Additional Supra-Self-Assembly of Human Serum Albumin under Amyloid-Like-Forming Solution Conditions

Abstract

Protein aggregation has a multitude of consequences ranging from affecting protein expression to its implication in different diseases. Of recent interest is the specific form of aggregation leading to the formation of amyloid fibrils, structures associated with diseases such as Alzheimer's disease. These fibrils can further associate in other more complex structures such as fibrillar gels, plaques, or spherulitic structures. In the present work, we describe the physical and structural properties of additional supraself-assembled structures of human serum albumin under solution conditions in which amyloid-like fibrils are formed. We have detected the formation of ordered aggregates of amyloid fibrils, i.e., spherulites which possess a radial arrangement of the fibrils around a disorganized protein core and sizes of several micrometers by means of polarized optical microscopy, laser confocal microscopy, and transmission electron microscopy. These spherulites are detected both in solution and embedded in an isotropic matrix of fibrillar gels. In this regard, we have also noted the formation of protein gels when the protein concentration and/or ionic strength exceds a threshold value (the gelation point) as observed by rheometry. Fibrillar gels are formed through intermolecular nonspecific association of amyloid fibrils at a pH far away from the isolectric point of the protein where protein molecules seem to display a "solid-like" behavior due to the existence of non-DLVO (Derjaguin-Landau-Verwey-Overbeck) intermolecular repulsive forces. As the solution ionic strength increases, a coarsening of this type of gel is observed by environmental scanning microscopy. In contrast, at pH close to the protein isoelectric point, particulate gels are formed due to a faster aggregation process, which does not allow substantial structural reorganization to enable the formation of ordered structures. This behavior also additionally corroborates that the existence of particulates might also be a generic property of all polypeptide chains as amyloid fibril formation under suitable conditions.

Spherulites of Amyloid-β42 In Vitro and in Alzheimer's Disease

“…Spherulites of the proteins insulin, β lactoglobulin and human serum albumin all form spontaneously in solution at either acidic pH or high temperature, conditions which favour the denaturation of the protein [1-3]. We postulated that spherulites of Aβ 42 would form under near-physiological conditions in saturated preparations of peptide which were aged for many months at 37°C.…”

“…1g,h). Thioflavin T has been used previously to suggest that spherulites of human serum albumin are composed of amyloid fibrils [3]. The anomaly in the core of the larger spherulites was not labelled with either Congo red or ThT and, if it is not an artifact, is assumed, as in observations of other amyloid spherulitic structures [1,3], to be non-proteinaceous or non β sheet Aβ 42 .…”

“…The first observations in vitro of the self-assembly of amyloid fibrils into spherulites [1-3] prompted further studies both into their mode of formation [4-8] and their physiological significance. The possibility of the latter was suggested by the occurrence of spherulitic structures in various tissue preparations associated with neurodegenerative conditions [9,10].…”

Human pro-islet amyloid polypeptide (ProIAPP1–48) forms amyloid fibrils and amyloid spherulites in vitro

“…Several amyloidogenic proteins including insulin, β-lactoglobulin and albumin, form spherulites in vitro under non-physiological conditions [15,16]. These micrometer-sized, roughly spherical structures are composed of ordered arrays of amyloid fibrils in radial arrangements which, characteristically, show a typical Maltese cross pattern of light extinction under the polarising microscope.…”

Early stage aggregation of human serum albumin in the presence of metal ions

“…Factors such as temperature, concentration of the protein, pH and ionic strength values of the dispersion media, stirring and aging of the samples, presence of metal ions and lipid membranes, play a key role in triggering and speeding up the inter-molecular association as well as the formation of insoluble fibril plaques in several proteins. Indeed, different forms of aggregates have been found in transthyretin [8,9], lysozyme [10,11], myoglobin [12], rabbit muscle creatine kinase [13], ␤-lactoglobulin [14][15][16] and albumin from different sources [17][18][19][20][21][22][23].…”