A phenotypic screen for compounds that reverse cAMP-mediated suppression of T cell functions.

The solid tumor microenvironment (TME) suppresses immune responses. Three alterations in the TME converge on a pathway triggered by elevated cyclic AMP (cAMP) that suppresses T cell receptor (TCR) signaling. We developed a phenotypic assay to screen for small molecules that interfere with this pathway using TALL-104 human leukemic cytotoxic T lymphocytes pretreated with prostaglandin E2 to elevate cAMP. Beads coated with anti-CD3 antibodies stimulate lytic granule exocytosis, which is detected via binding of an antibody against lysosome associated membrane protein 1 (LAMP-1) measured with flow cytometry. Confirming that the assay can find compounds with desired activity, treating cells with a phorbol ester restores exocytosis. The assay behaves well in 96-well format and we screened a collection of compounds expected to have effects on epigenetic regulatory proteins. Compounds in this collection affected lytic granule exocytosis after 24-hour treatment, but none prevented cAMP from suppressing lytic granule exocytosis. We used a fully automated 384-well version of the assay to screen the Prestwick Compound Library but obtained no confirmed hits. Analyzing this assay's performance reveals two points of interest. First, cytometry offers multiple ways to quantify signals. Z' was higher using percent positive cells than mean fluorescence because the relationship between the two measures saturates, but using percent positive could make it harder to find hits in some assays. Second, variance was higher in positive controls than in negative controls in this assay, which degrades assay performance less than if variance was higher in negative controls.

Inhibition of the HEG1-KRIT1 interaction increases KLF4 and KLF2 expression in endothelial cells.

The transmembrane protein heart of glass1 (HEG1) directly binds to and recruits Krev interaction trapped protein 1 (KRIT1) to endothelial junctions to form the HEG1-KRIT1 protein complex that establishes and maintains junctional integrity. Genetic inactivation or knockdown of endothelial HEG1 or KRIT1 leads to the upregulation of transcription factors Krüppel-like factors 4 and 2 (KLF4 and KLF2), which are implicated in endothelial vascular homeostasis; however, the effect of acute inhibition of the HEG1-KRIT1 interaction remains incompletely understood. Here, we report a high-throughput screening assay and molecular design of a small-molecule HEG1-KRIT1 inhibitor to uncover acute changes in signaling pathways downstream of the HEG1-KRIT1 protein complex disruption. The small-molecule HEG1-KRIT1 inhibitor 2 (HKi2) was demonstrated to be a bona fide inhibitor of the interaction between HEG1 and KRIT1 proteins, by competing orthosterically with HEG1 through covalent reversible interactions with the FERM (4.1, ezrin, radixin, and moesin) domain of KRIT1. The crystal structure of HKi2 bound to KRIT1 FERM revealed that it occupies the same binding pocket on KRIT1 as the HEG1 cytoplasmic tail. In human endothelial cells (ECs), acute inhibition of the HEG1-KRIT1 interaction by HKi2 increased KLF4 and KLF2 mRNA and protein levels, whereas a structurally similar inactive compound failed to do so. In zebrafish, HKi2 induced expression of klf2a in arterial and venous endothelium. Furthermore, genome-wide RNA transcriptome analysis of HKi2-treated ECs under static conditions revealed that, in addition to elevating KLF4 and KLF2 expression, inhibition of the HEG1-KRIT1 interaction mimics many of the transcriptional effects of laminar blood flow. Furthermore, HKi2-treated ECs also triggered Akt signaling in a phosphoinositide 3-kinase (PI3K)-dependent manner, as blocking PI3K activity blunted the Akt phosphorylation induced by HKi2. Finally, using an in vitro colocalization assay, we show that HKi6, an improved derivative of HKi2 with higher affinity for KRIT1, significantly impedes recruitment of KRIT1 to mitochondria-localized HEG1 in CHO cells, indicating a direct inhibition of the HEG1-KRIT1 interaction. Thus, our results demonstrate that early events of the acute inhibition of HEG1-KRIT1 interaction with HKi small-molecule inhibitors lead to: (i) elevated KLF4 and KLF2 gene expression; and (ii) increased Akt phosphorylation. Thus, HKi's provide new pharmacologic tools to study acute inhibition of the HEG1-KRIT1 protein complex and may provide insights to dissect early signaling events that regulate vascular homeostasis.

