Cdk5 Regulates Activity-Dependent Gene Expression and Dendrite Development

Neuronal CDK5RAP3 deficiency leads to encephalo-dysplasia via upregulation of N-glycosylases and glycogen deposition

CDK5RAP3 is a binding protein of CDK5 activating proteins and also one of the key co-factors of the E3 enzyme in the UFMylation system. Several reports have implicated the involvement of CDK5 and other components of the UFMylation system in neuronal development and multiple psychiatric disorders. However, the precise role of CDK5RAP3 in neurons remains elusive. In this study, we generated CDK5RAP3 neuron-specific knockout mice (CDK5RAPF/F: Nestin-Cre). CDK5RAP3 conditional knockout (CDK5RAP3 CKO) mice exhibited severe encephalo-dysplasia and a slower developmental trajectory compared to wild-type (WT) mice and succumbed to postnatal demise by day 14. Transcriptome sequencing unveiled that CDK5RAP3 deficiency affects synapse formation, transmembrane trafficking and physiological programs in the brain. Morphological analysis demonstrated that neuronal CDK5RAP3 deficiency leads to increased SLC17A6 and N-glycosylase (RPN1 and ALG2) protein expression, and while causing endoplasmic reticulum (ER) stress. In vitro experiments utilizing CDK5RAP3F/F: ROSA26-ERT2Cre MEFs were conducted to elucidate similar mechanism following CDK5RAP3 deletion. Both in vivo and in vitro, CDK5RAP3 deficiency significantly increased the expression of N-glycosylases (RPN1 and ALG2), as well as the total amount of glycoproteins. CDK5RAP3 may potentially maintain a balance by enhancing the degradation of RPN1 and ALG2 through proteolytic degradation pathways and autophagy. This study underscores the indispensable role of CDK5RAP3 in neuronal development and sheds new light on drug discovery endeavors targeting early brain abnormalities.

CDK5 Deficiency Does not Impair Neuronal Differentiation of Human Induced Pluripotent Stem Cells but Affects Neurite Outgrowth

Cyclin-dependent kinase 5 (CDK5) is a protein kinase involved in neuronal homeostasis and development critical for neuronal survival. Besides, its deregulation is linked to neurodegenerative pathologies such as Alzheimer's and Parkinson's diseases. For that reason, we aimed to generate a deficient CDK5 genetic model in neurons derived from human-induced pluripotent stem cells (hiPSCs) using CRISPR/Cas9 technology. We obtained a heterozygous CDK5+/- clone for the FN2.1 hiPSC line that retained hiPSC stemness and pluripotent potential. Then, neural stem cells (NSCs) and further neurons were derived from the CDK5+/- KO FN2.1 hiPSCs, and their phenotype was validated by immunofluorescence staining using antibodies that recognize lineage-specific markers (SOX-1, SOX-2, and NESTIN for NSCs and TUJ-1, MAP-5, and MAP-2 for neurons). We found that the proliferation rate increased in CDK5+/- KO hiPSC-derived neurons concomitantly with a reduction in NEUN and P35 expression levels. However, the morphometric analysis revealed that CDK5 deficiency caused an increase in the length of the main, primary, and secondary neurites and the neuronal soma area. As a whole, we found that a deficit in CDK5 does not impair hiPSC neuronal differentiation but deregulates proliferation and neurite outgrowth, favoring elongation. The misregulated activity of specific kinases leads to abnormalities such as impaired axonal connectivity in neurodegenerative diseases. Thus, therapeutic approaches aimed at normalizing the activity of kinases, such as CDK5, may help prevent the degeneration of vulnerable neurons.

Environmental Enrichment and Epigenetic Changes in the Brain: From the Outside to the Deep Inside

The brain plays a vital role in maintaining homeostasis and effective interaction with the environment, shaped by genetic and environmental factors throughout neurodevelopment and maturity. While genetic components dictate initial neurodevelopment stages, epigenetics-specifically neuroepigenetics-modulates gene expression in response to environmental influences, allowing for brain adaptability and plasticity. This interplay is particularly evident in neuropathologies like Rett syndrome and CDKL5 deficiency syndrome, where disruptions in neuroepigenetic processes underline significant cognitive and motor impairments. The environmental enrichment paradigm, introduced by Donald Hebb in the late 1940s, demonstrates how enriching stimuli-such as complex sensory, social, and cognitive inputs-affect brain structure and function. Despite methodological variability, evidence reveals that enriched environments catalyze beneficial changes in behavior and neuroanatomy, including increased synaptic plasticity, enhanced motor coordination, and improved cognitive performance in rodent models. Additionally, environmental enrichment induces epigenetic modifications that facilitate these outcomes, highlighting the necessity of understanding the mechanisms driving gene expression changes within the context of enriched experiences. Ultimately, this manifold relationship between environment, neuroepigenetic modulation, and brain function highlights the brain's capacity for change, reinforcing the importance of considering environmental factors in studies of neurodevelopment and therapy for neurological disorders.

