Succinate induces skeletal muscle fiber remodeling via SUNCR1 signaling

Abstract The conversion of skeletal muscle fiber from fast twitch to slow‐twitch is important for sustained and tonic contractile events, maintenance of energy homeostasis, and the alleviation of fatigue. Skeletal muscle remodeling is effectively induced by endurance or aerobic exercise, which also generates several tricarboxylic acid (TCA) cycle intermediates, including succinate. However, whether succinate regulates muscle fiber‐type transitions remains unclear. Here, we found that dietary succinate supplementation increased endurance exercise ability, myosin heavy chain I expression, aerobic enzyme activity, oxygen consumption, and mitochondrial biogenesis in mouse skeletal muscle. By contrast, succinate decreased lactate dehydrogenase activity, lactate production, and myosin heavy chain IIb expression. Further, by using pharmacological or genetic loss‐of‐function models generated by phospholipase Cβ antagonists, SUNCR1 global knockout, or SUNCR1 gastrocnemius‐specific knockdown, we found that the effects of succinate on skeletal muscle fiber‐type remodeling are mediated by SUNCR1 and its downstream calcium/NFAT signaling pathway. In summary, our results demonstrate succinate induces transition of skeletal muscle fiber via SUNCR1 signaling pathway. These findings suggest the potential beneficial use of succinate‐based compounds in both athletic and sedentary populations.

. Effects of succinate on muscle fiber and mitochondrial function of C2C12 cells (related to Fig 5).
C2C12 cells were treated with 0, 0.5 mM, or 2 mM SUC for 48 h.
Data information: Results are presented as mean AE SEM (n = 5-6). Different letters between bars mean P ≤ 0.05 in one-way ANOVA analyses followed by post hoc Tukey's tests.  Figure EV3. Role of SUNCR1/PLC-b in succinateinduced in vitro fiber-type transition in myotubes (related to Fig 6).
A Schematic representation of SUNCR1 KO by Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) strategy. The sgRNA sites were located in intron 1 and intron 2 of SUNCR1 gene. Four sgRNAs were designed to delete exon 2 of SUNCR1 gene. The DNA sequences contained sgRNA-binding regions are labeled with lines. B Immunoblots of SUNCR1 protein in liver, fat, soleus (sol), and gastrocnemius (gas) from WT and SUNCR1 KO mice. C Representative images for genotyping screen of WT, heterozygous (Het), and homozygous (KO) SUNCR1 KO mice. D-G (D) Cumulative food intake, (E) lean mass, (F) fat mass, and (G) body weight gain of WT or SUNCR1 KO mice after 6 weeks of dietary supplementation of 0 or 1% SUC. H, I Immunoblots and quantification of p-mTOR, mTOR, p-FoxO3a, FoxO3a, p-AKT, and AKT proteins in gastrocnemius from WT or SUNCR1 KO mice after 6 weeks of dietary supplementation of 0 or 1% SUC (n = 3).
A B C D Figure EV5. Effects of gastrocnemius-specific SUNCR1 knockdown on body weight (related to Fig 8).
A-D Male C57BL/6J mice were injected with LV-shScrambled or shSUNCR1 lentivirus specifically into the gastrocnemius at 6 weeks of age. After 2 weeks of recovery, mice were fed with chow diet supplemented with 0 or 1% SUC for 6 weeks. (A) Cumulative food intake, (B) body weight gain, (C) fat mass, and (D) lean mass of mice after 6 weeks of dietary SUC supplementation.
Data information: Results are presented as mean AE SEM (n = 7-8).