Munc13-4 Is a Rab11-binding Protein That Regulates Rab11-positive Vesicle Trafficking and Docking at the Plasma Membrane

Reconstitution of Rab11-FIP4 Expression Rescues Cellular Homeostasis in Cystinosis

Rab11 family interacting protein 4 (Rab11-FIP4) regulates endocytic trafficking. A possible role for Rab11-FIP4 in the regulation of lysosomal function has been proposed, but its precise function in the regulation of cellular homeostasis is unknown. By mRNA array and protein analysis, we found that Rab11-FIP4 is downregulated in the lysosomal storage disease cystinosis, which is caused by genetic defects in the lysosomal cystine transporter, cystinosin. Rescue of Rab11-FIP4 expression in Ctns-/- fibroblasts re-established normal autophagosome levels and decreased LC3B-II expression in cystinotic cells. Furthermore, Rab11-FIP4 reconstitution increased the localization of the chaperone-mediated autophagy receptor LAMP2A at the lysosomal membrane. Treatment with genistein, a phytoestrogen that upregulates macroautophagy, or the CMA activator QX77 (CA77) restored Rab11-FIP4 expression levels in cystinotic cells supporting a cross-regulation between two independent autophagic mechanisms, lysosomal function and Rab11-FIP4. Improved cellular homeostasis in cystinotic cells rescued by Rab11-FIP4 expression correlated with decreased endoplasmic reticulum stress, an effect that was potentiated by Rab11 and partially blocked by expression of a dominant negative Rab11. Restoring Rab11-FIP4 expression in cystinotic proximal tubule cells increased the localization of the endocytic receptor megalin at the plasma membrane, suggesting that Rab11-FIP4 reconstitution has the potential to improve cellular homeostasis and function in cystinosis.

Functional role of UNC13D in immune diseases and its therapeutic applications

UNC13 family (also known as Munc13) proteins are evolutionarily conserved proteins involved in the rapid and regulated secretion of vesicles, including synaptic vesicles and cytotoxic granules. Fast and regulated secretion at the neuronal and immunological synapses requires multiple steps, from the biogenesis of vesicles to membrane fusion, and a complex array of proteins for each step. Defects at these steps can lead to various genetic disorders. Recent studies have shown multiple roles of UNC13D in the secretion of cytotoxic granules by immune cells. Here, the molecular structure and detailed roles of UNC13D in the biogenesis, tethering, and priming of cytotoxic vesicles and in endoplasmic reticulum are summarized. Moreover, its association with immune diseases, including familial hemophagocytic lymphohistiocytosis type 3, macrophage activation syndrome, juvenile idiopathic arthritis, and autoimmune lymphoproliferative syndrome, is reviewed. Finally, the therapeutic application of CRISPR/Cas9-based gene therapy for genetic diseases is introduced.

Recycling machinery of integrin coupled with focal adhesion turnover via RAB11-UNC13D-FAK axis for migration of pancreatic cancer cells

BACKGROUND

Recycling of integrin via endosomal vesicles is critical for the migration of cancer cells, which leads to the metastasis of pancreatic cancer and devastating cancer-related death. So, new diagnostic and therapeutic molecules which target the recycling of endosomal vesicles need to be developed.

METHODS

Public databases including TCGA, ICGC, GSE21501, GSE28735, and GENT are analyzed to derive diagnostic and therapeutic targets. To reveal biological roles and underlying mechanisms of molecular targets, various molecular biological experiments were conducted.

RESULTS

First, we identified UNC13D's overexpression in patients with pancreatic cancer (n = 824) and its prognostic significance and high hazard ratio (HR) in four independent pancreatic cancer cohorts (TCGA, n = 178, p = 0.014, HR = 3.629; ICGC, n = 91, p = 0.000, HR = 4.362; GSE21501, n = 102, p = 0.002, HR = 2.339; GSE28735, n = 45, p = 0.022, HR = 2.681). Additionally, its expression is associated with the clinicopathological progression of pancreatic cancer. Further biological studies have shown that UNC13D regulates the migration of pancreatic cancer cells by coupling the exocytosis of recycling endosomes with focal adhesion turnover via the regulation of FAK phosphorylation. Immunoprecipitation and immunocytochemistry showed the formation of the RAB11-UNC13D-FAK axis in endosomes during integrin recycling. We observed that UNC13D directly interacted with the FERM domain of FAK and regulated FAK phosphorylation in a calcium-dependent manner. Finally, we found co-expression of UNC13D and FAK showed the poorest survival (TCGA, p = 0.000; ICGC, p = 0.036; GSE28735, p = 0.006).

CONCLUSIONS

We highlight that UNC13D, a novel prognostic factor, promotes pancreatic cancer progression by coupling integrin recycling with focal adhesion turnover via the RAB11-UNC13D-FAK axis for the migration of pancreatic cancer cells.

Defective endomembrane dynamics in Rab27a deficiency impairs nucleic acid sensing and cytokine secretion in immune cells

Endosomal Toll-like receptors (eTLRs) are essential for the sensing of non-self through RNA and DNA detection. Here, using spatiotemporal analysis of vesicular dynamics, super-resolution microscopy studies, and functional assays, we show that endomembrane defects associated with the deficiency of the small GTPase Rab27a cause delayed eTLR ligand recognition, defective early signaling, and impaired cytokine secretion. Rab27a-deficient neutrophils show retention of eTLRs in amphisomes and impaired ligand internalization. Extracellular signal-regulated kinase (ERK) signaling and β2-integrin upregulation, early responses to TLR7 and TLR9 ligands, are defective in Rab27a deficiency. CpG-stimulated Rab27a-deficient neutrophils present increased tumor necrosis factor alpha (TNF-α) secretion and decreased secretion of a selected group of mediators, including interleukin (IL)-10. In vivo, CpG-challenged Rab27a-null mice show decreased production of type I interferons (IFNs) and IFN-γ, and the IFN-α secretion defect is confirmed in Rab27a-null plasmacytoid dendritic cells. Our findings have significant implications for immunodeficiency, inflammation, and CpG adjuvant vaccination.

Murine Models of Familial Cytokine Storm Syndromes

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disease caused by mutations in effectors and regulators of cytotoxicity in cytotoxic T cells (CTL) and natural killer (NK) cells. The complexity of the immune system means that in vivo models are needed to efficiently study diseases like HLH. Mice with defects in the genes known to cause primary HLH (pHLH) are available. However, these mice only develop the characteristic features of HLH after the induction of an immune response (typically through infection with lymphocytic choriomeningitis virus). Nevertheless, murine models have been invaluable for understanding the mechanisms that lead to HLH. For example, the cytotoxic machinery (e.g., the transport of cytotoxic vesicles and the release of granzymes and perforin after membrane fusion) was first characterized in the mouse. Experiments in murine models of pHLH have emphasized the importance of cytotoxic cells, antigen-presenting cells (APC), and cytokines in hyperinflammatory positive feedback loops (e.g., cytokine storms). This knowledge has facilitated the development of treatments for human HLH, some of which are now being tested in the clinic.

Cytokine Storm Syndrome Associated with Systemic Juvenile Idiopathic Arthritis

The cytokine storm syndrome (CSS) associated with systemic juvenile idiopathic arthritis (sJIA) has widely been referred to as macrophage activation syndrome (MAS). In this chapter, we use the term sJIA-associated CSS (sJIA-CSS) when referring to this syndrome and use the term MAS when referencing publications that specifically report on sJIA-associated MAS.

