Abstract：
Tissue-specific deficiency of nicotinamide phosphoribosyl transferase (NAMPT), the rate-limiting enzyme of the nicotinamide adenine dinucleotide (NAD+)-salvage pathway, causes a decrease of NAD+ in the tissue, resulting in functional abnormalities. The NAD+-salvage pathway is drastically activated in the mammary gland during lactation, but the significance of this has not been established. To investigate the impact of NAD+ perturbation in the mammary gland, we generated two new lines of mammary gland epithelial-cell-specific Nampt-knockout mice (MGKO). LC-MS/MS analyses confirmed that the levels of NAD+ and its precursor nicotinamide mononucleotide (NMN) were significantly increased in lactating mammary glands. We found that murine milk contained a remarkably high level of NMN. MGKO exhibited a significant decrease in tissue NAD+ and milk NMN levels in the mammary gland during lactation periods. Despite the decline in NAD+ levels, the mammary glands of MGKO appeared to develop normally. Transcriptome analysis revealed that the gene profiles of MGKO were indistinguishable from those of their wild-type counterparts, except for Nampt. Although the NMN levels in milk from MGKO were decreased, the metabolomic profile of milk was otherwise unaltered. The mammary gland also contains adipocytes, but adipocyte-specific deficiency of Nampt did not affect mammary gland NAD+ metabolism or mammary gland development. These results demonstrate that the NAD+ -salvage pathway is activated in mammary epithelial cells during lactation and suggest that this activation is required for production of milk NMN rather than mammary gland development. Our MGKO mice could be a suitable model for exploring the potential roles of NMN in milk.
摘要：
烟酰胺磷酸核糖转移酶（NAMPT）的组织特异性缺陷，烟酰胺腺嘌呤二核苷酸（NAD+）-补救途径的限速酶，导致组织中NAD+的减少，导致功能异常。NAD+-挽救途径在哺乳期乳腺中急剧激活，但是这一点的意义还没有确立。为了研究NAD+扰动对乳腺的影响，我们产生了两个新的乳腺上皮细胞特异性Nampt基因敲除小鼠（MGKO）。LC-MS/MS分析证实，哺乳期乳腺中NAD及其前体烟酰胺单核苷酸（NMN）的水平显着增加。我们发现鼠奶含有非常高水平的NMN。在哺乳期,MGKO表现出乳腺中组织NAD+和乳NMN水平的显著降低。尽管NAD+水平下降，MGKO的乳腺似乎发育正常。转录组分析显示MGKO的基因谱与野生型的基因谱没有区别，除了Nampt.尽管MGKO牛奶中的NMN含量降低了，牛奶的代谢组学特征没有改变。乳腺也含有脂肪细胞，但是Nampt的脂肪细胞特异性缺乏并不影响乳腺NAD代谢或乳腺发育。这些结果表明,NAD+-补救途径在泌乳期间在乳腺上皮细胞中被激活,并且表明这种激活对于乳NMN的产生而不是乳腺发育是必需的。我们的MGKO小鼠可能是探索NMN在牛奶中的潜在作用的合适模型。
