Polo-like kinases in the nervous system

Polo-like kinase 2 promotes microglial activation via regulation of the HSP90α/IKKβ pathway

Polo-like kinase 2 (PLK2) is a serine/threonine protein kinase associated with the regulation of synaptic plasticity and centriole duplication. We identify PLK2 as a crucial early-response gene in lipopolysaccharide (LPS)-stimulated microglial cells. Knockdown or inhibition of PLK2 remarkably attenuates LPS-induced expression of proinflammatory factors in microglial cells by suppressing the inhibitor of nuclear factor kappa B kinase subunit beta (IKKβ)-nuclear factor (NF)-κB signaling pathway. We identify heat shock protein 90 alpha (HSP90α), a regulator of IKKβ activity, as a novel PLK2 substrate. Knockdown or pharmacological inhibition of HSP90α abolishes PLK2-mediated activation of NF-κB transcriptional activity and microglial inflammatory activation. Furthermore, phosphoproteomic analysis pinpoints Ser252 and Ser263 on HSP90α as novel phosphorylation targets of PLK2. Lastly, conditional knockout of PLK2 in microglial cells dramatically ameliorates neuroinflammation and subsequent dopaminergic neuron loss in an intracranial LPS-induced mouse Parkinson's disease (PD) model. The present study reveals that PLK2 promotes microglial activation through the phosphorylation of HSP90α and subsequent activation of the IKKβ-NF-κB signaling pathway.

Amyloid precursor protein combinatorial phosphorylation code regulates AMPA receptor removal during distinct forms of synaptic plasticity

Synaptic plasticity is essential for memory encoding and stabilization of neural network activity. Plasticity is impaired in neurodegenerative conditions including Alzheimer disease (AD). A central factor in AD is amyloid precursor protein (APP). Previous studies have suggested APP involvement in synaptic plasticity, but physiological roles of APP are not well understood. Here, we identified combinatorial phosphorylation sites within APP that regulate AMPA receptor trafficking during different forms of synaptic plasticity. Dual phosphorylation sites at threonine-668/serine-675 of APP promoted endocytosis of the GluA2 subunit of AMPA receptors during homeostatic synaptic plasticity. APP was also required for GluA2 internalization during NMDA receptor-dependent long-term depression, albeit via a distinct pair of phosphoresidues at serine-655/threonine-686. These data implicate APP as a central gate for AMPA receptor internalization during distinct forms of plasticity, unlocked by specific combinations of phosphoresidues, and suggest that APP may serve broad functions in learning and memory.

Conserved gene signatures shared among MAPT mutations reveal defects in calcium signaling

Introduction: More than 50 mutations in the MAPT gene result in heterogeneous forms of frontotemporal lobar dementia with tau inclusions (FTLD-Tau). However, early pathogenic events that lead to disease and the degree to which they are common across MAPT mutations remain poorly understood. The goal of this study is to determine whether there is a common molecular signature of FTLD-Tau. Methods: We analyzed genes differentially expressed in induced pluripotent stem cell-derived neurons (iPSC-neurons) that represent the three major categories of MAPT mutations: splicing (IVS10 + 16), exon 10 (p.P301L), and C-terminal (p.R406W) compared with isogenic controls. The genes that were commonly differentially expressed in MAPT IVS10 + 16, p.P301L, and p.R406W neurons were enriched in trans-synaptic signaling, neuronal processes, and lysosomal function. Many of these pathways are sensitive to disruptions in calcium homeostasis. One gene, CALB1, was significantly reduced across the three MAPT mutant iPSC-neurons and in a mouse model of tau accumulation. We observed a significant reduction in calcium levels in MAPT mutant neurons compared with isogenic controls, pointing to a functional consequence of this disrupted gene expression. Finally, a subset of genes commonly differentially expressed across MAPT mutations were also dysregulated in brains from MAPT mutation carriers and to a lesser extent in brains from sporadic Alzheimer disease and progressive supranuclear palsy, suggesting that molecular signatures relevant to genetic and sporadic forms of tauopathy are captured in a dish. The results from this study demonstrate that iPSC-neurons capture molecular processes that occur in human brains and can be used to pinpoint common molecular pathways involving synaptic and lysosomal function and neuronal development, which may be regulated by disruptions in calcium homeostasis.

Inhibition of PLK2 activity affects APP and tau pathology and improves synaptic content in a sex-dependent manner in a 3xTg mouse model of Alzheimer's disease

Converging lines of evidence suggest that abnormal accumulation of the kinase Polo-like kinase 2 (PLK2) might play a role in the pathogenesis of Alzheimer's disease (AD), possibly through its role in regulating the amyloid β (Aβ) cascade. In the present study, we investigated the effect of inhibiting PLK2 kinase activity in in vitro and in vivo models of AD neuropathology. First, we confirmed that PLK2 overexpression modulated APP and Tau protein levels and phosphorylation in cell culture, in a kinase activity dependent manner. Furthermore, a transient treatment of triple transgenic mouse model of AD (3xTg-AD) with a potent and specific PLK2 pharmacological inhibitor (PLK2i #37) reduced some neuropathological aspects in a sex-dependent manner. In 3xTg-AD males, treatment with PLK2i #37 led to lower Tau burden, higher synaptic protein content, and prevented learning and memory deficits. In contrast, treated females showed an exacerbation of Tau pathology, associated with a reduction in amyloid plaque accumulation. Overall, our findings suggest that PLK2 inhibition alters key components of AD neuropathology in a sex-dependent manner and might display a therapeutic potential for the treatment for AD and related dementia.

Tumor suppressor PLK2 may serve as a biomarker in triple-negative breast cancer for improved response to PLK1 therapeutics

Polo-like kinase (PLK) family members play important roles in cell cycle regulation. The founding member PLK1 is oncogenic and preclinically validated as a cancer therapeutic target. Paradoxically, frequent loss of chromosome 5q11-35 which includes PLK2 is observed in basal-like breast cancer. In this study, we found that PLK2 was tumor suppressive in breast cancer, preferentially in basal-like and triple-negative breast cancer (TNBC) subtypes. Knockdown of PLK1 rescued phenotypes induced by PLK2-loss both in vitro and in vivo. We also demonstrated that PLK2 directly interacted with PLK1 at prometaphase through the kinase but not the polo-box domains of PLK2, suggesting PLK2 functioned at least partially through the interaction with PLK1. Furthermore, an improved treatment response was seen in both Plk2-deleted/low mouse preclinical and PDX TNBC models using the PLK1 inhibitor volasertib alone or in combination with carboplatin. Re-expression of PLK2 in an inducible PLK2-null mouse model reduced the therapeutic efficacy of volasertib. In summary, this study delineates the effects of chromosome 5q loss in TNBC that includes PLK2, the relationship between PLK2 and PLK1, and how this may render PLK2-deleted/low tumors more sensitive to PLK1 inhibition in combination with chemotherapy.

Three decades of Cdk5

Cdk5 is a proline-directed serine/threonine protein kinase that governs a variety of cellular processes in neurons, the dysregulation of which compromises normal brain function. The mechanisms underlying the modulation of Cdk5, its modes of action, and its effects on the nervous system have been a great focus in the field for nearly three decades. In this review, we provide an overview of the discovery and regulation of Cdk5, highlighting recent findings revealing its role in neuronal/synaptic functions, circadian clocks, DNA damage, cell cycle reentry, mitochondrial dysfunction, as well as its non-neuronal functions under physiological and pathological conditions. Moreover, we discuss evidence underscoring aberrant Cdk5 activity as a common theme observed in many neurodegenerative diseases.

