An ACAP1-containing clathrin coat complex for endocytic recycling

PROPPINs and membrane fission in the endo-lysosomal system

PROPPINs constitute a conserved protein family with multiple members being expressed in many eukaryotes. PROPPINs have mainly been investigated for their role in autophagy, where they co-operate with several core factors for autophagosome formation. Recently, novel functions of these proteins on endo-lysosomal compartments have emerged. PROPPINs support the division of these organelles and the formation of tubulo-vesicular cargo carriers that mediate protein exit from them, such as those generated by the Retromer coat. In both cases, PROPPINs provide membrane fission activity. Integrating information from yeast and human cells this review summarizes the most important molecular features that allow these proteins to facilitate membrane fission and thus provide a critical element to endo-lysosomal protein traffic.

A conserved interaction between the effector Sca4 and host endocytic machinery suggests additional roles for Sca4 during rickettsial infection

Intracellular bacterial pathogens deploy secreted effector proteins that manipulate diverse host machinery and pathways to promote infection. Although many effectors carry out a single specific function or interaction, there are a growing number of secreted pathogen effectors capable of interacting with multiple host factors. However, few effectors secreted by obligate intracellular Rickettsia species have been linked to multiple host targets. Here, we investigated the conserved rickettsial secreted effector Sca4, which was previously shown to interact with host vinculin to promote cell-to-cell spread in the model Rickettsia species R. parkeri . We discovered that Sca4 also binds the host cell endocytic factor clathrin heavy chain (CHC, CLTC ) via a conserved segment in the Sca4 N-terminus. Ablation of CLTC expression or chemical inhibition of endocytosis reduced R. parkeri cell-to-cell spread, indicating that clathrin promotes efficient spread between mammalian cells. This activity was independent of Sca4 and appeared restricted to the recipient host cell, suggesting that the Sca4-clathrin interaction also regulates another aspect of the infectious lifecycle. Indeed, R. parkeri lacking Sca4 or expressing a Sca4 truncation unable to bind clathrin had markedly reduced burdens in tick cells, hinting at a cell-type specific function for the Sca4-clathrin interaction. Sca4 homologs from diverse Rickettsia species also bound clathrin, suggesting that the function of this novel effector-host interaction may be broadly important for rickettsial infection. We conclude that Sca4 has multiple targets during infection and that rickettsiae may manipulate host endocytic machinery to facilitate several stages of their life cycles.

Residence of the Nucleotide Sugar Transporter Family Members SLC35F1 and SLC35F6 in the Endosomal/Lysosomal Pathway

The SLC35 (Solute Carrier 35) family members acting as nucleotide sugar transporters are typically localized in the endoplasmic reticulum or Golgi apparatus. It is, therefore, intriguing that some reports document the presence of orphan transporters SLC35F1 and SLC35F6 within the endosomal and lysosomal system. Here, we compared the subcellular distribution of these proteins and found that they are concentrated in separate compartments; i.e., recycling endosomes for SLC35F1 and lysosomes for SLC35F6. Swapping the C-terminal tail of these proteins resulted in a switch of localization, with SLC35F1 being trafficked to lysosomes while SLC35F6 remained in endosomes. This suggested the presence of specific sorting signals in these C-terminal regions. Using site-directed mutagenesis, fluorescence microscopy, and cell surface biotinylation assays, we found that the EQERLL360 signal located in the cytoplasmic tail of human SLC35F6 is involved in its lysosomal sorting (as previously shown for this conserved sequence in mouse SLC35F6), and that SLC35F1 localization in the recycling pathway depends on two YXXΦ-type signals: a Y367KQF sequence facilitates its internalization from the plasma membrane, while a Y392TSL motif prevents its transport to lysosomes, likely by promoting SLC35F1 recycling to the cell surface. Taken together, these results support that some SLC35 members may function at different levels of the endosomal and lysosomal system.

Assembly and fission of tubular carriers mediating protein sorting in endosomes

Endosomes are central protein-sorting stations at the crossroads of numerous membrane trafficking pathways in all eukaryotes. They have a key role in protein homeostasis and cellular signalling and are involved in the pathogenesis of numerous diseases. Endosome-associated protein assemblies or coats collect transmembrane cargo proteins and concentrate them into retrieval domains. These domains can extend into tubular carriers, which then pinch off from the endosomal membrane and deliver the cargoes to appropriate subcellular compartments. Here we discuss novel insights into the structure of a number of tubular membrane coats that mediate the recruitment of cargoes into these carriers, focusing on sorting nexin-based coats such as Retromer, Commander and ESCPE-1. We summarize current and emerging views of how selective tubular endosomal carriers form and detach from endosomes by fission, highlighting structural aspects, conceptual challenges and open questions.

PTPN9 dephosphorylates FGFR2 pY656/657 through interaction with ACAP1 and ameliorates pemigatinib effect in cholangiocarcinoma

ABSTRACT AND AIM

Cholangiocarcinoma (CCA) is a highly aggressive and lethal cancer that originates from the biliary epithelium. Systemic treatment options for CCA are currently limited, and the first targeted drug of CCA, pemigatinib, emerged in 2020 for CCA treatment by inhibiting FGFR2 phosphorylation. However, the regulatory mechanism of FGFR2 phosphorylation is not fully elucidated.

APPROACH AND RESULTS

Here we screened the FGFR2-interacting proteins and showed that protein tyrosine phosphatase (PTP) N9 interacts with FGFR2 and negatively regulates FGFR2 pY656/657 . Using phosphatase activity assays and modeling the FGFR2-PTPN9 complex structure, we identified FGFR2 pY656/657 as a substrate of PTPN9, and found that sec. 14p domain of PTPN9 interacts with FGFR2 through ACAP1 mediation. Coexpression of PTPN9 and ACAP1 indicates a favorable prognosis for CCA. In addition, we identified key amino acids and motifs involved in the sec. 14p-APCP1-FGFR2 interaction, including the "YRETRRKE" motif of sec. 14p, Y471 of PTPN9, as well as the PH and Arf-GAP domain of ACAP1. Moreover, we discovered that the FGFR2 I654V substitution can decrease PTPN9-FGFR2 interaction and thereby reduce the effectiveness of pemigatinib treatment. Using a series of in vitro and in vivo experiments including patient-derived xenografts (PDX), we showed that PTPN9 synergistically enhances pemigatinib effectiveness and suppresses CCA proliferation, migration, and invasion by inhibiting FGFR2 pY656/657 .

CONCLUSIONS

Our study identifies PTPN9 as a negative regulator of FGFR2 phosphorylation and a synergistic factor for pemigatinib treatment. The molecular mechanism, oncogenic function, and clinical significance of the PTPN9-ACAP1-FGFR2 complex are revealed, providing more evidence for CCA precision treatment.

Akt may associate with insulin-responsive vesicles via interaction with sortilin

Insulin-responsive vesicles (IRVs) deliver the glucose transporter Glut4 to the plasma membrane in response to activation of the insulin signaling cascade: insulin receptor-IRS-PI3 kinase-Akt-TBC1D4-Rab10. Previous studies have shown that Akt, TBC1D4, and Rab10 are compartmentalized on the IRVs. Although functionally significant, the mechanism of Akt association with the IRVs remains unknown. Using pull-down assays, immunofluorescence microscopy, and cross-linking, we have found that Akt may be recruited to the IRVs via the interaction with the juxtamembrane domain of the cytoplasmic C terminus of sortilin, a major IRV protein. Overexpression of full-length sortilin increases insulin-stimulated phosphorylation of TBC1D4 and glucose uptake in adipocytes, while overexpression of the cytoplasmic tail of sortilin has the opposite effect. Our findings demonstrate that the IRVs represent both a scaffold and a target of insulin signaling.

