A revisited version of the apo structure of the ligand-binding domain of the human nuclear receptor retinoic X receptor α

De novo variants implicate chromatin modification, transcriptional regulation, and retinoic acid signaling in syndromic craniosynostosis

Craniosynostosis (CS) is the most common congenital cranial anomaly. Several Mendelian forms of syndromic CS are well described, but a genetic etiology remains elusive in a substantial fraction of probands. Analysis of exome sequence data from 526 proband-parent trios with syndromic CS identified a marked excess (observed 98, expected 33, p = 4.83 × 10-20) of damaging de novo variants (DNVs) in genes highly intolerant to loss-of-function variation (probability of LoF intolerance > 0.9). 30 probands harbored damaging DNVs in 21 genes that were not previously implicated in CS but are involved in chromatin modification and remodeling (4.7-fold enrichment, p = 1.1 × 10-11). 17 genes had multiple damaging DNVs, and 13 genes (CDK13, NFIX, ADNP, KMT5B, SON, ARID1B, CASK, CHD7, MED13L, PSMD12, POLR2A, CHD3, and SETBP1) surpassed thresholds for genome-wide significance. A recurrent gain-of-function DNV in the retinoic acid receptor alpha (RARA; c.865G>A [p.Gly289Arg]) was identified in two probands with similar CS phenotypes. CS risk genes overlap with those identified for autism and other neurodevelopmental disorders, are highly expressed in cranial neural crest cells, and converge in networks that regulate chromatin modification, gene transcription, and osteoblast differentiation. Our results identify several CS loci and have major implications for genetic testing and counseling.

A structural perspective of liver X receptors

Liver X receptors α and β are members of the nuclear receptor family, which comprise a flexible N-terminal domain, a DNA binding domain, a hinge linker, and a ligand binding domain. Liver X receptors are important regulators of cholesterol and lipid homeostasis by controlling the transcription of numerous genes. Key to their transcriptional role is synergetic interaction among the domains. DNA binding domain binds on DNA; ligand binding domain is a crucial switch to control the transcription activity through conformational change caused by ligand binding. The Liver X receptors form heterodimers with retinoid X receptor and then the liganded heterodimer may recruit other necessary transcription components to form an active transcription complex.

MLProbs: A Data-Centric Pipeline for Better Multiple Sequence Alignment

In this paper, we explore using the data-centric approach to tackle the Multiple Sequence Alignment (MSA) construction problem. Unlike the algorithm-centric approach, which reduces the construction problem to a combinatorial optimization problem based on an abstract mathematical model, the data-centric approach explores using classification models trained from existing benchmark data to guide the construction. We identified two simple classifications to help us choose a better alignment tool and determine whether and how much to carry out realignment. We show that shallow machine-learning algorithms suffice to train sensitive models for these classifications. Based on these models, we implemented a new multiple sequence alignment pipeline, called MLProbs. Compared with 10 other popular alignment tools over four benchmark databases (namely, BAliBASE, OXBench, OXBench-X and SABMark), MLProbs consistently gives the highest TC score. More importantly, MLProbs shows non-trivial improvement for protein families with low similarity; in particular, when evaluated against the 1,356 protein families with similarity ≤ 50%, MLProbs achieves a TC score of 56.93, while the next best three tools are in the range of [55.41, 55.91] (increased by more than 1.8%). We also compared the performance of MLProbs and other MSA tools in two real-life applications - Phylogenetic Tree Construction Analysis and Protein Secondary Structure Prediction - and MLProbs also had the best performance. In our study, we used only shallow machine-learning algorithms to train our models. It would be interesting to study whether deep-learning methods can help make further improvements, so we suggest some possible research directions in the conclusion section.

Deciphering the importance of MD descriptors in designing Vitamin D Receptor agonists and antagonists using machine learning

The Vitamin D Receptor (VDR) ligand-binding domain undergoes conformation change upon the binding of VDR agonists/antagonists. Helix 12 ((H)12) is one of the important helices at VDR ligand binding and its conformational changes are controlled by the binding of agonists and antagonists molecules. Various molecular modeling studies are available to explain the agonistic and antagonistic activity of vitamin D analogs. In this work, for the first time, we attempted to generate a machine learning model with fingerprints, 2D, 3D and MD descriptors that are specific to Vitamin D analogs and VDR. Initially, 2D and 3D descriptors and fingerprints of 1003 vitamin D analogs were calculated using CDK and RDKit. The machine learning model was generated using descriptors and fingerprints. Further, 80 Vitamin D analogs (40 VDR agonists + 40 VDR antagonists) were docked in the VDR active site. 50ns MD simulation was performed for each protein-ligand complex. Different MD descriptors such as Solvent Accessible Surface Area (SASA), radius of gyration, PC1 and PC2 were calculated and considered along with CDK and RDKit descriptors as features for machine learning calculations. A few other descriptors that are related to VDR conformational changes such as conformation of the (H)12, the angle at kink were considered for machine learning model generation. It was observed that the descriptors calculated from VDR conformational changes i) were able to distinguish between agonists and antagonists ii) provide key and comprehensive information about the unique binding characteristics of agonists and antagonists iii) provide a strong basis for the machine learning model generation. Overall, this study attempts the utilization of descriptors that are specific to a protein conformation will be helpful for the generation of an efficient machine learning model.

