Cyclin-dependent kinase 5 is required for associative learning

Molecular and Therapeutic Targets for Amyloid-beta Plaques in Alzheimer's Disease: A Review Study

Alzheimer's disease (AD) is characterized by progressive loss of cognition and a gradual decrease in memory. Although AD is considered the most persistent form of dementia and a global concern, no complete cure or agents that can completely halt the progression of AD have been found. In the past years, significant progress has been made in understanding the cellular and molecular changes associated with AD, and numerous drug targets have been identified for the development of drugs for this disease. Amyloid-beta (Aβ) plaques and neurofibrillary tangles (NFT) are the major attributes of AD. Symptomatic relief is the only possible treatment available at present and a disease-modifying drug is of utmost importance. The development of drugs that can inhibit different targets responsible for the formation of plaques is a potential area in AD research. This review is not a complete list of all possible targets for AD but serves to highlight the targets related to Aβ pathology and pathways concerned with the formation of Aβ fragments. This shall serve as a prospect in the identification of Aβ plaque inhibitors and pave the strategies for newer drug treatments. Nevertheless, substantial research is done in this area but to bridle, the clinical difficulty remains a concern.

Time-course analysis of frontal gene expression profiles in the rat model of posttraumatic stress disorder and a comparison with the conditioned fear model

Posttraumatic stress disorder (PTSD) is a complex disorder that involves physiological, emotional, and cognitive dysregulation that may occur after exposure to a life-threatening event. In contrast with the condition of learned fear with resilience to extinction, abnormal fear with impaired fear extinction and exaggeration are considered crucial factors for the pathological development of PTSD. The prefrontal cortex (mPFC) is considered a critical region of top-down control in fear regulation, which involves the modulation of fear expression and extinction. The pathological course of PTSD is usually chronic and persistent; a number of studies have indicated temporal progression in gene expression and phenotypes may be involved in PTSD pathology. In the current study, we use a well-established modified single-prolonged stress (SPS&FS) rat model to feature PTSD-like phenotypes and compared it with a footshock fear conditioning model (FS model); we collected the frontal tissue after extreme stress exposure or fear conditioning and extracted RNA for transcriptome-level gene sequencing. We compared the genetic profiling of the mPFC at early (<2 h after solely FS or SPS&FS exposure) and late (7 days after solely FS or SPS&FS exposure) stages in these two models. First, we identified temporal differences in the expressional patterns between these two models and found pathways such as protein synthesis factor eukaryotic initiation factor 2 (EIF2), transcription factor NF-E2-related factor 2 (NRF2)-mediated oxidative stress response, and acute phase responding signaling enriched in the early stage in both models with significant p-values. Furthermore, in the late stage, the sirtuin signaling pathway was enriched in both models; other pathways such as STAT3, cAMP, lipid metabolism, Gα signaling, and increased fear were especially enriched in the late stage of the SPS&FS model. However, pathways such as VDR/RXR, GP6, and PPAR signaling were activated significantly in the FS model's late stage. Last, the network analysis revealed the temporal dynamics of psychological disorder, the endocrine system, and also genes related to increased fear in the two models. This study could help elucidate the genetic temporal alteration and stage-specific pathways in these two models, as well as a better understanding of the transcriptome-level differences between them.

Hyperthyroidism leads learning and memory impairment possibly via GRIN2B expression alterations

The hippocampus as an important structure for learning and memory functions contains a high level of thyroid hormone receptors. Although there are numerous studies investigating the effects of thyroid hormones on cognitive dysfunction and psychiatric symptoms, the underlying molecular processes of these disorders have not yet been fully elucidated. In the present study, 24 male adult rats (4 months) were divided into 3 groups: control group, sham group and hyperthyroid group. Hyperthyroid group and sham group were treated with l-thyroxine or saline for 21 days. Each group was exposed to Morris water maze testing (MWMT), measuring their performance in a hidden-platform spatial task. After learning and memory tests, intracardiac blood was taken from the rats for serum thyroxine levels. Following blood collection, the rats were decapitated to isolate hippocampal tissue. GRIN2A, GRIN2B, BDNF, cFOS, Cdk5, cdk5r1 (p35), and cdk5r2 (p39) gene expression were evaluated using quantitative reverse transcriptase-PCR. Serum thyroxine level was found to be higher in hyperthyroid rats than in the control and sham groups. According to our MWMT findings, the memory performance of the hyperthyroid group was significantly impaired compared to the control and sham groups (p < 0.05). In the hippocampus, the GRIN2A gene expression level was decreased in the sham group, and the GRIN2B gene expression level was decreased in the sham and hyperthyroid groups compared to the control group (p < 0.05). There was no significant difference in other genes (p > 0.05). Hyperthyroidism impaired hippocampus-dependent spatial memory. Hyperthyroidism caused decreased level of GRIN2B gene expression in the hippocampus.

Traditional and Innovative Anti-seizure Medications Targeting Key Physiopathological Mechanisms: Focus on Neurodevelopment and Neurodegeneration

Despite the wide range of compounds currently available to treat epilepsy, there is still no drug that directly tackles the physiopathological mechanisms underlying its development. Indeed, antiseizure medications attempt to prevent seizures but are inefficacious in counteracting or rescuing the physiopathological phenomena that underlie their onset and recurrence, and hence do not cure epilepsy. Classically, the altered excitation/inhibition balance is postulated as the mechanism underlying epileptogenesis and seizure generation. This oversimplification, however, does not account for deficits in homeostatic plasticity resulting from either insufficient or excessive compensatory mechanisms in response to a change in network activity. In this respect, both neurodevelopmental epilepsies and those associated with neurodegeneration may share common underlying mechanisms that still need to be fully elucidated. The understanding of these molecular mechanisms shed light on the identification of new classes of drugs able not only to suppress seizures, but also to present potential antiepileptogenic effects or "disease-modifying" properties.

Decreased synapse-associated proteins are associated with the onset of epileptic memory impairment in endothelial CDK5-deficient mice

Accumulating evidence indicates that epilepsy has a higher risk of inducing memory impairment and dementia. However, the underlying onset mechanism remains unclear. Here, we found that mice with spontaneous epilepsy induced by endothelial CDK5 deficiency exhibited hippocampal-dependent memory impairment at 6 months of age, but not at 2 months of age. Moreover, the persistent epileptic seizures induce aberrant changes in phosphorylation of CaMKII protein in the hippocampus of spontaneous epileptic mice. Using genome-wide RNA sequencing and intergenic interaction analysis of STRING, we found that in addition to epilepsy-related genes, there are changes in synaptic organization pathway node genes, such as Bdnf and Grin1. The synapse-related proteins by Western blot analysis, such as NMDA receptors (NR1 and NR2B), PSD95, and the phosphorylation of synapsin1, are progressively decreased during epileptic seizures in Cdh5-CreERT2;CDK5f/f mice. Notably, we found that valproate (VPA) and phenytoin (PHT) augment mRNA expression and protein levels of synapse-related genes and ameliorate memory impairment in Cdh5-CreERT2;CDK5f/f mice. Our study elucidates a potential mechanism of memory deficits in epilepsy, and pharmacological reversal of synaptic pathology targeting might provide a new therapeutic intervention for epileptic memory deficits.

