Membrane flow through the Golgi apparatus: specific disassembly of the cis-Golgi network by ATP depletion

Early signs of neurodegenerative diseases: Possible mechanisms and targets for Golgi stress

The Golgi apparatus plays a crucial role in mediating the modification, transport, and sorting of intracellular proteins and lipids. The morphological changes occurring in the Golgi apparatus are exceptionally important for maintaining its function. When exposed to external pressure or environmental stimulation, the Golgi apparatus undergoes adaptive changes in both structure and function, which are known as Golgi stress. Although certain signal pathway responses or post-translational modifications have been observed following Golgi stress, further research is needed to comprehensively summarize and understand the related mechanisms. Currently, there is evidence linking Golgi stress to neurodegenerative diseases; however, the role of Golgi stress in the progression of neurodegenerative diseases such as Alzheimer's disease remains largely unexplored. This review focuses on the structural and functional alterations of the Golgi apparatus during stress, elucidating potential mechanisms underlying the involvement of Golgi stress in regulating immunity, autophagy, and metabolic processes. Additionally, it highlights the pivotal role of Golgi stress as an early signaling event implicated in the pathogenesis and progression of neurodegenerative diseases. Furthermore, this study summarizes prospective targets that can be therapeutically exploited to mitigate neurodegenerative diseases by targeting Golgi stress. These findings provide a theoretical foundation for identifying novel breakthroughs in preventing and treating neurodegenerative diseases.

Golgi apparatus dis- and reorganizations studied with the aid of 2-deoxy-D-glucose and visualized by 3D-electron tomography

We studied Golgi apparatus disorganizations and reorganizations in human HepG2 hepatoblastoma cells by using the nonmetabolizable glucose analogue 2-deoxy-d-glucose (2DG) and analyzing the changes in Golgi stack architectures by 3D-electron tomography. Golgi stacks remodel in response to 2DG-treatment and are replaced by tubulo-glomerular Golgi bodies, from which mini-Golgi stacks emerge again after removal of 2DG. The Golgi stack changes correlate with the measured ATP-values. Our findings indicate that the classic Golgi stack architecture is impeded, while cells are under the influence of 2DG at constantly low ATP-levels, but the Golgi apparatus is maintained in forms of the Golgi bodies and Golgi stacks can be rebuilt as soon as 2DG is removed. The 3D-electron microscopic results highlight connecting regions that interlink membrane compartments in all phases of Golgi stack reorganizations and show that the compact Golgi bodies mainly consist of continuous intertwined tubules. Connections and continuities point to possible new transport pathways that could substitute for other modes of traffic. The changing architectures visualized in this work reflect Golgi stack dynamics that may be essential for basic cell physiologic and pathologic processes and help to learn, how cells respond to conditions of stress.

Phenomenology based multiscale models as tools to understand cell membrane and organelle morphologies

An intriguing question in cell biology is "how do cells regulate their shape?" It is commonly believed that the observed cellular morphologies are a result of the complex interaction among the lipid molecules (constituting the cell membrane), and with a number of other macromolecules, such as proteins. It is also believed that the common biophysical processes essential for the functioning of a cell also play an important role in cellular morphogenesis. At the cellular scale-where typical dimensions are in the order of micrometers-the effects arising from the molecular scale can either be modeled as equilibrium or non-equilibrium processes. In this chapter, we discuss the dynamically triangulated Monte Carlo technique to model and simulate membrane morphologies at the cellular scale, which in turn can be used to investigate several questions related to shape regulation in cells. In particular, we focus on two specific problems within the framework of isotropic and anisotropic elasticity theories: namely, (i) the origin of complex, physiologically relevant, membrane shapes due to the interaction of the membrane with curvature remodeling proteins, and (ii) the genesis of steady state cellular shapes due to the action of non-equilibrium forces that are generated by the fission and fusion of transport vesicles and by the binding and unbinding of proteins from the parent membrane.

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.

The ceramide-enriched trans-Golgi compartments reorganize together with other parts of the Golgi apparatus in response to ATP-depletion

