Expression of liver X receptor beta is essential for formation of superficial cortical layers and migration of later-born neurons

Loss of ERβ in Aging LXRαβ Knockout Mice Leads to Colitis

Liver X receptors (LXRα and LXRβ) are oxysterol-activated nuclear receptors that play key roles in cholesterol homeostasis, the central nervous system, and the immune system. We have previously reported that LXRαβ-deficient mice are more susceptible to dextran sodium sulfate (DSS)-induced colitis than their WT littermates, and that an LXR agonist protects against colitis in mice mainly via the regulation of the immune system in the gut. We now report that both LXRα and LXRβ are expressed in the colonic epithelium and that in aging LXRαβ-/- mice there is a reduction in the intensity of goblet cells, mucin (MUC2), TFF3, and estrogen receptor β (ERβ) levels. The cytoplasmic compartment of the surface epithelial cells was markedly reduced and there was a massive invasion of macrophages in the lamina propria. The expression and localization of β-catenin, α-catenin, and E-cadherin were not changed, but the shrinkage of the cytoplasm led to an appearance of an increase in staining. In the colonic epithelium there was a reduction in the expression of plectin, a hemidesmosome protein whose loss in mice leads to spontaneous colitis, ELOVL1, a fatty acid elongase protein coding gene whose overexpression is found in colorectal cancer, and non-neuronal choline acetyltransferase (ChAT) involved in the regulation of epithelial cell adhesion. We conclude that in aging LXRαβ-/- mice, the phenotype in the colon is due to loss of ERβ expression.

Implication of Hippocampal Neurogenesis in Autism Spectrum Disorder: Pathogenesis and Therapeutic Implications

Autism spectrum disorder (ASD) is a cluster of heterogeneous neurodevelopmental conditions with atypical social communication and repetitive sensory-motor behaviors. The formation of new neurons from neural precursors in the hippocampus has been unequivocally demonstrated in the dentate gyrus of rodents and non-human primates. Accumulating evidence sheds light on how the deficits in the hippocampal neurogenesis may underlie some of the abnormal behavioral phenotypes in ASD. In this review, we describe the current evidence concerning pre-clinical and clinical studies supporting the significant role of hippocampal neurogenesis in ASD pathogenesis, discuss the possibility of improving hippocampal neurogenesis as a new strategy for treating ASD, and highlight the prospect of emerging pro-neurogenic therapies for ASD.

Npas3 regulates stemness maintenance of radial glial cells and neuronal migration in the developing mouse cerebral cortex

The neuronal PAS domain 3 (NPAS3) is a member of the basic helix-loop-helix (bHLH) PAS family of transcription factors and is implicated in psychiatric and neurodevelopmental disorders. NPAS3 is robustly expressed in the cortical ventricle zone (VZ), a transient proliferative zone containing progenitor cells, mainly radial glial cells, destined to give rise to cortical excitatory neurons. However, the role of NPAS3 in corticogenesis remains largely unknown. In this study, we knocked down Npas3 expression in the neural progenitor cells residing in the cortical VZ to investigate the role of Npas3 in cerebral cortical development in mice. We demonstrated that Npas3 knockdown profoundly impaired neuronal radial migration and changed the laminar cell fate of the cells detained in the deep cortical layers. Furthermore, the downregulation of Npas3 led to the stemness maintenance of radial glial cells and increased the proliferation rate of neural progenitor cells residing in the VZ/subventricular zone (SVZ). These findings underline the function of Npas3 in the development of the cerebral cortex and may shed light on the etiology of NPAS3-related disorders.

Liver X Receptor Regulation of Glial Cell Functions in the CNS

In this review, we discuss the role of liver X receptors (LXRs) in glial cells (microglia, oligodendrocytes and astrocytes) in the central nervous system (CNS). LXRs are oxysterol-activated nuclear receptors that, in adults, regulate genes involved in cholesterol homeostasis, the modulation of inflammatory responses and glutamate homeostasis. The study of LXR knockout mice has revealed that LXRβ plays a key role in maintaining the health of dopaminergic neurons in the substantia nigra, large motor neurons in the spinal cord and retinal ganglion cells in the eye. In the peripheral nervous system (PNS), LXRβ is responsible for the health of the spiral ganglion neurons (SGNs) in the cochlea. In addition, LXRs are essential for the homeostasis of the cerebrospinal fluid (CSF), and in LXRαβ^−/− mice, the lateral ventricles are empty and lined with lipid-laden cells. As LXRαβ^−/− mice age, lipid vacuoles accumulate in astrocytes surrounding blood vessels. By seven months of age, motor coordination becomes impaired, and there is a loss of motor neurons in the spinal cord of LXRβ^−/− mice. During development, migration of neurons in the cortex and cerebellum is retarded in LXRβ^−/− mice. Since LXRs are not expressed in dopaminergic or motor neurons in adult mice, the neuroprotective effects of LXRs appear to come from LXRs in glial cells where they are expressed. However, despite the numerous neurological deficits in LXR^−/− rodents, multiple sclerosis has the clear distinction of being the only human neurodegenerative disease in which defective LXR signaling has been identified. In this review, we summarize the regulation and functions of LXRs in glial cells and analyze how targeting LXRs in glial cells might, in the future, be used to treat neurodegenerative diseases and, perhaps, disorders caused by aberrant neuronal migration during development.

Ablation of Liver X receptor β in mice leads to overactive macrophages and death of spiral ganglion neurons

Age-related hearing loss is the most common type of hearing impairment, and is typically characterized by the loss of spiral ganglion neurons (SGNs). The two Liver X receptors (LXRs) are oxysterol-activated nuclear receptors which in adults, regulate genes involved in cholesterol homeostasis and modulation of macrophage activity. LXRβ plays a key role in maintenance of health of dopaminergic neurons in the substantia nigra, large motor neurons in the spinal cord, and retinal ganglion cells in adult mice. We now report that LXRβ is expressed in the SGNs of the cochlea and that loss of LXRβ leads to age-related cochlea degeneration. We found that in the cochlea of LXRβ-/- mice, there is loss of SGNs, activation of macrophages, demyelination in the spiral ganglion, decrease in glutamine synthetase (GS) expression and increase in glutamate accumulation in the cochlea. Part of the cause of damage to the SGNs might be glutamate toxicity which is known to be very toxic to these cells. Our study provides a so far unreported role of LXRβ in maintenance of SGNs whose loss is a very common cause of hearing impairment.

Structural overview and perspectives of the nuclear receptors, a major family as the direct targets for small-molecule drugs

The nuclear receptors (NRs) are an evolutionarily related family of transcription factors, which share certain common structural characteristics and regulate the expressions of various genes by recognizing different response elements. NRs play important roles in cell differentiation, proliferation, survival and apoptosis, rendering them indispensable in many physiological activities including growth and metabolism. As a result, dysfunctions of NRs are closely related to a variety of diseases, such as diabetes, obesity, infertility, inflammation, the Alzheimer's disease, cardiovascular diseases, prostate and breast cancers. Meanwhile, small-molecule drugs directly targeting NRs have been widely used in the treatment of above diseases. Here we summarize recent progress in the structural biology studies of NR family proteins. Compared with the dozens of structures of isolated DNA-binding domains (DBDs) and the striking more than a thousand of structures of isolated ligand-binding domains (LBDs) accumulated in the Protein Data Bank (PDB) over thirty years, by now there are only a small number of multi-domain NR complex structures, which reveal the integration of different NR domains capable of the allosteric signal transduction, or the detailed interactions between NR and various coregulator proteins. On the other hand, the structural information about several orphan NRs is still totally unavailable, hindering the further understanding of their functions. The fast development of new technologies in structural biology will certainly help us gain more comprehensive information of NR structures, inspiring the discovery of novel NR-targeting drugs with a new binding site beyond the classic LBD pockets and/or a new mechanism of action.

