Myofilament-based physiological regulatory compensation preserves diastolic function in failing hearts with severe Ca2+ handling deficits.

Severe dysfunction in cardiac muscle intracellular Ca2+ handling is a common pathway underlying heart failure. Here we used an inducible genetic model of severe Ca2+ cycling dysfunction by the targeted temporal gene ablation of the cardiac Ca2+ ATPase, SERCA2, in otherwise normal adult mice. In this model, in vivo heart performance was minimally affected initially, even though Serca2a protein was markedly reduced. The mechanism underlying the sustained in vivo heart performance in the weeks prior to complete heart pump failure and death is not clear and is important to understand. Studies were primarily focused on understanding how in vivo diastolic function could be relatively normal under conditions of marked Serca2a deficiency. Interestingly, data show increased cardiac troponin I (cTnI) serine 23/24 phosphorylation content in hearts upon Serca2a ablation in vivo. We report that hearts isolated from the Serca2-deficient mice retained near normal heart pump functional responses to ß-adrenergic stimulation. Unexpectedly, using genetic complementation models, in concert with inducible Serca2 ablation, data show that Serca2a-deficient hearts that also lacked the central ß-adrenergic signaling-dependent Serca2a negative regulator, phospholamban (PLN), had severe diastolic dysfunction that could still be corrected by ß-adrenergic stimulation. Notably, integrating a serines 23/24-to-alanine PKA-refractory sarcomere incorporated cTnI molecular switch complex in mice deficient in Serca2 showed blunting of ß-adrenergic stimulation-mediated enhanced diastolic heart performance. Taken together, these data provide evidence of a compensatory regulatory role of the myofilaments as a critical physiological bridging mechanism to aid in preserving heart diastolic performance in failing hearts with severe Ca2+ handling deficits.

Inducing positive inotropy in human iPSC-derived cardiac muscle by gene editing-based activation of the cardiac α-myosin heavy chain.

Human induced pluripotent stem cells and their differentiation into cardiac myocytes (hiPSC-CMs) provides a unique and valuable platform for studies of cardiac muscle structure-function. This includes studies centered on disease etiology, drug development, and for potential clinical applications in heart regeneration/repair. Ultimately, for these applications to achieve success, a thorough assessment and physiological advancement of the structure and function of hiPSC-CMs is required. HiPSC-CMs are well noted for their immature and sub-physiological cardiac muscle state, and this represents a major hurdle for the field. To address this roadblock, we have developed a hiPSC-CMs (ß-MHC dominant) experimental platform focused on directed physiological enhancement of the sarcomere, the functional unit of cardiac muscle. We focus here on the myosin heavy chain (MyHC) protein isoform profile, the molecular motor of the heart, which is essential to cardiac physiological performance. We hypothesized that inducing increased expression of α-MyHC in ß-MyHC dominant hiPSC-CMs would enhance contractile performance of hiPSC-CMs. To test this hypothesis, we used gene editing with an inducible α-MyHC expression cassette into isogeneic hiPSC-CMs, and separately by gene transfer, and then investigated the direct effects of increased α-MyHC expression on hiPSC-CMs contractility and relaxation function. Data show improved cardiac functional parameters in hiPSC-CMs induced with α-MyHC. Positive inotropy and relaxation was evident in comparison to ß-MyHC dominant isogenic controls both at baseline and during pacing induced stress. This approach should facilitate studies of hiPSC-CMs disease modeling and drug screening, as well as advancing fundamental aspects of cardiac function parameters for the optimization of future cardiac regeneration, repair and re-muscularization applications.

Rapid restitution of contractile dysfunction by synthetic copolymers in dystrophin-deficient single live skeletal muscle fibers.

