Regulation of genotoxic stress response by homeodomain-interacting protein kinase 2 through phosphorylation of cyclic AMP response element-binding protein at serine 271

Homeodomain Interacting Protein Kinase 2-Modified Rat Spinal Astrocytes Affect Neurofunctional Recovery After Spinal Cord Injury

BACKGROUND

Spinal cord injury (SCI) is regarded as an acute neurological disorder, and astrocytes play a role in the progression of SCI.

OBJECTIVE

Herein, we investigated the roles of homeodomain-interacting protein kinase 2 (HIPK2)- modified rat spinal astrocytes in neurofunctional recovery after SCI.

METHODS

Rat spinal astrocytes were cultured, isolated, and then identified through microscopic observation and immunofluorescence staining. Astrocytes were infected with the adenovirus vector overexpressing HIPK2 for modification, and proliferation and apoptosis of astrocytes were examined using Cell Counting Kit-8 method and flow cytometry. SCI rat models were established and treated with astrocytes or HIPK2-modified astrocytes. Subsequently, rat motor ability was analyzed via the Basso-Beattie-Bresnahan (BBB) scoring and inclined-plane test, and the damage to spinal cord tissues and neuronal survival were observed via Hematoxylin-eosin staining and Nissl staining. The levels of HIPK2, brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and nuclear factor erythroid 2- related transcription factor 2 (Nrf2)/antioxidant response element (ARE) pathway-related proteins were detected.

RESULTS

Rat spinal astrocytes were harvested successfully. HIPK2 overexpression accelerated the proliferation and repressed the apoptosis of rat spinal astrocytes. Rat spinal astrocytes treatment increased BBB points and the maximum angle at which SCI rats remained stable, ameliorated damage to spinal cord tissues, increased the number of neurons, and attenuated neural damage and inflammation, while the treatment of HIPK2-modified rat spinal astrocytes imparted more pronounced effects to the neurofunctional recovery of SCI rats. Meanwhile, HIPK2-modified rat spinal astrocytes further activated the Nrf2/ARE pathway.

CONCLUSION

HIPK2-modified rat spinal astrocytes facilitated neurofunctional recovery and activated the Nrf2/ARE pathway after SCI.

Multiomic analysis identifies CPT1A as a potential therapeutic target in platinum-refractory, high-grade serous ovarian cancer

Resistance to platinum compounds is a major determinant of patient survival in high-grade serous ovarian cancer (HGSOC). To understand mechanisms of platinum resistance and identify potential therapeutic targets in resistant HGSOC, we generated a data resource composed of dynamic (±carboplatin) protein, post-translational modification, and RNA sequencing (RNA-seq) profiles from intra-patient cell line pairs derived from 3 HGSOC patients before and after acquiring platinum resistance. These profiles reveal extensive responses to carboplatin that differ between sensitive and resistant cells. Higher fatty acid oxidation (FAO) pathway expression is associated with platinum resistance, and both pharmacologic inhibition and CRISPR knockout of carnitine palmitoyltransferase 1A (CPT1A), which represents a rate limiting step of FAO, sensitize HGSOC cells to platinum. The results are further validated in patient-derived xenograft models, indicating that CPT1A is a candidate therapeutic target to overcome platinum resistance. All multiomic data can be queried via an intuitive gene-query user interface (https://sites.google.com/view/ptrc-cell-line).

