Calcification processes in the chick otoconia and calcium binding proteins: patterns of tetracycline incorporation and calbindin-D28K distribution

Expression, functional, and structural analysis of proteins critical for otoconia development

Otoconia, developed during late gestation and perinatal stages, couple mechanic force to the sensory hair cells in the vestibule for motion detection and bodily balance. In the present work, we have investigated whether compensatory deposition of another protein(s) may have taken place to partially alleviate the detrimental effects of Oc90 deletion by analyzing a comprehensive list of plausible candidates, and have found a drastic increase in the deposition of Sparc-like 1 (aka Sc1 or hevin) in Oc90 null versus wt otoconia. We show that such up-regulation is specific to Sc1, and that stable transfection of Oc90 and Sc1 full-length expression constructs in NIH/3T3 cells indeed promotes matrix calcification. Analysis and modeling of Oc90 and Sc1 protein structures show common features that may be critical requirements for the otoconial matrix backbone protein. Such information will serve as the foundation for future regenerative purposes.

Meniere's disease in the elderly

Menière disease usually begins in adults from 20 to 60 years old, and occurs in more than 10% of patients older than 65. The treatment of Menière disease in the elderly represents a challenge because of polymedication. Antivertiginous drugs such as betahistine and cinnarizin give good results with minor secondary effects. In contrast, major vestibular suppressor drugs such as thiethylperazin must be avoided as long-term treatment because of their side effects. Definitive vestibular surgical deafferentations such as labyrinthectomy and selective vestibular neurectomy represent optional procedures but must be carefully evaluated from case to case. Ablative procedures remain the efficient treatment of drop attacks, which represent a high potential risk of severe injuries by older patients sometimes with important social consequences.

Regulation of cellular calcium in vestibular supporting cells by otopetrin 1

Otopetrin 1 (OTOP1) is a multitransmembrane domain protein, which is essential for mineralization of otoconia, the calcium carbonate biominerals required for vestibular function, and the normal sensation of gravity. The mechanism driving mineralization of otoconia is poorly understood, but it has been proposed that supporting cells and a mechanism to maintain high concentrations of calcium are critical. Using Otop1 knockout mice and a utricular epithelial organ culture system, we show that OTOP1 is expressed at the apex of supporting cells and functions to increase cytosolic calcium in response to purinergic agonists, such as adenosine 5'-triphosphate (ATP). This is achieved by blocking mobilization of calcium from intracellular stores in an extracellular calcium-dependent manner and by mediating influx of extracellular calcium. These data support a model in which OTOP1 acts as a sensor of the extracellular calcium concentration near supporting cells and responds to ATP in the endolymph to increase intracellular calcium levels during otoconia mineralization.

Morphological and biochemical analyses of otoliths of the ice-fish Chionodraco hamatus confirm a common origin with red-blooded species

The morphology and composition of the three otoliths of the Antarctic ice-fish Chionodraco hamatus were studied by scanning electron microscopy and X-ray diffraction. The composition of the sagitta, lapillus and asteriscus protein matrices was also analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, western blots and confocal laser scanning microscopy to reveal the presence of and to localize the calcium-binding proteins calmodulin, calbindin and S-100. Morphological results indicated that the otoliths in this ice-fish were similar to those of Trematomus bernacchii, a red-blooded Antarctic species [B. Avallone et al. (2003) J. Submicrosc. Cytol. Pathol. 35, 69-76], but rather different from those of other teleosts. These two Antarctic species possessed a completely vateritic asteriscus, whereas their sagitta and lapillus were made mostly of aragonite. Parallel analysis of protein patterns in C. hamatus and T. bernacchii revealed that the sagitta significantly differed from the lapillus and asteriscus in both species. The sagitta did not contain the S-100 protein and showed calmodulin and calbindin located in discontinuous or incremental zones, respectively. These results demonstrate that the otoliths of C. hamatus and T. bernacchii share more resemblances than differences and support the idea of a common origin of these species.

