Functional roles of ADP-ribosylation writers, readers and erasers

ADP-ribosylation is a reversible post-translational modification (PTM) tightly regulated by the dynamic interplay between its writers, readers and erasers. As an intricate and versatile PTM, ADP-ribosylation plays critical roles in various physiological and pathological processes. In this review, we discuss the major players involved in the ADP-ribosylation cycle, which may facilitate the investigation of the ADP-ribosylation function and contribute to the understanding and treatment of ADP-ribosylation associated disease.

Identification of the Mouse T Cell ADP-Ribosylome Uncovers ARTC2.2 Mediated Regulation of CD73 by ADP-Ribosylation

Mouse T cells express the ecto-ADP-ribosyltransferase ARTC2.2, which can transfer the ADP-ribose group of extracellular nicotinamide adenine dinucleotide (NAD⁺) to arginine residues of various cell surface proteins thereby influencing their function. Several targets of ARTC2.2, such as P2X7, CD8a and CD25 have been identified, however a comprehensive mouse T cell surface ADP-ribosylome analysis is currently missing. Using the Af1521 macrodomain-based enrichment of ADP-ribosylated peptides and mass spectrometry, we identified 93 ADP-ribsoylated peptides corresponding to 67 distinct T cell proteins, including known targets such as CD8a and CD25 but also previously unknown targets such as CD73. We evaluated the impact of ADP-ribosylation on the capability of CD73 to generate adenosine from adenosine monophosphate. Our results show that extracellular NAD⁺ reduces the enzymatic activity of CD73 HEK cells co-transfected with CD73/ARTC2.2. Importantly, NAD⁺ significantly reduced CD73 activity on WT CD8 T cells compared to ARTC2ko CD8 T cells or WT CD8 T cells treated with an ARTC2.2-blocking nanobody. Our study provides a comprehensive list of T cell membrane proteins that serve as targets for ADP-ribosylation by ARTC2.2 and whose function may be therefore affected by ADP-ribosylation.

Nanobodies as probes to investigate purinergic signaling

Nanobodies (VHHs) are the single variable immunoglobulin domains of heavy chain antibodies (hcAbs) that naturally occur in alpacas and other camelids. The two variable domains of conventional antibodies typically interact via a hydrophobic interface. In contrast, the corresponding surface area of nanobodies is hydrophilic, rendering these single immunoglobulin domains highly soluble, robust to harsh environments, and exceptionally easy to format into bispecific reagents. In homage to Geoffrey Burnstock, the pioneer of purinergic signaling, we provide a brief history of nanobody-mediated modulation of purinergic signaling, using our nanobodies targeting P2X7 and the NAD⁺-metabolizing ecto-enzymes CD38 and ARTC2.2 as examples.

Astrocytes and Microglia Are Resistant to NAD+-Mediated Cell Death Along the ARTC2/P2X7 Axis

ADP-ribosylation of the P2X7k splice variant on mouse T cells by Ecto-ADP-ribosyltransferase ARTC2.2 in response to its substrate extracellular nicotinamide adenine dinucleotide (NAD⁺) triggers cell death. Since NAD⁺ is released as a danger signal during tissue damage, this NAD⁺-induced cell death (NICD) may impact the survival of other cell populations co-expressing P2X7 and of one of the ARTC2 isoforms (ARTC2.1, ARTC2.2). NICD of brain-resident, non-T cell populations has only been rudimentarily investigated. In this study, we evaluated the susceptibility of two glia cell populations, astrocytes and microglia, towards NICD. We found that astrocytes and microglia strongly upregulate cell surface levels of ARTC2.1 and ADP-ribosylation of cell surface proteins in response to treatment with lipopolysaccharide (LPS) and the mitogen-activated protein kinase kinase (MEK) 1 and 2 inhibitor U0126, but do not respond to extracellular NAD⁺ with P2X7 activation and induction of cell death. Furthermore, we found that astrocytes and microglia preferentially express the ADP-ribosylation-insensitive P2X7a splice variant, likely accounting for the resistance of these cells to NICD.

