Peripheral markers of brain damage and blood-brain barrier dysfunction

Abstract

Purpose: Occurrence of brain damage is frequently associated with abnormal blood-brain barrier (BBB) function. Two brain-specific proteins, S100beta and neuron-specific enolase (NSE) are released systemically in a variety of neurological diseases, but S100beta levels sometimes rise in the absence of neuronal damage, suggesting that S100beta is a marker of BBB rather than neuronal damage.

Methods: We measured both proteins in the serum of patients undergoing iatrogenic BBB disruption with intrarterial mannitol, followed by chemotherapy.

Results: Serum S100beta increased significantly after mannitol infusion (p<0.05) while NSE did not. Furthermore, in a model of intracerebral hemorrhage, S100beta increases in CSF did not lead to serum changes at a time when the BBB was intact. Modeling of S100beta release from the CNS suggested that low (<0.34 ng/ml) serum levels of S100beta are consistent with BBB opening without CNS damage, while larger increases imply synthesis and release from presumable damaged glia.

Conclusions: Thus, S100beta in serum is an early marker of BBB openings that may precede neuronal damage and may influence therapeutic strategies. Secondary, massive elevations in S100beta are indicators of prior brain damage and bear clinical significance as predictors of poor outcome or diagnostic means to differentiate extensive damage from minor, transient impairment.

Fig. 1

Relationship between perivascular glia, neurons and cerebral blood vessels: distribution of immunoreactivity for astrocyte-specific markers S100β and GFAP. A) GFAP immunoreactivity (red), neuronal cell bodies (green) and capillaries in the rat hippocampus. The enlargements in the insets show capillaries and arterioles clearly outlined by GFAP immunoreactivity, emphasizing the proximity of perivascular glia to the lumen. B) GFAP immunoreactivity outlines large penetrating pial vessels in human cortex. C) S100β immunoreactivity (in red) co-localizes with intracellular GFAP (in green).

Fig. 2

S100β levels in serum correlate with BBB opening in the absence of neuronal damage. A) Serum levels of S100β rise as a result of osmotic opening of the blood-brain barrier, not of ongoing neuronal damage. The bar graph shows mean serum levels of S100β assessed after 32 blood-brain barrier disruptions in five patients. Also shown are mean serum levels of NSE measured in 18 openings (n = 3 patients). Error bars show standard error of the mean. Results show that S100β levels increased after the administration of intra-arterial mannitol and remained elevated, whereas levels of NSE did not change significantly. * indicates level was significantly different from level at induction, and # indicates level was significantly different from level at methotrexate administration (p < 0.05; paired t-test). B) Interpretation of results; see text for details.

Fig. 3

S100β levels after BBB disruption. A) Sigmoidal (Boltzmann) fit of S100β_serum levels measured after hemispheric BBBD. The average of 36 openings is shown reflecting leakage produced by opening of the BBB of one hemisphere. The asymptotic value determined was 0.176 ng/ml. B) Tri-dimensional representation of Eq. (2). The initial values of S100β_serum and S100β_CSF were 0.05 and 2 ng/ml respectively. Note that S100β_s−s obtained after hemispheric BBB disruption depend on both CSF and blood volumes. Similar plots were constructed at different S100β_CSF levels to estimate the contribution of neuronal damage to plasma levels (box 3 Fig. 3C) under conditions of breached BBB. C) shows the results of these calculations (see Eq. (3).

Fig. 4

S100β levels after intracerebral hemorrhage. A) FITC-labeled albumin is sequestered intraluminally 2 hrs after bilateral injection of autologous blood (as indicated in B). Note that even in close proximity of the clot (outlined by a dashed line) the BBB remained intact. BBB integrity was observed in both evacuated and non-evacuated hemispheres. B) Lack of correlation of S100β_serum with S100β_CSF after experimental intracerebral hemorrhage. Note that significant increases of CSF S100β were not accompanied by comparable changes in serum levels. The mean of six experiments is shown; * indicates p < 0.02. C) Interpretation of results. Under conditions of intact BBB, extravasation of biochemical markers of brain damage is limited by the low transendothelial permeability to macromolecules. Therefore, neuronal damage precedes appearance in peripheral blood of any proteic marker of brain damage. The latter will extravasate into plasma at later times if the BBB is breached.