COPII-Golgi protein interactions regulate COPII coat assembly and Golgi size

The Golgi Apparatus: A Voyage through Time, Structure, Function and Implication in Neurodegenerative Disorders

This comprehensive review article dives deep into the Golgi apparatus, an essential organelle in cellular biology. Beginning with its discovery during the 19th century until today's recognition as an important contributor to cell function. We explore its unique organization and structure as well as its roles in protein processing, sorting, and lipid biogenesis, which play key roles in maintaining homeostasis in cellular biology. This article further explores Golgi biogenesis, exploring its intricate processes and dynamics that contribute to its formation and function. One key focus is its role in neurodegenerative diseases like Parkinson's, where changes to the structure or function of the Golgi apparatus may lead to their onset or progression, emphasizing its key importance in neuronal health. At the same time, we examine the intriguing relationship between Golgi stress and endoplasmic reticulum (ER) stress, providing insights into their interplay as two major cellular stress response pathways. Such interdependence provides a greater understanding of cellular reactions to protein misfolding and accumulation, hallmark features of many neurodegenerative diseases. In summary, this review offers an exhaustive examination of the Golgi apparatus, from its historical background to its role in health and disease. Additionally, this examination emphasizes the necessity of further research in this field in order to develop targeted therapeutic approaches for Golgi dysfunction-associated conditions. Furthermore, its exploration is an example of scientific progress while simultaneously offering hope for developing innovative treatments for neurodegenerative disorders.

A SURF4-to-proteoglycan relay mechanism that mediates the sorting and secretion of a tagged variant of sonic hedgehog

SignificanceSonic Hedgehog (Shh) is a key signaling molecule that plays important roles in embryonic patterning, cell differentiation, and organ development. Although fundamentally important, the molecular mechanisms that regulate secretion of newly synthesized Shh are still unclear. Our study reveals a role for the cargo receptor, SURF4, in facilitating export of Shh from the endoplasmic reticulum (ER) via a ER export signal. In addition, our study provides evidence suggesting that proteoglycans promote the dissociation of SURF4 from Shh at the Golgi, suggesting a SURF4-to-proteoglycan relay mechanism. These analyses provide insight into an important question in cell biology: how do cargo receptors capture their clients in one compartment, then disengage at their destination?

ADP-Ribosylation Factor Family of Small GTP-Binding Proteins: Their Membrane Recruitment, Activation, Crosstalk and Functions

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide binding proteins play critical roles in various cellular processes, especially in regulating the secretory, and endocytic pathways. The fidelity of intracellular vesicular trafficking depends on proper activations and precise subcellular distributions of ARF family proteins regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Here we review recent progress in understanding the membrane recruitment, activation, crosstalk, and functions of ARF family proteins.

An in vitro vesicle formation assay reveals cargo clients and factors that mediate vesicular trafficking

Protein sorting in the secretory pathway is a fundamentally important cellular process, but the clients of a specific cargo sorting machinery remains largely underinvestigated. Here, utilizing a vesicle formation assay to profile proteins associated with vesicles, we identified cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner or that interact with GTP-bound Sar1A. We found that two of them, FAM84B and PRRC1, regulate anterograde trafficking. Moreover, we revealed specific clients of two export adaptors, SURF4 and ERGIC53. These analyses demonstrate that our approach is powerful to identify factors that regulate vesicular trafficking and to uncover clients of specific cargo receptors, providing a robust method to reveal insights into the secretory pathway.

The fidelity of protein transport in the secretory pathway relies on the accurate sorting of proteins to their correct destinations. To deepen our understanding of the underlying molecular mechanisms, it is important to develop a robust approach to systematically reveal cargo proteins that depend on specific sorting machinery to be enriched into transport vesicles. Here, we used an in vitro assay that reconstitutes packaging of human cargo proteins into vesicles to quantify cargo capture. Quantitative mass spectrometry (MS) analyses of the isolated vesicles revealed cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner. We found that two of them, FAM84B (also known as LRAT domain containing 2 or LRATD2) and PRRC1, contain proline-rich domains and regulate anterograde trafficking. Further analyses revealed that PRRC1 is recruited to endoplasmic reticulum (ER) exit sites, interacts with the inner COPII coat, and its absence increases membrane association of COPII. In addition, we uncovered cargo proteins that depend on GTP hydrolysis to be captured into vesicles. Comparing control cells with cells depleted of the cargo receptors, SURF4 or ERGIC53, we revealed specific clients of each of these two export adaptors. Our results indicate that the vesicle formation assay in combination with quantitative MS analysis is a robust and powerful tool to uncover novel factors that mediate vesicular trafficking and to uncover cargo clients of specific cellular factors.

Endoplasmic reticulum composition and form: Proteins in and out

The endoplasmic reticulum (ER) is the main harbor for newly synthesized proteins in eukaryotic cells. Through a continuous membrane network of sheets and tubules, the ER hosts secretory proteins, integral membrane proteins, and luminal proteins of the endomembrane system. These proteins are translated by ribosomes outside the ER and require subsequent integration into or translocation across the lipid bilayer of the ER. They are then modified post-translationally and folded in the ER. Some of these proteins are packaged into coat protein complex II-coated vesicles for export. Here, we review recent advances in understanding the mechanism of protein translocation and transmembrane domain insertion in the ER, summarize new insights into selective cargo packaging, and discuss the roles of ER morphological dynamics in these processes.