[Understanding the Underlying Mechanism of Xenobiotic-Sensing Nuclear Receptor Activation]

The nuclear receptor superfamily comprises 48 members in humans. In various organs, nuclear receptors regulate a variety of physiological functions through transcription of target genes. They are associated with the development and progression of endocrine and metabolic disorders, as well as with cancer development. Therefore, agonists and antagonists targeting nuclear receptors are currently being developed as therapeutic drugs for these diseases. Nuclear receptors can be activated through ligand binding or phosphorylation, which is mediated by various cellular signaling pathways. Activation of a nuclear receptor necessitates significant structural modifications in each of its domains. My research has been focused on unraveling the intricate mechanisms underlying the activation of nuclear receptors using constitutive androstane receptor (CAR) and pregnane X receptor (PXR) as model nuclear receptor proteins. CAR and PXR are highly expressed in the liver and are activated by a wide range of xenobiotics. Given their crucial roles in the metabolism and disposition of xenobiotics, as well as their potential in mediating drug-drug interactions, it is imperative to extensively study the mechanisms of xenobiotic-induced activation of these receptors. Such studies are essential for advancements in drug development, as well as for ensuring food and chemical safety. In this review, I elucidate the molecular basis underlying the activation of xenobiotic-responsive nuclear receptors.

Helix 12 stabilization contributes to basal transcriptional activity of PXR

Pregnane X receptor (PXR) plays an important role in xenobiotic metabolism. While ligand binding induces PXR-dependent gene transcription, PXR shows constitutive transcriptional activity in the absence of ligands when expressed in cultured cells. This constitutive activity sometimes hampers investigation of PXR activation by compounds of interest. In this study, we investigated the molecular mechanism of PXR activation. In the reported crystal structures of unliganded PXR, helix 12 (H12), including a coactivator binding motif, was stabilized, while it is destabilized in the unliganded structures of other nuclear receptors, suggesting a role for H12 stabilization in the basal activity of PXR. Since Phe420, located in the loop between H11 and H12, is thought to interact with Leu411 and Ile414 to stabilize H12, we substituted alanine at Phe420 (PXR-F420A) and separately inserted three alanine residues directly after Phe420 (PXR-3A) and investigated their influence on PXR-mediated transcription. Reporter gene assays demonstrated that the mutants showed drastically reduced basal activity and enhanced responses to various ligands, which was further enhanced by coexpression of the coactivator peroxisome proliferator-activated receptor gamma coactivator 1α. Mutations of both Leu411 and Ile414 to alanine also suppressed basal activity. Mammalian two-hybrid assays showed that PXR-F420A and PXR-3A bound to corepressors and coactivators in the absence and presence of ligands, respectively. We conclude that the intramolecular interactions of Phe420 with Leu411 and Ile414 stabilize H12 to recruit coactivators even in the absence of ligands, contributing to the basal transcriptional activity of PXR. We propose that the generated mutants might be useful for PXR ligand screening.

Mechanism of Vitamin D Receptor Ligand-Binding Domain Regulation Studied by gREST Simulations

The vitamin D receptor ligand-binding domain (VDR-LBD) undergoes conformational changes upon ligand binding. In this nuclear receptor family, agonistic or antagonistic activities are controlled by the conformation of the helix (H)12. However, all crystal structures of VDR-LBD reported to date correspond to the active H12 conformation, regardless of agonist/antagonist binding. To understand the mechanism of VDR-LBD regulation structurally, conformational samplings of agonist- and antagonist-bound rat VDR-LBD were performed using the generalized replica exchange with solute tempering (gREST) method. The gREST simulations demonstrated different structural responses of rat VDR-LBD to agonist or antagonist binding, whereas in conventional molecular dynamics simulations, the conformation was the same as that of the crystal structures, regardless of agonist/antagonist binding. In the gREST simulations, a spontaneous conformational change of H12 was observed only for the antagonist complex. The different responses to agonist/antagonist binding were attributed to hydrophobic core formation at the ligand-binding pocket and cooperative rearrangements of H11. The gREST method can be applied to the examination of structure-activity relationships for multiple VDR-LBD ligands.

