Functional Signature of LRP4 Antibodies in Myasthenia Gravis.

BACKGROUND AND OBJECTIVES: Antibodies (Abs) specific for the low-density lipoprotein receptor-related protein 4 (LRP4) occur in up to 5% of patients with myasthenia gravis (MG). The objective of this study was to profile LRP4-Ab effector actions. METHODS: We evaluated the efficacy of LRP4-specific compared with AChR-specific IgG to induce Ab-dependent cellular phagocytosis (ADCP), Ab-dependent cellular cytotoxicity (ADCC), and Ab-dependent complement deposition (ADCD). Functional features were additionally assessed in an independent AChR-Ab+ MG cohort. Levels of circulating activated complement proteins and frequency of Fc glycovariants were quantified and compared with demographically matched 19 healthy controls. RESULTS: Effector actions that required binding of Fc domains to cellular FcRs such as ADCC and ADCP were detectable for both LRP4-specific and AChR-specific Abs. In contrast to AChR-Abs, LRP4-binding Abs showed poor efficacy in inducing complement deposition. Levels of circulating activated complement proteins were not substantially increased in LRP4-Ab-positive MG. Frequency of IgG glycovariants carrying 2 sialic acid residues, indicative for anti-inflammatory IgG activity, was decreased in patients with LRP4-Ab-positive MG. DISCUSSION: LRP4-Abs are more effective in inducing cellular FcR-mediated effector mechanisms than Ab-dependent complement activation. Their functional signature is different from AChR-specific Abs.

Protein arginine methyltransferase 1 regulates B cell fate after positive selection in the germinal center in mice.

Positively selected germinal center B cells (GCBC) can either resume proliferation and somatic hypermutation or differentiate. The mechanisms dictating these alternative cell fates are incompletely understood. We show that the protein arginine methyltransferase 1 (Prmt1) is upregulated in murine GCBC by Myc and mTORC-dependent signaling after positive selection. Deleting Prmt1 in activated B cells compromises antibody affinity maturation by hampering proliferation and GCBC light zone to dark zone cycling. Prmt1 deficiency also results in enhanced memory B cell generation and plasma cell differentiation, albeit the quality of these cells is compromised by the GCBC defects. We further demonstrate that Prmt1 intrinsically limits plasma cell differentiation, a function co-opted by B cell lymphoma (BCL) cells. Consistently, PRMT1 expression in BCL correlates with poor disease outcome, depends on MYC and mTORC1 activity, is required for cell proliferation, and prevents differentiation. Collectively, these data identify PRMT1 as a determinant of normal and cancerous mature B cell proliferation and differentiation balance.

Erratum: The uracil-DNA glycosylase UNG protects the fitness of normal and cancer B cells expressing AID.

[This corrects the article DOI: 10.1093/narcan/zcaa019.].

N-glycosylation analysis of mouse immunoglobulin G isolated from dried blood spots.

The association of immunoglobulin G (IgG) glycosylation changes with various human diseases and physiological conditions is well established. Since the mechanistical explanation of the regulation of IgG glycosylation and its functional role in these various states is still missing, the eyes of the biomedical community are now turned towards animal models, which enable intervention studies necessary for conclusions on causality. Mice are recognized and used as a good experimental model for human IgG glycosylation. However, smaller blood volumes, low IgG concentrations at young ages (which are most often used in mice experiments) and multiple sampling protocols during the course of longitudinal studies would profit from a robust workflow for mouse IgG glycome analysis from minute amounts of starting material, collected through a simple sampling procedure. For this purpose, we have developed a protocol for analysis of total N-glycans of IgG isolated from mouse dried blood spots (DBS), which we report here. We show that mouse DBS are a good source of material for IgG N-glycan analysis by multiplexed capillary gel electrophoresis with laser-induced fluorescence (xCGE-LIF).

The uracil-DNA glycosylase UNG protects the fitness of normal and cancer B cells expressing AID.

In B lymphocytes, the uracil N-glycosylase (UNG) excises genomic uracils made by activation-induced deaminase (AID), thus underpinning antibody gene diversification and oncogenic chromosomal translocations, but also initiating faithful DNA repair. Ung-/- mice develop B-cell lymphoma (BCL). However, since UNG has anti- and pro-oncogenic activities, its tumor suppressor relevance is unclear. Moreover, how the constant DNA damage and repair caused by the AID and UNG interplay affects B-cell fitness and thereby the dynamics of cell populations in vivo is unknown. Here, we show that UNG specifically protects the fitness of germinal center B cells, which express AID, and not of any other B-cell subset, coincident with AID-induced telomere damage activating p53-dependent checkpoints. Consistent with AID expression being detrimental in UNG-deficient B cells, Ung-/- mice develop BCL originating from activated B cells but lose AID expression in the established tumor. Accordingly, we find that UNG is rarely lost in human BCL. The fitness preservation activity of UNG contingent to AID expression was confirmed in a B-cell leukemia model. Hence, UNG, typically considered a tumor suppressor, acquires tumor-enabling activity in cancer cell populations that express AID by protecting cell fitness.

