Improved reversal learning and altered fear conditioning in transgenic mice with regionally restricted p25 expression

Pharmacological Screening of Viola odorata L. for Memory-Enhancing Effect via Modulation of Oxidative Stress and Inflammatory Biomarkers

Purpose: Alzheimer disease (AD) is a progressive neurodegenerative disorder that is caused by neuroinflammation and oxidative stress. The present study aimed to characterize and then investigate the memory-enhancing potential of Viola odorata methanolic extract in lipopolysaccharide (LPS)–treated mice.

Methods:V. odorata characterization was done by using the GCMS technique. Neuroinflammation was induced by the intracerebroventricular administration of LPS at a dose of 12 µg. Animals were divided randomly into six groups (n = 10). Group I was normal control, which was given vehicle. Group II was disease control, which received LPS (12 µg) via the intracerebroventricular route. Group III was standard, which was administered with donepezil (3 µg) orally for 21 days. Groups IV–VI were the treatment groups, which were administered with the extract at 100, 200, and 400 mg/kg dose levels orally respectively for 21 days. Groups III–VI received LPS (12 µg) on the first day along with their treatments. During the treatment, the animals were assessed for memory retention by employing different behavioral paradigms namely elevated plus maze, passive avoidance, foot shock and open field. Various mediators [endogenous antioxidants, neurotransmitters, and acetylcholinesterase (AChE)] involved in the pathogenesis of AD were quantified by using the UV spectrophotometric method.

Results: Extract-treated groups showed a remarkable improvement in cognitive impairment in all behavioral paradigms. Oxidative stress biomarkers, that is, superoxide dismutase, catalase, and glutathione were raised dose-dependently in the treatment groups with a dose-dependent decrease in the malonaldehyde and AChE levels in the brains of the treated animals. The treatment groups showed decreased levels of inflammatory biomarkers, that is, tumor necrosis factor–alpha, nuclear factor kappa light-chain enhancer of activated β-cells, and cyclo-oxygenase, which supports the therapeutic effectiveness of the treatment.

Conclusion: Based on behavioral, oxidative stress biomarker, and neuroinflammatory data, it is concluded that V. odorata possesses memory-enhancing activity and may prove a beneficial role in the management of AD.

Cdk5 Inhibitory Peptide Prevents Loss of Neurons and Alleviates Behavioral Changes in p25 Transgenic Mice

BACKGROUND

Accumulation of p25 is thought to be a causative risk factor for Alzheimer's disease (AD). As a cleaved product of p35, p25 binds to cyclin-dependent kinase 5 (Cdk5) and leads to the hyperactivity of Cdk5. Then, Cdk5/p25 phosphorylates many pathological substrates related to neurodegenerative diseases. p25 transgenic (Tg) mouse model recaptures some pathological changes of AD, including tau hyperphosphorylation, neurofibrillary tangles, neuroinflammation, and neuronal death, which can be prevented by transgenic expression of Cdk5 inhibitory peptide (CIP) before the insult of p25.

OBJECTIVE

In the present study, we would like to know whether adeno-associated virus serotype-9 (AAV9)-mediated CIP can protect neurons after insult of p25 in p25Tg mice.

METHODS

Administration of AAV9-CIP or control virus were delivered in the brain of p25Tg mice via intracerebroventricular infusions following the induction of p25. Western blotting, immunohistochemistry and immunofluorescence assessment, and animal behavioral evaluation were performed.

RESULTS

Brain atrophy, neuronal death, tau phosphorylation and inflammation in the hippocampus, and cognitive decline were observed in p25Tg mice. Administration of CIP but not the control virus in p25Tg mice reduced levels of tau phosphorylation and inflammation in the hippocampus, which is correlated with inhibition of brain atrophy and neuronal apoptosis in the hippocampus, and improvement of cognitive decline.

CONCLUSION

Our results provide further evidence that the neurotoxicity of p25 can be alleviated by CIP.

Protein Deficiency-Induced Behavioral Abnormalities and Neurotransmitter Loss in Aged Mice Are Ameliorated by Essential Amino Acids

Nutritional epidemiology shows that insufficient protein intake is related to senile dementia. The levels of protein intake in aged people are positively associated with memory function, and elderly people with high protein intake have a low risk of mild cognitive impairment. Although the beneficial roles of protein nutrition in maintaining brain function in aged people are well demonstrated, little is known about the mechanism by which dietary intake of protein affects memory and brain conditions. We fed aged mice a low protein diet (LPD) for 2 months, which caused behavioral abnormalities, and examined the nutritional effect of essential amino acid administration under LPD conditions. The passive avoidance test revealed that LPD mice demonstrated learning and memory impairment. Similarly, the LPD mice showed agitation and hyperactive behavior in the elevated plus maze test. Moreover, LPD mice exhibited decreased concentrations of gamma-aminobutyric acid (GABA), glutamate, glycine, dopamine, norepinephrine, serotonin and aspartate in the brain. Interestingly, oral administration of seven essential amino acids (EAAs; valine, leucine, isoleucine, lysine, phenylalanine, histidine, and tryptophan) to LPD mice, which can be a source of neurotransmitters, reversed those behavioral changes. The oral administration of EAAs restored the brain concentration of glutamate, which is involved in learning and memory ability and may be associated with the observed behavioral changes. Although the details of the link between decreased amino acid and neurotransmitter concentrations and behavioral abnormalities must be examined in future studies, these findings suggest the importance of dietary protein and essential amino acids for maintaining brain function.

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).

