Cell polarity and migration: emerging role for the endosomal sorting machinery

Embryonic signals mediate extracellular vesicle biogenesis and trafficking at the embryo-maternal interface

BACKGROUND

Extracellular vesicles (EVs) are membrane-coated nanoparticles secreted by almost all cell types in living organisms. EVs, as paracrine mediators, are involved in intercellular communication, immune response, and several reproductive events, including the maintenance of pregnancy. Using a domestic animal model (Sus scrofa) with an epitheliochorial, superficial type of placentation, we focused on EV biogenesis pathway at the embryo-maternal interface, when the embryonic signaling occurs for maternal recognition and the maintenance of pregnancy.

RESULTS

Transmission electron microscopy was used during early pregnancy to visualize EVs and apocrine and/or merocrine pathways of secretion. Immunofluorescent staining localized proteins responsible for EV biogenesis and cell polarization at the embryo-maternal interface. The expression profiles of genes involved in biogenesis and the secretion of EVs pointed to the possible modulation of endometrial expression by embryonic signals. Further in vitro studies showed that factors of embryonic origin can regulate the expression of the ESCRT-II complex and EV trafficking within endometrial luminal epithelial cells. Moreover, miRNA-mediated rapid negative regulation of gene expression was abolished by delivered embryonic signals.

CONCLUSIONS

Our findings demonstrated that embryonic signals are potent modulators of ESCRT-dependent EV-mediated secretory activity of the endometrium during the critical stages of early pregnancy. Video Abstract.

miR-125b-5p impacts extracellular vesicle biogenesis, trafficking, and EV subpopulation release in the porcine trophoblast by regulating ESCRT-dependent pathway

Intercellular communication is a critical process that ensures cooperation between distinct cell types at the embryo-maternal interface. Extracellular vesicles (EVs) are considered to be potent mediators of this communication by transferring biological information in their cargo (e.g., miRNAs) to the recipient cells. miRNAs are small non-coding RNAs that affect the function and fate of neighboring and distant cells by regulating gene expression. Focusing on the maternal side of the dialog, we recently revealed the impact of embryonic signals, including miRNAs, on EV-mediated cell-to-cell communication. In this study, we show the regulatory mechanism of the miR-125b-5p ESCRT-mediated EV biogenesis pathway and the further secretion of EVs by trophoblasts at the time when the crucial steps of implantation are taking place. To test the ability of miR-125b-5p to influence the expression of genes involved in the generation and release of EV subpopulations in porcine conceptuses, we used an ex vivo approach. Next, in silico and in vitro analyses were performed to confirm miRNA-mRNA interactions. Finally, EV trafficking and release were assessed using several imaging and particle analysis tools. Our results indicated that conceptus development and implantation are accompanied by changes in the abundance of EV biogenesis and trafficking machinery. ESCRT-dependent EV biogenesis and the further secretion of EVs were modulated by miR-125b-5p, specifically impacting the ESCRT-II complex (via VPS36) and EV trafficking in primary porcine trophoblast cells. The identified miRNA-ESCRT interplay led to the generation and secretion of specific subpopulations of EVs. miRNA present at the embryo-maternal interface governs EV-mediated communication between the mother and the developing conceptus, leading to the generation, trafficking, and release of characteristic subpopulations of EVs.

Medium-throughput image-based phenotypic siRNA screen to unveil the molecular basis of B cell polarization

Cell polarity is an essential and highly conserved process governing cell function. Cell polarization is generally triggered by an external signal that induces the relocation of the centrosome, thus defining the polarity axis of the cell. Here, we took advantage of B cells as a model to study cell polarity and perform a medium-throughput siRNA-based imaging screen to identify new molecular regulators of polarization. We first identified candidates based on a quantitative proteomic analysis of proteins differentially associated with the centrosome of resting non-polarized and stimulated polarized B cells. We then targeted 233 candidates in a siRNA screen and identified hits regulating the polarization of the centrosome and/or lysosomes in B cells upon stimulation. Our dataset of proteomics, images, and polarity indexes provides a valuable source of information for a broad community of scientists interested in the molecular mechanisms regulating cell polarity.

