Imbalanced calcium homeostasis and endolymphatic hydrops

CACHD1-deficient mice exhibit hearing and balance deficits associated with a disruption of calcium homeostasis in the inner ear

CACHD1 recently was shown to be an α2δ-like subunit that can modulate the activity of some types of voltage-gated calcium channels, including the low-voltage activated, T-type Ca_V3 channels. CACHD1 is widely expressed in the central nervous system but its biological functions and relationship to disease states are unknown. Here, we report that mice with deleterious Cachd1 mutations are hearing impaired and have balance defects, demonstrating that CACHD1 is functionally important in the peripheral auditory and vestibular organs of the inner ear. The vestibular dysfunction of Cachd1 mutant mice, exhibited by leaning and head tilting behaviors, is related to a deficiency of calcium carbonate crystals (otoconia) in the saccule and utricle. The auditory dysfunction, shown by ABR threshold elevations and reduced DPOAEs, is associated with reduced endocochlear potentials and increased endolymph calcium concentrations. Paint-fills of mutant inner ears from prenatal and newborn mice revealed dilation of the membranous labyrinth caused by an enlarged volume of endolymph. These pathologies all can be related to a disturbance of calcium homeostasis in the endolymph of the inner ear, presumably caused by the loss of CACHD1 regulatory effects on voltage-gated calcium channel activity. Cachd1 expression in the cochlea appears stronger in late embryonic stages than in adults, suggesting an early role in establishing endolymph calcium concentrations. Our findings provide new insights into CACHD1 function and suggest the involvement of voltage-gated calcium channels in endolymph homeostasis, essential for normal auditory and vestibular function.

A possible mechanism of the formation of endolymphatic hydrops and its associated inner ear disorders

The pathology of Meniere's disease (MD) is well established to be endolymphatic hydrops. However, the mechanism underlying deafness and vertigo of MD or idiopathic endolymphatic hydrops is still unknown. In order to evaluate the pathogenesis of deafness and vertigo in MD, it seems to be rational to investigate the interrelationship between hydrops and inner ear disorders using animals with experimentally-induced endolymphatic hydrops. In spite of intense efforts by many researchers, the mechanism of vertiginous attack has been unexplained, because animals with experimental hydrops usually did not show vertiginous attack. Recently, there are two reports to succeed to evoke vertiginous attack in animals with experimental hydrops. In the present paper were first surveyed past proposals about underlying mechanism of the development of hydrops and inner ear disorders associated with hydrops, and were discussed the pathogenetic mechanism of vertiginous attack in hydrops. In conclusion, abrupt development of hydrops was thought to play a pivotal role in the onset of vertiginous seizure.

Endolymphatic hydrops: pathophysiology and experimental models

It is well established that endolymphatic hydrops plays a role in Ménière disease, even though the precise role is not fully understood and the presence of hydrops in the ear does not always result in symptoms of the disease. It nevertheless follows that a scientific understanding of how hydrops arises, how it affects the function of the ear, and how it can be manipulated or reversed could contribute to the development of effective treatments for the disease. Measurements in animal models in which endolymphatic hydrops has been induced have given numerous insights into the relationships between hydrops and other pathologic and electrophysiological changes, and how these changes influence the function of the ear. The prominent role of the endolymphatic sac in endolymph volume regulation, and the cascade of histopathological and electrophysiological changes that are associated with chronic endolymphatic hydrops, have now been established. An increasing number of models are now available that allow specific aspects of the interrelationships to be studied. The yclical nature of Ménière symptoms gives hope that treatments can be developed to maintain the ear in permanent state of remission, possibly by controlling endolymphatic hydrops, thereby avoiding the rogressive damage and secondary pathologic changes that may also contribute to the patient's symptoms.

Stria vascularis and vestibular dark cells: characterisation of main structures responsible for inner-ear homeostasis, and their pathophysiological relations

The regulation of inner-ear fluid homeostasis, with its parameters volume, concentration, osmolarity and pressure, is the basis for adequate response to stimulation. Many structures are involved in the complex process of inner-ear homeostasis. The stria vascularis and vestibular dark cells are the two main structures responsible for endolymph secretion, and possess many similarities. The characteristics of these structures are the basis for regulation of inner-ear homeostasis, while impaired function is related to various diseases. Their distinct morphology and function are described, and related to current knowledge of associated inner-ear diseases. Further research on the distinct function and regulation of these structures is necessary in order to develop future clinical interventions.

