Question:

Mast cells release histamine and further mediators of IgE-mediated allergic responses in the process of degranulation, which is triggered by the store operated Ca²⁺ (SOC) entry. Exogenously added ceramide has been shown to trigger mast cell apoptosis. However, effects of endogenously produced ceramide in mast cells remain undefined. Conversion of sphingomyelin to ceramide can be mediated by acid sphingomyelinase (aSM). The present study explored the impact of aSM on mast cell functions in vitro and in vivo.

Methods/Results:

Mast cells were isolated and cultured from bone marrow (BMMCs) of gene targeted mice lacking aSM ( asm^-/- ) and their wild-type littermates ( asm^+/+ ). Release of Ca²⁺ from internal stores and hence several Ca²⁺-dependent functions were strongly impaired in asm^-/- BMMCs. Thus, antigen-induced increase of [Ca²⁺]_i in IgE-sensitized cells was lower in asm^-/- BMMCs if compared to asm^+/+ BMMCs. Currents through Ca²⁺-activated K⁺ channels K_Ca3.1 induced by antigen but not ionomycin were also reduced in asm^-/- BMMCs. IgE/antigen- triggered degranulation, measured as b-hexosaminidase release, and antigen-induced mast cell migration were significantly decreased in asm^-/- BMMCs. Moreover, the observed reduction of antigen-induced increase of [Ca²⁺]_i, KCa3.1 currents, ß-hexosaminidase release and migration in asm^-/- BMMCs could be mimicked by pharmacological inhibition of aSM by amitriptylin (500 nM, 3h) in asm^+/+ BMMCs. Finally, aSM deficiency affects mast cell functions in vivo, as demonstrated by a significantly impaired decrease in body temperature upon induction of systemic anaphylaxis in asm^-/- if compared to asm^+/+ mice.

Conclusions:

Therefore, aSM is a novel important regulator of mast cell functions.