Modelling acquired resistance to DOT1L inhibition exhibits the adaptive potential of KMT2A-rearranged acute lymphoblastic leukemia.

In KMT2A-rearranged acute lymphoblastic leukemia (ALL), an aggressive malignancy, oncogenic KMT2A-fusion proteins inappropriately recruit DOT1L to promote leukemogenesis, highlighting DOT1L as an attractive therapeutic target. Unfortunately, treatment with the first-in-class DOT1L inhibitor pinometostat eventually leads to non-responsiveness. To understand this we established acquired pinometostat resistance in pediatric KMT2A::AFF1+ B-ALL cells. Interestingly, these cells became mostly independent of DOT1L-mediated H3K79 methylation, but still relied on the physical presence of DOT1L, HOXA9 and the KMT2A::AFF1 fusion. Moreover, these cells selectively lost the epigenetic regulation and expression of various KMT2A-fusion target genes such as PROM1/CD133, while other KMT2A::AFF1 target genes, including HOXA9 and CDK6 remained unaffected. Concomitantly, these pinometostat-resistant cells showed upregulation of several myeloid-associated genes, including CD33 and LILRB4/CD85k. Taken together, this model comprehensively shows the adaptive potential of KMT2A-rearranged ALL cells upon losing dependency on one of its main oncogenic properties.

Single-cell phenotypes of peripheral blood immune cells in early and late stages of non-alcoholic fatty liver disease.

BACKGROUND/AIMS: Immune and inflammatory cells respond to multiple pathological hits in the development of nonalcoholic steatohepatitis (NASH) and fibrosis. Relatively little is known about how their type and function change through the non-alcoholic fatty liver disease (NAFLD) spectrum. Here we used multi-dimensional mass cytometry and a tailored bioinformatic approach to study circulating immune cells sampled from healthy individuals and people with NAFLD. METHODS: Cytometry by time of flight using 36 metal-conjugated antibodies was applied to peripheral blood mononuclear cells (PBMCs) from biopsy-proven NASH fibrosis (late disease), steatosis (early disease), and healthy patients. Supervised and unsupervised analyses were used, findings confirmed, and mechanisms assessed using independent healthy and disease PBMC samples. RESULTS: Of 36 PBMC clusters, 21 changed between controls and disease samples. Significant differences were observed between diseases stages with changes in T cells and myeloid cells throughout disease and B cell changes in late stages. Semi-supervised gating and re-clustering showed that disease stages were associated with fewer monocytes with active signalling and more inactive NK cells; B and T cells bearing activation markers were reduced in late stages, while B cells bearing co-stimulatory molecules were increased. Functionally, disease states were associated with fewer activated mucosal-associated invariant T cells and reduced toll-like receptor-mediated cytokine production in late disease. CONCLUSION: A range of innate and adaptive immune changes begin early in NAFLD, and disease stages are associated with a functionally less active phenotype compared to controls. Further study of the immune response in NAFLD spectrum may give insight into mechanisms of disease with potential clinical application.

Flow cytometric minimal residual disease assessment in B-cell precursor acute lymphoblastic leukaemia patients treated with CD19-targeted therapies - a EuroFlow study.

The standardized EuroFlow protocol, including CD19 as primary B-cell marker, enables highly sensitive and reliable minimal residual disease (MRD) assessment in B-cell precursor acute lymphoblastic leukaemia (BCP-ALL) patients treated with chemotherapy. We developed and validated an alternative gating strategy allowing reliable MRD analysis in BCP-ALL patients treated with CD19-targeting therapies. Concordant data were obtained in 92% of targeted therapy patients who remained CD19-positive, whereas this was 81% in patients that became (partially) CD19-negative. Nevertheless, in both groups median MRD values showed excellent correlation with the original MRD data, indicating that, despite higher interlaboratory variation, the overall MRD analysis was correct.

m6A methylation potentiates cytosolic dsDNA recognition in a sequence-specific manner.

