Triggering receptor expressed on myeloid cells 2 (TREM2) represents a surface receptor without its own intracellular signaling motif. It therefore depends on the adaptor protein 12 (DAP12) for the initiation of signaling cascades. TREM2 has been primarily found and investigated in myeloid cells, and thus far, TREM2 and DAP12 mutations have only been known to be involved in progressive presenile dementia in patients suffering from polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (Nasu-Hakola disease), a rare autosomal recessive disorder.

The reports from Jonsson et al. and Guerreiro et al. now convincingly suggest that rare, heterozygous variants of the TREM2 gene, which most likely result in a reduced function of this receptor, are pathogenetically involved in sporadic AD. In the human cerebral cortex, TREM2 expression has been described in microglia and, to a smaller extent, in neurons (Sessa et al., 2004). Importantly, TREM2 has been described to be expressed by non-activated microglia (Schmid et al., 2002), but also can be found at the border of...  Read more

Triggering receptor expressed on myeloid cells 2 (TREM2) represents a surface receptor without its own intracellular signaling motif. It therefore depends on the adaptor protein 12 (DAP12) for the initiation of signaling cascades. TREM2 has been primarily found and investigated in myeloid cells, and thus far, TREM2 and DAP12 mutations have only been known to be involved in progressive presenile dementia in patients suffering from polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (Nasu-Hakola disease), a rare autosomal recessive disorder.

The reports from Jonsson et al. and Guerreiro et al. now convincingly suggest that rare, heterozygous variants of the TREM2 gene, which most likely result in a reduced function of this receptor, are pathogenetically involved in sporadic AD. In the human cerebral cortex, TREM2 expression has been described in microglia and, to a smaller extent, in neurons (Sessa et al., 2004). Importantly, TREM2 has been described to be expressed by non-activated microglia (Schmid et al., 2002), but also can be found at the border of amyloid-β (Aβ) plaque deposits in APP transgenic mice (Frank et al., 2008), thus implicating TREM2 in the innate immune response to Aβ accumulation and deposition in the brain.

Innate immune activation and neurodegenerative pathways interact at multiple levels, and this mutual interaction may ultimately function as an important motor of neurodegeneration (for a review see, e.g., (Lucin and Wyss-Coray, 2009; Heneka and O’Banion, 2007). Given the pro-phagocytic and anti-inflammatory role of TREM2 (Takahashi et al., 2005), expression of TREM2 by Aβ plaque-associated microglia may be interpreted as an effort to support Aβ clearance and to limit the proinflammatory cytokine expression in response to microglia activation by Aβ itself.

Knockdown of TREM2 in microglia increased the gene transcription of nitric oxide (NO) synthase 2 (NOS2) and tumor necrosis factor α (TNF-α) (Takahashi et al., 2005). As NOS2-derived NO may directly enhance Aβ aggregation (Kummer et al., 2011), and acts in concert with TNF-α to suppress hippocampal long-term potentiation (Wang et al., 2004; Wang et al., 2005), loss-of-function mutations in TREM2 may promote AD neuroinflammation and thereby drive neuronal degeneration. Likewise, TREM2 expression has been positively correlated with the phagocytic clearance of Aβ in APP transgenic mice (Melchior et al., 2010). Since impaired Aβ clearance has been suggested as the underlying cause of sporadic AD (Mawuenyega et al., 2010), loss of phagocytic efficacy due to TREM2 mutations could represent a possible mechanism by which the described mutations increase the risk to develop AD.

Further support for TREM2 being an important mediator in neuroinflammation comes from multiple sclerosis (MS), where soluble TREM2 has been found to be increased in patients suffering from relapsing-remitting and primary progressive MS (Piccio et al., 2008). Intriguingly, inhibition of TREM2 function potentiated pathology in the related murine model of experimental autoimmune encephalomyelitis (Piccio et al., 2007).

