A study published in the May issue of Neuron indicates that the brain microglia and TREM2 Microglia-Specific Gene may exert a protective role in Alzheimer´s disease (AD) by tightly wrapping around the early amyloid fibrils and plaques endorsing their compaction and insulation.
In AD, a striking feature of microglia is their universal clustering around amyloid-β (Aβ) deposits, one of the major pathological hallmarks of this condition. It has traditionally been thought that activated microglia remove Aβ deposits via phagocytosis.
However multiple recent lines of evidence indicate that microglia are ineffective at removing fibrillar Aβ in vivo. At the same time, given their ability to secrete cytokines and reactive oxygen species, microglia have the potential to be neurotoxic. Thus, it remains unknown whether certain aspects of microglial function play beneficial or detrimental roles that could be important for disease pathogenesis.
In Alzheimer´s diseasetwo hallmark protein aggregates, amyloid-β (Aβ) plaques and neurofibrillary tangles are the major players in neuroinflammation.
However, it appears that a new concept is emerging where microglia, the principal brain innate neuroimmune cells, play a dichotomous role in AD. Microglia can be protective, promoting phagocytosis and clearance of Aβ. But, with disease progression, microglia become dysfunctional, release neurotoxins, lose their ability to clear Aβ and produce pro-inflammatory cytokines that promote Aβ production and accumulation.
The TREM2 gene encodes Triggering Receptor Expressed on Myeloid cells 2 protein, which is a receptor expressed on innate immune cells. Of note, the TREM2 gene is specifically expressed by microglia in the central nervous system, and recent evidence indicates that several heterozygous variants of the TREM2 gene are associated with increased susceptibility to late-onset AD with an odds ratio close to that of ApoEε4 (cf. Hickman and El Khoury, Biochem Pharmacol. 2014, 88: 495).
In the Neuron study, Peng Yuan and colleagues from the Departments of Neurology and Neuroscience, Yale University, CT, USA, demonstrate that in AD-like mice and human AD tissue the microglia processes are in close contact with individual amyloid fibrils in early non-compact filamentous plaques.
The authors found that later, microglia form specialized protrusions rich in TREM2 that tightly envelop the surface of compact plaques. According to the authors, the process of tight envelopment of fibrils and early-stage plaques by the microglia processes contribute to amyloid fibril compaction.
Importantly, the authors demonstrate that in TREM2-haplodeficient mice and in humans with R47H TREM2 mutations, there was a significant reduction in amyloid plaque compaction that was associated with reduced microglia’s ability to envelop amyloid deposits.
The authors discuss that TREM2 deficiency disrupts this specialized barrier function and may thus constitute a previously unknown cellular mechanism linking TREM2 R47H variant with increased risk of dementia.
All this may indicate the complexity of the inflammatory process in Alzheimer´s disease, and opens the question or debate whether pro- or anti-inflammatory strategies should be applied in this condition.
Cover Image Credit: Triggering receptor expressed on myeloid cells 2 (TREM2) maintains microglial metabolism through the mammalian target of rapamycin (mTOR) pathway in Alzheimer’s disease (AD). TREM2 pairs with DAP12 through charge interactions in the transmembrane domain. Upon TREM2 ligand binding, DAP12 gets phosphorylated and recruits spleen tyrosine kinase (SYK), which initiates a cascade of signaling events, including phosphoinositide 3-kinase (PI3K) activation, which is composed of p85 and p110. The recruitment of p85 requires DAP10. One of the PI3K downstream targets AKT then activates mammalian target of rapamycin complex 1 (mTORC1) and mTORC2, which inhibits autophagy. These signaling events maintain microglia at high energy states so that in AD models, microglia are able to cluster around amyloid plaques. From: TREM2-Dependent Effects on Microglia in Alzheimer’s Disease, Front. Aging Neurosci., 09 July 2018, Open access, public domain.