Β-adrenergic Signaling Research Articles

P57Kip2 regulates embryonic blood stem cells by controlling sympathoadrenal progenitor expansion

AbstractHematopoietic stem cells (HSCs) are of major clinical importance, and finding methods for their in vitro generation is a prime research focus. We show here that the cell cycle inhibitor p57Kip2/Cdkn1c limits the number of emerging HSCs by restricting the size of the sympathetic nervous system (SNS) and the amount of HSC-supportive catecholamines secreted by these cells. This regulation occurs at the SNS progenitor level and is in contrast to the cell-intrinsic function of p57Kip2 in maintaining adult HSCs, highlighting profound differences in cell cycle requirements of adult HSCs compared with their embryonic counterparts. Furthermore, this effect is specific to the aorta-gonad-mesonephros (AGM) region and shows that the AGM is the main contributor to early fetal liver colonization, as early fetal liver HSC numbers are equally affected. Using a range of antagonists in vivo, we show a requirement for intact β2-adrenergic signaling for SNS-dependent HSC expansion. To gain further molecular insights, we have generated a single-cell RNA-sequencing data set of all Ngfr+ sympathoadrenal cells around the dorsal aorta to dissect their differentiation pathway. Importantly, this not only defined the relevant p57Kip2-expressing SNS progenitor stage but also revealed that some neural crest cells, upon arrival at the aorta, are able to take an alternative differentiation pathway, giving rise to a subset of ventrally restricted mesenchymal cells that express important HSC-supportive factors. Neural crest cells thus appear to contribute to the AGM HSC niche via 2 different mechanisms: SNS-mediated catecholamine secretion and HSC-supportive mesenchymal cell production.

Pan-Cancer Characterization of Intratumoral Autonomic Innervation in 32 Cancer Types in the Cancer Genome Atlas.

Simple SummaryThere have been growing interests in the roles of intratumoral innervation of the autonomic nervous system (ANS) as a mechanism linking psychosocial stress, β-adrenergic signaling pathways, and poor cancer outcomes, and a potential target for therapeutic purpose. Our current knowledge is being limited by the few cancer types where intratumoral ANS have been studied; it remains to be determined the extent of this mechanism existing in different cancer types. Our study provided the first pan-cancer characterization of intratumoral innervation across 32 cancer types, and further, their relationships with tumor histopathological and molecular characteristics and survival outcomes. We found wide variations in intratumoral ANS expression both within and across cancer types. The association of ANS signatures with tumor histopathological characteristics and survival outcomes also varied by cancer type. Our findings suggest that the potential benefits of cancer therapies targeting β-adrenergic receptor-mediated stress signaling pathways are likely dependent on cancer type.Over the past two decades, multiple studies have demonstrated the important role that the autonomic nervous system (ANS) plays in tumorigenesis and cancer progression. However, the mechanisms by which this process occurs have only recently begun to be elucidated. Further, the extent of autonomic innervation in various cancer types and its effects on tumor molecular, immunological, and histopathological features, as well as on patient outcomes, are not yet fully characterized. In this study, we analyzed intratumoral ANS gene expression signatures, including overall intratumoral neuron growth and sympathetic and parasympathetic markers, across 32 cancer types using tumor transcriptomic and clinical annotation data available from The Cancer Genome Atlas (TCGA). Our analysis revealed wide variations in intratumoral ANS expression both within and across cancer types. The association of ANS signatures with tumor histopathological characteristics and survival outcomes also varied by cancer type. We found intratumoral ANS expression to be commonly correlated with angiogenesis, TGF-β signaling, and immunosuppression in the tumor microenvironment of many cancer types, which provide mechanistic insights into the involvement of intratumoral innervation in cancer development and progression. Our findings suggest that the potential benefits of cancer therapies targeting β-adrenergic receptor-mediated stress signaling pathways are likely dependent on cancer type.

Antihypertensive Drugs and Cancer Risk.