The amphipathic helices of Arfrp1 and Arl14 are sufficient to determine subcellular localizations

The subcellular localization of Arf family proteins is generally thought to be determined by their corresponding guanine nucleotide exchange factors. By promoting GTP binding, guanine nucleotide exchange factors induce conformational changes of Arf proteins exposing their N-terminal amphipathic helices, which then insert into the membranes to stabilize the membrane association process. Here, we found that the N-terminal amphipathic motifs of the Golgi-localized Arf family protein, Arfrp1, and the endosome- and plasma membrane-localized Arf family protein, Arl14, play critical roles in spatial determination. Exchanging the amphipathic helix motifs between these two Arf proteins causes the switch of their localizations. Moreover, the amphipathic helices of Arfrp1 and Arl14 are sufficient for cytosolic proteins to be localized into a specific cellular compartment. The spatial determination mediated by the Arfrp1 helix requires its binding partner Sys1. In addition, the residues that are required for the acetylation of the Arfrp1 helix and the myristoylation of the Arl14 helix are important for the specific subcellular localization. Interestingly, Arfrp1 and Arl14 are recruited to their specific cellular compartments independent of GTP binding. Our results demonstrate that the amphipathic motifs of Arfrp1 and Arl14 are sufficient for determining specific subcellular localizations in a GTP-independent manner, suggesting that the membrane association and activation of some Arf proteins are uncoupled.

Molecular mechanisms that regulate export of the planar cell-polarity protein Frizzled-6 out of the endoplasmic reticulum

Planar cell polarity (PCP) is a process during which cells are polarized along the plane of the epithelium and is regulated by several transmembrane signaling proteins. After their synthesis, these PCP proteins are delivered along the secretory transport pathway to the plasma membrane, where they perform their physiological functions. However, the molecular mechanisms that regulate PCP protein transport remain largely unclear. Here, we found that the delivery of a PCP protein, Frizzled-6, to the cell surface is regulated by two conserved polybasic motifs: one located in its first intracellular loop and the other in its C-terminal cytosolic domain. We observed that the polybasic motif of Frizzled is also important for its surface localization in the Drosophila wing. Results from a mechanistic analysis indicated that Frizzled-6 packaging into vesicles at the endoplasmic reticulum (ER) is regulated by a direct interaction between the polybasic motif and the Glu-62 and Glu-63 residues on the secretion-associated Ras-related GTPase 1A (SAR1A) subunit of coat protein complex II (COPII). Moreover, we found that newly synthesized Frizzled-6 is associated with another PCP protein, cadherin EGF LAG seven-pass G-type receptor 1 (CELSR1), in the secretory transport pathway, and that this association regulates their surface delivery. Our results reveal insights into the molecular machinery that regulates the ER export of Frizzled-6. They also suggest that the association of CELSR1 with Frizzled-6 is important, enabling efficient Frizzled-6 delivery to the cell surface, providing a quality control mechanism that ensures the appropriate stoichiometry of these two PCP proteins at cell boundaries.

Mitochondrial origins of fractional control in regulated cell death

Individual cells in clonal populations often respond differently to environmental changes; for binary phenotypes, such as cell death, this can be measured as a fractional response. These types of responses have been attributed to cell-intrinsic stochastic processes and variable abundances of biochemical constituents, such as proteins, but the influence of organelles is still under investigation. We use the response to TNF-related apoptosis inducing ligand (TRAIL) and a new statistical framework for determining parameter influence on cell-to-cell variability through the inference of variance explained, DEPICTIVE, to demonstrate that variable mitochondria abundance correlates with cell survival and determines the fractional cell death response. By quantitative data analysis and modeling we attribute this effect to variable effective concentrations at the mitochondria surface of the pro-apoptotic proteins Bax/Bak. Further, our study suggests that inhibitors of anti-apoptotic Bcl-2 family proteins, used in cancer treatment, may increase the diversity of cellular responses, enhancing resistance to treatment.

Phenotypic cell-to-cell variability contributes to fractional killing, but the mechanisms are incompletely understood. Here the authors show that mitochondrial density correlates with cell survival in response to TRAIL, and that variable effective concentrations of Bax/Bak contribute to the effect.

Genome-wide identification of alternative splicing events that regulate protein transport across the secretory pathway

Alternative splicing (AS) strongly increases proteome diversity and functionality in eukaryotic cells. Protein secretion is a tightly-controlled process, especially in a tissue-specific and differentiation-dependent manner. While previous work has focussed on transcriptional and post-translational regulatory mechanisms, the impact of AS on the secretory pathway remains largely unexplored. Here we integrate a published screen for modulators of protein transport and RNA-Seq analyses to identify over 200 AS events as secretion regulators. We confirm that splicing events along all stages of the secretory pathway regulate the efficiency of membrane trafficking using Morpholinos and CRISPR/Cas9. We furthermore show that these events are highly tissue-specific and adapt the secretory pathway during T-cell activation and adipocyte differentiation. Our data substantially advance the understanding of AS functionality, add a new regulatory layer to a fundamental cell biological process and provide a resource of alternative isoforms that control the secretory pathway.