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin, a cytoskeletal protein essential for the preservation of the structural integrity of the muscle cell membrane. DMD patients develop severe skeletal muscle weakness, degeneration, and early death. We tested here amphiphilic synthetic membrane stabilizers in mdx skeletal muscle fibers (flexor digitorum brevis; FDB) to determine their effectiveness in restoring contractile function in dystrophin-deficient live skeletal muscle fibers. After isolating FDB fibers via enzymatic digestion and trituration from thirty-three adult male mice (9 C57BL10, 24 mdx), these were plated on a laminin-coated coverslip and treated with poloxamer 188 (P188; PEO75-PPO30-PEO75; 8400 g/mol), architecturally inverted triblock (PPO15-PEO200-PPO15, 10,700 g/mol), and diblock (PEO75-PPO16-C4, 4200 g/mol) copolymers. We assessed the twitch kinetics of sarcomere length (SL) and intracellular Ca2+ transient by Fura-2AM by field stimulation (25 V, 0.2 Hz, 25 °C). Twitch contraction peak SL shortening of mdx FDB fibers was markedly depressed to 30% of the dystrophin-replete control FDB fibers from C57BL10 (P < 0.001). Compared to vehicle-treated mdx FDB fibers, copolymer treatment robustly and rapidly restored the twitch peak SL shortening (all P < 0.05) by P188 (15 µM = + 110%, 150 µM = + 220%), diblock (15 µM = + 50%, 150 µM = + 50%), and inverted triblock copolymer (15 µM = + 180%, 150 µM = + 90%). Twitch peak Ca2+ transient from mdx FDB fibers was also depressed compared to C57BL10 FDB fibers (P < 0.001). P188 and inverted triblock copolymer treatment of mdx FDB fibers increased the twitch peak Ca2+ transient (P < 0.001). This study shows synthetic block copolymers with varied architectures can rapidly and highly effectively enhance contractile function in live dystrophin-deficient skeletal muscle fibers.

Cardiac Sarcomere Signaling in Health and Disease.

The cardiac sarcomere is a triumph of biological evolution wherein myriad contractile and regulatory proteins assemble into a quasi-crystalline lattice to serve as the central point upon which cardiac muscle contraction occurs. This review focuses on the many signaling components and mechanisms of regulation that impact cardiac sarcomere function. We highlight the roles of the thick and thin filament, both as necessary structural and regulatory building blocks of the sarcomere as well as targets of functionally impactful modifications. Currently, a new focus emerging in the field is inter-myofilament signaling, and we discuss here the important mediators of this mechanism, including myosin-binding protein C and titin. As the understanding of sarcomere signaling advances, so do the methods with which it is studied. This is reviewed here through discussion of recent live muscle systems in which the sarcomere can be studied under intact, physiologically relevant conditions.

Sarcomere dynamics revealed by a myofilament integrated FRET-based biosensor in live skeletal muscle fibers.

The sarcomere is the functional unit of skeletal muscle, essential for proper contraction. Numerous acquired and inherited myopathies impact sarcomere function causing clinically significant disease. Mechanistic investigations of sarcomere activation have been challenging to undertake in the context of intact, live skeletal muscle fibers during real time physiological twitch contractions. Here, a skeletal muscle specific, intramolecular FRET-based biosensor was designed and engineered into fast skeletal muscle troponin C (TnC) to investigate the dynamics of sarcomere activation. In transgenic animals, the TnC biosensor incorporated into the skeletal muscle fiber sarcomeres by stoichiometric replacement of endogenous TnC and did not alter normal skeletal muscle contractile form or function. In intact single adult skeletal muscle fibers, real time twitch contractile data showed the TnC biosensor transient preceding the peak amplitude of contraction. Importantly, under physiological temperatures, inactivation of the TnC biosensor transient decayed significantly more slowly than the Ca2+ transient and contraction. The uncoupling of the TnC biosensor transient from the Ca2+ transient indicates the biosensor is not functioning as a Ca2+ transient reporter, but rather reports dynamic sarcomere activation/ inactivation that, in turn, is due to the ensemble effects of multiple activating ligands within the myofilaments. Together, these findings provide the foundation for implementing this new biosensor in future physiological studies investigating the mechanism of activation of the skeletal muscle sarcomere in health and disease.

Physiologically-Based Pharmacokinetic Modeling to Predict Methylphenidate Exposure Affected by Interplay Among Carboxylesterase 1 Pharmacogenetics, Drug-Drug Interactions, and Sex.

