Signalling of the BCR is regulated by a lipid rafts-localised transcription factor, Bright

Post-transcriptional (re)programming of B lymphocyte development: From bench to bedside?

Hematopoiesis, a process which generates blood and immune cells, changes significantly during mammalian development. Definitive hematopoiesis is marked by the emergence of long-term hematopoietic stem cells (HSCs). Here, we will focus on the post-transcriptional differences between fetal liver (FL) and adult bone marrow (ABM) HSCs. It remains unclear how or why exactly FL HSCs transition to ABM HSCs, but we aim to leverage their differences to revive an old idea: in utero HSC transplantation. Unexpectedly, the expression of certain RNA-binding proteins (RBPs) play an important role in HSC specification, and can be employed to convert or reprogram adult HSCs back to a fetal-like state. Among other features, FL HSCs have a broad differentiation capacity that includes the ability to regenerate both conventional B and T cells, as well as innate-like or unconventional lymphocytes such as B-1a and marginal zone B (MzB) cells. This chapter will focus on RNA binding proteins, namely LIN28B and IGF2BP3, that are expressed during fetal life and how they promote B-1a cell development. Furthermore, this chapter considers a potential clinical application of synthetic co-expression of LIN28B and IGF2BP3 in HSCs.

Mechanisms and functions of SUMOylation in health and disease: a review focusing on immune cells

SUMOylation, which is a type of post-translational modification that involves covalent conjugation of small ubiquitin-like modifier (SUMO) proteins to target substrates, regulates various important molecular and cellular processes, including transcription, the cell cycle, cell signaling, and DNA synthesis and repair. Newly synthesized SUMO is immature and cleaved by the SUMO-specific protease family, resulting in exposure of the C-terminal Gly-Gly motif to become the mature form. In the presence of ATP, mature SUMO is conjugated with the activating enzyme E1 through the cysteine residue of E1, followed by transfer to the cysteine residue of E2-conjugating enzyme Ubc9 in humans that recognizes and modifies the lysine residue of a substrate protein. E3 SUMO ligases promote SUMOylation. SUMOylation is a reversible modification and mediated by SUMO-specific proteases. Cumulative studies have indicated that SUMOylation affects the functions of protein substrates in various manners, including cellular localization and protein stability. Gene knockout studies in mice have revealed that several SUMO cycling machinery proteins are crucial for the development and differentiation of various cell lineages, including immune cells. Aberrant SUMOylation has been implicated in several types of diseases, including cancers, cardiovascular diseases, and autoimmune diseases. This review summarizes the biochemistry of SUMO modification and the general biological functions of proteins involved in SUMOylation. In particular, this review focuses on the molecular mechanisms by which SUMOylation regulates the development, maturation, and functions of immune cells, including T, B, dendritic, and myeloid cells. This review also discusses the underlying relevance of disruption of SUMO cycling and site-specific interruption of SUMOylation on target proteins in immune cells in diseases, including cancers and infectious diseases.

ARID3a from the ARID family: structure, role in autoimmune diseases and drug discovery

The AT-rich interaction domain (ARID) family of DNA-binding proteins is a group of transcription factors and chromatin regulators with a highly conserved ARID domain that recognizes specific AT-rich DNA sequences. Dysfunction of ARID family members has been implicated in various human diseases including cancers and intellectual disability. Among them, ARID3a has gained increasing attention due to its potential involvement in autoimmunity. In this article we provide an overview of the ARID family, focusing on the structure and biological functions of ARID3a. It explores the role of ARID3a in autoreactive B cells and its contribution to autoimmune diseases such as systemic lupus erythematosus and primary biliary cholangitis. Furthermore, we also discuss the potential for drug discovery targeting ARID3a and present a plan for future research in this field.

Transcription factor TFII-I fine tunes innate properties of B lymphocytes

The ubiquitously expressed transcription factor TFII-I is a multifunctional protein with pleiotropic roles in gene regulation. TFII-I associated polymorphisms are implicated in Sjögren's syndrome and Lupus in humans and, germline deletion of the Gtf2i gene in mice leads to embryonic lethality. Here we report a unique role for TFII-I in homeostasis of innate properties of B lymphocytes. Loss of Gtf2i in murine B lineage cells leads to an alteration in transcriptome, chromatin landscape and associated transcription factor binding sites, which exhibits myeloid-like features and coincides with enhanced sensitivity to LPS induced gene expression. TFII-I deficient B cells also show increased switching to IgG3, a phenotype associated with inflammation. These results demonstrate a role for TFII-I in maintaining immune homeostasis and provide clues for GTF2I polymorphisms associated with B cell dominated autoimmune diseases in humans.

