Drug Discovery of Host CLK1 Inhibitors for Influenza Treatment

Cdc2-like kinases: structure, biological function, and therapeutic targets for diseases

The CLKs (Cdc2-like kinases) belong to the dual-specificity protein kinase family and play crucial roles in regulating transcript splicing via the phosphorylation of SR proteins (SRSF1-12), catalyzing spliceosome molecular machinery, and modulating the activities or expression of non-splicing proteins. The dysregulation of these processes is linked with various diseases, including neurodegenerative diseases, Duchenne muscular dystrophy, inflammatory diseases, viral replication, and cancer. Thus, CLKs have been considered as potential therapeutic targets, and significant efforts have been exerted to discover potent CLKs inhibitors. In particular, clinical trials aiming to assess the activities of the small molecules Lorecivivint on knee Osteoarthritis patients, and Cirtuvivint and Silmitasertib in different advanced tumors have been investigated for therapeutic usage. In this review, we comprehensively documented the structure and biological functions of CLKs in various human diseases and summarized the significance of related inhibitors in therapeutics. Our discussion highlights the most recent CLKs research, paving the way for the clinical treatment of various human diseases.

Roles of alternative splicing in infectious diseases: from hosts, pathogens to their interactions

ABSTRACT

Alternative splicing (AS) is an evolutionarily conserved mechanism that removes introns and ligates exons to generate mature messenger RNAs (mRNAs), extremely improving the richness of transcriptome and proteome. Both mammal hosts and pathogens require AS to maintain their life activities, and inherent physiological heterogeneity between mammals and pathogens makes them adopt different ways to perform AS. Mammals and fungi conduct a two-step transesterification reaction by spliceosomes to splice each individual mRNA (named cis -splicing). Parasites also use spliceosomes to splice, but this splicing can occur among different mRNAs (named trans -splicing). Bacteria and viruses directly hijack the host's splicing machinery to accomplish this process. Infection-related changes are reflected in the spliceosome behaviors and the characteristics of various splicing regulators (abundance, modification, distribution, movement speed, and conformation), which further radiate to alterations in the global splicing profiles. Genes with splicing changes are enriched in immune-, growth-, or metabolism-related pathways, highlighting approaches through which hosts crosstalk with pathogens. Based on these infection-specific regulators or AS events, several targeted agents have been developed to fight against pathogens. Here, we summarized recent findings in the field of infection-related splicing, including splicing mechanisms of pathogens and hosts, splicing regulation and aberrant AS events, as well as emerging targeted drugs. We aimed to systemically decode host-pathogen interactions from a perspective of splicing. We further discussed the current strategies of drug development, detection methods, analysis algorithms, and database construction, facilitating the annotation of infection-related splicing and the integration of AS with disease phenotype.

An overview of cdc2-like kinase 1 (Clk1) inhibitors and their therapeutic indications

Over the past decade, Clk1 has been identified as a promising target for the treatment of various diseases, in which deregulated alternative splicing plays a role. First small molecules targeting Clk1 are in clinical trials for the treatment of solid cancer, where variants of oncogenic proteins derived from alternative splicing promote tumor progression. Since many infectious pathogens hi-jack the host cell's splicing machinery to ensure efficient replication, further indications in this area are under investigation, such as Influenza A, HIV-1 virus, and Trypanosoma infections, and more will likely be discovered in the future. In addition, Clk1 was found to contribute to the progression of Alzheimer's disease through causing an imbalance of tau splicing products. Interestingly, homozygous Clk1 knockout mice showed a rather mild phenotype, opposed to what might be expected in view of the profound role of Clk1 in alternative splicing. A major drawback of most Clk1 inhibitors is their insufficient selectivity; in particular, Dyrk kinases and haspin were frequently identified as off-targets, besides the other Clk isoforms. Only few inhibitors were shown to be selective over Dyrk1A and haspin, whereas no Clk1 inhibitor so far achieved selectivity over the Clk4 isoform. In this review, we carefully compiled all Clk1 inhibitors from the scientific literature and summarized their structure-activity relationships (SAR). In addition, we critically discuss the available selectivity data and describe the inhibitor's efficacy in cellular models, if reported. Thus, we provide a comprehensive overview on the current state of Clk1 drug discovery and highlight the most promising chemotypes.

