Development of an Autofluorescent Translocation Biosensor System To Investigate Protein−Protein Interactions in Living Cells

Dual activity inhibition of threonine aspartase 1 by a single bisphosphate ligand

Therapy resistance remains a challenge for the clinics. Here, dual-active chemicals that simultaneously inhibit independent functions in disease-relevant proteins are desired though highly challenging. As a model, we here addressed the unique protease threonine aspartase 1, involved in various cancers. We hypothesized that targeting basic residues in its bipartite nuclear localization signal (NLS) by precise bisphosphate ligands inhibits additional steps required for protease activity. We report the bisphosphate anionic bivalent inhibitor 11d, selectively binding to the basic NLS cluster (²²⁰KKRR²²³) with high affinity (K _D = 300 nM), thereby disrupting its interaction and function with Importin α (IC₅₀ = 6 μM). Cell-free assays revealed that 11d additionally affected the protease's catalytic substrate trans-cleavage activity. Importantly, functional assays comprehensively demonstrated that 11d inhibited threonine aspartase 1 also in living tumor cells. We demonstrate for the first time that intracellular interference with independent key functions in a disease-relevant protein by an inhibitor binding to a single site is possible.

Nano Meets Micro-Translational Nanotechnology in Medicine: Nano-Based Applications for Early Tumor Detection and Therapy

Nanomaterials have great potential for the prevention and treatment of cancer. Circulating tumor cells (CTCs) are cancer cells of solid tumor origin entering the peripheral blood after detachment from a primary tumor. The occurrence and circulation of CTCs are accepted as a prerequisite for the formation of metastases, which is the major cause of cancer-associated deaths. Due to their clinical significance CTCs are intensively discussed to be used as liquid biopsy for early diagnosis and prognosis of cancer. However, there are substantial challenges for the clinical use of CTCs based on their extreme rarity and heterogeneous biology. Therefore, methods for effective isolation and detection of CTCs are urgently needed. With the rapid development of nanotechnology and its wide applications in the biomedical field, researchers have designed various nano-sized systems with the capability of CTCs detection, isolation, and CTCs-targeted cancer therapy. In the present review, we summarize the underlying mechanisms of CTC-associated tumor metastasis, and give detailed information about the unique properties of CTCs that can be harnessed for their effective analytical detection and enrichment. Furthermore, we want to give an overview of representative nano-systems for CTC isolation, and highlight recent achievements in microfluidics and lab-on-a-chip technologies. We also emphasize the recent advances in nano-based CTCs-targeted cancer therapy. We conclude by critically discussing recent CTC-based nano-systems with high therapeutic and diagnostic potential as well as their biocompatibility as a practical example of applied nanotechnology.

Discovery of Nonpeptide, Environmentally Sensitive Fluorescent Probes for Imaging p53-MDM2 Interactions in Living Cell Lines and Tissue Slice

Based on structural optimization work, probes 9-11 with practical activity and selectivity in tissue as well as living cell lines are well designed and synthesized. All the probes showed potent inhibitory and acceptable cell toxicity compared with the commercially available p53-MDM2 inhibitor Nutlin-3, and can increase the protein expression level of p53 and MDM2 in the A549 cell line; in particular, probes 10 and 11 can increase the protein expression level of p53 better than Nutlin-3. Moreover, their application in imaging and detecting wild-type p53-MDM2 protein-protein interactions have been well demonstrated in at the cell and tissue levels. Overall, these environmentally sensitive fluorescent turn-on probes are affordable and rapid for imaging, which is expected for applications in target drug screening as well as in pathologic diagnosis.

Is small smarter? Nanomaterial-based detection and elimination of circulating tumor cells: current knowledge and perspectives

Circulating tumor cells (CTCs) are disseminated cancer cells. The occurrence and circulation of CTCs seem key for metastasis, still the major cause of cancer-associated deaths. As such, CTCs are investigated as predictive biomarkers. However, due to their rarity and heterogeneous biology, CTCs’ practical use has not made it into the clinical routine. Clearly, methods for the effective isolation and reliable detection of CTCs are urgently needed. With the development of nanotechnology, various nanosystems for CTC isolation and enrichment and CTC-targeted cancer therapy have been designed. Here, we summarize the relationship between CTCs and tumor metastasis, and describe CTCs’ unique properties hampering their effective enrichment. We comment on nanotechnology-based systems for CTC isolation and recent achievements in microfluidics and lab-on-a-chip technologies. We discuss recent advances in CTC-targeted cancer therapy exploiting the unique properties of nanomaterials. We conclude by introducing developments in CTC-directed nanosystems and other advanced technologies currently in (pre)clinical research.

