Expression of anion exchangers in cultured human endolymphatic sac epithelia

“Reversed polarization” of Na/K-ATPase—a sign of inverted transport in the human endolymphatic sac: a super-resolution structured illumination microscopy (SR-SIM) study

The human endolymphatic sac (ES) is believed to regulate inner ear fluid homeostasis and to be associated with Meniere’s disease (MD). We analyzed the ion transport protein sodium/potassium-ATPase (Na/K-ATPase) and its isoforms in the human ES using super-resolution structured illumination microscopy (SR-SIM). Human vestibular aqueducts were collected during trans-labyrinthine vestibular schwannoma surgery after obtaining ethical permission. Antibodies against various isoforms of Na/K-ATPase and additional solute-transporting proteins, believed to be essential for ion and fluid transport, were used for immunohistochemistry. A population of epithelial cells of the human ES strongly expressed Na/K-ATPase α1, β1, and β3 subunit isoforms in either the lateral/basolateral or apical plasma membrane domains. The β1 isoform was expressed in the lateral/basolateral plasma membranes in mostly large cylindrical cells, while β3 and α1 both were expressed with “reversed polarity” in the apical cell membrane in lower epithelial cells. The heterogeneous expression of Na/K-ATPase subunits substantiates earlier notions that the ES is a dynamic structure where epithelial cells show inverted epithelial transport. Dual absorption and secretion processes may regulate and maintain inner ear fluid homeostasis. These findings may shed new light on the etiology of endolymphatic hydrops and MD.

Pathophysiologic Findings in the Human Endolymphatic Sac in Endolymphatic Hydrops: Functional and Molecular Evidence

BACKGROUND

The endolymphatic sac (ES) is a cystic structure situated on the posterior fossa dura and is connected to the luminal space of the vestibular organ through the endolymphatic duct, which branches into the utricular and saccular ducts. Unlike the cochlea and vestibule, the ES does not contain sensory epithelium in its luminal space, and a single layer of epithelial cells line the luminal surface area. The ES in the inner ear is thought to play a role in the regulation of inner ear homeostasis, fluid volume, and immune reaction. If these functions of the ES are disrupted, dysfunction of the inner ear may develop. The most well-known pathology arising from dysfunction of the ES is endolymphatic hydrops, characterized by an enlarged endolymphatic space due to the accumulation of excessive endolymphatic fluid. Although, molecular identities and functional evidence for the roles were identified in animal studies, basic studies of the human ES are relatively uncommon compared with those using animal tissues, because of limited opportunity to harvest the human ES.

METHODS

In this study, molecular and functional evidence for the role of the human ES in the development of endolymphatic hydrops are reviewed.

RESULTS AND CONCLUSIONS

Although evidence is insufficient, studies using the human ES have mostly produced findings similar to those of animal studies. This review may provide a basis for planning further studies to investigate the pathophysiology of disorders with the finding of endolymphatic hydrops.

The Human Vestibular Aqueduct: Anatomical Characteristics and Enlargement Criteria

HYPOTHESIS

The human vestibular aqueduct (VA) shows great anatomical variations, and imaging can be difficult, so we need more data on the normal anatomy of the VA for better radiologic evaluation of large vestibular aqueduct syndrome (LVAS).

BACKGROUND

The normal anatomy of the human VA was analyzed in micro-dissected human temporal bones.

METHODS

The study is based on two sets of human temporal bones. One set of 32 human temporal bones was selected from a collection of 50 micro-dissected specimens. The outline of the intraosseous portion of the VA was drawn and digitized, and dimensions were assessed. The other set of 20 plastic molds were randomly selected from a collection of 324 specimens, and the VA dimensions were assessed.

RESULTS

Measurements from this study are presented in means, standard deviations, and ranges. The results from these measurements are considered normal and compared with previously published data. The variations in the normal anatomy of the VA are presented and discussed.

CONCLUSION

The VA courses sagittal in the human skull. Therefore, we recommend the lateral projection (reformatted) to demonstrate the VA in LVAS patients. We advocate assessing: 1) the width (or height) of the external aperture (EA), 2) the width at the half distance between the EA and the common crus (CC), and if possible 3) the width of the proximal portion of the VA. Based on the measurements, our criteria for enlargement are 2.0 mm or greater, 1.5 mm or greater, and more than 1 mm at these sites.

Electrogenic transport and K(+) ion channel expression by the human endolymphatic sac epithelium

The endolymphatic sac (ES) is a cystic organ that is a part of the inner ear and is connected to the cochlea and vestibule. The ES is thought to be involved in inner ear ion homeostasis and fluid volume regulation for the maintenance of hearing and balance function. Many ion channels, transporters, and exchangers have been identified in the ES luminal epithelium, mainly in animal studies, but there has been no functional study investigating ion transport using human ES tissue. We designed the first functional experiments on electrogenic transport in human ES and investigated the contribution of K(+) channels in the electrogenic transport, which has been rarely identified, even in animal studies, using electrophysiological/pharmacological and molecular biological methods. As a result, we identified functional and molecular evidence for the essential participation of K(+) channels in the electrogenic transport of human ES epithelium. The identified K(+) channels involved in the electrogenic transport were KCNN2, KCNJ14, KCNK2, and KCNK6, and the K(+) transports via those channels are thought to play an important role in the maintenance of the unique ionic milieu of the inner ear fluid.