Stretch-activated ion channels modulate the resting membrane potential during early embryogenesis

Ion currents in embryo development

Ion channels are proteins expressed in the plasma membrane of electrogenic cells. In the zygote and blastomeres of the developing embryo, electrical modifications result from ion currents that flow through these channels. This phenomenon implies that ion current activity exerts a specific developmental function, and plays a crucial role in signal transduction and the control of embryogenesis, from the early cleavage stages and during growth and development of the embryo. This review describes the involvement of ion currents in early embryo development, from marine invertebrates to human, focusing on the occurrence, modulation, and dynamic role of ion fluxes taking place on the zygote and blastomere plasma membrane, and at the intercellular communication between embryo cell stages.

Cellular magnesium acquisition: an anomaly in embryonic cation homeostasis

The intracellular dominance of magnesium ion makes clinical assessment difficult despite the critical role of Mg(++) in many key functions of cells and enzymes. There is general consensus that serum Mg(++) levels are not representative of the growing number of conditions for which magnesium is known to be important. There is no consensus method or sample source for testing for clinical purposes. High intracellular Mg(++) in vertebrate embryos results in part from interactions of cations which influence cell membrane transport systems. These are functionally competent from the earliest stages, at least transiently held over from the unfertilized ovum. Kinetic studies with radiotracer cations, osmolar variations, media lacking one or more of the four biological cations, Na(+), Mg(++), K(+), and Ca(++), and metabolic poison 0.05 mEq/L NaF, demonstrated that: (1) all four cations influence the behavior of the others, and (2) energy is required for uptake and efflux on different time scales, some against gradient. Na(+) uptake is energy dependent against an efflux gradient. The rate of K(+) loss is equal with or without fluoride, suggesting a lack of an energy requirement at these stages. Ca(++) efflux took twice as long in the presence of fluoride, likely due in part to intracellular binding. Mg(++) is anomalous in that early teleost vertebrate embryos have an intracellular content exceeding the surrounding sea water, an isolated unaffected yolk compartment, and a clear requirement for energy for both uptake and efflux. The physiological, pathological, and therapeutic roles of magnesium are poorly understood. This will change: (1) when (28)Mg is once again generally available at a reasonable cost for both basic research and clinical assessment, and (2) when serum or plasma levels are determined simultaneously with intracellular values, preferably as part of complete four cation profiles. Atomic absorption spectrophotometry, energy-dispersive x-ray analysis, and inductively coupled plasma emission spectroscopy on sublingual mucosal and peripheral blood samples are potential methods of value for coordinated assessments.

Stretch-activated cation channels of leech neurons exhibit two activity modes

Single-channel recordings were used to characterize two activity modes of stretch activated channels (SACs) in identified neurons of the leech. Clear-cut differences in the activity pattern of SACs from freshly desheathed cell bodies and from cultured AP cells were observed. SACs of inside-out patches, made by 'gentle' sealing and excised from cell bodies of freshly desheathed ganglia exhibited spike-like (SL) activity, with a mean channel open time (MCOT) shorter than 10 ms. Fitting of dwell open-time distributions revealed time constants shorter than 2 and 10 ms. This activity was characterized by a chord conductance of about 115 pS. SACs from cultured cells often displayed activity just after excision. MCOT exceeded 200 ms and the time constants of open-time interval distributions were longer than 10 and 100 ms. Furthermore, this activity pattern was characterized by both sub- (about 80 and 40 pS) and super-conductance (150 pS) levels, hence denoted as multiconductance (MC) mode. The percentage of open time spent at the main subconductance level (80 pS) was significantly higher in patches isolated from growth cones than in those from cell bodies of cultured neurons. The two activity modes (SL and MC) should belong to the same channel because both modes have a common main conductance value and exhibit outward rectification, stretch sensitivity and blockage by Gd3+ and gentamicin. Cytochalasin D applied to the cytoplasmic side induced activation of SACs or increased their ongoing activity. Thus, the observed differences in the expression of the two activity modes of SACs might be associated with different arrangements of the cortical cytoskeleton.

Stretch-activated cation channels of leech neurons: characterization and role in neurite outgrowth

The goal of this study was to characterize the stretch-activated ion channels (SACs) of adult identified neurons of the leech Hirudo medicinalis and to test the role of SACs in neurite outgrowth of isolated cells. Using cell-attached patch recording, we established that SACs are densely distributed in the growth cone membrane of cultured neurons. In excised patches, we found that these channels are permeable to Ca2+, as well as to monovalent cations. The channels are blocked by the extracellular application of gadolinium (Gd3+), amiloride and gentamicin. Amiloride and gentamicin, respectively, induce a partial and complete voltage-dependent block. Time-lapse video recordings of neurite outgrowth from single cultured neurons were used to study the effects of blocking SACs with gentamicin. Within 20 h of plating in the presence of the aminoglycoside, the total length of neuronal arborization was significantly greater than that measured in its absence. The amount of assembled axon per unitary surface area remained constant over 40 h and did not differ significantly with or without gentamicin. Our findings show that SACs of leech neurons admit Ca2+, are densely distributed in the growth cone membrane and exhibit typical pharmacological features of mechanotransducer ion channels. In addition, our data suggest that these cation channels participate in the early interaction between growing neurites and culture substrate.

