Cyclin-dependent kinase 5 (CDK5) inhibitors in Parkinson disease.

Cyclin-dependent kinase 5 (Cdk5) is a protein expressed in postmitotic neurons in the central nervous system (CNS). Cdk5 is activated by p35 and p39 which are neuron regulatory subunits. Cdk5/p35 complex is activated by calpain protease to form Cdk5/p35 which has a neuroprotective effect by regulating the synaptic plasticity and memory functions. However, exaggerated Cdk5 is implicated in different types of neurodegenerative diseases including Parkinson disease (PD). Therefore, modulation of Cdk5 signalling may mitigate PD neuropathology. Therefore, the aim of the present review was to discuss the critical role of Cdk5 in the pathogenesis of PD, and how Cdk5 inhibitors are effectual in the management of PD. In conclusion, overactivated Cdk5 is involved the development of neurodegeneration, and Cdk5/calpain inhibitors such as statins, metformin, fenofibrates and rosiglitazone can attenuate the progression of PD neuropathology.

High-Caloric Diets in Adolescence Impair Specific GABAergic Subpopulations, Neurogenesis, and Alter Astrocyte Morphology.

We compared the effects of two different high-caloric diets administered to 4-week-old rats for 12 weeks: a diet rich in sugar (30% sucrose) and a cafeteria diet rich in sugar and high-fat foods. We focused on the hippocampus, particularly on the gamma-aminobutyric acid (GABA)ergic system, including the Ca2+-binding proteins parvalbumin (PV), calretinin (CR), calbindin (CB), and the neuropeptides somatostatin (SST) and neuropeptide Y (NPY). We also analyzed the density of cholinergic varicosities, brain-derived neurotrophic factor (BDNF), reelin (RELN), and cyclin-dependent kinase-5 (CDK-5) mRNA levels, and glial fibrillary acidic protein (GFAP) expression. The cafeteria diet reduced PV-positive neurons in the granular layer, hilus, and CA1, as well as NPY-positive neurons in the hilus, without altering other GABAergic populations or overall GABA levels. The high-sugar diet induced a decrease in the number of PV-positive cells in CA3 and an increase in CB-positive cells in the hilus and CA1. No alterations were observed in the cholinergic varicosities. The cafeteria diet also reduced the relative mRNA expression of RELN without significant changes in BDNF and CDK5 levels. The cafeteria diet increased the number but reduced the length of the astrocyte processes. These data highlight the significance of determining the mechanisms mediating the observed effects of these diets and imply that the cognitive impairments previously found might be related to both the neuroinflammation process and the reduction in PV, NPY, and RELN expression in the hippocampal formation.

CDK5-USP30 signaling pathway regulates MAVS-mediated inflammation via suppressing mitophagy in MPTP/MPP+ PD model.

The discovery of MPTP, an industrial chemical and contaminant of illicit narcotics, which causes parkinsonism in humans, non-human primates and rodents, has led to environmental pollutants exposure being convicted as key candidate in Parkinson's disease (PD) pathogenesis. Though MPTP-induced mitochondrial dysfunction and neuroinflammation are mainly responsible for the causative issue of MPTP neurotoxicity, the underlying mechanism involved remains unclear. Here, we reveal a novel signaling mechanism of CDK5-USP30-MAVS regulating MPTP/MPP+ induced PD. MPP+ (the toxic metabolite of MPTP) treatment not only led to the increased protein levels of USP30 but also to mitophagy inhibition, mitochondrial dysfunction, and MAVS-mediated inflammation in BV2 microglial cells. Both mitophagy stimulation (Urolithin A administration) and USP30 knockdown relieved MAVS-mediated inflammation via restoring mitophagy and mitochondrial function in MPP+-induced cell model. Notably, MPTP/MPP+-induced CDK5 activation regulated USP30 phosphorylation at serine 216 to stabilize USP30. Moreover, CDK5-USP30 pathway promoted MAVS-mediated inflammation in MPTP/MPP+-induced PD model. Inhibition of CDK5 not only had a protective effect on MPP+-induced cell model of PD via suppressing the upregulation of USP30 and the activation of MAVS inflammation pathway in vitro, but also prevented neurodegeneration in vivo and alleviated movement impairment in MPTP mouse model of PD. Overall, our study reveal that CDK5 blocks mitophagy through phosphorylating USP30 and activates MAVS inflammation pathway in MPTP/MPP+-induced PD model, which suggests that CDK5-USP30-MAVS signaling pathway represents a valuable treatment strategy for PD induced by environmental neurotoxic pollutants in relation to MPTP.