A Novel Flow Cytometric Assay to Identify Inhibitors of RBPJ-DNA Interactions.

Notch signaling is often involved in cancer cell initiation and proliferation. Aberrant Notch activation underlies more than 50% of T-cell acute lymphoblastic leukemia (T-ALL); accordingly, chemicals disrupting Notch signaling are of potential to treat Notch-dependent cancer. Here, we developed a flow cytometry-based high-throughput assay to identify compounds that disrupt the interactions of DNA and RBPJ, the major downstream effector of Notch signaling. From 1492 compounds, we identified 18 compounds that disrupt RBPJ-DNA interactions in a dose-dependent manner. Cell-based assays further revealed that auranofin downregulates Notch-dependent transcription and decreases RBPJ-chromatin interactions in cells. Most strikingly, T-ALL cells that depend on Notch signaling for proliferation are more sensitive to auranofin treatment, supporting the notion that auranofin downregulates Notch signaling by disrupting RBPJ-DNA interaction. These results validate the feasibility of our assay scheme to screen for additional Notch inhibitors and provide a rationale to further test the use of auranofin in treating Notch-dependent cancer.

Activation of Rho Family GTPases by Small Molecules.

Ras and Ras-related small GTPases are key regulators of diverse cellular functions that impact cell growth, survival, motility, morphogenesis, and differentiation. They are important targets for studies of disease mechanisms as well as drug discovery. Here, we report the characterization of small molecule agonists of one or more of six Rho, Rab, and Ras family GTPases that were first identified through flow cytometry-based, multiplexed high-throughput screening of 200000 compounds. The activators were categorized into three distinct chemical families that are represented by three lead compounds having the highest activity. Virtual screening predicted additional compounds with potential GTPase activating properties. Secondary dose-response assays performed on compounds identified through these screens confirmed agonist activity of 43 compounds. While the lead and second most active small molecules acted as pan activators of multiple GTPase subfamilies, others showed partial selectivity for Ras and Rab proteins. The compounds did not stimulate nucleotide exchange by guanine nucleotide exchange factors and did not protect against GAP-stimulated GTP hydrolysis. The activating properties were caused by a reversible stabilization of the GTP-bound state and prolonged effector protein interactions. Notably, these compounds were active both in vitro and in cell-based assays, and small molecule-mediated changes in Rho GTPase activities were directly coupled to measurable changes in cytoskeletal rearrangements that dictate cell morphology.

High-Throughput Flow Cytometry Screening of Multidrug Efflux Systems.

The resistance nodulation cell division (RND) family of proteins are inner membrane transporters that associate with periplasmic adaptor proteins and outer membrane porins to affect substrate transport from the cytosol and periplasm in Gram-negative bacteria. Various structurally diverse compounds are substrates of RND transporters. Along with their notable role in antibiotic resistance, these transporters are essential for niche colonization, quorum sensing, and virulence as well as for the removal of fatty acids and bile salts. As such, RNDs are an attractive target for antimicrobial development. However, while enhancing the utility of antibiotics with an RND inhibitor is an appealing concept, only a small core of chemotypes has been identified as efflux pump inhibitors (EPIs). Thus, our key objective is the development and validation of an efflux profiling and discovery strategy for RND model systems. Here we describe a flow cytometric dye accumulation assay that uses fluorescein diacetate (FDA) to interrogate the model Gram-negative pathogens Escherichia coli, Franscisella tularensis, and Burkholderia pseudomallei. Fluorochrome retention is increased in the presence of known efflux inhibitors and in RND deletion strains. The assay can be used in a high-throughput format to evaluate efflux of dye-substrate candidates and to screen chemical libraries for novel EPIs. Triaged compounds that inhibit efflux in pathogenic strains are tested for growth inhibition and antibiotic potentiation using microdilution culture plates in a select agent Biosafety Level-3 (BSL3) environment. This combined approach demonstrates the utility of flow cytometric analysis for efflux activity and provides a useful platform in which to characterize efflux in pathogenic Gram-negative bacteria. Screening small molecule libraries for novel EPI candidates offers the potential for the discovery of new classes of antibacterial compounds.

