Volume-activated DIDS-sensitive whole-cell chloride currents in trout red blood cells

Elevator-like movements of prestin mediate outer hair cell electromotility

The outstanding acuity of the mammalian ear relies on cochlear amplification, an active mechanism based on the electromotility (eM) of outer hair cells. eM is a piezoelectric mechanism generated by little-understood, voltage-induced conformational changes of the anion transporter homolog prestin (SLC26A5). We used a combination of molecular dynamics (MD) simulations and biophysical approaches to identify the structural dynamics of prestin that mediate eM. MD simulations showed that prestin samples a vast conformational landscape with expanded (ES) and compact (CS) states beyond previously reported prestin structures. Transition from CS to ES is dominated by the translational-rotational movement of prestin's transport domain, akin to elevator-type substrate translocation by related solute carriers. Reversible transition between CS and ES states was supported experimentally by cysteine accessibility scanning, cysteine cross-linking between transport and scaffold domains, and voltage-clamp fluorometry (VCF). Our data demonstrate that prestin's piezoelectric dynamics recapitulate essential steps of a structurally conserved ion transport cycle.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Patch-clamp analysis of the "new permeability pathways" in malaria-infected erythrocytes

The intraerythrocytic amplification of the malaria parasite Plasmodium falciparum induces new pathways of solute permeability in the host cell's membrane. These pathways play a pivotal role in parasite development by supplying the parasite with nutrients, disposing of the parasite's metabolic waste and organic osmolytes, and adapting the host's electrolyte composition to the parasite's needs. The "new permeability pathways" allow the fast electrogenic diffusion of ions and thus can be analyzed by patch-clamp single-channel or whole-cell recording. By employing these techniques, several ion-channel types with different electrophysiological profiles have been identified in P. falciparum-infected erythrocytes; they have also been identified in noninfected cells. This review discusses a possible contribution of these channels to the new permeability pathways on the one hand and their supposed functions in noninfected erythrocytes on the other.

Plasmodium Induces Swelling-activated ClC-2 Anion Channels in the Host Erythrocyte

Intraerythrocytic growth of the human malaria parasite Plasmodium falciparum depends on delivery of nutrients. Moreover, infection challenges cell volume constancy of the host erythrocyte requiring enhanced activity of cell volume regulatory mechanisms. Patch clamp recording demonstrated inwardly and outwardly rectifying anion channels in infected but not in control erythrocytes. The molecular identity of those channels remained elusive. We show here for one channel type that voltage dependence, cell volume sensitivity, and activation by oxidation are identical to ClC-2. Moreover, Western blots and FACS analysis showed protein and functional ClC-2 expression in human erythrocytes and erythrocytes from wild type (Clcn2(+/+)) but not from Clcn2(-/-) mice. Finally, patch clamp recording revealed activation of volume-sensitive inwardly rectifying channels in Plasmodium berghei-infected Clcn2(+/+) but not Clcn2(-/-) erythrocytes. Erythrocytes from infected mice of both genotypes differed in cell volume and inhibition of ClC-2 by ZnCl(2) (1 mm) induced an increase of cell volume only in parasitized Clcn2(+/+) erythrocytes. Lack of ClC-2 did not inhibit P. berghei development in vivo nor substantially affect the mortality of infected mice. In conclusion, activation of host ClC-2 channels participates in the altered permeability of Plasmodium-infected erythrocytes but is not required for intraerythrocytic parasite survival.

Multiple transport functions of a red blood cell anion exchanger, tAE1: its role in cell volume regulation

