Georgianna Guzman

Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy.

To study the effect of troponin (Tn) T mutations that cause familial hypertrophic cardiomyopathy (FHC) on cardiac muscle contraction, wild-type, and the following recombinant human cardiac TnT mutants were cloned and expressed: I79N, R92Q, F110I, E163K, R278C, and intron 16(G1 → A) (In16). These TnT FHC mutants were reconstituted into skinned cardiac muscle preparations and characterized for their effect on maximal steady state force activation, inhibition, and the Ca2+ sensitivity of force development. Troponin complexes containing these mutants were tested for their ability to regulate actin-tropomyosin(Tm)-activated myosin-ATPase activity. TnT(R278C) and TnT(F110I) reconstituted preparations demonstrated dramatically increased Ca2+sensitivity of force development, while those with TnT(R92Q) and TnT(I79N) showed a moderate increase. The deletion mutant, TnT(In16), significantly decreased both the activation and the inhibition of force, and substantially decreased the activation and the inhibition of actin-Tm-activated myosin-ATPase activity. ATPase activation was also impaired by TnT(F110I), while its inhibition was reduced by TnT(R278C). The TnT(E163K) mutation had the smallest effect on the Ca2+sensitivity of force; however, it produced an elevated activation of the ATPase activity in reconstituted thin filaments. These observed changes in the Ca2+ regulation of force development caused by these mutations would likely cause altered contractility and contribute to the development of FHC.

Cardiac troponin T isoforms affect the Ca2+ sensitivity and inhibition of force development. Insights into the role of troponin T isoforms in the heart.

At least four isoforms of troponin T (TnT) exist in the human heart, and they are expressed in a developmentally regulated manner. To determine whether the different N-terminal isoforms are functionally distinct with respect to structure, Ca2+ sensitivity, and inhibition of force development, the four known human cardiac troponin T isoforms, TnT1 (all exons present), TnT2 (missing exon 4), TnT3 (missing exon 5), and TnT4 (missing exons 4 and 5), were expressed, purified, and utilized in skinned fiber studies and in reconstituted actomyosin ATPase assays. TnT3, the adult isoform, had a slightly higher α-helical content than the other three isoforms. The variable region in the N terminus of cardiac TnT was found to contribute to the determination of the Ca2+ sensitivity of force development in a charge-dependent manner; the greater the charge the higher the Ca2+ sensitivity, and this was primarily because of exon 5. These studies also demonstrated that removal of either exon 4 or exon 5 from TnT increased the cooperativity of the pCa force relationship. Troponin complexes reconstituted with the four TnT isoforms all yielded the same maximal actin-tropomyosin-activated myosin ATPase activity. However, troponin complexes containing either TnT1 or TnT2 (both containing exon 5) had a reduced ability to inhibit this ATPase activity when compared with wild type troponin (which contains TnT3). Interestingly, fibers containing these isoforms also showed less relaxation suggesting that exon 5 of cardiac TnT affects the ability of Tn to inhibit force development and ATPase activity. These results suggest that the different N-terminal TnT isoforms would produce different functional properties in the heart that would directly affect myocardial contraction.

Familial Hypertrophic Cardiomyopathy Mutations in the Regulatory Light Chains of Myosin Affect Their Structure, Ca2+Binding, and Phosphorylation

The effect of the familial hypertrophic cardiomyopathy mutations, A13T, F18L, E22K, R58Q, and P95A, found in the regulatory light chains of human cardiac myosin has been investigated. The results demonstrate that E22K and R58Q, located in the immediate extension of the helices flanking the regulatory light chain Ca2+ binding site, had dramatically altered Ca2+ binding properties. The K Cavalue for E22K was decreased by ∼17-fold compared with the wild-type light chain, and the R58Q mutant did not bind Ca2+. Interestingly, Ca2+ binding to the R58Q mutant was restored upon phosphorylation, whereas the E22K mutant could not be phosphorylated. In addition, the α-helical content of phosphorylated R58Q greatly increased with Ca2+ binding. The A13T mutation, located near the phosphorylation site (Ser-15) of the human cardiac regulatory light chain, had 3-fold lowerK Ca than wild-type light chain, whereas phosphorylation of this mutant increased the Ca2+ affinity 6-fold. Whereas phosphorylation of wild-type light chain decreased its Ca2+ affinity, the opposite was true for A13T. The α-helical content of the A13T mutant returned to the level of wild-type light chain upon phosphorylation. The phosphorylation and Ca2+ binding properties of the regulatory light chain of human cardiac myosin are important for physiological function, and alteration any of these could contribute to the development of hypertrophic cardiomyopathy.

