Unraveling Sphingolipid Signaling in the Retina, one gene at a time- the role of Sphingosine kinase 2

Abstract

Purpose : Sphingolipids (SPL) are essential components of every cell membrane. Recently, a signaling role of bioactive sphingolipids such as ceramide (Cer) and sphingosine 1-phosphate (S1P) has been established. Cer induces apoptosis and S1P plays an anti-apoptotic role. S1P signals through G-protein coupled receptors. In mammalian cells, S1P is produced from sphingosine (Sph) by two kinases, sphingosine kinase 1 (SPHK1) and 2 (SPHK2). There is little or no information on the role of SPHKs in mammalian retina. Here we characterized the retina of Sphk2^-/- knock-out mice structurally and functionally. We also subjected Sphk2^-/- mice to light-induced retinal degeneration (LIRD) after FTY720 treatment to determine its mechanism of protection.

Methods : We analyzed eyes of both albino and pigmented Sphk2^-/- mice by electroretinography (ERG), OCT (optical coherence tomography), OKT (optokinetic tracking), histology and by electron microscopy at different age points up to 15 months. We subjected albino mice to LIRD at 500 lux for 10 h with and without FTY720 and analyzed their retina by histology, and ERG after 7 days. We determined SPL profile including FTY720 and FTY720-P in retina, plasma, and liver with and without FTY720 treatment in Sphk2^-/- mice. Littermate wild type mice in similar treatments/ conditions served as controls for each respective experiment.

Results : We detected no difference in retinal structure and function in Sphk2^-/- mice compared to their wild type littermates. However, OKT indicates measurable decrease in visual acuity. Albino Sphk2^-/- mice appear to be slightly resistant to LIRD. FTY720 is an analog of Sph and get phosphorylated in liver by SPHK2 to FTY720-P, which acts as S1P mimic and modulates S1P signaling. Sphk2^-/- mice plasma contains three-fold higher levels of S1P; significantly reduces FTY720 conversion to FTY720-P. Systemic FTY720 treatment immediately before LIRD protects both albino wild type retina and Sphk2^-/- mice to similar extent.

Conclusions : This first characterization of the retina of Sphk2^-/- mice suggest that there is a role of S1P in visual function of the retina. We previously showed that elevated Cer induces photoreceptor cell death and FTY720 prevents retinal degeneration by inhibiting Cer biosynthesis. This study provides further evidence that FTY720 mediated Cer modulation prevents retinal degeneration not S1P signaling modulation by FTY720-P.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.