Phosphorylation of CREB at Serine 142 and 143 Is Essential for Visual Cortex Plasticity

The transcription factor cAMP response element-binding protein (CREB) is involved in a myriad of cellular functions in the central nervous system. For instance, the role of CREB via phosphorylation at the amino-acid residue Serine (Ser)133 in expressing plasticity-related genes and activity-dependent neuronal plasticity processes has been extensively demonstrated. However, much less is known about the role of CREB phosphorylation at Ser142 and Ser143. Here, we employed a viral vector containing a dominant negative form of CREB, with serine-to-alanine mutations at residue 142 and 143 to specifically block phosphorylation at both sites. We then transfected this vector into primary neurons in vitro or intracortically injected it into mice in vivo, to test whether these phosphorylation events were important for activity-dependent plasticity. We demonstrated by immunohistochemistry of cortical neuronal cultures that the expression of Arc, a known plasticity-related gene, requires triple phosphorylation of CREB at Ser133, Ser142, and Ser143. Moreover, we recorded visually-evoked field potentials in awake mice before and after a 7-d period of monocular deprivation (MD) to show that, in addition to CREB phosphorylation at Ser133, ocular dominance plasticity (ODP) in the visual cortex also requires CREB phosphorylation at Ser142/143. Our findings suggest that Ser142/143 phosphorylation is an additional post-translational modification of CREB that triggers the expression of specific target genes and activity-dependent neuronal plasticity processes.

What turns CREB on? And off? And why does it matter?

Altered expression and function of the transcription factor cyclic AMP response-binding protein (CREB) has been identified to play an important role in cancer and is associated with the overall survival and therapy response of tumor patients. This review focuses on the expression and activation of CREB under physiologic conditions and in tumors of distinct origin as well as the underlying mechanisms of CREB regulation by diverse stimuli and inhibitors. In addition, the clinical relevance of CREB is summarized, including its use as a prognostic and/or predictive marker as well as a therapeutic target.

Deep Insights in Circular RNAs: from biogenesis to therapeutics

Abstract

Circular RNAs (circRNAs) have emerged as a universal novel class of eukaryotic non-coding RNA (ncRNA) molecules and are becoming a new research hotspot in RNA biology. They form a covalent loop without 5′ cap and 3′ tail, unlike their linear counterparts. Endogenous circRNAs in mammalian cells are abundantly conserved and discovered so far. In the biogenesis of circRNAs exonic, intronic, reverse complementary sequences or RNA-binding proteins (RBPs) play a very important role. Interestingly, the majority of them are highly conserved, stable, resistant to RNase R and show developmental-stage/tissue-specific expression. CircRNAs play multifunctional roles as microRNA (miRNA) sponges, regulators of transcription and post-transcription, parental gene expression and translation of proteins in various diseased conditions. Growing evidence shows that circRNAs play an important role in neurological disorders, atherosclerotic vascular disease, and cancer and potentially serve as diagnostic or predictive biomarkers due to its abundance in various biological samples. Here, we review the biogenesis, properties, functions, and impact of circRNAs on various diseases.

Graphical Abstract

Increased mAb production in amplified CHO cell lines is associated with increased interaction of CREB1 with transgene promoter

Most therapeutic monoclonal antibodies in biopharmaceutical processes are produced in Chinese hamster ovary (CHO) cells. Technological advances have rendered the selection procedure for higher producers a robust protocol. However, information on molecular mechanisms that impart the property of hyper-productivity in the final selected clones is currently lacking. In this study, an IgG-producing industrial cell line and its methotrexate (MTX)-amplified progeny cell line were analyzed using transcriptomic, proteomic, phosphoproteomic, and chromatin immunoprecipitation (ChIP) techniques. Computational prediction of transcription factor binding to the transgene cytomegalovirus (CMV) promoter by the Transcription Element Search System and upstream regulator analysis of the differential transcriptomic data suggested increased in vivo CMV promoter-cAMP response element binding protein (CREB1) interaction in the higher producing cell line. Differential nuclear proteomic analysis detected 1.3-fold less CREB1 in the nucleus of the high productivity cell line compared with the parental cell line. However, the differential abundance of multiple CREB1 phosphopeptides suggested an increase in CREB1 activity in the higher producing cell line, which was confirmed by increased association of the CMV promotor with CREB1 in the high producer cell line. Thus, we show here that the nuclear proteome and phosphoproteome have an important role in regulating final productivity of recombinant proteins from CHO cells, and that CREB1 may play a role in transcriptional enhancement. Moreover, CREB1 phosphosites may be potential targets for cell engineering for increased productivity.