A review on cyclin-dependent kinase 5: An emerging drug target for neurodegenerative diseases

Cyclin-dependent kinase 5 (CDK5) is the serine/threonine-directed kinase mainly found in the brain and plays a significant role in developing the central nervous system. Recent evidence suggests that CDK5 is activated by specific cyclins regulating its expression and activity. P35 and p39 activate CDK5, and their proteolytic degradation produces p25 and p29, which are stable products involved in the hyperphosphorylation of tau protein, a significant hallmark of various neurological diseases. Numerous high-affinity inhibitors of CDK5 have been designed, and some are marketed drugs. Roscovitine, like other drugs, is being used to minimize neurological symptoms. Here, we performed an extensive literature analysis to highlight the role of CDK5 in neurons, synaptic plasticity, DNA damage repair, cell cycle, etc. We have investigated the structural features of CDK5, and their binding mode with the designed inhibitors is discussed in detail to develop attractive strategies in the therapeutic targeting of CDK5 for neurodegenerative diseases. This review provides deeper mechanistic insights into the therapeutic potential of CDK5 inhibitors and their implications in the clinical management of neurodegenerative diseases.

Cdk5-p25 as a key element linking amyloid and tau pathologies in Alzheimer's disease: Mechanisms and possible therapeutic interventions

Despite the fact that the small atypical serine/threonine cyclin-dependent kinase 5 (Cdk5) is expressed in a number of tissues, its activity is restricted to the central nervous system due to the neuron-only localization of its activators p35 and p39. Although its importance for the proper development and function of the brain and its role as a switch between neuronal survival and death are unmistakable and unquestionable, Cdk5 is nevertheless increasingly emerging, as supported by a large number of publications on the subject, as a therapeutic target of choice in the fight against Alzheimer's disease. Thus, its aberrant over activation via the calpain-dependent conversion of p35 into p25 is observed during the pathogenesis of the disease where it leads to the hyperphosphorylation of the β-amyloid precursor protein and tau. The present review highlights the pivotal roles of the hyperactive Cdk5-p25 complex activity in contributing to the development of Alzheimer's disease pathogenesis, with a particular emphasis on the linking function between Aβ and tau that this kinase fulfils and on the fact that Cdk5-p25 is part of a deleterious feed forward loop giving rise to a molecular machinery runaway leading to AD pathogenesis. Additionally, we discuss the advances and challenges related to the possible strategies aimed at specifically inhibiting Cdk5-p25 activity and which could lead to promising anti-AD therapeutics.

Physical Interaction between Cyclin-Dependent Kinase 5 (CDK5) and Clock Factors Affects the Circadian Rhythmicity in Peripheral Oscillators

Circadian rhythms are self-sustained oscillators with a period of 24 h that is based on the output of transcriptional and post-translational feedback loops. Phosphorylation is considered one of the most important post-translational modifications affecting rhythmicity from cyanobacteria to mammals. For example, the lack of cyclin-dependent kinase 5 (CDK5) shortened the period length of the circadian oscillator in the Suprachiasmatic Nuclei (SCN) of mice via the destabilization of the PERIOD 2 (PER2) protein. Here, we show that CDK5 kinase activity and its interaction with clock components, including PER2 and CLOCK, varied over time in mouse embryonic fibroblast cells. Furthermore, the deletion of Cdk5 from cells resulted in a prolonged period and shifted the transcription of clock-controlled genes by about 2 to 4 h with a simple delay of chromatin binding of ARNTL (BMAL1) CLOCK. Taken together, our data indicate that CDK5 is critically involved in regulating the circadian clock in vitro at the molecular level.