Discovery of novel 5-methoxybenzothiophene hydrazides as metabolically stable Clk1 inhibitors with high potency and unprecedented Clk1 isoenzyme selectivity

Clk1 kinase is a key modulator of the pre-mRNA alternative splicing machinery which has been proposed as a promising target for treatment of various tumour types, Duchenne's muscular dystrophy and viral infections such as HIV-1 and influenza. Most reported Clk1 inhibitors showed significant co-inhibition of Clk2 and Clk4 in particular, which limits their usefulness for deciphering the individual roles of the Clk1 isoform in physiology and disease. Herein, we present a new 5-methoxybenzothiophene scaffold, enabling for the first time selective inhibition of Clk1 even among the isoenzymes. The 3,5-difluorophenyl and 3,5-dichlorophenyl derivatives 26a and 27a (Clk1 IC₅₀ = 1.4 and 1.7 nM, respectively) showed unprecedented selectivity factors of 15 and 8 over Clk4, and selectivity factors of 535 and 84 over Clk2. Furthermore, 26a and 27a exhibited good growth inhibitory activity in T24 cancer cells and long metabolic half-lives of almost 1 and 6.4 h, respectively. The overall favorable profile of our new Clk1 inhibitors suggests that they may be used in in vivo disease models or as probes to unravel the physiological or pathogenic roles of the Clk1 isoenzyme.

A new self-attenuated therapeutic influenza vaccine that uses host cell-restricted attenuation by artificial microRNAs

New strategies are urgently needed for developing vaccines and/or anti-viral drugs against influenza viruses, because antigenic shift and drift inevitably occurs in circulating strains each year, and new strains resistant to anti-viral drugs have recently emerged. In our study, we designed and incorporated artificial microRNAs (amiRNAs) into the NA segment of rescued influenza viruses to separately target two host genes, Cdc2-like kinase 1 (CLK1) and SON DNA binding protein (SON), which were found to play an essential role in virus replication. Mouse epithelial fibroblast (MEF) or human lung carcinoma A549 cells infected with engineered influenza PR8 viruses expressing amiR-30CLK1 (PR8-amiR-30CLK1) or amiR-93SON (PR8-amiR-93SON) had reduced expression of host proteins CLK1 and SON, respectively. All engineered influenza viruses functioned as attenuated vaccines, induced significantly higher antibody responses, and provided greater protective efficacy. In addition, they were found to be safe, based on the mouse weight changes and clinical signs observed. In contrast to the engineered viruses targeting SON, mice treated with engineered viruses targeting CLK1 recovered from weight loss and survived lethal infection by 6 h after lethal-dose PR8 infection, suggesting that our PR8-amiR-30CLK1 self-attenuated influenza virus (SAIV) could be used as a new therapeutic influenza vaccine.

In Silico Prediction and Bioactivity Evaluation of Chemical Ingredients Against Influenza A Virus From Isatis tinctoria L

Influenza A virus (IAV) is one of the major causes of seasonal endemic diseases and unpredictable periodic pandemics. Due to the high mutation rate and drug resistance, it poses a persistent threat and challenge to public health. Isatis tinctoria L. (Banlangen, BLG), a traditional herbal medicine widely used in Asian countries, has been reported to possess strong efficacy on respiratory viruses, including IAV. However, its effective anti-IAV components and the mechanism of actions (MOAs) are not yet fully elucidated. In this study, we first summarized the chemical components and corresponding contents in BLG according to current available chemical analysis literature. We then presented a network-based in silico framework for identifying potential drug candidates against IAV from BLG. A total of 269 components in BLG were initially screened by drug-likeness and ADME (absorption, distribution, metabolism, and excretion) evaluation. Thereafter, network predictive models were built via the integration of compound–target networks and influenza virus–host proteins. We highlighted 23 compounds that possessed high potential as anti-influenza virus agents. Through experimental evaluation, six compounds, namely, eupatorin, dinatin, linarin, tryptanthrin, indirubin, and acacetin, exhibited good inhibitory activity against wild-type H1N1 and H3N2. Particularly, they also exerted significant effects on drug-resistant strains. Finally, we explored the anti-IAV MOAs of BLG and showcased the potential biological pathways by systems pharmacology analysis. In conclusion, this work provides important information on BLG regarding its use in the development of anti-IAV drugs, and the network-based prediction framework proposed here also offers a powerfulful strategy for the in silico identification of novel drug candidates from complex components of herbal medicine.

