Tissue-specific expression of two human Ca(v)1.2 isoforms under the control of distinct 5' flanking regulatory elements

Alternative Splicing at N Terminus and Domain I Modulates CaV1.2 Inactivation and Surface Expression

The Ca_V1.2 L-type calcium channel is a key conduit for Ca²⁺ influx to initiate excitation-contraction coupling for contraction of the heart and vasoconstriction of the arteries and for altering membrane excitability in neurons. Its α_1C pore-forming subunit is known to undergo extensive alternative splicing to produce many Ca_V1.2 isoforms that differ in their electrophysiological and pharmacological properties. Here, we examined the structure-function relationship of human Ca_V1.2 with respect to the inclusion or exclusion of mutually exclusive exons of the N-terminus exons 1/1a and IS6 segment exons 8/8a. These exons showed tissue selectivity in their expression patterns: heart variant 1a/8a, one smooth-muscle variant 1/8, and a brain isoform 1/8a. Overall, the 1/8a, when coexpressed with Ca_Vβ_2a, displayed a significant and distinct shift in voltage-dependent activation and inactivation and inactivation kinetics as compared to the other three splice variants. Further analysis showed a clear additive effect of the hyperpolarization shift in V_1/2inact of Ca_V1.2 channels containing exon 1 in combination with 8a. However, this additive effect was less distinct for V_1/2act. However, the measured effects were β-subunit-dependent when comparing Ca_Vβ_2a with Ca_Vβ₃ coexpression. Notably, calcium-dependent inactivation mediated by local Ca²⁺-sensing via the N-lobe of calmodulin was significantly enhanced in exon-1-containing Ca_V1.2 as compared to exon-1a-containing Ca_V1.2 channels. At the cellular level, the current densities of the 1/8a or 1/8 variants were significantly larger than the 1a/8a and 1a/8 variants when coexpressed either with Ca_Vβ_2a or Ca_Vβ₃ subunit. This finding correlated well with a higher channel surface expression for the exon 1-Ca_V1.2 isoform that we quantified by protein surface-expression levels or by gating currents. Our data also provided a deeper molecular understanding of the altered biophysical properties of alternatively spliced human Ca_V1.2 channels by directly comparing unitary single-channel events with macroscopic whole-cell currents.

Protein kinase A regulates C-terminally truncated CaV 1.2 in Xenopus oocytes: roles of N- and C-termini of the α1C subunit

KEY POINTS

β-Adrenergic stimulation enhances Ca²⁺ entry via L-type Ca_V 1.2 channels, causing stronger contraction of cardiac muscle cells. The signalling pathway involves activation of protein kinase A (PKA), but the molecular details of PKA regulation of Ca_V 1.2 remain controversial despite extensive research. We show that PKA regulation of Ca_V 1.2 can be reconstituted in Xenopus oocytes when the distal C-terminus (dCT) of the main subunit, α_1C , is truncated. The PKA upregulation of Ca_V 1.2 does not require key factors previously implicated in this mechanism: the clipped dCT, the A kinase-anchoring protein 15 (AKAP15), the phosphorylation sites S1700, T1704 and S1928, or the β subunit of Ca_V 1.2. The gating element within the initial segment of the N-terminus of the cardiac isoform of α_1C is essential for the PKA effect. We propose that the regulation described here is one of two or several mechanisms that jointly mediate the PKA regulation of Ca_V 1.2 in the heart.

ABSTRACT

β-Adrenergic stimulation enhances Ca²⁺ currents via L-type, voltage-gated Ca_V 1.2 channels, strengthening cardiac contraction. The signalling via β-adrenergic receptors (β-ARs) involves elevation of cyclic AMP (cAMP) levels and activation of protein kinase A (PKA). However, how PKA affects the channel remains controversial. Recent studies in heterologous systems and genetically engineered mice stress the importance of the post-translational proteolytic truncation of the distal C-terminus (dCT) of the main (α_1C ) subunit. Here, we successfully reconstituted the cAMP/PKA regulation of the dCT-truncated Ca_V 1.2 in Xenopus oocytes, which previously failed with the non-truncated α_1C . cAMP and the purified catalytic subunit of PKA, PKA-CS, injected into intact oocytes, enhanced Ca_V 1.2 currents by ∼40% (rabbit α_1C ) to ∼130% (mouse α_1C ). PKA blockers were used to confirm specificity and the need for dissociation of the PKA holoenzyme. The regulation persisted in the absence of the clipped dCT (as a separate protein), the A kinase-anchoring protein AKAP15, and the phosphorylation sites S1700 and T1704, previously proposed as essential for the PKA effect. The Ca_V β_2b subunit was not involved, as suggested by extensive mutagenesis. Using deletion/chimeric mutagenesis, we have identified the initial segment of the cardiac long-N-terminal isoform of α_1C as a previously unrecognized essential element involved in PKA regulation. We propose that the observed regulation, that exclusively involves the α_1C subunit, is one of several mechanisms underlying the overall PKA action on Ca_V 1.2 in the heart. We hypothesize that PKA is acting on Ca_V 1.2, in part, by affecting a structural 'scaffold' comprising the interacting cytosolic N- and C-termini of α_1C .

MicroRNA-20b suppresses the expression of ZFP-148 in viral myocarditis

Viral myocarditis is a common cardiovascular disease, which seriously endangers the health of people and even leads to sudden unexpected death. MicroRNAs play very important roles in various physical and pathological processes including cardiogenesis and heart diseases. In recent years, miR-20b has been implicated in various diseases such as breast cancer, gastric cancer, hepatocellular carcinoma, cardiovascular diseases. However, the function of miR-20b in the pathological progress of viral myocarditis has not been reported. In this study, we found that miR-20b was up-regulated in mouse heart tissues post Coxsackievirus B3 (CVB3) infection. Bioinformatics analysis identified ZFP-148, a transcription factor that plays essential roles in the regulation of virus replication, is one of the predicted targets of miR-20b. MiR-20b expression was found to be up-regulated and ZFP-148 protein level was markedly repressed during viral myocarditis. Further studies demonstrated that miR-20b directly binds to the 3'-UTR of ZFP-148 and suppresses its translation. Moreover, aberrant expression of miR-20b promoted the expression of anti-apoptosis proteins Bcl-2 and Bcl-xL, suggesting that altered gene expression might promote cardiomyocytes survival in viral myocarditis. Our findings indicated that miR-20b might be a potential therapeutic target for CVB3-induced viral myocarditis and a useful marker for the diagnosis of viral myocarditis.