Loss of liver X receptor β in astrocytes leads to anxiety-like behaviors via regulating synaptic transmission in the medial prefrontal cortex in mice

Astrocytes are integral components of synaptic transmission, and their dysfunction leads to neuropsychiatric disorders such as anxiety and depression. Liver X receptor β (LXRβ) is expressed in astrocytes, and LXRβ global knockout mice shows impaired synaptic formation. In order to define the role of LXRβ in astrocytes, we used a conditional Cre-loxP system to specifically remove LXRβ from astrocytes. We found that this deletion caused anxiety-like but not depressive-like behaviors in adult male mice. This behavioral phenotype could be completely reproduced by selective deletion of LXRβ in astrocytes in the medial prefrontal cortex (mPFC). Pyramidal neurons in layer V of mPFC are involved in mood behaviors. We found that there was an increased spontaneous excitatory synaptic transmission in layer V pyramidal neurons of the mPFC of these mice. This was concurrent with increased dendritic complexity, despite normal appearance and number of dendritic spines. In addition, gene ontology analysis of RNA sequencing revealed that deletion of astrocytic LXRβ led to the enrichment of the process of synaptic transmission in mPFC. Finally, we also confirmed that renormalized excitatory synaptic transmission in layer V pyramidal neurons alleviated the anxiety in mice with astrocytic LXRβ deletion in mPFC. Together, our findings reveal that astrocytic LXRβ in mPFC is critical in the regulation of synaptic transmission, and this provides a potential new target for treatment of anxiety-like behavior.

The roles of liver X receptor α in inflammation and inflammation-associated diseases

Liver X receptor α (LXRα; also known as NR1H3), an isoform of LXRs, is a member of the nuclear receptor family of transcription factors and plays essential roles in the transcriptional control of cholesterol homeostasis. Previous in-depth phenotypic analyses of mouse models with deficient LXRα have also demonstrated various physiological functions of this receptor within inflammatory responses. LXRα activation exerts a combination of metabolic and anti-inflammatory actions resulting in the modulation and the amelioration of inflammatory disorders. The tight "repercussions" between LXRα and inflammation, as well as cholesterol homeostasis, have suggested that LXRα could be pharmacologically targeted in pathologies such as atherosclerosis, acute lung injury, and Alzheimer's disease. This review gives an overview of the recent advances in understanding the roles of LXRα in inflammation and inflammation-associated diseases, which will help in the design of future experimental researches on the potential of LXRα and advance the investigation of LXRα as pharmacological inflammatory targets.

Self-Reporting Transposons Enable Simultaneous Readout of Gene Expression and Transcription Factor Binding in Single Cells

Cellular heterogeneity confounds in situ assays of transcription factor (TF) binding. Single-cell RNA sequencing (scRNA-seq) deconvolves cell types from gene expression, but no technology links cell identity to TF binding sites (TFBS) in those cell types. We present self-reporting transposons (SRTs) and use them in single-cell calling cards (scCC), a novel assay for simultaneously measuring gene expression and mapping TFBS in single cells. The genomic locations of SRTs are recovered from mRNA, and SRTs deposited by exogenous, TF-transposase fusions can be used to map TFBS. We then present scCC, which map SRTs from scRNA-seq libraries, simultaneously identifying cell types and TFBS in those same cells. We benchmark multiple TFs with this technique. Next, we use scCC to discover BRD4-mediated cell-state transitions in K562 cells. Finally, we map BRD4 binding sites in the mouse cortex at single-cell resolution, establishing a new method for studying TF biology in situ.

Activation of LXRβ Signaling in the Amygdala Confers Anxiolytic Effects Through Rebalancing Excitatory and Inhibitory Neurotransmission upon Acute Stress

The balance of major excitatory (glutamate, Glu) and inhibitory (γ-aminobutyric acid, GABA), named as E/I neurotransmission, is critical for proper information processing. Anxiety-like responses upon stress are accompanied by abnormal alterations in the formation and function of synapses, resulting in the imbalance of E/I neurotransmission in the amygdala. Liver X receptors (LXRs), including LXRα and LXRβ isoforms, are nuclear receptors responsible for regulating central nervous system (CNS) functions besides maintaining metabolic homeostasis. However, little is known about the contribution of LXRs in E/I balance in regulating anxiety-related behaviors induced by stress. In this study, we found stress-induced anxiety led to the expression reduction of LXRβ not LXRα in mice amygdala. GW3965, a dual agonist for both LXRα and LXRβ, alleviated anxiety-like behaviors of stressed mice through activation of LXRβ, confirmed by the knockdown of LXRβ mediated by lentiviral shRNAs in the basolateral amygdala (BLA). This was paralleled by correcting the disequilibrium of E/I neurotransmission in the stressed BLA. Importantly, GW3965 exerted anxiolytic effects by correcting the promoted amplitude and frequency of miniature excitatory postsynaptic current (mEPSC), and augmenting the decreased that of miniature inhibitory postsynaptic current (mIPSC) in the stressed BLA. This suggests that stress-induced anxiety-like behaviors can largely be ascribed to the deficit of LXRβ signaling in E/I neurotransmission in BLA. These findings highlight the deficiency of LXRβ signaling in the amygdala linked to anxiety disorder, and LXRβ activation may represent a potential novel target for anxiety treatment with an alteration in synaptic transmission in the amygdala.

Loss of Wwox Perturbs Neuronal Migration and Impairs Early Cortical Development

Mutations in the WWOX gene cause a broad range of ultra-rare neurodevelopmental and brain degenerative disorders, associated with a high likelihood of premature death in animal models as well as in humans. The encoded Wwox protein is a WW domain-containing oxidoreductase that participates in crucial biological processes including tumor suppression, cell growth/differentiation and regulation of steroid metabolism, while its role in neural development is less understood. We analyzed the exomes of a family affected with multiple pre- and postnatal anomalies, including cerebellar vermis hypoplasia, severe neurodevelopmental impairment and refractory epilepsy, and identified a segregating homozygous WWOX mutation leading to a premature stop codon. Abnormal cerebral cortex development due to a defective architecture of granular and molecular cell layers was found in the developing brain of a WWOX-deficient human fetus from this family. A similar disorganization of cortical layers was identified in lde/lde rats (carrying a homozygous truncating mutation which disrupts the active Wwox C-terminal domain) investigated at perinatal stages. Transcriptomic analyses of Wwox-depleted human neural progenitor cells showed an impaired expression of a number of neuronal migration-related genes encoding for tubulins, kinesins and associated proteins. These findings indicate that loss of Wwox may affect different cytoskeleton components and alter prenatal cortical development, highlighting a regulatory role of the WWOX gene in migrating neurons across different species.

Compound K induces neurogenesis of neural stem cells in thrombin induced nerve injury through LXRα signaling in mice

Intracerebral hemorrhage (ICH) causes neurological function deficit due to the loss of neurons surrounding the hematoma. Increased neurogenesis of endogenous neural stem cells (EnNSCs) is believed to increase cell proliferation and differentiation, thereby improving the neurological deficit. However, there are still limited drugs that are effective for treating neurological deficit. So, the effects of compound K (CK) in EnNSCs were measured after thrombin-induced mice models both in vivo and in vitro, and investigated the probable mechanisms of CK during pro-neurogenesis. The results revealed that 10 μM CK promotes neurogenesis, proliferation and reduces apoptosis of EnNSCs after induction by thrombin. After that, CK treatment increased the neurogenesis of EnNSCs through liver X receptor α (LXRα) signaling pathway using adeno-associated virus knockdown and knocked out mice of LXRα gene. Finally, intraperitoneal injection of 10 mg/kg CK improved the neurogenesis of subventricular zone (SVZ), myelin repair and behavioral deficit after stereotaxic injection of thrombin in the basal ganglia of mice, and this process involved LXRα. These observations provided evidence regarding the effect of CK in pro-neurogenesis via LXRα activation, and suggested further evaluation of it due to its potential role as an effective modulator in the treatment of ICH.

Activation of liver X receptor promotes hippocampal neurogenesis and improves long-term cognitive function recovery in acute cerebral ischemia-reperfusion mice

Cerebral ischemia (CI) leads to cognitive dysfunction due to the loss of hippocampal neurons. Liver X receptors (LXRs), including the LXRα and LXRβ isoforms, are critical for neurogenesis, synaptic plasticity, neurodegeneration, and cholesterol metabolism. However, the potential role of LXRs in the pathogenesis of CI-induced cognitive impairment is unclear. Therefore, we investigated the effects of LXR activation on hippocampal neurogenesis and cognitive function in mice with CI. C57 mice were randomized into four groups that included a sham group and three treatment groups with CI [Vehicle, TO901317 (TO90, an agonist of LXRs) and GSK2033 (an antagonist of LXRs)]. Mice were subjected to bilateral common carotid artery occlusion for 20 min to induce transient CI. The Morris water maze test was executed to detect spatial learning and memory. Proliferation, differentiation, and immature neurons in the subgranular zone (SGZ) were examined using Immunofluorescence. Western blot assay was used to detect the expression of the Wnt/β-catenin signaling pathway-associated protein. TO90 significantly improved spatial learning and memory deficits induced by CI on 28 days. It enhanced the proliferation of neural stem cells, the number of immature neurons and the differentiation from nascent cells to neurons. The expression of the Wnt/β-catenin signaling pathway-associated protein level was totally increased. The forenamed effects of TO90 were decreased in GSK2033 group. Thus, our findings suggest that LXRs activation can improve long-term cognitive dysfunction caused by CI by increasing neurogenesis, and LXRs may serve as a potential therapeutic target for cerebral ischemia. Cover Image for this issue: doi: 10.1111/jnc.14753.

