Relations between ameboid movement and membrane-controlled electrical currents

Endogenous Bioelectrics in Development, Cancer, and Regeneration: Drugs and Bioelectronic Devices as Electroceuticals for Regenerative Medicine

A major frontier in the post-genomic era is the investigation of the control of coordinated growth and three-dimensional form. Dynamic remodeling of complex organs in regulative embryogenesis, regeneration, and cancer reveals that cells and tissues make decisions that implement complex anatomical outcomes. It is now essential to understand not only the genetics that specifies cellular hardware but also the physiological software that implements tissue-level plasticity and robust morphogenesis. Here, we review recent discoveries about the endogenous mechanisms of bioelectrical communication among non-neural cells that enables them to cooperate in vivo. We discuss important advances in bioelectronics, as well as computational and pharmacological tools that are enabling the taming of biophysical controls toward applications in regenerative medicine and synthetic bioengineering.

A bioenergetic mechanism for amoeboid-like cell motility profiles tested in a microfluidic electrotaxis assay

The amoeboid-like cell motility is known to be driven by the acidic enzymatic hydrolysis of ATP in the actin-myosin system. However, the electro-mechano-chemical coupling, whereby the free energy of ATP hydrolysis is transformed into the power of electrically polarized cell movement, is poorly understood. Previous experimental studies showed that actin filaments motion, cytoplasmic streaming, and muscle contraction can be reconstituted under actin-activated ATP hydrolysis by soluble non-filamentous myosin fragments. Thus, biological motility was demonstrated in the absence of a continuous protein network. These results lead to an integrative conceptual model for cell motility, which advocates an active role played by intracellular proton currents and cytoplasmic streaming (iPC-CS). In this model, we propose that protons and fluid currents develop intracellular electric polarization and pressure gradients, which generate an electro-hydrodynamic mode of amoeboid motion. Such energetic proton currents and active streaming are considered to be mainly driven by stereospecific ATP hydrolysis through myosin heads along oriented actin filaments. Key predictions of this model are supported by microscopy visualization and in-depth sub-population analysis of purified human neutrophils using a microfluidic electrotaxis assay. Three distinct phases in cell motility profiles, morphology, and cytoplasmic streaming in response to physiological ranges of chemoattractant stimulation and electric field application are revealed. Our results support an intrinsic electric dipole formation linked to different patterns of cytoplasmic streaming, which can be explained by the iPC-CS model. Collectively, this alternative biophysical mechanism of cell motility provides new insights into bioenergetics with relevance to potential new biomedical applications.

From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes

Eukaryotic cells convert external stimuli into membrane depolarization, which in turn triggers effector responses such as secretion and contraction. Here, we put forward an evolutionary hypothesis for the origin of the depolarization-contraction-secretion (DCS) coupling, the functional core of animal neuromuscular circuits. We propose that DCS coupling evolved in unicellular stem eukaryotes as part of an 'emergency response' to calcium influx upon membrane rupture. We detail how this initial response was subsequently modified into an ancient mechanosensory-effector arc, present in the last eukaryotic common ancestor, which enabled contractile amoeboid movement that is widespread in extant eukaryotes. Elaborating on calcium-triggered membrane depolarization, we reason that the first action potentials evolved alongside the membrane of sensory-motile cilia, with the first voltage-sensitive sodium/calcium channels (Nav/Cav) enabling a fast and coordinated response of the entire cilium to mechanosensory stimuli. From the cilium, action potentials then spread across the entire cell, enabling global cellular responses such as concerted contraction in several independent eukaryote lineages. In animals, this process led to the invention of mechanosensory contractile cells. These gave rise to mechanosensory receptor cells, neurons and muscle cells by division of labour and can be regarded as the founder cell type of the nervous system.

A large-scale screen reveals genes that mediate electrotaxis in Dictyostelium discoideum

Directional cell migration in an electric field, a phenomenon called galvanotaxis or electrotaxis, occurs in many types of cells, and may play an important role in wound healing and development. Small extracellular electric fields can guide the migration of amoeboid cells, and we established a large-scale screening approach to search for mutants with electrotaxis phenotypes from a collection of 563 Dictyostelium discoideum strains with morphological defects. We identified 28 strains that were defective in electrotaxis and 10 strains with a slightly higher directional response. Using plasmid rescue followed by gene disruption, we identified some of the mutated genes, including some previously implicated in chemotaxis. Among these, we studied PiaA, which encodes a critical component of TORC2, a kinase protein complex that transduces changes in motility by activating the kinase PKB (also known as Akt). Furthermore, we found that electrotaxis was decreased in mutants lacking gefA, rasC, rip3, lst8, or pkbR1, genes that encode other components of the TORC2-PKB pathway. Thus, we have developed a high-throughput screening technique that will be a useful tool to elucidate the molecular mechanisms of electrotaxis.