Osteopontin is not critical for otoconia formation or balance function

Unlike the structural and mechanical role of bone crystals, the inertial mass of otoconia crystals provides a shearing force to stimulate the mechanoreceptors of the utricle and saccule (the gravity receptor organ) under the stimuli of linear motion. It is not clear whether otoconia, composed primarily of CaCO3 and glycoproteins, go through similar calcification processes as bone. We have recently shown that otoconin-90 (Oc90) regulates the growth of otoconia crystals as osteopontin does bone crystals. Here, we analyzed the role of this non-collagenous bone matrix protein, osteopontin, in otoconia formation and balance function utilizing its knockout mice, whose inner ear phenotype has not been examined. Despite the presence of the protein in wild-type otoconia and vestibular hair cells, morphological, ultrastructural, and protein and calcium composition analyses of osteopontin null otoconia show that the protein is not needed for crystal formation, and no evidence of compensatory protein deposition is found. Employment of a wide spectrum of balance behavioral tests demonstrates that the protein is not critical for balance function either, which is confirmed by the normal function of the gravity receptor organ directly measured with linear vestibular-evoked potentials (VsEPs). When compared with findings on other otoconins, the data manifest a hierarchy of importance of proteins in crystallization and indicate mechanistic similarities and differences between bone and otoconia calcification.

Ultrastructural changes in otoconia of osteoporotic rats

The etiology of benign paroxysmal positional vertigo (BPPV) remains obscure in many cases and women are affected more often than men. A recent prospective study, performed in women >50 years of age suffering from recurrent BPPV, showed associated osteopenia or osteoporosis in a large percentage of these patients. These results suggested the possible relationship between recurrent BPPV and a decreased fixation of calcium in bone in women >50 years. To test this hypothesis, an experimental study was performed in adult female rats. Utricular otoconia of female rats in which osteopenia/osteoporosis was induced by bilateral ovariectomy (OVX) were compared to those of sham-operated adult females rats (SHAM), as control group. FIRST STUDY: The morphology of theutricles of OVX and SHAM rats was analyzed with scanning electron microscopy. In osteopenic/osteoporotic rats, the density of otoconia (i.e. the number of otoconia per unit area) was decreased (p = 0.036)and their size was increased (p = 0.036) compared to the control group. SECOND STUDY: To test the role of calcium turnover in such morphological changes, utricular otoconia of 2 other groups of OVX and SHAM rats, previously injected with calcein subcutaneously, were examined by conventional and epifluorescence microscopy. In epifluorescence microscopy, labeling with calcein showed no significant fluorescence in either group. This finding was interpreted as a lack of external calcium turnover into otoconia of adult female rats. The ultrastructural modifications of otoconia in osteopenic/osteoporotic female adult rats as well as the role of estrogenic receptors in the inner ear are discussed. The possible pathophysiological mechanisms which support the relationship between recurrent BPPV in women and the disturbance of the calcium metabolism of osteopenia/osteoporosis are debated.

Morphometric investigations of sensory vestibular structures in tadpoles (Xenopus laevis) after a spaceflight: implications for microgravity-induced alterations of the vestibuloocular reflex

In lower vertebrates, gravity deprivation by orbital flights modifies the vestibuloocular reflex. Using the amphibian Xenopus laevis, the experiments should clarify to which extent macular structures of the labyrinth are responsible for these modifications. In particular, the shape of otoconia and number and size of sensory macular cells expressing CalBindin were considered. CalBindin is common in mature sensory cells including vestibular hair cells and is probably involved in otoconia formation. Two developmental stages were used for this study: stage 26/27 embryos, which were unable to perform the roll-induced vestibuloocular reflex (rVOR) at onset of microgravity, and stage 45 tadpoles, which had already developed the reflex. The main observations were that the developmental progress of the animals was not affected by microgravity; that in the young tadpole group with normal body shape the rVOR was not modified by microgravity, while in the older group with microgravity experience, the rVOR was augmented; and that significant effects on the shape of otoconia and on the number and size of CalBindin-expressing cells of the labyrinthine maculae cells were absent. In addition, behavioural data were never significantly correlated with morphological features of macular structures such as size and number of CalBindin-expressing cells. It is postulated that mechanisms of vestibular adaptation to microgravity during early development are probably based on mechanisms located in central structures of the vestibular system.