miR-4645-3p attenuates podocyte injury and mitochondrial dysfunction in diabetic kidney disease by targeting Cdk5.

Podocyte injury plays a critical role in the progression of diabetic kidney disease (DKD), but the underlying cellular and molecular mechanisms remain poorly understanding. MicroRNAs (miRNAs) can disrupt gene expression by inducing translation inhibition and mRNA degradation, and recent evidence has shown that miRNAs may play a key role in many kidney diseases. In this study, we identified miR-4645-3p by global transcriptome expression profiling as one of the major downregulated miRNAs in high glucose-cultured podocytes. Moreover, whether DKD patients or STZ-induced diabetic mice, expression of miR-4645-3p was also significantly decreased in kidney. In the podocytes cultured by normal glucose, inhibition of miR-4645-3p expression promoted mitochondrial damage and podocyte apoptosis. In the podocytes cultured by high glucose (30 mM glucose), overexpression of miR-4645-3p significantly attenuated mitochondrial dysfunction and podocyte apoptosis induced by high glucose. Furthermore, we found that miR-4645-3p exerted protective roles by targeting Cdk5 inhibition. In vitro, miR-4645-3p obviously antagonized podocyte injury by inhibiting overexpression of Cdk5. In vivo of diabetic mice, podocyte injury, proteinuria, and impaired renal function were all effectively ameliorated by treatment with exogenous miR-4645-3p. Collectively, these findings demonstrate that miR-4645-3p can attenuate podocyte injury and mitochondrial dysfunction in DKD by targeting Cdk5. Sustaining the expression of miR-4645-3p in podocytes may be a novel strategy to treat DKD.

Verification of AKT and CDK5 Gene and RNA Interference Combined with Irradiation to Mediate Fertility Changes in Plutella xylostella (Linnaeus).

Plutella xylostella (Linnaeus) mainly damages cruciferous crops and causes huge economic losses. Presently, chemical pesticides dominate its control, but prolonged use has led to the development of high resistance. In contrast, the sterile insect technique provides a preventive and control method to avoid the development of resistance. We discovered two genes related to the reproduction of Plutella xylostella and investigated the efficacy of combining irradiation with RNA interference for pest management. The results demonstrate that after injecting PxAKT and PxCDK5, there was a significant decrease of 28.06% and 25.64% in egg production, and a decrease of 19.09% and 15.35% in the hatching rate compared to the control. The ratio of eupyrene sperm bundles to apyrene sperm bundles also decreased. PxAKT and PxCDK5 were identified as pivotal genes influencing male reproductive processes. We established a dose-response relationship for irradiation (0-200 Gy and 200-400 Gy) and derived the irradiation dose equivalent to RNA interference targeting PxAKT and PxCDK5. Combining RNA interference with low-dose irradiation achieved a sub-sterile effect on Plutella xylostella, surpassing either irradiation or RNA interference alone. This study enhances our understanding of the genes associated with the reproduction of Plutella xylostella and proposes a novel approach for pest management by combining irradiation and RNA interference.

Metformin inhibits nerve growth factor-induced sympathetic neuron differentiation through p35/CDK5 inhibition.