Hypertension is the most prevalent comorbidity in cancer patients. Consequently, many cancer patients are prescribed antihypertensive drugs before cancer diagnosis or during cancer treatment. However, whether antihypertensive drugs affect the incidence, treatment efficacy, or prognosis of cancer remains unanswered. For instance, renin-angiotensin and β-adrenergic signaling may be involved not only in blood pressure elevation but also in cell proliferation, angiogenesis, and tissue invasion. Therefore, the inhibition of these pathways may have beneficial effects on cancer prevention or treatment. In this article, we reviewed several studies regarding antihypertensive drugs and cancer. In particular, we focused on the results of clinical trials to evaluate whether the use of antihypertensive drugs affects future cancer risk and prognosis. Unfortunately, the results are somewhat inconsistent, and evidence demonstrating the effect of antihypertensive drugs remains limited. We indicate that the heterogeneity in the study designs makes it difficult to clarify the causal relationship between antihypertensive drugs and cancer. We also propose that additional experimental studies, including research with induced pluripotent cells derived from cancer patients, single-cell analyses of cancer cell clusters, and clinical studies using artificial intelligence electronic health record systems, might be helpful to reveal the precise association between antihypertensive drugs and cancer risk.

β2-Adrenoceptors in the Medial Prefrontal Cortex Excitatory Neurons Regulate Anxiety-like Behavior in Mice.

The medial prefrontal cortex (mPFC) and β-adrenoceptors (βARs) have been implicated in modulating anxiety-like behavior. However, the specific contributions of the β2-AR subtype in mPFC in anxiety are still unclear. To address this issue, we used optogenetic and microRNA-based (miRNA) silencing to dissect the role of β2-AR in mPFC in anxiety-like behavior. On the one hand, we use a chimeric rhodopsin/β2-AR (Opto-β2-AR) with in vivo optogenetic techniques to selectively activate β2-adrenergic signaling in excitatory neurons of the mPFC. We found that opto-activation of β2-AR is sufficient to induce anxiety-like behavior and reduce social interaction. On the other hand, we utilize the miRNA silencing technique to specifically knock down the β2-AR in mPFC excitatory neurons. We found that the β2-AR knock down induces anxiolytic-like behavior and promotes social interaction compared to the control group. These data suggest that β2-AR signaling in the mPFC has a critical role in anxiety-like states. These findings suggest that inhibiting of β2-AR signaling in the mPFC may be an effective treatment of anxiety disorders.

β-adrenergic signaling modulates the development and activity of erythroid suppressor cells

Abstract The neuroimmune response to stress has been linked to enhanced tumor proliferation, angiogenesis, and metastasis. In particular, signaling through β-adrenergic receptors has been demonstrated to promote a highly immunosuppressive tumor microenvironment by favoring the development and recruitment of suppressive immune cells, including regulatory T cells and myeloid-derived suppressive cells, that inhibit the cytotoxic CD8+ T cell response and enhance tumor progression. Interestingly, the complexity and scope of cancer immunosuppression extends beyond classical immune cells. Immunosuppressive erythroid progenitors, deemed erythroid suppressor cells (ESCs), have been shown to be prominent players in suppressing CD8+ T cell-mediated tumor responses; however, little is known about how these cells develop and are regulated. Thus, we examined if β-adrenergic signaling affects the development of ESCs and their role in tumor immunity. In this study, we show that β-adrenergic signaling promotes the accumulation of CD71+ Ter119+ erythroid progenitors consistent with the phenotypic description of ESCs in tumor-bearing mice. In characterizing the phenotype of these cells by flow cytometry and single-cell RNA sequencing, we elucidate unique features that endow these cells with immunosuppressive activity and illustrate their sensitivity to regulation by the sympathetic nervous system. We further demonstrate these erythroid precursors exhibit suppressive effects on the CD8 response. These findings elucidate a previously unappreciated axis of neuroimmune coordination in cancer and are pivotal to understanding crucial elements of tumor progression that will help shape the course of targeted immunotherapies in the future.