Nε-lysine acetylation in the endoplasmic reticulum - a novel cellular mechanism that regulates proteostasis and autophagy

Protein post-translational modifications (PTMs) take many shapes, have many effects and are necessary for cellular homeostasis. One of these PTMs, Nε-lysine acetylation, was thought to occur only in the mitochondria, cytosol and nucleus, but this paradigm was challenged in the past decade with the discovery of lysine acetylation in the lumen of the endoplasmic reticulum (ER). This process is governed by the ER acetylation machinery: the cytosol:ER-lumen acetyl-CoA transporter AT-1 (also known as SLC33A1), and the ER-resident lysine acetyltransferases ATase1 and ATase2 (also known as NAT8B and NAT8, respectively). This Review summarizes the more recent biochemical, cellular and mouse model studies that underscore the importance of the ER acetylation process in maintaining protein homeostasis and autophagy within the secretory pathway, and its impact on developmental and age-associated diseases.

An Overview of Protein Secretion in Yeast and Animal Cells

Protein secretion mediated by the secretory transport pathway is an important cellular process in eukaryotic cells. In the conventional secretory transport pathway, newly synthesized proteins pass through several endomembrane compartments en route to their specific destinations. Transport of secretory proteins between different compartments is shuttled by small, membrane-enclosed vesicles. To ensure the fidelity of transport, eukaryotic cells employ elaborate molecular machineries to accurately sort newly synthesized proteins into specific transport vesicles and precisely deliver these transport vesicles to distinct acceptor compartments. In this review, we summarize the molecular machineries that regulate each step of vesicular transport in the secretory transport pathway in yeast and animal cells.

Regulation of EGFR surface levels by COPII-dependent trafficking

Farhan discusses Scharaw et al.’s study about how the COPII machinery is used to replenish EGFR at the cell surface.

Cell surface levels of epidermal growth factor receptors (EGFRs) are thought to be controlled mainly by endocytic trafficking, with biosynthetic EGFR trafficking presumed to be a constitutive and unregulated process. However, Scharaw et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201601090) demonstrate a role for inducible COPII trafficking in controlling EGFR surface levels.

Growth of the Mammalian Golgi Apparatus during Interphase

During the cell cycle, genetic materials and organelles are duplicated to ensure that there is sufficient cellular content for daughter cells. While Golgi growth in interphase has been observed in lower eukaryotes, the elaborate ribbon structure of the mammalian Golgi apparatus has made it challenging to monitor. Here we demonstrate the growth of the mammalian Golgi apparatus in its protein content and volume during interphase. Through ultrastructural analyses, physical growth of the Golgi apparatus was revealed to occur by cisternal elongation of the individual Golgi stacks. By examining the timing and regulation of Golgi growth, we established that Golgi growth starts after passage through the cell growth checkpoint at late G1 phase and continues in a manner highly correlated with cell size growth. Finally, by identifying S6 kinase 1 as a major player in Golgi growth, we revealed the coordination between cell size and Golgi growth via activation of the protein synthesis machinery in early interphase.

TFG Promotes Organization of Transitional ER and Efficient Collagen Secretion

Collagen is the most abundant protein in the animal kingdom. It is of fundamental importance during development for cell differentiation and tissue morphogenesis as well as in pathological processes such as fibrosis and cancer cell migration. However, our understanding of the mechanisms of procollagen secretion remains limited. Here, we show that TFG organizes transitional ER (tER) and ER exit sites (ERESs) into larger structures. Depletion of TFG results in dispersion of tER elements that remain associated with individual ER-Golgi intermediate compartments (ERGICs) as largely functional ERESs. We show that TFG is not required for the transport and packaging of small soluble cargoes but is necessary for the export of procollagen from the ER. Our work therefore suggests a key relationship between the structure and function of ERESs and a central role for TFG in optimizing COPII assembly for procollagen export.

Dynamics of immature mAb glycoform secretion during CHO cell culture: An integrated modelling framework

Ensuring consistent glycosylation-associated quality of therapeutic monoclonal antibodies (mAbs) has become a priority in pharmaceutical bioprocessing given that the distribution and composition of the carbohydrates (glycans) bound to these molecules determines their therapeutic efficacy and immunogenicity. However, the interaction between bioprocess conditions, cellular metabolism and the intracellular process of glycosylation remains to be fully understood. To gain further insight into these interactions, we present a novel integrated modelling platform that links dynamic variations in mAb glycosylation with cellular secretory capacity. Two alternative mechanistic representations of how mAb specific productivity (qp ) influences glycosylation are compared. In the first, mAb glycosylation is modulated by the linear velocity with which secretory cargo traverses the Golgi apparatus. In the second, glycosylation is influenced by variations in Golgi volume. Within our modelling framework, both mechanisms accurately reproduce experimentally-observed dynamic changes in mAb glycosylation. In addition, an optimisation-based strategy has been developed to estimate the concentration of glycosylation enzymes required to minimise mAb glycoform variability. Our results suggest that the availability of glycosylation machinery relative to cellular secretory capacity may play a crucial role in mAb glycosylation. In the future, the modelling framework presented here may aid in selecting and engineering cell lines that ensure consistent mAb glycosylatio.

SEC16 in COPII coat dynamics at ER exit sites

Protein export from the endoplasmic reticulum (ER), the first step in protein transport through the secretory pathway, is mediated by coatomer protein II (COPII)-coated vesicles at ER exit sites. COPII coat assembly on the ER is well understood and the conserved large hydrophilic protein Sec16 clearly has a role to play in COPII coat dynamics. Sec16 localizes to ER exit sites, its loss of function impairs their functional organization in all species where it has been studied, and it interacts with COPII coat subunits. However, its exact function in COPII dynamics is debated, as Sec16 is proposed to act as a scaffold to recruit COPII components and as a device to regulate the Sar1 activity in uncoating, in such a way that the coat is released only when the vesicle is fully formed and loaded with cargo. Furthermore, Sec16 has been shown to respond to nutrient signalling, thus coupling environmental stimuli to secretory capacity.

