Regulation of KATP Channel Trafficking in Pancreatic β-Cells by Protein Histidine Phosphorylation

Etiology of the Neonatal Hypoglycemias

To provide a more appropriate foundation for dealing with the problem of hypoglycemia in newborn infants, this article focuses on the mechanisms which underlie the various forms of neonatal hypoglycemia and discusses their implications for newborn care. Evidence indicates that all of the major forms of neonatal hypoglycemia are the result of hyperinsulinism due to dysregulation of pancreatic islet insulin secretion. Based on these observations, the authors propose that routine measurement of B-hydroxybutyrate should be considered an essential part of glucose monitoring in newborn infants.

Histidine Phosphorylation: Protein Kinases and Phosphatases

Phosphohistidine (pHis) is a reversible protein post-translational modification (PTM) that is currently poorly understood. The P-N bond in pHis is heat and acid-sensitive, making it more challenging to study than the canonical phosphoamino acids pSer, pThr, and pTyr. As advancements in the development of tools to study pHis have been made, the roles of pHis in cells are slowly being revealed. To date, a handful of enzymes responsible for controlling this modification have been identified, including the histidine kinases NME1 and NME2, as well as the phosphohistidine phosphatases PHPT1, LHPP, and PGAM5. These tools have also identified the substrates of these enzymes, granting new insights into previously unknown regulatory mechanisms. Here, we discuss the cellular function of pHis and how it is regulated on known pHis-containing proteins, as well as cellular mechanisms that regulate the activity of the pHis kinases and phosphatases themselves. We further discuss the role of the pHis kinases and phosphatases as potential tumor promoters or suppressors. Finally, we give an overview of various tools and methods currently used to study pHis biology. Given their breadth of functions, unraveling the role of pHis in mammalian systems promises radical new insights into existing and unexplored areas of cell biology.

Association of Fetal Catecholamines With Neonatal Hypoglycemia

Importance

Perinatal stress and fetal growth restriction increase the risk of neonatal hypoglycemia. The underlying pathomechanism is poorly understood. In a sheep model, elevated catecholamine concentrations were found to suppress intrauterine insulin secretion, followed by hyperresponsive insulin secretion once the adrenergic stimulus subsided.

Objective

To determine whether neonates with risk factors for hypoglycemia have higher catecholamine concentrations in umbilical cord blood (UCB) and/or amniotic fluid (AF) and whether catecholamines are correlated with postnatal glycemia.

Design, Setting, and Participants

In a prospective cohort study of 328 neonates at a tertiary perinatal center from September 2020 through May 2022 in which AF and UCB were collected immediately during and after delivery, catecholamines and metanephrines were analyzed using liquid chromatography with tandem mass spectrometry. Participants received postnatal blood glucose (BG) screenings.

Exposure

Risk factor for neonatal hypoglycemia.

Main Outcomes and Measures

Comparison of catecholamine and metanephrine concentrations between at-risk neonates and control participants, and correlation of concentrations of catecholamines and metanephrines with the number and severity of postnatal hypoglycemic episodes.