Structural elucidation of how ARF small GTPases induce membrane tubulation for vesicle fission

The ADP-Ribosylation Factor (ARF) small GTPases have been found to act in vesicle fission through a direct ability to tubulate membrane. Here, we have used cryo-electron microscopy (EM) to solve the structure of an ARF6 protein lattice assembled on tubulated membrane to 3.9 Å resolution. ARF6 forms tetramers that polymerize into helical arrays to form this lattice. We identify, and confirm functionally, protein contacts critical for this lattice formation. The solved structure also suggests how the ARF amphipathic helix is positioned in the lattice for membrane insertion, and how a GTPase-activating protein (GAP) docks onto the lattice to catalyze ARF-GTP hydrolysis in completing membrane fission. As ARF1 and ARF6 are structurally conserved, we have also modeled ARF1 onto the ARF6 lattice, which has allowed us to pursue the reconstitution of Coat Protein I (COPI) vesicles to confirm more definitively that the ARF lattice acts in vesicle fission. Our findings are notable for having achieved the first detailed glimpse of how a small GTPase bends membrane and having provided a molecular understanding of how an ARF protein acts in vesicle fission.

Plasma membrane nanodeformations promote actin polymerization through CIP4/CDC42 recruitment and regulate type II IFN signaling

In their environment, cells must cope with mechanical stresses constantly. Among these, nanoscale deformations of plasma membrane induced by substrate nanotopography are now largely accepted as a biophysical stimulus influencing cell behavior and function. However, the mechanotransduction cascades involved and their precise molecular effects on cellular physiology are still poorly understood. Here, using homemade fluorescent nanostructured cell culture surfaces, we explored the role of Bin/Amphiphysin/Rvs (BAR) domain proteins as mechanosensors of plasma membrane geometry. Our data reveal that distinct subsets of BAR proteins bind to plasma membrane deformations in a membrane curvature radius-dependent manner. Furthermore, we show that membrane curvature promotes the formation of dynamic actin structures mediated by the Rho GTPase CDC42, the F-BAR protein CIP4, and the presence of PI(4,5)P2. In addition, these actin-enriched nanodomains can serve as platforms to regulate receptor signaling as they appear to contain interferon-γ receptor (IFNγ-R) and to lead to the partial inhibition of IFNγ-induced JAK/STAT signaling.

ACAP1 Deficiency Predicts Inferior Immunotherapy Response in Solid Tumors

BACKGROUND

ACAP1 plays a key role in endocytic recycling, which is essential for the normal function of lymphocytes. However, the expression and function of ACAP1 in lymphocytes have rarely been studied.

METHODS

Large-scale genomic data, including multiple bulk RNA-sequencing datasets, single-cell sequencing datasets, and immunotherapy cohorts, were exploited to comprehensively characterize ACAP1 expression, regulation, and function. Gene set enrichment analysis (GSEA) was used to uncover the pathways associated with ACAP1 expression. Eight algorithms, including TIMER, CIBERSORT, CIBERSORT-ABS, QUANTISEQ, xCELL, MCPCOUNTER, EPIC, and TIDE, were applied to estimate the infiltrating level of immune cells. Western blotting, qPCR, and ChIP-PCR were used to validate the findings from bioinformatic analyses. A T-cell co-culture killing assay was used to investigate the function of ACAP1 in lymphocytes.

RESULTS

ACAP1 was highly expressed in immune-related tissues and cells and minimally in other tissues. Moreover, single-cell sequencing analysis in tumor samples revealed that ACAP1 is expressed primarily in tumor-infiltrating lymphocytes (TILs), including T, B, and NK cells. ACAP1 expression is negatively regulated by promoter DNA methylation, with its promoter hypo-methylated in immune cells but hyper-methylated in other cells. Furthermore, SPI1 binds to the ACAP1 promoter and positively regulates its expression in immune cells. ACAP1 levels positively correlate with the infiltrating levels of TILs, especially CD8+ T cells, across a broad range of solid cancer types. ACAP1 deficiency is associated with poor prognosis and immunotherapeutic response in multiple cancer types treated with checkpoint blockade therapy (ICT). Functionally, the depletion of ACAP1 by RNA interference significantly impairs the T cell-mediated killing of tumor cells.

CONCLUSIONS

Our study demonstrates that ACAP1 is essential for the normal function of TILs, and its deficiency indicates an immunologically "cold" status of tumors that are resistant to ICT.

Coronin2A links actin-based endosomal processes to the EHD1 fission machinery

Fission of transport vesicles from endosomes is a crucial step in the recycling of lipids and receptors to the plasma membrane, but this process remains poorly understood. Although key components of the fission machinery, including the actin cytoskeleton and the ATPase Eps15 homology domain protein 1 (EHD1), have been implicated in endosomal fission, how this process is coordinately regulated is not known. We have identified the actin regulatory protein Coronin2A (CORO2A) as a novel EHD1 interaction partner. CORO2A localizes to stress fibers and actin microfilaments but also can be observed in partial overlap with EHD1 on endosomal structures. siRNA knockdown of CORO2A led to enlarged lamellae-like actin-rich protrusions, consistent with a role of other Coronin family proteins in attenuating actin-branching. Moreover, CORO2A depletion also caused a marked decrease in the internalization of clathrin-dependent cargo but had little impact on the uptake of clathrin-independent cargo, highlighting key differences in the role of branched actin for different modes of endocytosis. However, CORO2A was required for recycling of clathrin-independent cargo, and its depletion led to enlarged endosomes, supporting a role for CORO2A in the fission of endosomal vesicles. Our data support a novel role for CORO2A in coordinating endosomal fission and recycling with EHD1. [Media: see text].

GLUT4 On the move

Insulin rapidly stimulates GLUT4 translocation and glucose transport in fat and muscle cells. Signals from the occupied insulin receptor are translated into downstream signalling changes in serine/threonine kinases within timescales of seconds, and this is followed by delivery and accumulation of the glucose transporter GLUT4 at the plasma membrane. Kinetic studies have led to realisation that there are distinct phases of this stimulation by insulin. There is a rapid initial burst of GLUT4 delivered to the cell surface from a subcellular reservoir compartment and this is followed by a steady-state level of continuing stimulation in which GLUT4 recycles through a large itinerary of subcellular locations. Here, we provide an overview of the phases of insulin stimulation of GLUT4 translocation and the molecules that are currently considered to activate these trafficking steps. Furthermore, we suggest how use of new experimental approaches together with phospho-proteomic data may help to further identify mechanisms for activation of these trafficking processes.

Integrated analysis and validation reveal ACAP1 as a novel prognostic biomarker associated with tumor immunity in lung adenocarcinoma

ADP-ribosylation factor (Arf)-GTPase-activating protein (GAP) with coiled-coil, ankyrin repeat and PH domains 1 (ACAP1) has been reported to serve as an adaptor for clathrin coat complex playing a role in endocytic recycling and cellular migration. The potential role of ACAP1 in lung adenocarcinoma (LUAD) has not been yet completely defined. We performed the comprehensive analyses, including gene expression, survival analysis, genetic alteration, function enrichment, and immune characteristics. ACAP1 was remarkably downregulated in tumor tissues, and linked with the clinicopathologic features in LUAD patients. Prognostic analysis demonstrated that low ACAP1 expression was correlated with unsatisfactory overall survival (OS) and disease specific survival (DSS) in LUAD patients. Moreover, ACAP1 could be determined as a prognostic biomarker according to Cox proportional hazard model and nomogram model. We also confirmed that ACAP1 was downregulated in two LUAD cell lines, comparing to normal lung cell. Overexpression of ACAP1 caused a profound attenuation in cell proliferation, migration, invasion, and promoted cell apoptosis. Additionally, functional enrichment analyses confirmed that ACAP1 was highly correlated with T cell activation and immune response. Then, we further conducted immune landscape analyses, including single cell RNA sequencing, immune cells infiltration, and immune checkpoints. ACAP1 expression was positively associated with the infiltrating level of immune cells in TME and the expression of immune checkpoint molecules. This study first comprehensively analyzed molecular expression, clinical implication, and immune landscape features of ACAP1 in LUAD, suggesting that ACAP1 was predictive of prognosis and could serve as a potential biomarker predicting immunotherapy response for LUAD patients.

