IL-33/ST2 Signaling Regulates Synaptic Plasticity and Homeostasis in the Adult Hippocampal Circuitry

Inflammasome signaling in astrocytes modulates hippocampal plasticity

Emerging evidence indicates that a baseline level of controlled innate immune signaling is required to support proper brain function. However, little is known about the function of most innate immune pathways in homeostatic neurobiology. Here, we report a role for astrocyte-dependent inflammasome signaling in regulating hippocampal plasticity. Inflammasomes are multiprotein complexes that promote caspase-1-mediated interleukin (IL)-1 and IL-18 production in response to pathogens and tissue damage. We observed that inflammasome complex formation was regularly detected under homeostasis in hippocampal astrocytes and that its assembly is dynamically regulated in response to learning and regional activity. Conditional ablation of caspase-1 in astrocytes limited hyperexcitability in an acute seizure model and impacted hippocampal plasticity via modulation of synaptic protein density, neuronal activity, and perineuronal net coverage. Caspase-1 and IL-18 regulated hippocampal IL-33 production and related plasticity. These findings reveal a homeostatic function for astrocyte inflammasome activity in regulating hippocampal physiology in health and disease.

Innate neuroimmunity across aging and neurodegeneration: a perspective from amyloidogenic evolvability

In Alzheimer's Disease (AD), amyloidogenic proteins (APs), such as β-amyloid (Aβ) and tau, may act as alarmins/damage-associated molecular patterns (DAMPs) to stimulate neuroinflammation and cell death. Indeed, recent evidence suggests that brain-specific type 2 immune networks may be important in modulating amyloidogenicity and brain homeostasis. Central to this, components of innate neuroimmune signaling, particularly type 2 components, assume distinctly specialized roles in regulating immune homeostasis and brain function. Whereas balanced immune surveillance stems from normal type 2 brain immune function, appropriate microglial clearance of aggregated misfolded proteins and neurotrophic and synaptotrophic signaling, aberrant pro-inflammatory activity triggered by alarmins might disrupt this normal immune homeostasis with reduced microglial amyloid clearance, synaptic loss, and ultimately neurodegeneration. Furthermore, since increased inflammation may in turn cause neurodegeneration, it is predicted that AP aggregation and neuroinflammation could synergistically promote even more damage. The reasons for maintaining such adverse biological conditions which have not been weeded out during evolution remain unclear. Here, we discuss these issues from a viewpoint of amyloidogenic evolvability, namely, aEVO, a hypothetic view of an adaptation to environmental stress by AP aggregates. Speculatively, the interaction of AP aggregation and neuroinflammation for aEVO in reproduction, which is evolutionally beneficial, might become a co-activating relationship which promotes AD pathogenesis through antagonistic pleiotropy. If validated, simultaneously suppressing both AP aggregation and specific innate neuroinflammation could greatly increase therapeutic efficacy in AD. Overall, combining a better understanding of innate neuroimmunity in aging and disease with the aEVO hypothesis may help uncover novel mechanism of pathogenesis of AD, leading to improved diagnostics and treatments.

Molecular mechanisms underlying microglial sensing and phagocytosis in synaptic pruning

Microglia are the main non-neuronal cells in the central nervous system that have important roles in brain development and functional connectivity of neural circuits. In brain physiology, highly dynamic microglial processes are facilitated to sense the surrounding environment and stimuli. Once the brain switches its functional states, microglia are recruited to specific sites to exert their immune functions, including the release of cytokines and phagocytosis of cellular debris. The crosstalk of microglia between neurons, neural stem cells, endothelial cells, oligodendrocytes, and astrocytes contributes to their functions in synapse pruning, neurogenesis, vascularization, myelination, and blood-brain barrier permeability. In this review, we highlight the neuron-derived "find-me," "eat-me," and "don't eat-me" molecular signals that drive microglia in response to changes in neuronal activity for synapse refinement during brain development. This review reveals the molecular mechanism of neuron-microglia interaction in synaptic pruning and presents novel ideas for the synaptic pruning of microglia in disease, thereby providing important clues for discovery of target drugs and development of nervous system disease treatment methods targeting synaptic dysfunction.

