Processing of ADP-ribosylated integrin alpha 7 in skeletal muscle myotubes

Mono-ADP-ribosylation sites of human CD73 inhibit its adenosine-generating enzymatic activity

CD73-derived adenosine plays a major role in damage-induced tissue responses by inhibiting inflammation. Damage-associated stimuli, such as hypoxia and mechanical stress, induce the cellular release of ATP and NAD⁺ and upregulate the expression of the nucleotide-degrading purinergic ectoenzyme cascade, including adenosine-generating CD73. Extracellular NAD⁺ also serves as substrate for mono-ADP-ribosylation of cell surface proteins, which in human cells is mediated by ecto-ADP-ribosyltransferase 1 (ARTC1). Here we explored, whether human CD73 enzymatic activity is regulated by mono-ADP-ribosylation, using recombinant human CD73 in the presence of ARTC1 with etheno-labelled NAD⁺ as substrate. Multi-colour immunoblotting with an anti-etheno-adenosine antibody showed ARTC1-mediated transfer of ADP-ribose together with the etheno label to CD73. HPLC analysis of the enzymatic activity of in vitro-ribosylated CD73 revealed strong inhibition of adenosine generation in comparison to non-ribosylated CD73. Mass spectrometry of in vitro-ribosylated CD73 identified six ribosylation sites. 3D model analysis indicated that three of them (R328, R354, R545) can interfere with CD73 enzymatic activity. Our study identifies human CD73 as target for ARTC1-mediated mono-ADP-ribosylation, which can profoundly modulate its adenosine-generating activity. Thus, in settings with enhanced release of NAD⁺ as substrate for ARTC1, assessment of CD73 protein expression in human tissues may not be predictive of adenosine formation resulting in anti-inflammatory activity.

Nampt controls skeletal muscle development by maintaining Ca2+ homeostasis and mitochondrial integrity

Objective

NAD⁺ is a co-factor and substrate for enzymes maintaining energy homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT) controls NAD⁺ synthesis, and in skeletal muscle, NAD⁺ is essential for muscle integrity. However, the underlying molecular mechanisms by which NAD⁺ synthesis affects muscle health remain poorly understood. Thus, the objective of the current study was to delineate the role of NAMPT-mediated NAD⁺ biosynthesis in skeletal muscle development and function.

Methods

To determine the role of Nampt in muscle development and function, we generated skeletal muscle-specific Nampt KO (SMNKO) mice. We performed a comprehensive phenotypic characterization of the SMNKO mice, including metabolic measurements, histological examinations, and RNA sequencing analyses of skeletal muscle from SMNKO mice and WT littermates.

Results

SMNKO mice were smaller, with phenotypic changes in skeletal muscle, including reduced fiber area and increased number of centralized nuclei. The majority of SMNKO mice died prematurely. Transcriptomic analysis identified that the gene encoding the mitochondrial permeability transition pore (mPTP) regulator Cyclophilin D (Ppif) was upregulated in skeletal muscle of SMNKO mice from 2 weeks of age, with associated increased sensitivity of mitochondria to the Ca²⁺-stimulated mPTP opening. Treatment of SMNKO mice with the Cyclophilin D inhibitor, Cyclosporine A, increased membrane integrity, decreased the number of centralized nuclei, and increased survival.

Conclusions

Our study demonstrates that NAMPT is crucial for maintaining cellular Ca²⁺ homeostasis and skeletal muscle development, which is vital for juvenile survival.

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NAD⁺ salvage capacity is important for skeletal muscle development and survival.

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Skeletal muscle-specific Nampt knockout mice exhibit a dystrophy-like phenotype.

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Nampt deletion alters Ca²⁺ homeostasis and impairs mitochondrial function.

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Low NAD⁺ levels signals mitochondrial permeability transition pore opening.

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Cyclosporin A treatment improves sarcolemma integrity and increases survival rate.

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.

Proteomic Characterization of the Heart and Skeletal Muscle Reveals Widespread Arginine ADP-Ribosylation by the ARTC1 Ectoenzyme

The clostridium-like ecto-ADP-ribosyltransferase ARTC1 is expressed in a highly restricted manner in skeletal muscle and heart tissue. Although ARTC1 is well studied, the identification of ARTC1 targets in vivo and subsequent characterization of ARTC1-regulated cellular processes on the proteome level have been challenging and only a few ARTC1-ADP-ribosylated targets are known. Applying our recently developed mass spectrometry-based workflow to C2C12 myotubes and to skeletal muscle and heart tissues from wild-type mice, we identify hundreds of ARTC1-ADP-ribosylated proteins whose modifications are absent in the ADP-ribosylome of ARTC1-deficient mice. These proteins are ADP-ribosylated on arginine residues and mainly located on the cell surface or in the extracellular space. They are associated with signal transduction, transmembrane transport, and muscle function. Validation of hemopexin (HPX) as a ARTC1-target protein confirmed the functional importance of ARTC1-mediated extracellular arginine ADP-ribosylation at the systems level.

A need for NAD+ in muscle development, homeostasis, and aging

Skeletal muscle enables posture, breathing, and locomotion. Skeletal muscle also impacts systemic processes such as metabolism, thermoregulation, and immunity. Skeletal muscle is energetically expensive and is a major consumer of glucose and fatty acids. Metabolism of fatty acids and glucose requires NAD+ function as a hydrogen/electron transfer molecule. Therefore, NAD+ plays a vital role in energy production. In addition, NAD+ also functions as a cosubstrate for post-translational modifications such as deacetylation and ADP-ribosylation. Therefore, NAD+ levels influence a myriad of cellular processes including mitochondrial biogenesis, transcription, and organization of the extracellular matrix. Clearly, NAD+ is a major player in skeletal muscle development, regeneration, aging, and disease. The vast majority of studies indicate that lower NAD+ levels are deleterious for muscle health and higher NAD+ levels augment muscle health. However, the downstream mechanisms of NAD+ function throughout different cellular compartments are not well understood. The purpose of this review is to highlight recent studies investigating NAD+ function in muscle development, homeostasis, disease, and regeneration. Emerging research areas include elucidating roles for NAD+ in muscle lysosome function and calcium mobilization, mechanisms controlling fluctuations in NAD+ levels during muscle development and regeneration, and interactions between targets of NAD+ signaling (especially mitochondria and the extracellular matrix). This knowledge should facilitate identification of more precise pharmacological and activity-based interventions to raise NAD+ levels in skeletal muscle, thereby promoting human health and function in normal and disease states.

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.

Mono-ADP-ribosylation of histone 3 at arginine-117 promotes proliferation through its interaction with P300

Relatively little attention has been paid to ADP-ribosylated modifications of histones, especially to mono-ADP-ribosylation. As an increasing number of mono-ADP-ribosyltransferases have been identified in recent studies, the functions of mono-ADP-ribosylated proteins have aroused research interest. In particular, histones are substrates of some mono-ADP-ribosyltransferases and mono-ADP-ribosylated histone have been detected in physiological or pathological processes. In this research, arginine-117 (Arg-117; R-117) of hsitone3(H3) is identified as the a site of mono-ADP-ribosylation in colon carcinoma(the first such site to be identified); this posttranslational modification may promote the proliferation of colon carcinoma cells in vitro and in vivo. Using a point-mutant lentivirus transfection and using an activator of P300 allowed us to observe the mono-ADP-ribosylation at H3R117 and enhancement of the activity of P300 to up-regulate the level of acetylated β-catenin, which could increase the expression of c-myc and cyclin D1.