Suppression of MicroRNA 424 Levels by Human Papillomaviruses Is Necessary for Differentiation-Dependent Genome Amplification

High-risk human papillomaviruses (HPVs) link their life cycle to epithelial differentiation and require activation of DNA damage pathways for efficient replication. HPVs modulate the expression of cellular transcription factors, as well as cellular microRNAs (miRNAs) to control these activities. One miRNA that has been reported to be repressed in HPV-positive cancers of the cervix and oropharynx is miR-424. Our studies show that miR-424 levels are suppressed in cell lines that stably maintain HPV 31 or 16 episomes, as well as cervical cancer lines that contain integrated genomes such as SiHa. Introduction of expression vectors for miR-424 reduced both the levels of HPV genomes in undifferentiated cells and amplification upon differentiation. Our studies show that the levels of two putative targets of miR-424 that function in DNA damage repair, CHK1 and Wee1, are suppressed in HPV-positive cells, providing an explanation for why this microRNA is targeted in HPV-positive cells.IMPORTANCE We describe here for the first time a critical role for miR-424 in the regulation of HPV replication. HPV E6 and E7 proteins suppress the levels of miR-424, and this is important for controlling the levels of CHK1, which plays a central role in viral replication.

Ecto-ADP-ribosyltransferase ARTC2.1 functionally modulates FcγR1 and FcγR2B on murine microglia

Mammalian ecto-ADP-ribosyltransferases (ecto-ARTs or also ARTCs) catalyze the ADP-ribosylation of cell surface proteins using extracellular nicotinamide adenine dinucleotide (NAD⁺) as substrate. By this post-translational protein modification, ecto-ARTs modulate the function of various target proteins. A functional role of ARTC2 has been demonstrated for peripheral immune cells such as T cells and macrophages. Yet, little is known about the role of ecto-ARTs in the central nervous system and on microglia. Here, we identified ARTC2.1 as the major ecto-ART expressed on murine microglia. ARTC2.1 expression was strongly upregulated on microglia upon co-stimulation with LPS and an ERK1/2 inhibitor or upon IFNβ stimulation. We identified several target proteins modified by ARTC2.1 on microglia with a recently developed mass spectrometry approach, including two receptors for immunoglobulin G (IgG), FcγR1 and FcγR2B. Both proteins were verified as targets of ARTC2.1 in vitro using a radiolabeling assay with ³²P-NAD⁺ as substrate. Moreover, ADP-ribosylation of both targets strongly inhibited their capacity to bind IgG. In concordance, ARTC2.1 induction in WT microglia and subsequent cell surface ADP-ribosylation significantly reduced the phagocytosis of IgG-coated latex beads, which was unimpaired in NAD⁺/DTT treated microglia from ARTC2.1^-/- mice. Hence, induction of ARTC2.1 expression under inflammatory conditions, and subsequent ADP-ribosylation of cell surface target proteins could represent a hitherto unnoticed mechanism to regulate the immune response of murine microglia.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

STAT-5 Regulates Transcription of the Topoisomerase IIβ-Binding Protein 1 (TopBP1) Gene To Activate the ATR Pathway and Promote Human Papillomavirus Replication

The life cycle of high-risk human papillomaviruses (HPVs) is dependent upon epithelial differentiation. Following infection of basal cells, HPV genomes are stably maintained at low copy numbers, and productive replication or amplification is restricted to highly differentiated suprabasal cells. In high-risk HPV infections, the ATM pathway is constitutively activated in the absence of external DNA-damaging agents and is required for productive viral replication. The ataxia telangiectasia (ATM) pathway repairs double-strand breaks in DNA, while the ataxia telangiectasia and Rad3-related (ATR) pathway targets single-strand breaks. Our studies show that the ATR pathway, like the ATM pathway, is activated in HPV-positive cells and that inhibitors of ATR or CHK1 phosphorylation block both amplification and late viral gene expression in differentiated cells while moderately reducing stable copy numbers in undifferentiated cells. TopBP1 is a critical upstream activator of the ATR pathway and is expressed at elevated levels in HPV-positive cells. This increased expression of TopBP1 is necessary for ATR/CHK1 activation in HPV-positive cells, and knockdown blocks amplification. Furthermore, TopBP1 activation is shown to be regulated at the level of transcription initiation by the innate immune regulator STAT-5, which is activated by HPV proteins. STAT-5 has also been shown to be a regulator of the ATM response, demonstrating that these two pathways are coordinately regulated in HPV-positive cells. These findings identify a novel link between the innate immune response and activation of the ATR DNA damage response in regulating the life cycle of high-risk HPVs.