The link between intracellular calcium signaling and exosomal PD-L1 in cancer progression and immunotherapy

Exosomes are small membrane vesicles containing microRNA, RNA, DNA fragments, and proteins that are transferred from donor cells to recipient cells. Tumor cells release exosomes to reprogram the factors associated with the tumor microenvironment (TME) causing tumor metastasis and immune escape. Emerging evidence revealed that cancer cell-derived exosomes carry immune inhibitory molecule program death ligand 1 (PD-L1) that binds with receptor program death protein 1 (PD-1) and promote tumor progression by escaping immune response. Currently, some FDA-approved monoclonal antibodies are clinically used for cancer treatment by blocking PD-1/PD-L1 interaction. Despite notable treatment outcomes, some patients show poor drug response. Exosomal PD-L1 plays a vital role in lowering the treatment response, showing resistance to PD-1/PD-L1 blockage therapy through recapitulating the effect of cell surface PD-L1. To enhance therapeutic response, inhibition of exosomal PD-L1 is required. Calcium signaling is the central regulator of tumorigenesis and can regulate exosome biogenesis and secretion by modulating Rab GTPase family and membrane fusion factors. Immune checkpoints are also connected with calcium signaling and calcium channel blockers like amlodipine, nifedipine, lercanidipine, diltiazem, and verapamil were also reported to suppress cellular PD-L1 expression. Therefore, to enhance the PD-1/PD-L1 blockage therapy response, the reduction of exosomal PD-L1 secretion from cancer cells is in our therapeutic consideration. In this review, we proposed a therapeutic strategy by targeting calcium signaling to inhibit the expression of PD-L1-containing exosome levels that could reduce the anti-PD-1/PD-L1 therapy resistance and increase the patient's drug response rate.

Rab27a via its effector JFC1 localizes to Anaplasma inclusions and promotes Anaplasma proliferation in leukocytes

Anaplasma phagocytophilum is an obligatory intracellular bacterium that causes tick-borne zoonosis called human granulocytic anaplasmosis. Mechanisms by which Anaplasma replicates inside of the membrane-bound compartment called "inclusion" in neutrophils are incompletely understood. A small GTPase Rab27a is found in the secretory granules and multivesicular endosomes. In this study we found Rab27a-containing granules were localized to Anaplasma inclusions in guanine nucleotide-dependent manner, and constitutively active Rab27a enhanced Anaplasma infection and dominant-negative Rab27a inhibited Anaplasma infection. Rab27a effector, JFC1 is known to mediate docking/fusion of Rab27a-bearing granules for exocytosis in leukocytes. shRNA stable knockdown of Rab27a or JFC1 inhibited Anaplasma infection in HL-60 cells. Similar to Rab27a, both endogenous and transfected JFC1 were localized to Anaplasma inclusions by immunostaining or live cell imaging. The JFC1 C2A domain that binds 3'-phosphoinositides, was sufficient and required for JFC1 and Rab27a localization to Anaplasma inclusions which were enriched with phosphatidylinositol 3-phosphate. Nexinhib20, the small molecule inhibitor specific to Rab27a and JFC1 binding, inhibited Anaplasma infection. Taken together, these results imply elevated phosphatidylinositol 3-phosphate in the inclusion membrane recruits JFC1 to mediate Rab27a-bearing granules/vesicles to dock/fuse with Anaplasma inclusions, the lumen of which is topologically equivalent to the exterior of the cell to benefit Anaplasma proliferation.

Differences between migrasome, a 'new organelle', and exosome

The migrasome is a new organelle discovered by Professor Yu Li in 2015. When cells migrate, the membranous organelles that appear at the end of the retraction fibres are migrasomes. With the migration of cells, the retraction fibres which connect migrasomes and cells finally break. The migrasomes detach from the cell and are released into the extracellular space or directly absorbed by the recipient cell. The cytoplasmic contents are first transported to the migrasome and then released from the cell through the migrasome. This release mechanism, which depends on cell migration, is named 'migracytosis'. The main components of the migrasome are extracellular vesicles after they leave the cell, which are easy to remind people of the current hot topic of exosomes. Exosomes are extracellular vesicles wrapped by the lipid bimolecular layer. With extensive research, exosomes have solved many disease problems. This review summarizes the differences between migrasomes and exosomes in size, composition, property and function, extraction method and regulation mechanism for generation and release. At the same time, it also prospects for the current hotspot of migrasomes, hoping to provide literature support for further research on the generation and release mechanism of migrasomes and their clinical application in the future.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

The Obligate Intracellular Bacterial Pathogen Anaplasma phagocytophilum Exploits Host Cell Multivesicular Body Biogenesis for Proliferation and Dissemination

Anaplasma phagocytophilum is the etiologic agent of the emerging infection, granulocytic anaplasmosis. This obligate intracellular bacterium lives in a host cell-derived vacuole that receives membrane traffic from multiple organelles to fuel its proliferation and from which it must ultimately exit to disseminate infection. Understanding of these essential pathogenic mechanisms has remained poor. Multivesicular bodies (MVBs) are late endosomal compartments that receive biomolecules from other organelles and encapsulate them into intralumenal vesicles (ILVs) using endosomal sorting complexes required for transport (ESCRT) machinery and ESCRT-independent machinery. Association of the ESCRT-independent protein, ALIX, directs MVBs to the plasma membrane where they release ILVs as exosomes. We report that the A. phagocytophilum vacuole (ApV) is acidified and enriched in lysobisphosphatidic acid, a lipid that is abundant in MVBs. ESCRT-0 and ESCRT-III components along with ALIX localize to the ApV membrane. siRNA-mediated inactivation of ESCRT-0 and ALIX together impairs A. phagocytophilum proliferation and infectious progeny production. RNA silencing of ESCRT-III, which regulates ILV scission, pronouncedly reduces ILV formation in ApVs and halts infection by arresting bacterial growth. Rab27a and its effector Munc13-4, which drive MVB trafficking to the plasma membrane and subsequent exosome release, localize to the ApV. Treatment with Nexinhib20, a small molecule inhibitor that specifically targets Rab27a to block MVB exocytosis, abrogates A. phagocytophilum infectious progeny release. Thus, A. phagocytophilum exploits MVB biogenesis and exosome release to benefit each major stage of its intracellular infection cycle: intravacuolar growth, conversion to the infectious form, and exit from the host cell. IMPORTANCE Anaplasma phagocytophilum causes granulocytic anaplasmosis, a globally emerging zoonosis that can be severe, even fatal, and for which antibiotic treatment options are limited. A. phagocytophilum lives in an endosomal-like compartment that interfaces with multiple organelles and from which it must ultimately exit to spread within the host. How the bacterium accomplishes these tasks is poorly understood. Multivesicular bodies (MVBs) are intermediates in the endolysosomal pathway that package biomolecular cargo from other organelles as intralumenal vesicles for release at the plasma membrane as exosomes. We discovered that A. phagocytophilum exploits MVB biogenesis and trafficking to benefit all aspects of its intracellular infection cycle: proliferation, conversion to its infectious form, and release of infectious progeny. The ability of a small molecule inhibitor of MVB exocytosis to impede A. phagocytophilum dissemination indicates the potential of this pathway as a novel host-directed therapeutic target for granulocytic anaplasmosis.

Differential dysregulation of granule subsets in WASH-deficient neutrophil leukocytes resulting in inflammation

Dysregulated secretion in neutrophil leukocytes associates with human inflammatory disease. The exocytosis response to triggering stimuli is sequential; gelatinase granules modulate the initiation of the innate immune response, followed by the release of pro-inflammatory azurophilic granules, requiring stronger stimulation. Exocytosis requires actin depolymerization which is actively counteracted under non-stimulatory conditions. Here we show that the actin nucleator, WASH, is necessary to maintain azurophilic granules in their refractory state by granule actin entrapment and interference with the Rab27a-JFC1 exocytic machinery. On the contrary, gelatinase granules of WASH-deficient neutrophil leukocytes are characterized by decreased Rac1, shortened granule-associated actin comets and impaired exocytosis. Rac1 activation restores exocytosis of these granules. In vivo, WASH deficiency induces exacerbated azurophilic granule exocytosis, inflammation, and decreased survival. WASH deficiency thus differentially impacts neutrophil granule subtypes, impairing exocytosis of granules that mediate the initiation of the neutrophil innate response while exacerbating pro-inflammatory granule secretion.

Vesicle choreographies keep up cell-to-extracellular matrix adhesion dynamics in polarized epithelial and endothelial cells

In metazoans, cell adhesion to the extracellular matrix (ECM) drives the development, functioning, and repair of different tissues, organs, and systems. Disruption or dysregulation of cell-to-ECM adhesion promote the initiation and progression of several diseases, such as bleeding, immune disorders and cancer. Integrins are major ECM transmembrane receptors, whose function depends on both allosteric changes and exo-endocytic traffic, which carries them to and from the plasma membrane. In apico-basally polarized cells, asymmetric adhesion to the ECM is maintained by continuous targeting of the plasma membrane by vesicles coming from the trans Golgi network and carrying ECM proteins. Active integrin-bound ECM is indeed endocytosed and replaced by the exocytosis of fresh ECM. Such vesicular traffic is finely driven by the teamwork of microtubules (MTs) and their associated kinesin and dynein motors. Here, we review the main cytoskeletal actors involved in the control of the spatiotemporal distribution of active integrins and their ECM ligands, highlighting the key role of the synchronous (ant)agonistic cooperation between MT motors transporting vesicular cargoes, in the same or in opposite direction, in the regulation of traffic logistics, and the establishment of epithelial and endothelial cell polarity.