The role of septin 7 in physiology and pathological disease: A systematic review of current status

Septins are a conserved family of cytoskeletal GTPases present in different organisms, including yeast, drosophila, Caenorhabditis elegans and humans. In humans, septins are involved in various cellular processes, including exocytosis, apoptosis, leukemogenesis, carcinogenesis and neurodegeneration. Septin 7 is unique out of 13 human septins. Mammalian septin 6, septin 7, septin 2 and septin 9 coisolate together in complexes to form the core unit for the generation of the septin filaments. Physiological septin filaments are hetero‐oligomeric complexes consisting of core septin hexamers and octamers. Furthermore, septin 7 plays a crucial role in cytokinesis and mitosis. Septin 7 is localized to the filopodia and branches of developing hippocampal neurons, and is the most abundant septin in the adult rat forebrain as well as a structural component of the human and mouse sperm annuli. Septin 7 is crucial to the spine morphogenesis and dendrite growth in neurons, and is also a structural constituent of the annulus in human and mouse sperm. It can suppress growth of some tumours such as glioma and papillary thyroid carcinoma. However, the molecular mechanisms of involvement of septin 7 in human disease, especially in the development of cancer, remain unclear. This review focuses on the structure, function and mechanism of septin 7 in vivo, and summarizes the role of septin 7 in cell proliferation, cytokinesis, nervous and reproductive systems, as well as the underlying molecular events linking septin 7 to various diseases, such as Alzheimer's disease, schizophrenia, neuropsychiatric systemic lupus erythematosus, tumour and so on.

Anti-aggregant tau mutant promotes neurogenesis

BACKGROUND

The microtubule-associated protein Tau plays a role in neurodegeneration as well as neurogenesis. Previous work has shown that the expression of the pro-aggregant mutant Tau repeat domain causes strong aggregation and pronounced neuronal loss in the hippocampus whereas the anti-aggregant form has no deleterious effects. These two proteins differ mainly in their propensity to form ß structure and hence to aggregate.

METHODS

To elucidate the basis of these contrasting effects, we analyzed organotypic hippocampal slice cultures (OHSCs) from transgenic mice expressing the repeat domain (RD) of Tau with the anti-aggregant mutation (Tau^RDΔKPP) and compared them with slices containing pro-aggregant Tau^RDΔK. Transgene expression in the hippocampus was monitored via a sensitive bioluminescence reporter gene assay (luciferase).

RESULTS

The expression of the anti-aggregant Tau^RDΔKPP leads to a larger volume of the hippocampus at a young age due to enhanced neurogenesis, resulting in an increase in neuronal number. There were no signs of activation of microglia and astrocytes, indicating the absence of an inflammatory reaction. Investigation of signaling pathways showed that Wnt-5a was strongly decreased whereas Wnt3 was increased. A pronounced increase in hippocampal stem cell proliferation (seen by BrdU) was observed as early as P8, in the CA regions where neurogenesis is normally not observed. The increase in neurons persisted up to 16 months of age.

CONCLUSION

The data suggest that the expression of anti-aggregant Tau^RDΔKPP enhances hippocampal neurogenesis mediated by the canonical Wnt signaling pathway, without an inflammatory reaction. This study points to a role of tau in brain development and neurogenesis, in contrast to its detrimental role in neurodegeneration at later age.

Clinical role and biological function of CDK5 in hepatocellular carcinoma: A study based on immunohistochemistry, RNA-seq and in vitro investigation

To investigate the clinical role and biological function of cyclin-dependent kinase 5 (CDK5) in hepatocellular carcinoma (HCC), 412 surgically resected tissue samples (HCC, n=171; non-HCC=241) were obtained and analyzed with immunohistochemistry. The diagnostic and prognostic values of CDK5 expression levels in HCC were clarified. Moreover, RNA-seq data or microarray datasets from The Cancer Genome Atlas (TCGA) (HCC, n=374; normal, n=50) or other public databases (HCC, n=1864; non-tumor=1995) regarding CDK5 in HCC were extracted and examined. Several bioinformatic methods were performed to identify CDK5-regulated pathways. In vitro experiments were adopted to measure proliferation and apoptosis in HCC cells after CDK5 mRNA was inhibited in the HCC cell lines HepG2 and HepB3. Based on immunohistochemistry, CDK5 expression levels were notably increased in HCC tissues (n=171) compared with normal (n=33, P<0.001), cirrhosis (n=37, P<0.001), and adjacent non-cancerous liver (n=171, P<0.001) tissues. The up-regulation of CDK5 was associated with higher differentiation (P<0.001), metastasis (P<0.001), advanced clinical TNM stages (P<0.001), portal vein tumor embolus (P=0.003) and vascular invasion (P=0.004). Additionally, TCGA data analysis also revealed significantly increased CDK5 expression in HCC compared with non-cancerous hepatic tissues (P<0.001). The pooled standard mean deviation (SMD) based on 36 included datasets (HCC, n=2238; non-cancerous, n=2045) indicated that CDK5 was up-regulated in HCC (SMD=1.23, 95% CI: 1.00-1.45, P<0.001). The area under the curve (AUC) of the summary receiver operating characteristic (SROC) curve was 0.88. Furthermore, CDK5 knock-down inhibited proliferation and promoted apoptosis. In conclusion, CDK5 plays an essential role in the initiation and progression of HCC, most likely via accelerating proliferation and suppressing apoptosis in HCC cells by regulating the cell cycle and DNA replication pathways.

