Vps35‐dependent recycling of Trem2 regulates microglial function

Triggering receptor expressed on myeloid cells 2 (Trem2), an immune‐modulatory receptor, is preferentially expressed in microglia of central nervous system. Trem2 might be involved in the development of Alzheimer's disease (AD) through regulating the inflammatory responses and phagocytosis of microglia. However, the intracellular trafficking of Trem2 remains unclear. In this study, we showed that Trem2 in the plasma membrane underwent endocytosis and recycling. Trem2 is internalized in a clathrin‐dependent manner and then recycled back to the plasma membrane through vacuolar protein sorting 35 (Vps35), the key component of cargo recognition core of retromer complex, but not Rab11. When Vps35 is knocked down, Trem2 accumulated in the lysosomes but was not degraded. More importantly, Vps35 deficiency leads to excessive lipopolysaccharide (LPS)‐induced inducible nitric oxide synthase (iNOS) expression and IL‐6 production, which can be abolished by Trem2 overexpression. Furthermore, R47H Trem2, an AD‐associated mutant, failed to interact with Vps35 and became unstable compared with wild‐type Trem2. Our study suggests that Vps35/retromer is responsible for recycling of Trem2 in the regulation of microglial function such as proinflammatory responses, whereas R47H mutation impairs Trem2 trafficking, which might contribute to AD.      

More than just sugars: Conserved oligomeric Golgi complex deficiency causes glycosylation‐independent cellular defects

The conserved oligomeric Golgi (COG) complex controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle trafficking within the Golgi. Human COG defects lead to severe multisystemic diseases known as COG‐congenital disorders of glycosylation (COG‐CDG). To gain better understanding of COG‐CDGs, we compared COG knockout cells with cells deficient to 2 key enzymes, Alpha‐1,3‐mannosyl‐glycoprotein 2‐beta‐N‐acetylglucosaminyltransferase and uridine diphosphate‐glucose 4‐epimerase (GALE), which contribute to proper N‐ and O‐glycosylation. While all knockout cells share similar defects in glycosylation, these defects only account for a small fraction of observed COG knockout phenotypes. Glycosylation deficiencies were not associated with the fragmented Golgi, abnormal endolysosomes, defective sorting and secretion or delayed retrograde trafficking, indicating that these phenotypes are probably not due to hypoglycosylation, but to other specific interactions or roles of the COG complex. Importantly, these COG deficiency specific phenotypes were also apparent in COG7‐CDG patient fibroblasts, proving the human disease relevance of our CRISPR knockout findings. The knowledge gained from this study has important implications, both for understanding the physiological role of COG complex in Golgi homeostasis in eukaryotic cells, and for better understanding human diseases associated with COG/Golgi impairment.      

High‐Throughput Quantitation of Intracellular Trafficking and Organelle Disruption by Flow Cytometry

Current methods for the quantitation of membrane protein trafficking rely heavily on microscopy, which has limited quantitative capacity for analyses of cell populations and is cumbersome to perform. Here we describe a simple flow cytometry‐based method that circumvents these limitations. The method utilizes fluorescent pulse‐width measurements as a highly sensitive indicator to monitor the changes in intracellular distributions of a fluorescently labelled molecule in a cell. Pulse‐width analysis enabled us to discriminate cells with target proteins in different intracellular locations including Golgi, lyso‐endosomal network and the plasma membrane, as well as detecting morphological changes in organelles such as Golgi perturbation. The movement of endogenous and exogenous retrograde cargo was tracked from the plasma membrane‐to‐endosomes‐to‐Golgi, by decreasing pulse‐width values. A block in transport upon RNAi‐mediated ablation of transport machinery was readily quantified, demonstrating the versatility of this technique to identify pathway inhibitors. We also showed that pulse‐width can be exploited to sort and recover cells based on different intracellular staining patterns, e.g. early endosomes and Golgi, opening up novel downstream applications. Overall, the method provides new capabilities for viewing membrane transport in thousands of cells per minute, unbiased analysis of the trafficking of cargo, and the potential for rapid screening of inhibitors of trafficking pathways.      