RGD-Binding Integrins Revisited: How Recently Discovered Functions and Novel Synthetic Ligands (Re-)Shape an Ever-Evolving Field

Integrins have been extensively investigated as therapeutic targets over the last decades, which has been inspired by their multiple functions in cancer progression, metastasis, and angiogenesis as well as a continuously expanding number of other diseases, e.g., sepsis, fibrosis, and viral infections, possibly also Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). Although integrin-targeted (cancer) therapy trials did not meet the high expectations yet, integrins are still valid and promising targets due to their elevated expression and surface accessibility on diseased cells. Thus, for the future successful clinical translation of integrin-targeted compounds, revisited and innovative treatment strategies have to be explored based on accumulated knowledge of integrin biology. For this, refined approaches are demanded aiming at alternative and improved preclinical models, optimized selectivity and pharmacological properties of integrin ligands, as well as more sophisticated treatment protocols considering dose fine-tuning of compounds. Moreover, integrin ligands exert high accuracy in disease monitoring as diagnostic molecular imaging tools, enabling patient selection for individualized integrin-targeted therapy. The present review comprehensively analyzes the state-of-the-art knowledge on the roles of RGD-binding integrin subtypes in cancer and non-cancerous diseases and outlines the latest achievements in the design and development of synthetic ligands and their application in biomedical, translational, and molecular imaging approaches. Indeed, substantial progress has already been made, including advanced ligand designs, numerous elaborated pre-clinical and first-in-human studies, while the discovery of novel applications for integrin ligands remains to be explored.

Coordination of Grp1 recruitment mechanisms by its phosphorylation

The action of guanine nucleotide exchange factors (GEFs) on the ADP-ribosylation factor (ARF) family of small GTPases initiates intracellular transport pathways. This role requires ARF GEFs to be recruited from the cytosol to intracellular membrane compartments. An ARF GEF known as General receptor for 3-phosphoinositides 1 (Grp1) is recruited to the plasma membrane through its pleckstrin homology (PH) domain that recognizes phosphatidylinositol 3,4,5-trisphosphate (PIP3). Here, we find that the phosphorylation of Grp1 induces its PH domain to recognize instead phosphatidylinositol 4-phosphate (PI4P). This phosphorylation also releases an autoinhibitory mechanism that results in the coil–coil (CC) domain of Grp1 engaging two peripheral membrane proteins of the recycling endosome. Because the combination of these actions results in Grp1 being recruited preferentially to the recycling endosome rather than to the plasma membrane, our findings reveal the complexity of recruitment mechanisms that need to be coordinated in localizing an ARF GEF to an intracellular compartment to initiate a transport pathway. Our elucidation is also remarkable for having revealed that phosphoinositide recognition by a PH domain can be switched through its phosphorylation.

Vesicular Transport Machinery in Brain Endothelial Cells: What We Know and What We Do not

The vesicular transport machinery regulates numerous essential functions in cells such as cell polarity, signaling pathways, and the transport of receptors and their cargoes. From a pharmaceutical perspective, vesicular transport offers avenues to facilitate the uptake of therapeutic agents into cells and across cellular barriers. In order to improve receptor-mediated transcytosis of biologics across the blood-brain barrier and into the diseased brain, a detailed understanding of intracellular transport mechanisms is essential. The vesicular transport machinery is a highly complex network and involves an array of protein complexes, cytosolic adaptor proteins, and the subcellular structures of the endo-lysosomal system. The endo-lysosomal system includes several types of vesicular entities such as early, late, and recycling endosomes, exosomes, ectosomes, retromer-coated vesicles, lysosomes, trans-endothelial channels, and tubules. While extensive research has been done on the trafficking system in many cell types, little is known about vesicular trafficking in brain endothelial cells. Consequently, assumptions on the transport system in endothelial cells are based on findings in polarised epithelial cells, although recent studies have highlighted differences in the endothelial system. This review highlights aspects of the vesicular trafficking machinery in brain endothelial cells, including recent findings, limitations, and opportunities for further studies.

The protein kinase Akt acts as a coat adaptor in endocytic recycling

Coat proteins play a central role in vesicular transport by binding to cargoes for their sorting into intracellular pathways. Cargo recognition is mediated by components of the coat complex known as adaptor proteins^1–3. We previously showed that ACAP1 (ArfGAP with Coil-coil Ankyrin repeat Protein 1) functions as an adaptor for a clathrin coat complex acting in endocytic recycling^4–6. Here, we find that the protein kinase Akt acts as a co-adaptor in this complex, needed in conjunction with ACAP1 to bind cargo proteins for their recycling. Besides advancing the understanding of endocytic recycling, our findings uncover a fundamentally different way that a kinase acts, being an effector rather than a regulator in a cellular event.

A prognostic nomogram integrating novel biomarkers identified by machine learning for cervical squamous cell carcinoma

Background

Cervical cancer (CC) represents the fourth most frequently diagnosed malignancy affecting women all over the world. However, effective prognostic biomarkers are still limited for accurately identifying high-risk patients. Here, we provided a combination machine learning algorithm-based signature to predict the prognosis of cervical squamous cell carcinoma (CSCC).

Methods and materials

After utilizing RNA sequencing (RNA-seq) data from 36 formalin-fixed and paraffin-embedded (FFPE) samples, the most significant modules were highlighted by the weighted gene co-expression network analysis (WGCNA). A candidate genes-based prognostic classifier was constructed by the least absolute shrinkage and selection operator (LASSO) and then validated in an independent validation set. Finally, based on the multivariate analysis, a nomogram including the FIGO stage, therapy outcome, and risk score level was built to predict progression-free survival (PFS) probability.

Results

A mRNA-based signature was developed to classify patients into high- and low-risk groups with significantly different PFS and overall survival (OS) rate (training set: p < 0.001 for PFS, p = 0.016 for OS; validation set: p = 0.002 for PFS, p = 0.028 for OS). The prognostic classifier was an independent and powerful prognostic biomarker for PFS in both cohorts (training set: hazard ratio [HR] = 0.13, 95% CI 0.05–0.33, p < 0.001; validation set: HR = 0.02, 95% CI 0.01–0.04, p < 0.001). A nomogram that integrated the independent prognostic factors was constructed for clinical application. The calibration curve showed that the nomogram was able to predict 1-, 3-, and 5-year PFS accurately, and it performed well in the external validation cohorts (concordance index: 0.828 and 0.864, respectively).

Conclusion

The mRNA-based biomarker is a powerful and independent prognostic factor. Furthermore, the nomogram comprising our prognostic classifier is a promising predictor in identifying the progression risk of CSCC patients.

GLUT4 Storage Vesicles: Specialized Organelles for Regulated Trafficking

Fat and muscle cells contain a specialized, intracellular organelle known as the GLUT4 storage vesicle (GSV). Insulin stimulation mobilizes GSVs, so that these vesicles fuse at the cell surface and insert GLUT4 glucose transporters into the plasma membrane. This example is likely one instance of a broader paradigm for regulated, non-secretory exocytosis, in which intracellular vesicles are translocated in response to diverse extracellular stimuli. GSVs have been studied extensively, yet these vesicles remain enigmatic. Data support the view that in unstimulated cells, GSVs are present as a pool of preformed small vesicles, which are distinct from endosomes and other membrane-bound organelles. In adipocytes, GSVs contain specific cargoes including GLUT4, IRAP, LRP1, and sortilin. They are formed by membrane budding, involving sortilin and probably CHC22 clathrin in humans, but the donor compartment from which these vesicles form remains uncertain. In unstimulated cells, GSVs are trapped by TUG proteins near the endoplasmic reticulum - Golgi intermediate compartment (ERGIC). Insulin signals through two main pathways to mobilize these vesicles. Signaling by the Akt kinase modulates Rab GTPases to target the GSVs to the cell surface. Signaling by the Rho-family GTPase TC10α stimulates Usp25m-mediated TUG cleavage to liberate the vesicles from the Golgi. Cleavage produces a ubiquitin-like protein modifier, TUGUL, that links the GSVs to KIF5B kinesin motors to promote their movement to the cell surface. In obesity, attenuation of these processes results in insulin resistance and contributes to type 2 diabetes and may simultaneously contribute to hypertension and dyslipidemia in the metabolic syndrome.

