Monocytes release pro-cathepsin D to drive blood-to-brain transcytosis in diabetes

Microvascular complications are the major outcome of type 2 diabetes mellitus progression and the underlying mechanism remains to be determined. To explore the contribution of circulating monocytes to the microvascular lesions, high-throughput RNA sequencing was conducted and we found high-level expression of cathepsin D (CTSD) in the monocytes of type 2 diabetes mellitus patients. The transgenic mice expressing human CTSD in the monocytes showed increased brain microvascular permeability resembling the diabetic microvascular phenotype, accompanied with cognitive deficit. Mechanistically, we found the monocytes release non-enzymatic pro-CTSD to upregulate caveolin expression in brain endothelium triggering caveolae-mediated transcytosis, without affecting the paracellular route of brain microvasculature. The circulating pro-CTSD activated the caveolae-mediated transcytosis in brain endothelial cells via its binding with low density lipoprotein receptor-related protein 1 (LRP1). Importantly, genetic ablation of CTSD in the monocytes exhibited protective effect against the diabetes-enhanced brain microvascular transcytosis and the diabetes-induced cognitive impairment. Our findings thus uncover the novel role of circulatory pro-CTSD from monocytes in the pathogenesis of cerebral microvascular lesions in diabetes. The circulatory pro-CTSD is thus a potential target for the intervention of microvascular complications in diabetes. Overall design: To evaluate the transcriptional changes in the circulating monocytes of T2DM patients compared with healthy controls