Quantitative Analysis of Single Quantum Dot Trajectories

Quantum Dot Fluorescent Imaging: Using Atomic Structure Correlation Studies to Improve Photophysical Properties

Efforts to study intricate, higher-order cellular functions have called for fluorescence imaging under physiologically relevant conditions such as tissue systems in simulated native buffers. This endeavor has presented novel challenges for fluorescent probes initially designed for use in simple buffers and monolayer cell culture. Among current fluorescent probes, semiconductor nanocrystals, or quantum dots (QDs), offer superior photophysical properties that are the products of their nanoscale architectures and chemical formulations. While their high brightness and photostability are ideal for these biological environments, even state of the art QDs can struggle under certain physiological conditions. A recent method correlating electron microscopy ultrastructure with single-QD fluorescence has begun to highlight subtle structural defects in QDs once believed to have no significant impact on photoluminescence (PL). Specific defects, such as exposed core facets, have been shown to quench QD PL in physiologically accurate conditions. For QD-based imaging in complex cellular systems to be fully realized, mechanistic insight and structural optimization of size and PL should be established. Insight from single QD resolution atomic structure and photophysical correlative studies provides a direct course to synthetically tune QDs to match these challenging environments.

A Novel Biotinylated Homotryptamine Derivative for Quantum Dot Imaging of Serotonin Transporter in Live Cells

The serotonin transporter (SERT) is the primary target for selective serotonin reuptake inhibitor (SSRI) antidepressants that are thought to exert their therapeutic effects by increasing the synaptic concentration of serotonin. Consequently, probes that can be utilized to study cellular trafficking of SERT are valuable research tools. We have developed a novel ligand (IDT785) that is composed of a SERT antagonist (a tetrahydro pyridyl indole derivative) conjugated to a biotinylated poly ethylene glycol (PEG) via a phenethyl linker. This compound was determined to be biologically active and inhibited SERT-mediated reuptake of IDT307 with the half-maximal inhibitory concentration of 7.2 ± 0.3 μM. We demonstrated that IDT785 enabled quantum dot (QD) labeling of membrane SERT in transfected HEK-293 cultures that could be blocked using the high affinity serotonin reuptake inhibitor paroxetine. Molecular docking studies suggested that IDT785 might be binding to the extracellular vestibule binding site rather than the orthosteric substrate binding site, which could be attributable to the hydrophilicity of the PEG chain and the increased loss of degrees of freedom that would be required to penetrate into the orthosteric binding site. Using IDT785, we were able to study the membrane localization and membrane dynamics of YFP-SERT heterologously expressed in HEK-293 cells and demonstrated that SERT expression was enriched in the membrane edge and in thin cellular protrusions.