Flavonoids as Promising Antiviral Agents against SARS-CoV-2 Infection: A Mechanistic Review

A newly diagnosed coronavirus in 2019 (COVID-19) has affected all human activities since its discovery. Flavonoids commonly found in the human diet have attracted a lot of attention due to their remarkable biological activities. This paper provides a comprehensive review of the benefits of flavonoids in COVID-19 disease. Previously-reported effects of flavonoids on five RNA viruses with similar clinical manifestations and/or pharmacological treatments, including influenza, human immunodeficiency virus (HIV), severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and Ebola, were considered. Flavonoids act via direct antiviral properties, where they inhibit different stages of the virus infective cycle and indirect effects when they modulate host responses to viral infection and subsequent complications. Flavonoids have shown antiviral activity via inhibition of viral protease, RNA polymerase, and mRNA, virus replication, and infectivity. The compounds were also effective for the regulation of interferons, pro-inflammatory cytokines, and sub-cellular inflammatory pathways such as nuclear factor-κB and Jun N-terminal kinases. Baicalin, quercetin and its derivatives, hesperidin, and catechins are the most studied flavonoids in this regard. In conclusion, dietary flavonoids are promising treatment options against COVID-19 infection; however, future investigations are recommended to assess the antiviral properties of these compounds on this disease.

Transcriptome analysis of the bursa of Fabricius and thymus of laying ducks reveals immune gene expression changes underlying the impacts of stocking densities

The thymus and bursa of Fabricius are important immune organs in poultry as they play essential roles in sustaining the normal immune function to maintain health. The following trial investigated whether the stocking density affected gene expressions in immune organs.Jinding ducklings were raised in either low or high density (4 or 8 birds/m²) conditions from four to 14 weeks of age, and were then slaughtered and tissues removed. Samples were subjected to high-throughput sequencing to sequence RNA extraction. After filtering calculations with R software, a total of 508 (thymus) and 1,356 (bursa of Fabricius) differentially expressed genes (DEGs) were identified, suggesting that stocking density has an effect on gene expression in duck immune organs.A total of 112 immune factor genes and 112 immune pattern receptor genes in ducks, of which four thymus genes and 18 bursa of Fabricius genes were differentially expressed in ducks, which indicated that the change of stocking density could affect the expression of immune genes in poultry.

Dual-Specificity, Tyrosine Phosphorylation-Regulated Kinases (DYRKs) and cdc2-Like Kinases (CLKs) in Human Disease, an Overview

Dual-specificity tyrosine phosphorylation-regulated kinases (DYRK1A, 1B, 2-4) and cdc2-like kinases (CLK1-4) belong to the CMGC group of serine/threonine kinases. These protein kinases are involved in multiple cellular functions, including intracellular signaling, mRNA splicing, chromatin transcription, DNA damage repair, cell survival, cell cycle control, differentiation, homocysteine/methionine/folate regulation, body temperature regulation, endocytosis, neuronal development, synaptic plasticity, etc. Abnormal expression and/or activity of some of these kinases, DYRK1A in particular, is seen in many human nervous system diseases, such as cognitive deficits associated with Down syndrome, Alzheimer's disease and related diseases, tauopathies, dementia, Pick's disease, Parkinson's disease and other neurodegenerative diseases, Phelan-McDermid syndrome, autism, and CDKL5 deficiency disorder. DYRKs and CLKs are also involved in diabetes, abnormal folate/methionine metabolism, osteoarthritis, several solid cancers (glioblastoma, breast, and pancreatic cancers) and leukemias (acute lymphoblastic leukemia, acute megakaryoblastic leukemia), viral infections (influenza, HIV-1, HCMV, HCV, CMV, HPV), as well as infections caused by unicellular parasites (Leishmania, Trypanosoma, Plasmodium). This variety of pathological implications calls for (1) a better understanding of the regulations and substrates of DYRKs and CLKs and (2) the development of potent and selective inhibitors of these kinases and their evaluation as therapeutic drugs. This article briefly reviews the current knowledge about DYRK/CLK kinases and their implications in human disease.

