Rab11 regulates recycling through the pericentriolar recycling endosome

Autophagic and endo-lysosomal dysfunction in neurodegenerative disease

Due to their post-mitotic state, metabolic demands and often large polarised morphology, the function and survival of neurons is dependent on an efficient cellular waste clearance system both for generation of materials for metabolic processes and removal of toxic components. It is not surprising therefore that deficits in protein clearance can tip the balance between neuronal health and death. Here we discuss how autophagy and lysosome-mediated degradation pathways are disrupted in several neurological disorders. Both genetic and cell biological evidence show the diversity and complexity of vesicular clearance dysregulation in cells, and together may ultimately suggest a unified mechanism for neuronal demise in degenerative conditions. Causative and risk-associated mutations in Alzheimer's disease, Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, Parkinson's disease, Huntington's disease and others have given the field a unique mechanistic insight into protein clearance processes in neurons. Through their broad implication in neurodegenerative diseases, molecules involved in these genetic pathways, in particular those involved in autophagy, are emerging as appealing therapeutic targets for intervention in neurodegeneration.

PC1/3 KD Macrophages Exhibit Resistance to the Inhibitory Effect of IL-10 and a Higher TLR4 Activation Rate, Leading to an Anti-Tumoral Phenotype

During tumorigenesis, macrophages are recruited by tumors and orientated towards a pro-tumoral phenotype. One of the main anti-tumoral immunotherapy consists of their re-polarization in an anti-tumoral phenotype. We have demonstrated that the inhibition of proprotein convertase 1/3 combined with TLR4 activation in macrophages is a promising strategy. These macrophages display pro-inflammatory and anti-tumoral phenotypes. A hallmark is a stronger activation of the pro-inflammatory NFKB pathway. We believe that this can be explained by a modification of TLR4 expression at the cell surface or MYD88 cleavage since it exhibits a potential cleavage site for proprotein convertases. We tested these hypotheses through immunofluorescence and Western blot experiments. A proteomics study was also performed to test the sensitivity of these macrophages to IL-10. We demonstrated that these macrophages treated with LPS showed a quicker re-expression of TLR4 at the cell surface. The level of MYD88 was also higher when TLR4 was internalized. Moreover, these macrophages were resistant to the pro-tumoral effect of IL-10 and still produced pro-inflammatory factors. This established that the sensitivity to anti-inflammatory molecules and the length of TLR4 desensitization were reduced in these macrophages. Therefore, during antitumoral immunotherapy, a repeated stimulation of TLR4 may reactivate PC1/3 inhibited macrophages even in an anti-inflammatory environment.

The LMTK1-TBC1D9B-Rab11A Cascade Regulates Dendritic Spine Formation via Endosome Trafficking

Dendritic spines are postsynaptic protrusions at excitatory synapses that are critical for proper neuronal synaptic transmission. While lipid and protein membrane components are necessary for spine formation, it is largely unknown how they are recruited to developing spines. Endosomal trafficking is one mechanism that may influence this development. We recently reported that Lemur kinase 1A (LMTK1A), a membrane-bound Ser/Thr kinase, regulates trafficking of endosomes in neurons. LMTK1 has been shown to be a p35 Cdk5 activator-binding protein and a substrate for Cdk5-p35; however, its neuronal function has not been sufficiently studied. Here, we investigate the role of LMTK1 in spine formation. Depletion of LMTK1 increases spine formation, maturation, and density in primary cultured neurons and in mouse brain of either sex. Additionally, expression of kinase-negative LMTK1 stimulates spine formation in primary neurons and in vivo LMTK1 controls spine formation through Rab11, a regulator of recycling endosome trafficking. We identify TBC1D9B, a Rab11A GTPase-activating protein (Rab11A GAP), as a LMTK1 binding protein, and find that TBC1D9B mediates LMTK1 activity on Rab11A. TBC1D9B inactivates Rab11A under the control of LMTK1A. Further, by analyzing the effect of decreased TBC1D9B expression in primary neurons, we demonstrate that TBC1D9B indeed regulates spine formation. This is the first demonstration of the biological function of TBC1D9B. Together, with the regulation of LMTK1 by Cdk5-p35, we propose the Cdk5-LMTK1-TBC1D9B-Rab11A cascade as a novel signaling mechanism regulating endosomal transport for synapse formation and function.SIGNIFICANCE STATEMENT Dendritic spines are postsynaptic specializations essential for synaptic transmission. However, it is not known how critical membrane components are recruited to spines for their formation. Endosomal trafficking is one such mechanism that may mediate this process. Here we investigate regulators of endosomal trafficking and their contribution to spine formation. We identify two novel factors, LMTK1 and TBC1D9B, which regulate spine formation upstream of Rab11A, a small GTPase. LMTK1 is a membrane bound Ser/Thr kinase regulated by Cdk5-p35, and TBC1D9B is a recently identified Rab11 GAP. LMTK1 controls the GAP activity of TBC1D9B on Rab11A, and TBC1D9B mediates the LMTK1 activity on Rab11A. We propose the Cdk5-LMTK1-TBC1D9B-Rab11A cascade as a novel mechanism controlling spine formation and function.

Targeting of early endosomes by autophagy facilitates EGFR recycling and signalling

Despite recently uncovered connections between autophagy and the endocytic pathway, the role of autophagy in regulating endosomal function remains incompletely understood. Here, we find that the ablation of autophagy-essential players disrupts EGF-induced endocytic trafficking of EGFR. Cells lacking ATG7 or ATG16L1 exhibit increased levels of phosphatidylinositol-3-phosphate (PI(3)P), a key determinant of early endosome maturation. Increased PI(3)P levels are associated with an accumulation of EEA1-positive endosomes where EGFR trafficking is stalled. Aberrant early endosomes are recognised by the autophagy machinery in a TBK1- and Gal8-dependent manner and are delivered to LAMP2-positive lysosomes. Preventing this homeostatic regulation of early endosomes by autophagy reduces EGFR recycling to the plasma membrane and compromises downstream signalling and cell survival. Our findings uncover a novel role for the autophagy machinery in maintaining early endosome function and growth factor sensing.