Structures and nucleic acid-binding preferences of the eukaryotic ARID domain

The DNA-binding AT-rich interactive domain (ARID) exists in a wide range of proteins throughout eukaryotic kingdoms. ARID domain-containing proteins are involved in manifold biological processes, such as transcriptional regulation, cell cycle control and chromatin remodeling. Their individual domain composition allows for a sub-classification within higher mammals. ARID is categorized as binder of double-stranded AT-rich DNA, while recent work has suggested ARIDs as capable of binding other DNA motifs and also recognizing RNA. Despite a broad variability on the primary sequence level, ARIDs show a highly conserved fold, which consists of six α-helices and two loop regions. Interestingly, this minimal core domain is often found extended by helices at the N- and/or C-terminus with potential roles in target specificity and, subsequently function. While high-resolution structural information from various types of ARIDs has accumulated over two decades now, there is limited access to ARID-DNA complex structures. We thus find ourselves left at the beginning of understanding ARID domain target specificities and the role of accompanying domains. Here, we systematically summarize ARID domain conservation and compare the various types with a focus on their structural differences and DNA-binding preferences, including the context of multiple other motifs within ARID domain containing proteins.

Expression Signature of the AT-Rich Interactive Domain Gene Family Identified in Digestive Cancer

Background

The AT-rich interactive domain (ARID) gene family of 15 proteins has an important role in development and proliferation. Gene expression alterations of the ARID family are correlated with the pathogenesis of digestive cancer, but systematic research has not been conducted.

Methods

We obtained transcriptome sequencing data, clinical characteristics and stemness indices of the seven main types of digestive cancer (cholangiocarcinoma, colon adenocarcinoma, oesophageal carcinoma, liver hepatocellular carcinoma, pancreatic adenocarcinoma, rectum adenocarcinoma and stomach adenocarcinoma) from public pan-cancer data to combine the analysis of the expression and prognostic signature of the ARID gene family. The stromal and immune scores for each sample were calculated to explore the correlations between the ARID gene family members and the tumour microenvironment.

Results

After screening, 1,920 digestive cancer samples were included in our study. ARID3C was expressed at low levels throughout the digestive cancer samples. The expression levels of ARID1A and JARID1C were relatively high, but there was striking heterogeneity across the different cancer types for specific family members. The survival analysis indicated that many genes were significantly related to the prognosis of patients with liver hepatocellular carcinoma. The stemness indices, stromal score, and immune score analysis showed that the expression of a single ARID gene had characteristic consistency in each tumour, but the levels among the different genes still varied.

Conclusion

Our systematic study of the ARID gene family and its association with the immune infiltrate, tumour microenvironment and outcomes of digestive cancer patients focus on the complex relations and indicate the need to study each ARID member as an individual in a specific cancer type.

Lipid Rafts Interaction of the ARID3A Transcription Factor with EZRIN and G-Actin Regulates B-Cell Receptor Signaling

Several diseases originate via dysregulation of the actin cytoskeleton. The ARID3A/Bright transcription factor has also been implicated in malignancies, primarily those derived from hematopoietic lineages. Previously, we demonstrated that ARID3A shuttles between the nucleus and the plasma membrane, where it localizes within lipid rafts. There it interacts with components of the B-cell receptor (BCR) to reduce its ability to transmit downstream signaling. We demonstrate here that a direct component of ARID3A-regulated BCR signal strength is cortical actin. ARID3A interacts with actin exclusively within lipid rafts via the actin-binding protein EZRIN, which confines unstimulated BCRs within lipid rafts. BCR ligation discharges the ARID3A-EZRIN complex from lipid rafts, allowing the BCR to initiate downstream signaling events. The ARID3A-EZRIN interaction occurs almost exclusively within unpolymerized G-actin, where EZRIN interacts with the multifunctional ARID3A REKLES domain. These observations provide a mechanism by which a transcription factor directly regulates BCR signaling via linkage to the actin cytoskeleton with consequences for B-cell-related neoplasia.

Native-like membrane models of E. coli polar lipid extract shed light on the importance of lipid composition complexity

BACKGROUND

Lipid-protein interactions stabilize protein oligomers, shape their structure, and modulate their function. Whereas in vitro experiments already account for the functional importance of lipids by using natural lipid extracts, in silico methods lack behind by embedding proteins in single component lipid bilayers. However, to accurately complement in vitro experiments with molecular details at very high spatio-temporal resolution, molecular dynamics simulations have to be performed in natural(-like) lipid environments.

RESULTS

To enable more accurate MD simulations, we have prepared four membrane models of E. coli polar lipid extract, a typical model organism, each at all-atom (CHARMM36) and coarse-grained (Martini3) representations. These models contain all main lipid headgroup types of the E. coli inner membrane, i.e., phosphatidylethanolamines, phosphatidylglycerols, and cardiolipins, symmetrically distributed between the membrane leaflets. The lipid tail (un)saturation and propanylation stereochemistry represent the bacterial lipid tail composition of E. coli grown at 37∘C until 3/4 of the log growth phase. The comparison of the Simple three lipid component models to the complex 14-lipid component model Avanti over a broad range of physiologically relevant temperatures revealed that the balance of lipid tail unsaturation and propanylation in different positions and inclusion of lipid tails of various length maintain realistic values for lipid mobility, membrane area compressibility, lipid ordering, lipid volume and area, and the bilayer thickness. The only Simple model that was able to satisfactory reproduce most of the structural properties of the complex Avanti model showed worse agreement of the activation energy of basal water permeation with the here performed measurements. The Martini3 models reflect extremely well both experimental and atomistic behavior of the E. coli polar lipid extract membranes. Aquaporin-1 embedded in our native(-like) membranes causes partial lipid ordering and membrane thinning in its vicinity. Moreover, aquaporin-1 attracts and temporarily binds negatively charged lipids, mainly cardiolipins, with a distinct cardiolipin binding site in the crevice at the contact site between two monomers, most probably stabilizing the tetrameric protein assembly.

