Abstract The role of vacuolar H+-ATPase (V-ATPase) in exocytic and endocytic membrane transport processes was studied by using its specific inhibitor, bafilomycin A1 (Baf A1), as a tool. On the exocytic pathway, both brefeldin A- and nocodazole-induced retrograde transport of Golgi proteins to the endoplasmic reticulum (ER) were inhibited by Baf A1. Furthermore, p58/ERGIC-53, which normally cycles between the ER, the intermediate compartment (IC), and cis-Golgi, was arrested in pre-Golgi tubules and vacuoles, and the number of p58-positive 80-nm Golgi (COPI) vesicles was reduced, suggesting that the drug inhibits the vesicle-mediated retrieval of the protein from post-ER compartments. The small GTPase rab1p was efficiently recruited to the tubules, accumulating in the presence of Baf A1. In contrast, these tubules showed no enrichment of anterogradely transported proteins, indicating that they participate in retrograde transport. Interestingly, acidic lumenal pH could only be detected in the more central pre-Golgi elements. The forward (anterograde) transport of newly synthesized Semliki Forest virus (SFV) and vesicular stomatitis virus (VSV) glycoproteins from the ER to the cis-Golgi was largely unaffected by Baf A1. However, maturation processes occurring in the trans-Golgi were inhibited, and the amounts of viral glycoproteins appearing at the cell surface were reduced. Newly synthesized VSV glycoprotein accumulated into rab1p-positive Golgi membranes in the presence of Baf A1, indicating that the transport from cis-Golgi was affected. Furthermore, O-glycosylation of the expressed CD8 chimeras and lectin cytochemistry experiments indicate that Baf A1 affects the transport from cis-Golgi. Instead, Baf A1 did not affect the transport of viral glycoproteins from the trans-Golgi network to the cell surface. We propose, that anterograde intra-Golgi traffic may be affected indirectly by Baf A1, as it inhibits retrograde vesicle-mediated transport and thus cisternal maturation. Baf A1 inhibited the entry of SFV into BHK-21 cells. Thus, V-ATPase was responsible for the acidification of the endosomes needed for virus entry. In cells infected with VSV and subsequently treated with Baf A1, virus particles were found to be accumulated in tubular membrane structures, which also contained endocytosed BSA-gold. Neither VSV nor BSA-gold particles were detected in lysosomal glycoprotein (lgp) 120-positive lysosomes, however. Thus, secreted and further endocytosed virus particles accumulate into tubulated endocytic organelles, apparently early endosomes, in Baf A1-treated cells. We conclude that the transport from endosomes to lysosomes is inhibited by Baf A1. The bulk of rab7 GTPase, which participates in vesicle fusion to late endosomes, was localized to the ruffled border (RB) membrane of bone-resorbing osteoclast. This indicates that the membrane has some characteristics of late endosomal membranes and that endocytic membrane transport is oriented towards the RB. Consistently, both endocytosed lumenal horseradish peroxidase and receptor-bound transferrin were delivered to the RB. The delivery of membrane-associated transferrin to the RB further indicates that the RB has some endosomal characteristics and suggests that the endocytic pathway contributes to the maintenance of functional RB. The endocytic pathway could act in balancing the membrane traffic associated with transcytosis from the RB to the basal plasma membrane. Endocytic processes in osteoclasts appeared to be very sensitive to Baf A1. Thus, blocking of the endocytic membrane traffic towards the RB could explain the inactivation of cells by low concentrations of the drug.

