Low-Temperature-Induced Golgi Tubules Are Transient Membranes Enriched in Molecules Regulating Intra-Golgi Transport

A real-time cell-binding assay reveals dynamic features of STxB-Gb3 cointernalization and STxB-mediated cargo delivery into cancer cells

The interaction between the Shiga toxin B-subunit (STxB) and its globotriaosylceramide receptor (Gb3) has a high potential for being exploited for targeted cancer therapy. The primary goal of this study was to evaluate the capacity of STxB to carry small molecules and proteins as cargo into cells. For this purpose, an assay was designed to provide real-time information about the StxB-Gb3 interaction as well as the dynamics and mechanism of the internalization process. The assay revealed the ability to distinguish the process of binding to the cell surface from internalization and presented the importance of receptor and STxB clustering for internalization. The overall setup demonstrated that the binding mechanism is complex, and the concept of affinity is difficult to apply. Hence, time-resolved methods, providing detailed information about the interaction of STxB with cells, are critical for the optimization of intracellular delivery.

Commonly used trafficking blocks disrupt ARF1 activation and the localization and function of specific Golgi proteins

Cold temperature blocks used to synchronize protein trafficking inhibit GBF1 function, leading to a decrease in ARF1-GTP levels and mislocalization of the ARF1 effector golgin-160. Several other, but not all, Golgi proteins including ARL1 also mislocalize. ARF1 activity and golgin-160 localization require more than 30 min to recover from these blocks.

ADP-ribosylation factor (ARF) proteins are key regulators of the secretory pathway. ARF1, through interacting with its effectors, regulates protein trafficking by facilitating numerous events at the Golgi. One unique ARF1 effector is golgin-160, which promotes the trafficking of only a specific subset of cargo proteins through the Golgi. While studying this role of golgin-160, we discovered that commonly used cold temperature blocks utilized to synchronize cargo trafficking (20 and 16°C) caused golgin-160 dispersal from Golgi membranes. Here, we show that the loss of golgin-160 localization correlates with a decrease in the levels of activated ARF1, and that golgin-160 dispersal can be prevented by expression of a GTP-locked ARF1 mutant. Overexpression of the ARF1 activator Golgi brefeldin A–resistant guanine nucleotide exchange factor 1 (GBF1) did not prevent golgin-160 dispersal, suggesting that GBF1 may be nonfunctional at lower temperatures. We further discovered that several other Golgi resident proteins had altered localization at lower temperatures, including proteins recruited by ARF-like GTPase 1 (ARL1), a small GTPase that also became dispersed in the cold. Although cold temperature blocks are useful for synchronizing cargo trafficking through the Golgi, our data indicate that caution must be taken when interpreting results from these assays.

Protein kinase A activity is necessary for fission and fusion of Golgi to endoplasmic reticulum retrograde tubules

It is becoming increasingly accepted that together with vesicles, tubules play a major role in the transfer of cargo between different cellular compartments. In contrast to our understanding of the molecular mechanisms of vesicular transport, little is known about tubular transport. How signal transduction molecules regulate these two modes of membrane transport processes is also poorly understood. In this study we investigated whether protein kinase A (PKA) activity regulates the retrograde, tubular transport of Golgi matrix proteins from the Golgi to the endoplasmic reticulum (ER). We found that Golgi-to-ER retrograde transport of the Golgi matrix proteins giantin, GM130, GRASP55, GRASP65, and p115 was impaired in the presence of PKA inhibitors. In addition, we unexpectedly found accumulation of tubules containing both Golgi matrix proteins and resident Golgi transmembrane proteins. These tubules were still attached to the Golgi and were highly dynamic. Our data suggest that both fission and fusion of retrograde tubules are mechanisms regulated by PKA activity.

