Microglia play beneficial roles in multiple experimental seizure models

Contribution of microglia to the epileptiform activity that results from neonatal hypoxia

Microglia are described as the immune cells of the brain, their immune properties have been extensively studied since first described, however, their neural functions have only been explored over the last decade. Microglia have an important role in maintaining homeostasis in the central nervous system by surveying their surroundings to detect pathogens or damage cells. While these are the classical functions described for microglia, more recently their neural functions have been defined; they are critical to the maturation of neurons during embryonic and postnatal development, phagocytic microglia remove excess synapses during development, a process called synaptic pruning, which is important to overall neural maturation. Furthermore, microglia can respond to neuronal activity and, together with astrocytes, can regulate neural activity, contributing to the equilibrium between excitation and inhibition through a feedback loop. Hypoxia at birth is a serious neurological condition that disrupts normal brain function resulting in seizures and epilepsy later in life. Evidence has shown that microglia may contribute to this hyperexcitability after neonatal hypoxia. This review will summarize the existing data on the role of microglia in the pathogenesis of neonatal hypoxia and the plausible mechanisms that contribute to the development of hyperexcitability after hypoxia in neonates. This article is part of the Special Issue on "Microglia".

The cytokine receptor Fn14 is a molecular brake on neuronal activity that mediates circadian function in vivo

To survive, organisms must adapt to a staggering diversity of environmental signals, ranging from sensory information to pathogenic infection, across the lifespan. At the same time, organisms intrinsically generate biological oscillations, such as circadian rhythms, without input from the environment. While the nervous system is well-suited to integrate extrinsic and intrinsic cues, how the brain balances these influences to shape biological function system-wide is not well understood at the molecular level. Here, we demonstrate that the cytokine receptor Fn14, previously identified as a mediator of sensory experience-dependent synaptic refinement during brain development, regulates neuronal activity and function in adult mice in a time-of-day-dependent manner. We show that a subset of excitatory pyramidal (PYR) neurons in the CA1 subregion of the hippocampus increase Fn14 expression when neuronal activity is heightened. Once expressed, Fn14 constrains the activity of these same PYR neurons, suggesting that Fn14 operates as a molecular brake on neuronal activity. Strikingly, differences in PYR neuron activity between mice lacking or expressing Fn14 were most robust at daily transitions between light and dark, and genetic ablation of Fn14 caused aberrations in circadian rhythms, sleep-wake states, and sensory-cued and spatial memory. At the cellular level, microglia contacted fewer, but larger, excitatory synapses in CA1 in the absence of Fn14, suggesting that these brain-resident immune cells may dampen neuronal activity by modifying synaptic inputs onto PYR neurons. Finally, mice lacking Fn14 exhibited heightened susceptibility to chemically induced seizures, implicating Fn14 in disorders characterized by hyperexcitation, such as epilepsy. Altogether, these findings reveal that cytokine receptors that mediates inflammation in the periphery, such as Fn14, can also play major roles in healthy neurological function in the adult brain downstream of both extrinsic and intrinsic cues.

Repopulated spinal cord microglia exhibit a unique transcriptome and contribute to pain resolution

Microglia are implicated as primarily detrimental in pain models; however, they exist across a continuum of states that contribute to homeostasis or pathology depending on timing and context. To clarify the specific contribution of microglia to pain progression, we take advantage of a temporally controlled transgenic approach to transiently deplete microglia. Unexpectedly, we observe complete resolution of pain coinciding with microglial repopulation rather than depletion. We find that repopulated mouse spinal cord microglia are morphologically distinct from control microglia and exhibit a unique transcriptome. Repopulated microglia from males and females express overlapping networks of genes related to phagocytosis and response to stress. We intersect the identified mouse genes with a single-nuclei microglial dataset from human spinal cord to identify human-relevant genes that may ultimately promote pain resolution after injury. This work presents a comprehensive approach to gene discovery in pain and provides datasets for the development of future microglial-targeted therapeutics.

