Connexin 43 contributes to phenotypic robustness of the mouse skull

Abstract

Background We compared skull shape and variation among genetically modified mice that exhibit different levels of connexin43 (Cx43) channel function, to determine whether Cx43 contributes to craniofacial phenotypic robustness. Specifically, we used two heterozygous mutant mouse models (G60S/+ and I130T/+) that, when compared to their wildtype counterparts, have an ~80% and ~50% reduction in Cx43 function, respectively. Results Both mutant strains showed significant differences in skull shape compared to wildtype littermates and while these differences were more severe in the G60S/+ mouse, shape differences were localized to similar regions of the skull in both mutants. However, increased skull shape variation was observed in G60S/+ mutants only. Additionally, covariation of skull structures was disrupted in the G60S/+ mutants only, indicating that while a 50% reduction in Cx43 function is sufficient to cause a shift in mean skull shape, the threshold for Cx43 function for disrupting craniofacial phenotypic robustness is lower. Conclusions Collectively, our results indicate Cx43 can contribute to phenotypic robustness of the skull through a nonlinear relationship between Cx43 gap junctional function and phenotypic outcomes.

Discordant growth of nasal cartilage and bone contributes to phenotypic variability of the skull in a mouse model

“…To determine the tissue‐level developmental processes that link Cx43 function with nasal bone phenotypic variability, we compared development of nasal structures between G60S/+ mutant mice and their WT littermates from postnatal day 0 (P0) to 3 months. Our previous findings demonstrated that the nasal bone phenotype is not present at P0, but that many G60S/+ mice display deviated nasal bones by 3 months 4 . Given the postnatal timing of this aberrant development, and the well‐established role of Cx43 in bone remodeling, 21,22,24 we compared osteoblast and osteoclast numbers at the cortical surfaces of the nasal bones, as well as measures of nasal cortical bone formation using dynamic histology between G60S/+ and WT mice.…”

“…We found that while mean skull shape for both I130T/+ and G60S/+ mutant mice was different from their wild‐type (WT) littermates, increased variation of skull phenotype was observed only in the G60S/+ mice. These results indicated that skull shape variation is robust at 50% Cx43 channel function reduction, but is disrupted when channel function is reduced to 20%, demonstrating a threshold effect of Cx43 function on phenotypic variability of the skull 4 . The greatest shape variation in the G60S/+ mice was localized to the bones of the face, and this variation became more marked by 3 months of age compared to neonate mice.…”

“…For structures that are developmentally complex such as the skull and craniofacial region, constructing this map is extremely challenging. Several recent studies have demonstrated a nonlinear relationship between genotype and craniofacial phenotype, such that large differences in gene dosage result in negligible phenotypic differences until some minimal threshold is surpassed 1‐4 . These studies have provided important insights for evolutionary biology, by describing a mechanism that allows sufficient genetic variation to enable evolutionary change while minimizing maladaptive outcomes 5 .…”

“…Importantly, this deviated nasal bone phenotype was observed in only a subset of the G60S/+ mice, the severity of the deviation was variable, and it was present in 3-month mice but not neonates. 4 The role of Cx43 in bone development and remodeling is well established. [11][12][13] Clinically, altered Cx43 function through mutation manifests as bone disorders including oculodentodigital dysplasia (ODDD) and craniometaphysial dysplasia which largely affect bones of the skull.…”

“…The G60S/+ mutant is one of three mutant mouse models developed to phenocopy ODDD and accordingly presents with skull deformities such as depressed nasal bridge and lateral displacement of the frontal and occipital bones characteristic of the human disorder. 4,8,17,18 These mice have delayed ossification of the skull bones and this delay has been linked to abnormal osteoblast function. 9 Another consistent phenotype reported for mouse models of reduced Cx43, including G60S/+ mice, is tubular long bones, characterized by increased diaphyseal total cross-sectional area, increased cross-sectional marrow cavity area and thin cortical bone.…”