A high resolution ultrastructural study of experimental murine AA amyloid.

An essential and distinguishing feature of all amyloids is the presence of fibrillar structures of approximately 10-nm width. The precise nature of the fibril is not yet clearly understood, particularly in situ, and the ultrastructure of isolated fibrils differs significantly from that of fibrils observed in situ. The fibrils are generally believed to be composed of a protein specific to each type of amyloid, but increasing evidence suggests additional associations with other components such as heparan sulfate proteoglycan (HSPG) and amyloid P component (AP). Experimental AA amyloidosis was induced in mice by amyloid enhancing factor and an inflammatory stimulus (subcutaneous AgNO3); fibrils were thereafter examined in detail. Particular attention was paid to ultrastructural characteristics known to represent particular molecular components of basement membranes such as HSPG and AP. Additionally, rabbit anti-mouse AA antisera was used with 5-nm and 1-nm gold particles to establish the location of the AA protein in-situ. Amyloid fibrils could be identified in their mature form as well as at apparent intermediate stages of formation. The fibril contained an apparent core which is composed of an assembly of 3.5-nm wide pentosomal particles having the characteristics of AP. Wound around the AP assembly in a helical fashion is a "double tracked" ribbon-like entity, 3 nm wide, having the morphologic characteristics of chondroitin sulfate proteoglycan (CSPG). Covering the surface of this structure is a second ribbon-like double track structure, but this one is wider (4.6 nm vs 3.0 nm) than the CSPG. These have the ultrastructural characteristics of HSPG. Routine fixation and tissue preparation techniques that usually remove HSPG from microfibrils did not do so with amyloid fibrils, suggesting an alteration in affinity between these components. The AA protein could be identified as a 1 - to 2-nm filament network on the most exterior surface of the fibril. The ultrastructure of AA amyloid fibrils in situ resembles that of connective tissue microfibrils, and, in addition to AA protein, is likely composed of HSPG, CSPG, and AP. Amyloid fibrils can be distinguished from microfibrils by the apparently stronger binding of HSPG to the surface of the amyloid fibril and the presence of the AA filaments. A model of the in situ organization of AA amyloid fibrils is proposed.