A structural mechanism for maintaining the ‘on‐state’ of the CaMKII memory switch in the post‐synaptic density

Abstract

, providing a biochemical memory that is critical for plasticity. To understand the factors controlling the duration of this state of CaMKII, we studied dephosphorylation of CaMKII in the post-synaptic density (PSD), a structure that defines a neuronal subcompartment critical for plasticity. We found that PSD-resident PP1 can dephosphorylate many sites on CaMKII, but not the T286 site that produces Ca 2+ -independent activity. This, together with previous work showing that soluble PP2A cannot dephosphorylate PSD CaMKII, provides a novel explanation for the in vivo persistence of T286 phosphorylation: after activated CaMKII translocates from the cytoplasm to the PSD, structural constraints prevent phosphatases from dephosphorylating T286. These results also suggest that the PSD is more than a simple scaffold for synaptic proteins; it may act to regulate the activity of proteins by positioning them in orientations that either prevent or favor specific biochemical reactions. Keywords: learning and memory, long-term potentiation, phosphatase, PP1, protein kinase. Short-term and long-term synaptic modifications are triggered by brief periods of synaptic activity during which Ca 2+ elevation in the cytoplasm is critical. Such Ca 2+ elevations trigger biochemical networks that result in structural and functional changes at synapses. For instance, learning events produce long-term potentiation (LTP) of synapses within the hippocampus (Whitlock et al. 2006). Ca 2+ /calmodulindependent protein kinase II (CaMKII; EC number 2.7.11.17) has been implicated in both short-term and longterm synaptic plasticity (reviewed in Lisman et al. 2002;Elgersma et al. 2004;Colbran and Brown 2004). Importantly, the molecule itself is persistently altered by brief Ca 2+ elevation. Ca 2+ -dependent autophosphorylation at T286 (Lai et al. 1986;Miller and Kennedy 1986;Miller et al. 1988) makes the enzyme active even in the absence of sustained high levels of Ca 2+ . Such activation occurs during LTP induction and can persist for hours (Fukunaga et al. 1993). When this persistence is blocked by a mutation that prevents T286 phosphorylation, both learning and LTP are greatly reduced (Giese et al. 1998). It is thus crucial to understand the mechanisms that control persistent activation of synaptic CaMKII.This persistence is not straightforward to understand. An important pool of CaMKII is contained within the postsynaptic density (PSD), which also contains a high concentration of the phosphatase, PP1, an enzyme that can dephosphorylate CaMKII. Various theoretical models involving the balance of kinase autophosphorylation and phosphatase activities have been proposed to explain persistence (Miller et al. 2005); however, a recent study using reconstituted soluble CaMKII and PP1 was unable to demonstrate a persistent 'on-state ' (Bradshaw et al. 2003). One possible explanation is that persistence requires special conditions that occur in the PSD. To test this possibility, we examined the stability of switching within purified PSDs. Our results show

CaMKIIα interacts with multi-PDZ domain protein MUPP1 in spermatozoa and prevents spontaneous acrosomal exocytosis

“…The observation that lipid raft association and correct subcellular targeting are required for an effective and specific substrate phosphorylation of CaMKII has already been made (Strack et al, 1997;Tsui et al, 2005). Furthermore, binding to scaffolding proteins was found to prevent dephosphorylation and thus inactivation of CaMKII (Mullasseril et al, 2007). Although we do not know yet whether MUPP1 avoids kinase dephosphorlyation and whether CaMKII␣ substrates also bind to MUPP1 (Fig.…”

Differential expression of molecular markers of synaptic plasticity in the hippocampus, prefrontal cortex, and amygdala in response to spatial learning, predator exposure, and stress‐induced amnesia

“…predator stress, alone, reduced p-CaMKII in this cortical region. Interestingly, the group of rats given water maze training, in addition to cat exposure, did not exhibit this reduction, suggesting that spatial learning, perhaps via a training-induced translocation of CaMKII from the cytoplasm to the post-synaptic density (Mullasseril et al, 2007), prevented the predator stress-induced decrease of p-CaMKII expression.…”

Regulation of CaMKII In vivo: The Importance of Targeting and the Intracellular Microenvironment

“…CaMKII associated with PSDs is primarily dephosphorylated by PP1 [43,93,94,96], although phosphorylation specifically at Thr286 appears to be protected from dephosphorylation by PSD resident PP1 [97], presumably due to steric hindrance or conformational change caused by neighbouring proteins. By contrast, soluble CaMKII appears to be dephosphorylated by PP2A [43].…”