Li Enhui, Wang Rui, Li Yanli, Zan Xiang, Wu Shufen, Yin Yiru, Yang Xiaorong, Yin Litian, Zhang Yu, Li Jianguo, Zhao Xin, Zhang Ce
Key Laboratory of Cellular Physiology, Ministry of Education, Department of Physiology, Shanxi Medical University, Taiyuan, Shanxi, China.
Department of Neurology, First Hospital of Shanxi Medical University, Taiyuan, Shanxi, China.
J Cachexia Sarcopenia Muscle. 2025 Apr;16(2):e13797. doi: 10.1002/jcsm.13797.
Skeletal muscle function and mass continuously decrease during aging. Most studies target limb muscles owing to their direct impact on mobility and falls risk. The diaphragm (DIA), also a type of skeletal muscle with different phenotype, has received less attention. Comparative research of the DIA and limb muscles can reveal their distinct aging characteristics. Critically, the potential endogenous anti-aging mechanisms of DIA that may provide new insights into the mechanisms of sarcopenia in limb muscles remain scarce.
Treadmill and grip tests assessed limb muscle function, while a lung function system evaluated respiratory function in both adult (6-month-old) and old (22-month-old) mice. Histological assessments evaluated muscle mass in both the DIA and tibialis anterior (TA). Transcriptome sequencing identified differentially expressed genes (DEGs) between the DIA and TA with aging. Adeno-associated virus (AAV)-encoding short hairpin (sh) RNA targeting gene was injected into adult mice's TA muscles to knockdown target gene level in TA, and AAV-gene was injected into old mice's TA to overexpress target gene level.
Old mice displayed significantly reduced running distance (p = 0.0026), maximal speed (p = 0.0019), time to exhaustion (p = 0.0033) and grip strength (p = 0.0055) compared with adult mice, alongside TA's weight loss, decreased myofibre cross-sectional area (CSA) and autophagy deficiency. However, lung function indicators (respiratory rate, tidal volume, minute ventilation volume, forced vital capacity and ratio of forced expiratory volume in 100 or 200 ms to forced vital capacity), as well as DIA weight and morphology remained stable in old mice. Transcriptional analysis revealed 61 DEGs, with significant upregulation or downregulation observed in TA, but without changes in DIA during aging. Smox (spermine oxidase) is one of the DEGs, responsible for catalysing the conversion of spermine to spermidine. It was reported that in muscle atrophy models such as limb immobilisation, fasting and denervation, Smox's levels are positively correlated with muscle mass and function. Additionally, an increase in Smox also promotes mitochondrial biogenesis. In our study, AAV-shSmox adult mice decreased running distance, speed and time, myofibre CSA alongside mitochondrial function, compared with controls. In contrast, old mice with Smox overexpression showed enhanced mitochondrial function.
In conclusion, this study reveals aging diversities of TA and DIA, explores the sarcopenia of limb muscles based on the anti-aging properties of DIA, which offers a novel perspective on limb sarcopenia. Our findings suggest Smox as a potential target for developing strategies to mitigate sarcopenia progression.
骨骼肌功能和质量在衰老过程中持续下降。由于对肢体肌肉对活动能力和跌倒风险有直接影响，大多数研究都以肢体肌肉为目标。膈肌（DIA）也是一种具有不同表型的骨骼肌，但受到的关注较少。对膈肌和肢体肌肉进行比较研究可以揭示它们不同的衰老特征。至关重要的是，膈肌潜在的内源性抗衰老机制可能为肢体肌肉少肌症的机制提供新的见解，但目前仍然稀缺。
通过跑步机和握力测试评估肢体肌肉功能，同时使用肺功能系统评估成年（6个月大）和老年（22个月大）小鼠的呼吸功能。组织学评估评估膈肌和胫前肌（TA）的肌肉质量。转录组测序确定了随着衰老膈肌和胫前肌之间差异表达的基因（DEGs）。将编码靶向基因的短发夹（sh）RNA的腺相关病毒（AAV）注射到成年小鼠的胫前肌中，以降低胫前肌中靶基因水平，并将AAV基因注射到老年小鼠的胫前肌中以过表达靶基因水平。
与成年小鼠相比，老年小鼠的跑步距离（p = 0.0026）、最大速度（p = 0.0019）、疲劳时间（p = 0.0033）和握力（p = 0.0055）显著降低，同时胫前肌重量减轻、肌纤维横截面积（CSA）减小和自噬缺陷。然而，老年小鼠的肺功能指标（呼吸频率、潮气量、分钟通气量、用力肺活量以及100或200毫秒内用力呼气量与用力肺活量的比值）以及膈肌重量和形态保持稳定。转录分析揭示了61个差异表达基因，在衰老过程中胫前肌中观察到显著上调或下调，但膈肌中没有变化。Smox（精胺氧化酶）是差异表达基因之一，负责催化精胺转化为亚精胺。据报道，在诸如肢体固定、禁食和去神经支配等肌肉萎缩模型中，Smox的水平与肌肉质量和功能呈正相关。此外，Smox的增加还促进线粒体生物发生。在我们的研究中，与对照组相比，注射AAV-shSmox的成年小鼠的跑步距离、速度和时间、肌纤维CSA以及线粒体功能均下降。相反，过表达Smox的老年小鼠显示线粒体功能增强。
总之，本研究揭示了胫前肌和膈肌的衰老差异，基于膈肌的抗衰老特性探索了肢体肌肉少肌症，为肢体少肌症提供了新的视角。我们的研究结果表明Smox作为开发减缓少肌症进展策略的潜在靶点。
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