In vivo Roles of Rab27 and Its Effectors in Exocytosis

Immune cell infiltration into brain tumor microenvironment is mediated by Rab27-regulated vascular wall integrity

Aggressive brain tumors often exhibit immunologically 'cold' microenvironment, where the vascular barrier impedes effective immunotherapy in poorly understood ways. Tumor vasculature also plays a pivotal role in immunoregulation and antitumor immunity. Here, we show that small GTPase Rab27 controls the vascular morphogenesis and permeability for blood content and immune effectors. Thus, in Rab27a/b double knock out (Rab27-dKO) mice, the brain vasculature is abnormally scarce, while the blood vessels become dysmorphic and hyperpermeable in the context of brain tumors, including syngeneic glioblastoma. These defects are reflected in rearrangements of endothelial cell subpopulations with underlying diminution of venous endothelial subtype along with changes in gene and protein expression. Notably, Rab27-dKO brain endothelial cells exhibit deficient tight junctions, whereby they enable large-scale extravasation of cytotoxic T cells into the tumor mass. We show that Rab27-regulated vascular T cell infiltration can be exploited to enhance adoptive T cell therapy in syngeneic brain tumors.

Fetoplacental extracellular vesicles deliver conceptus-derived antigens to maternal secondary lymphoid tissues for immune recognition

Pregnancy is an immunological paradox where despite a competent maternal immune system, regulatory mechanisms at the fetoplacental interface and maternal secondary lymphoid tissues (SLTs) circumvent rejection of semi-allogeneic concepti. Small extracellular vesicles (sEVs) are a vehicle for intercellular communication; nevertheless, the role of fetoplacental sEVs in transport of antigens to maternal SLTs has not been conclusively demonstrated. Using mice in which the conceptus generates fluoroprobe-tagged sEVs shed by the plasma membrane or released from the endocytic compartment, we show that fetoplacental sEVs are delivered to immune cells in the maternal spleen. Injection of sEVs from placentas of females impregnated with Act-mOVA B6 males elicited suboptimal activation of OVA-specific CD8+ OT-I T cells in virgin females as occurs during pregnancy. Furthermore, when OVA+ concepti were deficient in Rab27a, a protein required for sEV secretion, OT-I cell proliferation in the maternal spleen was decreased. Proteomics analysis revealed that mouse trophoblast sEVs were enriched in antiinflammatory and immunosuppressive mediators. Translational relevance was tested in humanized mice injected using sEVs from cultures of human trophoblasts. Our findings show that sEVs deliver fetoplacental antigens to the mother's SLTs that are recognized by maternal T cells. Alterations of such a mechanism may lead to pregnancy disorders.

Mechanism of LINC01018/miR-182-5p/Rab27B in the immune escape through PD-L1-mediated CD8+ T cell suppression in glioma

BACKGROUND

Glioma is a malignant tumor associated with poorer prognosis. This study aims to elucidate the mechanism of LINC01018/miR-182-5p/Rab27B axis in PD-L1-mediated CD8+ T cell suppression in the progression of gliomas.

METHODS

LINC01018, miR-182-5p, and Rab27B expression levels in glioblastoma tissues were measured. The proportion of infiltrating macrophages and monocytes and CD8+ T cell function were assessed. The relationship between miR-182-5p and Rab27B was analyzed. Glioma cell activity, invasion, and migration were measured. The expression of E-cadherin, N-cadherin, Vimentin, PD-L1, iNOS, and CD206 was determined. Glioma cell-derived EVs were isolated, and the co-localization of Rab27B and PD-L1 and the binding of Rab27B to PD-L1 were analyzed. The endocytosis of EVs by microglia was assayed. The impact of LINC01018/miR-182-5p/Rab27B on glioma growth was observed. The function of macrophages and CD8+ T cells in tumors was analyzed.

RESULTS

Rab27B was downregulated, and infiltrating macrophages and monocytes were increased in glioblastoma. miR-182-5p inhibited Rab27B expression. Rab27B knockdown reverses the inhibitory effect of LINC01018 overexpression on glioma cell growth. Glioma cells-derived EVs with low Rab27B expression carried more PD-L1 to increase PD-L1 expression and M2 polarization in microglia. LINC01018 overexpression reduced macrophages in orthotopic tumors. CD8+ T cell numbers showed no significant difference, but TIM-3 increased and IFN-γ decreased. miR-182-5p inhibition enhanced the therapeutic effect of anti-PD-L1, which was reversed after glioma cell-derived EVs.

