A decade of CDK5

A Retrospective Tribute to Dr. Harish Pant (1938-2023) and His Seminal Work on Cyclin Dependent Kinase 5

Dr. Harish Chandra Pant was Chief of the Section on Neuronal Cytoskeletal Protein Regulation within the National Institute of Neurological Disorders and Stroke at the NIH. A main focus of his group was understanding the mechanisms regulating neuronal cytoskeletal phosphorylation. Phosphorylation of neurofilaments can increase filament stability and confer resistance to proteolysis, but aberrant hyperphosphorylation of neurofilaments can be found in the neurofibrillary tangles that are seen with neurodegenerative diseases like Alzheimer disease (AD). Through his work, Harish would inevitably come across cyclin dependent kinase 5 (Cdk5), a key kinase that can phosphorylate neurofilaments at KSPXK motifs. Cdk5 differs from other Cdks in that its activity is mainly in post-mitotic neurons rather than being involved in the cell cycle in dividing cells. With continued interest in Cdk5, Harish and his group were instrumental in identifying important roles for this neuronal kinase in not only neuronal cytoskeleton phosphorylation but also in neuronal development, synaptogenesis, and neuronal survival. Here, we review the accomplishments of Harish in characterizing the functions of Cdk5 and its involvement in neuronal health and disease.

Astrocyte-induced Cdk5 expedites breast cancer brain metastasis by suppressing MHC-I expression to evade immune recognition

Brain metastases (BrMs) evade the immune response to develop in the brain, yet the mechanisms of BrM immune evasion remains unclear. This study shows that brain astrocytes induce the overexpression of neuronal-specific cyclin-dependent kinase 5 (Cdk5) in breast cancer-derived BrMs, which facilitates BrM outgrowth in mice. Cdk5-overexpressing BrMs exhibit reduced expression and function of the class I major histocompatibility complex (MHC-I) and antigen-presentation pathway, which are restored by inhibiting Cdk5 genetically or pharmacologically, as evidenced by single-cell RNA sequencing and functional studies. Mechanistically, Cdk5 suppresses MHC-I expression on the cancer cell membrane through the Irf2bp1-Stat1-importin α-Nlrc5 pathway, enabling BrMs to avoid recognition by T cells. Treatment with roscovitine-a clinically applicable Cdk5 inhibitor-alone or combined with immune checkpoint inhibitors, significantly reduces BrM burden and increases tumour-infiltrating functional CD8+ lymphocytes in mice. Thus, astrocyte-induced Cdk5 overexpression endorses BrM immune evasion, whereas therapeutically targeting Cdk5 markedly improves the efficacy of immune checkpoint inhibitors and inhibits BrM growth.

S-Nitrosylation of p39 promotes its degradation and contributes to synaptic dysfunction induced by β-amyloid peptide

Alzheimer's disease (AD), characterized by cognitive decline, is increasingly recognized as a disorder marked by synaptic loss and dysfunction. Despite this understanding, the underlying pathophysiological mechanisms contributing to synaptic impairment remain largely unknown. In this study, we elucidate a previously undiscovered signaling pathway wherein the S-nitrosylation of the Cdk5 activator p39, a post-translational modification involving the addition of nitric oxide to protein cysteine residues, plays a crucial role in synaptic dysfunction associated with AD. Our investigation reveals heightened p39 S-nitrosylation in the brain of an amyloid precursor protein (APP)/presenilin 1 (PS1) transgenic mouse model of AD. Additionally, soluble amyloid-β oligomers (Aβ), implicated in synaptic loss in AD, induce p39 S-nitrosylation in cultured neurons. Notably, we uncover that p39 protein level is regulated by S-nitrosylation, with nitric oxide S-nitrosylating p39 at Cys265 and subsequently promoting its degradation. Furthermore, our study demonstrates that S-nitrosylation of p39 at Cys265 significantly contributes to amyloid-β (Aβ) peptide-induced dendrite retraction and spine loss. Collectively, our findings highlight S-nitrosylation of p39 as a novel aberrant redox protein modification involved in the pathogenesis of AD, suggesting its potential as a therapeutic target for the disease.

CDK5-cyclin B1 regulates mitotic fidelity

CDK1 has been known to be the sole cyclin-dependent kinase (CDK) partner of cyclin B1 to drive mitotic progression1. Here we demonstrate that CDK5 is active during mitosis and is necessary for maintaining mitotic fidelity. CDK5 is an atypical CDK owing to its high expression in post-mitotic neurons and activation by non-cyclin proteins p35 and p392. Here, using independent chemical genetic approaches, we specifically abrogated CDK5 activity during mitosis, and observed mitotic defects, nuclear atypia and substantial alterations in the mitotic phosphoproteome. Notably, cyclin B1 is a mitotic co-factor of CDK5. Computational modelling, comparison with experimentally derived structures of CDK-cyclin complexes and validation with mutational analysis indicate that CDK5-cyclin B1 can form a functional complex. Disruption of the CDK5-cyclin B1 complex phenocopies CDK5 abrogation in mitosis. Together, our results demonstrate that cyclin B1 partners with both CDK5 and CDK1, and CDK5-cyclin B1 functions as a canonical CDK-cyclin complex to ensure mitotic fidelity.

Proteomic and phosphoproteomic profiling of urinary small extracellular vesicles in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most prevalent form of primary liver cancer and a major cause of cancer-related mortality worldwide. Small extracellular vesicles (sEVs) are heterogeneous populations of membrane-structured vesicles that can be found in many biological fluids and are currently considered as a potential source of disease-associated biomarkers for diagnosis. The purpose of this study was to define the proteomic and phosphoproteomic landscape of urinary sEVs in patients with HCC. Mass spectrometry-based methods were used to detect the global proteome and phosphoproteome profiles of sEVs isolated by differential ultracentrifugation. Label-free quantitation analysis showed that 348 differentially expressed proteins (DEPs) and 548 differentially expressed phosphoproteins (DEPPs) were identified in the HCC group. Among them, multiple phosphoproteins related to HCC, including HSP90AA1, IQGAP1, MTOR, and PRKCA, were shown to be upregulated in the HCC group. Pathway enrichment analysis indicated that the upregulated DEPPs participate in the regulation of autophagy, proteoglycans in cancer, and the MAPK/mTOR/Rap1 signaling pathway. Furthermore, kinase-substrate enrichment analysis revealed activation of MTOR, AKT1, MAP2Ks, and MAPKs family kinases in HCC-derived sEVs, indicating that dysregulation of the MAPK and mTOR signaling pathways may be the primary sEV-mediated molecular mechanisms involved in the development and progression of HCC. This study demonstrated that urinary sEVs are enriched in proteomic and phosphoproteomic signatures that could be further explored for their potential use in early HCC diagnostic and therapeutic applications.