Inhibition of Cdk5 in PV Neurons Reactivates Experience-Dependent Plasticity in Adult Visual Cortex

Cyclin-dependent kinase 5 (Cdk5) has been shown to play a critical role in brain development, learning, memory and neural processing in general. Cdk5 is widely distributed in many neuron types in the central nervous system, while its cell-specific role is largely unknown. Our previous study showed that Cdk5 inhibition restored ocular dominance (OD) plasticity in adulthood. In this study, we specifically knocked down Cdk5 in different types of neurons in the visual cortex and examined OD plasticity by optical imaging of intrinsic signals. Downregulation of Cdk5 in parvalbumin-expressing (PV) inhibitory neurons, but not other neurons, reactivated adult mouse visual cortical plasticity. Cdk5 knockdown in PV neurons reduced the evoked firing rate, which was accompanied by an increment in the threshold current for the generation of a single action potential (AP) and hyperpolarization of the resting membrane potential. Moreover, chemogenetic activation of PV neurons in the visual cortex can attenuate the restoration of OD plasticity by Cdk5 inhibition. Taken together, our results suggest that Cdk5 in PV interneurons may play a role in modulating the excitation and inhibition balance to control the plasticity of the visual cortex.

Novel phospho-switch function of delta-catenin in dendrite development

In neurons, dendrites form the major sites of information receipt and integration. It is thus vital that, during development, the dendritic arbor is adequately formed to enable proper neural circuit formation and function. While several known processes shape the arbor, little is known of those that govern dendrite branching versus extension. Here, we report a new mechanism instructing dendrites to branch versus extend. In it, glutamate signaling activates mGluR5 receptors to promote Ckd5-mediated phosphorylation of the C-terminal PDZ-binding motif of delta-catenin. The phosphorylation state of this motif determines delta-catenin's ability to bind either Pdlim5 or Magi1. Whereas the delta:Pdlim5 complex enhances dendrite branching at the expense of elongation, the delta:Magi1 complex instead promotes lengthening. Our data suggest that these complexes affect dendrite development by differentially regulating the small-GTPase RhoA and actin-associated protein Cortactin. We thus reveal a "phospho-switch" within delta-catenin, subject to a glutamate-mediated signaling pathway, that assists in balancing the branching versus extension of dendrites during neural development.

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.

Cdk5-mediated Drp1 phosphorylation drives mitochondrial defects and neuronal apoptosis in radiation-induced optic neuropathy

Radiation-induced optic neuropathy (RION) is a devastating complication following external beam radiation therapy (EBRT) that leads to acute vision loss. To date, no efficient, available treatment for this complication, due partly to the lack of understanding regarding the developmental processes behind RION. Here, we report radiation caused changes in mitochondrial dynamics by regulating the mitochondrial fission proteins dynamin-related protein 1 (Drp1) and fission-1 (Fis1). Concurrent with an excessive production of reactive oxygen species (ROS), both neuronal injury and visual dysfunction resulted. Further, our findings delineate an important mechanism by which cyclin-dependent kinase 5 (Cdk5)-mediated phosphorylation of Drp1 (Ser616) regulates defects in mitochondrial dynamics associated with neuronal injury in the development of RION. Both the pharmacological inhibition of Cdk5 by roscovitine and the inhibition of Drp1 by mdivi-1 inhibited mitochondrial fission and the production of ROS associated with radiation-induced neuronal loss. Taken together, these findings may have clinical significance in preventing the development of RION.

Genetic and microstructural differences in the cortical plate of gyri and sulci during gyrification in fetal sheep

Gyrification of the cerebral cortex is a developmentally important process, but the mechanisms that drive cortical folding are not fully known. Theories propose that changes within the cortical plate (CP) cause gyrification, yet differences between the CP below gyri and sulci have not been investigated. Here we report genetic and microstructural differences in the CP below gyri and sulci assessed before (at 70 days of gestational age [GA] 70), during (GA 90), and after (GA 110) gyrification in fetal sheep. The areal density of BDNF, CDK5, and NeuroD6 immunopositive cells were increased, and HDAC5 and MeCP2 mRNA levels were decreased in the CP below gyri compared with sulci during gyrification, but not before. Only the areal density of BDNF-immunopositive cells remained increased after gyrification. MAP2 immunoreactivity and neurite outgrowth were also increased in the CP below gyri compared with sulci at GA 90, and this was associated with microstructural changes assessed via diffusion tensor imaging and neurite orientation dispersion and density imaging at GA 98. Differential neurite outgrowth may therefore explain the localized changes in CP architecture that result in gyrification.

