Temperature-dependent intracellular crystallization of firefly luciferase in mammalian cells is suppressed by D-luciferin and stabilizing inhibitors.

Firefly luciferase (Fluc) from Photinus pyralis is one of the most widely used reporter proteins in biomedical research. Despite its widespread use, Fluc's protein phase transition behaviors and phase separation characteristics have not received much attention. Current research uncovers Fluc's intrinsic property to phase separate in mammalian cells upon a simple cell culture temperature change. Specifically, Fluc spontaneously produced needle-shaped crystal-like inclusion bodies upon temperature shift to the hypothermic temperatures ranging from 25 °C to 31 °C. The crystal-like inclusion bodies were not associated with or surrounded by membranous organelles and were likely built from the cytosolic pool of Fluc. Furthermore, the crystal-like inclusion formation was suppressed when cells were cultured in the presence of D-luciferin and its synthetic analog, as well as the benzothiazole family of so-called stabilizing inhibitors. These two classes of compounds inhibited intracellular Fluc crystallization by different modes of action as they had contrasting effects on steady-state luciferase protein accumulation levels. This study suggests that, under substrate insufficient conditions, the excess Fluc phase separates into a crystal-like state that can modulate intracellular soluble enzyme availability and protein turnover rate.

Simultaneous induction of distinct protein phase separation events in multiple subcellular compartments of a single cell.

Intracellular protein crystallization occurs under various physiological and pathological settings, yet the underlying cellular processes remain enigmatic. After validating individual crystallization events using cellular proteins that readily crystallize in the ER (NEU1), cytosol (crystallin-γD mutant) or nucleus (CLC protein), I demonstrate three independent crystallization events can take place concurrently in different subcellular compartments of a single cell without compromising cell viability. By co-expressing NEU1 and previously reported two human monoclonal antibodies that undergo crystallization and liquid-liquid phase separation in the ER, I additionally demonstrate two independent phase separation events can be simultaneously induced in the ER lumen of a single cell without mixing or interfering each other's phase separation behaviors. Intracellular protein crystallization thus takes place in a crowded physiological cellular environment and does not require high protein purity. Furthermore, I report a simple method to increase the yield of intracellular protein crystals by treating the cells with a topoisomerase II inhibitor that blocks cell division without preventing cell size growth. This study not only presents accessible model tools for studying cryptic in vivo protein crystallization events, but also paves a way toward establishing the intracellular protein crystallization as a novel platform for recombinant protein expression and purification.

A rare IL33 loss-of-function mutation reduces blood eosinophil counts and protects from asthma.

IL-33 is a tissue-derived cytokine that induces and amplifies eosinophilic inflammation and has emerged as a promising new drug target for asthma and allergic disease. Common variants at IL33 and IL1RL1, encoding the IL-33 receptor ST2, associate with eosinophil counts and asthma. Through whole-genome sequencing and imputation into the Icelandic population, we found a rare variant in IL33 (NM_001199640:exon7:c.487-1G>C (rs146597587-C), allele frequency = 0.65%) that disrupts a canonical splice acceptor site before the last coding exon. It is also found at low frequency in European populations. rs146597587-C associates with lower eosinophil counts (ß = -0.21 SD, P = 2.5×10-16, N = 103,104), and reduced risk of asthma in Europeans (OR = 0.47; 95%CI: 0.32, 0.70, P = 1.8×10-4, N cases = 6,465, N controls = 302,977). Heterozygotes have about 40% lower total IL33 mRNA expression than non-carriers and allele-specific analysis based on RNA sequencing and phased genotypes shows that only 20% of the total expression is from the mutated chromosome. In half of those transcripts the mutation causes retention of the last intron, predicted to result in a premature stop codon that leads to truncation of 66 amino acids. The truncated IL-33 has normal intracellular localization but neither binds IL-33R/ST2 nor activates ST2-expressing cells. Together these data demonstrate that rs146597587-C is a loss of function mutation and support the hypothesis that IL-33 haploinsufficiency protects against asthma.

Membrane cholesterol modulates STEAP2 conformation during dynamic intracellular trafficking processes leading to broad subcellular distribution.

