TACI, an enigmatic BAFF/APRIL receptor, with new unappreciated biochemical and biological properties

BAFF is a B cell survival factor that binds to three receptors BAFF-R, TACI and BCMA. BAFF-R is the receptor triggering na?ve B cell survival and maturation while BCMA supports the survival of plasma cells in the bone marrow. Excessive BAFF production leads to autoimmunity, presumably as the consequence of inappropriate survival of self-reactive B cells. The function of TACI has been more elusive with TACI(-/-) mice revealing two sides of this receptor, a positive one driving T cell-independent immune responses and a negative one down-regulating B cell activation and expansion. Recent work has revealed that the regulation of TACI expression is intimately linked to the activation of innate receptors on B cells and that TACI signalling in response to multimeric BAFF and APRIL provides positive signals to plasmablasts. How TACI negatively regulates B cells remains elusive but may involve an indirect control of BAFF levels. The discovery of TACI mutations associated with common variable immunodeficiency (CVID) in humans not only reinforces its important role for humoral responses but also suggests a more complex role than first anticipated from knockout animals. TACI is emerging as an unusual TNF receptor-like molecule with a sophisticated mode of action.     

BAFF/APRIL系统研究进展

BAFF和APRIL是新近发现的TNF配体超家族2个关系密切的成员。虽然它们共用2个受体BCMA和TACI,但转基因和基因敲除小鼠模型揭示它们各自功能并不冗余。BAFF是外周血B细胞一个关键的存活与成熟因子,它的活性通过其特异性受体BAFF-R调节,其过表达与自身免疫性疾病发生和B细胞肿瘤形成有关。APRIL在不依赖T细胞的Ⅱ型抗原反应和T细胞存活过程中扮演重要角色,同时它也能诱导一些非淋巴细胞的增殖和存活,在许多肿瘤细胞株和肿瘤组织文库中发现有高表达的APRIL。针对BAFF/APRIL系统设计出治疗相关疾病的生物制剂有着非常令人乐观的应用前景。     

The role of BAFF and APRIL in rheumatoid arthritis

Development and activation of B cells quickly became clear after identifying new ligands and receptors in the tumor necrosis factor superfamily. B cell–activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are the members of membrane proteins Type 2 family released by proteolytic cleavage of furin to form active, soluble homotrimers. Except for B cells, ligands are expressed by all such immune cells like T cells, dendritic cells, monocytes, and macrophages. BAFF and APRIL have two common receptors, namely TNFR homolog transmembrane activator and Ca2+ modulator and CAML interactor (TACI) and B cell–maturation antigen. BAFF alone can also be coupled with a third receptor called BAFFR (also called BR3 or BLyS Receptor). These receptors are often expressed by immune cells in the B-cell lineage. The binding of BAFF or APRIL to their receptors supports B cells differentiation and proliferation, immunoglobulin production and the upregulation of B cell–effector molecules expression. It is possible that the overexpression of BAFF and APRIL contributes to the pathogenesis of autoimmune diseases. In BAFF transgenic mice, there is a pseudo-autoimmune manifestation, which is associated with an increase in B-lymphocytes, hyperglobulinemia, anti-single stranded DNA, and anti-double-stranded DNA antibodies, and immune complexes in their peripheral blood. Furthermore, overexpressing BAFF augments the number of peripheral B220+ B cells with a normal proliferation rate, high levels of Bcl2, and prolonged survival and hyperactivity. Therefore, in this review article, we studied BAFF and APRIL as important mediators in B-cell and discussed their role in rheumatoid arthritis.     

Expression and purification of APRIL by auto-induction

APRIL (A proliferation-inducing ligand) is a newly-identified member of the tumor necrosis factor family that induces pleiotropic biological responses, including immunological responses, IgA class switch and cell growth. It is associated with multiple diseases such as cancer and autoimmune diseases. High levels of APRIL mRNA can be detected in transformed cell lines and several malignant tumors; heparin sulfate proteoglycans (HSPG) are also involved in the APRIL tumor cell proliferation induction response. The interaction of APRIL and HSPG occurs through an N-terminal basic region on APRIL. We successfully expressed recombinant APRIL using an auto-induction system in Escherichia coli. By using in situ cleavage, we generated the mature form of APRIL, comprising its N-terminal basic region, which has full biological activity: receptor binding capability and proliferation induction activity.     

