Are Rab proteins the link between Golgi organization and membrane trafficking?

Role of Rab10 in cocaine-induced behavioral effects is associated with GABAB receptor membrane expression in the nucleus accumbens

Aim

Previous studies have demonstrated that Ras-related GTP-binding protein Rab10 (Rab10) plays a role in psychostimulant-induced behavioral effects. In this study, we showed that Rab10 in the nucleus accumbens (NAc) of male animals affects the development of cocaine-induced behavioral effects, which are associated with the plasma membrane expression of the GABAB heteroreceptor (GABABR).

Methods

We performed flow cytometry, immunoendocytosis, pHluorin activity analysis, electrophysiology analysis, and open-field testing to explore the role of Rab10 in modulating the membrane expression and function of GABABR and its regulatory effect on cocaine-induced behavioral effects.

Results

Transcriptomics analysis showed that Rab10 was elevated following acute cocaine treatment. Membrane levels of Rab10 increased within day 1 of the cocaine treatment, subsequently decreasing at later time points. Rab10 deficiency in NAc regions significantly increased cocaine-inhibited membrane GABABR levels and inhibited cocaine-induced hyperlocomotion and behavioral sensitization. In addition, GAD 67 + -expressing neurons from NAc regions treated with cocaine revealed a significant decrease in Rab10 membrane expression. Furthermore, NAc neuron-specific Rab10 knockout resulted in a significant increase in the cocaine-inhibited membrane expression of GABABR, along with increased miniature inhibitory postsynaptic current (mIPSC) amplitude and attenuation of baclofen-amplified Ca2+ influx.

Conclusion

These results uncover a new mechanism in which Rab10-GABABR signaling may serve as a potential pathway for regulating cocaine-induced behavioral effects.

Benzo[a]pyrene exposure disrupts the organelle distribution and function of mouse oocytes

Benzo[a]pyrene (BaP) is a polycyclic aromatic hydrocarbon compound that is generated during combustion processes, and is present in various substances such as foods, tobacco smoke, and burning emissions. BaP is extensively acknowledged as a highly carcinogenic substance to induce multiple forms of cancer, such as lung cancer, skin cancer, and stomach cancer. Recently it is shown to adversely affect the reproductive system. Nevertheless, the potential toxicity of BaP on oocyte quality remains unclear. In this study, we established a BaP exposure model via mouse oral gavage and found that BaP exposure resulted in a notable decrease in the ovarian weight, number of GV oocytes in ovarian, and oocyte maturation competence. BaP exposure caused ribosomal dysfunction, characterized by a decrease in the expression of RPS3 and HPG in oocytes. BaP exposure also caused abnormal distribution of the endoplasmic reticulum (ER) and induced ER stress, as indicated by increased expression of GRP78. Besides, the Golgi apparatus exhibited an abnormal localization pattern, which was confirmed by the GM130 localization. Disruption of vesicle transport processes was observed by the abnormal expression and localization of Rab10. Additionally, an enhanced lysosome and LC3 fluorescence intensity indicated the occurrence of protein degradation in oocytes. In summary, our results suggested that BaP exposure disrupted the distribution and functioning of organelles, consequently affecting the developmental competence of mouse oocytes.

Long-term ketogenic diet causes hyperlipidemia, liver dysfunction, and glucose intolerance from impaired insulin trafficking and secretion in mice

A ketogenic diet (KD) is a very low-carbohydrate, very high-fat diet proposed to treat obesity and type 2 diabetes. While KD grows in popularity, its effects on metabolic health are understudied. Here we show that, in male and female mice, while KD protects against weight gain and induces weight loss, over long-term, mice develop hyperlipidemia, hepatic steatosis, and severe glucose intolerance. Unlike high fat diet-fed mice, KD mice are not insulin resistant and have low levels of insulin. Hyperglycemic clamp and ex vivo GSIS revealed cell-autonomous and whole-body impairments in insulin secretion. Major ER/Golgi stress and disrupted ER-Golgi protein trafficking was indicated by transcriptomic profiling of KD islets and confirmed by electron micrographs showing a dilated Golgi network likely responsible for impaired insulin granule trafficking and secretion. Overall, our results suggest long-term KD leads to multiple aberrations of metabolic parameters that caution its systematic use as a health promoting dietary intervention.

Correlation between Rab3A Expression and Sperm Kinematic Characteristics

Ras-related (Rab) proteins, integral members of the monomeric G-protein family, play a pivotal role in regulating intracellular vesicular transport. These proteins contribute to male reproductive processes, specifically in acrosome formation, exocytosis, and sperm motility. Although a prior study indicated a correlation between Rab3A and sperm motility, including motion kinematic parameters such as mean dance, this association has only been explored within a limited sample size. Therefore, further verification is required to confirm the correlation between Rab3A and sperm motility parameters. In the present study, Rab3A expression, sperm motility, and motion kinematic parameters were analyzed in 150 boar spermatozoa. Additionally, correlations between Rab3A expression and sperm kinematic characteristics were evaluated statistically. The results revealed significant associations between Rab3A protein expression levels and various motion kinematic parameters. Specifically, Rab3A levels exhibited positive correlations with average path velocity (p <0.05), mean amplitude of lateral head displacement (p <0.05), and curvilinear velocity (p <0.01). Consequently, it is proposed that Rab3A protein plays a crucial role in male fertility through its correlation with sperm kinematic characteristics, making it a potential marker for sperm motility-related assessments.

Presynaptic Precursor Vesicles-Cargo, Biogenesis, and Kinesin-Based Transport across Species

The faithful formation and, consequently, function of a synapse requires continuous and tightly controlled delivery of synaptic material. At the presynapse, a variety of proteins with unequal molecular properties are indispensable to compose and control the molecular machinery concerting neurotransmitter release through synaptic vesicle fusion with the presynaptic membrane. As presynaptic proteins are produced mainly in the neuronal soma, they are obliged to traffic along microtubules through the axon to reach the consuming presynapse. This anterograde transport is performed by highly specialised and diverse presynaptic precursor vesicles, membranous organelles able to transport as different proteins such as synaptic vesicle membrane and membrane-associated proteins, cytosolic active zone proteins, ion-channels, and presynaptic membrane proteins, coordinating synaptic vesicle exo- and endocytosis. This review aims to summarise and categorise the diverse and numerous findings describing presynaptic precursor cargo, mode of trafficking, kinesin-based axonal transport and the molecular mechanisms of presynaptic precursor vesicles biogenesis in both vertebrate and invertebrate model systems.

Knockdown of TMEM30A in renal tubular epithelial cells leads to reduced glucose absorption

The kidney reabsorbs large amounts of glucose through Na+-glucose cotransporter 2 (SGLT2). P4-ATPase acts together with the β-subunit TMEM30A to mediate the asymmetric distribution of phosphatidylserine (PS), phosphatidylethanolamine (PE), and other amino phospholipids, promoting plasma membrane and internal vesicle fusion, and facilitating vesicle protein transport. We observed reduced TMEM30A expression in renal tubules of DKD and IgA patients, suggesting a potential role of TMEM30A in renal tubular cells. To investigate the role of TMEM30A in renal tubules, we constructed a TMEM30A knockdown cell model by transfecting mouse kidney tubular epithelium cells (TCMK-1) with TMEM30A shRNA. Knockdown of TMEM30A in TCMK-1 cells attenuated vesicle transporter protein synthesis, resulting in reduced transport and expression of SGLT2, which in turn reduced glucose absorption. These data suggested that TMEM30A plays a crucial role in renal tubules.