Chondroitin Sulfate Accelerates Trans‐Golgi‐to‐Surface Transport of Proteoglycan Amyloid Precursor Protein

The amyloid precursor protein (APP) is a membrane protein implicated in the pathogenesis of Alzheimer's disease. APP is a part‐time proteoglycan, as splice variants lacking exon 15 are modified by a chondroitin sulfate glycosaminoglycan (GAG) chain. Investigating the effect of the GAG chain on the trafficking of APP in non‐polarized cells, we found it to increase the steady‐state surface‐to‐intracellular distribution, to reduce the rate of endocytosis and to accelerate transport kinetics from the trans‐Golgi network (TGN) to the plasma membrane. Deletion of the cytosolic domain resulted in delayed surface arrival of GAG‐free APP, but did not affect the rapid export kinetics of the proteoglycan form. Protein‐free GAG chains showed the same TGN‐to‐cell surface transport kinetics as proteoglycan APP. Endosome ablation experiments were performed to distinguish between indirect endosomal and direct pathways to the cell surface. Surprisingly, TGN‐to‐cell surface transport of both GAG‐free and proteoglycan APP was found to be indirect via transferrin‐positive endosomes. Our results show that GAGs act as alternative sorting determinants in cellular APP transport that are dominant over cytoplasmic signals and involve distinct sorting mechanisms.      

Ferlins Show Tissue‐Specific Expression and Segregate as Plasma Membrane/Late Endosomal or Trans‐Golgi/Recycling Ferlins

Ferlins are a family of transmembrane‐anchored vesicle fusion proteins uniquely characterized by 5–7 tandem cytoplasmic C2 domains, Ca²⁺‐regulated phospholipid‐binding domains that regulate vesicle fusion in the synaptotagmin family. In humans, dysferlin mutations cause limb‐girdle muscular dystrophy type 2B (LGMD2B) due to defective Ca²⁺‐dependent, vesicle‐mediated membrane repair and otoferlin mutations cause non‐syndromic deafness due to defective Ca²⁺‐triggered auditory neurotransmission. In this study, we describe the tissue‐specific expression, subcellular localization and endocytic trafficking of the ferlin family. Studies of endosomal transit together with 3D‐structured illumination microscopy reveals dysferlin and myoferlin are abundantly expressed at the PM and cycle to Rab7‐positive late endosomes, supporting potential roles in the late‐endosomal pathway. In contrast, Fer1L6 shows concentrated localization to a specific compartment of the trans‐Golgi/recycling endosome, cycling rapidly between this compartment and the PM via Rab11 recycling endosomes. Otoferlin also shows trans‐Golgi to PM cycling, with very low levels of PM otoferlin suggesting either brief PM residence, or rare incorporation of otoferlin molecules into the PM. Thus, type‐I and type‐II ferlins segregate as PM/late‐endosomal or trans‐Golgi/recycling ferlins, consistent with different ferlins mediating vesicle fusion events in specific subcellular locations.      

Golgi‐mediated Glycosylation Determines Residency of the T2 RNase Rny1p in Saccharomyces cerevisiae

The role of glycosylation in the function of the T2 family of RNases is not well understood. In this work, we examined how glycosylation affects the progression of the T2 RNase Rny1p through the secretory pathway in Saccharomyces cerevisiae. We found that Rny1p requires entering into the ER first to become active and uses the adaptor protein Erv29p for packaging into COPII vesicles and transport to the Golgi apparatus. While inside the ER, Rny1p undergoes initial N‐linked core glycosylation at four sites, N37, N70, N103 and N123. Rny1p transport to the Golgi results in the further attachment of high‐glycans. Whereas modifications with glycans are dispensable for the nucleolytic activity of Rny1p, Golgi‐mediated modifications are critical for its extracellular secretion. Failure of Golgi‐specific glycosylation appears to direct Rny1p to the vacuole as an alternative destination and/or site of terminal degradation. These data reveal a previously unknown function of Golgi glycosylation in a T2 RNase as a sorting and secretion signal .      