Electrochemical regulation of budding yeast polarity

Cells are naturally surrounded by organized electrical signals in the form of local ion fluxes, membrane potential, and electric fields (EFs) at their surface. Although the contribution of electrochemical elements to cell polarity and migration is beginning to be appreciated, underlying mechanisms are not known. Here we show that an exogenous EF can orient cell polarization in budding yeast (Saccharomyces cerevisiae) cells, directing the growth of mating projections towards sites of hyperpolarized membrane potential, while directing bud emergence in the opposite direction, towards sites of depolarized potential. Using an optogenetic approach, we demonstrate that a local change in membrane potential triggered by light is sufficient to direct cell polarization. Screens for mutants with altered EF responses identify genes involved in transducing electrochemical signals to the polarity machinery. Membrane potential, which is regulated by the potassium transporter Trk1p, is required for polarity orientation during mating and EF response. Membrane potential may regulate membrane charges through negatively charged phosphatidylserines (PSs), which act to position the Cdc42p-based polarity machinery. These studies thus define an electrochemical pathway that directs the orientation of cell polarization.

Dissecting the Molecular Mechanisms of Electrotactic Effects

Significance: Steady electric fields (EFs) surround cells and tissues in vivo and may regulate cellular behavior during development, wound healing, or tissue regeneration. Application of exogenous EFs of similar magnitude as those found in vivo can direct migration, growth, and division in most cell types, ranging from bacteria to mammalian cells. These EF effects have therapeutic potential, for instance, in accelerating wound healing or improving nerve repair. EFs are thought to signal through the plasma membrane to locally activate or recruit components of the cytoskeleton and the polarity machinery. How EFs might function to steer polarity is, however, poorly understood at a molecular level. Recent Advances: Here, we review recent work introducing genetically tractable systems, such as yeast and Dictyostelium cells, that begin to identify proteins and pathways involved in this response both at the level of ion transport at the membrane and at the level of cytoskeleton regulation. Critical Issues: These studies highlight the complexity of these EF effects and bring important novel views on core polarity regulation. Future Directions: Future work pursuing initial screening in model organisms should generate broad mechanistic understanding of electrotactic effects.

Large electrical currents traverse developing Ceropia follicles

An intense (up to 20 muA/cm(2)) steady electrical current enters the anterior or nurse cell end of the growing follicle (or oocyte-nurse cell complex) of the Cecropia moth and is balanced by a more diffuse current leaving elsewhere. In late growth stages, the total transfollicular current is about 100 nA. Moreover, a separate small current, of about 1 nA, seems to leave the furrow between the oocyte and the nurse cells. After the nurse cells collapse, but before shell formation, the transfollicular current is redistributed so that a second relatively localized inward current appears at the posterior pole of the follicle. Thus, at this later stage currents enter both poles of the follicle and leave its sides. Previous measurements, with intracellular microelectrodes, seem to imply a very large (order of 1000 nA) back current across the cytoplasmic bridge between the oocyte and nurse cells. A simple model is presented that attributes the apparent bridge current, and the more directly measured transfollicular and furrow currents, to the action of an ion pump lying within the nurse cell face of the furrow membrane.

Calcium signalling during chemotaxis

The role of Ca2+ in chemotaxis of eosinophils from the newt Taricha granulosa was investigated using fluorescent indicators and digital imaging microscopy. In response to serum chemoattractant, cytoplasmic Ca2+ concentration ([Ca2+]i) rises prior to polarization. In polarized locomoting cells [Ca2+]i gradients (tail-high-front-low) are always seen, and when cells turn [Ca2+]i rises transiently and falls fastest and furthest in the new direction of cell motion. These Ca2+ signals, which are required for polarization and locomotion, arise from Ca2+ derived from internal stores released in response to inositol 1,4,5-trisphosphate (InsP3) (because microinjected heparin fully blocks them). 1,2-Diacyl-sn-glycerol (DAG), which is co-produced with InsP3, has an inhibitory effect on Ca2+ signals, an effect apparently mediated by protein kinase C. Studies with caged InsP3 reveal that InsP3-responsive stores appear to be concentrated in the nuclear and microtubule-organizing centre regions and that InsP3 moves so rapidly within the cell that it is effectively a global secondary messenger. Thus, stable [Ca2+] gradients observed during unidirectional migration appear to result from the concentration of InsP3-responsive Ca2+ stores in the rear of the cell. By contrast, we propose that reorientation of the [Ca2+] gradient prior to a change in direction of motion results from the joint actions of InsP3 and DAG, with InsP3 acting as a global secondary messenger stimulating Ca2+ release and DAG, through protein kinase C, acting as a spatially restricted secondary messenger inhibiting [Ca2+] increases locally near the site of chemotactic stimulation.