The nuclear export inhibitor aminoratjadone is a potent effector in extracellular-targeted drug conjugates

The concept of targeted drug conjugates has been successfully translated to clinical practice in oncology. Whereas the majority of cytotoxic effectors in drug conjugates are directed against either DNA or tubulin, our study aimed to validate nuclear export inhibition as a novel effector principle in drug conjugates. For this purpose, a semisynthetic route starting from the natural product ratjadone A, a potent nuclear export inhibitor, has been developed. The biological evaluation of ratjadones functionalized at the 16-position revealed that oxo- and amino-analogues had very high potencies against cancer cell lines (e.g. 16R-aminoratjadone 16 with IC50 = 260 pM against MCF-7 cells, or 19-oxoratjadone 14 with IC50 = 100 pM against A-549 cells). Mechanistically, the conjugates retained a nuclear export inhibitory activity through binding CRM1. To demonstrate a proof-of-principle for cellular targeting, folate- and luteinizing hormone releasing hormone (LHRH)-based carrier molecules were synthesized and coupled to aminoratjadones as well as fluorescein for cellular efficacy and imaging studies, respectively. The Trojan-Horse conjugates selectively addressed receptor-positive cell lines and were highly potent inhibitors of their proliferation. For example, the folate conjugate FA-7-Val-Cit-pABA-16R-aminoratjadone had an IC50 of 34.3 nM, and the LHRH conjugate d-Orn-Gose-Val-Cit-pABA-16R-aminoratjadone had an IC50 of 12.8 nM. The results demonstrate that nuclear export inhibition is a promising mode-of-action for extracellular-targeted drug conjugate payloads.

Nanosized food additives impact beneficial and pathogenic bacteria in the human gut: a simulated gastrointestinal study

Nanotechnology provides the food industry with new ways to modulate various aspects of food. Hence, engineered nanoparticles (NPs) are increasingly added to food and beverage products as functional ingredients. However, the impact of engineered as well as naturally occurring NPs on both commensal and pathogenic microorganisms within the gastrointestinal tract (GI) is not fully understood. Here, well-defined synthetic NPs and bacterial models were used to probe nanoparticle–bacteria interactions, from analytical to in situ to in vitro. NP–bacteria complexation occurred most efficiently for small NPs, independent of their core material or surface charge, but could be reduced by NPs’ steric surface modifications. Adsorption to bacteria could also be demonstrated for naturally occurring carbon NPs isolated from beer. Complex formation affected the (patho)biological behavior of both the NPs and bacteria, including their cellular uptake into epithelial cells and phagocytes, pathogenic signaling pathways, and NP-induced cell toxicity. NP–bacteria complex formation was concentration-dependently reduced when the NPs became coated with biomolecule coronas with sequential simulation of first oral uptake and then the GI. However, efficient NP adsorption was restored when the pH was sufficiently low, such as in simulating the conditions of the stomach. Collectively, NP binding to enteric bacteria may impact their (patho)biology, particularly in the stomach. Nanosized-food additives as well as naturally occurring NPs may be exploited to (rationally) shape the microbiome. The information contained in this article should facilitate a “safe by design” strategy for the development and application of engineered NPs as functional foods ingredients.

Engineered or naturally occurring nanoparticles could potentially affect the bacteria in the gut. A study led by Dana Westmeier and Roland Stauber from University Medical Center of Mainz, Germany probed the nanoparticle–bacteria interactions in situ. They found that NP–bacteria complex occurred most efficiently for small NPs, independent of their core material or surface charge. The complex formation affected the (patho)biological behavior of both the NPs and bacteria, particularly under conditions that simulate the stomach. The result shows that both engineered and naturally occurring nanoparticles could be exploited to shape the gut microbiome. The study can offer guidelines for future development and application of nanoparticles in food industry.

High Content Positional Biosensor Assay to Screen for Compounds that Prevent or Disrupt Androgen Receptor and Transcription Intermediary Factor 2 Protein-Protein Interactions

Transcriptional Intermediary Factor 2 (TIF2) is a key Androgen receptor (AR) coactivator that has been implicated in the development and progression of castration resistant prostate cancer (CRPC). This chapter describes the implementation of an AR-TIF2 protein-protein interaction (PPI) biosensor assay to screen for small molecules that can induce AR-TIF2 PPIs, inhibit the DHT-induced formation of AR-TIF2 PPIs, or disrupt pre-existing AR-TIF2 PPIs. The biosensor assay employs high content imaging and analysis to quantify AR-TIF2 PPIs and integrates physiologically relevant cell-based assays with the specificity of binding assays by incorporating structural information from AR and TIF2 functional domains along with intracellular targeting sequences using fluorescent protein reporters. Expression of the AR-Red Fluorescent Protein (RFP) "prey" and TIF2-Green Fluorescent Protein (GFP) "bait" components of the biosensor is directed by recombinant adenovirus (rAV) expression constructs that facilitated a simple co-infection protocol to produce homogeneous expression of both biosensors that is scalable for screening. In untreated cells, AR-RFP expression is localized predominantly to the cytoplasm and TIF2-GFP expression is localized only in the nucleoli of the nucleus. Exposure to DHT induces the co-localization of AR-RFP within the TIF2-GFP positive nucleoli of the nucleus. The AR-TIF2 biosensor assay therefore recapitulates the ligand-induced translocation of latent AR from the cytoplasm to the nucleus, and the PPIs between AR and TIF2 result in the colocalization of AR-RFP within TIF2-GFP expressing nucleoli. The AR-TIF2 PPI biosensor approach offers significant promise for identifying molecules with potential to modulate AR transcriptional activity in a cell-specific manner that may overcome the development of resistance and progression to CRPC.

