The amyloid P-component (protein AP): an integral part of the amyloid substance?

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Calcium-dependent and -independent binding of the pentraxin serum amyloid P component to glycosaminoglycans and amyloid proteins: enhanced binding at slightly acid pH

Serum amyloid P component (SAP), a member of the pentraxin family of proteins, binds calcium-dependently to several ligands including glycosaminoglycans (GAG's). We have investigated the influence of pH on the Ca2(+)-dependent binding of SAP to solid phase GAG's and amyloid fibril proteins (AA and beta2M) by ELISA. An increase in the dose-dependent binding of SAP to heparan sulfate, AA-protein and beta2M was observed as the pH decreased from 8.0 to 5.0. Furthermore, a lower, but significant Ca2(+)-independent binding of SAP to heparan sulfate, dermatan sulfate, AA protein and the amyloid precursor protein beta2M was observed. This binding was also enhanced at slightly acid pH, most pronounced at pH 5.0. The results of this study indicate that SAP can exhibit both Ca2(+)-dependent and -independent binding to ligands involved in amyloid fibril formation and that the binding is enhanced under conditions of slightly lowered pH.

Amyloidosis

Amyloidosis is a heterogenous group of diseases characterized by deposition of a fibrillar, proteinaceous material, amyloid, in various tissues and organs. Increasing knowledge about the different proteins that constitute the amyloid fibrils has made it possible to classify amyloidosis by the fibril protein, which appears more rational than the traditional classification by its clinical expression. A serum protein is the precursor of the amyloid fibril protein in the various systemic forms of amyloidosis. Although the chemical composition of amyloid is presently well known, the pathogenetic processes that convert such proteins into a fibrillar form and lay them down in the tissues are far from clarified. This review describes the amyloid deposits, some putative pathogenetic mechanisms, and the clinical, therapeutic, and prognostic aspects of the most important forms of amyloid disease.

Astroglia in Alzheimer's disease

Amyloid deposits are characteristic of Alzheimer's Disease (AD) and there is growing evidence that amyloid may play an important role in the genesis of this neurodegenerative disease. This review discusses data which suggests that reactive astrocytes and microglia may be a necessary concomitant with amyloid to produce the neuropathology which manifests as AD. Several hypotheses and supporting data for mechanisms by which reactive astrocytes may mediate this neuropathology are presented. These include the possibility that amyloid induces excitotoxicity by interferring with astrocytic glutamate uptake, the possibility that amyloid has this effect via an action on a tachykinin-related receptor and the possibility that proteoglycans released by astrocytes may facilitate the deposition of amyloid plaques. Both symptomatic treatment to enhance cognitive function and treatment to stop the progression of AD are needed. It is hoped that answers to some of the unique questions raised here may provide new insight into the etiology and treatment of AD.

High molecular weight glycosaminoglycans in AA type amyloid fibril extracts from human liver

Glycosaminoglycans have previously been identified in extracts of AA type hepatic amyloid fibril from a patient with amyloidosis associated with juvenile rheumatoid arthritis. The macromolecular properties of these polysaccharides are described here in more detail. By gel filtration and ion exchange chromatography glycosaminoglycans in the form of high molecular weight free chains were shown to coisolate with water extracted amyloid fibrils. About 60% of these were characterised as galactosamines (chondroitin sulphate/dermatan sulphate), whereas the remaining 40% consisted of N-sulphated glucosamines (heparin/heparan sulphate). The amyloid associated glycosaminoglycans were not part of intact proteoglycans in the fibril extracts.

