (PKA), which phosphorylates slow myosin binding protein-C. PKA significantly increased stretch activation during low (∼25%) Ca2+ activation and accelerated rates of delayed force development (kef) during both low and half-maximal Ca2+ activation. Following the step stretches and subsequent force development, fibers were rapidly shortened to original sarcomere length, which often elicited a shortening-induced transient force overshoot. After PKA, step shortening-induced transient force overshoot increased ∼10-fold following an ∼4% SL shortening during low Ca2+ activation levels. kdf following step shortening also increased after PKA during low and half-maximal Ca2+ activations. We next investigated thin filament regulation of stretch activation. We tested the interplay between cardiac troponin I (cTnI) phosphorylation at the canonical PKA and novel tyrosine kinase sites on stretch activation. Native slow-skeletal Tn complexes were exchanged with recombinant human cTn complex with different human cTnI N-terminal pseudo-phosphorylation molecules: 1) nonphosphorylated wild type (WT), 2) the canonical S22/23D PKA sites, 3) the tyrosine kinase Y26E site, and 4) the combinatorial S22/23D + Y26E cTnI. All three pseudo-phosphorylated cTnIs elicited greater stretch activation than WT. Following stretch activation, a new, elevated stretch-induced steady-state force was reached with pseudo-phosphorylated cTnI. Combinatorial S22/23D + Y26E pseudo-phosphorylated cTnI increased kdf. These results suggest that slow-skeletal myosin binding protein-C (sMyBP-C) phosphorylation modulates stretch activation by a combination of cross-bridge recruitment and faster cycling kinetics, whereas cTnI phosphorylation regulates stretch activation by both redundant and synergistic mechanisms; and, taken together, these sarcomere phosphoproteins offer precision targets for enhanced contractility.

A thermodynamically consistent monte carlo cross-bridge model with a trapping mechanism reveals the role of stretch activation in heart pumping

Changes in intracellular calcium concentrations regulate heart beats. However, the decline in the left ventricular pressure during early diastole is much sharper than that of the Ca²⁺ transient, resulting in a rapid supply of blood to the left ventricle during the diastole. At the tissue level, cardiac muscles have a distinct characteristic, known as stretch activation, similar to the function of insect flight muscles. Stretch activation, which is a delayed increase in force following a rapid muscle length increase, has been thought to be related to autonomous control in these muscles. In this numerical simulation study, we introduced a molecular mechanism of stretch activation and investigated the role of this mechanism in the pumping function of the heart, using the previously developed coupling multiple-step active stiffness integration scheme for a Monte Carlo (MC) cross-bridge model and a bi-ventricular finite element model. In the MC cross-bridge model, we introduced a mechanism for trapping the myosin molecule in its post-power stroke state. We then determined the rate constants of transitions for trapping and escaping in a thermodynamically consistent manner. Based on our numerical analysis, we draw the following conclusions regarding the stretch activation mechanism: (i) the delayed force becomes larger than the original isometric force because the population of trapped myosin molecules and their average force increase after stretching; (ii) the delayed force has a duration of more than a few seconds owing to a fairly small rate constant of escape from the trapped state. For the role of stretch activation in heart pumping, we draw the following conclusions: (iii) for the regions in which the contraction force decreases earlier than the neighboring region in the end-systole phase, the trapped myosin molecules prevent further lengthening of the myocytes, which then prevents further shortening of neighboring myocytes; (iv) as a result, the contraction forces are sustained longer, resulting in a larger blood ejection, and their degeneration is synchronized.

Shortening deactivation: quantifying a critical component of cyclical muscle contraction

A muscle undergoing cyclical contractions requires fast and efficient muscle activation and relaxation to generate high power with relatively low energetic cost. To enhance activation and increase force levels during shortening, some muscle types have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. SA's complementary phenomenon is shortening deactivation (SD), a delayed decrease in force following muscle shortening. SD increases muscle relaxation, which decreases resistance to subsequent muscle lengthening. While it might be just as important to cyclical power output, SD has received less investigation than SA. To enable mechanistic investigations into SD and quantitatively compare it to SA, we developed a protocol to elicit SA and SD from Drosophila and Lethocerus indirect flight muscles (IFM) and Drosophila jump muscle. When normalized to isometric tension, Drosophila IFM exhibited a 118% SD tension decrease, Lethocerus IFM dropped by 97%, and Drosophila jump muscle decreased by 37%. The same order was found for normalized SA tension: Drosophila IFM increased by 233%, Lethocerus IFM by 76%, and Drosophila jump muscle by only 11%. SD occurred slightly earlier than SA, relative to the respective length change, for both IFMs; but SD was exceedingly earlier than SA for jump muscle. Our results suggest SA and SD evolved to enable highly efficient IFM cyclical power generation and may be caused by the same mechanism. However, jump muscle SA and SD mechanisms are likely different, and may have evolved for a role other than to increase the power output of cyclical contractions.

Rapid frequency modulation in a resonant system: aerial perturbation recovery in hawkmoths

Centimetre-scale fliers must contend with the high power requirements of flapping flight. Insects have elastic elements in their thoraxes which may reduce the inertial costs of their flapping wings. Matching wingbeat frequency to a mechanical resonance can be energetically favourable, but also poses control challenges. Many insects use frequency modulation on long timescales, but wingstroke-to-wingstroke modulation of wingbeat frequencies in a resonant spring-wing system is potentially costly because muscles must work against the elastic flight system. Nonetheless, rapid frequency and amplitude modulation may be a useful control modality. The hawkmoth Manduca sexta has an elastic thorax capable of storing and returning significant energy. However, its nervous system also has the potential to modulate the driving frequency of flapping because its flight muscles are synchronous. We tested whether hovering hawkmoths rapidly alter frequency during perturbations with vortex rings. We observed both frequency modulation (32% around mean) and amplitude modulation (37%) occurring over several wingstrokes. Instantaneous phase analysis of wing kinematics revealed that more than 85% of perturbation responses required active changes in neurogenic driving frequency. Unlike their robotic counterparts that abdicate frequency modulation for energy efficiency, synchronous insects use wingstroke-to-wingstroke frequency modulation despite the power demands required for deviating from resonance.

Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental contractile properties, such as active force production and energy utilization. Factors that influence cross-bridge kinetics at the molecular level propagate through the sarcomeres, cells and tissue to modulate whole-muscle function. Conversely, movement and changes in the muscle length can influence cross-bridge kinetics on the molecular level. Reduced, single-molecule and single-fibre experiments have shown that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether these strain-dependent cycling mechanisms persist in the intact muscle tissue, which encompasses more complex organization and passive elements, remains unclear. To investigate this multi-scale relationship, we adapted traditional step-stretch protocols for use with mouse soleus muscle during isometric tetanic contractions, enabling novel estimates of length-dependent cross-bridge kinetics in the intact skeletal muscle. Compared to rates at the optimal muscle length (L_o), we found that cross-bridge detachment rates increased by approximately 20% at 90% of L_o (shorter) and decreased by approximately 20% at 110% of L_o (longer). These data indicate that cross-bridge kinetics vary with whole-muscle length during intact, isometric contraction, which could intrinsically modulate force generation and energetics, and suggests a multi-scale feedback pathway between whole-muscle function and cross-bridge activity.

