Abstract
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.
Highlights
Acid sphingomyelinase (Asm), which catalyzes the hydrolysis of sphingomyelin to ceramide, is a key enzyme in sphingolipid metabolism
Based on the observation that brain CD45 + leukocyte infiltration was increased by sphingomyelinase phosphodiesterase-1 (Smpd1)−/−, we hypothesized that intercellular adhesion molecule-1 (ICAM-1) abundance was elevated on cerebral microvessels of Smpd1−/− compared with Smpd1+/+ mice
polymorphonuclear neutrophils (PMN) depletion by antiLy6G antibody did not influence infarct size, brain swelling or brain CD45 + leukocyte infiltrates in Smpd1+/+ mice exposed to 30 min middle cerebral artery occlusion (MCAO) followed by 72 h reperfusion, but reversed the exacerbated infarct size and brain swelling (Fig. 6b, c) and reduced CD45 + leukocyte infiltrates by 61 ± 21% (Fig. 6d) in Smpd1−/− mice
Summary
Acid sphingomyelinase (Asm), which catalyzes the hydrolysis of sphingomyelin to ceramide, is a key enzyme in sphingolipid metabolism. Asm is translocated to the plasma membrane within a few seconds to minutes upon stress stimuli [1,2,3]. Ceramide clusters into membrane platforms that critically control the activity of signal pathways modifying cell survival and death [1, 2, 4, 5]. Several stress stimuli activating Asm and inducing ceramide-rich membrane domains were identified, including γ-irradiation, UV light exposure, ischemia/reperfusion (I/R), bacterial infection and chemotherapy [6,7,8,9,10,11]. Asm-dependent ceramide signaling was shown to control T cell CD3 signaling [12, 13], regulate CD4+ T cell activation and proliferation [12, 13], promote
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