Microglial depletion exacerbates motor impairment and dopaminergic neuron loss in a 6-OHDA model of Parkinson's disease

Deletion of the Transient Receptor Potential Melastatin 2 Gene Mitigates the 6-Hydroxydopamine-Induced Parkinson's Disease-Like Pathology

Pharmacological inhibition of the transient receptor potential melastatin 2 (TRPM2), an oxidative stress-activated calcium channel, was previously reported to be protective in Parkinson's disease (PD). However, the inhibitors used were not TRPM2 specific, so the involvement of this channel in PD remains unclear. Here, for the first time, Trpm2 partial (+ / -) and complete (- / -) knockout mice underwent stereotaxic surgery for PD induction. Six-hydroxydopamine was injected in the right striatum. On days 3 and 6, motor behavior tests (cylinder, apomorphine, and pole test) were performed. On day 7, brains were collected for dopaminergic neuron immunostaining. Our results showed that Trpm2 + / - male and female mice had reduced motor impairment and dopaminergic neuron death after PD induction. In addition, Trpm2 - / - male and female mice showed absent or lesser motor deficit and the dopaminergic neuronal loss was no longer observed. These findings suggest that TRPM2 is involved in the PD-like pathology and that targeting TRPM2 may possibly represent a potential neuroprotective strategy for PD.

Deciphering motor dysfunction and microglial activation in mThy1-α-synuclein mice: a comprehensive study of behavioral, gene expression, and methylation changes

Introduction

Growing recognition of microglia's role in neurodegenerative disorders has accentuated the need to characterize microglia profiles and their influence on pathogenesis. To understand changes observed in the microglial profile during the progression of synucleinopathies, microglial gene expression and DNA methylation were examined in the mThy1-α-synuclein mouse model.

Methods

Disease progression was determined using behavioral tests evaluating locomotor deficits before DNA and RNA extraction at 7 and 10 months from isolated microglia for enzymatic methyl-sequencing and RNA-sequencing.

Results

Pathway analysis of these changes at 7 months indicates a pro-inflammatory profile and changes in terms related to synaptic maintenance. Expression and methylation at both 7 and 10 months included terms regarding mitochondrial and metabolic stress. While behavior symptoms progressed at 10 months, we see many previously activated pathways being inhibited in microglia at a later stage, with only 8 of 53 shared pathways predicted to be directionally concordant. Despite the difference in pathway directionality, 21 of the 22 genes that were differentially expressed and annotated to differentially methylated regions at both 7 and 10 months had conserved directionality changes.

Discussion

These results highlight a critical period in disease progression, during which the microglia respond to α-synuclein, suggesting a transition in the role of microglia from the early to late stages of the disease.

Microglia modulate sleep/wakefulness under baseline conditions and under acute social defeat stress in adult mice

Although sleep is tightly regulated by multiple neuronal circuits in the brain, nonneuronal cells such as glial cells have been increasingly recognized as crucial sleep regulators. Recent studies have shown that microglia may act to maintain wakefulness. Here, we investigated the possible involvement of microglia in the regulation of sleep quantity and quality under baseline and stress conditions through electroencephalography (EEG)/electromyography (EMG) recordings, and by employing pharmacological methods to eliminate microglial cells in the adult mouse brain. We found that severe microglial depletion induced by the colony-stimulating factor 1 receptor (CSF1R) antagonist PLX5622 (PLX) reversibly decreased the total wake time and the wake episode duration and increased the EEG slow-wave power during wakefulness under baseline conditions. To examine the role of microglia in sleep/wake regulation under mental stress, we used the acute social defeat stress (ASDS) paradigm, an ethological model for psychosocial stress. Sleep analysis under ASDS revealed that microglial depletion exacerbated the stress-induced decrease in the total wake time and increase in anxiety-like behaviors in the open field test. These results demonstrate that microglia actively modulate sleep quantity and architecture under both baseline and stress conditions. Our findings suggest that microglia may potentially provide resilience against acute psychosocial stress by regulating restorative sleep.

Microglial inhibition alleviates alpha-synuclein propagation and neurodegeneration in Parkinson's disease mouse model

