A hippocampal Cdk5 pathway regulates extinction of contextual fear

Enhanced fear memory after social defeat in mice is dependent on interleukin-1 receptor signaling in glutamatergic neurons

Chronic stress is associated with increased anxiety, cognitive deficits, and post-traumatic stress disorder. Repeated social defeat (RSD) in mice causes long-term stress-sensitization associated with increased microglia activation, monocyte accumulation, and enhanced interleukin (IL)-1 signaling in endothelia and neurons. With stress-sensitization, mice have amplified neuronal, immune, and behavioral responses to acute stress 24 days later. This is clinically relevant as it shares key aspects with post-traumatic stress disorder. The mechanisms underlying stress-sensitization are unclear, but enhanced fear memory may be critical. The purpose of this study was to determine the influence of microglia and IL-1R1 signaling in neurons in the development of sensitization and increased fear memory after RSD. Here, RSD accelerated fear acquisition, delayed fear extinction, and increased cued-based freezing at 0.5 day. The enhancement in contextual fear memory after RSD persisted 24 days later. Next, microglia were depleted with a CSF1R antagonist prior to RSD and several parameters were assessed. Microglia depletion blocked monocyte recruitment to the brain. Nonetheless, neuronal reactivity (pCREB) and IL-1β RNA expression in the hippocampus and enhanced fear memory after RSD were microglial-independent. Because IL-1β RNA was prominent in the hippocampus after RSD even with microglia depletion, IL-1R1 mediated signaling in glutamatergic neurons was assessed using neuronal Vglut2+/IL-1R1-/- mice. RSD-induced neuronal reactivity (pCREB) in the hippocampus and enhancement in fear memory were dependent on neuronal IL-1R1 signaling. Furthermore, single-nuclei RNA sequencing (snRNAseq) showed that RSD influenced transcription in specific hippocampal neurons (DG neurons, CA2/3, CA1 neurons) associated with glutamate signaling, inflammation and synaptic plasticity, which were neuronal IL-1R1-dependent. Furthermore, snRNAseq data provided evidence that RSD increased CREB, BDNF, and calcium signaling in DG neurons in an IL-1R1-dependent manner. Collectively, increased IL-1R1-mediated signaling (monocytes/microglia independent) in glutamatergic neurons after RSD enhanced neuronal reactivity and fear memory.

Enhancing HIF-1α-P2X2 signaling in dorsal raphe serotonergic neurons promotes psychological resilience

Major depressive disorder (MDD) is a devastating condition. Although progress has been made in the past seven decades, patients with MDD continue to receive an inadequate treatment, primarily due to the late onset of first-line antidepressant drugs and to their acute withdrawal symptoms. Resilience is the ability to rebound from adversity in a healthy manner and many people have psychological resilience. Revealing the mechanisms and identifying methods promoting resilience will hopefully lead to more effective prevention strategies and treatments for depression. In this study, we found that intermittent hypobaric hypoxia training (IHHT), a method for training pilots and mountaineers, enhanced psychological resilience in adult mice. IHHT produced a sustained antidepressant-like effect in mouse models of depression by inducing long-term (up to 3 months after this treatment) overexpression of hypoxia-inducible factor (HIF)-1α in the dorsal raphe nucleus (DRN) of adult mice. Moreover, DRN-infusion of cobalt chloride, which mimics hypoxia increasing HIF-1α expression, triggered a rapid and long-lasting antidepressant-like effect. Down-regulation of HIF-1α in the DRN serotonergic (DRN5-HT) neurons attenuated the effects of IHHT. HIF-1α translationally regulated the expression of P2X2, and conditionally knocking out P2rx2 (encodes P2X2 receptors) in DRN5-HT neurons, in turn, attenuated the sustained antidepressant-like effect of IHHT, but not its acute effect. In line with these results, a single sub-anesthetic dose of ketamine enhanced HIF-1α-P2X2 signaling, which is essential for its rapid and long-lasting antidepressant-like effect. Notably, we found that P2X2 protein levels were significantly lower in the DRN of patients with MDD than that of control subjects. Together, these findings elucidate the molecular mechanism underlying IHHT promoting psychological resilience and highlight enhancing HIF-1α-P2X2 signaling in DRN5-HT neurons as a potential avenue for screening novel therapeutic treatments for MDD.

The Cdk5 inhibitor β-butyrolactone impairs reconsolidation of heroin-associated memory in the rat basolateral amygdala

The persistence of maladaptive heroin-associated memory, which is triggered by drug-related stimuli that remind the individual of the drug's pleasurable and rewarding effects, can impede abstinence efforts. Cyclin-dependent kinase 5 (Cdk5), a neuronal serine/threonine protein kinase that plays a role in multiple neuronal functions, has been demonstrated to be involved in drug addiction and learning and memory. Here, we aimed to investigate the role of cdk5 activity in the basolateral amygdala (BLA) in relapse to heroin seeking, using a self-administration rat model. Male rats underwent 10 days of heroin self-administration training, during which an active nose poke resulted in an intravenous infusion of heroin that was accompanied by a cue. The rats then underwent nose poke extinction for 10 days, followed by subsequent tests of heroin-seeking behaviour. We found that intra-BLA infusion of β-butyrolactone (100 ng/side), a Cdk5 inhibitor, administered 5 min after reactivation, led to a subsequent decrease in heroin-seeking behaviour. Further experiments demonstrated that the effects of β-butyrolactone are dependent on reactivated memories, temporal-specific and long-lasting on relapse of heroin-associated memory. Results provide suggestive evidence that the activity of Cdk5 in BLA is critical for heroin-associated memory and that the specific inhibitor, β-butyrolactone, may hold potential as a substance for the treatment of heroin abuse.

Peroxisome proliferator-activated receptor-α activation facilitates contextual fear extinction and modulates intrinsic excitability of dentate gyrus neurons

The dentate gyrus (DG) of the hippocampus encodes contextual information associated with fear, and cell activity in the DG is required for acquisition and extinction of contextual fear. However, the underlying molecular mechanisms are not fully understood. Here we show that mice deficient for peroxisome proliferator-activated receptor-α (PPARα) exhibited a slower rate of contextual fear extinction. Furthermore, selective deletion of PPARα in the DG attenuated, while activation of PPARα in the DG by local infusion of aspirin facilitated extinction of contextual fear. The intrinsic excitability of DG granule neurons was reduced by PPARα deficiency but increased by activation of PPARα with aspirin. Using RNA-Seq transcriptome we found that the transcription level of neuropeptide S receptor 1 (Npsr1) was tightly correlated with PPARα activation. Our results provide evidence that PPARα plays an important role in regulating DG neuronal excitability and contextual fear extinction.

A Cdk5-derived peptide inhibits Cdk5/p25 activity and improves neurodegenerative phenotypes

Aberrant activity of cyclin-dependent kinase (Cdk5) has been implicated in various neurodegenerative diseases. This deleterious effect is mediated by pathological cleavage of the Cdk5 activator p35 into the truncated product p25, leading to prolonged Cdk5 activation and altered substrate specificity. Elevated p25 levels have been reported in humans and rodents with neurodegeneration, and the benefit of genetically blocking p25 production has been demonstrated previously in rodent and human neurodegenerative models. Here, we report a 12-amino-acid-long peptide fragment derived from Cdk5 (Cdk5i) that is considerably smaller than existing peptide inhibitors of Cdk5 (P5 and CIP) but shows high binding affinity toward the Cdk5/p25 complex, disrupts the interaction of Cdk5 with p25, and lowers Cdk5/p25 kinase activity. When tagged with a fluorophore (FITC) and the cell-penetrating transactivator of transcription (TAT) sequence, the Cdk5i-FT peptide exhibits cell- and brain-penetrant properties and confers protection against neurodegenerative phenotypes associated with Cdk5 hyperactivity in cell and mouse models of neurodegeneration, highlighting Cdk5i's therapeutic potential.