STEAP2 is a member of the Six-Transmembrane Epithelial Antigen of the Prostate (STEAP) protein family that is proposed to function as metalloreductase. While STEAP2 shows a complex subcellular distribution pattern localizing to both secretory and endocytic pathway organelles, how such broad steady-state distribution is maintained is unknown. Similarly, whether STEAP2 undergoes any compartment-specific modulation during intracellular trafficking has not been reported. Leveraging a newly-identified monoclonal antibody that recognizes a conformation-sensitive epitope nested in the second extracellular loop of STEAP2, we demonstrate that the epitope formation was dependent on the cholesterol content of the membrane in which STEAP2 was embedded. Monitoring the STEAP2-dependent internalization of this antibody uncovered STEAP2's rapid internalization from the cell surface and their subsequence trafficking to the Golgi region and endosome-like puncta. Acute inhibition of endocytosis also increased the detectable amount of STEAP2 at the plasma membrane. Collectively, these experiments demonstrate that an intricate balance of membrane flux between the secretory and endocytic pathways underlies the characteristic broad subcellular localization of STEAP2. By using a cell-based assay that detects the metalloreductase functions of cell surface-localizing STEAP4, STEAP2's metalloreductase activities were not detectable, suggesting that its enzymatic function is suppressed at the plasma membrane. The conformational modulation of STEAP2 by the local membrane cholesterol content can therefore serve as a potential mechanism to modulate STEAP2 function in a compartment-restricted manner, by coupling a pre-existing difference in cholesterol content among different cellular membranes to a dynamic trafficking process leading to broad subcellular distribution.

Topogenesis and cell surface trafficking of GPR34 are facilitated by positive-inside rule that effects through a tri-basic motif in the first intracellular loop.

Protein folding, topogenesis and intracellular targeting of G protein-coupled receptors (GPCRs) must be precisely coordinated to ensure correct receptor localization. To elucidate how different steps of GPCR biosynthesis work together, we investigated the process of membrane topology determination and how it relates to the acquisition of cell surface trafficking competence in human GPR34. By monitoring a fused FLAG-tag and a conformation-sensitive native epitope during the expression of GPR34 mutant panel, a tri-basic motif in the first intracellular loop was identified as the key topogenic signal that dictates the orientation of transmembrane domain-1 (TM1). Charge disruption of the motif perturbed topogenic processes and resulted in the conformational epitope loss, post-translational processing alteration, and trafficking arrest in the Golgi. The placement of a cleavable N-terminal signal sequence as a surrogate topogenic determinant overcame the effects of tri-basic motif mutations and rectified the TM1 orientation; thereby restored the conformational epitope, post-translational modifications, and cell surface trafficking altogether. Progressive N-tail truncation and site-directed mutagenesis revealed that a proline-rich segment of the N-tail and all four cysteines individually located in the four separate extracellular regions must simultaneously reside in the ER lumen to muster the conformational epitope. Oxidation of all four cysteines was necessary for the epitope formation, but the cysteine residues themselves were not required for the trafficking event. The underlying biochemical properties of the conformational epitope was therefore the key to understand mechanistic processes propelled by positive-inside rule that simultaneously regulate the topogenesis and intracellular trafficking of GPR34.

A novel antibody engineering strategy for making monovalent bispecific heterodimeric IgG antibodies by electrostatic steering mechanism.

Producing pure and well behaved bispecific antibodies (bsAbs) on a large scale for preclinical and clinical testing is a challenging task. Here, we describe a new strategy for making monovalent bispecific heterodimeric IgG antibodies in mammalian cells. We applied an electrostatic steering mechanism to engineer antibody light chain-heavy chain (LC-HC) interface residues in such a way that each LC strongly favors its cognate HC when two different HCs and two different LCs are co-expressed in the same cell to assemble a functional bispecific antibody. We produced heterodimeric IgGs from transiently and stably transfected mammalian cells. The engineered heterodimeric IgG molecules maintain the overall IgG structure with correct LC-HC pairings, bind to two different antigens with comparable affinity when compared with their parental antibodies, and retain the functionality of parental antibodies in biological assays. In addition, the bispecific heterodimeric IgG derived from anti-HER2 and anti-EGF receptor (EGFR) antibody was shown to induce a higher level of receptor internalization than the combination of two parental antibodies. Mouse xenograft BxPC-3, Panc-1, and Calu-3 human tumor models showed that the heterodimeric IgGs strongly inhibited tumor growth. The described approach can be used to generate tools from two pre-existent antibodies and explore the potential of bispecific antibodies. The asymmetrically engineered Fc variants for antibody-dependent cellular cytotoxicity enhancement could be embedded in monovalent bispecific heterodimeric IgG to make best-in-class therapeutic antibodies.