Systems biology of the secretory pathway: what have we learned so far?

Several RNAi screens were performed in search for regulators of the secretory pathway. These screens were performed in different organisms and cell lines and relied on different readouts. Therefore, they have only little overlap among their hits, leading to the question of what we have learned from this approach so far and how these screens contributed towards an integrative understanding of the endomembrane system. The aim of this review is to revisit these screens and discuss their strengths and weaknesses as well as potential reasons for their failure to overlap with each other. As with secretory trafficking, RNAi screens were also performed on other cellular processes such as cell migration and autophagy, both of which were shown to be intimately linked to secretion. Another aim of this review is to compare the outcome of the RNAi screens on secretion, autophagy and cell migration and ask whether the functional genomic approaches have uncovered potential mechanistic insights into the links between these processes.

Regulation of Sec16 levels and dynamics links proliferation and secretion

We currently lack a broader mechanistic understanding of the integration of the early secretory pathway with other homeostatic processes such as cell growth. Here, we explore the possibility that Sec16A, a major constituent of endoplasmic reticulum exit sites (ERES), acts as an integrator of growth factor signaling. Surprisingly, we find that Sec16A is a short-lived protein that is regulated by growth factors in a manner dependent on Egr family transcription factors. We hypothesize that Sec16A acts as a central node in a coherent feed-forward loop that detects persistent growth factor stimuli to increase ERES number. Consistent with this notion, Sec16A is also regulated by short-term growth factor treatment that leads to increased turnover of Sec16A at ERES. Finally, we demonstrate that Sec16A depletion reduces proliferation, whereas its overexpression increases proliferation. Together with our finding that growth factors regulate Sec16A levels and its dynamics on ERES, we propose that this protein acts as an integrator linking growth factor signaling and secretion. This provides a mechanistic basis for the previously proposed link between secretion and proliferation.

Positive feedback between Golgi membranes, microtubules and ER exit sites directs de novo biogenesis of the Golgi

The Golgi complex is the central organelle of the secretory pathway. It undergoes dynamic changes during the cell cycle, but how it acquires and maintains its complex structure is unclear. To address this question, we have used laser nanosurgery to deplete BSC1 cells of the Golgi complex and have monitored its biogenesis by quantitative time-lapse microscopy and correlative electron microscopy. After Golgi depletion, endoplasmic reticulum (ER) export is inhibited and the number of ER exit sites (ERES) is reduced and does not increase for several hours. Occasional fusion of small post-ER carriers to form the first larger structures triggers a rapid and drastic growth of Golgi precursors, due to the capacity of these structures to attract more carriers by microtubule nucleation and to stimulate ERES biogenesis. Increasing the chances of post-ER carrier fusion close to ERES by depolymerizing microtubules results in the acceleration of Golgi and ERES biogenesis. Taken together, on the basis of our results, we propose a self-organizing principle of the early secretory pathway that integrates Golgi biogenesis, ERES biogenesis and the organization of the microtubule network by positive-feedback loops.

Reduction in Golgi apparatus dimension in the absence of a residential protein, N-acetylglucosaminyltransferase V

Various proteins are involved in the generation and maintenance of the membrane complex known as the Golgi apparatus. We have used mutant Chinese hamster ovary (CHO) cell lines Lec4 and Lec4A lacking N-acetylglucosaminyltransferase V (GlcNAcT-V, MGAT5) activity and protein in the Golgi apparatus to study the effects of the absence of a single glycosyltransferase on the Golgi apparatus dimension. Quantification of immunofluorescence in serial confocal sections for Golgi α-mannosidase II and electron microscopic morphometry revealed a reduction in Golgi volume density up to 49 % in CHO Lec4 and CHO Lec4A cells compared to parental CHO cells. This reduction in Golgi volume density could be reversed by stable transfection of Lec4 cells with a cDNA encoding Mgat5. Inhibition of the synthesis of β1,6-branched N-glycans by swainsonine had no effect on Golgi volume density. In addition, no effect on Golgi volume density was observed in CHO Lec1 cells that contain enzymatically active GlcNAcT-V, but cannot synthesize β1,6-branched glycans due to an inactive GlcNAcT-I in their Golgi apparatus. These results indicate that it may be the absence of the GlcNAcT-V protein that is the determining factor in reducing Golgi volume density. No dimensional differences existed in cross-sectioned cisternal stacks between Lec4 and control CHO cells, but significantly reduced Golgi stack hits were observed in cross-sectioned Lec4 cells. Therefore, the Golgi apparatus dimensional change in Lec4 and Lec4A cells may be due to a compaction of the organelle.

Binding of the vesicle docking protein p115 to the GTPase Rab1b regulates membrane recruitment of the COPI vesicle coat

Membrane recruitment of the COPI vesicle coat is fundamental to its function and contributes to compartment identity in the early secretory pathway. COPI recruitment is triggered by guanine nucleotide exchange activating the Arf1 GTPase, but the key exchange factor, GBF1, is a peripheral membrane component whose membrane association is dependent on another GTPase, Rab1. Inactive Rab GTPases are in a soluble complex with guanine nucleotide dissociation inhibitor (GDI) and activation of Rab GTPases by exchange factors can be enhanced by GDI dissociation factors (GDFs). In the present study, we investigated the vesicle docking protein p115 and it's binding to the Rab1 isoform Rab1b. Inhibition of p115 expression induced dissociation of Rab1b from Golgi membranes. Rab1b bound the cc2 domain of p115 and p115 lacking this domain failed to recruit Rab1b. Further, p115 inhibition blocked association of the COPI coat with Golgi membranes and this was suppressed by constitutive activation of Rab1b. These findings show p115 enhancement of Rab1b activation leading to COPI recruitment suggesting a connection between the vesicle docking machinery and the vesicle coat complex during the establishment of post-ER compartment identity.

