SEC23B is required for the maintenance of murine professional secretory tissues

Functional overlap between the mammalian Sar1a and Sar1b paralogs in vivo

Proteins carrying a signal peptide and/or a transmembrane domain enter the intracellular secretory pathway at the endoplasmic reticulum (ER) and are transported to the Golgi apparatus via COPII vesicles or tubules. SAR1 initiates COPII coat assembly by recruiting other coat proteins to the ER membrane. Mammalian genomes encode two SAR1 paralogs, SAR1A and SAR1B. While these paralogs exhibit ~90% amino acid sequence identity, it is unknown whether they perform distinct or overlapping functions in vivo. We now report that genetic inactivation of Sar1a in mice results in lethality during midembryogenesis. We also confirm previous reports that complete deficiency of murine Sar1b results in perinatal lethality. In contrast, we demonstrate that deletion of Sar1b restricted to hepatocytes is compatible with survival, though resulting in hypocholesterolemia that can be rescued by adenovirus-mediated overexpression of either SAR1A or SAR1B. To further examine the in vivo function of these two paralogs, we genetically engineered mice with the Sar1a coding sequence replacing that of Sar1b at the endogenous Sar1b locus. Mice homozygous for this allele survive to adulthood and are phenotypically normal, demonstrating complete or near-complete overlap in function between the two SAR1 protein paralogs in mice. These data also suggest upregulation of SAR1A gene expression as a potential approach for the treatment of SAR1B deficiency (chylomicron retention disease) in humans.

Functional overlap between the mammalian Sar1a and Sar1b paralogs in vivo

Proteins carrying a signal peptide and/or a transmembrane domain enter the intracellular secretory pathway at the endoplasmic reticulum (ER) and are transported to the Golgi apparatus via COPII vesicles or tubules. SAR1 initiates COPII coat assembly by recruiting other coat proteins to the ER membrane. Mammalian genomes encode two SAR1 paralogs, SAR1A and SAR1B. While these paralogs exhibit ~90% amino acid sequence identity, it is unknown whether they perform distinct or overlapping functions in vivo. We now report that genetic inactivation of Sar1a in mice results in lethality during mid-embryogenesis. We also confirm previous reports that complete deficiency of murine Sar1b results in perinatal lethality. In contrast, we demonstrate that deletion of Sar1b restricted to hepatocytes is compatible with survival, though resulting in hypocholesterolemia that can be rescued by adenovirus-mediated overexpression of either SAR1A or SAR1B. To further examine the in vivo function of these 2 paralogs, we genetically engineered mice with the Sar1a coding sequence replacing that of Sar1b at the endogenous Sar1b locus. Mice homozygous for this allele survive to adulthood and are phenotypically normal, demonstrating complete or near-complete overlap in function between the two SAR1 protein paralogs in mice. These data also suggest upregulation of SAR1A gene expression as a potential approach for the treatment of SAR1B deficiency (chylomicron retention disease) in humans.

The Deubiquitinase OTUD1 Suppresses Secretory Neutrophil Polarization And Ameliorates Immunopathology of Periodontitis

Tissue-infiltrating neutrophils (TINs) secrete various signaling molecules to establish paracrine communication within the inflammatory milieu. It is imperative to identify molecular mediators that control this secretory phenotype of TINs. The present study uncovers a secretory neutrophil subset that exhibits increased pro-inflammatory cytokine production and enhanced migratory capacity which is highly related with periodontal pathogenesis. Further analysis identifies the OTU domain-containing protein 1 (OTUD1) plays a regulatory role in this secretory neutrophil polarization. In human and mouse periodontitis, the waning of inflammation is correlated with OTUD1 upregulation, whereas severe periodontitis is induced when neutrophil-intrinsic OTUD1 is depleted. Mechanistically, OTUD1 interacts with SEC23B, a component of the coat protein II complex (COPII). By removing the K63-linked polyubiquitin chains on SEC23B Lysine 81, the deubiquitinase OTUD1 negatively regulates the COPII secretory machinery and limits protein ER-to-Golgi trafficking, thus restricting the surface expression of integrin-regulated proteins, CD9 and CD47. Accordingly, blockade of protein transport by Brefeldin A (BFA) curbs recruitment of Otud1-deficient TINs and attenuates inflammation-induced alveolar bone destruction. The results thus identify OTUD1 signaling as a negative feedback loop that limits the polarization of neutrophils with secretory phenotype and highlight the potential application of BFA in the treatment of periodontal inflammation.

The Sar1 GTPase is dispensable for COPII-dependent cargo export from the ER

Coat protein complex II (COPII) plays an integral role in the packaging of secretory cargoes within membrane-enclosed transport carriers that leave the endoplasmic reticulum (ER) from discrete subdomains. Lipid bilayer remodeling necessary for this process is driven initially by membrane penetration mediated by the Sar1 GTPase and further stabilized by assembly of a multilayered complex of several COPII proteins. However, the relative contributions of these distinct factors to transport carrier formation and protein trafficking remain unclear. Here, we demonstrate that anterograde cargo transport from the ER continues in the absence of Sar1, although the efficiency of this process is dramatically reduced. Specifically, secretory cargoes are retained nearly five times longer at ER subdomains when Sar1 is depleted, but they ultimately remain capable of being translocated to the perinuclear region of cells. Taken together, our findings highlight alternative mechanisms by which COPII promotes transport carrier biogenesis.

Mammalian cargo receptors for endoplasmic reticulum-to-Golgi transport: mechanisms and interactions

Proteins that are destined to enter the secretory pathway are synthesized on the rough endoplasmic reticulum (ER) and then translocated into the ER lumen, where they undergo posttranslational modifications, folding, and assembly. After passing a quality control system, the cargo proteins are packaged into coat protein complex II (COPII) vesicles to exit the ER. In metazoans, most COPII subunits have multiple paralogs, enabling COPII vesicles the flexibility to transport a diverse range of cargo. The cytoplasmic domains of transmembrane proteins can interact with SEC24 subunits of COPII to enter the ER exit sites. Some transmembrane proteins may also act as cargo receptors that bind soluble secretory proteins within the ER lumen, enabling them to enter COPII vesicles. The cytoplasmic domains of cargo receptors also contain coat protein complex I binding motifs that allow for their cycling back to the ER after unloading their cargo in the ER-Golgi intermediate compartment and cis-Golgi. Once unloaded, the soluble cargo proteins continue maturation through the Golgi before reaching their final destinations. This review provides an overview of receptor-mediated transport of secretory proteins from the ER to the Golgi, with a focus on the current understanding of two mammalian cargo receptors: the LMAN1-MCFD2 complex and SURF4, and their roles in human health and disease.

New Cases and Mutations in SEC23B Gene Causing Congenital Dyserythropoietic Anemia Type II

Congenital dyserythropoietic anemia type II (CDA II) is an inherited autosomal recessive blood disorder which belongs to the wide group of ineffective erythropoiesis conditions. It is characterized by mild to severe normocytic anemia, jaundice, and splenomegaly owing to the hemolytic component. This often leads to liver iron overload and gallstones. CDA II is caused by biallelic mutations in the SEC23B gene. In this study, we report 9 new CDA II cases and identify 16 pathogenic variants, 6 of which are novel. The newly reported variants in SEC23B include three missenses (p.Thr445Arg, p.Tyr579Cys, and p.Arg701His), one frameshift (p.Asp693GlyfsTer2), and two splicing variants (c.1512-2A>G, and the complex intronic variant c.1512-3delinsTT linked to c.1512-16_1512-7delACTCTGGAAT in the same allele). Computational analyses of the missense variants indicated a loss of key residue interactions within the beta sheet and the helical and gelsolin domains, respectively. Analysis of SEC23B protein levels done in patient-derived lymphoblastoid cell lines (LCLs) showed a significant decrease in SEC23B protein expression, in the absence of SEC23A compensation. Reduced SEC23B mRNA expression was only detected in two probands carrying nonsense and frameshift variants; the remaining patients showed either higher gene expression levels or no expression changes at all. The skipping of exons 13 and 14 in the newly reported complex variant c.1512-3delinsTT/c.1512-16_1512-7delACTCTGGAAT results in a shorter protein isoform, as assessed by RT-PCR followed by Sanger sequencing. In this work, we summarize a comprehensive spectrum of SEC23B variants, describe nine new CDA II cases accounting for six previously unreported variants, and discuss innovative therapeutic approaches for CDA II.