Rab2 stimulates LC3 lipidation on secretory membranes by noncanonical autophagy

The Golgi complex is a highly dynamic organelle that regulates various cellular activities and yet maintains a distinct structure. Multiple proteins participate in Golgi structure/organization including the small GTPase Rab2. Rab2 is found on the cis/medial Golgi compartments and the endoplasmic reticulum-Golgi intermediate compartment. Interestingly, Rab2 gene amplification occurs in a wide range of human cancers and Golgi morphological alterations are associated with cellular transformation. To learn how Rab2 'gain of function' influences the structure/activity of membrane compartments in the early secretory pathway that may contribute to oncogenesis, NRK cells were transfected with Rab2B cDNA. We found that Rab2B overexpression had a dramatic effect on the morphology of pre- and early Golgi compartments that resulted in a decreased transport rate of VSV-G in the early secretory pathway. We monitored the cells for the autophagic marker protein LC3 based on the findings that depressed membrane trafficking affects homeostasis. Morphological and biochemical studies confirmed that Rab2 ectopic expression stimulated LC3-lipidation on Rab2-containing membranes that was dependent on GAPDH and utilized a non-canonical LC3-conjugation mechanism that is nondegradative. Golgi structural alterations are associated with changes in Golgi-associated signalling pathways. Indeed, Rab2 overexpressing cells had elevated Src activity. We propose that increased Rab2 expression facilitates cis Golgi structural changes that are maintained and tolerated by the cell due to LC3 tagging, and subsequent membrane remodeling triggers Golgi associated signaling pathways that may contribute to oncogenesis.

Analysis of networks in the dorsolateral prefrontal cortex in chronic schizophrenia: Relevance of altered immune response

The dorsolateral prefrontal cortex (DLPFC) has a crucial role in cognitive functioning and negative symptoms in schizophrenia. However, limited information of altered protein networks is available in this region in schizophrenia. We performed a proteomic analysis using single-shot liquid chromatography-tandem mass spectrometry of grey matter of postmortem DLPFC in chronic schizophrenia subjects (n = 20) and unaffected subjects (n = 20) followed by bioinformatic analysis to identify altered protein networks in schizophrenia (PXD024939 identifier in ProteomeXchange repository). Our results displayed a proteome profile in the DLPFC of 1989 proteins. 43 proteins were found significantly altered in schizophrenia. Analysis of this panel showed an enrichment of biological processes implicated in vesicle-mediated transport, processing and antigen presentation via MHC class II, intracellular transport and selenium metabolism. The enriched identified pathways were MHC class II antigen presentation, vesicle-mediated transport, Golgi ER retrograde transport, Nef mediated CD8 downregulation and the immune system. All these enriched categories were found to be downregulated. Furthermore, our network analyses showed crosstalk between proteins involved in MHC class II antigen presentation, membrane trafficking, Golgi-to-ER retrograde transport, Nef-mediated CD8 downregulation and the immune system with only one module built by 13 proteins. RAB7A showed eight interactions with proteins of all these pathways. Our results provide an altered molecular network involved in immune response in the DLPFC in schizophrenia with a central role of RAB7A. These results suggest that RAB7A or other proteins of this network could be potential targets for novel pharmacological strategies in schizophrenia for improving cognitive and negative symptoms.

Conserved but not critical: Trafficking and function of NaV1.7 are independent of highly conserved polybasic motifs

Non-addictive treatment of chronic pain represents a major unmet clinical need. Peripheral voltage-gated sodium (NaV) channels are an attractive target for pain therapy because they initiate and propagate action potentials in primary afferents that detect and transduce noxious stimuli. NaV1.7 sets the gain on peripheral pain-signaling neurons and is the best validated peripheral ion channel involved in human pain, and previous work has shown that it is transported in vesicles in sensory axons which also carry Rab6a, a small GTPase known to be involved in vesicular packaging and axonal transport. Understanding the mechanism of the association between Rab6a and NaV1.7 could inform therapeutic modalities to decrease trafficking of NaV1.7 to the distal axonal membrane. Polybasic motifs (PBM) have been shown to regulate Rab-protein interactions in a variety of contexts. In this study, we explored whether two PBMs in the cytoplasmic loop that joins domains I and II of human NaV1.7 were responsible for association with Rab6a and regulate axonal trafficking of the channel. Using site-directed mutagenesis we generated NaV1.7 constructs with alanine substitutions in the two PBMs. Voltage-clamp recordings showed that the constructs retain wild-type like gating properties. Optical Pulse-chase Axonal Long-distance (OPAL) imaging in live sensory axons shows that mutations of these PBMs do not affect co-trafficking of Rab6a and NaV1.7, or the accumulation of the channel at the distal axonal surface. Thus, these polybasic motifs are not required for interaction of NaV1.7 with the Rab6a GTPase, or for trafficking of the channel to the plasma membrane.

Hesperetin Ameliorates Inhibition of Neuronal and Oligodendroglial Cell Differentiation Phenotypes Induced by Knockdown of Rab2b, an Autism Spectrum Disorder-Associated Gene Product

Autism spectrum disorder (ASD) is a central nervous system (CNS) neurodevelopmental disorder that includes autism, pervasive developmental disorder, and Asperger’s syndrome. ASD is characterized by repetitive behaviors and social communication deficits. ASD is thought to be a multifactorial disorder with a range of genetic and environmental factors/candidates. Among such factors is the rab2b gene, although it remains unclear how Rab2b itself is related to the CNS neuronal and glial developmental disorganization observed in ASD patients. Rab2 subfamily members regulate intracellular vesicle transport between the endoplasmic reticulum and the Golgi body. To the best of our knowledge, we are the first to report that Rab2b positively regulates neuronal and glial cell morphological differentiation. Knockdown of Rab2b inhibited morphological changes in N1E-115 cells, which are often used as the neuronal cell differentiation model. These changes were accomplished with decreased expression levels of marker proteins in neuronal cells. Similar results were obtained for FBD-102b cells, which are used as the model of oligodendroglial cell morphological differentiation. In contrast, knockdown of Rab2a, which is another Rab2 family member not known to be associated with ASD, affected only oligodendroglial and not neuronal morphological changes. In contrast, treatment with hesperetin, a citrus flavonoid with various cellular protective effects, in cells recovered the defective morphological changes induced by Rab2b knockdown. These results suggest that knockdown of Rab2b inhibits differentiation in neuronal and glial cells and may be associated with pathological cellular phenotypes in ASD and that hesperetin can recover their phenotypes at the in vitro level at least.

Clinical, radiological and molecular studies in 24 individuals with Dyggve-Melchior-Clausen dysplasia and Smith-McCort dysplasia from India

BACKGROUND

Dyggve-Melchior-Clausen dysplasia (DMC) and Smith-McCort dysplasia (SMC types 1 and 2) are rare spondyloepimetaphyseal dysplasias with identical radiological findings. The presence of intellectual disability in DMC and normal intellect in SMC differentiates the two. DMC and SMC1 are allelic and caused by homozygous or compound heterozygous variants in DYM. SMC2 is caused by variations in RAB33B. Both DYM and RAB33B are important in intravesicular transport and function in the Golgi apparatus.

METHODS

Detailed clinical phenotyping and skeletal radiography followed by molecular testing were performed in all affected individuals. Next-generation sequencing and Sanger sequencing were used to confirm DYM and RAB33B variants. Sanger sequencing of familial variants was done in all parents.

RESULTS

24 affected individuals from seven centres are described. 18 had DMC and 6 had SMC2. Parental consanguinity was present in 15 of 19 (79%). Height <3 SD and gait abnormalities were seen in 20 and 14 individuals, respectively. The characteristic radiological findings of lacy iliac crests and double-humped vertebral bodies were seen in 96% and 88% of the affected. Radiological findings became attenuated with age. 23 individuals harboured biallelic variants in either DYM or RAB33B. Fourteen different variants were identified, out of which 10 were novel. The most frequently occurring variants in this group were c.719 C>A (3), c.1488_1489del (2), c.1484dup (2) and c.1563+2T>C (2) in DYM and c.400C>T (2) and c.186del (2) in RAB33B. The majority of these have not been reported previously.