Proteoglycans in the pathogenesis of Alzheimer's disease and other amyloidoses

Proteoglycans and the amyloid P component are two constituents of amyloid that appear to be present regardless of the type of amyloid protein deposited, the extent of amyloid deposition and the tissue or organ involved. This article reviews the literature concerning proteoglycans and/or glycosaminoglycans in amyloidosis and describes recent studies which demonstrate their localization to the characteristic lesions of Alzheimer's disease and the amyloid plaques containing PrP protein in the prion diseases. Additionally, the possible interaction of proteoglycans with various amyloidogenic proteins, including the beta-amyloid protein in Alzheimer's disease is discussed. It is postulated that proteoglycans localized to a number of different amyloids play a common role in the pathogenesis of amyloidosis. Some of these hypothesized roles include 1) inducing amyloidogenic precursor proteins to form amyloid fibrils containing a predominant beta-pleated sheet structure, 2) influencing amyloid deposition to occur at specific anatomical sites within tissues and/or 3) aiding in prevention of amyloid degradation once amyloid has formed.

Cationic dyes reveal proteoglycans structurally integrated within the characteristic lesions of Alzheimer's disease

The cationic dyes ruthenium red (RR) and cuprolinic blue (CB) were used to preserve proteoglycans (PGs) for visualization at the ultrastructural level in brain tissue from seven cases of Alzheimer's disease (obtained at autopsy within 3-4 h after death). PGs were visualized as RR-positive granules specifically localized to the amyloid fibrils in neuritic plaques. In neurofibrillary tangles, RR granules were localized to the paired helical filaments and straight filaments usually at a consistent periodicity of 40-70 nm. CB, known to preserve PGs as short punctate filaments, also demonstrated PGs specifically localized to the amyloid fibrils in neuritic plaques and in association with paired helical filaments and straight filaments in neurofibrillary tangles. Persistent staining with CB at magnesium chloride concentrations of 0.3 and 0.7 M in the neuritic plaques suggested the presence of highly sulfated PGs, whereas abolishment of CB staining at 0.7 M magnesium chloride in the neurofibrillary tangles implied that different PGs and/or glycosaminoglycans were present in the neurofibrillary tangles. The specific ultrastructural localization of PGs to the characteristic lesions in Alzheimer's disease suggests that PGs are part of a complex structural network with amyloid fibrils in neuritic plaques and the filamentous structures present in neurofibrillary tangles.

Incidence and characterization of age related amyloid deposits in the human anterior pituitary gland

To identify amyloid deposits in the anterior pituitary gland, we have immunohistochemical, histochemical and alkaline Congo red staining. The anti-human P component reacted positively with these amyloid deposits, while antisera against prealbumin, AA type amyloid fibril protein and various anterior pituitary hormones were negative. A combination of Congo red and anti-human P component staining was most sensitive and reliable for detection of amyloid in the anterior pituitary glands of 300 randomly autopsied patients. Amyloid deposits increased in parallel with the age of the patients, however, they appeared earlier and more frequently than heretofore reported. Deposition of amyloid was seen initially in the 3rd decade and the positivity rate of amyloid deposits was 73% in the 5th decade. The histochemical characteristics of these pituitary amyloid deposits differed from those of cerebral and systemic deposits, particularly those found in the amyloid of senile systemic amyloidosis.

Binding characteristics of human serum amyloid P component

Serum amyloid P component (SAP), a normal human plasma glycoprotein, was found in a solid phase ELISA to have Ca2+-dependent binding for keyhole limpet haemocyanin (KLH), pectic acid, trinitrophenylated (TNP) macromolecules, and plastic surfaces. The binding to TNP-KLH was used to develop a sensitive ELISA. The binding of SAP to the ligands mentioned was inhibited by EDTA, KLH, pectic acid, TNP-conjugated macromolecules (bovine serum albumin, polyacrylhydrazide), and p-nitrophenylarsonic acid. Underivatized and DNP-conjugated macromolecules did not inhibit the SAP binding; arsenilic acid, picric acid, and dinitrophenyl were weak inhibitors. SAP bound to TNP-agarose was eluated by either EDTA or p-nitrophenylarsonic acid. Thus, a unique region of SAP is responsible for the polyspecific binding. We suggest that the polyspecific binding of SAP takes place through a Ca2+ bridge: half of the metal coordination sphere is occupied by SAP, with the other half available to interact with metal ligand.

