The effect of maternal diabetes on mouse fetal brown adipose tissue development

Ph.D.Offspring of mothers with diabetes mellitus (DM) are more prone to develop obesity and related metabolic diseases, such as type 2 DM. Recent studies show that brown adipose tissue (BAT), which serves to regulate body temperature in the newborn, is important in adults for preventing obesity via dissipating energy as heat. This thesis aims to investigate our hypothesis that maternal diabetes impairs fetal BAT development, such that offspring will be more inclined to develop obesity in later life.In the present study, a streptozotocin-induced pregestational type 1-like diabetic mouse model was used. Results showed that in fetuses of diabetic mice, the interscapular BAT (iBAT)-to-body weight ratio was significantly reduced. Moreover, there was a prominent decrease in lipid content in iBAT. Mitochondria, which are critical for BAT’s thermogenic function, exhibited abnormal subcellular structures, and significant reduction in density and DNA content. Several critical genes for mitochondrial biogenesis and BAT differentiation, which include uncoupling protein 1 (Ucp1) crucial for thermogenesis, failed to upregulate. In line with these findings, newborns of diabetic mice exhibited reduced basal body temperature and thermoregulatory ability. Despite a lower birthweight, offspring of diabetic mice underwent rapid catch-up growth. Upon challenged with a high-fat diet after weaning, male offspring of diabetic mice accumulated more subcutaneous and visceral fat. Moreover, they were more prone to develop hyperglycemia, and more insulin resistant and glucose intolerant. When subjected to cold, they showed attenuated upregulation of Ucp1 and several cold-inducible genes for thermogenesis in iBAT, and exhibited reduced thermoregulatory ability.All-trans retinoic acid (RA) is a diffusible signalling molecule important for mammalian embryogenesis. Results showed that fetuses of diabetic dams exhibited a significant reduction in RA synthesis and RA levels in tissues surrounding the iBAT, concomitant with a decrease in mRNA expression levels of RA receptors for RA signal transduction in iBAT in comparison to that in fetuses of non-diabetic mice. Administration of bisdiamine, a potent inhibitor of the RA synthesizing enzyme, reduced RA concentrations in iBAT-surrounding tissue, and significantly attenuated lipogenesis, mitochondrial biogenesis and Ucp1 levels, and downregulated the expression of crucial differentiation genes in iBAT. These changes were highly similar to those observed in iBAT of fetuses of diabetic mice. Conversely, maternal supplementation of RA to diabetic mice could rescue these developmental defects in iBAT of their fetuses. Together, these findings suggest that perturbation of RA homeostasis and signalling may be a critical mechanism contributing to impaired BAT development in fetuses of diabetic pregnancy.Increased oxidative stress has been implicated in diabetic embryopathy. Indeed, results showed that there was a marked increase in concentrations of the lipid peroxidation biomarker 8-isoprostane, and failure in upregulation of antioxidant genes in fetal iBAT exposed to maternal diabetes. Dietary supplementation of an antioxidant vitamin E to pregnant diabetic mice could improve lipogenesis, increase mitochondrial biosynthesis, and upregulate several BAT differentiation genes.To conclude, results of this thesis have demonstrated the adverse effects induced by maternal diabetes on fetal BAT development, and shed light on using an intrauterine therapeutic approach to correct defective fetal BAT development.糖尿病母親的後代會更易患上肥胖症及相關代謝疾病。研究顯示在新生兒中用於調節體溫的棕色脂肪組織 (簡稱BAT) 能將能量轉化為熱量，故可預防成年期肥胖。本文旨在研究我們的假設，即母體糖尿病會損害胎兒BAT的發育，導致後代更易患上肥胖症。本研究採用鏈脲黴素誘導的糖尿病小鼠模型。結果顯示在糖尿病孕鼠的胎兒中，其肩胛間BAT (簡稱iBAT) 相對於體重的比值顯著降低。在iBAT中，脂質含量也明顯下降。而對於BAT產熱功能至為關鍵的線粒體，則出現密度和DNA含量下降，以及異常的亞細胞結構。多種線粒體生物合成和負責BAT分化的關鍵基因無法上調其表達，包括負責BAT產熱的解偶聯蛋白1 (Ucp1)。與此一致的是，糖尿病鼠的新生兒出現基礎體溫和體溫調節能力下降。儘管出生體重較低，糖尿病鼠的後代表現出快速的追趕性生長。斷奶後用高脂膳食干預，其雄性後代蓄積更多的皮下和內臟脂肪。牠們亦更容易患上糖尿病，以及表現出更嚴重的胰島素抵抗和葡萄糖不耐受。暴露於寒冷刺激時，牠們BAT特異性基因表達的上調和體溫調節能力亦明顯受損。全反式視黃酸是對胚胎形成非常重要的信號分子。結果顯示與對照組相比，視黃酸的合成能力和濃度在糖尿病鼠胎兒的iBAT周圍組織中顯著降低，而視黃酸受體基因的表達在iBAT中亦較低。在非糖尿病孕鼠中，注射視黃酸合成酶抑制劑bisdiamine可顯著降低胎兒iBAT周圍組織的視黃酸濃度，並產生與糖尿病組高度相似的胎兒iBAT發育缺陷，即脂質生成，線粒體合成，Ucp1水平下降和分化基因顯著下調。相反，給予糖尿病母鼠補充視黃酸可挽救這些發育缺陷。因此，視黃酸平衡和信號傳導的擾亂可能是糖尿病妊娠中胎兒BAT發育受損的關鍵機制。氧化應激水平升高與糖尿病性胚胎病緊密相關。結果顯示在暴露於母體糖尿病的胎兒iBAT中，脂質過氧化標誌物8-異前列腺素水平顯著上升，而抗氧化酶基因的表達亦無法上調。於糖尿病孕鼠膳食中補充抗氧化劑維生素E可改善胎兒iBAT中受損的脂質生成和線粒體合成，以及上調部分BAT分化基因的表達。綜上所述，本研究表明母體糖尿病可損害胎兒BAT發育，並提供矯正其發育缺陷的宮內治療途徑。Lin, Nansheng.Thesis Ph.D. Chinese University of Hong Kong 2019.Includes bibliographical references (leaves 262-288).Abstracts also in Chinese.Title from PDF title page (viewed on 18, February, 2021)