Protein substrates for cGMP-dependent protein phosphorylation in cilia of wild type and atalanta mutants of Paramecium

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

Cyclic GMP and Cilia Motility

Motile cilia of the lungs respond to environmental challenges by increasing their ciliary beat frequency in order to enhance mucociliary clearance as a fundamental tenant of innate defense. One important second messenger in transducing the regulable nature of motile cilia is cyclic guanosine 3′,5′-monophosphate (cGMP). In this review, the history of cGMP action is presented and a survey of the existing data addressing cGMP action in ciliary motility is presented. Nitric oxide (NO)-mediated regulation of cGMP in ciliated cells is presented in the context of alcohol-induced cilia function and dysfunction.

Calcium regulation in the protozoan model, Paramecium tetraurelia

Early in eukaryotic evolution, the cell has evolved a considerable inventory of proteins engaged in the regulation of intracellular Ca(2+) concentrations, not only to avoid toxic effects but beyond that to exploit the signaling capacity of Ca(2+) by small changes in local concentration. Among protozoa, the ciliate Paramecium may now be one of the best analyzed models. Ciliary activity and exo-/endocytosis are governed by Ca(2+) , the latter by Ca(2+) mobilization from alveolar sacs and a superimposed store-operated Ca(2+) -influx. Paramecium cells possess plasma membrane- and endoplasmic reticulum-resident Ca(2+) -ATPases/pumps (PMCA, SERCA), a variety of Ca(2+) influx channels, including mechanosensitive and voltage-dependent channels in the plasma membrane, furthermore a plethora of Ca(2+) -release channels (CRC) of the inositol 1,4,5-trisphosphate and ryanodine receptor type in different compartments, notably the contractile vacuole complex and the alveolar sacs, as well as in vesicles participating in vesicular trafficking. Additional types of CRC probably also occur but they have not been identified at a molecular level as yet, as is the equivalent of synaptotagmin as a Ca(2+) sensor for exocytosis. Among established targets and sensors of Ca(2+) in Paramecium are calmodulin, calcineurin, as well as Ca(2+) /calmodulin-dependent protein kinases, all with multiple functions. Thus, basic elements of Ca(2+) signaling are available for Paramecium.

A genetic dissection of the photophobic response of Paramecium tetraurelia

Paramecium tetraurelia displayed two behavioral responses upon the initiation of a light stimulus at 7 x 10(4) lux. The cells exhibited a photophobic response in the form of behavioral avoiding reactions, followed by an increase in forward swimming velocity that was significantly higher than prior to the light stimulus activation. It was determined that an intensity of approximately 6.5 x 10(3) lux was required to initiate a moderate avoidance behavioral response. Following the avoiding response, a gradual increase in speed occurred as the intensity increased, indicating that increased swimming speeds are dependent on the light intensity. Two mutants, pawnA and Dancer, were utilized since they affect known Ca(2+)-currents of the cell. The use of pawnA cells, which lack voltage-dependent Ca(2+) channel activity, showed that the two responses to light could be genetically separated, in that the cells showed no avoiding reactions, but did increase their swimming speed. The Dancer cells, which display exaggerated Ca(2+) channel activity, exhibited similar initial avoiding responses as the wild type cells, however did not increase their swimming speed as the intensity of the light was increased. This phenotype as replicated in wildtype cells that had been placed in 25 μM 8-Br-cGMP. These data demonstrate that the photophobic light response of Paramecium tetraurelia can be genetically dissected as a means of elucidating the molecular mechanisms of the light response.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Regulation of ciliary motility: conserved protein kinases and phosphatases are targeted and anchored in the ciliary axoneme

Recent evidence has revealed that the dynein motors and highly conserved signaling proteins are localized within the ciliary 9+2 axoneme. One key mechanism for regulation of motility is phosphorylation. Here, we review diverse evidence, from multiple experimental organisms, that ciliary motility is regulated by phosphorylation/dephosphorylation of the dynein arms through kinases and phosphatases that are anchored immediately adjacent to their axonemal substrates.

