Wnt1 Lineage Specific Deletion of Gpr161 Results in Embryonic Midbrain Malformation and Failure of Craniofacial Skeletal Development

Regulatory role of primary cilia in oral and maxillofacial development and disease

Primary cilia have versatile functions, such as receiving signals from the extracellular microenvironment, mediating signaling transduction, and transporting ciliary substances, in tissue and organ development and clinical disease pathogenesis. During early development (embryos within 10 weeks) in the oral and maxillofacial region, defects in the structure and function of primary cilia can result in severe craniofacial malformations. For example, mice with mutations in the cilia-related genes Kif3a and IFT88 exhibit midline expansion and cleft lip/palate, which occur due to abnormalities in the fusion of the single frontonasal prominence and maxillary prominences. In the subsequent development of the oral and maxillofacial region, we discussed the regulatory role of primary cilia in the development of the maxilla, mandible, Meckel cartilage, condylar cartilage, lip, tongue, and tooth, among others. Moreover, primary cilia are promising regulators in some oral and maxillofacial diseases, such as tumors and malocclusion. We also summarize the regulatory mechanisms of primary cilia in oral and maxillofacial development and related diseases, including their role in various signaling transduction pathways. For example, aplasia of submandibular glands in the Kif3a mutant mice is associated with a decrease in SHH signaling within the glands. This review summarizes the similarities and specificities of the role of primary cilia in tissue and organ development and disease progression in the oral and maxillofacial region, which is expected to contribute several ideas for the treatment of primary cilia-related diseases.

GPR161 structure uncovers the redundant role of sterol-regulated ciliary cAMP signaling in the Hedgehog pathway

The orphan G protein-coupled receptor (GPCR) GPR161 plays a central role in development by suppressing Hedgehog signaling. The fundamental basis of how GPR161 is activated remains unclear. Here, we determined a cryogenic-electron microscopy structure of active human GPR161 bound to heterotrimeric Gs. This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, a sterol that binds adjacent to transmembrane helices 6 and 7 stabilizes a GPR161 conformation required for Gs coupling. Mutations that prevent sterol binding to GPR161 suppress Gs-mediated signaling. These mutants retain the ability to suppress GLI2 transcription factor accumulation in primary cilia, a key function of ciliary GPR161. By contrast, a protein kinase A-binding site in the GPR161 C terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the role of GPR161 function in other signaling pathways.

The novel linkage between Fuz and Gpr161 genes regulates sonic hedgehog signaling during mouse embryonic development

Sonic hedgehog (Shh) signaling regulates embryonic morphogenesis utilizing primary cilia, the cell antenna acting as a signaling hub. Fuz, an effector of planar cell polarity (PCP) signaling, involves Shh signaling via cilia formation, while the G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling. The range of phenotypic malformations observed in mice bearing mutations in either of these two genes is similar; however, their functional relations have not been previously explored. This study identified the genetic and biochemical link between Fuz and Gpr161 in mouse embryonic development. Fuz was genetically epistatic to Gpr161 via Shh signaling during mouse embryonic development. The FUZ biochemically interacted with GPR161, and Fuz regulated Gpr161 ciliary trafficking via β-arrestin2. Our study suggested the novel Gpr161-Fuz axis that regulates Shh signaling during mouse embryonic development.

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Sonic hedgehog (Shh) signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality associated with craniofacial defects and neural tube defects. However, the structural defects of later embryonic stages and cell lineages underlying abnormalities have not been well characterized due to the limited lifespan of Gpr161 null mice. We found that embryos with Pax3 lineage-specific deletion of Gpr161 presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest), vertebral abnormalities (somite) and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida was partly due to reduced Pax3 and Cdx4 gene expression in the posterior dorsal neural tubes of Gpr161 mutant embryos with decreased Wnt signaling, whereas Shh signaling was increased. We describe a previously unreported role for Gpr161 in the development of posterior neural tubes and confirm its role in cranial neural crest- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Shh signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality with craniofacial and neural tube defects (NTDs). However, the structural defects of later embryonic stages in Gpr161 null mice and cell lineages underlying abnormalities were not well characterized due to their limited lifespan. We found the Pax3 lineage-specific deletion of Gpr161 in mice presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest (CNC)), vertebral abnormalities (somite), and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida is partly due to the reduced Pax3 and Cdx4 gene expression of the posterior dorsal neural tubes of Gpr161 mutant embryos involving decreased Wnt signaling whereas Shh signaling was increased. This study provides the novel role of Gpr161 in the posterior neural tube development and confirms its role on CNC- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