The effects of MIBG on the invasive properties of HepG2 hepatocellular carcinoma cells

The aim of the present study was to investigate the effects of meta-iodobenzylguanidine (MIBG) on the invasive properties of hepatocellular carcinoma (HCC) cells and examine whether these effects are due to the ability of MIBG to inhibit arginine-specific mono-ADP-ribosylation. Samples from patients with HCC were divided into 2 groups, a metastatic group and a non-metastatic group. Immunohistochemistry and RT-PCR were used to detect the protein and mRNA expression of arginine-specific adenosine diphosphate-ribosyltransferase 1 (ART1) and integrin α7 in the HCC tissues. In addition, the expression of ART1 was measured in HepG2 HCC cells by immunofluorescence. The inhibition of the metastasis of HepG2 cells by MIBG at various concentrations was measured by MTT assay. In addition, the effects of MIBG on HepG2 cell metastasis were measured using a scratch wound assay and a transwell invasion assay. Western blot analysis was used to detect the protein expression of ART1, integrin α7, focal adhesion kinase (FAK), phosphatidylinositol 3-kinase (PI3K) and urokinase-type plasminogen activator (uPA) in the HepG2 cells. The mRNA and protein levels of ART1 and integrin α7 were higher in the metastatic HCC samples than in the non-metastatic HCC samples. ART1 expression was detected in the HepG2 cells. The half maximal inhibition concentration (IC50) of MIBG in the HepG2 cells was 200 µmol/l (P<0.05). Within a certain dose range, MIBG exerted inhibitory effects on HepG2 cell migration in a dose-dependent manner. Treatment with MIBG significantly inhibited the migration and invasion of the HepG2 cells relative to the control cells (P<0.05) and reduced the protein expression of ART1, integrin α7, FAK, PI3K and uPA (P<0.05). Our data demonstrate that ART1 and integrin α7 may be involved in the invasive and metastatic properties of HCC cells. MIBG inhibited the migration and invasion of HepG2 cells, possibly through the inhibition of arginine-specific single-adenosine diphosphate ribosylation and the suppression of the protein expression of integrin α7β1, FAK and PI3K and the secretion of uPA, leading to reduced invasion by HepG2 cells.

Chemical and biological methods to detect post-translational modifications of arginine

Post-translational modifications (PTMs) of protein embedded arginines are increasingly being recognized as playing an important role in both prokaryotic and eukaryotic biology, and it is now clear that these PTMs modulate a number of cellular processes including DNA binding, gene transcription, protein-protein interactions, immune system activation, and proteolysis. There are currently four known enzymatic PTMs of arginine (i.e., citrullination, methylation, phosphorylation, and ADP-ribosylation), and two non-enzymatic PTMs [i.e., carbonylation, advanced glycation end-products (AGEs)]. Enzymatic modification of arginine is tightly controlled during normal cellular function, and can be drastically altered in response to various second messengers and in different disease states. Non-enzymatic arginine modifications are associated with a loss of metabolite regulation during normal human aging. This abnormally large number of modifications to a single amino acid creates a diverse set of structural perturbations that can lead to altered biological responses. While the biological role of methylation has been the most extensively characterized of the arginine PTMs, recent advances have shown that the once obscure modification known as citrullination is involved in the onset and progression of inflammatory diseases and cancer. This review will highlight the reported arginine PTMs and their methods of detection, with a focus on new chemical methods to detect protein citrullination.

Structural and Functional Consequences Induced by Post-Translational Modifications in α-Defensins

HNP-1 is an antimicrobial peptide that undergoes proteolytic cleavage to become a mature peptide. This process represents the mechanism commonly used by the cells to obtain a fully active antimicrobial peptide. In addition, it has been recently described that HNP-1 is recognized as substrate by the arginine-specific ADP-ribosyltransferase-1. Arginine-specific mono-ADP-ribosylation is an enzyme-catalyzed post-translational modification in which NAD⁺ serves as donor of the ADP-ribose moiety, which is transferred to the guanidino group of arginines in target proteins. While the arginine carries one positive charge, the ADP-ribose is negatively charged at the phosphate moieties at physiological pH. Therefore, the attachment of one or more ADP-ribose units results in a marked change of cationicity. ADP-ribosylation of HNP-1 drastically reduces its cytotoxic and antibacterial activities. While the chemotactic activity of HNP-1 remains unaltered, its ability to induce interleukin-8 production is enhanced. The arginine 14 of HNP-1 modified by the ADP-ribose is in some cases processed into ornithine, perhaps representing a different modality in the regulation of HNP-1 activities.

ADP-ribosylarginine hydrolase regulates cell proliferation and tumorigenesis

Protein ADP-ribosylation is a reversible posttranslational modification of uncertain significance in cancer. In this study, we evaluated the consequences for cancer susceptibility in the mouse of a genetic deletion of the enzyme responsible for removing mono-ADP-ribose moieties from arginines in cellular proteins. Specifically, we analyzed cancer susceptibility in animals lacking the ADP-ribosylarginine hydrolase (ARH1) that cleaves the ADP ribose-protein bond. ARH1(-/-) cells or ARH1(-/-) cells overexpressing an inactive mutant ARH1 protein (ARH1(-/-)+dm) had higher proliferation rates than either wild-type ARH1(+/+) cells or ARH1(-/-) cells engineered to express the wild-type ARH1 enzyme. More significantly, ARH1(-/-) and ARH1(+/-) mice spontaneously developed lymphomas, adenocarcinomas, and metastases more frequently than wild-type ARH1(+/+) mice. In ARH1(+/-) mice, we documented in all arising tumors mutation of the remaining wild-type allele (or loss of heterozygosity), illustrating the strict correlation that existed between tumor formation and absence of ARH1 gene function. Our findings show that proper control of protein ADP-ribosylation levels affected by ARH1 is essential for cancer suppression.

ADP-ribosylation of arginine

Arginine adenosine-5′-diphosphoribosylation (ADP-ribosylation) is an enzyme-catalyzed, potentially reversible posttranslational modification, in which the ADP-ribose moiety is transferred from NAD⁺ to the guanidino moiety of arginine. At 540 Da, ADP-ribose has the size of approximately five amino acid residues. In contrast to arginine, which, at neutral pH, is positively charged, ADP-ribose carries two negatively charged phosphate moieties. Arginine ADP-ribosylation, thus, causes a notable change in size and chemical property at the ADP-ribosylation site of the target protein. Often, this causes steric interference of the interaction of the target protein with binding partners, e.g. toxin-catalyzed ADP-ribosylation of actin at R177 sterically blocks actin polymerization. In case of the nucleotide-gated P2X7 ion channel, ADP-ribosylation at R125 in the vicinity of the ligand-binding site causes channel gating. Arginine-specific ADP-ribosyltransferases (ARTs) carry a characteristic R-S-EXE motif that distinguishes these enzymes from structurally related enzymes which catalyze ADP-ribosylation of other amino acid side chains, DNA, or small molecules. Arginine-specific ADP-ribosylation can be inhibited by small molecule arginine analogues such as agmatine or meta-iodobenzylguanidine (MIBG), which themselves can serve as targets for arginine-specific ARTs. ADP-ribosylarginine specific hydrolases (ARHs) can restore target protein function by hydrolytic removal of the entire ADP-ribose moiety. In some cases, ADP-ribosylarginine is processed into secondary posttranslational modifications, e.g. phosphoribosylarginine or ornithine. This review summarizes current knowledge on arginine-specific ADP-ribosylation, focussing on the methods available for its detection, its biological consequences, and the enzymes responsible for this modification and its reversal, and discusses future perspectives for research in this field.