In type 1 diabetes (T1D), an autoimmune disease, autoantibodies are preferentially directed against proteins associated with Golgi and post-Golgi secretory vesicles, including insulin secretory granules and synaptic-like microvesicles. Thus, the study of beta-cell autoantigens with yet unknown function may provide novel insight into the secretory machinery of beta-cells and led to the discovery of novel pathways. Islet cell autoantigen of 69 kDa (ICA69) is a T1D autoantigen. It is a cytosolic protein of still unknown function. An impairment in neurotransmitter release upon mutation of its homologue in C. elegans suggests, however, an involvement of ICA69 in neurosecretion. Interestingly, ICA69 contains a BAR domain, present in several proteins involved in intracellular transport. The BAR domain functions as a dimerization motif, provides a general binding interface for different types of GTPases, and is a membrane binding/bending module. Its presence in ICA69 is a further hint supporting the putative involvement of ICA69 in intracellular membrane trafficking. The first part of this thesis was concerned with the characterization of ICA69, and the elucidation of its role in membrane traffic in pancreatic beta-cells. ICA69 was shown to be enriched in the perinuclear region, where also markers of the Golgi region are found. ICA69 was shown to interact with several membrane lipids, preferentially with PI(4)P, enriched on the Golgi complex. During the course of this thesis a combination of biochemical and imaging techniques were applied to investigate the interaction between ICA69 and Rab2, a small GTPase associated with the intermediate compartment and involved in the trafficking between the ER and the Golgi complex. ICA69 was shown to co-immunoprecipitate with Rab2 from INS-1 cells extracts. GST-pull down assays demonstrated that this interaction is GTP-dependent. Furthermore, confocal microscopy indicated that ICA69 and Rab2 extensively colocalize in particulate structures throughout the cytoplasm. Immunocytochemistry and subcellular fractionation experiments suggested that Rab2 recruits ICA69 to membranes. Functional studies indicated that ICA69 over-expression in INS-1 cells has effects that resemble, and in some cases amplify those observed upon Rab2 over-expression. Specifically, it impairs the trafficking between ER and Golgi, measured through the appearance and the conversion of the pro-form of ICA512 in the mature form of the protein. Moreover, it correlates with a redistribution of the beta-COP subunit of the coatomer, participating in the early secretory pathway, between membrane-bound compartments and the cytosol and it reduces stimulated insulin secretion. The data reported in this thesis conclusively point to ICA69 as a novel Rab2 effector, and may therefore contribute to the elucidation the yet poorly understood mechanism of action of Rab2 in the secretory pathway. The second part of the thesis was devoted to the study of an ICA69 paralogue gene, called ICA69-RP. Similarly to ICA69, ICA69-RP mRNA was shown to be primarily present in tissues such as brain and pancreatic islets, showing the expression pattern of a gene preferentially expressed in neuroendocrine cells. Unlike ICA69, however, and similar to other genes associated with the secretory machinery of beta-cells, ICA69-RP appeared to be glucose regulated, as shown by a 1.55 fold increase in mRNA levels upon stimulation of the cells with 25 mM glucose for two hours.Glucose stimulation of beta-cells prompts the activation of post-transcripional mechanisms which quickly up-regulate the expression of secretory granule genes and consequently renew granule stores. The increased expression of ICA69-RP upon glucose stimulation of cells may be part of this process. Unfortunately, all attempts to elucidate the intracellular localization of endogenous ICA69-RP failed, and it was not possible to obtain significant insights about its localization by over-expressing a fusion protein between ICA69-RP and GFP. Unlike other paralogues containing the BAR domain, such as amphiphysin 1 and 2 or Rvs167p and Rvs161p, ICA69 and ICA69-RP were shown not to form heterodimers. Furthermore, ICA69-RP did not show any interaction with Rab2 or Rab1, involved in the anterograde transport between ER and Golgi. Thus, its physiological role remains to be investigated.