Input-output relationship in galvanotactic response of Dictyostelium cells

Under a direct current electric field, Dictyostelium cells exhibit migration towards the cathode. To determine the input-output relationship of the cell's galvanotactic response, we developed an experimental instrument in which electric signals applied to the cells are highly reproducible and the motile response are analyzed quantitatively. With no electric field, the cells moved randomly in all directions. Upon applying an electric field, cell migration speeds became about 1.3 times faster than those in the absence of an electric field. Such kinetic effects of electric fields on the migration were observed for cells stimulated between 0.25 and 10 V/cm of the field strength. The directions of cell migrations were biased toward the cathode in a positive manner with field strength, showing galvanotactic response in a dose-dependent manner. Quantitative analysis of the relationship between field strengths and directional movements revealed that the biased movements of the cells depend on the square of electric field strength, which can be described by one simple phenomenological equation. The threshold strength for the galvanotaxis was between 0.25 and 1 V/cm. Galvanotactic efficiency reached to half-maximum at 2.6 V/cm, which corresponds to an approximate 8 mV voltage difference between the cathode and anode direction of 10 microm wide, round cells. Based on these results, possible mechanisms of galvanotaxis in Dictyostelium cells were discussed. This development of experimental system, together with its good microscopic accessibility for intracellular signaling molecules, makes Dictyostelium cells attractive as a model organism for elucidating stochastic processes in the signaling systems responsible for cell motility and its regulations.

A computerized 2-dimensional vibrating probe for mapping extracellular current patterns

We describe a computer-assisted 2-dimensional vibrating probe system for mapping endogenous electric current patterns in biological preparations. This system overcomes some of the main limitations of the original 1-dimensional vibrating probe design and adds several new capabilities. Two piezo-electric bender elements mounted perpendicularly are used to vibrate the probe in a circle by applying 2 sine waves (1 to each element) that are 90 degrees out-of-phase with each other. The circular rotation of the probe allows it to detect simultaneously the 2 orthogonal components of a current in the horizontal plane. The voltages measured by the probe are digitized and analyzed by a computer and are used to calculate a current vector. A graphical representation of the current vector is then superimposed on a video image of the experimental preparation. This probe system responds to known currents in the expected manner and exhibits a low inherent noise level. Also included in this paper are some preliminary measurements made with this instrument on neurulating Xenopus embryos and on transected larval sea lamprey (Petromyzon marinus) spinal cords.

Localized activity of Ca2+-stimulated ATPase and transcellular ionic currents during mesoderm induction in embryos ofLymnaea stagnalis (Mollusca)

InLymnaea stagnalis, mesoderm induction occurs at the 24-cell stage, when the apical tip of the macromere 3D establishes a close contact with a number of micromeres. Via its tip, the macromere 3D is supposed to receive an inductive signal from the micromeres, resulting in the determination of the mesodermal stem cell 4d at the next division. In view of the possibility that transcellular ionic currents might somehow be involved, either in the processes that bring about this particular configuration of blastomeres or in the induction process itself, we mapped the electric field around the embryo during the 24-cell stage, using a vibrating probe. We detected a reversal of the current direction as compared to the uncleaved egg, whilst the polarity of the field along the animal-vegetal axis was maintained. We also mapped the localization of Ca²⁺-stimulated AT-Pase, an enzyme that drives the Ca²⁺-efflux from the cell. We found that this enzyme is localized exclusively along the cytoplasmic face of the apical plasma membrane of macromere 3D, and that its presence is restricted to the period from 110 to 135 min after the fifth cleavage, when there is close contact between macormere 3D and the micromeres. Since the localization of the Ca²⁺-stimulated ATPase coincides both in time and space with the induction of the mesoderm-mother cell, we suggest that localized calcium fluxes may play a role in this induction process.

Cation distribution in the organ of Corti of the guinea pig

Cations were precipitated with potassium antimonate in the cochlea of the guinea pig and the distribution of the formed precipitates was studied by electron microscopy. The precipitate density in different cells of the organ of Corti was determined on electron micrographs by counting numbers of precipitates per unit area. The spatial distribution of the precipitates was also determined by electron spectroscopic imaging (ESI). Significant differences were found among the cells of the same tissue being analyzed. These precipitate-rich cells may play a role in a postulated current flow in the organ of Corti.

