Cdk5 and the non-catalytic arrest of the neuronal cell cycle

Eugenol as a potential adjuvant therapy for gingival squamous cell carcinoma

Adoption of plant-derived compounds for the management of oral cancer is encouraged by the scientific community due to emerging chemoresistance and conventional treatments adverse effects. Considering that very few studies investigated eugenol clinical relevance for gingival carcinoma, we ought to explore its selectivity and performance according to aggressiveness level. For this purpose, non-oncogenic human oral epithelial cells (GMSM-K) were used together with the Tongue (SCC-9) and Gingival (Ca9-22) squamous cell carcinoma lines to assess key tumorigenesis processes. Overall, eugenol inhibited cell proliferation and colony formation while inducing cytotoxicity in cancer cells as compared to normal counterparts. The recorded effect was greater in gingival carcinoma and appears to be mediated through apoptosis induction and promotion of p21/p27/cyclin D1 modulation and subsequent Ca9-22 cell cycle arrest at the G0/G1 phase, in a p53-independent manner. At these levels, distinct genetic profiles were uncovered for both cell lines by QPCR array. Moreover, it seems that our active component limited Ca9-22 and SCC-9 cell migration respectively through MMP1/3 downregulation and stimulation of inactive MMPs complex formation. Finally, Ca9-22 behaviour appears to be mainly modulated by the P38/STAT5/NFkB pathways. In summary, we can disclose that eugenol is cancer selective and that its mediated anti-cancer mechanisms vary according to the cell line with gingival squamous cell carcinoma being more sensitive to this phytotherapy agent.

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.

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

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

Cdk5 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.

Inhibition of cyclin-dependent kinase 5 activity alleviates diabetes-related cognitive deficits

Cognitive deficit is a prevalent and underestimated complication of diabetes, and the underlying cellular and molecular mechanisms are not well understood. Aberrant activity of cyclin-dependent kinase (Cdk)5 is implicated in a number of neurodegenerative diseases. The present study examined the role of Cdk5 in the progression of diabetes-related cognitive deficits. We showed that the Cdk5 protein expression and kinase activity were significantly increased in diabetic mice at 16 wk. In primary cultured hippocampal neurons exposed to 30 mM glucose, Cdk5 protein and kinase activity were also elevated in a time-dependent manner. Moreover, the high glucose exposure led to an aberrant Cdk5 activation due to its activator p25 that was cleaved from p35 by calpain. Both in diabetic mice and in cultured hippocampal neurons exposed to high glucose, inhibition of Cdk5 activity with roscovitine (Ros) or short hairpin RNA (shRNA) decreased the protein levels of cleaved caspase-3 and the ratio of Bax and Bcl-2. The apoptotic rate detected by TUNEL in vivo or Annexin V and propidium iodide staining for flow cytometry in vitro also had obvious reduction. In addition, high glucose exposure resulted in the increase of phosphorylated (phospho)-MAPK kinase (MKK)6, phospho-p38, and c-Jun, which were rescued by Ros or Cdk5 shRNA. It is more important that the cognitive deficits of diabetic mice were also effectively alleviated by Ros. These results indicate that aberrant Cdk5 activity triggered hippocampal neuron apoptosis by activating MKK6/p38 MAPK cascade in hyperglycemia. Inhibition of Cdk5 overactivation attenuates neuronal apoptosis and cognitive deficits and contributes to the relief of diabetic neurotoxicity in the brain.-Liu, W., Zhou, Y., Liang, R., Zhang, Y. Inhibition of cyclin-dependent kinase 5 activity alleviates diabetes-related cognitive deficits.

Emerging Perspectives on DNA Double-strand Breaks in Neurodegenerative Diseases

DNA double-strand breaks (DSBs) are common events that were recognized as one of the most toxic lesions in eu-karyotic cells. DSBs are widely involved in many physiological processes such as V(D)J recombination, meiotic recombina-tion, DNA replication and transcription. Deregulation of DSBs has been reported in multiple diseases in human beings, such as the neurodegenerative diseases, with which the underlying mechanisms are needed to be illustrated. Here, we reviewed the recent insights into the dysfunction of DSB formation and repair, contributing to the pathogenesis of neurodegenerative dis-orders including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD) and ataxia tel-angiectasia (A-T).

The stress response HPA-axis hormone, glucocorticoid, reduces cellular SKA complex gene expression

The Spindle- and Kinetochore-Associated (SKA) complex has been proven to be involved in many human mental behavioral disorders. Glucocorticoid, a hypothalamic-pituitary-adrenal (HPA) axis hormone, is a critical mediator of stress response in neurons. However, the underlying mechanisms of glucocorticoid's effects on human neuronal cells remain unclear. This study demonstrates that increased extracellular glucocorticoid levels significantly reduce neuronal cell SKA complex genes' expression levels, followed by altered neuronal cell viability and neurite development. The results suggest that the abnormality of this HPA-axis hormone could impact the neuronal cell functions through the alternation of SKA complex functions, which might induce cell death.