CONCLUSIONS

The here prepared and validated membrane models of E. coli polar lipids extract revealed that lipid tail complexity, in terms of double bond and cyclopropane location and varying lipid tail length, is key to stabilize membrane properties over a broad temperature range. In addition, they build a solid basis for manifold future simulation studies on more realistic lipid membranes bridging the gap between simulations and experiments.

Crucial Role of Increased Arid3a at the Pre-B and Immature B Cell Stages for B1a Cell Generation

The Lin28b⁺Let7⁻ axis in fetal/neonatal development plays a role in promoting CD5⁺ B1a cell generation as a B-1 B cell developmental outcome. Here we identify the Let7 target, Arid3a, as a crucial molecular effector of the B-1 cell developmental program. Arid3a expression is increased at pro-B cell stage and markedly increased at pre-B and immature B cell stages in the fetal/neonatal liver B-1 development relative to that in the Lin28b⁻Let7⁺ adult bone marrow (BM) B-2 cell development. Analysis of B-lineage restricted Lin28b transgenic (Tg) mice, Arid3a knockout and Arid3a Tg mice, confirmed that increased Arid3a allows B cell generation without requiring surrogate light chain (SLC) associated pre-BCR stage, and prevents MHC class II cell expression at the pre-B and newly generated immature B cell stages, distinct from pre-BCR dependent B development with MHC class II in adult BM. Moreover, Arid3a plays a crucial role in supporting B1a cell generation. The increased Arid3a leads higher Myc and Bhlhe41, and lower Siglec-G and CD72 at the pre-B and immature B cell stages than normal adult BM, to allow BCR signaling induced B1a cell generation. Arid3a-deficiency selectively blocks the development of B1a cells, while having no detectable effect on CD5⁻ B1b, MZ B, and FO B cell generation resembling B-2 development outcome. Conversely, enforced expression of Arid3a by transgene is sufficient to promote the development of B1a cells from adult BM. Under the environment change between birth to adult, altered BCR repertoire in increased B1a cells occurred generated from adult BM. However, crossed with B1a-restricted V_H/D/J IgH knock-in mice allowed to confirm that SLC-unassociated B1a cell increase and CLL/lymphoma generation can occur in aged from Arid3a increased adult BM. These results confirmed that in fetal/neonatal normal mice, increased Arid3a at the pre-B cell and immature B cell stages is crucial for generating B1a cells together with the environment for self-ligand reactive BCR selection, B1a cell maintenance, and potential for development of CLL/Lymphoma in aged mice.

Myosin 1g Contributes to CD44 Adhesion Protein and Lipid Rafts Recycling and Controls CD44 Capping and Cell Migration in B Lymphocytes

Cell migration and adhesion are critical for immune system function and involve many proteins, which must be continuously transported and recycled in the cell. Recycling of adhesion molecules requires the participation of several proteins, including actin, tubulin, and GTPases, and of membrane components such as sphingolipids and cholesterol. However, roles of actin motor proteins in adhesion molecule recycling are poorly understood. In this study, we identified myosin 1g as one of the important motor proteins that drives recycling of the adhesion protein CD44 in B lymphocytes. We demonstrate that the lack of Myo1g decreases the cell-surface levels of CD44 and of the lipid raft surrogate GM1. In cells depleted of Myo1g, the recycling of CD44 was delayed, the delay seems to be caused at the level of formation of recycling complex and entry into recycling endosomes. Moreover, a defective lipid raft recycling in Myo1g-deficient cells had an impact both on the capping of CD44 and on cell migration. Both processes required the transportation of lipid rafts to the cell surface to deliver signaling components. Furthermore, the extramembrane was essential for cell expansion and remodeling of the plasma membrane topology. Therefore, Myo1g is important during the recycling of lipid rafts to the membrane and to the accompanied proteins that regulate plasma membrane plasticity. Thus, Myosin 1g contributes to cell adhesion and cell migration through CD44 recycling in B lymphocytes.

Dual control of pcdh8l/PCNS expression and function in Xenopus laevis neural crest cells by adam13/33 via the transcription factors tfap2α and arid3a

Adam13/33 is a cell surface metalloprotease critical for cranial neural crest (CNC) cell migration. It can cleave multiple substrates including itself, fibronectin, ephrinB, cadherin-11, pcdh8 and pcdh8l (this work). Cleavage of cadherin-11 produces an extracellular fragment that promotes CNC migration. In addition, the adam13 cytoplasmic domain is cleaved by gamma secretase, translocates into the nucleus and regulates multiple genes. Here, we show that adam13 interacts with the arid3a/dril1/Bright transcription factor. This interaction promotes a proteolytic cleavage of arid3a and its translocation to the nucleus where it regulates another transcription factor: tfap2α. Tfap2α in turn activates multiple genes including the protocadherin pcdh8l (PCNS). The proteolytic activity of adam13 is critical for the release of arid3a from the plasma membrane while the cytoplasmic domain appears critical for the cleavage of arid3a. In addition to this transcriptional control of pcdh8l, adam13 cleaves pcdh8l generating an extracellular fragment that also regulates cell migration.