Stimulation of Neurite Outgrowth in Cerebrocortical Neurons by Sodium Channel Activator Brevetoxin-2 Requires Both N-Methyl-D-aspartate Receptor 2B (GluN2B) and p21 Protein (Cdc42/Rac)-Activated Kinase 1 (PAK1)

N-methyl-D-aspartate (NMDA) receptors play a critical role in activity-dependent dendritic arborization, spinogenesis, and synapse formation by stimulating calcium-dependent signaling pathways. Previously, we have shown that brevetoxin 2 (PbTx-2), a voltage-gated sodium channel (VGSC) activator, produces a concentration-dependent increase in intracellular sodium [Na+]I and increases NMDA receptor (NMDAR) open probabilities and NMDA-induced calcium (Ca2+) influxes. The objective of this study is to elucidate the downstream signaling mechanisms by which the sodium channel activator PbTx-2 influences neuronal morphology in murine cerebrocortical neurons. PbTx-2 and NMDA triggered distinct Ca2+-influx pathways, both of which involved the NMDA receptor 2B (GluN2B). PbTx-2-induced neurite outgrowth in day in vitro 1 (DIV-1) neurons required the small Rho GTPase Rac1 and was inhibited by both a PAK1 inhibitor and a PAK1 siRNA. PbTx-2 exposure increased the phosphorylation of PAK1 at Thr-212. At DIV-5, PbTx-2 induced increases in dendritic protrusion density, p-cofilin levels, and F-actin throughout the dendritic arbor and soma. Moreover, PbTx-2 increased miniature excitatory post-synaptic currents (mEPSCs). These data suggest that the stimulation of neurite outgrowth, spinogenesis, and synapse formation produced by PbTx-2 are mediated by GluN2B and PAK1 signaling.

Treatment with FRAX486 rescues neurobehavioral and metabolic alterations in a female mouse model of CDKL5 deficiency disorder

Introduction

CDKL5 deficiency disorder (CDD) is a rare neurodevelopmental condition, primarily affecting girls for which no cure currently exists. Neuronal morphogenesis and plasticity impairments as well as metabolic dysfunctions occur in CDD patients. The present study explored the potential therapeutic value for CDD of FRAX486, a brain‐penetrant molecule that was reported to selectively inhibit group I p21‐activated kinases (PAKs), serine/threonine kinases critically involved in the regulation of neuronal morphology and glucose homeostasis.

Methods

The effects of treatment with FRAX486 on CDD‐related alterations were assessed in vitro (100 nM for 48 h) on primary hippocampal cultures from Cdkl5‐knockout male mice (Cdkl5‐KO) and in vivo (20 mg/Kg, s.c. for 5 days) on Cdkl5‐KO heterozygous females (Cdkl5‐Het).

Results

The in vitro treatment with FRAX486 completely rescued the abnormal neuronal maturation and the number of PSD95‐positive puncta in Cdkl5‐KO mouse neurons. In vivo, FRAX486 normalized the general health status, the hyperactive profile and the fear learning defects of fully symptomatic Cdkl5‐Het mice. Systemically, FRAX486 treatment normalized the levels of reactive oxidizing species in the whole blood and the fasting‐induced hypoglycemia displayed by Cdkl5‐Het mice. In the hippocampus of Cdkl5‐Het mice, treatment with FRAX486 rescued spine maturation and PSD95 expression and restored the abnormal PAKs phosphorylation at sites which are critical for their activation (P‐PAK‐Ser144/141/139) or for the control cytoskeleton remodeling (P‐PAK1‐Thr212).

Conclusions

Present results provide evidence that PAKs may represent innovative therapeutic targets for CDD.

Inhibition of Rac1 in ventral hippocampal excitatory neurons improves social recognition memory and synaptic plasticity

Rac1 is critically involved in the regulation of the actin cytoskeleton, neuronal structure, synaptic plasticity, and memory. Rac1 overactivation is reported in human patients and animal models of Alzheimer’s disease (AD) and contributes to their spatial memory deficits, but whether Rac1 dysregulation is also important in other forms of memory deficits is unknown. In addition, the cell types and synaptic mechanisms involved remain unclear. In this study, we used local injections of AAV virus containing a dominant-negative (DN) Rac1 under the control of CaMKIIα promoter and found that the reduction of Rac1 hyperactivity in ventral hippocampal excitatory neurons improves social recognition memory in APP/PS1 mice. Expression of DN Rac1 also improves long-term potentiation, a key synaptic mechanism for memory formation. Our results suggest that overactivation of Rac1 in hippocampal excitatory neurons contributes to social memory deficits in APP/PS1 mice and that manipulating Rac1 activity may provide a potential therapeutic strategy to treat social deficits in AD.

Reduction of excitatory synaptic transmission efficacy in the infralimbic prefrontal cortex potentially contributes to impairment of contextual fear memory extinction in aged mice

Human beings are living longer than ever before and cognitive decline experienced by aged adults, such as compromise in cognitive flexibility, has been attracting more and more attention. One such example is the aging-related impairment of memory extinction. However, its underlying neural basis, especially the functional basis at the synapse level, is largely unknown. This study verifies that Pavlovian contextual fear memory extinction is impaired in aged mice. A large body of previous studies have shown that the infralimbic prefrontal cortex (ilPFC) plays a pivotal role in memory extinction. Correspondingly, this study reveals an aging-related reduction in the efficacy of excitatory synaptic transmission onto the ilPFC pyramidal neurons via electrophysiology recordings. This study further suggests that this reduced excitation potentially contributes to the aging-related impairment of contextual fear memory extinction: chemogenetically suppressing the activity of the ilPFC pyramidal neurons in young mice impairs contextual fear memory extinction, whereas chemogenetically compensating the reduced excitation of the ilPFC pyramidal neurons in aged mice restores contextual fear memory extinction. This study identifies a functional synaptic plasticity in the ilPFC pyramidal neurons that potentially contributes to the aging-related impairment of contextual fear memory extinction, which would potentially help to develop a therapy to treat related cognitive decline in aged human adults.

Involvement of Cdk5 activating subunit p35 in synaptic plasticity in excitatory and inhibitory neurons

Cyclin-dependent kinase 5 (Cdk5) /p35 is involved in many developmental processes of the central nervous system. Cdk5/p35 is also implicated in synaptic plasticity, learning and memory. Several lines of conditional Cdk5 knockout mice (KO) have been generated and have shown different outcomes for learning and memory. Here, we present our analysis of p35 conditional KO mice (p35cKO) in hippocampal pyramidal neurons or forebrain GABAergic neurons using electrophysiological and behavioral methods. In the fear conditioning task, CamKII-p35cKO mice showed impaired memory retention. Furthermore, NMDAR-dependent long-term depression (LTD) induction by low-frequency stimuli in hippocampal slices from CamkII-p35cKO mice was impaired compared to that in control mice. In contrast, Dlx-p35cKO mice showed no abnormalities in behavioral tasks and electrophysiological analysis in their hippocampal slices. These results indicated that Cdk5/p35 in excitatory neurons is important for the hippocampal synaptic plasticity and associative memory retention.

