Mechanism for long-term memory formation when synaptic strengthening is impaired

Abstract

Long-term memory (LTM) formation has been linked with functional strengthening of existing synapses and other processes including de novo synaptogenesis. However, it is unclear whether synaptogenesis can contribute to LTM formation. Here, using α-calcium/calmodulin kinase II autophosphorylation-deficient (T286A) mutants, we demonstrate that when functional strengthening is severely impaired, contextual LTM formation is linked with traininginduced PSD95 up-regulation followed by persistent generation of multiinnervated spines, a type of synapse that is characterized by several presynaptic terminals contacting the same postsynaptic spine. Both PSD95 up-regulation and contextual LTM formation in T286A mutants required signaling by the mammalian target of rapamycin (mTOR). Furthermore, we show that contextual LTM resists destabilization in T286A mutants, indicating that LTM is less flexible when synaptic strengthening is impaired. Taken together, we suggest that activation of mTOR signaling, followed by overexpression of PSD95 protein and synaptogenesis, contributes to formation of invariant LTM when functional strengthening is impaired.synaptic plasticity | hippocampus | immediate-early gene | reconsolidation A fundamental question in neuroscience concerns the cellular mechanisms that lead to long-term memory (LTM) formation (1-3). It has been shown that LTM formation is associated with and demands strengthening of existing synapses, socalled functional plasticity (3-5), but it may also be linked with a more overt form of structural plasticity, an increase in synapse density (6-9). It is, however, unclear whether de novo synaptogenesis is critical for LTM formation.The participation of synaptogenesis for LTM formation can be investigated when synaptic strengthening is fully blocked. It would be ideal to also block training-induced changes in neuronal excitability that have been implicated in memory formation (10). However, technically this block may not be achievable as, for example, there are distinct types of synaptic strengthening. As a proxy, here we have studied T286A missense mutant mice lacking autophosphorylation at threonine 286 of the α-isoform of calcium/calmodulindependent kinase II (αCaMKII), a major protein of glutamatergic synapses in the forebrain (11). The T286A mutants have fully blocked NMDA receptor-dependent synaptic strengthening at hippocampal CA1 synapses (11-14), and they have impaired regulation of the postburst afterhyperpolarization (AHP) in CA1 pyramidal neurons (15). Accordingly, T286A mutants are deficient in contextual fear conditioning after a single training trial (16). However, after five massed training trials, T286A mutants can form contextual LTM (16) and this type of LTM is hippocampus-dependent (14).Here, we have used T286A mutants as tools to investigate the mechanism of LTM formation when NMDA receptor-dependent synaptic strengthening is blocked. Our study provides evidence that activation of mTOR signaling in the hippocampus, followed by overexpression of PSD95 protein and...

The roles of protein kinases in learning and memory

“…Forebrain-restricted expression of a dominant-negative form of PAK impairs contextual fear LTM (Hayashi et al 2004). In these mutants basal synapse numbers are normal in the hippocampus (Hayashi et al 2004), but the authors did not test whether conditioning-induced synaptogenesis is impaired (compare with, Radwanska et al 2011).…”

Long-term potentiation can be induced in the CA1 region of hippocampus in the absence of αCaMKII T286-autophosphorylation

“…Specific LTP can be induced in T286A mice in the presence of rapamycin In T286A mice, immediate learning was impaired, but when the task was repeated in a massed training protocol of fear conditioning, learning occurred (Irvine et al 2005). Radwanska et al (2011) showed that this learning could be impaired by rapamycin.…”

“…Specific LTP can be induced in T286A mice in the presence of rapamycin In T286A mice, immediate learning was impaired, but when the task was repeated in a massed training protocol of fear conditioning, learning occurred (Irvine et al 2005). Radwanska et al (2011) showed that this learning could be impaired by rapamycin.We thus studied the effect of rapamycin on the LTP induced by four trains in mutant mice after a recovery period in submersion. Surprisingly, instead of inhibiting LTP, the incubation of slices with rapamycin increased the potentiation level of the stimulated pathway (Fig.…”

Generation of multi-innervated dendritic spines as a novel mechanism of long-term memory formation

“…These LTP impairments were demonstrated in hippocampal slices and in anaesthetised mice. However, despite the absence of NMDA (Irvine, Vernon, & Giese, 2005;Irvine et al, 2011;Radwanska et al, 2011). This was shown after training in contextual fear conditioning and the passive avoidance task.…”

“…aCaMKII T286A mice have a targeted point mutation in the aCaMKII gene that prevents phosphorylation at the autonomy site threonine-286 (Giese, Fedorov, Filipkowski, & Silva, 1998). Thorough electrophysiological investigation have established that the lack of T286 autophosphorylation completely abolishes NMDA receptor-dependent LTP at hippocampal CA1 synapses in adult, but not at neonatal age (Cooke et al, 2006;Giese et al, 1998;Irvine et al, 2011;Radwanska et al, 2011;Villers, Giese, & Ris, 2014;Yasuda, Barth, Stellwagen, & Malenka, 2003). These LTP impairments were demonstrated in hippocampal slices and in anaesthetised mice.…”