Synergistic roles of Wnt modulators R-spondin2 and R-spondin3 in craniofacial morphogenesis and dental development

Abstract

Wnt signaling plays a critical role in craniofacial patterning, as well as tooth and bone development. Rspo2 and Rspo3 are key regulators of Wnt signaling. However, their coordinated function and relative requirement in craniofacial development and odontogensis are poorly understood. We showed that in zebrafish rspo2 and rspo3 are both expressed in osteoprogenitors in the embryonic craniofacial skeleton. This is in contrast to mouse development, where Rspo3 is expressed in osteoprogenitors while Rspo2 expression is not observed. In zebrafish, rspo2 and rspo3 are broadly expressed in the pulp, odontoblasts and epithelial crypts. However, in the developing molars of the mouse, Rspo3 is largely expressed in the dental follicle and alveolar mesenchyme while Rspo2 expression is restricted to the tooth germ. While Rspo3 ablation in the mouse is embryonic lethal, zebrafish rspo3-/- mutants are viable with modest decrease in Meckel’s cartilage rostral length. However, compound disruption of rspo3 and rspo2 revealed synergistic roles of these genes in cartilage morphogenesis, fin development, and pharyngeal tooth development. Adult rspo3−/− zebrafish mutants exhibit a dysmorphic cranial skeleton and decreased average tooth number. This study highlights the differential functions of Rspo2 and Rspo3 in dentocranial morphogenesis in zebrafish and in mouse.

R-spondin 3 deletion favors Erk phosphorylation to enhance Wnt signaling and bone formation

“…We have recently reported that Rspo3 is expressed in osteoprogenitors in the craniofacial complex (33) 35 in both mice and zebrafish. However, rspo3 disruption in zebrafish has only mild effects on larval 36 craniofacial cartilage skeleton (33) and in mice its haplo-insufficiency does not appear to affect 37 craniofacial development. Human GWA studies have shown that RSPO3 might be specifically involved 38 in bone metabolism due to the strong association between RSPO3 common variants and bone mineral 39 density (34,35,(37)(38)(39).…”

“…Although all Rspos are expressed in bone during development, they appear to have specific functions and their deletion in mice and human leads to different phenotypes (10,13,(24)(25)(26)(27)(28)(29)(30)(31). Within the Rspo family, Rspo3 is of particular interest for bone because it is highly expressed in skeletal elements during mouse development (32,33) and several human GWA studies have shown that RSPO3 SNPs are strongly associated with bone mineral density and risk of fracture (34)(35)(36)(37)(38)(39). Not surprisingly, in vitro studies have shown that overexpression of, or treatment with, Rspos can enhance Wnt-ligand mediated osteoblast (OB) differentiation (10,28,40).…”

R-Spondin 3 Regulates Mammalian Dental and Craniofacial Development

“…A recent paper has shown that rspo2 and rspo3 have overlapping activities in craniofacial development in fish [22]. To uncover the roles of Rspo2 and Rspo3 in mice that were potentially masked by an overlap in their functions, we inactivated both genes in the craniofacial mesenchyme in Wnt1-Cre;Rspo2 −/− ;Rspo3 fl/− mutants (Rspo2 KO ;Rspo3 NCKO ).…”

“…The current study also provides the first description of the phenotypic consequences from the simultaneous loss of Rspo2 and Rspo3 during craniofacial development in mammals. A recent publication showed that Rspo2 and Rspo3 synergistically regulated craniofacial and dental development in fish [22]. However, they found significant differences between fish and mice in the expression patterns of the two Rspo genes, which made it important to examine mouse mutants.…”

“…However, one study noted that Rspo2 mutants showed no significant anomalies in the molars and incisors forming at their normal positions [20], and there has not been any other report on a dental phenotype of mouse Rspo mutants. A recent paper showed that rspo2 and rspo3 regulated tooth numbers in fish [22], but they also highlighted significant differences between fish and mice in Rspo expression patterns. Therefore, it remains unknown whether Rspo genes are required for tooth development in mammals.…”