Subtype Switching of T-Type Ca 2+ Channels From Cav3.2 to Cav3.1 During Differentiation of Embryonic Stem Cells to Cardiac Cell Lineage

Protein Kinase C Regulates Expression and Function of the Cav3.2 T-Type Ca2+ Channel during Maturation of Neonatal Rat Cardiomyocyte

Two distinct isoforms of the T-type Ca2+ channel, Cav3.1 and Cav3.2, play a pivotal role in the generation of pacemaker potentials in nodal cells in the heart, although the isoform switches from Cav3.2 to Cav3.1 during the early neonatal period with an unknown mechanism. The present study was designed to investigate the molecular system of the parts that are responsible for the changes of T-type Ca2+ channel isoforms in neonatal cardiomyocytes using the whole-cell patch-clamp technique and mRNA quantification. The present study demonstrates that PKC activation accelerates the Ni2+-sensitive beating rate and upregulates the Ni2+-sensitive T-type Ca2+ channel current in neonatal cardiomyocytes as a long-term effect, whereas PKC inhibition delays the Ni2+-sensitive beating rate and downregulates the Ni2+-sensitive T-type Ca2+ channel current. Because the Ni2+-sensitive T-type Ca2+ channel current is largely composed of the Cav3.2-T-type Ca2+ channel, it is accordingly assumed that PKC activity plays a crucial role in the maintenance of the Cav3.2 channel. The expression of Cav3.2 mRNA was highly positively correlated with PKC activity. The expression of a transcription factor Nkx2.5 mRNA, possibly corresponding to the Cav3.2 channel gene, was decreased by an inhibition of PKCβII. These results suggest that PKC activation, presumably by PKCβII, is responsible for the upregulation of CaV3.2 T-type Ca2+ channel expression that interacts with a cardiac-specific transcription factor, Nkx2.5, in neonatal cardiomyocytes.

NFAT5-mediated CACNA1C expression is critical for cardiac electrophysiological development and maturation

Entry of calcium into cardiomyocyte via L-type calcium channel (LTCC) is fundamental to cardiac contraction. CACNA1C, a type of LTCC and a hallmark of a matured ventricular myocyte, is developmentally regulated. Here, we identified 138 potential transcription factors by a comparative genomic study on 5-kb promoter regions of CACNA1C gene across eight vertebrate species, and showed that six factors were developmentally regulated with the expression of Cacna1c in mouse P19cl6 in vitro cardiomyocyte differentiation model. We further demonstrated that the nuclear factor of activated T cells 5 (Nfat5) bound to a consensus sequence TGGAAGCGTTC and activated the transcription of Cacna1c. The siRNA-mediated knockdown of Nfat5 suppressed the expression of Cacna1c and decreased L-type calcium current in mouse neonatal cardiomyocytes. Furthermore, morpholino-mediated knockdown of nfat5 in zebrafish prohibited the expression of cacna1c and resulted in a non-contractile ventricle, while over-expression of either cacna1c or nfat5 rescued this impaired phenotype. Thus, NFAT5-mediated expression of CACNA1C is evolutionarily conserved and critical for cardiac electrophysiological development and maturation of cardiomyocyte.

KEY MESSAGE

Nfat5 binds to a consensus sequence TGGAAGCGTTC in the promoter of Cacna1c. Nfat5 activates the transcription of Cacna1c. Nfat5 knockdown suppresses Cacna1c expression, decreases L-type calcium current, and results in non-beating ventricle. NFAT5-mediated expression of CACNA1C is evolutionarily conserved. NFAT5-mediated CACNA1C expression is critical for cardiac electrophysiological development and maturation.

mRNA regulation of cardiac iron transporters and ferritin subunits in a mouse model of iron overload

