The fidelity of signal transduction requires spatiotemporal control of the production of signaling agents. Phosphatidic acid (PA) is a pleiotropic lipid second messenger whose modes of action differ based on upstream stimulus, biosynthetic source, and site of production. How cells regulate the local production of PA to effect diverse signaling outcomes remains elusive. Unlike other second messengers, sites of PA biosynthesis cannot be accurately visualized with subcellular precision. Here, we describe a rapid, chemoenzymatic approach for imaging physiological PA production by phospholipase D (PLD) enzymes. Our method capitalizes on the remarkable discovery that bulky, hydrophilic trans-cyclooctene–containing primary alcohols can supplant water as the nucleophile in the PLD active site in a transphosphatidylation reaction of PLD’s lipid substrate, phosphatidylcholine. The resultant trans-cyclooctene–containing lipids are tagged with a fluorogenic tetrazine reagent via a no-rinse, inverse electron-demand Diels–Alder (IEDDA) reaction, enabling their immediate visualization by confocal microscopy in real time. Strikingly, the fluorescent reporter lipids initially produced at the plasma membrane (PM) induced by phorbol ester stimulation of PLD were rapidly internalized via apparent nonvesicular pathways rather than endocytosis, suggesting applications of this activity-based imaging toolset for probing mechanisms of intracellular phospholipid transport. By instead focusing on the initial 10 s of the IEDDA reaction, we precisely pinpointed the subcellular locations of endogenous PLD activity as elicited by physiological agonists of G protein-coupled receptor and receptor tyrosine kinase signaling. These tools hold promise to shed light on both lipid trafficking pathways and physiological and pathological effects of localized PLD signaling.

Phosphatidic acid (PA) is both a central phospholipid biosynthetic intermediate and a multifunctional lipid second messenger produced at several discrete subcellular locations. Organelle-specific PA pools are believed to play distinct physiological roles, but tools with high spatiotemporal control are lacking for unraveling these pleiotropic functions. Here, we present an approach to precisely generate PA on demand on specific organelle membranes. We exploited a microbial phospholipase D (PLD), which produces PA by phosphatidylcholine hydrolysis, and the CRY2–CIBN light-mediated heterodimerization system to create an optogenetic PLD (optoPLD). Directed evolution of PLD using yeast membrane display and IMPACT, a chemoenzymatic method for visualizing cellular PLD activity, yielded a panel of optoPLDs whose range of catalytic activities enables mimicry of endogenous, physiological PLD signaling. Finally, we applied optoPLD to elucidate that plasma membrane, but not intracellular, pools of PA can attenuate the oncogenic Hippo signaling pathway. OptoPLD represents a powerful and precise approach for revealing spatiotemporally defined physiological functions of PA.

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Phosphatidic acids (PAs) are glycerophospholipids that regulate key cell signaling pathways governing cell growth and proliferation, including the mTOR and Hippo pathways. Their acyl chains vary in tail length and degree of saturation, leading to marked differences in the signaling functions of different PA species. For example, in mTOR signaling, saturated forms of PA are inhibitory, whereas unsaturated forms are activating. To enable rapid control over PA signaling, we describe here the development of photoswitchable analogues of PA, termed AzoPA and dAzoPA , that contain azobenzene groups in one or both lipid tails, respectively. These photolipids enable optical control of their tail structure and can be reversibly switched between a straight trans form and a relatively bent cis form. We found that cis - dAzoPA selectively activates mTOR signaling, mimicking the bioactivity of unsaturated forms of PA. Further, in the context of Hippo signaling, whose growth-suppressing activity is blocked by PA, we found that the cis forms of both AzoPA and dAzoPA selectively inhibit this pathway. Collectively, these photoswitchable PA analogues enable optical control of mTOR and Hippo signaling, and we envision future applications of these probes to dissect the pleiotropic effects of physiological and pathological PA signaling.