Golgi enlargement in Arf-depleted yeast cells is due to altered dynamics of cisternal maturation

Regulation of the size and abundance of membrane compartments is a fundamental cellular activity. In Saccharomyces cerevisiae, disruption of the ADP-ribosylation factor 1 (ARF1) gene yields larger and fewer Golgi cisternae by partially depleting the Arf GTPase. We observed a similar phenotype with a thermosensitive mutation in Nmt1, which myristoylates and activates Arf. Therefore, partial depletion of Arf is a convenient tool for dissecting mechanisms that regulate Golgi structure. We found that in arf1Δ cells, late Golgi structure is particularly abnormal, with the number of late Golgi cisternae being severely reduced. This effect can be explained by selective changes in cisternal maturation kinetics. The arf1Δ mutation causes early Golgi cisternae to mature more slowly and less frequently, but does not alter the maturation of late Golgi cisternae. These changes quantitatively explain why late Golgi cisternae are fewer in number and correspondingly larger. With a stacked Golgi, similar changes in maturation kinetics could be used by the cell to modulate the number of cisternae per stack. Thus, the rates of processes that transform a maturing compartment can determine compartmental size and copy number.

Sec16 influences transitional ER sites by regulating rather than organizing COPII

During the budding of coat protein complex II (COPII) vesicles from transitional endoplasmic reticulum (tER) sites, Sec16 has been proposed to play two distinct roles: negatively regulating COPII turnover and organizing COPII assembly at tER sites. We tested these ideas using the yeast Pichia pastoris. Redistribution of Sec16 to the cytosol accelerates tER dynamics, supporting a negative regulatory role for Sec16. To evaluate a possible COPII organization role, we dissected the functional regions of Sec16. The central conserved domain, which had been implicated in coordinating COPII assembly, is actually dispensable for normal tER structure. An upstream conserved region (UCR) localizes Sec16 to tER sites. The UCR binds COPII components, and removal of COPII from tER sites also removes Sec16, indicating that COPII recruits Sec16 rather than the other way around. We propose that Sec16 does not in fact organize COPII. Instead, regulation of COPII turnover can account for the influence of Sec16 on tER sites.

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

Non-muscle myosin IIA transports a Golgi glycosyltransferase to the endoplasmic reticulum by binding to its cytoplasmic tail

The mechanism of the Golgi-to-ER transport of Golgi glycosyltransferases is not clear. We utilize a cell line expressing the core 2 N-acetylglucosaminyltransferase-M (C2GnT-M) tagged with c-Myc to explore this mechanism. By immunoprecipitation using anti-c-Myc antibodies coupled with proteomics analysis, we have identified several proteins including non-muscle myosin IIA (NMIIA), heat shock protein (HSP)-70 and ubiquitin activating enzyme E1 in the immunoprecipitate. Employing yeast-two-hybrid analysis and pulldown experiments, we show that the C-terminal region of the NMIIA heavy chain binds to the 1-6 amino acids in the cytoplasmic tail of C2GnT-M. We have found that NMIIA co-localizes with C2GnT-M at the periphery of the Golgi. In addition, inhibition or knockdown of NMIIA prevents the brefeldin A-induced collapse of the Golgi as shown by the inhibition of the migration of both Giantin, a Golgi matrix protein, and C2GnT-M, a Golgi non-matrix protein, to the ER. In contrast, knockdown of HSP70 retains Giantin in the Golgi but moves C2GnT-M to the ER, a process also blocked by inhibition or knockdown of NMIIA. Also, the intracellular distribution of C2GnT-M is not affected by knockdown of β-coatomer protein with or without inhibition of HSPs, suggesting that the Golgi-to-ER trafficking of C2GnT-M does not depend on coat protein complex-I. Further, inhibition of proteasome results in accumulation of ubiquitinated C2GnT-M, suggesting its degradation by proteasome. Therefore, NMIIA and not coat protein complex-I is responsible for transporting the Golgi glycosyltransferase to the ER for proteasomal degradation. The data suggest that NMIIA is involved in the Golgi remodeling.

Who needs microtubules? Myogenic reorganization of MTOC, Golgi complex and ER exit sites persists despite lack of normal microtubule tracks

A wave of structural reorganization involving centrosomes, microtubules, Golgi complex and ER exit sites takes place early during skeletal muscle differentiation and completely remodels the secretory pathway. The mechanism of these changes and their functional implications are still poorly understood, in large part because all changes occur seemingly simultaneously. In an effort to uncouple the reorganizations, we have used taxol, nocodazole, and the specific GSK3-β inhibitor DW12, to disrupt the dynamic microtubule network of differentiating cultures of the mouse skeletal muscle cell line C2. Despite strong effects on microtubules, cell shape and cell fusion, none of the treatments prevented early differentiation. Redistribution of centrosomal proteins, conditional on differentiation, was in fact increased by taxol and nocodazole and normal in DW12. Redistributions of Golgi complex and ER exit sites were incomplete but remained tightly linked under all circumstances, and conditional on centrosomal reorganization. We were therefore able to uncouple microtubule reorganization from the other events and to determine that centrosomal proteins lead the reorganization hierarchy. In addition, we have gained new insight into structural and functional aspects of the reorganization of microtubule nucleation during myogenesis.