CONCLUSION

LINC01018 promotes PD-L1-mediated CD8+ T cell suppression via the miR-182-5p/Rab27B axis in glioma cell-derived EVs, thereby contributing to immune escape in gliomas.

Role of the RAB27/SYTL Axis in Tumor Microenvironment Construction

Crosstalk between cancer cells and the tumor microenvironment (TME) is a key event in malignant progression and metastasis. The secretion of bioactive substances by cancer cells remodels the TME, affecting the activities of its components, including blood vessels, mesenchymal cells, and immune cells. These substances are effectively delivered through intracellular trafficking and exocytosis of cytoplasmic vesicles. The small guanosine triphosphatase (GTPase) RAB27 and its effectors, synaptotagmin-like (SYTL) family proteins, play essential roles in vesicle trafficking. Our recent research demonstrates the upregulation of RAB27A/B and SYTL1/2 in alveolar soft part sarcoma and acute myeloid leukemia. This enhanced trafficking promotes angiogenesis and the occupation of leukemia cells in the bone marrow niche. This review focuses on the role of the RAB27/SYTL axis in various cancer types associated with TME modifications, with a discussion on its importance as a therapeutic target.

Amphisome plays a role in HBV production and release through the endosomal and autophagic pathways

BACKGROUND

Autophagic and endosomal pathways coordinately contribute to HBV virions and subviral particles (SVPs) production. To date, limited evidence supports that HBV and exosomes have a common pathway for their biogenesis and secretion. The final steps of HBV production and release have not yet been well studied.

METHODS

We examined the production and release of HBV virions and SVPs by using GW4869 (N,N'-Bis[4-(4,5-dihydro-1H-imidazol-2-yl)phenyl]-3,3'-pht hal amide dihydrochloride), a small molecule inhibiting ceramide-mediated inward membrane budding. Neutral sphingomyelinase, the target of GW4869, and RAB27A and -B,  2 small GTPases involved in exosome release control, were silenced using gene silencing to confirm the results obtained. Western blot, immunofluorescence staining, and confocal microscopy were applied.

RESULTS

GW4869 inhibited HBV virion release, causing their accumulation along with SVPs in hepatocytes. This triggered cellular endoplasmic reticulum stress, leading to protein kinase B-mechanistic target of rapamycin kinase signaling pathway inactivation. GW4869 treatment increased autophagosome formation and impaired autophagic degradation by blocking autophagosome-lysosome fusion. Consequently, HBsAg is increasingly localized to autophagosomes and late endosomes/multivesicular bodies. Silencing neutral sphingomyelinase yielded consistent results. Similarly, RAB27A silencing inhibited HBV virion and SVP secretion, causing their accumulation within hepatoma cells. Notably, GW4869 treatment, as well as RAB27A and -B silencing, increased the presence of LC3+CD63+HBsAg+ complexes.

CONCLUSIONS

Our results demonstrate the involvement of the autophagosome-late endosome/multivesicular bodies-exosome axis in regulating HBV production and release, highlighting amphisomes as a potential platform for HBV release.

Revealing the role of RAB27 in HER receptor family expression and signaling in melanoma cells

BACKGROUND

Alterations in signalling pathways fuel the growth and progression of melanoma. Therefore, understanding these processes is essential for developing effective treatment strategies. RAB27A and RAB27B are known to possess oncogenic effects by modulating cancer cell proliferation, invasion and drug resistance in various types of cancer, including melanoma. These proteins are mostly acknowledged as coordinators of the vesicular trafficking, however, their function in cellular signaling is less recognized. Therefore we aimed to investigate the relationship between RAB27 and oncogenic or signalling proteins in melanoma cells.

METHODS

We generated RAB27A knockout (KO) in SkMel28, A375, and patient-derived DMBC12 melanoma cell lines. Additionally, a double RAB27A/B knockout (dKO) A375 cell line was created. Firstly, we applied the Proteome Profiler array to identify proteins differentially expressed upon RAB27A/B loss. Subsequently, we picked selected specific proteins for a further in-depth analysis using RT-PCR, Western blot, and flow cytometry.

