Efficient coupling of Sec23-Sec24 to Sec13-Sec31 drives COPII-dependent collagen secretion and is essential for normal craniofacial development

The structure of the Sec13/31 COPII cage bound to Sec23

Structural studies have revealed some of the organizing principles and mechanisms involved in the assembly of the COPII coat including the location of the Sec23/24 adapter layer. Previous studies, however, were unable to unambiguously determine the positions of Sec23 and Sec24 in the coat. Here, we have determined a cryogenic electron microscopic structure of Sec13/31 together with Sec23. Electron tomography revealed that the binding of Sec23 induces Sec13/31 to form a variety of different geometries including a cuboctahedron, as was previously characterized for Sec13/31 alone. Single-particle reconstruction of the Sec13/31-23 cuboctahedra revealed that the binding of Sec23 induces a conformational change in Sec13/31, resulting in a more extended conformation. Docking Sec23 crystal structures into the electron microscopy map suggested that Sec24 projects its cargo binding surface out into the large open faces of the coat. These results have implications for the mechanisms by which COPII transports large cargos, cargos with large intracellular domains, and for tethering complexes that must project out of the coat in order to interact with their binding partners. Furthermore, Sec23 binds Sec13/31 at two unique sites in the coat, which suggests that each site may have unique roles in the mechanisms of COPII vesiculation.

BBF2H7-mediated Sec23A pathway is required for endoplasmic reticulum-to-Golgi trafficking in dermal fibroblasts to promote collagen synthesis

Collagen fibers, structural elements responsible for mechanical strength in skin, are synthesized constitutively in response to cytokines such as IGF-I. However, little is known about their intracellular trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus during synthesis. We demonstrate herein that the BBF2 human homolog on chromosome 7 (BBF2H7)-mediated Sec23A pathway is involved in regulation of intracellular procollagen trafficking. The mRNA and protein expression of BBF2H7, Sec23A, and type I and III collagen (COL1 and COL3) was induced by IGF-I stimulation. In addition, the cleaved form of BBF2H7 was detected in IGF-I-treated cultures, indicating that activation occurs concurrently with its expression. Knockdown with small interfering RNAs targeting BBF2H7 caused a significant reduction in the expression of COL1 and COL3, regardless of IGF-I treatment. Both mitogen-activated protein kinase and phosphatidylinositol-3 kinase pathways via IGF-I receptor activation were required for BBF2H7 induction. Using immunofluorescence microscopy, we showed that Golgi apparatus dysmorphology is due to coat protein complex II vehicle hypoplasia caused by the absence of BBF2H7 and Sec23A. The BBF2H7-mediated Sec23A pathway was required for ER-to-Golgi procollagen trafficking to promote collagen synthesis. This role of growth factors such as IGF-I, which to our knowledge is previously unreported, suggests antiaging strategies.

Irradiation-induced protein inactivation reveals Golgi enzyme cycling to cell periphery

Acute inhibition is a powerful technique to test proteins for direct roles and order their activities in a pathway, but as a general gene-based strategy, it is mostly unavailable in mammalian systems. As a consequence, the precise roles of proteins in membrane trafficking have been difficult to assess in vivo. Here we used a strategy based on a genetically encoded fluorescent protein that generates highly localized and damaging reactive oxygen species to rapidly inactivate exit from the endoplasmic reticulum (ER) during live-cell imaging and address the long-standing question of whether the integrity of the Golgi complex depends on constant input from the ER. Light-induced blockade of ER exit immediately perturbed Golgi membranes, and surprisingly, revealed that cis-Golgi-resident proteins continuously cycle to peripheral ER-Golgi intermediate compartment (ERGIC) membranes and depend on ER exit for their return to the Golgi. These experiments demonstrate that ER exit and extensive cycling of cis-Golgi components to the cell periphery sustain the mammalian Golgi complex.

Ubiquitin-dependent regulation of COPII coat size and function

Packaging of proteins from the ER into COPII-vesicles is essential for secretion. In cells, most COPII-vesicles are ~60-80nm in diameter, yet some must increase their size to accommodate 300-400nm procollagen fibers or chylomicrons. Impaired COPII function results in collagen deposition defects, cranio-lenticulo-sutural dysplasia, or chylomicron retention disease, but mechanisms to enlarge COPII-coats have remained elusive. Here, we have identified the ubiquitin ligase Cul3^Klhl12 as a regulator of COPII coat formation. Cul3^Klhl12 catalyzes the monoubiquitination of the COPII-component Sec31 and drives the assembly of large COPII coats. As a result, ubiquitination by Cul3^Klhl12 is essential for collagen export, yet less important for the transport of small cargo. We conclude that monoubiquitination controls the size and function of a vesicle coat.

COPII and COPI traffic at the ER-Golgi interface

Protein traffic is necessary to maintain homeostasis in all eukaryotic organisms. All newly synthesized secretory proteins destined to the secretory and endolysosmal systems are transported from the endoplasmic reticulum to the Golgi before delivery to their final destinations. Here, we describe the COPII and COPI coating machineries that generate carrier vesicles and the tethers and SNAREs that mediate COPII and COPI vesicle fusion at the ER-Golgi interface.

