Song WM, Joshita S, Zhou Y, Ulland TK, Gilfillan S, Colonna M. Humanized TREM2 mice reveal microglia-intrinsic and -extrinsic effects of R47H polymorphism. J Exp Med. 2018 Mar 5;215(3):745-760. Epub 2018 Jan 10 PubMed.

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  1. The data from the Colonna group’s human TREM2 BAC transgenic mice provide another important layer of insight into the role of TREM2 and TREM2 variants in Alzheimer’s disease. The decreased plaque-associated microgliosis in both 5xFAD-TREM2 KO and 5xFAD-R47H mice compared to the 5xFAD-CV is consistent with the hypothesis that the R47H variant impairs TREM2 function leading to a reduction in the microglial response to amyloid pathology. These data are also in agreement with observed decreases in plaque-associated microgliosis and activation in postmortem R47H variant AD tissue (Krasemann et al., 2017; Yuan et al., 2016). The observation that the R47H and CV exhibited similar levels of surface expression is also in agreement with previous studies that characterized the effect of TREM2 variants on the protein’s trafficking and maturation (Kleinberger et al., 2014). 

    Perhaps the most intriguing set of data in the paper relates to the detection of TREM2 extracellular domain on neuronal cells and amyloid plaques. Analysis of sTREM2 levels in the CSF of AD cohorts would suggest that sTREM2 elevations occur subsequent to amyloid deposition and seem to best correlate with CSF tau levels rather than Aβ (Piccio et al., 2016; Suárez-Calvet et al., 2016; Suárez‐Calvet et al., 2016). Does the apparent increase in sTREM2 staining in BAC transgenic mice, particularly that fraction co-localizing with neurons, relate to the elevations in CSF sTREM2 that correlate with markers of synaptic damage? Even more provocatively, does sTREM2 have a physiological function in regard to neuronal health? It is interesting that sTREM2 was not detected on dystrophic neurites, but rather neuronal soma. Conceivably the axons of sTREM2-positive neurons could exhibit dystrophy, but the question remains as to why the sTREM2 would seem to accumulate at the cell body.

    Overall the hTREM2 mice produced by the Colonna lab will be very valuable as an animal model to understand the biology of AD-associated variants, test potential TREM2-targeted therapies, and investigate the regulation, and possible function, of sTREM2. It will also be interesting to compare the effect of variants in human TREM2 to the numerous TREM2 variant knock-in mice (T66M, Y38C, R47H, etc.) that are being generated. 

    References:

    Kleinberger G, Yamanishi Y, Suárez-Calvet M, Czirr E, Lohmann E, Cuyvers E, Struyfs H, Pettkus N, Wenninger-Weinzierl A, Mazaheri F, Tahirovic S, Lleó A, Alcolea D, Fortea J, Willem M, Lammich S, Molinuevo JL, Sánchez-Valle R, Antonell A, Ramirez A, Heneka MT, Sleegers K, van der Zee J, Martin JJ, Engelborghs S, Demirtas-Tatlidede A, Zetterberg H, Van Broeckhoven C, Gurvit H, Wyss-Coray T, Hardy J, Colonna M, Haass C. TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis. Sci Transl Med. 2014 Jul 2;6(243):243ra86. PubMed.

    Krasemann S, Madore C, Cialic R, Baufeld C, Calcagno N, El Fatimy R, Beckers L, O'Loughlin E, Xu Y, Fanek Z, Greco DJ, Smith ST, Tweet G, Humulock Z, Zrzavy T, Conde-Sanroman P, Gacias M, Weng Z, Chen H, Tjon E, Mazaheri F, Hartmann K, Madi A, Ulrich JD, Glatzel M, Worthmann A, Heeren J, Budnik B, Lemere C, Ikezu T, Heppner FL, Litvak V, Holtzman DM, Lassmann H, Weiner HL, Ochando J, Haass C, Butovsky O. The TREM2-APOE Pathway Drives the Transcriptional Phenotype of Dysfunctional Microglia in Neurodegenerative Diseases. Immunity. 2017 Sep 19;47(3):566-581.e9. PubMed.

