The muscle-specific MEF2D?2 isoform promotes muscle ketolysis and running capacity in mice

Ketone bodies are an alternate fuel source generated by the liver in response to low carbohydrate availability in neonates and after starvation and exhausting exercise in adulthood. The postnatal alternative splicing generates a highly conserved muscle-specific MEF2D?2 protein isoform of the transcription factor MEF2D. Here, we discovered that compared to WT mice, MEF2D?2 exon knockout (Eko) mice displayed reduced running capacity and muscle expression of all three ketolytic genes, BDH1, OXCT1, and ACAT1. Consistent with reduced muscle utilization of ketone bodies, MEF2D?2 Eko mice also showed increased ketone body levels in a tolerance test, after exercise, and upon feeding a ketogenic diet. Lastly, using mitochondria isolated from skeletal muscle, we showed reduced ketone body utilization and respiration in Eko compared to WT mice. Thus, we identified a new role of MEF2D?2 protein isoform in regulating skeletal muscle ketone body oxidation, exercise capacity, and its effect on systemic ketone body levels. Overall design: Quadrips were isolated from ~20-week old WT and Eko mice and subjected to polyA RNA sequencing.

酮体（Ketone bodies）是肝脏在新生儿碳水化合物供给不足时，以及成年个体饥饿或力竭运动后生成的替代能源物质。出生后可变剪接可产生转录因子MEF2D的肌肉特异性、高度保守的MEF2DΔ2蛋白亚型。本研究发现，与野生型（Wild Type, WT）小鼠相比，MEF2DΔ2外显子敲除（exon knockout, Eko）小鼠的跑步能力下降，且肌肉中三种酮解基因（ketolytic genes，BDH1、OXCT1及ACAT1）的表达量均显著降低。与肌肉酮体利用能力下降的实验结果一致，MEF2DΔ2 Eko小鼠在酮体耐受试验、运动后及喂食生酮饮食（ketogenic diet）后，体内酮体水平均出现升高。最后，本研究利用从骨骼肌中分离得到的线粒体开展实验，结果显示与WT小鼠相比，Eko小鼠的酮体利用能力及线粒体呼吸功能均有所下降。综上，本研究证实了MEF2DΔ2蛋白亚型在调控骨骼肌酮体氧化、运动能力以及其对全身酮体水平的影响方面的全新功能。整体实验设计：从约20周龄的WT及Eko小鼠体内分离股四头肌，随后进行polyA RNA测序（polyA RNA sequencing）。