Identification of structural determinants of nicotinamide phosphoribosyl transferase (NAMPT) activity and substrate selectivity.

NAD homeostasis in mammals requires the salvage of nicotinamide (Nam), which is cleaved from NAD+ by sirtuins, PARPs, and other NAD+-dependent signaling enzymes. Nam phosphoribosyltransferase (NAMPT) catalyzes the rate-limiting step in vitamin B3 salvage, whereby Nam reacts with phosphoribosyl pyrophosphate (PRPP) to form nicotinamide mononucleotide. NAMPT has a high affinity towards Nam, which is further enhanced by autophosphorylation of His247. The mechanism of this enhancement has remained unknown. Here, we present high-resolution crystal structures and biochemical data that provide reasoning for the increased affinity of the phosphorylated NAMPT for its substrate. Structural and kinetic analyses suggest a mechanism that includes Mg2+ coordination by phospho-His247, such that PRPP is stabilized in a position highly favorable for catalysis. Under these conditions, nicotinic acid (NA) can serve as a substrate. Moreover, we demonstrate that a stretch of 10 amino acids, present only in NAMPTs from deuterostomes, facilitates conformational plasticity and stabilizes the chemically unstable phosphorylation of His247. Thereby the apparent substrate affinity is considerably enhanced compared to prokaryotic NAMPTs. Collectively, our study provides a structural basis for the important function of NAMPT to recycle Nam into NAD biosynthesis with high affinity.

Identification of evolutionary and kinetic drivers of NAD-dependent signaling.

Nicotinamide adenine dinucleotide (NAD) provides an important link between metabolism and signal transduction and has emerged as central hub between bioenergetics and all major cellular events. NAD-dependent signaling (e.g., by sirtuins and poly-adenosine diphosphate [ADP] ribose polymerases [PARPs]) consumes considerable amounts of NAD. To maintain physiological functions, NAD consumption and biosynthesis need to be carefully balanced. Using extensive phylogenetic analyses, mathematical modeling of NAD metabolism, and experimental verification, we show that the diversification of NAD-dependent signaling in vertebrates depended on 3 critical evolutionary events: 1) the transition of NAD biosynthesis to exclusive usage of nicotinamide phosphoribosyltransferase (NamPT); 2) the occurrence of nicotinamide N-methyltransferase (NNMT), which diverts nicotinamide (Nam) from recycling into NAD, preventing Nam accumulation and inhibition of NAD-dependent signaling reactions; and 3) structural adaptation of NamPT, providing an unusually high affinity toward Nam, necessary to maintain NAD levels. Our results reveal an unexpected coevolution and kinetic interplay between NNMT and NamPT that enables extensive NAD signaling. This has implications for therapeutic strategies of NAD supplementation and the use of NNMT or NamPT inhibitors in disease treatment.

miR-34a expression, epigenetic regulation, and function in human placental diseases.

Preeclampsia (PE) is the major pregnancy-induced hypertensive disorder responsible for maternal and fetal morbidity and mortality that can be associated with intrauterine growth restriction (IUGR). PE and IUGR are thought to be due to a placental defect, occurring early during pregnancy. Several placental microRNAs (miRNAs) have been shown to be deregulated in the context of placental diseases and could thus play a role in the pathophysiology of PE. Here, we show that pri-miR-34a is overexpressed in preeclamptic placentas and that its placental expression is much higher during the first trimester of pregnancy than at term, suggesting a possible developmental role. We explored pri-miR-34a regulation and showed that P53, a known activator of miR-34a, is reduced in all pathological placentas and that hypoxia can induce pri-miR-34a expression in JEG-3 cells. We also studied the methylation status of the miR-34a promoter and revealed hypomethylation in all preeclamptic placentas (associated or not with IUGR), whereas hypoxia induced a hypermethylation in JEG-3 cells at 72 h. Despite the overexpression of pri-miR-34a in preeclampsia, there was a striking decrease of the mature miR-34a in this condition, suggesting preeclampsia-driven alteration of pri-miR-34a maturation. SERPINA3, a protease inhibitor involved in placental diseases, is elevated in IUGR and PE. We show here that miR-34a overexpression in JEG-3 downregulates SERPINA3. The low level of mature miR-34a could thus be an important mechanism contributing to SERPINA3 upregulation in placental diseases. Overall, our results support a role for miR-34a in the pathophysiology of preeclampsia, through deregulation of the pri-miRNA expression and its altered maturation.