Multiple routes of protein transport from endosomes to the trans Golgi network

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

Annexin A1 and A2: roles in retrograde trafficking of Shiga toxin

Annexins constitute a family of calcium and membrane binding proteins. As annexin A1 and A2 have previously been linked to various membrane trafficking events, we initiated this study to investigate the role of these annexins in the uptake and intracellular transport of the bacterial Shiga toxin (Stx) and the plant toxin ricin. Once endocytosed, both toxins are retrogradely transported from endosomes to the Golgi apparatus and the endoplasmic reticulum before being targeted to the cytosol where they inhibit protein synthesis. This study was performed to obtain new information both about toxin transport and the function of annexin A1 and annexin A2. Our data show that depletion of annexin A1 or A2 alters the retrograde transport of Stx but not ricin, without affecting toxin binding or internalization. Knockdown of annexin A1 increases Golgi transport of Stx, whereas knockdown of annexin A2 slightly decreases the same transport step. Interestingly, annexin A1 was found in proximity to cytoplasmic phospholipase A2 (cPLA₂), and the basal as well as the increased Golgi transport of Stx upon annexin A1 knockdown is dependent on cPLA₂ activity. In conclusion, annexin A1 and A2 have different roles in Stx transport to the trans-Golgi network. The most prominent role is played by annexin A1 which normally works as a negative regulator of retrograde transport from the endosomes to the Golgi network, most likely by complex formation and inhibition of cPLA₂.

Amyloid precursor protein (APP) traffics from the cell surface via endosomes for amyloid β (Aβ) production in the trans-Golgi network

Amyloid precursor protein (APP) is processed sequentially by the β-site APP cleaving enzyme and γ-secretase to generate amyloid β (Aβ) peptides, one of the hallmarks of Alzheimer's disease. The intracellular location of Aβ production-endosomes or the trans-Golgi network (TGN)-remains uncertain. We investigated the role of different postendocytic trafficking events in Aβ(40) production using an RNAi approach. Depletion of Hrs and Tsg101, acting early in the multivesicular body pathway, retained APP in early endosomes and reduced Aβ(40) production. Conversely, depletion of CHMP6 and VPS4, acting late in the pathway, rerouted endosomal APP to the TGN for enhanced APP processing. We found that VPS35 (retromer)-mediated APP recycling to the TGN was required for efficient Aβ(40) production. An interruption of the bidirectional trafficking of APP between the TGN and endosomes, particularly retromer-mediated retrieval of APP from early endosomes to the TGN, resulted in the accumulation of endocytosed APP in early endosomes with reduced APP processing. These data suggest that Aβ(40) is generated predominantly in the TGN, relying on an endocytosed pool of APP recycled from early endosomes to the TGN.

Retromer guides STxB and CD8-M6PR from early to recycling endosomes, EHD1 guides STxB from recycling endosome to Golgi

Retrograde trafficking transports proteins, lipids and toxins from the plasma membrane to the Golgi and endoplasmic reticulum (ER). To reach the Golgi, these cargos must transit the endosomal system, consisting of early endosomes (EE), recycling endosomes, late endosomes and lysosomes. All cargos pass through EE, but may take different routes to the Golgi. Retromer-dependent cargos bypass the late endosomes to reach the Golgi. We compared how two very different retromer-dependent cargos negotiate the endosomal sorting system. Shiga toxin B, bound to the external layer of the plasma membrane, and chimeric CD8-mannose-6-phosphate receptor (CI-M6PR), which is anchored via a transmembrane domain. Both appear to pass through the recycling endosome. Ablation of the recycling endosome diverted both of these cargos to an aberrant compartment and prevented them from reaching the Golgi. Once in the recycling endosome, Shiga toxin required EHD1 to traffic to the TGN, while the CI-M6PR was not significantly dependent on EHD1. Knockdown of retromer components left cargo in the EE, suggesting that it is required for retrograde exit from this compartment. This work establishes the recycling endosome as a required step in retrograde traffic of at least these two retromer-dependent cargos. Along this pathway, retromer is associated with EE to recycling endosome traffic, while EHD1 is associated with recycling endosome to TGN traffic of STxB.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