Polydatin as a therapeutic alternative for central nervous system disorders: A systematic review of animal studies

Polydatin, or piceid, is a natural stilbene found in grapes, peanuts, and wines. Polydatin presents pharmacological activities, including neuroprotective properties, exerting preventive and/or therapeutic effects in central nervous system (CNS) disorders. In the present study, we summarize and discuss the neuroprotective effects of polydatin in CNS disorders and related pathological conditions in preclinical animal studies. A systematic review was performed by searching online databases, returning a total of 110 records, where 27 articles were selected and discussed here. The included studies showed neuroprotective effects of polydatin in experimental models of neurological disorders, including cerebrovascular disorders, Parkinson's disease, traumatic brain injuries, diabetic neuropathy, glioblastoma, and neurotoxicity induced by chemical agents. Most studies were focused on stroke (22.2%) and conducted in male rodents. The intervention protocol with polydatin was mainly acute (66.7%), with postdamage induction treatment being the most commonly used regimen (55.2%). Overall, polydatin ameliorated behavioral dysfunctions and/or promoted neurological function by virtue of its antioxidant and antiinflammatory properties. In summary, this review offers important scientific evidence for the neuroprotective effects and distinct pharmacological mechanisms of polydatin that not only enhances the present understanding but is also useful for the development of future preclinical and clinical investigations.

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.

Transcriptome and chromatin alterations in social fear indicate association of MEG3 with successful extinction of fear

Social anxiety disorder is characterized by a persistent fear and avoidance of social situations, but available treatment options are rather unspecific. Using an established mouse social fear conditioning (SFC) paradigm, we profiled gene expression and chromatin alterations after the acquisition and extinction of social fear within the septum, a brain region important for social fear and social behaviors. Here, we particularly focused on the successful versus unsuccessful outcome of social fear extinction training, which corresponds to treatment responsive versus resistant patients in the clinics. Validation of coding and non-coding RNAs revealed specific isoforms of the long non-coding RNA (lncRNA) Meg3 regulated, depending on the success of social fear extinction. Moreover, PI3K/AKT was differentially activated with extinction success in SFC-mice. In vivo knockdown of specific Meg3 isoforms increased baseline activity of PI3K/AKT signaling, and mildly delayed social fear extinction. Using ATAC-Seq and CUT&RUN, we found alterations in the chromatin structure of specific genes, which might be direct targets of lncRNA Meg3.

Pharmacological Tools to Modulate Autophagy in Neurodegenerative Diseases

Considerable evidences suggest a link between autophagy dysfunction, protein aggregation, and neurodegenerative diseases. Given that autophagy is a conserved intracellular housekeeping process, modulation of autophagy flux in various model organisms have highlighted its importance for maintaining proteostasis. In postmitotic cells such as neurons, compromised autophagy is sufficient to cause accumulation of ubiquitinated aggregates, neuronal dysfunction, degeneration, and loss of motor coordination-all hallmarks of neurodegenerative diseases. Reciprocally, enhanced autophagy flux augments cellular and organismal health, in addition to extending life span. These genetic studies not-withstanding a plethora of small molecule modulators of autophagy flux have been reported that alleviate disease symptoms in models of neurodegenerative diseases. This review summarizes the potential of such molecules to be, perhaps, one of the first autophagy drugs for treating these currently incurable diseases.

Neuron-Specific Menin Deletion Leads to Synaptic Dysfunction and Cognitive Impairment by Modulating p35 Expression

Menin (MEN1) is a critical modulator of tissue development and maintenance. As such, MEN1 mutations are associated with multiple endocrine neoplasia type 1 (MEN1) syndrome. Although menin is abundantly expressed in the nervous system, little is known with regard to its function in the adult brain. Here, we demonstrate that neuron-specific deletion of Men1 (CcKO) affects dendritic branching and spine formation, resulting in defects in synaptic function, learning, and memory. Furthermore, we find that menin binds to the p35 promoter region to facilitate p35 transcription. As a primary Cdk5 activator, p35 is expressed mainly in neurons and is critical for brain development and synaptic plasticity. Restoration of p35 expression in the hippocampus and cortex of Men1 CcKO mice rescues synaptic and cognitive deficits associated with Men1 deletion. These results reveal a critical role for menin in synaptic and cognitive function by modulating the p35-Cdk5 pathway.

Sex-Specific Regulation of Fear Memory by Targeted Epigenetic Editing of Cdk5

BACKGROUND

Sex differences in the expression and prevalence of trauma- and stress-related disorders have led to a growing interest in the sex-specific molecular and epigenetic mechanisms underlying these diseases. Cyclin-dependent kinase 5 (CDK5) is known to underlie both fear memory and stress behavior in male mice. Given our recent finding that targeted histone acetylation of Cdk5 regulates stress responsivity in male mice, we hypothesized that such a mechanism may be functionally relevant in female mice as well.

METHODS

We applied epigenetic editing of Cdk5 in the hippocampus and examined the regulation of fear memory retrieval in male and female mice. Viral expression of zinc finger proteins targeting histone acetylation to the Cdk5 promoter was paired with a quantification of learning and memory of contextual fear conditioning, expression of CDK5, and enrichment of histone modifications of the Cdk5 gene.

RESULTS

We found that male mice exhibit stronger long-term memory retrieval than do female mice, and this finding was associated with male-specific epigenetic activation of hippocampal Cdk5 expression. Sex differences in behavior and epigenetic regulation of Cdk5 occurred after long-term, but not short-term, fear memory retrieval. Finally, targeted histone acetylation of hippocampal Cdk5 promoter attenuated fear memory retrieval and increased tau phosphorylation in female but not male mice.

CONCLUSIONS

Epigenetic editing uncovered a female-specific role of Cdk5 activation in attenuating fear memory retrieval. This finding may be attributed to CDK5 mediated hyperphosphorylation of tau only in the female hippocampus. Sex-specific epigenetic regulation of Cdk5 may reflect differences in the effect of CDK5 on downstream target proteins that regulate memory.