A kinase of many talents: non-neuronal functions of CDK5 in development and disease

The cyclin-dependent kinase 5 (CDK5) represents an unusual member of the family of cyclin-dependent kinases, which is activated upon binding to non-cyclin p35 and p39 proteins. The role of CDK5 in the nervous system has been very well established. In addition, there is growing evidence that CDK5 is also active in non-neuronal tissues, where it has been postulated to affect a variety of functions such as the immune response, angiogenesis, myogenesis, melanogenesis and regulation of insulin levels. Moreover, high levels of CDK5 have been observed in different tumour types, and CDK5 was proposed to play various roles in the tumorigenic process. In this review, we discuss these various CDK5 functions in normal physiology and disease, and highlight the therapeutic potential of targeting CDK5.

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.

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.

Recent advances in branching mechanisms underlying neuronal morphogenesis

Proper neuronal wiring is central to all bodily functions, sensory perception, cognition, memory, and learning. Establishment of a functional neuronal circuit is a highly regulated and dynamic process involving axonal and dendritic branching and navigation toward appropriate targets and connection partners. This intricate circuitry includes axo-dendritic synapse formation, synaptic connections formed with effector cells, and extensive dendritic arborization that function to receive and transmit mechanical and chemical sensory inputs. Such complexity is primarily achieved by extensive axonal and dendritic branch formation and pruning. Fundamental to neuronal branching are cytoskeletal dynamics and plasma membrane expansion, both of which are regulated via numerous extracellular and intracellular signaling mechanisms and molecules. This review focuses on recent advances in understanding the biology of neuronal branching.

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.

Structural insights and characterization of human Npas4 protein

Npas4 is an activity dependent transcription factor which is responsible for gearing the expression of target genes involved in neuro-transmission. Despite the importance of Npas4 in many neuronal diseases, the tertiary structure of Npas4 protein along with its physico-chemical properties is limited. In the current study, first we perfomed the phylogenetic analysis of Npas4 and determined the content of hydrophobic, flexible and order-disorder promoting amino acids. The protein binding regions, post-translational modifications and crystallization propensity of Npas4 were predicted through different in-silico methods. The three dimensional model of Npas4 was predicted through LOMET, SPARSKS-X, I-Tasser, RaptorX, MUSTER and Pyhre and the best model was selected on the basis of Ramachandran plot, PROSA, and Qmean scores. The best model was then subjected to further refinement though MODREFINER. Finally the interacting partners of Npas4 were identified through STRING database. The phylogenetic analysis showed the human Npas4 gene to be closely related to other primates such as chimpanzees, monkey, gibbon. The physiochemical properties of Npas4 showed that it is an intrinsically disordered protein with N-terminal ordered region. The post-translational modification analyses indicated absence of acetylation and mannosylation sites. Three potential phosphorylation sites (S108, T130 and T136) were found in PAS A domain whilst a single phosphorylation site (S273) was present in PAS B domain. The predicted tertiary structure of Npas4 showed that bHLH domain and PAS domain possess tertiary structures while the rest of the protein exhibited disorder property. Protein-protein interaction analysis revealed NPas4 interaction with various proteins which are mainly involved in nuclear trafficking of proteins to cytoplasm, activity regulated gene transcription and neurodevelopmental disorders. Moreover the analysis also highlighted the direct relation to proteins involved in promoting neuronal survival, plasticity and cAMP responsive element binding protein proteins. The current study helps in understanding the physicochemical properties and reveals the neuro-modulatory role of Npas4 in crucial pathways involved in neuronal survival and neural signalling hemostasis.

A Functional Role for the Epigenetic Regulator ING1 in Activity-induced Gene Expression in Primary Cortical Neurons

Epigenetic regulation of activity-induced gene expression involves multiple levels of molecular interaction, including histone and DNA modifications, as well as mechanisms of DNA repair. Here we demonstrate that the genome-wide deposition of inhibitor of growth family member 1 (ING1), which is a central epigenetic regulatory protein, is dynamically regulated in response to activity in primary cortical neurons. ING1 knockdown leads to decreased expression of genes related to synaptic plasticity, including the regulatory subunit of calcineurin, Ppp3r1. In addition, ING1 binding at a site upstream of the transcription start site (TSS) of Ppp3r1 depends on yet another group of neuroepigenetic regulatory proteins, the Piwi-like family, which are also involved in DNA repair. These findings provide new insight into a novel mode of activity-induced gene expression, which involves the interaction between different epigenetic regulatory mechanisms traditionally associated with gene repression and DNA repair.