Nucleic acid sensing through pattern recognition receptors is critical for immune recognition of microbial infections. Microbial DNA is frequently methylated at the N6 position of adenines (m6A), a modification that is rare in mammalian host DNA. We show here how that m6A methylation of 5'-GATC-3' motifs augments the immunogenicity of synthetic double-stranded (ds)DNA in murine macrophages and dendritic cells. Transfection with m6A-methylated DNA increased the expression of the activation markers CD69 and CD86, and of Ifnß, iNos and Cxcl10 mRNA. Similar to unmethylated cytosolic dsDNA, recognition of m6A DNA occurs independently of TLR and RIG-I signalling, but requires the two key mediators of cytosolic DNA sensing, STING and cGAS. Intriguingly, the response to m6A DNA is sequence-specific. m6A is immunostimulatory in some motifs, but immunosuppressive in others, a feature that is conserved between mouse and human macrophages. In conclusion, epigenetic alterations of DNA depend on the context of the sequence and are differentially perceived by innate cells, a feature that could potentially be used for the design of immune-modulating therapeutics.

Human VAPome Analysis Reveals MOSPD1 and MOSPD3 as Membrane Contact Site Proteins Interacting with FFAT-Related FFNT Motifs.

Membrane contact sites (MCS) are intracellular regions where two organelles come closer to exchange information and material. The majority of the endoplasmic reticulum (ER) MCS are attributed to the ER-localized tether proteins VAPA, VAPB, and MOSPD2. These recruit other proteins to the ER by interacting with their FFAT motifs. Here, we describe MOSPD1 and MOSPD3 as ER-localized tethers interacting with FFAT motif-containing proteins. Using BioID, we identify proteins interacting with VAP and MOSPD proteins and find that MOSPD1 and MOSPD3 prefer unconventional FFAT-related FFNT (two phenylalanines [FF] in a neutral tract) motifs. Moreover, VAPA/VAPB/MOSPD2 and MOSPD1/MOSPD3 assemble into two separate ER-resident complexes to interact with FFAT and FFNT motifs, respectively. Because of their ability to interact with FFNT motifs, MOSPD1 and MOSPD3 could form MCS between the ER and other organelles. Collectively, these findings expand the VAP family of proteins and highlight two separate complexes in control of interactions between intracellular compartments.

A molecular pore spans the double membrane of the coronavirus replication organelle.

Coronavirus genome replication is associated with virus-induced cytosolic double-membrane vesicles, which may provide a tailored microenvironment for viral RNA synthesis in the infected cell. However, it is unclear how newly synthesized genomes and messenger RNAs can travel from these sealed replication compartments to the cytosol to ensure their translation and the assembly of progeny virions. In this study, we used cellular cryo-electron microscopy to visualize a molecular pore complex that spans both membranes of the double-membrane vesicle and would allow export of RNA to the cytosol. A hexameric assembly of a large viral transmembrane protein was found to form the core of the crown-shaped complex. This coronavirus-specific structure likely plays a key role in coronavirus replication and thus constitutes a potential drug target.

A unifying structural and functional model of the coronavirus replication organelle: Tracking down RNA synthesis.

Zoonotic coronavirus (CoV) infections, such as those responsible for the current severe acute respiratory syndrome-CoV 2 (SARS-CoV-2) pandemic, cause grave international public health concern. In infected cells, the CoV RNA-synthesizing machinery associates with modified endoplasmic reticulum membranes that are transformed into the viral replication organelle (RO). Although double-membrane vesicles (DMVs) appear to be a pan-CoV RO element, studies to date describe an assortment of additional CoV-induced membrane structures. Despite much speculation, it remains unclear which RO element(s) accommodate viral RNA synthesis. Here we provide detailed 2D and 3D analyses of CoV ROs and show that diverse CoVs essentially induce the same membrane modifications, including the small open double-membrane spherules (DMSs) previously thought to be restricted to gamma- and delta-CoV infections and proposed as sites of replication. Metabolic labeling of newly synthesized viral RNA followed by quantitative electron microscopy (EM) autoradiography revealed abundant viral RNA synthesis associated with DMVs in cells infected with the beta-CoVs Middle East respiratory syndrome-CoV (MERS-CoV) and SARS-CoV and the gamma-CoV infectious bronchitis virus. RNA synthesis could not be linked to DMSs or any other cellular or virus-induced structure. Our results provide a unifying model of the CoV RO and clearly establish DMVs as the central hub for viral RNA synthesis and a potential drug target in CoV infection.

Applicability and reproducibility of acute myeloid leukaemia stem cell assessment in a multi-centre setting.

