Emerging roles for protein histidine phosphorylation in cellular signal transduction: lessons from the islet beta-cell

Orthogonal Translation for Site-Specific Installation of Post-translational Modifications

Post-translational modifications (PTMs) endow proteins with new properties to respond to environmental changes or growth needs. With the development of advanced proteomics techniques, hundreds of distinct types of PTMs have been observed in a wide range of proteins from bacteria, archaea, and eukarya. To identify the roles of these PTMs, scientists have applied various approaches. However, high dynamics, low stoichiometry, and crosstalk between PTMs make it almost impossible to obtain homogeneously modified proteins for characterization of the site-specific effect of individual PTM on target proteins. To solve this problem, the genetic code expansion (GCE) strategy has been introduced into the field of PTM studies. Instead of modifying proteins after translation, GCE incorporates modified amino acids into proteins during translation, thus generating site-specifically modified proteins at target positions. In this review, we summarize the development of GCE systems for orthogonal translation for site-specific installation of PTMs.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Association between the membrane transporter proteins and type 2 diabetes mellitus

Introduction: The prevalence rate of diabetes is increasing day by day and the current scenario of the available agents for its treatment has given rise to stimulation in the search for new therapeutic targets and agents. Therefore the present review will examine the role of membrane composition in the pathophysiology of Type 2 Diabetes and the possible therapeutic approaches for this.Areas covered: Glucose transporter proteins (GLUTs) are integral membrane proteins which are responsible for facilitated glucose transport over the plasma membrane into cells. Thus, this chapter is an attempt to interpret the co-relation between membrane transporter proteins and lipid molecules of cell membrane and their implications in type 2 diabetes mellitus. The relationship between the composition controlled flexibility of the membrane in the insertion of GLUTs into cell membrane as well as its fusion with the membrane is the focus of this chapter.Expert opinion: There is increasing data on the central role of phospholipid composition toward T2DM. Plasma membrane lipid composition plays a key role in maintaining the machinery for insulin-independent GLUT insertion into the membrane as well as insulin-dependent GLUT4 containing vesicles. As a therapeutic option, the designing of new chemical entities should be aimed to decrease saturated fatty acids of lipid bilayer phospholipids to target type 2 diabetes mellitus.

Role of G-proteins in islet function in health and diabetes

Glucose-stimulated insulin secretion (GSIS) involves interplay between metabolic and cationic events. Seminal contributions from multiple laboratories affirm essential roles for small G-proteins (Rac1, Cdc42, Arf6, Rab27A) in GSIS. Activation of these signalling proteins promotes cytoskeletal remodeling, transport and docking of insulin granules on the plasma membrane for exocytotic secretion of insulin. Evidence in rodent and human islets suggests key roles for lipidation (farnesylation and geranylgeranylation) of these G-proteins for their targeting to appropriate cellular compartments for optimal regulation of effectors leading to GSIS. Interestingly, however, inhibition of prenylation appears to cause mislocalization of non-prenylated, but (paradoxically) activated G-proteins, in "inappropriate" compartments leading to activation of stress kinases and onset of mitochondrial defects, loss in GSIS and apoptosis of the islet β-cell. This review highlights our current understanding of roles of G-proteins and their post-translational lipidation (prenylation) signalling networks in islet function in normal health, metabolic stress (glucolipotoxicity and ER stress) and diabetes. Critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signalling steps in β-cells in models of impaired insulin secretion and diabetes are also highlighted and discussed.

From phosphoproteins to phosphoproteomes: a historical account

The first phosphoprotein (casein) was discovered in 1883, yet the enzyme responsible for its phosphorylation was identified only 130 years later, in 2012. In the intervening time, especially in the last decades of the 1900s, it became evident that, far from being an oddity, phosphorylation affects the majority of eukaryotic proteins during their lifespan, and that this reaction is catalysed by the members of a large family of protein kinases, susceptible to a variety of stimuli controlling nearly every aspect of life and death. The aim of this review is to present a historical account of the main steps of this spectacular revolution, which transformed our conception of a biochemical reaction originally held as a sporadic curiosity into the master mechanism governing cell regulation, and, if it is perturbed, causing cell dysregulation.

Histidine kinases from bacteria to humans

It is more than 50 years since protein histidine phosphorylation was first discovered in 1962 by Boyer and co-workers; however, histidine kinases are still much less well recognized than the serine/threonine and tyrosine kinases. The best-known histidine kinases are the two-component signalling kinases that occur in bacteria, fungi and plants. The mechanisms and functions of these kinases, their cognate response regulators and associated phosphorelay proteins are becoming increasingly well understood. When genomes of higher eukaryotes began to be sequenced, it did not appear that they contained two-component histidine kinase system homologues, apart from a couple of related mitochondrial enzymes that were later shown not to function as histidine kinases. However, as a result of the burgeoning sequencing of genomes from a wide variety of eukaryotic organisms, it is clear that there are proteins that correspond to components of the two-component histidine kinase systems in higher eukaryotes and that operational two-component kinase systems are likely to occur in these organisms. There is unequivocal direct evidence that protein histidine phosphorylation does occur in mammals. So far, only nucleoside diphosphate kinases have been shown to be involved in protein histidine phosphorylation, but their mechanisms of action are not well understood. It is clear that other, yet to be identified, histidine kinases also exist in mammals and that protein histidine phosphorylation may play important roles in higher eukaryotes.

