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The brain capillary endothelial cell (BCEC) is a major functional component of the blood-brain barrier and is an underlying factor in the pathophysiology of various diseases, including brain ischemia, multiple sclerosis, and neurodegenerative disorders. We examined gene silencing in BCECs by using endogenous lipoprotein to introduce short-interfering RNA (siRNA) in vivo. A cholesterol-conjugated 21/23-mer siRNA targeting organic anion transporter 3 (OAT3) mRNA (Chol-siOAT3) was intravenously injected into mice after its incorporation into extracted endogenous lipoproteins. Chol-siOAT3 was not delivered to neurons or glia, but was successfully delivered into BCECs and resulted in a significant reduction of OAT3 mRNA levels when injected after its incorporation into high-density lipoprotein (HDL). Efficient delivery was not achieved, however, when Chol-siOAT3 was injected without any lipoproteins, or after its incorporation into low-density lipoprotein (LDL). Investigations in apolipoprotein E (ApoE)-deficient and LDL receptor (LDLR)-deficient mice revealed that the uptake of HDL-containing Chol-siOAT3 was mainly mediated by ApoE and LDLR in mice. These findings indicate that siRNA can be delivered into BCECs in vivo by using endogenous lipoprotein, which could make this strategy useful as a new gene silencing therapy for diseases involving BCECs. [ABSTRACT FROM AUTHOR]