Abstract
The endolymphatic sac (ES) is an inner ear organ that is connected to the cochleo-vestibular system through the endolymphatic duct. The luminal fluid of the ES contains a much higher concentration of proteins than any other compartment of the inner ear. This high protein concentration likely contributes to inner ear fluid volume regulation by creating an osmotic gradient between the ES lumen and the interstitial fluid. We characterized the protein profile of the ES luminal fluid of patients (n = 11) with enlarged vestibular aqueducts (EVA) by proteomics. In addition, we investigated differences in the protein profiles between patients with recent hearing deterioration and patients without hearing deterioration. The mean total protein concentration of the luminal fluid was 554.7±94.6 mg/dl. A total of 58 out of 517 spots detected by 2-DE were analyzed by MALDI-TOF MS. The protein profile of the luminal fluid was different from the profile of plasma. Proteins identified from 29 of the spots were also present in the MARC-filtered human plasma; however, the proteins identified from the other 25 spots were not detected in the MARC-filtered human plasma. The most abundant protein in the luminal fluid was albumin-like proteins, but most of them were not detected in MARC-filtered human plasma. The concentration of albumin-like proteins was higher in samples from patients without recent hearing deterioration than in patients with recent hearing deterioration. Consequently, the protein of ES luminal fluid is likely to be originated from both the plasma and the inner ear and considering that inner ear fluid volumes increase abnormally in patients with EVA following recent hearing deterioration, it is tempting to speculate that albumin-like proteins may be involved in the regulation of inner ear fluid volume through creation of an osmotic gradient during pathological conditions such as endolymphatic hydrops.
Highlights
The luminal part of the inner ear is filled with a low [Na+] and high [K+] fluid that is called endolymph [1]
The protein concentrations detected in patients with a definitive histories of sudden hearing deterioration within the prior 6 months tended to be significantly lower than the protein concentrations detected in patients without a recent hearing deterioration (P = 0.04, Fig. 2B)
One of the suggested mechanisms involved in endolymphatic volume regulation is the transport of water into or out of the endolymphatic sac (ES) lumen, which depends on the magnitude of the osmotic gradient between the lumen and the interstitial space
Summary
The luminal part of the inner ear is filled with a low [Na+] and high [K+] fluid that is called endolymph [1]. The ES is a small structure (,15 mm2) [2] that is an extension of the luminal compartment of the inner ear. It is situated on the posterior fossa dura and is connected to the cochleo-vestibular system through the endolymphatic duct (Fig. 1). If endolymphatic volume regulation is disturbed, serious derangement of inner ear function (i.e., hearing loss and dizziness) may occur. Representative diseases arising from disturbances of endolymphatic volume regulation are Meniere’s disease and enlarged vestibular aqueduct (EVA) syndrome. Most cases of EVA syndrome present with a distended ES, which may reflect increased endolymph volume [6,7,8]. The pathophysiological mechanism underlying the increases in endolymphatic volume that occur in these disease states is unclear
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