Epithelial organization and cyst lumen expansion require efficient Sec13-Sec31-driven secretion

Epithelial morphogenesis is directed by interactions with the underlying extracellular matrix. Secretion of collagen and other matrix components requires efficient coat complex II (COPII) vesicle formation at the endoplasmic reticulum. Here, we show that suppression of the outer layer COPII component, Sec13, in zebrafish embryos results in a disorganized gut epithelium. In human intestinal epithelial cells (Caco-2), Sec13 depletion causes defective epithelial polarity and organization on permeable supports. Defects are seen in the ability of cells to adhere to the substrate, form a monolayer and form intercellular junctions. When embedded in a three-dimensional matrix, Sec13-depleted Caco-2 cells form cysts but, unlike controls, are defective in lumen expansion. Incorporation of primary fibroblasts within the three-dimensional culture substantially restores normal morphogenesis. We conclude that efficient COPII-dependent secretion, notably assembly of Sec13-Sec31, is required to drive epithelial morphogenesis in both two- and three-dimensional cultures in vitro, as well as in vivo. Our results provide insight into the role of COPII in epithelial morphogenesis and have implications for the interpretation of epithelial polarity and organization assays in cell culture.

ER cargo properties specify a requirement for COPII coat rigidity mediated by Sec13p

Eukaryotic secretory proteins exit the endoplasmic reticulum (ER) via transport vesicles generated by the essential coat protein complex II (COPII) proteins. The outer coat complex, Sec13-Sec31, forms a scaffold that is thought to enforce curvature. By exploiting yeast bypass-of-sec-thirteen (bst) mutants, where Sec13p is dispensable, we probed the relationship between a compromised COPII coat and the cellular context in which it could still function. Genetic and biochemical analyses suggested that Sec13p was required to generate vesicles from membranes that contained asymmetrically distributed cargoes that were likely to confer opposing curvature. Thus, Sec13p may rigidify the COPII cage and increase its membrane-bending capacity; this function could be bypassed when a bst mutation renders the membrane more deformable.

The [corrected] SEC23-SEC31 [corrected] interface plays critical role for export of procollagen from the endoplasmic reticulum

COPII proteins are essential for exporting most cargo molecules from the endoplasmic reticulum. The membrane-facing surface of the COPII proteins (especially SEC23-SEC24) interacts directly or indirectly with the cargo molecules destined for exit. As we characterized the SEC23A mutations at the SEC31 binding site identified from patients with cranio-lenticulo-sutural dysplasia, we discovered that the SEC23-SEC31 interface can also influence cargo selection. Remarkably, M702V SEC23A does not compromise COPII assembly, vesicle size, and packaging of cargo molecules into COPII vesicles that we have tested but induces accumulation of procollagen in the endoplasmic reticulum when expressed in normal fibroblasts. We observed that M702V SEC23A activates SAR1B GTPase more than wild-type SEC23A when SEC13-SEC31 is present, indicating that M702V SEC23A causes premature dissociation of COPII from the membrane. Our results indicate that a longer stay of COPII proteins on the membrane is required to cargo procollagen than other molecules and suggest that the SEC23-SEC31 interface plays a critical role in capturing various cargo molecules.

COPII and the regulation of protein sorting in mammals

Secretory proteins are transported to the Golgi complex in vesicles that bud from the endoplasmic reticulum. The cytoplasmic coat protein complex II (COPII) is responsible for cargo sorting and vesicle morphogenesis. COPII was first described in Saccharomyces cerevisiae, but its basic function is conserved throughout all eukaryotes. Nevertheless, the COPII coat has adapted to the higher complexity of mammalian physiology, achieving more sophisticated levels of secretory regulation. In this review we cover aspects of mammalian COPII-mediated regulation of secretion, in particular related to the function of COPII paralogues, the spatial organization of cargo export and the role of accessory proteins.

Evidence for the involvement of the Arabidopsis SEC24A in male transmission

Eukaryotic cells use COPII-coated carriers for endoplasmic reticulum (ER)-to-Golgi protein transport. Selective cargo capture into ER-derived carriers is largely driven by the SEC24 component of the COPII coat. The Arabidopsis genome encodes three AtSEC24 genes with overlapping expression profiles but it is yet to be established whether the AtSEC24 proteins have overlapping roles in plant growth and development. Taking advantage of Arabidopsis thaliana as a model plant system for studying gene function in vivo, through reciprocal crosses, pollen characterization, and complementation tests, evidence is provided for a role for AtSEC24A in the male gametophyte. It is established that an AtSEC24A loss-of-function mutation is tolerated in the female gametophyte but that it causes defects in pollen leading to failure of male transmission of the AtSEC24A mutation. These data provide a characterization of plant SEC24 family in planta showing incompletely overlapping functions of the AtSEC24 isoforms. The results also attribute a novel role to SEC24 proteins in a multicellular model system, specifically in male fertility.

Characterization of human Sec16B: indications of specialized, non-redundant functions

The endoplasmic reticulum (ER) represents the entry point into the secretory pathway and from here newly synthesized proteins and lipids are delivered to the Golgi. The selective cargo export from the ER is mediated by COPII-assembly at specific sites of the ER, the so-called transitional ER (tER). The peripheral membrane protein Sec16, first identified in yeast, localizes to transitional ER and plays a key role in organization of these sites. Sec16 defines the tER and is thought to act as a scaffold for the COPII coat assembly. In humans two isoforms of Sec16 are present, the larger Sec16A and the smaller Sec16B. Nevertheless, the functional differences between the two isoforms are ill-defined. Here we describe characterization of the localization and dynamics of Sec16B relative to Sec16A, provide evidence that Sec16B is likely a minor or perhaps specialized form of Sec16, and that it is not functionally redundant with Sec16A.

