Interleukin-3 is renowned for its role in determining the fate of blood cell progenitors, but now researchers claim it protects against Alzheimer’s disease, as well. In the July 14 Nature, researchers led by Filip Swirski at Icahn School of Medicine at Mount Sinai, New York, and Rudolph Tanzi at Massachusetts General Hospital, Charlestown, report that microglia ramp up expression of the IL-3Rα receptor when they get their hooks into amyloid plaques. IL-3 then strengthens signaling downstream of TREM2, a cell-surface receptor that evokes microglial changes in AD. Knocking out IL-3 in a mouse model of amyloidosis led to more and bigger plaques and the animals labored to escape a water maze, suggesting they forgot where the hidden platform was. The kicker … in the brain, IL-3 does not come from immune cells, as one might suspect, but from astrocytes. Apparently, a teeny subset of these glia constitutively produces the cytokine, and the researchers believe that is what activates the microglia.
- Teeny subset of astrocytes makes IL-3.
- Microglial IL-3 receptor ramps up in AD and in mouse models.
- Without IL-3 signaling, amyloidosis worsens.
“We were very surprised to find a role for IL3 in the brain, because it had not been hypothesized before,” first author Cameron McAlpine told Alzforum. “That astrocytes produce the cytokine was even more striking,” he said.
In years past, Swirski and McAlpine have focused on the role of cytokines in immune responses and in inflammation, particularly of the cardiovascular system. So, what made them study AD? Though others had previously linked plasma IL-3 levels to a diagnosis of AD and to amyloid and tau biomarker levels, its role in the disease had not been investigated at the molecular level (October 2007 news; Soares et al., 2012; Britschgi et al., 2011; Kiddle et al., 2012). The authors decided to bring their expertise in immunology to bear.
McAlpine used cellular and mouse models to probe the role of IL-3 in the brain. He began by crossing IL-3 knockout mice with the 5xFAD mouse model of amyloidosis. He found that mice lacking the cytokine developed larger, more numerous plaques and that they had more trouble remembering the location of the hidden platform in the water maze. The data suggested that IL-3 somehow protects the brain from amyloid.
To find out how IL-3 does this, McAlpine asked which cells express its gene and its cognate receptor. Using CRISPR, he replaced the IL-3 gene with that for green fluorescent protein, then looked to see which cells fluoresced in the brain. Surprisingly, he found that about 4 percent of astrocytes did, but no other cells. Could these IL-3 producers represent some type of specialized astrocyte? McAlpine and Swirski don’t know, but that’s something they plan to study.
They do know that the proportion of IL-3-producing astrocytes varies across the brain, with the highest being in the lateral ventricles. Curiously, activating astrocytes did not increase IL-3, and knocking out IL-3 did not compromise astrocyte biology. From this, the authors concluded that the small subset of astrocytes that make IL-3 do so constitutively. In keeping with this, levels of IL-3 were similar across the mouse brain, increased with age, and were the same in wild-type and 5xFAD mice.
How does IL-3 reduce amyloidosis? To answer this, the authors looked for cells to which it talks. They zoned in on an age-dependent increase in IL-3Rα on mouse microglia, and found that this accelerated in 5xFAD mice. Only 8 percent of microglia expressed IL-3Rα in 8-month-old wild-type mice but, by that age in 5xFAD mice, already 50 percent of the cells were producing the receptor. Why so? A hint came when the researchers looked at where those IL-3Rα-positive microglia hung out. They mostly congregated around plaques, suggesting the uptick in the receptor was a response to Aβ.
All told, the findings suggest that constitutively expressed astrocyte IL-3 somehow activates microglia that have accumulated the IL-3Rα receptor in response to amyloid. How this signaling helps is not fully clear, but transcriptional profiling of microglia from 5xFAD mice and their counterparts that lack the IL-3 gene points the finger at cellular process that control motility and immune responses. Genes controlled by TREM2 were suppressed, but not TREM2 itself or its partner DAP12, suggesting IL-3 signaling works downstream of TREM2. Expression of APOE4, Clec7a, and other genes that make up the signature of disease-associated microglia, aka DAMs, were also suppressed when IL-3 was absent. In keeping with this, the authors found more IL-3Rα in DAMs.
Is IL-3 signaling important in the human brain? In postmortem cortical samples from 28 controls and 30 people who had had AD, the authors saw that IL-3 co-localized with astrocytes. The cytokines’ levels were similar between AD samples and controls, in keeping with the constitutive expression idea. As for IL-3Rα, it was higher in samples from people who had had AD or were APOE4/4 carriers. Receptor levels also rose with disease duration and correlated with plaque burden.
“These findings are an exciting advance in understanding the role of astrocytes and microglia in AD,” wrote Jerika Barron and Anna Molovsky, University of California, San Francisco, in a Nature News & Views. They raise the prospect of using this new signaling pathway for therapeutic development. In this vein, McAlpine and colleagues reported that stereotactically injecting recombinant IL-3 into the cortices of 5xFAD mice mobilized microglia within three days to surround plaques (see image above). Infusing the rIL-3 into the lateral ventricles over 28 days reduced plaque load and improved memory.
McAlpine is thinking of different ways to stimulate this signaling pathway therapeutically. “A mimetic or analog of IL-3 might be able to stimulate microglia, but genetic editing to genetically enhance expression of the receptor might be also useful,” he said. He believes that IL-3Rα is not the only target; a drug could go after molecules that signal downstream of it. Such a strategy may not be limited to AD, either. Swirski and McAlpine are studying if IL-3 signaling is important for mobilization of microglia in other neurodegenerative diseases.—Tom Fagan
Research Models Citations
- Soares HD, Potter WZ, Pickering E, Kuhn M, Immermann FW, Shera DM, Ferm M, Dean RA, Simon AJ, Swenson F, Siuciak JA, Kaplow J, Thambisetty M, Zagouras P, Koroshetz WJ, Wan HI, Trojanowski JQ, Shaw LM, . Plasma Biomarkers Associated With the Apolipoprotein E Genotype and Alzheimer Disease. Arch Neurol. 2012 Jul 16;:1-8. PubMed.
- Britschgi M, Rufibach K, Huang SL, Clark CM, Kaye JA, Li G, Peskind ER, Quinn JF, Galasko DR, Wyss-Coray T. Modeling of pathological traits in Alzheimer's disease based on systemic extracellular signaling proteome. Mol Cell Proteomics. 2011 Oct;10(10):M111.008862. PubMed.
- Kiddle SJ, Thambisetty M, Simmons A, Riddoch-Contreras J, Hye A, Westman E, Pike I, Ward M, Johnston C, Lupton MK, Lunnon K, Soininen H, Kloszewska I, Tsolaki M, Vellas B, Mecocci P, Lovestone S, Newhouse S, Dobson R, . Plasma based markers of [11C] PiB-PET brain amyloid burden. PLoS One. 2012;7(9):e44260. PubMed.
No Available Further Reading
- McAlpine CS, Park J, Griciuc A, Kim E, Choi SH, Iwamoto Y, Kiss MG, Christie KA, Vinegoni C, Poller WC, Mindur JE, Chan CT, He S, Janssen H, Wong LP, Downey J, Singh S, Anzai A, Kahles F, Jorfi M, Feruglio PF, Sadreyev RI, Weissleder R, Kleinstiver BP, Nahrendorf M, Tanzi RE, Swirski FK. Astrocytic interleukin-3 programs microglia and limits Alzheimer's disease. Nature. 2021 Jul;595(7869):701-706. Epub 2021 Jul 14 PubMed.