Unravelling biased signalling through a lysosomal ion channel

Communication is everything, as they say. And the same can be said of cells, which constantly generate and receive signals to function properly. Context In previous BBSRC-funded work, we discovered an ion channel protein called TPC2 which releases calcium from lysosomes. Lysosomes are organelles that are typically regarded as the cell's recycling centre. But they do much more and one such function is to act as a source of calcium that is tapped by ion channels such as TPC2. Calcium is a well known signal, vital for more or less all processes in the body from fertilisation (‘life’) to degeneration of nerves in diseases such as Alzheimer’s (‘death’). Understanding how calcium is regulated is therefore critical for understanding, and ultimately promoting, our well-being. But calcium signalling through lysosomes is much less well understood than signalling through other organelles. This application builds on our recent discovery, also funded by the BBSRC, that TPC2 can transform from being a calcium channel to being a sodium channel on demand. This is very unusual because textbooks, including my own (Cell Biology: A short course; ISBN-13, ?978-0470526996), tell us that the selectivity of an ion channel i.e. what ion(s) are permitted to flow, is fixed. This does not hold for TPC2 as it readily switches its ion selectivity depending on its activation stimulus, namely the messenger molecules, NAADP and PI(3,5)P2. Importantly, the two resulting modes of signalling have differential effects on the function of the lysosome. TPC2 is thus capable of what we term ‘biased’ signalling. Aims and objectives Our latest work has found individual amino acids in TPC2 which when mutated essentially convert TPC2 into a more ordinary, unbiased channel which does not switch its selectivity. This is a big step forward as we can now turn the switch on and off as we please, in order to study this process in detail. What we will do is pinpoint the gears and cogs within the protein which lead to such profound ion selectivity switching. This will establish a novel mechanism. We will also determine the impact of ion selectivity switching on the acidity and movement of lysosomes, as these are both essential features of the organelle. This will establish functional significance. And finally, we will relate ion channel switching to natural variants in the TPC2 gene and new cellular functions. This will establish physiological significance. Key to past, present and anticipated future success is collaboration. Co-applicants and an international project partner will bring much expertise allowing us all to go well beyond our individual capabilities. This adds considerable value to this proposal. Potential applications and benefits and relevance to the BBSRC Our findings will benefit many researchers including ‘signallers’ (not only calcium but sodium too), biophysicists studying ion channels and cell biologists studying lysosomes. More broadly, our findings will be of interest to Pharma and clinicians. This is because the insight we will provide might inform on developing drugs to specifically modulate bias. In turn, these could be used to combat diseases such as Parkinson’s and melanoma to which TPC2 has been linked. We will provide interdisciplinary training of people and connect excellence across places in the UK (and beyond) through an idea that will advance our understanding of, if not bend, the ‘rules of life’.