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.

NAD+ and ATP released from injured cells induce P2X7-dependent shedding of CD62L and externalization of phosphatidylserine by murine T cells

Extracellular NAD(+) and ATP trigger the shedding of CD62L and the externalization of phosphatidylserine on murine T cells. These events depend on the P2X(7) ion channel. Although ATP acts as a soluble ligand to activate P2X(7), gating of P2X(7) by NAD(+) requires ecto-ADP-ribosyltransferase ART2.2-catalyzed transfer of the ADP-ribose moiety from NAD(+) onto Arg125 of P2X(7). Steady-state concentrations of NAD(+) and ATP in extracellular compartments are highly regulated and usually are well below the threshold required for activating P2X(7). The goal of this study was to identify possible endogenous sources of these nucleotides. We show that lysis of erythrocytes releases sufficient levels of NAD(+) and ATP to induce activation of P2X(7). Dilution of erythrocyte lysates or incubation of lysates at 37 degrees C revealed that signaling by ATP fades more rapidly than that by NAD(+). We further show that the routine preparation of primary lymph node and spleen cells induces the release of NAD(+) in sufficient concentrations for ART2.2 to ADP-ribosylate P2X(7), even at 4 degrees C. Gating of P2X(7) occurs when T cells are returned to 37 degrees C, rapidly inducing CD62L-shedding and PS-externalization by a substantial fraction of the cells. The "spontaneous" activation of P2X(7) during preparation of primary T cells could be prevented by i.v. injection of either the surrogate ART substrate etheno-NAD or ART2.2-inhibitory single domain Abs 10 min before sacrificing mice.

Identification of two regulatory binding sites which confer myotube specific expression of the mono-ADP-ribosyltransferase ART1 gene

BACKGROUND

Mono-ADP-ribosyltransferase (ART) 1 belongs to a family of mammalian ectoenzymes that catalyze the transfer of ADP-ribose from NAD+ to a target protein. ART1 is predominantly expressed in skeletal and cardiac muscle. It ADP-ribosylates alpha7-integrin which together with beta1-integrin forms a dimer and binds to laminin, a protein of the extracellular matrix involved in cell adhesion. This posttranslational modification leads to an increased laminin binding affinity.

RESULTS

Using C2C12 and C3H-10T 1/2 cells as models of myogenesis, we found that ART1 expression was restricted to myotube formation. We identified a fragment spanning the gene 1.3 kb upstream of the transcriptional start site as the functional promoter of the ART1 gene. This region contains an E box and an A/T-rich element, two conserved binding sites for transcription factors found in the promoters of most skeletal muscle specific genes. Mutating the DNA consensus sequence of either the E box or the A/T-rich element resulted in a nearly complete loss of ART1 promoter inducibility, indicating a cooperative role of the transcription factors binding to those sites. Gel mobility shift analyses carried out with nuclear extracts from C2C12 and C3H-10T 1/2 cells revealed binding of myogenin to the E box and MEF-2 to the A/T-rich element, the binding being restricted to C2C12 and C3H-10T 1/2 myotubes.

CONCLUSION

Here we describe the molecular mechanism underlying the regulation of the ART1 gene expression in skeletal muscle cells. The differentiation-dependent upregulation of ART1 mRNA is induced by the binding of myogenin to an E box and of MEF-2 to an A/T-rich element in the proximal promoter region of the ART1 gene. Thus the transcriptional regulation involves molecular mechanisms similar to those used to activate muscle-specific genes.

Nicotinamide adenine dinucleotide: beyond a redox coenzyme

ADP-ribosylation using nicotinamide adenine dinucleotide (NAD+) is an important type of enzymatic reaction that affects many biological processes. A brief introductory review is given here to various ADP-ribosyltransferases, including poly(ADP-ribose) polymerase (PARPs), mono(ADP-ribosyl)-transferases (ARTs), NAD(+)-dependent deacetylases (sirtuins), tRNA 2'-phosphotransferases, and ADP-ribosyl cyclases (CD38 and CD157). Focus is given to the enzymatic reactions, mechanisms, structures, and biological functions.

Ecto-ADP-ribose transferases: cell-surface response to local tissue injury

Ecto-ADP-ribose transferases (ecto-ARTs) catalyze the transfer of ADP-ribose from NAD(+) to arginine residues in cell-surface proteins. Since the concentration of extracellular NAD(+) is very low under normal physiological conditions but rises significantly upon tissue injury or membrane stress, it is postulated that the main role of ecto-ARTs is to ADP-ribosylate and regulate the function of certain membrane receptors in response to elevated levels of NAD(+).

Physiological relevance of the endogenous mono(ADP-ribosyl)ation of cellular proteins

The mono(ADP-ribosyl)ation reaction is a post-translational modification that is catalysed by both bacterial toxins and eukaryotic enzymes, and that results in the transfer of ADP-ribose from betaNAD+ to various acceptor proteins. In mammals, both intracellular and extracellular reactions have been described; the latter are due to glycosylphosphatidylinositol-anchored or secreted enzymes that are able to modify their targets, which include the purinergic receptor P2X7, the defensins and the integrins. Intracellular mono(ADP-ribosyl)ation modifies proteins that have roles in cell signalling and metabolism, such as the chaperone GRP78/BiP, the beta-subunit of heterotrimeric G-proteins and glutamate dehydrogenase. The molecular identification of the intracellular enzymes, however, is still missing. A better molecular understanding of this reaction will help in the full definition of its role in cell physiology and pathology.

Purification, characterization and molecular cloning of glycosylphosphatidylinositol-anchored arginine-specific ADP-ribosyltransferases from chicken

Mono-ADP-ribosylation is a post-translational modification that regulates the functions of target proteins or peptides by attaching an ADP-ribose moiety. Here we report the purification, molecular cloning, characterization and tissue-specific distribution of novel arginine-specific Arts (ADP-ribosyltransferases) from chicken. Arts were detected in various chicken tissues as GPI (glycosylphosphatidylinositol)-anchored forms, and purified from the lung membrane fraction. By molecular cloning based on the partial amino acid sequence using 5'- and 3'-RACE (rapid amplification of cDNA ends), two full-length cDNAs of chicken GPI-anchored Arts, cgArt1 (chicken GPI-anchored Art1) and cgArt2, were obtained. The cDNA of cgArt1 encoded a novel polypeptide of 298 amino acids which shows a high degree of identity with cgArt2 (82.9%), Art6.1 (50.2%) and rabbit Art1 (42.1%). In contrast, the nucleotide sequence of cgArt2 was identical with that of Art7 cloned previously from chicken erythroblasts. cgArt1 and cgArt2 proteins expressed in DT40 cells were shown to be GPI-anchored Arts with a molecular mass of 45 kDa, and these Arts showed different enzymatic properties from the soluble chicken Art, Art6.1. RNase protection assays and real-time quantitative PCR revealed distinct expression patterns of the two Arts; cgArt1 was expressed predominantly in the lung, spleen and bone marrow, followed by the heart, kidney and muscle, while cgArt2 was expressed only in the heart and skeletal muscle. Thus GPI-anchored Arts encoded by the genes cgArt1 and cgArt2 are expressed extensively in chicken tissues. It may be worthwhile determining the functional roles of ADP-ribosylation in each tissue.

