Building an antibody factory: a job for the unfolded protein response

Enhancing the Antibody Production Efficiency of Chinese Hamster Ovary Cells through Improvement of Disulfide Bond Folding Ability and Apoptosis Resistance

The complex structure of monoclonal antibodies (mAbs) expressed in Chinese hamster ovary (CHO) cells may result in the accumulation of unfolded proteins, triggering endoplasmic reticulum (ER) stress and an unfolded protein response (UPR). If the protein folding ability cannot maintain ER homeostasis, the cell will shut down protein translation and ultimately induce apoptosis. We co-overexpressed HsQSOX1b and survivin proteins in the antibody-producing cell line CHO-PAb to obtain a new cell line, CHO-PAb-QS. Compared with CHO-PAb cells, the survival time of CHO-PAb-QS cells in batch culture was extended by 2 days, and the antibody accumulation and productivity were increased by 52% and 45%, respectively. The proportion of (HC-LC)2 was approximately doubled in the CHO-PAb-QS cells, which adapted to the accelerated disulfide bond folding capacity by upregulating the UPR's strength and increasing the ER content. The results of the apoptosis assays indicated that the CHO-PAb-QS cell line exhibited more excellent resistance to apoptosis induced by ER stress. Finally, CHO-PAb-QS cells exhibited mild oxidative stress but did not significantly alter the redox status. This study demonstrated that strategies based on HsQSOX1b and survivin co-overexpression could facilitate protein disulfide bond folding and anti-apoptosis ability, enhancing antibody production efficiency in CHO cell lines.

Machine learning-based endoplasmic reticulum-related diagnostic biomarker and immune microenvironment landscape for osteoarthritis

BACKGROUND

Osteoarthritis (OA) is the most common degenerative joint disease worldwide. Further improving the current limited understanding of osteoarthritis has positive clinical value.

METHODS

OA samples were collected from GEO database and endoplasmic reticulum related genes (ERRGs) were identified. The WGCNA network was further built to identify the crucial gene module. Based on the expression profiles of characteristic ERRGs, LASSO algorithm was used to select key factors according to the minimum λ value. Random forest (RF) algorithm was used to calculate the importance of ERRGs. Subsequently, overlapping genes based on LASSO and RF algorithms were identified as ERRGs-related diagnostic biomarkers. In addition, OA specimens were also collected and performed qRT-PCR quantitative analysis of selected ERRGs.

RESULTS

We identified four ERRGs associated with OA risk assessment through machine learning methods, and verified the abnormal expressions of these screened markers in OA patients through in vitro experiments. The influence of selected markers on OA immune infiltration was also evaluated.

CONCLUSIONS

Our results provide new evidence for the role of ER stress in the OA progression, as well as new markers and potential intervention targets for OA.

Lipid metabolism in regulation of B cell development and autoimmunity

B cells play an important role in adaptive immunity and participate in the process of humoral immunity mainly by secreting antibodies. The entire development and differentiation process of B cells occurs in multiple microenvironments and is regulated by a variety of environmental factors and immune signals. Differentiation biases or disfunction of B cells participate in the process of many autoimmune diseases. Emerging studies report the impact of altered metabolism in B cell biology, including lipid metabolism. Here, we discuss how extracellular lipid environment and metabolites, membrane lipid-related components, and lipid synthesis and catabolism programs coordinate B cell biology and describe the crosstalk of lipid metabolic programs with signal transduction pathways and transcription factors. We conclude with a summary of therapeutic targets for B cell lipid metabolism and signaling in autoimmune diseases and discuss important future directions.

Final step of B-cell differentiation into plasmablasts; the right time to activate plasma cell PIM2 kinase

The differentiation of B cells into antibody-secreting plasma cells is a complex process that involves extensive changes in morphology, lifespan, and cellular metabolism to support the high rates of antibody production. During the final stage of differentiation, B cells undergo significant expansion of their endoplasmic reticulum and mitochondria, which induces cellular stress and may lead to cell death in absence of effective inhibition of the apoptotic pathway. These changes are tightly regulated at transcriptional and epigenetic levels, as well as at post-translational level, with protein modifications playing a critical role in the process of cellular modification and adaptation. Our recent research has highlighted the pivotal role of the serine/threonine kinase PIM2 in B cell differentiation, from commitment stage to plasmablast and maintenance of expression in mature plasma cells. PIM2 has been shown to promote cell cycle progression during the final stage of differentiation and to inhibit Caspase 3 activation, raising the threshold for apoptosis. In this review, we examine the key molecular mechanisms controlled by PIM2 that contribute to plasma cell development and maintenance.

PDI inhibitor LTI6426 enhances panobinostat efficacy in preclinical models of multiple myeloma

The histone deacetylase inhibitor (HDACi), panobinostat (Pano), is approved by the United States Food and Drug Administration (FDA) and European Medicines Agency (EMA) for treatment of relapsed/refractory multiple myeloma (MM). Despite regulatory approvals, Pano is used on a limited basis in MM due largely to an unfavorable toxicity profile. The MM treatment landscape continues to evolve, and for Pano to maintain a place in that paradigm it will be necessary to identify treatment regimens that optimize its effectiveness, particularly those that permit dose reductions to eliminate unwanted toxicity. Here, we propose such a regimen by combining Pano with LTI6426, a first-in-class orally bioavailable protein disulfide isomerase (PDI) inhibitor. We show that LTI6426 dramatically enhances the anti-MM activity of Pano in vitro and in vivo using a proteasome inhibitor resistant mouse model of MM and a low dose of Pano that exhibited no signs of toxicity. We go on to characterize a transcriptional program that is induced by the LTI6426/Pano combination, demonstrating a convergence of the two drugs on endoplasmic reticulum (ER) stress pathway effectors ATF3 (Activating Transcription Factor 3), DDIT3/CHOP (DNA Damage Inducible Transcript 3, a.k.a. C/EBP Homologous Protein), and DNAJB1 (DnaJ homolog subfamily B member 1, a.k.a. HSP40). We conclude that LTI6426 may safely enhance low-dose Pano regimens and that ATF3, DDIT3/CHOP, and DNAJB1 are candidate pharmacodynamic biomarkers of response to this novel treatment regimen.

Early Aerobic Exercise Promotes Neurological Function Recovery of Rats after Cerebral Ischemia/Reperfusion by Upregulating the Expression of Heat Shock Protein A5

OBJECTIVE

The neuroprotective function of heat shock protein A5 (HSPA5) in ischemic stroke has been confirmed. This study aimed to investigate the effects of early aerobic exercise on neurological function recovery from cerebral ischemia/reperfusion and to determine whether these effects are associated with the expression level of HSPA5 in the ischemic penumbra.

