In neurons, mitogen-activated protein kinase (MAPK) regulates a variety of signal transduction pathways, including those involved in long-term memory and survival, so anything that modulates MAPK signaling could have important consequences for the working of the brain. In the March 23 Cell, Daniel Storm and colleagues at the University of Washington, Seattle, show that a protein called SCOP attenuates MAPK signaling in the hippocampus and must be proteolytically degraded before certain long-term memories can be formed. The work introduces a new level of complexity into the molecular biology of memories. It may also have implications for the pathology of Alzheimer disease since MAPK activity has been linked to tau and γ-secretase phosphorylation.
SCOP, or suprachiasmatic nucleus (SCN) circadian oscillatory protein, was first discovered because it undergoes rhythmic fluctuations in the SCN, a tiny region of the brain that houses the circadian clock. While SCOP does not undergo these oscillations in other regions of the brain, it is present, and Kimiko Shimizu and colleagues at Osaka University, Japan, showed that in PC-12 cells, it binds to K-ras, preventing stimulation of MAPK by neurotrophins (Shimizu et al., 2003). Now, working in Storm’s lab in Seattle, first author Shimizu and colleagues have delved deeper into SCOP biology to ask what role, if any, the protein may play in MAPK-mediated pathways in the hippocampus, a region of the brain that plays a crucial role in learning and memory and that is badly damaged by AD pathology.
Because MAPK-stimulated transcription through the cAMP response element (CRE) is necessary for memory consolidation, the researchers began by looking at the effects of SCOP on a CRE-mediated reporter system. Shimizu and colleagues found that the protein is expressed in the hippocampus and that in cultured hippocampal cells it attenuates expression of a CRE-driven luciferase gene. When they co-transfected hippocampal cells with reporter and SCOP expression constructs, brain-derived neurotrophic factor (BDNF)-induced luciferase expression was reduced by about 80 percent. In contrast, when they silenced SCOP expression by RNAi, CRE-dependent luciferase activity increased about fourfold, even in the absence of BDNF.
The findings suggest that SCOP regulates CRE-driven transcription, raising the question of how SCOP itself is regulated. Finding no evidence that the protein is phosphorylated, the researchers asked if its degradation might relieve suppression of CRE promoters. When they looked at SCOP levels in BDNF-treated primary hippocampal neurons, they found that the protein underwent rapid cycling. Five minutes after BDNF was added SCOP, levels fell by about half, but fully recovered after about 60 minutes. Test tube analysis of SCOP degradation revealed a calcium-dependent process, suggesting that caspases, the proteasome, or calpain might be the proteolytic culprit. Shimizu found that neither caspase-3 nor proteasome inhibitors prevented SCOP degradation. The calpain inhibitor calpeptin did, however, and since they also found that calpain is activated in the neurons on addition of BDNF, the findings suggest that calpain-induced degradation may provide short-term relief from SCOP suppression of MAPK signals.
What relevance might all of this have on neuronal activity in general and on memory in particular? To address this, Shimizu and colleagues trained mice in a novel object recognition paradigm of learning and memory. They found that this training led to a small (16 percent), though significant drop in SCOP levels after 5 minutes and an increase in MAPK activity. Furthermore, when they generated a mouse strain with a tetracycline-inducible SCOP expression system and tested it in the learning and memory paradigm, they found that induction of SCOP completely prevented the animals from learning new objects.
The findings link calcium influx, SCOP degradation, MAPK control of CRE-driven transcription, and learning and memory. “We hypothesize that calpain-catalyzed degradation of SCOP in response to activity-dependent Ca2+ increases is required for activation of MAPK during formation of hippocampus-dependent memory,” write the authors. SCOP regulation may have other far-reaching ramifications, too. MAPK has been implicated in tau (see Lambourne at al., 2005), presenilin (see Kim et al., 2006), and neurofilament phosphorylation (see Veeranna et al., 2004), which may all be important in AD. “Although this study focused on regulation of SCOP in hippocampal neurons and its relationship to memory formation, the data have broad implications because of the central role played by MAPK and CRE-mediated transcription in neuroplasticity,” write the authors.—Tom Fagan
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