Synaptic roles of Cdk5: implications in higher cognitive functions and neurodegenerative diseases

Biogenesis and reformation of synaptic vesicles

Communication within the nervous system relies on the calcium-triggered release of neurotransmitter molecules by exocytosis of synaptic vesicles (SVs) at defined active zone release sites. While decades of research have provided detailed insight into the molecular machinery for SV fusion, much less is known about the mechanisms that form functional SVs during the development of synapses and that control local SV reformation following exocytosis in the mature nervous system. Here we review the current state of knowledge in the field, focusing on the pathways implicated in the formation and axonal transport of SV precursor organelles and the mechanisms involved in the local reformation of SVs within nerve terminals in mature neurons. We discuss open questions and outline perspectives for future research.

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modeling

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modeling of postsynaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from postmortem RNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in the anterior cingulate cortex, lead to impaired protein kinase A (PKA)-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped electroencephalogram (EEG) dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

p39 Affects Myelin Formation in Cerebral Ischemic Injury

Stroke is a significant public health issue, and research has consistently focused on studying the mechanisms of injury and identifying new targets. As a CDK5 activator, p39 plays a crucial role in various diseases. In this article, we will explore the role and mechanism of p39 in cerebral ischemic injury. We measured the level of p39 using western blot and QPCR at various time points following cerebral ischemia-reperfusion (I/R) injury. The results indicated a significant reduction in the level of p39. TTC staining and behavioral results indicate that the knockout of p39 (p39KO) provides neuroprotection in the short-term. Interestingly, the behavioral dysfunction in p39KO mice was exacerbated after the repair phase of I/R. Further study revealed that this deterioration may be due to demyelination induced by elevated p35 levels. In summary, our study offers profound insights into the significance of p39 in both the acute and repair stages of ischemic injury recovery and a theoretical foundation for future therapeutic drug exploration.

P25/CDK5-mediated Tau Hyperphosphorylation in Both Ipsilateral and Contralateral Cerebra Contributes to Cognitive Deficits in Post-stroke Mice

OBJECTIVE

Post-stroke cognitive impairment (PSCI) develops in approximately one-third of stroke survivors and is associated with ingravescence. Nonetheless, the biochemical mechanisms underlying PSCI remain unclear. The study aimed to establish an ischemic mouse model by means of transient unilateral middle cerebral artery occlusions (MCAOs) and to explore the biochemical mechanisms of p25/cyclin-dependent kinase 5 (CDK5)-mediated tau hyperphosphorylation on the PSCI behavior.

METHODS

Cognitive behavior was investigated, followed by the detection of tau hyperphosphorylation, mobilization, activation of kinases and/or inhibition of phosphatases in the lateral and contralateral cerebrum of mice following ischemia in MACO mice. Finally, we treated HEK293/tau cells with oxygen-glucose deprivation (OGD) and a CDK5 inhibitor (Roscovitine) or a GSK3β inhibitor (LiCl) to the roles of CDK5 and GSK3β in mediating ischemia-reperfusion-induced tau phosphorylation.

RESULTS

Ischemia induced cognitive impairments within 2 months, as well as causing tau hyperphosphorylation and its localization to neuronal somata in both ipsilateral and contralateral cerebra. Furthermore, p25 that promotes CDK5 hyperactivation had significantly higher expression in the mice with MCAO than in the shamoperation (control) group, while the expression levels of protein phosphatase 2 (PP2A) and the phosphorylation level at Tyr307 were comparable between the two groups. In addition, the CDK5 inhibitor rescued tau from hyperphosphorylation induced by OGD.

CONCLUSION

These findings demonstrate that upregulation of CDK5 mediates tau hyperphosphorylation and localization in both ipsilateral and contralateral cerebra, contributing to the pathogenesis of PSCI.

Mitigating aberrant Cdk5 activation alleviates mitochondrial defects and motor neuron disease symptoms in spinal muscular atrophy

Spinal muscular atrophy (SMA), the top genetic cause of infant mortality, is characterized by motor neuron degeneration. Mechanisms underlying SMA pathogenesis remain largely unknown. Here, we report that the activity of cyclin-dependent kinase 5 (Cdk5) and the conversion of its activating subunit p35 to the more potent activator p25 are significantly up-regulated in mouse models and human induced pluripotent stem cell (iPSC) models of SMA. The increase of Cdk5 activity occurs before the onset of SMA phenotypes, suggesting that it may be an initiator of the disease. Importantly, aberrant Cdk5 activation causes mitochondrial defects and motor neuron degeneration, as the genetic knockout of p35 in an SMA mouse model rescues mitochondrial transport and fragmentation defects, and alleviates SMA phenotypes including motor neuron hyperexcitability, loss of excitatory synapses, neuromuscular junction denervation, and motor neuron degeneration. Inhibition of the Cdk5 signaling pathway reduces the degeneration of motor neurons derived from SMA mice and human SMA iPSCs. Altogether, our studies reveal a critical role for the aberrant activation of Cdk5 in SMA pathogenesis and suggest a potential target for therapeutic intervention.

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modelling

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modelling of post-synaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from post-mortem mRNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in anterior cingulate cortex, lead to impaired PKA-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped EEG dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials (VEP) were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

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

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

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.

Role of cyclin-dependent kinase 5 in psychosis and the modulatory effects of cannabinoids

Cyclin-dependent kinase 5 (CDK5) is a serine/threonine kinase that has emerged as a key regulator of neurotransmission in complex cognitive processes. Its expression is altered in treated schizophrenia patients, and cannabinoids modulate CDK5 levels in the brain of rodents. However, the role of this kinase, and its interaction with cannabis use in first-episode psychosis (FEP) patients is still not known. Hence, we studied the expression changes of CDK5 and its signaling partner, postsynaptic density protein 95 (PSD95) in olfactory neuroepithelial (ON) cells of FEP patients with (FEP/c) and without (FEP/nc) prior cannabis use, and in a dual-hit mouse model of psychosis. In this model, adolescent mice were exposed to the cannabinoid receptor 1 agonist (CB1R) WIN-55,212-2 (WIN: 1 mg/kg) during 21 days, and to the N-methyl-d-aspartate receptor (NMDAR) blocker phencyclidine (PCP: 10 mg/kg) during 10 days. FEP/c showed less social functioning deficits, lower CDK5 and higher PSD95 levels than FEP/nc. These changes correlated with social skills, but not cognitive deficits. Consistently, exposure of ON cells from FEP/nc patients to WIN in vitro reduced CDK5 levels. Convergent results were obtained in mice, where PCP by itself induced more sociability deficits, and PSD95/CDK5 alterations in the prefrontal cortex and hippocampus than exposure to PCP-WIN. In addition, central blockade of CDK5 activity with roscovitine in PCP-treated mice restored both sociability impairments and PSD95 levels. We provide translational evidence that increased CDK5 could be an early indicator of psychosis associated with social deficits, and that this biomarker is modulated by prior cannabis use.

