Arl13b and the non-muscle myosin heavy chain IIA are required for circular dorsal ruffle formation and cell migration

Membrane trafficking alterations in breast cancer progression

Breast cancer (BC) is the most common type of cancer in women, and remains one of the major causes of death in women worldwide. It is now well established that alterations in membrane trafficking are implicated in BC progression. Indeed, membrane trafficking pathways regulate BC cell proliferation, migration, invasion, and metastasis. The 22 members of the ADP-ribosylation factor (ARF) and the >60 members of the rat sarcoma (RAS)-related in brain (RAB) families of small GTP-binding proteins (GTPases), which belong to the RAS superfamily, are master regulators of membrane trafficking pathways. ARF-like (ARL) subfamily members are involved in various processes, including vesicle budding and cargo selection. Moreover, ARFs regulate cytoskeleton organization and signal transduction. RABs are key regulators of all steps of membrane trafficking. Interestingly, the activity and/or expression of some of these proteins is found dysregulated in BC. Here, we review how the processes regulated by ARFs and RABs are subverted in BC, including secretion/exocytosis, endocytosis/recycling, autophagy/lysosome trafficking, cytoskeleton dynamics, integrin-mediated signaling, among others. Thus, we provide a comprehensive overview of the roles played by ARF and RAB family members, as well as their regulators in BC progression, aiming to lay the foundation for future research in this field. This research should focus on further dissecting the molecular mechanisms regulated by ARFs and RABs that are subverted in BC, and exploring their use as therapeutic targets or prognostic markers.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

The Role of ARF Family Proteins and Their Regulators and Effectors in Cancer Progression: A Therapeutic Perspective

The Adenosine diphosphate-Ribosylation Factor (ARF) family belongs to the RAS superfamily of small GTPases and is involved in a wide variety of physiological processes, such as cell proliferation, motility and differentiation by regulating membrane traffic and associating with the cytoskeleton. Like other members of the RAS superfamily, ARF family proteins are activated by Guanine nucleotide Exchange Factors (GEFs) and inactivated by GTPase-Activating Proteins (GAPs). When active, they bind effectors, which mediate downstream functions. Several studies have reported that cancer cells are able to subvert membrane traffic regulators to enhance migration and invasion. Indeed, members of the ARF family, including ARF-Like (ARL) proteins have been implicated in tumorigenesis and progression of several types of cancer. Here, we review the role of ARF family members, their GEFs/GAPs and effectors in tumorigenesis and cancer progression, highlighting the ones that can have a pro-oncogenic behavior or function as tumor suppressors. Moreover, we propose possible mechanisms and approaches to target these proteins, toward the development of novel therapeutic strategies to impair tumor progression.

ARL13B regulates Sonic hedgehog signaling from outside primary cilia

ARL13B is a regulatory GTPase highly enriched in cilia. Complete loss of Arl13b disrupts cilia architecture, protein trafficking and Sonic hedgehog signaling. To determine whether ARL13B is required within cilia, we knocked in a cilia-excluded variant of ARL13B (V358A) and showed it retains all known biochemical function. We found that ARL13B^V358A protein was expressed but could not be detected in cilia, even when retrograde ciliary transport was blocked. We showed Arl13b^V358A/V358A mice are viable and fertile with normal Shh signal transduction. However, in contrast to wild type cilia, Arl13b^V358A/V358A cells displayed short cilia and lacked ciliary ARL3 and INPP5E. These data indicate that ARL13B's role within cilia can be uncoupled from its function outside of cilia. Furthermore, these data imply that the cilia defects upon complete absence of ARL13B do not underlie the alterations in Shh transduction, which is unexpected given the requirement of cilia for Shh transduction.