CONCLUSION

This large cohort from India contributes to the increasing knowledge of clinical and molecular findings in these rare 'Golgipathies'.

Prediction of male fertility using Ras-related proteins

Identifying effective biomarkers for the diagnosis of male fertility is crucial for improving animal production and treating male infertility in humans. Ras-related proteins (Rab) are associated with morphological and motion kinematic functions in spermatozoa. Moreover, Rab2A, a Rab protein, is a possible male fertility-related biomarker. The present study was designed to identify additional fertility-related biomarkers among the various Rab proteins. First, the expression of Rab proteins (Rab3A, 4, 5, 8A, 9, 14, 25, 27A, and 34A) from 31 duroc boar spermatozoa was measured before and after capacitation; correlation between Rab protein expression and litter size was evaluated by statistical analysis. The results showed that the expression of Rab3A, 4, 5, 8A, 9, and 25 before capacitation and Rab3A, 4, 5, 8A, 9, and 14 after capacitation were negatively correlated with litter size. Moreover, depending on the cut-off values calculated by receiver operating curves, an increase in litter size was observed when evaluating the ability of the Rab proteins to forecast litter size. Therefore, we suggest that Rab proteins may be potential fertility-related biomarkers that could help select superior sires in the livestock industry.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

A G-Protein-Coupled Receptor Modulates Gametogenesis via PKG-Mediated Signaling Cascade in Plasmodium berghei

Gametogenesis is essential for malaria parasite transmission, but the molecular mechanism of this process remains to be refined. Here, we identified a G-protein-coupled receptor 180 (GPR180) that plays a critical role in signal transduction during gametogenesis in Plasmodium. The P. berghei GPR180 was predominantly expressed in gametocytes and ookinetes and associated with the plasma membrane in female gametes and ookinetes. Knockout of pbgpr180 (Δpbgpr180) had no noticeable effect on blood-stage development but impaired gamete formation and reduced transmission of the parasites to mosquitoes. Transcriptome analysis revealed that a large proportion of the dysregulated genes in the Δpbgpr180 gametocytes had assigned functions in cyclic nucleotide signal transduction. In the Δpbgpr180 gametocytes, the intracellular cGMP level was significantly reduced, and the cytosolic Ca2+ mobilization showed a delay and a reduction in the magnitude during gametocyte activation. These results suggest that PbGPR180 functions upstream of the cGMP-protein kinase G-Ca2+ signaling pathway. In line with this functional prediction, the PbGPR180 protein was found to interact with several transmembrane transporter proteins and the small GTPase Rab6 in activated gametocytes. Allele replacement of pbgpr180 with the P. vivax ortholog pvgpr180 showed equal competence of the transgenic parasite in sexual development, suggesting functional conservation of this gene in Plasmodium spp. Furthermore, an anti-PbGPR180 monoclonal antibody and the anti-PvGPR180 serum possessed robust transmission-blocking activities. These results indicate that GPR180 is involved in signal transduction during gametogenesis in malaria parasites and is a promising target for blocking parasite transmission. IMPORTANCE Environmental changes from humans to mosquitoes activate gametogenesis of the malaria parasite, an obligative process for parasite transmission, but how the signals are relayed remains poorly understood. Here, we show the identification of a Plasmodium G-protein-coupled receptor, GPR180, and the characterization of its function in gametogenesis. In P. berghei, GPR180 is dispensable for asexual development and gametocytogenesis, but its deletion impairs gametogenesis and reduces transmission to mosquitoes. GPR180 appears to function upstream of the cGMP-protein kinase G-Ca2+ signaling pathway and is required for the maximum activity of this pathway. Genetic complementation shows that the GPR180 ortholog from the human malaria parasite P. vivax was fully functional in P. berghei, indicating functional conservation of GPR180 in Plasmodium spp. With predominant expression and membrane association of GPR180 in sexual stages, GPR180 is a promising target for blocking transmission, and antibodies against GPR180 possess robust transmission-blocking activities.

FKRP directed fibronectin glycosylation: A novel mechanism giving insights into muscular dystrophies?

The recently uncovered role of Fukutin-related protein (FKRP) in fibronectin glycosylation has challenged our understanding of the basis of disease pathogenesis in the muscular dystrophies. FKRP is a Golgi-resident glycosyltransferase implicated in a broad spectrum of muscular dystrophy (MD) pathologies that are not fully attributable to the well-described α-Dystroglycan hypoglycosylation. By revealing a new role for FKRP in the glycosylation of fibronectin, a modification critical for the development of the muscle basement membrane (MBM) and its associated muscle linkages, new possibilities for understanding clinical phenotype arise. This modification involves an interaction between FKRP and myosin-10, a protein involved in the Golgi organization and function. These observations suggest a FKRP nexus exists that controls two critical aspects to muscle fibre integrity, both fibre stability at the MBM and its elastic properties. This review explores the new potential disease axis in the context of our current knowledge of muscular dystrophies.

Ras-related proteins (Rab) play significant roles in sperm motility and capacitation status

Rab proteins are widely known for their involvement in establishing Golgi apparatus and controlling Golgi trafficking in eukaryotic cells. Specifically, Rab proteins play significant roles in acrosome formation and exocytosis. Furthermore, mechanisms involved in the regulation of Rab proteins during capacitation have been identified. However, there has been no direct evaluation to assess the correlation between Rab proteins and sperm function. Consequently, this study was designed to analyze the correlation between Rab proteins and sperm functions. Individually, we analyzed the sperm motility patterns, motion kinematics, capacitation status, and Rab protein expression levels of sperm samples from 31 boars before and after capacitation. As a result, we discovered that Rab3A, Rab5, Rab11, Rab14, and Rab27A correlated with various sperm motility patterns, motion kinematics before capacitation. Rab3A, Rab5, Rab11, Rab14, and Rab34 correlated with various sperm motility patterns, motion kinematics after capacitation. Moreover, Rab4 and Rab34 were associated with capacitation status before capacitation, and Rab3A, 25, and 27A correlated with capacitation status after capacitation. This is the first study to analyze the correlation between Rab proteins and sperm functions. Collectively, our results indicate that specific sperm motility and kinematics, as well as the structural condition of the sperm head and capacitation status, regulate individual Rab protein. Therefore, we expect that the current findings will be used to identify the etiology of idiopathic male infertility patients and to diagnose male fertility and that Rab proteins will be employed as biomarkers to predict and analyze male fertility.

Alcohol-Induced Liver Injury: Down-regulation and Redistribution of Rab3D Results in Atypical Protein Trafficking

Previous work from our laboratories has identified multiple defects in endocytosis, protein trafficking, and secretion, along with altered Golgi function after alcohol administration. Manifestation of alcohol-associated liver disease (ALD) is associated with an aberrant function of several hepatic proteins, including asialoglycoprotein receptor (ASGP-R), their atypical distribution at the plasma membrane (PM), and secretion of their abnormally glycosylated forms into the bloodstream, but trafficking mechanism is unknown. Here we report that a small GTPase, Rab3D, known to be involved in exocytosis, secretion, and vesicle trafficking, shows ethanol (EtOH)-impaired function, which plays an important role in Golgi disorganization. We used multiple approaches and cellular/animal models of ALD, along with Rab3D knockout (KO) mice and human tissue from patients with ALD. We found that Rab3D resides primarily in trans- and cis-faces of Golgi; however, EtOH treatment results in Rab3D redistribution from trans-Golgi to cis-medial-Golgi. Cells lacking Rab3D demonstrate enlargement of Golgi, especially its distal compartments. We identified that Rab3D is required for coat protein I (COPI) vesiculation in Golgi, and conversely, COPI is critical for intra-Golgi distribution of Rab3D. Rab3D/COPI association was altered not only in the liver of patients with ALD but also in the donors consuming alcohol without steatosis. In Rab3D KO mice, hepatocytes experience endoplasmic reticulum (ER) stress, and EtOH administration activates apoptosis. Notably, in these cells, ASGP-R, despite incomplete glycosylation, can still reach cell surface through ER-PM junctions. This mimics the effects seen with EtOH-induced liver injury. Conclusion: We revealed that down-regulation of Rab3D contributes significantly to EtOH-induced Golgi disorganization, and abnormally glycosylated ASGP-R is excreted through ER-PM connections, bypassing canonical (ER→Golgi→PM) anterograde transportation. This suggests that ER-PM sites may be a therapeutic target for ALD.