Sympathetic nervous system-mediated β-adrenergic signaling maintains the pool of mature natural killer cells

Abstract The sympathetic nervous system (SNS) innervates all lymphoid tissues and is, thus, well-positioned to influence the activity of immune cells. Accordingly, the coordinated responses of the SNS and immune system are crucial for immune responses to infection and cancer. Natural killer (NK) cells are essential for the immune response to certain viral infections and cancers in which they kill infected or cancerous cells and produce cytokines and chemokines to instruct the activity of other cells contributing to the response. We find here that steady state SNS signaling maintains the balance of mature and immature NK cells in the spleen and bone marrow. Using 6-hydroxydopamine to ablate sympathetic nerve terminals, we show by single cell RNA-sequencing and flow cytometry that SNS-ablated mice have decreased frequencies of mature CD27− CD11b+ NK cells and increased frequencies of immature CD27+ CD11b− and intermediate CD27+ CD11b+ NK cells. An increased ratio of immature NK cells to mature NK cells has been linked to decreased anti-tumor immunity. Along these lines, SNS-ablation also increases the growth rate of RMA-S tumors, the rejection of which depends on NK cells. This accelerated tumor growth was dependent on the absence of both α- and β-adrenergic signaling. Culture of early NK cell precursors in the presence of selective α- or β-adrenergic agonists suggests that signaling through β-adrenergic receptors promotes the maturation of NK cells. These data suggest that signaling through β-adrenergic receptors mediated by norepinephrine produced by the SNS maintains the pool of mature NK cells, allowing them to effectively survey peripheral tissues and participate in anti-tumor immunity.

PO-676-06 THE ROLE OF REPERFUSION IN MODULATING CYCLIC AMP SIGNALING FOLLOWING MYOCARDIAL INFARCTION

Reperfusion therapy is the standard of care for myocardial infarction (MI) patients, but it is not universally performed and is sometimes unsuccessful. Reperfusion impacts sympathetic innervation of the heart, with reperfused MIs remaining chronically hypo-innervated, whereas hyper-innervation is more common in non-reperfused MIs. We hypothesize that reperfusion status and resulting sympathetic innervation will differentially impact myocardial β-adrenergic signaling and electrophysiological responses.

Stress in Metastatic Breast Cancer: To the Bone and Beyond.

Simple SummaryBreast cancer is the most common cancer affecting women of all ages worldwide. In spite of the encouraging advances made in early diagnosis, 10% of breast cancer patients are still affected with metastatic breast cancer at the time of diagnosis. The available therapeutic options are predominantly palliative, and thus this unfavourable prognosis is associated with a low survival rate. Intriguingly, stress has been shown to promote the growth of breast tumours and the incidence of metastasis. Herein, we describe the contribution of the sympathetic hyperactivation induced by stress to the progression of breast cancer and its dissemination to distant organs, specifically to the bone, but also to the lung, liver and brain. The putative sympathetic adrenergic signalling mechanisms responsible for this modulation are also summarised. The knowledge gathered highlights the therapeutic potential of targeting sympathetic signalling to tackle cancer progression and metastasis.Breast cancer (BRCA) remains as one the most prevalent cancers diagnosed in industrialised countries. Although the overall survival rate is high, the dissemination of BRCA cells to distant organs correlates with a significantly poor prognosis. This is due to the fact that there are no efficient therapeutic strategies designed to overcome the progression of the metastasis. Over the past decade, critical associations between stress and the prevalence of BRCA metastases were uncovered. Chronic stress and the concomitant sympathetic hyperactivation have been shown to accelerate the progression of the disease and the metastases incidence, specifically to the bone. In this review, we provide a summary of the sympathetic profile on BRCA. Additionally, the current knowledge regarding the sympathetic hyperactivity, and the underlying adrenergic signalling pathways, involved on the development of BRCA metastasis to distant organs (i.e., bone, lung, liver and brain) will be revealed. Since bone is a preferential target site for BRCA metastases, greater emphasis will be given to the contribution of α2- and β-adrenergic signalling in BRCA bone tropism and the occurrence of osteolytic lesions.