Arl13b Regulates Breast Cancer Cell Migration and Invasion by Controlling Integrin-Mediated Signaling

Breast cancer is the first cause of cancer-related mortality among women worldwide, according to the most recent estimates. This mortality is mainly caused by the tumors' ability to form metastases. Cancer cell migration and invasion are essential for metastasis and rely on the interplay between actin cytoskeleton remodeling and cell adhesion. Therefore, understanding the mechanisms by which cancer cell invasion is controlled may provide new strategies to impair cancer progression. We investigated the role of the ADP-ribosylation factor (Arf)-like (Arl) protein Arl13b in breast cancer cell migration and invasion in vitro, using breast cancer cell lines and in vivo, using mouse orthotopic models. We show that Arl13b silencing inhibits breast cancer cell migration and invasion in vitro, as well as cancer progression in vivo. We also observed that Arl13b is upregulated in breast cancer cell lines and patient tissue samples. Moreover, we found that Arl13b localizes to focal adhesions (FAs) and interacts with β3-integrin. Upon Arl13b silencing, β3-integrin cell surface levels and FA size are increased and integrin-mediated signaling is inhibited. Therefore, we uncover a role for Arl13b in breast cancer cell migration and invasion and provide a new mechanism for how ARL13B can function as an oncogene, through the modulation of integrin-mediated signaling.

Ahi1 promotes Arl13b ciliary recruitment, regulates Arl13b stability and is required for normal cell migration

Mutations in the Abelson-helper integration site 1 (AHI1) gene are associated with neurological/neuropsychiatric disorders, and cause the neurodevelopmental ciliopathy Joubert syndrome (JBTS). Here, we show that deletion of the transition zone (TZ) protein Ahi1 in mouse embryonic fibroblasts (MEFs) has a small effect on cilia formation. However, Ahi1 loss in these cells results in: (1) reduced localization of the JBTS-associated protein Arl13b to the ciliary membrane, (2) decreased sonic hedgehog signaling, (3) and an abnormally elongated ciliary axoneme accompanied by an increase in ciliary IFT88 concentrations. While no changes in Arl13b levels are detected in crude cell membrane extracts, loss of Ahi1 significantly reduced the level of non-membrane-associated Arl13b and its stability via the proteasome pathway. Exogenous expression of Ahi1-GFP in Ahi1^-/- MEFs restored ciliary length, increased ciliary recruitment of Arl13b and augmented Arl13b stability. Finally, Ahi1^-/- MEFs displayed defects in cell motility and Pdgfr-α-dependent migration. Overall, our findings support molecular mechanisms underlying JBTS etiology that involve: (1) disruptions at the TZ resulting in defects of membrane- and non-membrane-associated proteins to localize to primary cilia, and (2) defective cell migration.This article has an associated First Person interview with the first author of the paper.

BD-2 and BD-3 increase skin flap survival in a model of ischemia and Pseudomonas aeruginosa infection

The main aim of this work was to study the usefulness of human β-defensins 2 (BD-2) and 3 (BD-3), which are part of the innate immune system, in the treatment of infected ischemic skin flaps. We investigated the effect of transducing rat ischemic skin flaps with lentiviral vectors encoding human BD-2, BD-3, or both BD-2 and BD-3, to increase flap survival in the context of a P. aeruginosa infection associated with a foreign body. The secondary endpoints assessed were: bacterial counts, and biofilm formation on the surface of the foreign body. A local ischemic environment was created by producing arterialized venous flaps in the left epigastric region of rats. Flaps were intentionally infected by placing underneath them two catheters with 10⁵ CFU of P. aeruginosa before the surgical wounds were hermetically closed. Flap biopsies were performed 3 and 7 days post-operatively, and the specimens submitted to immunohistochemical analysis for BD-2 and BD-3, as well as to bacterial quantification. Subsequently, the catheter segments were analyzed with scanning electron microscopy (SEM). Flaps transduced with BD-2 and BD-3 showed expression of these defensins and presented increased flap survival. Rats transduced with BD-3 presented a net reduction in the number of P. aeruginosa on the surface of the foreign body and lesser biofilm formation.

ARF GTPases and their GEFs and GAPs: concepts and challenges

Detailed structural, biochemical, cell biological, and genetic studies of any gene/protein are required to develop models of its actions in cells. Studying a protein family in the aggregate yields additional information, as one can include analyses of their coevolution, acquisition or loss of functionalities, structural pliability, and the emergence of shared or variations in molecular mechanisms. An even richer understanding of cell biology can be achieved through evaluating functionally linked protein families. In this review, we summarize current knowledge of three protein families: the ARF GTPases, the guanine nucleotide exchange factors (ARF GEFs) that activate them, and the GTPase-activating proteins (ARF GAPs) that have the ability to both propagate and terminate signaling. However, despite decades of scrutiny, our understanding of how these essential proteins function in cells remains fragmentary. We believe that the inherent complexity of ARF signaling and its regulation by GEFs and GAPs will require the concerted effort of many laboratories working together, ideally within a consortium to optimally pool information and resources. The collaborative study of these three functionally connected families (≥70 mammalian genes) will yield transformative insights into regulation of cell signaling.

