Two-photon microscopic analysis of acetylcholine-induced mucus secretion in guinea pig nasal glands

A revised conceptual framework for mouse vomeronasal pumping and stimulus sampling

The physiological performance of any sensory organ is determined by its anatomy and physical properties. Consequently, complex sensory structures with elaborate features have evolved to optimize stimulus detection. Understanding these structures and their physical nature forms the basis for mechanistic insights into sensory function. Despite its crucial role as a sensor for pheromones and other behaviorally instructive chemical cues, the vomeronasal organ (VNO) remains a poorly characterized mammalian sensory structure. Fundamental principles of its physico-mechanical function, including basic aspects of stimulus sampling, remain poorly explored. Here, we revisit the classical vasomotor pump hypothesis of vomeronasal stimulus uptake. Using advanced anatomical, histological, and physiological methods, we demonstrate that large parts of the lateral mouse VNO are composed of smooth muscle. Vomeronasal smooth muscle tissue comprises two subsets of fibers with distinct topography, structure, excitation-contraction coupling, and, ultimately, contractile properties. Specifically, contractions of a large population of noradrenaline-sensitive cells mediate both transverse and longitudinal lumen expansion, whereas cholinergic stimulation targets an adluminal group of smooth muscle fibers. The latter run parallel to the VNO's rostro-caudal axis and are ideally situated to mediate antagonistic longitudinal constriction of the lumen. This newly discovered arrangement implies a novel mode of function. Single-cell transcriptomics and pharmacological profiling reveal the receptor subtypes involved. Finally, 2D/3D tomography provides non-invasive insight into the intact VNO's anatomy and mechanics, enables measurement of luminal fluid volume, and allows an assessment of relative volume change upon noradrenergic stimulation. Together, we propose a revised conceptual framework for mouse vomeronasal pumping and, thus, stimulus sampling.

Bei Mu Gua Lou San facilitates mucus expectoration by increasing surface area and hydration levels of airway mucus in an air-liquid-interface cell culture model of the respiratory epithelium

BACKGROUND

Bei Mu Gua Lou San (BMGLS) is an ancient formulation known for its moisturizing and expectorant properties, but the underlying mechanisms remain unknown. We investigated concentration-dependent effects of BMGLS on its rehydrating and mucus-modulating properties using an air-liquid-interface (ALI) cell culture model of the Calu-3 human bronchial epithelial cell line and primary normal human bronchial epithelial cells (NHBE), and specifically focused on quantity and composition of the two major mucosal proteins MUC5AC and MUC5B.

METHODS

ALI cultures were treated with BMGLS at different concentrations over three weeks and evaluated by means of histology, immunostaining and electron microscopy. MUC5AC and MUC5B mRNA levels were assessed and quantified on protein level using an automated image-based approach. Additionally, expression levels of the major mucus-stimulating enzyme 15-lipoxygenase (ALOX15) were evaluated.

RESULTS

BMGLS induced concentration-dependent morphological changes in NHBE but not Calu-3 ALI cultures that resulted in increased surface area via the formation of herein termed intra-epithelial structures (IES). While cellular rates of proliferation, apoptosis or degeneration remained unaffected, BMGLS caused swelling of mucosal granules, increased the area of secreted mucus, decreased muco-glycoprotein density, and dispensed MUC5AC. Additionally, BMGLS reduced expression levels of MUC5AC, MUC5B and the mucus-stimulating enzyme 15-lipoxygenase (ALOX15).

CONCLUSIONS

Our studies suggest that BMGLS rehydrates airway mucus while stimulating mucus secretion by increasing surface areas and regulating goblet cell differentiation through modulating major mucus-stimulating pathways.