Direct targeting of Sec23a by miR-200s influences cancer cell secretome and promotes metastatic colonization

Although the role of miR-200s in regulating E-cadherin expression and epithelial-to-mesenchymal transition is well established, their influence on metastatic colonization remains controversial. Here we have used clinical and experimental models of breast cancer metastasis to discover a pro-metastatic role of miR-200s that goes beyond their regulation of E-cadherin and epithelial phenotype. Overexpression of miR-200s is associated with increased risk of metastasis in breast cancer and promotes metastatic colonization in mouse models, phenotypes that cannot be recapitulated by E-cadherin expression alone. Genomic and proteomic analyses revealed global shifts in gene expression upon miR-200 overexpression toward that of highly metastatic cells. miR-200s promote metastatic colonization partly through direct targeting of Sec23a, which mediates secretion of metastasis-suppressive proteins, including Igfbp4 and Tinagl1, as validated by functional and clinical correlation studies. Overall, these findings suggest a pleiotropic role of miR-200s in promoting metastatic colonization by influencing E-cadherin-dependent epithelial traits and Sec23a-mediated tumor cell secretome.

Molecular regulation of lumen morphogenesis

The asymmetric polarization of cells allows specialized functions to be performed at discrete subcellular locales. Spatiotemporal coordination of polarization between groups of cells allowed the evolution of metazoa. For instance, coordinated apical-basal polarization of epithelial and endothelial cells allows transport of nutrients and metabolites across cell barriers and tissue microenvironments. The defining feature of such tissues is the presence of a central, interconnected luminal network. Although tubular networks are present in seemingly different organ systems, such as the kidney, lung, and blood vessels, common underlying principles govern their formation. Recent studies using in vivo and in vitro models of lumen formation have shed new light on the molecular networks regulating this fundamental process. We here discuss progress in understanding common design principles underpinning de novo lumen formation and expansion.

ABA-dependent and -independent G-protein signaling in Arabidopsis roots revealed through an iTRAQ proteomics approach

Heterotrimeric G-proteins are important signal transducers in all eukaryotes. The plant hormone abscisic acid (ABA) has emerged as a key regulator of G-protein-mediated signaling pathways in plants. ABA-regulation of G-protein signaling involves both conventional and novel mechanisms. We have utilized the null mutant of the Arabidopsis G-protein α subunit gpa1 to evaluate to what extent ABA-dependent changes in the proteome are regulated by G-proteins. We used Arabidopsis root tissue as both ABA and G-proteins, individually and in combination, affect root growth and development. We identified 720 proteins, of which 42 showed GPA1-dependent and 74 showed ABA-dependent abundance changes. A majority of ABA-regulated proteins were also GPA1-dependent. Our data provide insight into how tissue specificity might be achieved in ABA-regulated G-protein signaling. A number of proteins related to ER body formation and intracellular trafficking were altered in gpa1 mutant, suggesting a novel role for GPA1 in these pathways. A potential link between ABA metabolism and ABA signaling was also revealed. The comparison of protein abundance changes in the absence of ABA offers clues to the role of GPA1 in ABA-independent signaling pathways, for example, regulation of cell division. These findings substantially contribute to our knowledge of G-protein signaling mechanisms in plants.

Global defects in collagen secretion in a Mia3/TANGO1 knockout mouse

Mia3’s contribution to protein secretion is broader than previously realized—its absence impairs collagen deposition and normal development of cartilage and bone.

Melanoma inhibitory activity member 3 (MIA3/TANGO1) is an evolutionarily conserved endoplasmic reticulum resident transmembrane protein. Recent in vitro studies have shown that it is required for the loading of collagen VII, but not collagen I, into COPII-coated transport vesicles. In this paper, we show that mice lacking Mia3 are defective for the secretion of numerous collagens, including collagens I, II, III, IV, VII, and IX, from chondrocytes, fibroblasts, endothelial cells, and mural cells. Collagen deposition by these cell types is abnormal, and extracellular matrix composition is compromised. These changes are associated with intracellular accumulation of collagen and the induction of a strong unfolded protein response, primarily within the developing skeleton. Chondrocyte maturation and bone mineralization are severely compromised in Mia3-null embryos, leading to dwarfism and neonatal lethality. Thus, Mia3’s role in protein secretion is much broader than previously realized, and it may, in fact, be required for the efficient secretion of all collagen molecules in higher organisms.

cTAGE5 mediates collagen secretion through interaction with TANGO1 at endoplasmic reticulum exit sites

The mechanism of collagen secretion is not completely understood. It is found that cTAGE5 binds to TANGO1, and it is suggested that collagen VII export from the ER is driven by a cTAGE5/TANGO1 complex.

Cutaneous T-cell lymphoma-–associated antigen 5 (cTAGE5), an originally identified tumor antigen, is overexpressed in various cancer cell lines. The cDNA encodes an integral membrane protein containing two coiled-coil motifs and a proline-rich domain. We show that cTAGE5 specifically localizes to the endoplasmic reticulum (ER) exit sites. In addition, cTAGE5 forms a complex with TANGO1 (MIA3), a previously characterized cargo receptor for collagen VII, by the interaction of their coiled-coil motifs. Of interest, cTAGE5, as well as TANGO1, is capable of interacting with the inner-layer coatomer of COPII Sec23/24 complex through their C-terminal proline-rich domains and required for collagen VII secretion. We propose that cTAGE5 acts as a coreceptor of TANGO1 for collagen VII export from the ER.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.

