Apelin-13 Suppresses Neuroinflammation Against Cognitive Deficit in a Streptozotocin-Induced Rat Model of Alzheimer’s Disease Through Activation of BDNF-TrkB Signaling Pathway

Interleukin-1 beta (IL-1 beta), interleukin-2 (IL-2), and interleukin-6 (IL-6) were measured in the cerebrospinal fluid (CSF) and plasma of 12 control subjects, 11 sporadic Alzheimer's disease (AD) and 22 de novo Parkinson's disease (PD) patients using high sensitivity enzyme-linked immunosorbent assays (ELISA). IL-1 beta and IL-6 contents were significantly elevated in the CSF of de novo PD and AD patients in comparison to the control group. In contrast, the plasma levels were not significantly affected. IL-2 contents in the CSF and plasma samples were unchanged in the three groups compared. Because the two cytokines IL-1 beta and IL-6 are known to play a key role in the interaction between the nervous and immune system, e.g. in the so-called acute phase response, our results support the involvement of immunological events in the complex process of neurodegeneration in AD and PD.

Inflammation is a key pathological hallmark of Alzheimer's disease (AD), although its impact on disease progression and neurodegeneration remains an area of active investigation. Among numerous inflammatory cytokines associated with AD, IL-1β in particular has been implicated in playing a pathogenic role. In this study, we sought to investigate whether inhibition of IL-1β signaling provides disease-modifying benefits in an AD mouse model and, if so, by what molecular mechanisms. We report that chronic dosing of 3xTg-AD mice with an IL-1R blocking Ab significantly alters brain inflammatory responses, alleviates cognitive deficits, markedly attenuates tau pathology, and partly reduces certain fibrillar and oligomeric forms of amyloid-β. Alterations in inflammatory responses correspond to reduced NF-κB activity. Furthermore, inhibition of IL-1 signaling reduces the activity of several tau kinases in the brain, including cdk5/p25, GSK-3β, and p38-MAPK, and also reduces phosphorylated tau levels. We also detected a reduction in the astrocyte-derived cytokine, S100B, and in the extent of neuronal Wnt/β-catenin signaling in 3xTg-AD brains, and provided in vitro evidence that these changes may, in part, provide a mechanistic link between IL-1 signaling and GSK-3β activation. Taken together, our results suggest that the IL-1 signaling cascade may be involved in one of the key disease mechanisms for AD.

There is a great deal of evidence suggesting an important role for systemic inflammation in the pathogenesis of Alzheimer's disease. The role of systemic inflammation, and indeed inflammation in general, is still largely considered to be as a contributor to the disease process rather than of aetiological importance although there is emerging evidence to suggest that its role may predate the deposition of amyloid. Therapies aimed at reducing inflammation in individuals with mild cognitive impairment and Alzheimer's disease have been disappointing and have largely focused on the need to ameliorate central inflammation with little attention to the importance of dampening down systemic inflammation. Novel approaches in this area require a greater understanding of the effects of systemic inflammation on the development and progression of Alzheimer's disease and of the communicating pathways between the systemic and central innate immune systems. © 2012 The Author. Neuropathology and Applied Neurobiology © 2012 British Neuropathological Society.