ARTICLE IN BRIEF
In a mouse model of Alzheimer disease, investigators developed a vaccine targeting oligomer tau, which resulted in the alleviation of motor deficits.
SAN DIEGO—In a mouse model of Alzheimer disease (AD), an antibody directed against a tau oligomer seemed to improve performance on a test of motor ability, the loss of which is used to model aspects of the human disease.
Although altered forms of the proteins amyloid beta and tau have been implicated in AD, scientists are still trying to determine the actual culprit. Recently, the focus has turned from amyloid to tau, and from longer fibrils to shorter oligomers, according to the study investigators who reported new data here at the annual meeting of the Society for Neuroscience.
“At least we know we were able to improve these mice; [deficits in] motor function were alleviated,” said Diana L. Castillo-Carranza, PhD, a postdoctoral fellow working with Rakez Kayed, PhD, George Jackson, MD, PhD, and others at the University of Texas Medical Branch in Galveston.
Dr. Jackson noted that “amyloid-based vaccination for Alzheimer disease had disappointing results, it didn't really work,” and that the side effects included encephalitis.
The approach these researchers are using is based on the concept of trans-synaptic spread of degenerative disease in the nervous system, similar to what is postulated to happen with synuclein in Parkinson disease, and in prion disease, said Dr. Jackson, professor of neurology and director of the Mitchell Center for Neurodegenerative Diseases at the University of Texas Medical Branch. “There is evidence that both tau and alpha synuclein act extrasynaptically, and spread the pathology,” Dr. Jackson said. “If the antibody can neutralize the oligomer, the immune system will take it apart.”
Dr. Kayed has developed a monoclonal antibody that recognizes specifically the oligomeric form of tau, but leaves the normal tau protein alone.
“This monoclonal can attack the pathological conformation while leaving the normal conformation alone,” Dr. Jackson said. “We have behavioral and biochemical evidence of protection with this vaccine,” he said, adding, “we should be able to bring this to fruition as an effective treatment for Alzheimer disease.”
Dr. Kayed has developed new antibodies against the oligomer forms of tau that he says people now think is the most toxic form of this Alzheimer associated protein. The antibody used in the mouse studies does not recognize the soluble, functional form of tau or mature tau tangles, said Dr. Kayed, an assistant professor of neurology at the University of Texas Medical Branch. This is why the investigators decided to try to use it to vaccinate animals that model AD in people.
“We know it goes to the brain,” Dr. Kayed said. The antibody may change the oligomer to the monomer forms of tau, he speculated, “or there could be some shift of the equilibrium” between the two forms, he said.
To test the antibody's use as a potential vaccine they used P301L mice expressing human tau, which is mutated at the 301 position; this form of tau cannot attach to microtubules. These mice have motor problems, so the researchers used the ability to stay walking on a rotating rod to test the effect of the vaccination.
After the researchers injected one microgram into the cerebral ventricle, and tested eight-month-old P301L transgenic mice four days later on the rotarod, they found improved performance. In the brain, they found that the oligomer form of tau is decreased and the monomer form of tau is increased after the vaccination.
Control mice were given an injection to the ventricle with nonspecific immunoglobulin G (IgG). Behavioral tests on the same animals were compared from three days before to four days after the injection. One week after the injection, brains were extracted for immunohistochemical and biochemical analyses.
Immunization with anti tau oligomer specific antibody was followed by improved time on the rotarod, which was statistically significant (p<0.01). The analysis also indicated a reduction in the burden of tau oligomers in the mice after vaccination. When the researchers injected (into the hippocampus) an antibody that recognizes the oligomer form of tau, a Western blot showed a decrease in the oligomers and an increase in the monomers, said Dr. Castillo-Carranza.
She counted the number of projections stained with the tau oligomers specific polyclonal antibody (T22), as compared to the histology in controls treated with nonspecific IgG. Dr. Castillo-Carranza said that 70 percent of the projections stained in the control model animals, but after the vaccine, 20 percent were stained, a significant decrease (p<0.01).
The amount of antibody staining for tau oligomers was reduced by about 80 percent in neuronal projections to the cortex after the intracerebroventricular injection of the tau antibody.
“This is just preliminary, we have so many things to do,” Dr. Castillo-Carranza said. She added the team will try IV injection using the same antibody. They will also test the tau oligomer specific antibody in Tg2576 Alzheimer model mice, which have both amyloid beta and tau alterations.
The main form of oligomeric tau appears to be a trimer, said Dr. Kayed. The group's antibody does not recognize monomer tau, but detects oligomeric tau. It has little overlap with AT8, a commercial antibody that recognizes phosphorylated tau, he said. “We see less than 10 percent of oligomeric tau are phosphorylated at this epitope, it looks like phosphorylation here is not the cause of the toxicity” in AD, he said. Or, it may be there is a different site of phosphorylation on the tau protein than what researchers have worked with so far.
Dr. Kayed said: “I am very positive, it gives us hope” that the studies will pave the way to a vaccine for the disease. “Over the next three to five years we will focus on production and evaluation of the humanized antibody to be tested in clinical trials,” the investigators said in a press statement.
“This is new, targeting the oligomer tau by an antibody against it, “said Karen Duff, PhD, professor of pathology and cell biology at the Taub Institute at Columbia University Medical Center in New York City. A vaccine to alpha synuclein was able to clear this protein from the brain, she said. “The alpha synuclein vaccine can somehow remove protein or otherwise ameliorate the situation; now this group is applying this strategy to tau.”
A few labs have been targeting phosphorylated tau, said Dr. Duff.
“We don't know what the toxic form of tau is,” Dr. Duff said. The aggregate protein, she said, may not be a toxic agent; as in many degenerative neurological diseases the amount of aggregates is not correlating with cell death. “People are starting to look again at what might be the toxic form of the proteins,” she said.
Dr. Duff noted that “Dr. Kayed was one of the first to show that the oligomers of tau are toxic. This is an early study,” Dr. Duff said, “but it is very suggestive.”