In this study, the ceramide-enriched trans-Golgi compartments representing sites of synthesis of sphingomyelin and higher organized lipids were visualized in control and ATP-depleted hepatoma and endothelial cells using internalization of BODIPY-ceramide and the diaminobenzidine photooxidation method for combined light-electron microscopical exploration. Metabolic stress induced by lowering the cellular ATP-levels leads to reorganizations of the Golgi apparatus and the appearance of tubulo-glomerular bodies and networks. The results obtained with three different protocols, in which BODIPY-ceramide either was applied prior to, concomitantly with, or after ATP-depletion, revealed that the ceramide-enriched compartments reorganize together with other parts of the Golgi apparatus under these conditions. They were found closely associated with and integrated in the tubulo-glomerular bodies formed in response to ATP-depletion. This is in line with the changes of the staining patterns obtained with the Helix pomatia lectin and the GM130 and TGN46 immuno-reactions occurring in response to ATP-depletion and is confirmed by 3D electron tomography. The 3D reconstructions underlined the glomerular character of the reorganized Golgi apparatus and demonstrated continuities of ceramide positive and negative parts. Most interestingly, BODIPY-ceramide becomes concentrated in compartments of the tubulo-glomerular Golgi bodies, even though the reorganization took place before BODIPY-ceramide administration. This indicates maintained functionalities although the regular Golgi stack organization is abolished; the results provide novel insights into Golgi structure-function relationships, which might be relevant for cells affected by metabolic stress.

Conserved motif of CDK5RAP2 mediates its localization to centrosomes and the Golgi complex

As the primary microtubule-organizing centers, centrosomes require gamma-tubulin for microtubule nucleation and organization. Located in close vicinity to centrosomes, the Golgi complex is another microtubule-organizing organelle in interphase cells. CDK5RAP2 is a gamma-tubulin complex-binding protein and functions in gamma-tubulin attachment to centrosomes. In this study, we find that CDK5RAP2 localizes to the Golgi complex in an ATP- and centrosome-dependent manner and associates with Golgi membranes independently of microtubules. CDK5RAP2 contains a centrosome-targeting domain with its core region highly homologous to the Motif 2 (CM2) of centrosomin, a functionally related protein in Drosophila. This sequence, referred to as the CM2-like motif, is also conserved in related proteins in chicken and zebrafish. Therefore, CDK5RAP2 may undertake a conserved mechanism for centrosomal localization. Using a mutational approach, we demonstrate that the CM2-like motif plays a crucial role in the centrosomal and Golgi localization of CDK5RAP2. Furthermore, the CM2-like motif is essential for the association of the centrosome-targeting domain to pericentrin and AKAP450. The binding with pericentrin is required for the centrosomal and Golgi localization of CDK5RAP2, whereas the binding with AKAP450 is required for the Golgi localization. Although the CM2-like motif possesses the activity of Ca(2+)-independent calmodulin binding, binding of calmodulin to this sequence is dispensable for centrosomal and Golgi association. Altogether, CDK5RAP2 may represent a novel mechanism for centrosomal and Golgi localization.

Organelle intermixing and membrane scrambling in cell death

In many cell types, intracellular organelles are involved along the progression of cell death. While many studies have focused on individual organelles such as mitochondria, evidence has accumulated that different organelles are simultaneously engaged in dynamic changes induced by death signaling before nuclear alterations are evident. This chapter examines approaches to evaluate dynamic aspects of organelle changes and intermixing during apoptosis. The methods presented here, which have been adapted from approaches used in the field of membrane traffic, enable the evaluation of mitochondrial intermixing with other organelles and the centrifugal movements of internal membranes that are associated, in particular, with death receptor-mediated apoptosis.

AMP-activated protein kinase phosphorylates Golgi-specific brefeldin A resistance factor 1 at Thr1337 to induce disassembly of Golgi apparatus

Sufficiency and depletion of nutrients regulate the cellular activities through the protein phosphorylation reaction; however, many protein substrates remain to be clarified. GBF1 (Golgi-specific brefeldin A resistance factor 1), a guanine nucleotide exchange factor for the ADP-ribosylation factor family associated with the Golgi apparatus, was isolated as a phosphoprotein from the glucose-depleted cells by using the phospho-Akt-substrate antibody, which recognizes the substrate proteins of several protein kinases. The phosphorylation of GBF1 was induced by 2-deoxyglucose (2-DG), which blocks glucose utilization and increases the intracellular AMP concentration, and by AICAR, an AMP-activated protein kinase (AMPK) activator. This phosphorylation was observed in the cells expressing the constitutively active AMPK. The 2-DG-induced phosphorylation of GBF1 was suppressed by Compound C, an AMPK inhibitor, and by the overexpression of the kinase-negative AMPK. Analysis using the deletion and point mutants identified Thr(1337) as the 2-DG-induced phosphorylation site in GBF1, which is phosphorylated by AMPK in vitro. ATP depletion is known to provoke the Golgi apparatus disassembly. Immunofluorescent microscopic analysis with the Golgi markers indicated that GBF1 associates with the fragmented Golgi apparatus in the cells treated with 2-DG and AICAR. The expression of the kinase-negative AMPK and the GBF1 mutant replacing Thr(1337) by Ala prevented the 2-DG-induced Golgi disassembly. These results indicate that GBF1 is a novel AMPK substrate and that the AMPK-mediated phosphorylation of GBF1 at Thr(1337) has a critical role, presumably by attenuating the function of GBF1, in the disassembly of the Golgi apparatus induced under stress conditions that lower the intracellular ATP concentration.

