Department of Physiology, Monash University, Clayton, Victoria, Australia; Metabolism, Diabetes and Obesity Program, Biomedicine Discovery Institute, Monash University, Clayton, Victoria, Australia.
Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia.
Mol Metab. 2018 May;11:47-58. doi: 10.1016/j.molmet.2018.01.024. Epub 2018 Feb 10.
The potential for brown adipose tissue (BAT) to be targeted as a therapeutic option to combat obesity has been heightened by the discovery of a brown-like form of inducible "beige" adipose tissue in white fat which has overlapping structural and functional properties to "classical" BAT. The likelihood that both beige and brown fat are recruited functionally by neural mechanisms, taken together with the lack of a detailed understanding of the nature of changes in the nervous system when white adipose tissue (WAT) is transformed to brown, provides the impetus for this study. Here, we aim to identify whether there is a shift in the gene expression profile in neurons directly innervating inguinal white adipose tissue (iWAT) that has undergone "beiging" to a signature that is more similar to neurons projecting to BAT.
Two groups of rats, one housed at thermoneutrality (27 °C) and the other exposed to cold (8 °C) for 7 days, were killed, and their T13/L1 ganglia, stellate ganglion (T1/T2), or superior cervical ganglion (SCG, C2/3) removed. This approach yielded ganglia containing neurons that innervate either beiged white fat (8 °C for 7 days), inguinal WAT (27 °C for 7 days), BAT (both 27 °C and 8 °C for 7 days) or non-WAT (8 °C for 7 days), the latter included to isolate changes in gene expression that were more aligned with a response to cold exposure than the transformation of white to beige adipocytes. Bioinformatics analyses of RNA sequencing data was performed followed by Ingenuity Pathway Analysis (IPA) to determine differential gene expression and recruitment of biosynthetic pathways.
When iWAT is "beiged" there is a significant shift in the gene expression profile of neurons in sympathetic ganglia (T13/L1) innervating this depot toward a gene neurochemical signature that is similar to the stellate ganglion projecting to BAT. Bioinformatics analyses of "beiging" related genes revealed upregulation of genes encoding neuropeptides proopiomelanocortin (POMC) and calcitonin-gene related peptide (CGRP) within ganglionic neurons. Treatment of differentiated 3T3L1 adipocytes with αMSH, one of the products cleaved from POMC, results in an elevation in lipolysis and the beiging of these cells as indicated by changes in gene expression markers of browning (Ucp1 and Ppargc1a).
These data indicate that, coincident with beiging, there is a shift toward a "brown-like" neurochemical signature of postganglionic neurons projecting to inguinal white fat, an increased expression of POMC, and, consistent with a causative role for this prohormone in beiging, an αMSH-mediated increase in beige gene markers in isolated adipocytes.
棕色脂肪组织（BAT）作为治疗肥胖的一种潜在治疗选择的可能性，由于在白色脂肪中发现了一种棕色样的诱导“米色”脂肪形式而增加，这种形式的脂肪具有与“经典”BAT 重叠的结构和功能特性。米色和棕色脂肪都有可能通过神经机制在功能上被招募，加上对白色脂肪组织（WAT）转化为棕色时神经系统变化的性质缺乏详细了解，这为这项研究提供了动力。在这里，我们旨在确定直接支配腹股沟白色脂肪组织（iWAT）的神经元的基因表达谱是否发生了变化，这些神经元已经“米色化”，表现出更类似于投射到 BAT 的神经元的特征。
将两组大鼠分别安置在热中性（27°C）和寒冷（8°C）环境中 7 天，然后处死，取出其 T13/L1 神经节、星状神经节（T1/T2）或颈上神经节（SCG，C2/3）。这种方法获得的神经节包含支配以下部位的神经元：接受过“米色化”处理的白色脂肪（7 天 8°C）、腹股沟 WAT（7 天 27°C）、BAT（7 天 27°C 和 8°C）或非 WAT（7 天 8°C），后者的目的是分离与冷暴露反应更相关的基因表达变化，而不是白色脂肪向米色脂肪的转化。对 RNA 测序数据进行生物信息学分析，然后进行 Ingenuity Pathway Analysis（IPA），以确定差异基因表达和生物合成途径的募集。
当 iWAT 被“米色化”时，支配该脂肪的交感神经节（T13/L1）中的神经元的基因表达谱会发生显著变化，向与投射到 BAT 的星状神经节相似的神经化学特征变化。对“米色化”相关基因的生物信息学分析显示，神经节神经元中编码神经肽促黑皮质素原（POMC）和降钙素基因相关肽（CGRP）的基因上调。用 POMC 切割产物之一αMSH 处理分化的 3T3L1 脂肪细胞，会导致脂肪分解增加，这些细胞的“米色化”，如 Browning 的基因标志物（Ucp1 和 Ppargc1a）的变化所示。
这些数据表明，与“米色化”同时发生的是，投射到腹股沟白色脂肪的节后神经元的“棕色样”神经化学特征发生了转变，POMC 的表达增加，并且，与这种前激素在“米色化”中起因果作用一致，αMSH 介导的分离脂肪细胞中米色基因标志物的增加。
