Faculty Bibliography

Photopharmacology relies on ligands that change their pharmacodynamics upon photoisomerization. Many of these ligands are azobenzenes that are thermodynamically more stable in their elongated trans-configuration. Often, they are biologically active in this form and lose activity upon irradiation and photoisomerization to their cis-isomer. Recently, cyclic azobenzenes, so-called diazocines, have emerged, which are thermodynamically more stable in their bent cis-form. Incorporation of these switches into a variety of photopharmaceuticals could convert dark-active ligands into dark-inactive ligands, which is preferred in most biological applications. This "pharmacological sign-inversion" is demonstrated for a photochromic blocker of voltage-gated potassium channels, termed CAL, and a photochromic opener of G protein-coupled inwardly rectifying potassium (GIRK) channels, termed CLOGO.

A short, biomimetic synthesis of the fungal metabolite preuisolactone A is described. Its key steps are a purpurogallin-type (5 + 2)-cycloaddition, followed by fragmentation, vinylogous aldol addition, oxidative lactonization, and a final benzilic acid rearrangement. Our work explains why preuisolactone A has been isolated as a racemate and suggests that the natural product is not a sesquiterpenoid but a phenolic polyketide.

Photopharmacology has yielded compounds that have potential to restore impaired visual responses resulting from outer retinal degeneration diseases such as retinitis pigmentosa. Here we evaluate two photoswitchable azobenzene ion channel blockers, DAQ and DAA for vision restoration. DAQ exerts its effect primarily on RGCs, whereas DAA induces light-dependent spiking primarily through amacrine cell activation. Degeneration-induced local field potentials remain a major challenge common to all vision restoration approaches. These 5-10 Hz rhythmic potentials increase the background firing rate of retinal ganglion cells (RGCs) and overlay the stimulated response, thereby reducing signal-to-noise ratio. Along with the bipolar cell-selective photoswitch DAD and second-generation RGC-targeting photoswitch PhENAQ, we investigated the effects of DAA and DAQ on rhythmic local field potentials (LFPs) occurring in the degenerating retina. We found that photoswitches targeting neurons upstream of RGCs, DAA (amacrine cells) and DAD (bipolar cells) suppress the frequency of LFPs, while DAQ and PhENAQ (RGCs) had negligible effects on frequency or spectral power of LFPs. Taken together, these results demonstrate remarkable diversity of cell-type specificity of photoswitchable channel blockers in the retina and suggest that specific compounds may counter rhythmic LFPs to produce superior signal-to-noise characteristics in vision restoration.

G protein-coupled receptors (GPCRs) are membrane proteins that play important roles in biology. However, our understanding of their function in complex living systems is limited because we lack tools that can target individual receptors with sufficient precision. State-of-the-art approaches, including DREADDs, optoXRs, and PORTL gated-receptors, control GPCR signaling with molecular, cell type, and temporal specificity. Nonetheless, these tools are based on engineered non-native proteins that may (i) express at nonphysiological levels, (ii) localize and turnover incorrectly, and/or (iii) fail to interact with endogenous partners. Alternatively, membrane-anchored ligands (t-toxins, DARTs) target endogenous receptors with molecular and cell type specificity but cannot be turned on and off. In this study, we used a combination of chemistry, biology, and light to control endogenous metabotropic glutamate receptor 2 (mGluR2), a Family C GPCR, in primary cortical neurons. mGluR2 was rapidly, reversibly, and selectively activated with photoswitchable glutamate tethered to a genetically targeted-plasma membrane anchor (membrane anchored Photoswitchable Orthogonal Remotely Tethered Ligand; maPORTL). Photoactivation was tuned by adjusting the length of the PORTL as well as the expression level and geometry of the membrane anchor. Our findings provide a template for controlling endogenous GPCRs with cell type specificity and high spatiotemporal precision.

Due its unique structure and its reported potent antifungal activity, the marine polyketide hippolachnin A (1) has attracted much attention in the synthetic community. Herein, we describe the development of a concise, diversifiable and scalable synthesis of the racemic natural product, which serves as a platform for the generation of bioactive analogues. Biological testing of our synthetic material did not confirm the reported antifungal activity of hippolachnin A but unraveled moderate activity against nematodes and microbes.

Ceramides are central intermediates of sphingolipid metabolism that also function as potent messengers in stress signaling and apoptosis. Progress in understanding how ceramides execute their biological roles is hampered by a lack of methods to manipulate their cellular levels and metabolic fate with appropriate spatiotemporal precision. Here, we report on clickable, azobenzene-containing ceramides, caCers, as photoswitchable metabolic substrates to exert optical control over sphingolipid production in cells. Combining atomic force microscopy on model bilayers with metabolic tracing studies in cells, we demonstrate that light-induced alterations in the lateral packing of caCers lead to marked differences in their metabolic conversion by sphingomyelin synthase and glucosylceramide synthase. These changes in metabolic rates are instant and reversible over several cycles of photoswitching. Our findings disclose new opportunities to probe the causal roles of ceramides and their metabolic derivatives in a wide array of sphingolipid-dependent cellular processes with the spatiotemporal precision of light.

Enterocin (vulgamycin) is a structurally remarkable natural product with significant antibiotic activity. The synthesis of a linear polyketide resembling a biosynthetic precursor was achieved using an unusual acyloin reaction. A diazo group was introduced as a protecting group for an enolizable ketone. We were unable to bring about the envisioned biomimetic aldol addition cascade and gained insights into the feasibility of this process by DFT calculations. As an alternative approach to enterocin, we developed a Cu-catalyzed intramolecular cyclopropanation followed by a MgI₂-induced fragmentation to install the 2-oxabicyclo[3.3.1]nonane core of the natural product.

The acremines are a family of meroterpenoids isolated from fungi of the genus Acremonium. Here, we present the asymmetric total synthesis of acremine F which hinges on a modestly enantioselective dihydroxylation and a subsequent kinetic resolution via a highly selective asymmetric reduction. Chemoselective oxidation of acremine F gave access to acremines A and B. The dimerization of acremine F to bisacremine E was investigated but could not be achieved, shedding light on the formation of the acremine dimers in nature.

Photoswitchable blockers of potassium channels can be used to optically control neuronal excitability and hold great promise for vision restoration. Here, we report a series of improved photoswitchable blockers that are furnished with a new pharmacophore based on the local anesthetic bupivacaine. These azobupivacaines (ABs) enable optical control over the delayed rectifier channel K_v2.1. and target the two-pore domain potassium channel TREK-1. For the first time, we have identified a compound that blocks conductance in the dark and potentiates it upon illumination. Using light as a trigger, ABs efficiently and reversibly silence action potential firing of hippocampal neurons in acute mouse brain slices.