Direct peptide profiling by mass spectrometry of single identified neurons reveals complex neuropeptide-processing pattern

Mass spectrometry-based characterization of endogenous peptides and metabolites in small volume samples

Technologies to assay single cells and their extracellular microenvironments are valuable in elucidating biological function, but there are challenges. Sample volumes are low, the physicochemical parameters of the analytes vary widely, and the cellular environment is chemically complex. In addition, the inherent difficulty of isolating individual cells and handling small volume samples complicates many experimental protocols. Here we highlight a number of mass spectrometry (MS)-based measurement approaches for characterizing the chemical content of small volume analytes, with a focus on methods used to detect intracellular and extracellular metabolites and peptides from samples as small as individual cells. MS has become one of the most effective means for analyzing small biological samples due to its high sensitivity, low analyte consumption, compatibility with a wide array of sampling approaches, and ability to detect a large number of analytes with different properties without preselection. Having access to a flexible portfolio of MS-based methods allows quantitative, qualitative, untargeted, targeted, multiplexed, and spatially resolved investigations of single cells and their similarly scaled extracellular environments. Combining MS with on-line and off-line sample conditioning tools, such as microfluidic and capillary electrophoresis systems, significantly increases the analytical coverage of the sample's metabolome and peptidome, and improves individual analyte characterization/identification. Small volume assays help to reveal the causes and manifestations of biological and pathological variability, as well as the functional heterogeneity of individual cells within their microenvironments and within cellular populations. This article is part of a Special Issue entitled: Neuroproteomics: Applications in Neuroscience and Neurology.

Mass spectrometry imaging and profiling of single cells

Mass spectrometry imaging and profiling of individual cells and subcellular structures provide unique analytical capabilities for biological and biomedical research, including determination of the biochemical heterogeneity of cellular populations and intracellular localization of pharmaceuticals. Two mass spectrometry technologies-secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption/ionization mass spectrometry (MALDI MS)-are most often used in micro-bioanalytical investigations. Recent advances in ion probe technologies have increased the dynamic range and sensitivity of analyte detection by SIMS, allowing two- and three-dimensional localization of analytes in a variety of cells. SIMS operating in the mass spectrometry imaging (MSI) mode can routinely reach spatial resolutions at the submicron level; therefore, it is frequently used in studies of the chemical composition of subcellular structures. MALDI MS offers a large mass range and high sensitivity of analyte detection. It has been successfully applied in a variety of single-cell and organelle profiling studies. Innovative instrumentation such as scanning microprobe MALDI and mass microscope spectrometers enables new subcellular MSI measurements. Other approaches for MS-based chemical imaging and profiling include those based on near-field laser ablation and inductively-coupled plasma MS analysis, which offer complementary capabilities for subcellular chemical imaging and profiling.

Probing neuropeptide signaling at the organ and cellular domains via imaging mass spectrometry

Imaging mass spectrometry (IMS) has evolved to be a promising technology due to its ability to detect a broad mass range of molecular species and create density maps for selected compounds. It is currently one of the most useful techniques to determine the spatial distribution of neuropeptides in cells and tissues. Although IMS is conceptually simple, sample preparation steps, mass analyzers, and software suites are just a few of the factors that contribute to the successful design of a neuropeptide IMS experiment. This review provides a brief overview of IMS sampling protocols, instrumentation, data analysis tools, technological advancements and applications to neuropeptide localization in neurons and endocrine tissues. Future perspectives in this field are also provided, concluding that neuropeptide IMS would greatly facilitate studies of neuronal network and biomarker discovery.

Recent advances in single-cell MALDI mass spectrometry imaging and potential clinical impact

Single-cell analysis is gaining popularity in the field of mass spectrometry as a method for analyzing protein and peptide content in cells. The spatial resolution of MALDI mass spectrometry (MS) imaging is by a large extent limited by the laser focal diameter and the displacement of analytes during matrix deposition. Owing to recent advancements in both laser optics and matrix deposition methods, spatial resolution on the order of a single eukaryotic cell is now achievable by MALDI MS imaging. Provided adequate instrument sensitivity, a lateral resolution of approximately 10 µm is currently attainable with commercial instruments. As a result of these advances, MALDI MS imaging is poised to become a transformative clinical technology. In this article, the crucial steps needed to obtain single-cell resolution are discussed, as well as potential applications to disease research.

AMASS: algorithm for MSI analysis by semi-supervised segmentation

Mass Spectrometric Imaging (MSI) is a molecular imaging technique that allows the generation of 2D ion density maps for a large complement of the active molecules present in cells and sectioned tissues. Automatic segmentation of such maps according to patterns of co-expression of individual molecules can be used for discovery of novel molecular signatures (molecules that are specifically expressed in particular spatial regions). However, current segmentation techniques are biased toward the discovery of higher abundance molecules and large segments; they allow limited opportunity for user interaction, and validation is usually performed by similarity to known anatomical features. We describe here a novel method, AMASS (Algorithm for MSI Analysis by Semi-supervised Segmentation). AMASS relies on the discriminating power of a molecular signal instead of its intensity as a key feature, uses an internal consistency measure for validation, and allows significant user interaction and supervision as options. An automated segmentation of entire leech embryo data images resulted in segmentation domains congruent with many known organs, including heart, CNS ganglia, nephridia, nephridiopores, and lateral and ventral regions, each with a distinct molecular signature. Likewise, segmentation of a rat brain MSI slice data set yielded known brain features and provided interesting examples of co-expression between distinct brain regions. AMASS represents a new approach for the discovery of peptide masses with distinct spatial features of expression. Software source code and installation and usage guide are available at http://bix.ucsd.edu/AMASS/ .