A novel conditional mouse model for Nkx2-5 reveals transcriptional regulation of cardiac ion channels

Nkx2-5 is one of the master regulators of cardiac development, homeostasis and disease. This transcription factor has been previously associated with a suite of cardiac congenital malformations and impairment of electrical activity. When disease causative mutations in transcription factors are considered, NKX2-5 gene dysfunction is the most common abnormality found in patients. Here we describe a novel mouse model and subsequent implications of Nkx2-5 loss for aspects of myocardial electrical activity. In this work we have engineered a new Nkx2-5 conditional knockout mouse in which flox sites flank the entire Nkx2-5 locus, and validated this line for the study of heart development, differentiation and disease using a full deletion strategy. While our homozygous knockout mice show typical embryonic malformations previously described for the lack of the Nkx2-5 gene, hearts of heterozygous adult mice show moderate morphological and functional abnormalities that are sufficient to sustain blood supply demands under homeostatic conditions. This study further reveals intriguing aspects of Nkx2-5 function in the control of cardiac electrical activity. Using a combination of mouse genetics, biochemistry, molecular and cell biology, we demonstrate that Nkx2-5 regulates the gene encoding Kcnh2 channel and others, shedding light on potential mechanisms generating electrical abnormalities observed in patients bearing NKX2-5 dysfunction and opening opportunities to the study of novel therapeutic targets for anti-arrhythmogenic therapies.

CACNA1C, schizophrenia and major depressive disorder in the Han Chinese population

BACKGROUND

Common psychiatric disorders are highly heritable, indicating that genetic factors play an important role in their aetiology. The CACNA1C gene, which codes for subunit alpha-1C of the Cav1.2 voltage-dependent L-type calcium channel, has been consistently found to be the shared risk gene for several kinds of mental disorder.

AIMS

To investigate whether CACNA1C is a susceptibility gene for schizophrenia and major depressive disorder in the Han Chinese population.

METHOD

We carried out a case-control study of 1235 patients with schizophrenia, 1045 with major depressive disorder and 1235 healthy controls. A tag single nucleotide polymorphism (SNP) rs1006737 along with another 10 tag SNPs in the CACNA1C gene were genotyped in all samples.

RESULTS

We found that rs1006737 was associated with both schizophrenia (P(allele) = 0.0014, P(genotype) = 0.006, odds ratio (OR) = 1.384, 95% CI 1.134-1.690) and major depressive disorder (P(allele) = 0.0007, P(genotype) = 0.003, OR = 1.425, 95% CI 1.160-1.752).

CONCLUSIONS

Our findings support CACNA1C being a risk gene for both schizophrenia and major depressive disorder in the Han Chinese population.

Downregulation of miR-151-5p contributes to increased susceptibility to arrhythmogenesis during myocardial infarction with estrogen deprivation

Estrogen deficiency is associated with increased incidence of cardiovascular diseases. But merely estrogen supplementary treatment can induce many severe complications such as breast cancer. The present study was designed to elucidate molecular mechanisms underlying increased susceptibility of arrhythmogenesis during myocardial infarction with estrogen deprivation, which provides us a new target to cure cardiac disease accompanied with estrogen deprivation. We successfully established a rat model of myocardial ischemia (MI) accompanied with estrogen deprivation by coronary artery ligation and ovariectomy (OVX). Vulnerability and mortality of ventricular arrhythmias increased in estrogen deficiency rats compared to non estrogen deficiency rats when suffered MI, which was associated with down-regulation of microRNA-151-5p (miR-151-5p). Luciferase Reporter Assay demonstrated that miR-151-5p can bind to the 3′-UTR of FXYD1 (coding gene of phospholemman, PLM) and inhibit its expression. We found that the expression of PLM was increased in (OVX+MI) group compared with MI group. More changes such as down-regulation of Kir2.1/I_K1, calcium overload had emerged in (OVX+MI) group compared to MI group merely. Transfection of miR-151-5p into primary cultured myocytes decreased PLM levels and [Ca²⁺]_i, however, increased Kir2.1 levels. These effects were abolished by the antisense oligonucleotides against miR-151-5p. Co-immunoprecipitation and immunofluorescent experiments confirmed the co-localization between Kir2.1 and PLM in rat ventricular tissue. We conclude that the increased ventricular arrhythmias vulnerability in response to acute myocardial ischemia in rat is critically dependent upon down-regulation of miR-151-5p. These findings support the proposal that miR-151-5p could be a potential therapeutic target for the prevention of ischemic arrhythmias in the subjects with estrogen deficiency.

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.

Multilayered regulation of cardiac ion channels

Essential to beat-to-beat heart function is the ability for cardiomyocytes to propagate electrical excitation and generate contractile force. Both excitation and contractility depend on specific ventricular ion channels, which include the L-type calcium channel (LTCC) and the connexin 43 (Cx43) gap junction. Each of these two channels is localized to a distinct subdomain of the cardiomyocyte plasma membrane. In this review, we focus on regulatory mechanisms that govern the lifecycles of LTCC and Cx43, from their biogenesis in the nucleus to directed delivery to T-tubules and intercalated discs, respectively. We discuss recent findings on how alternative promoter usage, tissue-specific transcription, and alternative splicing determine precise ion channel expression levels within a cardiomyocyte. Moreover, recent work on microtubule and actin-dependent trafficking for Cx43 and LTCC are introduced. Lastly, we discuss how human cardiac disease phenotypes can be attributed to defects in distinct mechanisms of channel regulation at the level of gene expression and channel trafficking. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

5E- and 5Z-farnesylacetones from Sargassum siliquastrum as novel selective L-type calcium channel blockers

