Effect of monochromatic light combinations on muscle fiber types in broilers after thyroidectomy

Our previous research indicated that monochromatic light combinations influence muscle fiber types in broilers, potentially through alterations in thyroid function, yet the underlying mechanisms remained elusive. Here, we employed a thyroidectomy (TX) model to investigate the role of thyroid hormone (T3) in mediating these light-induced effects. Broilers were reared under single-monochromatic light (white-white [WW], red-red [RR], green-green [GG], blue-blue [BB]) or different light combinations (green-blue [GB], blue-green [BG], red-blue [RB]). Our results demonstrate that monochromatic light combinations modulate serum thyroid hormone (TH) levels, thereby regulating oxidative stress (OS) and the composition of slow- (MyHC-I/IIa) and fast-twitch (MyHC-IIb) fibers in the gastrocnemius (GAS). Critically, TX-induced hypothyroidism not only reduced malondialdehyde (MDA) levels and the proportion of fast glycolytic fibers (MyHC-IIb) but also promoted a shift toward oxidative fibers (MyHC-I/IIa). Notably, this TX intervention abolished the intergroup differences in fiber-type composition caused by different light colors, confirming the indispensable mediating role of TH. Mechanistically, we found that TX significantly upregulated the inactivating deiodinase DIO3-a direct transcriptional target of the β-Catenin/TCF complex-while downregulating the activating deiodinase DIO2 and thyroid hormone receptor α (TRα). This was accompanied by enhanced phosphorylation of the PI3K/AKT/GSK3β axis, increased β-Catenin protein levels, and corresponding changes in gene expression: upregulation of slow-fiber markers (Pax7, Myf5, MYH7B) and downregulation of fast-fiber markers (MyoD, MyoG, Mstn, MYH1A). In conclusion, our study reveals that deiodinases DIO2 and DIO3 are critical mediators through which monochromatic light regulates intramuscular T3 levels and oxidative stress, driving fiber-type transitions via the PI3K/AKT-Wnt/β-Catenin signaling pathway.