Iontophoretic localization of Ca-sensitive sites controlling activation of ciliary beating in macrocilia of Beroë: the ciliary rete

Physiology and Evolution of Voltage-Gated Calcium Channels in Early Diverging Animal Phyla: Cnidaria, Placozoa, Porifera and Ctenophora

Voltage-gated calcium (Ca_v) channels serve dual roles in the cell, where they can both depolarize the membrane potential for electrical excitability, and activate transient cytoplasmic Ca²⁺ signals. In animals, Ca_v channels play crucial roles including driving muscle contraction (excitation-contraction coupling), gene expression (excitation-transcription coupling), pre-synaptic and neuroendocrine exocytosis (excitation-secretion coupling), regulation of flagellar/ciliary beating, and regulation of cellular excitability, either directly or through modulation of other Ca²⁺-sensitive ion channels. In recent years, genome sequencing has provided significant insights into the molecular evolution of Ca_v channels. Furthermore, expanded gene datasets have permitted improved inference of the species phylogeny at the base of Metazoa, providing clearer insights into the evolution of complex animal traits which involve Ca_v channels, including the nervous system. For the various types of metazoan Ca_v channels, key properties that determine their cellular contribution include: Ion selectivity, pore gating, and, importantly, cytoplasmic protein-protein interactions that direct sub-cellular localization and functional complexing. It is unclear when these defining features, many of which are essential for nervous system function, evolved. In this review, we highlight some experimental observations that implicate Ca_v channels in the physiology and behavior of the most early-diverging animals from the phyla Cnidaria, Placozoa, Porifera, and Ctenophora. Given our limited understanding of the molecular biology of Ca_v channels in these basal animal lineages, we infer insights from better-studied vertebrate and invertebrate animals. We also highlight some apparently conserved cellular functions of Ca_v channels, which might have emerged very early on during metazoan evolution, or perhaps predated it.

CEP290 tethers flagellar transition zone microtubules to the membrane and regulates flagellar protein content

Mutations in human CEP290 cause cilia-related disorders that range in severity from isolated blindness to perinatal lethality. Here, we describe a Chlamydomonas reinhardtii mutant in which most of the CEP290 gene is deleted. Immunoelectron microscopy indicated that CEP290 is located in the flagellar transition zone in close association with the prominent microtubule-membrane links there. Ultrastructural analysis revealed defects in these microtubule-membrane connectors, resulting in loss of attachment of the flagellar membrane to the transition zone microtubules. Biochemical analysis of isolated flagella revealed that the mutant flagella have abnormal protein content, including abnormal levels of intraflagellar transport proteins and proteins associated with ciliopathies. Experiments with dikaryons showed that CEP290 at the transition zone is dynamic and undergoes rapid turnover. The results indicate that CEP290 is required to form microtubule-membrane linkers that tether the flagellar membrane to the transition zone microtubules, and is essential for controlling flagellar protein composition.

Ciliary reorientation is evoked by a rise in calcium level over the entire cilium

Internal Ca2+ levels control the pattern of ciliary and flagellar beating in eukaryotes. In ciliates, ciliary reversal is induced by a rise in intra-ciliary Ca2+, but the mechanism by which Ca2+ induces reversal is not known. We injected the fluorescent Ca2+ indicator Calcium Green into a ciliate Didinium nasutum and observed the intra-ciliary Ca2+ level during the initial reversed stroke preceding spontaneous cyclic reversed beating. In D. nasutum, Ca2+ rose throughout the length of the cilia undergoing initial reversed stroke. Electron microscopy with a combined oxalate-pyroantimonate method showed Ca2+ deposits distributed throughout the reversed cilia. We injected caged Ca2+ into D. nasutum and irradiated the base or mid region of the cilia with UV to locally increase Ca2+ concentration. Uncaging Ca2+ in the middle of the cilia produced reversal distally, but not proximally to the site of Ca2+ release. These results strongly suggest that not only Ca2+ influx sites, but also Ca2+ binding sites and vectoral bending machineries for ciliary reversal, are distributed throughout the cilium.