Methylenebisphosphonate and triphosphate derivatives of the mevalonate pathway are substrates of yeast UTP:glucose-1-phosphate uridylyltransferase

UTP:glucose-1-phospate uridylyltransferase (EC 2.7.7.9) from Saccharomyces cerevisiae transfers the uridylyl moiety of UDP-glucose onto methylenebisphosphonate (pCH(2)p) yielding uridine 5'-(β,γ-methylenetriphosphate) (UppCH(2)p). The following bisphosphonates were not acceptors of UMP: alendronate, pamidronate, clodronate and etidronate. UDP-glucose serves as uridylyl donor to triphosphate derivatives of the mevalonate pathway: farnesyl (far-PPP), geranyl (ger-PPP) and isopentenyl (iso-PPP), with formation of farnesyl-tetraphosphouridine (far-ppppU); geranyl-tetraphosphouridine (ger-ppppU) and isopentenyl-tetraphosphouridine (iso-ppppU). The K(m) (mM) and V(max) (mU/mg protein) values determined for these substrates were: 0.32 ± 0.07 and 4.9 ± 0.6; 0.21 ± 0.06 and 5.7 ± 0.8; 0.51 ± 0.14 and 2.0 ± 0.2, respectively. The K(m) and V(max) values for methylenebisphosphonate were 1.1 ± 0.2 mM and 4055 ± 96 mU/mg protein, respectively.

Protein histidine [de]phosphorylation in insulin secretion: abnormalities in models of impaired insulin secretion

In the majority of cell types, including the islet β-cell, transduction of extracellular signals involves ligand binding to a receptor, often followed by the activation G proteins and their effector modules. The islet β-cell is unusual in that glucose lacks an extracellular receptor. Instead, events consequent to glucose metabolism promote insulin secretion via the generation of diffusible second messengers and mobilization of calcium. A selective increase in intracellular calcium has been shown to regulate the phosphorylation status key islet proteins thereby facilitating insulin secretion. In addition to classical protein kinases [e.g., protein kinases A and C], recent studies from our laboratory have focused on the expression and function of various forms of NDPK/nm23-like histidine kinases in clonal β-cells, normal rodent, and human islets. Further, we recently reported localization of a cytosolic protein histidine phosphatase [PHP] in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knock down of nm23-H1 and PHP in insulin-secreting INS 832/13 cells significantly attenuated glucose-induced insulin secretion. We also observed significant alterations in the expression and function of nm23-H1/PHP in β-cells chronically exposed to elevated levels of glucose and saturated fatty acids, such as palmitate (i.e., glucolipotoxicity). Similar changes were also noted in islets from the Goto-Kakizaki and Zucker Diabetic Fatty rats, two known models for type 2 diabetes. It is concluded that protein histidine phosphorylation-dephosphorylation cycles play novel regulatory roles in G protein-mediated physiological insulin secretion and that abnormalities in this signaling axis lead to impaired insulin secretion in glucolipotoxicity and type 2 diabetes.

Regulation of glucose- and mitochondrial fuel-induced insulin secretion by a cytosolic protein histidine phosphatase in pancreatic beta-cells

We report localization of a cytosolic protein histidine phosphatase (PHP; approximately 16 kDa) in INS 832/13 cells, normal rat islets, and human islets. siRNA-mediated knockdown of PHP markedly reduced glucose- or mitochondrial fuel-induced but not KCl-induced insulin secretion. siRNA-mediated knockdown of PHP also attenuated mastoparan-induced insulin secretion, suggesting its participation in G protein-sensitive signaling steps, leading to insulin secretion. Functional assays revealed that the beta-cell PHP catalyzes the dephosphorylation of ATP-citrate lyase (ACL). Silencing of PHP expression markedly reduced ACL activity, suggesting functional regulation of ACL by PHP in beta-cells. Coimmunoprecipitation studies revealed modest effects of glucose on the interaction between PHP and ACL. Confocal microscopic evidence indicated that glucose promotes association between ACL and nm23-H1, a known kinase histidine kinase, but not between PHP and ACL. Furthermore, metabolic viability of INS 832/13 cells was resistant to siRNA-PHP, suggesting no regulatory roles of PHP in cell viability. Finally, long-term exposure (24 h) of INS 832/13 cells or rat islets to high glucose (30 mM) increased the expression of PHP. Such increases in PHP expression were also seen in islets derived from the Zucker diabetic fatty rat compared with islets from the lean control animals. Together, these data implicate regulatory roles for PHP in a G protein-sensitive step involved in nutrient-induced insulin secretion. In light of the current debate on putative regulatory roles of ACL in insulin secretion, additional studies are needed to precisely identify the phosphoprotein substrate(s) for PHP in the cascade of events leading to nutrient-induced insulin secretion.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