BACKGROUND AND OBJECTIVE: The pharmacokinetics (PK) of methylphenidate (MPH) differ significantly among individuals. Carboxylesterase 1 (CES1) is the primary enzyme metabolizing MPH, and its function is affected by genetic variants, drug-drug interaction (DDI), and sex. The object of this study is to evaluate CES1 pharmacogenetics as related to MPH metabolism using human liver samples and develop a physiologically-based pharmacokinetic (PBPK) modeling approach to investigate the influence of CES1 genotypes and other factors on MPH PK. METHODS: The effect of the CES1 variant G143E (rs71647871) on MPH metabolism was studied utilizing 102 individual human liver S9 (HLS9) fraction samples. PBPK models were developed using the population-based PBPK software PK-Sim® by incorporating the HLS9 incubation data. The established models were applied to simulate MPH PK profiles under various clinical scenarios, including different genotypes, drug-alcohol interactions, and the difference between males and females. RESULTS: The HLS9 incubation study showed that subjects heterozygous for the CES1 variant G143E metabolized MPH at a rate of approximately 50% of that in non-carriers. The developed PBPK models successfully predicted the exposure alteration of MPH from the G143E genetic variant, ethanol-MPH DDI, and sex. Importantly, the study suggests that male G143E carriers who are alcohol consumers are at a higher risk of MPH overexposure. CONCLUSION: PBPK modeling provides a means for better understanding the mechanisms underlying interindividual variability in MPH PK and PD and could be utilized to develop a safer and more effective MPH pharmacotherapy regimen.

Oxidative stress pathogenically remodels the cardiac myocyte cytoskeleton via structural alterations to the microtubule lattice.

In the failing heart, the cardiac myocyte microtubule network is remodeled, which contributes to cellular contractile failure and patient death. However, the origins of this deleterious cytoskeletal reorganization are unknown. We now find that oxidative stress, a condition characteristic of heart failure, leads to cysteine oxidation of microtubules. Our electron and fluorescence microscopy experiments revealed regions of structural damage within the microtubule lattice that occurred at locations of oxidized tubulin. The incorporation of GTP-tubulin into these damaged, oxidized regions led to stabilized "hot spots" within the microtubule lattice, which suppressed the shortening of dynamic microtubules. Thus, oxidative stress may act inside of cardiac myocytes to facilitate a pathogenic shift from a sparse microtubule network into a dense, aligned network. Our results demonstrate how a disease condition characterized by oxidative stress can trigger a molecular oxidation event, which likely contributes to a toxic cellular-scale transformation of the cardiac myocyte microtubule network.

Sarcomere integrated biosensor detects myofilament-activating ligands in real time during twitch contractions in live cardiac muscle.

The sarcomere is the functional unit of cardiac muscle, essential for normal heart function. To date, it has not been possible to study, in real time, thin filament-based activation dynamics in live cardiac muscle. We report here results from a cardiac troponin C (TnC) FRET-based biosensor integrated into the cardiac sarcomere via stoichiometric replacement of endogenous TnC. The TnC biosensor provides, for the first time, evidence of multiple thin filament activating ligands, including troponin I interfacing with TnC and cycling myosin, during a cardiac twitch. Results show that the TnC FRET biosensor transient significantly precedes that of peak twitch force. Using small molecules and genetic modifiers known to alter sarcomere activation, independently of the intracellular Ca2+ transient, the data show that the TnC biosensor detects significant effects of the troponin I switch domain as a sarcomere-activating ligand. Interestingly, the TnC biosensor also detected the effects of load-dependent altered myosin cycling, as shown by a significant delay in TnC biosensor transient inactivation during the isometric twitch. In addition, the TnC biosensor detected the effects of myosin as an activating ligand during the twitch by using a small molecule that directly alters cross-bridge cycling, independently of the intracellular Ca2+ transient. Collectively, these results aid in illuminating the basis of cardiac muscle contractile activation with implications for gene, protein, and small molecule-based strategies designed to target the sarcomere in regulating beat-to-beat heart performance in health and disease.

Pharmacokinetics of gemcitabine and its amino acid ester prodrug following intravenous and oral administrations in mice.

Gemcitabine is an intravenously administered anti-cancer nucleoside analogue. Systemic exposure following oral administration of gemcitabine is limited by extensive first-pass metabolism via cytidine deaminase (CDA) and potentially by saturation of nucleoside transporter-mediated intestinal uptake. An amino acid ester prodrug of gemcitabine, 5'-l-valyl-gemcitabine (V-Gem), was previously shown to be a substrate of the intestinally expressed peptide transporter 1 (PEPT1) and stable against CDA-mediated metabolism. However, preliminary studies did not evaluate the in vivo oral performance of V-Gem as compared to parent drug. In the present study, we evaluated the pharmacokinetics and in vivo oral absorption of gemcitabine and V-Gem following intravenous and oral administrations in mice. These studies revealed that V-Gem undergoes rapid systemic elimination (half-life < 1 min) and has a low oral bioavailability (<1%). Most importantly, the systemic exposure of gemcitabine was not different following oral administration of equimolar doses of gemcitabine (gemcitabine bioavailability of 18.3%) and V-Gem (gemcitabine bioavailability of 16.7%). Single-pass intestinal perfusions with portal blood sampling in mice revealed that V-Gem undergoes extensive activation in intestinal epithelial cells and that gemcitabine undergoes first-pass metabolism in intestinal epithelial cells. Thus, formulation of gemcitabine as the prodrug V-Gem does not increase systemic gemcitabine exposure following oral dosing, due, in part, to the instability of V-Gem in intestinal epithelial cells.

