ESCRT proteins: Double-edged regulators of cellular signaling

Cellular ESCRT components are recruited to regulate the endocytic trafficking and RNA replication compartment assembly during classical swine fever virus infection

As the important molecular machinery for membrane protein sorting in eukaryotic cells, the endosomal sorting and transport complexes (ESCRT-0/I/II/III and VPS4) usually participate in various replication stages of enveloped viruses, such as endocytosis and budding. The main subunit of ESCRT-I, Tsg101, has been previously revealed to play a role in the entry and replication of classical swine fever virus (CSFV). However, the effect of the whole ESCRT machinery during CSFV infection has not yet been well defined. Here, we systematically determine the effects of subunits of ESCRT on entry, replication, and budding of CSFV by genetic analysis. We show that EAP20 (VPS25) (ESCRT-II), CHMP4B and CHMP7 (ESCRT-III) regulate CSFV entry and assist vesicles in transporting CSFV from Clathrin, early endosomes, late endosomes to lysosomes. Importantly, we first demonstrate that HRS (ESCRT-0), VPS28 (ESCRT-I), VPS25 (ESCRT-II) and adaptor protein ALIX play important roles in the formation of virus replication complexes (VRC) together with CHMP2B/4B/7 (ESCRT-III), and VPS4A. Further analyses reveal these subunits interact with CSFV nonstructural proteins (NS) and locate in the endoplasmic reticulum, but not Golgi, suggesting the role of ESCRT in regulating VRC assembly. In addition, we demonstrate that VPS4A is close to lipid droplets (LDs), indicating the importance of lipid metabolism in the formation of VRC and nucleic acid production. Altogether, we draw a new picture of cellular ESCRT machinery in CSFV entry and VRC formation, which could provide alternative strategies for preventing and controlling the diseases caused by CSFV or other Pestivirus.

ESCRT machinery can be responsible for virus budding and participate in regulating virus entry. However, little has been reported on its effects on VRC formation. Here, we uncover the novel roles of ESCRT-III and VPS4A in VRC assembly and update the additional subunits involved in the intracellular trafficking of CSFV. These data indicate that the ESCRT machinery promotes CSFV replication by forming VRC, which making it become nuclease-insensitive to avoid the recognition by the host antiviral surveillance system and the destruction of the viral RNA. Furthermore, we first demonstrate that the roles of ESCRT components in the formation of VRC in swine Pestivirus. Our findings highlight the growing evidence of diverse interactions between ESCRT subunits and viral factors of Flaviviridae family, and provide alternative strategies for preventing and controlling the diseases caused by CSFV or other Pestivirus.

Vesicular PtdIns(3,4,5)P3 and Rab7 are key effectors of sea urchin zygote nuclear membrane fusion

Regulation of nuclear envelope dynamics is an important example of the universal phenomena of membrane fusion. The signalling molecules involved in nuclear membrane fusion might also be conserved during the formation of both pronuclear and zygote nuclear envelopes in the fertilised egg. Here, we determine that class-I phosphoinositide 3-kinases (PI3Ks) are needed for in vitro nuclear envelope formation. We show that, in vivo, PtdIns(3,4,5)P₃ is transiently located in vesicles around the male pronucleus at the time of nuclear envelope formation, and around male and female pronuclei before membrane fusion. We illustrate that class-I PI3K activity is also necessary for fusion of the female and male pronuclear membranes. We demonstrate, using coincidence amplified Förster resonance energy transfer (FRET) monitored using fluorescence lifetime imaging microscopy (FLIM), a protein-lipid interaction of Rab7 GTPase and PtdIns(3,4,5)P₃ that occurs during pronuclear membrane fusion to create the zygote nuclear envelope. We present a working model, which includes several molecular steps in the pathways controlling fusion of nuclear envelope membranes.

TDP-43 loss of function inhibits endosomal trafficking and alters trophic signaling in neurons

Nuclear clearance of TDP-43 into cytoplasmic aggregates is a key driver of neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), but the mechanisms are unclear. Here, we show that TDP-43 knockdown specifically reduces the number and motility of RAB11-positive recycling endosomes in dendrites, while TDP-43 overexpression has the opposite effect. This is associated with delayed transferrin recycling in TDP-43-knockdown neurons and decreased β2-transferrin levels in patient CSF Whole proteome quantification identified the upregulation of the ESCRT component VPS4B upon TDP-43 knockdown in neurons. Luciferase reporter assays and chromatin immunoprecipitation suggest that TDP-43 represses VPS4B transcription. Preventing VPS4B upregulation or expression of its functional antagonist ALIX restores trafficking of recycling endosomes. Proteomic analysis revealed the broad reduction in surface expression of key receptors upon TDP-43 knockdown, including ErbB4, the neuregulin 1 receptor. TDP-43 knockdown delays the surface delivery of ErbB4. ErbB4 overexpression, but not neuregulin 1 stimulation, prevents dendrite loss upon TDP-43 knockdown. Thus, impaired recycling of ErbB4 and other receptors to the cell surface may contribute to TDP-43-induced neurodegeneration by blocking trophic signaling.