METHODS

A total of 72 male Sprague-Dawley rats were randomly assigned to the ischemia and exercise group [middle cerebral artery occlusion (MCAO)-Ex, n=18], ischemia and sedentary group (MCAO-St, n=18), sham-surgery and exercise group (Sham-Ex, n=18), or sham-surgery and sedentary group (Sham-St, n=18). The MCAO-Ex and MCAO-St groups were subjected to MCAO for 60 min, whereas the Sham-Ex and Sham-St groups were subjected to an identical operation without MCAO. Rats in the MCAO-Ex and Sham-Ex groups then ran on a treadmill for 30 min once a day for 5 consecutive days. After reperfusion, the motor function of the rats was scored by the Bederson neurological function test, balance beam test, and screen test. Nissl staining was conducted to assess morphological and structural change of nerve cells in the ischemic penumbra. The reverse transcription-quantitative polymerase chain reaction was applied to detect the mRNA expression of HSPA5. Western blot analysis was conducted to determine the protein expression of HSPA5. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining was carried out in the ischemic penumbra after MCAO.

RESULTS

Rats receiving early treadmill exercise had lower Bederson neurological function, balance beam, and screen test scores on the 3rd, 7th, and 14th days after MCAO; in addition, more neurons survived in the ischemic penumbra after MCAO, and higher mRNA and protein expression of HSPA5 and fewer TUNEL-positive stained cells were observed.

CONCLUSION

Our study demonstrated that early aerobic exercise can improve neurological function recovery after ischemia/reperfusion. Furthermore, the increased level of HSPA5 in the ischemic penumbra might be one of the mechanisms of enhanced neurological function recovery.

Immunity and Nutrition: The Right Balance in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is an increasingly urgent medical problem that strongly impairs quality of life for patients. A global rise in incidence has been observed over the last few decades, with the highest incidence rates recorded in North America and Europe. Still, an increased incidence has been reported in the last ten years in newly industrialized countries in Asia, including China and India, both with more than one billion inhabitants. These data underline that IBD is an urgent global health problem. In addition, it is estimated that between 20% and 30% of IBD patients will develop colorectal cancer (CRC) within their lifetime and CRC mortality is approximately 50% amongst IBD patients. Although the exact etiology of IBD is still being defined, it is thought to be due to a complex interaction between many factors, including defects in the innate and adaptive immune system; microbial dysbiosis, i.e., abnormal levels of, or abnormal response to, the gastrointestinal microbiome; a genetic predisposition; and several environmental factors. At present, however, it is not fully understood which of these factors are the initiators of inflammation and which are compounders. The purpose of this review is to analyze the complex balance that exists between these elements to maintain intestinal homeostasis and prevent IBD or limit adverse effects on people’s health.

The Immunity-malignancy equilibrium in multiple myeloma: lessons from oncogenic events in plasma cells

Multiple myeloma (MM) is a malignancy of plasma cells (PC) that grow within the bone marrow and maintain massive immunoglobulin (Ig) production. Disease evolution is driven by genetic lesions, whose effects on cell biology and fitness underlie addictions and vulnerabilities of myeloma cells. Several genes mutated in myeloma are strictly involved in dictating PC identity and antibody factory function. Here, we evaluate the impact of mutations in IRF4, PRDM1, and XBP1, essential transcription factors driving the B to PC differentiation, on MM cell biology and homeostasis. These factors are highly specialized, with limited overlap in their downstream transcriptional programs. Indeed, IRF4 sustains metabolism, survival, and proliferation, while PRDM1 and XBP1 are mainly responsible for endoplasmic reticulum expansion and sustained Ig secretion. Interestingly, IRF4 undergoes activating mutations and translocations, while PRDM1 and XBP1 are hit by loss-of-function events, raising the hypothesis that containment of the secretory program, but not its complete extinction, may be beneficial to malignant PCs. Finally, recent studies unveiled that also the PRDM1 target, FAM46C/TENT5C, an onco-suppressor uniquely and frequently mutated or deleted in myeloma, is directly and potently involved in orchestrating ER homeostasis and secretory activity. Inactivating mutations found in this gene and its interactors strengthen the notion that reduced secretory capacity confers advantage to myeloma cells. We believe that dissection of the evolutionary pressure on genes driving PC-specific functions in myeloma will disclose the cellular strategies by which myeloma cells maintain an equilibrium between antibody production and survival, thus unveiling novel therapeutic targets.

The Interaction of the Tumor Suppressor FAM46C with p62 and FNDC3 Proteins Integrates Protein and Secretory Homeostasis

FAM46C is a non-canonical poly(A) polymerase uniquely mutated in up to 20% of multiple myeloma (MM) patients, implying a tissue-specific tumor suppressor function. Here, we report that FAM46C selectively stabilizes mRNAs encoding endoplasmic reticulum (ER)-targeted proteins, thereby concertedly enhancing the expression of proteins that control ER protein import, folding, N-glycosylation, and trafficking and boosting protein secretion. This role requires the interaction with the ER membrane resident proteins FNDC3A and FNDC3B. In MM cells, FAM46C expression raises secretory capacity beyond sustainability, inducing ROS accumulation, ATP shortage, and cell death. FAM46C activity is regulated through rapid proteasomal degradation or the inhibitory interaction with the ZZ domain of the autophagic receptor p62 that hinders its association with FNDC3 proteins via sequestration in p62⁺ aggregates. Altogether, our data disclose a p62/FAM46C/FNDC3 circuit coordinating sustainable secretory activity and survival, providing an explanation for the MM-specific oncosuppressive role of FAM46C and uncovering potential therapeutic opportunities against cancer.

Early Emergence of Adaptive Mechanisms Sustaining Ig Production: Application to Antibody Therapy

Antibody therapy, where artificially-produced immunoglobulins (Ig) are used to treat pathological conditions such as auto-immune diseases and cancers, is a very innovative and competitive field. Although substantial efforts have been made in recent years to obtain specific and efficient antibodies, there is still room for improvement especially when considering a precise tissular targeting or increasing antigen affinity. A better understanding of the cellular and molecular steps of terminal B cell differentiation, in which an antigen-activated B cell becomes an antibody secreting cell, may improve antibody therapy. In this review, we use our recently published data about human B cell differentiation, to show that the mechanisms necessary to adapt a metamorphosing B cell to its new secretory function appear quite early in the differentiation process i.e., at the pre-plasmablast stage. After characterizing the molecular pathways appearing at this stage, we will focus on recent findings about two main processes involved in antibody production: unfolded protein response (UPR) and endoplasmic reticulum (ER) stress. We’ll show that many genes coding for factors involved in UPR and ER stress are induced at the pre-plasmablast stage, sustaining our hypothesis. Finally, we propose to use this recently acquired knowledge to improve productivity of industrialized therapeutic antibodies.

Mechanisms Targeting the Unfolded Protein Response in Asthma

Lung cells are constantly exposed to various internal and external stressors that disrupt protein homeostasis. To cope with these stimuli, cells evoke a highly conserved adaptive mechanism called the unfolded protein response (UPR). UPR stressors can impose greater protein secretory demands on the endoplasmic reticulum (ER), resulting in the development, differentiation, and survival of these cell types to meet these increasing functional needs. Dysregulation of the UPR leads to the development of the disease. The UPR and ER stress are involved in several human conditions, such as chronic inflammation, neurodegeneration, metabolic syndrome, and cancer. Furthermore, potent and specific compounds that target the UPR pathway are under development as future therapies. The focus of this review is to thoroughly describe the effects of both internal and external stressors on the ER in asthma. Furthermore, we discuss how the UPR signaling pathway is activated in the lungs to overcome cellular damage. We also present an overview of the pathogenic mechanisms, with a brief focus on potential strategies for pharmacological interventions.