Apical Trafficking Pathways of Influenza A Virus HA and NA via Rab17- and Rab23-Positive Compartments

The envelope proteins of influenza A virus, hemagglutinin (HA) and neuraminidase (NA), play critical roles in viral entry to host cells and release from the cells, respectively. After protein synthesis, they are transported from the trans-Golgi network (TGN) to the apical plasma membrane (PM) and assembled into virus particles. However, the post-TGN transport pathways of HA and NA have not been clarified. Temporal study by confocal microscopy revealed that HA and NA colocalized soon after their synthesis, and relocated together from the TGN to the upper side of the cell. Using the Rab family protein, we investigated the post-TGN transport pathways of HA and NA. HA partially colocalized with AcGFP-Rab15, Rab17, and Rab23, but rarely with AcGFP-Rab11. When analyzed in cells stably expressing AcGFP-Rab, HA/NA colocalized with Rab15 and Rab17, markers of apical sorting and recycling endosomes, and later colocalized with Rab23, which distributes to the apical PM and endocytic vesicles. Overexpression of the dominant-negative (DN) mutants of Rab15 and Rab17, but not Rab23, significantly delayed HA transport to the PM. However, Rab23DN impaired cell surface expression of HA. Live-cell imaging revealed that NA moved rapidly with Rab17 but not with Rab15. NA also moved with Rab23 in the cytoplasm, but this motion was confined at the upper side of the cell. A fraction of HA was localized to Rab17 and Rab23 double-positive vesicles in the cytoplasm. Coimmunoprecipitation indicated that HA was associated with Rab17 and Rab23 in lipid raft fractions. When cholesterol was depleted by methyl-β-cyclodextrin treatment, the motion of NA and Rab17 signals ceased. These results suggest that HA and NA are incorporated into lipid raft microdomains and are cotransported to the PM by Rab17-positive and followed by Rab23-positive vesicles.

The BEACH Domain Is Critical for Blue Cheese Function in a Spatial and Epistatic Autophagy Hierarchy

Drosophila blue cheese (bchs) encodes a BEACH domain adaptor protein that, like its human homolog ALFY, promotes clearance of aggregated proteins through its interaction with Atg5 and p62. bchs mutations lead to age-dependent accumulation of ubiquitinated inclusions and progressive neurodegeneration in the fly brain, but neither the influence of autophagy on bchs-related degeneration, nor bchs’ placement in the autophagic hierarchy have been shown. We present epistatic evidence in a well-defined larval motor neuron paradigm that in bchs mutants, synaptic accumulation of ubiquitinated aggregates and neuronal death can be rescued by pharmacologically amplifying autophagic initiation. Further, pharmacological rescue requires at least one intact BEACH-containing isoform of the two identified in this study. Genetically augmenting a late step in autophagy, however, rescues even a strong mutation which retains only a third, non-BEACH containing isoform. Using living primary larval brain neurons, we elucidate the primary defect in bchs to be an excess of early autophagic compartments and a deficit in mature compartments. Conversely, rescuing the mutants by full-length Bchs over-expression induces mature compartment proliferation and rescues neuronal death. Surprisingly, only the longest Bchs isoform colocalizes well with autophagosomes, and shuttles between different vesicular locations depending on the type of autophagic impetus applied. Our results are consistent with Bchs promoting autophagic maturation, and the BEACH domain being required for this function.

Developmental expression, co-localization and genetic interaction of exocyst component Sec15 with Rab11 during Drosophila development

Sec15, a component of an evolutionarily conserved octomeric exocyst complex, has been identified as an interactor of GTP-bound Rab11 in mammals and Drosophila which shows its role in secretion in yeast and intracellular vesicle transport. Here, we report the expression patterns of Drosophila Sec15 (DSec15) transcript and Sec15 protein during Drosophila development. At early embryonic stages, a profound level of maternally loaded DSec15 transcript and protein is found. At cellular blastoderm cells (stage 5 embryos); the expression is seen in pole cells, apical membrane and sub-apical region. The transcript is predominantly accumulated in mesoderm, tracheal pits, gut, LE cells, trachea, and ventral nerve cord as development proceeds. While, a robust expression of Sec15 is seen in amnioserosa (AS), lateral epidermis (LAE), developing trachea, gut, ventral nerve cord and epithelial cells. During larval development, the transcript is also found in all imaginal discs with a distinguished accumulation in the morphogenetic furrow of eye disc, gut, proventriculus and gastric ceacae, garland cells/nephrocytes, malpighian tubules, ovary and testis. Further, we show that Sec15 co-localizes with Rab11 during Drosophila embryonic and larval development. Finally, using a genetic approach, we demonstrate that Sec15 interacts with Rab11 in producing blister during Drosophila wing development.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Membrane curvature sensing of the lipid-anchored K-Ras small GTPase

Plasma membrane (PM) curvature defines cell shape and intracellular organelle morphologies and is a fundamental cell property. Growth/proliferation is more stimulated in flatter cells than the same cells in elongated shapes. PM-anchored K-Ras small GTPase regulates cell growth/proliferation and plays key roles in cancer. The lipid-anchored K-Ras form nanoclusters selectively enriched with specific phospholipids, such as phosphatidylserine (PS), for efficient effector recruitment and activation. K-Ras function may, thus, be sensitive to changing lipid distribution at membranes with different curvatures. Here, we used complementary methods to manipulate membrane curvature of intact/live cells, native PM blebs, and synthetic liposomes. We show that the spatiotemporal organization and signaling of an oncogenic mutant K-Ras G12V favor flatter membranes with low curvature. Our findings are consistent with the more stimulated growth/proliferation in flatter cells. Depletion of endogenous PS abolishes K-Ras G12V PM curvature sensing. In cells and synthetic bilayers, only mixed-chain PS species, but not other PS species tested, mediate K-Ras G12V membrane curvature sensing. Thus, K-Ras nanoclusters act as relay stations to convert mechanical perturbations to mitogenic signaling.

A Ciliary View of the Immunological Synapse

The primary cilium has gone from being a vestigial organelle to a crucial signaling hub of growing interest given the association between a group of human disorders, collectively known as ciliopathies, and defects in its structure or function. In recent years many ciliogenesis proteins have been observed at extraciliary sites in cells and likely perform cilium-independent functions ranging from regulation of the cytoskeleton to vesicular trafficking. Perhaps the most striking example is the non-ciliated T lymphocyte, in which components of the ciliary machinery are repurposed for the assembly and function of the immunological synapse even in the absence of a primary cilium. Furthermore, the specialization traits described at the immunological synapse are similar to those seen in the primary cilium. Here, we review common regulators and features shared by the immunological synapse and the primary cilium that document the remarkable homology between these structures.

Viroporins in the Influenza Virus

Influenza is a highly contagious virus that causes seasonal epidemics and unpredictable pandemics. Four influenza virus types have been identified to date: A, B, C and D, with only A–C known to infect humans. Influenza A and B viruses are responsible for seasonal influenza epidemics in humans and are responsible for up to a billion flu infections annually. The M2 protein is present in all influenza types and belongs to the class of viroporins, i.e., small proteins that form ion channels that increase membrane permeability in virus-infected cells. In influenza A and B, AM2 and BM2 are predominantly proton channels, although they also show some permeability to monovalent cations. By contrast, M2 proteins in influenza C and D, CM2 and DM2, appear to be especially selective for chloride ions, with possibly some permeability to protons. These differences point to different biological roles for M2 in types A and B versus C and D, which is also reflected in their sequences. AM2 is by far the best characterized viroporin, where mechanistic details and rationale of its acid activation, proton selectivity, unidirectionality, and relative low conductance are beginning to be understood. The present review summarizes the biochemical and structural aspects of influenza viroporins and discusses the most relevant aspects of function, inhibition, and interaction with the host.