In vitro isolation and cell culture of vestibular inner ear melanocytes

INTRODUCTION

Melanocytes of the membranous labyrinth of the inner ear have been described morphologically in various contexts. Nature and functions of these cells are as yet not completely clear, even though several hypotheses exist regarding the same. The limited knowledge is due in part to a lack of methods regarding in vitro cell culture. The aim of this study was to describe conditions for the successful cell culture of vestibular inner ear melanocytes (VIEM), to compare their growth properties with those of epidermal melanocytes, and to characterize them immunohistochemically.

MATERIALS AND METHODS

Membranous labyrinth cells from freshly slaughtered sheep were isolated, and melanocytes and fibroblasts subsequently cultured. In addition, melanocytes from the skin of the same sheep were cultured. Antibodies specific to tyrosinase, tyrosinase-related protein 1 (TRP-1/Mel-5), and melanoma-specific antigen A (Melan A) were used to analyze the cultured cells.

RESULTS

The proliferation of VIEM was retarded in comparison to epidermal melanocytes. After 14 days, VIEM began to proliferate for the first time, whereas epidermal melanocytes proliferated already after 7 days. In contrast to epidermal melanocytes, the culturing process of VIEM seemed to be dependent on the presence of fibroblasts, and VIEM often accumulated in the vicinity of fibroblasts forming three-dimensional clusters. Moreover, VIEM showed a higher ratio of highly pigmented cells with a round cell shape and small dendrites in comparison to epidermal melanocytes. Immunohistochemical techniques proved the VIEM to be positive for Melan A, TRP-1 and, in the majority of cases, also for tyrosinase.

CONCLUSION

We successfully cultured melanocytes of the inner ear vestibular labyrinth for the first time and demonstrated melanocytic characteristics of these cells. This accomplishment will provide the opportunity to investigate VIEM in more detail in future experiments.

Na+,K+-ATPase and Ca2+-ATPase activities in the cochlear lateral wall following surgical induction of hydrops

Na+,K+-ATPase and Ca2+-ATPase activities have not been studied quantitatively in the cochlea affected by endolymphatic hydrops. The present study was designed to measure quantitatively the Na+,K+-ATPase and Ca2+-ATPase activities in the cochlear lateral wall and the threshold of auditory brainstem response (ABR) for guinea pigs in the early stages (=2 months) of experimentally induced endolymphatic hydrops. A significant negative association was demonstrated between Ca2+-ATPase activity and the change in ABR threshold for hydropic cochleae (P=0.014), but not for control cochleae (P=0.123), although no such significant association was revealed between Na+,K+-ATPase activity and any change in ABR threshold for both hydropic cochleae (P=0.751) and control cochleae (P=0.352). A significant increase in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which normal ABR thresholds were maintained, as compared to the control ear. On the contrary, a mild decrease in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which ABR thresholds increased significantly. The present findings suggest that alterations of Ca2+-ATPase activity in the cochlear lateral wall may implicate disturbed calcium-homeostasis in the inner ear, resulting in hearing dysfunction in the early stages of experimentally induced endolymphatic hydrops.

Quantitative differences in endolymphatic calcium and endocochlear potential between pigmented and albino guinea pigs

A number of previous studies have suggested that melanin may play a role in Ca2+ homeostasis of endolymph. In the present study, endolymph Ca2+ levels and endocochlear potential (EP) were measured in all four cochlear turns of pigmented or albino guinea pigs. Auditory sensitivity was also evaluated using cochlear action potential (AP) thresholds. In pigmented animals we found that endolymph Ca2+ tended to increase from base to apex of the cochlea, while EP systematically decreased towards the apex. In contrast, no significant Ca2+ gradient was found in albinos and the EP decline was far less. As a result, the apical turn of albino animals had significantly lower Ca2+ and significantly higher EP than in pigmented animals. AP thresholds pooled across all test frequencies were significantly lower in albino animals although no differences at individual frequencies reached significance. Even after correction for EP differences, the endolymph Ca2+ levels in albino animals were significantly lower than in pigmented ones. These results confirm that there are significant physiologic differences between pigmented and albino animals, which are a likely consequence of the absence of melanin in the albino cochlea. They are consistent with the involvement of melanin in the active transport of Ca2+ into endolymph.