Development of novel methods for non-canonical myeloma protein analysis with an innovative adaptation of immunofixation electrophoresis, native top-down mass spectrometry, and middle-down de novo sequencing

Objectives

Multiple myeloma (MM) is a malignant plasma cell neoplasm, requiring the integration of clinical examination, laboratory and radiological investigations for diagnosis. Detection and isotypic identification of the monoclonal protein(s) and measurement of other relevant biomarkers in serum and urine are pivotal analyses. However, occasionally this approach fails to characterize complex protein signatures. Here we describe the development and application of next generation mass spectrometry (MS) techniques, and a novel adaptation of immunofixation, to interrogate non-canonical monoclonal immunoproteins.

Methods

Immunoprecipitation immunofixation (IP-IFE) was performed on a Sebia Hydrasys Scan2. Middle-down de novo sequencing and native MS were performed with multiple instruments (21T FT-ICR, Q Exactive HF, Orbitrap Fusion Lumos, and Orbitrap Eclipse). Post-acquisition data analysis was performed using Xcalibur Qual Browser, ProSight Lite, and TDValidator.

Results

We adapted a novel variation of immunofixation electrophoresis (IFE) with an antibody-specific immunosubtraction step, providing insight into the clonal signature of gamma-zone monoclonal immunoglobulin (M-protein) species. We developed and applied advanced mass spectrometric techniques such as middle-down de novo sequencing to attain in-depth characterization of the primary sequence of an M-protein. Quaternary structures of M-proteins were elucidated by native MS, revealing a previously unprecedented non-covalently associated hetero-tetrameric immunoglobulin.

Conclusions

Next generation proteomic solutions offer great potential for characterizing complex protein structures and may eventually replace current electrophoretic approaches for the identification and quantification of M-proteins. They can also contribute to greater understanding of MM pathogenesis, enabling classification of patients into new subtypes, improved risk stratification and the potential to inform decisions on future personalized treatment modalities.

Plasma cell targeting to prevent antibody-mediated rejection

Plasma cells (PCs) are the major source of pathogenic allo- and autoantibodies and have historically demonstrated resistance to therapeutic targeting. However, significant recent clinical progress has been made with the use of second-generation proteasome inhibitors (PIs). PIs provide efficient elimination of plasmablast-mediated humoral responses; however, long-lived bone marrow (BM) resident PCs (LLPCs) demonstrate therapeutic resistance, particularly to first-generation PIs. In addition, durability of antibody (Ab) reduction still requires improvement. More recent clinical trials have focused on conditions mediated by LLPCs and have included mechanistic studies of LLPCs from PI-treated patients. A recent clinical trial of carfilzomib (a second-generation irreversible PI) demonstrated improved efficacy in eliminating BM PCs and reducing anti-HLA Abs in chronically HLA-sensitized patients; however, Ab rebound was observed over several weeks to months following PI therapy. Importantly, recent murine studies have provided substantial insights into PC biology, thereby further enhancing our understanding of PC populations. It is now clear that BMPC populations, where LLPCs are thought to primarily reside, are heterogeneous and have distinct gene expression, metabolic, and survival signatures that enable identification and characterization of PC subsets. This review highlights recent advances in PC biology and clinical trials in transplant populations.

ER Stress and the UPR in Shaping Intestinal Tissue Homeostasis and Immunity

An imbalance in the correct protein folding milieu of the endoplasmic reticulum (ER) can cause ER stress, which leads to the activation of the unfolded protein response (UPR). The UPR constitutes a highly conserved and intricately regulated group of pathways that serve to restore ER homeostasis through adaptation or apoptosis. Numerous studies over the last decade have shown that the UPR plays a critical role in shaping immunity and inflammation, resulting in the recognition of the UPR as a key player in pathological processes including complex inflammatory, autoimmune and neoplastic diseases. The intestinal epithelium, with its many highly secretory cells, forms an important barrier and messenger between the luminal environment and the host immune system. It is not surprising, that numerous studies have associated ER stress and the UPR with intestinal diseases such as inflammatory bowel disease (IBD) and colorectal cancer (CRC). In this review, we discuss our current understanding of the roles of ER stress and the UPR in shaping immune responses and maintaining tissue homeostasis. Furthermore, the role played by the UPR in disease, with emphasis on IBD and CRC, is described here. As a key player in immunity and inflammation, the UPR has been increasingly recognized as an important pharmacological target in the development of therapeutic strategies for immune-mediated pathologies. We summarize available strategies targeting the UPR and their therapeutic implications. Understanding the balance between homeostasis and pathophysiology, as well as means of manipulating this balance, provides an important avenue for future research.

In the Right Place, at the Right Time: Spatiotemporal Conditions Determining Plasma Cell Survival and Function

Plasma cells (PCs), the B lineage cells responsible for producing and secreting antibodies (Abs), are critical cellular components of the humoral immune system. While most of the antibody-secreting cells in the body have a rather short lifetime of a few days, some of them can become long-lived and persist in the body over the entire life span of an individual. The majority of these long-lived plasma cells secretes protective antibodies against pathogens, and are thereby crucial for the humoral component of immunological memory. The generation of these protective antibody-secreting cells can be triggered by an exposure to pathogens, and also by vaccination. Although the majority of plasma cells are protective, sometimes long-lived plasma cells produce autoreactive antibodies, which contribute to the pathogenesis and perpetuation of chronic autoimmune diseases, including lupus erythematosus, rheumatoid arthritis, or multiple sclerosis. In order to promote the formation of protective antibody-secreting cells and to target pathogenic plasma cells, it is crucial to understand the signals which promote their longevity and allow them to exert their function. In recent years, it has become clear that plasma cells depend on extrinsic factors for their survival, leading to the concept that certain tissue microenvironments promote plasma cell retention and longevity. However, these niches are not static structures, but also have dynamic features with respect to their cellular composition. Here, we review what is known about the molecular and cellular composition of the niches, and discuss the impact of dynamic changes within these microenvironments on plasma cell function. As plasma cell metabolism is tightly linked to their function, we present new tools, which will allow us to analyze metabolic parameters in the plasma cell niches in vivo over time.

