Immunocytochemical studies of major gap junction proteins in rat salivary glands

Structural and functional analysis of salivary intercalated duct cells reveals a secretory phenotype

Currently, all salivary ducts (intercalated, striated and collecting) are assumed to function broadly in a similar manner, reclaiming ions that were secreted by the secretory acinar cells while preserving fluid volume and delivering saliva to the oral cavity. Nevertheless, there has been minimal investigation into the structural and functional differences between distinct types of salivary duct cells. Therefore, in this study, the expression profile of proteins involved in stimulus-secretion coupling, as well as the function of the intercalated duct (ID) and striated duct cells, was examined. Particular focus was placed on defining differences between distinct duct cell populations. To accomplish this, immunohistochemistry and in situ hybridization were utilized to examine the localization and expression of proteins involved in reabsorption and secretion of ions and fluid. Further, in vivo calcium imaging was employed to investigate cellular function. Based on the protein expression profile and functional data, marked differences between the IDs and striated ducts were observed. Specifically, the ID cells express proteins native to the secretory acinar cells while lacking proteins specifically expressed in the striated ducts. Further, the ID and striated duct cells display different calcium signalling characteristics, with the IDs responding to a neural stimulus in a manner similar to the acinar cells. Overall, our data suggest that the IDs have a distinct role in the secretory process, separate from the reabsorptive striated ducts. Instead, based on our evidence, the IDs express proteins found in secretory cells, generate calcium signals in a manner similar to acinar cells, and, therefore, are likely secretory cells. KEY POINTS: Current studies examining salivary intercalated duct cells are limited, with minimal documentation of the ion transport machinery and the overall role of the cells in fluid generation. Salivary intercalated duct cells are presumed to function in the same manner as other duct cells, reclaiming ions, maintaining fluid volume and delivering the final saliva to the oral cavity. Here we systematically examine the structure and function of the salivary intercalated duct cells using immunohistochemistry, in situ hybridization and by monitoring in vivo Ca2+ dynamics. Structural data revealed that the intercalated duct cells lack proteins vital for reabsorption and express proteins necessary for secretion. Ca2+ dynamics in the intercalated duct cells were consistent with those observed in secretory cells and resulted from GPCR-mediated IP3 production.

Gap junction-mediated cell-to-cell communication in oral development and oral diseases: a concise review of research progress

Homoeostasis depends on the close connection and intimate molecular exchange between extracellular, intracellular and intercellular networks. Intercellular communication is largely mediated by gap junctions (GJs), a type of specialized membrane contact composed of variable number of channels that enable direct communication between cells by allowing small molecules to pass directly into the cytoplasm of neighbouring cells. Although considerable evidence indicates that gap junctions contribute to the functions of many organs, such as the bone, intestine, kidney, heart, brain and nerve, less is known about their role in oral development and disease. In this review, the current progress in understanding the background of connexins and the functions of gap junctions in oral development and diseases is discussed. The homoeostasis of tooth and periodontal tissues, normal tooth and maxillofacial development, saliva secretion and the integrity of the oral mucosa depend on the proper function of gap junctions. Knowledge of this pattern of cell-cell communication is required for a better understanding of oral diseases. With the ever-increasing understanding of connexins in oral diseases, therapeutic strategies could be developed to target these membrane channels in various oral diseases and maxillofacial dysplasia.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Otoacoustic emissions at compensated middle ear pressure in children

Middle ear pathology has a negative effect on the detectability of otoacoustic emissions. In this study, we investigated the effect of compensating a deviant static middle ear pressure while measuring transient evoked otoacoustic emissions (TEOAEs). In 59 children (mean age 4 years, 5 months) TEOAEs were measured twice in one session: first at ambient pressure and than at compensated middle ear pressure. On average, TEOAE amplitudes increased by 1.9 dB as a result of middle ear pressure compensation. The amplitude increase was largest in frequency bands centred at 1 and 2 kHz and a statistically significant correlation was found between the amount of compensated pressure and the TEOAE amplitude increase. In the higher frequency bands centred at 3 and 4 kHz, TEOAE amplitudes were almost insensitive to pressure compensation. These results show that measuring OAEs at compensated middle ear pressure enhances the amplitude of TEOAEs, and thus improves the detectability.

