Novel culture system replicates course of Alzheimer’s disease, confirms amyloid hypothesis

An innovative laboratory culture system has succeeded, for the first time, in reproducing the full course of events underlying the development of Alzheimer’s disease. Using the system they developed, investigators from the Genetics and Aging Research Unit at Massachusetts General Hospital (MGH) now provide the first clear evidence supporting the hypothesis that deposition of beta-amyloid plaques in the brain is the first step in a cascade leading to the devastating neurodegenerative disease. They also identify the essential role in that process of an enzyme, inhibition of which could be a therapeutic target.

"Originally put forth in the mid-1980s, the amyloid hypothesis maintained that beta-amyloid deposits in the brain set off all subsequent events – the neurofibrillary tangles that choke the insides of neurons, neuronal cell death, and inflammation leading to a vicious cycle of massive cell death," says Rudolph Tanzi, PhD, director of the MGH Genetics and Aging Research Unit and co-senior author of the report receiving advance online publication in Nature. “One of the biggest questions since then has been whether beta-amyloid actually triggers the formation of the tangles that kill neurons. In this new system that we call ‘Alzheimer’s-in-a-dish,’ we’ve been able to show for the first time that amyloid deposition is sufficient to lead to tangles and subsequent cell death.”

While the mouse models of Alzheimer’s disease that express the gene variants causing the inherited early-onset form of the disease do develop amyloid plaques in their brains and memory deficits, the neurofibrillary tangles that cause most of the damage do not appear. Other models succeed in producing tangles but not plaques. Cultured neurons from human patients with Alzheimer’s exhibit elevated levels of the toxic form of amyloid found in plaques and the abnormal version of the tau protein that makes up tangles, but not actual plaques and tangles.

Genetics and Aging Research Unit investigator Doo Yeon Kim, PhD, co-senior author of the Nature paper, realized that the liquid two-dimensional systems usually used to grow cultured cells poorly represent the gelatinous three-dimensional environment within the brain. Instead the MGH team used a gel-based, three-dimensional culture system to grow human neural stem cells that carried variants in two genes – the amyloid precursor protein and presenilin 1 – known to underlie early-onset familial Alzheimer’s Disease (FAD). Both of those genes were co-discovered in Tanzi’s laboratory.

After growing for six weeks, the FAD-variant cells were found to have significant increases in both the typical form of beta-amyloid and the toxic form associated with Alzheimer’s. The variant cells also contained the neurofibrillary tangles that choke the inside of nerve cells causing cell death. Blocking steps known to be essential for the formation of amyloid plaques also prevented the formation of the tangles, confirming amyloid’s role in initiating the process. The version of tau found in tangles is characterized by the presence of excess phosphate molecules, and when the team investigated possible ways of blocking tau production, they found that inhibiting the action of an enzyme called GSK3-beta – known to phosphorylate tau in human neurons – prevented the formation of tau aggregates and tangles even in the presence of abundant beta-amyloid and amyloid plaques

"This new system – which can be adapted to other neurodegenerative disorders – should revolutionize drug discovery in terms of speed, costs and physiologic relevance to disease," says Tanzi. "Testing drugs in mouse models that typically have brain deposits of either plaques or tangles, but not both, takes more than a year and is very costly. With our three-dimensional model that recapitulates both plaques and tangles, we now can screen hundreds of thousands of drugs in a matter of months without using animals in a system that is considerably more relevant to the events occurring in the brains of Alzheimer’s patients."

How age opens the gates for Alzheimer’s

With advancing age, highly-evolved brain circuits become susceptible to molecular changes that can lead to neurofibrillary tangles — a hallmark of Alzheimer’s Disease, Yale researchers report the week of March 17 in the Proceedings of the National Academy of Sciences.

The findings not only help to explain why age is such a large risk factor for Alzheimer’s, but why the higher brain circuits regulating cognition are so vulnerable to degeneration while the sensory cortex remains unaffected.

“We hope that understanding the key molecular alterations that occur with advancing age can provide new strategies for disease prevention,” said Amy F.T. Arnsten, professor of neurobiology and one of the senior authors of the study.

Neurofibrillary tangles are made from a protein called tau, which becomes sticky and clumps together when modified in a process called phosphorylation. The Yale study found that phosphorylated tau collects in neurons in higher brain circuits of the aging primate brain, but does not accumulate in neurons of the sensory cortex. Phosphorylated tau collects in and near the excitatory connections called synapses where neurons communicate and can spread between cells in higher brain circuits, the study found.

The study led by Yale researchers Becky C. Carlyle, Angus Nairn, Arnsten and Constantinos D. Paspalas found clues about what causes tau to become phosphorylated with advancing age. They uncovered age-related changes in the molecular signals that control the strength of higher cortical connections. In young brains, an enzyme called phosphodiesterase PDE4A sits near the synapse where it inhibits a chemical “vicious cycle” that disconnects higher brain circuits when we are in danger, switching control of behavior to more primitive brain areas. They further found that PDE4A is lost in the aged prefrontal association cortex, unleashing a chemical cascade of events that increase the phosphorylation of tau. This process may be amplified in humans, where high order cortical neurons have even more excitatory connections, leading to tangle formation and ultimately cell death.

“This insight into one pathway by which tau may influence the onset and progression of Alzheimer’s disease takes us a step closer to unraveling this complex and devastating disorder,” said Dr. Molly Wagster, of the National Institutes of Health, a co-funder of the research.

The new study may also help to explain why head injury is a risk factor for Alzheimer’s, as it may also increase the activity of the chemical  “vicious cycle.”

“Now that we begin to see what makes neurons vulnerable, we may be able to protect cells with treatments that mimic the protective effects of PDE4A,” said Arnsten.

