Microglia may like to dine on amyloid plaques, but does the immunosuppressive environment in the brain quash their appetite? Apparently so. According to two recent studies, lifting the suppression unleashes the microglia’s craving, allowing them to feast on plaques, but without sampling neurons. The studies—one led by Todd Golde at the University of Florida in Gainesville and the other by Terrence Town at the University of Southern California in Los Angeles—used complementary approaches to reveal that the anti-inflammatory cytokine IL-10 worsens Alzheimer’s disease pathology and cognitive decline in mouse models. Removing the cytokine slowed disease pathology and kept synapses intact. Kevin Doty presented findings from the Town lab at “Neuroinflammation in Diseases of the Central Nervous System,” a Keystone symposium held January 25-30 in Taos, New Mexico. That work appeared in the February 4 Neuron, while Golde's was published in the January 7 issue of the same journal.
“It turns out that you can have your cake and eat it too,” Town told Alzforum. “Revving up microglia removes plaques without damaging neurons.”
Both labs had already shared preliminary results from their studies at conferences covered by Alzforum (see Mar 2013 and Dec 2013 conference coverage). In their published form, the studies also revealed that IL-10 boosts astroglial expression of ApoE, which the researchers reported decreases microglial appetite for Aβ in vitro. Other data presented at Keystone put ApoE as a major factor in promoting a pro-inflammatory microglial phenotype (see Part 3 of this series).
Neuroinflammation plays an important role in clearing the brain of debris, infectious pathogens, and sickly cells. However, if uncontrolled or chronic, it can also harm the brain, and this can occur during neurodegenerative disease (see Wyss-Coray and Mucke, 2002; Perry, 2010).
Neuroinflammation manifests as a complex yin-yang relationship between anti- and pro-inflammatory responses, and much previous work has focused on the latter. Some have reported that shutting down pro-inflammatory cytokines lessens AD symptoms (see Kiyota et al., 2010; Jun 2008 news; and Nov 2012 news). Other labs, including Golde’s, have reported that overexpressing pro-inflammatory cytokines leads to plaque clearance (see Boissonneault et al., 2009; Chakrabarty et al., 2010; Chakrabarty et al., 2010; Chakrabarty et al., 2011).
The quieter role of the yin—the anti-inflammatory cytokines—had been less explored. Two major anti-inflammatory cytokines, TGF-β and IL-10, are known to shut down immune responses before they spin out of control. In 2008, Town generated AD mice lacking TGF-β activity in macrophages and microglia. He reported that mice lacking this wet-blanket cytokine had fewer plaques and less cognitive decline than mice with TGF-β. Importantly, this occurred without damage to neurons (see Jun 2008 news). The result encouraged Town to go after IL-10.
In Town’s study, first author Marie-Victoire Guillot-Sestier and colleagues crossed IL-10-deficient mice with APP/PS1 mice. They found that year-old APP/PS1 mice harbored far fewer plaques when they lacked the cytokine. In biochemical analysis, soluble and insoluble Aβ40 and Aβ42 plummeted by more than half. In brain slices, activated microglia crowded around any remaining plaques, which appeared more diffuse than in animals with the cytokine. These cells expressed low levels of CD45, suggesting that they were resident microglia rather than infiltrating macrophages from outside the brain (see Part 2, Part 3, and Part 4 for more on the nature of immune cells in the AD brain). Immunostaining also revealed that these cells contained Aβ within phagolysosomes. When put into a dish, microglia harvested from IL-10-deficient mice more readily gobbled up aggregated Aβ42 than normal microglia did, and they sported more markers of activation.
Importantly, despite their penchant for destroying plaques, microglia in IL-10-deficient mice left neurons alone. In fact, these mice had wild-type levels of the synaptic marker synaptophysin, suggesting that synapses were preserved. This paid off in some cognitive benefits. The mice more readily recognized novel objects (a signature of episodic memory) than APP/PS1 mice, and they were less hyperactive. However, IL-10 deficiency did not rescue APP/PS1 mice from deficits in spatial working memory.
To take stock of the changes wrought in the IL-10 deficient mice, the researchers compared their gene expression with that of APP/PS1 mice. They identified 117 differentially expressed genes, the majority involved in innate immune function, chemoattraction, Aβ interaction, or phagocytosis. Surprisingly, the researchers found that IL-10-deficient APP/PS1 mice made less ApoE.
Could a dearth of ApoE explain the microglia's taste for Aβ? ApoE influences clearance of different forms of Aβ, but it is not clear how (see Apr 2013 news). The researchers tested whether ApoE would affect microglial uptake of Aβ. ApoE2 barely altered uptake of Aβ aggregates, whereas ApoE3 and ApoE4 reduced it by about 70 percent. Importantly, IL-10-deficient microglia efficiently consumed Aβ when mixed with either ApoE2 or ApoE3, but not ApoE4 (see Alzforum Webinar).
In Taos, Gary Landreth of Case Western Reserve University in Chicago noted that it was unclear how ApoE was inhibiting Aβ uptake in this tissue culture model, and he questioned whether the experiments were modeling bona fide phagocytosis.