Making waves: A proposed new role for myosin-binding protein C in regulating oscillatory contractions in vertebrate striated muscle

Myosin-binding protein C (MyBP-C) is a critical regulator of muscle performance that was first identified through its strong binding interactions with myosin, the force-generating protein of muscle. Almost simultaneously with its discovery, MyBP-C was soon found to bind to actin, the physiological catalyst for myosin’s activity. However, the two observations posed an apparent paradox, in part because interactions of MyBP-C with myosin were on the thick filament, whereas MyBP-C interactions with actin were on the thin filament. Despite the intervening decades since these initial discoveries, it is only recently that the dual binding modes of MyBP-C are becoming reconciled in models that place MyBP-C at a central position between actin and myosin, where MyBP-C alternately stabilizes a newly discovered super-relaxed state (SRX) of myosin on thick filaments in resting muscle and then prolongs the “on” state of actin on thin filaments in active muscle. Recognition of these dual, alternating functions of MyBP-C reveals how it is central to the regulation of both muscle contraction and relaxation. The purpose of this Viewpoint is to briefly summarize the roles of MyBP-C in binding to myosin and actin and then to highlight a possible new role for MyBP-C in inducing and damping oscillatory waves of contraction and relaxation. Because the contractile waves bear similarity to cycles of contraction and relaxation in insect flight muscles, which evolved for fast, energetically efficient contraction, the ability of MyBP-C to damp so-called spontaneous oscillatory contractions (SPOCs) has broad implications for previously unrecognized regulatory mechanisms in vertebrate striated muscle. While the molecular mechanisms by which MyBP-C can function as a wave maker or a wave breaker are just beginning to be explored, it is likely that MyBP-C dual interactions with both myosin and actin will continue to be important for understanding the new functions of this enigmatic protein.

CryoEM structure of Drosophila flight muscle thick filaments at 7 Å resolution

Striated muscle thick filaments are composed of myosin II and several non-myosin proteins. Myosin II's long α-helical coiled-coil tail forms the dense protein backbone of filaments, whereas its N-terminal globular head containing the catalytic and actin-binding activities extends outward from the backbone. Here, we report the structure of thick filaments of the flight muscle of the fruit fly Drosophila melanogaster at 7 Å resolution. Its myosin tails are arranged in curved molecular crystalline layers identical to flight muscles of the giant water bug Lethocerus indicus Four non-myosin densities are observed, three of which correspond to ones found in Lethocerus; one new density, possibly stretchin-mlck, is found on the backbone outer surface. Surprisingly, the myosin heads are disordered rather than ordered along the filament backbone. Our results show striking myosin tail similarity within flight muscle filaments of two insect orders separated by several hundred million years of evolution.

A myosin-based mechanism for stretch activation and its possible role revealed by varying phosphate concentration in fast and slow mouse skeletal muscle fibers

Stretch activation (SA) is a delayed increase in force following a rapid muscle length increase. SA is best known for its role in asynchronous insect flight muscle, where it has replaced calcium's typical role of modulating muscle force levels during a contraction cycle. SA also occurs in mammalian skeletal muscle but has previously been thought to be too low in magnitude, relative to calcium-activated (CA) force, to be a significant contributor to force generation during locomotion. To test this supposition, we compared SA and CA force at different P_i concentrations (0-16 mM) in skinned mouse soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscle fibers. CA isometric force decreased similarly in both muscles with increasing P_i, as expected. SA force decreased with P_i in EDL (40%), leaving the SA to CA force ratio relatively constant across P_i concentrations (17-25%). In contrast, SA force increased in soleus (42%), causing a quadrupling of the SA to CA force ratio, from 11% at 0 mM P_i to 43% at 16 mM P_i, showing that SA is a significant force modulator in slow-twitch mammalian fibers. This modulation would be most prominent during prolonged muscle use, which increases P_i concentration and impairs calcium cycling. Based upon our previous Drosophila myosin isoform studies and this work, we propose that in slow-twitch fibers a rapid stretch in the presence of P_i reverses myosin's power stroke, enabling quick rebinding to actin and enhanced force production, while in fast-twitch fibers, stretch and P_i cause myosin to detach from actin.

Through thick and thin: dual regulation of insect flight muscle and cardiac muscle compared

Both insect flight muscle and cardiac muscle contract rhythmically, but the way in which repetitive contractions are controlled is different in the two types of muscle. We have compared the flight muscle of the water bug, Lethocerus, with cardiac muscle. Both have relatively high resting elasticity and are activated by an increase in sarcomere length or a quick stretch. The larger response of flight muscle is attributed to the highly ordered lattice of thick and thin filaments and to an isoform of troponin C that has no exchangeable Ca²⁺-binding site. The Ca²⁺ sensitivity of cardiac muscle and flight muscle can be manipulated so that cardiac muscle responds to Ca²⁺ like flight muscle, and flight muscle responds like cardiac muscle, showing the malleability of regulation. The interactions of the subunits in flight muscle troponin are described; a model of the complex, using the structure of cardiac troponin as a template, shows an overall similarity of cardiac and flight muscle troponin complexes. The dual regulation by thick and thin filaments in skeletal and cardiac muscle is thought to operate in flight muscle. The structure of inhibited myosin heads folded back on the thick filament in relaxed Lethocerus fibres has not been seen in other species and may be an adaptation to the rapid contractions of flight muscle. A scheme for regulation by thick and thin filaments during oscillatory contraction is described. Cardiac and flight muscle have much in common, but the differing mechanical requirements mean that regulation by both thick and thin filaments is adapted to the particular muscle.

Effect of myofibril passive elastic properties on the mechanical communication between motor proteins on adjacent sarcomeres

Rapid sarcomere lengthening waves propagate along a single muscle myofibril during spontaneous oscillatory contraction (SPOC). In asynchronous insect flight muscles, SPOC is thought to be almost completely synchronized over the entire myofibril. This phenomenon does not require Ca²⁺ regulation of the dynamics of the motor proteins, and cannot be explained simply by the longitudinal mechanical equilibrium among sarcomeres in the myofibril. In the present study, we rationalize these phenomena by considering the lateral mechanical equilibrium, in which two tensions originating from the inverse relationship between sarcomere length and lattice spacing, along with the lattice alignment, play important roles in the mechanical communication between motor proteins on adjacent filaments via the Z-disc. The proposed model is capable of explaining various SPOC phenomena based on the stochastic power-stroke mechanism of motor proteins, which responds to temporal changes in longitudinal mechanical load.

Milk-whey diet substantially suppresses seizure-like phenotypes of paraShu, a Drosophila voltage-gated sodium channel mutant

The Drosophila mutant para^Shu harbors a dominant, gain-of-function allele of the voltage-gated sodium channel gene, paralytic (para). The mutant flies display severe seizure-like phenotypes, including neuronal hyperexcitability, spontaneous spasms, ether-induced leg shaking, and heat-induced convulsions. We unexpectedly found that two distinct food recipes used routinely in the Drosophila research community result in a striking difference in severity of the para^Shu phenotypes. Namely, when para^Shu mutants were raised on the diet originally formulated by Edward Lewis in 1960, they showed severe neurological defects as previously reported. In contrast, when they were raised on the diet developed by Frankel and Brousseau in 1968, these phenotypes were substantially suppressed. Comparison of the effects of these two well-established food recipes revealed that the diet-dependent phenotypic suppression is accounted for by milk whey, which is present only in the latter. Inclusion of milk whey in the diet during larval stages was critical for suppression of the adult para^Shu phenotypes, suggesting that this dietary modification affects development of the nervous system. We also found that milk whey has selective effects on other neurological mutants. Among the behavioral phenotypes of different para mutant alleles, those of para^GEFS+ and para^bss were suppressed by milk whey, while those of para^DS and para^ts1 were not significantly affected. Overall, our study demonstrates that different diets routinely used in Drosophila labs could have considerably different effects on neurological phenotypes of Drosophila mutants. This finding provides a solid foundation for further investigation into how dietary modifications affect development and function of the nervous system and, ultimately, how they influence behavior.