Iron Oxide Nanoparticles Induces Cell Cycle-Dependent Neuronal Apoptosis in Mice

Iron oxide (Fe₂O₃) nanoparticles (NPs) with its unique magnetic and paramagnetic properties are popular in biomedical applications. Some of their neurotoxic mechanisms due to repeated administration are proven. However, we speculate that the neuronal damage might be due to apoptosis resulting from unusual cell cycle entry. Moreover, iron accumulation has been shown to be closely associated with most of the neurodegenerative disorders. Thus, in the current study, mice were orally (po) treated with the Fe₂O₃-NPs to investigate cell cycle-associated events/components and occurrence of apoptosis. A subsequent increase in oxidant levels was observed with the iron accumulation due to Fe₂O₃-NPs exposure. The accumulated β-amyloid and reduced level of cdk5 seem to aid in the cell cycle entry and forcing progression towards apoptosis. Expression of Cyclin D1 and pRb (Ser 795) indicate the cell cycle re-entry of neurons. Overexpression of RNA Pol II and PARP cleavage suggests DNA damage due to Fe₂O₃-NPs exposure. Further, hyperphosphorylation of p38 (Thr 180/Tyr 182) confirms the activation of DNA damage-dependent checkpoint. Expression patterns of pro- and anti-apoptotic markers, TUNEL and TEM indicate the occurrences of apoptosis.

Why Cortical Neurons Cannot Divide, and Why Do They Usually Die in the Attempt?

Cortical neurons are prime examples of terminally differentiated, postmitotic cells. However, under experimental or pathological conditions, they can re-enter the cell cycle and replicate DNA but are unable to divide, dying by apoptosis or becoming either polyploid or aneuploid. Any cellular state that depends on the action of genes and their products can be reverted or bypassed by spontaneous or induced mutations, yet there are currently no reports of dividing cortical neurons. Thus, it seems unlikely that the remarkably stable postmitotic condition of cortical neurons depends on specific gene functions. This Review summarizes evidence that the postmitotic state of cortical neurons depends on the high stability of its underlying nuclear structure that results from an entropy-driven process aimed at dissipating the intrinsic structural stress present in chromosomal DNA in such a way that the structural stability of the neuronal nucleus becomes an insurmountable energy barrier for karyokinesis and mitosis. From this perspective, the integral properties of the nuclear higher order structure in neurons provide an explanation not only for why cortical neurons cannot divide but also for why they usually die if they happen to replicate their DNA. © 2016 Wiley Periodicals, Inc.

A Unified Hypothesis of Early- and Late-Onset Alzheimer's Disease Pathogenesis

Early-onset familial Alzheimer's disease (EOFAD) and late-onset sporadic AD (LOSAD) both follow a similar pathological and biochemical course that includes: neuron and synapse loss and dysfunction, microvascular damage, microgliosis, extracellular amyloid-β deposition, tau phosphorylation, formation of intracellular neurofibrillary tangles, endoreduplication and related cell cycle events in affected brain regions. Any mechanistic explanation of AD must accommodate these biochemical and neuropathological features for both forms of the disease. In this insight paper we provide a unifying hypothesis for EOFAD and LOSAD that proposes that the aberrant re-entry of terminally differentiated, post-mitotic neurons into the cell division cycle is a common pathway that explains both early and late-onset forms of AD. Cell cycle abnormalities appear very early in the disease process, prior to the appearance of plaques and tangles, and explain the biochemical (e.g. tau phosphorylation), neuropathological (e.g. neuron hypertrophy; polypoidy) and cognitive changes observed in EOFAD and LOSAD. Genetic mutations in AβPP, PSEN1, and PSEN2 that alter amyloid-β precursor protein and Notch processing drive reactivation of the cell cycle in EOFAD, while age-related reproductive endocrine dyscrasia that upregulates mitogenic TNF signaling and AβPP processing toward the amyloidogenic pathway drives reactivation of the cell cycle in LOSAD. In essence, AβPP and presenilin mutations initiate early, what endocrine dyscrasia initiates later: aberrant cell cycle re-entry of post-mitotic neurons leading to neurodegeneration and cognitive decline in AD. Inhibition of cell cycle re-entry in post-mitotic neurons may be a useful therapeutic strategy to prevent, slow or halt disease progression.