Phospholipase D2 is involved in the formation of Golgi tubules and ArfGAP1 recruitment

Lipids and lipid-modifying enzymes play a key role in the biogenesis, maintenance and fission of transport carriers in the secretory and endocytic pathways. In the present study we demonstrate that phosphatidic acid generated by phospholipase D2 (PLD2) is involved in the formation of Golgi tubules. The main evidence to support this is: 1) inhibitors of phosphatidic acid formation and PLD2 depletion inhibit the formation of tubules containing resident enzymes and regulators of intra-Golgi transport in a low temperature (15°C) model of Golgi tubulation but do not affect brefeldin A-induced tubules, 2) inhibition of PLD2 enzymatic activity and PLD2 depletion in cells cultured under physiological conditions (37°C) induce the formation of tubules specifically containing Golgi matrix proteins, and, 3) over-expression of PLD2 induces the formation of a tubular network. In addition, it was found that the generation of this lipid by the isoenzyme is necessary for ArfGAP1 recruitment to Golgi membranes. These results suggest that both proteins are involved in the molecular mechanisms which drive the formation of different types of Golgi tubules.

Physiological and transcriptional responses of anaerobic chemostat cultures of Saccharomyces cerevisiae subjected to diurnal temperature cycles

Diurnal temperature cycling is an intrinsic characteristic of many exposed microbial ecosystems. However, its influence on yeast physiology and the yeast transcriptome has not been studied in detail. In this study, 24-h sinusoidal temperature cycles, oscillating between 12°C and 30°C, were imposed on anaerobic, glucose-limited chemostat cultures of Saccharomyces cerevisiae. After three diurnal temperature cycles (DTC), concentrations of glucose and extracellular metabolites as well as CO2 production rates showed regular, reproducible circadian rhythms. DTC also led to waves of transcriptional activation and repression, which involved one-sixth of the yeast genome. A substantial fraction of these DTC-responsive genes appeared to respond primarily to changes in the glucose concentration. Elimination of known glucose-responsive genes revealed an overrepresentation of previously identified temperature-responsive genes as well as genes involved in the cell cycle and de novo purine biosynthesis. In-depth analysis demonstrated that DTC led to a partial synchronization of the cell cycle of the yeast populations in chemostat cultures, which was lost upon release from DTC. Comparison of DTC results with data from steady-state cultures showed that the 24-h DTC was sufficiently slow to allow S. cerevisiae chemostat cultures to acclimate their transcriptome and physiology at the DTC temperature maximum and to approach acclimation at the DTC temperature minimum. Furthermore, this comparison and literature data on growth rate-dependent cell cycle phase distribution indicated that cell cycle synchronization was most likely an effect of imposed fluctuations of the relative growth rate (μ/μmax) rather than a direct effect of temperature.

A model for the self-organization of vesicular flux and protein distributions in the Golgi apparatus

The generation of two non-identical membrane compartments via exchange of vesicles is considered to require two types of vesicles specified by distinct cytosolic coats that selectively recruit cargo, and two membrane-bound SNARE pairs that specify fusion and differ in their affinities for each type of vesicles. The mammalian Golgi complex is composed of 6-8 non-identical cisternae that undergo gradual maturation and replacement yet features only two SNARE pairs. We present a model that explains how distinct composition of Golgi cisternae can be generated with two and even a single SNARE pair and one vesicle coat. A decay of active SNARE concentration in aging cisternae provides the seed for a cis[Formula: see text]trans SNARE gradient that generates the predominantly retrograde vesicle flux which further enhances the gradient. This flux in turn yields the observed inhomogeneous steady-state distribution of Golgi enzymes, which compete with each other and with the SNAREs for incorporation into transport vesicles. We show analytically that the steady state SNARE concentration decays exponentially with the cisterna number. Numerical solutions of rate equations reproduce the experimentally observed SNARE gradients, overlapping enzyme peaks in cis, medial and trans and the reported change in vesicle nature across the Golgi: Vesicles originating from younger cisternae mostly contain Golgi enzymes and SNAREs enriched in these cisternae and extensively recycle through the Endoplasmic Reticulum (ER), while the other subpopulation of vesicles contains Golgi proteins prevalent in older cisternae and hardly reaches the ER.

