Protein kinase D1 (Prkd1) deletion in brown adipose tissue leads to altered myogenic gene expression after cold exposure, while thermogenesis remains intact

Brown adipose tissue (BAT) has in recent times been rediscovered in adult humans, and together with work from preclinical models, has shown to have the potential of providing a variety of positive metabolic benefits. These include lower plasma glucose, improved insulin sensitivity, and reduced susceptibility to obesity and its comorbidities. As such, its continued study could offer insights to therapeutically modulate this tissue to improve metabolic health. It has been reported that adipose-specific deletion of the gene for protein kinase D1 (Prkd1) in mice enhances mitochondrial respiration and improves whole-body glucose homeostasis. We sought to determine whether these effects were mediated specifically through brown adipocytes using a Prkd1 brown adipose tissue (BAT) Ucp1-Cre-specific knockout mouse model, Prkd1^BKO . We unexpectedly observed that upon both cold exposure and β₃ -AR agonist administration, Prkd1 loss in BAT did not alter canonical thermogenic gene expression or adipocyte morphology. We took an unbiased approach to assess whether other signaling pathways were affected. RNA from cold-exposed mice was subjected to RNA-Seq analysis. These studies revealed that myogenic gene expression is altered in Prkd1^BKO BAT after both acute and extended cold exposure. Given that brown adipocytes and skeletal myocytes share a common precursor cell lineage expressing myogenic factor 5 (Myf5), these data suggest that loss of Prkd1 in BAT may alter the biology of mature brown adipocytes and preadipocytes in this depot. The data presented herein clarify the role of Prkd1 in BAT thermogenesis and present new avenues for the further study of Prkd1 function in BAT.

Pacing Dynamics Determines the Arrhythmogenic Mechanism of the CPVT2-Causing CASQ2G112+5X Mutation in a Guinea Pig Ventricular Myocyte Computational Model

Calsequestrin Type 2 (CASQ2) is a high-capacity, low-affinity, Ca2+-binding protein expressed in the sarcoplasmic reticulum (SR) of the cardiac myocyte. Mutations in CASQ2 have been linked to the arrhythmia catecholaminergic polymorphic ventricular tachycardia (CPVT2) that occurs with acute emotional stress or exercise can result in sudden cardiac death (SCD). CASQ2G112+5X is a 16 bp (339-354) deletion CASQ2 mutation that prevents the protein expression due to premature stop codon. Understanding the subcellular mechanisms of CPVT2 is experimentally challenging because the occurrence of arrhythmia is rare. To obtain an insight into the characteristics of this rare disease, simulation studies using a local control stochastic computational model of the Guinea pig ventricular myocyte investigated how the mutant CASQ2s may be responsible for the development of an arrhythmogenic episode under the condition of β-adrenergic stimulation or in the slowing of heart rate afterward once β-adrenergic stimulation ceases. Adjustment of the computational model parameters based upon recent experiments explore the functional changes caused by the CASQ2 mutation. In the simulation studies under rapid pacing (6 Hz), electromechanically concordant cellular alternans appeared under β-adrenergic stimulation in the CPVT mutant but not in the wild-type nor in the non-β-stimulated mutant. Similarly, the simulations of accelerating pacing from slow to rapid and back to the slow pacing did not display alternans but did generate early afterdepolarizations (EADs) during the period of second slow pacing subsequent acceleration of rapid pacing.

Altered cardiac β1 responsiveness in hyperthermic older adults

Compared with younger adults, passive heating induced increases in cardiac output are attenuated by ∼50% in older adults. This attenuated response may be associated with older individuals' inability to maintain stroke volume through ionotropic mechanisms and/or through altered chronotropic mechanisms. The purpose of this study was to identify the interactive effect of age and hyperthermia on cardiac responsiveness to dobutamine-induced cardiac stimulation. Eleven young (26 ± 4 yr) and 8 older (68 ± 5 yr) participants underwent a normothermic and a hyperthermic (baseline core temperature +1.2°C) trial on the same day. In both thermal conditions, after baseline measurements, intravenous dobutamine was administered for 12 min at 5 µg/kg/min, followed by 12 min at 15 µg/kg/min. Primary measurements included echocardiography-based assessments of cardiac function, gastrointestinal and skin temperatures, heart rate, and mean arterial pressure. Heart rate responses to dobutamine were similar between groups in both thermal conditions (P > 0.05). The peak systolic mitral annular velocity (S'), i.e., an index of left ventricular longitudinal systolic function, was similar between groups for both thermal conditions at baseline. While normothermic, the increase in S' between groups was similar with dobutamine administration. However, while hyperthermic, the increase in S' was attenuated in the older participants with dobutamine (P < 0.001). Healthy, older individuals show attenuated inotropic, but maintained chronotropic responsiveness to dobutamine administration during hyperthermia. These data suggest that older individuals have a reduced capacity to increase cardiomyocyte contractility, estimated by changes in S', via β1-adrenergic mechanisms while hyperthermic.