Major differences in the larval anatomy of the digestive and excretory systems of three Oestridae species revealed by micro-CT

Oestrid flies (Diptera: Oestridae) do not feed during the adult stage, so they depend on an efficient assimilation and storage of nutrients during their parasitic larval stage. We describe the general morphology and provide volumetric data for the digestive and excretory organs of the three larval instars of the nasal bot fly Oestrus ovis L., using micro-computed tomography. The size of the digestive and excretory organs greatly increased across larval instars. In all instars, the two salivary glands were remarkably large and formed a 'glandular band' by coming together, but without lumina uniting, at their posterior ends. The distal region of the anterior Malpighian tubules was greatly enlarged and full of highly radio-opaque concretions. Moreover, the anatomy of O. ovis third-instar larva was compared to that of two species of, respectively, similar and different feeding habits: Cephenemyia stimulator (Clark) and Hypoderma actaeon Brauer. Whereas the general morphology and arrangement of the digestive and excretory systems of C. stimulator was similar to that of O. ovis, some differences were observed in H. actaeon: a swollen anterior region of the midgut, salivary glands shorter and not forming a 'band' and anterior Malpighian tubules narrowly uniform throughout their entire length.

Ultrasensitive imaging of Ca2+ dynamics in pancreatic acinar cells of yellow cameleon-nano transgenic mice

Yellow Cameleons are genetically encoded Ca²⁺ indicators in which cyan and yellow fluorescent proteins and calmodulin work together as a fluorescence (Förster) resonance energy transfer Ca²⁺-sensor probe. To achieve ultrasensitive Ca²⁺ imaging for low resting Ca²⁺ or small Ca²⁺ transients in various organs, we generated a transgenic mouse line expressing the highest-sensitive genetically encoded Ca²⁺ indicator (Yellow Cameleon-Nano 15) in the whole body. We then focused on the mechanism of exocytotic events mediated by intracellular Ca²⁺ signaling in acinar cells of the mice with an agonist and observed them by two-photon excitation microscopy. In the results, two-photon excitation imaging of Yellow Cameleon-Nano 15 successfully visualized intracellular Ca²⁺ concentration under stimulation with the agonist at nanomolar levels. This is the first demonstration for application of genetically encoded Ca²⁺ indicators to pancreatic acinar cells. We also simultaneously observed exocytotic events and an intracellular Ca²⁺ concentration under in vivo condition. Yellow Cameleon-Nano 15 mice are healthy and no significant deteriorative effect was observed on physiological response regarding the pancreatic acinar cells. The dynamic range of 165% was calculated from R_max and R_min values under in vivo condition. The mice will be useful for ultrasensitive Ca²⁺ imaging in vivo.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

A novel function of Noc2 in agonist-induced intracellular Ca2+ increase during zymogen-granule exocytosis in pancreatic acinar cells

Noc2, a putative Rab effector, contributes to secretory-granule exocytosis in neuroendocrine and exocrine cells. Here, using two-photon excitation live-cell imaging, we investigated its role in Ca²⁺-dependent zymogen granule (ZG) exocytosis in pancreatic acinar cells from wild-type (WT) and Noc2-knockout (KO) mice. Imaging of a KO acinar cell revealed an expanded granular area, indicating ZG accumulation. In our spatiotemporal analysis of the ZG exocytosis induced by agonist (cholecystokinin or acetylcholine) stimulation, the location and rate of progress of ZG exocytosis did not differ significantly between the two strains. ZG exocytosis from KO acinar cells was seldom observed at physiological concentrations of agonists, but was normal (vs. WT) at high concentrations. Flash photolysis of a caged calcium compound confirmed the integrity of the fusion step of ZG exocytosis in KO acinar cells. The decreased ZG exocytosis present at physiological concentrations of agonists raised the possibility of impaired elicitation of calcium spikes. When calcium spikes were evoked in KO acinar cells by a high agonist concentration: (a) they always started at the apical portion and traveled to the basal portion, and (b) calcium oscillations over the 10 µM level were observed, as in WT acinar cells. At physiological concentrations of agonists, however, sufficient calcium spikes were not observed, suggesting an impaired [Ca²⁺]_i-increase mechanism in KO acinar cells. We propose that in pancreatic acinar cells, Noc2 is not indispensable for the membrane fusion of ZG per se, but instead performs a novel function favoring agonist-induced physiological [Ca²⁺]_i increases.