Plasma levels of BAFF and APRIL are elevated in patients with asthma in Saudi Arabia

B-cell activation factor (BAFF) and a proliferation-inducing ligand (APRIL) are members of the tumor necrosis factor superfamily of cytokines and can induce B cell activation, differentiation, and antibody production via interaction with their receptors, including transmembrane activator, calcium modulator, and cyclophilin ligand interactor (TACI), B-cell maturation antigen (BCMA), and B-cell activating factor receptor (BAFF-R). Herein, we assessed the plasma protein levels of BAFF and APRIL in patients with asthma to determine whether their expression is correlated with total IgE production and examined the surface expression of BAFF/APRIL receptors on B cells. Blood samples were collected from 47 patients with controlled asthma symptoms and 20 healthy normal controls, and plasma levels of APRIL, BAFF, and total IgE protein were quantified by corresponding ELISA assays. Furthermore, lymphocytes were isolated and B cells were analyzed for the presence of BAFF-R, BCMA, and TACI receptors using flow cytometry. Our results showed that IgE, BAFF, and APRIL plasma levels were markedly increased in patients with asthma compared with healthy controls. Moreover, expression of BAFF-R and BCMA, but not that of TACI, was significantly increased in patients with asthma compared with healthy controls. Overall, the findings suggest BAFF and APRIL as key mediators of asthma, and determination of their plasma levels may be useful in monitoring asthma symptoms and treatment response.     

The BAFF/APRIL system: Emerging functions beyond B cell biology and autoimmunity

The BAFF system plays a key role in the development of autoimmunity, especially in systemic lupus erythematosus (SLE). This often leads to the assumption that BAFF is mostly a B cell factor with a specific role in autoimmunity. Focus on BAFF and autoimmunity, driven by pharmaceutical successes with the recent approval of a novel targeted therapy Belimumab, has relegated other potential roles of BAFF to the background. Far from being SLE-specific, the BAFF system has a much broader relevance in infection, cancer and allergy. In this review, we provide the latest views on additional roles of the BAFF system in health and diseases, as well as an update on BAFF and autoimmunity, with particular focus on current clinical trials.     

New roles for the BLyS/BAFF family in antigen-experienced B cell niches

BLyS family members govern selection and survival of cells in the pre-immune B cell compartment, and emerging evidence suggests similar roles in antigen-experienced B cell pools. We review the features of this family, with particular emphasis on recent findings of how BLyS influences affinity maturation in germinal centers, which lie at the intersection of the pre-immune and antigen-experienced B cell compartments. We propose a model whereby tolerogenic selection at the transitional stage and affinity maturation in the germinal center employ the same BLyS driven mechanism.     

Stoichiometry of Heteromeric BAFF and APRIL Cytokines Dictates Their Receptor Binding and Signaling Properties

The closely related TNF family ligands B cell activation factor (BAFF) and a proliferation-inducing ligand (APRIL) serve in the generation and maintenance of mature B-lymphocytes. Both BAFF and APRIL assemble as homotrimers that bind and activate several receptors that they partially share. However, heteromers of BAFF and APRIL that occur in patients with autoimmune diseases are incompletely characterized. The N and C termini of adjacent BAFF or APRIL monomers are spatially close and can be linked to create single-chain homo- or hetero-ligands of defined stoichiometry. Similar to APRIL, heteromers consisting of one BAFF and two APRILs (BAA) bind to the receptors B cell maturation antigen (BCMA), transmembrane activator and CAML interactor (TACI) but not to the BAFF receptor (BAFFR). Heteromers consisting of one APRIL and two BAFF (ABB) bind to TACI and BCMA and weakly to BAFFR in accordance with the analysis of the receptor interaction sites in the crystallographic structure of ABB. Receptor binding correlated with activity in reporter cell line assays specific for BAFFR, TACI, or BCMA. Single-chain BAFF (BBB) and to a lesser extent single-chain ABB, but not APRIL or single-chain BAA, rescued BAFFR-dependent B cell maturation in BAFF-deficient mice. In conclusion, BAFF-APRIL heteromers of different stoichiometries have distinct receptor-binding properties and activities. Based on the observation that heteromers are less active than BAFF, we speculate that their physiological role might be to down-regulate BAFF activity.     

The Design and Characterization of Receptor-selective APRIL Variants

A proliferation-inducing ligand (APRIL), a member of the TNF ligand superfamily with an important role in humoral immunity, is also implicated in several cancers as a prosurvival factor. APRIL binds two different TNF receptors, B cell maturation antigen (BCMA) and transmembrane activator and cylclophilin ligand interactor (TACI), and also interacts independently with heparan sulfate proteoglycans. Because APRIL shares binding of the TNF receptors with B cell activation factor, separating the precise signaling pathways activated by either ligand in a given context has proven quite difficult. In this study, we have used the protein design algorithm FoldX to successfully generate a BCMA-specific variant of APRIL, APRIL-R206E, and two TACI-selective variants, D132F and D132Y. These APRIL variants show selective activity toward their receptors in several in vitro assays. Moreover, we have used these ligands to show that BCMA and TACI have a distinct role in APRIL-induced B cell stimulation. We conclude that these ligands are useful tools for studying APRIL biology in the context of individual receptor activation.     

