SPHK1 and glaucoma: The trend of S1P receptor expression in eye tissue follows the general pattern of other tissues [63,64] such as S1pr1 > S1pr3 > S1pr2. The optic nerve has very high expression of S1pr3 and also the highest relative Sphk1 expression levels (Figure 2A,B), which could be of importance in terms of understanding the roles of S1P signaling in many forms of human optic nerve degeneration diseases and glaucoma.