ARTICLE IN BRIEF
In an animal model, a novel approach toward correct structural problems in apolipoprotein E 4 protein aims to prevent the misfolding, which triggers proteolysis, and the cascade leading to Alzheimer's disease.
A new treatment for Alzheimer's disease that focuses on correcting the structure of the apolipoprotein E4 (APOE4) protein has been shown to work in mice and could be ready for testing in humans within two years.
The treatment, tentatively named PY-101, would dissolve the bond that causes two domains of the APOE4 protein to fuse within neurons and cause misfolding, which triggers proteolysis. The breakdown of the protein then produces toxic fragments that disrupt mitochondrial function and promote cell death.
This process, rather than the accumulation of amyloid-beta (Abeta) accounts for the strong correlation between the APOE4 allele and numerous neurological disorders, including Alzheimer's disease, according to Robert W. Mahley, MD, PhD, president emeritus of the J. David Gladstone Institutes and professor of pathology and medicine at the University of California, San Francisco. Dr. Mahley described PY-101 during a presentation at the 2013 Society for Neuroscience (SfN) meeting in San Diego.
“We're still working on proof of concept in animals,” Dr. Mahley told Neurology Today. “We have to establish safety, and it will be 18-24 months before we will have a structure corrector that we can use in humans.”
Human tests would begin with people who have suffered traumatic brain injuries, such as soldiers and athletes, he said. If they have better outcomes than those not treated, that would be considered evidence of PY-101 blunting the toxic effects of APOE4.
Dr. Mahley said PY-101, which readily crosses the blood-brain barrier, would almost certainly be developed as a pill. “That's the only really efficient way to treat large numbers of patients with a complex disease,” he said. “We're working that out in our mice right now. We've already proven that PY-101 crosses the blood-brain barrier and gets into the brain.”
The APOE protein, which consists of 299 amino acids, contains two domains connected by a hinge-like bridge. The three isoforms of APOE — APOE2, APOE3, and APOE4 — differ from each other at only two sites, Dr. Mahley explained. APOE3, the most common isoform, has cysteine at residue 112 and arginine at residue 158, while APOE4 has arginine at both sites. APOE2, found in only about 7 percent of the population, has cysteine at both sites.
In APOE4, the arginine at site 112 causes the arginine at residue 61 to extend toward the other domain of the protein, enabling it to form a salt bridge with the glutamic acid at residue 255. In APOE2 and APOE3 this interaction between the two domains is less likely to occur. This salt bridge causes the protein to deform, which triggers proteolysis. When this occurs within a neuron, the process creates toxic fragments that disrupt mitochondria and the cytoskeleton. (Normally the vast majority of APOE is synthesized in astrocytes.)
This production of toxic fragments becomes an even bigger problem following brain injury, which causes upregulation of APOE4 protein within neurons. This leads to an even greater accumulation of toxic fragments within neurons, which accelerates their decline.
In a pair of papers published late in 2012, Dr. Mahley and his long-time colleague Yadong Huang, MD, PhD, described how APOE4 sets the stage for neurodegeneration following stress or injury, and how small-molecule structure correctors can obviate the protein's detrimental effects in mice.
“In my opinion, amyloid is not the only player and may, in fact, not even be the key player in Alzheimer's disease,” Dr. Mahley told the audience at his SfN presentation.
APOE4 has been associated with poorer outcomes in multiple sclerosis, Parkinson's disease, Lewy body and fronto-temporal dementia, delirium, and other disorders not attributed to amyloid buildup, he said. People who have an allele for APOE4 also tend to recover more slowly from traumatic brain injury, stroke, and other forms of brain damage.
APOE4 also disrupts synaptogenesis and reduces dendritic spine density in rodent models and in neuronal cultures, although researchers at the Gladstone Institute have found that rosiglitazone, an insulin sensitizer sold under the brand name Avandia, reverses the loss of dendritic spines — evidence that APOE4's detrimental effects on synaptogenesis are the result, at least in part, of mitochondrial damage.
The effects of APOE4 have been found in young adults and children who presumably have no significant accumulations of Abeta. And a paper published online Nov. 25 in the Journal of the American Medical Association Neurology (JAMA Neurology) reported white matter and gray matter differences in infants who carry an allele for ApoE4.
These effects of APOE4 do not undermine the amyloid cascade hypothesis — the reigning explanation for Alzheimer's disease—said veteran Alzheimer disease researcher William Jagust, MD, professor of public health and neuroscience at the University of California, Berkeley, who was not involved with Dr. Mahley's research.
“Our data, as well as data from many other laboratories around the world, show that people who have an E4 allele have more Abeta in their brains,” he said. “APOE4 also appears to produce toxic effects, as well as changes in structure and function, that are independent of Abeta accumulation. APOE4 might be affecting neural function through a mechanism that doesn't necessarily involve Abeta. But I don't think that suggests that Alzheimer's disease is independent of amyloid. I think what it suggests is that APOE4 might give you a double hit.”
Still, he finds the Gladstone Institute's pursuit of a structure corrector to be interesting and potentially helpful in treating Alzheimer's disease. “It's a very different approach that points toward mechanisms that may not directly involve amyloid, but I don't think these ideas are necessarily exclusive of the amyloid hypothesis,” Dr. Jagust said. “We think amyloid is likely to be a major culprit in the Alzheimer's story, but I think you can accept both approaches.”
People who carry a mutation that results in increased production of Abeta are highly susceptible to Alzheimer's — a piece of evidence that clearly implicates amyloid processing in the etiology of the disease.
“But is all Alzheimer's the same?” Dr. Jagust wondered. “Alterations in amyloid processing can cause Alzheimer's disease, but it's not inconceivable that you need other things to go with that. Older people who get Alzheimer's frequently have multiple pathologies, so maybe you need a double hit — a vascular pathology, for example, on top of the amyloid pathology — to cause dementia, and it may be that APOE4 causes a double hit.”
Also, a recent paper in the Journal of Neuroscience reported alterations in mitochondrial structure and function near dense plaques in a mouse model of Alzheimer's — a finding that suggests a potentially synergistic relationship between the accumulation of Abeta and the mitochondrial dysfunction found by Dr. Mahley and his colleagues in APOE4 carriers.
And APOE4 causes hyperphosphorylation of tau, which causes microtubules to fall apart and form the fibrillary tangles known to disrupt intercellular transport.
Dr. Mahley acknowledged that Abeta probably plays a role in Alzheimer's, but he believes the amyloid cascade hypothesis has constrained research into alternative possibilities.
“For 20 years the focus of most investigators has been on Abeta, but APOE4 alone can have detrimental effects causing neurodegeneration,” he said. “I'm not saying the amyloid hypothesis is totally wrong. This is a complex disease, and every complex disease has multiple pathways leading to pathology. But all the clinical trials that involved lowering Abeta have failed. It's time to think more broadly.”