GPR161 structure uncovers the redundant role of sterol-regulated ciliary cAMP signaling in the Hedgehog pathway

The orphan G protein-coupled receptor (GPCR) GPR161 is enriched in primary cilia, where it plays a central role in suppressing Hedgehog signaling1. GPR161 mutations lead to developmental defects and cancers2,3,4. The fundamental basis of how GPR161 is activated, including potential endogenous activators and pathway-relevant signal transducers, remains unclear. To elucidate GPR161 function, we determined a cryogenic-electron microscopy structure of active GPR161 bound to the heterotrimeric G protein complex Gs. This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, we identify a sterol that binds to a conserved extrahelical site adjacent to transmembrane helices 6 and 7 and stabilizes a GPR161 conformation required for Gs coupling. Mutations that prevent sterol binding to GPR161 suppress cAMP pathway activation. Surprisingly, these mutants retain the ability to suppress GLI2 transcription factor accumulation in cilia, a key function of ciliary GPR161 in Hedgehog pathway suppression. By contrast, a protein kinase A-binding site in the GPR161 C-terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how unique structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the broader role of GPR161 function in other signaling pathways.

Co-exposure to nanoplastics and acetaminophen causes skeletal dysplasia and behavioral abnormalities in zebrafish

Nanoplastics (NPs) and acetaminophen (APAP) are thought to be common contaminants and are invariably detected in the environment. Despite the increasing awareness of their toxicity to humans and animals, the embryonic toxicity, skeletal development toxicity, and mechanism of action of their combined exposure have not been clarified. This study was performed to investigate whether combined exposure to NPs and APAP induces abnormal embryonic and skeletal development in zebrafish and to explore the potential toxicological mechanisms. All zebrafish juveniles in the high-concentration compound exposure group showed some abnormal phenomena such as pericardial edema, spinal curvature, cartilage developmental abnormality and melanin inhibition together with a significant downward trend in body length. Behavioral data also implicated that the exposure of APAP alone, as well as the co-exposure of NPs and APAP, caused a depression in the total distance, swimming speed and the maximum acceleration. Furthermore, real-time polymerase chain reaction analysis showed that compared with exposure alone, the expression level of genes related to osteogenesis, runx2a, runx2b, Sp7, bmp2b and shh was significantly reduced with compound exposure. These results suggest that the compound exposure of NPs and APAP has adverse impacts on zebrafish embryonic development and skeletal growth.

Canonical Wnt/β-catenin signaling has positive effects on osteogenesis, but can have negative effects on cementogenesis

BACKGROUND

To date, therapeutic approaches for cementum regeneration are limited and outcomes remain unpredictable. A significant barrier to improve therapies for cementum regeneration is that the cementocyte and its intracellular signal transduction mechanisms remain poorly understood. This study aims to elucidate the regulatory mechanism of Wnt pathway in cementogenesis.

METHODS

The effects of canonical Wnt signaling were compared in vitro using immortalized murine cementocyte cell line IDG-CM6 and osteocyte cell line IDG-SW3 by qRT-PCR, Western blot, confocal microscopy, alkaline phosphatase (ALP) assay and Alizarin red S staining. In vivo, histological changes of cementum and bone formation were examined in transgenic mice in which constitutive activation of β-catenin is driven by Dmp1 promoter.

RESULTS

Expression of components of the Wnt/β-catenin pathway were much greater in the IDG-SW3 cells compared to the IDG-CM6 cells resulting in much lower expression of Sost/sclerostin in the IDG-SW3 cells. In the IDG-CM6 cells, low dose Wnt3a (20 ng/ml) had a modest effect while high dose (200 ng/ml) inhibited runt-related transcription factor 2 (Runx2), osterix (Osx), ALP and osteopontin (OPN) in contrast to the IDG-SW3 cells where high dose Wnt3a dramatically increased mRNA expression of these same markers. However, high Wnt3a significantly increased mRNA for components of Wnt/β-catenin signaling pathway in both IDG-CM6 and IDG-SW3 cells. In vivo, constitutive activation of β-catenin in the Dmp1-lineage cells in mice leads to bone hyperplasia and cementum hypoplasia.

CONCLUSION(S)

These findings indicate that Wnt signaling has distinct and different effects on the regulation of long bone as compared to cementum. This article is protected by copyright. All rights reserved.