PRMT5 is essential for B cell development and germinal center dynamics.

Mechanisms regulating B cell development, activation, education in the germinal center (GC) and differentiation, underpin the humoral immune response. Protein arginine methyltransferase 5 (Prmt5), which catalyzes most symmetric dimethyl arginine protein modifications, is overexpressed in B cell lymphomas but its function in normal B cells is poorly defined. Here we show that Prmt5 is necessary for antibody responses and has essential but distinct functions in all proliferative B cell stages in mice. Prmt5 is necessary for B cell development by preventing p53-dependent and p53-independent blocks in Pro-B and Pre-B cells, respectively. By contrast, Prmt5 protects, via p53-independent pathways, mature B cells from apoptosis during activation, promotes GC expansion, and counters plasma cell differentiation. Phenotypic and RNA-seq data indicate that Prmt5 regulates GC light zone B cell fate by regulating transcriptional programs, achieved in part by ensuring RNA splicing fidelity. Our results establish Prmt5 as an essential regulator of B cell biology.

A licensing step links AID to transcription elongation for mutagenesis in B cells.

Activation-induced deaminase (AID) mutates the immunoglobulin (Ig) genes to initiate somatic hypermutation (SHM) and class switch recombination (CSR) in B cells, thus underpinning antibody responses. AID mutates a few hundred other loci, but most AID-occupied genes are spared. The mechanisms underlying productive deamination versus non-productive AID targeting are unclear. Here we show that three clustered arginine residues define a functional AID domain required for SHM, CSR, and off-target activity in B cells without affecting AID deaminase activity or Escherichia coli mutagenesis. Both wt AID and mutants with single amino acid replacements in this domain broadly associate with Spt5 and chromatin and occupy the promoter of AID target genes. However, mutant AID fails to occupy the corresponding gene bodies and loses association with transcription elongation factors. Thus AID mutagenic activity is determined not by locus occupancy but by a licensing mechanism, which couples AID to transcription elongation.

Roles for APRIN (PDS5B) in homologous recombination and in ovarian cancer prediction.

APRIN (PDS5 cohesin associated factor B) interacts with both the cohesin complex and the BRCA2 tumor suppressor. How APRIN influences cohesion and DNA repair processes is not well understood. Here, we show that APRIN is recruited to DNA damage sites. We find that APRIN interacts directly with RAD51, PALB2 and BRCA2. APRIN stimulates RAD51-mediated DNA strand invasion. APRIN also binds DNA with an affinity for D-loop structures and single-strand (ss) DNA. APRIN is a new homologous recombination (HR) mediator as it counteracts the RPA inhibitory effect on RAD51 loading to ssDNA. We show that APRIN strongly improves the annealing of complementary-strand DNA and that it can stimulate this process in synergy with BRCA2. Unlike cohesin constituents, its depletion has no impact on class switch recombination, supporting a specific role for this protein in HR. Furthermore, we show that low APRIN expression levels correlate with a better survival in ovarian cancer patients and that APRIN depletion sensitizes cells to the PARP inhibitor Olaparib in xenografted zebrafish. Our findings establish APRIN as an important and specific actor of HR, with cohesin-independent functions.

Activation induced deaminase C-terminal domain links DNA breaks to end protection and repair during class switch recombination.

Activation-induced deaminase (AID) triggers antibody class switch recombination (CSR) in B cells by initiating DNA double strand breaks that are repaired by nonhomologous end-joining pathways. A role for AID at the repair step is unclear. We show that specific inactivation of the C-terminal AID domain encoded by exon 5 (E5) allows very efficient deamination of the AID target regions but greatly impacts the efficiency and quality of subsequent DNA repair. Specifically eliminating E5 not only precludes CSR but also, causes an atypical, enzymatic activity-dependent dominant-negative effect on CSR. Moreover, the E5 domain is required for the formation of AID-dependent Igh-cMyc chromosomal translocations. DNA breaks at the Igh switch regions induced by AID lacking E5 display defective end joining, failing to recruit DNA damage response factors and undergoing extensive end resection. These defects lead to nonproductive resolutions, such as rearrangements and homologous recombination that can antagonize CSR. Our results can explain the autosomal dominant inheritance of AID variants with truncated E5 in patients with hyper-IgM syndrome 2 and establish that AID, through the E5 domain, provides a link between DNA damage and repair during CSR.

Alternative end-joining and classical nonhomologous end-joining pathways repair different types of double-strand breaks during class-switch recombination.