Low Expression of CDK5 and p27 Are Associated with Poor Prognosis in Patients with Gastric Cancer

Several previous studies have demonstrated that CDK5 or p27 expression in gastric cancer are associated with overall survival. We have previously reported that tumor suppressive function of CDK5 is related to p27. The aim of this study was to investigate correlation between the clinicopathological parameters and overall survival with different CDK5/p27 expression statuses in 244 gastric cancer patients using immunohistochemistry. Low CDK5 expression was detected in 93 cases (38.11%) and low p27 in 157 cases (64.34%). The expression of CDK5 was significantly related to sex (P = 0.034) and Lauren's classification (P = 0.013). The expression of p27 was significantly related to sex (P = 0.012), differentiation (P = 0.003), TNM stage (P = 0.013) and lymph node metastasis (P = 0.001). Based on the combined expression of CDK5 and p27, we classified the patients into four subtypes: CDK5 Low/p27 Low (n = 69), CDK5 High/p27 Low (n = 88), CDK5 Low/p27 High (n = 24) and CDK5 High/p27 High (n = 63). The CDK5 Low/p27 Low expression was closely related to female (P = 0.026), diffuse type (P = 0.027) and lymph node metastasis (P = 0.010). The CDK5 Low/p27 Low patients displayed poorer survival in comparison with the rest of the patients in Kaplan-Meier analysis. No significant overall survival difference was observed among the patients with CDK5 High and/or p27 High expression. In the multivariate analysis, CDK5 and p27 co-expression status was identified as an independent prognostic factor for patients with gastric cancer.

The endocrine dyscrasia that accompanies menopause and andropause induces aberrant cell cycle signaling that triggers re-entry of post-mitotic neurons into the cell cycle, neurodysfunction, neurodegeneration and cognitive disease

This article is part of a Special Issue "SBN 2014". Sex hormones are physiological factors that promote neurogenesis during embryonic and fetal development. During childhood and adulthood these hormones support the maintenance of brain structure and function via neurogenesis and the formation of dendritic spines, axons and synapses required for the capture, processing and retrieval of information (memories). Not surprisingly, changes in these reproductive hormones that occur with menopause and during andropause are strongly correlated with neurodegeneration and cognitive decline. In this connection, much evidence now indicates that Alzheimer's disease (AD) involves aberrant re-entry of post-mitotic neurons into the cell cycle. Cell cycle abnormalities appear very early in the disease, prior to the appearance of plaques and tangles, and explain the biochemical, neuropathological and cognitive changes observed with disease progression. Intriguingly, a recent animal study has demonstrated that induction of adult neurogenesis results in the loss of previously encoded memories while decreasing neurogenesis after memory formation during infancy mitigated forgetting. Here we review the biochemical, epidemiological and clinical evidence that alterations in sex hormone signaling associated with menopause and andropause drive the aberrant re-entry of post-mitotic neurons into an abortive cell cycle that leads to neurite retraction, neuron dysfunction and neuron death. When the reproductive axis is in balance, gonadotropins such as luteinizing hormone (LH), and its fetal homolog, human chorionic gonadotropin (hCG), promote pluripotent human and totipotent murine embryonic stem cell and neuron proliferation. However, strong evidence supports menopausal/andropausal elevations in the LH:sex steroid ratio as driving aberrant mitotic events. These include the upregulation of tumor necrosis factor; amyloid-β precursor protein processing towards the production of mitogenic Aβ; and the activation of Cdk5, a key regulator of cell cycle progression and tau phosphorylation (a cardinal feature of both neurogenesis and neurodegeneration). Cognitive and biochemical studies confirm the negative consequences of a high LH:sex steroid ratio on dendritic spine density and human cognitive performance. Prospective epidemiological and clinical evidence in humans supports the premise that rebalancing the ratio of circulating gonadotropins:sex steroids reduces the incidence of AD. Together, these data support endocrine dyscrasia and the subsequent loss of cell cycle control as an important etiological event in the development of neurodegenerative diseases including AD, stroke and Parkinson's disease.

CDK5 phosphorylates DRP1 and drives mitochondrial defects in NMDA-induced neuronal death

Defects in mitochondrial fission and cyclin dependent kinase 5 (CDK5) activation are early events that precede neuronal loss following NMDA-induced neuronal death. Here, we report that the cytoplasmic CDK5 tightly regulates mitochondrial morphology defects associated with NMDA-induced neuronal injury via regulation of the mitochondrial fission protein, dynamin-related protein 1 (DRP1). We show that DRP1 is a direct target of CDK5. CDK5-mediated phosphorylation of DRP1 at a conserved Serine residue, S585, is elevated at the mitochondria and is associated with increased mitochondrial fission. Ectopic expression of a cytoplasmic CDK5 or mutant DRP1-S585D results in increased mitochondrial fragmentation in primary neurons. Conversely, expression of a dominant negative form of cytoplasmic CDK5 or mutant DRP1-S585A results in elongated mitochondria. In addition, pharmacological inhibition of CDK5 by Roscovitine inhibits DRP1 phosphorylation and mitochondrial fission associated with NMDA-induced neuronal loss. Importantly, conditional deletion of CDK5 significantly attenuates DRP1 phosphorylation at S585 and rescues mitochondrial fission defects in neurons exposed to NMDA. Our studies delineate an important mechanism by which CDK5 regulates mitochondrial morphology defects associated with neuronal injury.

