Mass spectrometric survey of interganglionically transported peptides in Aplysia

Current Challenges and Future Directions in Peptidomics

The field of peptidomics has been under development since its start more than 20 years ago. In this chapter we provide a personal outlook for future directions in this field. The applications of peptidomics technologies are spreading more and more from classical research of peptide hormones and neuropeptides towards commercial applications in plant and food-science. Many clinical applications have been developed to analyze the complexity of biofluids, which are being addressed with new instrumentation, automization, and data processing. Additionally, the newly developed field of immunopeptidomics is showing promise for cancer therapies. In conclusion, peptidomics will continue delivering important information in classical fields like neuropeptides and peptide hormones, benefiting from improvements in state-of-the-art technologies. Moreover, new directions of research such as immunopeptidomics will further complement classical omics technologies and may become routine clinical procedures. Taken together, discoveries of new substances, networks, and applications of peptides can be expected in different disciplines.

Molecular Characterization of Two Wamide Neuropeptide Signaling Systems in Mollusk Aplysia

Neuropeptides with the C-terminal Wamide (Trp-NH₂) are one of the last common ancestors of peptide families of eumetazoans and play various physiological roles. In this study, we sought to characterize the ancient Wamide peptides signaling systems in the marine mollusk Aplysia californica, i.e., APGWamide (APGWa) and myoinhibitory peptide (MIP)/Allatostatin B (AST-B) signaling systems. A common feature of protostome APGWa and MIP/AST-B peptides is the presence of a conserved Wamide motif in the C-terminus. Although orthologs of the APGWa and MIP signaling systems have been studied to various extents in annelids or other protostomes, no complete signaling systems have yet been characterized in mollusks. Here, through bioinformatics, molecular and cellular biology, we identified three receptors for APGWa, namely, APGWa-R1, APGWa-R2, and APGWa-R3. The EC₅₀ values for APGWa-R1, APGWa-R2, and APGWa-R3 are 45, 2100, and 2600 nM, respectively. For the MIP signaling system, we predicted 13 forms of peptides, i.e., MIP1-13 that could be generated from the precursor identified in our study, with MIP5 (WKQMAVWa) having the largest number of copies (4 copies). Then, a complete MIP receptor (MIPR) was identified and the MIP1-13 peptides activated the MIPR in a dose-dependent manner, with EC₅₀ values ranging from 40 to 3000 nM. Peptide analogs with alanine substitution experiments demonstrated that the Wamide motif at the C-terminus is necessary for receptor activity in both the APGWa and MIP systems. Moreover, cross-activity between the two signaling systems showed that MIP1, 4, 7, and 8 ligands could activate APGWa-R1 with a low potency (EC₅₀ values: 2800-22,000 nM), which further supported that the APGWa and MIP signaling systems are somewhat related. In summary, our successful characterization of Aplysia APGWa and MIP signaling systems represents the first example in mollusks and provides an important basis for further functional studies in this and other protostome species. Moreover, this study may be useful for elucidating and clarifying the evolutionary relationship between the two Wamide signaling systems (i.e., APGWa and MIP systems) and their other extended neuropeptide signaling systems.

Profiling 26,000 Aplysia californica neurons by single cell mass spectrometry reveals neuronal populations with distinct neuropeptide profiles

Neuropeptides are a chemically diverse class of cell-to-cell signaling molecules that are widely expressed throughout the central nervous system, often in a cell-specific manner. While cell-to-cell differences in neuropeptides is expected, it is often unclear how exactly neuropeptide expression varies among neurons. Here we created a microscopy-guided, high-throughput single cell matrix-assisted laser desorption/ionization mass spectrometry approach to investigate the neuropeptide heterogeneity of individual neurons in the central nervous system of the neurobiological model Aplysia californica, the California sea hare. In all, we analyzed more than 26,000 neurons from 18 animals and assigned 866 peptides from 66 prohormones by mass matching against an in silico peptide library generated from known Aplysia prohormones retrieved from the UniProt database. Louvain-Jaccard (LJ) clustering of mass spectra from individual neurons revealed 40 unique neuronal populations, or LJ clusters, each with a distinct neuropeptide profile. Prohormones and their related peptides were generally found in single cells from ganglia consistent with the prohormones' previously known ganglion localizations. Several LJ clusters also revealed the cellular colocalization of behaviorally related prohormones, such as an LJ cluster exhibiting achatin and neuropeptide Y, which are involved in feeding, and another cluster characterized by urotensin II, small cardiac peptide, sensorin A, and FRFa, which have shown activity in the feeding network or are present in the feeding musculature. This mass spectrometry-based approach enables the robust categorization of large cell populations based on single cell neuropeptide content and is readily adaptable to the study of a range of animals and tissue types.