Mass spectral imaging and profiling of neuropeptides at the organ and cellular domains

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is a rapid and sensitive analytical method that is well suited for determining molecular weights of peptides and proteins from complex samples. MALDI-MS can be used to profile the peptides and proteins from single-cell and small tissue samples without the need for extensive sample preparation. Furthermore, the recently developed MALDI imaging technique enables mapping of the spatial distribution of signaling molecules in tissue samples. Several examples of signaling molecule analysis at the single-cell and single-organ levels using MALDI-MS technology are highlighted followed by an outlook of future directions.

MALDI imaging mass spectrometry: state of the art technology in clinical proteomics

A decade after its inception, MALDI imaging mass spectrometry has become a unique technique in the proteomics arsenal for biomarker hunting in a variety of diseases. At this stage of development, it is important to ask whether we can consider this technique to be sufficiently developed for routine use in a clinical setting or an indispensable technology used in translational research. In this report, we consider the contributions of MALDI imaging mass spectrometry and profiling technologies to clinical studies. In addition, we outline new directions that are required to align these technologies with the objectives of clinical proteomics, including: 1) diagnosis based on profile signatures that complement histopathology, 2) early detection of disease, 3) selection of therapeutic combinations based on the individual patient's entire disease-specific protein network, 4) real time assessment of therapeutic efficacy and toxicity, 5) rational redirection of therapy based on changes in the diseased protein network that are associated with drug resistance, and 6) combinatorial therapy in which the signaling pathway itself is viewed as the target rather than any single "node" in the pathway.

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.

Molecular MALDI imaging: an emerging technology for neuroscience studies

Mass spectrometry (MS) has become an essential tool for the detection, identification, and characterization of the molecular components of biological processes, such as those responsible for the dynamic properties of the nervous system. Generally, the application of these powerful techniques requires the destruction of the specimen under study, but recent technological advances have made it possible to apply the matrix-assisted laser desorption/ionization (MALDI) MS technique directly to tissue sections. The major advantage of direct MALDI analysis is that it enables the acquisition of local molecular expression profiles, while maintaining the topographic integrity of the tissue and avoiding time-consuming extraction, purification, and separation steps, which have the potential for introducing artifacts. With automation and the ability to display complex spectral data using imaging software, it is now possible to create multiple 2D maps of selected biomolecules in register with tissue sections, a method now known as MALDI Imaging, or MSI (for Mass Spectrometry Imaging). This creates, for example, an opportunity to correlate functional states, determined a priori with live recording or imaging, with the corresponding molecular maps obtained at the time the tissue is frozen and analyzed with MSI. We review the increasing application of MALDI Imaging to the analysis of molecular distributions of proteins and peptides in nervous tissues of both vertebrates and invertebrates, focusing in particular on recent studies of neurodegenerative diseases and early efforts to implement assays of neuronal development.

Peptides in the brain: mass spectrometry-based measurement approaches and challenges

The function and activity of almost every circuit in the human brain are modified by the signaling peptides (SPs) surrounding the neurons. As the complement of peptides can vary even in adjacent neurons and their physiological actions can occur over a broad range of concentrations, the required figures of merit for techniques to characterize SPs are surprisingly stringent. In this review, we describe the formation and catabolism of SPs and highlight a range of mass spectrometric techniques used to characterize SPs. Approaches that supply high chemical information content, direct tissue profiling, spatially resolved data, and temporal information on peptide release are also described. Because of advances in measurement technologies, our knowledge of SPs has greatly increased over the last decade, and SP discoveries will continue as the capabilities of modern measurement approaches improve.

Direct identification of proteins from T47D cells and murine brain tissue by matrix-assisted laser desorption/ionization post-source decay/collision-induced dissociation

The purpose of this study is to determine the feasibility of the direct matrix-assisted laser desorption/ionization (MALDI) identification of proteins in fixed T47D breast cancer cells and murine brain tissues. The ability to identify proteins from cells and tissue may lead to biomarkers that effectively predict the onset of defined disease states, and their dynamic behavior could be an important hint for drug target discoveries. Direct tissue application of trypsin allows protein identification in cells and tissues, while maintaining spatial integrity and intracellular organization. Using a chemical printer, matrix was co-registered on trypsinized human T47D breast cancer cells and cryo-preserved sections of murine brain tissue, followed by MALDI post-source decay (PSD) or MALDI collision-induced dissociation (CID), respectively. Mass-to-charge (m/z) data from the cells and brain tissues were processed using Mascot software interrogation of the National Center for Biotechnology Information (NCBI) database. Histone H2B was identified from cultured T47D human breast cancer cells. Tubulin beta2 was identified from mouse brain cortex following an induced stroke. These results suggest that MALDI PSD/CID, combined with bioinformatics, can be used for the direct identification of proteins from cells and tissues. Refinements in preparation techniques may improve this approach to provide a tool for quantitative proteomics and clinical analysis.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Peptidomics: identification and quantification of endogenous peptides in neuroendocrine tissues

