Extravascular fibrinogen in the white matter of Alzheimer's disease and normal aged brains: implications for fibrinogen as a biomarker for Alzheimer's disease

Abstract

The blood-brain barrier (BBB) regulates cerebrovascular permeability and leakage of blood-derived fibrinogen. Dysfunction of the BBB has been associated with cerebral arteriolosclerosis small vessel disease (SVD) and white matter lesions (WML). Furthermore, BBB dysfunction is associated with the pathogenesis of Alzheimer's disease (AD) with the presence of CSF plasma proteins suggested to be a potential biomarker of AD. We aimed to determine if extravascular fibrinogen in the white matter was associated with the development of AD hallmark pathologies, i.e., hyperphosphorylated tau (HPτ) and amyloid-β (Aβ), as well as SVD, cerebral amyloid angiopathy (CAA) and measures of white matter damage. Using human post-mortem brains, parietal tissue from 20 AD and 22 non-demented controls was quantitatively assessed for HPτ, Aβ, white matter damage severity, axonal density, demyelination and the burden of extravascular fibrinogen in both WML and normal appearing white matter (NAWM). SVD severity was determined by calculating sclerotic indices. WML- and NAWM fibrinogen burden was not significantly different between AD and controls nor was it associated with the burden of HPτ or Aβ pathology, or any measures of white matter damage. Increasing severity of SVD was associated with and a predictor of both higher WML- and NAWM fibrinogen burden (all P < 0.05) in controls only. In cases with minimal SVD NAWM fibrinogen burden was significantly higher in the AD cases (P < 0.05). BBB dysfunction was present in both non-demented and AD brains and was not associated with the burden of AD-associated cortical pathologies. BBB dysfunction was strongly associated with SVD but only in the non-demented controls. In cases with minimal SVD, BBB dysfunction was significantly worse in AD cases possibly indicating the influence of CAA. In conclusion, extravascular fibrinogen is not associated with AD hallmark pathologies but indicates SVD, suggesting that the presence of fibrinogen in the CSF is not a surrogate marker for AD pathology.

Keywords: Alzheimer’s disease; biomarker; blood-brain barrier; cerebral small vessel disease; fibrinogen; white matter hyperintensities; white matter lesion.

Figure 1

A. WML areas were identified on LFB stained sections of parietal tissue and delineated (red dotted line). This was used to identify the same WML and NAWM (black dotted line) area on serial sections stained with fibrinogen B. B. The fibrinogen stained section of parietal tissue indicating the five randomly selected areas for a 3 × 3 image capture in the WML and NAWM areas (black boxes). C. A 3 × 3 large image acquisition in WML area. D. Image C again with applied bespoke fibrinogen threshold including region of interest for removal of vessel and PVS (green line). Scale bar, 50 mm, valid for all images in A and B; 100 μm, valid for C and D. WML, white matter lesions; LFB, luxol fast blue; NAWM, normal appearing white matter; PVS, perivascular spaces.

Figure 2

Photomicrographs illustrating fibrinogen immunoreactivity in white matter and its vasculature. A. A small artery exhibiting fibrinogen in the vessel wall in both the tunica media (arrow) and the tunica intima (arrowhead) indicating leakage of fibrinogen. B. Swollen white matter axons positive for extravascular fibrinogen (arrow). C. A capillary exhibiting fibrinogen immunopositivity contained within the endothelial wall in the transverse plane. Ci. A capillary exhibiting sustained leakage of fibrinogen into the endothelial wall, basement membrane and surrounding white matter parenchyma (arrowhead). D. A moderately dilated PVS (arrow) with accumulation of extraversion of fibrinogen in the white matter parenchyma (red arrowheads). Di. Magnified photomicrograph of extraverted fibrinogen in the surrounding white matter parenchyma (black arrowhead). E. Fibrinogen positive cells in the deep white matter (black arrowhead) and lining a dilated PVS (red arrowhead). Ei. Magnified photomicrograph of a deep white matter fibrinogen positive cell with the morphology of a hypertrophic fibrous astrocyte; immunoreactivity is present in both the cell soma (arrow) and cellular processes (arrow head). BBB, blood–brain barrier; PVS, perivascular space. Scale bars, 200 μm, valid for D and E; 100 μm, valid for A and B; 20 μm, valid for Di and Ei; 10 μm valid for C and Ci.

Figure 3

A, B. Regarding the whole cohort, WML and NAWM fibrinogen‐IR was not significantly higher in cases with moderate to severe SVD compared to cases with none‐mild SVD. C. WML fibrinogen‐IR was significantly higher in control cases with moderate‐severe SVD compared to control cases none‐mild SVD. D. NAWM fibrinogen‐IR was significantly higher in AD case with non‐mild SVD compared to control cases with none‐mild SVD. WML, white matter lesion; NAWM, normal appearing white matter; SVD, small vessel disease; mod‐sev, moderate to severe; AD, Alzheimer’s disease; IR, immunoreactivity. *P < 0.05.