Blood-brain barrier transcytosis genes, risk of dementia and stroke: a prospective cohort study of 74,754 individuals

To test whether genetic variants in PICALM, BIN1, CD2AP, and RIN3-suggested to be involved in blood-brain barrier amyloid-β transcytosis pathways-associate with Alzheimer's disease, all dementia, suggested vascular dementia, and stroke, and whether such associations are independent of the strong ε4 APOE risk allele. In a prospective cohort study of 74,754 individuals from the general population we genotyped PICALM (rs10792832), BIN1 (rs6733839), CD2AP (rs10948363), and RIN3 (rs10498633), and generated a weighted and a simple allele score. Multifactorially adjusted hazard ratios for the fourth quartile versus the first quartile of the weighted allele score were 1.42 (95% confidence interval 1.22-1.64) for Alzheimer's disease, and 1.33 (1.19-1.48) for all dementia. For suggested vascular dementia and stroke the corresponding estimates were 1.71 (1.18-2.49) and 1.12 (1.04-1.22), respectively. Hazard ratios were similar after APOE adjustment. Genetic variants in PICALM, BIN1, CD2AP, and RIN3 are associated with increased risk of Alzheimer's disease, all dementia, and suggested vascular dementia independent of the strong APOE ε4 allele. These findings may suggest that clathrin-mediated endocytosis in clearance of amyloid-β across the blood-brain barrier is important for the integrity of both brain tissue and cerebral vessels.

Gene replacement therapy in a model of Charcot-Marie-Tooth 4C neuropathy

Charcot-Marie-Tooth disease type 4C is the most common recessively inherited demyelinating neuropathy that results from loss of function mutations in the SH3TC2 gene. Sh3tc2-/- mice represent a well characterized disease model developing early onset progressive peripheral neuropathy with hypo- and demyelination, slowing of nerve conduction velocities and disturbed nodal architecture. The aim of this project was to develop a gene replacement therapy for treating Charcot-Marie-Tooth disease type 4C to rescue the phenotype of the Sh3tc2-/- mouse model. We generated a lentiviral vector LV-Mpz.SH3TC2.myc to drive expression of the human SH3TC2 cDNA under the control of the Mpz promoter specifically in myelinating Schwann cells. The vector was delivered into 3-week-old Sh3tc2-/- mice by lumbar intrathecal injection and gene expression was assessed 4-8 weeks after injection. Immunofluorescence analysis showed presence of myc-tagged human SH3TC2 in sciatic nerves and lumbar roots in the perinuclear cytoplasm of a subset of Schwann cells, in a dotted pattern co-localizing with physiologically interacting protein Rab11. Quantitative PCR analysis confirmed SH3TC2 mRNA expression in different peripheral nervous system tissues. A treatment trial was initiated in 3 weeks old randomized Sh3tc2-/- littermate mice which received either the full or mock (LV-Mpz.Egfp) vector. Behavioural analysis 8 weeks after injection showed improved motor performance in rotarod and foot grip tests in treated Sh3tc2-/- mice compared to mock vector-treated animals. Moreover, motor nerve conduction velocities were increased in treated Sh3tc2-/- mice. On a structural level, morphological analysis revealed significant improvement in g-ratios, myelin thickness, and ratios of demyelinated fibres in lumbar roots and sciatic nerves of treated Sh3tc2-/- mice. Finally, treated mice also showed improved nodal molecular architecture and reduction of blood neurofilament light levels, a clinically relevant biomarker for axonal injury/degeneration. This study provides a proof of principle for viral gene replacement therapy targeted to Schwann cells to treat Charcot-Marie-Tooth disease type 4C and potentially other similar demyelinating inherited neuropathies.

Lysosomal proteome analysis reveals that CLN3-defective cells have multiple enzyme deficiencies associated with changes in intracellular trafficking

Numerous lysosomal enzymes and membrane proteins are essential for the degradation of proteins, lipids, oligosaccharides, and nucleic acids. The CLN3 gene encodes a lysosomal membrane protein of unknown function, and CLN3 mutations cause the fatal neurodegenerative lysosomal storage disorder CLN3 (Batten disease) by mechanisms that are poorly understood. To define components critical for lysosomal homeostasis that are affected by this disease, here we quantified the lysosomal proteome in cerebellar cell lines derived from a CLN3 knock-in mouse model of human Batten disease and control cells. We purified lysosomes from SILAC-labeled, and magnetite-loaded cerebellar cells by magnetic separation and analyzed them by MS. This analysis identified 70 proteins assigned to the lysosomal compartment and 3 lysosomal cargo receptors, of which most exhibited a significant differential abundance between control and CLN3-defective cells. Among these, 28 soluble lysosomal proteins catalyzing the degradation of various macromolecules had reduced levels in CLN3-defective cells. We confirmed these results by immunoblotting and selected protease and glycosidase activities. The reduction of 11 lipid-degrading lysosomal enzymes correlated with reduced capacity for lipid droplet degradation and several alterations in the distribution and composition of membrane lipids. In particular, levels of lactosylceramides and glycosphingolipids were decreased in CLN3-defective cells, which were also impaired in the recycling pathway of the exocytic transferrin receptor. Our findings suggest that CLN3 has a crucial role in regulating lysosome composition and their function, particularly in degrading of sphingolipids, and, as a consequence, in membrane transport along the recycling endosome pathway.

A role for Rab11 in the homeostasis of the endosome-lysosomal pathway

The small GTPases Rab11a and 11b are key regulators of membrane transport, localised to the recycling endosomes and also early endosomes. The function of Rab11 within the recycling pathway has been well defined, however, the role of Rab11 at the early endosomes remains poorly characterised. Here, we have generated HeLa cell lines devoid of either Rab11a or Rab11b using CRISPR/Cas9 to functionally dissect the roles of these two Rab11 family members in recycling and in the endosomal-lysosomal system. Both Rab11a and Rab11b contribute to the dynamics of tubulation arising from recycling endosomes whereas Rab11a has the major role in recycling of transferrin receptor. Deletion of either Rab11a or Rab11b resulted in the formation of enlarged early endosomes and perturbation of the endosomal-lysosomal pathway. Strikingly, Rab11a knock-out cells showed an increased density of functional late endosomes/lysosomes as well as lysotracker-positive organelles which were primarily concentrated in a perinuclear location, indicating that the homeostasis of the endosome/lysosome pathway had been perturbed. Moreover, in Rab11a knockout cells there was a functional defect in the intracellular recycling of the cation-independent mannose 6-phosphate receptor (CI-M6PR) between the late endosomes and the TGN, a defect associated with enhanced degradation of CI-M6PR. Expression of wild-type Rab11a in Rab11a knockout cells rescued the late endosome/lysosome phenotype. Overall, these results indicate that Rab11a and Rab11b have overlapping and distinct functions and that Rab11a, unexpectedly, plays a central role in the homeostasis of endosomal-lysosomal biogenesis.