The authors' previous research has shown the pivotal roles of cyclin-dependent kinase 5 (CDK5) and its regulatory protein p35 in nerve growth factor (NGF)-induced differentiation of sympathetic neurons in PC12 cells. During the process of differentiation, neurons are susceptible to environmental influences, including the effects of drugs. Metformin is commonly used in the treatment of diabetes and its associated symptoms, particularly in diabetic neuropathy, which is characterized by dysregulation of the sympathetic neurons. However, the impacts of metformin on sympathetic neuronal differentiation remain unknown. In this study, we investigated the impact of metformin on NGF-induced sympathetic neuronal differentiation using rat pheochromocytoma PC12 cells as a model. We examined the regulation of TrkA-p35/CDK5 signaling in NGF-induced PC12 differentiation. Our results demonstrate that metformin reduces NGF-induced PC12 differentiation by inactivating the TrkA receptor, subsequently inhibiting ERK and EGR1. Inhibition of this cascade ultimately leads to the downregulation of p35/CDK5 in PC12 cells. Furthermore, metformin inhibits the activation of the presynaptic protein Synapsin-I, a substrate of CDK5, in PC12 differentiation. In addition, metformin alters axonal and synaptic bouton formation by inhibiting p35 at both the axons and axon terminals in fully differentiated PC12 cells. In summary, our study elucidates that metformin inhibits sympathetic neuronal differentiation in PC12 cells by disrupting TrkA/ERK/EGR1 and p35/CDK5 signaling. This research contributes to uncovering a novel signaling mechanism in drug response during sympathetic neuronal differentiation, enhancing our understanding of the intricate molecular processes governing this critical aspect of neurodevelopment.NEW & NOTEWORTHY This study unveils a novel mechanism influenced by metformin during sympathetic neuronal differentiation. By elucidating its inhibitory effects from the nerve growth factor (NGF) receptor, TrkA, to the p35/CDK5 signaling pathways, we advance our understanding of metformin's mechanisms of action and emphasize its potential significance in the context of drug responses during sympathetic neuronal differentiation.

Design, synthesis, and biological evaluation of dinaciclib and CAN508 hybrids as CDK inhibitors.

The scaffolds of two known CDK inhibitors (CAN508 and dinaciclib) were the starting point for synthesizing two series of pyarazolo[1,5-a]pyrimidines to obtain potent inhibitors with proper selectivity. The study presented four promising compounds; 10d, 10e, 16a, and 16c based on cytotoxic studies. Compound 16a revealed superior activity in the preliminary anticancer screening with GI % = 79.02-99.13 against 15 cancer cell lines at 10 µM from NCI full panel 60 cancer cell lines and was then selected for further investigation. Furthermore, the four compounds revealed good safety profile toward the normal cell lines WI-38. These four compounds were subjected to CDK inhibitory activity against four different isoforms. All of them showed potent inhibition against CDK5/P25 and CDK9/CYCLINT. Compound 10d revealed the best activity against CDK5/P25 (IC50 = 0.063 µM) with proper selectivity index against CDK1 and CDK2. Compound 16c exhibited the highest inhibitory activity against CDK9/CYCLINT (IC50 = 0.074 µM) with good selectivity index against other isoforms. Finally, docking simulations were performed for compounds 10e and 16c accompanied by molecular dynamic simulations to understand their behavior in the active site of the two CDKs with respect to both CAN508 and dinaciclib.

Impact of metformin on neocortical development during pregnancy: Involvement of ERK and p35/CDK5 pathways.

