In silico determination of intracellular glycosylation and phosphorylation sites in human selectins: implications for biological function

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

Regulation of the cardiac Na+ channel NaV1.5 by post-translational modifications

The cardiac voltage-gated Na(+) channel, Na(V)1.5, is responsible for the upstroke of the action potential in cardiomyocytes and for efficient propagation of the electrical impulse in the myocardium. Even subtle alterations of Na(V)1.5 function, as caused by mutations in its gene SCN5A, may lead to many different arrhythmic phenotypes in carrier patients. In addition, acquired malfunctions of Na(V)1.5 that are secondary to cardiac disorders such as heart failure and cardiomyopathies, may also play significant roles in arrhythmogenesis. While it is clear that the regulation of Na(V)1.5 protein expression and function tightly depends on genetic mechanisms, recent studies have demonstrated that Na(V)1.5 is the target of various post-translational modifications that are pivotal not only in physiological conditions, but also in disease. In this review, we examine the recent literature demonstrating glycosylation, phosphorylation by Protein Kinases A and C, Ca(2+)/Calmodulin-dependent protein Kinase II, Phosphatidylinositol 3-Kinase, Serum- and Glucocorticoid-inducible Kinases, Fyn and Adenosine Monophosphate-activated Protein Kinase, methylation, acetylation, redox modifications, and ubiquitylation of Na(V)1.5. Modern and sensitive mass spectrometry approaches, applied directly to channel proteins that were purified from native cardiac tissues, have enabled the determination of the precise location of post-translational modification sites, thus providing essential information for understanding the mechanistic details of these regulations. The current challenge is first, to understand the roles of these modifications on the expression and the function of Na(V)1.5, and second, to further identify other chemical modifications. It is postulated that the diversity of phenotypes observed with Na(V)1.5-dependent disorders may partially arise from the complex post-translational modifications of channel protein components.

In silico prediction of post-translational modifications

Methods for predicting protein post-translational modifications have been developed extensively. In this chapter, we review major post-translational modification prediction strategies, with a particular focus on statistical and machine learning approaches. We present the workflow of the methods and summarize the advantages and disadvantages of the methods.

Protein O-N-acetylglucosaminylation modulates promoter activities of cyclic AMP response element and activator protein 1 and enhances E-selectin expression on HuH-7 human hepatoma cells

High glucose accelerates O-N-acetylglucosaminylation (O-GlcNAcylation) of proteins and causes diabetic complications. In the present study, we found that treatment of HuH-7 human hepatoma cells with high glucose or the protein O-N-acetylglucosaminidase (O-GlcNAcase) inhibitor O-(2-acetoamide-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) increased the cell surface expression of E-selectin. A dual luciferase reporter assay indicated that high glucose and PUGNAc suppressed promoter activities of the cyclic AMP response element (CRE) and enhanced those of activator protein 1 (AP-1). Enhanced CRE promoter activities in HuH-7 cells treated with dibutyryl cAMP or co-transfected with a protein kinase A expression vector pFC-PKA that enhances the phosphorylation of CRE binding protein (CREB) were suppressed by PUGNAc. In contrast, PUGNAc further increased the enhanced AP-1 promoter activity in cells transfected with a mitogen-activated protein kinase kinase kinase expression vector pFC-MEKK that enhances c-Jun phosphorylation. Immuno-blotting using an anti-O-GlcNAc antibody revealed that high glucose and PUGNAc accelerated protein O-GlcNAcylation and that there were substantial differences in the O-GlcNAcylated proteins in the cytoplasmic and nuclear fractions. In addition, PUGNAc increased the nuclear import of O-GlcNAcylated CREB. These results suggest that protein O-GlcNAcylation modulates the promoter activities of E-selectin gene, suppression of CRE and enhancement of AP-1, and enhances E-selectin protein expression on hepatocytes.