Three decades of Cdk5

Cdk5 is a proline-directed serine/threonine protein kinase that governs a variety of cellular processes in neurons, the dysregulation of which compromises normal brain function. The mechanisms underlying the modulation of Cdk5, its modes of action, and its effects on the nervous system have been a great focus in the field for nearly three decades. In this review, we provide an overview of the discovery and regulation of Cdk5, highlighting recent findings revealing its role in neuronal/synaptic functions, circadian clocks, DNA damage, cell cycle reentry, mitochondrial dysfunction, as well as its non-neuronal functions under physiological and pathological conditions. Moreover, we discuss evidence underscoring aberrant Cdk5 activity as a common theme observed in many neurodegenerative diseases.

Inhibition of the CDK5/caspase-3 Pathway by p5-TAT Protects Hippocampal Neurogenesis and Alleviates Radiation-induced Cognitive Dysfunction

Radiation-induced cognitive dysfunction is a common complication associated with cranial radiation therapy. Inhibition of hippocampal neurogenesis and proliferation plays a critical role in this complication. Relieving hippocampal apoptosis may significantly protect hippocampal neurogenesis and proliferation. Previous studies have demonstrated that hyperactivity of cyclin-dependent kinase 5 (Cdk5) is closely related to apoptosis. The exact molecular changes and function of Cdk5 in radiation-induced cognitive dysfunction are still not clear. Whether inhibition of Cdk5 and the relevant caspase-3 could improve hippocampal neurogenesis and ameliorate radiation-induced cognitive dysfunction needs further exploration. We hypothesized that inhibition of the Cdk5/caspase-3 pathway by p5-TAT could protect hippocampal neurogenesis and alleviate radiation-induced cognitive dysfunction. In our study, we reported that radiation induced hyperactivity of Cdk5 accompanied by elevation of the levels of cleaved caspase-3, a marker of neuronal apoptosis. Inhibition of hippocampal neurogenesis and proliferation as well as cognitive dysfunction was also observed. p5-TAT, a specific inhibitor of Cdk5, decreased the overactivation of Cdk5 without affecting the levels of Cdk5 activators. Additionally, this treatment suppressed the expression of cleaved caspase-3. We further demonstrated that p5-TAT treatment reduced hippocampal dysfunction and improved behavioral performance. Therefore, Cdk5 inhibition by the small peptide p5-TAT is a promising therapeutic strategy for radiation-induced cognitive dysfunction.

Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

A persistent dogma in neuroscience supported the idea that terminally differentiated neurons permanently withdraw from the cell cycle. However, since the late 1990s, several studies have shown that cell cycle proteins are expressed in post-mitotic neurons under physiological conditions, indicating that the cell cycle machinery is not restricted to proliferating cells. Moreover, many studies have highlighted a clear link between cell cycle-related proteins and neurological disorders, particularly relating to apoptosis-induced neuronal death. Indeed, cell cycle-related proteins can be upregulated or overactivated in post-mitotic neurons in case of acute or degenerative central nervous system disease. Given the considerable lack of effective treatments for age-related neurological disorders, new therapeutic approaches targeting the cell cycle machinery might thus be considered. This review aims at summarizing current knowledge about the role of the cell cycle machinery in post-mitotic neurons in healthy and pathological conditions.

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

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

Transcriptional Silencing of MCL-1 Through Cyclin-Dependent Kinase Inhibition in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common adult acute leukemia. Survival remains poor, despite decades of scientific advances. Cytotoxic induction chemotherapy regimens are standard-of-care for most patients. Many investigations have highlighted the genomic heterogeneity of AML, and several new targeted therapeutic options have recently been approved. Additional novel therapies are showing promising clinical results and may rapidly transform the therapeutic landscape of AML. Despite the emerging clinical success of B-cell lymphoma (BCL)-2 targeting in AML and a large body of preclinical data supporting myeloid leukemia cell (MCL)-1 as an attractive therapeutic target for AML, MCL-1 targeting remains relatively unexplored, although novel MCL-1 inhibitors are under clinical investigation. Inhibitors of cyclin-dependent kinases (CDKs) involved in the regulation of transcription, CDK9 in particular, are being investigated in AML as a strategy to target MCL-1 indirectly. In this article, we review the basis for CDK inhibition in oncology with a focus on relevant preclinical mechanism-of-action studies of CDK9 inhibitors in the context of their therapeutic potential specifically in AML.

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.

Future Aspects of CDK5 in Prostate Cancer: From Pathogenesis to Therapeutic Implications

Cyclin-dependent kinase 5 (CDK5) is a unique member of the cyclin-dependent kinase family. CDK5 is activated by binding with its regulatory proteins, mainly p35, and its activation is essential in the development of the central nervous system (CNS) and neurodegeneration. Recently, it has been reported that CDK5 plays important roles in regulating various biological and pathological processes, including cancer progression. Concerning prostate cancer, the androgen receptor (AR) is majorly involved in tumorigenesis, while CDK5 can phosphorylate AR and promotes the proliferation of prostate cancer cells. Clinical evidence has also shown that the level of CDK5 is associated with the progression of prostate cancer. Interestingly, inhibition of CDK5 prevents prostate cancer cell growth, while drug-triggered CDK5 hyperactivation leads to apoptosis. The blocking of CDK5 activity by its small interfering RNAs (siRNA) or Roscovitine, a pan-CDK inhibitor, reduces the cellular AR protein level and triggers the death of prostate cancer cells. Thus, CDK5 plays a crucial role in the growth of prostate cancer cells, and AR regulation is one of the important pathways. In this review paper, we summarize the significant studies on CDK5-mediated regulation of prostate cancer cells. We propose that the CDK5–p35 complex might be an outstanding candidate as a diagnostic marker and potential target for prostate cancer treatment in the near future.

