Polyglutamine Tract Expansion Increases S-Nitrosylation of Huntingtin and Ataxin-1.

Expansion of the polyglutamine (polyQ) tract in the huntingtin (Htt) protein causes Huntington's disease (HD), a fatal inherited movement disorder linked to neurodegeneration in the striatum and cortex. S-nitrosylation and S-acylation of cysteine residues regulate many functions of cytosolic proteins. We therefore used a resin-assisted capture approach to identify these modifications in Htt. In contrast to many proteins that have only a single S-nitrosylation or S-acylation site, we identified sites along much of the length of Htt. Moreover, analysis of cells expressing full-length Htt or a large N-terminal fragment of Htt shows that polyQ expansion strongly increases Htt S-nitrosylation. This effect appears to be general since it is also observed in Ataxin-1, which causes spinocerebellar ataxia type 1 (SCA1) when its polyQ tract is expanded. Overexpression of nitric oxide synthase increases the S-nitrosylation of normal Htt and the frequency of conspicuous juxtanuclear inclusions of Htt N-terminal fragments in transfected cells. Taken together with the evidence that S-nitrosylation of Htt is widespread and parallels polyQ expansion, these subcellular changes show that S-nitrosylation affects the biology of this protein in vivo.

Convergence of G protein-coupled receptor and S-nitrosylation signaling determines the outcome to cardiac ischemic injury.

Heart failure caused by ischemic heart disease is a leading cause of death in the developed world. Treatment is currently centered on regimens involving G protein-coupled receptors (GPCRs) or nitric oxide (NO). These regimens are thought to target distinct molecular pathways. We showed that these pathways were interdependent and converged on the effector GRK2 (GPCR kinase 2) to regulate myocyte survival and function. Ischemic injury coupled to GPCR activation, including GPCR desensitization and myocyte loss, required GRK2 activation, and we found that cardioprotection mediated by inhibition of GRK2 depended on endothelial nitric oxide synthase (eNOS) and was associated with S-nitrosylation of GRK2. Conversely, the cardioprotective effects of NO bioactivity were absent in a knock-in mouse with a form of GRK2 that cannot be S-nitrosylated. Because GRK2 and eNOS inhibit each other, the balance of the activities of these enzymes in the myocardium determined the outcome to ischemic injury. Our findings suggest new insights into the mechanism of action of classic drugs used to treat heart failure and new therapeutic approaches to ischemic heart disease.

Trypanosoma rangeli: a possible role for ecto-phosphatase activity on cell proliferation.

Here we demonstrate for the first time that growth of Trypanosoma rangeli, a protozoa parasite, is strongly dependent on the presence of inorganic phosphate (Pi) in the culture medium and that the replacement of the inorganic phosphate in the culture medium by beta-glycerophosphate, a substrate for phosphatases lead the cells to achieve its maximal growth. The ecto-phosphatase activity present on the external surface of T. rangeli decreased during the growth phase of the parasite, suggesting that this enzyme could be important for the development. Accordingly, the inhibition of this ecto-phosphatase activity by sodium orthovanadate also inhibited the proliferation of T. rangeli. Parasites maintained in a Pi-starved culture medium (2 mM Pi) had 4-fold more ecto-phosphatase activity as compared to parasites maintained in a Pi-supplemented culture medium (50 mM Pi). Altogether, these results presented here suggest that this ecto-phosphatase activity leads to hydrolysis of phosphorylated compounds present in the extracellular medium, which could contribute to the acquisition of inorganic phosphate during the development of T. rangeli epimastigotes.

Trypanosoma brucei brucei: biochemical characterization of ecto-nucleoside triphosphate diphosphohydrolase activities.

