A single gene mutation that affects a potassium conductance and resting membrane potential in Paramecium

Isolation and characterization of magbane, a magnesium-lethal mutant of paramecium

Discerning the mechanisms responsible for membrane excitation and ionic control in Paramecium has been facilitated by the availability of genetic mutants that are defective in these pathways. Such mutants typically are selected on the basis of behavioral anomalies or resistance to ions. There have been few attempts to isolate ion-sensitive strains, despite the insights that might be gained from studies of their phenotypes. Here, we report isolation of "magbane," an ion-sensitive strain that is susceptible to Mg2+. Whereas the wild type tolerated the addition of > or =20 mm MgCl2 to the culture medium before growth was slowed and ultimately suppressed (at >40 mm), mgx mutation slowed growth at 10 mm. Genetic analysis indicated that the phenotype resulted from a recessive single-gene mutation that had not been described previously. We additionally noted that a mutant that was well described previously (restless) is also highly sensitive to Mg2+. This mutant is characterized by an inability to control membrane potential when extracellular K+ concentrations are lowered, due to inappropriate regulation of a Ca2+-dependent K+ current. However, comparing the mgx and rst mutant phenotypes suggested that two independent mechanisms might be responsible for their Mg2+ lethality. The possibility that mgx mutation may adversely affect a transporter that is required for maintaining low intracellular Mg2+ is considered.

Correlation between loss of a Mg2+ conductance and an adaptation defect in a mutant of Paramecium tetraurelia

Paramecium tetraurelia responds to chronic KCl-induced depolarization by swimming backward, but the ciliate recovers within seconds and then undergoes a prolonged adaptation period during which sensitivity to external stimuli is altered radically. We examined the role of Mg2+ in this phenomenon, prompted by finding that mutations in the eccentric-A gene both suppressed a Mg(2+)-specific conductance and prevented adaptation. Adaptation of the wild type proceeded normally when extracellular Mg2+ was varied from 0-20 mM, however, suggesting that channel-mediated Mg2+ fluxes were not involved. In seeking alternative explanations for the eccentric mutant phenotype, we ascertained that there was an osmotic component to adaptation but that K(+)-induced depolarization was the primary stimulus. We also noted that wild-type and eccentric mutant cells depolarized by equivalent amounts in KCl, suggesting that the genetic lesion must lie downstream of membrane-potential change. We also examined whether the adaptation-induced behavioral changes and, indeed, the defect in eccentric might be explained in terms of Mg2+ and Na+ efflux during behavioral testing, but experimental observations failed to support this notion. Finally, we consider the possibility that eccentric gene mutation prevents adaptation by interfering with intracellular free Mg2+ homeostasis in Paramecium.

Oscillating response to a purine nucleotide disrupted by mutation in Paramecium tetraurelia

The purine nucleotide GTP, when added extracellularly, induces oscillations in the swimming behaviour of the protist Paramecium tetraurelia. For periods as long as 10 min the cell swims backwards and forwards repetitively. The oscillations in swimming behaviour are driven by changes in membrane potential of the cell, which in turn are caused by periodic activation of inward Mg2+- and Na+-specific currents. We screened for and isolated mutants that are defective in this response, exploiting the fact that the net result of GTP on a population of cells is repulsion. One mutant, GTP-insensitive (gin A), is not repelled by GTP. In addition, GTP fails to induce repetitive backwards swimming in gin A mutants, although they swim backwards normally in response to other stimuli. GTP fails to evoke oscillations in membrane potential or Mg2+ and Na+ currents in the mutant, although the Mg2+ and Na+ conductances are not themselves measurably affected. A small, oscillating Ca2+ current induced by GTP in the wild type, which might be part of the mechanism that generates oscillations, is also missing from gin A cells. To our knowledge, gin A is the first example of a mutant defective in a purinergic response. We discuss the possibility that the gin A lesion affects the oscillator itself.

Regulation of adenylyl cyclase from Paramecium by an intrinsic potassium conductance

Hyperpolarization of the cell membrane of Paramecium stimulates adenosine 3',5'-monophosphate (cAMP) formation. Manipulations of the K+ resting conductance of the ciliate by adaptation in different buffers affected excitability of the cAMP generating system. Blockade of K+ channels inhibited hyperpolarization-stimulated cAMP formation. A mutant of Paramecium that is unable to control its K+ resting conductance had a defect in cAMP formation. Purified adenylyl cyclase, when incorporated into an artificial lipid bilayer membrane, revealed properties of a voltage-independent K+ channel. This indicates that the adenylyl cyclase of Paramecium has a secondary function as carrier of the K+ resting conductance. A hyperpolarization-activated K+ efflux appears to directly regulate adenylyl cyclase activity in vivo.

