Visualizing Single-Cell Secretion Dynamics with Single-Protein Sensitivity

Abstract

Cellular secretion of proteins into the extracellular environment is an essential mediator of critical biological mechanisms, including cell-to-cell communication, immunological response, targeted delivery, and differentiation. Here, we report a novel methodology that allows for the real-time detection and imaging of single unlabeled proteins that are secreted from individual living cells. This is accomplished via interferometric detection of scattered light (iSCAT) and is demonstrated with Laz388 cells, an Epstein-Barr virus (EBV)-transformed B cell line. We find that single Laz388 cells actively secrete IgG antibodies at a rate of the order of 100 molecules per second. Intriguingly, we also find that other proteins and particles spanning ca. 100 kDa-1 MDa are secreted from the Laz388 cells in tandem with IgG antibody release, likely arising from EBV-related viral proteins. The technique is general and, as we show, can also be applied to studying the lysate of a single cell. Our results establish label-free iSCAT imaging as a powerful tool for studying the real-time exchange between cells and their immediate environment with single-protein sensitivity.

Interferometric Scattering (iSCAT) Microscopy and Related Techniques

“…(i) Schematic of the experimental arrangement wherein a secreting cell is positioned near to the iSCAT field of view, the contrast distribution of proteins secreted by the cell as they bind to an anti-IgG functionalized surface (blank) agrees with that expected from pure IgG (red). iSCAT also reveals abundance of other secreted products (blue) [166]. Reproduced with permission from the American Chemical Society.…”

“…Another recent application of label-free single-protein detection was showcased in the context of real-time investigation of cellular secretion [135,166], shown in Fig. 12c.…”

Label-Free Imaging of Single Proteins Secreted from Living Cells via iSCAT Microscopy

“…The amount of light that a small bioparticle scatters scales with its polarizability. Assuming that the shape of a protein can be approximated by an effective scattering sphere 14,15,16 , and that different proteins have very similar refractive indices, the measured signal can be directly connected to the molecular weight (MW) of the protein. The empirical calibration of iSCAT contrast versus molecular weight by reference measurements allows one to distinguish proteins of different sizes.…”

“…Figure 4e and 4f show the results of background removal and differential post processing; two adsorbed proteins are visible as diffraction-limited spots in Figure 4f. Figure 6 shows a histogram of the detected proteins over the course of 125 s. These data were obtained by applying a peak-seeking algorithm to the captured images to count the binding events and catalog their contrast 16 . A total number of 503 proteins were detected.…”

“…Next, secreted species are identified by comparison with reference measurements carried out on purified protein solutions, or through additional measurements with functionalized glass surfaces 14,16 . The iSCAT data, thus, directly visualize cellular secretion dynamics on a subsecond scale .…”