Discovering new neuropeptides using single-cell mass spectrometry

Comparative Analysis of Neuropeptides in Homologous Interneurons and Prohormone Annotation in Nudipleuran Sea Slugs

Despite substantial research on neuronal circuits in nudipleuran gastropods, few peptides have been implicated in nudipleuran behavior. In this study, we expanded the understanding of peptides in this clade, using three species with well-studied nervous systems, Hermissenda crassicornis, Melibe leonina, and Pleurobranchaea californica. For each species, we performed sequence homology analysis of de novo transcriptome predictions to identify homologs to 34 of 36 prohormones previously characterized in the gastropods Aplysia californica and Lymnaea stagnalis. We then used single-cell mass spectrometry to characterize peptide profiles in homologous feeding interneurons: the multifunctional ventral white cell (VWC) in P. californica and the small cardioactive peptide B large buccal (SLB) cells in H. crassicornis and M. leonina. The neurons produced overlapping, but not identical, peptide profiles. The H. crassicornis SLB cells expressed peptides from homologs to the FMRFamide (FMRFa), small cardioactive peptide (SCP), LFRFamide (LFRFa), and feeding circuit activating peptides prohormones. The M. leonina SLB cells expressed peptides from homologs to the FMRFa, SCP, LFRFa, and MIP-related peptides prohormones. The VWC, previously shown to express peptides from the FMRFa and QNFLa (a homolog of A. californica pedal peptide 4) prohormones, was shown to also contain SCP peptides. Thus, each neuron expressed peptides from the FMRFa and SCP families, the H. crassicornis and M. leonina SLB cells expressed peptides from the LFRFa family, and each neuron contained peptides from a prohormone not found in the others. These data suggest each neuron performs complex co-transmission, which potentially facilitates a multifunctional role in feeding. Additionally, the unique feeding characteristics of each species may relate, in part, to differences in the peptide profiles of these neurons. These data add chemical insight to enhance our understanding of the neuronal basis of behavior in nudipleurans and other gastropods.

Neuropeptide Localization in Lymnaea stagnalis: From the Central Nervous System to Subcellular Compartments

Due to the relatively small number of neurons (few tens of thousands), the well-established multipurpose model organism Lymnaea stagnalis, great pond snail, has been extensively used to study the functioning of the nervous system. Unlike the more complex brains of higher organisms, L. stagnalis has a relatively simple central nervous system (CNS) with well-defined circuits (e.g., feeding, locomotion, learning, and memory) and identified individual neurons (e.g., cerebral giant cell, CGC), which generate behavioral patterns. Accumulating information from electrophysiological experiments maps the network of neuronal connections and the neuronal circuits responsible for basic life functions. Chemical signaling between synaptic-coupled neurons is underpinned by neurotransmitters and neuropeptides. This review looks at the rapidly expanding contributions of mass spectrometry (MS) to neuropeptide discovery and identification at different granularity of CNS organization. Abundances and distributions of neuropeptides in the whole CNS, eleven interconnected ganglia, neuronal clusters, single neurons, and subcellular compartments are captured by MS imaging and single cell analysis techniques. Combining neuropeptide expression and electrophysiological data, and aided by genomic and transcriptomic information, the molecular basis of CNS-controlled biological functions is increasingly revealed.

Development of Pico-ESI-MS for Single-Cell Metabolomics Analysis

In this chapter, we introduced a Pico-ESI strategy for metabolomics analysis with picoliter-level samples. This Pico-ESI strategy was technically achieved by pulsed direct current electrospray ionization source (Pulsed-DC-ESI). This source could collect MS signals for a few minutes from a cell, enabling us to obtain large-scale MS² data of metabolite IDs in single-cell analysis. Further identification of the single-cell metabolome such as the database match and chemical modification to metabolome was thereby achieved. Technically, this source could ionize sample with no need of sample and electrode contact, which can be potentially applied for high-throughput analysis. We also introduced several strategies related to Pico-ESI to reduce the matrix interference especially for extremely small samples developed in our group, including step-voltage nanoelectrospray, picoliter sample desalting method, droplet-based microextraction method, and probe-ESI, etc. All these strategies have been successfully applied to single-cell analysis.

