Monitoring drug-target interactions through target engagement-mediated amplification on arrays and in situ.

Drugs are designed to bind their target proteins in physiologically relevant tissues and organs to modulate biological functions and elicit desirable clinical outcomes. Information about target engagement at cellular and subcellular resolution is therefore critical for guiding compound optimization in drug discovery, and for probing resistance mechanisms to targeted therapies in clinical samples. We describe a target engagement-mediated amplification (TEMA) technology, where oligonucleotide-conjugated drugs are used to visualize and measure target engagement in situ, amplified via rolling-circle replication of circularized oligonucleotide probes. We illustrate the TEMA technique using dasatinib and gefitinib, two kinase inhibitors with distinct selectivity profiles. In vitro binding by the dasatinib probe to arrays of displayed proteins accurately reproduced known selectivity profiles, while their differential binding to fixed adherent cells agreed with expectations from expression profiles of the cells. We also introduce a proximity ligation variant of TEMA to selectively investigate binding to specific target proteins of interest. This form of the assay serves to improve resolution of binding to on- and off-target proteins. In conclusion, TEMA has the potential to aid in drug development and clinical routine by conferring valuable insights in drug-target interactions at spatial resolution in protein arrays, cells and in tissues.

A multiplex platform for digital measurement of circular DNA reaction products.

Digital PCR provides high sensitivity and unprecedented accuracy in DNA quantification, but current approaches require dedicated instrumentation and have limited opportunities for multiplexing. Here, we present an isothermal platform for digital enumeration of DNA reaction products in multiplex via standard fluorescence microscopy. Circular DNA strands, which may result from a wide range of molecular detection reactions, are captured on streptavidin-coated surfaces via hybridized biotinylated primers, followed by rolling circle amplification (RCA). The addition of 15% polyethylene glycol 4000 during RCA resulted in uniform, easily recorded reaction products. Immobilized DNA circles were visualized as RCA products with 100% efficiency, as determined by droplet digital PCR. We confirmed previous reports about the influence on RCA by sequence composition and size of RCA templates, and we developed an efficient one-step restaining procedure for sequential multiplexing using toehold-triggered DNA strand displacement. Finally, we exemplify applications of this digital readout platform by demonstrating more than three orders of magnitude improved sensitivity by digital measurement of prostate specific antigen (PSA) (detection threshold ∼100 pg/l), compared to a commercial enzyme-linked immunosorbent assay (ELISA) with analogue readout (detection threshold ∼500 ng/l), using the same antibody pair.

Close Encounters - Probing Proximal Proteins in Live or Fixed Cells.

The well-oiled machinery of the cellular proteome operates via variable expression, modifications, and interactions of proteins, relaying genomic and transcriptomic information to coordinate cellular functions. In recent years, a number of techniques have emerged that serve to identify sets of proteins acting in close proximity in the course of orchestrating cellular activities. These proximity-dependent assays, including BiFC, BioID, APEX, FRET, and isPLA, have opened up new avenues to examine protein interactions in live or fixed cells. We review herein the current status of proximity-dependent in situ techniques. We compare the advantages and limitations of the methods, underlining recent progress and the growing importance of these techniques in basic research, and we discuss their potential as tools for drug development and diagnostics.

PARP-1 attenuates Smad-mediated transcription.

The versatile cytokine transforming growth factor ß (TGF-ß) regulates cellular growth, differentiation, and migration during embryonic development and adult tissue homeostasis. Activation of TGF-ß receptors leads to phosphorylation of Smad2 and Smad3, which oligomerize with Smad4 and accumulate in the nucleus where they recognize gene regulatory regions and orchestrate transcription. Termination of Smad-activated transcription involves Smad dephosphorylation, nuclear export, or ubiquitin-mediated degradation. In an unbiased proteomic screen, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a Smad-interacting partner. PARP-1 dissociates Smad complexes from DNA by ADP-ribosylating Smad3 and Smad4, which attenuates Smad-specific gene responses and TGF-ß-induced epithelial-mesenchymal transition. Thus, our results identify ADP-ribosylation of Smad proteins by PARP-1 as a key step in controlling the strength and duration of Smad-mediated transcription.