High-Throughput Screening Approach for Identifying Compounds That Inhibit Nonhomologous End Joining.

DNA double-strand breaks (DSBs) are repaired primarily by homologous recombination (HR) or nonhomologous end joining (NHEJ). Compounds that modulate HR have shown promise as cancer therapeutics. The V(D)J recombination reaction, which assembles antigen receptor genes in lymphocytes, is initiated by the introduction of DNA DSBs at two recombining gene segments by the RAG endonuclease, followed by the NHEJ-mediated repair of these DSBs. Here, using HyperCyt automated flow cytometry, we develop a robust high-throughput screening (HTS) assay for NHEJ that utilizes engineered pre-B-cell lines where the V(D)J recombination reaction can be induced and monitored at a single-cell level. This approach, novel in processing four 384-well plates at a time in parallel, was used to screen the National Cancer Institute NeXT library to identify compounds that inhibit V(D)J recombination and NHEJ. Assessment of cell light scattering characteristics at the primary HTS stage (83,536 compounds) enabled elimination of 60% of apparent hits as false positives. Although all the active compounds that we identified had an inhibitory effect on RAG cleavage, we have established this as an approach that could identify compounds that inhibit RAG cleavage or NHEJ using new chemical libraries.

Inhibition of Dihydroorotate Dehydrogenase Overcomes Differentiation Blockade in Acute Myeloid Leukemia.

While acute myeloid leukemia (AML) comprises many disparate genetic subtypes, one shared hallmark is the arrest of leukemic myeloblasts at an immature and self-renewing stage of development. Therapies that overcome differentiation arrest represent a powerful treatment strategy. We leveraged the observation that the majority of AML, despite their genetically heterogeneity, share in the expression of HoxA9, a gene normally downregulated during myeloid differentiation. Using a conditional HoxA9 model system, we performed a high-throughput phenotypic screen and defined compounds that overcame differentiation blockade. Target identification led to the unanticipated discovery that inhibition of the enzyme dihydroorotate dehydrogenase (DHODH) enables myeloid differentiation in human and mouse AML models. In vivo, DHODH inhibitors reduced leukemic cell burden, decreased levels of leukemia-initiating cells, and improved survival. These data demonstrate the role of DHODH as a metabolic regulator of differentiation and point to its inhibition as a strategy for overcoming differentiation blockade in AML.

A high-throughput phenotypic screen of cytotoxic T lymphocyte lytic granule exocytosis reveals candidate immunosuppressants.

We screened the National Institutes of Health's Molecular Libraries Small Molecule Repository for inhibitors of cytotoxic T lymphocyte (CTL) lytic granule exocytosis by measuring binding of an antibody in the extracellular solution to a lysosomal membrane protein (LAMP-1) that is transferred to the plasma membrane by exocytosis. We used TALL-104 human leukemic CTLs stimulated with soluble chemicals. Using high-throughput cluster cytometry to screen 364,202 compounds in a 1536-well plate format, we identified 2404 initial hits: 161 were confirmed on retesting, and dose-response measurements were performed. Seventy-five of those compounds were obtained, and 48 were confirmed active. Experiments were conducted to determine the molecular mechanism of action (MMOA) of the active compounds. Fifteen blocked increases in intracellular calcium >50%. Seven blocked phosphorylation of extracellular signal-regulated kinase (ERK) by upstream mitogen-activated protein kinase kinases >50%. One completely blocked the activity of the calcium-dependent phosphatase calcineurin. None blocked ERK catalytic activity. Eight blocked more than one pathway. For 8 compounds, we were unable to determine an MMOA. The activity of 1 of these compounds was confirmed from powder resupply. We conclude that a screen based on antibody binding to CTLs is a good means of identifying novel candidate immunosuppressants with either known or unknown MMOAs.

FRET detection of lymphocyte function-associated antigen-1 conformational extension.

