Sundeep Malik

Differential Targeting of Gßγ-Subunit Signaling with Small Molecules

G protein βγ subunits have potential as a target for therapeutic treatment of a number of diseases. We performed virtual docking of a small-molecule library to a site on Gβγ subunits that mediates protein interactions. We hypothesized that differential targeting of this surface could allow for selective modulation of Gβγ subunit functions. Several compounds bound to Gβγ subunits with affinities from 0.1 to 60 μM and selectively modulated functional Gβγ-protein-protein interactions in vitro, chemotactic peptide signaling pathways in HL-60 leukocytes, and opioid receptor–dependent analgesia in vivo. These data demonstrate an approach for modulation of G protein–coupled receptor signaling that may represent an important therapeutic strategy.

Epac and phospholipase Cepsilon regulate Ca2+ release in the heart by activation of protein kinase Cepsilon and calcium-calmodulin kinase II.

Recently, we identified a novel signaling pathway involving Epac, Rap, and phospholipase C (PLC)ϵ that plays a critical role in maximal β-adrenergic receptor (βAR) stimulation of Ca2+-induced Ca2+ release (CICR) in cardiac myocytes. Here we demonstrate that PLCϵ phosphatidylinositol 4,5-bisphosphate hydrolytic activity and PLCϵ-stimulated Rap1 GEF activity are both required for PLCϵ-mediated enhancement of sarcoplasmic reticulum Ca2+ release and that PLCϵ significantly enhances Rap activation in response to βAR stimulation in the heart. Downstream of PLCϵ hydrolytic activity, pharmacological inhibition of PKC significantly inhibited both βAR- and Epac-stimulated increases in CICR in PLCϵ+/+ myocytes but had no effect in PLCϵ–/– myocytes. βAR and Epac activation caused membrane translocation of PKCϵ in PLCϵ+/+ but not PLCϵ–/– myocytes and small interfering RNA-mediated PKCϵ knockdown significantly inhibited both βAR and Epac-mediated CICR enhancement. Further downstream, the Ca2+/calmodulin-dependent protein kinase II (CamKII) inhibitor, KN93, inhibited βAR- and Epac-mediated CICR in PLCϵ+/+ but not PLCϵ–/– myocytes. Epac activation increased CamKII Thr286 phosphorylation and enhanced phosphorylation at CamKII phosphorylation sites on the ryanodine receptor (RyR2) (Ser2815) and phospholamban (Thr17) in a PKC-dependent manner. Perforated patch clamp experiments revealed that basal and βAR-stimulated peak L-type current density are similar in PLCϵ+/+ and PLCϵ–/– myocytes suggesting that control of sarcoplasmic reticulum Ca2+ release, rather than Ca2+ influx through L-type Ca2+ channels, is the target of regulation of a novel signal transduction pathway involving sequential activation of Epac, PLCϵ, PKCϵ, and CamKII downstream of βAR activation.

Epac-mediated activation of phospholipase C∈ plays a critical role in β-adrenergic receptor-dependent enhancement of Ca2+ mobilization in cardiac myocytes

Recently we demonstrated that PLCϵ plays an important role in β-adrenergic receptor (βAR) stimulation of Ca2+-induced Ca2+ release (CICR) in cardiac myocytes. Here we have reported for the first time that a pathway downstream of βAR involving the cAMP-dependent Rap GTP exchange factor, Epac, and PLCϵ regulates CICR in cardiac myocytes. To demonstrate a role for Epac in the stimulation of CICR, cardiac myocytes were treated with an Epac-selective cAMP analog, 8-4-(chlorophenylthio)-2′-O-methyladenosine-3′,5′-monophosphate (cpTOME). cpTOME treatment increased the amplitude of electrically evoked Ca2+ transients, implicating Epac for the first time in cardiac CICR. This response is abolished in PLCϵ-/- cardiac myocytes but rescued by transduction with PLCϵ, indicating that Epac is upstream of PLCϵ. Furthermore, transduction of PLCϵ+/+ cardiac myocytes with a Rap inhibitor, RapGAP1, significantly inhibited isoproterenol-dependent CICR. Using a combination of cpTOME and PKA-selective activators and inhibitors, we have shown that βAR-dependent increases in CICR consist of two independent components mediated by PKA and the novel Epac/PLCϵ pathway. We also show that Epac/PLCϵ-dependent effects on CICR are independent of sarcoplasmic reticulum loading and Ca2+ clearance mechanisms. These data define a novel endogenous PKA-independent βAR-signaling pathway through cAMP-dependent Epac activation, Rap, and PLCϵ that enhances intracellular Ca2+ release in cardiac myocytes.

