New NAD+ Pathway Discovered: Liver-Kidney Axis Maintains Cellular Energy
Scientists discover backup pathway where liver produces nicotinic acid riboside for kidneys to maintain NAD+ levels during aging.
Summary
Researchers discovered a previously unknown backup pathway for maintaining NAD+ levels in the body. When the liver's primary NAD+ production is impaired, it switches to producing nicotinic acid riboside (NaR), which travels to the kidneys where it's converted back to NAD+ precursors. This liver-kidney partnership maintains cellular energy levels even when the main pathway fails. Importantly, NaR levels decline with aging, but oral NaR supplementation in aged mice restored NAD+ levels across multiple organs and improved kidney function, reducing inflammation and protein leakage. This discovery reveals a new target for anti-aging interventions.
Detailed Summary
Scientists have uncovered a remarkable backup system that maintains cellular energy levels when the body's primary NAD+ production pathway fails. NAD+ is essential for cellular energy production and DNA repair, but levels decline with aging, contributing to various age-related diseases.
Researchers created mice lacking NMNAT1, a key enzyme for NAD+ production in the liver. Surprisingly, despite severely impaired liver NAD+ synthesis, these mice maintained normal NAD+ levels in other organs and showed no metabolic problems. The mystery was solved when scientists discovered the liver was producing massive amounts of a previously unknown molecule: nicotinic acid riboside (NaR).
The study revealed that NaR travels from the liver to the kidneys, where it's converted into nicotinamide—the same NAD+ precursor normally produced by the liver. This liver-kidney axis serves as a metabolic backup system, ensuring continuous NAD+ supply throughout the body. The researchers identified NT5C2 as the enzyme producing NaR in the liver and NRK1 as the kidney enzyme converting NaR back to NAD+ precursors.
Crucially, NaR levels decline with aging in both mice and humans. When aged mice received oral NaR supplements, their NAD+ levels increased across multiple organs including kidneys, heart, and muscle. The treatment also reduced kidney inflammation and albuminuria (protein in urine), suggesting improved kidney function.
This discovery opens new therapeutic possibilities for age-related NAD+ decline. Unlike other NAD+ precursors that primarily target individual organs, NaR appears to support whole-body NAD+ homeostasis through the liver-kidney axis, potentially making it a more effective anti-aging intervention.
Key Findings
- Liver produces nicotinic acid riboside (NaR) as backup when primary NAD+ synthesis fails
- Kidneys convert circulating NaR back to NAD+ precursors, maintaining whole-body energy levels
- NaR levels decline with aging in both mice and humans
- Oral NaR supplementation restores NAD+ levels across multiple organs in aged mice
- NaR treatment reduces kidney inflammation and improves kidney function markers
Methodology
Researchers used liver-specific NMNAT1 knockout mice, comprehensive metabolomics analysis, stable isotope tracing with labeled tryptophan, and LC-MS/MS to identify the novel NaR pathway. Aging studies included 18-month-old mice receiving oral NaR supplementation.
Study Limitations
Study conducted primarily in mice with limited human validation. Long-term safety and optimal dosing of NaR supplementation in humans remains unknown. The specific mechanisms of NaR transport and uptake by kidneys require further investigation.
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