The roles of Cdk5-mediated subcellular localization of FOXO1 in neuronal death

Deficiency of cyclin-dependent kinase 5 (Cdk5) has been linked to the death of postmitotic cortical neurons during brain development. We now report that, in mouse cortical neurons, Cdk5 is capable of phosphorylating the transcription factor FOXO1 at Ser249 in vitro and in vivo. Cellular stresses resulting from extracellular stimulation by H2O2 or β-amyloid promote hyperactivation of Cdk5, FOXO1 nuclear export and inhibition of its downstream transcriptional activity. In contrast, a loss of Cdk5 leads to FOXO1 translocation into the nucleus: a shift due to decreased AKT activity but independent of S249 phosphorylation. Nuclear FOXO1 upregulates transcription of the proapoptotic gene, BIM, leading to neuronal death, which can be rescued when endogenous FOXO1 was replaced by the cytoplasmically localized form of FOXO1, FOXO1-S249D. Cytoplasmic, but not nuclear, Cdk5 attenuates neuronal death by inhibiting FOXO1 transcriptional activity and BIM expression. Together, our findings suggest that Cdk5 plays a novel and unexpected role in the degeneration of postmitotic neurons through modulation of the cellular location of FOXO1, which constitutes an alternative pathway through which Cdk5 deficiency leads to neuronal death.

Cyclin-dependent kinase 5 decreases in gastric cancer and its nuclear accumulation suppresses gastric tumorigenesis

PURPOSE

As a cyclin-independent atypical CDK, the role of CDK5 in regulating cell proliferation in gastric cancer remains unknown.

EXPERIMENTAL DESIGN

Expression of CDK5 in gastric tumor and paired adjacent noncancerous tissues from 437 patients was measured by Western blotting, immunohistochemistry, and real-time PCR. The subcellular translocation of CDK5 was monitored during gastric cancer cell proliferation. The role of nuclear CDK5 in gastric cancer tumorigenic proliferation and ex vivo xenografts was explored. Furthermore, by screening for compounds in the PubChem database that disrupt CDK5 association with its nuclear export facilitator, we identified a small molecular (NS-0011) that inhibits gastric cancer cell growth.

RESULTS

CDK5 level was significantly decreased in the majority of gastric tumor tissues, and the reduction of CDK5 correlated with the severity of gastric cancer based on tumor and lymph node metastasis and patient 5-year fatality rate. Nuclear localization of CDK5 was found to be significantly decreased in tumor tissues and gastric cancer cell lines, whereas exogenously expression of nucleus-targeted CDK5 inhibited the proliferation and xenograft implantation of gastric cancer cells. Treatment with the small molecule NS-0011, which increases CDK5 accumulation in the nucleus, suppressed both cancer cell proliferation and xenograft tumorigenesis.

CONCLUSIONS

Our results suggest that low CDK5 expression is associated with poor overall survival in patients with gastric cancer, and nuclear accumulation of CDK5 inhibits the proliferation and tumorigenicity of human gastric cancer cells.

Mechanisms of HIV-1 Tat neurotoxicity via CDK5 translocation and hyper-activation: role in HIV-associated neurocognitive disorders

The advent of more effective antiretroviral therapies has reduced the frequency of HIV dementia, however the prevalence of milder HIV associated neurocognitive disorders [HAND] is actually rising. Neurodegenerative mechanisms in HAND might include toxicity by secreted HIV-1 proteins such as Tat, gp120 and Nef that could activate neuro-inflammatory pathways, block autophagy, promote excitotoxicity, oxidative stress, mitochondrial dysfunction and dysregulation of signaling pathways. Recent studies have shown that Tat could interfere with several signal transduction mechanisms involved in cytoskeletal regulation, cell survival and cell cycle re-entry. Among them, Tat has been shown to hyper-activate cyclin-dependent kinase [CDK] 5, a member of the Ser/Thr CDKs involved in cell migration, angiogenesis, neurogenesis and synaptic plasticity. CDK5 is activated by binding to its regulatory subunit, p35 or p39. For this manuscript we review evidence showing that Tat, via calcium dysregulation, promotes calpain-1 cleavage of p35 to p25, which in turn hyper-activates CDK5 resulting in abnormal phosphorylation of downstream targets such as Tau, collapsin response mediator protein-2 [CRMP2], doublecortin [DCX] and MEF2. We also present new data showing that Tat interferes with the trafficking of CDK5 between the nucleus and cytoplasm. This results in prolonged presence of CDK5 in the cytoplasm leading to accumulation of aberrantly phosphorylated cytoplasmic targets [e.g.: Tau, CRMP2, DCX] that impair neuronal function and eventually lead to cell death. Novel therapeutic approaches with compounds that block Tat mediated hyper-activation of CDK5 might be of value in the management of HAND.

