Cyclical morphodynamics of hair bundles in sea anemones: Second messenger pathways

High adenylyl cyclase activity and in vivo cAMP fluctuations in corals suggest central physiological role

Corals are an ecologically and evolutionarily significant group, providing the framework for coral reef biodiversity while representing one of the most basal of metazoan phyla. However, little is known about fundamental signaling pathways in corals. Here we investigate the dynamics of cAMP, a conserved signaling molecule that can regulate virtually every physiological process. Bioinformatics revealed corals have both transmembrane and soluble adenylyl cyclases (AC). Endogenous cAMP levels in live corals followed a potential diel cycle, as they were higher during the day compared to the middle of the night. Coral homogenates exhibited some of the highest cAMP production rates ever to be recorded in any organism; this activity was inhibited by calcium ions and stimulated by bicarbonate. In contrast, zooxanthellae or mucus had >1000-fold lower AC activity. These results suggest that cAMP is an important regulator of coral physiology, especially in response to light, acid/base disturbances and inorganic carbon levels.

Evidence for involvement of TRPA1 in the detection of vibrations by hair bundle mechanoreceptors in sea anemones

A homolog of TRPA1 was identified in the genome of the anemone, Nematostella vectensis (nv-TRPA1a), and predicted to possess six ankyrin repeat domains at the N-terminus and an ion channel domain near the C-terminus. Transmembrane segments of the ion channel domain are well conserved among several known TRPA1 polypeptides. Inhibitors of TRPA1 including ruthenium red decrease vibration-dependent discharge of nematocysts in N. vectensis and Haliplanella luciae. Activators of TRPA1 including URB-597 and polygodial increase nematocyst discharge in the absence of vibrations. Co-immunoprecipitation yields a band on SDS-PAGE gels at the predicted mass of the nv-TRPA1a polypeptide among other bands. Co-immunoprecipitation performed in the presence of antigenic peptide decreases the yield of this and several other polypeptides. In untreated controls, anti-nv-TRPA1a primarily labels the base of the hair bundle with some labeling also distributed along the length of stereocilia. Tissue immunolabeled in the presence of the antigenic peptide exhibits reduced labeling. Activating chemoreceptors for N-acetylated sugars induce immunolabel to distribute distally in stereocilia. In anemones, activating chemoreceptors for N-acetylated sugars induce hair bundles to elongate among several other structural and functional changes. Taken together, these results are consistent with the possibility that nv-TRPA1a participates in signal transduction of anemone hair bundles.

AA. The behavioral and developmental physiology of nematocysts

Nematocysts are the nonliving secretions of specialized cells, the nematocytes, which develop from multipotent stem cells. Nematocysts are the means by which coelenterates capture prey and defend against predation. The 25 or more known types of nematocysts can be divided into to four functional categories: those that pierce, ensnare, or adhere to prey, and those that adhere to the substrate. During development a collagenous cyst, which may contain toxins, forms; a hollow thread, which becomes coiled as it invaginates, develops. Maturing nematocyte–nematocyst complexes migrate to their discharge sites and are deployed in specific patterns. The mechanisms of pattern determination are not clear. Discharge of nematocysts appears to involve increases in intracapsular osmotic pressure consequent upon release of bound calcium within the capsule; the eversion of the filament may depend upon release of structural tension consequent upon a loss of zinc from the thread. Evidence exists that discharge is initiated as a calcium-dependent exocytosis, triggered by an electrical signal resulting from the transduction of mechanical stimuli received at the nematocyte's cnidocil. Chemical signals transduced in adjacent sensory cells alter the frequency response of the nematocyte. In opposition to the nematocyte–nematocyst independent effector hypothesis, excitatory and inhibitory neuronal input appears to regulate discharge.

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.

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.

