Chaperones are molecular machines designed to maintain proteins in a properly folded state (1). Misfolded or misassembled proteins that cannot be corrected by chaperones are ubiquitinated and thereby targeted for degradation by the proteasome (2). We and others proposed that the chaperone and proteasome systems may act in concert in clearing the toxic forms of phosphorylated tau (p-tau) that are associated with tauopathies, particularly Alzheimer disease (3-5). This same approach might also work for other neurodegenerative diseases.

Hsp90 inhibitors bind to the ATPase domain of Hsp90, which prevents any further attempts at refolding the client proteins, instead promoting their proteasomal degradation. Recently, we were the first to demonstrate that pharmacologic manipulation of Hsp90 activity through the use of Hsp90 inhibitors was able to selectively reduce “abnormal” p-tau species in cultured cells (3,6) and in mice (4). We identified EC102 as an Hsp90 inhibitor with the optimal properties to facilitate blood-brain barrier permeability. Once-daily peripheral administration...  Read more

Chaperones are molecular machines designed to maintain proteins in a properly folded state (1). Misfolded or misassembled proteins that cannot be corrected by chaperones are ubiquitinated and thereby targeted for degradation by the proteasome (2). We and others proposed that the chaperone and proteasome systems may act in concert in clearing the toxic forms of phosphorylated tau (p-tau) that are associated with tauopathies, particularly Alzheimer disease (3-5). This same approach might also work for other neurodegenerative diseases.

Hsp90 inhibitors bind to the ATPase domain of Hsp90, which prevents any further attempts at refolding the client proteins, instead promoting their proteasomal degradation. Recently, we were the first to demonstrate that pharmacologic manipulation of Hsp90 activity through the use of Hsp90 inhibitors was able to selectively reduce “abnormal” p-tau species in cultured cells (3,6) and in mice (4). We identified EC102 as an Hsp90 inhibitor with the optimal properties to facilitate blood-brain barrier permeability. Once-daily peripheral administration of this compound into transgenic tau mice for one week significantly and specifically reduced only abnormal tau species in the brains of these mice, preserving “normal” tau. In cell culture, we described a mechanism whereby knocking down Hsp90 with siRNA abolished the EC102 effect, indicating that Hsp90 is required as a structural component of a tau-refolding/degradation complex and therefore a "client" of Hsp90. We also highlight the role of the tau ubiquitin ligase CHIP as critical for this process, consistent with our previous findings in CHIP null mice (7). Most remarkable was our finding that the Hsp90 complex isolated from only those areas of the AD brain that accumulate amyloid and tau had a 1,000-fold higher affinity for Hsp90 inhibitors. Unaffected regions from these same patients as well as control brains did not have this high affinity complex. These results suggest that Hsp90 antagonists might selectively target areas of injury and act like a "magic bullet" for treatment of Alzheimer disease.

The most recent contribution to the area of Hsp90 inhibitor therapy for Alzheimer disease from Gabriela Chiosis’ group largely confirms these data. Their work strengthens the notion that Hsp90 inhibitors can preferentially degrade only abnormal tau species, the production of which is certainly enhanced by mutations in tau. They also describe the efficacy of these drugs over a relatively short dosing period in two distinct mouse models of tauopathy; the same hTau model used in our previous work, that overexpresses all six isoforms of wild-type human tau, and the JNPL3 mouse model that overexpresses a single human tau isoform harboring the P301L mutation that causes FTDP-17, a heritable form of amyloid-negative tauopathy. They also suggest that tau exists in a complex with Hsp90 and p35, a component of the Cdk5 complex which is known to aberrantly phosphorylate tau; however, this complex seems to predominate in CNS regions that lack tau pathology in the JNPL3 model.

Collectively, these studies provide the foundation for future investigation of these compounds to treat Alzheimer disease, using alternative transgenic mouse models. Specifically, determining whether Hsp90 inhibition can delay the pathological progression and memory loss in an inducible mouse model of tauopathy (rTg4510) will establish the framework for introduction to the clinic. With the recent advance in the field demonstrating that an amyloid-depositing mouse model does not develop memory deficits on a tau knockout background (8), it is clear that tau has finally emerged as a legitimate target for AD therapeutics.

