Molecular Spies in Action: Genetically Encoded Fluorescent Biosensors Light up Cellular Signals

Advancing biosensing through super-resolution fluorescence microscopy

Advancement of super-resolution fluorescence microscopy (SRM) has recently allowed applications to the biosensing by offering significant advantages over conventional methods. Its nanoscale spatial resolution and single-molecule sensitivity allow visualization and quantification of biomolecular targets without the need of signal amplification steps typically required in traditional biosensing methods. Moreover, recent innovations in probe design and imaging protocols have expanded SRM capabilities to enable dynamic biosensing in living cells, revealing molecular processes in their native cellular contexts. In this review, we discuss these applications of various SRM techniques to biosensing by highlighting their unique capabilities in providing spatial distribution information and high molecular sensitivity. We address several challenges that must be overcome for the broader application of SRM-based biosensing. Finally, we discuss perspectives on future directions for advancing this field towards practical applications.

Directed Evolution of Fluorescent Genetically Encoded Biosensors: Innovative Approaches for Development and Optimization of Biosensors

Fluorescent protein-based biosensors are indispensable molecular tools in cell biology and biomedical research, providing non-invasive dynamic measurements of metabolite concentrations and other cellular signals. Traditional methods for developing these biosensors rely on rational design, but directed evolution methods offer a more efficient alternative. This review discusses recent advancements in the development of biosensors using directed evolution, including methods for optimizing domain fusions, sequence optimization, and new screening and selection systems. Additionally, the incorporation of machine learning into the directed evolution process is explored, highlighting its potential to enhance the efficiency and cost reduction of biosensor development. Finally, emerging trends in the development of near-infrared biosensors and photochromic sensors are discussed, along with the opportunities presented by de novo design of sensing domains and biosensors.

Mechanism and application of thiol-disulfide redox biosensors with a fluorescence-lifetime readout

Genetically encoded biosensors with changes in fluorescence lifetime (as opposed to fluorescence intensity) can quantify small molecules in complex contexts, even in vivo. However, lifetime-readout sensors are poorly understood at a molecular level, complicating their development. Although there are many sensors that have fluorescence-intensity changes, there are currently only a few with fluorescence-lifetime changes. Here, we optimized two biosensors for thiol-disulfide redox (RoTq-Off and RoTq-On) with opposite changes in fluorescence lifetime in response to oxidation. Using biophysical approaches, we showed that the high-lifetime states of these sensors lock the chromophore more firmly in place than their low-lifetime states do. Two-photon fluorescence lifetime imaging of RoTq-On fused to a glutaredoxin (Grx1) enabled robust, straightforward monitoring of cytosolic glutathione redox state in acute mouse brain slices. The motional mechanism described here is probably common and may inform the design of other lifetime-readout sensors; the Grx1-RoTq-On fusion sensor will be useful for studying glutathione redox in physiology.

Far-red chemigenetic kinase biosensors enable multiplexed and super-resolved imaging of signaling networks

Fluorescent biosensors have advanced biomedical research by enabling direct live-cell measurements of signaling activities. However, current technology offers limited resolution and dimensionality, impeding our ability to resolve and interrogate spatiotemporally regulated networks of signaling activities. Here we introduce highly sensitive chemigenetic kinase activity biosensors that combine the genetically encodable self-labeling tag, HaloTag7, with far-red-emitting synthetic fluorophores. This technology enables both four-dimensional activity imaging and functional super-resolution imaging using stimulated emission depletion and other high-resolution microscopy techniques, permitting signaling activity to be detected across scales with the necessary resolution. Stimulated emission depletion imaging enabled the investigation of protein kinase A activity at individual clathrin-coated pits. We further demonstrate imaging of up to five analytes in single living cells, an increase in the dimensionality of biosensor multiplexing. Multiplexed imaging of cellular responses to the activation of different G-protein-coupled receptors (GPCRs) allowed quantitative measurements of spatiotemporal network states downstream of individual GPCR-ligand pairs.

Advancing super-resolution microscopy with metal complexes: functional imaging agents for nanoscale visualization

Super-resolution microscopy (SRM) has transformed biological imaging by overcoming the diffraction limit, offering nanoscale visualization of cellular structures and processes. However, the widespread use of organic fluorescent probes is often hindered by limitations such as photobleaching, short photostability, and inadequate performance in deep-tissue imaging. Metal complexes, with their superior photophysical properties, including exceptional photostability, tuneable luminescence, and extended excited-state lifetimes, address these challenges, enabling precise subcellular targeting and long-term imaging. Beyond imaging, their theranostic potential unlocks real-time diagnostics and treatments for diseases such as cancer and bacterial infections. This review explores recent advancements in applying metal complexes for SRM, focusing on their utility in visualizing intricate subcellular structures, capturing temporal dynamics in live cells and elucidating in vivo spatial organization. We emphasize how rational design strategies optimize biocompatibility, organelle specificity, and deep-tissue penetration, expanding their applicability in multiplexed imaging. Furthermore, we discuss the design of various metal nanoparticles (NPs) for SRM, along with emerging hybrid nanoscale probes that integrate metal complexes with gold (Au) scaffolds, offering promising avenues for overcoming current limitations. By highlighting both established successes and potential frontiers, this review provides a roadmap for leveraging metal complexes as versatile tools in advancing SRM applications.

State-dependent motion of a genetically encoded fluorescent biosensor

Genetically encoded biosensors can measure biochemical properties such as small-molecule concentrations with single-cell resolution, even in vivo. Despite their utility, these sensors are "black boxes": Very little is known about the structures of their low- and high-fluorescence states or what features are required to transition between them. We used LiLac, a lactate biosensor with a quantitative fluorescence-lifetime readout, as a model system to address these questions. X-ray crystal structures and engineered high-affinity metal bridges demonstrate that LiLac exhibits a large interdomain twist motion that pulls the fluorescent protein away from a "sealed," high-lifetime state in the absence of lactate to a "cracked," low-lifetime state in its presence. Understanding the structures and dynamics of LiLac will help to think about and engineer other fluorescent biosensors.