Modulation of in vivo IgG crystallization in the secretory pathway by heavy chain isotype class switching and N-linked glycosylation.

Crystalline bodies (CBs) can develop in the endoplasmic reticulum (ER) of antibody-producing cells. Although this phenotype is often reported in association with plasma cell dyscrasias and other hematological disorders, the details of CB biogenesis and CB's roles in pathophysiology remain poorly understood. Using an imaging-based screening method, we identified a secretion-competent human IgG2/λ clone that develops spindle-shaped intracellular crystals in transiently-transfected HEK293 cells upon Brefeldin A treatment. When stably overexpressed from CHO cells, the IgG2/λ clone spontaneously produced spindle-shaped CBs in the ER. Some CBs were released to the extracellular space while remaining enclosed by the membranes of secretory pathway origin. Structural modeling on the variable-region did not uncover prominent surface characteristics such as charge clusters. In contrast, alterations to the constant domain-encoded properties revealed their modulatory roles in CB-inducing propensities and CB morphology. For example, deletion of the entire Fc domain changed the morphology of CBs into thin filaments. Elimination of an N-linked glycan by a N297A mutation promoted Russell body biogenesis accompanied by marked reduction in IgG secretion. Isotype class switching from the original IgG2 to IgG1 and IgG4 changed the crystal morphology from spindle-shaped to long needle and acicular shaped, respectively. The IgG3 version, in contrast, suppressed the CB formation. Either the HC or LC alone or the Fc-domain alone did not trigger CB biogenesis. An IgG's in vivo crystal morphology and crystallization propensity can thus be modulated by the properties genetically and biochemically encoded in the HC constant region.

Understanding the biosynthesis of human IgM SAM-6 through a combinatorial expression of mutant subunits that affect product assembly and secretion.

Polymeric IgMs are secreted from plasma cells abundantly despite their structural complexity and intricate multimerization steps. To gain insights into IgM's assembly mechanics that underwrite such high-level secretion, we characterized the biosynthetic process of a natural human IgM, SAM-6, using a heterologous HEK293(6E) cell platform that allowed the production of IgMs both in hexameric and pentameric forms in a controlled fashion. By creating a series of mutant subunits that differentially disrupt secretion, folding, and specific inter-chain disulfide bond formation, we assessed their effects on various aspects of IgM biosynthesis in 57 different subunit chain combinations, both in hexameric and pentameric formats. The mutations caused a spectrum of changes in steady-state subcellular subunit distribution, ER-associated inclusion body formation, intracellular subunit detergent solubility, covalent assembly, secreted IgM product quality, and secretion output. Some mutations produced differential effects on product quality depending on whether the mutation was introduced to hexameric IgM or pentameric IgM. Through this systematic combinatorial approach, we consolidate diverse overlapping knowledge on IgM biosynthesis for both hexamers and pentamers, while unexpectedly revealing that the loss of certain inter-chain disulfide bonds, including the one between µHC and λLC, is tolerated in polymeric IgM assembly and secretion. The findings highlight the differential roles of underlying non-covalent protein-protein interactions in hexamers and pentamers when orchestrating the initial subunit interactions and maintaining the polymeric IgM product integrity during ER quality control steps, secretory pathway trafficking, and secretion.

Hair-follicle associated pluripotent (HAP)-cell-sheet implantation enhanced wound healing in diabetic db/db mice.

