Amyloid-β(1-42) protofibrils stimulate a quantum of secreted IL-1β despite significant intracellular IL-1β accumulation in microglia

Abstract

Neuroinflammation is a characteristic feature of the Alzheimer’s disease (AD) brain. Significant inflammatory markers such as activated microglia and cytokines can be found surrounding the extracellular senile plaques predominantly composed of amyloid-β protein (Aβ). Several innate immune pathways, including Toll-like receptors (TLRs) and the NLRP3 inflammasome, have been implicated in AD inflammation. Aβ plays a primary role in activating these pathways which likely contributes to the progressive neurodegeneration in AD. In order to better understand the complexities of this interaction we investigated the inflammatory response of primary microglia to Aβ(1–42) protofibrils. Aβ(1–42) protofibrils triggered a time- and MyD88-dependent process that produced tumor necrosis factor alpha (TNFα) and interleukin-1β (IL-1β) mRNA, and intracellular pro and mature forms of IL-1β protein. The accumulation of both IL-1β forms indicated that Aβ(1–42) protofibrils were able to prime and activate the NLRP3 inflammasome. Surprisingly, Aβ-induced accumulation of intracellular mature IL-1β did not translate into greater IL-1β secretion. Instead, we found that Aβ elicited a quantized burst of secreted IL-1β and this process occurred even prior to Aβ priming of the microglia suggesting a basal level of either pro or mature IL-1β in the cultured primary microglia. The IL-1β secretion burst was rapid but not sustained, yet could be re-evoked with additional Aβ stimulation. The findings from this study demonstrated multiple sites of IL-1β regulation by Aβ(1–42) protofibrils including TLR/MyD88-mediated priming, NLRP3 inflammasome activation, and modulation of the IL-1β secretory process. These results underscore the wide-ranging effects of Aβ on the innate immune response.

CD47 does not mediate amyloid-β(1–42) protofibril-stimulated microglial cytokine release

“…Primary microglia were isolated from both wild-type (WT) and CD47 −/− mice and the cytokine response to Aβ(1-42) protofibrils was assessed. We have previously investigated the time course for protofibril-induced TNFα secretion in primary microglia and found that 6 h is an appropriate time frame for time-dependent studies [37]. The protofibril response in the current study was consistent with our previous work in WT microglia showing little TNFα secretion at 2 h but a significant increase after 6 h of microglia exposure to Aβ(1-42) protofibrils (Figure 4A).…”

Insulin resistance and cognitive dysfunction

“…The inflammatory reaction is a core pathological mechanism of AD [83][84][85]. Inflammatory responses, which are present in obesity and T2DM, are closely related to the development of IR in both peripheral and central tissues, thus may increase the risk of AD [86].…”

Edaravone Attenuates the Proinflammatory Response in Amyloid-β-Treated Microglia by Inhibiting NLRP3 Inflammasome-Mediated IL-1β Secretion

“…Notably, increased IL-1β expression is observed in activated microglia surrounding Aβ plaques in patients with AD and in mouse models of AD. Furthermore, Aβ has been consistently shown to trigger IL-1β secretion in vitro and in vivo [1,24]. Importantly, the activation of the NLRP3 inflammasome is a critical upstream regulator of caspase-1 activation, which results in the maturation and secretion of IL-1β [31].…”

“…Primary microglia cultures were isolated from postnatal day 1 C57BL/6J mice and cultured as previously described [24]. The brains were removed, and cortices were freed from meninges and blood vessels.…”