Ever since rare variants in the microglial receptor TREM2 were identified as a major risk factor for Alzheimer’s, researchers have been trying to figure out how the protein fits into the pathogenesis of this long, slow disease. Two recent papers added an important clue. One, in the January 12 Acta Neuropathologica, comes from researchers led by Laura Piccio and Carlos Cruchaga at Washington University, St. Louis; the other, in the January 11 Molecular Neurodegeneration, from groups led by Amanda Heslegrave at University College London and Henrik Zetterberg at the University of Gothenburg, Mölndal, Sweden. Both papers reported higher levels of soluble TREM2 (sTREM2) in cerebrospinal fluid (CSF) in a combined 134 patients with mild AD compared with healthy controls. Importantly, sTREM2 correlated with CSF tau, hinting the protein might track neurodegeneration.
The findings appeared to contradict an earlier report of lower CSF sTREM2 in AD patients. Researchers speculated this might be due to different stages of disease, as some emerging findings now suggest that sTREM2 levels may peak early and then fall as disease progresses, as tau has been found to do. In addition, the WashU group analyzed 40 people who carried TREM2 variants associated with disease. Different variants had different effects on soluble TREM2, hinting at distinct disease mechanisms.
“These are important papers, and the data are believable,” said Monica Carson at the University of California, Riverside, who was not involved in the research. Figuring out how these data fit together may help researchers determine how TREM2 contributes to disease, she added.
Variants in TREM2 have been linked to AD, frontotemporal dementia, and ALS (see Nov 2012 news; Oct 2012 news; Feb 2014 news). TREM2 is believed to activate microglia, and can be cleaved to generate a soluble fragment of unknown function. Because the initial commercial ELISAs for sTREM2 were unreliable, few studies have attempted to measure sTREM2 in the CSF of Alzheimer’s patients. In the only previous paper on the subject, researchers led by Christian Haass at the German Center for Neurodegenerative Diseases in Munich reported that average CSF sTREM2 levels were lower in 56 patients with AD dementia than in 88 healthy controls, although values overlapped extensively between the groups. Haass and colleagues used an in-house ELISA to detect sTREM2 (see Kleinberger et al., 2014).
To replicate these findings, the WashU researchers analyzed CSF from 73 Alzheimer’s patients seen at the Knight Alzheimer’s Disease Research Center in St. Louis, along with samples from 107 healthy controls, using their own in-house ELISA. All participants were genotyped and confirmed to have wild-type TREM2. All but five of the AD patients had CDRs of 0.5 or 1, corresponding to prodromal or mild AD, respectively. In this cohort, sTREM2 levels averaged about 25 percent higher in AD patients than controls, although individual measurements again largely overlapped between the groups.
The London group took a different approach. Co-first authors Heslegrave and Wendy Heywood, along with Kevin Mills at UCL, developed a mass-spectrometry method to measure CSF sTREM2. They found that average sTREM2 levels ran about 18 percent higher in a cohort of 37 AD patients than in 22 controls. These patients had mild to moderate AD, with an average MMSE score of 22. The researchers replicated the findings in an independent cohort of 24 AD patients and 16 controls. None of the participants were genotyped, but as TREM2 variants are rare in the general population, most of them would be expected to have wild-type TREM2. Some of these data were previously presented at the 2015 Alzheimer’s Association International Conference in Washington, D.C. (see Aug 2015 conference news).
What explains these discrepant reports of lower or higher CSF sTREM2 in AD? Haass believes the levels depend on the stage of Alzheimer’s. His first paper did not include staging information or neuropsychological test data to estimate how advanced the dementia was in the initial cohort, but a new study does. Haass just completed a larger, multicenter cross-sectional study that compared 150 controls to 63 people with preclinical AD, 111 with prodromal AD, and 200 with AD dementia. It found that CSF sTREM2 levels peaked in the prodromal stage, then fell back toward control levels in those with dementia. As in the other studies, individual values overlapped greatly. The data are submitted for publication, led by first author Marc Suárez-Calvet.