ER Stress, CREB, and Memory: A Tangled Emerging Link in Disease

The brain undergoes several changes at structural, molecular, and cellular levels leading to alteration in its functions and these processes are primarily maintained by proteostasis in cells. However, an imbalance in proteostasis due to the abnormal accumulation of protein aggregates induces endoplasmic reticulum (ER) stress. This event, in turn, activate the unfolded protein response; however, in most neurodegenerative conditions and brain injury, an uncontrolled unfolded protein response elicits memory dysfunction. Although the underlying signaling mechanism for impairment of memory function following induction of ER stress remains elusive, recent studies have highlighted that inactivation of a transcription factor, CREB, which is essential for synaptic function and memory formation, plays an essential role for ER stress-induced synaptic and memory dysfunction. In this review, current studies and most updated view on how ER stress affects memory function in both physiological and pathological conditions will be highlighted.

An RNA interference screen identifies druggable regulators of MeCP2 stability

Alterations in gene dosage due to copy number variation are associated with autism spectrum disorder, intellectual disability (ID), and other psychiatric disorders. The nervous system is so acutely sensitive to the dose of methyl-CpG-binding protein 2 (MeCP2) that even a twofold change in MeCP2 protein-either increased or decreased-results in distinct disorders with overlapping features including ID, autistic behavior, and severe motor dysfunction. Rett syndrome is caused by loss-of-function mutations in MECP2, whereas duplications spanning the MECP2 locus result in MECP2 duplication syndrome (MDS), which accounts for ~1% of X-linked ID. Despite evidence from mouse models that restoring MeCP2 can reverse the course of disease, there are currently no U.S. Food and Drug Administration-approved therapies available to clinically modulate MeCP2 abundance. We used a forward genetic screen against all known human kinases and phosphatases to identify druggable regulators of MeCP2 stability. Two putative modulators of MeCP2, HIPK2 (homeodomain-interacting protein kinase 2) and PP2A (protein phosphatase 2A), were validated as stabilizers of MeCP2 in vivo. Further, pharmacological inhibition of PP2A in vivo reduced MeCP2 in the nervous system and rescued both overexpression and motor abnormalities in a mouse model of MDS. Our findings reveal potential therapeutic targets for treating disorders of altered MECP2 dosage.

A novel PDE9 inhibitor WYQ-C36D ameliorates corticosterone-induced neurotoxicity and depression-like behaviors by cGMP-CREB-related signaling

BACKGROUND

Major depressive disorder (MDD) is a mental disease characterized by depressed mood, lifetime anxiety, and deficits of learning and memory. Inhibition of phosphodiesterase 9 (PDE9) has been reported to improve rodent cognitive and memory function. However, the role of PDE9 in MDD, in particular its manifestations of depression and anxiety, has not been investigated.

METHODS

We examined the protective effects of WYQ-C36D (C36D), a novel PDE9 inhibitor, against corticosterone-induced cytotoxicity, pCREB/CREB and BDNF expression by cell viability, and immunoblot assays in HT-22 cells. The potential effects of C36D at doses of 0.1, 0.5, and 1 mg/kg on stress-induced depression- and anxiety-like behaviors and memory deficits were also examined in mice.

RESULTS

C36D significantly protected HT-22 cells against corticosterone-induced cytotoxicity and rescued corticosterone-induced decreases in cGMP, CREB phosphorylation, and BDNF expression. All these effects were otherwise blocked by the PKG inhibitor Rp-8-Br-PET-cGMPS (Rp8). In addition, when tested in vivo in stressed mice, C36D produced antidepressant-like effects on behavior, as shown by decreased immobility time both in the forced swimming and tail suspension tests. C36D also showed anxiolytic-like and memory-enhancing effects in the elevated plus-maze and novel object recognition tests.