PV network plasticity mediated by neuregulin1-ErbB4 signalling controls fear extinction

Neuroplasticity in the medial prefrontal cortex (mPFC) is essential for fear extinction, the process of which forms the basis of the general therapeutic process used to treat human fear disorders. However, the underlying molecules and local circuit elements controlling neuronal activity and concomitant induction of plasticity remain unclear. Here we show that sustained plasticity of the parvalbumin (PV) neuronal network in the infralimbic (IL) mPFC is required for fear extinction in adult male mice and identify the involvement of neuregulin 1-ErbB4 signalling in PV network plasticity-mediated fear extinction. Moreover, regulation of fear extinction by basal medial amygdala (BMA)-projecting IL neurons is dependent on PV network configuration. Together, these results uncover the local molecular circuit mechanisms underlying mPFC-mediated top-down control of fear extinction, suggesting alterative therapeutic approaches to treat fear disorders.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Chronic opipramol treatment extinguishes cocaine craving through Rac1 in responders: A rat model study

Ras-related C3 botulinum toxin substrate 1 (Rac1), of the Rho small GTPase family, is a key regulator of actin cytoskeleton rearrangement and plays an important role in dendritic morphogenesis. Cocaine produces neuronal alterations, including structural changes in dendritic number and morphology. Emerging data indicate sigma-1 receptors (σ-1Rs) as a promising candidate for the prevention of cocaine craving. Opipramol is a σ-1R agonist approved in some European countries for depression and anxiety. Here we report that opipramol, mediated by Rac1, attenuates cocaine-seeking behavior in a rat model of self-administration. The opipramol effect was shown in two phases. It decreased cocaine-seeking behavior throughout the withdrawal phase and, interestingly, showed a significant reduction of cocaine-primed reinstatement in 75% of the opipramol-treated group (termed 'responders'). All opipramol-treated rats showed a decrease in σ-1R mRNA expression levels in the nucleus accumbens (NAc) versus controls. Responders also exhibited significantly decreased NAc Rac1 mRNA expression levels, compared with non-responder rats. Hence, Rac1 differentiated responders from non-responders. Rac1 correlated positively with σ-1R mRNA levels in opipramol responders. In another experiment, Rac1 inhibitor injected directly into the NAc core decreased active lever presses on the first day of extinction, indicating the critical role of Rac1 in the opipramol effect on drug seeking. We postulate that chronic activation of σ-1R, through a dynamic interaction with Rac1, may suggest a new approach to treat substance use disorder (SUD). Rac1 inhibition is a prerequisite for decreasing drug seeking and rehabilitation, and this can be achieved by opipramol, a medication that can be given during detoxification.

Vcp Overexpression and Leucine Supplementation Increase Protein Synthesis and Improve Fear Memory and Social Interaction of Nf1 Mutant Mice

Neurofibromatosis type 1 (NF1) is a dominant genetic disorder manifesting, in part, as cognitive defects. Previous study indicated that neurofibromin (NF1 protein) interacts with valosin-containing protein (VCP)/P97 to control dendritic spine formation, but the mechanism is unknown. Here, using Nf1^+/- mice and transgenic mice overexpressing wild-type Vcp/p97, we demonstrate that neurofibromin acts with VCP to control endoplasmic reticulum (ER) formation and consequent protein synthesis and regulates dendritic spine formation, thereby modulating contextual fear memory and social interaction. To validate the role of protein synthesis, we perform leucine supplementation in vitro and in vivo. Our results suggest that leucine can effectively enter the brain and increase protein synthesis and dendritic spine density of Nf1^+/- neurons. Contextual memory and social behavior of Nf1^+/- mice are also restored by leucine supplementation. Our study suggests that the "ER-protein synthesis" pathway downstream of neurofibromin and VCP is a critical regulator of dendritic spinogenesis and brain function.

Impaired contextual fear conditioning in RasGRF2 mutant mice is likely Ras-ERK-dependent

Ras/Raf/MEK/ERK (Ras-ERK) signaling has been shown to play an important role in fear acquisition. However, little information is known regarding the mechanisms that contribute to the regulation of this pathway in terms of the learning of conditioned fears. Ras Guanine Nucleotide Releasing Factor 2 (RasGRF2) is one of two guanine nucleotide exchange factors (GEF) that regulates the Ras-ERK signaling pathway in a Ca²⁺-dependent manner via control of the cycling of Ras isoforms between an inactive and active state. Here we sought to determine the role of RasGRF2 on contextual fear conditioning in RasGRF2 knockout (KO) and their wild type (WT) counterparts. Male KO and WT mice underwent a single session of contextual fear conditioning (12 min, 4 unsignaled shocks), followed by either daily 12-min retention trials or the molecular analysis of Ras activation and pERK1/2 activity. KO mice showed an impaired acquisition of contextual fear, as demonstrated by reduced freezing during fear conditioning and 24-hr retention tests relative to WT mice. Ras analysis following fear conditioning demonstrated a reduction in Ras activation in the hippocampus as well as a reduction in pERK1/2 in the CA1 region of the hippocampus in KO mice, suggesting that the decrease in fear conditioning in KO mice is at least in part due to the impairment of Ras-ERK signaling in the hippocampus during learning. These data indicate a role for RasGRF2 in contextual fear conditioning in mice that may be Ras-ERK-dependent.

Understanding the dynamic and destiny of memories

Memory formation enables the retention of life experiences overtime. Based on previously acquired information, organisms can anticipate future events and adjust their behaviors to maximize survival. However, in an ever-changing environment, a memory needs to be malleable to maintain its relevance. In fact, substantial evidence suggests that a consolidated memory can become labile and susceptible to modifications after being reactivated, a process termed reconsolidation. When an extinction process takes place, a memory can also be temporarily inhibited by a second memory that carries information with opposite meaning. In addition, a memory can fade and lose its significance in a process known as forgetting. Thus, following retrieval, new life experiences can be integrated with the original memory trace to maintain its predictive value. In this review, we explore the determining factors that regulate the fate of a memory after its reactivation. We focus on three post-retrieval memory destinies (reconsolidation, extinction, and forgetting) and discuss recent rodent studies investigating the biological functions and neural mechanisms underlying each of these processes.

Optogenetic Stimulation of Prelimbic Pyramidal Neurons Maintains Fear Memories and Modulates Amygdala Pyramidal Neuron Transcriptome

Fear extinction requires coordinated neural activity within the amygdala and medial prefrontal cortex (mPFC). Any behavior has a transcriptomic signature that is modified by environmental experiences, and specific genes are involved in functional plasticity and synaptic wiring during fear extinction. Here, we investigated the effects of optogenetic manipulations of prelimbic (PrL) pyramidal neurons and amygdala gene expression to analyze the specific transcriptional pathways associated to adaptive and maladaptive fear extinction. To this aim, transgenic mice were (or not) fear-conditioned and during the extinction phase they received optogenetic (or sham) stimulations over photo-activable PrL pyramidal neurons. At the end of behavioral testing, electrophysiological (neural cellular excitability and Excitatory Post-Synaptic Currents) and morphological (spinogenesis) correlates were evaluated in the PrL pyramidal neurons. Furthermore, transcriptomic cell-specific RNA-analyses (differential gene expression profiling and functional enrichment analyses) were performed in amygdala pyramidal neurons. Our results show that the optogenetic activation of PrL pyramidal neurons in fear-conditioned mice induces fear extinction deficits, reflected in an increase of cellular excitability, excitatory neurotransmission, and spinogenesis of PrL pyramidal neurons, and associated to strong modifications of the transcriptome of amygdala pyramidal neurons. Understanding the electrophysiological, morphological, and transcriptomic architecture of fear extinction may facilitate the comprehension of fear-related disorders.

Functional Divergence of Mammalian TFAP2a and TFAP2b Transcription Factors for Bidirectional Sleep Control

Sleep is a conserved behavioral state. Invertebrates typically show quiet sleep, whereas in mammals, sleep consists of periods of nonrapid-eye-movement sleep (NREMS) and REM sleep (REMS). We previously found that the transcription factor AP-2 promotes sleep in Caenorhabditiselegans and Drosophila In mammals, several paralogous AP-2 transcription factors exist. Sleep-controlling genes are often conserved. However, little is known about how sleep genes evolved from controlling simpler types of sleep to govern complex mammalian sleep. Here, we studied the roles of Tfap2a and Tfap2b in sleep control in mice. Consistent with our results from C. elegans and Drosophila, the AP-2 transcription factors Tfap2a and Tfap2b also control sleep in mice. Surprisingly, however, the two AP-2 paralogs play contrary roles in sleep control. Tfap2a reduction of function causes stronger delta and theta power in both baseline and homeostasis analysis, thus indicating increased sleep quality, but did not affect sleep quantity. By contrast, Tfap2b reduction of function decreased NREM sleep time specifically during the dark phase, reduced NREMS and REMS power, and caused a weaker response to sleep deprivation. Consistent with the observed signatures of decreased sleep quality, stress resistance and memory were impaired in Tfap2b mutant animals. Also, the circadian period was slightly shortened. Taken together, AP-2 transcription factors control sleep behavior also in mice, but the role of the AP-2 genes functionally diversified to allow for a bidirectional control of sleep quality. Divergence of AP-2 transcription factors might perhaps have supported the evolution of more complex types of sleep.