Sec16A defines the site for vesicle budding from the endoplasmic reticulum on exit from mitosis

Mitotic inhibition of COPII-dependent export of proteins from the endoplasmic reticulum results in disassembly of the Golgi complex. This ensures ordered inheritance of organelles by the two daughter cells. Reassembly of the Golgi is intimately linked to the re-initiation of ER export on exit from mitosis. Here, we show that unlike all other COPII components, which are cytosolic during metaphase, Sec16A remains associated with ER exit sites throughout mitosis, and thereby could provide a template for the rapid assembly of functional export domains in anaphase. Full assembly of COPII at exit sites precedes reassembly of the Golgi in telophase.

LG186: An inhibitor of GBF1 function that causes Golgi disassembly in human and canine cells

Brefeldin A-mediated inhibition of ADP ribosylation factor (Arf) GTPases and their guanine nucleotide exchange factors, Arf-GEFs, has been a cornerstone of membrane trafficking research for many years. Brefeldin A (BFA) is relatively non-selective inhibiting at least three targets in human cells, Golgi brefeldin A resistance factor 1 (GBF1), brefeldin A inhibited guanine nucleotide exchange factor 1 (BIG1) and brefeldin A inhibited guanine nucleotide exchange factor 2 (BIG2). Here, we show that the previously described compound Exo2 acts through inhibition of Arf-GEF function, but causes other phenotypic changes that are not GBF1 related. We describe the engineering of Exo2 to produce LG186, a more selective, reversible inhibitor of Arf-GEF function. Using multiple-cell-based assays and GBF1 mutants, our data are most consistent with LG186 acting by selective inhibition of GBF1. Unlike other Arf-GEF and reported GBF1 inhibitors including BFA, Exo2 and Golgicide A, LG186 induces disassembly of the Golgi stack in both human and canine cells.

Cranio-lenticulo-sutural dysplasia associated with defects in collagen secretion

Cranio-lenticulo-sutural dysplasia (CLSD) is a rare autosomal recessive syndrome manifesting with large and late-closing fontanels and calvarial hypomineralization, Y-shaped cataracts, skeletal defects, and hypertelorism and other facial dysmorphisms. The CLSD locus was mapped to chromosome 14q13-q21 and a homozygous SEC23A F382L missense mutation was identified in the original family. Skin fibroblasts from these patients exhibit features of a secretion defect with marked distension of the endoplasmic reticulum (ER), consistent with SEC23A function in protein export from the ER. We report an unrelated family where a male proband presented with clinical features of CLSD. A heterozygous missense M702V mutation in a highly conserved residue of SEC23A was inherited from the clinically unaffected father, but no maternal SEC23A mutation was identified. Cultured skin fibroblasts from this new patient showed a severe secretion defect of collagen and enlarged ER, confirming aberrant protein export from the ER. Milder collagen secretion defects and ER distention were present in paternal fibroblasts, indicating that an additional mutation(s) is present in the proband. Our data suggest that defective ER export is the cause of CLSD and genetic element(s) besides SEC23A may influence its presentation.

Enhancement of procollagen biosynthesis by p180 through augmented ribosome association on the endoplasmic reticulum in response to stimulated secretion

A coiled-coil microtubule-bundling protein, p180, was originally reported as a ribosome-binding protein on the rough endoplasmic reticulum (ER) and is highly expressed in secretory tissues. Recently, we reported a novel role for p180 in the trans-Golgi network (TGN) expansion following stimulated collagen secretion. Here, we show that p180 plays a key role in procollagen biosynthesis and secretion in diploid fibroblasts. Depletion of p180 caused marked reductions of secreted collagens without significant loss of the ER membrane or mRNA. Metabolic labeling experiments revealed that the procollagen biosynthetic activity was markedly affected following p180 depletion. Moreover, loss of p180 perturbs ascorbate-stimulated de novo biosynthesis mainly in the membrane fraction with a preferential secretion defect of large proteins. At the EM level, one of the most prominent morphological features of p180-depleted cells was insufficient ribosome association on the ER membranes. In contrast, the ER of control cells was studded with numerous ribosomes, which were further enhanced by ascorbate. Similarly biochemical analysis confirmed that levels of membrane-bound ribosomes were altered in a p180-dependent manner. Taken together, our data suggest that p180 plays crucial roles in enhancing collagen biosynthesis at the entry site of the secretory compartments by a novel mechanism that mainly involves facilitating ribosome association on the ER.

Sar1 assembly regulates membrane constriction and ER export

While dynamin pinches vesicles from the plasma membrane, the Sar1 GTPase specializes in cinching ER membrane tubules.