A structural signature motif enlightens the origin and diversification of nuclear receptors

Nuclear receptors are ligand-activated transcription factors that modulate gene regulatory networks from embryonic development to adult physiology and thus represent major targets for clinical interventions in many diseases. Most nuclear receptors function either as homodimers or as heterodimers. The dimerization is crucial for gene regulation by nuclear receptors, by extending the repertoire of binding sites in the promoters or the enhancers of target genes via combinatorial interactions. Here, we focused our attention on an unusual structural variation of the α-helix, called π-turn that is present in helix H7 of the ligand-binding domain of RXR and HNF4. By tracing back the complex evolutionary history of the π-turn, we demonstrate that it was present ancestrally and then independently lost in several nuclear receptor lineages. Importantly, the evolutionary history of the π-turn motif is parallel to the evolutionary diversification of the nuclear receptor dimerization ability from ancestral homodimers to derived heterodimers. We then carried out structural and biophysical analyses, in particular through point mutation studies of key RXR signature residues and showed that this motif plays a critical role in the network of interactions stabilizing homodimers. We further showed that the π-turn was instrumental in allowing a flexible heterodimeric interface of RXR in order to accommodate multiple interfaces with numerous partners and critical for the emergence of high affinity receptors. Altogether, our work allows to identify a functional role for the π-turn in oligomerization of nuclear receptors and reveals how this motif is linked to the emergence of a critical biological function. We conclude that the π-turn can be viewed as a structural exaptation that has contributed to enlarging the functional repertoire of nuclear receptors.

The origin of novelties is a central topic in evolutionary biology. A fundamental question is how organisms constrained by natural selection can divert from existing schemes to set up novel structures or pathways. Among the most important strategies are exaptations, which represent pre-adaptation strategies. Many examples exist in biology, at both morphological and molecular levels, such as the one reported here that focuses on an unusual structural feature called the π-turn. It is found in the structure of the most ancestral nuclear receptors RXR and HNF4. The analyses trace back the complex evolutionary history of the π-turn to more than 500 million years ago, before the Cambrian explosion and show that this feature was essential for the heterodimerization capacity of RXR. Nuclear receptor lineages that emerged later in evolution lost the π-turn. We demonstrate here that this loss in nuclear receptors that heterodimerize with RXR was critical for the emergence of high affinity receptors, such as the vitamin D and the thyroid hormone receptors. On the other hand, the conserved π-turn in RXR allowed it to accommodate multiple heterodimer interfaces with numerous partners. This structural exaptation allowed for the remarkable diversification of nuclear receptors.

Stability of the Retinoid X Receptor-α Homodimer in the Presence and Absence of Rexinoid and Coactivator Peptide

Differential scanning calorimetry and differential scanning fluorimetry were used to measure the thermal stability of human retinoid X receptor-α ligand binding domain (RXRα LBD) homodimer in the absence or presence of rexinoid and coactivator peptide, GRIP-1. The apo-RXRα LBD homodimer displayed a single thermal unfolding transition with a Tm of 58.7 °C and an unfolding enthalpy (ΔH) of 673 kJ/mol (12.5 J/g), much lower than average value (35 J/g) of small globular proteins. Using a heat capacity change (ΔCp) of 15 kJ/(mol K) determined by measurements at different pH values, the free energy of unfolding (ΔG) of the native state was 33 kJ/mol at 37 °C. Rexinoid binding to the apo-homodimer increased Tm by 5 to 9 °C and increased the ΔG of the native homodimer by 12 to 20 kJ/mol at 37 °C, consistent with the nanomolar dissociation constant (Kd) of the rexinoids. GRIP-1 binding to holo-homodimers containing rexinoid resulted in additional increases in ΔG of 14 kJ/mol, a value that was the same for all three rexinoids. Binding of rexinoid and GRIP-1 resulted in a combined 50% increase in unfolding enthalpy, consistent with reduced structural fluidity and more compact folding observed in other published structural studies. The complexes of UAB110 and UAB111 are each more stable than the UAB30 complex by 8 kJ/mol due to enhanced hydrophobic interactions in the binding pocket because of their larger end groups. This increase in thermodynamic stability positively correlates with their improved RXR activation potency. Thermodynamic measurements are thus valuable in predicting agonist potency.