Repair of oxidative DNA damage, cell-cycle regulation and neuronal death may influence the clinical manifestation of Alzheimer's disease

Alzheimer’s disease (AD) is characterized by progressive cognitive decline associated with a featured neuropathology (neuritic plaques and neurofibrillary tangles). Several studies have implicated oxidative damage to DNA, DNA repair, and altered cell-cycle regulation in addition to cell death in AD post-mitotic neurons. However, there is a lack of studies that systematically assess those biological processes in patients with AD neuropathology but with no evidence of cognitive impairment. We evaluated markers of oxidative DNA damage (8-OHdG, H2AX), DNA repair (p53, BRCA1, PTEN), and cell-cycle (Cdk1, Cdk4, Cdk5, Cyclin B1, Cyclin D1, p27^Kip1, phospho-Rb and E2F1) through immunohistochemistry and cell death through TUNEL in autopsy hippocampal tissue samples arrayed in a tissue microarray (TMA) composed of three groups: I) “clinical-pathological AD” (CP-AD) - subjects with neuropathological AD (Braak≥IV and CERAD = B or C) and clinical dementia (CDR≥2, IQCODE>3.8); II) “pathological AD” (P-AD) - subjects with neuropathological AD (Braak≥IV and CERAD = B or C) and without cognitive impairment (CDR 0, IQCODE<3.2); and III) “normal aging” (N) - subjects without neuropathological AD (Braak≤II and CERAD 0 or A) and with normal cognitive function (CDR 0, IQCODE<3.2). Our results show that high levels of oxidative DNA damage are present in all groups. However, significant reductions in DNA repair and cell-cycle inhibition markers and increases in cell-cycle progression and cell death markers in subjects with CP-AD were detected when compared to both P-AD and N groups, whereas there were no significant differences in the studied markers between P-AD individuals and N subjects. This study indicates that, even in the setting of pathological AD, healthy cognition may be associated with a preserved repair to DNA damage, cell-cycle regulation, and cell death in post-mitotic neurons.

Nucleocytoplasmic protein translocation during mitosis in the social amoebozoan Dictyostelium discoideum

Mitosis is a fundamental and essential life process. It underlies the duplication and survival of all cells and, as a result, all eukaryotic organisms. Since uncontrolled mitosis is a dreaded component of many cancers, a full understanding of the process is critical. Evolution has led to the existence of three types of mitosis: closed, open, and semi-open. The significance of these different mitotic species, how they can lead to a full understanding of the critical events that underlie the asexual duplication of all cells, and how they may generate new insights into controlling unregulated cell division remains to be determined. The eukaryotic microbe Dictyostelium discoideum has proved to be a valuable biomedical model organism. While it appears to utilize closed mitosis, a review of the literature suggests that it possesses a form of mitosis that lies in the middle between truly open and fully closed mitosis-it utilizes a form of semi-open mitosis. Here, the nucleocytoplasmic translocation patterns of the proteins that have been studied during mitosis in the social amoebozoan D. discoideum are detailed followed by a discussion of how some of them provide support for the hypothesis of semi-open mitosis.

MiR-26b, upregulated in Alzheimer's disease, activates cell cycle entry, tau-phosphorylation, and apoptosis in postmitotic neurons

MicroRNA (miRNA) functions in the pathogenesis of major neurodegenerative diseases such as Alzheimer's disease (AD) are only beginning to emerge. We have observed significantly elevated levels of a specific miRNA, miR-26b, in the defined pathological areas of human postmortem brains, starting from early stages of AD (Braak III). Ectopic overexpression of miR-26b in rat primary postmitotic neurons led to the DNA replication and aberrant cell cycle entry (CCE) and, in parallel, increased tau-phosphorylation, which culminated in the apoptotic cell death of neurons. Similar tau hyperphosphorylation and CCE are typical features of neurons in pre-AD brains. Sequence-specific inhibition of miR-26b in culture is neuroprotective against oxidative stress. Retinoblastoma protein (Rb1), a major tumor suppressor, appears as the key direct miR-26b target, which mediates the observed neuronal phenotypes. The downstream signaling involves upregulation of Rb1/E2F cell cycle and pro-apoptotic transcriptional targets, including cyclin E1, and corresponding downregulation of cell cycle inhibitor p27/Kip1. It further leads to nuclear export and activation of Cdk5, a major kinase implicated in tau phosphorylation, regulation of cell cycle, and death in postmitotic neurons. Therefore, upregulation of miR-26b in neurons causes pleiotropic phenotypes that are also observed in AD. Elevated levels of miR-26b may thus contribute to the AD neuronal pathology.