References:
1. Dickey CA, Petrucelli L. Current strategies for the treatment of Alzheimer's disease and other tauopathies. Expert Opin Ther Targets. 2006 Oct;10(5):665-76. Review. Abstract

2. Ciechanover A. The ubiquitin-proteasome proteolytic pathway. Cell. 1994;79:13-21. Abstract

3. Dickey CA, Dunmore J, Lu B, Wang JW, Lee WC, Kamal A, Burrows F, Eckman C, Hutton M, Petrucelli L. HSP induction mediates selective clearance of tau phosphorylated at proline-directed Ser/Thr sites but not KXGS (MARK) sites. FASEB J. 2006 Apr;20(6):753-5. Epub 2006 Feb 7. Abstract

4. Dickey CA, Kamal A, Lundgren K, Klosak N, Bailey RM, Dunmore J, Ash P, Shoraka S, Zlatkovic J, Eckman CB, Patterson C, Dickson DW, Nahman NS Jr, Hutton M, Burrows F, Petrucelli L. The high-affinity HSP90-CHIP complex recognizes and selectively degrades phosphorylated tau client proteins. J Clin Invest. 2007 Mar;117(3):648-58. Epub 2007 Feb 15. Abstract

5. Dou F, Netzer WJ, Tanemura K, Li F, Hartl FU, Takashima A, Gouras GK, Greengard P, Xu H. Chaperones increase association of tau protein with microtubules. Proc Natl Acad Sci U S A. 2003 Jan 21;100(2):721-6. Epub 2003 Jan 9. Abstract

6. Dickey CA, Eriksen J, Kamal A, Burrows F, Kasibhatla S, Eckman CB, Hutton M, Petrucelli L. Development of a high throughput drug screening assay for the detection of changes in tau levels -- proof of concept with HSP90 inhibitors. Curr Alzheimer Res. 2005 Apr;2(2):231-8. Abstract

7. Dickey CA, Yue M, Lin WL, Dickson DW, Dunmore JH, Lee WC, Zehr C, West G, Cao S, Clark AM, Caldwell GA, Caldwell KA, Eckman C, Patterson C, Hutton M, Petrucelli L. Deletion of the ubiquitin ligase CHIP leads to the accumulation, but not the aggregation, of both endogenous phospho- and caspase-3-cleaved tau species. J Neurosci. 2006 Jun 28;26(26):6985-96. Abstract

8. Roberson ED, Scearce-Levie K, Palop JJ, Yan F, Cheng IH, Wu T, Gerstein H, Yu GQ, Mucke L. Reducing endogenous tau ameliorates amyloid beta-induced deficits in an Alzheimer's disease mouse model. Science. 2007 May 4;316(5825):750-4. Abstract

View all comments by Leonard Petrucelli

In this paper, Luo and colleagues showed that PU24FCI inhibited Hsp90 activity and accelerated tau degradation. It is quite surprising that this drug treatment in JNPL3 mice reduced the amount of sarcosyl-insoluble tau within hours. This result suggests the possibility that the drug treatment can resolve pre-existing PHF-tau aggregates through enhancing tau degradation.

Santacruz and colleagues showed previously that after turning off mutant tau expression, NFT formation continued while memory impairment and neuron loss were rescued (SantaCruz et al., 2005). These older results suggest that before but not during NFT formation, tau may be modified to affect neuronal function. Because PU24FCI treatment reduced both soluble and insoluble tau levels, Hsp90 may recognize a modified form of tau, which may be toxic, and reduce its level. If so, I wonder if PU24FCI treatment can prevent the severe motor dysfunction that is known to develop in the JNPL3 mouse strain?

View all comments by Akihiko Takashima

One provocative question of this paper is, What is the impact of altering HSF-1 activity in the CNS? This is of particular interest to the field because inhibitors of Hsp90 (inducers of the heat shock response) are beginning to emerge as a viable therapeutic approach to treat neurodegenerative disorders associated with abnormal protein accumulation. Molecular chaperones may protect against these diseases by promoting the formation of less toxic protein aggregates and/or by targeting toxic, misfolded proteins for degradation through the ubiquitin-proteasome system. Will ramping up the heat shock response in the CNS increase the risk for cancer and/or deregulate the cell cycle in postmitotic cells? Intriguing data from both the cancer and neuroscience fields suggests that Hsp90 inhibitors may act selectively in pathological cells, sparing normal cells. Yet how altered HSF-1 activity varies in peripheral tissue compared with the central nervous system is unknown.

View all comments by Leonard Petrucelli

I have a a question regarding Dr. Soares' talk about serum amyloid P component. Unlike CRP, the blood level of SAP is rather consistent. A lower blood level of SAP is found in chronic liver diseases such as cirrhosis and chronic active hepatitis. The question is whether the lowered SAP level Dr. Soares reported to be significant in atorvastatin-treated patients might be due to compromised liver function?

View all comments by Feng Chen

Reply to comment by Feng Chen
I'd like to thank the author for his thoughtful question. In answer, as part of the safety monitoring in the ADCLT study, blood-borne markers of liver transaminases (LFT panel) and altered muscle physiology (CPK) were evaluated quarterly as indicators of known adverse events that accompany statin use. Of the 64 subjects who were blinded and completed the first quarterly visit, five (all female) were instructed to discontinue based upon clinical chemistry safety monitoring related to findings in the LFT panel. The remaining patients did not experience alterations in liver enzymes, suggesting that serum amyloid P levels do not necessarily correlate with liver function in this population.

