The role of immunoglobulin heavy chain binding protein

RNA m6A modification orchestrates the rhythm of immune cell development from hematopoietic stem cells to T and B cells

RNA, one of the major building blocks of the cell, participates in many essential life processes. RNA stability is well-established to be closely related to various RNA modifications. To date, hundreds of different RNA modifications have been identified. N6-methyladenosine (m⁶A) is one of the most important RNA modifications in mammalian cells. An increasing body of evidence from recently published studies suggests that m⁶A modification is a novel immune system regulator of the generation and differentiation of hematopoietic stem cells (HSCs) and immune cells. In this review, we introduce the process and relevant regulatory mechanisms of m⁶A modification; summarize recent findings of m⁶A in controlling HSC generation and self-renewal, and the development and differentiation of T and B lymphocytes from HSCs; and discuss the potential mechanisms involved.

Deletion of mFICD AMPylase alters cytokine secretion and affects visual short-term learning in vivo

Fic domain-containing AMP transferases (fic AMPylases) are conserved enzymes that catalyze the covalent transfer of AMP to proteins. This posttranslational modification regulates the function of several proteins, including the ER-resident chaperone Grp78/BiP. Here we introduce a mouse FICD (mFICD) AMPylase knockout mouse model to study fic AMPylase function in vertebrates. We find that mFICD deficiency is well tolerated in unstressed mice. We also show that mFICD-deficient mouse embryonic fibroblasts are depleted of AMPylated proteins. mFICD deletion alters protein synthesis and secretion in splenocytes, including that of IgM, an antibody secreted early during infections, and the proinflammatory cytokine IL-1β, without affecting the unfolded protein response. Finally, we demonstrate that visual nonspatial short-term learning is stronger in old mFICD^−/− mice than in wild-type controls while other measures of cognition, memory, and learning are unaffected. Together, our results suggest a role for mFICD in adaptive immunity and neuronal plasticity in vivo.

Expression liabilities in a four-chain bispecific molecule

Multispecific antibodies, often composed of three to five polypeptide chains, have become increasingly relevant in the development of biotherapeutics. These molecules have mechanisms of action that include redirecting T cells to tumors and blocking multiple pathogenic mediators simultaneously. One of the major challenges for asymmetric multispecific antibodies is generating a high proportion of the correctly paired antibody during production. To understand the causes and effects of chain mispairing impurities in a difficult to express multispecific hetero-IgG, we investigated consequences of individual and pairwise chain expression in mammalian transient expression hosts. We found that one of the two light chains (LC) was not secretion competent when transfected individually or cotransfected with the noncognate heavy chain (HC). Overexpression of this secretion impaired LC reduced cell growth while inducing endoplasmic reticulum stress and CCAAT/enhancer-binding protein homologous protein (CHOP) expression. The majority of this LC was observed as monomer with incomplete intrachain disulfide bonds when expressed individually. Russell bodies (RB) were induced when this LC was co-expressed with the cognate HC. Moreover, one HC paired promiscuously with noncognate LC. These results identify the causes for the low product quality observed from stable cell lines expressing this heteroIgG and suggest mitigation strategies to improve overall process productivity of the correctly paired multispecific antibody. The approach described here provides a general strategy for identifying the molecular and cellular liabilities associated with difficult to express multispecific antibodies.

Linking Endoplasmic Reticular Stress and Alternative Splicing

RNA splicing patterns in antibody-secreting cells are shaped by endoplasmic reticulum stress, ELL2 (eleven-nineteen lysine-rich leukemia gene 2) induction, and changes in the levels of snRNAs. Endoplasmic reticulum stress induces the unfolded protein response comprising a highly conserved set of genes crucial for cell survival; among these is Ire1, whose auto-phosphorylation drives it to acquire a regulated mRNA decay activity. The mRNA-modifying function of phosphorylated Ire1 non-canonically splices Xbp1 mRNA and yet degrades other cellular mRNAs with related motifs. Naïve splenic B cells will activate Ire1 phosphorylation early on after lipopolysaccharide (LPS) stimulation, within 18 h; large-scale changes in mRNA content and splicing patterns result. Inhibition of the mRNA-degradation function of Ire1 is correlated with further differences in the splicing patterns and a reduction in the mRNA factors for snRNA transcription. Some of the >4000 splicing changes seen at 18 h after LPS stimulation persist into the late stages of antibody secretion, up to 72 h. Meanwhile some early splicing changes are supplanted by new splicing changes introduced by the up-regulation of ELL2, a transcription elongation factor. ELL2 is necessary for immunoglobulin secretion and does this by changing mRNA processing patterns of immunoglobulin heavy chain and >5000 other genes.

