Ketotifen is a microglial stabilizer by inhibiting secretory vesicle acidification

AIMS

Microglia survey the brain environment by sensing alarm signals to provide the first line of defense against injury or infection after which they acquire an activated phenotype, but they also respond to chemical signals sent from brain mast cells, sentinels of the immune system, when these are degranulated in response to noxious agents. Nevertheless, excessive microglia activation damages the surrounding healthy neural tissue causing progressive loss of neurons and inducing chronic inflammation. Thus, it would be of intense interest the development and application of agents which prevent mast cell mediator release and inhibit the actions of such mediators once released on microglia.

MAIN METHODS

Fluorescence measurements of fura-2 and quinacrine were used to measure intracellular Ca²⁺ signaling and exocytotic vesicle fusion in resting and activated microglia.

KEY FINDINGS

We show that treatment of microglia with a cocktail of mast cell mediators induces microglia activation, phagocytosis, and exocytosis, and reveal by the first-time microglia undergo a phase of vesicular acidification just before the exocytotic fusion occurs. This acidification is an important process for vesicular maturation and contributes with ∼25 % to the content that the vesicle can store and later release by exocytosis. Pre-incubation with ketotifen, a mast cell stabilizer and H1R antagonist completely abolished histamine-mediated calcium signaling and acidification of microglial organelles, and concomitantly reduced the discharge of vesicle contents.

SIGNIFICANCE

These results highlight a key role for vesicle acidification in microglial physiology and provide a potential therapeutic target for diseases related to mast cell and microglia-mediated neuroinflammation.

Analysis of centrosomal area actin reorganization and centrosome polarization upon lymphocyte activation at the immunological synapse

T cell receptor (TCR) and B cell receptor (BCR) stimulation of T and B lymphocytes, by antigen presented on an antigen-presenting cell (APC) induces the formation of the immunological synapse (IS). IS formation is associated with an initial increase in cortical filamentous actin (F-actin) at the IS, followed by a decrease in F-actin density at the central region of the IS, which contains the secretory domain. This is followed by the convergence of secretion vesicles towards the centrosome, and the polarization of the centrosome to the IS. These reversible, cortical actin cytoskeleton reorganization processes occur during lytic granule secretion in cytotoxic T lymphocytes (CTL) and natural killer (NK) cells, proteolytic granules secretion in B lymphocytes and during cytokine-containing vesicle secretion in T-helper (Th) lymphocytes. In addition, several findings obtained in T and B lymphocytes forming IS show that actin cytoskeleton reorganization also occurs at the centrosomal area. F-actin reduction at the centrosomal area appears to be associated with centrosome polarization. In this chapter we deal with the analysis of centrosomal area F-actin reorganization, as well as the centrosome polarization analysis toward the IS.

Live-Cell Imaging of Dynein-Mediated Cargo Transport in Aspergillus nidulans

Filamentous fungi have been used for studying long-distance transport of cargoes driven by cytoplasmic dynein. Aspergillus nidulans is a well-established genetic model organism used for studying dynein function and regulation in vivo. Here, we describe how we grow A. nidulans strains for live-cell imaging and how we observe the dynein-mediated distribution of early endosomes and secretory vesicles. Using an on-stage incubator and culture chambers for inverted microscopes, we can image fungal hyphae that naturally attach to the bottom of the chambers, using wide-field epifluorescence microscopes or the new Zeiss LSM 980 (with Airyscan 2) microscope. In addition to methods for preparing cells for imaging, a procedure for A. nidulans transformation is also described.

Measurements of Calcium in Chromaffin Cell Organelles Using Targeted Aequorins

The molecular mechanisms that mediate and regulate calcium (Ca²⁺) fluxes through the membranes of intracellular organelles play a key role in the generation and shaping of the local and global cytosolic Ca²⁺ signals triggering the process of regulated exocytosis in chromaffin cells. Beyond that role, intraorganellar Ca²⁺ homeostasis also regulates organelle-specific processes such as oxidative phosphorylation in mitochondria, maturation of secretory granules, or stress in the endoplasmic reticulum. In this chapter, we describe current methods to study mitochondrial, endoplasmic reticulum, and secretory vesicle calcium homeostasis in living chromaffin cells using engineered targeted aequorins.

