A photoactivatable small-molecule inhibitor for light-controlled spatiotemporal regulation of Rho kinase in live embryos.

To uncover the molecular mechanisms of embryonic development, the ideal loss-of-function strategy would be capable of targeting specific regions of the living embryo with both temporal and spatial precision. To this end, we have developed a novel pharmacological agent that can be light activated to achieve spatiotemporally limited inhibition of Rho kinase activity in vivo. A new photolabile caging group, 6-nitropiperonyloxymethyl (NPOM), was installed on a small-molecule inhibitor of Rho kinase, Rockout, to generate a 'caged Rockout' derivative. Complementary biochemical, cellular, molecular and morphogenetic assays in both mammalian cell culture and Xenopus laevis embryos validate that the inhibitory activity of the caged compound is dependent on exposure to light. Conveniently, this unique reagent retains many of the practical advantages of conventional small-molecule inhibitors, including delivery by simple diffusion in the growth medium and concentration-dependent tuneability, but can be locally activated by decaging with standard instrumentation. Application of this novel tool to the spatially heterogeneous problem of embryonic left-right asymmetry revealed a differential requirement for Rho signaling on the left and right sides of the primitive gut tube, yielding new insight into the molecular mechanisms that generate asymmetric organ morphology. As many aromatic/heterocyclic small-molecule inhibitors are amenable to installation of this caging group, our results indicate that photocaging pharmacological inhibitors might be a generalizable technique for engendering convenient loss-of-function reagents with great potential for wide application in developmental biology.