TFP5, a Peptide Inhibitor of Aberrant and Hyperactive Cdk5/p25, Attenuates Pathological Phenotypes and Restores Synaptic Function in CK-p25Tg Mice

It has been reported that cyclin-dependent kinase 5 (cdk5), a critical neuronal kinase, is hyperactivated in Alzheimer's disease (AD) and may be, in part, responsible for the hallmark pathology of amyloid plaques and neurofibrillary tangles (NFTs). It has been proposed by several laboratories that hyperactive cdk5 results from the overexpression of p25 (a truncated fragment of p35, the normal cdk5 regulator), which, when complexed to cdk5, induces hyperactivity, hyperphosphorylated tau/NFTs, amyloid-β plaques, and neuronal death. It has previously been shown that intraperitoneal (i.p.) injections of a modified truncated 24-aa peptide (TFP5), derived from the cdk5 activator p35, penetrated the blood-brain barrier and significantly rescued AD-like pathology in 5XFAD model mice. The principal pathology in the 5XFAD mutant, however, is extensive amyloid plaques; hence, as a proof of concept, we believe it is essential to demonstrate the peptide's efficacy in a mouse model expressing high levels of p25, such as the inducible CK-p25Tg model mouse that overexpresses p25 in CamKII positive neurons. Using a modified TFP5 treatment, here we show that peptide i.p. injections in these mice decrease cdk5 hyperactivity, tau, neurofilament-M/H hyperphosphorylation, and restore synaptic function and behavior (i.e., spatial working memory, motor deficit using Rota-rod). It is noteworthy that TFP5 does not inhibit endogenous cdk5/p35 activity, nor other cdks in vivo suggesting it might have no toxic side effects, and may serve as an excellent therapeutic candidate for neurodegenerative disorders expressing abnormally high brain levels of p25 and hyperactive cdk5.

Phosphorylation of K+ channels at single residues regulates memory formation

Phosphorylation is a ubiquitous post-translational modification of proteins, and a known physiological regulator of K⁺ channel function. Phosphorylation of K⁺ channels by kinases has long been presumed to regulate neuronal processing and behavior. Although circumstantial evidence has accumulated from behavioral studies of vertebrates and invertebrates, the contribution to memory of single phosphorylation sites on K⁺ channels has never been reported. We have used gene targeting in mice to inactivate protein kinase A substrate residues in the fast-inactivating subunit K_v4.2 (T38A mutants), and in the small-conductance Ca²⁺-activated subunit SK1 (S105A mutants). Both manipulations perturbed a specific form of memory, leaving others intact. T38A mutants had enhanced spatial memory for at least 4 wk after training, whereas performance in three tests of fear memory was unaffected. S105A mutants were impaired in passive avoidance memory, sparing fear, and spatial memory. Together with recent findings that excitability governs the participation of neurons in a memory circuit, this result suggests that the memory type supported by neurons may depend critically on the phosphorylation of specific K⁺ channels at single residues.

The Notch intracellular domain represses CRE-dependent transcription

Members of the cyclic-AMP response-element binding protein (CREB) transcription factor family regulate the expression of genes needed for long-term memory formation. Loss of Notch impairs long-term, but not short-term, memory in flies and mammals. We investigated if the Notch-1 (N1) exerts an effect on CREB-dependent gene transcription. We observed that N1 inhibits CREB mediated activation of cyclic-AMP response element (CRE) containing promoters in a γ-secretase-dependent manner. We went on to find that the γ-cleaved N1 intracellular domain (N1ICD) sequesters nuclear CREB1α, inhibits cAMP/PKA-mediated neurite outgrowth and represses the expression of specific CREB regulated genes associated with learning and memory in primary cortical neurons. Similar transcriptional effects were observed with the N2ICD, N3ICD and N4ICDs. Together, these observations indicate that the effects of Notch on learning and memory are, at least in part, via an effect on CREB-regulated gene expression.

Activity-dependent p25 generation regulates synaptic plasticity and Aβ-induced cognitive impairment

Cyclin-dependent kinase 5 regulates numerous neuronal functions with its activator, p35. Under neurotoxic conditions, p35 undergoes proteolytic cleavage to liberate p25, which has been implicated in various neurodegenerative diseases. Here, we show that p25 is generated following neuronal activity under physiological conditions in a GluN2B- and CaMKIIα-dependent manner. Moreover, we developed a knockin mouse model in which endogenous p35 is replaced with a calpain-resistant mutant p35 (Δp35KI) to prevent p25 generation. The Δp35KI mice exhibit impaired long-term depression and defective memory extinction, likely mediated through persistent GluA1 phosphorylation at Ser845. Finally, crossing the Δp35KI mice with the 5XFAD mouse model of Alzheimer's disease (AD) resulted in an amelioration of β-amyloid (Aβ)-induced synaptic depression and cognitive impairment. Together, these results reveal a physiological role of p25 production in synaptic plasticity and memory and provide new insights into the function of p25 in Aβ-associated neurotoxicity and AD-like pathology.

Memory enhancement by targeting Cdk5 regulation of NR2B

Many psychiatric and neurological disorders are characterized by learning and memory deficits, for which cognitive enhancement is considered a valid treatment strategy. The N-methyl-D-aspartate receptor (NMDAR) is a prime target for the development of cognitive enhancers because of its fundamental role in learning and memory. In particular, the NMDAR subunit NR2B improves synaptic plasticity and memory when overexpressed in neurons. However, NR2B regulation is not well understood and no therapies potentiating NMDAR function have been developed. Here, we show that serine 1116 of NR2B is phosphorylated by cyclin-dependent kinase 5 (Cdk5). Cdk5-dependent NR2B phosphorylation is regulated by neuronal activity and controls the receptor's cell surface expression. Disrupting NR2B-Cdk5 interaction via a small interfering peptide (siP) increases NR2B surface levels, facilitates synaptic transmission, and improves memory formation in vivo. Our results reveal a regulatory mechanism critical to NR2B function that can be targeted for the development of cognitive enhancers.