Impaired plasmalogen synthesis dysregulates liver X receptor-dependent transcription in cerebellum

Synthesis of ethanolamine plasmalogen (PlsEtn) is regulated by modulating the stability of fatty acyl-CoA reductase 1 (Far1) on peroxisomal membrane, a rate-limiting enzyme in plasmalogen synthesis. Dysregulation of plasmalogen homeostasis impairs cholesterol biosynthesis in cultured cells by altering the stability of squalene epoxidase (SQLE). However, regulation of PlsEtn synthesis and physiological consequences of plasmalogen homeostasis in tissues remain unknown. In the present study, we found that the protein but not the transcription level of Far1 in the cerebellum of the Pex14 mutant mouse expressing Pex14p lacking its C-terminal region (Pex14ΔC/ΔC) is higher than that from wild-type mouse, suggesting that Far1 is stabilized by the lowered level of PlsEtn. The protein level of SQLE was increased, whereas the transcriptional activity of the liver X receptors (LXRs), ligand-activated transcription factors of the nuclear receptor superfamily, is lowered in the cerebellum of Pex14ΔC/ΔC and the mice deficient in dihydroxyacetonephosphate acyltransferase, the initial enzyme for the synthesis of PlsEtn. These results suggest that the reduction of plasmalogens in the cerebellum more likely compromises the cholesterol homeostasis, thereby reducing the transcriptional activities of LXRs, master regulators of cholesterol homeostasis.

Expression of the RNA methyltransferase Nsun5 is essential for developing cerebral cortex

Nsun5 gene, encoding a cytosine-5 RNA methyltransferase, is deleted in about 95% patients with Williams-Beuren syndrome (WBS). WBS is a neurodevelopmental disorder and characterized by cognitive disorder. We generated single-gene Nsun5 knockout (Nsun5-KO) mice and reported that the Nsun5 deletion leads to deficit in spatial cognition. This study focused on investigating the influence of Nsun5 deficiency in the development of cerebral cortex. In comparison with wild-type littermates, the cortical thickness in postnatal day 10 Nsun5-KO mice was obviously reduced with an abnormal laminar organization, and the processes of pyramidal cells were shorter and finer. Nsun5 was selectively expressed in radial glial cells (RGCs) of cerebral cortex from embryonic day (E) 12.5 to E16.5, but not in intermediate progenitor cells (IPCs) or neocortical neurons. The Nsun5 deletion did not alter proliferation of RGCs or differentiation of RGCs into IPCs. Notably, the ablation of Nsun5 disrupted the growth of radial glial scaffolds, thus numerous basal processes of RGCs failed to reach pial basement membrane. Level of cell polarity regulator Cdc42 protein in radial glial scaffolds of E14.5 Nsun5-KO mice was reduced, but the level of Cdc42 mRNA was unchanged. The dysfunction of glial scaffolds impeded the radial migration of upper-layer and deeper-layer neurons to cause their subcortical accumulation and apoptosis, resulting in an obvious thinness of the cortical plate in E18.5 Nsun5-KO mice. These findings establish a critical role of Nsun5 in development of cerebral cortex through regulating radial glial scaffolds of RGCs to control migration of neocortical neurons.

Epigenetic suppression of liver X receptor β in anterior cingulate cortex by HDAC5 drives CFA-induced chronic inflammatory pain

Background

Liver X receptors (LXRs), including LXRα and LXRβ, are key regulators of transcriptional programs for both cholesterol homeostasis and inflammation in the brain. Here, the modes of action of LXRs and the epigenetic mechanisms regulating LXRβ expression in anterior cingulate cortex (ACC) of chronic inflammatory pain (CIP) are investigated.

Methods

The deficit of LXR isoform and analgesic effect of LXR activation by GW3965 were evaluated using the mouse model of CIP induced by hindpaw injection of complete Freund’s adjuvant (CFA). The mechanisms involved in GW-mediated analgesic effects were analyzed with immunohistochemical methods, ELISA, co-immunoprecipitation (Co-IP), Western blot, and electrophysiological recording. The epigenetic regulation of LXRβ expression was investigated by chromatin immunoprecipitation, quantitative real-time PCR, and sequencing.

Results

We revealed that CFA insult led to LXRβ reduction in ACC, which was associated with upregulated expression of histone deacetylase 5 (HDAC5), and knockdown of LXRβ by shRNA led to thermal hyperalgesia. Co-IP showed that LXRβ interacted with NF-κB p65 physically. LXRβ activation by GW3965 exerted analgesic effects by inhibiting the nuclear translocation of NF-κB, reducing the phosphorylation of mitogen-activated protein kinases (MAPKs) in ACC, and decreasing the promoted input-output and enhanced mEPSC frequency in ACC neurons after CFA exposure. In vitro experiments confirmed that HDAC5 triggered histone deacetylation on the promoter region of Lxrβ, resulting in downregulation of Lxrβ transcription.

Conclusion

These findings highlight an epigenetic mechanism underlying LXRβ deficits linked to CIP, and LXRβ activation may represent a potential novel target for the treatment of CIP with an alteration in inflammation responses and synaptic transmission in ACC.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1507-3) contains supplementary material, which is available to authorized users.

Activation of LXRβ inhibits proliferation, promotes apoptosis, and increases chemosensitivity of gastric cancer cells by upregulating the expression of ATF4

Recently, great advances have been achieved in both surgery and chemotherapy for the treatment of gastric cancer, but there is still poor prognosis for this disease. The aim of this study is to investigate the role of liver X receptor β (LXRβ) in chemosensitivity of gastric cancer SGC7901 cells. From 171 patients with gastric cancer, the gastric cancer and paracancerous tissues were selected to measure the expression of LXRβ and ATF4. Gastric cancer cell lines were cultured and screened to figure out the proliferation and apoptosis of gastric cancer SGC7901 cells with the treatment of LXRβ agonist (GW3965), ATF4 short hairpin RNA (shRNA), and chemotherapy drug paclitaxel. The expression of apoptosis-related gene cleaved caspase-3 was detected by Western blot analysis. First, we found that the expressions of LXRβ and ATF4 in gastric cancer tissues and cells were significantly lower than those in their paracancerous tissues and gastric mucosal epithelial cells. In addition, activation of LXRβ and paclitaxel treatment suppressed proliferation of SGC7901 cells, and the expression of ATF4 was upregulated in a concentration-dependent manner. Furthermore, shRNA significantly inhibited the expression of ATF4 and blocked the chemosensitivity of SGC7901 cells to LXRβ activation. Our study demonstrates that the expression of LXRβ was low in gastric cancer. In addition, activation of LXRβ may inhibit the proliferation of gastric cancer cells, promote apoptosis, and increase chemosensitivity by upregulating the expression of ATF4.