Clathrin Adaptor Complex-interacting Protein Irc6 Functions through the Conserved C-Terminal Domain

Clathrin coats drive transport vesicle formation from the plasma membrane and in pathways between the trans-Golgi network (TGN) and endosomes. Clathrin adaptors play central roles orchestrating assembly of clathrin coats. The yeast clathrin adaptor-interacting protein Irc6 is an orthologue of human p34, which is mutated in the inherited skin disorder punctate palmoplantar keratoderma type I. Irc6 and p34 bind to clathrin adaptor complexes AP-1 and AP-2 and are members of a conserved family characterized by a two-domain architecture. Irc6 is required for AP-1-dependent transport between the TGN and endosomes in yeast. Here we present evidence that the C-terminal two amino acids of Irc6 are required for AP-1 binding and transport function. Additionally, like the C-terminal domain, the N-terminal domain when overexpressed partially restores AP-1-mediated transport in cells lacking full-length Irc6. These findings support a functional role for Irc6 binding to AP-1. Negative genetic interactions with irc6∆ are enriched for genes related to membrane traffic and nuclear processes, consistent with diverse cellular roles for Irc6.

The rare mutation in the endosome-associated recycling protein gene VPS50 is associated with human neural tube defects

Background

Tight control of endosome trafficking is essential for the generation of a normally patterned embryo. Recent studies have found that VPS50 is a key ingredient in EARP which is required for recycling of internalized TfRs to the cell surface and dense-core vesicle maturation. However, the role of VPS50 in embryogenesis and human physiology are poorly understood.

Results

We identified a rare missense heterozygous VPS50 mutation (p. Gly169Val) in NTDs by high-throughput sequencing. In vitro functional analysis demonstrated that the p. Gly169Val was a loss-of-function mutation, delaying transferrin recycling and altering its interaction with VPS53. Using WISH during zebrafish embryogenesis, we demonstrated that vps50 gene was expressed throughout the early embryo, especially in the head. Abnormal body axis phenotypes were observed in those vps50 knock-down zebrafishes. Further rescue study in zebrafish suggested that the mutation displayed loss-of-function effects comparing with wild-type VPS50.

Conclusions

These findings thus demonstrated that the functional mutations in VPS50 might contribute to neurodevelopmental disorder and highlighted the critical importance of VPS50 function in cellular and organismal physiology.

Electronic supplementary material

The online version of this article (10.1186/s13039-019-0421-9) contains supplementary material, which is available to authorized users.

Rab10 regulates tubular endosome formation through KIF13A and KIF13B motors

Recycling endosomes are stations that sort endocytic cargoes to their appropriate destinations. Tubular endosomes have been characterized as a recycling endosomal compartment for clathrin-independent cargoes. However, the molecular mechanism by which tubular endosome formation is regulated is poorly understood. In this study, we identified Rab10 as a novel protein localized at tubular endosomes by using a comprehensive localization screen of EGFP-tagged Rab small GTPases. Knockout of Rab10 completely abolished tubular endosomal structures in HeLaM cells. We also identified kinesin motors KIF13A and KIF13B as novel Rab10-interacting proteins by means of in silico screening. The results of this study demonstrated that both the Rab10-binding homology domain and the motor domain of KIF13A are required for Rab10-positive tubular endosome formation. Our findings provide insight into the mechanism by which the Rab10-KIF13A (or KIF13B) complex regulates tubular endosome formation. This article has an associated First Person interview with the first author of the paper.

Crystal structure of Arabidopsis thaliana RabA1a

RabGTPase is a member of the Ras superfamily of small GTPases, which share a GTP-binding pocket containing highly conserved motifs that promote GTP hydrolysis. In Arabidopsis, the RabA group, which corresponds to the Rab11 group in animals, functions in the recycling of endosomes that control docking and fusion during vesicle transport. However, their molecular mechanisms remain unknown. In this study, we determined the crystal structures of the GDP-bound inactive form and both GppNHp- and GTP-bound active forms of RabA1a, at resolutions of 2.8, 2.6, and 2.6 Å, respectively. A bound sulfate ion in the active site of the GDP-bound structure stabilized Switch II by bridging the interaction between a magnesium ion and Arg74. Comparisons of the two states of RabA1a with Rab11 proteins revealed clear differences in the Switch I and II loops. These results suggested that conformational change of the Switch regions of RabA1a, derived by GTP or GDP binding, could maintain subcellular membrane traffic through the specific interaction of effector molecules.

Porcine sapelovirus enters PK-15 cells via caveolae-dependent endocytosis and requires Rab7 and Rab11

To comprehensively understand the endocytosis of Sapelovirus A (PSV) entry into PK-15 cells, we studied PSV infection in the context of cell perturbations through drug inhibition, siRNA silencing and overexpression of dominant negative (DN) mutants. We showed here that PSV infection of PK-15 cells was unaffected by pretreated with chlorpromazine, EIPA, knockdown of the clathrin heavy chain or overexpression of Eps15 DN mutant. Conversely, PSV infection was sensitive to NH₄Cl, chloroquine, dynasore, nystatin, MβCD and wortmannin with reduced PSV VP1 expression levels and virus titer. Additionally, PSV invasion leaded to rapid actin rearrangement and disruption of the cellular actin network enhanced PSV infection. After internalization the virus was transported to late endosomes and/or cycling endosomes that requires the participation of Rab7 and Rab11. Our findings demonstrate that PSV uses caveolae-dependent endocytosis as the predominant entry portal into PK-15 cells which requires low pH, dynamin, Rab7 and Rab11.

Multiple Roles of Rab GTPases at the Golgi

The Golgi apparatus is a central sorting station in the cell. It receives newly synthesized molecules from the endoplasmic reticulum and directs them to different subcellular destinations, such as the plasma membrane or the endocytic pathway. Importantly, in the last few years, it has emerged that the maintenance of Golgi structure is connected to the proper regulation of membrane trafficking. Rab proteins are small GTPases that are considered to be the master regulators of the intracellular membrane trafficking. Several of the over 60 human Rabs are involved in the regulation of transport pathways at the Golgi as well as in the maintenance of its architecture. This chapter will summarize the different roles of Rab GTPases at the Golgi, both as regulators of membrane transport, scaffold, and tethering proteins and in preserving the structure and function of this organelle.