Results

In this study of 328 neonates (234 in the risk group: median [IQR] gestational age, 270 [261-277] days; and 94 in the control group: median [IQR] gestational age, 273 [270-278] days), growth-restricted neonates showed increased UCB median (IQR) concentrations of norepinephrine (21.10 [9.15-42.33] vs 10.88 [5.78-18.03] nmol/L; P < .001), metanephrine (0.37 [0.13-1.36] vs 0.12 [0.08-0.28] nmol/L; P < .001), and 3-methoxytyramine (0.149 [0.098-0.208] vs 0.091 [0.063-0.149] nmol/L; P = .001). Neonates with perinatal stress had increased UCB median (IQR) concentrations of norepinephrine (22.55 [8.99-131.66] vs 10.88 [5.78-18.03] nmol/L; P = .001), normetanephrine (1.75 [1.16-4.93] vs 1.25 [0.86-2.56] nmol/L; P = .004), and 3-methoxytyramine (0.120 [0.085-0.228] vs 0.091 [0.063-0.149] nmol/L; P = .008) (P < .0083 was considered statistically significant). Concentrations of UCB norepinephrine, metanephrine, and 3-methoxytyramine were negatively correlated with AF C-peptide concentration (rs = -0.212, P = .005; rs = -0.182, P = .016; and rs = -0.183, P = .016, respectively [P < .017 was considered statistically significant]). Concentrations of UCB norepinephrine, metanephrine, and 3-methoxytyramine were positively correlated with the number of hypoglycemic episodes (BG concentration of 30-45 mg/dL) (rs = 0.146, P = .01; rs = 0.151, P = .009; and rs = 0.180, P = .002, respectively). Concentrations of UCB metanephrine and 3-methoxytyramine were negatively correlated with the lowest measured BG concentration (rs = -0.149, P = .01; and rs = -0.153, P = .008, respectively).

Conclusions and Relevance

Neonates at risk for hypoglycemia displayed increased catecholamine and metanephrine concentrations that were correlated with postnatal hypoglycemic episodes and lower BG levels; these results are consistent with findings in a sheep model that fetal catecholamines are associated with neonatal β-cell physiology and that perinatal stress or growth restriction is associated with subsequent neonatal hyperinsulinemic hypoglycemia. Improving the pathomechanistic understanding of neonatal hypoglycemia may help to guide management of newborns at risk for hypoglycemia.

Ion channels regulate energy homeostasis and the progression of metabolic disorders: Novel mechanisms and pharmacology of their modulators

The progression of metabolic diseases, featured by dysregulated metabolic signaling pathways, is orchestrated by numerous signaling networks. Among the regulators, ion channels transport ions across the membranes and trigger downstream signaling transduction. They critically regulate energy homeostasis and pathogenesis of metabolic diseases and are potential therapeutic targets for treating metabolic disorders. Ion channel blockers have been used to treat diabetes for decades by stimulating insulin secretion, yet with hypoglycemia and other adverse effects. It calls for deeper understanding of the largely elusive regulatory mechanisms, which facilitates the identification of new therapeutic targets and safe drugs against ion channels. In the article, we critically assess the two principal regulatory mechanisms, protein-channel interaction and post-translational modification on the activities of ion channels to modulate energy homeostasis and metabolic disorders through multiple novel mechanisms. Moreover, we discuss the multidisciplinary methods that provide the tools for elucidation of the regulatory mechanisms mediating metabolic disorders by ion channels. In terms of translational perspective, the mechanistic analysis of recently validated ion channels that regulate insulin resistance, body weight control, and adverse effects of current ion channel antagonists are discussed in details. Their small molecule modulators serve as promising new drug candidates to combat metabolic disorders.

New approaches to screening and management of neonatal hypoglycemia based on improved understanding of the molecular mechanism of hypoglycemia

For the past 70 years, controversy about hypoglycemia in newborn infants has focused on a numerical "definition of neonatal hypoglycemia", without regard to its mechanism. This ignores the purpose of screening newborns for hypoglycemia, which is to identify those with pathological forms of hypoglycemia and to prevent hypoglycemic brain injury. Recent clinical and basic research indicates that the three major forms of neonatal hypoglycemia are caused by hyperinsulinism (recognizing also that other rare hormonal or metabolic conditions may also present during this time frame). These include transitional hypoglycemia, which affects all normal newborns in the first few days after birth; perinatal stress-induced hypoglycemia in high-risk newborns, which afflicts ∼1 in 1,200 newborns; and genetic forms of congenital hyperinsulinism which afflict ∼1 in 10,000-40,000 newborns. (1) Transitional hyperinsulinism in normal newborns reflects persistence of the low glucose threshold for insulin secretion during fetal life into the first few postnatal days. Recent data indicate that the underlying mechanism is decreased trafficking of ATP-sensitive potassium channels to the beta-cell plasma membrane, likely a result of the hypoxemic state of fetal life. (2) Perinatal stress-induced hyperinsulinism in high-risk infants appears to reflect an exaggeration of this normal low fetal glucose threshold for insulin release due to more severe and prolonged exposure to perinatal hypoxemia. (3) Genetic hyperinsulinism, in contrast, reflects permanent genetic defects in various steps controlling beta-cell insulin release, such as inactivating mutations of the K _ATP-channel genes. The purpose of this report is to review our current knowledge of these three major forms of neonatal hyperinsulinism as a foundation for the diagnosis and management of hypoglycemia in newborn infants. This includes selection of appropriate interventions based on underlying disease mechanism; combined monitoring of both plasma glucose and ketone levels to improve screening for infants with persistent forms of hypoglycemia; and ultimately to ensure that infants at risk of persistent hyperinsulinemic hypoglycemia are recognized prior to discharge from the nursery.

