Dual roles of syndecan-4 in regulating chicken fibrosis in vitro

Wooden Breast (WB) is a myopathy affecting the skeletal breast muscle (Pectoralis major) in broiler chickens and is characterized by muscle fiber damage and varying degrees of fibrosis, ECM remodeling and inflammation. Several key factors such as pro-inflammatory cytokines like TGF-β1 and IL-1β, drive fibrosis in WB myopathy. We have previously shown that the expression of syndecan-4 (SDC4), a transmembrane proteoglycan, was increased in WB poultry skeletal muscle tissue. Furthermore, the ectodomain shedding of SDC4 by matrix metalloproteinases (MMPs) differed in the skeletal muscle satellite cells from isolated affected chickens compared with normal. While SDC4 has been previously implicated as a key driver for regulating myofibroblast activity in mechanically induced fibrosis in cardiac tissue, its specific role and shedding activity in chicken fibroblasts in relation to WB myopathy remain poorly understood.

In vitro the overexpression system was used to mimic the previously detected increased SDC4 levels in WB and to further investigate fibrotic markers and syndecans at the gene and protein levels. Furthermore, we used blocking peptides derived from the SDC4 ectodomain and investigated their effect on SDC4 shedding and fibrotic markers. Additionally, TGF-β1 treatment, the main trigger of myofibroblasts, fibrosis, and cytokines, was used to investigate the connection between SDC4 shedding and fibrosis.

Overexpression of SDC4 in chicken fibroblasts reduced the gene and protein expression of fibrotic markers such as collagen I, collagen III and MMP-2. At the same time, we observed an increase in the gene expression of TGFB1 and IL1B. SDC4 overexpression also modulated intracellular proteins connected to fibrosis-relevant signaling pathways, with increased phosphorylation of p38 MAPK and decreased phosphorylation of Akt. Moreover, we could observe a decreased production of ribosomal protein S6 and β-catenin. SDC4 overexpression induced shedding of a 15 kDa SDC4 fragment, while a 20 kDa fragment was produced at similar level regardless of overexpression. Modulation of ectodomain shedding using blocking peptides targeting various ectodomain regions (amino acids 76-106 and 100-130) significantly reduced the production of the 20 kDa endogenous fragment. However, it did not affect the expression level of the 15 kDa fragment. This contrasts with what we have seen with the same blocking peptides in chicken skeletal muscle satellite cells where the expression level of the 15 kDa fragment was reduced. When we stimulated the cells with TGF-β1, an increase in gene expression of fibrotic marker was observed as expected. No change in SDC4 gene expression was observed, while SDC4 fragment of 20 kDa was increased. Taken together, our results reveal a complex role of SDC4 and shedding in modulating fibrotic responses in chicken fibroblasts, suggesting a potential dual function where the full-length SDC4, produced by overexpression, may act as an anti-fibrotic regulator, while the TGF-β1 induced shed ectodomain fragments could promote pro-fibrotic and inflammatory processes.