Morphological and functional changes of the rat parotid glandular cells by clipping and reopening the parotid duct, using HAM8 antibody

The purpose of this experiment is to examine the proliferative process of rat acinar cells after parotid duct ligation and reopening. Two experimental groups were observed. The first group was killed from 0 to 14 days after the duct ligation. In the second group, the duct was clipped for 14 days, and it was reopened. Following a period of from 2 to 28 days after removal of the clip, the glands were removed to perform a histological analysis, including hematoxylin-eosin (HE), immunofluorescent staining using HAM8 antibody, which recognizes connexin 32, and transmission electron microscopy (TEM). In the experimental gland from the 1st group at 6 days after ligation (I-6D), the acinar cells disappeared. In the tissue from the 2nd group 8 days after reopening (II-8D), newly formed acinar cells were found again. Lobular structure of the parotid glands recovered in the II-21D. HAM8 signals were observed between normal acinar cells, while they declined in the tissue from I-1D, and they were not observed in the I-2D. HAM8 signals were first observed in the II-25D and then subsequently returned to normal levels in the II-28D. These results suggest that the intercellular communication and functional recovery was not complete 25 days after reopening of the duct.In conclusion, the recovery of the acinar structure was recognized during an extended period of duct ligation, however, a time lag between the morphological and functional recovery was found to exist.

Distortion product otoacoustic emissions at 6 months in term infants after perinatal hypoxia-ischaemia or with a low Apgar score

Distortion product otoacoustic emissions (DPOAEs) were analyzed at 6 months of age in term infants who had perinatal hypoxia-ischaemia (HI) and those who had a low Agar score alone to detect any abnormalities in cochlear function and any difference between the two groups of infants. The f2 primary tone was presented at ten frequencies (0.5-10 kHz). Both the left and right ears were tested. Compared to normal term controls, both the infants after perinatal HI and those with a low Apgar score alone showed lower DPOAE pass rates, mainly at 1-4 kHz at which the pass rates were decreased significantly (P < 0.05-0.01). The general pattern of DPOAE pass rates at different frequencies was similar in the two groups of infants. The pass rates at 1, 5 and 6 kHz tended to be lower in the infants after perinatal HI than those with a low Apgar score alone, although not statistically significant. Conclusions At 6 months of age, infants after perinatal HI or with a low Apgar score alone had a relatively poor cochlear function, mainly at 1-4 kHz. There are no major differences between the two groups of infants.

Gap junctional intercellular communication capacity by gap-FRAP technique: A comparative study

Gap junctions play an important role in vital functions, including the regulation of cell growth and cell differentiation. Connexins 43 (Cx43) are the most widely expressed gap junction proteins. Cellular localization of phosphorylated Cx43 has been implicated in the capacity of gap junctional intercellular communication (GJIC). To follow the functionality of GJIC of different cell types, in monolayer cultures, characterized by different patterns of phosphorylated Cx43, we used a fluorescence recovery after photobleaching (FRAP) technique, and compared two tracers, 5(6)-carboxyfluorescein diacetate (CFDA) and calcein acetoxymethylester (AM). The GJIC capacity was quantified by estimating fluorescence redistribution parameters. The functionality of GJIC was in relation with the staining localization of phosphorylated Cx43 to the cell-cell contact areas, corresponding to gap junctions between contacting cells. GJIC involvement in fluorescence restitution after photobleaching was checked by a gap junction channel inhibition assay. We demonstrated that the choice of the dye did not significantly influence the fluorescence recovery percentages despite a cell line-dependent CFDA release, whereas it had an important impact on fluorescence kinetic profiles. This study reinforces the interest of the gap-FRAP approach to quantify modifications in the functionality of gap junctions and, above all, argues about the limits of CFDA for 3-D future approaches.

