Studies suggest nicotinamide mononucleotide (NMN) is an essential molecule for nicotinamide adenine dinucleotide (NAD+) biosynthesis and can be a therapeutic agent in age-associated disease prevention.5  Scientists find declining NAD+ levels during aging in tissues, including muscle, liver, fat (adipose tissue), brain, pancreas, spleen, heart, kidney, and lung, which contribute to age-associated diseases.1,5,8,10,13  Studies report NMN has positive effects in improvement of diseases and combating age-associated physiological decline.2,3,4,5,6,7,9,11,12,14

The fast absorption of NMN into tissues and cells leads the scientists to propose an effective transporter exists in the body for direct absorption of NMN into tissues and organs.  The scientists want to identify such an NMN transporter in mammals. Previous studies indicate NMN absorption from the gut enters blood circulation within two to three minutes and transports to tissues within 10-30 minutes following ingestion.5,7,14  NMN is then immediately used for NAD+ biosynthesis and significantly increases NAD+ concentration in cells over the course of 60 minutes following ingestion.5  However, the physiological mechanisms through which NMN is transported into cells and tissues remains incompletely understood, according to previous research on NMN absorption.

To identify the proposed NMN transporter, scientists analyze gene expression in tissues treated with FK866, an inhibitor of NAD+ biosynthesis.  Previous studies suggest administering NMN to tissue treated with FK866 produces higher NAD+ level increases compared to NMN without FK866 treatment.  With reduced NAD+ biosynthesis from FK866 treatment, the authors of the study believe an increase in NAD+ production with NMN treatment in this tissue indicates an increase in quantities of the presumed NMN transporter.  The scientists study gene expression levels in liver cells, pancreas cells, and brain cells from the hippocampus, treated with FK866 to find what transporter genes might have increased expression. The research team discovered one gene for a transporter with increased expression levels– Slc12a8.

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Image from Grozio et al. (2019)

The scientists find closely related transporters exist in humans, mice, zebrafish, fruit flies, and roundworms.  The team locates high expression levels of the Slc12a8 transporter, in the small intestine and pancreas of the mice.  Moderate expression of the transporter exists in the liver and fat tissue. Since related transporters exist in humans, it is possible for  humans to have these NMN transporters in the gut.

Scientists run further experimentation to confirm the Slc12a8 gene encodes the NMN transporter.  To test this, the team inhibits the cellular pathways through which another NAD+ biosynthesis precursor, nicotinamide riboside (NR), might enter the cell in liver cells.  With inhibition of NR uptake, the scientists find NMN levels increase with NMN administration, indicating the presence of this NMN transporter. Under the same circumstances with genetically decreased levels of Slc12a8, the fast intake of NMN to the cell was lost.  These results suggest Slc12a8 is necessary for fast intake of NMN into hepatocytes.

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Images from Grozio et al., (2019)

For further evidence confirming the Slc12a8 gene encodes the NMN transporter, the team of scientists looked into NIH3T3 cells. These cells have very weak uptake of NMN under normal circumstances; however, their Slc12a8 transporters were altered genetically to express at higher levels.  With overexpression of Slc12a8 transporters in the NIH3T3 cells, scientists expect greater uptake of NMN. About five minutes from NMN intake, the cells overexpressing the Slc12a8 transporter gene have much higher levels of NMN in cells compared to NIH3T3 cells without overexpression of the transporter.  The scientists study the cells using biochemical analyses for evidence indicating the Slc12a8 transporter is present in the cells at higher concentrations in the genetic overexpression of Slc12a8 compared to NIH3T3 cells without overexpression.  The evidence indicates the Slc12a8 transporter is, in fact, present at higher concentrations in the genetic overexpression of Slc12a8 compared to the NIH3T3 cells without overexpression of the transporter.

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Image from Grozio et al. (2019)

The scientists of this study want to find out whether the Slc12a8 NMN transporter functions for NMN cellular uptake in live mice.  The scientists reduce levels of Slc12a8 NMN transporter in live mice. The group finds reduced NMN cellular intake following NMN administration with reduction of Slc12a8 NMN transporter levels.

The scientists of the study provide evidence of identification of the elusive NMN transporter.  “Thus, the NMN transporter encoded by the Slc12a8 gene functions to regulate NMN-driven NAD+ biosynthesis and maintain intestinal NAD+ in aged individuals,” says the authors.  “Because NMN conveys remarkable effects of mitigating age-associated physiological decline in mice,7,13 identifying compounds that could promote the NMN-transporting function of the Slc12a8 protein will provide an interesting opportunity to develop a more effective anti-ageing intervention, combined with NMN administration.5