A comparison of hair bundle mechanoreceptors in sea anemones and vertebrate systems

Periodic, moderate water flow reversibly increases hair bundle density and size in Nematostella vectensis

Animals employ hair bundles on hair cells to detect flow, vibrations and gravity. Hair bundles on sea anemone tentacles detect nearby vibrations in the water column produced by prey movements and then regulate discharge of cnidae to capture prey. This study investigated: (1) the progressive effects of periodic water flow on hair bundle morphology and density of hair bundles and cnidae in sea anemones, (2) the reversibility of the flow response and (3) the ability of the response to be expedited with increased flow duration. Linear density of hair bundles along tentacles and each hair bundle's dimensions was measured in anemones exposed to flow and in the absence of flow. With increasing numbers of days of flow, hair bundles in anemones exposed to flow for 1 h every weekday for 20 days increased in density and grew longer and wider at bases and middles, whereas controls did not. Time courses fit to a linear function exhibited significantly larger positive slopes from animals exposed to flow compared with controls. Hair bundles in anemones exposed to flow for 3 h each day increased in linear density, length, base width and middle width after 10 days of flow and returned to control levels after 10 days following cessation of flow. In addition, there was a trend for an increase in density of cnidae with flow. Therefore, anemone hair bundles are dynamically and reversibly modified by periodic, moderate flow to become more abundant and robust. These findings may have relevance to hair cells in acoustico-lateralis systems of higher animals.

Noise-induced hearing loss: new animal models

PURPOSE OF THE REVIEW

This article presents research findings from two invertebrate model systems with potential to advance both the understanding of noise-induced hearing loss mechanisms and the development of putative therapies to reduce human noise damage.

RECENT FINDINGS

Work on sea anemone hair bundles, which resemble auditory hair cells, has revealed secretions that exhibit astonishing healing properties not only for damaged hair bundles, but also for vertebrate lateral line neuromasts. We present progress on identifying functional components of the secretions, and their mechanisms of repair. The second model, the Johnston's organ in Drosophila, is also genetically homologous to hair cells and shows noise-induced hearing loss similar to vertebrates. Drosophila offers genetic and molecular insight into noise sensitivity and pathways that can be manipulated to reduce stress and damage from noise.

SUMMARY

Using the comparative approach is a productive avenue to understanding basic mechanisms, in this case cellular responses to noise trauma. Expanding study of these systems may accelerate identification of strategies to reduce or prevent noise damage in the human ear.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

Stem/progenitor cells derived from the cochlear sensory epithelium give rise to spheres with distinct morphologies and features

Nonmammalian vertebrates regenerate lost sensory hair cells by means of asymmetric division of supporting cells. Inner ear or lateral line supporting cells in birds, amphibians, and fish consequently serve as bona fide stem cells resulting in high regenerative capacity of hair cell-bearing organs. Hair cell regeneration does not happen in the mammalian cochlea, but cells with proliferative capacity can be isolated from the neonatal cochlea. These cells have the ability to form clonal floating colonies, so-called spheres, when cultured in nonadherent conditions. We noticed that the sphere population derived from mouse cochlear sensory epithelium cells was heterogeneous, consisting of morphologically distinct sphere types, hereby classified as solid, transitional, and hollow. Cochlear sensory epithelium-derived stem/progenitor cells initially give rise to small solid spheres, which subsequently transition into hollow spheres, a change that is accompanied by epithelial differentiation of the majority of sphere cells. Only solid spheres, and to a lesser extent, transitional spheres, appeared to harbor self-renewing stem cells, whereas hollow spheres could not be consistently propagated. Solid spheres contained significantly more rapidly cycling Pax-2-expressing presumptive otic progenitor cells than hollow spheres. Islet-1, which becomes upregulated in nascent sensory patches, was also more abundant in solid than in hollow spheres. Likewise, hair cell-like cells, characterized by the expression of multiple hair cell markers, differentiated in significantly higher numbers in cell populations derived from solid spheres. We conclude that cochlear sensory epithelium cell populations initially give rise to small solid spheres that have self-renewing capacity before they subsequently convert into hollow spheres, a process that is accompanied by loss of stemness and reduced ability to spontaneously give rise to hair cell-like cells. Solid spheres might, therefore, represent the most suitable sphere type for cell-based assays or animal model transplantation studies aimed at development of cell replacement therapies.

Ultrastructure of the abdominal sense organ of the scallop Mizuchopecten yessoensis (Jay)

The sensory epithelium of the abdominal sense organ (ASO) of the scallop Mizuchopecten yessoensis is composed of three cell types, sensory cells, mucous cells, and multiciliated cells. Sensory cells bear a single long (up to 250 microm) cilium surrounded by an inner ring of nine modified microvilli and an outer ring of ordinary microvilli paired with modified microvilli. Sensory cells make up about 90% of the total number of cells in the sensory epithelium. Mucous cells, which are much wider than sensory cells, bear only ordinary microvilli on their apical surface. Rare multiciliated cells with short (4-6 microm) cilia are scattered in the periphery of the sensory epithelium sheet. All hairs, cilium, and microvilli of each sensory cell are interconnected by a fibrous network. Nine modified microvilli of a single cell are interconnected by prominent laterally running fibrous links. Membrane-associated electron-dense material of modified microvilli is connected to the ciliary membrane-associated electron-dense material by fine string-like links. These links mechanically bridge the space between the cilium and modified microvilli, as do mechanical links, described for the stereocilia and kinocilium of vertebrate vestibular and cochlear hair cells. The proximal portion of a sensory cilium is about 100 microm long and has a typical 9 x 2+2 axoneme arrangement. The distal portion of a cilium is approximately 2 times thinner than the proximal one and is filled with homogeneous electron-dense material. Along the distal portion, diffuse material associated with the external surface of the membrane is found. The rigidity of distal portion of a cilium is much less than that of the proximal one.