Small GTPase Determinants for the Golgi Processing and Plasmalemmal Expression of Human Ether-a-go-go Related (hERG) K+ Channels

The pro-arrhythmic Long QT syndrome (LQT) is linked to 10 different genes (LQT1–10). Approximately 40% of genotype-positive LQT patients have LQT2, which is characterized by mutations in the human ether-a-go-go related gene (hERG). hERG encodes the voltage-gated K⁺ channel α-subunits that form the pore of the rapidly activating delayed rectifier K⁺ current in the heart. The purpose of this study was to elucidate the mechanisms that regulate the intracellular transport or trafficking of hERG, because trafficking is impaired for about 90% of LQT2 missense mutations. Protein trafficking is regulated by small GTPases. To identify the small GTPases that are critical for hERG trafficking, we coexpressed hERG and dominant negative (DN) GTPase mutations in HEK293 cells. The GTPases Sar1 and ARF1 regulate the endoplasmic reticulum (ER) export of proteins in COPII and COPI vesicles, respectively. Expression of DN Sar1 inhibited the Golgi processing of hERG, decreased hERG current (I_hERG) by 85% (n ≥ 8 cells per group, *, p < 0.01), and reduced the plasmalemmal staining of hERG. The coexpression of DN ARF1 had relatively small effects on hERG trafficking. Surprisingly, the coexpression of DN Rab11B, which regulates the endosomal recycling, inhibited the Golgi processing of hERG, decreased I_hERG by 79% (n ≥ 8 cells per group; *, p < 0.01), and reduced the plasmalemmal staining of hERG. These data suggest that hERG undergoes ER export in COPII vesicles and endosomal recycling prior to being processed in the Golgi. We conclude that hERG trafficking involves a pathway between the ER and endosomal compartments that influences expression in the plasmalemma.The human KCNH2 or ether-a-go-go related gene (hERG)³ encodes the voltage-gated K⁺ channel α-subunits that oligomerize to form the pore of the rapidly activating delayed rectifier K⁺ current (I_Kr) in cardiac myocytes (1–3). Hundreds of hERG mutations are linked to the congenital pro-arrhythmic Type 2 Long QT syndrome (LQT2) and functional studies suggest that these mutations result in a loss of normal hERG K⁺ channel (hERG) function (4, 5). In LQT2, missense mutations are the dominant abnormality and many LQT2 missense mutations reduce hERG K⁺ current (I_hERG) by decreasing the intracellular transport or trafficking of hERG to the Golgi apparatus (Golgi) and the cell surface membrane (plasmalemma) (6). Therefore, disruption of hERG K⁺ channel trafficking appears to be a principal mechanism for disease.Movement of proteins between membrane-bound intracellular compartments is mediated by small transport vesicles, which bud from a donor compartment to fuse with an appropriate acceptor compartment. The trafficking of many transmembrane and secretory proteins between the ER and Golgi compartments is dependent on the small GTPases ADP-ribosylation factor 1 (ARF1) and Sar1, which regulate the formation of coat-associated protein complex I (COPI) and II (COPII) vesicles, respectively (7–19). These small GTPases facilitate the polymerization of transport vesicle protein coats on the donor membrane. Vesicular cargo selection, docking, and fusion to the target membrane are regulated by adaptor proteins, SNARE proteins, and Rab GTPases. To rationally develop novel therapeutic targets that may increase the expression of trafficking-deficient LQT2 mutant channels, the molecular mechanisms that regulate the trafficking of hERG need to be explored. The purpose of this study is to identify transport proteins that regulate the trafficking of wild type (WT) hERG. We used a strategy of testing specific WT GTPases or ones containing dominant negative (DN) mutations to interfere with their function.     