β2-adrenergic stimulation potentiates spontaneous calcium release by increasing signal mass and co-activation of ryanodine receptor clusters

AIMS

It is unknown how β-adrenergic stimulation affects calcium dynamics in individual RyR2 clusters and leads to the induction of spontaneous calcium waves. To address this, we analysed spontaneous calcium release events in green fluorescent protein (GFP)-tagged RyR2 clusters.

METHODS

Cardiomyocytes from mice with GFP-tagged RyR2 or human right atrial tissue were subjected to immunofluorescent labelling or confocal calcium imaging.

RESULTS

Spontaneous calcium release from single RyR2 clusters induced 91.4% ± 2.0% of all calcium sparks while 8.0% ± 1.6% were caused by release from two neighbouring clusters. Sparks with two RyR2 clusters had 40% bigger amplitude, were 26% wider, and lasted 35% longer at half maximum. Consequently, the spark mass was larger in two- than one-cluster sparks with a median and interquartile range for the cumulative distribution of 15.7 ± 20.1 vs 7.6 ± 5.7 a.u. (P < .01). β2-adrenergic stimulation increased RyR2 phosphorylation at s2809 and s2815, tripled the fraction of two- and three-cluster sparks, and significantly increased the spark mass. Interestingly, the amplitude and mass of the calcium released from a RyR2 cluster were proportional to the SR calcium load, but the firing rate was not. The spark mass was also higher in 33 patients with atrial fibrillation than in 36 without (22.9 ± 23.4 a.u. vs 10.7 ± 10.9; P = .015).

CONCLUSIONS

Most sparks are caused by activation of a single RyR2 cluster at baseline while β-adrenergic stimulation doubles the mass and the number of clusters per spark. This mimics the shift in the cumulative spark mass distribution observed in myocytes from patients with atrial fibrillation.

Rap1A, Rap1B, and β-Adrenergic Signaling in Autologous HCT: A Randomized Controlled Trial of Propranolol

Successful hematopoietic cell transplantation (HCT) depends on rapid engraftment of the progenitor and stem cells that will reestablish hematopoiesis. Rap1A and Rap1B are two closely related small GTPases that may affect platelet and neutrophil engraftment during HCT through their roles in cell adhesion and migration. β-adrenergic signaling may regulate the participation of Rap1A and Rap1B in engraftment through their inhibition or activation. We conducted a correlative study of a randomized controlled trial evaluating the effects of the nonselective β-antagonist propranolol on expression and prenylation of Rap1A and Rap1B during neutrophil and platelet engraftment in 25 individuals receiving an autologous HCT for multiple myeloma. Propranolol was administered for 1 week prior to and 4 weeks following HCT. Blood was collected 7 days (baseline) and 2 days (Day -2) before HCT, and 28 days after HCT (Day +28). Circulating polymorphonuclear cells (PMNC) were isolated and analyzed via immunoblotting to determine levels of prenylated and total Rap1A versus Rap1B. Twelve participants were randomized to the intervention and 13 to the control. Rap1A expression significantly correlated with Rap1B expression. Rap1B expression significantly correlated with slower platelet engraftment; however, this association was not observed in the propranolol-treated group. There were no significant associations between neutrophil engraftment and Rap1A or Rap1B expression. Post hoc exploratory analyses did not reveal an association between social health variables and Rap1A or Rap1B expression. This study identifies a greater regulatory role for Rap1B than Rap1A in platelet engraftment and suggests a possible role for β-adrenergic signaling in modulating Rap1B function during HCT.

Propranolol Relieves L-Dopa-Induced Dyskinesia in Parkinsonian Mice

BACKGROUND

Parkinsonism is caused by dopamine (DA) insufficiency and results in a hypokinetic movement disorder. Treatment with L-Dopa can restore DA availability and improve motor function, but patients can develop L-Dopa-induced dyskinesia (LID), a secondary hyperkinetic movement disorder. The mechanism underlying LID remains unknown, and new treatments are needed. Experiments in mice have shown that DA deficiency promotes an imbalance between striatal acetylcholine (ACh) and DA that contributes to motor dysfunction. While treatment with L-Dopa improves DA availability, it promotes a paradoxical rise in striatal ACh and a further increase in the ACh to DA ratio may promote LID.