ACAP1 assembles into an unusual protein lattice for membrane deformation through multiple stages

Studies on the Bin-Amphiphysin-Rvs (BAR) domain have advanced a fundamental understanding of how proteins deform membrane. We previously showed that a BAR domain in tandem with a Pleckstrin Homology (PH domain) underlies the assembly of ACAP1 (Arfgap with Coil-coil, Ankryin repeat, and PH domain I) into an unusual lattice structure that also uncovers a new paradigm for how a BAR protein deforms membrane. Here, we initially pursued computation-based refinement of the ACAP1 lattice to identify its critical protein contacts. Simulation studies then revealed how ACAP1, which dimerizes into a symmetrical structure in solution, is recruited asymmetrically to the membrane through dynamic behavior. We also pursued electron microscopy (EM)-based structural studies, which shed further insight into the dynamic nature of the ACAP1 lattice assembly. As ACAP1 is an unconventional BAR protein, our findings broaden the understanding of the mechanistic spectrum by which proteins assemble into higher-ordered structures to achieve membrane deformation.

Transcytosis maintains CFTR apical polarity in the face of constitutive and mutation-induced basolateral missorting

Apical polarity of cystic fibrosis transmembrane conductance regulator (CFTR) is essential for solute and water transport in secretory epithelia and can be impaired in human diseases. Maintenance of apical polarity in the face of CFTR non-polarized delivery and inefficient apical retention of mutant CFTRs lacking PDZ-domain protein (NHERF1, also known as SLC9A3R1) interaction, remains enigmatic. Here, we show that basolateral CFTR delivery originates from biosynthetic (∼35%) and endocytic (∼65%) recycling missorting. Basolateral channels are retrieved via basolateral-to-apical transcytosis (hereafter denoted apical transcytosis), enhancing CFTR apical expression by two-fold and suppressing its degradation. In airway epithelia, CFTR transcytosis is microtubule-dependent but independent of Myo5B, Rab11 proteins and NHERF1 binding to its C-terminal DTRL motif. Increased basolateral delivery due to compromised apical recycling and accelerated internalization upon impaired NHERF1-CFTR association is largely counterbalanced by efficient CFTR basolateral internalization and apical transcytosis. Thus, transcytosis represents a previously unrecognized, but indispensable, mechanism for maintaining CFTR apical polarity that acts by attenuating its constitutive and mutation-induced basolateral missorting.

To degrade or not to degrade: mechanisms and significance of endocytic recycling

Newly endocytosed integral cell surface proteins are typically either directed for degradation or subjected to recycling back to the plasma membrane. The sorting of integral cell surface proteins, including signalling receptors, nutrient transporters, ion channels, adhesion molecules and polarity markers, within the endolysosomal network for recycling is increasingly recognized as an essential feature in regulating the complexities of physiology at the cell, tissue and organism levels. Historically, endocytic recycling has been regarded as a relatively passive process, where the majority of internalized integral proteins are recycled via a nonspecific sequence-independent 'bulk membrane flow' pathway. Recent work has increasingly challenged this view. The discovery of sequence-specific sorting motifs and the identification of cargo adaptors and associated coat complexes have begun to uncover the highly orchestrated nature of endosomal cargo recycling, thereby providing new insight into the function and (patho)physiology of this process.

GAPDH inhibits intracellular pathways during starvation for cellular energy homeostasis

Starvation poses a fundamental challenge to cell survival. Whereas the role of autophagy in promoting energy homeostasis in this setting has been extensively characterized1, other mechanisms are less well understood. Here we reveal that glyceraldehyde 3-phosphate dehydrogenase (GAPDH) inhibits coat protein I (COPI) transport by targeting a GTPase-activating protein (GAP) towards ADP-ribosylation factor 1 (ARF1) to suppress COPI vesicle fission. GAPDH inhibits multiple other transport pathways, also by targeting ARF GAPs. Further characterization suggests that this broad inhibition is activated by the cell during starvation to reduce energy consumption. These findings reveal a remarkable level of coordination among the intracellular transport pathways that underlies a critical mechanism of cellular energy homeostasis.

MiR-145 improves macrophage-mediated inflammation through targeting Arf6

PURPOSE

To explore the relationship between miR-145 and ADP ribosylation factor 6 (Arf6) in regulating macrophage-mediated inflammation.

METHODS

THP-1 cells were induced by 160 nM of phorbol 12^-myristate 13-acetate (PMA) for 48 h to differentiate to macrophages and then were treated with LPS (100 ng/ml) for 8 h to simulate chronic metabolic inflammation in vitro. Dual-luciferase reporter assay was performed. MiR-145 siRNA and LV-ARF6-RNAi were used to up or down regulate miR-145 and Arf6 expression in THP-1 cells, respectively. Omental adipose tissue from patients in surgical ward were collected to detect the expression of miR-145, Arf6 and production of proinflammatory cytokines. Patients were divided into three groups according to their body mass index and history of diabetes.

RESULTS

Dual-luciferase reporter assays showed the direct down-regulation of Arf6 by miR-145. Forty-eight-hour-transfection of miR-145 inhibitor resulted in significant increase of Arf6, IL-1beta, TNF-alpha and IL-6 as well as phosphorylation of p65 in NF-kappaB pathway in THP-1 cells, which, inversely, were reversed by overexpressing miR-145. In addition, down-regulation of Arf6 in macrophages reduced expression and secretion of cytokines. Expression of miR-145 was found to be attenuated in the omental adipose tissue of obese patients and diabetics with greater Arf6 expression, confirming the role of miR-145 in regulating macrophage-mediated inflammation targeting Arf6.

CONCLUSIONS

By means of reducing the expression of Arf6 and subsequent signal transduction via NF-kappaB, miR-145 plays a role in inhibiting the secretion of inflammatory factors and then improving the inflammatory status. MiR-145 might be one of the candidates for anti-inflammatory treatment for metabolic diseases.

Intracellular recycling and cross-presentation by MHC class I molecules

Cross-presentation of internalized antigens by dendritic cells requires efficient delivery of Major Histocompatibility Complex (MHC) class I molecules to peptide-loading compartments. Strong evidence suggests that such loading can occur outside of the endoplasmic reticulum; however, the trafficking pathways and sources of class I molecules involved are poorly understood. Examination of non-professional, non-phagocytic cells has revealed a clathrin-independent, Arf6-dependent recycling pathway likely traveled by internalized optimally loaded (closed) class I molecules. Some closed and all open MHC class I molecules travel to late endosomes to be degraded but might also partly be re-loaded with peptides and recycled. Studies of viral interference revealed pathways in which class I molecules are directed to degradation in lysosomes upon ubiquitination at the surface, or upon AP-1 and HIV-nef-dependent misrouting from the Golgi network to lysosomes. While many observations made in non-professional cells remain to be re-examined in dendritic cells, available evidence suggests that both recycling and neo-synthesized class I molecules can be loaded with cross-presented peptides. Recycling molecules can be recruited to phagosomes triggered by innate signals such as TLR4 ligands, and may therefore specialize in loading with phagocytosed antigens. In contrast, AP-1-dependent accumulation at, or trafficking through, a Golgi compartment of newly synthesized molecules appears to be important for cross-presentation of soluble proteins and possibly of long peptides that are processed in the so-called vacuolar pathway. However, significant cell biological work will be required to confirm this or any other model and to integrate knowledge on MHC class I biochemistry and trafficking in models of CD8(+) T-cell priming by dendritic cells.