Phosphorylation of LSD1 at serine 54 regulates genes involved in follicle selection by enhancing demethylation activity in chicken ovarian granulosa cells

Follicle selection in chicken refers to the process of selecting a follicle to enter hierarchy from a cohort of small yellow follicles (SY) with a diameter of 6 to 8 mm. The follicle being selected will develop rapidly and ovulate. Follicle selection is a key stage affecting chicken egg-laying performance. Our previous study showed that the phosphorylation level of lysine (K)-specific demethylase 1A (LSD1) at serine 54 (LSD1Ser54p) was significantly increased in F6 follicles compared to prehierarchal SY follicles, but its function was unclear. Here, the mechanism of this modification, the effect of LSD1Ser54p dephosphorylation on gene expression profile of chicken hierarchal granulosa cells and the function of fibroblast growth factor 9 (FGF9) that is regulated by LSD1Ser54p were further investigated. The modification of LSD1Ser54p was predicted to be mediated by cyclin-dependent kinase 5 (CDK5) and glycogen synthase kinase 3 (GSK3). Treatment of chicken hierarchal granulosa cells with CDK5 inhibitor significantly decreased LSD1Ser54p level (P < 0.05) and LSD1Ser54p interacted with CDK5, suggesting that, in the granulosa cells of chicken hierarchal follicles, LSD1Ser54p modification was carried out by CDK5. When the LSD1Ser54p level decreased in the granulosa cells of chicken hierarchal follicles, both the mRNA expression of FGF9 and α-actinin 2 (ACTN2) and the H3K4me2 level in their promoter regions significantly increased (P < 0.05), indicating that this phosphorylation modification enhanced the demethylation activity of LSD1. Moreover, in chicken hierarchal granulosa cells, overexpression of chicken FGF9 stimulated their proliferation and increased the mRNA expression of hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 1 (Hsd3b) and steroidogenic acute regulatory protein (StAR). This study collectively revealed that phosphorylation of LSD1 at serine 54 by CDK5 enhanced its demethylation activity in chicken ovarian granulosa cells and regulated genes including FGF9 that is engaged in chicken follicle selection.

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.

In Vivo Proximity Labeling Identifies a New Function for the Lifespan and Autophagy-regulating Kinase Pef1, an Ortholog of Human Cdk5

Cdk5 is a highly-conserved, noncanonical cell division kinase important to the terminal differentiation of mammalian cells in multiple organ systems. We previously identified Pef1, the Schizosaccharomyces pombe ortholog of cdk5, as regulator of chronological lifespan. To reveal the processes impacted by Pef1, we developed APEX2-biotin phenol-mediated proximity labeling in S. pombe. Efficient labeling required a short period of cell wall digestion and eliminating glucose and nitrogen sources from the medium. We identified 255 high-confidence Pef1 neighbors in growing cells and a novel Pef1-interacting partner, the DNA damage response protein Rad24. The Pef1-Rad24 interaction was validated by reciprocal proximity labeling and co-immunoprecipitation. Eliminating Pef1 partially rescued the DNA damage sensitivity of cells lacking Rad24. To monitor how Pef1 neighbors change under different conditions, cells induced for autophagy were labeled and 177 high-confidence Pef1 neighbors were identified. Gene ontology (GO) analysis of the Pef1 neighbors identified proteins participating in processes required for autophagosome expansion including regulation of actin dynamics and vesicle-mediated transport. Some of these proteins were identified in both exponentially growing and autophagic cells. Pef1-APEX2 proximity labeling therefore identified a new Pef1 function in modulating the DNA damage response and candidate processes that Pef1 and other cdk5 orthologs may regulate.

Ethaverine and Papaverine Target Cyclin-Dependent Kinase 5 and Inhibit Lung Cancer Cell Proliferation and Migration

CDK5 kinase plays a central role in the regulation of neuronal functions, and its hyperactivation has been associated with neurodegenerative pathologies and more recently with several human cancers, in particular lung cancer. However, ATP-competitive inhibitors targeting CDK5 are poorly selective and suffer limitations, calling for new classes of inhibitors. In a screen for allosteric modulators of CDK5, we identified ethaverine and closely related derivative papaverine and showed that they inhibit cell proliferation and migration of non small cell lung cancer cell lines. Moreover the efficacy of these compounds is significantly enhanced when combined with the ATP-competitive inhibitor roscovitine, suggesting an additive dual mechanism of inhibition targeting CDK5. These compounds do not affect CDK5 stability, but thermodenaturation studies performed with A549 cell extracts infer that they interact with CDK5 in cellulo. Furthermore, the inhibitory potentials of ethaverine and papaverine are reduced in A549 cells treated with siRNA directed against CDK5. Taken together, our results provide unexpected and novel evidence that ethaverine and papaverine constitute promising leads that can be repurposed for targeting CDK5 in lung cancer.

Maintaining Drosha expression with Cdk5 inhibitors as a potential therapeutic strategy for early intervention after TBI

Traumatic brain injury (TBI) is a major cause of death and disability in adults. The pathological process of TBI involves a multifactorial cascade in which kinases have been proven contribute to interactions between relevant factors and amplification of signaling cascades. Cyclin-dependent kinase 5 (Cdk5) is a promising kinase that has been implicated in various brain disorders, including TBI. However, the mechanism by which Cdk5 induces neuronal damage remains unclear. Here, we show for the first time that Drosha, a key enzyme in microRNA biogenesis, is a pivotal substrate of abnormally activated Cdk5. Cdk5-mediated phosphorylation decreases Drosha expression and exacerbates nerve injury in TBI. We proved that maintaining Drosha expression via the administration of repurposed Cdk5 inhibitors that were previously studied in clinical trials is a promising approach for the early treatment of TBI. Together, our work identifies Drosha as a novel target for neuroprotective strategies after TBI and suggests Cdk5-mediated regulation of Drosha expression as a potential therapeutic strategy for early TBI intervention.

Advances in synthesis and biological evaluation of CDK2 inhibitors for cancer therapy

One of the leading causes of mortality in the world is cancer. This disease occurs when responsible genes that regulate the cell cycle become inactive due to internal or external factors. Specifically, the G1/S and S/G2 transitions in the cell cycle are controlled by a protein called cyclin-dependent kinase 2 (CDK2). CDKs, which play a crucial role in managing the cell cycle, have been a wide area of research in cancer treatment. Over the past 11 years, significant research has been made in identifying potent, targeted, and efficient inhibitors of CDK2. In this summary, we have summarized recent developments in the synthesis and biological evaluation of CDK2 inhibitors.

Pharmacological Approaches Using Diabetic Drugs Repurposed for Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) are chronic, progressive disorders affecting the elderly, which fosters global healthcare concern with the growing aging population. Both T2DM and AD have been linked with increasing age, advanced glycosylation end products, obesity, and insulin resistance. Insulin resistance in the periphery is significant in the development of T2DM and it has been posited that insulin resistance in the brain plays a key role in AD pathogenesis, earning AD the name "type 3 diabetes". These clinical and epidemiological links between AD and T2DM have become increasingly pronounced throughout the years, and serve as a means to investigate the effects of antidiabetic therapies in AD, such as metformin, intranasal insulin, incretins, DPP4 inhibitors, PPAR-γ agonists, SGLT2 inhibitors. The majority of these drugs have shown benefit in preclinical trials, and have shown some promising results in clinical trials, with the improvement of cognitive faculties in participants with mild cognitive impairment and AD. In this review, we have summarize the benefits, risks, and conflicting data that currently exist for diabetic drugs being repurposed for the treatment of AD.

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.

CDK5 promotes apoptosis and attenuates chemoresistance in gastric cancer via E2F1 signaling

BACKGROUND

Chemoresistance is a major clinical challenge that leads to tumor metastasis and poor clinical outcome. The mechanisms underlying gastric cancer resistance to chemotherapy are still unclear.

METHODS

We conducted bioinformatics analyses of publicly available patient datasets to establish an apoptotic phenotype and determine the key pathways and clinical significance. In vitro cell models, in vivo mouse models, and numerous molecular assays, including western blotting, qRT-PCR, immunohistochemical staining, and coimmunoprecipitation assays were used to clarify the role of factors related to apoptosis in gastric cancer in this study. Differences between datasets were analyzed using the Student's t-test and two-way ANOVA; survival rates were estimated based on Kaplan-Meier analysis; and univariate and multivariate Cox proportional hazards models were used to evaluate prognostic factors.

RESULTS

Bulk transcriptomic analysis of gastric cancer samples established an apoptotic phenotype. Proapoptotic tumors were enriched for DNA repair and immune inflammatory signaling and associated with improved prognosis and chemotherapeutic benefits. Functionally, cyclin-dependent kinase 5 (CDK5) promoted apoptosis of gastric cancer cells and sensitized cells and mice to oxaliplatin. Mechanistically, we demonstrate that CDK5 stabilizes DP1 through direct binding to DP1 and subsequent activation of E2F1 signaling. Clinicopathological analysis indicated that CDK5 depletion correlated with poor prognosis and chemoresistance in human gastric tumors.