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

Hair bundle mechanoreceptors of the acousticolateralis system of vertebrates are similar to hair bundles found on tentacles of sea anemones, primitive marine invertebrates. In each case, hair bundles consist of actin-based stereocilia interconnected by extracellular linkages. Recently, considerable attention has been directed to one class of linkages called "tip links." Tip links interconnect the tip of one stereocilium to the adjacent, taller stereocilium. According to the currently favored hypothesis for signal transduction, tip links may be "gating springs" that gate cation channels opened during deflection of the hair bundle. To depolarize the membrane, deflections must be directed so as to induce strain on the tip links. Deflections in the opposite direction lead to hyperpolarization of the membrane. Hair cells adapt to prolonged deflection of hair bundles. Whereas in some vertebrates, adaptation is incomplete (i.e., the current fails to return to baseline), adaptation in anemones appears to be complete. Signal transduction is reversibly blocked by agents thought to interact with the transduction channel including streptomycin. In addition, signal transduction is abolished following exposure to agents thought to attack tip links including calcium-depleted buffers or elastase. Hair cells of lower vertebrates can be replaced by division and differentiation of supporting cells. In chickens, a repair system exists wherein tip links are replaced via a mechanism that does not involve protein synthesis. The repair mechanism of anemones involves synthesis of new proteins that may constitute replacement linkages and accessory proteins that attach the linkages to appropriate integral proteins.

Repair of hair bundles in sea anemones by secreted proteins

Sea anemones are sessile invertebrates that detect movements of prey using numerous hair bundles located on tentacles surrounding their mouth. Previously we found that hair bundles of anemones are structurally and functionally similar to those of vertebrates. After 10-15 min exposure to calcium depleted buffers, hair bundles in chickens suffer moderate damage from which they recover in 12 h without requiring new protein synthesis [Zhao, Yamoah and Gillespie, Proc. Natl. Acad. Sci. USA 94 (1996) 15469-15474]. We find that after 1 h exposure to calcium free seawater, hair bundles of anemones suffer extensive damage from which they recover in 4 h, apparently because of newly synthesized, secretory proteins called 'repair proteins'. Recovery is delayed in a dose dependent fashion by cycloheximide. In the presence of exogenously added repair proteins, recovery occurs within 8 min and is cycloheximide insensitive. Recovery is ascertained by a bioassay performed on intact specimens, by electrophysiology, and by timelapse video microscopy. Fraction beta, a chromatographic fraction with bioactivity comparable to the complete mixture of repair proteins, consists of complexes having an estimated mass of 2000 kDa. Avidin based cytochemistry suggests that biotinylated fraction beta binds to damaged hair bundles. SDS-PAGE gel electrophoresis demonstrates that fraction beta contains 8-10 polypeptides of 90 kDa or smaller. At least four of these polypeptides apparently are consumed during the repair process. Negatively stained samples of fraction beta are shown by transmission electron microscopy to include filamentous structures similar in length (150 nm) and width (6 nm) to linkages between stereocilia. The filamentous structures can be associated with globular structures (20 nm in diameter). A model is presented wherein repair proteins comprise replacement linkages and enzymes that attach linkages to appropriate membrane proteins.

Mechanotransduction of hair bundles arising from multicellular complexes in anemones

Sea anemones are among the simplest animals to use hair bundles to detect vibrations. Although we previously found anemone bundles to be morphologically similar to vertebrate hair bundles, only indirect evidence implicated anemone bundles in mechanotransduction. Here, we test mechanotransduction of these bundles using loose-patch current recording from apical membranes of cells at the base of deflected bundles. Step bundle deflection results in graded membrane currents that are inward in some cells (positive) and outward in other cells (negative). Positive responses range from 5 to 30 pA, abruptly saturate with stronger stimuli, and increase in duration with prolonged deflections. Negative responses range from 10 to 150 pA, show a logarithmic relation to stimulus strength, and attenuate with prolonged deflections. Additionally, responses are reversibly inhibited by streptomycin. We present a model for anemone bundle mechanotransduction modified from the gating spring model for vertebrate mechanotransduction. Because anemone bundles comprise stereocilia arising from a multicellular complex, we propose that supporting cells on opposite sides of a bundle function as oppositely polarized hair cells. Thus, deflection induces ion channels to open in cells on one side of the complex, while allowing channels to close in cells on the opposite side of the complex.

