An "R-spondin code" for multimodal signaling ON-OFF states

Area of Science:

• Cell Biology
• Molecular Biology
• Developmental Biology

Background:

• R-spondins (RSPOs) are secreted proteins that act as stem cell growth factors.
• RSPOs are known co-activators of Wnt signaling, amplifying its effects.
• RSPO2 and RSPO3 also bind BMP and FGF receptors, downregulating their respective signaling pathways.

Purpose of the Study:

• To identify new cell surface targets of R-spondins.
• To elucidate the mechanism by which RSPOs regulate diverse signaling pathways.
• To propose a novel framework, the "R-spondin code," for understanding combinatorial signaling.

Main Methods:

• Data mining of protein interaction databases.
• Cell surface binding assays to validate RSPO targets.
• Analysis of R-spondin modular domains and their functions.

Main Results:

• Identified SMO, PTC1,2, LGI1, ROBO4, and PTPR(F/S) as novel candidate RSPO targets.
• Confirmed that RSPOs function as "endocytosers," linking transmembrane proteins to ZNRF3/RNF43 E3 ligases for internalization.
• Demonstrated that distinct motifs in RSPO-TSP1 domains mediate target interaction and internalization.

Conclusions:

• RSPOs act as natural protein-targeting chimeras for cell surface proteins.
• The "R-spondin code" framework explains how RSPOs impart combinatorial signaling states.
• This code provides a new perspective on the coordinated regulation of signaling pathways in development and disease.