High-risk human papillomaviruses (HPVs) are the causative agents of cervical and other anogenital cancers, as well as many oral cancers. HPVs infect epithelial cells and restrict productive viral replication or amplification and virion production to differentiated cells. Our studies demonstrate that HPVs activate the ATR single-strand DNA repair pathway and this activation is necessary for HPV genome amplification. The innate immune regulator STAT-5 is shown to regulate transcription of the ATR binding factor TopBP1, and this is critical for the induction of the ATR pathway. Our study identifies important links between innate immune signaling, the ATR DNA damage pathway, and productive HPV replication that may lead to the characterization of new targets for the development of therapeutics to treat HPV-induced infections.

A functional genomics tool for the Pacific bluefin tuna: Development of a 44K oligonucleotide microarray from whole-genome sequencing data for global transcriptome analysis

Bluefin tunas are one of the most important fishery resources worldwide. Because of high market values, bluefin tuna farming has been rapidly growing during recent years. At present, the most common form of the tuna farming is based on the stocking of wild-caught fish. Therefore, concerns have been raised about the negative impact of the tuna farming on wild stocks. Recently, the Pacific bluefin tuna (PBT), Thunnus orientalis, has succeeded in completing the reproduction cycle under aquaculture conditions, but production bottlenecks remain to be solved because of very little biological information on bluefin tunas. Functional genomics approaches promise to rapidly increase our knowledge on biological processes in the bluefin tuna. Here, we describe the development of the first 44K PBT oligonucleotide microarray (oligo-array), based on whole-genome shotgun (WGS) sequencing and large-scale expressed sequence tags (ESTs) data. In addition, we also introduce an initial 44K PBT oligo-array experiment using in vitro grown peripheral blood leukocytes (PBLs) stimulated with immunostimulants such as lipopolysaccharide (LPS: a cell wall component of Gram-negative bacteria) or polyinosinic:polycytidylic acid (poly I:C: a synthetic mimic of viral infection). This pilot 44K PBT oligo-array analysis successfully addressed distinct immune processes between LPS- and poly I:C- stimulated PBLs. Thus, we expect that this oligo-array will provide an excellent opportunity to analyze global gene expression profiles for a better understanding of diseases and stress, as well as for reproduction, development and influence of nutrition on tuna aquaculture production.

The acetyltransferase Tip60 is a critical regulator of the differentiation-dependent amplification of human papillomaviruses

The life cycle of human papillomaviruses (HPVs) is dependent upon differentiation of the infected host epithelial cell as well as activation of the ataxia telangiectasia mutated (ATM) DNA repair pathway that in normal cells acts to repair double-strand DNA breaks. In normal cells, following DNA damage the acetyltransferase Tip60 must acetylate ATM proteins prior to their full activation by autophosphorylation. E6 proteins have been shown to induce the degradation of Tip60, suggesting that Tip60 action may not be required for activation of the ATM pathway in HPV-positive cells. We investigated what role, if any, Tip60 plays in regulating the differentiation-dependent HPV life cycle. Our study indicates that Tip60 levels and activity are increased in cells that stably maintain complete HPV genomes as episomes, while low levels are seen in cells that express only HPV E6 and E7 proteins. Knockdown of Tip60 with short hairpin RNAs in cells that maintain HPV episomes blocked ATM induction and differentiation-dependent genome amplification, demonstrating the critical role of Tip60 in the viral life cycle. The JAK/STAT transcription factor STAT-5 has previously been shown to regulate the phosphorylation of ATM. Our studies demonstrate that STAT-5 regulates Tip60 activation and this occurs in part by targeting glycogen synthase kinase 3β (GSK3β). Inhibition of either STAT-5, Tip60, or GSK3β blocked differentiation-dependent genome amplification. Taken together, our findings identify Tip60 to be an important regulator of HPV genome amplification whose activity during the viral life cycle is controlled by STAT-5 and the kinase GSK3β.

IMPORTANCE

Human papillomaviruses (HPVs) are the etiological agents of cervical and other anogenital cancers. HPVs regulate their differentiation-dependent life cycle by activation of DNA damage pathways. This study demonstrates that HPVs regulate the ATM DNA damage pathway through the action of the acetyltransferase Tip60. Furthermore, the innate immune regulator STAT-5 and the kinase GSK3β mediate the activation of Tip60 in HPV-positive cells. This study identifies critical regulators of the HPV life cycle.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