Unconventional secretion factor GRASP55 is increased by pharmacological unfolded protein response inducers in neurons

Accumulation of misfolded proteins in the endoplasmic reticulum (ER), defined as ER stress, results in activation of the unfolded protein response (UPR). UPR activation is commonly observed in neurodegenerative diseases. ER stress can trigger unconventional secretion mediated by Golgi reassembly and stacking proteins (GRASP) relocalization in cell lines. Here we study the regulation of GRASP55 by the UPR upon pharmacological induction of ER stress in primary mouse neurons. We demonstrate that UPR activation induces mRNA and protein expression of GRASP55, but not GRASP65, in cortical neurons. UPR activation does not result in relocalization of GRASP55. UPR-induced GRASP55 expression is reduced by inhibition of the PERK pathway of the UPR and abolished by inhibition of the endonuclease activity of the UPR transducer IRE1. Expression of the IRE1 target XBP1s in the absence of ER stress is not sufficient to increase GRASP55 expression. Knockdown of GRASP55 affects neither induction nor recovery of the UPR. We conclude that the UPR regulates the unconventional secretion factor GRASP55 via a mechanism that requires the IRE1 and the PERK pathway of the UPR in neurons.

Inhibitors of the protein disulfide isomerase family for the treatment of multiple myeloma

Multiple Myeloma (MM) is highly sensitive to disruptions in cellular protein homeostasis. Proteasome inhibitors (PIs) are initially effective in the treatment of MM, although cures are not achievable and the emergence of resistance limits the durability of responses. New therapies are needed for refractory patients, and those that combat resistance to standard of care agents would be particularly valuable. Screening of multiple chemical libraries for PI re-sensitizing compounds identified E61 as a potent enhancer of multiple PIs and MM specific activity. Using a tandem approach of click chemistry and peptide mass fingerprinting, we identified multiple protein disulfide isomerase (PDI) family members as the primary molecular targets of E61. PDIs mediate oxidative protein folding, and E61 treatment induced robust ER and oxidative stress responses as well as the accumulation of ubiquitinylated proteins. A chemical optimization program led to a new structural class of indene (exemplified by lead E64FC26), which are highly potent pan-style inhibitors of PDIs. In mice with MM, E64FC26 improved survival and enhanced the activity of bortezomib without any adverse effects. This work demonstrates the potential of E64FC26 as an early drug candidate and the strategy of targeting multiple PDI isoforms for the treatment of refractory MM and beyond.

The UPR Activator ATF6 Responds to Proteotoxic and Lipotoxic Stress by Distinct Mechanisms

The unfolded protein response (UPR) is induced by proteotoxic stress of the endoplasmic reticulum (ER). Here we report that ATF6, a major mammalian UPR sensor, is also activated by specific sphingolipids, dihydrosphingosine (DHS) and dihydroceramide (DHC). Single mutations in a previously undefined transmembrane domain motif that we identify in ATF6 incapacitate DHS/DHC activation while still allowing proteotoxic stress activation via the luminal domain. ATF6 thus possesses two activation mechanisms: DHS/DHC activation and proteotoxic stress activation. Reporters constructed to monitor each mechanism show that phenobarbital-induced ER membrane expansion depends on transmembrane domain-induced ATF6. DHS/DHC addition preferentially induces transcription of ATF6 target lipid biosynthetic and metabolic genes over target ER chaperone genes. Importantly, ATF6 containing a luminal achromatopsia eye disease mutation, unresponsive to proteotoxic stress, can be activated by fenretinide, a drug that upregulates DHC, suggesting a potential therapy for this and other ATF6-related diseases including heart disease and stroke.

Quality Control of Orphaned Proteins

The billions of proteins inside a eukaryotic cell are organized among dozens of sub-cellular compartments, within which they are further organized into protein complexes. The maintenance of both levels of organization is crucial for normal cellular function. Newly made proteins that fail to be segregated to the correct compartment or assembled into the appropriate complex are defined as orphans. In this review, we discuss the challenges faced by a cell of minimizing orphaned proteins, the quality control systems that recognize orphans, and the consequences of excess orphans for protein homeostasis and disease.

Increase of MZB1 in B cells in systemic lupus erythematosus: proteomic analysis of biopsied lymph nodes

Background

Systemic lupus erythematosus (SLE) is a prototypical autoimmune disease in which dysregulation of B cells has been recognized. Here, we searched for potential biomarkers of SLE using liquid chromatography-tandem mass spectrometry (LC-MS).

Methods

Lymph nodes from SLE patients and controls were analyzed by LC-MS. To validate the identified molecules, immunoblotting and immunohistochemistry were performed and B cells from SLE patients were analyzed by quantitative RT-PCR. B-cell subsets from NZB/W F1 mice, which exhibit autoimmune disease resembling human SLE, were analyzed by flow cytometry. Endoplasmic reticulum (ER) stress was induced by tunicamycin and the serum concentration of anti-dsDNA antibodies was determined by ELISA. TUNEL methods and immunoblotting were used to assess the effect of tunicamycin.

Results

MZB1, which comprises part of a B-cell-specific ER chaperone complex and is a key player in antibody secretion, was one of the differentially expressed proteins identified by LC-MS and confirmed by immunoblotting. Immunohistochemically, larger numbers of MZB1⁺ cells were located mainly in interfollicular areas and scattered in germinal centers in specimens from SLE patients compared with those from controls. MZB1 colocalized with CD138⁺ plasma cells and IRTA1⁺ marginal zone B cells. MZB1 mRNA was increased by 2.1-fold in B cells of SLE patients with active disease (SLE Disease Activity Index 2000 ≥ 6) compared with controls. In aged NZB/W F1 mice, splenic marginal zone B cells and plasma cells showed elevated MZB1 levels. Tunicamycin induced apoptosis of MZB1⁺ cells in target organs, resulting in decreased serum anti-dsDNA antibody levels. Additionally, MZB1⁺ cells were increased in synovial tissue specimens from patients with rheumatoid arthritis.

Conclusions

MZB1 may be a potential therapeutic target in excessive antibody-secreting cells in SLE.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1511-5) contains supplementary material, which is available to authorized users.

Shaping proteostasis at the cellular, tissue, and organismal level

The proteostasis network (PN) regulates protein synthesis, folding, transport, and degradation to maintain proteome integrity and limit the accumulation of protein aggregates, a hallmark of aging and degenerative diseases. In multicellular organisms, the PN is regulated at the cellular, tissue, and systemic level to ensure organismal health and longevity. Here we review these three layers of PN regulation and examine how they collectively maintain cellular homeostasis, achieve cell type-specific proteomes, and coordinate proteostasis across tissues. A precise understanding of these layers of control has important implications for organismal health and could offer new therapeutic approaches for neurodegenerative diseases and other chronic disorders related to PN dysfunction.