Endocytosis at the Crossroad of Polarity and Signaling Regulation: Learning from Drosophila melanogaster and Beyond

Cellular trafficking through the endosomal–lysosomal system is essential for the transport of cargo proteins, receptors and lipids from the plasma membrane inside the cells and across membranous organelles. By acting as sorting stations, vesicle compartments direct the fate of their content for degradation, recycling to the membrane or transport to the trans-Golgi network. To effectively communicate with their neighbors, cells need to regulate their compartmentation and guide their signaling machineries to cortical membranes underlying these contact sites. Endosomal trafficking is indispensable for the polarized distribution of fate determinants, adaptors and junctional proteins. Conversely, endocytic machineries cooperate with polarity and scaffolding components to internalize receptors and target them to discrete membrane domains. Depending on the cell and tissue context, receptor endocytosis can terminate signaling responses but can also activate them within endosomes that act as signaling platforms. Therefore, cell homeostasis and responses to environmental cues rely on the dynamic cooperation of endosomal–lysosomal machineries with polarity and signaling cues. This review aims to address advances and emerging concepts on the cooperative regulation of endocytosis, polarity and signaling, primarily in Drosophila melanogaster and discuss some of the open questions across the different cell and tissue types that have not yet been fully explored.

The ESCRT-III molecules regulate the apical targeting of bile salt export pump

Background

The bile salt export pump (BSEP) is a pivotal apical/canalicular bile salt transporter in hepatocytes that drives the bile flow. Defects in BSEP function and canalicular expression could lead to a spectrum of cholestatic liver diseases. One prominent manifestation of BSEP-associated cholestasis is the defective canalicular localization and cytoplasmic retention of BSEP. However, the etiology of impaired BSEP targeting to the canalicular membrane is not fully understood. Our goal was to discover what molecule could interact with BSEP and affect its post-Golgi sorting.

Methods

The human BSEP amino acids (a.a.) 491-630 was used as bait to screen a human fetal liver cDNA library through yeast two-hybrid system. We identified a BSEP-interacting candidate and showed the interaction and colocalization in the co-immunoprecipitation in hepatoma cell lines and histological staining in human liver samples. Temperature shift assays were used to study the post-Golgi trafficking of BSEP. We further determine the functional impacts of the BSEP-interacting candidate on BSEP in vitro. A hydrodynamically injected mouse model was established for in vivo characterizing the long-term impacts on BSEP.

Results

We identified that charged multivesicular body protein 5 (CHMP5), a molecule of the endosomal protein complex required for transport subcomplex-III (ESCRT-III), interacted and co-localized with BSEP in the subapical compartments (SACs) in developing human livers. Cholestatic BSEP mutations in the CHMP5-interaction region have defects in canalicular targeting and aberrant retention at the SACs. Post-Golgi delivery of BSEP and bile acid secretion were impaired in ESCRT-III perturbation or CHMP5-knockdown hepatic cellular and mouse models. This ESCRT-III-mediated BSEP sorting preceded Rab11A-regulated apical cycling of BSEP.

Conclusions

Our results showed the first example that ESCRT-III is essential for canalicular trafficking of apical membrane proteins, and provide new targets for therapeutic approaches in BSEP associated cholestasis.

The intralumenal fragment pathway mediates ESCRT-independent surface transporter down-regulation

Surface receptor and transporter protein down-regulation is assumed to be exclusively mediated by the canonical multivesicular body (MVB) pathway and ESCRTs (Endosomal Sorting Complexes Required for Transport). However, few surface proteins are known to require ESCRTs for down-regulation, and reports of ESCRT-independent degradation are emerging, suggesting that alternative pathways exist. Here, using Saccharomyces cerevisiae as a model, we show that the hexose transporter Hxt3 does not require ESCRTs for down-regulation conferring resistance to 2-deoxyglucose. This is consistent with GFP-tagged Hxt3 bypassing ESCRT-mediated entry into intralumenal vesicles at endosomes. Instead, Hxt3-GFP accumulates on vacuolar lysosome membranes and is sorted into an area that, upon fusion, is internalized as an intralumenal fragment (ILF) and degraded. Moreover, heat stress or cycloheximide trigger degradation of Hxt3-GFP and other surface transporter proteins (Itr1, Aqr1) by this ESCRT-independent process. How this ILF pathway compares to the MVB pathway and potentially contributes to physiology is discussed.