Post-translational control of protein function by disulfide bond cleavage

Protein action in nature is largely controlled by the level of expression and by post-translational modifications. Post-translational modifications result in a proteome that is at least two orders of magnitude more diverse than the genome. There are three basic types of post-translational modifications: covalent modification of an amino acid side chain, hydrolytic cleavage or isomerization of a peptide bond, and reductive cleavage of a disulfide bond. This review addresses the modification of disulfide bonds. Protein disulfide bonds perform either a structural or a functional role, and there are two types of functional disulfide: the catalytic and allosteric bonds. The allosteric disulfide bonds control the function of the mature protein in which they reside by triggering a change when they are cleaved. The change can be in ligand binding, substrate hydrolysis, proteolysis, or oligomer formation. The allosteric disulfides are cleaved by oxidoreductases or by thiol/disulfide exchange, and the configurations of the disulfides and the secondary structures that they link share some recurring features. How these bonds are being identified using bioinformatics and experimental screens and what the future holds for this field of research are also discussed.

The JAK-STAT transcriptional regulator, STAT-5, activates the ATM DNA damage pathway to induce HPV 31 genome amplification upon epithelial differentiation

High-risk human papillomavirus (HPV) must evade innate immune surveillance to establish persistent infections and to amplify viral genomes upon differentiation. Members of the JAK-STAT family are important regulators of the innate immune response and HPV proteins downregulate expression of STAT-1 to allow for stable maintenance of viral episomes. STAT-5 is another member of this pathway that modulates the inflammatory response and plays an important role in controlling cell cycle progression in response to cytokines and growth factors. Our studies show that HPV E7 activates STAT-5 phosphorylation without altering total protein levels. Inhibition of STAT-5 phosphorylation by the drug pimozide abolishes viral genome amplification and late gene expression in differentiating keratinocytes. In contrast, treatment of undifferentiated cells that stably maintain episomes has no effect on viral replication. Knockdown studies show that the STAT-5β isoform is mainly responsible for this activity and that this is mediated through the ATM DNA damage response. A downstream target of STAT-5, the peroxisome proliferator-activated receptor γ (PPARγ) contributes to the effects on members of the ATM pathway. Overall, these findings identify an important new regulatory mechanism by which the innate immune regulator, STAT-5, promotes HPV viral replication through activation of the ATM DNA damage response.

Over 120 types of human papillomavirus (HPV) have been identified, and approximately one-third of these infect epithelial cells of the genital mucosa. A subset of HPV types are the causative agents of cervical and other anogenital cancers. The infectious life cycle of HPV is dependent on differentiation of the host epithelial cell, with viral genome amplification and virion production restricted to differentiated suprabasal cells. While normal keratinocytes exit the cell cycle upon differentiation, HPV-positive suprabasal cells are able to re-enter S-phase to mediate productive replication. HPV induces an ATM-dependent DNA damage response in differentiating cells that is essential for viral genome amplification. Our studies describe an important mechanism by which human papillomaviruses activate a member of the JAK/STAT innate immune signaling pathway to induce the ATM DNA damage pathway. This is necessary for differentiation-dependent productive viral replication. HPVs must suppress the transcription of one member of the JAK/STAT pathway, STAT-1, while at the same time activating STAT-5 to regulate genome amplification in suprabasal cells. The E7 protein activates STAT-5 leading to induction of ATM phosphorylation through the PPARγ pathway. Our study identifies important links between innate immune signaling, the ATM DNA damage pathway and productive HPV replication that may lead to the characterization of new targets for the development of therapeutics to treat HPV-induced infections.

Alternative splicing of the N-terminal cytosolic and transmembrane domains of P2X7 controls gating of the ion channel by ADP-ribosylation

P2X7 is a homotrimeric ion channel with two transmembrane domains and a large extracellular ATP-binding domain. It plays a key role in the response of immune cells to danger signals released from cells at sites of inflammation. Gating of murine P2X7 can be induced by the soluble ligand ATP, as well as by NAD⁺-dependent ADP-ribosylation of arginine 125, a posttranslational protein modification catalyzed by the toxin-related ecto-enzymes ART2.1 and ART2.2. R125 is located at the edge of the ligand-binding crevice. Recently, an alternative splice variant of P2X7, designated P2X7(k), was discovered that differs from the previously described variant P2X7(a) in the N-terminal 42 amino acid residues composing the first cytosolic domain and most of the Tm1 domain. Here we compare the two splice variants of murine P2X7 with respect to their sensitivities to gating by ADP-ribosylation in transfected HEK cells. Our results show that the P2X7(k) variant is sensitive to activation by ADP-ribosylation whereas the P2X7(a) variant is insensitive, despite higher cell surface expression levels. Interestingly, a single point mutation (R276K) renders the P2X7(a) variant sensitive to activation by ADP-ribosylation. Residue 276 is located at the interface of neighboring subunits approximately halfway between the ADP-ribosylation site and the transmembrane domains. Moreover, we show that naive and regulatory T cells preferentially express the more sensitive P2X7(k) variant, while macrophages preferentially express the P2X7(a) variant. Our results indicate that differential splicing of alternative exons encoding the N-terminal cytosolic and transmembrane domains of P2X7 control the sensitivity of different immune cells to extracellular NAD⁺ and ATP.