    Piccio L, Deming Y, Del-Águila JL, Ghezzi L, Holtzman DM, Fagan AM, Fenoglio C, Galimberti D, Borroni B, Cruchaga C. Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status. Acta Neuropathol. 2016 Jun;131(6):925-33. Epub 2016 Jan 11 PubMed.

    Suárez-Calvet M, Araque Caballero MÁ, Kleinberger G, Bateman RJ, Fagan AM, Morris JC, Levin J, Danek A, Ewers M, Haass C, Dominantly Inherited Alzheimer Network. Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury. Sci Transl Med. 2016 Dec 14;8(369):369ra178. PubMed.

    Suárez-Calvet M, Kleinberger G, Araque Caballero MÁ, Brendel M, Rominger A, Alcolea D, Fortea J, Lleó A, Blesa R, Gispert JD, Sánchez-Valle R, Antonell A, Rami L, Molinuevo JL, Brosseron F, Traschütz A, Heneka MT, Struyfs H, Engelborghs S, Sleegers K, Van Broeckhoven C, Zetterberg H, Nellgård B, Blennow K, Crispin A, Ewers M, Haass C. sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers. EMBO Mol Med. 2016 May 2;8(5):466-76. PubMed.

    Yuan P, Condello C, Keene CD, Wang Y, Bird TD, Paul SM, Luo W, Colonna M, Baddeley D, Grutzendler J. TREM2 Haplodeficiency in Mice and Humans Impairs the Microglia Barrier Function Leading to Decreased Amyloid Compaction and Severe Axonal Dystrophy. Neuron. 2016 May 18;90(4):724-39. PubMed.

    View all comments by Jason Ulrich

  2. This is a very important paper. Colonna and colleagues provide clear evidence that the most relevant AD-associated variant (TREM2 p.R47H) causes a partial loss of function. The authors show this by an elegant approach. They rescue TREM2 KO phenotypes by transgenic overexpression of human wt TREM2 or the AD-associated variant p.R47H. Whereas wt TREM2 rescued microgliosis and microglia activation in 5xFAD mice, the mutant showed reduced rescuing activity. Thus the picture becomes more and more clear: TREM2 activity must have a protective function and microglial activation may be beneficial, at least initially. This is fully in line with our recent findings on the p.T66M mutation (Kleinberger et al., 2017), which strongly reduces microglial clustering and activation in vivo. Of note, these findings have important implications for therapeutic strategies aiming to modulate TREM2-dependent microglial activity.

    I also think that this paper shows nicely that we should not overlook the function of shed sTREM2. Colonna and his colleagues show that it accumulates in plaques and neurons. Increased shedding of sTREM2 during plaque accumulation also fits nicely with our CSF data from human patients (Suarez-Calvet et al., 2016; Suarez-Calvet et al., 2016), which showed that sTREM2 increases early during disease development. Now we need to learn what sTREM2 is doing on the surface of neurons and within plaques. And maybe this will finally refocus AD researchers on the function of soluble APP, which is shed by the same proteases as TREM2, namely ADAM 10 and 17 (Kleinberger et al., 2014; Schlepckow et al., 2017; Thornton et al., 2017). 

    References:

    Kleinberger G, Brendel M, Mracsko E, Wefers B, Groeneweg L, Xiang X, Focke C, Deußing M, Suárez-Calvet M, Mazaheri F, Parhizkar S, Pettkus N, Wurst W, Feederle R, Bartenstein P, Mueggler T, Arzberger T, Knuesel I, Rominger A, Haass C. The FTD-like syndrome causing TREM2 T66M mutation impairs microglia function, brain perfusion, and glucose metabolism. EMBO J. 2017 Jul 3;36(13):1837-1853. Epub 2017 May 30 PubMed.

    Suárez-Calvet M, Kleinberger G, Araque Caballero MÁ, Brendel M, Rominger A, Alcolea D, Fortea J, Lleó A, Blesa R, Gispert JD, Sánchez-Valle R, Antonell A, Rami L, Molinuevo JL, Brosseron F, Traschütz A, Heneka MT, Struyfs H, Engelborghs S, Sleegers K, Van Broeckhoven C, Zetterberg H, Nellgård B, Blennow K, Crispin A, Ewers M, Haass C. sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers. EMBO Mol Med. 2016 May 2;8(5):466-76. PubMed.

