M.Sc. Anja Armbruster

Recombinant adeno-associated viral (rAAV) vectors are a leading platform for in vivo gene therapy, valued for their excellent safety, broad serotype diversity, and scalable production. Targeted delivery through capsid display of ligands holds great promise, yet current retargeting strategies often rely on extensive capsid re-engineering and restrict the use of ligands incompatible with intracellular expression systems. Here, we present a modular AAV retargeting platform that, for the first time, employs the SpyTag/SpyCatcher system via genetic integration into the AAV2 capsid. SpyTag is a small peptide that forms a covalent, irreversible bond with its protein partner, SpyCatcher, allowing site-specific ligand coupling under physiological conditions. Inserting SpyTag into surface-exposed capsid sites enabled postassembly functionalization of AAVs with SpyCatcher-fused targeting proteins. As proof of concept, we used SpyCatcher fusions with designed ankyrin repeat proteins (DARPins) specific for EGFR, EpCAM, and HER2. This conferred highly specific transduction of corresponding cancer cell lines with minimal off-target activity. Therapeutic potential was demonstrated by delivering a suicide gene, inducing selective cancer cell killing upon prodrug administration. This “one-fits-all” platform allows rapid and flexible retargeting without significantly altering the underlying vectors genome or production process. It supports the incorporation of large or complex ligands not amenable to genetic fusion and facilitates high-throughput preclinical evaluation strategies. By uniting capsid engineering with modular ligand display, our approach provides a scalable and versatile framework for precision gene delivery, broadening the applicability of rAAV in both therapeutic and discovery settings.

Methods for the precise temporal control of cell surface receptor activation are indispensable for the investigation of signaling processes in mammalian cells. Optogenetics enables such precise control, but its application in primary cells is limited by the imperative for genetic manipulation of target cells. We here describe a method that overcomes this obstacle and enables the precise activation of the T cell receptor in nongenetically engineered human T cells by light. Our optogenetic receptor activation system OptoREACT employs a TCR-specific scFv fused to PIF6 that interacts with tetramerized PhyB in a light-dependent manner and thereby clusters and activates the T cell receptor in response to red light. OptoREACT not only omits genetic manipulation of the target cell but, because of its modular nature, is likely applicable to a broad range of oligomerization-activated cell surface receptors.

Optogenetics is a versatile and powerful tool for the control and analysis of cellular signaling processes. The activation of cellular receptors by light using optogenetic switches usually requires genetic manipulation of cells. However, this considerably limits the application in primary, nonengineered cells, which is crucial for the study of physiological signaling processes and for controlling cell fate and function for therapeutic purposes. To overcome this limitation, we developed a system for the light-dependent extracellular activation of cell surface receptors of nonengineered cells termed OptoREACT (Optogenetic Receptor Activation) based on the light-dependent protein interaction of A. thaliana phytochrome B (PhyB) with PIF6. In the OptoREACT system, a PIF6-coupled antibody fragment binds the T cell receptor (TCR) of Jurkat or primary human T cells, which upon illumination is bound by clustered phytochrome B to induce receptor oligomerization and activation. For clustering of PhyB, we either used tetramerization by streptavidin or immobilized PhyB on the surface of cells to emulate the interaction of a T cell with an antigen-presenting cell. We anticipate that this extracellular optogenetic approach will be applicable for the light-controlled activation of further cell surface receptors in primary, nonengineered cells for versatile applications in fundamental and applied research.

Molecular optogenetics utilizes genetically encoded, light-responsive protein switches to control the function of molecular processes. Over the last two years, there have been notable advances in the development of novel optogenetic switches, their utilization in elucidating intricate signaling pathways, and their progress toward practical applications in biotechnological processes, material sciences, and therapeutic applications. In this review, we discuss these areas, offer insights into recent developments, and contemplate future directions.