Metformin, the most commonly prescribed drug for the treatment of diabetes, is increasingly used during pregnancy to address various disorders such as diabetes, obesity, preeclampsia, and metabolic diseases. However, its impact on neocortex development remains unclear. Here, we investigated the direct effects of metformin on neocortex development, focusing on ERK and p35/CDK5 regulation. Using a pregnant rat model, we found that metformin treatment during pregnancy induces small for gestational age (SGA) and reduces relative cortical thickness in embryos and neonates. Additionally, we discovered that metformin inhibits neural progenitor cell proliferation in the sub-ventricular zone (SVZ)/ventricular zone (VZ) of the developing neocortex, a process possibly mediated by ERK inactivation. Furthermore, metformin induces neuronal apoptosis in the SVZ/VZ area of the developing neocortex. Moreover, metformin retards neuronal migration, cortical lamination, and differentiation, potentially through p35/CDK5 inhibition in the developing neocortex. Remarkably, compensating for p35 through in utero electroporation partially rescues metformin-impaired neuronal migration and development. In summary, our study reveals that metformin disrupts neocortex development by inhibiting neuronal progenitor proliferation, neuronal migration, cortical layering, and cortical neuron maturation, likely via ERK and p35/CDK5 inhibition. Consequently, our findings advocate for caution in metformin usage during pregnancy, given its potential adverse effects on fetal brain development.

Structural determinants for activation of the Tau kinase CDK5 by the serotonin receptor 5-HT7R.

BACKGROUND: Multiple neurodegenerative diseases are induced by the formation and deposition of protein aggregates. In particular, the microtubule-associated protein Tau leads to the development of so-called tauopathies characterized by the aggregation of hyperphosphorylated Tau within neurons. We recently showed that the constitutive activity of the serotonin receptor 7 (5-HT7R) is required for Tau hyperphosphorylation and aggregation through activation of the cyclin-dependent kinase 5 (CDK5). We also demonstrated physical interaction between 5-HT7R and CDK5 at the plasma membrane suggesting that the 5-HT7R/CDK5 complex is an integral part of the signaling network involved in Tau-mediated pathology. METHODS: Using biochemical, microscopic, molecular biological, computational and AI-based approaches, we investigated structural requirements for the formation of 5-HT7R/CDK5 complex. RESULTS: We demonstrated that 5-HT7R domains responsible for coupling to Gs proteins are not involved in receptor interaction with CDK5. We also created a structural model of the 5-HT7R/CDK5 complex and refined the interaction interface. The model predicted two conserved phenylalanine residues, F278 and F281, within the third intracellular loop of 5-HT7R to be potentially important for complex formation. While site-directed mutagenesis of these residues did not influence Gs protein-mediated receptor signaling, replacement of both phenylalanines by alanine residues significantly reduced 5-HT7R/CDK5 interaction and receptor-mediated CDK5 activation, leading to reduced Tau hyperphosphorylation and aggregation. Molecular dynamics simulations of 5-HT7R/CDK5 complex for wild-type and receptor mutants confirmed binding interface stability of the initial model. CONCLUSIONS: Our results provide a structural basis for the development of novel drugs targeting the 5-HT7R/CDK5 interaction interface for the selective treatment of Tau-related disorders, including frontotemporal dementia and Alzheimer's disease.

p35/CDK5 Regulates Bladder Cancer Proliferation and Migration and Promotes Higher Tumor Grade and Poor Survival Rate in Patients With Bladder Cancer.

BACKGROUND/AIM: Bladder cancer remains a significant global health concern, necessitating a deeper understanding of the molecular mechanisms underlying its progression. Cyclin-Dependent Kinase 5 (CDK5) has recently emerged as a potential player in bladder cancer pathogenesis. This study investigated the involvement of CDK5 in bladder cancer, emphasizing its potential as a therapeutic target. MATERIALS AND METHODS: The expression levels of CDK5 and p35 (CDK5 regulatory protein) and their roles in the tumor grade and malignancy of patient samples were evaluated using western blot analysis and immunohistochemistry. In addition, tumor cancer genome atlas (TCGA) was utilized to evaluate survival rate in patients with bladder cancer. We further confirmed the role of CDK5 with in vitro experiments using western blot analysis, immunocytochemistry, cell culture-based proliferation and migration assays. RESULTS: Higher CDK5 and p35 were associated with a higher tumor grade and poor survival rate in patients with bladder cancer. To confirm the role of CDK5 in vitro, we over-expressed CDK5 in bladder cancer cells. The results showed that the over-expression of CDK5 enhanced bladder cancer cell proliferation and migration. In addition, CDK5 inhibition by a pan-CDK inhibitor, Roscovitine (RV), significantly reduced proliferation of bladder cancer cells. Indeed, the migration and adhesion of bladder cancer cells were inhibited by RV treatment. CONCLUSION: CDK5 might play important roles in bladder cancer progression and be a potential diagnostic and therapeutic target in the near future.

