Why Do Some People Get Alzheimer’s Disease but Others Do Not?

Everyone has a brain that has the tools to develop Alzheimer’s disease. Why therefore do some people develop Alzheimer’s disease (AD) while others do not? An estimated five million Americans have AD – a number projected to triple by 2050– the vast majority of people do not and will not develop the devastating neurological condition. What is the difference between those whole develop AD and those who do not?

Subhojit Roy, associate professor in the Departments of Pathology and Neurosciences at the University of California, San Diego School of Medicine, asked this exact question. In a paper published in the journal Neuron, Roy and colleagues offer an explanation for this question. As it turns out, in most people there is a critical separation between a protein and an enzyme that, when combined, trigger the progressive cell degeneration and death characteristic of AD.

“It’s like physically separating gunpowder and match so that the inevitable explosion is avoided,” says principal investigator Roy, a cell biologist and neuropathologist in the Shiley-Marcos Alzheimer’s Disease Research Center at UC San Diego. “Knowing how the gunpowder and match are separated may give us new insights into possibly stopping the disease.”

Neurologists measure the severity of AD by the loss of functioning neurons. In terms of pathology, there are two tell-tale signs of AD: a) clumps of a protein called beta-amyloid “plaques” that accumulate outside neurons and, b) threads or “tangles” of ‘tau” protein found inside neurons. Most neuroscientists believe AD is caused by the accumulation of aggregates of beta-amyloid protein, which triggers a sequence of events that leads to impaired cell function and death. This so-called “amyloid cascade hypothesis” puts beta-amyloid protein at the center of AD pathology.

Creating beta-amyloid requires the convergence of a protein called amyloid precursor protein (APP) and an enzyme that cleaves APP into smaller toxic fragments called beta-secretase or BACE.

“Both of these proteins are highly expressed in the brain,” says Roy, “and if they were allowed to combine continuously, we would all have AD.”

It sounds inexorable, but it doesn’t always happen. Using cultured hippocampal neurons and tissue from human and mouse brains, Roy and Utpal Das, a postdoctoral fellow in Roy’s lab who was the first author of this paper, and other colleagues, discovered that healthy brain cells largely segregate APP and BACE-1 into distinct compartments as soon as they are manufactured, which ensures that these two proteins do not have much contact with each other.

“Nature seems to have come up with an interesting trick to separate co-conspirators,” says Roy.

What then brings APP and BACE together? Roy and his team found that those conditions that promote greater production of beta-amyloid protein also increase the convergence of APP and BACE. Specifically, an increase in neuronal electrical activity, which is known to increase the production of beta-amyloid, also increased the convergence of APP and BACE. Post-mortem examinations of AD patients have shown that the cellular locations of APP and BACE overlap, which lends credence to the pathophysiological significance of this phenomenon in human disease.

Neurons were cotransfected with APP:GFP and BACE-1:mCherry, neurons were stimulated with glycine or picrotoxin (PTX), and the colocalization of APP and BACE-1 fluorescence was analyzed (see Experimental Procedures for more details). (B) Note that stimulation with glycine greatly increased APP/BACE-1 colocalization in dendrites (overlaid images on right). (C and D) Quantification of APP/BACE-1 colocalization. Note that increases in glycine-induced APP/BACE-1 convergence can
Neurons were cotransfected with APP:GFP and BACE-1:mCherry, neurons were stimulated with glycine or
picrotoxin (PTX), and the colocalization of APP and BACE-1 fluorescence was analyzed (see Experimental Procedures for more details).
(B) Note that stimulation with glycine greatly increased APP/BACE-1 colocalization in dendrites (overlaid images on right).
(C and D) Quantification of APP/BACE-1 colocalization. Note that increases in glycine-induced APP/BACE-1 convergence can

Das says that their findings are fundamentally important because they elucidate some of the earliest molecular events triggering AD and show how a healthy brain naturally avoids them. In clinical terms, they point to a possible new avenue for ultimately treating or even preventing the disease.

“An exciting aspect is that we can perhaps screen for molecules that can physically keep APP and BACE-1 apart,” says Das. “It’s a somewhat unconventional approach.”

Drug Developers Increase Their Use of Stem Cells

Industries have increased their use of stem cells in research and development and product testing and the industrial use of stem cells will almost certainly increase in the future.