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

NK cell-mediated cytotoxicity is a critical element of our immune system required for protection from microbial infections and cancer. NK cells bind to and eliminate infected or cancerous cells via direct secretion of cytotoxic molecules toward the bound target cells. In this review, we summarize the current understanding of the molecular regulations of NK cell cytotoxicity, focusing on lytic granule development and degranulation processes. NK cells synthesize apoptosis-inducing proteins and package them into specialized organelles known as lytic granules (LGs). Upon activation of NK cells, LGs converge with the microtubule organizing center through dynein-dependent movement along microtubules, ultimately polarizing to the cytotoxic synapse where they subsequently fuse with the NK plasma membrane. From LGs biogenesis to degranulation, NK cells utilize several strategies to protect themselves from their own cytotoxic molecules. Additionally, molecular pathways that enable NK cells to perform serial killing are beginning to be elucidated. These advances in the understanding of the molecular pathways behind NK cell cytotoxicity will be important to not only improve current NK cell-based anti-cancer therapies but also to support the discovery of additional therapeutic opportunities.

The biogenesis and secretion of exosomes and multivesicular bodies (MVBs): Intercellular shuttles and implications in human diseases

Exosomes carry and transmit signaling molecules used for intercellular communication. The generation and secretion of exosomes is a multistep interlocking process that allows simultaneous control of multiple regulatory sites. Protein molecules, mainly RAB GTPases, cytoskeletal proteins and soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE), are specifically regulated in response to pathological conditions such as altered cellular microenvironment, stimulation by pathogenic factors, or gene mutation. This interferes with the smooth functioning of endocytosis, translocation, degradation, docking and fusion processes, leading to changes in the secretion of exosomes. Large numbers of secreted exosomes are disseminated by the flow of body fluids and absorbed by the recipient cells. By transmitting characteristic functional proteins and genetic information produced under disease conditions, exosomes can change the physiological state of the recipient cells and their microenvironment. The microenvironment, in turn, affects the occurrence and development of disease. Therefore, this review will discuss the mechanism by which exosome secretion is regulated in cells following the formation of mature secretory multivesicular bodies (MVBs). The overall aim is to find ways to eliminate disease-derived exosomes at their source, thereby providing an important new basis for the clinical treatment of disease.

In vivo Roles of Rab27 and Its Effectors in Exocytosis

The monomeric GTPase Rab27 regulates exocytosis of a broad range of vesicles in multicellular organisms. Several effectors bind GTP-bound Rab27a and/or Rab27b on secretory vesicles to execute a series of exocytic steps, such as vesicle maturation, movement along microtubules, anchoring within the peripheral F-actin network, and tethering to the plasma membrane, via interactions with specific proteins and membrane lipids in a local milieu. Although Rab27 effectors generally promote exocytosis, they can also temporarily restrict it when they are involved in the rate-limiting step. Genetic alterations in Rab27-related molecules cause discrete diseases manifesting pigment dilution and immunodeficiency, and can also affect common diseases such as diabetes and cancer in complex ways. Although the function and mechanism of action of these effectors have been explored, it is unclear how multiple effectors act in coordination within a cell to regulate the secretory process as a whole. It seems that Rab27 and various effectors constitutively reside on individual vesicles to perform consecutive exocytic steps. The present review describes the unique properties and in vivo roles of the Rab27 system, and the functional relationship among different effectors coexpressed in single cells, with pancreatic beta cells used as an example.Key words: membrane trafficking, regulated exocytosis, insulin granules, pancreatic beta cells.

DYNC1LI2 regulates localization of the chaperone-mediated autophagy receptor LAMP2A and improves cellular homeostasis in cystinosis

The dynein motor protein complex is required for retrograde transport but the functions of the intermediate-light chains that form the cargo-binding complex are not elucidated and the importance of individual subunits in maintaining cellular homeostasis is unknown. Here, using mRNA arrays and protein analysis, we show that the dynein subunit, DYNC1LI2 (dynein, cytoplasmic 1 light intermediate chain 2) is downregulated in cystinosis, a lysosomal storage disorder caused by genetic defects in CTNS (cystinosin, lysosomal cystine transporter). Reconstitution of DYNC1LI2 expression in ctns^-/- cells reestablished endolysosomal dynamics. Defective vesicular trafficking in cystinotic cells was rescued by DYNC1LI2 expression which correlated with decreased endoplasmic reticulum stress manifested as decreased expression levels of the chaperone HSPA5/GRP78, and the transcription factors ATF4 and DDIT3/CHOP. Mitochondrial fragmentation, membrane potential and endolysosomal-mitochondrial association in cystinotic cells were rescued by DYNC1LI2. Survival of cystinotic cells to oxidative stress was increased by DYNC1LI2 reconstitution but not by its paralog DYNC1LI1, which also failed to decrease ER stress and mitochondrial fragmentation. DYNC1LI2 expression rescued the localization of the chaperone-mediated autophagy (CMA) receptor LAMP2A, CMA activity, cellular homeostasis and LRP2/megalin expression in cystinotic proximal tubule cells, the primary cell type affected in cystinosis. DYNC1LI2 failed to rescue phenotypes in cystinotic cells when LAMP2A was downregulated or when co-expressed with dominant negative (DN) RAB7 or DN-RAB11, which impaired LAMP2A trafficking. DYNC1LI2 emerges as a regulator of cellular homeostasis and potential target to repair underlying trafficking and CMA in cystinosis, a mechanism that is not restored by lysosomal cystine depletion therapies.

Abbreviations: ACTB: actin, beta; ATF4: activating transcription factor 4; CMA: chaperone-mediated autophagy; DYNC1LI1: dynein cytoplasmic 1 light intermediate chain 1; DYNC1LI2: dynein cytoplasmic 1 light intermediate chain 2; ER: endoplasmic reticulum; LAMP1: lysosomal associated membrane protein 1; LAMP2A: lysosomal associated membrane protein 2A; LIC: light-intermediate chains; LRP2/Megalin: LDL receptor related protein 2; PTCs: proximal tubule cells; RAB: RAB, member RAS oncogene family; RAB11FIP3: RAB11 family interacting protein 3; RILP: Rab interacting lysosomal protein

Prickle promotes the formation and maintenance of glutamatergic synapses by stabilizing the intercellular planar cell polarity complex

Whether there exists a common signaling mechanism that assembles all glutamatergic synapses is unknown. We show here that knocking out Prickle1 and Prickle2 reduced the formation of the PSD-95–positive glutamatergic synapses in the hippocampus and medial prefrontal cortex in postnatal development by 70–80%. Prickle1 and Prickle2 double knockout in adulthood lead to the disassembly of 70 to 80% of the postsynaptic-density(PSD)-95–positive glutamatergic synapses. PSD-95–positive glutamatergic synapses in the hippocampus of Prickle2E8Q/E8Q mice were reduced by 50% at postnatal day 14. Prickle2 promotes synapse formation by antagonizing Vangl2 and stabilizing the intercellular complex of the planar cell polarity (PCP) components, whereas Prickle2 E8Q fails to do so. Coculture experiments show that the asymmetric PCP complexes can determine the presynaptic and postsynaptic polarity. In summary, the PCP components regulate the assembly and maintenance of a large number of glutamatergic synapses and specify the direction of synaptic transmission.