RESULTS

We found that silencing RAB27 markedly decreased the levels of various intracellular proteins linked with proliferation, invasion, angiogenesis, adhesion, or EMT at a cell-line dependent level. Among others, we observed a link between the expression of RAB27 and EGFR, HER2 and HER3. Altered levels of HER receptors disturbed the downstream signaling pathways by reducing the phosphorylation of AKT and ERK1/2 proteins.

CONCLUSIONS

Our findings present novel, previously unpublished data on the relationship between HER family receptor expression and potential activity, and the involvement of RAB27 in melanoma cells.

In situ architecture of the intercellular organelle reservoir between epididymal epithelial cells by volume electron microscopy

Mammalian epididymal epithelial cells are crucial for sperm maturation. Historically, vacuole-like ultrastructures in epididymal epithelial cells were observed via transmission electron microscopy but were undefined. Here, we utilize volume electron microscopy (vEM) to generate 3D reconstructions of epididymal epithelial cells and identify these vacuoles as intercellular organelle reservoirs (IORs) in the lateral intercellular space (LIS), which contains protein aggregates, autophagosomes, lysosome-related organelles and mitochondrial residues. Immunolabelling of organelle markers such as P62, LC3, LAMP1 and TOMM20 confirm these findings. The IOR size or number varies across four epididymal regions and decreases with age. Rab27a mutant mice exhibit reduced IORs in the caput epididymis and a subfertility phenotype, suggesting the involvement of Rab27a in the formation of IORs. Furthermore, we observe the presence of IORs between intestinal epithelial cells besides epididymis. Amino acid transporters at IOR edges suggest dynamic protein recycling. Our findings reveal that the IOR is an important structure critical for organelle turnover and recycling outside epithelial cells with limited self-degradation capabilities.

Multi-omic human pancreatic islet endoplasmic reticulum and cytokine stress response mapping provides type 2 diabetes genetic insights

Endoplasmic reticulum (ER) and inflammatory stress responses contribute to islet dysfunction in type 2 diabetes (T2D). Comprehensive genomic understanding of these human islet stress responses and whether T2D-associated genetic variants modulate them is lacking. Here, comparative transcriptome and epigenome analyses of human islets exposed ex vivo to these stressors revealed 30% of expressed genes and 14% of islet cis-regulatory elements (CREs) as stress responsive, modulated largely in an ER- or cytokine-specific fashion. T2D variants overlapped 86 stress-responsive CREs, including 21 induced by ER stress. We linked the rs6917676-T T2D risk allele to increased islet ER-stress-responsive CRE accessibility and allele-specific β cell nuclear factor binding. MAP3K5, the ER-stress-responsive putative rs6917676 T2D effector gene, promoted stress-induced β cell apoptosis. Supporting its pro-diabetogenic role, MAP3K5 expression correlated inversely with human islet β cell abundance and was elevated in T2D β cells. This study provides genome-wide insights into human islet stress responses and context-specific T2D variant effects.

Roles of small GTPases in cardiac hypertrophy (Review)

Cardiac hypertrophy results from the heart reacting and adapting to various pathological stimuli and its persistent development is a major contributing factor to heart failure. However, the molecular mechanisms of cardiac hypertrophy remain unclear. Small GTPases in the Ras, Rho, Rab, Arf and Ran subfamilies exhibit GTPase activity and play crucial roles in regulating various cellular responses. Previous studies have shown that Ras, Rho and Rab are closely linked to cardiac hypertrophy and that their overexpression can induce cardiac hypertrophy. Here, we review the functions of small GTPases in cardiac hypertrophy and provide additional insights and references for the prevention and treatment of cardiac hypertrophy.

N6-Methyladenosine Methyltransferase Component KIAA1429 Is a Potential Target of Cancer Therapy

N6-methyladenosine (m6A), the most abundant RNA modification in eukaryotes, has a crucial impact on tumorigenesis. KIAA1429 is the key component of the m6A methyltransferase complex, in which KIAA1429 functions as a scaffold to bridge the catalytic core proteins. KIAA1429 is often overexpressed in malignances, associated with patient prognosis, and required for tumorigenesis. KIAA1429 regulates the expression of a number of tumor-associated genes in an m6A -dependent manner, and thus, contributes to cell proliferation, migration, drug resistance, tumor formation and metastasis. This review focuses on recent progress in the understanding of roles and mechanisms of KIAA1429 in cancers, and offers ideas for potential anti-cancer therapeutic methods by targeting KIAA1429.