Arf6, JIP3, and dynein shape and mediate macropinocytosis

Macropinocytosis is an actin-driven form of clathrin-independent endocytosis that generates an enlarged structure, the macropinosome. Although many studies focus on signaling molecules and phosphoinositides involved in initiating macropinocytosis, the commitment to forming a macropinosome and the handling of that membrane have not been studied in detail. Here we show in HT1080 cells, a human fibrosarcoma cell line, a requirement for microtubules, dynein, the JIP3 microtubule motor scaffold protein, and Arf6, a JIP3 interacting protein, for the formation and inward movement of the macropinosome. While actin and myosin II also play critical roles in the formation of ruffling membrane, microtubules provide an important tract for initiation, sealing, and transport of the macropinosome through the actin- and myosin-rich lamellar region.

Primary Cilia-An Underexplored Topic in Major Mental Illness

Though much progress has been made in recent years towards understanding the function and physiology of primary cilia, they remain a somewhat elusive organelle. Some studies have explored the role of primary cilia in the developing nervous system, and their dysfunction has been linked with several neurosensory deficits. Yet, very little has been written on their potential role in psychiatric disorders. This article provides an overview of some of the functions of primary cilia in signalling pathways, and demonstrates that they are a worthy candidate in psychiatric research. The links between primary cilia and major mental illness have been demonstrated to exist at several levels, spanning genetics, signalling pathways, and pharmacology as well as cell division and migration. The primary focus of this review is on the sensory role of the primary cilium and the neurodevelopmental hypothesis of psychiatric disease. As such, the primary cilium is demonstrated to be a key link between the cellular environment and cell behaviour, and hence of key importance in the considerations of the nature and nurture debate in psychiatric research.

Lipid phosphatases SKIP and SHIP2 regulate fibronectin-dependent cell migration in glioblastoma

Cell migration is an important process that occurs during development and has also been linked to the motility of cancer cells. Cytoskeleton reorganization takes place in the migration process leading to lamellipodia formation. Understanding the molecular underpinnings of cell migration is particularly important in studies of glioblastoma, a highly invasive and aggressive cancer type. Two members of the phosphoinositide 5-phosphatase family, SKIP and SHIP2, have been associated with cell migration in glioblastoma; however, the precise role these enzymes play in the process-and whether they work in concert-remains unclear. Here, we compared phosphoinositide 5-phosphatases expression in glioblastoma primary cells and cell lines and showed that SHIP2 and SKIP expression greatly varies between different cell types, while OCRL, another phosphoinositide 5-phosphatase, is constitutively expressed. Upon adhesion of U-251 MG cells to fibronectin, SHIP2, SKIP, and PI(4,5)P2 colocalized in membrane ruffles. Upregulation of PI(4,5)P2 was observed in SKIP-depleted U-251 MG cells compared to control cells, but only when cells were adhered to fibronectin. Both PTEN-deficient (U-251) and PTEN-containing (LN229) glioblastoma cells showed a decrease in cell migration velocity in response to SKIP downregulation. Moreover, a SHIP2 catalytic inhibitor lowered cell migration velocity in the U-251 MG cell line. We conclude that integrin activation in U-251 cells leads to colocalization of both SKIP and SHIP2 in ruffles, where they act as potential drivers of cell migration. Depending on their expression levels in glioblastoma, phosphoinositide 5-phosphatases could cooperate and synergize in the regulation of cell migration and adhesion.

P311, Friend, or Foe of Tissue Fibrosis?

P311 was first identified by the group of Studler et al. (1993) in the developing brain. In healthy, but mainly in pathological tissues, P311 is implicated in cell migration and proliferation. Furthermore, evidence in models of tissue fibrosis points to the colocalization with and the stimulation of transforming growth factor β1 by P311. This review provides a comprehensive overview on P311 and discusses its potential as an anti-fibrotic target.