Disease-relevant signalling-pathways in head and neck cancer: Taspase1's proteolytic activity fine-tunes TFIIA function

Head and neck cancer (HNC) is the seventh most common malignancy in the world and its prevailing form, the head and neck squamous cell carcinoma (HNSCC), is characterized as aggressive and invasive cancer type. The transcription factor II A (TFIIA), initially described as general regulator of RNA polymerase II-dependent transcription, is part of complex transcriptional networks also controlling mammalian head morphogenesis. Posttranslational cleavage of the TFIIA precursor by the oncologically relevant protease Taspase1 is crucial in this process. In contrast, the relevance of Taspase1-mediated TFIIA cleavage during oncogenesis of HNSCC is not characterized yet. Here, we performed genome-wide expression profiling of HNSCC which revealed significant downregulation of the TFIIA downstream target CDKN2A. To identify potential regulatory mechanisms of TFIIA on cellular level, we characterized nuclear-cytoplasmic transport and Taspase1-mediated cleavage of TFIIA variants. Unexpectedly, we identified an evolutionary conserved nuclear export signal (NES) counteracting nuclear localization and thus, transcriptional activity of TFIIA. Notably, proteolytic processing of TFIIA by Taspase1 was found to mask the NES, thereby promoting nuclear localization and transcriptional activation of TFIIA target genes, such as CDKN2A. Collectively, we here describe a hitherto unknown mechanism how cellular localization and Taspase1 cleavage fine-tunes transcriptional activity of TFIIA in HNSCC.

Environment-sensitive turn-on fluorescent probes for p53-MDM2 protein-protein interaction

A series of probes with a turn-on switch for the p53-MDM2 protein-protein interaction were developed. After careful evaluation, these small molecule fluorescent probes exhibited high practical activity and selectivity in vitro and in cellulo. In particular probe 10, which had a K_i value of 0.03 μM, displayed much better binding affinity compared to the positive control Nutlin-3, which had a K_i value of 0.23 μM. These no-wash environment-sensitive turn-on fluorescent probes have been successfully applied to imaging p53-MDM2 interaction in the human lung cancer cell line A549 (wild-type p53) at the micromolar level. Therefore, these fluorescent probes are expected to be used in drug screening and cell staining in p53-MDM2 fields, as well as in pathological and physiological studies of the p53-MDM2 interaction.

High-content positional biosensor screening assay for compounds to prevent or disrupt androgen receptor and transcriptional intermediary factor 2 protein-protein interactions

The androgen receptor-transcriptional intermediary factor 2 (AR-TIF2) positional protein-protein interaction (PPI) biosensor assay described herein combines physiologically relevant cell-based assays with the specificity of binding assays by incorporating structural information of AR and TIF2 functional domains along with intracellular targeting sequences and fluorescent reporters. Expression of the AR-red fluorescent protein (RFP) "prey" and TIF2-green fluorescent protein (GFP) "bait" components of the biosensor was directed by recombinant adenovirus constructs that expressed the ligand binding and activation function 2 surface domains of AR fused to RFP with nuclear localization and nuclear export sequences, and three α-helical LXXLL motifs from TIF2 fused to GFP and an HIV Rev nucleolar targeting sequence. In unstimulated cells, AR-RFP was localized predominantly to the cytoplasm and TIF2-GFP was localized to nucleoli. Dihydrotestosterone (DHT) treatment induced AR-RFP translocation into the nucleus where the PPIs between AR and TIF2 resulted in the colocalization of both biosensors within the nucleolus. We adapted the translocation enhanced image analysis module to quantify the colocalization of the AR-RFP and TIF2-GFP biosensors in images acquired on the ImageXpress platform. DHT induced a concentration-dependent AR-TIF2 colocalization and produced a characteristic condensed punctate AR-RFP PPI nucleolar distribution pattern. The heat-shock protein 90 inhibitor 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) and antiandrogens flutamide and bicalutamide inhibited DHT-induced AR-TIF2 PPI formation with 50% inhibition concentrations (IC50s) of 88.5±12.5 nM, 7.6±2.4 μM, and 1.6±0.4 μM, respectively. Images of the AR-RFP distribution phenotype allowed us to distinguish between 17-AAG and flutamide, which prevented AR translocation, and bicalutamide, which blocked AR-TIF2 PPIs. We screened the Library of Pharmacologically Active Compounds (LOPAC) set for compounds that inhibited AR-TIF2 PPI formation or disrupted preexisting complexes. Eleven modulators of steroid family nuclear receptors (NRs) and 6 non-NR ligands inhibited AR-TIF2 PPI formation, and 10 disrupted preexisting complexes. The hits appear to be either AR antagonists or nonspecific inhibitors of NR activation and trafficking. Given that the LOPAC set represents such a small and restricted biological and chemical diversity, it is anticipated that screening a much larger and more diverse compound library will be required to find AR-TIF2 PPI inhibitors/disruptors. The AR-TIF2 protein-protein interaction biosensor (PPIB) approach offers significant promise for identifying molecules with potential to modulate AR transcriptional activity in a cell-specific manner that is distinct from the existing antiandrogen drugs that target AR binding or production. Small molecules that disrupt AR signaling at the level of AR-TIF2 PPIs may also overcome the development of resistance and progression to castration-resistant prostate cancer.