Mixing model systems: using zebrafish and mouse inner ear mutants and other organ systems to unravel the mystery of otoconial development

Human vestibular dysfunction is an increasing clinical problem. Degeneration or displacement of otoconia is a significant etiology of age-related balance disorders and Benign Positional Vertigo (BPV). In addition, commonly used antibiotics, such as aminoglycoside antibiotics, can lead to disruption of otoconial structure and function. Despite such clinical significance, relatively little information has been compiled about the development and maintenance of otoconia in humans. Recent studies in model organisms and other mammalian organ systems have revealed some of the proteins and processes required for the normal biomineralization of otoconia and otoliths in the inner ear of vertebrates. Orchestration of extracellular biomineralization requires bringing together ionic and proteinaceous components in time and space. Coordination of these events requires the normal formation of the otocyst and sensory maculae, specific secretion and localization of extracellular matrix proteins, as well as tight regulation of the endolymph ionic environment. Disruption of any of these processes can lead to the formation of abnormally shaped, or ectopic, otoconia, or otoconial agenesis. We propose that normal generation of otoconia requires a complex temporal and spatial control of developmental and biochemical events. In this review, we suggest a new hypothetical model for normal otoconial and otolith formation based on matrix vesicle mineralization in bone which we believe to be supported by information from existing mutants, morphants, and biochemical studies.

Assembly of the otoconia complex to the macular sensory epithelium of the vestibule

In the inner ear, specificity of stimulus perception is achieved by associating the sensory epithelia of the three mechanoreceptor organs, the utricle/saccule, cristae, and cochlea, with distinct types of acellular matrices. Only the utricle and saccule have an extremely dense matrix, the otoconial complex, which overlies the sensory epithelium (macula) and provides inertial mass to generate shearing forces essential for the mechanoreceptors to sense gravity and linear acceleration. Such sensation is necessary for spatial orientation and balance. The importance of otoconia is clearly demonstrated by the impact of balance disorders upon the elderly population that involve otoconia degeneration, as well as by canalithiasis and cupulolithiasis, in which otoconia are dislocated. This underscores the need to understand how otoconia are formed and maintained and how to prevent their degeneration. To date, a number of otoconia-related proteins have been identified mostly in mice and bony fish. Although most of these proteins are also present in other structures of the inner ear, a distinct collection of proteins in the macula plus the unique ionic microenvironment of the endolymph near its epithelium likely contribute to the site-specific calcification of otoconia. Based on the current literature and ongoing research, this mini-review postulates a working model of how the otoconia complex is assembled specifically above the macular sensory epithelium of the vestibule. The central hypothesis of this model is that proteins are critical in sequestering calcium for crystallization in the calcium-poor endolymph. The review also sets forth some issues that need to be resolved in the future.

Microscale analysis of proteins in inner ear tissues and fluids with emphasis on endolymphatic sac, otoconia, and organ of Corti

Here we describe preparatory techniques adapted for the study of proteins of inner ear tissues and fluids that have allowed us to apply state-of-the-art analytical techniques in spite of the minute size and anatomical complexities of this organ. Illustrative examples address unresolved issues of functional and clinical significance. First, we demonstrate how quick-freezing and freeze drying prevents artifacts that arise from sampling endolymphatic sac (ES) content in the liquid state. This set the stage for the generation of the first protein profile of the ES. Identification of crucial proteins will help elucidate mechanisms of endolymph volume regulation and pathogenesis of Meniere's disease. Second, we show how a unique situation allowed identification of otoconial proteins by mass spectrometric analysis without prior separation and we discuss possible roles for these minor otoconins in otoconial development and prevention of degenerative diseases that affect balance. Finally, we demonstrate techniques for the precise dissection of organ of Corti and its substructures, while preserving their near normal chemical state. We extended an earlier study in which we identified a novel calcium-binding protein by IEF, oncomodulin, localized in the outer hair cells and show here the applicability of prefractionation for the screening of calcium-binding proteins of organ of Corti. These studies demonstrate how advanced preparatory and analytical techniques can be applied to studies of the inner ear.