Phosphatidylcholine as a metabolic cue for determining B cell fate and function

In activated B cells, increased production of phosphatidylcholine (PtdCho), the most abundant cellular phospholipid, is handled primarily by the CDP-choline pathway. B cell-specific deletion of CTP:phosphocholine cytidylyltransferase α (CCTα), the rate-limiting enzyme in the CDP-choline pathway, led to augmented IgM secretion and reduced IgG production, suggesting that PtdCho synthesis is required for germinal center reactions. To specifically assess whether PtdCho influences B cell fate during germinal center responses, we examined immune responses in mice whereby PtdCho synthesis is disrupted in B cells that have undergone class switch recombination to IgG1 (referred to as either Cγ1^wt/wt, Cγ1^Cre/wt or Cγ1^Cre/Cre based on Cre copy number). Serum IgG1 was markedly reduced in naïve Cγ1^Cre/wt and Cγ1^Cre/Cre mice, while levels of IgM and other IgG subclasses were similar between Cγ1^Cre/wt and Cγ1^wt/wt control mice. Serum IgG2b titers were notably reduced and IgG3 titers were increased in Cγ1^Cre/Cre mice compared with controls. Following immunization with T cell-dependent antigen NP-KLH, control mice generated high titer IgG anti-NP while IgG anti-NP titers were markedly reduced in both immunized Cγ1^Cre/wt and Cγ1^Cre/Cre mice. Correspondingly, the frequency of NP-specific IgG antibody-secreting cells was also reduced in spleens and bone marrow of Cγ1^Cre/wt and Cγ. 1^Cre/Cre mice compared to control mice. Interestingly, though antigen-specific IgM B cells were comparable between Cγ1^Cre/wt, Cγ1^Cre/Cre and control mice, the frequency and number of IgG1 NP-specific B cells was reduced only in Cγ1^Cre/Cre mice. These data indicate that PtdCho is required for the generation of both germinal center-derived B cells and antibody-secreting cells. Further, the reduction in class-switched ASC but not B cells in Cγ1^Cre/wt mice suggests that ASC have a greater demand for PtdCho compared to germinal center B cells.

ER Stress, UPR and Virus Infections in Plants

The endoplasmic reticulum (ER) endomembrane is a central site for protein synthesis. Perturbation of ER homeostasis can result in an accumulation of unfolded proteins within the ER lumen, causing ER stress and the unfolded protein response (UPR). In humans, ER stress and UPR are closely associated with a vast number of diseases, including viral diseases. In plants, two arms that govern the UPR signaling network have been described: one that contains two ER membrane–associated transcription factors (bZIP17 and bZIP28) and the other that encompasses a dual protein kinase (RNA-splicing factor IRE1) and its target RNA (bZIP60). Although early studies mainly focus on the essential roles of the UPR in abiotic stresses, the significance of UPR in plant diseases caused by virus infections has recently drawn much attention. This chapter summarizes the latest scenario of ER stress and UPR in virus-infected plant cells, highlights the emerging roles of the IRE1 pathway in virus infections, and outlines exciting future directions to spark more research interest in the UPR field in plants.

Retrotransposon derepression leads to activation of the unfolded protein response and apoptosis in pro-B cells

The H3K9me3-specific histone methyltransferase Setdb1 impacts on transcriptional regulation by repressing both developmental genes and retrotransposons. How impaired retrotransposon silencing may lead to developmental phenotypes is currently unclear. Here, we show that loss of Setdb1 in pro-B cells completely abrogates B cell development. In pro-B cells, Setdb1 is dispensable for silencing of lineage-inappropriate developmental genes. Instead, we detect strong derepression of endogenous murine leukemia virus (MLV) copies. This activation coincides with an unusual change in chromatin structure, with only partial loss of H3K9me3 and unchanged DNA methylation, but strongly increased H3K4me3. Production of MLV proteins leads to activation of the unfolded protein response pathway and apoptosis. Thus, our data demonstrate that B cell development depends on the proper repression of retrotransposon sequences through Setdb1.

GCN5 inhibits XBP-1S-mediated transcription by antagonizing PCAF action

Cellular unfolded protein response (UPR) is induced when endoplasmic reticulum (ER) is under stress. XBP-1S, the active isoform of X-box binding protein 1 (XBP-1), is a key regulator of UPR. Previously, we showed that a histone acetyltransferase (HAT), p300/CBP-associated factor (PCAF), binds to XBP-1S and functions as an activator of XBP-1S. Here, we identify general control nonderepressible 5 (GCN5), a HAT with 73% identity to PCAF, as a novel XBP-1S regulator. Both PCAF and GCN5 bind to the same domain of XBP-1S. Surprisingly, GCN5 potently blocks the XBP-1S-mediated transcription, including cellular UPR genes and latent membrane protein 1 of Epstein-Barr virus. Unlike PCAF, GCN5 acetylates XBP-1S and enhances nuclear retention and protein stability of XBP-1S. However, such GCN5-mediated acetylation of XBP-1S shows no effects on XBP-1S activity. In addition, the HAT activity of GCN5 is not required for repression of XBP-1S target genes. We further demonstrate that GCN5 inhibits XBP-1S-mediated transcription by disrupting the PCAF-XBP-1S interaction and preventing the recruitment of XBP-1S to its target genes. Taken together, our results represent the first work demonstrating that GCN5 and PCAF exhibit different functions and antagonistically regulate the XBP-1S-mediated transcription.

Biological insights into the expression of translation initiation factors from recombinant CHOK1SV cell lines and their relationship to enhanced productivity

We show for translation initiation factors involved in formation of the closed loop mRNA, their expression is associated with recombinant antibody productivity in Chinese hamster ovary cells and maintaining these is important in determining the cells capacity for antibody productivity.

The unfolded protein response in multiple sclerosis

The unfolded protein response (UPR) occurs in response to endoplasmic reticulum (ER) stress caused by the accumulation of unfolded or misfolded proteins in the ER. The UPR is comprised of three signaling pathways that promote cytoprotective functions to correct ER stress; however, if ER stress cannot be resolved the UPR results in apoptosis of affected cells. The UPR is an important feature of various human diseases, including multiple sclerosis (MS). Recent studies have shown several components of the UPR are upregulated in the multiple cell types in MS lesions, including oligodendrocytes, T cells, microglia/macrophages, and astrocytes. Data from animal model studies, particularly studies of experimental autoimmune encephalomyelitis (EAE) and the cuprizone model, imply an important role of the UPR activation in oligodendrocytes in the development of MS. In this review we will cover current literature on the UPR and the evidence for its role in the development of MS.

NKG2D and DNAM-1 Ligands: Molecular Targets for NK Cell-Mediated Immunotherapeutic Intervention in Multiple Myeloma

A pivotal strategy to improve NK cell-mediated antitumor activity involves the upregulation of activating ligands on tumor cells. Enhancement of NK cell-mediated recognition of multiple myeloma cells was reported by us and others showing increased surface expression of NKG2D and DNAM-1 ligands on tumor cells following treatment with a number of chemotherapeutic agents, such as genotoxic drugs or inhibitors of proteasome, histone deacetylases, GSK3, and HSP-90. These compounds have the capability to affect tumor survival but also to activate specific transduction pathways associated with the upregulation of different NK cell activating ligands on the tumor cells. Here, we will summarize and discuss the molecular pathways whereby these drugs can regulate the expression of NK cell activating ligands in multiple myeloma cells.