miRNAs in B Cell Development and Lymphomagenesis

B lymphocytes are essential for an efficient immune response against a variety of pathogens. A large fraction of hematologic malignancies is of B cell origin, suggesting that the development and activation of B cells need to be tightly regulated. In recent years, increasing evidence has emerged demonstrating that microRNAs (miRNAs) - a class of non-coding RNAs that control gene expression - are involved in the regulation of B cell development and function. We provide here an overview of the current knowledge on the role of miRNAs and their relevant targets in B cell development, B cell activation, and B cell malignant transformation.

Pathophysiology of TFII-I: Old Guard Wearing New Hats

The biochemical properties of the signal-induced multifunctional transcription factor II-I (TFII-I) indicate that it is involved in a variety of gene regulatory processes. Although gene ablation in murine models and cell-based assays show that it is encoded by an essential gene, GTF2I/Gtf2i, its physiologic role in human disorders was relatively unknown until recently. Novel studies show that it is involved in an array of human diseases including neurocognitive disorders, systemic lupus erythematosus (SLE), and cancer. Here I bring together these diverse observations to illustrate its multiple pathophysiologic functions and further conjecture on how these could be related to its known biochemical properties. I expect that a better understanding of these 'structure-function' relationships would lead to future diagnostic and/or therapeutic potential.

Arid3a regulates mesoderm differentiation in mouse embryonic stem cells

Research into regulation of the differentiation of stem cells is critical to understanding early developmental decisions and later development growth. The transcription factor ARID3A previously was shown to be critical for trophectoderm and hematopoetic development. Expression of ARID3A increases during embryonic differentiation, but the underlying reason remained unclear. Here we show that Arid3a null embryonic stem (ES) cells maintain an undifferentiated gene expression pattern and form teratomas in immune-compromised mice. However, Arid3a null ES cells differentiated in vitro into embryoid bodies (EBs) significantly faster than control ES cells, and the majority forming large cystic embryoid EBs. Analysis of gene expression during this transition indicated that Arid3a nulls differentiated spontaneously into mesoderm and neuroectoderm lineages. While young ARID3A-deficient mice showed no gross tissue morphology, proliferative and structural abnormalities were observed in the kidneys of older null mice. Together these data suggest that ARID3A is not only required hematopoiesis, but is critical for early mesoderm differentiation.

LPS-Induced Macrophage Activation and Plasma Membrane Fluidity Changes are Inhibited Under Oxidative Stress

Macrophage activation is essential for a correct and efficient response of innate immunity. During oxidative stress membrane receptors and/or membrane lipid dynamics can be altered, leading to dysfunctional cell responses. Our aim is to analyze membrane fluidity modifications and cell function under oxidative stress in LPS-activated macrophages. Membrane fluidity of individual living THP-1 macrophages was evaluated by the technique two-photon microscopy. LPS-activated macrophage function was determined by TNFα secretion. It was shown that LPS activation causes fluidification of macrophage plasma membrane and production of TNFα. However, oxidative stress induces rigidification of macrophage plasma membrane and inhibition of cell activation, which is evidenced by a decrease of TNFα secretion. Thus, under oxidative conditions macrophage proinflammatory response might develop in an inefficient manner.

A Network Model to Describe the Terminal Differentiation of B Cells

Terminal differentiation of B cells is an essential process for the humoral immune response in vertebrates and is achieved by the concerted action of several transcription factors in response to antigen recognition and extracellular signals provided by T-helper cells. While there is a wealth of experimental data regarding the molecular and cellular signals involved in this process, there is no general consensus regarding the structure and dynamical properties of the underlying regulatory network controlling this process. We developed a dynamical model of the regulatory network controlling terminal differentiation of B cells. The structure of the network was inferred from experimental data available in the literature, and its dynamical behavior was analyzed by modeling the network both as a discrete and a continuous dynamical systems. The steady states of these models are consistent with the patterns of activation reported for the Naive, GC, Mem, and PC cell types. Moreover, the models are able to describe the patterns of differentiation from the precursor Naive to any of the GC, Mem, or PC cell types in response to a specific set of extracellular signals. We simulated all possible single loss- and gain-of-function mutants, corroborating the importance of Pax5, Bcl6, Bach2, Irf4, and Blimp1 as key regulators of B cell differentiation process. The model is able to represent the directional nature of terminal B cell differentiation and qualitatively describes key differentiation events from a precursor cell to terminally differentiated B cells.

Generation of antibody-producing cells through terminal B cell differentiation represents a good model to study the formation of multiple effector cells from a progenitor cell type. This process is controlled by the action of several molecules that maintain cell type specific programs in response to cytokines, antigen recognition and the direct contact with T helper cells, forming a complex regulatory network. While there is a large body of experimental data regarding some of the key molecules involved in this process and there have been several efforts to reconstruct the underlying regulatory network, a general consensus about the structure and dynamical behavior of this network is lacking. Moreover, it is not well understood how this network controls the establishment of specific B cell expression patterns and how it responds to specific external signals. We present a model of the regulatory network controlling terminal B cell differentiation and analyze its dynamical behavior under normal and mutant conditions. The model recovers the patterns of differentiation of B cells and describes a large set of gain- and loss-of-function mutants. This model provides an unified framework to generate qualitative descriptions to interpret the role of intra- and extracellular regulators of B cell differentiation.