Histidine dephosphorylation of the Gβ protein GPB-1 promotes axon regeneration in C. elegans

Histidine phosphorylation is an emerging noncanonical protein phosphorylation in animals, yet its physiological role remains largely unexplored. The protein histidine phosphatase (PHPT1) was recently identified for the first time in mammals. Here, we report that PHIP-1, an ortholog of PHPT1 in Caenorhabditis elegans, promotes axon regeneration by dephosphorylating GPB-1 Gβ at His-266 and inactivating GOA-1 Goα signaling, a negative regulator of axon regeneration. Overexpression of the histidine kinase NDK-1 also inhibits axon regeneration via GPB-1 His-266 phosphorylation. Thus, His-phosphorylation plays an antiregenerative role in C. elegans. Furthermore, we identify a conserved UNC-51/ULK kinase that functions in autophagy as a PHIP-1-binding protein. We demonstrate that UNC-51 phosphorylates PHIP-1 at Ser-112 and activates its catalytic activity and that this phosphorylation is required for PHIP-1-mediated axon regeneration. This study reveals a molecular link from ULK to protein histidine phosphatase, which facilitates axon regeneration by inhibiting trimeric G protein signaling.

Hyperinsulinemia: beneficial or harmful or both on glucose homeostasis

Insulin, a principal anabolic hormone produced by pancreatic β-cells, has a primary function of storage of nutrients following excessive energy intake. Pre- or early type 2 diabetes stages present hyperinsulinemia (β-cell dysfunction) and insulin resistance. Initiation of hyperinsulinemia is triggered by a loss of first-phase glucose-stimulated insulin secretion with altered membrane ion channel distribution. More factors, including insulin resistance and excessive proliferation of β-cells, deteriorate the hyperinsulinemia, whereas the hyperinsulinemia contributes to further development of insulin resistance and type 2 diabetes; to develop eventually late-stage diabetes with absolute insulin deficiency. In this mini-review, the major focus was put on the causes and pathophysiology of hyperinsulinemia, and the metabolic consequences and current treatment of hyperinsulinemia were discussed. The data used in this narrative review were collected mainly from relevant discoveries in the past 3 years.

A journey from phosphotyrosine to phosphohistidine and beyond

Protein phosphorylation is a reversible post-translational modification. Nine of the 20 natural amino acids in proteins can be phosphorylated, but most of what we know about the roles of protein phosphorylation has come from studies of serine, threonine, and tyrosine phosphorylation. Much less is understood about the phosphorylation of histidine, lysine, arginine, cysteine, aspartate, and glutamate, so-called non-canonical phosphorylations. Phosphohistidine (pHis) was discovered 60 years ago as a mitochondrial enzyme intermediate; since then, evidence for the existence of histidine kinases and phosphohistidine phosphatases has emerged, together with examples where protein function is regulated by reversible histidine phosphorylation. pHis is chemically unstable and has thus been challenging to study. However, the recent development of tools for studying pHis has accelerated our understanding of the multifaceted functions of histidine phosphorylation, revealing a large number of proteins that are phosphorylated on histidine and implicating pHis in a wide range of cellular processes.