Insights into Actin-Myosin Interactions within Muscle from 3D Electron Microscopy

Much has been learned about the interaction between myosin and actin through biochemistry, in vitro motility assays and cryo-electron microscopy (cryoEM) of F-actin, decorated with myosin heads. Comparatively less is known about actin-myosin interactions within the filament lattice of muscle, where myosin heads function as independent force generators and thus most measurements report an average signal from multiple biochemical and mechanical states. All of the 3D imaging by electron microscopy (EM) that has revealed the interplay of the regular array of actin subunits and myosin heads within the filament lattice has been accomplished using the flight muscle of the large water bug Lethocerus sp. The Lethocerus flight muscle possesses a particularly favorable filament arrangement that enables all the myosin cross-bridges contacting the actin filament to be visualized in a thin section. This review covers the history of this effort and the progress toward visualizing the complex set of conformational changes that myosin heads make when binding to actin in several static states, as well as the fast frozen actively contracting muscle. The efforts have revealed a consistent pattern of changes to the myosin head structures as determined by X-ray crystallography needed to explain the structure of the different actomyosin interactions observed in situ.

Comments to Recent Studies Showing Systemic Mechanisms Enabling Drosophila Larvae to Recover From Stress-Induced Damages

Compensational recovery from the damage created by stressors is important for all animals. However, how organisms recover from stress-induced negative impacts has been poorly understood. An 1-hour exposure to heat stress at 35°C led to reduced feeding activity of Drosophila melanogaster larvae, which caused reduction in body weight 2 hours after the stress, but not at other times. Such weight losses seem to be rescued by following enhanced feeding activities. We investigated the mechanisms underlying the accelerated feeding activity after the stress-induced reduction in feeding behavior. Our data showed increased expression of sweet taste gustatory receptor genes (Grs) and concomitant decreased expression of bitter taste Grs in the mouth parts 2 to 4 hours after the heat treatment for 1 hour. However, nontypical taste Gr expression was not changed. Furthermore, integration of both messenger RNA and protein expression analysis revealed that expression levels of tropomyosin and ATP (adenosine triphosphate) synthase β subunit were significantly increased in their mouths 3 to 5 hours after the heat stress. The increased expression of these genes would contribute to accelerated muscular movement of the mouth hooks. This study indicated that Drosophila larvae possess an efficient systemic mechanism that enables them to recover from growth delay caused by stress conditions.

Synchrotron Radiation X-ray Diffraction Techniques Applied to Insect Flight Muscle

X-ray fiber diffraction is a powerful tool used for investigating the molecular structure of muscle and its dynamics during contraction. This technique has been successfully applied not only to skeletal and cardiac muscles of vertebrates but also to insect flight muscle. Generally, insect flight muscle has a highly ordered structure and is often capable of high-frequency oscillations. The X-ray diffraction studies on muscle have been accelerated by the advent of 3rd-generation synchrotron radiation facilities, which can generate brilliant and highly oriented X-ray beams. This review focuses on some of the novel experiments done on insect flight muscle by using synchrotron radiation X-rays. These include diffraction recordings from single myofibrils within a flight muscle fiber by using X-ray microbeams and high-speed diffraction recordings from the flight muscle during the wing-beat of live insects. These experiments have provided information about the molecular structure and dynamic function of flight muscle in unprecedented detail. Future directions of X-ray diffraction studies on muscle are also discussed.

Coupling between myosin head conformation and the thick filament backbone structure

The recent high-resolution structure of the thick filament from Lethocerus asynchronous flight muscle shows aspects of thick filament structure never before revealed that may shed some light on how striated muscles function. The phenomenon of stretch activation underlies the function of asynchronous flight muscle. It is most highly developed in flight muscle, but is also observed in other striated muscles such as cardiac muscle. Although stretch activation is likely to be complex, involving more than a single structural aspect of striated muscle, the thick filament itself, would be a prime site for regulatory function because it must bear all of the tension produced by both its associated myosin motors and any externally applied force. Here we show the first structural evidence that the arrangement of myosin heads within the interacting heads motif is coupled to the structure of the thick filament backbone. We find that a change in helical angle of 0.16° disorders the blocked head preferentially within the Lethocerus interacting heads motif. This observation suggests a mechanism for how tension affects the dynamics of the myosin heads leading to a detailed hypothesis for stretch activation and shortening deactivation, in which the blocked head preferentially binds the thin filament followed by the free head when force production occurs.

The tymbal muscle of cicada has flight muscle-type sarcomeric architecture and protein expression

BACKGROUND

The structural and biochemical features of the tymbal (sound-producing) muscle of cicadas were studied by X-ray diffraction and immunochemistry, and compared with those of flight muscles from the same species.

RESULTS

The X-ray diffraction pattern of the tymbal muscle was very similar to that of the dorsal longitudinal flight muscle: In both muscles, the 2,0 equatorial reflection is much more intense than the 1,1, indicating that both muscles have a flight muscle-type myofilament lattice. In rigor, the first myosin/actin layer line reflection was finely lattice-sampled, indicating that the contractile proteins are arranged with a crystalline regularity as in asynchronous flight muscles. In contrast, the diffraction pattern from the tensor muscle, which modulates the sound by stressing the tymbal, did not show signs of such high regularity or flight muscle-type filament lattice. Electrophoretic patterns of myofibrillar proteins were also very similar in the tymbal muscle and flight muscles, but distinct from those from the tensor or leg muscles. The antibody raised against the flight muscle-specific troponin-I isoform reacted with an 80-kDa band from both tymbal and flight muscles, but with none of the bands from the tensor or leg muscles.

CONCLUSION

The close similarities of the structural and biochemical profiles between the tymbal and the flight muscles suggest the possibility that a set of flight muscle-specific proteins is diverted to the tymbal muscle to meet its demand for fast, repetitive contractions.

Stretch activation properties of Drosophila and Lethocerus indirect flight muscle suggest similar calcium-dependent mechanisms

Muscle stretch activation (SA) is critical for optimal cardiac and insect indirect flight muscle (IFM) power generation. The SA mechanism has been investigated for decades with many theories proposed, but none proven. One reason for the slow progress could be that multiple SA mechanisms may have evolved in multiple species or muscle types. Laboratories studying IFM SA in the same or different species have reported differing SA functional properties which would, if true, suggest divergent mechanisms. However, these conflicting results might be due to different experimental methodologies. Thus, we directly compared SA characteristics of IFMs from two SA model systems, Drosophila and Lethocerus, using two different fiber bathing solutions. Compared with Drosophila IFM, Lethocerus IFM isometric tension is 10- or 17-fold higher and SA tension was 5- or 10-fold higher, depending on the bathing solution. However, the rate of SA tension generation was 9-fold faster for Drosophila IFM. The inverse differences between rate and tension in the two species causes maximum power output to be similar, where Drosophila power is optimized in the bathing solution that favors faster muscle kinetics and Lethocerus in the solution that favors greater tension generation. We found that isometric tension and SA tension increased with calcium concentration for both species in both solutions, reaching a maximum plateau around pCa 5.0. Our results favor a similar mechanism for both species, perhaps involving a troponin complex that does not fully calcium activate the thin filament thus leaving room for further tension generation by SA.

