C1Q complexes with beta-2-microglobulin and amyloid P-component in sera of patients with dialysis-associated amyloidosis

beta(2)-microglobulin: from physiology to amyloidosis

beta(2)-microglobulin (beta(2)m) is capable of forming amyloid in osteoarticular structures in kidney failure patients that undergo chronic hemodialysis treatment. Although sophisticated analytical methods have yielded comprehensive data about the conformation of the native protein both as a monomer and as the light chain of the type I major histocompatibility complex, the cause and mechanisms leading to the transformation of beta(2)m into amyloid deposits in patients with dialysis-related amyloidosis are unsettled. The impact on conformational stability of various truncations, cleavages, amino acid substitutions, and divalent cations, especially Cu(2+), however, are highly relevant for understanding beta(2)m unfolding pathways leading to amyloid formation. This review describes the current knowledge about such conformationally destabilizing and amyloidogenic factors and links these to the structure and function of beta(2)m in normal physiology and pathology. Tables listing modifications of beta(2)m found in amyloid from patients and a systematic overview of laboratory conditions conducive to beta(2)m-fibrillogenesis are also included.

Molecular and cellular aspects of protein misfolding and disease

Proteins are essential elements for life. They are building blocks of all organisms and the operators of cellular functions. Humans produce a repertoire of at least 30,000 different proteins, each with a different role. Each protein has its own unique sequence and shape (native conformation) to fulfill its specific function. The appearance of incorrectly shaped (misfolded) proteins occurs on exposure to environmental changes. Protein misfolding and the subsequent aggregation is associated with various, often highly debilitating, diseases for which no sufficient cure is available yet. In the first part of this review we summarize the structural composition of proteins and the current knowledge of underlying forces that lead proteins to lose their native structure. In the second and third parts we describe the molecular and cellular mechanisms that are associated with protein misfolding in disease. Finally, in the last part we portray recent efforts to develop treatments for protein misfolding diseases.

Serum amyloid P component induces neuronal apoptosis and beta-amyloid immunoreactivity

Previously we have reported serum amyloid P component (SAP) induced cell death in cerebro-cortical cultures of rat brain. In this paper we studied the types of target cells and the molecular mechanism of SAP-induced cell death. Immuno-electron and light microscopy revealed that SAP penetrates the plasma membrane and translocates selectively into the nuclei of neurons. Neuronal cells with SAP immunoreactivity exhibit the morphological hallmarks of apoptosis in vitro. The apoptotic mechanism of cell death is also supported by the increased Bax/Bcl-2 ratio. In addition to neurotoxic effects, we detected elevated beta-amyloid (Abeta) immunoreactivity following SAP treatment. This study supports the thesis that SAP plays an important role in the pathomechanism of neurodegenerative diseases, including Alzheimer's disease by inducing neuronal apoptosis.

Complement activation by the amyloid proteins A beta peptide and beta 2-microglobulin

Complement activation (CA) has been reported to play a role in the pathogenesis of Alzheimer's disease (AD). To investigate whether CA may contribute to amyloidogenesis in general, the CA potential of different amyloid fibril proteins was tested. CA induced by A beta preparations containing soluble protein, protofilaments and some fibrils or only fibrils in a solid phase system (ELISA) was modest with a slow kinetics compared to the positive delta IgG control. Soluble A beta induced no detectable CA in a liquid phase system (complement consumption assay) while fibrillar A beta caused CA at 200 mg/ml and higher concentrations. Soluble beta 2-microglobulin (beta 2M) purified from peritoneal dialysates was found to be as potent a complement activator as A beta in both solid and liquid phase systems while beta 2M purified from urine exhibited lower activity, a difference which may be explained by differences observed in SDS-resistant oligomers and isoforms. Soluble Amyloid A-protein caused no significant CA. A beta and beta 2M activated complement via the classical pathway. The modifying influence by amyloid-associated molecules on A beta-induced CA was also investigated, but neither serum amyloid P component nor heparan sulfate did significantly alter the A beta-induced CA. The results indicate that not only fibrillar A beta but also oligomers of, in particular, beta 2M from patients with dialysis-associated amyloidosis are capable of inducing CA at supra-physiological concentrations.