On inflammatory cytokines and β-amyloid peptides in acute and chronic neurodegeneration
Abstract: Insults to the brain as well as neurodegenerative diseases are known to elicit inflammatory responses. Inflammation in the brain can on one hand initiate processes that are harmful to the injured tissue and exacerbate the damage, leading to neuronal degeneration and glial activation, and on the other hand activate processes that may be necessary for repair mechanisms and regeneration. Among the mediators of inflammatory response in the brain are the inflammatory cytokines. The most studied are interleukin-1 (IL)-1, IL-6 and tumor necrosis factor-alfa (TNF- ). Although the expression of these cytokines is low under normal conditions in the brain, it can be rapidly induced in response to injury.This thesis is focused on the role of IL-1 family of proteins, namely the agonists IL-1 and and the endogenous IL-1 receptor antagonist (IL-1ra), and IL-6, in different experimental models of neurodegeneration. In order to study the role of IL-1 family of proteins during inflammation in an excitotoxic model of brain injury, adult rats were injected systemically with kainic acid, a glutamate analogue known to evoke seizures and neuronal cell loss in the rat brain. Using the combined technique of reverse-transcriptase coupled to PCR (RT-PCR) and in situ hybridization histochemistry, an upregulation of microglial mRNA expression of IL-1 and IL-1ra was found in brain areas with neuronal degeneration, such as the hippocampus and amygdala. IL-1ra mRNA was induced at later time point than IL-1 mRNA and was identified as the transcript coding for the secreted isoform of IL-1ra. This suggets that upregulation of these cytokines is a part of an inflammtory response associated with neurodegeneration and that the effect of IL-1 may be regulated by the expression of IL-1ra in this model. In order to study the role played by IL-1 in inflammation associated with traumatic brain injury (TBI), an experimental model was inflicted on transgenic mice. Heterozygous overexpression of the human secreted isoform of IL-1ra in the brain decreased the induction of IL-1 and IL-6 after injury. Using a neurological severity score (NSS), which mainly reflects motor recovery, we found that these animals recovered faster as compared to their non-transgenic littermates.Furthermore, the proinflammatory cytokine expression was studied by RT-PCR in a mouse model of Alzheimer's disease (AD). The Tg2576 mice strain overexpress -amyloid (A ) precursor protein with the "Swedish" mutation linked to familiar AD and exhibits some of the neuropathology associated with AD, such as the deposition of insoluble extracellular amyloid fibrils (amyloid plaques) in specific brain regions. Analysis of expression of cytokines in the brain of Tg2576 mice revealed an early induction of IL-6 in the hippocampus and cerebral cortex that precedes the formation of amyloid plaques. This finding is interesting since in AD brain IL-6 is detected in microglia in the vicinity of diffuse plaques (non-fibrillar). Thus, the result from this study suggests that increased IL-6 expression may be an early event in AD inflammation.The main constituent of amyloid plaques in the AD brain is the A peptide. The synthetic peptide A (25-35), a neurotoxic fragment of the full-length A peptide was studied for its ability to activate glial cells in culture and induce cytokine expression, as well as for its influence on G-protein coupled signalling in rat brain tissue. A (25-35) treatment of mixed astroglial cultures resulted in marked induction of IL-6 mRNA as studied by RT-PCR. Together with the results from the Tg2576 mice these results suggest a role for IL-6 in AD pathogenesis.Alteration in cellular signal transduction has also been reported in AD brain. A (25-35) was shown to stimulate the enzymatic activities of GTPase and adenylate cyclase in membrane preparations from rat hippocampus and cerebral cortex, which are particularly affected regions in AD brain. Using Pertussis toxin treated membranes, the stimulatory effect on GTPase activity was totally abolished, suggesting that Gi/Go type of G-proteins mediated the effect of the A peptide.
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