**4.2 TREM2 regulates APOE mediating AD risk**

Although there is no difference in the quantity of activated microglia and reactive astrocytes between APOE4 carriers and noncarriers in the postmortem neocortex [99], relative transcriptomic studies have shown the connection between APOE and glia. Human APOE is expressed in three allelic variants, APOE2, APOE3, and APOE4, which exhibit different receptor binding properties [100]. APOE upregulation has been proved to be TREM2-dependent [101]. To some extent, TREM2 and APOE may have some special links [102], and the lack of TREM2 leads to a decrease of APOE4, while APOE3 remains unchanged [103]. Microglial plaque coverage and TREM2 are the highest in APOE3 male mice while significantly low in both APOE4 genotype and female sex [104], implicating a possible mechanism of AD between sex and APOE genotype. A reduction in plaque-associated APOE is also found in the brains of AD patients [105]. In another research, APOE3 is shown to promote the proliferation of microglia to injected Aβ, contribute to the uptake of Aβ, and improve cognition related to Aβ in preclinical models of AD [106]. Moreover, APOE was proven to stimulate different signal transduction cascades, ApoE4 > ApoE3 > ApoE2, in proportion to their AD risk [107]. This suggests that neuronal pathways may be related to the pathogenesis of AD. Human TREM2 (hTREM2) was bind to APOJ and APOE that are ligands of TREM2 under normal circumstances [73]. However, this binding is reduced in diseases or TREM2 KO mice microglia, leading to the impaired uptake of Aβ [73]. TREM2 is also an attractive target for drug regulation, but needs to be cautious because it is an important upstream mediator of microglia activation and phenotypic changes [53, 98]. In addition, single-cell transcriptomic studies pointing at microglia have shown a fascinating TREM2

ligand gal-3 that is related to neurodegenerative diseases [108]. Increased gal-3 is found in AD patients and 5xFAD mice, while decreased gal-3 shows improved cognitive ability and attenuates immune responses related to the TREM2-DAP pathway [109]. Therefore, suppressing gal-3 in the AD process may be a potential target in treatment.

## **5. sTREM2**

sTREM2, a soluble form of TREM2, is derived from the non-proteolyticmediated secretion of some TREM2 isoforms or due to extracellular domain of TREM2 being cleaved by different sheddases [110]. Years before the onset of dementia symptoms, sTREM2 increased in cerebrospinal fluid (CSF) of people with AD biomarker characteristics [111–114]. Recently, it is found that in preclinical AD, CSF sTREM2 changes are dynamic. In the absence of tau deposition and neurodegeneration, sTREM2 is decreased with Aβ pathology [115]. Different mutants of sTREM2 showed differences in concentration in CSF [114]. sTREM2 has a protective effect on Aβ and AD, such as reducing amyloid plaque load and restoring spatial memory [116]. Similarly, in the absence of TREM2 [105], sTREM2 enhances microglial proliferation, migration, clustering around Aβ, and contributing to the uptake and degradation of Aβ [116]. sTREM2 administration can also stimulate the expression of inflammatory cytokines and induce morphological changes of microglia such as decreased cell process and increased cell body size, thus enhancing microglial survival [117]. In TREM2 KO mice, administration of sTREM2 also showed positive feedback, like rescuing apoptosis upon colonystimulating factor (GM-CSF) withdrawal, inducing the proliferation and cell viability of the primary microglia [118, 119], compared with WT mice [117]. These results indicate the tremendous therapeutic potential of sTREM2, but warn that pro-inflammatory activation in the brain may lead to negative functional outcomes. Under stress, sTREM2 can promote myeloid cell survival too in a manner dependent on PI3K/AKT [117, 120]. Consequently, sTREM2 can be a target for AD therapy. But it is shown less potent for sTREM2-R47H and sTREM2-R62H variants to suppress apoptosis in AD context [117]. Among the three polymorphic forms (APOE2, APOE3, and APOE4), APOE4 proved to be more related to AD [121] and high levels of sTREM2 are associated with the decrease of APOE4 [122] and slower rates of Aβ accumulation [123]. In conclusion, the changes of sTREM2 can also be a biological hallmark for AD.
