Pathways that control, or can be exploited to alter, the increase in airway smooth muscle (ASM) mass and cellular remodeling that occur in asthma are not well defined. Here we report the expression of odorant receptors (ORs) belonging to the superfamily of G-protein coupled receptors (GPCRs), as well as the canonical olfaction machinery (Golf and AC3) in the smooth muscle of human bronchi. In primary cultures of isolated human ASM, we identified mRNA expression for multiple ORs. Strikingly, OR51E2 was the most highly enriched OR transcript mapped to the human olfactome in lung-resident cells. In a heterologous expression system, OR51E2 trafficked readily to the cell surface and showed ligand selectivity and sensitivity to the short chain fatty acids (SCFAs) acetate and propionate. These endogenous metabolic byproducts of the gut microbiota slowed the rate of cytoskeletal remodeling, as well as the proliferation of human ASM cells. These cellular responses in vitro were found in ASM from non-asthmatics and asthmatics, and were absent in OR51E2-deleted primary human ASM. These results demonstrate a novel chemo-mechanical signaling network in the ASM and serve as a proof-of-concept that a specific receptor of the gut-lung axis can be targeted to treat airflow obstruction in asthma.
Bitter taste receptors (TAS2Rs) are G-protein-coupled receptors now recognized to be expressed on extraoral cells, including airway smooth muscle (ASM) where they evoke relaxation. TAS2Rs are difficult to express in heterologous systems, with most receptors being trapped intracellularly. We find, however, that co-expression of β2-adrenergic receptors (β2AR) in HEK-293T routes TAS2R14 to the cell surface by forming receptor heterodimers. Cell surface TAS2R14 expression was increased by ∼5-fold when β2AR was co-expressed. Heterodimer formation was shown by co-immunoprecipitation with tagged receptors, biomolecular fluorescence complementation, and merged confocal images. The dynamic nature of this interaction was shown by: a gene-dose relationship between transfected β2AR and TAS2R14 expression, enhanced (up to 3-fold) TAS2R14 agonist stimulation of [Ca2+]i with β2AR co-transfection, ∼53% decrease in [Ca2+]i signaling with shRNA knockdown of β2AR in H292 cells, and ∼60% loss of [Ca2+]i responsiveness in βAR knock-out mouse ASM. Once expressed on the surface, we detected unidirectional, conformation-dependent, interaction within the heterodimer, with β2AR activation rapidly uncoupling TAS2R14 function (∼65% desensitization). Cross-talk was independent of β2AR internalization and cAMP/PKA, and not accompanied by TAS2R14 internalization. With prolonged β-agonist exposure, TAS2R14 internalized, consistent with slow recycling of naked TAS2R14 in the absence of the heterodimeric milieu. In studies of ASM mechanics, rapid cross-talk was confirmed at the physiologic level, where relaxation from TAS2R14 agonist was decreased by ∼50% with β-agonist co-treatment. Thus the β2AR acts as a double-edged sword: increasing TAS2R14 cell surface expression, but when activated by β-agonist, partially offsetting the expression phenotype by direct receptor:receptor desensitization of TAS2R14 function.
asthma; chronic obstructive pulmonary disease (COPD); cyclic AMP (cAMP); dimerization; G protein; receptor internalization; smooth muscle; bitter taste receptor
The field of genetics and genomics has advanced considerably with the achievement of recent milestones encompassing the identification of many loci for cardiovascular disease and variable drug responses. Despite this achievement, a gap exists in the understanding and advancement to meaningful translation that directly affects disease prevention and clinical care. The purpose of this scientific statement is to address the gap between genetic discoveries and their practical application to cardiovascular clinical care. In brief, this scientific statement assesses the current timeline for effective translation of basic discoveries to clinical advances, highlighting past successes. Current discoveries in the area of genetics and genomics are covered next, followed by future expectations, tools, and competencies for achieving the goal of improving clinical care.