METHODS

We used conditional Slc6a3^DTR/+ mice to model progressive DA deficiency and the β-adrenergic receptor (β-AR) antagonist propranolol to limit the activity of striatal cholinergic interneurons (ChIs). DA-deficient mice were treated with L-Dopa and the dopa decarboxylase inhibitor benserazide. LID and motor performance were assessed by rotarod, balance beam, and open field testing. Electrophysiological experiments characterized the effects of β-AR ligands on striatal ChIs.

RESULTS

LID was observed in a subset of DA-deficient mice. Treatment with propranolol relieved LID and motor hyperactivity. Electrophysiological experiments showed that β-ARs can effectively modulate ChI firing.

CONCLUSIONS

The work suggests that pharmacological modulation of ChIs by β-ARs might provide a therapeutic option for managing LID.

Effects of Propranolol on Bone, White Adipose Tissue, and Bone Marrow Adipose Tissue in Mice Housed at Room Temperature or Thermoneutral Temperature

Growing female mice housed at room temperature (22°C) weigh the same but differ in body composition compared to mice housed at thermoneutrality (32°C). Specifically, mice housed at room temperature have lower levels of white adipose tissue (WAT). Additionally, bone marrow adipose tissue (bMAT) and cancellous bone volume fraction in distal femur metaphysis are lower in room temperature-housed mice. The metabolic changes induced by sub-thermoneutral housing are associated with lower leptin levels in serum and higher levels of Ucp1 gene expression in brown adipose tissue. Although the precise mechanisms mediating adaptation to sub-thermoneutral temperature stress remain to be elucidated, there is evidence that increased sympathetic nervous system activity acting via β-adrenergic receptors plays an important role. We therefore evaluated the effect of the non-specific β-blocker propranolol (primarily β₁ and β₂ antagonist) on body composition, femur microarchitecture, and bMAT in growing female C57BL/6 mice housed at either room temperature or thermoneutral temperature. As anticipated, cancellous bone volume fraction, WAT and bMAT were lower in mice housed at room temperature. Propranolol had small but significant effects on bone microarchitecture (increased trabecular number and decreased trabecular spacing), but did not attenuate premature bone loss induced by room temperature housing. In contrast, propranolol treatment prevented housing temperature-associated differences in WAT and bMAT. To gain additional insight, we evaluated a panel of genes in tibia, using an adipogenesis PCR array. Housing temperature and treatment with propranolol had exclusive as well as shared effects on gene expression. Of particular interest was the finding that room temperature housing reduced, whereas propranolol increased, expression of the gene for acetyl-CoA carboxylase (Acacb), the rate-limiting step for fatty acid synthesis and a key regulator of β-oxidation. Taken together, these findings provide evidence that increased activation of β₁ and/or β₂ receptors contributes to reduced bMAT by regulating adipocyte metabolism, but that this pathway is unlikely to be responsible for premature cancellous bone loss in room temperature-housed mice.

Cardiovascular Effects of Micromeria graeca (L.) Benth. ex Rchb in Normotensive and Hypertensive Rats

AIMS

The present study was performed in order to analyze the antihypertensive activity of Micromeria graeca (L.) Benth. ex Rchb.

BACKGROUND

Micromeria graeca (L.) Benth. ex Rchb is an aromatic and medicinal plant belonging to the Lamiaceae family. This herb is used to treat various pathologies such as cardiovascular disorders. Meanwhile, its pharmacological effects on the cardiovascular system have not been studied.

OBJECTIVE

The present study aimed to evaluate the effect of aqueous extract of aerial parts of Micromeria graeca (AEMG) on the cardiovascular system in normotensive and hypertensive rats.

METHODS

In this study, the cardiovascular effect of AEMG was evaluated using in vivo and in vitro investigations. In order to assess the acute effect of AEMG on the cardiovascular system, anesthetized L-NAME-hypertensive and normotensive rats received AEMG (100 mg/kg) orally and arterial blood pressure parameters were monitored during six hours. In the sub-chronic study, rats were orally treated for one week, followed by blood pressure assessment during one week of treatment. Blood pressure was measured using a tail-cuff and a computer-assisted monitoring device. In the second experiment, isolated rat aortic ring pre-contracted with Epinephrine (EP) or KCl was used to assess the vasorelaxant effect of AEMG.