Advancing physiological maturation in human induced pluripotent stem cell-derived cardiac muscle by gene editing an inducible adult troponin isoform switch.

Advancing maturation of stem cell-derived cardiac muscle represents a major barrier to progress in cardiac regenerative medicine. Cardiac muscle maturation involves a myriad of gene, protein, and cell-based transitions, spanning across all aspects of cardiac muscle form and function. We focused here on a key developmentally controlled transition in the cardiac sarcomere, the functional unit of the heart. Using a gene-editing platform, human induced pluripotent stem cell (hiPSCs) were engineered with a drug-inducible expression cassette driving the adult cardiac troponin I (cTnI) regulatory isoform, a transition shown to be a rate-limiting step in advancing sarcomeric maturation of hiPSC cardiac muscle (hiPSC-CM) toward the adult state. Findings show that induction of the adult cTnI isoform resulted in the physiological acquisition of adult-like cardiac contractile function in hiPSC-CMs in vitro. Specifically, cTnI induction accelerated relaxation kinetics at baseline conditions, a result independent of alterations in the kinetics of the intracellular Ca2+ transient. In comparison, isogenic unedited hiPSC-CMs had no cTnI induction and no change in relaxation function. Temporal control of adult cTnI isoform induction did not alter other developmentally regulated sarcomere transitions, including myosin heavy chain isoform expression, nor did it affect expression of SERCA2a or phospholamban. Taken together, precision genetic targeting of sarcomere maturation via inducible TnI isoform switching enables physiologically relevant adult myocardium-like contractile adaptations that are essential for beat-to-beat modulation of adult human heart performance. These findings have relevance to hiPSC-CM structure-function and drug-discovery studies in vitro, as well as for potential future clinical applications of physiologically optimized hiPSC-CM in cardiac regeneration/repair.

Chemoproteomic Identification of Serine Hydrolase RBBP9 as a Valacyclovir-Activating Enzyme.

Prodrug discovery and development in the pharmaceutical industry have been hampered by a lack of knowledge of prodrug activation pathways. Such knowledge would minimize the risks of prodrug failure by enabling proper selection of preclinical animal models, prediction of pharmacogenomic variability, and identification of drug-drug interactions. Technologies for annotation of activating enzymes have not kept pace with the growing need. Activity-based protein profiling (ABPP) has matured considerably in recent decades, leading to widespread use in the pharmaceutical industry. Here, we report the extension of competitive ABPP (cABPP) to prodrug-activating enzyme identification in stable isotope-labeled cell lysates using a modified fluorophosphonate probe. Focusing on the antiviral ester prodrug valacyclovir (VACV), we identified serine hydrolase RBBP9 as an activating enzyme in Caco-2 cells via shotgun proteomics, validating the activity via the selective inhibitor emetine (EME). Kinetic characterization of RBBP9 revealed a catalytic efficiency (kcat·KM-1 = 104 mM-1·s-1) comparable to that of BPHL, the only known VACV-activating enzyme prior to this work. EME incubation in wild-type and Bphl-knockout jejunum and liver lysates demonstrated the near-exclusivity of VACV activation by RBBP9 in the intestine. Additionally, these studies showed that RBBP9 and BPHL are the two major and coequal VACV-activating enzymes in the liver. Single-pass intestinal perfusions of VACV ± EME in mice showed EME coperfusion significantly inhibited the intestinal activation of VACV, implying the in vivo relevance of RBBP9-mediated VACV activation. We envision that others might use the cABPP approach in the future for global, rapid, and efficient discovery of prodrug-activating enzymes.

Mechanisms of gemcitabine oral absorption as determined by in situ intestinal perfusions in mice.