NAD induces astrocyte calcium flux and cell death by ART2 and P2X7 pathway

Mono-ADP-ribosyltransferase 2 (ART2) is found in mouse T cells and has mediated NAD-induced cell death (NICD) alongside the P2X7 pathway. We determined whether ART2 was expressed in mouse brain astrocytes and the possible function of the NAD-ART2-P2X7 pathway in astrocytes. Our results demonstrate that ART2 existed both in cultured mouse astrocytes and mouse brain slices. Exposure of astrocytes to the ART2 substrate, NAD, induced calcium elevation, which was blocked by ART2 and P2X7 inhibitors. ATP and NAD had an additive effect on calcium elevation. NICD in low-calcium conditions was blocked by ART2 and P2X7 inhibitors. The harmful effect of ATP on astrocytes was inhibited by P2X7 and ART2 inhibitors, meaning that endogenous NAD release may occur. Both NICD function and oxygen-glucose deprivation injury in mouse brain slices were also involved in the ART2-P2X7 pathway. Collectively, to our knowledge, our study provides the first evidence that ART2 exists in mouse brain astrocytes and NAD induces calcium elevation and astrocyte death by an ART2 and P2X7-mediated mechanism. The results suggest a novel approach for manipulating astrocyte death.

Impact of ectoenzymes on p2 and p1 receptor signaling

P2 receptors that are activated by extracellular nucleotides (e.g., ATP, ADP, UTP, UDP, Ap(n)A) and P1 receptors activated by adenosine control a diversity of biological processes. The activation of these receptors is tightly regulated by ectoenzymes that metabolize their ligands. This review presents these enzymes as well as their roles in the regulation of P2 and P1 receptor activation. We focus specifically on the role of ectoenzymes in processes of our interest, that is, inflammation, vascular tone, and neurotransmission. An update on the development of ectonucleotidase inhibitors is also presented.

Suppression of STAT-1 expression by human papillomaviruses is necessary for differentiation-dependent genome amplification and plasmid maintenance

High-risk human papillomaviruses (HPVs) infect stratified epithelia to establish persistent infections that maintain low-copy-number episomes in infected basal cells. Amplification of viral genomes occurs upon keratinocyte differentiation, followed by virion synthesis. During persistent HPV infections, viral proteins act to evade surveillance by both innate and adaptive immune responses. One of the primary pathways regulating the innate immune response is the JAK/STAT pathway. Our studies indicate that the expression of STAT-1, but not other members of interferon (IFN)-stimulated gene factor 3 (ISGF-3) complex such as STAT-2 and IFN regulatory factor 9 (IRF9), is selectively suppressed by HPV proteins at the level of transcription. Both E6 and E7 oncoproteins independently suppress the expression of STAT-1, and mutational analyses indicate that the E6 targeting E6-associated protein (E6AP) is responsible for suppression. The levels of STAT-1 proteins increase upon differentiation of both normal and HPV-positive cells but are still significantly reduced in the latter cells. Transient restoration of STAT-1 levels in HPV-positive cells using recombinant retroviruses significantly impaired viral amplification upon differentiation while long-term increases abrogated maintenance of episomes. Similarly, increased levels of STAT-1 induced by gamma interferon treatment inhibited HPV genome amplification upon differentiation. Overall, our findings demonstrate that suppression of STAT-1 expression by HPV proteins is necessary for genome amplification and maintenance of episomes, suggesting an important role for this activity in viral pathogenesis.

