Lacrimal Gland Development is Controlled by the FGFR-ERK Signaling Activity

Abstract

Purpose: : Lacrimal Gland (LG) is responsible for secretion of the aqueous layer of the tear film which is essential for the health and function of the ocular surface. Previous studies showed that FGFR activation is required for LG induction and development, but the downstream signaling mechanism is unclear. In this study, we investigated the role of FGFR- and ERK-signaling activity in LG development.

Methods: : Le-Cre transgenic mice were used to delete Fgfr2 or Erk2 in the ocular surface ectodermal tissues. The conditional deletion mice carrying the following alleles were generated, LeCre;Fgfr2^flox/+, LeCre;Fgfr2^flox/flox, LeCre;Erk2^flox/flox, and Erk1^-/-;LeCre;Erk2^flox/flox , and were referred as Fgfr2^+/-, Fgfr2^CKO, Erk2^CKO , and Erk1/2^DKO respectively. LG development were monitored by GFP imaging and carmine staining. Cell proliferation and cell death were examined by phospho-histone H3 immunostaining and TUNEL assay respectively.

Results: : Normal LG budding occurs at the conjunctival fornix at embryonic day 14.5 (E14.5) in the wild type (WT), Fgfr2^+/-, and Erk2^CKO mice, but not in Fgfr2^CKO and Erk1/2^DKO mice. It was observed that, in the Fgfr2^CKO and Erk1/2^DKO mice, cell proliferation was severely decreased in the conjunctival fornix at E14.5 but cell survival did not seem to be affected. This result suggests that activation of FGFR-ERK pathway is essential for cell proliferation at the conjunctival fornix and is required for LG induction. During normal development, the LG buds elongate and then branch after E16.5. In the Fgfr2^+/- and Erk2^CKO mice, LG buds elongate normally but the number of LG branches was markedly reduced when compared to the WT mice. Further analysis showed that apoptosis was significantly increased in the LGs of these mutant mice. These results suggest that the level of FGFR- and ERK-signaling activity controls the extent of branching morphogenesis.

Conclusions: : We demonstrate that deletion of Fgfr2 or Erk1/2 blocks LG induction and deletion of one Fgfr2 allele or Erk2 results in a reduction in LG branching morphogenesis. Overall our data imply that different levels of FGFR-ERK signaling activity are required for LG morphogenesis and patterning during normal development.

Keywords: lacrimal gland • development • signal transduction