The unusual flagellar-targeting mechanism and functions of the trypanosome ortholog of the ciliary GTPase Arl13b

The small GTPase Arl13b is one of the most conserved and ancient ciliary proteins. In human and animals, Arl13b is primarily associated with the ciliary membrane, where it acts as a guanine-nucleotide-exchange factor (GEF) for Arl3 and is implicated in a variety of ciliary and cellular functions. We have identified and characterized Trypanosoma brucei (Tb)Arl13, the sole Arl13b homolog in this evolutionarily divergent, protozoan parasite. TbArl13 has conserved flagellar functions and exhibits catalytic activity towards two different TbArl3 homologs. However, TbArl13 is distinctly associated with the axoneme through a dimerization/docking (D/D) domain. Replacing the D/D domain with a sequence encoding a flagellar membrane protein created a viable alternative to the wild-type TbArl13 in our RNA interference (RNAi)-based rescue assay. Therefore, flagellar enrichment is crucial for TbArl13, but mechanisms to achieve this could be flexible. Our findings thus extend the understanding of the roles of Arl13b and Arl13b–Arl3 pathway in a divergent flagellate of medical importance.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: All roads lead to cilia – how the essential flagellar enrichment of Arl13 is achieved in trypanosome cells using a fundamentally different strategy compared with that of animal cells.

Axoneme polyglutamylation regulated by Joubert syndrome protein ARL13B controls ciliary targeting of signaling molecules

Tubulin polyglutamylation is a predominant axonemal post-translational modification. However, if and how axoneme polyglutamylation is essential for primary cilia and contribute to ciliopathies are unknown. Here, we report that Joubert syndrome protein ARL13B controls axoneme polyglutamylation, which is marginally required for cilia stability but essential for cilia signaling. ARL13B interacts with RAB11 effector FIP5 to promote cilia import of glutamylase TTLL5 and TTLL6. Hypoglutamylation caused by a deficient ARL13B-RAB11-FIP5 trafficking pathway shows no effect on ciliogenesis, but promotes cilia disassembly and, importantly, impairs cilia signaling by disrupting the proper anchoring of sensory receptors and trafficking of signaling molecules. Remarkably, depletion of deglutamylase CCP5, the predominant cilia deglutamylase, effectively restores hypoglutamylation-induced cilia defects. Our study reveals a paradigm that tubulin polyglutamylation is a major contributor for cilia signaling and suggests a potential therapeutic strategy by targeting polyglutamylation machinery to promote ciliary targeting of signaling machineries and correct signaling defects in ciliopathies.

Primary Cilia Reconsidered in the Context of Ciliopathies: Extraciliary and Ciliary Functions of Cilia Proteins Converge on a Polarity theme?

Once dismissed as vestigial organelles, primary cilia have garnered the interest of scientists, given their importance in development/signaling, and for their implication in a new disease category known as ciliopathies. However, many, if not all, "cilia" proteins also have locations/functions outside of the primary cilium. These extraciliary functions can complicate the interpretation of a particular ciliopathy phenotype: it may be a result of defects at the cilium and/or at extraciliary locations, and it could be broadly related to a unifying cellular process for these proteins, such as polarity. Assembly of a cilium has many similarities to the development of other polarized structures. This evolutionarily preserved process for the assembly of polarized cell structures offers a perspective on how the cilium may have evolved. We hypothesize that cilia proteins are critical for cell polarity, and that core polarity proteins may have been specialized to form various cellular protrusions, including primary cilia.

The Joubert syndrome protein ARL13B binds tubulin to maintain uniform distribution of proteins along the ciliary membrane

Cilia-mediated signal transduction involves precise targeting and localization of selected molecules along the ciliary membrane. However, the molecular mechanism underlying these events is unclear. The Joubert syndrome protein ARL13B is a membrane-associated G-protein that localizes along the cilium and functions in protein transport and signaling. We identify tubulin as a direct interactor of ARL13B and demonstrate that the association occurs via the G-domain and independently from the GTPase activity of ARL13B. The G-domain is necessary for the interaction of ARL13B with the axoneme both in vitro and in vivo We further show that exogenously expressed mutants lacking the tubulin-binding G-domain (ARL13B-ΔGD) or whose GTPase domain is inactivated (ARL13B-T35N) retain ciliary localization, but fail to rescue ciliogenesis defects of null Arl13bhnn mouse embryonic fibroblasts (MEFs). However, while ARL13B-ΔGD and the membrane proteins Smoothened (SMO) and Somatostatin receptor-3 (SSTR3) distribute unevenly along the cilium of Arl13bhnn MEFs, ARL13B-T35N distributes evenly along the cilium and enables the uniform distribution of SMO and SSTR3. Thus, we propose a so far unknown function of ARL13B in anchoring ciliary membrane proteins to the axoneme through the direct interaction of its G-domain with tubulin.

