Role of the sarcoplasmic reticulum in regulating the activity-dependent expression of the glycogen phosphorylase gene in contractile skeletal muscle cells

Comparison of the Biochemical Composition and Nutritional Quality Between Diploid and Triploid Hong Kong Oysters, Crassostrea hongkongensis

This study is the first systematic comparison of the biochemical composition and nutritional quality between diploid and triploid Hong Kong oysters, Crassostrea hongkongensis. Results showed that in the reproductive season, the glycogen content in five tissues (gill, mantle, adductor muscle, labial palps and gonad) was significantly higher (P < 0.05) in triploids than in diploids, with odds ratios (ORs) of 96.26, 60.17, 72.59, 53.56, and 128.52%, respectively. In the non-reproductive phase, significant differences in glycogen content (P < 0.05) between diploid and triploid oysters existed only in gill and gonad. In both diploid and triploid Hong Kong oysters, quantitative real-time PCR analysis of the glycogen synthesis gene (ChGS) and glycogen phosphorylase gene (ChGP) showed that the gene expression patterns matched the pattern of variation in glycogen content. Moreover, in both the reproductive and the non-reproductive phases, triploid Hong Kong oysters had a well balance of essential amino acids and were thus a well source of high-quality protein. Surprisingly, in both phases, significantly higher (P < 0.05) percentages of four essential fatty acids (α-linolenic acid, linoleic acid, eicosapentaenoic acid, and docosahexaenoic acid) were observed in triploids than in diploids. Additionally, the ratio of n-3/n-6 polyunsaturated fatty acids (PUFAs) was much higher in triploids than that in diploids. Variations in Biochemical composition were consistent with the relative expression of the citrate synthase gene (ChCS) and the α-ketoglutarate dehydrogenase gene (ChKD), which are key enzyme genes of the tricarboxylic acid cycle. Overall, the triploid Hong Kong oyster has a better nutritional value and taste than the diploid in terms of glycogen content, protein quality and fatty acid content.

PfIRR Interacts with HrIGF-I and Activates the MAP-kinase and PI3-kinase Signaling Pathways to Regulate Glycogen Metabolism in Pinctada fucata

The insulin-induced mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways are major intracellular signaling modules and conserved among eukaryotes that are known to regulate diverse cellular processes. However, they have not been investigated in the mollusk species Pinctada fucata. Here, we demonstrate that insulin-related peptide receptor of P. fucata (pfIRR) interacts with human recombinant insulin-like growth factor I (hrIGF-I), and stimulates the MAPK and PI3K signaling pathways in P. fucata oocytes. We also show that inhibition of pfIRR by the inhibitor PQ401 significantly attenuates the basal and hrIGF-I-induced phosphorylation of MAPK and PI3K/Akt at amino acid residues threonine 308 and serine 473. Furthermore, our experiments show that there is cross-talk between the MAPK and PI3K/Akt pathways, in which MAPK kinase positively regulates the PI3K pathway, and PI3K positively regulates the MAPK cascade. Intramuscular injection of hrIGF-I stimulates the PI3K and MAPK pathways to increase the expression of pfirr, protein phosphatase 1, glucokinase, and the phosphorylation of glycogen synthase, decreases the mRNA expression of glycogen synthase kinase-3 beta, decreases glucose levels in hemocytes, and increases glycogen levels in digestive glands. These results suggest that the MAPK and PI3K pathways in P. fucata transmit the hrIGF-I signal to regulate glycogen metabolism.