Mechanics of the thorax in flies

Insects represent more than 60% of all multicellular life forms, and are easily among the most diverse and abundant organisms on earth. They evolved functional wings and the ability to fly, which enabled them to occupy diverse niches. Insects of the hyper-diverse orders show extreme miniaturization of their body size. The reduced body size, however, imposes steep constraints on flight ability, as their wings must flap faster to generate sufficient forces to stay aloft. Here, we discuss the various physiological and biomechanical adaptations of the thorax in flies which enabled them to overcome the myriad constraints of small body size, while ensuring very precise control of their wing motion. One such adaptation is the evolution of specialized myogenic or asynchronous muscles that power the high-frequency wing motion, in combination with neurogenic or synchronous steering muscles that control higher-order wing kinematic patterns. Additionally, passive cuticular linkages within the thorax coordinate fast and yet precise bilateral wing movement, in combination with an actively controlled clutch and gear system that enables flexible flight patterns. Thus, the study of thoracic biomechanics, along with the underlying sensory-motor processing, is central in understanding how the insect body form is adapted for flight.

Analysis of spontaneous oscillations for a three-state power-stroke model

Our study considers the mechanism of the spontaneous oscillations of molecular motors that are driven by the power stroke principle by applying linear stability analysis around the stationary solution. By representing the coupling equation of microscopic molecular motor dynamics and mesoscopic sarcomeric dynamics by a rank-1 updated matrix system, we derived the analytical representations of the eigenmodes of the Jacobian matrix that cause the oscillation. Based on these analytical representations, we successfully derived the essential conditions for the oscillation in terms of the rate constants of the power stroke and the reversal stroke transitions of the molecular motor. Unlike the two-state model, in which the dependence of the detachment rates on the motor coordinates or the applied forces on the motors plays a key role for the oscillation, our three-state power stroke model demonstrates that the dependence of the rate constants of the power and reversal strokes on the strains in the elastic elements in the motor molecules plays a key role, where these rate constants are rationally determined from the free energy available for the power stroke, the stiffness of the elastic element in the molecular motor, and the working stroke size. By applying the experimentally confirmed values to the free energy, the stiffness, and the working stroke size, our numerical model reproduces well the experimentally observed oscillatory behavior. Furthermore, our analysis shows that two eigenmodes with real positive eigenvalues characterize the oscillatory behavior, where the eigenmode with the larger eigenvalue indicates the transient of the system of the quick sarcomeric lengthening induced by the collective reversal strokes, and the smaller eigenvalue correlates with the speed of sarcomeric shortening, which is much slower than lengthening. Applying the perturbation analyses with primal physical parameters, we find that these two real eigenvalues occur on two branches derived from a merge point of a pair of complex-conjugate eigenvalues generated by Hopf bifurcation.

The earliest molecular response to stretch of insect flight muscle as revealed by fast X-ray diffraction recording

Small insects drive their flight muscle at frequencies up to 1,000 Hz. This remarkable ability owes to the mechanism of stretch activation. However, it remains unknown as to what sarcomeric component senses the stretch and triggers the following force generation. Here we show that the earliest structural change after a step stretch is reflected in the blinking of the 111 and 201 reflections, as observed in the fast X-ray diffraction recording from isolated bumblebee flight muscle fibers. The same signal has also been observed in live bumblebee. We demonstrate that (1) the signal responds almost concomitantly to a quick step stretch, (2) the signal grows with increasing calcium levels as the stretch-activated force does, and (3) a full 3-dimensional model demonstrates that the signal is maximized when objects having a 38.7-nm actin periodicity travel by ~20 nm along the filament axis. This is the expected displacement if myosin heads are loosely associated with actin target zones (where actin monomers are favorably oriented), and are dragged by a 1.3% stretch, which effectively causes stretch-induced activation. These results support and strengthen our proposal that the myosin head itself acts as the stretch sensor, after calcium-induced association with actin in a low-force form.

Neural control and precision of flight muscle activation in Drosophila

Precision of motor commands is highly relevant in a large context of various locomotor behaviors, including stabilization of body posture, heading control and directed escape responses. While posture stability and heading control in walking and swimming animals benefit from high friction via ground reaction forces and elevated viscosity of water, respectively, flying animals have to cope with comparatively little aerodynamic friction on body and wings. Although low frictional damping in flight is the key to the extraordinary aerial performance and agility of flying birds, bats and insects, it challenges these animals with extraordinary demands on sensory integration and motor precision. Our review focuses on the dynamic precision with which Drosophila activates its flight muscular system during maneuvering flight, considering relevant studies on neural and muscular mechanisms of thoracic propulsion. In particular, we tackle the precision with which flies adjust power output of asynchronous power muscles and synchronous flight control muscles by monitoring muscle calcium and spike timing within the stroke cycle. A substantial proportion of the review is engaged in the significance of visual and proprioceptive feedback loops for wing motion control including sensory integration at the cellular level. We highlight that sensory feedback is the basis for precise heading control and body stability in flies.

The Drosophila indirect flight muscle myosin heavy chain isoform is insufficient to transform the jump muscle into a highly stretch-activated muscle type

Stretch activation (SA) is a delayed increase in force that enables high power and efficiency from a cyclically contracting muscle. SA exists in various degrees in almost all muscle types. In Drosophila, the indirect flight muscle (IFM) displays exceptionally high SA force production (F_SA), whereas the jump muscle produces only minimal F_SA We previously found that expressing an embryonic (EMB) myosin heavy chain (MHC) isoform in the jump muscle transforms it into a moderately SA muscle type and enables positive cyclical power generation. To investigate whether variation in MHC isoforms is sufficient to produce even higher F_SA, we substituted the IFM MHC isoform (IFI) into the jump muscle. Surprisingly, we found that IFI only caused a 1.7-fold increase in F_SA, less than half the increase previously observed with EMB, and only at a high Pi concentration, 16 mM. This IFI-induced F_SA is much less than what occurs in IFM, relative to isometric tension, and did not enable positive cyclical power generation by the jump muscle. Both isometric tension and F_SA of control fibers decreased with increasing Pi concentration. However, for IFI-expressing fibers, only isometric tension decreased. The rate of F_SA generation was ~1.5-fold faster for IFI fibers than control fibers, and both rates were Pi dependent. We conclude that MHC isoforms can alter F_SA and hence cyclical power generation but that isoforms can only endow a muscle type with moderate F_SA Highly SA muscle types, such as IFM, likely use a different or additional mechanism.