Altered calcium homeostasis in the rat cochlear duct and endogenous corticosteroid insufficiency

Free calcium concentration (CCa2+) profiles were evaluated in perilymph, endolymph, marginal cells, spiral ligament and blood serum of adrenalectomized (ADX) rats. Free CCa2+ was significantly greater in perilymph and significantly reduced in the serum of the ADX animals as compared to sham-operated animals. In addition, higher levels of free CCa2+ were found in the spiral ligament in ADX animals. Free CCa2+ did not appear to be affected by ADX in marginal cells and endolymph. These data suggest that marked reductions in endogenous levels of corticosteroids may have a systematic effect on free CCa2+ that is detectable in blood serum as well as cochlear fluids and tissues.

Expression of plasma membrane Ca-ATPase in the adult and developing gerbil cochlea

The distribution of the plasma membrane Ca-ATPase (PMCA) was mapped in the adult and developing gerbil cochlea by immunostaining with a monoclonal antibody against the human erythrocyte PMCA. In the mature cochlea, intense immunoreactivity was present at the surface of stereocilia of both inner (IHC) and outer (OHC) hair cells. The basolateral plasma membrane of IHCs but not OHCs stained strongly whereas that of strial marginal cells and the epithelial cell layer of Reissner's membrane showed only weak reactivity. Nerve terminals underlying IHCs were also selectively stained. At birth, strong to moderate reactivity for PMCA was present in the basolateral plasma membrane of IHCs and OHCs, strial marginal cells, and epithelial cells lining the scala media surface of Reissner's membrane and in the neurolemma of spiral ganglion cells. Immunostaining in the basolateral plasmalemma of OHCs, strial marginal cells, and epithelial cells lining Reissner's membrane remained strong to moderate up to 14 days after birth when it diminished or disappeared entirely, suggesting a developmental role for PMCA activity in these sites. Expression of PMCA at the surface of IHC and OHC stereocilia was first observed at 10 days after birth and staining reached adult levels by 14 days after birth. The abundance of PMCA in the stereociliary plasma membrane of mature hair cells supports the suggested involvement of Ca2+ in regulating transduction and adaptation mechanisms.

Endolymph volume changes during osmotic dehydration measured by two marker techniques

The processes underlying endolymph volume regulation during osmotic disturbances were investigated in vivo using ionic volume markers. The markers utilized were tetramethylammonium (TMA+) or hexafluoroarsenate (AsF6-). Both ions were used in concentrations low enough not to be toxic, but readily detectable by ion-selective microelectrodes (typically < 1 mM). Two marker techniques were developed. In one, termed the 'perfused volume marker' (PVM) method, the marker was loaded into endolymph throughout the cochlea by perfusion of the perilymphatic space. Concentration changes of the marker were measured with a double-barreled ion-selective microelectrode. These recordings were insensitive to longitudinal movements of endolymph. The second technique, termed the 'iontophoresed volume marker' (IVM) method, utilized a localized, iontophoretic injection of marker into endolymph. In this method, marker changes were recorded from two ion-selective electrodes, one placed basal and one placed apical to the injection site. These data were used to compute changes in cross-sectional area and longitudinal movements of endolymph. Changes in endolymph volume were induced by perfusion of the perilymphatic space with hypertonic media. The endolymph potassium increase produced by osmotic dehydration was of similar magnitude and time course to that of a volume marker loaded by the PVM method. Using the IVM method, it was shown that these concentration increases arose by two distinct processes. One component was the area decrease of scala media. A second component was a small apically directed movement of endolymph during dehydration, thereby concentrating the available electrolytes within a smaller volume. This latter component was estimated to contribute approximately one third of the electrolyte increase during dehydration. Both the present and previous studies show that in the undisturbed state, longitudinal endolymph movements are extremely small and cannot make a significant contribution to ionic homeostasis. However, when endolymph volume is disturbed, longitudinal movements contribute to the electrolyte changes and are part of the compensation process. This study provides the first direct evidence supporting the long-standing hypotheses that local, radial homeostasis and longitudinal volume corrections both occur in the mammalian cochlea.

Auditory and vestibular function in experimental hydrops

Endolymphatic hydrops is the characteristic morphopathology observed at postmortem examination of temporal bones of Meniere's disease. Surgical induction of endolymphatic hydrops in the guinea pig provides a reproducible animal model for the investigation of some effects of endolymphatic hydrops on inner ear function. Episodic and incapacitating vertigo are classic features of Meniere's disease, although very limited vestibular dysfunction has ever been described for the animal model. On the other hand, the auditory deficits peculiar to Meniere's disease, including fluctuant sensitivity losses and a peak audiogram, are also observed at different stages in the development of experimental hydrops. In future studies the model is likely to provide a means of investigating osmoregulatory function of the inner ear.

