Table_1_Adipose Tissue Myeloid-Lineage Neuroimmune Cells Express Genes Important for Neural Plasticity and Regulate Adipose Innervation.docx

Peripheral nerves allow a bidirectional communication between brain and adipose tissues, and many studies have clearly demonstrated that a loss of the adipose nerve supply results in tissue dysfunction and metabolic dysregulation. Neuroimmune cells closely associate with nerves in many tissues, including subcutaneous white adipose tissue (scWAT). However, in scWAT, their functions beyond degrading norepinephrine in an obese state remain largely unexplored. We previously reported that a myeloid-lineage knockout (KO) of brain-derived neurotrophic factor (BDNF) resulted in decreased innervation of scWAT, accompanied by an inability to brown scWAT after cold stimulation, and increased adiposity after a high-fat diet. These data underscored that adipose tissue neuroimmune cells support the peripheral nerve supply to adipose and impact the tissue’s metabolic functions. We also reported that a subset of myeloid-lineage monocyte/macrophages (Ly6c+CCR2+Cx3cr1+) is recruited to scWAT in response to cold, a process known to increase neurite density in adipose and promote metabolically healthy processes. These cold-induced neuroimmune cells (CINCs) also expressed BDNF. Here we performed RNAseq on CINCs from cold-exposed and room temperature-housed mice, which revealed a striking and coordinated differential expression of numerous genes involved in neuronal function, including neurotrophin signaling and axonal guidance, further supporting that CINCs fulfill a nerve-supporting role in adipose. The increased expression of leukocyte transendothelial migration genes in cold-stimulated CINCs also confirms prior evidence that they are recruited to scWAT and are not tissue resident. We now provide whole-depot imaging of scWAT from LysM-BDNF KO mice, revealing a striking reduction of innervation across the depot fitting with their reduced energy expenditure phenotype. By contrast, Cx3cr1-BDNF KO mice (a macrophage subset of LysM+ cells) exhibited increased thermogenesis and energy expenditure, with compensatory increased food intake and no change in adiposity or body weight. While these KO mice also exhibit a significantly reduced innervation of scWAT, especially around the subiliac lymph node, they displayed an increase in small fiber sympathetic neurite branching, which may underlie their increased thermogenesis. We propose a homeostatic role of scWAT myeloid-lineage neuroimmune cells together in nerve maintenance and neuro-adipose regulation of energy expenditure.

外周神经（peripheral nerves）介导大脑与脂肪组织间的双向信号交流，诸多研究已明确证实：脂肪组织神经支配缺失会引发组织功能障碍与代谢失调。神经免疫细胞与诸多组织内的神经紧密关联，其中包括皮下白色脂肪组织（subcutaneous white adipose tissue, scWAT）。然而，在皮下白色脂肪组织中，这类细胞在肥胖状态下除降解去甲肾上腺素外的其他功能，仍未得到充分探究。
我们此前曾报道：髓系谱系脑源性神经营养因子（brain-derived neurotrophic factor, BDNF）敲除（knockout, KO）会导致皮下白色脂肪组织的神经支配减少，同时使机体在冷刺激后无法将皮下白色脂肪组织棕色化，并在高脂饮食后加重肥胖表型。上述研究结果证实，脂肪组织神经免疫细胞可维持脂肪组织的外周神经支配，并影响该组织的代谢功能。
我们还曾报道：髓系谱系单核细胞/巨噬细胞（Ly6c+CCR2+Cx3cr1+）的一个亚群会在冷刺激下被招募至皮下白色脂肪组织，该过程可提升脂肪组织内的神经突密度，并促进代谢健康相关生理进程。这类冷诱导神经免疫细胞（cold-induced neuroimmune cells, CINCs）同样表达脑源性神经营养因子。
本研究对冷暴露及室温饲养小鼠的冷诱导神经免疫细胞进行了RNA测序（RNA sequencing, RNA-seq），结果显示诸多参与神经元功能的基因呈现出显著且协同的差异表达，其中包括神经营养因子信号通路与轴突导向相关基因，进一步证实了冷诱导神经免疫细胞在脂肪组织中发挥神经支持功能。冷刺激后的冷诱导神经免疫细胞中，白细胞跨内皮迁移相关基因的表达上调，也验证了此前的研究结论：这类细胞是被招募至皮下白色脂肪组织的外来细胞，而非组织驻留细胞。
本研究现已提供溶菌酶M-脑源性神经营养因子敲除（LysM-BDNF KO）小鼠的皮下白色脂肪组织全库成像结果，显示整个脂肪库的神经支配显著减少，这与其能量消耗降低的表型相符。与之相反，Cx3cr1-脑源性神经营养因子敲除小鼠（属于LysM+细胞的巨噬细胞亚群）表现出产热与能量消耗增加，同时伴随代偿性进食增多，但肥胖程度与体重未发生变化。尽管这类敲除小鼠的皮下白色脂肪组织神经支配同样显著减少，尤其是在髂下淋巴结周围，但其交感神经小纤维的分支却有所增加，这或许是其产热增加的分子基础。
我们据此提出：皮下白色脂肪组织的髓系神经免疫细胞共同发挥稳态作用，维持神经支配并通过神经-脂肪调控通路调节能量消耗。