Multi-omics investigation reveals benzalkonium chloride disinfectants alter sterol and lipid homeostasis in the mouse neonatal brain

Lipids are critical for neurodevelopment; therefore, disruption of lipid homeostasis by environmental chemicals is expected to have detrimental effects on this process. Previously, we demonstrated that the benzalkonium chlorides (BACs), a class of commonly used disinfectants, alter cholesterol biosynthesis and lipid homeostasis in neuronal cell cultures in a manner dependent on their alkyl chain length. However, the ability of BACs to reach the neonatal brain and alter sterol and lipid homeostasis during neurodevelopment in vivo has not been characterized. Therefore, the goal of this study was to use targeted and untargeted mass spectrometry and transcriptomics to investigate the effect of BACs on sterol and lipid homeostasis, and to predict the mechanism of toxicity of BACs on neurodevelopmental processes. After maternal dietary exposure to 120 mg BAC/kg body weight/day, we quantified BAC levels in the mouse neonatal brain, demonstrating for the first time that BACs can cross the blood-placental barrier and enter the developing brain. Transcriptomic analysis of neonatal brains using RNA sequencing revealed alterations in canonical pathways related to cholesterol biosynthesis, liver X receptor-retinoid X receptor (LXR/RXR) signaling, and glutamate receptor signaling. Mass spectrometry analysis revealed decreases in total sterol levels and downregulation of triglycerides and diglycerides, which were consistent with the upregulation of genes involved in sterol biosynthesis and uptake as well as inhibition of LXR signaling. In conclusion, these findings demonstrate that BACs target sterol and lipid homeostasis and provide new insights for the possible mechanisms of action of BACs as developmental neurotoxicants.

Axonogenesis Is Coordinated by Neuron-Specific Alternative Splicing Programming and Splicing Regulator PTBP2

How a neuron acquires an axon is a fundamental question. Piecemeal identification of many axonogenesis-related genes has been done, but coordinated regulation is unknown. Through unbiased transcriptome profiling of immature primary cortical neurons during early axon formation, we discovered an association between axonogenesis and neuron-specific alternative splicing. Known axonogenesis genes exhibit little expression alternation but widespread splicing changes. Axonogenesis-associated splicing is governed by RNA binding protein PTBP2, which is enriched in neurons and peaks around axonogenesis in the brain. Cortical depletion of PTBP2 prematurely induces axonogenesis-associated splicing, causes imbalanced expression of axonogenesis-associated isoforms, and specifically affects axon formation in vitro and in vivo. PTBP2-controlled axonogenesis-associated Shtn1 splicing determines SHTN1's capacity to regulate actin interaction, polymerization, and axon growth. Precocious Shtn1 isoform switch contributes to disorganized axon formation of Ptbp2^-/- neurons. We conclude that PTBP2-orchestrated alternative splicing programming is required for robust generation of a single axon in mammals.

Liver X receptor β in the hippocampus: A potential novel target for the treatment of major depressive disorder?

Liver X receptors (LXRs), including LXRα and LXRβ isoforms, have been implicated in multiple physiological functions including promoting neurogenesis, improving synaptic plasticity, preventing neurodegeneration, inhibiting inflammation as well as regulating cholesterol metabolism. However, a potential role of LXRs in the treatment of major depressive disorder (MDD) has never been investigated previously. Our present results demonstrated that levels of hippocampal LXRβ but not LXRα were down-regulated in rats exposed to chronic unpredictable stress (CUS) and were negatively correlated with the severity of CUS-induced depressive-like behaviors. Furthermore, rats with LXRβ knockdown by short hairpin RNA (shRNA) in hippocampus displayed depressive-like behaviors and impaired hippocampal neurogenesis similar to those observed after CUS exposure. Conversely, LXRs activation by GW3965 (GW), a synthetic dual agonist for both LXRα and LXRβ isoforms, could improve depression-like behaviors and reverse the impaired hippocampal neurogenesis in rats exposed to CUS. LXRβ knockdown by shRNA completely abrogated the antidepressant and hippocampal neurogenesis-promoting effects of GW, suggesting that LXRβ isoform mediated the antidepressant and hippocampal neurogenesis-promoting effects of the LXRα/β dual agonist. However, ablation of hippocampal neurogenesis with x-irradiation only partly but not completely abolished the antidepressant effects of GW in the behavioral tests, implying that the antidepressant effects mediated by LXRβ isoform are likely through both neurogenesis-dependent and -independent pathways. Thus, our findings suggest that LXRβ activation may represent a potential novel target for the treatment of MDD and also provide a novel insight into the underlying mechanisms of MDD.

Liver X receptor β regulates the development of the dentate gyrus and autistic-like behavior in the mouse

The dentate gyrus (DG) of the hippocampus is a laminated brain region in which neurogenesis begins during early embryonic development and continues until adulthood. Recent studies have implicated that defects in the neurogenesis of the DG seem to be involved in the genesis of autism spectrum disorders (ASD)-like behaviors. Liver X receptor β (LXRβ) has recently emerged as an important transcription factor involved in the development of laminated CNS structures, but little is known about its role in the development of the DG. Here, we show that deletion of the LXRβ in mice causes hypoplasia in the DG, including abnormalities in the formation of progenitor cells and granule cell differentiation. We also found that expression of Notch1, a central mediator of progenitor cell self-renewal, is reduced in LXRβ-null mice. In addition, LXRβ deletion in mice results in autistic-like behaviors, including abnormal social interaction and repetitive behavior. These data reveal a central role for LXRβ in orchestrating the timely differentiation of neural progenitor cells within the DG, thereby providing a likely explanation for its association with the genesis of autism-related behaviors in LXRβ-deficient mice.

Activation of liver X receptor β-enhancing neurogenesis ameliorates cognitive impairment induced by chronic cerebral hypoperfusion

Chronic cerebral hypoperfusion (CCH), a leading cause of various cerebrovascular diseases, leads to cognitive dysfunction due to neuron loss and impaired neurogenesis. Liver X receptors (LXRs), including LXRα and LXRβ isoforms, are crucial for cholesterol metabolism, synaptic plasticity as well as neurogenesis. However, it is not clear the potential roles of LXRs in the pathogenesis of cognitive impairment induced by CCH. In this study, we demonstrated that LXRβ expression decreased in hippocampus of CCH mice. GW3965, a synthetic dual agonist for both LXRα and LXRβ, ameliorated impairment of learning and memory in CCH mice by promoting neuronal survival and neural stem cells (NSCs) proliferation in dentate gyrus (DG) of CCH mice. The proliferative effects of GW3965 were further confirmed in cultured neural progenitor cells (NPCs) and showed in a concentration-dependent manner. Moreover, GW3965 phosphorylated protein kinase B (Akt) at Ser473 in a time- and concentration-dependent manner in NPCs. Furthermore, both LY294002, an inhibitor for phosphoinositide-3-kinase (PI3K), and short hairpin RNAs for LXRβ knockdown, abrogated GW3965-induced Akt phosphorylation, and therefore abolished GW3965-mediated proliferation-promoting of NPCs. All the data suggested that GW3965 ameliorated impaired cognitive functions in CCH by promoting NSC proliferation through PI3K/Akt pathway followed LXRβ activation. This study correlates a deficit of LXRβ in cognitive dysfunction in CCH with impaired neurogenesis in hippocampus, and LXRs may serve as a potential therapeutic target for chronic cerebral ischemia.

Activation of liver X receptor inhibits OCT2-mediated organic cation transport in renal proximal tubular cells

Liver X receptor (LXR) is transcriptional factor that plays an important role in the regulation of energy metabolism such as cholesterol, lipid, and glucose metabolism as well as membrane transporters and channels. Using both in vitro and in vivo models, LXR regulation of the expression and function of renal organic cation transporter 2 (OCT2) was observed. Synthetic LXR agonist (GW3965) and endogenous LXR agonist (22R-hydroxycholesterol) significantly reduced the uptake of ³H-MPP⁺, a prototypic substrate of OCT2, in both OCT2- Chinese hamster ovary K1 and human renal proximal tubular cells (RPTEC/TERT1). GW3965 decreased transport activity of OCT2 via a reduction of the maximal transport rate of MPP⁺ without affecting transporter affinity. The inhibitory effect of GW3965 was attenuated by co-treatment with LXR antagonist (fenofibrate) indicating the inhibition was LXR-dependent mechanism. In addition, co-treatment with a retinoic X receptor (RXR) ligand, 9-cis retinoic acid enhanced the inhibitory effect of GW3965, indicating negative regulation of OCT2 transport activity by the LXR/RXR complex. Treatment RPTEC/TERT1 cells with GW3965 significantly reduced OCT2 protein expression without changing mRNA expression. In parallel, the effect of LXR activation on OCT2 function was investigated in intact mouse kidney. Treating mice with 50 mg/kg BW T0901317 for 14 days significantly decreased ³H-MPP⁺ uptake into renal cortical slices, correlating with decreased OCT2 protein expression in renal cortex without changes in mRNA expression levels. Taken together, LXR/RXR activation downregulates the protein expression and function of OCT2 in renal proximal tubule, suggesting LXR might affect the total profile of renal excretion of cationic compounds.

