Single-molecule fluorescence quantification with a photobleached internal standard

In cellular and molecular biology, fluorophores are employed to aid in tracking and quantifying molecules involved in cellular function. We previously developed a sensitive single-molecule quantification technique to count the number of proteins and the variation of the protein number over the population of individual subcellular organelles. However, environmental effects on the fluorescent intensity of fluorophores can make it difficult to accurately quantify proteins using these sensitive techniques. In this letter, we demonstrate the use of photobleaching to extract an accurate single-molecule calibration intensity distribution from the sample directly to avoid any differences in environment that may alter the count. Using this technique, we were able to show that goat antimouse IgG antibody labeled with Alexa Fluor 488, an environmentally insensitive fluorophore, exhibited an average fluorescence equivalent to 4.6 single fluorophores. SynaptopHluorin vesicles, which contain the environmentally sensitive green fluorescent protein, exhibited an average of 4.4 single green fluorescent proteins per vesicle.

High-throughput fluorescence-activated nanoscale subcellular sorter with single-molecule sensitivity

Recent single-cell and single-molecule studies have shown that a variety of subpopulations exist within biological systems, such as synaptic vesicles, that have previously been overlooked in common bulk studies. By isolating and enriching these various subpopulations, detailed analysis with a variety of analytical techniques can be done to further understand the role that various subpopulations play in cellular dynamics and how alterations to these subpopulations affect the overall function of the biological system. Previous sorters lack the sensitivity, sorting speed, and efficiency to isolate synaptic vesicles and other nanoscale systems. This paper describes the development of a fluorescence-activated nanoscale subcellular sorter that can sort nearly 10 million objects per hour with single-molecule sensitivity. Utilizing a near-nanoscale channel system, we were able to achieve upward of 91% recovery of desired objects with a 99.7% purity.

Probing the interior of synaptic vesicles with internalized nanoparticles

Synaptic vesicles are subcellular organelles that are found in the synaptic bouton and are responsible for the propagation of signals between neurons. Synaptic vesicles undergo endo- and exocytosis with the neuronal membrane to load and release neurotransmitters. Here we discuss how we utilize this property to load nanoparticles as a means of probing the interior of synaptic vesicles. To probe the intravesicular region of synaptic vesicles, we have developed a highly sensitive pH-sensing polymer dot. We feel the robust nature of the pH-sensing polymer dot will provide insight into the dynamics of proton loading into synaptic vesicles.

Synaptosomes as a platform for loading nanoparticles into synaptic vesicles

Synaptosomes are intact, isolated nerve terminals that contain the necessary machinery to recycle synaptic vesicles via endocytosis and exocytosis upon stimulation. Here we use this property of synaptosomes to load quantum dots into synaptic vesicles. Vesicles are then isolated from the synaptosomes, providing a method to probe isolated, individual synaptic vesicles where each vesicle contains a single, encapsulated nanoparticle. This technique provided an encapsulation efficiency of ~16%, that is, ~16% of the vesicles contained a single quantum dot while the remaining vesicles were empty. The ability to load single nanoparticles into synaptic vesicles opens new opportunity for employing various nanoparticle-based sensors to study the dynamics of vesicular transporters.

Intracellular delivery of bioactive molecules using light-addressable nanocapsules

This paper describes a method by which molecules that are impermeable to cells are encapsulated in dye-sensitized lipid nanocapsules for delivery into cells via endocytosis. Once inside the cells, the molecules are released from the lipid nanocapsules into the cytoplasm with a single nanosecond pulse from a laser in the far red (645 nm). We demonstrate this method with the intracellular release of the second messenger IP(3) in CHO-M1 cells and report that calcium responses from the cells changed from a sustained increase to a transient spike when the average number of IP(3) released is decreased below 50 molecules per nanocapsule. We also demonstrate the delivery of a 23 kDa O(6)-alkylguanine-DNA alkyltransferase (AGT) fusion protein into Ba/F3 cells to inhibit a key player BCR-ABL in the apoptotic pathway. We show that an average of ∼8 molecules of the inhibitor is sufficient to induce apoptosis in the majority of Ba/F3 cells.

Sizing subcellular organelles and nanoparticles confined within aqueous droplets

This article describes two complementary techniques, single-particle tracking and correlation spectroscopy, for accurately sizing nanoparticles confined within picoliter volume aqueous droplets. Single-particle tracking works well with bright particles that can be continuously illuminated and imaged, and we demonstrated this approach for sizing single fluorescent beads. Fluorescence correlation spectroscopy detects small intensity bursts from particles or molecules diffusing through the confocal probe volume, which works well with dim and rapidly diffusing particles or molecules; we demonstrated FCS for sizing synaptic vesicles confined in aqueous droplets. In combination with recent advances in droplet manipulations and analysis, we anticipate this capability to size single nanoparticles and molecules in free solution will complement existing tools for probing cellular systems, subcellular organelles, and nanoparticles.