CDK5 in oncology: recent advances and future prospects

Selective abrogation of cyclin-dependent kinases (CDK) activity is a highly promising strategy in cancer treatment. The atypical CDK, CDK5 has long been known for its role in neurodegenerative diseases, and is becoming an attractive drug target for cancer therapy. Myriads of recent studies have uncovered that aberrant expression of CDK5 contributes to the oncogenic initiation and progression of multiple solid and hematological malignancies. CDK5 is also implicated in the regulation of cancer stem cell biology. In this review, we present the current state of knowledge of CDK5 as a druggable target for cancer treatment. We also provide a detailed outlook of designing selective and potent inhibitors of this enzyme.

Linking Network Activity to Synaptic Plasticity during Sleep: Hypotheses and Recent Data

Research findings over the past two decades have supported a link between sleep states and synaptic plasticity. Numerous mechanistic hypotheses have been put forth to explain this relationship. For example, multiple studies have shown structural alterations to synapses (including changes in synaptic volume, spine density, and receptor composition) indicative of synaptic weakening after a period of sleep. Direct measures of neuronal activity and synaptic strength support the idea that a period of sleep can reduce synaptic strength. This has led to the synaptic homeostasis hypothesis (SHY), which asserts that during slow wave sleep, synapses are downscaled throughout the brain to counteract net strengthening of network synapses during waking experience (e.g., during learning). However, neither the cellular mechanisms mediating these synaptic changes, nor the sleep-dependent activity changes driving those cellular events are well-defined. Here we discuss potential cellular and network dynamic mechanisms which could underlie reductions in synaptic strength during sleep. We also discuss recent findings demonstrating circuit-specific synaptic strengthening (rather than weakening) during sleep. Based on these data, we explore the hypothetical role of sleep-associated network activity patterns in driving synaptic strengthening. We propose an alternative to SHY—namely that depending on experience during prior wake, a variety of plasticity mechanisms may operate in the brain during sleep. We conclude that either synaptic strengthening or synaptic weakening can occur across sleep, depending on changes to specific neural circuits (such as gene expression and protein translation) induced by experiences in wake. Clarifying the mechanisms underlying these different forms of sleep-dependent plasticity will significantly advance our understanding of how sleep benefits various cognitive functions.

Cdk5-dependent phosphorylation of liprinα1 mediates neuronal activity-dependent synapse development

The experience-dependent modulation of brain circuitry depends on dynamic changes in synaptic connections that are guided by neuronal activity. In particular, postsynaptic maturation requires changes in dendritic spine morphology, the targeting of postsynaptic proteins, and the insertion of synaptic neurotransmitter receptors. Thus, it is critical to understand how neuronal activity controls postsynaptic maturation. Here we report that the scaffold protein liprinα1 and its phosphorylation by cyclin-dependent kinase 5 (Cdk5) are critical for the maturation of excitatory synapses through regulation of the synaptic localization of the major postsynaptic organizer postsynaptic density (PSD)-95. Whereas Cdk5 phosphorylates liprinα1 at Thr701, this phosphorylation decreases in neurons in response to neuronal activity. Blockade of liprinα1 phosphorylation enhances the structural and functional maturation of excitatory synapses. Nanoscale superresolution imaging reveals that inhibition of liprinα1 phosphorylation increases the colocalization of liprinα1 with PSD-95. Furthermore, disruption of liprinα1 phosphorylation by a small interfering peptide, siLIP, promotes the synaptic localization of PSD-95 and enhances synaptic strength in vivo. Our findings collectively demonstrate that the Cdk5-dependent phosphorylation of liprinα1 is important for the postsynaptic organization during activity-dependent synapse development.

p39 Is Responsible for Increasing Cdk5 Activity during Postnatal Neuron Differentiation and Governs Neuronal Network Formation and Epileptic Responses