Neuropeptides perform a large variety of functions as intercellular signaling molecules. While most proteomic studies involve digestion of the proteins with trypsin or other proteases, peptidomics studies usually analyze the native peptide forms. Neuropeptides can be studied by using mass spectrometry for identification and quantitation. In many cases, mass spectrometry provides an understanding of the precise molecular form of the native peptide, including post-translational cleavages and other modifications. Quantitative peptidomics studies generally use differential isotopic tags to label two sets of extracted peptides, as done with proteomic studies, except that the Cys-based reagents typically used for quantitation of proteins are not suitable because most peptides lack Cys residues. Instead, a number of amine-specific labels have been created and some of these are useful for peptide quantitation by mass spectrometry. In this review, peptidomics techniques are discussed along with the major findings of many recent studies and future directions for the field.

Peptidergic modulation of male sexual behavior in Lymnaea stagnalis: structural and functional characterization of -FVamide neuropeptides

In the simultaneous hermaphrodite snail Lymnaea stagnalis, copulation as a male is controlled by neurons that send axons to the male copulatory organs via a single penis nerve. Using direct mass spectrometry of a penis nerve sample, we show that one of the molecular ions has a mass corresponding to GAPRFVamide, previously identified from the buccal ganglia, and named Lymnaea inhibitory peptide (LIP). The identity of this peptide is confirmed by partial peptide purification from the penis nerve, followed by post source decay mass spectrometry. We cloned the LIP-encoding cDNA, which predicts a prohormone that gives rise to five copies of LIP (now re-named LIP A), two other -FVamide peptides (LIPs B and C), and five structurally unrelated peptides. The LIP gene is expressed in neurons of the right cerebral ventral lobe that send their axons into the penis nerve. We show that the LIP A peptide is present in these neurons and in the penis nerve, and confirmed the presence of LIP B and C in the penis nerve by post source decay mass spectrometry. Finally, we demonstrate that LIP A, B and C inhibit the contractions of the penis retractor muscle, thereby implicating their role in male copulation behavior.

Allatostatins of the tiger prawn, Penaeus monodon (Crustacea: Penaeidea)

More than 40 peptides belonging to the -Y/FXFGL-NH(2) allatostatin superfamily have been isolated and identified from the central nervous system (CNS) of the tiger prawn, Penaeus monodon (Crustacea: Penaeidea). The peptides can be arranged in seven sub-groups according to the variable post-tyrosyl residue represented by Ala, Gly, Ser, Thr, Asn, Asp, and Glu. Two of the residues (Thr and Glu) have not been observed in this position previously in either insects or crustaceans. Also reported for the first time for allatostatins, two of the peptides are N-terminally blocked by a pyroglutamic acid residue. The yields of certain peptides with similar amino acid sequences to each other were, in some instances, very different. As an example, the yield of ANQYTFGL-NH(2) was 2pmol, compared with ASQYTFGL-NH(2), with a yield of 156 pmol. There are several possibilities to account for this. If, as in all species so far investigated, there is a single allatostatin gene in P. monodon, then it would appear that different sub-populations have contributed mutant forms of particular peptides to the extract. Another, less likely possibility is that this species has more than one allatostatin gene, producing a variable array of peptides albeit in different molar ratios. Several peptides were present apparently as a result of the loss of one or more residues at the N-terminus of a larger form, either due to N-terminal degradation or specific post-translational processing. The number of peptides identified exceeds that for any other insect or crustacean species previously investigated. None is identical to any of the 60-70 insect allatostatins so far identified, and only three are common to other crustaceans. Immunohistochemical study of the CNS of P. monodon, with the same antisera as used to monitor the purification, confirms the widespread nature and complexity of allatostatinergic neural pathways in arthropods. Thus, all neuromeres of the brain, and all except one of the ventral cord ganglia, possess allatostatin neurons and extensive areas of allatostatin-innervated neuropile. In addition to the cytological evidence that the allatostatins act as neurotransmitters, associated with tissues as varied as eyes and legs, their presence in neurohemal areas such as the sinus gland and the perineural sheath of the thoracic ganglia suggests a neuroendocrine function. As well as posing a challenge to physiologists assigning specific functions to the allatostatins, their extensive intra-species multiplicity, linked to their inter-species variability, also presents a complex problem to geneticists and evolutionists.

Capillary sodium dodecyl sulfate-DALT electrophoresis of proteins in a single human cancer cell

Capillary Sodium dodlecyl sulfate (SDS)-DALT an (abbreviation for Dalton) electrophoresis was applied to analysis of proteins in single HT29 human colon adenocarcinoma cells. A vacuum pulse was employed to introduce a single cell into the coated capillary. Once the cell was lysed, proteins were denatured with SDS, fluorescantly labeled with 3-(2-furoyl)-quinoline-2-carboxaldehyde (FQ), and then separated by using 8% pullulan as the sieving matrix. This method offers a few advantages for single-cell protein analysis. First, it provides reproducible separation of single-cell proteins according to their size. Based on comparison with the migration time of standard proteins, most components from a single HT29 cancer cell have molecular masses within the range of 10-100 kDa. Second, as a one-dimensional separation method, it gives fairly good resolution for proteins. Typically, around 30 protein components of a single HT29 cell were resolved, indicating that this method has similar peak capacity to SDS-polyacrylamide gel electrophoresis (PAGE). Third, this method shows high detection sensitivity and wide dynamic range, which is important because of the wide range of protein expression in living systems. Detection limits for standard proteins ranged from 10(-10) to 10(-11) M. Finally, this method provides much higher speed than classical gel electrophoresis methods, and it provides automated anlysis of cellular proteins at the single-cell level; the separation is complete in 30 min and the entire analysis takes approximately 45 min.

