Quantum dots reveal heterogeneous membrane diffusivity and dynamic surface density polarization of dopamine transporter

Quantum Dot Imaging Agents: Haematopoietic Cell Interactions and Biocompatibility

Quantum dots (QDs) are semi-conducting nanoparticles that have been developed for a range of biological and non-biological functions. They can be tuned to multiple different emission wavelengths and can have significant benefits over other fluorescent systems. Many studies have utilised QDs with a cadmium-based core; however, these QDs have since been shown to have poor biological compatibility. Therefore, other QDs, such as indium phosphide QDs, have been developed. These QDs retain excellent fluorescent intensity and tunability but are thought to have elevated biological compatibility. Herein we discuss the applicability of a range of QDs to the cardiovascular system. Key disease states such as myocardial infarction and stroke are associated with cardiovascular disease (CVD), and there is an opportunity to improve clinical imaging to aide clinical outcomes for these disease states. QDs offer potential clinical benefits given their ability to perform multiple functions, such as carry an imaging agent, a therapy, and a targeting motif. Two key cell types associated with CVD are platelets and immune cells. Both cell types play key roles in establishing an inflammatory environment within CVD, and as such aid the formation of pathological thrombi. However, it is unclear at present how and with which cell types QDs interact, and if they potentially drive unwanted changes or activation of these cell types. Therefore, although QDs show great promise for boosting imaging capability, further work needs to be completed to fully understand their biological compatibility.

Dopamine transporter membrane mobility is bidirectionally regulated by phosphorylation and palmitoylation

The primary regulator of dopamine availability in the brain is the dopamine transporter (DAT), a plasma membrane protein that drives reuptake of released dopamine from the extracellular space into the presynaptic neuron. DAT activity is regulated by post-translational modifications that establish clearance capacity through impacts on transport kinetics, and dysregulation of these events may underlie dopaminergic imbalances in mood and psychiatric disorders. Here, using fluorescence recovery after photobleaching, we show that phosphorylation and palmitoylation induce opposing effects on DAT lateral membrane mobility, which may influence functional outcomes by regulating subcellular localization and binding partner interactions. Membrane mobility was also impacted by amphetamine and in polymorphic variant A559V in directions consistent with enhanced phosphorylation. These findings grow the list of DAT properties controlled by these post-translational modifications and highlight their role in establishment of dopaminergic tone in physiological and pathophysiological states.

Improvement of synaptic plasticity by nanoparticles and the related mechanisms: Applications and prospects

Synaptic plasticity is an important basis of learning and memory and participates in brain network remodelling after different types of brain injury (such as that caused by neurodegenerative diseases, cerebral ischaemic injury, posttraumatic stress disorder (PTSD), and psychiatric disorders). Therefore, improving synaptic plasticity is particularly important for the treatment of nervous system-related diseases. With the rapid development of nanotechnology, increasing evidence has shown that nanoparticles (NPs) can cross the blood-brain barrier (BBB) in different ways, directly or indirectly act on nerve cells, regulate synaptic plasticity, and ultimately improve nerve function. Therefore, to better elucidate the effect of NPs on synaptic plasticity, we review evidence showing that NPs can improve synaptic plasticity by regulating different influencing factors, such as neurotransmitters, receptors, presynaptic membrane proteins and postsynaptic membrane proteins, and further discuss the possible mechanism by which NPs improve synaptic plasticity. We conclude that NPs can improve synaptic plasticity and restore the function of damaged nerves by inhibiting neuroinflammation and oxidative stress, inducing autophagy, and regulating ion channels on the cell membrane. By reviewing the mechanism by which NPs regulate synaptic plasticity and the applications of NPs for the treatment of neurological diseases, we also propose directions for future research in this field and provide an important reference for follow-up research.

Confocal laser scanning microscopy study of intercellular events in filopodia using 3-mercaptopropoinc acid capped CdSe/ZnS quantum dots

Physico-chemical mobility of cells in three dimensions is dependent on the development of filipodia, which is the fundamental instinct for survival and other cellular functions in live cells. Specifically, our present research paper describes the synthesis of 3-Mercaptopropoinc acid (MPA) capped CdSe/ZnS quantum dots (QDs), which are biocompatible and utilized for cellular bioimaging applications. Using the pancreatic cell lines BXCP3 cells, we successfully demonstrated the applicability of MPA-capped QDs for intercellular filopodia imaging. Employing these QDs, we examined the dynamics of filopodia formation in real-time along the Z-axis by using confocal laser microscopy.

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.