Fly versus man: evolutionary impairment of nucleolar targeting affects the degradome of Drosophila's Taspase1

Human Taspase1 is essential for development and cancer by processing critical regulators, such as the mixed-lineage leukemia protein. Likewise, its ortholog, trithorax, is cleaved by Drosophila Taspase1 (dTaspase1), implementing a functional coevolution. To uncover novel mechanism regulating protease function, we performed a functional analysis of dTaspase1 and its comparison to the human ortholog. dTaspase1 contains an essential nucleophile threonine(195), catalyzing cis cleavage into its α- and β-subunits. A cell-based assay combined with alanine scanning mutagenesis demonstrated that the target cleavage motif for dTaspase1 (Q(3)[F/I/L/M](2)D(1)↓G(1')X(2')X(3')) differs significantly from the human ortholog (Q(3)[F,I,L,V](2)D(1)↓G(1')x(2')D(3')D(4')), predicting an enlarged degradome containing 70 substrates for Drosophila. In contrast to human Taspase1, dTaspase1 shows no discrete localization to the nucleus/nucleolus due to the lack of the importin-α/nucleophosmin1 interaction domain (NoLS) conserved in all vertebrates. Consequently, dTaspase1 interacts with neither the Drosophila nucleoplasmin-like protein nor human nucleophosmin1. The impact of localization on the protease's degradome was confirmed by demonstrating that dTaspase1 did not efficiently process nuclear substrates, such as upstream stimulatory factor 2. However, genetic introduction of the NoLS into dTaspase1 restored its nucleolar localization, nucleophosmin1 interaction, and efficient cleavage of nuclear substrates. We report that evolutionary functional divergence separating vertebrates from invertebrates can be achieved for proteases by a transport/localization-regulated mechanism.

High content screening biosensor assay to identify disruptors of p53-hDM2 protein-protein interactions

This chapter describes the implementation of the p53-hDM2 protein-protein interaction (PPI) biosensor (PPIB) HCS assay to identify disruptors of p53-hDM2 PPIs. Recombinant adenovirus expression constructs were generated bearing the individual p53-GFP and hDM2-RFP PPI partners. The N-terminal p53 transactivating domain that contains the binding site for hDM2 is expressed as a GFP fusion protein that is targeted and anchored in the nucleolus of infected cells by a nuclear localization (NLS) sequence. The p53-GFP biosensor is localized to the nucleolus to enhance and facilitate the image acquisition and analysis of the PPIs. The N-terminus of hDM2 encodes the domain for binding to the transactivating domain of p53, and is expressed as a RFP fusion protein that includes both an NLS and a nuclear export sequence (NES). In U-2 OS cells co-infected with both adenovirus constructs, the binding interactions between hDM2 and p53 result in both biosensors becoming co-localized within the nucleolus. Upon disruption of the p53-hDM2 PPIs, the p53-GFP biosensor remains in the nucleolus while the shuttling hDM2-RFP biosensor redistributes into the cytoplasm. p53-hDM2 PPIs are measured by acquiring fluorescent images of cells co-infected with both adenovirus biosensors on an automated HCS imaging platform and using an image analysis algorithm to quantify the relative distribution of the hDM2-RFP shuttling component of the biosensor between the cytoplasm and nuclear regions of compound treated cells.

A fluorescent probe for imaging p53-MDM2 protein-protein interaction

In this article, we describe a no-wash small-molecule fluorescent probe for detecting and imaging p53-MDM2 protein-protein interaction based on an environment-sensitive fluorescent turn-on mechanism. After extensive biological examination, this probe L1 exhibited practical activity and selectivity in vitro and in cellulo.