ADP-ribosylation of integrin alpha7 modulates the binding of integrin alpha7beta1 to laminin

The extracellular domain of integrin alpha7 is ADP-ribosylated by an arginine-specific ecto-ADP-ribosyltransferase after adding exogenous NAD+ to intact C2C12 skeletal muscle cells. The effect of ADP-ribosylation on the structure or function of integrin alpha7beta1 has not been explored. In the present study, we show that ADP-ribosylation of integrin alpha7 takes place exclusively in differentiated myotubes and that this post-translational modification modulates the affinity of alpha7beta1 dimer for its ligand, laminin. ADP-ribosylation in the 37-kDa 'stalk' region of alpha7 that takes place at micromolar NAD+ concentrations increases the binding of the alpha7beta1 dimer to laminin. Increased in vitro binding of integrin alpha7beta1 to laminin after ADP-ribosylation of the 37-kDa fragment of alpha7 requires the presence of Mn2+ and it is not observed in the presence of Mg2+. In contrast, ADP-ribosylation of the 63-kDa N-terminal region comprising the ligand-binding site of alpha7 that occurs at approx. 100 microM NAD+ inhibits the binding of integrin alpha7beta1 to laminin. Furthermore, incubation of C2C12 myotubes with NAD+ increases the expression of an epitope on integrin beta1 subunit recognized by monoclonal antibody 9EG7. We discuss our results based on the current models of integrin activation. We also hypothesize that ADP-ribosylation may represent a mechanism of regulation of integrin alpha7beta1 function in myofibres in vivo when the continuity of the membrane is compromised and NAD+ is available as a substrate for ecto-ADP-ribosylation.

Substrate binding and catalysis of ecto-ADP-ribosyltransferase 2.2 from rat

The structures of beta-methylenethiazole-4-carboxamide adenine dinucleotide (TAD), NAD(+), and NADH as bound to ecto-ADP-ribosyltransferase 2.2 from rat and to its mutants E189I and E189A, respectively, have been established. The positions and conformations of NAD(+) and its analogues agree in general with those in other ADP-ribosyltransferases. The kinetic constants for NAD(+) hydrolysis were determined by RP-HPLC. The specific activity amounts to 26 units/mg, which is 6000-fold higher than a previously reported rate and 500-fold higher than the hydrolysis rates of other ADP-ribosyltransferases, confirming that hydrolysis is the major function of this enzyme. On the basis of structures and mutant activities, a catalytic mechanism is proposed. The known auto-ADP-ribosylation of the enzyme at the suggested position R184 is supported by one of the crystal structures where the nucleophile position is occupied by an Neta atom of this arginine which in turn is backed up by the base E159.

Identification of regulatory domains in ADP-ribosyltransferase-1 that determine transferase and NAD glycohydrolase activities

Mono-ADP-ribosyltransferases (ART1-7) transfer ADP-ribose from NAD+ to proteins (transferase activity) or water (NAD glycohydrolase activity). The mature proteins contain two domains, an alpha-helical amino terminus and a beta-sheet-rich carboxyl terminus. A basic region in the carboxyl termini is encoded in a separate exon in ART1 and ART5. Structural motifs are conserved among ART molecules. Successive amino- or carboxyl-terminal truncations of ART1, an arginine-specific transferase, identified regions that regulated transferase and NAD glycohydrolase activities. In mouse ART1, amino acids 24-38 (ART-specific extension) were needed to inhibit both activities; amino acids 39-45 (common ART coil) were required for both. Successive truncations of the alpha-helical region reduced transferase and NAD glycohydrolase activities; however, truncation to residue 106 enhanced both. Removal of the carboxyl-terminal basic domain decreased transferase, but enhanced NAD glycohydrolase, activity. Thus, amino- and carboxyl-terminal regions of ART1 are required for transferase activity. The enhanced glycohydrolase activity of the shorter mutants indicates that sequences, which are not part of the NAD binding, core catalytic site, exert structural constraints, modulating substrate specificity and catalytic activity. These functional domains, defined by discrete exons or structural motifs, are found in ART1 and other ARTs, consistent with conservation of structure and function across the ART family.

Structure of the ecto-ADP-ribosyl transferase ART2.2 from rat

The mammalian extracellular ADP-ribosyl transferases ART1 through ART5 are sequence-related to each other. Among them ART2 is involved in immuno regulation. The variant ART2.2 was expressed in the periplasm of Escherichia coli and crystallized. Its structure was determined by X-ray diffraction at 1.7A resolution in one crystal form and at slightly lower resolutions in two others. The active center was indicated by a ligated nicotinamide analogue, which also revealed a small induced-fit. The centerpiece of the chainfold of ART2.2 agrees with those of all bacterial ADP-ribosyl transferases. This correspondence and the nicotinamide position were used to model the binding structure of the whole substrate NAD(+) at ART2.2. Two of the bacterial enzymes are structurally more closely related to ART2.2 while the others are more closely related to the eukaryotic poly(ADP-ribosyl)polymerase. This splits the ADP-ribosyl transferases into two distinct subfamilies. A special feature of ART2.2 is its long N-terminal extension and two disulfide bridges that are far away from the active center. They stabilize the protein against denaturation and presumably also against shearing forces parallel with the membrane where ART2.2 is anchored.

Integrin alpha 7 beta 1 in muscular dystrophy/myopathy of unknown etiology

To investigate the role of integrin alpha 7 in muscle pathology, we used a "candidate gene" approach in a large cohort of muscular dystrophy/myopathy patients. Antibodies against the intracellular domain of the integrin alpha 7A and alpha 7B were used to stain muscle biopsies from 210 patients with muscular dystrophy/myopathy of unknown etiology. Levels of alpha 7A and alpha 7B integrin were found to be decreased in 35 of 210 patients (approximately 17%). In six of these patients no integrin alpha 7B was detected. Screening for alpha 7B mutation in 30 of 35 patients detected only one integrin alpha 7 missense mutation (the mutation on the second allele was not found) in a patient presenting with a congenital muscular dystrophy-like phenotype. No integrin alpha 7 gene mutations were identified in all of the other patients showing integrin alpha 7 deficiency. In the process of mutation analysis, we identified a novel integrin alpha 7 isoform presenting 72-bp deletion. This isoform results from a partial deletion of exon 21 due to the use of a cryptic splice site generated by a G to A missense mutation at nucleotide position 2644 in integrin alpha 7 cDNA. This spliced isoform is present in about 12% of the chromosomes studied. We conclude that secondary integrin alpha 7 deficiency is rather common in muscular dystrophy/myopathy of unknown etiology, emphasizing the multiple mechanisms that may modulate integrin function and stability.

Endogenous ADP-ribosylation of the G protein beta subunit prevents the inhibition of type 1 adenylyl cyclase

Mono-ADP-ribosylation is a post-translational modification of cellular proteins that has been implicated in the regulation of signal transduction, muscle cell differentiation, protein trafficking, and secretion. In several cell systems we have observed that the major substrate of endogenous mono-ADP-ribosylation is a 36-kDa protein. This ADP-ribosylated protein was both recognized in Western blotting experiments and selectively immunoprecipitated by a G protein beta subunit-specific polyclonal antibody, indicating that this protein is the G protein beta subunit. The ADP-ribosylation of the beta subunit was due to a plasma membrane-associated enzyme, was sensitive to treatment with hydroxylamine, and was inhibited by meta-iodobenzylguanidine, indicating that the involved enzyme is an arginine-specific mono-ADP-ribosyltransferase. By mutational analysis, the target arginine was located in position 129. The ADP-ribosylated beta subunit was also deribosylated by a cytosolic hydrolase. This ADP-ribosylation/deribosylation cycle might be an in vivo modulator of the interaction of betagamma with specific effectors. Indeed, we found that the ADP-ribosylated betagamma subunit is unable to inhibit calmodulin-stimulated type 1 adenylyl cyclase in cell membranes and that the endogenous ADP-ribosylation of the beta subunit occurs in intact Chinese hamster ovary cells, where the NAD(+) pool was labeled with [(3)H]adenine. These results show that the ADP-ribosylation of the betagamma subunit could represent a novel cellular mechanism in the regulation of G protein-mediated signal transduction.