F110I and R278C Troponin T Mutations That Cause Familial Hypertrophic Cardiomyopathy Affect Muscle Contraction in Transgenic Mice and Reconstituted Human Cardiac Fibers

We have studied the physiological effects of the troponin T (TnT) F110I and R278C mutations associated with familial hypertrophic cardiomyopathy (FHC) in humans. Three to four-month-old transgenic (Tg) mice expressing F110I-TnT and R278C-TnT did not develop significant hypertrophy or ventricular fibrosis even after chronic exercise challenge. The F110I mutation impaired acute exercise tolerance, whereas R278C did not. Skinned papillary muscle fibers from transgenic mice expressing F110I-TnT demonstrated increased Ca2+ sensitivity of force and ATPase activity, and likewise an increased Ca2+ sensitivity of force was observed in F110I-TnT-reconstituted human cardiac muscle preparations. In contrast, no changes in force or the ATPase-pCa dependencies were observed in transgenic R278C fibers or in human fibers reconstituted with the R278C-TnT mutant. The maximal level of force development was dramatically decreased in both transgenic mice. However, the maximal ATPase was not different (R278C-TnT) or only slightly less (F110I-TnT) than that of non-Tg and WT-Tg littermates. Consequently, their ratios of ATPase/force (energy cost) at all Ca2+ concentrations were dramatically higher compared with non-Tg and WT-Tg fibers. This increase in energy cost most likely results from a decrease in force per myosin cross-bridge, because forcing all cross-bridges into the force generating state by substitution of MgADP for MgATP in maximum contracting solutions resulted in the same increase in maximal force (15%) in all transgenic and non-transgenic preparations. The combination of increased Ca2+ sensitivity and energy cost in the F110I hearts may be responsible for the greater severity of this phenotype compared with the R278C mutation.

The E22K mutation of myosin RLC that causes familial hypertrophic cardiomyopathy increases calcium sensitivity of force and ATPase in transgenic mice

Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease caused by mutations in all of the major sarcomeric proteins, including the ventricular myosin regulatory light-chain (RLC). The E22K-RLC mutation has been associated with a rare variant of cardiac hypertrophy defined by mid-left ventricular obstruction due to papillary muscle hypertrophy. This mutation was later found to cause ventricular and septal hypertrophy. We have generated transgenic (Tg) mouse lines of myc-WT (wild type) and myc-E22K mutant of human ventricular RLC and have examined the functional consequences of this FHC mutation in skinned cardiac-muscle preparations. In longitudinal sections of whole mouse hearts stained with hematoxylin and eosin, the E22K-mutant hearts of 13-month-old animals showed signs of inter-ventricular septal hypertrophy and enlarged papillary muscles with no filament disarray. Echo examination did not reveal evidence of cardiac hypertrophy in Tg-E22K mice compared to Tg-WT or Non-Tg hearts. Physiological studies utilizing skinned cardiac-muscle preparations showed an increase by ΔpCa50≥0.1 in Ca2+ sensitivity of myofibrillar ATPase activity and force development in Tg-E22K mice compared with Tg-WT or Non-Tg littermates. Our results suggest that E22K-linked FHC is mediated through Ca2+-dependent events. The FHC-mediated structural perturbations in RLC that affect Ca2+ binding properties of the mutated myocardium are responsible for triggering the abnormal function of the heart that in turn might initiate a hypertrophic process and lead to heart failure.

The role of the N-terminus of the myosin essential light chain in cardiac muscle contraction

To study the regulation of cardiac muscle contraction by the myosin essential light chain (ELC) and the physiological significance of its N-terminal extension, we generated transgenic (Tg) mice by partially replacing the endogenous mouse ventricular ELC with either the human ventricular ELC wild type (Tg-WT) or its 43-amino-acid N-terminal truncation mutant (Tg-Delta43) in the murine hearts. The mutant protein is similar in sequence to the short ELC variant present in skeletal muscle, and the ELC protein distribution in Tg-Delta43 ventricles resembles that of fast skeletal muscle. Cardiac muscle preparations from Tg-Delta43 mice demonstrate reduced force per cross-sectional area of muscle, which is likely caused by a reduced number of force-generating myosin cross-bridges and/or by decreased force per cross-bridge. As the mice grow older, the contractile force per cross-sectional area further decreases in Tg-Delta43 mice and the mutant hearts develop a phenotype of nonpathologic hypertrophy while still maintaining normal cardiac performance. The myocardium of older Tg-Delta43 mice also exhibits reduced myosin content. Our results suggest that the role of the N-terminal ELC extension is to maintain the integrity of myosin and to modulate force generation by decreasing myosin neck region compliance and promoting strong cross-bridge formation and/or by enhancing myosin attachment to actin.