Intraflagellar Transport Particles Participate Directly in Cilium-Generated Signaling in Chlamydomonas

Primary cilia are widely used for signal transduction during development and in homeostasis and are assembled and maintained by intraflagellar transport (IFT). Here, we have dissected the role of IFT in signaling within the flagella (structural and functional counterparts of cilia) of the biflagellated green alga Chlamydomonas. Using a conditional IFT mutant enables us to deplete the IFT machinery from intact, existing flagella. We identify a cGMP-dependent protein kinase (CrPKG) within flagella as the substrate of a protein tyrosine kinase activated by flagellar adhesion during fertilization. We demonstrate that flagellar adhesion stimulates association of CrPKG with a new flagellar compartment. Moreover, formation of the compartment requires IFT, and IFT particles themselves are part of the compartment. Our results lead to a model in which the IFT machinery is required not only for assembling cilia and flagella but also for organizing a signaling pathway within the organelles during cilium-generated signaling.

Multigene family encoding 3',5'-cyclic-GMP-dependent protein kinases in Paramecium tetraurelia cells

In the ciliate Paramecium tetraurelia, 3',5'-cyclic GMP (cGMP) is one of the second messengers involved in several signal transduction pathways. The enzymes for its production and degradation are well established for these cells, whereas less is known about the potential effector proteins. On the basis of a current Paramecium genome project, we have identified a multigene family with at least 35 members, all of which encode cGMP-dependent protein kinases (PKGs). They can be classified into 16 subfamilies with several members each. Two of the genes, PKG1-1 and PKG2-1, were analyzed in more detail after molecular cloning. They encode monomeric enzymes of 770 and 819 amino acids, respectively, whose overall domain organization resembles that in higher eukaryotes. The enzymes contain a regulatory domain of two tandem cyclic nucleotide-binding sites flanked by an amino-terminal region for intracellular localization and a catalytic domain with highly conserved regions for ATP binding and catalysis. However, some Paramecium PKGs show a different structure. In Western blots, PKGs are detected both as cytosolic and as structure-bound forms. Immunofluorescence labeling shows enrichment in the cell cortex, notably around the dense-core secretory vesicles (trichocysts), as well as in cilia. Immunogold electron microscopy analysis reveals consistent labeling of ciliary membranes, of the membrane complex composed of cell membrane and cortical Ca2+ stores, and of regions adjacent to ciliary basal bodies, trichocysts, and trafficking vesicles. Since PKGs (re)phosphorylate the exocytosis-sensitive phosphoprotein pp63/pf upon stimulation, the role of PKGs during stimulated exocytosis is discussed, in addition to a role in ciliary beat regulation.

Motility of a biflagellate sperm: waveform analysis and cyclic nucleotide activation

The sperm of the freshwater clam Corbicula fluminea are unusual in that they have two flagella, both of which are capable of beating. When Corbicula sperm are removed from the gonad and placed into freshwater, most remain immotile. Video microscopy was used to assess signaling molecules capable of activating Corbicula sperm motility. Experiments using the cAMP analogs dbcAMP or 8-Br-cAMP show that elevating cAMP activates flagellar motility. Treatments with 8-Br-cGMP activated motility in similar numbers of sperm. Treatments with the selective cAMP-dependent protein kinase (PKA) inhibitor H-89 block activation by 8-Br-cAMP but not by 8-Br-cGMP. Similar treatments with the cGMP-dependent protein kinase (PKG) inhibitor Rp-8-pCPT-cGMPS block activation by 8-Br-cGMP but not by 8-Br-cAMP. These results suggest that cAMP and cGMP each work through their specific kinase to activate flagellar motility. Analysis of spontaneously activated freely swimming sperm shows that the two flagella beat with different parameters. The A flagellum beats with a shorter wavelength and a higher frequency than the B flagellum. The observed differences in flagellar waveform indicate that the flagella are differentially controlled.