Comprehensive functional screening of miRNAs involved in fat cell insulin sensitivity among women

The key pathological link between obesity and type 2 diabetes is insulin resistance, but the molecular mechanisms are not entirely identified. micro-RNAs (miRNA) are dysregulated in obesity and may contribute to insulin resistance. Our objective was to detect and functionally investigate miRNAs linked to insulin sensitivity in human subcutaneous white adipose tissue (scWAT). Subjects were selected based on the insulin-stimulated lipogenesis response of subcutaneous adipocytes. Global miRNA profiling was performed in abdominal scWAT of 18 obese insulin-resistance (OIR), 21 obese insulin-sensitive (OIS), and 9 lean women. miRNAs demonstrating differential expression between OIR and OIS women were overexpressed in human in vitro-differentiated adipocytes followed by assessment of lipogenesis and identification of miRNA targets by measuring mRNA/protein expression and 3'-untranslated region analysis. Eleven miRNAs displayed differential expression between OIR and OIS states. Overexpression of miR-143-3p and miR-652-3p increased insulin-stimulated lipogenesis in human in vitro differentiated adipocytes and directly or indirectly affected several genes/proteins involved in insulin signaling at transcriptional or posttranscriptional levels. Adipose expression of miR-143-3p and miR-652-3p was positively associated with insulin-stimulated lipogenesis in scWAT independent of body mass index. In conclusion, miR-143-3p and miR-652-3p are linked to scWAT insulin resistance independent of obesity and influence insulin-stimulated lipogenesis by interacting at different steps with insulin-signaling pathways.

Arf GAPs and molecular motors

Arf GTPase-activating proteins (Arf GAPs) were first identified as regulators of the small GTP-binding proteins ADP-ribosylation factors (Arfs). The Arf GAPs are a large family of proteins in metazoans, outnumbering the Arfs that they regulate. The members of the Arf GAP family have complex domain structures and some have been implicated in particular cellular functions, such as cell migration, or with particular pathologies, such as tumor invasion and metastasis. The specific effects of Arfs sometimes depend on the Arf GAP involved in their regulation. These observations have led to speculation that the Arf GAPs themselves may affect cellular activities in capacities beyond the regulation of Arfs. Recently, 2 Arf GAPs, ASAP1 and AGAP1, have been found to bind directly to and influence the activity of myosins and kinesins, motor proteins associated with filamentous actin and microtubules, respectively. The Arf GAP-motor protein interaction is critical for cellular behaviors involving the actin cytoskeleton and microtubules, such as cell migration and other cell movements. Arfs, then, may function with molecular motors through Arf GAPs to regulate microtubule and actin remodeling.

Arf GAPs: A family of proteins with disparate functions that converge on a common structure, the integrin adhesion complex

ADP-ribosylation factors (Arfs) are members of the Ras GTPase superfamily. The function of Arfs is dependent on GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), which allow Arfs to cycle between the GDP-bound and GTP-bound forms. Arf GAPs have been shown to be present in integrin adhesion complexes, which include focal adhesions. Integrin adhesion complexes are composed of integrins, scaffolding proteins and signaling proteins and regulate cell proliferation, survival, differentiation and migration. Understanding the role of Arf GAPs in the regulation of integrin adhesion complexes is relevant to understanding normal physiology and cancer. In this review, we will discuss the contribution of the Arf GAP family members to the regulation of integrin adhesion complexes, examining the diverse mechanisms by which they control integrin adhesion complex formation, maturation and dissolution. GIT1 and ARAP2 serve as GAPs for Arf6, regulating Rac1 and other effectors by mechanisms still being defined. In contrast, GIT2 regulates Rac1 independent of Arf6. AGAP2 binds to and regulates focal adhesion kinase (FAK). ARAP2 and ACAP1, both Arf6 GAPs, regulate membrane trafficking of integrins through different endocytic pathways, exerting opposite effects on focal adhesions. ASAP1 not only regulates actin cytoskeleton remodeling through its interaction with nonmuscle myosin 2A, but is also important in integrin recycling. These examples illustrate the diversity and versatility of Arf GAPs as regulators of integrin adhesion complex structure and function.

Adaptor protein-3: A key player in RBL-2H3 mast cell mediator release

Mast cell (MC) secretory granules are Lysosome-Related Organelles (LROs) whose biogenesis is associated with the post-Golgi secretory and endocytic pathways in which the sorting of proteins destined for a specific organelle relies on the recognition of sorting signals by adaptor proteins that direct their incorporation into transport vesicles. The adaptor protein 3 (AP-3) complex mediates protein trafficking between the trans-Golgi network (TGN) and late endosomes, lysosomes, and LROs. AP-3 has a recognized role in LROs biogenesis and regulated secretion in several cell types, including many immune cells such as neutrophils, natural killer cells, and cytotoxic T lymphocytes. However, the relevance of AP-3 for these processes in MCs has not been previously investigated. AP-3 was found to be expressed and distributed in a punctate fashion in rat peritoneal mast cells ex vivo. The rat MC line RBL-2H3 was used as a model system to investigate the role of AP-3 in mast cell secretory granule biogenesis and mediator release. By immunofluorescence and immunoelectron microscopy, AP-3 was localized both to the TGN and early endosomes indicating that AP-3 dependent sorting of proteins to MC secretory granules originates in these organelles. ShRNA mediated depletion of the AP-3 δ subunit was shown to destabilize the AP-3 complex in RBL-2H3 MCs. AP-3 knockdown significantly affected MC regulated secretion of β-hexosaminidase without affecting total cellular enzyme levels. Morphometric evaluation of MC secretory granules by electron microscopy revealed that the area of MC secretory granules in AP-3 knockdown MCs was significantly increased, indicating that AP-3 is involved in MC secretory granule biogenesis. Furthermore, AP-3 knockdown had a selective impact on the secretion of newly formed and newly synthesized mediators. These results show for the first time that AP-3 plays a critical role in secretory granule biogenesis and mediator release in MCs.

Ankyrin-B is a PI3P effector that promotes polarized α5β1-integrin recycling via recruiting RabGAP1L to early endosomes

Endosomal membrane trafficking requires coordination between phosphoinositide lipids, Rab GTPases, and microtubule-based motors to dynamically determine endosome identity and promote long-range organelle transport. Here we report that ankyrin-B (AnkB), through integrating all three systems, functions as a critical node in the protein circuitry underlying polarized recycling of α5β1-integrin in mouse embryonic fibroblasts, which enables persistent fibroblast migration along fibronectin gradients. AnkB associates with phosphatidylinositol 3-phosphate (PI3P)-positive organelles in fibroblasts and binds dynactin to promote their long-range motility. We demonstrate that AnkB binds to Rab GTPase Activating Protein 1-Like (RabGAP1L) and recruits it to PI3P-positive organelles, where RabGAP1L inactivates Rab22A, and promotes polarized trafficking to the leading edge of migrating fibroblasts. We further determine that α5β1-integrin depends on an AnkB/RabGAP1L complex for polarized recycling. Our results reveal AnkB as an unexpected key element in coordinating polarized transport of α5β1-integrin and likely of other specialized endocytic cargos.