RESULTS

Oral administration of AEMG (100 mg/kg) provoked a decrease of arterial blood pressure parameters in hypertensive rats. In addition, AEMG induced a vasorelaxant effect in thoracic aortic rings pre-contracted with EP (10 μM) or KCl (80 mM). This effect was attenuated in the presence of propranolol and methylene blue. While in the presence of glibenclamide, L-NAME, nifedipine or Indomethacin, the vasorelaxant effect was not affected.

CONCLUSION

This study showed that Micromeria graeca possesses a potent antihypertensive effect and relaxes the vascular smooth muscle through β-adrenergic and cGMP pathways.

Transcriptional Regulation of ZNF638 in Thermogenic Cells by the cAMP Response Element Binding Protein in Male Mice

Zinc finger factors are implicated in a variety of cellular processes, including adipose tissue differentiation and thermogenesis. We have previously demonstrated that zinc finger protein 638 (ZNF638) is a transcriptional coactivator acting as an early regulator of adipogenesis in vitro. In this study, we show, to our knowledge for the first time, that, in vivo, ZNF638 abounds selectively in mature brown and subcutaneous fat tissues and in fully differentiated thermogenic adipocytes. Furthermore, gene expression studies revealed that ZNF638 is upregulated by cAMP modulators in vitro and by cold exposure and by pharmacological stimulation of β-adrenergic signaling in vivo. In silico analysis of the upstream regulatory region of the ZNF638 gene identified two putative cAMP response elements within 500 bp of the ZNF638 transcription start site. Detailed molecular analysis involving EMSA and chromatin immunoprecipitation assays demonstrated that cAMP response element binding protein (CREB) binds to these cAMP response element regions of the ZNF638 promoter, and functional studies revealed that CREB is necessary and sufficient to regulate the levels of ZNF638 transcripts. Taken together, these results demonstrate that ZNF638 is selectively expressed in mature thermogenic adipocytes and tissues and that its induction in response to classic stimuli that promote heat generation is mediated via CREB signaling, pointing to a possible novel role of ZNF638 in brown and beige fat tissues.

Effect of ractopamine and conjugated linoleic acid on performance of late finishing pigs

The dietary inclusion of feed additives to improve the carcass characteristics of the final product is of great importance for the pork production chain. The aim of our study was to evaluate the effects of the association of ractopamine (RAC) and conjugated linoleic acid (CLA) on the performance traits of finishing pigs during the last 26 days prior to slaughter. In total, 810 commercial hybrid barrows were used. Animals were distributed among treatments according to a randomised block design in a 3 × 3 factorial arrangement, with three RAC levels (0, 5 or 10 ppm) and three CLA levels (0, 0.3 or 0.6%). Pigs fed the diet with 5 ppm RAC had higher average daily feed intake (ADFI) (2.83 kg; P < 0.05) when compared with those fed 10 ppm RAC and the control diet (2.75 and 2.74 kg, respectively). Lower ADFI values (P < 0.01) were observed with the diets containing CLA compared with the control diet with no CLA (2.73 and 2.75 v. 2.85 kg/day, respectively). The average daily weight gain of pigs fed 5 and 10 ppm RAC was +148 and +173 g/dayhigher (P < 0.001), respectively, than those fed the control diet. Dietary RAC levels influenced (P < 0.001) feed conversion ratio (FCR), which was reduced as RAC levels increased, with the pigs fed 10, 5 and 0 ppm RAC presenting FCR values of 2.57, 2.71 and 3.05, respectively. FCR also improved (P < 0.05) with the inclusion of 0.6% CLA relative to the control diet (2.70 v. 2.84, respectively). There was a significant interaction between CLA × RAC levels (P < 0.01) for final BW, loin eye area (LEA) (P < 0.05) and backfat thickness (BT) (P < 0.05). The treatments containing 10 ppm RAC + 0.6% or 0.3% CLA increased LEA and reduced BT. In conclusion, the level of 10 ppm inclusion of RAC increased the overall performance parameters of pigs and therefore improved production efficiency. The combined use of RAC and CLA promoted a lower feed conversion ratio as well as better quantitative carcass traits, as demonstrated by the higher LEA and lower BT. The dietary inclusion of CLA at 0.3% improved feed efficiency, however, without affecting LEA or BT yields.