Golgi tubules: their structure, formation and role in intra-Golgi transport

Tubules are common Golgi elements that can form extensive networks associated with the cis-, lateral and trans-Golgi sides, but despite this, they have almost been forgotten for decades. The molecular mechanisms involved in their formation, elongation and fission are only just beginning to be understood. However, the role of these membranes is not well understood. In the present review, we analyze the mechanisms that induce Golgi tubulation or, conversely, disrupt tubules in order to throw some lights on the nature of these elements. The putative role of these elements in the framework of current models for intra-Golgi transport is also discussed.

Lipid phosphate phosphatase 3 participates in transport carrier formation and protein trafficking in the early secretory pathway

The inhibition of phosphatidic acid phosphatase (PAP) activity by propanolol indicates that diacylglycerol (DAG) is required for the formation of transport carriers at the Golgi and for retrograde trafficking to the ER. Here we report that the PAP2 family member lipid phosphate phosphatase 3 (LPP3, also known as PAP2b) localizes in compartments of the secretory pathway from ER export sites to the Golgi complex. The depletion of human LPP3: (i) reduces the number of tubules generated from the ER-Golgi intermediate compartment and the Golgi, with those formed from the Golgi being longer in LPP3-silenced cells than in control cells; (ii) impairs the Rab6-dependent retrograde transport of Shiga toxin subunit B from the Golgi to the ER, but not the anterograde transport of VSV-G or ssDsRed; and (iii) induces a high accumulation of Golgi-associated membrane buds. LPP3 depletion also reduces levels of de novo synthesized DAG and the Golgi-associated DAG contents. Remarkably, overexpression of a catalytically inactive form of LPP3 mimics the effects of LPP3 knockdown on Rab6-dependent retrograde transport. We conclude that LPP3 participates in the formation of retrograde transport carriers at the ER-Golgi interface, where it transitorily cycles, and during its route to the plasma membrane.

Intra-Golgi Transport: Roles for Vesicles, Tubules, and Cisternae

The Golgi complex is considered the central station of the secretory pathway where cargo proteins and lipids are properly modified, classified, packed into specific carriers and delivered to their final destinations. Early electron microscope studies showed the extraordinary structural complexity of this organelle. However, despite the large volume of incoming and outgoing traffic, it is able to maintain its architecture, although it is also flexible enough to adapt to the functional status of the cell. Many components of the molecular machinery involved in membrane traffic and other Golgi functions have been identified. However, some basic aspects of Golgi functioning remain unsolved. For instance, how cargo moves through the stack remains controversial and two classical models have been proposed: vesicular transport and cisternal maturation. Since neither of these models explains all the experimental data, a combination of these models as well as new models have been proposed. In this context, the specific role of the cisternae, vesicles and tubules needs to be clarified. In this review, we summarize our current knowledge of the Golgi organization and function, focusing on the mechanisms of intra-Golgi transport.

Morpho-functional architecture of the Golgi complex of neuroendocrine cells

In neuroendocrine cells, prohormones move from the endoplasmic reticulum to the Golgi complex (GC), where they are sorted and packed into secretory granules. The GC is considered the central station of the secretory pathway of proteins and lipids en route to their final destination. In most mammalian cells, it is formed by several stacks of cisternae connected by tubules, forming a continuous ribbon. This organelle shows an extraordinary structural and functional complexity, which is exacerbated by the fact that its architecture is cell type specific and also tuned by the functional status of the cell. It is, indeed, one the most beautiful cellular organelles and, for that reason, perhaps the most extensively photographed by electron microscopists. In recent decades, an exhaustive dissection of the molecular machinery involved in membrane traffic and other Golgi functions has been carried out. Concomitantly, detailed morphological studies have been performed, including 3D analysis by electron tomography, and the precise location of key proteins has been identified by immunoelectron microscopy. Despite all this effort, some basic aspects of Golgi functioning remain unsolved. For instance, the mode of intra-Golgi transport is not known, and two opposing theories (vesicular transport and cisternal maturation models) have polarized the field for many years. Neither of these theories explains all the experimental data so that new theories and combinations thereof have recently been proposed. Moreover, the specific role of the small vesicles and tubules which surround the stacks needs to be clarified. In this review, we summarize our current knowledge of the Golgi architecture in relation with its function and the mechanisms of intra-Golgi transport. Within the same framework, the characteristics of the GC of neuroendocrine cells are analyzed.