Cyclin-Dependent Kinase 5 Dysfunction Contributes to Depressive-like Behaviors in Huntington's Disease by Altering the DARPP-32 Phosphorylation Status in the Nucleus Accumbens

BACKGROUND

Depression is the most common psychiatric condition in Huntington's disease (HD), with rates more than twice those found in the general population. At the present time, there is no established molecular evidence to use as a basis for depression treatment in HD. Indeed, in some patients, classic antidepressant drugs exacerbate chorea or anxiety. Cyclin-dependent kinase 5 (Cdk5) has been involved in processes associated with anxiety and depression. This study evaluated the involvement of Cdk5 in the development and prevalence of depressive-like behaviors in HD and aimed to validate Cdk5 as a target for depression treatment.

METHODS

We evaluated the impact of pharmacological inhibition of Cdk5 in depressive-like and anxiety-like behaviors in Hdh^+/Q111 knock-in mutant mice by using a battery of behavioral tests. Biochemical and morphological studies were performed to define the molecular mechanisms acting downstream of Cdk5 activation. A double huntingtin/DARPP-32 (dopamine- and cAMP-regulated phosphoprotein 32) knock-in mutant mouse was generated to analyze the role of DARPP-32 in HD depression.

RESULTS

We found that Hdh^+/Q111 mutant mice exhibited depressive-like, but not anxiety-like, behaviors starting at 2 months of age. Cdk5 inhibition by roscovitine infusion prevented depressive-like behavior and reduced DARPP-32 phosphorylation at Thr75 in the nucleus accumbens. Hdh^+/Q111 mice heterozygous for DARPP-32 Thr75Ala point mutation were resistant to depressive-like behaviors. We identified β-adducin phosphorylation as a Cdk5 downstream mechanism potentially mediating structural spine plasticity changes in the nucleus accumbens and depressive-like behavior.

CONCLUSIONS

These results point to Cdk5 in the nucleus accumbens as a critical contributor to depressive-like behaviors in HD mice by altering DARPP-32/β-adducin signaling and disrupting the dendritic spine cytoskeleton.

Tissue-type plasminogen activator regulates p35-mediated Cdk5 activation in the postsynaptic terminal

Neuronal depolarization induces the synaptic release of tissue-type plasminogen activator (tPA). Cyclin-dependent kinase-5 (Cdk5) is a member of the family of cyclin-dependent kinases that regulates cell migration and synaptic function in postmitotic neurons. Cdk5 is activated by its binding to p35 (also known as Cdk5r1), a membrane-anchored protein that is rapidly degraded by the proteasome. Here, we show that tPA prevents the degradation of p35 in the synapse by a plasminogen-dependent mechanism that requires open synaptic N-methyl-D-aspartate (NMDA) receptors. We show that tPA treatment increases the abundance of p35 and its binding to Cdk5 in the postsynaptic density (PSD). Furthermore, our data indicate that tPA-induced p35-mediated Cdk5 activation does not induce cell death, but instead prevents NMDA-induced ubiquitylation of postsynaptic density protein-95 (PSD-95; also known as Dlg4) and the removal of GluR1 (also known as Gria1)-containing α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptors from the PSD. These results show that the interaction between tPA and synaptic NMDA receptors regulates the expression of AMPA receptor subunits in the PSD via p35-mediated Cdk5 activation. This is a novel role for tPA as a regulator of Cdk5 activation in cerebral cortical neurons.

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.

CB1R regulates CDK5 signaling and epigenetically controls Rac1 expression contributing to neurobehavioral abnormalities in mice postnatally exposed to ethanol

Fetal alcohol spectrum disorders (FASD) represent a wide array of defects that arise from ethanol exposure during development. However, the underlying molecular mechanisms are limited. In the current report, we aimed to further evaluate the cannabinoid receptor type 1 (CB1R)-mediated mechanisms in a postnatal ethanol-exposed animal model. We report that the exposure of postnatal day 7 (P7) mice to ethanol generates p25, a CDK5-activating peptide, in a time- and CB1R-dependent manner in the hippocampus and neocortex brain regions. Pharmacological inhibition of CDK5 activity before ethanol exposure prevented accumulation of cleaved caspase-3 (CC3) and hyperphosphorylated tau (PHF1) (a marker for neurodegeneration) in neonatal mice and reversed cAMP response element-binding protein (CREB) activation and activity-regulated cytoskeleton-associated protein (Arc) expression. We also found that postnatal ethanol exposure caused a loss of RhoGTPase-related, Rac1, gene expression in a CB1R and CDK5 activity-dependent manner, which persisted to adulthood. Our epigenetic analysis of the Rac1 gene promoter suggested that persistent suppression of Rac1 expression is mediated by enhanced histone H3 lysine 9 dimethylation (H3K9me2), a repressive chromatin state, via G9a recruitment. The inhibition of CDK5/p25 activity before postnatal ethanol exposure rescued CREB activation, Arc, chromatin remodeling and Rac1 expression, spatial memory, and long-term potentiation (LTP) abnormalities in adult mice. Together, these findings propose that the postnatal ethanol-induced CB1R-mediated activation of CDK5 suppresses Arc and Rac1 expression in the mouse brain and is responsible for persistent synaptic plasticity and learning and memory defects in adult mice. This CB1R-mediated activation of CDK5 signaling during active synaptic development may slow down the maturation of synaptic circuits and may cause neurobehavioral defects, as found in this FASD animal model.