CDK5 Targeting as a Therapy for Recovering Neurovascular Unit Integrity in Alzheimer's Disease

The neurovascular unit (NVU) is responsible for synchronizing the energetic demand, vasodynamic changes, and neurochemical and electrical function of the brain through a closed and interdependent interaction of cell components conforming to brain tissue. In this review, we will focus on cyclin-dependent kinase 5 (CDK5) as a molecular pivot, which plays a crucial role in the healthy function of neurons, astrocytes, and the endothelium and is implicated in the cross-talk of cellular adhesion signaling, ion transmission, and cytoskeletal remodeling, thus allowing the individual and interconnected homeostasis of cerebral parenchyma. Then, we discuss how CDK5 overactivation affects the integrity of the NVU in Alzheimer's disease (AD) and cognitive impairment; we emphasize how CDK5 is involved in the excitotoxicity spreading of glutamate and Ca2+ imbalance under acute and chronic injury. Additionally, we present pharmacological and gene therapy strategies for producing partial depletion of CDK5 activity on neurons, astrocytes, or endothelium to recover neuroplasticity and neurotransmission, suggesting that the NVU should be the targeted tissue unit in protective strategies. Finally, we conclude that CDK5 could be effective due to its intervention on astrocytes by its end feet on the endothelium and neurons, acting as an intermediary cell between systemic and central communication in the brain. This review provides integrated guidance regarding the pathogenesis of and potential repair strategies for AD.

Rho GTPases in the Amygdala-A Switch for Fears?

Fear is a fundamental evolutionary process for survival. However, excess or irrational fear hampers normal activity and leads to phobia. The amygdala is the primary brain region associated with fear learning and conditioning. There, Rho GTPases are molecular switches that act as signaling molecules for further downstream processes that modulate, among others, dendritic spine morphogenesis and thereby play a role in fear conditioning. The three main Rho GTPases—RhoA, Rac1, and Cdc42, together with their modulators, are known to be involved in many psychiatric disorders that affect the amygdala′s fear conditioning mechanism. Rich2, a RhoGAP mainly for Rac1 and Cdc42, has been studied extensively in such regard. Here, we will discuss these effectors, along with Rich2, as a molecular switch for fears, especially in the amygdala. Understanding the role of Rho GTPases in fear controlling could be beneficial for the development of therapeutic strategies targeting conditions with abnormal fear/anxiety-like behaviors.

Contributions of DNA Damage to Alzheimer's Disease

Alzheimer’s disease (AD) is the most common type of neurodegenerative disease. Its typical pathology consists of extracellular amyloid-β (Aβ) plaques and intracellular tau neurofibrillary tangles. Mutations in the APP, PSEN1, and PSEN2 genes increase Aβ production and aggregation, and thus cause early onset or familial AD. Even with this strong genetic evidence, recent studies support AD to result from complex etiological alterations. Among them, aging is the strongest risk factor for the vast majority of AD cases: Sporadic late onset AD (LOAD). Accumulation of DNA damage is a well-established aging factor. In this regard, a large amount of evidence reveals DNA damage as a critical pathological cause of AD. Clinically, DNA damage is accumulated in brains of AD patients. Genetically, defects in DNA damage repair resulted from mutations in the BRAC1 and other DNA damage repair genes occur in AD brain and facilitate the pathogenesis. Abnormalities in DNA damage repair can be used as diagnostic biomarkers for AD. In this review, we discuss the association, the causative potential, and the biomarker values of DNA damage in AD pathogenesis.

The Molecular Motor KIF21B Mediates Synaptic Plasticity and Fear Extinction by Terminating Rac1 Activation

Fear extinction is a component of cognitive flexibility that is relevant for important psychiatric diseases, but its molecular mechanism is still largely elusive. We established mice lacking the kinesin-4 motor KIF21B as a model for fear extinction defects. Postsynaptic NMDAR-dependent long-term depression (LTD) is specifically impaired in knockouts. NMDAR-mediated LTD-causing stimuli induce dynamic association of KIF21B with the Rac1GEF subunit engulfment and cell motility protein 1 (ELMO1), leading to ELMO1 translocation out of dendritic spines and its sequestration in endosomes. This process may essentially terminate transient activation of Rac1, shrink spines, facilitate AMPAR endocytosis, and reduce postsynaptic strength, thereby forming a mechanistic link to LTD expression. Antagonizing ELMO1/Dock Rac1GEF activity by the administration of 4-[3'-(2″-chlorophenyl)-2'-propen-1'-ylidene]-1-phenyl-3,5-pyrazolidinedione (CPYPP) significantly reverses the knockout phenotype. Therefore, we propose that KIF21B-mediated Rac1 inactivation is a key molecular event in NMDAR-dependent LTD expression underlying cognitive flexibility in fear extinction.

Different roles of Rac1 in the acquisition and extinction of methamphetamine-associated contextual memory in the nucleus accumbens

Rationale: Repeated methamphetamine (METH) exposure induces long-term cognitive deficits and pathological drug-associated memory that can be disrupted by manipulating memory reconsolidation and extinction. The nucleus accumbens (NAc) is the key region of the brain reward system and predominantly consists of two subtypes of medium spiny neurons (MSNs) based on the expression of D1 or D2 dopamine receptors (D1-MSNs or D2-MSNs). Spine structural plasticity in the NAc is critical for the acquisition, reconsolidation and extinction of drug-associated memory. However, the molecular mechanisms underlying METH-associated memory and spine remodelling in each type of MSNs in the NAc remain unknown. Here, we explored whether Rac1 in the NAc mediates METH-associated contextual memory and spine remodelling.

Methods: Pharmacological and genetic manipulations of Rac1 were used to investigate its role during the acquisition, reconsolidation and extinction of METH-associated contextual memory. Recombinant adeno-associated viruses expressing mCherry under the control of the dopamine D1 receptor gene promoter (Drd1-mCherry) or dopamine D2 receptor gene promoter (Drd2-mCherry) were used to specifically label D1-MSNs or D2-MSNs.

Results: Using viral-mediated gene transfer, we demonstrated that decreased Rac1 activity was required for the acquisition of METH-associated contextual memory and the METH-induced increase in thin spine density, whereas increased Rac1 signalling was important for the extinction of METH-associated contextual memory and the related elimination of thin spines. Moreover, the increase of dendritic spines was both found in D1-MSNs and D2-MSNs during the acquisition process, but extinction training selectively decreased the spine density in D1-MSNs. Interestingly, Rac1 was responsible for METH-induced spine plasticity in D1-MSNs but not in D2-MSNs. Additionally, we found that microinjection of a Rac1 inhibitor or activator into the NAc was not sufficient to disrupt reconsolidation, and the pharmacological activation of Rac1 in the NAc facilitated the extinction of METH-associated contextual memory. Regarding cognitive memory, decreased Rac1 activity improved the METH-induced impairment in object recognition memory.

Conclusion: Our findings indicate that Rac1 plays opposing roles in the acquisition and extinction of METH-associated contextual memory and reveal the cell-specific role of Rac1 in METH-associated spine remodelling, suggesting that Rac1 is a potential therapeutic target for reducing relapse in METH addiction and remediating METH-induced recognition memory impairment.

