Functional whole-genome analysis identifies Polo-like kinase 2 and poliovirus receptor as essential for neuronal differentiation upstream of the negative regulator alphaB-crystallin

Crosstalking interactions between P2X4 and 5-HT3A receptors

Ionotropic receptors are ligand-gated ion channels triggering fast neurotransmitter responses. Among them, P2X and 5-HT₃ receptors have been shown to physically interact each other and functionally inducing cross inhibitory responses. Nevertheless, despite the importance of P2X4 and 5-HT_3A receptors that mediate for example neuropathic pain and psychosis respectively, complementary evidence has recently started to move forward in the understanding of this interaction. In this review, we discuss current evidence supporting the mechanism of crosstalking between both receptors, from the structural to the transduction pathway level. We expect this work may guide the design of further experiments to obtain a comprehensive view for the neuropharmacological role of these interacting receptors.

ERK-dependent induction of the immediate-early gene Egr1 and the late gene Gpr50 contribute to two distinct phases of PACAP Gs-GPCR signaling for neuritogenesis

Gs-coupled GPCR-stimulated neuritogenesis in PC12 and NS-1 - cells depends on activation of the MAP kinase ERK. Here, we examine changes in ERK activation (phosphorylation), and the time course of ERK-dependent gene induction, to seek transcriptional determinants for this process. Quenching of ERK activation by inhibition of MEK with U0126 at any time point for at least 24 h following addition of PACAP resulted in arrest of neurite formation. Changes in the transcriptome profile throughout this time period revealed at least two phases of gene induction: an early phase dominated by induction of immediate-early genes, and a later phase of gene induction after 4-6 h of exposure to PACAP with persistent elevation of phospho-ERK levels. Genes induced by PACAP in both phases consisted in those whose induction was dependent on ERK (i.e., blocked by U0126), and some whose induction was blocked by the protein kinase A inhibitor H89. ERK-dependent "late gene" transcripts included Gpr50, implicated earlier in facilitation of NGF-induced neurite formation in NS-1 cells. Gpr50 induction by PACAP, but not NGF, was dependent on the guanine nucleotide exchange factor RapGEF2, which has been shown to be required for PACAP-induced neuritogenesis in NS-1 cells. Expression of a Gpr50-directed shRNA lowered basal levels of Gpr50 mRNA and attenuated Gpr50 mRNA and GPR50 protein induction by PACAP, with a corresponding attenuation of PACAP-induced neuritogenesis. Gs-GPCR-stimulated neuritogenesis first requires immediate-early gene induction, including that of Egr1 (Zif268/NGF1A/Krox24) as previously reported. This early phase of gene induction, however, is insufficient to maintain the neuritogenic process without ERK-dependent induction of additional late genes, including Gpr50, upon continuous exposure to neurotrophic neuropeptide. Early (Egr1) and late (Gpr50) gene induction by NGF, like that for PACAP, was inhibited by U0126, but was independent of RapGEF2, confirming distinct modes of ERK activation by Gs-coupled GPCRs and neurotrophic tyrosine receptor kinases, converging on a final common ERK-dependent signaling pathway for neuritogenesis.

Polo-Like Kinase 2: From Principle to Practice

Polo-like kinase (PLK) 2 is an evolutionarily conserved serine/threonine kinase that shares the n-terminal kinase catalytic domain and the C-terminal Polo Box Domain (PBD) with other members of the PLKs family. In the last two decades, mounting studies have focused on this and tried to clarify its role in many aspects. PLK2 is essential for mitotic centriole replication and meiotic chromatin pairing, synapsis, and crossing-over in the cell cycle; Loss of PLK2 function results in cell cycle disorders and developmental retardation. PLK2 is also involved in regulating cell differentiation and maintaining neural homeostasis. In the process of various stimuli-induced stress, including oxidative and endoplasmic reticulum, PLK2 may promote survival or apoptosis depending on the intensity of stimulation and the degree of cell damage. However, the role of PLK2 in immunity to viral infection has been studied far less than that of other family members. Because PLK2 is extensively and deeply involved in normal physiological functions and pathophysiological mechanisms of cells, its role in diseases is increasingly being paid attention to. The effect of PLK2 in inhibiting hematological tumors and fibrotic diseases, as well as participating in neurodegenerative diseases, has been gradually recognized. However, the research results in solid organ tumors show contradictory results. In addition, preliminary studies using PLK2 as a disease predictor and therapeutic target have yielded some exciting and promising results. More research will help people better understand PLK2 from principle to practice.

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.

Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

A persistent dogma in neuroscience supported the idea that terminally differentiated neurons permanently withdraw from the cell cycle. However, since the late 1990s, several studies have shown that cell cycle proteins are expressed in post-mitotic neurons under physiological conditions, indicating that the cell cycle machinery is not restricted to proliferating cells. Moreover, many studies have highlighted a clear link between cell cycle-related proteins and neurological disorders, particularly relating to apoptosis-induced neuronal death. Indeed, cell cycle-related proteins can be upregulated or overactivated in post-mitotic neurons in case of acute or degenerative central nervous system disease. Given the considerable lack of effective treatments for age-related neurological disorders, new therapeutic approaches targeting the cell cycle machinery might thus be considered. This review aims at summarizing current knowledge about the role of the cell cycle machinery in post-mitotic neurons in healthy and pathological conditions.

Screening of differentially expressed protein kinases in bone marrow endothelial cells and the protective effects of the p38a inhibitor SB203580 on bone marrow in liver fibrosis

Hematological abnormalities are frequently observed in patients with liver cirrhosis (LC). A previous study demonstrated that the apoptosis and damage of endothelial cells could cause the hematological abnormalities in LC. Protein kinases are one of the most important factors that regulate cell behavior, and are potential therapeutic targets for the treatment of a number of diseases. In a previous study, whole genome profiling was used to identify differentially expressed genes in human bone marrow endothelial cells treated with serum from 26 patients with LC. From this data set, the present study identified 14 differentially expressed kinase genes in human bone marrow endothelial cells in LC from the microarray data, including p38a, AKT1 and PDK1. Pathway analysis revealed that these kinase genes were enriched in certain important LC‑associated pathways (e.g. MAPK and WNT signaling pathway). Literature mining revealed that p38a was associated with bone marrow apoptosis; therefore, p38a and its inhibitor, SB203580, were selected as potential therapeutic targets in the present study. The results of hematoxylin‑eosin and Masson's trichrome staining of livers from a rat model of liver fibrosis (LF) that underwent ligation of the bile duct demonstrated that SB203580 reduced the degree of LF. In addition, SB203580‑treated rats with LF demonstrated a significantly higher number of platelets when compared with the untreated group. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis indicated that apoptosis of bone marrow tissue in rats with LF was inhibited by SB203580. In addition, the results from the immunohistochemical analysis demonstrated that SB203580 reduced the expression of von Willebrand factor and caspase 3 in the bone marrow of rats with LF. In conclusion, the results from the present study indicate that the p38a kinase inhibitor, SB203580, may exhibit a protective effect on bone marrow tissues in rats with LF. This suggests that protein kinases and their inhibitors may present novel therapeutic strategies for the treatment of hematological abnormalities in patients with LC.

Structural analysis of the polo-box domain of human Polo-like kinase 2

Polo-like kinases (Plks) are the key regulators of cell cycle progression, the members of which share a kinase domain and a polo-box domain (PBD) that serves as a protein-binding module. While Plk1 is a promising target for antitumor therapy, Plk2 is regarded as a tumor suppressor even though the two Plks commonly recognize the S-pS/T-P motif through their PBD. Herein, we report the crystal structure of the PBD of Plk2 at 2.7 Å. Despite the overall structural similarity with that of Plk1 reflecting their high sequence homology, the crystal structure also contains its own features including the highly ordered loop connecting two subdomains and the absence of 310 -helices in the N-terminal region unlike the PBD of Plk1. Based on the three-dimensional structure, we furthermore could model its interaction with two types of phosphopeptides, one of which was previously screened as the optimal peptide for the PBD of Plk2.

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.

The CD226/CD155 interaction regulates the proinflammatory (Th1/Th17)/anti-inflammatory (Th2) balance in humans

CD226 costimulatory signals strongly promote Th1 differentiation, enhancing IFN-γ production by naive T cells. We recently reported that knockdown of CD226 on human T cells resulted in a decrease in T-bet and IFN-γ expression. However, the role of CD226 on Th2 and Th17 cells remains unknown. In this study, we found that CD226 and its ligand CD155 were decreased on Th2-polarized naive T cells, whereas both were highly expressed under Th17 conditions. Most IFN-γ- and IL-17-producing cells expressed high levels of CD226, but production of IL-13 did not correlate with CD226 expression. CD226 knockdown by lentiviral transduction resulted in increased STAT-6 phosphorylation, enhanced GATA3 expression, and consequently higher production of IL-4 and IL-13. Under Th17 conditions, CD226-depleted cells showed slightly impaired IL-17 secretion, suggesting that CD226 contributes, in part, to IL-17 production but is dispensable for Th17 cell generation. In line with these results, CD226 blockade with neutralizing Abs efficiently inhibited T cell activation and proliferation and production of IFN-γ and IL-17, whereas IL-13 secretion remained functional. Taken together, our results establish an important role for CD226 in differentially regulating the proinflammatory (Th1/Th17)/anti-inflammatory (Th2) balance, suggesting that the CD226/CD155 interaction could potentially be targeted in therapeutic approaches to human autoimmune diseases.