Efficient CRISPR-rAAV engineering of endogenous genes to study protein function by allele-specific RNAi.

Gene knockout strategies, RNAi and rescue experiments are all employed to study mammalian gene function. However, the disadvantages of these approaches include: loss of function adaptation, reduced viability and gene overexpression that rarely matches endogenous levels. Here, we developed an endogenous gene knockdown/rescue strategy that combines RNAi selectivity with a highly efficient CRISPR directed recombinant Adeno-Associated Virus (rAAV) mediated gene targeting approach to introduce allele-specific mutations plus an allele-selective siRNA Sensitive (siSN) site that allows for studying gene mutations while maintaining endogenous expression and regulation of the gene of interest. CRISPR/Cas9 plus rAAV targeted gene-replacement and introduction of allele-specific RNAi sensitivity mutations in the CDK2 and CDK1 genes resulted in a >85% site-specific recombination of Neo-resistant clones versus ∼8% for rAAV alone. RNAi knockdown of wild type (WT) Cdk2 with siWT in heterozygotic knockin cells resulted in the mutant Cdk2 phenotype cell cycle arrest, whereas allele specific knockdown of mutant CDK2 with siSN resulted in a wild type phenotype. Together, these observations demonstrate the ability of CRISPR plus rAAV to efficiently recombine a genomic locus and tag it with a selective siRNA sequence that allows for allele-selective phenotypic assays of the gene of interest while it remains expressed and regulated under endogenous control mechanisms.

Transcriptional induction of salt-inducible kinase 1 by transforming growth factor ß leads to negative regulation of type I receptor signaling in cooperation with the Smurf2 ubiquitin ligase.

Transforming growth factor ß (TGFß) regulates many physiological processes and requires control mechanisms to safeguard proper and timely action. We have previously described how negative regulation of TGFß signaling is controlled by the serine/threonine kinase salt-inducible kinase 1 (SIK1). SIK1 forms complexes with the TGFß type I receptor and with the inhibitory Smad7 and down-regulates the type I receptor. We now demonstrate that TGFß induces SIK1 levels via a direct transcriptional mechanism that implicates the Smad proteins, and we have mapped a putative enhancer element on the SIK1 gene. We provide evidence that the ubiquitin ligase Smurf2 forms complexes and functionally cooperates with SIK1. Both the kinase activity of SIK1 and the ubiquitin ligase activity of Smurf2 are important for proper type I receptor turnover. We also show that knockdown of endogenous SIK1 and Smurf2 enhances physiological signaling by TGFß that leads to epithelial growth arrest. In conclusion, TGFß induces expression of Smad7, Smurf2, and SIK1, the products of which physically and functionally interlink to control the activity of this pathway.

Image-based high-throughput mapping of TGF-ß-induced phosphocomplexes at a single-cell level.

Protein interactions and posttranslational modifications orchestrate cellular responses to e.g. cytokines and drugs, but it has been difficult to monitor these dynamic events in high-throughput. Here, we describe a semi-automated system for large-scale in situ proximity ligation assays (isPLA), combining isPLA in microtiter wells with automated microscopy and computer-based image analysis. Phosphorylations and interactions are digitally recorded along with subcellular morphological features. We investigated TGF-ß-responsive Smad2 linker phosphorylations and complex formations over time and across millions of individual cells, and we relate these events to cell cycle progression and local cell crowding via measurements of DNA content and nuclear size of individual cells, and of their relative positions. We illustrate the suitability of this protocol to screen for drug effects using phosphatase inhibitors. Our approach expands the scope for image-based single cell analyses by combining observations of protein interactions and modifications with morphological details of individual cells at high throughput.

Detection of post-translational modifications using solid-phase proximity ligation assay.

