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Reduction of β2-microglobulin improves Alzheimer's disease symptoms in animal models


Misfolding and aggregation of β-amyloid (Aβ), as well as microtubule-associated tau protein, represent the main pathological features of Alzheimer's disease. The central role of Aβ in the pathogenesis of the disease is also supported by genetics, as mutations in the APP, PSEN1, and PSEN2 genes, all of which are associated with the production of Aβ peptides, are associated with early-onset familial AD. In these cases, Aβ accumulation in the brain is observed long before the manifestation of clinical symptoms; however, it also occurs in older people who do not have memory impairment and are thought to have protective factors that also appear to be inherited.


In addition, genome-wide association studies (GWAS) indicate strong associations between immunologically relevant genes and risk for AD. To date, more than 30 such gene loci have been identified and more than 50% of the variants associated with these genes are involved in immune responses. For example, they encode human leukocyte antigens (HLA) or are associated with the so-called inflammasome, such as apoptosis-associated speck-like protein (ASC), which promotes the formation of Aβ-oligomers. Clinical studies have shown that severe infections, particularly sepsis and pneumonia, are associated with an increased risk of dementia.


A study published by Xin Wang, Yini Zhao, and colleagues in Nature Neuroscience has now also identified β2-microglobulin (β2M) as an amyloidogenic protein. It forms the soluble subunit of the histocompatibility complex class I (MHC-I) light chain and is responsible for antigen presentation to activate cytotoxic T lymphocytes. Thus, it facilitates the differentiation between self and foreign in the context of infection. β2M is released from MHC-I, expressed particularly by microglia in the brain, and can assemble into amyloid fibrils.


The authors, working at Xiamen University in China, found that β2M is elevated in the brains of people with Alzheimer's disease and forms the core of amyloid plaques. Electron microscopy analysis showed that β2M and Aβ42, the Aβ-peptide particularly prone to aggregation, induce the formation of fibrillar aggregates. Stereotactic injections of adeno-associated viral vectors containing the β2M protein into the brains of 5×FAD mice enhanced AD-associated neuropathology (the 5xFAD mice are transgenic animals overexpressing genes for the Aβ-precursor protein APP and for presenilin 1, among others). Conversely, a reduction in β2M levels abolished Aβ neurotoxicity, as amyloid spread and cognitive deficits were reduced in the animals. Interestingly, deletion of β2M also significantly attenuated tau pathology in cultured neurons.


Using two-photon imaging, the authors further demonstrated that peripheral β2M located in blood vessels crossed the blood-brain barrier and accumulated in the brain. In this regard, the neuropathology of 5×FAD mice improved decisively when antibodies were injected that resulted in a reduction of peripheral β2M. It has been known for some time that β2M appears to function as a systemic pro-aging factor that can impair neurogenesis and lead to cognitive dysfunction (presumably via interference with the glutamta NMDA receptor).


In addition to antibodies, β2M antisense oligonucleotides (ASOs) were injected into 5×FAD mice and a reduction in amyloid pathology was subsequently observed. Combinations of anti-β2M antibodies and specific β2M peptides that prevent β2M-Aβ co-aggregation were also tested. They attenuated Aβ-neurotoxicity and significantly reduced cognitive impairment in mice. Unfortunately, because antibodies are usually administered in high doses in clinical trials to ensure that sufficient amounts cross the blood-brain barrier, side effects of antibody therapy, some of them severe, are also repeatedly observed. These include Aβ-antibody (Fc receptor)-mediated neuroinflammatory response and activation of immune cells in the brain. Therefore, alternative agents, such as ASOs or aggregation-preventing β2M-peptides, are of particular importance. Presumably, accurate structural characterization of the β2M-Aβ complex would also lead to the discovery of small molecule blockers that could be of therapeutic benefit in AD therapy as well.


References:


Xin Wang & Yini Zhao (2023) β2-Microglobulin boosts β-amyloid aggregation and neurotoxicity in an Alzheimer's disease model. Nature Neuroscience 26:1143


Zhao Y, Zheng Q, Hong Y, Gao Y, ..., Liu C, Wang X (2023) β2-Microglobulin coaggregates with Aβ and contributes to amyloid pathology and cognitive deficits in Alzheimer's disease model mice. Nature Neuroscience 26:1170


Image credit: iStock/Storman

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