CONCLUSION

Our results show that inhibition of PDE9 by C36D produces antidepressant- and anxiolytic-like behavioral effects and memory enhancement by activating cGMP/PKG signaling pathway. PDE9 inhibitors may have the potential as a novel class of drug to treat MDD.

Arsenic-Induced Activation of the Homeodomain-Interacting Protein Kinase 2 (HIPK2) to cAMP-Response Element Binding Protein (CREB) Axis

Cyclic AMP-response element-binding protein (CREB) plays key transcriptional roles in cell metabolism, proliferation, and survival. Ser133 phosphorylation by protein kinase A (PKA) is a well-characterized CREB activation mechanism. Homeodomain-interacting protein kinase (HIPK) 2, a nuclear serine/threonine kinase, activates CREB through Ser271 phosphorylation; however, the regulatory mechanism remains uncharacterized. Transfection of CREB in HEK293 cells together with the kinase demonstrated that HIPK2 phosphorylated CREB at Ser271 but not Ser133; likewise, PKA phosphorylated CREB at Ser133 but not Ser271, suggesting two distinct CREB regulatory mechanisms by HIPK2 and PKA. In vitro kinase assay revealed that HIPK2, and HIPK1 and HIPK3, directly phosphorylated CREB. Cells exposed to 10μM sodium arsenite increased the stability of HIPK1 and HIPK2 proteins, leading to CREB activation via Ser271 phosphorylation. Phospho-Ser271 CREB showed facilitated interaction with the TFIID subunit coactivator TAF4 assessed by immunoprecipitation. Furthermore, a focused gene array between cells transfected with CREB alone and CREB plus HIPK2 over empty vector-transfected control displayed 14- and 32-fold upregulation of cyclin A1, respectively, while no upregulation was displayed by HIPK2 alone. These results suggest that the HIPK2-phospho-Ser271 CREB axis is a new arsenic-responsive CREB activation mechanism in parallel with the PKA-phospho-Ser133 CREB axis.

Oxidative stress-induced CREB upregulation promotes DNA damage repair prior to neuronal cell death protection

cAMP response element-binding (CREB) protein is a cellular transcription factor that mediates responses to different physiological and pathological signals. Using a model of human neuronal cells we demonstrate herein, that CREB is phosphorylated after oxidative stress induced by hydrogen peroxide. This phosphorylation is largely independent of PKA and of the canonical phosphoacceptor site at ser-133, and is accompanied by an upregulation of CREB expression at both mRNA and protein levels. In accordance with previous data, we show that CREB upregulation promotes cell survival and that its silencing results in an increment of apoptosis after oxidative stress. Interestingly, we also found that CREB promotes DNA repair after treatment with hydrogen peroxide. Using a cDNA microarray we found that CREB is responsible for the regulation of many genes involved in DNA repair and cell survival after oxidative injury. In summary, the neuroprotective effect mediated by CREB appears to follow three essential steps following oxidative injury. First, the upregulation of CREB expression that allows sufficient level of activated and phosphorylated protein is the primordial event that promotes the induction of genes of the DNA Damage Response. Then and when the DNA repair is effective, CREB induces detoxification and survival genes. This kinetics seems to be important to completely resolve oxidative-induced neuronal damages.

Circular RNAs: the Emerging Class of Non-coding RNAs and Their Potential Role in Human Neurodegenerative Diseases