Gemcitabine is a widely used chemotherapeutic drug that is administered via intravenous infusion due to a low oral bioavailability of only 10%. This low oral bioavailability is believed to be the result of gemcitabine's low intestinal permeability and oral absorption, followed by significant presystemic metabolism. In the present study, we sought to define the mechanisms of gemcitabine intestinal permeability, the potential for saturation of intestinal uptake, and the transporter(s) responsible for mediating the oral absorption of drug using in situ single-pass intestinal perfusions in mice. Concentration-dependent studies were performed for gemcitabine over 0.5-2000 µM, along with studies of 5 µM gemcitabine in a sodium-containing buffer ±â€¯thymidine (which can inhibit concentrative (i.e., CNT1 and CNT3) and equilibrative (i.e., ENT1 and ENT2) nucleoside transporters) or dilazep (which can inhibit ENT1 and ENT2), or in a sodium-free buffer (which can inhibit CNT1 and CNT3). Our findings demonstrated that gemcitabine was, in fact, a high-permeability drug in the intestine at low concentrations, that jejunal uptake of gemcitabine was saturable and mediated almost exclusively by nucleoside transporters, and that jejunal flux was mediated by both high-affinity, low-capacity (Km = 27.4 µM, Vmax = 3.6 pmol/cm2/s) and low-affinity, high-capacity (Km = 700 µM, Vmax = 35.9 pmol/cm2/s) transport systems. Thus, CNTs and ENTs at the apical membrane allow for gemcitabine uptake from the lumen to enterocyte, whereas ENTs at the basolateral membrane allow for gemcitabine efflux from the enterocyte to portal venous blood.

Cytoplasmic nucleic acid-based XNAs directly enhance live cardiac cell function by a Ca2+ cycling-independent mechanism via the sarcomere.

Nucleic acid - protein interactions are critical for regulating gene activation in the nucleus. In the cytoplasm, however, potential nucleic acid-protein functional interactions are less clear. The emergence of a large and expanding number of non-coding RNAs and DNA fragments raises the possibility that the cytoplasmic nucleic acids may interact with cytoplasmic cellular components to directly alter key biological processes within the cell. We now show that both natural and synthetic nucleic acids, collectively XNAs, when introduced to the cytoplasm of live cell cardiac myocytes, markedly enhance contractile function via a mechanism that is independent of new translation, activation of the TLR-9 pathway or by altered intracellular Ca2+ cycling. Findings show a steep XNA oligo length-dependence, but not sequence dependence or nucleic acid moiety dependence, for cytoplasmic XNAs to hasten myocyte relaxation. XNAs localized to the sarcomere in a striated pattern and bound the cardiac troponin regulatory complex with high affinity in an electrostatic-dependent manner. Mechanistically, XNAs phenocopy PKA-based modified troponin to cause faster relaxation. Collectively, these data support a new role for cytoplasmic nucleic acids in directly modulating live cell cardiac performance and raise the possibility that cytoplasmic nucleic acid - protein interactions may alter functionally relevant pathways in other cell types.

Gene Transfer of Calcium-Binding Proteins into Adult Cardiac Myocytes.

Heart failure is the leading cause of combined morbidity and mortality in the USA with 50% of cases being diastolic heart failure. Diastolic heart failure results from poor myocardial relaxation and inadequate filling of the left ventricular chamber caused in part by calcium-handling dysregulation. In this chapter we describe methods to investigate new approaches of novel human Ca2+ binding protein motifs to restore normal Ca2+ handling function to diseased myocardium. Gene transfer of parvalbumin into adult cardiac myocytes has been studied as a potential therapeutic, specifically as a strategic Ca2+ buffer to correct cardiac mechanical dysfunction in disease. This chapter provides protocols for studying wild-type parvalbumin isoforms and parvalbumins with strategically designed EF-hand motifs in adult cardiac myocytes via acute adenoviral gene transfer. These protocols have been used extensively to optimize parvalbumin function as a potential therapeutic for failing heart muscle.

TnI Structural Interface with the N-Terminal Lobe of TnC as a Determinant of Cardiac Contractility.

