Direct and indirect roles of cyclin-dependent kinase 5 as an upstream regulator in the c-Jun NH2-terminal kinase cascade: relevance to neurotoxic insults in Alzheimer's disease

Antibody-free time-resolved terbium luminescence assays designed for cyclin-dependent kinase 5 (CDK5)

Novel time-resolved terbium luminescence assays were developed for CDK5 and CDK2 by designing synthetic substrates which incorporate phospho-inducible terbium sensitizing motifs with kinase substrate consensus sequences. Substrates designed for CDK5 showed no phosphorylation by CDK2, opening the possibility for CDK5-specific assay development for selective drug discovery.

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.

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.

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.

Reciprocal deregulation of NKX3.1 and AURKA axis in castration-resistant prostate cancer and NEPC models

Background

NKX3.1, a prostate-specific tumor suppressor, is either genomically lost or its protein levels are severely downregulated, which are invariably associated with poor prognosis in prostate cancer (PCa). Nevertheless, a clear disconnect exists between its mRNA and protein levels, indicating that its post-translational regulation may be critical in maintaining its protein levels. Similarly, AURKA is vastly overexpressed in all stages of prostate cancer (PCa), including castration-resistant PCa (CRPC) and neuroendocrine PCa (NEPC), although its transcripts are only increased in ~ 15% of cases, hinting at additional mechanisms of deregulation. Thus, identifying the upstream regulators that control AURKA and NKX3.1’s levels and/or their downstream effectors offer an alternative route to inhibit AURKA and upregulate NKX3.1 in highly fatal CRPC and NEPC. AURKA and NKX3.1 have not linked to each other in any study to date.

Methods

A chemical genetic screen revealed NKX3.1 as a direct target of AURKA. AURKA-NKX3.1 cross-talk was analyzed using several biochemical techniques in CRPC and NEPC cells.

Results

We uncovered a reciprocal loop between AURKA and NKX3.1 in CRPC and NEPC cells. We observed that AURKA-mediated NKX3.1 downregulation is a major mechanism that drives CRPC pathogenesis and NEPC differentiation. AURKA phosphorylates NKX3.1 at three sites, which degrades it, but AURKA does not regulate NKX3.1 mRNA levels. NKX3.1 degradation drives highly aggressive oncogenic phenotypes in cells. NKX3.1 also degrades AURKA in a feedback loop. NKX3.1-AURKA loop thus upregulates AKT, ARv7 and Androgen Receptor (AR)-signaling in tandem promoting highly malignant phenotypes. Just as importantly, we observed that NKX3.1 overexpression fully abolished synaptophysin and enolase expression in NEPC cells, uncovering a strong negative relationship between NKX3.1 and neuroendocrine phenotypes, which was further confirmed be measuring neurite outgrowth. While WT-NKX3.1 inhibited neuronal differentiation, 3A-NKX3.1 expression obliterated it.

Conclusions

NKX3.1 loss could be a major mechanism causing AURKA upregulation in CRPC and NEPC and vice versa. NKX3.1 genomic loss requires gene therapy, nonetheless, targeting AURKA provides a powerful tool to maintain NKX3.1 levels. Conversely, when NKX3.1 upregulation strategy using small molecules comes to fruition, AURKA inhibition should work synergistically due to the reciprocal loop in these highly aggressive incurable diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-021-00765-z.

LIMK2-NKX3.1 Engagement Promotes Castration-Resistant Prostate Cancer

Simple Summary

Prostate cancer is the principal cause of cancer-related mortality in men. While localized tumors can be successfully treated by orchiectomy or medical castration, most of the patients ultimately progress to the castration-resistant prostate cancer (CRPC) stage, which is incurable at present. Thus, uncovering the underlying mechanisms that cause CRPC could result in promising therapeutics. Our laboratory has identified LIMK2 kinase as an actionable target for CRPC. LIMK2 is vastly expressed in CRPC but minimally in normal prostates. LIMK2 knockout mice are healthy, indicating that LIMK2 inhibition should have minimal toxicity. LIMK2 is also expressed in other aggressive cancers; however, the molecular mechanisms leading to malignancy remain mostly unknown. This study identified that LIMK2 downregulates a prostate-specific tumor suppressor protein-NKX3.1 using two mechanisms. NKX3.1 loss is strongly associated with prostate cancer. Thus, LIMK2 inhibitor provides a powerful opportunity to rescue NKX3.1 loss, thereby preventing and/or delaying prostate cancer progression.

Abstract

NKX3.1’s downregulation is strongly associated with prostate cancer (PCa) initiation, progression, and CRPC development. Nevertheless, a clear disagreement exists between NKX3.1 protein and mRNA levels in PCa tissues, indicating that its regulation at a post-translational level plays a vital role. This study identified a strong negative relationship between NKX3.1 and LIMK2, which is critical in CRPC pathogenesis. We identified that NKX3.1 degradation by direct phosphorylation by LIMK2 is crucial for promoting oncogenicity in CRPC cells and in vivo. LIMK2 also downregulates NKX3.1 mRNA levels. In return, NKX3.1 promotes LIMK2’s ubiquitylation. Thus, the negative crosstalk between LIMK2-NKX3.1 regulates AR, ARv7, and AKT signaling, promoting aggressive phenotypes. We also provide a new link between NKX3.1 and PTEN, both of which are downregulated by LIMK2. PTEN loss is strongly linked with NKX3.1 downregulation. As NKX3.1 is a prostate-specific tumor suppressor, preserving its levels by LIMK2 inhibition provides a tremendous opportunity for developing targeted therapy in CRPC. Further, as NKX3.1 downregulates AR transcription and inhibits AKT signaling, restoring its levels by inhibiting LIMK2 is expected to be especially beneficial by co-targeting two driver pathways in tandem, a highly desirable requisite for developing effective PCa therapeutics.

The role of Bcl-2 proteins in modulating neuronal Ca2+ signaling in health and in Alzheimer's disease

The family of B-cell lymphoma-2 (Bcl-2) proteins exerts key functions in cellular health. Bcl-2 primarily acts in mitochondria where it controls the initiation of apoptosis. However, during the last decades, it has become clear that this family of proteins is also involved in controlling intracellular Ca2+ signaling, a critical process for the function of most cell types, including neurons. Several anti- and pro-apoptotic Bcl-2 family members are expressed in neurons and impact neuronal function. Importantly, expression levels of neuronal Bcl-2 proteins are affected by age. In this review, we focus on the emerging roles of Bcl-2 proteins in neuronal cells. Specifically, we discuss how their dysregulation contributes to the onset, development, and progression of neurodegeneration in the context of Alzheimer's disease (AD). Aberrant Ca2+ signaling plays an important role in the pathogenesis of AD, and we propose that dysregulation of the Bcl-2-Ca2+ signaling axis may contribute to the progression of AD and that herein, Bcl-2 may constitute a potential therapeutic target for the treatment of AD.

Phosphorylation-dependent regulation of SPOP by LIMK2 promotes castration-resistant prostate cancer

BACKGROUND

SPOP, an E3 ubiquitin ligase adaptor, can act either as a tumour suppressor or a tumour promoter. In prostate cancer (PCa), it inhibits tumorigenesis by degrading several oncogenic substrates. SPOP is the most altered gene in PCa (~15%), which renders it ineffective, promoting cancer. The remaining PCa tumours, which retain WT-SPOP, still progress to castration-resistant (CRPC) stage, indicating that other critical mechanisms exist for downregulating SPOP. SPOP is reduced in ~94% of WT-SPOP-bearing prostate tumours; however, no molecular mechanism is known for its downregulation.

METHODS

SPOP was identified as a direct target of LIMK2 using an innovative technique. The reciprocal relationship between SPOP and LIMK2 and its consequences on oncogenicity were analysed using a variety of biochemical assays. To probe this relationship in vivo, xenograft studies were conducted.