    Suárez-Calvet M, Araque Caballero MÁ, Kleinberger G, Bateman RJ, Fagan AM, Morris JC, Levin J, Danek A, Ewers M, Haass C, Dominantly Inherited Alzheimer Network. Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury. Sci Transl Med. 2016 Dec 14;8(369):369ra178. PubMed.

    Kleinberger G, Yamanishi Y, Suárez-Calvet M, Czirr E, Lohmann E, Cuyvers E, Struyfs H, Pettkus N, Wenninger-Weinzierl A, Mazaheri F, Tahirovic S, Lleó A, Alcolea D, Fortea J, Willem M, Lammich S, Molinuevo JL, Sánchez-Valle R, Antonell A, Ramirez A, Heneka MT, Sleegers K, van der Zee J, Martin JJ, Engelborghs S, Demirtas-Tatlidede A, Zetterberg H, Van Broeckhoven C, Gurvit H, Wyss-Coray T, Hardy J, Colonna M, Haass C. TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis. Sci Transl Med. 2014 Jul 2;6(243):243ra86. PubMed.

    Schlepckow K, Kleinberger G, Fukumori A, Feederle R, Lichtenthaler SF, Steiner H, Haass C. An Alzheimer-associated TREM2 variant occurs at the ADAM cleavage site and affects shedding and phagocytic function. EMBO Mol Med. 2017 Oct;9(10):1356-1365. PubMed.

    Thornton P, Sevalle J, Deery MJ, Fraser G, Zhou Y, Ståhl S, Franssen EH, Dodd RB, Qamar S, Gomez Perez-Nievas B, Nicol LS, Eketjäll S, Revell J, Jones C, Billinton A, St George-Hyslop PH, Chessell I, Crowther DC. TREM2 shedding by cleavage at the H157-S158 bond is accelerated for the Alzheimer's disease-associated H157Y variant. EMBO Mol Med. 2017 Oct;9(10):1366-1378. PubMed.

    View all comments by Christian Haass

  3. This is the second paper that back-translates human TREM2 mutations into mice. The first one was the T66M mutation by the Haass group (Kleinberger et al., 2017). 

    Besides offering important basic insights, this pivotal study adds supporting evidence on the directionality for potential TREM2-related therapeutic interventions in favor of TREM2 enhancing therapies. It seems that both membrane bound WT-TREM2 as well as shed WT-sTREM2 contribute to the positive modulatory role in the 5xFAD amyloidosis model when the R47H and WT TREM2 variants are compared. This picture gains importance by the fact that the authors introduced human TREM2 WT and R47H variants into a TREM2-deficient mouse background followed by a cross-breeding to the 5xFAD amyloidosis model.

    Given the role of TREM2 on microglia activation, as established in the paper, it might emerge as a target that could be effective in AD at later stages, although additional work is needed to substantiate this idea.

    Further, the binding of sTREM2 (WT, not for the R47H variant) on neurons is new and unexpected and will certainly stimulate efforts to identify the binding site and reveal its function (e.g., conferring neuroprotection?).

    View all comments by Dominik Feuerbach

  4. The new study by Song et al. describes the generation and analysis of transgenic mice expressing the human common variant (CV) of TREM2 or the R47H variant that has been associated with Alzheimer’s disease (AD). 

    This study is interesting not only because it is the first demonstration of transgenic mice expressing human R47H TREM2, but also reveals functional effects of this variant in brain resident microglia in vivo, despite limited effects on amyloid deposition.

    The Colonna group generated BAC transgenic mice expressing either the human common variant (CV) or the AD-associated R47H variant of TREM2 on a TREM2 knockout background. Detailed expression analysis of two lines showed overall similar expression of CV or R47H mutant TREM2 protein at the surface of isolated macrophages, although mRNA levels were about 20 percent higher and gene copy number was fivefold higher in the brains of CV TREM2 transgenic mice as compared to R47H TREM2 transgenic mice.

