Characterization of Organic Anion and Cation Transport in Three Human Renal Proximal Tubular Epithelial Models.

The polarised expression of specific transporters in proximal tubular epithelial cells is important for the renal clearance of many endogenous and exogenous compounds. Thus, ideally, the in vitro tools utilised for predictions would have a similar expression of apical and basolateral xenobiotic transporters as in vivo. Here, we assessed the functionality of organic cation and anion transporters in proximal tubular-like cells (PTL) differentiated from human induced pluripotent stem cells (iPSC), primary human proximal tubular epithelial cells (PTEC), and telomerase-immortalised human renal proximal tubular epithelial cells (RPTEC/TERT1). Organic cation and anion transport were studied using the fluorescent substrates 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP) and 6-carboxyfluorescein (6-CF), respectively. The level and rate of intracellular ASP accumulation in PTL following basolateral application were slightly lower but within a 3-fold range compared to primary PTEC and RPTEC/TERT1 cells. The basolateral uptake of ASP and its subsequent apical efflux could be inhibited by basolateral exposure to quinidine in all models. Of the three models, only PTL showed a modest preferential basolateral-to-apical 6-CF transfer. These results show that organic cation transport could be demonstrated in all three models, but more research is needed to improve and optimise organic anion transporter expression and functionality.

Development of a high-throughput in vitro screening method for the assessment of cell-damaging activities of snake venoms.

Snakebite envenoming is a globally important public health issue that has devastating consequences on human health and well-being, with annual mortality rates between 81,000 and 138,000. Snake venoms may cause different pathological effects by altering normal physiological processes such as nervous transfer and blood coagulation. In addition, snake venoms can cause severe (local) tissue damage that may result in life-long morbidities, with current estimates pointing towards an additional 450,000 individuals that suffer from permanent disabilities such as amputations, contractions and blindness. Despite such high morbidity rates, research to date has been mainly focusing on neurotoxic and haemotoxic effects of snake venoms and considerably less on venom-induced tissue damage. The molecular mechanisms underlaying this pathology include membrane disruption and extracellular matrix degradation. This research describes methods used to study the (molecular) mechanisms underlaying venom-induced cell- and tissue damage. A selection of cellular bioassays and fluorescent microscopy were used to study cell-damaging activities of snake venoms in multi-well plates, using both crude and fractionated venoms. A panel of 10 representative medically relevant snake species was used, which cover a large part of the geographical regions most heavily affected by snakebite. The study comprises both morphological data as well as quantitative data on cell metabolism and viability, which were measured over time. Based on this data, a distinction could be made in the ways by which viper and elapid venoms exert their effects on cells. We further made an effort to characterise the bioactive compounds causing these effects, using a combination of liquid chromatography methods followed by bioassaying and protein identification using proteomics. The outcomes of this study might prove valuable for better understanding venom-induced cell- and tissue-damaging pathologies and could be used in the process of developing and improving snakebite treatments.

Structural and Molecular Determinants for Isoform Bias at Human Histamine H3 Receptor Isoforms.

The human histamine H3 receptor (hH3R) is predominantly expressed in the CNS, where it regulates the synthesis and release of histamine and other neurotransmitters. Due to its neuromodulatory role, the hH3R has been associated with various CNS disorders, including Alzheimer's and Parkinson's disease. Markedly, the hH3R gene undergoes extensive splicing, resulting in 20 isoforms, of which 7TM isoforms exhibit variations in the intracellular loop 3 (IL3) and/or C-terminal tail. Particularly, hH3R isoforms that display variations in IL3 (e.g., hH3R-365) are shown to differentially signal via Gαi-dependent pathways upon binding of biased agonists (e.g., immepip, proxifan, imetit). Nevertheless, the mechanisms underlying biased agonism at hH3R isoforms remain unknown. Using a structure-function relationship study with a broad range of H3R agonists, we thereby explored determinants underlying isoform bias at hH3R isoforms that exhibit variations in IL3 (i.e., hH3R-445, -415, -365, and -329) in a Gαi-dependent pathway (cAMP inhibition). Hence, we systematically characterized hH3R isoforms on isoform bias by comparing various ligand properties (i.e., structural and molecular) to the degree of isoform bias. Importantly, our study provides novel insights into the structural and molecular basis of receptor isoform bias, highlighting the importance to study GPCRs with multiple isoforms to better tailor drugs.