A specific blocker of L-type Ca(2+) channels may be useful in decreasing arterial tone by reducing the open-state probability of L-type Ca(2+) channels. The aim of the present study was to evaluate the farnesylacetones, which are major active constituents of Sargassum siliquastrum, regarding their vasodilatation efficacies, selectivities toward L-type Ca(2+) channels, and in vivo antihypertensive activities. The application of 5E-(farnesylacetone 311) or 5Z-farnesylacetone (farnesylacetone 312) induced concentration-dependent vasodilatation effects on the basilar artery that was pre-contracted with depolarization and showed an ignorable potential role of endothelial-derived nitric oxide. We also tested farnesylacetone 311 or 312 to determine their pharmacological profiles for the blockade of native L-type Ca(2+) channels in basilar arterial smooth muscle cells (BASMCs) and ventricular myocytes (VMCs), cloned L- (α1C/β2a/α2δ), N- (α1B/β1b/α2δ), and T-type Ca(2+) channels (α1G, α1H, and α1I). Farnesylacetone 311 or 312 showed greater selectivity toward the L-type Ca(2+) channels among the tested voltage-gated Ca(2+) channels. The ranked order of the potency for farnesylacetone 311 was cloned α1C≒L-type (BASMC)≒L-type (VMCs)>α1B>α1H>α1I>α1G and that for farnesylacetone 312 was cloned α1C≒L-type (BASMCs)≒L-type (VMCs)>α1H>α1G>α1B>α1I. The oral administration of the farnesylacetone 311 (80mg/kg) conferred potent, long-lasting antihypertensive activity in spontaneous hypertensive rats, but it did not alter the heart rate.

Triton X-100 inhibits L-type voltage-operated calcium channels

Triton X-100 (TX-100) is a nonionic detergent frequently used at millimolar concentrations to disrupt cell membranes and solubilize proteins. At low micromolar concentrations, TX-100 has been reported to inhibit the function of potassium channels. Here, we have used electrophysiological and functional techniques to examine the effects of TX-100 on another class of ion channels, L-type voltage-operated calcium channels (VOCCs). TX-100 (30 nmol·L(-1) to 3 μmol·L(-1)) caused reversible concentration-dependent inhibition of recombinant L-type VOCC (CaV 1.2) currents and of native L-type VOCC currents recorded from rat vascular smooth muscle cells and cardiac myocytes, and murine and human pancreatic β-cells. In functional studies, TX-100 (165 nmol·L(-1) to 3.4 μmol·L(-1)) caused concentration-dependent relaxation of rat isolated mesenteric resistance arteries prestimulated with phenylephrine or KCl. This effect was independent of the endothelium. TX-100 (1.6 μmol·L(-1)) inhibited depolarization-induced exocytosis in both murine and human isolated pancreatic β-cells. These data indicate that at concentrations within the nanomolar to low micromolar range, TX-100 significantly inhibits L-type VOCC activity in a number of cell types, an effect paralleled by inhibition of cell functions dependent upon activation of these channels. This inhibition occurs at concentrations below those used to solubilize proteins and may compromise the use of solutions containing TX-100 in bioassays.

CACNA1C (Cav1.2) in the pathophysiology of psychiatric disease

One of the most consistent genetic findings to have emerged from bipolar disorder genome wide association studies (GWAS) is with CACNA1C, a gene that codes for the α(1C) subunit of the Ca(v)1.2 voltage-dependent L-type calcium channel (LTCC). Genetic variation in CACNA1C have also been associated with depression, schizophrenia, autism spectrum disorders, as well as changes in brain function and structure in control subjects who have no diagnosable psychiatric illness. These data are consistent with a continuum of shared neurobiological vulnerability between diverse-Diagnostic and Statistical Manual (DSM) defined-neuropsychiatric diseases. While involved in numerous cellular functions, Ca(v)1.2 is most frequently implicated in coupling of cell membrane depolarization to transient increase of the membrane permeability for calcium, leading to activation and, potentially, changes in intracellular signaling pathway activity, gene transcription, and synaptic plasticity. Ca(v)1.2 is involved in the proper function of numerous neurological circuits including those involving the hippocampus, amygdala, and mesolimbic reward system, which are strongly implicated in psychiatric disease pathophysiology. A number of behavioral effects of LTCC inhibitors have been described including antidepressant-like behavioral actions in rodent models. Clinical studies suggest possible treatment effects in a subset of patients with mood disorders. We review the genetic structure and variation of CACNA1C, discussing relevant human genetic and clinical findings, as well as the biological actions of Ca(v)1.2 that are most relevant to psychiatric illness.

Modulation of distinct isoforms of L-type calcium channels by G(q)-coupled receptors in Xenopus oocytes: antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca(2+) channels (L-VDCCs; Ca(v)1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G(q) enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G(q)-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of α(1C), whereas known smooth muscle short-NT isoforms were inhibited by PKC and G(q) activators. We report a novel regulation of the long-NT α(1C) isoform by Gβγ. Gβγ inhibited whereas a Gβγ scavenger protein augmented the G(q)--but not phorbol ester-mediated enhancement of channel activity, suggesting that Gβγ acts upstream from PKC. In vitro binding experiments reveal binding of both Gβγ and PKC to α(1C)-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT α(1C) sensitive to PKC stimulation and to Gβγ scavenging. Our results suggest a complex antagonistic interplay between G(q)-activated PKC and Gβγ in regulation of L-VDCC, in which multiple cytosolic segments of α(1C) are involved.

MicroRNA- 1 represses Cx43 expression in viral myocarditis

MicroRNAs (miRNAs) are increasingly reported to have important roles in diverse biological and pathological processes. Changes in abundance of muscle-specific microRNA, miR-1, have been implicated in cardiac disease, including arrhythmia and heart failure. However, the specific molecular targets and cellular mechanisms involved in the miR-1 function in the heart are only beginning to emerge. In this study, we investigated miR-1 expression and its potential role in the mouse model of viral myocarditis (VMC). The expression levels of miR-1 and its target gene Connexin 43 (Cx43) were measured by real-time PCR and western blotting, respectively. The miR-1 expression levels were significantly increased in cardiac myocytes from VMC mice in comparison with control samples (relative expression: 10 ± 2.5 vs. 31 ± 7.6, P < 0.05). Among the target genes of miR-1, the expression Cx43 protein was significantly reduced in such mice while there was no significant difference in the its mRNA levels. Our results revealed an inverse correlation between miR-1 levels and Cx43 protein expression in VMC samples. Using a bioinformatics-based approach, we found two identical potential binding sites were found in mouse miR-1 and Cx43 3'- untranslated region, this confirms a possible regulatory role of miR-1. In cultured, miRNA transfected myocardial cells, we show overexpression of miR-1 accompanied by a decrease in Cx43 protein's expression. There was only a slight (not statistically significant) drop in Cx43 mRNA levels. Our results indicate that miR-1 is involved in VMC via post-transcriptional repression of Cx43, and might constitute potentially valuable data for the development of a new approach in the treatment of this disease.