Principles and engineering of antibody folding and assembly

Antibodies are uniquely suited to serve essential roles in the human immune defense as they combine several specific functions in one hetero-oligomeric protein. Their constant regions activate effector functions and their variable domains provide a stable framework that allows incorporation of highly diverse loop sequences. The combination of non-germline DNA recombination and mutation together with heavy and light chain assembly allows developing variable regions that specifically recognize essentially any antigen they may encounter. However, this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully controlled before the protein is secreted from a plasma cell. Accordingly, the generic immunoglobulin fold β-barrel structure of antibody domains has been fine-tuned during evolution to fit the different requirements. Work over the past decades has identified important aspects of the folding and assembly of antibody domains and chains revealing domain specific variations of a general scheme. The most striking is the folding of an intrinsically disordered antibody domain in the context of its partner domain as the basis for antibody assembly and its control on the molecular level in the cell. These insights have not only allowed a better understanding of the antibody folding process but also provide a wealth of opportunities for rational optimization of antibody molecules. In this review, we summarize current concepts of antibody folding and assembly and discuss how they can be utilized to engineer antibodies with improved performance for different applications. This article is part of a Special Issue entitled: Recent advances in the molecular engineering of antibodies.

Regulatory crosstalk within the mammalian unfolded protein response

Increased demands on the protein folding capacity of the endoplasmic reticulum (ER) trigger the unfolded protein response (UPR). Comprised of a tripartite signaling system, the UPR regulates translation and gene transcription to manifest pro-adaptive and, if necessary, pro-apoptotic outcomes. The three UPR pathways, initiated by activating transcription factor 6, inositol requiring enzyme 1, and protein kinase RNA-activated-like ER kinase (PERK), direct distinct downstream signaling events. However, it is becoming increasingly clear that interplay between the cascades is vital in shaping the UPR. In particular, recent discoveries have revealed that PERK-dependent signals mediate both inter- and intra-pathway regulation within the UPR, underscoring the critical role of the PERK pathway in the cellular response to ER stress.

Role of endoplasmic reticulum stress in rheumatoid arthritis pathogenesis

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by abnormal proliferation of synoviocytes, leukocyte infiltration, and angiogenesis. The endoplasmic reticulum (ER) is the site of biosynthesis for all secreted and membrane proteins. The accumulation of unfolded proteins in the ER leads to a condition known as ER stress. Failure of the ER's adaptive capacity results in abnormal activation of the unfolded protein response. Recently, we have demonstrated that ER stress-associated gene signatures are highly expressed in RA synovium and synovial cells. Mice with Grp78 haploinsufficiency exhibit the suppression of experimentally induced arthritis, suggesting that the ER chaperone GRP78 is crucial for RA pathogenesis. Moreover, increasing evidence has suggested that GRP78 participates in antibody generation, T cell proliferation, and pro-inflammatory cytokine production, and is therefore one of the potential therapeutic targets for RA. In this review, we discuss the putative, pathophysiological roles of ER stress and GRP78 in RA pathogenesis.

Targeting HSP90 and monoclonal protein trafficking modulates the unfolded protein response, chaperone regulation and apoptosis in myeloma cells

Multiple myeloma is characterized by the production of substantial quantities of monoclonal protein. We have previously demonstrated that select inhibitors of the isoprenoid biosynthetic pathway (IBP) induce apoptosis of myeloma cells via inhibition of Rab geranylgeranylation, leading to disruption of monoclonal protein trafficking and induction of the unfolded protein response (UPR) pathway. Heat-shock protein 90 (HSP90) inhibitors disrupt protein folding and are currently under clinical investigation in myeloma. The effects of combining IBP and HSP90 inhibitors on cell death, monoclonal protein trafficking, the UPR and chaperone regulation were investigated in monoclonal protein-producing cells. An enhanced induction of cell death was observed following treatment with IBP and HSP90 inhibitors, which occurred through both ER stress and non-ER stress pathways. The HSP90 inhibitor 17-AAG abrogated the effects of the IBP inhibitors on intracellular monoclonal protein levels and localization as well as induction of the UPR in myeloma cells. Disparate effects on chaperone expression were observed in myeloma vs amyloid light chain cells. Here we demonstrate that the novel strategy of targeting MP trafficking in concert with HSP90 enhances myeloma cell death via a complex modulation of ER stress, UPR, and cell death pathways.

Improved antibody production in Chinese hamster ovary cells by ATF4 overexpression

To improve antibody production in Chinese hamster ovary (CHO) cells, the humanized antibody-producing CHO DP-12-SF cell line was transfected with the gene encoding activating transcription factor 4 (ATF4), a central factor in the unfolded protein response. Overexpression of ATF4 significantly enhanced the production of antibody in the CHO DP-12-SF cell line. The specific IgG production rate of in the ATF4-overexpressing CHO-ATF4-16 cells was approximately 2.4 times that of the parental host cell line. Clone CHO-ATF4-16 did not show any change in growth rate compared with the parental cells or mock-transfected CHO-DP12-SF cells. The expression levels of mRNAs encoding both the antibody heavy and light chains in the CHO-ATF4-16 clone were analyzed. This analysis showed that ATF4 overexpression improved the total production and specific production rate of antibody without affecting the mRNA transcription level. These results indicate that ATF4 overexpression is a promising method for improving recombinant IgG production in CHO cells.

Downregulation of miR-181b in mouse brain following ischemic stroke induces neuroprotection against ischemic injury through targeting heat shock protein A5 and ubiquitin carboxyl-terminal hydrolase isozyme L1

Understanding the molecular mechanism of cerebral hypoxic preconditioning (HPC)-induced endogenous neuroprotection may provide potential therapeutic targets for ischemic stroke. By using bioinformatics analysis, we found that miR-181b, one of 19 differentially expressed miRNAs, may target aconitate hydratase (ACO2), heat shock protein A5 (HSPA5), and ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1) among 26 changed protein kinase C isoform-specific interacting proteins in HPC mouse brain. In this study, the role of miR-181b in oxygen-glucose deprivation (OGD)-induced N2A cell ischemic injury in vitro and mouse middle cerebral artery occlusion (MCAO)-induced cerebral ischemic injury in vivo, and its regulation of ACO2, HSPA5, and UCHL1 were further determined. We found that miR-181b expression levels significantly decreased in mouse brain following MCAO and in OGD-treated N2A cells. Up- and downregulation of miR-181b by transfection of pre- or anti-miR-181b could negatively regulate HSPA5 and UCHL1 (but not ACO2) protein levels as well as N2A cell death and programmed cell death in OGD-treated N2A cells. By using a T7 promoter-driven control dual luciferase assay, we confirmed that miR-181b could bind to the 3'-untranslated rergions of HSPA5 and UCHL1 mRNAs and repress their translations. miR-181b antagomir reduced caspase-3 cleavage and neural cell loss in cerebral ischemic cortex and improved neurological deficit of mice after MCAO. In addition, HSPA5 and UCHL1 short interfering RNAs (siRNAs) blocked anti-miR-181b-mediated neuroprotection against OGD-induced N2A cell injury in vitro. These results suggest that the downregulated miR-181b induces neuroprotection against ischemic injury through negatively regulating HSPA5 and UCHL1 protein levels, providing a potential therapeutic target for ischemic stroke.