Ubiquitination of MBNL1 Is Required for Its Cytoplasmic Localization and Function in Promoting Neurite Outgrowth

The Muscleblind-like protein family (MBNL) plays an important role in regulating the transition between differentiation and pluripotency and in the pathogenesis of myotonic dystrophy type 1 (DM1), a CTG expansion disorder. How different MBNL isoforms contribute to the differentiation and are affected in DM1 has not been investigated. Here, we show that the MBNL1 cytoplasmic, but not nuclear, isoform promotes neurite morphogenesis and reverses the morphological defects caused by expanded CUG RNA. Cytoplasmic MBNL1 is polyubiquitinated by lysine 63 (K63). Reduced cytoplasmic MBNL1 in the DM1 mouse brain is consistent with the reduced extent of K63 ubiquitination. Expanded CUG RNA induced the deubiqutination of cytoplasmic MBNL1, which resulted in nuclear translocation and morphological impairment that could be ameliorated by inhibiting K63-linked polyubiquitin chain degradation. Our results suggest that K63-linked ubiquitination of MBNL1 is required for its cytoplasmic localization and that deubiquitination of cytoplasmic MBNL1 is pathogenic in the DM1 brain.

Conditional Deletion of CC2D1A Reduces Hippocampal Synaptic Plasticity and Impairs Cognitive Function through Rac1 Hyperactivation

Coiled-coil and C2 domain containing 1A (CC2D1A) is an evolutionarily conserved protein, originally identified as a nuclear factor-κB activator through a large-scale screen of human genes. Mutations in the human Cc2d1a gene result in autosomal recessive nonsyndromic intellectual disability. It remains unclear, however, how Cc2d1a mutation leads to alterations in brain function. Here, we have taken advantage of Cre/loxP recombinase-based strategy to conditionally delete Cc2d1a exclusively from excitatory neurons of male mouse forebrain to examine its role in hippocampal synaptic plasticity and cognitive function. We confirmed the expression of CC2D1A protein and mRNA in the mouse hippocampus. Double immunofluorescence staining showed that CC2D1A is expressed in both excitatory and inhibitory neurons of the adult hippocampus. Conditional deletion of Cc2d1a (cKO) from excitatory neurons leads to impaired performance in object location memory test and altered anxiety-like behavior. Consistently, cKO mice displayed a deficit in the maintenance of LTP in the CA1 region of hippocampal slices. Cc2d1a deletion also resulted in decreased complexity of apical and basal dendritic arbors of CA1 pyramidal neurons. An enhanced basal Rac1 activity was observed following Cc2d1a deletion, and this enhancement was mediated by reduced SUMO-specific protease 1 (SENP1) and SENP3 expression, thus increasing the amount of Rac1 SUMOylation. Furthermore, partial blockade of Rac1 activity rescued impairments in LTP and object location memory performance in cKO mice. Together, our results implicate Rac1 hyperactivity in synaptic plasticity and cognitive deficits observed in Cc2d1a cKO mice and reveal a novel role for CC2D1A in regulating hippocampal synaptic function.SIGNIFICANCE STATEMENT CC2D1A is abundantly expressed in the brain, but there is little known about its physiological function. Taking advantage of Cc2d1a cKO mice, the present study highlights the importance of CC2D1A in the maintenance of LTP at Schaffer collateral-CA1 synapses and the formation of hippocampus-dependent long-term object location memory. Our findings establish a critical link between elevated Rac1 activity, structural and synaptic plasticity alterations, and cognitive impairment caused by Cc2d1a deletion. Moreover, partial blockade of Rac1 activity rescues synaptic plasticity and memory deficits in Cc2d1a cKO mice. Such insights may have implications for the utility of Rac1 inhibitors in the treatment of intellectual disability caused by Cc2d1a mutations in human patients.

Kinase and Phosphatase Engagement Is Dissociated Between Memory Formation and Extinction

Associative long-term memories (LTMs) support long-lasting behavioral changes resulting from sensory experiences. Retrieval of a stable LTM by means of a large number of conditioned stimulus (CS) alone presentations produces inhibition of the original memory through extinction. Currently, there are two opposing hypotheses to account for the neural mechanisms supporting extinction. The unlearning hypothesis posits that extinction affects the original memory trace by reverting the synaptic changes supporting LTM. On the contrary, the new learning hypothesis proposes that extinction is simply the formation of a new associative memory that inhibits the expression of the original one. We propose that detailed analysis of extinction-associated molecular mechanisms could help distinguish between these hypotheses. Here we will review experimental evidence regarding the role of protein kinases and phosphatases (K&P) on LTM formation and extinction. Even though K&P regulate both memory processes, their participation appears to be dissociated. LTM formation recruits kinases, but is constrained by phosphatases. Memory extinction presents a more diverse molecular landscape, requiring phosphatases and some kinases, but also being constrained by kinase activity. Based on the available evidence, we propose a new theoretical model for memory extinction: a neuronal segregation of K&P supports a combination of time-dependent reversible inhibition of the original memory [CS-unconditioned stimulus (US)], with establishment of a new associative memory trace (CS-noUS).

Haploinsufficiency of autism causative gene Tbr1 impairs olfactory discrimination and neuronal activation of the olfactory system in mice

Background

Autism spectrum disorders (ASD) exhibit two clusters of core symptoms, i.e., social and communication impairment, and repetitive behaviors and sensory abnormalities. Our previous study demonstrated that TBR1, a causative gene of ASD, controls axonal projection and neuronal activation of amygdala and regulates social interaction and vocal communication in a mouse model. Behavioral defects caused by Tbr1 haploinsufficiency can be ameliorated by increasing neural activity via D-cycloserine treatment, an N-methyl-D-aspartate receptor (NMDAR) coagonist. In this report, we investigate the role of TBR1 in regulating olfaction and test whether D-cycloserine can also improve olfactory defects in Tbr1 mutant mice.

Methods

We used Tbr1^+/− mice as a model to investigate the function of TBR1 in olfactory sensation and discrimination of non-social odors. We employed a behavioral assay to characterize the olfactory defects of Tbr1^+/− mice. Magnetic resonance imaging (MRI) and histological analysis were applied to characterize anatomical features. Immunostaining was performed to further analyze differences in expression of TBR1 subfamily members (namely TBR1, TBR2, and TBX21), interneuron populations, and dendritic abnormalities in olfactory bulbs. Finally, C-FOS staining was used to monitor neuronal activation of the olfactory system upon odor stimulation.

Results

Tbr1^+/− mice exhibited smaller olfactory bulbs and anterior commissures, reduced interneuron populations, and an abnormal dendritic morphology of mitral cells in the olfactory bulbs. Tbr1 haploinsufficiency specifically impaired olfactory discrimination but not olfactory sensation. Neuronal activation upon odorant stimulation was reduced in the glomerular layer of Tbr1^+/− olfactory bulbs. Furthermore, although the sizes of piriform and perirhinal cortices were not affected by Tbr1 deficiency, neuronal activation was reduced in these two cortical regions in response to odorant stimulation. These results suggest an impairment of neuronal activation in olfactory bulbs and defective connectivity from olfactory bulbs to the upper olfactory system in Tbr1^+/− mice. Systemic administration of D-cycloserine, an NMDAR co-agonist, ameliorated olfactory discrimination in Tbr1^+/− mice, suggesting that increased neuronal activity has a beneficial effect on Tbr1 deficiency.

Conclusions

Tbr1 regulates neural circuits and activity in the olfactory system to control olfaction. Tbr1^+/− mice can serve as a suitable model for revealing how an autism causative gene controls neuronal circuits, neural activity, and autism-related behaviors.

Electronic supplementary material

The online version of this article (10.1186/s13229-019-0257-5) contains supplementary material, which is available to authorized users.

