Binding of ubiquitin to experimentally induced murine AA amyloid

Amyloid enhancing factor (AEF) activity has recently been demonstrated in ubiquitin purified from amyloidotic murine tissues and Alzheimer brain extract. Since AEF is known to bind to amyloid fibrils and 'fibril-AEF' on passive transfer induces accelerated amyloidogenesis in the recipient animals, it was of interest to investigate whether ubiquitin binds to amyloid. Immunohistological studies were carried out on liver sections from amyloidotic mice. Biotin-strepavidin-peroxidase methods using monospecific rabbit anti-mouse AA amyloid IgG (RAAG) and rabbit anti-bovine ubiquitin IgG (RABU) antibodies were employed to immunostain the amyloid and ubiquitin deposits, respectively. RABU-treated liver sections were counterstained with thioflavine S. RAAG reacted strongly with the amyloid, indicating that it is AA type, and RABU-positive immunodeposits were found bound to the thioflavine-S-positive AA deposits. Treatment of the liver sections with 0.1 M sodium acetate containing 0.5 M NaCl, pH 4, for 2-3 h at 37 degrees C nearly completely desorbed the AA amyloid-bound ubiquitin. Since ubiquitin demonstrates AEF activity in vivo and binds non-covalently to AA amyloid, we suggest that ubiquitin may indeed be 'fibril-AEF' and may play a crucial role in the pathogenesis of amyloidosis. To our knowledge, this is the first time that ubiquitin bound to extracellularly deposited amyloid has been demonstrated.