Phosphorylation of PBK at Thr9 by CDK5 correlates with invasion of prolactinomas.

CONTEXT: Prolactinomas are the most prevalent functional pituitary neuroendocrine tumors (PitNETs), and they are invasive to surrounding anatomic structures. The detailed mechanisms of invasion are not yet clear. OBJECTIVE: We explored the role of PBK phosphorylation in the proliferation and invasion of prolactinomas and its possible mechanism. RESULTS: We report that PBK directly binds to and is phosphorylated at Thr9 by cyclin-dependent kinase 5 (CDK5), which promotes GH3 cell EMT progression and proliferation. Phosphorylation of PBK at Thr9 (pPBK-T9) by CDK5 enhances the stability of PBK. p38 is one of the downstream targets of PBK, and its phosphorylation is reduced as pPBK-T9 increases in vivo and in vitro. Furthermore, we found that pPBK-T9 is highly expressed in invasive PitNETs and was significantly correlated with invasion by univariate and multivariate analyses. CONCLUSIONS: Phosphorylation of PBK at Thr9 by CDK5 promotes cell proliferation and EMT progression in prolactinomas.

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.

Cdk5-dependent rapid formation and stabilization of dendritic spines by corticotropin-releasing factor.

The neuropeptide corticotropin-releasing factor (CRF) exerts a pivotal role in modulating neuronal activity in the mammalian brain. The effects of CRF exhibit notable variations, depending on factors such as duration of exposure, concentration, and anatomical location. In the CA1 region of the hippocampus, the impact of CRF is dichotomous: chronic exposure to CRF impairs synapse formation and dendritic integrity, whereas brief exposure enhances synapse formation and plasticity. In the current study, we demonstrate long-term effects of acute CRF on the density and stability of mature mushroom spines ex vivo. We establish that both CRF receptors are present in this hippocampal region, and we pinpoint their precise subcellular localization within synapses by electron microscopy. Furthermore, both in vivo and ex vivo data collectively demonstrate that a transient surge of CRF in the CA1 activates the cyclin-dependent kinase 5 (Cdk5)-pathway. This activation leads to a notable augmentation in CRF-dependent spine formation. Overall, these data suggest that upon acute release of CRF in the CA1-SR synapse, both CRF-Rs can be activated and promote synaptic plasticity via activating different downstream signaling pathways, such as the Cdk5-pathway.

Dexmedetomidine ameliorates high glucose-induced epithelial-mesenchymal transformation in HK-2 cells through the Cdk5/Drp1/ROS pathway.

Epithelial-mesenchymal transformation (EMT) plays an important role in the progression of diabetic nephropathy. Dexmedetomidine (DEX) has shown renoprotective effects against ischemic reperfusion injury; however, whether and how DEX prevents high glucose-induced EMT in renal tubular epithelial cells is incompletely known. Here, we conduct in vitro experiments using HK-2 cells, a human tubular epithelial cell line. Our results demonstrate that high glucose increases the expressions of EMT-related proteins, including Vimentin, Slug, Snail and Twist, while decreasing the expression of E-cadherin and increasing Cdk5 expression in HK-2 cells. Both Cdk5 knockdown and inhibition by roscovitine increase the expressions of E-cadherin while decreasing the expressions of other EMT-related markers. DEX inhibits Cdk5 expression without affecting cell viability and changes the expressions of EMT-related markers, similar to effects of Cdk5 inhibition. Furthermore, Cdk5 is found to interact with Drp1 at the protein level and mediate the phosphorylation of Drp1. In addition, Drp1 inhibition with mdivi-1 could also restrain the high glucose-induced EMT process in HK-2 cells. Immunofluorescence results show that roscovitine, Mdivi-1 and DEX inhibit high glucose-induced intracellular ROS accumulation, while the oxidant H 2O 2 eliminates the protective effect of DEX on the EMT process. These results indicate that DEX mitigates high glucose-induced EMT progression in HK-2 cells via inhibition of the Cdk5/Drp1/ROS pathway.