Golgi requires a new casting in the screenplay of mucopolysaccharidosis II cytopathology

Lysosome (L), a hydrolytic compartment of the endo-lysosomal system (ELS), plays a central role in the metabolic regulation of eukaryotic cells. Furthermore, it has a central role in the cytopathology of several diseases, primarily in lysosomal storage diseases (LSDs). Mucopolysaccharidosis II (MPS II, Hunter disease) is a rare LSD caused by idunorate-2-sulphatase (IDS) enzyme deficiency. To provide a new platform for drug development and clarifying the background of the clinically observed cytopathology, we established a human in vitro model, which recapitulates all cellular hallmarks of the disease. Some of our results query the traditional concept by which the storage vacuoles originate from the endosomal system and suggest a new concept, in which endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and RAB2/LAMP positive Golgi (G) vesicles play an initiative role in the vesicle formation. In this hypothesis, Golgi is not only an indirectly affected organelle but enforced to be the main support of vacuole formation. The purposes of this minireview are to give a simple guide for understanding the main relationships in ELS, to present the storage vacuoles and their relation to ELS compartments, to recommend an alternative model for vacuole formation, and to place the Golgi in spotlight of MPS II cytopathology.

The multiple roles of RAB GTPases in female and male meiosis

BACKGROUND

RAB GTPases constitute the largest family of small GTPases and are found in all eukaryotes. RAB GTPases regulate components of the endomembrane system, the nucleus and the plasma membrane, and are involved in intracellular actin/tubulin-dependent vesicle movement, membrane fusion and cell growth in mitosis.

OBJECTIVE AND RATIONALE

RAB GTPases play multiple critical roles during both female and male meiosis. This review summarizes the progress made in our understanding of the role of RAB GTPases in female and male meiosis in different species. We also discuss the potential relationship between RAB GTPases and oocyte/sperm quality, which may help in understanding the mechanisms underlying oogenesis and spermatogenesis and potential genetic causes of infertility.

SEARCH METHODS

The PubMed database was searched for articles published between 1991 and 2020 using the following terms: 'RAB', 'RAB oocyte', 'RAB sperm' and 'RAB meiosis'.

OUTCOMES

An analysis of 126 relevant articles indicated that RAB GTPases are present in all eukaryotes, and ten subfamilies (almost 70 members) are expressed in human cells. The roles of 25 RAB proteins and orthologues in female meiosis and 12 in male meiosis have been reported. RAB proteins are essential for the accurate continuity of genetic material, successful fertilization and the normal growth of offspring. Distinct and crucial functions of RAB GTPases in meiosis have been reported. In oocytes, RAB GTPases are involved in spindle organization, kinetochore-microtubule attachment, chromosome alignment, actin filament-mediated spindle migration, cytokinesis, cell cycle and oocyte-embryo transition. RAB GTPases function in mitochondrial processes and Golgi-mediated vesicular transport during female meiosis, and are critical for cortical granule transport during fertilization and oocyte-embryo transition. In sperm, RAB GTPases are vital for cytoskeletal organization and successful cytokinesis, and are associated with Golgi-mediated acrosome formation, membrane trafficking and morphological changes of sperm cells, as well as the exocytosis-related acrosome reaction and zona reaction during fertilization.

WIDER IMPLICATIONS

Abnormal expression of RAB GTPases disrupts intracellular systems, which may induce diverse diseases. The roles of RAB proteins in female and male reproductive systems, thus, need to be considered. The mechanisms underlying the function of RAB GTPases and the binding specificity of their effectors during oogenesis, spermatogenesis and fertilization remain to be studied. This review should contribute to our understanding of the molecular mechanisms of oogenesis and spermatogenesis and potential genetic causes of infertility.

Rab18 binds PLIN2 and ACSL3 to mediate lipid droplet dynamics

Lipid droplet (LD) is a vital organelle governing lipid homeostasis and Rab18 has been linked to lipid metabolism. However, the mechanisms of Rab18-mediated LD dynamics in myoblast cells remain elusive. Here, we report that Rab18 plays an important role in oleic acid (OA)-induced LD accumulation in mouse myoblast C2C12 cells. Rab18 was translocated from the endoplasmic reticulum (ER) to LDs during LD accumulation, which was regulated by perilipin 2 (PLIN2), a major LD protein. LD-associated Rab18 bound with the C terminus of PLIN2 and the LD localization of Rab18 was diminished when PLIN2 was depleted. Moreover, loss of function of Rab18 led to reduced triacylglycerol (TAG) level and fewer but larger LDs. In contrast, overexpression of Rab18 resulted in elevated TAG content and LD number. Furthermore, LD-associated Rab18 interacted with acyl-CoA synthetase long-chain family member 3 (ACSL3), which in turn promoted the LD localization of this protein. These data show that Rab18 interacts with PLIN2 and forms a complex with PLIN2 and ACSL3, which plays a critical role in LD accumulation and dynamics of myoblast cells.

Rab2 regulates presynaptic precursor vesicle biogenesis at the trans-Golgi

Reliable delivery of presynaptic material, including active zone and synaptic vesicle proteins from neuronal somata to synaptic terminals, is prerequisite for successful synaptogenesis and neurotransmission. However, molecular mechanisms controlling the somatic assembly of presynaptic precursors remain insufficiently understood. We show here that in mutants of the small GTPase Rab2, both active zone and synaptic vesicle proteins accumulated in the neuronal cell body at the trans-Golgi and were, consequently, depleted at synaptic terminals, provoking neurotransmission deficits. Ectopic presynaptic material accumulations consisted of heterogeneous vesicles and short tubules of 40 × 60 nm, segregating in subfractions either positive for active zone or synaptic vesicle proteins and LAMP1, a lysosomal membrane protein. Genetically, Rab2 acts upstream of Arl8, a lysosomal adaptor controlling axonal export of precursors. Collectively, we identified a Golgi-associated assembly sequence of presynaptic precursor biogenesis dependent on a Rab2-regulated protein export and sorting step at the trans-Golgi.

Endosomal Rab GTPases regulate secretory granule maturation in Drosophila larval salivary glands

Secretory granules (SGs) are organelles responsible for regulated exocytosis of biologically active molecules in professional secretory cells. Maturation of SGs is a crucial process in which cargoes of SGs are processed and activated, allowing them to exert their function upon secretion. Nonetheless, the intracellular trafficking pathways required for SG maturation are not well defined. We recently performed an RNA interference (RNAi) screen in Drosophila larval salivary glands to identify trafficking components needed for SG maturation. From the screen, we identified several Rab GTPases (Rabs) that affect SG maturation. Expression of constitutively active (CA) and dominant-negative (DN) forms narrowed down the Rabs important for this process to Rab5, Rab9 and Rab11. However, none of these Rabs localizes to the limiting membrane of SGs. In contrast, examination of endogenously YFP-tagged Rabs (YRabs) in larval salivary glands revealed that YRab1 and YRab6 localize to the limiting membrane of immature SGs (iSGs) and SGs. These findings provide new insights into how Rab GTPases contribute to the process of SG maturation.