The atypical small GTPase GEM/Kir is a negative regulator of the NADPH oxidase and NETs production through macroautophagy

Despite the important function of neutrophils in the eradication of infections and induction of inflammation, the molecular mechanisms regulating the activation and termination of the neutrophil immune response is not well understood. Here, the function of the small GTPase from the RGK family, Gem, is characterized as a negative regulator of the NADPH oxidase through autophagy regulation. Gem knockout (Gem KO) neutrophils show increased NADPH oxidase activation and increased production of extracellular and intracellular reactive oxygen species (ROS). Enhanced ROS production in Gem KO neutrophils was associated with increased NADPH oxidase complex-assembly as determined by quantitative super-resolution microscopy, but normal exocytosis of gelatinase and azurophilic granules. Gem-deficiency was associated with increased basal autophagosomes and autolysosome numbers but decreased autophagic flux under phorbol ester-induced conditions. Neutrophil stimulation triggered the localization of the NADPH oxidase subunits p22phox and p47phox at LC3-positive structures suggesting that the assembled NADPH oxidase complex is recruited to autophagosomes, which was significantly increased in Gem KO neutrophils. Prevention of new autophagosome formation by treatment with SAR405 increased ROS production while induction of autophagy by Torin-1 decreased ROS production in Gem KO neutrophils, and also in wild-type neutrophils, suggesting that macroautophagy contributes to the termination of NADPH oxidase activity. Autophagy inhibition decreased NETs formation independently of enhanced ROS production. NETs production, which was significantly increased in Gem-deficient neutrophils, was decreased by inhibition of both autophagy and calmodulin, a known GEM interactor. Intracellular ROS production was increased in Gem KO neutrophils challenged with live Gram-negative bacteria Pseudomonas aeruginosa or Salmonella Typhimurium, but phagocytosis was not affected in Gem-deficient cells. In vivo analysis in a model of Salmonella Typhimurium infection indicates that Gem-deficiency provides a genetic advantage manifested as a moderate increased in survival to infections. Altogether, the data suggest that Gem-deficiency leads to the enhancement of the neutrophil innate immune response by increasing NADPH oxidase assembly and NETs production and that macroautophagy differentially regulates ROS and NETs in neutrophils.

Lack of Cdk5 activity is involved on Dopamine Transporter expression and function: Evidences from an animal model of Attention-Deficit Hyperactivity Disorder

Attention deficit/Hyperactivity disorder (ADHD) is one of the most diagnosed psychiatric disorders nowadays. The core symptoms of the condition include hyperactivity, impulsiveness and inattention. The main pharmacological treatment consists of psychostimulant drugs affecting Dopamine Transporter (DAT) function. We have previously shown that genetically modified mice lacking p35 protein (p35KO), which have reduced Cdk5 activity, present key hallmarks resembling those described in animal models useful for studying ADHD. The p35KO mouse displays spontaneous hyperactivity and shows a calming effect of methylphenidate or amphetamine treatment. Interestingly, dopaminergic neurotransmission is altered in these mice as they have an increased Dopamine (DA) content together with a low DA turnover. This led us to hypothesize that the lack of Cdk5 activity affects DAT expression and/or function in this animal model. In this study, we performed biochemical assays, cell-based approaches, quantitative fluorescence analysis and functional studies that allowed us to demonstrate that p35KO mice exhibit decreased DA uptake and reduced cell surface DAT expression levels in the striatum (STR). These findings are supported by in vitro observations in which the inhibition of Cdk5 activity in N2a cells induced a significant increase in constitutive DAT endocytosis with a concomitant increase in DAT localization to recycling endosomes. Taken together, these data provide evidences regarding the role of Cdk5/p35 in DAT expression and function, thus contributing to the knowledge of DA neurotransmission physiology and also providing therapeutic options for the treatment of DA pathologies such as ADHD.

Using single-vesicle technologies to unravel the heterogeneity of extracellular vesicles

Extracellular vesicles (EVs) are heterogeneous lipid containers with a complex molecular cargo comprising several populations with unique roles in biological processes. These vesicles are closely associated with specific physiological features, which makes them invaluable in the detection and monitoring of various diseases. EVs play a key role in pathophysiological processes by actively triggering genetic or metabolic responses. However, the heterogeneity of their structure and composition hinders their application in medical diagnosis and therapies. This diversity makes it difficult to establish their exact physiological roles, and the functions and composition of different EV (sub)populations. Ensemble averaging approaches currently employed for EV characterization, such as western blotting or 'omics' technologies, tend to obscure rather than reveal these heterogeneities. Recent developments in single-vesicle analysis have made it possible to overcome these limitations and have facilitated the development of practical clinical applications. In this review, we discuss the benefits and challenges inherent to the current methods for the analysis of single vesicles and review the contribution of these approaches to the understanding of EV biology. We describe the contributions of these recent technological advances to the characterization and phenotyping of EVs, examination of the role of EVs in cell-to-cell communication pathways and the identification and validation of EVs as disease biomarkers. Finally, we discuss the potential of innovative single-vesicle imaging and analysis methodologies using microfluidic devices, which promise to deliver rapid and effective basic and practical applications for minimally invasive prognosis systems.

Roles of neutrophil granule proteins in orchestrating inflammation and immunity

Neutrophil granulocytes form the first line of host defense against invading pathogens and tissue injury. They are rapidly recruited from the blood to the affected sites, where they deploy an impressive arsenal of effectors to eliminate invading microbes and damaged cells. This capacity is endowed in part by readily mobilizable proteins acquired during granulopoiesis and stored in multiple types of cytosolic granules with each granule type containing a unique cargo. Once released, granule proteins contribute to killing bacteria within the phagosome or the extracellular milieu, but are also capable of inflicting collateral tissue damage. Neutrophil-driven inflammation underlies many common diseases. Research over the last decade has documented neutrophil heterogeneity and functional versatility far beyond their antimicrobial function. Emerging evidence indicates that neutrophils utilize granule proteins to interact with innate and adaptive immune cells and orchestrate the inflammatory response. Granule proteins have been identified as important modulators of neutrophil trafficking, reverse transendothelial migration, phagocytosis, neutrophil life span, neutrophil extracellular trap formation, efferocytosis, cytokine activity, and autoimmunity. Hence, defining their roles within the inflammatory locus is critical for minimizing damage to the neighboring tissue and return to homeostasis. Here, we provide an overview of recent advances in the regulation of degranulation, granule protein functions, and signaling in modulating neutrophil-mediated immunity. We also discuss how targeting granule proteins and/or signaling could be harnessed for therapeutic benefits.

Endosomal Rab GTPases regulate secretory granule maturation in Drosophila larval salivary glands

Secretory granules (SGs) are organelles responsible for regulated exocytosis of biologically active molecules in professional secretory cells. Maturation of SGs is a crucial process in which cargoes of SGs are processed and activated, allowing them to exert their function upon secretion. Nonetheless, the intracellular trafficking pathways required for SG maturation are not well defined. We recently performed an RNA interference (RNAi) screen in Drosophila larval salivary glands to identify trafficking components needed for SG maturation. From the screen, we identified several Rab GTPases (Rabs) that affect SG maturation. Expression of constitutively active (CA) and dominant-negative (DN) forms narrowed down the Rabs important for this process to Rab5, Rab9 and Rab11. However, none of these Rabs localizes to the limiting membrane of SGs. In contrast, examination of endogenously YFP-tagged Rabs (YRabs) in larval salivary glands revealed that YRab1 and YRab6 localize to the limiting membrane of immature SGs (iSGs) and SGs. These findings provide new insights into how Rab GTPases contribute to the process of SG maturation.

Novel and Converging Ways of NOX2 and SOD3 in Trafficking and Redox Signaling in Macrophages

Macrophages and related tissue macrophage populations use the classical NADPH oxidase (NOX2) for the regulated production of superoxide and derived oxidants for pathogen combat and redox signaling. With an emphasis on macrophages, we discuss how sorting into secretory storage vesicles, agonist-responsive membrane trafficking, and segregation into sphingolipid and cholesterol-enriched microdomains (lipid rafts) determine the subcellular distribution and spatial organization of NOX2 and superoxide dismutase-3 (SOD3). We discuss how inflammatory activation of macrophages, in part through small GTPase Rab27A/B regulation of the secretory compartments, mediates the coalescence of these two proteins on the cell surface to deliver a focalized hydrogen peroxide output. In interplay with membrane-embedded oxidant transporters and redox sensitive target proteins, this arrangement allows for the autocrine and paracrine signaling, which govern macrophage activation states and transcriptional programs. By discussing examples of autocrine and paracrine redox signaling, we highlight why formation of spatiotemporal microenvironments where produced superoxide is rapidly converted to hydrogen peroxide and conveyed immediately to reach redox targets in proximal vicinity is required for efficient redox signaling. Finally, we discuss the recent discovery of macrophage-derived exosomes as vehicles of NOX2 holoenzyme export to other cells.