Import oligomers induce positive feedback to promote peroxisome differentiation and control organelle abundance

A fundamental question in cell biology is how cells control organelle composition and abundance. Woronin bodies are fungal peroxisomes centered on a crystalline core of the self-assembled HEX protein. Despite using the canonical peroxisome import machinery for biogenesis, Woronin bodies are scarce compared to the overall peroxisome population. Here, we show that HEX oligomers promote the differentiation of a subpopulation of peroxisomes, which become enlarged and highly active in matrix protein import. HEX physically associates with the essential matrix import peroxin, PEX26, and promotes its enrichment in the membrane of differentiated peroxisomes. In addition, a PEX26 mutant that disrupts differentiation produces increased numbers of aberrantly small Woronin bodies. Our data suggest a mechanism where HEX oligomers recruit a key component of the import machinery, which promotes the import of additional HEX. This type of positive feedback provides a basic mechanism for the production of an organelle subpopulation of distinct composition and abundance.

ERK7 is a negative regulator of protein secretion in response to amino-acid starvation by modulating Sec16 membrane association

RNAi screening for kinases regulating the functional organization of the early secretory pathway in Drosophila S2 cells has identified the atypical Mitotic-Associated Protein Kinase (MAPK) Extracellularly regulated kinase 7 (ERK7) as a new modulator. We found that ERK7 negatively regulates secretion in response to serum and amino-acid starvation, in both Drosophila and human cells. Under these conditions, ERK7 turnover through the proteasome is inhibited, and the resulting higher levels of this kinase lead to a modification in a site within the C-terminus of Sec16, a key ER exit site component. This post-translational modification elicits the cytoplasmic dispersion of Sec16 and the consequent disassembly of the ER exit sites, which in turn results in protein secretion inhibition. We found that ER exit site disassembly upon starvation is TOR complex 1 (TORC1) independent, showing that under nutrient stress conditions, cell growth is not only inhibited at the transcriptional and translational levels, but also independently at the level of secretion by inhibiting the membrane flow through the early secretory pathway. These results reveal the existence of new signalling circuits participating in the complex regulation of cell growth.

A novel laser nanosurgery approach supports de novo Golgi biogenesis in mammalian cells

The Golgi complex has a central role in the secretory pathway of all higher organisms. To explain the synthesis of its unique stacked structure in mammalian cells, two major models have been proposed. One suggests that it is synthesized de novo from the endoplasmic reticulum. The second model postulates a pre-existing Golgi template that serves as a scaffold for its biogenesis. To test these hypotheses directly, we have developed an approach in which we deplete the Golgi complex from living cells by laser nanosurgery, and subsequently analyze the 'Golgi-depleted' karyoplast using time-lapse and electron microscopy. We show that biosynthetic transport is blocked after Golgi depletion, but is restored 12 hours later. This recovery of secretory transport coincides with an ordered assembly of stacked Golgi structures, and we also observe the appearance of matrix proteins before that of Golgi enzymes. Functional experiments using RNA interference-mediated knockdown of GM130 further demonstrate the importance of the matrix during Golgi biogenesis. By contrast, the centrosome, which can also be removed by laser nanosurgery and is not reformed within the considered time frame, is not required for this process. Altogether, our data provide evidence that de novo Golgi biogenesis can occur in mammalian cells.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

Cellular and molecular biology of glycosphingolipid glycosylation

Brain tissue is characterized by its high glycosphingolipid content, particularly those containing sialic acid (gangliosides). As a result of this observation, brain tissue was a focus for studies leading to the characterization of the enzymes participating in ganglioside biosynthesis, and their participation in driving the compositional changes that occur in glycolipid expression during brain development. Later on, this focus shifted to the study of cellular aspects of the synthesis, which lead to the identification of the site of synthesis in the neuronal soma and their axonal transport toward the periphery. In this review article, we will focus in subcellular aspects of the biosynthesis of glycosphingolipid oligosaccharides, particularly the mechanisms underlying the trafficking of glycosphingolipid glycosyltransferases from the endoplasmic reticulum to the Golgi, those that promote their retention in the Golgi and those that participate in their topological organization as part of the complex membrane bound machinery for the synthesis of glycosphingolipids.

Organization of the synthesis of glycolipid oligosaccharides in the Golgi complex

Glycolipids constitute a complex family of amphipathic molecules structurally characterized by a hydrophilic mono- or oligo-saccharide moiety linked to a hydrophobic ceramide moiety. Due to their asymmetric distribution in cell membranes, exposing the saccharide moiety to the extracytoplasmic side of the cell, glycolipids participate in a variety of cell-cell and cell-ligand interactions. Here we summarize aspects of the cell biology of the stepwise synthesis of the saccharide moiety in the Golgi complex of cells from vertebrates. In particular we refer to the participant glycosyltransferases, with emphasis on their trafficking along the secretory pathway, their retention and organization in the Golgi complex membranes and their dependence on the Golgi complex ultra structural organization for proper function.