CDC2-like (CLK) protein kinase inhibition as a novel targeted therapeutic strategy in prostate cancer

Dysregulation of alternative splicing is a feature of cancer, both in aetiology and progression. It occurs because of mutations in splice sites or sites that regulate splicing, or because of the altered expression and activity of splice factors and of splice factor kinases that regulate splice factor activity. Recently the CDC2-like kinases (CLKs) have attracted attention due to their increasing involvement in cancer. We measured the effect of the CLK inhibitor, the benzothiazole TG003, on two prostate cancer cell lines. TG003 reduced cell proliferation and increased apoptosis in PC3 and DU145 cells. Conversely, the overexpression of CLK1 in PC3 cells prevented TG003 from reducing cell proliferation. TG003 slowed scratch closure and reduced cell migration and invasion in a transwell assay. TG003 decisively inhibited the growth of a PC3 cell line xenograft in nude mice. We performed a transcriptomic analysis of cells treated with TG003. We report widespread and consistent changes in alternative splicing of cancer-associated genes including CENPE, ESCO2, CKAP2, MELK, ASPH and CD164 in both HeLa and PC3 cells. Together these findings suggest that targeting CLKs will provide novel therapeutic opportunities in prostate cancer.

Interplay Between CMGC Kinases Targeting SR Proteins and Viral Replication: Splicing and Beyond

Protein phosphorylation constitutes a major post-translational modification that critically regulates the half-life, intra-cellular distribution, and activity of proteins. Among the large number of kinases that compose the human kinome tree, those targeting RNA-binding proteins, in particular serine/arginine-rich (SR) proteins, play a major role in the regulation of gene expression by controlling constitutive and alternative splicing. In humans, these kinases belong to the CMGC [Cyclin-dependent kinases (CDKs), Mitogen-activated protein kinases (MAPKs), Glycogen synthase kinases (GSKs), and Cdc2-like kinases (CLKs)] group and several studies indicate that they also control viral replication via direct or indirect mechanisms. The aim of this review is to describe known and emerging activities of CMGC kinases that share the common property to phosphorylate SR proteins, as well as their interplay with different families of viruses, in order to advance toward a comprehensive knowledge of their pro- or anti-viral phenotype and better assess possible translational opportunities.

Anti-Influenza Drug Discovery and Development: Targeting the Virus and Its Host by All Possible Means

Infections by influenza virus constitute a major and recurrent threat for human health. Together with vaccines, antiviral drugs play a key role in the prevention and treatment of influenza virus infection and disease. Today, the number of antiviral molecules approved for the treatment of influenza is relatively limited, and their use is threatened by the emergence of viral strains with resistance mutations. There is therefore a real need to expand the prophylactic and therapeutic arsenal. This chapter summarizes the state of the art in drug discovery and development for the treatment of influenza virus infections, with a focus on both virus-targeting and host cell-targeting strategies. Novel antiviral strategies targeting other viral proteins or targeting the host cell, some of which are based on drug repurposing, may be used in combination to strengthen our therapeutic arsenal against this major pathogen.

Viral-induced alternative splicing of host genes promotes influenza replication

Viral infection induces the expression of numerous host genes that impact the outcome of infection. Here, we show that infection of human lung epithelial cells with influenza A virus (IAV) also induces a broad program of alternative splicing of host genes. Although these splicing-regulated genes are not enriched for canonical regulators of viral infection, we find that many of these genes do impact replication of IAV. Moreover, in several cases, specific inhibition of the IAV-induced splicing pattern also attenuates viral infection. We further show that approximately a quarter of the IAV-induced splicing events are regulated by hnRNP K, a host protein required for efficient splicing of the IAV M transcript in nuclear speckles. Finally, we find an increase in hnRNP K in nuclear speckles upon IAV infection, which may alter accessibility of hnRNP K for host transcripts thereby leading to a program of host splicing changes that promote IAV replication.

