Diphospho-inositol polyphosphates (InsP7) are second messengers involved in essential cell signalling pathways. A distinct difference of InsP7 compared with other inositol polyphosphates is the presence of a phosphoanhydride bond in, for example, the 5-position, rendering them a structurally unique class of second messengers. This special feature is also the reason for their nickname ‘inositol pyrophosphates’.
InsP7 are implicated in the regulation of diverse cellular and metabolic functions in different kingdoms of life. It has been proposed that InsP7 bind to the pleckstrin homology (PH) domain of protein kinase B (Akt), and competitively suppress its specific phosphatidylinositol 3,4,5-trisphosphate (PIP3) association at the plasma membrane, thereby inhibiting phosphoinositide-dependent kinase 1 (PDK1)-mediated phosphorylation of Akt.
However, there remains uncertainty as to whether the reduced phosphorylation of Akt is a result of the inhibition of its membrane association via ts PH-domain, since the in vitro assays that have been performed do not contain any membrane or membrane mimics. Inhibition of the Akt pathway by InsP7 has an impact on glucose uptake and insulin sensitivity, as exemplified by a mouse model that lacks inositol hexakisphosphate-kinase 1 (IP6K1).
These knockout mice have reduced levels of InsP7 and show a lean phenotype on high-fat diet concomitant with increased insulin sensitivity. As a consequence, IP6K1 has recently been proposed as a novel target in the treatment of diabetes and obesity.
To address fundamental questions about the mechanism of action of these potent signalling molecules and their subcellular localization, the development of new chemical tools is required. To understand cellular signalling mediated by second messengers, photocaged analogues that can be activated on demand inside living cells with spatiotemporal resolution have attracted great interest.
Unfortunately, preparation of such analogues often requires lengthy synthetic sequences. Potocaged derivatives are currently available for the more complex diphospho-myo-inositol pentakisphosphates
Researchers report the design, step-economical synthesis, photophysical and metabolic evaluation of photocaged 5-InsP7, and significantly, a general solution to the delivery of unmodified polyphosphate probes into cells using guanidinium-rich molecular transporters.
The caged molecule is stable and releases InsP7 only on irradiation. While photocaged InsP7 does not enter cells, its cellular uptake is achieved using nanoparticles formed by association with a guanidinium-rich molecular transporter.
This novel synthesis and unprecedented polyphosphate delivery strategy enable the first studies required to understand InsP7 signalling in cells with controlled spatiotemporal resolution. It is shown herein that cytoplasmic photouncaging of InsP7 leads to translocation of the PH-domain of Akt, an important signalling-node kinase involved in glucose homeostasis, from the membrane into the cytoplasm.