G9a/GLP Complex Acts as a Bidirectional Switch to Regulate Metabotropic Glutamate Receptor-Dependent Plasticity in Hippocampal CA1 Pyramidal Neurons

Synapses tagged, memories kept: synaptic tagging and capture hypothesis in brain health and disease

The synaptic tagging and capture (STC) hypothesis lays the framework on the synapse-specific mechanism of protein synthesis-dependent long-term plasticity upon synaptic induction. Activated synapses will display a transient tag that will capture plasticity-related products (PRPs). These two events, tag setting and PRP synthesis, can be teased apart and have been studied extensively-from their electrophysiological and pharmacological properties to the molecular events involved. Consequently, the hypothesis also permits interactions of synaptic populations that encode different memories within the same neuronal population-hence, it gives rise to the associativity of plasticity. In this review, the recent advances and progress since the experimental debut of the STC hypothesis will be shared. This includes the role of neuromodulation in PRP synthesis and tag integrity, behavioural correlates of the hypothesis and modelling in silico. STC, as a more sensitive assay for synaptic health, can also assess neuronal aberrations. We will also expound how synaptic plasticity and associativity are altered in ageing-related decline and pathological conditions such as juvenile stress, cancer, sleep deprivation and Alzheimer's disease. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Inhibitory Metaplasticity in Juvenile Stressed Rats Restores Associative Memory in Adulthood by Regulating Epigenetic Complex G9a/GLP

BACKGROUND

Exposure to juvenile stress was found to have long-term effects on the plasticity and quality of associative memory in adulthood, but the underlying mechanisms are still poorly understood.

METHODS

Three- to four week-old male Wistar rats were subjected to a 3-day juvenile stress paradigm. Their electrophysiological correlates of memory using the adult hippocampal slice were inspected to detect alterations in long-term potentiation and synaptic tagging and capture model of associativity. These cellular alterations were tied in with the behavioral outcome by subjecting the rats to a step-down inhibitory avoidance paradigm to measure strength in their memory. Given the role of epigenetic response in altering plasticity as a repercussion of juvenile stress, we aimed to chart out the possible epigenetic marker and its regulation in the long-term memory mechanisms using quantitative reverse transcription polymerase chain reaction.

RESULTS

We demonstrate that even long after the elimination of actual stressors, an inhibitory metaplastic state is evident, which promotes synaptic competition over synaptic cooperation and decline in latency of associative memory in the behavioral paradigm despite the exposure to novelty. Mechanistically, juvenile stress led to a heightened expression of the epigenetic marker G9a/GLP complex, which is thus far ascribed to transcriptional silencing and goal-directed behavior.

CONCLUSIONS

The blockade of the G9a/GLP complex was found to alleviate deficits in long-term plasticity and associative memory during the adulthood of animals exposed to juvenile stress. Our data provide insights on the long-term effects of juvenile stress that involve epigenetic mechanisms, which directly impact long-term plasticity, synaptic tagging and capture, and associative memory.

Post-Treatment Sevoflurane Protects Against Hypoxic-Ischemic Brain Injury in Neonatal Rats by Downregulating Histone Methyltransferase G9a and Upregulating Nuclear Factor Erythroid 2-Related Factor 2 (NRF2)

BACKGROUND Perinatal hypoxia and subsequent reduction of cerebral blood flow leads to neonatal hypoxic-ischemic brain injury (HIBI), resulting in severe disability and even death. Preconditioning or post-conditioning with sevoflurane protects against cerebral injury. This study investigated the mechanism of sevoflurane in HIBI. MATERIAL AND METHODS The HIBI model of neonatal rats was established and the model rats were post-treated with sevoflurane. The oxygen-glucose deprivation (OGD) cell model was established, and the OGD cells were transfected with NRF2-siRNA plasmid and post-treated with sevoflurane. The Morris water maze test was used to detect the motor activity, spatial learning, and memory ability of HIBI rats. Histological stainings were performed to observe the area of cerebral infarction, record the number of neurons in the hippocampus, and assess neuron apoptosis. The levels of inflammatory factors were detected by ELISA. The protein levels of histone methyltransferase G9a and histone H3 lysine 9 (H3K9me2) were detected by western blot assay. The apoptosis was detected by flow cytometry. RESULTS Sevoflurane post-treatment significantly shortened the escape latency of HIBI neonatal rats, increased the density of neurons, reduced the area of cerebral infarction, and decreased the levels of inflammatory factors and neuronal apoptosis. Sevoflurane post-treatment decreased G9a and H3K9me2 levels, and G9a level was negatively correlated with NRF2 level. NRF2 silencing reversed the alleviation of sevoflurane post-treatment on OGD-induced cell injury. CONCLUSIONS Sevoflurane post-treatment promotes NRF2 expression by inhibiting G9a and H3K9me2, thus alleviating HIBI in neonatal rats.