New functions of a long-known molecule. Emerging roles of NAD in cellular signaling

Over the past decades, the pyridine nucleotides have been established as important molecules in signaling pathways, besides their well known function in energy transduction. Similarly to another molecule carrying such dual functions, ATP, NAD(P)+ may serve as substrate for covalent protein modification or as precursor of biologically active compounds. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific amino-acid residues of the acceptor proteins. For a number of ADP ribosylation reactions the specific transferases and their target proteins have been identified. As a result of the modification, the biological activity of the acceptor proteins may be severely changed. The cell nucleus contains enzymes catalyzing the transfer of ADP-ribose polymers (polyADP-ribose) onto the acceptor proteins. The best known enzyme of this type is poly(ADP-ribose) polymerase 1 (PARP1), which has been implicated in the regulation of several important processes including DNA repair, transcription, apoptosis, neoplastic transformation and others. The second group of reactions leads to the synthesis of an unusual cyclic nucleotide, cyclic ADP-ribose (cADPR). Moreover, the enzymes catalyzing this reaction may also replace the nicotinamide of NADP+ by nicotinic acid resulting in the synthesis of nicotinic acid adenine dinucleotide phosphate (NAADP+). Both cADPR and NAADP+ have been reported to be potent intracellular calcium-mobilizing agents. In concert with inositol 1,4,5-trisphosphate, they participate in cytosolic calcium regulation by releasing calcium from intracellular stores.

Characterization of glycosylphosphatidylinositiol-anchored, secreted, and intracellular vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation is a posttranslational modification of proteins in which the ADP-ribose moiety of nicotinamide adenine dinucleotide is transferred to an acceptor amino acid. Five mammalian ADP-ribosyltransferases (ART1--ART5) have been cloned and expression is restricted to tissues such as cardiac and skeletal muscle, leukocytes, brain, and testis. ART1 and ART2 are glycosylphosphatidylinositol (GPI)-anchored ectoenzymes. ART5 appears not to be GPI-linked and may be secreted. In skeletal muscle and lymphocytes, ART1 modifies specific members of the integrin family of adhesion molecules, suggesting that ADP-ribosylation affects cell-matrix or cell-cell interactions. In lymphocytes, ADP-ribosylation of surface proteins is associated with changes in p56lck tyrosine kinase-mediated signaling. The catalytic sites of bacterial toxins and vertebrate transferases have conserved structural features, consistent with a common reaction mechanism. ADP-ribosylation can be reversed by ADP-ribosylarginine hydrolases, resulting in the regeneration of free arginine. Thus, an ADP-ribosylation cycle may play a regulatory role in vertebrate tissues.

The effects of mono-ADP-ribosylation on desmin assembly-disassembly

Previous studies have shown that desmin, the muscle-specific intermediate filament protein, is a substrate for the endogenous muscle arginine-specific mono-ADP-ribosyltransferase and that ADP-ribosylation inhibits assembly of desmin into intermediate filaments (Huang et al., Exp. Cell Res. 226, 147-153, 1996). In this paper, the effects of mono-ADP-ribosylation on assembly and disassembly of desmin intermediate filaments were further characterized. First, it was found that ADP-ribosylated desmin does not coassemble with unmodified desmin and has no effect on assembly of unmodified desmin. Second, incubation of assembled desmin filaments with mono-ADP-ribosyltransferase and NAD+ results in disassembly of the filaments. Finally, the structural components of the attached ADP-ribose moiety responsible for altering the assembly of desmin into filaments were investigated by a stepwise cleavage of ADP-ribose with snake venom phosphodiesterase and alkaline phophatase, followed by analysis of assembly. The reactions catalyzed by these two enzymes were established using a desmin peptide as a substrate. Our results show that ribosylated desmin, but not phosphoribosylated desmin, was able to self-assemble into intermediate filaments, suggesting that the presence of a phosphate group is needed to alter desmin's assembly ability.

Suppression of cell adhesion and spreading activities of fibronectin by arginine-specific ADP-ribosyltransferase from chicken polymorphonuclear leukocytes

Arginine-specific ADP-ribosyltransferase present in secretory granules of chicken polymorphonuclear leukocytes (so-called heterophils) was shown to be released into the extracellular space by secretagogues (Terashima et al., J. Biochem. 120 (1996) 1209-1215). In the present work, we examined fibronectin as an extracellular target protein of the released transferase. Fibronectin was ADP-ribosylated by purified transferase and stoichiometry of ADP-ribose incorporation into fibronectin was 1.0 mol/mol of fibronectin. Cell adhesion and spreading assays revealed that ADP-ribosylation of fibronectin markedly inhibited the adhesion activity of fibronectin. A proteolytic peptide map of ADP-ribosylated fibronectin demonstrated that the modification occurs in the cell binding domain of fibronectin. ADP-ribosylation of the RGD peptide suggests that the RGD sequence is the modification site in the domain. ADP-ribosylation of fibronectin in plasma means that fibronectin can probably serve as the substrate for extracellularly released ADP-ribosyltransferase in vivo. Thus, in the extracellular space, ADP-ribosyltransferase released from polymorphonuclear leukocytes may perhaps be involved in regulation of cell adhesion process by interfering with the activity of fibronectin.

Expression of ADP-Ribosyltransferase on Normal T Lymphocytes and Effects of Nicotinamide Adenine Dinucleotide on Their Function

ADP-ribosyltransferase (ADPRT) is a glycosylphosphatidylinositol-anchored cell surface enzyme on CTL. Expression of this enzyme correlates with suppression of CTL functions in the presence of its substrate β-nicotinamide adenine dinucleotide (NAD). To investigate the immunoregulatory importance of ADPRT on normal lymphocytes in vivo, NAD was injected into mice and the effects on cell-mediated and humoral immunity were assessed. Induction of both delayed-type hypersensitivity and CTL, but not Ab responses, are shown to be suppressed by NAD. Consistent with this, mature T cells, but not B cells or macrophages, express ADPRT and are able to ADP-ribosylate cell surface proteins. ADP-ribosylated molecules were identified as LFA-1, CD8, CD27, CD43, CD44, and CD45. Concomitant to ADP-ribosylation of these molecules, T cell trafficking to secondary lymphoid organs is suppressed by NAD. To examine whether this is due to effects of NAD on cell activation, Ag-stimulated responses were assayed in vitro. NAD is shown to inhibit induction of cell proliferation, cytotoxicity, and cytokine secretion. It is suggested that ADPRT regulates T cells on the level of transmembrane signaling via ADP-ribosylation of cell surface molecules. This effect is reported to be indirect, as it involves transmission of signals through TCRs, which are not ADP-ribosylated.

