An in-depth paper was released in Nature Communications this month with an impressive line up of researchers incl. Charles Brenner discoverer of Nicotinamide Riboside and Johan Auwerx whose lab released this flagship study. The paper describes in detail the metabolism of nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) in mammalian cells.
NR’s conversion to NAD+ starts with phosphorylation of NR to NMN by NR kinases (NRKs). NRKs, encoded by the Nmrk genes, are highly conserved enzymes in all eukaryotes. In mammals there are two NRK enzymes, NRK1 and NRK2. Already in his 2004 research paper Brenner outlines the possible role of NRKs in the metabolism path but real research into their physiological roles has been missing. In this study the researchers explored how modulation of NRK activity influences the action of NAD+ precursors.
The researchers created NRK gain- and loss-of function cellular models as well as an Nmrk1-deficient mouse model (NRK1KO). The initial tests in NIH/3T3 cell lines (those have low NRK levels and therefore more likely to react to stimulus) with constructs that allowed for over expression of NRK1 or NRK2 enzymes established that increasing NRK activity is sufficient to boost NR-driven NAD+ synthesis, which suggests a rate-limiting role for NRK1 and/or NRK2 in NR utilization.
It was also noted that, inline with previous research, NRK2 expression is very tissue-specific despite seeing increases at the mRNA level. The researchers speculated that NRK2 requires muscle-specific factors for stability where it is known to synthesize NR. NRK1 expression showed very significant dose-dependent increases in NAD+ accumulation when NR was added and apparently there are no additional dependencies like in the case of NRK2. This confirmation of previous reported research indications is good news by itself. NRK1 is expressed in many tissues in the body which means that those can all synthesize NR and thus may be helped with NR supplementation, you can see that in the so called Expression Atlas (= provides information on gene expression patterns based on experimental data from ArrayExpress). Select “Show anatomograms” on the left side of the page to get a “graphic of the body expression” overview.
The next experiment delved into one of the hot topics around NMN. Does NMN need to be converted into NR to enter the cell or can it pass straight into the cell via a so far unknown transporter or diffusion mechanism. If that latter were the case then NMN should be able to increase NAD+ independently of NRK1 (which is the enzyme that synthesizes NR to NMN, see above). It was observed that only in the constructs that allowed for over expression of NRK1, NMN increased NAD+ synthesis but did so less potently than NR. The researchers also tested NAM which enhanced NAD+ synthesis irrespective of NRK1 overexpression. This experiment strongly suggests that NMN is first converted to NR. The researchers followed this up with an impressive range of experiments including labeling NMN and NR to enable stable isotope tracer analyses to track compounds. The accumulation and appearance of the labels around and in the cell supported the hypotheses of extracellular NMN conversion to NR. For example when cells were treated with labelled NMN, there was a rapid appearance of extracellular NR that included the NMN label as well as enrichment of intracellular NR that included the NMN label. Overall the experiments combined delivered very strong proof that NMN is indeed converted to NR before entering the cell.
The expansive study delivered numerous other data points and helps to clarify the roles of NR and NMN which have both been shown to deliver overlapping benefits on metabolic health. The research team made the following conclusions:
– NMN converts to NR, which then enters the cell for NAD+ synthesis. There is no signs of a direct transport path into the cell for NMN.
– NRK1 is necessary and rate-limiting for the use of exogenous NR and NMN for NAD+ synthesis
– NRK1 is not needed for other NAD+ precursors (nicotinamide or nicotinic acid) to be converted to NAD+
– the fact that extracellular NMN converts to NR for cellular uptake and NAD+ synthesis, explains the overlapping metabolic effects
observed with the two compounds.
Currently nicotinamide riboside is the only supplement available for consumers and the study supports the notion that NR is a leading or possibly the best pre-curser to boost NAD+. Having said that this very interesting recently published study about NMN indicates there are large differences in bio-availability of each, with NMN acting extremely fast. That may lead to complementary roles for each. In addition the confirmation that NRK1 is a rate limiter leads to the question should we try to boost (selectively) expression for tissues and organs in trouble and if so how can that be done?
But the article has already become too long, more about the NMN publication next time.
CORRECTION: A reader forwarded me the following link to a press release about a NMN supplement that is on the market. Thank you! Considering the producer is a Japanese pharmaceutical and selling in the home market it would appear that the NMN supplement has passed local safety regulations and is available to consumers. Price indication is massive.