Modulation of the blood–brain barrier permeability by plasma glycoprotein orosomucoid

Previous studies have shown that the glycoprotein orosomucoid modulates permeability of peripheral microvessels to charged molecules by contributing to the net charge on the microvessel wall. To investigate whether or not orosomucoid also modulates the permeability of the blood–brain barrier (BBB) by a similar mechanism, we measured the permeability (P) of rat pial microvessels to similar-sized molecules with different charges: α-lactalbumin (−10, Stokes radius 2.08 nm) and ribonuclease (+ 4, Stokes radius 2.01 nm). Tests were performed under control conditions with a Ringer-BSA (bovine serum albumin) perfusate and with 0.1 mg/ml orosomucoid in Ringer-BSA perfusate. The pial circulation was observed through a section of frontoparietal bones thinned with a micro-grinder, and P was determined using a quantitative fluorescence video microscopy. In the absence of orosomucoid, the permeability of pial microvessels to positively charged ribonuclease was 4-fold that to negatively charged α-lactalbumin. In contrast, in the presence of orosomucoid, permeability to ribonuclease was 12-fold that to α-lactalbumin. On the basis of these experimental data, our theoretical model predicted that the charge density of the endothelial glycocalyx layer at the luminal surface of the BBB increased 2.8-fold in the presence of 0.1 mg/ml orosomucoid, while the charge density of the BBB basement membrane increased 1.8-fold, compared to their control values. Our results indicate that orosomucoid can modulate the permeability of the BBB to charged molecules by adding negative charge to the matrix components of the BBB.