The proteasome in terminal plasma cell differentiation

The ability of eukaryotic cells to adapt to changing environmental conditions, respond to stimuli, and differentiate relies on their capacity to control the concentration, conformation, localization, and interaction of proteins, thereby reshaping their proteome. Protein degradation plays a critical role in maintaining protein homeostasis, and hence is carefully regulated. During the spectacular and demanding metamorphosis of activated B lymphocytes, expression programs are launched in coordinated waves, and adaptive strategies are deployed to prepare for antibody secretion. Surprisingly, though, despite increased demand for proteolysis, proteasome capacity collapses. As a result, antibody-secreting cells show symptoms of proteotoxic stress, and become extremely vulnerable to proteasome inhibition. The emerging concept that proteostenosis naturally follows B-cell activation has biological and immune implications, for it provides a model to dissect the integrated regulation of protein homeostasis, and a molecular counter limiting antibody responses, of use against autoimmune diseases. Mounting evidence linking proteotoxicity with proteasome vulnerability in malignant plasma cells visualizes strategies to understand responsiveness and obviate resistance to proteasome inhibition, with implications for the biology and therapy of plasma cell dyscrasias, namely, light chain amyloidosis and multiple myeloma.

Diversity in the origins of proteostasis networks--a driver for protein function in evolution

Although the sequence of a protein largely determines its function, proteins can adopt different folding states in response to changes in the environment, some of which may be deleterious to the organism. All organisms--Bacteria, Archaea and Eukarya--have evolved a protein homeostasis, or proteostasis, network comprising chaperones and folding factors, degradation components, signalling pathways and specialized compartmentalized modules that manage protein folding in response to environmental stimuli and variation. Surveying the origins of proteostasis networks reveals that they have co-evolved with the proteome to regulate the physiological state of the cell, reflecting the unique stresses that different cells or organisms experience, and that they have a key role in driving evolution by closely managing the link between the phenotype and the genotype.

Plant transducers of the endoplasmic reticulum unfolded protein response

The unfolded protein response (UPR) activates a set of genes to overcome accumulation of unfolded proteins in the endoplasmic reticulum (ER), a condition termed ER stress, and constitutes an essential part of ER protein quality control that ensures efficient maturation of secretory and membrane proteins in eukaryotes. Recent studies on Arabidopsis and rice identified the signaling pathway in which the ER membrane-localized ribonuclease IRE1 (inositol-requiring enzyme 1) catalyzes unconventional cytoplasmic splicing of mRNA, thereby producing the active transcription factor Arabidopsis bZIP60 (basic leucine zipper 60) and its ortholog in rice. Here we review recent findings identifying the molecular components of the plant UPR, including IRE1/bZIP60 and the membrane-bound transcription factors bZIP17 and bZIP28, and implicating its importance in several physiological phenomena such as pathogen response.

Plant transducers of the endoplasmic reticulum unfolded protein response

The unfolded protein response (UPR) activates a set of genes to overcome accumulation of unfolded proteins in the endoplasmic reticulum (ER), a condition termed ER stress, and constitutes an essential part of ER protein quality control that ensures efficient maturation of secretory and membrane proteins in eukaryotes. Recent studies on Arabidopsis and rice identified the signaling pathway in which the ER membrane-localized ribonuclease IRE1 (inositol-requiring enzyme 1) catalyzes unconventional cytoplasmic splicing of mRNA, thereby producing the active transcription factor Arabidopsis bZIP60 (basic leucine zipper 60) and its ortholog in rice. Here we review recent findings identifying the molecular components of the plant UPR, including IRE1/bZIP60 and the membrane-bound transcription factors bZIP17 and bZIP28, and implicating its importance in several physiological phenomena such as pathogen response.

IRE1α activation protects mice against acetaminophen-induced hepatotoxicity

The mammalian stress sensor IRE1α plays a central role in the unfolded protein, or endoplasmic reticulum (ER), stress response by activating its downstream transcription factor XBP1 via an unconventional splicing mechanism. IRE1α can also induce the degradation of a subset of mRNAs in a process termed regulated IRE1-dependent decay (RIDD). Although diverse mRNA species can be degraded by IRE1α in vitro, the pathophysiological functions of RIDD are only beginning to be explored. Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure in young adults in the United States and is primarily caused by CYP1A2-, CYP2E1-, and CYP3A4-driven conversion of APAP into hepatotoxic metabolites. We demonstrate here that genetic ablation of XBP1 results in constitutive IRE1α activation in the liver, leading to RIDD of Cyp1a2 and Cyp2e1 mRNAs, reduced JNK activation, and protection of mice from APAP-induced hepatotoxicity. A pharmacological ER stress inducer that activated IRE1α suppressed the expression of Cyp1a2 and Cyp2e1 in WT, but not IRE1α-deficient mouse liver, indicating the essential role of IRE1α in the down-regulation of these mRNAs upon ER stress. Our study reveals an unexpected function of RIDD in drug metabolism.

Virus-induced ER stress and the unfolded protein response

The accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum (ER) results in ER stress that triggers cytoprotective signaling pathways, termed the unfolded protein response (UPR), to restore and maintain homeostasis in the ER or to induce apoptosis if ER stress remains unmitigated. The UPR signaling network encompasses three core elements, i.e., PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring protein-1 (IRE1). Activation of these three branch pathways of the UPR leads to the translation arrest and degradation of misfolded proteins, the expression of ER molecular chaperones, and the expansion of the ER membrane to decrease the load of proteins and increase the protein-folding capacity in the ER. Recently, the essential roles of the UPR have been implicated in a number of mammalian diseases, particularly viral diseases. In virus-infected cells, the cellular translation machinery is hijacked by the infecting virus to produce large amounts of viral proteins, which inevitably perturbs ER homeostasis and causes ER stress. This review summarizes current knowledge about the UPR signaling pathways, highlights two identified UPR pathways in plants, and discuss progress in elucidating the UPR in virus-infected cells and its functional roles in viral infection.

In vivo crystallization of human IgG in the endoplasmic reticulum of engineered Chinese hamster ovary (CHO) cells

Protein synthesis and secretion are essential to cellular life. Although secretory activities may vary in different cell types, what determines the maximum secretory capacity is inherently difficult to study. Increasing protein synthesis until reaching the limit of secretory capacity is one strategy to address this key issue. Under highly optimized growth conditions, recombinant CHO cells engineered to produce a model human IgG clone started housing rod-shaped crystals in the endoplasmic reticulum (ER) lumen. The intra-ER crystal growth was accompanied by cell enlargement and multinucleation and continued until crystals outgrew cell size to breach membrane integrity. The intra-ER crystals were composed of correctly folded, endoglycosidase H-sensitive IgG. Crystallizing propensity was due to the intrinsic physicochemical properties of the model IgG, and the crystallization was reproduced in vitro by exposing a high concentration of IgG to a near neutral pH. The striking cellular phenotype implicated the efficiency of IgG protein synthesis and oxidative folding exceeded the capacity of ER export machinery. As a result, export-ready IgG accumulated progressively in the ER lumen until a threshold concentration was reached to nucleate crystals. Using an in vivo system that reports accumulation of correctly folded IgG, we showed that the ER-to-Golgi transport steps became rate-limiting in cells with high secretory activity.

