Cathepsin K Knockout Exacerbates Haemorrhagic Transformation Induced by Recombinant Tissue Plasminogen Activator after Focal Cerebral Ischaemia in Mice

Severe haemorrhagic transformation (HT), a common complication of recombinant tissue plasminogen activator (rtPA) treatment, predicts poor clinical outcomes in acute ischaemic stroke. The search for agents to mitigate this effect includes investigating biomolecules involved in neovascularization. This study examines the role of Cathepsin K (Ctsk) in rtPA-induced HT after focal cerebral ischaemia in mice. After knockout of Ctsk, the gene encoding Ctsk, the outcomes of Ctsk+/+ and Ctsk−/− mice were compared 24 h after rtPA-treated cerebral ischaemia with respect to HT severity, neurological deficits, brain oedema, infarct volume, number of apoptotic neurons and activated microglia/macrophage, blood–brain barrier integrity, vascular endothelial growth factor (VEGF) expression and Akt-mTOR pathway activation. We observed that haemoglobin levels, brain oedema and infarct volume were significantly greater and resulted in more severe neurological deficits in Ctsk−/− than in Ctsk+/+ mice. Consistent with our hypothesis, the number of NeuN-positive neurons was lower and the number of TUNEL-positive apoptotic neurons and activated microglia/macrophage was higher in Ctsk−/− than in Ctsk+/+ mice. Ctsk knockout mice exhibited more severe blood–brain barrier (BBB) disruption, with microvascular endothelial cells exhibiting greater VEGF expression and lower ratios of phospo-Akt/Akt and phospo-mTOR/mTOR than in Ctsk+/+ mice. This study is the first to provide molecular insights into Ctsk-regulated HT after cerebral ischaemia, suggesting that Ctsk deficiency may disrupt the BBB via Akt/mTOR/VEGF signalling, resulting in neurological deficits and neuron apoptosis. Ctsk administration has the potential as a novel modality for improving the safety of rtPA treatment following stroke.