Title: β3 Adrenergic Receptor Signaling in the Human Myometrium

Preterm labor leading to preterm birth is the leading cause of infant morbidity and mortality. Although β2 adrenergic agonists fail to provide adequate tocolysis, the expression of the β3 adrenergic receptor in myometrium and its unique signaling suggest a role for β3 agonist in the management of preterm labor. Western blot analysis showed that the β3 adrenergic receptor expression increased in human pregnancy myometrium compared to nonpregnant tissues (p < 0.0001). There was no difference in β3 adrenergic receptor expression throughout pregnancy (p > 0.05). The addition of the β3 agonist mirabegron in the tissue bath relaxed oxytocin contracted myometrium with an EC50 of 41.5 µM. Relaxation was partially blocked by the addition of the eNOS blocker Nω-nitro-L-arginine, or the large conductance potassium channel blocker paxilline. Combination of Nω-nitro-L-arginine and paxilline prevented mirabegron-mediated relaxation. Imaging revealed that the β3 adrenergic receptors are expressed by both myocyte and microvascular endothelial cells isolated from human myometrium. Nitric oxide production measured by 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate revealed that mirabegron stimulated nitric oxide production in myometrial endothelial cells. These data suggest that both endothelial and smooth muscle cells contribute to relaxation through disparate signaling pathways. Repurposing of approved medications tested in human myometrium as uterine tocolytics can advance prevention of preterm birth. These data argue that further examination of β3 adrenergic receptor signaling in myometrium may reveal mirabegron as a useful tocolytic in combination tocolysis regimens.

MicroRNA miR-133a-3p Facilitates Adrenergic Proarrhythmic Ectopy in Rat Pulmonary Vein Myocardium by Increasing cAMP Content.

Cardiac-specific microRNA miR-133a-3p modulates adrenergic signaling. Adrenergic receptors and their intracellular pathways are the key players in proarrhythmic ectopy derived from the myocardial sleeves of the pulmonary veins. We studied the effect of miR-133a-3p on ectopy induced by norepinephrine in myocardial tissue of rat pulmonary veins. Using microelectrode technique, we revealed facilitation of proarrhythmic pattern of spontaneous bursts of action potentials induced by norepinephrine in tissue preparations of the pulmonary veins isolated from rats in 24 h after injection of a transfection mixture containing miR-133a-3p (1 mg/kg) in vivo. According to ELISA data, the cAMP level in the pulmonary vein myocardium of rats receiving miR-133a-3p was 2-fold higher than in control animals. Bioinformatic analysis showed that mRNA of protein phosphatases and some phosphodiesterases are most probable targets of miR-133a-3p. The proarrhythmic effect of miR-133a-3p can be related to inhibition of the expression of phosphodiesterases accompanied by cAMP accumulation and increased intracellular β-adrenergic signaling.

REEP6 knockout leads to defective β-adrenergic signaling in adipocytes and promotes obesity-related metabolic dysfunction.

IntroductionThe mobilization and catabolism of lipid energy is a central function of adipocytes that is under the control of the β-adrenergic signaling pathway, and defects in β-adrenergic signaling in adipocytes have been linked to obesity and obesity-related metabolic diseases. Receptor expression-enhancing proteins (REEPs) are endoplasmic reticulum (ER) proteins that play critical roles in subcellular targeting of receptor signaling complexes. Examination of gene expression profiles indicates that, among REEPs expressed in adipocytes, REEP6 expression is uniquely upregulated by sympathetic nervous system activation, suggesting involvement in regulating adrenergic signal transduction. ObjectiveThe aim of this study was to assess the contribution of REEP6 to the thermogenic activation of adipocytes and characterize the metabolic consequences of REEP6 deficiency in vivo. MethodsExpression levels of Reep6 in adipose tissue were examined by using public transcriptomic data and validated by Western blot and qPCR analyses. Adipocyte-specific regulatory roles of REEP6 were investigated in vitro in C3H10T1/2 adipocytes and in primary adipocytes obtained from REEP6 KO mice. Effects of in vivo REEP6 deficiency on energy expenditure were measured by indirect calorimetry. Mitochondrial content in adipose tissue was accessed by immunoblot, mitochondrial DNA analysis, and confocal and electron microscopy. Effects of REEP6 KO on obesity-induced metabolic dysfunction were tested in a high-fat diet-induced obesity mouse model by glucose tolerance test, Western blot, and histological analyses. ResultsREEP6 expression is highly enriched in murine adipocytes and is sharply upregulated upon adipocyte differentiation and by cold exposure. Inactivation of REEP6 in mice increased adiposity, and reduced energy expenditure and cold tolerance. REEP6 KO severely reduced protein kinase A-mediated signaling in BAT and greatly reduced mitochondrial mass. The effect of REEP6 inactivation on diminished β-adrenergic signaling was reproduced in cultured adipocytes, indicating that this effect is cell-autonomous. REEP6 KO also suppressed expression of adenylate cyclase 3 (Adcy3) in brown adipose tissue and knockdown of REEP6 in adipocytes reduced targeting of ADCY3 to the plasma membrane. Lastly, REEP6 KO exacerbated high-fat diet-induced insulin resistance and inflammation in adipose tissue. ConclusionsThis study indicates that REEP6 plays an important role in β-adrenergic signal transduction in adipocytes involving the expression and trafficking of Adcy3. Genetic inactivation of REEP6 reduces energy expenditure, increases adiposity, and the susceptibility to obesity-related metabolic dysfunction.

