[On amyloid genesis]

Bovine glomerular amyloidosis: morphological studies

Bovine kidney material with advanced glomerular deposits of amyloid was studied immunohistochemically using the avidin biotin complex immunoperoxidase method, with rabbit anti-horse AA serum as primary antibody. Severely affected glomeruli consisted of strongly reacting positive material, obscuring all cellular structures. Transmission electron microscopy revealed that the amyloidotic areas, with evident amyloid fibrils, also contained a considerable admixture of cellular remnants. In this investigation it was found that such material was more abundant in the bovine glomerular amyloid masses than in amyloid laden organs from other animal species, and it is discussed to what extent this observation may explain the varying tinctorial properties of amyloid deposits in bovine tissues, and the relative high content of histones in bovine amyloid proteins.

The perisinusoidal functional unit in amyloidosis

The close morphological and functional relations between the various components of the perisinusoidal functional unit may be demonstrated by combination of ultrastructural results with the latest findings in the context of intra-hepatic synthesis and transformation of amyloid. The precursors of amyloid, synthesised in hepatocytes in the region of the endoplasmic reticulum, lysosomes, and plasma membrane of the vascular pole, are moved into the vascular system together with high-density lipoproteins which are probably identical with lipoprotein vesicles close to plasma membranes and in Disse space, and in the vascular system they are transformed to Serum Amyloid A (SAA). A certain role is probably played by Kupffer cells in polymerisation of amyloid A to fibrils. Growing accumulation of fibrils causes ligature and melt-off of peripheral parts from the vascular pole of hepatocytes, with its volume being substantively reduced, so that the spacing between plasma membrane and nucleus may reduce from something between 10 and 12 micron to below 1 micron. There is neither any compression of cytoplasm nor translocation of organelles. Hence, the process can be interpreted as a specific form of atrophy.

Impaired Kupffer cell function precedes development of secondary amyloidosis

It has been demonstrated previously that the acute phase reactant, serum amyloid A (SAA), is subject to degradation by surface membrane-associated proteinases of peripheral blood monocytes. However, monocytes obtained from the blood of patients with amyloidosis degraded SAA incompletely, leaving a cleavage product that, biochemically and immunologically, resembled the amyloid protein A (AA) deposited in their tissues. To investigate the role of fixed macrophages in amyloidogenesis and to establish more definitively that amyloid deposition is attributable to faulty processing of the precursor protein rather than aberrant synthesis, secondary amyloidosis was induced in C57BL/6J mice by serial injections of casein. Kupffer cells (KC) were isolated from livers of mice that had received 0, 8, 13, 18, and greater than 30 injections of the stimulant. The cells were cultured with SAA for 4, 8, and 18 h and then subjected to electron microscopy and enzyme analyses. The medium was analyzed by SDS-PAGE to determine the amount of residual SAA and/or the appearance of AA. KC of healthy animals degraded SAA completely whereas KC of stimulated mice showed increasing amounts of residual SAA and the appearance of the AA cleavage product. The AA peptide appeared in KC cultures early during the course of casein injections and before any amyloid could be demonstrated in the organs of the stimulated mice. The addition of KC isolated from healthy mice to cultures that had produced AA eliminated the abnormal peptide. The results, indicate that defective KC function precedes amyloidosis. The abnormal AA cleavage product formed by such cells is still susceptible to hydrolysis by normal cells. In addition, ultrastructural evidence is presented that suggests that KC may also play a role in fibrillogenesis of the AA protein.