Classical nonhomologous end-joining (C-NHEJ) and alternative end-joining (A-EJ) are the main DNA double-strand break (DSB) repair pathways when a sister chromatid is not available. However, it is not clear how one pathway is chosen over the other to process a given DSB. To address this question, we studied in mouse splenic B cells and CH12F3 cells how C-NHEJ and A-EJ repair DSBs initiated by the activation-induced deaminase during IgH (Igh) class-switch recombination (CSR). We show in this study that lowering the deamination density at the Igh locus increases DSB resolution by microhomology-mediated repair while decreasing C-NHEJ activity. This process occurs without affecting 53BP1 and γH2AX levels during CSR. Mechanistically, lowering deamination density increases exonuclease I recruitment and single-stranded DNA at the Igh locus and promotes C-terminal binding protein interacting protein and MSH2-dependent DSB repair during CSR. Indeed, reducing activation-induced deaminase levels increases CSR efficiency in C-NHEJ-defective cells, suggesting enhanced use of an A-EJ pathway. Our results establish a mechanism by which C-NHEJ and this C-terminal binding protein interacting protein/MSH2-dependent pathway that relies on microhomology can act concurrently but independently to repair different types of DSBs and reveal that the density of DNA lesions influences the choice of DSB repair pathway during CSR.

MSH6- or PMS2-deficiency causes re-replication in DT40 B cells, but it has little effect on immunoglobulin gene conversion or on repair of AID-generated uracils.

The mammalian antibody repertoire is shaped by somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) loci of B lymphocytes. SHM and CSR are triggered by non-canonical, error-prone processing of G/U mismatches generated by activation-induced deaminase (AID). In birds, AID does not trigger SHM, but it triggers Ig gene conversion (GC), a 'homeologous' recombination process involving the Ig variable region and proximal pseudogenes. Because recombination fidelity is controlled by the mismatch repair (MMR) system, we investigated whether MMR affects GC in the chicken B cell line DT40. We show here that Msh6(-/-) and Pms2(-/-) DT40 cells display cell cycle defects, including genomic re-replication. However, although IgVλ GC tracts in MMR-deficient cells were slightly longer than in normal cells, Ig GC frequency, donor choice or the number of mutations per sequence remained unaltered. The finding that the avian MMR system, unlike that of mammals, does not seem to contribute towards the processing of G/U mismatches in vitro could explain why MMR is unable to initiate Ig GC in this species, despite initiating SHM and CSR in mammalian cells. Moreover, as MMR does not counteract or govern Ig GC, we report a rare example of 'homeologous' recombination insensitive to MMR.

Regulation of activation-induced deaminase stability and antibody gene diversification by Hsp90.

Activation-induced deaminase (AID) is the mutator enzyme that initiates somatic hypermutation and isotype switching of the antibody genes in B lymphocytes. Undesired byproducts of AID function are oncogenic mutations. AID expression levels seem to correlate with the extent of its physiological and pathological functions. In this study, we identify AID as a novel Hsp90 (heat shock protein 90 kD) client. We find that cytoplasmic AID is in a dynamic equilibrium regulated by Hsp90. Hsp90 stabilizes cytoplasmic AID, as specific Hsp90 inhibition leads to cytoplasmic polyubiquitination and proteasomal degradation of AID. Consequently, Hsp90 inhibition results in a proportional reduction in antibody gene diversification and off-target mutation. This evolutionarily conserved regulatory mechanism determines the functional steady-state levels of AID in normal B cells and B cell lymphoma lines. Thus, Hsp90 assists AID-mediated antibody diversification by stabilizing AID. Hsp90 inhibition provides the first pharmacological means to down-regulate AID expression and activity, which could be relevant for therapy of some lymphomas and leukemias.

The mechanisms regulating the subcellular localization of AID.

Activation induced deaminase (AID) is a unique enzyme that directly introduces mutations in the immunoglobulin genes to generate antibody diversity during the humoral immune response. Since this mutator enzyme poses a measurable risk of off-target mutation, which can be deleterious or transforming for a cell, several regulatory mechanisms exist to control its activity. At least three of these mechanisms affect AID subcellular localization. It was recently found that AID is actively imported into the nucleus, most likely through importin-α/ß recognizing a structural nuclear localization signal. However, AID is largely excluded from the nucleus in steady state thanks to two mechanisms. In addition to nuclear export through the exportin CRM1, a mechanism retaining AID in the cytoplasm exists. Cytoplasmic retention hinders the passive diffusion of AID into the nucleus playing an important role in the nuclear exclusion of AID. Subcellular localization of AID also determines its stability. The regulation of the nuclear fraction of AID by these many mechanisms has functional implications for antibody diversification.

Active nuclear import and cytoplasmic retention of activation-induced deaminase.