A model for flagellar motility

Experimental investigation has provided a wealth of structural, biochemical, and physiological information regarding the motile mechanism of eukaryotic flagella/cilia. This chapter surveys the available literature, selectively focusing on three major objectives. First, it attempts to identify those conserved structural components essential to providing motile function in eukaryotic axonemes. Second, it examines the relationship between these structural elements to determine the interactions that are vital to the mechanism of flagellar/ciliary beating. Third, the vital principles of these interactions are incorporated into a tractable theoretical model, referred to as the Geometric Clutch, and this hypothetical scheme is examined to assess its compatibility with experimental observations.

Visualization of calcium transients controlling orientation of ciliary beat

To image changes in intraciliary Ca controlling ciliary motility, we microinjected Ca Green dextran, a visible wavelength fluorescent Ca indicator, into eggs or two cell stages of the ctenophore Mnemiopsis leidyi. The embryos developed normally into free-swimming, approximately 0.5 mm cydippid larvae with cells and ciliary comb plates (approximately 100 microns long) loaded with the dye. Comb plates of larvae, like those of adult ctenophores, undergo spontaneous or electrically stimulated reversal of beat direction, triggered by Ca influx through voltage-sensitive Ca channels. Comb plates of larvae loaded with Ca Green dextran emit spontaneous or electrically stimulated fluorescent flashes along the entire length of their cilia, correlated with ciliary reversal. Fluorescence intensity peaks rapidly (34-50 ms), then slowly falls to resting level in approximately 1 s. Electrically stimulated Ca Green emissions often increase in steps to a maximum value near the end of the stimulus pulse train, and slowly decline in 1-2 s. In both spontaneous and electrically stimulated flashes, measurements at multiple sites along a single comb plate show that Ca Green fluorescence rises within 17 ms (1 video field) and to a similar relative extent above resting level from base to tip of the cilia. The decline of fluorescence intensity also begins simultaneously and proceeds at similar rates along the ciliary length. Ca-free sea water reversibly abolishes spontaneous and electrically stimulated Ca Green ciliary emissions as well as reversed beating. Calculations of Ca diffusion from the ciliary base show that Ca must enter the comb plate along the entire length of the ciliary membranes. The voltage-dependent Ca channels mediating changes in beat direction are therefore distributed over the length of the comb plate cilia. The observed rapid and virtually instantaneous Ca signal throughout the intraciliary space may be necessary for reprogramming the pattern of dynein activity responsible for reorientation of the ciliary beat cycle.

On the localization of voltage-sensitive calcium channels in the flagella of Chlamydomonas reinhardtii

This study was undertaken to prove that voltage-sensitive calcium channels controlling the photophobic stop response of the unicellular green alga Chlamydomonas reinhardtii are exclusively found in the flagellar region of the cell and to answer the question as to their exact localization within the flagellar membrane. The strategy used was to amputate flagella to a variable degree without perturbing the electrical properties of the cell and measure flagellar currents shortly after amputation and during the subsequent regeneration process. Under all conditions, a close correlation was found between current size and flagellar length, strongly suggesting that the channels that mediate increases in intraflagellar calcium concentration are confined to and distributed over the total flagellar length. Bald mutants yielded tiny flagellar currents, in agreement with the existence of residual flagellar stubs. In the presence of the protein synthesis inhibitor cycloheximide, flagellar length and flagellar currents also recovered in parallel. Recovery came to an earlier end, however, leveling off at a time when in the absence of cycloheximide only half maximal values were achieved. This suggests the existence of a pool of precursors, which permits the maintenance of a constant ratio between voltage-sensitive calcium channels and other intraflagellar proteins.

