There are approximately 55 million people living with Alzheimer's disease (AD) worldwide. As the population ages, this number doubles every 20 years. Among people over the age of 85, one in three people has Alzheimer's disease. Among them, sporadic and late-onset Alzheimer's disease (LOAD) is the most common type of Alzheimer's disease, accounting for more than 95% of the total cases. The symptoms experienced by people with late-onset Alzheimer's disease usually begin to appear when they are 65 years old or older. Before patients develop symptoms such as amnesia, the disease has actually been developing in the brain for decades. Unfortunately, we currently have no cure for Alzheimer's disease. Because the cause of late-onset Alzheimer's disease is very complex and is not determined by genetic inheritance alone, there are also many non-genetic factors that affect the occurrence of the disease, so the factors for treating each patient are different. Therefore, it is difficult to simulate such diseases in laboratory studies using traditional methods of highly expressing genes. Until now, we have no suitable model to study sporadic, late-onset Alzheimer's disease, which has led to our understanding of the mechanism of the disease and restricted our ability to find new treatments.

Recently, Dr. Sun Zhao and other researchers from the team of Andrew Yoo, a senior professor of developmental biology at Washington University School of Medicine in St. Louis, published a research paper in Science titled: Endogenous recapitulation of late-onset Alzheimer's disease neuropathology via direct neural reprogramming. The paper has made a breakthrough in establishing a disease model for late-onset Alzheimer's disease. Dr. Sun Zhao and others have developed a method based on highly expressed microRNAs to directly reprogram/directly transform patient skin cells into neuronal cells. The discovery makes it possible to study brain cells in Alzheimer's patients without conducting a human brain biopsy. What's more interesting is that these in vitro induced neurons act like a mirror, accurately displaying the main pathological characteristics of the brain of patients with Alzheimer's disease: including clinically seen beta amyloid (Aβ), tau protein deposition, and neuron death. The study also used this model to discover a new way to alleviate neuronal death in Alzheimer's disease: by preventing the dysregulation of transposon elements ("skipping genes"), the researchers successfully reduced the death of patient-derived neuronal cells, making them healthier. This major discovery is crucial for the study of the pathology and mechanism of Alzheimer's disease, as well as the development of personalized treatment methods and drug screening for Alzheimer's patients.

In detail, the research team used microRNAs (miRNAs) enriched in the human brain: miR-9/9* and miR-124, as factors that efficiently induce skin cells to become neurons. In a three-dimensional (3D) culture environment, the patient's skin fibroblasts were converted into neurons, establishing a powerful conversion platform that can capture age and AD phenotype. The researchers found that neurons generated through direct reprogramming retained the cellular age factor of the affected individuals, laying the foundation for this cellular model to be able to reproduce the pathological and degenerative processes associated with late-onset Alzheimer's disease. This has a huge technical advantage compared with neurons derived from induced pluripotent stem cells (iPSCs) that lose the original age characteristics of the patient from which they originated and become neurons in the embryonic stage.

As a proof of principle, the researchers first used skin cells from the familial, dominant inherited form of Alzheimer's disease (ADAD) for in vitro reprogramming. Because this kind of familial Alzheimer's disease is caused entirely by a single gene mutation. Compared with late-onset Alzheimer's disease, its etiology is relatively simple, so it is relatively easy to simulate in theory. But this familial type of Alzheimer's disease is rare (accounting for only 5% or less of the total number of Alzheimer's patients). Researchers overexpressed miR-9/9*-124, NEUROD2 and MYT1L in skin cells derived from patients with dominant inherited Alzheimer's disease to induce neuron production. There are two methods for 3D culture of neurons induced by this miRNA: 1) The neurons are cultured in a thin gel composed of Matrigel. 2) Culturing neurons in high cell density, self-assembled neuronal spheres. By comparing them with control neurons from normal cognitive individuals of similar age, the researchers found that neurons derived from patients with dominant inheritance of Alzheimer's disease showed Aβ amyloid protein, a contagious, insoluble tau protein formation, abnormal swelling of neurites, and nerve death. More importantly, when researchers applied this in vitro 3D neural reprogramming method to fibroblasts from individuals with sporadic, non-inherited, late-onset Alzheimer's disease, it also effectively demonstrated the hallmark pathological features of AD. Interestingly, deinhibiting beta amyloid (Aβ) production in the early stages of in vitro neural induction reduces Aβ deposition, tauopathy, and nerve death, while intervening after Aβ deposition has begun to form has less effect. This is consistent with the conclusions of clinical Aβ antibody treatment. The importance of early treatment or intervention was emphasized.

In addition, in vitro transformed neurons derived from late-onset Alzheimer's disease patients showed changes in gene expression associated with neuroinflammation compared to age-matched controls. Interestingly, compared to neurons derived from control groups of young healthy individuals (ages 36-61), neurons from older healthy controls (66-90) and late-onset Alzheimer's disease patients (66-90) showed abnormal expression of retrotransposable elements (RTE), a small piece of DNA that can jump to different positions in the genome. After adding an inhibitor of reverse transcriptase called lamivudine (3TC) to neurons derived from patients with late-onset Alzheimer's disease, researchers found that the inhibitor reduced age-related RTE disorders in neurons with late-onset Alzheimer's disease, resulting in effective reductions in Aβ, tau protein, neuron death and DNA damage. Analysis of differentially expressed genes in RNA sequencing results also proved that changes in the expression of some genes were associated with inflammatory responses. Interestingly, lamivudine has no significant effect on the degradation of neurons derived from patients with early-onset, hereditary Alzheimer's disease (ADAD). This proves that sporadic, late-onset Alzheimer's disease (LOAD) is different in terms of molecular characteristics and pathogenesis from early-onset, hereditary Alzheimer's disease (ADAD).

In summary: The results of this study demonstrate the feasibility and adequacy of inducing skin cells derived from patients with late-onset Alzheimer's disease to transform into neurons in a 3D environment, and using this neuron cells to model late-onset Alzheimer's disease. These directly transformed and cultured neurons in vitro will provide a good research platform for understanding how aging affects neurodegeneration in patients with late-onset Alzheimer's disease. The platform will also help develop drugs and methods that can effectively treat Alzheimer's disease and provide an important tool for understanding the complex mechanisms of the disease.

Dr. Mattia Maroso, senior editor of the journal Science, said: "This method of modeling late-onset Alzheimer's disease using direct reprogramming technology is expected to help scientists better understand the complex pathology of late-onset Alzheimer's disease and provide effective treatment strategies for the development of this currently incurable disease."

Dr. Zhao Sun, Washington University School of Medicine, St. Louis, is the lead author and Dr. Andrew Yoo is the corresponding author.

Original link: science.org/doi/10.1126/science.adl2992