A role for Rab30 in retrograde trafficking and maintenance of endosome-TGN organization

Rab30 is a poorly characterized small GTPase. Here we show that Rab30 is localised primarily to the TGN and recycling endosomes in a range of cell types, including primary neurons; minor levels of Rab30 were also detected throughout the Golgi stack and early endosomes. Silencing of Rab30 resulted in the dispersal of both early and recycling endosomes and TGN compartments in HeLa cells. By analyzing cargo trafficking in Rab30-silenced and Rab30-overexpressing HeLa cells, we demonstrate that Rab30 plays a role in retrograde trafficking of TGN38 from endosomes to the Golgi, but has no apparent role in the endocytic recycling of the transferrin receptor to the plasma membrane. Five interactive partners with Rab30 were identified by pull-down and MS analysis using GFP-tagged Rab30 mutant, Rab30(Q68L). Two of the interactive partners identified were Arf1 and Arf4, known regulators of endosome to TGN retrograde transport. Knockdown of Arf1 and Arf4 results in GFP-Rab30 decorated tubules arising from the recycling endosomes, suggesting association of Rab30 with tubular carriers. Overall our data demonstrates a role for Rab30 in regulating retrograde transport to the TGN and maintenance of endosomal-TGN organization.

Super-Resolution Microscopy and Particle-Tracking Approaches for the Study of Vesicular Trafficking in Primary Neutrophils

Neutrophils are short-lived cells after isolation. The analysis of neutrophil vesicular trafficking requires rapid and gentle handling. Recently developed super-resolution microscopy technologies have generated unparalleled opportunities to help understand the molecular mechanisms regulating neutrophil vesicular trafficking, exocytosis, and associated functions at the molecular level. Here, we describe super-resolution and total internal reflection fluorescence (TIRF) microscopy approaches for the analysis of vesicular trafficking and associated functions of primary neutrophils.

Focus on the morphogenesis, fate and the role in tumor progression of multivesicular bodies

Multivesicular bodies (MVBs) are endosome organelles that are gradually attracting research attention. Initially, MVBs were considered as important components of the endosomal-lysosomal degradation pathway. In recent years, with an increase in extracellular vesicle (EV) research, the biogenesis, fate, and pathological effects of MVBs have been increasingly studied. However, the mechanisms by which MVBs are sorted to the lysosome and plasma membrane remain unclear. In addition, whether the trafficking of MVBs can determine whether exosomes are released from cells, the factors are involved in cargo loading and regulating the fate of MVBs, and the roles that MVBs play in the development of disease are unknown. Consequently, this review focuses on the mechanism of MVB biogenesis, intraluminal vesicle formation, sorting of different cargoes, and regulation of their fate. We also discuss the mechanisms of emerging amphisome-dependent secretion and degradation. In addition, we highlight the contributions of MVBs to the heterogeneity of EVs, and their important roles in cancer. Thus, we attempt to unravel the various functions of MVBs in the cell and their multiple roles in tumor progression.

News about non-secretory exocytosis: mechanisms, properties, and functions

The fusion by exocytosis of many vesicles to the plasma membrane induces the discharge to the extracellular space of their abundant luminal cargoes. Other exocytic vesicles, however, do not contain cargoes, and thus, their fusion is not followed by secretion. Therefore, two distinct processes of exocytosis exist, one secretory and the other non-secretory. The present review deals with the knowledge of non-secretory exocytosis developed during recent years. Among such developments are the dual generation of the exocytic vesicles, initially released either from the trans-Golgi network or by endocytosis; their traffic with activation of receptors, channels, pumps, and transporters; the identification of their tethering and soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes that govern membrane fusions; the growth of axons and the membrane repair. Examples of potential relevance of these processes for pathology and medicine are also reported. The developments presented here offer interesting chances for future progress in the field.

Alternative UNC13D Promoter Encodes a Functional Munc13-4 Isoform Predominantly Expressed in Lymphocytes and Platelets

Autosomal recessive mutations in genes required for cytotoxicity are causative of a life-threatening, early-onset hyperinflammatory syndrome termed familial hemophagocytic lymphohistiocytosis (FHL). Mutations in UNC13D cause FHL type 3. UNC13D encodes Munc13-4, a member of the Unc13 protein family which control SNARE complex formation and vesicle fusion. We have previously identified FHL3-associated mutations in the first intron of UNC13D which control transcription from an alternative transcriptional start site. Using isoform specific antibodies, we demonstrate that this alternative Munc13-4 isoform with a unique N-terminus is preferentially expressed in human lymphocytes and platelets, as compared to the conventional isoform that was mostly expressed in monocytes and neutrophils. The distinct N-terminal of the two isoforms did not impact on Munc13-4 localization or trafficking to the immunological synapse of cytotoxic T cells. Moreover, ectopic expression of both isoforms efficiently restored exocytosis by FHL3 patient-derived Munc13-4 deficient T cells. Thus, we demonstrate that the conventional and alternative Munc13-4 isoforms have different expression pattern in hematopoietic cell subsets, but display similar localization and contribution to T cell exocytosis. The use of an alternative transcriptional starting site (TSS) in lymphocytes and platelets could be selected for increasing the overall levels of Munc13-4 expression for efficient secretory granule release.

Super-STORM: Molecular Modeling to Achieve Single-molecule Localization with STORM Microscopy

This protocol introduces the SuperSTORM technique, combining stochastic optical reconstruction microscopy (STORM) and molecular modeling. SuperSTORM is optimized for acquiring and processing STORM images of neutrophil integrins but can be used for any cell-surface molecule with known structure and antibody-binding site(s). SuperSTORM identifies molecular cut-offs for eliminating multiple blinks of STORM imaging, determines colocalization, identifies clusters, and reveals molecular orientations and distributions. This protocol extends STORM imaging to cells in microfluidic systems. Improved resolution is achieved by using biomolecule-inherent parameters. For complete information on the generation and use of this protocol, please refer to the paper by Fan et al. (2019).

Neutrophil recruitment and intracellular vesicle transport: A short overview

Recruitment of neutrophils from the intravascular compartment into injured tissue is an essential component of the inflammatory response. It involves intracellular trafficking of vesicles within neutrophils and endothelial cells, both containing numerous proteins that have to be distributed in a tightly controlled and precise spatiotemporal fashion during the recruitment process. Rab proteins, a family of small GTPases, together with their effectors, are the key players in guiding and regulating the intracellular vesicle trafficking machinery during neutrophil recruitment. This review will provide a short overview on this process and highlight new findings as well as current controversies in the field.

N-GSDMD trafficking to neutrophil organelles facilitates IL-1β release independently of plasma membrane pores and pyroptosis

Gasdermin-D (GSDMD) in inflammasome-activated macrophages is cleaved by caspase-1 to generate N-GSDMD fragments. N-GSDMD then oligomerizes in the plasma membrane (PM) to form pores that increase membrane permeability, leading to pyroptosis and IL-1β release. In contrast, we report that although N-GSDMD is required for IL-1β secretion in NLRP3-activated human and murine neutrophils, N-GSDMD does not localize to the PM or increase PM permeability or pyroptosis. Instead, biochemical and microscopy studies reveal that N-GSDMD in neutrophils predominantly associates with azurophilic granules and LC3+ autophagosomes. N-GSDMD trafficking to azurophilic granules causes leakage of neutrophil elastase into the cytosol, resulting in secondary cleavage of GSDMD to an alternatively cleaved N-GSDMD product. Genetic analyses using ATG7-deficient cells indicate that neutrophils secrete IL-1β via an autophagy-dependent mechanism. These findings reveal fundamental differences in GSDMD trafficking between neutrophils and macrophages that underlie neutrophil-specific functions during inflammasome activation.

