The Hedgehog (HH) pathway controls multiple aspects of craniofacial development. HH ligands signal through the canonical receptor PTCH1, and three co-receptors: GAS1, CDON and BOC. Together, these co-receptors are required during embryogenesis to mediate proper HH signaling. Here we investigated the individual and combined contributions of GAS1, CDON and BOC to HH-dependent mammalian craniofacial development. Notably, individual deletion of either Gas1 or Cdon results in variable holoprosencephaly phenotypes, even on a congenic background. In contrast, we find that Boc deletion results in facial widening that correlates with increased HH target gene expression. Additionally, Boc deletion in a Gas1 null background partially ameliorates the craniofacial defects observed in Gas1 single mutants; a phenotype that persists over developmental time, resulting in significant improvements to a subset of craniofacial structures. This contrasts with HH-dependent phenotypes in other tissues that significantly worsen following combined deletion of Gas1 and Boc. Together, these data indicate that BOC acts as a multi-functional regulator of HH signaling during craniofacial development, alternately promoting or restraining HH pathway activity in a tissue-specific fashion.

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statementHere we identify dual, tissue-specific roles for the Hedgehog co-receptor BOC in both the promotion and antagonism of Hedgehog signaling during craniofacial development. Abstract 1The Hedgehog (HH) pathway controls multiple aspects of craniofacial development. HH ligands 2 signal through the canonical receptor PTCH1, and three co-receptors-GAS1, CDON and BOC. Together, 3 these co-receptors are required during embryogenesis to mediate proper HH signaling. Here we investigated 4 the individual and combined contributions of GAS1, CDON and BOC to HH-dependent mammalian 5 craniofacial development. Individual deletion of either Gas1 or Cdon results in variable holoprosencephaly 6 phenotypes, characterized by the failure to divide and form the telencephalon and midfacial structures. In 7 contrast, we find that Boc deletion results in facial widening consistent with increased HH pathway activity. 8

Hedgehog (HH) signaling is essential for correct patterning of vertebrate embryos, including during craniofacial development. HH ligands signal through the canonical receptor Patched1 (PTCH1), and three co‐receptors–growth arrest specific 1 (GAS1), cell adhesion molecule‐related/down‐regulated by oncogenes (CDON), and brother of CDON (BOC). Together, these co‐receptors are required during embryogenesis to mediate proper HH signaling. However, recent genetic studies suggest tissue‐specific, antagonistic roles for these proteins. For example, Gas1 restricts the HH pathway during mouse tooth development, while Boc antagonizes HH signaling during zebrafish jaw formation. Here I explored possible antagonistic roles for GAS1, CDON and BOC in HH‐dependent mammalian craniofacial development. Genetic deletion of these co‐receptors results in variable holoprosencephaly (HPE) phenotypes (failure to divide the forebrain into two cerebral hemispheres). Gas1 mutant mice display a spectrum of HPE phenotypes, and significant patterning defects in the neural tube and limb. In contrast, genetic deletion of Boc does not perturb patterning in any of these tissues. Strikingly, the combined deletion of Boc and Gas1 results in more severe neural tube and limb patterning defects than those observed in Gas1 single mutants. However, the Gas1 craniofacial phenotype is partially ameliorated in Gas1;Boc double mutant embryos as determined by the levels of NKX2.1, Gli1, Ptch1 and internasal distance quantitation. Micro‐computed tomography and skeletal analyses reveal that the rescue of the craniofacial defects in Gas1;Boc double mutants is restricted to the nasal bone and nasal capsule; other craniofacial bones are more severely affected, similar to what is observed in the neural tube and the limb. This effect does not appear to be due to differential GAS1 and BOC function in the forebrain neuroepithelium as overexpression of these co‐receptors in the developing chicken forebrain both promote HH signaling. Instead, I hypothesize that these co‐receptors have distinct, tissue‐specific functions in the craniofacial mesenchyme or the surface ectoderm. To further explore an antagonistic role for BOC during craniofacial development, I am currently deleting Boc in a sensitized HH genetic background through breeding with Ptch1 and Shh heterozygous mutant animals. These findings suggest that GAS1 and BOC do not function simply as redundant HH co‐receptors, but instead display distinct, tissue‐specific functions, particularly during craniofacial development.Support or Funding InformationNational Sciences Foundation Graduate Research Fellowship Program (DGE 1256260), Rackham Pre‐Candidate Graduate Student Research Grant, Bradley Merill Patten Fellowship and Organogenesis Training Grant (NIH T32HD007505)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.