Cellular Protrusions Rely on Specialized Receptors to Guide Embryonic Signals

Embryonic development relies on precise cellular communication networks that transport morphogens, which are specialized signaling molecules directing cell fate, across growing tissues. Researchers investigating how cells build and stabilize the microscopic, actin-based cellular protrusions known as cytonemes have uncovered a vital molecular relay involving cell surface proteins. These specialized communication extensions, which span significant distances to deliver vital growth instructions, depend on specific receptor components to organize the internal cellular skeleton required for elongation.

Proteins Repurposed to Build Signaling Filopodia

During tissue morphogenesis, cells must form dedicated transport extensions that restrict themselves to carrying specific signaling proteins or receptors. Scientists focused on Sonic Hedgehog signaling, a crucial pathway controlling embryonic patterning and cell differentiation, wanted to understand how signal-producing cells orchestrate the formation of these dynamic filaments. Prior work established that transmembrane proteins Brother of CDON and Cell adhesion molecule-related, Down-regulated by Oncogenes act as co-receptors in signal-receiving cells, but their role in ligand-producing sources remained less clear. The new investigation demonstrates that these proteins, along with Dispatched 1, form a functional deployment complex that actively drives the outward growth of cellular extensions.

To determine the necessity of these co-receptors, the researchers utilized CRISPR-Cas9 gene editing to generate various knockout mouse embryonic fibroblast cell lines lacking specific combinations of receptor genes. Through high-resolution fluorescence microscopy, the team evaluated the cytoneme-forming potential of these engineered cells after expressing fluorescently tagged Sonic Hedgehog ligands. The analysis revealed that while single or triple knockouts involving related receptors still produced extensions, removing both Brother of CDON and Cell adhesion molecule-related, Down-regulated by Oncogenes severely impaired cytoneme induction. Introducing exogenous versions of either protein successfully restored extension outgrowth in the deficient cells, indicating that either molecule is sufficient to drive biogenesis when properly expressed.

In Vivo Validation in Developing Neural Tube Tissues

To confirm that these observations reflect natural physiological processes, the team analyzed embryonic mouse neural tubes using live confocal fluorescence microscopy. By crossing conditional mutant mouse lines, the investigators selectively deleted the gene encoding Cell adhesion molecule-related, Down-regulated by Oncogenes specifically within Sonic Hedgehog-producing neuroepithelial cells of the ventral floor plate and notochord. Time-lapse imaging of vibratome-sectioned neural tissues revealed a significant drop in both the average number and maximum length of dynamic cellular extensions in the conditional mutants compared to control embryos. These structural deficits correlated with noticeable alterations in the spatial patterning of neighboring progenitor cell populations, demonstrating that impaired extension dynamics disrupt normal morphogen gradient establishment.

Further domain-mapping experiments investigated how these cell surface proteins communicate with the internal actin cytoskeleton. By engineering various truncation mutants lacking specific extracellular or intracellular domains, the authors established that the intracellular domain is essential for transducing the extension-promoting signal. Additional biochemical assays and proximity labeling techniques identified the guanine nucleotide exchange factor FERM, RhoGEF, and Pleckstrin domain-containing protein 1, known as FARP1, as a key downstream effector interacting with the intracellular region.

Targeting the Actin Regulatory Network for Extension Growth

The study, published in Science Advances, tracked how FARP1 localizes to the tips of active cellular protrusions and recruits the small Rho GTPase Cdc42. Using a Förster resonance energy transfer biosensor to monitor real-time enzyme dynamics within live cells, the researchers detected elevated Cdc42 activity specifically concentrated inside the growing extensions. Inhibiting Cdc42 pharmacologically eliminated the stimulatory effect of the ligand on protrusion formation, confirming that this GTPase acts downstream of the co-receptor complex to stimulate the actin polymerization necessary for structural elongation.

These findings provide a clear mechanism for how specialized tissue sources deploy morphogenic signals with high spatial precision. By linking ligand packaging pathways directly with cytoskeletal extension machinery, these dual-function cell surface proteins ensure that embryonic signals reach their intended targets across developing tissues without diffusing randomly.

The research was published in Science advances on November 11, 2026.