Shedding Light on Lysosomal Malondialdehyde Affecting Vitamin B12 Transport during Cerebral Ischemia/Reperfusion Injury

Cerebral ischemia-reperfusion injury (CIRI) is often accompanied by upregulation of homocysteine (Hcy). Excessive Hcy damages cerebral vascular endothelial cells and neurons, inducing neurotoxicity and even neurodegeneration. Normally, supplementation of vitamin B₁₂ is an ideal intervention to reduce Hcy. However, vitamin B₁₂ therapy is clinically inefficacious for CIRI. Considering oxidative stress is closely related to CIRI, the lysosome is the pivotal site for vitamin B₁₂ transport. Lysosomal oxidative stress might hinder the transport of vitamin B₁₂. Whether lysosomal malondialdehyde (lysosomal MDA), as the authoritative biomarker of lysosomal oxidative stress, interferes with the transport of vitamin B₁₂ has not been elucidated. This is ascribed to the absence of effective methods for real-time and in situ measurement of lysosomal MDA within living brains. Herein, a fluorescence imaging agent, Lyso-MCBH, was constructed to specifically monitor lysosomal MDA by entering the brain and targeting the lysosome. Erupting the lysosomal MDA level in living brains of mice under CIRI was first observed using Lyso-MCBH. Excessive lysosomal MDA was found to affect the efficacy of vitamin B₁₂ by blocking the transport of vitamin B₁₂ from the lysosome to the cytoplasm. More importantly, the expression and function of the vitamin B₁₂ transporter LMBD1 were proved to be associated with excessive lysosomal MDA. Altogether, the revealing of the lysosomal MDA-LMBD1 axis provides a cogent interpretation of the inefficacy of vitamin B₁₂ in CIRI, which could be a prospective therapeutic target.