Two distinct protein cofactors, p35 and p39, independently activate Cyclin-dependent kinase 5 (Cdk5), which plays diverse roles in normal brain function and the pathogenesis of many neurological diseases. The initial discovery that loss of p35 impairs neuronal migration in the embryonic brain prompted intensive research exploring the function of p35-dependent Cdk5 activity. In contrast, p39 expression is restricted to the postnatal brain and its function remains poorly understood. Despite the robustly increased Cdk5 activity during neuronal differentiation, which activator is responsible for enhancing Cdk5 activation and how the two distinct activators direct Cdk5 signaling to govern neuronal network formation and function still remains elusive. Here we report that p39, but not p35, is selectively upregulated by histone acetylation-mediated transcription, which underlies the robust increase of Cdk5 activity during rat and mouse neuronal differentiation. The loss of p39 attenuates overall Cdk5 activity in neurons and preferentially affects phosphorylation of specific Cdk5 targets, leading to aberrant axonal growth and impaired dendritic spine and synapse formation. In adult mouse brains, p39 deficiency results in dysregulation of p35 and Cdk5 targets in synapses. Moreover, in contrast to the proepileptic phenotype caused by the lack of p35, p39 loss leads to deficits in maintaining seizure activity and induction of immediate early genes that control hippocampal excitability. Together, our studies demonstrate essential roles of p39 in neuronal network development and function. Furthermore, our data support a model in which Cdk5 activators play nonoverlapping and even opposing roles to govern balanced Cdk5 signaling in the postnatal brain.

SIGNIFICANCE STATEMENT

Neuronal network development requires tightly regulated activation of Cyclin-dependent kinase 5 (Cdk5) by two distinct cofactors, p35 and p39. Despite the well-known p35-dependent Cdk5 function, why postnatal neurons express abundant p39 in addition to p35 remained unknown for decades. In this study, we discovered that selective upregulation of p39 is the underlying mechanism that accommodates the increased functional requirement of Cdk5 activation during neuronal differentiation. In addition, we demonstrated that p39 selectively directs Cdk5 to phosphorylate protein substrates essential for axonal development, dendritic spine formation, and synaptogenesis. Moreover, our studies suggest opposing roles of p39 and p35 in synaptic Cdk5 function and epileptic responses, arguing that cooperation between Cdk5 activators maintains balanced Cdk5 signing, which is crucial for postnatal brain function.

Cdk5 activity is required for Purkinje cell dendritic growth in cell-autonomous and non-cell-autonomous manners

Cyclin-dependent kinase 5 (Cdk5) is recognized as a unique member among other Cdks due to its versatile roles in many biochemical processes in the nervous system. The proper development of neuronal dendrites is required for the formation of complex neural networks providing the physiological basis of various neuronal functions. We previously reported that sparse dendrites were observed on cultured Cdk5-null Purkinje cells and Purkinje cells in Wnt1^cre -mediated Cdk5 conditional knockout (KO) mice. In the present study, we generated L7^cre -mediated p35; p39 double KO (L7^cre -p35^f/f ; p39^-/- ) mice whose Cdk5 activity was eliminated specifically in Purkinje cells of the developing cerebellum. Consequently, these mice exhibited defective Purkinje cell migration, motor coordination deficiency and a Purkinje dendritic abnormality similar to what we have observed before, suggesting that dendritic growth of Purkinje cells was cell-autonomous in vivo. We found that mixed and overlay cultures of WT cerebellar cells rescued the dendritic deficits in Cdk5-null Purkinje cells, however, indicating that Purkinje cell dendritic development was also supported by non-cell-autonomous factors. We then again rescued these abnormalities in vitro by applying exogenous brain-derived neurotrophic factor (BDNF). Based on the results from culture experiments, we attempted to rescue the developmental defects of Purkinje cells in L7^cre -p35^f/f ; p39^-/- mice by using a TrkB agonist. We observed partial rescue of morphological defects of dendritic structures of Purkinje cells. These results suggest that Cdk5 activity is required for Purkinje cell dendritic growth in cell-autonomous and non-cell-autonomous manners. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1175-1187, 2017.

Using the shared genetics of dystonia and ataxia to unravel their pathogenesis

In this review we explore the similarities between spinocerebellar ataxias and dystonias, and suggest potentially shared molecular pathways using a gene co-expression network approach. The spinocerebellar ataxias are a group of neurodegenerative disorders characterized by coordination problems caused mainly by atrophy of the cerebellum. The dystonias are another group of neurological movement disorders linked to basal ganglia dysfunction, although evidence is now pointing to cerebellar involvement as well. Our gene co-expression network approach identified 99 shared genes and showed the involvement of two major pathways: synaptic transmission and neurodevelopment. These pathways overlapped in the two disorders, with a large role for GABAergic signaling in both. The overlapping pathways may provide novel targets for disease therapies. We need to prioritize variants obtained by whole exome sequencing in the genes associated with these pathways in the search for new pathogenic variants, which can than be used to help in the genetic counseling of patients and their families.