BDNF release and signaling are required for the antidepressant actions of GLYX-13

Conventional antidepressant medications, which act on monoaminergic systems, display significant limitations, including a time lag of weeks to months and low rates of therapeutic efficacy. GLYX-13 is a novel glutamatergic compound that acts as an N-methyl-D-aspartate (NMDA) modulator with glycine-like partial agonist properties; like the NMDA receptor antagonist ketamine GLYX-13 produces rapid antidepressant actions in depressed patients and in preclinical rodent models. However, the mechanisms underlying the antidepressant actions of GLYX-13 have not been characterized. Here we use a combination of neutralizing antibody (nAb), mutant mouse and pharmacological approaches to test the role of brain-derived neurotrophic factor-tropomyosin-related kinase B (BDNF-TrkB) signaling in the actions of GLYX-13. The results demonstrate that the antidepressant effects of GLYX-13 are blocked by intra-medial prefrontal cortex (intra-mPFC) infusion of an anti-BDNF nAb or in mice with a knock-in of the BDNF Val66Met allele, which blocks the processing and activity-dependent release of BDNF. We also demonstrate that pharmacological inhibitors of BDNF-TrkB signaling or of L-type voltage-dependent Ca2+ channels (VDCCs) block the antidepressant behavioral actions of GLYX-13. Finally, we examined the role of the Rho GTPase proteins by injecting a selective inhibitor into the mPFC and found that activation of Rac1 but not RhoA is involved in the antidepressant effects of GLYX-13. Together, these findings indicate that enhanced release of BDNF through exocytosis caused by activation of VDCCs and subsequent TrkB-Rac1 signaling is required for the rapid and sustained antidepressant effects of GLYX-13.

PSD-95-nNOS Coupling Regulates Contextual Fear Extinction in the Dorsal CA3

Fear extinction depends on N-methyl-D-aspartate glutamate receptors (NMDARs) and brain-derived neurotrophic factor (BDNF) activation in the limbic system. However, postsynaptic density-95 (PSD-95) and neuronal nitric oxide synthase (nNOS) coupling, the downstream signaling of NMDARs activation, obstructs the BDNF signaling transduction. Thus, we wondered distinct roles of NMDAR activation and PSD-95-nNOS coupling on fear extinction. To explore the mechanisms, we detected protein-protein interaction using coimmunoprecipitation and measured protein expression by western blot. Contextual fear extinction induced a shift from PSD-95-nNOS to PSD-95-TrkB association in the dorsal hippocampus and c-Fos expression in the dorsal CA3. Disrupting PSD-95-nNOS coupling in the dorsal CA3 up-regulated phosphorylation of extracellular signal-regulates kinase (ERK) and BDNF, enhanced the association of BDNF-TrkB signaling with PSD-95, and promoted contextual fear extinction. Conversely, blocking NMDARs in the dorsal CA3 down-regulated BDNF expression and hindered contextual fear extinction. NMDARs activation and PSD-95-nNOS coupling play different roles in modulating contextual fear extinction in the hippocampus. Because inhibitors of PSD-95-nNOS interaction produce antidepressant and anxiolytic effect without NMDAR-induced side effects, PSD-95-nNOS could be a valuable target for PTSD treatment.

Acoustically targeted chemogenetics for the non-invasive control of neural circuits

Neurological and psychiatric disorders are often characterized by dysfunctional neural circuits in specific regions of the brain. Existing treatment strategies, including the use of drugs and implantable brain stimulators, aim to modulate the activity of these circuits. However, they are not cell-type-specific, lack spatial targeting or require invasive procedures. Here, we report a cell-type-specific and non-invasive approach based on acoustically targeted chemogenetics that enables the modulation of neural circuits with spatiotemporal specificity. The approach uses ultrasound waves to transiently open the blood-brain barrier and transduce neurons at specific locations in the brain with virally encoded engineered G-protein-coupled receptors. The engineered neurons subsequently respond to systemically administered designer compounds to activate or inhibit their activity. In a mouse model of memory formation, the approach can modify and subsequently activate or inhibit excitatory neurons within the hippocampus, with selective control over individual brain regions. This technology overcomes some of the key limitations associated with conventional brain therapies.

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

Rho-class small GTPases are implicated in basic cellular processes at nearly all brain developmental steps, from neurogenesis and migration to axon guidance and synaptic plasticity. GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Rho GTPases are highly regulated by a complex set of activating (GEFs) and inactivating (GAPs) partners, via protein:protein interactions (PPI). Misregulated RhoA, Rac1/Rac3 and cdc42 activity has been linked with intellectual disability (ID) and other neurodevelopmental conditions that comprise ID. All genetic evidences indicate that in these disorders the RhoA pathway is hyperactive while the Rac1 and cdc42 pathways are consistently hypoactive. Adopting cultured neurons for in vitro testing and specific animal models of ID for in vivo examination, the endophenotypes associated with these conditions are emerging and include altered neuronal networking, unbalanced excitation/inhibition and altered synaptic activity and plasticity. As we approach a clearer definition of these phenotype(s) and the role of hyper- and hypo-active GTPases in the construction of neuronal networks, there is an increasing possibility that selective inhibitors and activators might be designed via PPI, or identified by screening, that counteract the misregulation of small GTPases and result in alleviation of the cognitive condition. Here we review all knowledge in support of this possibility.

Formin 2 links neuropsychiatric phenotypes at young age to an increased risk for dementia

Age-associated memory decline is due to variable combinations of genetic and environmental risk factors. How these risk factors interact to drive disease onset is currently unknown. Here we begin to elucidate the mechanisms by which post-traumatic stress disorder (PTSD) at a young age contributes to an increased risk to develop dementia at old age. We show that the actin nucleator Formin 2 (Fmn2) is deregulated in PTSD and in Alzheimer's disease (AD) patients. Young mice lacking the Fmn2 gene exhibit PTSD-like phenotypes and corresponding impairments of synaptic plasticity, while the consolidation of new memories is unaffected. However, Fmn2 mutant mice develop accelerated age-associated memory decline that is further increased in the presence of additional risk factors and is mechanistically linked to a loss of transcriptional homeostasis. In conclusion, our data present a new approach to explore the connection between AD risk factors across life span and provide mechanistic insight to the processes by which neuropsychiatric diseases at a young age affect the risk for developing dementia.

Persistent histone modifications at the BDNF and Cdk-5 promoters following extinction of nicotine-seeking in rats

Drugs of addiction lead to a wide range of epigenetic changes at the promoter regions of genes directly implicated in learning and memory processes. We have previously shown that the histone deactylase inhibitor, sodium butyrate (NaB), accelerates the extinction of nicotine-seeking and provides resistance to relapse. Here, we explore the potential molecular mechanisms underlying this effect. Rats received intravenous nicotine or saline self-administration, followed by 6 days of extinction training, with each extinction session followed immediately by treatment with NaB or vehicle. On the last day of extinction, rats were killed and the medial ventral prefrontal cortex retained for chromatin immunoprecipitation and quantitative polymerase chain reaction (qPCR). A history of nicotine exposure significantly decreased H3K14 acetylation at the brain-derived neurotrophic factor (BDNF) exon IV promoter, and this effect was abolished with NaB treatment. In contrast, nicotine self-administration alone, resulted in a significant decrease in histone methylation at the H3K27me3 and H3K9me2 marks in the promoter regions of BDNF exon IV and cyclin-dependent kinase 5 (Cdk-5). Quantitative PCR-identified changes in several genes associated with NaB treatment that were independent of nicotine exposure; however, an interaction of nicotine history and NaB treatment was detected only in the expression of BDNF IV and BDNF IX. Together these results suggest that nicotine self-administration leads to a number of epigenetic changes at both the BDNF and Cdk-5 promoters, and that these changes may contribute to the enhanced extinction of nicotine-seeking by NaB.