Potentiometric study of resting potential, contributing K+ channels and the onset of Na+ channel excitability in embryonic rat cortical cells

Resting membrane potential (RMP), K+ channel contribution to RMP and the development of excitability were investigated in the entire population of acutely dissociated embryonic (E) rat cortical cells over E11-22 using a voltage-sensitive fluorescent indicator dye and flow cytometry. During the period of intense proliferation (E11-13), two cell subpopulations with distinct estimated RMPs were recorded: one polarized at approximately -70 mV and the other relatively less-polarized at approximately -40 mV. Ca2+o was critical in sustaining the RMP of the majority of less-polarized cells, while the well-polarized cells were characterized by membrane potentials exhibiting a approximately Nernstian relationship between RMP and [K+]o. Analysis of these two subpopulations revealed that > 80% of less-polarized cells were proliferative, while > 90% of well-polarized cells were postmitotic. Throughout embryonic development, the disappearance of Ca2+o-sensitive, less-polarized cells correlated with the disappearance of the proliferating population, while the appearance of the K+o-sensitive, well-polarized population correlated with the appearance of terminally postmitotic neurons, immuno-identified as BrdU-, tetanus toxin+ cells. Differentiating neurons were estimated to contain increased K+i relative to less-polarized cells, coinciding with the developmental expression of Cs+/Ba2+-sensitive and Ca2+-dependent K+ channels. Both K+ channels contributed to the RMP of well-polarized cells, which became more negative toward the end of neurogenesis. Depolarizing effects of veratridine, first observed at E11, progressively changed from Ca2+o-dependent and tetrodotoxin-insensitive to Na+o-dependent and tetrodotoxin-sensitive response by E18. The results reveal a dynamic development of RMP, contributing K+ channels and voltage-dependent Na+ channels in the developing cortex as it transforms from proliferative to primarily differentiating tissue.

A cytoplasmic cell cycle controls the activity of a K+ channel in pre-implantation mouse embryos

We previously have reported that the activity of a 240 pS K+ channel varies during the cell cycle in pre-implantation mouse embryos. In the present study, we show that: (i) the cycling of channel activity is not prevented by inhibiting protein synthesis and hence does not involve cyclin-dependent kinase 1 (cdk1)-cyclin B; and (ii) the cycling of channel activity continues in anucleate zygote fragments with a time course similar to that observed in nucleate fragments. We further demonstrate that: (i) persistent activation of the K+ channel in one-cell embryos arrested in metaphase requires the maintenance of an active cdk1-cyclin B complex; and (ii) both DNA synthesis inhibition with aphidicolin and DNA damage produced by mitomycin C prevent the down-regulation of the channel at the start of S phase by a mechanism that requires tyrosine kinase activation. Thus, the 240 pS K+ channel in these cells is controlled by a previously unsuspected cytoplasmic clock that functions independently of the well-known clock controlling the chromosomal cell cycle, but can interact with it.

Membrane stretch activates a potassium channel in pig articular chondrocytes

Activity of stretch-activated potassium channels has been recorded in articular chondrocytes using patch-clamp technique. Pressure dependence is described by a sigmoidal function with a half-maximum effect at -20.5 mbar. Selectivity for potassium is demonstrated by agreement between the reversal potential measured at different [K+]o and the prediction of Nernst equation and by block of these channels by caesium.

Molecular dissection of the large mechanosensitive ion channel (MscL) of E. coli: mutants with altered channel gating and pressure sensitivity

In the search for the essential functional domains of the large mechanosensitive ion channel (MscL) of E. coli, we have cloned several mutants of the mscL gene into a glutathione S-transferase fusion protein expression system. The resulting mutated MscL proteins had either amino acid additions, substitutions or deletions in the amphipathic N-terminal region, and/or deletions in the amphipathic central or hydrophilic C-terminal regions. Proteolytic digestion of the isolated fusion proteins by thrombin yielded virtually pure recombinant MscL proteins that were reconstituted into artificial liposomes and examined for function by the patch-clamp technique. The addition of amino acid residues to the N-terminus of the MscL did not affect channel activity, whereas N-terminal deletions or changes to the N-terminal amino acid sequence were poorly tolerated and resulted in channels exhibiting altered pressure sensitivity and gating. Deletion of 27 amino acids from the C-terminus resulted in MscL protein that formed channels similar to the wild-type, while deletion of 33 C-terminal amino acids extinguished channel activity. Similarly, deletion of the internal amphipathic region of the MscL abolished activity. In accordance with a recently proposed spatial model of the MscL, our results suggest that (i) the N-terminal portion participates in the channel activation by pressure, and (ii) the essential channel functions are associated with both, the putative central amphipathic alpha-helical portion of the protein and the six C-terminal residues RKKEEP forming a charge cluster following the putative M2 membrane spanning alpha-helix.