Discovery and SAR of Natural-Product-Inspired RXR Agonists with Heterodimer Selectivity to PPARδ-RXR

A known natural product, magnaldehyde B, was identified as an agonist of retinoid X receptor (RXR) α. Magnaldehyde B was isolated from Magnolia obovata (Magnoliaceae) and synthesized along with more potent analogs for screening of their RXRα agonistic activities. Structural optimization of magnaldehyde B resulted in the development of a candidate molecule that displayed a 440-fold increase in potency. Receptor-ligand docking simulations indicated that this molecule has the highest affinity with the ligand binding domain of RXRα among the analogs synthesized in this study. Furthermore, the selective activation of the peroxisome proliferator-activated receptor (PPAR) δ-RXR heterodimer with a stronger efficacy compared to those of PPARα-RXR and PPARγ-RXR was achieved in luciferase reporter assays using the PPAR response element driven reporter (PPRE-Luc). The PPARδ activity of the molecule was significantly inhibited by the antagonists of both RXR and PPARδ, whereas the activity of GW501516 was not affected by the RXR antagonist. Furthermore, the molecule exhibited a particularly weak PPARδ agonistic activity in reporter gene assays using the Gal4 hybrid system. The obtained data therefore suggest that the weak PPARδ agonistic activity of the optimized molecule is synergistically enhanced by its own RXR agonistic activity, indicating the potent agonistic activity of the PPARδ-RXR heterodimer.

Classical pathways of gene regulation by retinoids

Retinoic acid receptors were discovered during early studies of the actions and mechanisms of essential vitamins. Vitamin A is metabolized in the body to retinoic acid (RA) which is a key compound in the control of many developmental processes in chordates. These functions are mediated by a subfamily of nuclear receptors, divided into two classes, the retinoic acid receptors (RAR) and the retinoid X receptors (RXR). Each class is encoded by three closely related genes that are located on different chromosomes. The three proteins in each class are designated α, β and γ, respectively. A wealth of structural studies have shown that they all share the same architecture including a DNA-binding domain connected by a flexible linker to the ligand and co-activator binding domain. Retinoic acid incorporation into the ligand-binding domain leads to a conformational change enabling the formation of RAR homodimers or RAR/RXR heterodimers that in turn bind specifically to target DNA sequences. The consensus sequences located on the promotors of regulated genes are known as retinoic acid response elements (RARE). The activated RAR/RXR homodimers recruit co-activators with histone acetylase activity leading to an opening of the chromatin structure and enabling downstream transcription of regulated genes. These canonical pathways describe the control mechanism for the majority of developmental processes mediated by retinoic acid and its derivatives.

A molecular switch regulating transcriptional repression and activation of PPARγ

Nuclear receptor (NR) transcription factors use a conserved activation function-2 (AF-2) helix 12 mechanism for agonist-induced coactivator interaction and NR transcriptional activation. In contrast, ligand-induced corepressor-dependent NR repression appears to occur through structurally diverse mechanisms. We report two crystal structures of peroxisome proliferator-activated receptor gamma (PPARγ) in an inverse agonist/corepressor-bound transcriptionally repressive conformation. Helix 12 is displaced from the solvent-exposed active conformation and occupies the orthosteric ligand-binding pocket enabled by a conformational change that doubles the pocket volume. Paramagnetic relaxation enhancement (PRE) NMR and chemical crosslinking mass spectrometry confirm the repressive helix 12 conformation. PRE NMR also defines the mechanism of action of the corepressor-selective inverse agonist T0070907, and reveals that apo-helix 12 exchanges between transcriptionally active and repressive conformations—supporting a fundamental hypothesis in the NR field that helix 12 exchanges between transcriptionally active and repressive conformations.

Structural studies of nuclear receptor transcription factors revealed that nearly all nuclear receptors share a conserved helix 12 dependent transcriptional activation mechanism. Here the authors present two crystal structures of peroxisome proliferator-activated receptor gamma (PPARγ) in an inverse agonist/corepressor-bound transcriptionally repressive conformation, where helix 12 is located within the orthosteric ligand-binding pocket instead, and discuss mechanistic implications.

Definition of functionally and structurally distinct repressive states in the nuclear receptor PPARγ

The repressive states of nuclear receptors (i.e., apo or bound to antagonists or inverse agonists) are poorly defined, despite the fact that nuclear receptors are a major drug target. Most ligand bound structures of nuclear receptors, including peroxisome proliferator-activated receptor γ (PPARγ), are similar to the apo structure. Here we use NMR, accelerated molecular dynamics and hydrogen-deuterium exchange mass spectrometry to define the PPARγ structural ensemble. We find that the helix 3 charge clamp positioning varies widely in apo and is stabilized by efficacious ligand binding. We also reveal a previously undescribed mechanism for inverse agonism involving an omega loop to helix switch which induces disruption of a tripartite salt-bridge network. We demonstrate that ligand binding can induce multiple structurally distinct repressive states. One state recruits peptides from two different corepressors, while another recruits just one, providing structural evidence of ligand bias in a nuclear receptor.

The repressive states of peroxisome proliferator-activated receptor γ (PPARγ) are ill-defined, despite nuclear receptors being a major drug target. Here authors demonstrate multiple structurally distinct repressive states, providing a structural rationale for ligand bias in a nuclear receptor.