Studies On The Role Of Liver X Receptorβ In The Transformation Of Radial Glial Cells Into Astrocytes During Cerebral Cortex And Hippocampus Development

The liver X receptors, LXRα and LXRβ, are members of the ligand-dependent nuclearreceptor superfamily. LXRα is highly expressed in the organs involved in lipid homeostasissuch as liver, whereas LXRβ is predominantly expressed in the central nervous system. Themammalian cerebral corticogenesis involves layering of neurons in an “inside-out” manner,with the earlier generated neurons located in the deeper layers and later generated neuronsmigrating beyond previously established layers to settle at progressively more superficiallevels. We have previously demonstrated that the expression of LXRβ in the brain isdevelopmentally regulated and LXRβ is essential for formation of superficial cortical layersand migration of later born neurons. We further demonstrated that LXR agonist promotes themigration of granule neurons from external granular layer into internal granular layer duringthe development of the cerebellar. These indicate that LXRβ plays an important role in theradial migration of newborn neurons during the cerebral cortex development.The radial glia cells(RGCs) arise from neuroepithelial cells(NCs), early incorticogenesis, the RGCs serve as stem cell pool to produce new born neurons, later theyserve as scaffolds for the migration of the newborn neurons inward the cortical laminar duringcerebral cortex development. It has been determined that the transformation of the RGCs intoastrocytes began at late embryogenesis and terminated at two weeks postnatal during thecerebral cortex development. If the RGCs were acceleratly transformed into astrocytes, themigration of newborn neuron and the development of cortex would be halted. We havepreviously observed that loss of LXRβ resulted in the disruption of radial glia fibers duringcerebral cortex development. We also found that T0901317, a potent LXR receptor agonist,inhibited premature differentiation of Bergmann glia during cerebellum development. Theseindicate that LXRβ affects the transformation of radial glia cells into astrocytes.It has been proved that TGFβ1/Smad4signaling pathway was involved in thetransformation of RGCs into astrocytes. Our recent study has confirmed that TGFβ1/Smad4 signaling expression profile in the cerebellum is downregulated by the LXR agonistT0901317. However, it is not clear whether LXRβ knockout can accelerate the transformationof RGCs into astrocytes, the underlying mechanism has not been clarified yet.Besides cerebral cortex and cerebellum, hippocampus is a classical lamellar structureand composed of molecular layer, granule cell layer and polymorphic layer. During thehippocampus development, RGCs work as guideline cells and guide new born neuronsmigration from the subgranular layers into granular layers. We have previously found thatLXRβ affected the radial migration of later born neurons during cerebral cortex development.However, it is not clear whether LXRβ affects the morphology of RGCs in the developinghippocampus.In this study, the expression patterns of LXRβ were detected in the cerebral cortex andthe hippocampus. The effects of LXRβ on RGCs in cerebral cortex and hippocampus wereinvestigated by measurement of BLBP and GFAP using Western Blot andimmunofluorescence staining. The percentage of transformation of RGCs into astrocyteswas compared between LXRβ knockout mice and age matched WT littermates. Themolecules involved the effect of LXRβ on the transformation of RGCs into astrocytes weredetected with RT-PCR. The results were as follows:1. LXRβ was highly expressed in the cerebral cortex and the dentate gyrus of thehippocampus.2. Compared to the age matched WT littermates, the number of Nestin positive cellswere decreased markedly in the LXRβ knockout mice.3. RGCs was gradually transformed into mature astrocytes with the development ofcerebral cortex, compared to the age matched WT littermates, LXRβ knockout inducedsignificant decrease in the number of RGCs and increase in the number of astrocytes in thecerebral cortex.4. Compared to the age matched WT littermates, the level of BLBP was significantlydecreased and the level of GFAP was markedly increased in the cerebral cortex of LXRβknockout mice at P2and P7.5. Compared to the age matched WT littermates, the number of BLBP and GFAPdouble staining positive cells were increased in the LXRβ knockout mice at P7and P10.6. The level of TGFβ1and Smad4mRNA in the cerebral cortex were significantly increased in the LXRβ knockout mice at P2and P7compared to the age matched WTlittermates. There was no significant difference in the TGFβ1and Smad4mRNA level inthe cerebral cortex at P10and P14between LXRβ knockout mice and age matched WTlittermates.7. The PCNA positive cells and the density of radial glia fibers were decreasedmarkedly in the hippocampus of the LXRβ knockout mice compared to the age matchedWT littermates.Our results suggest that LXRβ inhibits the transformation of RGCs into astrocytesthrough TGFβ1/Smad4signaling pathway during the development of cerebral cortex andhippocampus.