L-type calcium channel auto-regulation of transcription

L-type calcium channels (LTCC) impact the function of nearly all excitable cells. The classical LTCC function is to mediate trans-sarcolemmal Ca(2+) flux. This review focuses on the contribution of a mobile segment of the LTCC that regulates ion channel function, and also serves as a regulator of transcription in the nucleus. Specifically we highlight recent work demonstrating an auto-feedback regulatory pathway whereby the LTCC transcription factor regulates the LTCC. Also discussed is acute and long-term regulation of function by the LTCC-transcription regulator.

CaBP1 regulates voltage-dependent inactivation and activation of Ca(V)1.2 (L-type) calcium channels

CaBP1 is a Ca(2+)-binding protein that regulates the gating of voltage-gated (Ca(V)) Ca(2+) channels. In the Ca(V)1.2 channel α(1)-subunit (α(1C)), CaBP1 interacts with cytosolic N- and C-terminal domains and blunts Ca(2+)-dependent inactivation. To clarify the role of the α(1C) N-terminal domain in CaBP1 regulation, we compared the effects of CaBP1 on two alternatively spliced variants of α(1C) containing a long or short N-terminal domain. In both isoforms, CaBP1 inhibited Ca(2+)-dependent inactivation but also caused a depolarizing shift in voltage-dependent activation and enhanced voltage-dependent inactivation (VDI). In binding assays, CaBP1 interacted with the distal third of the N-terminal domain in a Ca(2+)-independent manner. This segment is distinct from the previously identified calmodulin-binding site in the N terminus. However, deletion of a segment in the proximal N-terminal domain of both α(1C) isoforms, which spared the CaBP1-binding site, inhibited the effect of CaBP1 on VDI. This result suggests a modular organization of the α(1C) N-terminal domain, with separate determinants for CaBP1 binding and transduction of the effect on VDI. Our findings expand the diversity and mechanisms of Ca(V) channel regulation by CaBP1 and define a novel modulatory function for the initial segment of the N terminus of α(1C).

Angiotensin II upregulates Ca(V)1.2 protein expression in cultured arteries via endothelial H(2)O(2) production

BACKGROUND

We previously reported that angiotensin II caused an endothelial-dependent increase in L-type voltage-dependent Ca(2+) channel (Ca(V)1.2) in cultured arteries, but the signaling pathways are not clear.

METHODS

Endothelial damage was generated by brief intra-arterial perfusion with 0.3% CHAPS. Ca(V)1.2 expression, function and H(2)O(2) were measured by Western blot, tension recording and Amplex Red H(2)O(2) assay kit, respectively.

RESULTS

Angiotensin II dose-dependently upregulated Ca(V)1.2 expression in endothelium-intact arteries. The angiotensin II upregulation of Ca(V)1.2 expression in endothelium-intact arteries was blocked by NAD(P)H oxidase inhibitor diphenyleneiodonium (DPI), apocynin, a more specific NAD(P)H oxidase inhibitor gp91ds-tat and also by catalase. H(2)O(2) similarly upregulated Ca(V)1.2 expression in endothelium-intact and endothelium-damaged arteries, and the latter effect was also blocked by DPI and apocynin. Angiotensin II increased H(2)O(2) production by endothelium-intact but not by endothelium-damaged arteries, and this effect was blocked by apocynin, catalase and gp91ds-tat. The upregulation of Ca(V)1.2 by angiotensin II and H(2)O(2) is accompanied by an increased tension response to KCl and the Ca(2+) channel activator FPL 64176, and this effect was also attenuated by gp91ds-tat.

CONCLUSION

These results suggest that angiotensin II stimulates endothelial NAD(P)H oxidase-produced H(2)O(2,) which may additionally act through vascular smooth muscle NAD(P)H oxidase, to upregulate vascular Ca(V)1.2 protein.

Autoregulation of cardiac l-type calcium channels

l-Type calcium channels (LTCCs) are major contributors to electrical and contractile function of the heart. They regulate action potential duration, enable calcium entry into cardiac myocytes for contraction, and regulate growth-related signaling in the heart. In cardiac development and in mature heart disease, LTCCs are regulated at levels of acute function and transcription. In addition, LTCCs are clinically relevant therapeutic targets for antihypertensive medications. In this review, we discuss LTCC homeostasis whereby cardiac myocytes maintain LTCC expression via a novel transcriptionally regulated pathway that includes a segment of the LTCC that moves between surface membrane and nucleus.

Developmental control of CaV1.2 L-type calcium channel splicing by Fox proteins

CaV1.2 voltage-gated calcium channels play critical roles in the control of membrane excitability, gene expression, and muscle contraction. These channels show diverse functional properties generated by alternative splicing at multiple sites within the CaV1.2 pre-mRNA. The molecular mechanisms controlling this splicing are not understood. We find that two exons in the CaV1.2 channel are controlled in part by members of the Fox family of splicing regulators. Exons 9* and 33 confer distinct electrophysiological properties on the channel and show opposite patterns of regulation during cortical development, with exon 9* progressively decreasing its inclusion in the CaV1.2 mRNA over time and exon 33 progressively increasing. Both exons contain Fox protein binding elements within their adjacent introns, and Fox protein expression is induced in cortical neurons in parallel with the changes in CaV1.2 splicing. We show that knocking down expression of Fox proteins in tissue culture cells has opposite effects on exons 9* and 33. The loss of Fox protein increases exon 9* splicing and decreases exon 33, as predicted by the positions of the Fox binding elements and by the pattern of splicing in development. Conversely, overexpression of Fox1 and Fox2 proteins represses exon 9* and enhances exon 33 splicing in the endogenous CaV1.2 mRNA. These effects of Fox proteins on exons 9* and 33 can be recapitulated in transfected minigene reporters. Both the repressive and the enhancing effects of Fox proteins are dependent on the Fox binding elements within and adjacent to the target exons, indicating that the Fox proteins are directly regulating both exons. These results demonstrate that the Fox protein family is playing a key role in tuning the properties of CaV1.2 calcium channels during neuronal development.