Leukaemic stem cells (LSC) have been experimentally defined as the leukaemia-propagating population and are thought to be the cellular reservoir of relapse in acute myeloid leukaemia (AML). Therefore, LSC measurements are warranted to facilitate accurate risk stratification. Previously, we published the composition of a one-tube flow cytometric assay, characterised by the presence of 13 important membrane markers for LSC detection. Here we present the validation experiments of the assay in several large AML research centres, both in Europe and the United States. Variability within instruments and sample processing showed high correlations between different instruments (Rpearson  > 0·91, P < 0·001). Multi-centre testing introduced variation in reported LSC percentages but was found to be below the clinical relevant threshold. Clear gating protocols resulted in all laboratories being able to perform LSC assessment of the validation set. Participating centres were nearly unanimously able to distinguish LSChigh (>0·03% LSC) from LSClow (<0·03% LSC) despite inter-laboratory variation in reported LSC percentages. This study proves that the LSC assay is highly reproducible. These results together with the high prognostic impact of LSC load at diagnosis in AML patients render the one-tube LSC assessment a good marker for future risk classification.

ADAM17-Mediated Reduction in CD14++CD16+ Monocytes ex vivo and Reduction in Intermediate Monocytes With Immune Paresis in Acute Pancreatitis and Acute Alcoholic Hepatitis.

Impaired immune responses and increased susceptibility to infection characterize acute inflammatory conditions such as pancreatitis and alcoholic hepatitis and are major causes of morbidity and mortality. However, the mechanisms that drive this apparent immune paresis remain poorly understood. Monocytes mediate host responses to damage and pathogens in health and disease, and three subsets of monocytes have been defined based on CD14 and CD16 expression. We sought to determine the changes in monocyte subsets in acute pancreatitis (AP) and acute alcoholic hepatitis (AAH), together with functional consequences and mechanisms that underlie this change. Peripheral blood mononuclear cells (PBMCs) from patients with AP or AAH were compared with healthy controls. Monocyte subsets were defined by HLA-DR, CD14, and CD16 expression. Changes in surface and intracellular protein expression and phosphorylation were determined by flow cytometry. Phenotype and function were assessed following stimulation with lipopolysaccharide (LPS) or other agonists in the presence of specific inhibitors of TNFα and a disintegrin and metalloproteinase 17 (ADAM17). Patients with AP and AAH had reduced CD14++CD16+ intermediate monocytes compared to controls. Reduction of intermediate monocytes was recapitulated ex vivo by stimulating healthy control PBMCs with Toll-like receptor (TLR) agonists LPS, flagellin or polyinosilic:polycytidylic acid (poly I:C). Stimulation caused shedding of CD14 and CD16, which could be reversed using the ADAM17 inhibitor, TMI005 but not direct inhibitors of TNFα, a known ADAM17-target. Culturing PBMCs from healthy controls resulted in expansion of intermediate monocytes, which did not occur when LPS was in the culture medium. Cultured intermediate monocytes showed reduced expression of CX3CR1, CCR2, TLR4, and TLR5. We found reduced migratory responses, intracellular signaling and pro-inflammatory cytokine production, and increased expression of IL-10. Stimulation with TLR agonists results in ADAM17-mediated shedding of phenotypic markers from CD16+ monocytes, leading to apparent "loss" of intermediate monocytes. Reduction in CD14++CD16- monocytes and increased CD14++CD16+ is associated with altered responses in functional assays ex vivo. Patients with AP and AAH had reduced proportions of CD14++CD16+ monocytes and reduced phosphorylation of NFκB and IL-6 production in response to bacterial LPS. Together, these processes may contribute to the susceptibility to infection observed in AP and AAH.

Origins of Enterovirus Replication Organelles Established by Whole-Cell Electron Microscopy.