Repurposing of pexidartinib for microglia depletion and renewal

Pexidartinib (PLX3397) is a small molecule receptor tyrosine kinase inhibitor of colony stimulating factor 1 receptor (CSF1R) with moderate selectivity over other members of the platelet derived growth factor receptor family. It is approved for treatment of tenosynovial giant cell tumors (TGCT). CSF1R is highly expressed by microglia, which are macrophages of the central nervous system (CNS) that defend the CNS against injury and pathogens and contribute to synapse development and plasticity. Challenged by pathogens, apoptotic cells, debris, or inflammatory molecules they adopt a responsive state to propagate the inflammation and eventually return to a homeostatic state. The phenotypic switch may fail, and disease-associated microglia contribute to the pathophysiology in neurodegenerative or neuropsychiatric diseases or long-lasting detrimental brain inflammation after brain, spinal cord or nerve injury or ischemia/hemorrhage. Microglia also contribute to the growth permissive tumor microenvironment of glioblastoma (GBM). In rodents, continuous treatment for 1-2 weeks via pexidartinib food pellets leads to a depletion of microglia and subsequent repopulation from the remaining fraction, which is aided by peripheral monocytes that search empty niches for engraftment. The putative therapeutic benefit of such microglia depletion or forced renewal has been assessed in almost any rodent model of CNS disease or injury or GBM with heterogeneous outcomes, but a tendency of partial beneficial effects. So far, microglia monitoring e.g. via positron emission imaging is not standard of care for patients receiving Pexidartinib (e.g. for TGCT), so that the depletion and repopulation efficiency in humans is still largely unknown. Considering the virtuous functions of microglia, continuous depletion is likely no therapeutic option but short-lasting transient partial depletion to stimulate microglia renewal or replace microglia in genetic disease in combination with e.g. stem cell transplantation or as part of a multimodal concept in treatment of glioblastoma appears feasible. The present review provides an overview of the preclinical evidence pro and contra microglia depletion as a therapeutic approach.

Chemogenetic approaches reveal dual functions of microglia in seizures

Microglia are key players in maintaining brain homeostasis and exhibit phenotypic alterations in response to epileptic stimuli. However, it is still relatively unknown if these alterations are pro- or anti-epileptic. To unravel this dilemma, we employed chemogenetic manipulation of microglia using the artificial Gi-Dreadd receptor within a kainic acid (KA) induced murine seizure model. Our results indicate that acute Gi-Dreadd activation with Clozapine-N-Oxide can reduce seizure severity. Additionally, we observed increased interaction between microglia and neuronal soma, which correlated with reduced neuronal hyperactivity. Interestingly, prolonged activation of microglial Gi-Dreadds by repeated doses of CNO over 3 days, arrested microglia in a less active, homeostatic-like state, which associated with increased neuronal loss after KA induced seizures. RNAseq analysis revealed that prolonged activation of Gi-Dreadd interferes with interferon β signaling and microglia proliferation. Thus, our findings highlight the importance of microglial Gi signaling not only during status epilepticus (SE) but also within later seizure induced pathology.

Research Progress on the Immune-Inflammatory Mechanisms of Posttraumatic Epilepsy

Posttraumatic epilepsy (PTE) is a severe complication arising from a traumatic brain injury caused by various violent actions on the brain. The underlying mechanisms for the pathogenesis of PTE are complex and have not been fully defined. Approximately, one-third of patients with PTE are resistant to antiepileptic therapy. Recent research evidence has shown that neuroinflammation is critical in the development of PTE. This article reviews the immune-inflammatory mechanisms regarding microglial activation, astrocyte proliferation, inflammatory signaling pathways, chronic neuroinflammation, and intestinal flora. These mechanisms offer novel insights into the pathophysiological mechanisms of PTE and have groundbreaking implications in the prevention and treatment of PTE. Immunoinflammatory cross-talk between glial cells and gut microbiota in posttraumatic epilepsy. This graphical abstract depicts the roles of microglia and astrocytes in posttraumatic epilepsy, highlighting the influence of the gut microbiota on their function. TBI traumatic brain injury, AQP4 aquaporin-4, Kir4.1 inward rectifying K channels.

