Distinct signaling pathways mediate touch and osmosensory responses in a polymodal sensory neuron.
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The Caenorhabditis elegans ASH sensory neurons mediate responses to nose touch, hyperosmolarity, and volatile repellent chemicals. We show here that distinct signaling pathways mediate the responses to touch and hyperosmolarity. ASH neurons distinguish between these stimuli because habituation to nose touch has no effect on the response to high osmolarity or volatile chemicals (1-octanol). Mutations in osm-10 eliminate the response to hyperosmolarity but have no effect on responses to nose touch or to volatile repellents. OSM-10 is a novel cytosolic protein expressed in ASH and three other classes of sensory neurons. Mutations in two other osmosensory-defective genes, eos-1 and eos-2, interact genetically with osm-10. Our analysis suggests that nose touch sensitivity and osmosensation occur via distinct signaling pathways in ASH and that OSM-10 is required for osmosensory signaling. (+info)
Osmosensing by bacteria: signals and membrane-based sensors.
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Bacteria can survive dramatic osmotic shifts. Osmoregulatory responses mitigate the passive adjustments in cell structure and the growth inhibition that may ensue. The levels of certain cytoplasmic solutes rise and fall in response to increases and decreases, respectively, in extracellular osmolality. Certain organic compounds are favored over ions as osmoregulatory solutes, although K+ fluxes are intrinsic to the osmoregulatory response for at least some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting changes in cell structure (indirect osmosensing). Those located in the cytoplasmic membranes and nucleoids of bacteria are positioned for indirect osmosensing. Cytoplasmic membrane-based osmosensors may detect changes in the periplasmic and/or cytoplasmic solvent by experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular crowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experience changes in mechanical strain in response to imposed osmotic shifts. The osmosensory mechanisms controlling membrane-based K+ transporters, transcriptional regulators, osmoprotectant transporters, and mechanosensitive channels intrinsic to the cytoplasmic membrane of Escherichia coli are under intensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstitution in proteoliposomes. Evidence that sensor kinase KdpD receives multiple sensory inputs is consistent with the effects of K+ fluxes on nucleoid structure, cellular energetics, cytoplasmic ionic strength, and ion composition as well as on cytoplasmic osmolality. Thus, osmoregulatory responses accommodate and exploit the effects of individual cosolvents on cell structure and function as well as the collective contribution of cosolvents to intracellular osmolality. (+info)
Functional consensus for mammalian osmotic response elements.
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The molecular mechanisms underlying adaptation to hyperosmotic stress through the accumulation of organic osmolytes are largely unknown. Yet, among organisms, this is an almost universal phenomenon. In mammals, the cells of the renal medulla are uniquely exposed to high and variable salt concentrations; in response, renal cells accumulate the osmolyte sorbitol through increased transcription of the aldose reductase (AR) gene. In cloning the rabbit AR gene, we found the first evidence of an osmotic response region in a eukaryotic gene. More recently, we functionally defined a minimal essential osmotic response element (ORE) having the sequence CGGAAAATCAC(C) (bp -1105 to -1094). In the present study, we systematically replaced each base with every other possible nucleotide and tested the resulting sequences individually in reporter gene constructs. Additionally, we categorized hyperosmotic response by electrophoretic mobility shift assays of a 17-bp sequence (-1108 to -1092) containing the native ORE as a probe against which the test constructs would compete for binding. In this manner, binding activity was assessed for the full range of osmotic responses obtained. Thus we have arrived at a functional consensus for the mammalian ORE, NGGAAAWDHMC(N). This finding should accelerate the discovery of genes previously unrecognized as being osmotically regulated. (+info)
Regulation of renal urea transporters.
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Urea is important for the conservation of body water due to its role in the production of concentrated urine in the renal inner medulla. Physiologic data demonstrate that urea is transported by facilitated and by active urea transporter proteins. The facilitated urea transporter (UT-A) in the terminal inner medullary collecting duct (IMCD) permits very high rates of transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. Four isoforms of the UT-A urea transporter family have been cloned to date. The facilitated urea transporter (UT-B) in erythrocytes permits these cells to lose urea rapidly as they traverse the ascending vasa recta, thereby preventing loss of urea from the medulla and decreasing urine-concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in Jk null individuals (who lack Kidd antigen). In addition to these facilitated urea transporters, three sodium-dependent, secondary active urea transport mechanisms have been characterized functionally in IMCD subsegments: (1) active urea reabsorption in the apical membrane of initial IMCD from low-protein fed or hypercalcemic rats; (2) active urea reabsorption in the basolateral membrane of initial IMCD from furosemide-treated rats; and (3) active urea secretion in the apical membrane of terminal IMCD from untreated rats. This review focuses on the physiologic, biophysical, and molecular evidence for facilitated and active urea transporters, and integrative studies of their acute and long-term regulation in rats with reduced urine-concentrating ability. (+info)
Physiology and pathophysiology of renal aquaporins.