RESULTS

LIMK2 degrades SPOP by direct phosphorylation at three sites. SPOP promotes LIMK2's ubiquitylation, creating a feedback loop. SPOP's degradation stabilises AR, ARv7 and c-Myc promoting oncogenicity. Phospho-resistant SPOP completely suppresses tumorigenesis in vivo, indicating that LIMK2-mediated SPOP degradation is a key event in PCa progression.

CONCLUSIONS

While genomically altered SPOP-bearing tumours require gene therapy, uncovering LIMK2-SPOP relationship provides a powerful opportunity to retain WT-SPOP by inhibiting LIMK2, thereby halting disease progression.

Molecular Interplay between AURKA and SPOP Dictates CRPC Pathogenesis via Androgen Receptor

SPOP, an adaptor protein for E3 ubiquitin ligase can function as a tumor-suppressor or a tumor-enhancer. In castration-resistant prostate cancer (CRPC), it inhibits tumorigenesis by degrading many oncogenic targets, including androgen receptor (AR). Expectedly, SPOP is the most commonly mutated gene in CRPC (15%), which closely correlates with poor prognosis. Importantly, 85% of tumors that retain wild-type SPOP show reduced protein levels, indicating that SPOP downregulation is an essential step in CRPC progression. However, the underlying molecular mechanism remains unknown. This study uncovered the first mechanism of SPOP regulation in any type of cancer. We identified SPOP as a direct substrate of Aurora A (AURKA) using an innovative technique. AURKA directly phosphorylates SPOP at three sites, causing its ubiquitylation. SPOP degradation drives highly aggressive oncogenic phenotypes in cells and in vivo including stabilizing AR, ARv7 and c-Myc. Further, SPOP degrades AURKA via a feedback loop. SPOP upregulation is one of the mechanisms by which enzalutamide exerts its efficacy. Consequently, phospho-resistant SPOP fully abrogates tumorigenesis and EMT in vivo, and renders CRPC cells sensitive to enzalutamide. While genomic mutations of SPOP can be treated with gene therapy, identification of AURKA as an upstream regulator of SPOP provides a powerful opportunity for retaining WT-SPOP in a vast majority of CRPC patients using AURKA inhibitors ± enzalutamide, thereby treating the disease and inhibiting its progression.

Negative cross talk between LIMK2 and PTEN promotes castration resistant prostate cancer pathogenesis in cells and in vivo

Androgen deprivation therapy (ADT) and androgen receptor (AR) signaling inhibitors are front-line treatments for highly aggressive prostate cancer. However, prolonged inhibition of AR triggers a compensatory activation of PI3K pathway, most often due to the genomic loss of tumor suppressor PTEN, driving progression to the castration-resistant prostate cancer (CRPC) stage, which has very poor prognosis. We uncovered a novel mechanism of PTEN downregulation triggered by LIMK2, which contributes significantly to CRPC pathogenesis. LIMK2 is a CRPC-specific target. Its depletion fully reverses tumorigenesis in vivo. LIMK2 phosphorylates PTEN at five sites, degrading and inhibiting its activity, thereby driving highly aggressive oncogenic phenotypes in cells and in vivo. PTEN also degrades LIMK2 in a feedback loop, which was confirmed in prostates from PTEN^-/- and PTEN^+/+ mice. LIMK2 is also the missing link between hypoxia and PTEN degradation in CRPC. This is the first study to show a feedback loop between PTEN and its regulator. Uncovering the LIMK2-PTEN loop provides a powerful therapeutic opportunity to retain the activity and stability of PTEN protein by inhibiting LIMK2, thereby halting the progression to CRPC, ADT-resistance and drug-resistance.

Aurora Kinase A-YBX1 Synergy Fuels Aggressive Oncogenic Phenotypes and Chemoresistance in Castration-Resistant Prostate Cancer

Multifunctional protein YBX1 upregulation promotes castration-resistant prostate cancer (CRPC). However, YBX1 protein abundance, but not its DNA status or mRNA levels, predicts CRPC recurrence, although the mechanism remains unknown. Similarly, the mechanism by which YBX1 regulates androgen receptor (AR) signaling remains unclear. We uncovered the first molecular mechanism of YBX1 upregulation at a post-translational level. YBX1 was identified as an Aurora Kinase-A (AURKA) substrate using a chemical screen. AURKA phosphorylates YBX1 at two key residues, which stabilizes it and promotes its nuclear translocation. YBX1 reciprocates and stabilizes AURKA, thereby initiating a synergistic loop. Notably, phospho-resistant YBX1 is dominant-negative and fully inhibits epithelial to mesenchymal transition, chemoresistance, drug-resistance and tumorigenesis in vivo. Unexpectedly, we further observed that YBX1 upregulates AR post-translationally by preventing its ubiquitylation, but not by increasing its transcription as reported before. Uncovering YBX1-mediated AR stabilization is highly significant due to AR's critical role in both androgen-sensitive prostate cancer and CRPC. As YBX1 inhibitors are unknown, AURKA inhibitors provide a potent tool to degrade both YBX1 and AR simultaneously. Finally, this is the first study to show a reciprocal loop between YBX1 and its kinase, indicating that their concomitant inhibition will be act synergistically for CRPC therapy.

Site-specific phosphorylation of Fbxw7 by Cdk5/p25 and its resulting decreased stability are linked to glutamate-induced excitotoxicity

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine protein kinase that regulates brain development and neurodegeneration. Cdk5 is activated by p25 that is generated from calpain-dependent cleavage of p35. The generation of p25 is responsible for the aberrant hyper-activation of Cdk5, which causes neurodegeneration. Using in vitro assays, we discovered that F-box/WD repeat-containing protein 7 (Fbxw7) is a new substrate of Cdk5. Additionally, Cdk5-dependent phosphorylation of Fbxw7 was detected in the presence of p25, and two amino acid residues (S349 and S372) were determined to be major phosphorylation sites. This phosphorylation was eventually linked to decreased stability of Fbxw7. Using a culture model of cortical neurons challenged with glutamate, we confirmed that decreased stability of Fbxw7 was indeed Cdk5-dependent. Furthermore, diminished levels of Fbxw7 led to increased levels of transcription factor AP-1 (c-Jun), a known substrate of Fbxw7. Given that previous reports demonstrate that c-Jun plays a role in accelerating neuronal apoptosis in these pathological models, our data support the concepts of a molecular cascade in which Cdk5-mediated phosphorylation of Fbxw7 negatively regulates Fbxw7 expression, thereby contributing to neuronal cell death following glutamate-mediated excitotoxicity.

Identification of LIMK2 as a therapeutic target in castration resistant prostate cancer

This study identified LIMK2 kinase as a disease-specific target in castration resistant prostate cancer (CRPC) pathogenesis, which is upregulated in response to androgen deprivation therapy, the current standard of treatment for prostate cancer. Surgical castration increases LIMK2 expression in mouse prostates due to increased hypoxia. Similarly, human clinical specimens showed highest LIMK2 levels in CRPC tissues compared to other stages, while minimal LIMK2 was observed in normal prostates. Most notably, inducible knockdown of LIMK2 fully reverses CRPC tumorigenesis in castrated mice, underscoring its potential as a clinical target for CRPC. We also identified TWIST1 as a direct substrate of LIMK2, which uncovered the molecular mechanism of LIMK2-induced malignancy. TWIST1 is strongly associated with CRPC initiation, progression and poor prognosis. LIMK2 increases TWIST1 mRNA levels upon hypoxia; and stabilizes TWIST1 by direct phosphorylation. TWIST1 also stabilizes LIMK2 by inhibiting its ubiquitylation. Phosphorylation-dead TWIST1 acts as dominant negative and fully prevents EMT and tumor formation in vivo, thereby highlighting the significance of LIMK2-TWIST1 signaling axis in CRPC. As LIMK2 null mice are viable, targeting LIMK2 should have minimal collateral toxicity, thereby improving the overall survival of CRPC patients.

