A role for the orphan nuclear receptor TLX in the interaction between neural precursor cells and microglia

Exercise mitigates a gut microbiota-mediated reduction in adult hippocampal neurogenesis and associated behaviours in rats

Lifestyle factors, especially exercise, impact the manifestation and progression of psychiatric and neurodegenerative disorders such as depression and Alzheimer's disease, mediated by changes in hippocampal neuroplasticity. The beneficial effects of exercise may be due to its promotion of adult hippocampal neurogenesis (AHN). Gut microbiota has also been showed to be altered in a variety of brain disorders, and disturbances of the microbiota have resulted in alterations in brain and behaviour. However, whether exercise can counteract the negative effects of altered gut microbiota on brain function remains under explored. To this end, chronic disruption of the gut microbiota was achieved using an antibiotic cocktail in rats that were sedentary or allowed voluntary access to running wheels. Sedentary rats with disrupted microbiota displayed impaired performance in hippocampal neurogenesis-dependent tasks: the modified spontaneous location recognition task and the novelty suppressed feeding test. Performance in the elevated plus maze was also impaired due to antibiotics treatment. These behaviours, and an antibiotics-induced reduction in AHN were attenuated by voluntary exercise. The effects were independent of changes in the hippocampal metabolome but were paralleled by caecal metabolomic changes. Taken together these data highlight the importance of the gut microbiota in AHN-dependent behaviours and demonstrate the power of lifestyle factors such as voluntary exercise to attenuate these changes.

Dysfunctional synaptic pruning by microglia correlates with cognitive impairment in sleep-deprived mice: Involvement of CX3CR1 signaling

Microglia are involved in sleep/wake cycles and the response to sleep loss. Synaptic pruning by microglia is necessary for central nervous system circuit refinement and contributes to cognitive function. Here, we investigated whether and how microglia-mediated synaptic pruning may be involved in cognitive deficits induced by sleep deprivation in mice. Mice were deprived of sleep by leaving them in a spontaneously rotating rod for 72 h, after which their cognitive function was assessed using an object location test, Y maze, and novel object recognition test. Sleep deprivation lowered the discrimination index for familiar locations in the object location test and Y maze. Microglial morphology was assessed using immunostaining Iba1, while microglia-mediated synaptic pruning was examined based on immunostaining PSD95, CD68, and Iba1. Sleep deprivation also activated microglial cells in the hippocampus, as reflected in bigger soma as well as fewer and shorter branches than normal sleep. Sleep deprivation downregulated phagocytic markers and internalization of postsynaptic protein 95 (PSD95), suggesting impaired synaptic pruning. CX3C motif chemokine receptor 1 (CX3CR1) signaling was detected in in vitro experiments. Sleep deprivation also downregulated CX3CR1. Activation of CX3CR1 signaling increased phagocytosis activity of BV2 microglia in vitro. Sleep deprivation dysregulates microglial CX3CR1 signaling and inhibits synaptic pruning, contributing to associated cognitive deficits. These findings identify CX3CR1-dependent synaptic pruning as a potential therapeutic target in which sleep deprivation causes recognition impairments.

The Dialogue Between Neuroinflammation and Adult Neurogenesis: Mechanisms Involved and Alterations in Neurological Diseases

Adult neurogenesis occurs mainly in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. Evidence supports the critical role of adult neurogenesis in various conditions, including cognitive dysfunction, Alzheimer's disease (AD), and Parkinson's disease (PD). Several factors can alter adult neurogenesis, including genetic, epigenetic, age, physical activity, diet, sleep status, sex hormones, and central nervous system (CNS) disorders, exerting either pro-neurogenic or anti-neurogenic effects. Compelling evidence suggests that any insult or injury to the CNS, such as traumatic brain injury (TBI), infectious diseases, or neurodegenerative disorders, can provoke an inflammatory response in the CNS. This inflammation could either promote or inhibit neurogenesis, depending on various factors, such as chronicity and severity of the inflammation and underlying neurological disorders. Notably, neuroinflammation, driven by different immune components such as activated glia, cytokines, chemokines, and reactive oxygen species, can regulate every step of adult neurogenesis, including cell proliferation, differentiation, migration, survival of newborn neurons, maturation, synaptogenesis, and neuritogenesis. Therefore, this review aims to present recent findings regarding the effects of various components of the immune system on adult neurogenesis and to provide a better understanding of the role of neuroinflammation and neurogenesis in the context of neurological disorders, including AD, PD, ischemic stroke (IS), seizure/epilepsy, TBI, sleep deprivation, cognitive impairment, and anxiety- and depressive-like behaviors. For each disorder, some of the most recent therapeutic candidates, such as curcumin, ginseng, astragaloside, boswellic acids, andrographolide, caffeine, royal jelly, estrogen, metformin, and minocycline, have been discussed based on the available preclinical and clinical evidence.