Biophysical analysis of the effect of chemical modification by 4-oxononenal on the structure, stability, and function of binding immunoglobulin protein (BiP)

Binding immunoglobulin protein (BiP) is a molecular chaperone important for the folding of numerous proteins, which include millions of immunoglobulins in human body. It also plays a key role in the unfolded protein response (UPR) in the endoplasmic reticulum. Free radical generation is a common phenomenon that occurs in cells under healthy as well as under stress conditions such as ageing, inflammation, alcohol consumption, and smoking. These free radicals attack the cell membranes and generate highly reactive lipid peroxidation products such as 4-oxononenal (4-ONE). BiP is a key protein that is modified by 4-ONE. In this study, we probed how such chemical modification affects the biophysical properties of BiP. Upon modification, BiP shows significant tertiary structural changes with no changes in its secondary structure. The protein loses its thermodynamic stability, particularly, that of the nucleotide binding domain (NBD) where ATP binds. In terms of function, the modified BiP completely loses its ATPase activity with decreased ATP binding affinity. However, modified BiP retains its immunoglobulin binding function and its chaperone activity of suppressing non-specific protein aggregation. These results indicate that 4-ONE modification can significantly affect the structure-function of key proteins such as BiP involved in cellular pathways, and provide a molecular basis for how chemical modifications can result in the failure of quality control mechanisms inside the cell.

The Sel1L-Hrd1 Endoplasmic Reticulum-Associated Degradation Complex Manages a Key Checkpoint in B Cell Development

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a principal mechanism that targets ER-associated proteins for cytosolic proteasomal degradation. Here, our data demonstrate a critical role for the Sel1L-Hrd1 complex, the most conserved branch of ERAD, in early B cell development. Loss of Sel1L-Hrd1 ERAD in B cell precursors leads to a severe developmental block at the transition from large to small pre-B cells. Mechanistically, we show that Sel1L-Hrd1 ERAD selectively recognizes and targets the pre-B cell receptor (pre-BCR) for proteasomal degradation in a BiP-dependent manner. The pre-BCR complex accumulates both intracellularly and at the cell surface in Sel1L-deficient pre-B cells, leading to persistent pre-BCR signaling and pre-B cell proliferation. This study thus implicates ERAD mediated by Sel1L-Hrd1 as a key regulator of B cell development and reveals the molecular mechanism underpinning the transient nature of pre-BCR signaling.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

High-level production of recombinant proteins in CHO cells using a dicistronic DHFR intron expression vector

We have constructed expression vectors for Chinese hamster ovary (CHO) cells that produce both selectable marker and recombinant cDNA from a single primary transcript via differential splicing. These vectors produce stable CHO cell clones that, when pooled, produce abundant amounts of secreted recombinant proteins compared with the amounts produced by conventional expression approaches that have selectable marker and the cDNA of interest under control of separate transcription units. Our vectors divert most of the transcript to product expression while linking it, at a fixed ratio, to dihydrofolate reductase (DHFR) expression to allow selection of stable transfectants. Pools of clones with increased expression of the product gene can be efficiently generated by selection in methotrexate. The high level of expression from pools allows convenient and rapid production of milligram amounts of recombinant proteins.