Pregnane X Receptor Deletion Modifies Recognition Memory and Electroencephalographic Activity

Nuclear receptors (NR) are emerging as key players in the central nervous system (CNS) with reported implications in physiological and pathophysiological conditions. While a number of NR has been studied, it is unknown whether invalidation of the pregnane xenobiotic receptor (PXR, NR1I2) corresponds to neurological modifications in the adult brain. PXR^-/- C57BL/6J and wild-type mice were used to investigate: (i) recognition memory, motor coordination, and anxiety-like behaviors; (ii) longitudinal video-electroencephalographic (EEG) recordings and frequency wave analysis; (iii) neurovascular structures by histological evaluation and expression of the cerebrovascular tight junctions ZO1 and CLDN5. Absence of PXR was associated with anxiety-like behavior and recognition memory impairment in adult mice. The latter was simultaneous to an EEG signature of lower theta frequency during sleep and abnormal delta waves. Neurophysiological changes did not correspond to significant structural changes in the adult brain, expect for a localized and minor increase in the fronto-parietal neurovascular density and reduced ZO1, but not CLDN5, expression in isolated brain capillaries. Our results converge with existing evidence supporting a link between NR expression and brain physiology. Although the exact modalities remain to be elucidated, the possibility that extra-physiological modulation of PXR may constitute a pathophysiological entry point or a molecular target for brain diseases is proposed.

Liver X Receptors differentially modulate central myelin gene mRNA levels in a region-, age- and isoform-specific manner

Liver X Receptors (LXRs) α and β are nuclear receptors able to bind oxidative forms of cholesterol. They play important roles in the central nervous system (CNS), through their implication in a large variety of physiological and pathological processes among which modulation of cholesterol homeostasis and inflammation. Besides, we recently revealed their crucial role in myelination and remyelination in the cerebellum. Given the pleiotropic effects of such receptors on CNS functioning, we studied here the influence of LXRs on myelin gene mRNA accumulation in the major myelinated regions of the CNS in vivo. We show that both LXR isoforms differentially affect mRNA amount of myelin genes (PLP and MBP) in highly myelinated structures such as spinal cord, corpus callosum, optic nerve and cerebellum. In the adult, LXR activation by the synthetic agonist TO901317 significantly increases myelin gene mRNA amount in the cerebellum but not in the other regions studied. Invalidation of the sole LXRβ isoform leads to decreased PLP and MBP mRNA levels in all the structures except the spinal cord, while the knock out of both isoforms (LXR dKO) decreases myelin gene mRNA amounts in all the regions tested except the corpus callosum. Interestingly, during myelination process (post-natal day 21), both cerebellum and optic nerve display a decrease in myelin gene mRNA levels in LXR dKO mice. Concomitantly, PLP and MBP mRNA accumulation in the spinal cord is increased. Relative expression level of LXR isoforms could account for the differential modulation of myelin gene expression in the CNS. Altogether our results suggest that, within the CNS, each LXR isoform differentially influences myelin gene mRNA levels in a region- and age-dependant manner, participating in the fine regulation of myelin gene expression.

Active liver X receptor signaling in phagocytes in multiple sclerosis lesions

OBJECTIVE

We sought to determine the liver X receptor (LXR) ligands present in human macrophages after myelin phagocytosis and whether LXRs are activated in multiple sclerosis (MS) lesions.

METHODS

We used real-time quantitative polymerase chain reaction (PCR) and immunohistochemistry to determine expression of LXRs and their response genes in human phagocytes after myelin phagocytosis and in active MS lesions. We used gas chromatographic/mass spectrometric analysis to determine LXR-activating oxysterols and cholesterol precursors present and formed in myelin and myelin-incubated cells, respectively.

RESULTS

Myelin induced LXR response genes ABCA1 and ABCG1 in human monocyte-derived macrophages. In active MS lesions, we found that both gene expression and protein levels of ABCA1 and apolipoprotein E ( APOE) are upregulated in foamy phagocytes. Moreover, we found that the LXR ligand 27-hydroxycholesterol (27OHC) is significantly increased in human monocyte-derived macrophages after myelin uptake.

CONCLUSION

LXR response genes are upregulated in phagocytes present in active MS lesions, indicating that LXRs are activated in actively demyelinating phagocytes. In addition, we have shown that myelin contains LXR ligands and that 27OHC is generated in human monocyte-derived macrophages after myelin processing. This suggests that LXRs in phagocytes in active MS lesions are activated at least partially by (oxy)sterols present in myelin and the generation thereof during myelin processing.

LXR agonists promote the proliferation of neural progenitor cells through MEK-ERK pathway

The liver X receptors (LXRs) are transcriptional regulators of lipid homeostasis and may be critical for neurodegeneration and neurogenesis in vivo. However, it remains largely unknown about the role of LXRs and its agonists in the in vitro proliferation of neural progenitor cells (NPCs). Here we revealed for the first time that LXRs were markedly expressed in mouse NPCs and were critical for the in vitro proliferation. LXR agonists GW3965 and LXR623 promoted the proliferation of wildtype NPCs, but not NPCs from LXR double-knockout mice. Mechanistically, phosphorylation of MEK1/2 and ERK1/2 in NPCs was enhanced upon LXR agonist treatment, while abrogation of MEK/ERK phosphorylation by the inhibitors PD98059 and U0126 impaired the proliferation of wildtype NPCs in the presence or absence of LXR agonists. Collectively, our findings suggest that LXR agonists GW3965 and LXR623 can stimulate the NPC proliferation in LXR- and MEK/ERK-dependent manner.

Estrogen Receptor β as a Pharmaceutical Target

A major issue in clinical endocrinology today is how to use hormones to achieve the health benefits that they clearly can provide but avoid the negative side effects, that is, how to develop more precise medicines. This problem of how to use hormones is pervasive in clinical endocrinology. It is true for estrogen, progesterone, androgen, vitamin D, and thyroid hormone, and the problem is amplified in the case of new ligands for the more recently discovered nuclear receptors. Selective targeting of hormone receptor subtypes is one attractive way to harness the beneficial effects of hormones while reducing unwanted side effects. Here, we focus on estrogen receptor (ER)β, which has promise as a selective target in hormone replacement therapy, and in breast and prostate cancers.

Arsenic trioxide suppresses liver X receptor β and enhances cholesteryl ester transfer protein expression without affecting the liver X receptor α in HepG2 cells

Chronic arsenic exposure is associated with cerebrovascular disease and the formation of atherosclerotic lesions. Our previous study demonstrated that arsenic trioxide (ATO) exposure was associated with atherosclerotic lesion formation through alterations in lipid metabolism in the reverse cholesterol transport process. In mouse livers, the expression of the liver X receptor β (LXR-β) and the cholesteryl ester transfer protein (CETP) was suppressed without any changes to the lipid profile. The aim of this study was to elucidate whether ATO contributes to atherosclerotic lesions by suppressing LXR-β and CETP levels in hepatocytes. HepG2 cells, human hepatocytes, were exposed to different ATO concentrations in vitro. Cell viability was determined by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide assay. The liver X receptor α (LXR-α), LXR-β, sterol regulatory element-binding protein-1c (SREBP-1c) and CETP protein levels were measured by Western blotting, and their mRNA levels were measured by real-time PCR. Cholesterol efflux was analyzed by flow cytometry. The results showed ATO inhibited LXR-β mRNA and protein levels with a subsequent decrease in SREBP-1c protein levels and reduced cholesterol efflux from HepG2 cells into the extracellular space without influencing LXR-α mRNA and protein levels. CETP protein levels of HepG2 cells were significantly elevated under arsenic exposure. Transfection of LXR-β shRNA did not change CETP protein levels, implying that there is no cross-talk between LXR-β and CETP. In conclusion, arsenic not only inhibits LXR-β and SREBP-1c mRNA and protein levels but also independently increases CETP protein levels in HepG2 cells.