A voltage-gated chloride channel in ascidian embryos modulated by both the cell cycle clock and cell volume

1. Eggs of the ascidian Boltenia villosa have an inwardly rectifying Cl- current whose amplitude varies by more than 10-fold during each cell cycle, the largest amplitude being at exit from M-phase. We examined whether this current was also sensitive to changes in cell volume. 2. Cell swelling, produced by direct inflation through a whole-cell recording pipette, greatly increased the amplitude of the Cl- current at all stages of the cell cycle in activated eggs. Swelling was much less effective in unfertilized eggs. 3. The increase in Cl- current amplitude continued for 10-20 min after an increase in diameter that was complete in 10 s, suggesting the involvement of a second messenger system in the response. 4. Treatment of unfertilized eggs with 6-dimethylaminopurine (DMAP), an inhibitor of cell cycle-dependent protein kinases, increased the amplitude of the Cl- current and its sensitivity to swelling to levels characteristic of fertilized eggs. 5. Osmotically produced swelling also increased Cl- current amplitude in unfertilized eggs. 6. We propose that dephosphorylation renders the Cl- channel functional, and that swelling or activation of the egg increases the sensitivity of the channel to dephosphorylation, perhaps by disrupting its links to the cytoskeleton.

Novel cation-selective mechanosensitive ion channel in the atrial cell membrane

Stretch of atrial muscle causes the release of atrial natriuretic peptide, but no stretch-sensitive membrane sensors have been clearly identified so far. The existence of an ion channel that could mediate stretch-induced Ca2+ influx and subsequent release of the peptide was examined in rat atrial cells. In this report, the discovery of a novel atrial ion channel whose opening probability is extremely sensitive to either positive or negative pressure (half-maximal activation pressure, approximately 1.5 mm Hg) is described. Activation of the current by pressure applied to the pipette was observed both at the whole-cell and single-channel levels. The channel was permeable to cations including Ca2+. The channels were clustered with six to nine channels in each cluster, and several channels tended to open simultaneously in response to a graded increase in pressure for the inwardly passing current. Hypotonic swelling also activated these channels. These results show that mechanosensitive nonselective cation channels exist in atrial cells and suggest that they could be involved in beat-to-beat regulation of the atrial contraction as well as the stretch-induced expression of proto-oncogenes and secretion of atrial natriuretic peptide via increased Ca2+ entry.

Are stretch-sensitive channels in molluscan cells and elsewhere physiological mechanotransducers?

Single-channel recordings of dozens of cell types, including invertebrate (molluscan) and vertebrate heart cells, reveal stretch-sensitive ion channels. The physiological roles of these channels are undoubtedly diverse but it is usually assumed that the roles they play are related to the channels' mechanosensitive gating. Whether this assumption is valid remains to be seen. Attempts to connect the single-channel observations with the mechanical aspects of physiological or developmental processes are discussed. In the case of molluscan cells, recent work suggests that their stretch channels have physiological functions unrelated to mechanosensitive gating.

Stretch activation of the Aplysia S-channel

The S-channel, a receptor-mediated K+ channel of Aplysia sensory neurons which functions in neuromodulation, bears a strong resemblance to the ubiquitous stretch-activated channels of snail neurons. Snail neuron stretch channels are stretch sensitive only in the patch, not at the macroscopic level, a situation which leaves open the question of their physiological role. If S-channels resemble snail stretch channels because both belong to the same general class of channels, the S-channel, too, should display stretch sensitivity in the patch. We show, using single-channel recording, that the S-channel can be activated by stretch. Furthermore, we show that Aplysia neurons in general have stretch-activated K+ channels. We suggest that the stretch-sensitive K+ channels of molluscan neurons and other preparations (e.g., Drosophila muscle, snail heart) are S-like channels, i.e., receptor-mediated channels which adventitiously exhibit mechanosensitivity in the patch.