PLK2 protects retinal ganglion cells from oxidative stress by potentiating Nrf2 signaling via GSK-3β

Oxidative stress of retinal ganglion cells (RGCs) has been established as a main contributor to retinal degeneration in the pathogenesis of glaucoma. Polo-like kinase 2 (PLK2) has recently been reported to be a potent antioxidant protein that enhances cell survival in response to oxidative stress. To date, the involvement of PLK2 in RGC-associated oxidative stress is undermined. In the present work, we evaluated whether PLK2 regulates oxidative stress evoked by hydrogen peroxide (H₂ O₂ ) in RGCs. PLK2 expression was induced by H₂ O₂ stimulation in RGCs. Upregulation of PLK2 had a profoundly cytoprotective effect on H₂ O₂ -stimulated RGCs by attenuating cellular apoptosis and reactive oxygen species (ROS) level. Further data revealed that upregulation of PLK2 strikingly enhanced the activation of Nrf2 signaling. Moreover, PLK2 overexpression promoted glycogen synthase kinase (GSK)-3β phosphorylation, whereas PLK2 knockdown reduced the levels of GSK-3β phosphorylation. Notably, GSK-3β inhibition using a chemical inhibitor markedly abrogated the suppressive effects of PLK2 knockdown on Nrf2 activation. Repression of Nrf2 blocked the PLK2 overexpression-induced protective effects in H₂ O₂ -stimulated RGCs. Overall, this study elucidates that upregulation of PLK2 protects RGCs against H₂ O₂ -induced oxidative stress injury by upregulating Nrf2 activation via modulation of GSK-3β phosphorylation. These findings underline the pivotal role of PLK2 in mediating oxidative stress-evoked retinal degeneration in the pathogenesis of glaucoma.

Genetic deletion of Polo-like kinase 2 reduces alpha-synuclein serine-129 phosphorylation in presynaptic terminals but not Lewy bodies

Phosphorylation of alpha-synuclein at serine-129 is an important marker of pathologically relevant, aggregated forms of the protein in several important human diseases, including Parkinson's disease, Dementia with Lewy bodies, and Multiple system atrophy. Although several kinases have been shown to be capable of phosphorylating alpha-synuclein in various model systems, the identity of the kinase that phosphorylates alpha-synuclein in the Lewy body remains unknown. One member of the Polo-like kinase family, PLK2, is a strong candidate for being the Lewy body kinase. To examine this possibility, we have used a combination of approaches, including biochemical, immunohistochemical, and in vivo multiphoton imaging techniques to study the consequences of PLK2 genetic deletion on alpha-synuclein phosphorylation in both the presynaptic terminal and preformed fibril-induced Lewy body pathology in mouse cortex. We find that PLK2 deletion reduces presynaptic terminal alpha-synuclein serine-129 phosphorylation, but has no effect on Lewy body phosphorylation levels. Serine-129 mutation to the phosphomimetic alanine or the unphosphorylatable analog aspartate does not change the rate of cell death of Lewy inclusion-bearing neurons in our in vivo multiphoton imaging paradigm, but PLK2 deletion does slow the rate of neuronal death. Our data indicate that inhibition of PLK2 represents a promising avenue for developing new therapeutics, but that the mechanism of neuroprotection by PLK2 inhibition is not likely due to reducing alpha-synuclein serine-129 phosphorylation and that the true Lewy body kinase still awaits discovery.

Transcriptome Remodeling in Gradual Development of Inverse Resistance between Paclitaxel and Cisplatin in Ovarian Cancer Cells

Resistance to anti-cancer drugs is the main challenge in oncology. In pre-clinical studies, established cancer cell lines are primary tools in deciphering molecular mechanisms of this phenomenon. In this study, we proposed a new, transcriptome-focused approach, utilizing a model of isogenic cancer cell lines with gradually changing resistance. We analyzed trends in gene expression in the aim to find out a scaffold of resistance development process. The ovarian cancer cell line A2780 was treated with stepwise increased concentrations of paclitaxel (PTX) to generate a series of drug resistant sublines. To monitor transcriptome changes we submitted them to mRNA-sequencing, followed by the identification of differentially expressed genes (DEGs), principal component analysis (PCA), and hierarchical clustering. Functional interactions of proteins, encoded by DEGs, were analyzed by building protein-protein interaction (PPI) networks. We obtained human ovarian cancer cell lines with gradually developed resistance to PTX and collateral sensitivity to cisplatin (CDDP) (inverse resistance). In their transcriptomes, we identified two groups of DEGs: (1) With fluctuations in expression in the course of resistance acquiring; and (2) with a consistently changed expression at each stage of resistance development, constituting a scaffold of the process. In the scaffold PPI network, the cell cycle regulator-polo-like kinase 2 (PLK2); proteins belonging to the tumor necrosis factor (TNF) ligand and receptor family, as well as to the ephrin receptor family were found, and moreover, proteins linked to osteo- and chondrogenesis and the nervous system development. Our cellular model of drug resistance allowed for keeping track of trends in gene expression and studying this phenomenon as a process of evolution, reflected by global transcriptome remodeling. This approach enabled us to explore novel candidate genes and surmise that abrogation of the osteomimic phenotype in ovarian cancer cells might occur during the development of inverse resistance between PTX and CDDP.

TNF-Mediated Homeostatic Synaptic Plasticity: From in vitro to in vivo Models

Since it was first described almost 30 years ago, homeostatic synaptic plasticity (HSP) has been hypothesized to play a key role in maintaining neuronal circuit function in both developing and adult animals. While well characterized in vitro, determining the in vivo roles of this form of plasticity remains challenging. Since the discovery that the pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) mediates some forms of HSP, it has been possible to probe some of the in vivo contribution of TNF-mediated HSP. Work from our lab and others has found roles for TNF-HSP in a variety of functions, including the developmental plasticity of sensory systems, models of drug addiction, and the response to psychiatric drugs.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

Maternal immune activation alters brain microRNA expression in mouse offspring

Objective

Maternal immune activation (MIA) is associated with an increased risk of autism spectrum disorder (ASD) in offspring. Herein, we investigate the altered expression of microRNAs (miRNA), and that of their target genes, in the brains of MIA mouse offspring.

Methods

To generate MIA model mice, pregnant mice were injected with polyriboinosinic:polyribocytidylic acid on embryonic day 12.5. We performed miRNA microarray and mRNA sequencing in order to determine the differential expression of miRNA and mRNA between MIA mice and controls, at 3 weeks of age. We further identified predicted target genes of dysregulated miRNAs, and miRNA‐target interactions, based on the inverse correlation of their expression levels.

Results

Mice prenatally subjected to MIA exhibited behavioral abnormalities typical of ASD, such as a lack of preference for social novelty and reduced prepulse inhibition. We found 29 differentially expressed miRNAs (8 upregulated and 21 downregulated) and 758 differentially expressed mRNAs (542 upregulated and 216 downregulated) in MIA offspring compared to controls. Based on expression levels of the predicted target genes, 18 downregulated miRNAs (340 target genes) and three upregulated miRNAs (60 target genes) were found to be significantly enriched among the differentially expressed genes. miRNA and target gene interactions were most significant between mmu‐miR‐466i‐3p and Hfm1 (ATP‐dependent DNA helicase homolog), and between mmu‐miR‐877‐3p and Aqp6 (aquaporin 6).

Interpretation

Our results provide novel information regarding miRNA expression changes and their putative targets in the early postnatal period of brain development. Further studies will be needed to evaluate potential pathogenic roles of the dysregulated miRNAs.

Lupus-Associated Functional Polymorphism in PNP Causes Cell Cycle Abnormalities and Interferon Pathway Activation in Human Immune Cells

OBJECTIVE

Systemic lupus erythematosus (SLE) is frequently characterized by activation of the type I interferon (IFN) pathway. We previously observed that a missense single-nucleotide polymorphism (rs1049564) in the purine nucleoside phosphorylase (PNP) gene was associated with high levels of IFN in SLE. PNP is a key enzyme involved in purine metabolism. In this study, we performed functional follow-up of this polymorphism in human cells.