Structure of myosin filaments from relaxed Lethocerus flight muscle by cryo-EM at 6 Å resolution

We describe a cryo-electron microscopy three-dimensional image reconstruction of relaxed myosin II-containing thick filaments from the flight muscle of the giant water bug Lethocerus indicus. The relaxed thick filament structure is a key element of muscle physiology because it facilitates the reextension process following contraction. Conversely, the myosin heads must disrupt their relaxed arrangement to drive contraction. Previous models predicted that Lethocerus myosin was unique in having an intermolecular head-head interaction, as opposed to the intramolecular head-head interaction observed in all other species. In contrast to the predicted model, we find an intramolecular head-head interaction, which is similar to that of other thick filaments but oriented in a distinctly different way. The arrangement of myosin's long α-helical coiled-coil rod domain has been hypothesized as either curved layers or helical subfilaments. Our reconstruction is the first report having sufficient resolution to track the rod α helices in their native environment at resolutions ~5.5 Å, and it shows that the layer arrangement is correct for Lethocerus. Threading separate paths through the forest of myosin coiled coils are four nonmyosin peptides. We suggest that the unusual position of the heads and the rod arrangement separated by nonmyosin peptides are adaptations for mechanical signal transduction whereby applied tension disrupts the myosin heads as a component of stretch activation.

Toward Understanding the Molecular Bases of Stretch Activation: A STRUCTURAL COMPARISON OF THE TWO TROPONIN C ISOFORMS OF LETHOCERUS

Muscles are usually activated by calcium binding to the calcium sensory protein troponin-C, which is one of the three components of the troponin complex. However, in cardiac and insect flight muscle activation is also produced by mechanical stress. Little is known about the molecular bases of this calcium-independent activation. In Lethocerus, a giant water bug often used as a model system because of its large muscle fibers, there are two troponin-C isoforms, called F1 and F2, that have distinct roles in activating the muscle. It has been suggested that this can be explained either by differences in structural features or by differences in the interactions with other proteins. Here we have compared the structural and dynamic properties of the two proteins and shown how they differ. We have also mapped the interactions of the F2 isoform with peptides spanning the sequence of its natural partner, troponin-I. Our data have allowed us to build a model of the troponin complex and may eventually help in understanding the specialized function of the F1 and F2 isoforms and the molecular mechanism of stretch activation.

Historical perspective on heart function: the Frank-Starling Law

More than a century of research on the Frank-Starling Law has significantly advanced our knowledge about the working heart. The Frank-Starling Law mandates that the heart is able to match cardiac ejection to the dynamic changes occurring in ventricular filling and thereby regulates ventricular contraction and ejection. Significant efforts have been attempted to identify a common fundamental basis for the Frank-Starling heart and, although a unifying idea has still to come forth, there is mounting evidence of a direct relationship between length changes in individual constituents (cardiomyocytes) and their sensitivity to Ca²⁺ ions. As the Frank-Starling Law is a vital event for the healthy heart, it is of utmost importance to understand its mechanical basis in order to optimize and organize therapeutic strategies to rescue the failing human heart. The present review is a historic perspective on cardiac muscle function. We "revive" a century of scientific research on the heart's fundamental protein constituents (contractile proteins), to their assemblies in the muscle (the sarcomeres), culminating in a thorough overview of the several synergistically events that compose the Frank-Starling mechanism. It is the authors' personal beliefs that much can be gained by understanding the Frank-Starling relationship at the cellular and whole organ level, so that we can finally, in this century, tackle the pathophysiologic mechanisms underlying heart failure.

Myofilament proteins in the synchronous flight muscles of Manduca sexta show both similarities and differences to Drosophila melanogaster

Insect flight muscles have been classified as either synchronous or asynchronous based on the coupling between excitation and contraction. In the moth Manduca sexta, the flight muscles are synchronous and do not display stretch activation, which is a property of asynchronous muscles. We annotated the M. sexta genes encoding the major myofibrillar proteins and analyzed their isoform pattern and expression. Comparison with the homologous genes in Drosophila melanogaster indicates both difference and similarities. For proteins such as myosin heavy chain, tropomyosin, and troponin I the availability and number of potential variants generated by alternative spicing is mostly conserved between the two insects. The exon usage associated with flight muscles indicates that some exon sets are similarly used in the two insects, whereas others diverge. For actin the number of individual genes is different and there is no evidence for a flight muscle specific isoform. In contrast for troponin C, the number of genes is similar, as well as the isoform composition in flight muscles despite the different calcium regulation. Both troponin I and tropomyosin can include COOH-terminal hydrophobic extensions similar to tropomyosinH and troponinH found in D. melanogaster and the honeybee respectively.

Elastic proteins in the flight muscle of Manduca sexta

The flight muscles (DLM1) of the Hawkmoth, Manduca sexta are synchronous, requiring a neural spike for each contraction. Stress/strain curves of skinned DLM1 showed hysteresis indicating the presence of titin-like elastic proteins. Projectin and kettin are titin-like proteins previously identified in Lethocerus and Drosophila flight muscles. Analysis of Manduca muscles with 1% SDS-agarose gels and western blots showed two bands near 1 MDa that cross-reacted with antibodies to Drosophila projectin. Antibodies to Drosophila kettin cross-reacted to bands at ∼500 and ∼700 kDa, but also to bands at ∼1.6 and ∼2.1 MDa, that had not been previously observed in insect flight muscles. Mass spectrometry identified the 2.1 MDa protein as a product of the Sallimus (sls) gene. Analysis of the gene sequence showed that all 4 putative Sallimus and kettin isoforms could be explained as products of alternative splicing of the single sls gene. Both projectin and sallimus isoforms were expressed to higher levels in ventrally located DLM1 subunits, primarily responsible for active work production, as compared to dorsally located subunits, which may act as damped springs. The different expression levels of the 2 projectin isoforms and 4 sallimus/kettin isoforms may be adaptations to the specific requirements of individual muscle subunits.

Biomechanical basis of wing and haltere coordination in flies

The spectacular success and diversification of insects rests critically on two major evolutionary adaptations. First, the evolution of flight, which enhanced the ability of insects to colonize novel ecological habitats, evade predators, or hunt prey; and second, the miniaturization of their body size, which profoundly influenced all aspects of their biology from development to behavior. However, miniaturization imposes steep demands on the flight system because smaller insects must flap their wings at higher frequencies to generate sufficient aerodynamic forces to stay aloft; it also poses challenges to the sensorimotor system because precise control of wing kinematics and body trajectories requires fast sensory feedback. These tradeoffs are best studied in Dipteran flies in which rapid mechanosensory feedback to wing motor system is provided by halteres, reduced hind wings that evolved into gyroscopic sensors. Halteres oscillate at the same frequency as and precisely antiphase to the wings; they detect body rotations during flight, thus providing feedback that is essential for controlling wing motion during aerial maneuvers. Although tight phase synchrony between halteres and wings is essential for providing proper timing cues, the mechanisms underlying this coordination are not well understood. Here, we identify specific mechanical linkages within the thorax that passively mediate both wing-wing and wing-haltere phase synchronization. We demonstrate that the wing hinge must possess a clutch system that enables flies to independently engage or disengage each wing from the mechanically linked thorax. In concert with a previously described gearbox located within the wing hinge, the clutch system enables independent control of each wing. These biomechanical features are essential for flight control in flies.

