Characterization of NAD salvage pathways and their role in virulence in Streptococcus pneumoniae

From bacteria to biomedicine: Developing therapies exploiting NAD+ metabolism

Nicotinamide adenine dinucleotide (NAD+) serves as a critical cofactor in cellular metabolism and redox reactions. Bacterial pathways rely on NAD+ participation, where its stability and concentration govern essential homeostasis and functions. This review delves into the role and metabolic regulation of NAD+ in bacteria, highlighting its influence on physiology and virulence. Notably, we explore enzymes linked to NAD+ metabolism as antibacterial drug targets and vaccine candidates. Moreover, we scrutinize NAD+'s medical potential, offering insights for its application in biomedicine. This comprehensive assessment informs future research directions in the dynamic realm of NAD+ and its biomedical significance.

Contribution of nadR to the cell growth and virulence of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) has been reported to be a highly invasive pathogen in swine and a zoonotic agent for humans. Although many bacterial virulence factors have been identified, our an insightful understanding of SS2 pathogenicity is lacking. The gene nadR, encoding nicotinamide-nucleotide adenylyltransferase, was first reported as a regulator and transporter of the intracellular NAD synthesis pathway in Salmonella typhimurium. In this study, we constructed a mutant strain of nadR (ΔnadR) to test the phenotypic and virulence variations between the deletion mutant and the wild-type strain ZY05719. The phenotypic experimental results showed that ΔnadR obviously inhibited cell growth and exhibited shorter chains than WT. The growth defect of ΔnadR was caused by the loss of the function of nadR for transporting the substrates nicotinamide mononucleotide and nicotinamide riboside in the intracellular NAD synthesis pathway. In the process of interaction with the host, ΔnadR participated in adherence and invasion to the host cells, and it was more easily cleared by RAW264.7 macrophages. More importantly, both zebrafish and BALB/c mice in vivo virulence experimental results showed that ΔnadR dramatically attenuated the virulence of SS2, and the ability of ΔnadR to colonize tissues was notably reduced in comparison with that of WT in the BALB/c mice infection model. To the best of our knowledge, this is the first time to demonstrate that nadR not only plays an important role in bacterial growth, but also in connection with the virulence of SS2 as a global transcriptional regulator.

NAD+ metabolism is a key modulator of bacterial respiratory epithelial infections

Lower respiratory tract infections caused by Streptococcus pneumoniae (Spn) are a leading cause of death globally. Here we investigate the bronchial epithelial cellular response to Spn infection on a transcriptomic, proteomic and metabolic level. We found the NAD+ salvage pathway to be dysregulated upon infection in a cell line model, primary human lung tissue and in vivo in rodents, leading to a reduced production of NAD+. Knockdown of NAD+ salvage enzymes (NAMPT, NMNAT1) increased bacterial replication. NAD+ treatment of Spn inhibited its growth while growth of other respiratory pathogens improved. Boosting NAD+ production increased NAD+ levels in immortalized and primary cells and decreased bacterial replication upon infection. NAD+ treatment of Spn dysregulated the bacterial metabolism and reduced intrabacterial ATP. Enhancing the bacterial ATP metabolism abolished the antibacterial effect of NAD+. Thus, we identified the NAD+ salvage pathway as an antibacterial pathway in Spn infections, predicting an antibacterial mechanism of NAD+.

Symbiosis of Carpenter Bees with Uncharacterized Lactic Acid Bacteria Showing NAD Auxotrophy

Eusocial bees (such as honey bees and bumble bees) harbor core gut microbiomes that are transmitted through social interaction between nestmates. Carpenter bees are not eusocial; however, recent microbiome analyses found that Xylocopa species harbor distinctive core gut microbiomes. In this study, we analyzed the gut microbiomes of three Xylocopa species in Japan between 2016 and 2021 by V1 to V2 region-based 16S rDNA amplicon sequencing, and 14 candidate novel species were detected based on the full-length 16S rRNA gene sequences. All Xylocopa species harbor core gut microbiomes consisting of primarily lactic acid bacteria (LAB) that were phylogenetically distant from known species. Although they were difficult to cultivate, two LAB species from two different Xylocopa species were isolated by supplementing bacterial culture supernatants. Both genomes exhibited an average LAB genome size with a large set of genes for carbohydrate utilization but lacked genes to synthesize an essential coenzyme NAD, which is unique among known insect symbionts. Our findings of phylogenetically distinct core LAB of NAD auxotrophy reflected the evolution of Xylocopa-restricted bacteria retention and maintenance through vertical transmission of microbes during solitary life. We propose five candidate novel species belonging to the families Lactobacillaceae and Bifidobacteriaceae, including a novel genus, and their potential functions in carbohydrate utilization. IMPORTANCE Recent investigations found unique microbiomes in carpenter bees, but the description of individual microbes, including isolation and genomics, remains largely unknown. Here, we found that the Japanese Xylocopa species also harbor core gut microbiomes. Although most of them were difficult to isolate a pure colony, we successfully isolated several strains. We performed whole-genome sequencing of the isolated candidate novel species and found that the two Lactobacillaceae strains belonging to the Xylocopa-specific novel LAB clade lack the genes for synthesizing NAD, a coenzyme central to metabolism in all living organisms. Here, we propose a novel genus for the two LAB species based on very low 16S rRNA gene sequence similarities and genotypic characters.

