Osmosensitivity through the PIP2availability: just add water

Abstract

Every cell in our body contains an aqueous cytosol enwrapped by the hydrophobic plasma membrane perforated by the specific permeability pathways for water (water channels) and different inorganic and organic ions (ion channels, transporters and pumps). The clockwork regulation of these transport pathways is one of the fundamental principles of life as it underlies such basic cellular functions as excitability, contractility and metabolism. However, high permeability of cellular plasma membrane to water in combination with restricted permeability to ions creates certain thermodynamic difficulties for maintaining an osmotic equilibrium and cell volume. Thus, when extracellular osmolarity exceeds an intracellular one, water exits the cells leading to cell shrinkage; equally, when extracellular osmolarity falls below the intracellular level, water enters the cells causing cell swelling. Thus, without proper mechanisms for adjusting intracellular osmolarity cells would not be able to maintain their integrity. In very general terms, these mechanisms are known as regularly volume increase (RVI) and regulatory volume decrease (RVD). When a cell is subjected to a hypoosmotic condition it takes up water and swells, and thus the RVD is activated; generally it requires extrusion of cellular ions (e.g. by activation of K+ and/or anion channels; Lang, 2007) to reduce intracellular osmolarity. In contrast, when a cell is surrounded by a hyperosmotic milieu, it shrinks, which triggers RVI, which aims to increase intracellular osmolarity by the accumulation of ions (e.g. by the activation of Na+ channels, inhibition of K+ channels and also by concerted action of several transporters and exchangers (reviewed in Lang, 2007). Obviously, there has got to be mechanisms, linking cellular osmolarity to the activity of ion transport proteins involved in RVI and RVD. Indeed a number of such mechanisms have been identified including a direct regulation of channel activity by the membrane tension or cytoskeleton, osmodependent changes in channel abundance, or regulation by osmodependent second messengers (reviewed in Piron et al. 2010). In a recent issue of The Journal of Physiology, an interesting new mechanism which may prove to be one of the general principles of ion channel (or any plasma membrane protein) osmosensitivity was put forward by Piron et al. (2010). The authors studied osmosensitivity of the cardiac potassium current IKs conducted by the heteromeric Kv7.1(KCNQ1)/KCNE1 channels. They noted that Kv7.1–KCNE1 concatemers overexpressed in COS-7 cells were activated by hypoosmotic treatment and inhibited by hyperosmotic treatment. Intriguingly, the features of this osmotic reactivity closely resembled the regulation of IKs by the availability of plasma membrane phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP2). Particularly, hypoosmotic activation of Kv7.1–KCNE1 was accompanied by a negative shift of channel voltage dependence and by slowing of channel deactivation; a similar effects were seen in response to the increase in membrane PIP2 (Loussouarn et al. 2003; Piron et al. 2010). Conversely, hyperosmotic inhibition of Kv7.1–KCNE1 was accompanied by inverted changes in channel activity and kinetics – quite similarly to the effects produced by the PIP2 depletion (Loussouarn et al. 2003; Piron et al. 2010). The authors hypothesized that the true mechanism underlying the IKs osmosensitivity lies in the changes in the apparent PIP2 availability induced by the osmotic challenges (Fig. 1). Kv7.1, like other members of the Kv7 channel family (Gamper & Shapiro, 2007), requires PIP2 to support channel opening (Loussouarn et al. 2003). Under the basal conditions some of the membrane PIP2 is bound to the intracellular divalent and polyvalent cations such as Mg2+ and polyamines making a pool of ‘chelated’ PIP2 which is not available for interaction with the IKs channels. Osmotic cell swelling is mediated by the cellular water uptake, which dilutes intracellular cations and thus ‘frees’ some of the bound PIP2 leading to the channel activation. On the opposite side, osmotic cell shrinkage is brought about by the loss of cellular water and results in the increased concentration of intracellular cations which, in turn, bind more PIP2 competing it off the IKs channels and causing channel inhibition. This hypothesis is supported by the following observations: (1) saturation of membrane PIP2 by the dialysis of the water-soluble analogue diC8-PIP2 prevented IKs osmosensitivity; (2) Chelation of Mg2+ and polyamines by EDTA prevented IKs osmosensitivity; (3) isoosmotic changes in intracellular concentrations of Mg2+ and polyamines (spermine and spermidine) mimicking these produced by hypo- or hyperosmotic conditions fully reproduced osmotic regulation of IKs amplitude and kinetics. What makes the proposed mechanism for osmosensitivity particularly attractive is the fact that many membrane ion channels, transporters and pumps are regulated by PIP2 (Gamper & Shapiro, 2007), and thus, this mechanism can provide a much needed general principle for the complex and precise ion flux regulation in response to a sudden change in the extracellular osmolarity. Figure 1 Osmosensitivity of membrane ion channel (cylinders) can be mediated by changes in PIP2 (green shapes labelled ‘–’) avilability brought about by the concentration/dilution of cytosolic polycations (blue shapes labelled ‘++’, ... The challenge ahead is twofold. First, the hypothesis will need to be tested against the other PIP2-sensitive ion channels and transporters; thus far it is not clear if it stands true even for the Kv7 channels all of which are PIP2 sensitive: while Kv7.4 seems to ‘obey’ the principle (Hougaard et al. 2004), the native current conducted by neuronal Kv7 channels (‘IM’) in CA1 pyramidal neurons does not seem to (Caspi et al. 2009). Second, some ion transport proteins that need PIP2 for their activity are expected to be regulated reciprocally upon osmotic challenges (e.g. K+ channels and Na+/H+ exchanger; Gamper & Shapiro, 2007; Lang, 2007), and thus, their osmosensitivity cannot be straightforwardly deduced from their PIP2 sensitivity. Future research should rigorously address these questions before the hypothesis of ‘osmosensitivity through the PIP2 availability’ can be accepted as a general principle of cellular osmoregulation.