The molecular mechanisms underlying early/recycling endosomes-to-TGN transport are still not understood. We identified interactions between the TGN-localized putative t-SNAREs syntaxin 6, syntaxin 16, and Vti1a, and two early/recycling endosomal v-SNAREs, VAMP3/cellubrevin, and VAMP4. Using a novel permeabilized cell system, these proteins were functionally implicated in the post-Golgi retrograde transport step. The function of Rab6a' was also required, whereas its closely related isoform, Rab6a, has previously been implicated in Golgi-to-endoplasmic reticulum transport. Thus, our study shows that membrane exchange between the early endocytic and the biosynthetic/secretory pathways involves specific components of the Rab and SNARE machinery, and suggests that retrograde transport between early/recycling endosomes and the endoplasmic reticulum is critically dependent on the sequential action of two members of the Rab6 subfamily.

The most prevalent disease-causing mutation of CFTR is the deletion of Phe508 (ΔF508), which leads to defects in conventional Golgi-mediated exocytosis and cell surface expression. We report that ΔF508-CFTR surface expression can be rescued in vitro and in vivo by directing it to an unconventional GRASP-dependent secretion pathway. An integrated molecular and physiological analysis indicates that mechanisms associated with ER stress induce cell surface trafficking of the ER core-glycosylated wild-type and ΔF508-CFTR via the GRASP-dependent pathway. Phosphorylation of a specific site of GRASP and the PDZ-based interaction between GRASP and CFTR are critical for this unconventional surface trafficking. Remarkably, transgenic expression of GRASP in ΔF508-CFTR mice restores CFTR function and rescues mouse survival without apparent toxicity. These findings provide insight into how unconventional protein secretion is activated, and offer a potential therapeutic strategy for the treatment of cystic fibrosis and perhaps diseases stemming from other misfolded proteins.

Fragmented or otherwise miniaturized plastic materials in the form of micro- or nanoplastics have been of nagging environmental concern. Perturbation of organismal physiology and behavior by micro- and nanoplastics have been widely documented for marine invertebrates. Some of these effects are also manifested by larger marine vertebrates such as fishes. More recently, possible effects of micro- and nanoplastics on mammalian gut microbiota as well as host cellular and metabolic toxicity have been reported in mouse models. Human exposure to micro- and nanoplastics occurs largely through ingestion, as these are found in food or derived from food packaging, but also in a less well-defined manner though inhalation. The pathophysiological consequences of acute and chronic micro- and nanoplastics exposure in the mammalian system, particularly humans, are yet unclear. In this review, we focus on the recent findings related to the potential toxicity and detrimental effects of micro- and nanoplastics as demonstrated in mouse models as well as human cell lines. The prevailing data suggest that micro- and nanoplastics accumulation in mammalian and human tissues would likely have negative, yet unclear long-term consequences. There is a need for cellular and systemic toxicity due to micro- and nanoplastics to be better illuminated, and the underlying mechanisms defined by further work.

Poliovirus (PV) replicates its genome in association with membranous vesicles in the cytoplasm of infected cells. To elucidate the origin and mode of formation of PV vesicles, immunofluorescence labeling with antibodies against the viral vesicle marker proteins 2B and 2BC, as well as cellular markers of the endoplasmic reticulum (ER), anterograde transport vesicles, and the Golgi complex, was performed in BT7-H cells. Optical sections obtained by confocal laser scanning microscopy were subjected to a deconvolution process to enhance resolution and signal-to-noise ratio and to allow for a three-dimensional representation of labeled membrane structures. The mode of formation of the PV vesicles was, on morphological grounds, similar to the formation of anterograde membrane traffic vesicles in uninfected cells. ER-resident membrane markers were excluded from both types of vesicles, and the COPII components Sec13 and Sec31 were both found to be colocalized on the vesicular surface, indicating the presence of a functional COPII coat. PV vesicle formation during early time points of infection did not involve the Golgi complex. The expression of PV protein 2BC or the entire P2 and P3 genomic region led to the production of vesicles carrying a COPII coat and showing the same mode of formation as vesicles produced after PV infection. These results indicate that PV vesicles are formed at the ER by the cellular COPII budding mechanism and thus are homologous to the vesicles of the anterograde membrane transport pathway.Poliovirus (PV), the prototype member of the family Picornaviridae, contains an RNA genome of plus polarity with a single large open reading frame (ORF) coding for a polyprotein of 265 kDa. The translation product is cleaved by intrinsic viral proteases into about 20 proteins, of which intermediate as well as end products are functional (38, 83, 88; reviewed in reference 35). The proteins encoded by the P1 genomic region form the capsid, whereas most of the nonstructural proteins, encoded by the P2 and P3 genomic region, are involved in viral RNA replication (2,27,28,37,48,55,58,82). The viral genome replicates asymmetrically with an excess of plus strands produced in multistranded replicative intermediates (2,17,31,49). PV genome replication depends on the template RNA molecule carrying a cis-acting replicative element (see references 32 and 54), a primer (54), viral and cellular proteins and their proper interactions (2,29,30,53), and the presence of membranes (10,12,14,26,74).Genome replication of all plus-strand RNA viruses investigated so far takes place in membrane-bound replication complexes (see references 19 and 65 and references therein). However, the morphology of such replication complexes of different viruses is diverse, and different cellular compartments provide membranes for and are involved in viral replication (44,56,65). PV replication complexes are built up from individual membranous vesicles (PV vesicles) which are assembled into specific higher-order structures (11,12,15).The PV replicat...