Cdk5 levels oscillate during the neuronal cell cycle: Cdh1 ubiquitination triggers proteosome-dependent degradation during S-phase

When cell cycle re-activation occurs in post-mitotic neurons it places them at increased risk for death. The cell cycle/cell death association has been reported in many neurodegenerative diseases including Alzheimer disease (AD), yet the mechanisms by which a normal neuron suppresses the cycle remain largely unknown. Recently, our laboratory has shown that Cdk5 (cyclin-dependent kinase 5) is a key player in this protective function. When a neuron is under stress, Cdk5 is transported to the cytoplasm; this eliminates its cell cycle suppression activity and the neuron re-enters S-phase. In the current study we show that a similar principle applies during a normal cell cycle. When a neuronal cell enters S phase, Cdk5 is transported to the cytoplasm where it is ubiquitinated by the E3 ligase APC-Cdh1. Ubiquitinated Cdk5 is then rapidly degraded by the proteasome. The ubiquitination site of Cdk5 appears to be in the p35 binding area; in the presence of high levels of p35, the ubiquitination of Cdk5 was blocked, and the degradation in S phase was attenuated. The data suggest an unsuspected role for Cdk5 during the progression of a normal cell cycle and offer new pharmaceutical targets for regulating neuronal cell cycling and cell death.

Cyclin-dependent kinase 5 regulates E2F transcription factor through phosphorylation of Rb protein in neurons

Recent studies have shown the involvement of cyclin-dependent kinase 5 (Cdk5) in cell cycle regulation in postmitotic neurons. In this study, we demonstrate that Cdk5 and its co-activator p35 were detected in the nuclear fraction in neurons and Cdk5/p35 phosphorylated retinoblastoma (Rb) protein, a key protein controlling cell cycle re-entry. Cdk5/p35 phosphorylates Rb at the sites similar to those phosphorylated by Cdk4 and Cdk2. Furthermore, increased Cdk5 activity elevates activity of E2F transcription factor, which can trigger cell cycle re-entry, leading to neuronal cell death. A normal Cdk5 activity in neurons did not induce E2F activation, suggesting that Cdk5 does not induce cell cycle re-entry under normal conditions. Taken together, these results indicate that Cdk5 can regulate cell cycle by its ability to phosphorylate Rb. Most importantly, increased Cdk5 activity induces cell cycle re-entry, which is especially detrimental for survival of postmitotic neurons.

The post-mitotic state in neurons correlates with a stable nuclear higher-order structure

Neurons become terminally differentiated (TD) post-mitotic cells very early during development yet they may remain alive and functional for decades. TD neurons preserve the molecular machinery necessary for DNA synthesis that may be reactivated by different stimuli but they never complete a successful mitosis. The non-reversible nature of the post-mitotic state in neurons suggests a non-genetic basis for it since no set of mutations has been able to revert it. Comparative studies of the nuclear higher-order structure in neurons and cells with proliferating potential suggest that the non-reversible nature of the post-mitotic state in neurons has a structural basis in the stability of the nuclear higher-order structure.

The secret life of kinases: functions beyond catalysis

Protein phosphorylation participates in the regulation of all fundamental biological processes, and protein kinases have been intensively studied. However, while the focus was on catalytic activities, accumulating evidence suggests that non-catalytic properties of protein kinases are essential, and in some cases even sufficient for their functions. These non-catalytic functions include the scaffolding of protein complexes, the competition for protein interactions, allosteric effects on other enzymes, subcellular targeting, and DNA binding. This rich repertoire often is used to coordinate phosphorylation events and enhance the specificity of substrate phosphorylation, but also can adopt functions that do not rely on kinase activity. Here, we discuss such kinase independent functions of protein and lipid kinases focussing on kinases that play a role in the regulation of cell proliferation, differentiation, apoptosis, and motility.

CYY-1/cyclin Y and CDK-5 differentially regulate synapse elimination and formation for rewiring neural circuits

The assembly and maturation of neural circuits require a delicate balance between synapse formation and elimination. The cellular and molecular mechanisms that coordinate synaptogenesis and synapse elimination are poorly understood. In C. elegans, DD motoneurons respecify their synaptic connectivity during development by completely eliminating existing synapses and forming new synapses without changing cell morphology. Using loss- and gain-of-function genetic approaches, we demonstrate that CYY-1, a cyclin box-containing protein, drives synapse removal in this process. In addition, cyclin-dependent kinase-5 (CDK-5) facilitates new synapse formation by regulating the transport of synaptic vesicles to the sites of synaptogenesis. Furthermore, we show that coordinated activation of UNC-104/Kinesin3 and Dynein is required for patterning newly formed synapses. During the remodeling process, presynaptic components from eliminated synapses are recycled to new synapses, suggesting that signaling mechanisms and molecular motors link the deconstruction of existing synapses and the assembly of new synapses during structural synaptic plasticity.

