The diagnosis of limbal or corneal stem cell deficiency is often based on the expression of conjunctival keratins and the presence of goblet cells on the cornea. It has been assumed that goblet cells migrate onto the cornea from the neighboring bulbar conjunctiva. Our data show that the mouse limbus has both conventional stem cell niches and compound niches. Each BALB/c mouse cornea has an average of 29 compound niches composed of an average of 9 K8+ cells. The cells within these compound niches are in one of several differentiation states: K8+slow-cycling corneal progenitor cells, K8+K12+ cells which can proliferate, and K8+ post-mitotic differentiated goblet cells expressing Muc5ac. During homeostasis and after small debridement wounds, conventional and compound niche progenitor cells generate progeny that differentiate primarily into corneal epithelial cells. However, after corneal wounds that remove more than 70% of the corneal epithelium, many of the compound niches at the limbus migrate from the limbal:corneal border onto the cornea. Goblet cells from within the conjunctiva are not observed migrating onto the cornea. Compound niches express the Hh signaling protein patched 1 and the activated Wnt signaling protein phosphorylated ??catenin suggesting that Hh and Wnt signaling may be involved in the maintenance and migration of compound niches in response to injury. Once the K8+ progenitor cells have migrated onto the cornea, they proliferate and reform goblet cell clusters containing non-proliferating goblet cells. The studies proposed investigate the hypothesis that corneal stem cell deficiency in the mouse actually reflects excess proliferation of K8+ corneal epithelial progenitor cells. The specific hypothesis we will be testing is whether Hedgehog (Hh) and Wnt signaling regulate the formation, maintenance, and expansion of the cells comprising the compound niche during homeostatsis and progression of goblet cell hyperplasia.
Our Aims i n this exploratory grant application are to determine the roles played by the Hh and Wnt signaling pathways in the formation and maintenance of compound niches in BALB/c, TOPGAL and BATGAL mice. We will use pathway specific agonists and antagonists and Hh and Wnt pathway PCR arrays to determine whether altering Hh or Wnt signaling can reduce the differentiation of goblet cells on the cornea. The insight we gain through the research proposed will provide the basis for targeted studies investigating the signaling pathways that control the differentiation of goblet cells on the cornea from progenitor cells located at the limbus. Data from these studies will improve our ability to target treatments that aim to reduce the proliferation of goblet cells n the ocular surface.
Goblet cells are present on the cornea in mouse models of corneal epithelial stem cell deficiency. Our data indicate that the goblet cells on the mouse cornea 4 weeks after 2.5 mm debridement wounds are derived from cells normally found at the limbus. The proposed studies will identify the signaling pathways that control the migration of progenitor cells from the limbus onto the cornea and enhance their differentiation into goblet cells.
|Stepp, Mary Ann; Zieske, James D; Trinkaus-Randall, Vickery et al. (2014) Wounding the cornea to learn how it heals. Exp Eye Res 121:178-93|