The acetylation of cyclin-dependent kinase 5 at lysine 33 regulates kinase activity and neurite length in hippocampal neurons

Cyclin-dependent kinase 5 (CDK5) plays a pivotal role in neural development and neurodegeneration. CDK5 activity can be regulated by posttranslational modifications, including phosphorylation and S-nitrosylation. In this study, we demonstrate a novel mechanism by which the acetylation of CDK5 at K33 (Ac-CDK5) results in the loss of ATP binding and impaired kinase activity. We identify GCN5 and SIRT1 as critical factor controlling Ac-CDK5 levels. Ac-CDK5 achieved its lowest levels in rat fetal brains but was dramatically increased during postnatal periods. Intriguingly, nuclear Ac-CDK5 levels negatively correlated with neurite length in embryonic hippocampal neurons. Either treatment with the SIRT1 activator SRT1720 or overexpression of SIRT1 leads to increases in neurite length, whereas SIRT1 inhibitor EX527 or ectopic expression of acetyl-mimetic (K33Q) CDK5 induced the opposite effect. Furthermore, the expression of nuclear-targeted CDK5 K33Q abolished the SRT1720-induced neurite outgrowth, showing that SIRT1 positively regulates neurite outgrowth via deacetylation of nuclear CDK5. The CDK5 activity-dependent increase of neurite length was mediated by enhanced transcriptional regulation of BDNF via unknown mechanism(s). Our findings identify a novel mechanism by which acetylation-mediated regulation of nuclear CDK5 activity plays a critical role in determining neurite length in embryonic neurons.

Cross-species systems analysis identifies gene networks differentially altered by sleep loss and depression

To understand the transcriptomic organization underlying sleep and affective function, we studied a population of (C57BL/6J × 129S1/SvImJ) F2 mice by measuring 283 affective and sleep phenotypes and profiling gene expression across four brain regions. We identified converging molecular bases for sleep and affective phenotypes at both the single-gene and gene-network levels. Using publicly available transcriptomic datasets collected from sleep-deprived mice and patients with major depressive disorder (MDD), we identified three cortical gene networks altered by the sleep/wake state and depression. The network-level actions of sleep loss and depression were opposite to each other, providing a mechanistic basis for the sleep disruptions commonly observed in depression, as well as the reported acute antidepressant effects of sleep deprivation. We highlight one particular network composed of circadian rhythm regulators and neuronal activity-dependent immediate-early genes. The key upstream driver of this network, Arc, may act as a nexus linking sleep and depression. Our data provide mechanistic insights into the role of sleep in affective function and MDD.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Inducible Knockout of the Cyclin-Dependent Kinase 5 Activator p35 Alters Hippocampal Spatial Coding and Neuronal Excitability

p35 is an activating co-factor of Cyclin-dependent kinase 5 (Cdk5), a protein whose dysfunction has been implicated in a wide-range of neurological disorders including cognitive impairment and disease. Inducible deletion of the p35 gene in adult mice results in profound deficits in hippocampal-dependent spatial learning and synaptic physiology, however the impact of the loss of p35 function on hippocampal in vivo physiology and spatial coding remains unknown. Here, we recorded CA1 pyramidal cell activity in freely behaving p35 cKO and control mice and found that place cells in the mutant mice have elevated firing rates and impaired spatial coding, accompanied by changes in the temporal organization of spiking both during exploration and rest. These data shed light on the role of p35 in maintaining cellular and network excitability and provide a physiological correlate of the spatial learning deficits in these mice.

Synaptic roles of cyclin-dependent kinase 5 & its implications in epilepsy

There is an urgent need to understand the molecular mechanisms underlying epilepsy to find novel prognostic/diagnostic biomarkers to prevent epilepsy patients at risk. Cyclin-dependent kinase 5 (CDK5) is involved in multiple neuronal functions and plays a crucial role in maintaining homeostatic synaptic plasticity by regulating intracellular signalling cascades at synapses. CDK5 deregulation is shown to be associated with various neurodegenerative diseases such as Alzheimer's disease. The association between chronic loss of CDK5 and seizures has been reported in animal models of epilepsy. Genetic expression of CDK5 at transcriptome level has been shown to be abnormal in intractable epilepsy. In this review various possible mechanisms by which deregulated CDK5 may alter synaptic transmission and possibly lead to epileptogenesis have been discussed. Further, CDK5 has been proposed as a potential biomarker as well as a pharmacological target for developing treatments for epilepsy.

TFP5 peptide, derived from CDK5-activating cofactor p35, provides neuroprotection in early-stage of adult ischemic stroke

Cyclin-dependent kinase 5 (CDK5) is a multifaceted protein shown to play important roles in the central nervous system. Abundant evidence indicates that CDK5 hyperactivities associated with neuronal apoptosis and death following ischemic stroke. CDK5 activity increases when its cofactor p35 cleaves into p25 during ischemia. Theoretically, inhibition of CDK5/p25 activity or reduction of p25 would be neuroprotective. TFP5, a modified 24-aa peptide (Lys254-Ala277) derived from p35, was found to effectively inhibit CDK5 hyperactivity and improve the outcomes of Alzheimer's disease and Parkinson's disease in vivo. Here, we showed that intraperitoneal injection of TFP5 significantly decreased the size of ischemia in early-stage of adult ischemic stroke rats. Relative to controls, rats treated with TFP5 displayed reduced excitotoxicity, neuroinflammation, apoptosis, astrocytes damage, and blood-brain barrier disruption. Our findings suggested that TFP5 might serve as a potential therapeutic candidate for acute adult ischemic stroke.

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.

CDK5 downregulation enhances synaptic plasticity

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

Loss of Cdk5 function in the nucleus accumbens decreases wheel running and may mediate age-related declines in voluntary physical activity

KEY POINTS

Physical inactivity, which drastically increases with advancing age, is associated with numerous chronic diseases. The nucleus accumbens (the pleasure and reward 'hub' in the brain) influences wheel running behaviour in rodents. RNA-sequencing and subsequent bioinformatics analysis led us to hypothesize a potential relationship between the regulation of dendritic spine density, the molecules involved in synaptic transmission, and age-related reductions in wheel running. Upon completion of follow-up studies, we developed the working model that synaptic plasticity in the nucleus accumbens is central to age-related changes in voluntary running. Testing this hypothesis, inhibition of Cdk5 (comprising a molecule central to the processes described above) in the nucleus accumbens reduced wheel running. The results of the present study show that reductions in synaptic transmission and Cdk5 function are related to decreases in voluntary running behaviour and provide guidance for understanding the neural mechanisms that underlie age-dependent reductions in the motivation to be physically active.