Flagellar radial spokes contain a Ca2+-stimulated nucleoside diphosphate kinase

The radial spokes are required for Ca(2+)-initiated intraflagellar signaling, resulting in modulation of inner and outer arm dynein activity. However, the mechanochemical properties of this signaling pathway remain unknown. Here, we describe a novel nucleoside diphosphate kinase (NDK) from the Chlamydomonas flagellum. This protein (termed p61 or RSP23) consists of an N-terminal catalytic NDK domain followed by a repetitive region that includes three IQ motifs and a highly acidic C-terminal segment. We find that p61 is missing in axonemes derived from the mutants pf14 (lacks radial spokes) and pf24 (lacks the spoke head and several stalk components) but not in those from pf17 (lacking only the spoke head). The p61 protein can be extracted from oda1 (lacks outer dynein arms) and pf17 axonemes with 0.5 M KI, and copurifies with radial spokes in sucrose density gradients. Furthermore, p61 contains two classes of calmodulin binding site: IQ1 interacts with calmodulin-Sepharose beads in a Ca(2+)-independent manner, whereas IQ2 and IQ3 show Ca(2+)-sensitive associations. Wild-type axonemes exhibit two distinct NDKase activities, at least one of which is stimulated by Ca(2+). This Ca(2+)-responsive enzyme, which accounts for approximately 45% of total axonemal NDKase, is missing from pf14 axonemes. We found that purified radial spokes also exhibit NDKase activity. Thus, we conclude that p61 is an integral component of the radial spoke stalk that binds calmodulin and exhibits Ca(2+)-controlled NDKase activity. These observations suggest that nucleotides other than ATP may play an important role in the signal transduction pathway that underlies the regulatory mechanism defined by the radial spokes.

Ethanol stimulates ciliary beating by dual cyclic nucleotide kinase activation in bovine bronchial epithelial cells

Previously, we have shown that ethanol (EtOH) stimulates a rapid increase in the ciliary beat frequency (CBF) of bovine bronchial epithelial cells (BBECs) via the activation of PKA. We have also shown that inhibitors of nitric oxide synthase block EtOH-stimulated increases in CBF. We hypothesize that EtOH acutely stimulates CBF via the activation of both PKA and PKG pathways. Using chemiluminescence detection of nitric oxide (NO), we directly measured increases in NO production in BBECs treated with 100 mmol/L of EtOH beginning at 25 minutes. Pretreatment of BBECs with guanylyl cyclase inhibitors, ODQ or LY83583, resulted in the inhibition of EtOH-stimulated CBF. Low concentrations (1 nmol/L) of cyclic nucleotide analogues do not stimulate CBF increases. However, a combination of both 1 nmol/L of 8Br-cAMP and 8Br-cGMP stimulates a significant increase over baseline CBF. This effect could be blocked by pretreating BBECs with inhibitors of either PKA or PKG. Very high concentrations of either 8Br-cAMP or 8Br-cGMP (> or =100 micromol/L) were required to cross-activate both PKA and PKG. This suggests that cross-activation of PKA by cGMP is not occurring at the concentrations (1 nmol/L) capable of stimulating CBF. 8-pCPT-cGMPS, an antagonist analogue to cGMP, blocked EtOH-stimulated PKA activity increases. These data support that EtOH-stimulated increases in CBF require the dual activation of both PKA (via cAMP) and PKG (via NO).

Relaxin stimulates bronchial epithelial cell PKA activation, migration, and ciliary beating

Relaxin is an insulin-like serum protein secreted during pregnancy and found in many tissues, including the lung. Relaxin is reported to stimulate epithelial cell proliferation, but the effects of relaxin on airway epithelium are unknown. We tested the hypothesis that relaxin would stimulate the increased migration of bronchial epithelial cells (BEC) in response to wounding. Using monolayers of BEC in a wound-healing model, relaxin augmented wound closure with maximal closure occurring at 12 hr (1 micro M). Unlike cytokines, relaxin did not stimulate increased BEC interleukin-8 (IL-8) release. Relaxin caused a significant stimulation of ciliary beat frequency (CBF) in BEC. Because protein kinase (PKA) activation increases CBF and relaxin can elevate intracellular cAMP levels, we measured PKA activity in BEC treated with relaxin. Relaxin increased PKA activity 3-4 fold by approximately 4 hr, with a return to baseline levels by 8-10 hr. Relaxin-stimulated PKA activity differs temporally from the rapid (1 hr) beta-adrenergic activation of PKA in BEC. These data suggest that relaxin augments epithelial repair by increasing airway cell migration and CBF via PKA-dependent mechanisms.

