The senescence in mammals is generally concomitant with the dysregulation of intestinal homeostasis and the accumulation of mitochondrial DNA (mtDNA) mutations. High-burden mtDNA mutations lead to NAD+ depletion and activate the transcription factor ATF5-dependent UPRmt, which in turn promotes and exacerbates the intestinal senescence phenotype. By supplementation with the NAD+ precursor NMN, this intestinal senescence phenotype can be rescued to some extent, as evidenced by the recovery of intestinal organoid differentiation and the increased number of intestinal stem cells.
2. NAD+ depletion during intestinal senescence caused by mtDNA mutations
There is impairment of NADH/NAD+ redox in Mut/Mut*** intestines, as manifested by the enriched NADH dehydrogenase complex assembly pathway. Through transfection of intestinal crypt cells with SoNar (a NADH/NAD+ sensor), a higher NADH/NAD+ ratio is observed in Mut/Mut*** mice, hinting the perturbed redox potential. Likewise, following transfection of intestinal crypt cells with FiNad (a NAD+ sensor), less NAD+ content is discovered in the Mut/Mut*** cells. All of these findings mirror NAD+ depletion in the intestinal senescence triggered by mtDNA mutations.
Note: mtDNA mutations are classified into four types: negligible (WT/WT), low (WT/WT*), moderate (WT/Mut**) and high (Mut/Mut***).
3. The link between mtDNA mutation content and physiological intestinal senescence
The small intestine of aged mouse intestine is characterized by decreased intestinal crypt number, increased villus length, higher expression of CDKN1A/p21 (a well-known senescence marker) and shorter telomere length, which is accompanied by accumulation of mtDNA mutations, primarily low-frequency (less than 0.05) point mutations.
4. LONP1 protein as a candidate marker for intestinal senescence caused by accumulated mtDNA mutations
Mitochondrial unfolded protein response (UPRmt) is activated by a variety of mitochondrial stresses, including protein imbalances between mitochondria and the nucleus as well as impaired mitochondrial protein transport. The hallmarks of UPRmt are increased protein expression levels of LONP1, HSP60 and ClpP. Noteworthily, only LONP1 protein is specifically upregulated in senescent UPRmt activation triggered by accumulated mtDNA mutations, which may be a candidate biomarker for intestinal senescence.
5. The role of NAD+ in intestinal senescence induced by elevated mtDNA mutations.
NAD+ repletion in vivo alleviates the small intestine senescent phenotypes caused by mtDNA mutation burden, and rescues the decreased colony formation efficiency in Mut/Mut*** intestinal organoids. NAD+-dependent UPRmt triggered by mtDNA mutations regulates intestinal senescence. These data further indicate that NAD+ depletion functions as a key mediator of the intestinal senescence induced by accumulated mtDNA mutations.
6. The role of NAD+ in the signal pathways regulating intestinal senescence caused by increased mtDNA mutations
NAD+ repletion rescues the Foxl1 downregulation and Notch1 upregulation in Mut/Mut*** mice, suggesting that mtDNA mutation burden can regulate the function or number of niche cells through NAD+ depletion. In addition, NAD+ depletion caused by increased mtDNA mutation burden induces the decline of LGR5-positive intestinal cells via impairment of the Wnt/β-catenin pathway.
NAD+ repletion is significant for the regulation of intestinal homeostasis, playing a critical role in rescuing the intestinal senescence phenotype caused by accumulated mtDNA mutations.
Yang, Liang et al. “NAD+ dependent UPRmt activation underlies intestinal aging caused by mitochondrial DNA mutations.” Nature communications vol. 15,1 546. 16 Jan. 2024, doi:10.1038/s41467-024-44808-z
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