Spontaneous assembly of bivalent single chain antibody fragments in Escherichia coli

The ability of immunoglobulin Fab and single chain (ScFv) fragments to penetrate effectively into tissue from the vascular system has made these molecules excellent candidates as drug delivery systems and imaging tools. This study investigates the use of single chain antibody fragment bacterial expression vectors as a possible strategy for the production of these molecules. We have modified the pSW1-VHD1.3-VKD1.3-TAG1 vector [Ward et al. (1989) Nature 341, 544-546] which originally, when expressed in E. coli, produced an Fab fragment. In an effort to improve the affinity of the parent vector product a novel single chain antibody construct which encodes a protein with anti-P. aeruginosa activity was generated using a 14 amino acid linker [Chaudhary et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 1066-1070]. In addition to the heavy and light chain variable domain genes, our construct also contained the light chain kappa constant domain gene to aid purification of the fragments. To underline this difference from the conventional ScFv fragment we have described this protein as a ScAb. The ScAb generated had an antigen binding capacity similar to the parent anti-P. aeruginosa antibody but was superior to the recombinant anti-P. aeruginosa Fab fragment. On HPLC and non-denaturing gel electrophoresis analysis, the ScAb was found to exist in multimeric forms while the Fab fragment existed only as a single unit. Dimeric ScAb had a similar antigen binding profile to the parent antibody.

Normal B lymphocyte differentiation

Normal differentiation of B lineage cells has been the subject of intensive investigation over the past three decades. Current models of this process in humans are melded from the results of studies in a variety of organisms, including humans, mice and birds. Several recent developments have significantly reshaped and refined these models. The technique of homologous recombination in embryonic stem cells has allowed the production of mice with selectively disrupted genes that are important for B cell development in mice. At the same time, functional studies of human B cell differentiation, together with analysis of naturally occurring mutations that disrupt this process, have progressed rapidly. This has provided insight into the pathogenesis of lymphoproliferative and immunodeficiency diseases as well as a clearer view of normal developmental events. In this chapter we have reviewed human B cell differentiation with particular emphasis on newly emerging concepts. We also discussed CD5, a pan-T cell antigen that is expressed in low levels on a subpopulation of B cells implicated in the pathogenesis of chronic lymphocytic leukaemia (CLL). Finally, we discussed the issue of restricted variable region gene usage during B cell ontogeny and in CLL.

High level expression on a chimeric anti-ganglioside GD2 antibody: genomic kappa sequences improve expression in COS and CHO cells

We report a flexible strategy for the high level expression of a recombinant human monoclonal antibody (mAb) in Chinese hamster ovary (CHO) cells, initially using COS monkey kidney cell transfections to evaluate rapidly modifications to immunoglobulin (Ig) DNA constructs. Using sequential transfections with two amplifiable markers, we generated CHO cell lines and clones that secrete 80-110 micrograms/10(6) cells/24 hours of a mouse-human chimeric IgG1 kappa mAb. This cellular productivity is considerably greater than most murine hybridomas and transfected myelomas. Our data also demonstrate that genomic kappa sequences can improve mAb expression in COS and CHO cells. As a paradigm, we focused our expression studies on a human chimeric form of 3F8, a murine mAb that binds to ganglioside GD2 on neuroblastoma and melanoma tumor cells.

Role of the transmembrane and cytoplasmic domains of surface IgM in endocytosis and signal transduction

The cross-linking of membrane IgM (mIgM) triggers the activation and differentiation of B lymphocytes. One very rapid result of the cross-linking is the activation of phospholipase C, the subsequent mobilization of free calcium from internal stores and the activation of protein kinase C. This is followed by a redistribution of the receptor-ligand complexes to a small cap on the B cell surface, the first step in endocytosis and antigen processing. Cross-linking of major histocompatibility complex (MHC) class I neither stimulates the release of intracellular calcium nor does it induce capping and endocytosis of the cell surface receptors. In this study, we sought to determine the role of the two carboxyterminal domains of the mu heavy chain in signal transduction, capping and endocytosis of mIgM. We took advantage of the clear differences between MHC class I molecules and mIgM, replacing the transmembrane and cytoplasmic domains of mu by their MHC class I equivalents. Our results show that the hybrid heavy chain could still associate with light chains and assemble into a tetramer on the cell surface. However, cross-linking of the hybrid cell receptor produced neither release of calcium from internal stores, nor capping and endocytosis. These observations demonstrate that the two carboxy-terminal domains of mu are critical to both signal transduction and modulation of the mIgM-ligand complexes from the surface of B lymphocytes.