METHODS

Type I IFN was measured in patient sera, using a reporter cell assay. Structural modeling of the PNP variant was performed using PyMOL software. PNP messenger RNA (mRNA) and protein levels and type I IFN-induced gene expression were measured in lymphoblastoid cell lines with known PNP rs1049564 genotypes. The cell cycle was assayed using flow cytometry.

RESULTS

Structural modeling indicated no major disruption in folding related to rs1049564. We observed that homozygous rs1049564 TT lymphoblastoid cells had decreased PNP mRNA expression and protein levels, and that cells with the TT genotype had reduced PNP enzymatic activity even when the amount of PNP was controlled. Cells with the TT genotype had a 2-fold increase in S-phase block as compared with cells with the homozygous CC phenotype. The S-phase block could be pharmacologically reversed with hypoxanthine and adenosine, supporting the notion that relative PNP deficiency is the cause of the S-phase block. Type I IFN-induced transcripts were increased in a dose-response manner related to the rs1049564 T allele, at both baseline and after type I IFN stimulation.

CONCLUSION

The PNP rs1049564 T allele is a loss-of-function variant that induces S-phase block and IFN pathway activation in lymphocytes. The S-phase block could be rescued in our in vitro experiments, suggesting the potential for personalized treatment.

The SNK and SPAR signaling pathway changes in hippocampal neurons treated with amyloid-beta peptide in vitro

Amyloid-β peptide (Aβ) is believed to be a primary cause of Alzheimer's disease. Many studies have demonstrated that Aβ causes morphological and functional alterations of dendritic spines, leading to synaptic dysfunction, but the effect of Aβ on damage to synaptic functions is not fully understood. Spine-associated Rap guanosine triphosphatase-activating protein (SPAR) is an important regulator of activity-dependent remodeling of synapses and is critically involved in both mature dendritic spine formation and the maintenance of spine maturity. Serum-inducible kinase (SNK) is an activity-inducible member of the polo-like family of serine/threonine kinases. Coordinated regulation of Ras and Rap by SNK is critical for homeostatic plasticity and memory. A previous study in which rats were injected with Aβ_1-40 into the hippocampus showed that the SNK and SPAR signaling pathway may play a crucial role in Aβ-induced excitotoxic damage in the central nervous system by regulating synaptic stability. The present study was designed to investigate whether the SNK and SPAR signaling pathway was involved in Aβ-induced neurotoxicity in rat primary neurons. We measured mRNA and protein expression levels of SNK and SPAR in primary hippocampal neurons following Aβ treatment and used RNA interference to knockdown SNK to investigate the underlying mechanism. Expression of SNK and SPAR was altered by Aβ treatment, indicating that the SNK and SPAR signaling pathways may be involved in the damage to dendritic spines in hippocampal neurons induced by Aβ.

Polo-like kinase 2 phosphorylation of amyloid precursor protein regulates activity-dependent amyloidogenic processing

Alzheimer's disease (AD) is a neurodegenerative disorder with cognitive deficits. Amyloidogenic processing of amyloid precursor protein (APP) produces amyloid β (Aβ), the major component of hallmark AD plaques. Synaptic activity stimulates APP cleavage, whereas APP promotes excitatory synaptic transmission, suggesting APP participates in neuronal homeostasis. However, mechanisms linking synaptic activity to APP processing are unclear. Here we show that Polo-like kinase 2 (Plk2), an activity-inducible regulator of homeostatic plasticity, directly binds and phosphorylates threonine-668 and serine-675 of APP in vitro and associates with APP in vivo. Plk2 accelerates APP amyloidogenic cleavage by β-secretase at synapses and is required for neuronal overactivity-stimulated Aβ secretion. These findings implicate Plk2 as a novel mediator of activity-dependent APP amyloidogenic processing.

Dissecting the Molecular Pathway Involved in PLK2 Kinase-mediated α-Synuclein-selective Autophagic Degradation

Increasing lines of evidence support the causal link between α-synuclein (α-syn) accumulation in the brain and Parkinson's disease (PD) pathogenesis. Therefore, lowering α-syn protein levels may represent a viable therapeutic strategy for the treatment of PD and related disorders. We recently described a novel selective α-syn degradation pathway, catalyzed by the activity of the Polo-like kinase 2 (PLK2), capable of reducing α-syn protein expression and suppressing its toxicity in vivo However, the exact molecular mechanisms underlying this degradation route remain elusive. In the present study we report that among PLK family members, PLK3 is also able to catalyze α-syn phosphorylation and degradation in living cells. Using pharmacological and genetic approaches, we confirmed the implication of the macroautophagy on PLK2-mediated α-syn turnover, and our observations suggest a concomitant co-degradation of these two proteins. Moreover, we showed that the N-terminal region of α-syn is important for PLK2-mediated α-syn phosphorylation and degradation and is implicated in the physical interaction between the two proteins. We also demonstrated that PLK2 polyubiquitination is important for PLK2·α-syn protein complex degradation, and we hypothesize that this post-translational modification may act as a signal for the selective recognition by the macroautophagy machinery. Finally, we observed that the PD-linked mutation E46K enhances PLK2-mediated α-syn degradation, suggesting that this mutated form is a bona fide substrate of this degradation pathway. In conclusion, our study provides a detailed description of the new degradation route of α-syn and offers new opportunities for the development of therapeutic strategies aiming to reduce α-syn protein accumulation and toxicity.

Implication of Alpha-Synuclein Phosphorylation at S129 in Synucleinopathies: What Have We Learned in the Last Decade?

Abnormal accumulation of proteinaceous intraneuronal inclusions called Lewy bodies (LBs) is the neurpathological hallmark of Parkinson’s disease (PD) and related synucleinopathies. These inclusions are mainly constituted of a presynaptic protein, α-synuclein (α-syn). Over the past decade, growing amounts of studies reported an aberrant accumulation of phosphorylated α-syn at the residue S129 (pS129) in the brain of patients suffering from PD, as well as in transgenic animal models of synucleinopathies. Whereas only a small fraction of α-syn (<4%) is phosphorylated in healthy brains, a dramatic accumulation of pS129 (>90%) has been observed within LBs, suggesting that this post-translational modification may play an important role in the regulation of α-syn aggregation, LBs formation and neuronal degeneration. However, whether phosphorylation at S129 suppresses or enhances α-syn aggregation and toxicity in vivo remains a subject of active debate. The answer to this question has important implications for understanding the role of phosphorylation in the pathogenesis of synucleinopathies and determining if targeting kinases or phosphatases could be a viable therapeutic strategy for the treatment of these devastating neurological disorders. In the present review, we explore recent findings from in vitro, cell-based assays and in vivo studies describing the potential implications of pS129 in the regulation of α-syn physiological functions, as well as its implication in synucleinopathies pathogenesis and diagnosis.

A model for regulation by SynGAP-α1 of binding of synaptic proteins to PDZ-domain 'Slots' in the postsynaptic density

SynGAP is a Ras/Rap GTPase-activating protein (GAP) that is a major constituent of postsynaptic densities (PSDs) from mammalian forebrain. Its α1 isoform binds to all three PDZ (PSD-95, Discs-large, ZO-1) domains of PSD-95, the principal PSD scaffold, and can occupy as many as 15% of these PDZ domains. We present evidence that synGAP-α1 regulates the composition of the PSD by restricting binding to the PDZ domains of PSD-95. We show that phosphorylation by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and Polo-like kinase-2 (PLK2) decreases its affinity for the PDZ domains by several fold, which would free PDZ domains for occupancy by other proteins. Finally, we show that three critical postsynaptic signaling proteins that bind to the PDZ domains of PSD-95 are present in higher concentration in PSDs isolated from mice with a heterozygous deletion of synGAP.

