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In the \"A Model of a Metaplastic Synapse\" section and the \"The Effect of Use-dependent Synaptic Potentiation: Coupling to a Stochastic and Bistable Switch\" section, \"3 pt\" should not appear after the bullets. Please see the correct equation 7 here: f Please see the correct equation 12 here: f Please see the correct equation 13 here: f Please see the correct equation 18 here: f Citation: Mehta A, Luck J-M, Luk CC, Syed NI (2014) Correction: Synaptic Metaplasticity Underlies Tetanic Potentiation in Lymnaea: A Novel Paradigm.

Microglia, the resident immune cell of the brain, can be eliminated via pharmacological inhibition of the colony‐stimulating factor 1 receptor (CSF1R). Withdrawal of CSF1R inhibition then stimulates microglial repopulation, effectively replacing the microglial compartment. In the aged brain, microglia take on a “primed” phenotype and studies indicate that this coincides with age‐related cognitive decline. Here, we investigated the effects of replacing the aged microglial compartment with new microglia using CSF1R inhibitor‐induced microglial repopulation. With 28 days of repopulation, replacement of resident microglia in aged mice (24 months) improved spatial memory and restored physical microglial tissue characteristics (cell densities and morphologies) to those found in young adult animals (4 months). However, inflammation‐related gene expression was not broadly altered with repopulation nor the response to immune challenges. Instead, microglial repopulation resulted in a reversal of age‐related changes in neuronal gene expression, including expression of genes associated with actin cytoskeleton remodeling and synaptogenesis. Age‐related changes in hippocampal neuronal complexity were reversed with both microglial elimination and repopulation, while microglial elimination increased both neurogenesis and dendritic spine densities. These changes were accompanied by a full rescue of age‐induced deficits in long‐term potentiation with microglial repopulation. Thus, several key aspects of the aged brain can be reversed by acute noninvasive replacement of microglia.

MicroRNAs play a pivotal role in rapid, dynamic, and spatiotemporal modulation of synaptic functions. Among them, recent emerging evidence highlights that microRNA‐181a (miR‐181a) is particularly abundant in hippocampal neurons and controls the expression of key plasticity‐related proteins at synapses. We have previously demonstrated that miR‐181a was upregulated in the hippocampus of a mouse model of Alzheimer's disease (AD) and correlated with reduced levels of plasticity‐related proteins. Here, we further investigated the underlying mechanisms by which miR‐181a negatively modulated synaptic plasticity and memory. In primary hippocampal cultures, we found that an activity‐dependent upregulation of the microRNA‐regulating protein, translin, correlated with reduction of miR‐181a upon chemical long‐term potentiation (cLTP), which induced upregulation of GluA2, a predicted target for miR‐181a, and other plasticity‐related proteins. Additionally, Aβ treatment inhibited cLTP‐dependent induction of translin and subsequent reduction of miR‐181a, and cotreatment with miR‐181a antagomir effectively reversed the effects elicited by Aβ but did not rescue translin levels, suggesting that the activity‐dependent upregulation of translin was upstream of miR‐181a. In mice, a learning episode markedly decreased miR‐181a in the hippocampus and raised the protein levels of GluA2. Lastly, we observed that inhibition of miR‐181a alleviated memory deficits and increased GluA2 and GluA1 levels, without restoring translin, in the 3xTg‐AD model. Taken together, our results indicate that miR‐181a is a major negative regulator of the cellular events that underlie synaptic plasticity and memory through AMPA receptors, and importantly, Aβ disrupts this process by suppressing translin and leads to synaptic dysfunction and memory impairments in AD. In the hippocampus, neuronal stimulation produces upregulation of translin, reduction of miR‐181a, and an increase in the protein levels of its target GluA2 leading to synaptic plasticity. This plasticity mechanism is impaired by amyloid‐beta (Aβ) toxic species.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial surface receptor genetically linked to the risk for Alzheimer’s disease (AD). A proteolytic product, soluble TREM2 (sTREM2), is abundant in the cerebrospinal fluid and its levels positively correlate with neuronal injury markers. To gain insights into the pathological roles of sTREM2, we studied sTREM2 in the brain of 5xFAD mice, a model of AD, by direct stereotaxic injection of recombinant sTREM2 protein or by adeno-associated virus (AAV)-mediated expression. We found that sTREM2 reduces amyloid plaque load and rescues functional deficits of spatial memory and long-term potentiation. Importantly, sTREM2 enhances microglial proliferation, migration, clustering in the vicinity of amyloid plaques and the uptake and degradation of Aβ. Depletion of microglia abolishes the neuroprotective effects of sTREM2. Our study demonstrates a protective role of sTREM2 against amyloid pathology and related toxicity and suggests that increasing sTREM2 can be explored for AD therapy. TREM2 is a genetic risk factor for Alzheimer’s disease, and soluble TREM2 (sTREM2) in the CSF correlates with AD progression. Here the authors study the role of sTREM2 in a mouse model of Alzheimer’s disease, and find it reduces amyloid accumulation and increases the numbers of plaque-associated microglia which correlates with improved behavioural function in the mice.

