Summary Familial hypertrophic cardiomyopathy (HCM) is a prevalent hereditary cardiac disorder linked to arrhythmia and sudden cardiac death. While the causes of HCM have been identified as genetic mutations in the cardiac sarcomere, the pathways by which sarcomeric mutations engender myocyte hypertrophy and electrophysiological abnormalities are not understood. To elucidate the mechanisms underlying HCM development, we generated patient-specific induced pluripotent stem cell cardiomyocytes (iPSC-CMs) from a ten-member family cohort carrying a hereditary HCM missense mutation (Arg663His) in the MYH7 gene. Diseased iPSC-CMs recapitulated numerous aspects of the HCM phenotype including cellular enlargement and contractile arrhythmia at the single-cell level. Calcium (Ca2+) imaging indicated dysregulation of Ca2+ cycling and elevation in intracellular Ca2+ ([Ca2+]i) are central mechanisms for disease pathogenesis. Pharmacological restoration of Ca2+ homeostasis prevented development of hypertrophy and electrophysiological irregularities. We anticipate that these findings will help elucidate the mechanisms underlying HCM development and identify novel therapies for the disease.

Dilated cardiomyopathy (DCM) is the most common cardiomyopathy, characterized by ventricular dilatation, systolic dysfunction, and progressive heart failure. DCM is the most common diagnosis leading to heart transplantation and places a significant burden on healthcare worldwide. The advent of induced pluripotent stem cells (iPSCs) offers an exceptional opportunity for creating disease-specific models, investigating underlying mechanisms, and optimizing therapy. Here we generated cardiomyocytes (CMs) from iPSCs derived from patients of a DCM family carrying a point mutation (R173W) in the gene encoding sarcomeric protein cardiac troponin T. Compared to the control healthy individuals in the same family cohort, DCM iPSC-CMs exhibited altered Ca2+ handling, decreased contractility, and abnormal sarcomeric α-actinin distribution. When stimulated with β-adrenergic agonist, DCM iPSC-CMs showed characteristics of failure such as reduced beating rates, compromised contraction, and significantly more cells with abnormal sarcomeric α-actinin distribution. β-adrenergic blocker treatment and over-expression of sarcoplasmic reticulum Ca2+ ATPase (Serca2a) improved DCM iPSC-CMs function. Our study demonstrated that human DCM iPSC-CMs recapitulated to some extent the disease phenotypes morphologically and functionally, and thus can serve as a useful platform for exploring molecular and cellular mechanisms and optimizing treatment of this particular disease.

Background Drug-induced arrhythmia is the most common cause of drug development failure and withdrawal from market. This study tested whether human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) combined with a low-impedance microelectrode array (MEA) system could improve upon industry-standard, preclinical cardiotoxicity screening methods, identify the effects of well-characterized drugs, and elucidate underlying risk factors for drug-induced arrhythmia. Human iPSC-CMs may be advantageous over immortalized cell lines because they possess similar functional characteristics as primary human cardiomyocytes and can be generated in unlimited quantities. Methods and Results Pharmacological responses of beating embryoid bodies (EBs) exposed to a comprehensive panel of drugs at 65 to 95 days post-induction were determined. Responses of hiPSC-CMs to drugs were qualitatively and quantitatively consistent with the reported drug effects in literature. Torsadogenic hERG blockers such as sotalol and quinidine produced statistically and physiologically significant effects, consistent with patch-clamp studies on human embryonic stem cell-derived cardiomyocytes (hESC-CMs). False negative and false positive hERG blockers were identified accurately. Consistent with published studies using animal models, early afterdepolarizations (EADs) and ectopic beats were observed in 33% and 40% of embryoid bodies treated with sotalol and quinidine, respectively, compared to negligible EADs and ectopic beats in untreated controls. Conclusions We found that drug-induced arrhythmias can be recapitulated in hiPSC-CMs and documented with MEA. Our data indicate that the MEA/hiPSC-CM assay is a sensitive, robust, and efficient platform for testing drug effectiveness and for arrhythmia screening. We believe that this system holds great potential for reducing drug development costs and may provide significant advantages over current industry standard assays that use immortalized cell lines or animal models.