Flight and seizure motor patterns in Drosophila mutants: simultaneous acoustic and electrophysiological recordings of wing beats and flight muscle activity

Abstract Tethered flies allow studies of biomechanics and electrophysiology of flight control. We performed microelectrode recordings of spikes in an indirect flight muscle (the dorsal longitudinal muscle, DLMa) coupled with acoustic analysis of wing beat frequency (WBF) via microphone signals. Simultaneous electrophysiological recording of direct and indirect flight muscles has been technically challenging; however, the WBF is thought to reflect in a one-to-one relationship with spiking activity in a subset of direct flight muscles, including muscle m1b. Therefore, our approach enables systematic mutational analysis for changes in temporal features of electrical activity of motor neurons innervating subsets of direct and indirect flight muscles. Here, we report the consequences of specific ion channel disruptions on the spiking activity of myogenic DLMs (firing at ∼5 Hz) and the corresponding WBF (∼200 Hz). We examined mutants of the genes enconding: 1) voltage-gated Ca(2+) channels (cacophony, cac), 2) Ca(2+)-activated K(+) channels (slowpoke, slo), and 3) voltage-gated K(+) channels (Shaker, Sh) and their auxiliary subunits (Hyperkinetic, Hk and quiver, qvr). We found flight initiation in response to an air puff was severely disrupted in both cac and slo mutants. However, once initiated, slo flight was largely unaltered, whereas cac displayed disrupted DLM firing rates and WBF. Sh, Hk, and qvr mutants were able to maintain normal DLM firing rates, despite increased WBF. Notably, defects in the auxiliary subunits encoded by Hk and qvr could lead to distinct consequences, that is, disrupted DLM firing rhythmicity, not observed in Sh. Our mutant analysis of direct and indirect flight muscle activities indicates that the two motor activity patterns may be independently modified by specific ion channel mutations, and that this approach can be extended to other dipteran species and additional motor programs, such as electroconvulsive stimulation-induced seizures.

The roles of troponin C isoforms in the mechanical function of Drosophila indirect flight muscle

Stretch activation (SA) is a fundamental property of all muscle types that increases power output and efficiency, yet its mechanism is unknown. Recently, studies have implicated troponin isoforms as important in the SA mechanism. The highly stretch-activated Drosophila IFMs express two isoforms of the Ca(2+)-binding subunit of troponin (TnC). TnC1 (TnC-F2 in Lethocerus IFM) has two calcium binding sites, while an unusual isoform, TnC4 (TnC-F1 in Lethocerus IFM), has only one binding site. We investigated the roles of these two TnC isoforms in Drosophila IFM by targeting RNAi to each isoform. IFMs with TnC4 expression (normally ~90% of total TnC) replaced by TnC1 did not generate isometric tension, power or display SA. However, TnC4 knockdown resulted in sarcomere ultrastructure disarray, which could explain the lack of mechanical function and thus make interpretation of the influence of TnC4 on SA difficult. Elimination of TnC1 expression (normally ~10% of total TnC) by RNAi resulted in normal muscle structure. In these IFMs, fiber power generation, isometric tension, stretch-activated force and calcium sensitivity were statistically identical to wild type. When TnC1 RNAi was driven by an IFM specific driver, there was no decrease in flight ability or wing beat frequency, which supports our mechanical findings suggesting that TnC1 is not essential for the mechanical function of Drosophila IFM. This finding contrasts with previous work in Lethocerus IFM showing TnC1 is essential for maximum isometric force generation. We propose that differences in TnC1 function in Lethocerus and Drosophila contribute to the ~40-fold difference in IFM isometric tension generated between these species.

Inhibitory motoneurons in arthropod motor control: organisation, function, evolution

Miniaturisation of somatic cells in animals is limited, for reasons ranging from the accommodation of organelles to surface-to-volume ratio. Consequently, muscle and nerve cells vary in diameters by about two orders of magnitude, in animals covering 12 orders of magnitude in body mass. Small animals thus have to control their behaviour with few muscle fibres and neurons. Hexapod leg muscles, for instance, may consist of a single to a few 100 fibres, and they are controlled by one to, rarely, 19 motoneurons. A typical mammal has thousands of fibres per muscle supplied by hundreds of motoneurons for comparable behavioural performances. Arthopods—crustaceans, hexapods, spiders, and their kin—are on average much smaller than vertebrates, and they possess inhibitory motoneurons for a motor control strategy that allows a broad performance spectrum despite necessarily small cell numbers. This arthropod motor control strategy is reviewed from functional and evolutionary perspectives and its components are described with a focus on inhibitory motoneurons. Inhibitory motoneurons are particularly interesting for a number of reasons: evolutionary and phylogenetic comparison of functional specialisations, evolutionary and developmental origin and diversification, and muscle fibre recruitment strategies.

An embryonic myosin isoform enables stretch activation and cyclical power in Drosophila jump muscle

The mechanism behind stretch activation (SA), a mechanical property that increases muscle force and oscillatory power generation, is not known. We used Drosophila transgenic techniques and our new muscle preparation, the jump muscle, to determine if myosin heavy chain isoforms influence the magnitude and rate of SA force generation. We found that Drosophila jump muscles show very low SA force and cannot produce positive power under oscillatory conditions at pCa 5.0. However, we transformed the jump muscle to be moderately stretch-activatable by replacing its myosin isoform with an embryonic isoform (EMB). Expressing EMB, jump muscle SA force increased by 163% and it generated net positive power. The rate of SA force development decreased by 58% with EMB expression. Power generation is Pi dependent as >4 mM Pi was required for positive power from EMB. Pi increased EMB SA force, but not wild-type SA force. Our data suggest that when muscle expressing EMB is stretched, EMB is more easily driven backward to a weakly bound state than wild-type jump muscle. This increases the number of myosin heads available to rapidly bind to actin and contribute to SA force generation. We conclude that myosin heavy chain isoforms influence both SA kinetics and SA force, which can determine if a muscle is capable of generating oscillatory power at a fixed calcium concentration.

An inverse power-law distribution of molecular bond lifetimes predicts fractional derivative viscoelasticity in biological tissue

Viscoelastic characteristics of many materials falling under the category of soft glassy substances, including biological tissue, often exhibit a mechanical complex modulus Y(ω) well described by a fractional derivative model: Y(ω) = E(iω/ϕ)k, where E = a generalized viscoelastic stiffness; i = (-1)1/2; ω = angular frequency; ϕ = scaling factor; and k = an exponent valued between 0 and 1. The term "fractional derivative" refers to the value of k: when k = 0 the viscoelastic response is purely elastic, and when k = 1 the response is purely viscous. We provide an analytical derivation of the fractional derivative complex modulus based on the hypothesis that the viscoelastic response arises from many intermittent molecular crosslinks, whose lifetimes longer than a critical threshold lifetime, tcrit, are distributed with an inverse power law proportional to t-(k+2). We demonstrate that E is proportional to the number and stiffness of crosslinks formed at any moment; the scaling factor ϕ is equivalent to reciprocal of tcrit; and the relative mean lifetime of the attached crosslinks is inversely proportional to the parameter k. To test whether electrostatic molecular bonds could be responsible for the fractional derivative viscoelasticity, we used chemically skinned human skeletal muscle as a one-dimensional model of a soft glassy substance. A reduction in ionic strength from 175 to 110 mEq resulted in a larger E with no change in k, consistent with a higher probability of interfilament molecular interactions. Thick to thin filament spacing was reduced by applying 4% w/v of the osmolyte Dextran T500, which also resulted in a larger E, indicating a greater probability of crosslink formation in proportion to proximity. A 10°C increase in temperature resulted in an increase in k, which corresponded to a decrease in cross-bridge attachment lifetime expected with higher temperatures. These theoretical and experimental results suggest that the fractional derivative viscoelasticity observed in some biological tissue arises as a mechanical consequence of electrostatic interactions, whose longest lifetimes are distributed with an inverse power law.