Iron cardiomyopathy is the leading cause of death in iron overload. Men have twice the mortality rate of women, though the cause is unknown. In hemojuvelin-knockout mice, a model of the disease, males load more cardiac iron than females. We postulated that sex differences in cardiac iron import cause differences in cardiac iron concentration. Reverse transcription polymerase chain reaction was used to measure mRNA of cardiac iron transporters in hemojuvelin-knockout mice. No sex differences were discovered among putative importers of nontransferrin-bound iron (L-type and T-type calcium channels, ZRT/IRT-like protein 14 zinc channels). Transferrin-bound iron transporters were also analyzed; these are controlled by the iron regulatory element/iron regulatory protein (IRE/IRP) system. There was a positive relationship between cardiac iron and ferroportin mRNA in both sexes, but it was significantly steeper in females (p < 0.05). Transferrin receptor 1 and divalent metal transporter 1 were more highly expressed in females than males (p < 0.01 and p < 0.0001, respectively), consistent with their lower cardiac iron levels, as predicted by IRE/IRP regulatory pathways. Light-chain ferritin showed a positive correlation with cardiac iron that was nearly identical in males and females (R(2) = 0.41, p < 0.01; R(2) = 0.56, p < 0.05, respectively), whereas heavy-chain ferritin was constitutively expressed in both sexes. This represents the first report of IRE/IRP regulatory pathways in the heart. Transcriptional regulation of ferroportin was suggested in both sexes, creating a potential mechanism for differential set points for iron export. Constitutive heavy-chain-ferritin expression suggests a logical limit to cardiac iron buffering capacity at levels known to produce heart failure in humans.

Sex differences and steroid modulation of cardiac iron in a mouse model of iron overload

Iron cardiomyopathy is the leading cause of death in transfusional iron overload, and men have twice the mortality of women. Because the prevalence of cardiac iron overload increases rapidly during the second decade of life, we postulated that there are steroid-dependent sex differences in cardiac iron uptake. To test this hypothesis, we manipulated sex steroids in mice with constitutive iron absorption (homozygous hemojuvelin knockout); this model mimics the myocyte iron deposition observed in humans. At 4 weeks of age, female mice were ovariectomized (OVX) and male mice were castrated (OrchX). Female mice received an estrogen implant (OVX + E) or a cholesterol control (OVX), whereas male mice received an implant containing testosterone (OrchX + T), dihydrotestosterone (OrchX + DHT), estrogen (OrchX + E), or cholesterol (OrchX). All animals received a high-iron diet for 8 weeks. OrchX, OVX, and OVX + E mice all had similar cardiac iron loads. However, OrchX + E males had a significant increase in cardiac iron concentration compared with OrchX mice (P < 0.01), whereas the OrchX + T and OrchX + DHT groups only trended higher (P < 0.06 and P < 0.15, respectively). Hormone treatments did not impact liver iron concentration in either sex. When data were pooled across hormone therapies, liver iron concentration was 25% greater in males than females (P < 0.01). In summary, we found that estrogen increased cardiac iron loading in male mice, but not in females. Male mice loaded 25% more hepatic iron than female mice regardless of the hormone treatment.

miRNA in the regulation of ion channel/transporter expression

Ion channels and transporters are expressed in every living cell, where they participate in controlling a plethora of biological processes and physiological functions, such as excitation of cells in response to stimulation, electrical activities of cells, excitation-contraction coupling, cellular osmolarity, and even cell growth and death. Alterations of ion channels/transporters can have profound impacts on the cellular physiology associated with these proteins. Expression of ion channels/transporters is tightly regulated and expression deregulation can trigger abnormal processes, leading to pathogenesis, the channelopathies. While transcription factors play a critical role in controlling the transcriptome of ion channels/transporters at the transcriptional level by acting on the 5'-flanking region of the genes, microribonucleic acids (miRNAs), a newly discovered class of regulators in the gene network, are also crucial for expression regulation at the posttranscriptional level through binding to the 3'untranslated region of the genes. These small noncoding RNAs fine tune expression of genes involved in a wide variety of cellular processes. Recent studies revealed the role of miRNAs in regulating expression of ion channels/transporters and the associated physiological functions. miRNAs can target ion channel genes to alter cardiac excitability (conduction, repolarization, and automaticity) and affect arrhythmogenic potential of heart. They can modulate circadian rhythm, pain threshold, neuroadaptation to alcohol, brain edema, etc., through targeting ion channel genes in the neuronal systems. miRNAs can also control cell growth and tumorigenesis by acting on the relevant ion channel genes. Future studies are expected to rapidly increase to unravel a new repertoire of ion channels/transporters for miRNA regulation.