AHA Scientific Statements; adrenergic beta-antagonists; DNA; genetics; genome-wide association study; HapMap Project; Human Genome Project; PCSK9 protein; mouse; polymorphism; single nucleotide
Bitter taste receptors (TAS2Rs) have recently been found to be expressed on human airway smooth muscle (HASM), and their activation results in marked relaxation. These agents have been proposed as a new class of bronchodilators in the treatment of obstructive lung diseases because they act via a different mechanism than β-agonists. The TAS2R signal transduction pathway in HASM has multiple elements that are potentially subject to regulation by inflammatory, genetic, and epigenetic mechanisms associated with asthma. To address this, expression, signaling, and physiologic functions of the three major TAS2Rs (subtypes 10, 14, and 31) on HASM were studied. Transcript expression of these TAS2Rs was not decreased in HASM cells derived from donors with asthma compared with those without asthma (n = 6 from each group). In addition, intracellular calcium ([Ca2+]i) signaling using TAS2R subtype–specific agonists (diphenhydramine, chloroquine, saccharin, and flufenamic acid) was not impaired in the cells derived from donors with asthma, nor was the response to quinine, which activates all three subtypes. HASM cell mechanics measured by magnetic twisting cytometry revealed equivalent TAS2R-mediated relaxation of methacholine-treated cells between the two groups. Human precision-cut lung slices treated with IL-13 caused a decrease in β-agonist (formoterol)-mediated relaxation of carbachol-contracted airways compared with control slices. In contrast, TAS2R-mediated relaxation was unaffected by IL-13. We conclude that TAS2R expression or function is unaffected in HASM cells derived from patients with asthma or the IL-13 inflammatory environment.
receptor; asthma; bronchodilator; β-agonist; bitter taste
The stress-activated transcription factor, heat shock factor-1 (HSF1), regulates many genes including cytoprotective heat shock proteins (HSPs). We hypothesized that polymorphisms in HSF1 may alter the level or function of HSF1 protein accounting for interindividual viability in disease susceptibility or prognosis. We searched for exomic variants in HSF1 by querying human genome databases and directly sequencing DNA from 80 anonymous genomic DNA samples. Overall, HSF1 sequence was highly conserved, with no common variations. We found 31 validated deviations from a reference sequence in the dbSNP database and an additional 5 novel variants by sequencing, with allele frequencies that were 0.06 or less. Of these 36, 2 were in 5′-untranslated region (5′UTR), 10 in 3′UTR, and 24 in the coding region. The potential effects of 5′UTR on secondary structure, protein structure/function, and 3′UTR targets of microRNAs were analyzed using RNAFold, PolyPhen-2, SIFT, and MicroSNiper. One of the 5′UTR variants was predicted to strengthen secondary structure. Eight of 3′UTR variants were predicted to modify microRNA target sequences. Eight of the coding region variants were predicted to modify HSF1 structure/function. Reducing HSF1 levels in A549 cells using short hairpin RNA (shRNA) increased sensitivity to heat-induced killing demonstrating the impact that genetic variants that reduce HSF1 levels might have. Using the pmirGLO expression system, we found that the wild-type HSF1 3′UTR suppressed translation of a firefly luciferase reporter plasmid by 65 %. Introducing two of four 3′UTR single nucleotide polymorphisms (SNPs) increased HSF1 3′UTR translational suppression by 27–44 % compared with the wild-type HSF1 3′UTR sequence while a third SNP reduced suppression by 25 %. HSF1 variants may alter HSF1 protein levels or function with potential effects on cell functions, including sensitivity to stress.