Perspectives on fetal derived CD5+ B1 B cells

CD5(+) B-cell origins and their predisposition to lymphoma are long-standing issues. Transfer of fetal and adult liver BM Pro-B cells generates B cells with distinct phenotypes: fetal cells generate IgM(high) IgD(low) CD5(+) , whereas adult cells IgM(low) IgD(high) CD5(-) . This suggests a developmental switch in B lymphopoiesis, similar to the switch in erythropoiesis. Comparison of mRNA and miRNA expression in fetal and adult Pro-B cells revealed differential expression of Lin28b mRNA and Let-7 miRNA, providing evidence that this regulatory axis functions in the switch. Recent work has shown that Arid3a is a key transcription factor mediating fetal-type B-cell development. Lin28b-promoted fetal development generates CD5(+) B cells as a consequence of positively selected self-reactivity. CD5(+) B cells play important roles in clearance of apoptotic cells and in protective immune responses, but also pose a risk of progression to leukemia/lymphoma. Differential Lin28b expression in fetal and adult human B-cell precursors showed that human B-cell development may resemble mouse, with self-reactive "innate-like" B cells generated early in life. It remains to be determined whether such human B cells have a higher propensity to leukemic progression. This review describes our recent research with CD5(+) B cells and presents our perspective on their role in disease.

Entry of Francisella tularensis into Murine B Cells: The Role of B Cell Receptors and Complement Receptors

Francisella tularensis, the etiological agent of tularemia, is an intracellular pathogen that dominantly infects and proliferates inside phagocytic cells but can be seen also in non-phagocytic cells, including B cells. Although protective immunity is known to be almost exclusively associated with the type 1 pathway of cellular immunity, a significant role of B cells in immune responses already has been demonstrated. Whether their role is associated with antibody-dependent or antibody-independent B cell functions is not yet fully understood. The character of early events during B cell–pathogen interaction may determine the type of B cell response regulating the induction of adaptive immunity. We used fluorescence microscopy and flow cytometry to identify the basic requirements for the entry of F. tularensis into B cells within in vivo and in vitro infection models. Here, we present data showing that Francisella tularensis subsp. holarctica strain LVS significantly infects individual subsets of murine peritoneal B cells early after infection. Depending on a given B cell subset, uptake of Francisella into B cells is mediated by B cell receptors (BCRs) with or without complement receptor CR1/2. However, F. tularensis strain FSC200 ΔiglC and ΔftdsbA deletion mutants are defective in the ability to enter B cells. Once internalized into B cells, F. tularensis LVS intracellular trafficking occurs along the endosomal pathway, albeit without significant multiplication. The results strongly suggest that BCRs alone within the B-1a subset can ensure the internalization process while the BCRs on B-1b and B-2 cells need co-signaling from the co receptor containing CR1/2 to initiate F. tularensis engulfment. In this case, fluidity of the surface cell membrane is a prerequisite for the bacteria’s internalization. The results substantially underline the functional heterogeneity of B cell subsets in relation to F. tularensis.

The nanoscale organization of signaling domains at the plasma membrane

In this chapter, we present an overview of the role of the nanoscale organization of signaling domains in regulating key cellular processes. In particular, we illustrate the importance of protein and lipid nanodomains as triggers and mediators of cell signaling. As particular examples, we summarize the state of the art of understanding the role of nanodomains in the mounting of an immune response, cellular adhesion, intercellular communication, and cell proliferation. Thus, this chapter underlines the essential role the nanoscale organization of key signaling proteins and lipid domains. We will also see how nanodomains play an important role in the lifecycle of many pathogens relevant to human disease and therefore illustrate how these structures may become future therapeutic targets.

Lin28b promotes fetal B lymphopoiesis through the transcription factor Arid3a

Zhou et al. demonstrate a requirement for the Let-7–Lin28b axis regulating a shift in development between fetal liver and bone marrow B lymphocyte progenitors in the generation of B1 versus B2 B cells. Specifically, the transcription factor Arid3a, induced by Lin28b and a target of Let-7 miRNA, is sufficient to recapitulate fetal B cell development from bone marrow progenitors.

Mouse B cell precursors from fetal liver and adult bone marrow (BM) generate distinctive B cell progeny when transplanted into immunodeficient recipients, supporting a two-pathway model for B lymphopoiesis, fetal “B-1” and adult “B-2.” Recently, Lin28b was shown to be important for the switch between fetal and adult pathways; however, neither the mechanism of Lin28b action nor the importance of B cell antigen receptor (BCR) signaling in this process was addressed. Here, we report key advances in our understanding of the regulation of B-1/B-2 development. First, modulation of Let-7 in fetal pro-B cells is sufficient to alter fetal B-1 development to produce B cells resembling the progeny of adult B-2 development. Second, intact BCR signaling is required for the generation of B1a B cells from Lin28b-transduced BM progenitors, supporting a requirement for ligand-dependent selection, as is the case for normal B1a B cells. Third, the V_H repertoire of Lin28b-induced BM B1a B cells differs from that of normal B1a, suggesting persisting differences from fetal progenitors. Finally, we identify the Arid3a transcription factor as a key target of Let-7, whose ectopic expression is sufficient to induce B-1 development in adult pro-B cells and whose silencing by knockdown blocks B-1 development in fetal pro-B cells.

