A HIGH-RESOLUTION ULTRASTRUCTURAL-STUDY OF EXPERIMENTAL MURINE AA AMYLOID

An essential and distinguishing feature of all amyloids is the presenc e of fibrillar structures of approximately 10-nm width. The precise na ture of the fibril is not yet clearly understood, particularly in situ , and the ultrastructure of isolated fibrils differs significantly fro m that of fibrils observed in situ. The fibrils are generally believed to be composed of a protein specific to each type of amyloid, but inc reasing evidence suggests additional associations with other component s such as heparan sulfate proteoglycan (HSPG) and amyloid P component (AP). Experimental PLA amyloidosis was induced in mice by amyloid enha ncing factor and an inflammatory stimulus (subcutaneous AgNO3); fibril s were thereafter examined in detail. Particular attention was paid to ultrastructural characteristics known to represent particular molecul ar components of basement membranes such as HSPG and AP. Additionally, rabbit anti-mouse AA antisera was used with 5-nm and 1-nm gold partic les to establish the location of the AA protein in-situ. Amyloid fibri ls could be identified in their mature form as well as at apparent int ermediate stages of formation. The fibril contained an apparent core w hich is composed of an assembly of 3.5-nm wide pentosomal particles ha ving the characteristics of AP. Wound around the AP assembly in a heli cal fashion is a ''double tracked'' ribbon-like entity, 3 nm wide, hav ing the morphologic characteristics of chondroitin sulfate proteoglyca n (CSPG). Covering the surface of this structure is a second ribbon-li ke double track structure, but this one is wider (4.6 nm vs 3.0 nm) th an the CSPG, These have the ultrastructural characteristics of HSPG. R outine 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 cou ld 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 si tu resembles that of connective tissue microfibrils, and, in addition to AA protein, is likely composed of HSPG, CSPG, and AP. Amyloid fibri ls can be distinguished from microfibrils by the apparently stronger b inding of HSPG to the surface of the amyloid fibril and the presence o f the AA filaments. A model of the in situ organization of AA amyloid fibrils is proposed.