Ubiquitination of human AP-endonuclease 1 (APE1) enhanced by T233E substitution and by CDK5

Apurinic/apyrimidinic endonuclease-1 (APE1) is a multifunctional DNA repair/gene regulatory protein in mammalian cells, and was recently reported to be phosphorylated at Thr233 by CDK5. We here report that ubiquitination of T233E APE1, a mimicry of phospho-T233 APE1, was markedly increased in multiple cell lines. Expression of CDK5 enhanced monoubiquitination of endogenous APE1. Polyubiquitinated APE1 was decreased when K48R ubiquitin was expressed, suggesting that polyubiquitination was mediated mainly through Lys48 of ubiquitin. The ubiquitination activity of MDM2, consistent in its role for APE1 ubiquitination, was increased for T233E APE1 compared to the wild-type APE1. In mouse embryonic fibroblasts lacking the MDM2 gene, ubiquitination of T233E APE1 was still observed probably because of the decreased degradation activity for monoubiquitinated APE1 and because of backup E3 ligases in the cells. Monoubiquitinated APE1 was present in the nucleus, and analyzing global gene expression profiles with or without induction of a ubiquitin-APE1 fusion gene suggested that monoubiquitination enhanced the gene suppression activity of APE1. These data reveal a delicate balance of ubiquitination and phosphorylation activities that alter the gene regulatory function of APE1.

Continued stabilization of the nuclear higher-order structure of post-mitotic neurons in vivo

Background

Cellular terminal differentiation (TD) correlates with a permanent exit from the cell cycle and so TD cells become stably post-mitotic. However, TD cells express the molecular machinery necessary for cell proliferation that can be reactivated by experimental manipulation, yet it has not been reported the stable proliferation of any type of reactivated TD cells. Neurons become post-mitotic after leaving the ventricular zone. When neurons are forced to reenter the cell cycle they invariably undergo cell death. Wider evidence indicates that the post-mitotic state cannot solely depend on gene products acting in trans, otherwise mutations in the corresponding genes may lead to reentry and completion of the cell cycle in TD cells, but this has not been observed. In the interphase, nuclear DNA of metazoan cells is organized in supercoiled loops anchored to a nuclear nuclear matrix (NM). The DNA-NM interactions define a higher-order structure in the cell nucleus (NHOS). We have previously compared the NHOS of aged rat hepatocytes with that of early post-mitotic rat neurons and our results indicated that a very stable NHOS is a common feature of both senescent and post-mitotic cells in vivo.

Principal Findings

In the present work we compared the NHOS in rat neurons from different post-natal ages. Our results show that the trend towards further stabilization of the NHOS in neurons continues throughout post-natal life. This phenomenon occurs in absence of overt changes in the post-mitotic state and transcriptional activity of neurons, suggesting that it is independent of functional constraints.

Conclusions

Apparently the continued stabilization of the NHOS as a function of time is basically determined by thermodynamic and structural constraints. We discuss how the resulting highly stable NHOS of neurons may be the structural, non-genetic basis of their permanent and irreversible post-mitotic state.

Cell cycle activation and spinal cord injury

Traumatic spinal cord injury (SCI) evokes a complex cascade of events with initial mechanical damage leading to secondary injury processes that contribute to further tissue loss and functional impairment. Growing evidence suggests that the cell cycle is activated following SCI. Up-regulation of cell cycle proteins after injury appears to contribute not only to apoptotic cell death of postmitotic cells, including neurons and oligodendrocytes, but also to post-traumatic gliosis and microglial activation. Inhibition of key cell cycle regulatory pathways reduces injury-induced cell death, as well as microglial and astroglial proliferation both in vitro and in vivo. Treatment with cell cycle inhibitors in rodent SCI models prevents neuronal cell death and reduces inflammation, as well as the surrounding glial scar, resulting in markedly reduced lesion volumes and improved motor recovery. Here we review the effects of SCI on cell cycle pathways, as well as the therapeutic potential and mechanism of action of cell cycle inhibitors for this disorder.

Phosphorylation of p27Kip1 at Thr187 by cyclin-dependent kinase 5 modulates neural stem cell differentiation

Cdk5 plays a role in nervous system development; its role in the initial stages of neural differentiation is poorly understood. We isolated neural stem cells from E13 Cdk5 WT and KO mouse and observed them as they switched from proliferating stage to neural differentiation. We show that Cdk5 phosphorylation of p27^kip1 at Thr187 is crucial to neural differentiation.