Fast surface acoustic wave-matrix-assisted laser desorption ionization mass spectrometry of cell response from islets of Langerhans

A desire for higher speed and performance in molecular profiling analysis at a reduced cost is driving a trend in miniaturization and simplification of procedures. Here we report the use of a surface acoustic wave (SAW) atomizer for fast sample handling in matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) peptide and protein profiling of Islets of Langerhans, for future type 2 diabetes (T2D) studies. Here the SAW atomizer was used for ultrasound (acoustic) extraction of insulin and other peptide hormones released from freshly prepared islets, stimulated directly on a membrane. A high energy propagating SAW atomizes the membrane-bound liquid into approximately 2 μm diameter droplets, rich in cell-released molecules. Besides acting as a sample carrier, the membrane provides a purification step by entrapping cell clusters and other impurities within its fibers. A new SAW-based sample-matrix deposition method for MALDI MS was developed and characterized by a strong insulin signal, and a limit of detection (LOD) lower than 100 amol was achieved. Our results support previous work reporting the SAW atomizer as a fast and inexpensive tool for ultrasound, membrane-based sample extraction. When interfaced with MALDI MS, the SAW atomizer constitutes a valuable tool for rapid cell studies. Other biomedical applications of SAW-MALDI MS are currently being developed, aiming at fast profiling of biofluids. The membrane sampling is a simplistic and noninvasive collection method of limited volume biofluids such as the gingival fluid and the tearfilm.

Collection of peptides released from single neurons with particle-embedded monolithic capillaries followed by detection with matrix-assisted laser desorption/ionization mass spectrometry

Characterization of the stimulated release of neuropeptides from brain slices and individual cultured neurons requires efficient collection of the releasate from relatively large volumes of physiological saline. Here, several collection approaches are optimized using particle-embedded monolithic capillaries (PEMCs) with poly(stearyl methacrylate-co-ethylene glycol dimethacrylate) monolith acting as a "glue". Two distinct extraction particles, with either pyrrolidone (PY) or ethylenediamine (EDA) as the functional group on polystyrene backbone, have been embedded into capillaries having an inner diameter of 250 μm. The capillaries act as collection devices for sampling neuropeptide release; the collection protocols are described, and the extraction efficiency of the probes are characterized. Specifically, the binding of angiotensin II from a peptide mixture onto the PY and EDA columns was 16 and 28 pmol, respectively, in a volume of 20 μL of saline. The peptides released from these columns have been characterized via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with low femtomole detection limits. When the PEMC columns were positioned in close proximity to individual neurons and 50 mM KCl was used as the secretagogue, peptides released from individual identified cultured neurons isolated from Aplysia californica were collected and characterized.

Bioanalytical tools for single-cell study of exocytosis

Regulated exocytosis is a fundamental biological process used to deliver chemical messengers for cell-cell communication via membrane fusion and content secretion. A plethora of cell types employ this chemical-based communication to achieve crucial functions in many biological systems. Neurons in the brain and platelets in the circulatory system are representative examples utilizing exocytosis for neurotransmission and blood clotting. Single-cell studies of regulated exocytosis in the past several decades have greatly expanded our knowledge of this critical process, from vesicle/granule transport and docking at the early stages of exocytosis to membrane fusion and to eventual chemical messenger secretion. Herein, four main approaches that have been widely used to study single-cell exocytosis will be highlighted, including total internal reflection fluorescence microscopy, capillary electrophoresis, single-cell mass spectrometry, and microelectrochemistry. These techniques are arranged in the order following the route of a vesicle/granule destined for secretion. Within each section, the basic principles and experimental strategies are reviewed and representative examples are given revealing critical spatial, temporal, and chemical information of a secretory vesicle/granule at different stages of its lifetime. Lastly, an analytical chemist's perspective on potential future developments in this exciting field is discussed.

