Chemical probes for phosphatidylinositol phosphates used as tools to interrogate lipid-protein interactions

Phosphatidylinositol phosphates (PIPs) are membrane phospholipids and have been studied owing to their crucial role in a wide range of cellular functions. Amongst the seven naturally occurring PIPs, PI(4,5)P2 has been primarily investigated and linked to cellular events related to cell growth, proliferation and apoptosis. Changes of cellular PIP levels and a disruption of the equilibrium between the different PIPs has been linked to the development of a host of diseases, such as Alzheimer's, bipolar disorder or cancer. The development of a chemical tool which is capable to control these PIP levels can thus have the potential for diagnostic and therapeutic application. Previously, it was shown by our group that small molecules which bind to PIPs can be employed to disrupt protein-PIP interactions. By binding to the PIP headgroups, the small molecule prevents the protein from interacting with PIPs, resulting in an apparent reduction of target PIP available and as a consequence, proteins are not recruited to the membrane and downstream signalling pathways can be disrupted. Following from these results, the aim of this project was to investigate the potential of these small molecules to be developed into optical probes and also further expand the library of small molecules for PIP targets. A series of small molecule-receptors were synthesised based on previous leads, incorporating the same recognition motifs for PIPs. Additionally, fluorophores were attached to the scaffold with the aim of studying the host-guest interaction as well as to monitor the cellular uptake of the new receptors. Additionally, we investigated the effect of introducing solubilising substituents on the receptor to facilitate both synthesis and application of these receptors. Moreover, a receptor based on a tripodal scaffold was synthesised, which differed from previously investigated receptor arrangements. In vitro investigations showed that the inclusion of an optical unit changes both binding affinity and PIP-binding selectivity compared to previous small molecule-receptors. Additionally, it was shown that the tripodal scaffold, containing an extra target recognition motif, is capable of binding PIP with higher affinity compared to receptors based on one set of recognition motifs and additionally, PIP-binding selectivity is shifted compared to previously investigated lead compounds. Preliminary results suggest this tripodal receptor is able to enter cells and bind its preferred PIP target in vivo. Combined with its in vitro binding affinities, this suggests that the tripodal arrangement has the potential to generate potent PIP binders, which could be further investigated both for diagnostic or therapeutic application.