Explanation Mono-ADP-ribosylation is a post-translational protein modification, in which ADP-ribose is transferred from NAD to an acceptor amino acid. It was first identified as a mechanism of disease pathogenesis in the bacterial diseases such as cholera, diphtheria and pertussis, where toxins ADP-ribosylate critical regulatory and biosynthetic proteins. Mammalian tissues have enzymatic activities that mimic those of the bacterial toxins. Mammalian arginine-specific mono-ADP-ribosylation however, is a reversible modification of protein. Arginine-specific mono-ADP-ribosyltrans- ferases (ARTs) (e.g., ART1, ART5), transfer ADP-ribose from NAD to arginine residues of target proteins and ADP-ribosylarginine hydrolase 1 (ARH1) reverses the reaction by cleaving the ADP-ribose-(arginine)-protein bond. Data are consistent with ART and ARH1 serving as opposing arms of an arginine ADP-ribosylation cycle. 1. TRIM72 is an abundant in heart and skeletal muscle and in lung and kidney epithelial cells where it is involved in membrane repair. Effective membrane repair protects cardiomyocytes from ischemic damage. Oligomerization of TRIM72 was required acutely for membrane repair, bringing TRIM72 to the site of injury. TRIM72 also countered cell damage due to ischemia-reperfusion injury. TRIM72 in complex with caveolin-3 (Cav-3) activated phosphatidylinositol-3-kinases (PI3K)-dependent reperfusion injury salvage kinase (RISK), thereby enhancing cell survival. The regulatory enzymes ART1 and ARH1 and their substrate TRIM72 were found in complexes, which were co-immunoprecipitated from mouse heart lysates. The complexes found in association with TRIM72 appeared to be relatively heterogeneous and could be resolved by FPLC and HPLC. Some of the complexes included Cav-3. It has been reported that TRIM72 oligomerization serves as a key mechanism for regulation of acute membrane repair. Oligomerization of TRIM72, observed in WT mice, was delayed in ARH1-deficient heart lysates, where a dimer of TRIM72 rather than a trimer was seen. In addition, at a cellular level, oligomerization of TRIM72 at the sites of injury required the presence of the mono-ADP-ribosylation cycle, that is, ART1 and ARH1. 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. 2. Enhanced sensitivity to cholera toxin in female ADP-ribosylarginine hydrolase (ARH1)-deficient mice. 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 Gs 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 Gs 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. 3. PARP1 inhibition alleviates injury in ARH3-deficient mice and human cells. 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 (H2O2) 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 H2O2 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. 4. Collabotative studies are ongoing with Michael Hottiger and In-Kwon Kim.
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