Heat shock; Heat shock transcription factor-1; Single nucleotide variation; MicroRNA
Asthma is characterized by airway inflammation and airflow obstruction from human airway smooth muscle (HASM) constriction due to increased local bronchoconstrictive substances. We have recently found bitter taste receptors (TAS2Rs) on HASM, which increase [Ca2+]i and relax the muscle. We report here that some, but not all, TAS2R agonists decrease [Ca2+]i and relax HASM contracted by G-protein coupled receptors (GPCRs) that stimulate [Ca2+]i. This suggests both a second pathway by which TAS2Rs relax, and, a heterogeneity of the response phenotype. We utilized eight TAS2R agonists and five procontractile GPCR agonists in cultured HASM cells. We find that heterogeneity in the inhibitory response hinges on which procontractile GPCR is activated. For example, chloroquine inhibits [Ca2+]i increases from histamine, but failed to inhibit [Ca2+]i increases from endothelin-1. Conversely, aristolochic acid inhibited [Ca2+]i increases from endothelin-1 but not histamine. Other dichotomous responses were found when [Ca2+]i was stimulated by bradykinin, angiotensin, and acetylcholine. There was no association between [Ca2+]i inhibition and TAS2R subtype, nor whether [Ca2+]i was increased by Gq- or Gi-coupled GPCRs. Selected studies revealed a correlation between [Ca2+]i inhibition and HASM cell-membrane hyperpolarization. To demonstrate physiologic correlates, ferromagnetic beads were attached to HASM cells and cell stiffness measured by magnetic twisting cytometry. Consistent with the [Ca2+]i inhibition results, chloroquine abolished the cell stiffening response (contraction) evoked by histamine but not by endothelin-1, while aristolochic acid inhibited cell stiffening from endothelin-1, but not from histamine. In studies using intact human bronchi, these same differential responses were found. Those TAS2R agonists that decreased [Ca2+]i, promoted hyperpolarization, and decreased HASM stiffness, caused relaxation of human airways. Thus TAS2Rs relax HASM in two ways: a low-efficiency de novo [Ca2+]i stimulation, and, a high-efficiency inhibition of GPCR-stimulated [Ca2+]i. Furthermore, there is an interaction between TAS2Rs and some GPCRs that facilitates this [Ca2+]i inhibition limb.
There is an unmet need for a new class of direct bronchodilators for the treatment of asthma and chronic obstructive lung disease. Unexpectedly, bitter taste receptors (TAS2Rs) have been localized on airway smooth muscle and when activated cause marked smooth muscle relaxation via a mechanism that is distinct from β2-adrenegic receptors. Thus TAS2R agonists have emerged as a novel class of bronchodilator.
A synopsis of the TAS2R family and its biology for bitter taste perception on the tongue is provided, followed by a review of the identification and molecular and physiological characterization of TAS2R subtypes on human and mouse airway smooth muscle. The proposed molecular mechanisms leading to the relaxation response are provided, along with gaps in our understanding at certain points in the signaling cascade. Unresolved issues that may need to be considered for drug development are discussed.
TAS2R agnosts show promise as a new class of highly efficacious bronchodilators for treatment of obstructive lung disease. With tens of thousands of known natural and synthetic bitter compounds, there is substantial diversity within the known agonists, and, a ready source of agents for screening and further development of an inhaled TAS2R agonist for therapeutic purposes.
asthma; COPD; smooth muscle; calcium; bronchospasm; β-agonist
A three-stage approach was undertaken using genome-wide, case-control, and case-only association studies to identify genetic variants associated with heart failure mortality. In an Amish founder population (n = 851), cardiac hypertrophy, a trait integral to the adaptive response to failure, was found to be heritable (h2 = 0.28, p = 0.0002) and GWAS revealed 21 candidate hypertrophy SNPs. In a case (n = 1,610)-control (n = 463) study in unrelated Caucasians, one of the SNPs associated with hypertrophy (rs2207418, p = 8 × 10−6), was associated with heart failure, RR = 1.85(1.25–2.73, p = 0.0019). In heart failure cases rs2207418 was associated with increased mortality, HR = 1.51(1.20–1.97, p = 0.0004). There was consistency between studies, with the GG allele being associated with increased ventricular mass (~13 g/m2) in the Amish, heart failure risk, and heart failure mortality. This SNP is in a gene desert of chromosome 20p12. Five genes are within 2.0 mbp of rs2207418 but with low LD between their SNPs and rs2207418. A region near this SNP is highly conserved in multiple vertebrates (lod score = 1,208). This conservation and the internal consistency across studies suggests that this region has biologic importance in heart failure, potentially acting as an enhancer or repressor element. rs2207418 may be useful for predicting a more progressive form of heart failure that may require aggressive therapy.