Truncated mini LRP1 transports cargo from luminal to basolateral side across the blood brain barrier

BACKGROUND

The most crucial area to focus on when thinking of novel pathways for drug delivery into the CNS is the blood brain barrier (BBB). A number of nanoparticulate formulations have been shown in earlier research to target receptors at the BBB and transport therapeutics into the CNS. However, no mechanism for CNS entrance and movement throughout the CNS parenchyma has been proposed yet. Here, the truncated mini low-density lipoprotein receptor-related protein 1 mLRP1_DIV* was presented as blood to brain transport carrier, exemplified by antibodies and immunoliposomes using a systematic approach to screen the receptor and its ligands' route across endothelial cells in vitro.

METHODS

The use of mLRP1_DIV* as liposomal carrier into the CNS was validated based on internalization and transport assays across an in vitro model of the BBB using hcMEC/D3 and bEnd.3 cells. Trafficking routes of mLRP1_DIV* and corresponding cargo across endothelial cells were analyzed using immunofluorescence. Modulation of γ-secretase activity by immunoliposomes loaded with the γ-secretase modulator BB25 was investigated in co-cultures of bEnd.3 mLRP1_DIV* cells and CHO cells overexpressing human amyloid precursor protein (APP) and presenilin 1 (PSEN1).

RESULTS

We showed that while expressed in vitro, mLRP1_DIV* transports both, antibodies and functionalized immunoliposomes from luminal to basolateral side across an in vitro model of the BBB, followed by their mLRP1_DIV* dependent release of the cargo. Importantly, functionalized liposomes loaded with the γ-secretase modulator BB25 were demonstrated to effectively reduce toxic Aß42 peptide levels after mLRP1_DIV* mediated transport across a co-cultured endothelial monolayer.

CONCLUSION

Together, the data strongly suggest mLRP1_DIV* as a promising tool for drug delivery into the CNS, as it allows a straight transport of cargo from luminal to abluminal side across an endothelial monolayer and it's release into brain parenchyma in vitro, where it exhibits its intended therapeutic effect.

IL-1β Induces LDL Transcytosis by a Novel Pathway Involving LDLR and Rab27a

BACKGROUND

In early atherosclerosis, circulating LDLs (low-density lipoproteins) traverse individual endothelial cells by an active process termed transcytosis. The CANTOS trial (Canakinumab Antiinflammatory Thrombosis Outcome Study) treated advanced atherosclerosis using a blocking antibody for IL-1β (interleukin-1β); this significantly reduced cardiovascular events. However, whether IL-1β regulates early disease, particularly LDL transcytosis, remains unknown.

METHODS

We used total internal reflection fluorescence microscopy to quantify transcytosis by human coronary artery endothelial cells exposed to IL-1β. To investigate transcytosis in vivo, we injected wild-type and knockout mice with IL-1β and LDL to visualize acute LDL deposition in the aortic arch.

RESULTS

Exposure to picomolar concentrations of IL-1β induced transcytosis of LDL but not of albumin by human coronary artery endothelial cells. Surprisingly, expression of the 2 known receptors for LDL transcytosis, ALK-1 (activin receptor-like kinase-1) and SR-BI (scavenger receptor BI), was unchanged or decreased. Instead, IL-1β increased the expression of the LDLR (LDL receptor); this was unexpected because LDLR is not required for LDL transcytosis. Overexpression of LDLR had no effect on basal LDL transcytosis. However, knockdown of LDLR abrogated the effect of IL-1β on transcytosis rates while the depletion of Cav-1 (caveolin-1) did not. Since LDLR was necessary but overexpression had no effect, we reasoned that another player must be involved. Using public RNA sequencing data to curate a list of Rab (Ras-associated binding) GTPases affected by IL-1β, we identified Rab27a. Overexpression of Rab27a alone had no effect on basal transcytosis, but its knockdown prevented induction by IL-1β. This was phenocopied by depletion of the Rab27a effector JFC1 (synaptotagmin-like protein 1). In vivo, IL-1β increased LDL transcytosis in the aortic arch of wild-type but not Ldlr-/- or Rab27a-deficient mice. The JFC1 inhibitor nexinhib20 also blocked IL-1β-induced LDL accumulation in the aorta.