Golgi-associated Rab GTPases implicated in autophagy

Autophagy is a conserved cellular degradation process in eukaryotes that facilitates the recycling and reutilization of damaged organelles and compartments. It plays a pivotal role in cellular homeostasis, pathophysiological processes, and diverse diseases in humans. Autophagy involves dynamic crosstalk between different stages associated with intracellular vesicle trafficking. Golgi apparatus is the central organelle involved in intracellular vesicle trafficking where Golgi-associated Rab GTPases function as important mediators. This review focuses on the recent findings that highlight Golgi-associated Rab GTPases as master regulators of autophagic flux. The scope for future research in elucidating the role and mechanism of Golgi-associated Rab GTPases in autophagy and autophagy-related diseases is discussed further.

The role of the Golgi apparatus in disease (Review)

The Golgi apparatus is known to underpin many important cellular homeostatic functions, including trafficking, sorting and modifications of proteins or lipids. These functions are dysregulated in neurodegenerative diseases, cancer, infectious diseases and cardiovascular diseases, and the number of disease-related genes associated with Golgi apparatus is on the increase. Recently, many studies have suggested that the mutations in the genes encoding Golgi resident proteins can trigger the occurrence of diseases. By summarizing the pathogenesis of these genetic diseases, it was found that most of these diseases have defects in membrane trafficking. Such defects typically result in mislocalization of proteins, impaired glycosylation of proteins, and the accumulation of undegraded proteins. In the present review, we aim to understand the patterns of mutations in the genes encoding Golgi resident proteins and decipher the interplay between Golgi resident proteins and membrane trafficking pathway in cells. Furthermore, the detection of Golgi resident protein in human serum samples has the potential to be used as a diagnostic tool for diseases, and its central role in membrane trafficking pathways provides possible targets for disease therapy. Thus, we also introduced the clinical value of Golgi apparatus in the present review.

Rab6 is required for rapid, cisternal-specific, intra-Golgi cargo transport

Rab6, the most abundant Golgi associated small GTPase, consists of 2 equally common isoforms, Rab6A and Rab6A′, that differ in 3 amino acids and localize to trans Golgi cisternae. The two isoforms are largely redundant in function and hence are often referred to generically as Rab6. Rab6 loss-of-function inhibits retrograde Golgi trafficking, induces an increase in Golgi cisternal number in HeLa cells and delays the cell surface appearance of the anterograde cargo protein, VSVG. We hypothesized that these effects are linked and might be explained by a cisternal-specific delay in cargo transport. In pulse chase experiments using a deconvolved, confocal line scanning approach to score the distribution of the tsO45 mutant of VSVG protein in Rab6 depleted cells, we found that anterograde transport at 32 °C, permissive conditions, through the Golgi apparatus was locally delayed, almost tenfold, between medial and trans Golgi cisterna. Cis to medial transport was nearly normal as was trans Golgi to TGN transport. TGN exit was unaffected by Rab6 depletion. These effects were the same with either of two siRNAs. Similar intra-Golgi transport delays were seen at 37 °C with RUSH VSVG or a RUSH GPI-anchored construct using a biotin pulse to release the marker proteins from the ER. Using 3D-SIM, a super resolution approach, we found that RUSH VSVG transport was delayed pre-trans Golgi. These visual approaches suggest a selective slowing of anterograde transport relative to 3 different marker proteins downstream of the trans Golgi. Using a biochemical approach, we found that the onset of VSVG endoglycosidase H resistance in Rab6 depleted cells was delayed. Depletion of neither Rab6A or Rab6A′ isoforms alone had any effect on anterograde transport through the Golgi suggesting that Rab6A and Rab6A′ act coordinately. Delayed cargo transport conditions correlate strongly with a proliferation of Golgi cisternae observed in earlier electron microscopy. Our results strongly indicate that Rab6 is selectively required for rapid anterograde transport from the medial to trans Golgi. We suggest that the observed correlation with localized cisternal proliferation fits best with a cisternal progression model of Golgi function.

Golgi Apparatus: A Potential Therapeutic Target for Autophagy-Associated Neurological Diseases

Autophagy has dual effects in human diseases: appropriate autophagy may protect cells from stress, while excessive autophagy may cause cell death. Additionally, close interactions exist between autophagy and the Golgi. This review outlines recent advances regarding the role of the Golgi apparatus in autophagy. The signaling processes of autophagy are dependent on the normal function of the Golgi. Specifically, (i) autophagy-related protein 9 is mainly located in the Golgi and forms new autophagosomes in response to stressors; (ii) Golgi fragmentation is induced by Golgi-related proteins and accompanied with autophagy induction; and (iii) the endoplasmic reticulum-Golgi intermediate compartment and the reticular trans-Golgi network play essential roles in autophagosome formation to provide a template for lipidation of microtubule-associated protein 1A/1B-light chain 3 and induce further ubiquitination. Golgi-related proteins regulate formation of autophagosomes, and disrupted formation of autophagy can influence Golgi function. Notably, aberrant autophagy has been demonstrated to be implicated in neurological diseases. Thus, targeted therapies aimed at protecting the Golgi or regulating Golgi proteins might prevent or ameliorate autophagy-related neurological diseases. Further studies are needed to investigate the potential application of Golgi therapy in autophagy-based neurological diseases.

Unlocking Golgi: Why Does Morphology Matter?

The mammalian Golgi apparatus is a highly dynamic organelle, which is normally localized in the juxtanuclear space and plays an essential role in the regulation of cellular homeostasis. While posttranslational modification of cargo is mediated by the resident enzymes (glycosyltransferases, glycosidases, and kinases), the ribbon structure of Golgi and its cisternal stacking mostly rely on the cooperation of coiled-coil matrix golgins. Among them, giantin, GM130, and GRASPs are unique, because they form a tripartite complex and serve as Golgi docking sites for cargo delivered from the endoplasmic reticulum (ER). Golgi undergoes significant disorganization in many pathologies associated with a block of the ER-to-Golgi or intra-Golgi transport, including cancer, different neurological diseases, alcoholic liver damage, ischemic stress, viral infections, etc. In addition, Golgi fragments during apoptosis and mitosis. Here, we summarize and analyze clinically relevant observations indicating that Golgi fragmentation is associated with the selective loss of Golgi residency for some enzymes and, conversely, with the relocation of some cytoplasmic proteins to the Golgi. The central concept is that ER and Golgi stresses impair giantin docking site but have no impact on the GM130-GRASP65 complex, thus inducing mislocalization of giantin-sensitive enzymes only. This cardinally changes the processing of proteins by eliminating the pathways controlled by the missing enzymes and by activating the processes now driven by the GM130-GRASP65-dependent proteins. This type of Golgi disorganization is different from the one induced by the cytoskeleton alteration, which despite Golgi de-centralization, neither impairs function of golgins nor alters trafficking.

Post-ER Stress Biogenesis of Golgi Is Governed by Giantin

BACKGROUND

The Golgi apparatus undergoes disorganization in response to stress, but it is able to restore compact and perinuclear structure under recovery. This self-organization mechanism is significant for cellular homeostasis, but remains mostly elusive, as does the role of giantin, the largest Golgi matrix dimeric protein.

METHODS

In HeLa and different prostate cancer cells, we used the model of cellular stress induced by Brefeldin A (BFA). The conformational structure of giantin was assessed by proximity ligation assay and atomic force microscopy. The post-BFA distribution of Golgi resident enzymes was examined by 3D SIM high-resolution microscopy.

RESULTS

We detected that giantin is rather flexible than an extended coiled-coil dimer and BFA-induced Golgi disassembly was associated with giantin monomerization. A fusion of the nascent Golgi membranes after BFA washout is forced by giantin re-dimerization via disulfide bond in its luminal domain and assisted by Rab6a GTPase. GM130-GRASP65-dependent enzymes are able to reach the nascent Golgi membranes, while giantin-sensitive enzymes appeared at the Golgi after its complete recovery via direct interaction of their cytoplasmic tail with N-terminus of giantin.