Common Membrane Trafficking Defects of Disease‐Associated Dynamin 2 Mutations

Dynamin (Dyn) is a multidomain and multifunctional GTPase best known for its essential role in clathrin‐mediated endocytosis (CME). Dyn2 mutations have been linked to two human diseases, centronuclear myopathy (CNM) and Charcot‐Marie‐Tooth (CMT) disease. Paradoxically, although Dyn2 is ubiquitously expressed and essential for embryonic development, the disease‐associated Dyn2 mutants are autosomal dominant, but result in slowly progressing and tissue‐specific diseases. Thus, although the cellular defects that cause disease remain unclear, they are expected to be mild. To gain new insight into potential pathogenic mechanisms, we utilized mouse Dyn2 conditional knockout cells combined with retroviral‐mediated reconstitution to mimic both heterozygous and homozygous states and characterized cellular phenotypes using quantitative assays for several membrane trafficking events. Surprisingly, none of the four mutants studied exhibited a defect in CME, but all were impaired in their ability to support p75/neurotrophin receptor export from the Golgi, the raft‐dependent endocytosis of cholera toxin and the clathrin‐independent endocytosis of epidermal growth factor receptor (EGFR). While it will be important to study these mutants in disease‐relevant muscle and neuronal cells, given the importance of neurotrophic factors and lipid rafts in muscle physiology, we speculate that these common cellular defects might contribute to the tissue‐specific diseases caused by a ubiquitously expressed protein.     

Dynamin‐2 in nervous system disorders

Dynamin‐2 is a pleiotropic GTPase whose best‐known function is related to membrane scission during vesicle budding from the plasma or Golgi membranes. In the nervous system, dynamin‐2 participates in synaptic vesicle recycling, post‐synaptic receptor internalization, neurosecretion, and neuronal process extension. Some of these functions are shared with the other two dynamin isoforms. However, the involvement of dynamin‐2 in neurological illnesses points to a critical function of this isoform in the nervous system. In this regard, mutations in the dynamin‐2 gene results in two congenital neuromuscular disorders. One of them, Charcot‐Marie‐Tooth disease, affects myelination and peripheral nerve conduction, whereas the other, Centronuclear Myopathy, is characterized by a progressive and generalized atrophy of skeletal muscles, yet it is also associated with abnormalities in the nervous system. Furthermore, single nucleotide polymorphisms located in the dynamin‐2 gene have been associated with sporadic Alzheimer's disease. In the present review, we discuss the pathogenic mechanisms implicated in these neurological disorders.

     

Abnormal Rab11‐Rab8‐vesicles cluster in enterocytes of patients with microvillus inclusion disease

Microvillus inclusion disease (MVID) is a congenital enteropathy characterized by accumulation of vesiculo‐tubular endomembranes in the subapical cytoplasm of enterocytes, historically termed “secretory granules.” However, neither their identity nor pathophysiological significance is well defined. Using immunoelectron microscopy and tomography, we studied biopsies from MVID patients (3× Myosin 5b mutations and 1× Syntaxin3 mutation) and compared them to controls and genome‐edited CaCo2 cell models, harboring relevant mutations. Duodenal biopsies from 2 patients with novel Myosin 5b mutations and typical clinical symptoms showed unusual ultrastructural phenotypes: aberrant subapical vesicles and tubules were prominent in the enterocytes, though other histological hallmarks of MVID were almost absent (ectopic intra‐/intercellular microvilli, brush border atrophy). We identified these enigmatic vesiculo‐tubular organelles as Rab11‐Rab8‐positive recycling compartments of altered size, shape and location harboring the apical SNARE Syntaxin3, apical transporters sodium‐hydrogen exchanger 3 (NHE3) and cystic fibrosis transmembrane conductance regulator. Our data strongly indicate that in MVID disrupted trafficking between cargo vesicles and the apical plasma membrane is the primary cause of a defect of epithelial polarity and subsequent facultative loss of brush border integrity, leading to malabsorption. Furthermore, they support the notion that mislocalization of transporters, such as NHE3 substantially contributes to the reported sodium loss diarrhea.      