Roscovitine effectively enhances antitumor activity of temozolomide in vitro and in vivo mediated by increased autophagy and Caspase-3 dependent apoptosis

Gliomas are incurable solid tumors with extremely high relapse rate and definite mortality. As gliomas readily acquire resistance to only approved drug, temozolomide (TMZ), there is increasing need to overcome drug resistance by novel therapeutics or by repurposing the existing therapy. In the current study, we investigated antitumor efficacy of roscovitine, a Cdk inhibitor, in combination with TMZ in vitro (U87, U373, LN 18 and C6 cell lines) and in vivo (orthotopic glioma model in Wistar rats) glioma models. We observed that TMZ treatment following a pre-treatment with RSV significantly enhanced chemo-sensitivity and suppressed the growth of glioma cells by reducing Cdk-5 activity and simultaneous induction of autophagy and Caspase-3 mediated apoptosis. Additionally, reduced expression of Ki67, GFAP and markers of angiogenesis (CD31, VEGF) was observed in case of TMZ + RSV treatments. Also, presence of reactive astrocytes in peri-tumoral areas and in areas around blood vessels was completely diminished in TMZ + RSV treated brain sections. Taken together, results in the current study provide evidence that RSV in conjunction with TMZ restricts glioma growth, reduces angiogenesis and also eliminates reactive astrocytes thereby preventing the spread of glioma to adjacent healthy brain tissues and thus might be more potent therapeutic option for glioma.

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).

Telencephalon transcriptome analysis of chronically stressed adult zebrafish

Chronic stress leads to disruptions in learning and memory processes. The effects of chronic stress experience on the adult zebrafish brain, particularly the memory associated telencephalon brain region, is unclear. The goal of this study was to identify gene expression changes in the adult zebrafish brain triggered by chronic unpredictable stress. Transcriptome analysis of the telencephalon revealed 155 differentially expressed genes. Of these genes, some are critical genes involved in learning and memory, such as cdk5 and chrna7, indicating effects of chronic unpredictable stress on zebrafish memory. Interestingly, several genes were annotated in the Orange domain, which is an amino acid sequence present in eukaryotic DNA-binding transcription repressors. Furthermore, we identified hsd11b2, a cortisol inactivating gene, as chronic stress-responsive in the whole zebrafish brain. Collectively, these findings suggest that memory associated gene expression changes in adult zebrafish telencephalon are affected by chronic stress experience.

Multiple Layers of CDK5R1 Regulation in Alzheimer's Disease Implicate Long Non-Coding RNAs

Cyclin-dependent kinase 5 regulatory subunit 1 (CDK5R1) gene encodes for p35, the main activator of Cyclin-dependent kinase 5 (CDK5). The active p35/CDK5 complex is involved in numerous aspects of brain development and function, and its deregulation is closely associated to Alzheimer's disease (AD) onset and progression. We recently showed that miR-15/107 family can negatively regulate CDK5R1 expression modifying mRNA stability. Interestingly, miRNAs belonging to miR-15/107 family are downregulated in AD brain while CDK5R1 is upregulated. Long non-coding RNAs (lncRNAs) are emerging as master regulators of gene expression, including miRNAs, and their dysregulation has been implicated in the pathogenesis of AD. Here, we evaluated the existence of an additional layer of CDK5R1 expression regulation provided by lncRNAs. In particular, we focused on three lncRNAs potentially regulating CDK5R1 expression levels, based on existing data: NEAT1, HOTAIR, and MALAT1. We demonstrated that NEAT1 and HOTAIR negatively regulate CDK5R1 mRNA levels, while MALAT1 has a positive effect. We also showed that all three lncRNAs positively control miR-15/107 family of miRNAs. Moreover, we evaluated the expression of NEAT1, HOTAIR, and MALAT1 in AD and control brain tissues. Interestingly, NEAT1 displayed increased expression levels in temporal cortex and hippocampus of AD patients. Interestingly, we observed a strong positive correlation between CDK5R1 and NEAT1 expression levels in brain tissues, suggesting a possible neuroprotective role of NEAT1 in AD to compensate for increased CDK5R1 levels. Overall, our work provides evidence of another level of CDK5R1 expression regulation mediated by lncRNAs and points to NEAT1 as a biomarker, as well as a potential pharmacological target for AD therapy.

Neonatal Neurodegeneration in Alzheimer's Disease Transgenic Mouse Model

Alzheimer’s disease (AD) is a progressive disorder characterized by a variety of molecular pathologies causing cortical dementia with a prominent memory deficit. Formation of the pathology, which begins decades before the diagnosis of the disease, is highly correlated with the clinical symptoms. Several proteomics studies were performed using animal models to monitor the alterations of the brain tissue proteome at different stages of AD. However, proteome changes in the brain regions of newborn transgenic mouse model have not been investigated yet. To this end, we analyzed protein expression alterations in cortex, hippocampus and cerebellum of transgenic mice carrying five familial AD mutations (5XFAD) at neonatal day-1. Our results indicate a remarkable difference in protein expression profile of newborn 5XFAD brain with region specific variations. Additionally, the proteins, which show similar expression alteration pattern in postmortem human AD brains, were determined. Bioinformatics analysis showed that the molecular alterations were mostly related to the cell morphology, cellular assembly and organization, and neuroinflammation. Moreover, morphological analysis revealed that there is an increase in neurite number of 5XFAD mouse neurons in vitro. We suggest that, molecular alterations in the AD brain exist even at birth, and perhaps the disease is silenced until older ages when the brain becomes vulnerable.

Cdk5 Is Essential for Amphetamine to Increase Dendritic Spine Density in Hippocampal Pyramidal Neurons

Psychostimulant drugs of abuse increase dendritic spine density in reward centers of the brain. However, little is known about their effects in the hippocampus, where activity-dependent changes in the density of dendritic spine are associated with learning and memory. Recent reports suggest that Cdk5 plays an important role in drug addiction, but its role in psychostimulant's effects on dendritic spines in hippocampus remain unknown. We used in vivo and in vitro approaches to demonstrate that amphetamine increases dendritic spine density in pyramidal neurons of the hippocampus. Primary cultures and organotypic slice cultures were used for cellular, molecular, pharmacological and biochemical analyses of the role of Cdk5/p25 in amphetamine-induced dendritic spine formation. Amphetamine (two-injection protocol) increased dendritic spine density in hippocampal neurons of thy1-green fluorescent protein (GFP) mice, as well as in hippocampal cultured neurons and organotypic slice cultures. Either genetic or pharmacological inhibition of Cdk5 activity prevented the amphetamine-induced increase in dendritic spine density. Amphetamine also increased spine density in neurons overexpressing the strong Cdk5 activator p25. Finally, inhibition of calpain, the protease necessary for the conversion of p35 to p25, prevented amphetamine's effect on dendritic spine density. We demonstrate, for the first time, that amphetamine increases the density of dendritic spine in hippocampal pyramidal neurons in vivo and in vitro. Moreover, we show that the Cdk5/p25 signaling and calpain activity are both necessary for the effect of amphetamine on dendritic spine density. The identification of molecular mechanisms underlying psychostimulant effects provides novel and promising therapeutic approaches for the treatment of drug addiction.

