Cog2 null mutant CHO cells show defective sphingomyelin synthesis

Vti1a/b support distinct aspects of TGN and cis-/medial Golgi organization

Retrograde trafficking towards the trans-Golgi network (TGN) is important for dense core vesicle (DCV) biogenesis. Here, we used Vti1a/b deficient neurons to study the impact of disturbed retrograde trafficking on Golgi organization and cargo sorting. In Vti1a/b deficient neurons, staining intensity of cis-/medial Golgi proteins (e.g., GM130 and giantin) was increased, while the intensity of two recycling TGN proteins, TGN38 and TMEM87A, was decreased and the TGN-resident protein Golgin97 was normal. Levels and localization of DCV cargo markers, LAMP1 and KDEL were also altered. This phenotype was not caused by reduced Golgi size or absence of a TGN compartment. The phenotype was partially phenocopied by disturbing sphingolipid homeostasis, but was not rescued by overexpression of sphingomyelin synthases or the sphingolipid synthesis inhibitor myriocin. We conclude that Vti1a/b are important for distinct aspects of TGN and cis-/medial Golgi organization. Our data underline the importance of retrograde trafficking for Golgi organization, DCV cargo sorting and the distribution of proteins of the regulated secretory pathway.

Maintaining order: COG complex controls Golgi trafficking, processing, and sorting

The conserved oligomeric Golgi (COG) complex, a multisubunit tethering complex of the CATCHR (complexes associated with tethering containing helical rods) family, controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle targeting within the Golgi. In humans, COG defects lead to severe multisystemic diseases known as COG-congenital disorders of glycosylation (COG-CDG). The COG complex both physically and functionally interacts with all classes of molecules maintaining intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, and vesicular coats. Here, we review our current knowledge of COG-related trafficking and glycosylation defects in humans and model organisms, and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting.

More than just sugars: Conserved oligomeric Golgi complex deficiency causes glycosylation-independent cellular defects

The conserved oligomeric Golgi (COG) complex controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle trafficking within the Golgi. Human COG defects lead to severe multisystemic diseases known as COG-congenital disorders of glycosylation (COG-CDG). To gain better understanding of COG-CDGs, we compared COG knockout cells with cells deficient to 2 key enzymes, Alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase and uridine diphosphate-glucose 4-epimerase (GALE), which contribute to proper N- and O-glycosylation. While all knockout cells share similar defects in glycosylation, these defects only account for a small fraction of observed COG knockout phenotypes. Glycosylation deficiencies were not associated with the fragmented Golgi, abnormal endolysosomes, defective sorting and secretion or delayed retrograde trafficking, indicating that these phenotypes are probably not due to hypoglycosylation, but to other specific interactions or roles of the COG complex. Importantly, these COG deficiency specific phenotypes were also apparent in COG7-CDG patient fibroblasts, proving the human disease relevance of our CRISPR knockout findings. The knowledge gained from this study has important implications, both for understanding the physiological role of COG complex in Golgi homeostasis in eukaryotic cells, and for better understanding human diseases associated with COG/Golgi impairment.

Role and Function of Sphingomyelin Biosynthesis in the Development of Cancer

Sphingomyelin (SM) biosynthesis represents a complex, finely regulated process, mostly occurring in vertebrates. It is intimately linked to lipid transport and it is ultimately carried out by two enzymes, SM synthase 1 and 2, selectively localized in the Golgi and plasma membrane. In the course of the SM biosynthetic reaction, various lipids are metabolized. Because these lipids have both structural and signaling functions, the SM biosynthetic process has the potential to affect diverse important cellular processes (such as cell proliferation, cell survival, and migration). Thus defects in SM biosynthesis might directly or indirectly impact the normal physiology of the cell and eventually of the organism. In this chapter, we will focus on evidence supporting a role for SM biosynthesis in specific cellular functions and how its dysregulation can affect neoplastic transformation.