Post-translational modifications (PTMs) regulate protein activities to help orchestrate and fine-tune cellular processes. Dysregulation of PTMs is often related with disorders and malignancies, and may serve as a precise biomarker of disease. Developing sensitive tools to measure and monitor low-abundant PTMs in tissue lysates or serum will be instrumental for opening up new PTM-based diagnostic avenues. Here, we investigate the use of solid-phase proximity ligation assay (SP-PLA) for detection of different PTMs. The assay depends on the recognition of the target protein molecule and its modification by three affinity binders. Using antibodies and lectins, we applied the method for detection of glycosylated CD44 and E-Cadherin, and phosphorylated p53 and EGFR. The assay was found to have superior dynamic range and limit of detection compared to standard ELISAs. In summary, we have established the use of SP-PLA as an appropriate method for sensitive detection of PTMs in lysates and sera, which may provide a basis for future PTM-based diagnostic and prognostic biomarkers.

Enhancing Endosomal Escape for Intracellular Delivery of Macromolecular Biologic Therapeutics.

Bioactive macromolecular peptides and oligonucleotides have significant therapeutic potential. However, due to their size, they have no ability to enter the cytoplasm of cells. Peptide/Protein transduction domains (PTDs), also called cell-penetrating peptides (CPPs), can promote uptake of macromolecules via endocytosis. However, overcoming the rate-limiting step of endosomal escape into the cytoplasm remains a major challenge. Hydrophobic amino acid R groups are known to play a vital role in viral escape from endosomes. Here we utilize a real-time, quantitative live cell split-GFP fluorescence complementation phenotypic assay to systematically analyze and optimize a series of synthetic endosomal escape domains (EEDs). By conjugating EEDs to a TAT-PTD/CPP spilt-GFP peptide complementation assay, we were able to quantitatively measure endosomal escape into the cytoplasm of live cells via restoration of GFP fluorescence by intracellular molecular complementation. We found that EEDs containing two aromatic indole rings or one indole ring and two aromatic phenyl groups at a fixed distance of six polyethylene glycol (PEG) units from the TAT-PTD-cargo significantly enhanced cytoplasmic delivery in the absence of cytotoxicity. EEDs address the critical rate-limiting step of endosomal escape in delivery of macromolecular biologic peptide, protein and siRNA therapeutics into cells.

Cationic PTD/CPP-mediated macromolecular delivery: charging into the cell.

INTRODUCTION: Macromolecular therapeutics, including enzymes, transcription factors, siRNAs, peptides and large synthetic molecules, can potentially be used to treat human diseases by targeting intracellular molecular pathways and modulating biological responses. However, large macromolecules have no ability to enter cells and require delivery vehicles. Protein transduction domains (PTDs), also known as cell-penetrating peptides (CPPs), are a diverse class of peptides that can deliver macromolecules into cells. AREAS COVERED: In this review, we cover the uptake and usage of arginine-rich PTDs/CPPs (TAT-PTD, Penetratin/Antp and 8R). We review the endocytosis-mediated uptake of these peptides and highlight three important steps: i) cell association; ii) internalization and iii) endosomal escape. We also discuss the array of different cargos that have been delivered by cationic PTDs/CPPs as well as cellular processes and biological responses that have been modulated. EXPERT OPINION: PTDs/CPPs have shown great potential to deliver otherwise undeliverable macromolecular therapeutics into cells for experimentation in cell culture and in animal disease models in vivo. Moreover, over 25 clinical trials have been performed predominantly using the TAT-PTD. However, more work is still needed. Endosomal escape and target-cell specificity remain two of the major future challenges.

Cellular activity of siRNA oligonucleotides containing synthetic isomorphic nucleoside surrogates.

Singly and multiply modified synthetic siRNA oligonucleotides, containing isomorphic surrogate nucleobases, show high interference potency in cell culture, suggesting the highly isomorphic RNA alphabet, based on a thieno[3,4-d]-pyrimidine core, is tolerated well by the cellular silencing machinery.

Crosstalk between Hippo and TGFß: Subcellular Localization of YAP/TAZ/Smad Complexes.