TRP Channels as Molecular Targets to Relieve Endocrine-Related Diseases

Transient receptor potential (TRP) channels are polymodal channels capable of sensing environmental stimuli, which are widely expressed on the plasma membrane of cells and play an essential role in the physiological or pathological processes of cells as sensors. TRPs often form functional homo- or heterotetramers that act as cation channels to flow Na⁺ and Ca²⁺, change membrane potential and [Ca²⁺]_i (cytosolic [Ca²⁺]), and change protein expression levels, channel attributes, and regulatory factors. Under normal circumstances, various TRP channels respond to intracellular and extracellular stimuli such as temperature, pH, osmotic pressure, chemicals, cytokines, and cell damage and depletion of Ca²⁺ reserves. As cation transport channels and physical and chemical stimulation receptors, TRPs play an important role in regulating secretion, interfering with cell proliferation, and affecting neural activity in these glands and their adenocarcinoma cells. Many studies have proved that TRPs are widely distributed in the pancreas, adrenal gland, and other glands. This article reviews the specific regulatory mechanisms of various TRP channels in some common glands (pancreas, salivary gland, lacrimal gland, adrenal gland, mammary gland, gallbladder, and sweat gland).

Advances in Understanding the Mechanism of Transitional Neonatal Hypoglycemia and Implications for Management

Our lack of basic knowledge about the basic mechanisms of transitional hypoglycemia and other forms of hypoglycemia in newborns underlies the ongoing controversies over standards for managing these conditions. To address this deficiency, the authors evaluated regulation of insulin secretion in fetal, newborn, and adult rats. The results demonstrate that transitional hypoglycemia in normal neonates and persistent hypoglycemia in high-risk infants both reflect altered beta-cell insulin regulation. These findings provide a new foundation for improving detection and management and preventing hypoglycemic brain injury in normal neonates and, especially, in infants with persistent hypoglycemia and genetic forms of congenital hyperinsulinism.

Decreased KATP Channel Activity Contributes to the Low Glucose Threshold for Insulin Secretion of Rat Neonatal Islets

Transitional hypoglycemia in normal newborns occurs in the first 3 days of life and has clinical features consistent with hyperinsulinism. We found a lower threshold for glucose-stimulated insulin secretion from freshly isolated embryonic day (E) 22 rat islets, which persisted into the first postnatal days. The threshold reached the adult level by postnatal day (P) 14. Culturing P14 islets also decreased the glucose threshold. Freshly isolated P1 rat islets had a lower threshold for insulin secretion in response to 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid, a nonmetabolizable leucine analog, and diminished insulin release in response to tolbutamide, an inhibitor of β-cell KATP channels. These findings suggested that decreased KATP channel function could be responsible for the lower glucose threshold for insulin secretion. Single-cell transcriptomic analysis did not reveal a lower expression of KATP subunit genes in E22 compared with P14 β cells. The investigation of electrophysiological characteristics of dispersed β cells showed that early neonatal and cultured cells had fewer functional KATP channels per unit membrane area. Our findings suggest that decreased surface density of KATP channels may contribute to the observed differences in glucose threshold for insulin release.

LHPP-Mediated Histidine Dephosphorylation Suppresses the Self-Renewal of Mouse Embryonic Stem Cells