CONCLUSION

Our findings reveal that CDK5 promotes cell apoptosis by stabilizing DP1 and activating E2F1 signaling, suggesting its potential role in the prognosis and therapeutic decisions for patients with gastric cancer.

Protein modification regulated autophagy in Bombyx mori and Drosophila melanogaster

Post-translational modifications refer to the chemical alterations of proteins following their biosynthesis, leading to changes in protein properties. These modifications, which encompass acetylation, phosphorylation, methylation, SUMOylation, ubiquitination, and others, are pivotal in a myriad of cellular functions. Macroautophagy, also known as autophagy, is a major degradation of intracellular components to cope with stress conditions and strictly regulated by nutrient depletion, insulin signaling, and energy production in mammals. Intriguingly, in insects, 20-hydroxyecdysone signaling predominantly stimulates the expression of most autophagy-related genes while concurrently inhibiting mTOR activity, thereby initiating autophagy. In this review, we will outline post-translational modification-regulated autophagy in insects, including Bombyx mori and Drosophila melanogaster, in brief. A more profound understanding of the biological significance of post-translational modifications in autophagy machinery not only unveils novel opportunities for autophagy intervention strategies but also illuminates their potential roles in development, cell differentiation, and the process of learning and memory processes in both insects and mammals.

Regulation of NLGN3 and the Synaptic Rho-GEF Signaling Pathway by CDK5

Neuroligins (NLGNs) are postsynaptic cell adhesion molecules that are involved in synapse assembly and function. The NLGN gene family consists of 5 genes (NLGN1-3, 4X, and 4Y). NLGN3 forms heterodimers with other NLGNs and is expressed at both excitatory and inhibitory synapses, although the distinct role at different synapses is not fully understood. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that targets various neuronal substrates to impact neuronal migration, neurite outgrowth, synaptic transmission, and plasticity. Both NLGNs and their presynaptic binding partners neurexins are highly associated with neurodevelopmental disorders. The NLGN3 gene is on the X chromosome and variants in NLGN3 have been linked to the pathophysiology in neurodevelopmental disorders. To better understand the endogenous modulation of NLGN3, we generated an HA-tagged knock-in mouse. We found that Cdk5 associates with NLGN3 in vivo and phosphorylates NLGN3 on serine 725 (S725) in the knock-in mouse of either sex. The phosphorylation affects the NLGN3 association with Kalirin-7, a postsynaptic guanine nucleotide exchange factors for Rho GTPase family proteins. We further observed that the phosphorylation modulates NLGN3 surface expression and NLGN3-mediated synaptic currents in cultured rat neurons. Thus, we characterized NLGN3 as a novel Cdk5 substrate and revealed the functional consequences of NLGN3 S725 phosphorylation in neurons. Our study provides a novel molecular mechanism underlying Cdk5-mediated regulation of postsynaptic cell adhesion molecules.SIGNIFICANCE STATEMENT NLGN3 is involved in synapse assembly and function at both excitatory and inhibitory synapses and has been associated with the pathophysiology of neurodevelopmental disorders. Cdk5 has brain-specific activity and is involved in neuronal transmission, synapse function, and plasticity. Here, we characterize NLGN3 as a Cdk5 substrate for the first time and show that Cdk5-mediated phosphorylation regulates NLGN3 function. We demonstrate that NLGN3 S725 is a Cdk5 phosphorylation site, and reveal that the site is important for NLGN3 association with Kalirin-7, NLGN3 surface expression, and NLGN3-mediated synaptic transmission.

Discovery of small molecule degraders for modulating cell cycle

The cell cycle is a complex process that involves DNA replication, protein expression, and cell division. Dysregulation of the cell cycle is associated with various diseases. Cyclin-dependent kinases (CDKs) and their corresponding cyclins are major proteins that regulate the cell cycle. In contrast to inhibition, a new approach called proteolysis-targeting chimeras (PROTACs) and molecular glues can eliminate both enzymatic and scaffold functions of CDKs and cyclins, achieving targeted degradation. The field of PROTACs and molecular glues has developed rapidly in recent years. In this article, we aim to summarize the latest developments of CDKs and cyclin protein degraders. The selectivity, application, validation and the current state of each CDK degrader will be overviewed. Additionally, possible methods are discussed for the development of degraders for CDK members that still lack them. Overall, this article provides a comprehensive summary of the latest advancements in CDK and cyclin protein degraders, which will be helpful for researchers working on this topic.

Cyclers' kinases in cell division: from molecules to cancer therapy

Faithful eucaryotic cell division requires spatio-temporal orchestration of multiple sequential events. To ensure the dynamic nature of these molecular and morphological transitions, a swift modulation of key regulatory pathways is necessary. The molecular process that most certainly fits this description is phosphorylation, the post-translational modification provided by kinases, that is crucial to allowing the progression of the cell cycle and that culminates with the separation of two identical daughter cells. In detail, from the early stages of the interphase to the cytokinesis, each critical step of this process is tightly regulated by multiple families of kinases including the Cyclin-dependent kinases (CDKs), kinases of the Aurora, Polo, Wee1 families, and many others. While cell-cycle-related CDKs control the timing of the different phases, preventing replication machinery errors, the latter modulate the centrosome cycle and the spindle function, avoiding karyotypic abnormalities typical of chromosome instability. Such chromosomal abnormalities may result from replication stress (RS) and chromosome mis-segregation and are considered a hallmark of poor prognosis, therapeutic resistance, and metastasis in cancer patients. Here, we discuss recent advances in the understanding of how different families of kinases concur to govern cell cycle, preventing RS and mitotic infidelity. Additionally, considering the growing number of clinical trials targeting these molecules, we review to what extent and in which tumor context cell-cycle-related kinases inhibitors are worth exploiting as an effective therapeutic strategy.

Desmodin and isopongachromene as potential inhibitors of cyclin-dependent kinase 5: phytoconstituents targeting anticancer and neurological therapy

Cyclin-dependent kinase 5 (CDK5) is a proline-directed serine-threonine protein kinase vital for neuronal cell cycle arrest and differentiation. It activates by binding with p35 and p39 and is important for the functioning of the nervous system. A growing body of evidence suggests that CDK5 contributes to the onset and progression of neurodegeneration and tumorigenesis and represents itself as a potential therapeutic target. Our research illustrates virtual screening of phytochemicals from the IMPPAT (Indian Medicinal Plants, Phytochemistry and Therapeutics) library to search for potential inhibitors of CDK5. Initially, the compounds from the parent library were filtered out via their physicochemical properties following the Lipinski rule of five. Then sequentially, molecular docking-based virtual screening, PAINS filter, ADMET, PASS analysis, and molecular dynamics (MD) simulation were done using various computational tools to rule out adversities that can cause hindrances in the identification of potential inhibitors of CDK5. Finally, two compounds were selected via the extensive screening showing significant binding with CDK5 ATP-binding pocket and ultimately were selected as potent ATP-competitive inhibitors of CDK5. Finally, we propose that the elucidated compounds Desmodin and Isopongachromene can be used further in the drug discovery process and act as therapeutics in the medical industry to treat certain complex diseases, including cancer and neurodegeneration.Communicated by Ramaswamy H. Sarma.