Multiple Novel Classes of APRIL-specific Receptor-blocking Peptides Isolated by Phage Display

A proliferation-inducing ligand (APRIL) is a member of the tumor necrosis factor (TNF) ligand superfamily and has a proliferative effect on both normal and tumor cells. The TNF family receptors (B-cell maturation antigen (BCMA), transmembrane activator and CAML-interactor (TACI), and BAFF receptor-3 (BR3)) for APRIL and the closely related ligand, B-cell activating factor of the TNF family (BAFF), bind these ligands through a highly conserved six residue DXL motif ((F/Y/W)-D-X-L-(V/T)-(R/G)). Panning peptide phage display libraries led to the identification of several novel classes of APRIL-binding peptides, which could be grouped by their common sequence motifs. Interestingly, only one of these ten classes consisted of peptides containing the DXL motif. Nevertheless, all classes of peptides prevented APRIL, but not BAFF, from binding BCMA, their shared receptor. Synthetic peptides based on selected sequences inhibited APRIL binding to BCMA with IC₅₀ values of 0.49-27 μM. An X-ray crystallographic structure of APRIL bound to one of the phage-derived peptides showed that the peptide, lacking the DXL motif, was nevertheless bound in the DXL pocket on APRIL. Our results demonstrate that even though a focused, highly conserved motif is required for APRIL-receptor interaction, remarkably, many novel and distinct classes of peptides are also capable of binding APRIL at the ligand receptor interface.     

BAFF inhibition: a new class of drugs for the treatment of autoimmunity

BAFF (BLyS) and APRIL are TNF-like cytokines that support survival and differentiation of B cells. Recent studies have discovered a role for BAFF in augmenting both innate and adaptive immune responses as well as in collaborating with other inflammatory cytokines to promote the activation and differentiation of effector immune cells. BAFF is an important pathogenic factor in lupus mouse models and BAFF inhibition successfully delays disease onset in these mice, although the responsiveness to BAFF inhibition varies among different strains. These results have led to the development of inhibitors targeting BAFF and APRIL in humans. An anti-BAFF antibody has shown significant but modest efficacy in two Phase III clinical trials for moderately active SLE and other inhibitors are being developed or at early stages of clinical testing.     

BLyS受体TACI、BCMA与人IgG1 Fc融合蛋白在哺乳动物细胞中的表达与鉴定

从人肝脏和肾脏cDNA文库中扩增了人IgG1Fc与B淋巴细胞刺激因子 (BLyS)的受体TACI及BCMA的胞外编码区 ,构建了TACI Fc及BCMA Fc融合表达质粒pSec1 Fc TACI与 pSec1 Fc BCMA ,并使用电穿孔法转染COS 7细胞 ,从 1L无血清培养上清中可纯化得到分泌表达的TACI Fc与BCMA Fc融合蛋白约 2mg。为获得TACI Fc与BCMA Fc的稳定来源 ,构建了TACI Fc与BCMA Fc的CHO稳定表达细胞株。免疫沉淀和ELISA结果显示 ,TACI Fc及BCMA Fc能特异性地结合其配体BLyS而不与BLyS同家族的TNF结合。TACI Fc及BCMA Fc可阻断BLyS对体外培养的小鼠B淋巴细胞的促增殖作用     

Selectivity of BAFF/BLyS and APRIL for binding to the TNF family receptors BAFFR/BR3 and BCMA

BAFF (B cell activating factor of the TNF family, also known as BlyS and TALL-1), a TNF family cytokine critical for the development and function of B cells, has been reported to bind to three receptors, BCMA (B cell maturation protein), TACI (transmembrane activator and CAML [calcium-modulator and cyclophilin ligand] interactor), and BAFFR (BAFF receptor), but with widely conflicting values for the affinity and selectivity of binding. BCMA and TACI additionally bind APRIL (a proliferation-inducing ligand), the TNF family ligand most homologous to BAFF. Using soluble, monomeric forms of the receptors, we demonstrate that BAFFR binds BAFF with K(D) approximately 16 nM, while BCMA binds with K(D) approximately 1.6 microM, indicating a approximately 100-fold selectivity for binding to BAFFR over BCMA. APRIL shows the opposite selectivity, binding to BCMA with K(D) approximately 16 nM while showing no detectable affinity for BAFFR (K(D) > 3 microM). The binding of BAFF or APRIL to these receptors is highly sensitive to assay-dependent avidity effects, likely explaining the widely ranging affinity values reported in the literature. Binding of BAFF to BCMA-Fc, a bivalent fusion protein consisting of the extracellular domain of BCMA fused to the hinge and CH1 and CH2 domains of human IgG1, in solution or coated onto an ELISA plate gave apparent binding affinities of approximately 0.63 and approximately 0.15 nM, respectively, compared to values of K(D(app)) 相似文献   