Golde’s lab took the reciprocal approach. Rather than knocking out IL-10, first author Paramita Chakrabarty and colleagues overexpressed the cytokine in the mouse brain. The researchers used a method they had previously pioneered to overexpress other cytokines. They injected adeno-associated virus 2/1 (AAV2/1) expressing IL-10 directly into the brain of neonatal CRND8 mice.
For the most part, the results of Golde’s study mirrored Town’s. Overexpression of IL-10 elevated plaque burden, damaged synapses, and accelerated cognitive decline. Microglia huddled around plaques but cleared little Aβ. A comparison of whole-brain gene-expression profiles revealed 140 genes differentially expressed between CRND8 mice with or without IL-10 overexpression, including chemokines, complement, and immune signaling genes. ApoE was upregulated 1.7-fold in CRND8 mice overexpressing IL-10.
As had Town’s group, Chakrabarty and colleagues decided to investigate the link between IL-10 and ApoE. The researchers found that ApoE protein levels rose nearly fourfold in the insoluble fraction of brain extracts of mice overexpressing IL-10, and immunofluorescence showed that ApoE was expressed by astrocytes and intermixed with plaques. To measure ApoE’s effects on Aβ uptake by microglia, the researchers incubated the cells in media taken from IL-10-deficient astrocyte cultures, which are chock-full of ApoE. When they fed the cells aggregated, fluorescent Aβ, they saw a reduction in microglial uptake compared to cells grown in normal astrocyte media.
Town said it was exceedingly rare that two papers agreed to this extent. “It’s really uncanny,” he said. Both Town and Golde propose a mechanism whereby microglia secrete IL-10, which then prevents them from taking up Aβ. In parallel, IL-10 boosts ApoE expression, which could reduce Aβ uptake even more.
Could blocking IL-10 work as a therapy for AD? Milan Fiala of the University of California, Los Angeles, found the results intriguing, but commented that blocking IL-10 in people would likely have inconsistent effects, as background levels of inflammatory and anti-inflammatory cytokines vary markedly throughout the population.
Richard Ransohoff of Biogen Idec in Cambridge, Massachusetts, agreed with Fiala, noting that blocking IL-10 could trigger harmful inflammatory effects. He added that results based on the manipulation of a single cytokine, such as IL-10, could be too simplistic to support therapeutic conclusions.—Jessica Shugart
- Blessing or Curse? Peripheral Cytokines in the Brain
- Innate Immune Cells Enlisted to Clear Amyloid, Fight Disease
- Microglia in Disease: Innocent Bystanders, or Agents of Destruction?
- Complement: AD Friend or Foe? New Work Tips Balance to Former
- Soothing Neuroinflammation Quells Plaques in Mice
- Macrophages Storm Blood-brain Barrier, Clear Plaques—or Do They?
- Nature Versus Nurture: What Gives Microglia Their Identity?
- United in Confusion: TREM2 Puzzles Researchers in Taos
- ApoE Does Not Bind Aβ, Competes for Clearance
Research Models Citations
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- Perry VH. Contribution of systemic inflammation to chronic neurodegeneration. Acta Neuropathol. 2010 Sep;120(3):277-86. PubMed.
- Kiyota T, Okuyama S, Swan RJ, Jacobsen MT, Gendelman HE, Ikezu T. CNS expression of anti-inflammatory cytokine interleukin-4 attenuates Alzheimer's disease-like pathogenesis in APP+PS1 bigenic mice. FASEB J. 2010 Aug;24(8):3093-102. PubMed.
- Boissonneault V, Filali M, Lessard M, Relton J, Wong G, Rivest S. Powerful beneficial effects of macrophage colony-stimulating factor on beta-amyloid deposition and cognitive impairment in Alzheimer's disease. Brain. 2009 Apr;132(Pt 4):1078-92. Epub 2009 Jan 17 PubMed.
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- Chakrabarty P, Ceballos-Diaz C, Beccard A, Janus C, Dickson D, Golde TE, Das P. IFN-gamma promotes complement expression and attenuates amyloid plaque deposition in amyloid beta precursor protein transgenic mice. J Immunol. 2010 May 1;184(9):5333-43. PubMed.
- Chakrabarty P, Herring A, Ceballos-Diaz C, Das P, Golde TE. Hippocampal expression of murine TNFα results in attenuation of amyloid deposition in vivo. Mol Neurodegener. 2011;6:16. PubMed.
- Chakrabarty P, Li A, Ceballos-Diaz C, Eddy JA, Funk CC, Moore B, DiNunno N, Rosario AM, Cruz PE, Verbeeck C, Sacino A, Nix S, Janus C, Price ND, Das P, Golde TE. IL-10 alters immunoproteostasis in APP mice, increasing plaque burden and worsening cognitive behavior. Neuron. 2015 Feb 4;85(3):519-33. Epub 2015 Jan 22 PubMed.
- Guillot-Sestier MV, Doty KR, Gate D, Rodriguez J Jr, Leung BP, Rezai-Zadeh K, Town T. Il10 deficiency rebalances innate immunity to mitigate Alzheimer-like pathology. Neuron. 2015 Feb 4;85(3):534-48. Epub 2015 Jan 22 PubMed.