Marked enhancement of lysosomal targeting and efficacy of ErbB2-targeted drug delivery by HSP90 inhibition

Targeted delivery of anticancer drugs to tumor cells using monoclonal antibodies against oncogenic cell surface receptors is an emerging therapeutic strategy. These strategies include drugs directly conjugated to monoclonal antibodies through chemical linkers (Antibody-Drug Conjugates, ADCs) or those encapsulated within nanoparticles that in turn are conjugated to targeting antibodies (Antibody-Nanoparticle Conjugates, ANPs). The recent FDA approval of the ADC Trastuzumab-TDM1 (Kadcyla; Genentech; San Francisco) for the treatment of ErbB2-overexpressing metastatic breast cancer patients has validated the strong potential of these strategies. Even though the activity of ANPs and ADCs is dependent on lysosomal traffic, the roles of the endocytic route traversed by the targeted receptor and of cancer cell-specific alterations in receptor dynamics on the efficiency of drug delivery have not been considered in these new targeted therapies. For example, constitutive association with the molecular chaperone HSP90 is thought to either retard ErbB2 endocytosis or to promote its recycling, traits undesirable for targeted therapy with ANPs and ADCs. HSP90 inhibitors are known to promote ErbB2 ubiquitination, targeting to lysosome and degradation. We therefore hypothesized that ErbB2-targeted drug delivery using Trastuzumab-conjugated nanoparticles could be significantly improved by HSP90 inhibitor-promoted lysosomal traffic of ErbB2. Studies reported here validate this hypothesis and demonstrate, both in vitro and in vivo, that HSP90 inhibition facilitates the intracellular delivery of Trastuzumab-conjugated ANPs carrying a model chemotherapeutic agent, Doxorubicin, specifically into ErbB2-overexpressing breast cancer cells, resulting in improved antitumor activity. These novel findings highlight the need to consider oncogene-specific alterations in receptor traffic in the design of targeted drug delivery strategies. We suggest that combination of agents that enhance receptor endocytosis and lysosomal routing can provide a novel strategy to significantly improve therapy with ANPs and ADCs.

Drosophila Vps4 promotes Epidermal growth factor receptor signaling independently of its role in receptor degradation

Endocytic trafficking of signaling receptors is an important mechanism for limiting signal duration. Components of the Endosomal Sorting Complexes Required for Transport (ESCRT), which target ubiquitylated receptors to intra-lumenal vesicles (ILVs) of multivesicular bodies, are thought to terminate signaling by the epidermal growth factor receptor (EGFR) and direct it for lysosomal degradation. In a genetic screen for mutations that affect Drosophila eye development, we identified an allele of Vacuolar protein sorting 4 (Vps4), which encodes an AAA ATPase that interacts with the ESCRT-III complex to drive the final step of ILV formation. Photoreceptors are largely absent from Vps4 mutant clones in the eye disc, and even when cell death is genetically prevented, the mutant R8 photoreceptors that develop fail to recruit surrounding cells to differentiate as R1-R7 photoreceptors. This recruitment requires EGFR signaling, suggesting that loss of Vps4 disrupts the EGFR pathway. In imaginal disc cells mutant for Vps4, EGFR and other receptors accumulate in endosomes and EGFR target genes are not expressed; epistasis experiments place the function of Vps4 at the level of the receptor. Surprisingly, Vps4 is required for EGFR signaling even in the absence of Shibire, the Dynamin that internalizes EGFR from the plasma membrane. In ovarian follicle cells, in contrast, Vps4 does not affect EGFR signaling, although it is still essential for receptor degradation. Taken together, these findings indicate that Vps4 can promote EGFR activity through an endocytosis-independent mechanism.