Calpain-2/p35-p25/Cdk5 pathway is involved in the neuronal apoptosis induced by polybrominated diphenyl ether-153

Polybrominated diphenyl ethers (PBDEs) have been demonstrated to induce neurotoxicity in experimental rats and mice, with neuronal apoptosis as one of the major mechanisms, however, the mechanisms underlying PBDEs-induced neuronal apoptosis remain unclear. In this study, we aimed to investigate the role of calpain/p35-p25/Cdk5 pathway in BDE-153-induced neuronal apoptosis in the hippocampus and primary neurons in rats. Results showed that compared to the controls, neuronal apoptosis was significantly increased in vivo and ex vivo, as manifested by the increased hippocampus TUNEL-positive cell rates, apoptotic neurons in Hoechst and AO/EB staining, and the increased LDH activity and percentage of Annexin V-positive cells in rat hippocampus and primary neurons. Calpain activity was significantly increased in all the BDE-153-treated groups in vivo and ex vivo when compared to non-treatment controls. In addition, we showed that calpain-2 accounted for the calpain activation instead of calpain-1, as demonstrated by the up-regulated mRNA and protein expressions in calpain-2 but not calpain-1. Activated calpain truncated p35 into p25, which resulted in the p25/Cdk5 formation and activation. Calpain inhibitor PD150606 or p25/Cdk5 inhibitor Roscovitine relieved neuronal apoptosis mainly via inhibiting the p25/Cdk5 activation. Overall, the findings suggested that calpain-2/p35-p25/Cdk5 pathway was involved in BDE-153-induced neuronal apoptosis, which provides novel insight into the mechanisms of PBDE neurotoxicity.

Phosphorylation of CRMP2 by Cdk5 Regulates Dendritic Spine Development of Cortical Neuron in the Mouse Hippocampus

Proper density and morphology of dendritic spines are important for higher brain functions such as learning and memory. However, our knowledge about molecular mechanisms that regulate thedevelopment and maintenance of dendritic spines is limited. We recently reported that cyclin-dependent kinase 5 (Cdk5) is required for the development and maintenance of dendritic spines of cortical neurons in the mouse brain. Previous in vitro studies have suggested the involvement of Cdk5 substrates in the formation of dendritic spines; however, their role in spine development has not been tested in vivo. Here, we demonstrate that Cdk5 phosphorylates collapsin response mediator protein 2 (CRMP2) in the dendritic spines of cultured hippocampal neurons and in vivo in the mouse brain. When we eliminated CRMP2 phosphorylation in CRMP2^KI/KI mice, the densities of dendritic spines significantly decreased in hippocampal CA1 pyramidal neurons in the mouse brain. These results indicate that phosphorylation of CRMP2 by Cdk5 is important for dendritic spine development in cortical neurons in the mouse hippocampus.

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.

The role of ventral striatal cAMP signaling in stress-induced behaviors

The cAMP and cAMP-dependent protein kinase A (PKA) signaling cascade is a ubiquitous pathway acting downstream of multiple neuromodulators. We found that the phosphorylation of phosphodiesterase-4 (PDE4) by cyclin-dependent protein kinase 5 (Cdk5) facilitated cAMP degradation and homeostasis of cAMP/PKA signaling. In mice, loss of Cdk5 throughout the forebrain elevated cAMP levels and increased PKA activity in striatal neurons, and altered behavioral responses to acute or chronic stressors. Ventral striatum- or D1 dopamine receptor-specific conditional knockout of Cdk5, or ventral striatum infusion of a small interfering peptide that selectively targeted the regulation of PDE4 by Cdk5, produced analogous effects on stress-induced behavioral responses. Together, our results demonstrate that altering cAMP signaling in medium spiny neurons of the ventral striatum can effectively modulate stress-induced behavioral states. We propose that targeting the Cdk5 regulation of PDE4 could be a new therapeutic approach for clinical conditions associated with stress, such as depression.

Cdk5 activity in the brain - multiple paths of regulation

Cyclin dependent kinase-5 (Cdk5), a family member of the cyclin-dependent kinases, plays a pivotal role in the central nervous system. During embryogenesis, Cdk5 is indispensable for brain development and, in the adult brain, it is essential for numerous neuronal processes, including higher cognitive functions such as learning and memory formation. However, Cdk5 activity becomes deregulated in several neurological disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease, which leads to neurotoxicity. Therefore, precise control over Cdk5 activity is essential for its physiological functions. This Commentary covers the various mechanisms of Cdk5 regulation, including several recently identified protein activators and inhibitors of Cdk5 that control its activity in normal and diseased brains. We also discuss the autoregulatory activity of Cdk5 and its regulation at the transcriptional, post-transcriptional and post-translational levels. We finally highlight physiological and pathological roles of Cdk5 in the brain. Specific modulation of these protein regulators is expected to provide alternative strategies for the development of effective therapeutic interventions that are triggered by deregulation of Cdk5.

Barrier Function of the Repaired Skin Is Disrupted Following Arrest of Dicer in Keratinocytes

Tissue injury transiently silences miRNA-dependent posttranscriptional gene silencing in its effort to unleash adult tissue repair. Once the wound is closed, miRNA biogenesis is induced averting neoplasia. In this work, we report that Dicer plays an important role in reestablishing the barrier function of the skin post-wounding via a miRNA-dependent mechanism. MicroRNA expression profiling of skin and wound-edge tissue revealed global upregulation of miRNAs following wound closure at day 14 post-wounding with significant induction of Dicer expression. Barrier function of the skin, as measured by trans-epidermal water loss, was compromised in keratinocyte-specific conditional (K14/Lox-Cre) Dicer-ablated mice because of malformed cornified epithelium lacking loricrin expression. Studies on human keratinocytes recognized that loricrin expression was inversely related to the expression of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Compared to healthy epidermis, wound-edge keratinocytes from Dicer-ablated skin epidermis revealed elevated p21(Waf1/Cip1) expression. Adenoviral and pharmacological suppression of p21(Waf1/Cip1) in keratinocyte-specific conditional Dicer-ablated mice improved wound healing indicating a role of Dicer in the suppression of p21(Waf1/Cip1). This work upholds p21(Waf1/Cip1) as a druggable target to restore barrier function of skin suffering from loss of Dicer function as would be expected in diabetes and other forms of oxidant insult.

