26 June 2007. Evidence that stress can increase the risk for Alzheimer disease (AD) is growing. We recently reported that physical stress, in the form of ischemic trauma (see ARF related news story), and psychological stress (see ARF related news story) can increase the production of amyloid-β (Aβ) in the brain. In the latter case, researchers led by David Holtzman at Washington University, St. Louis, found that Aβ increases may be related to corticotropin releasing factor (CRF) signaling. We can now add tau phosphorylation to stress-related CRF activity. In the June 13 Journal of Neuroscience, Robert Rissman and colleagues at The SALK Institute for Biological Studies and Foundation for Medical Research, La Jolla, California, reported that stress-induced phosphorylation of tau is mediated by CRF receptor 1 (CRFR1). Conversely, the researchers found that CRFR2 receptors attenuate tau phosphorylation. Together with Holtzman’s data, these findings suggest CRF is an important mediator of stress-related AD pathology. “Collectively, I think that our data and that from Holtzman’s lab show that exposure to repeated stress can serve as a trigger for AD neuropathology,” said Rissman.

The microtubule-binding protein tau is the major component of neurofibrillary tangles, a hallmark of AD and other dementias. Tau hyperphosphorylation is the first step on the road to tau aggregates, and it has been well established that stress can activate tau kinases and increase tau phosphorylation in the brain, particularly the hippocampus. But how stress and tau phosphorylation are connected has been unclear. To address this, Rissman and colleagues, including Paul Sawchenko, tested if adrenal glucocorticoids, potent mediators of stress, have any effect on phosphorylation of tau in response to acute restraint. When they examined hippocampal extracts by Western blot, they found that even in adrenalectomized animals, this type of emotional stress led to increased phosphorylation of tau at several sites linked to AD pathology, including serines 202/205 (recognized by the AT8 antibody), and serines 396/404 (recognized by PHF-1), suggesting that glucocorticoids are not major regulators of tau phosphorylation.

Because corticotropin releasing factors have also been implicated in stress responses, Rissman and colleagues next looked to see if mice deficient in CRF receptors respond differently to stress. When they repeated their restraint experiments in CRFR1 knockout animals, they failed to detect any increase in tau phosphorylation, even though baseline levels of phospho-tau detected by the AT8 antibody were slightly higher than normal. In the case of CRFR2, the researchers found just the opposite. In CRFR2-negative animals, baseline phospho-tau levels were unchanged but after acute restraint jumped much higher than normal at five different sites: S181, S199, T212, T231, and the PHF-1 epitope. The AT8 and S422 sites were not modified to any greater extent than in control animals.

Though the results indicate that CRFR signaling mediates the effects of stress on tau phosphorylation, this effect could be indirect, given that CRFR knockouts may have drastically altered physiology or development. However, Rissman and colleagues found that the selective CRFR1 blocker antalarmin blocked stress-induced increases in tau phosphorylation at PHF-1 and AT8 sites, suggesting a more direct role for CRF signaling in the stress response. The researchers were also able to recapitulate the effects of stress by injecting CRF directly into the brain. This local administration yielded a similar pattern of tau phosphorylation as seen in stressed animals—CRFR2 knockout animals had greater increases in phospho-tau than controls, whereas CRFR1 knockouts had less.

How does CRF signaling affect tau phosphorylation? The simplest explanations would be either an increase in kinase or a decrease in phosphatase activity. The authors examined both. They found that active forms of GSK-3β, JNK, and ERK2 kinases were upregulated by stress, and the time course of their activation was similar to that for tau phosphorylation, peaking about 20-40 minutes after a 30-minute period of restraint. Kinase activation also seems dependent on CRFR status. Active GSK-3β was not elevated in stressed CRFR1-negative mice, for example, but was much higher in stressed CRFR2-negative animals than in controls, as was active JNK kinases. “Overall, these results identify several tau kinases as potential effectors of CRFR-dependent effects on acute emotional stress on tau-P,” write the authors.

The researchers also found that levels of protein phosphatase 2A catalytic subunit (PP2A-c) in soluble hippocampal extracts increased in response to repeated stress, but whether this is related to phospho-tau turnover is unclear. “Although additional characterization is needed, these findings identify PP2A as a potential contributor to alterations in tau-P under repeated stress conditions,” they write.

Whether these latest findings may help people who are prone to stress reduce their risk for AD is difficult to say at present. “These are very complex issues to deal with. The responses our bodies have to stress play adaptive roles in preparing our bodies to deal with challenges and stress-induced tau phosphorylation is most likely an integral component of stress neuroplasticity,” said Rissman. Nevertheless, there are companies actively pursuing the development of small molecules that bind CRF receptors, and some are already in phase 2 clinical trials for depression and other mood disorders. “We may have discovered another application. Such drugs could have a prophylactic effect or delay the progression of Alzheimer’s disease,” said Sawchenko.—Tom Fagan.

Reference:
Rissman RA, Lee K-F, Vale W, Sawchenko PE. Corticotropin-releasing factor receptors differentially regulate stress-induced tau phosphorylation. J. Neurosci. 2007, June 13;27:6552-6562. Abstract