VIDEO ABSTRACT

Conditional Loss of Arx From the Developing Dorsal Telencephalon Results in Behavioral Phenotypes Resembling Mild Human ARX Mutations

Mutations in the Aristaless-Related Homeobox (ARX) gene cause structural anomalies of the brain, epilepsy, and neurocognitive deficits in children. During forebrain development, Arx is expressed in both pallial and subpallial progenitor cells. We previously demonstrated that elimination of Arx from subpallial-derived cortical interneurons generates an epilepsy phenotype with features overlapping those seen in patients with ARX mutations. In this report, we have selectively removed Arx from pallial progenitor cells that give rise to the cerebral cortical projection neurons. While no discernable seizure activity was recorded, these mice exhibited a peculiar constellation of behaviors. They are less anxious, less social, and more active when compared with their wild-type littermates. The overall cortical thickness was reduced, and the corpus callosum and anterior commissure were hypoplastic, consistent with a perturbation in cortical connectivity. Taken together, these data suggest that some of the structural and behavioral anomalies, common in patients with ARX mutations, are specifically due to alterations in pallial progenitor function. Furthermore, our data demonstrate that some of the neurobehavioral features found in patients with ARX mutations may not be due to on-going seizures, as is often postulated, given that epilepsy was eliminated as a confounding variable in these behavior analyses.

Generation of the Cdk5 activator p25 is a memory mechanism that is affected in early Alzheimer's disease

About 15 years ago it was proposed that generation of the truncated protein p25 contributes to toxicity in Alzheimer's disease (AD). p25 is a calcium-dependent degradation product of p35, the principal activator of cyclin-dependent kinase 5 (Cdk5). The biochemical properties of p25 suggested that its generation would cause Cdk5 overactivation and tau hyperphosphorylation, a prerequisite for neurofibrillary tangle (NFT) formation. Whilst this model was appealing as it explained NFT formation, many laboratories could not confirm the finding of increased p25 generation in brain from AD patients. On the contrary, it emerged that p25 levels are reduced in AD. This reduction occurs primarily in the early stages of the disease. Further, p25 generation in the mouse hippocampus is associated with normal memory formation and p25 overexpression enhances synaptogenesis. Therefore, it transpires that p25 generation is a molecular memory mechanism that is impaired in early AD. I discuss the prospect that investigation of p25-regulated proteins will shed light into mechanisms underlying synaptic degeneration associated with memory decline in AD.

Using mice to model Alzheimer's dementia: an overview of the clinical disease and the preclinical behavioral changes in 10 mouse models

The goal of this review is to discuss how behavioral tests in mice relate to the pathological and neuropsychological features seen in human Alzheimer's disease (AD), and present a comprehensive analysis of the temporal progression of behavioral impairments in commonly used AD mouse models that contain mutations in amyloid precursor protein (APP). We begin with a brief overview of the neuropathological changes seen in the AD brain and an outline of some of the clinical neuropsychological assessments used to measure cognitive deficits associated with the disease. This is followed by a critical assessment of behavioral tasks that are used in AD mice to model the cognitive changes seen in the human disease. Behavioral tests discussed include spatial memory tests [Morris water maze (MWM), radial arm water maze (RAWM), Barnes maze], associative learning tasks (passive avoidance, fear conditioning), alternation tasks (Y-Maze/T-Maze), recognition memory tasks (Novel Object Recognition), attentional tasks (3 and 5 choice serial reaction time), set-shifting tasks, and reversal learning tasks. We discuss the strengths and weaknesses of each of these behavioral tasks, and how they may correlate with clinical assessments in humans. Finally, the temporal progression of both cognitive and non-cognitive deficits in 10 AD mouse models (PDAPP, TG2576, APP23, TgCRND8, J20, APP/PS1, TG2576 + PS1 (M146L), APP/PS1 KI, 5×FAD, and 3×Tg-AD) are discussed in detail. Mouse models of AD and the behavioral tasks used in conjunction with those models are immensely important in contributing to our knowledge of disease progression and are a useful tool to study AD pathophysiology and the resulting cognitive deficits. However, investigators need to be aware of the potential weaknesses of the available preclinical models in terms of their ability to model cognitive changes observed in human AD. It is our hope that this review will assist investigators in selecting an appropriate mouse model, and accompanying behavioral paradigms to investigate different aspects of AD pathology and disease progression.

The role of Cdk5 in neuroendocrine thyroid cancer

Medullary thyroid carcinoma (MTC) is a neuroendocrine cancer that originates from calcitonin-secreting parafollicular cells, or C cells. We found that Cdk5 and its cofactors p35 and p25 are highly expressed in human MTC and that Cdk5 activity promotes MTC proliferation. A conditional MTC mouse model was generated and corroborated the role of aberrant Cdk5 activation in MTC. C cell-specific overexpression of p25 caused rapid C cell hyperplasia leading to lethal MTC, which was arrested by repressing p25 overexpression. A comparative phosphoproteomic screen between proliferating and arrested MTC identified the retinoblastoma protein (Rb) as a crucial Cdk5 downstream target. Prevention of Rb phosphorylation at Ser807/Ser811 attenuated MTC proliferation. These findings implicate Cdk5 signaling via Rb as critical to MTC tumorigenesis and progression.

A touch screen-automated cognitive test battery reveals impaired attention, memory abnormalities, and increased response inhibition in the TgCRND8 mouse model of Alzheimer's disease

Transgenic mouse models of Alzheimer's disease (AD) with abundant β-amyloid develop memory impairments. However, multiple nonmnemonic cognitive domains such as attention and executive control are also compromised early in AD individuals, but have not been routinely assessed in animal models. Here, we assessed the cognitive abilities of TgCRND8 mice—a widely used model of β-amyloid pathology—with a touch screen-based automated test battery. The test battery comprises highly translatable tests of multiple cognitive constructs impaired in human AD, such as memory, attention, and response control, as well as appropriate control tasks. We found that familial AD mutations affect not only memory, but also cause significant alterations of sustained attention and behavioral flexibility. Because changes in attention and response inhibition may affect performance on tests of other cognitive abilities including memory, our findings have important consequences for the assessment of disease mechanisms and therapeutics in animal models of AD. A more comprehensive phenotyping with specialized, multicomponent cognitive test batteries for mice might significantly advance translation from preclinical mouse studies to the clinic.