ABSTRACT

Increases in age are often associated with reduced levels of physical activity, which, in turn, associates with the development of numerous chronic diseases. We aimed to assess molecular differences in the nucleus accumbens (NAc) (a specific brain nucleus postulated to influence rewarding behaviour) with respect to wheel running and sedentary female Wistar rats at 8 and 14 weeks of age. RNA-sequencing was used to interrogate transcriptomic changes between 8- and 14-week-old wheel running rats, and select transcripts were later analysed by quantitative RT-PCR in age-matched sedentary rats. Voluntary wheel running was greatest at 8 weeks and had significantly decreased by 12 weeks. From 619 differentially expressed mRNAs, bioinformatics suggested that cAMP-mediated signalling, dopamine- and cAMP-regulated neuronal phosphoprotein of 32 kDa feedback, and synaptic plasticity were greater in 8- vs. 14-week-old rats. In depth analysis of these networks showed significant (∼20-30%; P < 0.05) decreases in cell adhesion molecule (Cadm)4 and p39 mRNAs, as well as their proteins from 8 to 14 weeks of age in running and sedentary rats. Furthermore, Cadm4, cyclin-dependent kinase 5 (Cdk5) and p39 mRNAs were significantly correlated with voluntary running distance. Analysis of dendritic spine density in the NAc showed that wheel access increased spine density (P < 0.001), whereas spine density was lower in 14- vs. 8-week-old sedentary rats (P = 0.03). Intriguingly, intra-NAc injection of the Cdk5 inhibitor roscovitine, dose-dependently decreased wheel running. Collectively, these experiments suggest that an age-dependent loss in synaptic function and Cdk5/p39 activity in the NAc may be partially responsible for age-related declines in voluntary running behaviour.

Cdk5 Modulates Long-Term Synaptic Plasticity and Motor Learning in Dorsolateral Striatum

The striatum controls multiple cognitive aspects including motivation, reward perception, decision-making and motor planning. In particular, the dorsolateral striatum contributes to motor learning. Here we define an approach for investigating synaptic plasticity in mouse dorsolateral cortico-striatal circuitry and interrogate the relative contributions of neurotransmitter receptors and intracellular signaling components. Consistent with previous studies, we show that long-term potentiation (LTP) in cortico-striatal circuitry is facilitated by dopamine, and requires activation of D1-dopamine receptors, as well as NMDA receptors (NMDAR) and their calcium-dependent downstream effectors, including CaMKII. Moreover, we assessed the contribution of the protein kinase Cdk5, a key neuronal signaling molecule, in cortico-striatal LTP. Pharmacological Cdk5 inhibition, brain-wide Cdk5 conditional knockout, or viral-mediated dorsolateral striatal-specific loss of Cdk5 all impaired dopamine-facilitated LTP or D1-dopamine receptor-facilitated LTP. Selective loss of Cdk5 in dorsolateral striatum increased locomotor activity and attenuated motor learning. Taken together, we report an approach for studying synaptic plasticity in mouse dorsolateral striatum and critically implicate D1-dopamine receptor, NMDAR, Cdk5, and CaMKII in cortico-striatal plasticity. Furthermore, we associate striatal plasticity deficits with effects upon behaviors mediated by this circuitry. This approach should prove useful for the study of the molecular basis of plasticity in the dorsolateral striatum.

S-nitrosylation-dependent proteasomal degradation restrains Cdk5 activity to regulate hippocampal synaptic strength

Precise regulation of synaptic strength requires coordinated activity and functions of synaptic proteins, which is controlled by a variety of post-translational modification. Here we report that S-nitrosylation of p35, the activator of cyclin-dependent kinase 5 (Cdk5), by nitric oxide (NO) is important for the regulation of excitatory synaptic strength. While blockade of NO signalling results in structural and functional synaptic deficits as indicated by reduced mature dendritic spine density and surface expression of glutamate receptor subunits, phosphorylation of numerous synaptic substrates of Cdk5 and its activity are aberrantly upregulated following reduced NO production. The results show that the NO-induced reduction in Cdk5 activity is mediated by S-nitrosylation of p35, resulting in its ubiquitination and degradation by the E3 ligase PJA2. Silencing p35 protein in hippocampal neurons partially rescues the NO blockade-induced synaptic deficits. These findings collectively demonstrate that p35 S-nitrosylation by NO signalling is critical for regulating hippocampal synaptic strength.

Phosphorylation of synaptic substrates by Cdk5 plays an essential role in synapse development. Here Zhang et al. reveal that S-nitrosylation of the activator of Cdk5, p35, by nitric oxide results in its degradation and reduced Cdk5 activity, leading to alterations in synaptic strength.

Mechanisms Regulating Protein Localization

Cellular functions are dictated by protein content and activity. There are numerous strategies to regulate proteins varying from modulating gene expression to post-translational modifications. One commonly used mode of regulation in eukaryotes is targeted localization. By specifically redirecting the localization of a pool of existing protein, cells can achieve rapid changes in local protein function. Eukaryotic cells have evolved elegant targeting pathways to direct proteins to the appropriate cellular location or locations. Here, we provide a general overview of these localization pathways, with a focus on nuclear and mitochondrial transport, and present a survey of the evolutionarily conserved regulatory strategies identified thus far. We end with a description of several specific examples of proteins that exploit localization as an important mode of regulation.

Phosphorylation of CHIP at Ser20 by Cdk5 promotes tAIF-mediated neuronal death

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase and its dysregulation is implicated in neurodegenerative diseases. Likewise, C-terminus of Hsc70-interacting protein (CHIP) is linked to neurological disorders, serving as an E3 ubiquitin ligase for targeting damaged or toxic proteins for proteasomal degradation. Here, we demonstrate that CHIP is a novel substrate for Cdk5. Cdk5 phosphorylates CHIP at Ser20 via direct binding to a highly charged domain of CHIP. Co-immunoprecipitation and ubiquitination assays reveal that Cdk5-mediated phosphorylation disrupts the interaction between CHIP and truncated apoptosis-inducing factor (tAIF) without affecting CHIP's E3 ligase activity, resulting in the inhibition of CHIP-mediated degradation of tAIF. Lentiviral transduction assay shows that knockdown of Cdk5 or overexpression of CHIP(S20A), but not CHIP(WT), attenuates tAIF-mediated neuronal cell death induced by hydrogen peroxide. Thus, we conclude that Cdk5-mediated phosphorylation of CHIP negatively regulates its neuroprotective function, thereby contributing to neuronal cell death progression following neurotoxic stimuli.