Lymphocyte function-associated antigen 1 (LFA-1, CD11a/CD18, αLß2-integrin) and its ligands are essential for adhesion between T-cells and antigen-presenting cells, formation of the immunological synapse, and other immune cell interactions. LFA-1 function is regulated through conformational changes that include the modulation of ligand binding affinity and molecular extension. However, the relationship between molecular conformation and function is unclear. Here fluorescence resonance energy transfer (FRET) with new LFA-1-specific fluorescent probes showed that triggering of the pathway used for T-cell activation induced rapid unquenching of the FRET signal consistent with extension of the molecule. Analysis of the FRET quenching at rest revealed an unexpected result that can be interpreted as a previously unknown LFA-1 conformation.

Small Molecule Inhibitors Limit Endothelial Cell Invasion by Staphylococcus aureus.

Staphylococcus aureus is a leading causative agent in sepsis, endocarditis, and pneumonia. An emerging concept is that prognosis worsens when the infecting S. aureus strain has the capacity to not only colonize tissue as an extracellular pathogen, but to invade host cells and establish intracellular bacterial populations. In previous work, we identified host CDC42 as a central regulator of endothelial cell invasion by S. aureus. In the current work, we report that ML 141, a first-in-class CDC42 inhibitor, decreases invasion and resultant pathogenesis in a dose-dependent and reversible manner. Inhibition was found to be due in part to decreased remodeling of actin that potentially drives endocytic uptake of bacteria/fibronectin/integrin complexes. ML 141 decreased binding to fibronectin at these complexes, thereby limiting a key pathogenic mechanism used by S. aureus to invade. Structural analogs of ML 141 were synthesized (designated as the RSM series) and a subset identified that inhibit invasion through non-cytotoxic and non-bactericidal mechanisms. Our results support the development of adjunctive therapeutics targeting host CDC42 for mitigating invasive infection at the level of the host.

Flow cytometry enables a high-throughput homogeneous fluorescent antibody-binding assay for cytotoxic T cell lytic granule exocytosis.

We developed a homogeneous phenotypic fluorescence end-point assay for cytotoxic T lymphocyte lytic granule exocytosis. This flow cytometric assay measures binding of an antibody to a luminal epitope of a lysosomal membrane protein (LAMP-1) that is exposed by exocytosis to the extracellular solution. Washing to remove unbound antibody is not required. Confirming the assay's ability to detect novel active compounds, we screened at a concentration of 50 µM a synthetic diversity library of 91 compounds in a 96-well plate format, identifying 17 compounds that blocked by 90% or more. The actions of six structurally related tetracyano-hexahydroisoindole compounds that inhibited by ~90% at a concentration of 10 µM were investigated further. Four reduced elevations in intracellular Ca(2+); it is likely that depolarization of the cells' membrane potential underlies the effect for at least two of the compounds. Another compound was found to be a potent inhibitor of the activation of the mitogen-activated protein (MAP) kinase ERK. Finally, we transferred the assay to a 384-well format and screened the Prestwick Compound Library using high-throughput flow cytometry. Our results indicate that our assay will likely be a useful means of screening libraries for novel compounds with important biological activities.

A novel flow cytometric HTS assay reveals functional modulators of ATP binding cassette transporter ABCB6.

ABCB6 is a member of the adenosine triphosphate (ATP)-binding cassette family of transporter proteins that is increasingly recognized as a relevant physiological and therapeutic target. Evaluation of modulators of ABCB6 activity would pave the way toward a more complete understanding of the significance of this transport process in tumor cell growth, proliferation and therapy-related drug resistance. In addition, this effort would improve our understanding of the function of ABCB6 in normal physiology with respect to heme biosynthesis, and cellular adaptation to metabolic demand and stress responses. To search for modulators of ABCB6, we developed a novel cell-based approach that, in combination with flow cytometric high-throughput screening (HTS), can be used to identify functional modulators of ABCB6. Accumulation of protoporphyrin, a fluorescent molecule, in wild-type ABCB6 expressing K562 cells, forms the basis of the HTS assay. Screening the Prestwick Chemical Library employing the HTS assay identified four compounds, benzethonium chloride, verteporfin, tomatine hydrochloride and piperlongumine, that reduced ABCB6 mediated cellular porphyrin levels. Validation of the identified compounds employing the hemin-agarose affinity chromatography and mitochondrial transport assays demonstrated that three out of the four compounds were capable of inhibiting ABCB6 mediated hemin transport into isolated mitochondria. However, only verteporfin and tomatine hydrochloride inhibited ABCB6's ability to compete with hemin as an ABCB6 substrate. This assay is therefore sensitive, robust, and suitable for automation in a high-throughput environment as demonstrated by our identification of selective functional modulators of ABCB6. Application of this assay to other libraries of synthetic compounds and natural products is expected to identify novel modulators of ABCB6 activity.

