Age- and Obesity Induced Decline of Brown Fat Function as Consequence of Impaired miR-328 Dependent Silencing of Bace1

Activated brown adipose tissue contributes to control of energy and glucose homeostasis in rodents and humans. Defining cell-autonomous processes underlying BAT differentiation and activation may thus reveal novel therapeutic targets for obesity and type 2 diabetes mellitus intervention. Here we show that ageing- and obesity-associated demises in BAT function coincide with down-regulation of mature microRNAs in BAT in the presence of reduced expression of the critical microRNA processing enzyme Dicer1. To mimic this partial down-regulation of microRNA processing in obesity and ageing, we inactivated one allele of Dicer1 selectively in BAT of mice. BAT- restricted heterozygosity of Dicer1 caused glucose intolerance in lean mice and aggravated diet-induced-obesity (DIO)-evoked deterioration of glucose homeostasis. Using combinatorial analyses of altered microRNA-expression in BAT during in vitro preadipocyte commitment and mouse models of progeria, longevity and DIO, we identified 23 microRNAs dysregulated among these conditions. Of these, we identified miR-328 as a novel regulator of BAT differentiation. miR-328 over-expression promotes BAT-differentiation and impairs muscle progenitor commitment, while reducing miR-328 expression blocks BAT specification. We validated the ß-Secretase Bace1 as a target of miR-328, which is consequently over-expressed in BAT of obese and premature ageing mice. Reducing Bace1 expression enhances brown adipocyte, while impairing myogenic differentiation in vitro. In vivo small-molecule Bace1 inhibition in obese mice delayed DIO-induced weight gain, ameliorated obesity-associated deterioration of glucose metabolism and improved insulin sensitivity. Collectively, these experiments reveal reduced Dicer1-miR-328-Bace1 axis in presence of generalized impairment of microRNA processing in ageing and obesity as a novel determinant of ageing- and obesity-associated decline in BAT function. This may define in vivo Bace1-inhibition as an innovative therapeutic approach to not only target age-related neurodegenerative diseases but at the same time improving age-related impairment of BAT-function and metabolism.

活化的棕色脂肪组织（brown adipose tissue, BAT）可调控啮齿类动物与人类的能量及葡萄糖稳态。阐明棕色脂肪细胞分化与激活的细胞自主调控过程，或可为肥胖与2型糖尿病的干预发掘全新治疗靶点。本研究发现，衰老与肥胖相关的棕色脂肪组织功能衰退，与成熟微小核糖核酸（microRNA, miRNA）的表达下调同时发生，且伴随关键miRNA加工酶Dicer1的表达降低。为模拟肥胖与衰老状态下miRNA加工的部分下调现象，我们在小鼠的棕色脂肪组织中选择性灭活了Dicer1的一个等位基因。棕色脂肪组织特异性的Dicer1杂合缺失，可导致瘦小鼠出现糖耐量异常，并加剧饮食诱导肥胖（diet-induced obesity, DIO）引发的葡萄糖稳态紊乱。通过对体外前体脂肪细胞定向分化过程以及早衰、长寿与饮食诱导肥胖小鼠模型的棕色脂肪组织中差异表达miRNA进行联合分析，我们在上述条件中筛选出23个异常表达的miRNA。其中，我们发现miR-328是棕色脂肪细胞分化的新型调控因子。过表达miR-328可促进棕色脂肪细胞分化，并抑制肌肉祖细胞的定向分化；而敲低miR-328的表达则会阻断棕色脂肪细胞的特化过程。我们验证了β-分泌酶Bace1是miR-328的靶基因，该基因在肥胖与早衰小鼠的棕色脂肪组织中呈高表达状态。体外实验证实，下调Bace1的表达可促进棕色脂肪细胞生成，同时抑制肌源性分化过程。在肥胖小鼠体内进行小分子Bace1抑制剂干预，可延缓饮食诱导肥胖所致的体重增加，改善肥胖相关的糖代谢紊乱，并提升胰岛素敏感性。综合上述实验结果，我们发现：在衰老与肥胖状态下，miRNA加工广泛受损，伴随Dicer1-miR-328-Bace1信号通路的功能减弱，这是衰老与肥胖相关的棕色脂肪组织功能衰退的全新调控机制。该发现提示，体内靶向抑制Bace1不仅可用于治疗年龄相关性神经退行性疾病，同时还能改善衰老相关的棕色脂肪组织功能损伤与代谢异常。