TFP5, a Peptide Derived from Cdk5 Activator p35, Protects Pancreatic ß Cells from Glucose Toxicity.

We studied the effect of TFP5 on MIN6 cells (cultured mouse islet ß cells) treated with different concentrations of glucose (5 or 25 mM). The results were verified in C57BL/6J mice (control; n=12) and db/db mice with type 2 diabetes mellitus (n=12). To synthesize TFP5, peptide p5 (a derivative of p35 protein, activator of cyclin-dependent kinase 5, Cdk5) was conjugated with a FITC tag at the N-terminus and an 11-amino acid TAT protein transduction domain at the C-terminus. TFP5 was employed to inhibit Cdk5 activity and then to evaluate its efficiency in treating experimental type 2 diabetes mellitus. TFP5 effectively inhibited the pathological hyperactivity of Cdk5, enhanced insulin secretion, and protected pancreatic ß cells from apoptosis in vitro and in vivo. In addition, TFP5 inhibited inflammation in pancreatic islets by reducing the expression of inflammatory cytokines TGF-ß1, TNFα, and IL-1ß. These novel data indicates that TFP5 is a promising candidate for treatment of type 2 diabetes mellitus.

Shortwave radiation-induced reproductive organ damage in male rats by enhanced expression of molecules associated with the calpain/Cdk5 pathway and oxidative stress.

Shortwave radiation has been reported to have harmful effects on several organs in humans and animals. However, the biological effects of 27 MHz shortwave on the reproductive system are not clear. In this study, we investigated the effects of shortwave whole-body exposure at a frequency of 27 MHz on structural and functional changes in the testis. Male Wistar rats were exposed to 27 MHz continuous shortwaves at average power densities of 0, 5, 10, or 30 mW/cm2 for 6 min. The levels of insulin-like factor 3 (INSL3) and anti-sperm antibodies (AsAb) in the peripheral serum, sperm motility, sperm malformation rate, and testicular tissue structure of rats were analyzed. Furthermore, the activity of superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA) content, calpain, and Cdk5 expression were analyzed at 1, 7, 14, and 28 days after exposure. We observed that the rats after radiation had decreased serum INSL3 levels (p < 0.01), increased AsAb levels (p < 0.05), decreased percentage of class A+B sperm (p < 0.01 or p < 0.05), increased sperm malformation (p < 0.01 or p < 0.05), injured testicular tissue structure, decreased SOD and CAT activities (p < 0.01 or p < 0.05), increased MDA content (p < 0.01), and testicular tissue expressions of calpain1, calpain2, and Cdk5 were increased (p < 0.01 or p < 0.05). In conclusion, Shortwave radiation caused functional and structural damage to the reproductive organs of male rats. Furthermore, oxidative stress and key molecules in the calpain/Cdk5 pathway are likely involved in this process.

Shortwave radiation has been used in communications, medical and military applications, and its damaging effects on several organs of the human body have been reported in the literature. However, the biological effects of shortwave radiation on the male reproductive system are unknown. The present study, by constructing an animal model of short-wave radiation and analyzing the experimental results, revealed that shortwave radiation could cause functional and structural damage to the reproductive organs of male rats, and that oxidative stress and key molecules in the calpain/Cdk5 pathway might be involved in this process. It will provide organizational data for further studies on the mechanisms of male reproductive damage by shortwave radiation.

