Regulation of membrane-bound PKC in adult cardiac ventricular myocytes

Understanding the impact of nuclear-localized GPCRs on cellular signalling

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

High-Field Asymmetric Waveform Ion Mobility Spectrometry: Practical Alternative for Cardiac Proteome Sample Processing

Heart tissue sample preparation for mass spectrometry (MS) analysis that includes prefractionation reduces the cellular protein dynamic range and increases the relative abundance of nonsarcomeric proteins. We previously described "IN-Sequence" (IN-Seq) where heart tissue lysate is sequentially partitioned into three subcellular fractions to increase the proteome coverage more than a single direct tissue analysis by mass spectrometry. Here, we report an adaptation of the high-field asymmetric ion mobility spectrometry (FAIMS) coupled to mass spectrometry, and the establishment of a simple one step sample preparation coupled with gas-phase fractionation. The FAIMS approach substantially reduces manual sample handling, significantly shortens the MS instrument processing time, and produces unique protein identification and quantification approximating the commonly used IN-Seq method in less time.

Regulation of cardiac nitric oxide signaling by nuclear β-adrenergic and endothelin receptors

At the cell surface, βARs and endothelin receptors can regulate nitric oxide (NO) production. β-adrenergic receptors (βARs) and type B endothelin receptors (ETB) are present in cardiac nuclear membranes and regulate transcription. The present study investigated the role of the NO pathway in the regulation of gene transcription by these nuclear G protein-coupled receptors. Nitric oxide production and transcription initiation were measured in nuclei isolated from the adult rat heart. The cell-permeable fluorescent dye 4,5-diaminofluorescein diacetate (DAF2 DA) was used to provide a direct assessment of nitric oxide release. Both isoproterenol and endothelin increased NO production in isolated nuclei. Furthermore, a β3AR-selective agonist, BRL 37344, increased NO synthesis whereas the β1AR-selective agonist xamoterol did not. Isoproterenol increased, whereas ET-1 reduced, de novo transcription. The NO synthase inhibitor l-NAME prevented isoproterenol from increasing either NO production or de novo transcription. l-NAME also blocked ET-1-induced NO-production but did not alter the suppression of transcription initiation by ET-1. Inhibition of the cGMP-dependent protein kinase (PKG) using KT5823 also blocked the ability of isoproterenol to increase transcription initiation. Furthermore, immunoblotting revealed eNOS, but not nNOS, in isolated nuclei. Finally, caged, cell-permeable isoproterenol and endothelin-1 analogs were used to selectively activate intracellular β-adrenergic and endothelin receptors in intact adult cardiomyocytes. Intracellular release of caged ET-1 or isoproterenol analogs increased NO production in intact adult cardiomyocytes. Hence, activation of the NO synthase/guanylyl cyclase/PKG pathway is necessary for nuclear β3ARs to increase de novo transcription. Furthermore, we have demonstrated the potential utility of caged receptor ligands in selectively modulating signaling via endogenous intracellular G protein-coupled receptors.

Signaling pathways regulating murine cardiac CREB phosphorylation

Using the mouse Langendorff heart perfusion model, the signaling pathways that regulate cardiac CREB-S133 phosphorylation have been defined. In mouse hearts stimulated with isoproterenol (ISO) (10(-8) M), endothelin-1 (ET-1) (10(-8) M), and phorbol 12-myristate 13-acetate (TPA) (10(-7) M), CREB-S133 phosphorylation was attained only by TPA-treatment. Activation of protein kinase A (PKA) was achieved by ISO. ISO- and ET-1-stimulation activated Ca2+/calmodulin-dependent kinase II (CaMKII). Protein kinase C (PKC) and p90(RSK) were activated with all three stimuli. Inhibition of ERK1/2 with PD98059 (10(-5) M) completely inhibited the activation of p90(RSK), but did not block CREB-S133 phosphorylation in TPA-perfused heart, indicating that PKA, CaMKII, and p90(RSK) do not phosphorylate CREB-S133 in the murine heart. PKC activation is signal specific. Analyses of PKC isoforms suggest that CREB phosphorylation is mediated by PKC epsilon translocating into nucleus only with TPA stimulation. These results, unlike those reported in other tissues, demonstrate that cardiac CREB is not a multi-signal target.

Isolation and characterization of subcellular protein fractions from mouse heart

In this study, we report different protocols used to obtain highly enriched and well-characterized protein fractions that could be used to determine the subcellular localization of proteins. Different protein fractions (total, cytosolic, total membrane, sarcolemmal, and nuclear) were isolated from mouse heart by a combination of either polytron homogenization or liquid nitrogen pulverization followed by density gradient centrifugation. Triton X-100 was used in specific fractions to help in the solubilization of proteins obtained with fractionation protocols. Following the isolation, enzymatic assays and Western blot analysis were used to evaluate the enrichment and/or cross-contamination of these protein fractions. Glucose-6-phosphate dehydrogenase, Na+/K+-ATPase, mitochondrial Ca2+-ATPase, sarco-endoplasmic reticulum Ca2+-ATPase, glucose-regulated protein, and nucleoporin P62 were used as specific markers for the cytosol, sarcolemma, mitochondria, sarco-endoplasmic reticulum, endoplasmic reticulum, and nucleus, respectively. The results show that we obtained enriched protein fractions with little to no cross-contamination. These purification protocols allow us to obtain different protein fractions that could be used in a wide variety of studies.

Salicylic acid alters endothelin-1 binding in intact adult rat ventricular myocytes

Endothelin receptors ETAR and ETBR form tight receptor–ligand complexes that complicate our understanding of the physiological, pharmacological, and biochemical properties of these receptors. Although radioligand–binding studies have demonstrated the binding of endothelin-1 (ET-1) to ETAR to be essentially irreversible, ETAR internalize in a ligand-dependent manner, release ET-1, and then recycle to the cell surface. Salicylic acid (SA) reduces ET-1 binding (IC50= 10 mmol/L) to recombinant ETAR in isolated membranes by promoting dissociation of [125I]ET-1. In the present study, SA (5 mmol SA/L) did not alter [125I]ET-1 binding to intact adult rat ventricular myocytes. The lack of effect was not due to internalization of receptor–ligand complexes. However, 100 mmol SA/L significantly reduced [125I]ET-1 binding to both intact myocytes and isolated membranes. SA induced the phosphorylation p42/44 extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase and an unidentified 40-kDa protein on the activating threonine-glutamic acid-tyrosine (T-E-Y) motif. ERK phosphorylation was reduced by a MAP kinase kinase (MEK) inhibitor, PD98059. Phosphorylation of p40 was reduced by the p38 MAP kinase inhibitor SB203580, but not PD98059. However, inhibition of ERK or p38 MAP kinases did not alter the ability of 100 mmol SA/L to induce dissociation of [125I]ET-1. These results suggest that, in the ventricular myocyte, salicylic acid alters the kinetics of ET-1 binding. The results also suggest an allosteric binding site may be present that modulates the dissociation of ET-1 receptor–ligand complexes in response to an as-of-yet unidentified mediator.Key words: cell communication, endothelin, endothelin receptor, inotropic agents, signal transduction, heart, ventricular myocyte.