The glucose transporter 2 undergoes plasma membrane endocytosis and lysosomal degradation in a secretagogue-dependent manner

In beta-cells of the pancreas, the glucose transporter (GLUT)-2 facilitative glucose transporter protein is localized to the plasma membrane and functions as part of the glucose sensing mechanism for the stimulation of insulin secretion. We observed that expressed GLUT2 protein in the cultured Min6B1 cell line undergoes enhanced endocytosis at high extracellular glucose concentrations that stimulate insulin secretion. Moreover, the internalized GLUT2 protein undergoes rapid degradation induced by chronic high-glucose or arginine stimulation but does not undergo plasma membrane recycling or accumulation in any microscopically apparent intracellular membrane compartment. The rapid degradation of GLUT2 was prevented by lysosomal inhibition (chloroquine) concomitant with the accumulation of GLUT2 in endomembrane structures. In contrast, neither endocytosis nor the lack of internal membrane localized GLUT2 remained completely unaffected by proteosomal inhibition (lactacystin) or an heat shock protein-90 inhibitor (geldanamycin). Moreover, the endocytosis and degradation of GLUT2 was specific for beta-cells because expression of GLUT2 in 3T3L1 adipocytes remained cell surface localized and did not display a rapid rate of degradation. Together, these data demonstrate that hyperglycemia directly affects beta-cell function and activates a trafficking pathway that results in the rapid endocytosis and degradation of the cell surface GLUT2 glucose transporter.

Down-regulation of expression and function of nucleoside diphosphate kinase in insulin-secreting beta-cells under in vitro conditions of glucolipotoxicity

Previously, we reported a significant reduction in expression and the activity of nucleoside diphosphate kinase (NDP kinase) in islets derived from the Goto-Kakizaki rat (GK rat), an animal model for type 2 diabetes. Herein, we examined the effects of chronic exposure of insulin-secreting beta-(INS 832/13) cells to high glucose (a model for glucotoxicity), palmitate (a model for lipotoxicity), or glucose plus palmitate (a model for glucolipotoxicity) on the expression and activity of nm23-H1 (NDP kinase A) and nm23-H2 (NDP kinase B). Our findings indicate a marked reduction in the expression of both nm23-H1 and nm23-H2 and the associated NDP kinase activity under each of these conditions. A cell-permeable analog of ceramide (CER) also mimicked the effects of palmitate in significantly reducing the expression of nm23-H1 and nm23-H2 and NDP kinase activity in these cells. These findings suggest that de novo generation of intracellular CER from palmitate might represent at least one of the signaling steps involved in lipid-induced effects on NDP kinase expression and function in beta-cells. Based on these data, we conclude that glucolipotoxic conditions significantly impair expression and function of NDP kinase in pancreatic beta-cells. Potential significance of these findings, specifically at the level of abnormal G-protein activation and impaired insulin secretion under glucolipotoxic conditions is discussed.

Regulatory roles for Tiam1, a guanine nucleotide exchange factor for Rac1, in glucose-stimulated insulin secretion in pancreatic beta-cells

Using various biochemical, pharmacological and molecular biological approaches, we have recently reported regulatory roles for Rac1, a small G-protein, in glucose-stimulated insulin secretion (GSIS). However, little is understood with respect to localization of, and regulation by, specific regulatory factors of Rac1 in GSIS. Herein, we investigated regulatory roles for Tiam1, a specific nucleotide exchange factor (GEF) for Rac1, in GSIS in pancreatic beta-cells. Western blot analysis indicated that Tiam1 is predominantly cytosolic in distribution. NSC23766, a specific inhibitor of Tiam1-mediated activation of Rac1, markedly attenuated glucose-induced, but not KCl-induced insulin secretion in INS 832/13 cells and normal rat islets. Further, NSC23766 significantly reduced glucose-induced activation (i.e. GTP-bound form) and membrane association of Rac1 in INS 832/13 cells and rat islets. Moreover, siRNA-mediated knock-down of Tiam1 markedly inhibited glucose-induced membrane trafficking and activation of Rac1 in INS 832/13 cells. Interestingly, however, in contrast to the inhibitory effects of NSC23766, Tiam1 gene depletion potentiated GSIS in these cells; such a potentiation of GSIS was sensitive to extracellular calcium. Together, our studies present the first evidence for a regulatory role for Tiam1/Rac1-sensitive signaling step in GSIS. They also provide evidence for the existence of a potential Rac1/Tiam1-independent, but calcium-sensitive component for GSIS in these cells.