Human iPSC-derived microglia sense and dampen hyperexcitability of cortical neurons carrying the epilepsy-associated SCN2A-L1342P mutation

Neuronal hyperexcitability is a hallmark of seizures. It has been recently shown in rodent models of seizures that microglia, the brain's resident immune cells, can respond to and modulate neuronal excitability. However, how human microglia interacts with human neurons to regulate hyperexcitability mediated by epilepsy-causing genetic mutation found in human patients remains unknown. The SCN2A genetic locus is responsible for encoding the voltage-gated sodium channel Nav1.2, recognized as one of the leading contributors to monogenic epilepsies. Previously, we demonstrated that the recurring Nav1.2-L1342P mutation identified in patients with epilepsy leads to hyperexcitability in a hiPSC-derived cortical neuron model from a male donor. While microglia play an important role in the brain, these cells originate from a different lineage (yolk sac) and thus are not naturally present in hiPSCs-derived neuronal culture. To study how microglia respond to diseased neurons and influence neuronal excitability, we established a co-culture model comprising hiPSC-derived neurons and microglia. We found that microglia display altered morphology with increased branch length and enhanced calcium signal when co-cultured with neurons carrying the Nav1.2-L1342P mutation. Moreover, the presence of microglia significantly lowers the action potential firing of neurons carrying the mutation. Interestingly, we further demonstrated that the current density of sodium channels in neurons carrying the epilepsy-associated mutation was reduced in the presence of microglia. Taken together, our work reveals a critical role of human iPSCs-derived microglia in sensing and dampening hyperexcitability mediated by an epilepsy-causing mutation present in human neurons, highlighting the importance of neuron-microglia interactions in human pathophysiology.

Distinguishing the effects of systemic CSF1R inhibition by PLX3397 on microglia and peripheral immune cells

Microglia, the primary immune cells of the central nervous system (CNS), are derived from the yolk sac and populate the brain during development. Once microglia migrate to the CNS, they are self-renewing and require CSF1R signaling for their maintenance. Pexidartinib (PLX3397, PLX), a small molecule inhibitor of the CSF1R, has been shown to effectively deplete microglia since microglial maintenance is CSF1R-dependent. There have, however, been several conflicting reports that have shown the potential off-target effects of PLX on peripheral immune cells particularly those of lymphoid origin. Given this controversy in the use of the PLX family of drugs, it has become important to ascertain to what extent PLX affects the peripheral immune profile in lymphoid (spleen, and bone marrow) and non-lymphoid (kidney, lungs, and heart) organs. PLX3397 chow treatment at 660 mg/kg for 7 days significantly reduced CD45+ macrophages, CX3CR1-GFP cells, CD11b+CD45intermediate cells, and P2RY12 expression in the brain. However, there were minimal effects on peripheral immune cells from both lymphoid and non-lymphoid organs except in the heart where there was a significant decrease in CD3+ cells, inflammatory and patrolling monocytes, and CD11b+Ly6G+ neutrophils. We then stimulated the immune system with 1 mg/kg of LPS which resulted in a significant reduction in the number of innate immune cells. In this context, PLX did not alter the cytokine profile in the serum and the brain of naïve mice but did so in the LPS-stimulated group resulting in a significant reduction in TNFα, IL-1α, IFN-γ and IL-1β. Furthermore, PLX did not alter locomotor activity in the open field test suggesting that microglia do not contribute to LPS-induced sickness behavior. Our results provide an assessment of immune cell populations with PLX3397 treatment on brain, lymphoid and non-lymphoid organs without and during LPS treatment that can serve as a resource for understanding consequences of such approaches.

EpiPro, a Novel, Synthetic, Activity-Regulated Promoter That Targets Hyperactive Neurons in Epilepsy for Gene Therapy Applications

Epileptogenesis is characterized by intrinsic changes in neuronal firing, resulting in hyperactive neurons and the subsequent generation of seizure activity. These alterations are accompanied by changes in gene transcription networks, first with the activation of early-immediate genes and later with the long-term activation of genes involved in memory. Our objective was to engineer a promoter containing binding sites for activity-dependent transcription factors upregulated in chronic epilepsy (EpiPro) and validate it in multiple rodent models of epilepsy. First, we assessed the activity dependence of EpiPro: initial electrophysiology studies found that EpiPro-driven GFP expression was associated with increased firing rates when compared with unlabeled neurons, and the assessment of EpiPro-driven GFP expression revealed that GFP expression was increased ~150× after status epilepticus. Following this, we compared EpiPro-driven GFP expression in two rodent models of epilepsy, rat lithium/pilocarpine and mouse electrical kindling. In rodents with chronic epilepsy, GFP expression was increased in most neurons, but particularly in dentate granule cells, providing in vivo evidence to support the "breakdown of the dentate gate" hypothesis of limbic epileptogenesis. Finally, we assessed the time course of EpiPro activation and found that it was rapidly induced after seizures, with inactivation following over weeks, confirming EpiPro's potential utility as a gene therapy driver for epilepsy.