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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for a global pandemic that resulted in more than 6-million deaths worldwide. The virus encodes several non-structural proteins (Nsps) that contain elements capable of disrupting cellular processes. Among these Nsp proteins, Nsp3 contains macrodomains, e.g., Mac1, Mac2, Mac3, with potential effects on host cells. Mac1 has been shown to increase SARS-CoV-2 virulence and disrupt ADP-ribosylation pathways in mammalian cells. ADP-ribosylation results from the transfer of the ADP-ribose moiety of NAD+to various acceptors, e.g., proteins, DNA, RNA, contributing on a cell’s biological processes. ADP-ribosylation is the mechanism of action of bacterial toxins, e.g., Pseudomonas toxins, diphtheria toxin that disrupt protein biosynthetic and signaling pathways. On the other hand, some viral macrodomains cleavage ADP-ribose-acceptor bond, generating free ADP-ribose. By this reaction, the macrodomain-containing proteins interfere ADP-ribose homeostasis in host cells. Here, we examined potential hydrolytic activities of SARS-CoV-2 Mac1, 2, and 3 on substrates containing ADP-ribose. Mac1 cleaved α-NAD+, but not β-NAD+, consistent with stereospecificity at the C-1” bond. In contrast to ARH1 and ARH3, Mac1 did not require Mg2+for optimal activity. Mac1 also hydrolyzedO-acetyl-ADP-ribose and ADP-ribose-1”-phosphat, but not Mac2 and Mac3. However, Mac1 did not cleave α-ADP-ribose-(arginine) and ADP-ribose-(serine)-histone H3 peptide, suggesting that Mac1 hydrolyzes ADP-ribose attached to O- and N-linked functional groups, with specificity at the catalytic site in the ADP-ribose moiety. We conclude that SARS-CoV-2 Mac1 may exert anti-viral activity by reversing host-mediated ADP-ribosylation. New insights on Nsp3 activities may shed light on potential SARS-CoV-2 therapeutic targets.IMPORTANCESARS-CoV-2, the virus responsible for COVID-19, encodes 3 macrodomain-containing proteins, e.g., Mac1, Mac2, Mac3, within non-structural proteins 3 (Nsp3). Mac1 was shown previously to hydrolyze ADP-ribose-phosphate. Inactivation of Mac1 reduced viral proliferation. Here we report that Mac1, but not Mac2 and Mac3, has multiple activities, i.e., Mac1 hydrolyzed. α-NAD+andO-acetyl-ADP-ribose. However, Mac1 did not hydrolyze β-NAD+, ADP-ribose-serine on a histone 3 peptide (aa1-21), and ADP-ribose-arginine, exhibiting substrate selectivity. These data suggest that Mac1 may have multi-function as a α-NAD+consumer for viral replication and a disruptor of host-mediated ADP-ribosylation pathways. Understanding Mac1’s mechanisms of action is important to provide possible therapeutic targets for COVID-19.

Arginine-specific mono-adenosine diphosphate (ADP)-ribosylation is a nicotinamide adenine dinucleotide (NAD)+-dependent, reversible post-translational modification involving the transfer of an ADP-ribose from NAD+ by bacterial toxins and eukaryotic ADP-ribosyltransferases (ARTs) to arginine on an acceptor protein or peptide. ADP-ribosylarginine hydrolase 1 (ARH1) catalyzes the cleavage of the ADP-ribose-arginine bond, regenerating (arginine)protein. Arginine-specific mono-ADP-ribosylation catalyzed by bacterial toxins was first identified as a mechanism of disease pathogenesis. Cholera toxin ADP-ribosylates and activates the α subunit of Gαs, a guanine nucleotide-binding protein that stimulates adenylyl cyclase activity, increasing cyclic adenosine monophosphate (cAMP), and resulting in fluid and electrolyte loss. Arginine-specific mono-ADP-ribosylation in mammalian cells has potential roles in membrane repair, immunity, and cancer. In mammalian tissues, ARH1 is a cytosolic protein that is ubiquitously expressed. ARH1 deficiency increased tumorigenesis in a gender-specific manner. In the myocardium, in response to cellular injury, an arginine-specific mono-ADP-ribosylation cycle, involving ART1 and ARH1, regulated the level and cellular distribution of ADP-ribosylated tripartite motif-containing protein 72 (TRIM72). Confirmed substrates of ARH1 in vivo are Gαs and TRIM72, however, more than a thousand proteins, ADP-ribosylated on arginine, have been identified by proteomic analysis. This review summarizes the current understanding of the properties of ARH1, e.g., bacterial toxin action, myocardial membrane repair following injury, and tumorigenesis.

Poly(ADP-ribosyl)ation refers to the covalent attachment of ADP-ribose to protein, generating branched, long chains of ADP-ribose moieties, known as poly(ADP-ribose) (PAR). Poly(ADP-ribose) polymerase 1 (PARP1) is the main polymerase and acceptor of PAR in response to DNA damage. Excessive intracellular PAR accumulation due to PARP1 activation leads cell death in a pathway known as parthanatos. PAR degradation is mainly controlled by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribose-acceptor hydrolase 3 (ARH3). Our previous results demonstrated that ARH3 confers protection against hydrogen peroxide (H(2)O(2)) exposure, by lowering cytosolic and nuclear PAR levels and preventing apoptosis-inducing factor (AIF) nuclear translocation. We identified a family with an ARH3 gene mutation that resulted in a truncated, inactive protein. The 8-year-old proband exhibited a progressive neurodegeneration phenotype. In addition, parthanatos was observed in neurons of the patient’s deceased sibling, and an older sibling exhibited a mild behavioral phenotype. Consistent with the previous findings, the patient’s fibroblasts and ARH3-deficient mice were more sensitive, respectively, to H(2)O(2) stress and cerebral ischemia/reperfusion-induced PAR accumulation and cell death. Further, PARP1 inhibition alleviated cell death and injury resulting from oxidative stress and ischemia/reperfusion. PARP1 inhibitors may attenuate the progression of neurodegeneration in affected patients with ARH3 deficiency.