Endocytic Trafficking of Integrins in Cell Migration

Integrins are a family of heterodimeric receptors that bind to components of the extracellular matrix and influence cellular processes as varied as proliferation and migration. These effects are achieved by tight spatiotemporal control over intracellular signalling pathways, including those that mediate cytoskeletal reorganisation. The ability of integrins to bind to ligands is governed by integrin conformation, or activity, and this is widely acknowledged to be an important route to the regulation of integrin function. Over the last 15 years, however, the pathways that regulate endocytosis and recycling of integrins have emerged as major players in controlling integrin action, and studying integrin trafficking has revealed fresh insight into the function of this fascinating class of extracellular matrix receptors, in particular in the context of cell migration and invasion. Here, we review our current understanding of the contribution of integrin trafficking to cell motility.

ACAP3 regulates neurite outgrowth through its GAP activity specific to Arf6 in mouse hippocampal neurons

ACAP3 (ArfGAP with coiled-coil, ankyrin repeat and pleckstrin homology domains 3) belongs to the ACAP family of GAPs (GTPase-activating proteins) for the small GTPase Arf (ADP-ribosylation factor). However, its specificity to Arf isoforms and physiological functions remain unclear. In the present study, we demonstrate that ACAP3 plays an important role in neurite outgrowth of mouse hippocampal neurons through its GAP activity specific to Arf6. In primary cultured mouse hippocampal neurons, knockdown of ACAP3 abrogated neurite outgrowth, which was rescued by ectopically expressed wild-type ACAP3, but not by its GAP activity-deficient mutant. Ectopically expressed ACAP3 in HEK (human embryonic kidney)-293T cells showed the GAP activity specific to Arf6. In support of this observation, the level of GTP-bound Arf6 was significantly increased by knockdown of ACAP3 in hippocampal neurons. In addition, knockdown and knockout of Arf6 in mouse hippocampal neurons suppressed neurite outgrowth. These results demonstrate that ACAP3 positively regulates neurite outgrowth through its GAP activity specific to Arf6. Furthermore, neurite outgrowth suppressed by ACAP3 knockdown was rescued by expression of a fast cycle mutant of Arf6 that spontaneously exchanges guanine nucleotides on Arf6, but not by that of wild-type, GTP- or GDP-locked mutant Arf6. Thus cycling between active and inactive forms of Arf6, which is precisely regulated by ACAP3 in concert with a guanine-nucleotide-exchange factor(s), seems to be required for neurite outgrowth of hippocampal neurons.

Regulation of Cell Migration and β1 Integrin Trafficking by the Endosomal Adaptor GGA3

The integrin family of cell adhesion receptors link extracellular matrices to intracellular signaling pathways and the actin cytoskeleton; and regulate cell migration, proliferation and survival in normal and diseased tissues. The subcellular location of integrin receptors is critical for their function and deregulated trafficking is implicated in various human diseases. Here we identify a role for Golgi-localized gamma-ear containing Arf-binding protein 3 (GGA3), in regulating trafficking of β1 integrin. GGA3 knockdown reduces cell surface and total levels of α2, α5 and β1 integrin subunits, inhibits cell spreading, reduces focal adhesion number, as well as cell migration. In the absence of GGA3, integrins are increasingly retained inside the cell, traffic toward the perinuclear lysosomal compartment and their degradation is enhanced. Integrin traffic and maintenance of integrin levels are dependent on the integrity of the Arf binding site of GGA3. Furthermore, sorting nexin 17 (SNX17), a critical regulator of integrin recycling, becomes mislocalized to enlarged late endosomes upon GGA3 depletion. These data support a model whereby GGA3, through its ability to regulate SNX17 endosomal localization and through interaction with Arf6 diverts integrins from the degradative pathway supporting cell migration.

Glut4 Is Sorted from a Rab10 GTPase-independent Constitutive Recycling Pathway into a Highly Insulin-responsive Rab10 GTPase-dependent Sequestration Pathway after Adipocyte Differentiation

The RabGAP AS160/TBC1D4 controls exocytosis of the insulin-sensitive glucose transporter Glut4 in adipocytes. Glut4 is internalized and recycled through a highly regulated secretory pathway in these cells. Glut4 also cycles through a slow constitutive endosomal pathway distinct from the fast transferrin (Tf) receptor recycling pathway. This slow constitutive pathway is the only Glut4 cycling pathway in undifferentiated fibroblasts. The α2-macroglobulin receptor LRP1 cycles with Glut4 and the Tf receptor through all three exocytic pathways. To further characterize these pathways, the effects of knockdown of AS160 substrates on the trafficking kinetics of Glut4, LRP1, and the Tf receptor were measured in adipocytes and fibroblasts. Rab10 knockdown decreased cell surface Glut4 in insulin-stimulated adipocytes by 65%, but not in basal adipocytes or in fibroblasts. This decrease was due primarily to a 62% decrease in the rate constant of Glut4 exocytosis (kex), although Rab10 knockdown also caused a 1.4-fold increase in the rate constant of Glut4 endocytosis (ken). Rab10 knockdown in adipocytes also decreased cell surface LRP1 by 30% by decreasing kex 30-40%. There was no effect on LRP1 trafficking in fibroblasts or on Tf receptor trafficking in either cell type. These data confirm that Rab10 is an AS160 substrate that limits exocytosis through the highly insulin-responsive specialized secretory pathway in adipocytes. They further show that the slow constitutive endosomal (fibroblast) recycling pathway is Rab10-independent. Thus, Rab10 is a marker for the specialized pathway in adipocytes. Interestingly, mathematical modeling shows that Glut4 traffics predominantly through the specialized Rab10-dependent pathway both before and after insulin stimulation.

A Role for the Non-Canonical Wnt-β-Catenin and TGF-β Signaling Pathways in the Induction of Tolerance during the Establishment of a Salmonella enterica Serovar Enteritidis Persistent Cecal Infection in Chickens

Non-typhoidal Salmonella enterica induce an early pro-inflammatory response in chickens. However, the response is short-lived, asymptomatic of disease, resulting in a persistent colonization of the ceca, and fecal shedding of bacteria. The underlying mechanisms that control this persistent infection of chickens by Salmonella are unknown. Recently, we found an expansion of the Treg population and subsequent increased in vitro immunosuppressive functions of the CD4(+)CD25(+) cells isolated from the ceca of the Salmonella-infected chickens by day 4 post-infection that increased steadily throughout the course of the 14 days of infection, whereas the number of CD4(+)CD25(+) cells in the non-infected controls remained steady throughout the study. CD4(+)CD25(+) cells from cecal tonsils of S. enteritidis-infected birds had greater expression of IL-10 mRNA content than the CD4(+)CD25(+) cells from the non-infected controls at all the time points studied. These results suggest the development of a tolerogenic immune response in the cecum of Salmonella-infected chickens may contribute to the persistance of Salmonella cecal colonization. Using a chicken-specific kinome peptide immune array, we have analyzed the signaling pathways altered during the establishment of this tolerogenic state. This analysis has revealed a role for the non-canonical Wnt signaling pathway in the cecum at 4 days post-infection. Infection induced the significant (p < 0.01) phosphorylation of the G-protein-coupled transmembrane protein, Frizzled 1 (FZD1), resulting in an influx of intracellular Ca(2+) and the phosphorylation of the Ca(2+)-dependent effector molecules calcium/calmodulin-dependent kinase II (CamKII), β-catenin, protein kinase C, and the activation of the transcription factor, NFAT. Nuclear translocation of NFAT resulted in a significant increase in the expression of the anti-inflammatory cytokines IL-10 and TGF-β. Increased expression of TGF-β4 mRNA activates the TGF-β signaling pathway that phosphorylates the receptor-activated Smads, Smad2 and Smad3. Combined with the results from our Treg studies, these studies describe kinome-based phenotypic changes in the cecum of chickens during Salmonella Enteritidis infection starting 4 days post-infection that leads to an anti-inflammatory, tolerogenic local environment, and results in the establishment of persistent intestinal colonization.