Liver X receptor reduces proliferation of human oral cancer cells by promoting cholesterol efflux via up-regulation of ABCA1 expression

Liver X receptors (LXRs) contribute not only to maintain cholesterol homeostasis but also to control cell growth. However, the molecular mechanisms behind the LXR-mediated anti-proliferative effects are largely unknown. Here we show, by immunohistochemistry, that LXRα and LXRβ are differentially distributed in oral stratified squamous epithelia. By immunohistochemical and Western blot analyses, we also reveal that LXRα is abundantly expressed in human oral squamous cell carcinoma (HOSCC) tissues and cell lines. Cell counting, BrdU labeling and cell cycle assay indicated that LXR stimulation led to significant reduction of proliferation in HOSCC cells. Importantly, our study highlights, by using RNA interference, that the ATP-binding cassette transporter A1 (ABCA1)-accelerated cholesterol efflux is critical for the growth inhibitory action of LXRs in HOSCC cells. Moreover, we demonstrate that LXR activation reduces the growth of xenograft tumour of HOSCC cells in mice accompanied by the upregulation of ABCA1 expression and the decline of cholesterol levels in the tumour. These findings strongly suggested that targeting the LXR-regulated cholesterol transport, yielding in lowering intracellular cholesterol levels, could be a promising therapeutic option for certain types of cancers.

Down-Regulated Expression of Liver X Receptor beta in Cortical Lesions of Patients with Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is strongly associated with medically intractable epilepsy. Studies suggest that liver X receptor beta (LXRβ) may participate in the pathogenesis of FCD. The present study investigated the expression pattern of LXRβ in FCD and the distribution of LXRβ in different neural precursor cells. Twenty-five surgical specimens from FCD patients and 11 age-matched control samples from autopsies were included in our study. Protein levels and distribution were detected by western blot, immunohistochemistry, and immunofluorescence staining. We found that (1) the level of LXRβ protein was markedly reduced in FCD. (2) LXRβ staining was weaker in the dysplastic cortices of FCD and was mainly observed in neuronal microcolumns, and malformed cells. (3) LXRβ was co-localized with radial glial cells (RGCs) markers and oligodendrocyte precursor cells (OPCs) markers in malformed cells. (4) RGCs marker and OPCs marker were down-regulated while LXRβ downstream factors were up-regulated in FCD specimens. Taken together, our results indicate that LXRβ may interact with β-catenin to regulate the generation of OPCs and the transformation of RGCs. LXRβ therefore potentially contributes to the pathogenesis of FCD.

Liver X receptors regulate cerebrospinal fluid production

Of the two isoforms of Liver X receptor (LXR), LXRβ has been shown to have major effects in the central nervous system (CNS) and on the regulation of aquaporins while LXRα has its most marked effects on cholesterol homeostasis. Both receptors have immunomodulatory functions. In LXRαβ knockout (ko) mice, the CNS phenotype is much more severe than in the LXRβ ko mice, suggesting a contribution of LXRα in CNS functions. One of the most striking abnormalities in the brains of LXRαβ ko mice is the occlusion of the lateral ventricles with age. In the present study, we have found by immunohistochemical staining that both LXRα and LXRβ are expressed in the cell nuclei of the epithelium of the choroid plexus and in the ependymal cells surrounding the lateral ventricles. The two receptors regulate several genes and can compensate for each other on expression of genes involved in structural integrity (E-cadherin, P-cadherin and β-catenin) and function (aquaporin 1 and carbonic anhydrase IX). Aquaporin 4 (AQ4) is not expressed in the choroid plexus but is expressed in the astrocytic end feet and ependymal cells. AQP4 expression was increased in white matter around lateral ventricles but not in neurons of LXRαβ ko mice. The data show that LXR is a regulator of cerebrospinal fluid (CSF) both at the choroid plexus and at the astrocytic end feet and defects in the synthesis of cerebrospinal fluid may be targeted by LXR agonists to facilitate CSF production, turnover and clearance in CNS diseases.

LXR Regulation of Brain Cholesterol: From Development to Disease

Liver X receptors (LXRs) are master regulators of cholesterol homeostasis and inflammation in the central nervous system (CNS). The brain, which contains a disproportionately large amount of the body's total cholesterol (∼25%), requires a complex and delicately balanced cholesterol metabolism to maintain neuronal function. Dysregulation of cholesterol metabolism has been implicated in numerous neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Due to their cholesterol-sensing and anti-inflammatory activities, LXRs are positioned centrally in the everyday maintenance of CNS function. This review focuses on recent research into the role of LXRs in the CNS during normal development and homeostasis and in disease states.

Lack of CAR impacts neuronal function and cerebrovascular integrity in vivo

Nuclear receptors (NRs) are a group of transcription factors emerging as players in normal and pathological CNS development. Clinically, an association between the constitutive androstane NR (CAR) and cognitive impairment was proposed, however never experimentally investigated. We wished to test the hypothesis that the impact of CAR on neurophysiology and behavior is underlined by cerebrovascular-neuronal modifications. We have used CAR(-/-) C57BL/6 and wild type mice and performed a battery of behavioral tests (recognition, memory, motor coordination, learning and anxiety) as well as longitudinal video-electroencephalographic recordings (EEG). Brain cell morphology was assessed using 2-photon or electron microscopy and fluorescent immunohistochemistry. We observed recognition memory impairment and increased anxiety-like behavior in CAR(-/-) mice, while locomotor activity was not affected. Concomitantly to memory deficits, EEG monitoring revealed a decrease in 3.5-7Hz waves during the awake/exploration and sleep periods. Behavioral and EEG abnormalities in CAR(-/-) mice mirrored structural changes, including tortuous fronto-parietal penetrating vessels. At the cellular level we found reduced ZO-1, but not CLDN5, tight junction protein expression in cortical and hippocampal isolated microvessel preparations. Interestingly, the neurotoxin kainic acid, when injected peripherally, provoked a rapid onset of generalized convulsions in CAR(-/-) as compared to WT mice, supporting the hypothesis of vascular permeability. The morphological phenotype of CAR(-/-) mice also included some modifications of GFAP/IBA1 glial cells in the parenchymal or adjacent to collagen-IV(+) or FITC(+) microvessels. Neuronal defects were also observed including increased cortical NEUN(+) cell density, hippocampal granule cell dispersion and increased NPY immunoreactivity in the CA1 region in CAR(-/-) mice. The latter may contribute to the in vivo phenotype. Our results indicate that behavioral and electroencephalographic changes in adult CAR(-/-) mice are concomitant to discrete developmental or structural brain defects. The latter could increase the vulnerability to neurotoxins. The possibility that interfering with nuclear receptors during development could contribute to adulthood brain changes is proposed.

Liver X receptor-β improves autism symptoms via downregulation of β-amyloid expression in cortical neurons

Background

We study the effect of liver X receptor β (LXRβ) on β-amyloid (Aβ) peptide generation and autism behaviors by conducting an animal experiment.

Methods

In autistic mice treated with LXRβ agonist T0901317, enzyme linked immunosorbent assay was used to measure Aβ in brain tissue homogenates. Western blot was used to detect Aβ precursors, Aβ degradation and secretase enzymes, and expression of autophagy-related proteins and Ras/Raf/Erkl/2 signaling pathway proteins in brain tissue. Changes in autism spectrum disorder syndromes of the BTBR mice were compared before and after T0901317 treatment.

Results

Compared with the control group, autistic mice treated with LXRβ agonist T0901317 showed significantly lower Aβ level in brain tissue (P < 0.05), significantly higher Aβ degradation enzyme (NEP, IDE proteins) levels (all P < 0.05), significantly lower Aβ secretase enzyme BACE1 protein level (P < 0.05), and significantly lower Ras, P-C-Raf, C-Raf, P-Mekl/2, P-Erkl/2 protein levels (all P < 0.05). BTBR mice treated with T0901317 showed improvements in repetitive stereotyped behavior, inactivity, wall-facing standing time, self-combing time and center stay time, stayed longer in platform quadrant, and crossed the platform more frequently (all P < 0.05).

Conclusions

LXRβ could potentially reduce brain Aβ generation by inhibiting Aβ production and promoting Aβ degradation, thereby increasing the expression of autophagy-related proteins, reducing Ras/Raf/Erkl/2 signaling pathway proteins, and improving autism behaviors.