Primary cilia proteins: ciliary and extraciliary sites and functions

Primary cilia are immotile organelles known for their roles in development and cell signaling. Defects in primary cilia result in a range of disorders named ciliopathies. Because this organelle can be found singularly on almost all cell types, its importance extends to most organ systems. As such, elucidating the importance of the primary cilium has attracted researchers from all biological disciplines. As the primary cilia field expands, caution is warranted in attributing biological defects solely to the function of this organelle, since many of these "ciliary" proteins are found at other sites in cells and likely have non-ciliary functions. Indeed, many, if not all, cilia proteins have locations and functions outside the primary cilium. Extraciliary functions are known to include cell cycle regulation, cytoskeletal regulation, and trafficking. Cilia proteins have been observed in the nucleus, at the Golgi apparatus, and even in immune synapses of T cells (interestingly, a non-ciliated cell). Given the abundance of extraciliary sites and functions, it can be difficult to definitively attribute an observed phenotype solely to defective cilia rather than to some defective extraciliary function or a combination of both. Thus, extraciliary sites and functions of cilia proteins need to be considered, as well as experimentally determined. Through such consideration, we will understand the true role of the primary cilium in disease as compared to other cellular processes' influences in mediating disease (or through a combination of both). Here, we review a compilation of known extraciliary sites and functions of "cilia" proteins as a means to demonstrate the potential non-ciliary roles for these proteins.

Arl13b Promotes Gastric Tumorigenesis by Regulating Smo Trafficking and Activation of the Hedgehog Signaling Pathway

Inhibitors of the Hedgehog (Hh) pathway transducer Smoothened (Smo) have been approved for cancer treatment, but Smo mutations often lead to tumor resistance and it remains unclear how Smo is regulated. In this study, we identified the small GTPase Arl13b as a novel partner and regulator of Smo. Arl13b regulated Smo stability, trafficking, and localization, which are each crucial for Hh signaling. In gastric cancer cells, Arl13b stimulated proliferation, migration, and invasion in vitro and in vivo In clinical specimens of gastric cancer, Arl13b expression correlated strongly with tumor size and depth of invasion; patients with high levels of Arl13b had a poor prognosis. Our results show how Arl13b participates in Hh pathway activation in gastric cancer. Cancer Res; 77(15); 4000-13. ©2017 AACR.

Arl13b regulates Shh signaling from both inside and outside the cilium

The regulatory GTPase Arl13b localizes to primary cilia, where it regulates Sonic hedgehog (Shh) signaling. Missense mutations in ARL13B can cause the ciliopathy Joubert syndrome, while the mouse null allele is embryonic lethal. We used mouse embryonic fibroblasts as a system to determine the effects of Arl13b mutations on Shh signaling. We tested a total of seven different mutants, three JS-causing variants, two point mutants predicted to alter guanine nucleotide handling, one that disrupts cilia localization, and one that prevents palmitoylation and thus membrane binding, in assays of transcriptional and non-transcriptional Shh signaling. We found that mutations disrupting Arl13b's palmitoylation site, cilia localization signal, or GTPase handling altered the Shh response in distinct assays of transcriptional or non-transcriptional signaling. In contrast, JS-causing mutations in Arl13b did not affect Shh signaling in these same assays, suggesting these mutations result in more subtle defects, likely affecting only a subset of signaling outputs. Finally, we show that restricting Arl13b from cilia interferes with its ability to regulate Shh-stimulated chemotaxis, despite previous evidence that cilia themselves are not required for this non-transcriptional Shh response. This points to a more complex relationship between the ciliary and non-ciliary roles of this regulatory GTPase than previously envisioned.

