Abstract：
Neuromuscular junctions are innervated by motor and sympathetic nerves. The sympathetic modulation of motor innervation shows functional decline during aging, but the cellular and molecular mechanism of this change is not fully known. This study aimed to evaluate the effect of aging on sympathetic nerves and synaptic proteins at mouse neuromuscular junctions. Sympathetic nerves, presynaptic, and postsynaptic proteins of sympathetic nerves at neuromuscular junctions were visualized using immunohistochemistry, and aging-related changes were compared between adult-, aged-, and nicotinamide mononucleotide (NMN) administered aged mice. Sympathetic nerves were detected by anti-tyrosine hydroxylase antibody, and presynaptic protein vesicular monoamine transporter 2 colocalized with the sympathetic nerves. These two signals surrounded motor nerve terminals and acetylcholine receptor clusters. Postsynaptic neurotransmitter receptor β2-adrenergic receptors colocalized with motor nerve terminals and resided in reduced density at extrasynaptic sarcolemma. The signal intensity of the sympathetic nerve marker did not show a significant difference at neuromuscular junctions between 8.5-month-old adult mice and 25-month-old aged mice. However, the signal intensity of vesicular monoamine transporter 2 and β2-adrenergic receptors showed age-related decline at neuromuscular junctions. Interestingly, both age-related declines reverted to the adult level after 1 month of oral administration of NMN by drinking water. In contrast, NMN administration did not alter the expression level of sympathetic marker tyrosine hydroxylase at neuromuscular junctions. The results suggest a functional decline of sympathetic nerves at aged neuromuscular junctions due to decreases in presynaptic and postsynaptic proteins, which can be reverted to the adult level by NMN administration.
摘要：
神经肌肉接头由运动和交感神经支配。运动神经支配的交感神经调制显示衰老过程中的功能下降，但是这种变化的细胞和分子机制还不完全清楚。本研究旨在评估衰老对小鼠神经肌肉接头处交感神经和突触蛋白的影响。交感神经,突触前,神经肌肉接头处的交感神经突触后蛋白用免疫组织化学方法可视化，与衰老相关的变化进行了比较，aged-,和烟酰胺单核苷酸（NMN）给予老年小鼠。抗酪氨酸羟化酶抗体检测交感神经,和突触前蛋白囊泡单胺转运蛋白2与交感神经共定位。这两个旌旗灯号环绕运动神经末梢和乙酰胆碱受体簇。突触后神经递质受体β2-肾上腺素能受体与运动神经末梢共定位，并在突触外肌膜处密度降低。在8.5个月大的成年小鼠和25个月大的小鼠之间，交感神经标记物的信号强度在神经肌肉接头处没有显着差异。然而,囊泡单胺转运蛋白2和β2-肾上腺素能受体的信号强度在神经肌肉接头处显示出与年龄相关的下降。有趣的是,通过饮用水口服NMN1个月后,两种年龄相关的下降均恢复至成人水平.相比之下,NMN给药不会改变神经肌肉接头处交感神经标志物酪氨酸羟化酶的表达水平。结果表明，由于突触前和突触后蛋白的减少，老年神经肌肉接头的交感神经功能下降。可以通过NMN管理恢复到成人水平。