Role of Rab1b in COPII dynamics and function

In eukaryotic cells, proteins destined for secretion are translocated into the endoplasmic reticulum (ER) and packaged into so-called COPII-coated vesicles. In the ER exit sites (ERES), COPII has the capacity of deforming the lipid bilayer, where it modulates the selective sorting and concentration of cargo proteins. In this study, we analyze the involvement of Rab1b in COPII dynamics and function by expressing either the Rab1b negative-mutant (Rab1N121I) or the Rab1b GTP restricted mutant (Rab1Q67L), or performing short interference RNA-based knockdown. We show that Rab1b interacts with the COPII components Sec23, Sec24 and Sec31 and that Rab1b inhibition changes the COPII phenotype. FRAP assays reveal that Rab1b modulates COPII association/dissociation kinetics at the ERES interface. Furthermore, Rab1b inhibition delays cargo sorting at the ER exit sites. We postulate that Rab1b is a key regulatory component of COPII dynamics and function.

Identification of a site in Sar1 involved in the interaction with the cytoplasmic tail of glycolipid glycosyltransferases

Glycolipid glycosyltransferases (GGT) are transported from the endoplasmic reticulum (ER) to the Golgi, their site of residence, via COPII vesicles. An interaction of a (R/K)X(R/K) motif at their cytoplasmic tail (CT) with Sar1 is critical for the selective concentration in the transport vesicles. In this work using computational docking, we identify three putative binding pockets in Sar1 (sites A, B, and C) involved in the interaction with the (R/K)X(R/K) motif. Sar1 mutants with alanine replacement of amino acids in site A were tested in vitro and in cells. In vitro, mutant versions showed a reduced ability to bind immobilized peptides with the CT sequence of GalT2. In cells, Sar1 mutants (Sar1(D198A)) specifically affect the exiting of GGT from the ER, resulting in an ER/Golgi concentration ratio favoring the ER. Neither the typical Golgi localization of GM130 nor the exiting and transport of the G protein of the vesicular stomatitis virus were affected. The protein kinase inhibitor H89 produced accumulation of Sec23, Sar1, and GalT2 at the ER exit sites; Sar1(D189A) also accumulated at these sites, but in this case GalT2 remained disperse along ER membranes. The results indicate that amino acids in site A of Sar1 are involved in the interaction with the CT of GGT for concentration at ER exiting sites.

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.

Scyl1 regulates Golgi morphology

Background

Membrane trafficking is a defining feature of eukaryotic cells, and is essential for the maintenance of organelle homeostasis and identity. We previously identified Scy1-like 1 (Scyl1), a member of the Scy1-like family of catalytically inactive protein kinases, as a high-affinity binding partner of COPI coats. COPI-coated vesicles control Golgi to endoplasmic reticulum trafficking and we observed that disruption of Scyl1 function leads to a decrease in trafficking of the KDEL receptor via the COPI pathway. We reasoned that if Scyl1 plays a major role in COPI trafficking its disruption could influence Golgi homeostasis.

Methodology/Principal Findings

We performed Scyl1 knock down in cultured cells using previously established methods and observed an alteration in Golgi morphology. Both the surface area and volume of the Golgi is increased in Scyl1-depleted cells, but the continuity and polarity of the organelle is unperturbed. At the ultrastructural level we observe a decrease in the orderly structure of the Golgi with an increase in cisternal luminal width, while the number of Golgi cisternae remains unchanged. The golgin family of proteins forms a detergent resistant network that controls Golgi homeostasis. Disruption of this protein network by knock down of the golgin p115 disrupts the Golgi localization of Scyl1. Moreover, we find that Scyl1 interacts with 58K/formiminotransferase cyclodeaminase (FTCD), a protein that is tightly associated with the cis face of the Golgi.

Conclusions/Significance

Our results place Scyl1 at an interface between the golgin network and COPI trafficking and demonstrate that Scyl1 is required for the maintenance of Golgi morphology. Coupled with the observation from others that Scyl1 is the gene product responsible for the neurodegenerative mouse model mdf, our results additionally implicate the regulation of COPI trafficking and Golgi homeostasis in neurodegeneration.

Localization of Golgi-resident glycosyltransferases

For many glycosyltransferases, the information that instructs Golgi localization is located within a relatively short sequence of amino acids in the N-termini of these proteins comprising: the cytoplasmic tail, the transmembrane spanning region, and the stem region (CTS). Also, one enzyme may be more reliant on a particular region in the CTS for its localization than another. The predominance of these integral membrane proteins in the Golgi has seen these enzymes become central players in the development of membrane trafficking models of transport within this organelle. It is now understood that the means by which the characteristic distributions of glycosyltransferases arise within the subcompartments of the Golgi is inextricably linked to the mechanisms that cells employ to direct the flow of proteins and lipids within this organelle.

Discrete, continuous, and stochastic models of protein sorting in the Golgi apparatus

The Golgi apparatus plays a central role in processing and sorting proteins and lipids in eukaryotic cells. Golgi compartments constantly exchange material with each other and with other cellular components, allowing them to maintain and reform distinct identities despite dramatic changes in structure and size during cell division, development, and osmotic stress. We have developed three minimal models of membrane and protein exchange in the Golgi-a discrete, stochastic model, a continuous ordinary differential equation model, and a continuous stochastic differential equation model-each based on two fundamental mechanisms: vesicle-coat-mediated selective concentration of cargoes and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins during vesicle formation and SNARE-mediated selective fusion of vesicles. By exploring where the models differ, we hope to discover whether the discrete, stochastic nature of vesicle-mediated transport is likely to have appreciable functional consequences for the Golgi. All three models show similar ability to restore and maintain distinct identities over broad parameter ranges. They diverge, however, in conditions corresponding to collapse and reassembly of the Golgi. The results suggest that a continuum model provides a good description of Golgi maintenance but that considering the discrete nature of vesicle-based traffic is important to understanding assembly and disassembly of the Golgi. Experimental analysis validates a prediction of the models that altering guanine nucleotide exchange factor expression levels will modulate Golgi size.