The glucocorticoid receptor in osteoprogenitors regulates bone mass and marrow fat

Excess fat within bone marrow is associated with lower bone density. Metabolic stressors such as chronic caloric restriction (CR) can exacerbate marrow adiposity, and increased glucocorticoid signaling and adrenergic signaling are implicated in this phenotype. The current study tested the role of glucocorticoid signaling in CR-induced stress by conditionally deleting the glucocorticoid receptor (GR) in bone marrow osteoprogenitors (Osx1-Cre) of mice subjected to CR and ad libitum diets. Conditional knockout of the GR (GR-CKO) reduced cortical and trabecular bone mass as compared to wildtype (WT) mice under both ad libitum and CR conditions. No interaction was detected between genotype and diet, suggesting that the GR is not required for CR-induced skeletal changes. The lower bone mass in GR-CKO mice, and the further suppression of bone by CR, resulted from suppressed bone formation. Interestingly, treatment with the -adrenergic receptor antagonist propranolol mildly but selectively improved metrics of cortical bone mass in GR-CKO mice during CR, suggesting interaction between adrenergic and glucocorticoid signaling pathways that affects cortical bone. GR-CKO mice dramatically increased marrow fat under both ad libitum and CR-fed conditions, and surprisingly propranolol treatment was unable to rescue CR-induced marrow fat in either WT or GR-CKO mice. Additionally, serum corticosterone levels were selectively elevated in GR-CKO mice with CR, suggesting the possibility of bone-hypothalamus-pituitary-adrenal crosstalk during metabolic stress. This work highlights the complexities of glucocorticoid and β-adrenergic signaling in stress-induced changes in bone mass, and the importance of GR function in suppressing marrow adipogenesis while maintaining healthy bone mass.

Increasing T-type calcium channel activity by β-adrenergic stimulation contributes to β-adrenergic regulation of heart rates

KEY POINTS

Cav3.1 T-type Ca2+ channel current (ICa-T ) contributes to heart rate genesis but is not known to contribute to heart rate regulation by the sympathetic/β-adrenergic system (SAS). We show that the loss of Cav3.1 makes the beating rates of the heart in vivo and perfused hearts ex vivo, as well as sinoatrial node cells, less sensitive to β-adrenergic stimulation; it also renders less conduction acceleration through the atrioventricular node by β-adrenergic stimulation. Increasing Cav3.1 in cardiomyocytes has the opposite effects. ICa-T in sinoatrial nodal cells can be upregulated by β-adrenergic stimulation. The results of the present study add a new contribution to heart rate regulation by the SAS system and provide potential new mechanisms for the dysregulation of heart rate and conduction by the SAS in the heart. T-type Ca2+ channel can be a target for heart disease treatments that aim to slow down the heart rate ABSTRACT: Cav3.1 (α1G ) T-type Ca2+ channel (TTCC) is expressed in mouse sinoatrial node cells (SANCs) and atrioventricular (AV) nodal cells and contributes to heart rate (HR) genesis and AV conduction. However, its role in HR regulation and AV conduction acceleration by the β-adrenergic system (SAS) is unclear. In the present study, L- (ICa-L ) and T-type (ICa-T ) Ca2+ currents were recorded in SANCs from Cav3.1 transgenic (TG) and knockout (KO), and control mice. ICa-T was absent in KO SANCs but enhanced in TG SANCs. In anaesthetized animals, different doses of isoproterenol (ISO) were infused via the jugular vein and the HR was recorded. The EC50 of the HR response to ISO was lower in TG mice but higher in KO mice, and the maximal percentage of HR increase by ISO was greater in TG mice but less in KO mice. In Langendorff-perfused hearts, ISO increased HR and shortened PR intervals to a greater extent in TG but to a less extent in KO hearts. KO SANCs had significantly slower spontaneous beating rates than control SANCs before and after ISO; TG SANCs had similar basal beating rates as control SANCs probably as a result of decreased ICa-L but a greater response to ISO than control SANCs. ICa-T in SANCs was significantly increased by ISO. ICa-T upregulation by β-adrenergic stimulation contributes to HR and conduction regulation by the SAS. TTCC can be a target for slowing the HR.

Propranolol produces short-term facilitation of extinction in a rabbit model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a learning-based anxiety disorder with significant public health challenges due to difficulties in treating the complex, multiple symptomology. We have developed an animal model of PTSD, based on Pavlovian eyeblink conditioning in rabbits, that addresses two key features: conditioned responses (CRs) to cues associated with an aversive event and a form of conditioned hyperarousal referred to as conditioning-specific reflex modification (CRM). We have found previously that unpaired extinction is ideal for reducing both CRs and CRM simultaneously and shows sensitivity to systemic serotonergic and glutamatergic manipulations. The following study aimed to extend our work to examine the role of the noradrenergic system, dysregulation of which is strongly implicated as part of the neurobiology of PTSD and which may also play a role in the balance shift from fear reconsolidation to extinction during treatment. The goal of the following two studies was to examine whether the β-adrenergic receptor antagonist propranolol combined with either a full or brief course of unpaired extinction treatment could enhance extinction of CRs and/or CRM. Results showed a within-session facilitation of propranolol on extinction of CRs, particularly during the first extinction session, and a short-term enhancement of extinction of CRM when extinction treatment was brief. However, neither benefit translated to long-term extinction retention for the majority of subjects. Findings suggest that propranolol may provide the most therapeutic benefit in situations of high arousal early in treatment, which may be more important for future patient compliance rather than long-term treatment outcomes.