Enhancement of cognitive function in models of brain disease through environmental enrichment and physical activity

This review will provide an overview of the non-drug based approaches that have been demonstrated to enhance cognitive function of the compromised brain, primarily focussed on the two most widely adopted paradigms of environmental enrichment and enhanced physical exercise. Environmental enrichment involves the generation of novelty and complexity in animal housing conditions which facilitates enhanced sensory and cognitive stimulation as well as physical activity. In a wide variety of animal models of brain disorders, environmental enrichment and exercise have been found to have beneficial effects, including cognitive enhancement, delayed disease onset, enhanced cellular plasticity and associated molecular processes. Potential cellular and molecular mechanisms will also be discussed, which have relevance for the future development of 'enviromimetics', drugs which could mimic or enhance the beneficial effects of environmental stimulation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Cyclin-dependent kinases in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multifaceted serine/threonine kinase protein with important roles in the nervous system. Two related proteins, p35 and p39, activate Cdk5 upon direct binding. Over the past decade, Cdk5 activity has been demonstrated to regulate many events during brain development, including neuronal migration as well as axon and dendrite development. Recent evidence also suggests a pivotal role for Cdk5 in synaptic plasticity, behavior, and cognition. Dysfunction of Cdk5 has been implicated in a number of neurological disorders and neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Niemann-Pick type C disease, and ischemia. Hyperactivation of Cdk5 due to the conversion of p35 to p25 by the calcium-dependent protease calpain during neurotoxicity also contributes to the pathological state. This review surveys recent literature surrounding Cdk5 in synaptic plasticity and homeostasis, with particular emphasis on Cdk5 kinase activity under neurodegenerative conditions.

Silencing of CDK5 as potential therapy for Alzheimer's disease

Neurodegeneration is one of the greatest public health challenges for the 21st century. Among neurodegenerative diseases, Alzheimer's disease (AD) is the most prevalent and best characterized. Nevertheless, despite the large investment in AD research, currently there is no effective therapeutic option. In the present review, we highlight a novel alternative, which takes advantage of the biotechnological outbreak deployed by the discovery of the RNA interference-based gene silencing mechanism, and its application as a tool for neurodegeneration treatment. Here, we highlight cyclin-dependent kinase 5 (CDK5) as a key candidate target for therapeutic gene silencing. Unlike other members of the cyclin-dependent kinase family, CDK5 does not seem to play a crucial role in cell cycle regulation. By contrast, CDK5 participates in multiple functions during nervous system development and has been established as a key mediator of Tau hyperphosphorylation and neurofibrillary pathology, thus serving as an optimal candidate for targeted therapy in the adult nervous system. We propose that the use of RNA interference for CDK5 silencing presents an attractive and specific therapeutic alternative for AD and perhaps against other tauopathies.

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.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Insulin receptor substrate 2 is a negative regulator of memory formation

Insulin has been shown to impact on learning and memory in both humans and animals, but the downstream signaling mechanisms involved are poorly characterized. Insulin receptor substrate-2 (Irs2) is an adaptor protein that couples activation of insulin- and insulin-like growth factor-1 receptors to downstream signaling pathways. Here, we have deleted Irs2, either in the whole brain or selectively in the forebrain, using the nestin Cre- or D6 Cre-deleter mouse lines, respectively. We show that brain- and forebrain-specific Irs2 knockout mice have enhanced hippocampal spatial reference memory. Furthermore, NesCreIrs2KO mice have enhanced spatial working memory and contextual- and cued-fear memory. Deletion of Irs2 in the brain also increases PSD-95 expression and the density of dendritic spines in hippocampal area CA1, possibly reflecting an increase in the number of excitatory synapses per neuron in the hippocampus that can become activated during memory formation. This increase in activated excitatory synapses might underlie the improved hippocampal memory formation observed in NesCreIrs2KO mice. Overall, these results suggest that Irs2 acts as a negative regulator on memory formation by restricting dendritic spine generation.

Striatal-enriched protein tyrosine phosphatase (STEP) knockout mice have enhanced hippocampal memory

Striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific phosphatase that opposes synaptic strengthening by the regulation of key synaptic signaling proteins. Previous studies suggest a possible role for STEP in learning and memory. To demonstrate the functional importance of STEP in learning and memory, we generated STEP knockout (KO) mice and examined the effect of deletion of STEP on behavioral performance, as well as the phosphorylation and expression of its substrates. Here we report that loss of STEP leads to significantly enhanced performance in hippocampal-dependent learning and memory tasks. In addition, STEP KO mice displayed greater dominance behavior, although they were normal in their motivation, motor coordination, visual acuity and social interactions. STEP KO mice displayed enhanced tyrosine phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2), the NR2B subunit of the N-methyl-D-aspartate receptor (NMDAR) and proline-rich tyrosine kinase (Pyk2), as well as an increased phosphorylation of ERK1/2 substrates. Concomitant with the increased phosphorylation of NR2B, synaptosomal expression of NR1/NR2B NMDARs was increased in STEP KO mice, as was the GluR1/GluR2 containing α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs), providing a potential molecular mechanism for the improved cognitive performance. The data support a role for STEP in the regulation of synaptic strengthening. The absence of STEP improves cognitive performance, and may do so by the regulation of downstream effectors necessary for synaptic transmission.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Does Huntingtin play a role in selective macroautophagy?