Cell surface receptors are thought to be internalized via the multivesicular bodies (MVBs) in an ESCRT-dependent pathway. Here, the authors report that in yeast, a hexose transporter is internalized via an ESCRT-independent pathway into intralumenal fragments (ILF).

From top to bottom: Cell polarity in Hedgehog and Wnt trafficking

Spatial organization of membrane domains within cells and cells within tissues is key to the development of organisms and the maintenance of adult tissue. Cell polarization is crucial for correct cell-cell signalling, which, in turn, promotes cell differentiation and tissue patterning. However, the mechanisms linking internal cell polarity to intercellular signalling are just beginning to be unravelled. The Hedgehog (Hh) and Wnt pathways are major directors of development and their malfunction can cause severe disorders like cancer. Here we discuss parallel advances into understanding the mechanism of Hedgehog and Wnt signal dissemination and reception. We hypothesize that cell polarization of the signal-sending and signal-receiving cells is crucial for proper signal spreading and activation of the pathway and, thus, fundamental for development of multicellular organisms.

VPS4 is a dynamic component of the centrosome that regulates centrosome localization of γ-tubulin, centriolar satellite stability and ciliogenesis

The hexameric AAA ATPase VPS4 facilitates ESCRT III filament disassembly on diverse intracellular membranes. ESCRT III components and VPS4 have been localized to the ciliary transition zone and spindle poles and reported to affect centrosome duplication and spindle pole stability. How the canonical ESCRT pathway could mediate these events is unclear. We studied the association of VPS4 with centrosomes and found that GFP-VPS4 was a dynamic component of both mother and daughter centrioles. A mutant, VPS4^EQ, which can’t hydrolyze ATP, was less dynamic and accumulated at centrosomes. Centrosome localization of the VPS4^EQ mutant, caused reduced γ-tubulin levels at centrosomes and consequently decreased microtubule growth and altered centrosome positioning. In addition, preventing VPS4 ATP hydrolysis nearly eliminated centriolar satellites and paused ciliogensis after formation of the ciliary vesicle. Zebrafish embryos injected with GFP-VPS4^EQ mRNA were less viable, exhibited developmental defects and had fewer cilia in Kupffer’s vesicle. Surprisingly, ESCRT III proteins seldom localized to centrosomes and their depletion did not lead to these phenotypes. Our data support an ESCRT III-independent function for VPS4 at the centrosome and reveal that this evolutionary conserved AAA ATPase influences diverse centrosome functions and, as a result, global cellular architecture and development.

GTPases Rac1 and Ras Signaling from Endosomes

The endocytic compartment is not only the functional continuity of the plasma membrane but consists of a diverse collection of intracellular heterogeneous complex structures that transport, amplify, sustain, and/or sort signaling molecules. Over the years, it has become evident that early, late, and recycling endosomes represent an interconnected vesicular-tubular network able to form signaling platforms that dynamically and efficiently translate extracellular signals into biological outcome. Cell activation, differentiation, migration, death, and survival are some of the endpoints of endosomal signaling. Hence, to understand the role of the endosomal system in signal transduction in space and time, it is therefore necessary to dissect and identify the plethora of decoders that are operational in the different steps along the endocytic pathway. In this chapter, we focus on the regulation of spatiotemporal signaling in cells, considering endosomes as central platforms, in which several small GTPases proteins of the Ras superfamily, in particular Ras and Rac1, actively participate to control cellular processes like proliferation and cell mobility.

