TLX is an intrinsic regulator of the negative effects of IL-1ß on proliferating hippocampal neural progenitor cells.

Hippocampal neurogenesis is a lifelong process whereby new neurons are produced and integrate into the host circuitry within the hippocampus. It is regulated by a multitude of extrinsic and intrinsic regulators and is believed to contribute to certain hippocampal-dependent cognitive tasks. Hippocampal neurogenesis and associated cognition have been demonstrated to be impaired after increases in the levels of proinflammatory cytokine IL-1ß in the hippocampus, such as that which occurs in various neurodegenerative and psychiatric disorders. IL-1ß also suppresses the expression of TLX (orphan nuclear receptor tailless homolog), which is an orphan nuclear receptor that functions to promote neural progenitor cell (NPC) proliferation and suppress neuronal differentiation; therefore, manipulation of TLX represents a potential strategy with which to prevent the antiproliferative effects of IL-1ß. In this study, we assessed the mechanism that underlies IL-1ß-induced changes in TLX expression and determined the protective capacity of TLX to mitigate the effects of IL-1ß on embryonic rat hippocampal neurosphere expansion. We demonstrate that IL-1ß activated the NF-κB pathway in proliferating NPCs and that this activation was responsible for IL-1ß-induced changes in TLX expression. In addition, we report that enhancing TLX expression prevented the IL-1ß-induced suppression of neurosphere expansion. Thus, we highlight TLX as a potential protective regulator of the antiproliferative effects of IL-1ß on hippocampal neurogenesis.-Ó'Léime, C. S., Kozareva, D. A., Hoban, A. E., Long-Smith, C. M., Cryan, J. F., Nolan, Y. M. TLX is an intrinsic regulator of the negative effects of IL-1ß on proliferating hippocampal neural progenitor cells.

The orphan nuclear receptor TLX regulates hippocampal transcriptome changes induced by IL-1ß.

TLX is an orphan nuclear receptor highly expressed within neural progenitor cells (NPCs) in the hippocampus where is regulates proliferation. Inflammation has been shown to have negative effects on hippocampal function as well as on NPC proliferation. Specifically, the pro-inflammatory cytokine IL-1ß suppresses NPC proliferation as well as TLX expression in the hippocampus. However, it is unknown whether TLX itself is involved in regulating the inflammatory response in the hippocampus. To explore the role of TLX in inflammation, we assessed changes in the transcriptional landscape of the hippocampus of TLX knockout mice (TLX-/-) compared to wildtype (WT) littermate controls with and without intrahippocampal injection of IL-1ß using a whole transcriptome RNA sequencing approach. We demonstrated that there is an increase in the transcription of genes involved in the promotion of inflammation and regulation of cell chemotaxis (Tnf, Il1b, Cxcr1, Cxcr2, Tlr4) and a decrease in the expression of genes relating to synaptic signalling (Lypd1, Syt4, Cplx2) in cannulated TLX-/- mice compared to WT controls. We demonstrate that mice lacking in TLX share a similar increase in 176 genes involved in regulating inflammation (e.g. Cxcl1, Tnf, Il1b) as WT mice injected with IL-1ß into the hippocampus. Moreover, TLX-/- mice injected with IL-1ß displayed a blunted transcriptional profile compared to WT mice injected with IL-1ß. Thus, TLX-/- mice, which already have an exaggerated inflammatory profile after cannulation surgery, are primed to respond differently to an inflammatory stimulus such as IL-1ß. Together, these results demonstrate that TLX regulates hippocampal inflammatory transcriptome response to brain injury (in this case cannulation surgery) and cytokine stimulation.

Nuclear deterrents: Intrinsic regulators of IL-1ß-induced effects on hippocampal neurogenesis.

Hippocampal neurogenesis, the process by which new neurons are born and develop into the host circuitry, begins during embryonic development and persists throughout adulthood. Over the last decade considerable insights have been made into the role of hippocampal neurogenesis in cognitive function and the cellular mechanisms behind this process. Additionally, an increasing amount of evidence exists on the impact of environmental factors, such as stress and neuroinflammation on hippocampal neurogenesis and subsequent impairments in cognition. Elevated expression of the pro-inflammatory cytokine interleukin-1ß (IL-1ß) in the hippocampus is established as a significant contributor to the neuronal demise evident in many neurological and psychiatric disorders and is now known to negatively regulate hippocampal neurogenesis. In order to prevent the deleterious effects of IL-1ß on neurogenesis it is necessary to identify signalling pathways and regulators of neurogenesis within neural progenitor cells that can interact with IL-1ß. Nuclear receptors are ligand regulated transcription factors that are involved in modulating a large number of cellular processes including neurogenesis. In this review we focus on the signalling mechanisms of specific nuclear receptors involved in regulating neurogenesis (glucocorticoid receptors, peroxisome proliferator activated receptors, estrogen receptors, and nuclear receptor subfamily 2 group E member 1 (NR2E1 or TLX)). We propose that these nuclear receptors could be targeted to inhibit neuroinflammatory signalling pathways associated with IL-1ß. We discuss their potential to be therapeutic targets for neuroinflammatory disorders affecting hippocampal neurogenesis and associated cognitive function.

Absence of the neurogenesis-dependent nuclear receptor TLX induces inflammation in the hippocampus.

The orphan nuclear receptor TLX (Nr2e1) is a key regulator of hippocampal neurogenesis. Impaired adult hippocampal neurogenesis has been reported in neurodegenerative and psychiatric conditions including dementia and stress-related depression. Neuroinflammation is also implicated in the neuropathology of these disorders, and has been shown to negatively affect hippocampal neurogenesis. To investigate a role for TLX in hippocampal neuroinflammation, we assessed microglial activation in the hippocampus of mice with a spontaneous deletion of TLX. Results from our study suggest that a lack of TLX is implicated in deregulation of microglial phenotype and that consequently, the survival and function of newborn cells in the hippocampus is impaired. TLX may be an important target in understanding inflammatory-associated impairments in neurogenesis.