Evolution and Comparative Physiology of Luqin-Type Neuropeptide Signaling

Luqin is a neuropeptide that was discovered and named on account of its expression in left upper quadrant cells of the abdominal ganglion in the mollusc Aplysia californica. Subsequently, luqin-type peptides were identified as cardio-excitatory neuropeptides in other molluscs and a cognate receptor was discovered in the pond snail Lymnaea stagnalis. Phylogenetic analyses have revealed that orthologs of molluscan luqin-type neuropeptides occur in other phyla; these include neuropeptides in ecdysozoans (arthropods, nematodes) that have a C-terminal RYamide motif (RYamides) and neuropeptides in ambulacrarians (echinoderms, hemichordates) that have a C-terminal RWamide motif (RWamides). Furthermore, precursors of luqin-type neuropeptides typically have a conserved C-terminal motif containing two cysteine residues, although the functional significance of this is unknown. Consistent with the orthology of the neuropeptides and their precursors, phylogenetic and pharmacological studies have revealed that orthologous G-protein coupled receptors (GPCRs) mediate effects of luqin-type neuropeptides in spiralians, ecdysozoans, and ambulacrarians. Luqin-type signaling originated in a common ancestor of the Bilateria as a paralog of tachykinin-type signaling but, unlike tachykinin-type signaling, luqin-type signaling was lost in chordates. This may largely explain why luqin-type signaling has received less attention than many other neuropeptide signaling systems. However, insights into the physiological actions of luqin-type neuropeptides (RYamides) in ecdysozoans have been reported recently, with roles in regulation of feeding and diuresis revealed in insects and roles in regulation of feeding, egg laying, locomotion, and lifespan revealed in the nematode Caenorhabditis elegans. Furthermore, characterization of a luqin-type neuropeptide in the starfish Asterias rubens (phylum Echinodermata) has provided the first insights into the physiological roles of luqin-type signaling in a deuterostome. In conclusion, although luqin was discovered in Aplysia over 30 years ago, there is still much to be learnt about luqin-type neuropeptide signaling. This will be facilitated in the post-genomic era by the emerging opportunities for experimental studies on a variety of invertebrate taxa.

Discovery and functional characterisation of a luqin-type neuropeptide signalling system in a deuterostome

Neuropeptides are diverse and evolutionarily ancient regulators of physiological/behavioural processes in animals. Here we have investigated the evolution and comparative physiology of luqin-type neuropeptide signalling, which has been characterised previously in protostomian invertebrates. Phylogenetic analysis indicates that luqin-type receptors and tachykinin-type receptors are paralogous and probably originated in a common ancestor of the Bilateria. In the deuterostomian lineage, luqin-type signalling has been lost in chordates but interestingly it has been retained in ambulacrarians. Therefore, here we characterised luqin-type signalling for the first time in an ambulacrarian – the starfish Asterias rubens (phylum Echinodermata). A luqin-like neuropeptide with a C-terminal RWamide motif (ArLQ; EEKTRFPKFMRW-NH₂) was identified as the ligand for two luqin-type receptors in A. rubens, ArLQR1 and ArLQR2. Furthermore, analysis of the expression of the ArLQ precursor using mRNA in situ hybridisation revealed expression in the nervous system, digestive system and locomotory organs (tube feet) and in vitro pharmacology revealed that ArLQ causes dose-dependent relaxation of tube feet. Accordingly, previous studies have revealed that luqin-type signalling regulates feeding and locomotor activity in protostomes. In conclusion, our phylogenetic analysis combined with characterisation of luqin-type signalling in a deuterostome has provided new insights into neuropeptide evolution and function in the animal kingdom.