Ca2+-binding proteins of cilia and infraciliary lattice ofParamecium tetraurelia: their phosphorylation by purified endogenous Ca2+-dependent protein kinases

We purified two small, acidic calcium-binding proteins(ParameciumCa2+-binding proteins, PCBP-25α and PCBP-25β) from Paramecium tetraurelia by Ca2+-dependent chromatography on phenyl-Sepharose and by anion-exchange chromatography. The proteins were immunologically distinct. Monoclonal antibodies against PCBP-25β did not react with PCBP-25α, and antibodies against centrin from Chlamydomonas reacted with PCBP-25α but not with PCBP-25β. Like the centrins described previously, both PCBPs were associated with the infraciliary lattice (ICL), a fibrillar cytoskeletal element in Paramecium. Both were also present in isolated cilia, from which they could be released (with dynein) by a high-salt wash, and both PCBPs cosedimented with dynein in a sucrose gradient. PCBP-25β was especially prominent in cilia and in the deciliation supernatant, a soluble fraction released during the process of deciliation. The results of immunoreactivity and localization experiments suggest that PCBP-25α is a Paramecium centrin and that PCBP-25β is a distinct Ca2+-binding protein that confers Ca2+ sensitivity on some component of the cilium, ciliary basal body or ICL.

We characterized these proteins and Paramecium calmodulin as substrates for two Ca2+-dependent protein kinases purified from Paramecium. PCBP-25α and calmodulin were in vitro substrates for one of the two Ca2+-dependent protein kinases (CaPK-2), but only PCBP-25α was phosphorylated by CaPK-1. These results raise the possibility that the biological activities of PCBP-25α and calmodulin are regulated by phosphorylation.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Simple fluctuation of Ca2+ elicits the complex circadian dynamics of cyclic AMP and cyclic GMP in Paramecium

The circadian dynamics of cyclic adenosine 3′,5′-monophosphate (cAMP) and cyclic guanosine 3′,5′-monophosphate (cGMP) were simulated in Paramecium multimicronucleatum. The mathematical functions determined closely mimic the Ca2+ dependence of adenylate cyclase (AC) and guanylate cyclase (GC) activities as documented in P. tetraurelia. Patterns of cAMP concentration ([cAMP]), cGMP concentration ([cGMP]), and the ratio [cGMP]/[cAMP] were calculated with respect to Ca2+ concentrations ([Ca2+]) fluctuating sinusoidally with a period of 24 hours at three different levels: low, medium, and high. The functions displayed varying patterns of [cAMP] characteristic for [Ca2+] fluctuating at each level, while patterns of [cGMP] and [cGMP]/[cAMP] almost paralleled [Ca2+] fluctuations. Similar patterns were observed for actual [cAMP] and [cGMP] measured during the light/dark cycle in P. multimicronucleatum, grown in axenic media additionally containing [Ca2+] at 25 (low), 100 (medium), or 400 (high) microM, respectively. The coincidence between simulated and measured fluctuations of [cAMP] and [cGMP] suggests that the circadian fluctuations of intracellular [Ca2+] primarily stimulate activities of AC and GC via their different degrees of Ca2+ dependence, which are ultimately responsible for the circadian spatiotemporal organization of various physiological functions in Paramecium.

Regulation of ciliary beat frequency by both PKA and PKG in bovine airway epithelial cells

Ciliary beating is required for the maintenance of lung mucociliary transport. We investigated the role of cyclic nucleotide-dependent protein kinases in stimulating ciliary beat frequency (CBF) in bovine bronchial epithelial cells (BBECs). cAMP-dependent protein kinase (PKA) activity and cGMP-dependent protein kinase (PKG) activity were distinguished after DEAE-Sephacel chromatography of BBEC extracts. cAMP levels and PKA activity are increased in BBECs stimulated with 0.01-1 mM isoproterenol, with a corresponding increase in CBF. cGMP levels and PKG activity are increased in BBECs stimulated with 0.1-10 microM sodium nitroprusside, with a corresponding increase in CBF. Direct protein kinase-activating analogs of cAMP and cGMP (dibutyryl cAMP and 8-bromo-cGMP, respectively) also activate their specific kinases and stimulate CBF. Preincubation of BBECs with inhibitors of PKA or PKG [KT-5720 or Rp-8-(p-chlorophenylthio)-guanosine 3',5'-cyclic monophosphothioate] results in the inhibition of specific kinase activity as well as in the inhibition of CBF. These studies suggest that the activation of either PKA or PKG can lead to the stimulation of CBF in bovine airway epithelium.