Ceramide-1-phosphate promotes the migration of retina Müller glial cells.

Müller glial cells, the major glial cell type in the retina, are activated by most retina injuries, leading to an increased proliferation and migration that contributes to visual dysfunction. The molecular cues involved in these processes are still ill defined. We demonstrated that sphingosine-1-phosphate (S1P), a bioactive sphingolipid, promotes glial migration. We now investigated whether ceramide-1-phosphate (C1P), also a bioactive sphingolipid, was involved in Müller glial cell migration. We evaluated cell migration in primary Müller glial cultures, prepared from newborn rat retinas, by the scratch wound assay. Addition of either 10 µM C8-ceramide-1-phosphate (C8-C1P) or 5 µM C16-C1P (a long chain, natural C1P) stimulated glial migration. Inhibiting PI3K almost completely blocked C8-C1P-elicited migration whereas inhibition of ERK1-2/MAPK pathway diminished it and p38MAPK inhibition did not affect it. Pre-treatment with a cytoplasmic phospholipase A2 (cPLA2) inhibitor markedly reduced C8-C1P-induced migration. Inhibiting ceramide kinase (CerK), the enzyme catalyzing C1P synthesis, partially decreased glial migration. Combined addition of S1P and C8-C1P promoted glial migration to the same extent as when they were added separately, suggesting they converge on their downstream signaling to stimulate Müller glia migration. These results suggest that C1P addition stimulated migration of glial Müller cells, promoting the activation of cPLA2, and the PI3K and ERK/MAPK pathways. They also suggest that CerK-dependent C1P synthesis was one of the factors contributing to glial migration, thus uncovering a novel role for C1P in controlling glial motility.

Sphingolipids as Emerging Mediators in Retina Degeneration.

The sphingolipids ceramide (Cer), sphingosine-1-phosphate (S1P), sphingosine (Sph), and ceramide-1-phosphate (C1P) are key signaling molecules that regulate major cellular functions. Their roles in the retina have gained increasing attention during the last decade since they emerge as mediators of proliferation, survival, migration, neovascularization, inflammation and death in retina cells. As exacerbation of these processes is central to retina degenerative diseases, they appear as crucial players in their progression. This review analyzes the functions of these sphingolipids in retina cell types and their possible pathological roles. Cer appears as a key arbitrator in diverse retinal pathologies; it promotes inflammation in endothelial and retina pigment epithelium (RPE) cells and its increase is a common feature in photoreceptor death in vitro and in animal models of retina degeneration; noteworthy, inhibiting Cer synthesis preserves photoreceptor viability and functionality. In turn, S1P acts as a double edge sword in the retina. It is essential for retina development, promoting the survival of photoreceptors and ganglion cells and regulating proliferation and differentiation of photoreceptor progenitors. However, S1P has also deleterious effects, stimulating migration of Müller glial cells, angiogenesis and fibrosis, contributing to the inflammatory scenario of proliferative retinopathies and age related macular degeneration (AMD). C1P, as S1P, promotes photoreceptor survival and differentiation. Collectively, the expanding role for these sphingolipids in the regulation of critical processes in retina cell types and in their dysregulation in retina degenerations makes them attractive targets for treating these diseases.

Ceramide Induces the Death of Retina Photoreceptors Through Activation of Parthanatos.

Ceramide (Cer) has a key role inducing cell death and has been proposed as a messenger in photoreceptor cell death in the retina. Here, we explored the pathways induced by C2-acetylsphingosine (C2-Cer), a cell-permeable Cer, to elicit photoreceptor death. Treating pure retina neuronal cultures with 10 µM C2-Cer for 6 h selectively induced photoreceptor death, decreasing mitochondrial membrane potential and increasing the formation of reactive oxygen species (ROS). In contrast, amacrine neurons preserved their viability. Noteworthy, the amount of TUNEL-labeled cells and photoreceptors expressing cleaved caspase-3 remained constant and pretreatment with a pan-caspase inhibitor did not prevent C2-Cer-induced death. C2-Cer provoked polyADP ribosyl polymerase-1 (PARP-1) overactivation. Inhibiting PARP-1 decreased C2-Cer-induced photoreceptor death; C2-Cer increased polyADP ribose polymer (PAR) levels and induced the translocation of apoptosis inducing factor (AIF) from mitochondria to photoreceptor nuclei, which was prevented by PARP-1 inhibition. Pretreatment with a calpain and cathepsin inhibitor and with a calpain inhibitor reduced photoreceptor death, whereas selective cathepsin inhibitors granted no protection. Combined pretreatment with a PARP-1 and a calpain inhibitor evidenced the same protection as each inhibitor by itself. Neither autophagy nor necroptosis was involved in C2-Cer-elicited death; no increase in LDH release was observed upon C2-Cer treatment and pretreatment with inhibitors of necroptosis and autophagy did not rescue photoreceptors. These results suggest that C2-Cer induced photoreceptor death by a novel, caspase-independent mechanism, involving activation of PARP-1, decline of mitochondrial membrane potential, calpain activation, and AIF translocation, all of which are biochemical features of parthanatos.