How do cells shape and interpret PIP3 signals?

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Multi-cellular organisms rely on a large array of different transmitter substances to allow certain cells to control the behavior of others. The more sophisticated the organism the more complex the cell to cell communication. In mammals this language probably involves hundreds of fundamentally different types of transmitter. Clearly such systems need a large collection of specialized receptor molecules that can detect the individual presence of any particular transmitter. Further, these receptors, typically found on the outer surface of the cell's limiting membrane, have to signal their specific stimulation by passing a molecular message into the cells interior, effectively informing the cell that the receptor has been activated. Clearly, if a cell has many different types of receptors on its surface the molecular signal generated inside the cell by each different receptor (often called an intracellular message) must identify and distinguish which specific receptor has been stimulated. Otherwise the cell could not discriminate between the transmitters present on the outside of the cell and could not respond correctly. Hence, mammalian cells have vastly complex intracellular signalling mechanisms continuously informing the cell of what is happening in other parts of the organism or its environment. One such intracellular signalling molecule or 'message' is PIP3. It is a phospholipid molecule found on the inside surface of the cell's limiting membrane. Levels of PIP3 rise rapidly on activation of a large number of receptors. This is surprising given the problems the cell faces in knowing precisely which receptor has been activated when it detects an intracellular signal. This grant application is to understand how it is possible that rises in PIP3 can encode specific messages from so many different receptors. We have performed some experiments that have, in fact, shown that PIP3 in cells is not a single type of molecule. At least four tiny variants of PIP3 can be detected, called molecular species of PIP3. Interestingly, we find that these different molecular species of PIP3 do not respond equivalently to different ways of activating the cells we work with. We and others have also found that the different receptors can make the levels of PIP3 rise for different times and to different maximum levels. We propose that these small differences are very important inside the cell for discriminating whether a certain receptor has been stimulated. This is a 'clever' economy or efficiency on the part of the cell and allows it to use similar mechanisms to perform many different jobs. Although on the surface these might appear trivial details in the business of understanding biology, it has recently been discovered that many different cancers are caused by mutations in genes that regulate PIP3 levels in cells. Mutations that by chance cause the production of PIP3 to be increased without any need for receptor stimulation make cancers much more likely to occur. Mutations that by chance stop the enzymes that normally break down PIP3 from working also make cancer more likely to occur. As a result it is clear that understanding how PIP3 is made and then interpreted by cells is crucial for us to better understand how cancer occurs and how to treat it. Many companies are already trying to design drugs that will reduce PIP3 levels to fight cancer. This work will help us understand how to make better drugs of that type.