The role of cyclic nucleotides in modulation of crayfish neuromuscular junctions by a neuropeptide /

DF2, a heptapeptide, is a member of the family of FMRFamide-like peptides and has been shown to increase the amount of transmitter released at neuromuscular junctions of the crayfish, Procambarus clarkit Recent evidence has shown that protein kinase C (PKC), calcium/calmodulin-dependent protein kinase II (CaMKII) and the cAMPdependent protein kinase (PKA) play a role in the neuromodulatory pathway of DF2. The involvement of these kinases led to the prediction that a G-protein-coupled receptor (GPCR) is activated by DF2 due to the role that each kinase plays in traditional GPCR pathways seen in other organisms and in other cells. G-proteins can also act on an enzyme that generates cyclic guanosine monophosphate (cGMP) which mediates its effects through a cGMP-dependent protein kinase (PKG). This thesis addresses the question of whether or not DF2's effects on synaptic transmission in crayfish are mediated by the cyclic nucleotides cAMP and cGMP. The effects of DF2 on synaptic transmission were examined using deep abdominal extensor muscles of the crayfish Procambarus clarkii. An identified motor neuron was stimulated, and excitatory post-synaptic potentials (EPSPs) were recorded in abdominal extensor muscle LI . A number of activators and inhibitors were used to determine whether or not cAMP, PKA, cGMP and PKG mediate the effect of this peptide. Chemicals that are known to activate PKA (Sp-cAMPS) and/or PKG (8-pCPTcGMP) mimic and potentiate DF2's effect by increasing EPSP amplitude. Inhibitors of either PKA (Rp-cAMPS) or PKG (Rp-8-pCPT-cGMPS) block a portion of the increase in EPSP amplitude induced by the peptide. When both kinase inhibitors are applied simultaneously, the entire effect of DF2 on EPSPs is blocked. The PKG inhibitor blocks the effects of a PKG activator but does not alter the effect of a PKA activator on EPSP amplitude. Thus, the PKG inhibitor appears to be relatively specific for PKG. A trend in the data suggests that the PKA inhibitor blocks a portion of the response elicited by the PKG activator. Thus, the PKA inhibitor may be less specific for PKA. Phosphodiesterase inhibitors, which are known to inhibit the breakdown of cAMP (IBMX) and/or cGMP (mdBAMQ), potentiate the effect of the peptide. These results support the hypothesis that cAMP and cGMP, acting through their respective protein kinase enzymes, mediate the ability of DFi to increase transmitter output.

Adaptations of skeletal muscle pyruvate dehydrogenase kinase in response to food-restriction in mitochondrial subpopulations

University, 2006 Dr. Sandra J. Peters Pyruvate dehydrogenase (PDH) catalyses the decarboxylation of pyruvate, to form acetyl-CoA. PDH activity is down-regulated by intrinsic PDH kinases (predominantly PDK2 and PDK4 isoforms), but the understanding of the PDK isoform distribution and adaptation to nutritional stresses has been restricted to mixed mitochondrial populations, and not delineated between subsarcolemmal (SS) and intermyofibrillar (IMF) subpopulations. SS and IMF mitochondria exhibit distinct morphological and biochemical properties; however the functional differences are not well understood. This study investigated the effect of fed (FED) versus 48 h total foodrestriction (FR) on rat red gastrocnemius muscle PDK2 and 4 isoform content in SS and IMF mitochondria. PDK4 content was ~3-5 fold higher in SS mitochondria compared to IMF (p=0.001), and increased with FR -3-4- fold in both subpopulations (p<0.001). PDK2 was -2.5-4 fold higher in SS mitochondria compared to IMF (p=0.001), but PDK2 was unaltered with FR. Citrate synthase activity (|imol/min/mg mitochondrial protein) was not different between either subpopulation. As well there were no significant differences between mitochondrial subpopulations in PDH complex components in both fed and FR states. These results demonstrate that there is a markedly higher content of both PDK isofonns in SS compared to IMF mitochondria. Although PDK2 does not increase in either subpopulation in response to FR, PDK4 increases to a similar extent in both SS and IMF after 48 h food-restriction.