Diabetes often results in chronic ulcers that fail to heal. Effective treatment for diabetic wounds has not been achieved, although stem-cell-treatment has shown promise. Hair-follicle-associated-pluripotent (HAP)-stem-cells from bulge area of mouse hair follicle have been shown to differentiate into keratinocytes, vascular endothelial cells, smooth muscle cells, and some other types of cells. In the present study, we developed HAP-cell-sheets to determine their effects on wound healing in type-2 diabetes mellitus (db/db) C57BL/6 mouse model. Flow cytometry analysis showed cytokeratin 15 expression in 64% of cells and macrophage expression in 3.6% of cells in HAP-cell-sheets. A scratch cell migration assay in vitro showed the ability of fibroblasts to migrate and proliferate was enhanced when co-cultured with HAP-cell-sheets. To investigate in vivo effects of the HAP-cell-sheets, they were implanted into 10 mm circular full-thickness resection wounds made on the back of db/db mice. Wound closure was facilitated in the implanted group until day 16. The thickness of epithelium and granulation tissue volume at day 7 were significantly increased by the implantation. CD68 positive area and TGF-ß1 positive area were significantly increased; meanwhile, iNOS positive area was reduced at day 7 in the HAP-cell-sheets implanted group. After 21 days, CD68 positive areas in the implanted group were reduced to under the control group level, and TGF-ß1 positive area had no difference between the two groups. These observations strongly suggest that the HAP-cell-sheets implantation is efficient to facilitate early macrophage activity and to suppress inflammation level. Using immuno-double-staining against CD34 and α-SMA, we found more vigorous angiogenesis in the implanted wound tissue. The present results suggest autologous HAP-cell-sheets can be used to heal refractory diabetic ulcers and have clinical promise.

Russell body inducing threshold depends on the variable domain sequences of individual human IgG clones and the cellular protein homeostasis.

Russell bodies are intracellular aggregates of immunoglobulins. Although the mechanism of Russell body biogenesis has been extensively studied by using truncated mutant heavy chains, the importance of the variable domain sequences in this process and in immunoglobulin biosynthesis remains largely unknown. Using a panel of structurally and functionally normal human immunoglobulin Gs, we show that individual immunoglobulin G clones possess distinctive Russell body inducing propensities that can surface differently under normal and abnormal cellular conditions. Russell body inducing predisposition unique to each immunoglobulin G clone was corroborated by the intrinsic physicochemical properties encoded in the heavy chain variable domain/light chain variable domain sequence combinations that define each immunoglobulin G clone. While the sequence based intrinsic factors predispose certain immunoglobulin G clones to be more prone to induce Russell bodies, extrinsic factors such as stressful cell culture conditions also play roles in unmasking Russell body propensity from immunoglobulin G clones that are normally refractory to developing Russell bodies. By taking advantage of heterologous expression systems, we dissected the roles of individual subunit chains in Russell body formation and examined the effect of non-cognate subunit chain pair co-expression on Russell body forming propensity. The results suggest that the properties embedded in the variable domain of individual light chain clones and their compatibility with the partnering heavy chain variable domain sequences underscore the efficiency of immunoglobulin G biosynthesis, the threshold for Russell body induction, and the level of immunoglobulin G secretion. We propose that an interplay between the unique properties encoded in variable domain sequences and the state of protein homeostasis determines whether an immunoglobulin G expressing cell will develop the Russell body phenotype in a dynamic cellular setting.

R-SNARE ykt6 resides in membrane-associated protease-resistant protein particles and modulates cell cycle progression when over-expressed.

BACKGROUND INFORMATION: The arginine-type soluble N-ethylmaleimide-sensitive factor attachment protein receptor (R-SNARE) ykt6 possesses several atypical properties including selective high expression in neurons, a lipidated C-terminus, localization to punctae that do not correspond with known endomembrane markers, a potent ability to protect the secretory pathway from alpha-synuclein over-expression and specific up-regulation in tumors. We have followed up on several of these features that together suggest nontraditional SNARE structures and functions. RESULTS: A significant portion of ykt6 in PC12 cells was found in a protease-resistant state suggestive of a large complex or aggregate. Other endoplasmic reticulum/Golgi SNAREs were not protease resistant, demonstrating that SNARE complexes per se did not cause protease resistance. Mutagenesis indicated that lipidation of the ykt6 C-terminus was also not involved, implicating its longin domain in particle formation. Immunogold electron microscopy revealed ykt6 labeling of ∼100 nm electron densities associated with diverse membranes. Density gradient analysis of the protease-resistant structures confirmed their tight association with membranes. Since excess ykt6 has been correlated with tumorigenesis, we tested whether ykt6 over-expression in normal rat kidney cells that normally express little ykt6 affected the cell cycle. Ykt6 over-expression was found to result in altered cell division cycles as evidenced by significantly smaller cells, a higher mitotic index and increased DNA synthesis. Mutagenesis studies dis-correlated SNARE function with the cell cycle effects; instead, the cell cycle effects correlated better with ykt6 properties related to the longin domain or particle formation. CONCLUSIONS: The ykt6 particles/aggregates may represent ykt6 engaged in a non-SNARE function(s) or else nonfunctional, stored and/or excess ykt6. Whether the particulate ykt6 structures represent a means of buffering the apparent proliferative activity or are in fact mechanistically related to this activity will be of future interest in neuroscience and cancer biology.