The accumulation of alpha-synuclein (αSyn) is widely recognized as the main pathological process in Parkinson's disease (PD). Additionally, neuroinflammation is considered to be one of the contributing mechanisms in the development of PD. In light of this, it is hypothesized that the reactive microglia exacerbate the propagation of αSyn and neurodegeneration, while the inhibition of microglial activity may mitigate these effects. To test this hypothesis, αSyn preformed fibrils (PFF)-injected PD mouse model was employed. Co-injection of lipopolysaccharide (LPS) and PFF was performed to investigate if microglial reactivity intensified αSyn propagation and neurodegeneration. Additionally, oral administration of PLX5622, a microglial inhibitor that targets the colony-stimulating factor 1 receptor, was given for two weeks before and after PFF injection each to explore if microglial inhibition could prevent or reduce αSyn pathology. Intrastriatal co-injection of LPS and PFF resulted in increased microglial reactivity, αSyn accumulation, and neurodegeneration compared to PFF injection alone. However, treatment with PLX5622 significantly suppressed microglial reactivity, reduced αSyn pathology, and alleviated dopaminergic neuron degeneration in the PD mouse model injected with PFF. Based on these findings, it is evident that microglial reactivity plays a crucial role in the progression of αSyn pathology and neurodegeneration in PD. Furthermore, the results suggest that microglial inhibition may hold promise as a therapeutic strategy to delay the progression of αSyn pathology in PD.

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.

Microglial depletion exacerbates axonal damage and motor dysfunction in mice with cuprizone-induced demyelination

The cuprizone (CPZ)-induced demyelination model, an animal model of Multiple sclerosis (MS), is characterized by demyelination and motor dysfunction due to microglial-mediated neuroinflammation. To determine the contribution of microglia to motor function during CPZ-induced demyelination, the microglia of mice in the CPZ-model were depleted using PLX3397 (PLX), an orally bioavailable selective colony stimulating factor 1 receptor inhibitor. PLX treatment aggravated motor dysfunction as shown by the pole, beam walk, ladder walk, and rotarod tests. PLX treatment removed microglia from the superior cerebellar peduncle (SCP), but not from the corpus callosum (CC). Although PLX treatment did not affect the degree of demyelination in both of CC and SCP, the expression of axonal damage marker APP (amyloid precursor protein) was increased. Increased TNF-α, IL-1β, and iNOS expressions were observed in PLX-treated mice. These results suggest that microglial depletion exacerbates axonal damage and motor dysfunction in CPZ model mice. In this study, we found that microglia contribute to motor function and axon-protective effects in CPZ-induced demyelination.

In Vitro Models of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is one of the commonest neurodegenerative diseases of adult-onset, which is characterized by the progressive death of motor neurons in the cerebral cortex, brain stem and spinal cord. The dysfunction and death of motor neurons lead to the progressive muscle weakness, atrophy, fasciculations, spasticity and ultimately the whole paralysis of body. Despite the identification of several genetic mutations associated with the pathogenesis of ALS, including mutations in chromosome 9 open reading frame 72 leading to the abnormal expansion of GGGGCC repeat sequence, TAR DNA-binding protein 43, fused in sarcoma/translocated in liposarcoma, copper/zinc superoxide dismutase 1 (SOD1) and TANK-binding kinase 1, the exact mechanisms underlying the specific degeneration of motor neurons that causes ALS remain incompletely understood. At present, since the transgenic model expressed SOD1 mutants was established, multiple in vitro models of ALS have been developed for studying the pathology, pathophysiology and pathogenesis of ALS as well as searching the effective neurotherapeutics. This review reviewed the details of present established in vitro models used in studying the pathology, pathophysiology and pathogenesis of ALS. Meanwhile, we also discussed the advantages, disadvantages, cost and availability of each models.

Cytokine Profiling in Human iPSC-Derived Dopaminergic Neuronal and Microglial Cultures

Aside from the degeneration of dopaminergic neurons, inflammation is a key component in the movement disorder Parkinson's disease (PD). Microglia activation as well as elevated cytokine levels were observed in the brains of PD patients, but the specific role of microglia in the disease process is unknown. Here, we generate human cellular models by differentiating iPSCs into dopaminergic neurons and microglia. We combine these cells in co-culture to perform cytokine profiling, representing the final functional outcome of various signaling pathways. For this, we used unstimulated conditions and treatment with inflammatory stressors. Importantly, only co-cultures but not the monocultures responded to IL-1β treatment suggesting co-culture-related crosstalk. Moreover, we identified the main types of released cytokines and chemokines in this model system and found a preference for the activation of the chemotaxis pathway in response to all treatments, which informs future studies on the cell-type-specific reaction to inflammatory stimulation. Finally, we detected protein level changes in PD risk factor GPNMB upon stress in microglia, further confirming the link between PD-associated genes and inflammation in human-derived cellular models.

The systemic immune response in Parkinson's disease: focus on the peripheral immune component

During Parkinson's disease (PD), both the central nervous system (CNS) and peripheral nervous system (PNS) are affected. In parallel, innate immune cells respond early to neuronal changes and alpha-synuclein (α-syn) pathology. Moreover, some of the affected neuronal groups innervate organs with a relevant role in immunity. Consequently, not only microglia, but also peripheral immune cells are altered, resulting in a systemic immune response. Innate and adaptive immune cells may participate in the neurodegenerative process by acting peripherally, infiltrating the brain, or releasing mediators that can protect or harm neurons. However, the sequence of the changes and the significance of each immune compartment in the disease remain to be clarified. In this review, we describe current understanding of the peripheral immune response in PD and discuss the road ahead.

