Article In Brief
Working in PS19 mice, which carry a transgenic mutant tau gene, researchers observed abundant epichaperome formation at three months in the hippocampus, amygdala, and other brain structures implicated in Alzheimer's disease, which preceded the appearance of tau pathology.
Pathologic changes in the network of chaperone proteins, which regulate protein folding, may be among the earliest drivers of Alzheimer's disease (AD), according to a study published online on January 16 in Nature Communications.
The study—led by Wenjie Luo, PhD, assistant professor of neuroscience at the Brain and Mind Institute of Weill Cornell Medical College, and Gabriela Chiosis, PhD, of the chemical biology program at Memorial Sloan Kettering Cancer Center—shows that interrupting these pathologic changes has therapeutic potential for Alzheimer's disease, a possibility currently being tested in early-phase clinical trials.
The understanding of the role of chaperone proteins in AD comes, in part, from insights gained from research in cancer biology. Through her study of chaperone networks in cancer cells, Dr. Chiosis has developed a more complex view of the function of chaperone behavior as assistants in protein folding or degradation. In addition to these functions, she said, the more than 300 chaperones plus co-chaperones and other co-factors—collectively known as the “chaperome”—also serve critical roles in mediating interactions among other proteins, through formation of multiprotein platforms, or scaffolds. This function goes awry in disease, she explained, when these normally dynamic and highly transient scaffolds become stabilized.
“At that point, they begin to misregulate how these other proteins interact, leading to dysfunction in several cellular pathways,” Dr. Chiosis said.
Dr. Chiosis has dubbed this aberrant, stabilized set of platforms the “epichaperome,” to stress that the change is not due to changes in chaperone gene expression, but rather in the type and strength of interactions within the existing network of chaperones. (For more information, see “The Science Explained: Chaperones, Chaperome, and the Epichaperome.”)
The structural details of these complexes, either the normal dynamic structures or the pathologically stable ones, are not fully worked out, she noted, but some features are clear. There are core chaperones, for instance the chaperone HSP90, around which perhaps a dozen other chaperones coalesce, followed by yet more proteins surrounding those. The identity of these outer binding partners determines the precise function of the platform.
Study Design, Findings
To explore the implications of epichaperome formation in AD, the team stained for the presence of stable HSP90-based complexes using PU-AD, a purine derivative developed in Dr. Chiosis's lab. Normally, she said, PU-AD will bind and release HSP90 extremely rapidly. “But in the epichaperome, it is stuck, and thus the compound can be used to image the epichaperome,” including with PET imaging using radiolabeled PU-AD.
Working in PS19 mice, which carry a transgenic mutant tau gene, the team found abundant epichaperome formation at three months in the hippocampus, amygdala, and other brain structures implicated in AD, but little in other brain regions. Staining levels increased in intensity over the next five months, and became more widely distributed, so that by 11 months, they were found throughout the brain.
Importantly, Dr. Chiosis said, the staining showed that epichaperome deposition preceded the appearance of tau pathology. “These data suggest that the switch of the chaperome to epichaperome appears before the tau pathology.”
PU-AD is not only a marker for epichaperome structures, Dr. Chiosis said, but, through its interaction with HSP90, it helps break them apart and inhibits their reformation. She found that mice given the compound displayed reduced tau pathology, suggesting that epichaperomes not only precede tau pathology, “but support processes that ultimately lead to it,” she said.
In human AD brains, the team found that much more of the total chaperone content was retained in epichaperome structures, compared with normal brains. In vitro, PU-AD also facilitated dissolution of epichaperomes in AD neurons.
The functional consequences of epichaperome formation in the human brain are significant, Dr. Chiosis said. Using PU-AD beads as bait, the team captured epichaperomes and the many proteins they interact with, comparing this “interactome” between normal and AD brains, between PS19 and wild-type mice, and similarly in two other AD-related models.
In normal neurons, chaperome interactions were focused on activities of folding and transport, but in the diseased neurons, in each case, those same chaperone proteins, now part of epichaperomes, interacted preferentially with proteins involved in synaptic function, memory, and learning.
The involvement of epichaperomes in both AD, a disease of cell death, and cancer, one of cell proliferation, is likely explained by differences in the specific suite of binding partners within the cell, Dr. Chiosis said. In the former, these binding partners determine dysfunction in several cellular pathways that result in cell death, while in the latter they support uncontrolled proliferation and survival.
