Characterization of the Physicochemical Parameters of Dense Core Atrial Gland and Lucent Red Hemiduct Vesicles in Aplysia californica

Image-guided MALDI mass spectrometry for high-throughput single-organelle characterization

Peptidergic dense-core vesicles are involved in packaging and releasing neuropeptides and peptide hormones-critical processes underlying brain, endocrine and exocrine function. Yet, the heterogeneity within these organelles, even for morphologically defined vesicle types, is not well characterized because of their small volumes. We present image-guided, high-throughput mass spectrometry-based protocols to chemically profile large populations of both dense-core vesicles and lucent vesicles for their lipid and peptide contents, allowing observation of the chemical heterogeneity within and between these two vesicle populations. The proteolytic processing products of four prohormones are observed within the dense-core vesicles, and the mass spectral features corresponding to the specific peptide products suggest three distinct dense-core vesicle populations. Notable differences in the lipid mass range are observed between the dense-core and lucent vesicles. These single-organelle mass spectrometry approaches are adaptable to characterize a range of subcellular structures.

Single-cell lipidomics: characterizing and imaging lipids on the surface of individual Aplysia californica neurons with cluster secondary ion mass spectrometry

Neurons isolated from Aplysia californica , an organism with a well-defined neural network, were imaged with secondary ion mass spectrometry, C(60)-SIMS. A major lipid component of the neuronal membrane was identified as 1-hexadecyl-2-octadecenoyl-sn-glycero-3-phosphocholine [PC(16:0e/18:1)] using tandem mass spectrometry (MS/MS). The assignment was made directly off the sample surface using a C(60)-QSTAR instrument, a prototype instrument that combines an ion source with a commercial electrospray ionization/matrix-assisted laser desorption ionization (ESI/MALDI) mass spectrometer. Normal phase liquid chromatography mass spectrometry (NP-LC-MS) was used to confirm the assignment. Cholesterol and vitamin E were also identified with in situ tandem MS analyses that were compared to reference spectra obtained from purified compounds. In order to improve sensitivity on the single-cell level, the tandem MS spectrum of vitamin E reference material was used to extract and compile all the vitamin E related peaks from the cell image. The mass spectrometry images reveal heterogeneous distributions of intact lipid species, PC(16:0e/18:1), vitamin E, and cholesterol on the surface of a single neuron. The ability to detect these molecules and determine their relative distribution on the single-cell level shows that the C(60)-QSTAR is a potential platform for studying important biochemical processes, such as neuron degeneration.

Mechanical tension modulates local and global vesicle dynamics in neurons

Growing experimental evidence suggests that mechanical tension plays a significant role in determining the growth, guidance, and function of neurons. Mechanical tension in axons contributes to neurotransmitter clustering at the Drosophila neuromuscular junction (NMJ) and is actively regulated by neurons both in vitro and in vivo. In this work, we applied mechanical strain on in vivo Drosophila neurons and in vitro Aplysia neurons and studied their vesicle dynamics by live-imaging. Our experiments show that mechanical stretch modulates the dynamics of vesicles in two different model systems: (1) The global accumulation of synaptic vesicles (SV) at the Drosophila NMJ and (2) the local motion of individual large dense core vesicles (LDCV) in Aplysia neurites. Specifically, a sustained stretch results in enhanced SV accumulation in the Drosophila NMJ. This increased SV accumulation occurs in the absence of extracellular Ca(2+), plateaus after approximately 50 min, and persists for at least 30 min after stretch is reduced. On the other hand, mechanical compression in Aplysia neurites immediately disrupts LDCV motion, leading to decreased range and processivity. This impairment of LDCV motion persists for at least 15 min after tension is restored. These results show that mechanical stretch modulates both local and global vesicle dynamics and strengthens the notion that tension serves a role in regulating neuronal function.

Electrochemical probes for detection and analysis of exocytosis and vesicles

Unraveling the mechanistic details of neurotransmitter exocytosis is arguably among the most important molecular problems in neuroscience today. Investigations at single cells, particularly with electrochemical methods, have given unique chemical and biological insight into this process at the fundamental level. The rapid response time (submillisecond) of microelectrodes makes them well suited for monitoring the dynamic process of exocytosis. We review here recent developments in electrochemical techniques to spatially and simultaneously detect exocytosis across a single cell and to measure the transmitter content of single vesicles removed from cells. The former method is used to demonstrate dynamic heterogeneity in release across a cell, and in the latter work comparison is made between vesicle content and release to conclude that only a fraction of the transmitter is released during full exocytosis.

Analytical approaches to investigate transmitter content and release from single secretory vesicles

The vesicle serves as the primary intracellular unit for the highly efficient storage and release of chemical messengers triggered during signaling processes in the nervous system. This review highlights conventional and emerging analytical methods that have used microscopy, electrochemistry, and spectroscopy to resolve the location, time course, and quantal content characteristics of neurotransmitter release. Particular focus is on the investigation of the synaptic vesicle and its involvement in the fundamental molecular mechanisms of cell communication.