Factor VIII delivered by haematopoietic stem cell-derived B cells corrects the phenotype of haemophilia A mice

The main impediments to clinical application of haematopoietic stem cell (HSC) gene therapy for treatment of haemophilia A are the bone marrow transplant-related risks and the potential for insertional mutagenesis caused by retroviral vectors. To circumvent these limitations, we have adapted a non-myeloablative conditioning regimen and directed factor VIII (FVIII) protein synthesis to B lineage cells using an insulated lentiviral vector containing an immunoglobulin heavy chain enhancer-promoter. Transplantation of lentiviral vector-modified HSCs resulted in therapeutic levels of FVIII in the circulation of all transplanted mice for the duration of the study (six months). Immunostaining of spleen cells showed that the majority of FVIII was synthesised by B220+ B cells and CD138+ plasma cells. Subsequent challenge with recombinant FVIII elicited at most a minor anti-FVIII antibody response, demonstrating induction of immune hyporesponsiveness. All transplant recipients exhibited clot formation and survived tail clipping, indicating correction of their haemophilic phenotype. Therapeutic levels of FVIII could be transferred to secondary recipients by bone marrow transplantation, confirming gene transfer into long-term repopulating HSCs. Moreover, short-term therapeutic FVIII levels could also be achieved in secondary recipients by adoptive transfer of HSC-derived splenic B cells. Our findings support pursuit of B cell-directed protein delivery as a potential clinical approach to treat haemophilia A and other disorders correctable by systemically distributed proteins.

Proteostenosis and plasma cell pathophysiology

Plasma cells differentiate from B lymphocytes to sustain antibody production. As professional secretors, they allow dissecting proteostasis in the exocytic compartment, the stresses that protein production entails and their possible roles in signaling. Most plasma cells are short-lived to limit antibody responses. After a few days of intense immunoglobulin production, they undergo apoptosis, offering a unique model of cellular senescence. Recent observations reveal that proteotoxic stresses physiologically contribute to regulate their biogenesis, function and lifespan, explaining partly the sensitivity of multiple myeloma cells to proteasome inhibitors. This essay summarizes these plasma cell lessons, and their general implications for the regulation of proteostasis, cell senescence and cancer therapeutics.

Increased B cell proliferation and reduced Ig production in DREAM transgenic mice

DREAM/KChIP-3 is a calcium-dependent transcriptional repressor highly expressed in immune cells. Transgenic mice expressing a dominant active DREAM mutant show reduced serum Ig levels. In vitro assays show that reduced Ig secretion is an intrinsic defect of transgenic B cells that occurs without impairment in plasma cell differentiation, class switch recombination, or Ig transcription. Surprisingly, transgenic B cells show an accelerated entry in cell division. Transcriptomic analysis of transgenic B cells revealed that hyperproliferative B cell response could be correlated with a reduced expression of Klf9, a cell-cycle regulator. Pulse-chase experiments demonstrated that the defect in Ig production is associated with reduced translation rather than with increased protein degradation. Importantly, transgenic B cells showed reduced expression of the Eif4g3 gene, which encodes a protein related to protein translation. Our results disclose, to our knowledge, a novel function of DREAM in proliferation and Ig synthesis in B lymphocytes.

Isoprenoid biosynthetic pathway inhibition disrupts monoclonal protein secretion and induces the unfolded protein response pathway in multiple myeloma cells

Myeloma is characterized by the overproduction and secretion of monoclonal protein. Inhibitors of the isoprenoid biosynthetic pathway (IBP) have pleiotropic effects in myeloma cells. To investigate whether IBP inhibition interferes with monoclonal protein secretion, human myeloma cells were treated with specific inhibitors of the IBP or prenyltransferases. These studies demonstrate that agents that inhibit Rab geranylgeranylation disrupt light chain trafficking, lead to accumulation of light chain in the endoplasmic reticulum, activate the unfolded protein response pathway and induce apoptosis. These studies provide a novel mechanism of action for IBP inhibitors and suggest that further exploration of Rab-targeted agents in myeloma is warranted.

PCAF interacts with XBP-1S and mediates XBP-1S-dependent transcription

X-box binding protein 1 (XBP-1) is a key regulator required for cellular unfolded protein response (UPR) and plasma cell differentiation. In addition, involvement of XBP-1 in host cell–virus interaction and transcriptional regulation of viruses, such as human T-lymphotropic virus type 1 (HTLV-1), has been revealed recently. Two XBP-1 isoforms, XBP-1U and XBP-1S, which share an identical N-terminal domain, are present in cells. XBP-1S is a transcription activator while XBP-1U is the inactive isoform. Although the transactivation domain of XBP-1S has been identified within the XBP-1S-specific C-terminus, molecular mechanism of the transcriptional activation by XBP-1S still remains unknown. Here we report the interaction between p300/CBP-associated factor (PCAF) and XBP-1S through the C-terminal domain of XBP-1S. No binding between XBP-1U and PCAF is detected. In a cell-based reporter assay, overexpression of PCAF further stimulates the XBP-1S-mediated cellular and HTLV-1 transcription while knockdown of PCAF exhibits the opposite effect. Expression of endogenous XBP-1S cellular target genes, such as BiP and CHOP, is significantly inhibited when PCAF is knocked down. Furthermore, PCAF is recruited to the promoters of XBP-1S target genes in vivo, in a XBP-1S-dependent manner. Collectively, our results demonstrate that PCAF mediates the XBP-1S-dependent transcription through the interaction with XBP-1S.

Regulation of XBP-1 signaling during transient and stable recombinant protein production in CHO cells

X-box binding protein 1 (XBP-1) is a key regulator of cellular unfolded protein response (UPR). The spliced isoform of XBP-1, XBP-1S, is a transcription activator, which is expressed only when UPR is induced. However, the impact of recombinant protein production on the regulation of XBP-1 signaling in CHO cells is not well understood. In this report, we cloned the Chinese hamster XBP-1 homolog to aid the investigation of the interplay between protein productivity, culture conditions, and endogenous XBP-1 signaling in CHO cells. Interestingly, expression of XBP-1S is detected in the non-producing and unstressed CHO-K1 cells. Transient expression of recombinant erythropoietin reveals a positive correlation between XBP-1 mRNA abundance and protein production level. However, such a correlation is not observed in batch cultivation of stable producing cell lines. The increased XBP-1 splicing is detected in late-phase cultures, suggesting that induction of XBP-1S may be a result of nutrient limitations or other environmental stresses rather than that of increased intracellular accumulation of recombinant proteins. Our data suggest that XBP-1 is a key determinant for the secretory capacity of CHO cells. Understanding its dynamic regulation hence provides a rational basis for cellular engineering strategies to improve recombinant protein secretion.