Reduction of epileptiform activity by valproic acid in a mouse model of Alzheimer's disease is not long-lasting after treatment discontinuation

Patients with Alzheimer's disease are at increased risk for unprovoked seizures and epilepsy compared with age-matched controls. Experimental evidence suggests that neuronal hyperexcitability and epilepsy can be triggered by amyloid-β (Aβ), the main component of amyloid plaques. Previous studies demonstrated that the administration of an anticonvulsant and histone deacetylase inhibitor, valproic acid, leads to a long-lasting reduction in Aβ levels. Here we used an APdE9 mouse model of Alzheimer's disease with overproduction of Aβ to assess whether treatment with valproic acid initiated immediately after epilepsy onset modifies the occurrence of epileptiform activity. We also analyzed whether the effect is long-lasting and associated with antiamyloidogenesis and histone-modifications. Male APdE9 mice (15 week old) received daily intraperitoneal injections of 30mg/kg valproic acid for 1 week. After a 3-week wash-out, the same animals received injections of a higher dose of valproic acid (300mg/kg) daily for 1 week. Long-term video-electroencephalography monitoring was performed prior to, during, and after the treatments. Aβ and total histone H3 and H4 acetylation levels were measured at 1 month after the final valproic acid treatment. While 30mg/kg valproic acid reduced spontaneous seizures in APdE9 mice (p<0.05, chi-square), epileptiform discharges were not reduced. Administration of 300mg/kg valproic acid, however, reduced epileptiform discharges in APdE9 mice for at least 1 week after treatment discontinuation (p<0.05, Wilcoxon test), but there was no consistent long-term effects on epileptiform activity after treatment withdrawal. Further, we found no long-lasting effect on Aβ levels (p>0.05, Mann-Whitney test), only a meager increase in global acetylation of histone H3 (p<0.05), and no effects on H4 acetylation (p>0.05). In conclusion, valproic acid treatment of APdE9 mice at the stage when amyloid plaques are beginning to develop and epileptiform activity is detected reduced the amount of epileptiform activity, but the effect disappeared after treatment discontinuation.

Networks of protein kinases and phosphatases in the individual phases of contextual fear conditioning in the C57BL/6J mouse

Although protein kinases and phosphatases have been reported to be involved in fear memory, information about these signalling molecules in the individual phases of contextual fear conditioning (cFC) is limited. C57BL/6J mice were tested in cFC, sacrificed and hippocampi were used for screening of approximately 800 protein kinases and phosphatases by protein microarrays with subsequent Western blot confirmation of threefold higher or lower hippocampal levels as compared to foot shock controls. Immunoblotting of the protein kinases and phosphatases screened out was carried out by Western blotting. A network of protein kinases and phosphatases was generated (STRING 9.1). Animals learned the task in the paradigm and protein kinase and phosphatase levels were determined in the individual phases acquisition, consolidation and retrieval and compared to foot shock controls. Protein kinases discoidin containing receptor 2 (DDR2), mitogen activated protein kinase kinase kinase 7 (TAK1), protein phosphatases dual specificity protein phosphatase (PTEN) and protein phosphatase 2a (PP2A) were modulated in the individual phases of cFC. Phosphatidyl-inositol-3,4,5-triphosphate 3-phosphatase and phosphatidylinositol-3 kinase (PI3K) that is interacting with PTEN were modulated as well. Freezing time was correlating with PP2A levels in the retrieval phase of cFC. The abovementioned protein kinases, phosphatases and inositol-signalling enzymes were not reported so far in cFC and the results are relevant for interpretation of previous and design of future studies in cFC or fear memory. Protein phosphatase PP2A was, however, the only signalling compound tested that was directly linked to retrieval in the cFC.

Inhibiting cyclin-dependent kinase 5 in the nucleus accumbens enhances the expression of amphetamine-induced locomotor conditioning

When psychostimulant drugs like amphetamine are administered repeatedly in the presence of a contextual stimulus complex, long-lasting associations form between the unconditioned effects of the drug and the contextual stimuli. Here we assessed the role played by the proline-directed serine/threonine kinase cyclin-dependent kinase 5 (Cdk5) in the nucleus accumbens (NAcc) on the expression of the conditioned locomotion normally observed when rats are returned to a context previously paired with amphetamine. Infusing the Cdk5 inhibitor roscovitine (40nmol/0.5μl/side) into the NAcc 30-min before the test for conditioning significantly enhanced the conditioned locomotor response observed in rats previously administered amphetamine in the test environment. This effect was specific to the expression of a conditioned response as inhibiting Cdk5 produced no effect in control rats previously administered saline or previously administered amphetamine elsewhere. As inhibiting Cdk5 during exposure to amphetamine has been found to block the accrual of locomotor conditioning, the present results suggest distinct roles for NAcc Cdk5 in the induction and expression of excitatory conditioning by amphetamine.

Locomotor conditioning by amphetamine requires cyclin-dependent kinase 5 signaling in the nucleus accumbens

Intermittent systemic exposure to psychostimulants leads to several forms of long-lasting behavioral plasticity including nonassociative sensitization and associative conditioning. In the nucleus accumbens (NAcc), the protein serine/threonine kinase cyclin-dependent kinase 5 (Cdk5) and its phosphorylation target, the guanine-nucleotide exchange factor kalirin-7 (Kal7), may contribute to the neuroadaptations underlying the formation of conditioned associations. Pharmacological inhibition of Cdk5 in the NAcc prevents the increases in dendritic spine density normally observed in this site following repeated cocaine. Mice lacking the Kal7 gene display similar effects. As increases in spine density may relate to the formation of associative memories and both Cdk5 and Kal7 regulate the generation of spines following repeated drug exposure, we hypothesized that either inhibiting Cdk5 or preventing its phosphorylation of Kal7 in the NAcc may prevent the induction of drug conditioning. In the present experiments, blockade in rats of NAcc Cdk5 activity with roscovitine (40 nmol/0.5 μl/side) prior to each of 4 injections of amphetamine (1.5 mg/kg; i.p.) prevented the accrual of contextual locomotor conditioning but spared the induction of locomotor sensitization as revealed on tests conducted one week later. Similarly, transient viral expression in the NAcc exclusively during amphetamine exposure of a threonine-alanine mutant form of Kal7 [mKal7(T1590A)] that is not phosphorylated by Cdk5 also prevented the accrual of contextual conditioning and spared the induction of sensitization. These results indicate that signaling via Cdk5 and Kal7 in the NAcc is necessary for the formation of context-drug associations, potentially through the modulation of dendritic spine dynamics in this site.