Enterovirus genome replication occurs at virus-induced structures derived from cellular membranes and lipids. However, the origin of these replication organelles (ROs) remains uncertain. Ultrastructural evidence of the membrane donor is lacking, suggesting that the sites of its transition into ROs are rare or fleeting. To overcome this challenge, we combined live-cell imaging and serial block-face scanning electron microscopy of whole cells to capture emerging enterovirus ROs. The first foci of fluorescently labeled viral protein correlated with ROs connected to the endoplasmic reticulum (ER) and preceded the appearance of ROs stemming from the trans-Golgi network. Whole-cell data sets further revealed striking contact regions between ROs and lipid droplets that may represent a route for lipid shuttling to facilitate RO proliferation and genome replication. Our data provide direct evidence that enteroviruses use ER and then Golgi membranes to initiate RO formation, demonstrating the remarkable flexibility with which enteroviruses usurp cellular organelles.IMPORTANCE Enteroviruses are causative agents of a range of human diseases. The replication of these viruses within cells relies on specialized membranous structures termed replication organelles (ROs) that form during infection but whose origin remains elusive. To capture the emergence of enterovirus ROs, we use correlative light and serial block-face scanning electron microscopy, a powerful method to pinpoint rare events in their whole-cell ultrastructural context. RO biogenesis was found to occur first at ER and then at Golgi membranes. Extensive contacts were found between early ROs and lipid droplets (LDs), which likely serve to provide LD-derived lipids required for replication. Together, these data establish the dual origin of enterovirus ROs and the chronology of their biogenesis at different supporting cellular membranes.

Adaptive Mutations in Replicase Transmembrane Subunits Can Counteract Inhibition of Equine Arteritis Virus RNA Synthesis by Cyclophilin Inhibitors.

Previously, the cyclophilin inhibitors cyclosporine (CsA) and alisporivir (ALV) were shown to inhibit the replication of diverse RNA viruses, including arteriviruses and coronaviruses, which both belong to the order Nidovirales In this study, we aimed to identify arterivirus proteins involved in the mode of action of cyclophilin inhibitors and to investigate how these compounds inhibit arterivirus RNA synthesis in the infected cell. Repeated passaging of the arterivirus prototype equine arteritis virus (EAV) in the presence of CsA revealed that reduced drug sensitivity is associated with the emergence of adaptive mutations in nonstructural protein 5 (nsp5), one of the transmembrane subunits of the arterivirus replicase polyprotein. Introduction of singular nsp5 mutations (nsp5 Q21R, Y113H, or A134V) led to an ∼2-fold decrease in sensitivity to CsA treatment, whereas combinations of mutations further increased EAV's CsA resistance. The detailed experimental characterization of engineered EAV mutants harboring CsA resistance mutations implicated nsp5 in arterivirus RNA synthesis. Particularly, in an in vitro assay, EAV RNA synthesis was far less sensitive to CsA treatment when nsp5 contained the adaptive mutations mentioned above. Interestingly, for increased sensitivity to the closely related drug ALV, CsA-resistant nsp5 mutants required the incorporation of an additional adaptive mutation, which resided in nsp2 (H114R), another transmembrane subunit of the arterivirus replicase. Our study provides the first evidence for the involvement of nsp2 and nsp5 in the mechanism underlying the inhibition of arterivirus replication by cyclophilin inhibitors.IMPORTANCE Currently, no approved treatments are available to combat infections with nidoviruses, a group of positive-stranded RNA viruses, including important zoonotic and veterinary pathogens. Previously, the cyclophilin inhibitors cyclosporine (CsA) and alisporivir (ALV) were shown to inhibit the replication of diverse nidoviruses (both arteriviruses and coronaviruses), and they may thus represent a class of pan-nidovirus inhibitors. In this study, using the arterivirus prototype equine arteritis virus, we have established that resistance to CsA and ALV treatment is associated with adaptive mutations in two transmembrane subunits of the viral replication machinery, nonstructural proteins 2 and 5. This is the first evidence for the involvement of specific replicase subunits of arteriviruses in the mechanism underlying the inhibition of their replication by cyclophilin inhibitors. Understanding this mechanism of action is of major importance to guide future drug design, both for nidoviruses and for other RNA viruses inhibited by these compounds.

How do Dutch primary care providers overcome barriers to advance care planning with older people? A qualitative study.

BACKGROUND: Few older people benefit from advance care planning (ACP), due to several barriers related to primary care professionals, such as insufficient knowledge, negative beliefs and a lack of time. Information on overcoming these barriers is limited. We assumed primary care professionals experienced in ACP with older patients are likely to have learned how to overcome these barriers. OBJECTIVE: To investigate how Dutch primary care professionals experienced in ACP with older patients overcome these barriers. METHODS: A qualitative study, based on semi-structured interviews, among a purposive sample of 14 Dutch primary care professionals experienced in ACP with older people. Transcripts were thematically analysed. RESULTS: We interviewed eight general practitioners (GPs), three nurses and three elderly care physicians, experienced in ACP with older people. Respondents overcame their own insufficient knowledge and skills, as well as their negative attitudes and beliefs by gaining experience through practicing ACP in their daily practices, exchanging and reflecting on those experiences with peers, pursuing continuing education, teaching and participating in research. To overcome patients' and families' lack of initiative and openness to ACP, respondents prepared them for further steps in ACP. To overcome a lack of time, respondents used tools and information communication technology, delegated parts of ACP to other primary care professionals, acquired financing and systematized documentation of ACP. CONCLUSION: Primary care professionals can overcome barriers to ACP with older patients by practicing, reflecting on experiences and pursuing continuing education, by preparing patients and involving family and by investing in support to approach ACP more efficiently.