The guanosine triphosphatase Sar1 controls the assembly and fission of COPII vesicles. Sar1 utilizes an amphipathic N-terminal helix as a wedge that inserts into outer membrane leaflets to induce vesicle neck constriction and control fission. We hypothesize that Sar1 organizes on membranes to control constriction as observed with fission proteins like dynamin. Sar1 activation led to membrane-dependent oligomerization that transformed giant unilamellar vesicles into small vesicles connected through highly constricted necks. In contrast, membrane tension provided through membrane attachment led to organization of Sar1 in ordered scaffolds that formed rigid, uniformly nonconstricted lipid tubules to suggest that Sar1 organization regulates membrane constriction. Sar1 organization required conserved residues located on a unique C-terminal loop. Mutations in this loop did not affect Sar1 activation or COPII recruitment and enhanced membrane constriction, yet inhibited Sar1 organization and procollagen transport from the endoplasmic reticulum (ER). Sar1 activity was directed to liquid-disordered lipid phases. Thus, lipid-directed and tether-assisted Sar1 organization controls membrane constriction to regulate ER export.

Aging and chronic sun exposure cause distinct epigenetic changes in human skin

Epigenetic changes are widely considered to play an important role in aging, but experimental evidence to support this hypothesis has been scarce. We have used array-based analysis to determine genome-scale DNA methylation patterns from human skin samples and to investigate the effects of aging, chronic sun exposure, and tissue variation. Our results reveal a high degree of tissue specificity in the methylation patterns and also showed very little interindividual variation within tissues. Data stratification by age revealed that DNA from older individuals was characterized by a specific hypermethylation pattern affecting less than 1% of the markers analyzed. Interestingly, stratification by sun exposure produced a fundamentally different pattern with a significant trend towards hypomethylation. Our results thus identify defined age-related DNA methylation changes and suggest that these alterations might contribute to the phenotypic changes associated with skin aging.

Although a role of epigenetic mechanisms in aging and in the adaptation to environmental exposures has been widely assumed, research in this area has been hampered by major methodological challenges. We have now used a novel platform for genome-scale methylation analysis to determine the methylation patterns of human skin samples. Skin represents a particularly suitable model for this study because of its well-known phenotype changes associated with aging and sun exposure, and because skin samples are characterized by a very high degree of cellular homogeneity. By examining 50 samples, and analyzing 50 million data points, we show that aging and sun exposure are associated with comparably small, but significant changes in the DNA methylation patterns of human epidermis and dermis samples. Interestingly, aging was not associated with a general variation in DNA methylation patterns, but rather with a directed DNA hypermethylation shift. Importantly, our results also suggest that epigenetic mechanisms may be functionally important for the phenotypic changes associated with aging and chronic sun exposure.

Trafficking through COPII stabilises cell polarity and drives secretion during Drosophila epidermal differentiation

Background

The differentiation of an extracellular matrix (ECM) at the apical side of epithelial cells implies massive polarised secretion and membrane trafficking. An epithelial cell is hence engaged in coordinating secretion and cell polarity for a correct and efficient ECM formation.

Principal Findings

We are studying the molecular mechanisms that Drosophila tracheal and epidermal cells deploy to form their specific apical ECM during differentiation. In this work we demonstrate that the two genetically identified factors haunted and ghost are essential for polarity maintenance, membrane topology as well as for secretion of the tracheal luminal matrix and the cuticle. We show that they code for the Drosophila COPII vesicle-coating components Sec23 and Sec24, respectively, that organise vesicle transport from the ER to the Golgi apparatus.

Conclusion

Taken together, epithelial differentiation during Drosophila embryogenesis is a concerted action of ECM formation, plasma membrane remodelling and maintenance of cell polarity that all three rely mainly, if not absolutely, on the canonical secretory pathway from the ER over the Golgi apparatus to the plasma membrane. Our results indicate that COPII vesicles constitute a central hub for these processes.

Regulation of coat assembly--sorting things out at the ER

The small GTPase Sar1 resides at the core of a regulatory cycle that controls protein export from the ER in COPII vesicles. Recent advances in minimally reconstituted systems indicate continual flux of Sar1 through GTPase cycles facilitates cargo concentration into forming vesicles that ultimately bud from membranes. During export from ER membranes, this GTPase cycle is harnessed through the combinatorial power of multiple coat subunits and cargo adaptors to sort an expanding array of proteins into ER-derived vesicles. The COPII budding machinery is further organized into higher-order structures at transitional zones on the ER surface where the large multi-domain Sec16 protein appears to perform a central function.

Sec24D-dependent transport of extracellular matrix proteins is required for zebrafish skeletal morphogenesis

Protein transport from endoplasmic reticulum (ER) to Golgi is primarily conducted by coated vesicular carriers such as COPII. Here, we describe zebrafish bulldog mutations that disrupt the function of the cargo adaptor Sec24D, an integral component of the COPII complex. We show that Sec24D is essential for secretion of cartilage matrix proteins, whereas the preceding development of craniofacial primordia and pre-chondrogenic condensations does not depend on this isoform. Bulldog chondrocytes fail to secrete type II collagen and matrilin to extracellular matrix (ECM), but membrane bound receptor β1-Integrin and Cadherins appear to leave ER in Sec24D-independent fashion. Consequently, Sec24D-deficient cells accumulate proteins in the distended ER, although a subset of ER compartments and Golgi complexes as visualized by electron microscopy and NBD C₆-ceramide staining appear functional. Consistent with the backlog of proteins in the ER, chondrocytes activate the ER stress response machinery and significantly upregulate BiP transcription. Failure of ECM secretion hinders chondroblast intercalations thus resulting in small and malformed cartilages and severe craniofacial dysmorphology. This defect is specific to Sec24D mutants since knockdown of Sec24C, a close paralog of Sec24D, does not result in craniofacial cartilage dysmorphology. However, craniofacial development in double Sec24C/Sec24D-deficient animals is arrested earlier than in bulldog/sec24d, suggesting that Sec24C can compensate for loss of Sec24D at initial stages of chondrogenesis, but Sec24D is indispensable for chondrocyte maturation. Our study presents the first developmental perspective on Sec24D function and establishes Sec24D as a strong candidate for cartilage maintenance diseases and craniofacial birth defects.

