Faculty Bibliography

To better understand the physiologic excretion and/or catabolism of circulating peripheral amyloid beta (Abeta), we labeled human Abeta40 (monomeric, with predominant unordered structure) and Abeta42 (mixture of monomers and oligomers in approximately 50:50 ratio, rich in beta-sheet conformation) with either Na(125)I or (125)I-tyramine cellobiose, also known as the cell-trapping ligand procedure, testing their blood clearance and organ uptake in B6SJLF1/J mice. Irrespective of the labeling protocol, the peptide conformation, and the degree of oligomerization, both Abeta40 and Abeta42 showed a short half-life of 2.5-3.0 min. The liver was the major organ responsible for plasma clearance, accounting for >60% of the peptide uptake, followed by the kidney. In vivo, hepatocytes captured >90% of the radiolabeled peptides which, after endocytosis, were preferentially catabolized and excreted into the bile. Biliary excretion of intact as well as partially degraded Abeta species became obviously relevant at doses above 10 microg. The use of biotin-labeled Abeta allowed the visualization of the interaction with HepG2 cells in culture, whereas competitive inhibition experiments with unlabeled Abeta demonstrated the specificity of the binding. The capability of the liver to uptake, catabolize, and excrete large doses of Abeta, several orders of magnitude above its physiologic concentration, may explain not only the femtomolar plasma levels of Abeta but the little fluctuation observed with age and disease stages

Amyloid deposition can take place in the walls of arteries, arterioles, and, less often, capillaries and veins of the central nervous system, a phenomenon known as cerebral amyloid angiopathy (CAA). The major clinicopathological manifestations of CAA include cerebral hemorrhage, ischemic lesions, and dementia. CAA may be classified according to the amyloid protein deposited. In the most common form, sporadic CAA, and in CAA related to sporadic Alzheimer disease (AD). A beta deposition is characteristic. CAA can also be severe in variants of familial AD caused by mutations of the amyloid-beta precursor protein or presenilin-1 genes in which deposition of A beta variants and/or wild-type A beta occurs. Other amyloid proteins involved in familial CAAs include 1) the mutant cystatin C (ACys) in hereditary cerebral hemorrhage with amyloidosis of Icelandic type, 2) variant transthyretins (ATTR) in meningo-vascular amyloidoses, 3) mutated gelsolin (AGel) in familial amyloidosis of Finnish type, 4) disease-associated prion protein (PrP(Sc)) in a variant of the Gerstmann-Straussler-Scheinker syndrome, and 5) ABri and ADan in CAAs observed in the recently described BRI2 gene-related dementias, familial British dementia and familial Danish dementia, respectively. This review addresses issues related to the correlation between morphology, biochemistry, and genetics, and briefly discusses both the pathogenesis and animal models of CAAs

P-component is a member of the pentraxin family of proteins that has been found associated with amyloid deposits in vivo in both systemic and localized forms of amyloidosis including AD. Recently, two novel familial forms of cerebrovascular amyloidosis have been described. These hereditary conditions, familial British dementia (FBD) and familial Danish dementia (FDD), both result from genetic alterations in the BRI2 gene and show striking clinical and neuropathological similarities with AD. Despite structural differences among the amyloid subunits (ABri in FBD, ADan in FDD, and Abeta in AD) all these disorders are characterized by the presence of neurofibrillary tangles and parenchymal and vascular amyloid deposits co-localizing with reactive microglia and activation products of the complement cascade. Immunohistochemical studies of FBD and FDD brain tissue identified P-component associated with the amyloid lesions. Using affinity chromatography, ELISA binding assays and electrophoretic techniques we were able to demonstrate a specific binding interaction between P-component and ABri/ADan peptides, study the biochemical parameters of the interaction, and compare them with that of Abeta amyloid species. Our studies revealed a high affinity, Calcium dependent, saturable binding between P-component and ABri/ADan peptides with Kd values in the low nanomolar range and in the same order of magnitude to those resulting from the interaction of P-component with Abeta1-40 and Abeta1-42. The high affinity interaction of P-component with ABri and ADan peptides in vitro suggests a likely mechanism for their in vivo co-localization in FBD and FDD lesions. Whether the presence of P-component protects the ABri/ADan amyloid deposits from enzymatic degradation as demonstrated for other amyloids is being investigated

Several mutations within the Abeta sequence of the APP gene are associated with autosomal dominant cerebral amyloid angiopathy. An Asp to Asn mutation at position 23 of Abeta due to a single nucleotide change at codon 694 of the APP gene causes early onset dementia with leukoencephalopathy and cortical calcification in the Iowa kindred. Severe cerebral amyloid angiopathy and cortical pre-amyloid deposits as well as widespread neurofibrillary tangles are the main neuropathological features of the disease. We have extracted the deposited Abeta species from affected brain areas and biochemically analyzed them using a combination of immunoprecipitation, mass spectrometry, amino acid sequence and western blot analysis. Corroborating previous histological data, western blots with C-terminal specific antibodies revealed an extensive accumulation of Abetax-40 in cortical lesions whereas Abetax-42 was only a minor component. Amino acid sequence of the formic-acid soluble extracts showed high degree of N-terminal heterogeneity, being Abeta starting at position 2 (Ala) the predominant species followed by Abeta4 (Phe) and Abeta1 (Asp) in a 4:2:1 ratio, respectively. IP/mass spect confirmed the presence of Abeta1-40, Abeta2-40 and Abeta4-40 as well as minor components starting at Asp1 and Ala2 but ending at Gly38. Interestingly, amino acid sequence analysis demonstrated the presence of both Asp and Asn at position 23 of the Abeta sequence at a 1:4 ratio, respectively, indicating that the deposited amyloid is a mixture of mutant and wild-type Abeta. Whether one of them is an innocent bystander being recruited by the other (conformational mimicry) or both mutated and non-mutated Abeta peptides are important for the amyloidogenesis process in the Iowa family is being currently investigated

Chromosome 13 dementias, familial British dementia (FBD) and familial Danish dementia (FDD), are associated with neurodegeneration and cerebrovascular amyloidosis, with striking neuropathological similarities to Alzheimer's disease (AD). Despite the structural differences among the amyloid subunits (ABri in FBD, ADan in FDD, and Abeta in AD), these disorders are all characterized by the presence of neurofibrillary tangles and parenchymal and vascular amyloid deposits co-localizing with markers of glial activation, suggestive of local inflammation. Proteins of the complement system and their pro-inflammatory activation products are among the inflammation markers associated with AD lesions. Immunohistochemistry of FBD and FDD brain sections demonstrated the presence of complement activation components of the classical and alternative pathways as well as the neo-epitope of the membrane attack complex. Hemolytic experiments and enzyme-linked immunosorbent assays specific for the activation products iC3b, C4d, Bb, and C5b-9 indicated that ABri and ADan are able to fully activate the complement cascade at levels comparable to those generated by Abeta1-42. ABri and ADan specifically bound C1q with high affinity and formed stable complexes in physiological conditions. Activation proceeds approximately 70-75% through the classical pathway while only approximately 25-30% seems to occur through the alternative pathway. The data suggest that the chronic inflammatory response generated by the amyloid peptides in vivo might be a contributing factor for the pathogenesis of FBD and FDD and, in more general terms, to other neurodegenerative conditions