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Microglia as effector cells in brain damage and repair: focus on prostanoids and nitric oxide
1Neurobiology Section, Istituto Superiore di Sanità Viale Regina Elena, Rome, Italy.
Abstract:
Microglial cells are believed to play an active role in brain inflammatory, immune and degenerative processes. Depending on the magnitude of microglial reaction, on the type of stimulus and on the concurrence of other local factors, microglia can contribute to host defence and repair, or to the establishment and maintenance of brain damage. Many of the effects of microglial cells can be ascribed to the numerous substances that these cells can synthesize and release in response to a variety of stimuli (cytokines, pro-inflammatory substances, neurotransmitters, toxins, etc.). The present article deals with two classes of compounds that activated microglial cells can produce in large amounts: prostanoids (that derive from arachidonic acid through the cyclooxygenase pathway), and nitric oxide (that is synthesized from arginine by nitric oxide synthase). Prostanoids and nitric oxide have a number of common targets, on which they may exert similar or opposite actions, and have a crucial role in the regulation of inflammation, immune responses and cell viability. Their synthesis can massively increase when the inducible isoforms of cyclooxygenase and nitric oxide synthase are expressed. The metabolic pathways of prostanoids and nitric oxide are finely tuned by the respective end-products, by cyclic AMP and by a number of exogenous factors, such as cytokines, glucocorticoids, lipocortin-1 and others. Some of these factors (e.g. transforming growth factor-beta 1, interleukin-10, lipocortin-1) may be secreted by microglial cells themselves, and act in an autocrine-paracrine way. In view of the neuroprotective role attributed to some prostaglandins and to the cytotoxicity of excessive levels of nitric oxide or its derivatives, the balance between prostanoid and nitric oxide levels may be crucial for orienting microglial reactions towards neuroprotection or neurotoxicity.
Insights
Microglia produce prostanoids and nitric oxide, influencing brain inflammation and repair. Balancing these molecules is key to directing microglial responses toward neuroprotection or neurotoxicity.
Area of Science:
- Neuroscience
- Immunology
- Cell Biology
Background:
- Microglial cells are key players in brain inflammation, immune responses, and neurodegeneration.
- Their functions range from host defense and repair to contributing to brain damage.
- Microglial activity is mediated by substances like cytokines, neurotransmitters, and toxins.
Purpose of the Study:
- To examine the role of prostanoids and nitric oxide produced by activated microglial cells.
- To understand how these molecules influence inflammation, immune responses, and cell viability.
- To explore the factors regulating their synthesis and their impact on neuroprotection versus neurotoxicity.
Main Methods:
- Focuses on the synthesis of prostanoids via the cyclooxygenase (COX) pathway and nitric oxide via nitric oxide synthase (NOS).
- Discusses the role of inducible isoforms of COX and NOS in amplifying synthesis.
- Examines regulatory mechanisms involving end-products, cyclic AMP, and exogenous factors like cytokines and glucocorticoids.
Main Results:
- Activated microglia synthesize large amounts of prostanoids and nitric oxide.
- These compounds have diverse and sometimes opposing effects on common targets.
- Their synthesis is significantly upregulated by inducible COX and NOS isoforms.
- Metabolic pathways are regulated by end-products, cyclic AMP, and various exogenous factors, some secreted by microglia themselves.
Conclusions:
- The balance between prostanoids and nitric oxide is critical for microglial function.
- Prostaglandins can be neuroprotective, while excessive nitric oxide can be cytotoxic.
- Fine-tuning the levels of these molecules may determine whether microglial reactions are neuroprotective or neurotoxic.