Endocytosis controls spreading and effective signaling range of Fgf8 protein

Area of Science:

• Developmental Biology
• Molecular Biology
• Cell Biology

Background:

• Secreted signaling molecules are crucial for embryonic development, regulating cell induction, proliferation, differentiation, and patterning.
• Fibroblast Growth Factor 8 (Fgf8) is a key signaling molecule involved in the development of various vertebrate structures, including the brain, heart, and mesoderm.
• The precise mechanism controlling the signaling range of morphogens like Fgf8 in developing tissues remains largely unknown.

Purpose of the Study:

• To investigate the mechanism controlling the extracellular signaling range of Fgf8 in living zebrafish embryos.
• To determine if Fgf8 acts as a morphogen and how its spreading is regulated within target tissues.

Main Methods:

• Studied the spreading of epitope-tagged Fgf8 in the nascent neuroectoderm of living zebrafish embryos.
• Investigated the role of endocytosis and lysosomal degradation in controlling Fgf8 extracellular availability.
• Manipulated endocytosis rates to observe effects on Fgf8 spreading and target gene activation.

Main Results:

• Fgf8 spreading is actively controlled by endocytosis and subsequent lysosomal degradation, termed 'restrictive clearance'.
• Inhibition of endocytosis leads to increased extracellular accumulation and a broader signaling range of Fgf8.
• Enhanced endocytosis reduces Fgf8 uptake and shortens its effective signaling range.

Conclusions:

• Fgf8 spreads extracellularly via a diffusion-based mechanism.
• Target cells actively regulate Fgf8 availability through endocytosis and degradation, influencing signaling range.
• This 'restrictive clearance' mechanism provides a novel insight into morphogen gradient formation during embryonic development.