In type 1 diabetes (T1D), an autoimmune disease, autoantibodies are preferentially directed against proteins associated with Golgi and post-Golgi secretory vesicles, including insulin secretory granules and synaptic-like microvesicles. Thus, the study of beta-cell autoantigens with yet unknown function may provide novel insight into the secretory machinery of beta-cells and led to the discovery of novel pathways. Islet cell autoantigen of 69 kDa (ICA69) is a T1D autoantigen. It is a cytosolic protein of still unknown function. An impairment in neurotransmitter release upon mutation of its homologue in C. elegans suggests, however, an involvement of ICA69 in neurosecretion. Interestingly, ICA69 contains a BAR domain, present in several proteins involved in intracellular transport. The BAR domain functions as a dimerization motif, provides a general binding interface for different types of GTPases, and is a membrane binding/bending module. Its presence in ICA69 is a further hint supporting the putative involvement of ICA69 in intracellular membrane trafficking. The first part of this thesis was concerned with the characterization of ICA69, and the elucidation of its role in membrane traffic in pancreatic beta-cells. ICA69 was shown to be enriched in the perinuclear region, where also markers of the Golgi region are found. ICA69 was shown to interact with several membrane lipids, preferentially with PI(4)P, enriched on the Golgi complex. During the course of this thesis a combination of biochemical and imaging techniques were applied to investigate the interaction between ICA69 and Rab2, a small GTPase associated with the intermediate compartment and involved in the trafficking between the ER and the Golgi complex. ICA69 was shown to co-immunoprecipitate with Rab2 from INS-1 cells extracts. GST-pull down assays demonstrated that this interaction is GTP-dependent. Furthermore, confocal microscopy indicated that ICA69 and Rab2 extensively colocalize in particulate structures throughout the cytoplasm. Immunocytochemistry and subcellular fractionation experiments suggested that Rab2 recruits ICA69 to membranes. Functional studies indicated that ICA69 over-expression in INS-1 cells has effects that resemble, and in some cases amplify those observed upon Rab2 over-expression. Specifically, it impairs the trafficking between ER and Golgi, measured through the appearance and the conversion of the pro-form of ICA512 in the mature form of the protein. Moreover, it correlates with a redistribution of the beta-COP subunit of the coatomer, participating in the early secretory pathway, between membrane-bound compartments and the cytosol and it reduces stimulated insulin secretion. The data reported in this thesis conclusively point to ICA69 as a novel Rab2 effector, and may therefore contribute to the elucidation the yet poorly understood mechanism of action of Rab2 in the secretory pathway. The second part of the thesis was devoted to the study of an ICA69 paralogue gene, called ICA69-RP. Similarly to ICA69, ICA69-RP mRNA was shown to be primarily present in tissues such as brain and pancreatic islets, showing the expression pattern of a gene preferentially expressed in neuroendocrine cells. Unlike ICA69, however, and similar to other genes associated with the secretory machinery of beta-cells, ICA69-RP appeared to be glucose regulated, as shown by a 1.55 fold increase in mRNA levels upon stimulation of the cells with 25 mM glucose for two hours.Glucose stimulation of beta-cells prompts the activation of post-transcripional mechanisms which quickly up-regulate the expression of secretory granule genes and consequently renew granule stores. The increased expression of ICA69-RP upon glucose stimulation of cells may be part of this process. Unfortunately, all attempts to elucidate the intracellular localization of endogenous ICA69-RP failed, and it was not possible to obtain significant insights about its localization by over-expressing a fusion protein between ICA69-RP and GFP. Unlike other paralogues containing the BAR domain, such as amphiphysin 1 and 2 or Rvs167p and Rvs161p, ICA69 and ICA69-RP were shown not to form heterodimers. Furthermore, ICA69-RP did not show any interaction with Rab2 or Rab1, involved in the anterograde transport between ER and Golgi. Thus, its physiological role remains to be investigated.

Myotubularins are lipid phosphatases that dephosphorylate phosphatidylinositol 3-phosphate and phosphatidylinositol 3,5-bisphosphate the position three of the inositol ring. This allows them to regulate the structure of the lipid layer of the membrane compartment. The first member of the family was described in association with a severe hereditary myopathy. From that point on, another thirteen members have been added to the family. The catalytically inactive MTMR9 carrying the conserved mutation in the phosphatase domain regulates the localization of the marker of the early secretory pathway, RAB1A, the cis-Golgi structure and the secretion. MTMR9 interacts with the catalytically active MTMR6 and MTMR8 that specifically localizes and increases their phophatase activity. The aim of this diploma thesis was to find out whether the phenotype observed in cells with altered MTMR9 levels is dependent on the catalytically active phosphatases MTMR6 and MTMR8. We proved the influence of MTMR6 and MTMR8 on the distribution of tranfected RAB1A between the intermediate compartment and the Golgi apparatus. MTMR6 and MTMR8 also take part in regulating the cis-Golgi structure. By the use of two different approaches we did not manage to clarify the influence of MTMR6 and MTMR8 on secretion. Changes in the catalytic...