How Camillo Golgi became “the Golgi”

On April 1898 Camillo Golgi communicated to the Medical-Surgical Society of Pavia, the discovery of the “internal reticular apparatus”, a novel intracellular organelle which he observed in nerve cells with the silver impregnation he had introduced for the staining of the nervous system. Soon after the discovery it became evident that this cellular component, which was also named the “Golgi apparatus”, was a ubiquitous structure in eukaryotic cells. However the reality of the organelle was questioned for years and many cytologists considered the internal reticular apparatus as an artefact due to the fixation and/or metallic impregnation procedure. The controversy was finally solved in the mid-1950s by electron microscopy when the Golgi apparatus definitely acquired its dignity of being a genuine cell organelle. The designation of “Golgi complex” entered officially in the literature in 1956. Both the terms Golgi apparatus and Golgi complex are currently interchangeable. However a quick “the Golgi” and the introduction of Golgi in adjectival form are now prevalent in the blooming scientific literature on the organelle. Thus Camillo Golgi underwent his final transformation and, becoming the eponym of the organelle he had discovered, he found a way to immortality.     

Golgi disassembly in apoptosis: cause or effect?

The Golgi complex undergoes a dramatic disassembly process during apoptosis. Some Golgi proteins implicated in Golgi structure and vesicle transport are cleaved during apoptosis, and expression of noncleavable mutants of these proteins delays Golgi disassembly after pro-apoptotic stimuli. Cleavage of Golgi structural proteins and subsequent disassembly of the organelle could simply be the result of the apoptotic process. However, recent studies raise the intriguing possibility that cleavage of Golgi proteins during apoptosis might be required for more than disassembly of the organelle.     

Golgi biogenesis

The Golgi is an essential membrane-bound organelle in the secretary pathway of eukaryotic cells. In mammalian cells, the Golgi stacks are integrated into a continuous perinuclear ribbon, which poses a challenge for the daughter cells to inherit this membrane organelle during cell division. To facilitate proper partitioning, the mammalian Golgi ribbon is disassembled into vesicles in early mitosis. Following segregation into the daughter cells, a functional Golgi is reformed. Here we summarize our current understanding of the molecular mechanisms that control the mitotic Golgi disassembly and postmitotic reassembly cycle in mammalian cells.     

A brefeldin A-like phenotype is induced by the overexpression of a human ERD-2-like protein, ELP-1.

Brefeldin A (BFA) is a unique drug affecting the molecular mechanisms that regulate membrane traffic and organelle structure. BFA's ability to alter retrograde traffic from the Golgi to the endoplasmic reticulum (ER) led us to ask whether the ERD-2 retrieval receptor, proposed to return escaped ER resident proteins from the Golgi, might either interfere with or mimic the effects of the drug. When either human ERD-2 or a novel human homolog (referred to as ELP-1) is overexpressed in a variety of cell types, the effects are phenotypically indistinguishable from the addition of BFA. These include the redistribution of the Golgi coat protein, beta-COP, to the cytosol, the loss of the Golgi apparatus as a distinct organelle, the mixing of this organelle with the ER, the addition of complex oligosaccharides to resident ER glycoproteins, and the block of anterograde traffic. Thus, these receptors may provide signals that regulate retrograde traffic between the Golgi and the ER.     