Exocytosis in islet beta-cells

The development of technologies that allow for live optical imaging of exocytosis from beta-cells has greatly improved our understanding of insulin secretion. Two-photon imaging, in particular, has enabled researchers to visualize the exocytosis of large dense-core vesicles (LDCVs) containing insulin from beta-cells in intact islets of Langerhans. These studies have revealed that high glucose levels induce two phases of insulin secretion and that this release is dependent upon cytosolic Ca(2+) and cAMP. This technology has also made it possible to examine the spatial profile of insulin exocytosis in these tissues and compare that profile with those of other secretory glands. Such studies have led to the discovery of the massive exocytosis of synaptic-like microvesicles (SLMVs) in beta-cells. These imaging studies have also helped clarify facets of insulin exocytosis that cannot be properly addressed using the currently available electrophysiological techniques. This chapter provides a concise introduction to the field of optical imaging for those researchers who wish to characterize exocytosis from beta-cells in the islets of Langerhans.

Visualizing form and function in organotypic slices of the adult mouse parotid gland

An organotypic slice preparation of the adult mouse parotid salivary gland amenable to a variety of optical assessments of fluid and protein secretion dynamics is described. The semi-intact preparation rendered without the use of enzymatic treatment permitted live-cell imaging and multiphoton analysis of cellular and supracellular signals. Toward this end we demonstrated that the parotid slice is a significant addition to the repertoire of tools available to investigators to probe exocrine structure and function since there is currently no cell culture system that fully recapitulates parotid acinar cell biology. Importantly, we show that a subpopulation of the acinar cells of parotid slices can be maintained in short-term culture and retain their morphology and function for up to 2 days. This in vitro model system is a significant step forward compared with enzymatically dispersed acini that rapidly lose their morphological and functional characteristics over several hours, and it was shown to be long enough for the expression and trafficking of exogenous protein following adenoviral infection. This system is compatible with a variety of genetic and physiological approaches used to study secretory function.

Decreased olfactory mucus secretion and nasal abnormality in mice lacking type 2 and type 3 IP3 receptors

Although nasal mucus is thought to play important roles in the mammalian olfactory system, the mechanisms of secretion of it and its physiological roles are poorly understood. Here we show that type 2 and type 3 IP3 receptors (IP3R2 and IP3R3) play critical roles in olfactory mucus secretion. Histological studies showed that IP3R2 and IP3R3 are predominantly expressed in two types of nasal glands, the anterior glands of the nasal septum and the lateral nasal glands (LNG), which contain mucosal proteins secreted to the main olfactory epithelium. We therefore examined LNG acinar cells, and found that acetylcholine-mediated calcium responses and fluid- and protein- secretion in the acinar cells were markedly decreased in IP3R2-R3 double-knockout (KO) mice. We also found nasal inflammation and a decrease in olfactory capacity in IP3R2-R3 KO mice. Despite intact signal transduction in the olfactory epithelium, IP3R2-R3 KO mice exhibited elevated threshold sensitivity to odorants on in vivo imaging of olfactory glomerular responses and behavioral tests. Our findings suggest that IP3R2 and IP3R3 mediate nasal mucus secretion, which is important for the maintenance of nasal tissue as well as the perception of odors.

[Recent progress in membrane dynamics research by two-photon microscopy]

Two-photon microscopy is a less-invasive cross-sectional imaging technique for long-term visualization of living cells within deeper layers of organs. This microscopy is based on the multi-photon excitation process and has been used widely in medical and biological sciences. An attractive property of two-photon microscopy, multicolor excitation capability has enabled quantification of spatiotemporal patterns of [Ca(2+)]i, ion transport and single episodes of fusion pore openings during exocytosis. In pancreatic acinar cells, we have successfully demonstrated the existence of "sequential compound exocyotosis" for the first time. Sequential compound exocytosis has subsequently been identified in a wide variety of secretory cells including exocrine, endocrine and blood cells. Further exploration has revealed dynamics and physiological roles of actin cytoskeleton, and soluble NSF attachment receptor (SNARE) proteins. In addition, our newly developed method (TEPIQ method) can be used to determine fusion pores and the diameters of vesicles smaller than the diffraction-limited resolution. Recently, we have successfully observed neurons deeper than 0.9 mm from the brain cortex surface in an anesthetized mouse. We have also improved the spatial resolution needed to visualize fine structures of basal dendrites in layer V in vivo. This microscopy also can be used to visualize dendritic spines, axon terminals and miroglia cells, suggesting that we can follow long-term changes of neural or glial cells in a living mouse. Two-photon microscopy will thus be important in advancing the study of the molecular basis of physiological and pathological events in the human body.

