Nicotinamide mononucleotide suppresses cellular senescence and increases aquaporin 5 expression in the submandibular gland of aged male mice to ameliorate aging-related dry mouth.

Dry mouth results from decreased saliva secretion due to aging or drug side effects. Decreased saliva secretion causes dryness in the oral cavity that makes swallowing difficult and increases the risk of aspiration pneumonia. There are few fundamental treatments for dry mouth. Here we investigated whether treatment of old mice with nicotinamide mononucleotide (NMN) improved factors associated with dry mouth. Young (16-week-old) and old (113-week-old) male mice were treated subcutaneously with saline or NMN (300 mg/kg) once every two days for four weeks and saliva secretion was measured. The amount of nicotinamide adenine dinucleotide (NAD⁺) in salivary gland tissues was measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Gene expression in the intestinal tract and salivary glands was measured by real-time PCR. The population of cells with acetylation in the submandibular gland was quantified by immunohistological staining. SA-β-gal activity in the submandibular gland was measured to assess cell senescence. Statistical analysis was performed by one-way analysis of variance with Tukey post hoc analysis. The submandibular glands from old mice treated with NMN exhibited increased saliva secretion and NAD⁺ levels, which both decrease with aging. In addition, the submandibular glands from NMN-treated old mice had decreased acetylation, numbers of senescent cells, and levels of senescence-associated secretory phenotype (SASP) factors, which all increase with aging, as well as increased aquaporin5 (AQP5) mRNA expression. NMN administration may improve dry mouth by regulating cellular senescence in the submandibular gland and increasing expression of AQP5, a water channel involved in saliva secretion, to inhibit age-related decreases in saliva secretion. It is necessary to elucidate further mechanism and confirm its effectiveness in humans.