Scientists know that the ravages of Alzheimer disease cripple the ability to lay down new memories, but the precise molecular dance that determines the success or failure of memory formation is still a mystery. Ongoing research is slowly pulling back the veil, however. Two new papers detail specific, discrete pathways of memory formation that share a common element, namely, signaling by the transcription factor cAMP-response element binding protein (CREB), which choreographs expression of many genes, including that for brain-derived neurotrophic factor (BDNF). In the July 14 issue of The Journal of Neuroscience, researchers led by Carlos Saura of Universitat Autònoma de Barcelona, Spain, report the discovery of a molecular mechanism that acts through CREB and BDNF and may help explain memory deficits in a mouse model of AD. Meanwhile, scientists working with Li-Huei Tsai at Massachusetts Institute of Technology in Cambridge show, in a July 11 letter to Nature, that a sirtuin protein, SIRT1, also promotes learning and memory through a CREB- and BDNF-mediated mechanism. Both of these findings may have implications for the treatment of AD and cognitive impairment.
CREB has long been known to be a star player in memory. In healthy brains, synaptic activity induces specific genes in neurons to turn on or off, which in turn leads to the long-lasting synaptic changes that underlie learning and memory. CREB plays a crucial role, in part, by turning on the gene for BDNF, a powerful neuronal growth and protective protein with numerous effects in the brain. BDNF levels are low in the brains of people with AD and in AD mouse models (see Phillips et al., 1991 and ARF related news story on Dickey et al., 2003), and a dearth of BDNF is associated with cognitive decline (see ARF related news story on Li et al., 2009). More excitingly, BDNF has demonstrated therapeutic potential for AD in animal models (see ARF related news story on Nagahara et al., 2009 and ARF related news story on Blurton-Jones et al., 2009).
The mechanisms behind AD’s effect on BDNF remain shrouded, however. To investigate, first author Judit España in Saura’s group used an AD mouse model that expresses human mutant APP with the Swedish and Indiana mutation (J9 line). España and colleagues demonstrated that CREB does not do its job properly in the mutant APP primary neurons. BDNF and other genes downstream of CREB were activated much less in transgenic neurons in response to neuronal activity than in wild-type neurons. By adding soluble Aβ to the cell cultures, the authors found that Aβ oligomers, but not monomers, reduced transcription of CREB-dependent genes. In addition, CREB signaling and BDNF expression could be fully rescued by using the γ-secretase inhibitor DAPT to reduce Aβ levels in the APP neurons. The findings suggest that oligomers of Aβ modulate activity of the transcription factor.
How does Aβ dampen CREB signaling? The authors linked the two to disrupted influx through L-type calcium channels, which normally allow calcium into neurons in response to electrical activity. They found that lack of calcium cripples calcineurin, a phosphatase needed to activate the CREB-regulated transcription coactivator 1 (CRTC1). CRTC1 normally cooperates with CREB to turn on genes related to learning and memory. In mutant APP neurons, CRTC1 is not activated, the authors found, and genes, including BDNF, do not turn on in response to synaptic activity.
Finally, the authors showed that the mechanism also occurs in vivo by examining APP mice at six months old, the age when Aβ begins to accumulate in the brain. At that age, mutant mice also demonstrate poorer learning and memory abilities in the Morris water maze than do wild-type animals. The researchers compared gene expression after water maze training and found that BDNF and other memory-related genes controlled by CREB were less active in the mutant mice compared to wild type, agreeing with the results from the cell culture experiments.
The findings suggest that decreased calcium influx, resulting from Aβ toxicity, inactivates CRTC1 and limits the action of CREB on memory formation. The authors would next like to show a direct link between CRTC1 and memory, Saura said, by testing if activation of CRTC1 can rescue the memory deficits in the APP transgenic mice. Eventually, his group would like to see if the CRTC1 pathway is also disturbed in humans with AD, which will require the analysis of postmortem brain samples from people at both early and late stages of the disease. If the results hold in humans, activation of the CRTC1 pathway may hold promise as a therapeutic strategy for AD, Saura said, but the first challenge will be to develop a drug that can specifically activate CRTC1 without producing toxic side effects.
The second paper investigated a different mechanism behind CREB signaling and BDNF expression, focusing on the role of SIRT1. This deacetylase is known to have a role in the cardiac system and DNA repair, and recent reports suggest it may have a function in the brain as well. The authors previously showed that SIRT1 increases neuronal survival in mouse models of AD and ALS (see Kim et al., 2007). The deacetylase is also activated by the small molecule resveratrol, found in red wine, which likewise improves neuronal survival.
To investigate the mechanisms behind SIRT1’s neuroprotective effects, first author Jun Gao generated conditional knockout (cKO) mice that lack SIRT1 only in brain cells. These mice showed poorer memory abilities than wild-type mice in fear-conditioning tests, novel object recognition tasks, and the Morris water maze. The impaired memory corresponded to physical changes in the brain: SIRT1 cKO mice had fewer synapses in their hippocampi than did wild-type mice, and their hippocampal synapses failed to acquire long-term potentiation (LTP) after stimulation.
The authors demonstrated that the SIRT1 KOs had less CREB protein in their hippocampi, leading to reduced binding of CREB to BDNF promoters, and lower levels of BDNF. They traced the CREB deficit to a microRNA, miRNA-134, that was inhibiting translation of CREB mRNA. They then showed that SIRT1 binds to DNA regions upstream of miRNA-134 and reduces its expression. In the SIRT1 KOs, higher expression of miRNA-134 inhibits CREB production, and ultimately leads to less BDNF in the brain. To nail down this mechanism, the authors showed that the presence of excess miRNA-134 in wild-type mice could abolish LTP and impair memory, and that knockdown of miRNA-134 in SIRT1 KO mice was sufficient to rescue LTP and memory formation.
Tsai said they would next like to look at SIRT1’s role in specific neuronal populations, for example, in excitatory neurons versus inhibitory neurons. The authors also want to determine the mechanism behind SIRT1’s role in neuroprotection. The preliminary results already suggest that SIRT1 activation could be beneficial for neurodegenerative disorders, Tsai said, and indicate the importance of further research into SIRT1’s actions in the brain. “It’s pretty clear that SIRT1 has evolved to play a very important role in regulating synaptic plasticity, learning, and memory in mammalian brain.”—Madolyn Bowman Rogers
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