Otolith matrix proteins OMP-1 and Otolin-1 are necessary for normal otolith growth and their correct anchoring onto the sensory maculae

Fish otoliths are highly calcified concretions deposited in the inner ear and serve as a part of the hearing and balance systems. They consist mainly of calcium carbonate and a small amount of organic matrix. The latter component is considered to play important roles in otolith formation. Previously, we identified two major otolith matrix proteins, OMP-1 (otolith matrix protein-1) and Otolin-1, from salmonid species. To assess the function of these proteins in otolith formation, we performed antisense morpholino oligonucleotide (MO)-mediated knockdown of omp-1 and otolin-1 in zebrafish embryos. We first identified zebrafish cDNA homologs of omp-1 (zomp-1) and otolin-1 (zotolin-1). Whole-mount in situ hybridization then revealed that the expression of both zomp-1 and zotolin-1 mRNAs is restricted to the otic vesicles. zomp-1 mRNA was expressed from the 14-somite stage in the otic placode, but the zOMP-1 protein was detected only from 26-somite stage onwards. In contrast, zotolin-1 mRNA expression became clear around 72 hpf. MOs designed to inhibit zomp-1 and zotolin-1 mRNA translation, respectively, were injected into 1-2 cell stage embryos. zomp-1 MO caused a reduction in otolith size and an absence of zOtolin-1 deposition, while zotolin-1 MO caused a fusion of the two otoliths, and an increased instability of otoliths after fixation. We conclude that zOMP-1 is required for normal otolith growth and deposition of zOtolin-1 in the otolith, while zOtolin-1, a collagenous protein, is involved in the correct arrangement of the otoliths onto the sensory epithelium of the inner ear and probably in stabilization of the otolith matrix.

Localization of calbindin D-28K in the otoconia of lizard Podarcis sicula

The membranous labyrinth of lizard Podarcis sicula contains calcite and aragonite crystals. Saccule, utricle and lagena contain calcite crystals while aragonite crystals are present only in the saccule where they are very abundant. We have recently demonstrated the presence of calbindin D-28K in the organic matrix of lizard P. sicula otoconia. In order to define its localization, since calbindin modulates cellular Ca2+ level, otoconia from utricle and lagena were collected separately from those from saccule and then otoconial proteins were extracted. Immunoblot assay on proteins extracted from the otoconia and confocal laser scanning microscope analyses of otoconia using monoclonal anti-calbindin D-28K antibodies indicated that calbindin D-28K is a protein typical of aragonite crystals.

Calcium-tracers disclose the site of biomineralization in inner ear otoliths of fish

Since changing gravity (concerning direction and amplitude) strongly affects inner ear otolith growth and otolithic calcium incorporation in developing fish, it was the aim of the present study to locate the site of mineralization in order to gain cues and insights into the provenance of the otoliths inorganic compounds. Therefore, larval cichlid fish (Oreochromis mossambicus) were incubated in the calcium-tracer alizarin complexone (AC; red fluorescence). After maintenance in aquarium water for various periods (1, 2, 3, 6, 9 and 12 h; 1, 2, 3, 5, 6, 7, 15, 29, 36 and 87 d), the animals were incubated in the calcium-tracer calcein (CAL; green fluorescence). AC thus labeled calcium being incorporated at the beginning of the experiment and would subsequently accompany calcium in the course of a possible dislocation, whereas CAL visualized calcium being deposited right at the end of the test. Subsequently, the otoliths were analyzed using a laser scanning microscope and it was shown that the initial site of calcium incorporation was located directly adjacent to the sensory epithelium and the otolithic membrane. Later, calcium deposits were also found on further regions of the otoliths' surface area, where they had been shifted to in the course of dislocation. This finding strongly indicates that the sensory epithelium plays a prominent role in otolithic biomineralization, which is in full agreement with an own electron microscopical study [ELGRA News 23 (2003) 63].

