Endothelial protein C receptor-assisted transport of activated protein C across the mouse blood|[ndash]|brain barrier

Activated protein C (APC), a serine-protease with anticoagulant, anti-inflammatory, and cytoprotective activities, is neuroprotective and holds potential to treat different neurologic disorders. It is unknown whether APC crosses the blood–brain barrier (BBB) to reach its therapeutic targets in the brain. By using a brain vascular perfusion technique, we show that 125I-labeled plasma-derived mouse APC enters the brain from cerebrovascular circulation by a concentration-dependent mechanism. The permeability surface area product of 125I-APC (0.1 nmol/L) in different forebrain regions ranged from 3.11 to 4.13 μL/min/g brain. This was approximately 80- to 110-fold greater than for 14C-inulin, a simultaneously infused reference tracer. The Km value for APC BBB cortical transport was 1.6±0.2 nmol/L. Recombinant APC variants with reduced anticoagulant activity, 5A-APC and 3K3A-APC, but not protein C, exhibited high affinity for the APC BBB transport system. Blockade of APC-binding site on endothelial protein C receptor (EPCR), but not blockade of its protease-activated receptor-1 (PAR1) catalytic site, inhibited by >85% APC entry into the brain. APC brain uptake was reduced by 64% in severely deficient EPCR mice, but not in PAR1 null mice. These data suggest that APC and its variants with reduced anticoagulant activity cross the BBB via EPCR-mediated saturable transport.