Functional and phenotypical analysis of IL-6-secreting CD4+ T cells in human adipose tissue.

Emerging evidence indicates that a dynamic interplay between the immune system and adipocytes contributes to the disturbed homeostasis in adipose tissue of obese subjects. Recently, we observed IL-6-secretion by CD4+ T cells from the stromal vascular fraction (SVF) of the infrapatellar fat pad (IFP) of knee osteoarthritis patients directly ex vivo. Here we show that human IL-6+ CD4+ T cells from SVF display a more activated phenotype than the IL-6- T cells, as evidenced by the expression of the activation marker CD69. Analysis of cytokines secretion, as well as expression of chemokine receptors and transcription factors associated with different Th subsets (Treg, Th1, Th2, Th17 and Tfh) revealed that IL-6-secreting CD4+ T cells cannot be assigned to a conventional Th subset. TCRß gene analysis revealed that IL-6+ and IL-6- CD4+ T cells appear clonally unrelated to each other, suggesting a different specificity of these cells. In line with these observations, adipocytes are capable of enhancing IL-6 production by CD4+ T cells. Thus, IL-6+ CD4+ T cells are TCRαß T cells expressing an activated phenotype potentially resulting from an interplay with adipocytes that could be involved in the inflammatory processes in the OA joint.

Lack of high BMI-related features in adipocytes and inflammatory cells in the infrapatellar fat pad (IFP).

BACKGROUND: Obesity is associated with the development and progression of osteoarthritis (OA). Although the infrapatellar fat pad (IFP) could be involved in this association, due to its intracapsular localization in the knee joint, there is currently little known about the effect of obesity on the IFP. Therefore, we investigated cellular and molecular body mass index (BMI)-related features in the IFP of OA patients. METHODS: Patients with knee OA (N = 155, 68% women, mean age 65 years, mean (SD) BMI 29.9 kg/m2 (5.7)) were recruited: IFP volume was determined by magnetic resonance imaging in 79 patients with knee OA, while IFPs and subcutaneous adipose tissue (SCAT) were obtained from 106 patients undergoing arthroplasty. Crown-like structures (CLS) were determined using immunohistochemical analysis. Adipocyte size was determined by light microscopy and histological analysis. Stromal vascular fraction (SVF) cells were characterized by flow cytometry. RESULTS: IFP volume (mean (SD) 23.6 (5.4) mm3) was associated with height, but not with BMI or other obesity-related features. Likewise, volume and size of IFP adipocytes (mean 271 pl, mean 1933 µm) was not correlated with BMI. Few CLS were observed in the IFP, with no differences between overweight/obese and lean individuals. Moreover, high BMI was not associated with higher SVF immune cell numbers in the IFP, nor with changes in their phenotype. No BMI-associated molecular differences were observed, besides an increase in TNFα expression with high BMI. Macrophages in the IFP were mostly pro-inflammatory, producing IL-6 and TNFα, but little IL-10. Interestingly, however, CD206 and CD163 were associated with an anti-inflammatory phenotype, were the most abundantly expressed surface markers on macrophages (81% and 41%, respectively) and CD163+ macrophages had a more activated and pro-inflammatory phenotype than their CD163- counterparts. CONCLUSIONS: BMI-related features usually observed in SCAT and visceral adipose tissue could not be detected in the IFP of OA patients, a fat depot implicated in OA pathogenesis.

Global hypoxia-ischemia induced inflammation and structural changes in the preterm ovine gut which were not ameliorated by mesenchymal stem cell treatment.