Genetic dissection of Ca2(+)-dependent ion channel function in Paramecium

The ciliated protozoan, Paramecium, broadcasts the activity of its individual ion channel classes through its swimming behaviour. This fact has made it possible to isolate mutants with defective ion currents, simply by selecting individuals with abnormal swimming patterns. At least four of Paramecium's ion currents are activated by rising intracellular calcium concentration, including two K+ currents and a Na+ current. A variety of cell lines with defects in these Ca2(+)-dependent currents have been isolated: in several cases, the defects have been traced to mutations in the structural gene for calmodulin. Sequence analysis of calmodulins from these and other Ca2(+)-dependent ion-current mutants may enable a detailed mapping of putative channel interaction domains on the surface of the calmodulin molecule.

Evidence for two K+ currents activated upon hyperpolarization of Paramecium tetraurelia

Hyperpolarization of voltage-clamped Paramecium tetraurelia in K+ solutions elicits a complex of Ca2+ and K+ currents. The tail current that accompanies a return to holding potential (-40 mV) contains two K+ components. The tail current elicited by a step to -110 mV of greater than or equal to 50-msec duration contains fast-decaying (tau approximately 3.5 msec) and slow-decaying (tau approximately 20 msec) components. The reversal potential of both components shifts by 55-57 mV/10-fold change in external [K+], suggesting that they represent pure K+ currents. The dependence of the relative amplitudes of the two tail currents on duration of hyperpolarization suggests that the slow K+ current activates slowly and is sustained, whereas the fast current activates rapidly during hyperpolarization and then rapidly inactivates. Iontophoretic injection of a Ca2+ chelator, EGTA, specifically reduces slow tail-current amplitude without affecting the fast tail component. Both K+ currents are inhibited by extracellular TEA+ in a concentration-dependent, noncooperative manner, whereas the fast K+ current alone is inhibited by 0.7 mM quinidine.

Interactions between mutants with defects in two Ca2(+)-dependent K+ currents of Paramecium tetraurelia

Paramecium tetraurelia possesses two Ca2(+)-dependent K+ currents, activated upon depolarization IK(Ca,d), or upon hyperpolarization IK(Ca,h). The two currents are mediated by pharmacologically distinct ion channel populations. Three mutations of P. tetraurelia affect these currents. Pantophobiac A mutations (pntA) cause calmodulin sequence defects, resulting in the loss of both Ca2(+)-dependent K+ currents. A second mutation, TEA-insensitive A (teaA), greatly enhances IK(Ca,d) but has no affect on IK(Ca,h). A third mutation, restless (rst), also increases IK(Ca,d) slightly, but its principle effect is in causing an early activation of IK(Ca,h). Interactions between the products of these three genes were investigated by constructing three double mutants. Both teaA and rst restore IK(Ca,d) and IK(Ca,h) in pantophobiac A1, but the phenotypes of teaA and rst are not corrected by a second mutation. These observations may indicate a role for the gene products of teaA and rst in regulating the activity of IK(Ca,d) and IK(Ca,h), respectively.

Calcium-dependent potassium channel in Paramecium studied under patch clamp

We have studied a class of Ca2+i-dependent K channels in inside-out excised membrane patches from Paramecium under patch clamp. single channels had a conductance of 72 +/- 9.0 pS in a solution containing 100 mM K+. The channels were selective for K+ over Rb+ with the permeability ratio of 1: 0.56, and over Na+, Cs+ or NH+4 with a ratio 1: less than 0.1. The channel activity was dependent on Ca2+i, which was applied to the cytoplasmic side; the Ca2+i concentration for the half maximal activation was 2 microM. The Hill coefficient for the Ca2+i dependence of the channel activity was 2.58, indicating that more than two Ca2+i bindings are necessary for full activation. Unlike most Ca2+i-dependent K channels in other organisms, the channels in Paramecium were slightly more active upon hyperpolarization than upon depolarization. The voltage dependence was fitted to a Boltzmann curve with 41.2 mV per e-fold change in channel activity. While a high Ca2+i concentration activated the channels, it also irreversibly reduced the channel activity over time. The decay of channel activity occurred faster at higher Ca2+i concentrations. Quaternary ammonium ions suppressed ion passage through the channel; more highly alkylated quaternary ammonium ions were more efficient in blocking. Ba2+i and Ca2+i were relatively ineffective in blockage. it was concluded that these Ca2+i-dependent K channels in Paramecium are different from the previously described Ca2+i-dependent K channels, and are perhaps of a novel class.

