Timothy J. Kamp

Functional Cardiomyocytes Derived From Human Induced Pluripotent Stem Cells

Human induced pluripotent stem (iPS) cells hold great promise for cardiovascular research and therapeutic applications, but the ability of human iPS cells to differentiate into functional cardiomyocytes has not yet been demonstrated. The aim of this study was to characterize the cardiac differentiation potential of human iPS cells generated using OCT4, SOX2, NANOG, and LIN28 transgenes compared to human embryonic stem (ES) cells. The iPS and ES cells were differentiated using the embryoid body (EB) method. The time course of developing contracting EBs was comparable for the iPS and ES cell lines, although the absolute percentages of contracting EBs differed. RT-PCR analyses of iPS and ES cell–derived cardiomyocytes demonstrated similar cardiac gene expression patterns. The pluripotency genes OCT4 and NANOG were downregulated with cardiac differentiation, but the downregulation was blunted in the iPS cell lines because of residual transgene expression. Proliferation of iPS and ES cell–derived cardiomyocytes based on 5-bromodeoxyuridine labeling was similar, and immunocytochemistry of isolated cardiomyocytes revealed indistinguishable sarcomeric organizations. Electrophysiology studies indicated that iPS cells have a capacity like ES cells for differentiation into nodal-, atrial-, and ventricular-like phenotypes based on action potential characteristics. Both iPS and ES cell–derived cardiomyocytes exhibited responsiveness to β-adrenergic stimulation manifest by an increase in spontaneous rate and a decrease in action potential duration. We conclude that human iPS cells can differentiate into functional cardiomyocytes, and thus iPS cells are a viable option as an autologous cell source for cardiac repair and a powerful tool for cardiovascular research.

Human Embryonic Stem Cells Develop Into Multiple Types of Cardiac Myocytes Action Potential Characterization

Abstract— Human embryonic stem (hES) cells can differentiate in vitro, forming embryoid bodies (EBs) composed of derivatives of all three embryonic germ layers. Spontaneously contracting outgrowths from these EBs contain cardiomyocytes (CMs); however, the types of human CMs and their functional properties are unknown. This study characterizes the contractions and action potentials (APs) from beating EB outgrowths cultured for 40 to 95 days. Spontaneous and electrical field-stimulated contractions were measured with video edge-detection microscopy. &bgr;-Adrenergic stimulation with 1.0 &mgr;mol/L isoproterenol resulted in a significant increase in contraction magnitude. Intracellular electrical recordings using sharp KCl microelectrodes in beating EB outgrowths revealed three distinct classes of APs: nodal-like, embryonic atrial-like, and embryonic ventricular-like. The APs were described as embryonic based on the relatively depolarized resting membrane potential and slow AP upstroke. Repeated impalements of an individual beating outgrowth revealed a reproducible AP morphology recorded from different cells, suggesting that each outgrowth is composed of a predominant cell type. Complex functional properties typical of cardiac muscle were observed in the hES cell-derived CMs including rate adaptation of AP duration and provoked early and delayed afterdepolarizations. Repolarization of the AP showed a significant role for IKr based on E-4031 induced prolongation of AP duration as anticipated for human CMs. In conclusion, hES cells can differentiate into multiple types of CMs displaying functional properties characteristic of embryonic human cardiac muscle. Thus, hES provide a renewable source of distinct types of human cardiac myocytes for basic research, pharmacological testing, and potentially therapeutic applications.

Regulation of Cardiac L-Type Calcium Channels by Protein Kinase A and Protein Kinase C

