Vignesh Subramanian ’24
Alzheimer’s disease (AD) is a neurodegenerative disorder and form of dementia that causes progressive loss of memory, critical thinking skills, and behavioral capabilities. The disease is characterized by the extracellular aggregation of beta (β)-amyloid plaques. These buildups gradually interfere with proteostasis, the regulation of protein synthesis and degradation, and decrease levels of neurotransmitter signaling between neurons. However, the failure of microglia – specialized cells responsible for clearing away damaged neurons and pathogens via phagocytosis, the consumption of such detritus – to regulate proteotoxicity raises the question of why this line of immune defense leaves certain plaques intact. The two major types of such plaques – diffused and dense-core – are believed to spontaneously form upon the breakdown of amyloid precursor protein (APP), which ought to subject them to detection by microglia at surface receptors just as deceased cells are. To better understand the relationship between microglial activation and the improbable survival of such plaques, a study at the Salk Institute explored the cleanup mechanism’s interactions with the TAM receptors of engaged plaques.
Researchers subjected C57BL/6J WT mice of various Axl and Mer strains – providing for their respective TAM receptor tyrosine kinases (RTKs) – to antibody-based perfusion procedures and then stained their cortical and hippocampal plaques to quantify their densities. Immunohistochemical analyses of APP/PS1 and WT mice involving purification of brain immune single cells and stereotaxic injections prepared them for cell sequencing and sorting that engineered some to lack TAM receptors and others to upregulate them, as well as PCR, scRNA-seq, two-photon microscopy, and confocal imaging of fluorescent tracer dyes. Finally, surgical procedures readied the mouse brains for extraction of soluble Aβ42 (an APP breakdown product) for assay quantification, immunoblot analysis, and comparison of immunolabeled localizations to human postmortem AD brain sections.
Researchers demonstrated that TAM receptor signaling was significantly correlated with upregulation of the microglial transcriptomic response, and that activated microglia did in fact consume amyloid plaques upon engagement of TAM receptors with the assistance of expressed ligands (Axl, Mer, Gas6). More surprisingly, researchers discovered that conversely reduced levels of phagocytosis and microglia presence decreased the quantity of detectable dense-core plaques, which should have thrived; subsequent live imaging found that the fibril remnants of engulfed diffuse plaques were organized into dense-core structures. The active shielding of plaque formation from acidic environments friendly to lysosomal activity ultimately suggests it may be incorporated as part of the intercellular cleaning process.
 Y. Huang, et al., Microglia use TAM receptors to detect and engulf amyloid β plaques. Nature Immunology (2021). DOI: 10.1038/s41590-021-00913-5