Eukaryotic unicells: how useful in studying chemoreception?

The description of the chemoreception pathway in Paramecium is incomplete, but the technical means are available to study these pathways at the molecular level. The hallmark of ciliates is their versatility and their most important attribute is the availability of useful mutants. It is just this versatility and amenability to genetic manipulation that will move the study of Paramecium chemoreception forward and provide useful information for chemoreceptor cell function in general.

A calcium-dependent potassium current is increased by a single-gene mutation in Paramecium

The membrane currents of wild type Paramecium tetraurelia and the behavioral mutant teaA were analyzed under voltage clamp. The teaA mutant was shown to have a greatly increased outward current which was blocked completely by the combined use of internally delivered Cs+ and external TEA+. This, along with previous work (Satow, Y., Kung, C., 1976, J. Exp. Biol. 65:51-63) identified this as a K+ current. It was further found to be a calcium-activated K+ current since this increased outward K+ current cannot be elicited when the internal calcium is buffered with injected EGTA. The mutation pwB, which blocks the inward calcium current, also blocks this increased outward K+ current in teaA. This shows that this mutant current is activated by calcium through the normal depolarization-sensitive calcium channel. While tail current decay kinetic analysis showed that the apparent inactivation rates for this calcium-dependent K+ current are the same for mutant and wild type, the teaA current activates extremely rapidly. It is fully activated within 2 msec. This early activation of such a large outward current causes a characteristic reduction in the amplitude of the action potential of the teaA mutant. The teaA mutation had no effect on any of the other electrophysiological parameters examined. The phenotype of the teaA mutant is therefore a general decrease in responsiveness to depolarizing stimuli because of a rapidly activating calcium-dependent K+ current which prematurely repolarizes the action potential.

Chemoreception in Paramecium tetraurelia: acetate and folate-induced membrane hyperpolarization

Acetic and folic acids hyperpolarize the membrane potential of Paramecium tetraurelia in a concentration-dependent manner. The membrane responses are accompanied by small changes in cell resistance, and are significantly reduced by increasing extracellular cation concentrations, suggesting that the attractants bring about the membrane potential change by increasing cell permeability to cations. The inability to show a reversal potential for the hyperpolarization to attractants suggests that the effects of cations on the response are non-specific, however. The possible roles of Ca++, K+, and Na+ in the attractant-induced responses were further investigated by applying acetate and folate to cells with genetic defects in specific ion conductances, by collapsing the driving forces for these ions, and by testing the effects of ion channel blockers on the responses. These studies suggest that the membrane responses to attractants are not due to the direct effects of increased or decreased membrane permeability to cations. Attempts to block the acetate and folate-induced hyperpolarization by collapsing surface potential or using a mutant with reduced surface charge were inconclusive, as were studies on the possible role of attractant transport in the membrane responses. We hypothesize that the membrane hyperpolarization may be due to either the indirect effects of increased calcium permeability, to extrusion of calcium through activation of a calcium pump, or to a proton efflux.

A mutation that increases a novel calcium-activated potassium conductance of Paramecium tetraurelia

Under two-electrode voltage clamp, a mutant of P. tetraurelia, restless (rst/rst), showed a large increase in induced current and an outward tail current when compared to the wild-type cell for hyperpolarizing voltage steps. An increase in the induced and tail currents is also observed for depolarizing voltage steps. The larger current during voltage steps and tail in the mutant were eliminated by the use of CsCl-filled electrodes and tetraethylammonium ion (TEA+) in the bath solution, characterizing the lesion as affecting a K+ conductance. Ionophoretic injection of ethylene glycol bis-(beta-aminoethyl ether) n,n,n',n-tetraacetic acid (EGTA) to buffer internal Ca2+ concentration reduced the increased K+ current and tail of the restless cell, indicating Ca2+ activation of the K+ current. Time course and amplitude of remaining currents after blockage of K+ conductances with Cs+ and TEA+ were similar in wild-type and restless cells suggesting no restless defect in entry of calcium. The Ca2+-activated sodium current was similar in the mutant to that in wild type arguing against a defect in calcium regulation activating the K+ channel in the restless cell. We conclude that the restless mutation alters a Ca2+-activated potassium conductance other than the one previously described. The multiplicity of Ca2+-activated potassium conductances in Paramecium is discussed.