The accumulation of protein aggregates in neurons appears to be a basic feature of neurodegenerative disease. In Huntington's Disease (HD), a progressive and ultimately fatal neurodegenerative disorder caused by an expansion of the polyglutamine repeat within the protein Huntingtin (Htt), the immediate proximal cause of disease is well understood. However, the cellular mechanisms which modulate the rate at which fragments of Htt containing polyglutamine accumulate in neurons is a central issue in the development of approaches to modulate the rate and extent of neuronal loss in this disease. We have recently found that Htt is phosphorylated by the kinase IKK on serine (S) 13, activating its phosphorylation on S16 and its acetylation and poly-SUMOylation, modifications that modulate its clearance by the proteasome and lysosome in cells. In the discussion here I suggest that Htt may have a normal function in the lysosomal mechanism of selective macroautophagy involved in its own degradation which may share some similarity with the yeast cytoplasm to vacuole targeting (Cvt) pathway. Pharmacologic activation of this pathway may be useful early in disease progression to treat HD and other neurodegenerative diseases characterized by the accumulation of disease proteins.

Predictably irrational: assaying cognitive inflexibility in mouse models of schizophrenia

The development of sophisticated, translatable mouse-based assays modeling the behavioral manifestations of neuropsychiatric diseases, such as schizophrenia, has lagged the advances in molecular and genomic techniques. Our laboratory has made efforts to fill this gap by investing in the development of novel assays, including adapting a touchscreen-based method for measuring cognitive and executive functions for use in mice. As part of these efforts, a recent study by Brigman et al. (2009) investigated the effects of subchronic phencyclidine treatment on mouse touchscreen-based pairwise visual discrimination and reversal learning. Here, we summarize the results of that study, and place them in the larger context of ongoing efforts to develop valid mouse "models" of schizophrenia, with a focus on reversal learning and other measures of cognitive flexibility. Touchscreen-based systems could provide a tractable platform for fully utilizing the mouse to elucidate the pathophysiology of cognitive inflexibility in schizophrenia and other neuropsychiatric disorders.

Towards a molecular understanding of sex differences in memory formation

Sex differences exist in brain function and behavior. However, the underlying molecular mechanisms are only beginning to emerge. Recent studies in rodents have revealed molecular mechanisms underlying sex differences in memory formation. It is becoming clear that sex differences are not simply reflective of differences in sex hormones, but also reflect distinctions in synaptic signaling mechanisms including the role of synaptic kinases. Furthermore, there are sex differences in the activation of transcription factors and gene transcription during memory formation. This review discusses emerging evidence in the field and how these findings are providing a first step towards a molecular understanding of how sex differences impact on memory formation both in health and disease.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Crosstalk between Cdk5 and GSK3beta: Implications for Alzheimer's Disease

GSK3β and Cdk5 are the two kinases in the center of research on Alzheimer's disease (AD), involved in the pathological symptoms of AD, Aβ plaque formation, tau hyperphosphorylation and neurodegeneration. So far, both kinases have mostly been examined in isolation, leading to a schism of the research field into defenders of the GSK3β-versus the Cdk5 hypotheses of AD. However, in this debate the fact that activities of GSK3β and Cdk5 can influence each other deserves more attention. Recent evidence from p25 transgenic mice suggests that there is a dynamic crosstalk: during aging or prolonged overactivation of Cdk5, GSK3β activity may alter in favor of AD pathogenesis. In this review we summarize the connections between GSK3β and Cdk5 and discuss implications for AD hypotheses.

Nicotinamide restores cognition in Alzheimer's disease transgenic mice via a mechanism involving sirtuin inhibition and selective reduction of Thr231-phosphotau

Memory loss is the signature feature of Alzheimer's disease, and therapies that prevent or delay its onset are urgently needed. Effective preventive strategies likely offer the greatest and most widespread benefits. Histone deacetylase (HDAC) inhibitors increase histone acetylation and enhance memory and synaptic plasticity. We evaluated the efficacy of nicotinamide, a competitive inhibitor of the sirtuins or class III NAD(+)-dependent HDACs in 3xTg-AD mice, and found that it restored cognitive deficits associated with pathology. Nicotinamide selectively reduces a specific phospho-species of tau (Thr231) that is associated with microtubule depolymerization, in a manner similar to inhibition of SirT1. Nicotinamide also dramatically increased acetylated alpha-tubulin, a primary substrate of SirT2, and MAP2c, both of which are linked to increased microtubule stability. Reduced phosphoThr231-tau was related to a reduction of monoubiquitin-conjugated tau, suggesting that this posttranslationally modified form of tau may be rapidly degraded. Overexpression of a Thr231-phospho-mimic tau in vitro increased clearance and decreased accumulation of tau compared with wild-type tau. These preclinical findings suggest that oral nicotinamide may represent a safe treatment for AD and other tauopathies, and that phosphorylation of tau at Thr231 may regulate tau stability.

Unravelling the complex role of Cdk5 in the developing cerebral cortex

The normal development of the mammalian CNS is entirely dependent on the coordinated behavior of its cellular components. Particular importance is attributed to the correct morphology, migration and communication of neurons. Recent years have seen the identification of many extracellular, cell surface and intracellular signaling molecules that are important for normal CNS development, consequently triggering huge progress in our understanding of the complex processes involved. A key molecule to emerge is Cdk5. To date, Cdk5 has been functionally linked with controlled neuronal morphology, migration, synaptic function, cognition, drug addiction, neuronal death and neurodegeneration. The complexity of its function has been confirmed by the ever increasing number of diverse upstream regulators, protein substrates and biological consequences of altered catalytic function. The aim of this review is to consolidate recent findings concerning the role of Cdk5 in the developing nervous system, particularly the cerebral cortex, where its importance is most clearly evidenced.

