By cleaving sphingomyelin into ceramide, which is an essential component of plasma membrane microdomains, acid sphingomyelinase (Asm) pivotally controls cell signaling. To define how the activation of the Asm/ceramide pathway, which occurs within seconds to minutes upon stress stimuli, influences brain ischemia/reperfusion (I/R) injury, we exposed male and female wildtype mice carrying both alleles of Asm's gene sphingomyelinase phosphodiesterase-1 (Smpd1+/+), heterozygously Asm-deficient mice (Smpd1+/-) and homozygously Asm-deficient mice (Smpd1-/-) of different age (8, 12 or 16 weeks) to 30, 60 or 90 min intraluminal middle cerebral artery occlusion (MCAO). For studying the contribution of brain-invading polymorphonuclear neutrophils (PMN) to I/R injury, PMNs were depleted by delivery of a PMN-specific Ly6G antibody. In male and female mice exposed to 30 min, but not 60 or 90 min MCAO, homozygous Smpd1-/- consistently increased I/R injury, blood-brain barrier permeability and brain leukocyte and PMN infiltration, whereas heterozygous Smpd1+/- reduced I/R injury. Increased abundance of the intercellular leukocyte adhesion molecule ICAM-1 was noted on cerebral microvessels of Smpd1-/- mice. PMN depletion by anti-Ly6G delivery prevented the exacerbation of I/R injury in Smpd1-/- compared with wildtype mice and reduced brain leukocyte infiltrates. Our results show that Asm tempers leukocyte entry into the reperfused ischemic brain, thereby attenuating I/R injury.
Keywords: Ceramide-rich membrane microdomain; Focal cerebral ischemia; Inflammation; Intercellular adhesion molecule-1; Leukocyte; Lipid raft; Middle cerebral artery occlusion; Sphingolipid.