The blood clotting Factor XIIIa forms unique complexes with amyloid-beta (Aβ) and colocalizes with deposited Aβ in cerebral amyloid angiopathy

AIMS

Cerebral amyloid angiopathy (CAA) is a key pathological hallmark of Alzheimer's disease (AD) characterized by accumulation of amyloid-beta (Aβ) protein in blood vessel walls. CAA impairs vessel functioning, affects blood brain barrier integrity and accelerates cognitive decline of AD patients. Unfortunately, mechanisms underlying Aβ deposition in the vessel wall remain largely unknown. Factor XIIIa (FXIIIa) is a blood-derived transglutaminase crucial in blood coagulation by cross-linking fibrin molecules. Evidence is mounting that blood-derived factors are present in CAA and may play a role in protein deposition in the vessel wall. We therefore investigated whether FXIIIa is present in CAA and if FXIIIa cross-link activity affects Aβ aggregation.

METHODS

Using immunohistochemistry, we investigated the distribution of FXIIIa, its activator thrombin and in situ FXIIIa activity in CAA in post-mortem AD tissue. We used surface plasmon resonance and Western blot analysis to study binding of FXIIIa to Aβ and the formation of FXIIIa-Aβ complexes, respectively. In addition, we studied cytotoxicity of FXIIIa-Aβ complexes to cerebrovascular cells.

RESULTS

FXIIIa, thrombin and in situ FXIIIa activity colocalize with the Aβ deposition in CAA. Furthermore, FXIIIa binds to Aβ with a higher binding affinity for Aβ1-42 compared with Aβ1-40 . Moreover, highly stable FXIIIa-Aβ complexes are formed independently of FXIIIa cross-linking activity that protected cerebrovascular cells from Aβ-induced toxicity in vitro.

CONCLUSIONS

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.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Tissue transglutaminase colocalizes with extracellular matrix proteins in cerebral amyloid angiopathy

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.

Increased prothrombin, apolipoprotein A-IV, and haptoglobin in the cerebrospinal fluid of patients with Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease caused by an unstable CAG trinucleotide repeat expansion. The need for biomarkers of onset and progression in HD is imperative, since currently reliable outcome measures are lacking. We used two-dimensional electrophoresis and mass spectrometry to analyze the proteome profiles in cerebrospinal fluid (CSF) of 6 pairs of HD patients and controls. Prothrombin, apolipoprotein A-IV (Apo A-IV) and haptoglobin were elevated in CSF of the HD patients in comparison with the controls. We used western blot as a semi-quantified measurement for prothrombin and Apo A-IV, as well as enzyme linked immunosorbent assay (ELISA) for measurement of haptoglobin, in 9 HD patients and 9 controls. The albumin quotient (Qalb), a marker of blood-brain barrier (BBB) function, was not different between the HD patients and the controls. The ratios of CSF prothrombin/albumin (prothrombin/Alb) and Apo A-IV/albumin (Apo A-IV/Alb), and haptoglobin level were significantly elevated in HD. The ratio of CSF prothrombin/Alb significantly correlated with the disease severity assessed by Unified Huntington's Disease Rating Scale (UHDRS). The results implicate that increased CSF prothrombin, Apo A-IV, and haptoglobin may be involved in pathogenesis of HD and may serve as potential biomarkers for HD.

Neurochemical approaches in the laboratory diagnosis of Parkinson and Parkinson dementia syndromes: a review

The diagnosis of Parkinson disease (PD) is rendered on the basis of clinical parameters, whereby laboratory chemical tests or morphological imaging is only called upon to exclude other neurodegenerative diseases. The differentiation between PD and other diseases of the basal ganglia, especially the postsynaptic Parkinson syndromes multisystem atrophy (MSA) and progressive supranuclear palsy (PSP), is of decisive importance, on the one hand, for the response to an appropriate therapy, and on the other hand, for the respective prognosis of the disease. However, particularly at the onset of symptoms, it is difficult to precisely distinguish these diseases from each other, presenting with an akinetic‐rigid syndrome. It is not yet possible to conduct a neurochemical differentiation of Parkinson syndromes. Therefore, a reliable biomarker is still to be found that might predict the development of Parkinson dementia. Since this situation is currently the subject of various different studies, the following synopsis is intended to provide a brief summary of the investigations addressing the field of the early neurochemical differential diagnosis of Parkinson syndromes and the early diagnosis of Parkinson dementia, from direct α‐synuclein detection to proteomic approaches. In addition, an overview of the tested biomarkers will be given with regard to their possible introduction as a screening method.

Increased levels of gamma-glutamylamines in Huntington disease CSF

Transglutaminases (TGases) catalyze several reactions with protein substrates, including formation of gamma-glutamyl-epsilon-lysine cross-links and gamma-glutamylpolyamine residues. The resulting gamma-glutamylamines are excised intact during proteolysis. TGase activity is altered in several diseases, highlighting the importance of in situ enzymatic determinations. Previous work showed that TGase activity (as measured by an in vitro assay) and free gamma-glutamyl-epsilon-lysine levels are elevated in Huntington disease (HD) and that gamma-glutamyl-epsilon-lysine is increased in HD CSF. Although free gamma-glutamyl-epsilon-lysine was used in these studies as an index of in situ TGase activity, gamma-glutamylpolyamines may also be diagnostic. We have devised methods for the simultaneous determination of four gamma-glutamylamines in CSF: gamma-glutamyl-epsilon-lysine, gamma-glutamylspermidine, gamma-glutamylputrescine, and bis-gamma-glutamylputrescine and showed that all are present in normal human CSF at concentrations of approximately 150, 670, 40, and 240 nM, respectively. The high gamma-glutamylspermidine/gamma-glutamylputrescine and gamma-glutamylspermidine/bis-gamma-glutamylputrescine ratios presumably reflect in part the large spermidine to putrescine mole ratio in human brain. We also showed that all four gamma-glutamylamines are elevated in HD CSF. Our findings support the hypotheses that (i) gamma-glutamylpolyamines are reflective of TGase activity in human brain, (ii) polyamination is an important post-translational modification of brain proteins, and (iii) TGase-catalyzed modification of proteins is increased in HD brain.