Multifaceted Regulation of ALDH1A1 by Cdk5 in Alzheimer's Disease Pathogenesis

This study revealed multifaceted regulation of ALDH1A1 by Cdk5 in Alzheimer's disease (AD) pathogenesis. ALDH1A1 is a multifunctional enzyme with dehydrogenase, esterase, and anti-oxidant activities. ALDH1A1 is also a major regulator of retinoic acid (RA) signaling, which is critical for normal brain homeostasis. We identified ALDH1A1 as both physiological and pathological target of Cdk5. First, under neurotoxic conditions, Cdk5-induced oxidative stress upregulates ALDH1A1 transcription. Second, Cdk5 increases ALDH1A1 levels by preventing its ubiquitylation via direct phosphorylation. Third, ALDH1A1 phosphorylation increases its dehydrogenase activity by altering its tetrameric state to a highly active monomeric state. Fourth, persistent oxidative stress triggered by deregulated Cdk5 inactivates ALDH1A1. Thus, initially, the good Cdk5 attempts to mitigate ensuing oxidative stress by upregulating ALDH1A1 via phosphorylation and paradoxically by increasing oxidative stress. Later, sustained oxidative stress generated by Cdk5 inhibits ALDH1A1 activity, leading to neurotoxicity. ALDH1A1 upregulation is highly neuroprotective. In human AD tissues, ALDH1A1 levels increase with disease severity. However, ALDH1A1 activity was highest at mild and moderate stages, but declines significantly at severe stage. These findings confirm that during the initial stages, neurons attempt to upregulate and activate ALDH1A1 to protect from accruing oxidative stress-induced damage; however, persistently deleterious conditions inactivate ALDH1A1, further contributing to neurotoxicity. This study thus revealed two faces of Cdk5, good and bad in neuronal function and survival, with a single substrate, ALDH1A1. The bad Cdk5 prevails in the end, overriding the good Cdk5 act, suggesting that Cdk5 is an effective therapeutic target for AD.

A Stem Cell-Based Screening Platform Identifies Compounds that Desensitize Motor Neurons to Endoplasmic Reticulum Stress

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease selectively targeting motor neurons in the brain and spinal cord. The reasons for differential motor neuron susceptibility remain elusive. We developed a stem cell-based motor neuron assay to study cell-autonomous mechanisms causing motor neuron degeneration, with implications for ALS. A small-molecule screen identified cyclopiazonic acid (CPA) as a stressor to which stem cell-derived motor neurons were more sensitive than interneurons. CPA induced endoplasmic reticulum stress and the unfolded protein response. Furthermore, CPA resulted in an accelerated degeneration of motor neurons expressing human superoxide dismutase 1 (hSOD1) carrying the ALS-causing G93A mutation, compared to motor neurons expressing wild-type hSOD1. A secondary screen identified compounds that alleviated CPA-mediated motor neuron degeneration: three kinase inhibitors and tauroursodeoxycholic acid (TUDCA), a bile acid derivative. The neuroprotective effects of these compounds were validated in human stem cell-derived motor neurons carrying a mutated SOD1 allele (hSOD1A4V). Moreover, we found that the administration of TUDCA in an hSOD1G93A mouse model of ALS reduced muscle denervation. Jointly, these results provide insights into the mechanisms contributing to the preferential susceptibility of ALS motor neurons, and they demonstrate the utility of stem cell-derived motor neurons for the discovery of new neuroprotective compounds.

Downregulated apoptosis and autophagy after anti-Aβ immunotherapy in Alzheimer's disease

Aβ immunization of Alzheimer's disease (AD) patients in the AN1792 (Elan Pharmaceuticals) trial caused Aβ removal and a decreased density of neurons in the cerebral cortex. As preservation of neurons may be a critical determinant of outcome after Aβ immunization, we have assessed the impact of previous Aβ immunization on the expression of a range of apoptotic proteins in post-mortem human brain tissue. Cortex from 13 AD patients immunized with AN1792 (iAD) and from 27 nonimmunized AD (cAD) cases was immunolabeled for proapoptotic proteins implicated in AD pathophysiology: phosphorylated c-Jun N-terminal kinase (pJNK), activated caspase3 (a-casp3), phosphorylated GSK3β on tyrosine 216 (GSK3βtyr216 ), p53 and Cdk5/p35. Expression of these proteins was analyzed in relation to immunization status and other clinical data. The antigen load of all of these proapoptotic proteins was significantly lower in iAD than cAD (P < 0.0001). In cAD, significant correlations (P < 0.001) were observed between: Cdk5/p35 and GSK3βtyr216 ; a-casp3 and Aβ42 ; p53 and age at death. In iAD, significant correlations were found between GSK3βtyr216 and a-casp3; both spongiosis and neuritic curvature ratio and Aβ42 ; and Cdk5/p35 and Aβ-antibody level. Although neuronal loss was increased by immunization with AN1792, our present findings suggest downregulation of apoptosis in residual neurons and other cells.

Inhibition of cyclin-dependent kinase 5 affects early neuroinflammatory signalling in murine model of amyloid beta toxicity

BACKGROUND

Cyclin-dependent kinase 5 (Cdk5) belongs to the family of proline-directed serine/threonine kinases and plays a critical role in neuronal differentiation, migration, synaptogenesis, plasticity, neurotransmission and apoptosis. The deregulation of Cdk5 activity was observed in post mortem analysis of brain tissue of Alzheimer's disease (AD) patients, suggesting the involvement of Cdk5 in the pathomechanism of this neurodegenerative disease. However, our recent study demonstrated the important function of Cdk5 in regulating inflammatory reaction.

METHODS

Since the role of Cdk5 in regulation of inflammatory signalling in AD is unknown, we investigated the involvement of Cdk5 in neuroinflammation induced by single intracerebroventricular (icv) injection of amyloid beta protein (Aβ) oligomers in mouse. The brain tissue was analysed up to 35 days post injection. Roscovitine (intraperitoneal administration) was used as a potent Cdk5 inhibitor. The experiments were also performed on human neuroblastoma SH-SY5Y as well as mouse BV2 cell lines treated with exogenous oligomeric Aβ.

RESULTS

Our results demonstrated that single injection of Aβ oligomers induces long-lasting activation of microglia and astrocytes in the hippocampus. We observed also profound, early inflammatory response in the mice hippocampus, leading to the significant elevation of pro-inflammatory cytokines expression (e.g. TNF-α, IL-1β, IL-6). Moreover, Aβ oligomers elevated the formation of truncated protein p25 in mouse hippocampus and induced overactivation of Cdk5 in neuronal cells. Importantly, administration of roscovitine reduced the inflammatory processes evoked by Aβ in the hippocampus, leading to the significant decrease of cytokines level.

CONCLUSIONS

These studies clearly show the involvement of Cdk5 in modulation of brain inflammatory response induced by Aβ and may indicate this kinase as a novel target for pharmacological intervention in AD.