Dysgenesis of melanocytes and cochlear dysfunction in mutant microphthalmia (mi) mice

In order to evaluate the cytological homology of intermediate cells and melanocytes, and to investigate the function of melanocytes in the inner ear, hearing acuity and cochlear pathology were studied in three strains of mice, namely, wild type mice (+/+), albino mice without melanin (c2J/c2J), and microphthalmia mice with no melanocytes (mibw/mibw). Our histochemical data indicated that intermediate cells showed cytological characteristics almost identical to those of melanocytes and that disorders of melanin and/or melanocytes were reflected in the stria vascularis of each mouse. While c2J/c2J presented the same normal hearing acuity and normal structure of the stria vascularis as +/+, the hearing acuity of mibw/mibw mutants was severely impaired. Their stria vascularis was abnormally thin, lacking intermediate cells. According to these results, lack of melanin has little influence on hearing acuity; however, the absence of intermediate cells or melanocytes causes severe hearing loss, presumably due to a strial dysfunction.

Transepithelial voltage and resistance of vestibular dark cell epithelium from the gerbil ampulla

Transepithelial voltage (Vt) and resistance (Rt) were measured across the dark cell epithelium of the gerbil ampulla using a micro Ussing chamber of improved design in order to test the view that the histologically similar epithelia in the utricle and in the ampullae exhibit similar electrophysiologic functions. Vt was found to be 8.0 +/- 0.3 mV and Rt was 11.6 +/- 0.4 ohm-cm2 (N = 179) when both sides of the tissue were perfused with symmetric perilymph-like solution. The equivalent short circuit current (Isc = Vt/Rt) was 712 +/- 18 microA/cm2 (N = 179). Isc was reduced from 638 +/- 60 to 48 +/- 16 microA/cm2 (N = 14) by basolateral perfusion of 10(-3) M ouabain and from 538 +/- 27 to 27 +/- 4 microA/cm2 (N = 15) by basolateral perfusion of 5 x 10(-5) M bumetanide. Basolateral K+ steps (Na+ substitution) from 3.6 to 25 mM increased Vt from 6.5 +/- 0.5 to 12.2 +/- 0.6 mV and reduced Rt from 9.7 +/- 0.7 to 7.4 +/- 0.5 ohm-cm2 (N = 43). Apical K+ steps from 3.6 to 25, to 100 mM or to 145 mM led to a decrease in both Vt and Rt. The steady state Vt during apical perfusion of 145 mM K+ was near zero. Upon return to 3.6 mM K+, Vt transiently overshot its original level. Apical Cl- steps from 150 to 50 mM (gluconate substitution) monophasically decreased Vt from 5.9 +/- 0.7 to 4.1 +/- 0.8 mV (N = 15) and increased Rt from 9.6 +/- 1.3 to 12.0 +/- 1.5 ohm-cm2 (N = 14).(ABSTRACT TRUNCATED AT 250 WORDS)

Early bone formation in the human fetal otic capsule. A methodological approach

The present study was concentrated on the use of energy-dispersive X-ray analysis and backscattered electron imaging as practical tools for advanced autopsy of human fetuses when diagnostic evaluation of ear pathology is required. These methods were used to revisit the primary calcification front of the fetal otic capsule between 18 and 36 weeks' gestational age. Energy-dispersive X-ray analysis indicates an equal Ca/P ratio in all the three layers of the otic capsule. These results are discussed in view of calcium homeostasis and inner ear function.

Early ossification within the human fetal otic capsule: morphological and microanalytical findings

Besides the use of conventional techniques such as light and polarization microscopy, the present paper proposes the combined use of transmission electron microscopy, secondary and backscattered electron imaging, energy dispersive X-ray analysis and computed tomography for the diagnostic evaluation of ear pathology in the human fetus. These methods were used to revisit the primary calcification front of the fetal otic capsule between 16 and 23 weeks gestational age. Ultramicroscopic evaluation demonstrates similar fetal bone formation to that found in other bones of the human fetus. The formation of the endosteal and periosteal layers is a typical example of early intra-membranous ossification. The enchondral layer is made up of fibrillar bone, laid down around the calcified cartilage remnants. Microchemical analysis indicates a significantly higher Ca/P ratio in the endochondral layer with respect to the endosteum and periosteum. The consequences of a lower Ca/P ratio in the endosteal layer are discussed in view of calcium homeostasis and inner ear function.