β-Adrenergic Signaling Regulates a Concerted Thermogenic Response in Brown Adipose Tissue and White Adipose Tissue

Brown adipose tissue (BAT) is important for thermoregulation via uncoupling of oxidative phosphorylation. In addition to BAT-driven thermogenesis, mammals use induced browning of subcutaneous white adipose tissue (scWAT) as a mechanism to cope with chronic cold stress. Under acute cold stress in mammals, JMJD1A, a histone H3 lysine 9 (H3K9) demethylase, upregulates thermogenic genes via β-adrenergic signaling in BAT through higher-order chromatin structural changes that occur independent of histone demethylase activity. Following exposure to chronic cold stress, scWAT browning occurs via a two-step process that requires both β-adrenergic-dependent phosphorylation of S265 and JMJD1A-induced demethylation of H3K9me2.

Pathophysiology of Takotsubo syndrome-a joint scientific statement from the Heart Failure Association Takotsubo Syndrome Study Group and Myocardial Function Working Group of the European Society of Cardiology-Part 1: overview and the central role for catecholamines and sympathetic nervous system.

This is the first part of a scientific statement from the Heart Failure Association (HFA) of the European Society of Cardiology focused upon the pathophysiology of Takotsubo syndrome and is complimentary to the previous HFA position statement on Takotsubo syndrome which focused upon clinical management. In part 1 we provide an overview of the pathophysiology of Takotsubo syndrome and fundamental questions to consider. We then review and discuss the central role of catecholamines and the sympathetic nervous system in the pathophysiology, and the direct effects of high surges in catecholamines upon myocardial biology including β-adrenergic receptor signalling, G-protein coupled receptor kinases, cardiomyocyte calcium physiology, myofilament physiology, cardiomyocyte gene expression, myocardial electrophysiology and arrhythmogenicity, myocardial inflammation, metabolism and energetics. The integrated effects upon ventricular haemodynamics are discussed and integrated into the pathophysiological model.

β-Adrenergic signaling in skin cancer.

Activation of the sympathetic nervous system releases catecholamines that can interact with β‐adrenergic receptors on tumor cells. Preclinical models have shown that the signaling processes initiated by activation of β‐adrenergic receptors increase tumorigenesis, stimulate cell proliferation, and inhibit apoptosis. Indeed, preclinical studies have also shown that β‐adrenergic blockade can decrease tumor burden. Researchers have been studying the effects of β‐adrenergic receptor blockers on tumor cells and how they may slow the progression of melanoma, basal cell carcinoma, and squamous cell carcinoma. Moreover, clinical data have shown improved prognosis in patients with skin cancer who take β‐blockers. This review discusses the mechanisms of β‐adrenergic signaling in cancer and immune cells, details preclinical models of sympathetic blockade, and considers clinical evidence of the effects of β‐adrenergic blockade in skin cancers.

Nanoscale Organization, Regulation, and Dynamic Reorganization of Cardiac Calcium Channels.