genetics; heart failure; hypertrophy; mortality; signal transduction
Bitter taste receptors (TAS2Rs) were shown to be expressed in human airway smooth muscle (ASM). They couple to specialized [Ca2+]i release, leading to membrane hyperpolarization, the relaxation of ASM, and marked bronchodilation. TAS2Rs are G-protein–coupled receptors, known to undergo rapid agonist-promoted desensitization that can limit therapeutic efficacy. Because TAS2Rs represent a new drug target for treating obstructive lung disease, we investigated their capacity for rapid desensitization, and assessed their potential mechanisms. The pretreatment of human ASM cells with the prototypic TAS2R agonist quinine resulted in a 31% ± 5.1% desensitization of the [Ca2+]i response from a subsequent exposure to quinine. No significant change in the endothelin-stimulated [Ca2+]i response was attributed to the short-term use of quinine, indicating a homologous form of desensitization. The TAS2R agonist saccharin also evoked desensitization, and cross-compound desensitization with quinine was evident. Desensitization of the [Ca2+]i response was attenuated by a dynamin inhibitor, suggesting that receptor internalization (a G-protein coupled receptor kinase [GRK]-mediated, β-arrestin–mediated process) plays an integral role in the desensitization of TAS2R. Desensitization was insensitive to antagonists of the second messenger kinases protein kinase A and protein kinase C. Using intact airways, short-term, agonist-promoted TAS2R desensitization of the relaxation response was also observed. Thus these receptors, which represent a potential novel target for direct bronchodilators, undergo a modest degree of agonist-promoted desensitization that may affect clinical efficacy. Collectively, the results of these mechanistic studies, along with the multiple serines and threonines in intracellular loop 3 and the cytoplasmic tail of TAS2Rs, suggest a GRK-mediated mode of desensitization.
airway smooth muscle relaxation; taste receptors; tachyphylaxis; phosphorylation; G-protein–coupled receptor kinases
Because of its relatively recent evolution, Homo sapiens exhibits relatively little within-species genomic diversity. However, because of genome size, a proportionally small amount of variation creates ample opportunity for both rare mutations that may be disease-causative as well as more common genetic variation that may be important in disease modification or pharmacogenetics. Primarily because of the East African origin of modern humans, individuals of African ancestry (AA) exhibit greater degrees of genetic diversity than more recently established populations, such as those of European ancestry (EA) or Asian ancestry. These population effects extend to differences in the frequency of common gene variants that may be important in heart failure natural history or therapy. For cell-signaling mechanisms important in heart failure, we review and present new data on genetic variation between AA and EA populations. The data indicate that 1) neurohormonal signaling mechanisms frequently (16 of the 19 investigated polymorphisms) exhibit racial differences in the allele frequencies of variants comprising key constituents, 2) some of these differences in allele frequency may differentially affect the natural history of heart failure in AA vs. EA individuals, and 3) in many cases these differences likely play a role in observed racial differences in drug or device response.
Genetic polymorphisms; pharmacogenetics; racial ancestry; heart failure
Human Rhinovirus (HRV) infection is the cause of about one-half of asthma and COPD exacerbations. With >100 serotypes in the HRV reference set an effort was undertaken to sequence their complete genomes so as to understand diversity, structural variation, and evolution of the virus. Analysis revealed conserved motifs, hypervariable regions, a potential fourth HRV species, within-serotype variation in field isolates, a non-scanning internal ribosome entry site, and evidence for HRV recombination. Techniques have now been developed using next generation sequencing to generate complete genomes from patient isolates with high throughput, deep coverage, and low costs. Thus relationships can now be sought between obstructive lung phenotypes and variation in HRV genomes in infected patients, and, potential novel therapeutic strategies developed based on HRV sequence.
Asthma; COPD; inflammation; virus
This study assessed the impact of bucindolol, a beta-blocker/sympatholytic agent, on the development of atrial fibrillation (AF) in advanced chronic heart failure with reduced left ventricular ejection fraction (HFREF) patients enrolled in the BEST (Beta-Blocker Evaluation of Survival Trial).
β-Blockers have modest efficacy for AF prevention in HFREF patients. Bucindolol’s effects on HF and ventricular arrhythmic endpoints are genetically modulated by β1- and α2c-adrenergic receptor (AR) polymorphisms that can be used to subdivide HFREF populations into those with bucindolol effectiveness levels that are enhanced, unchanged, or lost.
