Liver X receptor β delays transformation of radial glial cells into astrocytes during mouse cerebral cortical development

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.

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.

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.

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 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.

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.