Imaging of the immune system - towards a subcellular and molecular understanding

Immune responses involve many types of leukocytes that traffic to the site of injury, recognize the insult and respond appropriately. Imaging of the immune system involves a set of methods and analytical tools that are used to visualize immune responses at the cellular and molecular level as they occur in real time. We will review recent and emerging technological advances in optical imaging, and their application to understanding the molecular and cellular responses of neutrophils, macrophages and lymphocytes. Optical live-cell imaging provides deep mechanistic insights at the molecular, cellular, tissue and organism levels. Live-cell imaging can capture quantitative information in real time at subcellular resolution with minimal phototoxicity and repeatedly in the same living cells or in accessible tissues of the living organism. Advanced FRET probes allow tracking signaling events in live cells. Light-sheet microscopy allows for deeper tissue penetration in optically clear samples, enriching our understanding of the higher-level organization of the immune response. Super-resolution microscopy offers insights into compartmentalized signaling at a resolution beyond the diffraction limit, approaching single-molecule resolution. This Review provides a current perspective on live-cell imaging in vitro and in vivo with a focus on the assessment of the immune system.

Molecular regulation of the plasma membrane-proximal cellular steps involved in NK cell cytolytic function

Natural killer (NK) cells, cytolytic lymphocytes of the innate immune system, play a crucial role in the immune response against infection and cancer. NK cells kill target cells through exocytosis of lytic granules that contain cytotoxic proteins, such as perforin and granzymes. Formation of a functional immune synapse, i.e. the interface between the NK cell and its target cell enhances lysis through accumulation of polymerized F-actin at the NK cell synapse, leading to convergence of lytic granules to the microtubule organizing center (MTOC) and its subsequent polarization along microtubules to deliver the lytic granules to the synapse. In this review, we focus on the molecular mechanisms regulating the cellular processes that occur after the lytic granules are delivered to the cytotoxic synapse. We outline how - once near the synapse - the granules traverse the clearings created by F-actin remodeling to dock, tether and fuse with the plasma membrane in order to secrete their lytic content into the synaptic cleft through exocytosis. Further emphasis is given to the role of Ca²⁺ mobilization during degranulation and, whenever applicable, we compare these mechanisms in NK cells and cytotoxic T lymphocytes (CTLs) as adaptive immune system effectors.

Mycobacterium tuberculosis Lipoarabinomannan Activates Human Neutrophils via a TLR2/1 Mechanism Distinct from Pam3CSK4

Neutrophils, polymorphonuclear (PMN) leukocytes, play an important role in the early innate immune response to Mycobacterium tuberculosis infection in the lung. Interactions between PMN and mycobacterial lipids impact the activation state of these migrated cells with consequences for the surrounding tissue in terms of resolution versus ongoing inflammation. We hypothesized that lipoarabinomannan from M. tuberculosis (Mtb LAM) would prime human PMN in a TLR2-dependent manner and investigated this with specific comparison with the purified synthetic TLR2 agonists, Pam₃CSK₄ and FSL-1. In contrast to Pam₃CSK₄ and FSL-1, we found Mtb LAM did not induce any of the classical PMN priming phenotypes, including enhancement of NADPH oxidase activity, shedding of l-selectin, or mobilization of CD11b. However, exposure of PMN to Mtb LAM did elicit pro- and anti-inflammatory cytokine production and release in a TLR2/1-dependent manner, using the TLR1 single-nucleotide polymorphism rs5743618 (1805G/T) as a marker for TLR2/1 specificity. Moreover, Mtb LAM did not elicit p38 MAPK phosphorylation or endocytosis, although these processes occurred with Pam₃CSK₄ stimulation, and were necessary for the early priming events to occur. Interestingly, Mtb LAM did not abrogate priming responses elicited by Pam₃CSK₄ Notably, subfractionation of light membranes from Pam₃CSK₄ versus Mtb LAM-stimulated cells demonstrated differential patterns of exocytosis. In summary, Mtb LAM activates PMN via TLR2/1, resulting in the production of cytokines but does not elicit early PMN priming responses, as seen with Pam₃CSK₄ We speculate that the inability of Mtb LAM to prime PMN may be due to differential localization of TLR2/1 signaling.

Unbiased Thiol-Labeling and Top-Down Proteomic Analyses Implicate Multiple Proteins in the Late Steps of Regulated Secretion

Regulated exocytosis enables temporal and spatial control over the secretion of biologically active compounds; however, the mechanism by which Ca²⁺ modulates different stages of exocytosis is still poorly understood. For an unbiased, top-down proteomic approach, select thiol- reactive reagents were used to investigate this process in release-ready native secretory vesicles. We previously characterized a biphasic effect of these reagents on Ca²⁺-triggered exocytosis: low doses potentiated Ca²⁺ sensitivity, whereas high doses inhibited Ca²⁺ sensitivity and extent of vesicle fusion. Capitalizing on this novel potentiating effect, we have now identified fluorescent thiol- reactive reagents producing the same effects: Lucifer yellow iodoacetamide, monobromobimane, and dibromobimane. Top-down proteomic analyses of fluorescently labeled proteins from total and cholesterol-enriched vesicle membrane fractions using two-dimensional gel electrophoresis coupled with mass spectrometry identified several candidate targets, some of which have been previously linked to the late steps of regulated exocytosis and some of which are novel. Initial validation studies indicate that Rab proteins are involved in the modulation of Ca²⁺ sensitivity, and thus the efficiency of membrane fusion, which may, in part, be linked to their previously identified upstream roles in vesicle docking.

Lysosome-related organelles as functional adaptations of the endolysosomal system

Unique functions of specialised cells such as those of the immune and haemostasis systems, skin, blood vessels, lung, and bone require specialised compartments, collectively referred to as lysosome-related organelles (LROs), that share features of endosomes and lysosomes. LROs harbour unique morphological features and cell type-specific contents, and most if not all undergo regulated secretion for diverse functions. Ongoing research, largely driven by analyses of inherited diseases and their model systems, is unravelling the mechanisms involved in LRO generation, maturation, transport and secretion. A molecular understanding of these features will provide targets and markers that can be exploited for diagnosis and therapy of a myriad of diseases.

Cross-regulation of defective endolysosome trafficking and enhanced autophagy through TFEB in UNC13D deficiency

Several lines of evidence support the occurrence of cross-regulation between the endocytic pathway and autophagy, but the molecular mechanisms regulating this process are not well-understood. Here, we show that the calcium sensor UNC13D regulates the molecular mechanism of late endosomal trafficking and endosomal maturation, and defects in UNC13D lead to macroautophagy upregulation. unc13d-null cells showed impaired endosomal trafficking and defective endocytic flux. The defective phenotypes were rescued by the expression of UNC13D but not by its STX7-binding-deficient mutant. This defective endosomal function in UNC13D-deficient cells resulted in increased autophagic flux, increased long-lived protein degradation, decreased SQSTM1/p62 protein levels and increased autolysosome formation as determined by biochemical, microscopy and structural methods. The autophagic phenotype was not associated with increased recruitment of the UNC13D-binding proteins and autophagy regulators, RAB11 or VAMP8, but was caused, at least in part, by TFEB-mediated upregulation of a subset of autophagic and lysosomal genes, including Atg9b. Downregulation of TFEB decreased Atg9b levels and decreased macroautophagy in unc13d-null cells. UNC13D upregulation corrected the defects in endolysosomal trafficking and decreased the number of accumulated autophagosomes in a cellular model of the lysosomal-storage disorder cystinosis, under both fed and starvation conditions, identifying UNC13D as an important new regulatory molecule of autophagy regulation in cells with lysosomal disorders. Abbreviations ACTB: actin, beta; CTSB: cathepsin B; EEA1: early endosome antigen 1; ESCRT: endosomal sorting complex required for transport; FHL3: familial hemophagocytic; lymphohistiocytosis type 3; HEX: hexosaminidase; HLH: hemophagocytic lymphohistiocytosis; LSD: lysosomal storage disorder; MEF: mouse embryonic fibroblast; SEM: standard errors of the mean; SNARE: soluble n-ethylmaleimide-sensitive-factor attachment receptor; STX: syntaxin; SYT7: synaptotagmin VII; TFE3: transcription factor E3; TFEB: transcription factor EB; TIRF: total internal reflection fluorescence ULK1: unc-51 like kinase 1; UNC13D: unc-13 homolog d; VAMP: vesicle-associate membrane protein; WT: wild-type.