CONCLUSIONS

IL-1β induces LDL transcytosis by a distinct pathway requiring LDLR and Rab27a; this route differs from basal transcytosis. We speculate that induction of transcytosis by IL-1β may contribute to the acceleration of early disease.

mTOR activation induces endolysosomal remodeling and nonclassical secretion of IL-32 via exosomes in inflammatory reactive astrocytes

Astrocytes respond and contribute to neuroinflammation by adopting inflammatory reactive states. Although recent efforts have characterized the gene expression signatures associated with these reactive states, the cell biology underlying inflammatory reactive astrocyte phenotypes remains under-explored. Here, we used CRISPR-based screening in human iPSC-derived astrocytes to identify mTOR activation a driver of cytokine-induced endolysosomal system remodeling, manifesting as alkalinization of endolysosomal compartments, decreased autophagic flux, and increased exocytosis of certain endolysosomal cargos. Through endolysosomal proteomics, we identified and focused on one such cargo-IL-32, a disease-associated pro-inflammatory cytokine not present in rodents, whose secretion mechanism is not well understood. We found that IL-32 was partially secreted in extracellular vesicles likely to be exosomes. Furthermore, we found that IL-32 was involved in the polarization of inflammatory reactive astrocyte states and was upregulated in astrocytes in multiple sclerosis lesions. We believe that our results advance our understanding of cell biological pathways underlying inflammatory reactive astrocyte phenotypes and identify potential therapeutic targets.

Dynamic changes in the proximitome of neutral sphingomyelinase-2 (nSMase2) in TNFα stimulated Jurkat cells

Ceramides generated by the activity of the neutral sphingomyelinase 2 (nSMase2) play a pivotal role in stress responses in mammalian cells. Dysregulation of sphingolipid metabolism has been implicated in numerous inflammation-related pathologies. However, its influence on inflammatory cytokine-induced signaling is yet incompletely understood. Here, we used proximity labeling to explore the plasma membrane proximal protein network of nSMase2 and TNFα-induced changes thereof. We established Jurkat cells stably expressing nSMase2 C-terminally fused to the engineered ascorbate peroxidase 2 (APEX2). Removal of excess biotin phenol substantially improved streptavidin-based affinity purification of biotinylated proteins. Using our optimized protocol, we determined nSMase2-proximal biotinylated proteins and their changes within the first 5 min of TNFα stimulation by quantitative mass spectrometry. We observed significant dynamic changes in the nSMase2 microenvironment in response to TNFα stimulation consistent with rapid remodeling of protein networks. Our data confirmed known nSMase2 interactors and revealed that the recruitment of most proteins depended on nSMase2 enzymatic activity. We measured significant enrichment of proteins related to vesicle-mediated transport, including proteins of recycling endosomes, trans-Golgi network, and exocytic vesicles in the proximitome of enzymatically active nSMase2 within the first minutes of TNFα stimulation. Hence, the nSMase2 proximal network and its TNFα-induced changes provide a valuable resource for further investigations into the involvement of nSMase2 in the early signaling pathways triggered by TNFα.

Exosomes encapsulated in hydrogels for effective central nervous system drug delivery

The targeted delivery of pharmacologically active molecules, metabolites, and growth factors to the brain parenchyma has become one of the major challenges following the onset of neurodegeneration and pathological conditions. The therapeutic effect of active biomolecules is significantly impaired after systemic administration in the central nervous system (CNS) because of the blood-brain barrier (BBB). Therefore, the development of novel therapeutic approaches capable of overcoming these limitations is under discussion. Exosomes (Exo) are nano-sized vesicles of endosomal origin that have a high distribution rate in biofluids. Recent advances have introduced Exo as naturally suitable bio-shuttles for the delivery of neurotrophic factors to the brain parenchyma. In recent years, many researchers have attempted to regulate the delivery of Exo to target sites while reducing their removal from circulation. The encapsulation of Exo in natural and synthetic hydrogels offers a valuable strategy to address the limitations of Exo, maintaining their integrity and controlling their release at a desired site. Herein, we highlight the current and novel approaches related to the application of hydrogels for the encapsulation of Exo in the field of CNS tissue engineering.