Quantification of Secretory Granule Exocytosis by TIRF Imaging and Capacitance Measurements

Hormones and neurotransmitters are released from (neuro)endocrine cells by regulated exocytosis of secretory granules. During exocytosis, the granule membrane fuses with the plasma membrane, which allows release of the stored content into the bloodstream or the surrounding tissue. Here, we give a detailed description of two complementary methods to observe and quantify exocytosis in single cells: high-resolution TIRF microscopy and patch-clamp capacitance recordings. Precise stimulation of exocytosis is achieved by local pressure application or voltage-clamp depolarizations. While the chapter is focused on insulin-secreting cells as an accessible and disease-relevant model system, the methodology is applicable to a wide variety of secretory cells including chromaffin and PC12 cells.

Mesenchymal Stromal Cell Secretome for Therapeutic Application in Skin Wound Healing: A Systematic Review of Preclinical Studies

Non-healing skin wounds remain a challenge in the healthcare system. In this sense, it is suggested that the secretome of mesenchymal stromal cells (MSCs) can be effective as a therapeutic strategy for regenerative medicine. Therefore, this systematic review aimed to determine the effects of treatment with a secretome derived from MSCs on the healing of skin wounds in a preclinical model of rodents (mice and rats). Studies were systematically retrieved from 6 databases and gray literature that provided 1,172 records, of which 25 met the inclusion criteria for qualitative analysis. Results revealed substantial heterogeneity among studies concerning experimental designs and methodologies, resulting in a high risk of bias. Together, the selected studies reported that treatment improved wound healing by (1) accelerating wound closure and improving skin repair quality; (2) reducing inflammation by decreasing the number of cells and inflammatory cytokines, accompanied by polarization of the M2 macrophage; (3) complete re-epithelialization and epidermal reorganization; (4) neovascularization promoted by proliferation of endothelial cells (CD34+) and increased levels of pro-angiogenic mediators; (5) better scar quality promoted by increased expression of collagen types I and III, as well as improved deposition and remodeling of collagen fibers. In conclusion, despite the need for alignment of methodological protocols and transparent reports in future studies, results show that the secretome of MSCs from different tissue sources corresponds to a promising tool of regenerative medicine for the treatment of skin wounds.

TGFβ1 and TGFβ2 proteins in corneas with and without stromal fibrosis: Delayed regeneration of apical epithelial growth factor barrier and the epithelial basement membrane in corneas with stromal fibrosis

The purpose of this study was to investigate the expression and localization of transforming growth factor (TGF) β1 and TGFβ2 in rabbit corneas that healed with and without stromal fibrosis, and to further study defective perlecan incorporation in the epithelial basement membrane (EBM) in corneas with scarring fibrosis. A total of 120 female rabbits had no surgery, -4.5D PRK, or -9D PRK. Immunohistochemistry (IHC) was performed at time points from unwounded to eight weeks after surgery, with four corneas at each time point in each group. Multiplex IHC was performed for TGFβ1 or TGFβ2, with Image-J quantitation, and keratocan, vimentin, alpha-smooth muscle actin (SMA), perlecan, laminin-alpha 5, nidogen-1 or CD11b. Corneas at the four-week peak for myofibroblast and fibrosis development were evaluated using Imaris 3D analysis. Delayed regeneration of both an apical epithelial growth factor barrier and EBM barrier function, including defective EBM perlecan incorporation, was greater in high injury -9D PRK corneas compared to -4.5D PRK corneas without fibrosis. Defective apical epithelial growth factor barrier and EBM allowed epithelial and tear TGFβ1 and tear TGFβ2 to enter the corneal stroma to drive myofibroblast generation in the anterior stroma from vimentin-positive corneal fibroblasts, and likely fibrocytes. Vimentin-positive cells and unidentified vimentin-negative, CD11b-negative cells also produce TGFβ1 and/or TGFβ2 in the stroma in some corneas. TGFβ1 and TGFβ2 were at higher levels in the anterior stroma in the weeks preceding myofibroblast development in the -9D group. All -9D corneas (beginning two to three weeks after surgery), and four -4.5D PRK corneas developed significant SMA + myofibroblasts and stromal fibrosis. Both the apical epithelial growth factor barrier and/or EBM barrier functions tended to regenerate weeks earlier in -4.5D PRK corneas without fibrosis, compared to -4.5D or -9D PRK corneas with fibrosis. SMA-positive myofibroblasts were markedly reduced in most corneas by eight weeks after surgery. The apical epithelial growth factor barrier and EBM barrier limit TGFβ1 and TGFβ2 entry into the corneal stroma to modulate corneal fibroblast and myofibroblast development associated with scarring stromal fibrosis. Delayed regeneration of these barriers in corneas with more severe injuries promotes myofibroblast development, prolongs myofibroblast viability and triggers stromal scarring fibrosis.