Attenuation of skeletal muscle FFA-induced insulin resistance by the polyphenol resveratrol. Elucidation of the mechanisms involved

Excess plasma free fatty acids (FFA) are correlated with insulin resistance and are a risk factor for the development of type 2 diabetes. In this study we examined the effect of the polyphenol resveratrol on FF A-induced insulin resistance in skeletal muscle cells and the mechanisms involved. Incubation of L6 myotubes with the FF A palmitate significantly decreased the insulin-stimulated glucqse uptake. Importantly, the effect of palmitate was ameliorated by resveratrol. Palmitate significantly increased serine phosphorylation of IRS..; 1 and reduced insulin-stimulated Akt phosphorylation, an effect that was abolished by resveratrol. We then investigated the effect of palmitate and resveratrol on the expression and phosphorylation of JNK, mTOR, p70-S6K, and AMPK kinases. The results demonstrated that our treatments had no effect on the expression of these proteins. However, palmitate increased the phosphorylation of mTOR and p70- S6K, whereas resveratrol abolished this effect and increased the phosphorylation of AMPK. Furthermore, all effects of resveratrol were abolished with sirtuin inhibitors, sirtinol and nicotinamide. These results indicate that resveratrol ameliorated FF A-induced insulin resistance by regulating mTOR and p70-S6K phosphorylation in skeletal muscle cells, through a mechanism involving sirtuins.

Pyruvate dehydrogenase activity in response to skeletal muscle contraction at two stimulation frequencies in pyruvate dehydrogenase kinase 2 knockout mice

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PD H is deactivated by a set of PD H kinases (PD K 1-4) with PDK2 and 4 being the predominant isoforms in skeletal muscle. PDK2 is highly sensitive to pyruvate inhibition, and is the most abundant isoform, while PDKI and 4 protein content are normally lower. This study examined the PDK isoform content and PDHa activation in muscle at rest and 10 and 40 Hz stimulation from PDK2 knockout (PDK2KO) mice to delineate the role of PDK2 in activating the PDH complex during low and moderate intensity muscle contraction. PDHa activity was lower in PDK2KO mice during contraction while total PDK actitvity was -4 fold lower. PDK4 protein was not different, however PDKI partially compensated for the lack of PDK2 and was -56% higher than WT. PDKI is a very potent inhibitor of the PDH complex due to its phosphorylation site specificity and allosteric regulation. These results suggest that the site specificity and allosteric regulatory properties of the individual PDK isoforms are more important than total PDK activity in determining transformation of the complex and PDHa activity during acute muscle contraction.

Mode of action of FMRFamide-like peptide on drosophila body wall muscle conractions

Neuropeptides are the largest group of signalling chemicals that can convey the information from the brain to the cells of all tissues. DPKQDFMRFamide, a member of one of the largest families of neuropeptides, FMRFamide-like peptides, has modulatory effects on nerve-evoked contractions of Drosophila body wall muscles (Hewes et aI.,1998) which are at least in part mediated by the ability of the peptide to enhance neurotransmitter release from the presynaptic terminal (Hewes et aI., 1998, Dunn & Mercier., 2005). However, DPKQDFMRFamide is also able to act directly on Drosophila body wall muscles by inducing contractions which require the influx of extracellular Ca 2+ (Clark et aI., 2008). The present study was aimed at identifying which proteins, including the membrane-bound receptor and second messenger molecules, are involved in mechanisms mediating this myotropic effect of the peptide. DPKQDFMRFamide induced contractions were reduced by 70% and 90%, respectively, in larvae in which FMRFamide G-protein coupled receptor gene (CG2114) was silenced either ubiquitously or specifically in muscle tissue, when compared to the response of the control larvae in which the expression of the same gene was not manipulated. Using an enzyme immunoassay (EIA) method, it was determined that at concentrations of 1 ~M- 0.01 ~M, the peptide failed to increase cAMP and cGMP levels in Drosophila body wall muscles. In addition, the physiological effect of DPKQDFMRFamide at a threshold dose was not potentiated by 3-lsobutyl-1-methylxanthine, a phosphodiesterase inhibitor, nor was the response to 1 ~M peptide blocked or reduced by inhibitors of cAMP-dependent or cGMP-dependent protein kinases. The response to DPKQDFMRFamide was not affected in the mutants of the phosholipase C-~ (PLC~) gene (norpA larvae) or IP3 receptor mutants, which suggested that the PLC-IP3 pathway is not involved in mediat ing the peptide's effects. Alatransgenic flies lacking activity of calcium/calmodul in-dependent protein kinase (CamKII showed an increase in muscle tonus following the application of 1 JlM DPKQDFMRFamide similar to the control larvae. Heat shock treatment potentiated the response to DPKQDFMRFamide in both ala1 and control flies by approximately 150 and 100 % from a non heat-shocked larvae, respectively. Furthermore, a CaMKII inhibitor, KN-93, did not affect the ability of peptide to increase muscle tonus. Thus, al though DPKQDFMRFamide acts through a G-protein coupled FMRFamide receptor, it does not appear to act via cAMP, cGMP, IP3, PLC or CaMKl1. The mechanism through which the FMRFamide receptor acts remains to be determined.