L-type calcium channel C terminus autoregulates transcription

Calcium homeostasis is critical for cardiac myocyte function and must be tightly regulated. The guiding hypothesis of this study is that a carboxyl-terminal cleavage product of the cardiac L-type calcium channel (Ca(V)1.2) autoregulates expression. First, we confirmed that the Ca(V)1.2 C terminus (CCt) is cleaved in murine cardiac myocytes from mature and developing ventricle. Overexpression of full-length CCt caused a 34+/-8% decrease of Ca(V)1.2 promoter activity, and truncated CCt caused an 80+/-3% decrease of Ca(V)1.2 promoter (n=12). The full-length CCt distributes into cytosol and nucleus. A deletion mutant of CCt has a greater relative affinity for the nucleus than full-length CCt, and this is consistent with increased repression of Ca(V)1.2 promoter activity by truncated CCt. Chromatin immunoprecipitation analysis revealed that CCt interacts with the Ca(V)1.2 promoter in adult ventricular cardiac myocytes at promoter modules containing Nkx2.5/Mef2, C/EBp, and a cis regulatory module. The next hypothesis tested was that CCt contributes to transcriptional signaling associated with cellular hypertrophy. We explored whether fetal cardiac myocyte Ca(V)1.2 was regulated by serum in vitro. We tested atrial natriuretic factor promoter activity as a positive control and measured the serum response of Ca(V)1.2 promoter, protein, and L-type current (I(Ca,L)) from fetal mouse ventricular myocytes. Serum increased atrial natriuretic factor promoter activity and cell size as expected. Serum withdrawal increased Ca(V)1.2 promoter activity, mRNA, and I(Ca,L). Moreover, serum withdrawal decreased the relative nuclear localization of CCt. A combination of promoter deletion mutant analyses, and the response of promoter mutants to serum withdrawal support the conclusion that CCt, a proteolytic fragment of Ca(V)1.2, autoregulates Ca(V)1.2 expression in cardiac myocytes. These data support the novel mechanism that a mobile segment of Ca(V)1.2 links Ca handling to nuclear signaling.

Vascular smooth muscle-specific knockdown of the noncardiac form of the L-type calcium channel by microRNA-based short hairpin RNA as a potential antihypertensive therapy

In different rodent models of hypertension, vascular voltage-gated L-type calcium channel (Ca(L)) current and vascular tone is increased because of increased expression of the noncardiac form of the Ca(L) (Ca(v)1.2). The objective of this study was to develop a small interfering RNA (siRNA) expression system against the noncardiac form of Ca(v)1.2 to reduce its expression in vascular smooth muscle cells (VSMCs). siRNAs expressing plasmids and appropriate controls were constructed and first screened in human embryonic kidney (HEK) 293 cells cotransfected with a rat Ca(v)1.2 expression vector. The most effective gene silencing was achieved with a modified mir-30a-based short hairpin RNA (shRNAmir) driven by the cytomegalovirus promoter. In A7r5 cells, a vascular smooth muscle cell line, two copies of shRNAmir driven by a chimeric VSMC-specific enhancer/promoter reduced endogenous Ca(v)1.2 expression by 61% and decreased the Ca(L) current carried by barium by 47%. Moreover, the chimeric vascular smooth muscle-specific enhancer/promoter displayed almost no activity in non-VSMCs (PC-12 and HEK 293). Because the proposed siRNA was designed to only target the noncardiac form of Ca(v)1.2, it did not affect the Ca(L) expression and function in cultured cardiomyocytes, even when driven by a stronger cytomegalovirus promoter. In conclusion, vascular Ca(v)1.2 expression and function were effectively reduced by VSMC-specific delivery of the noncardiac form of Ca(v)1.2 siRNA without similarly affecting cardiac Ca(L) expression and function. When coupled with a viral vector, this molecular intervention in vivo may provide a novel long-term vascular-specific gene therapy for hypertension.

A single anti-microRNA antisense oligodeoxyribonucleotide (AMO) targeting multiple microRNAs offers an improved approach for microRNA interference

Anti-miRNA antisense inhibitors (AMOs) have demonstrated their utility in miRNA research and potential in miRNA therapy. Here we report a modified AMO approach in which multiple antisense units are engineered into a single unit that is able to simultaneously silence multiple-target miRNAs, the multiple-target AMO or MTg-AMO. We validated the technique with two separate MTg-AMOs: anti-miR-21/anti-miR-155/anti-miR-17-5p and anti-miR-1/anti-miR-133. We first verified the ability of the MTg-AMOs to antagonize the repressive actions of their target miRNAs using luciferase reporter activity assays and to specifically knock down the levels of their target miRNAs using real-time RT-PCR methods. We then used the MTg-AMO approach to identify several tumor suppressors-TGFBI, APC and BCL2L11 as the target genes for oncogenic miR-21, miR-155 and miR-17-5p, respectively, and two cardiac ion channel genes HCN2 (encoding a subunit of cardiac pacemaker channel) and CACNA1C (encoding the alpha-subunit of cardiac L-type Ca(2+) channel) for the muscle-specific miR-1 and miR-133. We further demonstrated that the MTg-AMO targeting miR-21, miR-155 and miR-17-5p produced a greater inhibitory effect on cancer cell growth, compared with the regular single-target AMOs. Moreover, while using the regular single-target AMOs excluded HCN2 as a target gene for either miR-1 or miR-133, the MTg-AMO approach is able to reveal HCN2 as the target for both miR-1 and miR-133. Our findings suggest the MTg-AMO as an improved approach for miRNA target finding and for studying function of miRNAs. This approach may find its broad application for exploring biological processes involving multiple miRNAs and multiple genes.

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.