References:
Sessa G, Podini P, Mariani M, Meroni A, Spreafico R, Sinigaglia F, Colonna M, Panina P, Meldolesi J. Distribution and signaling of TREM2/DAP12, the receptor system mutated in human polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy dementia. Eur J Neurosci. 2004 Nov;20(10):2617-28. Abstract

Schmid CD, Sautkulis LN, Danielson PE, Cooper J, Hasel KW, Hilbush BS, Sutcliffe JG, Carson MJ. Heterogeneous expression of the triggering receptor expressed on myeloid cells-2 on adult murine microglia. J Neurochem. 2002 Dec;83(6):1309-20. Abstract

Frank S, Burbach GJ, Bonin M, Walter M, Streit W, Bechmann I, Deller T. TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice. Glia. 2008 Oct;56(13):1438-47. Abstract

Lucin KM, Wyss-Coray T. Immune activation in brain aging and neurodegeneration: too much or too little? Neuron. 2009 Oct 15;64(1):110-22. Abstract

Heneka MT, O'Banion MK. Inflammatory processes in Alzheimer's disease. J Neuroimmunol. 2007 Mar;184(1-2):69-91. Abstract

Takahashi K, Rochford CD, Neumann H. Clearance of apoptotic neurons without inflammation by microglial triggering receptor expressed on myeloid cells-2. J Exp Med. 2005 Feb 21;201(4):647-57. Abstract

Kummer MP, Hermes M, Delekarte A, Hammerschmidt T, Kumar S, Terwel D, Walter J, Pape HC, König S, Roeber S, Jessen F, Klockgether T, Korte M, Heneka MT. Nitration of tyrosine 10 critically enhances amyloid β aggregation and plaque formation. Neuron. 2011 Sep 8;71(5):833-44. Abstract

Wang Q, Rowan MJ, Anwyl R. Beta-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide. J Neurosci. 2004 Jul 7;24(27):6049-56. Abstract

Wang Q, Wu J, Rowan MJ, Anwyl R. Beta-amyloid inhibition of long-term potentiation is mediated via tumor necrosis factor. Eur J Neurosci. 2005 Dec;22(11):2827-32. Abstract

Melchior B, Garcia AE, Hsiung BK, Lo KM, Doose JM, Thrash JC, Stalder AK, Staufenbiel M, Neumann H, Carson MJ. Dual induction of TREM2 and tolerance-related transcript, Tmem176b, in amyloid transgenic mice: implications for vaccine-based therapies for Alzheimer's disease. ASN Neuro. 2010;2(3):e00037. Abstract

Mawuenyega KG, Sigurdson W, Ovod V, Munsell L, Kasten T, Morris JC, Yarasheski KE, Bateman RJ. Decreased clearance of CNS beta-amyloid in Alzheimer's disease. Science. 2010 Dec 24;330(6012):1774. Abstract

Piccio L, Buonsanti C, Cella M, Tassi I, Schmidt RE, Fenoglio C, Rinker J, Naismith RT, Panina-Bordignon P, Passini N, Galimberti D, Scarpini E, Colonna M, Cross AH. Identification of soluble TREM-2 in the cerebrospinal fluid and its association with multiple sclerosis and CNS inflammation. Brain. 2008 Nov;131(Pt 11):3081-91. Abstract

Piccio L, Buonsanti C, Mariani M, Cella M, Gilfillan S, Cross AH, Colonna M, Panina-Bordignon P. Blockade of TREM-2 exacerbates experimental autoimmune encephalomyelitis. Eur J Immunol. 2007 May;37(5):1290-301. Abstract

View all comments by Michael T. Heneka

The present work can be viewed as key support for the neuroinflammation hypothesis of Alzheimer’s disease. In my mind, at least, it dovetails well with earlier work demonstrating genomewide association between another microglial proinflammatory gene, complement receptor 1, and Alzheimer’s disease (Lambert et al., 2009). Together, these and other studies mark a turning of the tide in how we fundamentally view the disease—specifically, as an inflammatory syndrome.