Intranasal Monophosphoryl Lipid a Administration Ameliorates depression-like Behavior in Chronically Stressed Mice Through Stimulation of Microglia

We and others have reported that systematic stimulation of the central innate immune system by a low dose of lipopolysaccharide (LPS) can improve depression-like behavior in chronically stressed animals. However, it is unclear whether similar stimulation by intranasal administration could improve depression-like behavior in animals. We investigated this question using monophosphoryl lipid A (MPL), a derivative of LPS that lacks the adverse effects of LPS but is still immuno-stimulatory. We found that a single intranasal administration of MPL at a dose of 10 or 20 µg/mouse, but not at a dose of 5 µg/mouse, ameliorated chronic unpredictable stress (CUS)-induced depression-like behavior in mice, as evidenced by the decrease in immobility time in tail suspension test and forced swimming test and the increase in sucrose intake in sucrose preference test. In the time-dependent analysis, the antidepressant-like effect of a single intranasal MPL administration (20 µg/mouse) was observed 5 and 8 h but not 3 h after drug administration and persisted for at least 7 days. Fourteen days after the first intranasal MPL administration, a second intranasal MPL administration (20 µg/mouse) still showed an antidepressant-like effect. The innate immune response mediated by microglia might mediate the antidepressant-like effect of intranasal MPL administration, because both inhibition of microglial activation by pretreatment with minocycline and depletion of microglia by pretreatment with PLX3397 prevented the antidepressant-like effect of intranasal MPL administration. These results suggest that intranasal administration of MPL can produce significant antidepressant-like effects in animals under chronic stress conditions via stimulation of microglia.

A 14-day pulse of PLX5622 modifies α-synucleinopathy in preformed fibril-infused aged mice of both sexes

Reactive microglia are observed with aging and in Lewy body disorders, including within the olfactory bulb of men with Parkinson's disease. However, the functional impact of microglia in these disorders is still debated. Resetting these reactive cells by a brief dietary pulse of the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 may hold therapeutic potential against Lewy-related pathologies. To our knowledge, withdrawal of PLX5622 after short-term exposure has not been tested in the preformed α-synuclein fibril (PFF) model, including in aged mice of both sexes. Compared to aged female mice, we report that aged males on the control diet showed higher numbers of phosphorylated α-synuclein+ inclusions in the limbic rhinencephalon after PFFs were injected in the posterior olfactory bulb. However, aged females displayed larger inclusion sizes compared to males. Short-term (14-day) dietary exposure to PLX5622 followed by control chow reduced inclusion numbers and levels of insoluble α-synuclein in aged males-but not females-and unexpectedly raised inclusion sizes in both sexes. Transient delivery of PLX5622 also improved spatial reference memory in PFF-infused aged mice, as evidenced by an increase in novel arm entries in a Y-maze. Superior memory was positively correlated with inclusion sizes but negatively correlated with inclusion numbers. Although we caution that PLX5622 delivery must be tested further in models of α-synucleinopathy, our data suggest that larger-sized-but fewer-α-synucleinopathic structures are associated with better neurological outcomes in PFF-infused aged mice.

Intranasal administration of lipopolysaccharide reverses chronic stress-induced depression-like behavior in mice by microglial stimulation

We recently reported that intraperitoneal injection of a low dose of lipopolysaccharide (LPS) reversed depression-like behavior in mice induced by chronic stress by stimulating microglia in the hippocampus. In this study, we found that a single intranasal administration of LPS at a dose of 5 or 10 μg/mouse, but not at a dose of 1 μg/mouse, rapidly reversed depression-like behavior in mice stimulated with chronic unpredictable stress (CUS). In the time-dependent experiment, a single intranasal administration of LPS (10 μg/mouse) reversed CUS-induced depression-like behavior in mice 5 and 8 h but not 3 h after drug administration. The antidepressant effect of a single intranasal LPS administration (10 μg/mouse) lasted at least 10 days and disappeared 14 days after administration. Fourteen days after the first intranasal LPS administration, a second intranasal LPS administration (10 μg/mouse) still reversed the increased immobility time in TST and FST and the decreased sucrose uptake in SPT in CUS mice, which again exhibited depression-like behaviors 5 h after LPS administration. The antidepressant effect of intranasal LPS administration was dependent on microglial activation, because inhibition of microglia by pretreatment with minocycline (40 mg/kg) or depletion of microglia by pretreatment with PLX3397 (290 mg/kg) prevented the antidepressant effect of intranasal LPS administration in CUS mice. These results suggest that stimulation of the microglia-mediated innate immune response by intranasal administration of LPS can produce rapid and sustained antidepressant effects in animals under chronic stress conditions.