This chapter presents an overview of the current status of studies on the structural and molecular biology of the retinoid X receptor subtypes α, β, and γ (RXRs, NR2B1–3), their nuclear and cytoplasmic functions, post-transcriptional processing, and recently reported ligands. Points of interest are the different changes in the ligand-binding pocket induced by variously shaped agonists, the communication of the ligand–bound pocket with the coactivator binding surface and the heterodimerization interface, and recently identified ligands that are natural products, those that function as environmental toxins or drugs that had been originally designed to interact with other targets, as well as those that were deliberately designed as RXR-selective transcriptional agonists, synergists, or antagonists. Of these synthetic ligands, the general trend in design appears to be away from fully aromatic rigid structures to those containing partial elements of the flexible tetraene side chain of 9-cis-retinoic acid.

Rationale Human embryonic stem cells (hESCs) can form cardiomyocytes when cultured under differentiation conditions. Although the initiating step of mesoderm formation is well characterized, the subsequent steps that enrich for cardiac lineages are poorly understood and limit the yield of cardiomyocytes. Objective Our aim was to develop a hESC-based high content screening (HCS) assay to discover small molecules that drive cardiogenic differentiation after mesoderm is established to improve our understanding of the biology. Screening of libraries of small molecule pathway modulators was predicted to provide insight into the cellular proteins and signaling pathways that control stem cell cardiogenesis. Methods and results About 550 known pathway modulators were screened in a HCS assay with hits being called out by the appearance of a red fluorescent protein driven by the promoter of the cardiac specific MYH6 gene. One potent small molecule was identified that inhibits transduction of the canonical Wnt response within the cell, demonstrating that Wnt inhibition alone is sufficient for deriving cardiomyocytes from hESC originating mesoderm cells. Transcriptional profiling of inhibitor-treated compared to vehicle-treated samples further indicated that inhibition of Wnt does not induce other mesoderm lineages. Notably, several other Wnt inhibitors are very efficient in inducing cardiogenesis, including a molecule that prevents Wnts from being secreted by the cell, confirming Wnt inhibition as the relevant biological activity. Conclusions Pharmacological inhibition of Wnt signaling is sufficient to drive human mesoderm cells to form cardiomyocytes, yielding novel tools for the benefit of pharmaceutical and clinical applications.

We designed a minilibrary of 55 small molecule peptidomimetics based on beta-turns of the neurotrophin growth factor polypeptides neurotrophin-3 (NT-3) and nerve growth factor (NGF). Direct binding, binding competition, and biological screens identified agonistic ligands of the ectodomain of the neurotrophin receptors TrkC and TrkA. Agonism is intrinsic to the peptidomimetic ligand (in the absence of neurotrophins), and/or can also be detected as potentiation of neurotrophin action. Remarkably, some peptidomimetics afford both neurotrophic activities of cell survival and neuronal differentiation, while others afford discrete signals leading to either survival or differentiation. The high rate of hits identified suggests that focused minilibraries may be desirable for developing bioactive ligands of cell surface receptors. Small, selective, proteolytically stable ligands with defined biological activity may have therapeutic potential.

Apoptotic and antiproliferative activities of small heterodimer partner (SHP) nuclear receptor ligand (E)-4-[3′-(1-adamantyl)-4′-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC), which was derived from 6-[3′-(1-adamantyl)-4′-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN), and several carboxyl isosteric or hydrogen bond-accepting analogues were examined. 3-Cl-AHPC continued to be the most effective apoptotic agent, whereas tetrazole, thiazolidine-2,4-dione, methyldinitrile, hydroxamic acid, boronic acid, 2-oxoaldehyde, and ethyl phosphonic acid hydrogen bond-acceptor analogues were inactive or less efficient inducers of KG-1 acute myeloid leukemia and MDA-MB-231 breast, H292 lung, and DU-145 prostate cancer cell apoptosis. Similarly, 3-Cl-AHPC was the most potent inhibitor of cell proliferation. 4-[3′-(1-Adamantyl)-4′-hydroxyphenyl]-3chlorophenyltetrazole, (2E)-5-{2-[3′-(1-adamantyl)-2-chloro-4′-hydroxy-4-biphenyl]-ethenyl}-1H