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The secretions of osteocalcin and bone morphogenetic protein 2 (BMP2) from living osteoblastic cells were visualized for the first time using a method of video-rate bioluminescence imaging. The fusion proteins with Gaussia luciferase (GLase) for mouse osteocalcin and BMP2 (OC-GLase and BMP2-GLase, respectively) expressed in osteoblastic MC3T3-E1 cells were correctly processed and secreted. In the video images of exocytotic secretion, the luminescence spots of OC-GLase and BMP2-GLase disappeared rapidly and gradually, respectively, indicating different manners of these proteins in diffusion. Notably, a deletion mutant of BMP2 (Δ3BMP2-GLase) lacking three basic amino acid residues in the N-terminal region for binding to heparan sulfate showed rapidly disappearing luminescence spots. In our imaging conditions, the half-life of luminescence for the spots of Δ3BMP2-GLase (1.61 ± 0.20 s) was similar to that of OC-GLase (1.22 ± 0.14 s) but not to that of BMP2-GLase (4.31 ± 0.41 s). These results suggest that, in contrast to osteocalcin, the diffusion of BMP2 from cells occurred slowly after exocytosis. Thus, our bioluminescence imaging method is useful to study the diffusion properties of secreted proteins in exocytosis., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Cell adhesion molecule 1 (CADM1) on mast cells promotes attachment and communication with neurons by homophilic binding. However, we found that mast cell CADM1 was responsible for both the attachment of mast cells to dorsal root ganglia (DRG) neurites and their calcium responses to activated DRG neurites, despite the low expression of CADM1 in DRG. Instead, nectin-3 was expressed on DRG neurons and localized to regions of cell-cell contact. A neutralizing antibody to nectin-3 inhibited both mast cell attachment and subsequent calcium responses. This suggests that heterophilic binding between CADM1 and nectin-3 mediates functional DRG-mast cell interactions in vitro., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Close apposition of nerve and mast cells is viewed as a functional unit of neuro-immune mechanisms, and it is sustained by trans-homophilic binding of cell adhesion molecule-1 (CADM1), an Ig superfamily member. Cerebral nerve-mast cell interaction might be developmentally modulated, because the alternative splicing pattern of four (a-d) types of CADM1 transcripts drastically changed during development of the mouse cerebrum: developing cerebrums expressed CADM1b and CADM1c exclusively, while mature cerebrums expressed CADM1d additionally and predominantly. To probe how individual isoforms are involved in nerve-mast cell interaction, Neuro2a neuroblastoma cells that express CADM1c endogenously were modified to express additionally either CADM1b (Neuro2a-CADM1b) or CADM1d (Neuro2a-CADM1d), and they were cocultured with mouse bone marrow-derived mast cells (BMMCs) and BMMC-derived cell line IC-2 cells, both of which expressed CADM1c. BMMCs were found to adhere to Neuro2a-CADM1d neurites more firmly than to Neuro2a-CADM1b neurites when the adhesive strengths were estimated from the femtosecond laser-induced impulsive forces minimally required for detaching BMMCs. GFP-tagging and crosslinking experiments revealed that the firmer adhesion site consisted of an assembly of CADM1d cis-homodimers. When Neuro2a cells were specifically activated by histamine, intracellular Ca(2+) concentration was increased in 63 and 38% of CADM1c-expressing IC-2 cells that attached to the CADM1d assembly site and elsewhere, respectively. These results indicate that CADM1d is a specific neuronal isoform that enhances nerve-mast cell interaction, and they suggest that nerve-mast cell interaction may be reinforced as the brain grows mature because CADM1d becomes predominant.

A team of researchers from Nagoya, Tokyo and Hamilton developed a unique technique for studying neuro-immune interaction with confocal laser scanning fluorescence microscopy several years ago. It relies on guiding immune and nerve cell interaction by creating an adhesive environment using an in vitro coculture dish. With their technique, they are able to study details of the mechanism of how nerve cells communicate with immune cells (mast cells and T lymphocytes) and vice versa. They showed that nerve-mast cell communication could occur in the absence of an intermediary transducing cell and that the neuropeptide substance P, operating via NK-1 receptors, was a soluble factor of this communication. In addition, recently, they showed that ATP which was released from activated mast cells mediated the activation of nerve cells. Further, with their technique, Nagoya's group was able to study details of the molecular mechanism of nerve-mast cell interaction. N-cadherin and CADM1 (cell adhesion molecule 1) appeared to mediate attachment and promoted the communication between mast cells and nerves predominantly. It would lead to new therapeutic modalities for diseases based on neuroimmune interaction such as neurogenic inflammation, intestinal bowel diseases, asthma, and autoimmune disorders.

Using an in vitro co-culture approach comprising cultured murine superior cervical ganglia and MC3T3-E1 osteoblast-like cells, we found that the addition of scorpion venom (SV) elicited neurite activation via intracellular Ca2+ mobilization and, after a lag period, osteoblastic Ca2+ mobilization. SV did not have any direct effect on the osteoblastic cells in the absence of neurites. The addition of an alpha1-adrenergic receptor (AR) antagonist, prazosin, dose-dependently prevented the osteoblastic activation that resulted as a consequence of neural activation by SV. These results demonstrate that osteoblastic activation occurred as a direct response to neuronal activation, which activation was mediated by alpha1-ARs in the osteoblastic cells.