Self-renewal of embryonic stem cells (ESCs) is orchestrated by a vast number of genes at the transcriptional and translational levels. However, the molecular mechanisms of post-translational regulatory factors in ESC self-renewal remain unclear. Histidine phosphorylation, also known as hidden phosphorylation, cannot be detected by conventional experimental methods. A recent study defined phospholysine phosphohistidine inorganic pyrophosphate phosphatase (LHPP) as a histidine phosphatase, which regulates various biological behaviors in cells via histidine dephosphorylation. In this study, the doxycycline (DOX)-induced hLHPP-overexpressing mouse ESCs and mouse LHPP silenced mESCs were constructed. Quantitative polymerase chain reaction (qPCR), western blotting analysis, immunofluorescence, Flow cytometry, colony formation assays, alkaline phosphatase (AP) and bromodeoxyuridine (Brdu) staining were performed. We found that the histidine phosphorylation level was strikingly reduced following LHPP overexpression. Besides, the expression of Oct4 and Lefty1, indispensable genes in the process of ESCs self-renewal, was significantly down-regulated, while markers related to the differentiation were markedly elevated. Moreover, LHPP-mediated histidine dephosphorylation induced G_0/G₁ phase arrest in mESCs, suggesting LHPP was implicated in cell proliferation and cell cycle. Conversely, silencing of Lhpp promoted the self-renewal of mESCs and reversed the RA induced increased expression of genes associated with differentiation. Mechanistically, our findings suggested that the enzymatic active site of LHPP was the cysteine residue at position 226, not 53. LHPP-mediated histidine dephosphorylation lowered the expression levels of β-catenin and the cell cycle-related genes CDK4 and CyclinD1, while it up-regulated the cell cycle suppressor genes P21 and P27. Taken together, our findings reveal that LHPP-mediated histidine dephosphorylation plays a role in the self-renewal of ESCs. LHPP-mediated histidine dephosphorylation inhibited the self-renewal of ESCs by negatively regulating the Wnt/β-catenin pathway and downstream cell cycle-related genes, providing a new perspective and regulatory target for ESCs self-renewal.

The phosphohistidine phosphatase SixA dephosphorylates the phosphocarrier NPr

Histidine phosphorylation is a posttranslational modification that alters protein function and also serves as an intermediate of phosphoryl transfer. Although phosphohistidine is relatively unstable, enzymatic dephosphorylation of this residue is apparently needed in some contexts, since both prokaryotic and eukaryotic phosphohistidine phosphatases have been reported. Here we identify the mechanism by which a bacterial phosphohistidine phosphatase dephosphorylates the nitrogen-related phosphotransferase system, a broadly conserved bacterial pathway that controls diverse metabolic processes. We show that the phosphatase SixA dephosphorylates the phosphocarrier protein NPr and that the reaction proceeds through phosphoryl transfer from a histidine on NPr to a histidine on SixA. In addition, we show that Escherichia coli lacking SixA are outcompeted by wild-type E. coli in the context of commensal colonization of the mouse intestine. Notably, this colonization defect requires NPr and is distinct from a previously identified in vitro growth defect associated with dysregulation of the nitrogen-related phosphotransferase system. The widespread conservation of SixA, and its coincidence with the phosphotransferase system studied here, suggests that this dephosphorylation mechanism may be conserved in other bacteria.

NME/NM23/NDPK and Histidine Phosphorylation

The NME (Non-metastatic) family members, also known as NDPKs (nucleoside diphosphate kinases), were originally identified and studied for their nucleoside diphosphate kinase activities. This family of kinases is extremely well conserved through evolution, being found in prokaryotes and eukaryotes, but also diverges enough to create a range of complexity, with homologous members having distinct functions in cells. In addition to nucleoside diphosphate kinase activity, some family members are reported to possess protein-histidine kinase activity, which, because of the lability of phosphohistidine, has been difficult to study due to the experimental challenges and lack of molecular tools. However, over the past few years, new methods to investigate this unstable modification and histidine kinase activity have been reported and scientific interest in this area is growing rapidly. This review presents a global overview of our current knowledge of the NME family and histidine phosphorylation, highlighting the underappreciated protein-histidine kinase activity of NME family members, specifically in human cells. In parallel, information about the structural and functional aspects of the NME family, and the knowns and unknowns of histidine kinase involvement in cell signaling are summarized.