Abstract— Voltage-dependent L-type Ca2+ channels are multisubunit transmembrane proteins, which allow the influx of Ca2+ (ICa) essential for normal excitability and excitation-contraction coupling in cardiac myocytes. A variety of different receptors and signaling pathways provide dynamic regulation of ICa in the intact heart. The present review focuses on recent evidence describing the molecular details of regulation of L-type Ca2+ channels by protein kinase A (PKA) and protein kinase C (PKC) pathways. Multiple G protein–coupled receptors act through cAMP/PKA pathways to regulate L-type channels. &bgr;-Adrenergic receptor stimulation results in a marked increase in ICa, which is mediated by a cAMP/PKA pathway. Growing evidence points to an important role of localized signaling complexes involved in the PKA-mediated regulation of ICa, including A-kinase anchor proteins and binding of phosphatase PP2a to the carboxyl terminus of the &agr;1C (Cav1.2) subunit. Both &agr;1C and &bgr;2a subunits of the channel are substrates for PKA in vivo. The regulation of L-type Ca2+ channels by Gq-linked receptors and associated PKC activation is complex, with both stimulation and inhibition of ICa being observed. The amino terminus of the &agr;1C subunit is critically involved in PKC regulation. Crosstalk between PKA and PKC pathways occurs in the modulation of ICa. Ultimately, precise regulation of ICa is needed for normal cardiac function, and alterations in these regulatory pathways may prove important in heart disease.

Mutant Caveolin-3 Induces Persistent Late Sodium Current and Is Associated With Long-QT Syndrome

Background— Congenital long-QT syndrome (LQTS) is a primary arrhythmogenic syndrome stemming from perturbed cardiac repolarization. LQTS, which affects ≈1 in 3000 persons, is 1 of the most common causes of autopsy-negative sudden death in the young. Since the sentinel discovery of cardiac channel gene mutations in LQTS in 1995, hundreds of mutations in 8 LQTS susceptibility genes have been identified. All 8 LQTS genotypes represent primary cardiac channel defects (ie, ion channelopathy) except LQT4, which is a functional channelopathy because of mutations in ankyrin-B. Approximately 25% of LQTS remains unexplained pathogenetically. We have pursued a “final common pathway” hypothesis to elicit novel LQTS-susceptibility genes. With the recent observation that the LQT3-associated, SCN5A-encoded cardiac sodium channel localizes in caveolae, which are known membrane microdomains whose major component in the striated muscle is caveolin-3, we hypothesized that mutations in caveolin-3 may represent a novel pathogenetic mechanism for LQTS. Methods and Results— Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, we performed open reading frame/splice site mutational analysis on CAV3 in 905 unrelated patients referred for LQTS genetic testing. CAV3 mutations were engineered by site-directed mutagenesis and the molecular phenotype determined by transient heterologous expression into cell lines that stably express the cardiac sodium channel hNav1.5. We identified 4 novel mutations in CAV3-encoded caveolin-3 that were absent in >1000 control alleles. Electrophysiological analysis of sodium current in HEK293 cells stably expressing hNav1.5 and transiently transfected with wild-type and mutant caveolin-3 demonstrated that mutant caveolin-3 results in a 2- to 3-fold increase in late sodium current compared with wild-type caveolin-3. Our observations are similar to the increased late sodium current associated with LQT3-associated SCN5A mutations. Conclusions— The present study reports the first CAV3 mutations in subjects with LQTS, and we provide functional data demonstrating a gain-of-function increase in late sodium current.

Directed cardiomyocyte differentiation from human pluripotent stem cells by modulating Wnt/β-catenin signaling under fully defined conditions

The protocol described here efficiently directs human pluripotent stem cells (hPSCs) to functional cardiomyocytes in a completely defined, growth factor– and serum-free system by temporal modulation of regulators of canonical Wnt signaling. Appropriate temporal application of a glycogen synthase kinase 3 (GSK3) inhibitor combined with the expression of β-catenin shRNA or a chemical Wnt inhibitor is sufficient to produce a high yield (0.8–1.3 million cardiomyocytes per cm2) of virtually pure (80–98%) functional cardiomyocytes in 14 d from multiple hPSC lines without cell sorting or selection. Qualitative (immunostaining) and quantitative (flow cytometry) characterization of differentiated cells is described to assess the expression of cardiac transcription factors and myofilament proteins. Flow cytometry of BrdU incorporation or Ki67 expression in conjunction with cardiac sarcomere myosin protein expression can be used to determine the proliferative capacity of hPSC-derived cardiomyocytes. Functional human cardiomyocytes differentiated via these protocols may constitute a potential cell source for heart disease modeling, drug screening and cell-based therapeutic applications.