Exclusion of integrins from CNS axons is regulated by Arf6 activation and the AIS

Integrins are adhesion and survival molecules involved in axon growth during CNS development, as well as axon regeneration after injury in the peripheral nervous system (PNS). Adult CNS axons do not regenerate after injury, partly due to a low intrinsic growth capacity. We have previously studied the role of integrins in axon growth in PNS axons; in the present study, we investigate whether integrin mechanisms involved in PNS regeneration may be altered or lacking from mature CNS axons by studying maturing CNS neurons in vitro. In rat cortical neurons, we find that integrins are present in axons during initial growth but later become restricted to the somato-dendritic domain. We investigated how this occurs and whether it can be altered to enhance axonal growth potential. We find a developmental change in integrin trafficking; transport becomes predominantly retrograde throughout axons, but not dendrites, as neurons mature. The directionality of transport is controlled through the activation state of ARF6, with developmental upregulation of the ARF6 GEF ARNO enhancing retrograde transport. Lowering ARF6 activity in mature neurons restores anterograde integrin flow, allows transport into axons, and increases axon growth. In addition, we found that the axon initial segment is partly responsible for exclusion of integrins and removal of this structure allows integrins into axons. Changing posttranslational modifications of tubulin with taxol also allows integrins into the proximal axon. The experiments suggest that the developmental loss of regenerative ability in CNS axons is due to exclusion of growth-related molecules due to changes in trafficking.

Formation of Tubulovesicular Carriers from Endosomes and Their Fusion to the trans-Golgi Network

Endosomes undergo extensive spatiotemporal rearrangements as proteins and lipids flux through them in a series of fusion and fission events. These controlled changes enable the concentration of cargo for eventual degradation while ensuring the proper recycling of other components. A growing body of studies has now defined multiple recycling pathways from endosomes to the trans-Golgi network (TGN) which differ in their molecular machineries. The recycling process requires specific sets of lipids, coats, adaptors, and accessory proteins that coordinate cargo selection with membrane deformation and its association with the cytoskeleton. Specific tethering factors and SNARE (SNAP (Soluble NSF Attachment Protein) Receptor) complexes are then required for the docking and fusion with the acceptor membrane. Herein, we summarize some of the current knowledge of the machineries that govern the retrograde transport from endosomes to the TGN.

Transport of the cholera toxin B-subunit from recycling endosomes to the Golgi requires clathrin and AP-1

The retrograde pathway is defined by the transport of proteins and lipids from the plasma membrane through endosomes to the Golgi complex, and is essential for a variety of cellular activities. Recycling endosomes are important sorting stations for some retrograde cargo. SMAP2, a GTPase-activating protein (GAP) for Arf1 with a putative clathrin-binding domain, has previously been shown to participate in the retrograde transport of the cholera toxin B-subunit (CTxB) from recycling endosomes. Here, we found that clathrin, a vesicle coat protein, and clathrin adaptor protein complex 1 (AP-1) were present at recycling endosomes and were needed for the retrograde transport of CTxB from recycling endosomes to the Golgi, but not from the plasma membrane to recycling endosomes. SMAP2 immunoprecipitated clathrin and AP-1 through a putative clathrin-binding domain and a CALM-binding domain, and SMAP2 mutants that did not interact with clathrin or AP-1 could not localize to recycling endosomes. Moreover, knockdown of Arf1 suppressed the retrograde transport of CTxB from recycling endosomes to the Golgi. These findings suggest that retrograde transport is mediated by clathrin-coated vesicles from recycling endosomes and that the role of the coat proteins is in the recruitment of Arf GAP to transport vesicles.

The cytohesin guanosine exchange factors (GEFs) are required to promote HGF-mediated renal recovery after acute kidney injury (AKI) in mice

The lack of current treatment and preventable measures for acute kidney injury (AKI) in hospitalized patients results in an increased mortality rate of up to 80% and elevated health costs. Additionally, if not properly repaired, those who survive AKI may develop fibrosis and long-term kidney damage. The molecular aspects of kidney injury and repair are still uncertain. Hepatocyte growth factor (HGF) promotes recovery of the injured kidney by inducing survival and migration of tubular epithelial cells to repopulate bare tubule areas. HGF-stimulated kidney epithelial cell migration requires the activation of ADP-ribosylation factor 6 (Arf6) and Rac1 via the cytohesin family of Arf-guanine-nucleotide exchange factors (GEFs), in vitro. We used an ischemia and reperfusion injury (IRI) mouse model to analyze the effects of modulating this signaling pathway on kidney recovery. We treated IRI mice with either HGF, the cytohesin inhibitor SecinH3, or a combination of both. As previously reported, HGF treatment promoted rapid improvement of kidney function as evidenced by creatinine (Cre) and blood urea nitrogen (BUN) levels. In contrast, simultaneous treatment with SecinH3 and HGF blocks the ability of HGF to promote kidney recovery. Immunohistochemistry showed that HGF treatment promoted recovery of tubule structure, and had enhanced levels of active, GTP-bound Arf6 and GTP-Rac1. SecinH3 treatment, however, caused a dramatic decrease in GTP-Arf6 and GTP-Rac1 levels when compared to kidney sections from HGF-treated IRI mice. Additionally, SecinH3 counteracted the renal reparative effects of HGF. Our results support the conclusion that cytohesin function is required for HGF-stimulated renal IRI repair.

An ACAP1 coat complex acting in endocytic recycling

A key function of coat proteins is the sorting of protein cargoes into intracellular transport pathways. For many years, however, it has been unclear whether this role of coat proteins would apply to pathways of endocytic recycling. This issue has been clarified in recent years through the identification of multiple coat complexes acting in the recycling pathways. Leading this charge have been studies on a coat complex defined by ACAP1 (adenosine diphosphate ribosylation factor GTPase-activating proteins with Coiled-coil, Ankryin repeat and PH domains 1), which acts in the sorting of cargoes at the recycling endosome for their return to the plasma membrane. This chapter describes the methods used to characterize this role of ACAP1.

Nipah virus fusion protein: Importance of the cytoplasmic tail for endosomal trafficking and bioactivity

Nipah virus (NiV) is a highly pathogenic paramyxovirus which encodes two surface glycoproteins: the receptor-binding protein G and the fusion protein F. As for all paramyxoviruses, proteolytic activation of the NiV-F protein is an indispensable prerequisite for viral infectivity. Interestingly, proteolytic activation of NiV-F differs principally from other paramyxoviruses with respect to protease usage (cathepsins instead of trypsin- or furin-like proteases), and the subcellular localization where cleavage takes place (endosomes instead of Golgi or plasma membrane). To allow efficient F protein activation needed for productive virus replication and cell-to-cell fusion, the NiV-F cytoplasmic tail contains a classical tyrosine-based endocytosis signal (Y₅₂₅RSL) that we have shown earlier to be needed for F uptake and proteolytic activation. In this report, we furthermore revealed that an intact endocytosis signal alone is not sufficient for full bioactivity. The very C-terminus of the cytoplasmic tail is needed in addition. Deletions of more than four residues did not affect F uptake or endosomal cleavage but downregulated the surface expression, likely by delaying the intracellular trafficking through endosomal-recycling compartments. Given that the NiV-F cytoplasmic tail is needed for timely and correct intracellular trafficking, endosomal cleavage and fusion activity, the influence of tail truncations on NiV-mediated cell-to-cell fusion and on pseudotyping lentiviral vectors is discussed.

A novel twist in membrane dePHormation

Bin-Amphiphysin-Rvs (BAR) domain-containing proteins form oligomeric assemblies that aid membrane remodeling. In this issue of Developmental Cell, Pang et al. (2014) show that the BAR domain of ACAP1, although architecturally similar to other BAR domains, cooperates with its neighboring pleckstrin homology domain to deform membranes and facilitate endosomal recycling.