Neurodegeneration and neuroinflammation in cdk5/p25-inducible mice: a model for hippocampal sclerosis and neocortical degeneration

The cyclin-dependent kinase cdk5 is atypically active in postmitotic neurons and enigmatic among the kinases proposed as molecular actors in neurodegeneration. We generated transgenic mice to express p25, the N-terminally truncated p35 activator of cdk5, in forebrain under tetracycline control (TET-off). Neuronal expression of p25 (p25(ON)) caused high mortality postnatally and early in life. Mortality was completely prevented by administration of doxycycline in the drinking water of pregnant dams and litters until P42, allowing us to study the action of p25 in adult mouse forebrain. Neuronal p25 triggered neurodegeneration and also microgliosis, rapidly and intensely in hippocampus and cortex. Progressive neurodegeneration was severe with marked neuron loss, causing brain atrophy (40% loss at age 5 months) with nearly complete elimination of the hippocampus. Neurodegeneration did not involve phosphorylation of protein tau or generation of amyloid peptide. Degenerating neurons did not stain for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling or activated caspase-3 but were marked by FluoroJadeB in early stages. Diseased neurons were always closely associated with activated microglia already very early in the disease process. Primary neurons derived from p25 embryos were more prone to apoptosis than wild-type neurons, and they activated microglial cells in co-culture. The inducible p25 mice present as a model for neurodegeneration in hippocampal sclerosis and neocortical degeneration, with important contributions of activated microglia.

The potential role of cyclin-dependent kinase 5 in focal cortical dysplasia

Focal cortical dysplasia (FCD) is the most common malformation of cortical development found in epilepsy surgical series. Characterised by cortical mislamination, dysplastic neurons and, in a subgroup of cases, balloon cells, FCD is potently epileptogenic. Despite decades of study, the underlying aetiology of FCD remains uncertain and research has been hampered by the lack of a good animal model in which to simulate the condition. In this article we review some of the potential molecular mechanisms that might underpin human FCD. In particular we examine the potential role of cyclin-dependent kinase 5 and its principal activator p35 in FCD and estimate the contribution that deregulation of cyclin-dependent kinase 5 might make to the pathogenesis of this condition.

Sex-dependent up-regulation of two splicing factors, Psf and Srp20, during hippocampal memory formation

Gene transcription is required for long-term memory (LTM) formation. LTM formation is impaired in a male-specific manner in mice lacking either of the two Ca(2+)/calmodulin-dependent kinase kinase (Camkk) genes. Since altered transcription was suggested to cause these impairments in LTM formation, we used microarrays to screen for CaMKKbeta-dependent gene expression changes. Here we show that the hippocampal mRNA expression of two splicing factors, splicing factor arginine/serine-rich 3 (Sfrs3/Srp20) and polypyrimidine tract-binding protein-associated splicing factor (Psf), is altered in CaMKKbeta-deficient males. In wild-type (WT) mice, the basal expression level in the hippocampus is higher in males than in females, and the sex difference in Srp20 expression is detectable before puberty. Training in two hippocampus-dependent learning tasks, the spatial version of the Morris water maze (MWM) and background contextual fear conditioning, increases the hippocampal mRNA expression of both splicing factors in WT males. However, the increase in Srp20 mRNA expression occurs only in males and not in females, whereas the up-regulation of Psf expression occurs in both sexes. Importantly, control experiments demonstrate that the up-regulation of both splicing factors is specific for the learned associations after contextual fear conditioning. In summary, we provide the first evidence for a regulation of splicing factors during LTM formation and we suggest that alternative splicing contributes to sex differences in LTM formation.

Alternative roles for Cdk5 in learning and synaptic plasticity

Protein kinases mediate the intracellular signal transduction pathways controlling synaptic plasticity in the central nervous system. While the majority of protein kinases achieve this function via the phosphorylation of synaptic substrates, some kinases may contribute through alternative mechanisms in addition to enzymatic activity. There is growing evidence that protein kinases may often play structural roles in plasticity as well. Cyclin-dependent kinase 5 (Cdk5) has been implicated in learning and synaptic plasticity. Initial scrutiny focused on its enzymatic activity using pharmacological inhibitors and genetic modifications of Cdk5 cofactors. Quite recently Cdk5 has been shown to govern learning and plasticity via regulation of glutamate receptor degradation, a function that may not dependent on phosphorylation of downstream effectors. From these new studies, two roles emerge for Cdk5 in plasticity: one in which it controls structural plasticity via phosphorylation of synaptic substrates, and a second where it regulates functional plasticity via protein-protein interactions.

Collapsin response mediator protein-2 hyperphosphorylation is an early event in Alzheimer's disease progression

Collapsin response mediator protein 2 (CRMP2) is an abundant brain-enriched protein that can regulate microtubule assembly in neurons. This function of CRMP2 is regulated by phosphorylation by glycogen synthase kinase 3 (GSK3) and cyclin-dependent kinase 5 (Cdk5). Here, using novel phosphospecific antibodies, we demonstrate that phosphorylation of CRMP2 at Ser522 (Cdk5-mediated) is increased in Alzheimer's disease (AD) brain, while CRMP2 expression and phosphorylation of the closely related isoform CRMP4 are not altered. In addition, CRMP2 phosphorylation at the Cdk5 and GSK3 sites is increased in cortex and hippocampus of the triple transgenic mouse [presenilin-1 (PS1)(M146V)KI; Thy1.2-amyloid precursor protein (APP)(swe); Thy1.2tau(P301L)] that develops AD-like plaques and tangles, as well as the double (PS1(M146V)KI; Thy1.2-APP(swe)) transgenic mouse. The hyperphosphorylation is similar in magnitude to that in human AD and is evident by 2 months of age, ahead of plaque or tangle formation. Meanwhile, there is no change in CRMP2 phosphorylation in two other transgenic mouse lines that display elevated amyloid beta peptide levels (Tg2576 and APP/amyloid beta-binding alcohol dehydrogenase). Similarly, CRMP2 phosphorylation is normal in hippocampus and cortex of Tau(P301L) mice that develop tangles but not plaques. These observations implicate hyperphosphorylation of CRMP2 as an early event in the development of AD and suggest that it can be induced by a severe APP over-expression and/or processing defect.

