Perspectives on phenotypic screening-Screen Design and Assay Technology Special Interest Group.

Here we offer perspectives on phenotypic screening based on a wide-ranging discussion entitled "Phenotypic screening, target ID, and multi-omics: enabling more disease relevance in early discovery?" at the Screen Design and Assay Technology Special Interest Group Meeting at the 2023 SLAS Conference. During the session, the authors shared their own experience from within their respective organizations, followed by an open discussion with the audience. It was recognized that while substantial progress has been made towards translating disease-relevant phenotypic early discovery into clinical success, there remain significant operational and scientific challenges to implementing phenotypic screening efforts, and improving translation of screening hits comes with substantial resource demands and organizational commitment. This Perspective assesses progress, highlights pitfalls, and offers possible solutions to help unlock the therapeutic potential of phenotypic drug discovery. Areas explored comprise screening and hit validation strategy, choice of cellular model, moving beyond 2D cell culture into three dimensions, and leveraging high-dimensional data sets downstream of phenotypic screens.

Hit Triage and Validation in Phenotypic Screening: Considerations and Strategies.

The promise of phenotypic screening resides in its track record of novel biology and first-in-class therapies. However, challenges stemming from major differences between target-based and phenotypic screening do exist. These challenges prompted us to rethink the critical stage of hit triage and validation on the road to clinical candidates and novel drug targets. Whereas this process is usually straightforward for target screening hits, phenotypic screening hits act through a variety of mostly unknown mechanisms within a large and poorly understood biological space. Our analysis suggests successful hit triage and validation is enabled by three types of biological knowledge-known mechanisms, disease biology, and safety-while structure-based hit triage may be counterproductive.

Dimethadione embryotoxicity in the rat is neither correlated with maternal systemic drug concentrations nor embryonic tissue levels.

Pregnant rats treated with dimethadione (DMO), the N-demethylated metabolite of the anticonvulsant trimethadione, produce offspring having a 74% incidence of congenital heart defects (CHD); however, the incidence of CHD has high inter-litter variability (40-100%) that presents a challenge when studying the initiating events prior to the presentation of an abnormal phenotype. We hypothesized that the variability in CHD incidence was the result of differences in maternal systemic concentrations or embryonic tissue concentrations of DMO. To test this hypothesis, dams were administered 300 mg/kg DMO every 12h from the evening of gestational day (GD) 8 until the morning of GD 11 (six total doses). Maternal serum levels of DMO were assessed on GD 11, 12, 13, 14, 15, 18 and 21. Embryonic tissue concentrations of DMO were assessed on GD 11, 12, 13 and 14. In a separate cohort of GD 12 embryos, DMO concentrations and parameters of growth and development were assessed to determine if tissue levels of DMO were correlated with these endpoints. Embryos were exposed directly to different concentrations of DMO with whole embryo culture (WEC) and their growth and development assessed. Key findings were that neither maternal systemic concentrations nor tissue concentrations of DMO identified embryos that were sensitive or resistant to DMO in vivo. Direct exposure of embryos to DMO via WEC also failed to show correlations between embryonic concentrations of DMO with developmental outcomes in vitro. We conclude that neither maternal serum nor embryonic tissue concentrations of DMO predict embryonic outcome.

Noninvasive high-resolution ultrasound reveals structural and functional deficits in dimethadione-exposed fetal rat hearts in utero.