Interaction of integrin alpha 7 beta 1 in C2C12 myotubes and in solution with laminin

The dimer of integrin alpha 7 and beta 1 is a major laminin-binding receptor in skeletal muscle. We studied interactions of integrin alpha 7 beta 1 with the extracellular matrix protein laminin in solution and in intact cells. Integrin alpha 7 beta 1 bound to EHS laminin (laminin-1, composed of alpha 1, beta 1, and gamma 1 chains), but not to endogenous laminin expressed in C2C12 myotubes. Northern blot analysis demonstrated that C2C12 myotubes synthesized laminin-1 alpha, beta, and gamma subunits mRNAs. C2C12 laminin was, however, immunologically distinct from EHS laminin; it was not recognized by 5D3 anti-laminin-1 monoclonal antibody, whereas 5A2 and LT3 antibodies reacted equally well with C2C12 and EHS laminins. Following deglycosylation of EHS laminin, separation of the subunits by SDS-PAGE, Western blotting, and partial amino acid sequencing of the protein bands, the epitope recognized by 5D3 antibody was localized to the gamma 1 laminin chain. Following binding in vitro, the complex of EHS laminin and integrin alpha 7 beta 1 was subject to chemical cross-linking. The two proteins did not undergo cross-linking at the cell surface, consistent with the fact that in intact, resting myotubes integrin alpha 7 beta 1 interacted poorly with EHS laminin, which may reflect a limited accessibility of integrin alpha 7 beta 1 in the membrane to laminin or an inactive state of the integrin.

A novel extracellular domain variant of the human integrin alpha 7 subunit generated by alternative intron splicing

The integrin alpha 7 beta 1 laminin receptor, which is expressed on replicating myoblasts, and upregulated during myogenic differentiation, is involved in cell adhesion and communication between muscle cells and the extracellular matrix. It is a major cell-surface substrate in skeletal muscle cells for the cell-surface, argininespecific, ADP-ribosyltransferase. Both the extracellular and cytoplasmic domains of the mouse alpha 7 subunit undergo alternative splicing during development, generating differentially expressed variants with presumably unique ligand-binding and signalling properties. Here human cDNA clones isolated from a fetal heart lambda gt10 cDNA library encoded the complete sequence of the alpha 7 subunit and hybridised to a single major 4.4 kb alpha 7 subunit transcript abundantly expressed in human skeletal muscle, moderately expressed in heart, and weakly expressed in most other tissues. One clone out of four contained a novel 225-nucleotide in-frame deletion corresponding to 75 amino acids in the C-terminal region of the extracellular domain. The variant, whose expression appears to be tissue-specific, is created by alternative splicing at sites flanking an intron in the alpha 7 gene. A related mouse form was identified in P19 embryonal carcinoma cells. Deletion of the spliced region, which either contains or is in very close proximity to the major ADP-ribosylation site of the alpha 7 subunit, may serve to modulate the effects of ADP-ribosylation, or alternatively molecular associations, and receptor-ligand affinity.

Specificity and target proteins of arginine-specific mono-ADP-ribosylation in T-tubules of rabbit skeletal muscle

In order to specify that protein labeling is the result of mono-ADP ribosylation, a careful evaluation of the reaction conditions and products is necessary. To investigate the specificity and target proteins of the arginine-specific mono-ADP-ribosyltransferase (mADP-RT) in rabbit skeletal muscle T-tubules (TT) biotin- or digoxigenin-coupled NAD-derivatives were synthesized. They were used for the nonradioactive labeling of proteins and compared with radioactive mono-ADP-ribosylation. According to the results of our studies, they cannot be used as substrates to detect arginine-specific or pertussis toxin-dependent mono-ADP-ribosylation of target proteins in skeletal muscle. In contrast, radioactive NAD can be used to monitor these reactions. Under the appropriate reaction conditions, the radioactive [adenylate-14C]NAD and [32P]NAD were found to be solely consumed by the arginine-specific mADP-RT of skeletal muscle TT. The incorporation studies confirmed earlier data on the localization of the mADP-RT and its targets in TT. The T-tubular targets were purified in a single-step procedure using phenylboronate affinity chromatography. Of 18 target proteins delineated by autoradiography of electrophoretically separated T-tubular proteins, a 42-kDa protein was suggested to be the stimulatory G protein (Gsalpha). Mono-ADP-ribosylation of Gsalpha resulted in an inhibition of the T-tubular adenylate cyclase activity as proven by the suppression of this inhibition using novobiocin as a specific inhibitor of mADP-RT.

The alpha7beta1 integrin mediates adhesion and migration of skeletal myoblasts on laminin

Many aspects of myogenesis are believed to be regulated by myoblast interactions with specific components of the extracellular matrix. For example, laminin has been found to promote adhesion, migration, and proliferation of mammalian myoblasts. Based on affinity chromatography, the alpha7beta1 integrin has been presumed to be the major receptor mediating myoblast interactions with laminin. We have prepared a monoclonal antibody, O26, that specifically reacts with both the X1 and the X2 extracellular splice variants of the alpha7 integrin chain. This antibody completely and selectively blocks adhesion and migration of rat L8E63 myoblasts on laminin-1, but not on fibronectin. In contrast, a polyclonal antibody to the fibronectin receptor, alpha5beta1 integrin, blocks myoblast adhesion on fibronectin, but not on laminin-1. The alpha7beta1 integrin also binds to a mixture of laminin-2 and laminin-4, the major laminin isoforms in developing and adult skeletal muscle, but O26 is a much less potent inhibitor of myoblast adhesion on the laminin-2/4 mixture than on laminin-1. Based on affinity chromatography, we suggest that this may be due to higher affinity binding of alpha7X1 to laminin-2/4 than to laminin-1.

ADP-ribosylation of tuftsin suppresses its receptor-binding capacity and phagocytosis-stimulating activity to murine peritoneal macrophages

Arginine-specific ADP-ribosyltransferase present in granules of chicken polymorphonuclear leukocytes (so-called heterophils) is released into the extracellular space by stimulus of calcium ionophore A23187 or opsonized zymosan [Terashima et al. (1996) J. Biochem. 120, 1209-1215]. In the present work, we examined extracellular targets of the released transferase and identified tuftsin, a phagocytosis-stimulating tetrapeptide derived from leukokinin, as a preferential substrate of the enzyme in chicken plasma. Specific binding of FITC-tuftsin to murine peritoneal macrophages, observed under a fluorescent microscope, was impaired by ADP-ribosylation of the labelled peptide. Phagocytic assay analyzed by flow cytometry revealed that ADP-ribosylation of tuftsin decreased its phagocytosis-stimulating activity towards the macrophages. Thus, the ADP-ribosylation of tuftsin apparently decreases its biological activity and ADP-ribosylation may possibly be involved in inflammatory processes through alterations in tuftsin activity.