Dual inhibition of sodium-mediated proton and calcium efflux triggers non-apoptotic cell death in malignant gliomas

Malignant glioma cells maintain an elevated intracellular pH (pH(i)) within hypoxic-ischemic tumor microenvironments through persistent activation of sodium-proton transport (McLean et al., 2000). Amiloride has been reported to selectively kill human malignant glioma cell lines but not primary astrocytes (Hegde et al., 2004). While amiloride reduces pH(i) of malignant gliomas by inhibiting isoform 1 of sodium-proton exchange (NHE1), direct acidification was shown to be cytostatic rather than cytotoxic. At cytotoxic concentrations, amiloride has multiple drug targets including inhibition of NHE1 and sodium-calcium exchange. Amiloride's glioma cytotoxicity can be explained, at least in part, by dual inhibition of NHE1 and of Na(+)-dependent calcium efflux by isoform 1.1 of the sodium-calcium exchanger (NCX1.1), which increases [Ca(2+)](i) and initiates glioma cell demise. As a result of persistent NHE1 activity, cytosolic free levels of sodium ([Na(+)](i)) in U87 and C6 glioma cells are elevated 3-fold, as compared with normal astrocytes. Basal cytosolic free calcium levels ([Ca(2+)](i)) also are increased 5-fold. 2', 4'-dichlorobenzamil (DCB) inhibits the sodium-dependent calcium transporter (NCX1.1) much more potently than NHE1. DCB was employed in a concentration-dependent fashion in glioma cells to selectively inhibit the forward mode of NCX1.1 at ≤1μM, while dually inhibiting both NHE1 and NCX1.1 at ≥20μM. DCB (1μM) was not cytotoxic to glioma cells, while DCB (20μM) further increased basal elevated levels of [Ca(2+)](i) in glioma cells that was followed by cell demise. Cariporide and SEA0400 are more selective inhibitors of NHE1 and NCX1.1 than amiloride or DCB, respectively. Individually, Cariporide and SEA0400 are not cytotoxic, but in combination induced glioma cell death. Like amiloride, the combination of Cariporide and SEA0400 produced glioma cell death in the absence of demonstrable caspase activation.

NF-kappaB activation by depolarization of skeletal muscle cells depends on ryanodine and IP3 receptor-mediated calcium signals

Depolarization of skeletal muscle cells by either high external K(+) or repetitive extracellular field potential pulses induces calcium release from internal stores. The two components of this release are mediated by either ryanodine receptors or inositol 1,4,5-trisphosphate (IP(3)) receptors and show differences in kinetics, amplitude, and subcellular localization. We have reported that the transcriptional regulators including ERKs, cAMP/Ca(2+)-response element binding protein, c-fos, c-jun, and egr-1 are activated by K(+)-induced depolarization and that their activation requires IP(3)-dependent calcium release. We presently describe the activation of the nuclear transcription factor NF-kappaB in response to depolarization by either high K(+) (chronic) or electrical pulses (fluctuating). Calcium transients of relative short duration activate an NF-kappaB reporter gene to an intermediate level, whereas long-lasting calcium increases obtained by prolonged electrical stimulation protocols of various frequencies induce maximal activation of NF-kappaB. This activation is independent of extracellular calcium, whereas calcium release mediated by either ryanodine or IP(3) receptors contribute in all conditions tested. NF-kappaB activation is mediated by IkappaBalpha degradation and p65 translocation to the nucleus. Partial blockade by N-acetyl-l-cysteine, a general antioxidant, suggests the participation of reactive oxygen species. Calcium-dependent signaling pathways such as those linked to calcineurin and PKC also contribute to NF-kappaB activation by depolarization, as assessed by blockade through pharmacological agents. These results suggest that NF-kappaB activation in skeletal muscle cells is linked to membrane depolarization and depends on the duration of elevated intracellular calcium. It can be regulated by sequential activation of calcium release mediated by the ryanodine and by IP(3) receptors.

Molecular cloning and seasonal expression of oyster glycogen phosphorylase and glycogen synthase genes