The Cdk5-Mcl-1 axis promotes mitochondrial dysfunction and neurodegeneration in a model of Alzheimer's disease

Cdk5 deregulation is highly neurotoxic in Alzheimer's disease (AD). We identified Mcl-1 as a direct Cdk5 substrate using an innovative chemical screen in mouse brain lysates. Our data demonstrate that Mcl-1 levels determine the threshold for cellular damage in response to neurotoxic insults. Mcl-1 is a disease-specific target of Cdk5, which associates with Cdk5 under basal conditions, but is not regulated by it. Neurotoxic insults hyperactivate Cdk5 causing Mcl-1 phosphorylation at T92. This phosphorylation event triggers Mcl-1 ubiquitylation, which directly correlates with mitochondrial dysfunction. Consequently, ectopic expression of phosphorylation-dead T92A-Mcl-1 fully prevents mitochondrial damage and subsequent cell death triggered by neurotoxic treatments in neuronal cells and primary cortical neurons. Notably, enhancing Mcl-1 levels offers comparable neuroprotection to that observed upon Cdk5 depletion, suggesting that Mcl-1 degradation by direct phosphorylation is a key mechanism by which Cdk5 promotes neurotoxicity in AD. The clinical significance of the Mcl-1-Cdk5 axis was investigated in human AD clinical specimens, revealing an inverse correlation between Mcl-1 levels and disease severity. These results emphasize the potential of Mcl-1 upregulation as an attractive therapeutic strategy for delaying or preventing neurodegeneration in AD.

Tale of the Good and the Bad Cdk5: Remodeling of the Actin Cytoskeleton in the Brain

Cdk5 kinase, a cyclin-dependent kinase family member, is a key regulator of cytoskeletal remodeling in the brain. Cdk5 is essential for brain development during embryogenesis. After birth, it is essential for numerous neuronal processes such as learning and memory formation, drug addiction, pain signaling, and long-term behavior changes, all of which rely on rapid alterations in the cytoskeleton. Cdk5 activity is deregulated in various brain disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and ischemic stroke, resulting in profound remodeling of the neuronal cytoskeleton, loss of synapses, and ultimately neurodegeneration. This review focuses on the "good and bad" Cdk5 in the brain and its pleiotropic contribution in regulating neuronal actin cytoskeletal remodeling. A vast majority of physiological and pathological Cdk5 substrates are associated with the actin cytoskeleton. Thus, our special emphasis is on the numerous Cdk5 substrates identified in the past two decades such as ephexin1, p27, Mst3, CaMKv, kalirin-7, RasGRF2, Pak1, WAVE1, neurabin-1, TrkB, 5-HT6R, talin, drebrin, synapsin I, synapsin III, CRMP1, GKAP, SPAR, PSD-95, and LRRK2. These substrates have unraveled the molecular mechanisms by which Cdk5 plays divergent roles in regulating neuronal actin cytoskeletal dynamics both in healthy and diseased states.

The Aurora-A-Twist1 axis promotes highly aggressive phenotypes in pancreatic carcinoma

We uncovered a crucial role for the Aurora kinase A (AURKA)-Twist1 axis in promoting epithelial-to-mesenchymal transition (EMT) and chemoresistance in pancreatic cancer. Twist1 is the first EMT-specific target of AURKA that was identified using an innovative screen. AURKA phosphorylates Twist1 at three sites, which results in its multifaceted regulation - AURKA inhibits its ubiquitylation, increases its transcriptional activity and favors its homodimerization. Twist1 reciprocates and prevents AURKA degradation, thereby triggering a feedback loop. Ablation of either AURKA or Twist1 completely inhibits EMT, highlighting both proteins as central players in EMT progression. Phosphorylation-dead Twist1 serves as a dominant-negative and fully reverses the EMT phenotype induced by Twist1, underscoring the crucial role of AURKA-mediated phosphorylation in mediating Twist1-induced malignancy. Likewise, Twist1-overexpressing BxPC3 cells formed large tumors in vivo, whereas expression of phosphorylation-dead Twist1 fully abrogated this effect. Furthermore, immunohistochemical analysis of pancreatic cancer specimens revealed a 3-fold higher level of Twist1 compared to that seen in healthy normal tissues. This is the first study that links Twist1 in a feedback loop with its activating kinase, which indicates that concurrent inhibition of AURKA and Twist1 will be synergistic in inhibiting pancreatic tumorigenesis and metastasis.

Phosphorylation-dependent regulation of ALDH1A1 by Aurora kinase A: insights on their synergistic relationship in pancreatic cancer

Background

Epithelial-to-mesenchymal transition (EMT) and cancer stem cell (CSC) formation are key underlying causes that promote extensive metastasis, drug resistance, and tumor recurrence in highly lethal pancreatic cancer. The mechanisms leading to EMT and CSC phenotypes are not fully understood, which has hindered the development of effective targeted therapies capable of improving treatment outcomes in patients with pancreatic cancer.

Results

We show a central role of Aurora kinase A (AURKA) in promoting EMT and CSC phenotypes via ALDH1A1, which was discovered as its direct substrate using an innovative chemical genetic screen. AURKA phosphorylates ALDH1A1 at three critical residues which exert a multifaceted regulation over its level, enzymatic activity, and quaternary structure. While all three phosphorylation sites contribute to its increased stability, T267 phosphorylation primarily regulates ALDH1A1 activity. AURKA-mediated phosphorylation rapidly dissociates tetrameric ALDH1A1 into a highly active monomeric species. ALDH1A1 also reciprocates and prevents AURKA degradation, thereby triggering a positive feedback activation loop which drives highly aggressive phenotypes in cancer. Phospho-resistant ALDH1A1 fully reverses EMT and CSC phenotypes, thus serving as dominant negative, which underscores the clinical significance of the AURKA-ALDH1A1 signaling axis in pancreatic cancer.

Conclusions

While increased levels and activity of ALDH1A1 are hallmarks of CSCs, the underlying molecular mechanism remains unclear. We show the first phosphorylation-dependent regulation of ALDH1A1, which increases its levels and activity via AURKA. Recent global phospho-proteomic screens have revealed increased phosphorylation of ALDH1A1 at the T267 site in human cancers and healthy liver tissues where ALDH1A1 is highly expressed and active, indicating that this regulation is likely crucial both in normal and diseased states. This is also the first study to demonstrate oligomer-dependent activity of ALDH1A1, signifying that targeting its oligomerization state may be an effective therapeutic approach for counteracting its protective functions in cancer. Finally, while AURKA inhibition provides a potent tool to reduce ALDH1A1 levels and activity, the reciprocal loop between them ensures that their concurrent inhibition will be highly synergistic when inhibiting tumorigenesis, chemoresistance, and metastasis in highly aggressive pancreatic cancer and beyond.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0335-5) contains supplementary material, which is available to authorized users.

Cyclin-dependent kinase 5 contributes to endoplasmic reticulum stress induced podocyte apoptosis via promoting MEKK1 phosphorylation at Ser280 in diabetic nephropathy

Endoplasmic reticulum (ER) stress has been reported to be associated with podocyte apoptosis in diabetic nephropathy, but the mechanism of ER signaling in podocyte apoptosis hasn't been fully understood. Our previous studies have demonstrated that Cyclin-dependent kinase 5 (Cdk5) was associated with podocyte apoptosis in diabetic nephropathy. The present study was designed to examine whether and how Cdk5 activity plays a role in ER stress induced podocyte apoptosis in diabetic nephropathy. The results showed that along with induction of Cdk5 and apoptosis, GRP78 and its two sensors as well as CHOP and cleaved caspase-12 were induced in high glucose treated podocytes. These responses were attenuated by treated salubrinal. The ER stress inducer, tunicamycin, also up-regulated the kinase activity and protein expression of Cdk5 in podocytes accompanied with the increasing of GRP78. On the other hand, Cdk5 phosphorylates MEKK1 at Ser280 in tunicamycin treated podocytes, and together, they increase the JNK phosphorylation. Moreover, disruption of this pathway can decrease the podocyte apoptosis induced by tunicamycin. Therefore, our study proved that Cdk5 may play an important role in ER stress induced podocyte apoptosis through MEKK1/JNK pathway in diabetic nephropathy.