The heterotrimeric cardiac troponin complex is a key regulator of contraction and plays an essential role in conferring Ca2+ sensitivity to the sarcomere. During ischemic injury, rapidly accumulating protons acidify the myoplasm, resulting in markedly reduced Ca2+ sensitivity of the sarcomere. Unlike the adult heart, sarcomeric Ca2+ sensitivity in fetal cardiac tissue is comparatively pH insensitive. Replacement of the adult cardiac troponin I (cTnI) isoform with the fetal troponin I (ssTnI) isoform renders adult cardiac contractile machinery relatively insensitive to acidification. Alignment and functional studies have determined histidine 132 of ssTnI to be the predominant source of this pH insensitivity. Substitution of histidine at the cognate position 164 in cTnI confers the same pH insensitivity to adult cardiac myocytes. An alanine at position 164 of cTnI is conserved in all mammals, with the exception of the platypus, which expresses a proline. Prolines are biophysically unique because of their innate conformational rigidity and helix-disrupting function. To provide deeper structure-function insight into the role of the TnC-TnI interface in determining contractility, we employed a live-cell approach alongside molecular dynamics simulations to ascertain the chemo-mechanical implications of the disrupted helix 4 of cTnI where position 164 exists. This important motif belongs to the critical switch region of cTnI. Substitution of a proline at position 164 of cTnI in adult rat cardiac myocytes causes increased contractility independent of alterations in the Ca2+ transient. Free-energy perturbation calculations of cTnC-Ca2+ binding indicate no difference in cTnC-Ca2+ affinity. Rather, we propose the enhanced contractility is derived from new salt bridge interactions between cTnI helix 4 and cTnC helix A, which are critical in determining pH sensitivity and contractility. Molecular dynamics simulations demonstrate that cTnI A164P structurally phenocopies ssTnI under baseline but not acidotic conditions. These findings highlight the evolutionarily directed role of the TnI-cTnC interface in determining cardiac contractility.

Functional Equivalence of Retroviral MA Domains in Facilitating Psi RNA Binding Specificity by Gag.

Retroviruses specifically package full-length, dimeric genomic RNA (gRNA) even in the presence of a vast excess of cellular RNA. The "psi" (Ψ) element within the 5'-untranslated region (5'UTR) of gRNA is critical for packaging through interaction with the nucleocapsid (NC) domain of Gag. However, in vitro Gag binding affinity for Ψ versus non-Ψ RNAs is not significantly different. Previous salt-titration binding assays revealed that human immunodeficiency virus type 1 (HIV-1) Gag bound to Ψ RNA with high specificity and relatively few charge interactions, whereas binding to non-Ψ RNA was less specific and involved more electrostatic interactions. The NC domain was critical for specific Ψ binding, but surprisingly, a Gag mutant lacking the matrix (MA) domain was less effective at discriminating Ψ from non-Ψ RNA. We now find that Rous sarcoma virus (RSV) Gag also effectively discriminates RSV Ψ from non-Ψ RNA in a MA-dependent manner. Interestingly, Gag chimeras, wherein the HIV-1 and RSV MA domains were swapped, maintained high binding specificity to cognate Ψ RNAs. Using Ψ RNA mutant constructs, determinants responsible for promoting high Gag binding specificity were identified in both systems. Taken together, these studies reveal the functional equivalence of HIV-1 and RSV MA domains in facilitating Ψ RNA selectivity by Gag, as well as Ψ elements that promote this selectivity.

Sarcomere neutralization in inherited cardiomyopathy: small-molecule proof-of-concept to correct hyper-Ca2+-sensitive myofilaments.

The sarcomere is the functional unit of the heart. Alterations in sarcomere activation lead to disease states such as hypertrophic and restrictive cardiomyopathy (HCM/RCM). Mutations in many of the sarcomeric genes are causal for HCM/RCM. In most cases, these mutations result in increased Ca(2+) sensitivity of the sarcomere, giving rise to altered systolic and diastolic function. There is emerging evidence that small-molecule sarcomere neutralization is a potential therapeutic strategy for HCM/RCM. To pursue proof-of-concept, W7 was used here because of its well-known Ca(2+) desensitizer biochemical effects at the level of cardiac troponin C. Acute treatment of adult cardiac myocytes with W7 caused a dose-dependent (1-10 µM) decrease in contractility in a Ca(2+)-independent manner. Alkalosis was used as an in vitro experimental model of acquired heightened Ca(2+) sensitivity, resulting in increased live cell contractility and decreased baseline sarcomere length, which were rapidly corrected with W7. As an inherited cardiomyopathy model, R193H cardiac troponin I (cTnI) transgenic myocytes showed significant decreased baseline sarcomere length and slowed relaxation that were rapidly and dose-dependently corrected by W7. Langendorff whole heart pacing stress showed that R193H cTnI transgenic hearts had elevated end-diastolic pressures at all pacing frequencies compared with hearts from nontransgenic mice. Acute treatment with W7 rapidly restored end-diastolic pressures to normal values in R193H cTnI hearts, supporting a sarcomere intrinsic mechanism of dysfunction. The known off-target effects of W7 notwithstanding, these results provide further proof-of-concept that small-molecule-based sarcomere neutralization is a potential approach to remediate hyper-Ca(2+)-sensitive sarcomere function.