BEST enrolled 2,708 New York Heart Association (NYHA) class III to IV HFREF patients. A substudy in which 1,040 patients’ DNA was genotyped for the β1-AR position 389 Arg/Gly and the α2c322–325 wild type (Wt)/deletion (Del) polymorphisms, and new-onset AF was assessed from adverse event case report forms or electrocardiograms at baseline and at 3 and 12 months.
In the entire cohort, bucindolol reduced the rate of new-onset AF compared to placebo by 41% (hazard ratio [HR]: 0.59 [95% confidence interval (CI): 0.44 to 0.79], p = 0.0004). In the 493 β1389 arginine homozygotes (Arg/Arg) in the DNA substudy, bucindolol reduced new-onset AF by 74% (HR: 0.26 [95% CI: 0.12 to 0.57]), with no effect in β1389 Gly carriers (HR: 1.01 [95% CI: 0.56 to 1.84], interaction test = 0.008). When β1389 Gly carriers were subdivided by α2c Wt homozygotes (n = 413, HR: 0.94 [95% CI: 0.48 to 1.82], p = 0.84) or Del variant carriers (n = 134, HR: 1.33 [95% CI: 0.32 to 5.64], p = 0.70), there was a positive interaction test (p = 0.016) when analyzed with β1389 Arg homozygotes.
Bucindolol prevented new-onset AF; β1 and α2c polymorphisms predicted therapeutic response; and the 47% of patients who were β1389 Arg homozygotes had an enhanced effect size of 74%. (Beta-Blocker Evaluation in Survival Trial [BEST]; NCT00000560)
arrhythmia; beta adrenergic receptors; genetics; heart failure; norepinephrine
The major pathologic β-adrenergic (βAR) subtype in heart failure is the β1AR. Our laboratory has thus pursued genetic variation of the β1AR gene at the molecular, cellular, physiologic and clinical levels as the potential basis for interindividual variability in the response to β-blocker treatment in heart failure. This chapter will review these findings, with an emphasis on mechanism of action and future directions.
adrenergic; adenylyl cyclase; myocardium; β-blocker; transgenic
Genome sequences of human Rhinoviruses (HRV) have primarily been from stocks collected in the 1960’s, with genomes and phylogeny of modern HRVs remaining undefined. Here, two modern isolates (hrv-A101 and hrv-A101-v1) collected ~8 years apart were sequenced in their entirety. Incorporation into our full-genome HRV alignment with subsequent phylogenetic network inference indicated that these represent a unique HRV-A, localized within an early diverging clade. They appear to have resulted from recombination of the hrv-65 and hrv-78 lineages. These results support our contention that there are unrecognized distinct HRV-A strains, and that recombination is evident in currently circulating strains.
Full-length or nearly full-length RNA genome sequences for 98 rhinovirus (RV) A isolates (from the Enterovirus genus of the Picornaviridae family), representing 43 different genotypes, were resolved as part of ongoing studies to define RV genetic diversity and its potential link to respiratory disease.
Nearly full-length RNA genome sequences for 39 rhinovirus B isolates (RV-B), representing 13 different genotypes, were resolved as part of ongoing studies at the University of Wisconsin that attempt to link rhinovirus (RV) diversity and respiratory disease in infants.
Human rhinovirus (RV) isolates from the RV-C species are recently discovered infectious agents that are closely linked to asthma and wheezing etiologies in infants. Clinical study samples collected at the University of Wisconsin–Madison describe 41 nearly complete genome sequences representing 21 RV-C genotypes.
β-agonist treatment of asthma displays substantial interindividual variation, which has prompted polymorphism discovery and characterization of β2-adrenergic (β2AR) signaling genes. β2AR function undergoes desensitization during persistent agonist exposure due to receptor phosphorylation by G-protein coupled receptor kinases (GRKs). GRK5 was found to be highly expressed in airway smooth muscle, the tissue target for β-agonists. The coding region is polymorphic at codon 41, where Gln can be substituted by Leu (minor allele), but almost exclusively in those of African descent. In transfected cells, GRK5-Leu41 evoked a greater degree of agonist-promoted desensitization of adenylyl cyclase compared to GRK5-Gln41. Consistent with this functional effect, agonist-promoted β2AR phosphorylation was greater in cells expressing GRK5-Leu41, as was the rate of agonist-promoted receptor internalization. In studies with mutated β2AR lacking PKA-phosphorylation sites, this phenotype was confirmed as being GRK-specific. So, GRK5-Leu41 represents a gain-of-function polymorphism that evokes enhanced loss-of-function of β2AR during persistent agonist exposure, and thus may contribute to β-agonist variability in asthma treatment of African-Americans.