Mitochondrial dysfunction and cell death in neurodegenerative diseases through nitroxidative stress

Mitochondria are important for providing cellular energy ATP through the oxidative phosphorylation pathway. They are also critical in regulating many cellular functions including the fatty acid oxidation, the metabolism of glutamate and urea, the anti-oxidant defense, and the apoptosis pathway. Mitochondria are an important source of reactive oxygen species leaked from the electron transport chain while they are susceptible to oxidative damage, leading to mitochondrial dysfunction and tissue injury. In fact, impaired mitochondrial function is commonly observed in many types of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, alcoholic dementia, brain ischemia-reperfusion related injury, and others, although many of these neurological disorders have unique etiological factors. Mitochondrial dysfunction under many pathological conditions is likely to be promoted by increased nitroxidative stress, which can stimulate post-translational modifications (PTMs) of mitochondrial proteins and/or oxidative damage to mitochondrial DNA and lipids. Furthermore, recent studies have demonstrated that various antioxidants, including naturally occurring flavonoids and polyphenols as well as synthetic compounds, can block the formation of reactive oxygen and/or nitrogen species, and thus ultimately prevent the PTMs of many proteins with improved disease conditions. Therefore, the present review is aimed to describe the recent research developments in the molecular mechanisms for mitochondrial dysfunction and tissue injury in neurodegenerative diseases and discuss translational research opportunities.

Cdk5-Foxo3 axis: initially neuroprotective, eventually neurodegenerative in Alzheimer's disease models

Deregulated Cdk5 causes neurotoxic amyloid beta peptide (Aβ) processing and cell death, two hallmarks of Alzheimer's disease, through the Foxo3 transcriptional factor in hippocampal cells, primary neurons and an Alzheimer's disease mouse model. Using an innovative chemical genetic screen, we identified Foxo3 as a direct substrate of Cdk5 in brain lysates. Cdk5 directly phosphorylates Foxo3, which increased its levels and nuclear translocation. Nuclear Foxo3 initially rescued cells from ensuing oxidative stress by upregulating MnSOD (also known as SOD2). However, following prolonged exposure, Foxo3 upregulated Bim (also known as BCL2L11) and FasL (also known as FASLG) causing cell death. Active Foxo3 also increased Aβ(1-42) levels in a phosphorylation-dependent manner. These events were completely inhibited either by expressing phosphorylation-resistant Foxo3 or by depleting Cdk5 or Foxo3, highlighting a key role for Cdk5 in regulating Foxo3. These results were confirmed in an Alzheimer's disease mouse model, which exhibited increased levels and nuclear localization of Foxo3 in hippocampal neurons, which preceded neurodegeneration and Aβ plaque formation, indicating this phenomenon is an early event in Alzheimer's disease pathogenesis. Collectively, these results show that Cdk5-mediated phospho-regulation of Foxo3 can activate several genes that promote neuronal death and aberrant Aβ processing, thereby contributing to the progression of neurodegenerative pathologies.

A Tale of the Good and Bad: Remodeling of the Microtubule Network in the Brain by Cdk5

Cdk5, a cyclin-dependent kinase family member, is a global orchestrator of neuronal cytoskeletal dynamics. During embryogenesis, Cdk5 is indispensable for brain development. In adults, it is essential for numerous neuronal processes, including higher cognitive functions such as learning and memory formation, drug addiction, pain signaling, and long-term behavior changes through long-term potentiation and long-term depression, all of which rely on rapid alterations in the cytoskeleton. Cdk5 activity becomes deregulated in various brain disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, attention-deficit hyperactivity disorder, epilepsy, schizophrenia, and ischemic stroke; these all result in profound remodeling of the neuronal cytoskeleton. This Commentary specifically focuses on the pleiotropic contribution of Cdk5 in regulating neuronal microtubule remodeling. Because the vast majority of the physiological substrates of Cdk5 are associated with the neuronal cytoskeleton, our emphasis is on the Cdk5 substrates, such as CRMP2, stathmin, drebrin, dixdc1, axin, MAP2, MAP1B, doublecortin, kinesin-5, and tau, that have allowed to unravel the molecular mechanisms through which Cdk5 exerts its divergent roles in regulating neuronal microtubule dynamics, both in healthy and disease states.

Cyclin-Dependent Kinase Inhibitor P1446A Induces Apoptosis in a JNK/p38 MAPK-Dependent Manner in Chronic Lymphocytic Leukemia B-Cells

CDK (cyclin-dependent kinase) inhibitors have shown remarkable activity in CLL, where its efficacy has been linked to inhibition of the transcriptional CDKs (7 and 9) and deregulation of RNA polymerase and short-lived pro-survival proteins such as MCL1. Furthermore, ER (endoplasmic reticulum) stress has been implicated in CDK inhibition in CLL. Here we conducted a pre-clinical study of a novel orally active kinase inhibitor P1446A in CLL B-cells. P1446A inhibited CDKs at nanomolar concentrations and induced rapid apoptosis of CLL cells in vitro, irrespective of chromosomal abnormalities or IGHV mutational status. Apoptosis preceded inactivation of RNA polymerase, and was accompanied by phosphorylation of stress kinases JNK (c-Jun N-terminal kinase) and p38 MAPK (mitogen-activated protein kinase). Pharmacologic inhibitors of JNK/p38 MAPK conferred protection from P1446A-mediated apoptosis. Treatment with P1446A led to a dramatic induction of NOXA in a JNK-dependent manner, and sensitized CLL cells to ABT-737, a BH3-mimetic. We observed concurrent activation of apoptosis stress-inducing kinase 1 (ASK1) and its interaction with inositol-requiring enzyme 1 (IRE1) and tumor necrosis factor receptor-associated factor 2 (TRAF2) in CLL cells treated with P1446A, providing insights into upstream regulation of JNK in this setting. Consistent with previous reports on limited functionality of ER stress mechanism in CLL cells, treatment with P1446A failed to induce an extensive unfolded protein response. This study provides rationale for additional investigations of P1446A in CLL.

Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses

Cyclin-dependent kinase 5 (Cdk5) is involved in proper neurodevelopment and brain function and serves as a switch between neuronal survival and death. Overactivation of Cdk5 is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important link between disease-initiating factors and cell death effectors. A common hallmark of neurodegenerative disorders is incorrect folding of specific proteins, thus leading to their intra- and extracellular accumulation in the nervous system. Abnormal Cdk5 signaling contributes to dysfunction of individual proteins and has a substantial role in either direct or indirect interactions of proteins common to, and critical in, different neurodegenerative diseases. While the roles of Cdk5 in α-synuclein (ASN) - tau or β-amyloid peptide (Aβ) - tau interactions are well documented, its contribution to many other pertinent interactions, such as that of ASN with Aβ, or interactions of the Aβ - ASN - tau triad with prion proteins, did not get beyond plausible hypotheses and remains to be proven. Understanding of the exact position of Cdk5 in the deleterious feed-forward loop critical for development and progression of neurodegenerative diseases may help designing successful therapeutic strategies of several fatal neurodegenerative diseases. Cyclin-dependent kinase 5 (Cdk5) is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important factor involved in protein misfolding, toxicity and interaction. We suggest that Cdk5 may contribute to the vicious circle of neurotoxic events involved in the pathogenesis of different neurodegenerative diseases.