Regulation of Epidermal Growth Factor-induced Connexin 43 Gap Junction Communication by Big Mitogen-activated Protein Kinase 1/ERK5 but Not ERK1/2 Kinase Activation

The gap junction protein, Cx43, plays a pivotal role in coupling cells electrically and metabolically, and the putative phosphorylation sites that modulate its function are reflected as changes in gap junction communication. Growth factor stimulation has been correlated with a decrease in gap junction communication and a parallel activation of ERK1/2; the inhibition of epidermal growth factor (EGF)-induced Cx43 gap junction uncoupling was observed by using the MEK1/2 inhibitor, PD98059. Because 1) BMK1/ERK5, another MAPK family member also activated by growth factors, possesses a phosphorylation motif similar to ERK1/2, and 2) it has been reported that PD98059 can inhibit not only MEK1/2-ERK1/2 but also MEK5-BMK1 activation, we investigated whether BMK1 can regulate EGF-induced Cx43 gap junction uncoupling and phosphorylation, comparing this to the role of ERK1/2 on Cx43 function and phosphorylation induced by EGF. Selective activation or inactivation of ERK1/2 by using a constitutively active form or a dominant negative form of MEK1 did not regulate Cx43 gap junction coupling. In contrast, we found that BMK1, selectively activated by constitutively active MEK5α, induced gap junction uncoupling, and the inhibition of BMK1 activation by transfection of dominant negative BMK1 prevented EGF-induced gap junction uncoupling. Activated BMK1 selectively phosphorylates Cx43 on Ser-255 in vitro and in vivo, but not on S279/S282, which are reported as the consensus phosphorylation sites for MAPK. Furthermore, by co-immunoprecipitation, we found that BMK1 directly associates with Cx43 in vivo. These data indicate that BMK1 is more important than ERK1/2 in EGF-mediated Cx43 gap junction uncoupling by association and Cx43 Ser- 255 phosphorylation.

Phospholipase Cε hydrolyzes perinuclear phosphatidylinositol 4-phosphate to regulate cardiac hypertrophy.

Phospholipase Cε (PLCε) is a multifunctional enzyme implicated in cardiovascular, pancreatic, and inflammatory functions. Here we show that conditional deletion of PLCε in mouse cardiac myocytes protects from stress-induced pathological hypertrophy. PLCε small interfering RNA (siRNA) in ventricular myocytes decreases endothelin-1 (ET-1)-dependent elevation of nuclear calcium and activation of nuclear protein kinase D (PKD). PLCε scaffolded to muscle-specific A kinase-anchoring protein (mAKAP), along with PKCε and PKD, localizes these components at or near the nuclear envelope, and this complex is required for nuclear PKD activation. Phosphatidylinositol 4-phosphate (PI4P) is identified as a perinuclear substrate in the Golgi apparatus for mAKAP-scaffolded PLCε. We conclude that perinuclear PLCε, scaffolded to mAKAP in cardiac myocytes, responds to hypertrophic stimuli to generate diacylglycerol (DAG) from PI4P in the Golgi apparatus, in close proximity to the nuclear envelope, to regulate activation of nuclear PKD and hypertrophic signaling pathways.

