Novel TREM2 splicing isoform that lacks the V-set immunoglobulin domain is abundant in the human brain.

Triggering receptor expressed on myeloid cells 2 (TREM2) is an immunoglobulin-like receptor expressed by certain myeloid cells, such as macrophages, dendritic cells, osteoclasts, and microglia. In the brain, TREM2 plays an important role in the immune function of microglia, and its dysfunction is linked to various neurodegenerative conditions in humans. Ablation of TREM2 or its adaptor protein TYROBP causes polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (also known as Nasu-Hakola disorder) with early onset of dementia, whereas some missense variants in TREM2 are associated with an increased risk of late-onset Alzheimer's disease. The human TREM2 gene is subject to alternative splicing, and its major, full-length canonic transcript encompasses 5 exons. Herein, we report a novel alternatively spliced TREM2 isoform without exon 2 (Δe2), which constitutes a sizable fraction of TREM2 transcripts and has highly variable inter-individual expression in the human brain (average frequency 10%; range 3.7-35%). The protein encoded by Δe2 lacks a V-set immunoglobulin domain from its extracellular part but retains its transmembrane and cytoplasmic domains. We demonstrated Δe2 protein expression in TREM2-positive THP-1 cells, in which the expression of full-length transcript was precluded by CRISPR/Cas9 disruption of the exon 2 coding frame. Similar to the full-length TREM2, Δe2 is sorted to the plasma membrane and is subject to receptor shedding. In "add-back" experiments, Δe2 TREM2 had diminished capacity to restore phagocytosis of amyloid beta peptide and promote IFN-I response as compared to full-length TREM2. Our findings suggest that changes in the balance of two mutually exclusive TREM2 isoforms may modify the dosage of full-length transcript potentially weakening some TREM2 receptor functions in the human brain.

R47H Variant of TREM2 Associated With Alzheimer Disease in a Large Late-Onset Family: Clinical, Genetic, and Neuropathological Study.

IMPORTANCE: The R47H variant in the triggering receptor expressed on myeloid cells 2 gene (TREM2), a modulator of the immune response of microglia, is a strong genetic risk factor for Alzheimer disease (AD) and possibly other neurodegenerative disorders. OBJECTIVE: To investigate a large family with late-onset AD (LOAD), in which R47H cosegregated with 75% of cases. DESIGN, SETTING, AND PARTICIPANTS: This study includes genetic and pathologic studies of families with LOAD from 1985 to 2014. A total of 131 families with LOAD (751 individuals) were included from the University of Washington Alzheimer Disease Research Center. To identify LOAD genes/risk factors in the LOAD123 family with 21 affected members and 12 autopsies, we sequenced 4 exomes. Candidate variants were tested for cosegregation with the disease. TREM2 R47H was genotyped in an additional 130 families with LOAD. We performed clinical and neuropathological assessments of patients with and without R47H and evaluated the variant's effect on brain pathology, cellular morphology, and expression of microglial markers. MAIN OUTCOMES AND MEASURES: We assessed the effect of TREM2 genotype on age at onset and disease duration. We compared Braak and Consortium to Establish a Registry for Alzheimer's Disease scores, presence of α-synuclein and TAR DNA-binding protein 43 aggregates, and additional vascular or Parkinson pathology in TREM2 R47H carriers vs noncarriers. Microglial activation was assessed by quantitative immunohistochemistry and morphometry. RESULTS: Twelve of 16 patients with AD in the LOAD123 family carried R47H. Eleven patients with dementia had apolipoprotein E 4 (ApoE4) and R47H genotypes. We also found a rare missense variant, D353N, in a nominated AD risk gene, unc-5 homolog C (UNC5C), in 5 affected individuals in the LOAD123 family. R47H carriers demonstrated a shortened disease duration (mean [SD], 6.7 [2.8] vs 11.1 [6.6] years; 2-tailed t test; P = .04) and more frequent α-synucleinopathy. The panmicroglial marker ionized calcium-binding adapter molecule 1 was decreased in all AD cases and the decrease was most pronounced in R47H carriers (mean [SD], in the hilus: 0.114 [0.13] for R47H_AD vs 0.574 [0.26] for control individuals; 2-tailed t test; P = .005 and vs 0.465 [0.32] for AD; P = .02; in frontal cortex gray matter: 0.006 [0.004] for R47H_AD vs 0.016 [0.01] for AD; P = .04 and vs 0.033 [0.013] for control individuals; P < .001). Major histocompatibility complex class II, a marker of microglial activation, was increased in all patients with AD (AD: 2.5, R47H_AD: 2.7, and control: 1.0; P < .01). CONCLUSIONS AND RELEVANCE: Our results demonstrate a complex genetic landscape of LOAD, even in a single pedigree with an apparent autosomal dominant pattern of inheritance. ApoE4, TREM2 R47H, and rare variants in other genes, such as UNC5C D353N, are likely responsible for the notable occurrence of AD in this family. Our findings support the role of the TREM2 receptor in microglial clearance of aggregation-prone proteins that is compromised in R47H carriers and may accelerate the course of disease.