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The discovery of aquaporin membrane water channels by Agre and coworkers answered a long-standing biophysical question of how water specifically crosses biologic membranes, and provided insight, at the molecular level, into the fundamental physiology of water balance and the pathophysiology of water balance disorders. Of nine aquaporin isoforms, at least six are known to be present in the kidney at distinct sites along the nephron and collecting duct. Aquaporin-1 (AQP1) is extremely abundant in the proximal tubule and descending thin limb, where it appears to provide the chief route for proximal nephron water reabsorption. AQP2 is abundant in the collecting duct principal cells and is the chief target for vasopressin to regulate collecting duct water reabsorption. Acute regulation involves vasopressin-regulated trafficking of AQP2 between an intracellular reservoir and the apical plasma membrane. In addition, AQP2 is involved in chronic/adaptational regulation of body water balance achieved through regulation of AQP2 expression. Importantly, multiple studies have now identified a critical role of AQP2 in several inherited and acquired water balance disorders. This concerns inherited forms of nephrogenic diabetes insipidus and several, much more common acquired types of nephrogenic diabetes insipidus where AQP2 expression and/or targeting are affected. Conversely, AQP2 expression and targeting appear to be increased in some conditions with water retention such as pregnancy and congestive heart failure. AQP3 and AQP4 are basolateral water channels located in the kidney collecting duct, and AQP6 and AQP7 appear to be expressed at lower abundance at several sites including the proximal tubule. This review focuses mainly on the role of AQP2 in water balance regulation and in the pathophysiology of water balance disorders. (+info)
The osmoprotectant glycine betaine inhibits salt-induced cross-tolerance towards lethal treatment in Enterococcus faecalis.
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The response of Enterococcus faecalis ATCC 19433 to salt stress has been characterized previously in complex media. In this report, it has been demonstrated that this bacterium actively accumulates the osmoprotectant glycine betaine (GB) from salt-enriched complex medium BHI. To further understand the specific effects of GB and other osmoprotective compounds in salt adaptation and salt-induced cross-tolerance to lethal challenges, a chemically defined medium lacking putative osmoprotectants was used. In this medium, bacterial growth was significantly reduced by increasing concentrations of NaCl. At 0.75 M NaCl, 90% inhibition of the growth rate was observed; GB and its structural analogues restored growth to the non-salt-stressed level. In contrast, proline, pipecolate and ectoine did not allow growth recovery of stressed cells. Kinetic studies showed that the uptake of betaines shows strong structural specificity and occurs through a salt-stress-inducible high-affinity porter [Km = 3.3 microM; Vmax = 130 nmol min(-1) (mg protein)(-1); the uptake activity increased 400-fold in the presence of 0.5 M NaCl]. Moreover, GB and its analogues were accumulated as non-metabolizable cytosolic osmolytes and reached intracellular levels ranging from 1-3 to 1.5 micromol (mg protein)(-1). In contrast to the beneficial effect of GB on the growth of salt-stressed cultures of E. faecalis, its accumulation inhibits the salt-induced cross-tolerance to a heterologous lethal challenge. Indeed, pretreatment of bacterial cells with 0.5 M NaCl induced resistance to 0.3% bile salts (survival of adapted cells increased by a factor of 6800). The presence of GB in the adaptation medium reduced the acquisition of bile salts resistance 680-fold. The synthesis of 11 of the 13 proteins induced during salt adaptation was significantly reduced in the presence of GB. These results raise questions about the actual beneficial effect of GB in natural environments where bacteria are often subjected to various stresses. (+info)
Cell shrinkage regulates Src kinases and induces tyrosine phosphorylation of cortactin, independent of the osmotic regulation of Na+/H+ exchangers.
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The signaling pathways by which cell volume regulates ion transporters, e.g. Na+/H+ exchangers (NHEs), and affects cytoskeletal organization are poorly understood. We have previously shown that shrinkage induces tyrosine phosphorylation in CHO cells, predominantly in an 85-kDa band. To identify volume-sensitive kinases and their substrates, we investigated the effect of hypertonicity on members of the Src kinase family. Hyperosmolarity stimulated Fyn and inhibited Src. Fyn activation was also observed in nystatin-permeabilized cells, where shrinkage cannot induce intracellular alkalinization. In contrast, osmotic inhibition of Src was prevented by permeabilization or by inhibiting NHE-1. PP1, a selective Src family inhibitor, strongly reduced the hypertonicity-induced tyrosine phosphorylation. We identified one of the major targets of the osmotic stress-elicited phosphorylation as cortactin, an 85-kDa actin-binding protein and well known Src family substrate. Cortactin phosphorylation was triggered by shrinkage and not by changes in osmolarity or pHi and was abrogated by PP1. Hyperosmotic cortactin phosphorylation was reduced in Fyn-deficient fibroblasts but remained intact in Src-deficient fibroblasts. To address the potential role of the Src family in the osmotic regulation of NHEs, we used PP1. The drug affected neither the hyperosmotic stimulation of NHE-1 nor the inhibition of NHE-3. Thus, members of the Src family are volume-sensitive enzymes that may participate in the shrinkage-related reorganization of the cytoskeleton but are probably not responsible for the osmotic regulation of NHE. (+info)
Chronic lithium treatment inhibits amiloride-sensitive sodium transport in the rat distal nephron.
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Chronic treatment of rats with lithium leads to Na+ loss and a reduced antinatriuretic response to aldosterone, suggesting that lithium reduces conductive Na+ transport in the distal nephron. This was investigated in the present study by measuring the renal response to aldosterone infusion followed by amiloride in chronically instrumented conscious rats given lithium for 3 to 4 weeks to achieve plasma Li+ concentrations of approximately 0.5 mM. A servo-controlled infusion system was used to maintain sodium and water homeostasis, thereby preventing misinterpretation of the findings as a consequence of drug-induced changes in Na+ balance. In a control group of rats, Na+ excretion decreased in response to aldosterone (p <.01) and subsequent amiloride administration led to a marked increase in Na+ excretion (p <.001). In contrast, in the lithium-treated group, there was no significant response to either aldosterone or amiloride. It is concluded that long-term treatment with lithium, even when plasma Li+ concentrations are below 1 mM, reduces aldosterone-stimulated Na+ transport through the amiloride-sensitive Na+ channels in the principal cells of the distal nephron. (+info)