Embryonic stem cell-derived CD166+ precursors develop into fully functional sinoatrial-like cells

RATIONALE

A cell-based biological pacemaker is based on the differentiation of stem cells and the selection of a population displaying the molecular and functional properties of native sinoatrial node (SAN) cardiomyocytes. So far, such selection has been hampered by the lack of proper markers. CD166 is specifically but transiently expressed in the mouse heart tube and sinus venosus, the prospective SAN.

OBJECTIVE

We have explored the possibility of using CD166 expression for isolating SAN progenitors from differentiating embryonic stem cells.

METHODS AND RESULTS

We found that in embryonic day 10.5 mouse hearts, CD166 and HCN4, markers of the pacemaker tissue, are coexpressed. Sorting embryonic stem cells for CD166 expression at differentiation day 8 selects a population of pacemaker precursors. CD166+ cells express high levels of genes involved in SAN development (Tbx18, Tbx3, Isl-1, Shox2) and function (Cx30.2, HCN4, HCN1, CaV1.3) and low levels of ventricular genes (Cx43, Kv4.2, HCN2, Nkx2.5). In culture, CD166+ cells form an autorhythmic syncytium composed of cells morphologically similar to and with the electrophysiological properties of murine SAN myocytes. Isoproterenol increases (+57%) and acetylcholine decreases (-23%) the beating rate of CD166-selected cells, which express the β-adrenergic and muscarinic receptors. In cocultures, CD166-selected cells are able to pace neonatal ventricular myocytes at a rate faster than their own. Furthermore, CD166+ cells have lost pluripotency genes and do not form teratomas in vivo.

CONCLUSIONS

We demonstrated for the first time the isolation of a nonteratogenic population of cardiac precursors able to mature and form a fully functional SAN-like tissue.

T-type calcium channel blockade improves survival and cardiovascular function in thalassemic mice

OBJECTIVES

Iron-overload cardiomyopathy is a major cause of morbidity and mortality in patients with thalassemia. However, the precise mechanisms of iron entry and sequestration in the heart are still unclear. Our previous study showed that Fe(2+) uptake in thalassemic cardiomyocytes are mainly mediated by T-type calcium channels (TTCC). Nevertheless, the role of TTCC as well as other transporters such as divalent metal transporter1 (DMT1) and L-type calcium channels (LTCC) as possible portals for iron entry into the heart in in vivo thalassemic mice under an iron-overload condition has not been investigated.

METHODS

An iron-overload condition was induced in genetically altered β-thalassemic mice and adult wild-type mice by feeding them with an iron diet (0.2% ferrocene w/w) for 3 months. Then, blockers for LTCC (verapamil and nifedipine), TTCC (efonidipine), and DMT1 (ebselen) as well as iron chelator desferoxamine (DFO) were given for 1 month with continuous iron feeding.

RESULTS

Treatment with LTCC, TTCC, DMT1 blockers, and DFO reduced cardiac iron deposit, cardiac malondialdehyde (MDA), plasma non-transferrin-bound iron, and improved heart rate variability and left ventricular (LV) function in thalassemic mice with iron overload. Only TTCC and DMT1 blockers and DFO reduced liver iron accumulation, liver MDA, plasma MDA, and decreased mortality rate in iron-overloaded thalassemic mice.