Capturing time-dependent activation of genes and stress-response pathways using transcriptomics in iPSC-derived renal proximal tubule cells.

Transcriptomic analysis is a powerful method in the utilization of New Approach Methods (NAMs) for identifying mechanisms of toxicity and application to hazard characterization. With this regard, mapping toxicological events to time of exposure would be helpful to characterize early events. Here, we investigated time-dependent changes in gene expression levels in iPSC-derived renal proximal tubular-like cells (PTL) treated with five diverse compounds using TempO-Seq transcriptomics with the aims to evaluate the application of PTL for toxicity prediction and to report on temporal effects for the activation of cellular stress response pathways. PTL were treated with either 50 µM amiodarone, 10 µM sodium arsenate, 5 nM rotenone, or 300 nM tunicamycin over a temporal time course between 1 and 24 h. The TGFß-type I receptor kinase inhibitor GW788388 (1 µM) was used as a negative control. Pathway analysis revealed the induction of key stress-response pathways, including Nrf2 oxidative stress response, unfolding protein response, and metal stress response. Early response genes per pathway were identified much earlier than 24 h and included HMOX1, ATF3, DDIT3, and several MT1 isotypes. GW788388 did not induce any genes within the stress response pathways above, but showed deregulation of genes involved in TGFß inhibition, including downregulation of CYP24A1 and SERPINE1 and upregulation of WT1. This study highlights the application of iPSC-derived renal cells for prediction of cellular toxicity and sheds new light on the temporal and early effects of key genes that are involved in cellular stress response pathways.

SAR exploration of the non-imidazole histamine H3 receptor ligand ZEL-H16 reveals potent inverse agonism.

Histamine H3 receptor (H3 R) agonists without an imidazole moiety remain very scarce. Of these, ZEL-H16 (1) has been reported previously as a high-affinity non-imidazole H3 R (partial) agonist. Our structure-activity relationship analysis using derivatives of 1 identified both basic moieties as key interaction motifs and the distance of these from the central core as a determinant for H3 R affinity. However, in spite of the reported H3 R (partial) agonism, in our hands, 1 acts as an inverse agonist for Gαi signaling in a CRE-luciferase reporter gene assay and using an H3 R conformational sensor. Inverse agonism was also observed for all of the synthesized derivatives of 1. Docking studies and molecular dynamics simulations suggest ionic interactions/hydrogen bonds to H3 R residues D1143.32 and E2065.46 as essential interaction points.

Fusicoccin-A Targets Cancerous Inhibitor of Protein Phosphatase 2A by Stabilizing a C-Terminal Interaction with 14-3-3.

The cancerous inhibitor of protein phosphatase 2A (CIP2A) is an oncoprotein found overexpressed in many types of cancer. CIP2A has been shown to stabilize oncoproteins such as cMYC by shielding them from PP2A-mediated dephosphorylation. Here we report that the penultimate residue Ser904 in the C-terminus of CIP2A can be phosphorylated to create a binding site for the regulatory protein 14-3-3. We demonstrate that 14-3-3 is a new interaction partner of CIP2A. The 14-3-3/CIP2A C-terminal interaction complex can be targeted by the protein-protein interaction (PPI) stabilizer fusicoccin-A (FC-A), resulting in enhanced levels of phosphorylated Ser904. FC-A treatment of TNBC cells leads to the increased association of CIP2A with 14-3-3. We show that the composite interface between 14 and 3-3 and CIP2A's C-terminus can be targeted by the PPI stabilizer FC-A, providing a new interface that could potentially be exploited to modulate CIP2A's activity.

Fragment Screening Yields a Small-Molecule Stabilizer of 14-3-3 Dimers That Modulates Client Protein Interactions.