Bottom up proteomics identifies neuronal differentiation pathway networks activated by cathepsin inhibition treatment in neuroblastoma cells that are enhanced by concurrent 13-cis retinoic acid treatment

Neuroblastoma is the second most common pediatric cancer involving the peripheral nervous system in which stage IVS metastatic tumors regress due to spontaneous differentiation. 13-cis retinoic acid (13-cis RA) is currently used in the clinic for its differentiation effects and although it improves outcomes, relapse is seen in half of high-risk patients. Combinatorial therapies have been shown to be more effective in oncotherapy and since cathepsin inhibition reduces tumor growth, we explored the potential of coupling 13-cis RA with a cathepsin inhibitor (K777) to enhance therapeutic efficacy against neuroblastoma. Shotgun proteomics was used to identify proteins affected by K777 and dual (13-cis RA/K777) treatment in neuroblastoma SK-N-SH cells. Cathepsin inhibition was more effective in increasing proteins involved in neuronal differentiation and neurite outgrowth than 13-cis RA alone, but the combination of both treatments enhanced the neuronal differentiation effect. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of more accurate diagnostic markers and more effective treatments. In this study, we established a differentiation proteomic map of SK-N-SH cells treated with a cathepsin inhibitor (K777) and K777/13-cis RA (dual). Bioinformatic analysis revealed these treatments enhanced neuronal differentiation and axonogenesis pathways. The most affected proteins in these pathways may become valuable biomarkers of efficacy of drugs designed to enhance differentiation of neuroblastoma [1].

Geoffrey Burnstock, our friend and magister: the diadenosine polyphosphate connection

Development of science needs the cooperation of many creative brains. Sometimes, ideas on a specific area get suddenly exhausted and then it is the time for a privileged mind to think in a different way and reach the turning point to introduce a new paradigm. This happened to Geoffrey Burnstock, a heterodox thinker and nonconformist scientist that has been the paladin of purinergic signalling since 1972, opening neuroscience to the understanding of organs and tissues functioning and development of a new pharmacology. This review summarizes the contribution of our group to the understanding of the role of the diadenosine polyphosphates, Ap_nA, as signalling molecules, describing their tissue and organ distribution, their transport and storage in secretory vesicles and their release and interaction with purinergic receptors. We also have to acknowledge the friendly and kindly support of Professor Burnstock that showed a great interest in the field from our initial findings and actively stimulated our efforts to establish the extracellular roles and biological significance of these dinucleotides.

Effects of aging on the secretory apparatus in the right atrial cardiomyocytes of rats

The cardiac atria secrets polypeptide hormones usually called natriuretic peptides (NPs). These substances play a relevant role in the blood pressure regulation. The objective of the study was to estimate the effects of aging on the secretory apparatus of NPs in cardiomyocytes of the right atrium. Twenty male Wistar rats were studied: 10 young animals aged 3 months old (237 ± 27 g; mean ± SD) and 10 old animals aged 20 months old (450 ± 68 g; mean ± SD). The systolic blood pressure was verified instants before the moment of the euthanasia. Electron micrographs were prepared to quantify the area and density of the NP granules and the relative volumes of the endoplasmic reticulum, Golgi complex, and mitochondria. In addition, the number of pores per 10 μm of karyotheca was another variable evaluated. The significance of the results between the two groups evaluated was analyzed by the Student's t test (p < 0.05). The cardiomyocytes obtained from animals of the old group showed decreased in sectional area and density of secretory granules of NP and lower relative volume of endoplasmic reticulum, Golgi complex, and mitochondria compared with the young rats. Moreover, the quantitative density of nuclear pores was significantly lower compared with the youngers. CONCLUSION: Aging causes hypotrophy of the cardiomyocytes of right atrium, similar to what occurs in ventricular cardiomyocytes.

