Calcium- and magnesium-dependent interactions between the C-terminus of troponin I and the N-terminal, regulatory domain of troponin C

Molecular dynamics provides new insights into the mechanism of calcium signal transduction and interdomain interactions in cardiac troponin

Understanding the regulation of cardiac muscle contraction at a molecular level is crucial for the development of therapeutics for heart conditions. Despite the availability of atomic structures of the protein components of cardiac muscle thin filaments, detailed insights into their dynamics and response to calcium are yet to be fully depicted. In this study, we used molecular dynamics simulations of the core domains of the cardiac muscle protein troponin to characterize the equilibrium dynamics of its calcium-bound and calcium-free forms, with a focus on elements of cardiac muscle contraction activation and deactivation, that is, calcium binding to the cardiac troponin Ca²⁺ -binding subunit (TnC) and the release of the switch region of the troponin inhibitory subunit (TnI) from TnC. The process of calcium binding to the TnC binding site is described as a three-step process commencing with calcium capture by the binding site residues, followed by cooperative residue interplay bringing the calcium ion to the binding site, and finally, calcium-water exchange. Furthermore, we uncovered a set of TnC-TnI interdomain interactions that are critical for TnC N-lobe hydrophobic pocket dynamics. Absence of these interactions allows the closure of the TnC N-lobe hydrophobic pocket while the TnI switch region remains expelled, whereas if the interactions are maintained, the hydrophobic pocket remains open. Modification of these interactions may fine-tune the ability of the TnC N-lobe hydrophobic pocket to close or remain open, modulate cardiac contractility and present potential therapy-relevant targets.

Effects of Graphene Oxide Nanofilm and Chicken Embryo Muscle Extract on Muscle Progenitor Cell Differentiation and Contraction

Finding an effective muscle regeneration technique is a priority for regenerative medicine. It is known that the key factors determining tissue formation include cells, capable of proliferating and/or differentiating, a niche (surface) allowing their colonization and growth factors. The interaction between these factors, especially between the surface of the artificial niche and growth factors, is not entirely clear. Moreover, it seems that the use of a complex of complementary growth factors instead of a few strictly defined ones could increase the effectiveness of tissue maturation, including muscle tissue. In this study, we evaluated whether graphene oxide (GO) nanofilm, chicken embryo muscle extract (CEME), and GO combined with CEME would affect the differentiation and functional maturation of muscle precursor cells, as well as the ability to spontaneously contract a pseudo-tissue muscle. CEME was extracted on day 18 of embryogenesis. Muscle cells obtained from an 8-day-old chicken embryo limb bud were treated with GO and CEME. Cell morphology and differentiation were observed using different microscopy methods. Cytotoxicity and viability of cells were measured by lactate dehydrogenase and Vybrant Cell Proliferation assays. Gene expression of myogenic regulatory genes was measured by Real-Time PCR. Our results demonstrate that CEME, independent of the culture surface, was the main factor influencing the intense differentiation of muscle progenitor cells. The present results, for the first time, clearly demonstrated that the cultured tissue-like structure was capable of inducing contractions without externally applied impulses. It has been indicated that a small amount of CEME in media (about 1%) allows the culture of pseudo-tissue muscle capable of spontaneous contraction. The study showed that the graphene oxide may be used as a niche for differentiating muscle cells, but the decisive influence on the maturation of muscle tissue, especially muscle contractions, depends on the complexity of the applied growth factors.

Colorimetric detection of magnesium (II) ions using tryptophan functionalized gold nanoparticles

The functional nanoparticles with specific molecular probe appear to be a promising approach for developing colorimetric nanosensor. In this work, we have synthesized tryptophan capped gold nanoparticles (AuNPs) and used to establish colorimetric detection of magnesium (Mg²⁺). The colorimetric response of the AuNPs toward Mg²⁺ was noticed with naked eyes, and spectral changes were monitored by using UV-Vis spectrophotometer. The detection response was rapid (less than 1 min), with a detection limit (LOD) about 0.2 µmol L⁻¹. The proposed nanoprobe shows characteristic red-shift of the AuNPs at 620 nm and high selectivity for Mg²⁺ due to the binding affinity of the tryptophan with Mg²⁺. The real-time response of the UV-Vis spectrum was monitored at three different concentrations of Mg²⁺ (0.45, 0.50, and 0.55 µmol L⁻¹). The AuNPs probe was suitable to provide a molecular platform for selective coordination with Mg²⁺ over Ca²⁺ ions, thus it could be facile to establish a practically viable sensing system. Furthermore, experimental results were confirmed to exhibit excellent linear curve for urine and serum samples spiked with Mg²⁺. Thus, this nanosensor is practically useful for the detection of Mg²⁺, without using expensive instruments, enzymes and/or DNA molecules.