Effect of N-n-butyl haloperidol iodide on ROS/JNK/Egr-1 signaling in H9c2 cells after hypoxia/reoxygenation

Reactive oxygen species (ROS)-induced oxidative stress in cells is an important pathophysiological process during myocardial ischemia/reperfusion (I/R) injury, and the transcription factor Egr-1 is a master switch for various damage pathways during reperfusion injury. An in vitro model of myocardial I/R injury and H9c2 cardiomyoblast cells hypoxia/reoxygenation (H/R) was used to assess whether there is abnormal intracellular ROS/JNK/Egr-1 signaling. We also assessed whether N-n-butyl haloperidol (F2), which exerts protective effects during myocardial I/R injury, can modulate this pathway. H/R induced ROS generation, JNK activation, and increased the expression of Egr-1 protein in H9c2 cells. The ROS scavengers edaravone (EDA) and N-acetyl-L-cysteine (NAC) reduced ROS level, downregulated JNK activation, and Egr-1 expression in H9c2 cells after H/R. The JNK inhibitor SP600125 inhibited Egr-1 overexpression in H9c2 cells caused by H/R. F2 could downregulate H/R-induced ROS level, JNK activation, and Egr-1 expression in H9c2 cells in a dose-dependent manner. The ROS donor hypoxanthine-xanthine oxidase (XO/HX) and the JNK activator ANISO antagonized the effects of F2. Therefore, H/R activates ROS/Egr-1 signaling pathway in H9c2 cells, and JNK activation plays an important role in this pathway. F2 regulates H/R-induced ROS/JNK/Egr-1 signaling, which might be an important mechanism by which it antagonizes myocardial I/R injury.

Cdk5 activity in the brain - multiple paths of regulation

Cyclin dependent kinase-5 (Cdk5), a family member of the cyclin-dependent kinases, plays a pivotal role in the central nervous system. During embryogenesis, Cdk5 is indispensable for brain development and, in the adult brain, it is essential for numerous neuronal processes, including higher cognitive functions such as learning and memory formation. However, Cdk5 activity becomes deregulated in several neurological disorders, such as Alzheimer's disease, Parkinson's disease and Huntington's disease, which leads to neurotoxicity. Therefore, precise control over Cdk5 activity is essential for its physiological functions. This Commentary covers the various mechanisms of Cdk5 regulation, including several recently identified protein activators and inhibitors of Cdk5 that control its activity in normal and diseased brains. We also discuss the autoregulatory activity of Cdk5 and its regulation at the transcriptional, post-transcriptional and post-translational levels. We finally highlight physiological and pathological roles of Cdk5 in the brain. Specific modulation of these protein regulators is expected to provide alternative strategies for the development of effective therapeutic interventions that are triggered by deregulation of Cdk5.

Loss of mitofusin 2 links beta-amyloid-mediated mitochondrial fragmentation and Cdk5-induced oxidative stress in neuron cells

Mitochondrial dysfunction is implicated in age-related degenerative disorders such as Alzheimer's disease (AD). Maintenance of mitochondrial dynamics is essential for regulating mitochondrial function. Aβ oligomers (AβOs), the typical cause of AD, lead to mitochondrial dysfunction and neuronal loss. AβOs have been shown to induce mitochondrial fragmentation, and their inhibition suppresses mitochondrial dysfunction and neuronal cell death. Oxidative stress is one of the earliest hallmarks of AD. Cyclin-dependent kinase 5 (Cdk5) may cause oxidative stress by disrupting the antioxidant system, including Prx2. Cdk5 is also regarded as a modulator of mitochondrial fission; however, a precise mechanistic link between Cdk5 and mitochondrial dynamics is lacking. We estimated mitochondrial morphology and alterations in mitochondrial morphology-related proteins in Neuro-2a (N2a) cells stably expressing the Swedish mutation of amyloid precursor protein (APP), which is known to increase AβO production. We demonstrated that mitochondrial fragmentation by AβOs accompanies reduced mitofusin 1 and 2 (Mfn1/2) levels. Interestingly, the Cdk5 pathway, including phosphorylation of the Prx2-related oxidative stress, has been shown to regulate Mfn1 and Mfn2 levels. Furthermore, Mfn2, but not Mfn1, over-expression significantly inhibits the AβO-mediated cell death pathway. Therefore, these results indicate that AβO-mediated oxidative stress triggers mitochondrial fragmentation via decreased Mfn2 expression by activating Cdk5-induced Prx2 phosphorylation. Mitochondrial fragmentation induced by amyloid-beta oligomer (AβOs) which is generated from the Swedish mutation of amyloid precursor protein (APP) accompanies reduced Mfn1/2 levels. Interestingly, the Cdk5 pathway, including phosphorylation of the Prx2-related oxidative stress, has been shown to regulate Mfn1/2. Furthermore, Mfn2 over-expression significantly inhibits the AβO-mediated neuronal cells death pathway, but not Mfn1 over-expression. Therefore, these results indicate that AβO-mediated oxidative stress triggers mitochondrial fragmentation via decreased Mfn2 expression by activating Cdk5-induced Prx2 phosphorylation. ATP, adenosine triphosphate; Bax, Bcl-2-associated X protein; Bcl-2, B-cell lymphoma 2; Cdk5, Cyclin-dependent kinase; Cyt C, cytochrome C; Mfn2, mitofusin 2; Prx2, peroxiredoxin 2; ROS, reactive oxygen species.

Searching for novel Cdk5 substrates in brain by comparative phosphoproteomics of wild type and Cdk5-/- mice

Protein phosphorylation is the most common post-translational modification that regulates several pivotal functions in cells. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase which is mostly active in the nervous system. It regulates several biological processes such as neuronal migration, cytoskeletal dynamics, axonal guidance and synaptic plasticity among others. In search for novel substrates of Cdk5 in the brain we performed quantitative phosphoproteomics analysis, isolating phosphoproteins from whole brain derived from E18.5 Cdk5^+/+ and Cdk5^−/− embryos, using an Immobilized Metal-Ion Affinity Chromatography (IMAC), which specifically binds to phosphorylated proteins. The isolated phosphoproteins were eluted and isotopically labeled for relative and absolute quantitation (iTRAQ) and mass spectrometry identification. We found 40 proteins that showed decreased phosphorylation at Cdk5^−/− brains. In addition, out of these 40 hypophosphorylated proteins we characterized two proteins, :MARCKS (Myristoylated Alanine-Rich protein Kinase C substrate) and Grin1 (G protein regulated inducer of neurite outgrowth 1). MARCKS is known to be phosphorylated by Cdk5 in chick neural cells while Grin1 has not been reported to be phosphorylated by Cdk5. When these proteins were overexpressed in N2A neuroblastoma cell line along with p35, serine phosphorylation in their Cdk5 motifs was found to be increased. In contrast, treatments with roscovitine, the Cdk5 inhibitor, resulted in an opposite effect on serine phosphorylation in N2A cells and primary hippocampal neurons transfected with MARCKS. In summary, the results presented here identify Grin 1 as novel Cdk5 substrate and confirm previously identified MARCKS as a a bona fide Cdk5 substrate.

LIMK2 is a crucial regulator and effector of Aurora-A-kinase-mediated malignancy

Aurora A is overexpressed in majority of breast carcinomas. With the exception of BRCA1 and PHLDA1, no oncogenic Aurora A substrates are known in breast cancer. In this study, a chemical genetic approach was used to identify malignant targets of Aurora A, which revealed LIMK2 as a novel Aurora A substrate. Aurora A regulates LIMK2 kinase activity, subcellular localization and protein levels by direct phosphorylation at S283, T494 and T505. In response, LIMK2 also positively regulates the level of Aurora A, thereby engaging in a positive-feedback loop, promoting Aurora-A-mediated oncogenic pathways. Most importantly, LIMK2 ablation fully abrogates Aurora-A-mediated tumorigenesis in nude mice, suggesting that LIMK2 is a key oncogenic effector of Aurora A. Furthermore, LIMK2 ablation acts synergistically with inhibition of Aurora A in promoting cell death. Finally, Aurora-A-mediated upregulation of LIMK2 appears to be a common mechanism in many cancers. LIMK2 inhibition or ablation is therefore an alternative approach for modulating Aurora A deregulation in cancer.

