Investigating the regulatory role of the nuclear receptor TLX in IL-1β-induced changes in hippocampal neurogenesis

Hippocampal neurogenesis is the process by which new neurons are born within the dentate gyrus (DG). This process begins during embryonic development and persists throughout life. Neurogenesis encompasses proliferation, differentiation and integration of neural progenitor cells (NPCs) into the surrounding neural network. Each stage is regulated by a host of intrinsic and extrinsic factor such as intracellular signalling molecules, exercise, environmental enrichment, diet and learning. TLX is an orphan nuclear receptor and transcription factor, which promotes the proliferation of NPCs, maintains the neurogenic pool of cells within the DG, and has been shown to promote hippocampal neurogenesis-associated cognition. Conversely, the proinflammatory cytokine IL-1β is a major mediator of the anti-neurogenic effects of hippocampal neuroinflammation, and previous work from the group has shown that IL-1β can suppress the expression of TLX within proliferating NPCs. The aims of this thesis were to investigate the interactions between TLX and IL-1β both in vitro and in vivo, and to determine the behavioural outcome of enhancing TLX and IL-1β, as well as well as in response to dietary intervention in vivo. We demonstrate that IL-1β suppresses TLX expression and neurogenesis (neurosphere expansion) in vitro, and that these effects are mediated by the NF-κB pathway. Restoration of TLX expression is sufficient to attenuate the negative effects of IL-1β on neurogenesis. We have shown using an RNA sequencing approach that TLX expression maintains a reduced inflammatory transcriptional profile in the hippocampus at baseline, and regulates the transcriptional response to IL-1β in vivo. We demonstrate that lentiviral-mediated overexpression of TLX does not enhance hippocampal neurogenesis-associated cognitive processes in vivo but that it impairs object recognition memory in rats. This suggests that enhancing cell proliferation is not sufficient to promote certain hippocampal-associated cognitive processes, and may even have a detrimental effect on cognitive behaviour. Finally, we show that an adolescent cafeteria diet which induces negative effects on hippocampal-associated memory, does not induce lasting cognitive defects when rats are switched to standard chow diet in adulthood. Lentiviral-mediated overexpression of IL-1β does not impact upon cognitive behaviours in rats fed a cafeteria diet throughout adolescence. However, we show that this chronic low-grade hippocampal IL-1β-mediated inflammation promotes fear memory in adulthood. In summary, TLX and IL-1β can enhance and repress hippocampal neurogenesis respectively. Determining the role that TLX has on neurogenesis-associated cognition and how it can interact with IL-1β may position TLX as a novel therapeutic target for the treatment of neuroinflammatory-associated disorders where hippocampal neurogenesis is impaired

Enduring effects of an unhealthy diet during adolescence on systemic but not neurobehavioural measures in adult rats

Introduction: Adolescence is an important stage of maturation for various brain structures. It is during this time therefore that the brain may be more vulnerable to environmental factors such as diet that may influence mood and memory. Diets high in fat and sugar (termed a cafeteria diet) during adolescence have been shown to negatively impact upon cognitive performance, which may be reversed by switching to a standard diet during adulthood. Consumption of a cafeteria diet increases both peripheral and central levels of interleukin-1β (IL-1β), a pro-inflammatory cytokine which is also implicated in cognitive impairment during the ageing process. It is unknown whether adolescent exposure to a cafeteria diet potentiates the negative effects of IL-1β on cognitive function during adulthood. Methods: Male Sprague-Dawley rats consumed a cafeteria diet during adolescence after which time they received a lentivirus injection in the hippocampus to induce chronic low-grade overexpression of IL-1β. After viral integration, metabolic parameters, circulating and central pro-inflammatory cytokine levels, and cognitive behaviours were assessed. Results: Our data demonstrate that rats fed the cafeteria diet exhibit metabolic dysregulations in adulthood, which were concomitant with low-grade peripheral and central inflammation. Overexpression of hippocampal IL-1β in adulthood impaired spatial working memory. However, adolescent exposure to a cafeteria diet, combined with or without hippocampal IL-1β in adulthood did not induce any lasting cognitive deficits when the diet was replaced with a standard diet in adulthood. Discussion: These data demonstrate that cafeteria diet consumption during adolescence induces metabolic and inflammatory changes, but not behavioural changes in adulthood

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

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-1β has been shown to suppress 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-1β 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-1β into the hippocampus. Moreover, TLX-/- mice injected with IL-1β display a blunted transcriptional profile compared to WT mice injected with IL-1β. 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-1β. Together, these results demonstrate that TLX regulates hippocampal inflammatory transcriptome response to brain injury (in this case cannulation surgery) and cytokine stimulation