The Hippo pathway plays a crucial role in growth control, proliferation and tumor suppression. Activity of the signaling pathway is associated with cell density sensing and tissue organization. Furthermore, the Hippo pathway helps to coordinate cellular processes through crosstalk with growth-factor-mediated signaling pathways such as TGFß. Here we have examined the localization of interactions between proteins of the Hippo pathway (YAP/TAZ) and TGFß (Smad2/3) signaling pathway by using in situ proximity ligation assays. We investigated the formation of protein complexes between YAP/TAZ and Smad2/3 and examined how these interactions were affected by TGFß stimulation and cell density in HaCaT keratinocytes and in Smad4-deficient HT29 colon cancer cells. We demonstrate that TGFß induces formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells. Under sparse cell conditions, the complexes were detected to a higher degree and were predominantly located in the nucleus, while under dense culture conditions, the complexes were fewer and mainly located in the cytoplasm. Surprisingly, we could not detect any YAP/TAZ-Smad2/3 complexes in HT29 cells. To examine if Smad4 deficiency was responsible for the absence of interactions, we treated HaCaT cells with siRNA targeting Smad4. However, we could still observe complex formation in the siRNA-treated cells, suggesting that Smad4 is not essential for the YAP-Smad2/3 interaction. In conclusion, this study shows localized, density-dependent formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells and provides evidence supporting a crosstalk between the Hippo and the TGFß signaling pathways.

Functional loss of IκBε leads to NF-κB deregulation in aggressive chronic lymphocytic leukemia.

NF-κB is constitutively activated in chronic lymphocytic leukemia (CLL); however, the implicated molecular mechanisms remain largely unknown. Thus, we performed targeted deep sequencing of 18 core complex genes within the NF-κB pathway in a discovery and validation CLL cohort totaling 315 cases. The most frequently mutated gene was NFKBIE (21/315 cases; 7%), which encodes IκBε, a negative regulator of NF-κB in normal B cells. Strikingly, 13 of these cases carried an identical 4-bp frameshift deletion, resulting in a truncated protein. Screening of an additional 377 CLL cases revealed that NFKBIE aberrations predominated in poor-prognostic patients and were associated with inferior outcome. Minor subclones and/or clonal evolution were also observed, thus potentially linking this recurrent event to disease progression. Compared with wild-type patients, NFKBIE-deleted cases showed reduced IκBε protein levels and decreased p65 inhibition, along with increased phosphorylation and nuclear translocation of p65. Considering the central role of B cell receptor (BcR) signaling in CLL pathobiology, it is notable that IκBε loss was enriched in aggressive cases with distinctive stereotyped BcR, likely contributing to their poor prognosis, and leading to an altered response to BcR inhibitors. Because NFKBIE deletions were observed in several other B cell lymphomas, our findings suggest a novel common mechanism of NF-κB deregulation during lymphomagenesis.

Fine-tuning of Smad protein function by poly(ADP-ribose) polymerases and poly(ADP-ribose) glycohydrolase during transforming growth factor ß signaling.