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

Minimal information for studies of extracellular vesicles (MISEV2023): From basic to advanced approaches

Extracellular vesicles (EVs), through their complex cargo, can reflect the state of their cell of origin and change the functions and phenotypes of other cells. These features indicate strong biomarker and therapeutic potential and have generated broad interest, as evidenced by the steady year-on-year increase in the numbers of scientific publications about EVs. Important advances have been made in EV metrology and in understanding and applying EV biology. However, hurdles remain to realising the potential of EVs in domains ranging from basic biology to clinical applications due to challenges in EV nomenclature, separation from non-vesicular extracellular particles, characterisation and functional studies. To address the challenges and opportunities in this rapidly evolving field, the International Society for Extracellular Vesicles (ISEV) updates its 'Minimal Information for Studies of Extracellular Vesicles', which was first published in 2014 and then in 2018 as MISEV2014 and MISEV2018, respectively. The goal of the current document, MISEV2023, is to provide researchers with an updated snapshot of available approaches and their advantages and limitations for production, separation and characterisation of EVs from multiple sources, including cell culture, body fluids and solid tissues. In addition to presenting the latest state of the art in basic principles of EV research, this document also covers advanced techniques and approaches that are currently expanding the boundaries of the field. MISEV2023 also includes new sections on EV release and uptake and a brief discussion of in vivo approaches to study EVs. Compiling feedback from ISEV expert task forces and more than 1000 researchers, this document conveys the current state of EV research to facilitate robust scientific discoveries and move the field forward even more rapidly.

A conserved electrostatic membrane-binding surface in synaptotagmin-like proteins revealed using molecular phylogenetic analysis and homology modeling

Protein structure prediction has emerged as a core technology for understanding biomolecules and their interactions. Here, we combine homology-based structure prediction with molecular phylogenetic analysis to study the evolution of electrostatic membrane binding among the vertebrate synaptotagmin-like protein (Slp) family. Slp family proteins play key roles in the membrane trafficking of large dense-core secretory vesicles. Our previous experimental and computational study found that the C2A domain of Slp-4 (also called granuphilin) binds with high affinity to anionic phospholipids in the cytoplasmic leaflet of the plasma membrane through a large positively charged protein surface centered on a cluster of phosphoinositide-binding lysine residues. Because this surface contributes greatly to Slp-4 C2A domain membrane binding, we hypothesized that the net charge on the surface might be evolutionarily conserved. To test this hypothesis, the known C2A sequences of Slp-4 among vertebrates were organized by class (from mammalia to pisces) using molecular phylogenetic analysis. Consensus sequences for each class were then identified and used to generate homology structures, from which Poisson-Boltzmann electrostatic potentials were calculated. For comparison, homology structures and electrostatic potentials were also calculated for the five human Slp protein family members. The results demonstrate that the charge on the membrane-binding surface is highly conserved throughout the evolution of Slp-4, and more highly conserved than many individual residues among the human Slp family paralogs. Such molecular phylogenetic-driven computational analysis can help to describe the evolution of electrostatic interactions between proteins and membranes which are crucial for their function.

Vesicular Trafficking, a Mechanism Controlled by Cascade Activation of Rab Proteins: Focus on Rab27

Vesicular trafficking is essential for the cell to internalize useful proteins and soluble substances, for cell signaling or for the degradation of pathogenic elements such as bacteria or viruses. This vesicular trafficking also enables the cell to engage in secretory processes for the elimination of waste products or for the emission of intercellular communication vectors such as cytokines, chemokines and extracellular vesicles. Ras-related proteins (Rab) and their effector(s) are of crucial importance in all of these processes, and mutations/alterations to them have serious pathophysiological consequences. This review presents a non-exhaustive overview of the role of the major Rab involved in vesicular trafficking, with particular emphasis on their involvement in the biogenesis and secretion of extracellular vesicles, and on the role of Rab27 in various pathophysiological processes. Therefore, Rab and their effector(s) are central therapeutic targets, given their involvement in vesicular trafficking and their importance for cell physiology.