Rab-mediated trafficking in the secondary cells of Drosophila male accessory glands and its role in fecundity

The male seminal fluid contains factors that affect female post‐mating behavior and physiology. In Drosophila, most of these factors are secreted by the two epithelial cell types that make up the male accessory gland: the main and secondary cells. Although secondary cells represent only ~4% of the cells of the accessory gland, their contribution to the male seminal fluid is essential for sustaining the female post‐mating response. To better understand the function of the secondary cells, we investigated their molecular organization, particularly with respect to the intracellular membrane transport machinery. We determined that large vacuole‐like structures found in the secondary cells are trafficking hubs labeled by Rab6, 7, 11 and 19. Furthermore, these organelles require Rab6 for their formation and many are essential in the process of creating the long‐term postmating behavior of females. In order to better serve the intracellular membrane and protein trafficking communities, we have created a searchable, online, open‐access imaging resource to display our complete findings regarding Rab localization in the accessory gland.

Changes in GABA and glutamate receptors on auditory cortical excitatory neurons in a rat model of salicylate-induced tinnitus

Tinnitus is associated with neural hyperactivity, which is regulated by neuronal plasticity in the auditory central system, especially the auditory cortex (AC). Excitatory neurons constitute approximately 70-85% of the total populations of neuronal cells. However, few reports have focused on the AMPA receptor (AMPAR) and the GABA_A receptor (GABA_AR) on the excitatory neuron in animal model of tinnitus. In this study, we gave rats a single or long-term of salicylate administrations. The tinnitus-like behavior was assessed by combination of the gap prepulse inhibition of acoustic startle (GPIAS) and the pre-pulse inhibition (PPI) tests. Using immunofluorescent staining, we examined whether the AMPAR and the GABA_AR on the calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) -labeled excitatory neurons in the auditory cortex underwent changes following salicylate treatment. The rats with 14 days of salicylate administration showed evidence of experiencing tinnitus, while the rats receiving a single dose of salicylate manifested no tinnitus-like behavior. Furthermore, the AMPAR and GABA_AR responded in a homeostatic manner after a single dose of salicylate while those showing in a Hebbian way after long-term salicylate administration. Thus, the different patterns of plasticity change in cortical excitatory neurons might affect the generating of salicylate-induced tinnitus.

Endocytic Adaptor Protein HIP1R Controls Intracellular Trafficking of Epidermal Growth Factor Receptor in Neuronal Dendritic Development

Huntington-interacting protein 1-related protein (HIP1R) was identified on the basis of its structural homology with HIP1. Based on its domain structure, HIP1R is a putative endocytosis-related protein. Our previous study had shown that knockdown of HIP1R induces a dramatic decrease of dendritic growth and branching in cultured rat hippocampal neurons. However, the underlying mechanism remains elucidative. In this study, we found that knockdown of HIP1R impaired the endocytosis of activated epidermal growth factor receptor (EGFR) and the consequent activation of the downstream ERK and Akt proteins. Meanwhile, it blocked the EGF-induced dendritic outgrowth. We also showed that the HIP1R fragment, amino acids 633–822 (HIP1R_633–822), interacted with EGFR and revealed a dominant negative effect in disrupting the HIP1R-EGFR interaction-mediated neuronal development. Collectively, these results reveal a novel mechanism that HIP1R plays a critical role in neurite initiation and dendritic branching in cultured hippocampal neurons via mediating the endocytosis of EGFR and downstream signaling.

Relevance of Rab Proteins for the Life Cycle of Hepatitis C Virus

Although potent direct-acting antiviral drugs for the treatment of chronic hepatitis C virus (HCV) infection are licensed, there are more than 70 million individuals suffering from chronic HCV infection. In light of the limited access to these drugs, high costs, and a lot of undiagnosed cases, it is expected that the number of HCV cases will not decrease worldwide in the next years. Therefore, and due to the paradigmatic character of HCV for deciphering the crosstalk between viral pathogens and the host cell, characterization of HCV life cycle remains a challenge. HCV belongs to the family of Flaviviridae. As an enveloped virus HCV life cycle depends in many steps on intracellular trafficking. Rab GTPases, a large family of small GTPases, play a central role in intracellular trafficking processes controlling fusion, uncoating, vesicle budding, motility by recruiting specific effector proteins. This review describes the relevance of various Rab proteins for the different steps of the HCV life cycle.

Mason-Pfizer Monkey Virus Envelope Glycoprotein Cycling and Its Vesicular Co-Transport with Immature Particles

The envelope glycoprotein (Env) plays a crucial role in the retroviral life cycle by mediating primary interactions with the host cell. As described previously and expanded on in this paper, Env mediates the trafficking of immature Mason-Pfizer monkey virus (M-PMV) particles to the plasma membrane (PM). Using a panel of labeled RabGTPases as endosomal markers, we identified Env mostly in Rab7a- and Rab9a-positive endosomes. Based on an analysis of the transport of recombinant fluorescently labeled M-PMV Gag and Env proteins, we propose a putative mechanism of the intracellular trafficking of M-PMV Env and immature particles. According to this model, a portion of Env is targeted from the trans-Golgi network (TGN) to Rab7a-positive endosomes. It is then transported to Rab9a-positive endosomes and back to the TGN. It is at the Rab9a vesicles where the immature particles may anchor to the membranes of the Env-containing vesicles, preventing Env recycling to the TGN. These Gag-associated vesicles are then transported to the plasma membrane.

Rab GTPases take centre stage in understanding Entamoeba histolytica biology

Rab GTPases constitute the largest subgroup in the Ras superfamily of GTPases. It is well established that different Rab GTPases are localized in discrete subcellular localization and regulate the membrane trafficking in nearly all eukaryotic cells. Rab GTPase diversity is often regarded as an expression of vesicular trafficking complexity. The pathogenic amoeba Entamoeba histolytica harbours 91 Rab GTPases which is the highest among the currently available genome sequences from the eukaryotic kingdom. Here, we review the current status of amoebic Rab GTPases diversity, unique biochemical and structural features and summarise their predicted regulators. We discuss how amoebic Rab GTPases are involved in cellular processes such as endocytosis, phagocytosis, and invasion of host cellular components, which are essential for parasite survival and virulence.