Diversity in genetic in vivo methods for protein-protein interaction studies: from the yeast two-hybrid system to the mammalian split-luciferase system

The yeast two-hybrid system pioneered the field of in vivo protein-protein interaction methods and undisputedly gave rise to a palette of ingenious techniques that are constantly pushing further the limits of the original method. Sensitivity and selectivity have improved because of various technical tricks and experimental designs. Here we present an exhaustive overview of the genetic approaches available to study in vivo binary protein interactions, based on two-hybrid and protein fragment complementation assays. These methods have been engineered and employed successfully in microorganisms such as Saccharomyces cerevisiae and Escherichia coli, but also in higher eukaryotes. From single binary pairwise interactions to whole-genome interactome mapping, the self-reassembly concept has been employed widely. Innovative studies report the use of proteins such as ubiquitin, dihydrofolate reductase, and adenylate cyclase as reconstituted reporters. Protein fragment complementation assays have extended the possibilities in protein-protein interaction studies, with technologies that enable spatial and temporal analyses of protein complexes. In addition, one-hybrid and three-hybrid systems have broadened the types of interactions that can be studied and the findings that can be obtained. Applications of these technologies are discussed, together with the advantages and limitations of the available assays.

Allosteric inhibition of Taspase1's pathobiological activity by enforced dimerization in vivo

Taspase1 mediates cleavage of the mixed lineage leukemia (MLL) protein and leukemia-provoking MLL fusions and promotes solid malignancies. Currently, no effective and specific Taspase1 inhibitors are available, precluding its therapeutic exploitation. As the Taspase1 proenzyme is autoproteolytically cleaved and assumed to assemble into an active αββα heterodimer, we attempted to interfere with its activity by targeting Taspase1's dimerization. Notably, enforced expression of inactive Taspase1 mutants, aiming to inhibit formation of active protease dimers, was not inhibitory. Immunoprecipitation, gel filtration, and in vivo protein interaction assays revealed that active Taspase1 exists predominantly as an αβ monomer in living cells, providing an explanation why overexpression of inactive mutants was not trans-dominant. To alternatively test the biological consequences of enforced dimerization, we engineered Taspase1 variants containing the Jun/Fos dimerization motif. In absence of the respective interaction partners, the protease fusions were fully active, while enforcing dimerization by coexpression significantly inhibited processing of several target proteins in living cells. Our study provides the first evidence that Taspase1 is already active as an αβ monomer, arguing against heterocomplex formation being required for its pathobiological activity. Thus, it clearly supports strategies aiming to inhibit the cancer-promoting activity of Taspase1 by the identification of chemical decoys enforcing its dimerization.

Overexpression of the catalytically impaired Taspase1 T234V or Taspase1 D233A variants does not have a dominant negative effect in T(4;11) leukemia cells

Background

The chromosomal translocation t(4;11)(q21;q23) is associated with high-risk acute lymphoblastic leukemia of infants. The resulting AF4•MLL oncoprotein becomes activated by Taspase1 hydrolysis and is considered to promote oncogenic transcriptional activation. Hence, Taspase1’s proteolytic activity is a critical step in AF4•MLL pathophysiology. The Taspase1 proenzyme is autoproteolytically processed in its subunits and is assumed to assemble into an αββα-heterodimer, the active protease. Therefore, we investigated here whether overexpression of catalytically inactive Taspase1 variants are able to interfere with the proteolytic activity of the wild type enzyme in AF4•MLL model systems.

Methodology/Findings

The consequences of overexpressing the catalytically dead Taspase1 mutant, Taspase1^T234V, or the highly attenuated variant, Taspase1^D233A, on Taspase1’s processing of AF4•MLL and of other Taspase1 targets was analyzed in living cancer cells employing an optimized cell-based assay. Notably, even a nine-fold overexpression of the respective Taspase1 mutants neither inhibited Taspase1’s cis- nor trans-cleavage activity in vivo. Likewise, enforced expression of the α- or β-subunits showed no trans-dominant effect against the ectopically or endogenously expressed enzyme. Notably, co-expression of the individual α- and β-subunits did not result in their assembly into an enzymatically active protease complex. Probing Taspase1 multimerization in living cells by a translocation-based protein interaction assay as well as by biochemical methods indicated that the inactive Taspase1 failed to assemble into stable heterocomplexes with the wild type enzyme.

Conclusions

Collectively, our results demonstrate that inefficient heterodimerization appears to be the mechanism by which inactive Taspase1 variants fail to inhibit wild type Taspase1’s activity in trans. Our work favours strategies targeting Taspase1’s catalytic activity rather than attempts to block the formation of active Taspase1 dimers to interfere with the pathobiological function of AF4•MLL.

Bioassays to monitor Taspase1 function for the identification of pharmacogenetic inhibitors

Background

Threonine Aspartase 1 (Taspase1) mediates cleavage of the mixed lineage leukemia (MLL) protein and leukemia provoking MLL-fusions. In contrast to other proteases, the understanding of Taspase1's (patho)biological relevance and function is limited, since neither small molecule inhibitors nor cell based functional assays for Taspase1 are currently available.