Dissecting novel virulent determinants in the Burkholderia cepacia complex.

Prevention and control of infectious diseases remains a major public health challenge and a number of highly virulent pathogens are emerging both in and beyond the hospital setting. Despite beneficial aspects such as use in biocontrol and bioremediation exhibited by members of the Burkholderia cepacia complex (Bcc) some members of this group have recently gained attention as significant bacterial pathogens due to their high levels of intrinsic antibiotic resistance, transmissibility in nosocomial settings, persistence in the presence of antimicrobials and intracellular survival capabilities. The Bcc are opportunistic pathogens and their arsenal of virulence factors includes proteases, lipases and other secreted exoproducts, including secretion system-associated effectors. Deciphering the function of virulence factors and assessment of novel therapeutic strategies has been facilitated by use of diverse non-vertebrate hosts (the fly Drosophila melanogaster, the microscopic nematode Caenorhabditis elegans, the zebrafish and the greater Galleria mellonella wax moth caterpillar larvae). Researchers are now employing sophisticated approaches to dissect the virulence determinants of Bcc with the ultimate goal being the development of novel anti-infective countermeasures. This editorial will highlight selected recent research endeavors aimed at dissecting adaptive responses and the virulence factor portfolio of Burkholderia species.

Real-time analysis of the inside-out regulation of lymphocyte function-associated antigen-1 revealed similarities to and differences from very late antigen-4.

Ten years ago, we introduced a fluorescent probe that shed light on the inside-out regulation of one of the major leukocyte integrins, very late antigen-4 (VLA-4, CD49d/CD29). Here we describe the regulation of another leukocyte integrin, lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18) using a novel small fluorescent probe in real time on live cells. We found that multiple signaling mechanisms regulate LFA-1 conformation in a manner analogous to VLA-4. LFA-1 can be rapidly activated by Gα(i)-coupled G protein-coupled receptors (GPCRs) and deactivated by Gα(s)-coupled GPCRs. The effects of Gα(s)-coupled GPCR agonists can be reversed in real time by receptor-specific antagonists. The specificity of the fluorescent probe binding has been assessed in a competition assay using the natural LFA-1 ligand ICAM-1 and the LFA-1-specific α I allosteric antagonist BIRT0377. Similar to VLA-4 integrin, modulation of the ligand dissociation rate can be observed for different LFA-1 affinity states. However, we also found a striking difference in the binding of the small fluorescent ligand. In the absence of inside-out activation ligand, binding to LFA-1 is extremely slow, at least 10 times slower than expected for diffusion-limited binding. This implies that an additional structural mechanism prevents ligand binding to inactive LFA-1. We propose that such a mechanism explains the inability of LFA-1 to support cell rolling, where the absence of its rapid engagement by a counterstructure in the inactive state leads to a requirement for a selectin-mediated rolling step.

Drug Repurposing from an Academic Perspective.