Expression, purification and crystallization of CLK1 kinase - A potential target for antiviral therapy

Cdc-like kinase 1 (CLK1) is a dual-specificity kinase capable of autophosphorylation on tyrosine residues and Ser/Thr phosphorylation of its substrates. CLK1 belongs to the CLK kinase family that regulates alternative splicing through phosphorylation of serine-arginine rich (SR) proteins. Recent studies have demonstrated that CLK1 has an important role in the replication of influenza A and chikungunya viruses. Furthermore, CLK1 was found to be relevant for the replication of HIV-1 and the West Nile virus, making CLK1 an interesting cellular candidate for the development of a host-directed antiviral therapy that might be efficient for treatment of newly emerging viruses. We describe here our attempts and detailed procedures to obtain the recombinant kinase domain of CLK1 in suitable amounts for crystallization in complex with specific inhibitors. The key solution for the reproducibility of crystals resides in devising and refining expression and purification protocols leading to homogeneous protein. Co-expression of CLK1 with λ-phosphatase and careful purification has yielded crystals of CLK1 complexed with the KH-CB19 inhibitor that diffracted to 1.65 Å. These results paved the path to the screening of more structures of CLK1 complexed compounds, leading to further optimization of their inhibitory activity. Moreover, since kinases are desired targets in numerous pathologies, the approach we report here, the co-expression of kinases with λ-phosphatase, previously used in other kinases, can be adopted as a general protocol in numerous kinase targets for obtaining reproducible and homogenic non-phosphorylated (inactive) forms suitable for biochemical and structural studies thus facilitating the development of novel inhibitors.

Host-Directed Antiviral Therapy

Antiviral drugs have traditionally been developed by directly targeting essential viral components. However, this strategy often fails due to the rapid generation of drug-resistant viruses. Recent genome-wide approaches, such as those employing small interfering RNA (siRNA) or clustered regularly interspaced short palindromic repeats (CRISPR) or those using small molecule chemical inhibitors targeting the cellular "kinome," have been used successfully to identify cellular factors that can support virus replication. Since some of these cellular factors are critical for virus replication, but are dispensable for the host, they can serve as novel targets for antiviral drug development. In addition, potentiation of immune responses, regulation of cytokine storms, and modulation of epigenetic changes upon virus infections are also feasible approaches to control infections. Because it is less likely that viruses will mutate to replace missing cellular functions, the chance of generating drug-resistant mutants with host-targeted inhibitor approaches is minimized. However, drug resistance against some host-directed agents can, in fact, occur under certain circumstances, such as long-term selection pressure of a host-directed antiviral agent that can allow the virus the opportunity to adapt to use an alternate host factor or to alter its affinity toward the target that confers resistance. This review describes novel approaches for antiviral drug development with a focus on host-directed therapies and the potential mechanisms that may account for the acquisition of antiviral drug resistance against host-directed agents.

Identification of Chebulinic Acid and Chebulagic Acid as Novel Influenza Viral Neuraminidase Inhibitors

The influenza A virus (IAV) causes seasonal epidemics and occasional but devastating pandemics, which are of a major public health concern. Although several antiviral drugs are currently available, there is an urgent need to develop novel antiviral therapies with different mechanisms of action due to emergence of drug resistance. In this study, two related compounds, chebulagic acid (CHLA) and chebulinic acid (CHLI), were identified as novel inhibitors against IAV replication. A reporter virus-based infection assay demonstrated that CHLA and CHLI exhibit no inhibitory effect on IAV entry or RNA replication during the virus replication cycle. Results of viral release inhibition assay and neuraminidase (NA) inhibition assay indicated that CHLA and CHLI exert their inhibitory effect on the NA-mediated viral release. Moreover, oseltamivir-resistance mutation NA/H274Y of NA is susceptible to CHLA or CHLI, suggesting a different mechanism of action for CHLA and CHLI. In summary, CHLA and CHLI are promising new NA inhibitors that may be further developed as novel antivirals against IAVs.

Discovery of Multitarget-Directed Ligands Against Influenza A Virus From Compound Yizhihao Through a Predictive System for Compound-Protein Interactions