CONCLUSION

Post-stress recovery of Golgi is conducted by giantin dimer and Golgi proteins refill membranes according to their docking affiliation rather than their intra-Golgi location.

Ypt/Rab GTPases and their TRAPP GEFs at the Golgi

The conserved Ypt/Rab GTPases regulate the different steps of all intracellular trafficking pathways. Ypt/Rabs are activated by their specific nucleotide exchangers termed GEFs, and when GTP bound, they recruit their downstream effectors, which mediate vesicular transport substeps. In the yeast exocytic pathway, Ypt1 and Ypt31/32 regulate traffic through the Golgi and the conserved modular TRAPP complex acts a GEF for both Ypt1 and Ypt31/32. However, the precise localization and function of these Ypts have been under debate, as is the identity of their corresponding GEFs. We have established that Ypt1 and Ypt31 reside on the two sides of the Golgi, early and late, respectively, and regulate Golgi cisternal progression. We and others have shown that whereas a single TRAPP complex, TRAPP II, activates Ypt31, three TRAPP complexes can activate Ypt1: TRAPPs I, III, and IV. We propose that TRAPP I and II activate Ypt1 and Ypt31, respectively, at the Golgi, whereas TRAPP III and IV activate Ypt1 in autophagy. Resolving these issues is important because both Rabs and TRAPPs are implicated in multiple human diseases, ranging from cancer to neurodegenerative diseases.

Multitasking Rab Proteins in Autophagy and Membrane Trafficking: A Focus on Rab33b

Autophagy (particularly macroautophagy) is a bulk degradation process used by eukaryotic cells in order to maintain adequate energy levels and cellular homeostasis through the delivery of long-lived proteins and organelles to the lysosome, resulting in their degradation. It is becoming increasingly clear that many of the molecular requirements to fulfil autophagy intersect with those of conventional and unconventional membrane trafficking pathways. Of particular interest is the dependence of these processes on multiple members of the Rab family of small GTP binding proteins. Rab33b is a protein that localises to the Golgi apparatus and has suggested functions in both membrane trafficking and autophagic processes. Interestingly, mutations in the RAB33B gene have been reported to cause the severe skeletal disorder, Smith–McCort Dysplasia; however, the molecular basis for Rab33b in this disorder remains to be determined. In this review, we focus on the current knowledge of the participation of Rab33b and its interacting partners in membrane trafficking and macroautophagy, and speculate on how its function, and dysfunction, may contribute to human disease.

A Kinetic View of Membrane Traffic Pathways Can Transcend the Classical View of Golgi Compartments

A long-standing assumption is that the cisternae of the Golgi apparatus can be grouped into functionally distinct compartments, yet the molecular identities of those compartments have not been clearly described. The concept of a compartmentalized Golgi is challenged by the cisternal maturation model, which postulates that cisternae form de novo and then undergo progressive biochemical changes. Cisternal maturation can potentially be reconciled with Golgi compartmentation by defining compartments as discrete kinetic stages in the maturation process. These kinetic stages are distinguished by the traffic pathways that are operating. For example, a major transition occurs when a cisterna stops producing COPI vesicles and begins producing clathrin-coated vesicles. This transition separates one kinetic stage, the "early Golgi," from a subsequent kinetic stage, the "late Golgi" or "trans-Golgi network (TGN)." But multiple traffic pathways drive Golgi maturation, and the periods of operation for different traffic pathways can partially overlap, so there is no simple way to define a full set of Golgi compartments in terms of kinetic stages. Instead, we propose that the focus should be on the series of transitions experienced by a Golgi cisterna as various traffic pathways are switched on and off. These traffic pathways drive changes in resident transmembrane protein composition. Transitions in traffic pathways seem to be the fundamental, conserved determinants of Golgi organization. According to this view, the initial goal is to identify the relevant traffic pathways and place them on the kinetic map of Golgi maturation, and the ultimate goal is to elucidate the logic circuit that switches individual traffic pathways on and off as a cisterna matures.

Characteristics and Functions of the Yip1 Domain Family (YIPF), Multi-Span Transmembrane Proteins Mainly Localized to the Golgi Apparatus

Yip1 domain family (YIPF) proteins are multi-span, transmembrane proteins mainly localized in the Golgi apparatus. YIPF proteins have been found in virtually all eukaryotes, suggesting that they have essential function(s). Saccharomyces cerevisiae contains four YIPFs: Yip1p, Yif1p, Yip4p, and Yip5p. Early analyses in S. cerevisiae indicated that Yip1p and Yif1p bind to each other and play a role in budding of transport vesicles and/or fusion of vesicles to target membranes. However, the molecular basis of their functions remains unclear. Analysis of YIPF proteins in mammalian cells has yielded significant clues about the function of these proteins. Human cells have nine family members that appear to have overlapping functions. These YIPF proteins are divided into two sub-families: YIPFα/Yip1p and YIPFβ/Yif1p. A YIPFα molecule forms a complex with a specific partner YIPFβ molecule. In the most broadly hypothesized scenario, a basic tetramer complex is formed from two molecules of each partner YIPF protein, and this tetramer forms a higher order oligomer. Three distinct YIPF protein complexes are formed from pairs of YIPFα and YIPFβ proteins. These are differently localized in either the early, middle, or late compartments of the Golgi apparatus and are recycled between adjacent compartments. Because a YIPF protein is predicted to have five transmembrane segments, a YIPF tetramer complex is predicted to have 20 transmembrane segments. This high number of transmembrane segments suggests that YIPF complexes function as channels, transporters, or transmembrane receptors. Here, the evidence from functional studies of YIPF proteins obtained during the last two decades is summarized and discussed.

Enhanced autophagic-lysosomal activity and increased BAG3-mediated selective macroautophagy as adaptive response of neuronal cells to chronic oxidative stress

Oxidative stress and a disturbed cellular protein homeostasis (proteostasis) belong to the most important hallmarks of aging and of neurodegenerative disorders. The proteasomal and autophagic-lysosomal degradation pathways are key measures to maintain proteostasis. Here, we report that hippocampal cells selected for full adaptation and resistance to oxidative stress induced by hydrogen peroxide (oxidative stress-resistant cells, OxSR cells) showed a massive increase in the expression of components of the cellular autophagic-lysosomal network and a significantly higher overall autophagic activity. A comparative expression analysis revealed that distinct key regulators of autophagy are upregulated in OxSR cells. The observed adaptive autophagic response was found to be independent of the upstream autophagy regulator mTOR but is accompanied by a significant upregulation of further downstream components of the canonical autophagy network such as Beclin1, WIPI1 and the transmembrane ATG9 proteins. Interestingly, the expression of the HSP70 co-chaperone BAG3, mediator of BAG3-mediated selective macroautophagy and highly relevant for the clearance of aggregated proteins in cells, was found to be increased in OxSR cells that were consequently able to effectively overcome proteotoxic stress. Overexpression of BAG3 in oxidative stress-sensitive HT22 wildtype cells partly established the vesicular phenotype and the enhanced autophagic flux seen in OxSR cells suggesting that BAG3 takes over an important part in the adaptation process. A full proteome analysis demonstrated additional changes in the expression of mitochondrial proteins, metabolic enzymes and different pathway regulators in OxSR cells as consequence of the adaptation to oxidative stress in addition to autophagy-related proteins. Taken together, this analysis revealed a wide variety of pathways and players that act as adaptive response to chronic redox stress in neuronal cells.