Benign paroxysmal positional vertigo in older women may be related to osteoporosis and osteopenia

Benign paroxysmal positional vertigo (BPPV), so-called canalolithiasis and cupulolithiasis, usually occurs after head trauma or viral vestibular neuritis. In many cases, the cause remains obscure, and it often affects women more than 50 years old. The goal of this work was to study a possible relationship between BPPV and osteopenia or osteoporosis. Thirty-two women, whose ages ranged from 50 to 85 years (median age, 69 years), who had BPPV and were free of any other otoneurologic history, were selected. The diagnosis of osteopenia or osteoporosis was confirmed by a bone mineral density measurement made with dual x-ray absorptiometry of spine and hip (T-score). The BPPV was unilateral in 26 patients and bilateral in 6 patients. Our results showed osteopenia or osteoporosis in 24 of the 32 patients (75%) with BPPV. The T-scores were compared in 3 age groups to those of 83 healthy women. The patients with BPPV had a significantly lower (p < .026) T-score in all groups. Possible pathophysiological mechanisms are discussed to explain the apparent correlation between BPPV and osteopenia or osteoporosis.

Histological preparation of developing vestibular otoconia for scanning electron microscopy

The unique nature of vestibular otoconia as calcium carbonate biominerals makes them particularly susceptible to chemical deformation during histological processing. We fixed and stored otoconia from all three otolith endorgans of embryonic, hatchling and adult Japanese quail in glutaraldehyde containing either phosphate or non-phosphate buffers for varying lengths of time and processed them for scanning electron microscopy. Otoconia from all age groups and otolith endorgans processed in 0.1 M phosphate buffer (pH 7.4) showed abnormal surface morphology when compared to acetone fixed controls. Otoconia processed in 0.1 M sodium cacodylate or HEPES buffered artificial endolymph (pH 7.4) showed normal morphology that was similar to controls. The degree of otoconial deformation was directly related to the time exposed to phosphate buffer. Short duration exposure produced particulate deformations while longer exposures resulted in fused otoconia that formed solid sheets. Otoconial surface deformation and fusing was independent of the glutaraldehyde component of the histological processing. These findings should help vestibular researchers to develop appropriate histological processing protocols in future studies of otoconia.

Calbindin D28K is a component of the organic matrix of lizard Podarcis sicula otoconia

The factors controlling otoconia growth are not well known but it seems that the type of proteins contained in the otoconia regulates the initiation and/or the subsequent rates of crystal growth determining the morphology and the size of the final crystal. In order to clarify the mechanism of otoconia formation and their turnover, major proteins contained in the otoconia from the maculae of the saccule, utricle and lagena of inner ear of lizard Podarcis sicula were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Coomassie staining of SDS-PAGE resulted in a major broad band of 15 kDa and four other bands of 21, 28, 45 and 97 kDa. The proteins of 15, 21, 28 and 45 kDa were separated by high-pressure liquid chromatography on a C-4-reverse-phase column and the incubation of blots with monoclonal anti-Calbindin D28K antibodies indicated that the band of 28 kDa was Calbindin D28K, a calcium-binding protein.

Transgenic and gene targeting studies of hair cell function in mouse inner ear

Despite the rapid discovery of a large number of genes in sensory hair cells of the inner ear, the functional roles of these genes in hair cells remain largely undetermined. Recent advances in transgenic and gene targeting technologies in mice have offered unprecedented opportunities to genetically manipulate the expression of these genes and to study their functional roles in hair cells in vivo. Transgenic analyses have revealed the presence of hair-cell-specific promoters in the genes encoding Math1, myosin VIIa, Pou4f3, and the alpha9 subunit of the acetylcholine receptor (alpha9 AChR). Targeted inactivation using embryonic stem cell technology and transgenic expression studies have revealed the roles of several genes involved in hair cell lineage (Math1), differentiation (Pou4f3), mechanotransduction (Myo1c, and Myo7a), electromotility (Prestin), and efferent modulation (Chrna9, encoding alpha9 AChR). Although many of these genes also play roles in other tissues, inactivation of these genes in hair cells alone will soon be possible by using the Cre-loxP system. Also imminent is the development of genetic methods to inactivate genes specifically in mouse hair cells at a desired time, by using inducible systems established in other types of neurons. Combining these types of manipulation of gene expression will enable hearing researchers to elucidate some of the fundamental and unique features of hair cell function such as mechanotransduction, frequency tuning, active mechanical amplification, and efferent modulation.