A Non-Cell-Autonomous Role of BEC-1/BECN1/Beclin1 in Coordinating Cell-Cycle Progression and Stem Cell Proliferation during Germline Development

The decision of stem cells to proliferate and differentiate is finely controlled. The Caenorhabditis elegans germline provides a tractable system for studying the mechanisms that control stem cell proliferation and homeostasis [1-4]. Autophagy is a conserved cellular recycling process crucial for cellular homeostasis in many different contexts [5], but its function in germline stem cell proliferation remains poorly understood. Here, we describe a function for autophagy in germline stem cell proliferation. We found that autophagy genes such as bec-1/BECN1/Beclin1, atg-16.2/ATG16L, atg-18/WIPI1/2, and atg-7/ATG7 are required for the late larval expansion of germline stem cell progenitors in the C. elegans gonad. We further show that BEC-1/BECN1/Beclin1 acts independently of the GLP-1/Notch or DAF-7/TGF-β pathways but together with the DAF-2/insulin IGF-1 receptor (IIR) signaling pathway to promote germline stem cell proliferation. Similar to DAF-2/IIR, BEC-1/BECN1/Beclin1, ATG-18/WIPI1/2, and ATG-16.2/ATG16L all promote cell-cycle progression and are negatively regulated by the phosphatase and tensin homolog DAF-18/PTEN. However, whereas BEC-1/BECN1/Beclin1 acts through the transcriptional regulator SKN-1/Nrf1, ATG-18/WIPI1/2 and ATG-16.2/ATG16L exert their function through the DAF-16/FOXO transcription factor. In contrast, ATG-7 functions in concert with the DAF-7/TGF-β pathway to promote germline proliferation and is not required for cell-cycle progression. Finally, we report that BEC-1/BECN1/Beclin1 functions non-cell-autonomously to facilitate cell-cycle progression and stem cell proliferation. Our findings demonstrate a novel non-autonomous role for BEC-1/BECN1/Beclin1 in the control of stem cell proliferation and cell-cycle progression, which may have implications for the understanding and development of therapies against malignant cell growth in the future.

Mitochondrial dysfunction and mitochondrial dynamics-The cancer connection

Mitochondrial dysfunction is a hallmark of many diseases. The retrograde signaling initiated by dysfunctional mitochondria can bring about global changes in gene expression that alters cell morphology and function. Typically, this is attributed to disruption of important mitochondrial functions, such as ATP production, integration of metabolism, calcium homeostasis and regulation of apoptosis. Recent studies showed that in addition to these factors, mitochondrial dynamics might play an important role in stress signaling. Normal mitochondria are highly dynamic organelles whose size, shape and network are controlled by cell physiology. Defective mitochondrial dynamics play important roles in human diseases. Mitochondrial DNA defects and defective mitochondrial function have been reported in many cancers. Recent studies show that increased mitochondrial fission is a pro-tumorigenic phenotype. In this paper, we have explored the current understanding of the role of mitochondrial dynamics in pathologies. We present new data on mitochondrial dynamics and dysfunction to illustrate a causal link between mitochondrial DNA defects, excessive fission, mitochondrial retrograde signaling and cancer progression. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Inhibition of the Expression of the Small Heat Shock Protein αB-Crystallin Inhibits Exosome Secretion in Human Retinal Pigment Epithelial Cells in Culture

Exosomes carry cell type-specific molecular cargo to extracellular destinations and therefore act as lateral vectors of intercellular communication and transfer of genetic information from one cell to the other. We have shown previously that the small heat shock protein αB-crystallin (αB) is exported out of the adult human retinal pigment epithelial cells (ARPE19) packaged in exosomes. Here, we demonstrate that inhibition of the expression of αB via shRNA inhibits exosome secretion from ARPE19 cells indicating that exosomal cargo may have a role in exosome biogenesis (synthesis and/or secretion). Sucrose density gradient fractionation of the culture medium and cellular extracts suggests continued synthesis of exosomes but an inhibition of exosome secretion. In cells where αB expression was inhibited, the distribution of CD63 (LAMP3), an exosome marker, is markedly altered from the normal dispersed pattern to a stacked perinuclear presence. Interestingly, the total anti-CD63(LAMP3) immunofluorescence in the native and αB-inhibited cells remains unchanged suggesting continued exosome synthesis under conditions of impaired exosome secretion. Importantly, inhibition of the expression of αB results in a phenotype of the RPE cell that contains an increased number of vacuoles and enlarged (fused) vesicles that show increased presence of CD63(LAMP3) and LAMP1 indicating enhancement of the endolysosomal compartment. This is further corroborated by increased Rab7 labeling of this compartment (RabGTPase 7 is known to be associated with late endosome maturation). These data collectively point to a regulatory role for αB in exosome biogenesis possibly via its involvement at a branch point in the endocytic pathway that facilitates secretion of exosomes.