OCRL controls trafficking through early endosomes via PtdIns4,5P₂-dependent regulation of endosomal actin

Mutations in the phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P(2)) 5-phosphatase OCRL cause Lowe syndrome, which is characterised by congenital cataracts, central hypotonia, and renal proximal tubular dysfunction. Previous studies have shown that OCRL interacts with components of the endosomal machinery; however, its role in endocytosis, and thus the pathogenic mechanisms of Lowe syndrome, have remained elusive. Here, we show that via its 5-phosphatase activity, OCRL controls early endosome (EE) function. OCRL depletion impairs the recycling of multiple classes of receptors, including megalin (which mediates protein reabsorption in the kidney) that are retained in engorged EEs. These trafficking defects are caused by ectopic accumulation of PtdIns4,5P(2) in EEs, which in turn induces an N-WASP-dependent increase in endosomal F-actin. Our data provide a molecular explanation for renal proximal tubular dysfunction in Lowe syndrome and highlight that tight control of PtdIns4,5P(2) and F-actin at the EEs is essential for exporting cargoes that transit this compartment.

Tumour suppressive microRNA-874 regulates novel cancer networks in maxillary sinus squamous cell carcinoma

BACKGROUND

On the basis of the microRNA (miRNA) expression signature of maxillary sinus squamous cell carcinoma (MSSCC), we found that miR-874 was significantly reduced in cancer cells. We focused on the functional significance of miR-874 in cancer cells and identification of miR-874-regulated novel cancer networks in MSSCC.

METHODS

We used PCR-based methods to investigate the downregulated miRNAs in clinical specimens of MSSCC. Our signature analyses identified 23 miRNAs that were significantly reduced in cancer cells, such as miR-874, miR-133a, miR-375, miR-204, and miR-1. We focused on miR-874 as the most downregulated novel miRNA in our analysis.

RESULTS

We found potential tumour suppressive functions such as inhibition of cancer cell proliferation and invasion. A molecular target search of miR-874 revealed that PPP1CA was directly regulated by miR-874. Overexpression of PPP1CA was observed in MSSCC clinical specimens. Silencing of the PPP1CA gene significantly inhibited cancer cell proliferation and invasion.

CONCLUSION

The downregulation of miR-874 was a frequent event in MSSCC, which suggests that miR-874 functions as a tumour suppressive miRNA, directly regulating PPP1CA that has a potential role of an oncogene. The identification of novel miR-874-regulated cancer pathways could provide new insights into potential molecular mechanisms of MSSCC oncogenesis.

Endosome maturation

Being deeply connected to signalling, cell dynamics, growth, regulation, and defence, endocytic processes are linked to almost all aspects of cell life and disease. In this review, we focus on endosomes in the classical endocytic pathway, and on the programme of changes that lead to the formation and maturation of late endosomes/multivesicular bodies. The maturation programme entails a dramatic transformation of these dynamic organelles disconnecting them functionally and spatially from early endosomes and preparing them for their unidirectional role as a feeder pathway to lysosomes.

Dynamic remodeling of the actin cytoskeleton by FMNL1γ is required for structural maintenance of the Golgi complex

Formin-like 1 (FMNL1) is a member of the formin family of actin nucleators, and is one of the few formins for which in vitro activities have been well characterized. However, the functional roles of this mammalian formin remain ill-defined. In particular, it is unclear how the unique in vitro biochemical properties of FMNL1 relate to its regulation of cellular processes. Here, we demonstrate that FMNL1 depletion caused a dramatic increase in cellular F-actin content, which resulted in Golgi complex fragmentation. Moreover, increased F-actin and maintenance of Golgi structure were distinctly regulated by the gamma isoform of FMNL1, which required binding to actin. Importantly, in addition to Golgi fragmentation, increased F-actin content in the absence of FMNL1 also led to cation-independent mannose 6-phosphate receptor dispersal, lysosomal enlargement and missorting of cathepsin D. Taken together, our data support a model in which FMNL1 regulates cellular F-actin levels required to maintain structural integrity of the Golgi complex and lysosomes.