Cdk5-Mediated Brain Unfolded Protein Response Upregulation Associated with Cognitive Impairments in Type 2 Diabetes and Ameliorative Action of NAC

The role of cyclin-dependent kinase 5 (Cdk5) in the normal functioning of the central nervous system and synaptic plasticity is well established. However, dysregulated kinase activity can have a significant impact on neurodegeneration and cognitive impairment. Cdk5 hyperactivation is linked to diabetes-associated neurodegeneration, but the underlying mechanism is not fully understood. Our study reveals that oxidative stress can lead to Cdk5 hyperactivity, which in turn is linked to neurodegeneration and cognitive impairment. Specifically, our experiments with N2A cells overexpressing Cdk5 and its activators p35 and p25 show ER stress, resulting in activation of the unfolded protein response (UPR) pathway. We identified Cdk5 as the epicenter of this regulatory process, leading to the activation of the CDK5-IRE1-XBP1 arm of UPR. Moreover, our study demonstrated that Cdk5 hyperactivation can lead to ER stress and activation of the UPR pathway, which may contribute to cognitive impairments associated with diabetes. Our findings also suggest that antioxidants such as NAC and GSH can decrease deregulated Cdk5 kinase activity and rescue cells from UPR-mediated ER stress. The accumulation of phosphorylated Tau protein in AD brain protein has been widely described earlier. Notably, we observed that oral treatment with NAC decreased Cdk5 kinase activity in the hippocampus, attenuated high levels of phospho-tau (ser396), and ameliorated memory and learning impairments in a type 2 diabetic (T2D) mouse model. Additionally, the high-fat-induced T2D model exhibits elevated phospho-tau levels, which are rescued by the NAC treatment. Taken together, these results suggest that targeting Cdk5 may be a promising therapeutic strategy for treating diabetes-associated cognitive impairments.

TP5: A Novel Therapeutic Approach Targeting Aberrant and Hyperactive CDK5/p25 for the Treatment of Colorectal Carcinoma

Colorectal carcinoma (CRC) is a prevalent cancer worldwide with a high mortality rate. Evidence suggests that increased expression of Cyclin-dependent kinase 5 (CDK5) contributes to cancer progression, making it a promising target for treatment. This study examined the efficacy of selectively inhibiting CDK5 in colorectal carcinoma using TP5, a small peptide that selectively inhibits the aberrant and hyperactive CDK5/p25 complex while preserving physiological CDK5/p35 functions. We analyzed TP5's impact on CDK5 activity, cell survival, apoptosis, the cell cycle, DNA damage, ATM phosphorylation, and reactive oxygen species (ROS) signaling in mitochondria, in CRC cell lines, both alone and in combination with chemotherapy. We also assessed TP5's efficacy on a xenograft mouse model with HCT116 cells. Our results showed that TP5 decreased CDK5 activity, impaired cell viability and colony formation, induced apoptosis, increased DNA damage, and led to the G1 phase arrest of cell cycle progression. In combination with irinotecan, TP5 demonstrated a synergy by leading to the accumulation of DNA damage, increasing the γH2A.X foci number, and inhibiting G2/M arrest induced by Sn38 treatment. TP5 alone or in combination with irinotecan increased mitochondrial ROS levels and inhibited tumor growth, prolonging mouse survival in the CRC xenograft animal model. These results suggest that TP5, either alone or in combination with irinotecan, is a promising therapeutic option for colorectal carcinoma.

p35 is a Crucial Player in NK-cell Cytotoxicity and TGFβ-mediated NK-cell Dysfunction

Natural killer (NK) cells are innate lymphocytes with cytotoxic activity. Understanding the factors regulating cytotoxicity is crucial for improving NK-cell adoptive therapies. Here, we studied a previously unknown role of p35 (CDK5R1), a coactivator of cyclin-dependent kinase 5 (CDK5) in NK-cell function. p35 expression was thought to be neuronal-specific and the majority of studies are still focused on neuronal cells. Here, we show that CDK5 and p35 are expressed in NK cells and are kinase-active. NK cells from p35 knockout mice were analyzed and showed significantly increased cytotoxicity against murine cancer cells, while they did not show any differences in cell numbers or maturation stages. We confirmed this using human NK cells transduced with p35 short hairpin RNA (shRNA), showing similar increase in cytotoxicity against human cancer cells. Overexpression of p35 in NK cells resulted in moderate decrease in cytotoxicity, while expressing a kinase-dead mutant of CDK5 displayed increased cytotoxicity. Together, these data suggest that p35 negatively regulates NK-cell cytotoxicity. Surprisingly, we found that TGFβ, a known negative regulator of NK-cell cytotoxicity, induces p35 expression in NK cells. NK cells cultured with TGFβ exhibit reduced cytotoxicity, while NK cells transduced with p35 shRNA or mutant CDK5 expression exhibited partial reversal of this inhibitory effect pointing to an interesting hypothesis that p35 plays an important role in TGFβ-mediated NK-cell exhaustion.

Significance

This study reports a role for p35 in NK-cell cytotoxicity and this might help to improve NK-cell adoptive therapy.

Stigmasterol isolated from Azadirachta indica flowers attenuated glutamate-induced neurotoxicity via downregulation of the Cdk5/p35/p25 signaling pathway in the HT-22 cells

BACKGROUND

Glutamate, an excitatory neurotransmitter, was elevated in the brain of neurodegenerative disease (ND) patients. The excessive glutamate induces Ca²⁺ influx and reactive oxygen species (ROS) production which exacerbates mitochondrial function, leading to mitophagy aberration, and hyperactivates Cdk5/p35/p25 signaling leading to neurotoxicity in ND. Stigmasterol, a phytosterol, has been reported for its neuroprotective effects; however, the underlying mechanism of stigmasterol on restoring glutamate-induced neurotoxicity is not fully investigated.

PURPOSE

We investigated the effect of stigmasterol, a compound isolated from Azadirachta indica (AI) flowers, on ameliorating glutamate-induced neuronal apoptosis in the HT-22 cells.

STUDY DESIGN

To further understand the underlying molecular mechanisms of stigmasterol, we investigated the effect of stigmasterol on Cdk5 expression, which was aberrantly expressed in glutamate-treated cells. Cell viability, Western blot analysis, and immunofluorescence are employed.

RESULTS

Stigmasterol significantly inhibited glutamate-induced neuronal cell death via attenuating ROS production, recovering mitochondrial membrane depolarization, and ameliorating mitophagy aberration by decreasing mitochondria/lysosome fusion and the ratio of LC3-II/LC3-I. In addition, stigmasterol treatment downregulated glutamate-induced Cdk5, p35, and p25 expression via enhancement of Cdk5 degradation and Akt phosphorylation. Although stigmasterol demonstrated neuroprotective effects on inhibiting glutamate-induced neurotoxicity, the efficiency of stigmasterol is limited due to its poor water solubility. We conjugated stigmasterol to soluble soybean polysaccharides with chitosan nanoparticles to overcome the limitations. We found that the encapsulated stigmasterol increased water solubility and enhanced the protective effect on attenuating the Cdk5/p35/p25 signaling pathway compared with free stigmasterol.

CONCLUSION

Our findings illustrate the neuroprotective effect and the improved utility of stigmasterol in inhibiting glutamate-induced neurotoxicity.

CDK5-PRMT1-WDR24 signaling cascade promotes mTORC1 signaling and tumor growth

The mammalian target of rapamycin complex1 (mTORC1) is a central regulator of metabolism and cell growth by sensing diverse environmental signals, including amino acids. The GATOR2 complex is a key component linking amino acid signals to mTORC1. Here, we identify protein arginine methyltransferase 1 (PRMT1) as a critical regulator of GATOR2. In response to amino acids, cyclin-dependent kinase 5 (CDK5) phosphorylates PRMT1 at S307 to promote PRMT1 translocation from nucleus to cytoplasm and lysosome, which in turn methylates WDR24, an essential component of GATOR2, to activate the mTORC1 pathway. Disruption of the CDK5-PRMT1-WDR24 axis suppresses hepatocellular carcinoma (HCC) cell proliferation and xenograft tumor growth. High PRMT1 protein expression is associated with elevated mTORC1 signaling in patients with HCC. Thus, our study dissects a phosphorylation- and arginine methylation-dependent regulatory mechanism of mTORC1 activation and tumor growth and provides a molecular basis to target this pathway for cancer therapy.