CONCLUSIONS

DMT1, LTCC, and TTCC played important roles for iron entry in the thalassemic heart under an iron-overloaded condition. Unlike LTCC blocker, TTCC blocker provided all benefits including attenuating iron deposit in both the heart and liver, reduced oxidative stress, and decreased mortality in iron-overloaded mice.

Tracking stem cell differentiation in the setting of automated optogenetic stimulation

Membrane depolarization has been shown to play an important role in the neural differentiation of stem cells and in the survival and function of mature neurons. Here, we introduce a microbial opsin into ESCs and develop optogenetic technology for stem cell engineering applications, with an automated system for noninvasive modulation of ESC differentiation employing fast optogenetic control of ion flux. Mouse ESCs were stably transduced with channelrhodopsin-2 (ChR2)-yellow fluorescent protein and purified by fluorescence activated cell sorting (FACS). Illumination of resulting ChR2-ESCs with pulses of blue light triggered inward currents. These labeled ESCs retained the capability to differentiate into functional mature neurons, assessed by the presence of voltage-gated sodium currents, action potentials, fast excitatory synaptic transmission, and expression of mature neuronal proteins and neuronal morphology. We designed and tested an apparatus for optically stimulating ChR2-ESCs during chronic neuronal differentiation, with high-speed optical switching on a custom robotic stage with environmental chamber for automated stimulation and imaging over days, with tracking for increased expression of neural and neuronal markers. These data point to potential uses of ChR2 technology for chronic and temporally precise noninvasive optical control of ESCs both in vitro and in vivo, ranging from noninvasive control of stem cell differentiation to causal assessment of the specific contribution of transplanted cells to tissue and network function.

Different distribution of Cav3.2 and Cav3.1 transcripts encoding T-type Ca(2+) channels in the embryonic heart of mice

We investigated the distribution of T-type Ca(2+) channel mRNAs in the mouse embryonic heart. Cav3.2, but not Cav3.1, was expressed in the E8.5 embryonic heart along with cardiac progenitor markers (Nkx2.5, Tbx5, Isl-1) and contractile proteins (alpha and beta MHC). In the E10.5 heart, the distribution of Cav3.1 mRNA was confirmed in the AV-canal and overlapped with that of MinK or Tbx2. Cav3.2 mRNA was observed not only in the AV-canal but also in the outflow tract, along with MinK and Isl-1, indicating the expression of Cav3.2 in the secondary heart field. Thus, Cav3.2 may contribute to the development of the outflow tract from the secondary heart field in the embryonic heart, whereas Cav3.1 may be involved in the development of the cardiac conduction-system together with Cav3.2.

Hypertrophic phenotype in cardiac cell assemblies solely by structural cues and ensuing self-organization

In vitro models of cardiac hypertrophy focus exclusively on applying "external" dynamic signals (electrical, mechanical, and chemical) to achieve a hypertrophic state. In contrast, here we set out to demonstrate the role of "self-organized" cellular architecture and activity in reprogramming cardiac cell/tissue function toward a hypertrophic phenotype. We report that in neonatal rat cardiomyocyte culture, subtle out-of-plane microtopographic cues alter cell attachment, increase biomechanical stresses, and induce not only structural remodeling, but also yield essential molecular and electrophysiological signatures of hypertrophy. Increased cell size and cell binucleation, molecular up-regulation of released atrial natriuretic peptide, altered expression of classic hypertrophy markers, ion channel remodeling, and corresponding changes in electrophysiological function indicate a state of hypertrophy on par with other in vitro and in vivo models. Clinically used antihypertrophic pharmacological treatments partially reversed hypertrophic behavior in this in vitro model. Partial least-squares regression analysis, combining gene expression and functional data, yielded clear separation of phenotypes (control: cells grown on flat surfaces; hypertrophic: cells grown on quasi-3-dimensional surfaces and treated). In summary, structural surface features can guide cardiac cell attachment, and the subsequent syncytial behavior can facilitate trophic signals, unexpectedly on par with externally applied mechanical, electrical, and chemical stimulation.