Genetic marker identification of SEC13 gene for milk production traits in Chinese holstein

SEC13 homolog, nuclear pore and COPII coat complex component (SEC13) is the core component of the cytoplasmic COPII complex, which mediates material transport from the endoplasmic reticulum to the Golgi complex. Our preliminary work found that SEC13 gene was differentially expressed in dairy cows during different stages of lactation, and involved in metabolic pathways of milk synthesis such as citric acid cycle, fatty acid, starch and sucrose metabolisms, so we considered that the SEC13 might be a candidate gene affecting milk production traits. In this study, we detected the polymorphisms of SEC13 gene and verified their genetic effects on milk yield and composition traits in a Chinese Holstein cow population. By sequencing the whole coding and partial flanking regions of SEC13, we found four single nucleotide polymorphisms (SNPs). Subsequent association analysis showed that these four SNPs were significantly associated with milk yield, fat yield, protein yield or protein percentage in the first and second lactations (p ≤.0351). We also found that two SNPs in SEC13 formed one haplotype block by Haploview4.2, and the block was significantly associated with milk yield, fat yield, fat percentage, protein yield or protein percentage (p ≤ .0373). In addition, we predicted the effect of SNP on 5'region on transcription factor binding sites (TFBSs), and found that the allele A of 22:g.54362761A>G could bind transcription factors (TFs) GATA5, GATA3, HOXD9, HOXA10, CDX1 and Hoxd13; and further dual-luciferase reporter assay verified that the allele A of this SNP inhibited the fluorescence activity. We speculate that the A allele of 22:g.54362761A>G might inhibit the transcriptional activity of SEC13 gene by binding the TFs, which may be a cause mutation affecting the formation of milk production traits in dairy cows. In summary, we proved that SEC13 has a significant genetic effect on milk production traits and the identified significant SNPs could be used as candidate genetic markers for GS SNP chips development; on the other hand, we verified the transcriptional regulation of 22:g.54362761A>G on SEC13 gene, providing research direction for further function validation tests.

Cargo selection in endoplasmic reticulum-to-Golgi transport and relevant diseases

Most proteins destined for the extracellular space or various intracellular compartments must traverse the intracellular secretory pathway. The first step is the recruitment and transport of cargoes from the endoplasmic reticulum (ER) lumen to the Golgi apparatus by coat protein complex II (COPII), consisting of five core proteins. Additional ER transmembrane proteins that aid cargo recruitment are referred to as cargo receptors. Gene duplication events have resulted in multiple COPII paralogs present in the mammalian genome. Here, we review the functions of each COPII protein, human disorders associated with each paralog, and evidence for functional conservation between paralogs. We also provide a summary of current knowledge regarding two prototypical cargo receptors in mammals, LMAN1 and SURF4, and their roles in human health and disease.

Functional screening of congenital heart disease risk loci identifies 5 genes essential for heart development in zebrafish

Congenital heart disease (CHD) is the most common birth defect worldwide and a main cause of perinatal and infant mortality. Our previous genome-wide association study identified 53 SNPs that associated with CHD in the Han Chinese population. Here, we performed functional screening of 27 orthologous genes in zebrafish using injection of antisense morpholino oligos. From this screen, 5 genes were identified as essential for heart development, including iqgap2, ptprt, ptpn22, tbck and maml3. Presumptive roles of the novel CHD-related genes include heart chamber formation (iqgap2 and ptprt) and atrioventricular canal formation (ptpn22 and tbck). While deficiency of maml3 led to defective cardiac trabeculation and consequent heart failure in zebrafish embryos. Furthermore, we found that maml3 mutants showed decreased cardiomyocyte proliferation which caused a reduction in cardiac trabeculae due to inhibition of Notch signaling. Together, our study identifies 5 novel CHD-related genes that are essential for heart development in zebrafish and first demonstrates that maml3 is required for Notch signaling in vivo.

The congenital dyserythropoieitic anemias: genetics and pathophysiology

PURPOSE OF REVIEW

The congenital dyserythropoietic anemias (CDA) are hereditary disorders characterized by ineffective erythropoiesis. This review evaluates newly developed CDA disease models, the latest advances in understanding the pathogenesis of the CDAs, and recently identified CDA genes.

RECENT FINDINGS

Mice exhibiting features of CDAI were recently generated, demonstrating that Codanin-1 (encoded by Cdan1) is essential for primitive erythropoiesis. Additionally, Codanin-1 was found to physically interact with CDIN1, suggesting that mutations in CDAN1 and CDIN1 result in CDAI via a common mechanism. Recent advances in CDAII (which results from SEC23B mutations) have also been made. SEC23B was found to functionally overlap with its paralogous protein, SEC23A, likely explaining the absence of CDAII in SEC23B-deficient mice. In contrast, mice with erythroid-specific deletion of 3 or 4 of the Sec23 alleles exhibited features of CDAII. Increased SEC23A expression rescued the CDAII erythroid defect, suggesting a novel therapeutic strategy for the disease. Additional recent advances included the identification of new CDA genes, RACGAP1 and VPS4A, in CDAIII and a syndromic CDA type, respectively.

SUMMARY

Establishing cellular and animal models of CDA is expected to result in improved understanding of the pathogenesis of these disorders, which may ultimately lead to the development of new therapies.

SEC23A rescues SEC23B-deficient congenital dyserythropoietic anemia type II

Congenital dyserythropoietic anemia type II (CDAII) results from loss-of-function mutations in SEC23B. In contrast to humans, SEC23B-deficient mice deletion do not exhibit CDAII but die perinatally with pancreatic degeneration. Here, we demonstrate that expression of the full SEC23A protein (the SEC23B paralog) from the endogenous regulatory elements of Sec23b completely rescues the SEC23B-deficient mouse phenotype. Consistent with these data, while mice with erythroid-specific deletion of either Sec23a or Sec23b do not exhibit CDAII, we now show that mice with erythroid-specific deletion of all four Sec23 alleles die in mid-embryogenesis with features of CDAII and that mice with deletion of three Sec23 alleles exhibit a milder erythroid defect. To test whether the functional overlap between the SEC23 paralogs is conserved in human erythroid cells, we generated SEC23B-deficient HUDEP-2 cells. Upon differentiation, these cells exhibited features of CDAII, which were rescued by increased expression of SEC23A, suggesting a novel therapeutic strategy for CDAII.