A new model of interactive effects of alcohol and high-fat diet on hepatic fibrosis

BACKGROUND

Alcoholic steatohepatitis (ASH) and nonalcoholic steatohepatitis (NASH) are the most frequent conditions leading to elevated liver enzymes and liver cirrhosis, respectively, in the Western world. However, despite strong epidemiological evidence for combined effects on the progression of liver injury, the mutual interaction of the pathophysiological mechanisms is incompletely understood. The aim of this study was to establish and analyze an experimental murine model, where we combined chronic alcohol administration with a NASH-inducing high-fat (HF) diet.

METHODS

Balb/c mice were randomly allocated into 4 experimental groups receiving (i) standard chow, (ii) an HF diet, (iii) alcohol in drinking water (increasing concentrations up to 5%), or (iv) an HF diet and alcohol ad libitum for 6 weeks.

RESULTS

An HF diet significantly induced hepatic triglyceride accumulation and expression of proinflammatory genes (p47(phox) and tumor necrosis factor), while the effects of alcohol alone were less pronounced. However, in combination with HF diet, alcohol significantly enhanced proinflammatory gene expression compared to the HF diet alone. Furthermore, alcohol as well as HF diet led to a marked increase in profibrogenic genes (collagen type I and transforming growth factor-β), activation of hepatic stellate cells, and extracellular matrix deposition in the liver tissue, and noteworthy, the combination of both alcohol and HF diet led to a further marked induction of hepatic fibrosis. Moreover, endotoxin levels in the portal circulation were significantly elevated in mice that received alcohol or HF diet and were further significantly increased in those receiving both. Furthermore and surprisingly, HF diet alone and in combination with alcohol led to a markedly increased hepatic expression of the endotoxin receptor Toll-like receptor 4 (TLR4), which is known to play a crucial role in hepatic fibrosis.

CONCLUSIONS

In summary, this new model allows the investigation of isolated or joint effects of alcohol and HF diet on hepatic injury, where alcohol and HF diet appear to act synergistically on the development of hepatic fibrosis, potentially via enhanced TLR4 signaling.

Testing the role of P2X7 receptors in the development of type 1 diabetes in nonobese diabetic mice

Although P2rx7 has been proposed as a type 1 diabetes (T1D) susceptibility gene in NOD mice, its potential pathogenic role has not been directly determined. To test this possibility, we generated a new NOD stock deficient in P2X(7) receptors. T1D development was not altered by P2X(7) ablation. Previous studies found CD38 knockout (KO) NOD mice developed accelerated T1D partly because of a loss of CD4(+) invariant NKT (iNKT) cells and Foxp3(+) regulatory T cells (Tregs). These immunoregulatory T cell populations are highly sensitive to NAD-induced cell death activated by ADP ribosyltransferase-2 (ART2)-mediated ADP ribosylation of P2X(7) receptors. Therefore, we asked whether T1D acceleration was suppressed in a double-KO NOD stock lacking both P2X(7) and CD38 by rescuing CD4(+) iNKT cells and Tregs from NAD-induced cell death. We demonstrated that P2X(7) was required for T1D acceleration induced by CD38 deficiency. The CD38 KO-induced defects in homeostasis of CD4(+) iNKT cells and Tregs were corrected by coablation of P2X(7). T1D acceleration in CD38-deficient NOD mice also requires ART2 expression. If increased ADP ribosylation of P2X(7) in CD38-deficient NOD mice underlies disease acceleration, then a comparable T1D incidence should be induced by coablation of both CD38 and ART2, or CD38 and P2X(7). However, a previously established NOD stock deficient in both CD38 and ART2 expression is T1D resistant. This study demonstrated the presence of a T1D resistance gene closely linked to the ablated Cd38 allele in the previously reported NOD stock also lacking ART2, but not in the newly generated CD38/P2X(7) double-KO line.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

Regulation of vascular smooth muscle cell calcification by extracellular pyrophosphate homeostasis: synergistic modulation by cyclic AMP and hyperphosphatemia