High purity human-induced pluripotent stem cell-derived cardiomyocytes: electrophysiological properties of action potentials and ionic currents

Human-induced pluripotent stem cells (hiPSCs) can differentiate into functional cardiomyocytes; however, the electrophysiological properties of hiPSC-derived cardiomyocytes have yet to be fully characterized. We performed detailed electrophysiological characterization of highly pure hiPSC-derived cardiomyocytes. Action potentials (APs) were recorded from spontaneously beating cardiomyocytes using a perforated patch method and had atrial-, nodal-, and ventricular-like properties. Ventricular-like APs were more common and had maximum diastolic potentials close to those of human cardiac myocytes, AP durations were within the range of the normal human electrocardiographic QT interval, and APs showed expected sensitivity to multiple drugs (tetrodotoxin, nifedipine, and E4031). Early afterdepolarizations (EADs) were induced with E4031 and were bradycardia dependent, and EAD peak voltage varied inversely with the EAD take-off potential. Gating properties of seven ionic currents were studied including sodium (I(Na)), L-type calcium (I(Ca)), hyperpolarization-activated pacemaker (I(f)), transient outward potassium (I(to)), inward rectifier potassium (I(K1)), and the rapidly and slowly activating components of delayed rectifier potassium (I(Kr) and I(Ks), respectively) current. The high purity and large cell numbers also enabled automated patch-clamp analysis. We conclude that these hiPSC-derived cardiomyocytes have ionic currents and channel gating properties underlying their APs and EADs that are quantitatively similar to those reported for human cardiac myocytes. These hiPSC-derived cardiomyocytes have the added advantage that they can be used in high-throughput assays, and they have the potential to impact multiple areas of cardiovascular research and therapeutic applications.

Differentiation of Human Embryonic Stem Cells and Induced Pluripotent Stem Cells to Cardiomyocytes: A Methods Overview

Since human embryonic stem cells (hESCs) were first differentiated to beating cardiomyocytes a decade ago, interest in their potential applications has increased exponentially. This has been further enhanced over recent years by the discovery of methods to induce pluripotency in somatic cells, including those derived from patients with hereditary cardiac diseases. Human pluripotent stem cells have been among the most challenging cell types to grow stably in culture but advances in reagent development now mean that most laboratories can expand both embryonic and induced pluripotent stem cells robustly using commercially available products. However, differentiation protocols have lagged behind and, in many cases, only produce the cell types required with low efficiency. Cardiomyocyte differentiation techniques were also initially inefficient and not readily transferable across cell lines, but there are now a number of more robust protocols available. Here we review the basic biology underlying the differentiation of pluripotent cells to cardiac lineages and describe current state-of-the-art protocols as well as ongoing refinements. This should provide a useful entry for laboratories new to this area to start their research. Ultimately, efficient and reliable differentiation methodologies are essential to generate desired cardiac lineages in order to realize the full promise of human pluripotent stem cells for biomedical research, drug development, and clinical applications.Since human embryonic stem cells were first differentiated to beating cardiomyocytes a decade ago, interest in their potential applications has increased exponentially. This has been further enhanced over recent years by the discovery of methods to induce pluripotency in somatic cells, including those derived from patients with hereditary cardiac diseases. Human pluripotent stem cells have been among the most challenging cell types to grow stably in culture, but advances in reagent development now mean that most laboratories can expand both embryonic and induced pluripotent stem cells robustly using commercially available products. However, differentiation protocols have lagged behind and in many cases only produce the cell types required with low efficiency. Cardiomyocyte differentiation techniques were also initially inefficient and not readily transferable across cell lines, but there are now a number of more robust protocols available. Here, we review the basic biology underlying the differentiation of pluripotent cells to cardiac lineages and describe current state-of-the-art protocols, as well as ongoing refinements. This should provide a useful entry for laboratories new to this area to start their research. Ultimately, efficient and reliable differentiation methodologies are essential to generate desired cardiac lineages to realize the full promise of human pluripotent stem cells for biomedical research, drug development, and clinical applications.