Functional analysis of the Aspergillus nidulans kinome

The filamentous fungi are an ecologically important group of organisms which also have important industrial applications but devastating effects as pathogens and agents of food spoilage. Protein kinases have been implicated in the regulation of virtually all biological processes but how they regulate filamentous fungal specific processes is not understood. The filamentous fungus Aspergillus nidulans has long been utilized as a powerful molecular genetic system and recent technical advances have made systematic approaches to study large gene sets possible. To enhance A. nidulans functional genomics we have created gene deletion constructs for 9851 genes representing 93.3% of the encoding genome. To illustrate the utility of these constructs, and advance the understanding of fungal kinases, we have systematically generated deletion strains for 128 A. nidulans kinases including expanded groups of 15 histidine kinases, 7 SRPK (serine-arginine protein kinases) kinases and an interesting group of 11 filamentous fungal specific kinases. We defined the terminal phenotype of 23 of the 25 essential kinases by heterokaryon rescue and identified phenotypes for 43 of the 103 non-essential kinases. Uncovered phenotypes ranged from almost no growth for a small number of essential kinases implicated in processes such as ribosomal biosynthesis, to conditional defects in response to cellular stresses. The data provide experimental evidence that previously uncharacterized kinases function in the septation initiation network, the cell wall integrity and the morphogenesis Orb6 kinase signaling pathways, as well as in pathways regulating vesicular trafficking, sexual development and secondary metabolism. Finally, we identify ChkC as a third effector kinase functioning in the cellular response to genotoxic stress. The identification of many previously unknown functions for kinases through the functional analysis of the A. nidulans kinome illustrates the utility of the A. nidulans gene deletion constructs.

The expanding genomic landscape of autism: discovering the 'forest' beyond the 'trees'

Autism spectrum disorders are neurodevelopmental disorders characterized by significant deficits in reciprocal social interactions, impaired communication and restricted, repetitive behaviors. As autism spectrum disorders are among the most heritable of neuropsychiatric disorders, much of autism research has focused on the search for genetic variants in protein-coding genes (i.e., the 'trees'). However, no single gene can account for more than 1% of the cases of autism spectrum disorders. Yet, genome-wide association studies have often identified statistically significant associations of genetic variations in regions of DNA that do not code for proteins (i.e., intergenic regions). There is increasing evidence that such noncoding regions are actively transcribed and may participate in the regulation of genes, including genes on different chromosomes. This article summarizes evidence that suggests that the research spotlight needs to be expanded to encompass far-reaching gene-regulatory mechanisms that include a variety of epigenetic modifications, as well as noncoding RNA (i.e., the 'forest'). Given that noncoding RNA represents over 90% of the transcripts in most cells, we may be observing just the 'tip of the iceberg' or the 'edge of the forest' in the genomic landscape of autism.

The Polo-like kinase PLKA in Aspergillus nidulans is not essential but plays important roles during vegetative growth and development

The Polo-like kinases (Plks) are conserved, multifunctional cell cycle regulators that are induced in many forms of cancer and play additional roles in metazoan development. We previously identified plkA in Aspergillus nidulans, the only Plk investigated in filamentous fungi to date, and partially characterized its function through overexpression. Here, we report the plkA null phenotype. Surprisingly, plkA was not essential, unlike Plks in other organisms that contain a single homologue. A subset of cells lacking PLKA contained defects in spindle formation and chromosome organization, supporting some conservation in cell cycle function. However, septa were present, suggesting that PLKA, unlike other Plks, is not a central regulator of septation. Colonies lacking PLKA were compact with multibranched hyphae, implying a role for this factor in aspects of hyphal morphogenesis. These defects were suppressed by high temperature or low concentrations of benomyl, suggesting that PLKA may function during vegetative growth by influencing microtubule dynamics. However, the colonies also showed reduced conidiation and precocious formation of sexual Hülle cells in a benomyl- and temperature-insensitive manner. This result suggests that PLKA may influence reproduction through distinct mechanisms and represents the first example of a link between Plk function and development in fungi. Finally, filamentous fungal Plks have distinct features, and phylogenetic analyses reveal that they may group more closely with metazoan PLK4. In contrast, yeast Plks are more similar to metazoan proteins PLK1 to PLK3. Thus, A. nidulans PLKA shows some conservation in cell cycle function but may also play novel roles during hyphal morphogenesis and development.

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.

