Department of Medicine, Division of Bone and Mineral Diseases, Washington University, Saint Louis, MO, USA.
Department of Neuroscience, Washington University, Saint Louis, MO, USA; Center of Regenerative Medicine, Washington University School of Medicine, Saint Louis, MO, USA; Hope Center for Neurological Disorders, Washington University School of Medicine, Saint Louis, MO, USA.
Mol Metab. 2023 Feb;68:101664. doi: 10.1016/j.molmet.2022.101664. Epub 2022 Dec 28.
Obesity and nutrient oversupply increase mammalian target of rapamycin (mTOR) signaling in multiple cell types and organs, contributing to the onset of insulin resistance and complications of metabolic disease. However, it remains unclear when and where mTOR activation mediates these effects, limiting options for therapeutic intervention. The objective of this study was to isolate the role of constitutive mTOR activation in Nav1.8-expressing peripheral neurons in the onset of diet-induced obesity, bone loss, and metabolic disease.
In humans, loss of function mutations in tuberous sclerosis complex 2 (TSC2) lead to maximal constitutive activation of mTOR. To mirror this in mice, we bred Nav1.8-Cre with TSC2 animals to conditionally delete TSC2 in Nav1.8-expressing neurons. Male and female mice were studied from 4- to 34-weeks of age and a subset of animals were fed a high-fat diet (HFD) for 24-weeks. Assays of metabolism, body composition, bone morphology, and behavior were performed.
By lineage tracing, Nav1.8-Cre targeted peripheral sensory neurons, a subpopulation of postganglionic sympathetics, and several regions of the brain. Conditional knockout of TSC2 in Nav1.8-expressing neurons (Nav1.8-TSC2) selectively upregulated neuronal mTORC1 signaling. Male, but not female, Nav1.8-TSC2 mice had a 4-10% decrease in body size at baseline. When challenged with HFD, both male and female Nav1.8-TSC2 mice resisted diet-induced gains in body mass. However, this did not protect against HFD-induced metabolic dysfunction and bone loss. In addition, despite not gaining weight, Nav1.8-TSC2 mice fed HFD still developed high body fat, a unique phenotype previously referred to as 'normal weight obesity'. Nav1.8-TSC2 mice also had signs of chronic itch, mild increases in anxiety-like behavior, and sex-specific alterations in HFD-induced fat distribution that led to enhanced visceral obesity in males and preferential deposition of subcutaneous fat in females.
Knockout of TSC2 in Nav1.8+ neurons increases itch- and anxiety-like behaviors and substantially modifies fat storage and metabolic responses to HFD. Though this prevents HFD-induced weight gain, it masks depot-specific fat expansion and persistent detrimental effects on metabolic health and peripheral organs such as bone, mimicking the 'normal weight obesity' phenotype that is of growing concern. This supports a mechanism by which increased neuronal mTOR signaling can predispose to altered adipose tissue distribution, adipose tissue expansion, impaired peripheral metabolism, and detrimental changes to skeletal health with HFD - despite resistance to weight gain.
肥胖和营养过剩会增加哺乳动物雷帕霉素靶蛋白（mTOR）在多种细胞类型和器官中的信号转导，导致胰岛素抵抗和代谢疾病并发症的发生。然而，mTOR 激活何时以及在何处介导这些效应仍不清楚，这限制了治疗干预的选择。本研究的目的是分离 Nav1.8 表达的周围神经元中组成型 mTOR 激活在饮食诱导肥胖、骨丢失和代谢疾病发生中的作用。
在人类中，结节性硬化复合物 2（TSC2）的功能丧失突变导致 mTOR 的最大组成型激活。为了在小鼠中模拟这种情况，我们将 Nav1.8-Cre 与 TSC2 动物杂交，以在 Nav1.8 表达的神经元中条件性缺失 TSC2。从 4 到 34 周龄对雄性和雌性小鼠进行研究，并对一部分动物进行高脂肪饮食（HFD）喂养 24 周。进行代谢、体成分、骨形态和行为检测。
通过谱系追踪，Nav1.8-Cre 靶向周围感觉神经元、节后交感神经的一个亚群和大脑的几个区域。Nav1.8 表达神经元中 TSC2 的条件性缺失（Nav1.8-TSC2）选择性地上调神经元 mTORC1 信号。雄性，但不是雌性，Nav1.8-TSC2 小鼠在基线时的体重减少了 4-10%。当受到 HFD 挑战时，雄性和雌性 Nav1.8-TSC2 小鼠都抵抗了饮食引起的体重增加。然而，这并不能防止 HFD 引起的代谢功能障碍和骨丢失。此外，尽管没有体重增加，Nav1.8-TSC2 小鼠在 HFD 喂养下仍会发展出高体脂肪，这是一种以前被称为“正常体重肥胖”的独特表型。Nav1.8-TSC2 小鼠还表现出慢性瘙痒、焦虑样行为轻度增加，以及 HFD 诱导的脂肪分布的性别特异性改变，导致雄性内脏肥胖增加，雌性皮下脂肪优先沉积。
Nav1.8+神经元中 TSC2 的缺失会增加瘙痒和焦虑样行为，并显著改变对 HFD 的脂肪储存和代谢反应。虽然这可以防止 HFD 引起的体重增加，但它掩盖了特定部位脂肪的扩张以及对代谢健康和外周器官（如骨骼）的持续不良影响，模拟了日益受到关注的“正常体重肥胖”表型。这支持了神经元 mTOR 信号增加可导致脂肪组织分布改变、脂肪组织扩张、外周代谢受损以及骨骼健康恶化的机制，尽管对体重增加有抵抗力。
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