Hemophagocytic lymphohistiocytosis in children with Griscelli syndrome type 2: genetics, laboratory findings and treatment

Griscelli syndrome is a rare inherited autosomal recessive syndrome that causes immunodeficiency. Hemophagocytic lymphohistiocytosis (HLH), which is characterized by a high mortality rate, may develop because of Griscelli syndrome type 2 (GS2). We aimed to share our experience with the diagnosis and treatment methods of patients who developed HLH secondary to GS2. Patients with GS2 who were diagnosed and treated for HLH between 2017 and 2022 at the Cukurova University Division of Pediatric Allergy & Immunology and Division of Pediatric Hematology were included in the study. Microscopic examination of the hair shaft and next-generation sequencing for molecular genetic testing of RAB27A helped in the diagnosis of GS2. The first clinical presentation of 8 patients was HLH. One patient presented with CNS involvement and two patients presented with recurrent fever. Over 5 years, GS2 was diagnosed in 15 patients, of whom 11 (73.3%) developed HLH. The HLH-2004 protocol was used to treat these patients. Hematopoietic stem cell transplantation (HSCT) was performed in five patients who were matched with suitable donors. While all patients who underwent HSCT were alive, three patients who could not undergo HSCT because no donor could be found died. Deletion of CAAGC at nucleotides 514_518 in GS2 patients is associated with CNS involvement and a poor prognosis. HLH may be the first sign of presentation in patients with GS2. Although further research is needed, regardless of the conditioning regimen utilized, early HSCT remains the primary therapy option for preventing GS2-induced mortality in HLH.

A Conserved Electrostatic Membrane-Binding Surface in Synaptotagmin-Like Proteins Revealed Using Molecular Phylogenetic Analysis and Homology Modeling

Protein structure prediction has emerged as a core technology for understanding biomolecules and their interactions. Here, we combine homology-based structure prediction with molecular phylogenetic analysis to study the evolution of electrostatic membrane binding among vertebrate synaptotagmin-like proteins (Slps). Slp family proteins play key roles in the membrane trafficking of large dense-core secretory vesicles. Our previous experimental and computational study found that the C2A domain of Slp-4 (also called granuphilin) binds with high affinity to anionic phospholipids in the cytoplasmic leaflet of the plasma membrane through a large positively charged protein surface centered on a cluster of phosphoinositide-binding lysine residues. Because this surface contributes greatly to Slp-4 C2A domain membrane binding, we hypothesized that the net charge on the surface might be evolutionarily conserved. To test this hypothesis, the known C2A sequences of Slp-4 among vertebrates were organized by class (from mammalia to pisces) using molecular phylogenetic analysis. Consensus sequences for each class were then identified and used to generate homology structures, from which Poisson-Boltzmann electrostatic potentials were calculated. For comparison, homology structures and electrostatic potentials were also calculated for the five human Slp protein family members. The results demonstrate that the charge on the membrane-binding surface is highly conserved throughout the evolution of Slp-4, and more highly conserved than many individual residues among the human Slp family paralogs. Such molecular phylogenetic-driven computational analysis can help to describe the evolution of electrostatic interactions between proteins and membranes which are crucial for their function.

Osh-dependent and -independent Regulation of PI4P Levels During Polarized Growth of Saccharomyces cerevisiae

Polarized secretion facilitates polarized cell growth. For a secretory vesicle to dock at the plasma membrane, it must mature with a progressive association or dissociation of molecules that are, respectively, necessary for or inhibitory to vesicle docking, including an exchange of Rab GTPases. In current models, oxysterol-binding protein homologue 4 (Osh4p) establishes a phosphatidylinositol 4-phosphate (PI4P) gradient along the secretory trafficking pathway such that vesicles have higher PI4P levels after budding from the trans-Golgi relative to when vesicles arrive at the plasma membrane. In this study, using the lipid-binding domain P4M and live-cell imaging, we show that secretory vesicle-associated PI4P levels remain constant when vesicles traffic from the trans-Golgi to the plasma membrane. We also show that deletion of OSH4 does not alter vesicle-associated PI4P levels, though loss of any individual member of the OSH family or complete loss of OSH family function alters the intracellular distribution of PI4P. We propose a model in which the Rab GTPases Ypt32p and Sec4p remain associated with a secretory vesicle during trafficking, independent of PI4P levels and Osh4p. Together these data indicate the necessity of experiments revealing the location and timing of events required for vesicle maturation.

