Effect of insulin insufficiency on ultrastructure and function in skeletal muscle

Decreased insulin availability and high blood glucose levels, the hallmark features of poorly controlled diabetes, drive disease progression and are associated with decreased skeletal muscle mass. We have shown that mice with beta-cell dysfunction and normal insulin sensitivity have decreased skeletal muscle mass. This project asks how insulin deficiency impacts on the structure and function of the remaining skeletal muscle in these animals. Methods: Skeletal muscle function was determined by measuring exercise capacity and specific muscle strength prior to and after insulin supplementation for 28 days in 12-week-old mice with conditional beta-cell deletion of the ATP binding cassette transporters ABCA1 and ABCG1 (beta-DKO mice). Abca1 and Abcg1 floxed (fl/fl) mice were used as controls. RNAseq was used to quantify changes in transcripts in soleus and extensor digitorum longus muscles. Skeletal muscle and mitochondrial morphology were assessed by transmission electron microscopy. Myofibrillar Ca2+ sensitivity and maximum isometric single muscle fibre force were assessed using MyoRobot biomechatronics technology. Results: RNA transcripts were significantly altered in β-DKO mice compared to fl/fl controls (32 in extensor digitorum longus and 412 in soleus). Exercise capacity and muscle strength were significantly decreased in beta-DKO mice compared to fl/fl controls (p = 0.012), and a loss of structural integrity was also observed in skeletal muscle from the beta-DKO mice. Supplementation of beta-DKO mice with insulin restored muscle integrity, strength and expression of 13 and 16 of the dysregulated transcripts in and extensor digitorum longus and soleus muscles, respectively. Conclusions: Insulin insufficiency due to beta-cell dysfunction perturbs the structure and function of skeletal muscle. These adverse effects are rectified by insulin supplementation.胰岛素可用性降低与高血糖水平是糖尿病控制不佳的标志性特征，可推动疾病进展，并与骨骼肌质量下降相关。本研究已证实，β细胞（beta-cell）功能障碍但胰岛素敏感性正常的小鼠存在骨骼肌质量下降的情况。本项目旨在探究胰岛素缺乏对该类小鼠剩余骨骼肌的结构与功能产生的影响。 方法：选取携带ATP结合盒转运蛋白（ATP binding cassette transporters）ABCA1与ABCG1条件性β细胞敲除的12周龄小鼠（β-DKO小鼠，beta-DKO mice），在其补充胰岛素28天前后，分别测定运动能力与特定肌肉力量以评估骨骼肌功能。本研究以Abca1与Abcg1 floxed（fl/fl）小鼠作为对照。采用RNA测序（RNAseq）定量比目鱼肌（soleus）与趾长伸肌（extensor digitorum longus）的转录本表达变化；通过透射电子显微镜（transmission electron microscopy）评估骨骼肌与线粒体形态；利用MyoRobot生物机电一体化（biomechatronics）技术测定肌原纤维钙离子敏感性与最大等长单肌纤维张力。 结果：与fl/fl对照组相比，β-DKO小鼠的转录本表达发生显著改变，其中趾长伸肌中有32个转录本出现差异表达，比目鱼肌中有412个。相较于对照组，β-DKO小鼠的运动能力与肌肉力量显著下降（p=0.012），且其骨骼肌的结构完整性出现受损。为β-DKO小鼠补充胰岛素后，其肌肉完整性、肌肉力量以及趾长伸肌与比目鱼肌中分别有13个和16个失调转录本的表达均得到恢复。 结论：β细胞功能障碍引发的胰岛素不足会扰乱骨骼肌的结构与功能，而胰岛素补充可纠正这些不良影响。