Schistosoma mansoni NAD(+) catabolizing enzyme: identification of key residues in catalysis

Schistosoma mansoni and the purinergic halo

Intravascular schistosomes may control immune and hemostatic responses by regulating the nature and amount of selected host purinergic signaling molecules - such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and nicotinamide adenine dinucleotide (NAD) - surrounding them. Such metabolites are collectively known as the worm's 'purinergic halo'. Host-interactive, membrane-bound, tegumental ectonucleotidases, notably SmATPDase1, SmNPP5, SmAP and SmNACE, can degrade proinflammatory, prothrombotic and immunomodulatory purinergic metabolites like those listed. A common catabolic product is the anti-inflammatory metabolite adenosine that can additionally be taken in by the worms as food. We envision the tegumental ectonucleotidases as having a twofold role at the worm surface: first, they degrade potentially harmful host signaling molecules, and second, they generate vital nutrients around the worms from where these can be conveniently imported.

NAD-catabolizing ectoenzymes of Schistosoma mansoni

Infection with schistosomes (blood flukes) can result in the debilitating disease schistosomiasis. These parasites survive in their host for many years, and we hypothesize that proteins on their tegumental surface, interacting with the host microenvironment, facilitate longevity. One such ectoenzyme - the nucleotide pyrophosphatase/phosphodiesterase SmNPP5 can cleave ADP (to prevent platelet aggregation) and NAD (likely preventing Treg apoptosis). A second tegumental ectoenzyme, the glycohydrolase SmNACE, also catabolizes NAD. Here, we undertake a comparative biochemical characterization of these parasite ectoenzymes. Both are GPI-linked and exhibit different optimal pH ranges.  While SmNPP5 requires divalent cations, SmNACE does not. The Km values of the two enzymes for NAD at physiological pH differ: SmNPP5, Km=340µM±44; SmNACE, Km=49µM±4. NAD cleavage by each enzyme yields different products. SmNPP5 cleaves NAD to form nicotinamide mononucleotide (NMN) and AMP, whereas SmNACE cleaves NAD to generate nicotinamide (NAM) and adenosine diphosphate ribose (ADPR). Each enzyme can process the other's reaction product. Thus, SmNACE cleaves NMN (to yield NAM and ribose phosphate) and SmNPP5 cleaves ADPR (yielding AMP and ribose phosphate). Metabolomic analysis of plasma containing adult worms supports the idea that these cleavage pathways are active in vivo. We hypothesize that a primary function of SmNPP5 is to cleave NAD to control host immune cell function and a primary function of SmNACE is to cleave NMN to generate the vital nutrient nicotinamide (vitamin B3) for convenient uptake by the worms. Chemical inhibition of one or both ectoenzymes could upset worm metabolism and control schistosome infection.

The essential schistosome tegumental ectoenzyme SmNPP5 can block NAD-induced T cell apoptosis

Infection with intravascular platyhelminths of the genus Schistosoma can result in the debilitating disease schistosomiasis. Schistosomes (blood flukes) can survive in the host for many years. We hypothesize that proteins on their host-interactive surface modify the worm's external environment to help insure worm survival. Previously, we have shown that a surface ectoenzyme of Schistosoma mansoni, SmNPP5 - a nucleotide pyrophosphatase/phosphodiesterase - can cleave ADP and block platelet aggregation in vitro. In this work, we show that both adult schistosomes and recombinant SmNPP5 can cleave the exogenous purinergic signaling molecule nicotinamide adenine dinucleotide (NAD). In doing so, worms and rSmNPP5 can prevent NAD-induced apoptosis of T cells in vitro. Since regulatory T cells (Tregs) are especially prone to such NAD-induced cell death (NICD), we hypothesize that schistosome cleavage of NAD promotes Treg survival which creates a more immunologically hospitable environment for the worms in vivo. In addition to SmNPP5, schistosomes express another host-interactive NAD-degrading enzyme, SmNACE. We successfully suppressed the expression of SmNPP5 and SmNACE (singly or together) using RNAi. Only SmNPP5-suppressed worms, and not SmNACE-suppressed worms, were significantly impaired in their ability to cleave exogenous NAD compared to controls. Therefore, we contend that ectoenzyme SmNPP5 on the surface of the worm is primarily responsible for extracellular NAD cleavage and that this helps modulate the host immune environment by preventing Treg cell death.

Efficient Inhibition of SmNACE by Coordination Complexes Is Abolished by S. mansoni Sequestration of Metal

SmNACE is a NAD catabolizing enzyme expressed on the outer tegument of S. mansoni, a human parasite that is one of the major agents of the neglected tropical disease schistosomiasis. Recently, we identified aroylhydrazone derivatives capable of inhibiting the recombinant form of the enzyme with variable potency (IC₅₀ ranging from 88 μM to 33 nM). In the present study, we investigated the mechanism of action of the least potent micromolar inhibitor (compound 1) and the most potent nanomolar inhibitor (compound 2) in the series on both the recombinant and native SmNACE enzymes. Using mass spectroscopy, spectrophotometry, and activity assays under different experimental conditions, we demonstrated that the >3 log gain in potency against recombinant SmNACE by this class of compounds is dependent on the formation of a coordination complex with metal cations, such as Ni(II), Zn(II), and Fe(II), that are loaded on the protein surface. Testing the compounds on live parasites, we observed that only the weak micromolar compound 1 was active on the native enzyme. We showed that S. mansoni effectively sequesters the metal from the coordination complex, resulting in the loss of inhibitory activity of the potent nanomolar compound 2. Importantly, the modeling of the transition complex between Zn(II) and compound 2 enabled the discovery of a new metal-independent aroylhydrazone analogue, which is now the most potent and selective inhibitor of native SmNACE known.