Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Medicine

Major Professor

Chad Dickey, Ph.D.

Committee Member

David Kang, Ph.D.

Committee Member

Jaya Padmanabhan, Ph.D.

Committee Member

Edwin Weeber, Ph.D.

Keywords

chaperones, FKBP5, Hsp90, oligomers

Abstract

Tau is a protein which regulates microtubule stability and is heavily involved in axonal transport. This stability is dynamically controlled in part by over 40 phosphorylation sites across the tau protein which allows for binding and release from the microtubules. However, if abnormal hyperphosphorylation occurs, tau dissociates from the microtubules. Once released, the microtubules become unstable and the aberrant tau mislocalizes from the axon to the somatodendric compartment, where it aggregates. These aggregates are made of many pathological forms of tau including oligomeric species, paired helical filaments, and neurofibrillary tangles, all of which have associated toxicities. Tau pathology is a hallmark of Alzheimer's disease, one of over 15 diseases known as tauopathies which present with tau pathology, all of which lack effective treatments. Heat shock protein 90 kDa (Hsp90) is a major adenosine triphosphate (ATP)-dependent regulator of non-native proteins, like misfolded tau. Although Hsp90 is able to effectively refold and degrade many aberrant proteins, it has been associated with preserving aberrant tau. In fact, inhibiting the Hsp90 ATPase activity leads to the degradation of tau, which has been demonstrated in a number of models with the use of various Hsp90 inhibitors. However, there are many side-effects associated with the use of these inhibitors including toxicity and heat shock factor 1 (HSF1) activation. Although improvements on Hsp90 inhibitors are still in progress, this study explores targeting Hsp90 through a slightly different mechanism, by targeting Hsp90 co-chaperones. Hsp90 is involved in almost every pathway in each cell throughout the body. Co-chaperone proteins assist Hsp90 in these various processes, but are each only involved in a subset of the total Hsp90 interactome. Therefore, targeting Hsp90 co-chaperones could lead to improved efficacy, potency, and safety of drugs designed toward Hsp90 for the treatment of tauopathies. We previously showed one of these co-chaperones, FK506 binding protein 51 kDa (FKBP51), a tetratricopeptide repeat (TPR) domain containing immunophilin, coordinates with Hsp90 to regulate tau metabolism. More specifically, we found that increases and decreases in FKBP51 levels correlated with increases and decreases in tau levels, respectively.

FKBP51 knockout mice have been extensively studied and have shown no negative phenotypes in these characterizations. In this study, we found that this mouse model has decreased endogenous tau levels. Furthermore, this study demonstrates that FKPB51 colocalizes with pathological tau in the AD brain, and synergizes with Hsp90 to preserve tau from proteasomal degradation. Additionally, FKBP51 overexpression in mouse model of tau pathology leads to the preservation of tau. We went on to characterize this accumulated tau as being neurotoxic and oligomeric in nature, while being low in silver positive, β-sheet structure. In the human brain, we found that FKBP51 is strikingly increased with aging and even further in the AD brain. In support of these findings, we also found age-associated decreased methylation in the FKBP5 gene, which encodes FKBP51. Moreover, we found that increasing levels of FKBP51 caused other co-chaperone to have reduced Hsp90 binding and led to tau preservation. This supports a model where age-related increases in FKBP51 lead to the preservation of misfolded tau species and ultimately disease.

In order to model the high FKBP51 expression found in the aging brain, we generated the first FKBP5 overexpressing mouse model, which is tet-regulatable. This mouse, rTgFKBP5, was made by targeted, single insertion of the human FKBP5 gene into the HIP11 locus of the mouse genome crossed with CamKIIα tTa mice. We have now confirmed high FKBP51 levels in the forebrain and hippocampus of this mouse, which will serve as a testing platform for FKBP51 regulating drugs.

Overall, this work exemplifies FKBP51 as an important regulator of tau metabolism through Hsp90. With the absence of a negative phenotype in mice ablated of FKBP51 and the development of this novel, FKBP51 overexpressing mouse model, strategies designed to decrease FKPB51 levels or to disrupt the FKBP51/Hsp90 complex could be relevant for the treatment of tauopathies, like AD.

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