The architectural specializations and targeted delivery pathways of cardiomyocytes ensure that L-type Ca2+ channels (CaV1.2) are concentrated on the t-tubule sarcolemma within nanometers of their intracellular partners the type 2 ryanodine receptors (RyR2) which cluster on the junctional sarcoplasmic reticulum (jSR). The organization and distribution of these two groups of cardiac calcium channel clusters critically underlies the uniform contraction of the myocardium. Ca2+ signaling between these two sets of adjacent clusters produces Ca2+ sparks that in health, cannot escalate into Ca2+ waves because there is sufficient separation of adjacent clusters so that the release of Ca2+ from one RyR2 cluster or supercluster, cannot activate and sustain the release of Ca2+ from neighboring clusters. Instead, thousands of these Ca2+ release units (CRUs) generate near simultaneous Ca2+ sparks across every cardiomyocyte during the action potential when calcium induced calcium release from RyR2 is stimulated by depolarization induced Ca2+ influx through voltage dependent CaV1.2 channel clusters. These sparks summate to generate a global Ca2+ transient that activates the myofilaments and thus the electrical signal of the action potential is transduced into a functional output, myocardial contraction. To generate more, or less contractile force to match the hemodynamic and metabolic demands of the body, the heart responds to β-adrenergic signaling by altering activity of calcium channels to tune excitation-contraction coupling accordingly. Recent accumulating evidence suggests that this tuning process also involves altered expression, and dynamic reorganization of CaV1.2 and RyR2 channels on their respective membranes to control the amplitude of Ca2+ entry, SR Ca2+ release and myocardial function. In heart failure and aging, altered distribution and reorganization of these key Ca2+ signaling proteins occurs alongside architectural remodeling and is thought to contribute to impaired contractile function. In the present review we discuss these latest developments, their implications, and future questions to be addressed.

Clenbuterol, a Selective β2-Adrenergic Receptor Agonist, Inhibits or Limits Post-Stroke Pneumonia, but Increases Infarct Volume in MCAO Mice.

BackgroundLarge ischemic stroke provokes an inflammatory response, promoting the release of norepinephrine (NE) by intensifying the sympathetic nervous system. This augmented sympathetic outflow was deemed to act on β2-adrenergic receptors (β2-ARs) expressed by immune cells, rendering organisms to post-stroke infections, like pneumonia. To clarify this issue, we introduced selective β2-ARs agonist clenbuterol (CLEN) to stroke mice to investigate how β2-adrenergic signaling augmentation after stroke affects immune response and post-stroke outcomes, including central and peripheral.MethodsWe developed a middle cerebral artery occlusion (MCAO) stroke model in mice to induce large ischemic stroke and administered CLEN 24 h after the onset of MCAO stroke. First, we assessed infarct volume and NE levels in plasma and spleen 3ds later. Next, the immune state was identified by analyzing the spleen index, immune cell populations, and immune cytokines. Finally, peripheral outcomes were assessed by measuring signs of pneumonia, such as pathology, bacterial burden, and lung cytokines.ResultsWe report that CLEN treatment 24 h after MCAO stroke causes an enlarged infarct volume and a decrease in NE levels at 3ds after stroke. Consistent with a reduction of total T cells, T helper cells, and increase of cytotoxic T cells, the immune milieu after CLEN treatment presents an anti-inflammatory landscape, showing raised expression of anti-inflammatory cytokines: IL-4, IL-10, and TGF-β1, and decreased expression of pro-inflammatory cytokines, such as TNF-α, IL-1β, and IFN-γ, with a dramatically reduced percentage of Gr-1+ neutrophils and B cells but an increased percentage of NK cells. In our study, CLEN treatment results in no higher risk of pneumonia but relieves bacterial burden, inhibits or limits pneumonia, and diminishes TNF-α expression in lung tissues after MCAO.ConclusionWe identified increased β2-adrenergic signaling after MCAO stroke, inhibits or limits post-stroke pneumonia but enlarges stroke volume in MCAO mice. Thus, careful consideration must be taken to improve post-stroke outcomes by manipulation over β2-adrenergic receptors.

Adopting a Phosphoproteomics Approach to Investigate a Role for Protein Phosphatase 2A (PP2A) Regulatory Subunit B55α in Cardiac β-Adrenergic Receptor Signalling

Protein kinases are recognised as integral regulators of phosphorylation within beta-adrenergic receptor (β-AR) signalling pathways in the heart. In contrast, the role of protein phosphatases and dephosphorylation in β-AR pathways is less well defined. In vitro evidence suggests that protein phosphatase 2A (PP2A) regulatory subunit B55α may modulate gene transcription as a key downstream effector of β-AR signalling in cardiomyocytes. The goal of this study was to determine how loss of PP2A-B55α impacts β-AR signalling in cardiomyocytes.

