Usage of sphingosine kinase isoforms in mast cells is species and/or cell type determined

FcεRI engagement in mast cells (MCs) induces the activation of two distinct sphingosine kinase isoforms (SphK1 and SphK2) to produce sphingosine-1-phosphate, a mediator essential for MC responses. Whereas embryonic-derived SphK2-null MCs showed impaired responses to Ag, RNA silencing studies on other MC types indicated a dominant role for SphK1. Given the known functional heterogeneity of MCs, we explored whether the reported differences in SphK1 or SphK2 usage could be reflective of phenotypic differences between MC populations. Using lentiviral-based short hairpin RNA to silence SphK1 or SphK2, we found that SphK2 is required for murine MC degranulation, calcium mobilization, and cytokine and leukotriene production, irrespective of the tissue from which the MC progenitors were derived, the stage of MC granule maturity, or the conditions used for differentiation. This finding was consistent with the lack of a full allergic response in SphK2-null mice challenged to undergo passive cutaneous anaphylaxis. A redundant role for both SphKs was uncovered, however, in chemotaxis toward Ag in all MC types tested and in TNF-α production in certain MC types. In contrast, human MC responses were dependent only on SphK1, associating with a more robust expression of this isoform and a more varied representation of SphK variants relative to murine MCs. The findings show that the function of SphK1 and SphK2 can be interchangeable in MCs; however, an important determinant of SphK isoform usage is the species of origin and an influencing factor, the tissue from which MCs may be derived and/or their differentiation state.