In regard to the present work, I find it interesting that TREM2 expression was increased in the TgCRND8 mouse model of cerebral amyloidosis. One logical follow-on question is whether interrupting TREM2 function will impact disease progression in rodent models. Given that TREM2 plays a role in mediating proinflammatory innate immune signaling by microglia, as Michael Heneka nicely pointed out, one could hypothesize that blunting neuroinflammation by knocking out TREM2 would mitigate AD-like pathology in transgenic mice. By corollary, this would suggest that blocking TREM2-mediated neuroinflammation, as opposed to...  Read more

The present work can be viewed as key support for the neuroinflammation hypothesis of Alzheimer’s disease. In my mind, at least, it dovetails well with earlier work demonstrating genomewide association between another microglial proinflammatory gene, complement receptor 1, and Alzheimer’s disease (Lambert et al., 2009). Together, these and other studies mark a turning of the tide in how we fundamentally view the disease—specifically, as an inflammatory syndrome.

In regard to the present work, I find it interesting that TREM2 expression was increased in the TgCRND8 mouse model of cerebral amyloidosis. One logical follow-on question is whether interrupting TREM2 function will impact disease progression in rodent models. Given that TREM2 plays a role in mediating proinflammatory innate immune signaling by microglia, as Michael Heneka nicely pointed out, one could hypothesize that blunting neuroinflammation by knocking out TREM2 would mitigate AD-like pathology in transgenic mice. By corollary, this would suggest that blocking TREM2-mediated neuroinflammation, as opposed to promoting it, would be the therapeutic approach here.

References:
Lambert JC, Heath S, Even G, Campion D, Sleegers K, Hiltunen M, Combarros O, Zelenika D, Bullido MJ, Tavernier B, Letenneur L, Bettens K, Berr C, Pasquier F, Fiévet N, Barberger-Gateau P, Engelborghs S, De Deyn P, Mateo I, Franck A, Helisalmi S, Porcellini E, Hanon O; European Alzheimer's Disease Initiative Investigators, de Pancorbo MM, Lendon C, Dufouil C, Jaillard C, Leveillard T, Alvarez V, Bosco P, Mancuso M, Panza F, Nacmias B, Bossù P, Piccardi P, Annoni G, Seripa D, Galimberti D, Hannequin D, Licastro F, Soininen H, Ritchie K, Blanché H, Dartigues JF, Tzourio C, Gut I, Van Broeckhoven C, Alpérovitch A, Lathrop M, Amouyel P. Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer's disease. Nat Genet. 2009 Oct;41(10):1094-9. Abstract

View all comments by Terrence Town

These studies provide further evidence that the genetic architecture of Alzheimer's disease does indeed include rare variants of intermediate penetrance, analogous to our work on rare variants in the GRN (Brouwers et al., 2008) and CLU genes (Bettens et al., 2012). Moreover, these NEJM papers demonstrate the strength of techniques such as whole-exome sequencing to detect rare variants implicated in Alzheimer's disease, although it should be noted that large numbers of samples are required before there is sufficient basis to draw first conclusions. Of note, there was no evidence of association of the TREM2 variant in several individual replication cohorts, with the variant only reaching significance when pooling results.

One of the caveats when combining exome or genome sequence data from several sites, with controls and patients coming from different centers/platforms, is population stratification. Population allele frequency estimates seem to differ among countries. Nevertheless, association was also observed in one single large cohort from Iceland.

Discovery of a rare...  Read more

These studies provide further evidence that the genetic architecture of Alzheimer's disease does indeed include rare variants of intermediate penetrance, analogous to our work on rare variants in the GRN (Brouwers et al., 2008) and CLU genes (Bettens et al., 2012). Moreover, these NEJM papers demonstrate the strength of techniques such as whole-exome sequencing to detect rare variants implicated in Alzheimer's disease, although it should be noted that large numbers of samples are required before there is sufficient basis to draw first conclusions. Of note, there was no evidence of association of the TREM2 variant in several individual replication cohorts, with the variant only reaching significance when pooling results.

One of the caveats when combining exome or genome sequence data from several sites, with controls and patients coming from different centers/platforms, is population stratification. Population allele frequency estimates seem to differ among countries. Nevertheless, association was also observed in one single large cohort from Iceland.