Methodology/Findings

Efficient cell-based assays to probe Taspase1 function in vivo are presented here. These are composed of glutathione S-transferase, autofluorescent protein variants, Taspase1 cleavage sites and rational combinations of nuclear import and export signals. The biosensors localize predominantly to the cytoplasm, whereas expression of biologically active Taspase1 but not of inactive Taspase1 mutants or of the protease Caspase3 triggers their proteolytic cleavage and nuclear accumulation. Compared to in vitro assays using recombinant components the in vivo assay was highly efficient. Employing an optimized nuclear translocation algorithm, the triple-color assay could be adapted to a high-throughput microscopy platform (Z'factor = 0.63). Automated high-content data analysis was used to screen a focused compound library, selected by an in silico pharmacophor screening approach, as well as a collection of fungal extracts. Screening identified two compounds, N-[2-[(4-amino-6-oxo-3H-pyrimidin-2-yl)sulfanyl]ethyl]benzenesulfonamide and 2-benzyltriazole-4,5-dicarboxylic acid, which partially inhibited Taspase1 cleavage in living cells. Additionally, the assay was exploited to probe endogenous Taspase1 in solid tumor cell models and to identify an improved consensus sequence for efficient Taspase1 cleavage. This allowed the in silico identification of novel putative Taspase1 targets. Those include the FERM Domain-Containing Protein 4B, the Tyrosine-Protein Phosphatase Zeta, and DNA Polymerase Zeta. Cleavage site recognition and proteolytic processing of these substrates were verified in the context of the biosensor.

Conclusions

The assay not only allows to genetically probe Taspase1 structure function in vivo, but is also applicable for high-content screening to identify Taspase1 inhibitors. Such tools will provide novel insights into Taspase1's function and its potential therapeutic relevance.

An improved bimolecular fluorescence complementation tool based on superfolder green fluorescent protein

Bimolecular fluorescence complementation (BiFC) has been widely used in the analysis of protein-protein interactions (PPIs) in recent years. There are many notable advantages of BiFC such as convenience and direct visualization of PPI in cells. However, BiFC has one common limitation: the separated non-fluorescent fragments can be spontaneously self-assembled into an intact protein, which leads to false-positive results. In this study, a pair of complementary fragments (sfGFPN and sfGFPC) was constructed by splitting superfolder GFP (sfGFP) between the 214 and 215 amino acid residue, and sfGFPC was mutated by site-directed gene mutagenesis to decrease the signal of negative control. Our results showed that mutations in sfGFPC (sfGFPC(m12)) can effectively decrease the signal of negative control. Thus, we provide an improved BiFC tool for the analysis of PPI. Further, since the self-assembly problem is a common shortcoming for application of BiFC, our research provides a feasible strategy for other BiFC candidate proteins with the same problem.

Cell-based analysis of structure-function activity of threonine aspartase 1

Taspase1 is a threonine protease responsible for cleaving intracellular substrates. As such, (de)regulated Taspase1 function is expected not only to be vital for ordered development but may also be relevant for disease. However, the full repertoires of Taspase1 targets as well as the exact biochemical requirements for its efficient and substrate-specific cleavage are not yet resolved. Also, no cellular assays for this protease are currently available, hampering the exploitation of the (patho)biological relevance of Taspase1. Here, we developed highly efficient cell-based translocation biosensor assays to probe Taspase1 trans-cleavage in vivo. These modular sensors harbor variations of Taspase1 cleavage sites and localize to the cytoplasm. Expression of Taspase1 but not of inactive Taspase1 mutants or of unrelated proteases triggers proteolytic cleavage and nuclear accumulation of the biosensors. Employing our assay combined with scanning mutagenesis, we identified the sequence and spatial requirements for efficient Taspase1 processing in liquid and solid tumor cell lines. Collectively, our results defined an improved Taspase1 consensus recognition sequence, Q(3)(F/I/L/V)(2)D(1)↓G(1)'X(2)'D(3)'D(4)', allowing the first genome-wide bioinformatic identification of the human Taspase1 degradome. Among the 27 most likely Taspase1 targets are cytoplasmic but also nuclear proteins, such as the upstream stimulatory factor 2 (USF2) or the nuclear RNA export factors 2/5 (NXF2/5). Cleavage site recognition and proteolytic processing of selected targets were verified in the context of the biosensor and for the full-length proteins. We provide novel mechanistic insights into the function and bona fide targets of Taspase1 allowing for a focused investigation of the (patho)biological relevance of this type 2 asparaginase.

Time-resolved luminescence resonance energy transfer imaging of protein-protein interactions in living cells