A bioinformatic approach to identify pathogenic variants for Stevens-Johnson syndrome

Stevens-Johnson syndrome (SJS) produces a severe hypersensitivity reaction caused by Herpes simplex virus or mycoplasma infection, vaccination, systemic disease, or other agents. Several studies have investigated the genetic susceptibility involved in SJS. To provide further genetic insights into the pathogenesis of SJS, this study prioritized high-impact, SJS-associated pathogenic variants through integrating bioinformatic and population genetic data. First, we identified SJS-associated single nucleotide polymorphisms from the genome-wide association studies catalog, followed by genome annotation with HaploReg and variant validation with Ensembl. Subsequently, expression quantitative trait locus (eQTL) from GTEx identified human genetic variants with differential gene expression across human tissues. Our results indicate that two variants, namely rs2074494 and rs5010528, which are encoded by the HLA-C (human leukocyte antigen C) gene, were found to be differentially expressed in skin. The allele frequencies for rs2074494 and rs5010528 also appear to significantly differ across continents. We highlight the utility of these population-specific HLA-C genetic variants for genetic association studies, and aid in early prognosis and disease treatment of SJS.

Oral supplementation of nicotinamide riboside alters intestinal microbial composition in rats and mice, but not humans

The gut microbiota impacts systemic levels of multiple metabolites including NAD⁺ precursors through diverse pathways. Nicotinamide riboside (NR) is an NAD⁺ precursor capable of regulating mammalian cellular metabolism. Some bacterial families express the NR-specific transporter, PnuC. We hypothesized that dietary NR supplementation would modify the gut microbiota across intestinal sections. We determined the effects of 12 weeks of NR supplementation on the microbiota composition of intestinal segments of high-fat diet-fed (HFD) rats. We also explored the effects of 12 weeks of NR supplementation on the gut microbiota in humans and mice. In rats, NR reduced fat mass and tended to decrease body weight. Interestingly, NR increased fat and energy absorption but only in HFD-fed rats. Moreover, 16S rRNA gene sequencing analysis of intestinal and fecal samples revealed an increased abundance of species within Erysipelotrichaceae and Ruminococcaceae families in response to NR. PnuC-positive bacterial strains within these families showed an increased growth rate when supplemented with NR. The abundance of species within the Lachnospiraceae family decreased in response to HFD irrespective of NR. Alpha and beta diversity and bacterial composition of the human fecal microbiota were unaltered by NR, but in mice, the fecal abundance of species within Lachnospiraceae increased while abundances of Parasutterella and Bacteroides dorei species decreased in response to NR. In conclusion, oral NR altered the gut microbiota in rats and mice, but not in humans. In addition, NR attenuated body fat mass gain in rats, and increased fat and energy absorption in the HFD context.

Antibiotic resistance in Neisseria gonorrhoeae: broad-spectrum drug target identification using subtractive genomics

Neisseria gonorrhoeae is a Gram-negative aerobic diplococcus bacterium that primarily causes sexually transmitted infections through direct human sexual contact. It is a major public health threat due to its impact on reproductive health, the widespread presence of antimicrobial resistance, and the lack of a vaccine. In this study, we used a bioinformatics approach and performed subtractive genomic methods to identify potential drug targets against the core proteome of N. gonorrhoeae (12 strains). In total, 12,300 protein sequences were retrieved, and paralogous proteins were removed using CD-HIT. The remaining sequences were analyzed for non-homology against the human proteome and gut microbiota, and screened for broad-spectrum analysis, druggability, and anti-target analysis. The proteins were also characterized for unique interactions between the host and pathogen through metabolic pathway analysis. Based on the subtractive genomic approach and subcellular localization, we identified one cytoplasmic protein, 2Fe-2S iron-sulfur cluster binding domain-containing protein (NGFG RS03485), as a potential drug target. This protein could be further exploited for drug development to create new medications and therapeutic agents for the treatment of N. gonorrhoeae infections.