The exciting world of research with RNAs has to its credit some breakthrough findings that led to newer insights on multiple problems including that of human diseases. After the advent of siRNA, microRNA, and lncRNA, exciting novel molecules called circular RNAs (circRNAs) have been recently described. circRNAs are a class of non-coding RNAs, which are produced by scrambling of exons at the time of splicing. They are primarily produced in the brain region and are naturally present inside the cell. The best known ones so far include a particular type of circRNA namely "circular RNA sponge for miR-7" (ciRS-7 and CDR1as) which is the inhibitor of miR-7 microRNA-known to regulate various diseases like, cancer, neurodegenerative diseases, diabetes, and atherosclerosis. Similarly, another circRNA molecule called circmbl modulates the ratio of linear mRNA by competing with linear muscleblind gene through which it is synthesized. Considering the complex association of these molecules with critical microRNAs and gene families, circRNAs might have important roles in the cause and progression of human diseases. In particular, the multi-factorial nature of neurodegenerative diseases does warrant studies employing novel approaches towards identifying underlying root causes of these ailments. The non-coding RNAs, like circRNAs and microRNAs, could well present a common genetic trigger to multiple factors associated with neurodegenerative diseases. A specific fingerprint of a combination of various marker circRNAs could be explored for early diagnostic purpose as well. Herein, we review the possibility of exploring the role of circRNAs in the context of the central nervous system (CNS) and age-associated neurodegenerative diseases.

HIPK family kinases bind and regulate the function of the CCR4-NOT complex

Down-regulation of the HIPK interactor CNOT2 leads to reduced HIPK2 protein levels, identifying the CCR4-NOT complex as a new regulator of HIPK2 abundance. Functional assays reveal that HIPK2 and HIPK1 restrict CNOT2-dependent mRNA decay, thus extending the regulatory potential of these kinases to the level of posttranscriptional gene regulation.

The serine/threonine kinase HIPK2 functions as a regulator of developmental processes and as a signal integrator of a wide variety of stress signals, such as DNA damage, hypoxia, and reactive oxygen intermediates. Because the kinase is generated in a constitutively active form, its expression levels are restricted by a variety of different mechanisms. Here we identify the CCR4-NOT complex as a new regulator of HIPK2 abundance. Down-regulation or knockout of the CCR4-NOT complex member CNOT2 leads to reduced HIPK2 protein levels without affecting the expression level of HIPK1 or HIPK3. A fraction of all HIPK family members associates with the CCR4-NOT components CNOT2 and CNOT3. HIPKs also phosphorylate the CCR4-NOT complex, a feature that is shared with their yeast progenitor kinase, YAK1. Functional assays reveal that HIPK2 and HIPK1 restrict CNOT2-dependent mRNA decay. HIPKs are well known regulators of transcription, but the mutual regulation between CCR4-NOT and HIPKs extends the regulatory potential of these kinases by enabling posttranscriptional gene regulation.

Complex regulation of CREB-binding protein by homeodomain-interacting protein kinase 2

CREB-binding protein (CBP) and p300 are transcriptional coactivators involved in numerous biological processes that affect cell growth, transformation, differentiation, and development. In this study, we provide evidence of the involvement of homeodomain-interacting protein kinase 2 (HIPK2) in the regulation of CBP activity. We show that HIPK2 interacts with and phosphorylates several regions of CBP. We demonstrate that serines 2361, 2363, 2371, 2376, and 2381 are responsible for the HIPK2-induced mobility shift of CBP C-terminal activation domain. Moreover, we show that HIPK2 strongly potentiates the transcriptional activity of CBP. However, our data suggest that HIPK2 activates CBP mainly by counteracting the repressive action of cell cycle regulatory domain 1 (CRD1), located between amino acids 977 and 1076, independently of CBP phosphorylation. Our findings thus highlight a complex regulation of CBP activity by HIPK2, which might be relevant for the control of specific sets of target genes involved in cellular proliferation, differentiation and apoptosis.

Decrease of phosphorylated proto-oncogene CREB at Ser 133 site inhibits growth and metastatic activity of renal cell cancer

OBJECTIVE

Cyclic-AMP-responsive element-binding protein (CREB) is a proto-oncogenic transcription factor. The authors' previous reports showed that blocking the CREB binding site at Ser 133 inhibited the expression of target genes, which related to the progression of some tumors. In this study, the authors investigated the role of phosphorylated CREB (pCREB) at Ser133 in renal cell carcinoma (RCC) growth and metastases.