Familial amyloid polyneuropathy

Amyloid fibrils were isolated from the myocardium of two patients with familial amyloid polyneuropathy. The solubilized amyloid fibril whole protein shared immunologic determinants with normal human serum prealbumin (transthyretin), but revealed subtle differences on immunoelectrophoresis and radial immunodiffusion. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, amyloid fibril whole protein was resolved into numerous bands that reacted with antitransthyretin on immunoblots. The whole protein also contained peptide fragments of fibronectin, but was devoid of amyloid P protein. An antiserum raised against the whole protein was suitable for immunocytochemistry of amyloid in paraffin sections. In contrast, commercial antitransthyretin, raised against the intact tetrameric protein failed to react with tissue amyloid. Immunochemical and immunocytochemical results support the concept that familial amyloid polyneuropathy with cardiomyopathy is due to infiltration of susceptible tissues by an anomalous transthyretin.

Human amyloid P component: an elastase inhibitor

Amyloid deposits almost invariably contain a pentagonal-shaped protein (a so-called pentraxin), termed amyloid P component (AP), in close apposition to the amyloid fibrils. AP is also detected alongside normal elastin fibres in skin and basement membrane. In the present studies, purified human AP was shown to inhibit the activity of porcine pancreatic elastase. The inhibition of elastolytic activity was not abolished by heating AP to 70 degrees C. Furthermore, two other human serum proteins used as controls did not inhibit elastase activity: albumin, which has a similarly acidic pI, and C-reactive protein, which is a pentraxin, sharing 50% sequence homology with AP. Enzyme kinetic studies showed that elastase treated with AP had a slower elastolytic rate than untreated elastase. The inhibitory effect of AP was reversed by high substrate (fivefold) concentration. These observations suggest that AP may function in vivo to protect elastin and amyloid fibrils from proteolytic cleavage. Indeed, this may in part account for the relative resistance of amyloid deposits to resorption and proteolysis.

A second component in bovine AA amyloid fibrils not identical with protein AA is essential for AA amyloid fibrillogenesis

Amyloid fibrils were isolated from the renal papillae and glomeruli of cows with spontaneous AA amyloidosis. The fibrils were solubilized by treatment with guanidine hydrochloride (Gu HCl) and subjected to gel filtration on Sephacryl S-200. Two other fractions were obtained beside the void volume and the AA fractions. Reaggregation studies were performed by dialysing the fractions, separately or in combinations, against Gu-HCl-free solutions. Protein AA alone (about 10 kd) appeared not to precipitate. The other fractions alone and the combinations of fractions tested formed precipitates. The precipitates containing all fractions (including protein AA) or protein AA plus a fraction containing a 19- and a 23-kd protein revealed congophilic green birefringent fibrillar material. Dialysis against acidic and calcium-containing solutions gave the best results. Amyloid fibril-like material was visible on electron microscopic examination. The amino acid composition of the 19 + 23-kd material appeared to be slightly different from protein AA and evidently unlike SAP. On immunofluorescence-absorbance studies the 19 + 23-kd material appeared evidently unlike protein AA and SAP. From these findings it is concluded that for spontaneous formation of AA amyloid fibrils other non-AA proteins are necessary.

Human amyloid P component: a circulating lectin that modulates immunological responses

Amyloid P component (AP/SAP), a glycoprotein, precipitated with purified snail galactans from Helix pomatia and Arianta arbustorum in a dose-dependent manner. Radiolabelled AP binds to human peripheral blood mononuclear cells (PBMC), erythrocytes, and cells derived from human non-T, non-B acute lymphocytic leukaemia. The AP cell binding is specific in that it is dose-dependent and can be blocked both by excess cold AP and by Helix pomatia galactan, although it cannot be blocked by an equal amount of the monosaccharide galactose. In vitro studies of human PBMC immune responses demonstrated that AP inhibits PBMC proliferation responses to mycobacterial purified protein derivative and to phytohaemagglutinin and the humoral, antibody response to pokeweed mitogen. The AP-induced suppression of non-specific antibody production by human PBMC was dependent on the time at which AP was added to the culture. AP was suppressive if added in the first 48 h of the 7-day culture, and the suppression could not be reversed by washing the cells after the exposure to AP. The mechanism of AP-induced immunosuppression is still unclear, but human SAP circulates as a pair of pentameric rings, having ten identical subunits that bind to galactose polymers, and our present data suggest that AP affects the immune response through its properties as a lectin.