Mono-ADP-ribosylation of an (arginine) protein catalyzed by ADP-ribosyltransferase 1 (ART1) - i.e., transfer of ADP-ribose from NAD to arginine - is reversed by ADP-ribosylarginine hydrolase 1 (ARH1) cleavage of the ADP-ribose-arginine bond. ARH1-deficient mice developed cardiomyopathy with myocardial fibrosis, decreased myocardial function under dobutamine stress, and increased susceptibility to ischemia/reperfusion injury. The membrane repair protein TRIM72 was identified as a substrate for ART1 and ARH1; ADP-ribosylated TRIM72 levels were greater in ARH1-deficient mice following ischemia/reperfusion injury. To understand better the role of TRIM72 and ADP-ribosylation, we used C2C12 myocytes. ARH1 knockdown in C2C12 myocytes increased ADP-ribosylation of TRIM72 and delayed wound healing in a scratch assay. Mutant TRIM72 (R207K, R260K) that is not ADP-ribosylated interfered with assembly of TRIM72 repair complexes at a site of laser-induced injury. The regulatory enzymes ART1 and ARH1 and their substrate TRIM72 were found in multiple complexes, which were coimmunoprecipitated from mouse heart lysates. In addition, the mono-ADP-ribosylation inhibitors vitamin K1 and novobiocin inhibited oligomerization of TRIM72, the mechanism by which TRIM72 is recruited to the site of injury. We propose that a mono-ADP-ribosylation cycle involving recruitment of TRIM72 and other regulatory factors to sites of membrane damage is critical for membrane repair and wound healing following myocardial injury.

Anosmin is an extracellular matrix protein, and genetic defects in anosmin result in human Kallmann syndrome. It functions in neural crest formation, cell adhesion, and neuronal migration. Anosmin consists of multiple domains, and it has been reported to bind heparan sulfate, FGF receptor, and UPA. In this study, we establish cell adhesion/spreading assays for anosmin and use them for antibody inhibition analyses to search for an integrin adhesion receptor. We find that α5β1, α4β1, and α9β1 integrins are needed for effective adhesive receptor function in cell adhesion and cell spreading on anosmin; adhesion is inhibited by both RGD and α4β1 CS1-based peptides. This identification of anosmin-integrin adhesion receptors should facilitate studies of anosmin function in cell and developmental biology.

Key roles for fibronectin and its integrin receptors have been postulated in the multiple cell-matrix interactions essential for chick embryo morphogenesis. However, mechanistic studies of these processes have been hampered by the current absence of sequence data and chicken cDNA clones for the major fibronectin receptor subunit, integrin α5 (ITGA5). We report here the sequence, endogenous expression pattern, and transfection of full-length chicken integrin α5. During early chicken embryonic development, α5 is highly expressed in cranial neural folds and migrating neural crest cells, suggesting potential roles in neural crest formation and migration. In fact, over-expression of this integrin in early neural tube selectively induces BMP5, a growth factor recently implicated in neural crest formation. Availability of these α5 integrin tools should facilitate studies of its functions in early embryonic development.

Cholera toxin, an 84-kDa multimeric protein and a major virulence factor of Vibrio cholerae, uses the ADP-ribosyltransferase activity of its A subunit to intoxicate host cells. ADP-ribosylation is a posttranslational modification of proteins, in which the ADP-ribose moiety of NAD+ is transferred to an acceptor. In mammalian cells, ADP-ribosylation of acceptors appears to be reversible. ADP-ribosyltransferases (ARTs) catalyze the modification of acceptor proteins, and ADP-ribose-acceptor hydrolases (ARHs) cleave the ADP-ribose-acceptor bond. ARH1 specifically cleaves the ADP-ribose-arginine bond. We previously demonstrated a role for endogenous ARH1 in regulating the extent of cholera toxin-mediated fluid and electrolyte abnormalities in a mouse model of intoxication. Murine ARH1-knockout (KO) cells and ARH1-KO mice exhibited increased sensitivity to cholera toxin compared to their wild-type (WT) counterparts. In the current report, we examined the sensitivity to cholera toxin of male and female ARH1-KO and WT mice. Intestinal loops derived from female ARH1-KO mice when injected with cholera toxin showed increased fluid accumulation compared to male ARH1-KO mice. WT mice did not show gender differences in fluid accumulation, ADP-ribosylarginine content, and ADP-ribosyl Gαs levels. Injection of 8-Bromo-cAMP into the intestinal loops also increased fluid accumulation, however, there was no significant difference between female and male mice or in WT and KO mice. Female ARH1-KO mice showed greater amounts of ADP-ribosylated Gαs protein and increased ADP-ribosylarginine content both in whole intestine and in epithelial cells than did male ARH1-KO mice. These results demonstrate that female ARH1-KO mice are more sensitive to cholera toxin than male mice. Loss of ARH1 confers gender sensitivity to the effects of cholera toxin but not of cyclic AMP. These observations may in part explain the finding noted in some clinical reports of enhanced symptoms of cholera and/or diarrhea in women than men.