Gliocrine System: Astroglia as Secretory Cells of the CNS

Astrocytes are secretory cells, actively participating in cell-to-cell communication in the central nervous system (CNS). They sense signaling molecules in the extracellular space, around the nearby synapses and also those released at much farther locations in the CNS, by their cell surface receptors, get excited to then release their own signaling molecules. This contributes to the brain information processing, based on diffusion within the extracellular space around the synapses and on convection when locales relatively far away from the release sites are involved. These functions resemble secretion from endocrine cells, therefore astrocytes were termed to be a part of the gliocrine system in 2015. An important mechanism, by which astrocytes release signaling molecules is the merger of the vesicle membrane with the plasmalemma, i.e., exocytosis. Signaling molecules stored in astroglial secretory vesicles can be discharged into the extracellular space after the vesicle membrane fuses with the plasma membrane. This leads to a fusion pore formation, a channel that must widen to allow the exit of the Vesiclal cargo. Upon complete vesicle membrane fusion, this process also integrates other proteins, such as receptors, transporters and channels into the plasma membrane, determining astroglial surface signaling landscape. Vesiclal cargo, together with the whole vesicle can also exit astrocytes by the fusion of multivesicular bodies with the plasma membrane (exosomes) or by budding of vesicles (ectosomes) from the plasma membrane into the extracellular space. These astroglia-derived extracellular vesicles can later interact with various target cells. Here, the characteristics of four types of astroglial secretory vesicles: synaptic-like microvesicles, dense-core vesicles, secretory lysosomes, and extracellular vesicles, are discussed. Then machinery for vesicle-based exocytosis, second messenger regulation and the kinetics of exocytotic vesicle content discharge or release of extracellular vesicles are considered. In comparison to rapidly responsive, electrically excitable neurons, the receptor-mediated cytosolic excitability-mediated astroglial exocytotic vesicle-based transmitter release is a relatively slow process.

Sphingolipids modulate docking, Ca2+ sensitivity and membrane fusion of native cortical vesicles

Docking, priming, and membrane fusion of secretory vesicles (i.e. regulated exocytosis) requires lipids and proteins. Sphingolipids, in particular, sphingosine and sphingosine-1-phosphate, have been implicated in the modulation of exocytosis. However, the specific exocytotic steps that sphingolipids modulate and the enzymes that regulate sphingolipid concentrations on native secretory vesicle membranes remain unknown. Here we use tightly coupled functional and molecular analyses of fusion-ready cell surface complexes and cortical vesicles isolated from oocytes to assess the role of sphingolipids in the late, Ca²⁺-triggered steps of exocytosis. The molecular changes resulting from treatments with sphingolipid modifying compounds coupled with immunoblotting analysis revealed the presence of sphingosine kinase on native vesicles; the presence of a sphingosine-1-phosphate phosphatase is also indicated. Changes in sphingolipid concentrations on vesicles altered their docking/priming, Ca²⁺-sensitivity, and ability to fuse, indicating that sphingolipid concentrations are tightly regulated and maintained at optimal levels and ratios to ensure efficient exocytosis.

