Real-time imaging of cell-surface proteins with antibody-based fluorogenic probes

Visualizing the cellular internalization of therapeutic antibodies via pH-sensitive release of AIEgen

Among the fastest-growing bio-pharmaceuticals, therapeutic antibodies have achieved unprecedented success in treating various diseases. Though powerful, issues such as inefficacy or acquired resistance are waiting to be addressed to benefit more patients with improved therapeutic outcomes. In addition to in vivo distribution, the cellular spatiotemporal information including the antibody-antigen interaction and subsequent internalization is found to be important for the therapeutic effects. To better understand the cellular fate of therapeutic antibodies, especially the cellular internalization process, we employed a pH-sensitive linker to attach a red-emissive AIEgen onto the antibody. The resulting antibody conjugate will undergo AIEgen release to liberate brilliant fluorescence inside acidic endo/lysosomes, allowing wash-free visualization of the internalization process and facilitating the evaluation of antibody-drug efficacy.

Multimodal techniques and strategies for chemical and metabolic imaging at the single-cell level

Single-cell chemical and metabolic imaging technologies provide unprecedented insights into individual cell dynamics, advancing our understanding of cellular processes, molecular interactions, and metabolic activities. Advances in fluorescence, Raman, optoacoustic (photoacoustic), or mass spectrometry methods have paved the way to characterize metabolites, signaling molecules, and other moieties within individual cells. These modalities can also lead to single-cell imaging capabilities by targeting endogenous cell contrast or by employing exogenous contrast generation techniques, including contrast agents that target specific cell structure or function. In this review, we present key developments, summarize recent applications in single-cell interrogation and imaging, and illustrate their advantages, limitations, and outlook.

Targeted Degradation of Cell-Surface Proteins via Chaperone-Mediated Autophagy by Using Peptide-Conjugated Antibodies

Cell-surface proteins are important drug targets but historically have posed big challenges for the complete elimination of their functions. Herein, we report antibody-peptide conjugates (Ab-CMAs) in which a peptide targeting chaperone-mediated autophagy (CMA) was conjugated with commercially available monoclonal antibodies for specific cell-surface protein degradation by taking advantage of lysosomal degradation pathways. Unique features of Ab-CMAs, including cell-surface receptor- and E3 ligase-independent degradation, feasibility towards different cell-surface proteins (e.g., epidermal growth factor receptor (EGFR), programmed cell death ligand 1 (PD-L1), human epidermal growth factor receptor 2 (HER2)) by a simple change of the antibody, and successful tumor inhibition in vivo, make them attractive protein degraders for biomedical research and therapeutic applications. As the first example employing CMA to degrade proteins from the outside in, our findings may also shed new light on CMA, a degradation pathway typically targeting cytosolic proteins.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Reduction-Activatable Fluorogenic Nanobody for Targeted and Low-Background Bioimaging

Environment-sensitive fluorogenic antibodies enable target-specific bioimaging with reduced unspecific background signal and improved spatiotemporal resolution. However, current strategies for the construction of fluorogenic antibodies are hard to handle due to challenges that lie in the prior design of fluorogenic probes and subsequent antibody labeling. Here, we report a simple strategy to generate a fluorogenic nanobody, which we term D-body, by in situ incorporation of a reduction-responsive Nile blue foldamer which is self-quenched via a dimerization-caused quenching mechanism. The D-body can be efficiently internalized by cells with high epidermal growth factor receptor expression levels and is highly fluorogenic upon lysosomal activation, allowing wash-free cell imaging with exquisite specificity and fast in vivo imaging with a high tumor-to-background ratio. The modular D-body is readily available and easy to handle, offering a platform that is highly tunable for bioimaging applications.

Rapid and Highly Selective Fluorescent Labeling of Peptides via a Thia-Diels-Alder Cycloaddition: Application to Apelin

Herein, we describe a catalyst-free thia-Diels-Alder cycloaddition for the chemoselective labeling of fully deprotected phosphonodithioester-peptides in solution with fluorophores functionalized with an exocyclic diene. The reaction was optimized on the model tripeptide 1 containing a lysine residue, which enabled its rapid and straightforward labeling with three different fluorophores (fluorescein, lissamine rhodamine B, and squaraine) in very mild conditions (H₂O/iPrOH, 37 °C, 1 h). The reaction was then successfully applied to the chemoselective labeling of fully deprotected apelin-13 with squaraine dye. The resulting fluorescent ligand 18 exhibited a high affinity (0.17 ± 0.03 nM) for apelinR. It enabled the development of time-resolved FRET-based competition assays for high-throughput screening and drug discovery. Thanks to its fluorogenic properties, ligand 18 was also successfully involved in the live-cell optical imaging of apelinR in no-wash conditions.

In vivo targeted delivery of antibodies into cancer cells with pH-responsive cell-penetrating poly(disulfide)s

Cell-penetrating poly(disulfide)s (CPDs) are promising vehicles for cytosolic delivery of proteins. However, currently available arginine-rich CPD has rarely been reported for systemic delivery due to its "always" positive charge. Herein, we developed pH-responsive CPDIMD that executes tumor targeting delivery via protonation of imidazole groups within the acidic tumor microenvironment.

Intra Q-body: an antibody-based fluorogenic probe for intracellular proteins that allows live cell imaging and sorting

Although intracellular biomarkers can be imaged with fluorescent dye(s)-labeled antibodies, the use of such probes for precise imaging of intracellular biomarkers in living cells remains challenging due to background noise from unbound probes. Herein, we describe the development of a conditionally active Fab-type Quenchbody (Q-body) probe derived from a monoclonal antibody (DO-1) with the ability to both target and spatiotemporally visualize intracellular p53 in living cells with low background signal. p53 is a key tumor suppressor and validated biomarker for cancer diagnostics and therapeutics. The Q-body displayed up to 27-fold p53 level-dependent fluorescence enhancement in vitro with a limit of detection of 0.72 nM. In fixed and live cells, 8.3- and 8.4-fold enhancement was respectively observed. Furthermore, we demonstrate live-cell sorting based on p53 expression. This study provides the first evidence of the feasibility and applicability of Q-body probes for the live-cell imaging of intrinsically intracellular proteins and opens a novel avenue for research and diagnostic applications on intracellular target-based live-cell sorting.

A fluorescent immunosensor that lights up tumor biomarker p53 in living cells was developed based on the Q-body technology. The technology was further applied to the live cell monitoring of p53 levels, and live cell sorting based on p53 expression.