To investigate the control at the mRNA level of glycogen metabolism in the cupped oyster Crassostrea gigas, we report in the present paper the cloning and characterization of glycogen phosphorylase and synthase cDNAs (Cg-GPH and Cg-GYS, respectively, transcripts of main enzymes for glycogen use and storage), and their first expression profiles depending on oyster tissues and seasons. A strong expression of both genes was observed in the labial palps and the gonad in accordance with specific cells located in both tissues and ability to store glucose. Cg-GPH expression was also found mainly in muscle suggesting ability to use glycogen as readily available glucose to supply its activity. For seasonal examinations, expression of Cg-GYS and Cg-GPH genes appeared to be regulated according to variation in glycogen content. Relative levels of Cg-GYS transcripts appeared highest in October corresponding to glycogen storage and resting period. Relative levels of Cg-GPH transcripts were highest in May corresponding to mobilization of glycogen needed for germ cell maturation. Expression of both genes would likely be driven by the oyster's reproductive cycle, reflecting the central role of glycogen in energy storage and gametogenic development in C. gigas. Both genes are useful molecular markers in the regulation of glycogen metabolism and reproduction in C. gigas but enzymatic regulation of glycogen phosphorylase and synthase remains to be elucidated.

Amiloride peptide conjugates: prodrugs for sodium-proton exchange inhibition

Inhibition of the sodium-proton exchanger (NHE) plays an important role in reducing tissue damage during ischemic reperfusion injury; however, pharmacological inhibitors of NHE have restricted access to acutely ischemic tissues because of severely compromised tissue perfusion. We describe the syntheses, characterization, and NHE inhibitory activities of a novel class of amiloride derivatives where peptides are conjugated to the amiloride C(5) amino group. These new peptide-C(5)-amiloride conjugates are inactive; however, peptide residues were chosen such that selective cleavage by neutral endopeptidase 24.11 (enkephalinase) liberates an amino acid-C(5)-amiloride conjugate that inhibits NHE in a glial cell line. These results confirm the feasibility of using peptide-amiloride conjugates as NHE inhibitor prodrugs. We envision the design of analogous peptide-amiloride prodrugs that can be administered prior to ischemic events and subsequently activated by endopeptidases selectively expressed by ischemic tissues.

Intracellular calcium and myosin isoform transitions. Calcineurin and calcium-calmodulin kinase pathways regulate preferential activation of the IIa myosin heavy chain promoter

Intracellular calcium levels can have profound effects on muscle biology via alterations in gene expression. In particular, intracellular calcium levels increase during muscle activation and are thought to underlie fast-to-slow shifts in muscle gene expression. In the present work, we determined that increased intracellular calcium has a significant effect on the activity of the adult fast myosin heavy chain (MyHC) promoters in the order of MyHC IIa>> IId/x > IIb. We have identified the pathways by which the calcium signal mediates increased activation of the MyHC IIa promoter. Inhibition of calcineurin or calcium-calmodulin kinase greatly attenuates ionophore-induced activation of the MyHC IIa promoter, whereas protein kinase C inhibitors have no effect. Inhibition and overexpression studies with members of the mitogen-activated protein kinase family reveal roles for MEK1/MEK2 and MEKK1, but not p38 or phosphatidylinositol 3-kinase. Downstream mediators of these effects are the activities of the MEF-2 and NFAT transcription factors, whose binding sites in the MyHC IIa promoter are required for calcium-induced activation of the MyHC IIa promoter.

Reduction in intracellular calcium levels inhibits myoblast differentiation

In myocytes, calcium plays an important role in intracellular signaling and contraction. However, the ability of calcium to modulate the differentiation of striated muscle cells is poorly understood. To examine this issue we studied C2C12 cells, which is a myoblast cell line that differentiates in vitro. First, we observed that the L-type calcium channel blockers nifedipine and verapamil effectively inhibited electrically induced calcium transients. Next, C2C12 cells were exposed to these agents during conditions that induce myocyte differentiation. In the presence of nifedipine and verapamil, myoblasts failed to form myotubes. Dantrolene and thapsigargin, which decrease intracellular calcium by different mechanisms, also inhibited differentiation. In addition, nifedipine and verapamil inhibited the expression of myosin heavy chain and myogenin, two markers of skeletal myoblast differentiation. In contrast, levels of the transcriptional factor Myf5, which is expressed in undifferentiated myoblasts, did not decline. Calcium channel blockade also prevented the expression of a reporter driven by the skeletal muscle alpha-actin promoter. These data demonstrate that lowering intracellular calcium levels inhibits the differentiation of skeletal myoblasts into mature myotubes.