Extracellular superoxide dismutase is present in secretory vesicles of human neutrophils and released upon stimulation

Extracellular superoxide dismutase (EC-SOD) is an antioxidant enzyme present in the extracellular matrix (ECM), where it provides protection against oxidative degradation of matrix constituents including type I collagen and hyaluronan. The enzyme is known to associate with macrophages and polymorphonuclear leukocytes (neutrophils) and increasing evidence supports a role for EC-SOD in the development of an inflammatory response. Here we show that human EC-SOD is present at the cell surface of isolated neutrophils as well as stored within secretory vesicles. Interestingly, we find that EC-SOD mRNA is absent throughout neutrophil maturation indicating that the protein is synthesized by other cells and subsequently endocytosed by the neutrophil. When secretory vesicles were mobilized by neutrophil stimulation using formyl-methionyl-leucyl-phenylalanine (fMLF) or phorbol 12-myristate 13-acetate (PMA), the protein was released into the extracellular space and found to associate with DNA released from stimulated cells. The functional consequences were evaluated by the use of neutrophils isolated from wild-type and EC-SOD KO mice, and showed that EC-SOD release significantly reduce the level of superoxide in the extracellular space, but does not affect the capacity to generate neutrophil extracellular traps (NETs). Consequently, our data signifies that EC-SOD released from activated neutrophils affects the redox conditions of the extracellular space and may offer protection against highly reactive oxygen species such as hydroxyl radicals otherwise generated as a result of respiratory burst activity of activated neutrophils.

ER to Golgi-Dependent Protein Secretion: The Conventional Pathway

Secretion is the cellular process present in every organism that delivers soluble proteins and cargoes to the extracellular space. In eukaryotes, conventional protein secretion (CPS) is the trafficking route that secretory proteins undertake when are transported from the endoplasmic reticulum (ER) to the Golgi apparatus (GA), and subsequently to the plasma membrane (PM) via secretory vesicles or secretory granules. This book chapter recalls the fundamental steps in cell biology research contributing to the elucidation of CPS; it describes the most prominent examples of conventionally secreted proteins in eukaryotic cells and the molecular mechanisms necessary to regulate each step of this process.

Neuropeptidomics Mass Spectrometry Reveals Signaling Networks Generated by Distinct Protease Pathways in Human Systems

Neuropeptides regulate intercellular signaling as neurotransmitters of the central and peripheral nervous systems, and as peptide hormones in the endocrine system. Diverse neuropeptides of distinct primary sequences of various lengths, often with post-translational modifications, coordinate and integrate regulation of physiological functions. Mass spectrometry-based analysis of the diverse neuropeptide structures in neuropeptidomics research is necessary to define the full complement of neuropeptide signaling molecules. Human neuropeptidomics has notable importance in defining normal and dysfunctional neuropeptide signaling in human health and disease. Neuropeptidomics has great potential for expansion in translational research opportunities for defining neuropeptide mechanisms of human diseases, providing novel neuropeptide drug targets for drug discovery, and monitoring neuropeptides as biomarkers of drug responses. In consideration of the high impact of human neuropeptidomics for health, an observed gap in this discipline is the few published articles in human neuropeptidomics compared with, for example, human proteomics and related mass spectrometry disciplines. Focus on human neuropeptidomics will advance new knowledge of the complex neuropeptide signaling networks participating in the fine control of neuroendocrine systems. This commentary review article discusses several human neuropeptidomics accomplishments that illustrate the rapidly expanding diversity of neuropeptides generated by protease processing of pro-neuropeptide precursors occurring within the secretory vesicle proteome. Of particular interest is the finding that human-specific cathepsin V participates in producing enkephalin and likely other neuropeptides, indicating unique proteolytic mechanisms for generating human neuropeptides. The field of human neuropeptidomics has great promise to solve new mechanisms in disease conditions, leading to new drug targets and therapeutic agents for human diseases. Graphical Abstract ᅟ.

Secretory proteins without a transport signal are retained in secretory granules during maturation in rat parotid acinar cells

OBJECTIVE

The acinar cells of the parotid gland are filled with numerous secretory granules (SGs), which accumulate the digestion enzyme amylase. SGs mature accompanied with membrane remodelling such as fusion and budding of small vesicles. However, little is understood about the mechanism of the condensation of SG contents during maturation. In this study, we examined whether secretory proteins need a specific signal to be retained in SGs.

DESIGN

To induce internalization of the luminal membrane after exocytosis, we injected the β-adrenergic agonist isoproterenol into rats. Acinar cells were then incubated with Lucifer Yellow (LY) dye as a tracer for 3h for uptake into immature secretory granules (ISGs). To observe whether LY was retained in SGs after maturation, we continued incubating the cultured acinar cells for 2 days.

