Six2 exhibits a temporal-spatial expression profile in the developing mouse palate and impacts cell proliferation during murine palatogenesis

Abstract

Cleft palate is one of the most common congenital malformations in humans which occurs at a frequency of approximately 1:700 live births worldwide. Sine Oculis-related homeobox 2 (Six2) is a member of the vertebrate Six gene family that encode proteins that are transcription factors. Six2 has been reported to be a downstream target of Homeobox a2 (Hoxa2), a gene that plays a direct a role in mouse secondary palate (SP) development. In my thesis, I utilized quantitative real time Polymerase Chain Reaction (qPCR), Western blot analysis and fluorescence immunohistochemisrty (IHC) to characterize the spatial and temporal distribution patterns of Six2 in the developing SP. Additionally, I also employed in vivo cell counting analysis and in vitro cell proliferation assays to investigate the role of Six2 during palate mesenchymal cell proliferation. My study examined the temporal and spatial distribution of Six2 in the developing mouse palatal mesenchyme and epithelia in both wild-type and Hoxa2 null mice. Six2 was expressed throughout the period of embryonic palatogenesis, with the highest levels of Six2 mRNA and protein observed in palatal shelves at E13.5 in both wild-type and Hoxa2 null mice. Six2 protein expression at all stages of SP development (E12.5 to E15.5) increased in the anterior to posterior (A-P) direction with highest expression in the posterior regions of the developing SP. In addition, expression of Six2 protein was higher in the oral half of the palatal mesenchyme compared to the nasal half of the palatal mesenchyme. Interestingly, Six2 protein was expressed in the nasal palatal epithelium but was completely absent from the oral palatal epithelium. Loss of the Hoxa2 gene induced up regulation of Six2 protein and mRNA in the developing palate across all stages of palatogenesis. In the Hoxa2 null mice, there was a significant increase in cell proliferation (Ki-67 positive cells) and the percentage of actively proliferating cells that were co-expressing Six2 protein (Six2/Ki-67 double positive cells) along both the A-P and oral-nasal (O-N) axes of the developing SP. Also, the highest percentage of actively proliferating cells and Six2/Ki-67 double positive cells was observed in the nasal half of the posterior palatal mesenchyme. Furthermore, Six2 siRNA knock down in mouse embryonic palatal mesenchyme (MEPM) cell cultures restored cell proliferation and Cyclin D1 expression in the Hoxa2 null cell cultures to wild-type levels. Collectively, my data reveals a novel spatial and temporal expression profile for Six2 in the developing mouse SP and the potential role it might play during the epithelial-mesenchymal cross talk that drives palatal shelf cell proliferation and out growth.