Relay of cyclin-dependent kinases in the regulation of axonal growth

One of the most perplexing problems in neuronal morphogenesis is how local polarity signals echo genetic instructions to establish structural and functional asymmetry of neuronal compartments, i.e., axons, dendrites, and synapses. However studying these phenomena is complicated because both genes and the local environment influence the phenotype of developing neurons. Cell cycle-associated nuclear transcription regulators involved in axon extension, for example Cdk12 and Cdk13, thus provide ideal models for connecting spatially separated events at specific developmental time points.

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

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

Regulation of neuronal cav3.1 channels by cyclin-dependent kinase 5 (Cdk5)

Low voltage-activated (LVA) T-type Ca²⁺ channels activate in response to subthreshold membrane depolarizations and therefore represent an important source of Ca²⁺ influx near the resting membrane potential. In neurons, these proteins significantly contribute to control relevant physiological processes including neuronal excitability, pacemaking and post-inhibitory rebound burst firing. Three subtypes of T-type channels (Ca_v3.1 to Ca_v3.3) have been identified, and using functional expression of recombinant channels diverse studies have validated the notion that T-type Ca²⁺ channels can be modulated by various endogenous ligands as well as by second messenger pathways. In this context, the present study reveals a previously unrecognized role for cyclin-dependent kinase 5 (Cdk5) in the regulation of native T-type channels in N1E-115 neuroblastoma cells, as well as recombinant Ca_v3.1channels heterologously expressed in HEK-293 cells. Cdk5 and its co-activators play critical roles in the regulation of neuronal differentiation, cortical lamination, neuronal cell migration and axon outgrowth. Our results show that overexpression of Cdk5 causes a significant increase in whole cell patch clamp currents through T-type channels in N1E-115 cells, while siRNA knockdown of Cdk5 greatly reduced these currents. Consistent with this, overexpression of Cdk5 in HEK-293 cells stably expressing Ca_v3.1channels upregulates macroscopic currents. Furthermore, using site-directed mutagenesis we identified a major phosphorylation site at serine 2234 within the C-terminal region of the Ca_v3.1subunit. These results highlight a novel role for Cdk5 in the regulation of T-type Ca²⁺ channels.

The inhibition of Cdk5 activity after hypoxia/ischemia injury reduces infarct size and promotes functional recovery in neonatal rats

Recent studies indicate that over-activation of Cdk5 is a crucial pro-death signal and Cdk5 activity inhibition provides neuroprotection in animal stroke models. However, Cdk5 inhibitors are reported to affect physiological functions of Cdk5 and lead to serious side effects. Therefore, targeting Cdk5 or its activators without affecting physiological functions of Cdk5 is a therapeutic strategy for ischemic brain injury. In this study, we examined Cdk5 activity in a rat hypoxia/ischemia (HI) injury model. Cdk5 expression was not changed after HI injury, but Cdk5 activity significantly increased, which was demonstrated by the increased phorsphorylation-phosphorylation of Tau and glucocorticoid receptor (GR), two downstream signals of Cdk5. We further showed that the levels of Cdk5 activators p35 and p39 decreased after HI injury, while p25, which is converted from p35 and has a higher activator activity on Cdk5, increased markedly after HI injury. P5, a 24-residue mimetic peptide of p35, was reported to specifically inhibit the p25/Cdk5 signal pathway in an Alzheimer's disease model. P5-TAT, which can cross the blood-brain barrier and cell membrane facilitated by TAT protein, was used in our study. We found that p5-TAT treatment did not change the levels of p35, p39, and p25, but reduced the phorsphorylation of Tau and GR, suggesting the inhibition of the p25/Cdk5 by the peptide p5-TAT. This was supported by the fact that p5 interacted with Cdk5, but not with Cdk5 activators. In addition, p5-TAT reduced cleaved caspase-3 level, a marker of neuronal apoptosis. We further demonstrated that p5-TAT pre-treatment reduced cerebral infarct volume; even when p5-TAT was delayed to be administered at 24h after HI injury, p5-TAT still promoted long-term functional recovery. Therefore, Cdk5 inhibition by the small peptide p5-TAT or its derivatives is a promising therapeutic strategy for the treatment of ischemic brain injury including hypoxic-ischemic encephalopathy and stroke.

The neuroprotective mechanism of puerarin in the treatment of acute spinal ischemia-reperfusion injury is linked to cyclin-dependent kinase 5

Puerarin is shown to exert a variety of pharmacological effects including neuroprotective properties. However, mechanisms of the action are not fully understood. This study was designed to explore the mechanism of puerarin in treatment of acute spinal ischemia-reperfusion injury in rats. Acute spinal ischemia-reperfusion injury was conducted by aortic occlusion in twenty-eight male Sprague-Dawley rats, weighting 230-250 g. The animals were randomly divided into four groups. In the animals with puerarin treatment, 50 mg/kg of puerarin was injected intraperitoneally after reperfusion, and followed by the same dose of injection every 24h for 2 days. In the animals with roscovitine pre-treatment, 30 mg/kg roscovitine was intravenously administrated 60 min before spinal ischemia started. After spinal ischemia for 60 min followed by 48 h of reperfusion, the motor function, spinal infarction volume, apoptosis indices and the activities of Cdk5 and p25 were examined. Acute spinal ischemia-reperfusion resulted in an injury of the spines associated with motor deficit, elevation of Cdk5 and p25 activities, and increase in the spinal apoptosis number and spinal infarction volume. Puerarin improved motor function associated with the decreased apoptosis number, spinal infarction volume, and Cdk5 and p25 activities. The present study indicated that reduction of spinal injury was associated with inhibition of Cdk5 and p25, and that inhibition of Cdk5 and p25 was one of the neuroprotective mechanisms in the puerarin treatment of acute ischemia/reperfusion-induced spinal injury in rats.