Traumatic brain injury (TBI) is a leading cause of long-term neurological disability, yet the mechanisms underlying the chronic cognitive deficits associated with TBI remain unknown. Consequently, there are no effective treatments for patients suffering from the long-lasting symptoms of TBI. Here, we show that TBI persistently activates the integrated stress response (ISR), a universal intracellular signaling pathway that responds to a variety of cellular conditions and regulates protein translation via phosphorylation of the translation initiation factor eIF2α. Treatment with ISRIB, a potent drug-like small-molecule inhibitor of the ISR, reversed the hippocampaldependent cognitive deficits induced by TBI in two different injury mouse models—focal contusion and diffuse concussive injury. Surprisingly, ISRIB corrected TBI-induced memory deficits when administered weeks after the initial injury and maintained cognitive improvement after treatment was terminated. At the physiological level, TBI suppressed long-term potentiation in the hippocampus, which was fully restored with ISRIB treatment. Our results indicate that ISR inhibition at time points late after injury can reverse memory deficits associated with TBI. As such, pharmacological inhibition of the ISR emerges as a promising avenue to combat head traumainduced chronic cognitive deficits.

Learning is primarily mediated by activity-dependent modifications of synaptic strength within neuronal circuits. We discovered that place fields in hippocampal area CA1 are produced by a synaptic potentiation notably different from Hebbian plasticity. Place fields could be produced in vivo in a single trial by potentiation of input that arrived seconds before and after complex spiking. The potentiated synaptic input was not initially coincident with action potentials or depolarization. This rule, named behavioral time scale synaptic plasticity, abruptly modifies inputs that were neither causal nor close in time to postsynaptic activation. In slices, five pairings of subthreshold presynaptic activity and calcium (Ca2+) plateau potentials produced a large potentiation with an asymmetric seconds-long time course. This plasticity efficiently stores entire behavioral sequences within synaptic weights to produce predictive place cell activity.

LTP is the most intensively studied cellular model of the memory and generally divided at least two distinct phases as early and late. E-LTP requires activation of CaMKII that initiates biochemical events and trafficking of proteins, which eventually potentiate synaptic transmission, and is independent of de novo protein synthesis. In contrast, L-LTP requires gene expression and local protein synthesis regulated via TrkB receptor- and functional prions CPEB2-3-mediated translation. Maintenance of LTP for longer periods depends on constitutively active PKMζ. Throughout this review, current knowledge about early and late phases of LTP will be reviewed.

