Cerebral amyloid angiopathy (CAA) is a key histopathological hallmark of Alzheimer's disease (AD) and hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D). CAA is characterized by amyloid-beta (Aβ) depositions and remodeling of the extracellular matrix (ECM) in brain vessels and plays an important role in the development and progression of both AD and HCHWA-D. Tissue transglutaminase (tTG) modulates the ECM by molecular cross-linking of ECM proteins. Here, we investigated the distribution pattern, cellular source, and activity of tTG in CAA in control, AD, and HCHWA-D cases. We observed increased tTG immunoreactivity and colocalization with Aβ in the vessel wall in early stage CAA, whereas in later CAA stages, tTG and its cross-links were present in halos enclosing the Aβ deposition. In CAA, tTG and its cross-links at the abluminal side of the vessel were demonstrated to be either of astrocytic origin in parenchymal vessels, of fibroblastic origin in leptomeningeal vessels, and of endothelial origin at the luminal side of the deposited Aβ. Furthermore, the ECM proteins fibronectin and laminin colocalized with the tTG-positive halos surrounding the deposited Aβ in CAA. However, we observed that in situ tTG activity was present throughout the vessel wall in late stage CAA. Together, our data suggest that tTG and its activity might play a differential role in the development and progression of CAA, possibly evolving from direct modulation of Aβ aggregation to cross-linking of ECM proteins resulting in ECM restructuring.

Protein misfolding and the formation of stable insoluble protein complexes by self-interacting proteins, in particular amyloid-β and tau protein, play a central role in the pathogenesis of Alzheimer's disease (AD). Unfortunately, the underlying mechanisms that trigger the misfolding of self-interacting proteins that eventually results in formation of neurotoxic dimers, oligomers, and aggregates remain unclear. Elucidation of the driving forces of protein complex formation in AD is of crucial importance for the development of disease-modifying therapies. Tissue transglutaminase (tTG) is a calcium-dependent enzyme that induces the formation of covalent ε-(γ-glutamyl)lysine isopeptide bonds, which results in both intra- and intermolecular protein cross-links. These tTG-catalyzed intermolecular cross-links induce stable, rigid, and insoluble protein complexes, whereas intramolecular cross-links change the conformation of proteins. Inhibition of tTG-catalyzed cross-linking counteracts the formation of protein aggregates, as observed in disease-models of other protein misfolding diseases, in particular Parkinson's and Huntington's diseases. Although data of tTG activity in AD models is limited, there is compelling evidence from both in vitro and postmortem human brain tissue of AD patients that point toward a crucial role for tTG in the pathogenesis of AD. Here, we review these data on the role of tTG in the initiation and development of protein aggregates in AD, and discuss the possibility to use inhibitors of the cross-linking activity of tTG as a new therapeutic approach for AD.

More than 80% of Alzheimer's disease (AD) patients have some degree of cerebral amyloid angiopathy (CAA). In addition to arteries and veins, capillaries can also be affected. Capillary CAA (capCAA), rather than CAA in larger vessels, is associated with flame-like amyloidbeta (A␤) deposits that may extend beyond the vessel wall and radiate into the neuropil, a phenomenon also known as "dyshoric angiopathy." A␤ deposits in AD, parenchymal as well as (cap)CAA and dyshoric angiopathy, are associated with a local inflammatory reaction, including activation of microglial cells and astrocytes that, among others, produce cytokines and reactive oxygen species. This neuroinflammatory reaction may account for at least part of the cognitive decline. In previous studies we observed that small heat shock proteins (sHsps) are associated with A␤ deposits in AD. In this study the molecular chaperones Hsp20, HspB8 and HspB2B3 were found to colocalize with CAA and capCAA in AD brains. In addition, Hsp20, HspB8 and HspB2B3 colocalized with intercellular adhesion molecule 1 (ICAM-1) in capCAA-associated dyshoric angiopathy. Furthermore, we demonstrated that Hsp20, HspB8 and HspB2B3 induced production of interleukin 8, soluble ICAM-1 and monocyte chemoattractant protein 1 by human leptomeningeal smooth muscle cells and human brain astrocytes in vitro and that Hsp27 inhibited production of transforming growth factor beta 1 and CD40 ligand. Our results suggest a central role for sHsps in the neuroinflammatory reaction in AD and CAA and thus in contributing to cognitive decline.

Alzheimer’s disease (AD) is characterised by amyloid-beta (Aβ) protein deposition in the brain. Posttranslational modifications in Aβ play an important role in Aβ deposition. Tissue transglutaminase (tTG) is an enzyme involved in posttranslational cross-linking of proteins. tTG levels and activity are increased in AD brains, and tTG is associated with Aβ deposits and lesion-associated astrocytes in AD cases. Furthermore, Aβ is a substrate of tTG-catalysed cross-linking. To study the role of tTG in Aβ pathology, we compared tTG distribution and activity in both the APPSWE/PS1ΔE9 and APP23 mice models with human AD. Using immunohistochemistry, we found association of both tTG and in situ active tTG with Aβ plaques and vascular Aβ, in early and late stages of Aβ deposition. In addition, tTG staining colocalised with Aβ-associated reactive astrocytes. Thus, alike human AD cases, tTG was associated with Aβ depositions in these AD models. Although, distribution pattern and spatial overlay of both tTG and its activity with Aβ pathology was substantially different from human AD cases, our findings provide evidence for an early role of tTG in Aβ pathology. Yet, species differences should be taken into account when using these models to study the role of tTG in Aβ pathology.

Our data showed that FXIIIa colocalizes with Aβ in CAA and that FXIIIa forms unique protein complexes with Aβ that might play an important role in Aβ deposition and persistence in the vessel wall.