Competitive coordination of the dual roles of the Hedgehog co-receptor in homophilic adhesion and signal reception
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Evaluation Summary:
This is a well-conceived and well-presented study that sheds light on two critical questions related to Hedgehog signaling, namely, the dual function of the Hedgehog co-receptor Ihog in Hh signal transduction and homophilic adhesion, and the regulation of cytoneme structure.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #3 agreed to share their name with the authors.)
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Abstract
Hedgehog (Hh) signaling patterns embryonic tissues and contributes to homeostasis in adults. In Drosophila , Hh transport and signaling are thought to occur along a specialized class of actin-rich filopodia, termed cytonemes. Here, we report that Interference hedgehog (Ihog) not only forms a Hh receptor complex with Patched to mediate intracellular signaling, but Ihog also engages in trans -homophilic binding leading to cytoneme stabilization in a manner independent of its role as the Hh receptor. Both functions of Ihog ( trans -homophilic binding for cytoneme stabilization and Hh binding for ligand sensing) involve a heparin-binding site on the first fibronectin repeat of the extracellular domain. Thus, the Ihog-Ihog interaction and the Hh-Ihog interaction cannot occur simultaneously for a single Ihog molecule. By combining experimental data and mathematical modeling, we determined that Hh-Ihog heterophilic interaction dominates and Hh can disrupt and displace Ihog molecules involved in trans -homophilic binding. Consequently, we proposed that the weaker Ihog-Ihog trans interaction promotes and stabilizes direct membrane contacts along cytonemes and that, as the cytoneme encounters secreted Hh ligands, the ligands trigger release of Ihog from trans Ihog-Ihog complex enabling transport or internalization of the Hh ligand-Ihog-Patched -receptor complex. Thus, the seemingly incompatible functions of Ihog in homophilic adhesion and ligand binding cooperate to assist Hh transport and reception along the cytonemes.
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Reviewer #3 (Public Review):
Yang et al. build on earlier studies from the Zheng lab and show in tissues that (i) the Hedgehog (Hh) co-receptor Ihog mediates homophilic interactions that enable cytoneme bundling and (ii) that Ihog-Hh interactions are stronger than and displace Ihog-Ihog interactions during signaling, consistent with biochemical and cell-based studies of relative affinities of Ihog for itself or Hh. These studies are bolstered by modeling and experiments showing co-localization of Dally and Dlp, which presumably supply the heparan sulfate chains needed to promotes homo- and hetero-philic interactions involving Ihog. I found the studies convincing, interesting, and an important extension of biochemical/cellular work on Ihog to tissue behavior.
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Reviewer #2 (Public Review):
This well-conceived and well-presented work has both originality and substance, and contributes important new ideas to the Hh signaling field with wonderful clarity.
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Reviewer #1 (Public Review):
This study builds upon previous findings by the authors and others that the Hedgehog (Hh) co-receptor Ihog not only binds Hh to trigger Hh signal transduction, but also engages trans-homophilic interactions in cell-cell adhesion. Using experimental manipulation and mathematical modeling, the authors assessed the role of Ihog trans-homophilic binding in stabilizing cytoneme structure and the relative strengths of Ihog-Ihog and Hh-Ihog binding. These findings led to a model whereby the weaker Ihog-Ihog trans interaction promotes direct membrane contacts along cytonemes and that Hh-Ihog binding releases Ihog from trans Ihog-Ihog complex. The studies are well designed and executed, and the findings are convincing.
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Evaluation Summary:
This is a well-conceived and well-presented study that sheds light on two critical questions related to Hedgehog signaling, namely, the dual function of the Hedgehog co-receptor Ihog in Hh signal transduction and homophilic adhesion, and the regulation of cytoneme structure.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #3 agreed to share their name with the authors.)
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