Earlier this year, I reported that scientists had made induced pluripotent stem cells iPSCs from the skin cells of Alzheimer’s patients in order to differentiate them into neurons and study the effects of Alzheimer’s disease on the function of those neurons. An update on this research is now available from talks presented at the Alzheimer’s Association International Conference.
Current mouse models of Alzheimer’s disease use animals in which particular genes known to play a role in the onset and pathology of Alzheimer’s disease are overexpressed in the brains of the animals. For example 3xTg-AD mice overexpress a mutant version of the beta-amyloid precursor protein (beta-APP), a mutant version of the presenilin gene (PS1M146V), and a mutant version of the tau protein (tauP301L). Al three of these alleles play important roles in Alzheimer’s disease (AD) etiology. For example tauP301L is the most common mutation found in the tau protein-encoding gene associated with neurodegenerative diseases. Mice that mice that overexpress the mutant human tauP301L have, in their brains, neurofibrillary tangles (NFTs), neuronal cell losses and memory disturbances (see see Wakasaya Y., et al., J. Neurosci 2011 89(4):576-84). The PS1M146V mutation in humans is responsible for one of the more aggressive forms of early onset AD. Finally, the double Swedish mutation of the beta-APP protein gene (Lys to Asn at residue 595 plus Met to Leu at position 596) increases production of amyloid protein, the material found in the plaques in the brains of AD, 6-8 times. The 3XTg-AD mouse show much of the pathology found in the brains of AD patients, and have been useful in AD research.
However, according to Alzheimer’s Association scientist William Thies: “Current animal models of Alzheimer’s are highly engineered to express elements of the disease, and, while valuable for research, incompletely represent how the disease form and progresses in people. In order to develop better therapies and eventually prevent Alzheimer’s, we need better, more accurate animal and cellular models of the disease. This newly reported research (i.e., the production of iPSCs from AD patients) is a significant step forward in that direction.”
Amyloid plaque formation is a hallmark of AD, and the present mouse models do not form amyloid plaques in their brains in the same way as the brains of human AD patients. Furthermore, significant brain cell death does not occur in the present mouse models even though they do occur in human AD patients.
Amyloid plaques form as a result of the processing of beta-APP. beta-APPis processed in one of two ways. If an enzyme called alpha-secretase clips beta-APP, it forms a soluble N-terminal fragment (sAPPa) and a C-terminal fragment (CTFa). The sAPPa protein seems to enhance synapse formation, neurite outgrowth and neuronal survival. CTFa remains in the membrane and is cleaved by presenilin-containing gamma-secretase to produce a soluble N-terminal fragment (p3) and a membrane-bound C-terminal fragment (AICD, or APP intracellular domain). AICD might be involved in nuclear signaling. This mode of beta-APP processing does not produce amyloid protein for plaque formation.
If beta-APP is first cleaved by beta-secretase, it produces a soluble N-terminal fragment (sAPPb) and a membrane-bound C-terminal fragment (CTFb). CTFb is longer than CTFa, and cleavage of CTFb by gamma-secretase produces AICD and a soluble fragment called amyloid-beta, or Abeta. If Ab accumulates in the extracellular spaces of the brain, it can aggregate to form amyloid plaques. Abeta is ‘stickier’ than other APP fragments. It accumulates bit by bit, into microscopic plaques by means of a multi-step mechanism by which Ab peptides aggregate into oligomers that cluster together to form fibrils with a regular b-sheet structure. The fibrils adhere to form mats, which clump together with other substances to eventually form plaques. Ab plaques may kill off brain cells and trigger inflammation, which also kills cells. Environmental conditions, age, genetic predisposition, and health in general influence of production of Abeta.
Andrew Sproul works as a postdoctoral research fellow at the New York Stem Cell Foundation in the laboratory of Scott Noggle. Sproul used iPSCs to model AD and reported on his results at this conference. Sproul used skin cells from AD patients and skin cells from unaffected relatives as controls and then made iPSCs from these skin cells.
“One advantage of this technology is that we get a near infinite supply of disease and control patient stem cells,” noted Sproul. “Another is that we can then turn the iSPCs into any tissue in the body. This allows us to investigate the role of various cells in Alzheimer’s disease progression by manipulating the iPSCs for form different types of brain cells (forebrain nerve cells, neural stem cells, glial cells) that we and others believe are involved in Alzheimer’s.”
Researchers made iPSCs from 12 young people with early-onset Alzheimer’s and healthy controls from families that harbored the genetic predisposition for young-onset AD. The iPSC lines have been tested to ensure that they are pluripotent
“We have made both the control and Alzheimer’s iPSCs into brain cells and have demonstrated that they are electrically active. These brain cells include forebrain cholinergic neurons (neurons that re;ease acetylcholine as a neurotransmitter) which are particularly vulnerable in Alzheimer’s disease.”
“We have also begun to use the iPSC-derived neurons and neural stem cells to compare differences in cellular function between people with Alzheimer’s and their unaffected relatives. For example, we, in conjunction with Dr. Sam Gandy’s group at Mount Sinai School of Medicine, have demonstrated that Alzheimer’s neurons produce more of the toxic form of beta amyloid, the protein fragment that makes up amyloid plaques, though this aspect of the research is preliminary,” Sproul said.
Much of the research in this group is concerned with Presenilin-1. Mutations in PSM1 are responsible for the most common form of rare, inherited young-onset AD (less than 2% of all cases). The iPSC platform might provide the best system to drug testing those new compounds that could mitigate the effects of this devastating disease.
Because the majority of AD patients have the sporadic form of AD, scientists plan to expand their research to include large-scale production of iPSCs from people with different forms of AD.
Sproul added, “We have begun to extend this work by collaborating with four different institutions in New York City; the Mount Sinai School of Medicine, Columbia University, New York University, and Rockefeller University. Over the next few years, we expect to provide substantial insight into Alzheimer’s and valuable tools to help create the next generation of therapeutics.”