Proteolytic cleavage of chromogranin A (CgA) by plasmin. Selective liberation of a specific bioactive CgA fragment that regulates catecholamine release

Chromogranin A (CgA), the major soluble protein in catecholamine storage vesicles, serves as a prohormone that is cleaved into bioactive peptides that inhibit catecholamine release, providing an autocrine, negative feedback mechanism for regulating catecholamine responses during stress. However, the proteases responsible for the processing of CgA and release of bioactive peptides have not been established. Recently, we found that chromaffin cells express components of the plasmin(ogen) system, including tissue plasminogen activator, which is targeted to catecholamine storage vesicles and released with CgA and catecholamines in response to sympathoadrenal stimulation, and high affinity cell surface receptors for plasminogen, to promote plasminogen activation at the cell surface. In the present study, we investigated processing of CgA by plasmin and sought to identify specific bioactive CgA peptides produced by plasmin proteolysis. Highly purified human CgA (hCgA) was produced by expression in Escherichia coli and purification using metal affinity chromatography. hCgA was digested with plasmin. Matrix-assisted laser desorption/ionization mass spectrometry identified a major peptide produced with a mass/charge ratio (m/z) of 1546, corresponding uniquely to hCgA-(360-373), the identity of which was confirmed by reverse phase high pressure liquid chromatography and amino-terminal microsequencing. hCgA-(360-373) was selectively liberated by plasmin from hCgA at early time points and was stable even after prolonged exposure to plasmin. The corresponding synthetic peptide markedly inhibited nicotine-induced catecholamine release from pheochromocytoma cells. These results identify plasmin as a protease, present in the local environment of the chromaffin cell, that selectively cleaves CgA to generate a bioactive fragment, hCgA-(360-373), that inhibits nicotinic-mediated catecholamine release. These results suggest that the plasminogen/plasmin system through its interaction with CgA may play a major role in catecholaminergic function and suggest a specific mechanism as well as a discrete CgA peptide through which this effect is mediated.

Matrix-assisted laser desorption/ionization time-of-flight mass analysis of peptides

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is one of the most useful techniques for determining the mass of biomolecules, with exceptional capabilities for mass analysis of peptides. Relative to other ionization techniques, it provides high sensitivity and excellent tolerance of salt and other common buffer components. Routine detection limits for peptides are in the subpicomole range. The ions commonly observed are the protonated molecules (M+H(+)), which makes data analysis relatively easy. This overview discusses instrument configuration and calibration, sample preparation, along with specific approaches for analyzing peptide mixtures, synthetic peptides, and chemical modifications of peptides.

Multimeric CREB-binding sites in the promoter regions of a family of G-protein-coupled receptors related to the vertebrate galanin and nociceptin/orphanin-FQ receptor families

Four related genes encoding a family of G-protein-coupled receptors (GPCRs) have been isolated from the mollusc Lymnaea stagnalis. The coding regions of this family of receptors share 97-99% sequence similarity at both the protein and nucleotide level, and they also share high sequence identity with vertebrate galanin and orphanin-FQ/nociceptin GPCR families. Analysis of the promoter regions reveals shared domains, some of which encode highly conserved repeating units. One 27-bp repeating unit, which encodes a c-AMP response element (CRE) and binds CREB protein, is repeated 14 times in one promoter. In situ hybridization showed expression of these receptors in identified neurons of several behaviourly important networks including those involved in feeding and ion and water regulation. These Lymnaea receptors are likely to represent members of a novel family of invertebrate neuropeptide receptors extensively regulated in response to intracellular signalling cascades.

Characterization of the molecular species of phosphatidylethanolamine from kidney of the fresh water snail Lymneae stagnalis by mass spectrometry

The structural analysis of sixteen molecular species of diacyl glycerophosphoethanolamine from fresh water snail Lymneae stagnalis kidney using chromatography and mass spectrometry is described in this paper. 1-eicosadienoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 20:2-20:4), 1-eicosapentaenoyl-2-eicosadienoyl-sn-glycero-3-phosphoethanolamine (PE 20:5-20:2) and 1-eicosatrienoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 20:3-20:4) as well as 1-octadecanoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 18:0-20:4), 1-ocetadecenoyl-2-eicosatetraenoyl-sn-glycero-3-phosphoethanolamine (PE 18:1-20:4) and 1-octadecanoyl-2-eicosapentaneoyl-sn-glycero-3-phosphoethanolamine (PE 18:0-20:5) were found as the major molecular species, and the first three were tentatively identified as the novel species present in this biological material. The presence of a relatively high content of 1,2-dieicosenoyl-sn-glycero-3-phosphoethanolamine species (approximately 27% of total species) as well as the absence of 22-carbon fatty acid containing and plasmalogen PE molecular species are remarkable in healthy Lymneae stagnalis kidney. Copyright 1999 John Wiley & Sons, Ltd.