A novel missense mutation of TNNI2 in a Chinese family cause distal arthrogryposis type 1

The distal arthrogryposis (DA) syndromes are a group of disorders characterized by congenital contractures of limbs. According to phenotypical characteristics, DA syndromes have been clinically classified into 10 types. Currently, at least nine disease causing genes have been identified for different types of DA. Here, we report a 3-generation Chinese pedigree with three DA affected members. We performed whole exome sequencing on two affected and one unaffected individuals of this family and successfully identified a novel missense mutation in TNNI2 as the pathogenic mutation. The TNNI2 gene encodes a subunit of the troponin complex, a contractile machinery of the muscle. The mutation p.F178C that could change the H-bond formation of a neighboring residue occurs at a highly conserved position, suggesting that this variation probably affects the TNNI2 protein function. Our study also demonstrates the power of whole exome sequencing in causal mutation identification for phenotypically variable and genetically heterogeneous disorders.

A gain-of-function mutation in Tnni2 impeded bone development through increasing Hif3a expression in DA2B mice

Distal arthrogryposis type 2B (DA2B) is an important genetic disorder in humans. However, the mechanisms governing this disease are not clearly understood. In this study, we generated knock-in mice carrying a DA2B mutation (K175del) in troponin I type 2 (skeletal, fast) (TNNI2), which encodes a fast-twitch skeletal muscle protein. Tnni2^K175del mice (referred to as DA2B mice) showed typical DA2B phenotypes, including limb abnormality and small body size. However, the current knowledge concerning TNNI2 could not explain the small body phenotype of DA2B mice. We found that Tnni2 was expressed in the osteoblasts and chondrocytes of long bone growth plates. Expression profile analysis using radii and ulnae demonstrated that Hif3a expression was significantly increased in the Tnni2^K175del mice. Chromatin immunoprecipitation assays indicated that both wild-type and mutant tnni2 protein can bind to the Hif3a promoter using mouse primary osteoblasts. Moreover, we showed that the mutant tnni2 protein had a higher capacity to transactivate Hif3a than the wild-type protein. The increased amount of hif3a resulted in impairment of angiogenesis, delay in endochondral ossification, and decrease in chondrocyte differentiation and osteoblast proliferation, suggesting that hif3a counteracted hif1a-induced Vegf expression in DA2B mice. Together, our data indicated that Tnni2^K175del mutation led to abnormally increased hif3a and decreased vegf in bone, which explain, at least in part, the small body size of Tnni2^K175del mice. Furthermore, our findings revealed a new function of tnni2 in the regulation of bone development, and the study of gain-of-function mutation in Tnni2 in transgenic mice opens a new avenue to understand the pathological mechanism of human DA2B disorder.

Distal arthrogryposis type 2B (DA2B) is an autosomal dominant genetic disorder. The typical clinical features of DA2B include hand and/or foot contracture and shortness of stature in patients. To date, mutations in TNNI2 can explain approximately 20% of familial incidences of DA2B. TNNI2 encodes a subunit of the Tn complex, which is required for calcium-dependent fast twitch muscle fiber contraction. In the absence of Ca²⁺ ions, TNNI2 impedes sarcomere contraction. Here, we reported a knock-in mouse carrying a DA2B mutation TNNI2 (K175del) had typical limb abnormality and small body size that observed in human DA2B. However, the small body did not seem to be convincingly explained using the present knowledge of TNNI2 associated skeletal muscle contraction. Our findings showed that the Tnni2^K175del mutation impaired bone development of Tnni2^K175del mice. Our data further showed that the mutant tnni2 protein had a higher capacity to transactivate Hif3a than the wild-type protein and led to a reduction in Vegf expression in bone of DA2B mice. Taken together, our findings demonstrated that the disease-associated Tnni2^K175del mutation caused bone defects, which accounted for, at least in part, the small body size of Tnni2^K175del mice. Our data also suggested, for the first time, a novel role of tnni2 in the regulation of bone development of mice by affecting Hif-vegf signaling.

Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I

To understand the molecular mechanisms whereby cardiomyopathy-related cardiac troponin I (cTnI) mutations affect myofilament activity, we have investigated the Ca2+ binding properties of various assemblies of the regulatory components that contain one of the cardiomyopahty-related mutant cTnI. Acto-S1 ATPase activities in reconstituted systems were also determined. We investigated R145G and R145W mutations from the inhibitory region and D190H and R192H mutations from the second actin-tropomyosin-binding site. Each of the four mutations sensitized the acto-S1 ATPase to Ca2+. Whereas the mutations from the inhibitory region increased the basal level of ATPase activity, those from the second actin-tropomyosin-binding site did not. The effects on the Ca2+ binding properties of the troponin ternary complex and the troponin-tropomyosin complex with one of four mutations were either desensitization or no effect compared with those with wild-type cTnI. All of the mutations, however, affected the Ca2+ sensitivities of the reconstituted thin filaments in the same direction as the acto-S1 ATPase activity. Also the thin filaments with one of the mutant cTnIs bound Ca2+ with less cooperativity compared with those with wild-type cTnI. These data indicate that the mutations found in the inhibitory region and those from the second actin-tropomyosin site shift the equilibrium of the states of the thin filaments differently. Moreover, the increased Ca2+ bound to myofilaments containing the mutant cTnIs may be an important factor in triggered arrhythmias associated with the cardiomyopathy.

Mutations in genes encoding fast-twitch contractile proteins cause distal arthrogryposis syndromes

The distal arthrogryposes (DAs) are a group of disorders characterized by multiple congenital contractures of the limbs. We previously mapped a locus for DA type 2B (DA2B), the most common of the DAs, to chromosome 11. We now report that DA2B is caused by mutations in TNNI2 that are predicted to disrupt the carboxy-terminal domain of an isoform of troponin I (TnI) specific to the troponin-tropomyosin (Tc-Tm) complex of fast-twitch myofibers. Because the DAs are genetically heterogeneous, we sought additional candidate genes by examining modifiers of mutant Drosophila isoforms of TnI. One of these modifiers, Tm2, encodes tropomyosin, another component of the Tc-Tm complex. A human homologue of Tm2, TPM2, encodes beta-tropomyosin and maps to the critical interval of DA type 1 (DA1). We discovered that DA1 is caused by substitution of a highly conserved amino acid residue in beta-tropomyosin. These findings suggest that DAs, in general, may be caused by mutations in genes encoding proteins of the contractile apparatus specific to fast-twitch myofibers. This provides a new opportunity to directly study the etiology and pathogenesis of multiple-congenital-contracture syndromes.

Correlation between normal modes in the 20-200 cm-1 frequency range and localized torsion motions related to certain collective motions in proteins

In certain biologically relevant collective motions, such as protein domain motions and sub-domain motions, large amplitude movements are localized in one or a few flexible regions consisting of a small number of residues. This paper explores the possible use of normal mode analysis in probing localized vibrational torsion motions in these flexible regions that may be related to certain collective motions. The normal modes of 10 structures of five proteins in different conformation (TRP repressor, calmodulin, calbindin D(9k), HIV-1 protease and troponin C), known to have shear or hinge domain or sub-domain motion, respectively, are analyzed. Our study identifies, for each structure, unique normal modes in the 20-200 cm-1 frequency range, whose corresponding motions are primarily concentrated in the region where large amplitude torsion movements of a known domain or sub-domain motion occur. This suggests possible correlation between normal modes at 20-200 cm-1 frequency range and initial fluctuational motions leading to localized collective motions in proteins, and thus the potential application of normal mode analysis in facilitating the study of biologically important localized motions in biomolecules.

Two mutations in troponin I that cause hypertrophic cardiomyopathy have contrasting effects on cardiac muscle contractility

We investigated the effects of two mutations in human cardiac troponin I, Arg(145)-->Gly and Gly(203)-->Ser, that are reported to cause familial hypertrophic cardiomyopathy. Mutant and wild-type troponin I, overexpressed in Escherichia coli, were used to reconstitute troponin complexes in vanadate-treated guinea pig cardiac trabeculae skinned fibres, and thin filaments were reconstituted with human cardiac troponin and tropomyosin along with rabbit skeletal muscle actin for in vitro motility and actomyosin ATPase assays. Troponin containing the Arg(145)-->Gly mutation inhibited force in skinned trabeculae less than did the wild-type, and had almost no inhibitory function in the in vitro motility assay. There was an enhanced inhibitory function with mixtures of 10-30% [Gly(145)]troponin I with the wild-type protein. Skinned trabeculae reconstituted with troponin I containing the Gly(203)-->Ser mutation and troponin C produced less Ca(2+)-activated force (64+/-8% of wild-type) and demonstrated lower Ca(2+) sensitivity [Delta(p)Ca(50) (log of the Ca(2+) concentration that gave 50% of maximal activation) 0.25 unit (P<0.05)] compared with wild-type troponin I, but thin filaments containing [Ser(203)]-troponin I were indistinguishable from those containing the wild-type protein in in vitro motility and ATPase assays. Thus these two mutations each result in hypertrophic cardiomyopathy, but have opposite effects on the overall contractility of the muscle in the systems we investigated, indicating either that we have not yet identified the relevant alteration in contractility for the Gly(203)->Ser mutation, or that the disease does not result directly from any particular alteration in contractility.