Alpha-synuclein delays endoplasmic reticulum (ER)-to-Golgi transport in mammalian cells by antagonizing ER/Golgi SNAREs

This work demonstrates that α-synuclein inhibits the biosynthetic secretory pathway of mammalian cells potently and directly under nontoxic conditions and in the absence of insoluble α-synuclein aggregates. A potential mechanism involving α-synuclein binding to ER/Golgi SNAREs and inhibiting fusogenic SNARE complex assembly is elucidated.

Toxicity of human α-synuclein when expressed in simple organisms can be suppressed by overexpression of endoplasmic reticulum (ER)-to-Golgi transport machinery, suggesting that inhibition of constitutive secretion represents a fundamental cause of the toxicity. Whether similar inhibition in mammals represents a cause of familial Parkinson's disease has not been established. We tested elements of this hypothesis by expressing human α-synuclein in mammalian kidney and neuroendocrine cells and assessing ER-to-Golgi transport. Overexpression of wild type or the familial disease-associated A53T mutant α-synuclein delayed transport by up to 50%; however, A53T inhibited more potently. The secretory delay occurred at low expression levels and was not accompanied by insoluble α-synuclein aggregates or mistargeting of transport machinery, suggesting a direct action of soluble α-synuclein on trafficking proteins. Co-overexpression of ER/Golgi arginine soluble N-ethylmaleimide-sensitive factor attachment protein receptors (R-SNAREs) specifically rescued transport, indicating that α-synuclein antagonizes SNARE function. Ykt6 reversed α-synuclein inhibition much more effectively than sec22b, suggesting a possible neuroprotective role for the enigmatic high expression of ykt6 in neurons. In in vitro reconstitutions, purified α-synuclein A53T protein specifically inhibited COPII vesicle docking and fusion at a pre-Golgi step. Finally, soluble α-synuclein A53T directly bound ER/Golgi SNAREs and inhibited SNARE complex assembly, providing a potential mechanism for toxic effects in the early secretory pathway.

Apolipoprotein B100 biogenesis: a complex array of intracellular mechanisms regulating folding, stability, and lipoprotein assemblyThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Apolipoprotein B100 (apoB) is a large amphipathic lipid-binding protein that is synthesized by hepatocytes and used to assemble and stabilize very low density lipoproteins (VLDL). It may have been derived through evolution from other lipid-associating proteins such as microsomal triglyceride transfer protein or vitellogenin. The correct folding of apoB requires assistance from chaperone proteins in co-translational lipidation, disulfide bond formation, and glycosylation. Any impairment in these processes results in co-translational targeting of the misfolded apoB molecule for proteasomal degradation. In fact, most of the regulation of apoB production is mediated by intracellular degradation. ApoB that misfolds post-translationally, perhaps as a result of oxidative stress, may be eliminated through autophagy. This review focuses on the proposed pentapartite domain structure of apoB, the role that each domain plays in the binding of lipid species and regulation of apoB synthesis, and the process of VLDL assembly. The factors involved in the recognition, ubiquitination, and proteasomal delivery of defective apoB molecules are also discussed.

Specific functions of BIG1 and BIG2 in endomembrane organization

Background

Transport of molecules from one subcellular compartment to another involves the recruitment of cytosolic coat protein complexes to a donor membrane to concentrate cargo, deform the membrane and ultimately to form an independent carrier. Small-GTP-binding proteins of the Arf family are central to many membrane trafficking events. Arfs are activated by guanine nucleotide exchange factors (GEFs) which results in their recruitment to membranes and subsequent engagement with Arf-effectors, many of which are coat proteins. Among the human BFA-sensitive large Arf-GEFs, the function of the two closely related BIG1 and BIG2 is still not clear, and recent studies have raised the question of functional redundancy between the two proteins.

Methodology/Principal Findings

Here we have used small-interfering RNA on human cells and a combination of fixed and live-cell imaging to investigate the differential functions of BIG1 and BIG2 in endomembrane organization and function. Importantly, in this direct comparative study, we show discrete functions for BIG1 and BIG2. Our results show that depletion of BIG2 but not of BIG1 induces a tubulation of the recycling endosomal compartment, consistent with a specific role for BIG2 here. In contrast, suppression of BIG1 induces the formation of Golgi mini-stacks still polarized and functional in terms of cargo export.

Conclusions

A key finding from our work is that suppression of BIG1 expression results in a fragmentation of the Golgi apparatus. Our data indicate that the human BFA-sensitive large Arf-GEFs have non-redundant functions in cell organization and membrane trafficking. BIG1 is required to maintain the normal morphology of the Golgi; BIG2 is important for endosomal compartment integrity and cannot replace the function of BIG1 in Golgi organization.