TLX, an Orphan Nuclear Receptor With Emerging Roles in Physiology and Disease

TLX (NR2E1), an orphan member of the nuclear receptor superfamily, is a transcription factor that has been described to be generally repressive in nature. It has been implicated in several aspects of physiology and disease. TLX is best known for its ability to regulate the proliferation of neural stem cells and retinal progenitor cells. Dysregulation, overexpression, or loss of TLX expression has been characterized in numerous studies focused on a diverse range of pathological conditions, including abnormal brain development, psychiatric disorders, retinopathies, metabolic disease, and malignant neoplasm. Despite the lack of an identified endogenous ligand, several studies have described putative synthetic and natural TLX ligands, suggesting that this receptor may serve as a therapeutic target. Therefore, this article aims to briefly review what is known about TLX structure and function in normal physiology, and provide an overview of TLX in regard to pathological conditions. Particular emphasis is placed on TLX and cancer, and the potential utility of this receptor as a therapeutic target.

Glial PAMPering and DAMPening of Adult Hippocampal Neurogenesis

Adult neurogenesis represents a mature brain's capacity to integrate newly generated neurons into functional circuits. Impairment of neurogenesis contributes to the pathophysiology of various mood and cognitive disorders such as depression and Alzheimer's Disease. The hippocampal neurogenic niche hosts neural progenitors, glia, and vasculature, which all respond to intrinsic and environmental cues, helping determine their current state and ultimate fate. In this article we focus on the major immune communication pathways and mechanisms through which glial cells sense, interact with, and modulate the neurogenic niche. We pay particular attention to those related to the sensing of and response to innate immune danger signals. Receptors for danger signals were first discovered as a critical component of the innate immune system response to pathogens but are now also recognized to play a crucial role in modulating non-pathogenic sterile inflammation. In the neurogenic niche, viable, stressed, apoptotic, and dying cells can activate danger responses in neuroimmune cells, resulting in neuroprotection or neurotoxicity. Through these mechanisms glial cells can influence hippocampal stem cell fate, survival, neuronal maturation, and integration. Depending on the context, such responses may be appropriate and on-target, as in the case of learning-associated synaptic pruning, or excessive and off-target, as in neurodegenerative disorders.

Retinal Degeneration: Short-Term Options and Long-Term Vision for Future Therapy

The leading cause blindness is the loss of retinal ganglion cells which connect the retina to the brain. Degenerative retinal diseases include retinal dystrophy, macular degeneration and diabetic retinopathy, which are currently incurable as the mammalian retina has no intrinsic regenerative capacity. By utilizing insight gained from retinal regeneration in simpler species we define an approach that may unlock regenerative programs in the mammalian retina that potentially facilitate the clinical restoration of retinal function.

Neuronal Signaling: A reflection on the Biochemical Society's newest journal and an exciting outlook on its next steps

The inaugural Editor-in-Chief of Neuronal Signaling, Aideen M. Sullivan, reflects on the journal's journey so far and welcomes the new Editor-in-Chief, Clare Stanford, as she shares some of the exciting initiatives and plans for its future.

The orphan nuclear receptor TLX: an emerging master regulator of cross-talk between microglia and neural precursor cells

Neuroinflammation and neurogenesis have both been the subject of intensive investigation over the past 20 years. The sheer complexity of their regulation and their ubiquity in various states of health and disease have sometimes obscured the progress that has been made in unraveling their mechanisms and regulation. A recent study by Kozareva et al. (Neuronal Signaling (2019) 3), provides evidence that the orphan nuclear receptor TLX is central to communication between microglia and neural precursor cells and could help us understand how inflammation, mediated by microglia, influences the development of new neurons in the adult hippocampus. Here, we put recent studies on TLX into the context of what is known about adult neurogenesis and microglial activation in the brain, along with the many hints that these processes must be inter-related.