Serum amyloid A forms stable oligomers that disrupt vesicles at lysosomal pH and contribute to the pathogenesis of reactive amyloidosis

Abstract

Serum amyloid A (SAA) is an acute-phase plasma protein that functions in innate immunity and lipid homeostasis. SAA is a protein precursor of reactive AA amyloidosis, the major complication of chronic inflammation and one of the most common human systemic amyloid diseases worldwide. Most circulating SAA is protected from proteolysis and misfolding by binding to plasma high-density lipoproteins. However, unbound soluble SAA is intrinsically disordered and is either rapidly degraded or forms amyloid in a lysosome-initiated process. Although acidic pH promotes amyloid fibril formation by this and many other proteins, the molecular underpinnings are unclear. We used an array of spectroscopic, biochemical, and structural methods to uncover that at pH 3.5-4.5, murine SAA1 forms stable soluble oligomers that are maximally folded at pH 4.3 with ∼35% α-helix and are unusually resistant to proteolysis. In solution, these oligomers neither readily convert into mature fibrils nor bind lipid surfaces via their amphipathic α-helices in a manner typical of apolipoproteins. Rather, these oligomers undergo an α-helix to β-sheet conversion catalyzed by lipid vesicles and disrupt these vesicles, suggesting a membranolytic potential. Our results provide an explanation for the lysosomal origin of AA amyloidosis. They suggest that high structural stability and resistance to proteolysis of SAA oligomers at pH 3.5-4.5 help them escape lysosomal degradation, promote SAA accumulation in lysosomes, and ultimately damage cellular membranes and liberate intracellular amyloid. We posit that these soluble prefibrillar oligomers provide a missing link in our understanding of the development of AA amyloidosis. prefibrillar amyloid oligomers | intrinsically disordered proteins | pH-induced conformational changes | hydrophobic cavities | acute-phase reactant

Serum amyloid A promotes LPS clearance and suppresses LPS ‐induced inflammation and tissue injury

“…In addition to the inducible feature, our approach allows for the production of lipid-free or lipid-poor SAA1 proteins that are crucial to studies of its interaction with cell surface receptors and pathogen-derived molecules such as LPS. The forms and sites of SAA production are important to experimental design, given that SAA in lipid-bound and lipid-free forms may produce different biological functions [26,27]. Using the SAA1 transgenic mice, we report for the first time that acute-phase SAA1 offers partial protection against LPS-induced inflammation and acute lung injury through direct binding to LPS and promotion of its clearance.…”

“…In addition to the inducible feature, our approach allows for the production of lipid‐free or lipid‐poor SAA1 proteins that are crucial to studies of its interaction with cell surface receptors and pathogen‐derived molecules such as LPS. The forms and sites of SAA production are important to experimental design, given that SAA in lipid‐bound and lipid‐free forms may produce different biological functions .…”

“…The well-documented beta-pleated sheets in amyloid A were, however, entirely missing from the solved crystal structure. A recent study reports that, under low pH conditions (pH 3.5-4.5), SAA1 forms soluble oligomers that undergo an ahelix to b-sheet conversion catalyzed by lipid vesicles [27]. Pep 2 (a.a. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] that disrupts SAA1-LPS interaction overlaps with the second helix of mature SAA1, suggesting that helix II or a 3D structure containing helix II is involved in LPS binding.…”

Real-Time 3D Imaging and Inhibition Analysis of Various Amyloid Aggregations Using Quantum Dots

“…The global amyloidosis epidemic is a major complex combination of many diseases involving chronic inflammatory and misfolding of proteins that are characterized by the accumulation of amyloid-plaques and neurofibrillary tangles in tissues and organs [27][28][29]. Although there have been breakthroughs related to the structure and molecular mechanisms of amyloid proteins [30][31][32], the pathogenic mechanism of amyloidosis is still largely unknown.…”

Lipid membranes accelerate amyloid formation in the mouse model of AA amyloidosis

“…In a recent study on murine lipid free SAA, it was shown that SAA forms unusually stable soluble oligomers at pH 3.5-4.5 that may escape from lysosomal degradation and contribute to the SAA accumulation in lysosomes and eventually participate in the formation of intracellular amyloid. In addition, lipid vesicles accelerate the conversion of these oligomers from a-helix to b-sheet which involves structural remodelling of both protein and lipid as well as disruption of lipid membrane [45]. When SAA concentration is elevated, it dissociates from HDL and consequently a fraction of SAA might circulate in plasma as lipid-free SAA [46].…”