Mechanistic insights into the synergistic activation of the RXR-PXR heterodimer by endocrine disruptor mixtures.

Humans are chronically exposed to mixtures of xenobiotics referred to as endocrine-disrupting chemicals (EDCs). A vast body of literature links exposure to these chemicals with increased incidences of reproductive, metabolic, or neurological disorders. Moreover, recent data demonstrate that, when used in combination, chemicals have outcomes that cannot be predicted from their individual behavior. In its heterodimeric form with the retinoid X receptor (RXR), the pregnane X receptor (PXR) plays an essential role in controlling the mammalian xenobiotic response and mediates both beneficial and detrimental effects. Our previous work shed light on a mechanism by which a binary mixture of xenobiotics activates PXR in a synergistic fashion. Structural analysis revealed that mutual stabilization of the compounds within the ligand-binding pocket of PXR accounts for the enhancement of their binding affinity. In order to identify and characterize additional active mixtures, we combined a set of cell-based, biophysical, structural, and in vivo approaches. Our study reveals features that confirm the binding promiscuity of this receptor and its ability to accommodate bipartite ligands. We reveal previously unidentified binding mechanisms involving dynamic structural transitions and covalent coupling and report four binary mixtures eliciting graded synergistic activities. Last, we demonstrate that the robust activity obtained with two synergizing PXR ligands can be enhanced further in the presence of RXR environmental ligands. Our study reveals insights as to how low-dose EDC mixtures may alter physiology through interaction with RXR-PXR and potentially several other nuclear receptor heterodimers.

Insights into the activation mechanism of human estrogen-related receptor γ by environmental endocrine disruptors.

The estrogen-related receptor γ (ERRγ, NR3B3) is a constitutively active nuclear receptor which has been proposed to act as a mediator of the low-dose effects of a number of environmental endocrine-disrupting chemicals (EDCs) such as the xenoestrogen bisphenol-A (BPA). To better characterize the ability of exogenous compounds to bind and activate ERRγ, we used a combination of cell-based, biochemical, structural and computational approaches. A purposely created stable cell line allowed for the determination of the EC50s for over 30 environmental ERRγ ligands, including previously unknown ones. Interestingly, affinity constants (Kds) of the most potent compounds measured by isothermal titration calorimetry were in the 50-500 nM range, in agreement with their receptor activation potencies. Crystallographic analysis of the interaction between the ERRγ ligand-binding domain (LBD) and compounds of the bisphenol, alkylphenol and naphthol families revealed a partially shared binding mode and minimal alterations of the receptor conformation upon ligand binding. Further biophysical characterizations coupled to molecular dynamics simulations suggested a mechanism through which ERRγ ligands would exhibit their agonistic properties by preserving the transcriptionally active form of the receptor while rigidifying some loop regions with associated functions. This unique mechanism contrasts with the classical one involving a ligand-induced repositioning and stabilization of the C-terminal activation helix H12.

Design and in vitro characterization of RXR variants as tools to investigate the biological role of endogenous rexinoids.

Retinoid X receptors (RXRα, ß, and γ) are essential members of the nuclear receptor (NR) superfamily of ligand-dependent transcriptional regulators that bind DNA response elements and control the expression of large gene networks. As obligate heterodimerization partners of many NRs, RXRs are involved in a variety of pathophysiological processes. However, despite this central role in NR signaling, there is still no consensus regarding the precise biological functions of RXRs and the putative role of the endogenous ligands (rexinoids) previously proposed for these receptors. Based on available crystal structures, we introduced a series of amino acid substitutions into the ligand-binding pocket of all three RXR subtypes in order to alter their binding properties. Subsequent characterization using a battery of cell-based and in vitro assays led to the identification of a double mutation abolishing the binding of any ligand while keeping the other receptor functions intact and a triple mutation that selectively impairs interaction with natural rexinoids but not with some synthetic ligands. We also report crystal structures that help understand the specific ligand-binding capabilities of both variants. These RXR variants, either fully disabled for ligand binding or retaining the property of being activated by synthetic compounds, represent unique tools that could be used in future studies to probe the presence of active endogenous rexinoids in tissues/organs and to investigate their role in vivo. Last, we provide data suggesting a possible involvement of fatty acids in the weak interaction of RXRs with corepressors.

A special type of head stereotypies in children with developmental (?cerebellar) disorder: description of 8 cases and literature review.

The authors report eight children, who presented in the first year of life with isolated head stereotypies, that corresponded neither to the usual normal 'rhythmic habit patterns of infancy', nor to various types of abnormal repetitive head movements described in young children. Their head stereotypies closely resembled those described in bobble-head doll syndrome. The neurological status evidenced axial hypotonia, ataxia, oculomotor abnormalities, motor and language delay. The patients were followed for several years clinically and with video recordings. No single aetiology was found. Computed tomography (CT) or magnetic resonance imaging (MRI) show a congenital cerebellar abnormality in two children but no hydrocephalus. The outcome of these children showed in all but one patient a normal cognitive and psychosocial development, even though the head stereotypies are still present in six of eight patients and all remain significantly clumsy. The association of head stereotypies and motor delay should prompt a search for cerebellar congenital malformation. The outcome of those patients was much better than originally anticipated and these head stereotypies are not related either to mental retardation, or to psychopathology. The possible mechanisms involved are also discussed.

Immune system dysregulation in adolescent major depressive disorder.

BACKGROUND: A large body of evidence suggests that immune system dysregulation is associated with Major Depressive Disorder (MDD) in adults. This study extends this work to adolescent MDD to examine the hypotheses of immune system dysregulation in adolescents with MDD, as manifested by significantly: (i) elevated plasma levels of cytokines (interferon [IFN]-gamma, tumor necrosis factor-alpha, interleukin [IL]-6, IL-1beta, and IL-4); and (ii) Th1/Th2 cytokine imbalance shifted toward Th1 as indexed by increased IFN-gamma/IL-4. METHOD: Thirty adolescents with MDD (19 females; 13 medication-free/naïve; ages 12-19) of at least 6 weeks duration and a minimum severity score of 40 on the Children's Depression Rating Scale-Revised, and 15 healthy comparisons (8 females), group-matched for age, were enrolled. Plasma cytokines were examined using enzyme-linked immunosorbent assay. Mann-Whitney test was used to compare subjects with MDD and controls. RESULTS: Adolescents with MDD had significantly elevated plasma IFN-gamma levels (3.38+/-11.8 pg/ml versus 0.37+/-0.64 pg/ml; p<0.003), and IFN-gamma/IL-4 ratio (16.6+/-56.5 versus 1.76+/-2.28; p=0.007). A trend for IL-6 to be elevated in the MDD group was also observed (1.52+/-2.88 pg/ml versus 0.49+/-0.90 pg/ml; p=0.09). Importantly, findings remained evident when medicated subjects were excluded. CONCLUSIONS: Findings suggest that immune system dysregulation may be associated with adolescent MDD, with an imbalance of Th1/Th2 shifted toward Th1, as documented in adult MDD. Larger studies with medication-free adolescents should follow.