1. It was previously shown that expressed in Xenopus oocyte the mouse (mAE1) and the trout (tAE1) anion exchanger behave differently: both elicit anion exchange activity but only tAE1 induces a transport of organic solutes correlated with a chloride channel activity. The present data, obtained by measurement of Xenopus oocyte membrane permeability and conductance, provide evidence that tAE1 also induces a large increase in Na(+) and K(+) permeability inhibited by several AE1 inhibitors. 2. This inhibition does not result from an effect on the driving force for electrodiffusion but represents a direct effect on the cation pathway. 3. As a control, expression of cystic fibrosis transmembrane conductance regulator (CFTR) having, once stimulated by 3-isobutyl-1-methylxanthine (IBMX), the same anion conductance magnitude as tAE1 did not induce any cation movement. 4. Chloride exchange, channel activity and cation transport induced by anion exchanger expression are inhibited by free or covalently bound H2DIDS as well. This covalent inhibition is reversed by the point mutation of Lys-522, the covalent binding site of H2DIDS to the protein. These data reveal that tAE1 itself acts both as an anion exchanger and as a channel of broad selectivity. 5. All results obtained by expression of AE1 isoforms in Xenopus oocytes and those obtained in erythrocytes are consistent with the proposal that, in nucleated erythrocytes, tAE1 functions as the swelling-activated osmolyte anion channel involved in cell volume regulation. In contrast AE1 from mammalian red cells, which do not regulate their volume, lacks swelling-activated osmolyte channel properties. 6. tAE1 illustrates the ability of a specific transport system to be a multifunctional protein exhibiting other transport functions when submitted to regulation.

Malaria parasite Plasmodium gallinaceum up-regulates host red blood cell channels

The properties of the malaria parasite-induced permeability pathways in the host red blood cell have been a major area of interest particularly in the context of whether the pathways are host- or parasite-derived. In the present study, the whole-cell configuration of the patch-clamp technique has been used to show that, compared with normal cells, chicken red blood cells infected by Plasmodium gallinaceum exhibited a 5-40-fold larger membrane conductance, which could be further increased up to 100-fold by raising intracellular Ca(2+) levels. The increased conductance was not due to pathways with novel electrophysiological properties. Rather, the parasite increased the activity of endogenous 24 pS stretch-activated non-selective cationic (NSC) and 62 pS calcium-activated NSC channels, and, in some cases, of endogenous 255 pS anionic channels.

The role of anion and cation channels in volume regulatory responses in trout red blood cells

(1) An outwardly rectifying chloride channel (ORCC) of large conductance has been detected under isotonic conditions (320 mosM 1(-1)) in the plasma membrane of trout red blood cells (RBCs) using the excised inside-out configuration. The channel, with a permeability ratio P(Cl)/Pcation of 12, was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) (50 microM), and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (100 microM) in the bathing solution. (2) In hypotonic conditions (215 mosM 1(-1)), 44% of cell-attached patches showed spontaneous single channel activity identified as nonselective cationic (NSC) channels. A second group, corresponding to 7% of cell-attached patches, showed spontaneous activity corresponding to a channel type presenting outward rectification and anionic selectivity. Finally, 49% of patches displayed a complex spontaneous signal corresponding to the superimposition of inward and outward currents probably due to activation of different channel types. (3) Giga-seals obtained without suction in intact cells under isotonic conditions possessed NSC channels that were quiescent but which could be activated either by mechanical deformation of cell membrane or by hypotonic cell swelling. (4) Hypotonically swollen RBCs exhibited regulatory volume decrease (RVD) over 3 h, which was linked to a fivefold to sixfold increase in unidirectional fluxes of K+, a net loss of intracellular K+ and net gain of extracellular Na+. RVD and the hypotonically activated, unidirectional K+ influx continued after replacement of Cl- by methylsulfonate (MeSF) albeit more slowly. (5) The NSC channel inhibitor, barium, and the Cl- channel inhibitor, NPPB, both inhibited the RVD response by approximately 50% in Cl- containing saline. When Cl- was replaced by MeSF, the inhibition was > 90% suggesting that NSC channels and ORCC play key roles in the chloride-independent component of RVD.

