Oct. 2, 2000 Calcium handling pathway may be powerful target for drug development
A research team based at Massachusetts General Hospital (MGH) has discovered a key link between two cellular abnormalities associated with early-onset Alzheimer's disease - mutations in genes for proteins called presenilins and an altered handling of calcium inside cells - and have tied a specific calcium pathway to the production of amyloid-beta42, the sticky protein fragments that make up the plaques found in the brains of people with the disease. The findings, which also suggest a normal function for the presenilin proteins, identify a new target for drugs that may prevent or treat the devastating disease.
In the paper in the September issue of Neuron, the scientists from the MGH Genetics and Aging Unit and their colleagues report that the presenilins appear to control a process that regulates the level of calcium in a cellular compartment called the endoplasmic reticulum and that presenilin mutations known to be associated with inherited Alzheimer's disease slow down this regulatory process. They also show that inhibiting this particular calcium pathway increases production of amyloid-beta42 (A-beta42).
"We have found the molecular basis of the altered calcium metabolism previously observed in neurons expressing Alzheimer's-associated versions of presenilin," says Tae-Wan Kim, PhD, the paper's senior author. "We also identified a link between a specific calcium influx pathway and aberrant, presenilin-mediated processing of the amyloid precursor protein." Kim, who had been a member of the MGH Genetics and Aging Unit, recently joined the Taub Institute for Research on Alzheimer's Disease and the Aging Brain at Columbia University College of Physicians and Surgeons.
Rudolph Tanzi, PhD, director of the MGH Genetics and Aging Unit and a co-author of the Neuron paper, adds, "Our work predicts that, if you could develop a drug that stimulates this specific calcium pathway, you could lower levels of A-beta42, which is the name of the game in Alzheimer's disease drug therapy development."
Mutations in two presenilin proteins - dubbed PS1 and PS2 - have been identified as causing most cases of inherited, early-onset Alzheimer's disease, also called familial Alzheimer's disease (FAD). The function of the presenilins is as yet unknown, although some researchers have found evidence that one of both of them may be a sought-after enzyme called gamma-secretase that clips the large amyloid precursor protein (APP) to form A-beta fragments. Other recent Alzheimer's research has shown that brain cells from people with FAD or cells into which mutated presenilin genes have been inserted show alterations in their handling of calcium, a process that is key to normal cellular metabolism.
Many cellular activities are triggered when signalling molecules bind to receptors on the cell surface, setting off a process called signal transduction. A key step in signal transduction involves release of calcium stored in the endoplasmic reticulum (ER) into the body of the cell, which helps translate the signal into a cellular activity. Once the calcium has been released, the supply in the ER must be replenished - much as an electronic camera flash has to recharge after firing. Movement of calcium from outside the cell into the ER utilizes a recently recognized pathway called capacitated calcium entry (CCE). The processes that control CCE, which differs from other modes of moving calcium into or out of the cell, are still being investigated.
Because earlier studies at other centers suggested that cells with FAD-associated mutations in their presenilins showed elevated release of calcium from the ER into the cell, the MGH team decided to examine whether these presenilin mutation had any effect on CCE. In a number of experiments carried out by Andrew Yoo and Issac Cheng, the researchers first found that cells in which the PS1 protein was either absent or inactivated showed accelerated CCE, suggesting that the protein may act to control the process. In cells with the FAD-associated mutations in either PS1 or PS2, CCE was slowed down, suggesting that those mutations increased the proteins' regulatory effect. When the FAD mutant cells were treated with substances known to inhibit CCE, the altered calcium handling disappeared. However two substances that inhibit other calcium handling pathways did not affect the FAD mutant cells, confirming that CCE was the process impacted by the FAD-associated presenilin mutations!
To test whether inhibition of CCE had an impact on the deposition of A-beta, the researchers used cells that overproduce the amyloid precursor protein and as a result A-beta. They applied the three calcium pathways inhibitors and found that inhibition of the CCE pathway led to increased production of A-beta42, compared with the more common version of the fragment, called A-beta40. The other calcium pathway inhibitors had no effect on the cells' A-beta production.
"If we can find compounds that increase CCE just enough to lower A-beta42, we may be able to offer a way to prevent or slow down the progression of Alzheimer's to members of families effected by these mutations, some of whom begin to show symptoms at a very young age," Tanzi says. "While we currently don't know whether this information also might be applied to the late-onset, non-inherited type of Alzheimer's, we would assume that the pathways that go wrong in early-onset illness also are involved in late-onset disease."
Tanzi and Kim are currently working with Neurogenetics Inc., of San Diego to identify compounds that may lower A-beta42 by stimulating CCE. Both researchers are scientific founders and have equity interests in the company, which has licensed from the MGH the concept of treating Alzheimer's through increasing CCE.
Other co-authors of the Neuron paper are Tallessyn Grenfell, Eunju Pack-Chung, Giuseppina Tesco, MD, and Aleister Saunders, PhD, of the MGH Genetics and Aging Unit; Sungkwon Chung, and Hanmi Lee of Sungkyunkwan University School of Medicine in Korea; and Melissa Handler, PhD, Jie Shen, PhD, Weiming Xia, PhD, Kai Ding, and Matthew Frosch, MD, PhD, of the Center for Neurologic Diseases at Brigham and Women's Hospital. The study was supported by grants from the National Institute on Aging, the American Health Assistance Foundation, the Alzheimer's Association, Partners HealthCare System and the Brain Science and Engineering Research Program of the Korean Ministry of Science and Technology.
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