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.

Regulation of TFEB nuclear localization by HSP90AA1 promotes autophagy and longevity

TFEB (transcription factor EB) regulates multiple genes involved in the process of macroautophagy/autophagy and plays a critical role in lifespan determination. However, the detailed mechanisms that regulate TFEB activity are not fully clear. In this study, we identified a role for HSP90AA1 in modulating TFEB. HSP90AA1 was phosphorylated by CDK5 at Ser 595 under basal condition. This phosphorylation inhibited HSP90AA1, disrupted its binding to TFEB, and impeded TFEB's nuclear localization and subsequent autophagy induction. Pro-autophagy signaling attenuated CDK5 activity and enhanced TFEB function in an HSP90AA1-dependent manner. Inhibition of HSP90AA1 function or decrease in its expression significantly attenuated TFEB's nuclear localization and transcriptional function following autophagy induction. HSP90AA1-mediated regulation of a TFEB ortholog was involved in the extended lifespan of Caenorhabditis elegans in the absence of its food source bacteria. Collectively, these findings reveal that this regulatory process plays an important role in modulation of TFEB, autophagy, and longevity.Abbreviations : AL: autolysosome; AP: autophagosome; ATG: autophagy related; BafA1: bafilomycin A1; CDK5: cyclin-dependent kinase 5; CDK5R1: cyclin dependent kinase 5 regulatory subunit 1; CR: calorie restriction; FUDR: 5-fluorodeoxyuridine; HSP90AA1: heat shock protein 90 alpha family class A member 1; MAP1LC3: microtubule associated protein 1 light chain 3; NB: novobiocin sodium; SQSTM1: sequestosome 1; TFEB: transcription factor EB; WT: wild type.

Phosphorylation of the Human DNA Glycosylase NEIL2 Is Affected by Oxidative Stress and Modulates Its Activity

The DNA glycosylase NEIL2 plays a central role in maintaining genome integrity, in particular during oxidative stress, by recognizing oxidized base lesions and initiating repair of these via the base excision repair (BER) pathway. Post-translational modifications are important molecular switches that regulate and coordinate the BER pathway, and thereby enable a rapid and fine-tuned response to DNA damage. Here, we report for the first time that human NEIL2 is regulated by phosphorylation. We demonstrate that NEIL2 is phosphorylated by the two kinases cyclin-dependent kinase 5 (CDK5) and protein kinase C (PKC) in vitro and in human SH-SY5Y neuroblastoma cells. The phosphorylation of NEIL2 by PKC causes a substantial reduction in NEIL2 repair activity, while CDK5 does not directly alter the enzymatic activity of NEIL2 in vitro, suggesting distinct modes of regulating NEIL2 function by the two kinases. Interestingly, we show a rapid dephosphorylation of NEIL2 in response to oxidative stress in SH-SY5Y cells. This points to phosphorylation as an important modulator of NEIL2 function in this cellular model, not least during oxidative stress.

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.

Focusing on cyclin-dependent kinases 5: A potential target for neurological disorders

Cyclin-dependent kinases 5 (Cdk5) is a special member of proline-directed serine threonine kinase family. Unlike other Cdks, Cdk5 is not directly involved in cell cycle regulation but plays important roles in nervous system functions. Under physiological conditions, the activity of Cdk5 is tightly controlled by p35 or p39, which are specific activators of Cdk5 and highly expressed in post-mitotic neurons. However, they will be cleaved into the corresponding truncated forms namely p25 and p29 under pathological conditions, such as neurodegenerative diseases and neurotoxic insults. The binding to truncated co-activators results in aberrant Cdk5 activity and contributes to the initiation and progression of multiple neurological disorders through affecting the down-stream targets. Although Cdk5 kinase activity is mainly regulated through combining with co-activators, it is not the only way. Post-translational modifications of Cdk5 including phosphorylation, S-nitrosylation, sumoylation, and acetylation can also affect its kinase activity and then participate in physiological and pathological processes of nervous system. In this review, we focus on the regulatory mechanisms of Cdk5 and its roles in a series of common neurological disorders such as neurodegenerative diseases, stroke, anxiety/depression, pathological pain and epilepsy.

Extracellular vesicles derived from the choroid plexus trigger the differentiation of neural stem cells

The choroid plexus secrets cerebrospinal fluid (CSF) composed of electrolytes, cytokines, growth factors, metabolites and extracellular vesicles (EVs) that flow through the interconnected brain ventricles. On their course, CSF components can act as signals that affect, for example, neural stem cells (NSCs) residing in niches of the ventricular wall. We studied EV-born CSF signals in an in vitro culture system. We purified EVs from the secretome of a choroid plexus cell line (Z310 cells), and from primary choroid plexus cultures and co-cultured those EVs with NSCs isolated from the niche of the lateral and the third ventricle. EVsZ310 and EVsCHP were purified by differential centrifugation. This yielded fractions of EVs of 50-150-nm diameter that induced a complex multicellular network formation and NSC differentiation. Both types of EV converted the round NSCs to cells that extended long processes that contacted nearby, alike-shaped cells. Mass spectrometry showed that the differentiation-inducing EVZ310 were enriched for membrane and membrane-associated proteins involved in cell differentiation, membrane trafficking, and membrane organization. We hypothesize that this type of EV Z310 cargo causes changes of stem cell morphology that leads to multicellular networks in the niches. This cell-shape transition may represent an initial step in NSC differentiation.

Cdk5 regulates IP3R1-mediated Ca2+ dynamics and Ca2+-mediated cell proliferation

Loss of cyclin-dependent kinase 5 (Cdk5) in the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) increases ER–mitochondria tethering and ER Ca²⁺ transfer to the mitochondria, subsequently increasing mitochondrial Ca²⁺ concentration ([Ca²⁺]_mt). This suggests a role for Cdk5 in regulating intracellular Ca²⁺ dynamics, but how Cdk5 is involved in this process remains to be explored. Using ex vivo primary mouse embryonic fibroblasts (MEFs) isolated from Cdk5^−/− mouse embryos, we show here that loss of Cdk5 causes an increase in cytosolic Ca²⁺concentration ([Ca²⁺]_cyt), which is not due to reduced internal Ca²⁺ store capacity or increased Ca²⁺ influx from the extracellular milieu. Instead, by stimulation with ATP that mediates release of Ca²⁺ from internal stores, we determined that the rise in [Ca²⁺]_cyt in Cdk5^−/− MEFs is due to increased inositol 1,4,5-trisphosphate receptor (IP3R)-mediated Ca²⁺ release from internal stores. Cdk5 interacts with the IP3R1 Ca²⁺ channel and phosphorylates it at Ser₄₂₁. Such phosphorylation controls IP3R1-mediated Ca²⁺ release as loss of Cdk5, and thus, loss of IP3R1 Ser₄₂₁ phosphorylation triggers an increase in IP3R1-mediated Ca²⁺ release in Cdk5^−/− MEFs, resulting in elevated [Ca²⁺]_cyt. Elevated [Ca²⁺]_cyt in these cells further induces the production of reactive oxygen species (ROS), which upregulates the levels of Nrf2 and its targets, Prx1 and Prx2. Cdk5^−/− MEFs, which have elevated [Ca²⁺]_cyt, proliferate at a faster rate compared to wt, and Cdk5^−/− embryos have increased body weight and size compared to their wt littermates. Taken together, we show that altered IP3R1-mediated Ca²⁺ dynamics due to Cdk5 loss correspond to accelerated cell proliferation that correlates with increased body weight and size in Cdk5^−/− embryos.

