Regulation of ion channels and transporters by phosphatidylinositol 4,5-bisphosphate


Phosphatidylinositol-4,5-bisphosphate, or more prosaically, PIP2, doesn't exactly roll off the tongue does it? But who would have thought such a dreary sounding molecule, a mere lipid at that, could provide such an interesting and powerful regulator of key signalling molecules? For someone weaned on the famous Singer and Nicholson cartoon, where the crucial proteins such as ion channels and receptors floated in a sea of otherwise rather dull supporting cast lipids, the following Journal of Physiology symposium proved fascinating and revelatory. The symposium ‘Regulation of ion channels and transporters by phosphatidylinositol 4,5-bisphosphate (PIP2)’, held in conjunction with the 51st Biophysical Society Annual Meeting in Baltimore, proved a great success, with most of the great and the good in the PIP2 field presenting outstanding seminars, which have become reports of current theories and cutting-edge developments in this issue of The Journal of Physiology. The only thing lacking is the stimulating and often colourful discussions and questions and answers, but these reports will provide the reader with a most valuable introduction to this fascinating field.

Don Hilgeman (2007) reminds us how he and his colleagues were the first to observe roles for PIP2 on ion channels and exchangers aside from its canonical function in phosphoinositide signalling (where it is a precursor for DAG and IP3). He discovered that PIP2 modulated Kir channels and Na+/Ca2+ exchangers in cardiac muscle, and in his present report shows that the membrane surface availability of PIP2 acts in a permissive manner, allowing these signalling and transport proteins to be active, whilst the absence of PIP2 in intracellular membranes keeps these channels ‘sleeping’ whilst they are being trafficked or processed deep inside the cell. His review also deals with interesting experiments on internalization and potential compartmentalization of PIP2 in cellular membranes (the latter being a recurring theme throughout the meeting).

Bertil Hille, as always, provided an authoritative review of his group's tremendous efforts in unravelling the regulation of voltage-gated KCNQ/Kv7 channels (some of which underlie the famous ‘M’ current – see below) by PIP2. Suh & Hille (2007) review some of the recent elegant experiments using sophisticated optical probes and kinetic modelling showing how dynamic changes in PIP2 concentration can control M current amplitude in model cells transfected with KCNQ2 and KCNQ3 channel subunits. This work was nicely echoed and extended by David Brown, who as the Godfather of the M current, provided a magisterial review of the regulation of M current in neurons by PIP2 (Brown et al. 2007). Once again, using a variety of techniques, including the new powerful optical probes, he compared and contrasted the actions of bradykinin and oxotremorine (a standard mAchR agonist) on sympathetic ganglion cells, showing how distinct signalling pathways involving PIP2 as a master regulator allow neurones to subtly alter their overall output. The ever-increasing subtleties in modulation of neurotransmission serve to remind us, like J. B. S. Haldane's conjecture, that neuronal signalling after agonist binding is not only more complex than we suppose but perhaps more complicated than we can suppose!

Some of the difficulties in testing hypotheses about PIP2 action have been overcome with tools developed by Tobias Meyer and Tamas Balla. Here, Balla (2007) comprehensively illuminates his group's progress in developing optical probes (with different phosphoinositide binding domains fused to fluorescent indicators) including a ‘new generation’ of PIP2 tools – inducible regulators of PIP2 turnover – to dissect out PIP2's functional roles and to alter PIP2 concentration inside cells.

The report of Voets & Nilius (2007) focuses on modulation of the highly fashionable TRP channels, a class of membrane protein which appears to be affected by a new chemical entity or physical force almost weekly. In particular, they home in on the dramatic modulation of the TRPM4 channel's voltage and calcium dependence by PIP2– the former seeing a leftward shift in V1/2 to more physiological voltages and the latter having an almost 100-fold increase in apparent affinity.

Leslie Loew gave a beautifully illustrated talk entitled ‘Where does all the PIP2 come from?’ (Loew, 2007) in which he uses the ‘Virtual Cell’ portal, an online facility run at the University of Connecticut Health Center (where he is also Director), which provides a computational modelling and simulation problem solving environment for cell biology. Loew used this powerful tool to model kinetics of PIP2 breakdown and release of its metabolites, to test existing models and propose further experiments, to help us to better understand what is really going on with phosphoinositide turnover when one particular reaction in the cycle, for example, PLC-induced PIP2 hydrolysis, is up-regulated.

Using powerful molecular modelling, with mapping of PIP2 onto the three-dimensional atomic scale models of Kir channels, Diomedes Logothetis gave a beautiful and compelling presentation. The report here shows some of these models, which also gives data from his group's site-directed mutagenesis experiments, resulting in a more than plausible scheme which can explain channel activation by PIP2 (Logothetis et al. 2007). Furthermore, because of the proximity of the PIP2 binding site to those sites of action of a variety of modulators, Logothetis et al. convincingly argue the hypothesis that PIP2 might serve as a merging point for multiple modulatory pathways.

Mark Shapiro gave a stimulating and informative talk on ‘Regulation of voltage-gated Ca2+ channels by phosphoinositides’, which outlined his and his colleagues' recent efforts in deciphering the control of N and P/Q type calcium channels by different Gq/11 coupled receptors. In their present review, Gamper & Shapiro (2007) take the opportunity to expand upon that theme, discussing more generally how cellular and receptor specificity might be achieved with PIP2 signalling – for instance, are there membrane microdomains? Importantly, they also point out that all of those fabulous indicators (such as GFP-tagged plekstrin homology domains) currently employed in this expanding field may bring their own problems to the measurement of PIP2, since by their very nature they can change the concentration of PIP2 in tiny enclosed regions – leading to a sort of ‘Uncertainty PIPrinciple’. Luckily, they point out that a broad based approach to unravel the complexities of PIP2 signalling will be required, perhaps necessitating another such high-quality symposium and symposium proceedings in the near future.

For my own part, I had great expectations of this PIP meeting (a joke which died betwixt my lips and several score biophysicists ears), which thanks to the excellent speakers and the expert Chairs, Gamper and Shapiro, were well exceeded. Thank you to all, and I hope the readers enjoy some of the results.

The interested reader is also encouraged to read the following research papers on the fascinating and growing physiology of PIP2, which are also published in the present volume (Crowder et al. 2007; Nam et al. 2007; Nielsen et al. 2007; Shen et al. 2007; Sohn et al. 2007; Yaradanakul et al. 2007).