Mental strength and history of winning play an important role in the determination of social dominance. However, the neural circuits mediating these intrinsic and extrinsic factors have remained unclear. Working in mice, we identified a dorsomedial prefrontal cortex (dmPFC) neural population showing “effort”-related firing during moment-to-moment competition in the dominance tube test. Activation or inhibition of the dmPFC induces instant winning or losing, respectively. In vivo optogenetic-based long-term potentiation and depression experiments establish that the mediodorsal thalamic input to the dmPFC mediates long-lasting changes in the social dominance status that are affected by history of winning. The same neural circuit also underlies transfer of dominance between different social contests. These results provide a framework for understanding the circuit basis of adaptive and pathological social behaviors.

It is assumed that synaptic strengthening and weakening balance throughout learning to avoid runaway potentiation and memory interference. However, energetic and informational considerations suggest that potentiation should occur primarily during wake, when animals learn, and depression should occur during sleep. We measured 6920 synapses in mouse motor and sensory cortices using three-dimensional electron microscopy. The axon-spine interface (ASI) decreased ~18% after sleep compared with wake. This decrease was proportional to ASI size, which is indicative of scaling. Scaling was selective, sparing synapses that were large and lacked recycling endosomes. Similar scaling occurred for spine head volume, suggesting a distinction between weaker, more plastic synapses (~80%) and stronger, more stable synapses. These results support the hypothesis that a core function of sleep is to renormalize overall synaptic strength increased by wake.

It was proposed that protein kinase M-ζ (PKM-ζ) is a key factor in long-term potentiation (LTP) and memory maintenance on the basis of the disruption of LTP and memory by inhibitors of PKM-ζ; however, here mice that do not express PKM-ζ are shown to have normal LTP and memory, thus casting doubts on a critical role for PKM-ζ in these processes. Rethink on memory mechanisms Long-term potentiation (LTP), a persistent enhancement of signalling between nerve cells, has long been considered the likely cellular correlate of memory, but only now are the specific molecular mechanisms underlying the maintenance of LTP beginning to emerge. Some time ago, it was proposed that sustained activity of protein kinase M-ζ (PKM-ζ) may be a key factor in sustaining LTP, based mainly on experiments using pharmacological inhibitors. Two groups have now engineered mice lacking PKM-ζ to test more directly for its role in LTP and memory. Studies from the labs of Richard Huganir and Robert Messing find that loss of PKM-ζ has no effect on LTP or memory formation. And despite the absence of this kinase, pharmacological inhibitors of PKM-ζ still disrupt memory in these mutant mice. These data cast doubt on the role of PKM-ζ in LTP maintenance and re-open the exploration for key molecules regulating long-term plasticity. Long-term potentiation (LTP), a well-characterized form of synaptic plasticity, has long been postulated as a cellular correlate of learning and memory. Although LTP can persist for long periods of time 1 , the mechanisms underlying LTP maintenance, in the midst of ongoing protein turnover and synaptic activity, remain elusive. Sustained activation of the brain-specific protein kinase C (PKC) isoform protein kinase M-ζ (PKM-ζ) has been reported to be necessary for both LTP maintenance and long-term memory 2 . Inhibiting PKM-ζ activity using a synthetic zeta inhibitory peptide (ZIP) based on the PKC-ζ pseudosubstrate sequence reverses established LTP in vitro and in vivo 3 , 4 . More notably, infusion of ZIP eliminates memories for a growing list of experience-dependent behaviours, including active place avoidance 4 , conditioned taste aversion 5 , fear conditioning and spatial learning 6 . However, most of the evidence supporting a role for PKM-ζ in LTP and memory relies heavily on pharmacological inhibition of PKM-ζ by ZIP. To further investigate the involvement of PKM-ζ in the maintenance of LTP and memory, we generated transgenic mice lacking PKC-ζ and PKM-ζ. We find that both conventional and conditional PKC-ζ/PKM-ζ knockout mice show normal synaptic transmission and LTP at Schaffer collateral–CA1 synapses, and have no deficits in several hippocampal-dependent learning and memory tasks. Notably, ZIP still reverses LTP in PKC-ζ/PKM-ζ knockout mice, indicating that the effects of ZIP are independent of PKM-ζ.