Despite the image of stem cells in the popular imagination as the stalwarts of regenerative medicine, stem cells have revolutionized drug development and testing. James Thomson, director of regenerative biology at the Morgridge Institute for Research in Madison, Wisconsin, and one of the founders of Cellular Dynamics International, also in Madison, said, “I think there are tremendous parallels to the early days of recombinant DNA in this field. I don’t think people appreciated what a broad-ranging tool recombinant DNA was in the middle ’70s.” Thomson also thinks that people also seriously underestimate the tremendous number of hurdles that must be overcome in order to use such technologies in clinical treatments. Stem cells, according to Thomson, are in a similar situation. While the therapeutic use of these cells might eventually come to fruition, “people underappreciate how broadly enabling a research tool it is.”

About two years ago, drug companies began to investigate the use of stem cells in testing and evaluating new drugs. Today, the pharmaceutical industries all over the world are increasingly using stem cell lines to test drug toxicity and identify and evaluate potential new therapies. For example, Thomson’s company, Cellular Dynamics, sells human heart cells called cardiomyocytes, which are made from induced pluripotent stem (iPS) cells. Thomson says that “essentially all the major pharma companies” have purchased these cells for use in their laboratories. The company also produces brain cells and cells that line blood vessels, and is about to release a line of human liver cells.

Cellular Dynamics is not the only company that makes stem cell lines for drug testing. Three years ago, a stem-cell biologist named Stephen Minger left his job in at a United Kingdom university to be the head of General Electric Healthcare’s push into stem cells. This medical-technology company, which is headquartered in Chalfont St. Giles, UK, has been selling human heart cells made from embryonic stem (ES) cells for well over a year, and is due to start selling ES cell-derived liver cells soon.

Minger’s team at GE Healthcare assessed their ES-derived heart muscle cells in a blind trial against a set of unnamed drug compounds to determine if they could determine which compounds were toxic. Once the tests were completed, Minger said that they found that the cells had been affected by those compounds that are known to be toxic. However, the stem cells also identified a problem that had only been discovered after the drugs had reached the market (after they had been approved by the US Food and Drug Administration). According to Minger, “These are compounds which went all the way through animal testing, then went through phase I, II, III and then were licensed in many cases by the FDA.”

Stem cell lines can do more than identify drugs with dangerous side effects’ they can save the industry millions of dollars in wasted development costs. However, they might also be tools for drug development. Cellular Dynamics and GE Healthcare even market their cells from this very purpose. Adam Rosenthal, senior director for strategic and corporate development at iPierian, a biopharmaceutical company based in San Francisco, California, said, “Many of the animal models out there are poor, demonstrating great efficacy in the mouse, but not repeating in man during late-stage clinical trials. Therefore having an in vitro model years before, which can actually recapitulate human disease, would be a huge advantage.

iPierian has a different strategy than other stem cell companies, since it has its own proprietary in-house stem cell lines that it uses. It does not sell those cell lines, but uses them to develop treatments for neurodegenerative diseases; e.g., Alzheimer’s. This same company has recently announced that they are going to move forward with their development of monoclonal antibodies that target the tau proteins thought to be important in the onset of Alzhiemer’s disease. iPierian made this decision based on information that came from stem-cell work.

Lee Rubin, co-founder of iPierian and director of translational medicine at the Harvard Stem Cell Institute in Cambridge, Massachusetts, says that there is debate within industry if stem cells serve as appropriate model systems to study certain diseases. This is particularly the case with particularly non-genetic or late-onset disorders or conditions that result from pathological interactions between different tissues. Rubin has used stem cells in his research to model a disease called spinal muscular atrophy, which is actually a group of early onset genetic disorders. Rubin makes it clear that the only way to definitively demonstrate that stem cells are a superior model system from drug discovery is to show that the drugs developed from stem cell-based models works in people. Rubin put it this way, “That’s a long-term project. That’s the ultimate test.”

Thomson notes that stem cells will almost certainly find even wider uses than drug-development work. “What human ES cells and iPS cells now do is give you access to the basic building blocks of the human body, just for basic study. We will understand the human body at a much greater detail because of these cells.” How stem cells will be used are not clear, but Thomson added, “But I do think it will profoundly change human medicine.”