A Salvaging Strategy Enables Stable Metabolite Provisioning among Free-Living Bacteria

All organisms rely on complex metabolites such as amino acids, nucleotides, and cofactors for essential metabolic processes. Some microbes synthesize these fundamental ingredients of life de novo, while others rely on uptake to fulfill their metabolic needs. Although certain metabolic processes are inherently "leaky," the mechanisms enabling stable metabolite provisioning among microbes in the absence of a host remain largely unclear. In particular, how can metabolite provisioning among free-living bacteria be maintained under the evolutionary pressure to economize resources? Salvaging, the process of "recycling and reusing," can be a metabolically efficient route to obtain access to required resources. Here, we show experimentally how precursor salvaging in engineered Escherichia coli populations can lead to stable, long-term metabolite provisioning. We find that salvaged cobamides (vitamin B₁₂ and related enzyme cofactors) are readily made available to nonproducing population members, yet salvagers are strongly protected from overexploitation. We also describe a previously unnoted benefit of precursor salvaging, namely, the removal of the nonfunctional, proliferation-inhibiting precursor. As long as compatible precursors are present, any microbe possessing the terminal steps of a biosynthetic process can, in principle, forgo de novo biosynthesis in favor of salvaging. Consequently, precursor salvaging likely represents a potent, yet overlooked, alternative to de novo biosynthesis for the acquisition and provisioning of metabolites in free-living bacterial populations. IMPORTANCE Recycling gives new life to old things. Bacteria have the ability to recycle and reuse complex molecules they encounter in their environment to fulfill their basic metabolic needs in a resource-efficient way. By studying the salvaging (recycling and reusing) of vitamin B₁₂ precursors, we found that metabolite salvaging can benefit others and provide stability to a bacterial community at the same time. Salvagers of vitamin B₁₂ precursors freely share the result of their labor yet cannot be outcompeted by freeloaders, likely because salvagers retain preferential access to the salvaging products. Thus, salvaging may represent an effective, yet overlooked, mechanism of acquiring and provisioning nutrients in microbial populations.

The global emergence of a novel Streptococcus suis clade associated with human infections

Streptococcus suis, a ubiquitous bacterial colonizer in pigs, has recently extended host range to humans, leading to a global surge of deadly human infections and three large outbreaks since 1998. To better understand the mechanisms for the emergence of cross-species transmission and virulence in human, we have sequenced 366 S. suis human and pig isolates from 2005 to 2016 and performed a large-scale phylogenomic analysis on 1,634 isolates from 14 countries over 36 years. We show the formation of a novel human-associated clade (HAC) diversified from swine S. suis isolates. Phylogeographic analysis identified Europe as the origin of HAC, coinciding with the exportation of European swine breeds between 1960s and 1970s. HAC is composed of three sub-lineages and contains several healthy-pig isolates that display high virulence in experimental infections, suggesting healthy-pig carriers as a potential source for human infection. New HAC-specific genes are identified as promising markers for pathogen detection and surveillance. Our discovery of a human-associated S. suis clade provides insights into the evolution of this emerging human pathogen and extend our understanding of S. suis epidemics worldwide.

Functional definition of NrtR, a remnant regulator of NAD+ homeostasis in the zoonotic pathogen Streptococcus suis

NAD+ is an enzyme cofactor required for the 3 domains of life. However, little is known about the NAD+ biosynthesis and salvage pathways in the opportunistic pathogen Streptococcus suis. A genome-wide search allows us to identify the NAD+ salvage pathway encoded by an operon of nadR-pnuC-nrtR (from SSU05_1973 to SSU05_1971 on the reverse strand) in the S. suis 05ZYH33 that causes streptococcal toxin shock-like syndrome. The regulator of this pathway is Nudix-related transcriptional regulator (NrtR), a transcription regulator of the Nudix family comprising an N-terminal Nudix-like effector domain, and a C-terminal DNA-binding winged helix-turn-helix-like domain. Intriguingly, the S. suis NrtR naturally contains a single amino acid substitution (K92E) in the catalytic site of its Nudix domain that renders it catalytically inactive but does not influence its ability to bind DNA. Despite its lack of enzymatic activity, DNA-binding activity of NrtR is antagonized by the effector ADP-ribose. Furthermore, nrtR knockout in S. suis serotype 2 reduces its capacity to form biofilms and attenuates its virulence in a mouse infection model. Genome mining indicates that nrtR appears in a strain-specific manner whose occupancy is correlated to bacterial infectivity. Unlike the paradigmatic member of NrtR family having 2 unrelated functions (Nudix hydrolase and DNA binding), S. suis 2 retains a single regulatory role in the modulation of NAD+ salvage. This control of NAD+ homeostasis contributes to S. suis virulence.-Wang, Q., Hassan, B. H., Lou, N., Merritt, J., Feng, Y. Functional definition of NrtR, a remnant regulator of NAD+ homeostasis in the zoonotic pathogen Streptococcus suis.

