Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Medicine

Major Professor

Chad A. Dickey

Keywords

FKBP5, Glucocorticoid Receptor, Hsp70, Hsp90, Methylene Blue, Tau

Abstract

The elderly are at increased risk for developing psychiatric diseases, which include Alzheimer's disease, depression, anxiety and suicide. The probability of multiple disease comorbidity is also increased in the elderly. At the cellular level, the loss of protein homeostasis is often at the root of disease emergence, and thus the scientific community is searching for ways to help maintain this balance. A vast group of proteins that are paramount to balancing and counterbalancing protein levels is the molecular chaperone protein group, which has evolved a tremendous variety of functions in the cell. They aid in protein trafficking, folding, receptor signaling, neurotransmission, vesicle forming and fusion, protein degradation, and apoptosis, among other activities. Despite their best efforts, disease still ensues, but because of their vast number and multiple abilities, it may be possible to modulate these proteins as a way to treat and prevent disease. Chaperones are of particular interest in diseases of aging, because chaperone induction and effectiveness is reduced with age. In addition, many diseases of the elderly are brought on by aberrant protein accumulation, like Alzheimer's disease.

As a result, the hypothesis of this dissertation is whether the

modulation of molecular chaperones changes disease pathology. A molecular chaperone family that is important to protein degradation is the Hsp70 chaperone complex. Hsp70 proteins have specialized function depending on cell type and cellular compartment, but Hsp70 proteins are very important for protein synthesis and degradation. As a result, they are in a position to contribute to the regulation of proteins that become aberrant.

In recent years scientific literature has indicated that compounds that inhibit the enzymatic ATP hydrolysis of these proteins promote tau degradation, which accumulates in Alzheimer's disease. Alzheimer's disease is the sixth leading cause of death in the U.S., it is a progressive neurodegenerative disease, and is caused by the aberrant accumulation of the amyloid beta and tau proteins. Here, we show that treatment with the Hsp70 inhibitor methylene blue, reduces tau, saves neurons, and restores cognition, in a mouse model of tau accumulation (rTg4510). Cognitive rescue occurred despite a severe tangle load, equal to control treated tau transgenic mice. This study shows that reducing soluble tau can restore cognition, reducing tangles is not necessarily to ameliorate cognition, and saving neurons is not sufficient to increase cognition if they are burdened with soluble tau.

This work shows that methylene blue does not affect the the number

of tau tangles in this model, as suggested by in vitro data. It also suggests that further work into the development of Hsp70 ATPase inhibitors may find success in alleviating the soluble tau burden found in Alzheimer's disease.

The co-chaperone FKBP5 is also of extreme importance, not because it is essential, but because research has implicated this protein with a host of psychiatric diseases. Single nucleotide polymorphisms in this gene, which increase the levels of FKBP5, interact with averse traumatic events to enhance the likelihood of developing mood and anxiety disorders, including major depressive disorder, post-traumatic stress disorder, bipolar disorder, and suicide. Moreover, we have found that FKBP5 protein levels increase with age in the human brain, increasing the risk for the elderly of developing disease if exposed to traumatic stress. Here, we tested the hypothesis that FKBP5 negatively regulates resilient behavior. We found that FKBP5 levels increase with age in the wild type mouse brain, and that wild type mice display reduced resiliency with age. FKBP5-/- mice, on the other hand, show enhanced resiliency to stress at all ages tested, and are protected from aging-induced despair. At the molecular level, FKBP5 is a robust inhibitor of the glucocorticoid receptor, which is responsible for the shut-off of the hypothalamic-pituitary-adrenal axis.

In addition, excess glucocorticoid levels in the blood is a robust marker of psychiatric disease. Consequently, FKBP5 may be causing disease through enhanced levels of glucocorticoids. FKBP5-/- mice display reduced corticosterone after stress. Moreover, corticosterone production increases with age, and FKBP5-/- mice are protected from this increase. These studies are the first to show that reducing the levels of FKBP5 is a promising therapeutic option for the treatment of mood disorders in the elderly, resiliency naturally declines with age due to FKBP5, corticosterone levels after stress rise due to FKBP5, and that the ablation of this gene increases resiliency and prevents aging- induced despair.

As a whole, these data show that the modulation of chaperone proteins has the potential for developing new therapies for the treatment of psychiatric diseases of the elderly.

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