Reduced transglutaminase-catalyzed protein aggregation is observed in the presence of creatine using sedimentation velocity

Transglutaminases (TGases) are enzymes that catalyze covalent isopeptide crosslinks between reactive lysine and glutamine residues in proteins. Higher than normal local concentrations of TGase have been correlated with increased protein aggregation in vivo. These insoluble protein aggregates are the hallmark of several neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases, although each aggregating protein involved is disease specific. Because TGase is implicated in protein aggregation, there is evidence that its regulation may retard disease progression. Here we report on a laser light transmission technique as an in vitro tool to gauge the efficacy of creatine, a candidate inhibitor, to regulate aggregation. Sedimentation velocities of protein-coated particles in TGase-containing water-glycerol solutions were tracked with different levels of creatine. Sedimentation velocities were converted to apparent aggregate sizes using Stoke's law of sedimentation. The results indicated that creatine promoted up to a 20% reduction in protein aggregation in vitro. This technique may prove to be useful in identifying other functional TGase inhibitors.

Development of a mechanism-based assay for tissue transglutaminase—results of a high-throughput screen and discovery of inhibitors

Tissue transglutaminase (TGase) is a Ca(2+)-dependent enzyme that catalyzes cross-linking of intracellular proteins through a mechanism that involves isopeptide bond formation between Gln and Lys residues. In addition to its transamidation activity, TGase can bind guanosine 5'-triphosphate (GTP) and does so in a manner that is antagonized by calcium. Once bound, GTP undergoes hydrolysis to form guanosine 5'-diphosphate and inorganic phosphate. TGase is thought to play a pathogenic role in neurodegenerative diseases by promoting aggregation of disease-specific proteins that accumulate in these disorders. Thus, this enzyme represents a viable target for drug discovery. We now report the development of a mechanism-based assay for TGase and the results of a screen using this assay in which we tested 56,500 drug-like molecules for their ability to inhibit TGase. In this assay, the Gln- and Lys-donating substrates are N,N-dimethylated casein (NMC) and N-Boc-Lys-NH-CH(2)-CH(2)-NH-dansyl (KXD), respectively. Through a combination of steady state kinetic experiments and reaction progress curve simulations, we were able to calculate values for the initial concentrations of NMC, KXD, and Ca(2+) that would produce a steady state situation in which all thermodynamically significant forms of substrate-bound TGase exist in equal concentration. Under these conditions, the assay is sensitive to both competitive and mixed active-site inhibitors and to inhibitors that bind to the GTP site. The assay was optimized for automated screening in 384-well format and was then used to test our compound library. From among these compounds, 104 authentic hits that represent several mechanistic classes were identified.

Elevated concentration of cerebrospinal fluid tissue transglutaminase in Parkinson's disease indicating apoptosis

Tissue transglutaminase (tTG) is activated during the apoptotic cell death cascade and plays a key role in the formation of apoptotic bodies. We found significant elevation of tTG concentration in the cerebrospinal fluid (CSF) of 54 patients with Parkinson's disease (PD) compared with that measured in 34 control subjects, thus showing increased neural cell apoptosis in patients with PD.

Cross-linking of ubiquitin, HSP27, parkin, and alpha-synuclein by gamma-glutamyl-epsilon-lysine bonds in Alzheimer's neurofibrillary tangles

The accumulation of misfolded proteins in intracellular inclusions is a generic feature of neurodegenerative disorders. Although heavily ubiquitylated, the aggregated proteins are not degraded by the proteasomes. A possible reason for this phenomenon may be a modification of deposited proteins by transglutaminases forming gamma-glutamyl-epsilon-lysine (GGEL) cross-links between distinct proteins. Here, we show that the frequency of GGEL cross-links is an order of magnitude higher in Alzheimer's brain cortex than in age-matched or younger controls. This difference is due to the accumulation of GGEL cross-links in ubiquitin-immunopositive protein particles present in both Alzheimer's brains and those from aged individuals. The highly cross-linked protein aggregates show immunoreactivity to antibodies against tau and neurofilament proteins, and partially also to alpha-synuclein, indicating that these structures are inherent in Alzheimer's neurofibrillary tangles and Lewy bodies. Using mass sequence analysis, we identified the same six pairs of peptide sequences cross-linked in both senile and Alzheimer's specimens: Gln31 and Gln190 of HSP27 protein are cross-linked with Lys29 and Lys48 of ubiquitin and HSP27 therefore may cross-link two (poly)ubiquitin chains. One lysine residue of parkin and one of alpha-synuclein were also found to be cross-linked. The data suggest that cross-linking of (poly)ubiquitin moieties via HSP27 may have a role in the stabilization of the intraneuronal protein aggregates by interference with the proteasomal elimination of unfolded proteins.