A characteristic hallmark of Alzheimer's disease (AD) is the deposition of amyloid plaques formed primarily from the amyloid-beta protein. Monomeric amyloid-beta self-associates to form the fibrillar amyloid-beta that is deposited as plaques. Initial aggregation of amyloid-beta monomer occurs via a rate-limiting nucleation step. Following nucleus formation, rapid growth ensues and proceeds through a soluble intermediate known as a protofibril, which will then progress to form amyloid-beta fibrils via two mechanisms of protofibril growth: elongation by monomer deposition and direct protofibril association [1]. Multiple lines of evidence suggest a pathogenic role for amyloid-beta assembly in the progression of AD, and increasing evidence points more specifically toward soluble protofibril intermediates as a pathogenic species.

Endogenous proteins bind to various assembly forms of amyloid-beta, including monomer, protofibril, and fibril, serving as carrier proteins as well as inhibitors for their self association [1]. In particular, the highly abundant plasma protein albumin is associated with amyloid plaques and is capable of binding the amyloid-beta protein. In fact, almost 90% of amyloid-beta protein in plasma is bound to albumin [2]. Furthermore, binding of albumin to amyloid-beta has been shown to inhibit fibril formation [3]. These endogenous interactions may have implications in disease progression, as well as toward the in vivo detection of amyloid-beta protein and inhibition of fibril formation which have emerged as strategies for the diagnosis and prevention of AD.

To further understand the relationship between albumin and amyloid-beta, we have investigated the interactions between bovine serum albumin (BSA) and the amyloid-beta protein, with a focus on differential interactions between BSA and the various aggregation species of amyloid-beta (monomer, soluble protofibril intermediates, and mature fibrils). Monomeric amyloid-beta was isolated using size exclusion chromatography. Subsequent aggregation of amyloid-beta monomer yielded aggregated mixtures from which mature fibrils were separated from soluble species via centrifugation and soluble protofibril intermediates were isolated from unreacted monomer using size exclusion chromatography [1]. The ability of BSA to bind different assembly species of amyloid-beta was quantified using a modified ELISA. Here, binding was measured for varying concentrations of amyloid-beta in the presence of a constant BSA concentration to determine dissociation constants (Kd). Subsequently, the ability of BSA to inhibit aggregation of amyloid-beta monomer, as well as subsequent growth of amyloid-beta aggregation intermediates, was explored using in vitro assays that isolate various mechanisms of fibril formation.

Experimental data illustrate that BSA binds amyloid-beta with high affinity, and these interactions result in the inhibition of aggregate formation and subsequent aggregate growth. Future experiments will probe whether the binding of BSA to amyloid-beta and the subsequent inhibition of amyloid-beta aggregation attenuates physiological responses induced by amyloid-beta. These findings will provide insight into the interactions between BSA and the amyloid-beta protein that could impact disease progression, as well as design of therapeutic agents and development of assays for the in vivo detection of amyloid-beta in disease diagnosis.

References:

1. Nichols, M.R., et al., Growth of beta-amyloid(1-40) protofibrils by monomer elongation and lateral association. Characterization of distinct products by light scattering and atomic force microscopy. Biochemistry, 2002. 41(19): 6115-6127.

2. Biere, A.L., et al., Amyloid beta-peptide is transported on lipoproteins and albumin in human plasma. J Biol Chem, 1996. 271(51): 32916-22.

3. Bohrmann, B., et al., Endogenous proteins controlling amyloid beta-peptide polymerization. Possible implications for beta-amyloid formation in the central nervous system and in peripheral tissues. J Biol Chem, 1999. 274(23): 15990-15995.