AMPA Receptor Trafficking for Postsynaptic Potentiation

Long-term potentiation (LTP) of excitatory synaptic strength, which has long been considered a synaptic correlate for learning and memory, requires a fast recruitment of additional α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (AMPARs) to the postsynaptic sites. As cell biological concepts have been applied to the field and genetic manipulation and microscopic imaging technologies have been advanced, visualization of the trafficking of AMPARs to synapses for LTP has been investigated intensively over the last decade. Recycling endosomes have been reported as intracellular storage organelles to supply AMPARs for LTP through the endocytic recycling pathway. In addition, exocytic domains in the spine plasma membrane, where AMPARs are inserted from the intracellular compartment, and nanodomains, where diffusing AMPARs are trapped and immobilized inside synapses for LTP, have been described. Furthermore, cell surface lateral diffusion of AMPARs from extrasynaptic to synaptic sites has been reported as a key step for AMPAR location to the synaptic sites for LTP. This review article will discuss recent findings and views on the reservoir(s) of AMPARs and their trafficking for LTP expression by focusing on the exocytosis and lateral diffusion of AMPARs, and provide some future directions that need to be addressed in the field of LTP.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

In silico analysis of putative dormancy genes in Plasmodium vivax

Plasmodium vivax is the most widely spread species causing human malaria. The control of malaria caused by P. vivax has been largely hampered by its ability to develop a dormant liver stage that can generate a new blood infection at different periods of time. Unfortunately, the mechanisms of dormancy in P. vivax have not been thoroughly elucidated to date. In this study, the putative dormancy genes were analyzed to select genes with less genetic variability to maintain the function of relapsing. Expression data concerning these genes were searched to support the selection. Protein interactions among selected gene products were identified based on known and predicted protein-protein interaction using String database. Potentially interacting proteins (n = 15) were used to propose a mechanism involved in dormancy based on the differential vesicular transport due to the iron available in the hepatocyte.

TRAPPopathies: An emerging set of disorders linked to variations in the genes encoding transport protein particle (TRAPP)-associated proteins

The movement of proteins between cellular compartments requires the orchestrated actions of many factors including Rab family GTPases, Soluble NSF Attachment protein REceptors (SNAREs) and so-called tethering factors. One such tethering factor is called TRAnsport Protein Particle (TRAPP), and in humans, TRAPP proteins are distributed into two related complexes called TRAPP II and III. Although thought to act as a single unit within the complex, in the past few years it has become evident that some TRAPP proteins function independently of the complex. Consistent with this, variations in the genes encoding these proteins result in a spectrum of human diseases with diverse, but partially overlapping, phenotypes. This contrasts with other tethering factors such as COG, where variations in the genes that encode its subunits all result in an identical phenotype. In this review, we present an up-to-date summary of all the known disease-related variations of genes encoding TRAPP-associated proteins and the disorders linked to these variations which we now call TRAPPopathies.

rebuff regulates apical luminal matrix to control tube size in Drosophila trachea

The Drosophila embryonic tracheal network is an excellent model to study tube size. The chitin-based apical luminal matrix and cell polarity are well known to regulate tube size in Drosophila trachea. Defects in luminal matrix and cell polarity lead to tube overexpansion. Here, we address the novel function of the rebuff (reb) gene, which encodes an evolutionarily conserved Smad-like protein. In reb mutants, tracheal tubes are moderately over-elongated. Despite the establishment of normal cell polarity, we observed significantly reduced apical luminal matrix in reb mutants. Among various luminal components, luminal Obstructor-A (ObstA) is drastically reduced. Interestingly, ObstA is localized in vesicle-like structures that are apically concentrated in reb mutants. To investigate the possibility that reb is involved in the endocytosis of ObstA, we analyzed the co-localization of ObstA and endocytic markers in reb mutants. We observed that ObstA is localized in late endosomes and recycling endosomes. This suggests that in reb mutant trachea, endocytosed ObstA is degraded or recycled back to the apical region. However, ObstA vesicles are retained in the apical region and are failed to be secreted to the lumen. Taken together, these results suggest one function of reb is regulating the endocytosis of luminal matrix components.

Summary: Novel function of Smad-like protein Rebuff in regulating tube size of Drosophila trachea through endocytosis of luminal matrix components.

Distinct mechanisms enable inward or outward budding from late endosomes/multivesicular bodies

Regulating the residence time of membrane proteins on the cell surface can modify their response to extracellular cues and allow for cellular adaptation in response to changing environmental conditions. The fate of membrane proteins that are internalized from the plasma membrane and arrive at the limiting membrane of the late endosome/multivesicular body (MVB) is dictated by whether they remain on the limiting membrane, bud into internal MVB vesicles, or bud outwardly from the membrane. The molecular details underlying the disposition of membrane proteins that transit this pathway and the mechanisms regulating these trafficking events are unclear. We established a cell-free system that reconstitutes budding of membrane protein cargo into internal MVB vesicles and onto vesicles that bud outwardly from the MVB membrane. Both budding reactions are cytosol-dependent and supported by Saccharomyces cerevisiae (yeast) cytosol. We observed that inward and outward budding from the MVB membrane are mechanistically distinct but may be linked, such that inhibition of inward budding triggers a re-routing of cargo from inward to outward budding vesicles, without affecting the number of vesicles that bud outwardly from MVBs.

CMTM4 regulates angiogenesis by promoting cell surface recycling of VE-cadherin to endothelial adherens junctions

Vascular endothelial (VE) cadherin is a key component of endothelial adherens junctions (AJs) and plays an important role in maintaining vascular integrity. Endocytosis of VE-cadherin regulates junctional strength and a decrease of surface VE-cadherin reduces vascular stability. However, disruption of AJs is also a requirement for vascular sprouting. Identifying novel regulators of endothelial endocytosis could enhance our understanding of angiogenesis. Here, we evaluated the angiogenic potential of (CKLF-like MARVEL transmembrane domain 4) CMTM4 and assessed in which molecular pathway CMTM4 is involved during angiogenesis. Using a 3D vascular assay composed of GFP-labeled HUVECs and dsRED-labeled pericytes, we demonstrated in vitro that siRNA-mediated CMTM4 silencing impairs vascular sprouting. In vivo, CMTM4 silencing by morpholino injection in zebrafish larvae inhibits intersomitic vessel growth. Intracellular staining revealed that CMTM4 colocalizes with Rab4⁺ and Rab7⁺ vesicles, both markers of the endocytic trafficking pathway. CMTM4 colocalizes with both membrane-bound and internalized VE-cadherin. Adenovirus-mediated CMTM4 overexpression enhances the endothelial endocytic pathway, in particular the rapid recycling pathway, shown by an increase in early endosomal antigen-1 positive (EEA1⁺), Rab4⁺, Rab11⁺ , and Rab7⁺ vesicles. CMTM4 overexpression enhances membrane-bound VE-cadherin internalization, whereas CMTM4 knockdown decreases internalization of VE-cadherin. CMTM4 overexpression promotes endothelial barrier function, shown by an increase in recovery of transendothelial electrical resistance (TEER) after thrombin stimulation. We have identified in this study a novel regulatory function for CMTM4 in angiogenesis. CMTM4 plays an important role in the turnover of membrane-bound VE-cadherin at AJs, mediating endothelial barrier function and controlling vascular sprouting.