Tissue imaging using MALDI-MS: a new frontier of histopathology proteomics

Modern pathology is an amalgam of many disciplines, such as microbiology, biochemistry and immunology, which historically have been intermingled with the practice of clinical medicine. For centuries, the pre-eminent pathological tool, at least in the context of patients, was a post-mortem examination. With the advent of optical microscopes, morphology became a predominant means of developing tissue classification. A further paradigm shift occurred in the attempt to understand the nature and origin of disease; the recognition that, ultimately, it is the derangement in the structure and function of genes and proteins that causes human disease. More recent progress in pathology has led to the use of genomics and molecular technologies, including DNA sequencing, microarray analysis, PCR, in situ hybridization and proteomics. Today, the newest frontier appears to be histopathology proteomics, which adds the mass spectrometer to the arsenal of tools for the direct analysis of tissue biopsies and molecular diagnosis. Typically called MALDI imaging, this technique takes mass spectral snapshots of intact tissue slices, revealing how proteins and peptides are spatially distributed within a given sample. In this review, MALDI imaging technology is presented as well as applications of such technology in cancer or neurodegenerative diseases.

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.

Peptidomics: a logical sequel to proteomics

Rapid progress of separation techniques as well as methods of structural analysis provided conditions in the past decade for total screening of complex biologic mixtures for any given class of biomolecules. The present review updates the reader with the modern state of peptidomics, a chapter of chemical biology that deals with structure and biologic properties of sets of peptides present in biologic tissues, cells or fluids. Scope and limitations of currently employed experimental techniques are considered and the main results are outlined. Considerable attention will be afforded to the biologic role of peptides formed in vivo by proteolysis of nonspecialized precursor proteins with other well-defined functions. In conclusion, the connection is discussed between peptidomics and the much more mature and still closely related field of proteomics.

Identification of mammalian cell lines using MALDI-TOF and LC-ESI-MS/MS mass spectrometry

Direct mass spectrometric analysis of complex biological samples is becoming an increasingly useful technique in the field of proteomics. Matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS) is a rapid and sensitive analytical tool well suited for obtaining molecular weights of peptides and proteins from complex samples. Here, a fast and simple approach to cellular protein profiling is described in which mammalian cells are lysed directly in the MALDI matrix 2,5-dihydroxybenzoic acid (DHB) and mass analyzed using MALDI-time of flight (TOF). Using the unique MALDI mass spectral "fingerprint" generated in these analyses, it is possible to differentiate among several different mammalian cell lines. A number of techniques, including MALDI-post source decay (PSD), MALDI tandem time-of-flight (TOF-TOF), MALDI-Fourier transform ion cyclotron resonance (FTICR), and nanoflow liquid chromatography followed by electrospray ionization and tandem mass spectrometry (LC-ESI-MS/MS) were employed to attempt to identify the proteins represented in the MALDI spectra. Performing a tryptic digestion of the supernatant of the cells lysed in DHB with subsequent LC-ESI-MS/MS analysis was by far the most successful method to identify proteins.

The detection of nitrated tyrosine in neuropeptides: a MALDI matrix-dependent response

Neuropeptides are a diverse class of signaling molecules that typically have one or more posttranslational modifications. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is an effective tool for identification and characterization of neuropeptides from samples as small as individual neurons. However, the detection of one particular posttranslational modification-nitrotyrosine-has been problematic because of the lability of the nitro group of nitrotyrosine under MALDI-MS conditions. The detection of nitrated tyrosine in peptide standards was dependent on the MALDI matrix used for the analysis. Specifically, sinapinic acid was the optimum matrix tested to observe this modification while it was not consistently detected with matrices such as 2,5-dihydroxybenzoic acid. Using the optimized procedures, several identified nitric-oxide-synthase positive neurons from Lymnaea stagnalis were tested to determine if the neuropeptides present were nitrated. In all cases, the nitrated form of the neuropeptide was not observed. The dependence on the sample-preparation procedures of observing this particular chemical modification demonstrates the need for careful selection of sample-preparation methods with MALDI or the use of other ionization methods.

Techniques for neuropeptide determination

Because immunoassay responds to epitopes, and many molecules share the same peptide epitope, it is very difficult to obtain an accurate understanding of peptides, their creation and hydrolysis, in biological systems. Separate-and-detect approaches have merit in that the many active peptides and inactive fragments of a particular system can be separately determined. This review discusses the separation, by chromatography and capillary electrophoresis, and detection, by absorbance, fluorescence, electrochemistry, and immunoassay techniques. When separation pre-concentration is accompanied by laser-induced fluorescence or biuret-based electrochemical detection, nM-pM detection limits are obtained.