Influenza A virus (IAV) is a threat to public health due to its high mutation rate and resistance to existing drugs. In this investigation, 15 targets selected from an influenza virus–host interaction network were successfully constructed as a multitarget virtual screening system for new drug discovery against IAV using Naïve Bayesian, recursive partitioning, and CDOCKER methods. The predictive accuracies of the models were evaluated using training sets and test sets. The system was then used to predict active constituents of Compound Yizhihao (CYZH), a Chinese medicinal compound used to treat influenza. Twenty-eight compounds with multitarget activities were selected for subsequent in vitro evaluation. Of the four compounds predicted to be active on neuraminidase (NA), chlorogenic acid, and orientin showed inhibitory activity in vitro. Linarin, sinensetin, cedar acid, isoliquiritigenin, sinigrin, luteolin, chlorogenic acid, orientin, epigoitrin, and rupestonic acid exhibited significant effects on TNF-α expression, which is almost consistent with predicted results. Results from a cytopathic effect (CPE) reduction assay revealed acacetin, indirubin, tryptanthrin, quercetin, luteolin, emodin, and apigenin had protective effects against wild-type strains of IAV. Quercetin, luteolin, and apigenin had good efficacy against resistant IAV strains in CPE reduction assays. Finally, with the aid of Gene Ontology biological process analysis, the potential mechanisms of CYZH action were revealed. In conclusion, a compound-protein interaction-prediction system was an efficient tool for the discovery of novel compounds against influenza, and the findings from CYZH provide important information for its usage and development.

Disordered protein interactions for an ordered cellular transition: Cdc2-like kinase 1 is transported to the nucleus via its Ser-Arg protein substrate

Serine-arginine (SR) proteins are essential splicing factors that promote numerous steps associated with mRNA processing and whose biological function is tightly regulated through multi-site phosphorylation. In the nucleus, the cdc2-like kinases (CLKs) phosphorylate SR proteins on their intrinsically disordered Arg-Ser (RS) domains, mobilizing them from storage speckles to the splicing machinery. The CLKs have disordered N termini that bind tightly to RS domains, enhancing SR protein phosphorylation. The N termini also promote nuclear localization of CLKs, but their transport mechanism is presently unknown. To explore cytoplasmic-nuclear transitions, several classical nuclear localization sequences in the N terminus of the CLK1 isoform were identified, but their mutation had no effect on subcellular localization. Rather, we found that CLK1 amplifies its presence in the nucleus by forming a stable complex with the SR protein substrate and appropriating its NLS for transport. These findings indicate that, along with their well-established roles in mRNA splicing, SR proteins use disordered protein-protein interactions to carry their kinase regulator from the cytoplasm to the nucleus.

Anti-influenza effect and action mechanisms of the chemical constituent gallocatechin-7-gallate from Pithecellobium clypearia Benth

Host cdc2-like kinase 1 (CLK1) is responsible for the alternative splicing of the influenza virus M2 gene during influenza virus infection and replication that has been recognized as a potential anti-influenza virus target. In this study, we showed that gallocatechin-7-gallate (J10688), a novel CLK1 inhibitor isolated from Pithecellobium clypearia Benth, exerted potent anti-influenza virus activity in vivo and in vitro. ICR mice were intranasally infected with a lethal dose of H1N1. Administration of J10688 (30 mg·kg^-1·d^-1, iv, for 5 days) significantly increased the survival rate of the H1N1-infected mice to 91.67% and prolong their mean survival time from 5.83 ± 1.74 days to 13.66 ± 1.15 days. J10688 administration also slowed down body weight loss, significantly alleviated influenza-induced acute lung injury, reduced lung virus titer, elevated the spleen and thymus indexes, and enhanced the immunological function. We further explored its anti-influenza mechanisms in the H1N1-infected A549 cells: as a novel CLK1 inhibitor, J10688 (3, 10, 30 μmol/L) dose-dependently impaired synthesis of the viral proteins NP and M2, and significantly downregulated the phosphorylation of splicing factors SF2/ASF and SC35, which regulate virus M2 gene alternative splicing. As a novel CLK1 inhibitor with potent anti-influenza activity in vitro and in vivo, J10688 could be a promising antiviral drug for the therapy of influenza A virus infection.

Advancements in Host-Based Interventions for Influenza Treatment

Influenza is a major acute respiratory infection that causes mortality and morbidity worldwide. Two classes of conventional antivirals, M2 ion channel blockers and neuraminidase inhibitors, are mainstays in managing influenza disease to lessen symptoms while minimizing hospitalization and death in patients with severe influenza. However, the development of viral resistance to both drug classes has become a major public health concern. Vaccines are prophylaxis mainstays but are limited in efficacy due to the difficulty in matching predicted dominant viral strains to circulating strains. As such, other potential interventions are being explored. Since viruses rely on host cellular functions to replicate, recent therapeutic developments focus on targeting host factors involved in virus replication. Besides controlling virus replication, potential targets for drug development include controlling virus-induced host immune responses such as the recently suggested involvement of innate lymphoid cells and NADPH oxidases in influenza virus pathogenesis and immune cell metabolism. In this review, we will discuss the advancements in novel host-based interventions for treating influenza disease.