TREM2 and β-catenin regulate bone homeostasis by controlling the rate of osteoclastogenesis

TREM2 is an immunoreceptor expressed on osteoclasts (OC) and microglia that transmits intracellular signals through the adaptor DAP12. Individuals with genetic mutations inactivating TREM2 or DAP12 develop the Nasu-Hakola disease (NHD) with cystic-like lesions of the bone and brain demyelination that lead to fractures and presenile dementia. The mechanisms of this disease are poorly understood. In this study, we report that TREM2-deficient mice have an osteopenic phenotype reminiscent of NHD. In vitro, lack of TREM2 impairs proliferation and β-catenin activation in osteoclast precursors (OcP) in response to M-CSF. This defect results in accelerated differentiation of OcP into mature OC. Corroborating the importance of a balanced proliferation and differentiation of OcP for bone homeostasis, we show that conditional deletion of β-catenin in OcP also results in reduced OcP proliferation and accelerated osteoclastogenesis in vitro as well as osteopenia in vivo. These results reveal that TREM2 regulates the rate of osteoclastogenesis and provide a mechanism for the bone pathology in NHD.     

Autosomal‐dominant Alzheimer's disease mutations at the same codon of amyloid precursor protein differentially alter Aβ production

Autosomal‐dominant Alzheimer's disease (ADAD) is a genetic disorder caused by mutations in Amyloid Precursor Protein (APP) or Presenilin (PSEN) genes. Studying the mechanisms underlying these mutations can provide insight into the pathways that lead to AD pathology. The majority of biochemical studies on APP mutations to‐date have focused on comparing mechanisms between mutations at different codons. It has been assumed that amino acid position is a major determinant of protein dysfunction and clinical phenotype. However, the differential effect of mutations at the same codon has not been sufficiently addressed. In the present study we compared the effects of the aggressive ADAD‐associated APP I716F mutation with I716V and I716T on APP processing in human neuroglioma and CHO‐K1 cells. All APP I716 mutations increased the ratio of Aβ42/40 and changed the product line preference of γ‐secretase towards Aβ38 production. In addition, the APP I716F mutation impaired the ε‐cleavage and the fourth cleavage of γ‐secretase and led to abnormal APP β‐CTF accumulation at the plasma membrane. Taken together, these data indicate that APP mutations at the same codon can induce diverse abnormalities in APP processing, some resembling PSEN1 mutations. These differential effects could explain the clinical differences observed among ADAD patients bearing different APP mutations at the same position.

     

Analysis of COPII Vesicles Indicates a Role for the Emp47–Ssp120 Complex in Transport of Cell Surface Glycoproteins

Coat protein complex II (COPII) vesicle formation at the endoplasmic reticulum (ER) transports nascent secretory proteins forward to the Golgi complex. To further define the machinery that packages secretory cargo and targets vesicles to Golgi membranes, we performed a comprehensive proteomic analysis of purified COPII vesicles. In addition to previously known proteins, we identified new vesicle proteins including Coy1, Sly41 and Ssp120, which were efficiently packaged into COPII vesicles for trafficking between the ER and Golgi compartments. Further characterization of the putative calcium‐binding Ssp120 protein revealed a tight association with Emp47 and in emp47Δ cells Ssp120 was mislocalized and secreted. Genetic analyses demonstrated that EMP47 and SSP120 display identical synthetic positive interactions with IRE1 and synthetic negative interactions with genes involved in cell wall assembly. Our findings support a model in which the Emp47–Ssp120 complex functions in transport of plasma membrane glycoproteins through the early secretory pathway.      