Fast skeletal muscle regulatory light chain is required for fast and slow skeletal muscle development

In skeletal muscle, the myosin molecule contains two sets of noncovalently attached low molecular weight proteins, the regulatory (RLC) and essential (ELC) light chains. To assess the functional and developmental significance of the fast skeletal isoform of the RLC (RLC‐f), the murine fast skeletal RLC gene (Mylpf) was disrupted by homologous recombination. Heterozygotes containing an intronic neo cassette (RLC−/+) had approximately one‐half of the amount of the RLC‐f mRNA compared to wild‐type (WT) mice but their muscles were histologically normal in both adults and neonates. In contrast, homozygous mice (RLC−/−) had no RLC‐f mRNA or protein and completely lacked both fast and slow skeletal muscle. This was likely due to interference with mRNA processing in the presence of the neo cassette. These RLC‐f null mice died immediately after birth, presumably due to respiratory failure since their diaphragms lacked skeletal muscle. The body weight of newborn RLC‐f null mice was decreased 30% compared to heterozygous or WT newborn mice. The lack of skeletal muscle formation in the null mice did not affect the development of other organs including the heart. In addition, we found that WT mice did not express the ventricular/slow skeletal RLC isoform (RLC‐v/s) until after birth, while it was expressed normally in the embryonic heart. The lack of skeletal muscle formation observed in RLC‐f null mice indicates the total dependence of skeletal muscle development on the presence of RLC‐f during embryogenesis. This observation, along with the normal function of the RLC‐v/s in the heart, implicates a coupled, diverse pathway for RLC‐v/s and RLC‐f during embryogenesis, where RLC‐v/s is responsible for heart development and RLC‐f is necessary for skeletal muscle formation. In conclusion, in this study we demonstrate that the Mylpf gene is critically important for fast and slow skeletal muscle development.–Wang Y., Szczesna‐Cordary, D., Craig, R., Diaz‐Perez, Z., Guzman, G., Miller, T., Potter J. D. Fast skeletal muscle regulatory light chain is required for fat and slow skeletal muscle development. FASEB J. 21, 2205–2214 (2007)

Characterisation of a mutant of barnacle troponin C lacking Ca2+-binding sites at positions II and IV

Abstract This study investigates a mutant barnacle troponin C (TnC) protein (BTnC2–4-) in which the Ca2+-binding sites (sites II and IV) have been rendered non-functional. Eliminating Ca2+ binding at both Ca2+-binding sites of barnacle TnC did not prevent the incorporation of BTnC2–4- into TnC-depleted myofibrillar bundles, although, as expected, the mutant was not able to effect muscle regulation. We conclude that the Mg2+ involved in stabilising the interaction between TnC and TnI in the barnacle must bind at a separate location to the Ca2+-binding sites. Competition experiments between BTnC2–4- and wild-type barnacle TnC (BTnCWT) or the native isoform BTnC2 indicate that BTnC2–4- has an approximately fourfold higher affinity for barnacle TnI than BTnCWT but a lower affinity for TnI compared to BTnC2. These results indicate that disabling sites II and IV changes the affinity of BTnC2–4- for TnC-denuded barnacle myofibrils, altering the stability of the bond formed between TnC and the thin filament.

Investigation of a genetically engineered mutant of barnacle troponin C containing a central helix deletion.

Abstract. To examine the importance of the central α-helix of troponin C (TnC) we have bacterially expressed one of the isoforms of barnacle TnC (BTnC2), BTnCWT, but without the aspartate residue at position 80 in the central helix (BTnC80–). This manipulation is expected to produce an approximately 100° axial rotation of the C-domain with respect to the N-domain, and a net charge change of –1. BTnC80– mutant was able to restore force to TnC-depleted skinned barnacle myofibrillar bundles to a greater extent than wild-type protein (≅170%). Competition experiments between BTnC80– and BTnC2–4-, a mutant lacking both of the calcium-specific sites (sites II and IV), shows that deletion of a single amino acid in the central helix results in a protein with increased affinity for the thin filament and one that is bound preferentially compared to BTnC2–4- when at equimolar concentrations.