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.

Diverse functions of reactive cysteines facilitate unique biosynthetic processes of aggregate-prone interleukin-31.

Interleukin-31 (IL-31) is a member of the four helical-bundle gp130/IL-6 cytokine family. Despite its implicated roles in inflammatory diseases, the biosynthetic processes of IL-31 have been poorly investigated. A detailed understanding of IL-31 biosynthesis and the nature of ligand-receptor interactions can provide insights into effective strategies for the design of therapeutic approaches. By using various heterologous protein expression systems, we demonstrated that murine IL-31 was secreted as inter-molecularly disulfide-bonded covalent aggregates. Covalently aggregated IL-31 appeared while trafficking in the secretory pathway, but was not actively retained in the ER. The aggregate formation was not caused by a dysfunctional ER quality control mechanism or an intrinsic limitation in protein folding capacity. Furthermore, secreted IL-31 aggregates were part of a large complex composed of various pleiotropic secretory factors and immune-stimulators. The extent and the heterogeneous nature of aggregates may imply that IL-31 was erroneously folded, but it was capable of signaling through cognate receptors. Mutagenesis revealed the promiscuity of all five cysteines in inter-molecular disulfide formation with components of the hetero-aggregates, but no cysteine was required for IL-31 secretion itself. Our present study not only illustrated various functions that cysteines perform during IL-31 biosynthesis and secretion, but also highlighted their potential roles in cytokine effector functions.

Native IL-32 is released from intestinal epithelial cells via a non-classical secretory pathway as a membrane-associated protein.

Although IL-32 has been shown to be induced under various pathological conditions, a detailed understanding of native IL-32 intracellular distribution and mechanism of release from cells has not been reported. We examined the expression of IL-32 in the intestinal epithelial cell line HT-29 following TNFα and IFNγ co-stimulation. The subcellular localization of induced IL-32 was associated with the membrane of lipid droplet-like structures and vacuolar structures that co-localized with markers of endosomes and lysosomes. Prolonged co-stimulation resulted in cell death and appearance of IL-32 in the culture medium. IL-32 released from co-stimulated HT-29 cells was found in a detergent-sensitive particulate fraction, and in a step density gradient the IL-32 particulate was buoyant, suggesting association with a membrane-bound vesicle. Upon Triton X-114 partitioning, most of the IL-32 partitioned to the detergent phase, suggesting hydrophobic characteristics. When IL-32-containing vesicles were subjected to protease K treatment, a protease resistant ∼12kDa fragment was generated from ∼24kDa IL-32. We propose that under these conditions, native IL-32 is released via a non-classical secretory route perhaps involving multi-vesicular bodies and exosomes. Demonstration of membrane association for both intracellular and released IL-32 suggests this unique cytokine may have a complex biosynthetic pathway and mechanism of action.

Light chain subunit of a poorly soluble human IgG2λ crystallizes in physiological pH environment both in cellulo and in vitro.

Prominent inclusion bodies can develop in the endoplasmic reticulum (ER) when overexpressed antibodies possess intrinsically high condensation propensities. These observations suggest that antibodies deemed to show notable solubility problems may reveal such characteristics preemptively in the form of ER-associated inclusion bodies during antibody overexpression. To define the relationships between solubility problems and inclusion body phenotypes, we investigated the biosynthesis of a model human IgG2λ that shows severe opalescence in an acidic formulation buffer yet retains high solubility at physiological pH. Consistent with the pH-dependent solubility characteristics, the model antibody did not induce notable inclusion body in the physiological pH environment of the ER lumen. However, when individual subunit chains of the antibody were expressed separately, the light chain (LC) spontaneously induced notable crystal-like inclusion bodies in the ER. The LC crystallization event was readily reproducible in vitro by simply concentrating the purified LC protein at physiological pH. Two independent structural determinants for the LC crystallization were identified through rational mutagenesis approach by monitoring the effect of amino acid substitutions on intracellular LC crystallogenesis. The effect of mutations on crystallization was also recapitulated in vitro using purified LC proteins. Importantly, when introduced directly into the model antibody, a mutation that prevents the LC crystallization remediated the antibody's solubility problem without compromising the secretory output or antigen binding. These results illustrate that the ER can serve as a "physiological test tube" that not only reports secretory cargo's high condensation propensity at physiological pH, but also provides an orthogonal method that guides antibody engineering strategy.

