BackgroundTREM2 is a transmembrane receptor that is predominantly expressed by microglia in the central nervous system. Rare variants in the TREM2 gene increase the risk for late-onset Alzheimer’s disease (AD). Soluble TREM2 (sTREM2) resulting from shedding of the TREM2 ectodomain can be detected in the cerebrospinal fluid (CSF) and is a surrogate measure of TREM2-mediated microglia function. CSF sTREM2 has been previously reported to increase at different clinical stages of AD, however, alterations in relation to Amyloid β-peptide (Aβ) deposition or additional pathological processes in the amyloid cascade (such as tau pathology or neurodegeneration) remain unclear. In the current cross-sectional study, we employed the biomarker-based classification framework recently proposed by the NIA-AA consensus guidelines, in combination with clinical staging, in order to examine the CSF sTREM2 alterations at early asymptomatic and symptomatic stages of AD.MethodsA cross-sectional study of 1027 participants of the Alzheimer’s Disease Imaging Initiative (ADNI) cohort, including 43 subjects carrying TREM2 rare genetic variants, was conducted to measure CSF sTREM2 using a previously validated enzyme-linked immunosorbent assay (ELISA). ADNI participants were classified following the A/T/N framework, which we implemented based on the CSF levels of Aβ1-42 (A), phosphorylated tau (T) and total tau as a marker of neurodegeneration (N), at different clinical stages defined by the clinical dementia rating (CDR) score.ResultsCSF sTREM2 differed between TREM2 variants, whereas the p.R47H variant had higher CSF sTREM2, p.L211P had lower CSF sTREM2 than non-carriers. We found that CSF sTREM2 increased in early symptomatic stages of late-onset AD but, unexpectedly, we observed decreased CSF sTREM2 levels at the earliest asymptomatic phase when only abnormal Aβ pathology (A+) but no tau pathology or neurodegeneration (TN-), is present.ConclusionsAβ pathology (A) and tau pathology/neurodegeneration (TN) have differing associations with CSF sTREM2. While tau-related neurodegeneration is associated with an increase in CSF sTREM2, Aβ pathology in the absence of downstream tau-related neurodegeneration is associated with a decrease in CSF sTREM2.Electronic supplementary materialThe online version of this article (10.1186/s13024-018-0301-5) contains supplementary material, which is available to authorized users.

Heterozygous loss-of-function mutations in the progranulin (GRN) gene and the resulting reduction of GRN levels is a common genetic cause for frontotemporal lobar degeneration (FTLD) with accumulation of TAR DNA-binding protein (TDP)-43. Recently, it has been shown that a complete GRN deficiency due to a homozygous GRN loss-of-function mutation causes neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disorder. These findings suggest that lysosomal dysfunction may also contribute to some extent to FTLD. Indeed, Grn(-/-) mice recapitulate not only pathobiochemical features of GRN-associated FTLD-TDP (FTLD-TDP/GRN), but also those which are characteristic for NCL and lysosomal impairment. In Grn(-/-) mice the lysosomal proteins cathepsin D (CTSD), LAMP (lysosomal-associated membrane protein) 1 and the NCL storage components saposin D and subunit c of mitochondrial ATP synthase (SCMAS) were all found to be elevated. Moreover, these mice display increased levels of transmembrane protein (TMEM) 106B, a lysosomal protein known as a risk factor for FTLD-TDP pathology. In line with a potential pathological overlap of FTLD and NCL, Ctsd(-/-) mice, a model for NCL, show elevated levels of the FTLD-associated proteins GRN and TMEM106B. In addition, pathologically phosphorylated TDP-43 occurs in Ctsd(-/-) mice to a similar extent as in Grn(-/-) mice. Consistent with these findings, some NCL patients accumulate pathologically phosphorylated TDP-43 within their brains. Based on these observations, we searched for pathological marker proteins, which are characteristic for NCL or lysosomal impairment in brains of FTLD-TDP/GRN patients. Strikingly, saposin D, SCMAS as well as the lysosomal proteins CTSD and LAMP1/2 are all elevated in patients with FTLD-TDP/GRN. Thus, our findings suggest that lysosomal storage disorders and GRN-associated FTLD may share common features.

