A new study published in Nature Metabolism has brought to light a new cellular membrane transporter specific to NMN. (1) Encoded by the Slc12a8 gene, it allows NMN to become available inside the cells for direct NAD+ production. Research done in the field of cell metabolism has identified an age-related decline of NAD+ that has a negative effect on intracellular energy production. This decline in NAD+ availability over time is thought to be associated with many of the chronic conditions commonly believed to be a natural consequence of aging. For this reason, the last few years have seen an increased interest in maintaining levels of NAD+ to preserve metabolic function and prevent the development of age-related diseases.
Until recently, several of the steps for production of NAD+ remained a mystery. For example, even though it was clear that NMN was indispensable for the production of NAD+, the exact process by which NMN was able to get inside cells was not known. There was also no clarity on how NMN became bioavailable for use in the biochemical reactions that produce NAD+.
Scientists knew that production of NAD+ was dependent on two biosynthetic precursors, Nicotinamide Riboside (NR) and NMN. Several theories were put forth to explain how NMN had to be transformed before being able to enter into cells. Eventually, a path was discovered. The biosynthesis of NAD+ was explained through an indirect and inefficient series of cellular processes.
Through this path, metabolization of NMN would take place on the surface of the cells. NMN would lose some of its phosphate groups through the work of some equilibrative nucleotide transporters, and be converted back into NR to be able to pass through the cellular membrane. Once inside, another series of biosynthetic reactions would take place to restore the phosphate groups, so that NR could be reassembled into NMN for use in the production of NAD+.
Researchers have identified the Slc12a8 gene as being responsible for encoding the transporter specific to NMN. Thanks to this, it has become evident that NMN can be delivered directly into the cells in a much more efficient manner. The indirect path that takes place on the surface of cells where NMN is broken down into NR and later reassembled still takes place, but the transporter mechanism is now understood to be the main path of delivery and absorption.
In addition, scientists believe that expression of Slc12a8 in the gut also facilitates the uptake of NAD+ precursors as well as NAD+ itself from naturally occurring food sources. Due to the low concentration of NMN in foods, it is theorized that some NAD+ precursors may be biosynthesized by bacteria present in the gut.
The Slc12a8 transporter facilitates rapid uptake of NMN and explains how the vital NAD+ precursor becomes bioavailable. It is important to note the quick metabolic response to the administration of NMN. A myriad of metabolic processes take place within minutes, with the purpose of utilizing this essential nutrient in a fast and efficient manner.
Once NMN has been ingested, it can be detected in the blood within a matter of minutes. Based on studies done in the lab, NMN can be detected in the blood 2-3 minutes after consumption. More importantly, NMN reaches peripheral target tissues within 10-30 minutes of ingestion. In addition, the end-goal of NMN supplementation, increased levels of NAD+, can be detected in tissues within 60 minutes of administration of NMN.
Through a series of experiments carried out in animal models, scientists have discovered that Slc12a8 is not just a specific transporter for NMN, but also actively helps regulate and maintain the overall production levels of NAD+. Data from these experiments show that when levels of NAD+ decline, a compensatory mechanism is set in motion, where the transporter is upregulated by the Slc12a8 gene, so that more NMN can be delivered into cells. Closer examination of the Slc12a8 gene shows that it has high levels of activity in the small intestine and pancreas, as well as some action in the liver and fatty tissues.
Levels of NMN inside cells were measured when the transporter was present, and results demonstrated a significant increase of the NAD+ precursor. In contrast, there was no effect observed for intracellular levels of NR in the presence of the transporter, which confirmed specificity for NMN. The actions of the transporter were found to be dependent on the presence of sodium ion. Further experimentation done in animal models and in the lab showed that when expression of the Slc12a8 gene is blocked, the uptake of NMN is decreased.
Researchers now understand that the major place where NMN is absorbed is the small intestine. Expression of the Slc12a8 gene is about 100-fold higher in the small intestine than in other tissues. To prove this, the Slc12a8 gene was manipulated to block or knock-out its effect in the gut, thus reducing the amount of transporter in the small intestine. As a result, the researchers observed dramatically decreased NMN concentrations in cells. Consistent with these findings, NAD+ levels were found to be decreased as well, which confirms the importance of Slc12a8 for transportation of NMN from the gut to the circulation. NMN supplementation that can bypass the digestive processes that take place within the stomach, will be metabolized more efficiently in the small intestine.