Disrupted neuronal trafficking in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, adult-onset neurodegenerative disease caused by degeneration of motor neurons in the brain and spinal cord leading to muscle weakness. Median survival after symptom onset in patients is 3-5 years and no effective therapies are available to treat or cure ALS. Therefore, further insight is needed into the molecular and cellular mechanisms that cause motor neuron degeneration and ALS. Different ALS disease mechanisms have been identified and recent evidence supports a prominent role for defects in intracellular transport. Several different ALS-causing gene mutations (e.g., in FUS, TDP-43, or C9ORF72) have been linked to defects in neuronal trafficking and a picture is emerging on how these defects may trigger disease. This review summarizes and discusses these recent findings. An overview of how endosomal and receptor trafficking are affected in ALS is followed by a description on dysregulated autophagy and ER/Golgi trafficking. Finally, changes in axonal transport and nucleocytoplasmic transport are discussed. Further insight into intracellular trafficking defects in ALS will deepen our understanding of ALS pathogenesis and will provide novel avenues for therapeutic intervention.

Claudin7-dependent exosome-promoted reprogramming of nonmetastasizing tumor cells

Claudin7 (cld7) is a cancer-initiating cell (CIC) marker in gastrointestinal tumors, a cld7-knockdown (kd) being accompanied by loss of tumor progression. Tumor exosomes (TEX) restoring CIC activities, we explored the contribution of cld7. This became particularly interesting, as tight junction (TJ)- and glycolipid-enriched membrane domain (GEM)-derived cld7 is recruited into distinct TEX. TEXs were derived from CIC or cld7kd cells of a rat pancreatic and a human colon cancer line. TEX derived from pancreatic cancer cld7kd cells rescued with palmitoylation site-deficient cld7 (cld7mP) allowed selectively evaluating the contribution of GEM-derived TEX, only palmitoylated cld7 being integrated into GEM. Cld7 CIC-TEX promoted tumor cell dissemination and metastatic growth without a major impact on proliferation, apoptosis resistance and epithelial-mesenchymal transition. Instead, migration, invasion and (lymph)angiogenesis were strongly supported, only migration being selectively fostered by GEM-derived cld7 TEX. CIC-TEX coculture of cld7kd cells uncovered significant changes in the cld7kd cell protein and miRNA profiles. However, changes did not correspond to the CIC-TEX profile, CIC-TEX rather initiating integrin, protease and RTK, particularly lymphangiogenic receptor activation. CIC-TEX preferentially rescuing cld7kd-associated defects in signal transduction was backed up by an RTK inhibitor neutralizing the impact of CIC-TEX on tumor progression. In conclusion, cld7 contributes to selective steps of the metastatic cascade. Defects of cld7kd and cld7mP cells in migration, invasion and (lymph)angiogenesis are effaced by CIC-TEX that act by signaling cascade activation. Accordingly, RTK inhibitors are an efficient therapeutic defeating CIC-TEX.

Multiple Roles of Rab GTPases at the Golgi

The Golgi apparatus is a central sorting station in the cell. It receives newly synthesized molecules from the endoplasmic reticulum and directs them to different subcellular destinations, such as the plasma membrane or the endocytic pathway. Importantly, in the last few years, it has emerged that the maintenance of Golgi structure is connected to the proper regulation of membrane trafficking. Rab proteins are small GTPases that are considered to be the master regulators of the intracellular membrane trafficking. Several of the over 60 human Rabs are involved in the regulation of transport pathways at the Golgi as well as in the maintenance of its architecture. This chapter will summarize the different roles of Rab GTPases at the Golgi, both as regulators of membrane transport, scaffold, and tethering proteins and in preserving the structure and function of this organelle.

Rab-mediated trafficking in the secondary cells of Drosophila male accessory glands and its role in fecundity

The male seminal fluid contains factors that affect female post‐mating behavior and physiology. In Drosophila, most of these factors are secreted by the two epithelial cell types that make up the male accessory gland: the main and secondary cells. Although secondary cells represent only ~4% of the cells of the accessory gland, their contribution to the male seminal fluid is essential for sustaining the female post‐mating response. To better understand the function of the secondary cells, we investigated their molecular organization, particularly with respect to the intracellular membrane transport machinery. We determined that large vacuole‐like structures found in the secondary cells are trafficking hubs labeled by Rab6, 7, 11 and 19. Furthermore, these organelles require Rab6 for their formation and many are essential in the process of creating the long‐term postmating behavior of females. In order to better serve the intracellular membrane and protein trafficking communities, we have created a searchable, online, open‐access imaging resource to display our complete findings regarding Rab localization in the accessory gland.

Rab-H1b is essential for trafficking of cellulose synthase and for hypocotyl growth in Arabidopsis thaliana

Cell-wall deposition of cellulose microfibrils is essential for plant growth and development. In plant cells, cellulose synthesis is accomplished by cellulose synthase complexes located in the plasma membrane. Trafficking of the complex between endomembrane compartments and the plasma membrane is vital for cellulose biosynthesis; however, the mechanism for this process is not well understood. We here report that, in Arabidopsis thaliana, Rab-H1b, a Golgi-localized small GTPase, participates in the trafficking of CELLULOSE SYNTHASE 6 (CESA6) to the plasma membrane. Loss of Rab-H1b function resulted in altered distribution and motility of CESA6 in the plasma membrane and reduced cellulose content. Seedlings with this defect exhibited short, fragile etiolated hypocotyls. Exocytosis of CESA6 was impaired in rab-h1b cells, and endocytosis in mutant cells was significantly reduced as well. We further observed accumulation of vesicles around an abnormal Golgi apparatus having an increased number of cisternae in rab-h1b cells, suggesting a defect in cisternal homeostasis caused by Rab-H1b loss function. Our findings link Rab GTPases to cellulose biosynthesis, during hypocotyl growth, and suggest Rab-H1b is crucial for modulating the trafficking of cellulose synthase complexes between endomembrane compartments and the plasma membrane and for maintaining Golgi organization and morphology.

Modulation of cis- and trans- Golgi and the Rab9A-GTPase during infection by Besnoitia besnoiti, Toxoplasma gondii and Neospora caninum

Like most intracellular pathogens, the apicomplexan parasites Besnoitia besnoiti, Toxoplasma gondii and Neospora caninum scavenge metabolites from their host cells. Recruitment of the Golgi complex to the vicinity of the parasitophorous vacuole (PV) is likely to aid in this process. In this work, we comparatively assessed B. besnoiti, T. gondii and N. caninum infected human retinal pigmented epithelial (hTERT-RPE-1) cells at 24 h post-infection and used antibodies to confirm Golgi ribbon compaction in B. besnoiti, and Golgi ribbon dispersion in T. gondii, while no alteration in Golgi morphology was seen in N. caninum infected cells. In either case, the Golgi stacks of infected cells contained both cis- (GM130) and trans- (TGN46) Golgi proteins. The localization of Rab9A, an important regulator of endosomal trafficking, was also studied. GFP-tagged Rab9A was recruited to the vicinity of the PV of all three parasites. Toxoplasma-infected cells exhibited increased expression of Rab9A in comparison to non-infected cells. However, Rab9A expression levels remained unaltered upon infection with N. caninum and B. besnoiti tachyzoites. In contrast to Rab9A, a GFP-tagged dominant negative mutant form of Rab9A (Rab9A DN), was not recruited to the PV, and the expression of Rab9A DN did not affect host cell invasion nor replication by all three parasites. Thus, B. besnoiti, T. gondii and N. caninum show similarities but also differences in how they affect constituents of the endosomal/secretory pathways.

trans-Golgi network-bound cargo traffic

Cargo following the retrograde trafficking are sorted at endosomes to be targeted the trans-Golgi network (TGN), a central receiving organelle. Though molecular requirements and their interaction networks have been somewhat established, the complete understanding of the intricate nature of their action mechanisms in every step of the retrograde traffic pathway remains unachieved. This review focuses on elucidating known functions of key regulators, including scission factors at the endosome and tethering/fusion mediators at the receiving dock, TGN, as well as a diverse range of cargo.