Actin pegs and ultrastructure of presumed sensory receptors of Beroë (Ctenophora)

We have investigated the actin content and ultrastructure of two kinds of presumed sensory projections on the lip epidermis of beroid ctenophores. Transmission electron microscopy showed that conical pegs contain a large bundle of densely packed, parallel microfilaments. Rhodamine-phalloidin brightly stained the pegs, confirming that they contain filamentous actin. Epidermal cells with actin pegs also bear a single long cilium with an onion-root structure, previously described as arising from a different type of cell. The actin peg and onion-root cilium project side-by-side, defining a polarized axis of the cell which is shared by neighboring cells. The onion-root body is surrounded by a flattened membrane sac which lies immediately below the plasma membrane. The perimeter of the membrane sac is encircled by aggregates of dense material. An extra layer of dense material is found along the side of the membrane sac facing the peg; this material often makes direct contact with the adjacent actin filament bundle. Cells with actin pegs and onion-root cilia synapse onto adjacent neurites and secretory gland cells, indicating that one or both types of projections are sensory elements. Since the feeding responses of beroids are reported to depend on chemical and tactile stimuli to the lips, the cells bearing pegs and cilia may function as both mechanoreceptors and chemoreceptors, that is, as double sensory receptors.

Ca/Ba/Sr-induced conformational changes of ciliary axonemes

Macrocilia of the ctenophore Beroë undergo Ca/Ba/Sr-dependent activation of ciliary beating and microtubule sliding disintegration [Tamm, J. Comp. Physiol. A163:23-31, 1988a; Tamm, Cell Motil. Cytoskeleton 11:126-138, 1988b; Tamm, Cell Motil. Cytoskeleton 12:104-112, 1989; Tamm and Tamm, Proc. Natl. Acad. Sci. U.S.A. 86:6987-6991, 1989]. Here we report that detergent-extracted macrocilia show an ATP-independent conformational change in response to high concentrations of Ca, Ba, or Sr ions. When applied locally by iontophoresis, these ions induce a rapid planar curvature of the distal end of the macrociliary shaft, followed by a slower relaxation to the rest position. Tip curling occurs in a direction opposite to the physiological Ca/Ba/Sr response. When applied uniformly in the bath, a threshold concentration of 10(-1) M Sr is required to induce curling of the tip, and the distal ends remain curved. Calmodulin antagonists do not inhibit the tip curling response. Previous workers found that Ca induces changes in the helical shape of isolated doublet microtubules [Miki-Noumura and Kamiya, Exp. Cell Res. 97:451-453, 1976; Miki-Noumura and Kamiya, J. Cell Biol. 81:355-360, 1979; Takasaki and Miki-Noumura, J. Mol. Biol. 158:317-324, 1982] and sperm axonemes [Okuno and Brokaw, Cell Motil. 1:349-362, 1981] and suggested that conformational changes in microtubules may play a role in Ca regulation of ciliary motility. We propose that the Ca/Ba/Sr-induced curling of the macrociliary tip is due to similar helical changes of doublet microtubules, some of which in macrocilia are prevented from sliding by permanent connections (compartmenting lamellae) between adjacent axonemes within the shaft. Although the tip curling response does not appear to be directly relevant to the physiological Ca response of macrocilia, it provides a novel system for investigating Ca-induced conformational changes of microtubules independent of dynein-powered active sliding.

Calcium sensitivity extends the length of ATP-reactivated ciliary axonemes

We use the Ca-dependent activation response of macrocilia of the ctenophore Beroë to map the distribution of Ca sensitivity along axonemes of detergent-extracted ATP-reactivated models. Local iontophoretic application of Ca (or Sr or Ba) to any site along the length of demembranated macrocilia in ATP-Mg solution elicits oscillatory bending. Bending responses are localized to the site of application of these cations and do not propagate. Ca sensitivity for initiating bends is, therefore, distributed along the entire length of the axonemes. Since Ca triggers ATP-dependent microtubule sliding disintegration of macrociliary axonemes, a Ca-sensitive mechanism for activating microtubule sliding extends the length of the axonemes. In contrast, local application of Ca to living dissociated macrociliary cells elicits beating only when applied to the base of the macrocilium, indicating that the effective site of Ca entry is localized to the membrane at the ciliary base. Therefore, the spatial distributions of membrane Ca permeability and axonemal Ca sensors do not coincide.

