Animal models clearly illustrate that the maintenance of skeletal muscle mass depends on the function and interaction of a heterogeneous population of resident and infiltrating mononuclear cells. Several lines of evidence suggest that mononuclear cells also play a role in muscle wasting in humans, and targeting these cells may open new treatment options for intervention or prevention in sarcopenia. Methodological and ethical constraints have perturbed exploration of the cellular characteristics and function of mononuclear cells in human skeletal muscle. Thus, investigations of cellular phenotypes often depend on immunohistochemical analysis of small tissue samples obtained by needle biopsies, which do not match the deep phenotyping of mononuclear cells obtained from animal models. Here, we have developed a protocol for Fluorescence Activated Cell Sorting (FACS), based on single-cell RNA-sequencing data, for quantifying and characterizing mononuclear cell populations in human skeletal muscle. Muscle stem cells, fibro-adipogenic progenitors, and two subsets of macrophages (CD11c^+/-) are present in needle biopsies in comparable quantities per milligram tissue to open surgical biopsies. We find that direct cell isolation is preferable due to a substantial shift in transcriptome when using pre-culture before the FACS procedure. Finally, in vitro validation of the cellular phenotype of muscle stem cells, fibro-adipogenic progenitors, and macrophages confirms population specific traits. This study demonstrates that mononuclear cell populations can be quantified and subsequently analyzed from needle biopsy material and opens the perspective for future clinical studies of cellular mechanisms in muscle wasting.

中文翻译：

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从人类骨骼肌活检中分离和表征肌肉干细胞、纤维脂肪生成祖细胞和巨噬细胞

动物模型清楚地表明，骨骼肌质量的维持取决于常驻和浸润性单核细胞的异质群体的功能和相互作用。多项证据表明，单核细胞也在人类肌肉萎缩中发挥作用，靶向这些细胞可能为干预或预防肌肉减少症开辟新的治疗选择。方法学和伦理学的限制扰乱了对人类骨骼肌中单核细胞的细胞特征和功能的探索。因此，细胞表型的研究通常依赖于通过针活检获得的小组织样本的免疫组织化学分析，这与从动物模型中获得的单核细胞的深层表型不匹配。这里，我们开发了一种基于单细胞 RNA 测序数据的荧光激活细胞分选 (FACS) 协议，用于量化和表征人类骨骼肌中的单核细胞群。肌肉干细胞、纤维脂肪生成祖细胞和两个巨噬细胞亚群 (CD11c^+/- ) 以每毫克组织与开放手术活检相当的数量存在于针活检中。我们发现直接细胞分离更可取，因为在 FACS 程序之前使用预培养时转录组发生了实质性变化。最后，肌肉干细胞、纤维脂肪生成祖细胞和巨噬细胞的细胞表型的体外验证证实了群体特异性特征。这项研究表明，可以对单核细胞群进行量化，然后从针活检材料中对其进行分析，并为未来肌肉萎缩细胞机制的临床研究开辟了前景。