The Screening And Applications Of TREM2 Peptides For Alzheimer’s Disease Therapy

TREM2(Triggering receptor expressed on myeloid 2)is a type I transmembrane receptor composed of 230 amino acids and mainly expresses in microglia and binds to DAP12 to activate downstream signaling in the central nervous system.Studies have reported that TREM2 played an important role in regulating various functions of microglia,including phagocytosis,inflammatory response,and migration.Furthermore,increased TREM2 expression was found to remodel microglia responsiveness and ameliorate pathological and behavioral deficits in a mouse model of Alzheimer’s Disease(AD).The extracellular segment of TREM2 is cleaved by metalloprotease ADAM10/17,releasing soluble TREM2(soluble TREM2,sTREM2)into the extracellular matrix.Studies have reported that AD patients with higher levels of sTREM2 in the cerebrospinal fluid had better prognosis.The above results suggest that increasing the level of TREM2 or sTREM2 may ameliorate AD pathology.Part Ⅰ Previous studies in our laboratory found that sTREM2 could alleviate Aβpathological and behavioral cognitive deficits in AD mice.The full-length sTREM2 protein contains 157 amino acids and contains complex glycosylation modifications,making it difficult to cross the blood-brain barrier due to the high molecular weight(Results not showing).Therefore,we performed a series of N-terminal and C-terminal truncations on the full-length sTREM2,with the hope to identify a minimal sTREM2 peptide that recapitulates the function of fulllength sTREM2.We found that both sTREM2 fragments 41-81 and 51-81 enhance cell viability and inflammatory responses in primary microglia.When administrated to the 5×FAD mice brain,the sTREM2 fragment 41-81,but not 51-81,increased the number of plaque-associated microglia and reduced the plaque deposition.Interestingly,the fragment 41-81 was more efficient than the physiological form of sTREM2 in ameliorating Aβ-related pathology.Part Ⅱ As an important risk gene for AD,TREM2 and its interaction proteins have been extensively studied in the AD field.Previous studies in our laboratory found that the extracellular domain of TREM2 physically interacts with complement component 1q(C1q).It has been reported that the complement pathway was abnormally activated under AD pathology,resulting in synaptic loss and neurological dysfunction.Therefore,we speculated that TREM2 might be involved in the maintenance of synaptic homeostasis under AD pathology by binding to C1q.Through in vitro experiments,we found that TREM2 significantly inhibits C1q-dependent activation of the classical complement pathway.To further explore how TREM2 binding to C1q affects AD pathology in vivo,we performed serial truncation on the extracellular domain of TREM2 and screened for small functional peptide that binds to C1q.We found that the TREM2 31-71 specifically binds to C1q,but not Aβ,thus excluding the role of this peptide in alleviating AD pathology by binding to Aβ.Our in vitro studies showed that TREM2 31-71 significantly inhibits the activation of the C1qdependent classical complement pathway.Interestingly,in different AD pathological models,exogenous administration of the TREM2 31-71 peptide significantly inhibited complement activity in the brain and decreased synaptic phagocytosis by microglia.Ultimately,the TREM2 31-71 peptide increased synaptic density in AD mice and ameliorated synaptic and cognitive impairment.The above study demonstrated for the first time that TREM2 participates in the regulation of complement activity and synaptic homeostasis by binding to the complement protein C1q.Furthermore,the TREM2 31-71 obtained by screening inhibits complement-mediated synaptic loss,thus holding the potential for AD therapeutic applications.Taken together,we sought to screen small molecular peptides of TREM2 and explore their capacity in reducing AD-related pathological phenotypes.Interestingly,we uncovered two peptides of TREM2 that ameliorate AD pathology by binding to two different AD pathological proteins,Aβ and C1q,respectively.The sTREM2 41-81 ameliorated Aβ-related pathology by binding to Aβ.The TREM2 31-71 inhibited complement-mediated synaptic loss in AD mouse models by binding to C1q.These studies further expand our understanding of the biological functions of TREM2 and might provide new strategies for TREM2-based AD therapy.