Roles of GASP-1 and GDF-11 in Dental and Craniofacial Development

Purpose: Growth and differentiation factor (GDF)-11 is a transforming growth factor-${\beta}$ family member that plays important regulatory roles in development of multiple tissues which include axial skeletal patterning, palatal closure, and tooth formation. Proteins that have been identified as GDF-11 inhibitors include GDF-associated serum protein (GASP)-1 and GASP-2. Recently, we found that mice genetically engineered to lack both Gasp1 and Gdf11 have an increased frequency of cleft palate. The goal of this study was to investigate the roles of GDF-11 and its inhibitors, GASP-1 and GASP-2, during dental and craniofacial development and growth. Methods: Mouse genetic studies were used in this study. Homozygous knockout mice for Gasp1 ($Gasp1^{-/-}$) and Gasp2 ($Gasp2^{-/-}$) were viable and fertile, but Gdf11 homozygous knockout ($Gdf11^{-/-}$) mice died within 24 hours after birth. The effect of either Gasp1 or Gasp2 deletion in $Gdf11^{-/-}$ mice during embryogenesis was evaluated in $Gasp1^{-/-}$;$Gdf11^{-/-}$ and $Gasp2^{-/-}$;$Gdf11^{-/-}$ mouse embryos at 18.5 days post-coitum (E18.5). For the analysis of adult tissues, we used $Gasp1^{-/-}$;$Gdf11^{+/-}$ and $Gasp2^{-/-}$;$Gdf11^{+/-}$ mice to evaluate the potential haploinsufficiency of Gdf11 in $Gasp1^{-/-}$ and $Gasp2^{-/-}$ mice. Results: Although Gasp2 expression decreased after E10.5, Gasp1 expression was readily detected in various ectodermal tissues at E17.5, including hair follicles, epithelium in nasal cavity, retina, and developing tooth buds. Interestingly, $Gasp1^{-/-}$;$Gdf11^{-/-}$ mice had abnormal formation of lower incisors: tooth buds for lower incisors were under-developed or missing. Although $Gdf11^{+/-}$ mice were viable and had mild transformations of the axial skeleton, no specific defects in the craniofacial development have been observed in $Gdf11^{+/-}$ mice. However, loss of Gasp1 in $Gdf11^{+/-}$ mice occasionally resulted in small and abnormally shaped auricles. Conclusions: These findings suggest that both GASP-1 and GDF-11 play important roles in dental and craniofacial development both during embryogenesis and in adult tissues.