Review on Pharmacogenetics and Pharmacogenomics Applied to the Study of Asthma

Nearly one-half of asthmatic patients do not respond to the most common therapies. Evidence suggests that genetic factors may be involved in the heterogeneity in therapeutic response and adverse events to asthma therapies. We focus on the three major classes of asthma medication: β-adrenergic receptor agonist, inhaled corticosteroids, and leukotriene modifiers. Pharmacogenetics and pharmacogenomics studies have identified several candidate genes associated with drug response.In this chapter, the main pharmacogenetic and pharmacogenomic studies in addition to the future perspectives in personalized medicine will be reviewed. The ideal treatment of asthma would be a tailored approach to health care in which adverse effects are minimized and the therapeutic benefit for an individual asthmatic is maximized leading to a more cost-effective care.

A meta-analysis of the effects of β-adrenergic blockers in chronic heart failure

Adrenergic β-blockers are drugs that bind to, but do not activate β-adrenergic receptors. Instead they block the actions of β-adrenergic agonists and are used for the treatment of various diseases such as cardiac arrhythmias, angina pectoris, myocardial infarction, hypertension, headache, migraines, stress, anxiety, prostate cancer, and heart failure. Several meta-analysis studies have shown that β-blockers improve the heart function and reduce the risks of cardiovascular events, rate of mortality, and sudden death through chronic heart failure (CHF) of patients. The present study identified results from recent meta-analyses of β-adrenergic blockers and their usefulness in CHF. Databases including Medline/Embase/Cochrane Central Register of Controlled Trials (CENTRAL), and PubMed were searched for the periods May, 1985 to March, 2011 and June, 2013 to August, 2015, and a number of studies identified. Results of those studies showed that use of β-blockers was associated with decreased sudden cardiac death in patients with heart failure. However, contradictory results have also been reported. The present meta-analysis aimed to determine the efficacy of β-blockers on mortality and morbidity in patients with heart failure. The results showed that mortality was significantly reduced by β-blocker treatment prior to the surgery of heart failure patients. The results from the meta-analysis studies showed that β-blocker treatment in heart failure patients correlated with a significant decrease in long-term mortality, even in patients that meet one or more exclusion criteria of the MERIT-HF study. In summary, the findings of the current meta-analysis revealed beneficial effects different β-blockers have on patients with heart failure or related heart disease.

Troponin I Mutations R146G and R21C Alter Cardiac Troponin Function, Contractile Properties, and Modulation by Protein Kinase A (PKA)-mediated Phosphorylation

Two hypertrophic cardiomyopathy-associated cardiac troponin I (cTnI) mutations, R146G and R21C, are located in different regions of cTnI, the inhibitory peptide and the cardiac-specific N terminus. We recently reported that these regions may interact when Ser-23/Ser-24 are phosphorylated, weakening the interaction of cTnI with cardiac TnC. Little is known about how these mutations influence the affinity of cardiac TnC for cTnI (KC-I) or contractile kinetics during β-adrenergic stimulation. Here, we tested how cTnI(R146G) or cTnI(R21C) influences contractile activation and relaxation and their response to protein kinase A (PKA). Both mutations significantly increased Ca(2+) binding affinity to cTn (KCa) and KC-I. PKA phosphorylation resulted in a similar reduction of KCa for all complexes, but KC-I was reduced only with cTnI(WT). cTnI(WT), cTnI(R146G), and cTnI(R21C) were complexed into cardiac troponin and exchanged into rat ventricular myofibrils, and contraction/relaxation kinetics were measured ± PKA phosphorylation. Maximal tension (Tmax) was maintained for cTnI(R146G)- and cTnI(R21C)-exchanged myofibrils, and Ca(2+) sensitivity of tension (pCa50) was increased. PKA phosphorylation decreased pCa50 for cTnI(WT)-exchanged myofibrils but not for either mutation. PKA phosphorylation accelerated the early slow phase relaxation for cTnI(WT) myofibrils, especially at Ca(2+) levels that the heart operates in vivo. Importantly, this effect was blunted for cTnI(R146G)- and cTnI(R21C)-exchanged myofibrils. Molecular dynamics simulations suggest both mutations inhibit formation of intra-subunit contacts between the N terminus and the inhibitory peptide of cTnI that is normally seen with WT-cTn upon PKA phosphorylation. Together, our results suggest that cTnI(R146G) and cTnI(R21C) blunt PKA modulation of activation and relaxation kinetics by prohibiting cardiac-specific N-terminal interaction with the cTnI inhibitory peptide.

