The hair follicle cycle in goat is divided into three phases: the growth phase (anagen), the regression phase (catagen), and the resting phase (telogen). Fibroblast growth factor 5 (FGF5) and fibroblast growth factor 21 (FGF21) are important regulators in goat hair cycle. However, the precise localization of FGF5 and FGF21 in goat skin tissue and their regulatory mechanisms remain unclear during hair follicle cycle transition. Dermal papilla cells (DPCs) located in the dermal papilla region, are essential for hair follicle development. When DPCs lose their function, hair follicle remains in the telogen, leading to hair loss.

Therefore, this study aimed to identify the expression and localization of FGF5 and FGF21 in Cashmere goat skin tissue, understand their roles in DPCs, and to analyze their regulatory mechanisms. This would enrich the knowledge of fibroblast growth factor family proteins in hair follicle cycle regulation and provide a theoretical basis for further elucidating the molecular mechanisms of Cashmere goat hair follicle cycle transition.

Real-time quantitative PCR (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA) were used to measure the expression levels of FGF5, FGF21 and their receptor 1 (FGFR1) in the skin tissue of Cashmere goats at anagen and telogen. Single-cell transcriptome sequencing data were analyzed to determine the expression and localization of FGF5, FGF21 and FGFR1, which were validated through immunofluorescence methods. Adenovirus was explored to overexpress FGF5 and FGF21 in DPCs. The effects on DPCs proliferation were measured using MTT, EdU and flow cytometry. The expressions of FGFR1 and proliferation-related genes were detected using RT-qPCR, immunofluorescence and western blotting methods. RT-qPCR, immunofluorescence and western blotting were used to investigate the effect of FGF5 and FGF21 on the proliferation of DPCs.

FGF5, FGF21 and FGFR1 exhibited higher expression levels in anagen than that in telogen, all of which were located within DPCs. Both FGF5 and FGF21 restrained DPCs viability, which significantly reduced the number of EdU-positive cells. FGF5 decreased the percentage of DPCs in G2 and S phases, while FGF21 specifically affected the G2 phase. The expression of FGFR1 was significantly increased following the overexpression of FGF5 and FGF21. Moreover, the expressions of cell proliferation markers KI67 and PCNA were also significantly inhibited with the overexpression of FGF5 and FGF21. Additionally, the expression of β-catenin, a key component of Wnt/β-catenin signaling pathway, was reduced. Thereby the activity of transcription factors TCF3 and JUN was inhibited, which were the downstream effectors of Wnt/β-catenin signaling pathway. Thus the inhibition further led to the decreased expression of proliferation-related genes MYC and CYCLIND1.

FGF5 and FGF21 played a critical role as regulatory factors in controlling the biological function of DPCs. This study provided the first evidences that FGF21, similar to FGF5, restrained the proliferation of DPCs by suppressing the Wnt/β-catenin signaling pathway. These findings contributes to our understanding of how fibroblast growth factors influence hair follicle cycle transition, particularly in the context of Cashmere goat hair growth and cashmere production.