BACKGROUND: We previously showed dimethadione (DMO), the N-demethylated metabolite of the anticonvulsant trimethadione, induces ventricular septation defects (VSD) and other heart anomalies in rat (Weston et al., 2011). Because of the relationship between cardiac structure and function, we hypothesized that DMO-induced structural defects of the heart are associated with in utero functional deficits. To test the hypothesis, the goals were (1) define the parameters for ultrasound in the rat conceptus, and; (2) use ultrasound to identify structural and functional deficits following DMO treatment. METHODS: Different ultrasound modes (B-mode, M-mode, and Pulse-wave Doppler) using four high-resolution ultrasound transducer heads of varying frequency (25-40 MHz) were tested on gestational day (GD) 14, 15, 16, 17, and 21. Having identified the optimal conditions, pregnant Sprague-Dawley rats were administered six 300 mg/kg doses of DMO every 12 hr beginning at 19:00 hr on GD 8 to generate conceptuses with a high incidence of VSD. RESULTS: The three ultrasound modalities were used to identify VSD and several novel and rare structural heart anomalies (cardiac effusions and bifurcated septum) in live rat fetuses. DMO-treated hearts had an array of functional deficits including a decrease in mean heart rate, ejection fraction, and cardiac output and increased incidence of bradycardia and dysrhythmia. CONCLUSIONS: The ultrasound biomicroscope is an effective tool for the real-time characterization of the structure and function of embryo/fetal rat hearts. DMO causes significant deficits to in utero heart function for up to ten days (GD 21) following its final administration, suggesting long-term or possible permanent changes cardiac function.

Co-variation in frequency and severity of cardiovascular and skeletal defects in Sprague-Dawley rats after maternal administration of dimethadione, the N-demethylated metabolite of trimethadione.

BACKGROUND: The anticonvulsant trimethadione is a potent inducer of ventricular septation defects, both clinically and in rodents. Teratogenicity requires its N-demethylation to dimethadione, the proximate teratogen. It was previously demonstrated trimethadione only induced membranous ventricular septation defects in rat (Fleeman et al., 2004), and our present goal is to determine whether direct administration of dimethadione increases the incidence and severity of septation defects. METHODS: Pregnant Sprague-Dawley rats were divided into five groups and administered either distilled water (control) or four different regimens of dimethadione. The core treatment was 300 mg/kg dimethadione b.i.d. on gestation day 9, 10 with additional groups given one additional dose of dimethadione 12 hr earlier, 12 hr later or two additional doses 12 hr earlier and later. Caesarian sections occurred on gestation day 21 and fetuses were examined for standard developmental toxicity endpoints. RESULTS: The broadest dosing regimen yielded the highest incidence and the most severe heart and axioskeletal findings with a decrease in mean fetal body weight. The overall incidence of ventricular septation defects was 74%, of which 68% were membranous and 9% muscular. Outflow tract anomalies (17%) were also observed, as were malformations of the axioskeleton (97%), but not of the long bones, and of particular interest was the high incidence of sternoschesis. CONCLUSIONS: Unlike trimethadione, dimethadione induces more serious muscular septation defects that are believed to be more clinically relevant. This, when taken together with the high incidence of total septation anomalies suggests dimethadione is useful for the study of chemically induced ventricular septation defects.

A role for cyclic nucleotide monophosphates in synaptic modulation by a crayfish neuropeptide.

DF2 (DRNFLRFamide), a FMRFamide-like peptide, has been shown to increase the amount of transmitter released at crayfish neuromuscular junctions. Here, we examined a possible role for the cyclic nucleotide monophosphates, cAMP and cGMP, in DF2's effects on synaptic transmission. The effects of DF2 on synaptic transmission were monitored by recording excitatory postsynaptic potentials (EPSPs) in the deep abdominal extensor muscles of the crayfish, Procambarus clarkii. A number of activators and inhibitors were used to determine whether or not cAMP, cGMP, protein kinase A (PKA) and protein kinase G (PKG) mediate the effect of this neuropeptide. Phosphodiesterase inhibitors, known to inhibit the breakdown of cAMP (IBMX) and/or cGMP (mdBAMQ), potentiate the effect of DF2 on synaptic transmission. Activators of PKA (Sp-cAMPS) and PKG (8-pCPT-cGMP) increase EPSP amplitude, mimicking the effects of DF2. Inhibitors of PKA (Rp-cAMPS) and PKG (Rp-8-pCPT-cGMPS) each block a portion of the response to the peptide, and when applied together these two inhibitors completely block the response. Taken together, these results indicate that cyclic nucleotides and cyclic nucleotide-dependent protein kinases are necessary components of the pathway underlying modulation by this neuropeptide.

Thoracic skeletal defects and cardiac malformations: a common epigenetic link?