Mammalian ykt6 is a neuronal SNARE targeted to a specialized compartment by its profilin-like amino terminal domain.

SNAREs are required for specific membrane fusion throughout the endomembrane system. Here we report the characterization of rat ykt6, a prenylated SNARE selectively expressed in brain neurons. Immunofluorescence microscopy in neuronal and neuroendocrine cell lines revealed that membrane-associated ykt6 did not colocalize significantly with any conventional markers of endosomes, lysosomes, or the secretory pathway. However, ykt6-containing membranes displayed very minor overlaps with lysosomes and dense-core secretory granules and were similar to lysosomes in buoyant density. Thus, ykt6 appears to be specialized for the trafficking of a unique membrane compartment, perhaps related to lysosomes, involved in aspects of neuronal function. Targeting of this SNARE to the ykt6 compartment was mediated by its profilin-like amino-terminal domain, even in the absence of protein prenylation. Although several other R-SNAREs contain related amino-terminal domains, only the ykt6 version was able to confer the specialized localization. Rat ykt6, which contains an arginine in its SNARE motif zero-layer, was found to behave like other R-SNAREs in its SNARE assembly properties. Interestingly, cytosolic ykt6, constituting more than half of the total cellular pool, appeared to be conformationally inactive for SNARE complex assembly, perhaps indicative of a regulatory mechanism that prevents promiscuous and potentially deleterious SNARE interactions.

Single amino acid substitution in LC-CDR1 induces Russell body phenotype that attenuates cellular protein synthesis through eIF2α phosphorylation and thereby downregulates IgG secretion despite operational secretory pathway traffic.

Amino acid sequence differences in the variable region of immunoglobulin (Ig) cause wide variations in secretion outputs. To address how a primary sequence difference comes to modulate Ig secretion, we investigated the biosynthetic process of 2 human IgG2κ monoclonal antibodies (mAbs) that differ only by one amino acid in the light chain complementarity-determining region 1 while showing ∼20-fold variance in secretion titer. Although poorly secreted, the lower-secreting mAb of the 2 was by no means defective in terms of its folding stability, antigen binding, and in vitro biologic activity. However, upon overexpression in HEK293 cells, the low-secreting mAb revealed a high propensity to aggregate into enlarged globular structures called Russell bodies (RBs) in the endoplasmic reticulum. While Golgi morphology was affected by the formation of RBs, secretory pathway membrane traffic remained operational in those cells. Importantly, cellular protein synthesis was severely suppressed in RB-positive cells through the phosphorylation of eIF2α. PERK-dependent signaling was implicated in this event, given the upregulation and nuclear accumulation of downstream effectors such as ATF4 and CHOP. These findings illustrated that the underlining process of poor Ig secretion in RB-positive cells was due to downregulation of Ig synthesis instead of a disruption or blockade of secretory pathway trafficking. Therefore, RB formation signifies an end of active Ig production at the protein translation level. Consequently, depending on how soon and how severely an antibody-expressing cell develops the RB phenotype, the productive window of Ig secretion can vary widely among the cells expressing different mAbs.

Intermolecular interactions involving an acidic patch on immunoglobulin variable domain and the γ2 constant region mediate crystalline inclusion body formation in the endoplasmic reticulum.