Bloodstream form Trypanosoma brucei do not require mRPN1 for gRNA processing.

Mitochondrial RNA processing in the kinetoplastid parasite Trypanosoma brucei involves numerous specialized catalytic activities that are incompletely understood. The mitochondrial genome consists of maxicircles that primarily encode rRNAs and mRNAs, and minicircles that encode a diverse array of guide RNAs (gRNAs). RNA editing uses these gRNAs as templates to recode mRNAs by insertion and deletion of uridine (U) residues. While the multiprotein complex that catalyzes RNA editing has been extensively studied, other players involved in mitochondrial RNA processing have remained enigmatic. The proteins required for processing mitochondrial polycistronic transcripts into mature species was essentially unknown until an RNase III endonuclease, called mRPN1, was reported to be involved in gRNA processing in procyclic form parasites. In this work, we examine the role of mRPN1 in gRNA processing in bloodstream form parasites, and show that complete elimination of mRPN1 by gene knockout does not alter gRNA maturation. These results indicate that another enzyme must be involved in gRNA processing.

Mutational analysis of Trypanosoma brucei editosome proteins KREPB4 and KREPB5 reveals domains critical for function.

The transcriptome of kinetoplastid mitochondria undergoes extensive RNA editing that inserts and deletes uridine residues (U's) to produce mature mRNAs. The editosome is a multiprotein complex that provides endonuclease, TUTase, exonuclease, and ligase activities required for RNA editing. The editosome's KREPB4 and KREPB5 proteins are essential for editosome integrity and parasite viability and contain semi-conserved motifs corresponding to zinc finger, RNase III, and PUF domains, but to date no functional analysis of these domains has been reported. We show here that various point mutations to KREPB4 and KREPB5 identify essential domains, and suggest that these proteins do not themselves perform RNase III catalysis. The zinc finger of KREPB4 but not KREPB5 is essential for editosome integrity and parasite viability, and mutation of the RNase III signature motif in KREPB5 prevents integration into editosomes, which is lethal. Isolated TAP-tagged KREPB4 and KREPB5 complexes preferentially associate with components of the deletion subcomplex, providing additional insights into editosome architecture. A new alignment of editosome RNase III sequences from several kinetoplastid species implies that KREPB4 and KREPB5 lack catalytic activity and reveals that the PUF motif is present in the editing endonucleases KREN1, KREN2, and KREN3. The data presented here are consistent with the hypothesis that KREPB4 and KREPB5 form intermolecular heterodimers with the catalytically active editing endonucleases, which is unprecedented among known RNase III proteins.

Loss-of-Function Screen Reveals Novel Regulators Required for Drosophila Germline Stem Cell Self-Renewal.

The germline stem cells (GSCs) of Drosophila melanogaster ovary provide an excellent model system to study the molecular mechanisms of stem cell self-renewal. To reveal novel factors required for Drosophila female GSC maintenance and/or division, we performed a loss-of-function screen in GSCs by using a collection of P-element-induced alleles of essential genes. Mutations in genes of various functional groups were identified to cause defects in GSC self-renewal. Here we report that a group of mutations affecting various ubiquitin-conjugating enzymes cause significant GSCs loss, including Plenty of SH3s (POSH), Ubiquitin-conjugating enzyme 10 (UbcD10), and pineapple eye (pie). Ubiquitin-mediated protein degradation plays a variety of roles in the regulation of many developmental processes, including mediating stem cell division through degradation of cell cycle regulators. We demonstrated that pie, sharing highly conserved RING domains with human E3 ubiquitin ligase G2E3 that are critical for early embryonic development, is specifically required for GSC maintenance, possibly through regulation of bone morphogenetic protein signaling pathway. Despite the previously reported role in imaginal disc cell survival, pie loss-of-function induced GSC loss is not to the result of caspase-involved cell death. Further efforts are needed to elucidate the functions of ubiquitin ligases in GSC maintenance, which will ultimately contribute to a better understanding of how the ubiquitin-conjugating enzymes regulate stem cell biology in mammalian systems.