Deciphering the Golgi apparatus: from imaging to genes

The Golgi apparatus is a vital organelle in eukaryotic cells. It grabs and processes secretory materials synthesized by the endoplasmic reticulum (ER) before sorting them to their destination. The Golgi also receives materials from vacuoles/lysosomes and the plasma membrane for further recycling to other compartments within the cell (1) (Figure 1). Given the vital role of the Golgi in a cell, it is important to understand how this organelle attains and maintains its structural and functional integrity during the intense processes of membrane traffic. Despite an equally central role of the Golgi in membrane traffic in eukaryotes, the organization of this organelle has some unique features in each cell system. Therefore, the wealth of information available on the structure and activity of the Golgi in one system is not always directly transferable to others. However, certain morphological and functional aspects are common among cell systems. Therefore, studying the factors that regulate organelle biogenesis and organization of the Golgi apparatus is important in basic cell biology of eukaryotes and may also contribute to a better understanding of how different cell systems have evolved. In this study, we report on the identification of Golgi mutants in plant cells. We have developed a screen that is a promising strategy not only for the identification of genes responsible for the morphological and functional integrity of the plant Golgi but could also provide fundamental information on other multicellular systems for which the power of forward genetics cannot be exploited as easily as in Arabidopsis.     

Multiple activities for Arf1 at the Golgi complex

The Arf family of GTPases regulates membrane traffic and organelle structure. At the Golgi complex, Arf proteins facilitate membrane recruitment of many cytoplasmic coat proteins to allow sorting of membrane proteins for transport, stimulate the activity of enzymes that modulate the lipid composition of the Golgi, and assemble a cytoskeletal scaffold on the Golgi. Arf1 is the Arf family member most closely studied for its function at the Golgi complex. A number of regulators that activate and inactivate Arf1 on the Golgi have been described that localize to different regions of the organelle. This spatial distribution of Arf regulators may facilitate the recruitment of the coat proteins and other Arf effectors to different regions of the Golgi complex.     

Global approaches to study Golgi function

Enormous insights into Golgi function have been provided by yeast genetics, biochemical assays and immuno-labeling methods and the emerging picture is of a very complex organelle with multiple levels of regulation. Despite many elegant experimental approaches, it remains unclear what mechanisms transport secretory proteins and lipids through the Golgi, and even the basic structure of the organelle is debated. Recently, new, global approaches such as proteomics and functional genomics have been applied to study the Golgi and its matrix. The data produced reveals great complexity and has potential to help address major unresolved questions concerning Golgi function.     

Golgi apparatus partitioning during cell division

This review discusses the mitotic segregation of the Golgi apparatus. The results from classical biochemical and morphological studies have suggested that in mammalian cells this organelle remains distinct during mitosis, although highly fragmented through the formation of mitotic Golgi clusters of small tubules and vesicles. Shedding of free Golgi-derived vesicles would consume Golgi clusters and disperse this organelle throughout the cytoplasm. Vesicles could be partitioned in a stochastic and passive way between the two daughter cells and act as a template for the reassembly of this key organelle. This model has recently been modified by results obtained using GFP- or HRP-tagged Golgi resident enzymes, live cell imaging and electron microscopy. Results obtained with these techniques show that the mitotic Golgi clusters are stable entities throughout mitosis that partition in a microtubule spindle-dependent fashion. Furthermore, a newer model proposes that at the onset of mitosis, the Golgi apparatus completely loses its identity and is reabsorbed into the endoplasmic reticulum. This suggests that the partitioning of the Golgi apparatus is entirely dependent on the partitioning of the endoplasmic reticulum. We critically discuss both models and summarize what is known about the molecular mechanisms underlying the Golgi disassembly and reassembly during and after mitosis. We will also review how the study of the Golgi apparatus during mitosis in other organisms can answer current questions and perhaps reveal novel mechanisms.     

Golgi membrane dynamics and lipid metabolism

The striking morphology of the Golgi complex has fascinated cell biologists since its discovery over 100 years ago. Yet, despite intense efforts to understand how membrane flow relates to Golgi form and function, this organelle continues to baffle cell biologists and biochemists alike. Fundamental questions regarding Golgi function, while hotly debated, remain unresolved. Historically, Golgi function has been described from a protein-centric point of view, but we now appreciate that conceptual frameworks for how lipid metabolism is integrated with Golgi biogenesis and function are essential for a mechanistic understanding of this fascinating organelle. It is from a lipid-centric perspective that we discuss the larger question of Golgi dynamics and membrane trafficking. We review the growing body of evidence for how lipid metabolism is integrally written into the engineering of the Golgi system and highlight questions for future study.     

