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MIF and neuroinflammation in models of Alzheimer’s disease

An increasing body of evidence indicates that inflammatory processes are involved in the pathophysiology of Alzheimer’s disease (AD). A central event in these processes appears to be the activation of microglia by a variety of factors, including β-amyloid (Aβ) and pro-inflammatory cytokines. Activated microglia in turn release pro-inflammatory cytokines, such as interleukin (IL)-1- beta, IL-6, IL-12, and TNF-α, that may lead to neuronal death and dysfunction by a variety of mechanisms, including enhancement of glutamate-induced excitotoxicity, inhibition of long-term potentiation (which limits functional plasticity after neuronal injury and inhibition of hippocampal neurogenesis. A few recent clinical studies including patients with mild cognitive impairment (MCI), strongly suggest that inflammatory processes precede the development of dementia. In addition, higher serum levels of systemic inflammation markers have been shown to predict cognitive decline or dementia in subjects with normal cognition. The assumption that inflammation is an early event in AD is further supported by histological studies showing very early focal glia activation in conjunction with increased expression of IL-1 and IL-6, increased production and up-regulated activity of BACE1, and with early amyloid plaque stages. Together, these results provide convincing evidence that neuroinflammation occurs early in AD, and may contribute to disease progression already at pre-dementia stages.

Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine that is highly expressed in many tissues and disease states. MIF has potent autocrine and paracrine effects and promotes the production or expression of several inflammatory mediators which includes TNF-α, IL-1, IFN-γ, IL-6, IL-8, IL-12, and nitric oxide. Furthermore, MIF counter-regulates immunosuppressive effects of gluco-corticoids, for example the inhibition of TNF-α, IL-1, IL-6 and IL-8.}. There is a significant level of baseline MIF expression in the neurons of the hippocampus as well as in other regions of the brain, and pro-inflammatory stimuli lead to a marked upregulation of neuronal MIF mRNA and protein.

Interestingly, MIF also has been isolated in association with Aβ, which is the main constituent of AD plaques, thereby supporting an emerging theory of a proinflammatory etiology for this neurodegenerative disease. In a recent report, Professor Bacher’s group were the first to show that MIF levels were elevated significantly in comparison to age matched healthy control samples of AD patients. Of note, in this study the highest MIF level in patients with amnestic mild cognitive impairment (aMCI) was obtained, suggesting that MIF is involved in inflammation occurring early in AD. The possible involvement of MIF in AD has also been provided by a recent report showing a significant upregulation of CD74, which acts as a receptor binding site for MIF. Biochemical assays revealed that MIF bound to CXCR2 and CXCR4 with nanomolar affinity, and elicited internalization of both receptors. Finally, it was shown that CXCR2 colocalizes with and physically interacts with CD74, demonstrating that a functional MIF receptor complex involves chemokine receptors and CD74. A further link between the biological activity of MIF and the pathogenesis of AD may also be provided by a study showing that in vivo depletion of CXCR2 in APP mice inhibits Aβ production and reduces γ-secretase activity.

In a recent study, it became significantly clear that MIF concentration increases in the CSF of AD and of MCI patients compared to age- and gender-matched controls. There was no significant difference between the MIF levels in MCI and in AD patients, although there was trend towards higher levels in MCI patients. The CSF concentrations of TNF-α, IL-6 and IFN-γ were not increased in MCI and AD compared to the control s group. 

As MIF has been established to promote the production of several inflammatory mediators including TNF-α, IL-6 and, IFN-γ, MIF may play a central role in the regulation of these processes, and substantially contribute to the deleterious effects of inflammation in AD. A previous study has demonstrated that MIF is able to bind Aβ, and found MIF to be co-localized with microglia surrounding amyloid plaques in the AD brain. Together with these findings, the results of elevated levels of MIF in the CSF of patients with AD strongly suggest that MIF is implicated in the disease-related inflammatory process.

In addition, the increase of MIF in this group was not related to the dementia severity or the degree of cognitive impairment as measured by MMSE, indicating that MIF-mediated neuroinflammation may persist during all clinical stages of AD. Interestingly, in this study, the MIF CSF concentrations were also markedly increased in participants with MCI compared to controls. Subjects with aMCI are considered at high risk to develop dementia over time. Several studies have shown that AD-related biological alteration, like hippocampal atrophy and CSF biomarker changes, are already detectable in MCI. A few recent studies including patients with MCI strongly suggest that inflammatory processes may precede the dementia stage.

In conclusion, these studies provide additional evidence that neuroinflammation occurs at pre-dementia stages of AD with first results suggesting that MIF is implicated in the disease-related inflammatory process. Follow-up studies including subjects with aMCI are required to determine whether inflammatory activity as measured by the early increase of MIF CSF levels may predict cognitive decline, and to confirm the involvement of MIF in the pathological changes taking place in AD.

 In another recent report, further evidence has been provided for the involvement of MIF in AD pathology using a transgenic murine model of AD (CDRN8) and in vitro experiments. Our initial MIF immunostaining using a transgenic APP mouse model confirmed and extended the observation of a co-localization of MIF and Aβ plaques. A possible cellular source for the MIF is activated microglia in close proximity to Aβ plaques. 

The scope of this study is to provide a functional link between MIF and Aβ-induced neurotoxicity. In vitro data using a classical neuroblastoma cell line (SHSY5y) provided clear evidence that the inhibition of endogenous MIF using a small molecule-specific MIF inhibitor almost completely antagonized the neurotoxic effects of Aβ. A similar effect of ISO-1 was detected in murine BV2 microglial cells. In summary, evidence has been provided that a functional link between the pro -inflammatory MIF and the toxicity of Aβ aggregates, which are considered to play a central role in the pathogenesis of AD. Blocking the endogenous MIF in neuronal and microglial cells prevented the Aβ-dependent toxicity in both cell types. These results suggest that a similar link between MIF and the progression of AD might also exist in vivo and that the Aβ toxicity can be attributed directly to the increased expression of MIF. The observations will be confirmed and extended by in-vivo experiments on mice models for sporadic AD. In case we obtain clear evidence for the involvement of MIF for the progress of this disease, we would reintroduce MIF expression into the brain by bone marrow engraftment.