Cyclin-dependent kinase 5 (Cdk5) plays a key role in the development of the mammalian nervous system; it phosphorylates a number of targeted proteins involved in neuronal migration during development to synaptic activity in the mature nervous system. Its role in the initial stages of neuronal commitment and differentiation of neural stem cells (NSCs), however, is poorly understood. In this study, we show that Cdk5 phosphorylation of p27^Kip1 at Thr187 is crucial to neural differentiation because 1) neurogenesis is specifically suppressed by transfection of p27^Kip1 siRNA into Cdk5^+/+ NSCs; 2) reduced neuronal differentiation in Cdk5^−/− compared with Cdk5^+/+ NSCs; 3) Cdk5^+/+ NSCs, whose differentiation is inhibited by a nonphosphorylatable mutant, p27/Thr187A, are rescued by cotransfection of a phosphorylation-mimicking mutant, p27/Thr187D; and 4) transfection of mutant p27^Kip1 (p27/187A) into Cdk5^+/+ NSCs inhibits differentiation. These data suggest that Cdk5 regulates the neural differentiation of NSCs by phosphorylation of p27^Kip1 at theThr187 site. Additional experiments exploring the role of Ser10 phosphorylation by Cdk5 suggest that together with Thr187 phosphorylation, Ser10 phosphorylation by Cdk5 promotes neurite outgrowth as neurons differentiate. Cdk5 phosphorylation of p27^Kip1, a modular molecule, may regulate the progress of neuronal differentiation from cell cycle arrest through differentiation, neurite outgrowth, and migration.

Many roads lead to primary autosomal recessive microcephaly

Autosomal recessive primary microcephaly (MCPH), historically referred to as Microcephalia vera, is a genetically and clinically heterogeneous disease. Patients with MCPH typically exhibit congenital microcephaly as well as mental retardation, but usually no further neurological findings or malformations. Their microcephaly with grossly preserved macroscopic organization of the brain is a consequence of a reduced brain volume, which is evident particularly within the cerebral cortex and thus results to a large part from a reduction of grey matter. Some patients with MCPH further provide evidence of neuronal heterotopias, polymicrogyria or cortical dysplasia suggesting an associated neuronal migration defect. Genetic causes of MCPH subtypes 1-7 include mutations in genes encoding microcephalin, cyclin-dependent kinase 5 regulatory associated protein 2 (CDK5RAP2), abnormal spindle-like, microcephaly associated protein (ASPM), centromeric protein J (CENPJ), and SCL/TAL1-interrupting locus (STIL) as well as linkage to the two loci 19q13.1-13.2 and 15q15-q21. Here, we provide a timely overview of current knowledge on mechanisms leading to microcephaly in humans with MCPH and abnormalities in cell division/cell survival in corresponding animal models. Understanding the pathomechanisms leading to MCPH is of high importance not only for our understanding of physiologic brain development (particularly of cortex formation), but also for that of trends in mammalian evolution with a massive increase in size of the cerebral cortex in primates, of microcephalies of other etiologies including environmentally induced microcephalies, and of cancer formation.

Conditional deletion of neuronal cyclin-dependent kinase 5 in developing forebrain results in microglial activation and neurodegeneration

Neuronal migration disorders are often identified in patients with epilepsy refractory to medical treatment. The prolonged or repeated seizures are known to cause neuronal death; however, the mechanism underlying seizure-induced neuronal death remains to be elucidated. An essential role of cyclin-dependent kinase 5 (Cdk5) in brain development has been demonstrated in Cdk5(-/-) mice, which show neuronal migration defects and perinatal lethality. Here, we show the consequences of Cdk5 deficiency in the postnatal brain by generating Cdk5 conditional knockout mice, in which Cdk5is selectively eliminated from neurons in the developing forebrain. The conditional mutant mice were viable, but exhibited complex neurological deficits including seizures, tremors, and growth retardation. The forebrain not only showed disruption of layering, but also neurodegenerative changes accompanied by neuronal loss and microglial activation. The neurodegenerative changes progressed with age and were accompanied by up-regulation of the neuronal tissue-type plasminogen activator, a serine protease known to mediate microglial activation. Thus age-dependent neurodegeneration in the Cdk5 conditional knockout mouse brain invoked a massive inflammatory reaction. These findings indicate an important role of Cdk5 in inflammation, and also provide a mouse model to examine the possible involvement of inflammation in the pathogenesis of progressive cognitive decline in patients with neuronal migration disorders.

Cell-intrinsic signals that regulate adult neurogenesis in vivo: insights from inducible approaches

The process by which adult neural stem cells generate new and functionally integrated neurons in the adult mammalian brain has been intensely studied, but much more remains to be discovered. It is known that neural progenitors progress through distinct stages to become mature neurons, and this progression is tightly controlled by cell-cell interactions and signals in the neurogenic niche. However, less is known about the cell-intrinsic signaling required for proper progression through stages of adult neurogenesis. Techniques have recently been developed to manipulate genes specifically in adult neural stem cells and progenitors in vivo, such as the use of inducible transgenic mice and viral-mediated gene transduction. A critical mass of publications utilizing these techniques has been reached, making it timely to review which molecules are now known to play a cell-intrinsic role in regulating adult neurogenesis in vivo. By drawing attention to these isolated molecules (e.g. Notch), we hope to stimulate a broad effort to understand the complex and compelling cascades of intrinsic signaling molecules important to adult neurogenesis. Understanding this process opens the possibility of understanding brain functions subserved by neurogenesis, such as memory, and also of harnessing neural stem cells for repair of the diseased and injured brain.