Investigating Complex Samples with Molograms of Low-Affinity Binders

Focal Molography Allows for Affinity and Concentration Measurements of Proteins in Complex Matrices with High Accuracy

Characterizing biomolecular receptor-ligand interactions is critical for research and development. However, performing analyses in complex, biologically relevant matrices, such as serum, remains challenging due to non-specific binding that often impairs measurements. Here, we evaluated Focal Molography (FM) for determining KD and kinetic constants in comparison to gold-standard methods using single-domain heavy-chain antibodies in various systems. FM provided kinetic constants highly comparable to SPR and BLI in standard buffers containing blocking proteins, with KDs of soluble CD4 (sCD4) interactions within a 2.4-fold range across technologies. In buffers lacking blocking proteins, FM demonstrated greater robustness against non-specific binding and rebinding effects. In serum, FM exhibited stable baseline signals, unlike SPR and BLI, and yielded KDs of sCD4 interaction in 50% Bovine Serum within a 1.8-fold range of those obtained in standard buffers. For challenging molecules prone to non-specific binding (Granzyme B), FM successfully determined kinetic constants without external referencing. Finally, FM enabled direct analyte quantification in complex matrices. sCD4 quantification in cell culture media and 50% FBS showed recovery rates of 97.8-100.3% with an inter-assay CV below 1.3%. This study demonstrates the high potential of FM for kinetic affinity determination and biomarker quantification in complex matrices, enabling reliable measurements under biologically relevant conditions.

Advanced Materials and Sensors for Microphysiological Systems: Focus on Electronic and Electrooptical Interfaces

Advanced in vitro cell culture systems or microphysiological systems (MPSs), including microfluidic organ-on-a-chip (OoC), are breakthrough technologies in biomedicine. These systems recapitulate features of human tissues outside of the body. They are increasingly being used to study the functionality of different organs for applications such as drug evolutions, disease modeling, and precision medicine. Currently, developers and endpoint users of these in vitro models promote how they can replace animal models or even be a better ethically neutral and humanized alternative to study pathology, physiology, and pharmacology. Although reported models show a remarkable physiological structure and function compared to the conventional 2D cell culture, they are almost exclusively based on standard passive polymers or glass with none or minimal real-time stimuli and readout capacity. The next technology leap in reproducing in vivo-like functionality and real-time monitoring of tissue function could be realized with advanced functional materials and devices. This review describes the currently reported electronic and optical advanced materials for sensing and stimulation of MPS models. In addition, an overview of multi-sensing for Body-on-Chip platforms is given. Finally, one gives the perspective on how advanced functional materials could be integrated into in vitro systems to precisely mimic human physiology.

An Approach for the Real-Time Quantification of Cytosolic Protein-Protein Interactions in Living Cells

In recent years, cell-based assays have been frequently used in molecular interaction analysis. Cell-based assays complement traditional biochemical and biophysical methods, as they allow for molecular interaction analysis, mode of action studies, and even drug screening processes to be performed under physiologically relevant conditions. In most cellular assays, biomolecules are usually labeled to achieve specificity. In order to overcome some of the drawbacks associated with label-based assays, we have recently introduced "cell-based molography" as a biosensor for the analysis of specific molecular interactions involving native membrane receptors in living cells. Here, we expand this assay to cytosolic protein-protein interactions. First, we created a biomimetic membrane receptor by tethering one cytosolic interaction partner to the plasma membrane. The artificial construct is then coherently arranged into a two-dimensional pattern within the cytosol of living cells. Thanks to the molographic sensor, the specific interactions between the coherently arranged protein and its endogenous interaction partners become visible in real time without the use of a fluorescent label. This method turns out to be an important extension of cell-based molography because it expands the range of interactions that can be analyzed by molography to those in the cytosol of living cells.

Ultra-Stable Molecular Sensors by Sub-Micron Referencing and Why They Should Be Interrogated by Optical Diffraction-Part I. The Concept of a Spatial Affinity Lock-in Amplifier

Label-free optical biosensors, such as surface plasmon resonance, are sensitive and well-established for the characterization of molecular interactions. Yet, these sensors require stabilization and constant conditions even with the use of reference channels. In this paper, we use tools from signal processing to show why these sensors are so cross-sensitive and how to overcome their drawbacks. In particular, we conceptualize the spatial affinity lock-in as a universal design principle for sensitive molecular sensors even in the complete absence of stabilization. The spatial affinity lock-in is analogous to the well-established time-domain lock-in. Instead of a time-domain signal, it modulates the binding signal at a high spatial frequency to separate it from the low spatial frequency environmental noise in Fourier space. In addition, direct sampling of the locked-in sensor's response in Fourier space enabled by diffraction has advantages over sampling in real space as done by surface plasmon resonance sensors using the distributed reference principle. This paper and part II hint at the potential of spatially locked-in diffractometric biosensors to surpass state-of-the-art temperature-stabilized refractometric biosensors. Even simple, miniaturized and non-stabilized sensors might achieve the performance of bulky lab instruments. This may enable new applications in label-free analysis of molecular binding and point-of-care diagnostics.