Augmented expression of Polo-like kinase 1 is a strong predictor of shorter cancer-specific overall survival in early stage breast cancer at 15-year follow-up

Polo-like kinase 1 (PLK1) is a serine-threonine kinase that plays a crucial role in the regulation of cell division. In addition, it acts as a modulator of the DNA damage response and as a novel factor in the maintenance of genome stability during DNA replication. The present study aimed to reveal the associations between PLK1 expression and clinicopathological features of patients with breast cancer (BC), particularly patient survival at 5-, 10- and 15-year follow-up. PLK1 expression was evaluated immunohistochemically in routine diagnostic tissue specimens from 83 patients treated radically for stage II BC. Kaplan-Meier analysis revealed a correlation between PLK1 overexpression and long-term survival. High PLK1 immunoreactivity was associated with shorter cancer-specific overall survival (CSOS) and disease-free survival (P=0.00001 and 0.00013, respectively). Multivariate analysis confirmed the negative prognostic significance of PLK1 overexpression for CSOS in all 83 patients (P=0.00030). Furthermore, analogous correlations were observed in both subgroups with and without nodal metastases (P=0.01400 and 0.01200, respectively). The present results indicate that PLK1 expression has a prognostic role in early BC. Immunohistochemical assessment of PLK1 reactivity may potentially become a qualifier for inclusion of PLK1 inhibitor therapy.

Spotlight on Volasertib: Preclinical and Clinical Evaluation of a Promising Plk1 Inhibitor

Considering the important side effects of conventional microtubule targeting agents, more and more research focuses on regulatory proteins for the development of mitosis-specific agents. Polo-like kinase 1 (Plk1), a master regulator of several cell cycle events, has arisen as an intriguing target in this research field. The observed overexpression of Plk1 in a broad range of human malignancies has given rise to the development of several potent and specific small molecule inhibitors targeting the kinase. In this review, we focus on volasertib (BI6727), the lead agent in category of Plk1 inhibitors at the moment. Numerous preclinical experiments have demonstrated that BI6727 is highly active across a variety of carcinoma cell lines, and the inhibitor has been reported to induce tumor regression in several xenograft models. Moreover, volasertib has shown clinical efficacy in multiple tumor types. As a result, Food and Drug Administration (FDA) has recently awarded volasertib the Breakthrough Therapy status after significant benefit was observed in acute myeloid leukemia (AML) patients treated with the Plk1 inhibitor. Here, we discuss both preclinical and clinical data available for volasertib administered as monotherapy or in combination with other anticancer therapies in a broad range of tumor types.

Structure-based design and confirmation of peptide ligands for neuronal polo-like kinase to promote neuroregeneration

Neuronal polo-like kinase (nPLK) is an essential regular of cell cycle and differentiation in nervous system, and targeting nPLK has been established as a promising therapeutic strategy to treat neurological disorders and to promote neuroregeneration. The protein contains an N-terminal kinase domain (KD) and a C-terminal Polo-box domain (PBD) that are mutually inhibited by each other. Here, the intramolecular KD-PBD complex in nPLK was investigated at structural level via bioinformatics analysis, molecular dynamics (MD) simulation and binding affinity scoring. From the complex interface two regions representing separately two continuous peptide fragments in PBD domain were identified as the hot spots of KD-PBD interaction. Structural and energetic analysis suggested that one (PBD peptide 1) of the two peptides can bind tightly to a pocket nearby the active site of KD domain, which is thus potential as self-inhibitory peptide to target and suppress nPLK kinase activity. The knowledge harvesting from computational studies were then used to guide the structural optimization and mutation of PBD peptide 1. Consequently, two of three peptide mutants separately exhibited moderately and considerably increased affinity as compared to the native peptide. The computationally modeled complex structures of KD domain with these self-inhibitory peptides were also examined in detail to unravel the structural basis and energetic property of nPLK-peptide recognition and interaction.

Polo-Like Kinase 3 Appears Dispensable for Normal Retinal Development Despite Robust Embryonic Expression

During retinogenesis seven different cell types are generated in distinct yet overlapping timepoints from a population of retinal progenitor cells. Previously, we performed single cell transcriptome analyses of retinal progenitor cells to identify candidate genes that may play roles in the generation of early-born retinal neurons. Based on its expression pattern in subsets of early retinal cells, polo-like kinase 3 (Plk3) was identified as one such candidate gene. Further characterization of Plk3 expression by in situ hybridization revealed that this gene is expressed as cells exit the cell cycle. We obtained a Plk3 deficient mouse and investigated changes in the retina’s morphology and transcriptome through immunohistochemistry, in situ hybridization and gene expression profiling. These experiments have been performed initially on adult mice and subsequently extended throughout retinal development. Although morphological studies revealed no consistent changes in retinogenesis upon Plk3 loss, microarray profiling revealed potential candidate genes altered in Plk3-KO mice. Further studies will be necessary to understand the connection between these changes in gene expression and the loss of a protein kinase such as Plk3.

Recent Advances and New Strategies in Targeting Plk1 for Anticancer Therapy

Polo-like kinase 1 (Plk1) plays key roles in regulating mitotic processes that are crucial for cellular proliferation. Overexpression of Plk1 is tightly associated with the development of particular cancers in humans, and a large body of evidence suggests that Plk1 is an attractive target for anticancer therapeutic development. Drugs targeting Plk1 can potentially be directed at two distinct sites: the N-terminal catalytic kinase domain (KD), which phosphorylates substrates, and the C-terminal polo-box domain (PBD) which is essential for protein-protein interactions. In this review we summarize recent advances and new challenges in the development of Plk1 inhibitors targeting these two domains. We also discuss novel strategies for designing and developing next-generation inhibitors to effectively treat Plk1-associated human disorders.

Prion-induced neurotoxicity: Possible role for cell cycle activity and DNA damage response

Protein misfolding neurodegenerative diseases arise through neurotoxicity induced by aggregation of host proteins. These conditions include Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, motor neuron disease, tauopathies and prion diseases. Collectively, these conditions are a challenge to society because of the increasing aged population and through the real threat to human food security by animal prion diseases. It is therefore important to understand the cellular and molecular mechanisms that underlie protein misfolding-induced neurotoxicity as this will form the basis for designing strategies to alleviate their burden. Prion diseases are an important paradigm for neurodegenerative conditions in general since several of these maladies have now been shown to display prion-like phenomena. Increasingly, cell cycle activity and the DNA damage response are recognised as cellular events that participate in the neurotoxic process of various neurodegenerative diseases, and their associated animal models, which suggests they are truly involved in the pathogenic process and are not merely epiphenomena. Here we review the role of cell cycle activity and the DNA damage response in neurodegeneration associated with protein misfolding diseases, and suggest that these events contribute towards prion-induced neurotoxicity. In doing so, we highlight PrP transgenic Drosophila as a tractable model for the genetic analysis of transmissible mammalian prion disease.

Estrogen-related receptor gamma regulates dopaminergic neuronal phenotype by activating GSK3β/NFAT signaling in SH-SY5Y cells

The orphan nuclear receptor estrogen-related receptor gamma (ERRγ) is highly expressed in the nervous system during embryogenesis and in adult brains, but its physiological role in neuronal development remains unknown. In this study, we evaluated the relevance of ERRγ in regulating dopaminergic (DAergic) phenotype and the corresponding signaling pathway. We used retinoic acid (RA) to differentiate human neuroblastoma SH-SY5Y cells. RA induced neurite outgrowth of SH-SY5Y cells with an increase in DAergic neuron-like properties, including up-regulation of tyrosine hydroxylase, dopamine transporter, and vesicular monoamine transporter 2. ERRγ, but not ERRα, was up-regulated by RA, and participated in RA effect on SH-SY5Y cells. ERRγ over-expression enhanced mature DAergic neuronal phenotype with neurite outgrowth as with RA treatment; and RA-induced increase in DAergic phenotype was attenuated by silencing ERRγ expression. ERRγ appears to have a crucial role in morphological and functional regulation of cells that is selective for DAergic neurons. Polo-like kinase 2 was up-regulated in ERRγ-over-expressing SH-SY5Y cells, which was involved in phosphorylation of glycogen synthase kinase 3β and resulting downstream activation of nuclear factor of activated T cells. The likely involvement of ERRγ in regulating the DAergic neuronal phenotype makes this orphan nuclear receptor a novel target for understanding DAergic neuronal differentiation. We propose the relevance of estrogen-related receptor gamma (ERRγ) in regulating dopaminergic neuronal phenotype: ERRγ is up-regulated by retinoic acid in SH-SY5Y cells, and enhances dopaminergic phenotypes and induces neurite outgrowth; Polo-like kinase 2 (PLK2) and glycogen synthase kinase 3 beta/nuclear factor of activated T cells (GSK3β/NFAT) signaling are responsible for the ERRγ effect. Our findings provide the first insights into the role of ERRγ in the brain, as a novel approach toward understanding dopaminergic differentiation.