Flight muscle-specific Pro-Ala-rich extension of troponin is important for maintaining the insect-type myofilament lattice integrity

Insect flight muscle (IFM) can oscillate at frequencies up to 1000Hz, owing to its capability of stretch activation (SA). It is a highly specialized form of cross striated muscles, and its peculiar features include the IFM-specific isoform of troponin-I (troponin-H or TnH) with an unusually long Pro-Ala-rich extension at the C-terminus. Although we have shown that this extension does not directly take part in SA, questions remain as to what its real role is and why it is expressed only in IFM. Here we explored the structural role of the extension, be comparing X-ray diffraction patterns and electron micrographs of bumblebee IFM fibers before and after enzymatic removal of the extension. The removal had a dramatic effect on diffraction patterns: In IFMs in general, the equatorial 2,0 reflection is much stronger than the 1,1 reflection, but after removal, their intensities became almost equal (stronger 1,1 is a feature of vertebrate skeletal muscle). Electron micrographs revealed that a substantial fraction of the thin filaments showed a tendency to move towards the vertebrate position (the trigonal position between three thick filaments), while the rest of the thin filaments remained in their original insect position (midway between two neighboring thick filaments). Therefore, one of the roles of the extension is suggested to keep the filament lattice in the correct configuration for IFM. This insect-type lattice structure is preserved among IFMs from varied insect orders but not in body muscles, suggesting that the maintenance of this lattice structure is important for flight functions.

Calcium signalling indicates bilateral power balancing in the Drosophila flight muscle during manoeuvring flight

Manoeuvring flight in animals requires precise adjustments of mechanical power output produced by the flight musculature. In many insects such as fruit flies, power generation is most likely varied by altering stretch-activated tension, that is set by sarcoplasmic calcium levels. The muscles reside in a thoracic shell that simultaneously drives both wings during wing flapping. Using a genetically expressed muscle calcium indicator, we here demonstrate in vivo the ability of this animal to bilaterally adjust its calcium activation to the mechanical power output required to sustain aerodynamic costs during flight. Motoneuron-specific comparisons of calcium activation during lift modulation and yaw turning behaviour suggest slightly higher calcium activation for dorso-longitudinal than for dorsoventral muscle fibres, which corroborates the elevated need for muscle mechanical power during the wings' downstroke. During turning flight, calcium activation explains only up to 54 per cent of the required changes in mechanical power, suggesting substantial power transmission between both sides of the thoracic shell. The bilateral control of muscle calcium runs counter to the hypothesis that the thorax of flies acts as a single, equally proportional source for mechanical power production for both flapping wings. Collectively, power balancing highlights the precision with which insects adjust their flight motor to changing energetic requirements during aerial steering. This potentially enhances flight efficiency and is thus of interest for the development of technical vehicles that employ bioinspired strategies of power delivery to flapping wings.

The long C-terminal extension of insect flight muscle-specific troponin-I isoform is not required for stretch activation

Stretch-induced enhancement of active force (stretch activation, SA) is observed in striated muscles in general, and most conspicuously in insect flight muscle (IFM). It remains unclear whether a common mechanism underlies the SA of all muscle types, or the SA of IFM relies on its highly specialized features. Recent studies suggest that IFM-specific isoforms of thin filament regulatory proteins (troponin and tropomyosin) are implicated in SA. Among others, IFM-specific troponin-I (troponin-H or TnH), with an unusually long Pro-Ala-rich extension at the C-terminus, has been speculated to transmit the mechanical signal of stretch to the troponin complex. To verify this hypothesis, it was removed by a specific endoproteinase in bumblebee IFM, expecting that it would eliminate SA while leaving intact the capacity for Ca(2+)-activated isometric force. Electrophoretic data showed that the extension was almost completely (97%) removed from IFM fibers after treatment. Unexpectedly, SA force was still conspicuous, and its rate of rise was not affected. Therefore, the results preclude the possibility that the extension is a main part of the mechanism of SA. This leaves open the possibility that SAs of IFM and vertebrate striated muscles, which lack the extension, operate under common basic mechanisms.

Cooperative regulation of myosin-S1 binding to actin filaments by a continuous flexible Tm-Tn chain

The regulation of striated muscle contraction involves cooperative interactions between actin filaments, myosin-S1 (S1), tropomyosin (Tm), troponin (Tn), and calcium. These interactions are modeled by treating overlapping tropomyosins as a continuous flexible chain (CFC), weakly confined by electrostatic interactions with actin. The CFC is displaced locally in opposite directions on the actin surface by the binding of either S1 or Troponin I (TnI) to actin. The apparent rate constants for myosin and TnI binding to and detachment from actin are then intrinsically coupled via the CFC model to the presence of neighboring bound S1s and TnIs. Monte Carlo simulations at prescribed values of the CFC stiffness, the CFC’s degree of azimuthal confinement, and the angular displacements caused by the bound proteins were able to predict the stopped-flow transients of S1 binding to regulated F-actin. The transients collected over a large range of calcium concentrations could be well described by adjusting a single calcium-dependent parameter, the rate constant of TnI detachment from actin, k_−I. The resulting equilibrium constant \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ K_{\text{B}} \equiv 1/K_{\text{I}} $$\end{document} varied sigmoidally with the free calcium, increasing from 0.12 at low calcium (pCa >7) to 12 at high calcium (pCa <5.5) with a Hill coefficient of ~2.15. The similarity of the curves for excess-actin and excess-myosin data confirms their allosteric relationship. The spatially explicit calculations confirmed variable sizes for the cooperative units and clustering of bound myosins at low calcium concentrations. Moreover, inclusion of negative cooperativity between myosin units predicted the observed slowing of myosin binding at excess-myosin concentrations.

Actin filaments as tension sensors

The field of mechanobiology has witnessed an explosive growth over the past several years as interest has greatly increased in understanding how mechanical forces are transduced by cells and how cells migrate, adhere and generate traction. Actin, a highly abundant and anomalously conserved protein, plays a large role in forming the dynamic cytoskeleton that is so essential for cell form, motility and mechanosensitivity. While the actin filament (F-actin) has been viewed as dynamic in terms of polymerization and depolymerization, new results suggest that F-actin itself may function as a highly dynamic tension sensor. This property may help explain the unusual conservation of actin's sequence, as well as shed further light on actin's essential role in structures from sarcomeres to stress fibers.

On mechanosensation, acto/myosin interaction, and hypertrophy

Current concepts of mechanosensation are general and applicable to almost every cell type. However, striated muscle cells are distinguished by their ability to generate strong forces via actin/myosin interaction, and this process is fine-tuned for optimum contractility. This aspect, unique for actively contracting cells, may be defined as "sensing of the magnitude and dynamics of contractility," as opposed to the well-known concepts of the "perception of extracellular mechanical stimuli." The acto/myosin interaction, by producing changes in ATP, ADP, Pi, and force on a millisecond timescale, may be regarded as a novel and previously unappreciated mechanosensory mechanism. In addition, sarcomeric mechanosensory structures, such as the Z-disc, are directly linked to autophagy, survival, and cell death-related pathways. One emerging example is telethonin and its ability to interfere with p53 metabolism and hence apoptosis (mechanoptosis). In this article, we introduce contractility per se as an important mechanosensory mechanism, and we differentiate extracellular from intracellular mechanosensory effects.