The role of Cdk5 in neurological disorders

Neurological disorders are a group of disorders with motor, sensory or cognitive damage, caused by dysfunction of the central or peripheral nervous system. Cyclin-dependent kinases 5 (Cdk5) is of vital significance for the development of the nervous system, including the migration and differentiation of neurons, the formation of synapses, and axon regeneration. However, when the nervous system is subject to pathological stimulation, aberrant activation of Cdk5 will induce abnormal phosphorylation of a variety of substrates, resulting in a cascade signaling pathway, and thus lead to pathological changes. Cdk5 is intimately related to the pathological mechanism of a variety of neurological disorders, such as A-β protein formation in Alzheimer’s disease, mitochondrial fragmentation in cerebral ischemia, and apoptosis of dopaminergic neurons in Parkinson’s disease. It is worth noting that Cdk5 inhibitors have been reported to have neuroprotective effects by inhibiting related pathological processes. Therefore, in this review, we will briefly introduce the physiological and pathological mechanisms of Cdk5 in the nervous system, focusing on the recent advances of Cdk5 in neurological disorders and the prospect of targeted Cdk5 for the treatment of neurological disorders.

The Role of CDK Pathway Dysregulation and Its Therapeutic Potential in Soft Tissue Sarcoma

Soft tissue sarcomas (STSs) are tumors that are challenging to treat due to their pathologic and molecular heterogeneity and their tumor biology that is not yet fully understood. Recent research indicates that dysregulation of cyclin-dependent kinase (CDK) signaling pathways can be a strong driver of sarcogenesis. CDKs are enzyme forms that play a crucial role in cell-cycle control and transcription. They belong to the protein kinases group and to the serine/threonine kinases subgroup. Recently identified CDK/cyclin complexes and established CDK/cyclin complexes that regulate the cell cycle are involved in the regulation of gene expression through phosphorylation of critical components of transcription and pre-mRNA processing mechanisms. The current and continually growing body of data shows that CDKs play a decisive role in tumor development and are involved in the proliferation and growth of sarcoma cells. Since the abnormal expression or activation of large numbers of CDKs is considered to be characteristic of cancer development and progression, dysregulation of the CDK signaling pathways occurs in many subtypes of STSs. This review discusses how reversal and regulation can be achieved with new therapeutics and summarizes the current evidence from studies regarding CDK modulation for STS treatment.

BmCDK5 Affects Cell Proliferation and Cytoskeleton Morphology by Interacting with BmCNN in Bombyx mori

The ordered cell cycle is important to the proliferation and differentiation of living organisms. Cyclin-dependent kinases (CDKs) perform regulatory functions in different phases of the cell cycle process to ensure order. We identified a homologous gene of the Cyclin-dependent kinase family, BmCDK5, in Bombyx mori. BmCDK5 contains the STKc_CDK5 domain. The BmCDK5 gene was highly expressed in S phase. Overexpression of the BmCDK5 gene accelerates the process of the cell cycle's mitotic period (M) and promotes cell proliferation; knocking out the BmCDK5 gene inhibited cell proliferation. Furthermore, we identified a protein, BmCNN, which can interact with BmCDK5 and represents the same express patterns as the BmCDK5 gene in the cell cycle phase and the spatial-temporal expression of B. mori. This study revealed that BmCDK5 and BmCNN play roles in promoting cell proliferation and regulating cytoskeleton morphology, but do not induce expression changes in microtubule protein. Therefore, our findings provide a new insight; the BmCDK5 gene has a regulatory effect on the cell cycle and proliferation of B. mori, which is presumably due to the interaction between BmCDK5 and BmCNN regulating changes in the cytoskeleton.

Computational Study of the Allosteric Effects of p5 on CDK5-p25 Hyperactivity as Alternative Inhibitory Mechanisms in Neurodegeneration

The cyclin-dependent kinase (CDK5) forms a stable complex with its activator p25, leading to the hyperphosphorylation of tau proteins and to the formation of plaques and tangles that are considered to be one of the typical causes of Alzheimer's disease (AD). Hence, the pathological CDK5-p25 complex is a promising therapeutic target for AD. Small peptides, obtained from the truncation of CDK5 physiological activator p35, have shown promise in inhibiting the pathological complex effectively while also crossing the blood-brain barrier. One such small 24-residue peptide, p5, has shown selective inhibition toward the pathological complex in vivo. Our previous research focused on the characterization of a computationally predicted CDK5-p5 binding mode and of its pharmacophore, which was consistent with competitive inhibition. In continuation of our previous work, herein, we investigate four additional binding modes to explore other possible mechanisms of interaction between CDK5 and p5. The quantitative description of the pharmacophore is consistent with both competitive and allosteric p5-induced inhibition mechanisms of CDK5-p25 pathology. The gained insights can direct further in vivo/in vitro tests and help design small peptides, linear or cyclic, or peptidomimetic compounds as adjuvants of orthosteric inhibitors or as part of a cocktail of drugs with enhanced effectiveness and lower side effects.

d-Pinitol promotes tau dephosphorylation through a cyclin-dependent kinase 5 regulation mechanism: A new potential approach for tauopathies?

Background and Purpose

Recent evidence links brain insulin resistance with neurodegenerative diseases, where hyperphosphorylated tau protein contributes to neuronal cell death. In the present study, we aimed to evaluate if d‐pinitol inositol, which acts as an insulin sensitizer, affects the phosphorylation status of tau protein.

Experimental Approach

We studied the pharmacological effect of d‐pinitol on insulin signalling and tau phosphorylation in the hippocampus of Wistar and Zucker rats. To this end, we evaluated by western blotting the Akt pathway and its downstream proteins as being one of the main insulin‐mediator pathways. Also, we explored the functional status of additional kinases phosphorylating tau, including PKA, ERK1/2, AMPK and CDK5. We utilized the 3xTg mouse model as a control for tauopathy, since it carries tau mutations that promote phosphorylation and aggregation.

Key Results

Surprisingly, we discovered that oral d‐pinitol treatment lowered tau phosphorylation significantly, but not through the expected kinase GSK‐3 regulation. An extensive search for additional kinases phosphorylating tau revealed that this effect was mediated through a mechanism dependent on the reduction of the activity of the CDK5, affecting both its p35 and p25 subunits. This effect disappeared in leptin‐deficient Zucker rats, uncovering that the association of leptin deficiency, obesity, dyslipidaemia and hyperinsulinaemia abrogates d‐pinitol actions on tau phosphorylation. The 3xTg mice confirmed d‐pinitol effectiveness in a genetic AD‐tauopathy.

Conclusion and Implications

The present findings suggest that d‐pinitol, by regulating CDK5 activity through a decrease of CDK5R1, is a potential drug for developing treatments for neurological disorders such as tauopathies.

CRISPR/Cas9 gene editing: a new approach for overcoming drug resistance in cancer

The CRISPR/Cas9 system is an RNA-based adaptive immune system in bacteria and archaea. Various studies have shown that it is possible to target a wide range of human genes and treat some human diseases, including cancers, by the CRISPR/Cas9 system. In fact, CRISPR/Cas9 gene editing is one of the most efficient genome manipulation techniques. Studies have shown that CRISPR/Cas9 technology, in addition to having the potential to be used as a new therapeutic approach in the treatment of cancers, can also be used to enhance the effectiveness of existing treatments. Undoubtedly, the issue of drug resistance is one of the main obstacles in the treatment of cancers. Cancer cells resist anticancer drugs by a variety of mechanisms, such as enhancing anticancer drugs efflux, enhancing DNA repair, enhancing stemness, and attenuating apoptosis. Mutations in some proteins of different cellular signaling pathways are associated with these events and drug resistance. Recent studies have shown that the CRISPR/Cas9 technique can be used to target important genes involved in these mechanisms, thereby increasing the effectiveness of anticancer drugs. In this review article, studies related to the applications of this technique in overcoming drug resistance in cancer cells will be reviewed. In addition, we will give a brief overview of the limitations of the CRISP/Cas9 gene-editing technique.

