KIF3B promotes a PI3K signaling gradient causing changes in a Shh protein gradient and suppressing polydactyly in...

Area of Science:

• Developmental Biology
• Molecular Biology
• Genetics

Background:

• Digit determination in limb buds relies on a Sonic hedgehog (Shh) protein gradient.
• The precise mechanism governing Shh gradient formation remains largely unknown.
• KIF3B motor protein plays a role in limb development, as suggested by preaxial polydactyly phenotypes in hypomorphic mice.

Purpose of the Study:

• To elucidate the mechanism of Sonic hedgehog (Shh) gradient formation during limb bud development.
• To investigate the role of KIF3B, fibroblast growth factor (FGF), and phosphatidylinositol 3-kinase (PI3K) signaling in Shh gradient regulation.
• To propose a diffusion-and-trapping hypothesis for Shh gradient formation.

Main Methods:

• Analysis of limb buds from KIF3B motor hypomorphic mice exhibiting preaxial polydactyly.
• Experimentation involving the transplantation of fibroblast growth factor 8b (FGF8b)-soaked beads to mimic developmental phenotypes.
• Subcellular investigation of KIF3B transport of Talpid3 and its effect on PI3K signaling.
• Differential sorting analysis of endocytosed Shh based on PI3K signaling strength.

Main Results:

• Disorganization of distal-to-proximal FGF and PI3K signaling gradients, and posterior-to-anterior Shh gradients in KIF3B hypomorphic mouse limb buds.
• Successful reproduction of the observed phenotype by FGF8b bead transplantation.
• KIF3B-mediated transport of Talpid3 to regulate PI3K signaling.
• Differential sorting of Shh into exosome-like particles (high PI3K) or cytonemal punctata (low PI3K).

Conclusions:

• Shh gradient formation is regulated by a diffusion-and-trapping mechanism.
• PI3K signaling strength dictates the sorting of Shh-containing particles.
• Shh particles undergo peripheral diffusion or central cytonemal trapping, establishing the spatial gradient essential for limb development.