Serum Transthyretin Monomer As a Possible Marker of Blood-to-CSF Barrier Disruption

The CNS is shielded from systemic influences by two separate barriers, the blood-brain barrier (BBB) and the blood-to-CSF barrier. Failure of either barrier bears profound significance in the etiology and diagnosis of several neurological diseases. Furthermore, selective opening of BBB tight junctions provides an opportunity for delivery of otherwise BBB impermeant drugs. Peripheral assessment of BBB opening can be achieved by detection in blood of brain-specific proteins that extravasate when these endothelial junctions are breached. We developed a proteomic approach to discover clusters of CNS-specific proteins with extravasation into serum that correlates with BBB openings. Protein profiles from blood samples obtained from patients undergoing iatrogenic BBB disruption (BBBD) with intra-arterial hyperosmotic mannitol were compared with pre-BBB opening serum. A low molecular weight protein (14 kDa) identified by mass spectroscopy as transthyretin (TTR) consistently correlated with BBBD. Protein gel electrophoresis and immunodetection confirmed that TTR was indeed extravasated in its monomeric form when CNS barriers were breached. The time course of TTR extravasation was compared with release from the brain of another BBB integrity marker, S-100beta (11 kDa). Kinetic analysis revealed that the appearance of S-100beta, presumably originating from perivascular astrocytic end feet, preceded extravasation of TTR by several minutes. Because TTR is localized primarily in choroid plexus and, as a soluble monomer, in CSF, we concluded that although S-100beta is a marker of BBB integrity, TTR instead may be a peripheral tracer of blood-to-cerebrospinal barrier.