Autoregulation of the human splice factor kinase CLK1 through exon skipping and intron retention

Alternative splicing is a key process required for the regulation of gene expression in normal development and physiology. It is regulated by splice factors whose activities are in turn regulated by splice factor kinases and phosphatases. The CDC-like protein kinases are a widespread family of splice factor kinases involved in normal physiology and in several diseases including cancer. In humans they include the CLK1, CLK2, CLK3 and CLK4 genes. The expression of CLK1 is regulated through alternative splicing producing both full-length catalytically active and truncated catalytically inactive isoforms, CLK^T1 (arising from exon 4 skipping) and CLK^T2 (arising from intron 4 retention). We examined CLK1 alternative splicing in a range of cancer cell lines, and report widespread and highly variable rates of exon 4 skipping and intron 4 retention. We also examined the effect of severe environmental stress including heat shock, osmotic shock, and exposure to the alkaloid drug harmine on CLK1 alternative splicing in DU145 prostate cancer cells. All treatments rapidly reduced exon 4 skipping and intron 4 retention, shifting the balance towards full-length CLK1 expression. We also found that the inhibition of CLK1 with the benzothiazole TG003 reduced exon 4 skipping and intron 4 retention suggesting an autoregulatory mechanism. CLK1 inhibition with TG003 also resulted in modified alternative splicing of five cancer-associated genes.

Comparative Protein Interaction Network Analysis Identifies Shared and Distinct Functions for the Human ROCO Proteins

Signal transduction cascades governed by kinases and GTPases are a critical component of the command and control of cellular processes, with the precise outcome partly determined by direct protein-protein interactions (PPIs). Here, we use the human ROCO proteins as a model for investigating PPI signaling events-taking advantage of the unique dual kinase/GTPase activities and scaffolding properties of these multidomain proteins. PPI networks are reported that encompass the human ROCO proteins, developed using two complementary approaches. First, using the recently developed weighted PPI network analysis (WPPINA) pipeline, a confidence-weighted overview of validated ROCO protein interactors is obtained from peer-reviewed literature. Second, novel ROCO PPIs are assessed experimentally via protein microarray screens. The networks derived from these orthologous approaches are compared to identify common elements within the ROCO protein interactome; functional enrichment analysis of this common core of the network identified stress response and cell projection organization as shared functions within this protein family. Despite the presence of these commonalities, the results suggest that many unique interactors and therefore some specialized cellular roles have evolved for different members of the ROCO proteins. Overall, this multi-approach strategy to increase the resolution of protein interaction networks represents a prototype for the utility of PPI data integration in understanding signaling biology.

Hypoxia leads to significant changes in alternative splicing and elevated expression of CLK splice factor kinases in PC3 prostate cancer cells

BACKGROUND

Mounting evidence suggests that one of the ways that cells adapt to hypoxia is through alternative splicing. The aim of this study was firstly to examine the effect of hypoxia on the alternative splicing of cancer associated genes using the prostate cancer cell line PC3 as a model. Secondly, the effect of hypoxia on the expression of several regulators of splicing was examined.

METHODS

PC3 cells were grown in 1% oxygen in a hypoxic chamber for 48 h, RNA extracted and sent for high throughput PCR analysis at the RNomics platform at the University of Sherbrooke, Canada. Genes whose exon inclusion rate PSI (ψ) changed significantly were identified, and their altered exon inclusion rates verified by RT-PCR in three cell lines. The expression of splice factors and splice factor kinases in response to hypoxia was examined by qPCR and western blotting. The splice factor kinase CLK1 was inhibited with the benzothiazole TG003.