Rab proteins as major determinants of the Golgi complex structure

GTP-ases of the Rab family (about 70 in human) are key regulators of intracellular transport and membrane trafficking in eukaryotic cells. Remarkably, almost one third associate with membranes of the Golgi complex and TGN (trans-Golgi network). Through interactions with a variety of effectors that include molecular motors, tethering complexes, scaffolding proteins and lipid kinases, they play an important role in maintaining Golgi architecture.

Export of membrane proteins from the Golgi complex to the primary cilium requires the kinesin motor, KIFC1

Microtubule-based motors contribute to the efficiency and selectivity of Golgi exit and post-Golgi transport of membrane proteins that are targeted to distinct compartments. Cytoplasmic dynein moves post-Golgi vesicles that carry rhodopsin toward the base of the connecting cilium in photoreceptor cells; however, the identity of the motors that are involved in the vesicular trafficking of ciliary membrane proteins in nonphotoreceptor cells remains unclear. Here, we demonstrate that the minus end-directed kinesin KIFC1 (kinesin family member C1) is required for both ciliary membrane protein transport and serum starvation-induced ciliogenesis in retinal pigmented epithelial 1 cells. Although KIFC1 is known as a mitotic motor that is sequestered in the nucleus during interphase, KIFC1 immunoreactivity appeared in the Golgi region after serum starvation. Knockdown of KIFC1 inhibited the export of ciliary receptors from the Golgi complex. KIFC1 overexpression affected the Golgi localization of GMAP210 (Golgi microtubule-associated protein 210) and IFT20 (intraflagellar transport 20), which are involved in membrane protein transport to cilia. Moreover, KIFC1 physically interacted with ASAP1 (ADP-ribosylation factor GTPase-activating protein with SH3 domain, ankyrin repeat and PH domain 1), which regulates the budding of rhodopsin transport carriers from the Golgi complex, and KIFC1 depletion caused Golgi accumulation of ASAP1. A decrease in the centrosomal levels of IFT20 and TTBK2 (τ-tubulin kinase 2) was associated with ciliogenesis defects in KIFC1-depleted cells. Our results suggest that KIFC1 plays roles in the Golgi exit of ciliary receptors and in the recruitment of ciliogenesis regulators.-Lee, S.-H., Joo, K., Jung, E. J., Hong, H., Seo, J., Kim, J. Export of membrane proteins from the Golgi complex to the primary cilium requires the kinesin motor, KIFC1.

Kingdom Chromista and its eight phyla: a new synthesis emphasising periplastid protein targeting, cytoskeletal and periplastid evolution, and ancient divergences

In 1981 I established kingdom Chromista, distinguished from Plantae because of its more complex chloroplast-associated membrane topology and rigid tubular multipartite ciliary hairs. Plantae originated by converting a cyanobacterium to chloroplasts with Toc/Tic translocons; most evolved cell walls early, thereby losing phagotrophy. Chromists originated by enslaving a phagocytosed red alga, surrounding plastids by two extra membranes, placing them within the endomembrane system, necessitating novel protein import machineries. Early chromists retained phagotrophy, remaining naked and repeatedly reverted to heterotrophy by losing chloroplasts. Therefore, Chromista include secondary phagoheterotrophs (notably ciliates, many dinoflagellates, Opalozoa, Rhizaria, heliozoans) or walled osmotrophs (Pseudofungi, Labyrinthulea), formerly considered protozoa or fungi respectively, plus endoparasites (e.g. Sporozoa) and all chromophyte algae (other dinoflagellates, chromeroids, ochrophytes, haptophytes, cryptophytes). I discuss their origin, evolutionary diversification, and reasons for making chromists one kingdom despite highly divergent cytoskeletons and trophic modes, including improved explanations for periplastid/chloroplast protein targeting, derlin evolution, and ciliary/cytoskeletal diversification. I conjecture that transit-peptide-receptor-mediated 'endocytosis' from periplastid membranes generates periplastid vesicles that fuse with the arguably derlin-translocon-containing periplastid reticulum (putative red algal trans-Golgi network homologue; present in all chromophytes except dinoflagellates). I explain chromist origin from ancestral corticates and neokaryotes, reappraising tertiary symbiogenesis; a chromist cytoskeletal synapomorphy, a bypassing microtubule band dextral to both centrioles, favoured multiple axopodial origins. I revise chromist higher classification by transferring rhizarian subphylum Endomyxa from Cercozoa to Retaria; establishing retarian subphylum Ectoreta for Foraminifera plus Radiozoa, apicomonad subclasses, new dinozoan classes Myzodinea (grouping Colpovora gen. n., Psammosa), Endodinea, Sulcodinea, and subclass Karlodinia; and ranking heterokont Gyrista as phylum not superphylum.

Identification of Rab18 as an Essential Host Factor for BK Polyomavirus Infection Using a Whole-Genome RNA Interference Screen

Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

BK polyomavirus (BKPyV) is a human pathogen first isolated in 1971. BKPyV infection is ubiquitous in the human population, with over 80% of adults worldwide being seropositive for BKPyV. BKPyV infection is usually asymptomatic; however, BKPyV reactivation in immunosuppressed transplant patients causes two diseases, polyomavirus-associated nephropathy and hemorrhagic cystitis. To establish a successful infection in host cells, BKPyV must travel in retrograde transport vesicles to reach the nucleus. To make this happen, BKPyV requires the cooperation of host cell proteins. To further identify host factors associated with BKPyV entry and intracellular trafficking, we performed a whole-genome small interfering RNA screen on BKPyV infection of primary human renal proximal tubule epithelial cells. The results revealed the importance of Ras-related protein Rab18 and syntaxin 18 for BKPyV infection. Our subsequent experiments implicated additional factors that interact with this pathway and suggest a more detailed model of the intracellular trafficking process, indicating that BKPyV reaches the endoplasmic reticulum (ER) lumen through a retrograde transport pathway between the late endosome and the ER.

IMPORTANCE Polyomaviruses bind to a group of specific gangliosides on the plasma membrane of the cell prior to being endocytosed. They then follow a retrograde trafficking pathway to reach the endoplasmic reticulum (ER). The viruses begin to disassemble in the ER and then exit the ER and move to the nucleus. However, the details of intracellular trafficking between the endosome and the ER are largely unknown. By implementing a whole human genome small interfering RNA screen, we identified Rab18, syntaxin 18, and the NRZ complex as key components in endosome-ER trafficking of the human polyomavirus BKPyV. These results serve to further elucidate the route BKPyV takes from outside the cell to its site of replication in the nucleus.

Expression and trafficking of MR1

MHC class I-related gene protein (MR1) is a non-polymorphic MHC class IB antigen-presenting molecule that is the restricting molecule for mucosal-associated invariant T (MAIT) cells, a prominent population of innate-like antibacterial T cells. The MAIT cell-MR1 axis represents a new paradigm in antigen presentation, with the MR1 ligand derived from vitamin B compounds or their metabolic precursors. Many bacteria and some fungi produce the activating ligand for MR1. In evolution, MR1 is highly conserved in most, but not all, mammals. In humans and rodents it is expressed in a broad range of cell types, both haematopoietic and non-haematopoietic, although cell surface expression has been difficult to detect. Although MR1 trafficking shares features with both the MHC class I and MHC class II pathways, it is distinct. Several strands of evidence suggest that the intracellular location where MR1 is loaded differs for soluble ligand and for ligand derived from intact bacteria. The regulation of MR1 surface expression may also vary between different cell types. This paper will review what is currently known about the expression and trafficking of MR1 and propose a model for the loading and trafficking of MR1.