Polymorphism; tachyphylaxis; β-agonist; kinases; desensitization; asthma
Infection by human rhinovirus (HRV) is a major cause of upper and lower respiratory tract disease worldwide and displays considerable phenotypic variation. We examined diversity by completing the genome sequences for all known serotypes (n = 99). Superimposition of capsid crystal structure and optimal-energy RNA configurations established alignments and phylogeny. These revealed conserved motifs; clade-specific diversity, including a potential newly identified species (HRV-D); mutations in field isolates; and recombination. In analogy with poliovirus, a hypervariable 5′ untranslated region tract may affect virulence. A configuration consistent with nonscanning internal ribosome entry was found in all HRVs and may account for rapid translation. The data density from complete sequences of the reference HRVs provided high resolution for this degree of modeling and serves as a platform for full genome-based epidemiologic studies and antiviral or vaccine development.
Like other intronless G protein–coupled receptor genes, the β2-adrenergic receptor (β2AR) has minimal genetic space for population variability, and has attained such via multiple coding and noncoding polymorphisms. Yet most clinical studies use the two nonsynonymous polymorphisms of the coding region for association analysis despite low levels of linkage disequilibrium with some promoter and 5′UTR polymorphisms. To assess the potential for allele-specific transcription factor binding to β2AR 5′-flanking sequence, 3′-biotin–labeled oligonucleotide duplexes were synthesized. Each was centered on variable sites representing major or minor alleles found in the human population with frequencies of 5% or greater (20 polymorphic sites). Electrophoretic mobility shift assays were performed using human airway smooth muscle or airway epithelial cell nuclear extracts. Many of these polymorphisms resulted in an alteration in binding, and both major allele and minor allele dominance were observed. For example, in airway smooth muscle nuclear extracts, 10 polymorphisms decreased and 2 increased binding, whereas 5 showed no differences. Concordance between airway smooth muscle and epithelial cell nuclear extract binding to polymorphic alleles was found in only ∼ 50% of cases. There was no tendency for the rare variants to be more likely to have altered nuclear extract binding compared to the more common variants. Taken together, these results provide potential mechanisms by which β2AR 5′-flanking polymorphisms affect obstructive lung phenotypes.
asthma; β; -agonist; polymorphism
β-Agonists used for treatment of obstructive lung disease have a variety of different structures but are typically classified by their intrinsic activities for stimulation of cAMP, and predictions are made concerning other downstream signals based on such a classification. We generated modified β2-adrenergic receptors with insertions of energy donor and acceptor moieties to monitor agonist-promoted conformational changes of the receptor using intramolecular bioluminescence resonance energy transfer in live cells. These studies suggested unique conformations stabilized by various agonists that were not based on their classic intrinsic activities. To address the cellular consequences of these differences, Gs-coupling, Gi-coupling (p44/p42 activation), G protein–coupled receptor kinase–mediated receptor phosphorylation, internalization, and down-regulation were assessed in response to isoproterenol, albuterol, terbutaline, metaproterenol, salmeterol, formoterol, and fenoterol. In virtually every case, agonists did not maintain the classic rank order, indicating that distinct signaling is evoked by β-agonists of different structures, which is unrelated to intrinsic activity. The extensive pleiotropy of agonist responses shown here suggests that classification of agonists by cAMP-based intrinsic activity is inadequate as it pertains to other intracellular events and that it may be possible to engineer a β-agonist that stabilizes conformations that evoke an ideal portfolio of signals for therapeutic purposes.