Novel approaches for gene-specific interference via manipulating actions of microRNAs: examination on the pacemaker channel genes HCN2 and HCN4

Recent evidence has suggested microRNAs as viable therapeutic targets for a wide range of human disease. However, lack of gene-specificity of microRNA actions may hinder this application. Here we developed two new approaches, the gene-specific microRNA mimic and microRNA-masking antisense approaches, to explore the possibility of using microRNA's principle of actions in a gene-specific manner. We examined the value of these strategies as rational approaches to develop heart rate-reducing agents and "biological pacemakers" by manipulating the expression of the cardiac pacemaker channel genes HCN2 and HCN4. We showed that the gene-specific microRNA mimics, 22-nt RNAs designed to target the 3'untranslated regions (3'UTRs) of HCN2 and HCN4, respectively, were efficient in abrogating expression and function of HCN2 and HCN4. The gene-specific microRNA mimics repressed protein levels, accompanied by depressed f-channel conductance and the associated rhythmic activity, without affecting mRNA levels of HCN2 and HCN4. Meanwhile, we also designed the microRNA-masking antisense based on the miR-1 and miR-133 target sites in the 3'UTRs of HCN2 and HCN4 and found that these antisense oligodeoxynucleotides markedly enhanced HCN2/HCN4 expression and function, as reflected by increased protein levels of HCN2/HCN4 and If conductance, by removing the repression of HCN2/HCN4 expression induced by endogenous miR-1/miR-133. The experimental examination of these techniques and the resultant findings not only indicate feasibility of interfering miRNA action in a gene-specific fashion but also may provide a new research tool for studying function of miRNAs. The new approaches also have the potential of becoming alternative gene therapy strategies.

Transcriptional activation by stimulating protein 1 and post-transcriptional repression by muscle-specific microRNAs of IKs-encoding genes and potential implications in regional heterogeneity of their expressions

In cardiac cells, KCNQ1 assembles with KCNE1 and forms a channel complex constituting the slow delayed rectifier current I(Ks). Expression of KCNQ1 and KCNE1 are regionally heterogeneous and changes with pathological states of the heart. The aims of this study were to decipher the molecular mechanisms for transcriptional and post-transcriptional regulation expression of KCNQ1 and KCNE1 genes and to shed light on the molecular mechanisms for their spatial heterogeneity of distribution. We cloned the 5'-flanking region and identified the transcription start sites of the KCNQ1 gene. We characterized the core promoters of KCNQ1 and KCNE1 and revealed the stimulating protein (Sp1) as a common transactivator of KCNQ1 and KCNE1 by interacting with the Sp1 cis-acting elements in the core promoter regions of these genes. We also characterized the 3' untranslated regions (3'UTRs) of the genes and experimentally established KCNQ1 and KCNE1 as targets for repression by the muscle-specific microRNAs miR-133 and miR-1, respectively. We demonstrated spatial heterogeneity of KCNQ1 and KCNE1 distributions at three axes (interventricular, transmural and apical-basal) and disparity between mRNA and protein expressions of these genes. We also found characteristic regional differences of expressions of Sp1 and miR-1/miR-133 in the heart. Our study unraveled a novel aspect of the cellular function of miRNAs and suggests that the I(Ks)-encoding genes KCNQ1 and KCNE1 expressions are dynamically balanced by transcription factor regulation and miRNA repression. The heterogeneities of Sp1 and miR-1/miR-133 offer an explanation for the well-recognized regional differences and disparity between mRNA and protein expressions of KCNQ1 and KCNE1.

Overexpression HERG K(+) channel gene mediates cell-growth signals on activation of oncoproteins SP1 and NF-kappaB and inactivation of tumor suppressor Nkx3.1

The long QT syndrome gene human ether-a-go-go related gene (HERG) encodes a K(+) channel critical to cardiac repolarization. It peculiarly overexpresses in cancer cells of different histogenesis and promotes tumorigenesis. To decipher the molecular mechanisms for HERG overexpression, we identified and characterized the promoter region of the HERG gene, which contains cis-elements for multiple oncoproteins and tumor suppressors. Oncoprotein Sp1 was found to be essential to driving the HERG promoter thereby transcription. Another oncoprotein NF-kappaB transactivated, while tumor suppressor Nkx3.1 repressed HERG promoter activity and endogenous HERG transcription. Loss-of-function mutations in the corresponding cis-elements rendered a loss of the ability of the oncoproteins Sp1 and NF-kappaB to transactivate, and of the tumor repressor Nkx3.1 to repress, HERG transcription. Either activation of Sp1 and NF-kappaB or silencing of Nkx3.1 promoted tumor cell growth, and the effects were abrogated by HERG inhibition or knockdown, but facilitated by overexpression of HERG, indicating that HERG mediates the cell growth signals generated by activation of oncoproteins or inactivation of tumor suppressors. Binding of Sp1, NF-kappaB, and Nkx3.1 to their respective cis-elements in the HERG promoter in vitro and their presence on the HERG promoter in vivo were confirmed. Therefore, the HERG promoter region is characterized by multiple Sp1 binding sites that are responsible for transcription initiation of the HERG gene and by binding sites for multiple other oncogenes and tumor suppressor genes being important for regulating HERG expression. The HERG K(+) channel is likely a mediator of growth-promoting processes induced by oncoproteins and/or by silencing of tumor suppressors.

Signature combinatorial splicing profiles of rat cardiac- and smooth-muscle Cav1.2 channels with distinct biophysical properties

l-type (Ca(v)1.2) voltage-gated calcium channels play an essential role in muscle contraction in the cardiovascular system. Alternative splicing of the pore-forming Ca(v)1.2 subunit provides potent means to enrich the functional diversity of the channels. There are 11 alternatively spliced exons identified in rat Ca(v)1.2 gene and random rearrangements may generate up to hundreds of combinatorial splicing profiles. Due to such complexity, the real combinatorial splicing profiles of Ca(v)1.2 have not been solved. This study investigated whether the 11 alternatively spliced exons are spliced randomly or linked and if linked, how many combinatorial splicing profiles can be arranged in cardiac- and smooth-muscle cells. By examining three full-length cDNA libraries of the Ca(v)1.2 transcripts isolated from rat heart and aorta, our results showed that the arrangements of some of the alternatively spliced exons are tissue-specific and tightly linked, giving rise to only 41 alternative combinatorial profiles, of which 29 have not been reported. Interestingly, the 41 combinatorial profiles were distinctively distributed in the three Ca(v)1.2 libraries and the one named "heart 1-50" contained unexpected splice variants. Significantly, the tissue-specific cardiac- and smooth-muscle combinatorial splicing profiles of Ca(v)1.2 channels demonstrated distinct electrophysiological properties that may help rationalize the differences observed in native currents. The unique sequences in these tissue-specific splice variants may provide the potential targets for drug design and screening.