Cyclin I-like (CCNI2) is a cyclin-dependent kinase 5 (CDK5) activator and is involved in cell cycle regulation

In contrast to conventional cyclin-dependent kinases that are important for mitotic cell division, cyclin-dependent kinase 5 (CDK5) is predominantly activated in post-mitotic cells and is involved in various cellular events. The kinase activity of CDK5 is tightly regulated by specific activators including p35, p39, and cyclin I (CCNI). Here we show that cyclin I-like (CCNI2), a homolog of CCNI, interacts with CDK5 and activates the kinase activity of CDK5. Different from CCNI, which colocalizes with CDK5 in the nuclei in transfected cells, CCNI2 mainly retains CDK5 in the cytoplasm as well as on the cell membrane. Furthermore, although the expression level of CCNI2 mRNA and CCNI2 protein do not change significantly during cell cycle, depletion of CCNI2 with siRNA affects cell cycle progression as well as cell proliferation. In conclusion, our data strongly suggest that CCNI2 is a novel CDK5 activator and is involved in cell cycle regulation.

CDK5 downregulation enhances synaptic plasticity

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

The role of cAMP and its downstream targets in neurite growth in the adult nervous system

Injured neurons in the adult mammalian central nervous system (CNS) have a very limited capacity for axonal regeneration and neurite outgrowth. This inability to grow new axons or to regrow injured axons is due to the presence of molecules that inhibit axonal growth, and age related changes in the neuron's innate growth capabilities. Available levels of cAMP are thought to have an important role in linking both of these factors. Elevated levels of cAMP in the developing nervous system are important for the guidance and stability of growth cones. As the nervous system matures, cAMP levels decline and the growth promoting effects of cAMP diminish. It has frequently been demonstrated that increasing neuronal cAMP can enhance neurite growth and regeneration. Some methods used to increase cAMP include administration of cAMP agonists, conditioning lesions, or electrical stimulation. Furthermore, it has been proposed that multiple stages of cAMP induced growth exist, one directly caused by its downstream effector Protein Kinase A (PKA) and one caused by the eventual upregulation of gene transcription. Although the role cAMP in promoting axon growth is well accepted, the downstream pathways that mediate cAMP-mediated axonal growth are less clear. This is partly because various key studies that explored the link between PKA and axonal outgrowth relied on the PKA inhibitors KT5720 and H89. More recent studies have shown that both of these drugs are less specific than initially thought and can inhibit a number of other signalling molecules including the Exchange Protein Activated by cAMP (EPAC). Consequently, it has recently been shown that a number of intracellular signalling pathways previously attributed to PKA can now be attributed solely to activation of EPAC specific pathways, or the simultaneous co-activation of PKA and EPAC specific pathways. These new studies open the door to new potential treatments for repairing the injured spinal cord.

Stress-induced nuclear translocation of CDK5 suppresses neuronal death by downregulating ERK activation via VRK3 phosphorylation

Although extracellular signal-related kinase 1/2 (ERK 1/2) activity is generally associated with cell survival, prolonged ERK activation induced by oxidative stress also mediates neuronal cell death. Here we report that oxidative stress-induced cyclin-dependent kinase 5 (CDK5) activation stimulates neuroprotective signaling via phosphorylation of vaccinia-related kinase 3 (VRK3) at Ser 108. The binding of vaccinia H1-related (VHR) phosphatase to phosphorylated VRK3 increased its affinity for phospho-ERK and subsequently downregulated ERK activation. Overexpression of VRK3 protected human neuroblastoma SH-SY5Y cells against hydrogen peroxide (H₂O₂)-induced apoptosis. However the CDK5 was unable to phosphorylate mutant VRK3, and thus the mutant forms of VRK3 could not attenuate apoptotic process. Suppression of CDK5 activity results in increase of ERK activation and elevation of proapoptotic protein Bak expression in mouse cortical neurons. Results from VRK3-deficient neurons were further confirmed the role of VRK3 phosphorylation in H₂O₂-evoked ERK regulation. Importantly, we showed an association between phospho-VRK3 levels and the progression of human Alzheimer’s disease (AD) and Parkinson’s disease (PD). Together our work reveals endogenous protective mechanism against oxidative stress-induced neuronal cell death and suggest VRK3 as a potential therapeutic target in neurodegenerative diseases.