Polo-like kinase 3 (PLK3) mediates the clearance of the accumulated PrP mutants transiently expressed in cultured cells and pathogenic PrP(Sc) in prion infected cell line via protein interaction

Polo-like kinases (PLKs) family has long been known to be critical for cell cycle and recent studies have pointed to new dimensions of PLKs function in the nervous system. Our previous study has verified that the levels of PLK3 in the brain are severely downregulated in prion-related diseases. However, the associations of PLKs with prion protein remain unclear. In the present study, we confirmed that PrP protein constitutively interacts with PLK3 as determined by both in vitro and in vivo assays. Both the kinase domain and polo-box domain of PLK3 were proved to bind PrP proteins expressed in mammalian cell lines. Overexpression of PLK3 did not affect the level of wild-type PrP, but significantly decreased the levels of the mutated PrPs in cultured cells. The kinase domain appeared to be responsible for the clearance of abnormally aggregated PrPs, but this function seemed to be independent of its kinase activity. RNA-mediated knockdown of PLK3 obviously aggravated the accumulation of cytosolic PrPs. Moreover, PLK3 overexpression in a scrapie infected cell line caused notable reduce of PrP(Sc) level in a dose-dependent manner, but had minimal effect on the expression of PrP(C) in its normal partner cell line. Our findings here confirmed the molecular interaction between PLK3 and PrP and outlined the regulatory activity of PLK3 on the degradation of abnormal PrPs, even its pathogenic isoform PrP(Sc). We, therefore, assume that the recovery of PLK3 in the early stage of prion infection may be helpful to prevent the toxic accumulation of PrP(Sc) in the brain tissues.

Current Update on Synopsis of miRNA Dysregulation in Neurological Disorders

Aberrant expression of microRNAs (miRNAs) has been implicated in various neurological disorders (NDs) of the central nervous system such as Alzheimer disease, Parkinson's disease, Huntington disease, amyotrophic lateral sclerosis, schizophrenia and autism. If dysregulated miRNAs are identified in patients suffering from NDs, this may serve as a biomarker for the earlier diagnosis and monitoring of disease progression. Identifying the role of miRNAs in normal cellular processes and understanding how dysregulated miRNA expression is responsible for their neurological effects is also critical in the development of new therapeutic strategies for NDs. miRNAs hold great promise from a therapeutic point of view especially if it can be proved that a single miRNA has the ability to influence several target genes, making it possible for the researchers to potentially modify a whole disease phenotype by modulating a single miRNA molecule. Hence, better understanding of the mechanisms by which miRNA play a role in the pathogenesis of NDs may provide novel targets to scientists and researchers for innovative therapies.

Crystal structure of the polo-box domain of polo-like kinase 2

Polo-like kinase 2 (PLK2) is a crucial regulator in cell cycle progression, DNA damage response, and neuronal activity. PLK2 is characterized by the conserved N-terminal kinase domain and the unique C-terminal polo-box domain (PBD). The PBD mediates diverse functions of PLK2 by binding phosphorylated Ser-pSer/pThr motifs of its substrates. Here, we report the first crystal structure of the PBD of PLK2. The overall structure of the PLK2 PBD is similar to that of the PLK1 PBD, which is composed by two polo boxes each contain β6α structures that form a 12-stranded β sandwich domain. The edge of the interface between the two polo boxes forms the phosphorylated Ser-pSer/pThr motifs binding cleft. On the hand, the peripheral regions around the core binding cleft of the PLK2 PBD is distinct from that of the PLK1 PBD, which might confer the substrate specificity of the PBDs of the polo-like kinase family.

Plk2 Raps up Ras to subdue synapses

We recently identified the activity-inducible protein kinase Plk2 as a novel overseer of the balance between Ras and Rap small GTPases. Plk2 achieves a profound level of regulatory control by interacting with and phosphorylating at least four Ras and Rap guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). Combined, these actions result in synergistic suppression of Ras and hyperstimulation of Rap signaling. Perturbation of Plk2 function abolished homeostatic adaptation of synapses to enhanced activity and impaired behavioral adaptation in various learning tasks, indicating that this regulation was critical for maintaining appropriate Ras/Rap levels. These studies provide insights into the highly cooperative nature of Ras and Rap regulation in neurons. However, different GEF and GAP substrates of Plk2 also controlled specific aspects of dendritic spine morphology, illustrating the ability of individual GAPs/GEFs to assemble microdomains of Ras and Rap signaling that respond to different stimuli and couple to distinct output pathways.

Parkinson's disease-implicated kinases in the brain; insights into disease pathogenesis

Substantial evidence implicates abnormal protein kinase function in various aspects of Parkinson’s disease (PD) etiology. Elevated phosphorylation of the PD-defining pathological protein, α-synuclein, correlates with its aggregation and toxic accumulation in neurons, whilst genetic missense mutations in the kinases PTEN-induced putative kinase 1 and leucine-rich repeat kinase 2, increase susceptibility to PD. Experimental evidence also links kinases of the phosphoinositide 3-kinase and mitogen-activated protein kinase signaling pathways, amongst others, to PD. Understanding how the levels or activities of these enzymes or their substrates change in brain tissue in relation to pathological states can provide insight into disease pathogenesis. Moreover, understanding when and where kinase dysfunction occurs is important as modulation of some of these signaling pathways can potentially lead to PD therapeutics. This review will summarize what is currently known in regard to the expression of these PD-implicated kinases in pathological human postmortem brain tissue.

Protein phosphorylation in neurodegeneration: friend or foe?

Protein misfolding and aggregation is a common hallmark in neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), and fronto-temporal dementia (FTD). In these disorders, the misfolding and aggregation of specific proteins occurs alongside neuronal degeneration in somewhat specific brain areas, depending on the disorder and the stage of the disease. However, we still do not fully understand the mechanisms governing protein aggregation, and whether this constitutes a protective or detrimental process. In PD, alpha-synuclein (aSyn) forms protein aggregates, known as Lewy bodies, and is phosphorylated at serine 129. Other residues have also been shown to be phosphorylated, but the significance of phosphorylation in the biology and pathophysiology of the protein is still controversial. In AD and in FTD, hyperphosphorylation of tau protein causes its misfolding and aggregation. Again, our understanding of the precise consequences of tau phosphorylation in the biology and pathophysiology of the protein is still limited. Through the use of a variety of model organisms and technical approaches, we are now gaining stronger insight into the effects of phosphorylation in the behavior of these proteins. In this review, we cover recent findings in the field and discuss how targeting phosphorylation events might be used for therapeutic intervention in these devastating diseases of the nervous system.