An explanation for the defect in secretion of IgM Mott cells and their predominant occurrence in the Ly-1 B cell compartment

Mott cells are a variant form of plasma cell in which the immunoglobulin (Ig), rather than being secreted, accumulates in rough endoplasmic reticulum-derived vesicles called Russell bodies. We have examined the molecular cause of this defect and the in vivo origin of IgM Mott cells. Our examination of the Ig variable region gene sequences of two IgM Mott hybridomas derived from C.B-20 Ly-1 B cells showed all to be germ line. In a series of mix and match transfection experiments, the Mott phenotype was only reconstituted when the original Mott specificity was expressed as an IgM, suggesting that both the specificity and the isotype were critical to the formation of Russell bodies. Based on our finding that Russell body formation was dependent on the Ig isotype being IgM, we suggest that the Mott phenotype is apparent only after differentiation of B cells into plasma cells and that probably the major cause of the IgM Mott phenotype is low-affinity interaction of the Mott Ig with some as yet unknown intracellular component(s) being stabilized by the intrinsic high avidity of the pentameric secreted form of IgM. Consistent with this proposal was the finding that after in vitro lipopolysaccharide (LPS) stimulation of sorted Ly-1 B cells derived from C.B-20 mice, Mott cells represented up to 5% of the IgM plasma cells in the culture. LPS stimulation of conventional B cells also induced the appearance of IgM Mott cells, but at the much reduced level of 0.1%, suggesting that the major, if not the only, source of Mott cells in vivo is the Ly-1 B cell population. A possible causal relationship between the elevated frequency of Mott cells in the Ly-1 B cell-derived LPS blasts and the repertoire selection inherent in the development of these B cells is discussed.

Protein folding in the cell

In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.

The signal peptide of the rotavirus glycoprotein VP7 is essential for its retention in the ER as an integral membrane protein

The rotavirus glycoprotein VP7 has a cleavable signal peptide and is normally resident as an integral membrane protein in the ER of infected cells. A gene was constructed in which the VP7 H2 signal peptide was replaced by one from influenza hemagglutinin. COS cells transfected with this gene produced VP7 with the correct amino terminus, but the protein was rapidly secreted. Uncleaved VP7 from either precursor was not detected in cells after brief pulse-labeling, suggesting that the signal peptide was not acting as a temporary anchor; rather, it exerted its effect despite rapid cleavage. By splicing the H2 signal peptide onto another reporter protein, the malaria S-antigen, we demonstrated that H2 was necessary, but not itself sufficient, for targeting and retention. We propose that an interaction between the cleaved signal peptide and other downstream sequences in VP7 is required for retention of this protein in the ER as an integral membrane polypeptide.

Induction of expression of protein disulphide-isomerase during lymphocyte maturation stimulated by bacterial lipopolysaccharide

Protein disulphide-isomerase (PDI) activity, and the level of immunodetectable PDI protein, were monitored in splenic lymphocytes and in BCL1 cells during culture in the presence of various activating factors. Bacterial lipopolysaccharide stimulated induction of PDI in splenic B cells and BCL1 cells. The time-course and specificity of induction indicated that the increase in expression of PDI is closely coupled to the final stages of B cell differentiation into antibody-producing plasma cells. The system will prove valuable in studies on the control of expression of PDI.

Homology of the 70-kilodalton antigens from Mycobacterium leprae and Mycobacterium bovis with the Mycobacterium tuberculosis 71-kilodalton antigen and with the conserved heat shock protein 70 of eucaryotes