Valproic Acid-Induced Anxiety and Depression Behaviors are Ameliorated in p39 Cdk5 Activator-Deficient Mice

Valproic acid (VPA) is a drug used for the treatment of epilepsy, seizures, migraines, and bipolar disorders. Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase activated by p35 or p39 in neurons and plays a role in a variety of neuronal functions, including psychiatric behaviors. We previously reported that VPA suppressed Cdk5 activity by reducing the expression of p35 in cultured cortical neurons, leaving p39 unchanged. In this study, we asked for the role of Cdk5 in VPA-induced anxiety and depression behaviors. Wild-type (WT) mice displayed increased anxiety and depression after chronic administration of VPA for 14 days, when the expression of p35 was decreased. To clarify their relationship, we used p39 knockout (KO) mice, in which p35 is the only Cdk5 activator. When p39 KO mice were treated chronically with VPA, unexpectedly, they exhibited fewer anxiety and depression behaviors than WT mice. The effects were p39 cdk5r2 gene-dosage dependent. Together, these results indicate that Cdk5-p39 plays a specific role in VPA-induced anxiety and depression behaviors.

Cdk5-mediated JIP1 phosphorylation regulates axonal outgrowth through Notch1 inhibition

BACKGROUND

Activated Cdk5 regulates a number of processes during nervous system formation, including neuronal differentiation, growth cone stabilization, and axonal growth. Cdk5 phosphorylates its downstream substrates located in axonal growth cones, where the highly expressed c-Jun N-terminal kinase (JNK)-interacting protein1 (JIP1) has been implicated as another important regulator of axonal growth. In addition, stringent control of the level of intracellular domain of Notch1 (Notch1-IC) plays a regulatory role in axonal outgrowth during neuronal differentiation. However, whether Cdk5-JIP1-Notch1 cooperate to regulate axonal outgrowth, and the mechanism of such joint contribution to this pathway, is presently unknown, and here we explore their potential interaction.

RESULTS

Our interactome screen identified JIP1 as an interactor of p35, a Cdk5 activator, and we sought to explore the relationship between Cdk5 and JIP1 on the regulation of axonal outgrowth. We demonstrate that JIP1 phosphorylated by Cdk5 at Thr205 enhances axonal outgrowth and a phosphomimic JIP1 rescues the axonal outgrowth defects in JIP1^-/- and p35^-/- neurons. Axonal outgrowth defects caused by the specific increase of Notch1 in JIP1^-/- neurons are rescued by Numb-mediated inhibition of Notch1. Finally, we demonstrate that Cdk5 phosphorylation of JIP1 further amplifies the phosphorylation status of yet another Cdk5 substrate E3-ubiquitin ligase Itch, resulting in increased Notch1 ubiquitination.

CONCLUSIONS

Our findings identify a potentially critical signaling axis involving Cdk5-JIP1-Itch-Notch1, which plays an important role in the regulation of CNS development. Future investigation into the way this pathway integrates with additional pathways regulating axonal growth will further our knowledge of normal central nervous system development and pathological conditions.

Systemic Administration of a Brain Permeable Cdk5 Inhibitor Alters Neurobehavior

Cyclin-dependent kinase 5 (Cdk5) is a crucial regulator of neuronal signal transduction. Cdk5 activity is implicated in various neuropsychiatric and neurodegenerative conditions such as stress, anxiety, depression, addiction, Alzheimer's disease, and Parkinson's disease. While constitutive Cdk5 knockout is perinatally lethal, conditional knockout mice display resilience to stress-induction, enhanced cognition, neuroprotection from stroke and head trauma, and ameliorated neurodegeneration. Thus, Cdk5 represents a prime target for treatment in a spectrum of neurological and neuropsychiatric conditions. While intracranial infusions or treatment of acutely dissected brain tissue with compounds that inhibit Cdk5 have allowed the study of kinase function and corroborated conditional knockout findings, potent brain-penetrant systemically deliverable Cdk5 inhibitors are extremely limited, and no Cdk5 inhibitor has been approved to treat any neuropsychiatric or degenerative diseases to date. Here, we screened aminopyrazole-based analogs as potential Cdk5 inhibitors and identified a novel analog, 25-106, as a uniquely brain-penetrant anti-Cdk5 drug. We characterize the pharmacokinetic and dynamic responses of 25-106 in mice and functionally validate the effects of Cdk5 inhibition on open field and tail-suspension behaviors. Altogether, 25-106 represents a promising preclinical Cdk5 inhibitor that can be systemically administered with significant potential as a neurological/neuropsychiatric therapeutic.

Substrate-selective positive allosteric modulation of PTPRD’s phosphatase by flavonols

The receptor type protein tyrosine phosphatase D (PTPRD) is expressed by neurons and implicated in interesting phenotypes that include reward from addictive substances, restless leg syndrome and neurofibrillary tangle densities in Alzheimer's disease (AD-NFTs). However, the brain phosphotyrosine phosphoprotein (PTPP) substrates for PTPRD's phosphatase have not been clearly defined. Although we have identified small molecule inhibitors of PTPRD's phosphatase that are candidates for reducing reward from addictive substances, no positive allosteric modulators of this phosphatase that might be candidates for reducing AD-NFTs have been reported. We now report identification of candidate brain substrates for PTPRD based on their increased phosphorylation in knockout vs wildtype animals, coexpression with PTPRD in neuronal subtypes and brisk dephosphorylation by recombinant human PTPRD phosphatase. We also report discovery that quercetin and other flavonols, though not closely-related flavones, enhance rates of PTPRD's dephosphorylation of a group of these candidate substrate PTPPs but not others. This substrate-selective positive allosteric modulation provides a novel pharmacological action. Flavonol-mediated increases in PTPRD's dephosphorylation of the GSK3 β and α kinases that hyperphosphorylate tau, the major component of AD-NFTs, could help to explain recent data concerning genetic and dietary impacts on Alzheimer's disease.

Alzheimer’s Disease and Tau Self-Assembly: In the Search of the Missing Link

Alzheimer’s disease (AD) is a multifactorial neurodegenerative disease characterized by progressive cognitive impairment, apathy, and neuropsychiatric disorders. Two main pathological hallmarks have been described: neurofibrillary tangles, consisting of tau oligomers (hyperphosphorylated tau) and Aβ plaques. The influence of protein kinases and phosphatases on the hyperphosphorylation of tau is already known. Hyperphosphorylated tau undergoes conformational changes that promote its self-assembly. However, the process involving these mechanisms is yet to be elucidated. In vitro recombinant tau can be aggregated by the action of polyanions, such as heparin, arachidonic acid, and more recently, the action of polyphosphates. However, how that process occurs in vivo is yet to be understood. In this review, searching the most accurate and updated literature on the matter, we focus on the precise molecular events linking tau modifications, its misfolding and the initiation of its pathological self-assembly. Among these, we can identify challenges regarding tau phosphorylation, the link between tau heteroarylations and the onset of its self-assembly, as well as the possible metabolic pathways involving natural polyphosphates, that may play a role in tau self-assembly.