Centrin-mediated microtubule severing during flagellar excision in Chlamydomonas reinhardtii

Chlamydomonas cells excise their flagella in response to a variety of experimental conditions (e.g., extremes of temperature or pH, alcohol or detergent treatment, and mechanical shear). Here, we show that flagellar excision is an active process whereby microtubules are severed at select sites within the transition zone. The transition zone is located between the flagellar axoneme and the basal body; it is characterized by a pair of central cylinders that have an H shape when viewed in longitudinal section. Both central cylinders are connected to the A tubule of each microtubule doublet of the transition zone by fibers (approximately 5 nm diam). When viewed in cross section, these fibers are seen to form a distinctive stellate pattern characteristic of the transition zone (Manton, I. 1964. J. R. Microsc. Soc. 82:279-285; Ringo. D. L. 1967. J. Cell Biol. 33:543-571). We demonstrate that at the time of flagellar excision these fibers contract and displace the microtubule doublets of the axoneme inward. We believe that the resulting shear force and torsional load act to sever the axonemal microtubules immediately distal to the central cylinder. Structural alterations of the transition zone during flagellar excision occur both in living cells and detergent-extracted cell models, and are dependent on the presence of calcium (greater than or equal to 10(-6) M). Immunolocalization using monoclonal antibodies against the calcium-binding protein centrin demonstrate the presence of centrin in the fiber-based stellate structure of the transition zone of wild-type cells. Examination of the flagellar autotomy mutant, fa-1, which fails to excise its flagella (Lewin, R., and C. Burrascano. 1983. Experientia. 39:1397-1398), demonstrates that the fa-1 lacks the ability to completely contract the fibers of the stellate structure. We conclude that flagellar excision in Chlamydomonas involves microtubule severing that is mediated by the action of calcium-sensitive contractile fibers of the transition zone. These observations have led us to question whether microtubule severing may be a more general phenomenon than previously suspected and to suggest that microtubule severing may contribute to the dynamic behavior of cytoplasmic microtubules in other cells.

Control of reactivation and microtubule sliding by calcium, strontium, and barium in detergent-extracted macrocilia of Beroë

Macrocilia of the ctenophore Beroë are activated to beat continuously in the normal direction by membrane-mediated Ca2+ influx (Tamm: Journal of Comparative Physiology [A] 163:23-31, 1988a). Using saponin or Brij-58 permeabilized models of macrocilia, we show that ATP-reactivation of beating requires microM levels of free Ca2+, Ba2+, or Sr2+. Isolated macrocilia beat initially in reactivation solution (RS) containing Ca2+, Ba2+, or Sr2+ and then undergo microtubule sliding disintegration without added proteases. Addition of protease inhibitors to RS + 10(-5) M Ca2+ prevents sliding disruption. Pretreatment in wash solution (containing 1 mM EGTA) without protease inhibitors, followed by RS + 10(-5) M Ca2+ with protease inhibitors results in extensive sliding disintegration. However, treatment in wash solution followed by RS + protease inhibitors does not induce sliding. Therefore, Ca2+ is not required for proteolysis by endogenous proteases, but is necessary for sliding disintegration. Local iontophoretic application of Ca2+, Ba2+, or Sr2+ to permeabilized macrocilia in RS lacking these cations triggers motility and/or sliding disintegration. Extrusion of microtubules occurs from the tip or the base, depending on whether or not the macrocilium remains attached to its large actin bundle. Thin sheets of microtubules telescope out initially, due to synchronized sliding of subsets of doublet microtubules from parallel rows of axonemes. Macrocilia are one of the first examples of ATP-induced microtubule sliding which retains Ca2+ sensitivity. In addition, the finding that Ba2+ and Sr2+ also trigger active sliding provides an additional method for investigating the control of dynein-powered microtubule movements.