The structure of a glycosylated protein hormone responsible for sex determination in the isopod, Armadillidium vulgare

Two glycoforms (AH1 and AH2) of androgenic hormone, and its corresponding hormone precursor derived from HPLC-purified androgenic gland extract from the woodlouse Armadillidium vulgare were fully characterized by microsequencing and mass spectrometry. The amino-acid sequences of the two glycoforms were identical; they consist of two peptide chains, A and B, of 29 and 44 amino acids, respectively, with chain A carrying one N-glycosylated moiety on Asn18. The two chains are linked by two disulfide bridges. Glycoforms were only differentiated by the size and heterogeneity of the glycan chain. The androgenic hormone precursor (16.5 kDa) was shown to contain the sequence of chains A and B from the androgenic hormone, connected by a C-peptide (50 amino acids). These results were confirmed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis performed on a single hypertrophied androgenic gland. When injected into young females, both glycoforms of the androgenic hormone were able to override genetic sex-determination. In invertebrates, there is no other example where sex-differentiation is controlled by a protein hormone that is not synthesized by the gonads but by a special gland. A functional comparison with two other hormones which are believed to play a role in sex determination, i.e. ecdysone in insects and anti-Müllerian hormone in mammals, is presented. Work is in progress to clone and characterize the gene encoding androgenic hormone, moreover special attention is devoted to its regulatory regions, putative targets for the Wolbachia action.

A molluscan peptide alpha-amidating enzyme precursor that generates five distinct enzymes

Mechanisms underlying the specificity and efficiency of enzymes, which modify peptide messengers, especially with the variable requirements of synthesis in the neuronal secretory pathway, are poorly understood. Here, we examine the process of peptide alpha-amidation in individually identifiable Lymnaea neurons that synthesize multiple proproteins, yielding complex mixtures of structurally diverse peptide substrates. The alpha-amidation of these peptide substrates is efficiently controlled by a multifunctional Lymnaea peptidyl glycine alpha-amidating monooxygenase (LPAM), which contains four different copies of the rate-limiting Lymnaea peptidyl glycine alpha-hydroxylating monooxygenase (LPHM) and a single Lymnaea peptidyl alpha-hydroxyglycine alpha-amidating lyase. Endogenously, this zymogen is converted to yield a mixture of monofunctional isoenzymes. In vitro, each LPHM displays a unique combination of substrate affinity and reaction velocity, depending on the penultimate residue of the substrate. This suggests that the different isoenzymes are generated in order to efficiently amidate the many peptide substrates that are present in molluscan neurons. The cellular expression of the LPAM gene is restricted to neurons that synthesize amidated peptides, which underscores the critical importance of regulation of peptide alpha-amidation.

Characterization of the Aplysia californica cerebral ganglion F cluster

The cerebral ganglia neurons of Aplysia californica are involved in the development and modulation of many behaviors. The medially located F cluster has been characterized using morphological, electrophysiological and biochemical techniques and contains at least three previously uncharacterized neuronal population. As the three subtypes are located in three distinct layers, they are designated as top, middle, and bottom layer F-cluster neurons (CFT, CFM, and CFB). The CFT cells are large (92 +/- 25 microm), white, nonuniformly shaped, and located partially in the sheath surrounding the ganglion. These neurons exhibit weak electrical coupling, the presence of synchronized spontaneous changes in membrane potential, and a generalized inhibitory input upon electrical stimulation of the anterior tentacular (AT) nerve. Similar to the CFT neurons, the CFM neurons (46 +/- 12 microm) are mainly silent but do not show electrical coupling or synchronized changes in membrane potential. Unlike the CFT neurons, the CFM neurons exhibit weak action potential broadening during constant current injection. Comparison of the peptide profiles of CFT, CFM, and CFB (10-30 microm) neurons using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry demonstrates distinct peptide molecular weights for each neuronal subtype with the masses of these peptides not matching any previously characterized peptides from A. californica. The mass spectra obtained from the AT nerve are similar to the CFT neuron mass spectra, while upper labial nerve contains many peptides observed in the CFM neurons located in nongranular neuron region.

Nanoliter chemistry combined with mass spectrometry for peptide mapping of proteins from single mammalian cell lysates

A nanoliter-chemistry station combined with matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was developed to characterize proteins at the attomole level. Chemical reactions including protein digestion were carried out in nanoliter or subnanoliter volumes, followed by microspot sample deposition of the digest to a MALDI-TOF mass spectrometer. Accurate mass determination of the peptides from the enzyme digest, in conjunction with protein database searching, allowed the identification of the proteins in the protein database. This method is particularly useful for handling small-volume samples such as in single-cell analysis. The high sensitivity and specificity of this method were demonstrated by peptide mapping and identifying hemoglobin variants of sickle cell disease from a single red blood cell. The approach of combining nanoliter chemistry with highly sensitive mass spectrometric analysis should find general use in characterizing proteins from biological systems where only a limited amount of material is available for interrogation.

