Therapeutic effect of intranasal nicotinamide adenine dinucleotide in the restoration of olfactory dysfunction

Abstract Olfactory dysfunction is a debilitating condition with no established treatment. This study evaluated the efficacy of intranasal (i.n.) NAD administration in restoring olfactory function. Cultured human olfactory stem cells (hOSCs) were treated with NAD and assessed by immunofluorescence staining, PCR, and western blot analyses. In vivo, mice with ZnSO 4 -induced anosmia were treated with i.n. NAD, intraperitoneal dexamethasone, or PBS and evaluated by histological analysis, behavioral tests, bulk RNA-sequencing (RNA-seq), and in situ hybridization. NAD promoted hOSC differentiation into olfactory sensory neurons (OSNs), evidenced by increased stem cell (SOX2 and nestin) and OSN markers (Tuj1 and OMP) expression, and upregulated neuronal differentiation-related genes ( SOX2, NESTIN, NEUROD1, NEUROG1 , and OMP ). In vivo, the NAD group showed significant olfactory function improvement and marked olfactory epithelium repair. Bulk RNA-seq of the olfactory turbinate tissue identified 113 differentially expressed genes (cluster T1) upregulated in control and NAD groups. The Gene Ontology (GO) term “modulation of chemical synaptic transmission” was associated with cluster T1, and 25 genes implicated in this GO were upregulated in the NAD group. Integration with publicly available single-cell RNA-seq data identified six neuronal marker genes — ABHD2, DLGAP2, FOXO3, HIPK2, KCNMA1 , and PCDH17 — upregulated by NAD. Protein expression of DLGAP2 and PCDH17 was higher in differentiated hOSCs treated with NAD. In situ hybridization confirmed that Dlgap2 , Foxo3 , and Pcdh17 expression was restored in anosmic mice treated with i.n. NAD. The potential therapeutic efficacy of i.n. NAD administration was demonstrated by showing regeneration of OSNs in hOSCs and restoring olfactory function in an anosmia mouse model.