A novel multiprotein complex is required to generate the prechylomicron transport vesicle from intestinal ER

Dietary lipid absorption is dependent on chylomicron production whose rate-limiting step across the intestinal absorptive cell is the exit of chylomicrons from the endoplasmic reticulum (ER) in its ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). This study addresses the composition of the budding complex for PCTV. Immunoprecipitation (IP) studies from rat intestinal ER solubilized in Triton X-100 suggested that vesicle-associated membrane protein 7 (VAMP7), apolipoprotein B48 (apoB48), liver fatty acid-binding protein (L-FABP), CD36, and the COPII proteins were associated on incubation of the ER with cytosol and ATP. This association was confirmed by chromatography of the solubilized ER over Sephacryl S400-HR in which these constituents cochromatographed with an apparent kDa of 630. No multiprotein complex was detected when the ER was chromatographed in the absence of PCTV budding activity (resting ER or PKCzeta depletion of ER and cytosol). Treatment of the ER with anti-apoB48 or anti-VAMP7 antibodies or using gene disrupted L-FABP or CD36 mice all significantly inhibited PCTV generation. A smaller complex (no COPII proteins) was formed when only rL-FABP was used to bud PCTV. The data support the conclusion that the PCTV budding complex in intestinal ER is composed of VAMP7, apoB48, CD36, and L-FABP, plus the COPII proteins.

New directions in craniofacial morphogenesis

The vertebrate head is an extremely complicated structure: development of the head requires tissue-tissue interactions between derivates of all the germ layers and coordinated morphogenetic movements in three dimensions. In this review, we highlight a number of recent embryological studies, using chicken, frog, zebrafish and mouse, which have identified crucial signaling centers in the embryonic face. These studies demonstrate how small variations in growth factor signaling can lead to a diversity of phenotypic outcomes. We also discuss novel genetic studies, in human, mouse and zebrafish, which describe cell biological mechanisms fundamental to the growth and morphogenesis of the craniofacial skeleton. Together, these findings underscore the complex interactions leading to species-specific morphology. These and future studies will improve our understanding of the genetic and environmental influences underlying human craniofacial anomalies.

ER exit sites--localization and control of COPII vesicle formation

The first membrane trafficking step in the biosynthetic secretory pathway, the export of proteins and lipids from the endoplasmic reticulum (ER), is mediated by COPII-coated vesicles. In mammalian cells, COPII vesicle budding occurs at specialized sites on the ER, the so-called transitional ER (tER). Here, we discuss aspects of the formation and maintenance of these sites, the mechanisms by which cargo becomes segregated within them, and the propagation of ER exit sites (ERES) during cell division. All of these features are inherently linked to the formation, maintenance and function of the Golgi apparatus underlining the importance of ERES to Golgi function and more widely in terms of intracellular organization and cellular function.

Differentially expressed genes strongly correlated with femur strength in rats

The region of chromosome 1q33-q54 harbors quantitative trait loci (QTL) for femur strength in COPxDA and F344xLEW F2 rats. The purpose of this study is to identify the genes within this QTL region that contribute to the variation in femur strength. Microarray analysis was performed using RNA extracted from femurs of COP, DA, F344 and LEW rats. Genes differentially expressed in the 1q33-q54 region among these rat strains were then ranked based on the strength of correlation with femur strength in F2 animals derived from these rats. A total of 214 genes in this QTL region were differentially expressed among all rat strains, and 81 genes were found to be strongly correlated (r(2)>0.50) with femur strength. Of these, 12 candidate genes were prioritized for further validation, and 8 of these genes (Ifit3, Ppp2r5b, Irf7, Mpeg1, Bloc1s2, Pycard, Sec23ip, and Hps6) were confirmed by quantitative PCR (qPCR). Ingenuity Pathway Analysis suggested that these genes were involved in interferon alpha, nuclear factor-kappa B (NFkB), extracellular signal-related kinase (ERK), hepatocyte nuclear factor 4 alpha (HNF4A) and tumor necrosis factor (TNF) pathways.

Visualization of cargo concentration by COPII minimal machinery in a planar lipid membrane

Selective protein export from the endoplasmic reticulum is mediated by COPII vesicles. Here, we investigated the dynamics of fluorescently labelled cargo and non-cargo proteins during COPII vesicle formation using single-molecule microscopy combined with an artificial planar lipid bilayer. Single-molecule analysis showed that the Sar1p-Sec23/24p-cargo complex, but not the Sar1p-Sec23/24p complex, undergoes partial dimerization before Sec13/31p recruitment. On addition of a complete COPII mixture, cargo molecules start to assemble into fluorescent spots and clusters followed by vesicle release from the planar membrane. We show that continuous GTPase cycles of Sar1p facilitate cargo concentration into COPII vesicle buds, and at the same time, non-cargo proteins are excluded from cargo clusters. We propose that the minimal set of COPII components is required not only to concentrate cargo molecules, but also to mediate exclusion of non-cargo proteins from the COPII vesicles.

Vesicle coating and uncoating: controlling the formation of large COPII-coated carriers

The basic mechanisms underlying the formation of coated vesicles are now defined in considerable detail. This article highlights recent developments in our understanding of the problem of exporting large macromolecular cargo such as procollagen from the endoplasmic reticulum and discusses the implications that this has for cell and tissue organisation and human disease.

