Targeting NAD Homeostasis: Compartmentalization, Quantification, and Modulation.

Nicotinamide adenine dinucleotide (NAD⁺) and its reduced form, NADH, are essential coenzymes that play central roles in cellular redox homeostasis, energy metabolism, DNA repair, and signaling. Cellular NAD⁺ levels are maintained by a dynamic balance between the de novo Preiss-Handler, and salvage synthesis pathways, and consumption by enzymes like sirtuins, PARPs, and CD38. Among these, the nicotinamide Phosphoribosyltransferase (NAMPT)-driven salvage pathway represents the predominant route of NAD+ synthesis. The specific regulation of NAD (NAD⁺ and NADH) levels across distinct subcellular compartments has emerged as a critical determinant of cellular function but it remains poorly understood. Dysregulation of NAD metabolism is a hallmark of aging and various pathologies, including cancer, neurodegenerative disorders, and metabolic diseases, making strategies to modulate NAD levels a promising therapeutic frontier. This review provides the first integrated overview of NAD concentrations across cellular compartments (cytosol, mitochondria, nucleus, endoplasmic reticulum, Golgi, peroxisomes, and the extracellular space) together with measurement and modulation strategies. We summarize current knowledge on NAD distribution within organelles, address key challenges in accurate quantification, and highlight established and emerging approaches for both global and compartment-specific analysis. Finally, we discuss therapeutic strategies, from NAD⁺ precursor supplementation to enzyme modulators and gene therapy, highlighting both their translational potential and current limitations in treating diverse diseases and prolonging life and health span.