The development of protein-protein interaction (PPI) inhibitors has been a successful strategy in drug development. However, the identification of PPI stabilizers has proven much more challenging. Here we report a fragment-based drug screening approach using the regulatory hub-protein 14-3-3 as a platform for identifying PPI stabilizers. A homogenous time-resolved FRET assay was used to monitor stabilization of 14-3-3/peptide binding using the known interaction partner estrogen receptor alpha. Screening of an in-house fragment library identified fragment 2 (VUF15640) as a putative PPI stabilizer capable of cooperatively stabilizing 14-3-3 PPIs in a cooperative fashion with Fusicoccin-A. Mechanistically, fragment 2 appears to enhance 14-3-3 dimerization leading to increased client-protein binding. Functionally, fragment 2 enhanced potency of 14-3-3 in a cell-free system inhibiting the enzyme activity of the nitrate reductase. In conclusion, we identified a general PPI stabilizer targeting 14-3-3, which could be used as a tool compound for investigating 14-3-3 client protein interactions.

Generation and characterization of iPSC-derived renal proximal tubule-like cells with extended stability.

The renal proximal tubule is responsible for re-absorption of the majority of the glomerular filtrate and its proper function is necessary for whole-body homeostasis. Aging, certain diseases and chemical-induced toxicity are factors that contribute to proximal tubule injury and chronic kidney disease progression. To better understand these processes, it would be advantageous to generate renal tissues from human induced pluripotent stem cells (iPSC). Here, we report the differentiation and characterization of iPSC lines into proximal tubular-like cells (PTL). The protocol is a step wise exposure of small molecules and growth factors, including the GSK3 inhibitor (CHIR99021), the retinoic acid receptor activator (TTNPB), FGF9 and EGF, to drive iPSC to PTL via cell stages representing characteristics of early stages of renal development. Genome-wide RNA sequencing showed that PTL clustered within a kidney phenotype. PTL expressed proximal tubular-specific markers, including megalin (LRP2), showed a polarized phenotype, and were responsive to parathyroid hormone. PTL could take up albumin and exhibited ABCB1 transport activity. The phenotype was stable for up to 7 days and was maintained after passaging. This protocol will form the basis of an optimized strategy for molecular investigations using iPSC derived PTL.

4-(3-Aminoazetidin-1-yl)pyrimidin-2-amines as High-Affinity Non-imidazole Histamine H3 Receptor Agonists with in Vivo Central Nervous System Activity.

Despite the high diversity of histamine H3 receptor (H3R) antagonist/inverse agonist structures, partial or full H3R agonists have typically been imidazole derivatives. An in-house screening campaign intriguingly afforded the non-imidazole 4-(3-azetidin-1-yl)pyrimidin-2-amine 11b as a partial H3R agonist. Here, the design, synthesis, and structure-activity relationships of 11b analogues are described. This series yields several non-imidazole full agonists with potencies varying with the alkyl substitution pattern on the basic amine following the in vitro evaluation of H3R agonism using a cyclic adenosine monophosphate response element-luciferase reporter gene assay. The key compound VUF16839 (14d) combines nanomolar on-target activity (pKi = 8.5, pEC50 = 9.5) with weak activity on cytochrome P450 enzymes and good metabolic stability. The proposed H3R binding mode of 14d indicates key interactions similar to those attained by histamine. In vivo evaluation of 14d in a social recognition test in mice revealed an amnesic effect at 5 mg/kg intraperitoneally. The excellent in vitro and in vivo pharmacological profiles and the non-imidazole structure of 14d make it a promising tool compound in H3R research.

A Photoswitchable Agonist for the Histamine H3 Receptor, a Prototypic Family A G-Protein-Coupled Receptor.

Spatiotemporal control over biochemical signaling processes involving G protein-coupled receptors (GPCRs) is highly desired for dissecting their complex intracellular signaling. We developed sixteen photoswitchable ligands for the human histamine H3 receptor (hH3 R). Upon illumination, key compound 65 decreases its affinity for the hH3 R by 8.5-fold and its potency in hH3 R-mediated Gi protein activation by over 20-fold, with the trans and cis isomer both acting as full agonist. In real-time two-electrode voltage clamp experiments in Xenopus oocytes, 65 shows rapid light-induced modulation of hH3 R activity. Ligand 65 shows good binding selectivity amongst the histamine receptor subfamily and has good photolytic stability. In all, 65 (VUF15000) is the first photoswitchable GPCR agonist confirmed to be modulated through its affinity and potency upon photoswitching while maintaining its intrinsic activity, rendering it a new chemical biology tool for spatiotemporal control of GPCR activation.