Cytoimmunofluorescent localization of severin in Dictyostelium amoebae

Severin is a 40-kDa Ca2+-activated protein from Dictyostelium that rapidly fragments and disassembles actin filaments in vitro (S.S. Brown, K. Yamamoto, and J.A. Spudich, J. Cell Biol. 93, 205-210, 1982; and K. Yamamoto, J.D. Pardee, J. Reidler, L. Stryer, and J.A. Spudich. J. Cell Biol. 95, 711-719, 1982). To determine if severin is colocalized with actin filaments in vivo, we have used the agar-overlay technique of S. Yumura, H. Mori, and Y. Fukui (J. Cell Biol. 99, 894-899, 1984) to examine the intracellular locations of severin and F-actin in vegetative Dictyostelium amoebae. In rounded cells taken from suspension culture severin colocalized with F-actin at cortical edges while maintaining an endoplasmic presence. Both severin and F-actin were present throughout nascent pseudopods of motile cells, while severin appeared concentrated at the leading edge of fully developed pseudopods. Amoebae feeding on a bacterial lawn formed large phagocytic vesicles that were surrounded by an extensive cell cortex rich in severin. Streaming cells entering aggregates during the Dictyostelium developmental cycle showed severin staining throughout the cytoplasm with F-actin at the cortex. The preferential localization of severin in cytoplasmic regions of vegetative cells undergoing extensive actin cytoskeletal rearrangement prompts consideration of a role for severin-mediated disruption of actin filament networks during pseudopod extension and phagocytosis.

Pulsed subcutaneous electrical stimulation in spinal cord injury: preliminary results

The treatment of long-term, stable para- and quadriplegics with pulsed electrical stimulation for pain control resulted in, anecdotally, a significant number of these individuals showing increased motor function as well as sensory awareness. This small pilot study was conducted in order to assess the hypothesis that pulsed electrical fields can effect diseased neurological function. Thirteen para- and quadriplegic subjects with 18 months of stable neurological signs and symptoms were exposed daily to pulsed electrical stimulation for a 6-month period and assessed for any improvement in motor function or sensory perception. The hypothesis is that pulsed electromagnetic fields can normalize viable but dysfunctional neuronal structures. Results were encouraging.

Electrophysiological properties of Achlya hyphae: ionic currents studied by intracellular potential recording

The electrical properties of the water mold Achlya bisexualis were investigated using intracellular microelectrodes. Hyphae growing in a defined medium maintained a membrane potential (Vm) of -150 to -170 mV, interior negative. Under the conditions used here, this potential was insensitive to changes in the inorganic ion composition of the medium. Changes in external pH did affect Vm, but only outside the physiological pH range. By contrast, the addition of respiratory inhibitors caused a rapid depolarization without affecting the conductance of the plasma membrane. Taken together these findings strongly suggest that the membrane potential is governed by an electrogenic ion pump rather than by an ionic diffusion potential. Previous work from this laboratory showed that Achlya hyphae generate a transcellular proton current that enters the growing tip, flows along the hyphal length, and exits distally from the trunk. These initial experiments used an extracellular vibrating electrode, and I now report intracellular electrical recordings which support the hypothesis that protons enter the tip by symport with amino acids and are expelled distally by a proton-translocating ATPase. Most significantly, current flowing intracellularly along the hyphal length is associated with a cytoplasmic electric field of 0.2 V/cm or greater. Conditions that inhibit the current also abolish the internal field, suggesting that these two phenomena are closely linked.

Chemotaxis and cell motility in the cellular slime molds

Chemotaxis and cell motility have essential roles to play throughout the developmental cycle of the cellular slime molds. The particular emphasis of this review, however, will be on the amoeboid stages of the life cycle. The nature of the chemoattractants and their detection will be discussed as will the possible mechanisms that may account for the directed locomotion of amoebae. Intracellular chemoattractant-elicited molecular responses thought to play a role in transduction of extracellular signals into a motility response will also be examined. Furthermore, relationships of these transduction pathway components with changes in assembly states of the cytoskeletal proteins contributing to shape change and cell movement will be assessed. Theories of amoeboid movement involving these cytoskeletal proteins will be compared and discussed in terms of their relevance to cellular slime mold motility.