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

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

CDK5: an oncogene or an anti-oncogene: location location location.

Recent studies have uncovered various physiological functions of CDK5 in many nonneuronal tissues. Upregulation of CDK5 and/or its activator p35 in neurons promotes healthy neuronal functions, but their overexpression in nonneuronal tissues is causally linked to cancer of many origins. This review focuses on the molecular mechanisms by which CDK5 recruits diverse tissue-specific substrates to elicit distinct phenotypes in sixteen different human cancers. The emerging theme suggests that CDK5's role as an oncogene or anti-oncogene depends upon its subcellular localization. CDK5 mostly acts as an oncogene, but in gastric cancer, it is a tumor suppressor due to its unique nuclear localization. This indicates that CDK5's access to certain nuclear substrates converts it into an anti-oncogenic kinase. While acting as a bonafide oncogene, CDK5 also activates a few cancer-suppressive pathways in some cancers, presumably due to the mislocalization of nuclear substrates in the cytoplasm. Therefore, directing CDK5 to the nucleus or exporting tumor-suppressive nuclear substrates to the cytoplasm may be promising approaches to combat CDK5-induced oncogenicity, analogous to neurotoxicity triggered by nuclear CDK5. Furthermore, while p35 overexpression is oncogenic, hyperactivation of CDK5 by inducing p25 formation results in apoptosis, which could be exploited to selectively kill cancer cells by dialing up CDK5 activity, instead of inhibiting it. CDK5 thus acts as a molecular rheostat, with different activity levels eliciting distinct functional outcomes. Finally, as CDK5's role is defined by its substrates, targeting them individually or in conjunction with CDK5 should create potentially valuable new clinical opportunities.

CDK5 destabilizes PD-L1 via chaperon-mediated autophagy to control cancer immune surveillance in hepatocellular carcinoma.

BACKGROUND: In the past few years, immunotherapies of hepatocellular carcinoma (HCC) targeting programmed cell death protein 1 (PD-1) and its ligand programmed cell death ligand 1 (PD-L1), have achieved durable clinical benefits. However, only a fraction of HCC patients showed objective clinical response to PD-1/PD-L1 blockade alone. Despite the impact on post-translational modifications of PD-L1 being substantial, its significance in resistance to HCC immunotherapy remains poorly defined. METHODS: Cyclin-dependent kinase 5 (CDK5) expression was knocked down in HCC cells, CDK5 and PD-L1 protein levels were examined by Western blot. Coimmunoprecipitation was conducted to evaluate the interaction between proteins. Preclinical HCC mice model was constructed to evaluate the effect of CDK5 inhibitor alone or in combination with PD-1 antibody. Clinical HCC samples were used to elucidate the clinical relevance of CDK5, PD-L1, and PD-L1 T290 phosphorylation in HCC. RESULTS: We find that CDK5 deficiency upregulates PD-L1 protein expression in HCC cells and decipher a novel molecular mechanism under which PD-L1 is downregulated by CDK5, that is, CDK5 mediated PD-L1 phosphorylation at T290 promotes its binding with chaperon protein heat-shock cognate protein 70 (HSC70) and degradation through chaperon-mediated autophagy. Notably, treatment of CDK5 inhibitor, PNU112455A, effectively upregulates the tumorous PD-L1 level, promotes the response to anti-PD-1 immunotherapy,and prolongs the survival time of mice bearing HCC tumors. What is more, the T290 phosphorylation status of PD-L1 correlates with the prognosis of HCC. CONCLUSIONS: Targeting CDK5 can synergize with PD-1 blockade to suppress HCC growth, which may have clinical benefits. Our study reveals a unique regulation of the degradation of PD-L1 in HCC, and provides an attractive therapeutic target, a potential drug, and a new prognostic marker for the clinical treatment of HCC.

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 GSK3ß inhibitor (LiCl) to the roles of CDK5 and GSK3ß 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.