Equilibrium distribution of skeletal actin-tropomyosin-troponin states, determined by pyrene-tropomyosin fluorescence

Hypertrophic Cardiomyopathy Mutations of Troponin Reveal Details of Striated Muscle Regulation

Striated muscle contraction is inhibited by the actin associated proteins tropomyosin, troponin T, troponin I and troponin C. Binding of Ca²⁺ to troponin C relieves this inhibition by changing contacts among the regulatory components and ultimately repositioning tropomyosin on the actin filament creating a state that is permissive for contraction. Several lines of evidence suggest that there are three possible positions of tropomyosin on actin commonly called Blocked, Closed/Calcium and Open or Myosin states. These states are thought to correlate with different functional states of the contractile system: inactive-Ca²⁺-free, inactive-Ca²⁺-bound and active. The inactive-Ca²⁺-free state is highly occupied at low free Ca²⁺ levels. However, saturating Ca²⁺ produces a mixture of inactive and active states making study of the individual states difficult. Disease causing mutations of troponin, as well as phosphomimetic mutations change the stabilities of the states of the regulatory complex thus providing tools for studying individual states. Mutants of troponin are available to stabilize each of three structural states. Particular attention is given to the hypertrophic cardiomyopathy causing mutation, Δ14 of TnT, that is missing the last 14 C-terminal residues of cardiac troponin T. Removal of the basic residues in this region eliminates the inactive-Ca²⁺-free state. The major state occupied with Δ14 TnT at inactivating Ca²⁺ levels resembles the inactive-Ca²⁺-bound state in function and in displacement of TnI from actin-tropomyosin. Addition of Ca²⁺, with Δ14TnT, shifts the equilibrium between the inactive-Ca²⁺-bound and the active state to favor that latter state. These mutants suggest a unique role for the C-terminal region of Troponin T as a brake to limit Ca²⁺ activation.

M-class hypertrophic cardiomyopathy cardiac actin mutations increase calcium sensitivity of regulated thin filaments

Hypertrophic cardiomyopathy is a commonly occurring cardiovascular disease resulting primarily from changes in proteins participating in muscle contraction in the heart, including the cardiac actin protein. Changes in cardiac actin located exclusively in the myosin binding site are called M-class variants and include the H88Y, R95C, and E99K substitutions and F90Δ deletion. The prevailing hypothesis for these mutations is that hypertrophic cardiomyopathy is the result of increased calcium sensitivity of contraction in the myocardium. To test this hypothesis, we studied the activity of myosin at varying calcium concentrations in the presence of regulated thin filaments containing M-class cardiac actin variants. We found that all M-class cardiac actin variants exhibit increased calcium sensitivity, with the R95C variant also displaying significant decreases in maximal myosin activity. This work represents the first characterization of all M-class variant proteins and suggests that drugs targeting contraction specifically to treat hypertrophic cardiomyopathy must consider the impact on both calcium sensitivity and maximal myosin activity on overall heart health.

Disease causing mutations of troponin alter regulated actin state distributions

Striated muscle contraction is regulated primarily through the action of tropomyosin and troponin that are bound to actin. Activation requires Ca(2+) binding to troponin and/or binding of high affinity myosin complexes to actin. Mutations within components of the regulatory complex may lead to familial cardiomyopathies and myopathies. In several cases examined, either physiological or pathological changes in troponin alter the distribution among states of actin-tropomyosin-troponin that differ in their abilities to stimulate myosin ATPase activity. These observations open possibilities for managing disorders of the troponin complex. Furthermore, analyses of mutant forms of troponin give insights into the regulation of striated muscle contraction.

Kinetics of regulated actin transitions measured by probes on tropomyosin

Changes in the muscle regulatory protein complex, troponin, are important for modulation of activity and may occur as a result of disease-causing mutations. Both increases and decreases in the rate of ATP hydrolysis by myosin may occur as dictated by changes in the distribution of actin-tropomyosin-troponin among its different states. It is important to measure the rates of transition among these states to study physiological adaptation and disease processes. We show here that acrylodan or pyrene probes on tropomyosin can be used to monitor the transition from active to intermediate and inactive states of actin-tropomyosin-troponin. Transitions measured in the absence of calcium had two phases, as previously reported for some other probes on troponin and actin. The first step was a rapid equilibrium that favored the formation of the intermediate state and had an apparent rate constant less than that of S1-ATP dissociation. The second fluorescence transition was slower, with an apparent constant that increased from approximately 5 to 80/s over a range of 1-37 degrees C. Only the initial rapid transition was seen in the presence of saturating calcium. The acrylodan probe had the advantage of yielding a larger signal than the pyrene probe. Furthermore, the acrylodan signal decreased in going from the active state to the intermediate state, and then increased upon going to the inactive state.

Troponin regulatory function and dynamics revealed by H/D exchange-mass spectrometry

Muscle contraction is tightly regulated by Ca(2+) binding to the thin filament protein troponin. The mechanism of this regulation was investigated by detailed mapping of the dynamic properties of cardiac troponin using amide hydrogen exchange-mass spectrometry. Results were obtained in the presence of either saturation or non-saturation of the regulatory Ca(2+) binding site in the NH(2) domain of subunit TnC. Troponin was found to be highly dynamic, with 60% of amides exchanging H for D within seconds of exposure to D(2)O. In contrast, portions of the TnT-TnI coiled-coil exhibited high protection from exchange, despite 6 h in D(2)O. The data indicate that the most stable portion of the trimeric troponin complex is the coiled-coil. Regulatory site Ca(2+) binding altered dynamic properties (i.e. H/D exchange protection) locally, near the binding site and in the TnI switch helix that attaches to the Ca(2+)-saturated TnC NH(2) domain. More notably, Ca(2+) also altered the dynamic properties of other parts of troponin: the TnI inhibitory peptide region that binds to actin, the TnT-TnI coiled-coil, and the TnC COOH domain that contains the regulatory Ca(2+) sites in many invertebrate as opposed to vertebrate troponins. Mapping of these affected regions onto the troponin highly extended structure suggests that cardiac troponin switches between alternative sets of intramolecular interactions, similar to previous intermediate resolution x-ray data of skeletal muscle troponin.

Some cardiomyopathy-causing troponin I mutations stabilize a functional intermediate actin state

We examined four cardiomyopathy-causing mutations of troponin I that appear to disturb function by altering the distribution of thin filament states. The R193H (mouse) troponin I mutant had greater than normal actin-activated myosin-S1 ATPase activity in both the presence and absence of calcium. The rate of ATPase activity was the same as that of the wild-type at near-saturating concentrations of the activator, N-ethylmaleimide-S1. This mutant appeared to function by stabilizing the active state of thin filaments. Mutations D191H, R146G, and R146W had lower ATPase activities in the presence of calcium, but higher activities in the absence of calcium. These effects were most pronounced with mutations at position 146. For all three mutants the rates were similar to those of the wild-type at near-saturating concentrations of N-ethylmaleimide-S1. These results, combined with previous results, show that any alteration in the normal distribution of actomyosin states is capable of producing cardiomyopathy. The results of the D191H, R146G, and R146W mutations are most readily explained if the intermediate state of regulated actin has a unique function. The intermediate state appears to have an ability to accelerate the rate of ATP hydrolysis by myosin that exceeds that of the inactive state.