CSF levels of the neuronal injury biomarker visinin‐like protein‐1 in Alzheimer's disease and dementia with Lewy bodies

The overlapping clinical features of Alzheimer's disease (AD) and Dementia with Lewy bodies (DLB) make differentiation difficult in the clinical environment. Evaluating the CSF levels of biomarkers in AD and DLB patients could facilitate clinical diagnosis. CSF Visinin‐like protein‐1 (VILIP‐1), a calcium‐mediated neuronal injury biomarker, has been described as a novel biomarker for AD. The aim of this study was to investigate the diagnostic utility of CSF VILIP‐1 and VILIP‐1/Aβ_1–42 ratio to distinguish AD from DLB. Levels of CSF VILIP‐1, t‐tau, p‐tau_181P, Aβ_1–42, and α‐synuclein were measured in 61 AD patients, 32 DLB patients, and 40 normal controls using commercial ELISA kits. The results showed that the CSF VILIP‐1 level had significantly increased in AD patients compared with both normal controls and DLB patients. The CSF VILIP‐1 and VILIP‐1/Aβ_1–42 levels had enough diagnostic accuracy to allow the detection and differential diagnosis of AD. Additionally, CSF VILIP‐1 levels were positively correlated with t‐tau and p‐tau_181P within each group and with α‐synuclein in the AD and control groups. We conclude that CSF VILIP‐1 could be a diagnostic marker for AD, differentiating it from DLB. The analysis of biomarkers, representing different neuropathologies, is an important approach reflecting the heterogeneous features of AD and DLB.

     

Apolipoprotein E Is a Ligand for Triggering Receptor Expressed on Myeloid Cells 2 (TREM2)

Several heterozygous missense mutations in the triggering receptor expressed on myeloid cells 2 (TREM2) have recently been linked to risk for a number of neurological disorders including Alzheimer disease (AD), Parkinson disease, and frontotemporal dementia. These discoveries have re-ignited interest in the role of neuroinflammation in the pathogenesis of neurodegenerative diseases. TREM2 is highly expressed in microglia, the resident immune cells of the central nervous system. Along with its adaptor protein, DAP12, TREM2 regulates inflammatory cytokine release and phagocytosis of apoptotic neurons. Here, we report apolipoprotein E (apoE) as a novel ligand for TREM2. Using a biochemical assay, we demonstrated high-affinity binding of apoE to human TREM2. The functional significance of this binding was highlighted by increased phagocytosis of apoE-bound apoptotic N2a cells by primary microglia in a manner that depends on TREM2 expression. Moreover, when the AD-associated TREM2-R47H mutant was used in biochemical assays, apoE binding was vastly reduced. Our data demonstrate that apoE-TREM2 interaction in microglia plays critical roles in modulating phagocytosis of apoE-bound apoptotic neurons and establish a critical link between two proteins whose genes are strongly linked to the risk for AD.     

PLCγ1 Participates in Protein Transport and Diacylglycerol Production Triggered by cargo Arrival at the Golgi

Diacylglycerol (DAG) is required for membrane traffic and structural organization at the Golgi. DAG is a lipid metabolite of several enzymatic reactions present at this organelle, but the mechanisms by which they are regulated are still unknown. Here, we show that cargo arrival at the Golgi increases the recruitment of the DAG‐sensing constructs C1‐PKCθ‐GFP and the PKD‐wt‐GFP. The recruitment of both constructs was reduced by PLCγ1 silencing. Post‐Golgi trafficking of transmembrane and soluble proteins was impaired in PLCγ1‐silenced cells. Under basal conditions, PLCγ1 contributed to the maintenance of the pool of DAG associated with the Golgi and to the structural organization of the organelle. Finally, we show that cytosolic phospholipase C (PLC) can hydrolyse phosphatidylinositol 4‐phosphate in isolated Golgi membranes. Our results indicate that PLCγ1 is part of the molecular mechanism that couples cargo arrival at the Golgi with DAG production to co‐ordinate the formation of transport carriers for post‐Golgi traffic.      

