Research ArticleDevelopmental Biology

Sphingosine 1-phosphate stimulates eyelid closure in the developing rat by stimulating EGFR signaling

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Sci. Signal.  23 Oct 2018:
Vol. 11, Issue 553, eaat1470
DOI: 10.1126/scisignal.aat1470

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A signaling lipid directs eyelid morphogenesis

To prevent the eye from being damaged during birth and early postnatal development, the eyelids of many mammals migrate over the cornea and fuse during embryonic development. Failure of this process causes an eyes-open-at-birth (EOB) phenotype, which leads to permanent corneal damage. Bian et al. identified an insertional mutation in Spns2, which encodes a sphingosine 1-phosphate (S1P) transporter, in a strain of transgenic rats exhibiting the EOB phenotype. Experiments in vivo and with eyelid tissue explants in vitro showed that S1P stimulated eyelid closure by promoting the activation of proteins involved in cell migration and by stimulating epidermal growth factor receptor (EGFR) signaling. These results identify a complex network of signaling and feedback events through which S1P stimulates the metalloprotease-dependent mobilization of EGFR ligands to coordinate the migration and fusion of the eyelids during embryogenesis.

Abstract

In many mammals, the eyelids migrate over the eye and fuse during embryogenesis to protect the cornea from damage during birth and early life. Loss-of-function mutations affecting the epidermal growth factor receptor (EGFR) signaling pathway cause an eyes-open-at-birth (EOB) phenotype in rodents. We identified an insertional mutation in Spinster homolog 2 (Spns2) in a strain of transgenic rats exhibiting the EOB phenotype. Spns2, a sphingosine 1-phosphate (S1P) transporter that releases S1P from cells, was enriched at the tip of developing eyelids in wild-type rat embryos. Spns2 expression or treatment with S1P or any one of several EGFR ligands rescued the EOB Spns2 mutant phenotype in vivo and in tissue explants in vitro and rescued the formation of stress fibers in primary keratinocytes from mutants. S1P signaled through the receptors S1PR1, S1PR2, and S1PR3 to activate extracellular signal–regulated kinase (ERK) and EGFR-dependent mitogen-activated protein kinase kinase kinase 1 (MEKK1)–c-Jun signaling. S1P also induced the nuclear translocation of the transcription factor MAL in a manner dependent on EGFR signaling. MAL and c-Jun stimulated the expression of the microRNAs miR-21 and miR-222, both of which target the metalloprotease inhibitor TIMP3, thus promoting metalloprotease activity. The metalloproteases ADAM10 and ADAM17 stimulated EGFR signaling by cleaving a membrane-anchored form of EGF to release the ligand. Our results outline a network by which S1P transactivates EGFR signaling through a complex mechanism involving feedback between several intra- and extracellular molecules to promote eyelid fusion in the developing rat.

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