The function of the M-line protein obscurin in controlling the symmetry of the sarcomere in the flight muscle of Drosophila

Obscurin (also known as Unc-89 in Drosophila) is a large modular protein in the M-line of Drosophila muscles. Drosophila obscurin is similar to the nematode protein UNC-89. Four isoforms are found in the muscles of adult flies: two in the indirect flight muscle (IFM) and two in other muscles. A fifth isoform is found in the larva. The larger IFM isoform has all the domains that were predicted from the gene sequence. Obscurin is in the M-line throughout development of the embryo, larva and pupa. Using P-element mutant flies and RNAi knockdown flies, we have investigated the effect of decreased obscurin expression on the structure of the sarcomere. Embryos, larvae and pupae developed normally. In the pupa, however, the IFM was affected. Although the Z-disc was normal, the H-zone was misaligned. Adults were unable to fly and the structure of the IFM was irregular: M-lines were missing and H-zones misplaced or absent. Isolated thick filaments were asymmetrical, with bare zones that were shifted away from the middle of the filaments. In the sarcomere, the length and polarity of thin filaments depends on the symmetry of adjacent thick filaments; shifted bare zones resulted in abnormally long or short thin filaments. We conclude that obscurin in the IFM is necessary for the development of a symmetrical sarcomere in Drosophila IFM.

Regulating the contraction of insect flight muscle

The rapid movement of the wings in small insects is powered by the indirect flight muscles. These muscles are capable of contracting at up to 1,000 Hz because they are activated mechanically by stretching. The mechanism is so efficient that it is also used in larger insects like the waterbug, Lethocerus. The oscillatory activity of the muscles occurs a low concentration of Ca(2+), which stays constant as the muscles contract and relax. Activation by stretch requires particular isoforms of tropomyosin and the troponin complex on the thin filament. We compare the tropomyosin and troponin of Lethocerus and Drosophila with that of vertebrates. The characteristics of the flight muscle regulatory proteins suggest ways in which stretch-activation works. There is evidence for bridges between troponin on thin filaments and myosin crossbridges on the thick filaments. Recent X-ray fibre diffraction results suggest that a pull on the bridges activates the thin filament by shifting tropomyosin from a blocking position on actin. The troponin bridges are likely to contain extended sequences of tropomyosin or troponin I (TnI). Flight muscle has two isoforms of TnC with different Ca(2+)-binding properties: F1 TnC is needed for stretch-activation and F2 TnC for isometric contractions. In this review, we describe the structural changes in both isoforms on binding Ca(2+) and TnI, and discuss how the steric model of muscle regulation can apply to insect flight muscle.

Millisecond-scale biochemical response to change in strain

Muscle fiber contraction involves the cyclical interaction of myosin cross-bridges with actin filaments, linked to hydrolysis of ATP that provides the required energy. We show here the relationship between cross-bridge states, force generation, and Pi release during ramp stretches of active mammalian skeletal muscle fibers at 20°C. The results show that force and Pi release respond quickly to the application of stretch: force rises rapidly, whereas the rate of Pi release decreases abruptly and remains low for the duration of the stretch. These measurements show that biochemical change on the millisecond timescale accompanies the mechanical and structural responses in active muscle fibers. A cross-bridge model is used to simulate the effect of stretch on the distribution of actomyosin cross-bridges, force, and Pi release, with explicit inclusion of ATP, ADP, and Pi in the biochemical states and length-dependence of transitions. In the simulation, stretch causes rapid detachment and reattachment of cross-bridges without release of Pi or ATP hydrolysis.

Dynamical behavior of molecular motor assemblies in the rigid and crossbridge models

We present a detailed analysis of the dynamical instabilities appearing in two kinetic theories for the collective behavior of molecular motors: the rigid two-state model and the two-state crossbridge (or power-stroke) model with continuous binding sites. We calculate force-velocity relations, discuss their stability, plot a diagram that summarizes the oscillation regimes, identify the location of the Hopf bifurcation with a memory effect, discuss the oscillation frequency and make a link with single-molecule experiments. We show that the instabilities present in these models naturally translate into non-linearities in force-displacement relations, and at linear order give forces that are similar to the delayed stretch activation observed in oscillating muscles. We also find that instabilities can appear for both apparent load-decelerated and load-accelerated detachment rates in a 3-state crossbridge model.

Measuring myosin cross-bridge attachment time in activated muscle fibers using stochastic vs. sinusoidal length perturbation analysis

The average time myosin cross bridges remain bound to actin (t(on)) can be measured by sinusoidal length perturbations (sinusoidal analysis) of striated muscle fibers using recently developed analytic methods. This approach allows measurements of t(on) in preparations possessing a physiologically relevant myofilament lattice. In this study, we developed an approach to measure t(on) in 5-10% of the time required for sinusoidal analysis by using stochastic length perturbations (white noise analysis). To compare these methods, we measured the influence of MgATP concentration ([MgATP]) on t(on) in demembranated myocardial strips from mice, sampling muscle behavior from 0.125 to 200 Hz with a 20-s burst of white noise vs. a 300-s series of sinusoids. Both methods detected a similar >300% increase in t(on) as [MgATP] decreased from 5 to 0.25 mM, differing by only 3-14% at any [MgATP]. Additional experiments with Drosophila indirect flight muscle fibers demonstrated that faster cross-bridge cycling kinetics permit further reducing of the perturbation time required to measure t(on). This reduced sampling time allowed strain-dependent measurements of t(on) in flight muscle fibers by combining 10-s bursts of white noise during periods of linear shortening and lengthening. Analyses revealed longer t(on) values during shortening and shorter t(on) values during lengthening. This asymmetry may provide a mechanism that contributes to oscillatory energy transfer between the flight muscles and thoracic cuticle to power flight. This study demonstrates that white noise analysis can detect underlying molecular processes associated with dynamic muscle contraction comparable to sinusoidal analysis, but in a fraction of the time.

Binding properties of the calcium-activated F2 isoform of Lethocerus troponin C

While in most muscles contraction is triggered by calcium effluxes, insect flight muscles are also activated by mechanical stretch. We are interested in understanding the role that the troponin C protein, usually the calcium sensor, plays in stretch activation. In the flight muscles of Lethocerus, a giant water bug often used as a model system, there are two isoforms of TnC, F1 and F2, present in an approximately 10:1 ratio. F1 TnC is responsible for activating the muscle following a stretch, whereas F2 TnC produces a sustained contraction, the magnitude of which depends on the concentration of Ca²⁺ in the fiber. We have previously shown that F1 TnC binds only one Ca²⁺ ion in its C-terminal domain and that interaction with troponin H, the insect ortholog of troponin I, is insensitive to Ca²⁺. Here, we have studied the effect of Ca²⁺ and Mg²⁺ on the affinities of the interaction of F2 TnC with troponin H peptides. We show that the presence of two Ca²⁺ ions, one in each of the globular domains, increases the affinity for TnH by at least 1 order of magnitude. The N lobe has a lower affinity for Ca²⁺, but it is also sensitive to Mg²⁺. The C lobe is insensitive to Mg²⁺ as previously demonstrated by mutations of the individual EF-hands. The interaction with TnH seems also to have significant structural differences from that observed for the F1 TnC isoform. We discuss how our findings could account for stretch activation.