Liver X receptor β: new player in the regulatory network of thyroid hormone and 'browning' of white fat

The recent discovery of browning of white adipose tissue (WAT) has raised great research interest because of its significant potential in counteracting obesity and type II diabetes. However, the mechanisms underlying browning are still poorly understood. Liver X receptors (LXRs) are one class of nuclear receptors, which play a vital role in regulating cholesterol, triglyceride and glucose metabolism. Following our previous finding that LXRs serve as repressors of UCP1 in classic brown adipose tissue in female mice, we found that LXRs, especially LXRβ, also repress the browning process of subcutaneous adipose tissue (SAT) in male rodents fed a normal diet. Depletion of LXRs activated thyrotropin releasing hormone positive neurons in the paraventricular area of the hypothalamus, and thus stimulated secretion of thyroid-stimulating hormone from the pituitary. Consequently production of thyroid hormones in the thyroid gland and circulating thyroid hormone level were increased. Moreover, the activity of thyroid signaling in SAT was markedly increased. One unexpected finding of our study is that LXRs are indispensable in the thyroid hormone negative feedback loop at the level of the hypothalamus. LXRs maintain expression of thyroid receptors in the brain and when they are inactivated there is no negative feedback of thyroid hormone in the hypothalamus. Together, our findings have uncovered the basis of increased energy expenditure in male LXR knock-out mice and provided support for targeting LXRs in treatment of obesity.

Treatment with TO901317, a synthetic liver X receptor agonist, reduces brain damage and attenuates neuroinflammation in experimental intracerebral hemorrhage

Background

Intracerebral hemorrhage (ICH) induces a series of inflammatory processes that contribute to neuronal damage and neurological deterioration. Liver X receptors (LXRs) are nuclear receptors that negatively regulate transcriptional processes involved in inflammatory responses, but their role in the pathology following ICH remains unclear. The present study investigated the neuroprotective effects and anti-inflammatory actions of TO901317, a synthetic LXR agonist, in a model of collagenase-induced ICH and in microglial cultures.

Methods

Mice subjected to collagenase-induced ICH injury were injected with either TO901317 (30 mg/kg) or vehicle 10 min after ICH and subsequently daily for 2 days. Behavioral studies, histology analysis, and assessments of hematoma volumes, brain water content, and blood-brain barrier (BBB) permeability were performed. The protein expression of LXR-α, LXR-β, ATP binding cassette transporter-1 (ABCA-1), and inflammatory molecules was analyzed. The anti-inflammatory mechanism of TO901317 was investigated in cultured microglia that were stimulated with either lipopolysaccharide (LPS) or thrombin.

Results

ICH induced an increase in LXR-α protein levels in the hemorrhagic hemisphere at 6 h whereas LXR-β expression remained unaffected. Both LXR-α and LXR-β were expressed in neurons and microglia in the peri-ICH region and but rarely in astrocytes. TO901317 significantly attenuated functional deficits and brain damage up to 28 days post-ICH. TO901317 also reduced neuronal death, BBB disruption, and brain edema at day 4 post-ICH. These changes were associated with marked reductions in microglial activation, neutrophil infiltration, and expression levels of inflammatory mediators at 4 and 7 days. However, TO901317 had no effect on matrix metalloproteinase-9 activity. In BV2 microglial cultures, TO901317 attenuated LPS- and thrombin-stimulated nitric oxide production and reduced LPS-induced p38, JNK, MAPK, and nuclear factor-kappa B (NF-κB) signaling. Moreover, delaying administration of TO901317 to 3 h post-ICH reduced brain tissue damage and neuronal death.

Conclusions

Our results suggest that enhancing LXR activation may provide a potential therapy for ICH by modulating the cytotoxic functions of microglia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0524-8) contains supplementary material, which is available to authorized users.

Liver X Receptor β Is Involved in Formalin-Induced Spontaneous Pain

Increasing evidence indicates that the liver X receptor(LXR) β modulates inflammatory pain. However, the molecular mechanisms through which LXRβ modulates pain are unclear. Here, we found that LXRβ-null mice responded more strongly to acute noxious stimuli than wild-type (WT) littermates (in the hot plate and Hargreaves tests) and had augmented tonic inflammatory pain (in the formalin test). This increased reactivity to inflammatory pain was accompanied by enhanced formalin-evoked Fos and pERK staining of second-order nociceptive neurons. Immunohistochemistry showed that the expression of CGRP, SP, and IB4 was increased in the lamina I-II of the lumbar dorsal horns in formalin-injected LXRβ knockout (KO) mice compared with the WT controls. In addition, LXRβ deletion in the mice enhanced the formalin-induced inflammation with more activated microglia and astrocytes in the spinal cord. Furthermore, the levels of pro-inflammatory cytokines (IL-1β ,TNF-α) as well as NFκB in the formalin-injected paw were elevated by the loss of LXRβ. Taken together, these data indicate that LXRβ is involved in acute as well as inflammatory pain, and thus, it may be considered as a new target for the development of analgesics.

Lxr regulates lipid metabolic and visual perception pathways during zebrafish development

The Liver X Receptors (LXRs) play important roles in multiple metabolic pathways, including fatty acid, cholesterol, carbohydrate and energy metabolism. To expand the knowledge of the functions of LXR signaling during embryonic development, we performed a whole-genome microarray analysis of Lxr target genes in zebrafish larvae treated with either one of the synthetic LXR ligands T0901317 or GW3965. Assessment of the biological processes enriched by differentially expressed genes revealed a prime role for Lxr in regulating lipid metabolic processes, similarly to the function of LXR in mammals. In addition, exposure to the Lxr ligands induced changes in expression of genes in the neural retina and lens of the zebrafish eye, including the photoreceptor guanylate cyclase activators and lens gamma crystallins, suggesting a potential novel role for Lxr in modulating the transcription of genes associated with visual function in zebrafish. The regulation of expression of metabolic genes was phenotypically reflected in an increased absorption of yolk in the zebrafish larvae, and changes in the expression of genes involved in visual perception were associated with morphological alterations in the retina and lens of the developing zebrafish eye. The regulation of expression of both lipid metabolic and eye specific genes was sustained in 1 month old fish. The transcriptional networks demonstrated several conserved effects of LXR activation between zebrafish and mammals, and also identified potential novel functions of Lxr, supporting zebrafish as a promising model for investigating the role of Lxr during development.

Liver X receptors alpha and beta promote myelination and remyelination in the cerebellum

The identification of new pathways governing myelination provides innovative avenues for remyelination. Liver X receptors (LXRs) α and β are nuclear receptors activated by oxysterols that originated from the oxidation of cholesterol. They are crucial for cholesterol homeostasis, a major lipid constituent of myelin sheaths that are formed by oligodendrocytes. However, the role of LXRs in myelin generation and maintenance is poorly understood. Here, we show that LXRs are involved in myelination and remyelination processes. LXRs and their ligands are present in oligodendrocytes. We found that mice invalidated for LXRs exhibit altered motor coordination and spatial learning, thinner myelin sheaths, and reduced myelin gene expression. Conversely, activation of LXRs by either 25-hydroxycholesterol or synthetic TO901317 stimulates myelin gene expression at the promoter, mRNA, and protein levels, directly implicating LXRα/β in the transcriptional control of myelin gene expression. Interestingly, activation of LXRs also promotes oligodendroglial cell maturation and remyelination after lysolecithin-induced demyelination of organotypic cerebellar slice cultures. Together, our findings represent a conceptual advance in the transcriptional control of myelin gene expression and strongly support a new role of LXRs as positive modulators in central (re)myelination processes.

Targeting cholesterol homeostasis to fight hearing loss: a new perspective

Sensorineural hearing loss (SNHL) is a major pathology of the inner ear that affects nearly 600 million people worldwide. Despite intensive researches, this major health problem remains without satisfactory solutions. The pathophysiological mechanisms involved in SNHL include oxidative stress, excitotoxicity, inflammation, and ischemia, resulting in synaptic loss, axonal degeneration, and apoptosis of spiral ganglion neurons. The mechanisms associated with SNHL are shared with other neurodegenerative disorders. Cholesterol homeostasis is central to numerous pathologies including neurodegenerative diseases and cholesterol regulates major processes involved in neurons survival and function. The role of cholesterol homeostasis in the physiopathology of inner ear is largely unexplored. In this review, we discuss the findings concerning cholesterol homeostasis in neurodegenerative diseases and whether it should be translated into potential therapeutic strategies for the treatment of SNHL.

Activation of liver-X-receptor α but not liver-X-receptor β protects against myocardial ischemia/reperfusion injury

BACKGROUND

Liver-X-receptors, LXRα (NR1H3) and LXRβ (NR1H2), encode 2 different but highly homologous isoforms of transcription factors belonging to the nuclear receptor superfamily. Whether LXRα and LXRβ subtypes have discrete roles in the regulation of cardiac physiology/pathology is unknown. We determine the role of each LXR subtype in myocardial ischemia/reperfusion (MI/R) injury.