Multiple pathways and independent functional pools in insulin granule exocytosis

In contrast to synaptic vesicle exocytosis, secretory granule exocytosis follows a much longer time course, and thus allows for different prefusion states prior to stimulation. Indeed, total internal reflection fluorescence microscopy in living pancreatic β cells reveals that, prior to stimulation, either visible or invisible granules fuse in parallel during both early (first) and late (second) phases after glucose stimulation. Therefore, fusion occurs not only from granules predocked to the plasma membrane but also from those translocated from the cell interior during ongoing stimulation. Recent findings suggest that such heterogeneous exocytosis is conducted by a specific set of multiple Rab27 effectors that appear to operate on the same granule; namely, exophilin-8, granuphilin, and melanophilin play differential roles in distinct secretory pathways to final fusion. Furthermore, the exocyst, which is known to tether secretory vesicles to the plasma membrane in constitutive exocytosis, cooperatively functions with these Rab27 effectors in regulated exocytosis. In this review, the basic nature of insulin granule exocytosis will be described as a representative example of secretory granule exocytosis, followed by a discussion of the means by which different Rab27 effectors and the exocyst coordinate to regulate the entire exocytic processes in β cells.

Functional hierarchy among different Rab27 effectors involved in secretory granule exocytosis

The Rab27 effectors are known to play versatile roles in regulated exocytosis. In pancreatic beta cells, exophilin-8 anchors granules in the peripheral actin cortex, whereas granuphilin and melanophilin mediate granule fusion with and without stable docking to the plasma membrane, respectively. However, it is unknown whether these coexisting effectors function in parallel or in sequence to support the whole insulin secretory process. Here, we investigate their functional relationships by comparing the exocytic phenotypes in mouse beta cells simultaneously lacking two effectors with those lacking just one of them. Analyses of prefusion profiles by total internal reflection fluorescence microscopy suggest that melanophilin exclusively functions downstream of exophilin-8 to mobilize granules for fusion from the actin network to the plasma membrane after stimulation. The two effectors are physically linked via the exocyst complex. Downregulation of the exocyst component affects granule exocytosis only in the presence of exophilin-8. The exocyst and exophilin-8 also promote fusion of granules residing beneath the plasma membrane prior to stimulation, although they differentially act on freely diffusible granules and those stably docked to the plasma membrane by granuphilin, respectively. This is the first study to diagram the multiple intracellular pathways of granule exocytosis and the functional hierarchy among different Rab27 effectors within the same cell.

Munc13b stimulus-dependently accumulates on granuphilin-mediated, docked granules prior to fusion

The Rab27 effector granuphilin plays an indispensable role in stable docking of secretory granules to the plasma membrane by interacting with the complex of Munc18-1 and the fusion-incompetent, closed form of syntaxins-1~3. Although this process prevents spontaneous granule exocytosis, those docked granules actively fuse in parallel with other undocked granules after stimulation. Therefore, it is postulated that the closed form of syntaxins must be converted into the fusion-competent open form in a stimulus-dependent manner. Although Munc13 family proteins are generally thought to prime docked vesicles by facilitating conformational change in syntaxins, it is unknown which isoform acts in granuphilin-mediated, docked granule exocytosis. In the present study, we show that, although both Munc13a and Munc13b are expressed in mouse pancreatic islets and their beta-cell line MIN6, the silencing of Munc13b, but not that of Munc13a, severely affects glucose-induced insulin secretion. Furthermore, Munc13b accumulates on a subset of granules beneath the plasma membrane just prior to fusion during stimulation, whereas Munc13a is translocated to the plasma membrane where granules do not exist. When fluorescently labeled granuphilin was introduced to discriminate between molecularly docked granules and other undocked granules in living cells, Munc13b downregulation was observed to preferentially decrease the fusion of granuphilin-positive granules immobilized to the plasma membrane. These findings suggest that Munc13b promotes insulin exocytosis by clustering on molecularly docked granules in a stimulus-dependent manner.Key words: docking, insulin, live cell imaging, priming, TIRF microscopy.