Background: TMEM106B, a major risk factor for FTLD, is a protein of unknown function and cellular properties. Results: TMEM106B is a glycosylated type 2 membrane protein that localizes to late endosomes/lysosomes. Conclusion: The cellular properties of TMEM106B suggest a function in protein turnover in endosomes/lysosomes. Significance: These findings provide the biochemical and cell biological basis for elucidating the pathological role of TMEM106B in FTLD.

Loss of function mutations in progranulin cause tau-negative frontotemporal lobar degeneration with ubiquitin-positive inclusions. A major protein component of these inclusions is TDP-43, which becomes hyperphosphorylated, ubiquitinated, and cleaved to generate C-terminal fragments, which apparently translocate from nuclei to the cytoplasm. Most progranulin mutations are nonsense mutations resulting in nonsensemediated mRNA decay and consequently reduced progranulin protein levels. However, some missense mutations are described that occur within the signal sequence and mature progranulin. We now demonstrate that a progranulin mutation located within the signal sequence (PGRN A9D) results in cytoplasmic missorting with extremely low expression. In contrast, two other progranulin mutations (PGRN P248L and R432C) are expressed as immature proteins but are inefficiently transported through and partially degraded within the secretory pathway, resulting in a significantly reduced secretion. Thus apparently all progranulin mutations cause reduced protein expression or secretion, although by different cellular mechanisms. To investigate a putative relationship between reduced expression of progranulin and TDP-43 relocalization and deposition, we down-regulated progranulin in human cell lines and in zebrafish. Upon reduction of progranulin, neither a major redistribution of TDP-43 nor proteolytic processing to disease-characterizing C-terminal fragments could be observed.Dementias are a major health problem in our aging society. The most frequent forms of dementia, namely Alzheimer disease, frontotemporal lobar degeneration (FTLD), 3 as well as dementia with Lewy bodies and related disorders are associated with selective neuronal cell loss. In these neurodegenerative disorders, proteins, which are normally soluble are known to misfold because of proteolytic processing and/or abnormal posttranslational modifications. Such insoluble amyloidogenic proteins are often deposited and may form reservoirs for neurotoxic oligomers (1). FTLD, which accounts approximately for 15% of all dementias, is characterized by two different types of cellular inclusions. About 40% of FTLD cases have tau-positive inclusions (2, 3). Genetic linkage led to the identification of more than 40 different mutations in the microtubule-associated protein tau gene locus on chromosome 17 (4). However, a number of familial FTLD cases failed to exhibit mutations in the tau gene, although strong linkage to chromosome 17 was observed (5). These cases were characterized by tau-and ␣-synuclein-negative, ubiquitin-positive cytoplasmic and intranuclear inclusions (3). The inclusions are observed in the frontotemporal cortex, the temporal neocortex, and the hippocampus and define the frontotemporal lobar degeneration with ubiquitin-immunoreactive inclusions (FTLD-U). FTLD-U is the most frequent neuropathological form of FTLD and presents with progressive social, behavioral symptoms, and language dysfunction. Patients may also develop typical symptoms of motoneuron...

Microglia adopt numerous fates with homeostatic microglia ( HM ) and a microglial neurodegenerative phenotype ( MG nD) representing two opposite ends. A number of variants in genes selectively expressed in microglia are associated with an increased risk for neurodegenerative diseases such as Alzheimer's disease ( AD ) and frontotemporal lobar degeneration ( FTLD ). Among these genes are progranulin ( GRN ) and the triggering receptor expressed on myeloid cells 2 ( TREM 2 ). Both cause neurodegeneration by mechanisms involving loss of function. We have now isolated microglia from Grn −/− mice and compared their transcriptomes to those of Trem2 −/− mice . Surprisingly, while loss of Trem2 enhances the expression of genes associated with a homeostatic state, microglia derived from Grn −/− mice showed a reciprocal activation of the MG nD molecular signature and suppression of gene characteristic for HM . The opposite mRNA expression profiles are associated with divergent functional phenotypes. Although loss of TREM 2 and progranulin resulted in opposite activation states and functional phenotypes of microglia, FDG (fluoro‐2‐deoxy‐ d ‐glucose)‐μ PET of brain revealed reduced glucose metabolism in both conditions, suggesting that opposite microglial phenotypes result in similar wide spread brain dysfunction.