Synaptic optical imaging platforms: Examining pharmacological modulation of neurotransmitter release at discrete synapses

Chemical Targeting of Voltage Sensitive Dyes to Specific Cells and Molecules in the Brain

Voltage sensitive fluorescent dyes (VSDs) are important tools for probing signal transduction in neurons and other excitable cells. The impact of these highly lipophilic sensors has, however, been limited due to the lack of cell-specific targeting methods in brain tissue or living animals. We address this key challenge by introducing a nongenetic molecular platform for cell- and molecule-specific targeting of synthetic VSDs in the brain. We employ a dextran polymer particle to overcome the inherent lipophilicity of VSDs by dynamic encapsulation and high-affinity ligands to target the construct to specific neuronal cells utilizing only native components of the neurotransmission machinery at physiological expression levels. Dichloropane, a monoamine transporter ligand, enables targeting of dense dopaminergic axons in the mouse striatum and sparse noradrenergic axons in the mouse cortex in acute brain slices. PFQX in conjunction with ligand-directed acyl imidazole chemistry enables covalent labeling of AMPA-type glutamate receptors in the same brain regions. Probe variants bearing either a classical electrochromic ANEP dye or state-of-the-art VoltageFluor-type dye respond to membrane potential changes in a similar manner to the parent dyes, as shown by whole-cell patch recording. We demonstrate the feasibility of optical voltage recording with our probes in brain tissue with one-photon and two-photon fluorescence microscopy and define the signal limits of optical voltage imaging with synthetic sensors under a low photon budget determined by the native expression levels of the target proteins. This work demonstrates the feasibility of a chemical targeting approach and expands the possibilities of cell-specific imaging and pharmacology.

A Fluorescent False Neurotransmitter as a Dual Electrofluorescent Probe for Secretory Cell Models

A dual electrofluorescent probe (FFN42) belonging to the fluorescent false neurotransmitter family was rationally designed for investigating cell secretion. This probe, which comprises a coumarin core with one amino and two hydroxy groups, is very promising due to its electroactive and fluorescent properties. The optimal excitation and emission wavelengths (380 nm and 470 nm respectively) make this probe adapted for use in fluorescence microscopy. FFN42 has a quantum yield of 0.18, a molar absorption coefficient of 12000 M^-1 cm^-1 and pKa values of 5.4 and 6.7 for the hydroxy groups. The electroactivity of FFN42 was evidenced on carbon fiber and ITO electrodes at relatively low oxidation potentials (0.24 V and 0.45 V vs Ag/AgCl respectively). Epifluorescence observations showed that FFN42 accumulated into secretory vesicles of PC12 and N13 cells. Toxicity tests further revealed that FFN42 had no lethal effect on these cells. Amperometric data obtained on carbon fiber electrodes proved that the probe is released by N13 cells.

Identification of Fluorescent Small Molecule Compounds for Synaptic Labeling by Image-Based, High-Content Screening

Few tools are available for noninvasive imaging of synapses in the living mammalian brain. Current paradigms require the use of genetically modified mice or viral delivery of genetic material to the brain. To develop an alternative chemical approach, utilizing the recognition of synaptic components by organic small molecules, we designed an imaging-based, high-content screen in cultured cortical neurons to identify molecules based on their colocalization with fluorescently tagged synaptic proteins. We used this approach to screen a library of ∼7000 novel fluorescent dyes, and identified a series of compounds in the xanthone family that exhibited consistent synaptic labeling. Follow-up studies with one of these compounds, CX-G3, demonstrated its ability to label acidic organelles and in particular synaptic vesicles at glutamatergic synapses in cultured neurons and murine brain tissue, indicating the potential of this screening approach to identify promising lead compounds for use as synaptic markers, sensors, and targeting devices.

A Dual Functional Electroactive and Fluorescent Probe for Coupled Measurements of Vesicular Exocytosis with High Spatial and Temporal Resolution

In this work, Fluorescent False Neurotransmitter 102 (FFN102), a synthesized analogue of biogenic neurotransmitters, was demonstrated to show both pH-dependent fluorescence and electroactivity. To study secretory behaviors at the single-vesicle level, FFN102 was employed as a new fluorescent/electroactive dual probe in a coupled technique (amperometry and total internal reflection fluorescence microscopy (TIRFM)). We used N13 cells, a stable clone of BON cells, to specifically accumulate FFN102 into their secretory vesicles, and then optical and electrochemical measurements of vesicular exocytosis were experimentally achieved by using indium tin oxide (ITO) transparent electrodes. Upon stimulation, FFN102 started to diffuse out from the acidic intravesicular microenvironment to the neutral extracellular space, leading to fluorescent emissions and to the electrochemical oxidation signals that were simultaneously collected from the ITO electrode surface. The correlation of fluorescence and amperometric signals resulting from the FFN102 probe allows real-time monitoring of single exocytotic events with both high spatial and temporal resolution. This work opens new possibilities in the investigation of exocytotic mechanisms.