Murine SEC24D can substitute functionally for SEC24C during embryonic development

The COPII component SEC24 mediates the recruitment of transmembrane cargos or cargo adaptors into newly forming COPII vesicles on the ER membrane. Mammalian genomes encode four Sec24 paralogs (Sec24a-d), with two subfamilies based on sequence homology (SEC24A/B and C/D), though little is known about their comparative functions and cargo-specificities. Complete deficiency for Sec24d results in very early embryonic lethality in mice (before the 8 cell stage), with later embryonic lethality (E7.5) observed in Sec24c null mice. To test the potential overlap in function between SEC24C/D, we employed dual recombinase mediated cassette exchange to generate a Sec24cc-d allele, in which the C-terminal 90% of SEC24C has been replaced by SEC24D coding sequence. In contrast to the embryonic lethality at E7.5 of SEC24C-deficiency, Sec24cc-d/c-d pups survive to term, though dying shortly after birth. Sec24cc-d/c-d pups are smaller in size, but exhibit no other obvious developmental abnormality by pathologic evaluation. These results suggest that tissue-specific and/or stage-specific expression of the Sec24c/d genes rather than differences in cargo export function explain the early embryonic requirements for SEC24C and SEC24D.

Sec22b determines Weibel-Palade body length by controlling anterograde ER-Golgi transport

Von Willebrand factor (VWF) is a multimeric hemostatic protein that is synthesized in endothelial cells, where it is stored for secretion in elongated secretory organelles called Weibel-Palade bodies (WPB). The hemostatic activity of VWF is strongly related to the length of these bodies, but how endothelial cells control the dimensions of their WPB is unclear. In this study, using a targeted short hairpin RNA screen, we identified longin-SNARE Sec22b as a novel determinant of WPB size and VWF trafficking. We found that Sec22b depletion resulted in loss of the typically elongated WPB morphology together with disintegration of the Golgi and dilation of rough endoplasmic reticulum cisternae. This was accompanied by reduced proteolytic processing of VWF, accumulation of VWF in the dilated rough endoplasmic reticulum and reduced basal and stimulated VWF secretion. Our data demonstrate that the elongation of WPB, and thus adhesive activity of their cargo VWF, is determined by the rate of anterograde transport between endoplasmic reticulum and Golgi, which depends on Sec22b-containing SNARE complexes.

Defects in early secretory pathway transport machinery components and neurodevelopmental disorders

The early secretory pathway, provisionally comprising of vesicular traffic between the endoplasmic reticulum (ER) and the Golgi apparatus, occurs constitutively in mammalian cells. Critical for a constant supply of secretory and plasma membrane (PM) materials, the pathway is presumably essential for general cellular function and survival. Neurons exhibit a high intensity in membrane dynamics and protein/lipid trafficking, with differential and polarized trafficking towards the somatodendritic and axonal PM domains. Mutations in genes encoding early secretory pathway membrane trafficking machinery components are known to result in neurodevelopmental or neurological disorders with disease manifestation in early life. Here, such rare disorders associated with autosomal recessive mutations in coat proteins, membrane tethering complexes and membrane fusion machineries responsible for trafficking in the early secretory pathway are summarily discussed. These mutations affected genes encoding subunits of coat protein complex I and II, subunits of transport protein particle (TRAPP) complexes, members of the YIP1 domain family (YIPF) and a SNAP receptor (SNARE) family member. Why the ubiquitously present and constitutively acting early secretory pathway machinery components could specifically affect neurodevelopment is addressed, with the plausible underlying disease etiologies and neuropathological mechanisms resulting from these mutations explored.

Congenital dyserythropoietic anemias

Congenital dyserythropoietic anemias (CDAs) are a heterogeneous group of inherited anemias that affect the normal differentiation-proliferation pathways of the erythroid lineage. They belong to the wide group of ineffective erythropoiesis conditions that mainly result in monolinear cytopenia. CDAs are classified into the 3 major types (I, II, III), plus the transcription factor-related CDAs, and the CDA variants, on the basis of the distinctive morphological, clinical, and genetic features. Next-generation sequencing has revolutionized the field of diagnosis of and research into CDAs, with reduced time to diagnosis, and ameliorated differential diagnosis in terms of identification of new causative/modifier genes and polygenic conditions. The main improvements regarding CDAs have been in the study of iron metabolism in CDAII. The erythroblast-derived hormone erythroferrone specifically inhibits hepcidin production, and its role in the mediation of hepatic iron overload has been dissected out. We discuss here the most recent advances in this field regarding the molecular genetics and pathogenic mechanisms of CDAs, through an analysis of the clinical and molecular classifications, and the complications and clinical management of patients. We summarize also the main cellular and animal models developed to date and the possible future therapies.

Non-canonical role of wild-type SEC23B in the cellular stress response pathway

While germline recessive loss-of-function mutations in SEC23B in humans cause a rare form of anaemia, heterozygous change-of-function mutations result in increased predisposition to cancer. SEC23B encodes SEC23 homologue B, a component of coat protein complex II (COPII), which canonically transports proteins from the endoplasmic reticulum (ER) to the Golgi. Despite the association of SEC23B with anaemia and cancer, the precise pathophysiology of these phenotypic outcomes remains unknown. Recently, we reported that mutant SEC23B has non-canonical COPII-independent function, particularly within the ER stress and ribosome biogenesis pathways, and that may contribute to the pathobiology of cancer predisposition. In this study, we hypothesized that wild-type SEC23B has a baseline function within such cellular stress response pathways, with the mutant protein reflecting exaggerated effects. Here, we show that the wild-type SEC23B protein localizes to the nucleus in addition to classical distribution at the ER/Golgi interface and identify multiple putative nuclear localization and export signals regulating nuclear-cytoplasmic transport. Unexpectedly, we show that, independently of COPII, wild-type SEC23B can also localize to cell nucleoli under proteasome inhibition conditions, with distinct distribution patterns compared to mutant cells. Unbiased proteomic analyses through mass spectrometry further revealed that wild-type SEC23B interacts with a subset of nuclear proteins, in addition to central proteins in the ER stress, protein ubiquitination, and EIF2 signalling pathways. We validate the genotype-specific differential SEC23B-UBA52 (ribosomal protein RPL40) interaction. Finally, utilizing patient-derived lymphoblastoid cell lines harbouring either wild-type or mutant SEC23B, we show that SEC23B levels increase in response to ER stress, further corroborating its role as a cellular stress response sensor and/or effector. Overall, these observations suggest that SEC23B, irrespective of mutation status, has unexplored roles in the cellular stress response pathway, with implications relevant to cancer and beyond that, CDAII and normal cell biology.

ER-to-Golgi transport and SEC23-dependent COPII vesicles regulate T cell alloimmunity

T cell-mediated responses are dependent on their secretion of key effector molecules. However, the critical molecular determinants of the secretion of these proteins are largely undefined. Here, we demonstrate that T cell activation increases trafficking via the ER-to-Golgi pathway. To study the functional role of this pathway, we generated mice with a T cell-specific deletion in SEC23B, a core subunit of coat protein complex II (COPII). We found that SEC23B critically regulated the T cell secretome following activation. SEC23B-deficient T cells exhibited a proliferative defect and reduced effector functions in vitro, as well as in experimental models of allogeneic and xenogeneic hematopoietic cell transplantation in vivo. However, T cells derived from 3 patients with congenital dyserythropoietic anemia II (CDAII), which results from Sec23b mutation, did not exhibit a similar phenotype. Mechanistic studies demonstrated that unlike murine KO T cells, T cells from patients with CDAII harbor increased levels of the closely related paralog, SEC23A. In vivo rescue of murine KO by expression of Sec23a from the Sec23b genomic locus restored T cell functions. Together, our data demonstrate a critical role for the COPII pathway, with evidence for functional overlap in vivo between SEC23 paralogs in the regulation of T cell immunity in both mice and humans.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

Mutations in the coat complex II component SEC23B promote colorectal cancer metastasis

Metastasis is the leading cause of death for colorectal cancer (CRC). However, the protein transport process involved in CRC metastasis remains unclear. In this report, we use whole-exome sequencing and bioinformatics analysis to identify somatic mutations in CRC samples and found mutations of the protein transport gene Sec23 homolog B (SEC23B) in patients with metachronous liver metastasis. We show that deletion of SEC23B suppresses the membrane localization of adhesion proteins and augments cell mobility. SEC23B mutations either cause a premature stop (C649T) or impair its protein transport activity (C1467G and T488C + G791A + G2153A). Furthermore, SEC23B mutations inhibit the transport of epithelial cell adhesion molecule (EPCAM) and CD9 molecule, thereby attenuating cell adhesion and promoting invasiveness both in vitro and in vivo. Taken together, these data demonstrate the important impact of SEC23B mutations on metastasis, and we propose that SEC23B is a potential suppressor of CRC metastasis.