METHODS AND RESULTS

Mice (wild type; those genetically depleted of LXRα, LXRβ, or both; and those overexpressing LXRα or LXRβ by in vivo intramyocardial adenoviral vector) were subjected to MI/R injury. Both LXRα and LXRβ were detected in wild-type mouse heart. LXRα, but not LXRβ, was significantly upregulated after MI/R. Dual activation of LXRα and LXRβ by natural and synthetic agonists reduced myocardial infarction and improved contractile function after MI/R. Mechanistically, LXR activation inhibited MI/R-induced oxidative stress and nitrative stress, attenuated endoplasmic reticulum stress and mitochondrial dysfunction, and reduced cardiomyocyte apoptosis in ischemic/reperfused myocardium. The aforementioned cardioprotective effects of LXR agonists were impaired in the setting of cardiac-specific gene silencing of LXRα, but not LXRβ subtype. Moreover, LXRα/β double-knockout and LXRα-knockout mice, but not LXRβ-knockout mice, increased MI/R injury, exacerbated MI/R-induced oxidative/nitrative stress, and aggravated endoplasmic reticulum stress and mitochondrial dysfunction. Furthermore, cardiac LXRα, not LXRβ, overexpression via adenoviral transfection suppressed MI/R injury.

CONCLUSIONS

Our study provides the first direct evidence that the LXRα, but not LXRβ, subtype is a novel endogenous cardiac protective receptor against MI/R injury. Drug development strategies specifically targeting LXRα may be beneficial in treating ischemic heart disease.

An evo-devo approach to thyroid hormones in cerebral and cerebellar cortical development: etiological implications for autism

The morphological alterations of cortical lamination observed in mouse models of developmental hypothyroidism prompted the recognition that these experimental changes resembled the brain lesions of children with autism; this led to recent studies showing that maternal thyroid hormone deficiency increases fourfold the risk of autism spectrum disorders (ASD), offering for the first time the possibility of prevention of some forms of ASD. For ethical reasons, the role of thyroid hormones on brain development is currently studied using animal models, usually mice and rats. Although mammals have in common many basic developmental principles regulating brain development, as well as fundamental basic mechanisms that are controlled by similar metabolic pathway activated genes, there are also important differences. For instance, the rodent cerebral cortex is basically a primary cortex, whereas the primary sensory areas in humans account for a very small surface in the cerebral cortex when compared to the associative and frontal areas that are more extensive. Associative and frontal areas in humans are involved in many neurological disorders, including ASD, attention deficit-hyperactive disorder, and dyslexia, among others. Therefore, an evo-devo approach to neocortical evolution among species is fundamental to understand not only the role of thyroid hormones and environmental thyroid disruptors on evolution, development, and organization of the cerebral cortex in mammals but also their role in neurological diseases associated to thyroid dysfunction.

Lipids in the nervous system: from biochemistry and molecular biology to patho-physiology

Lipids in the nervous system accomplish a great number of key functions, from synaptogenesis to impulse conduction, and more. Most of the lipids of the nervous system are localized in myelin sheaths. It has long been known that myelin structure and brain homeostasis rely on specific lipid-protein interactions and on specific cell-to-cell signaling. In more recent years, the growing advances in large-scale technologies and genetically modified animal models have provided valuable insights into the role of lipids in the nervous system. Key findings recently emerged in these areas are here summarized. In addition, we briefly discuss how this new knowledge can open novel approaches for the treatment of diseases associated with alteration of lipid metabolism/homeostasis in the nervous system. This article is part of a Special Issue entitled Linking transcription to physiology in lipidomics.

Liver X receptor β is essential for the differentiation of radial glial cells to oligodendrocytes in the dorsal cortex

Several psychiatric disorders are associated with aberrant white matter development, suggesting oligodendrocyte and myelin dysfunction in these diseases. There are indications that radial glial cells (RGCs) are involved in initiating myelination, and may contribute to the production of oligodendrocyte progenitor cells (OPCs) in the dorsal cortex. Liver X receptors (LXRs) are involved in maintaining normal myelin in the central nervous system (CNS), however, their function in oligodendrogenesis and myelination is not well understood. Here, we demonstrate that loss of LXRβ function leads to abnormality in locomotor activity and exploratory behavior, signs of anxiety and hypomyelination in the corpus callosum and optic nerve, providing in vivo evidence that LXRβ deletion delays both oligodendrocyte differentiation and maturation. Remarkably, along the germinal ventricular zone-subventricular zone and corpus callosum there is reduced OPC production from RGCs in LXRβ(-/-) mice. Conversely, in cultured RGC an LXR agonist led to increased differentiation into OPCs. Collectively, these results suggest that LXRβ, by driving RGCs to become OPCs in the dorsal cortex, is critical for white matter development and CNS myelination, and point to the involvement of LXRβ in psychiatric disorders.

Frontiers in therapeutic development of allopregnanolone for Alzheimer's disease and other neurological disorders

Allopregnanolone (Allo), a neurosteroid, has emerged as a promising promoter of endogenous regeneration in brain. In a mouse model of Alzheimer’s disease, Allo induced neurogenesis, oligodendrogenesis, white matter generation and cholesterol homeostasis while simultaneously reducing β-amyloid and neuroinflammatory burden. Allo activates signaling pathways and gene expression required for regeneration of neural stem cells and their differentiation into neurons. In parallel, Allo activates systems to sustain cholesterol homeostasis and reduce β-amyloid generation. To advance Allo into studies for chronic human neurological conditions, we examined translational and clinical parameters: dose, regimen, route, formulation, outcome measures, and safety regulations. A treatment regimen of once per week at sub-sedative doses of Allo was optimal for regeneration and reduction in Alzheimer’s pathology. This regimen had a high safety profile following chronic exposure in aged normal and Alzheimer’s mice. Formulation of Allo for multiple routes of administration has been developed for both preclinical and clinical testing. Preclinical evidence for therapeutic efficacy of Allo spans multiple neurological diseases including Alzheimer’s, Parkinson’s, multiple sclerosis, Niemann-Pick, diabetic neuropathy, status epilepticus, and traumatic brain injury. To successfully translate Allo as a therapeutic for multiple neurological disorders, it will be necessary to tailor dose and regimen to the targeted therapeutic mechanisms and disease etiology. Treatment paradigms conducted in accelerated disease models in young animals have a low probability of successful translation to chronic diseases in adult and aged humans. Gender, genetic risks, stage and burden of disease are critical determinants of efficacy. This review focuses on recent advances in development of Allo for Alzheimer’s disease (AD) that have the potential to accelerate therapeutic translation for multiple unmet neurological needs.

Sex differences in LXR expression in normal offspring and in rats born to diabetic dams

Gestational diabetes (GD) alters normal fetal development and is related to a diabetogenic effect in the progeny. Liver X receptors (LXRs) are considered to be potential drug targets for the regulation, treatment, or prevention of diabetes. The aim of this study was to evaluate early and late changes of LXR in the hippocampus and hypothalamus of the male and female offspring of control (CO) and diabetic (DO) mothers. We used an experimental model of streptozotocin-induced GD to assess the protein expression of LXRα (NR1H3) and LXRβ (NR1H2) by western blotting. The tissues were obtained from CO and DO animals at postnatal day 1 (1D), day 10 (10D), and day 35 (35D) and 9 months (9M). In CO, the LXR expression showed significant differences among the groups, which were tissue- and receptor-specific (P<0.05). Sex differences in CO were found only in the hypothalamus for LXRβ expression at 35D and 9M (P<0.05). When CO and DO were compared, differences between them were observed in the majority of the studied groups at 1D (male hippocampus, LXRα 31% and LXRβ 161%; female hippocampus, LXRβ 165%; male hypothalamus, LXRβ 182%; and female hypothalamus, LXRα 85%; P<0.05). However, these differences disappeared later with the exception of LXRβ expression in the male hypothalamus (P<0.05). The area under the curve during the glucose tolerance test correlated negatively with LXRβ in CO but not in DO animals. Moreover, in a male DO subpopulation this correlation was positive as it occurs in intolerant animals. These results indicate that GD affects hypothalamic LXR expression differently in male and female offspring.