Endothelial-pericyte interactions activate insulin signaling and its implications for blood-brain barrier dysfunction in Alzheimer's disease.

Purpose: This study aimed to investigate how pericyte degeneration contributes to BBB disruption in Alzheimer's disease, focusing on the roles of insulin signaling and the imbalance between matrix metalloproteinases (MMPs) and endogenous tissue inhibitors of MMPs (TIMPs). Methods: We employed an in vitro BBB model by co-culturing brain-specific microvascular endothelial-like cells (iBMECs) differentiated from human induced pluripotent stem cells (hiPSCs) and primary human brain vasculature pericytes (hBVPs). Protein expression under solo- versus co-culture conditions was assessed by western blot. MMP enzymatic activity in the culture media was measured by fluorometric assay. Exosomes were isolated from conditioned media and brain derived neurotrophic growth factor (BDNF) concentrations were determined using ELISA assays. Results: TIMP1 and collagen-IV expression was significantly increased in co-cultured BBB endothelial cells and pericytes compared to solo-cultures. However, a greater effect was observed in cells co-cultured for 2 days than 7 days. Elevated TIMP1 in co-culture media significantly inhibited MMP activity. The AKT and ERK pathways were activated in both cell types after 7 days of co-culture, and the ERK signaling mediated TIMP1 upregulation in endothelial cells. BDNF was significantly enriched in exosomes isolated from co-culture media on the abluminal side compared to the solo-cultures. Endothelial cells also protected pericytes from accumulation of toxic amyloid-beta 42 by downregulating low density lipoprotein receptor-related protein 1 (LRP1) expression. Conclusions: These findings provide mechanistic insights into BBB disruption due to pericyte degeneration and highlight the important role of BBB insulin resistance in causing cerebrovascular dysfunction in AD.