Upregulated Hepatic Lipogenesis from Dietary Sugars Supplies Palmitic Acid to the Developing Brain of Mice fed Low Palmitic Acid from Birth

Background: Palmitic acid (PAM) can be provided in the diet or synthesized?via de novo lipogenesis (DNL) primarily from glucose. Preclinical work on the origin of brain PAM during development is scarce and contrasts results on the origin of adult brain PAM. Here, we utilize naturally occurring carbon isotope ratios (13C/12C; d13C) and RNA sequencing to uncover the origin of brain PAM, and pathways involved in maintaining brain PAM, respectively, during development. maintaining brain PAM, respectively, during development. Methods: Dams were equilibrated onto diets low (<2%), medium (47%) or high (>95%) in PAM prior to breeding. Dietary PAM was depleted in d13C, while dietary sugars were enriched. Offspring stayed on the respective dam diet and were euthanized at postnatal day 0, 10, 21, and day 35. Pup liver and brain fatty acids were quantified, after which, tissue d13C-PAM was measured by compound specific isotope analysis. Day 35 tissue RNA was sequenced on a NovaSeq S4 Flowcell. Results: Although PAM levels in the liver reflected levels of dietary PAM, PAM was maintained in total and individual brain phospholipid fractions across diet groups at all timepoints. Tissue d13C-PAM was enriched overall and augmented in mice fed low PAM, compared to medium and high PAM suggesting that DNL from dietary sugars maintained the majority of the brain PAM pool. Furthermore, DNL pathways were upregulated in mice fed low compared to high PAM in the liver, but not the brain at day 35. Conclusions: Lipogenesis from dietary sugars maintains the majority of brain PAM during development and is augmented in mice fed low PAM from birth. Importantly, hepatic lipogenesis from dietary sugars determines PAM availability to the developing brain when low in the diet – a compensatory mechanism identified to maintain total brain PAM pools compared to periphery which ultimately suggests an importance of brain PAM regulation during development. Overall design: Pairwise comparisons of RNAseq data of brain and liver tissue from 35-day old BALB/c mice fed custom diets containing low (<2%), medium (~47%) and high (>95%) PAM

背景：棕榈酸（Palmitic acid, PAM）可通过膳食摄入，亦可主要由葡萄糖经从头脂肪生成（de novo lipogenesis, DNL）合成。目前针对发育阶段脑内棕榈酸来源的临床前研究较为匮乏，且其与成年脑内棕榈酸来源的相关研究结果存在分歧。本研究利用天然存在的碳同位素比值（13C/12C; δ13C）与RNA测序技术，分别揭示发育阶段脑内棕榈酸的来源及维持脑内棕榈酸水平的相关通路。
方法：受孕母鼠先在棕榈酸含量分别为低（<2%）、中（47%）、高（>95%）的饲料中适应饲养，于配种前完成适应。膳食棕榈酸的δ13C值偏低（贫化），而膳食糖类的δ13C值偏高（富集）。子代小鼠维持饲喂对应母鼠的饲料，并分别在出生后第0、10、21及35天实施安乐死。定量检测幼鼠肝脏与脑组织中的脂肪酸含量，随后采用化合物特异性同位素分析技术检测组织内δ13C标记的棕榈酸水平。对出生后第35天的组织样本进行RNA测序，测序平台为NovaSeq S4流动槽（Flowcell）。
结果：尽管肝脏内的棕榈酸水平与膳食棕榈酸含量呈对应关系，但各膳食组在所有时间点的脑组织总磷脂及各单一磷脂组分中的棕榈酸水平均保持稳定。与中、高棕榈酸膳食组相比，低棕榈酸膳食组小鼠的组织内δ13C标记棕榈酸整体水平更高且显著升高，提示膳食糖类经从头脂肪生成途径合成的棕榈酸是脑内棕榈酸池的主要来源。此外，出生后第35天时，低棕榈酸膳食组小鼠肝脏内的从头脂肪生成通路表达上调，而脑组织中未见该通路的表达变化。
结论：发育阶段脑内绝大多数棕榈酸来源于膳食糖类的脂肪生成，且自出生起饲喂低棕榈酸膳食的小鼠该通路活性显著增强。值得注意的是，当膳食棕榈酸含量较低时，肝脏通过膳食糖类进行的脂肪生成可决定发育中大脑可获取的棕榈酸水平——这一维持脑内棕榈酸池稳定的代偿机制与外周组织形成鲜明对比，最终提示发育阶段脑内棕榈酸调控的重要性。
实验整体设计：对饲喂定制化低（<2%）、中（~47%）、高（>95%）棕榈酸膳食的35日龄BALB/c小鼠的脑组织与肝脏组织的RNA测序数据进行两两比较分析。