METHODS

Immunohistochemistry, xenograft model in nude mice, cell proliferation assay, cell invasion/migration assay, fluorescent immunocytochemistry and Western analysis were performed in an immortalized proximal tubule epithelial cell line and clear-cell RCC.

RESULTS

The authors' results showed that knockdown of pCREB inhibited kidney cancer cells growth in vivo. Furthermore, suppression of the pCREB level blunted the capabilities of cell migration and invasion in vitro and was accompanied with significantly decreased expression of MMP-2 and MMP-9, the filopodia formation and epithelial-mesenchymal transition-related proteins. Surprisingly, no changes of expression or location of vimentin were revealed in the experiment. Bioinformatic software explained the possible reason for this is that the promoter of vimentin does not contain the CRE sequence.

CONCLUSIONS

These data suggest that decreasing the level of pCREB inhibits the growth and metastasis of RCC by targeting the Ser 133 site.

Cyclin-dependent kinase 1-dependent phosphorylation of cAMP response element-binding protein decreases chromatin occupancy

The cyclic AMP response element-binding protein (CREB) initiates transcriptional responses to a wide variety of stimuli. CREB activation involves its phosphorylation on Ser-133, which promotes interaction between the CREB kinase-inducible domain (KID) and the KID-interacting domain of the transcriptional coactivator, CREB-binding protein (CBP). The KID also contains a highly conserved phosphorylation cluster, termed the ATM/CK cluster, which is processively phosphorylated in response to DNA damage by the coordinated actions of ataxia-telangiectasia-mutated (ATM) and casein kinases (CKs) 1 and 2. The ATM/CK cluster phosphorylation attenuates CBP binding and CREB transcriptional activity. Paradoxically, it was recently reported that DNA damage activates CREB through homeodomain-interacting protein kinase 2-dependent phosphorylation of Ser-271 near the CREB bZIP DNA binding domain. In this study we sought to further clarify DNA damage-dependent CREB phosphorylation as well as to explore the possibility that the ATM/CK cluster and Ser-271 synergistically or antagonistically modulate CREB activity. We show that, rather than being induced by DNA damage, Ser-270 and Ser-271 of CREB cophosphorylated in a CDK1-dependent manner during G2/M phase. Functionally, we show that phosphorylation of CREB on Ser-270/Ser-271 during mitosis correlated with reduced CREB chromatin occupancy. Furthermore, CDK1-dependent phosphorylation of CREB in vitro inhibited its DNA binding activity. The combined results suggest that CDK1-dependent phosphorylation of CREB on Ser-270/Ser-271 facilitates its dissociation from chromatin during mitosis by reducing its intrinsic DNA binding potential.

The PML-Interacting Protein DAXX: Histone Loading Gets into the Picture

The promyelocytic leukemia (PML) protein has been implicated in regulation of multiple key cellular functions, from transcription to calcium homeostasis. PML pleiotropic role is in part related to its ability to localize to both the nucleus and cytoplasm. In the nucleus, PML is known to regulate gene transcription, a role linked to its ability to associate with transcription factors as well as chromatin-remodelers. A new twist came from the discovery that the PML-interacting protein death-associated protein 6 (DAXX) acts as chaperone for the histone H3.3 variant. H3.3 is found enriched at active genes, centromeric heterochromatin, and telomeres, and has been proposed to act as important carrier of epigenetic information. Our recent work has implicated DAXX in regulation of H3.3 loading and transcription in the central nervous system (CNS). Remarkably, driver mutations in H3.3 and/or its loading machinery have been identified in brain cancer, thus suggesting a role for altered H3.3 function/deposition in CNS tumorigenesis. Aberrant H3.3 deposition may also play a role in leukemia pathogenesis, given DAXX role in PML-RARα-driven transformation and the identification of a DAXX missense mutation in acute myeloid leukemia. This review aims to critically discuss the existing literature and propose new avenues for investigation.