A quantitative approach for analyzing the spatio-temporal distribution of 3D intracellular events in fluorescence microscopy

Analysis of the spatial distribution of endomembrane trafficking is fundamental to understand the mechanisms controlling cellular dynamics, cell homeostasy, and cell interaction with its external environment in normal and pathological situations. We present a semi-parametric framework to quantitatively analyze and visualize the spatio-temporal distribution of intracellular events from different conditions. From the spatial coordinates of intracellular features such as segmented subcellular structures or vesicle trajectories, QuantEv automatically estimates weighted densities that are easy to interpret and performs a comprehensive statistical analysis from distribution distances. We apply this approach to study the spatio-temporal distribution of moving Rab6 fluorescently labeled membranes with respect to their direction of movement in crossbow- and disk-shaped cells. We also investigate the position of the generating hub of Rab11-positive membranes and the effect of actin disruption on Rab11 trafficking in coordination with cell shape.

Proteins are the workhorses of the body, performing a range of roles that are essential for life. Often, this requires these molecules to move from one location to another inside a cell. Scientists are interested in following an individual protein in a living cell ‘in real time’, as this helps understand what this protein does.

Scientists can track the whereabouts of a protein by ‘tagging’ it with a fluorescent molecule that emits light which can be picked up by a powerful microscope. This process is repeated many times on different samples. Finally, researchers have to analyze all the resulting images, and conduct statistical analysis to draw robust conclusions about the overall trajectories of the proteins. This process often relies on experts assessing the images, and it is therefore time-consuming and not easily scalable or applied to other experiments.

To help with this, Pécot et al. have developed QuantEV, a free algorithm that can analyze proteins’ paths within a cell, and then return statistical graphs and 3D visualizations. The program also gives access to the statistical procedure that was used, which means that different experiments can be compared.

Pécot et al. used the method to follow the Rab6 protein in cells of different shapes, and found that the conformation of the cell influences where Rab6 is located. For example, in crossbow-shaped cells, Rab6 is found more often toward the three tips of the crossbow, while its distribution is uniform in cells that look like disks. Another experiment examined where the protein Rab11 is normally placed, and how this changes when the cell’s skeleton is artificially disrupted. Both studies help to gain an insight into the behavior of the cellular structures in which Rab6 and Rab11 are embedded.

Following proteins in the cell is an increasingly popular method, and there is therefore a growing amount of data to process. QuantEV should make it easier for biologists to analyze their results, which could help them to have a better grasp on how cells work in various circumstances.

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation.

The enigmatic endosome - sorting the ins and outs of endocytic trafficking

The early endosome (EE), also known as the sorting endosome (SE) is a crucial station for the sorting of cargoes, such as receptors and lipids, through the endocytic pathways. The term endosome relates to the receptacle-like nature of this organelle, to which endocytosed cargoes are funneled upon internalization from the plasma membrane. Having been delivered by the fusion of internalized vesicles with the EE or SE, cargo molecules are then sorted to a variety of endocytic pathways, including the endo-lysosomal pathway for degradation, direct or rapid recycling to the plasma membrane, and to a slower recycling pathway that involves a specialized form of endosome known as a recycling endosome (RE), often localized to the perinuclear endocytic recycling compartment (ERC). It is striking that 'the endosome', which plays such essential cellular roles, has managed to avoid a precise description, and its characteristics remain ambiguous and heterogeneous. Moreover, despite the rapid advances in scientific methodologies, including breakthroughs in light microscopy, overall, the endosome remains poorly defined. This Review will attempt to collate key characteristics of the different types of endosomes and provide a platform for discussion of this unique and fascinating collection of organelles. Moreover, under-developed, poorly understood and important open questions will be discussed.

The recycling endosome and bacterial pathogens

Bacterial pathogens have developed a wide range of strategies to survive within human cells. A number of pathogens multiply in a vacuolar compartment, whereas others can rupture the vacuole and replicate in the host cytosol. A common theme among many bacterial pathogens is the use of specialised secretion systems to deliver effector proteins into the host cell. These effectors can manipulate the host's membrane trafficking pathways to remodel the vacuole into a replication-permissive niche and prevent degradation. As master regulators of eukaryotic membrane traffic, Rab GTPases are principal targets of bacterial effectors. This review highlights the manipulation of Rab GTPases that regulate host recycling endocytosis by several bacterial pathogens, including Chlamydia pneumoniae, Chlamydia trachomatis, Shigella flexneri, Salmonella enterica serovar Typhimurium, Uropathogenic Escherichia coli, and Legionella pneumophila. Recycling endocytosis plays key roles in a variety of cellular aspects such as nutrient uptake, immunity, cell division, migration, and adhesion. Though much remains to be understood about the molecular basis and the biological relevance of bacterial pathogens exploiting Rab GTPases, current knowledge supports the notion that endocytic recycling Rab GTPases are differentially targeted to avoid degradation and support bacterial replication. Thus, future studies of the interactions between bacterial pathogens and host endocytic recycling pathways are poised to deepen our understanding of bacterial survival strategies.

Ascorbic acid increases SVCT2 localization at the plasma membrane by accelerating its trafficking from early secretory compartments and through the endocytic-recycling pathway

Ascorbic acid (Asc) is an antioxidant molecule essential for physiological functions. The concentration of extracellular Asc increases during synaptic transmission and renal reabsorption. These phenomena induce an increase of the Sodium-dependent-Vitamin-C-transporter 2 (SVCT2) at plasma membrane (PM) localization, as we previously demonstrated in neuronal and non-neuronal cells. Hence, the aim of this study was to evaluate intracellular SVCT2 trafficking kinetics in response to Asc. We observed two peaks of SVCT2 localization and function at the PM (at 5-10 min, "acute response", and 30-60 min, "post-acute response") when cells were incubated with Asc. We defined that the post-acute response was dependent on SVCT2 located in early secretory compartments, and its trafficking was abolished with Tunicamycin and Brefeldin A treatment. Moreover, using the RUSH system to retain and synchronize cargo secretion through the secretory pathway we demonstrated that the post-acute response increases SVCT2 trafficking kinetics from the ER to the PM suggesting the retention of SVCT2 at the early secretory pathway when Asc is absent. However, these observations do not explain the increased SVCT2 levels at the PM during the "acute" response, suggesting the involvement of a faster mechanism in close proximity with the PM. To investigate the possible role of endosomal compartments, we tested the effect of endocytosis inhibition. Expression of dominant-negative (DN) versions of the GTPase-dynamin II and clathrin-accessory protein AP180 showed a significant increase in SVCT2 levels at the PM. Moreover, expression of Rab11-DN, a GTPase implicated in cargo protein recycling from endosomes to the PM showed a similar outcome, strongly indicating that Asc impacts SVCT2 trafficking during the acute response. Therefore, our results revealed two mechanisms by which Asc modulates SVCT2 levels at the PM, one at the early secretory pathway and another at the endocytic compartments. We propose that these two mechanisms have key protective implications in the homeostasis of metabolically active and specialized tissues.