The Histone Methyltransferase G9a Controls Axon Growth by Targeting the RhoA Signaling Pathway

The generation of axonal and dendritic domains is critical for brain circuitry assembly and physiology. Negative players, such as the RhoA-Rho coiled-coil-associated protein kinase (ROCK) signaling pathway, restrain axon development and polarization. Surprisingly, the genetic control of neuronal polarity has remained largely unexplored. Here, we report that, in primary cultured neurons, expression of the histone methyltransferase G9a and nuclear translocation of its major splicing isoform (G9a/E10+) peak at the time of axon formation. RNAi suppression of G9a/E10+ or pharmacological blockade of G9a constrains neuronal migration, axon initiation, and the establishment of neuronal polarity in situ and in vitro. Inhibition of G9a function upregulates RhoA-ROCK activity by increasing the expression of Lfc, a guanine nucleotide exchange factor (GEF) for RhoA. Together, these results identify G9a as a player in neuronal polarization.

Age-related changes in hippocampal-dependent synaptic plasticity and memory mediated by p75 neurotrophin receptor

The plasticity mechanisms in the nervous system that are important for learning and memory are greatly impacted during aging. Notably, hippocampal-dependent long-term plasticity and its associative plasticity, such as synaptic tagging and capture (STC), show considerable age-related decline. The p75 neurotrophin receptor (p75NTR ) is a negative regulator of structural and functional plasticity in the brain and thus represents a potential candidate to mediate age-related alterations. However, the mechanisms by which p75NTR affects synaptic plasticity of aged neuronal networks and ultimately contribute to deficits in cognitive function have not been well characterized. Here, we report that mutant mice lacking the p75NTR were resistant to age-associated changes in long-term plasticity, associative plasticity, and associative memory. Our study shows that p75NTR is responsible for age-dependent disruption of hippocampal homeostatic plasticity by modulating several signaling pathways, including BDNF, MAPK, Arc, and RhoA-ROCK2-LIMK1-cofilin. p75NTR may thus represent an important therapeutic target for limiting the age-related memory and cognitive function deficits.

Antisense Oligodeoxynucleotide Perfusion Blocks Gene Expression of Synaptic Plasticity-related Proteins without Inducing Compensation in Hippocampal Slices

The elucidation of the molecular mechanisms of long-term synaptic plasticity has been hindered by both the compensation that can occur after chronic loss of the core plasticity molecules and by ex vivo conditions that may not reproduce in vivo plasticity. Here we describe a novel method to rapidly suppress gene expression by antisense oligodeoxynucleotides (ODNs) applied to rodent brain slices in an "Oslo-type" interface chamber. The method has three advantageous features: 1) rapid blockade of new synthesis of the targeted proteins that avoids genetic compensation, 2) efficient oxygenation of the brain slice, which is critical for reproducing in vivo conditions of long-term synaptic plasticity, and 3) a recirculation system that uses only small volumes of bath solution (< 5 ml), reducing the amount of reagents required for long-term experiments lasting many hours. The method employs a custom-made recirculation system involving piezoelectric micropumps and was first used for the acute translational blockade of protein kinase Mζ (PKMζ) synthesis during long-term potentiation (LTP) by Tsokas et al., 2016. In that study, applying antisense-ODN rapidly prevents the synthesis of PKMζ and blocks late-LTP without inducing the compensation by other protein kinase C (PKC) isoforms that occurs in PKCζ/PKMζ knockout mice. In addition, we show that in a low-oxygenation submersion-type chamber, applications of the atypical PKC inhibitor, zeta inhibitory peptide (ZIP), can result in unstable baseline synaptic transmission, but in the high-oxygenation, "Oslo-type" interface electrophysiology chamber, the drug reverses late-LTP without affecting baseline synaptic transmission. This comparison reveals that the interface chamber, but not the submersion chamber, reproduces the effects of ZIP in vivo. Therefore, the protocol combines the ability to acutely block new synthesis of specific proteins for the study of long-term synaptic plasticity, while maintaining properties of synaptic transmission that reproduce in vivo conditions relevant for long-term memory.

Social Environment and Epigenetics

Our social environment, from the microscopic to the macro-social, affects us for the entirety of our lives. One integral line of research to examine how interpersonal and societal environments can get "under the skin" is through the lens of epigenetics. Epigenetic mechanisms are adaptations made to our genome in response to our environment which include tags placed on and removed from the DNA itself to how our DNA is packaged, affecting how our genes are read, transcribed, and interact. These tags are affected by social environments and can persist over time; this may aid us in responding to experiences and exposures, both the enriched and the disadvantageous. From memory formation to immune function, the experience-dependent plasticity of epigenetic modifications to micro- and macro-social environments may contribute to the process of learning from comfort, pain, and stress to better survive in whatever circumstances life has in store.