Hair bundle motility induced by chemoreceptors in anemones

Most hair bundles are essentially fixed with respect to frequency specificity. However, hair bundles in sea anemones are dynamically tuned by actin-dependent changes in length. Tuning to low frequencies is accomplished by activation of chemoreceptors to N-acetylated sugars resulting in hair bundle elongation. We report here that following sugar-induced tuning of hair bundles, membrane currents reverse polarity in hair cells during unidirectional mechanical stimulation. Reversal in polarity of currents with sugar stimulation is inhibited if hair bundle elongation is blocked by pretreatment with cytochalasin D. A re-examination of morphological changes to hair bundles reveals a sugar-induced reorientation of stereocilia in addition to elongation with chemosensitization. In controls, hair bundles are noticeably twisted. With sugar stimulation stereocilia become oriented more parallel to the long axis of the hair bundle. This sugar-induced change in orientation is similarly inhibited by cytochalasin D pretreatment. Based on these results, we present a model wherein anemone hair bundle twisting serves as a built-in safety mechanism to preserve linkages likely to be subjected to potentially damaging tension during tuning. The twisted hair bundles can untwist while elongating to relieve excessive tension on extracellular linkages between stereocilia critical to mechanosensitivity.

Immunological evidence that anemone repair proteins include replacement linkages

In response to damage to hair bundles caused by exposure to calcium free buffers, sea anemones secrete large protein complexes named 'repair proteins' that rapidly restore structural integrity and function to hair bundles. A specific chromatographic fraction of the repair protein mixture, named 'fraction beta', has biological activity comparable to the complete repair protein mixture (Watson et al., 1998, Hear. Res. 115, 119-128). In this study, we find that polyclonal antibodies raised against deglycosylated fraction beta specifically bind fraction beta on Western blots. Anti-fraction beta delays the normal recovery of vibration sensitivity in experimental animals (i.e., those with hair bundles damaged by calcium free buffers). Moreover, anti-fraction beta disrupts vibration sensitivity in control animals (i.e., those with healthy hair bundles). Experimentally damaged hair bundles subsequently exposed to repair protein and then processed for immunoelectron microscopy show labeled linkages interconnecting stereocilia of the hair bundle. Immunofluorescence microscopy confirms strong labeling of hair bundles treated with repair proteins and only weak labeling of tips of hair bundles from control animals. Immunofluorescence microscopy indicates stores of repair proteins in gland cells of the body column in control animals and in gland cells of the mouth in experimental animals. Repair biological activity is confirmed in column purified homogenates of these tissues. Apparently repair proteins are delivered to damaged hair bundles in mucus carried by beating cilia.

Gap junctional communication in the vibration-sensitive response of sea anemones

Although gap junctions occur in auditory and vestibular systems, their function is unclear. Here we present evidence for gap junctional communication in transmitting mechanosensory signals in a sea anemone model system. Hair bundles on anemone tentacles are vibration-sensitive mechanoreceptors that regulate discharge of nematocyst from effector cells. We find that vibration-dependent nematocyst discharge is selectively and reversibly blocked by the gap junction uncouplers, heptanol and arachidonic acid. Epidermal cells within excised tentacles exhibit a low level of dye coupling which is significantly enhanced upon deflection of overlying hair bundles. Dye coupling is inhibited both by gap junction uncouplers and by agents that interfere with mechanotransduction, including streptomycin and elastase. Electrophysiological data suggest gap junctional communication between cells giving rise to different hair bundles. When hair bundles are stimulated with a sweep of vibrations, individual cells show responses to five to eight frequencies. The number of responsive frequencies is reduced to one or two by heptanol and essentially abolished with streptomycin treatment. Immunoreactivity to the gap junction protein, connexin 43, is abundant in the tentacle epidermis and localized to membranes at junctions between several cell types. Small areas of close membrane apposition are observed between these cell types with intermembrane clefts of 4-7 nm. Of the several membrane proteins isolated from tentacles, immunoreactivity to connexin 43 is observed in a single band with an apparent molecular weight of approximately 46 kDa.

ATP enhances repair of hair bundles in sea anemones

Hair bundle mechanoreceptors of sea anemones are similar to those of the acousticolateralis system of vertebrates (Watson, Mire and Hudson, 1997, Hear. Res. 107, 53-63). Anemone hair bundles are repaired by 'repair proteins' secreted following a complete loss of structural integrity and loss of function caused by 1 h exposure to calcium free seawater. Exogenously supplied repair proteins (RP) restore structural integrity to hair bundles and restore vibration sensitivity in 7-8 min (Watson, Mire and Hudson, 1998, Hear. Res. 115, 119-128). We here report that exogenously supplied ATP enhances the rate by which RP restore vibration sensitivity. A bimodal dose response to ATP indicates maximal enhancement at picomolar and micromolar concentrations of ATP. At these concentrations of ATP, vibration sensitivity is restored in 2 min. These data suggest that at least two ATPases exhibiting different binding affinities for ATP are involved in the repair process. Whereas the higher affinity site is specific for ATP, the lower affinity site does not discriminate between ATP and ADP. Nucleotidase cytochemistry localizes ATPase activity in isolated repair proteins. In the absence of exogenously added RP, sea anemones secrete and consume ATP during the 4 h recovery period after 1 h exposure to calcium free seawater. In the presence of exogenously added RP, ATP is secreted and then consumed within 10 min. Quinacrine cytochemistry localizes possible stores of ATP in the apical cytoplasm of sensory neurons located at the center of the hair bundle. According to our model, ATP is secreted by the sensory neuron after its hair bundle loses structural integrity. Hydrolysis of ATP by repair proteins is essential to the repair process.