"Doctor, do I have Alzheimer's?"
A person in the very early stages of Alzheimer's disease-perhaps experiencing some memory problems but still essentially healthy-would have a hard time getting a straight answer to this simple question. In fact, the only foolproof way to diagnose Alzheimer's is to autopsy the brain, looking for the clumps and tangles of abnormal protein that accumulate as the disease progresses. In a living person, it is only until the symptoms become relatively obvious that a doctor can offer a reliable a diagnosis of Alzheimer's dementia. Unfortunately, at that point the brain has already suffered a lot of damage. To intervene earlier would require a way to check the brain for the first signs of disease.
Currently, the leading explanation for what causes Alzheimer's disease is the amyloid cascade hypothesis. The idea is that the brain begins to overproduce a sticky protein called beta amyloid, which forms clumpy "plaques" between neurons. The formation of plaques is thought to be the beginning of a process that ends with permanent damage to the brain and, eventually, dementia. Stringy deposits of "tau" protein, or neurofibrillary tangles, form inside brain cells, but according to the amyloid hypothesis these aren't the primary cause of the dementia.
Right now, the plaques and tangles-and therefore the disease process itself-remain invisible to standard medical imaging, but that may change. Multiple teams of researchers from UCLA in Los Angeles to the BF Research Institute in Osaka, Japan, are racing to find a way to unmask Alzheimer's disease. They hope to do this by rendering beta amyloid plaques visible to brain-imaging technologies such as PET and MRI. If the amyloid hypothesis is correct, imaging plaques could help scientists to solve the riddle of what causes Alzheimer's, develop drugs and other treatments to combat it, and diagnose the disease early enough to prevent major damage.
In 2004, researchers William Klunk, M.D., Ph.D., and Chester Mathis, Ph.D., at the University of Pittsburgh School of Medicine reported an important milestone on the road to beta amyloid imaging. After more than a decade of work, Klunk and Mathis had developed a tracer molecule that could be injected into a living person and home in on Alzheimer's plaques. Mildly radioactive carbon atoms attached to the tracer allow researchers to estimate the volume of plaque in the brain using a positron emission tomography (PET) scanner, although the view isn't sharp enough to make out individual plaques. The precise role that the plaques play in Alzheimer's disease is not yet fully proven, but one thing is certain: All people with Alzheimer's have plaques in their brains. If you can see amyloid-beta, you may also be seeing the disease itself.
The new tracer was dubbed Pittsburgh Compound-B, or PiB. Working with Klunk and Mathis, scientists in Uppsala, Sweden, tested PiB on 16 people diagnosed with Alzheimer's disease and a comparison group of 9 people without the illness. In the people with Alzheimer's, the PET scans showed accumulations of PiB in the same regions of the brain where amyloid-beta is usually found to build up. Klunk and his colleagues reported their success in the January 21, 2004, Annals of Neuroscience.
PiB's success has created traction in the field for developing a practical tracer to image amyloid-beta with PET scanners. PiB itself is being tested in at least 18 different research centers. Other teams are developing new PET tracers known by an alphabet soup of acronyms such as FDDNP, IMPY, SB13, and BF168. Work on PiB has continued, too, because the experimental version used in the Swedish study has an important limitation: The radioactive portion of the tracer, which makes the plaques visible to PET, loses half its potency in about 20 minutes. This provides a relatively short window of time in which to make a brain scan.
Also, only about one in 10 PET facilities have the capability to cook up the radioactive carbon tracers. Most PET scanning facilities are set up to use a radioactive tracer of fluorine atoms, with a half-life of 102 minutes-a larger window for creating a scan. And the fluorine tracer is widely available and PET scan facilities are experienced using it. Klunk and Mathis are working with GE Healthcare to develop a version of PiB with a standard fluorine tracer. The new PiB is expected to be ready for clinical trials by the end of 2006.
Amyloid on MRI: Hold Still!
PET is not the only technology scientists are tapping to image plaques. At the Mayo Clinic College of Medicine in Rochester, Minnesota, radiology professor Clifford Jack, Jr., M.D., working with Michael Garwood PhD at the University of Minnesota and Joseph Poduslo at the Mayo Clinic, has harnessed magnetic resonance imaging (MRI) to see plaques in mice bred to produce amyloid-beta in their brains. The research, published in the December 2004 issue of Magnetic Resonance in Medicine, was the first time anyone had seen plaques in a living brain with MRI. The images are sharp enough to show individual plaques.
The advantage of MRI is that it shows you actual brain structure, whereas PET measures how much radiation different regions of the brain are giving off. The strength of the radiation indicates the amount of beta amyloid plaques present. But for imaging tiny plaques, MRI's strength is also a weakness. As in photography with an ordinary camera, imaging plaques with MRI requires that the subject hold still-very, very still.