But were these changes in interactome necessarily pathologic, or might they be a sign of the struggle to regain protein homeostasis in the diseased neuron? To address that question, the team examined the effect of PU-AD treatment in PS19 mice. They found that treatment beginning at either three months, representing early disease, or eight to nine months, representing later-stage disease, restored both protein networks and cognitive performance.
The conclusion, Dr. Chiosis said, is that the chaperome-to-epichaperome switch and its consequent alteration of multiple biochemical pathways in the neuron are implicated in the cause of the cognitive decline. “The changes are not homeostatic, they are pathological,” she said. “These data also tell us that when you inhibit the epichaperome, the mice can recover.”
That insight has led to a recently completed successful phase 1 trial of the compound in humans and plans for a phase 2 trial later this year, under the auspices of Samus Therapeutics, a company Dr. Chiosis co-founded.
“This is a great paper, one that opens up a whole new field of inquiry,” commented Kurt Brunden, PhD, research professor of pathology and laboratory medicine at the University of Pennsylvania Perelman School of Medicine, who studies AD mechanisms and treatment strategies.
“The concept of the epichaperome is so new that it is not widely recognized,” he said, “but the results are pretty fascinating. And the fact that they have shown that a small molecule can reverse the consequences of the epichaperome, I find to be quite interesting from a translational perspective.”
One caveat, he noted, was that the models in which the experiments were done were mostly ones of protein overexpression, and it would be of interest to further confirm these observations in models that do not rely on such overexpression. On the other hand, he noted, the human AD brains also showed epichaperome formation, suggesting this may be a broadly important mechanism in the disease.
Joel Buxbaum, MD, professor emeritus in the department of molecular medicine at Scripps Research in La Jolla, CA, questioned whether the study adequately demonstrated that the changes in the epichaperome appeared before tau-induced neuronal pathology, since potentially toxic oligomeric forms may not have been detected with the antibodies used in the study.
“The epichaperome might come first,” he said, “but I don't think their argument is absolutely convincing. In transgenic mice, even in the embryo, the neurons are not normal anymore, with changes in the transcriptome that precede any appearance of plaques or tangles. The real test of the importance of the epichaperome in AD will come from the clinical trials.”
Dr. Chiosis holds equity interests in Samus, serves as a member of its board of directors, and has intellectual property interests related to PU-AD and PU-H71 that MSK has licensed to Samus. MSK has institutional financial interests related to Samus in the form of radiolabeled PU-AD. Drs. Buxbaum and Brunden had no disclosures.
The Science Explained: Chaperones, Chaperome, and the Epichaperome
WHAT THEY ARE
Chaperones are proteins that are best known as regulators of protein folding. Chaperones help new proteins fold correctly, help refold misfolded proteins, and regulate disposal of proteins that cannot be refolded. Collectively, chaperones, co-chaperones, and other factors make up the so-called chaperome, which is responsible for protein homeostasis in the cell.
Two chaperones, heat shock proteins 70 (HSP70) and 90 (HSP90) play especially critical roles in protein homeostasis, since they act as the core of short-lived multi-protein scaffolds that carry out much of the folding and refolding work of the chaperome.
Work by Gabriela Chiosis, PhD, of the chemical biology program at Memorial Sloan Kettering Cancer Center in New York City, has revealed that in many types of cancer cells, these normally short-lived scaffolds become stabilized, a condition she has termed the epichaperome, to stress that, akin to epigenetic (post-translational) changes in other proteins, the change is not in the level of expression of the individual chaperones, but in the structure and behavior of the complex.
HOW IT WORKS
Because the chaperome is composed of over 200 dynamically interacting proteins, much remains to be learned about the detailed function of the chaperome in cellular health and disease. Research by Dr. Chiosis suggests that prolonged cellular stress is a trigger for stabilization of chaperone platforms—that is, creation of an epichaperome complex. The epichaperome then affects a variety of cellular pathways, depending on the cell, the type of stress, and other context-dependent factors. Neurons are among the most sensitive cell types to disruptions of internal processes, and Dr. Chiosis's research suggests that epichaperome-mediated dysfunction can affect pathways that contribute to development of Alzheimer's disease.
HOW IT IS APPLIED
Because misfolded amyloid-beta and tau are the central pathology of Alzheimer's disease, attempts have been made to manipulate individual chaperones for therapy in the disease, including through upregulating and downregulating their activity or expression. To date, these attempts have not led to clinical success. An epichaperome inhibitor is currently undergoing early clinical trials in three types of cancer. Identification of the role of the epichaperome in AD models has led to development and early clinical testing of an inhibitor of epichaperome stabilization in that disease as well.