By Jamshed Arslan Pharm.D.
Microglia are brain's macrophages. In Alzheimer's disease (AD), microglia clear up protein aggregates called amyloid beta plaques. The connection between immune activation and AD is unclear, but a major sensor for danger-signals, called NLRP3 inflammasome, is known to be activated in the brains of patients and transgenic mice (APP/PS1) that overproduce amyloid beta.1 Activated NLRP3 inflammasome leads to the release of pro-inflammatory cytokine (IL-1 beta) and ASC (apoptosis-associated speck-like protein containing a caspase-recruitment domain) speck into the intercellular space. ASC speck refers to a micrometer-sized, paranuclear, signaling complex between NLRP3 and adapter protein ASC. NLRP3 deficiency prevents APP/PS1 mice from microglial IL-1 beta production and amyloid beta related pathology. Nevertheless, how NLRP3 inflammasome and ASC speck may contribute to amyloid beta pathology remained unknown.
Now, a study headed by German institutes shows that inflammasomes trigger the release of ASC specks from microglia and these ASC specks then promote amyloid beta aggregation in AD.2
Genetic Strategies Validation. Immunohistochemistry: beta Amyloid Antibody (MOAB-2) [NBP2-13075] - IHC analysis of beta Amyloid on normal mouse brain (left) and 5xFAD mouse brain (right) using DAB with hematoxylin counterstain. The MOAB-2 antibody was used at 1:20 on normal mouse brain and at 1:400 on 5xFAD mouse brain.
Inducing inflammasomes triggers ASC specks
Flow cytometric analysis revealed that exposing mouse microglia to nigericin, an activator of NLRP3 inflammasomes, induces the formation and release of ASC specks in vitro. The postmortem hippocampal sections of patients and APP/PS1 transgenic mice showed ASC specks within microglia and in the extracellular space. As the APP/PS1 mice aged, their ASC expression and amyloid beta plaques were found to be enhanced. These findings imply that inflammasomes enhance microglia-derived ASC specks in AD (which is the major type of elderly dementia).
Microglia-derived ASC specks increase amyloid beta aggregation
To observe the association of ASC with amyloid beta, fractionation and immunoprecipitation experiments were performed on postmortem brain samples. Fractions of amyloid beta plaques, especially the plaque cores, contained ASC. Experiments with speck-inhibiting mutations in the PYD and CARD domains of ASC showed that only the PYD part is necessary for ASC-induced increase in amyloid beta aggregates.
To study the role of ASC in vivo, researchers injected brain extracts from the APP/PS1 mice into one hippocampus of 3-month old APP/PS1 mice. They injected brain homogenate from wild-type mice to the other (contralateral) hippocampus. As expected, immunohistochemistry and ELISA in the 5-month old mice showed that the side injected with APP/PS1 brain extract had about double the number of plaques as compared to the wild-type brain extract. Intriguingly, APP/PS1 brain extract did not increase plaques in the APP/PS1 mice lacking ASC. Likewise, brain lysates of APP/PS1 mice lacking ASC exhibited a reduced capacity to enhance amyloid beta pathology. This means that amyloid beta pathology is dependent on ASC.
Blocking ASC-specks reduces amyloid beta deposition
To confirm ASC's pathogenic role, amyloid beta were co-incubated with ASC specks and anti-ASC-speck antibody. Thioflavin-T fluorescence assay showed that the antibody dose-dependently prevented ASC-speck-induced amyloid beta aggregates. Likewise, both anti-ASC-speck antibody or ASC knockout were found to reduce the disease progression in APP/PS1 mice.
This research indicates how the body's immune response (microglia-induced activation of inflammasomes) can play a negative role in AD. At a time when anti-inflammatory drugs are failing as a therapeutic approach in AD due to non-specific effects, this study identifies inflammasomes and ASC specks as specific targets for reversing amyloid beta pathology.
Learn more with our Alzheimer’s poster
Jamshed Arslan, Pharm D.
University of Alabama at Birmingham, School of Medicine
Dr. Arslan studies cell signaling in mitochondrial defects in C. elegans
and transgenic mice.
- Heneka, M., et al. “NLRP3 is Activated in Alzheimer's Disease and Contributes to Pathology in APP/PS1 Mice.” Nature, vol. 493, no. 7434, 2013, pp. 674–678. doi: 10.1038/nature11729.
- Venegas, Carmen, et al. “Microglia-Derived ASC Specks Cross-Seed Amyloid Beta in Alzheimer's Disease.” Nature, vol. 552, no. 7685, 2017, pp. 355–361. doi:10.1038/nature25158