Photoswitching the Efficacy of a Small-Molecule Ligand for a Peptidergic GPCR: from Antagonism to Agonism.

For optical control of GPCR function, we set out to develop small-molecule ligands with photoswitchable efficacy in which both configurations bind the target protein but exert distinct pharmacological effects, that is, stimulate or antagonize GPCR activation. Our design was based on a previously identified efficacy hotspot for the peptidergic chemokine receptor CXCR3 and resulted in the synthesis and characterization of five new azobenzene-containing CXCR3 ligands. G protein activation assays and real-time electrophysiology experiments demonstrated photoswitching from antagonism to partial agonism and even to full agonism (compound VUF16216). SAR evaluation suggests that the size and electron-donating properties of the substituents on the inner aromatic ring are important for the efficacy photoswitching. These compounds are the first GPCR azo ligands with a nearly full efficacy photoswitch and may become valuable pharmacological tools for the optical control of peptidergic GPCR signaling.

Synthesis and Characterization of a Bidirectional Photoswitchable Antagonist Toolbox for Real-Time GPCR Photopharmacology.

Noninvasive methods to modulate G protein-coupled receptors (GPCRs) with temporal and spatial precision are in great demand. Photopharmacology uses photons to control in situ the biological properties of photoswitchable small-molecule ligands, which bodes well for chemical biological precision approaches. Integrating the light-switchable configurational properties of an azobenzene into the ligand core, we developed a bidirectional antagonist toolbox for an archetypical family A GPCR, the histamine H3 receptor (H3R). From 16 newly synthesized photoswitchable compounds, VUF14738 (28) and VUF14862 (33) were selected as they swiftly and reversibly photoisomerize and show over 10-fold increased or decreased H3R binding affinities, respectively, upon illumination at 360 nm. Both ligands combine long thermal half-lives with fast and high photochemical trans-/ cis conversion, allowing their use in real-time electrophysiology experiments with oocytes to confirm dynamic photomodulation of H3R activation in repeated second-scale cycles. VUF14738 and VUF14862 are robust and fatigue-resistant photoswitchable GPCR antagonists suitable for spatiotemporal studies of H3R signaling.

Interaction of 14-3-3 proteins with the estrogen receptor alpha F domain provides a drug target interface.

Estrogen receptor alpha (ERα) is involved in numerous physiological and pathological processes, including breast cancer. Breast cancer therapy is therefore currently directed at inhibiting the transcriptional potency of ERα, either by blocking estrogen production through aromatase inhibitors or antiestrogens that compete for hormone binding. Due to resistance, new treatment modalities are needed and as ERα dimerization is essential for its activity, interference with receptor dimerization offers a new opportunity to exploit in drug design. Here we describe a unique mechanism of how ERα dimerization is negatively controlled by interaction with 14-3-3 proteins at the extreme C terminus of the receptor. Moreover, the small-molecule fusicoccin (FC) stabilizes this ERα/14-3-3 interaction. Cocrystallization of the trimeric ERα/14-3-3/FC complex provides the structural basis for this stabilization and shows the importance of phosphorylation of the penultimate Threonine (ERα-T(594)) for high-affinity interaction. We confirm that T(594) is a distinct ERα phosphorylation site in the breast cancer cell line MCF-7 using a phospho-T(594)-specific antibody and by mass spectrometry. In line with its ERα/14-3-3 interaction stabilizing effect, fusicoccin reduces the estradiol-stimulated ERα dimerization, inhibits ERα/chromatin interactions and downstream gene expression, resulting in decreased cell proliferation. Herewith, a unique functional phosphosite and an alternative regulation mechanism of ERα are provided, together with a small molecule that selectively targets this ERα/14-3-3 interface.