Vascular calcification is a multifaceted process involving gain of calcification inducers and loss of calcification inhibitors. One such inhibitor is inorganic pyrophosphate (PP(i)), and regulated generation and homeostasis of extracellular PP(i) is a critical determinant of soft-tissue mineralization. We recently described an autocrine mechanism of extracellular PP(i) generation in cultured rat aortic vascular smooth muscle cells (VSMC) that involves both ATP release coupled to the ectophosphodiesterase/pyrophosphatase ENPP1 and efflux of intracellular PP(i) mediated or regulated by the plasma membrane protein ANK. We now report that increased cAMP signaling and elevated extracellular inorganic phosphate (P(i)) act synergistically to induce calcification of these VSMC that is correlated with progressive reduction in ability to accumulate extracellular PP(i). Attenuated PP(i) accumulation was mediated in part by cAMP-dependent decrease in ANK expression coordinated with cAMP-dependent increase in expression of TNAP, the tissue nonselective alkaline phosphatase that degrades PP(i). Stimulation of cAMP signaling did not alter ATP release or ENPP1 expression, and the cAMP-induced changes in ANK and TNAP expression were not sufficient to induce calcification. Elevated extracellular P(i) alone elicited only minor calcification and no significant changes in ANK, TNAP, or ENPP1. In contrast, combined with a cAMP stimulus, elevated P(i) induced decreases in the ATP release pathway(s) that supports ENPP1 activity; this resulted in markedly reduced rates of PP(i) accumulation that facilitated robust calcification. Calcified VSMC were characterized by maintained expression of multiple SMC differentiation marker proteins including smooth muscle (SM) alpha-actin, SM22alpha, and calponin. Notably, addition of exogenous ATP (or PP(i) per se) rescued cAMP + phosphate-treated VSMC cultures from progression to the calcified state. These observations support a model in which extracellular PP(i) generation mediated by both ANK- and ATP release-dependent mechanisms serves as a critical regulator of VSMC calcification.

A recombinant heavy chain antibody approach blocks ART2 mediated deletion of an iNKT cell population that upon activation inhibits autoimmune diabetes

The ectoenzyme ADP-ribosyltransferase 2.2 (ART2.2) can apoptotically delete various T-cell subsets. Depending on the involved apoptotic T-cell subset, enhanced ART2.2 activity could result in immunosuppression or autoimmunity. Diminished activity of the CD38 ectoenzyme that normally represents a counter-regulatory competitor for the NAD substrate represents one mechanism enhancing ART2.2 activity. Hence, it would be desirable to develop an agent that efficiently blocks ART2.2 activity in vivo. While the llama derived recombinant s+16 single domain antibody overcame the difficulty of specifically targeting the ART2.2 catalytic site potential therapeutic use of this reagent is limited due to short in vivo persistence. Thus, we tested if a modified version of s+16 incorporating the murine IgG1 Fc tail (s+16Fc) mediated long-term efficient in vivo suppression of ART2.2. We reasoned an ideal model to test the s+16Fc reagent were NOD mice in which genetic ablation of CD38 results in an ART2.2 mediated reduction in already sub-normal numbers of immunoregulatory natural killer T-(NKT) cells to a level that no longer allows them when activated by the super-agonist alpha-galactosylceramide (alpha-GalCer) to elicit effects inhibiting autoimmune type 1 diabetes (T1D) development. Treatment with s+16Fc efficiently mediated long-term in vivo inhibition of ART2.2 activity in NOD.CD38(null) mice, restoring their iNKT cell numbers to levels that upon alpha-GalCer activation were capable of inhibiting T1D development.

Differential regulation of P2X7 receptor activation by extracellular nicotinamide adenine dinucleotide and ecto-ADP-ribosyltransferases in murine macrophages and T cells

Extracellular NAD induces the ATP-independent activation of the ionotropic P2X(7) purinergic receptor (P2X(7)R) in murine T lymphocytes via a novel covalent pathway involving ADP-ribosylation of arginine residues on the P2X(7)R ectodomain. This modification is catalyzed by ART2.2, a GPI-anchored ADP-ribosyltransferase (ART) that is constitutively expressed in murine T cells. We previously reported that ART2.1, a related ecto-ART, is up-regulated in inflammatory murine macrophages that constitutively express P2X(7)R. Thus, we tested the hypothesis that extracellular NAD acts via ART2.1 to regulate P2X(7)R function in murine macrophages. Coexpression of the cloned murine P2X(7)R with ART2.1 or ART2.2 in HEK293 cells verified that P2X(7)R is an equivalent substrate for ADP-ribosylation by either ART2.1 or ART2.2. However, in contrast with T cells, the stimulation of macrophages or HEK293 cells with NAD alone did not activate the P2X(7)R. Rather, NAD potentiated ATP-dependent P2X(7)R activation as indicated by a left shift in the ATP dose-response relationship. Thus, extracellular NAD regulates the P2X(7)R in both macrophages and T cells but via distinct mechanisms. Although ADP-ribosylation is sufficient to gate a P2X(7)R channel opening in T cells, this P2X(7)R modification in macrophages does not gate the channel but decreases the threshold for gating in response to ATP binding. These findings indicate that extracellular NAD and ATP can act synergistically to regulate P2X(7)R signaling in murine macrophages and also suggest that the cellular context in which P2X(7)R signaling occurs differs between myeloid and lymphoid leukocytes.

