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Page 198 Walker. Neuroimmunol Neuroinflammation 2020;7:194-214 I http://dx.doi.org/10.20517/2347-8659.2020.09

Table 1. List of key genes described associated with disease-associated microglia
Gene Gene Gene
Homeostatic Microglia Stage 1 DAM Stage 2 DAM
a b c
Hexb CD33 Trem2
a b c
Cst3 Cx3cr1 Ankh
a b c
Cx3cr1 P2ry12 Cd63
Ctsd b P2ry13 c Cd9
Csf1r b Tgfbr1 c Serpine2
Ctss b Txnip c Ctsz
Sparc b Glu1 c Cd68
Tmsb4x b Tmem119 c Cadm1
Tmem119 c Tyrobp c Spp1
P2ry12 c Ctsb c Cd52
C1qa c Cstb c Ctsa
C1qb c Ctsd c Clec7a
c Apoe c Axl
c B2m c Cts1
c Fth1 c Lpl
c Timp2 c Ccl6
c H2-d1 c Csf1
c Lyz2 c Hif1a
c Cusb
c Itgax
a
b
c
See text for gene identification. Core homeostatic Genes; Down regulated genes; Upregulated genes. DAM: disease-associated
microglia
Cst3 remained unchanged between the different classes of microglia. Transition to stage 2 DAM involved
upregulation of Trem2, Ankh (Progressive ankylosis protein), Cd9 (Tetraspanin), Cd63 (Tetraspanin-30),
Serpine2 (Serine Protease inhibitor-2), Ctsz, Cd68 (macrosialin), Cadm1 (Cell-adhesion molecule-1), Spp1
(Secreted phosphoprotein-1), Cd52 (CD52), Ctsa, Clec7a (C-type lectin domain family 7 member A),
Axl (AXL receptor tyrosine kinase), Ctsl, Lpl (Lipoprotein lipase), Ccl6 (C-C chemokine 6), Csf1 (Colony
stimulating factor-1), Hif1a (Hypoxia inducible factor-1 alpha), Cusb (Cation efflux system protein
CusB), Itgax (Integrin Subunit Alpha X) and Cst7 and downregulation of Cx3cr1, P2ry12, CD33 and
TMEM119 [32,33] . This study showed that Trem2 expression levels increased progressively upon activation
from homeostatic to stage 1 and stage 2 DAM. This study chose Lpl (lipoprotein lipase), Timp2 (Tissue
inhibitor of metalloprotease-2) and Itgax (CD11c) for antibody validation in human AD brain sections
and showed strong expression in AD plaque-associated microglia. Lipoprotein lipase functions as a
homodimer, and has the dual functions of triglyceride hydrolase and ligand/bridging factor for receptor-
mediated lipoprotein uptake. Its expression is not specific for microglia/macrophage cells. Other stage 2
DAM markers that have been studied in human AD brains include TREM2 (see separate section below),
CD68 and ITGAX (CD11c). There has been controversy about the significance of CD68 expression by
microglia. This is a monocyte specific lysosomal-associated membrane protein that becomes upregulated
with increased phagocytosis . It has been considered an activation marker, but this does raise the
[34]
question whether phagocytosis markers are genuine proinflammatory markers or whether increased
phagocytosis is a reparative response. CD11c (a complement C3b integrin receptor CR4) is a cell adhesion
and phagocytosis marker for dendritic cells. An earlier study had shown that CD11c was constitutively
expressed by microglia in brain with some upregulation in reactive microglia in AD brains [35,36] . It has been
considered an activation marker, but this does raise the Table 1. List of key genes associated with Disease-
Associated Microglia.

The identification of ApoE as a microglial activation marker does not concur with previous
immunohistochemistry results. Possession of the APOE allele e4 is the most significant risk factor for
developing sporadic AD [37,38] . Subjects homozygous for APOE e4 have up to a 7-fold greater risk of