Conserved molecular mechanisms underlying homeostasis of the Golgi complex

The Golgi complex performs a central function in the secretory pathway in the sorting and sequential processing of a large number of proteins destined for other endomembrane organelles, the plasma membrane, or secretion from the cell, in addition to lipid metabolism and signaling. The Golgi apparatus can be regarded as a self-organizing system that maintains a relatively stable morphofunctional organization in the face of an enormous flux of lipids and proteins. A large number of the molecular players that operate in these processes have been identified, their functions and interactions defined, but there is still debate about many aspects that regulate protein trafficking and, in particular, the maintenance of these highly dynamic structures and processes. Here, we consider how an evolutionarily conserved underlying mechanism based on retrograde trafficking that uses lipids, COPI, SNAREs, and tethers could maintain such a homeodynamic system.

Regulation of ER-Golgi intermediate compartment tubulation and mobility by COPI coats, motor proteins and microtubules

Little is known about the formation and regulation of endoplasmic reticulum (ER)-Golgi transport intermediates, although previous studies suggest that cargo is the main regulator of their morphology. In this study, we analyze the role of coat protein I (COPI) and cytoskeleton in the formation of tubular ER-Golgi intermediate compartment (ERGIC) and also show that partial COPI detachment by means of low temperature (15 degrees C) or brefeldin A induces the formation of transient tubular ERGIC elements. Most of them moved from the cell periphery to the perinuclear area and were 2.5x slower than vesicles. Time-lapse analysis of living cells demonstrates that the ERGIC elements are able to shift very fast from tubular to vesicular forms and vice versa, suggesting that the amount of cargo is not the determining factor for ERGIC morphology. Both the partial microtubule depolymerization and the inhibition of uncoating of the membranes result in the formation of long tubules that grow from round ERGICs and form at complex network. Interestingly, both COPI detachment and microtubule depolymerization induce a redistribution of kinesin from peripheral ERGIC elements to the Golgi area, while dynein distribution is not affected. However, both kinesin and dynein downregulation by RNA interference induced ERGIC tubulation. The tubules induced by kinesin depletion were static, whereas those resulting from dynein depletion were highly mobile. Our results strongly suggest that the interaction of motor proteins with COPI-coated membranes and microtubules is a key regulator of ERGIC morphology and mobility.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

Heterotypic tubular connections at the endoplasmic reticulum-Golgi complex interface

Electron microscopy and cryoimmunocytochemistry were used to characterize tubular connections in the secretory pathway using rat spermatids as model. Our results support the existence of a complex tubular network enriched in the Golgi matrix protein GM130 that transiently joins the cis-Golgi side and the endoplasmic reticulum. These tubules occasionally contain the endoplasmic reticulum resident protein PDI but not COPII complexes or KDEL receptor. At the lateral edges of the stacks tubules were seen to connect cisternae belonging to the same or adjacent stacks. These connections were observed in all cisternae but preferentially on the cis side. Giantin, Gos28 and Rab6 were detected in the tubules; importantly, we reported the presence of cis-trans heterotypic connections between cisternae. On the trans-Golgi side, we occasionally observed tubules highly immunoreactive for Rab6 connecting the stack with the forming acrosome. Together, our results support the existence of transient continuities throughout the secretory pathways.

Low temperature (15 degrees C) induces COPII dissociation from membranes and slow exit from the endoplasmic reticulum in HeLa cells

Low temperature induces a transport blockade at the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) in cultured cells. Our previous studies support that the primary effect of low temperature is the detachment of COPI complexes from membranes. In the present study, we have used immunofluorescence and cryoimmunoelectron microscopy to investigate the effects of low temperature on both COPII and clathrin coat complexes in HeLa cells. Strikingly, COPII proteins moved from membranes to the cytosol at 15 degrees C, accumulating into electron-dense areas. In agreement with this observation, we also showed that ER exit is delayed in cells cultured at this temperature. In contrast, clathrin coat is not affected. Together, our results demonstrate that low temperature induces COPII dissociation from membranes and slow exit from the endoplasmic reticulum.