The trafficking protein JFC1 regulates Rac1-GTP localization at the uropod controlling neutrophil chemotaxis and in vivo migration

Neutrophil chemotaxis is essential in responses to infection and underlies inflammation. In neutrophils, the small GTPase Rac1 has discrete functions at both the leading edge and in the retraction of the trailing structure at the cell's rear (uropod), but how Rac1 is regulated at the uropod is unknown. Here, we identified a mechanism mediated by the trafficking protein synaptotagmin-like 1 (SYTL1 or JFC1) that controls Rac1-GTP recycling from the uropod and promotes directional migration of neutrophils. JFC1-null neutrophils displayed defective polarization and impaired directional migration to N-formyl-methionine-leucyl-phenylalanine in vitro, but chemoattractant-induced actin remodeling, calcium signaling and Erk activation were normal in these cells. Defective chemotaxis was not explained by impaired azurophilic granule exocytosis associated with JFC1 deficiency. Mechanistically, we show that active Rac1 localizes at dynamic vesicles where endogenous JFC1 colocalizes with Rac1-GTP. Super-resolution microscopy (STORM) analysis shows adjacent distribution of JFC1 and Rac1-GTP, which increases upon activation. JFC1 interacts with Rac1-GTP in a Rab27a-independent manner to regulate Rac1-GTP trafficking. JFC1-null cells exhibited Rac1-GTP accumulation at the uropod and increased tail length, and Rac1-GTP uropod accumulation was recapitulated by inhibition of ROCK or by interference with microtubule remodeling. In vivo, neutrophil dynamic studies in mixed bone marrow chimeric mice show that JFC1^-/- neutrophils are unable to move directionally toward the source of the chemoattractant, supporting the notion that JFC1 deficiency results in defective neutrophil migration. Our results suggest that defective Rac1-GTP recycling from the uropod affects directionality and highlight JFC1-mediated Rac1 trafficking as a potential target to regulate chemotaxis in inflammation and immunity.

A Comprehensive Review on the Interaction Between the Host GTPase Rab11 and Influenza A Virus

This year marks the 100th anniversary of one of the deadliest pandemic outbreaks, commonly referred as the Spanish Flu, that was caused by influenza A virus (IAV). Since then, IAV has been in governmental agendas worldwide, and a lot of effort has been put into understanding the pathogen's lifecycle, predict and mitigate the emergence of the strains that provoke yearly epidemics and pandemic events. Despite decades of research and seminal contributions there is still a lot to be investigated. In particular for this review, IAV lifecycle that takes place inside the host cell is not fully understood. Two steps that need clarification include genome transport to budding sites and genome assembly, the latter a complex process challenged by the nature of IAV genome that is divided into eight distinct parts. Assembly of such segmented genome is crucial to form fully infectious viral particles but is also critical for the emergence of viruses with pandemic potential that arise when avian and human IAV strains co-infect a host. The host GTPase Rab11 was separately implicated in both steps, and, interestingly these processes are beginning to emerge as being intimately related. Rab11 was initially proposed to be involved in the budding/release of IAV virions. It was subsequently shown to transport progeny genome, and later proposed to promote assembly of viral genome, but the underlying bridging mechanism the two is far from clear. For simplicity, this Rab11-centric review provides an initial separate account of Rab11 involvement in genome transport and in assembly. IAV genome assembly is a complicated molecular biology process, and therefore earned a dedicated section on how/if the viral genome forms a genomic supramolecular complex. Both topics present intricate challenges, outstanding questions, and unique controversies. At the end of the review, I will explore possible mechanisms intertwining IAV vRNP transport and genome assembly. Importantly, Rab11 has recently emerged as a key factor subverted by evolutionary unrelated viral families (Paramyxo, Bunya, and Orthomyxoviruses, among many others) and bacteria (Salmonella and Shigella) relevant to human health. This review provides a framework to identify common biological principles among the lifecycles of these pathogens.

Roles of the fission yeast UNC-13/Munc13 protein Ync13 in late stages of cytokinesis

Cytokinesis is a complicated yet conserved step of the cell-division cycle that requires the coordination of multiple proteins and cellular processes. Here we describe a previously uncharacterized protein, Ync13, and its roles during fission yeast cytokinesis. Ync13 is a member of the UNC-13/Munc13 protein family, whose animal homologues are essential priming factors for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex assembly during exocytosis in various cell types, but no roles in cytokinesis have been reported. We find that Ync13 binds to lipids in vitro and dynamically localizes to the plasma membrane at cell tips during interphase and at the division site during cytokinesis. Deletion of Ync13 leads to defective septation and exocytosis, uneven distribution of cell-wall enzymes and components of cell-wall integrity pathway along the division site and massive cell lysis during cell separation. Interestingly, loss of Ync13 compromises endocytic site selection at the division plane. Collectively, we find that Ync13 has a novel function as an UNC-13/Munc13 protein in coordinating exocytosis, endocytosis, and cell-wall integrity during fission yeast cytokinesis.

Fibronectin rescues estrogen receptor α from lysosomal degradation in breast cancer cells

Estrogen receptor α (ERα) is expressed in tissues as diverse as brains and mammary glands. In breast cancer, ERα is a key regulator of tumor progression. Therefore, understanding what activates ERα is critical for cancer treatment in particular and cell biology in general. Using biochemical approaches and superresolution microscopy, we show that estrogen drives membrane ERα into endosomes in breast cancer cells and that its fate is determined by the presence of fibronectin (FN) in the extracellular matrix; it is trafficked to lysosomes in the absence of FN and avoids the lysosomal compartment in its presence. In this context, FN prolongs ERα half-life and strengthens its transcriptional activity. We show that ERα is associated with β1-integrin at the membrane, and this integrin follows the same endocytosis and subcellular trafficking pathway triggered by estrogen. Moreover, ERα⁺ vesicles are present within human breast tissues, and colocalization with β1-integrin is detected primarily in tumors. Our work unravels a key, clinically relevant mechanism of microenvironmental regulation of ERα signaling.

Effect of metabolic status on conceptus-maternal interactions on day 19 in dairy cattle: II. Effects on the endometrial transcriptome

The aim of this study was to test the hypothesis that the metabolic stresses associated with lactation alter the ability of the endometrium to respond appropriately to the conceptus by examining endometrial gene expression on day 19 of pregnancy. Immediately after calving, primiparous Holstein cows with similar production and fertility estimated breeding values were randomly divided into two groups and either dried off (i.e. never milked) immediately or milked twice daily. Approximately 65-75 days postpartum, grade 1 blastocysts recovered from superovulated Holstein heifer donors (n = 5) were transferred (1 per recipient) into lactating (n = 11) and nonlactating (n = 11) recipients. Control nulliparous Holstein heifers (n = 6) were artificially inseminated. RNA-sequencing was performed on intercaruncular endometrial samples recovered at slaughter from confirmed pregnant animals on day 19 (n = 5 lactating and nonlactating cows; n = 4 heifers). Differentially expressed genes (DEGs) were identified between both postpartum groups compared to heifers and between lactating and nonlactating cows. Functional annotation of DEGs between cows and heifers revealed over-representation of categories, including endosome, cytoplasmic vesicle, endocytosis, regulation of exocytosis, and cytokine receptor activity. Functional categories including transcription factor binding sites, cell motility, and cell migration were enriched for DEGs between endometria from lactating and nonlactating cows. In conclusion, while the evidence for a major effect of lactation on the endometrial transcriptome is relatively weak, these data suggest that the metabolic status of the animal (heifer vs cow) modulates the response of the endometrium to the developing conceptus.

A Ca2+-stimulated exosome release pathway in cancer cells is regulated by Munc13-4

Cancer cells secrete copious amounts of exosomes, and elevated intracellular Ca²⁺ is critical for tumor progression and metastasis, but the underlying cellular mechanisms are unknown. Munc13-4 is a Ca²⁺-dependent SNAP receptor- and Rab-binding protein required for Ca²⁺-dependent membrane fusion. Here we show that acute elevation of Ca²⁺ in cancer cells stimulated a fivefold increase in CD63⁺, CD9⁺, and ALIX⁺ exosome release that was eliminated by Munc13-4 knockdown and not restored by Ca²⁺ binding-deficient Munc13-4 mutants. Direct imaging of CD63-pHluorin exosome release confirmed its Munc13-4 dependence. Depletion of Munc13-4 in highly aggressive breast carcinoma MDA-MB-231 cells reduced the size of CD63⁺ multivesicular bodies (MVBs), indicating a role for Munc13-4 in MVB maturation. Munc13-4 used a Rab11-dependent trafficking pathway to generate MVBs competent for exosome release. Membrane type 1 matrix metalloproteinase trafficking to MVBs by a Rab11-dependent pathway was also Munc13-4 dependent, and Munc13-4 depletion reduced extracellular matrix degradation. These studies identify a novel Ca²⁺- and Munc13-4-dependent pathway that underlies increased exosome release by cancer cells.