Core fucosylation of IgG B cell receptor is required for antigen recognition and antibody production

Ag recognition and Ab production in B cells are major components of the humoral immune response. In the current study, we found that the core fucosylation catalyzed by α1,6-fucosyltransferase (Fut8) was required for the Ag recognition of BCR and the subsequent signal transduction. Moreover, compared with the 3-83 B cells, the coalescing of lipid rafts and Ag-BCR endocytosis were substantially reduced in Fut8-knockdown (3-83-KD) cells with p31 stimulation and then completely restored by reintroduction of the Fut8 gene to the 3-83-KD cells. Indeed, Fut8-null (Fut8(-/-)) mice evoked a low immune response following OVA immunization. Also, the frequency of IgG-producing cells was significantly reduced in the Fut8(-/-) spleen following OVA immunization. Our results clearly suggest an unexpected mode of BCR function, in which the core fucosylation of IgG-BCR mediates Ag recognition and, concomitantly, cell signal transduction via BCR and Ab production.

Role of Bruton's tyrosine kinase inhibitors in HIV-1-infected cells

Many cellular cofactors have been documented to be critical for various stages of viral replication. Using high-throughput proteomic assays, we have previously identified Bruton's tyrosine kinase (BTK) as a host protein that was uniquely upregulated in the plasma membrane of human immunodeficiency virus (HIV-1)-infected T cells. Here, we have further characterized the BTK expression in HIV-1 infection and show that this cellular factor is specifically expressed in infected myeloid cells. Significant upregulation of the phosphorylated form of BTK was observed in infected cells. Using size exclusion chromatography, we found BTK to be virtually absent in the uninfected U937 cells; however, new BTK protein complexes were identified and distributed in both high molecular weight (∼600 kDa) and a small molecular weight complex (∼60-120 kDa) in the infected U1 cells. BTK levels were highest in cells either chronically expressing virus or induced/infected myeloid cells and that BTK translocated to the membrane following induction of the infected cells. BTK knockdown in HIV-1-infected cells using small interfering RNA (siRNA) resulted in selective death of infected, but not uninfected, cells. Using BTK-specific antibody and small-molecule inhibitors including LFM-A13 and a FDA-approved compound, ibrutinib (PCI-32765), we have found that HIV-1-infected cells are sensitive to apoptotic cell death and result in a decrease in virus production. Overall, our data suggests that HIV-1-infected cells are sensitive to treatments targeting BTK expressed in infected cells.

Role of tissue-specific AT-rich DNA sequence-binding proteins in lymphocyte differentiation

A great many transcription factors, cytokines, and cytokine receptors have been identified as indispensable elements in lymphocyte differentiation, but the molecular mechanism that orchestrates the expression and function of these molecular factors is unknown. The process of lymphocyte differentiation involves both the simultaneous activation of lymphoid-related genes and the inactivation of non-lymphoid lineage-related genes, suggesting that there should be critical molecules that regulate such gene expression in both temporal and spatial dimensions. Recent studies of chromatin-remodeling proteins shed light on this complex process. In particular, special AT-rich sequence-binding protein 1 has been studied extensively. In this article, we review the wealth of information characterizing this protein.

The Bright Side of Hematopoiesis: Regulatory Roles of ARID3a/Bright in Human and Mouse Hematopoiesis

ARID3a/Bright is a DNA-binding protein that was originally discovered for its ability to increase immunoglobulin transcription in antigen-activated B cells. It interacts with DNA as a dimer through its ARID, or A/T-rich interacting domain. In association with other proteins, ARID3a increased transcription of the immunoglobulin heavy chain and led to improved chromatin accessibility of the heavy chain enhancer. Constitutive expression of ARID3a in B lineage cells resulted in autoantibody production, suggesting its regulation is important. Abnormal ARID3a expression has also been associated with increased proliferative capacity and malignancy. Roles for ARID3a in addition to interactions with the immunoglobulin locus were suggested by transgenic and knockout mouse models. Over-expression of ARID3a resulted in skewing of mature B cell subsets and altered gene expression patterns of follicular B cells, whereas loss of function resulted in loss of B1 lineage B cells and defects in hematopoiesis. More recent studies showed that loss of ARID3a in adult somatic cells promoted developmental plasticity, alterations in gene expression patterns, and lineage fate decisions. Together, these data suggest new regulatory roles for ARID3a. The genes influenced by ARID3a are likely to play pivotal roles in lineage decisions, highlighting the importance of this understudied transcription factor.