Basal and inducible expression of the thiol-sensitive ART2.1 ecto-ADP-ribosyltransferase in myeloid and lymphoid leukocytes

ADP-ribosylation of cell surface proteins in mammalian cells is a post-translational modification by which ecto-ADP-ribosyltransferases (ARTs) transfer ADP-ribose from extracellular NAD to protein targets. The ART2 locus at murine chromosome 7 encompasses the tandem Art2a and Art2b genes that encode the distinct ART2.1 and ART2.2 proteins. Although both ecto-enzymes share 80% sequence identity, ART2.1 activity is uniquely regulated by an allosteric disulfide bond that is reducible in the presence of extracellular thiols, such as cysteine and glutathione, that accumulate in hypoxic and ischemic tissues. Previous studies have characterized the expression of ART2.1 and ART2.2 in murine T lymphocytes but not in other major classes of lymphoid and myeloid leukocytes. Here, we describe the expression of ART2.1 activity in a wide range of freshly isolated or tissue-cultured murine myeloid and lymphoid leukocytes. Spleen-derived macrophages, dendritic cells (DC), and B cells constitutively express ART2.1 as their predominant ART while spleen T cells express both ART2.1 and the thiol-independent ART2.2 isoform. Although bone-marrow-derived macrophages (BMDM) and dendritic cells (BMDC) constitutively express ART2.1 at low levels, it is markedly up-regulated when these cells are stimulated in vitro with IFNbeta or IFNgamma. ART2.1 expression and activity in splenic B cells is modestly up-regulated during incubation in vitro for 24 h, a condition that promotes B cell apoptosis. This increase in ART2.1 is attenuated by IL-4 (a B cell survival factor), but is not affected by IFNbeta/gamma, suggesting a possible induction of ART2.1 as an ancillary response to B cell apoptosis. In contrast, ART2.1 and ART2.2, which are highly expressed in freshly isolated splenic T cells, are markedly down-regulated when purified T cells are incubated in vitro for 12-24 h. Studies with the BW5147 mouse thymocyte line verified basal expression of ART2.1 and ART2.2, as in primary spleen T cells, and demonstrated that both isoforms can be up-regulated when T cells are maintained in the presence of IFNs. Comparison of the surface proteins which are ADP-ribosylated by ART2.1 in the different leukocyte subtypes indicated both shared and cell-specific proteins as ART2.1 substrates. The LFA-1 integrin, a major target for ART2.2 in T cells, is also ADP-ribosylated by the ART2.1 expressed in macrophages. Thus, ART2.1, in contrast to ART2.2, is expressed in a broad range of myeloid and lymphoid leukocytes. The thiol redox-sensitive nature of this ecto-enzyme suggests an involvement in purinergic signaling that occurs in the combined context of inflammation and hypoxia/ischemia.

Extracellular NAD is a regulator for FcgammaR-mediated phagocytosis in murine macrophages

NAD is available in the extracellular environment and elicits immune modulation such as T cell apoptosis by being used as the substrate of cell surface ADP-ribosyl transferase. However, it is unclear whether extracellular NAD affects function of macrophages expressing cell surface ADP-ribosyl transferase. Here we show that extracellular NAD enhances Fcgamma receptor (FcgammaR)-mediated phagocytosis in J774A.1 macrophages via the conversion into cyclic ADP-ribose (cADPR), a potent calcium mobilizer, by CD38, an ADP-ribosyl cyclase. Extracellular NAD increased the phagocytosis of IgG-coated sheep red blood cells (IgG-SRBC) in J774A.1 macrophages, which was completely abolished by pretreatment of 8-bromo-cADPR, an antagonist of cADPR, or CD38 knockdown. Extracellular NAD increased basal intracellular Ca(2+) concentration, which also was abolished by pretreatment of 8-bromo-cADPR or CD38 knockdown. Moreover, the chelation of intracellular calcium abolished NAD-induced enhancement of phagocytosis of IgG-SRBC. Our results suggest that extracellular NAD act as a regulator for FcgammaR-mediated phagocytosis in macrophages.