Amyloid P component in human thyroid

The distribution of amyloid P component in the adult human thyroid was studied by direct immunofluorescence on frozen sections of surgically removed tissue. Amyloid P component shows a striking fibrillary and broken linear distribution in the interfollicular areas. This pattern corresponds to that of reticulin fibres. Amyloid P is also localised to the small amount of elastic tissue in blood vessels but is not demonstrable within cells or in acinar basement membranes.

[New aspects of amyloidosis (author's transl)]

Amyloid is a substance that has the same composition of the basic qualities even in the different patterns of amyloidosis. Electron microscopic investigations have revealed that all forms of amyloidosis do not only exhibit homogeneous basic qualities but also common principle of accumulation. This may be commented as follows: Fibrils of amyloid are always found in close connection with basement membranes or basement membrane-like substances respectively produced by cells with the property of contracting (myocytic or "myopotent' cells). Collagen fibres of different types do not display a regular relation to the substance of amyloid. The origin and development of the various forms of amyloidosis depends on the three following factors: 1. On the extent of the production of amyloidogenic proteins which may belong to different types of proteins according to the basic disease; 2. On the way of protein silting (hematogenic silting-generalized amyloidosis; local enrichment--local amyloidosis); 3. On the site of predilection of all deposits of amyloid in the areas of basement membranes or basement membrane-like substances resp. produced by cells with the properties of contracting. A new classification should be made on the basis of these principles.

The association of amyloid P-component (AP) with the amyloid fibril: an updated method for amyloid fibril protein isolation

An amyloid fibril isolation procedure is proposed which uses citrate as well as saline washes to dissociate the calcium dependent linkage of amyloid P-component (AP) from the amyloid fibril. In two amyloid rich tissues, the amount of AP was quantitated in each saline and citrate wash and totalled 13.8% and 20.8% of the amyloid fibrils isolated. The amount of AP removed from these and 22 additional amyloid rich tissues was greater than had previously been recognized. AP protein was present in tissue only when amyloid fibrils were present. It could not be found in normal non-amyloidotic tissue, nor could it be found in tissue sediment after the fibrils were removed.

Amyloidosis of the stomach. Report of a case with ultrastructure

The author reports on a rare case of local amyloidosis (amyloid tumour) of the stomach. The first electron-microscopic study of such case was performed by her. She observed a large number of microfilaments in the mucus producing cells of the stomach mucosa, and assumed this to be indicative of a pathologic mucus secretion. The epithelial cells involved also changes indicating the disorder of protein secretion. She treats in detail the electron-microscopic characteristics of cellular elements found in amyloid. The closest relationship to amyloid deposits was shown by myofibroblasts. They probably play an important role in the formation of local amyloid.

Demonstration and partial characterization of an atypical protein in the urine of a patient with primary amyloidosis

A paraprotein has been isolated from the urine of a patient with primary amyloidosis. Immunologically it was classified as a free lambda light chain. The molecular weight was 22500 daltons. N-terminal amino acid analysis demonstrated homology with lambda IV variable subgroup in 19 of the first 20 amino acids. Extensive homology with lambda IV chains was demonstrated also in the hypervariable region of the light chain. An antiserum produced against the paraprotein was rendered idiotype-specific by absorption with pooled human light chains. This antiserum stained tissue specimen from the rectum and liver of the patient by the indirect immunofluorescence technique. This strongly indicates that the free lambda light chains that can be isolated from the urine are also deposited in the tissues as amyloid substance.