Förster resonance energy transfer (FRET) with fluorescent proteins permits high spatial resolution imaging of protein-protein interactions in living cells. However, substantial non-FRET fluorescence background can obscure small FRET signals, making many potential interactions unobservable by conventional FRET techniques. Here we demonstrate time-resolved microscopy of luminescence resonance energy transfer (LRET) for live-cell imaging of protein-protein interactions. A luminescent terbium complex, TMP-Lumi4, was introduced into cultured cells using two methods: (i) osmotic lysis of pinocytic vesicles; and (ii) reversible membrane permeabilization with streptolysin O. Upon intracellular delivery, the complex was observed to bind specifically and stably to transgenically expressed Escherichia coli dihydrofolate reductase (eDHFR) fusion proteins. LRET between the eDHFR-bound terbium complex and green fluorescent protein (GFP) was detected as long-lifetime, sensitized GFP emission. Background signals from cellular autofluorescence and directly excited GFP fluorescence were effectively eliminated by imposing a time delay (10 micros) between excitation and detection. Background elimination made it possible to detect interactions between the first PDZ domain of ZO-1 (fused to eDHFR) and the C-terminal YV motif of claudin-1 (fused to GFP) in single microscope images at subsecond time scales. We observed a highly significant (P<10(-6)), six-fold difference between the mean, donor-normalized LRET signal from cells expressing interacting fusion proteins and from control cells expressing noninteracting mutants. The results show that time-resolved LRET microscopy with a selectively targeted, luminescent terbium protein label affords improved speed and sensitivity over conventional FRET methods for a variety of live-cell imaging and screening applications.

The nuclear translocation assay for intracellular protein-protein interactions and its application to the Bcr coiled-coil domain

Protein interactions are critical for normal biological processes and molecular pathogenesis. While it is important to study these interactions, there are limited assays that are performed inside the cell, in the native cell environment, where the majority of protein-protein interactions take place. Here we present a method of studying protein interactions intracellularly using one protein of interest fused to a localization-controllable enhanced GFP (EGFP) construct and the other protein of interest fused to the red fluorescent protein, DsRed. Nuclear translocation of the EGFP construct is induced by addition of a ligand, and the difference in nuclear localization between the induced and noninduced states of the DsRed construct provides an indication of the interaction between the two proteins. This assay, the nuclear translocation assay (NTA), is introduced here as broadly applicable for studying protein interactions in the native environment inside cells and is demonstrated using forms of the coiled-coil domain from the breakpoint cluster region (Bcr) protein.

A flow cytometric assay for the study of E3 ubiquitin ligase activity

Current methods for monitoring E3 ubiquitin ligase activity in cell culture or in vivo are limited. As a result, the degradation of cellular targets by many E3 ubiquitin ligases in live cells has not yet been examined. For this study, a target of an E3 ubiquitin ligase was expressed as a fluorescently labeled protein in cell culture. If the E3 ubiquitin ligase mediates the degradation of a target protein in cell culture, it is expected that the target will show a reduced fluorescence signal by FCM analysis. We initially used the E3 ubiquitin ligase, herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) and one of its targets, promyelocytic leukemia (PML) protein, to determine the feasibility of our approach. Cells expressing a PML-GFP fusion protein were selected by cell sorting and infected with an adenoviral vector expressing ICP0. In contrast to mock-infected cells, only PML-GFP-expressing cells infected with the ICP0 adenoviral vector led to a significant decrease in the fluorescence signal of PML-GFP when examined by fluorescence microscopy and FCM analysis. Our results suggest that it is possible to examine the live activity of an E3 ubiquitin ligase (via one of its targets) in cell culture by FCM analysis.

Identification of inhibitors for MDM2 ubiquitin ligase activity from natural product extracts by a novel high-throughput electrochemiluminescent screen

High-throughput screening technologies have revolutionized the manner in which potential therapeutics are identified. Although they are the source of lead compounds for ~65% of anticancer and antimicrobial drugs approved by the Food and Drug Administration between 1981 and 2002, natural products have largely been excluded from modern screening programs. This is due, at least in part, to the inherent difficulties in testing complex extract mixtures, which often contain nuisance compounds, in modern bioassay systems. In this article, the authors present a novel electrochemiluminescent assay system for inhibition of MDM2 activity that is suitable for testing natural product extracts in high-throughput screening systems. The assay was used to screen more than 144,000 natural product extracts. The authors identified 1 natural product, sempervirine, that inhibited MDM2 auto-ubiquitination, MDM2-mediated p53 degradation, and led to accumulation of p53 in cells. Sempervirine preferentially induced apoptosis in transformed cells expressing wild-type p53, suggesting that it could be a potential lead for anticancer therapeutics.

Phosphorylation of nm23-H1 by CKI induces its complex formation with h-prune and promotes cell motility

The combination of an increase in the cAMP-phosphodiesterase activity of h-prune and its interaction with nm23-H1 have been shown to be key steps in the induction of cellular motility in breast cancer cells. Here we present the molecular mechanisms of this interaction. The region of the nm23-h-prune interaction lies between S120 and S125 of nm23, where missense mutants show impaired binding; this region has been highly conserved throughout evolution, and can undergo serine phosphorylation by casein kinase I. Thus, the casein kinase I delta-epsilon specific inhibitor IC261 impairs the formation of the nm23-h-prune complex, which translates 'in vitro' into inhibition of cellular motility in a breast cancer cellular model. A competitive permeable peptide containing the region for phosphorylation by casein kinase I impairs cellular motility to the same extent as IC261. The identification of these two modes of inhibition of formation of the nm23-H1-h-prune protein complex pave the way toward new challenges, including translational studies using IC261 or this competitive peptide 'in vivo' to inhibit cellular motility induced by nm23-H1-h-prune complex formation during progression of breast cancer.