Abortive cell cycle events in the brains of scrapie-infected hamsters with remarkable decreases of PLK3/Cdc25C and increases of PLK1/cyclin B1

Polo-like kinases (PLKs) consist of a family of kinases which play critical roles during multiple stages of cell cycle progression. Increase of PLK1 and decrease of PLK3 are associated with the developments and metastases of many types of human malignant tumors; however, the situations of PLKs in prion diseases are less understood. Using Western blots and immunohistochemical and immunofluorescent assays, marked increase of PLK1 and decrease of PLK3 were observed in the brains of scrapie strain 263K-infected hamsters, presenting obviously a time-dependent phenomenon along with disease progression. Similar alterations of PLKs were also detected in a scrapie infectious cell line SMB-S15. Both PLK1 and PLK3 were observed in neurons by confocal microscopy. Accompanying with the changes of PLKs in the brains of 263K-infected hamsters, Cdc25C and its phosphorylated forms (p-Cdc25C-Ser198 and p-Cdc25C-Ser216) were significantly down-regulated, whereas Cyclin B1 and PCNA were obviously up-regulated, while phospho-histone H3 remained almost unchanged. Moreover, exposure of the cytotoxic peptide PrP106-126 on the primary cultured cortical neuron cells induced similar changes of cellular PLKs and some cell cycle-related proteins, such as Cdc25C and its phosphorylated forms, phospho-histone H3. Those results illustrate obviously aberrant expressions of cell cycle regulatory proteins in the prion-infected neurons, which may lead to the cell cycle arrest at M phase. Possibly due to the ill-regulation of some key cell cycle events during prion infection, together with the fact that neurons are unable to complete mitosis, the cell cycle reentry in prion-infected neurons is definitely abortive, which may lead to neuron apoptosis and neuron degeneration.

In vivo modulation of polo-like kinases supports a key role for PLK2 in Ser129 α-synuclein phosphorylation in mouse brain

α-Synuclein is the major component of Lewy bodies. α-Synuclein phosphorylated at Ser 129 (Phospho-α-Syn) is the most common synuclein modification observed in Parkinson's disease pathology and transgenic animal models. Polo-like kinase 2 (PLK2) was previously proposed as an important kinase in α-synuclein phosphorylation at Ser129. To better understand the role of PLK2 in α-synuclein phosphorylation in vivo, we further evaluated the effect of PLK2 genetic knockdown and pharmacological inhibition on Phospho-α-Syn levels in different brain regions of PLK2 knockout (KO), heterozygous (Het) and wild-type (WT) mice. Whereas PLK2 knockdown had no effect on Total-α-synuclein brain levels, it resulted in a gene-dosage dependent, albeit incomplete, reduction of endogenous Phospho-α-Syn levels in all brain regions investigated. No compensatory induction of other α-synuclein kinases (PLK3, casein kinase-2, G-protein-coupled receptor kinase 5 (GRK5) and GRK6) was observed at the mRNA level in the PLK2 KO mouse brain. To determine whether increased activity of another PLK family member is responsible for the residual Phospho-α-Syn levels in the PLK2 KO mouse brain, the pan-PLK inhibitor BI 2536 was tested in PLK2 KO mice. Whereas BI 2536 reduced Phospho-α-Syn levels in WT mice, it did not further reduce the residual endogenous Phospho-α-Syn levels in PLK2 KO and Het mice, suggesting that a kinase other than PLK1-3 accounts for the remaining PLK inhibitor-resistant pool in the mouse brain. Moreover, PLK3 KO in mice had no effect on both Total- and Phospho-α-Syn brain levels. These results support a significant role for a PLK kinase in phosphorylating α-synuclein at Ser129 in the brain, and suggest that PLK2 is responsible for this activity under physiological conditions.

Polo-like kinase 2 regulates selective autophagic α-synuclein clearance and suppresses its toxicity in vivo

An increase in α-synuclein levels due to gene duplications/triplications or impaired degradation is sufficient to trigger its aggregation and cause familial Parkinson disease (PD). Therefore, lowering α-synuclein levels represents a viable therapeutic strategy for the treatment of PD and related synucleinopathies. Here, we report that Polo-like kinase 2 (PLK2), an enzyme up-regulated in synucleinopathy-diseased brains, interacts with, phosphorylates and enhances α-synuclein autophagic degradation in a kinase activity-dependent manner. PLK2-mediated degradation of α-synuclein requires both phosphorylation at S129 and PLK2/α-synuclein complex formation. In a rat genetic model of PD, PLK2 overexpression reduces intraneuronal human α-synuclein accumulation, suppresses dopaminergic neurodegeneration, and reverses hemiparkinsonian motor impairments induced by α-synuclein overexpression. This PLK2-mediated neuroprotective effect is also dependent on PLK2 activity and α-synuclein phosphorylation. Collectively, our findings demonstrate that PLK2 is a previously undescribed regulator of α-synuclein turnover and that modulating its kinase activity could be a viable target for the treatment of synucleinopathies.

Prenatal stress increased Snk Polo-like kinase 2, SCF β-TrCP ubiquitin ligase and ubiquitination of SPAR in the hippocampus of the offspring at adulthood

Exposure to excessive glucocorticoids during fetal development period contributes to later life psychopathology. Prenatal stress decreases dendritic spine density and impair LTP in the hippocampus of rat pups, however, the mechanisms regulating these changes are still unclear. Glutamate receptors are localized in the postsynaptic density. PSD-95 is a postsynaptic scaffolding protein that plays a role in synaptic maturation and regulation of the synaptic strength and plasticity. PSD-95 interacts with other proteins to form the protein networks that promote dendritic spine formation. The present study investigated the effect of prenatal stress on the levels of scaffolding proteins of NMDA receptor in the hippocampus in order to explain how prenatal stress alters the amount of NMDA receptor in the pups' brain. Pregnant rats were randomly assigned to either the prenatal stress (PS) or the control group (C). The pregnant rats in the PS group were restrained in a plexiglas restrainer for 4h/day during the GD 14-21. Control rats were left undisturbed for the duration of their pregnancies. The amount of PSD-95, SPAR, NR2A and NR2B, as well as the levels of Snk Polo-like kinase 2 and the SCF β-TrCP ubiquitin ligase were measured in the hippocampus of the offspring. The results show that prenatal stress induces a reduction in the amount of NR2B and NR2A subunits in the hippocampus of rat pups, parallel to the decrease in PSD-95 and SPAR at P40 and P60. Moreover, prenatal stress increases Snk and β-TrCP in the hippocampus of rat pups, and the timing correlates with the decrease of SPAR and PSD-95. Prenatal stress also induces a significantly increases in the level of ubiquitinated SPAR in the hippocampus of rat pups at adulthood. The results suggest that degradation of SPAR via UPS system may contribute to the loss of PSD-95 and NMDA receptor subunits in the hippocampus of rat pups at adulthood. In conclusion, the present work demonstrates that the developing brain is critically influenced by glucocorticoids, especially during pre- and early postnatal period, which can have long-term effects on brain development. In addition, an involvement of the UPS system in the prenatal stress model has led to a greater understanding of the effects of prenatal stress later on in life.

Mammalian target of rapamycin complex 1 activation negatively regulates Polo-like kinase 2-mediated homeostatic compensation following neonatal seizures

Homeostatic plasticity is characterized by compensatory changes in synaptic strength and intrinsic membrane properties in response to chronic changes in neuronal activity. Neonatal seizures are a naturally occurring source of neuronal overactivation and can lead to long-term epilepsy and cognitive deficits. Using a rodent model of hypoxia-induced neonatal seizures that results in a persistent increase in AMPA receptor (AMPAR) function in hippocampal CA1 pyramidal neurons, we aimed to determine whether there was any evidence of an opposing endogenous homeostatic antiepileptic response. Given that this model results in long-term epilepsy, we also examined mechanisms whereby this homeostasis fails. Whole-cell patch-clamp recordings from neurons in slices removed at intervals following seizure onset revealed an initial up-regulation of AMPAR function that was followed by a transient dynamic attenuation of this enhancement by 48-72 h, although AMPAR function was still increased compared with nonseizure control baseline. This secondary down-regulation of enhanced AMPAR function was coincident with a marked transient increase in expression and function of the Polo-like kinase 2 (PLK2), which has previously been implicated in homeostatic down-regulation of neuronal excitability in cell/slice culture models. The effects were transient and at 1 wk AMPAR function once again became up-regulated, simultaneous with a decrease in PLK2 expression and function. This negative regulation was mediated by subacute postseizure increases in mammalian target of rapamycin (mTOR). Application of the mTOR inhibitor rapamycin prevented post-hypoxic seizure impairment of homeostasis, suggesting that homeostatic plasticity mechanisms may be potentially modifiable therapeutic targets in epileptogenesis.