A novel muscle protein located inside the terminal cisternae of the sarcoplasmic reticulum

An immunofluorescence study of adult rat muscle tissues with a polyclonal antibody against the RGD-directed fibronectin receptor of Friend's erythroleukemia cells (alpha5beta1-integrin) unexpectedly revealed a pattern of intracellular antigen distribution. Western blotting analysis of rat and rabbit membrane fractions indicated that the antibody recognizes a 167-kDa protein expressed both in heart and in skeletal muscle (relative abundance: heart > slow muscle > fast muscle), but not in liver and kidney. The 167-kDa protein did not show altered electrophoretic mobility upon reduction and failed to bind several lectins, including wheat germ agglutinin. A study of its subcellular distribution in rabbit skeletal muscle revealed that the 167-kDa protein is mostly associated with the terminal cisternae of the sarcoplasmic reticulum (SR) and, to a smaller extent, with the sarcolemma, while it is absent in the longitudinal tubules of the SR. The 167-kDa protein is not an integral membrane protein since it can be extracted at pH >/=10. This protein can be proteolytically cleaved only in the presence of detergent, indicating that it resides on the luminal side of the SR. The 167-kDa protein could be resolved from the closely spaced sarcalumenin and histidine-rich protein by column chromatography followed by detergent dialysis and two-dimensional gel electrophoresis. The N terminus and the internal sequences did not match any known sequence in protein and DNA data bases, indicating that the 167-kDa protein is a novel muscle protein selectively localized to the SR. Integrins from rat kidney fibroblasts were not recognized by either (i) a polyclonal antiserum against the purified 167-kDa protein or (ii) the anti-alpha5beta1-integrin antiserum after affinity purification onto the 167-kDa protein. These data indicate that the 167-kDa protein is not immunologically cross-reactive with integrins, despite its reaction with a polyclonal anti-integrin antibody.

Molecular cloning of a functional murine arginine-specific mono-ADP-ribosyltransferase and its expression in lymphoid cells

A protein mono-ADP-ribosyltransferase (ADPRT), anchored in the cell membrane as a glycosylphosphatidylinositol (GPI)-anchored cell-surface enzyme, was recently described on murine cytotoxic T cells (CTL). Expression of this enzyme was shown to exert regulatory functions on CTL proliferation and cytotoxic activity, presumably by modulating activity of the protein tyrosine kinase p56(lck), which is associated with the CTL co-receptor CD8. Here we report on the molecular cloning and expression of this important regulatory enzyme. The ADPRT coding sequence was derived by making use of ADPRT sequence homologies from different vertebrate species. A cDNA fragment of the enzyme coding sequence was generated by reverse transcription polymerase chain reaction (RT-PCR) from murine T-cell lymphoma SL12, which expresses the cell-surface ADPRT. The cDNA fragment was found to share extensive homology with the corresponding sequences of human and rabbit muscle ADPRT. In Northern blot hybridization, this cDNA fragment generates a strong hybridization signal with RNA from murine heart and skeletal muscle. Weak signals are seen with SL12, thymus, and spleen. Therefore, a murine skeletal muscle cDNA library was used to identify and obtain the coding sequence of the ADPRT gene. It is shown that the nucleic acid open reading frame sequence of the murine skeletal muscle gene shares 80.3% and 76.3% homology with the sequences of the human and rabbit muscle genes, respectively. Semiquantitative RT-PCR with intron-spanning primers shows that the ADPRT mRNA is present in lymphoid organs, cytotoxic T cells, and T-cell lines. Transfection of the ADPRT coding sequence into EL4 cells results in expression of the enzyme as a functional GPI-anchored cell-surface protein, able to ADP-ribosylate the arginine analog agmatine as well as cell-surface molecules.

ADP-ribosylarginine hydrolases and ADP-ribosyltransferases. Partners in ADP-ribosylation cycles

Mono-ADP-ribosylation is a reversible modification of arginine residues in proteins, with NAD:arginine ADP-ribosyltransferases and ADP-ribosylarginine hydrolases constituting opposing arms of a putative ADP-ribosylation cycle. The enzymatic components of an ADP-ribosylation cycle have been identified in both prokaryotic and eukaryotic systems. The regulatory significance of the cycle has been best documented in prokaryotes. As shown by Ludden and coworkers, ADP-ribosylation controls the activity of dinitrogenase reductase in the phototropic bacterium Rhodospirillum rubrum. ADP-ribosylation of other amino acids, such as cysteine, has also been demonstrated, lending credence to the hypothesis that this modification is heterogeneous. In eukaryotes, the functional relationship between ADP-ribosyltransferases and ADP-ribosylarginine hydrolases is less well documented. The transferase-catalyzed reaction results in sterospecific formation of alpha-ADP-ribosylarginine from beta-NAD; ADP-ribosylarginine hydrolases specifically cleave the alpha-anomer, leading to release of ADP-ribose and regeneration of the free guanidino group of arginine. The two reactions can thus be coupled in vitro. Coupling in vivo is dependent on cellular localization. The deduced amino acid sequences of ADP-ribosyltransferases from avian and mammalian tissues have common consensus sequences involved in catalytic activity but, in some instances, enzyme-specific cellular localization signals. The presence of amino- and carboxy-terminal signal sequences is consistent with the glycosylphosphatidylinositol(GPI)-anchoring to the cell surface. The muscle and lymphocyte transferases ADP-ribosylate integrins. Some transferases lack the carboxy- terminal signal sequence needed for GPI-anchoring. Most ADP-ribosylarginine hydrolase activity is cytosolic, although perhaps some is located at the cell surface. Deduced amino acid sequences of hydrolases from a number of mammalian species are consistent with their cytoplasmic localization. Katada and coworkers have determined, however, that auto-ADP-ribosylated RT6, a GPI-linked protein, is metabolized by a hydrolase-like activity, consistent with the existence of an ADP-ribosylation cycle. ADP-ribosyl RT6 may be internalized, thereby coming in contact with the cytosolic hydrolase; alternatively, a novel form of the hydrolase may be located at the surface. The mechanism of coupling of ADP-ribosyltransferases and hydrolases in eukaryotic ADP-ribosylation cycles has yet to be clarified.

Regulatory role of arginine-specific mono(ADP-ribosyl)transferase in muscle cells

Earlier we demonstrated that meta-iodobenzylguanidine (MIBG), a specific inhibitor of arginine mono-ADP-ribosylation blocks proliferation and differentiation of chick skeletal myogenic cells in culture (Exp. Cell Res., 1992, 201:33-42). Membrane fractions from 4-day, myotube cultures of embryonic chick muscle cells were incubated with 32P-NAD+. Several proteins were labeled, but labeling of two hands of about 53 and 36 kDa appeared to be due to arginyl ADP-ribosylation. Immunoprecipitation with D3 monoclonal antibody to the intermediate filament protein desmin, SDS-PAGE and autoradiography demonstrated that the 53 kDa band contained desmin, and that this desmin is ADP-ribosylated by the endogenous arginine-specific mono(ADP-ribosyl)transferase (Exp. Cell Res., 1996, in press). Desmin is the muscle-specific intermediate filament protein, and it appears to be one of the first muscle-specific proteins expressed during terminal myogenic differentiation. We have examined whether desmin can be ADP-ribosylated in muscle cells by use of polyclonal antibodies for ADP-ribosylated arginyl residues. We have found that soluble desmin is present in 5-6 day myogenic cell cultures and that this desmin contains ADP-ribose, demonstrating that desmin is ADP-ribosylated in skeletal muscle cells. We also found that purified avian desmin contains antigenic material that reacts with these antibodies. In both cases, NaCl had no effect on the reactivity, but NH2OH did, which is consistent with an arginine-ADPR linkage. In summary, these results suggest that ADP-ribosylation is an important regulatory mechanism in differentiating muscle cells, and that the intermediate filament protein desmin is an important substrate for modification in muscle cells.