Congenital heart defects (CHDs) are the most common birth defects in humans. In addition, cardiac malformations represent the most frequently identified anomaly in teratogenicity experiments with laboratory animals. To explore the mechanisms of these drug-induced defects, we developed a model in which pregnant rats are treated with dimethadione, resulting in a high incidence of heart malformations. Interestingly, these heart defects were accompanied by thoracic skeletal malformations (cleft sternum, fused ribs, extra or missing ribs, and/or wavy ribs), which are characteristic of anterior-posterior (A/P) homeotic transformations and/or disruptions at one or more stages in somite development. A review of other teratogenicity studies suggests that the co-occurrence of these two disparate malformations is not unique to dimethadione, rather it may be a more general phenomenon caused by various structurally unrelated agents. The coexistence of cardiac and thoracic skeletal malformations has also presented clinically, suggesting a mechanistic link between cardiogenesis and skeletal development. Evidence from genetically modified mice reveals that several genes are common to heart development and to formation of the axial skeleton. Some of these genes are important in regulating chromatin architecture, while others are tightly controlled by chromatin-modifying proteins. This review focuses on the role of these epigenetic factors in development of the heart and axial skeleton, and examines the hypothesis that posttranslational modifications of core histones may be altered by some developmental toxicants.

A data integration methodology for systems biology: experimental verification.

The integration of data from multiple global assays is essential to understanding dynamic spatiotemporal interactions within cells. In a companion paper, we reported a data integration methodology, designated Pointillist, that can handle multiple data types from technologies with different noise characteristics. Here we demonstrate its application to the integration of 18 data sets relating to galactose utilization in yeast. These data include global changes in mRNA and protein abundance, genome-wide protein-DNA interaction data, database information, and computational predictions of protein-DNA and protein-protein interactions. We divided the integration task to determine three network components: key system elements (genes and proteins), protein-protein interactions, and protein-DNA interactions. Results indicate that the reconstructed network efficiently focuses on and recapitulates the known biology of galactose utilization. It also provided new insights, some of which were verified experimentally. The methodology described here, addresses a critical need across all domains of molecular and cell biology, to effectively integrate large and disparate data sets.

A data integration methodology for systems biology.

Different experimental technologies measure different aspects of a system and to differing depth and breadth. High-throughput assays have inherently high false-positive and false-negative rates. Moreover, each technology includes systematic biases of a different nature. These differences make network reconstruction from multiple data sets difficult and error-prone. Additionally, because of the rapid rate of progress in biotechnology, there is usually no curated exemplar data set from which one might estimate data integration parameters. To address these concerns, we have developed data integration methods that can handle multiple data sets differing in statistical power, type, size, and network coverage without requiring a curated training data set. Our methodology is general in purpose and may be applied to integrate data from any existing and future technologies. Here we outline our methods and then demonstrate their performance by applying them to simulated data sets. The results show that these methods select true-positive data elements much more accurately than classical approaches. In an accompanying companion paper, we demonstrate the applicability of our approach to biological data. We have integrated our methodology into a free open source software package named POINTILLIST.

Systems biology, proteomics, and the future of health care: toward predictive, preventative, and personalized medicine.

The emergence of systems biology is bringing forth a new set of challenges for advancing science and technology. Defining ways of studying biological systems on a global level, integrating large and disparate data types, and dealing with the infrastructural changes necessary to carry out systems biology, are just a few of the extraordinary tasks of this growing discipline. Despite these challenges, the impact of systems biology will be far-reaching, and significant progress has already been made. Moving forward, the issue of how to use systems biology to improve the health of individuals must be a priority. It is becoming increasingly apparent that the field of systems biology and one of its important disciplines, proteomics, will have a major role in creating a predictive, preventative, and personalized approach to medicine. In this review, we define systems biology, discuss the current capabilities of proteomics and highlight some of the necessary milestones for moving systems biology and proteomics into mainstream health care.

Revisiting the role of retinoid signaling in skeletal development.