Glycans modify mesenchymal stem cell differentiation to impact on the function of resulting osteoblasts

Glycans are inherently heterogeneous, yet glycosylation is essential in eukaryotes, and glycans show characteristic cell type-dependent distributions. By using an immortalized human mesenchymal stromal cell (MSC) line model, we show that both N- and O-glycan processing in the Golgi functionally modulates early steps of osteogenic differentiation. We found that inhibiting O-glycan processing in the Golgi prior to the start of osteogenesis inhibited the mineralization capacity of the formed osteoblasts 3 weeks later. In contrast, inhibition of N-glycan processing in MSCs altered differentiation to enhance the mineralization capacity of the osteoblasts. The effect of N-glycans on MSC differentiation was mediated by the phosphoinositide-3-kinase (PI3K)/Akt pathway owing to reduced Akt phosphorylation. Interestingly, by inhibiting PI3K during the first 2 days of osteogenesis, we were able to phenocopy the effect of inhibiting N-glycan processing. Thus, glycan processing provides another layer of regulation that can modulate the functional outcome of differentiation. Glycan processing can thereby offer a novel set of targets for many therapeutically attractive processes.

Summary: Both N- and O-glycan processing modulate MSC differentiation early during osteogenesis to influence mineral formation. Inhibition of N-glycan processing increases mineralization.

Identification of a new B4GalNAcT1 (GM2/GD2/GA2 synthase) isoform, and regulation of enzyme stability and intracellular transport by arginine-based motif

Glycosphingolipids (GSLs) are abundant in plasma membranes of mammalian cells, and their synthesis is strictly regulated in the Golgi apparatus. Disruption of GSL homeostasis is the cause of numerous diseases. Hundreds of molecular species of GSLs exist, and the detailed mechanisms underlying their homeostasis remain unclear. We investigated the physiological significance of isoform production for β1,4-N-acetyl-galactosaminyl transferase 1/B4GALNT1 (B4GN1), an enzyme involved in synthesis of ganglio-series GSLs GM2/GD2/GA2. We discovered a new mRNA variant (termed variant 2) of B4GN1 through EST clone search. A new isoform, M1-B4GN1, which has an NH₂-terminal cytoplasmic tail longer than that of previously-known isoform M2-B4GN1, is translated from variant 2. M1-B4GN1 has R-based motif (a retrograde transport signal) in the cytoplasmic tail. M1-B4GN1 is partially localized in the endoplasmic reticulum (ER) depending on the R-based motif, whereas M2-B4GN1 is localized in the Golgi. Stability of M1-B4GN1 is higher than that of M2-B4GN1 because of the R-based motif. M2-B4GN1 forms a homodimer via disulfide bonding. When M1-B4GN1 and M2-B4GN1 were co-expressed in CHO-K1 cells, the two isoforms formed a heterodimer. The M1/M2-B4GN1 heterodimer was more stable than the M2-B4GN1 homodimer, but the heterodimer was not transported from the Golgi to the ER. Our findings indicate that stabilization of M1-B4GN1 homodimer and M1/M2-B4GN1 heterodimer by R-based motif is related to prolongation of Golgi retention, but not to retrograde transport from the Golgi to the ER. Coexistence of several B4GN1 isoforms having distinctive characteristics presumably helps maintain overall enzyme stability and GSL homeostasis.

Defects in the COG complex and COG-related trafficking regulators affect neuronal Golgi function

The Conserved Oligomeric Golgi (COG) complex is an evolutionarily conserved hetero-octameric protein complex that has been proposed to organize vesicle tethering at the Golgi apparatus. Defects in seven of the eight COG subunits are linked to Congenital Disorders of Glycosylation (CDG)-type II, a family of rare diseases involving misregulation of protein glycosylation, alterations in Golgi structure, variations in retrograde trafficking through the Golgi and system-wide clinical pathologies. A troublesome aspect of these diseases are the neurological pathologies such as low IQ, microcephaly, and cerebellar atrophy. The essential function of the COG complex is dependent upon interactions with other components of trafficking machinery, such as Rab-GTPases and SNAREs. COG-interacting Rabs and SNAREs have been implicated in neurodegenerative diseases like Alzheimer's disease and Parkinson's disease. Defects in Golgi maintenance disrupts trafficking and processing of essential proteins, frequently associated with and contributing to compromised neuron function and human disease. Despite the recent advances in molecular neuroscience, the subcellular bases for most neurodegenerative diseases are poorly understood. This article gives an overview of the potential contributions of the COG complex and its Rab and SNARE partners in the pathogenesis of different neurodegenerative disorders.

Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins

In recent decades, many sphingolipid enzymes, sphingolipid-metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide-1-phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four-phosphate adaptor protein 2 (FAPP2) and ceramide-1-phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.

2-Deoxy-D-glucose treatment changes the Golgi apparatus architecture without blocking synthesis of complex lipids

The classic Golgi apparatus organization, an arrangement of highly ordered cisternal stacks with tubular-vesicular membrane specializations on both sides, is the functional image of a continuous flow of contents and membranes with input, metabolization, and output in a dynamic steady state. In response to treatment with 2-deoxy-D-glucose (2-DG), which lowers the cellular ATP level by about 70% within minutes, this organization is rapidly replaced by tubular-glomerular membrane convolutes described as Golgi networks and bodies. 2-DG is a non-metabolizable glucose analogue and competitive inhibitor of glycolysis, which has become attractive in the context of therapeutic approaches for several kinds of tumors specifically targeting glycolysis in cancer. With the question of whether the functions of the Golgi apparatus in lipid synthesis would be influenced by the 2-DG-induced Golgi apparatus reorganization, we focused on lipid metabolism within the Golgi bodies. For this, we applied a fluorophore-labeled short-chain ceramide (BODIPY-Cer) in various combinations with 2-DG treatment to HepG2 cell cultures and followed uptake, enrichment and metabolization to higher ordered lipids. The cellular ATP status in each experiment was controlled with a bioluminescence assay, and the response of the Golgi apparatus was tracked by immunostaining of the trans-Golgi network protein TGN46. For electron microscopy, the fluorescent BODIPY-Cer signals were converted into electron-dense precipitates by photooxidation of diaminobenzidine (DAB); DAB precipitates labeled trans-Golgi areas in control cultures but also compartments at the periphery of the Golgi bodies formed in response to 2-DG treatment, thus indicating that concentration of ceramide takes place in spite of the Golgi apparatus reorganization. Lipid analyses by thin-layer chromatography (TLC) performed in parallel showed that BODIPY-Cer is not only concentrated in compartments of the 2-DG-induced Golgi bodies but is partly metabolized to BODIPY-sphingomyelin. Both, uptake and condensation of BODIPY-Cer and its conversion to complex lipids indicate that functions of the Golgi apparatus in the cellular lipid metabolism persist although the classic Golgi apparatus organization is abolished.

Target silencing of components of the conserved oligomeric Golgi complex impairs HIV-1 replication

All viruses require host cell factors to replicate. A large number of host factors have been identified that participate at numerous points of the human immunodeficiency virus 1 (HIV-1) life cycle. Recent evidence supports a role for components of the trans-Golgi network (TGN) in mediating early steps in the HIV-1 life cycle. The conserved oligomeric Golgi (COG) complex is a heteroctamer complex that functions in coat protein complex I (COPI)-mediated intra-Golgi retrograde trafficking and plays an important role in the maintenance of Golgi structure and integrity as well as glycosylation enzyme homeostasis. The targeted silencing of components of lobe B of the COG complex, namely COG5, COG6, COG7 and COG8, inhibited HIV-1 replication. This inhibition of HIV-1 replication preceded late reverse transcription (RT) but did not affect viral fusion. Silencing of the COG interacting protein the t-SNARE syntaxin 5, showed a similar defect in late RT product formation, strengthening the role of the TGN in HIV replication.