Phospholipase Cϵ Scaffolds to Muscle-specific A Kinase Anchoring Protein (mAKAPβ) and Integrates Multiple Hypertrophic Stimuli in Cardiac Myocytes

To define a role for phospholipase Cϵ (PLCϵ) signaling in cardiac myocyte hypertrophic growth, PLCϵ protein was depleted from neonatal rat ventricular myocytes (NRVMs) using siRNA. NRVMs with PLCϵ depletion were stimulated with endothelin (ET-1), norepinephrine, insulin-like growth factor-1 (IGF-1), or isoproterenol and assessed for development of hypertrophy. PLCϵ depletion dramatically reduced hypertrophic growth and gene expression induced by all agonists tested. PLCϵ catalytic activity was required for hypertrophy development, yet PLCϵ depletion did not reduce global agonist-stimulated inositol phosphate production, suggesting a requirement for localized PLC activity. PLCϵ was found to be scaffolded to a muscle-specific A kinase anchoring protein (mAKAPβ) in heart and NRVMs, and mAKAPβ localizes to the nuclear envelope in NRVMs. PLCϵ-mAKAP interaction domains were defined and overexpressed to disrupt endogenous mAKAPβ-PLCϵ complexes in NRVMs, resulting in significantly reduced ET-1-dependent NRVM hypertrophy. We propose that PLCϵ integrates multiple upstream signaling pathways to generate local signals at the nucleus that regulate hypertrophy.

Phospholipase Cε is a nexus for Rho and Rap-mediated G protein-coupled receptor-induced astrocyte proliferation

Phospholipase Cε (PLCε) has been suggested to transduce signals from small GTPases, but its biological function has not yet been clarified. Using astrocytes from PLCε-deficient mice, we demonstrate that endogenous G protein-coupled receptors (GPCRs) for lysophosphatidic acid, sphingosine 1-phosphate, and thrombin regulate phosphoinositide hydrolysis primarily through PLCε. Stimulation by lysophospholipids occurs through Gi, whereas thrombin activates PLC through Rho. Further studies reveal that PLCε is required for thrombin- but not LPA-induced sustained ERK activation and DNA synthesis, providing a novel mechanism for GPCR and Rho signaling to cell proliferation. The requirement for PLCε in this pathway can be explained by its role as a guanine nucleotide exchange factor for Rap1. Thus, PLCε serves to transduce mitogenic signals through a mechanism distinct from its role in generation of PLC-derived second messengers.

Epac2‐dependent mobilization of intracellular Ca2+ by glucagon‐like peptide‐1 receptor agonist exendin‐4 is disrupted in β‐cells of phospholipase C‐ɛ knockout mice

Calcium can be mobilized in pancreatic β‐cells via a mechanism of Ca2+‐induced Ca2+ release (CICR), and cAMP‐elevating agents such as exendin‐4 facilitate CICR in β‐cells by activating both protein kinase A and Epac2. Here we provide the first report that a novel phosphoinositide‐specific phospholipase C‐ɛ (PLC‐ɛ) is expressed in the islets of Langerhans, and that the knockout (KO) of PLC‐ɛ gene expression in mice disrupts the action of exendin‐4 to facilitate CICR in the β‐cells of these mice. Thus, in the present study, in which wild‐type (WT) C57BL/6 mouse β‐cells were loaded with the photolabile Ca2+ chelator NP‐EGTA, the UV flash photolysis‐catalysed uncaging of Ca2+ generated CICR in only 9% of the β‐cells tested, whereas CICR was generated in 82% of the β‐cells pretreated with exendin‐4. This action of exendin‐4 to facilitate CICR was reproduced by cAMP analogues that activate protein kinase A (6‐Bnz‐cAMP‐AM) or Epac2 (8‐pCPT‐2′‐O‐Me‐cAMP‐AM) selectively. However, in β‐cells of PLC‐ɛ KO mice, and also Epac2 KO mice, these test substances exhibited differential efficacies in the CICR assay such that exendin‐4 was partly effective, 6‐Bnz‐cAMP‐AM was fully effective, and 8‐pCPT‐2′‐O‐Me‐cAMP‐AM was without significant effect. Importantly, transduction of PLC‐ɛ KO β‐cells with recombinant PLC‐ɛ rescued the action of 8‐pCPT‐2′‐O‐Me‐cAMP‐AM to facilitate CICR, whereas a K2150E PLC‐ɛ with a mutated Ras association (RA) domain, or a H1640L PLC‐ɛ that is catalytically dead, were both ineffective. Since 8‐pCPT‐2′‐O‐Me‐cAMP‐AM failed to facilitate CICR in WT β‐cells transduced with a GTPase activating protein (RapGAP) that downregulates Rap activity, the available evidence indicates that a signal transduction ‘module’ comprised of Epac2, Rap and PLC‐ɛ exists in β‐cells, and that the activities of Epac2 and PLC‐ɛ are key determinants of CICR in this cell type.