Alzheimer’s Disease Mouse Models Point To A Potential Therapeutic Approach

Building on research published eight years ago in the journal Chemistry and Biology, Kenneth S. Kosik, Harriman Professor in Neuroscience and co-director of the Neuroscience Research Institute (NRI) at UC Santa Barbara, and his team have now applied their findings to two distinct, well-known mouse models, demonstrating a new potential target in the fight against Alzheimer’s and other neurodegenerative diseases.

The results were published online June 4 as the Paper of the Week in the Journal of Biological Chemistry. As a Paper of the Week, Kosik’s work is among the top 2 percent of manuscripts the journal reviews in a year. Based on significance and overall importance, between 50 and 100 papers are selected for this honor from the more than 6,600 published each year.

Kosik and his research team focused on tau, a protein normally present in the brain, which can develop into neurofibrillary tangles (NFTs) that, along with plaques containing amyloid-ß protein, characterize Alzheimer’s disease. When tau becomes pathological, many phosphate groups attach to it, causing it to become dysfunctional and intensely phosphorylated, or hyperphosphorylated. Aggregations of hyperphosphorylated tau are also referred to as paired helical filaments.

"What struck me most while working on this project was how so many people I’d never met came to me to share their stories and personal anxieties about Alzheimer’s disease," said Xuemei Zhang, lead co-author and an assistant specialist in the Kosik Lab. "There is no doubt that finding therapeutic treatment is the only way to help this fast-growing population." Israel Hernandez, a postdoctoral scholar of the NRI and UCSB’s Department of Molecular, Cellular and Developmental Biology, is the paper’s other lead co-author.

Treatments for hyperphosphorylated tau, one of the main causes of Alzheimer’s disease, do not exist. Current treatment is restricted to drugs that increase the concentration of neurotransmitters to promote signaling between neurons.

However, this latest research explores the possibility that a small class of molecules called diaminothiazoles can act as inhibitors of kinase enzymes that phosphorylate tau. Kosik’s team studied the toxicity and immunoreactivity of several diaminothiazoles that targeted two key kinases, CDK5/p25 and GSK3ß, in two Alzheimer’s disease mouse models. The investigators found that the compounds can efficiently inhibit the enzymes with hardly any toxic effects in the therapeutic dose range.

Treatment with the lead compound in this study, LDN-193594, dramatically affected the prominent neuronal cell loss that accompanies increased CDK5 activity. Diaminothiazole kinase inhibitors not only reduced tau phosphorylation but also exerted a neuroprotective effect in vivo. In addition to reducing the amount of the paired helical filaments in the mice’s brains, they also restored their learning and memory abilities during a fear-conditioning assay.

According to the authors, the fact that treatment with diaminothiazole kinase inhibitors reduced the phosphorylation of tau provides strong evidence that small molecular kinase inhibitor treatment could slow the progression of tau pathology. “Given the contribution of both CDK5 and GSK3ß to tau phosphorylation,” said Kosik, “effective treatment of tauopathies may require dual kinase targeting.”

Madison Cornwell, a Beckman Scholar with UCSB’s Center for Science and Engineering Partnerships who worked in Kosik’s lab, added: “As a beginning step, we demonstrated that two of these compounds were successful in clearing the brain of tau tangles in a mouse model, but someday inhibitors of these kinases may serve to ameliorate the symptoms of Alzheimer’s disease in patients.”

From trauma to tau - Researchers tie brain injury to toxic form of protein

University of Texas Medical Branch at Galveston researchers have uncovered what may be a key molecular mechanism behind the lasting damage done by traumatic brain injury.

The discovery centers on a particular form of a protein that neuroscientists call tau, which has also been associated with Alzheimer’s disease and other neurodegenerative conditions. Under ordinary conditions, tau is essential to neuron health, but in Alzheimer’s the protein aggregates into two abnormal forms: so-called “neurofibrillary tangles,” and collections of two, three, or four or more tau units known as “oligomers.”

Neurofibrillary tangles are not believed to be harmful, but tau oligomers are toxic to nerve cells. They are also are thought to have an additional damaging property — when they come into contact with healthy tau proteins, they cause them to also clump together into oligomers, and so spread toxic tau oligomers to other parts of the brain.

Now, in experiments with laboratory rats, using novel antibodies developed at UTMB, scientists have found that traumatic brain injuries also generate tau oligomers. The destructive protein assemblages formed within four hours after injury and persisted for at least two weeks — long enough to suggest that they might contribute to lasting brain damage.

Significantly, the rats used in the experiments were normal, unlike the genetically modified animals used in most tau research. The findings are thus likely to be more relevant to human traumatic brain injuries.

“Although people have given some attention to the formation of neurofibrillary tangles after traumatic brain injury, we were the first to look at tau oligomers, because we have an antibody that allows us to separate them out and see how much of the total tau is the toxic species,” said Bridget Hawkins, lead author of a paper on the research now online in the Journal of Biological Chemistry. “We saw that it’s a substantial amount — enough to play an important role in the effects of traumatic brain injury.”

Those effects can include memory deficits, which have been recently shown by UTMB researchers to be induced by tau oligomers. Other long-term ramifications of TBI include seizures, and disruptions in the sleep-wake cycle. The UTMB scientists hypothesize that these problems could be avoided if physicians had a way to stop the process of tau oligomerization.

One possibility is a treatment based on the antibodies used to label tau oligomers in this project, which were developed as part of an effort to develop a vaccine against different neurodegenerative disorders.

“We have antibodies that can specifically target these tau oligomers without interfering with the function of healthy tau,” said UTMB associate professor Rakez Kayed, the senior author on the paper. “This is a new approach — we’re starting by targeting them in animals — but we hope to eventually humanize these antibodies for clinical trials.”