Trafficking of Acetyl‐C16‐Ceramide‐NBD with Long‐Term Stability and No Cytotoxicity into the Golgi Complex

The Golgi complex plays a prominent role in the modification and sorting of lipids and proteins, and is a highly dynamic organelle that is dispersed and rearranged before and after mitosis. Several reagents including 4‐nitrobenzo‐2‐oxa‐1,3‐diazole‐labeled C6‐ceramide (NBD‐C6‐ceramide, a ceramide having an NBD‐bound C6‐N‐acyl chain) and Golgi‐specific proteins that emit fluorescence are used as Golgi markers. In the present study, we synthesized a new ceramide analog, acetyl‐C16‐ceramide‐NBD (a ceramide having an acetylated C‐1 hydroxyl group, C16‐N‐acyl chain, and NBD‐bound C15‐sphingosine), and showed that it preferentially accumulated in the Golgi complex without cytotoxicity for over 24 h. Pathways for cellular uptake and interorganelle trafficking of acetyl‐C16‐ceramide‐NBD were investigated. Acetyl‐C16‐ceramide‐NBD was transported to the Golgi complex via ceramide transport proteins. In contrast to NBD‐C6‐ceramide, acetyl‐C16‐ceramide‐NBD was resistant to ceramide metabolic enzymes such as sphingomyelin synthase and glucosylceramide synthase. Because of its weaker cytotoxicity and resistance to ceramide metabolic enzymes, the localization of the Golgi complex could be observed in acetyl‐C16‐ceramide‐NBD‐labeled cells before and after mitosis.      

The Arl3 and Arl1 GTPases co‐operate with Cog8 to regulate selective autophagy via Atg9 trafficking

The Arl3‐Arl1 GTPase cascade plays important roles in vesicle trafficking at the late Golgi and endosomes. Subunits of the conserved oligomeric Golgi (COG) complex, a tethering factor, are important for endosome‐to‐Golgi transport and contribute to the efficient functioning of the cytoplasm‐to‐vacuole targeting (Cvt) pathway, a well‐known selective autophagy pathway. According to our findings, the Arl3‐Arl1 GTPase cascade co‐operates with Cog8 to regulate the Cvt pathway via Atg9 trafficking. arl3cog8Δ and arl1cog8Δ exhibit profound defects in aminopeptidase I maturation in rich medium. In addition, the Arl3‐Arl1 cascade acts on the Cvt pathway via dynamic nucleotide binding. Furthermore, Atg9 accumulates at the late Golgi in arl3cog8Δ and arl1cog8Δ cells under normal growth conditions but not under starvation conditions. Thus, our results offer insight into the requirement for multiple components in the Golgi‐endosome system to determine Atg9 trafficking at the Golgi, thereby regulating selective autophagy.      

The Triggering Receptor Expressed on Myeloid Cells 2 Binds Apolipoprotein E

The triggering receptor expressed on myeloid cells 2 (TREM2) is an Ig-like V-type receptor expressed by populations of myeloid cells in the central nervous system and periphery. Loss-of-function mutations in TREM2 cause a progressive, fatal neurodegenerative disorder called Nasu-Hakola disease. In addition, a TREM2 R47H coding variant was recently identified as a risk factor for late-onset Alzheimer disease. TREM2 binds various polyanionic molecules but no specific protein ligands have been identified. Here we show that TREM2 specifically binds apolipoprotein E, a well established participant in Alzheimer disease. TREM2-Ig fusions efficiently precipitate ApoE from cerebrospinal fluid and serum. TREM2 also binds recombinant ApoE in solution and immobilized ApoE as detected by ELISA. Furthermore, the Alzheimer disease-associated R47H mutation, and other artificial mutations introduced in the same location, markedly reduced the affinity of TREM2 for ApoE. These findings reveal a link between two Alzheimer disease risk factors and may provide important clues to the pathogenesis of Nasu-Hakola disease and other neurodegenerative disorders.