Molecular Regulations and Functions of the Transient Receptor Potential Channels of the Islets of Langerhans and Insulinoma Cells

Insulin secretion from the β-cells of the islets of Langerhans is triggered mainly by nutrients such as glucose, and incretin hormones such as glucagon-like peptide-1 (GLP-1). The mechanisms of the stimulus-secretion coupling involve the participation of the key enzymes that metabolize the nutrients, and numerous ion channels that mediate the electrical activity. Several members of the transient receptor potential (TRP) channels participate in the processes that mediate the electrical activities and Ca²⁺ oscillations in these cells. Human β-cells express TRPC1, TRPM2, TRPM3, TRPM4, TRPM7, TRPP1, TRPML1, and TRPML3 channels. Some of these channels have been reported to mediate background depolarizing currents, store-operated Ca²⁺ entry (SOCE), electrical activity, Ca²⁺ oscillations, gene transcription, cell-death, and insulin secretion in response to stimulation by glucose and GLP1. Different channels of the TRP family are regulated by one or more of the following mechanisms: activation of G protein-coupled receptors, the filling state of the endoplasmic reticulum Ca²⁺ store, heat, oxidative stress, or some second messengers. This review briefly compiles our current knowledge about the molecular mechanisms of regulations, and functions of the TRP channels in the β-cells, the α-cells, and some insulinoma cell lines.

Immunohistochemistry (IHC): Chromogenic Detection of 3-Phosphohistidine Proteins in Formaldehyde-Fixed, Frozen Mouse Liver Tissue Sections

The development of antibodies that specifically detect histidine-phosphorylated proteins is a recent achievement and allows potential roles of histidine phosphorylated proteins in pathological and physiological conditions to be characterized. Immunohistochemical analyses enable the detection of proteins in tissues and can reveal alterations to the quantity and/or localization of these proteins through comparisons of normal and diseased specimens. However, the sensitivity of phosphohistidine modifications to phosphatases, acidic pH, and elevated temperatures poses unique challenges to the detection process and requires a protocol that bypasses traditional procedures utilizing paraffin-embedding and antigen-retrieval methods. Here, we detail a method for a brief fixation by 4% (v/v) paraformaldehyde on freshly collected tissues in the presence of PhosSTOP to block phosphatase activity, followed by a float on sucrose to protect the tissue prior to freezing. Specimens are then embedded in a cryopreservation medium in molds and frozen using an isoflurane, dry ice bath to best preserve the tissue morphology and phosphohistidine signal. We validate this technique in normal mouse liver using SC44-1, a monoclonal anti-3-pHis antibody used to uncover a role for a protein histidine phosphatase as a tumor suppressor in the liver. Furthermore, we demonstrate that the antibody signal can be eliminated by preincubating SC44-1 with a peptide treated with phosphoramidate to phosphorylate histidine residues. Thus, we present an IHC protocol suitable for specific detection of 3-phosphohistidine proteins in mouse liver tissue, and suggest that this can be used as a starting point for optimization of IHC using other phosphohistidine antibodies or in other tissue types, generating information that will enhance our understanding of phosphohistidine in models of disease.

What do archaeal and eukaryotic histidine kinases sense?

Signal transduction systems configured around a core phosphotransfer step between a histidine kinase and a cognate response regulator protein occur in organisms from all domains of life. These systems, termed two-component systems, constitute the majority of multi-component signaling pathways in Bacteria but are less prevalent in Archaea and Eukarya. The core signaling domains are modular, allowing versatility in configuration of components into single-step phosphotransfer and multi-step phosphorelay pathways, the former being predominant in bacteria and the latter in eukaryotes. Two-component systems regulate key cellular regulatory processes that provide adaptive responses to environmental stimuli and are of interest for the development of antimicrobial therapeutics, biotechnology applications, and biosensor engineering. In bacteria, two-component systems have been found to mediate responses to an extremely broad array of extracellular and intracellular chemical and physical stimuli, whereas in archaea and eukaryotes, the use of two-component systems is more limited. This review summarizes recent advances in exploring the repertoire of sensor histidine kinases in the Archaea and Eukarya domains of life.