TSG101, a tumor susceptibility gene, bidirectionally modulates cell invasion through regulating MMP-9 mRNA expression

Background

Tumor susceptibility gene 101 (TSG101) was initially identified in fibroblasts as a tumor suppressor gene but subsequent studies show that TSG101 also functions as a tumor-enhancing gene in some epithelial tumor cells. Although previous studies have unraveled diverse biological functions of TSG101, the precise mechanism by which TSG101 is involved in carcinogenesis and tumor progression in a bidirectional and multifaceted manner remains unclear.

Methods

To reveal the mechanism underlying bidirectional modulation of cell invasion by TSG101, we used RNA interference to examine whether TSG101 depletion bidirectionally modulated matrix metalloproteinase (MMP)-9 expression in different cell types.

Results

TSG101 depletion promoted cell invasion of HT1080 cells but contrarily reduced cell invasion of HeLaS3 cells. In HT1080 cells, TSG101 depletion increased both baseline and phorbol 12-myristate 13-acetate (PMA)-induced MMP-9 secretion through enhancing MMP-9 mRNA expression, but did not affect the expression or activation of MMP-2. In contrast, TSG101 depletion decreased PMA-induced MMP-9 secretion through reducing MMP-9 mRNA expression in HeLaS3 cells. TSG101 depletion had little impact on the signaling pathways required for the activation of transcription of MMP-9 or MMP-9 mRNA stability in either cell line.

Conclusion

TSG101 bidirectionally modulates cell invasion through regulating MMP-9 mRNA expression in different cell types. Our results provide a mechanistic context for the role of TSG101 in cell invasion as a multifaceted gene.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1942-1) contains supplementary material, which is available to authorized users.

Dysregulated Inflammatory Signaling upon Charcot-Marie-Tooth Type 1C Mutation of SIMPLE Protein

Endosomal trafficking is a key mechanism to modulate signal propagation and cross talk. Ubiquitin adaptors, along with endosomal sorting complex required for transport (ESCRT) complexes, are also integrated to terminate ligand-receptor activation in late endosomes and multivesicular bodies (MVBs). Within these pathways, we recently demonstrated that the protein SIMPLE is a novel player in MVB regulation. SIMPLE is also clinically important and its mutation accounts for the Charcot-Marie-Tooth type 1C (CMT1C) disease. MVB defects of mutation and deletion of SIMPLE, however, are distinct. Here, we show that MVB defects found in mutation but not deletion of SIMPLE lead to impaired turnover and accumulation of ESCRT-0 protein Hrs punctain late endosomes. We further uncover increased colocalization of ubiquitin ligase TRAF6 and Hrs in late endosomes. Upon stimulation with interkeukin-1 or transforming growth factor , prolonged activation of p38 kinase/JNK is detected, while nuclear accumulation of NF-κB and phosphorylation of SMAD2 is reduced with CMT1C mutation. The aberrant kinetics we observed in inflammatory signaling may contribute to increased tumor susceptibility and changes in the levels of chemokines/cytokines that result from CMT1C mutation. We propose that altered endosomal trafficking due to malformations of MVBs and subsequent atypical signaling kinetic may account for a toxic gain of function in CMT1C pathogenesis.