Several years ago, it was discovered that an imbalance of vitamin A during embryonic development has dramatic teratogenic effects. These effects have since been attributed to vitamin A's most active metabolite, retinoic acid (RA), which itself profoundly influences the development of multiple organs including the skeleton. After decades of study, researchers are still uncovering the molecular basis whereby retinoids regulate skeletal development. Retinoid signaling involves several components, from the enzymes that control the synthesis and degradation of RA, to the cytoplasmic RA-binding proteins, and the nuclear receptors that modulate gene transcription. As new functions for each component continue to be discovered, their developmental roles appear increasingly complex. Interestingly, each component has been implicated in skeletal development. Moreover, retinoid signaling comes into play at distinct stages throughout the developmental sequence of skeletogenesis, highlighting a fundamental role for this pathway in forming the adult skeleton. Consistent with these roles, manipulation of the retinoid signaling pathway significantly affects the expression of the skeletogenic master regulatory factors, Sox9 and Cbfa1. In addition to the fact that we now have a greater understanding of the retinoid signaling pathway on a molecular level, much more information is now available to begin placing retinoid signaling within the context of other factors that regulate skeletogenesis. Here we review these recent advances and describe our current understanding of how retinoid signaling functions to coordinate skeletal development. We also discuss future directions and clinical implications in this field.

Inhibition of p38 MAPK signaling promotes late stages of myogenesis.

Signaling through the p38 mitogen-activated protein kinases (MAPKs) is essential for cartilage formation in primary cultures of limb mesenchyme. Here we show that, concurrent with a decrease in chondrogenesis, inhibition of p38 in limb bud cultures dramatically promotes muscle development. Specifically, treatment of primary limb bud cultures with p38 inhibitors increases the expression of myogenic markers and causes a striking increase in formation of myotubes, which were detected using antibodies specific for myosin heavy chain. These results are surprising in that they contrast with several previous reports describing a requirement for p38 during myogenesis. Nonetheless, the enhanced myogenesis leads to the formation of an extensive network of contractile myofibers, and this enhanced myogenesis can be conferred upon myogenic cells from clonal populations, such as G8 or C2C12 cells, if they are co-cultured with the limb mesenchymal cells. We provide evidence for the maintenance and rapid organization of existing, somitic-derived limb myoblasts in response to p38 inhibitors. These findings imply a novel and unexpected role for p38 MAPK inhibition in myogenesis and highlight the importance of the limb bud microenvironment in promoting the progression of limb myoblasts.

Molecular mechanisms regulating chondroblast differentiation.

BACKGROUND: Formation of the cartilage template involves a multi-step process in which prechondrogenic mesenchymal cells form condensations prior to differentiating into matrix-producing chondroblasts. Retinoids, particularly retinoic acid, are among the numerous signaling molecules that have been implicated in this process. A proper balance of retinoids is essential for normal skeletal development in that too much or too little negatively impacts skeletogenesis. During the past few years, substantial advances have been made in our understanding of the role of retinoid signaling in these processes, which is reviewed in this report. METHODS: To examine the function of retinoid signaling in skeletal development, transgenic mice that overexpressed a weak, constitutively active retinoic acid receptor (retinoic acid receptor-alpha) in their developing limbs were generated. The mice presented with a range of skeletal abnormalities. To examine the mechanisms responsible for these abnormalities, primary limb mesenchymal cultures from the transgenic mice were compared with cultures from wild-type mice. In addition, to address the molecular basis of retinoic acid receptor action, retinoic acid receptor activity in the primary cultures was manipulated with use of retinoic acid receptor-selective agonists and antagonists. The evaluation of the response to the manipulation of retinoic acid receptors was followed by histological studies and by the use of Northern blot analysis and reporter assays to analyze changes in the expression of chondrocytic markers and to monitor transcription factor activity, respectively. RESULTS: The evidence reviewed here indicates that retinoids maintain cells within condensations in a prechondrogenic, mesenchymal cell state, which prevents the cells from differentiating into chondroblasts. More recent studies have demonstrated that the inhibition of receptor-mediated retinoid signaling induces the expression of Sox9, a transcription factor that is considered a "master switch" for the differentiation of chondroblasts. These effects are largely mediated by the activation of the p38 MAPK signaling cascade. CONCLUSIONS: These findings demonstrate that retinoid receptor-mediated repression is both necessary and sufficient for chondroblast differentiation. Moreover, retinoic acid receptor repression acts downstream of BMP signaling or in a distinct pathway to activate p38 MAPK, which in turn induces chondroblast differentiation.

