Introduction: Stroke is a leading cause of mortality and morbidity. Several drugs with neuroprotective properties have been proposed for stroke treatment but many have failed in clinical trials. These failures may be due to limited drug permeability across the blood-brain barrier (BBB). Targeting endogenous BBB uptake transporters (i.e., organic anion transporting polypeptides (Oatps)) is a novel approach that can improve CNS delivery of drugs relevant to stroke therapy (i.e., statins). Optimal CNS drug delivery via Oatp in the setting of stroke requires characterization of regulatory pathways such as transforming growth factor (TGF)-β signaling. The goal of the present study was to investigate, in vivo, involvement of TGF-β signaling via the activin-like kinase (ALK)-1 receptor on Oatp1a4 expression at the BBB.
Methods: Female Sprague-Dawley rats (200-250 g) were administered bone morphogenic protein-9 (BMP-9; 0-5 μg/kg, i.p.), an ALK1 agonist, or vehicle (i.e., 0.9% saline). Inhibition experiments were performed using LDN193189 (0-5 mg/kg, i.p.), an ALK1 antagonist. Western blot analysis and fluorescence microscopy of isolated brain microvessels were used to determine protein expression and localization in rat brain microvessels respectively.
Results: Fluorescence staining demonstrated localization of Oatp1a4 and ALK1 in rat brain microvessels. Western blot analysis showed a dose dependent increase in Oatp1a4 protein expression in brain microvessels isolated from BMP-9 treated rats as compared to controls. Treatment with 0.5 μg/kg and 5 μg/kg BMP-9 resulted in a 55% and 116% increase in Oatp1a4 protein expression, implying that activation of ALK1 signaling can up-regulate Oatp1a4 at the brain microvasculature. In contrast, 6 h treatment with LDN193189 did not alter Oatp1a4 expression across a dose range of 0-5 mg/kg, suggesting that ALK1 inhibition does not modulate basal Oatp1a4 expression at the BBB.
Conclusions: Taken together, our data implies that TGF-β/ALK1 signaling may play a role in altering Oatp1a4 protein expression at the BBB. Studies are currently being undertaken in our laboratory to fully characterize the role of TGF-β/ALK1 signaling in determining CNS delivery of drugs relevant to stroke treatment (i.e., statins).