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Abstract
Mammalian Sphingosine kinase 2 is the primary enzyme responsible for phosphorylating FTY720 to its active form, FTY720-P. Systemic FTY720 treatment confers significant protection to murine retinas from light- and disease-mediated photoreceptor cell death. It is not clear whether FTY720-P, FTY720, or both are responsible for this photoreceptor protection. We investigated Sphingosine kinase 2 knockout (Sphk2 KO) mouse retinas, tested their sensitivity to light, and measured what degree of protection from light-induced damage they receive from systemic FTY720 treatment. Sphk2 KO retinas were found to be similar to their wild-type counterparts in sensitivity to light damage. Additionally, FTY720 treatment protected Sphk2 KO retinas from light-induced damage despite significant retardation of FTY720 phosphorylation in Sphk2 KO mice. We conclude that FTY720 serves an active role in preventing photoreceptor cell death. Furthermore, we conclude that the phosphorylation of FTY720 is not necessary to provide this protective effect.
Highlights
We have shown previously that systemic dosing of FTY720 before exposure to intense, damaging light provides significant protection to photoreceptor cells from apoptotic cell death[22,23]
We found that Cer plays a crucial role in light-induced degeneration of photoreceptor cells and were able to conclude that FTY720 prevents excess Cer formation during light exposure[22]
Retinal photoreceptor cell death by apoptosis is the hallmark of major retinal degenerative diseases such as retinitis pigmentosa (RP), and is a major component of diabetic retinopathy (DR) and age-related macular degeneration (AMD)[25,26,27]
Summary
We have shown previously that systemic dosing of FTY720 before exposure to intense, damaging light provides significant protection to photoreceptor cells from apoptotic cell death[22,23]. We have shown in our in vitro studies that reduction of Cer by overexpression of acid Ceramidase (ASAH1), a lysosomal enzyme which degrades excess Cer, in retinal pigment epithelial (RPE) cells provides significant protection from oxidative stress-induced cell death, further highlighting the role of Cer in retinal degeneration[24]. Our findings that FDA-approved FTY720 plays an active, protective role in the prevention of retinal cell death opened the door to many potential avenues for further investigation of the complex pathophysiology of human retinal degenerative diseases and potential therapy development. Our experiments provided convincing evidence that FTY720 itself can provide protection from light-induced damage to retinal cells without conversion to FTY720-P
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