The differentiation antigen NY-BR-1 is a potential target for antibody-based therapies in breast cancer

Antibody-based cancer immunotherapy relies on the identification and characterization of target antigens and the development of potent antibodies recognizing the target. Here we report the expression analysis and molecular characterization of the differentiation antigen NY-BR-1, which we previously identified by using the SEREX (serological analysis of recombinant cDNA expression libraries) method. Corroborating methodologies, including mRNA quantitation and immunoblotting show that NY-BR-1 is strongly expressed in >70% of 129 breast tumors. Application of a NY-BR-1 specific antibody demonstrated NY-BR-1 expression in primary and metastastic breast cancers. In contrast, most of the breast cancer cell lines tested, expressed only low NY-BR-1 levels. Importantly, confocal microscopy revealed that ectopically expressed NY-BR-1 localizes to the cytoplasm and the cell membrane. NY-BR-1 localization in breast cancer specimens was also confirmed by immunohistochemistry. Bioinformatic analysis and deletion mutagenesis further show that NY-BR-1 membrane localization is mediated by 2 cis-active membrane targeting domains. Biochemical surface labeling and FACS analysis of live cells further characterize NY-BR-1 as a transmembrane protein, which can be specifically recognized by the anti-NY-BR-1 antibody on the surface of vital cells. The strong expression of NY-BR-1 in breast tumors, its cytoplasmic and membrane localization and accessibility to an ectopically applied antibody now suggest to pursue NY-BR-1 as a potential target for antibody-based therapies in breast cancer patients.

Chemical modifications of inert organic monolayers with oxygen plasma for biosensor applications

In this paper we present a study of using oxygen plasma for chemically modifying inert hydrocarbon self-assembled monolayers of octadecyltrichlorosilane (OTS-SAMs) and rendering active surfaces for protein immobilization. Detailed surface modification and protein immobilization were characterized by using ellipsometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance spectroscopy, and fluorescence microscopy. Our XPS results showed that the surface reaction between OTS-SAMs and oxygen plasma can generate new surface functional groups such as alcohol (C-O), aldehyde (C=O), and carboxylic acid (O-C=O), and their compositions can be controlled by using different treatment times and powers. A short treatment time ( approximately 1 s) and high power (10 W) can lead to a higher density of aldehyde groups, which can serve as linker groups for protein immobilization through the formation of Schiff bases with the amine groups of proteins. By using the fluorescence immunostaining method, we confirmed that human immunoglobulin (IgG) can be immobilized on a glass slide, only if the surface was decorated with OTS-SAMs and if the OTS-SAMs were pretreated with oxygen plasma. The protein immobilized on the oxygen-plasma-treated surface can only be recognized by using a highly specific antibody, FITC-anti-IgG, but not FITC-anti-biotin.

Dynamic survivin in head and neck cancer: Molecular mechanism and therapeutic potential

Although disease management of head and neck squamous cell carcinomas (HNSCC) has improved significantly, therapy resistance leading to tumor recurrence still counteracts improvement of long-term survival. Consequently, identification of molecular markers that signal increased risk of treatment failure or, which can be exploited by targeted therapy, is urgently needed. Survivin is strongly expressed in HNSCC, and its proposed dual role as an apoptosis inhibitor and a mitotic effector positioned survivin in the front line of cancer research. Notably, survivin is detected as a cytoplasmic and as a nuclear protein in HNSCC patients, which stimulated numerous studies to investigate and to speculate on the functional and prognostic significance of its dynamic localization. This review focuses on our current understanding of the molecular mechanisms regulating survivin's intracellular localization and discusses its potential prognostic and therapeutic relevance for head and neck cancer.

Nucleocytoplasmic shuttling and the biological activity of mouse survivin are regulated by an active nuclear export signal

Survivin appears to function as a regulator of cell division and as an apoptosis inhibitor in many species. Here, we characterized the nucleocytoplasmic transport of mouse survivin(140), and its splice variants survivin(121) and survivin(40). We show that the dynamic intracellular localization of survivin(140) is mediated by a Crm1-dependent nuclear export signal (NES) present also in survivin(121), but absent in survivin(40). In contrast, neither survivin nor survivin splice variants contain an active nuclear import signal and seem to enter the nucleus by passive diffusion. The activity of the NES is required for survivin-mediated protection against cell death inducing stimuli and influences protein degradation. During mitosis, NES-deficient survivin variants fail to correctly localize to the mitotic machinery and promote proper cell division. In vivo and in vitro protein interaction assays show that survivin(140) and survivin(121) as well as their export-deficient mutants are able to form homo- as well as heterodimers. The trans-dominant negative phenotype observed upon expression of export-deficient survivin appears, therefore, to be mediated by the formation of inactive survivin heterodimers. The survivin-Crm1 axis is essential for the biological activities of murine survivin, and mouse models will allow investigating its functional implications during development and tumorigenesis.