In this work we describe the ability of living cells of Trypanosoma brucei brucei to hydrolyze extracellular ATP. In these intact parasites there was a low level of ATP hydrolysis in the absence of any divalent metal (4.72+/-0.51 nmol Pi x 10(-7) cells x h(-1)). The ATP hydrolysis was stimulated by MgCl(2) and the Mg-dependent ecto-ATPase activity was 27.15+/-2.91 nmol Pi x 10(-7) cells x h(-1). This stimulatory activity was also observed when MgCl(2) was replaced by MnCl(2). CaCl(2) and ZnCl(2) were also able to stimulate the ATPase activity, although less than MgCl(2). The apparent K(m) for ATP was 0.61 mM. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities. To confirm that this Mg-dependent ATPase activity is an ecto-ATPase activity, we used an impermeable inhibitor, DIDS (4, 4'-diisothiocyanostylbene 2'-2'-disulfonic acid), as well as suramin, an antagonist of P(2) purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. Living cells sequentially hydrolyzed the ATP molecule generating ADP, AMP and adenosine, and supplementation of the culture medium with ATP was able to sustain the proliferation of T. brucei brucei as well as adenosine supplementation. Furthermore, the E-NTPDase activity of T. brucei brucei is modulated by the availability of purines in the medium. These results indicate that this surface enzyme may play a role in the salvage of purines from the extracellular medium in T. brucei brucei.

Trypanosoma rangeli: characterization of a Mg-dependent ecto ATP-diphosphohydrolase activity.

In this work we describe the ability of living Trypanosoma rangeli to hydrolyze extracellular ATP. In these intact parasites whose viability was assessed before and after the reactions by motility and by Trypan blue dye exclusion, there was a low level of ATP hydrolysis in the absence of any divalent metal (1.53+/-0.12 nmol P(i)/h x 10(7) cells). The ATP hydrolysis was stimulated by MgCl(2) and the Mg-dependent ecto-ATPase activity was 5.24+/-0.64 nmol P(i)/h x 10(7) cells. The Mg-dependent ecto-ATPase activity was linear with cell density and with time for at least 60 min. This stimulatory effect on the ATP hydrolysis was also observed when MgCl(2) was replaced by MnCl(2), but not by CaCl(2), SrCl(2), and ZnCl(2). The apparent K(m) for Mg-ATP2- was 0.53+/-0.11 mM. The optimum pH for the T. rangeli Mg-dependent ecto-ATPase activity lies in the alkaline range. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities, such as oligomycin, sodium azide, bafilomycin A1, ouabain, furosemide, vanadate, molybdate, sodium fluoride, tartrate, and levamizole. To confirm that this Mg-dependent ATPase was an ecto-ATPase, we used an impermeant inhibitor, DIDS (4, 4'-diisothiocyanostylbene 2'-2'-disulfonic acid) as well as suramin, an antagonist of P2 purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. This ecto-ATPase activity was stimulated by carbohydrates involved in the attachment/invasion of salivary glands of Rhodnius prolixus and by lipophorin, an insect lipoprotein circulating in the hemolymph.

A metallo phosphatase activity present on the surface of Trypanosoma brucei procyclic forms.

In this work, we describe how living cells of Trypanosoma brucei procyclic forms were able to hydrolyze extracellular p-nitrophenylphosphate (pNPP). These intact parasites, which had their viability determined by motility and the Trypan blue method, presented a low level of pNPP hydrolysis in the absence of any divalent metal (0.72+/-0.07 nmol pNP/mg min). Interestingly, in the presence of 5mM MgCl(2), ectophosphatase activity of 1.91+/-0.21 nmol pNP/mg min was observed. The ectophosphatase activity was also stimulated by MnCl(2), CoCl(2) and CuCl(2) but not by CaCl(2) and CdCl(2) and was inhibited by ZnCl(2). The addition of Mg(2+), Mn(2+), Co(2+) and Cu(2+) to extracellular medium increased the ectophosphatase activity in a dose-dependent manner. At 5mM pNPP, half-maximal stimulation of pNPP hydrolysis was obtained with 0.39+/-0.05 mM MgCl(2), 0.33+/-0.03 mM MnCl(2), 1.63+/-0.12 mM CoCl(2) and 2.04+/-0.33 mM CuCl(2). In the absence of any divalent metal (basal activity) the apparent K(m) for pNPP was 0.66+/-0.09 mM, while at saturating MgCl(2) concentrations the corresponding apparent K(m) for pNPP for Mg(2+)-stimulated phosphatase activity (difference between total minus basal phosphatase activity) was 0.27+/-0.03 mM. The Mg(2+)-stimulated pNPP hydrolysis was strongly inhibited by ZnCl(2) and vanadate, while the metal-independent basal phosphatase activity was less inhibited by these phosphotyrosyl phosphatase inhibitors.