Academia and small business research units are poised to play an increasing role in drug discovery, with drug repurposing as one of the major areas of activity. Here we summarize project status for a number of drugs or classes of drugs: raltegravir, cyclobenzaprine, benzbromarone, mometasone furoate, astemizole, R-naproxen, ketorolac, tolfenamic acid, phenothiazines, methylergonovine maleate and beta-adrenergic receptor drugs, respectively. Based on this multi-year, multi-project experience we discuss strengths and weaknesses of academic-based drug repurposing research. Translational, target and disease foci are strategic advantages fostered by close proximity and frequent interactions between basic and clinical scientists, which often result in discovering new modes of action for approved drugs. On the other hand, lack of integration with pharmaceutical sciences and toxicology, lack of appropriate intellectual coverage and issues related to dosing and safety may lead to significant drawbacks. The development of a more streamlined regulatory process world-wide, and the development of pre-competitive knowledge transfer systems such as a global healthcare database focused on regulatory and scientific information for drugs world-wide, are among the ideas proposed to improve the process of academic drug discovery and repurposing, and to overcome the "valley of death" by bridging basic to clinical sciences.

Detection of intracellular granularity induction in prostate cancer cell lines by small molecules using the HyperCyt high-throughput flow cytometry system.

Prostate cancer is a leading cause of death among men due to the limited number of treatment strategies available for advanced disease. Discovery of effective chemotherapeutics involves the identification of agents that inhibit cancer cell growth. Increases in intracellular granularity have been observed during physiological processes that include senescence, apoptosis, and autophagy, making this phenotypic change a useful marker for identifying small molecules that induce cellular growth arrest or death. In this regard, epithelial-derived cancer cell lines appear uniquely susceptible to increased intracellular granularity following exposure to chemotherapeutics. We have established a novel flow cytometry approach that detects increases in side light scatter in response to morphological changes associated with intracellular granularity in the androgen-sensitive LNCaP and androgen-independent PC3 human prostate cancer cell lines. A cell-based assay was developed to screen for small molecule inducers of intracellular granularity using the HyperCyt high-throughput flow cytometry platform. Validation was performed using the Prestwick Chemical Library, where known modulators of LNCaP intracellular granularity, such as testosterone, were identified. Nonandrogenic inducers of granularity were also detected. A further screen of approximately 25,000 small molecules led to the identification of a class of aryl-oxazoles that increased intracellular granularity in both cell lines, often leading to cell death. The most potent agents exhibited submicromolar efficacy in LNCaP and PC3 cells.

Phenotypic characterization of inflammatory cells from osteoarthritic synovium and synovial fluids.

Osteoarthritis (OA) is considered a degenerative joint disorder caused by mechanical wear to the articular surface. However, while joint injury, obesity, and mutations in collagen increase the risk of developing OA, evidence implicates inflammatory mechanisms in disease progression and chronicity. To address this question we used FACS analysis, immunohistochemistry, and in vitro cell culture to evaluate inflammatory mechanisms in synovial fluids and joint tissues obtained after arthrocentesis or knee replacement surgery. Immunohistochemistry revealed a significant T cell infiltrate in six of nine tissue specimens. T cells were present throughout the synovial membrane and were particularly localized around vasculature and in large cellular aggregates. Cells within the aggregates expressed markers associated with immune activation and antigen presentation. T cells from OA synovial fluids expressed an activated phenotype and synthesized interferon-gamma following in vitro stimulation. These data support the hypothesis that inflammatory cells play a significant role in OA disease progression and chronicity.

Rheumatoid arthritis--a molecular understanding.

The application of molecular immunology techniques in the study of rheumatoid arthritis has resulted in an explosion of knowledge on the risk factors for the disease, predictors of disease severity, the molecular mechanisms of inflammatory responses, and mechanisms of tissue destruction. We know, for example, that inheriting certain genes in the major histocompatibility complex partly dictates susceptibility and severity of rheumatoid arthritis. These genes and others in the major histocompatibility complex are critical for the occurrence of immune responses both constructive (prevention of infection, surveillance for malignant cells) and destructive (development of autoimmune diseases). We also now understand mechanisms of cell communication, regulation of immune responses, how the cells that mediate immune responses and tissue injury accumulate in tissues, and how the injury occurs. The knowledge itself is satisfying, but more important, based on this knowledge, effective and reasonably safe treatments that address basic mechanisms of the disease process have been developed and are now widely used. In fact, the newer treatments represent the "tip of the iceberg," and as our basic knowledge increases, so too will the armamentarium with which we can fight rheumatoid arthritis and other similar autoimmune diseases.