Arf proteins in cancer cell migration

Members of the ADP-ribosylation factor (Arf) family of small GTP-binding (G) proteins regulate several aspects of membrane trafficking, such as vesicle budding, tethering and cytoskeleton organization. Arf family members, including Arf-like (Arl) proteins have been implicated in several essential cellular functions, like cell spreading and migration. These functions are used by cancer cells to disseminate and invade the tissues surrounding the primary tumor, leading to the formation of metastases. Indeed, Arf and Arl proteins, as well as their guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) have been found to be abnormally expressed in different cancer cell types and human cancers. Here, we review the current evidence supporting the involvement of Arf family proteins and their GEFs and GAPs in cancer progression, focusing on 3 different mechanisms: cell-cell adhesion, integrin internalization and recycling, and actin cytoskeleton remodeling.

The cell surface environment for pathogen recognition and entry

The surface of mammalian cells offers an interface between the cell interior and its surrounding milieu. As part of the innate immune system, macrophages have cell surface features optimised for probing and sampling as they patrol our tissues for pathogens, debris or dead cells. Their highly dynamic and constantly moving cell surface has extensions such as lamellipodia, filopodia and dorsal ruffles that help detect pathogens. Dorsal ruffles give rise to macropinosomes for rapid, high volume non-selective fluid sampling, receptor internalisation and plasma membrane turnover. Ruffles can also generate phagocytic cups for the receptor-mediated uptake of pathogens or particles. The membrane lipids, actin cytoskeleton, receptors and signalling proteins that constitute these cell surface domains are discussed. Although the cell surface is designed to counteract pathogens, many bacteria, viruses and other pathogens have evolved to circumvent or hijack these cell structures and their underlying machinery for entry and survival. Nevertheless, these features offer important potential for developing vaccines, drugs and preventative measures to help fight infection.

The ciliary GTPase Arl13b regulates cell migration and cell cycle progression

The GTPase ARL13B is localized to primary cilia; small cellular protrusions that act as antennae. Its defective ARL13B hennin (HNN) variant is linked causally with Joubert Syndrome, a developmental ciliopathy attributed to poor sensing of extracellular chemical gradients. We tested the hypothesis that impaired detection of extracellular voltage gradients also contributes to the HNN phenotype. In vitro, extracellular electric fields stimulated migration of wild type (WT) and HNN fibroblasts toward the cathode but the field only increased the migration speed of WT cells. Cilia on WT cells did not align to the field vector. HNN cells divided more slowly than WT cells, arresting at the G2/M phase. Mechanistically, HNN cells had reduced phospho-ERK1/2 signaling and elevated levels of Suppressor of Fused protein. These suggest that cells may not be able to read extracellular chemical cues appropriately, resulting in deficits in cell migration and proliferation. Finally, an increase in tubulin stabilization (more detyrosinated tubulin) confirmed the general stagnation of HNN cells, which may further contribute to slower migration and cell cycle progression. We conclude that Arl13b dysfunction resulted in HNN cell stagnation due to poor growth factor signaling and impaired detection of extracellular electrical gradients, and that the role of Arl13b in cell proliferation may be understated.

Actin Dynamics Regulates Dendritic Cell-Mediated Transfer of HIV-1 to T Cells

Whereas human dendritic cells (DCs) are largely resistant to productive infection with HIV-1, they have a unique ability to take up the virus and transmit it efficiently to T lymphocytes through a process of trans-infection or trans-enhancement. To elucidate the molecular and cell biological mechanism for trans-enhancement, we performed an shRNA screen of several hundred genes involved in organelle and membrane trafficking in immature human monocyte-derived dendritic cells (MDDCs). We identified TSPAN7 and DNM2, which control actin nucleation and stabilization, as having important and distinct roles in limiting HIV-1 endocytosis and in maintaining virus particles on dendrites, which is required for efficient transfer to T lymphocytes. Further characterization of this process may provide insights not only into the role of DCs in transmission and dissemination of HIV-1 but also more broadly into mechanisms controlling capture and internalization of pathogens.