Epigenetic modulation of Cdk5 contributes to memory deficiency induced by amyloid fibrils

Alzheimer's disease (AD) is a frequent neurodegenerative disorder with progressive neuroinflammation, loss of synaptic plasticity in central neurons and memory deficiency. Numerous studies demonstrated the epigenetic modification of the expression of specific genes involved in the pathogenesis of amyloid-associated memory deficiency. It was also reported that dysregulation of cyclin-dependent kinase 5 (Cdk5) activity critically contributed to the synaptic dysfunction and memory deficiency in the rodent model of AD. The present study aims to study the epigenetic mechanism underlying the altered Cdk5 activity and its functional significance in the rats with hippocampal infusion of amyloid fibrils. Significantly increased mRNA and expression of Cdk5 were observed in the hippocampal CA1 in the rats injected with amyloid fibrils. Increased acetylation of histone H3 was detected in the Cdk5 promoter region in hippocampal CA1 in these rats. Further chromatin immunoprecipitation and bisulfite sequencing studies illustrated the decreased cytosine methylation in the Cdk5 promoter region in hippocampal CA1 in the modeled rats. Administration with Cdk5 inhibitor roscovitine significantly attenuated the phosphorylation of tau, recovered the synaptic dysfunction of hippocampal CA1 neurons, and improved the behavioral performance in the Morris water maze test and novel object recognition test in the rats injected with amyloid fibrils. These results elucidate the potential epigenetic mechanism underlying the upregulated expression of Cdk5 induced by amyloid fibrils and provided novel insights into the pathogenic mechanism of Alzheimer's disease.

Protection after stroke: cellular effectors of neurovascular unit integrity

Neurological disorders are prevalent worldwide. Cerebrovascular diseases (CVDs), which account for 55% of all neurological diseases, are the leading cause of permanent disability, cognitive and motor disorders and dementia. Stroke affects the function and structure of blood-brain barrier, the loss of cerebral blood flow regulation, oxidative stress, inflammation and the loss of neural connections. Currently, no gold standard treatments are available outside the acute therapeutic window to improve outcome in stroke patients. Some promising candidate targets have been identified for the improvement of long-term recovery after stroke, such as Rho GTPases, cell adhesion proteins, kinases, and phosphatases. Previous studies by our lab indicated that Rho GTPases (Rac and RhoA) are involved in both tissue damage and survival, as these proteins are essential for the morphology and movement of neurons, astrocytes and endothelial cells, thus playing a critical role in the balance between cell survival and death. Treatment with a pharmacological inhibitor of RhoA/ROCK blocks the activation of the neurodegeneration cascade. In addition, Rac and synaptic adhesion proteins (p120 catenin and N-catenin) play critical roles in protection against cerebral infarction and in recovery by supporting the neurovascular unit and cytoskeletal remodeling activity to maintain the integrity of the brain parenchyma. Interestingly, neuroprotective agents, such as atorvastatin, and CDK5 silencing after cerebral ischemia and in a glutamate-induced excitotoxicity model may act on the same cellular effectors to recover neurovascular unit integrity. Therefore, future efforts must focus on individually targeting the structural and functional roles of each effector of neurovascular unit and the interactions in neural and non-neural cells in the post-ischemic brain and address how to promote the recovery or prevent the loss of homeostasis in the short, medium and long term.

Ischemic stroke injury is mediated by aberrant Cdk5

Ischemic stroke is one of the leading causes of morbidity and mortality. Treatment options are limited and only a minority of patients receive acute interventions. Understanding the mechanisms that mediate neuronal injury and death may identify targets for neuroprotective treatments. Here we show that the aberrant activity of the protein kinase Cdk5 is a principal cause of neuronal death in rodents during stroke. Ischemia induced either by embolic middle cerebral artery occlusion (MCAO) in vivo or by oxygen and glucose deprivation in brain slices caused calpain-dependent conversion of the Cdk5-activating cofactor p35 to p25. Inhibition of aberrant Cdk5 during ischemia protected dopamine neurotransmission, maintained field potentials, and blocked excitotoxicity. Furthermore, pharmacological inhibition or conditional knock-out (CKO) of Cdk5 prevented neuronal death in response to ischemia. Moreover, Cdk5 CKO dramatically reduced infarctions following MCAO. Thus, targeting aberrant Cdk5 activity may serve as an effective treatment for stroke.

Cdk12 and Cdk13 regulate axonal elongation through a common signaling pathway that modulates Cdk5 expression

Cdk12 and Cdk13 are Cdc2-related proteins that share 92% identity in their kinase domains. Using in situ hybridization and Western blot analysis, we detected the expression of Cdk12 and Cdk13 mRNAs and their proteins in developing mouse embryos, especially during development of the nervous system. We explored the roles of Cdk12 and Cdk13 in neuronal differentiation using the P19 neuronal differentiation model. Upon knockdown of Cdk12 or Cdk13, no effect on differentiated cell numbers was detected, but a substantial decrease of numbers of neurons with long neurites was identified. Similarly, knockdown of Cdk12 or Cdk13 in primarily cultured cortical neurons shortens the averaged axonal length. A microarray analysis was used to examine changes in gene expression after knockdown or overexpression of Cdk12 and we identified Cdk5 as a molecule potentially involved in mediating the effect of Cdk12 and Cdk13. Depletion of Cdk12 or Cdk13 in P19 cells significantly reduces Cdk5 expression at both the mRNA and protein levels. Expression of Cdk5 protein in the developing mouse brain is also reduced in conditional Cdk12-knockout mice in proportion to the residual amount of Cdk12 protein present. This suggests that the reduced axonal outgrowth after knockdown of Cdk12 or Cdk13 might be due to lower Cdk5 expression. Furthermore, overexpression of Cdk5 protein in P19 cells was able to partially rescue the neurite outgrowth defect observed when Cdk12 or Cdk13 is depleted. Together, these findings suggest that Cdk12 and Cdk13 regulate axonal elongation through a common signaling pathway that modulates Cdk5 expression.