    Interestingly, bone marrow-derived macrophages from R47H TREM2 mice showed decreased viability upon CSF1 withdrawal from the culture media as compared to that of CV TREM2 mice, suggesting decreased activity of the R47H TREM2 variant in the promotion of cell survival.

    To assess the role of human CV and R47H TREM2 during amyloid deposition, these mice were crossed with a transgenic (5XFAD) amyloidosis mouse model. The mice expressing human CV TREM2 had higher levels of microgliosis (higher number of microglia) around plaques as compared to R47H TREM2 transgenic mice. However, the plaque load was comparable in both CV and R47H TREM2 transgenic mice. The higher microgliosis around plagues might be related to increased viability and higher activation of microglia expressing CV TREM2 than that expressing the R47H variant. Indeed, authors show profound upregulation of several transcripts related to activation of microglia, including Cst/, Spp1, and Gpnmb in brains of CV TREM2 transgenic mice as compared to that expressing the R47H variant.

    Another interesting finding in this study is that soluble TREM2 appears to bind to plaques and to neurons. Here, binding of soluble CV TREM2 was higher as compared to R47H TREM2. It remains to be determined to which structures soluble TREM2 binds on neurons and plaques. As discussed in the paper, phospholipids and Aβ are potential candidates. The binding of soluble TREM2 to neurons might also underlie earlier findings of TREM2 reactivity on neurons in APP transgenic mice (Guerreiro et al., 2013), and it will be interesting to assess whether the binding of soluble TREM2 has a functional effect on neurons or on plaque dynamics. A previous paper showed positive effects of sTREM2 in microglial proliferation (Zhong et al., 2017). 

    In a last set of in vitro experiments with cultured macrophage-like cells, authors showed that LPS, TNFα, and IFNγ stimulates shedding of TREM2 by ADAM17, suggesting that ongoing inflammation increases release of soluble TREM2. Shedding of TREM2 by proteolytic processing has been consistently described previously in cultured cells, and soluble TREM2 variants are detected in human cerebrospinal fluid (Kleinberger et al., 2014; Wunderlich et al., 2013; Suárez-Calvet et al., 2016; Wood, 2017; Heslegrave et al., 2016). 

    It will be interesting to test whether increased shedding contributes to increased levels of sTREM2 observed during the pathogenesis of AD (Henjum et al., 2016; Heslegrave et al., 2016; Piccio et al., 2016; Suárez-Calvet et al., 2016). However, the shedding of TREM2 was similar for the CV and R47H form. These data support some earlier findings that the R47H has more subtle if any effect on shedding as compared to other variants associated with Nasu-Hakola disease or frontotemporal dementia (Feuerbach et al., 2017; Kleinberger et al., 2014). 

    The combined findings of the paper strongly support a partial loss of function of the R47H variant in several aspects, including decreased binding to certain ligands and impaired activation of microglia. Thus, it will be interesting to further investigate how TREM2 variants could contribute to the pathogenesis of AD.

    References:

    Guerreiro R, Wojtas A, Bras J, Carrasquillo M, Rogaeva E, Majounie E, Cruchaga C, Sassi C, Kauwe JS, Younkin S, Hazrati L, Collinge J, Pocock J, Lashley T, Williams J, Lambert JC, Amouyel P, Goate A, Rademakers R, Morgan K, Powell J, St George-Hyslop P, Singleton A, Hardy J, Alzheimer Genetic Analysis Group. TREM2 variants in Alzheimer's disease. N Engl J Med. 2013 Jan 10;368(2):117-27. Epub 2012 Nov 14 PubMed.

    Zhong L, Chen XF, Wang T, Wang Z, Liao C, Wang Z, Huang R, Wang D, Li X, Wu L, Jia L, Zheng H, Painter M, Atagi Y, Liu CC, Zhang YW, Fryer JD, Xu H, Bu G. Soluble TREM2 induces inflammatory responses and enhances microglial survival. J Exp Med. 2017 Mar 6;214(3):597-607. Epub 2017 Feb 16 PubMed.