Active repression by unliganded retinoid receptors in development: less is sometimes more.

The retinoid receptors have major roles throughout development, even in the absence of ligand. Here, we summarize an emerging theme whereby gene repression, mediated by unliganded retinoid receptors, can dictate cell fate. In addition to activating transcription, retinoid receptors actively repress gene transcription by recruiting cofactors that promote chromatin compaction. Two developmental processes for which gene silencing by the retinoid receptors is essential are head formation in Xenopus and skeletal development in the mouse. Inappropriate repression, by oncogenic retinoic acid (RA)**Abbreviations used in this paper: APL, acute promyelocytic leukemia; dnRARalpha, dominant-negative version of the RARalpha; E, embryonic age; HDAC, histone deacetylase; LCoR, ligand-dependent corepressor; NCoR, nuclear receptor corepressor; RA, retinoic acid; RAR, RA receptor; RARE, RXR homodimer bound to bipartite response element; RXR, retinoid X receptor; TSA, trichostatin A; CYP26, cytochrome p450, 26; TR, thyroid hormone receptor. receptor (RAR) fusion proteins, blocks myeloid differentiation leading to a rare form of leukemia. Our current understanding of the developmental role of retinoid repression and future perspectives in this field are discussed.

Requirement for RAR-mediated gene repression in skeletal progenitor differentiation.

Chondrogenesis is a multistep process culminating in the establishment of a precisely patterned template for bone formation. Previously, we identified a loss in retinoid receptor-mediated signaling as being necessary and sufficient for expression of the chondroblast phenotype (Weston et al., 2000. J. Cell Biol. 148:679-690). Here we demonstrate a close association between retinoic acid receptor (RAR) activity and the transcriptional activity of Sox9, a transcription factor required for cartilage formation. Specifically, inhibition of RAR-mediated signaling in primary cultures of mouse limb mesenchyme results in increased Sox9 expression and activity. This induction is attenuated by the histone deacetylase inhibitor, trichostatin A, and by coexpression of a dominant negative nuclear receptor corepressor-1, indicating an unexpected requirement for RAR-mediated repression in skeletal progenitor differentiation. Inhibition of RAR activity results in activation of the p38 mitogen-activated protein kinase (MAPK) and protein kinase A (PKA) pathways, indicating their potential role in the regulation of chondrogenesis by RAR repression. Accordingly, activation of RAR signaling, which attenuates differentiation, can be rescued by activation of p38 MAPK or PKA. In summary, these findings demonstrate a novel role for active RAR-mediated gene repression in chondrogenesis and establish a hierarchical network whereby RAR-mediated signaling functions upstream of the p38 MAPK and PKA signaling pathways to regulate emergence of the chondroblast phenotype.

An intensive hands-on course designed to teach molecular biology techniques to physiology graduate students.

To address a growing need to make research trainees in physiology comfortable with the tools of molecular biology, we have developed a laboratory-intensive course designed for graduate students. This course is offered to a small group of students over a three-week period and is organized such that comprehensive background lectures are coupled with extensive hands-on experience. The course is divided into seven modules, each organized by a faculty member who has particular expertise in the area covered by that module. The modules focus on basic methods such as cDNA subcloning, sequencing, gene transfer, polymerase chain reaction, and protein and RNA expression analysis. Each module begins with a lecture that introduces the technique in detail by providing a historical perspective, describing both the uses and limitations of that technique, and comparing the method with others that yield similar information. Most of the lectures are followed by a laboratory session during which students follow protocols that were carefully designed to avoid pitfalls. Throughout these laboratory sessions, students are given an appreciation of the importance of proper technique and accuracy. Communication among the students, faculty, and the assistant coordinator is focused on when and why each procedure would be used, the importance of each step in the procedure, and approaches to troubleshooting. The course ends with an exam that is designed to test the students' general understanding of each module and their ability to apply the various techniques to physiological questions.