Matrix-assisted laser desorption/ionization time of flight mass spectrometric analysis of the pattern of peptide expression in single neurons resulting from alternative mRNA splicing of the FMRFamide gene

MALDI-ToF MS (matrix-assisted laser desorption/ionization time of flight mass spectrometry) has become a fast, reliable and sensitive technique for the identification of neuropeptides in biological tissues. Here, we applied this technique to identified neurons of the cardioregulatory network in the snail Lymnaea that express the FMRFamide gene. This enabled us to study the complex processing of the FMRFamide gene at the level of single identified neurons. In the CNS of Lymnaea, FMRFamide-like and additional peptides are encoded by a common, multiexon gene. Alternate mRNA splicing of the FMRFamide gene leads to the production of two different mRNAs. Type 1 mRNA (exon II) encodes for the tetrapeptides (FLRF/FMRFamide), whereas Type 2 (exons III-V) encodes for the heptapeptides (SDPFLRFamide/GDPFLRFamide). Previous in situ hybridization and immunocytochemical studies indicated that these two transcripts are expressed in the CNS neurons of Lymnaea in a differential and mutually exclusive manner. Two single identified neurons of the cardiorespiratory network, the Ehe neuron and the visceral white interneuron (VWI), were known to express the FMRFamide gene (Ehe, type 1 mRNA; VWI, type 2 mRNA). MALDI-ToF MS analysis of these neurons and other neurons expressing the FMRFamide gene confirmed the mutually exclusive expression of the distinct sets of peptides encoded on the two transcripts and revealed the pattern of post-translational processing of both protein precursors. From the gene sequence it was predicted that 16 final peptide products from the two precursor proteins could possibly exist. We showed that most of these peptides were indeed present in the identified neurons (13) while others were not (three), suggesting that not all of the potential cleavage sites within the two precursors are utilized. In this way, the neuronal expression of the full range of the peptide products resulting from alternative mRNA splicing was revealed for the first time.

Neural modulation of gut motility by myomodulin peptides and acetylcholine in the snail Lymnaea

Families of peptide neuromodulators are believed to play important roles in neural networks that control behaviors. Here, we investigate the expression and role of one such group of modulators, the myomodulins, in the feeding system of Lymnaea stagnalis. Using a combination of in situ hybridization and antibody staining, expression of the myomodulin gene was confirmed in a number of identified behaviorally significant neuronal types, including the paired B2 motor neurons. The B2 cells were shown to project axons to the proesophagus, where they modulate foregut contractile activity. The presence of the five myomodulin peptide structures was confirmed in the B2 cells, the proesophagus, and the intervening nerve by mass spectrometry. Using a sensitive cell culture assay, evidence that the B2 cells are cholinergic also is presented. Application of four of the five myomodulin peptides to the isolated foregut increased both contraction frequency and tonus, whereas the main effect of acetylcholine (ACh) application was a large tonal contraction. The fifth myomodulin peptide (pQIPMLRLamide) appeared to have little or no effect on gut motility. Coapplication of all five myomodulin peptides gave a greater increase in tonus than that produced by the peptides applied individually, suggesting that corelease of the peptides onto the gut would produce an enhanced response. The combined effects that the myomodulin peptides and ACh have on foregut motility can mimic the main actions of B2 cell stimulation.

Proteolytic processing of the Aplysia egg-laying hormone prohormone

By using matrix-assisted laser desorption/ionization time-of-flight MS, individual peptidergic neurons from Aplysia are assayed. A semiquantitative method is developed for comparing single-cell profiles by using spectral normalization, and peptides are localized to specific cells by mass spectrometric cell mapping. In addition to all previously identified products of the egg-laying hormone (ELH) gene, other peptides are formed from proteolytic hydrolysis of Leu-Leu residues within ELH and acidic peptide (AP). AP exhibits further processing to yield AP1-20 and AP9-27. These peptides appear to be colocalized in vesicles with ELH, transported to specific neuronal targets, and released in a Ca2+-dependent manner. A differential peptide distribution is observed at a specific target cell, and a low-frequency variation of AP, [Thr21]AP, is detected in a single animal.

Mass spectrometry of peptides in neuroscience

This review focuses on the contributions of modern mass spectrometry to neuropeptide research. An introduction to newer mass spectrometric techniques is provided. Also, the use of mass spectrometry in combination with high-resolution separation techniques for neuropeptide identification in biological samples is illustrated. The amino acid sequence information that is important for the identification and analysis of known, novel, or chemically modified neuropeptides may be obtained using mass spectrometric techniques. Because mass spectrometry techniques can be used to reflect the dynamic properties associated with neuropeptide processing in biological systems, they may be used in the future to monitor peptide profiles within organisms in response to environmental challenges such as disease and stress.

Mass spectrometric survey of interganglionically transported peptides in Aplysia

The major ganglionic connectives in Aplysia are assayed to determine putative neuropeptides. Matrix-assisted laser desorption/ionization mass spectrometry allows direct measurement of peptides in a nerve. Many previously characterized peptides are observed, including APGWamide, buccalins, small cardioactive peptides, and egg-laying hormone. Several unreported peptides are detected in specific nerves, suggesting they may have important physiological roles. Furthermore, novel processing products of the L5-67 precursor peptide and the APGWamide/cerebral peptide 1 prohormone are strongly suggested, and their interganglionic transport demonstrated.