Regulatory interactions between the amino terminus of G-protein βγ subunits and the catalytic domain of phospholipase Cβ2

We previously identified a 10-amino acid region from the Y domain of phospholipase Cβ2 (PLCβ2) that associates with G-protein βγ subunits (Sankaran, B., Osterhout, J., Wu, D., and Smrcka, A. V. (1998) J. Biol. Chem. 273, 7148–7154). We mapped the site for cross-linking of a synthetic peptide (N20K) corresponding to this Y domain region to Cys25 within the amino-terminal coiled-coil domain of Gβγ (Yoshikawa, D. M., Bresciano, K., Hatwar, M., and Smrcka, A. V. (2001) J. Biol. Chem. 276, 11246–11251). Here, further experiments with a series of variable length cross-linking agents refined the site of N20K binding to within 4.4–6.7 Å of Cys25. A mutant within the amino terminus of the Gβ subunit, Gβ1(23–27)γ2, activated PLCβ2 more effectively than wild type, with no significant change in the EC50, indicating that this region is directly involved in the catalytic regulation of PLCβ2. This mutant was deficient in cross-linking to N20K, suggesting that a binding site for the peptide had been eliminated. Surprisingly, N20K could still inhibit Gβ1(23–27)γ2-dependent activation of PLC, suggesting a second N20K binding site. Competition analysis with a peptide that binds to the Gα subunit switch II binding surface of Gβγ indicates a second N20K binding site at this surface. Furthermore, mutations to the N20K region within the Y-domain of full-length PLCβ2 inhibited Gβγ-dependent regulation of the enzyme, providing further evidence for aGβγ binding site within the catalytic domain of PLCβ2. The data support a model with two modes of PLC binding to Gβγ through the catalytic domain, where interactions with the amino-terminal coiled-coil domain are inhibitory, and interactions with the Gα subunit switch II binding surface are stimulatory.

Knock down of spinal NMDA receptors reduces NMDA and formalin evoked behaviors in rat.

Chronic pain remains a major health problem afflicting an estimated 70% of patients with advanced cancer and inflammatory disorders, and up to 94% of patients with spinal cord injuries. Although progress has been made in the pharmacotherapy of chronic pain management, such as usage of adjuvant drugs and more effective methods of drug delivery, the mainstay of clinical pain management still depends on opiates. NMDA receptor activation, at the level of the spinal cord has been shown to play an important role in the facilitation of nociception (pain) in several animal models. Unfortunately, potent NMDA receptor antagonists, such as MK-801 and APV, have toxic properties and low safety margins that preclude their clinical use. We present evidence which indicates that the use of antisense oligonucleotides targeted to the NMDA-R1 receptor subunit (AS-NMDA-R1), but not sense, abolishes NMDA and formalin induced behaviors. Moreover, we demonstrate that spinal administration of AS-NMDA-R1 results in the abolition of staining for immunoreactive NMDA-R1 in the spinal cord. These data provide novel evidence supporting the feasibility of the use of gene therapy approaches in the management of neuropathic pain.