Beat-to-beat dynamic regulation of intracellular pH in cardiomyocytes

SummaryThe mammalian heart beats incessantly with rhythmic mechanical activities generating acids that need to be buffered to maintain a stable intracellular pH (pHi) for normal cardiac function. Even though spatial pHi non-uniformity in cardiomyocytes has been documented, it remains unknown how pHi is regulated to match the dynamic cardiac contractions. Here, we demonstrated beat-to-beat intracellular acidification, termed pHi transients, in synchrony with cardiomyocyte contractions. The pHi transients are regulated by pacing rate, Cl−/HCO3- transporters, pHi buffering capacity, and β-adrenergic signaling. Mitochondrial electron-transport chain inhibition attenuates the pHi transients, implicating mitochondrial activity in sculpting the pHi regulation. The pHi transients provide dynamic alterations of H+ transport required for ATP synthesis, and a decrease in pHi may serve as a negative feedback to cardiac contractions. Current findings dovetail with the prevailing three known dynamic systems, namely electrical, Ca2+, and mechanical systems, and may reveal broader features of pHi handling in excitable cells.

Exercise increases phosphorylation of the putative mTORC2 activity readout NDRG1 in human skeletal muscle.

In mice, exercise is suggested to activate the mechanistic target of rapamycin complex 2 (mTORC2) in skeletal muscle, and mTORC2 is required for normal muscle glucose uptake during exercise. Whether this translates to human skeletal muscle and what signaling pathways facilitate the exercise-induced mTORC2 activation is unknown. We herein tested the hypothesis that exercise increases mTORC2 activity in human skeletal muscle and investigated if β2-adrenergic receptor (AR) activation mediates exercise-induced mTORC2 activation. We examined several mTORC2 activity readouts (p-NDRG1 Thr346, p-Akt Ser473, p-mTOR S2481, and p-Akt Thr450) in human skeletal muscle biopsies after uphill walking or cycling exercise. In mouse muscles, we assessed mTORC2 activity readouts following acute activation of muscle β2-adrenergic or GS signaling and during in vivo and ex vivo muscle contractions. Exercise increased phosphorylation of NDRG1 Thr346 in human soleus, gastrocnemius, and vastus lateralis muscle, without changing p-Akt Ser473, p-Akt Thr450, and p-mTOR Ser2481. In mouse muscle, stimulation of β2-adrenergic or GS signaling and ex vivo contractions failed to increase p-NDRG1 Thr346, whereas in vivo contractions were sufficient to induce p-NDRG1 Thr346. In conclusion, the mTORC2 activity readout p-NDRG1 Thr346 is a novel exercise-responsive signaling protein in human skeletal muscle. Notably, contraction-induced p-NDRG1 Thr346 appears to require a systemic factor. Unlike exercise, and in contrast to published data obtained in cultured muscles cells, stimulation of β2-adrenergic signaling is not sufficient to trigger NDRG1 phosphorylation in mature mouse skeletal muscle.NEW & NOTEWORTHY The mTORC2 readout p-NDRG Thr346 is a novel exercise-responsive protein in human skeletal muscle. β2-AR and GS signaling are not sufficient to induce mTORC2 signaling in adult muscle. In vivo, but not ex vivo, contraction induced p-NDRG Thr346, which indicates requirement of a systemic factor for exercise-induced mTORC2 activation.

Dominant Role of PI3K p110α over p110β in Insulin and β-Adrenergic Receptor Signalling.

Attribution of specific roles to the two ubiquitously expressed PI 3-kinase (PI3K) isoforms p110α and p110β in biological functions they have been implicated, such as in insulin signalling, has been challenging. While p110α has been demonstrated to be the principal isoform activated downstream of the insulin receptor, several studies have provided evidence for a role of p110β. Here we have used isoform-selective inhibitors to estimate the relative contribution of each of these isoforms in insulin signalling in adipocytes, which are a cell type with essential roles in regulation of metabolism at the systemic level. Consistent with previous genetic and pharmacological studies, we found that p110α is the principal isoform activated downstream of the insulin receptor under physiological conditions. p110α interaction with Ras enhanced the strength of p110α activation by insulin. However, this interaction did not account for the selectivity for p110α over p110β in insulin signalling. We also demonstrate that p110α is the principal isoform activated downstream of the β-adrenergic receptor (β-AR), another important signalling pathway in metabolic regulation, through a mechanism involving activation of the cAMP effector molecule EPAC1. This study offers further insights in the role of PI3K isoforms in the regulation of energy metabolism with implications for the therapeutic application of selective inhibitors of these isoforms.