Control of cell migration through mRNA localization and local translation

Cell migration plays an important role in many normal and pathological functions such as development, wound healing, immune defense, and tumor metastasis. Polarized migrating cells exhibit asymmetric distribution of many cytoskeletal proteins, which is believed to be critical for establishing and maintaining cell polarity and directional cell migration. To target these proteins to the site of function, cells use a variety of mechanisms such as protein transport and messenger RNA (mRNA) localization-mediated local protein synthesis. In contrast to the former which is intensively investigated and relatively well understood, the latter has been understudied and relatively poorly understood. However, recent advances in the study of mRNA localization and local translation have demonstrated that mRNA localization and local translation are specific and effective ways for protein localization and are crucial for embryo development, neuronal function, and many other cellular processes. There are excellent reviews on mRNA localization, transport, and translation during development and other cellular processes. This review will focus on mRNA localization-mediated local protein biogenesis and its impact on somatic cell migration.

RhoGTPase-binding proteins, the exocyst complex and polarized vesicle trafficking

Cell polarity, the asymmetric distribution of proteins and lipids, is essential for a variety of cellular functions. One mechanism orchestrating cell polarity is polarized vesicle trafficking; whereby cargo loaded secretory vesicles are specifically transported to predetermined areas of the cell. The evolutionarily conserved exocyst complex and its small GTPase regulators play crucial roles in spatiotemporal control of polarized vesicle trafficking. In studies on neuronal membrane remodeling and synaptic plasticity, conserved mechanisms of exocyst regulation and cargo recycling during polarized vesicle trafficking are beginning to emerge as well. Recently, our lab demonstrated that RhoGTPase-binding proteins in both yeast (Bem3) and mammals (Ocrl1) are also required for the efficient traffic of secretory vesicles to sites of polarized growth and signaling. Together with our studies, we highlight the evolutionary conservation of the basic elements essential for polarized vesicle traffic across different cellular functions and model systems. In conclusion, we emphasize that studies on RhoGTPase-binding proteins in these processes should be included in the next level of investigation, for a more complete understanding of their hitherto unknown roles in polarized membrane traffic and exocyst regulation.

Dynamics of KRas on endosomes: involvement of acidic phospholipids in its association

The endocytic compartment is emerging as a functional platform for controlling important cellular processes. We have found that ∼10 to 15% of total KRas, a protein that is frequently mutated in cancer, is present on endosomes, independent of its activation state. The dynamics of GFP-KRas wild-type (WT) and constitutively active or inactive mutants on endosomes were analyzed by fluorescence recovery after photobleaching (FRAP) microscopy. The measurements revealed an extraordinarily fast recovery of KRas WT [half-time (HT), ∼1.3 s] compared to HRas, Rab5, and EGFR, with the active KRasG12V mutant being significantly faster and more mobile (HT, ∼1 s, and ∼82% of exchangeable fraction) than the inactive KRasS17N (HT, ∼1.6 s, and ∼60% of exchangeable fraction). KRas rapidly switches from the cytoplasm to the endosomal membranes by an electrostatic interaction between its polybasic region and the endosomal acidic phospholipids, mainly phosphatidylserine.-Gelabert-Baldrich, M., Soriano-Castell, D., Calvo, M., Lu, A., Viña-Vilaseca, A., Rentero, C., Pol, A., Grinstein, S. Enrich, C., Tebar, F. Dynamics of KRas on endosomes: involvement of acidic phospholipids in its association.

Reprogramming of lysosomal gene expression by interleukin-4 and Stat6

Background

Lysosomes play important roles in multiple aspects of physiology, but the problem of how the transcription of lysosomal genes is coordinated remains incompletely understood. The goal of this study was to illuminate the physiological contexts in which lysosomal genes are coordinately regulated and to identify transcription factors involved in this control.

Results

As transcription factors and their target genes are often co-regulated, we performed meta-analyses of array-based expression data to identify regulators whose mRNA profiles are highly correlated with those of a core set of lysosomal genes. Among the ~50 transcription factors that rank highest by this measure, 65% are involved in differentiation or development, and 22% have been implicated in interferon signaling. The most strongly correlated candidate was Stat6, a factor commonly activated by interleukin-4 (IL-4) or IL-13. Publicly available chromatin immunoprecipitation (ChIP) data from alternatively activated mouse macrophages show that lysosomal genes are overrepresented among Stat6-bound targets. Quantification of RNA from wild-type and Stat6-deficient cells indicates that Stat6 promotes the expression of over 100 lysosomal genes, including hydrolases, subunits of the vacuolar H⁺ ATPase and trafficking factors. While IL-4 inhibits and activates different sets of lysosomal genes, Stat6 mediates only the activating effects of IL-4, by promoting increased expression and by neutralizing undefined inhibitory signals induced by IL-4.