Linking pathways in the developing and aging brain with neurodegeneration

The molecular and cellular mechanisms, which coordinate the critical stages of brain development to reach a normal structural organization with appropriate networks, are progressively being elucidated. Experimental and clinical studies provide evidence of the occurrence of developmental alterations induced by genetic or environmental factors leading to the formation of aberrant networks associated with learning disabilities. Moreover, evidence is accumulating that suggests that also late-onset neurological disorders, even Alzheimer's disease, might be considered disorders of aberrant neural development with pathological changes that are set up at early stages of development before the appearance of the symptoms. Thus, evaluating proteins and pathways that are important in age-related neurodegeneration in the developing brain together with the characterization of mechanisms important during brain development with relevance to brain aging are of crucial importance. In the present review we focus on (1) aspects of neurogenesis with relevance to aging; (2) neurodegenerative disease (NDD)-associated proteins/pathways in the developing brain; and (3) further pathways of the developing or neurodegenerating brains that show commonalities. Elucidation of complex pathogenetic routes characterizing the earliest stage of the detrimental processes that result in pathological aging represents an essential first step toward a therapeutic intervention which is able to reverse these pathological processes and prevent the onset of the disease. Based on the shared features between pathways, we conclude that prevention of NDDs of the elderly might begin during the fetal and childhood life by providing the mothers and their children a healthy environment for the fetal and childhood development.

Protein profiles associated with context fear conditioning and their modulation by memantine

Analysis of the molecular basis of learning and memory has revealed details of the roles played by many genes and the proteins they encode. Because most individual studies focus on a small number of proteins, many complexities of the relationships among proteins and their dynamic responses to stimulation are not known. We have used the technique of reverse phase protein arrays (RPPA) to assess the levels of more than 80 proteins/protein modifications in subcellular fractions from hippocampus and cortex of mice trained in Context Fear Conditioning (CFC). Proteins include components of signaling pathways, several encoded by immediate early genes or involved in apoptosis and inflammation, and subunits of glutamate receptors. At one hour after training, levels of more than half the proteins had changed in one or more fractions, among them multiple components of the Mitogen-activated protein kinase, MAPK, and Mechanistic Target of Rapamycin, MTOR, pathways, subunits of glutamate receptors, and the NOTCH pathway modulator, NUMB homolog (Drosophila). Levels of 37 proteins changed in the nuclear fraction of hippocampus alone. Abnormalities in levels of thirteen proteins analyzed have been reported in brains of patients with Alzheimer's Disease. We therefore further investigated the protein profiles of mice treated with memantine, a drug approved for treatment of AD. In hippocampus, memantine alone induced many changes similar to those seen after CFC and altered the levels of seven proteins associated with Alzheimer's Disease abnormalities. Lastly, to further explore the relevance of these datasets, we superimposed responses to CFC and memantine onto components of the long term potentiation pathway, a process subserving learning and memory formation. Fourteen components of the long term potentiation pathway and 26 proteins interacting with components responded to CFC and/or memantine. Together, these datasets provide a novel view of the diversity and complexity in protein responses and interactions following normal learning.

Forebrain-specific deletion of Cdk5 in pyramidal neurons results in mania-like behavior and cognitive impairment

Cyclin-dependent kinase 5 (Cdk5) is associated with synaptic plasticity and cognitive function. Previous reports have demonstrated that Cdk5 is necessary for memory formation, although others have reported Cdk5 conditional knockout mouse models exhibiting enhanced learning and memory. Furthermore, how Cdk5 acts in specific cell populations to affect behavior and cognitive outcomes remains unclear. Here we conduct a behavioral characterization of a forebrain-specific Cdk5 conditional knockout mouse model under the αCaMKII promoter, in which Cdk5 is ablated in excitatory pyramidal neurons of the forebrain. The Cdk5 conditional knockouts exhibit hyperactivity in the open field, reduced anxiety, and reduced behavioral despair. Moreover, the Cdk5 conditional knockouts also display impaired spatial learning in the Morris water maze and are severely impaired in contextual fear memory, which correspond to deficits in synaptic transmission. Remarkably, the hyperactivity of the Cdk5 conditional knockouts can be ameliorated by the administration of lithium chloride, an inhibitor of GSK3β signaling. Collectively, our data reveal that Cdk5 ablation from forebrain excitatory neurons results in deleterious effects on emotional and cognitive behavior and highlight a key role for Cdk5 in regulating the GSK3β signaling pathway.

Deregulated Cdk5 activity is involved in inducing Alzheimer's disease

Alzheimer's disease (AD), the most devastating chronic neurodegenerative disease in adults, causes dementia and eventually, death of the affected individuals. Clinically, AD is characterized as late-onset, age-dependent cognitive decline due to loss of neurons in cortex and hippocampus. The pathologic corollary of these symptoms is the formation of senile plaques and neurofibrillary tangles. Senile plaques are formed due to accumulation of oligomeric amyloid beta (Aβ) forming plaques. This occurs due to the amyloidogenic processing of the amyloid precursor protein (APP) by various secretases. On the other hand, neurofibrillary tangles are formed due to hyperphosphorylation of cytoskeleton proteins like tau and neurofilament. Both are hyperphosphorylated by cyclin-dependent kinase-5 (Cdk5) and are part of the paired helical filament (PHF), an integral part of neurofibrillary tangles. Unlike other cyclin-dependent kinases, Cdk5 plays a very important role in the neuronal development. Cdk5 gets activated by its neuronal activators p35 and p39. Upon stress, p35 and p39 are cleaved by calpain resulting in truncated products as p25 and p29. Association of Cdk5/p25 is longer and uncontrolled causing aberrant hyperphosphorylation of various substrates of Cdk5 like APP, tau and neurofilament, leading to neurodegenerative pathology like AD. Additionally recent evidence has shown increased levels of p25, Aβ, hyperactivity of Cdk5, phosphorylated tau and neurofilament in human AD brains. This review briefly describes the above-mentioned aspects of involvement of Cdk5 in the pathology of AD and at the end summarizes the advances in Cdk5 as a therapeutic target.