Investigation on PLK2 and PLK3 substrate recognition

Analyses of human phosphoproteome based on primary structure of the aminoacids surrounding the phosphor Ser/Thr suggest that a significant proportion of phosphosites is generated by a restricted number of acidophilic kinases, among which protein kinase CK2 plays a prominent role. Recently, new acidophilic kinases belonging to the Polo like kinase family have been characterized, with special reference to PLK1, PLK2, and PLK3 kinases. While some progress has been made in deciphering the PLK1-dependent phosphoproteome, very little is known about the targets of PLK2 and PLK3 kinases. In this report by using an in vitro approach, consisting of cell lysate phosphorylation, phosphoprotein separation by 2D gel electrophoresis and mass spectrometry, we describe the identification of new potential substrates of PLK2 and PLK3 kinases. We have identified and validated as in vitro PLK2 and PLK3 substrates HSP90, GRP-94, β-tubulin, calumenin, and 14-3-3 epsilon. The phosphosites generated by PLK3 in these proteins have been identified by mass spectrometry analysis to get new insights about PLKs specificity determinants. These latter have been further corroborated by an in silico analysis of the PLKs substrate binding region.

Roles of Polo-like kinase 3 in suppressing tumor angiogenesis

Angiogenesis is essential for promoting growth and metastasis of solid tumors by ensuring blood supply to the tumor mass. Targeting angiogenesis is therefore an attractive approach to therapeutic intervention of cancer. Tumor angiogenesis is a process that is controlled by a complex network of molecular components including sensors, signaling transducers, and effectors, leading to cellular responses under hypoxic conditions. Positioned at the center of this network are the hypoxia-inducible factors (HIFs). HIF-1 is a major transcription factor that consists of two subunits, HIF-1α and HIF-1β. It mediates transcription of a spectrum of gene targets whose products are essential for mounting hypoxic responses. HIF-1α protein level is very low in the normoxic condition but is rapidly elevated under hypoxia. This dramatic change in the cellular HIF-1α level is primarily regulated through the proteosome-mediated degradation process. In the past few years, scientific progress has clearly demonstrated that HIF-1α phosphorylation is mediated by several families of protein kinases including GSK3β and ERKs both of which play crucial roles in the regulation of HIF-1α stability. Recent research progress has identified that Polo-like kinase 3 (Plk3) phosphorylates HIF-1α at two previously unidentified serine residues and that the Plk3-mediated phosphorylation of these residues results in destabilization of HIF-1α. Plk3 has also recently been found to phosphorylate and stabilize PTEN phosphatase, a known regulator of HIF-1α and tumor angiogenesis. Given the success of targeting protein kinases and tumor angiogenesis in anti-cancer therapies, Plk3 could be a potential molecular target for the development of novel and effective therapeutic agents for cancer treatment.

NMS-P937, an orally available, specific small-molecule polo-like kinase 1 inhibitor with antitumor activity in solid and hematologic malignancies

Polo-like kinase 1 (PLK1) is a serine/threonine protein kinase considered to be the master player of cell-cycle regulation during mitosis. It is indeed involved in centrosome maturation, bipolar spindle formation, chromosome separation, and cytokinesis. PLK1 is overexpressed in a variety of human tumors and its overexpression often correlates with poor prognosis. Although five different PLKs are described in humans, depletion or inhibition of kinase activity of PLK1 is sufficient to induce cell-cycle arrest and apoptosis in cancer cell lines and in xenograft tumor models. NMS-P937 is a novel, orally available PLK1-specific inhibitor. The compound shows high potency in proliferation assays having low nanomolar activity on a large number of cell lines, both from solid and hematologic tumors. NMS-P937 potently causes a mitotic cell-cycle arrest followed by apoptosis in cancer cell lines and inhibits xenograft tumor growth with clear PLK1-related mechanism of action at well-tolerated doses in mice after oral administration. In addition, NMS-P937 shows potential for combination in clinical settings with approved cytotoxic drugs, causing tumor regression in HT29 human colon adenocarcinoma xenografts upon combination with irinotecan and prolonged survival of animals in a disseminated model of acute myelogenous leukemia in combination with cytarabine. NMS-P937, with its favorable pharmacologic parameters, good oral bioavailability in rodent and nonrodent species, and proven antitumor activity in different preclinical models using a variety of dosing regimens, potentially provides a high degree of flexibility in dosing schedules and warrants investigation in clinical settings.

Systematic discovery of TLR signaling components delineates viral-sensing circuits

Deciphering the signaling networks that underlie normal and disease processes remains a major challenge. Here, we report the discovery of signaling components involved in the Toll-like receptor (TLR) response of immune dendritic cells (DCs), including a previously unkown pathway shared across mammalian antiviral responses. By combining transcriptional profiling, genetic and small-molecule perturbations, and phosphoproteomics, we uncover 35 signaling regulators, including 16 known regulators, involved in TLR signaling. In particular, we find that Polo-like kinases (Plk) 2 and 4 are essential components of antiviral pathways in vitro and in vivo and activate a signaling branch involving a dozen proteins, among which is Tnfaip2, a gene associated with autoimmune diseases but whose role was unknown. Our study illustrates the power of combining systematic measurements and perturbations to elucidate complex signaling circuits and discover potential therapeutic targets.

Neurodegeneration and neuroregeneration in Chagas disease

Autonomic dysfunction plays a significant role in the development of chronic Chagas disease (CD). Destruction of cardiac parasympathetic ganglia can underlie arrhythmia and heart failure, while lesions of enteric neurons in the intestinal plexuses are a direct cause of aperistalsis and megasyndromes. Neuropathology is generated by acute infection when the parasite, though not directly damaging to neuronal cells, elicits immune reactions that can become cytotoxic, inducing oxidative stress and neurodegeneration. Anti-neuronal autoimmunity may further contribute to neuropathology. Much less clear is the mechanism of subsequent neuronal regeneration in patients that survive acute infection. Morphological and functional recovery of the peripheral neurons in these patients correlates with the absence of CD clinical symptoms, while persistent neuronal deficiency is observed for the symptomatic group. The discovery that Trypanosoma cruzi trans-sialidase can moonlight as a parasite-derived neurotrophic factor (PDNF) suggests that the parasite might influence the balance between neuronal degeneration and regeneration. PDNF functionally mimics mammalian neurotrophic factors in that it binds and activates neurotrophin Trk tyrosine kinase receptors, a mechanism which prevents neurodegeneration. PDNF binding to Trk receptors triggers PI3K/Akt/GSK-3β and MAPK/Erk/CREB signalling cascades which in neurons translates into resistance to oxidative and nutritional stress, and inhibition of apoptosis, whereas in the cytoplasm of infected cells, PDNF represents a substrate-activator of the host Akt kinase, enhancing host-cell survival until completion of the intracellular cycle of the parasite. Such dual activity of PDNF provides sustained activation of survival mechanisms which, while prolonging parasite persistence in host tissues, can underlie distinct outcomes of CD.

Identification and functional characterization of polo-like kinase 2 autoregulatory sites

Polo family kinases play important roles in cellular proliferation as well as neuronal synaptic plasticity. However, the posttranslational regulation of these kinases is not fully understood. Here, we identified several novel Plk2 phosphorylation sites stimulated by Plk2 itself. By site-directed mutagenesis, we uncovered three additional hyperactivating Plk2 mutations as well as a series of residues regulating Plk2 steady-state expression level. Because of the established role of Plk2 in homeostatic negative control of excitatory synaptic strength, these phosphorylation sites could play an important role in the rapid activation, expansion, and prolongation of Plk2 signaling in this process.

