In particular, attention has focused on CHIP as a key regulator of protein fate and a link between chaperones such as Hsp70 and the proteasome protein degradation pathway. When a misfolded protein binds Hsp70, the chaperone tries to refold the protein and release it. But if the protein cannot be refolded, then the ubiquitin ligase activity of CHIP tags the abnormal protein, sending it to be destroyed.

A paper in the March 23 issue of Nature shows a new side to CHIP, as a regulator of the Hsp70 protein stability, and thus the cell stress response. The work, from the lab of Cam Patterson at the University of North Carolina at Chapel Hill, reveals that once CHIP has dispatched with abnormal proteins, it turns its ubiquitin ligase activity on Hsp70, causing destruction of the chaperone. The results show a new and unsuspected mechanism for regulating the duration of inducible Hsp expression. The central role of CHIP in the response to misfolded proteins raises the possibility that it could play a part in causing the buildup of toxic, disease-causing proteins, or might be enlisted to clear them.

CHIP, first cloned by Patterson in 1999, was known to regulate the transcriptional induction of Hsp70 after heat shock, independently of its effects on protein clearance. Efforts to study this regulation in more detail yielded a paradoxical observation: First author Shu-Bing Qian discovered that either overexpression or underexpression of CHIP in cultured cells resulted in higher levels of Hsp70 protein. The first result was explained by CHIP’s ability to enhance Hsp70 gene transcription, but the second observation was puzzling. By pulling apart CHIP function using transfection studies and CHIP-/- cells, Qian et al. showed that CHIP enhanced the ubiquitination and degradation of Hsp70. However, in the presence of a misfolded protein substrate, Hsp70 ubiquitination and degradation is inhibited. It is only when misfolded proteins are depleted that CHIP ubiquitinates Hsp70 and speeds its degradation. In this way, the authors explain, CHIP maintains homeostasis by shutting down the Hsp70 response after misfolded proteins have been cleared from cells.

In the last few years, several studies have established that Hsp70/CHIP activity affects the levels or toxicity of tau (Shimura et al., 2004; Petrucelli et al., 2004; Sahara et al., 2005), polyglutamine-containing proteins (Miller et al., 2005), α-synuclein (Shin et al., 2005), and mutant SOD1 (Urushitani et al., 2004). In addition, CHIP has been reported to be associated with parkin, and to enhance that protein’s ubiquitin ligase activity (Imai et al., 2002). Understanding how CHIP regulates the stress response and protein degradation could lead to new ways to help neuronal cells clean up their act when these toxic proteins enter the picture.—Pat McCaffrey.

Reference:
Qian S, McDonough H, Boellmann F, Cyr DM, Patterson C. CHIP-mediated stress recovery by sequential ubiquitination of substrates and Hsp70. Nature. 23 March 2006; 440: 551-555. Abstract

Q&A with Cam Patterson.

Q: CHIP activity has been shown to enhance the degradation of several mutated or misfolded proteins involved in neurodegeneration, for example, tau, α-synuclein, SOD mutants, polyglutamine-expanded proteins. Do your results tell us something new about the potential role of CHIP in neurodegeneration?
A: Our new results tell us a lot of things, and I think that in the context of neurodegenerative diseases, there are a couple of particularly salient points. First, it is clear that CHIP plays an important role in removing many of the proteins that contribute to neurodegenerative diseases— the ones you describe above—because they are prone to misfold. Our new studies point out that the stress response that depends on Hsp70 to repair and refold proteins is turned off in a coordinated fashion when damaged proteins are cleared by the ubiquitin ligase CHIP. In situations where the misfolded proteins cannot be cleared, and this may particularly apply to neurodegenerative diseases, the stress response will remain continually activated, and this will ultimately have adverse consequences at the cellular level. Thus, impairment of this new mechanism we have discovered to shut down the stress response may play an important role in perpetuating injury to neurons that express these misfolded mutant proteins. This may be one important reason why neurons die and brains atrophy in these horrible diseases.

Q: A few years ago you generated CHIP knockout mice. Do they show evidence of any kind of neurodegeneration?
A: This is an interesting question. Our mice have indeed generated a lot of attention from investigators who study mouse models of neurodegenerative disease, and we have gone out of our way to share our mice with these investigators and to collaborate with them when we can. I don't want to diminish the impact of these studies, since they haven't yet been published, but will say that there are some very fundamental and exciting observations that will soon be reported about acceleration of neurodegeneration in our mice that lack CHIP.

Q: You suggest that increasing CHIP activity might be a way to address protein misfolding diseases. Any ideas how you might do that?
A: Gene therapy would be one way, and some of our collaborators are working on this in animal models of neurodegenerative disease. Perhaps even more exciting in this regard is the recent report of the crystal structure of CHIP that was published by Laurence Pearl's group in a recent edition of Molecular Cell (Zhang et al., 2005). The structure of CHIP is beautiful and very surprising, and we are looking at clues from the structure that might help us to enhance CHIP's activity using pharmacologic approaches. This could be a practical and exciting way to modulate outcomes in individuals who are at risk for neurodegenerative diseases.