Prophylactic Ketamine Attenuates Learned Fear

Ketamine has been reported to be an efficacious antidepressant for major depressive disorder and posttraumatic stress disorder. Most recently, ketamine has also been shown to be prophylactic against stress-induced depressive-like behavior in mice. It remains unknown, however, when ketamine should be administered relative to a stressor in order to maximize its antidepressant and/or prophylactic effects. Moreover, it is unknown whether ketamine can be prophylactic against subsequent stressors. We systematically administered ketamine at different time points relative to a fear experience, in order to determine when ketamine is most effective at reducing fear expression or preventing fear reactivation. Using a contextual fear conditioning (CFC) paradigm, mice were administered a single dose of saline or ketamine (30 mg/kg) at varying time points before or after CFC. Mice administered prophylactic ketamine 1 week, but not 1 month or 1 h before CFC, exhibited reduced freezing behavior when compared with mice administered saline. In contrast, ketamine administration following CFC or during extinction did not alter subsequent fear expression. However, ketamine administered before reinstatement increased the number of rearing bouts in an open field, possibly suggesting an increase in attentiveness. These data indicate that ketamine can buffer a fear response when given a week before as prophylactic, but not when given immediately before or after a stress-inducing episode. Thus, ketamine may be most useful in the clinic if administered in a prophylactic manner 1 week before a stressor, in order to protect against heightened fear responses to aversive stimuli.

Adiponectin regulates contextual fear extinction and intrinsic excitability of dentate gyrus granule neurons through AdipoR2 receptors

Post-traumatic stress disorder (PTSD) is characterized by exaggerated fear expression and impaired fear extinction. The underlying molecular and cellular mechanisms of PTSD are largely unknown. The current pharmacological and non-pharmacological treatments for PTSD are either ineffective or temporary with high relapse rates. Here we report that adiponectin-deficient mice exhibited normal contextual fear conditioning but displayed slower extinction learning. Infusions of adiponectin into the dentate gyrus (DG) of the hippocampus in fear-conditioned mice facilitated extinction of contextual fear. Whole-cell patch-clamp recordings in brain slices revealed that intrinsic excitability of DG granule neurons was enhanced by adiponectin deficiency and suppressed after treatment with the adiponectin mimetic AdipoRon, which were associated with increased input resistance and hyperpolarized resting membrane potential, respectively. Moreover, deletion of AdipoR2, but not AdipoR1 in the DG, resulted in augmented fear expression and reduced extinction, accompanied by intrinsic hyperexcitability of DG granule neurons. Adiponectin and AdipoRon failed to induce facilitation of fear extinction and elicit inhibition of intrinsic excitability of DG neurons in AdipoR2 knockout mice. These results indicated that adiponectin action via AdipoR2 was both necessary and sufficient for extinction of contextual fear and intrinsic excitability of DG granule neurons, implying that enhancing or dampening DG neuronal excitability may cause resistance to or facilitation of extinction. Therefore, our findings provide a functional link between adiponectin/AdipoR2 activation, DG neuronal excitability and contextual fear extinction, and suggest that targeting adiponectin/AdipoR2 may be used to strengthen extinction-based exposure therapies for PTSD.

Reciprocal Interaction of Dendrite Geometry and Nuclear Calcium-VEGFD Signaling Gates Memory Consolidation and Extinction

Nuclear calcium is an important signaling end point in synaptic excitation-transcription coupling that is critical for long-term neuroadaptations. Here, we show that nuclear calcium acting via a target gene, VEGFD, is required for hippocampus-dependent fear memory consolidation and extinction in mice. Nuclear calcium-VEGFD signaling upholds the structural integrity and complexity of the dendritic arbor of CA1 neurons that renders those cells permissive for the efficient generation of synaptic input-evoked nuclear calcium transients driving the expression of plasticity-related genes. Therefore, the gating of memory functions rests on the reciprocally reinforcing maintenance of an intact dendrite geometry and a functional synapse-to-nucleus communication axis. In psychiatric and neurodegenerative disorders, therapeutic application of VEGFD may help to stabilize dendritic structures and network connectivity, which may prevent cognitive decline and could boost the efficacy of extinction-based exposure therapies.SIGNIFICANCE STATEMENT This study uncovers a reciprocal relationship between dendrite geometry, the ability to generate nuclear calcium transients in response to synaptic inputs, and the subsequent induction of expression of plasticity-related and dendritic structure-preserving genes. Insufficient nuclear calcium signaling in CA1 hippocampal neurons and, consequently, reduced expression of the nuclear calcium target gene VEGFD, a dendrite maintenance factor, leads to reduced-complexity basal dendrites of CA1 neurons, which severely compromises the animals' consolidation of both memory and extinction memory. The structure-protective function of VEGFD may prove beneficial in psychiatric disorders as well as neurodegenerative and aging-related conditions that are associated with loss of neuronal structures, dysfunctional excitation-transcription coupling, and cognitive decline.

CDK5 downregulation enhances synaptic plasticity

CDK5 is a serine/threonine kinase that is involved in the normal function of the adult brain and plays a role in neurotransmission and synaptic plasticity. However, its over-regulation has been associated with Tau hyperphosphorylation and cognitive deficits. Our previous studies have demonstrated that CDK5 targeting using shRNA-miR provides neuroprotection and prevents cognitive deficits. Dendritic spine morphogenesis and forms of long-term synaptic plasticity-such as long-term potentiation (LTP)-have been proposed as essential processes of neuroplasticity. However, whether CDK5 participates in these processes remains controversial and depends on the experimental model. Using wild-type mice that received injections of CDK5 shRNA-miR in CA1 showed an increased LTP and recovered the PPF in deficient LTP of APPswe/PS1Δ9 transgenic mice. On mature hippocampal neurons CDK5, shRNA-miR for 12 days induced increased dendritic protrusion morphogenesis, which was dependent on Rac activity. In addition, silencing of CDK5 increased BDNF expression, temporarily increased phosphorylation of CaMKII, ERK, and CREB; and facilitated calcium signaling in neurites. Together, our data suggest that CDK5 downregulation induces synaptic plasticity in mature neurons involving Ca2+ signaling and BDNF/CREB activation.

The Regulation of Cytokine Networks in Hippocampal CA1 Differentiates Extinction from Those Required for the Maintenance of Contextual Fear Memory after Recall

We investigated the distinctiveness of gene regulatory networks in CA1 associated with the extinction of contextual fear memory (CFM) after recall using Affymetrix GeneChip Rat Genome 230 2.0 Arrays. These data were compared to previously published retrieval and reconsolidation-attributed, and consolidation datasets. A stringent dual normalization and pareto-scaled orthogonal partial least-square discriminant multivariate analysis together with a jack-knifing-based cross-validation approach was used on all datasets to reduce false positives. Consolidation, retrieval and extinction were correlated with distinct patterns of gene expression 2 hours later. Extinction-related gene expression was most distinct from the profile accompanying consolidation. A highly specific feature was the discrete regulation of neuroimmunological gene expression associated with retrieval and extinction. Immunity-associated genes of the tyrosine kinase receptor TGFβ and PDGF, and TNF families' characterized extinction. Cytokines and proinflammatory interleukins of the IL-1 and IL-6 families were enriched with the no-extinction retrieval condition. We used comparative genomics to predict transcription factor binding sites in proximal promoter regions of the retrieval-regulated genes. Retrieval that does not lead to extinction was associated with NF-κB-mediated gene expression. We confirmed differential NF-κBp65 expression, and activity in all of a representative sample of our candidate genes in the no-extinction condition. The differential regulation of cytokine networks after the acquisition and retrieval of CFM identifies the important contribution that neuroimmune signalling plays in normal hippocampal function. Further, targeting cytokine signalling upon retrieval offers a therapeutic strategy to promote extinction mechanisms in human disorders characterised by dysregulation of associative memory.