Membrane Nanoscopic Organization of D2L Dopamine Receptor Probed by Quantum Dot Tracking

The role of lateral mobility and nanodomain organization of G protein-coupled receptors in modulating subcellular signaling has been under increasing scrutiny. Investigation of D2 dopamine receptor diffusion dynamics is of particular interest, as these receptors have been linked to altered neurotransmission in affective disorders and represent the primary target for commonly prescribed antipsychotics. Here, we applied our single quantum dot tracking approach to decipher intrinsic diffusion patterns of the wild-type long isoform of the D2 dopamine receptor and its genetic variants previously identified in several cohorts of schizophrenia patients. We identified a subtle decrease in the diffusion rate of the Val96Ala mutant that parallels its previously reported reduced affinity for potent neuroleptics clozapine and chlorpromazine. Slower Val96Ala variant diffusion was not accompanied by a change in receptor-receptor transient interactions as defined by the diffraction-limited quantum dot colocalization events. In addition, we implemented a Voronoї tessellation-based algorithm to compare nanoclustering of the D2 dopamine receptor to the dominant anionic phospholipid phosphatidylinositol 4,5-bisphosphate in the plasma membrane of live cells.

Determining the Efficiency of Single Molecule Quantum Dot Labeling of HER2 in Breast Cancer Cells

Quantum dots exhibit unique properties compared to other fluorophores, such as bright fluorescence and lack of photobleaching, resulting in their widespread utilization as fluorescent protein labels in the life sciences. However, their application is restricted to relative quantifications due to lacking knowledge about the labeling efficiency. We here present a strategy for determining the labeling efficiency of quantum dot labeling of HER2 in overexpressing breast cancer cells. Correlative light- and liquid-phase electron microscopy of whole cells was used to convert fluorescence intensities into the underlying molecular densities of the quantum dots. The labeling procedure with small affinity proteins was optimized yielding a maximal labeling efficiency of 83%, which was applicable to the high amount of ∼1.5 × 10⁶ HER2 per cell. With the labeling efficiency known, it is now possible to derive the absolute protein expression levels in the plasma membrane and its variation within a cell and between cells.

Novel Fluorescent Ligands Enable Single-Molecule Localization Microscopy of the Dopamine Transporter

The dopamine transporter (DAT) is critical for spatiotemporal control of dopaminergic neurotransmission and is the target for therapeutic agents, including ADHD medications, and abused substances, such as cocaine. Here, we develop new fluorescently labeled ligands that bind DAT with high affinity and enable single-molecule detection of the transporter. The cocaine analogue MFZ2-12 (1) was conjugated to novel rhodamine-based Janelia Fluorophores (JF₅₄₉ and JF₆₄₆). High affinity binding of the resulting ligands to DAT was demonstrated by potent inhibition of [³H]dopamine uptake in DAT transfected CAD cells and by competition radioligand binding experiments on rat striatal membranes. Visualization of binding was substantiated by confocal or TIRF microscopy revealing selective binding of the analogues to DAT transfected CAD cells. Single particle tracking experiments were performed with JF₅₄₉-conjugated DG3-80 (3) and JF₆₄₆-conjugated DG4-91 (4) on DAT transfected CAD cells enabling quantification and categorization of the dynamic behavior of DAT into four distinct motion classes (immobile, confined, Brownian, and directed). Finally, we show that the ligands can be used in direct stochastic optical reconstruction microscopy (dSTORM) experiments permitting further analyses of DAT distribution on the nanoscale. In summary, these novel fluorescent ligands are promising new tools for studying DAT localization and regulation with single-molecule resolution.

Single Quantum Dot Tracking Unravels Agonist Effects on the Dopamine Receptor Dynamics

D2 dopamine receptors (DRD2s) belong to a family of G protein-coupled receptors that modulate synaptic dopaminergic tone via regulation of dopamine synthesis, storage, and synaptic release. DRD2s are the primary target for traditional antipsychotic medications; dysfunctional DRD2 signaling has been linked to major depressive disorder, attention-deficit hyperactivity disorder, addiction, Parkinson's, and schizophrenia. DRD2 lateral diffusion appears to be an important post-translational regulatory mechanism; however, the dynamic response of DRD2s to ligand-induced activation is poorly understood. Dynamic imaging of the long isoform of DRD2 (D2L) fused to an N-terminal antihemagglutinin (HA) epitope and transiently expressed in HEK-293 cells was achieved through a combination of a high-affinity biotinylated anti-HA antigen-binding fragment (Fab) and streptavidin-conjugated quantum dots (QD). Significant reduction (∼40%) in the rate of lateral diffusion of QD-tagged D2L proteins was observed under agonist (quinpirole; QN)-stimulated conditions compared to basal conditions. QN-induced diffusional slowing was accompanied by an increase in frequency, lifetime, and confinement of temporary arrest of lateral diffusion (TALL), an intrinsic property of single receptor lateral motion. The role of the actin cytoskeleton in QN-induced diffusional slowing of D2L was also explored. The observed dynamic changes appear to be a sensitive indicator of the receptor activity status and might also spatially and temporally shape the receptor-mediated downstream signaling. This dynamic information could potentially be useful in informing drug discovery efforts based on single-molecule pharmacology.