Release of inflammatory cytokines implicated in Alzheimer’s disease requires signaling through inflammasomes. What regulates these innate immune complexes? A June 30 Nature Immunology paper highlights the essential role of CD36, an Aβ-binding scavenger receptor expressed on microglia. Challenging the assumption that inflammasomes rev up in response to extracellular amyloid, the new study suggests that fibrils assembled within lysosomes activate these complexes. Aβ piles up in the lysosomes after entering microglia via CD36, the research suggests. “Rather than accumulating outside the cell, the ‘danger molecules’ form within the macrophage itself,” said senior author Kathryn Moore at New York University School of Medicine. The study, which focused initially on inflammasome activation in atherosclerosis, identifies CD36 as a central regulator of innate immune responses that drive pathogenesis in other inflammatory diseases, including AD and type 2 diabetes, a risk factor for Alzheimer's.
Research implicates interleukin-1β (IL-1β) in many inflammatory disorders. AD, type 2 diabetes, and atherosclerosis each feature build up of insoluble material at sites of inflammation, be it Aβ plaques, amyloid-containing amylin-islet amyloid polypeptide (IAPP), or cholesterol crystals. The prevailing view holds that microglia “get frustrated” trying to engulf these extracellular aggregates and, in turn, trigger NLRP3 (NOD-like receptor family, pyrin domain-containing 3) inflammasomes to turn on caspases that promote IL-1β secretion. In AD, support for that theory comes from reports of fibrillar amyloid activating NLRP3 in cultured mouse microglial cells (see Halle et al., 2008 ). Moreover, silencing this molecular complex improved cognition and reduced brain Aβ deposition in APP/PS1 mice (ARF related news story), stimulating further interest in the molecular signals that set off the inflammasome.
In their atherosclerosis experiments, lead author Frederick Sheedy and colleagues saw that extracellular aggregates seemed unnecessary for activation of the inflammasome. They were able to activate NLRP3 in vitro by culturing macrophages with soluble precursors to cholesterol aggregates. Within hours, the macrophages took up these precursors (oxidized lipoproteins) and formed cholesterol crystals within lysosomes, as revealed by confocal microscopy. This paralleled intracellular build-up of IL-1β and required CD36, since macrophages lacking this receptor did not form crystals or make IL-1β. Strengthening the in-vitro data, inflammasome activity tanked in CD36-deficient mice and in animals treated with CD36 oligonucleotides that block expression of the gene.
Given that CD36 binds soluble Aβ in vitro (Wilkinson et al., 2011), and mediates microglial and macrophage responses to Aβ in mice (see El Khoury et al., 2003; ARF related news story), the scientists wondered if NLRP3 activation in AD occurs by a similar mechanism.
To test that possibility, the researchers monitored fibril formation and IL-1β release by primary mouse macrophages cultured with soluble synthetic Aβ1-42. Within three hours, thioflavin-S-positive amyloid formed within lysosomes, and IL-1β secretion rose as well. These effects did not show up in macrophages treated with a nonamyloidogenic control peptide, or in cells isolated from CD36 knockout mice. Interestingly, the researchers could prevent IL-1β release by pre-loading lysosomes with Congo red to block fibril formation. The data suggests that NLRP3 inflammasome activation and IL-1β production requires fibrillization of soluble amyloid internalized via CD36, and fits with prior work suggesting that amyloid fibrils form intracellularly (see Friedrich et al., 2010; Walsh et al., 2000; Haass et al., 1992). In addition, the scientists showed that inflammasome activation by IAPP occurs in vitro by a similar mechanism.
Frank Heppner of Charité–Universitätsmedizin Berlin, Germany, found the work “important and encouraging.” It fleshes out an innate immune mechanism to curb amyloidosis by identifying CD36 as a key upstream regulator of NLRP3 activation in three major diseases, he wrote in an email to Alzforum (see full comment below).
Others said that while the findings point to CD36 as a therapeutic target in atherosclerosis, the implications for AD seem less clear. For example, “one would not wish to hinder Aβ clearance by microglia,” Richard Ransohoff of Cleveland Clinic, Ohio, wrote in an email to Alzforum. “On the other hand, if [the CD36 pathway] leads to microglial dysfunction, targeting intracellular events downstream of CD36 might attenuate that while allowing for Abeta clearance and other effector and surveillance functions.”
Moore said ongoing analyses in CD36-deficient AD transgenic mice should help discern the effects of CD36 on inflammasome activity and pathogenesis in vivo.—Esther Landhuis
- Microglia and AD—Does the Inflammasome Drive Aβ Pathology?
- Bad Blood—Scavenger Receptor Links Aβ to Oxidative Stress in Mice
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