    Kleinberger G, Yamanishi Y, Suárez-Calvet M, Czirr E, Lohmann E, Cuyvers E, Struyfs H, Pettkus N, Wenninger-Weinzierl A, Mazaheri F, Tahirovic S, Lleó A, Alcolea D, Fortea J, Willem M, Lammich S, Molinuevo JL, Sánchez-Valle R, Antonell A, Ramirez A, Heneka MT, Sleegers K, van der Zee J, Martin JJ, Engelborghs S, Demirtas-Tatlidede A, Zetterberg H, Van Broeckhoven C, Gurvit H, Wyss-Coray T, Hardy J, Colonna M, Haass C. TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis. Sci Transl Med. 2014 Jul 2;6(243):243ra86. PubMed.

    Wunderlich P, Glebov K, Kemmerling N, Tien NT, Neumann H, Walter J. Sequential proteolytic processing of the triggering receptor expressed on myeloid cells-2 (TREM2) by ectodomain shedding and γ-secretase dependent intramembranous cleavage. J Biol Chem. 2013 Nov 15;288(46):33027-36. PubMed.

    Suárez-Calvet M, Araque Caballero MÁ, Kleinberger G, Bateman RJ, Fagan AM, Morris JC, Levin J, Danek A, Ewers M, Haass C, Dominantly Inherited Alzheimer Network. Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury. Sci Transl Med. 2016 Dec 14;8(369):369ra178. PubMed.

    Wood H. Alzheimer disease: Soluble TREM2 in CSF sheds light on microglial activation in AD. Nat Rev Neurol. 2017 Feb;13(2):65. Epub 2017 Jan 13 PubMed.

    Heslegrave A, Heywood W, Paterson R, Magdalinou N, Svensson J, Johansson P, Öhrfelt A, Blennow K, Hardy J, Schott J, Mills K, Zetterberg H. Increased cerebrospinal fluid soluble TREM2 concentration in Alzheimer's disease. Mol Neurodegener. 2016 Jan 12;11:3. PubMed.

    Henjum K, Almdahl IS, Årskog V, Minthon L, Hansson O, Fladby T, Nilsson LN. Cerebrospinal fluid soluble TREM2 in aging and Alzheimer's disease. Alzheimers Res Ther. 2016 Apr 27;8(1):17. PubMed.

    Piccio L, Deming Y, Del-Águila JL, Ghezzi L, Holtzman DM, Fagan AM, Fenoglio C, Galimberti D, Borroni B, Cruchaga C. Cerebrospinal fluid soluble TREM2 is higher in Alzheimer disease and associated with mutation status. Acta Neuropathol. 2016 Jun;131(6):925-33. Epub 2016 Jan 11 PubMed.

    Suárez-Calvet M, Kleinberger G, Araque Caballero MÁ, Brendel M, Rominger A, Alcolea D, Fortea J, Lleó A, Blesa R, Gispert JD, Sánchez-Valle R, Antonell A, Rami L, Molinuevo JL, Brosseron F, Traschütz A, Heneka MT, Struyfs H, Engelborghs S, Sleegers K, Van Broeckhoven C, Zetterberg H, Nellgård B, Blennow K, Crispin A, Ewers M, Haass C. sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers. EMBO Mol Med. 2016 May 2;8(5):466-76. PubMed.

    Feuerbach D, Schindler P, Barske C, Joller S, Beng-Louka E, Worringer KA, Kommineni S, Kaykas A, Ho DJ, Ye C, Welzenbach K, Elain G, Klein L, Brzak I, Mir AK, Farady CJ, Aichholz R, Popp S, George N, Neumann U. ADAM17 is the main sheddase for the generation of human triggering receptor expressed in myeloid cells (hTREM2) ectodomain and cleaves TREM2 after Histidine 157. Neurosci Lett. 2017 Nov 1;660:109-114. Epub 2017 Sep 18 PubMed.

    View all comments by Jochen Walter

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This paper appears in the following:

News

  1. New Mouse Models Reveal Unexpected Property of TREM2
  2. In Mice, Activating TREM2 Tempers Plaque Toxicity, not Load

Research Models

  1. TREM2, humanized (common variant) X 5XFAD
  2. TREM2, humanized (R47H) X 5XFAD
  3. TREM2, humanized (common variant)
  4. TREM2, humanized (R47H)

Mutations

  1. TREM2 R47H