Identification, isolation and characterization of HCN4-positive pacemaking cells derived from murine embryonic stem cells during cardiac differentiation

BACKGROUND

Development of biological pacemaker is a potential treatment for bradyarrhythmias. Pacemaker cells could be extracted from differentiated embryonic stem (ES) cells based on their specific cell marker hyperpolarization-activated cyclic nucleotide-gated (HCN)4. The goal of this study was to develop a method of identification, isolation, and characterization of pacemaking cells derived from differentiated ES cells with GFP driven by HCN4 promoter.

METHODS AND RESULTS

Polymerase chain reaction (PCR) screening and southern blot analysis revealed that HCN4p-EGFP trans-gene was stably integrated into the chromosome of mouse AB1 ES cells. RT-PCR and immunostaining results showed similar expression of the specific cardiac pacemaker markers of the HCN4p-EGFP ES cells and its parental AB1 ES cell lines. Although HCN4p-EGFP trans-gene may have slight effect on the general mesodermal differentiation, it had no effect on the pluripotency of ES cells, on the transcription of cardiac specific factors and cardiac contractile proteins, and on the capability of ES cells to differentiate into pacemaker cells. Electrophysiological study indicated that HCN4p-GFP-positive cells revealed the spontaneous action potential, which was slowed by the treatment with 2 mM Cs(+), and expressed the hyperpolarization-activeted cation current I(f) encoded by HCN4 gene.

CONCLUSION

By the approach of using stable transfectant of HCN4p-EGFP gene, the identification, isolation, and characterization of ES cell-derived pacemaking cells could be carried out.

T-type calcium channels are regulated by hypoxia/reoxygenation in ventricular myocytes

Low-voltage-activated calcium channels are reexpressed in ventricular myocytes in pathological conditions associated with hypoxic episodes, but a direct relation between oxidative stress and T-type channel function and regulation in cardiomyocytes has not been established. We aimed to investigate low-voltage-activated channel regulation under oxidative stress in neonatal rat ventricular myocytes. RT-PCR measurements of voltage-gated Ca(2+) (Ca(v))3.1 and Ca(v)3.2 mRNA levels in oxidative stress were compared with whole cell patch-clamp recordings of T-type calcium current. The results indicate that hypoxia reduces T-type current density at -30 mV (the hallmark of this channel) based on the shift of the voltage dependence of activation to more depolarized values and downregulation of Ca(v)3.1 at the mRNA level. Upon reoxygenation, both Ca(v)3.1 mRNA levels and the voltage dependence of total T-type current are restored, although differently for activation and inactivation. Using Ni(2+), we distinguished different effects of hypoxia/reoxygenation on the two current components. Long-term incubation in the presence of 100 microM CoCl(2) reproduced the effects of hypoxia on T-type current activation and inactivation, indicating that the chemically induced oxidative state is sufficient to alter T-type calcium current activity, and that hypoxia-inducible factor-1alpha is involved in Ca(v)3.1 downregulation. Our results demonstrate that Ca(v)3.1 and Ca(v)3.2 T-type calcium channels are differentially regulated by hypoxia/reoxygenation injury, and, therefore, they may serve different functions in the myocyte in response to hypoxic injury.

The Ca V 3.2 T-Type Ca 2+ Channel Is Required for Pressure Overload–Induced Cardiac Hypertrophy in Mice