As a Potential Therapeutic Target, C1q Induces Synapse Loss Via Inflammasome-activating Apoptotic and Mitochondria Impairment Mechanisms in Alzheimer's Disease

C1q, the initiator of the classical pathway of the complement system, is activated during Alzheimer's disease (AD) development and progression and is especially associated with the production and deposition of β-amyloid protein (Aβ) and phosphorylated tau in β-amyloid plaques (APs) and neurofibrillary tangles (NFTs). Activation of C1q is responsible for induction of synapse loss, leading to neurodegeneration in AD. Mechanistically, C1q could activate glial cells, which results in the loss of synapses via regulation of synapse pruning and phagocytosis in AD. In addition, C1q induces neuroinflammation by inducing proinflammatory cytokine secretion, which is partially mediated by inflammasome activation. Activation of inflammasomes might mediate the effects of C1q on induction of synapse apoptosis. On the other hand, activation of C1q impairs mitochondria, which hinders the renovation and regeneration of synapses. All these actions of C1q contribute to the loss of synapses during neurodegeneration in AD. Therefore, pharmacological, or genetic interventions targeting C1q may provide potential therapeutic strategies for combating AD.

Integrated meta-analysis, data mining, and animal experiments to investigate the efficacy and potential pharmacological mechanism of a TCM tonic prescription, Jianpi Tongmai formula, in depression

BACKGROUND

Depression is a common psychiatric disorder and has become a growing public health issue. Traditional Chinese medicine (TCM) tonic prescriptions have been clinically proven to be an effective treatment for depression.

PURPOSE

This study aimed to identify the core prescription to improve depression among the numerous TCM tonic prescriptions.

METHODS AND RESULTS

First, we used meta-analysis to clarify the efficacy and safety of tonic prescriptions in depression among 37 studies and identified 16 effective tonic prescriptions. Second, we conducted data mining to analyze the tonic prescriptions and identified important nourishing herbs. Third, based on the data mining results, we constructed a Delphi experiment to investigate the effects of these important nourishing herbs in depression. Combining the results of Delphi expert questionnaires and weight analysis, a core TCM tonic prescription, Jianpi Tongmai formula (JPTMF) for the treatment of depression, was constructed and was composed of invigorating Spleen qi herbs. Fourth, we verified that JPTMF can improve chronic unpredictable mild stress (CUMS) induced depression-like behaviors in mice. Fifth, we predicted that the mechanism of JPTMF in the treatment of depression was mainly associated with chemical synaptic transmission and neuroinflammation through network pharmacology and determined preliminary confirmation through animal experiments.

CONCLUSION

This study was undertaken to evaluate the efficacy of TCM tonic prescriptions on depression and construct a core TCM tonic prescription, JPTMF, through a progressive analysis. Network pharmacology and animal experiments verified the reliability of JPTMF. The proposal of JPTMF is of innovative significance, and may provide far-reaching implications for improving depression by using nourishing herbs. Furthermore, the integrated methods applied in this study provide an innovative paradigm for the standardization and scientific basis of TCM research.

Dual roles of interleukin-33 in cognitive function by regulating central nervous system inflammation

With the advent of an aging society, the incidence of dementia is increasing, resulting in a vast burden on society. It is increasingly acknowledged that neuroinflammation is implicated in various neurological diseases with cognitive dysfunction such as Alzheimer’s disease, multiple sclerosis, ischemic stroke, traumatic brain injury, and central nervous system infections. As an important neuroinflammatory factor, interleukin-33 (IL-33) is highly expressed in various tissues and cells in the mammalian brain, where it plays a role in the pathogenesis of a number of central nervous system conditions. Reams of previous studies have shown that IL-33 has both pro- and anti-inflammatory effects, playing dual roles in the progression of diseases linked to cognitive impairment by regulating the activation and polarization of immune cells, apoptosis, and synaptic plasticity. This article will summarize the current findings on the effects IL-33 exerts on cognitive function by regulating neuroinflammation, and attempt to explore possible therapeutic strategies for cognitive disorders based on the adverse and protective mechanisms of IL-33.