Alpha2-macroglobulin is a potential facilitator of prion protein transformation

The Emerging Roles of Extracellular Chaperones in Complement Regulation

The immune system is essential to protect organisms from internal and external threats. The rapidly acting, non-specific innate immune system includes complement, which initiates an inflammatory cascade and can form pores in the membranes of target cells to induce cell lysis. Regulation of protein homeostasis (proteostasis) is essential for normal cellular and organismal function, and has been implicated in processes controlling immunity and infection. Chaperones are key players in maintaining proteostasis in both the intra- and extracellular environments. Whilst intracellular proteostasis is well-characterised, the role of constitutively secreted extracellular chaperones (ECs) is less well understood. ECs may interact with invading pathogens, and elements of the subsequent immune response, including the complement pathway. Both ECs and complement can influence the progression of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis, as well as other diseases including kidney diseases and diabetes. This review will examine known and recently discovered ECs, and their roles in immunity, with a specific focus on the complement pathway.

Alpha-2-Macroglobulin, a Hypochlorite-Regulated Chaperone and Immune System Modulator

Alpha-macroglobulins are ancient proteins that include monomeric, dimeric, and tetrameric family members. In humans, and many other mammals, the predominant alpha-macroglobulin is alpha-2-macroglobulin (α₂M), a tetrameric protein that is constitutively abundant in biological fluids (e.g., blood plasma, cerebral spinal fluid, synovial fluid, ocular fluid, and interstitial fluid). α₂M is best known for its remarkable ability to inhibit a broad spectrum of proteases, but the full gamut of its activities affects diverse biological processes. For example, α₂M can stabilise and facilitate the clearance of the Alzheimer's disease-associated amyloid beta (Aβ) peptide. Additionally, α₂M can influence the signalling of cytokines and growth factors including neurotrophins. The results of several studies support the idea that the functions of α₂M are uniquely regulated by hypochlorite, an oxidant that is generated during inflammation, which induces the native α₂M tetramer to dissociate into dimers. This review will discuss the evidence for hypochlorite-induced regulation of α₂M and the possible implications of this in neuroinflammation and neurodegeneration.

Extracellular chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Extracellular Chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Prion protein paralog doppel protein interacts with alpha-2-macroglobulin: a plausible mechanism for doppel-mediated neurodegeneration

Doppel protein (Dpl) is a paralog of the cellular form of the prion protein (PrP^C), together sharing common structural and biochemical properties. Unlike PrP^C, which is abundantly expressed throughout the central nervous system (CNS), Dpl protein expression is not detectable in the CNS. Interestingly, its ectopic expression in the brain elicits neurodegeneration in transgenic mice. Here, by combining native isoelectric focusing plus non-denaturing polyacrylamide gel electrophoresis and mass spectrometry analysis, we identified two Dpl binding partners: rat alpha-1-inhibitor-3 (α₁I₃) and, by sequence homology, alpha-2-macroglobulin (α₂M), two known plasma metalloproteinase inhibitors. Biochemical investigations excluded the direct interaction of PrP^C with either α₁I₃ or α₂M. Nevertheless, enzyme-linked immunosorbent assays and surface plasmon resonance experiments revealed a high affinity binding occurring between PrP^C and Dpl. In light of these findings, we suggest a mechanism for Dpl-induced neurodegeneration in mice expressing Dpl ectopically in the brain, linked to a withdrawal of natural inhibitors of metalloproteinase such as α₂M. Interestingly, α₂M has been proven to be a susceptibility factor in Alzheimer's disease, and as our findings imply, it may also play a relevant role in other neurodegenerative disorders, including prion diseases.

Senile systemic amyloidosis affects 25% of the very aged and associates with genetic variation in alpha2-macroglobulin and tau: a population-based autopsy study

BACKGROUND

Senile systemic amyloidosis (SSA) is characterized by deposition of wild-type transthyretin (TTR)-based amyloid in parenchymal organs in elderly individuals. Previously, no population-based studies have been performed on SSA.

METHODS

Here we have studied the prevalence and risk factors for SSA in a Finnish autopsied population aged 85 or over, as part of the population-based Vantaa 85+ Autopsy Study (n = 256). The diagnosis of SSA was based on histological examination of myocardial samples stained with Congo red and anti-TTR immunohistochemistry. The genotype frequencies of 20 polymorphisms in 9 genes in subjects with and without SSA were compared.

RESULTS

The prevalence of SSA was 25%. SSA was associated with age, myocardial infarctions, the G/G (Val/Val) genotype of the exon 24 polymorphism in the alpha2-macroglobulin (alpha2M), and the H2 haplotype of the tau gene (P-values 0.002, 0.004, 0.042, and 0.016).

CONCLUSION

This population-based study shows that SSA is very common in old individuals, affecting one-quarter of people aged over 85 years. Myocardial infarctions and variation in the genes for alpha2M and tau may be associated with SSA.