Consequences of mutations in the genes of the ER export machinery COPII in vertebrates

Coat protein complex II (COPII) plays an essential role in the export of cargo molecules such as secretory proteins, membrane proteins, and lipids from the endoplasmic reticulum (ER). In yeast, the COPII machinery is critical for cell viability as most COPII knockout mutants fail to survive. In mice and fish, homozygous knockout mutants of most COPII genes are embryonic lethal, reflecting the essentiality of the COPII machinery in the early stages of vertebrate development. In humans, COPII mutations, which are often hypomorphic, cause diseases having distinct clinical features. This is interesting as the fundamental cellular defect of these diseases, that is, failure of ER export, is similar. Analyses of humans and animals carrying COPII mutations have revealed clues to why a similar ER export defect can cause such different diseases. Previous reviews have focused mainly on the deficit of secretory or membrane proteins in the final destinations because of an ER export block. In this review, we also underscore the other consequence of the ER export block, namely ER stress triggered by the accumulation of cargo proteins in the ER.

Murine Surf4 is essential for early embryonic development

Newly synthesized proteins co-translationally inserted into the endoplasmic reticulum (ER) lumen may be recruited into anterograde transport vesicles by their association with specific cargo receptors. We recently identified a role for the cargo receptor SURF4 in facilitating the secretion of PCSK9 in cultured cells. To examine the function of SURF4 in vivo, we used CRISPR/Cas9-mediated gene editing to generate mice with germline loss-of-function mutations in Surf4. Heterozygous Surf4^+/- mice exhibit grossly normal appearance, behavior, body weight, fecundity, and organ development, with no significant alterations in circulating plasma levels of PCSK9, apolipoprotein B, or total cholesterol, and a detectable accumulation of intrahepatic apoliprotein B. Homozygous Surf4^-/- mice exhibit embryonic lethality, with complete loss of all Surf4^-/- offspring between embryonic days 3.5 and 9.5. In contrast to the milder murine phenotypes associated with deficiency of known SURF4 cargoes, the embryonic lethality of Surf4^-/- mice implies the existence of additional SURF4 cargoes or functions that are essential for murine early embryonic development.

Regulation of glucose and lipid metabolism in health and disease

Glucose and fatty acids are the major sources of energy for human body. Cholesterol, the most abundant sterol in mammals, is a key component of cell membranes although it does not generate ATP. The metabolisms of glucose, fatty acids and cholesterol are often intertwined and regulated. For example, glucose can be converted to fatty acids and cholesterol through de novo lipid biosynthesis pathways. Excessive lipids are secreted in lipoproteins or stored in lipid droplets. The metabolites of glucose and lipids are dynamically transported intercellularly and intracellularly, and then converted to other molecules in specific compartments. The disorders of glucose and lipid metabolism result in severe diseases including cardiovascular disease, diabetes and fatty liver. This review summarizes the major metabolic aspects of glucose and lipid, and their regulations in the context of physiology and diseases.

The Functional Role of SEC23 in Vesicle Transportation, Autophagy and Cancer

SEC23, the core component of the coat protein complex II (COPII), functions to transport newly synthesized proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus in cells for secretion. SEC23 protein has two isoforms (SEC23A and SEC23B) and their aberrant expression and mutations were reported to cause human diseases and oncogenesis, whereas SEC23A and SEC23B may have the opposite activity in human cancer, for a reason that remains unclear. This review summarizes recent research in SEC23, COPII-vesicle transportation, autophagy, and cancer.

How to Avoid a No-Deal ER Exit

Efficiency and fidelity of protein secretion are achieved thanks to the presence of different steps, located sequentially in time and space along the secretory compartment, controlling protein folding and maturation. After entering into the endoplasmic reticulum (ER), secretory proteins attain their native structure thanks to specific chaperones and enzymes. Only correctly folded molecules are allowed by quality control (QC) mechanisms to leave the ER and proceed to downstream compartments. Proteins that cannot fold properly are instead retained in the ER to be finally destined to proteasomal degradation. Exiting from the ER requires, in most cases, the use of coated vesicles, departing at the ER exit sites, which will fuse with the Golgi compartment, thus releasing their cargoes. Protein accumulation in the ER can be caused by a too stringent QC or by ineffective transport: these situations could be deleterious for the organism, due to the loss of the secreted protein, and to the cell itself, because of abnormal increase of protein concentration in the ER. In both cases, diseases can arise. In this review, we will describe the pathophysiology of protein folding and transport between the ER and the Golgi compartment.

Coupling of COPII vesicle trafficking to nutrient availability by the IRE1α-XBP1s axis

The cytoplasmic coat protein complex-II (COPII) is evolutionarily conserved machinery that is essential for efficient trafficking of protein and lipid cargos. How the COPII machinery is regulated to meet the metabolic demand in response to alterations of the nutritional state remains largely unexplored, however. Here, we show that dynamic changes of COPII vesicle trafficking parallel the activation of transcription factor X-box binding protein 1 (XBP1s), a critical transcription factor in handling cellular endoplasmic reticulum (ER) stress in both live cells and mouse livers upon physiological fluctuations of nutrient availability. Using live-cell imaging approaches, we demonstrate that XBP1s is sufficient to promote COPII-dependent trafficking, mediating the nutrient stimulatory effects. Chromatin immunoprecipitation (ChIP) coupled with high-throughput DNA sequencing (ChIP-seq) and RNA-sequencing analyses reveal that nutritional signals induce dynamic XBP1s occupancy of promoters of COPII traffic-related genes, thereby driving the COPII-mediated trafficking process. Liver-specific disruption of the inositol-requiring enzyme 1α (IRE1α)-XBP1s signaling branch results in diminished COPII vesicle trafficking. Reactivation of XBP1s in mice lacking hepatic IRE1α restores COPII-mediated lipoprotein secretion and reverses the fatty liver and hypolipidemia phenotypes. Thus, our results demonstrate a previously unappreciated mechanism in the metabolic control of liver protein and lipid trafficking: The IRE1α-XBP1s axis functions as a nutrient-sensing regulatory nexus that integrates nutritional states and the COPII vesicle trafficking.

Non-canonical role of cancer-associated mutant SEC23B in the ribosome biogenesis pathway

SEC23B is a component of coat protein complex II (COPII) vesicles that transport secretory proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. Loss-of-function SEC23B mutations cause a rare form of anemia, resulting from decreased SEC23B levels. We recently identified germline heterozygous SEC23B variants as potentially cancer-predisposing. Mutant SEC23B associated with ER stress-mediated tumorigenesis, without decreased SEC23B expression. However, our understanding of the processes behind these observations remain limited. Here, we show mutant SEC23B exists within nucleoli, in addition to classical distribution at the ER/Golgi. This occurs independent of other COPII proteins and does not compromise secretory function. Mutant cells have increased ribosomal protein and translation-related gene expression, and enhanced translational capacity, in the presence of ER stress. We show that mutant SEC23B binds to UBF transcription factor, with increased UBF transcription factor binding at the ribosomal DNA promoter. Our data indicate SEC23B has potential non-canonical COPII-independent function, particularly within the ribosome biogenesis pathway, and that may contribute to the pathogenesis of cancer-predisposition.

