Sall1 plays pivotal roles for lens fiber cell differentiation in mouse

Mammals possess four Sall transcription factors that play various roles in organogenesis. Previously, we found that Sall1 is expressed in microglia in the central nervous system, and it plays pivotal roles in microglia maturation. In the eye, Sall1 was also expressed in the developing lens, and we examined its role in lens development. A knock-in mouse harboring the EGFP gene in the Sall1 locus (Sall1-gfp) was used to analyze the Sall1 expression pattern. In Sall1-gfp/wild, EGFP was expressed throughout the presumptive lens at E11.5, and subsequently the expression in the lens epithelium became weaker. After birth, signals were observed in the equator region. The effects of Sall1 knockout on lens development were examined in Sall1-gfp/gfp. Lens sections revealed small vacuole-like holes and gaps in the center of the lens fibers at E14.5. Subsequently, the vacuoles appeared in most regions of the fiber cells. Electron microscopic analysis indicated that the vacuoles were between the fiber cells, leading to huge gaps. In addition, contact between the lens epithelium and apical end of the fiber cell was disrupted, and there were gaps between the adjoining lens epithelial cells. However, gap junction structure was observed by electron microscopic analysis, and immunostaining of Zo1 showed rather appropriate expression pattern. Immunohistochemistry indicated that the major lens transcription factors Prox1 and Pax6 were expressed in relatively normal patterns. However, although the expression of Prox1 and Pax6 decreased in nuclei in the control lens, it remained in Sall1-gfp/gfp. In addition, lower expression level of c-Maf protein was observed. Therefore, Sall1 is strongly expressed in the lens from the early developmental stage and plays an essential role in the maintenance of fiber cell and lens epithelium adhesion.