Golgi apparatus in parasitic protists (review of the literature)

This review summarizes modem data on Golgi apparatus of parasitic protists and demonstrates how the parasitic lifestyle determines functional and structural peculiarities of secretory systems in unrelated groups of unicellular parasites, in comparison to ones of "model systems", mammalian and yeast cells. The review covers the most well-studied protists, predominantly of high medical importance, belonging to following taxons: Parabasalia (Trichomonas), Diplomonada (Giardia), Entamoebidae (Entamoeba), parasitic Alveolata of the phyllum Apicomplexa (Toxoplasma and Plasmodium), and Kinetoplastida (Trypanosoma and Leishmania). Numerous recent publications demonstrated that studies on intracellular traffic in the mentioned above parasites essentially advanced our knowledge of Golgi function, traditionally based on research of cultured mammalian and yeast cells. Morphology of Golgi organelle in eukaryotes from various taxonomic groups has been compared. Within three of total six the highest taxons of Eukatyota (Adl et al., 2005) there exist at minimum eight groups represented by species lacking Golgi dictiosomes. However, biochemical and (or) molecular (genomic) evidences indicate that the organelle with functions of Golgi was present in every studied so far lineage of eukaryotes. Loss of Golgi organelle is a secondary event, which has been proven by identification of Golgi genes in the genomes of Golgi-lacking lineages. This loss might have occurred independently several times in the course of evolution. Neither the number of stacks, nor the size of the organelle correlates with intensity of secretion, or the position of the species on the evolutionary tree (in terms of presumably early/lately diverged lineages).     

Maintenance of Golgi structure and function depends on the integrity of ER export.

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitative fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We found that all classes of Golgi components are dynamically associated with this organelle, contrary to the prediction of the stable organelle model. Enzymes and recycling components are continuously exiting and reentering the Golgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without disrupting COPII-dependent ER export machinery (by brefeldin A treatment or expression of Arf1[T31N]), the Golgi structure disassembles, leaving no residual Golgi membranes. Rather, all Golgi components redistribute into the ER, the cytoplasm, or to ER exit sites still active for recruitment of selective membrane-bound and peripherally associated cargos. A similar phenomenon is induced by the constitutively active Sar1[H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a juxtanuclear region. In cells expressing Sar1[T39N], a constitutively inactive form of Sar1 that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These results argue against the hypothesis that the Golgi apparatus contains stable components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose membranes can be nucleated and are maintained by the activities of the Sar1-COPII and Arf1-coatomer systems.     

The Rab6 GTPase regulates recruitment of the dynactin complex to Golgi membranes

Dynactin is a multisubunit protein complex required for the activity of dynein in diverse intracellular motility processes, including membrane transport. Dynactin can bind to vesicles and liposomes containing acidic phospholipids, but general properties such as this are unlikely to explain the regulated recruitment of dynactin to specific sites on organelle membranes. Additional factors must therefore exist to control this process. Candidates for these factors are the Rab GTPases, which function in the tethering of vesicles to their target organelle prior to membrane fusion. In particular, Rab27a tethers melanosomes to the actin cytoskeleton. Other Rabs have been implicated in microtubule-dependent organelle motility; Rab7 controls lysosomal transport, and Rab6 is involved in microtubule-dependent transport pathways through the Golgi and from endosomes to the Golgi. We demonstrate that dynactin binds to Rab6 and shows a Rab6-dependent recruitment to Golgi membranes. Other Golgi Rabs do not bind to dynactin and are unable to support its recruitment to membranes. Rab6 therefore functions as a specificity or tethering factor controlling the recruitment of dynactin to membranes.     

