Brain and Peripheral Atypical Inflammatory Mediators Potentiate Neuroinflammation and NeurodegenerationFigure 1. Mast cells in inflammatory diseases. Mast cells play a crucial role in the pathogenesis of many inflammatory diseases of various organs. Certain systemic inflammatory conditions could affect and augment neuroinflammation and neurodegeneration. Therefore, targeting mast cells represents a potentially novel approach to developing the next generation of precision guided therapeutic strategies for the treatment of inflammatory diseases. IBD, inflammatory bowel disease; CAD, coronary artery disease; BBB, blood-brain barrier.
Figure 2. Schematic diagram showing peripheral inflammatory factors and cells on neuroinflammation and neurodegeneration. Peripheral mast cell activation releases proinflammatory and neurotoxic mediators such as histamine, glia maturation factor (GMF), ?-synuclein, corticotropin-releasing hormone (CRH), proteases, cytokines and chemokines. These mediators can induce neuroinflammation by inducing BBB breakdown, entering into the brain and activating glia and neurons to secrete various additional inflammatory mediators. Peripheral mast cells and T-cells enter into the brain, proliferate and secrete proinflammatory mediators that activate glia and neurons to secrete more inflammatory mediators, reduce uncoupling protein (UCP) expression, and induce neurodegeneration. Further, glia and mast cells reactivate each other in the brain through co-stimulatory molecules CD40/CD154 or inflammatory mediators such as TNF-?, IL-1? or IL-33. Mast cell tryptase acts on the neurons through PAR2.
Mast cells can reactivate by their own mediators in an autocrine and paracrine manner to exacerbate inflammatory mechanisms. ?-synuclein or MPP+ from glia/neuron or extracellular A?1–42 can further activate mast cells to release neuroinflammatory mediators in Alzheimer’s disease (AD) or Parkinson’s disease (PD). Additionally several inflammatory mediators from the peripheral system can alter BBB, enter the brain and activate the neuroinflammatory pathways. Inflammatory mediators released from activated microglia and astrocytes can enter into peripheral system through defective BBB; then, they can activate and recruit immune and inflammatory cells towards the inflammatory site in the brain. MC, mast cell; MCP, mast cell progenitor; TC, T-cell; PAR2, protease activated receptor-2.