RESULTS

The localization of LY into ISGs was confirmed by the following four methods: (1) co-localization of the fluorescence of LY and amylase by confocal laser microscopy, (2) detection of the fluorescence from purified ISGs, (3) secretion of the fluorescence together with amylase upon stimulation, and (4) observation of the intracellular localization of LY by electron microscopy. Moreover, we observed co-localization of some of the SGs with the fluorescence of LY after cell culture.

CONCLUSIONS

Although the fusion and budding of small vesicles may contribute to the process of granule maturation, LY remained in the SGs even after maturation. These results suggest that secretory proteins that have no transport signal are not excluded from SGs, and they are retained in SGs during granule maturation in exocrine parotid glands.

Insights into the Nanobiology of Growth Hormone Secretion

The fact that partially empty vesicles are generated following cell secretion suggested that secretory vesicles do not collapse at the cell plasma membrane but, rather, transiently dock and fuse at the plasma membrane to expel a portion of their contents before retracting or undergoing endocytosis into the cell. Such a process has also been referred to in the literature as a "kiss-and-run" mechanism. This mechanism of cell secretion was conclusively demonstrated following the discovery of permanent cup-shaped lipoprotein structures at the cell plasma membrane, called "porosomes", where secretory vesicles transiently dock and fuse to expel intravesicular contents from the cell. Porosomes are present in all secretory cells, from the digestive enzyme-secreting pancreatic acinar cells, to the hormone-releasing growth hormone cells, mast cells, chromaffin cells, hair cells of the inner ear, to neurons secreting neurotransmitters. Hence, it can be asserted that porosomes are the universal secretory machinery in the plasma membrane of secretory cells. Therefore, the discovery of the porosome has resulted in a paradigm shift in our understanding of cell secretion. Rapid transport of secretory vesicles containing hormones to the plasma membrane is powered by high-energy molecules such as ATP, GTP or NADH. Immunogold labeled transmission electron microscopy (TEM) was used to determine the total number of secretory vesicles in resting and in GH-stimulated porcine pituitary cells. We identified three categories of vesicles: filled, empty, and partly empty. Resting GH cells contained more than twice as many filled vesicles than did the stimulated ones. However, stimulated cells contained nearly twice as many empty vesicles and 2.5 times more partly empty vesicles than did resting cells. Secretory vesicles in GH cells further revealed the localization of GH only in electron dense vesicles in both resting and stimulated cells. No change in the total number of secretory vesicles following secretion was observed. These results are consistent with a mechanism that, after stimulation of secretion, vesicles transiently dock and fuse at the porosome to establish a fusion pore, through which intravesicular contents are released.

Domain 3a of Munc18-1 plays a crucial role at the priming stage of exocytosis

Munc18-1 is believed to prime or stimulate SNARE-mediated membrane fusion/exocytosis through binding to the SNARE complex, in addition to chaperoning its cognate syntaxins. Nevertheless, a Munc18-1 mutant that selectively loses the priming function while retaining the syntaxin chaperoning activity has not been identified. As a consequence, the mechanism that mediates Munc18-1-dependent priming remains unclear. In the course of analyzing the functional outcomes of a variety of point mutations in domain 3a of Munc18-1, we discovered insertion mutants (K332E/K333E with insertions of 5 or 39 residues). These mutants completely lose their ability to rescue secretion whereas they effectively restore syntaxin-1 expression at the plasma membrane as well as dense-core vesicle docking in Munc18-1 and Munc18-2 double-knockdown PC12 cells. The mutants can bind syntaxin-1A in a stoichiometric manner. However, binding to the SNARE complex is impaired compared with the wild type or the hydrophobic pocket mutant (F115E). Our results suggest that the domain 3a of Munc18-1 plays a crucial role in priming of exocytosis, which is independent of its syntaxin-1 chaperoning activity and is downstream of dense-core vesicle docking. We also suggest that the priming mechanism of Munc18-1 involves its domain-3a-dependent interaction with the SNARE complex.