New advances in nanomedicine: diagnosis and preventive medicine

This article describes the use of nanobiologic techniques in diagnosis and preventive medicine. It discusses the engineering of fluorescent and bioluminescent proteins to visualize biologic functions; single-molecule measurements of protein structure and function; two-photon microscopy for molecular imaging of the structure and function in living cells and tissues; and imaging ligand-induced protein conformations and interactions by fluorescence in single living cells.

Exocytic process analyzed with two-photon excitation imaging in endocrine pancreas

To elucidate the spatiotemporal profiles of final secretory stage, we have established two-photon extracellular polar tracer (TEP) imaging, with which we can quantify all exocytic events in the plane of focus within the intact tissues. With such technique, we can estimate the precise diameters of vesicles independently of the spatial resolution of optical microscope, and measure the fusion pore dynamics at nanometer resolution. At insulin exocytosis in the pancreatic islets, it took two seconds for the fusion pore to dilate from 1.4 nm in diameter to 6 nm in diameter, and such unusual stability of the pore may be due to the crystallization of the intragranular contents. Opening of the pore was preceded by unrestricted lateral diffusion of lipids along the inner wall of the pores, supporting the idea that this structure was mainly composed of membrane lipids. TEP imaging has been also applied to other representative secretory glands, and has revealed hitherto unexpected diversity in spatial organizations of exocytosis and endocytosis, which are relevant for physiology and pathology of secretory tissues. In the pancreatic islet, compound exocytosis was characteristically inhibited (<5%), partly due to the rarity of SNAP25 redistribution into the exocytosed vesicle membrane. Such mechanisms necessitate transport of insulin granules to the cell surface for fusion, and possibly rendering exocytosis more sensitive to metabolic state. Two-photon imaging will be powerful tools to elucidate molecular and cellular mechanisms of exocytosis and related disease, and to develop new therapeutic agencies as well as diagnostic tools.

Two cAMP-dependent pathways differentially regulate exocytosis of large dense-core and small vesicles in mouse beta-cells

It has been reported that cAMP regulates Ca(2+)-dependent exocytosis via protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac) in neurons and secretory cells. It has, however, never been clarified how regulation of Ca(2+)-dependent exocytosis by cAMP differs depending on the involvement of PKA and Epac, and depending on two types of secretory vesicles, large dense-core vesicles (LVs) and small vesicles (SVs). In this study, we have directly visualized Ca(2+)-dependent exocytosis of both LVs and SVs with two-photon imaging in mouse pancreatic beta-cells. We found that marked exocytosis of SVs occurred with a time constant of 0.3 s, more than three times as fast as LV exocytosis, on stimulation by photolysis of a caged-Ca(2+) compound. The diameter of SVs was identified as approximately 80 nm with two-photon imaging, which was confirmed by electron-microscopic investigation with photoconversion of diaminobenzidine. Calcium-dependent exocytosis of SVs was potentiated by the cAMP-elevating agent forskolin, and the potentiating effect was unaffected by antagonists of PKA and was mimicked by the Epac-selective agonist 8-(4-chlorophenylthio)-2'-O-methyl cAMP, unlike that on LVs. Moreover, high-glucose stimulation induced massive exocytosis of SVs in addition to LVs, and photolysis of caged cAMP during glucose stimulation caused potentiation of exocytosis with little delay for SVs but with a latency of 5 s for LVs. Thus, Epac and PKA selectively regulate exocytosis of SVs and LVs, respectively, in beta-cells, and Epac can regulate exocytosis more rapidly than PKA.

Two-photon excitation imaging of exocytosis and endocytosis and determination of their spatial organization

Two-photon excitation imaging is the least invasive optical approach to study living tissues. We have established two-photon extracellular polar-tracer (TEP) imaging with which it is possible to visualize and quantify all exocytic events in the plane of focus within secretory tissues. This technology also enables estimate of the precise diameters of vesicles independently of the spatial resolution of the optical microscope, and determination of the fusion pore dynamics at nanometer resolution using TEP-imaging based quantification (TEPIQ). TEP imaging has been applied to representative secretory glands, e.g., exocrine pancreas, endocrine pancreas, adrenal medulla and a pheochromocytoma cell line (PC12), and has revealed unexpected diversity in the spatial organization of exocytosis and endocytosis crucial for the physiology and pathology of secretory tissues and neurons. TEP imaging and TEPIQ analysis are powerful tools for elucidating the molecular and cellular mechanisms of exocytosis and certain related diseases, such as diabetes mellitus, and the development of new therapeutic agents and diagnostic tools.

