Dr. Jingeon Bae
Genes are the underlying factor in many human diseases. The amyloid precursor protein (APP) gene encodes amyloid precursor protein (APP), a transmembrane protein that produces the amyloid Aβ peptide. Mutations in APP are an intrinsic factor in familial forms of early-onset Alzheimer’s disease and cerebral amyloid angiopathy (CAA). These pathogenic mutations typically alter processing by secretory enzymes, resulting in an overall increase in Aβ production or changes in the proportion of specific Aβ peptides.
Uppsala is widely known as Uppsala University, the oldest university in Sweden. It is the capital of the province of Uppsala, about 70 km north of Stockholm, and has a population of about 130,000 inhabitants. Uppsala’s association with Alzheimer’s occurred five years ago, when scientists discovered a deletion of six amino acids (triangles in center of image) in the APP gene and called it the “Uppsala deletion.”
Figure 1: The “Uppsala deletion,” one of the APP mutations, has sequences 19-24 removed in the middle of the Aβ amino acids.
Three members of a Swedish family, each with a copy of the Uppsala fruit, reported developing Alzheimer’s disease in their 40s, relatively early in life. This mutation removes residues 19–24 in the middle of the Aβ sequence, making APP more susceptible to rapid aggregation of β-cleavage and extrusion peptides than full-length Aβ.
First, the “Uppsala deletion” was first discovered in a family living in Uppsala, Sweden, and now researchers led by Dag Sehlin and Martin Ingelsson of Uppsala University have created mice that express the deletion APP Uppsala gene. Mice with this “Uppsala deletion,” reported in the Feb. 5 issue of Acta Neuropathologica Communications, produce Aβ fibrils that cause poor responses from microglia and astrocytes. It is reported to be difficult to detect using PiB PET (positron emission tomography), which can identify amyloid plaques, and does not bind well to lecanemab, a beta-amyloid antibody, but binds tightly to bapineuzumab. This is because bapinezumab is an antibody that targets the Aβ N-terminus. “[이 쥐들은] “We summarize several pathological features of Uppsala APP mutation carriers,” the authors write in the article.
First author María Pagnon de la Vega engineered mice to express human APP, including the Uppsala deletion and the Swedish mutation (ArcSwe). It is cleaved more easily by β-secretase, and the latter has been used extensively in mice to accelerate Aβ pathology. Sehlin said he had bred mice with only the Uppsala defect, but all of these rodents died for unknown reasons.
At six months of age, the mouse’s plaques, called “tg-UppSwe,” were almost entirely made up of AβUpp42. In other words, 6 amino acids 1-42 were not eliminated. Immunostaining showed that small, diffuse plaques of Aβ accumulated first in the prefrontal cortex at 4 months, then in the hippocampus and cerebral cortex at 8 and 13 months, and finally in the thalamus at 18 months (image below). Mice of all ages had little soluble or plaque Aβ40. This reproduces in animal models the Aβ42-rich, Aβ40-poor plaques observed in the autopsy of a family member with the “Uppsala deletion.”
Figure 2: Uppsala spread. Tg-UppSwe mice accumulated deficient Aβ40 aggregates with increasing age (left column). Aβ42 plaques originate in the prefrontal cortex, spread through the cerebral cortex, and then invade the thalamus (right row). [Pagnon de la Vega 저자 외, Acta Neuropathologica Communications, 2024.]
For other pathologies, fewer synapses around Aβ fibrils were detected in array tomography of brain tissue from 18-month-old “tg-UppSwe” mice. Interestingly, these aggregates showed little response by astrocytes or microglia, as judged by GFAP and Iba1 immunostaining of brain tissue and by ELISA of soluble GFAP and TREM2 in brain extracts. The few activated astrocytes and microglia that the mice selected appeared unable to detect the fibrils because they barely stained with the plaques in the brain slices. Sehlin attributed this to mice having poorly soluble Aβ oligomers, which attract glial cells to pack the Aβ into dense nuclear plaques. These soluble aggregates can be found in high-throughput supernatants of mouse brain extracts, but Pagnon de la Vega rarely found them in tg-UppSwe mice.
Indeed, Andrew Stern of Brigham and Women’s Hospital in Boston agrees that the lack of soluble forms of Aβ may explain the poor microglial response. “Glial access to adequately sized diffusible Aβ aggregates can result in inflammation, synapse loss, and tau aggregation,” he wrote (described below). In addition to exhibiting almost no gliosis, like most amyloid models, the tg-UppSwe mice had no tau pathology.
Figure 3: Invisible plaque? Amyloid plaques from 18-month-old UppSwe (left) and ArcSwe (right) mice bind bapinezumab (top), but only ArcSwe plaques bind lecanemab (bottom right). [Pagnon de la Vega 저자 외, Acta Neuropathologica Communications, 2024.]
Unlike other amyloid models, including age-matched mice expressing APP with the Arctic and Swedish (ArcSwe) mutations, 18-month-old tg-UppSwe mice showed no amyloid PET in an in vivo scan with PiB binding to dense central plates. no signal. Amyloid tg-UppSwe did not bind to radiolabeled mAb158, the mouse equivalent of lecanemab, which binds specifically to Aβ fibrils (top image). These findings reflect the weak PiB PET signal observed in “Uppsala deletion” carriers and the barely detectable levels of soluble Aβ fibrils in Uppsala brains at autopsy. In contrast, the brains of UppSwe mice were labeled with mAb3D6, a predecessor of bapineuzumab that, like lecanemab, binds to the N-terminus of Aβ (top image). “The results… show that mutations distant from this region can lead to stable differences in structure that are reflected in the ability of antibodies to bind to this region,” concluded Charles Glabe of the University of California, Irvine.
“Uppsala amyloid deposits are detectable in vivo, but their shape is antibody-specific,” Sehlin told Alzforum. According to Glabe, this means that “pan” Aβ antibodies may not be present. “This may limit the effectiveness of individual monoclonal antibodies,” she wrote. “This principle is important for drug development, especially in the era of antibody therapies,” Sehlin said. She hesitated to speculate on what these findings might mean for the two living APPUpp carriers. Because they also have a copy of the regular APP.
Interestingly, Sehlin and colleagues found that Aβ aggregates in the soluble fraction of tg-UPPSWe mice bound better to lecanemab than plaques or aggregates released from the detergent, suggesting that the peptide changes shape when it becomes insoluble. “I think there is not yet sufficient evidence to reach this conclusion,” Stern wrote. “We hope that the high-resolution aggregate structures obtained from all fractions of the tg-UppSwe tissue and the human Uppsala mutant tissue will allow us to test the following hypotheses: can different polymorphs be identified in different fractions or are they mostly the same ? length; Are the structures different only in the compression and/or ligation partners?” he asked. Sehlin, in collaboration with Gunnar Schröder from Forschungszentrum Jülich, Germany, analyzed the structure of AβUpp fibrils from tg-UppSwe mice and autopsied the APPUpp carriers using cryoEM.
To summarize this study, the researchers first created APP-bearing mice with the “Uppsala deletion.” These mice accumulate Aβ fibrils that elicit little response from glial cells. These Aβ fibrils evaded PiB PET and the Aβ antibody lecanemab, but bound to bapineuzumab.
References
Altered amyloid-β structure markedly reduces gliosis in the brains of mice harboring the Uppsala APP deletion. Acta Neuropathol Commun. 2024 Feb 5;12(1):22. PubMed. ‘Alzforum’ 16 February 2024
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