Characterizing Sphingosine Kinases and Sphingosine 1-Phosphate Receptors in the Mammalian Eye and Retina

Abstract

Sphingosine 1-phosphate (S1P) signaling regulates numerous biological processes including neurogenesis, inflammation and neovascularization. However, little is known about the role of S1P signaling in the eye. In this study, we characterize two sphingosine kinases (SPHK1 and SPHK2), which phosphorylate sphingosine to S1P, and three S1P receptors (S1PR1, S1PR2 and S1PR3) in mouse and rat eyes. We evaluated sphingosine kinase and S1P receptor gene expression at the mRNA level in various rat tissues and rat retinas exposed to light-damage, whole mouse eyes, specific eye structures, and in developing retinas. Furthermore, we determined the localization of sphingosine kinases and S1P receptors in whole rat eyes by immunohistochemistry. Our results unveiled unique expression profiles for both sphingosine kinases and each receptor in ocular tissues. Furthermore, these kinases and S1P receptors are expressed in mammalian retinal cells and the expression of SPHK1, S1PR2 and S1PR3 increased immediately after light damage, which suggests a function in apoptosis and/or light stress responses in the eye. These findings have numerous implications for understanding the role of S1P signaling in the mechanisms of ocular diseases such as retinal inflammatory and degenerative diseases, neovascular eye diseases, glaucoma and corneal diseases.

Sphingolipids as Emerging Mediators in Retina Degeneration

“…SphK1 is expressed in photoreceptors whereas SphK1 and SphK2 are expressed both in ARPE19 and in human fetal RPE cells, implying these cells have the molecular machinery required for S1P synthesis (Abrahan et al, 2010; Zhu et al, 2010). A recent work has dissected the distribution of SphKs and S1PRs in mouse retina (Porter et al, 2018). SphK1 is low in most mouse ocular tissues and highest in the retina and optic nerve, whereas higher levels of SphK2 are observed in all mouse eye tissues.…”

“…S1p1 and S1p3 are expressed in the retina, while expression of S1p2 and S1p5 is minimal; S1P3 expression remains high and constant during retina development, whereas S1P1 increases gradually. Photoreceptors express both S1P1 and S1P3; S1P1 is highly expressed in RPE cells while S1P3 is localized in ganglion cells (Porter et al, 2018). Müller glial cells also show expression of SphK1 and S1P3 (Simón et al, 2015).…”

“…S1P1 contributes to survival and axonal sprout of injured retinal ganglion cells after damage to the optic nerve (Joly and Pernet, 2016) and FTY720 has neuroprotective effects in experimental glaucoma in rats (You et al, 2014). Moreover, the expression of SphK1, S1P2, and S1P3 immediately increases in light-stressed retinas, with S1PRs localizing to the pyknotic nuclei, suggesting the up-regulation of a cytoprotective S1P signaling to counteract the onset of apoptosis (Porter et al, 2018).…”

“…S1P is indispensable for establishing retina structure, regulating proliferation of retinal progenitors and their later laminar distribution in the retina (Miranda et al, 2009; Fang et al, 2018). It is also a pro-survival factor for neurons, promoting their differentiation, preserving normal functioning and granting neuroprotection (Crousillac et al, 2009; Miranda et al, 2009; Abrahan et al, 2010; Joly and Pernet, 2016; Porter et al, 2018). On the other hand, S1P unleashes threatening processes in Müller glial cells and RPE cells, such as secretion of pro- inflammatory cytokines, proliferation, migration and transdifferentiation that promote and/or enhance inflammation and fibrosis (Swaney et al, 2008; Qiao et al, 2012; Simón et al, 2015; Terao et al, 2017).…”

Light Stress-Induced Increase of Sphingosine 1-Phosphate in Photoreceptors and Its Relevance to Retinal Degeneration

“…However, we did not clarify the presence and activation dynamics of S1P in the retina. A report recently characterized the expression of SphKs and S1P receptors in rat and murine eyes, and found that the expression of SphK1, S1P2, and S1P3 in the retina is upregulated by light [15]. The findings strongly implied that S1P is associated with cell death and retinal degeneration in the mechanism of light-induced retinal degeneration.…”