Global expression analysis identified a preferentially nerve growth factor-induced transcriptional program regulated by sustained mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) and AP-1 protein activation during PC12 cell differentiation

Neuronal differentiation of PC12 cells in response to NGF is a prototypical model in which signal duration determines a biological response. Sustained ERK activity induced by NGF, as compared with transient activity induced by EGF, is critical to the differentiation of these cells. To characterize the transcriptional program activated preferentially by NGF, we compared global gene expression profiles between cells treated with NGF and EGF for 2-4 h, when sustained ERK signaling in response to NGF is most distinct from the transient signal elicited by EGF. This analysis identified 69 genes that were preferentially up-regulated in response to NGF. As expected, up-regulation of these genes was mediated by sustained ERK signaling. In addition, they were up-regulated in response to other neuritogenic treatments (pituitary adenylate cyclase-activating polypeptide and 12-O-tetradecanoylphorbol-13-acetate plus dbcAMP) and were enriched for genes related to neuronal differentiation/function. Computational analysis and chromatin immunoprecipitation identified binding of CREB and AP-1 family members (Fos, FosB, Fra1, JunB, JunD) upstream of >30 and 50%, respectively, of the preferentially NGF-induced genes. Expression of several AP-1 family members was induced by both EGF and NGF, but their induction was more robust and sustained in response to NGF. The binding of Fos family members to their target genes was similarly sustained in response to NGF and was reduced upon MEK inhibition, suggesting that AP-1 contributes significantly to the NGF transcriptional program. Interestingly, Fra1 as well as two other NGF-induced AP-1 targets (HB-EGF and miR-21) function in positive feedback loops that may contribute to sustained AP-1 activity.

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.

Timing-dependent actions of NGF required for cell differentiation

Background

Continuous NGF stimulation induces PC12 cell differentiation. However, why continuous NGF stimulation is required for differentiation is unclear. In this study, we investigated the underlying mechanisms of the timing-dependent requirement of NGF action for cell differentiation.

Methodology/Principal Findings

To address the timing-dependency of the NGF action, we performed a discontinuous stimulation assay consisting of a first transient stimulation followed by an interval and then a second sustained stimulation and quantified the neurite extension level. Consequently, we observed a timing-dependent action of NGF on cell differentiation, and discontinuous NGF stimulation similarly induced differentiation. The first stimulation did not induce neurite extension, whereas the second stimulation induced fast neurite extension; therefore, the first stimulation is likely required as a prerequisite condition. These observations indicate that the action of NGF can be divided into two processes: an initial stimulation-driven latent process and a second stimulation-driven extension process. The latent process appears to require the activities of ERK and transcription, but not PI3K, whereas the extension-process requires the activities of ERK and PI3K, but not transcription. We also found that during the first stimulation, the activity of NGF can be replaced by PACAP, but not by insulin, EGF, bFGF or forskolin; during the second stimulation, however, the activity of NGF cannot be replaced by any of these stimulants. These findings allowed us to identify potential genes specifically involved in the latent process, rather than in other processes, using a microarray.

Conclusions/Significance

These results demonstrate that NGF induces the differentiation of PC12 cells via mechanically distinct processes: an ERK-driven and transcription-dependent latent process, and an ERK- and PI3K-driven and transcription-independent extension process.

Phosphorylation of synucleins by members of the Polo-like kinase family

Phosphorylation of alpha-synuclein (alpha-syn) at Ser-129 is a hallmark of Parkinson disease and related synucleinopathies. However, the identity of the natural kinases and phosphatases responsible for regulating alpha-syn phosphorylation remain unknown. Here we demonstrate that three closely related members of the human Polo-like kinase (PLK) family (PLK1, PLK2, and PLK3) phosphorylate alpha-syn and beta-syn specifically at Ser-129 and Ser-118, respectively. Unlike other kinases reported to partially phosphorylate alpha-syn at Ser-129 in vitro, phosphorylation by PLK2 and PLK3 is quantitative (>95% conversion). Only PLK1 and PLK3 phosphorylate beta-syn at Ser-118, whereas no phosphorylation of gamma-syn was detected by any of the four PLKs (PLK1 to -4). PLK-mediated phosphorylation was greatly reduced in an isolated C-terminal fragment (residues 103-140) of alpha-syn, suggesting substrate recognition via the N-terminal repeats and/or the non-amyloid component domain of alpha-syn. PLKs specifically co-localized with phosphorylated Ser-129 (Ser(P)-129) alpha-syn in various subcellular compartments (cytoplasm, nucleus, and membranes) of mammalian cell lines and primary neurons as well as in alpha-syn transgenic mice, especially cortical brain areas involved in synaptic plasticity. Furthermore, we report that the levels of PLK2 are significantly increased in brains of Alzheimer disease and Lewy body disease patients. Taken together, these results provide biochemical and in vivo evidence of alpha-syn and beta-syn phosphorylation by specific PLKs. Our results suggest a need for further studies to elucidate the potential role of PLK-syn interactions in the normal biology of these proteins as well as their involvement in the pathogenesis of Parkinson disease and other synucleinopathies.