Physiological relevance of cell cycle kinases

The basic biology of the cell division cycle and its control by protein kinases was originally studied through genetic and biochemical studies in yeast and other model organisms. The major regulatory mechanisms identified in this pioneer work are conserved in mammals. However, recent studies in different cell types or genetic models are now providing a new perspective on the function of these major cell cycle regulators in different tissues. Here, we review the physiological relevance of mammalian cell cycle kinases such as cyclin-dependent kinases (Cdks), Aurora and Polo-like kinases, and mitotic checkpoint regulators (Bub1, BubR1, and Mps1) as well as other less-studied enzymes such as Cdc7, Nek proteins, or Mastl and their implications in development, tissue homeostasis, and human disease. Among these functions, the control of self-renewal or asymmetric cell division in stem/progenitor cells and the ability to regenerate injured tissues is a central issue in current research. In addition, many of these proteins play previously unexpected roles in metabolism, cardiovascular function, or neuron biology. The modulation of their enzymatic activity may therefore have multiple therapeutic benefits in human disease.

From Plk1 to Plk5: functional evolution of polo-like kinases

Mammalian polo-like kinases (Plks) are characterized by the presence of an N-terminal protein kinase domain and a C-terminal polo-box domain (PBD) involved in substrate binding and regulation of kinase activity. Plk1-4 have traditionally been linked to cell cycle progression, genotoxic stress and, more recently, neuron biology. Recently, a fifth mammalian Plk family member, Plk5, has been characterized in murine and human cells. Plk5 is expressed mainly in differentiated tissues such as the cerebellum. Despite apparent loss of catalytic activity and a stop codon in the middle of the human gene, Plk5 proteins retain important functions in neuron biology. Notably, its expression is silenced by epigenetic alterations in brain tumors, such as glioblastomas, and its re-expression prevents cell proliferation of these tumor cells. In this review, we will focus on the non-cell cycle roles of Plks, the biology of the new member of the family and the possible kinase- and PBD-independent functions of polo-like kinases.

Serum inducible kinase is a positive regulator of cortical dendrite development and is required for BDNF-promoted dendritic arborization

Serum inducible kinase (SNK), also known as polo-like kinase 2 (PLK2), is a known regulator of mitosis, synaptogenesis and synaptic homeostasis. However, its role in early cortical development is unknown. Herein, we show that snk is expressed in the cortical plate from embryonic day 14, but not in the ventricular/subventricular zones (VZ/SVZ), and SNK protein localizes to the soma and dendrites of cultured immature cortical neurons. Loss of SNK impaired dendritic but not axonal arborization in a dose-dependent manner and overexpression had opposite effects, both in vitro and in vivo. Overexpression of SNK also caused abnormal branching of the leading process of migrating cortical neurons in electroporated cortices. The kinase activity was necessary for these effects. Extracellular signal-regulated kinase (ERK) pathway activity downstream of brain-derived neurotrophic factor (BDNF) stimulation led to increases in SNK protein expression via transcriptional regulation, and this upregulation was necessary for the growth-promoting effect of BDNF on dendritic arborization. Taken together, our results indicate that SNK is essential for dendrite morphogenesis in cortical neurons.

Repeated carbenoxolone injections during late pregnancy alter Snk-SPAR and PSD-95 expression in the hippocampus of rat pups

Homeostasis of circulating cortisol is maintained by the 11β-HSD2 enzyme which inactivates cortisol into cortisone. It is abundantly expressed in the placenta where it protects the fetus from high levels of maternal glucocorticoids (GCs). Maternal administration of Carbenoxolone (Cbx), a powerful 11β-HSD2 inhibitor, leads to an increase in fetal cortisol. Previous data showed that intrauterine environment plays a crucial role in determining hippocampal structure and function. Exposure of pregnant rats to high levels of GC leads to low birth weight in offspring and an increased risk of age related memory and cognitive deficits later in life. Glutamate receptors are localized in the postsynaptic density (PSD), where many signaling proteins, cytoskeleton proteins, and ion channels are found. Any change in the number of these molecules can influence the morphology and function of the dendritic spine. We proposed that repeated Cbx injections during late pregnancy may alter the scaffolding proteins of the NMDA receptor in the pup's brain. We investigated the effects of repeated maternal Cbx injections on the scaffolding proteins of NMDA receptor in the hippocampus of rat pups. We showed that injecting pregnant rats with Cbx injections (30mg/kg) during GD 14-21 leads to a significant decrease in SPAR (Spine Associated Rap Guanylate kinase activating protein) (p<0.001) and PSD-95 (p<0.05) but a significant increase in Snk (Serum inducible kinase) (p<0.001) in the pup's hippocampus at P40. In general, Snk is induced by neuronal activity and plays an important role in phosphorylating SPAR. The phosphorylated SPAR is then recognized and degraded by ubiquitin proteasome system (UPS), causing the depletion of SPAR and PSD-95 from the spines. The results suggest that fetal exposure to excessive GC levels may activate the Snk/SPAR pathway and lead to the depletion of SPAR and PSD-95. Since GCs drugs are commonly used in various obstetric and pediatric conditions, it is important to consider the risks and benefits of prenatal GCs exposure in order to prevent neurodevelopmental delay in the offspring.

Plk5, a polo box domain-only protein with specific roles in neuron differentiation and glioblastoma suppression

Polo-like kinases (Plks) are characterized by the presence of a specific domain, known as the polo box (PBD), involved in protein-protein interactions. Plk1 to Plk4 are involved in centrosome biology as well as the regulation of mitosis, cytokinesis, and cell cycle checkpoints in response to genotoxic stress. We have analyzed here the new member of the vertebrate family, Plk5, a protein that lacks the kinase domain in humans. Plk5 does not seem to have a role in cell cycle progression; in fact, it is downregulated in proliferating cells and accumulates in quiescent cells. This protein is mostly expressed in the brain of both mice and humans, and it modulates the formation of neuritic processes upon stimulation of the brain-derived neurotrophic factor (BDNF)/nerve growth factor (NGF)-Ras pathway in neurons. The human PLK5 gene is significantly silenced in astrocytoma and glioblastoma multiforme by promoter hypermethylation, suggesting a tumor suppressor function for this gene. Indeed, overexpression of Plk5 has potent apoptotic effects in these tumor cells. Thus, Plk5 seems to have evolved as a kinase-deficient PBD-containing protein with nervous system-specific functions and tumor suppressor activity in brain cancer.

Calmyrin1 binds to SCG10 protein (stathmin2) to modulate neurite outgrowth

Calmyrin1 (CaMy1) is an EF-hand Ca(2+)-binding protein expressed in several cell types, including brain neurons. Using a yeast two-hybrid screen of a human fetal brain cDNA library, we identified SCG10 protein (stathmin2) as a CaMy1 partner. SCG10 is a microtubule-destabilizing factor involved in neuronal growth during brain development. We found increased mRNA and protein levels of CaMy1 during neuronal development, which paralleled the changes in SCG10 levels. In developing primary rat hippocampal neurons in culture, CaMy1 and SCG10 colocalized in cell soma, neurites, and growth cones. Pull-down, coimmunoprecipitation, and proximity ligation assays demonstrated that the interaction between CaMy1 and SCG10 is direct and Ca(2+)-dependent in vivo and requires the C-terminal domain of CaMy1 (residues 99-192) and the N-terminal domain of SCG10 (residues 1-35). CaMy1 did not interact with stathmin1, a protein that is homologous with SCG10 but lacks the N-terminal domain characteristic of SCG10. CaMy1 interfered with SCG10 inhibitory activity in a microtubule polymerization assay. Moreover, CaMy1 overexpression inhibited SCG10-mediated neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells. This CaMy1 activity did not occur when an N-terminally truncated SCG10 mutant unable to interact with CaMy1 was expressed. Altogether, these data suggest that CaMy1 via SCG10 couples Ca(2+) signals with the dynamics of microtubules during neuronal outgrowth in the developing brain. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.