Emerging role of the scaffolding protein Dlg1 in vesicle trafficking

Discs large 1 (Dlg1) is a modular scaffolding protein implicated in the control of cell polarity through assembly of specific multiprotein complexes, including receptors, ion channels and signaling proteins, at specialized zones of the plasma membrane. Recent data have shown that in addition to these well-known interaction partners, Dlg1 may also recruit components of the vesicle trafficking machinery either to the plasma membrane or to transport vesicles. Here, we discuss Dlg1 function in vesicle formation, targeting, tethering and fusion, in both the exocytotic and endocytotic pathways. These pathways contribute to cell functions as major and diverse as glutamatergic activity in the neurons, membrane homeostasis in Schwann cell myelination, insulin stimulation of glucose transport in adipocytes, or endothelial secretion of the hemostatic protein, von Willebrand factor (VWF).

Endocytic regulation of alkali metal transport proteins in mammals, yeast and plants

The relative concentrations of ions and solutes inside cells are actively maintained by several classes of transport proteins, in many cases against their concentration gradient. These transport processes, which consume a large portion of cellular energy, must be constantly regulated. Many structurally distinct families of channels, carriers, and pumps have been characterized in considerable detail during the past decades and defects in the function of some of these proteins have been linked to a growing list of human diseases. The dynamic regulation of the transport proteins present at the cell surface is vital for both normal cellular function and for the successful adaptation to changing environments. The composition of proteins present at the cell surface is controlled on both the transcriptional and post-translational level. Post-translational regulation involves highly conserved mechanisms of phosphorylation- and ubiquitylation-dependent signal transduction routes used to modify the cohort of receptors and transport proteins present under any given circumstances. In this review, we will summarize what is currently known about one facet of this regulatory process: the endocytic regulation of alkali metal transport proteins. The physiological relevance, major contributors, parallels and missing pieces of the puzzle in mammals, yeast and plants will be discussed.

Exosomal Proteome Profiling: A Potential Multi-Marker Cellular Phenotyping Tool to Characterize Hypoxia-Induced Radiation Resistance in Breast Cancer

Radiation and drug resistance are significant challenges in the treatment of locally advanced, recurrent and metastatic breast cancer that contribute to mortality. Clinically, radiotherapy requires oxygen to generate cytotoxic free radicals that cause DNA damage and allow that damage to become fixed in the genome rather than repaired. However, approximately 40% of all breast cancers have hypoxic tumor microenvironments that render cancer cells significantly more resistant to irradiation. Hypoxic stimuli trigger changes in the cell death/survival pathway that lead to increased cellular radiation resistance. As a result, the development of noninvasive strategies to assess tumor hypoxia in breast cancer has recently received considerable attention. Exosomes are secreted nanovesicles that have roles in paracrine signaling during breast tumor progression, including tumor-stromal interactions, activation of proliferative pathways and immunosuppression. The recent development of protocols to isolate and purify exosomes, as well as advances in mass spectrometry-based proteomics have facilitated the comprehensive analysis of exosome content and function. Using these tools, studies have demonstrated that the proteome profiles of tumor-derived exosomes are indicative of the oxygenation status of patient tumors. They have also demonstrated that exosome signaling pathways are potentially targetable drivers of hypoxia-dependent intercellular signaling during tumorigenesis. This article provides an overview of how proteomic tools can be effectively used to characterize exosomes and elucidate fundamental signaling pathways and survival mechanisms underlying hypoxia-mediated radiation resistance in breast cancer.

BioArchitecture: the organization and regulation of biological space

BioArchitecture is a term used to describe the organization and regulation of biological space. It applies to the principles which govern the structure of molecules, polymers and mutiprotein complexes, organelles, membranes and their organization in the cytoplasm and the nucleus. It also covers the integration of cells into their three dimensional environment at the level of cell-matrix, cell-cell interactions, integration into tissue/organ structure and function and finally into the structure of the organism. This review will highlight studies at all these levels which are providing a new way to think about the relationship between the organization of biological space and the function of biological systems.

Multivesicular body morphogenesis

Multivesicular bodies (MVBs) are unique organelles in the endocytic pathway that contain vesicles in their lumen. Sorting and incorporation of material into such vesicles is a critical cellular process that has been intensely studied following discovery of the ESCRT (endosomal sorting complex required for transport) machinery just more than a decade ago. In this review, we summarize current understanding of the cellular functions of MVBs and how the ESCRT machinery contributes to MVB morphogenesis. We also highlight the importance of MVBs and ESCRTs in human health. We identify critical areas in which further mechanistic and spatiotemporal studies in living cells will advance this exciting area of research.