β-adrenergic effects on cardiac myofilaments and contraction in an integrated rabbit ventricular myocyte model

A five-state model of myofilament contraction was integrated into a well-established rabbit ventricular myocyte model of ion channels, Ca(2+) transporters and kinase signaling to analyze the relative contribution of different phosphorylation targets to the overall mechanical response driven by β-adrenergic stimulation (β-AS). β-AS effect on sarcoplasmic reticulum Ca(2+) handling, Ca(2+), K(+) and Cl(-) currents, and Na(+)/K(+)-ATPase properties was included based on experimental data. The inotropic effect on the myofilaments was represented as reduced myofilament Ca(2+) sensitivity (XBCa) and titin stiffness, and increased cross-bridge (XB) cycling rate (XBcy). Assuming independent roles of XBCa and XBcy, the model reproduced experimental β-AS responses on action potentials and Ca(2+) transient amplitude and kinetics. It also replicated the behavior of force-Ca(2+), release-restretch, length-step, stiffness-frequency and force-velocity relationships, and increased force and shortening in isometric and isotonic twitch contractions. The β-AS effect was then switched off from individual targets to analyze their relative impact on contractility. Preventing β-AS effects on L-type Ca(2+) channels or phospholamban limited Ca(2+) transients and contractile responses in parallel, while blocking phospholemman and K(+) channel (IKs) effects enhanced Ca(2+) and inotropy. Removal of β-AS effects from XBCa enhanced contractile force while decreasing peak Ca(2+) (due to greater Ca(2+) buffering), but had less effect on shortening. Conversely, preventing β-AS effects on XBcy preserved Ca(2+) transient effects, but blunted inotropy (both isometric force and especially shortening). Removal of titin effects had little impact on contraction. Finally, exclusion of β-AS from XBCa and XBcy while preserving effects on other targets resulted in preserved peak isometric force response (with slower kinetics) but nearly abolished enhanced shortening. β-AS effects on XBCa and XBcy have greater impact on isometric and isotonic contraction, respectively.

Basal and β-adrenergic regulation of the cardiac calcium channel CaV1.2 requires phosphorylation of serine 1700

L-type calcium (Ca(2+)) currents conducted by voltage-gated Ca(2+) channel CaV1.2 initiate excitation-contraction coupling in cardiomyocytes. Upon activation of β-adrenergic receptors, phosphorylation of CaV1.2 channels by cAMP-dependent protein kinase (PKA) increases channel activity, thereby allowing more Ca(2+) entry into the cell, which leads to more forceful contraction. In vitro reconstitution studies and in vivo proteomics analysis have revealed that Ser-1700 is a key site of phosphorylation mediating this effect, but the functional role of this amino acid residue in regulation in vivo has remained uncertain. Here we have studied the regulation of calcium current and cell contraction of cardiomyocytes in vitro and cardiac function and homeostasis in vivo in a mouse line expressing the mutation Ser-1700-Ala in the CaV1.2 channel. We found that preventing phosphorylation at this site decreased the basal L-type CaV1.2 current in both neonatal and adult cardiomyocytes. In addition, the incremental increase elicited by isoproterenol was abolished in neonatal cardiomyocytes and was substantially reduced in young adult myocytes. In contrast, cellular contractility was only moderately reduced compared with wild type, suggesting a greater reserve of contractile function and/or recruitment of compensatory mechanisms. Mutant mice develop cardiac hypertrophy by the age of 3-4 mo, and maximal stress-induced exercise tolerance is reduced, indicating impaired physiological regulation in the fight-or-flight response. Our results demonstrate that phosphorylation at Ser-1700 alone is essential to maintain basal Ca(2+) current and regulation by β-adrenergic activation. As a consequence, blocking PKA phosphorylation at this site impairs cardiovascular physiology in vivo, leading to reduced exercise capacity in the fight-or-flight response and development of cardiac hypertrophy.