A truncated peptide from p35, a Cdk5 activator, prevents Alzheimer's disease phenotypes in model mice

Alzheimer's disease (AD), one of the leading neurodegenerative disorders of older adults, which causes major socioeconomic burdens globally, lacks effective therapeutics without significant side effects. Besides the hallmark pathology of amyloid plaques and neurofibrillary tangles (NFTs), it has been reported that cyclin-dependent kinase 5 (Cdk5), a critical neuronal kinase, is hyperactivated in AD brains and is, in part, responsible for the above pathology. Here we show that a modified truncated 24-aa peptide (TFP5), derived from the Cdk5 activator p35, penetrates the blood-brain barrier after intraperitoneal injections, inhibits abnormal Cdk5 hyperactivity, and significantly rescues AD pathology (up to 70-80%) in 5XFAD AD model mice. The mutant mice, injected with TFP5 exhibit behavioral rescue, whereas no rescue was observed in mutant mice injected with either saline or scrambled peptide. However, TFP5 does not inhibit cell cycle Cdks or normal Cdk5/p35 activity, and thereby has no toxic side effects (even at 200 mg/kg), a common problem in most current therapeutics for AD. In addition, treated mice displayed decreased inflammation, amyloid plaques, NFTs, cell death, and an extended life by 2 mo. These results suggest TFP5 as a potential therapeutic, toxicity-free candidate for AD.

Increased Cdk5/p35 activity in the dentate gyrus mediates depressive-like behaviour in rats

Depression is one of the most pervasive and debilitating psychiatric diseases, and the molecular mechanisms underlying the pathophysiology of depression have not been elucidated. Cyclin-dependent kinase 5 (Cdk5) has been implicated in synaptic plasticity underlying learning, memory, and neuropsychiatric disorders. However, whether Cdk5 participates in the development of depressive diseases has not been examined. Using the chronic mild stress (CMS) procedure, we examined the effects of Cdk5/p35 activity in the hippocampus on depressive-like behaviour in rats. We found that CMS increased Cdk5 activity in the hippocampus, accompanied by translocation of neuronal-specific activator p35 from the cytosol to the membrane in the dentate gyrus (DG) subregion. Inhibition of Cdk5 in DG but not in the cornu ammonis 1 (CA1) or CA3 hippocampal subregions inhibited the development of depressive-like symptoms. Overexpression of p35 in DG blocked the antidepressant-like effect of venlafaxine in the CMS model. Moreover, the antidepressants venlafaxine and mirtazapine, but not the antipsychotic aripiprazole, reduced Cdk5 activity through the redistribution of p35 from the membrane to the cytosol in DG. Our results showed that the development of depressive-like behaviour is associated with increased Cdk5 activity in the hippocampus and that the Cdk5/p35 complex plays a key role in the regulation of depressive-like behaviour and antidepressant actions.

GABAA receptor activation in the lateral septum reduces the expression of conditioned defeat and increases aggression in Syrian hamsters

Exposure to social stressors can cause profound changes in an individual's physiology and behavior. In Syrian hamsters, even a single social defeat results in conditioned defeat, which includes an abolishment of territorial aggression and the emergence of high levels of submissive behavior. The purpose of the current study was to determine whether the lateral septum (LS) is a component of the putative neural circuit underlying conditioned defeat. Experiment 1 explored the possibility that plasticity in the LS is necessary for the induction of conditioned defeat. Infusions of the protein synthesis inhibitor, anisomycin, prior to defeat training, however, failed to alter conditioned defeat during testing on the following day, suggesting that synaptic plasticity in the LS is not critical for defeat-induced suppression of aggression. Experiment 2 tested whether the LS is necessary for the expression of conditioned defeat. Infusions of the GABA(A) agonist muscimol into the LS prior to testing significantly increased aggression and decreased submission in previously defeated animals suggesting that the LS is an important component of the neural circuit mediating the expression of both aggression and submission in conditioned defeat. Experiment 3 examined whether the effects of muscimol on aggression were dependent on prior social defeat. Non-defeated animals receiving muscimol infusions prior to testing with a non-aggressive intruder displayed significantly more aggression than did hamsters receiving control injections. Thus, these data suggest that the activation of GABA(A) receptors in the LS increases aggression regardless of whether or not a hamster has previously experienced social defeat.

Venlafaxine facilitates between-session extinction and prevents reinstatement of auditory-cue conditioned fear

Anxiety disorders, characterized by anxiety and fearfulness, are found to be able to cause abnormal emotional responses' associated with memories of negative events, which implicate pressure on society with an increasingly large burden. Better treatment has been of concern to the community. Venlafaxine (VEN), a nonclassical antidepressant agent, is applied in the treatment of social phobia, major depression (MD) and general anxiety disorder (GAD) and, to a certain extent, posttraumatic stress disorder (PTSD), which improves working memory and spatial memory as well as ameliorates emotion by affecting specified brain regions. In this study, we committed to seek a new way for using VEN on treatment of anxiety disorders. To investigate the effect of VEN on extinction of auditory-cue conditioned fear, conditioned rats received a treatment with VEN before extinction training and tests for freezing level of within-session and between-session extinction. To investigate the effect of VEN on reinstatement, all conditioned rats received a treatment with VEN over a period for 21 days. After a rest for 7 days, two tests for freezing level were conducted. We found that: (1) VEN (40mg/kg) treatment at 30min prior to extinction training significantly facilitated the between-session extinction, but not the within-session extinction; (2) chronic administration with VEN (40mg/kg) prevented the return of extinguished auditory-cue fear. These data elucidate the critical role of VEN in auditory-cue fear memory, suggesting that VEN may be an ideal choice for the exposure-based drug treatment and maintenance treatment in patients with GAD, SAD and PTSD.

Cdk5: a multifaceted kinase in neurodegenerative diseases

Since the identification of cyclin-dependent kinase-5 (Cdk5) as a tau kinase and member of the Cdk family almost 20 years ago, deregulation of Cdk5 activity has been linked to an array of neurodegenerative diseases. As knowledge on the etiopathological mechanisms of these diseases evolved through the years, Cdk5 has also been implicated in additional cellular events that are affected under these pathological conditions. From the role of Cdk5 in the regulation of synaptic functions to its involvement in autophagy deregulation, significant insights have been obtained regarding the role of Cdk5 as a key regulator of neurodegeneration. Here, we summarize recent findings on the involvement of Cdk5 in the pathophysiological mechanisms underlying various neurodegenerative diseases.

Fear conditioning and extinction: emotional states encoded by distinct signaling pathways

Conditioning and extinction of fear have traditionally been viewed as two independent learning processes for encoding representations of contexts or cues (conditioned stimuli, CS), aversive events (unconditioned stimuli, US), and their relationship. Based on the analysis of protein kinase signaling patterns in neurons of the fear circuit, we propose that fear and extinction are best conceptualized as emotional states triggered by a single CS representation with two opposing values: aversive and non-aversive. These values are conferred by the presence or absence of the US and encoded by distinct sets of kinase signaling pathways and their downstream targets. Modulating specific protein kinases thus has the potential to modify emotional states, and hence, may emerge as a promising treatment for anxiety disorders.