Chemo-enzymatic synthesis of isotopically labeled nicotinamide riboside

As a cofactor for numerous reactions, NAD+ is found widely dispersed across many maps of cellular metabolism. This core redox role alone makes the biosynthesis of NAD+ of great interest. Recent studies have revealed new biological roles for NAD+ as a substrate for diverse enzymes that regulate a broad spectrum of key cellular tasks. These NAD+-consuming enzymes further highlight the importance of understanding NAD+ biosynthetic pathways. In this study, we developed a chemo-enzymatic synthesis of isotopically labeled NAD+ precursor, nicotinamide riboside (NR). The synthesis of NR isotopomers allowed us to unambiguously determine that NR is efficiently converted to NAD+ in the cellular environment independent of degradation to nicotinamide, and it is incorporated into NAD+ in its intact form. The versatile synthetic method along with the isotopically labeled NRs will provide powerful tools to further decipher the important yet complicated NAD+ metabolism.

NADH-Mediated Gene Expression in Streptococcus pneumoniae and Role of Rex as a Transcriptional Repressor of the Rex-Regulon

Nicotinamide adenine dinucleotides (NAD(H)) play a vital role in various biological processes, including keeping the cellular redox balance. In this study, we investigate the regulatory responses of Streptococcus pneumoniae D39 to NADH and characterize the role of the Rex protein as a transcriptional repressor of the gapN, fba, pncB, adhB2, gap, and adhE genes. Transcriptomic analysis was used to observe the response of S. pneumoniae D39 to NADH. Our microarray studies revealed elevated expression of various genes/operons involved in transport and biosynthesis of niacin (gapN, fba, pncB, adhB2, gap, and adhE). Promoter lacZ-fusion assays and microarray studies with the rex mutant revealed the role of Rex as a transcriptional repressor of gapN, fba, pncB, adhB2, gap, and adhE involved in niacin uptake and biosynthesis, in the presence of NADH. We predict the operator site (5′-TTGTKAWAAWWTTCACAA-3′) of Rex in the regulatory regions of Rex-regulated genes that was subsequently validated by promoter mutational experiments.

Niacin-mediated Gene Expression and Role of NiaR as a Transcriptional Repressor of niaX, nadC, and pnuC in Streptococcus pneumoniae

NAD (Nicotinamide Adenine Dinucleotide) biosynthesis is vital for bacterial physiology and plays an important role in cellular metabolism. A naturally occurring vitamin B complex, niacin (nicotinic acid), is a precursor of coenzymes NAD and NADP. Here, we study the impact of niacin on global gene expression of Streptococcus pneumoniae D39 and elucidate the role of NiaR as a transcriptional regulator of niaX, nadC, and pnuC. Transcriptome comparison of the D39 wild-type grown in chemically defined medium (CDM) with 0 to 10 mM niacin revealed elevated expression of various genes, including niaX, nadC, pnuC, fba, rex, gapN, pncB, gap, adhE, and adhB2 that are putatively involved in the transport and utilization of niacin. Niacin-dependent expression of these genes is confirmed by promoter lacZ-fusion studies. Moreover, the role of transcriptional regulator NiaR in the regulation of these genes is explored by DNA microarray analysis. Our transcriptomic comparison of D39 ΔniaR to D39 wild-type revealed that the transcriptional regulator NiaR acts as a transcriptional repressor of niaX, pnuC, and nadC. NiaR-dependent regulation of niaX, nadC, and pnuC is further confirmed by promoter lacZ-fusion studies. The putative operator site of NiaR (5′-TACWRGTGTMTWKACASYTRWAW-3′) in the promoter regions of niaX, nadC, and pnuC is predicted and further confirmed by promoter mutational experiments.

NRK1 controls nicotinamide mononucleotide and nicotinamide riboside metabolism in mammalian cells

NAD⁺ is a vital redox cofactor and a substrate required for activity of various enzyme families, including sirtuins and poly(ADP-ribose) polymerases. Supplementation with NAD⁺ precursors, such as nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR), protects against metabolic disease, neurodegenerative disorders and age-related physiological decline in mammals. Here we show that nicotinamide riboside kinase 1 (NRK1) is necessary and rate-limiting for the use of exogenous NR and NMN for NAD⁺ synthesis. Using genetic gain- and loss-of-function models, we further demonstrate that the role of NRK1 in driving NAD⁺ synthesis from other NAD⁺ precursors, such as nicotinamide or nicotinic acid, is dispensable. Using stable isotope-labelled compounds, we confirm NMN is metabolized extracellularly to NR that is then taken up by the cell and converted into NAD⁺. Our results indicate that mammalian cells require conversion of extracellular NMN to NR for cellular uptake and NAD⁺ synthesis, explaining the overlapping metabolic effects observed with the two compounds.

Raising cellular levels of the metabolic cofactor NAD⁺ reverses key indicators of aging. Here, Ratajczak et al. show that cellular levels of NAD⁺ depend on the extracellular catalytic activity of NRK1, which processes two NAD⁺ precursors, nicotinamide mononucleotide and nicotinamide riboside, in mice.