The Role of Pyruvate Dehydrogenase Kinase-4 in Post-Exercise Glycogen Recovery

The pyruvate dehydrogenase (PDH) complex regulates the oxidation of carbohydrates in mammals. Decreased activation of PDH following exhaustive exercise may aid the resynthesis of glycogen through increased activity of PDH kinase-4 (PDK4), one of four kinases that decrease the activity of the PDH complex. The purpose of this study was to examine the role of PDK4 in post-exercise glycogen resynthesis. Wild-type (WT) and PDK4-knockout (PDK4-KO mice) were exercised to exhaustion and were sampled at rest (Rest), at exercise exhaustion (Exh), and after two-hours post-exercise (Rec). Differences in feeding post-exercise led to the addition of a PDK4-KO group, pair-fed (PF) with WT mice. Glycogen fully recovered in all Rec groups in muscle however remained low in the PF group in liver. Flux through PDH was elevated in PDK4-KO muscle with feeding and low in the PF group in both tissues. This suggests PDK4 may fine-tune flux through PDH during exercise recovery.

Mode of action of a Drosophila FMRFamide in inducing muscle contraction

Drosophila melanogaster is a model system for examining the mechanisms of action of neuropeptides. DPKQDFMRFamide was previously shown to induce contractions in Drosophila body wall muscle fibres in a Ca(2+)-dependent manner. The present study examined the possible involvement of a G-protein-coupled receptor and second messengers in mediating this myotropic effect after removal of the central nervous system. DPKQDFMRFamide-induced contractions were reduced by 70% and 90%, respectively, in larvae with reduced expression of the Drosophila Fmrf receptor (FR) either ubiquitously or specifically in muscle tissue, compared with the response in control larvae in which expression was not manipulated. No such effect occurred in larvae with reduced expression of this gene only in neurons. The myogenic effects of DPKQDFMRFamide do not appear to be mediated through either of the two Drosophila myosuppressin receptors (DmsR-1 and DmsR-2). DPKQDFMRFamide-induced contractions were not reduced in Ala1 transgenic flies lacking activity of calcium/calmodulin-dependent protein kinase (CamKII), and were not affected by the CaMKII inhibitor KN-93. Peptide-induced contractions in the mutants of the phospholipase C-β (PLCβ) gene (norpA larvae) and in IP3 receptor mutants were similar to contractions elicited in control larvae. The peptide failed to increase cAMP and cGMP levels in Drosophila body wall muscles. Peptide-induced contractions were not potentiated by 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, and were not antagonized by inhibitors of cAMP-dependent or cGMP-dependent protein kinases. Additionally, exogenous application of arachidonic acid failed to induce myogenic contractions. Thus, DPKQDFMRFamide induces contractions via a G-protein coupled FMRFamide receptor in muscle cells but does not appear to act via cAMP, cGMP, IP3, PLC, CaMKII or arachidonic acid.