BACKGROUND: Initiation, amplitude, duration and termination of transforming growth factor ß (TGFß) signaling via Smad proteins is regulated by post-translational modifications, including phosphorylation, ubiquitination and acetylation. We previously reported that ADP-ribosylation of Smads by poly(ADP-ribose) polymerase 1 (PARP-1) negatively influences Smad-mediated transcription. PARP-1 is known to functionally interact with PARP-2 in the nucleus and the enzyme poly(ADP-ribose) glycohydrolase (PARG) can remove poly(ADP-ribose) chains from target proteins. Here we aimed at analyzing possible cooperation between PARP-1, PARP-2 and PARG in regulation of TGFß signaling. METHODS: A robust cell model of TGFß signaling, i.e. human HaCaT keratinocytes, was used. Endogenous Smad3 ADP-ribosylation and protein complexes between Smads and PARPs were studied using proximity ligation assays and co-immunoprecipitation assays, which were complemented by in vitro ADP-ribosylation assays using recombinant proteins. Real-time RT-PCR analysis of mRNA levels and promoter-reporter assays provided quantitative analysis of gene expression in response to TGFß stimulation and after genetic perturbations of PARP-1/-2 and PARG based on RNA interference. RESULTS: TGFß signaling rapidly induces nuclear ADP-ribosylation of Smad3 that coincides with a relative enhancement of nuclear complexes of Smads with PARP-1 and PARP-2. Inversely, PARG interacts with Smads and can de-ADP-ribosylate Smad3 in vitro. PARP-1 and PARP-2 also form complexes with each other, and Smads interact and activate auto-ADP-ribosylation of both PARP-1 and PARP-2. PARP-2, similar to PARP-1, negatively regulates specific TGFß target genes (fibronectin, Smad7) and Smad transcriptional responses, and PARG positively regulates these genes. Accordingly, inhibition of TGFß-mediated transcription caused by silencing endogenous PARG expression could be relieved after simultaneous depletion of PARP-1. CONCLUSION: Nuclear Smad function is negatively regulated by PARP-1 that is assisted by PARP-2 and positively regulated by PARG during the course of TGFß signaling.

Efficient delivery of RNAi prodrugs containing reversible charge-neutralizing phosphotriester backbone modifications.

RNA interference (RNAi) has great potential to treat human disease. However, in vivo delivery of short interfering RNAs (siRNAs), which are negatively charged double-stranded RNA macromolecules, remains a major hurdle. Current siRNA delivery has begun to move away from large lipid and synthetic nanoparticles to more defined molecular conjugates. Here we address this issue by synthesis of short interfering ribonucleic neutrals (siRNNs) whose phosphate backbone contains neutral phosphotriester groups, allowing for delivery into cells. Once inside cells, siRNNs are converted by cytoplasmic thioesterases into native, charged phosphodiester-backbone siRNAs, which induce robust RNAi responses. siRNNs have favorable drug-like properties, including high synthetic yields, serum stability and absence of innate immune responses. Unlike siRNAs, siRNNs avidly bind serum albumin to positively influence pharmacokinetic properties. Systemic delivery of siRNNs conjugated to a hepatocyte-specific targeting domain induced extended dose-dependent in vivo RNAi responses in mice. We believe that siRNNs represent a technology that will open new avenues for development of RNAi therapeutics.

Regulating the stability of TGFbeta receptors and Smads.

Transforming growth factor beta (TGFbeta) controls cellular behavior in embryonic and adult tissues. TGFbeta binding to serine/threonine kinase receptors on the plasma membrane activates Smad molecules and additional signaling proteins that together regulate gene expression. In this review, mechanisms and models that aim at explaining the coordination between several components of the signaling network downstream of TGFbeta are presented. We discuss how the activity and duration of TGFbeta receptor/Smad signaling can be regulated by post-translational modifications that affect the stability of key proteins in the pathway. We highlight links between these mechanisms and human diseases, such as tissue fibrosis and cancer.

TGFbeta induces SIK to negatively regulate type I receptor kinase signaling.

Signal transduction by transforming growth factor beta (TGFbeta) coordinates physiological responses in diverse cell types. TGFbeta signals via type I and type II receptor serine/threonine kinases and intracellular Smad proteins that regulate transcription. Strength and duration of TGFbeta signaling is largely dependent on a negative-feedback program initiated during signal progression. We have identified an inducible gene target of TGFbeta/Smad signaling, the salt-inducible kinase (SIK), which negatively regulates signaling together with Smad7. SIK and Smad7 form a complex and cooperate to down-regulate the activated type I receptor ALK5. We further show that both the kinase and ubiquitin-associated domain of SIK are required for proper ALK5 degradation, with ubiquitin functioning to enhance SIK-mediated receptor degradation. Loss of endogenous SIK results in enhanced gene responses of the fibrotic and cytostatic programs of TGFbeta. We thus identify in SIK a negative regulator that controls TGFbeta receptor turnover and physiological signaling.