Department of Internal Medicine, Section on Gerontology and Geriatric Medicine, Wake Forest School of Medicine, Winston-Salem, NC, USA.
Neuroscience Program, Wake Forest School of Medicine, Winston-Salem, NC, USA.
J Cachexia Sarcopenia Muscle. 2021 Feb;12(1):91-108. doi: 10.1002/jcsm.12644. Epub 2020 Nov 30.
Sarcopenia, or age-dependent decline in muscle force and power, impairs mobility, increasing the risk of falls, institutionalization, co-morbidity, and premature death. The discovery of adrenoceptors, which mediate the effects of the sympathetic nervous system (SNS) neurotransmitter norepinephrine on specific tissues, sparked the development of sympathomimetics that have profound influence on skeletal muscle mass. However, chronic administration has serious side effects that preclude their use for muscle-wasting conditions. Interventions that can adjust neurotransmitter release to changing physiological demands depend on understanding how the SNS affects neuromuscular transmission, muscle motor innervation, and muscle mass.
We examined age-dependent expression of the heart and neural crest derivative 2 (Hand2), a critical transcription factor for SN maintenance, and we tested the possibility that inducing its expression exclusively in sympathetic neurons (SN) will prevent (i) motor denervation, (ii) impaired neuromuscular junction (NMJ) transmission, and (iii) loss of muscle mass and function in old mice. To test this hypothesis, we delivered a viral vector carrying Hand2 expression or an empty vector exclusively in SNs by vein injection in 16-month-old C57BL/6 mice that were sacrificed 6 months later. Techniques include RNA-sequencing, real-time PCR, genomic DNA methylation, viral vector construct, tissue immunohistochemistry, immunoblot, confocal microscopy, electrophysiology, and in vivo mouse physical performance.
Hand2 expression declines throughout life, but inducing its expression increased (i) the number and size of SNs, (ii) muscle sympathetic innervation, (iii) muscle weight and force and whole-body strength, (iv) myofiber size but not muscle fibre-type composition, (v) NMJ transmission and nerve-evoked muscle force, and (vi) motor innervation in old mice. Additionally, the SN controls a set of genes to reduce inflammation and to promote transcription factor activity, cell signalling, and synapse in the skeletal muscle. Hand2 DNA methylation may contribute, at least partially, to gene silencing.
Selective expression of Hand2 in the mouse SNs from middle age through old age increases muscle mass and force by (i) regulating skeletal muscle sympathetic and motor innervation; (ii) improving acetylcholine receptor stability and NMJ transmission; (iii) preventing inflammation and myofibrillar protein degradation; (iv) increasing autophagy; and (v) probably enhancing protein synthesis.
肌肉减少症，或随年龄增长导致的肌肉力量和功能下降，会损害活动能力，增加跌倒、住院、合并症和过早死亡的风险。肾上腺素能受体的发现，介导了交感神经系统（SNS）神经递质去甲肾上腺素对特定组织的作用，这激发了拟交感药的发展，它们对骨骼肌质量有深远的影响。然而，慢性给药有严重的副作用，使其不能用于肌肉消耗性疾病。能够根据生理需求调整神经递质释放的干预措施，取决于对 SNS 如何影响神经肌肉传递、肌肉运动神经支配和肌肉质量的理解。
我们研究了心脏和神经嵴衍生 2（Hand2）的年龄依赖性表达，Hand2 是维持 SN 的关键转录因子，我们测试了专门在交感神经元（SN）中诱导其表达是否会防止（i）运动神经支配丧失、（ii）神经肌肉接头（NMJ）传递受损以及（iii）老年小鼠的肌肉质量和功能丧失。为了验证这一假设，我们通过静脉注射将携带 Hand2 表达的病毒载体或空载体专门递送到 16 个月大的 C57BL/6 小鼠的 SN 中，6 个月后处死这些小鼠。所使用的技术包括 RNA 测序、实时 PCR、基因组 DNA 甲基化、病毒载体构建、组织免疫组织化学、免疫印迹、共聚焦显微镜、电生理学和体内小鼠体能表现。
Hand2 的表达在整个生命过程中都会下降，但诱导其表达会增加（i）SN 的数量和大小、（ii）肌肉交感神经支配、（iii）肌肉重量和力量以及全身力量、（iv）肌纤维大小但不改变肌肉纤维类型组成、（v）NMJ 传递和神经诱发的肌肉力量以及（vi）老年小鼠的运动神经支配。此外，SN 控制了一组基因，以减少炎症并促进转录因子活性、细胞信号转导和骨骼肌肉中的突触。Hand2 的 DNA 甲基化可能至少部分导致基因沉默。
从中年到老年，在小鼠的 SN 中选择性表达 Hand2，通过以下方式增加肌肉质量和力量：（i）调节骨骼肌交感神经和运动神经支配；（ii）改善乙酰胆碱受体稳定性和 NMJ 传递；（iii）防止炎症和肌原纤维蛋白降解；（iv）增加自噬；以及（v）可能增强蛋白质合成。