ARF6 and Rab11 as intrinsic regulators of axon regeneration

Adult central nervous system (CNS) axons do not regenerate after injury because of extrinsic inhibitory factors, and a low intrinsic capacity for axon growth. Developing CNS neurons have a better regenerative ability, but lose this with maturity. This mini-review summarises recent findings which suggest one reason for regenerative failure is the selective distribution of growth machinery away from axons as CNS neurons mature. These studies demonstrate roles for the small GTPases ARF6 and Rab11 as intrinsic regulators of polarised transport and axon regeneration. ARF6 activation prevents the axonal transport of integrins in Rab11 endosomes in mature CNS axons. Decreasing ARF6 activation permits axonal transport, and increases regenerative ability. The findings suggest new targets for promoting axon regeneration after CNS injury.

Novel Regulation of Integrin Trafficking by Rab11-FIP5 in Aggressive Prostate Cancer

The laminin-binding integrins, α3β1 and α6β1, are needed for tumor metastasis and their surface expression is regulated by endocytic recycling. β1 integrins share the Rab11 recycling machinery, but the trafficking of α3β1 and α6β1 are distinct by an unknown mechanism. Using a mouse PDX tumor model containing human metastatic prostate cancer, Rab11 family interacting protein 5 (Rab11-FIP5) was identified as a lead candidate for α6β1 trafficking. Rab11-FIP5 and its membrane-binding domain were required for α6β1 recycling, without affecting the other laminin-binding integrin (i.e., α3β1) or unrelated membrane receptors like CD44, transferrin receptor, or E-cadherin. Depletion of Rab11-FIP5 resulted in the intracellular accumulation of α6β1 in the Rab11 recycling compartment, loss of cell migration on laminin, and an unexpected loss of α6β1 recycling in cell-cell locations. Taken together, these data demonstrate that α6β1 is distinct from α3β1 via Rab11-FIP5 recycling and recycles in an unexpected cell-cell location.Implications: Rab11-FIP5-dependent α6β1 integrin recycling may be selectively targeted to limit migration of prostate cancer cells into laminin-rich tissues. Mol Cancer Res; 16(8); 1319-31. ©2018 AACR.

Essential Role of the a3 Isoform of V-ATPase in Secretory Lysosome Trafficking via Rab7 Recruitment

Secretory lysosomes are required for the specialised functions of various types of differentiated cells. In osteoclasts, the lysosomal proton pump V-ATPase (vacuolar-type ATPase) is targeted to the plasma membrane via secretory lysosomes and subsequently acidifies the extracellular compartment, providing optimal conditions for bone resorption. However, little is known about the mechanism underlying this trafficking of secretory lysosomes. Here, we demonstrate that the lysosome-specific a3 isoform of the V-ATPase a subunit plays an indispensable role in secretory lysosome trafficking, together with Rab7, a small GTPase involved in organelle trafficking. In osteoclasts lacking a3, lysosomes were not transported to the cell periphery, and Rab7 was not localised to lysosomes but diffused throughout the cytoplasm. Expression of dominant-negative (GDP-bound form) Rab7 inhibited lysosome trafficking in wild-type cells. Furthermore, a3 directly interacted with the GDP-bound forms of Rab7 and Rab27A. These findings reveal a novel role for the proton pump V-ATPase in secretory lysosome trafficking and an unexpected mechanistic link with Rab GTPases.

Development of a Series of Practical Fluorescent Chemical Tools To Measure pH Values in Living Samples

In biological systems, the pH in intracellular organelles or tissues is strictly regulated, and differences of pH are deeply related to key biological events such as protein degradation, intracellular trafficking, renal failure, and cancer. Ratiometric fluorescence imaging is useful for determination of precise pH values, but existing fluorescence probes have substantial limitations, such as inappropriate p K_a for imaging in the physiological pH range, inadequate photobleaching resistance, and insufficiently long excitation and emission wavelengths. Here we report a versatile scaffold for ratiometric fluorescence pH probes, based on asymmetric rhodamine. To demonstrate its usefulness for biological applications, we employed it to develop two probes. (1) SiRpH5 has suitable p K_a and water solubility for imaging in acidic intracellular compartments; by using transferrin tagged with SiRpH5, we achieved time-lapse imaging of pH in endocytic compartments during protein trafficking for the first time. (2) Me-pEPPR is a near-infrared (NIR) probe; by using dextrin tagged with Me-pEPPR, we were able to image extracellular pH of renal tubules and tumors in situ. These chemical tools should be useful for studying the influence of intra- and extracellular pH on biological processes, as well as for in vivo imaging.

Targeting GLP-1 receptor trafficking to improve agonist efficacy

Glucagon-like peptide-1 receptor (GLP-1R) activation promotes insulin secretion from pancreatic beta cells, causes weight loss, and is an important pharmacological target in type 2 diabetes (T2D). Like other G protein-coupled receptors, the GLP-1R undergoes agonist-mediated endocytosis, but the functional and therapeutic consequences of modulating GLP-1R endocytic trafficking have not been clearly defined. Here, we investigate a series of biased GLP-1R agonists with variable propensities for GLP-1R internalization and recycling. Compared to a panel of FDA-approved GLP-1 mimetics, compounds that retain GLP-1R at the plasma membrane produce greater long-term insulin release, which is dependent on a reduction in β-arrestin recruitment and faster agonist dissociation rates. Such molecules elicit glycemic benefits in mice without concomitant increases in signs of nausea, a common side effect of GLP-1 therapies. Our study identifies a set of agents with specific GLP-1R trafficking profiles and the potential for greater efficacy and tolerability as T2D treatments.

Glucagon-like peptide-1 receptor (GLP-1R) promotes insulin secretion from pancreatic beta cells and undergoes agonist-mediated endocytosis. Here, authors study GLP-1R endocytosis caused by different agonists and show that a longer plasma membrane retention time of GLP-1R results in greater long-term insulin release.

Autophagy as a pharmacological target in hematopoiesis and hematological disorders

Autophagy is involved in many cellular processes, including cell homeostasis, cell death/survival balance and differentiation. Autophagy is essential for hematopoietic stem cell survival, quiescence, activation and differentiation. The deregulation of this process is associated with numerous hematological disorders and pathologies, including cancers. Thus, the use of autophagy modulators to induce or inhibit autophagy emerges as a potential therapeutic approach for treating these diseases and could be particularly interesting for differentiation therapy of leukemia cells. This review presents therapeutic strategies and pharmacological agents in the context of hematological disorders. The pros and cons of autophagy modulators in therapy will also be discussed.