Cell biology of sarcomeric protein engineering: disease modeling and therapeutic potential.

The cardiac sarcomere is the functional unit for myocyte contraction. Ordered arrays of sarcomeric proteins, held in stoichiometric balance with each other, respond to calcium to coordinate contraction and relaxation of the heart. Altered sarcomeric structure-function underlies the primary basis of disease in multiple acquired and inherited heart disease states. Hypertrophic and restrictive cardiomyopathies are caused by inherited mutations in sarcomeric genes and result in altered contractility. Ischemia-mediated acidosis directly alters sarcomere function resulting in decreased contractility. In this review, we highlight the use of acute genetic engineering of adult cardiac myocytes through stoichiometric replacement of sarcomeric proteins in these disease states with particular focus on cardiac troponin I. Stoichiometric replacement of disease causing mutations has been instrumental in defining the molecular mechanisms of hypertrophic and restrictive cardiomyopathy in a cellular context. In addition, taking advantage of stoichiometric replacement through gene therapy is discussed, highlighting the ischemia-resistant histidine-button, A164H cTnI. Stoichiometric replacement of sarcomeric proteins offers a potential gene therapy avenue to replace mutant proteins, alter sarcomeric responses to pathophysiologic insults, or neutralize altered sarcomeric function in disease.

Molecular determinants of cardiac myocyte performance as conferred by isoform-specific TnI residues.

Troponin I (TnI) is the molecular switch of the sarcomere. Cardiac myocytes express two isoforms of TnI during development. The fetal heart expresses the slow skeletal TnI (ssTnI) isoform and shortly after birth ssTnI is completely and irreversibly replaced by the adult cardiac TnI (cTnI) isoform. These two isoforms have important functional differences; broadly, ssTnI is a positive inotrope, especially under acidic/hypoxic conditions, whereas cTnI facilitates faster relaxation performance. Evolutionary directed changes in cTnI sequence suggest cTnI evolved to favor relaxation performance in the mammalian heart. To investigate the mechanism, we focused on several notable TnI isoform and trans-species-specific residues located in TnI's helix 4 using structure/function and molecular dynamics analyses. Gene transduction of adult cardiac myocytes by cTnIs with specific helix 4 ssTnI substitutions, Q157R/A164H/E166V/H173N (QAEH), and A164H/H173N (AH), were investigated. cTnI QAEH is similar in these four residues to ssTnI and nonmammalian chordate cTnIs, whereas cTnI AH is similar to fish cTnI in these four residues. In comparison to mammalian cTnI, cTnI QAEH and cTnI AH showed increased contractility and slowed relaxation, which functionally mimicked ssTnI expressing myocytes. cTnI QAEH molecular dynamics simulations demonstrated altered intermolecular interactions between TnI helix 4 and cTnC helix A, specifically revealing a new, to our knowledge, electrostatic interaction between R171of cTnI and E15 of cTnC, which structurally phenocopied the ssTnI conformation. Free energy perturbation calculation of cTnC Ca(2+) binding for these conformations showed relative increased calcium binding for cTnI QAEH compared to cTnI. Taken together, to our knowledge, these new findings provide evidence that the evolutionary-directed coordinated acquisition of residues Q157, A164, E166, H173 facilitate enhanced relaxation performance in mammalian adult cardiac myocytes.

Fatty acid flux in adipocytes: the in's and out's of fat cell lipid trafficking.

The trafficking of fatty acids into and out of adipocytes is regulated by a complex series of proteins and enzymes and is under control by a variety of hormonal and metabolic factors. The biochemical basis of fatty acid influx, despite its widespread appreciation, remains enigmatic with regard to the biophysical and biochemical properties that facilitate long-chain fatty acid uptake. Fatty acid efflux is initiated by hormonally controlled lipolysis of the droplet stores and produces fatty acids that must transit from their site of production to the plasma membrane and subsequently out of the cells. This review will focus on the "in's and out's" of fatty acid trafficking and summarize the current concepts in the field.