The alpha 7 integrin as a target protein for cell surface mono-ADP-ribosylation in muscle cells

A membrane-associated arginine-specific mono-ADP-ribosyltransferase was purified 215,000-fold from rabbit skeletal muscle and its gene was isolated from a skeletal muscle cDNA library. The enzyme was a glycosylphosphatidyl-inositol-linked protein, present on the surface of differentiated skeletal muscle myoblasts (myotubes). Following incubation of cultured, intact myotubes with [adenylate-32P]NAD and analysis by SDS-PAGE, a major radiolabeled protein of 97/140 kDa (reduced/nonreduced conditions) was observed. It was identified as integrin alpha 7 based on its size, binding to a laminin affinity column, immunoprecipitation with a monoclonal antibody, and partial amino acid sequencing. Since ADP-ribosylarginine hydrolase, the enzyme responsible for cleavage of the ADP-ribosylarginine bond and a component with the transferase of a putative ADP-ribosylation cycle, is cytosolic, whereas the transferase is attached via a GPI-anchor to the cell surface, the processing of ADP-ribosylated integrin alpha 7 was investigated. 32P label was rapidly removed from [32P]ADP-ribosylated integrin alpha 7, a process inhibited by free ADP-ribose or p-nitrophenylthymidine-5'-monophosphate, alternative substrates for 5'-nucleotide phosphodiesterase. The processed integrin alpha 7 was not susceptible to subsequent ADP-ribosylation, although the amount of surface integrin alpha 7 remained constant. During the processing, no loss of label was observed from integrin alpha 7 radiolabeled with [14C]NAD, containing 14C in the nicotinamide-proximal ribose, consistent with a degradation of the ADP-ribose moiety by a cell surface 5'-nucleotide phosphodiesterase. Thus, cell surface ADP-ribosylation, in contrast to intracellular ADP-ribosylation, is not readily reversed by the presently known ADP-ribosylarginine hydrolase and seems to operate outside the postulated ADP-ribosylation cycle.

Arginine-specific mono(ADP-ribosyl)transferase activity on the surface of human polymorphonuclear neutrophil leucocytes

An Arg-specific mono(ADP-ribosyl)transferase activity on the surface of human polymorphonuclear neutrophil leucocytes (PMNs) was confirmed by the use of diethylamino-(benzylidineamino)guanidine (DEA-BAG) as an ADP-ribose acceptor. Two separate HPLC systems were used to separate ADP-ribosyl-DEA-BAG from reaction mixtures, and its presence was confirmed by electrospray mass spectrometry. ADP-ribosyl-DEA-BAG was produced in the presence of PMNs, but not in their absence. Incubation of DEA-BAG with ADP-ribose (0.1-10 mM) did not yield ADP-ribosyl-DEA-BAG, which indicates that ADP-ribosyl-DEA-BAG formed in the presence of PMNs was not simply a product of a reaction between DEA-BAG and free ADP-ribose, due possibly to the hydrolysis of NAD+ by an NAD+ glycohydrolase. The assay of mono(ADP-ribosyl)transferase with agmatine as a substrate was modified for intact PMNs, and the activity was found to be approx. 50-fold lower than that in rabbit cardiac membranes. The Km of the enzyme for NAD+ was 100.1 30.4 microM and the Vmax 1.4 0.2 pmol of ADP-ribosylagmatine/h per 10(6) cells. The enzyme is likely to be linked to the cell surface via a glycosylphosphatidylinositol anchor, since incubation of intact PMNs with phosphoinositol-specific phospholipase C (PI-PLC) led to a 98% decrease in mono(ADP-ribosyl)transferase activity in the cells. Cell surface proteins were labelled after exposure of intact PMNs to [32P]NAD+. Their molecular masses were 79, 67, 46, 36 and 26 kDa. The time course for labelling was non-linear under these conditions over a period of 4 h. The labelled products were identified as mono(ADP-ribosyl)ated proteins by hydrolysis with snake venom phosphodiesterase to yield 5'-AMP.

Mono-ADP-ribosylation: a reversible posttranslational modification of proteins

Mono-ADP-ribosyltransferase activity has been detected in numerous vertebrate tissues and transferase cDNAs from a few species have recently been cloned. In vitro ADP-ribosylation has been demonstrated with diverse substrates such as phosphorylase kinase, actin, and Gs alpha resulting in the alteration of substrate function. ADP-ribosylation of endogenous target proteins has been observed in chicken heterophils, rat brain, and human platelets, and integrin alpha 7 was found to be the endogenous substrate of the GPI-anchored rabbit skeletal muscle transferase. The reversibility of ADP-ribosylation is made possible by ADP-ribosylarginine hydrolases which have been isolated and cloned from rodent and human tissues. The transferases and hydrolases could in principle form an intracellular ADP-ribosylation regulatory cycle. In the case of the skeletal muscle transferases, however, processing of ADP-ribosylated integrin alpha 7 is carried out by phosphodiesterases and possibly phosphatases (Fig. 1). Most bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 family of proteins, share a common catalytic-site structure despite a lack of overall sequence identity. The transferases that have been studied thus far possess a critical glutamic acid and other amino acids at the catalytic cleft which function to position NAD for nucleophilic attack at the N-glycosidic linkage for either ADP-ribose transfer or NAD hydrolysis. The amino acid differences among transferases at the active site may reflect different catalytic mechanisms of ADP-ribosylation or may be required for accommodating the different ADP-ribose acceptor molecules.

Structure and function of eukaryotic mono-ADP-ribosyltransferases

ADP-ribosylation of proteins has been observed in numerous animal tissues including chicken heterophils, rat brain, human platelets, and mouse skeletal muscle. ADP-ribosylation in these tissues is thought to modulate critical cellular functions such as muscle cell development, actin polymerization, and cytotoxic T lymphocyte proliferation. Specific substrates of the ADP-ribosyltransferases have been identified; the skeletal muscle transferase ADP-ribosylates integrin alpha 7 whereas the chicken heterophil enzyme modifies the heterophil granule protein p33 and the CTL enzyme ADP-ribosylates the membrane-associated protein p40. Transferase sequence has been determined which should assist in elucidating the role of ADP-ribosylation in cells. There is sequence similarity among the vertebrate transferases and the rodent RT6 alloantigens. The RT6 family of proteins are NAD glycohydrolases that have been shown to possess auto-ADP-ribosyltransferase activity whereas the mouse Rt6-1 is also capable of ADP-ribosylating histone. Absence of RT6+ T cells has been associated with the development of an autoimmune-mediated diabetes in rodents. Humans have an RT6 pseudogene and do not express RT6 proteins. The reversal of ADP-ribosylation is catalyzed by ADP-ribosylarginine hydrolases, which have been purified and cloned from rodent and human tissues. In principle, the transferases and hydrolases could form an intracellular ADP-ribosylation regulatory cycle. In skeletal muscle and lymphocytes, however, the transferases and their substrates are extracellular membrane proteins whereas the hydrolases described thus far are cytoplasmic. In cultured mouse skeletal muscle cells, processing of the ADP-ribosylated integrin alpha 7 was carried out by phosphodiesterases and possibly phosphatases, leaving a residual ribose attached to the (arginine)protein. Several bacterial toxin and eukaryotic mono-ADP-ribosyltransferases, and perhaps other NAD-utilizing enzymes such as the RT6 alloantigens share regions of amino acid sequence similarity, which form, in part, the catalytic site. The catalytic cleft, found in the bacterial toxins that have been studied thus far, contains a critical glutamate and other amino acids that function to position NAD for nucleophilic attack at the N-glycosidic linkage, for either ADP-ribose transfer or NAD hydrolysis. Amino acid differences among the transferases at the active site may be required for accommodating the different ADP-ribose acceptor molecules.