Voltage-gated T-type Ca 2+ channels (T-channels) are normally expressed during embryonic development in ventricular myocytes but are undetectable in adult ventricular myocytes. Interestingly, T-channels are reexpressed in hypertrophied or failing hearts. It is unclear whether T-channels play a role in the pathogenesis of cardiomyopathy and what the mechanism might be. Here we show that the α 1H voltage-gated T-type Ca 2+ channel (Ca v 3.2) is involved in the pathogenesis of cardiac hypertrophy via the activation of calcineurin/nuclear factor of activated T cells (NFAT) pathway. Specifically, pressure overload–induced hypertrophy was severely suppressed in mice deficient for Ca v 3.2 (Ca v 3.2 −/− ) but not in mice deficient for Ca v 3.1 (Ca v 3.1 −/− ). Angiotensin II–induced cardiac hypertrophy was also suppressed in Ca v 3.2 −/− mice. Consistent with these findings, cultured neonatal myocytes isolated from Ca v 3.2 −/− mice fail to respond hypertrophic stimulation by treatment with angiotensin II. Together, these results demonstrate the importance of Ca v 3.2 in the development of cardiac hypertrophy both in vitro and in vivo. To test whether Ca v 3.2 mediates the hypertrophic response through the calcineurin/NFAT pathway, we generated Ca v 3.2 −/− , NFAT-luciferase reporter mice and showed that NFAT-luciferase reporter activity failed to increase after pressure overload in the Ca v 3.2 −/− /NFAT-Luc mice. Our results provide strong genetic evidence that Ca v 3.2 indeed plays a pivotal role in the induction of calcineurin/NFAT hypertrophic signaling and is crucial for the activation of pathological cardiac hypertrophy.

Lentiviral vectors bearing the cardiac promoter of the Na+-Ca2+ exchanger report cardiogenic differentiation in stem cells

Cardiosphere-derived resident cardiac stem cells (CDCs) are readily isolated from adult hearts and confer functional benefit in animal models of heart failure. To study cardiogenic differentiation in CDCs, we developed a method to genetically label and selectively enrich for cells that have acquired a cardiac phenotype. Lentiviral vectors achieved significantly higher transduction efficiencies in CDCs than any of the nine adeno-associated viral (AAV) serotypes tested. To define the most suitable vector system for reporting cardiogenic differentiation, we compared the cell specificity of five commonly-used cardiac-specific promoters in the context of lentiviral vectors. The promoter of the cardiac sodium-calcium exchanger (NCX1) conveyed the highest degree of cardiac specificity, as assessed by transducing seven cell types with each vector and measuring fluorescence intensity by flow cytometry. NCX1-GFP-positive CDC subpopulations, demonstrating prolonged expression of a variety of cardiac markers, could be isolated and expanded in vitro. Finally, we used chemical biology to validate that lentiviral vectors bearing the cardiac NCX1-promoter can serve as a highly accurate biosensor of cardiogenic small molecules in stem cells. The ability to accurately report cardiac fate and selectively enrich for cardiomyocytes and their precursors has important implications for drug discovery and the development of cell-based therapies.

Fibroblasts from human postmyocardial infarction scars acquire properties of cardiomyocytes after transduction with a recombinant myocardin gene

Myocardial scar formation impairs heart function by inducing cardiac remodeling, decreasing myocardial compliance, and compromising normal electrical conduction. Conversion of myocardial scar fibroblasts (MSFs) into (functional) cardiomyocytes may be an effective alternative treatment to limit loss of cardiac performance after myocardial injury. In this study, we investigated whether the phenotype of MSFs can be modified by gene transfer into cells with properties of cardiomyocytes. To this end, fibroblasts from postmyocardial infarction scars of human left ventricles were isolated and characterized by cell biological, immunological, and molecular biological assays. Cultured human MSFs express GATA4 and connexin 43 and display adipogenic differentiation potential. Infection of human MSFs with a lentivirus vector encoding the potent cardiogenic transcription factor myocardin renders them positive for a wide variety of cardiomyocyte-specific proteins, including sarcomeric components, transcription factors, and ion channels, and induces the expression of several smooth muscle marker genes. Forced myocardin expression also endowed human MSFs with the ability to transmit an action potential and to repair an artificially created conduction block in cardiomyocyte cultures. These finding indicate that in vivo myocardin gene transfer may potentially limit cardiomyocyte loss, myocardial fibrosis, and disturbances in electrical conduction caused by myocardial infarction.