Involvement of gap junctional communication in secretion

Glands were the first type of tissues in which the permissive role of gap junctions in the cell-to-cell transfer of membrane-impermeant molecules was shown. During the 40 years that have followed this seminal finding, gap junctions have been documented in all types of multicellular secretory systems, whether of the exocrine, endocrine or pheromonal nature. Also, compelling evidence now indicates that gap junction-mediated coupling, and/or the connexin proteins per se, play significant regulatory roles in various aspects of gland functions, ranging from the biosynthesis, storage and release of a variety of secretory products, to the control of the growth and differentiation of secretory cells, and to the regulation of gland morphogenesis. This review summarizes this evidence in the light of recent reports.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Expression of connexin 32 and 43 in developing rat submandibular salivary glands

The expression of the gap-junction proteins connexin 32 and 43 in the developing rat submandibular gland was determined using reverse transcription-polymerase chain reaction and immunohistochemistry. Sprague-Dawley rats from the 17th gestational day through to the 28th postnatal day were used. Connexin 43 gene expression was detected on the 17th gestational day. Connexin 32 gene expression was also detected on the 17th gestational day but less intensely. Immunolabelling for connexin 43 was also found in the developing submandibular gland from the 17th gestational day. Prenatally the most immunoreactive areas for connexin 43 were located in the periphery of terminal tubules, but some staining was discernible between the cells in terminal buds. In the postnatal period, reactive areas were located at both the periphery of acinus-like structures and around the intercalated duct. This is consistent with the known association of connexin 43 with myoepithelial cells. Connexin 32 immunostaining was first detected in the developing submandibular glands on the 18th day of prenatal development. Positive staining was present on the lateral side of the proacinar and mature acinar cells. The number of immunoreactive areas per cell increased during early development followed by a significant decrease perinatally. During postnatal development the density of areas again showed a pattern of increase. These results suggest that connexin 43 is associated with growth and differentiation in the pre- and perinatal periods and also with the contractile function of myoepithelial cells in the postnatal period of the developing submandibular gland. It is also implied that the number of connexin 32-positive areas may correspond to an increase or decrease in the number of proacinar and mature acinar cells.

Effect of negative middle-ear pressure on transient-evoked otoacoustic emissions

OBJECTIVE

The purpose of the study was to illustrate the effect of negative middle-ear pressure (MEP) on both the stimulus and response of transient-evoked otoacoustic emissions (TEOAEs) and the effect of compensating for negative pressure in the middle ear by pneumatically introducing pressure into the ear canal. Simulation of negative MEP by introducing positive pressure into the ear canal also was examined.

DESIGN

TEOAEs were measured over 6 mo in a subject who frequently had negative MEP out to -150 daPa. Compensation was done for MEPs of -105, -135, and -165 daPa. Simulation of negative pressure was done for these same pressures. The effect of a pressure differential across the eardrum on the stimulus spectrum was measured at 100, 200, and 300 daPa. All measurements were made on the same subject.

RESULTS

Small amounts of negative MEP significantly affected both stimulus and response spectra. The simulated negative MEP approximated actual MEP at MEPs of -105 and -135 daPa. At -165 daPa, a divergence between the two spectra occurred below 2.0 kHz. Compensation for negative MEP by pneumatically introducing pressure into the ear canal essentially returned both spectra to that seen when the MEP was close to ambient pressure, at least for frequencies above 1.5 to 2.0 kHz. At lower frequencies, compensation resulted in increased TEOAE amplitude relative to the amplitude at ambient pressure.

CONCLUSIONS

Small amounts of negative MEP may affect TEOAE spectra and potentially influence the reliability of the test. For long-term monitoring of TEOAEs, MEPs either should be near ambient pressure or should be compensated for by an equivalent pressure in the ear canal.