[Inhibition of type 3 deiodinase expression can improve mitochondrial function in skeletal muscle of sepsis by up-regulating peroxisome proliferator-activated receptor-γ coactivator-1α].

Objective: To investigate the protective effects and mechanisms of targeted inhibition of type 3 deiodinase (Dio3) on skeletal muscle mitochondria in sepsis.

Methods: (1) In vivo experiments: adeno-associated virus (AAV) was employed to specifically target Dio3 expression in the anterior tibial muscle of rats, and a septic rat model was generated using cecal ligation and puncture (CLP). The male Sprague-Dawley (SD) rats were divided into shNC+Sham group, shD3+Sham group, shNC+CLP group, and shD3+CLP group by random number table method, with 8 rats in each group. After CLP modeling, tibial samples were collected and Western blotting analysis was conducted to assess the protein levels of Dio3, peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), and silence-regulatory protein 1 (SIRT1). Real-time fluorescence quantitative polymerase chain reaction (RT-qPCR) was utilized to examine mRNA expression of genes including thyroid hormone receptors (THRα, THRβ), monocarboxylate transporter 10 (MCT10), mitochondrial DNA (mtDNA), and PGC1α. Transmission electron microscopy was employed to investigate mitochondrial morphology. (2) In vitro experiments: involved culturing C2C12 myoblasts, interfering with Dio3 expression using lentivirus, and constructing an endotoxin cell model by treating cells with lipopolysaccharide (LPS). C2C12 cells were divided into shNC group, shD3 group, shNC+LPS group, and shD3+LPS group. Immunofluorescence colocalization analysis was performed to determine the intracellular distribution of PGC1α. Co-immunoprecipitation assay coupled with Western blotting was carried out to evaluate the acetylation level of PGC1α.

Results: (1) In vivo experiments: compared with the shNC+Sham group, the expression of Dio3 protein in skeletal muscle of the shNC+CLP group was significantly increased (Dio3/β-Tubulin: 3.32±0.70 vs. 1.00±0.49, P < 0.05), however, there was no significant difference in the shD3+Sham group. Dio3 expression in the shD3+CLP group was markedly reduced relative to the shNC+CLP group (Dio3/β-Tubulin: 1.42±0.54 vs. 3.32±0.70, P < 0.05). Compared with the shNC+CLP group, the expression of T3-regulated genes in the shD3+CLP group were restored [THRα mRNA (2^-ΔΔCt): 0.67±0.05 vs. 0.33±0.01, THRβ mRNA (2^-ΔΔCt): 0.94±0.05 vs. 0.67±0.02, MCT10 mRNA (2^-ΔΔCt): 0.65±0.03 vs. 0.57±0.02, all P < 0.05]. Morphology analysis by electron microscopy suggested prominent mitochondrial damage in the skeletal muscle of the shNC+CLP group, while the shD3+CLP group exhibited a marked improvement. Compared with the shNC+Sham group, the shNC+CLP group significantly reduced the number of mitochondria (cells/HP: 10.375±1.375 vs. 13.750±2.063, P < 0.05), while the shD3+CLP group significantly increased the number of mitochondria compared to the shNC+CLP group (cells/HP: 11.250±2.063 vs. 10.375±1.375, P < 0.05). The expression of mtDNA in shNC+CLP group was markedly reduced compared with shNC+Sham group (copies: 0.842±0.035 vs. 1.002±0.064, P < 0.05). Although no difference was detected in the mtDNA expression between shD3+CLP group and shNC+CLP group, but significant increase was found when compared with the shD3+Sham group (copies: 0.758±0.035 vs. 0.474±0.050, P < 0.05). In the shD3+CLP group, PGC1α expression was significantly improved at both transcriptional and protein levels relative to the shNC+CLP group [PGC1α mRNA (2^-ΔΔCt): 1.49±0.13 vs. 0.68±0.06, PGC1α/β-Tubulin: 0.76±0.02 vs. 0.62±0.04, both P < 0.05]. (2) In vitro experiments: post-24-hour LPS treatment of C2C12 cells, the cellular localization of PGC1α became diffuse; interference with Dio3 expression promoted PGC1α translocation to the perinuclear region and nucleus. Moreover, the acetylated PGC1α level in the shD3+LPS group was significantly lower than that in the shNC+LPS group (acetylated PGC1α/β-Tubulin: 0.59±0.01 vs. 1.24±0.01, P < 0.05), while the expression of the deacetylating agent SIRT1 was substantially elevated following Dio3 inhibition (SIRT1/β-Tubulin: 1.04±0.04 vs. 0.58±0.03, P < 0.05). When SIRT1 activity was inhibited by using EX527, PGC1α protein expression was notably decreased compared to the shD3+LPS group (PGC1α/β-Tubulin: 0.92±0.03 vs. 1.58±0.03, P < 0.05).

Conclusions: Inhibition of Dio3 in skeletal muscle reduced the acetylation of PGC1α through activating SIRT1, facilitating nuclear translocation of PGC1α, thereby offering protection against sepsis-induced skeletal muscle mitochondrial damage.