At the same time, when the Slc12a8 transporter was over expressed, full capacity of NMN transport was achieved, even in tissues where there is minimal NMN metabolic activity under normal conditions. Animal model studies have also demonstrated that in most cases, total body deficiency of the Slc12a8 is not compatible with survival.
The rapid series of events that allow NMN to be transported inside cells for quick transformation into NAD+ are thought to be part of a controlled response to meet an urgent need for NAD+ production. Interestingly, observation of the guts of older study subjects showed that expression of Slc12a8 is upregulated in response to decreased NAD+ content. Researchers concluded that the NMN transporter encoded by the Slc12a8 gene functions to regulate NMN-driven NAD+ biosynthesis and maintain intestinal NAD+ levels in aged individuals.
This feedback function of the Slc12a8 transporter is crucial to maintaining adequate levels of NAD+ in old age. When enough NMN+ is supplied, adequate levels of NAD+ can be maintained, closer to the ideal production seen in young age. Extended release of NMN into the gut would provide a long-acting supply for prolonged production of NAD+.
Several studies show that interventions that use NMN to preserve or increase NAD+ production delay or even reduce the appearance of disease, as well as having a mitigating effect on the metabolic decline that occurs as a part of aging. (2-10)
These recent findings on Slc12a8, the specific transporter for NMN, provide good insight into the ideal path for NMN delivery. Focusing delivery into the small intestine, the major place where NMN is absorbed, allows better availability for consistent production of NAD+. Likewise, extending the release of NMN will allow for more constant output of NAD+. By making the delivery of NMN more direct, metabolization is made more efficient, and distribution among peripheral tissues can be assured, preserving physiological function, and preventing age related decline.
- Grozio A, Mills KF, Yoshino J. Slc12a8 is a nicotinamide mononucleotide transporter. Nature Metabolism, 2019 1(1), 47–57. doi: 10.1038/s42255-018-0009-4.
- Yoshino, J., Mills, K. F., Yoon, M. J. & Imai, S. Nicotinamide mononucleotide, a key NAD+ intermediate, treats the pathophysiology of diet- and age induced diabetes in mice. Cell Metab. 14, 528–536 (2011).
- Caton, P. W., Kieswich, J., Yaqoob, M. M., Holness, M. J. & Sugden, M. C. Nicotinamide mononucleotide protects against pro-inflammatory cytokine-mediated impairment of mouse islet function. Diabetologia 54, 3083–3092 (2011).
- De Picciotto, N. E. et al. Nicotinamide mononucleotide supplementation reverses vascular dysfunction and oxidative stress with aging in mice. Aging Cell 15, 522–530 (2016).
- Gomes, A. P. et al. Declining NAD+ induces a pseudohypoxic state disrupting nuclear-mitochondrial communication during aging. Cell 155, 1624–1638 (2013).
- Long, A. N. et al. Effect of nicotinamide mononucleotide on brain mitochondrial respiratory deficits in an Alzheimer’s disease-relevant murine model. BMC Neurology 15, 19 (2015).
- Mills, K. F. et al. Long-term administration of nicotinamide mononucleotide mitigates age-associated physiological decline in mice. Cell Metab. 24, 795–806 (2016).
- . Stein, L. R. & Imai, S. Specific ablation of Nampt in adult neural stem cells recapitulates their functional defects during aging. EMBO J. 33, 1321–1340 (2014).
- Wang, X., Hu, X., Yang, Y., Takata, T. & Sakurai, T. Nicotinamide mononucleotide protects against β-amyloid oligomer-induced cognitive impairment and neuronal death. Brain Res. 1643, 1–9 (2016).
- Yamamoto, T. et al. Nicotinamide mononucleotide, an intermediate of NAD+ synthesis, protects the heart from ischemia and reperfusion. PLoS ONE 9, e98972 (2014).