Arl13b and the exocyst interact synergistically in ciliogenesis

Arl13b belongs to the ADP-ribosylation factor family within the Ras superfamily of regulatory GTPases. Mutations in Arl13b cause Joubert syndrome, which is characterized by congenital cerebellar ataxia, hypotonia, oculomotor apraxia, and mental retardation. Arl13b is highly enriched in cilia and is required for ciliogenesis in multiple organs. Nevertheless, the precise role of Arl13b remains elusive. Here we report that the exocyst subunits Sec8, Exo70, and Sec5 bind preferentially to the GTP-bound form of Arl13b, consistent with the exocyst being an effector of Arl13b. Moreover, we show that Arl13b binds directly to Sec8 and Sec5. In zebrafish, depletion of arl13b or the exocyst subunit sec10 causes phenotypes characteristic of defective cilia, such as curly tail up, edema, and abnormal pronephric kidney development. We explored this further and found a synergistic genetic interaction between arl13b and sec10 morphants in cilia-dependent phenotypes. Through conditional deletion of Arl13b or Sec10 in mice, we found kidney cysts and decreased ciliogenesis in cells surrounding the cysts. Moreover, we observed a decrease in Arl13b expression in the kidneys from Sec10 conditional knockout mice. Taken together, our results indicate that Arl13b and the exocyst function together in the same pathway leading to functional cilia.

Insights into immune responses in oral cancer through proteomic analysis of saliva and salivary extracellular vesicles

The development and progression of oral cavity squamous cell carcinoma (OSCC) involves complex cellular mechanisms that contribute to the low five-year survival rate of approximately 20% among diagnosed patients. However, the biological processes essential to tumor progression are not completely understood. Therefore, detecting alterations in the salivary proteome may assist in elucidating the cellular mechanisms modulated in OSCC and improve the clinical prognosis of the disease. The proteome of whole saliva and salivary extracellular vesicles (EVs) from patients with OSCC and healthy individuals were analyzed by LC-MS/MS and label-free protein quantification. Proteome data analysis was performed using statistical, machine learning and feature selection methods with additional functional annotation. Biological processes related to immune responses, peptidase inhibitor activity, iron coordination and protease binding were overrepresented in the group of differentially expressed proteins. Proteins related to the inflammatory system, transport of metals and cellular growth and proliferation were identified in the proteome of salivary EVs. The proteomics data were robust and could classify OSCC with 90% accuracy. The saliva proteome analysis revealed that immune processes are related to the presence of OSCC and indicate that proteomics data can contribute to determining OSCC prognosis.

Primary cilia found on HeLa and other cancer cells

For many years now, researchers have known of a sensory appendage on the surface of most differentiated cell types called primary cilium. Primary cilia are both chemo- and mechano-sensory in function and have an obvious role in cell cycle control. Because of this, it has been thought that primary cilia are not found on rapidly proliferating cells, for example, cancer cells. Here we report using immunofluorescent staining for the ciliary protein Arl13b that primary cilia are frequently found on HeLa (human epithelial adenocarcinoma) and other cancer cell lines such as MG63 (human osteosarcoma) commonly used for cell culture studies and that the ciliated population is significantly higher (ave. 28.6% and 46.5%, respectively in starved and 15.7-18.6% in un-starved cells) than previously anticipated. Our finding impacts the current perception of primary cilia formed in highly proliferative cells.

Charming neighborhoods on the cell surface: plasma membrane microdomains regulate receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTK) are an important family of growth factor and hormone receptors that regulate many aspects of cellular physiology. Ligand binding by RTKs at the plasma membrane elicits activation of many signaling intermediates. The spatial and temporal regulation of RTK signaling within cells is an important determinant of receptor signaling outcome. In particular, the compartmentalization of the plasma membrane into a number of microdomains allows context-specific control of RTK signaling. Indeed various RTKs are recruited to and enriched within specific plasma membrane microdomains under various conditions, including lipid-ordered domains such as caveolae and lipid rafts, clathrin-coated structures, tetraspanin-enriched microdomains, and actin-dependent protrusive membrane microdomains such as dorsal ruffles and invadosomes. We examine the evidence for control of RTK signaling by each of these plasma membrane microdomains, as well as molecular mechanisms for how this spatial organization controls receptor signaling.

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt(T308) and P-Akt(S473), and previous work showed decreased P-Akt(S473) in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt(T308) and P-Akt(S473) in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt(T308). Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt(S473) or P-Akt(T308) upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA-dependent phosphorylation of Akt(S473) and Akt(T308) in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.