Phosphodiesterase-2 inhibitor reverses corticosterone-induced neurotoxicity and related behavioural changes via cGMP/PKG dependent pathway

Phosphodiesterase 2 (PDE2) is an enzyme responsible for hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) to restrict intracellular signalling of these second messenger molecules. This study investigated how PDE2 inhibitor Bay 60-7550 affects the dysregulated glucocorticoid signalling in neuronal cells and regulates depressive behaviours after chronic stress in mice. We found that exposure of hippocampal neurons to corticosterone resulted in time- and concentration-dependent increases in PDE2 expression. These intriguing findings were confirmed in the hippocampal cell line HT-22. After corticosterone exposure for 24 h, HT-22 cells showed a concentration-dependent increase in mRNA levels for PDE2 subtypes, PDE2A1 and 2A3, as well as for the total PDE2A protein expression. Bay 60-7550 was found to reverse the cell lesion induced by corticosterone (50 μm). This neuroprotective effect was blocked by pretreatment with protein kinase G inhibitor KT5823, but not protein kinase A inhibitor H89, suggesting the involvement of cGMP-dependent signalling. Although Bay 60-7550 treatment for 24 h did not change the levels of phosphorylated mitogen-activated protein kinases ERK1/2 (pERK) and phosphorylated cAMP response element-binding protein (pCREB), it down-regulated pERK at 2 h and up-regulated a CREB co-activator, CREB-binding protein, at 24 h. Both of these effects were blocked by KT 5823. Furthermore, Bay 60-7550 reversed corticosterone-induced down-regulation of brain-derived neurotrophic factor protein levels 24 h after corticosterone exposure. In behavioural testing, Bay 60-7550 produced antidepressant-like effects and reduced corticosterone levels in stressed mice, further supporting the involvement of a PDE2-dependent pathway in mediating Bay 60-7550's effect during stress hormone insults.

Intragenic complementation of hepatitis C virus NS5A RNA replication-defective alleles

Hepatitis C virus NS5A has three structural domains, is required for RNA replication and virion assembly, and exists in hypo- and hyperphosphorylated forms. Accumulated data suggest that phosphorylation is involved in modulating NS5A functions. We performed a mutational analysis of highly conserved serine residues in the linker region between domains I and II of genotype 2a JFH1 NS5A. As with genotype 1b Con1 NS5A, we found that specific serine residues were important for efficient hyperphosphorylation of JFH1 NS5A. However, in contrast with Con1 replicons, we observed a positive correlation between hyperphosphorylation and JFH1 replicon replication. We previously demonstrated trans-complementation of a hyperphosphorylation-deficient, replication-defective JFH1 replicon. Our results suggested that the defective NS5A encoded by this replicon, while lacking one NS5A function, was capable of performing a separate replication function. In this report, we examined an additional set of replication-defective NS5A mutations in trans-complementation assays. While some behaved similarly to the S232I replicon, others displayed a unique trans-complementation phenotype, suggesting that NS5A trans-complementation can occur by two distinct modes. Moreover, we were able, for the first time, to demonstrate intragenic complementation of replication-defective NS5A alleles. Our results identified three complementation groups: group A, comprising mutations within NS5A domain I; group B, comprising mutations affecting serine residues important for hyperphosphorylation and a subset of the domain I mutations; and group C, comprising a single mutation within the C-terminal region of domain II. We postulate that these complementation groups define three distinct and genetically separable functions of NS5A in RNA replication.