In the outer hair cell (OHC), membrane capacitance principally derives from two components - that associated with lateral membrane sensor/motor charge movement, and that proportional to the membrane surface area (C(sa)). We used measures of membrane capacitance to test a model hypothesis that OHC lateral membrane molecular motors, recently identified as the protein prestin, fluctuate between two area states. By measuring membrane capacitance in native OHCs or prestin-transfected HEK cells at extreme voltages (+/-200 mV) where motor-derived charge movement is small or absent, we observed that C(sa) depends on the state of the motors, or correspondingly on membrane voltage. Deiters cells or control HEK cells, which lack motors, do not show this dependence. We modeled the voltage-dependent change in C(sa) as a Boltzmann process with the same parameters that describe the charge movement of the motors' voltage sensors. C(sa) is 3.28+/-0.75 pF (mean +/-SD; n=23) larger during extreme hyperpolarization, and the number of motors in OHCs and prestin-transfected HEK cells correlates with the magnitude of Delta C(sa)( r=0.78). Although these data are consistent with the area motor model, the corresponding area change, assuming 0.5 microF/cm(2) under constant membrane thickness, is unphysiologically large, and indicates that the capacitance change must result from changes not only in lateral membrane area but also specific capacitance. Thus, we conclude that a conformational change in the lateral membrane motor, prestin, additionally alters the dielectric constant and/or thickness of the lateral plasma membrane.

Prestin is a unique molecular-motor protein expressed in the lateral plasma membrane of outer hair cells (OHC) in the organ of Corti of the mammalian cochlea. It is thought that prestin undergoes conformational changes driven by the cell's membrane potential. The resulting alterations in OHC-length are assumed to constitute the cochlear amplifier. Prestin is a member of the anion solute carrier family 26 (SCL26A), but it is different from other family members in its unique function of voltage-driven motility. Because the C-terminus is the least conserved region in the family, we investigated its influence with a series of deletion, point and chimeric mutants. The function and cellular expression of mutants were examined in a heterologous expression system by measurement of nonlinear capacitance (NLC) and immunofluorescence. Each mutant produced a unique mixture of patterns of cell morphologies, which were classified as to the location of prestin within the cell. The data from deletion mutants (Del516, Del525, Del630, Del590, Del709, Del719) revealed that nearly the full length (>708 amino acids) of the protein was required for normal prestin expression and function. Since most deletion mutations eliminated plasma membrane targeting, chimeric proteins were constructed by fusing prestin, at amino acid 515 or 644, with the homologous portion of the C-terminus from the two most closely related SLC26A members, pendrin and putative anion exchanger 1. These chimeric proteins were again improperly (but differently) targeted than simple truncation mutants, and all lacked functional phenotype. When two of the potential basolateral membrane-targeting motifs were mutated (Y520A/Y526A), incomplete plasma membrane expression was seen. We also show that some double point mutations (V499G/Y501H) fully express in the plasma membrane but lack NLC. These non-charged amino acids may have unrevealed important roles in prestin's function. Together, these data suggest that certain specific sequences and individual amino acids in the C-terminus are necessary for correct cellular distribution and function.

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Targeted deletion of the prestin gene reduces cochlear sensitivity and eliminates both frequency selectivity and outer hair cell (OHC) somatic electromotility. In addition, it has been reported by Liberman and colleagues that F2 generation heterozygotes exhibit a 6 dB reduction in sensitivity, as well as a decrease in protein and electromotility. Considering that the active process is non-linear, a halving of somatic electromotility would be expected to produce a much larger change in sensitivity. We therefore re-evaluated comparisons between heterozygotes and wildtype mice using both in vivo and in vitro electrophysiology, as well as molecular biology. Data reported here for F3-F5 generation mice indicate that compound action potential thresholds and tuning curves, as well as the cochlear microphonic, are similar in heterozygotes and wildtype controls. Measurements of non-linear capacitance in isolated OHCs demonstrate that charge density, as well as the voltage dependence and sensitivity of motor function, is indistinguishable in the two genotypes, as is somatic electromotility. In addition, both immunocytochemistry and western blot analysis in young adult mice suggest that prestin protein in heterozygotes is near normal. In contrast, prestin mRNA is always less than in wildtype mice at all ages tested. Results from F3-F5 generation mice suggest that one copy of the prestin gene is capable of compensating for the deleted copy and that heterozygous mice do not suffer peripheral hearing impairment.

Outer hair cells provide amplification within the mammalian cochlea to enhance audition. The mechanism is believed to reside within the lateral membrane of the cell that houses an expansive array of molecular motors, identified as prestin, which drives somatic electromotility. By measuring nonlinear capacitance, the electrical signature of electromotility, at kilohertz rates we have uncovered new details of the early molecular events that arise from voltage perturbations of prestin. We show that dynamic changes in motor state probability occur within the kilohertz range, and signify an amplificatory event. Additionally, we show a lack of effect of Cl driving force, an absence of cell length effect (indicating that the kinetics does not vary across auditory frequency), and the first demonstration of the time dependence of tension induced amplificatory shifts. The process we have identified, where the stimulus-response function shifts in time along the stimulus axis in a multi-exponential manner, bears similarities to those components of adaptation found in the OHC stereociliar transducer identified recently. As with the forward transducer, the speed of the reverse transducer amplificatory event consequently impacts on high frequency peripheral auditory processing.