IL-33 ameliorates Alzheimer's disease-like pathology and cognitive decline

Alzheimer's disease (AD) is a devastating condition with no known effective treatment. AD is characterized by memory loss as well as impaired locomotor ability, reasoning, and judgment. Emerging evidence suggests that the innate immune response plays a major role in the pathogenesis of AD. In AD, the accumulation of β-amyloid (Aβ) in the brain perturbs physiological functions of the brain, including synaptic and neuronal dysfunction, microglial activation, and neuronal loss. Serum levels of soluble ST2 (sST2), a decoy receptor for interleukin (IL)-33, increase in patients with mild cognitive impairment, suggesting that impaired IL-33/ST2 signaling may contribute to the pathogenesis of AD. Therefore, we investigated the potential therapeutic role of IL-33 in AD, using transgenic mouse models. Here we report that IL-33 administration reverses synaptic plasticity impairment and memory deficits in APP/PS1 mice. IL-33 administration reduces soluble Aβ levels and amyloid plaque deposition by promoting the recruitment and Aβ phagocytic activity of microglia; this is mediated by ST2/p38 signaling activation. Furthermore, IL-33 injection modulates the innate immune response by polarizing microglia/macrophages toward an antiinflammatory phenotype and reducing the expression of proinflammatory genes, including IL-1β, IL-6, and NLRP3, in the cortices of APP/PS1 mice. Collectively, our results demonstrate a potential therapeutic role for IL-33 in AD.

Post-training activation of Rac1 in the basolateral amygdala is required for the formation of both short-term and long-term auditory fear memory

Rac1, a member of the Rho family of small GTPases, is crucial for morphological changes of the mature neuronal synapse including spine formation and activity-dependent spine enlargement, while its role in the formation of associated memories, such as conditioned fear memory, is not clear. Here, we report that selective deletion of Rac1 in excitatory neurons, but not in parvalbumin inhibitory neurons, impaired short- and long-term memories (STM and LTM) of fear conditioning. Conditional knockout of Rac1 before associative fear training in the basolateral amygdala (BLA), a key area for fear memory acquisition and storage, impaired fear memory. The expression of dominant-negative mutant of Rac1, or infusion of Rac1 inhibitor NSC23766 into BLA blocked both STM and LTM of fear conditioning. Furthermore, selective inhibition of Rac1 activation in BLA immediately following fear conditioning impaired STM and LTM, demonstrating that fear conditioning-induced Rac1 activation in BLA plays a critical role in the formation of both STM and LTM of conditioned fear.

Light exposure before learning improves memory consolidation at night

Light is recently recognized as a modulator able to activate the hippocampus and modulate memory processing, but little is known about the molecular mechanisms. Here, we report that in mice, a short pulse of white light before learning dramatically improves consolidation of contextual fear memory during the night. The light exposure increases hippocampal active p21-activated kinase 1 (PAK1) and CA1 long-term potentiation (LTP). These light effects are abolished in PAK1 knockout and dominant-negative transgenic mice, but preserved by expression of constitutively active PAK1 in the hippocampus. Our results indicate that light can act as a switch of PAK1 activity that modulate CA1 LTP and thereby memory consolidation without affecting learning and short-term memory.

Beneficial effects of polydatin on learning and memory in rats with chronic ethanol exposure

The purpose of this paper is to examine the effects of polydatin on cognitive function in rats self-administered with chronic ethanol levels. The levels of cyclin-dependent kinase 5 (Cdk5) were also determined. In the in vivo study, adult male Sprague-Dawley rats were used to establish an ethanol-administered rat model. Cognitive function was measured using the Morris water maze and the level of Cdk5 expression was measured to evaluate the effect of polydatin treatment. Cdk5 kinase activity and cell survival rate in primary hippocampal neuron cultures treated with ethanol or ethanol and polydatin were measured in the in vitro study. Polydatin reversed the performance impairments in chronic ethanol treated rats in Morris water maze test, and decreased unregulated Cdk5 expression. Moreover, polydatin increased cell survival rate, and decreased Cdk5 activity in the ethanol-treated primary culture of hippocampal neurons. The study results suggest that polydatin exhibits neuroprotective potential for ethanol induced neurotoxicity, both in vivo and in vitro, which is most likely related to its ability to target Cdk5 in neurons.

Mapping fear memory consolidation and extinction-specific expression of JunB

Understanding the molecular and cellular process specifically regulated during fear memory consolidation and extinction is a critical step toward development of new strategies in the treatment of human fear disorders. Here we used inhibitory component of AP-1 transcription factor, JunB, in order to map brain regions where JunB-dependent transcription is regulated during consolidation and extinction of contextual fear memory. We found that contextual fear memory consolidation induced JunB expression in the medial nucleus and intercalated cells of the amygdala while extinction training induced JunB in the CA1 and CA3 areas of the dorsal hippocampus. JunB upregulation induced by contextual fear memory extinction was absent in alphaCaMKII autophosphorylation-deficient mice which have impaired contextual fear memory extinction. Thus, our data suggest that JunB expression in the medial nucleus and intercalated cells of the amygdala is involved in fear memory consolidation while alphaCaMKII-autophosphorylation-dependent JunB expression in the areas CA1 and CA3 of the dorsal hippocampus regulates fear memory extinction.

p35 and Rac1 underlie the neuroprotection and cognitive improvement induced by CDK5 silencing

CDK5 plays an important role in neurotransmission and synaptic plasticity in the normal function of the adult brain, and dysregulation can lead to Tau hyperphosphorylation and cognitive impairment. In a previous study, we demonstrated that RNAi knock down of CDK5 reduced the formation of neurofibrillary tangles (NFT) and prevented neuronal loss in triple transgenic Alzheimer's mice. Here, we report that CDK5 RNAi protected against glutamate-mediated excitotoxicity using primary hippocampal neurons transduced with adeno-associated virus 2.5 viral vector eGFP-tagged scrambled or CDK5 shRNA-miR during 12 days. Protection was dependent on a concomitant increase in p35 and was reversed using p35 RNAi, which affected the down-stream Rho GTPase activity. Furthermore, p35 over-expression and constitutively active Rac1 mimicked CDK5 silencing-induced neuroprotection. In addition, 3xTg-Alzheimer's disease mice (24 months old) were injected in the hippocampus with scrambled or CDK5 shRNA-miR, and spatial learning and memory were performed 3 weeks post-injection using 'Morris' water maze test. Our data showed that CDK5 knock down induced an increase in p35 protein levels and Rac activity in triple transgenic Alzheimer's mice, which correlated with the recovery of cognitive function; these findings confirm that increased p35 and active Rac are involved in neuroprotection. In summary, our data suggest that p35 acts as a mediator of Rho GTPase activity and contributes to the neuroprotection induced by CDK5 RNAi.

Modulation of actin dynamics by Rac1 to target cognitive function

The small GTPase Rac1 is well known for regulating actin cytoskeleton reorganization in cells. Formation of extensions at the surface of the cell is required for migration and even for cell invasion and metastases. Because an elevated level and hyperactivation of this protein has been associated with metastasis in cancer, direct regulators of Rac1 are currently envisioned as a potential strategy to treat certain cancers. Less research, however, has been done regarding the role of this small GTP-binding protein in brain development, where it has an important role in dendritic spine morphogenesis through the regulation of actin. Alteration of dendritic development and spinogenesis has been often associated with mental disorders. Rac1 is associated with and required for learning and the formation of memories in the brain. Rac1 appears to be dysregulated in certain neurodevelopmental disorders that present all these three alterations: mental retardation, atypical synaptic plasticity and aberrant spine morphology. Thus, to develop novel therapies for rescuing cognitive impairment, a reasonable approach might be to target this protein, Rac1, which plays a pivotal role in directing signals that regulate actin dynamics, which in turn might have an effect in spine cytoarchitecture and synaptic function. It is possible that novel drugs that regulate Rac1 activation and function could modulate actin cytoskeleton and spine dynamics, representing potential candidates to repair intellectual disability in disorders associated with spine abnormalities. Herein, we present a list of the current Rac1 inhibitors that might fulfill this role together with a summary of the latest findings concerning their function as they relate to neuronal studies. While the small GTPase Rac1 is well known for regulating actin cytoskeleton reorganization in different type of cells, it appears to be also required for learning and the formation of memories in the brain. Abnormal regulation of this protein has been associated with cognitive disabilities, atypical synaptic plasticity and abnormal morphology of dendritic spines in certain neurodevelopmental disorders. Thus, modulation of Rac1 activity using novel inhibitors might be a strategy to reestablish cognitive function.