Nerve-mast cell interaction is involved in both homeostatic and pathologic regulations. The molecules that sustain this association have not been identified. Because synaptic cell adhesion molecule (SynCAM), alternatively named spermatogenic Ig superfamily (SgIGSF), is expressed on both nerves and mast cells and because it binds homophilically, this molecule may be a candidate. To examine this possibility, mast cells with or without SgIGSF/SynCAM were cocultured with superior cervical ganglion neurons that express SgIGSF/SynCAM, and the number of mast cells attached to neurites was counted. The attachment of mast cells with SgIGSF/SynCAM, i.e., bone marrow-derived mast cells (BMMC) from wild-type mice, was inhibited dose-dependently by blocking Ab to SgIGSF/SynCAM. Mast cells without SgIGSF/SynCAM, i.e., BMMC from microphthalmia transcription factor-deficient mice and BMMC-derived cell line IC-2 cells, were defective in attachment to neurite, and transfection with SgIGSF/SynCAM normalized this. When the nerves were specifically activated by scorpion venom, one-quarter of the attached IC-2 cells mobilized Ca(2+) after a few dozen seconds, and ectopic SgIGSF/SynCAM doubled this proportion. At points of contact between neurites and wild-type BMMC, SgIGSF/SynCAM was locally concentrated in both neurites and BMMC. SgIGSF/SynCAM on mast cells appeared to predominantly mediate attachment and promote communication with nerves.

Mast cell-neurite interaction serves as a model for neuroimmune interaction. We have shown that neurite-mast cell communication can occur via substance P interacting with neurokinin (NK)-1 receptors on the mucosal mast cell-like cell, the rat basophilic leukemia (RBL) cell. Neurite (murine superior cervical ganglia) and RBL cell [expressing the granule-associated antigen CD63-green fluorescent protein (GFP) conjugate] cocultures were established and stimulated with bradykinin (BK; 10 nM) or scorpion venom (SV; 10 pg/ml), both of which activate only neurites. Cell activation was assessed by confocal imaging of Ca2+ (cells preloaded with fluo 3), and analyses of RBL CD63-GFP+ granule movement were conducted. Neurite activation by BK or SV was followed by RBL Ca2+ mobilization, which was inhibited by an NK-1 receptor antagonist (NK-1 RA). Moreover, membrane ruffling was observed on RBL pseudopodial extensions in contact with the activated neurite, but not on noncontacting pseudopodia. RBL membrane ruffling was inhibited by NK-1 RA, but not NK-2 RA, and was accompanied by a significant increase in granule movement (0.13 +/- 0.04 vs. 0.05 +/- 0.01 microm/s) that was most evident at the point of neurite contact: many of the granules moved toward the plasmalemma. This is the first documentation of such precise (restricted to the membrane's contact site) transfer of information between nerves and mast cells that could allow for very subtle in vivo communication between these two cell types.

Communication between nerves and mast cells is a prototypic demonstration of neuro-immune interaction. Recently, we used an in vitro co-culture approach comprising cultured murine superior cervical ganglia (SCG) and rat basophilic leukemia (RBL) cells to study this interaction. Previously, we concentrated mainly on the activation signal from neurites to mast cells (RBL). However, it is proposed that mast cell-nerve communication is not a one-sided relationship but a bi-directional one. In the present work, we studied the communication from mast cells to neurites. We observed that binding of anti-IgE receptor antibodies to mast cells increases calcium ion concentration [Ca2+]i in SCG neurites. This indicates that mast cell-nerve communication is bi-directional. Confocal fluorescence microscopic images indicated that [Ca2+]i in neurites increased after an increase of [Ca2+]i in mast cells. The lag-time of neurite activation was several times longer than that of mast cell activation. The correlation coefficient between the lag-times for mast cell and nerve activation was calculated to be 0.81. In addition, the fluorescence images showed that calcium signals in SCG neurites were able to extend to a long distance (100-200 microm) from the site where mast cells (RBL) attached to neurites.

Communication between nerves and mast cells is a prototypic demonstration of neuroimmune interaction. We used an in vitro co-culture approach comprising cultured murine superior cervical ganglia (SCG) and rat basophilic leukemia (RBL-2H3) cells. Atomic force microscopy (AFM) showed how neurites attached to a pseudopodium or a cell body of an RBL cell. After stimulation of SCG neurites with bradykinin or scorpion venom, RBL cells attached to neurites spread and flattened, and several discharged granules (0. 5-1.0 microm in diameter) were found on the surface of the RBL cells. A neurokinin (NK)-1 receptor (i.e. substance P receptor) antagonist prevented the RBL degranulation. The results showed that activation of the SCG neurites with bradykinin or scorpion venom was able to elicit degranulation in RBL cells which were attached to neurites.

Communication between nerves and mast cells is a prototypic demonstration of neuroimmune interaction. However, whether mast cell activation occurs as a direct response to neuronal activation or requires an intermediary cell is unclear. Addressing this issue, we used an in vitro coculture approach comprising cultured murine superior cervical ganglia and rat leukemia basophilic cells (RBLs; possesses properties of mucosal-type mast cells). Following loading with the calcium fluorophore, Fluo-3, neurite-RBL units (separated by <50 nm) were examined by confocal laser scanning microscopy. Addition of bradykinin, or scorpion venom, dose-dependently elicited neurite activation (i.e., Ca2+ mobilization) and, after a lag period, RBL Ca2+ mobilization. Neither bradykinin nor scorpion venom had any direct effect on the RBLs in the absence of neurites. Addition of a neutralizing substance P Ab or a neurokinin (NK)-1 receptor antagonist, but not an NK-2 receptor antagonist, dose-dependently prevented the RBL activation that resulted as a consequence of neural activation by either bradykinin or scorpion venom. These data illustrate that nerve-mast cell cross-talk can occur in the absence of an intermediary transducing cell and that the neuropeptide substance P, operating via NK-1 receptors, is an important mediator of this communication. Our findings have implications for the neuroimmune signaling cascades that are likely to occur during airways inflammation, intestinal hypersensitivity, and other conditions in which mast cells feature.