Preferential targeting of p39-activated Cdk5 to Rac1-induced lamellipodia

Cdk5 is a member of the cyclin-dependent kinase (Cdk) family that plays a role in various neuronal activities including brain development, synaptic regulation, and neurodegeneration. Cdk5 requires the neuronal specific activators, p35 and p39 for subcellular compartmentalization. However, it is not known how active Cdk5 is recruited to F-actin cytoskeleton, which is a Cdk5 target. Here we found p35 and p39 localized to F-actin rich regions of the plasma membrane and investigated the underlying targeting mechanism in vitro by expressing them with Rho family GTPases in Neuro2A cells. Both p35 and p39 accumulated at the cell peripheral lamellipodia and perinuclear regions, where active Rac1 is localized. Interestingly, p35 and p39 displayed different localization patterns as p35 was found more at the perinuclear region and p39 was found more in peripheral lamellipodia. We then confirmed this distinct localization in primary hippocampal neurons. We also determined that the localization of p39 to lamellipodia requires myristoylation and Lys clusters within the N-terminal p10 region. Additionally, we found that p39-Cdk5, but not p35-Cdk5 suppressed lamellipodia formation by reducing Rac1 activity. These results suggest that p39-Cdk5 has a dominant role in Rac1-dependent lamellipodial activity.

Cyclin e1 regulates Kv2.1 channel phosphorylation and localization in neuronal ischemia

Kv2.1 is a major delayed rectifying K(+) channel normally localized to highly phosphorylated somatodendritic clusters in neurons. Excitatory stimuli induce calcineurin-dependent dephosphorylation and dispersal of Kv2.1 clusters, with a concomitant hyperpolarizing shift in the channel's activation kinetics. We showed previously that sublethal ischemia, which renders neurons transiently resistant to excitotoxic cell death, can also induce Zn(2+)-dependent changes in Kv2.1 localization and activation kinetics, suggesting that activity-dependent modifications of Kv2.1 may contribute to cellular adaptive responses to injury. Recently, cyclin-dependent kinase 5 (Cdk5) was shown to phosphorylate Kv2.1, with pharmacological Cdk5 inhibition being sufficient to decluster channels. In another study, cyclin E1 was found to restrict neuronal Cdk5 kinase activity. We show here that cyclin E1 regulates Kv2.1 cellular localization via inhibition of Cdk5 activity. Expression of cyclin E1 in human embryonic kidney cells prevents Cdk5-mediated phosphorylation of Kv2.1, and cyclin E1 overexpression in rat cortical neurons triggers dispersal of Kv2.1 channel clusters. Sublethal ischemia in neurons induces calcineurin-dependent upregulation of cyclin E1 protein expression and cyclin E1-dependent Kv2.1 channel declustering. Importantly, overexpression of cyclin E1 in neurons is sufficient to reduce excitotoxic cell death. These results support a novel role for neuronal cyclin E1 in regulating the phosphorylation status and localization of Kv2.1 channels, a likely component of signaling cascades leading to ischemic preconditioning.

Intrathecal administration of roscovitine prevents remifentanil-induced postoperative hyperalgesia and decreases the phosphorylation of N-methyl-D-aspartate receptor and metabotropic glutamate receptor 5 in spinal cord

N-methyl-D-aspartate receptor (NMDAR) and metabotropic glutamate receptor 5 (mGluR5) play an important role in nociceptive processing and central sensitization. Our previous study showed that tyrosine phosphorylation of NMDAR subunit 2B (NR2B) at Tyr1472 in spinal dorsal horn contributes to the postoperative hyperalgesia induced by remifentanil. Cyclin-dependent kinase 5 (Cdk5) has been implicated in synaptic plasticity, learning, memory and pain signaling via regulating the phosphorylation of NMDAR and mGluR5. In the present study, a rat model of postoperative pain was used to investigate the role of Cdk5 in spinal dorsal horn in remifentanil-induced hyperalgesia and the intervention of pretreatment with Cdk5 inhibitor roscovitine. Intraoperative infusion of remifentanil (0.04 mg/kg, subcutaneous) significantly enhanced mechanical allodynia and thermal hyperalgesia induced by plantar incision during the postoperative period (each lasting between 2 h and 48 h), which were attenuated by pretreatment with roscovitine. Correlated with the pain behavior changes, Western blotting revealed that there was a significant increase in the expression of Cdk5 and its activator p35/p25, and further the kinase activity of Cdk5 in spinal dorsal horn after intraoperative infusion of remifentanil. The phosphorylation of NR2A at Ser1232, the phosphorylation of NR2B at Tyr1472 and the phosphorylation of mGluR5 at Ser1167 were also significantly up-regulated. Furthermore, these increases were attenuated by pretreatment with roscovitine. These results suggested that Cdk5 may contribute to remifentanil-induced postoperative hyperalgesia via regulating the phosphorylation of NMDAR and mGluR5 in spinal dorsal horn. These findings provide experimental evidence for the further application of Cdk5 inhibitor in preventing remifentanil-induced hyperalgesia.

Altered expression of c-Abl in patients with epilepsy and in a rat model

c-Abl is an ubiquitous nonreceptor tyrosine kinase involved in signal transduction pathways that promote cytoskeleton remodeling and apoptosis. In brain, c-Abl plays important roles in neuronal development, neurogenesis, neuronal migration, neurite outgrowth, and synaptic plasticity. Neuronal death, gliosis and synaptic remodeling are thought to be involved in the development of epilepsy. Here we investigated the expression pattern and distribution of total and phosphorylated c-Abl in patients with temporal lobe epilepsy (TLE) and a rat model of epilepsy to explore the probable relationship between c-Abl expression and TLE. Double immunolabeling, Immunohistochemistry, and immunoblotting results showed that both total and phosphorylated c-Abl were upregulated in the temporal neocortex of 26 patients with TLE compared to nonepileptic controls. In the temporal neocortex of pilocarpine-treated rats, upregulation of total and phosphorylated c-Abl began 6 hours after seizures, with relatively high expression for 60 days. In the hippocampus of experimental rats, total unphosphorylated c-Abl elevated from 6 hours to 30 days after seizures, the expression then returned to normal levels at 60 days, while phosphorylated c-Abl increased along with the time and maintained at significant high levels for up to 60 days. These results indicate that c-Abl may play an important role in the development of TLE.