Pattern changes of pituitary peptides in rat after salt-loading as detected by means of direct, semiquantitative mass spectrometric profiling

We have established a differential peptide display method, based on a mass spectrometric technique, to detect peptides that show semiquantitative changes in the neurointermediate lobe (NIL) of individual rats subjected to salt-loading. We employed matrix-assisted laser desorption/ionization mass spectrometry, using a single-reference peptide in combination with careful scanning of the whole crystal rim of the matrix-analyte preparation, to detect in a semiquantitative manner the molecular ions present in the unfractionated NIL homogenate. Comparison of the mass spectra generated from NIL homogenates of salt-loaded and control rats revealed a selective and significant decrease in the intensities of several molecular ion species of the NIL homogenates from salt-loaded rats. These ion species, which have masses that correspond to the masses of oxytocin, vasopressin, neurophysins, and an unidentified putative peptide, were subsequently chemically characterized. We confirmed that the decreased molecular ion species are peptides derived exclusively from propressophysin and prooxyphysin (i.e., oxytocin, vasopressin, and various neurophysins). The putative peptide is carboxyl-terminal glycopeptide. The carbohydrate moiety of the latter peptide was determined by electrospray tandem MS as bisected biantennary Hex3HexNAc5Fuc. This posttranslational modification accounts for the mass difference between the predicted mass of the peptide based on cDNA studies and the measured mass of the mature peptide.

A human gene encoding morphine modulating peptides related to NPFF and FMRFamide

FMRFamide-related peptides have been isolated from both invertebrates and vertebrates and exhibit a wide range of biological effects in rats. We show here that in humans 2 FMRFamide-related peptides are encoded by a single gene expressed as a spliced mRNA. The larger predicted peptide (AGEGLNSQFWSLAAPQRFamide) differs from the peptide isolated from bovines (AGEGLSSPFWSLAAPQRFamide) by the substitutions of 2 amino acids. The shorter predicted peptide (NPSF, SQAFLFQPQRFamide) is 3 amino acids longer than the bovine 8 amino-acid NPFF (FLFQPQRFamide) or the human NPFF peptide isolated from serum [5], suggesting that the encoded protein is subject to cleavage by a tripeptidyl peptidase or by a novel processing mechanism. On rat spinal cord, the larger peptide is indistinguishable in activity from the equivalent bovine peptide whereas the smaller extended peptide is inactive.

Isolation and identification of intact chromogranin A and two N-terminal processing products, vasostatin I and II, from bovine adrenal medulla chromaffin granules by chromatographic and mass spectrometric methods

Chromogranin A (CGA) is the most abundant protein of the bovine adrenal medulla and plays an important role as precursor protein of several peptides that act as modulators for endocrine cell secretory activity. Furthermore, it is presumed to play a role in the targeting of peptide hormones and neurotransmitters to granules of the regulated pathway. However, its complete primary structure and proteolytic processing have not yet been identified. This study describes a rapid and efficient procedure for the high yield isolation of bovine CGA and its N-terminal processing products, vasostatin I and II. Using the lysate from bovine adrenal medulla chromaffin granules, the soluble proteins were purified by three consecutive HPLC steps, thereby avoiding the use of buffer solutions. The protein fractions were isolated and characterized by SDS-PAGE and Western blot analysis as well as by mass spectrometry. In the latter analysis, the efficiency of matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) was demonstrated, enabling the unequivocal and sensitive characterization of proteins from crude mixtures. Sufficient amounts of pure protein were obtained by the present procedure to form the basis for detailed structural studies by spectroscopic methods and X-ray crystallography.

Diversity in cell specific co-expression of four neuropeptide genes involved in control of male copulation behaviour in Lymnaea stagnalis

We report here the neuron-specific co-expression of four genes coding for neuropeptides involved in the control of male behaviour. These neurons are located in the anterior lobe of the right cerebral ganglion in the central nervous system of Lymnaea stagnalis and project via the penis nerve to the penial complex. In order to accomplish optimal assurance we applied in situ hybridization, immunocytochemistry and matrix-assisted laser desorption ionization mass spectrometry. The anterior lobe neurons express the gene encoding the amidated tetrapeptide APGWamide. Subsets of these cells are now shown to co-express the APGWamide gene exclusively with one of three other neuropeptide genes, encoding Lymnaea neuropeptide Y, conopressin or pedal peptide, respectively. All four genes are also expressed in other neurons in other centres projecting to the penial complex, but in these cells co-expression was not observed. The neuropeptides encoded by the genes could be identified in the anterior lobe cell bodies on the basis of immunocytochemistry and mass spectrometrical analysis. The neuropeptides APGWamide and Lymnaea neuropeptide Y, which are co-localized in the anterior lobe cells as well as in axons innervating the penis retractor muscle, do not induce muscle contraction but have a modulatory action by affecting the relaxation rate and amplitude of the contraction. APGWamide and conopressin had earlier been suggested to modulate peristalsis of the vas deferens. Thus, it seems that the neurons co-expressing the various combinations of neuropeptide genes in the anterior lobe represent functional units, each acting in the fine tuning of different muscles involved in specific aspects of male copulation behaviour.