Lipid Raft is required for PSGL-1 ligation induced HL-60 cell adhesion on ICAM-1

P-selectin glycoprotein ligand-1 (PSGL-1) and integrins are adhesion molecules that play critical roles in host defense and innate immunity. PSGL-1 mediates leukocyte rolling and primes leukocytes for integrin-mediated adhesion. However, the mechanism that PSGL-1 as a rolling receptor in regulating integrin activation has not been well characterized. Here, we investigate the function of lipid raft in regulating PSGL-1 induced β2 integrin-mediated HL-60 cells adhesion. PSGL-1 ligation with antibody enhances the β2 integrin activation and β2 integrin-dependent adhesion to ICAM-1. Importantly, with the treatment of methyl-β-cyclodextrin (MβCD), we confirm the role of lipid raft in regulating the activation of β2 integrin. Furthermore, we find that the protein level of PSGL-1 decreased in raft fractions in MβCD treated cells. PSGL-1 ligation induces the recruitment of spleen tyrosine kinase (Syk), a tyrosine kinase and Vav1 (the pivotal downstream effector of Syk signaling pathway involved in cytoskeleton regulation) to lipid raft. Inhibition of Syk activity with pharmacologic inhibitor strongly reduces HL-60 cells adhesion, implicating Syk is crucial for PSGL-1 mediated β2 integrin activation. Taken together, we report that ligation of PSGL-1 on HL-60 cells activates β2 integrin, for which lipid raft integrity and Syk activation are responsible. These findings have shed new light on the mechanisms that connect leukocyte initial rolling with subsequent adhesion.

Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT₁A receptor: characterization and subcellular distribution of sumoylated 5-HT₁A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT1A-R is ∼55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT1A-Rs examined. Acute stimulation of 5-HT1A-Rs significantly increased SUMO1-5-HT1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT1A-Rs are an inactive form of 5-HT1A-Rs, a finding further supported by results showing minimal 5-HT1A-R agonist binding to SUMO1-5-HT1A-Rs. Furthermore, SUMO1-5-HT1A-Rs in the DRM were increased by treatment with a 5-HT1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT1A-Rs may be related to 5-HT1A-R trafficking and internalization, which may contribute to 5-HT1A-R desensitization. Since 5-HT1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.

Biochemistry and biology of the inducible multifunctional transcription factor TFII-I: 10 years later

Exactly twenty years ago TFII-I was discovered as a biochemical entity that was able to bind to and function via a core promoter element called the Initiator (Inr). Since then several different properties of this signal-induced multifunctional factor were discovered. Here I update these ever expanding functions of TFII-I--focusing primarily on the last ten years since the first review appeared in this journal.

Characterization of a new ARID family transcription factor (Brightlike/ARID3C) that co-activates Bright/ARID3A-mediated immunoglobulin gene transcription

Two members, Bright/ARID3A and Bdp/ARID3B, of the ARID (AT-Rich Interaction Domain) transcription family are distinguished by their ability to specifically bind to DNA and to self-associate via a second domain, REKLES. Bright and Bdp positively regulate immunoglobulin heavy chain gene (IgH) transcription by binding to AT-rich motifs within Matrix Associating Regions (MARs) residing within a subset of V(H) promoters and the Eμ intronic enhancer. In addition, REKLES provides Bright nuclear export function, and a small pool of Bright is directed to plasma membrane sub-domains/lipid rafts where it associates with and modulates signaling of the B cell antigen receptor (BCR). Here, we characterize a third, highly conserved, physically condensed ARID3 locus, Brightlike/ARID3C. Brightlike encodes two alternatively spliced, SUMO-I-modified isoforms that include or exclude (Δ6) the REKLES-encoding exon 6. Brightlike transcripts and proteins are expressed preferentially within B lineage lymphocytes and coordinate with highest Bright expression in activated follicular B cells. Brightlike, but not BrightlikeΔ6, undergoes nuclear-cytoplasmic shuttling with a fraction localizing within lipid rafts following BCR stimulation. Brightlike, but not BrightlikeΔ6, associates with Bright in solution, at common DNA binding sites in vitro, and is enriched at Bright binding sites in chromatin. Although possessing little transactivation capacity of its own, Brightlike significantly co-activates Bright-dependent IgH transcription with maximal activity mediated by the unsumoylated form. In sum, this report introduces Brightlike as an additional functional member of the family of ARID proteins, which should be considered in regulatory circuits, previously ascribed to be mediated by Bright.

Enhancer-promoter communication and transcriptional regulation of Igh

Transcriptional regulation of eukaryotic protein-coding genes requires the participation of site-specific transcription factors that bind distal regulatory elements, as well as factors that, together with RNA polymerase II, form the basal transcription machinery at the core promoter. Gene regulation requires proper communication between promoters and enhancers, often over great distances. Therefore, it is important to understand the potentially inter-related transcription factor interactions at both of these elements. How this is achieved on tissue-specific genes, such as the immunoglobulin heavy chain (IgH) in B cells remains unclear. Here, we review known interactions at the Igh variable region (V(H)) promoters and present our perspective on promoter-enhancer interactions that are likely important for Ig gene regulation in B cells.