A single decoy oligodeoxynucleotides targeting multiple oncoproteins produces strong anticancer effects

Cancer in general is a multifactorial process. Targeting a single factor may not be optimal in therapy, because single agents are limited by incomplete efficacy and dose-limiting adverse effects. Combination pharmacotherapy or "drug cocktail" therapy has value against many diseases, including cancers. We report an innovative decoy oligodeoxynucleotide (dODN) technology that we term complex decoy oligodeoxynucleotide (cdODNs) in which multiple cis elements are engineered into single dODNs attacking multiple target transcription factors, mimicking the drug cocktail approach. We designed dODNs targeting NF-kappaB, E2F, and Stat3 separately and a cdODN targeting NF-kappaB, E2F, and Stat3 concomitantly. We evaluated effects of this cdODN on expression of cancer-related genes, viability of human cancer cell lines, and in vivo tumor growth in nude mice. The cdODN targeting all NF-kappaB, E2F, and Stat3 together demonstrated enhancement of efficacy of more than 2-fold and increases in potency of 2 orders of magnitude compared with each of the dODNs or the combination of all three dODNs. The cdODN also showed earlier onset and longer-lasting action. Most strikingly, the cdODN acquired the ability to attack multiple molecules critical to cancer progression via multiple mechanisms, leading to elimination of regression. Real-time reverse transcription-polymerase chain reaction revealed that the cdODNs knocked down expression of the genes regulated by the target transcription factors. The cdODN strategy offers resourceful combinations of varying cis elements for concomitantly targeting multiple molecules in cancer biological processes and opens the door to "one-drug, multiple-target" therapy for a broad range of human cancers.

Regulation of maximal open probability is a separable function of Ca(v)beta subunit in L-type Ca2+ channel, dependent on NH2 terminus of alpha1C (Ca(v)1.2alpha)

β subunits (Ca_vβ) increase macroscopic currents of voltage-dependent Ca²⁺ channels (VDCC) by increasing surface expression and modulating their gating, causing a leftward shift in conductance–voltage (G-V) curve and increasing the maximal open probability, P_o,max. In L-type Ca_v1.2 channels, the Ca_vβ-induced increase in macroscopic current crucially depends on the initial segment of the cytosolic NH₂ terminus (NT) of the Ca_v1.2α (α_1C) subunit. This segment, which we term the “NT inhibitory (NTI) module,” potently inhibits long-NT (cardiac) isoform of α_1C that features an initial segment of 46 amino acid residues (aa); removal of NTI module greatly increases macroscopic currents. It is not known whether an NTI module exists in the short-NT (smooth muscle/brain type) α_1C isoform with a 16-aa initial segment. We addressed this question, and the molecular mechanism of NTI module action, by expressing subunits of Ca_v1.2 in Xenopus oocytes. NT deletions and chimeras identified aa 1–20 of the long-NT as necessary and sufficient to perform NTI module functions. Coexpression of β_2b subunit reproducibly modulated function and surface expression of α_1C, despite the presence of measurable amounts of an endogenous Ca_vβ in Xenopus oocytes. Coexpressed β_2b increased surface expression of α_1C approximately twofold (as demonstrated by two independent immunohistochemical methods), shifted the G-V curve by ∼14 mV, and increased P_o,max 2.8–3.8-fold. Neither the surface expression of the channel without Ca_vβ nor β_2b-induced increase in surface expression or the shift in G-V curve depended on the presence of the NTI module. In contrast, the increase in P_o,max was completely absent in the short-NT isoform and in mutants of long-NT α_1C lacking the NTI module. We conclude that regulation of P_o,max is a discrete, separable function of Ca_vβ. In Ca_v1.2, this action of Ca_vβ depends on NT of α_1C and is α_1C isoform specific.

Restoring depressed HERG K+ channel function as a mechanism for insulin treatment of abnormal QT prolongation and associated arrhythmias in diabetic rabbits

Abnormal QT prolongation (QT-P) in diabetic patients has become a nonnegligible clinical problem and has attracted increasing attention from basic scientists, because it increases the risk of lethal ventricular arrhythmias. Correction of QT-P may be an important measure in minimizing sudden cardiac death in diabetic patients. Here we report the efficacy of insulin in preventing QT-P and the associated arrhythmias and the mechanisms underlying the effects in a rabbit model of type 1 insulin-dependent diabetes mellitus (IDDM). The heart rate-corrected QT (QTc) interval and action potential duration were considerably prolonged, with frequent ventricular tachycardias. The rapid delayed rectifier K+ current (IKr) was markedly reduced in IDDM hearts, and hyperglycemia depressed the function of the human ether-a-go-go-related gene (HERG), which conducts IKr. The impairment was primarily ascribed to the enhanced oxidative damage to the myocardium, as indicated by the increased intracellular level of reactive oxygen species and simultaneously decreased endogenous antioxidant reserve and by the increased lipid peroxidation and protein oxidation. Moreover, IDDM or hyperglycemia resulted in downregulation of HERG protein level. Insulin restored the depressed IKr/HERG and prevented QTc/action potential duration prolongation and the associated arrhythmias, and the beneficial actions of insulin are partially due to its antioxidant ability. Our study represents the first documentation of oxidative stress as the major metabolic mechanism for HERG K+ dysfunction, which causes diabetic QT-P, and suggests IKr/HERG as a potential therapeutic target for treatment of the disorder.

Vascular calcium channels and high blood pressure: pathophysiology and therapeutic implications

Long-lasting Ca(2+) (Ca(L)) channels of the Ca(v)1.2 gene family are heteromultimeric structures that are minimally composed of a pore-forming alpha(1C) subunit and regulatory beta and alpha(2)delta subunits in vascular smooth muscle cells. The Ca(L) channels are the primary pathways for voltage-gated Ca(2+) influx that trigger excitation-contraction coupling in small resistance vessels. Notably, vascular smooth muscle cells of hypertensive rats show an increased expression of Ca(L) channel alpha(1C) subunits, which is associated with elevated Ca(2+) influx and the development of abnormal arterial tone. Indeed, blood pressure per se appears to promote Ca(L) channel expression in small arteries, and even short-term rises in pressure may alter channel expression. Membrane depolarization has been shown to be one stimulus associated with elevated blood pressure that promotes Ca(L) channel expression at the plasma membrane. Future studies to define the molecular processes that regulate Ca(L) channel expression in vascular smooth muscle cells will provide a rational basis for designing antihypertensive therapies to normalize Ca(L) channel expression and the development of anomalous vascular tone in hypertensive pathologies.