To get a clear image of the plaques in the mouse brain, Jack and his team had to prevent the animals from moving while probed their diminutive brains with electromagnetic pulses in an MRI scanner designed especially for mice. To prevent blurring, the animals had to be anesthetized and their heads held in a special brace. And the individual pulses had to be synchronized to their breathing and pulse. It took an hour and 40 minutes to obtain a decent image of plaque in the mice. Gradually, this built up enough individual snapshots to reconstruct a three-dimensional image of the plaques.
The trouble is this: Imagine your grandmother lying with her head in an MRI machine for 100 minutes. "We can hold a mouse still for an hour and forty minutes, but there's no way that's going to happen in humans," Jack says. "The real question is what's the maximum time a person can lay dead still." Working with a physicist Michael Garwood, Jack hopes to make the MRI scanning procedure much more efficient, shortening the scan time enough to make it practical in humans. A scan time of 15-20 minutes would be ideal.
Another way around the problem is to inject the mouse (or person) with a chemical, called a contrast agent, that binds to the plaques and makes them much more visible to the MRI. Jack is working with another Mayo researcher, biochemist and neuroscientist Joseph Poduslo, Ph.D., to develop such an MRI contrast agent. Even with the contrast agent, they still have to time the MRI scans to the mouse heartbeat and breathing and hold its head still.
As Klunk continues his efforts to develop PET imaging of amyloid, he is also collaborating with scientists at Massachusetts General Hospital in Charlestown and M.I.T. in Cambridge to take PiB in a new direction. When exposed to certain wavelengths of light, PiB emits fluorescent light. This means that, theoretically, it may be possible to probe the brain with light to make images of amyloid-beta.
The "photographer" in this optical imaging project is Brian Bacskai, Ph.D., who works in Mass General's Alzheimer's Disease Research Laboratory. He's also an expert in a technology called multiphoton miscoscopy, which uses laser light to scan living tissue at different depths. After tagging the plaques in the mice with fluorescent PiB, the multiphoton microscope produced images of individual plaques in the brains of living mice.
If the light-scanning technique could work on living people, it would have some advantages over PET or MRI. Unlike with PET, it does not require short-lived and expensive radioactive tracers. It is also fundamentally lower-tech and less expensive than the massive scanning apparatus required for PET and MRI. But light-scanning will first have to overcome one major impediment that does not apply to PET and MRI: the human skull.
To scan mouse brains, Bacskai and his team had to create "cranial windows" in the brains of the mice-literally a hole in the skull with a piece of glass cemented over it. Otherwise the laser light could not penetrate the head far enough and with enough intensity to light up the fluorescent-labeled plaques. Also, without the cranial windows, the light given off by the fluorescent plaques could get back out of the head to be converted into an image.
But it is theoretically possible to scan through the skull, Bacskai says, if they used light farther in the infrared region of the light spectrum. At these wavelengths, the light penetrates farther and with less loss of power. That takes care of the scanning part of the system. Then, Bacskai says, they would need to make a fluorescent tag that sticks to the plaque and gives off light in the infrared when stimulated, enabling it to exit the skull and be turned into a picture. "It's a matter of finding the right compound and tuning it, but the chemistry is extremely challenging."
Optical scanning for Alzheimer's may be a way off, but research to unmask Alzheimer's will pay off when someone figures out how to prevent plaques from accumulating in the brain-and if doing so actually prevents the disease from progressing. "Amyloid imaging will facilitate the development of drugs by being able to identify people who have amyloid in their brains to start with," Klunk says. "Then it would directly assess the effect of the amyloid drug on its target." Or to put it another way: In the search for a cure to Alzheimer's disease, seeing is believing.
- "In vivo visualization of Alzheimer's amyloid plaques by MRI in transgenic mice without a contrast agent," by Clifford R. Jack and others. (Magnetic Resonance in Medicine, December 2004, Volume 52, Number 6, pp. 1263-1271.)
- "In vivo magnetic resonance microimaging of individual amyloid plaques in Alzheimer's transgenic mice," by Clifford R. Jack and others. (Journal of Neuroscience, October 26, 2005, Volume 25, Number 43, pp. 10,041-10,048.)
- "In vivo optical imaging of amyloid aggregates in brain: design of new fluorescent markers," by Evgueni E. Nesterov and others. (Angewandte Chemie, August 26, 2005, Volume 44, Number 34, pp. 5452-5462.)
- "Design and chemical synthesis of a magnetic resonance contrast agent with enhanced in vitro binding, high blood-brain barrier permeability, and in vivo targeting to Alzheimer's disease amyloid plaques," by Joseph F. Poduslo and others. (Biochemistry, May 25; 2004, Volume 43, Number 20, pp. 6064-6075.)
- "Imaging brain amyloid in Alzheimer's disease with Pittsburgh Compound-B," by William E. Klunk and others. (Annals of Neurology, March 2004, Volume 55, Number 3, pp. 306-319.)