Sensitivity of stretch-activated K+ channels changes during cell-cleavage cycle and may be regulated by cAMP-dependent protein kinase

The properties of stretch-activated K+ channels in the membrane of loach (Misgurnus fossilis) embryos were studied using the patch-clamp technique. It was found that in the early stages of embryogenesis (2-256 cells) the stretch sensitivity of stretch-activated (SA) channels changes dramatically during the cell cleavage cycle. At the beginning of interphase the stretch sensitivity of SA channels and the probability of being in the open state (P0) were minimal, whereas at prometaphase they were increased 10-100-fold. Application of ATP to the cytoplasmic surface of excised inside-out patches induced a reversible increase in resting P0 and of stretch sensitivity of the SA channels in 50% of the patches, but the non-hydrolysable analogue of ATP, 5'-adenylylimidodiphosphate (AMP-PNP), was not effective. Phosphatase inhibitors (orthovanadate and para-nitrophenyl phosphate) prolonged the effect of ATP. Combined application of ATP, cAMP and cAMP-dependent protein kinase (PK) induced a reversible increase in the SA channel activity in 70% of those excised patches which did not respond to ATP. Inhibitors of PK prevented its activating effect. Dibutyryl-cAMP (dB cAMP) transiently increased activity of SA channels in intact cells. These results suggest that activity of SA channels may be regulated through cAMP-dependent phosphorylation and thus provide the basis for explanation of stretch sensitivity modulation during the cell cycle.

Amiloride block of the mechanosensitive cation channel in Xenopus oocytes

1. Patch clamp recording techniques have been used to investigate the block by amiloride of the mechanosensitive cation-selective channel in frog (Xenopus laevis) oocytes. 2. Cell-attached and outside-out patch recording configurations were employed to study the differences in block produced when amiloride was present at either the extracellular (external) or intracellular (internal) membrane face. 3. External amiloride causes a highly voltage-dependent 'flickery' block of single mechanosensitive channel currents in which inward mechanosensitive current recorded at negative potentials is reduced in amplitude but outward mechanosensitive current recorded at positive potentials is almost unaffected. 4. At -100 mV the apparent dissociation constant (Kd) for external amiloride block is 0.5 mM. The extracellular concentration dependence of amiloride block yields a Hill coefficient equal to 2, inconsistent with a single site blocking stoichiometry. 5. The shapes of current-voltage relationships measured in different external amiloride concentrations also indicate deviations from a simple channel plug model in which a single blocking cation is driven into the channel by the membrane potential. 6. Internal amiloride causes a voltage-independent 'flickery' block of mechanosensitive channel currents which equally reduces both inward and outward mechanosensitive currents. 7. The present data indicate that a minimum of two amiloride binding sites are necessary to predict external amiloride block. A model involving a voltage-dependent conformational change with subsequent voltage-independent co-operative binding of two amiloride molecules is found to explain the data. 8. The relevance of the present actions of amiloride on mechanosensitive channels is discussed in relation to reports of amiloride-inhibitable cation flux pathways involved in a number of basic physiological functions including mechanosensitivity of sensory cells, volume regulation and fertilization.

Structure and function of the interfaces in biotrophic symbioses as they relate to nutrient transport

In this review we compare the structure and function of the interfaces between symbionts in biotrophic associations. The emphasis is on biotrophic fungal parasites and on mycorrhizas, although necrotrophic parasitic associations and the Rhizobium/legume symbiosis are mentioned briefly. We take as a starting point the observations that in the parasitic associations nutrient transport is polarized towards the parasite, whereas in mutualistic associations it is bidirectional. The structure and function of the interfaces are then compared. An important common feature is that in nearly all cases the heterotrophic symbiont (whether mutualistic or parasitic) is located topologically outside the cytoplasm of the host cells, in an apoplastic compartment. This means that nutrient movements across the interface must involve transport into and out of this apoplastic region through membranes of both organisms. Basic principles of membrane transport in uninfected cells are briefly reviewed to set the scene for a discussion of transport mechanisms which may operate in parasitic and mycorrhizal symbioses. The presence and possible roles of ATPases associated with membranes at the interfaces are discussed. We conclude that cytochemical techniques (used to demonstrate the activity of these enzymes) need to he extended and complemented by biochemical and biophysical studies in order to confirm that the activity is due to transport ATPases. Nevertheless, the distribution of activity appears to he in accord with polarized transport mechanisms in some pathogens and with bidirectional transport in mycorrhizas. The absence of ATPases on many fungal membranes needs re-examination. We emphasize that transport mechanisms between mycorrhizal symbionts cannot be viewed simply as the exchange of carbon for phosphate. Additional features include provision for transport of carbon and nitrogen as amino acids or amides and for ions such as K⁺ and H⁺ involved in the maintenance of charge balance and pH regulation, processes which also occur in parasitic associations. Interplant transport of nutrients via mycorrhizal hyphae is discussed in the context of these complexities. Some suggestions for the directions of future work are made. CONTENTS Summary 1 I. Introduction 2 II. The availability of nutrients to the symbionts 3 III. Structure of interfaces between symbionts 4 IV. Identity of nutrients transferred: an overview 12 V. Membrane transport: basic principles 14 VI. Transport at the interface of biotrophic symbioses 15 VII. Regulation of pH in biotrophic symbioses 25 VIII. Conclusions: 26.