Polarity of isolated blastomeres from mouse morulae: detection of transcellular ion currents

Eight- to sixteen-cell stage mouse morulae were dissociated with Ca2+-free medium into blastomeres that were labeled with fluoresceinated-succinylated Con A (FS-Con A) to mark their apical-basal axes. The vibrating probe was then used to map their extracellular current patterns. The average current density around normal blastomeres approached the resolution of the probe system (0.2 microA/cm2) and was undetectable in the majority of blastomeres. Since the current density at the measuring point outside the cell is known to increase with cell size in other systems, enlarged blastomeres were created by fusing together blastomeres of 4-cell stage embryos in 45% polyethylene glycol. Enlarged blastomeres were then aggregated with normal blastomeres using phytohemagglutinin and cultured to the 8- to 16-cell stage to allow them to become polarized. Such aggregates were then dissociated with Ca2+-free medium to recover polarized, enlarged blastomeres. The enlarged blastomeres were 30-65 microns in diameter and 70% of them generated a detectable current; currents were detected around 83% of those blastomeres larger than 40 micron in diameter. The current pattern in these most reliable cases was predominantly inward apical (11/16 or 69%) and outward basal (15/16 or 94%), with lateral currents about three-fold smaller in amplitude than these apical-basal currents. Lateral currents were undetectable in 53% of the cases. Preliminary data suggest that the inward current is carried in part by Na+ influx and is independent of the Na+,K+-ATPase over the short term. Transcellular ion currents were detectable as long as 4 hr after dissociation, and the apical-basal current pattern was usually stable during that time. In contrast, the fluorescent cap of FS-Con A faded within 7-30 min at 35 degrees C but remained stable in 0.1% azide or 1.5 micrograms/ml cytochalasin D. The electrical polarity therefore persisted after the apical cap of Con A fluorescence was no longer visible. We propose that these transcellular ion currents may be involved in the establishment of blastomere polarity and describe a mechanism of action in an "ion current polarization" hypothesis.

Perpendicular orientation and directional migration of amphibian neural crest cells in dc electrical fields

The behavior of cultured neural crest cells of Ambystoma mexicanum and Xenopus laevis in dc electrical fields was studied. In fields of 1-5 V/cm, isolated or confluent cells retract both their anode- and cathode-facing margins. Subsequently, the cells elongate, with protrusive activity confined to their narrow ends. In larger fields (greater than or equal to 5 V/cm), protrusions form on the cathode-facing sides of the perpendicularly oriented cells. The cells then begin migrating laterally, perpendicular to their long axes, towards the cathode. We suggest that the perpendicular alignment and cathode-directed migrations result from cytoskeletal changes mediated by modified ion fluxes through the anode-facing (hyperpolarized) and cathode-facing (depolarized) cell membranes. The breaking of cellular confluence in response to dc electric fields is also discussed.

Quantitative analysis of changes in cell shape of Amoeba proteus during locomotion and upon responses to salt stimuli

A new parameter expressing the complexity of cell shape defined as (periphery)2/(area) in 2D projection was found useful for a quantitative analysis of changes in the cell shape of Amoeba proteus and potentially of any amoeboid cells. During locomotion the complexity and the motive force of the protoplasmic streaming in amoeba varied periodically, and the Fourier analysis of the two showed a similar pattern in the power spectrum, giving a rather broad peak at about 2.5 X 10(-3) Hz. The complexity increased mainly due to elongation of the cell as external Ca2+ increased. This effect was blocked by La3+, half the inhibition being attained at 1/200 amount of the coexisting Ca2+. On the other hand, the complexity decreased due to rounding up of the cell as the concentration of other cations, such as Sr2+, Mg2+, Co2+, Ni2+, Na+, K+ etc., increased. Irrespective of the opposite effects of Ca2+ and other cations on the cell shape, the ATP concentration in amoeba decreased in both cases with increase of all these cations. The irregularity in amoeboid motility is discussed in terms of a dynamic system theory.

Ca2+-sensitive isolation of a cortical actin matrix from Dictyostelium amoebae

A cortical actin matrix has been isolated from amoebae of Dictyostelium discoideum grown in liquid culture. The existence of this actin matrix in whole cells is indicated in electron micrographs as an area free of cytoplasmic organelles. The actin beneath the membrane is more clearly visible in sections of cells that are lysed gently with 0.5% Triton X-100 and fixed with 1% glutaraldehyde. Such Triton-lysed cells have fragments of plasma membrane associated with the cortical actin matrix. Isolation of the actin matrix, which sediments at 400 g, is inhibited by Ca2+. As much as 50% of the actin of the cell and about 12% of the total protein is found in the matrix isolated in lysis buffer containing no added Ca2+ and 2.5 mM EGTA, whereas less than 15% of the actin of the cell is recovered in a 400 g pellet when cells are lysed in buffer containing 2.5 mM Ca2+ and 2.5 mM EGTA. A 40 000 molecular weight protein that fragments F-actin in a Ca2+-dependent manner is not found in the isolated cortical actin matrix.