Rab35-dependent extracellular nanovesicles are required for induction of tumour supporting stroma

Communication between diseased cells and the microenvironment is a complex yet crucial element in progression of varied pathological processes. Recent studies in cancer highlight an important role for small extracellular nanovesicles secreted by cancer cells as modulators of cancer-associated stroma, leading to enhanced angiogenesis and metastatic priming. The intrinsic factors regulating extracellular nanovesicle biogenesis and secretion are therefore relevant in studies of nano-communication in the cancer milieu. We generated prostate cancer cells bearing stable knockdown of several candidate vesicle regulating factors and examined the impact on cell health, vesicle secretion and on communication with fibroblastic stromal cells. We highlight that RAB11B and RAB35 regulate phenotypically distinct nanovesicle populations, each accounting for only around 20% of the total. Depleting RAB35, but not RAB11B leaves a remaining population of vesicles whose phenotype is insufficient for driving fibroblast to myofibroblast differentiation, leading to attenuated motile behaviours in 3D in vitro models. Co-implantation of tumour cells with stromal fibroblasts in xenografts similarly showed that RAB11B knockdown had little effect on growth rates in vivo. In contrast, significant attenuation in growth, and attenuation of myofibroblasts at the tumour site was evident when using RAB35-knockdown cells. The study concludes that a RAB35 regulated nanovesicle sub-population is particularly important for communication between cancer and stromal cells, and is required for generating a tumour-supportive microenvironment.

Armed for destruction: formation, function and trafficking of neutrophil granules

Neutrophils respond nearly instantly to infection, rapidly deploying a potent enzymatic and chemical arsenal immediately upon entering an infected site. This capacity for rapid and potent responses is endowed by stores of antimicrobial proteins contained in readily mobilizable granules. These granules contain the proteins necessary to mediate the recruitment, chemotaxis, antimicrobial function and NET formation of neutrophils. Four granule types exist, and are sequentially deployed as neutrophils enter infected sites. Secretory vesicles are released first, enabling recruitment of neutrophils out of the blood. Next, specific and gelatinase granules are released to enable neutrophil migration and begin the formation of an antimicrobial environment. Finally, azurophilic granules release potent antimicrobial proteins at the site of infection and into phagosomes. The step-wise mobilization of these granules is regulated by calcium signaling, while specific trafficking regulators and membrane fusion complexes ensure the delivery of granules to the correct subcellular site. In this review, we describe neutrophil granules from their formation through to their deployment at the site of infection, focusing on recent developments in our understanding of the signaling pathways and vesicular trafficking mechanisms which mediate neutrophil degranulation.

Rab GTPases in Immunity and Inflammation

Strict spatiotemporal control of trafficking events between organelles is critical for maintaining homeostasis and directing cellular responses. This regulation is particularly important in immune cells for mounting specialized immune defenses. By controlling the formation, transport and fusion of intracellular organelles, Rab GTPases serve as master regulators of membrane trafficking. In this review, we discuss the cellular and molecular mechanisms by which Rab GTPases regulate immunity and inflammation.

Molecular mechanisms regulating secretory organelles and endosomes in neutrophils and their implications for inflammation

Neutrophils constitute the first line of cellular defense against invading microorganisms and modulate the subsequent innate and adaptive immune responses. In order to execute a rapid and precise response to infections, neutrophils rely on preformed effector molecules stored in a variety of intracellular granules. Neutrophil granules contain microbicidal factors, the membrane-bound components of the respiratory burst oxidase, membrane-bound adhesion molecules, and receptors that facilitate the execution of all neutrophil functions including adhesion, transmigration, phagocytosis, degranulation, and neutrophil extracellular trap formation. The rapid mobilization of intracellular organelles is regulated by vesicular trafficking mechanisms controlled by effector molecules that include small GTPases and their interacting proteins. In this review, we focus on recent discoveries of mechanistic processes that are at center stage of the regulation of neutrophil function, highlighting the discrete and selective pathways controlled by trafficking modulators. In particular, we describe novel pathways controlled by the Rab27a effectors JFC1 and Munc13-4 in the regulation of degranulation, reactive oxygen species and neutrophil extracellular trap production, and endolysosomal signaling. Finally, we discuss the importance of understanding these molecular mechanisms in order to design novel approaches to modulate neutrophil-mediated inflammatory processes in a targeted fashion.

BAIAP3, a C2 domain-containing Munc13 protein, controls the fate of dense-core vesicles in neuroendocrine cells

Dense-core vesicle (DCV) exocytosis is a SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor)-dependent anterograde trafficking pathway that requires multiple proteins for regulation. Several C2 domain-containing proteins are known to regulate Ca2+-dependent DCV exocytosis in neuroendocrine cells. In this study, we identified others by screening all (∼139) human C2 domain-containing proteins by RNA interference in neuroendocrine cells. 40 genes were identified, including several encoding proteins with known roles (CAPS [calcium-dependent activator protein for secretion 1], Munc13-2, RIM1, and SYT10) and many with unknown roles. One of the latter, BAIAP3, is a secretory cell-specific Munc13-4 paralog of unknown function. BAIAP3 knockdown caused accumulation of fusion-incompetent DCVs in BON neuroendocrine cells and lysosomal degradation (crinophagy) of insulin-containing DCVs in INS-1 β cells. BAIAP3 localized to endosomes was required for Golgi trans-Golgi network 46 (TGN46) recycling, exhibited Ca2+-stimulated interactions with TGN SNAREs, and underwent Ca2+-stimulated TGN recruitment. Thus, unlike other Munc13 proteins, BAIAP3 functions indirectly in DCV exocytosis by affecting DCV maturation through its role in DCV protein recycling. Ca2+ rises that stimulate DCV exocytosis may stimulate BAIAP3-dependent retrograde trafficking to maintain DCV protein homeostasis and DCV function.

Cystinosin, the small GTPase Rab11, and the Rab7 effector RILP regulate intracellular trafficking of the chaperone-mediated autophagy receptor LAMP2A

The lysosomal storage disease cystinosis, caused by cystinosin deficiency, is characterized by cell malfunction, tissue failure, and progressive renal injury despite cystine-depletion therapies. Cystinosis is associated with defects in chaperone-mediated autophagy (CMA), but the molecular mechanisms are incompletely understood. Here, we show CMA substrate accumulation in cystinotic kidney proximal tubule cells. We also found mislocalization of the CMA lysosomal receptor LAMP2A and impaired substrate translocation into the lysosome caused by defective CMA in cystinosis. The impaired LAMP2A trafficking and localization were rescued either by the expression of wild-type cystinosin or by the disease-associated point mutant CTNS-K280R, which has no cystine transporter activity. Defective LAMP2A trafficking in cystinosis was found to associate with decreased expression of the small GTPase Rab11 and the Rab7 effector RILP. Defective Rab11 trafficking in cystinosis was rescued by treatment with small-molecule CMA activators. RILP expression was restored by up-regulation of the transcription factor EB (TFEB), which was down-regulated in cystinosis. Although LAMP2A expression is independent of TFEB, TFEB up-regulation corrected lysosome distribution and lysosomal LAMP2A localization in Ctns^-/- cells but not Rab11 defects. The up-regulation of Rab11, Rab7, or RILP, but not its truncated form RILP-C33, rescued LAMP2A-defective trafficking in cystinosis, whereas dominant-negative Rab11 or Rab7 impaired LAMP2A trafficking. Treatment of cystinotic cells with a CMA activator increased LAMP2A localization at the lysosome and increased cell survival. Altogether, we show that LAMP2A trafficking is regulated by cystinosin, Rab11, and RILP and that CMA up-regulation is a potential clinically relevant mechanism to increase cell survival in cystinosis.