Class II pyrethroids: noninhibitors calcineurin

Type II pyrethroid insecticides have been reported to be potent inhibitors of bovine brain calcineurin (EC 3.1.3.16, Enan E and Matsumara F, Biochem Pharmacol 43: 1777-1784, 1992). In concentrations up to 10(-5) M, none of the pyrethroid insecticides used in this study caused inhibition of the calcineurin-dependent dephosphorylation of the 19-amino acid phosphopeptide derived from the regulatory subunit R-II of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase, which has been established as a good substrate for this enzyme. Neither did any of the compounds tested cause a shift in the inhibitory activity of okadaic acid (apparent Ki of 5 microM). The assumption that calcineurin is generally inhibited by pyrethroid insecticides is incorrect, and the interpretation of cellular experiments in which this assumption has been made must be revised.

Hair bundles of sea anemones as a model system for vertebrate hair bundles

Sea anemones are marine invertebrates that use hair bundles to detect swimming movements of prey. Prey are captured by nematocysts (stinging capsules) that discharge into the prey. To further characterize anemone hair bundles and to compare hair bundles in anemones with hair bundles in vertebrates, we investigated fine structure and cytochemistry of anemone hair bundles. In addition, using a biological assay based on counting nematocysts discharged into vibrating test probes, we examined sensitivity of vibration detection to aminoglycoside antibiotics, Ca(2+)-free seawater, and amiloride. Like vertebrate hair bundles, anemone hair bundles are composed of stereocilia, possess lateral linkages between stereocilia whose preservation for transmission electron microscopy is enhanced by ruthenium red, and possess tip links morphologically similar to vertebrate tip links. Furthermore, vibration-dependent discharge of nematocysts is reversibly inhibited by 10(-4) M streptomycin and abolished by brief exposure to Ca(2+)-free seawater. However, unlike vertebrate hair bundles, anemone hair bundles appear to be insensitive to amiloride since vibration-dependent discharge of nematocysts is unaffected by up to mM amiloride. Thus, anemone hair bundles may serve as a useful model system for vertebrate hair bundles with the interesting feature of being insensitive to amiloride.

Chemoreceptor-mediated polymerization and depolymerization of actin in hair bundles of sea anemones

Hair bundles located on tentacles of sea anemones are morphodynamic mechanoreceptors employed to regulate discharge of nematocysts into swimming prey. Activation of chemoreceptors for N-acetylated sugars is known to induce anemone hair bundles to elongate while shifting discharge to lower frequencies matching those produced by calmly swimming prey. In the continued presence of N-acetylated sugars, activation of proline receptors is known to induce hair bundles to shorten while shifting nematocyst discharge to higher frequencies presumed to correspond to movements produced by wounded, struggling prey. In the present study, N-acetylneuraminic acid (NANA) causes stereocilia to become more intensely fluorescent in confocal optical sections of phalloidin-stained specimens, suggesting that receptors for N-acetylated sugars initiate processes to increase the density of F-actin within stereocilia. Computer analysis of electron micrographs is consistent with this interpretation for large diameter stereocilia but not for small diameter stereocilia. In the continued presence of NANA, proline causes fluorescence intensity of phalloidin to decrease to or below control levels. DNaseI uniformly stains large diameter stereocilia, suggesting that these stereocilia contain a pool of G-actin. Fluorescence intensity of DNaseI in stereocilia is significantly less bright in specimens exposed to NANA alone than in specimens exposed to proline in the continued presence of NANA. It appears that whereas activated receptors for NANA induce G-actin to polymerize in large diameter stereocilia, activated receptors for proline induce F-actin to depolymerize, restoring G-actin pools.