Transcription factors Csx/Nkx2.5 and GATA4 distinctly regulate expression of Ca2+ channels in neonatal rat heart

The cardiac transcription factors Csx/Nkx2.5 and GATA4 play important roles in vertebrate heart development. Although mutations of Csx/Nkx2.5 or GATA4 are associated with various congenital heart diseases, their mechanism of action on cardiomyocyte function is not completely elucidated. In this study, we therefore investigated the actions of these transcription factors on the electrophysiological features and expression of ion channels in cardiomyocytes. Genes for transcription factors Csx/Nkx2.5 and GATA4 were transfected into rat neonatal cardiomyocytes by adenoviral infection. Action potentials, L-, T-type Ca(2+) channels and hyperpolarization-activated cation current (I(h)) of rat neonatal myocytes were recorded by patch clamp technique after adenoviral infection. Expression of ion channels was confirmed by real-time PCR. In Csx/Nkx2.5 overexpression myocytes, the spontaneous beating rate was markedly increased with an up-regulation of the Ca(v)3.2 T-type Ca(2+) channel, while in GATA4 overexpression myocytes, the T-type Ca(2+) channel was unchanged. On the other hand, the L-type Ca(2+) channel was down-regulated by both Csx/Nkx2.5 and GATA4 overexpression; the level of Ca(v)1.3 mRNA was dramatically decreased by Csx/Nkx2.5 overexpression. These results indicate that Csx/Nkx2.5 and GATA4 play important roles on the generation of pacemaker potentials modulating voltage-dependent Ca(2+) channels in the neonatal cardiomyocyte.

Delayed onset of beating and decreased expression of T-type Ca2+ channel in mouse ES cell-derived cardiocytes carrying human chromosome 21

The mouse ES cell line hcgp7/#21, which carries a human chromosome 21 (hChr.21), was used as an in vitro model to examine the effects of hChr.21 on cardiomyocyte differentiation. Cardiomyocytes derived from hcgp7/#21 showed a significant delay in the onset of spontaneous beating. The number of Nkx2.5/GFP(+) cardiac progenitor cells was comparable to that in control ES cells and they also expressed comparable mRNA levels for mesodermal markers, cardiac specific transcription factors, contractile proteins, and L-type Ca(2+) channels. However, cells from hcgp7/#21 expressed significantly reduced levels of mRNA for Cav3.1 and Cav3.2, which was consistent with the decreased number of cells expressing T-type Ca(2+) channels and decreased T-type Ca(2+) channel currents. These findings suggest that the presence of human chromosome 21 suppresses expression of T-type Ca(2+) channels in cardiomyocytes during differentiation, which may be responsible for delayed onset of spontaneous beating.

Design, synthesis, and biological evaluation of 1,3-dioxoisoindoline-5-carboxamide derivatives as T-type calcium channel blockers

A small molecule library of 1,3-dioxoisoindoline-5-carboxamides 4 was designed based on the pharmacophore model, synthesized and biologically evaluated as potential T-type calcium channel blockers. The most active compounds 4d and 4n show T-type calcium channel blocking activity with IC50 values of 0.93 and 0.96 microM, respectively.

Molecular and Electrophysiological Differences in the L-Type Ca2+ Channel of the Atrium and Ventricle of Rat Hearts

BACKGROUND

Many pathological conditions induce electrical remodeling, possibly through intracellular Ca2+ overload, but the currently available L-type Ca2+ channel blockers may be detrimental because of their global negative inotropic effects.

METHODS AND RESULTS

To determine whether the L-type Ca2+ channel is identical throughout the heart, the distribution of the mRNAs and proteins comprising the L-type Ca2+ channel and its electrophysiological properties were analyzed in rat atria and ventricles. The mRNA of alpha2delta-2 (Cacna2d2) was more abundantly expressed in the atrium (approximately 5-fold) than in the ventricle. In contrast, alpha1C (Cacna1c) (Cav1.2) mRNA was significantly less abundant in the atrium. The level of the alpha1C (Cacna1c) (Cav1.2) protein was decreased (approximately 0.5-fold) and that of alpha2 delta-1 (Cacna2d1) was increased (approximately 2-fold) in the atrium compared with the ventricle. Although the peak ICa,L density showed no significant differences, voltage dependence of inactivation and activation of the current showed a more depolarized shift in the atrium than in the ventricle.

