FGFR2 signaling in normal and limbless chick limb buds

Area of Science:

• Developmental Biology
• Molecular Biology
• Genetics

Background:

• Fibroblast Growth Factor 10 (FGF10) and FGF8 are key signaling molecules in developing limb buds.
• They signal through FGFR2b and FGFR2c receptor isoforms, respectively.
• A proposed model involves a reciprocal paracrine signaling loop between FGF10 and FGF8.

Purpose of the Study:

• To investigate the expression patterns of FGFR2b and FGFR2c during chick limb development.
• To test the proposed paracrine signaling loop model for FGF10 and FGF8.
• To determine the role of the limbless gene in this signaling pathway.

Main Methods:

• Examined FGFR2b and FGFR2c mRNA expression using isoform-specific probes in developing chick limb buds.
• Analyzed gene expression in wild-type and limbless mutant embryos.
• Screened for mutations in FGFR2b and FGFR2c specific exons in limbless embryos.

Main Results:

• FGFR2b is expressed in limb ectoderm, including the apical ectoderm ridge (AER).
• FGFR2c is expressed in both mesoderm and ectoderm, suggesting autocrine and paracrine functions for FGF8.
• FGFR2c expression in mesoderm decreases during outgrowth, while remaining in the AER.
• FGF10, FGFR2b, and FGFR2c transcripts are present in limbless mutant limb buds, but FGF8 is absent.
• No mutations in FGFR2b or FGFR2c specific exons were found in limbless embryos.

Conclusions:

• The expression patterns support FGF10 signaling via FGFR2b in the ectoderm.
• FGFR2c expression suggests FGF8 can act in an autocrine manner on ectoderm and paracrine manner on mesoderm.
• The limbless gene product is essential for FGF10 to induce FGF8 expression, highlighting its critical role in initiating limb bud signaling.
• This implies the limbless gene acts upstream of the FGF10-FGF8 signaling cascade for limb initiation.