Chemical modulation of memory formation in larval zebrafish

Whole organism-based small-molecule screens have proven powerful in identifying novel therapeutic chemicals, yet this approach has not been exploited to identify new cognitive enhancers. Here we present an automated high-throughput system for measuring nonassociative learning behaviors in larval zebrafish. Using this system, we report that spaced training blocks of repetitive visual stimuli elicit protein synthesis-dependent long-term habituation in larval zebrafish, lasting up to 24 h. Moreover, repetitive acoustic stimulation induces robust short-term habituation that can be modulated by stimulation frequency and instantaneously dishabituated through cross-modal stimulation. To characterize the neurochemical pathways underlying short-term habituation, we screened 1,760 bioactive compounds with known targets. Although we found extensive functional conservation of short-term learning between larval zebrafish and mammalian models, we also discovered several compounds with previously unknown roles in learning. These compounds included a myristic acid analog known to interact with Src family kinases and an inhibitor of cyclin dependent kinase 2, demonstrating that high-throughput chemical screens combined with high-resolution behavioral assays provide a powerful approach for the discovery of novel cognitive modulators.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

Schizophrenia is associated with dysregulation of a Cdk5 activator that regulates synaptic protein expression and cognition

Cyclin-dependent kinase 5 is activated by small subunits, of which p35 is the most abundant. The functions of cyclin-dependent kinase 5 signalling in cognition and cognitive disorders remains unclear. Here, we show that in schizophrenia, a disorder associated with impaired cognition, p35 expression is reduced in relevant brain regions. Additionally, the expression of septin 7 and OPA1, proteins downstream of truncated p35, is decreased in schizophrenia. Mimicking a reduction of p35 in heterozygous knockout mice is associated with cognitive endophenotypes. Furthermore, a reduction of p35 in mice results in protein changes similar to schizophrenia post-mortem brain. Hence, heterozygous p35 knockout mice model both cognitive endophenotypes and molecular changes reminiscent of schizophrenia. These changes correlate with reduced acetylation of the histone deacetylase 1 target site H3K18 in mice. This site has previously been shown to be affected by truncated p35. By restoring H3K18 acetylation with the clinically used specific histone deacetylase 1 inhibitor MS-275 both cognitive and molecular endophenotypes of schizophrenia can be rescued in p35 heterozygous knockout mice. In summary, we suggest that reduced p35 expression in schizophrenia has an impact on synaptic protein expression and cognition and that these deficits can be rescued, at least in part, by the inhibition of histone deacetylase 1.

Cyclin-dependent kinase 5 activator p25 is generated during memory formation and is reduced at an early stage in Alzheimer's disease

BACKGROUND

The cyclin-dependent kinase 5 activator p35 can be cleaved into p25. Formation of p25 has been suggested to contribute to neurodegeneration in Alzheimer's disease (AD). However, overexpression of low levels of p25 in mice enhances memory formation. Therefore, it has been suggested that p25 formation might be an event early in AD to compensate for impairments in synaptic plasticity. Ongoing p25 formation has been hypothesized to contribute to neurodegeneration at the later stages of AD.

METHODS

Here, we tested the early compensation hypothesis by analyzing the levels of p25 and its precursor p35 in AD postmortem samples from different brain regions at different stages of tau pathology, using quantitative Western blots. Furthermore, we studied p35 and p25 during spatial memory formation. By employing quantitative mass spectrometry, we identified proteins downstream of p25, which were then studied in AD samples.

RESULTS

We found that p25 is generated during spatial memory formation. Furthermore, we demonstrate that overexpression of p25 in the physiological range increases the expression of two proteins implicated in spine formation, septin 7 and optic atrophy 1. We show that the expression of p35 and p25 is reduced as an early event in AD. Moreover, expression of the p25-regulated protein optic atrophy 1 was reduced in a time course similar to p25 expression.

CONCLUSIONS

Our findings suggest that p25 generation is a mechanism underlying hippocampal memory formation that is impaired in the early stages of AD. Our findings argue against the previously raised early compensation hypothesis and they propose that p25-mediated neurotoxicity does not occur in AD.

Cdk5: mediator of neuronal development, death and the response to DNA damage

In the central nervous system, cyclin-dependent kinase 5 (Cdk5), an unusual member of the Cdk family, is implicated in the regulation of various physiological processes ranging from neuronal survival, migration and differentiation, to synaptogenesis, synaptic plasticity and neurotransmission. Dysregulation of this kinase has been demonstrated to play a critical role in the pathogenic process of neurodegenerative disorders. DNA damage is emerging as an important pathological component in various neurodegenerative conditions. In this review, we discuss the recent progress regarding the regulation and roles of Cdk5 under physiological conditions, and its dysregulation under pathological conditions, especially in neuronal death mediated by DNA damage.

Metabotropic glutamate receptor 5/Homer interactions underlie stress effects on fear

BACKGROUND

Glutamatergic transmission is one of the main components of the stress response; nevertheless, its role in the emotional stress sequelae is not known. Here, we investigated whether interactions between group I metabotropic glutamate receptors (metabotropic glutamate receptor 1 and metabotropic glutamate receptor 5 [mGluR5]) and Homer proteins mediate the delayed and persistent enhancement of fear induced by acute stress.

METHODS

Antagonists and inverse agonists of metabotropic glutamate receptor 1 and mGluR5 were injected into the hippocampus after immobilization stress and before contextual fear conditioning. Metabotropic glutamate receptor 5 was displaced from constitutive Homer scaffolds by viral transfection of Homer1a or injection of Tat decoy peptides. The effects of these manipulations on stress-enhanced fear were determined.

RESULTS

We show that stress induces interactions between hippocampal mGluR5 and Homer1a; causes a sustained, ligand-independent mGluR5 activity; and enhances contextual fear. Consistent with this mechanism, enhancement of fear was abolished by delayed poststress application of inverse agonists, but not antagonists, of mGluR5. The effect of stress was mimicked by virally transfected Homer1a or injection of Tat-metabotropic glutamate receptor C-tail decoy peptides into the hippocampus.

CONCLUSIONS

Constitutive activation of mGluR5 is identified as a principal hippocampal mechanism underlying the delayed stress effects on emotion and memory. Inverse agonists, but not antagonists, of mGluR5 are therefore proposed as a preventive treatment option for acute and posttraumatic stress disorders.