For details on withdrawal adverse events in the ADCLT study that were based upon clinical chemistry, specifically in reference to liver enzymes, see Sparks et al., 2003.

View all comments by Holly D. Soares

This is an interesting paper. Of additional relevance is the role of aluminum, usually neglected, in the obstruction of tau degradation. Walton recently demonstrated in a rat model that aluminum inhibited activity of PP2A. PP2A is the major phosphate-removing enzyme in the brain against tau and neurofilament hyperphosphorylation, and against tau accumulation and NFT formation.

Walton also demonstrated using the Walton stain in autopsied human brain that aluminum participates within the human hippocampal neurons to facilitate accumulation of NFTs and eventually kill the cells by enucleation. Control of aluminum offers inexpensive prevention of AD according to many published epidemiology studies.

References:
Walton JR, An aluminum-based rat model for Alzheimer's disease exhibits oxidative inhibition of PP2A activity, hyperphosphorylated tau, and granulovacular degeneration. J Inorg Biochem. 2007 Sept;101(9):1275-84. Abstract

Walton JR, Aluminum in hippocampal neurons from humans with Alzheimer's disease. Neurotoxicology. 2006 May;27(3):385-94. Abstract

View all comments by Erik Jansson

This is an interesting set of results that seems to be suggesting that providing additional Bag2 somehow promotes a ubiquitin-independent proteasomal turnover of tau. I'm wondering how strongly expressed Bag2 might be in this context, relative to its basal level. Regardless, this is probing some interesting circuitry that deserves close attention.

View all comments by Arthur Horwich

This is a very informative paper from Carretierro et al. describing a novel relationship between BAG2 and tau. It further demonstrates the growing complexity of the chaperone network, moving us away from the idea that the chaperones are merely housekeeping genes that act in an unregulated, automated fashion. It seems that the route of tau clearance will be as complex, if not more complex, than the road to its hyperphosphorylation.

Members of the degradation process may also offer us more appropriate drug targets for therapeutic intervention in tauopathies and perhaps other diseases of protein misfolding. Identifying which chaperones are most specific for tau degradation could provide us with very novel clinical strategies for Alzheimer disease.

It should be noted that KNK437 does not inhibit Hsp70 activity but rather its levels via transcriptional repression of not only the Hsp70 gene, but also other heat shock genes (Yokota et al., 2000; Koishi et al., 2001). Changing the expression of Hsp70 levels could have very different consequences from directly inhibiting its...  Read more

This is a very informative paper from Carretierro et al. describing a novel relationship between BAG2 and tau. It further demonstrates the growing complexity of the chaperone network, moving us away from the idea that the chaperones are merely housekeeping genes that act in an unregulated, automated fashion. It seems that the route of tau clearance will be as complex, if not more complex, than the road to its hyperphosphorylation.

Members of the degradation process may also offer us more appropriate drug targets for therapeutic intervention in tauopathies and perhaps other diseases of protein misfolding. Identifying which chaperones are most specific for tau degradation could provide us with very novel clinical strategies for Alzheimer disease.

It should be noted that KNK437 does not inhibit Hsp70 activity but rather its levels via transcriptional repression of not only the Hsp70 gene, but also other heat shock genes (Yokota et al., 2000; Koishi et al., 2001). Changing the expression of Hsp70 levels could have very different consequences from directly inhibiting its ATPase function when clients are already bound.

One concern from many of these studies, including our own, is that the majority of models we work with overexpress tau. This is quite different from endogenous levels becoming dysfunctional, which is what occurs in human disease. It is quite possible that overexpressed chaperone clients, like tau, are processed quite differently from those that become dysfunctional at endogenous levels. Defining this could be critical for our understanding of the nature of chaperone biology in all protein misfolding diseases.

References:
Yokota S, Kitahara M, Nagata K. Benzylidene lactam compound, KNK437, a novel inhibitor of acquisition of thermotolerance and heat shock protein induction in human colon carcinoma cells. Cancer Res. 2000 Jun 1;60(11):2942-8. Abstract

Koishi M, Yokota S, Mae T, Nishimura Y, Kanamori S, Horii N, Shibuya K, Sasai K, Hiraoka M. The effects of KNK437, a novel inhibitor of heat shock protein synthesis, on the acquisition of thermotolerance in a murine transplantable tumor in vivo. Clin Cancer Res. 2001 Jan;7(1):215-9. Abstract

View all comments by Chad Dickey