Analysis of MCFD2- and LMAN1-deficient mice demonstrates distinct functions in vivo

The LMAN1-MCFD2 complex serves as a cargo receptor for efficient transport of factor V (FV) and FVIII from the endoplasmic reticulum (ER) to the Golgi. Genetic deficiency of LMAN1 or MCFD2 in humans results in the moderate bleeding disorder combined FV and FVIII deficiency, with a similar phenotype previously observed in LMAN1-deficient mice. We now report that MCFD2-deficient mice generated by gene targeting also demonstrate reduced plasma FV and FVIII, with levels lower than those in LMAN1-deficient mice, similar to previous observations in LMAN1- and MCDF2-deficient humans. Surprisingly, FV and FVIII levels in doubly deficient mice match the higher levels observed in LMAN1-deficient mice. In contrast to the strain-specific partial lethality previously observed in LMAN1-null mice, MCFD2-null mice demonstrate normal survival in different genetic backgrounds, although doubly deficient mice exhibit partial embryonic lethality comparable to LMAN1-deficient mice. These results suggest that an alternative pathway is responsible for FV/FVIII secretion in doubly deficient mice and distinct cargo-specific functions for LMAN1 and MCFD2 within the ER-to-Golgi secretory pathway. We also observed decreased plasma levels of α1-antitrypsin (AAT) in male mice for all 3 groups of deficient mice. Comparable accumulation of AAT was observed in hepatocyte ER of singly and doubly deficient mice, demonstrating a role for LMAN1 and MCFD2 in efficient ER exit of AAT.

Molecular Regulatory Mechanism of Exocytosis in the Salivary Glands

Every day, salivary glands produce about 0.5 to 1.5 L of saliva, which contains salivary proteins that are essential for oral health. The contents of saliva, 0.3% proteins (1.5 to 4.5 g) in fluid, help prevent oral infections, provide lubrication, aid digestion, and maintain oral health. Acinar cells in the lobular salivary glands secrete prepackaged secretory granules that contain salivary components such as amylase, mucins, and immunoglobulins. Despite the important physiological functions of salivary proteins, we know very little about the regulatory mechanisms of their secretion via exocytosis, which is a process essential for the secretion of functional proteins, not only in salivary glands, but also in other secretory organs, including lacrimal and mammary glands, the pancreas, and prostate. In this review, we discuss recent findings that elucidate exocytosis by exocrine glands, especially focusing on the salivary glands, in physiological and pathological conditions.

Functions of the COPII gene paralogs SEC23A and SEC23B are interchangeable in vivo

Approximately one-third of the mammalian proteome is transported from the endoplasmic reticulum-to-Golgi via COPII-coated vesicles. SEC23, a core component of coat protein-complex II (COPII), is encoded by two paralogous genes in vertebrates (Sec23a and Sec23b). In humans, SEC23B deficiency results in congenital dyserythropoietic anemia type-II (CDAII), while SEC23A deficiency results in a skeletal phenotype (with normal red blood cells). These distinct clinical disorders, together with previous biochemical studies, suggest unique functions for SEC23A and SEC23B. Here we show indistinguishable intracellular protein interactomes for human SEC23A and SEC23B, complementation of yeast Sec23 by both human and murine SEC23A/B, and rescue of the lethality of sec23b deficiency in zebrafish by a sec23a-expressing transgene. We next demonstrate that a Sec23a coding sequence inserted into the murine Sec23b locus completely rescues the lethal SEC23B-deficient pancreatic phenotype. We show that SEC23B is the predominantly expressed paralog in human bone marrow, but not in the mouse, with the reciprocal pattern observed in the pancreas. Taken together, these data demonstrate an equivalent function for SEC23A/B, with evolutionary shifts in the transcription program likely accounting for the distinct phenotypes of SEC23A/B deficiency within and across species, a paradigm potentially applicable to other sets of paralogous genes. These findings also suggest that enhanced erythroid expression of the normal SEC23A gene could offer an effective therapeutic approach for CDAII patients.

Cluster-assembled zirconia substrates promote long-term differentiation and functioning of human islets of Langerhans

Ex vivo expansion and differentiation of human pancreatic β-cell are enabling steps of paramount importance for accelerating the development of therapies for diabetes. The success of regenerative strategies depends on their ability to reproduce the chemical and biophysical properties of the microenvironment in which β-cells develop, proliferate and function. In this paper we focus on the biophysical properties of the extracellular environment and exploit the cluster-assembled zirconia substrates with tailored roughness to mimic the nanotopography of the extracellular matrix. We demonstrate that β-cells can perceive nanoscale features of the substrate and can convert these stimuli into mechanotransductive processes which promote long-term in vitro human islet culture, thus preserving β-cell differentiation and function. Proteomic and quantitative immunofluorescence analyses demonstrate that the process is driven by nanoscale topography, via remodelling of the actin cytoskeleton and nuclear architecture. These modifications activate a transcriptional program which stimulates an adaptive metabolic glucose response. Engineered cluster-assembled substrates coupled with proteomic approaches may provide a useful strategy for identifying novel molecular targets for treating diabetes mellitus and for enhancing tissue engineering in order to improve the efficacy of islet cell transplantation therapies.

The COPII cargo adapter SEC24C is essential for neuronal homeostasis

SEC24 family members are components of the coat protein complex II (COPII) machinery that interact directly with cargo or with other adapters to ensure proper sorting of secretory cargo into COPII vesicles. SEC24C is 1 of 4 mammalian SEC24 paralogs (SEC24A-D), which segregate into 2 subfamilies on the basis of sequence homology (SEC24A/SEC24B and SEC24C/SEC24D). Here, we demonstrate that postmitotic neurons, unlike professional secretory cells in other tissues, are exquisitely sensitive to loss of SEC24C. Conditional KO of Sec24c in neural progenitors during embryogenesis caused perinatal mortality and microcephaly, with activation of the unfolded protein response and apoptotic cell death of postmitotic neurons in the murine cerebral cortex. The cell-autonomous function of SEC24C in postmitotic neurons was further highlighted by the loss of cell viability caused by disrupting Sec24c expression in forebrain neurons of mice postnatally and in differentiated neurons derived from human induced pluripotent stem cells. The neuronal cell death associated with Sec24c deficiency was rescued in knockin mice expressing Sec24d in place of Sec24c. These data suggest that SEC24C is a major cargo adapter for COPII-dependent transport in postmitotic neurons in developing and adult brains and that its functions overlap at least partially with those of SEC24D in mammals.

GRP94 Is an Essential Regulator of Pancreatic β-Cell Development, Mass, and Function in Male Mice

Deficiencies in pancreatic β-cell mass contribute to both type 1 and type 2 diabetes. We investigated the role of the glucose-regulated protein (GRP) 94, an endoplasmic reticulum protein abundantly expressed in the pancreatic acini and islets, in β-cell development, survival, and function. We used a conditional knockout (KO) mouse in which the GRP94 gene, Hsp90b1, was specifically deleted in pancreatic and duodenal homeobox 1 (Pdx1)-expressing cells. These Hsp90b1 flox/flox;Pdx1Cre KO mice exhibited pancreatic hypoplasia at embryonic day (E) 16.5 to E18.5 and had significantly reduced β-cell mass at 4 weeks after birth. Further mechanistic studies showed that deletion of GRP94 reduced β-cell proliferation with increased cell apoptosis in both Pdx1+ endocrine progenitor cells and differentiated β cells. Although Hsp90b1 flox/flox;Pdx1Cre KO mice remained euglycemic at 8 weeks of age, they exhibited impaired glucose tolerance. In aggregate, these findings indicate that GRP94 is an essential regulator of pancreatic β-cell development, mass, and function.