Putative terminator and/or effector functions of Arf GAPs in the trafficking of clathrin-coated vesicles

The role of ArfGAP1 as a terminator or effector in COPi-vesicle formation has been the subject of ongoing discussions. Here, the discussion on the putative terminator/effector functions has been enlarged to include Arf GAP members involved in the formation of clathrin-coated vesicles. ACAP1, whose role has been studied extensively, enhances the recycling of endocytosed proteins to the plasma membrane. Importantly, this positive role appears to be an overall reflection of both the terminator and effector activities attributed to ACAP1. Other Arf GAP subtypes have also been suggested to possess both terminator and effector activities. Interestingly, while most Arf GAP proteins regulate membrane trafficking by acting as facilitators, a few Arf GAP subtypes act as inhibitors.

A PH domain in ACAP1 possesses key features of the BAR domain in promoting membrane curvature

The BAR (Bin-Amphiphysin-Rvs) domain undergoes dimerization to produce a curved protein structure, which superimposes onto membrane through electrostatic interactions to sense and impart membrane curvature. In some cases, a BAR domain also possesses an amphipathic helix that inserts into the membrane to induce curvature. ACAP1 (Arfgap with Coil coil, Ankyrin repeat, and PH domain protein 1) contains a BAR domain. Here, we show that this BAR domain can neither bind membrane nor impart curvature, but instead requires a neighboring PH (Pleckstrin Homology) domain to achieve these functions. Specific residues within the PH domain are responsible for both membrane binding and curvature generation. The BAR domain adjacent to the PH domain instead interacts with the BAR domains of neighboring ACAP1 proteins to enable clustering at the membrane. Thus, we have uncovered the molecular basis for an unexpected and unconventional collaboration between PH and BAR domains in membrane bending.

Defective lysosome maturation and Legionella pneumophila replication in Dictyostelium cells mutant for the Arf GAP ACAP-A

Dictyostelium discoideum ACAP-A is an Arf GTPase-activating protein (GAP) involved in cytokinesis, cell migration and actin cytoskeleton dynamics. In mammalian cells, ACAP family members regulate endocytic protein trafficking. Here, we explored the function of ACAP-A in the endocytic pathway of D. discoideum. In the absence of ACAP-A, the efficiency of fusion between post-lysosomes and the plasma membrane was reduced, resulting in the accumulation of post-lysosomes. Moreover, internalized fluid-phase markers showed extended intracellular transit times, and the transfer kinetics of phagocyted particles from lysosomes to post-lysosomes was reduced. Neutralization of lysosomal pH, one essential step in lysosome maturation, was also delayed. Whereas expression of ACAP-A-GFP in acapA(-) cells restored normal particle transport kinetics, a mutant ACAP-A protein with no GAP activity towards the small GTPase ArfA failed to complement this defect. Taken together, these data support a role for ACAP-A in maturation of lysosomes into post-lysosomes through an ArfA-dependent mechanism. In addition, we reveal that ACAP-A is required for efficient intracellular growth of Legionella pneumophila, a pathogen known to subvert the endocytic host cell machinery for replication. This further emphasizes the role of ACAP-A in the endocytic pathway.

ArfGAPs: key regulators for receptor sorting

Mammalian cells have many membranous organelles that require proper composition of proteins and lipids. Cargo sorting is a process required for transporting specific proteins and lipids to appropriate organelles, and if this process is disrupted, organelle function as well as cell function is disrupted. ArfGAP family proteins have been found to be critical for receptor sorting. In this review, we summarize our recent knowledge about the mechanism of cargo sorting that require function of ArfGAPs in promoting the formation of transport vesicles, and discuss the involvement of specific ArfGAPs for the sorting of a variety of receptors, such as MPR, EGFR, TfR, Glut4, TRAIL-R1/DR4, M₅-muscarinic receptor, c-KIT, rhodopsin and β1-integrin. Given the importance of many of these receptors to human disease, the studies of ArfGAPs may provide novel therapeutic strategies in addition to providing mechanistic insight of receptor sorting.

Role of dynamin and clathrin in the cellular trafficking of flotillins

Flotillin-1 and flotillin-2 are highly conserved, membrane-microdomain-associated proteins that have been shown to be involved in signal transduction, membrane trafficking and cell adhesion. Upon growth factor stimulation, flotillins are tyrosine phosphorylated and become endocytosed from the plasma membrane into endosomes from which they are recycled back to the plasma membrane. Although a role for flotillin-1 in the endocytosis of certain cargo proteins has been suggested, it is not known how the growth-factor-induced endocytosis of flotillins is regulated and which endocytosis pathway is used. However, this is likely to be different from the pathway used by flotillin-dependent cargo. In this study, we have addressed the mechanistic details of flotillin trafficking during growth factor signaling. We show that dynamin-2 activity is required for the uptake of flotillins from the plasma membrane upon epidermal growth factor stimulation, and inhibition of dynamin-2 GTPase activity impairs flotillin endocytosis. Surprisingly, recycling of flotillins from endosomes to the plasma membrane appears to require both dynamin-2 and clathrin. Upon overexpression of dynamin-2 mutants or depletion of clathrin heavy chain, flotillins are permanently trapped in endosomes. These data show that clathrin and dynamin are required for the endosomal sorting of flotillins, and the study provides a mechanistic dissection of the thus far poorly characterized endosomal trafficking of flotillins.

Membrane-associated cargo recycling by tubule-based endosomal sorting

The endosome system is a collection of organelles that sort membrane-associated proteins and lipids for lysosomal degradation or recycling back to their target organelle. Recycling cargo is captured in a network of membrane tubules emanating from endosomes where tubular carriers pinch off. These tubules are formed and stabilized through the scaffolding properties of cytosolic Bin/Amphiphysin/Rvs (BAR) proteins that comprise phosphoinositide-detecting moieties, recruiting these proteins to specific endosomal membrane areas. These include the protein family of sorting nexins that remodel endosome membrane into tubules by an evolutionary conserved mechanism of dimerization, local membrane curvature detection/induction and oligomerization. How the formation of such a tubular membrane carrier is coordinated with cargo capture is largely unknown. The tubular structure of the membrane carriers could sequester membrane-bound cargo through an iterative mechanism of geometric sorting. Furthermore, the recent identification of cargo adaptors for the endosome protein sorting complex retromer has expanded the sorting signals that retrieve specific sets of cargo away from lysosomal degradation through distinct membrane trafficking pathways.

A proteolytic pathway that controls glucose uptake in fat and muscle

Insulin regulates glucose uptake by controlling the subcellular location of GLUT4 glucose transporters. GLUT4 is sequestered within fat and muscle cells during low-insulin states, and is translocated to the cell surface upon insulin stimulation. The TUG protein is a functional tether that sequesters GLUT4 at the Golgi matrix. To stimulate glucose uptake, insulin triggers TUG endoproteolytic cleavage. Cleavage accounts for a large proportion of the acute effect of insulin to mobilize GLUT4 to the cell surface. During ongoing insulin exposure, endocytosed GLUT4 recycles to the plasma membrane directly from endosomes, and bypasses a TUG-regulated trafficking step. Insulin acts through the TC10α GTPase and its effector protein, PIST, to stimulate TUG cleavage. This action is coordinated with insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases, and with other signals to direct overall GLUT4 targeting. Data support the idea that the N-terminal TUG cleavage product, TUGUL, functions as a novel ubiquitin-like protein modifier to facilitate GLUT4 movement to the cell surface. The C-terminal TUG cleavage product is extracted from the Golgi matrix, which vacates an "anchoring" site to permit subsequent cycles of GLUT4 retention and release. Together, GLUT4 vesicle translocation and TUG cleavage may coordinate glucose uptake with physiologic effects of other proteins present in the GLUT4-containing vesicles, and with potential additional effects of the TUG C-terminal product. Understanding this TUG pathway for GLUT4 retention and release will shed light on the regulation of glucose uptake and the pathogenesis of type 2 diabetes.