The enzyme activation-induced deaminase (AID) triggers antibody diversification in B cells by catalyzing deamination and consequently mutation of immunoglobulin genes. To minimize off-target deamination, AID is restrained by several regulatory mechanisms including nuclear exclusion, thought to be mediated exclusively by active nuclear export. Here we identify two other mechanisms involved in controlling AID subcellular localization. AID is unable to passively diffuse into the nucleus, despite its small size, and its nuclear entry requires active import mediated by a conformational nuclear localization signal. We also identify in its C terminus a determinant for AID cytoplasmic retention, which hampers diffusion to the nucleus, competes with nuclear import and is crucial for maintaining the predominantly cytoplasmic localization of AID in steady-state conditions. Blocking nuclear import alters the balance between these processes in favor of cytoplasmic retention, resulting in reduced isotype class switching.

Genetic interaction between members of the Vangl family causes neural tube defects in mice.

Neural tube defects (NTDs) are very frequent congenital abnormalities in humans. Recently, we have documented independent association of Vangl1 and Vangl2 gene mutations with NTDs. In the Looptail mouse, homozygosity (but not heterozygosity) for loss-of-function alleles at Vangl2 causes the severe NTD craniorachischisis, whereas heterozygosity for mutant variants of VANGL1 is associated with NTDs in a human cohort of sporadic and familial cases. To understand the role of Vangl1 in normal development, we created a mouse mutant with an inactivating mutation at Vangl1 (Vangl1(gt)). Vangl1 shows a dynamic pattern of expression in the developing neural tube and notochord at the time of neural tube closure. Vangl1(gt/+) heterozygotes and Vangl1(gt/gt) homozygotes are viable and fertile, although Vangl1(gt/gt) display subtle alterations in polarity of inner hair cells of the cochlea. Remarkably, and as opposed to healthy Vangl1(gt/+) and Vangl2(lp/+) heterozygotes, Vangl1(gt/+);Vangl2(lp/+) double heterozygotes show profound developmental defects that include severe craniorachischisis, inner ear defects (disorganization of the stereociliary bundles of hair cells of the organ of Corti), and cardiac abnormality (aberrant right subclavian artery). These results show that genetic interaction between Vangl1 and Vangl2 genes causes neural tube defects and raise the possibility that interaction between individual Vangl genes and other genetic loci and/or environmental factors may additionally contribute to the etiology of NTDs.

Canonical WNT signaling during kidney development.

The canonical WNT signaling pathway plays a crucial role in patterning of the embryo during development, but little is known about the specific developmental events which are under WNT control. To understand more about how the WNT pathway orchestrates mammalian organogenesis, we studied the canonical beta-catenin-mediated WNT signaling pathway in kidneys of mice bearing a beta-catenin-responsive TCF/betaGal reporter transgene. In metanephric kidney, intense canonical WNT signaling was evident in epithelia of the branching ureteric bud and in nephrogenic mesenchyme during its transition into renal tubules. WNT signaling activity is rapidly downregulated in maturing nephrons and becomes undetectable in postnatal kidney. Sites of TCF/betaGal activity are in proximity to the known sites of renal WNT2b and WNT4 expression, and these WNTs stimulate TCF reporter activity in kidney cell lines derived from ureteric bud and metanephric mesenchyme lineages. When fetal kidney explants from HoxB7/GFP mice were exposed to the canonical WNT signaling pathway inhibitor, Dickkopf-1, arborization of the ureteric bud was significantly reduced. We conclude that restricted zones of intense canonical WNT signaling drive branching nephrogenesis in fetal kidney.

Tissue, cellular and sub-cellular localization of the Vangl2 protein during embryonic development: effect of the Lp mutation.

Loop-tail (Lp) mice show a very severe neural tube defect, craniorachischisis, which is caused by mis-sense mutations in the Vangl2 gene. The membrane protein Vangl2 belongs to a highly conserved group of proteins that regulate planar polarity in certain epithelia, and that are also important for convergent extension movements during gastrulation and neurulation. A specific anti-Vangl2 antiserum was produced and used to examine the tissue, cell type, and sub-cellular localization of Vangl2 during embryogenesis. Vangl2 protein is expressed at high levels in the neural tube and shows a dynamic expression profile during neurulation. After neural tube closure, robust Vangl2 staining is detected in several neural and neurosensory tissues, including cerebral cortex, dorsal root ganglia, olfactory epithelium, retina, mechanosensory hair cells of the cochlea, and optic nerve. Vangl2 is also expressed during organogenesis in a number of tubular epithelia, including the bronchial tree, intestinal crypt/villus axis, and renal tubular segments derived from ureteric bud and from metanephric mesenchyme. Examination of Vangl2 localization in the neural tubes and cochleas of the normal and Lp/Lp embryos shows disruption of normal membrane localization of Vangl2 in independent alleles at Lp (Lp, Lp(m1Jus)) as well as overall decrease in the expression level.