The COG complex interacts directly with Syntaxin 6 and positively regulates endosome-to-TGN retrograde transport

The conserved oligomeric Golgi (COG) complex interacts with the t-SNARE Syntaxin 6 and promotes endosome-to-TGN retrograde trafficking.

The conserved oligomeric Golgi (COG) complex has been implicated in the regulation of endosome to trans-Golgi network (TGN) retrograde trafficking in both yeast and mammals. However, the exact mechanisms by which it regulates this transport route remain largely unknown. In this paper, we show that COG interacts directly with the target membrane SNARE (t-SNARE) Syntaxin 6 via the Cog6 subunit. In Cog6-depleted cells, the steady-state level of Syntaxin 6 was markedly reduced, and concomitantly, endosome-to-TGN retrograde traffic was significantly attenuated. Cog6 knockdown also affected the steady-state levels and/or subcellular distributions of Syntaxin 16, Vti1a, and VAMP4 and impaired the assembly of the Syntaxin 6–Syntaxin16–Vti1a–VAMP4 SNARE complex. Remarkably, overexpression of VAMP4, but not of Syntaxin 6, bypassed the requirement for COG and restored endosome-to-TGN trafficking in Cog6-depleted cells. These results suggest that COG directly interacts with specific t-SNAREs and positively regulates SNARE complex assembly, thereby affecting their associated trafficking steps.

Mechanisms of protein retention in the Golgi

The protein composition of the Golgi is intimately linked to its structure and function. As the Golgi serves as the major protein-sorting hub for the secretory pathway, it faces the unique challenge of maintaining its protein composition in the face of constant influx and efflux of transient cargo proteins. Much of our understanding of how proteins are retained in the Golgi has come from studies on glycosylation enzymes, largely because of the compartment-specific distributions these proteins display. From these and other studies of Golgi membrane proteins, we now understand that a variety of retention mechanisms are employed, the majority of which involve the dynamic process of iterative rounds of retrograde and anterograde transport. Such mechanisms rely on protein conformation and amino acid-based sorting signals as well as on properties of transmembrane domains and their relationship with the unique lipid composition of the Golgi.

GD1b-derived gangliosides modulate FcεRI endocytosis in mast cells

The role of the mast cell-specific gangliosides in the modulation of the endocytic pathway of FcεRI was investigated in RBL-2H3 cells and in the ganglioside-deficient cell lines, E5 and D1. MAb BC4, which binds to the α subunit of FcεRI, was used in the analysis of receptor internalization. After incubation with BC4-FITC for 30 min, endocytic vesicles in RBL-2H3 and E5 cells were dispersed in the cytoplasm. After 1 hr, the endocytic vesicles of the RBL-2H3 cells had fused and formed clusters, whereas in the E5 cells, the fusion was slower. In contrast, in D1 cells, the endocytic vesicles were smaller and remained close to the plasma membrane even after 3 hr of incubation. When incubated with BC4-FITC and subsequently imunolabeled for markers of various endocytic compartments, a defect in the endocytic pathway in the E5 and D1 cells became evident. In the D1 cells, this defect was observed at the initial steps of endocytosis. Therefore, the ganglioside derivatives from GD1b are important in the endocytosis of FcεRI in mast cells. Because gangliosides may play a role in mast cell-related disease processes, they provide an attractive target for drug therapy and diagnosis.

Retrograde transport: two (or more) roads diverged in an endosomal tree?