Full-length immunoglobulins (Igs) are widely considered difficult to crystallize because of their large size, N-linked glycosylation, and flexible hinge region. However, numerous cases of intracellular Ig crystallization are reported in plasma cell dyscrasias. What makes some Ig clones more prone to crystallize during biosynthesis as well as the biochemical and cell biological requirements for this cryptic event are poorly understood. To investigate the underlying process of intracellular Ig crystallization we searched for model IgGs that can induce crystalline inclusions during recombinant overexpression. By testing various subunit combinations through mixing and matching of individual subunit chains derived from a panel of human IgG clones, we identified one secretion competent IgG2λ that induced needle-like crystalline inclusions in transfected HEK293 cells. Ig crystallization rarely occurred at steady-state cell growth conditions but was easily induced when ER-to-Golgi transport was pharmacologically blocked. Homology modeling revealed the presence of a prominent negatively-charged patch on the variable domain surface. The patch was composed of eight aspartic acids, of which five were in the heavy chain variable region and three were in the light chain. Crystallization occurred only when the two subunits were co-transfected and the intracellular crystals co-localized with ER resident proteins. Furthermore, subtype switching from IgG2 to IgG1 and stepwise neutralization of the acidic patch independently abrogated Ig crystallization events. The evidence supported that the formation of needle-like crystalline inclusions in the ER was underscored by multivalent intermolecular interactions between the acidic patch and undefined determinants present on the γ2 subunit constant region.

Overexpression of cryoglobulin-like single-chain antibody induces morular cell phenotype via liquid-liquid phase separation in the secretory pathway organelles.

Cryoprecipitation of immunoglobulins is often reported in association with B-cell lymphoproliferative disorders and plasma cell dyscrasias. However, the biochemical basis of such cryoglobulin behaviors is not well understood because of a general lack of suitable experimental systems. Here, we report the identification and characterization of a single-chain antibody (scFv-Fc) that recapitulates cryoglobulin-like properties. When model scFv-Fc protein was engineered to multimerize, by appending the secretory tailpiece (stp) of human immunoglobulin µ-chain to the C terminus, the resulting oligomeric scFv-Fc-stp protein acquired two unexpected properties: the induction of a morular cell phenotype during protein biosynthesis and the cryoprecipitation of secreted proteins in harvested cell culture media. The turbidity of the culture media and the inclusion bodies that gave morular appearances were attributed to microscopic spherical protein droplet formation, a hallmark characteristic of liquid-liquid phase separation (LLPS) event. Mutagenesis approaches revealed that these two phenomena were independent of covalent protein oligomerization induced by stp. Disruption of the N-linked glycosylation motif in the stp region enhanced morular phenotype propensity but reduced protein secretion. Intermolecular disulfide bonds that stabilize Fc dimers and oligomers were necessary for efficient induction of LLPS, but their simultaneous elimination could not abrogate the LLPS propensity completely. Noncovalent protein-protein interactions between scFv-Fc-stp chains sufficiently established a basis for LLPS induction. Morular cell phenotypes and cryoprecipitation were clearly underpinned by intrinsic physicochemical properties embedded in the overexpressed cargo protein. Overproduction of condensation-prone secretory proteins that culminate in LLPS in the endoplasmic reticulum therefore serves as a path to produce morular Russell body phenotype.

Russell body phenotype is preferentially induced by IgG mAb clones with high intrinsic condensation propensity: relations between the biosynthetic events in the ER and solution behaviors in vitro.

The underlying reasons for why some mAb (monoclonal antibody) clones are much more inclined to induce a Russell body (RB) phenotype during immunoglobulin biosynthesis remain elusive. Although RBs are morphologically understood as enlarged globular aggregates of immunoglobulins deposited in the endoplasmic reticulum (ER), little is known about the properties of the RB-inducing mAb clones as secretory cargo and their physical behaviors in the extracellular space. To elucidate how RB-inducing propensities, secretion outputs, and the intrinsic physicochemical properties of individual mAb clones are interrelated, we used HEK293 cells to study the biosynthesis of 5 human IgG mAbs for which prominent solution behavior problems were known a priori. All 5 model mAbs with inherently high condensation propensities induced RB phenotypes both at steady state and under ER-to-Golgi transport block, and resulted in low secretion titer. By contrast, one reference mAb that readily crystallized at neutral pH in vitro produced rod-shaped crystalline bodies in the ER without inducing RBs. Another reference mAb without notable solution behavior issues did not induce RBs and was secreted abundantly. Intrinsic physicochemical properties of individual IgG clones thus directly affected the biosynthetic steps in the ER, and thereby produced distinctive cellular phenotypes and influenced IgG secretion output. The findings implicated that RB formation represents a phase separation event or a loss of colloidal stability in the secretory pathway organelles. The process of RB induction allows the cell to preemptively reduce the extracellular concentration of potentially pathogenic, highly aggregation-prone IgG clones by selectively storing them in the ER.