The Role Of Liver X Receptor β In The Regulation Of Hippocampal Dentate Gyrus Neurogenesis

The hippocampus is an important center involved in learning and memory. The dentategyrus (DG) is one of important brain regions in mammals including humans undergoingcontinued neurogenesis after birth and adult. DG has a typical laminated structurecomposed of molecular layer, granule cell layer and polymorphic layer. Different fromproduction of hippocampal pyramidal neurons, granule cells in the DG generated from thefirst10days of the embryonic period extending to20days after birth in a unique‘outside–in’ pattern. Formation of hippocampal DG is a multiple and complex process,including cell proliferation, migration and differentiation. In vivo studies have shown thatradial glial cells (RGCs) play important roles in the morphogenesis of the DG of thehippocampus. According to the characteristics of RGCs, RGCs can be classified intoprimary and secondary RGCs. The structure of primary RGCs is similar to the RGCs in thecerebral cortex and mainly located in the prenatal of development hippocampus. The radialprocesses are oriented parallel to the pial surface and have focal contacts to the pialbasement membrane of DG，which provides scaffold for newborn neurons and neuralprecursor cells migration from the epithelium to the DG primordia.Secondary RGCs aregenerated from early non-radial precursor cells, committed to glial production in thepostnatal DG. Experimental studies have confirmed that RGCs maintaining depends onmany factors such as nuclear transcription factors, secreting proteins, microRNAs, andinflammatory cues.Liver X Receptor (LXRs) belong to nuclear receptor superfamily. LXRs have twosubtypes LXRα (NRIH3) and LXRβ (NRIH2), which are located in11p11.2and19q13.3, inthe DNA-binding domain and the ligand has a77%amino acid sequence homology. LXRαexpression was usally confined to the organs related to lipid metabolism, such as liver,intestine, and brown adipose tissue. LXRα is involved in the regulation of sterolmetabolism, fat synthesis. LXRβ is more widely expressed in the body including thenervous system and endocrine system. Current studies have indicated that LXRβ has multiple functions in the CNS development. Our previous studies have found that LXRβexpression in the cerebral cortex is developmentally regulated, referred to its role in thelamination of cerebral cortex. We further demonstrated that LXRβ is essential for the radialmigration of later born neurons via long processes of RGCs. Furthermore, the underlyingmechanisms involved in role of LXRβ maintaining RGCs are related to delaytransformation of RGCs into astrocytes during cerebral cortex development. Similarly,LXRs agonist treatment promoted the migration of cerebellar granule neurons in thedeveloping cerebellum and and suppression RGCs transformation into astrocytes viainhibition of TGFβ and Smad4signaling pathways. This might suggest that LXRβ have animportant role in the development of hippocampal DG through modulation of RGCsdevelopment.To clarify the real ways and mechanisms of LXRβ in the regulation DGdevelopment, probably provide new therapeutic strategies for treatment ofneurodegenerative diseases with inormal adult hippocampal neurogenesis, such asAlzheimer ’s disease, depression.First of all, the spatial and temporal expression pattern of LXRβ in the hippocampusdevelopment was detected to explore the possible correlation between LXRβ and DGmorphogenesis. By comparing the differences in proliferation, differentiation, andmigration of dental neuroprogenitor cells between WT and LXRβ knockout mice from P2toP14(the critical period for DG development), roles of endogenous LXRβ in the DGformation and RGCs development was further explored. Both Noggin and Notch1arepivocal signaling pathways involved in the RGCs development, were also analyzed in thehippocampus in both WT and LXRβ knockout mice at P7. To correlate the DG developmentwith hippocampal related functions, behaviour tests such as nest building, morris watermaze and new object recognition were also performed in male mice at3months of ages.Finally, LXRs agonist TO901317(TO) was administrated into neonatal mice to checkwhether endogenous LXRβ activation protect hippocampal DG from damage induced byalcohol treatment.The main results are as follows:1.Effects of LXRβ on hippocampal DG Development of mice.(1) LXRβ appeared in the DG from embryonic stages (E16.5, E18.5), mainly locatedin the anlage of the DG. Specially, LXRβ was expressed in the subgranular zone (SGZ) at P14and P2M, suggesting that LXRβ be closely related to the morphogenesis of the DG,and possibly contribute to production of granule cells in the postnatal DG.(2) HE staining on hippocampus sections from WT and LXRβ knockout mouse fromP2to P14has shown LXRβ deletion in mice at P2, P7and P10caused a significantreduction in the DG area and the number of granule cells; however the decline almostdisappeared by P14. Sox2as a marker for neural precursor cells (NPCs) was also decreasedin the DG by loss of LXRβ in mice at P2and P7. Noticeably, there were a significiantreduction of Sox2positive cells in the SGZ of LXRβ knockout mice at P10and P14,although no alterations in the total number of DG. BrdU were used to analyze proliferationof NPCs. Consistent with finding in Sox2staining, loss of LXRβ caused a significantreduction in the BrdU positive cells in the DG at P2and P7.(3) Using RGCs specific markers such as BLBP, GFAP and Sox2, we found thatLXRβ deletion lead to reduced primary RGCs at P2and secondary RGCs at P14. Meawhile,more secondary RGCs were observed in the granle cellular layer in LXRβ knockout mice atP7compared to WT littlematters.(4) It has been shown that long processes of RGCs partially contribute to NPCmigration and granule neuron maturation. It has been found that loss of LXRβ delayedNPCs migration from hilus to GCL, when BrdU labeling NPCs at P2were analyzed at P7.Calbindin (CB) and Calretinin (CR)-positive cells were also significantly reduced indentate gyrus of LXRβ knockout mice at P7and P10compared to WT controls. Golgistaining has further confirmed LXRβ deletion caused a siginificant reduction in thedendritic spines of hippocampal granule neuron at P14.2.LXRβ is involed in the regulation of adult hippocampus neurogenesisHippocampal DG is one of important brain regions with ongoing neurogenesisthroughout postnatal and adult life. We found LXRβ deletion caused loss of DCX markednewborn granule cells and Neun and CB marked mature neurons. This has been furtherconfirmed with behavior tests referred to hippocampus–relatedfunction.3.Activation of endogenous LXR could protect NPC in the DG of neonatal mice fromdamage induced by alcohol exposureHippocampal DG of neonatal mice is particularly sensitive to alcohol exposure. NPCsproliferation was significantly reduced in mice exposed to alcohol, which could be rescued by TO pretreatment. Moreover, loss of long processes of RGCs induced by alcoholexposure were also reversed effectively by TO pretreatment, which imply typical protectionof TO on RGCs.Conclusion:LXRβ has mutipal functions in the regulation of DG morphogenesis and is involved inkey events of NPCs such as proliferation, migration, differentiation and maturation.Meanwhile, LXRβ has important roles in the RGCs development via up-regulation ofNotch1and Noggin signaling pathways, and thus contribute to migration of NPCs andmaturation of granule cells. As suggested roles in the DG development, LXRβ alsopromotes adult hippocampus neurogenesis, which thereby provide new strategy to treatthose of neurodegenerative diseases which are characterized by dysfunction of thehippocampus.