Directional protrusive pseudopodial activity and motility in macrophages induced by extracellular electric fields

Extracellularly applied electric fields (less than 12 V/cm) strongly influence murine resident peritoneal macrophages (M phi) to undergo directional protrusive pseudopodial activity towards the positive pole of the electric fields in the absence of exogenously applied chemotactic ligands. Internal and external morphological features were not grossly disrupted by the fields. Directional motility induced by the electric fields was inhibited in the presence of 1.0 mM La3+ or 2.5 mM Mg2+ and 5.0 mM EGTA. Effects of the fields were latent in the inhibited cells and directional motility was expressed after termination of the field and removal of the inhibitors. Receptors for the lectins concanavalin A (Con A) and phytohemagglutinin (PHA-L) were uniformly distributed on the surfaces of M phi with no exposure to electric fields. After exposure to the fields, Con A receptors were preferentially distributed on regions of the M phi surface facing the negative pole and PHA-L receptors were preferentially distributed on those regions facing the positive pole. The possibility that directional M phi motility is regulated by the molecular topography of the cell surface is discussed.

Inward rectification in mouse macrophages: evidence for a negative resistance region

The electrical properties of cultured mouse thioglycollate-induced peritoneal macrophages were investigated using intracellular recording techniques. Thirty-five percent of the cells studied had membrane potentials ranging from -65 to -95 mV and exhibited S-shaped, steady-state current-voltage (I-V) relationships containing a transitional region. Analysis of currents in the transitional region from the rate of rise and fall of the voltage responses to current pulses indicated the presence of a negative resistance region in this area. Tetrodotoxin (3 X 10(-5) M), cobalt chloride (3 mM), 4-aminopyridine (4 mM), and tetraethylammonium chloride (8 mM) did not eliminate the transitional region of the I-V curves, whereas addition of barium chloride (4 mM) and rubidium chloride (3 mM) did. Increasing the external concentration of potassium shifted the I-V relationship horizontally along the current axis but did not eliminate the transitional region. These data indicate that the inward rectification and the negative resistance region probably result from a voltage-dependent potassium conductance.

Microtubule assembly and disassembly at alkaline pH

Although it is now apparent that the intracellular pH may rise considerably above neutrality under physiological conditions, information on the effect of alkaline pH on microtubule assembly and disassembly is still quite fragmentay. We have studied the assembly/disassembly of bovine brain microtubule protein at alkaline pH in vitro. When microtubules are assembled to a new steady state at pH less than 7 and pH is then made more alkaline, they undergo a rapid disassembly to a new steady state. This disassembly is reversed by acidification. The degree of disassembly is determined largely by the pH- dependence of the critical concentration, which increases five to eight times, from pH 7 to 8. A fraction of assembly-incompetent tubulin is identified that increases with pH, but its incompetency is largely reversed with acidification. Measurements of microtubule lengths are used to indicate that disassembly occurs by uniform shortening of microtubules. A comparison of shortening by alkalinization with dilution suggests that the intrinsic rate of disassembly is accelerated by increasing pH. The capacity for initiating assembly is progressively lost with incubation at alkaline pH (although some protection is afforded by sulfhydryl-reducing agents). However, direct assembly from depolymerized mixtures is possible at least up to pH 8.3, and the steady state achieved at these alkaline pH values is stable. Such preparations are readily disassembled by cold and podophyllotoxin (PLN). Disassembly induced by PLN is also markedly enhanced at alkaline pH, suggesting a corresponding enhancement of "treadmilling." The implications of physiological events leading to alkaline shifts of pH for microtubule assembly/disassembly are discussed, particularly in the light of recent hypotheses regarding treadmilling and its role in controlling the distribution of microtubules in vivo.

Pinocytosis and locomotion of amoebae. XV. Visualization of Ca++-dynamics by chlorotetracycline (CTC) fluorescence during induced pinocytosis in living Amoeba proteus

The dynamics of Ca++ during induced pinocytosis were studied in Amoeba proteus using chlorotetracycline (CTC). The fluorescence of the Ca++ - CTC-complex was monitored by an image intensification system, which has certain advantages over standard equipment: (1) Living cells are not subjected to the damaging influence of intensive microscopic illumination, (2) fluorescent probes are not bleached during observation, and (3) the rapid dynamics of the Ca++ -fluxes can be recorded using short exposure times. The results demonstrate the existence of Ca++ bound to intracellular and extracellular sites of the cell membrane complex in normal locomoting and pinocytotic Amoeba proteus. The application of cations inducing pinocytosis causes a rapid decrease in the external CTC-fluorescence probably due to a release of Ca++ from the mucous layer. The degree of fluorescence intensity is correlated with the capacity of pinocytotic channel formation, i.e., the fluorescence decreases as the number of channels increases. During the phase of vesiculation a distinct fluorescence mainly restricted to the basal region of the channels is observed. Intracellular Ca++ was detected in close vicinity to the plasma membrane after both microinjection and external application of CTC. The internal CTC-fluorescence is slightly decreased during the induction phase of pinocytosis. The observations are in good agreement with previous results on the localization of Ca++ -binding sites at the plasma membrane of Amoeba proteus and demonstrate the important role of Ca++ -fluxes for the process of pinocytosis.