Golgi Tubule Traffic and the Effects of Brefeldin A Visualized in Living Cells

The Golgi complex is a dynamic organelle engaged in both secretory and retrograde membrane traffic. Here, we use green fluorescent protein–Golgi protein chimeras to study Golgi morphology in vivo. In untreated cells, membrane tubules were a ubiquitous, prominent feature of the Golgi complex, serving both to interconnect adjacent Golgi elements and to carry membrane outward along microtubules after detaching from stable Golgi structures. Brefeldin A treatment, which reversibly disassembles the Golgi complex, accentuated tubule formation without tubule detachment. A tubule network extending throughout the cytoplasm was quickly generated and persisted for 5–10 min until rapidly emptying Golgi contents into the ER within 15–30 s. Both lipid and protein emptied from the Golgi at similar rapid rates, leaving no Golgi structure behind, indicating that Golgi membranes do not simply mix but are absorbed into the ER in BFA-treated cells. The directionality of redistribution implied Golgi membranes are at a higher free energy state than ER membranes. Analysis of its kinetics suggested a mechanism that is analogous to wetting or adsorptive phenomena in which a tension-driven membrane flow supplements diffusive transfer of Golgi membrane into the ER. Such nonselective, flow-assisted transport of Golgi membranes into ER suggests that mechanisms that regulate retrograde tubule formation and detachment from the Golgi complex are integral to the existence and maintenance of this organelle.     

The Golgi apparatus in parasitic protists

This review summarizes the current reports on the Golgi apparatus of parasitic protists. Numerous recent publications have demonstrated that studies on intracellular traffic in parasites essentially advanced our knowledge on the Golgi structure and function, which has been traditionally based on research on yeast and mammalian cultured cells. It has been reported that the parasitic lifestyle determines the functional and structural peculiarities of the secretory systems in unrelated groups of unicellular parasites that make them different from those in mammalian and yeast cells. This review covers the best-studied protists, predominantly those of high medical importance, belonging to the following taxa: Parabasalia (Trichomonas), Diplomonada (Giardia), Entamoebidae (Entamoeba), parasitic Alveolata of the phyllum Apicomplexa (Toxoplasma, Plasmodium), and Kinetoplastida (Trypanosoma, Leishmania). The morphology of the Golgi organelle in eukaryotes from various taxonomic groups has been compared. Within three of the six highest taxa of Eukaryota (Adl et al., 2005) a minimum of eight groups are represented by species lacking Golgi dictiosomes. However, biochemical and/or molecular (genomic) evidence indicate that an organelle with the functions of the Golgi was present in every lineage of eukaryotes studied thus far. Loss of the Golgi organelle is a secondary event as proven by identification of Golgi genes in the genomes of Golgi-lacking lineages. The loss might have occurred independently several times in evolution. Neither the number of stacks, nor the size of the organelle correlates with the intensity of secretion or the position of the species on the evolutionary tree (in terms of presumably early/lately diverged lineages).     

Cell cycle maintenance and biogenesis of the Golgi complex

How organelle identity is established and maintained, and how organelles divide and partition between daughter cells, are central questions of organelle biology. For the membrane-bound organelles of the secretory and endocytic pathways [including the endoplasmic reticulum (ER), Golgi complex, lysosomes, and endosomes], answering these questions has proved difficult because these organelles undergo continuous exchange of material. As a result, many "resident" proteins are not localized to a single site, organelle boundaries overlap, and when interorganellar membrane flow is interrupted, organelle structure is altered. The existence and identity of these organelles, therefore, appears to be a product of the dynamic processes of membrane trafficking and sorting. This is particularly true for the Golgi complex, which resides and functions at the crossroads of the secretory pathway. The Golgi receives newly synthesized proteins from the ER, covalently modifies them, and then distributes them to various final destinations within the cell. In addition, the Golgi recycles selected components back to the ER. These activities result from the Golgi's distinctive membranes, which are organized as polarized stacks (cis to trans) of flattened cisternae surrounded by tubules and vesicles. Golgi membranes are highly dynamic despite their characteristic organization and morphology, undergoing rapid disassembly and reassembly during mitosis and in response to perturbations in membrane trafficking pathways. How Golgi membranes fragment and disperse under these conditions is only beginning to be clarified, but is central to understanding the mechanism(s) underlying Golgi identity and biogenesis. Recent work, discussed in this review, suggests that membrane recycling pathways operating between the Golgi and ER play an indispensable role in Golgi maintenance and biogenesis, with the Golgi dispersing and reforming through the intermediary of the ER both in mitosis and in interphase when membrane cycling pathways are disrupted.     