Compound Exocytosis: Mechanisms and Functional Significance

Compound exocytosis occurs in many cell types. It represents a specialized form of secretion in which vesicles undergo fusion with each other as well as with the plasma membrane. In most cases, compound exocytosis occurs sequentially, with deeper-lying vesicles fusing, after a delay, with vesicles that have already fused with the plasma membrane. However, in some cells, vesicles can also apparently fuse with each other intracellularly before any interaction with the plasma membrane. In this review, we discuss the general features of compound exocytosis, and the features that are specific to particular cells. We consider mechanisms that might impose the requirement for vesicles to fuse with the plasma membrane before they become able to fuse with each other, the possibility that there are biochemical differences between vesicle-plasma membrane fusion events and subsequent secondary homotypic vesicle fusion events, and the role that cytoskeletal elements might play in the stabilization of fused vesicles, in order to permit secondary fusion events. Finally, we discuss the likely physiological significance of compound exocytosis in the various cell types in which it exists.

Sequential compound exocytosis of large dense-core vesicles in PC12 cells studied with TEPIQ (two-photon extracellular polar-tracer imaging-based quantification) analysis

We investigated exocytosis of PC12 cells using two-photon excitation imaging and extracellular polar tracers (TEP imaging) at the basal region of PC12 cells adjacent to the glass cover slip. TEPIQ (two-photon extracellular polar-tracer imaging-based quantification) analysis revealed that most exocytosis was mediated by large dense-core vesicles (LVs) with a mean diameter of 220 nm, and that exocytosis of LVs occurred slowly with a mean latency of approximately 7 s even though exocytosis was induced with large increases in cytosolic Ca2+ concentration by uncaging of a caged-Ca2+ compound. We also found that 97% of exocytic LVs remained poised at the plasma membrane, 72% maintained their fusion pores in an open conformation for more than 30 s, and 76% triggered sequential compound exocytosis of vesicles that were located deeper in the cytosol. Sequential compound exocytosis by PC12 cells was confirmed by electron microscopic investigation with photoconversion of diaminobenzidine by FM1-43 (a polar membrane tracer). Our data suggest that pre-stimulus docking of LVs to the plasma membrane does not necessarily hasten the fusion reaction, while docking and resulting stability of exocytic LVs facilitates sequential compound exocytosis, and thereby allowing mobilization of deep vesicles.

A new quantitative (two-photon extracellular polar-tracer imaging-based quantification (TEPIQ)) analysis for diameters of exocytic vesicles and its application to mouse pancreatic islets

We have developed an imaging approach to estimate the diameter of exocytic vesicles that are smaller than the resolution of an optical microscope and present within intact tissue. This approach is based on two-photon excitation imaging of polar tracers in the extracellular medium, is designated TEPIQ (two-photon extracellular polar-tracer imaging-based quantification), and has three variants. TEPIQ analysis of DeltaV measures vesicle volume with a fluid-phase tracer, sulforhodamine B (SRB). TEPIQ analysis of DeltaS determines vesicle surface area with a polar membrane tracer, FM1-43. TEPIQ analysis of DeltaV/DeltaS estimates vesicle diameter from the SRB/FM1-43 fluorescence ratio. TEPIQ analysis is insensitive to microscope settings because the same setup is used for calibration and actual experiments. We tested the validity of TEPIQ with glucose-induced exocytosis from beta-cells within pancreatic islets. The three TEPIQ variants yielded estimates for the mean diameter of exocytic vesicles of between 340 and 390 nm, consistent with the size of insulin granules. TEPIQ analysis relies on the combination of two-photon excitation imaging, the narrow intercellular spaces of intact tissue, and the presence of diffusible polar tracers in the extracellular medium. It allows quantitative imaging of exocytosis within secretory organs, yielding estimates of vesicle diameter with nanometer resolution.