Dynamic Glycosylation Governs the Vertebrate COPII Protein Trafficking Pathway

The COPII coat complex, which mediates secretory cargo trafficking from the endoplasmic reticulum, is a key control point for subcellular protein targeting. Because misdirected proteins cannot function, protein sorting by COPII is critical for establishing and maintaining normal cell and tissue homeostasis. Indeed, mutations in COPII genes cause a range of human pathologies, including cranio-lenticulo-sutural dysplasia (CLSD), which is characterized by collagen trafficking defects, craniofacial abnormalities, and skeletal dysmorphology. Detailed knowledge of the COPII pathway is required to understand its role in normal cell physiology and to devise new treatments for disorders in which it is disrupted. However, little is known about how vertebrates dynamically regulate COPII activity in response to developmental, metabolic, or pathological cues. Several COPII proteins are modified by O-linked β-N-acetylglucosamine (O-GlcNAc), a dynamic form of intracellular protein glycosylation, but the biochemical and functional effects of these modifications remain unclear. Here, we use a combination of chemical, biochemical, cellular, and genetic approaches to demonstrate that site-specific O-GlcNAcylation of COPII proteins mediates their protein-protein interactions and modulates cargo secretion. In particular, we show that individual O-GlcNAcylation sites of SEC23A, an essential COPII component, are required for its function in human cells and vertebrate development, because mutation of these sites impairs SEC23A-dependent in vivo collagen trafficking and skeletogenesis in a zebrafish model of CLSD. Our results indicate that O-GlcNAc is a conserved and critical regulatory modification in the vertebrate COPII-dependent trafficking pathway.

A Whole-Genome RNA Interference Screen Reveals a Role for Spry2 in Insulin Transcription and the Unfolded Protein Response

Insulin production by the pancreatic β-cell is required for normal glucose homeostasis. While key transcription factors that bind to the insulin promoter are known, relatively little is known about the upstream regulators of insulin transcription. Using a whole-genome RNA interference screen, we uncovered 26 novel regulators of insulin transcription that regulate diverse processes including oxidative phosphorylation, vesicle traffic, and the unfolded protein response (UPR). We focused on Spry2-a gene implicated in human type 2 diabetes by genome-wide association studies but without a clear connection to glucose homeostasis. We showed that Spry2 is a novel UPR target and its upregulation is dependent on PERK. Knockdown of Spry2 resulted in reduced expression of Serca2, reduced endoplasmic reticulum calcium levels, and induction of the UPR. Spry2 deletion in the adult mouse β-cell caused hyperglycemia and hypoinsulinemia. Our study greatly expands the compendium of insulin promoter regulators and demonstrates a novel β-cell link between Spry2 and human diabetes.

SEC23B is required for pancreatic acinar cell function in adult mice

Inactivation of Sec23b exclusively in the pancreatic acinar cells of adult mice results in loss of pancreatic mass, with evidence of cell loss, degeneration of exocrine cells (with smaller-than-normal zymogen granules and ER dilation), ER stress, and increased pancreatic cell apoptosis.

Mice with germline absence of SEC23B die perinatally, exhibiting massive pancreatic degeneration. We generated mice with tamoxifen-inducible, pancreatic acinar cell–specific Sec23b deletion. Inactivation of Sec23b exclusively in the pancreatic acinar cells of adult mice results in decreased overall pancreatic weights from pancreatic cell loss (decreased pancreatic DNA, RNA, and total protein content), as well as degeneration of exocrine cells, decreased zymogen granules, and alterations in the endoplasmic reticulum (ER), ranging from vesicular ER to markedly expanded cisternae with accumulation of moderate-density content or intracisternal granules. Acinar Sec23b deletion results in induction of ER stress and increased apoptosis in the pancreas, potentially explaining the loss of pancreatic cells and decreased pancreatic weight. These findings demonstrate that SEC23B is required for normal function of pancreatic acinar cells in adult mice.

Loss of Arf4 causes severe degeneration of the exocrine pancreas but not cystic kidney disease or retinal degeneration

Arf4 is proposed to be a critical regulator of membrane protein trafficking in early secretory pathway. More recently, Arf4 was also implicated in regulating ciliary trafficking, however, this has not been comprehensively tested in vivo. To directly address Arf4’s role in ciliary transport, we deleted Arf4 specifically in either rod photoreceptor cells, kidney, or globally during the early postnatal period. Arf4 deletion in photoreceptors did not cause protein mislocalization or retinal degeneration, as expected if Arf4 played a role in protein transport to the ciliary outer segment. Likewise, Arf4 deletion in kidney did not cause cystic disease, as expected if Arf4 were involved in general ciliary trafficking. In contrast, global Arf4 deletion in the early postnatal period resulted in growth restriction, severe pancreatic degeneration and early death. These findings are consistent with Arf4 playing a critical role in endomembrane trafficking, particularly in the pancreas, but not in ciliary function.

Primary cilia are sensory organelles found on most cells and contain specific receptors that detect extracellular stimuli. Defects in trafficking receptors to cilia cause a diverse set of diseases called ciliopathies, which include polycystic kidney disease, obesity, cerebral anomalies and retinal degeneration. Based mostly on in vitro studies, the small GTPase Arf4 was thought to be critically important for localizing rhodopsin to the outer segment of photoreceptor cells and cystoproteins to kidney cilia. Here we genetically remove Arf4 from mice in either a tissue specific or time dependent manner. To our surprise, the loss of Arf4 does not cause retinal degeneration or cystic kidney disease. Since ciliary dysfunction causes retinal degeneration and cystic disease, our findings indicate that Arf4 does not play a role in ciliary function. Instead, mice have zymogen granule defects and degeneration of the exocrine pancreas supporting roles for Arf4 in regulating endomembrane trafficking in specific cells.

Zebrafish Developmental Models of Skeletal Diseases

The zebrafish skeleton shares many similarities with human and other vertebrate skeletons. Over the past years, work in zebrafish has provided an extensive understanding of the basic developmental mechanisms and cellular pathways directing skeletal development and homeostasis. This review will focus on the cell biology of cartilage and bone and how the basic cellular processes within chondrocytes and osteocytes function to assemble the structural frame of a vertebrate body. We will discuss fundamental functions of skeletal cells in production and secretion of extracellular matrix and cellular activities leading to differentiation of progenitors to mature cells that make up the skeleton. We highlight important examples where findings in zebrafish provided direction for the search for genes causing human skeletal defects and also how zebrafish research has proven important for validating candidate human disease genes. The work we cover here illustrates utility of zebrafish in unraveling molecular mechanisms of cellular functions necessary to form and maintain a healthy skeleton.

Sec16 alternative splicing dynamically controls COPII transport efficiency

The transport of secretory proteins from the endoplasmic reticulum (ER) to the Golgi depends on COPII-coated vesicles. While the basic principles of the COPII machinery have been identified, it remains largely unknown how COPII transport is regulated to accommodate tissue- or activation-specific differences in cargo load and identity. Here we show that activation-induced alternative splicing of Sec16 controls adaptation of COPII transport to increased secretory cargo upon T-cell activation. Using splice-site blocking morpholinos and CRISPR/Cas9-mediated genome engineering, we show that the number of ER exit sites, COPII dynamics and transport efficiency depend on Sec16 alternative splicing. As the mechanistic basis, we suggest the C-terminal Sec16 domain to be a splicing-controlled protein interaction platform, with individual isoforms showing differential abilities to recruit COPII components. Our work connects the COPII pathway with alternative splicing, adding a new regulatory layer to protein secretion and its adaptation to changing cellular environments.

