Objective: This study aimed to investigate the effects of moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) on glucose metabolism in the skeletal muscles of T2DM mice, and to explore the potential regulatory mechanisms of MSTN-AMPK/PGC-1α signaling on mitochondrial quality control and its relationship with glucose metabolism.
Methods: Bioinformatics analysis was used to mine data from databases of T2DM patients to identify potential molecular targets for exercise intervention in T2DM. Eight-week-old male db/db mice were randomly assigned to three parallel experimental groups: diabetic control group (DC), moderate-intensity continuous training group (MICT), and high-intensity interval training group (HIIT). Age-matched wild-type (db/m) mice served as the normal control group (NC), with 12 mice in each group. The NC and DC groups did not undergo any exercise, while the MICT group underwent 10 weeks of moderate-intensity continuous exercise, and the HIIT group underwent 10 weeks of high-intensity interval exercise. Various techniques, including staining, transmission electron microscopy (TEM), enzyme activity assays, qRT-PCR, and Western blotting, were used to evaluate glucose metabolism, muscle structure, mitochondrial quality, and protein expression in the skeletal muscles of the mice.
Results: From the gene screening results of bioinformatics analysis, MSTN was identified as a key driver of exercise intervention in improving glucose metabolism in T2DM. After 10 weeks, the body weight and random blood glucose levels of the DC group were significantly higher than those of the NC group (P < 0.001), while both the MICT and HIIT groups showed significant reductions compared to the DC group (P < 0.05 or P < 0.01). Muscle fibers in the NC group were dense and orderly, with densely and uniformly stained glycogen granules. In contrast, the DC group exhibited sparse and disordered muscle fibers with sparse and unevenly distributed deep purple glycogen granules. The MICT and HIIT groups showed improved muscle fiber density and reduced inter-fiber spaces, with the HIIT group displaying densely and uniformly stained glycogen granules similar to the normal group, while the MICT group had intermediate glycogen granule staining intensity between the HIIT and DC groups. The cross-sectional area (CSA) and glycogen content in the DC group were significantly lower than those in the NC group (P < 0.001), while both the MICT and HIIT groups showed significant increases compared to the DC group (P < 0.05 or P < 0.001), with the HIIT group showing a more significant increase compared to the MICT group (P < 0.05 or P < 0.001). The expression of GLUT4 protein and G6PDH enzyme activity in the DC group were significantly lower than those in the NC group (P < 0.01 or P < 0.001), while both the MICT and HIIT groups showed significant increases compared to the DC group (P < 0.001), with the HIIT group showing a more significant increase compared to the MICT group (P < 0.05 or P < 0.001). L-LDH enzyme activity in the DC group was significantly higher than that in the NC group (P < 0.001), while both the MICT and HIIT groups showed significant decreases compared to the DC group (P < 0.05 or P < 0.01), with the HIIT group showing a more significant decrease compared to the MICT group (P < 0.001). Under TEM, mitochondria in the NC group appeared as typical ellipsoids or rods, arranged tightly and orderly, with clear and complete cristae and uniformly dense matrix. In the DC group, most mitochondria were swollen and vacuolated, with disrupted and dissolved cristae, and some membrane structures were damaged. Both the MICT and HIIT groups showed improved mitochondrial morphology with reduced swelling and partially restored cristae, although some cristae remained fragmented, and membrane integrity was somewhat improved. The number of mtDNA copies and the expression of NRF2, TFAM, OPA1, MFN2, DRP1, PINK1, and PARKIN proteins in the DC group were significantly lower than those in the NC group (P < 0.01 or P < 0.001), while both the MICT and HIIT groups showed significant increases compared to the DC group (P < 0.001). Except for the lack of significant changes in MFN2 protein expression, the HIIT group showed significantly higher expression compared to the MICT group (P < 0.05 or P < 0.01 or P < 0.001). The expression of MSTN protein in the DC group was significantly higher than that in the NC group (P < 0.01), while both the MICT and HIIT groups showed significant decreases compared to the DC group (P < 0.001). The phosphorylation level of AMPK and the expression of PGC-1α and AMPK proteins in the DC group were significantly lower than those in the NC group (P < 0.01 or P < 0.001), while both the MICT and HIIT groups showed significant increases compared to the DC group (P < 0.01 or P < 0.001), with the HIIT group showing a more significant increase compared to the MICT group (P < 0.05 or P < 0.01).
Conclusion: T2DM mice exhibit impaired glucose metabolism and imbalanced mitochondrial quality control in their skeletal muscles. Both HIIT and MICT interventions improve these conditions by downregulating the expression of MSTN in skeletal muscles, activating the AMPK/PGC-1α signaling pathway, and increasing the expression of proteins related to mitochondrial quality control, thereby ameliorating glucose metabolism disorders in T2DM skeletal muscles.