RESULTS

In PC3 cells the exon inclusion rate PSI (ψ) was seen to change by > 25% in 12 cancer-associated genes; MBP, APAF1, PUF60, SYNE2, CDC42BPA, FGFR10P, BTN2A2, UTRN, RAP1GDS1, PTPN13, TTC23 and CASP9 (caspase 9). The expression of the splice factors SRSF1, SRSF2, SRSF3, SAM68, HuR, hnRNPA1, and of the splice factor kinases SRPK1 and CLK1 increased significantly in hypoxia. We also observed that the splice factor kinase CLK3, but not CLK2 and CLK4, was also induced in hypoxic DU145 prostate, HT29 colon and MCF7 breast cancer cell lines. Lastly, we show that the inhibition of CLK1 in PC3 cells with the benzothiazole TG003 increased expression of the anti-apoptotic isoform caspase 9b.

CONCLUSIONS

Significant changes in alternative splicing of cancer associated genes occur in prostate cancer cells in hypoxic conditions. The expression of several splice factors and splice factor kinases increases during hypoxia, in particular the Cdc-like splice factor kinases CLK1 and CLK3. We suggest that in hypoxia the elevated expression of these regulators of splicing helps cells adapt through alternative splicing of key cancer-associated genes. We suggest that the CLK splice factor kinases could be targeted in cancers in which hypoxia contributes to resistance to therapy.

Synthesis of novel pentacyclic triterpene-Neu5Ac2en derivatives and investigation of their in vitro anti-influenza entry activity

Sialic acid derivatives, analogs, and their conjugates are important pharmacophores. Modification of the C-4 hydroxyl group of sialic acid can lead to derivatives, such as zanamivir, with potent anti-influenza activities. Herein, we described the synthesis of C-4-modified sialic acid derivatives via conjugation with naturally derived pentacyclic triterpenes, which are active ingredients of traditional Chinese medicine, and the evaluation of their in vitro anti-influenza virus activity in MDCK cells. Interestingly, a set of configurational isomers was obtained during the de-O-acetylation reaction of two pentacyclic triterpene-sialic acid conjugates under Zemplén conditions, and a mechanism was proposed. Owing to the attachment of the Neu5Ac2en moiety, all synthesized conjugates displayed lower hydrophobicity than their parent compounds. In comparison with ursane- and lupane-type triterpenes, oleanane-type triterpene-functionalized Neu5Ac2en conjugates were most promising. The insertion of a (1,2,3-triazol-4-yl)-methyl between the amide bond and Neu5Ac2en caused a substantial decrease in activity. Compound 15a exhibited the highest inhibitory activity (IC50 = 8.3 μM) and selectivity index (SI = 22.7). Further studies involving hemagglutination inhibition and neuraminidase inhibition suggested that compound 15a inhibited virus-induced hemagglutination with no effect on the enzymatic activity of neuraminidase, indicating that the antiviral activity appeared to be mediated via interaction with hemagglutinin at the initial stage of viral infection.

Novel CLK1 inhibitors based on N-aryloxazol-2-amine skeleton - A possible way to dual VEGFR2 TK/CLK ligands

BACKGROUND

Inhibitors of CLK protein kinases suppress cell growth and induce apoptosis by modulating pre-mRNA splicing in cancer. CLK family kinases are also involved in alternative splicing and RNA processing in Duchenne muscular dystrophy, Alzheimer's disease, HIV-1, and influenza virus. Small inhibitors are valuable tools for better understanding the molecular mechanisms of splicing and may serve as seeds for a novel class of therapeutics.

ACHIEVEMENTS

Here we describe a discovery of four novel CLK1 inhibitors possessing N-aryloxazol-2-amine skeleton. Their activity against CLK1 (IC₅₀: 20, 30, 40 and 80 nM) and some other CMGC kinases, predicted CLK binding poses, synthesis and physico-chemical characteristics are also stated. Additionally analysis of all PDB available CLK structures and interactions of their ligands was performed. There are only few powerful dual CLK/VEGFR inhibitors known in the literature. We proposed that our inhibitors have similar binding places and interactions in CLK1, 3 and VEGFR2 TK mostly due to the joint N-aryloxazol-2-amine pharmacophoric fragment. One of our N-aryloxazol-2-amines already proved a good activity against both VEGFR2 and CLK1 enzymes (23/80 nM, resp). We proposed that the presented class of compounds has a potential to be developed in dual VEGFR2/CLK clinical compounds with prospective synergy to treat cancer.