Only a Fraction of Quantal Content is Released During Exocytosis as Revealed by Electrochemical Cytometry of Secretory Vesicles

The primary method for neuronal communication involves the release of chemical messengers that are packaged intracellularly in vesicles. Although experiments measuring release at single cells have classically been thought to assess the entire content of vesicles, there is evidence in the literature that suggests that the total transmitter stored in vesicles is not expelled during exocytosis. In this work, we introduce a novel technology using a microfluidic-based platform to electrochemically probe individual PC12 cell vesicles isolated from the cell environment. We measure the total vesicular content using methodology that circumvents the biophysical processes of the cell associated with exocytosis. Direct comparisons of amperometric data from release experiments at single PC12 cells versus our cell-free model reveal that on average vesicles release only 40% of their total transmitter load. The data support the intriguing hypothesis that the average vesicle does not open all the way during the normal exocytosis process, resulting in incomplete distention of the vesicular contents. In addition, we have shown that vesicular catecholamine levels can be altered with pharmacological manipulation and variances observed from these treatments can be resolved at the single vesicle level in a high-throughput manner, a process that we have termed electrochemical cytometry. Upon establishing that release in exocytotic processes proceeds in an incomplete manner, electrochemical data quantified from both single cell release experiments and electrochemical cytometry of vesicles were related to vesicular volume from electron microscopy measurements to investigate the location of intravesicular catecholamine stores retained post-fusion.

Time-dependent interactions of the two porphyrinic compounds chlorin e6 and mono-L-aspartyl-chlorin e6 with phospholipid vesicles probed by NMR spectroscopy

The distribution processes of chlorin e6 (CE) and monoaspartyl-chlorin e6 (MACE) between the outer and inner phospholipid monolayers of 1,2-dioleoyl-phosphatidylcholine (DOPC) vesicles were monitored by 1H NMR spectroscopy through analysis of chemical shifts and line widths of the DOPC vesicle resonances. Chlorin adsorption to the outer vesicle monolayer induced changes in the DOPC 1H NMR spectrum. Most pronounced was a split of the N-methyl choline resonance, allowing for separate analysis of inner and outer vesicle layers. Transbilayer distribution of the chlorin compounds was indicated by time-dependent characteristic spectral changes of the DOPC resonances. Kinetic parameters for the flip-flop processes, that is, half-lives and rate constants, were obtained from the experimental data points. In comparison to CE, MACE transbilayer movement was significantly reduced, with MACE remaining more or less attached to the outer membrane layer. The distribution coefficients for CE and MACE between the vesicular and aqueous phase were determined. Both CE and MACE exhibited a high affinity for the vesicular phase. For CE, a positive correlation was found between transfer rate and increasing molar ratio CE/DOPC. Enhanced membrane rigidity induced by increasing amounts of cholesterol into the model membrane was accompanied by a decrease of CE flip-flop rates across the membrane. The present study shows that the movement of porphyrins across membranes can efficiently be investigated by 1H NMR spectroscopy and that small changes in porphyrin structure can have large effects on membrane kinetics.

Characterizing intercellular signaling peptides in drug addiction

Intercellular signaling peptides (SPs) coordinate the activity of cells and influence organism behavior. SPs, a chemically and structurally diverse group of compounds responsible for transferring information between neurons, are broadly involved in neural plasticity, learning and memory, as well as in drug addiction phenomena. Historically, SP discovery and characterization has tracked advances in measurement capabilities. Today, a suite of analytical technologies is available to investigate individual SPs, as well as entire intercellular signaling complements, in samples ranging from individual cells to entire organisms. Immunochemistry and in situ hybridization are commonly used for following preselected SPs. Discovery-type investigations targeting the transcriptome and proteome are accomplished using high-throughput characterization technologies such as microarrays and mass spectrometry. By integrating directed approaches with discovery approaches, multiplatform studies fill critical gaps in our knowledge of drug-induced alterations in intercellular signaling. Throughout the past 35 years, the National Institute on Drug Abuse has made significant resources available to scientists that study the mechanisms of drug addiction. The roles of SPs in the addiction process are highlighted, as are the analytical approaches used to detect and characterize them.

Within the cell: analytical techniques for subcellular analysis

This review covers recent developments in the preparation, manipulation, and analyses of subcellular environments. In particular, it highlights approaches for (1) separation and detection of individual organelles, (2) preparation of ultra-pure organelle fractions, and (3) utilization of novel labeling strategies. These approaches, based on innovative technologies such as microfluidics, immunoisolation, mass spectrometry and electrophoresis, suggest that subcellular analyses will soon become as commonplace as single cell and bulk cellular assays.