Deregulated Cdk5 triggers aberrant activation of cell cycle kinases and phosphatases inducing neuronal death

Aberrant activation of cell cycle proteins is believed to play a critical role in Alzheimer's disease (AD) pathogenesis; although, the molecular mechanisms leading to their activation in diseased neurons remain elusive. The goal of this study was to investigate the mechanistic link between Cdk5 deregulation and cell cycle re-activation in β-amyloid(1-42) (Aβ(1-42))-induced neurotoxicity. Using a chemical genetic approach, we identified Cdc25A, Cdc25B and Cdc25C as direct Cdk5 substrates in mouse brain lysates. We show that deregulated Cdk5 directly phosphorylates Cdc25A, Cdc25B and Cdc25C at multiple sites, which not only increases their phosphatase activities but also facilitates their release from 14-3-3 inhibitory binding. Cdc25A, Cdc25B and Cdc25C in turn activate Cdk1, Cdk2 and Cdk4 kinases causing neuronal death. Selective inhibition of Cdk5 abrogates Cdc25 and Cdk activations in Aβ(1-42)-treated neurons. Similarly, phosphorylation-resistant mutants of Cdc25 isoforms at Cdk5 sites are defective in activating Cdk1, Cdk2 and Cdk4 in Aβ(1-42)-treated primary cortical neurons, emphasizing a major role of Cdk5 in the activation of Cdc25 isoforms and Cdks in AD pathogenesis. These results were further confirmed in human AD clinical samples, which had higher Cdc25A, Cdc25B and Cdc25C activities that were coincident with increased Cdk5 activity, as compared to age-matched controls. Inhibition of Cdk5 confers the highest neuroprotection against Aβ(1-42) toxicity, whereas inhibition of Cdc25 isoforms was partially neuroprotective, further emphasizing a decisive role of Cdk5 deregulation in cell-cycle-driven AD neuronal death.

Crosstalk between cdk5 and MEK-ERK signalling upon opioid receptor stimulation leads to upregulation of activator p25 and MEK1 inhibition in rat brain

Cyclin-dependent kinase 5 (cdk5) participates in opioid receptor signalling through complex molecular mechanisms. The acute effects of selective μ-(fentanyl) and δ-(SNC-80) opioid receptor agonists, as well as the chronic effects of morphine (the prototypic opiate agonist mainly acting at μ-receptors), modulating cdk5 and activators p35/p25 and their interactions with neurotoxic/apoptotic factors, dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32) and extracellular signal-regulated kinase (ERK) were quantified (Western Blot analyses) in the rat corpus striatum and/or cerebral cortex. To assess the involved mechanisms, MDL28170 was used to inhibit calpain activity and SL327 to disrupt MEK (ERK kinase)-ERK activation. Acute fentanyl (0.1mg/kg) and SNC-80 (10mg/kg) induced rapid (7-60 min) 2- to 4-fold increases of p25 content, without induction of cdk5/p25 pro-apoptotic c-Jun NH(2)-terminal protein kinase or aberrant cleavage of poly(ADP-ribose)-polymerase-1, a hallmark of apoptosis. In contrast, fentanyl and SNC-80 stimulated cdk5-mediated p-Thr75 DARPP-32 (+116-166%; PKA inhibition) and p-Thr286 MEK1 (+21-82%; MEK inactivation), and this latter effect resulted in uncoupling of MEK to ERK signals. Calpain inhibition with MDL28170 (cleavage of p35 to p25) attenuated fentanyl-induced p25 accumulation (-57%), but not the stimulation of p-Thr286 MEK1 or p-Thr75 DARPP-32. MEK-ERK inhibition with SL327 fully prevented fentanyl-induced p25 upregulation. Notably, chronic morphine treatment (10-100mg/kg for 6 days) also increased p25 content and p25/p35 ratio (and activated/inactivated MEK1) in rat brain cortex, which indicated that p25 upregulation persisted under the sustained stimulation of μ-opioid receptors. The results demonstrate that the acute stimulation of opioid receptors leads to upregulation of p25 activator through a MEK-ERK and calpain-dependent pathway, and to disruption of MEK-ERK signalling by a cdk5/p35-induced MEK1 inhibition. Moreover, the effects induced by the sustained stimulation of μ-receptors with morphine suggest the participation of cdk5/p25 complex in opiate-induced long-term neuroplasticity.

Cdk5: a multifaceted kinase in neurodegenerative diseases

Since the identification of cyclin-dependent kinase-5 (Cdk5) as a tau kinase and member of the Cdk family almost 20 years ago, deregulation of Cdk5 activity has been linked to an array of neurodegenerative diseases. As knowledge on the etiopathological mechanisms of these diseases evolved through the years, Cdk5 has also been implicated in additional cellular events that are affected under these pathological conditions. From the role of Cdk5 in the regulation of synaptic functions to its involvement in autophagy deregulation, significant insights have been obtained regarding the role of Cdk5 as a key regulator of neurodegeneration. Here, we summarize recent findings on the involvement of Cdk5 in the pathophysiological mechanisms underlying various neurodegenerative diseases.

PHLDA1 is a crucial negative regulator and effector of Aurora A kinase in breast cancer

Aurora A kinase is overexpressed in the majority of breast carcinomas. A chemical genetic approach was used to identify the malignant targets of Aurora A, which revealed pleckstrin-homology-like domain protein PHLDA1 as an Aurora A substrate. PHLDA1 downregulation is a powerful prognostic predictor for breast carcinoma, which was confirmed in our study. We further show that downregulation of PHLDA1 is associated with estrogen receptor (ER) expression in breast carcinoma. Aurora A directly phosphorylates PHLDA1 leading to its degradation. PHLDA1 also negatively regulates Aurora A, thereby triggering a feedback loop. We demonstrate the underlying mechanisms by which PHLDA1 upregulation strongly antagonizes Aurora-A-mediated oncogenic pathways, thereby revealing PHLDA1 degradation as a key mechanism by which Aurora A promotes breast malignancy. Thus, not surprisingly, PHLDA1 upregulation acts synergistically with Aurora A inhibition in promoting cell death. PHLDA1 overexpression might therefore be an alternative method to modulate Aurora A deregulation in breast carcinoma. Finally, this study led to the discovery of a mutation in the Aurora A active site that renders it amenable to the chemical genetic approach. Similar mutations are required for Aurora B, suggesting that this modified approach can be extended to other kinases that have hitherto not been amenable to this methodology.

c-Jun N-terminal kinase 2 phosphorylates endothelial nitric oxide synthase at serine 116 and regulates nitric oxide production

The c-Jun N-terminal kinases (JNKs) belonging to the mitogen-activated protein kinase (MAPK) superfamily play important roles in foam-cell formation, hypercholesterolemia-mediated endothelial dysfunction, and the development of obesity. Although decreased nitric oxide (NO) production via decreased phosphorylation of endothelial NO synthase at serine 1179 (eNOS-Ser(1179)) was reported to be partly involved in JNK2-derived endothelial dysfunction, JNK2 seems likely to be indirectly involved in this signaling pathway. Here, using bovine aortic endothelial cells, we examined whether JNK2 directly phosphorylated eNOS-Ser(116), a putative substrate site for the MAPK superfamily, and this phosphorylation resulted in decreased NO release. JNK inhibitor SP60012 increased NO release in a time- and dose-dependent manner, which was accompanied by increased eNOS-Ser(116) phosphorylation. Purified JNK2 directly phosphorylated eNOS-Ser(116)in vitro. Ectopic expression of dominant negative JNK2 repressed eNOS-Ser(116) phosphorylation and increased NO production. Coimmunoprecipitation and confocal microscopy studies revealed a colocalization of eNOS and JNK2. However, all these observed effects were not manifested when JNK1 probes were used. Overall, this study indicates that JNK2 is a physiological kinase responsible for eNOS-Ser(116) phosphorylation and regulates NO production.