The transport of secretory proteins from the endoplasmic reticulum to the Golgi depends on COPII-coated vesicles. Here, the authors show that activation-induced alternative splicing of Sec16 controls adaptation of COPII transport to increased secretory cargo upon T cell activation.

Pancreatic SEC23B deficiency is sufficient to explain the perinatal lethality of germline SEC23B deficiency in mice

In humans, loss of function mutations in SEC23B result in Congenital Dyserythropoietic Anemia type II (CDAII), a disease limited to defective erythroid development. Patients with two nonsense SEC23B mutations have not been reported, suggesting that complete SEC23B deficiency might be lethal. We previously reported that SEC23B-deficient mice die perinatally, exhibiting massive pancreatic degeneration and that mice with hematopoietic SEC23B deficiency do not exhibit CDAII. We now show that SEC23B deficiency restricted to the pancreas is sufficient to explain the lethality observed in mice with global SEC23B-deficiency. Immunohistochemical stains demonstrate an acinar cell defect but normal islet cells. Mammalian genomes contain two Sec23 paralogs, Sec23A and Sec23B. The encoded proteins share ~85% amino acid sequence identity. We generate mice with pancreatic SEC23A deficiency and demonstrate that these mice survive normally, exhibiting normal pancreatic weights and histology. Taken together, these data demonstrate that SEC23B but not SEC23A is essential for murine pancreatic development. We also demonstrate that two BAC transgenes spanning Sec23b rescue the lethality of mice homozygous for a Sec23b gene trap allele, excluding a passenger gene mutation as the cause of the pancreatic lethality, and indicating that the regulatory elements critical for Sec23b pancreatic function reside within the BAC transgenes.

Germline Heterozygous Variants in SEC23B Are Associated with Cowden Syndrome and Enriched in Apparently Sporadic Thyroid Cancer

Cancer-predisposing genes associated with inherited cancer syndromes help explain mechanisms of sporadic carcinogenesis and often inform normal development. Cowden syndrome (CS) is an autosomal-dominant disorder characterized by high lifetime risks of epithelial cancers, such that ∼50% of affected individuals are wild-type for known cancer-predisposing genes. Using whole-exome and Sanger sequencing of a multi-generation CS family affected by thyroid and other cancers, we identified a pathogenic missense heterozygous SEC23B variant (c.1781T>G [p.Val594Gly]) that segregates with the phenotype. We also found germline heterozygous SEC23B variants in 3/96 (3%) unrelated mutation-negative CS probands with thyroid cancer and in The Cancer Genome Atlas (TCGA), representing apparently sporadic cancers. We note that the TCGA thyroid cancer dataset is enriched with unique germline deleterious SEC23B variants associated with a significantly younger age of onset. SEC23B encodes Sec23 homolog B (S. cerevisiae), a component of coat protein complex II (COPII), which transports proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. Interestingly, germline homozygous or compound-heterozygous SEC23B mutations cause an unrelated disorder, congenital dyserythropoietic anemia type II, and SEC23B-deficient mice suffer from secretory organ degeneration due to ER-stress-associated apoptosis. By characterizing the p.Val594Gly variant in a normal thyroid cell line, we show that it is a functional alteration that results in ER-stress-mediated cell-colony formation and survival, growth, and invasion, which reflect aspects of a cancer phenotype. Our findings suggest a different role for SEC23B, whereby germline heterozygous variants associate with cancer predisposition potentially mediated by ER stress "addiction."

Neural tube opening and abnormal extraembryonic membrane development in SEC23A deficient mice

COPII (coat protein complex-II) vesicles transport proteins from the endoplasmic reticulum (ER) to the Golgi. Higher eukaryotes have two or more paralogs of most COPII components. Here we characterize mice deficient for SEC23A and studied interactions of Sec23a null allele with the previously reported Sec23b null allele. SEC23A deficiency leads to mid-embryonic lethality associated with defective development of extraembryonic membranes and neural tube opening in midbrain. Secretion defects of multiple collagen types are observed in different connective tissues, suggesting that collagens are primarily transported in SEC23A-containing vesicles in these cells. Other extracellular matrix proteins, such as fibronectin, are not affected by SEC23A deficiency. Intracellular accumulation of unsecreted proteins leads to strong induction of the unfolded protein response in collagen-producing cells. No collagen secretion defects are observed in SEC23B deficient embryos. We report that E-cadherin is a cargo that accumulates in acini of SEC23B deficient pancreas and salivary glands. Compensatory increase of one paralog is observed in the absence of the second paralog. Haploinsufficiency of the remaining Sec23 paralog on top of homozygous inactivation of the first paralog leads to earlier lethality of embryos. Our results suggest that mammalian SEC23A and SEC23B transport overlapping yet distinct spectra of cargo in vivo.

Mechanisms for exporting large-sized cargoes from the endoplasmic reticulum

Cargo proteins exported from the endoplasmic reticulum to the Golgi apparatus are typically transported in coat protein complex II (COPII)-coated vesicles of 60–90 nm diameter. Several cargo molecules including collagens and chylomicrons form structures that are too large to be accommodated by these vesicles, but their secretion still requires COPII proteins. Here, we first review recent progress on large cargo secretions derived especially from animal models and human diseases, which indicate the importance of COPII proteins. We then discuss the recent isolation of specialized factors that modulate the process of COPII-dependent cargo formation to facilitate the exit of large-sized cargoes from the endoplasmic reticulum. Based on these findings, we propose a model that describes the importance of the GTPase cycle for secretion of oversized cargoes. Next, we summarize reports that describe the structures of COPII proteins and how these results provide insight into the mechanism of assembly of the large cargo carriers. Finally, we discuss what issues remain to be solved in the future.

Mammalian COPII coat component SEC24C is required for embryonic development in mice

COPII-coated vesicles mediate the transport of newly synthesized proteins from the endoplasmic reticulum to the Golgi. SEC24 is the COPII component primarily responsible for recruitment of protein cargoes into nascent vesicles. There are four Sec24 paralogs in mammals, with mice deficient in SEC24A, -B, and -D exhibiting a wide range of phenotypes. We now report the characterization of mice with deficiency in the fourth Sec24 paralog, SEC24C. Although mice haploinsufficient for Sec24c exhibit no apparent abnormalities, homozygous deficiency results in embryonic lethality at approximately embryonic day 7. Tissue-specific deletion of Sec24c in hepatocytes, pancreatic cells, smooth muscle cells, and intestinal epithelial cells results in phenotypically normal mice. Thus, SEC24C is required in early mammalian development but is dispensable in a number of tissues, likely as a result of compensation by other Sec24 paralogs. The embryonic lethality resulting from loss of SEC24C occurs considerably later than the lethality previously observed in SEC24D deficiency; it is clearly distinct from the restricted neural tube phenotype of Sec24b null embryos and the mild hypocholesterolemic phenotype of adult Sec24a null mice. Taken together, these results demonstrate that the four Sec24 paralogs have developed unique functions over the course of vertebrate evolution.