Selective targeting of ER exit sites supports axon development

During neuron development, the biosynthetic needs of the axon initially outweigh those of dendrites. However, although a localized role for the early secretory pathway in dendrite development has been observed, such a role in axon growth remains undefined. We therefore studied the localization of Sar1, a small GTPase that controls ER export, during early stages of neuronal development that are characterized by selective and robust axon growth. At these early stages, Sar1 was selectively targeted to the axon where it gradually concentrated within varicosities in which additional proteins that function in the early secretory pathway were detected. Sar1 targeting to the axon followed axon specification and was dependent on localized actin instability. Changes in Sar1 expression levels at these early development stages modulated axon growth. Specifically, reduced expression of Sar1, which was initially only detectable in the axon, correlated with reduced axon growth, where as overexpression of Sar1 supported the growth of longer axons. In support of the former finding, expression of dominant negative Sar1 inhibited axon growth. Thus, as observed in lower organisms, mammalian cells use temporal and spatial regulation of endoplasmic reticulum exit site (ERES) to address developmental biosynthetic demands. Furthermore, axons, such as dendrites, rely on ERES targeting and assembly for growth.

Organisation of human ER-exit sites: requirements for the localisation of Sec16 to transitional ER

The COPII complex mediates the selective incorporation of secretory cargo and relevant machinery into budding vesicles at specialised sites on the endoplasmic reticulum membrane called transitional ER (tER). Here, we show using confocal microscopy, immunogold labelling of ultrathin cryosections and electron tomography that in human cells at steady state, Sec16 localises to cup-like structures of tER that are spatially distinct from the localisation of other COPII coat components. We show that Sec16 defines the tER, whereas Sec23-Sec24 and Sec13-Sec31 define later structures that precede but are distinct from the intermediate compartment. Steady-state localisation of Sec16 is independent of the localisation of downstream COPII components Sec23-Sec24 and Sec13-Sec31. Sec16 cycles on and off the membrane at a slower rate than other COPII components with a greater immobile fraction. We define the region of Sec16A that dictates its robust localisation of tER membranes and find that this requires both a highly charged region as well as a central domain that shows high sequence identity between species. The central conserved domain of Sec16 binds to Sec13 linking tER membrane localisation with COPII vesicle formation. These data are consistent with a model where Sec16 acts as a platform for COPII assembly at ERES.

Role of syntaxin 18 in the organization of endoplasmic reticulum subdomains

The presence of subdomains in the endoplasmic reticulum (ER) enables this organelle to perform a variety of functions, yet the mechanisms underlying their organization are poorly understood. In the present study, we show that syntaxin 18, a SNAP (soluble NSF attachment protein) receptor localized in the ER, is important for the organization of two ER subdomains, smooth/rough ER membranes and ER exit sites. Knockdown of syntaxin 18 caused a global change in ER membrane architecture, leading to the segregation of the smooth and rough ER. Furthermore, the organization of ER exit sites was markedly changed concomitantly with dispersion of the ER-Golgi intermediate compartment and the Golgi complex. These morphological changes in the ER were substantially recovered by treatment of syntaxin-18-depleted cells with brefeldin A, a reagent that stimulates retrograde membrane flow to the ER. These results suggest that syntaxin 18 has an important role in ER subdomain organization by mediating the fusion of retrograde membrane carriers with the ER membrane.

Specificity of cytoplasmic dynein subunits in discrete membrane-trafficking steps

The cytoplasmic dynein motor complex is known to exist in multiple forms, but few specific functions have been assigned to individual subunits. A key limitation in the analysis of dynein in intact mammalian cells has been the reliance on gross perturbation of dynein function, e.g., inhibitory antibodies, depolymerization of the entire microtubule network, or the use of expression of dominant negative proteins that inhibit dynein indirectly. Here, we have used RNAi and automated image analysis to define roles for dynein subunits in distinct membrane-trafficking processes. Depletion of a specific subset of dynein subunits, notably LIC1 (DYNC1LI1) but not LIC2 (DYNC1LI2), recapitulates a direct block of ER export, revealing that dynein is required to maintain the steady-state composition of the Golgi, through ongoing ER-to-Golgi transport. Suppression of LIC2 but not of LIC1 results in a defect in recycling endosome distribution and cytokinesis. Biochemical analyses also define the role of each subunit in stabilization of the dynein complex; notably, suppression of DHC1 or IC2 results in concomitant loss of Tctex1. Our data demonstrate that LIC1 and LIC2 define distinct dynein complexes that function at the Golgi versus recycling endosomes, respectively, suggesting that functional populations of dynein mediate discrete intracellular trafficking pathways.

TANGO1 facilitates cargo loading at endoplasmic reticulum exit sites

A genome-wide screen revealed previously unidentified components required for transport and Golgi organization (TANGO). We now provide evidence that one of these proteins, TANGO1, is an integral membrane protein localized to endoplasmic reticulum (ER) exit sites, with a luminal SH3 domain and a cytoplasmic proline-rich domain (PRD). Knockdown of TANGO1 inhibits export of bulky collagen VII from the ER. The SH3 domain of TANGO1 binds to collagen VII; the PRD binds to the COPII coat subunits, Sec23/24. In this scenario, PRD binding to Sec23/24 subunits could stall COPII carrier biogenesis to permit the luminal domain of TANGO1 to guide SH3-bound cargo into a growing carrier. All cells except those of hematopoietic origin express TANGO1. We propose that TANGO1 exports other cargoes in cells that do not secrete collagen VII. However, TANGO1 does not enter the budding carrier, which represents a unique mechanism to load cargo into COPII carriers.