Receptor regulation of osmolyte homeostasis in neural cells

Abstract

The capacity of cells to correct their volume in response to hyposmotic stress via the efflux of inorganic and organic osmolytes is well documented. However, the ability of cell-surface receptors, in particular G-protein-coupled receptors (GPCRs), to regulate this homeostatic mechanism has received much less attention. Mechanisms that underlie the regulation of cell volume are of particular importance to cells in the central nervous system because of the physical restrictions of the skull and the adverse impact that even small increases in cell volume can have on their function. Increases in brain volume are seen in hyponatraemia, which can arise from a variety of aetiologies and is the most frequently diagnosed electrolyte disorder in clinical practice. In this review we summarize recent evidence that the activation of GPCRs facilitates the volume-dependent efflux of osmolytes from neural cells and permits them to more efficiently respond to small, physiologically relevant, reductions in osmolarity. The characteristics of receptor-regulated osmolyte efflux, the signalling pathways involved and the physiological significance of receptor activation are discussed. In addition, we propose that GPCRs may also regulate the re-uptake of osmolytes into neural cells, but that the influx of organic and inorganic osmolytes is differentially regulated. The ability of neural cells to closely regulate osmolyte homeostasis through receptor-mediated alterations in both efflux and influx mechanisms may explain, in part at least, why the brain selectively retains its complement of inorganic osmolytes during chronic hyponatraemia, whereas its organic osmolytes are depleted.

Figure 1. General schematic representation of events leading to swelling-activated (basal)- and GPCR-mediated osmolyte efflux

A reduction in osmolarity results in an influx of water and an increase in cell volume, which in turn triggers a hypothetical ‘volume sensor’. It has been proposed that the latter may represent changes in macromolecular crowding and/or changes in the cytoskeleton. Activation of the volume sensor elicits a tyrosine kinase (TK)-dependent activation of K⁺ channels (both Ca²⁺-dependent and -independent), VSOAC and, on occasions, the KCC transporter. This results in the efflux of K⁺, Cl⁻, organic osmolytes and osmotically obligated water. The latter results in a restoration of cell volume or ‘regulatory volume decrease’. Volume-dependent efflux of inorganic and organic osmolytes can also be significantly increased by activation of GPCRs, the magnitude of which is often dependent on Ca²⁺ availability and PKC activity. Note that GPCR activation of osmolyte release is strongly dependent on prior activation of the volume sensor and little or no increase is seen under isotonic or hypertonic conditions. Pharmacological evidence indicates that the same or similar channels mediate osmolyte efflux under basal- and GPCR-stimulated conditions.

Figure 2. Basal- and oxotremorine-M (Oxo-M)-stimulated efflux of taurine and Rb⁺ as a function of osmolarity

SH-SY5Y neuroblastoma cells, prelabelled overnight with either [¹⁴C]taurine or ⁸⁶Rb⁺, were incubated in isotonic, hypertonic or hypotonic media for either 20 min (taurine) or 5 min (Rb⁺) in the absence (open bars) or presence (filled bars) of 100 μm Oxo-M, a muscarinic cholinergic agonist. Results are expressed as taurine or Rb⁺ efflux (percentage of total soluble radioactivity) and are the means ±s.e.m. for 4–13 experiments. A, taurine efflux; B, Rb⁺ efflux. *Different from basal efflux, P < 0.05 by paired Student's t test. **Different from basal efflux monitored under isotonic conditions, P < 0.01 by one-way ANOVA, followed by Dunnett's multiple comparison test. Data are taken from Heacock et al. (2004) and Foster et al. (2008).

Figure 3. Diagrammatic representation of the ability of GPCRs present in SH-SY5Y cells to regulate the efflux and uptake of K⁺ (A), taurine (B) or glutamate (C) under isotonic (Iso) or hypotonic (Hypo) conditions

Although only a limited efflux of osmolytes is observed following the activation of GPCRs under isotonic conditions, the efflux of all three is markedly increased by hypotonicity (as indicated by the relative size of the arrows). The efflux of K⁺ is mediated primarily by K⁺ channel(s) whereas taurine (Tau) and glutamate (Glu) are released via the VSOAC. K⁺ uptake, which is principally mediated by Na⁺ −K⁺-ATPase and the NKCC transporters in this cell line, is facilitated by GPCR activation under conditions of isotonicity, and further enhanced by hyposmolarity. In contrast, under isotonic conditions, GPCR activation results in an inhibition of taurine uptake, which is mediated via the taurine transporter (TauT). This inhibition of TauT is further enhanced by hyposmolarity. Glutamate uptake, which is mediated by EAAT3, is markedly increased by GPCR activation under isotonic conditions. However, receptor-mediated stimulation of Glu uptake is progressively inhibited as osmolarity is reduced.