Two lambda gt11 recombinant clones, JKL2 and JKL15, each containing an insert coding for part of the highly immunogenic 70-kilodalton (kDa) protein antigen, were isolated from a Mycobacterium leprae genomic library by immunoscreening with the monoclonal antibody L7. Clone JKL2 contained the largest insert, 2.3 kilobase pairs. Nonoverlapping fragments of this insert were used as probes and showed strong hybridization to a number of Mycobacterium tuberculosis-lambda gt11 recombinants producing proteins recognized by an anti-M. tuberculosis 71-kDa monoclonal antibody, IT11. One clone from a recombinant Mycobacterium bovis library was also characterized by using L7, and the insert from this clone, B5bt, hybridized strongly to the M. leprae probes as well. The nucleotide sequence of the 1,037-base-pair coding region of the JKL2 M. leprae clone which encodes the carboxy-terminal half of the 70-kDa protein had extensive homology with genes from a number of species. In all cases, these genes, including the recently described Ag63 and Ag361 of Plasmodium falciparum, were found to be members of the heat shock protein 70 (hsp 70) family of genes. At the amino acid level, homology was maximal between amino acids 83 through 107 and 159 through 184, which showed extreme conservation (92 and 85% identity) with Escherichia coli DnaK amino acids 386 through 409 and 460 through 485, respectively, and was 51% homologous over the entire coding region (amino acids 1 through 344 of JKL2). In contrast, amino acids 129 through 158 had maximal homology with the phylogenetically more distant Xenopus laevis hsp70. Homology declined substantially in the carboxy-terminal 34 amino acids. The predicted ATP-binding functional activity of the 70-kDa antigen from M. bovis was confirmed with affinity purification of the antigen by binding to ATP-agarose and elution with ATP. In view of the conservation of sequences between these mycobacterial antigens and mammalian endogenous cellular enzymes, further evaluation of these molecules in vivo may aid in understanding tolerance to self-antigens as well as provide potentially useful immunodiagnostic reagents.

The 'molten globule' state is involved in the translocation of proteins across membranes?

Strong evidence exists that the translocation of proteins across a variety of membranes involves a non-native or denatured conformational states. On the other hand a compact state having secondary but not rigid tertiary structure and called the 'molten globule' state has been identified as being stable under mild denaturing conditions. A similar state has been shown to accumulate on the folding pathway of globular proteins. These states are compact though sufficiently expanded to include water, and they are internally mobile. It is proposed that these molten globule states may be suitable candidates for protein translocation across biological membranes.

Assembly of a functional immunoglobulin Fv fragment in Escherichia coli

An expression system was developed that allows the production of a completely functional antigen-binding fragment of an antibody in Escherichia coli. The variable domains of the phosphorylcholine-binding antibody McPC603 were secreted together into the periplasmic space, where protein folding as well as heterodimer association occurred correctly. Thus, the assembly pathway for the Fv fragment in E. coli is similar to that of a whole antibody in the eukaryotic cell. The Fv fragment of McPC603 was purified to homogeneity with an antigen-affinity column in a single step. The correct processing of both signal sequences was confirmed by amino-terminal protein sequencing. The functionality of the recombinant Fv fragment was demonstrated by equilibrium dialysis. These experiments showed that the affinity constant of the Fv fragment is identical to that of the native antibody McPC603, that there is one binding site for phosphorylcholine in the Fv fragment, and that there is no inactive protein in the preparation. This expression system should facilitate future protein engineering experiments on antibodies.

The presence of malfolded proteins in the endoplasmic reticulum signals the induction of glucose-regulated proteins

Two glucose-regulated proteins, GRP78 and GRP94, are major constituents of the endoplasmic reticulum (ER) of mammalian cells. These proteins are synthesized constitutively in detectable amounts under normal growth conditions; they can also be induced under a variety of conditions of stress including glucose starvation and treatment with drugs that inhibit cellular glycosylation, with calcium ionophores or with amino-acid analogues. Unlike the closely-related heat shock protein (HSP) family, the GRPs are not induced significantly by high temperature. Recently, GRP78 has been identified as the immunoglobulin heavy chain binding protein (BiP) (ref. 5 and Y.K. et al., in preparation) which binds transiently to a variety of nascent, wild-type secretory and transmembrane proteins and permanently to malfolded proteins that accumulate within the ER. We have tested the hypothesis that the presence of malfolded proteins may be the primary signal for induction of GRPs by expressing wild-type and mutant forms of influenza virus haemagglutinin (HA) in simian cells. Only malfolded HAs, whose transport from the ER is blocked, induced the synthesis of GRPs 78 and 94. Additional evidence is presented that malfolding per se, rather than abnormal glycosylation, is the proximal inducer of this family of stress proteins.