Conclusions

The current data establish Stat6 as a broadly acting regulator of lysosomal gene expression in mouse macrophages. Other regulators whose expression correlates with lysosomal genes suggest that lysosome function is frequently re-programmed during differentiation, development and interferon signaling.

Membrane fission reactions of the mammalian ESCRT pathway

The endosomal sorting complexes required for transport (ESCRT) pathway was initially defined in yeast genetic screens that identified the factors necessary to sort membrane proteins into intraluminal endosomal vesicles. Subsequent studies have revealed that the mammalian ESCRT pathway also functions in a series of other key cellular processes, including formation of extracellular microvesicles, enveloped virus budding, and the abscission stage of cytokinesis. The core ESCRT machinery comprises Bro1 family proteins and ESCRT-I, ESCRT-II, ESCRT-III, and VPS4 complexes. Site-specific adaptors recruit these soluble factors to assemble on different cellular membranes, where they carry out membrane fission reactions. ESCRT-III proteins form filaments that draw membranes together from the cytoplasmic face, and mechanistic models have been advanced to explain how ESCRT-III filaments and the VPS4 ATPase can work together to catalyze membrane fission.

PIP2 signaling, an integrator of cell polarity and vesicle trafficking in directionally migrating cells

Cell migration is a fundamental cellular process required for embryonic development to wound healing and also plays a key role in tumor metastasis and atherosclerosis. Migration is regulated at multiple strata, from cytoskeletal reorganization to vesicle trafficking. In migrating cells, signaling pathways are integrated with vesicle trafficking machineries in a highly coordinated fashion to accomplish the recruitment and trafficking of the trans-membrane proteins toward the leading edge. Different signaling molecules regulate cell migration in different physio-pathological contexts, among them, phosphatidylinositol-4,5-biphosphate (PIP2) is an integral component of the plasma membrane and pleiotropic lipid signaling molecule modulating diverse biological processes, including actin cytoskeletal dynamics and vesicle trafficking required for cell migration. In this commentary, we provide a brief overview of our current understandings on the phosphoinositide signaling and its implication in regulation of cell polarity and vesicle trafficking in migrating cells. In addition, we highlight the coordinated role of PIPKIγi2, a focal adhesion-targeted enzyme that synthesizes PIP2, and the exocyst complex, a PIP2-effector, in the trafficking of E-cadherin in epithelial cells and integrins in migrating cancer cells.

Cdc28-Cln3 phosphorylation of Sla1 regulates actin patch dynamics in different modes of fungal growth

A dynamic balance between targeted transport and endocytosis is critical for polarized cell growth. However, how actin-mediated endocytosis is regulated in different growth modes remains unclear. Here we report differential regulation of cortical actin patch dynamics between the yeast and hyphal growth in Candida albicans. The mechanism involves phosphoregulation of the endocytic protein Sla1 by the cyclin-dependent kinase (CDK) Cdc28-Cln3 and the actin-regulating kinase Prk1. Mutational studies of the CDK phosphorylation sites of Sla1 revealed that Cdc28-Cln3 phosphorylation of Sla1 enhances its further phosphorylation by Prk1, weakening Sla1 association with Pan1, an activator of the actin-nucleating Arp2/3 complex. Sla1 is rapidly dephosphorylated upon hyphal induction and remains so throughout hyphal growth. Consistently, cells expressing a phosphomimetic version of Sla1 exhibited markedly reduced actin patch dynamics, impaired endocytosis, and defective hyphal development, whereas a nonphosphorylatable Sla1 had the opposite effect. Taken together, our findings establish a molecular link between CDK and a key component of the endocytic machinery, revealing a novel mechanism by which endocytosis contributes to cell morphogenesis.