Endoplasmic reticulum chaperones and their roles in the immunogenicity of cancer vaccines

The endoplasmic reticulum (ER) is a major site of passage for proteins en route to other organelles, to the cell surface, and to the extracellular space. It is also the transport route for peptides generated in the cytosol by the proteasome into the ER for loading onto major histocompatibility complex class I (MHC I) molecules for eventual antigen presentation at the cell surface. Chaperones within the ER are critical for many of these processes; however, outside the ER certain of those chaperones may play important and direct roles in immune responses. In some cases, particular ER chaperones have been utilized as vaccines against tumors or infectious disease pathogens when purified from tumor tissue or recombinantly generated and loaded with antigen. In other cases, the cell surface location of ER chaperones has implications for immune responses as well as possible tumor resistance. We have produced heat-shock protein/chaperone protein-based cancer vaccines called “chaperone-rich cell lysate” (CRCL) that are conglomerates of chaperones enriched from solid tumors by an isoelectric focusing technique. These preparations have been effective against numerous murine tumors, as well as in a canine with an advanced lung carcinoma treated with autologous CRCL. We also published extensive proteomic analyses of CRCL prepared from human surgically resected tumor samples. Of note, these preparations contained at least 10 ER chaperones and a number of other residents, along with many other chaperones/heat-shock proteins. Gene ontology and network analyses utilizing these proteins essentially recapitulate the antigen presentation pathways and interconnections. In conjunction with our current knowledge of cell surface/extracellular ER chaperones, these data collectively suggest that a systems-level view may provide insight into the potent immune stimulatory activities of CRCL with an emphasis on the roles of ER components in those processes.

Altered chromatin occupancy of master regulators underlies evolutionary divergence in the transcriptional landscape of erythroid differentiation

Erythropoiesis is one of the best understood examples of cellular differentiation. Morphologically, erythroid differentiation proceeds in a nearly identical fashion between humans and mice, but recent evidence has shown that networks of gene expression governing this process are divergent between species. We undertook a systematic comparative analysis of six histone modifications and four transcriptional master regulators in primary proerythroblasts and erythroid cell lines to better understand the underlying basis of these transcriptional differences. Our analyses suggest that while chromatin structure across orthologous promoters is strongly conserved, subtle differences are associated with transcriptional divergence between species. Many transcription factor (TF) occupancy sites were poorly conserved across species (∼25% for GATA1, TAL1, and NFE2) but were more conserved between proerythroblasts and cell lines derived from the same species. We found that certain cis-regulatory modules co-occupied by GATA1, TAL1, and KLF1 are under strict evolutionary constraint and localize to genes necessary for erythroid cell identity. More generally, we show that conserved TF occupancy sites are indicative of active regulatory regions and strong gene expression that is sustained during maturation. Our results suggest that evolutionary turnover of TF binding sites associates with changes in the underlying chromatin structure, driving transcriptional divergence. We provide examples of how this framework can be applied to understand epigenomic variation in specific regulatory regions, such as the β-globin gene locus. Our findings have important implications for understanding epigenomic changes that mediate variation in cellular differentiation across species, while also providing a valuable resource for studies of hematopoiesis.

The process whereby blood progenitor cells differentiate into red blood cells, known as erythropoiesis, is very similar between mice and humans. Yet, while studies of this process in mouse have substantially improved our knowledge of human erythropoiesis, recent work has shown a significant divergence in global gene expression across species, suggesting that extrapolation from mouse models to human is not always straightforward. In order to better understand these differences, we have performed a comparative epigenomic analysis of six histone modifications and four master transcription factors. By globally comparing chromatin structure across primary cells and model cell lines in both species, we discovered that while chromatin structure is well conserved at orthologous promoters, subtle changes are predictive of species-specific gene expression. Furthermore, we discovered that the genomic localizations of master transcription factors are poorly conserved, and species-specific losses or gains are associated with changes to the underlying chromatin structure and concomitant gene expression. By using our comparative epigenomics framework, we identified a putative human-specific cis-regulatory module that drives expression of human, but not mouse, GDF15, a gene implicated in iron homeostasis. Our results provide a resource to aid researchers in interpreting genetic and epigenetic differences between species.

Retrospective cohort study of 205 cases with congenital dyserythropoietic anemia type II: definition of clinical and molecular spectrum and identification of new diagnostic scores

Congenital Dyserythropoietic Anemia II (CDA II) is a rare hyporegenerative anemia of variable degree, whose causative gene is SEC23B. More than 60 causative mutations in 142 independent pedigrees have been described so far. However, the prevalence of the CDA II is probably underestimated, since its clinical spectrum was not yet well-defined and thus it is often misdiagnosed with more frequent clinically-related anemias. This study represents the first meta-analysis on clinical and molecular spectrum of CDA II from the largest cohort of cases ever described. We characterized 41 new cases and 18 mutations not yet associated to CDA II, thus expanding the global series to 205 cases (172 unrelated) and the total number of causative variants to 84. The 68.3% of patients are included in our International Registry of CDA II (Napoli, Italy). A genotype-phenotype correlation in three genotypic groups of patients was assessed. To quantify the degree of severity in each patient, a method based on ranking score was performed. We introduced a clinical index to easily discriminate patients with a well-compensated hemolytic anemia from those with ineffective erythropoiesis. Finally, the worldwide geographical distribution of SEC23B alleles highlighted the presence of multiple founder effects in different areas of the world.

Absence of a red blood cell phenotype in mice with hematopoietic deficiency of SEC23B

Congenital dyserythropoietic anemia type II (CDAII) is an autosomal recessive disease of ineffective erythropoiesis characterized by increased bi/multinucleated erythroid precursors in the bone marrow. CDAII results from mutations in SEC23B. The SEC23 protein is a core component of coat protein complex II-coated vesicles, which transport secretory proteins from the endoplasmic reticulum to the Golgi apparatus. Though the genetic defect underlying CDAII has been identified, the pathophysiology of this disease remains unknown. We previously reported that SEC23B-deficient mice die perinatally, exhibiting massive pancreatic degeneration, with this early mortality limiting evaluation of the adult hematopoietic compartment. We now report that mice with SEC23B deficiency restricted to the hematopoietic compartment survive normally and do not exhibit anemia or other CDAII characteristics. We also demonstrate that SEC23B-deficient hematopoietic stem cells (HSC) do not exhibit a disadvantage at reconstituting hematopoiesis when compared directly to wild-type HSC in a competitive repopulation assay. Secondary bone marrow transplants demonstrated continued equivalence of SEC23B-deficient and WT HSC in their hematopoietic reconstitution potential. The surprising discordance in phenotypes between SEC23B-deficient mice and humans may reflect an evolutionary shift in SEC23 paralog function and/or expression, or a change in a specific COPII cargo critical for erythropoiesis.

Erythro-megakaryocytic transcription factors associated with hereditary anemia

Most heritable anemias are caused by mutations in genes encoding globins, red blood cell (RBC) membrane proteins, or enzymes in the glycolytic and hexose monophosphate shunt pathways. A less common class of genetic anemia is caused by mutations that alter the functions of erythroid transcription factors (TFs). Many TF mutations associated with heritable anemia cause truncations or amino acid substitutions, resulting in the production of functionally altered proteins. Characterization of these mutant proteins has provided insights into mechanisms of gene expression, hematopoietic development, and human disease. Mutations within promoter or enhancer regions that disrupt TF binding to essential erythroid genes also cause anemia and heritable variations in RBC traits, such as fetal hemoglobin content. Defining the latter may have important clinical implications for de-repressing fetal hemoglobin synthesis to treat sickle cell anemia and β thalassemia. Functionally important alterations in genes encoding TFs or their cognate cis elements are likely to occur more frequently than currently appreciated, a hypothesis that will soon be tested through ongoing genome-wide association studies and the rapidly expanding use of global genome sequencing for human diagnostics. Findings obtained through such studies of RBCs and associated diseases are likely generalizable to many human diseases and quantitative traits.