Endogenous electrical currents in the water mold Blastocladiella emersonii during growth and sporulation

We have explored the pattern of electrical currents generated by single cells of the water mold Blastocladiella emersonii at several stages of its life cycle. Extracellular currents were measured with a vibrating probe constructed after the design of Jaffe and Nuccitelli [Jaffe, L. F. & Nuccitelli, R. (1974) J. Cell Biol. 63, 614-628]. In growing cells positive current, of the order of 1 microA/cm2, enters the rhizoid and leaves from the thallus; circumstantial evidence suggests that protons carry much of the current. Sporulation is associated with reversal of the current pattern, such that positive current enters the thallus and leaves from the rhizoidal region; the ions that carry the current have not been identified. These current patterns appear to play a role in the spatial localization of fungal growth and development.

Intracellular pH in single motile cells

Cytoplasmic pH in single living specimens of Chaos carolinensis is determined microfluorometrically by measuring the ratio of fluorescence intensity of microinjected fluorescein-thiocarbamyl (FTC)-ovalbumin at two different excitation wavelengths. The probe is evenly distributed throughout, and confined to, the cytoplasm, and the fluorescence intensity ratio depends only upon pH. It is independent of pathlength, concentration of probe, divalent cations, and ionic strength. Ratios are calibrated with a standard curve generated in situ by adjusting internal pH of FTC-ovalbumin-containing amebae with weak acid and weak base or by injection of strong buffers. With this technique, the average cytoplasmic pH of freely moving ameba is found to be 6.75 (SD +/- 0.3). The pH of a given spot relative to the morphology of a moving ameba remains fairly constant (+/- 0.05 U), whereas the pH of two different spots in the same cell may differ by as much as 0.4 U, and average pH in different amebae ranges from 6.3 to 7.4, with a suggestion of clustering about pH 6.5 and 6.8. During wound healing, there is a local, transient drop in pH (as great as 0.35 U) at the wound site upon puncture, proportional in extent to the degree of damage. Comparison of tails and advancing pseudopod tips reveals no significant difference in cytoplasmic pH at this level of spatial (50 microns diameter spot) and temporal (1.3 s) resolution. Fluctuations in intracellular pH and/or intracellular free Ca++ may be involved in regulation of cytoplasmic structure and contractility.

Contractile basis of ameboid movement. VII. Aequorin luminescence during ameboid movement, endocytosis, and capping

Aequorin luminescence has been utilized to determine the spatial and temporal fluctuations of the free calcium ion concentration [Ca++] in Chaos carolinensis during ameboid movement, pinocytosis, and capping. The [Ca++] increases above approximately 10(-7) M during normal ameboid movement. Three types of luminescent signals are detected in cells: continuous luminescence, spontaneous pulses, and stimulated pulses. Continuous luminescence is localized in the tails of actively motile cells, and spontaneous pulses occur primarily over the anterior regions of cells. We are sometimes able to correlate the spontaneous pulses with extending pseudopods, whereas stimulated pulses are induced by mechanical damage, electrical stimulation, concanavalin A-induced capping, and pinocytosis. The localization of both distinct actin structures and sites where [Ca++] increases suggests cellular sites of contractile activity. The independent evidence from localizing actin structures and the distribution of [Ca++] can also be viewed in relation to the solation-contraction coupling hypothesis defined in vitro.

Contractile basis of ameboid movement. VII. The distribution of fluorescently labeled actin in living amebas

The technique of molecular cytochemistry has been used to follow the distribution of fluorescently labeled actin in living Chaos carolinensis and Amoeba proteus during ameboid movement and various cellular processes. The distribution of 5-iodoacetamidofluorescein-labeled actin was compared with that of Lissamine rhodamine B sulfonyl chloride-labeled ovalbumin microinjected into the same cell and recorded with an image intensification microscope system. Actively motile cells demonstrated a rather uniform distribution of actin throughout most of the cytoplasm, except in the tail ectoplasm and in plasma gel sheets, where distinct actin structures were observed. In addition, actin-containing structures were induced in the cortex during wound healing, concanavalin A capping, pinocytosis, and contractions elicited by phalloidin injections. The formation of distinct fluorescent actin structures has been correlated with contractile activities.