β-agonist; β-adrenergic receptor; adenylyl cyclase; G-protein; asthma
Receptor-mediated airway smooth muscle (ASM) contraction via Gαq, and relaxation via Gαs, underlie the bronchospastic features of asthma and its treatment. Asthma models show increased ASM Gαi expression, considered the basis for the proasthmatic phenotypes of enhanced bronchial hyperreactivity to contraction mediated by M3-muscarinic receptors and diminished relaxation mediated by β2-adrenergic receptors (β2ARs). A causal effect between Gi expression and phenotype has not been established, nor have mechanisms whereby Gi modulates Gq/Gs signaling. To delineate isolated effects of altered Gi, transgenic mice were generated overexpressing Gαi2 or a Gαi2 peptide inhibitor in ASM. Unexpectedly, Gαi2 overexpression decreased contractility to methacholine, while Gαi2 inhibition enhanced contraction. These opposite phenotypes resulted from different crosstalk loci within the Gq signaling network: decreased phospholipase C and increased PKCα, respectively. Gαi2 overexpression decreased β2AR-mediated airway relaxation, while Gαi2 inhibition increased this response, consistent with physiologically relevant coupling of this receptor to both Gs and Gi. IL-13 transgenic mice (a model of asthma), which developed increased ASM Gαi, displayed marked increases in airway hyperresponsiveness when Gαi function was inhibited. Increased Gαi in asthma is therefore a double-edged sword: a compensatory event mitigating against bronchial hyperreactivity, but a mechanism that evokes β-agonist resistance. By selective intervention within these multipronged signaling modules, advantageous Gs/Gq activities could provide new asthma therapies.
Tropomyosin (TM) plays a central role in calcium mediated striated muscle
contraction. There are three muscle TM isoforms: α-TM,
β-TM, and γ-TM.
α-TM is the predominant cardiac and skeletal muscle
isoform. β-TM is expressed in skeletal and embryonic
cardiac muscle. γ-TM is expressed in slow-twitch
musculature, but is not found in the heart. Our previous work established that
muscle TM isoforms confer different physiological properties to the cardiac
sarcomere. To determine whether one of these isoforms is dominant in dictating
its functional properties, we generated single and double transgenic mice
expressing β-TM and/or γ-TM in
the heart, in addition to the endogenously expressed
α-TM. Results show significant TM protein expression in
the βγ-DTG hearts:
α-TM: 36%, β-TM:
32%, and γ-TM: 32%. These
βγ-DTG mice do not develop pathological
abnormalities; however, they exhibit a hyper contractile phenotype with
decreased myofilament calcium sensitivity, similar to
γ-TM transgenic hearts. Biophysical studies indicate
that γ-TM is more rigid than either
α-TM or β-TM. This is the
first report showing that with approximately equivalent levels of expression
within the same tissue, there is a functional dominance of
γ-TM over α-TM or
β-TM in regulating physiological performance of the
striated muscle sarcomere. In addition to the effect expression of
γ-TM has on Ca2+ activation of
the cardiac myofilaments, our data demonstrates an effect on cooperative
activation of the thin filament by strongly bound rigor cross-bridges. This is
significant in relation to current ideas on the control mechanism of the steep
relation between Ca2+ and tension.
Tropomyosin isoforms; Thin filament regulation; Calcium sensitivity; Muscle contraction; Transgenic mice
A pro-asthmatic culture milieu and β2-agonist (isoproterenol) were previously shown to regulate the expression of select transcription factors (TFs) within human airway epithelial and smooth muscle cells. This study tests 1116 single nucleotide polymorphisms (SNPs) across 98 of these TF genes for association with bronchodilator response (BDR) in asthma patients. Genotyping was conducted using the Illumina HumanHap550v3 Beadchip in 403 non-Hispanic White asthmatic children and their parents. SNPs were evaluated for association with BDR using family and population-based analyses. Forty-two SNPs providing p values < 0.1 in both analyses were then genotyped in three adult asthma trials. One SNP 5’ of the thyroid hormone receptor beta gene was associated with BDR in the childhood population and two adult populations (p value = 0.0012). This investigation identified a novel locus for inter-individual variability in BDR and represents a translation of a cellular drug-response study to potential personalization of clinical asthma management.
Bronchodilator response; transcription factor; association; thyroid hormone receptor β; asthma; pharmacogenetics