A computational investigation of cardiac caveolae as a source of persistent sodium current

Recent studies of cholesterol-rich membrane microdomains, called caveolae, reveal that caveolae are reservoirs of “recruitable” sodium ion channels. Caveolar channels constitute a substantial and previously unrecognized source of sodium current in cardiac cells. In this paper we model for the first time caveolar sodium currents and their contributions to cardiac action potential morphology. We show that the β-agonist-induced opening of caveolae may have substantial impacts on peak overshoot, maximum upstroke velocity, and ultimately conduction velocity. Additionally, we show that prolonged action potentials and the formation of potentially arrhythmogenic afterdepolarizations, can arise if caveolae open intermittently throughout the action potential. Our simulations suggest that caveolar sodium current may constitute a route, which is independent of channelopathies, to delayed repolarization and the arrhythmias associated with such delays.

A requirement for memory retrieval during and after long-term extinction learning

Current learning theories are based on the idea that learning is driven by the difference between expectations and experience (the delta rule). In extinction, one learns that certain expectations no longer apply. Here, we test the potential validity of the delta rule by manipulating memory retrieval (and thus expectations) during extinction learning. Adrenergic signaling is critical for the time-limited retrieval (but not acquisition or consolidation) of contextual fear. Using genetic and pharmacologic approaches to manipulate adrenergic signaling, we find that long-term extinction requires memory retrieval but not conditioned responding. Identical manipulations of the adrenergic system that do not affect memory retrieval do not alter extinction. The results provide substantial support for the delta rule of learning theory. In addition, the timing over which extinction is sensitive to adrenergic manipulation suggests a model whereby memory retrieval occurs during, and several hours after, extinction learning to consolidate long-term extinction memory.

Cyclic AMP-dependent protein kinase phosphorylates group III metabotropic glutamate receptors and inhibits their function as presynaptic receptors

Recent evidence suggests that the functions of presynaptic metabotropic glutamate receptors (mGluRs) are tightly regulated by protein kinases. We previously reported that cAMP-dependent protein kinase (PKA) directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail region of the receptor, and that phosphorylation of this site inhibits coupling of mGluR2 to GTP-binding proteins. This may be the mechanism by which the adenylyl cyclase activator forskolin inhibits presynaptic mGluR2 function at the medial perforant path-dentate gyrus synapse. We now report that PKA also directly phosphorylates several group III mGluRs (mGluR4a, mGluR7a, and mGluR8a), as well as mGluR3 at single conserved serine residues on their C-terminal tails. Furthermore, activation of PKA by forskolin inhibits group III mGluR-mediated responses at glutamatergic synapses in the hippocampus. Interestingly, beta-adrenergic receptor activation was found to mimic the inhibitory effect of forskolin on both group II and III mGluRs. These data suggest that a common PKA-dependent mechanism may be involved in regulating the function of multiple presynaptic group II and group III mGluRs. Such regulation is not limited to the pharmacological activation of adenylyl cyclase but can also be elicited by the stimulation of endogenous G(s)-coupled receptors, such as beta-adrenergic receptors.

Depressed cyclic AMP levels in airways smooth muscle from asthmatic dogs

We tested mongrel dogs by intradermal injection and tracheobronchial aerosol challenge with Ascaris suum antigen extract. All dogs were skin-test positive but could be segregated, on the basis of airways resistance measurements, into "asthmatic" (bronchoreactive) and "nonasthmatic" (nonbronchoreactive) groups. By using tracheal rings from these dogs, we measured the abilities of the beta-adrenergic agonist, isoproterenol, to relax tracheal smooth muscle contracted by methacholine and to cause cyclic AMP (cAMP) accumulation in the presence and absence of methacholine. The magnitude of relaxation induced by any concentration of isoproterenol was always less in the smooth muscle from "asthmatic" dogs. In the same tissues the concentrations of cAMP after in vitro equilibration, but prior to isoproterenol addition, were significantly less in the "asthmatic" than "nonasthmatic" samples. The accumulation of cAMP due to isoproterenol was similar in both groups for every dose of isoproterenol so that the initial difference between groups in cAMP concentration was maintained in an additive fashion over the entire dose-response curve. Total protein content of trachealis muscles from both groups of dogs was the same. We conclude that beta-adrenergically sensitive adenylate cyclase is not impaired in tracheal smooth muscle from "asthmatic" dogs; rather, the basal concentration of cAMP is depressed in "asthmatic" airway smooth muscle, and this difference is maintained throughout the isoproterenol dose-response curve. The depressed intracellular cAMP concentrations may be related to the decreased relaxation induced by isoproterenol in the "asthmatic" tracheal smooth muscle.