NAD replenishment with nicotinamide mononucleotide protects blood–brain barrier integrity and attenuates delayed tissue plasminogen activator‐induced haemorrhagic transformation after cerebral ischaemia

Background and Purpose Tissue plasminogen activator (tPA) is the only approved pharmacological therapy for acute brain ischaemia; however, a major limitation of tPA is the haemorrhagic transformation that follows tPA treatment. Here, we determined whether nicotinamide mononucleotide (NMN), a key intermediate of nicotinamide adenine dinucleotide biosynthesis, affects tPA-induced haemorrhagic transformation. Experimental Approach Middle cerebral artery occlusion (MCAO) was achieved in CD1 mice by introducing a filament to the left MCA for 5 h. When the filament was removed for reperfusion, tPA was infused via the tail vein. A single dose of NMN was injected i.p. (300 mg·kg−1). Mice were killed at 24 h post ischaemia, and their brains were evaluated for brain infarction, oedema, haemoglobin content, apoptosis, neuroinflammation, blood–brain barrier (BBB) permeability, the expression of tight junction proteins (TJPs) and the activity/expression of MMPs. Key Results In the mice infused with tPA at 5 h post ischaemia, there were significant increases in mortality, brain infarction, brain oedema, brain haemoglobin level, neural apoptosis, Iba-1 staining (microglia activation) and myeloperoxidase staining (neutrophil infiltration). All these tPA-induced alterations were significantly prevented by NMN administration. Mechanistically, the delayed tPA treatment increased BBB permeability by down-regulating TJPs, including claudin-1, occludin and zonula occludens-1, and enhancing the activities and protein expression of MMP9 and MMP2. Similarly, NMN administration partly blocked these tPA-induced molecular changes. Conclusions and Implications Our results demonstrate that NMN ameliorates tPA-induced haemorrhagic transformation in brain ischaemia by maintaining the integrity of the BBB.