Some proteins and lipids traffic from the plasma membrane to the trans Golgi network (TGN)/Golgi apparatus and the endoplasmic reticulum, via the retrograde transport route. Endosomes are an obligatory through station. Whether early, recycling and late endosomes all hand off material to the TGN have remained a matter of debate. In this review, we give a short historical overview on how retrograde transport was discovered and explored. We then summarize and critically discuss data that have been put forward in favour of the existence of trafficking interfaces between each of the different endocytic localizations and the TGN. We finally point out some conceptual and technological challenges that will have to be met to establish definite conclusions for each of these scenarios.

Entry at the trans-face of the Golgi

The trans-Golgi network (TGN) receives a select set of proteins from the endocytic pathway-about 5% of total plasma membrane glycoproteins (Duncan and Kornfeld 1988). Proteins that are delivered include mannose 6-phosphate receptors (MPRs), TGN46, sortilin, and various toxins that hitchhike a ride backward through the secretory pathway to intoxicate cells after they exit into the cytoplasm from the endoplasmic reticulum (ER). This article will review work on the molecular players that drive protein transport from the endocytic pathway to the TGN. Distinct requirements have revealed multiple routes for retrograde transport; in addition, the existence of multiple, potential coat proteins and/or cargo adaptors imply that multiple vesicular transfers are likely involved. Several comprehensive reviews have appeared recently and should be sought for additional details (Bonifacino and Rojas 2006; Johannes and Popoff 2008).

HIV-1 Vpu blocks recycling and biosynthetic transport of the intrinsic immunity factor CD317/tetherin to overcome the virion release restriction

The intrinsic immunity factor CD317 (BST-2/HM1.24/tetherin) imposes a barrier to HIV-1 release at the cell surface that can be overcome by the viral protein Vpu. Expression of Vpu results in a reduction of CD317 surface levels; however, the mechanism of this Vpu activity and its contribution to the virological antagonism are incompletely understood. Here, we characterized the influence of Vpu on major CD317 trafficking pathways using quantitative antibody-based endocytosis and recycling assays as well as a microinjection/microscopy-based kinetic de novo expression approach. We report that HIV-1 Vpu inhibited both the anterograde transport of newly synthesized CD317 and the recycling of CD317 to the cell surface, while the kinetics of CD317 endocytosis remained unaffected. Vpu trapped trafficking CD317 molecules at the trans-Golgi network, where the two molecules colocalized. The subversion of both CD317 transport pathways was dependent on the highly conserved diserine S52/S56 motif of Vpu; however, it did not require recruitment of the diserine motif interactor and substrate adaptor of the SCF-E3 ubiquitin ligase complex, β-TrCP. Treatment of cells with the malaria drug primaquine resulted in a CD317 trafficking defect that mirrored that induced by Vpu. Importantly, primaquine could functionally replace Vpu as a CD317 antagonist and rescue HIV-1 particle release.

IMPORTANCE

HIV efficiently replicates in the human host and induces the life-threatening immunodeficiency AIDS. Mammalian genomes encode proteins such as CD317 that can inhibit viral replication at the cellular level. As a countermeasure, HIV has evolved genes like vpu that can antagonize these intrinsic immunity factors. Investigating the mechanism by which Vpu overcomes the virion release restriction imposed by CD317, we find that Vpu subverts recycling and anterograde trafficking pathways of CD317, resulting in surface levels of the restriction factor insufficient to block HIV-1 spread. This describes a novel mechanism of immune evasion by HIV.

Transport at the recycling endosome

The recycling endosome (RE) has long been considered as a sub-compartment of the early endosome that recycles internalized cargoes to the plasma membrane. The RE is now appreciated to participate in a more complex set of intracellular itineraries. Key cargo molecules and transport factors that act in these pathways are being identified. These advancements are beginning to reveal complexities in pathways involving the RE, and also suggest ways of further delineating functional domains of this compartment.