Biogenesis of secretory organelles during B cell differentiation

The differentiation of B cells into Ig-secreting plasma cells requires the expansion of secretory organelles to cope with the increased cargo load. To evaluate the timeline of this process, we have quantitated the kinetics of secretory organelle expansion relative to Ig secretion and examined regulatory components of secretory transport following in vitro activation of human B lymphocytes. Unstimulated B cells contain minimal endomembranes. After activation, ER membrane induction appears as tightly packed spherical structures of 0.5-1 mum diameter concentrated in a juxtanuclear position. When the cells differentiate into plasmablasts, there is dramatic cell-size increase, but the ER remains concentrated close to the nucleus and only later fills the entire cell. In sharp contrast, previous studies in other cell types have found that the ER expands in synchrony with increasing cell size during interphase, by extension of ER tubules under the PM. In this study, the Golgi remains consistently as a single juxtanuclear structure but linearly expands sixfold in volume during B cell activation. Furthermore, following active cell proliferation, ER exit sites proliferate rapidly, increasing almost fourfold in number, in parallel with a sharp increase in Ig secretion. These findings demonstrate that the control of organelle biogenesis and expansion in primary human B cells are differentially regulated by cargo flux caused by Ig synthesis.

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.

Active mask segmentation of fluorescence microscope images

We propose a new active mask algorithm for the segmentation of fluorescence microscope images of punctate patterns. It combines the (a) flexibility offered by active-contour methods, (b) speed offered by multiresolution methods, (c) smoothing offered by multiscale methods, and (d) statistical modeling offered by region-growing methods into a fast and accurate segmentation tool. The framework moves from the idea of the "contour" to that of "inside and outside," or masks, allowing for easy multidimensional segmentation. It adapts to the topology of the image through the use of multiple masks. The algorithm is almost invariant under initialization, allowing for random initialization, and uses a few easily tunable parameters. Experiments show that the active mask algorithm matches the ground truth well and outperforms the algorithm widely used in fluorescence microscopy, seeded watershed, both qualitatively, as well as quantitatively.

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.

Chronic stress elicits prolonged activation of alpha-MSH secretion and subsequent degeneration of melanotroph

Prolonged stress affects homeostasis in various organs and induces stress-associated disorders. We examined the cellular changes of pituitary gland under the continuous stress condition using a rat model in which rats were kept in a cage filled with water to a height of 1.5 cm for up to 5 days. Among the pituitary hormone mRNAs, proopiomelanocortin mRNA was up-regulated specifically in the intermediate lobe (IL) of this rat model. Additionally, the peripheral blood levels of alpha-melanocyte stimulating hormone (alpha-MSH), a major product of proopiomelanocortin in IL were increased. The alpha-MSH secreting cells, melanotrophs, showed a markedly developed endoplasmic reticulum and Golgi apparatus in the early phase of the experiment. Subsequent continuous stress caused remarkable dilation of the endoplasmic reticulum, disruption of the Golgi structure, and the degeneration of some melanotrophs. In addition the dopaminergic nerve fibers from hypothalamus were markedly decreased in IL. A dopamine antagonist elicited the similar morphologic changes of melanotroph in normal rat. These findings suggest that prolonged stress suppressed hypothalamus-derived dopamine release in IL, which elicited over-secretion of alpha-MSH from the melanotrophs. The present study also suggests that prolonged hyperactivation of endocrine cells could lead to disorder of secretion mechanisms and eventual degeneration.

Dynamic organization of COPII coat proteins at endoplasmic reticulum export sites in plant cells

Protein export from the endoplasmic reticulum (ER) is mediated by the accumulation of COPII proteins such as Sar1, Sec23/24 and Sec13/31 at specialized ER export sites (ERES). Although the distribution of COPII components in mammalian and yeast systems is established, a unified model of ERES dynamics has yet to be presented in plants. To investigate this, we have followed the dynamics of fluorescent fusions to inner and outer components of the coat, AtSec24 and AtSec13, in three different plant model systems: tobacco and Arabidopsis leaf epidermis, as well as tobacco BY-2 suspension cells. In leaves, AtSec24 accumulated at Golgi-associated ERES, whereas AtSec13 showed higher levels of cytosolic staining compared with AtSec24. However, in BY-2 cells, both AtSec13 and AtSec24 labelled Golgi-associated ERES, along with AtSec24. To correlate the distribution of the COPII coat with the dynamics of organelle movement, quantitative live-cell imaging analyses demonstrated that AtSec24 and AtSec13 maintained a constant association with Golgi-associated ERES, irrespective of their velocity. However, recruitment of AtSec24 and AtSec13 to ERES, as well as the number of ERES marked by these proteins, was influenced by export of membrane cargo proteins from the ER to the Golgi. Additionally, the increased availability of AtSec24 affected the distribution of AtSec13, inducing recruitment of this outer COPII coat component to ERES. These results provide a model that, in plants, protein export from the ER occurs via sequential recruitment of inner and outer COPII components to form transport intermediates at mobile, Golgi-associated ERES.