Sirt1 is the most prominent and extensively studied member of sirtuins, the family of mammalian class III histone deacetylases heavily implicated in health span and longevity. Although primarily a nuclear protein, Sirt1’s deacetylation of Peroxisome proliferator-activated receptor Gamma Coactivator-1α (PGC-1α) has been extensively implicated in metabolic control and mitochondrial biogenesis, which was proposed to partially underlie Sirt1’s role in caloric restriction and impacts on longevity. The notion of Sirt1’s regulation of PGC-1α activity and its role in mitochondrial biogenesis has, however, been controversial. Interestingly, Sirt1 also appears to be important for the turnover of defective mitochondria by mitophagy. I discuss here evidences for Sirt1’s regulation of mitochondrial biogenesis and turnover, in relation to PGC-1α deacetylation and various aspects of cellular physiology and disease.

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We report here the characterization of gp27 (hp24␥ 3 ), a glycoprotein of the p24 family of small and abundant transmembrane proteins of the secretory pathway. Immunoelectron and confocal scanning microscopy show that at steady state, gp27 localizes to the cis side of the Golgi apparatus. In addition, some gp27 was detected in COPI-and COPII-coated structures throughout the cytoplasm. This indicated cycling that was confirmed in three ways. First, 15°C temperature treatment resulted in accumulation of gp27 in pre-Golgi structures colocalizing with anterograde cargo. Second, treatment with brefeldin A caused gp27 to relocate into peripheral structures positive for both KDEL receptor and COPII. Third, microinjection of a dominant negative mutant of Sar1p trapped gp27 in the endoplasmic reticulum (ER) by blocking ER export. Together, this shows that gp27 cycles extensively in the early secretory pathway. Immunoprecipitation and coexpression studies further revealed that a significant fraction of gp27 existed in a hetero-oligomeric complex. Three members of the p24 family, GMP25 (hp24␣ 2 ), p24 (hp24␤ 1 ), and p23 (hp24␦ 1 ), coprecipitated in what appeared to be stochiometric amounts. This heterocomplex was specific. Immunoprecipitation of p26 (hp24␥ 4 ) failed to coprecipitate GMP25, p24, or p23. Also, very little p26 was found coprecipitating with gp27. A functional requirement for complex formation was suggested at the level of ER export. Transiently expressed gp27 failed to leave the ER unless other p24 family proteins were coexpressed. Comparison of attached oligosaccharides showed that gp27 and GMP25 recycled differentially. Only a very minor portion of GMP25 displayed complex oligosaccharides. In contrast, all of gp27 showed modifications by medial and trans enzymes at steady state. We conclude from these data that a portion of gp27 exists as hetero-oligomeric complexes with GMP25, p24, and p23 and that these complexes are in dynamic equilibrium with individual p24 proteins to allow for differential recycling and distributions.

An in vitro transport assay, established with a modified Shiga toxin B subunit (STxB) as a marker, has proved to be useful for the study of transport from the early/recycling endosome (EE/RE) to the trans-Golgi network (TGN). Here, we modified this assay to test antibodies to all known soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) that have been shown to localize in the Golgi and found that syntaxin 5, GS28, Ykt6, and GS15 antibodies specifically inhibited STxB transport. Because syntaxin 5, GS28, Ykt6, and GS15 exist as a unique SNARE complex, our observation indicates that these four SNAREs function as a complex in EE/RE-TGN transport. The importance of GS15 in EE/RE-TGN transport was further demonstrated by a block in recombinant STxB transport in HeLa cells when GS15 expression was knocked down by its small interfering iRNA. Morphological analyses showed that some GS15 and Ykt6 were redistributed from the Golgi to the endosomes when the recycling endosome was perturbed by SNX3-overexpression, suggesting that GS15 and Ykt6 might cycle between the endosomes and the Golgi apparatus. Further studies indicated that syntaxin 5 and syntaxin 16 exerted their role in EE/RE-TGN transport in an additive manner. The kinetics of inhibition exhibited by syntaxin 16 and syntaxin 5 antibodies is similar. INTRODUCTIONMammalian cells endocytose a variety of molecules. Some of them escape from the lysosomal degradative pathway and are instead delivered to the Golgi apparatus. Examples of these are some protein toxins such as cholera toxin, Shiga toxin, and ricin (Sandvig and van Deurs, 2002) as well as some endogenous proteins such as TGN38 (Ghosh et al., 1998;Mallet and Maxfield, 1999), mannose 6-phosphate receptor (Goda and Pfeffer, 1988), furin (Mallet and Maxfield, 1999), GLUT4 (Shewan et al., 2003), and some glycosylphosphatidylinositol (GPI)-anchored proteins (Nichols et al., 2001;Nichols, 2002). To date, several retrograde transport pathways in the endocytic route to the trans-Golgi network (TGN) have been identified. These include a well-studied pathway from the late endosome to the TGN taken by mannose 6-phosphate receptors (Goda and Pfeffer, 1988) and furin (Mallet and Maxfield, 1999), and a newly discovered direct pathway from the early/recycling endosome to the TGN taken by TGN38 (Ghosh et al., 1998;Mallet and Maxfield, 1999), GLUT4 (Shewan et al., 2003), and exogenously added Shiga toxin B subunit (Mallard et al., 1998). Similar transport from the late endosome (prevacuolar compartment) and the early/recycling endosome (post-Golgi compartment) to the TGN (late Golgi) has been described in the yeast Saccharomyces cerevisiae (Bensen et al., 2001;Siniossoglou et al., 2001). Recently, GPI-anchored green fluorescent protein (GFP), CD59, and a fraction of cholera toxin B subunit have been found to accumulate in a discrete population of endosomes en route to the Golgi apparatus (Nichols, 2002). These endosomes are devoid of markers for classical early and recycling endosomes, but they do...