Analysis of fructans from higher plants by matrix-assisted laser desorption/ionization mass spectrometry

In this communication both matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and high-performance anion-exchange chromatography (HPAEC) have been applied to analyze fructans from higher plants. Size distribution of a commercially available fructan preparation from Dahlia variabilis L. was determined by MALDI-MS. Molecular masses ranged from 2,000 up to 6,000 Da with a peak value of distribution at 2,635 Da. Essentially the same pattern was obtained using HPAEC. Low-molecular-weight fructans from onion bulbs (Allium cepa L.) were studied in more detail. Tissue extracts were analyzed by MALDI-MS without any analyte purification. Mass-spectra of both proteins and oligosaccharides were obtained. For identification, metastable ion scanning was performed. Neither deproteinization nor deionization of the samples affected the oligosaccharide pattern. Using HPAEC, a more complex oligosaccharide pattern was obtained because isomeric glycans were differentiated. However, the overall size distribution was similar to that obtained by MALDI-MS. In further experiments epidermal or parenchyma cell layers of the onion bulb were placed into matrix solution and were then subjected to MALDI-MS and metastable ion scanning as well. By taking this approach, analyte desorption was achieved immediately from plant tissue. Oligosaccharide mass spectra were essentially the same as those of the extracts. To our knowledge, this is the first time that MALDI-MS has been applied as a microprobe to plant tissue. Finally MALDI-MS analysis was performed using single-cell extracts from onion tissues without any purification of the analyte.

Intracellular degradation of C-peptides in molluscan neurons producing insulin-related hormones

Single Light Green Cells (LGC) of Lymnaea stagnalis, expressing four genes encoding insulin-related peptides (MIPs) and C-peptides, and sections from the median lip nerve (MLN) were subjected to MALDI-MS. Mass spectra of LGCs and MLNs were almost identical. Masses corresponding to those of the MIPs and some C alpha-peptides could be distinguished. ProMIP III C alpha-peptide and C beta-peptides were not found. The spectra showed additional masses matching those of carboxyterminally truncated C alpha-peptides. Peptides with similar masses were isolated from MLN extracts by HPLC, using electrospray-MS screening. Amino acid sequence analysis revealed intact proMIP I, II and V C alpha-peptides and I, II C alpha-peptide 1-24, 1-22 and 1-15.

Identification of the dipteran Leu-callatostatin peptide family: the pattern of precursor processing revealed by isolation studies in Calliphora vomitoria

Information from the Leu-callatostatin gene sequences of the blowflies Calliphora vomitoria and Lucilia cuprina was used to develop antisera specific for the variable post-tyrosyl amino-acid residues Ser, Ala and Asn of the common Leu-callatostatin C-terminal pentapeptide sequence -YXFGL-NH2. Radioimmunoassays based on these antisera were used to purify peptides from an extract of 40000 blowfly heads. Five neuropeptides of the Leu-callatostatin family were identified. Three have a seryl residue in the post-tyrosyl position. Two of these are octapeptides that differ only at the N-terminal residue; NRPYSFGL-NH2 and ARPYSFGL-NH2, whilst the third is the heptapeptide derived by N-terminal trimming; RPYSFGL-NH2. Two octapeptides in which X is Ala and Asn were also identified; VERYAFGL-NH2 and LPVYNFGL-NH2. The latter peptide is derived by processing at the internal dibasic site of a putative heneicosapeptide encoded by the DNA. These findings stress the necessity to have putative structures verified at the peptide level. Potent, reversible inhibitory effects on the spontaneous contractile activity of the blowfly rectum were recorded for ARPYSFGL-NH2 (monophasic dose-response curve with an IC50 = 10 fM) and for LPVYNFGL-NH2 (biphasic dose-response curve with IC50 values of approximately 1 fM and 1 nM). It is suggested that regulation of gut motility in insects, rather than an allatostatic function, may represent an ancestral and universal function of the allatostatins. One of the reasons for the large number of members of the Leu-callatostatin family appears to be in the provision of an integrated form of gut motility control, with different peptides controlling specific regions of the gut.

Structure, localization and action of a novel inhibitory neuropeptide involved in the feeding of Lymnaea

A neuropeptide that strongly inhibits the spontaneous contractions of the oesophagus in Lymnaea has been characterized as GAPRFVamide. Direct mass spectrometry of nervous tissues and immunocytochemical studies show that the peptide is synthesized by neurones in the buccal ganglia and transported to the oesophagus via the dorso-buccal nerve. In accordance with the function of the peptide, immunoreactive fibres are detected within the muscle layer of the oesophagus. Finally, mass spectrometry reveals the presence of a number of unidentified peptides in the nerves that innervate the oesophagus, which suggests that oesophageal activities may be modified by multiple peptides.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS)

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been responsible for solving many problems in structural biology. Mass analysis is now used routinely to confirm proper expression and processing of proteins, and to locate and identify post-translational modifications. Innovative advances in instrumentation have led to higher mass resolution and mass accuracy. New sample preparation methods are likewise yielding higher sensitivity plus greater tolerance for buffer components that have in the past suppressed signals at higher concentrations. Advancements in the technique have also led to new or improved applications in many areas, including peptide sequencing and the identification of proteins by database searching with peptide masses. Instruments with lower cost, smaller size, and higher performance are making mass measurements available to an increasing number of laboratories. MALDI-MS is poised to continue to improve in performance and in its usefulness for current and new applications.