CONCLUSION

These results indicate that in the rat heart the L-type Ca2+ channel differs between the atrium and ventricle with regard to gene expression and electrophysiological properties.

Hallmarks of ion channel gene expression in end-stage heart failure

Electrical conductance is greatly altered in end-stage heart failure, but little is known about the underlying events. We therefore investigated the expression of genes coding for major inward and outward ion channels, calcium binding proteins, ion receptors, ion exchangers, calcium ATPases, and calcium/calmodulin-dependent protein kinases in explanted hearts (n=13) of patients diagnosed with end-stage heart failure. With the exception of Kv11.1 and Kir3.1 and when compared with healthy controls, major sodium, potassium, and calcium ion channels, ion transporters, and exchangers were significantly repressed, but expression of Kv7.1, HCN4, troponin C and I, SERCA1, and phospholamban was elevated. Hierarchical gene cluster analysis provided novel insight into regulated gene networks. Significant induction of the transcriptional repressor m-Bop and the translational repressor NAT1 coincided with repressed cardiac gene expression. The statistically significant negative correlation between repressors and ion channels points to a mechanism of disease. We observed coregulation of ion channels and the androgen receptor and propose a role for this receptor in ion channel regulation. Overall, the reversal of repressed ion channel gene expression in patients with implanted assist devices exemplifies the complex interactions between pressure load/stretch force and heart-specific gene expression.

Functional expression of the hyperpolarization-activated, non-selective cation current I(f) in immortalized HL-1 cardiomyocytes

HL-1 cells are adult mouse atrial myocytes induced to proliferate indefinitely by SV40 large T antigen. These cells beat spontaneously when confluent and express several adult cardiac cell markers including the outward delayed rectifier K(+) channel. Here, we examined the presence of a hyperpolarization-activated I(f) current in HL-1 cells using the whole-cell patch-clamp technique on isolated cells enzymatically dissociated from the culture at confluence. Cell membrane capacitance (C(m)) ranged from 5 to 53 pF. I(f) was detected in about 30% of the cells and its occurrence was independent of the stage of the culture. I(f) maximal slope conductance was 89.7 +/- 0.4 pS pF(-1) (n = 10). I(f) current in HL-1 cells showed typical characteristics of native cardiac I(f) current: activation threshold between -50 and -60 mV, half-maximal activation potential of -83.1 +/- 0.7 mV (n = 50), reversal potential at -20.8 +/- 1.5 mV (n = 10), time-dependent activation by hyperpolarization and blockade by 4 mM Cs(+). In half of the cells tested, activation of adenylyl cyclase by the forskolin analogue L858051 (20 microM) induced both an approximately 6 mV positive shift of the half-activation potential and an approximately 37 % increase in the fully activated I(f) current. RT-PCR analysis of the hyperpolarization-activated, cyclic nucleotide-gated channels (HCN) expressed in HL-1 cells demonstrated major contributions of HCN1 and HCN2 channel isoforms to I(f) current. Cytosolic Ca(2+) oscillations in spontaneously beating HL-1 cells were measured in Fluo-3 AM-loaded cells using a fast-scanning confocal microscope. The oscillation frequency ranged from 1.3 to 5 Hz and the spontaneous activity was stopped in the presence of 4 mM Cs(+). Action potentials from HL-1 cells had a triangular shape, with an overshoot at +15 mV and a maximal diastolic potential of -69 mV, i.e. more negative than the threshold potential for I(f) activation. In conclusion, HL-1 cells display a hyperpolarization-activated I(f) current which might contribute to the spontaneous contractile activity of these cells.