Various methods and compositions for treating age-associated conditions and other medical conditions, such as muscle diseases, type 2 diabetes, and/or obesity are described. Methods of enhancing cellular uptake of NMN and stimulating NAD+ production are further described. Various mammalian cells and mammalian cell lines are described including those comprising a cDNA encoding a Slc12a8 protein. Gene therapy vectors comprising a nucleic acid encoding Slc12a8 and non-human animals comprising an inactivating mutation in a Slc12a8 gene are also disclosed. Also described are methods for screening a candidate compound to identify compounds that promote NMN transport.