Histone Deacetylases 1 and 2 in Memory Function

Histone deacetylases (HDACs) have been implicated in learning and memory, and their dysregulation has been linked to cognitive impairment in brain aging and neurodegenerative diseases. In this review, we focus on HDAC1 and HDAC2, highlighting recent progress on their roles in regulating brain function through distinct mechanisms, including gene repression and DNA repair pathways. Moreover, we discuss evidence demonstrating how HDAC1 and HDAC2 could be modulated and their potential as targets to combat memory deficits.

ccd-5, a novel cdk-5 binding partner, regulates pioneer axon guidance in the ventral nerve cord of Caenorhabditis elegans

During nervous system development, axons navigate complex environments to reach synaptic targets. Early extending axons must interact with guidance cues in the surrounding tissue, while later extending axons can interact directly with earlier "pioneering" axons, "following" their path. In Caenorhabditis elegans, the AVG neuron pioneers the right axon tract of the ventral nerve cord. We previously found that aex-3, a rab-3 guanine nucleotide exchange factor, is essential for AVG axon navigation in a nid-1 mutant background and that aex-3 might be involved in trafficking of UNC-5, a receptor for the guidance cue UNC-6/netrin. Here, we describe a new gene in this pathway: ccd-5, a putative cdk-5 binding partner. ccd-5 mutants exhibit increased navigation defects of AVG pioneer as well as interneuron and motor neuron follower axons in a nid-1 mutant background. We show that ccd-5 acts in a pathway with cdk-5, aex-3, and unc-5. Navigation defects of follower interneuron and motoneuron axons correlate with AVG pioneer axon defects. This suggests that ccd-5 mostly affects pioneer axon navigation and that follower axon defects are largely a secondary consequence of pioneer navigation defects. To determine the consequences for nervous system function, we assessed various behavioral and movement parameters. ccd-5 single mutants have no significant movement defects, and nid-1 ccd-5 double mutants are less responsive to mechanosensory stimuli compared with nid-1 single mutants. These surprisingly minor defects indicate either a high tolerance for axon guidance defects within the motor circuit and/or an ability to maintain synaptic connections among commonly misguided axons.

Kai-Xin-San Inhibits Tau Pathology and Neuronal Apoptosis in Aged SAMP8 Mice

Alzheimer’s disease (AD) is an age-related neurological disorder. Currently, there is no effective cure for AD due to its complexity in pathogenesis. In light of the complex pathogenesis of AD, the traditional Chinese medicine (TCM) formula Kai-Xin-San (KXS), which was used for amnesia treatment, has been proved to improve cognitive function in AD animal models. However, the active ingredients and the mechanism of KXS have not yet been clearly elucidated. In this study, network pharmacology analysis predicts that KXS yields 168 candidate compounds acting on 863 potential targets, 30 of which are associated with AD. Enrichment analysis revealed that the therapeutic mechanisms of KXS for AD are associated with the inhibition of Tau protein hyperphosphorylation, inflammation, and apoptosis. Therefore, we chose 7-month-old senescence-accelerated mouse prone 8 (SAMP8) mice as AD mouse model, which harbors the behavioral and pathological hallmarks of AD. Subsequently, the potential underlying action mechanisms of KXS on AD predicted by the network pharmacology analyses were experimentally validated in SAMP8 mice after intragastric administration of KXS for 3 months. We observed that KXS upregulated AKT phosphorylation, suppressed GSK3β and CDK5 activation, and inhibited the TLR4/MyD88/NF-κB signaling pathway to attenuate Tau hyperphosphorylation and neuroinflammation, thus suppressing neuronal apoptosis and improving the cognitive impairment of aged SAMP8 mice. Taken together, our findings reveal a multi-component and multi-target therapeutic mechanism of KXS for attenuating the progression of AD, contributing to the future development of TCM modernization, including KXS, and broader clinical application.

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.

Lessons Learned from Past Cyclin-Dependent Kinase Drug Discovery Efforts

Inhibition of cyclin-dependent kinases (CDKs) has become an effective therapeutic strategy for treating various diseases, especially cancer. Over almost three decades, although great efforts have been made to discover CDK inhibitors, many of which have entered clinical trials, only four CDK inhibitors have been approved. In the process of CDK inhibitor development, many difficulties and misunderstandings have hampered their discovery and clinical applications, which mainly include inadequate understanding of the biological functions of CDKs, less attention paid to pan- and multi-CDK inhibitors, nonideal isoform selectivity of developed selective CDK inhibitors, overlooking the metabolic stability of early discovered CDK inhibitors, no effective resistance solutions, and a lack of available combination therapy and effective biomarkers for CDK therapies. After reviewing the mechanisms of CDKs and the research progress of CDK inhibitors, this perspective summarizes and discusses these difficulties or lessons, hoping to facilitate the successful discovery of more useful CDK inhibitors.

Inhibition of Cdk5 Ameliorates Skeletal Bone Loss in Glucocorticoid-Treated Mice

Glucocorticoids (GCs) are widely used to treat inflammatory diseases. However, their long-term use leads to glucocorticoid-induced osteoporosis, increasing morbidity and mortality. Both anabolic and anti-resorptive drugs are used to counteract GC-induced bone loss, however, they are expensive and/or have major side effects. Therefore, identifying new targets for cost-effective, small-molecule inhibitors is essential. We recently identified cyclin-dependent kinase 5 (Cdk5) as a suppressor of osteoblast differentiation and showed that its inhibition with roscovitine promoted osteoblastogenesis, thus improving the skeletal bone mass and fracture healing. Here, we assessed whether Cdk5 knockdown or inhibition could also reverse the GC-mediated suppression of osteoblast differentiation, bone loss, and fracture healing. We first demonstrated that Cdk5 silencing abolished the dexamethasone (Dex)-induced downregulation of alkaline phosphatase (Alp) activity, osteoblast-specific marker gene expression (Runx2, Sp7, Alpl, and Bglap), and mineralization. Similarly, Cdk5 inhibition rescued Dex-induced suppression of Alp activity. We further demonstrated that Cdk5 inhibition reversed prednisolone (Pred)-induced bone loss in mice, due to reduced osteoclastogenesis rather than improved osteoblastogenesis. Moreover, we revealed that Cdk5 inhibition failed to improve Pred-mediated impaired fracture healing. Taken together, we demonstrated that Cdk5 inhibition with roscovitine ameliorated GC-mediated bone loss but did not reverse GC-induced compromised fracture healing in mice.

Phosphorylation of p53 by Cdk5 contributes to benzo[a]pyrene-induced neuronal apoptosis

Benzo[a]pyrene (B[a]P) is a ubiquitous carcinogenic pollutant in the environment, however, the potential neurotoxic effects of B[a]P has not been elucidated clearly. In the present study, we explored the potential involvement of p53 phosphorylation by Cdk5 in B[a]P-induced neuronal apoptosis at both in vitro and in vivo settings. For in vitro studies, primary cortical neurons isolated from the brains of Sprague Dawley (SD) rat pup were exposed to 0, 10, 20, and 40 μM of B[a]P for 12, 24, or 48 h. For in vivo studies, SD rats were injected intraperitoneally with 0, 1.0, 2.5, and 6.25 mg/kg of B[a]P every other day for 1, 2, or 3 months. Our results demonstrated that exposure to B[a]P caused a dose- and a time-dependent increase in neuronal apoptotic ratio in both in vitro and in vivo studies. There was also a dose- and a time-dependent upregulation of p35, p25, Cdk5, and phosphorylated p53 at Ser15 after B[a]P exposure. In order to explore whether B[a]P-induced increased neuronal apoptosis was through Cdk5/p53 pathway, roscovitine, a specific Cdk5 inhibitor, was applied to pretreat neurons prior to B[a]P exposure. The results showed that pretreatment of neurons with roscovitine partially rescued cells from B[a]P-induced apoptosis, and alleviated B[a]P-induced upregulation of phosphorylated p53 at Ser15. Our results suggest that Cdk5/p53 signaling pathway may be involved in B[a]P-induced neuronal apoptosis, which will provide information to further elucidate the molecular mechanisms of B[a]P-induced neurotoxicity.