Discovery of a rare variant associated with AD, like the one described in TREM2, has a small epidemiological impact, but may have significant impact in understanding the pathomechanisms leading to AD.

References:
Brouwers N, Sleegers K, Engelborghs S, Maurer-Stroh S, Gijselinck I, van der Zee J, Pickut BA, Van den Broeck M, Mattheijssens M, Peeters K, Schymkowitz J, Rousseau F, Martin JJ, Cruts M, De Deyn PP, Van Broeckhoven C. Genetic variability in progranulin contributes to risk for clinically diagnosed Alzheimer disease. Neurology. 2008 Aug 26;71(9):656-64. Abstract

Bettens K, Brouwers N, Engelborghs S, Lambert JC, Rogaeva E, Vandenberghe R, Le Bastard N, Pasquier F, Vermeulen S, Van Dongen J, Mattheijssens M, Peeters K, Mayeux R, St George-Hyslop P, Amouyel P, De Deyn PP, Sleegers K, Van Broeckhoven C. Both common variations and rare non-synonymous substitutions and small insertion/deletions in CLU are associated with increased Alzheimer risk. Mol Neurodegener. 2012;7:3. Abstract

View all comments by Kristel Sleegers

With the increasing acceptance (1) that Alzheimer’s disease is a multifactorial disorder (2), it might be a mistake to interpret every new finding on Alzheimer’s disease through the amyloid lens (see Michael Heneka’s comment above). As Neumann and Daly pointed out in their NEJM commentary, amyloid plaques have not been reported in Nasu-Hakola patients despite a near-complete loss of function of TREM2 and progressive presenile inflammatory neurodegeneration. Thus, increased AD risk in TREM2 variants is just as likely to be independent of amyloid, and future investigations should be designed/interpreted accordingly.

Looking back, we could argue that a similar mistake was made when epidemiological data on NSAIDs’ protective effects against developing AD were interpreted in terms of NSAIDs' ability to inhibit γ-secretase cleavage (3). This resulted in subsequent confusing data on various NSAIDs and Aβ production, and perhaps muddied NSAIDs' potential as therapeutic drugs. Perhaps we need to ponder whether our inability to think outside the amyloid framework is inhibiting the...  Read more

With the increasing acceptance (1) that Alzheimer’s disease is a multifactorial disorder (2), it might be a mistake to interpret every new finding on Alzheimer’s disease through the amyloid lens (see Michael Heneka’s comment above). As Neumann and Daly pointed out in their NEJM commentary, amyloid plaques have not been reported in Nasu-Hakola patients despite a near-complete loss of function of TREM2 and progressive presenile inflammatory neurodegeneration. Thus, increased AD risk in TREM2 variants is just as likely to be independent of amyloid, and future investigations should be designed/interpreted accordingly.

Looking back, we could argue that a similar mistake was made when epidemiological data on NSAIDs’ protective effects against developing AD were interpreted in terms of NSAIDs' ability to inhibit γ-secretase cleavage (3). This resulted in subsequent confusing data on various NSAIDs and Aβ production, and perhaps muddied NSAIDs' potential as therapeutic drugs. Perhaps we need to ponder whether our inability to think outside the amyloid framework is inhibiting the field from making faster progress towards an effective therapeutic treatment.

References:
1. Storandt M, Head D, Fagan AM, Holtzman DM, Morris JC. Toward a multifactorial model of Alzheimer disease. Neurobiol Aging. 2012 Oct;33(10):2262-71. Abstract

2. Pimplikar SW. Reassessing the amyloid cascade hypothesis of Alzheimer's disease. Int J Biochem Cell Biol. 2009 Jun;41(6):1261-8. Abstract

3. Weggen S, Eriksen JL, Das P, Sagi SA, Wang R, Pietrzik CU, Findlay KA, Smith TE, Murphy MP, Bulter T, Kang DE, Marquez-Sterling N, Golde TE, Koo EH. A subset of NSAIDs lower amyloidogenic Abeta42 independently of cyclooxygenase activity. Nature. 2001 Nov 8;414(6860):212-6. Abstract

View all comments by Sanjay W. Pimplikar