Reduction in density of transverse tubules and L-type Ca2+ channels in canine tachycardia-induced heart failure

OBJECTIVE Persistent supraventricular tachycardia leads to the development of a dilated cardiomyopathy with impairment of excitation-contraction (EC) coupling. Since the initial trigger for EC coupling in ventricular muscle is the influx of Ca(2+) through L-type Ca(2+) channels (I(Ca)) in the transverse tubules (T-tubules), we determined if the density of the T-tubule system and L-type Ca(2+) channels change in canine tachycardia pacing-induced cardiomyopathy. METHODS Confocal imaging of isolated ventricular myocytes stained with the membrane dye Di-8-ANEPPS was used to image the T-tubule system, and standard whole-cell patch clamp techniques were used to measure I(Ca) and intramembrane charge movement. RESULTS A complex staining pattern of interconnected tubules including prominent transverse components spaced every approximately 1.6 microm was present in control ventricular myocytes, but failing cells demonstrated a far less regular T-tubule system with a relative loss of T-tubules. In confocal optical slices, the average % of the total cell area staining for T-tubules decreased from 11.5+/-0.4 in control to 8.7+/-0.4% in failing cells (P<0.001). Whole-cell patch clamp studies revealed that I(Ca) density was unchanged. Since whole-cell I(Ca) is due to both the number of channels as well as the functional properties of those channels, we measured intramembrane charge movement as an assay for changes in channel number. The saturating amount of charge that moves due to gating of L-type Ca(2+) channels, Q(on,max), was decreased from 6.5+/-0.6 in control to 2.8+/-0.3 fC/pF in failing myocytes (P<0.001). CONCLUSIONS Cellular remodeling in heart failure results in decreased density of T-tubules and L-type Ca(2+) channels, which contribute to abnormal EC coupling.

Differentiation of Human Embryonic Stem Cells and Induced Pluripotent Stem Cells to Cardiomyocytes

Since human embryonic stem cells (hESCs) were first differentiated to beating cardiomyocytes a decade ago, interest in their potential applications has increased exponentially. This has been further enhanced over recent years by the discovery of methods to induce pluripotency in somatic cells, including those derived from patients with hereditary cardiac diseases. Human pluripotent stem cells have been among the most challenging cell types to grow stably in culture but advances in reagent development now mean that most laboratories can expand both embryonic and induced pluripotent stem cells robustly using commercially available products. However, differentiation protocols have lagged behind and, in many cases, only produce the cell types required with low efficiency. Cardiomyocyte differentiation techniques were also initially inefficient and not readily transferable across cell lines, but there are now a number of more robust protocols available. Here we review the basic biology underlying the differentiation of pluripotent cells to cardiac lineages and describe current state-of-the-art protocols as well as ongoing refinements. This should provide a useful entry for laboratories new to this area to start their research. Ultimately, efficient and reliable differentiation methodologies are essential to generate desired cardiac lineages in order to realize the full promise of human pluripotent stem cells for biomedical research, drug development, and clinical applications.Since human embryonic stem cells were first differentiated to beating cardiomyocytes a decade ago, interest in their potential applications has increased exponentially. This has been further enhanced over recent years by the discovery of methods to induce pluripotency in somatic cells, including those derived from patients with hereditary cardiac diseases. Human pluripotent stem cells have been among the most challenging cell types to grow stably in culture, but advances in reagent development now mean that most laboratories can expand both embryonic and induced pluripotent stem cells robustly using commercially available products. However, differentiation protocols have lagged behind and in many cases only produce the cell types required with low efficiency. Cardiomyocyte differentiation techniques were also initially inefficient and not readily transferable across cell lines, but there are now a number of more robust protocols available. Here, we review the basic biology underlying the differentiation of pluripotent cells to cardiac lineages and describe current state-of-the-art protocols, as well as ongoing refinements. This should provide a useful entry for laboratories new to this area to start their research. Ultimately, efficient and reliable differentiation methodologies are essential to generate desired cardiac lineages to realize the full promise of human pluripotent stem cells for biomedical research, drug development, and clinical applications.