Perinatal asphyxia, a condition of impaired gas exchange during birth, leads to fetal hypoxia-ischemia (HI) and is associated with postnatal adverse outcomes including intestinal dysmotility and necrotizing enterocolitis (NEC). Evidence from adult animal models of transient, locally-induced intestinal HI has shown that inflammation is essential in HI-induced injury of the gut. Importantly, mesenchymal stem cell (MSC) treatment prevented this HI-induced intestinal damage. We therefore assessed whether fetal global HI induced inflammation, injury and developmental changes in the gut and whether intravenous MSC administration ameliorated these HI-induced adverse intestinal effects. In a preclinical ovine model, fetuses were subjected to umbilical cord occlusion (UCO), with or without MSC treatment, and sacrificed 7 days after UCO. Global HI increased the number of myeloperoxidase positive cells in the mucosa, upregulated mRNA levels of interleukin (IL)-1ß and IL-17 in gut tissue and caused T-cell invasion in the intestinal muscle layer. Intestinal inflammation following global HI was associated with increased Ki67+ cells in the muscularis and subsequent muscle hyperplasia. Global HI caused distortion of glial fibrillary acidic protein immunoreactivity in the enteric glial cells and increased synaptophysin and serotonin expression in the myenteric ganglia. Intravenous MSC treatment did not ameliorate these HI-induced adverse intestinal events. Global HI resulted in intestinal inflammation and enteric nervous system abnormalities which are clinically associated with postnatal complications including feeding intolerance, altered gastrointestinal transit and NEC. The intestinal histopathological changes were not prevented by intravenous MSC treatment directly after HI, indicating that alternative treatment regimens for cell-based therapies should be explored.

Fatty acids, lipid mediators, and T-cell function.

Research toward the mechanisms underlying obesity-linked complications has intensified during the last years. As a consequence, it has become clear that metabolism and immunity are intimately linked. Free fatty acids and other lipids acquired in excess by current feeding patterns have been proposed to mediate this link due to their immune modulatory capacity. The functional differences between saturated and unsaturated fatty acids, in combination with their dietary intake are believed to modulate the outcome of immune responses. Moreover, unsaturated fatty acids can be oxidized in a tightly regulated and specific manner to generate either potent pro-inflammatory or pro-resolving lipid mediators. These oxidative derivatives of fatty acids have received detailed attention during the last years, as they have proven to have strong immune modulatory capacity, even in pM ranges. Both fatty acids and oxidized fatty acids have been studied especially in relation to macrophage and T-cells functions. In this review, we propose to focus on the effect of fatty acids and their oxidative derivatives on T-cells, as it is an active area of research during the past 5 years. The effect of fatty acids and their derivatives on activation and proliferation of T-cells, as well as the delicate balance between stimulation and lipotoxicity will be discussed. Moreover, the receptors involved in the interaction between free fatty acids and their derivatives with T-cells will be summarized. Finally, the mechanisms involved in modulation of T-cells by fatty acids will be addressed, including cellular signaling and metabolism of T-cells. The in vitro results will be placed in context of in vivo studies both in humans and mice. In this review, we summarize the latest findings on the immune modulatory function of lipids on T-cells and will point out novel directions for future research.

Homodimerization of the Wnt receptor DERAILED recruits the Src family kinase SRC64B.

Ryk pseudokinase receptors act as important transducers of Wnt signals, particularly in the nervous system. Little is known, however, of their interactions at the cell surface. Here, we show that a Drosophila Ryk family member, DERAILED (DRL), forms cell surface homodimers and can also heterodimerize with the two other fly Ryks, DERAILED-2 and DOUGHNUT ON 2. DERAILED homodimerization levels increase significantly in the presence of its ligand, WNT5. In addition, DERAILED displays ligand-independent dimerization mediated by a motif in its transmembrane domain. Increased dimerization of DRL upon WNT5 binding or upon the replacement of DERAILED's extracellular domain with the immunoglobulin Fc domain results in an increased recruitment of the Src family kinase SRC64B, a previously identified downstream pathway effector. Formation of the SRC64B/DERAILED complex requires SRC64B's SH2 domain and DERAILED's PDZ-binding motif. Mutations in DERAILED's inactive tyrosine kinase-homologous domain also disrupt the formation of DERAILED/SRC64B complexes, indicating that its conformation is likely important in facilitating its interaction with SRC64B. Finally, we show that DERAILED's function during embryonic axon guidance requires its Wnt-binding domain, a putative juxtamembrane extracellular tetrabasic cleavage site, and the PDZ-binding domain, indicating that DERAILED's activation involves a complex set of events including both dimerization and proteolytic processing.