Swelling activation of transport pathways in erythrocytes: effects of Cl-, ionic strength, and volume changes

If swelling of a cell is induced by a decrease in external medium tonicity, the regulatory response is more complex than if swelling of similar magnitude is due to salt uptake. The present results provide an explanation. In fish erythrocytes, two distinct transport pathways were swelling activated: a channel of broad specificity and a K+-Cl- cotransporter. Each was activated by a specific signal: the channel by a decrease in intracellular ionic strength and the K+-Cl- cotransporter by cell enlargement. A decrease in ionic strength also affected K+-Cl- cotransport activity, but by acting as a negative modulator of the cotransport. Thus cells swollen by salt accumulation respond by activating exclusively the K+-Cl- cotransport, leading to a Cl--dependent K+ loss. By contrast, cells swollen by electrolyte dilution respond by activating both pathways, leading to a reduced loss of electrolytes and a large loss of taurine. Thus two swelling-sensitive pathways, differently regulated, would allow control of the ionic composition of a cell exposed to different volume perturbations.

Transport of uncharged organic solutes in Xenopus oocytes expressing red cell anion exchangers (AE1s)

When expressed in Xenopus oocytes, the trout red cell anion exchanger tAE1, but not the mouse exchanger mAE1, elicited a transport of electroneutral solutes (sorbitol, urea) in addition to the expected anion exchange activity. Chimeras constructed from mAE1 and tAE1 allowed us to identify the tAE1 domains involved in the induction of these transports. Expression of tAE1 (but not mAE1) is known to generate an anion conductance associated with a taurine transport. The present data provide evidence that (i) the capacity of tAE1 and tAE1 chimeras to generate urea and sorbitol permeability also was associated with an anion conductance; (ii) the same inhibitors affected both the permeability of solutes and anion conductance; and (iii) no measurable water transport was associated with the tAE1-dependent conductance. These results support the view that fish red blood cells, to achieve cell volume regulation in response to hypotonic swelling, activate a tAE1-associated anion channel that can mediate the passive transport of taurine and electroneutral solutes.

Chloride and non-selective cation channels in unstimulated trout red blood cells

1. The cell-attached and excised inside-out configurations of the patch-clamp technique were used to demonstrate the presence of two different types of ion channels in the membrane of trout red blood cells under isotonic and normoxic conditions, in the absence of hormonal stimulation. The large majority (93%) of successful membrane seals allowed observation of at least one channel type. 2. In the cell-attached mode with Ringer solution in the bath and Ringer solution, 145 mM KCl or 145 NaCl in the pipette, a channel of intermediate conductance (15-25 pS at clamped voltage, Vp = 0 mV) was present in 85% of cells. The single channel activity reversed between 5 and 7 mV positive to the spontaneous membrane potential. A small conductance channel of 5-6 pS and +5 mV reversal potential was also present in 62% of cells. 3. After excision into the inside-out configuration (with 145 mM KCl or NaCl, pCa 8 in the bath, 145 mM KCl or NaCl, pCa 3 in the pipette) the intermediate conductance channel was present in 439 out of 452 successful seals. This channel was spontaneously active in 90% of patches and in the other 10% of patches the channel was activated by suction. The current-voltage relationship showed slight inward rectification. The channel conductance was in the range 15-20 pS between -60 and 0 mV and increased to 25-30 pS between 0 and 60 mV, with a reversal potential close to zero. Substitution of K+ for Na+ in the pipette or in the bath did not significantly change the single channel conductance. Dilution of the bathing solution KCl concentration shifted the reversal potential towards the Nernst equilibrium for cations. Substitution of N-methyl-D-glucamine (NMDG) for K+ or Na+ in the bath almost abolished the outward current whilst the divalent cation Ca2+ permeated the channel with a higher permeability than K+ and Na+. Inhibition of channel openings was obtained with flufenamic acid, quinine, gadolinium or barium. Taken together these data demonstrate that the intermediate conductance channel belongs to a class of non-selective cation (NSC) channels. 4. In excised patches, under the same control conditions, the conductance of the small conductance non-rectifying channel was 8.6 +/- 0.8 pS (n = 12) between -60 and +60 mV and the reversal potential was close to 0 mV. This channel could be blocked by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) but not by flufenamic acid, DIDS, barium or gadolinium. Selectivity and substitution experiments made it possible to identify this channel as a non-rectifying small conductance chloride (SCC) channel.