Inhibition of the NAD salvage pathway in schistosomes impairs metabolism, reproduction, and parasite survival

The enigmatic heptalaminate surface membrane of intravascular schistosomes

Intravascular schistosomes have an unusual outer, heptalaminate (seven-layered) covering consisting of not one but two lipid bilayers. Here, we present an updated model of the molecular composition of these bilayers in Schistosoma mansoni that places most identified proteins in the outer, and not the inner, membrane. Here, enzymes would have access to their recently described (non-membrane-permeable) substrates. By contrast, nutrient transporter proteins must be in both membranes to facilitate uptake into the worm's inner tissues. Ectoenzyme activities displayed by living worms suggest the presence on their outer surface of several noncanonically extracellular proteins. The advantages of having a double-bilayered covering may relate to impeding host immunological attack and/or to the worm's ability to acquire selected host molecules onto their exterior.

Synthesis, Biological, and Computational Evaluations of Conformationally Restricted NAD-Mimics as Discriminant Inhibitors of Human NMN-Adenylyltransferase Isozymes

Nicotinamide adenine dinucleotide (NAD) cofactor metabolism plays a significant role in cancer development. Tumor cells have an increased demand for NAD and ATP to support rapid growth and proliferation. Limiting the amount of available NAD by targeting critical NAD biosynthesis enzymes has emerged as a promising anticancer therapeutic approach. In mammals, the enzyme nicotinamide/nicotinic acid adenylyltransferase (NMNAT) catalyzes a crucial downstream reaction for all known NAD synthesis routes. Novel nicotinamide/nicotinic acid adenine dinucleotide (NAD/NaAD) analogues 1-4, containing a methyl group at the ribose 2'-C and 3'-C-position of the adenosine moiety, were synthesized as inhibitors of the three isoforms of human NMN-adenylyltransferase, named hNMNAT-1, hNMNAT-2, and hNMNAT-3. An NMR-based conformational analysis suggests that individual NAD-analogues (1-4) have distinct conformational preferences. Biological evaluation of dinucleotides 1-4 as inhibitors of hNMNAT isoforms revealed structural relationships between different conformations (North-anti and South-syn) and enzyme-inhibitory activity. Among the new series of NAD analogues synthesized and tested, the 2'-C-methyl-NAD analogue 1 (Ki = 15 and 21 µM towards NMN and ATP, respectively) emerged as the most potent and selective inhibitor of hNMNAT-2 reported so far. Finally, we rationalized the in vitro bioactivity and selectivity of methylated NAD analogues with in silico studies, helping to lay the groundwork for rational scaffold optimization.

Macrophage-mediated trogocytosis contributes to destroying human schistosomes in a non-susceptible rodent host, Microtus fortis

Schistosoma parasites, causing schistosomiasis, exhibit typical host specificity in host preference. Many mammals, including humans, are susceptible to infection, while the widely distributed rodent, Microtus fortis, exhibits natural anti-schistosome characteristics. The mechanisms of host susceptibility remain poorly understood. Comparison of schistosome infection in M. fortis with the infection in laboratory mice (highly sensitive to infection) offers a good model system to investigate these mechanisms and to gain an insight into host specificity. In this study, we showed that large numbers of leukocytes attach to the surface of human schistosomes in M. fortis but not in mice. Single-cell RNA-sequencing analyses revealed that macrophages might be involved in the cell adhesion, and we further demonstrated that M. fortis macrophages could be mediated to attach and kill schistosomula with dependence on Complement component 3 (C3) and Complement receptor 3 (CR3). Importantly, we provided direct evidence that M. fortis macrophages could destroy schistosomula by trogocytosis, a previously undescribed mode for killing helminths. This process was regulated by Ca2+/NFAT signaling. These findings not only elucidate a novel anti-schistosome mechanism in M. fortis but also provide a better understanding of host parasite interactions, host specificity and the potential generation of novel strategies for schistosomiasis control.

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 Prospective Synergy of Antitubercular Drugs With NAD Biosynthesis Inhibitors

Given the upsurge of drug-resistant tuberculosis worldwide, there is much focus on developing novel drug combinations allowing shorter treatment duration and a lower toxicity profile. Nicotinamide adenine dinucleotide (NAD) biosynthesis targeting is acknowledged as a promising strategy to combat drug-susceptible, drug-resistant, and latent tuberculosis (TB) infections. In this review, we describe the potential synergy of NAD biosynthesis inhibitors with several TB-drugs in prospective novel combination therapy. Despite not directly targeting the essential NAD cofactor's biosynthesis, several TB prodrugs either require a NAD biosynthesis enzyme to be activated or form a toxic chemical adduct with NAD(H) itself. For example, pyrazinamide requires the action of nicotinamidase (PncA), often referred to as pyrazinamidase, to be converted into its active form. PncA is an essential player in NAD salvage and recycling. Since most pyrazinamide-resistant strains are PncA-defective, a combination with downstream NAD-blocking molecules may enhance pyrazinamide activity and possibly overcome the resistance mechanism. Isoniazid, ethionamide, and delamanid form NAD adducts in their active form, partly perturbing the redox cofactor metabolism. Indeed, NAD depletion has been observed in Mycobacterium tuberculosis (Mtb) during isoniazid treatment, and activation of the intracellular NAD phosphorylase MbcT toxin potentiates its effect. Due to the NAD cofactor's crucial role in cellular energy production, additional synergistic correlations of NAD biosynthesis blockade can be envisioned with bedaquiline and other drugs targeting energy-metabolism in mycobacteria. In conclusion, future strategies targeting NAD metabolism in Mtb should consider its potential synergy with current and other forthcoming TB-drugs.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

Nicotinamide adenine dinucleotide (NAD+) and its metabolites function as critical regulators to maintain physiologic processes, enabling the plastic cells to adapt to environmental changes including nutrient perturbation, genotoxic factors, circadian disorder, infection, inflammation and xenobiotics. These effects are mainly achieved by the driving effect of NAD+ on metabolic pathways as enzyme cofactors transferring hydrogen in oxidation-reduction reactions. Besides, multiple NAD+-dependent enzymes are involved in physiology either by post-synthesis chemical modification of DNA, RNA and proteins, or releasing second messenger cyclic ADP-ribose (cADPR) and NAADP+. Prolonged disequilibrium of NAD+ metabolism disturbs the physiological functions, resulting in diseases including metabolic diseases, cancer, aging and neurodegeneration disorder. In this review, we summarize recent advances in our understanding of the molecular mechanisms of NAD+-regulated physiological responses to stresses, the contribution of NAD+ deficiency to various diseases via manipulating cellular communication networks and the potential new avenues for therapeutic intervention.