Use of differentiated pluripotent stem cells in replacement therapy for treating disease

BACKGROUND Decades of laboratory and clinical investigation have led to successful therapies using hematopoietic stem cells (HSCs), but few other cell therapies have transitioned from experimental to standard clinical care. Providing patients with autologous rather than allogeneic HSCs reduces morbidity and mortality, and in some circumstances broader use could expand the range of conditions amenable to HSC transplantation. The availability of a homogeneous supply of mature blood cells would also be advantageous. An unlimited supply of pluripotent stem cells (PSCs) directed to various cell fates holds great promise as source material for cell transplantation and minimally invasive therapies to treat a variety of disorders. In this Review, we discuss past experience and challenges ahead and examine the extent to which hematopoietic stem cell transplantation and cell therapy for diabetes, liver disease, muscular dystrophies, neurodegenerative disorders, and heart disease would be affected by the availability of precisely differentiated PSCs. Unlimited populations of differentiated PSCs should facilitate blood therapies and hematopoietic stem cell transplantation, as well as the treatment of heart, pancreas, liver, muscle, and neurologic disorders. However, successful cell transplantation will require optimizing the best cell type and site for engraftment, overcoming limitations to cell migration and tissue integration, and possibly needing to control immunologic reactivity (challenges indicated in red). iPSC, induced PSC; ES cells, embryonic stem cells. Unlimited populations of differentiated PSCs should facilitate blood therapies and hematopoietic stem cell transplantation, as well as the treatment of heart, pancreas, liver, muscle, and neurologic disorders. However, successful cell transplantation will require optimizing the best cell type and site for engraftment, overcoming limitations to cell migration and tissue integration, and possibly needing to control immunologic reactivity (challenges indicated in red). iPSC, induced PSC; ES cells, embryonic stem cells. ADVANCES Although it is not yet possible to differentiate PSCs to cells with characteristics identical to those in the many organs that need replacement, it is likely a matter of time before these “engineering” problems can be overcome. Experience with cell therapies, both in the laboratory and the clinic, however, indicate that many challenges remain for treatment of diseases other than those involving the hematopoietic system. There are issues of immunity, separate from controlling graft rejection, and identifying the optimal cell type for treatment in the case of muscular dystrophies and heart disease. Optimization is also needed for the transplant site, as in diabetes, or when dealing with disruption of the extracellular matrix in treating degenerative diseases, such as chronic liver and heart disease. Finally, when the pathologic process is diffuse and migration of transplanted cells is limited, as is the case with Alzheimer’s disease, amyotrophic lateral sclerosis, and the muscular dystrophies, identifying the best means and location for cell delivery will require further study. OUTLOOK Considering the pace of progress in generating transplantable cells with a mature phenotype, and the availability of PSC-derived lineages in sufficient mass to treat some patients already, the challenges to scaling up production and eliminating cells with tumor-forming potential are probably within reach. However, generation of enough cells to treat an individual patient requires time for expansion, differentiation, selection, and testing to exclude contamination by tumorigenic precursors. Current methods are far too long and costly to address the treatment of acute organ injury or decompensated function. Immune rejection of engrafted cells, however, is likely to be overcome through transplantation of autologous cells from patient-derived PSCs. Availability of PSC-derived cell populations will have a dramatic effect on blood cell transfusion and the use of hematopoietic stem cell transplantation, and it will likely facilitate treatment of diabetes, some forms of liver disease and neurologic disorders, retinal diseases, and possibly heart disease. Close collaboration between scientists and clinicians—including surgeons and interventional radiologists—and between academia and industry will be critical to overcoming challenges and to bringing new therapies to patients in need. Challenges for stem cell–based therapies Patient-derived pluripotent stem cells (PSCs) hold promise in the treatment of injury and disease. An ever-increasing number of specific cell types can be generated from PSCs, but technical challenges remain in applying these cells in the clinic. Fox et al. review the challenges in attaining this goal. These include gene modification, cell rejection, and delivery and localization issues involved in transplantation of cells for the treatment of diabetes and disorders of the blood, liver, heart, and brain. Science, this issue 10.1126/science.1247391 Pluripotent stem cells (PSCs) directed to various cell fates holds promise as source material for treating numerous disorders. The availability of precisely differentiated PSC-derived cells will dramatically affect blood component and hematopoietic stem cell therapies and should facilitate treatment of diabetes, some forms of liver disease and neurologic disorders, retinal diseases, and possibly heart disease. Although an unlimited supply of specific cell types is needed, other barriers must be overcome. This review of the state of cell therapies highlights important challenges. Successful cell transplantation will require optimizing the best cell type and site for engraftment, overcoming limitations to cell migration and tissue integration, and occasionally needing to control immunologic reactivity, as well as a number of other challenges. Collaboration among scientists, clinicians, and industry is critical for generating new stem cell–based therapies.