Role of lipid rafts in liver health and disease

Liver diseases are an increasingly common cause of morbidity and mortality; new approaches for investigation of mechanisms of liver diseases and identification of therapeutic targets are emergent. Lipid rafts (LRs) are specialized domains of cellular membranes that are enriched in saturated lipids; they are small, mobile, and are key components of cellular architecture, protein partition to cellular membranes, and signaling events. LRs have been identified in the membranes of all liver cells, parenchymal and non-parenchymal; more importantly, LRs are active participants in multiple physiological and pathological conditions in individual types of liver cells. This article aims to review experimental-based evidence with regard to LRs in the liver, from the perspective of the liver as a whole organ composed of a multitude of cell types. We have gathered up-to-date information related to the role of LRs in individual types of liver cells, in liver health and diseases, and identified the possibilities of LR-dependent therapeutic targets in liver diseases.

Monte Carlo study of B-cell receptor clustering mediated by antigen crosslinking and directed transport

It is known from experiments that in the presence of soluble antigen, B-cell receptors (BCRs) assemble into microclusters and then collect into a macrocluster known as a 'cap'. However, the mechanisms of BCR cluster formation during recognition of soluble antigens remain unclear. In previous work, we demonstrated that effective intrinsic attractions among BCRs can lead to the formation of small microclusters of BCR molecules. The effective intrinsic attractions could be caused by multivalent antigen binding, association with lipid rafts, or other biochemical factors. In the present study, we have developed and studied a Monte Carlo model of BCR clustering mediated by explicit binding and crosslinking of soluble bivalent antigens. Antigen crosslinking is shown to microcluster BCRs in an affinity-dependent manner and also in a biologically relevant timescale; however, antigen crosslinking alone does not appear to be sufficient for the formation of a single macrocluster of receptor molecules. We show that directed transport of BCRs is needed to drive the formation of large macroclusters. We constructed a simple model of directed transport, where BCR molecules diffuse towards the largest cluster or towards a random BCR microcluster, which results in a single macrocluster of receptor molecules. The mechanisms for both types of directed transport are compared using network-based metrics. We also develop and use appropriate network measures to analyze the effect of BCR and antigen concentration on BCR clustering, the stability of the formed clusters over time and the size of BCR-antigen crosslinked chains.

The ARID family transcription factor bright is required for both hematopoietic stem cell and B lineage development

Bright/Arid3a has been characterized both as an activator of immunoglobulin heavy-chain transcription and as a proto-oncogene. Although Bright expression is highly B lineage stage restricted in adult mice, its expression in the earliest identifiable hematopoietic stem cell (HSC) population suggests that Bright might have additional functions. We showed that >99% of Bright(-/-) embryos die at midgestation from failed hematopoiesis. Bright(-/-) embryonic day 12.5 (E12.5) fetal livers showed an increase in the expression of immature markers. Colony-forming assays indicated that the hematopoietic potential of Bright(-/-) mice is markedly reduced. Rare survivors of lethality, which were not compensated by the closely related paralogue Bright-derived protein (Bdp)/Arid3b, suffered HSC deficits in their bone marrow as well as B lineage-intrinsic developmental and functional deficiencies in their peripheries. These include a reduction in a natural antibody, B-1 responses to phosphocholine, and selective T-dependent impairment of IgG1 class switching. Our results place Bright/Arid3a on a select list of transcriptional regulators required to program both HSC and lineage-specific differentiation.

Palmitoylation of prohibitin at cysteine 69 facilitates its membrane translocation and interaction with Eps 15 homology domain protein 2 (EHD2)

Plasma membrane translocation of specific cytosolic proteins plays an important role in cell signaling pathways. We have recently shown that prohibitin (PHB) , a protein present in the plasma membranes of various cell types, interacts with Eps 15 homology domain protein 2 (EHD2), a lipid raft protein. However, the mechanism involved in membrane translocation of PHB is not known.We report that PHB undergoes palmitoylation at cysteine 69 (Cys69), and that this palmitoylation is required for PHB's membrane translocation. Furthermore, we demonstrate that membrane translocation of PHB facilitates tyrosine phosphorylation and its interaction with EHD2. Thus, the palmitoylation and membrane translocation of PHB and its interaction with EHD2 may play a role in cell signaling.

The network of receptors characterize B cell receptor micro- and macroclustering in a Monte Carlo model

During the recognition of soluble antigens, B cell receptors (BCR) are known to form signaling clusters that can crucially modulate intracellular activation pathways and B cell response. Little is known about the precise nature of receptor cluster and its formation mechanism for the case of soluble antigens. Initial experiments have shown that B cell receptors first microcluster upon ligation with soluble antigens, and then coarsen into a macroscopic cap structure at one pole of a B cell. Such a mutual receptor-receptor attraction can arise locally due to cross-linking by soluble antigens among other possibilities. We develop an energy based Monte Carlo model to investigate the mechanism of B-cell receptor clustering upon ligation with soluble antigens. Our results show that mutual attraction between nearest neighbor receptor pairs can lead to microclustering of B cell receptors, but it is not sufficient for receptor macroclustering. A simple model of biased diffusion where BCR molecules experience a biased directed motion toward the largest cluster is then applied, which results in a single macrocluster of receptor molecules. The various types of receptor clusters are analyzed using the developed network-based metrics such as the average distance between any pairs of receptors.