Negative transcriptional regulation of human colonic smooth muscle Cav1.2 channels by p50 and p65 subunits of nuclear factor-kappaB

BACKGROUND & AIMS

The expression of Cav1.2 channels in colonic circular smooth muscle cells and the contractility of these cells are suppressed in inflammation. Our aim was to investigate whether the activation of p50 and p65 nuclear factor-kappaB subunits mediates these effects.

METHODS

Primary cultures of human colonic circular smooth muscle cells and muscle strips were used.

RESULTS

The messenger RNA and protein expression of the pore-forming alpha1C subunit of Cav1.2 channels decreased time dependently in response to tumor necrosis factor alpha. This effect was blocked by prior transient transfection of the cells with antisense oligonucleotides to p50 or p65. The overexpression of p50 and p65 inhibited the constitutive expression of alpha1C. Three putative kappaB binding motifs were identified on the 5' flanking region of exon 1b of the human L-type calcium channel alpha1C gene. Progressive 5' deletions of the promoter and point mutations of the kappaB binding motifs indicated that the two 5' binding sites, but not the third 3' binding site, were essential for the suppression of alpha1C. Transient transfection of human colonic circular muscle strips with antisense oligonucleotides to p50 and p65 decreased expression of the 2 nuclear factor-kappaB units and reversed the suppression of alpha1C, as well as that of the contractile response to acetylcholine, by 24 hours of treatment with tumor necrosis factor alpha.

CONCLUSIONS

The activation of p50 and p65 by tumor necrosis factor alpha suppresses the expression of the alpha1C subunit of Cav1.2 channels in human colonic circular smooth muscle cells and their contractile response to acetylcholine. Nuclear factor-kappaB must bind concurrently to the two 5' kappaB motifs on the promoter of alpha1C to produce this effect.

Molecular Mechanism for Divergent Regulation of Cav1.2 Ca2+ Channels by Calmodulin and Ca2+-binding Protein-1

Ca(2+)-binding protein-1 (CaBP1) and calmodulin (CaM) are highly related Ca(2+)-binding proteins that directly interact with, and yet differentially regulate, voltage-gated Ca(2+) channels. Whereas CaM enhances inactivation of Ca(2+) currents through Ca(v)1.2 (L-type) Ca(2+) channels, CaBP1 completely prevents this process. How CaBP1 and CaM mediate such opposing effects on Ca(v)1.2 inactivation is unknown. Here, we identified molecular determinants in the alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2) that distinguish the effects of CaBP1 and CaM on inactivation. Although both proteins bind to a well characterized IQ-domain in the cytoplasmic C-terminal domain of alpha(1)1.2, mutations of the IQ-domain that significantly weakened CaM and CaBP1 binding abolished the functional effects of CaM, but not CaBP1. Pulldown binding assays revealed Ca(2+)-independent binding of CaBP1 to the N-terminal domain (NT) of alpha(1)1.2, which was in contrast to Ca(2+)-dependent binding of CaM to this region. Deletion of the NT abolished the effects of CaBP1 in prolonging Ca(v)1.2 Ca(2+) currents, but spared Ca(2+)-dependent inactivation due to CaM. We conclude that the NT and IQ-domains of alpha(1)1.2 mediate functionally distinct interactions with CaBP1 and CaM that promote conformational alterations that either stabilize or inhibit inactivation of Ca(v)1.2.

Expression of multiple CaV1.2 transcripts in rat tissues mediated by different promoters

The expression of two different transcripts for Ca(V)1.2 in rat tissues mirrors that which has previously been described for human tissue, in that expression of transcripts expressing exon 1a is predominant only in heart, whereas expression of transcripts expressing exon 1b is greater in smooth muscle rich tissues such as aorta and intestine. Transcripts expressing exon 1b also predominate in brain and in diaphragm. Western blots indicate that the N-terminus coded for by exon 1b is present in much of the protein in all these tissues except heart. The promoter just upstream of exon 1b has been cloned, sequenced and utilized to drive expression of luciferase in smooth muscle A7r5 cells, cardiac HL-1 cells, skeletal muscle L6 cells and neuronal PC12 cells. The nucleotide sequence of the promoter exhibits 80% identity with the equivalent promoter previously identified in humans and 94% identity with the sequence of the equivalent region of the mouse genome. Evidence in favor of still another promoter upstream of exon 2 has been uncovered.

Transcript scanning reveals novel and extensive splice variations in human l-type voltage-gated calcium channel, Cav1.2 alpha1 subunit

The L-type (Cav1.2) voltage-gated calcium channels play critical roles in membrane excitability, gene expression, and muscle contraction. The generation of splice variants by the alternative splicing of the poreforming Cav1.2 alpha1-subunit (alpha(1)1.2) may thereby provide potent means to enrich functional diversity. To date, however, no comprehensive scan of alpha(1)1.2 splice variation has been performed, particularly in the human context. Here we have undertaken such a screen, exploiting recently developed "transcript scanning" methods to probe the human gene. The degree of variation turns out to be surprisingly large; 19 of the 55 exons comprising the human alpha(1)1.2 gene were subjected to alternative splicing. Two of these are previously unrecognized exons and two others were not known to be spliced. Comparisons of fetal and adult heart and brain uncovered a large IVS3-S4 variability resulting from combinatorial utilization of exons 31-33. Electrophysiological characterization of such IVS3-S4 variation revealed unmistakable shifts in the voltage dependence of activation, according to an interesting correlation between increased IVS3-S4 linker length and activation at more depolarized potentials. Steady-state inactivation profiles remained unaltered. This systematic portrait of splice variation furnishes a reference library for comprehending combinatorial arrangements of Cav1.2 splice exons, especially as they impact development, physiology, and disease.