Cytoplasmic free calcium and amoeboid movement

Measurements with the Ca2+ -sensitive photoprotein aequorin show that locomotion in the amoeba Chaos carolinense occurs without changes in the aequorin signal and that not more than 0.025% of the cytoplasm can exist at the micromolar threshold concentration for contraction. The results do not support the hypothesis that cytoplasmic streaming is under the control of changes in the cytoplasmic free Ca2+ concentration.

HCO 3 (-) and OH (-)transport across the plasmalemma ofChara : Spatial resolution obtained using extracellular vibrating probe

Internodal and whorl (branch) cells of the green alga,Chara corallina Klein ex Willd., em. R.D.W., were studied with the extracellular vibrating probe for measuring transmembrane ion currents, and with an extracellular pH microprobe for measuring the surface pH profile. Bands of positive inward current (OH(-) efflux) 1-3 mm wide were separated by wider bands of outward current (HCO 3 (-) influx) along the length of the cell. The measured peaks of inward current ranged from 20 to 60 μA cm(-2) (98 μm from the cell surface) which would correspond to a surface ionic flux of 270-800 pmol cm(-2) s(-1). The peaks of outward current (HCO 3 (-) influx) ranged from 10 to 30 μA cm(-2) which would correspond to a surface ionic flux of 140-400 pmol cm(-2) s(-1). The inward current bands matched the regions of surface alkalinity very well. The outward current (HCO 3 (-) influx) was reduced at least 10-fold in low-HCO 3 (-) medium, with a commensurate readjustment in the strength and pattern of inward current (OH(-) efflux). (Although these experiments involved a manipulation of the external pH, it is felt that the main adjustment in current patterns was in response to the reduction in exogenous HCO 3 (-) ). The presence of the vibrating probe perturbed the inward current region when vibrating with a 26-μm amplitude, but this perturbation was eliminated when a 7-μm amplitude was used. The perturbation was usually observed as a reduction in the number of inward current peaks with an increase (approximate doubling) in the amplitudes of the one or two remaining peaks. Both the inward and outward currents were light-dependent, falling off within seconds of light removal.

The contractile basis of ameboid movement. VI. The solation-contraction coupling hypothesis

The contracted pellets derived from a high-speed supernate of Dictyostelium discoideum (S3) were investigated to determine the functional activity associated with this specific subset of the cellular motile apparatus. A partially purified model system of gelation and contraction (S6) was prepared from the contracted pellets, and the presence of calcium- and pH-sensitive gelation and contraction in this model demonstrated that a functional cytoskeletal-contratile complex remained at least partially associated with the actin and myosin during contraction. Semi-quantitative assays of gelation and solation in the myosin-free preparation S6 included measurements of turbidity, relative viscosity, and strain birefringence. The extent of gelation was optimal at pH 6.8 and a free calcium ion concentration of approximately 3.0 x 10(-8) M. Solation was favored when the free calcium ion concentration was greater than 7.6 x 10(-7) M or when the pH was increased or decreased from pH 6.8. Gelation was reversibly inhibited by increasing the free calcium ion concentration to approxomately 4.6 x 10(-6) M at pH 6.8. The solation-gelation process of this model has been interpreted to involve the reversible cross-linking of actin filaments. The addition of purified D. discoideum myosin to S6 served to reconstitute calcium- and pH-regulated contraction. The results from this study indicate that contraction is coupled functionally to the local breakdown (solation) of the gel. Therefore, solation has been identified as a structural requirement for extensive shortening during contraction. We have called this concept the solation-contraction coupling hypothesis. Fractionation of a preparation derived from the contracted pellets yielded a fraction consisting of actin and a 95,000-dalton polypeptide that exhibited calcium-sensitive gelation at 28 degrees C and a fraction composed of actin and 30,000- and 18,000-dalton polypeptides that demonstrated calcium-sensitive genlation at 0 degrees C.

Electrophysiology of phagocytic membranes. II. Membrane potential and induction of slow hyperpolarizations in activated macrophages

The potential differences measured on the cell surface and after penetration into the cytoplasm of activated macrophages are described. Linear regressions are made of the measured potential differences as functions of the tip potential of each microelectrode. The surface potential of the macrophage is not significantly different from zero. Mouse macrophages have a transmembrane potential of--26 mV, whereas in guinea-pig cells this value is--18 mV. The input resistances of guinea-pig cells are higher than those of mouse macrophages. The cytoplasmic location of the electrode was characterized both by fluorescent dye injection and by electric criteria. Slow membrane hyperpolarizations are directly elicited by mechanical stimulation. Electric responses evoked by current pulses were further characterized. Our results lead to the extablishment of objective criteria to validate intracellular recordings from macrophage.