Antiapoptotic effects of roscovitine on camptothecin-induced DNA damage in neuroblastoma cells

In the present study dopaminergic neuroblastoma B65 cells were exposed to Camptothecin (CPT) (0.5-10 μM), either alone or in the presence of roscovitine (ROSC). The results show that CPT induces apoptosis through the activation of ataxia telangiectasia mutated (ATM)-induced cell-cycle alteration in neuroblastoma B65 cells. The apoptotic process is mediated through the activation of cystein proteases, namely calpain/caspases. However, whereas a pan-caspase inhibitor, zVADfmk, inhibited CPT-mediated apoptosis, a calpain inhibitor, calpeptin, did not prevent cell death. Interestingly, CPT also induces CDK5 activation and ROSC (25 μM) blocked CDK5, ATM activation and apoptosis (as measured by caspase-3 activation). By contrast, selective inhibition of ATM, by KU55933, and non-selective inhibition, by caffeine, did not prevent CPT-mediated apoptosis. Thus, we conclude that CDK5 is activated in response to DNA damage and that CDK5 inhibition prevents ATM and p53ser15 activation. However, pharmacological inhibition of ATM using KU55933 and caffeine suggests that ATM inhibition by ROSC is not the only mechanism that might explain the anti-apoptotic effects of this drug in this apoptosis model. Our findings have a potential clinical implication, suggesting that combinatory drugs in the treatment of cancer activation should be administered with caution.

Glutathione-S-transferase P1 is a critical regulator of Cdk5 kinase activity

Cyclin dependent kinase-5 (Cdk5) activity is deregulated in Alzheimer's disease (AD) and contributes to all three hallmarks: neurotoxic β-amyloid formation, neurofibrillary tangles, and neuronal death. However, the mechanism leading to Cdk5 deregulation remains controversial. Cdk5 deregulation in AD is usually linked to the formation of p25, a proteolysis product of Cdk5 activator p35, which leads to Cdk5 mislocalization and hyperactivation. A few studies have indeed shown increased p25 levels in AD brains; however, others have refuted this observation. These contradictory findings suggest that additional factors contribute to Cdk5 deregulation. This study identified glutathione-S-transferase pi 1 (GSTP1) as a novel Cdk5 regulatory protein. We demonstrate that it is a critical determinant of Cdk5 activity in human AD brains and various cancer and neuronal cells. Increased GSTP1 levels were consistently associated with reduced Cdk5 activity. GSTP1 directly inhibits Cdk5 by dislodging p25/p35, and indirectly by eliminating oxidative stress. Cdk5 promotes and is activated by oxidative stress, thereby engaging a feedback loop which ultimately leads to cell death. Not surprisingly, GSTP1 transduction conferred a high degree of neuroprotection under neurotoxic conditions. Given the critical role of oxidative stress in AD pathogenesis, an increase in GSTP1 level may be an alternative way to modulate Cdk5 signaling, eliminate oxidative stress, and prevent neurodegeneration.

Nuclear envelope dispersion triggered by deregulated Cdk5 precedes neuronal death

Nuclear fragmentation is a common feature in many neurodegenerative diseases, including Alzheimer's disease (AD). In this study, we show that nuclear lamina dispersion is an early and irreversible trigger for cell death initiated by deregulated Cdk5, rather than a consequence of apoptosis. Cyclin-dependent kinase 5 (Cdk5) activity is significantly increased in AD and contributes to all three hallmarks: neurotoxic amyloid-β (Aβ), neurofibrillary tangles (NFT), and extensive cell death. Using Aβ and glutamate as the neurotoxic stimuli, we show that deregulated Cdk5 induces nuclear lamina dispersion by direct phosphorylation of lamin A and lamin B1 in neuronal cells and primary cortical neurons. Phosphorylation-resistant mutants of lamins confer resistance to nuclear dispersion and cell death on neurotoxic stimulation, highlighting this as a major mechanism for neuronal death. Rapid alteration of lamin localization pattern and nuclear membrane change are further supported by in vivo data using an AD mouse model. After p25 induction, the pattern of lamin localization was significantly altered, preceding neuronal death, suggesting that it is an early pathological event in p25-inducible transgenic mice. Importantly, lamin dispersion is coupled with Cdk5 nuclear localization, which is highly neurotoxic. Inhibition of nuclear dispersion rescues neuronal cells from cell death, underscoring the significance of this event to Cdk5-mediated neurotoxicity.

Monocyte-mediated regulation of genes by the amyloid and prion peptides in SH-SY5Y neuroblastoma cells

Alzheimer's disease as well as prion-related encephalopathies are neurodegenerative disorders of the central nervous system, which cause mental deterioration and progressive dementia. Both pathologies appear to be primarily associated with the pathological accumulation and deposit of β-amyloid or prion peptides in the brain, and it has been even suggested that neurotoxicity induced by these peptides would be associated to essentially similar pathogenic mechanisms, in particular to those that follow the activation of microglial cells. To probe whether the neurotoxic effects induced by the β-amyloid and prion peptides are actually mediated by similar glial-associated mechanisms, we have examined the differential expression of genes in SH-SY5Y neuroblastoma cells incubated with conditioned media from β-amyloid or prion-stimulated THP-1 monocytic cells. According to microarray analysis, not many coincidences are observed and only four genes (Hint3, Psph, Daam1 and c-Jun) appear to be commonly upregulated by both peptides. Furthermore, c-Jun appears to be involved in the cell death mediated by both peptides.

Regulation and role of cyclin-dependent kinase activity in neuronal survival and death

Cyclin-dependent kinase (Cdk)5 is a proline-directed Ser/Thr protein kinase that functions mainly in neurons and is activated by binding to a regulatory subunit, p35 or p39. Kinase activity is mainly determined by the amount of p35 available, which is controlled by a balance between synthesis and degradation. Kinase activity is also regulated by Cdk5 phosphorylation, but the activity of phosphorylated Cdk5 is in contrast to that of cycling Cdks. Cdk5 is a versatile protein kinase that regulates multiple neuronal activities including neuronal migration and synaptic signaling. Further, Cdk5 plays a role in both survival and death of neurons. Long-term inactivation of Cdk5 triggers cell death, and the survival activity of Cdk5 is apparent when neurons suffer from stress. In contrast, hyper-activation of Cdk5 by p25 promotes cell death, probably by reactivating cell-cycle machinery in the nucleus. The pro-death activity is suppressed by membrane association of Cdk5 via myristoylation of p35. Appropriate activity, localization, and regulation of Cdk5 may be critical for long-term survival of neurons, which is more than 80 years in the case of humans.

Cdk5 is a major regulator of p38 cascade: relevance to neurotoxicity in Alzheimer's disease

Cyclin-dependent kinase (Cdk) 5 and p38 activities are significantly increased in Alzheimer's Disease (AD). Both p38 and Cdk5 promote neurodegeneration upon deregulation. However, to date the mechanistic link between Cdk5 and p38 remains unclear. This study presents the first mechanism showing Cdk5 as a major regulator of p38 cascade in neurons and in transgenic mouse model of AD. Using beta-amyloid and glutamate as the neurotoxic stimuli, our results show that deregulated Cdk5 induces p38 activation by increasing reactive oxygen species (ROS) in neuronal cells and in primary cortical neurons. Elimination of ROS inhibits p38 activation, revealing ROS as major stimuli of the p38 cascade. Importantly, Cdk5-mediated p38 activation increases c-Jun expression, thereby revealing a mechanistic link between deregulated Cdk5 and c-Jun level in AD brains. c-Jun is over-expressed in AD, and is believed to contribute significantly to neurodegeneration. Based on the proposed mechanism, Cdk5 inhibition is more neuroprotective relative to p38 and c-Jun, suggesting that Cdk5 is an upstream regulator of neurodegenerative pathways triggered by p38 and a preferable therapeutic target for AD.