Detection of the TNFSF members BAFF, APRIL, TWEAK and their receptors in normal kidney and renal cell carcinomas

In advanced renal cell carcinoma (RCC), surgery combined with systemic chemotherapy and immunotherapy have had limited effectiveness. Therapeutic modalities targeting VEGF, PDGF, and c-kit using tyrosine kinase inhibitors and m-TOR using specific biologic factors are in development. Therapeutic approaches targeting TNF-alpha have shown limited efficacy, while anti-TRAIL (TNFSF10) antibodies have shown enhanced activity. The presence and potential significance of other members of the TNFSF has not been investigated. Here, we assayed the TNFSF members APRIL, BAFF, TWEAK and their receptors (BCMA, TACI, BAFFR, Fn14) in 86 conventional type clear cell RCC, using immunohistochemistry and correlated our findings with histological data and, in a limited series, follow-up of patients. We observed a differential expression of these TNFSF ligands and receptors in cancerous and non-cancerous structures. BAFF was found in all RCC; APRIL expression is associated with an aggressive phenotype, correlating negatively with patients' disease-free survival, while TWEAK and its receptor Fn14 are heterogeneously expressed, correlating negatively with the grade and survival of RCC patients. This is the first study, presenting together the TNFSF members APRIL, BAFF, TWEAK and their receptors in different areas of normal renal tissue and RCC, suggesting a potential role of these TNFSF members in renal tumor biology.     

Molecular Characterization of Equine APRIL and its Expression Analysis During the Adipogenic Differentiation of Equine Adipose-Derived Stem Cell In Vitro

A proliferation inducing ligand (APRIL) is a member of the TNF superfamily. It shares two receptors with B-cell activating factor (BAFF), B-cell maturation antigen (BCMA), and transmembrane activator and CAML interactor (TACI). Herein, the equine APRIL was identified from equine adipose-derived stem cell (ASC), and the protein expression of APRIL and its related molecules were detected during the adipogenic differentiation of equine ASC in vitro. The equine APRIL gene was located on chromosome 11, spans 1852 base pairs (bp). Its open reading frame covers 753 bp, encoding a 250-amino acid protein with the typical TNF structure domain. During the two weeks’ adipogenic differentiation of equine ASC, although the protein expression of APRIL and TACI had an insignificant change, that of BCMA increased significantly. Moreover, with the addition of recombinant protein His₆-sAPRIL, a reduced differentiation of equine ASC toward adipocyte was detected. These results may provide the basis for investigating the role of APRIL in ASC adipogenic differentiation.     

谷氨酸受体分子的多样性及其分子机理

谷氨酸是中枢神经系统一种重要的兴奋性神经递质,它与相应受体分子相互作用,通过细胞膜对阳离子通透性的改变或与G蛋白和第二信使系统相偶联,从而引起一系列复杂的信号转导反应。近年有关谷氨酸受体分子及其基因的研究表明:由于多基因家族、选择性剪接、RNA编辑以及异聚体形成等分子机理,使谷氨酸受体分子的结构和功能具有多样性,这种多样性是生物多样性的分子基础,也在微观水平上证明了生物多样性的原理。这方面的深入探索必将为中枢神经系统该受体表达及调控以及相关神经精神疾病发病的分子机理和治疗性药物设计提供新的线索。     

Investigation of ligand selectivity in CYP3A7 by molecular dynamics simulations

Cytochrome P450 (CYP) 3A7 plays a crucial role in the biotransformation of the metabolized endogenous and exogenous steroids. To compare the metabolic capabilities of CYP3A7–ligands complexes, three endogenous ligands were selected, namely dehydroepiandrosterone (DHEA), estrone, and estradiol. In this study, a three-dimensional model of CYP3A7 was constructed by homology modeling using the crystal structure of CYP3A4 as the template and refined by molecular dynamics simulation (MD). The docking method was adopted, combined with MD simulation and the molecular mechanics generalized born surface area method, to probe the ligand selectivity of CYP3A7. These results demonstrate that DHEA has the highest binding affinity, and the results of the binding free energy were in accordance with the experimental conclusion that estrone is better than estradiol. Moreover, several key residues responsible for substrate specificity were identified on the enzyme. Arg372 may be the most important residue due to the low interaction energies and the existence of hydrogen bond with DHEA throughout simulation. In addition, a cluster of Phe residues provides a hydrophobic environment to stabilize ligands. This study provides insights into the structural features of CYP3A7, which could contribute to further understanding of related protein structures and dynamics.