Inactivation of ceramide transfer protein during pro-apoptotic stress by Golgi disassembly and caspase cleavage

The mammalian Golgi apparatus is composed of multiple stacks of cisternal membranes organized laterally into a polarized ribbon. Furthermore, trans-Golgi membranes come in close apposition with ER (endoplasmic reticulum) membranes to form ER-trans-Golgi contact sites, which may facilitate transfer of newly synthesized ceramide from the ER to SM (sphingomyelin) synthase at the trans-Golgi via CERT (ceramide transfer protein). CERT interacts with both ER and Golgi membranes, and together with Golgi morphology contributes to efficient SM synthesis. In the present study, we show that Golgi disassembly during pro-apoptotic stress induced by TNFα (tumour necrosis factor α) and anisomycin results in decreased levels of CERT at the Golgi region. This is accompanied by a caspase-dependent loss of full-length CERT and reduction in de novo SM synthesis. In vitro, CERT is cleaved by caspases 2, 3 and 9. Truncated versions of CERT corresponding to fragments generated by caspase 2 cleavage at Asp213 were mislocalized and did not promote efficient de novo SM synthesis. Thus it is likely that during cellular stress, disassembly of Golgi structure together with inactivation of CERT by caspases causes a reduction in ceramide trafficking and SM synthesis, and could contribute to the cellular response to pro-apoptotic stress.

Chlamydia trachomatis hijacks intra-Golgi COG complex-dependent vesicle trafficking pathway

Chlamydia spp. are obligate intracellular bacteria that replicate inside the host cell in a bacterial modified unique compartment called the inclusion. As other intracellular pathogens, chlamydiae exploit host membrane trafficking pathways to prevent lysosomal fusion and to acquire energy and nutrients essential for their survival and replication. The Conserved Oligomeric Golgi (COG) complex is a ubiquitously expressed membrane-associated protein complex that functions in a retrograde intra-Golgi trafficking through associations with coiled-coil tethers, SNAREs, Rabs and COPI proteins. Several COG complex-interacting proteins, including Rab1, Rab6, Rab14 and Syntaxin 6 are implicated in chlamydial development. In this study, we analysed the recruitment of the COG complex and GS15-positive COG complex-dependent vesicles to Chlamydia trachomatis inclusion and their participation in chlamydial growth. Immunofluorescent analysis revealed that both GFP-tagged and endogenous COG complex subunits associated with inclusions in a serovar-independent manner by 8 h post infection and were maintained throughout the entire developmental cycle. Golgi v-SNARE GS15 was associated with inclusions 24 h post infection, but was absent on the mid-cycle (8 h) inclusions, indicating that this Golgi SNARE is directed to inclusions after COG complex recruitment. Silencing of COG8 and GS15 by siRNA significantly decreased infectious yield of chlamydiae. Further, membranous structures likely derived from lysed bacteria were observed inside inclusions by electron microscopy in cells depleted of COG8 or GS15. Our results showed that C. trachomatis hijacks the COG complex to redirect the population of Golgi-derived retrograde vesicles to inclusions. These vesicles likely deliver nutrients that are required for bacterial development and replication.

Mislocalization of large ARF-GEFs as a potential mechanism for BFA resistance in COG-deficient cells

Defects in subunits of the conserved oligomeric Golgi (COG) complex represent a growing subset of congenital disorders of glycosylation (CDGs). In addition to altered protein glycosylation and vesicular trafficking, Cog-deficient patient fibroblasts exhibit a striking delay in the Golgi-disrupting effects of brefeldin A (BFA). Despite the diagnostic value of this BFA resistance, the molecular basis of this response is not known. To investigate potential mechanisms of resistance, we analyzed the localization of the large ARF-GEF, GBF1, in several Cog-deficient cell lines. Our results revealed mislocalization of GBF1 to non-Golgi compartments, in particular the ERGIC, within these cells. Biochemical analysis of GBF1 in control and BFA-treated fibroblasts demonstrated that the steady-state level and membrane recruitment is not substantially affected by COG deficiency, supporting a role for the COG complex in the localization but not membrane association of GBF1. We also showed that pretreatment of fibroblasts with bafilomycin resulted in a GBF1-independent BFA resistance that appears additive with the resistance associated with COG deficiency. These data provide new insight into the mechanism of BFA resistance in Cog-deficient cells by suggesting a role for impaired ARF-GEF localization.

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.