HIPK2: A tumour suppressor that controls DNA damage-induced cell fate and cytokinesis

In response to DNA-damage, cells have to decide between different cell fate programmes. Activation of the tumour suppressor HIPK2 specifies the DNA damage response (DDR) and tips the cell fate balance towards an apoptotic response. HIPK2 is activated by the checkpoint kinase ATM, and triggers apoptosis through regulatory phosphorylation of a set of cellular key molecules including the tumour suppressor p53 and the anti-apoptotic corepressor CtBP. Recent work has identified HIPK2 as a regulator of the ultimate step in cytokinesis: the abscission of the mother and daughter cells. Since proper cytokinesis is essential for genome stability and maintenance of correct ploidy, this finding sheds new light on the tumour suppressor function of HIPK2. Here we highlight the molecular mechanisms coordinating HIPK2 function and discuss its emerging role as a tumour suppressor.

Drosophila Smt3 negatively regulates JNK signaling through sequestering Hipk in the nucleus

Post-translational modification by the small ubiquitin-related modifier (SUMO) is important for a variety of cellular and developmental processes. However, the precise mechanism(s) that connects sumoylation to specific developmental signaling pathways remains relatively less clear. Here, we show that Smt3 knockdown in Drosophila wing discs causes phenotypes resembling JNK gain of function, including ectopic apoptosis and apoptosis-induced compensatory growth. Smt3 depletion leads to an increased expression of JNK target genes Mmp1 and puckered. We show that, although knockdown of the homeodomain-interacting protein kinase (Hipk) suppresses Smt3 depletion-induced activation of JNK, Hipk overexpression synergistically enhances this type of JNK activation. We further demonstrate that Hipk is sumolylated in vivo, and its nuclear localization is dependent on the sumoylation pathway. Our results thus establish a mechanistic connection between the sumoylation pathway and the JNK pathway through the action of Hipk. We propose that the sumoylation-controlled balance between cytoplasmic and nuclear Hipk plays a crucial role in regulating JNK signaling.

CREB: a multifaceted regulator of neuronal plasticity and protection

Since its initial characterization over 20 years ago, there has been intense and unwavering interest in understanding the role of the transcription factor cAMP-responsive element binding protein (CREB) in nervous system physiology. Through an array of experimental approaches and model systems, researchers have begun to unravel the complex and multifaceted role of this transcription factor in such diverse processes as neurodevelopment, synaptic plasticity, and neuroprotection. Here we discuss current insights into the molecular mechanisms by which CREB couples synaptic activity to long-term changes in neuronal plasticity, which is thought to underlie learning and memory. We also discuss work showing that CREB is a critical component of the neuroprotective transcriptional network, and data indicating that CREB dysregulation contributes to an array of neuropathological conditions.

Transcriptional regulation of ferritin and antioxidant genes by HIPK2 under genotoxic stress

ATF1 (activating transcription factor 1), a stimulus-induced CREB family transcription factor, plays important roles in cell survival and proliferation. Phosphorylation of ATF1 at Ser63 by PKA (cAMP-dependent protein kinase) and related kinases was the only known post-translational regulatory mechanism of ATF1. Here, we found that HIPK2 (homeodomain-interacting protein kinase 2), a DNA-damage-responsive nuclear kinase, is a new ATF1 kinase that phosphorylates Ser198 but not Ser63. ATF1 phosphorylation by HIPK2 activated ATF1 transcription function in the GAL4-reporter system. ATF1 is a transcriptional repressor of ferritin H, the major intracellular iron storage gene, through an ARE (antioxidant-responsive element). HIPK2 overrode the ATF1-mediated ARE repression in a kinase-activity-dependent manner in HepG2 cells. Furthermore, DNA-damage-inducing agents doxorubicin, etoposide and sodium arsenite induced ferritin H mRNA expression in HIPK2(+/+) MEF cells, whereas it was significantly impaired in HIPK2(-/-) MEF cells. Induction of other ARE-regulated detoxification genes such as NQO1 (NADPH quinone oxidoreductase 1), GST (glutathione S-transferase) and HO1 (heme oxygenase 1) by genotoxic stress was also decreased in HIPK2-deficient cells. Taken together, these results suggest that HIPK2 is a new ATF1 kinase involved in the regulation of ferritin H and other antioxidant detoxification genes in genotoxic stress conditions.