In addition, Haass is wrapping up a cross-sectional study measuring sTREM2 in CSF from the Dominantly Inherited Alzheimer Network (DIAN). Suárez-Calvet told Alzforum that there, too, sTREM2 levels peak in people around their estimated age of symptom onset but trend lower at later stages. Haass envisions an inverted U-shaped curve of CSF sTREM2 values along the time axis of AD pathogenesis.
Other researchers agree that disease stage might be a factor. “Alzheimer’s is a dynamic disease; different things happen at different stages,” Cruchaga noted. Zetterberg wrote to Alzforum, “I think this idea of stage-specific changes in soluble TREM2 is very interesting, but we ourselves so far have no data to support it. It needs further investigation.” Cruchaga plans to analyze longitudinal CSF data from DIAN, ADNI, or the Adult Children Study at WashU to see how values change over time in individual participants.
What do soluble TREM2 levels mean? They could signal microglial activation and immune response, researchers suggested. “To me, it looks like a protective response to the initial brain injury,” Haass said. Carson noted that sTREM2 rises initially and later drops in many injury states, for example in mice during remission in demyelinating conditions such as experimental autoimmune encephalomyelitis (EAE). In people with multiple sclerosis—the neuroinflammatory disease that EAE models—CSF sTREM2 levels shoot up to threefold higher than in healthy controls (see Piccio et al., 2008; Öhrfelt et al., 2016). By comparison, the sTREM2 increase in early AD seen thus far is only about 25 percent.
Another possibility is that CSF sTREM2 indicates neurodegeneration. All three groups report that CSF sTREM2 correlates with CSF total tau and phosphorylated tau, which are considered markers of cell death. Ongoing brain inflammation may lead to neuron death and tau release, Haass suggested. Intriguingly, longitudinal studies suggest CSF tau also drops as disease progresses (see Mar 2014 news).
What sort of imprint do genetic TREM2 variants leave in the CSF? The WashU researchers examined 40 carriers. Several pathological variants, including T66M, Q33X, R136Q, and D87N, cause the bone disease Nasu-Hakola or a frontotemporal dementia-like syndrome when homozygous, and increase dementia risk when heterozygous. The variants R47H and R62H have been linked to a higher risk of AD, whereas the co-inherited variants T96K, L211P, and W191X associate with AD risk only in African populations. These variants produced widely varying effects on soluble TREM2. Five people who were heterozygous for Nasu-Hakola mutations had about half the normal level of CSF sTREM2, while 16 carriers of T96K/L211P/W191X had about 25 percent less sTREM2, and 10 carriers of R62H had normal levels. Finally, nine carriers of R47H had roughly double the normal levels of CSF sTREM2. Clinically, carriers in each group spanned the gamut from cognitively normal to demented, so if stage of disease is important, it could muddle this group comparison between the variants.
The findings emphasize that these disease variants act via different mechanisms, Cruchaga said. Haass and others have previously reported that Nasu-Hakola mutations prevent TREM2 from maturing and reaching the cell surface, consistent with the low CSF levels seen in these carriers (see Jul 2014 webinar). The R47H variant, on the other hand, seems to mature and be shed normally (see April 2015 conference news). It is unclear how this variant contributes to disease; suggestions range from faulty ligand binding to a dosage effect, where too much TREM2 might be as bad as too little. Knock-in mouse models of R47H will be required to investigate this, researchers said.
In search of clues to TREM2’s mechanism of action, the WashU group analyzed genotype data from cases and controls to find SNPs in other genes that associated with CSF TREM2 levels. The strongest signal came from MS4A, which encodes a receptor highly expressed in microglia. Possibly, this receptor may interact directly or indirectly with TREM2, the authors suggested. MS4A also associates with AD risk. “By studying the genetics of CSF TREM2, we may identify additional variants that increase the risk for AD,” Cruchaga said.
Researchers agreed that many questions remain about what TREM2 does and how it may influence disease. “One of the more consistent findings across studies is that there is a lot of variability in sTREM2 levels in AD patients. Does that relate to the clinical features or prognosis of the disease? Addressing this will be key,” said Suárez-Calvet.—Madolyn Bowman Rogers
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- Microglia—Who Are You Really? New Clues Emerge
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