A hippocampal Cdk5 pathway regulates extinction of contextual fear

Treatment of emotional disorders involves the promotion of extinction processes, which are defined as the learned reduction of fear. The molecular mechanisms underlying extinction have only begun to be elucidated. By employing genetic and pharmacological approaches in mice, we show here that extinction requires downregulation of Rac-1 and cyclin-dependent kinase 5 (Cdk5), and upregulation of p21 activated kinase-1 (PAK-1) activity. This is physiologically achieved by a Rac-1-dependent relocation of the Cdk5 activator p35 from the membrane to the cytosol and dissociation of p35 from PAK-1. Moreover, our data suggest that Cdk5/p35 activity prevents extinction in part by inhibition of PAK-1 activity in a Rac-1-dependent manner. We propose that extinction of contextual fear is regulated by counteracting components of a molecular pathway involving Rac-1, Cdk5 and PAK-1. Our data suggest that this pathway could provide a suitable target for therapeutic treatment of emotional disorders.

Structure-activity relationships of 3,4-dihydro-1H-quinazolin-2-one derivatives as potential CDK5 inhibitors

Cyclin-dependent kinase 5 (CDK5) is a serine/threonine kinase that plays a critical role in the early development of the nervous system. Deregulation of CDK5 is believed to contribute to the abnormal phosphorylation of various cellular substrates associated with neurodegenerative disorders such as Alzheimer's disease, amyotrophic lateral sclerosis, and ischemic stroke. Acyclic urea 3 was identified as a potent CDK5 inhibitor and co-crystallographic data of urea 3/CDK2 enzyme were used to design a novel series of 3,4-dihydroquinazolin-2(1H)-ones as CDK5 inhibitors. In this investigation we present our synthetic studies toward this series of compounds and discuss their biological relevance as CDK5 inhibitors.

Activation of cyclin-dependent kinase 5 by calpains contributes to human immunodeficiency virus-induced neurotoxicity

Although the specific mechanism of neuronal damage in human immunodeficiency virus (HIV) -associated dementia is not known, a prominent role for NMDA receptor (NMDAR)-induced excitotoxicity has been demonstrated in neurons exposed to HIV-infected/activated macrophages. We hypothesized NMDAR-mediated activation of the calcium-dependent protease, calpain, would contribute to cell death by induction of cyclin-dependent kinase 5 (CDK5) activity. Using an in vitro model of HIV neurotoxicity, in which primary rat cortical cultures are exposed to supernatants from primary human HIV-infected macrophages, we have observed increased calpain-dependent cleavage of the CDK5 regulatory subunit, p35, to the constitutively active isoform, p25. Formation of p25 is dependent upon NMDAR activation and calpain activity and is coincident with increased CDK5 activity in this model. Further, inhibition of CDK5 by roscovitine provided neuroprotection in our in vitro model. Consistent with our observations in vitro, we have observed a significant increase in calpain activity and p25 levels in midfrontal cortex of patients infected with HIV, particularly those with HIV-associated cognitive impairment. Taken together, our data suggest calpain activation of CDK5, a pathway activated in HIV-infected individuals, can mediate neuronal damage and death in a model of HIV-induced neurotoxicity.

Cyclin-dependent kinase 5 governs learning and synaptic plasticity via control of NMDAR degradation

Learning is accompanied by modulation of postsynaptic signal transduction pathways in neurons. Although the neuronal protein kinase cyclin-dependent kinase 5 (Cdk5) has been implicated in cognitive disorders, its role in learning has been obscured by the perinatal lethality of constitutive knockout mice. Here we report that conditional knockout of Cdk5 in the adult mouse brain improved performance in spatial learning tasks and enhanced hippocampal long-term potentiation and NMDA receptor (NMDAR)-mediated excitatory postsynaptic currents. Enhanced synaptic plasticity in Cdk5 knockout mice was attributed to reduced NR2B degradation, which caused elevations in total, surface and synaptic NR2B subunit levels and current through NR2B-containing NMDARs. Cdk5 facilitated the degradation of NR2B by directly interacting with both it and its protease, calpain. These findings reveal a previously unknown mechanism by which Cdk5 facilitates calpain-mediated proteolysis of NR2B and may control synaptic plasticity and learning.

Cyclin-dependent kinase 5 in synaptic plasticity, learning and memory

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with a multitude of functions. Although Cdk5 is widely expressed, it has been studied most extensively in neurons. Since its initial characterization, the fundamental contribution of Cdk5 to an impressive range of neuronal processes has become clear. These phenomena include neural development, dopaminergic function and neurodegeneration. Data from different fields have recently converged to provide evidence for the participation of Cdk5 in synaptic plasticity, learning and memory. In this review, we consider recent data implicating Cdk5 in molecular and cellular mechanisms underlying synaptic plasticity. We relate these findings to its emerging role in learning and memory. Particular attention is paid to the activation of Cdk5 by p25, which enhances hippocampal synaptic plasticity and memory, and suggests formation of p25 as a physiological process regulating synaptic plasticity and memory.