Tunicamycin preserves intercellular junctions, cytoarchitecture, and cell-substratum interactions in ATP-depleted epithelial cells

Pretreatment with the nucleoside antibiotic tunicamycin was found to protect cultured renal epithelial cells in the face of ATP-depletion, in large part by preserving junctional and cellular architecture. Tunicamycin pretreatment of Madin-Darby canine kidney cells not only preserved E-cadherin staining at the plasma membrane, but also inhibited ATP-depletion-mediated E-cadherin degradation. Electron microscopic analysis, together with the preservation of the staining patterns of the tight junction marker ZO-1, the apical/microvillar marker gp135, and basolateral marker Na/K-ATPase suggested that tunicamycin preserved the junctional complex and the polarized epithelial cell phenotype. Tunicamycin pretreatment also prevented reductions in the filamentous actin content of the cells, as well as preserving Golgi architecture. Moreover, a quantitative measure of cell adhesion demonstrated that tunicamycin pretreatment resulted in a fivefold increase in attachment of cells to the substratum (77% versus 16%). Thus, pretreatment with tunicamycin protects polarized epithelial cells from ischemic injury through the preservation of epithelial cell architecture, intercellular junctions, and cell-substratum interactions in the setting of intracellular ATP-depletion.

Fluoride causes reversible dispersal of Golgi cisternae and matrix in neuroendocrine cells

A role for heterotrimeric G proteins in the regulation of Golgi function and formation of secretory granules is generally accepted. We set out to study the effect of activation of heterotrimeric G proteins by aluminum fluoride on secretory granule formation in AtT-20 corticotropic tumor cells and in melanotrophs from the rat pituitary. In AtT-20 cells, treatment with aluminum fluoride or fluoride alone for 60 min induced complete dispersal of Golgi, ER-Golgi intermediate compartment and Golgi matrix markers, while betaCOP immunoreactiviy retained a juxtanuclear position and TGN38 was unaffected. Electron microscopy showed compression of Golgi cisternae followed by conversion of the Golgi stacks into clusters of tubular and vesicular elements. In the melanotroph of the rat pituitary a similar compression of Golgi cisternae was observed, followed by a progressive loss of cisternae from the stacks. As shown in other cells, brefeldin A induced redistribution of the Golgi matrix protein GM130 to punctate structures in the cytoplasm in AtT-20 cells, while mannosidase II immunoreactivity was completely dispersed. Fluoride induced a complete dispersal of mannosidase II and GM130 immunoreactivity. The effect of fluoride was fully reversible with reestablishment of normal mannosidase II and GM130 immunoreactivity within 2 h. After 1 h of recovery, showing varying stages of reassembly, the patterns of mannosidase II and GM130 immunoreactivity were identical in individual cells, indicating that Golgi matrix and cisternae reassemble with similar kinetics during recovery from fluoride treatment. Instead of a specific aluminum fluoride effect on secretory granule formation in the trans-Golgi network, we thus observe a unique form of Golgi dispersal induced by fluoride alone, possibly via its action as a phosphatase inhibitor.

Nucleotide depletion increases trafficking of gentamicin to the Golgi complex in LLC-PK1 cells

Having shown rapid trafficking of aminoglycosides to the Golgi complex in cell culture, we focused on the injurious interaction that occurs when gentamicin administration is preceded by renal ischemia. Using Texas red-labeled gentamicin as a tracer, we determined that 15 min of cellular nucleotide depletion did not significantly increase subsequent uptake. However, cells previously depleted of nucleotides accumulated significantly more Texas red-labeled gentamicin within a dispersed Golgi complex. Using Ricinus communis and Lens culinaris lectins, which label specific compartments of the Golgi complex (trans-Golgi network/trans and medial/cis compartments, respectively), we determined that the medial/cis compartment dispersed after 15 min of nucleotide depletion but the trans-Golgi network/trans compartment remained unaffected. An increase in the number of cells exhibiting disrupted medial/cis-Golgi morphology after repletion in physiological media containing gentamicin was also seen. In summary, the increase in nephrotoxicity seen when ischemia precedes aminoglycoside uptake may be part of a complex mechanism initially involving increased Golgi accumulation and prolonged Golgi dispersion. The Golgi complex must then endure the effects of gentamicin accumulated in larger quantities in an aberrant physiological state.