Mobile factories: Golgi dynamics in plant cells

The plant Golgi apparatus plays a central role in the synthesis of cell wall material and the modification and sorting of proteins destined for the cell surface and vacuoles. Earlier perceptions of this organelle were shaped by static transmission electron micrographs and by its biosynthetic functions. However, it has become increasingly clear that many Golgi activities can only be understood in the context of its dynamic organization. Significant new insights have been gained recently into the molecules that mediate this dynamic behavior, and how this machinery differs between plants and animals or yeast. Most notable is the discovery that plant Golgi stacks can actively move through the cytoplasm along actin filaments, an observation that has major implications for trafficking to, through and from this organelle.     

Targeting of proteins to the Golgi apparatus

 The proteins that reside in the Golgi carry out functions associated with post-translational modifications, including glycosylation and proteolytic processing, membrane transport, recycling of endoplasmic reticulum proteins and maintenance of the structural organisation of the organelle itself. The latter includes Golgi stacking, interconnections between stacks and the microtubule-dependent positioning of the organelle within the cell. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recycling trans-Golgi network (TGN) proteins, peripheral membrane proteins and receptors. Considerable effort has been directed at understanding the basis of the localisation of Golgi glycosyltransferases and recycling TGN proteins; in both cases there is increasing evidence that multiple signals may be involved in their specific localisation. A number of models for the Golgi retention of glycosyltransferases have been proposed including oligomerisation, lipid-mediated sorting and intra-Golgi retrograde transport. More information is required to determine the contribution of each of these potential mechanisms in the targeting of different glycosyltransferases. Future work is also likely to focus on the relationship between the localisation of resident Golgi proteins and the maintenance of Golgi structure. Accepted: 15 October 1997     

The plant Golgi apparatus--going with the flow

The plant Golgi apparatus is composed of many separate stacks of cisternae which are often associated with the endoplasmic reticulum and which in many cell types are motile. In this review, we discuss the latest data on the molecular regulation of Golgi function. The concept of the Golgi as a distinct organelle is challenged and the possibility of a continuum between the endoplasmic reticulum and Golgi is proposed.     

The plant Golgi apparatus--going with the flow

The plant Golgi apparatus is composed of many separate stacks of cisternae which are often associated with the endoplasmic reticulum and which in many cell types are motile. In this review, we discuss the latest data on the molecular regulation of Golgi function. The concept of the Golgi as a distinct organelle is challenged and the possibility of a continuum between the endoplasmic reticulum and Golgi is proposed.     

Gut thoughts on the Golgi complex

The new millennium coincides within 1 year of Camillo Golgi's centennial celebrations. It is quite remarkable that the structure and formation of this organelle is as controversial today as was its mere existence from Golgi's time to the 1950s, when EM approaches were introduced. Since the late 1950s, two opposing models of Golgi structure and function have split the Golgi scientific community, namely vesicular transport versus organelle maturation. Although a few years ago Golgi maturation seemed to be 'out for the count', it has recently seen an almost messianic revival. In this review, I argue that this large-scale desertion from the vesicle transport model to the maturation camp is premature. I propose an alternative, dynamic steady-state model, in which transient tubular connections function in parallel to vesicular transport and that the biosynthetic pathway is made up of three major distinct compartments: the ER, the Golgi and the TGN.