Discovery of leucokinin-like neuropeptides that modulate a specific parameter of feeding motor programs in the molluscan model, Aplysia

A better understanding of neuromodulation in a behavioral system requires identification of active modulatory transmitters. Here, we used identifiable neurons in a neurobiological model system, the mollusc Aplysia, to study neuropeptides, a diverse class of neuromodulators. We took advantage of two types of feeding neurons, B48 and B1/B2, in the Aplysia buccal ganglion that might contain different neuropeptides. We performed a representational difference analysis (RDA) by subtraction of mRNAs in B48 versus mRNAs in B1/B2. The RDA identified an unusually long (2025 amino acids) peptide precursor encoding Aplysia leucokinin-like peptides (ALKs; e.g. ALK-1 and ALK-2). Northern blot analysis revealed that, compared with other ganglia (e.g. the pedal-pleural ganglion), ALK mRNA is predominantly present in the buccal ganglion, which controls feeding behavior. We then used in situ hybridization and immunohistochemistry to localize ALKs to specific neurons, including B48. MALDI-TOF MS on single buccal neurons revealed expression of 40 ALK precursor-derived peptides. Among these, ALK-1 and ALK-2 are active in the feeding network; they shortened the radula protraction phase of feeding motor programs triggered by a command-like neuron. We also found that this effect may be mediated by the ALK-stimulated enhancement of activity of an interneuron, which has previously been shown to terminate protraction. We conclude that our multipronged approach is effective for determining the structure and defining the diverse functions of leucokinin-like peptides. Notably, the ALK precursor is the first verified nonarthropod precursor for leucokinin-like peptides with a novel, marked modulatory effect on a specific parameter (protraction duration) of feeding motor programs.

Differential peptide expression in the central nervous system of the land snail Theba pisana, between active and aestivated

Hypometabolism is a physiological state of dormancy entered by many animals in times of environmental stress. There are gaps in our understanding of the molecular components used by animals to achieve this metabolic state. The availability of genomic and transcriptome data can be useful to study the process of hypometabolism at the molecular level. In this study, we use the land snail Theba pisana to identify peptides that may be involved in the hypometabolic state known as aestivation. We found a total of 22 neuropeptides in the central nervous system (CNS) that were differentially produced during activity and aestivation based on mass spectral-based neuropeptidome analysis. Of these, 4 were upregulated in active animals and 18 were upregulated in aestivation. A neuropeptide known to regulate muscle contractions in a variety of molluscs, the small cardioactive peptide A (sCAPA), and a peptide of yet unknown function (termed Aestivation Associated Peptide 12) were chosen for further investigation using temporal and spatial expression analysis of the precursor gene and peptide. Both peptides share expression within regions of the CNS cerebral ganglia and suboesophageal ganglia. Relative transcript abundance suggests that regulation of peptide synthesis and secretion is post-transcriptional. In summary, we provide new insights into the molecular basis of the regulation of aestivation in land snails through CNS peptide control.

Mass Spectrometry Imaging and GC-MS Profiling of the Mammalian Peripheral Sensory-Motor Circuit

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) has evolved to become an effective discovery tool in science and clinical diagnostics. Here, chemical imaging approaches are applied to well-defined regions of the mammalian peripheral sensory-motor system, including the dorsal root ganglia (DRG) and adjacent nerves. By combining several MSI approaches, analyte coverage is increased and 195 distinct molecular features are observed. Principal component analysis suggests three chemically different regions within the sensory-motor system, with the DRG and adjacent nerve regions being the most distinct. Investigation of these regions using gas chromatography-mass spectrometry corroborate these findings and reveal important metabolic markers related to the observed differences. The heterogeneity of the structurally, physiologically, and functionally connected regions demonstrates the intricate chemical and spatial regulation of their chemical composition.

Imaging and Mapping of Tissue Constituents at the Single-Cell Level Using MALDI MSI and Quantitative Laser Scanning Cytometry

For nearly a century, histopathology involved the laborious morphological analyses of tissues stained with broad-spectrum dyes (i.e., eosin to label proteins). With the advent of antibody-labeling, immunostaining (fluorescein and rhodamine for fluorescent labeling) and immunohistochemistry (DAB and hematoxylin), it became possible to identify specific immunological targets in cells and tissue preparations. Technical advances, including the development of monoclonal antibody technology, led to an ever-increasing palate of dyes, both fluorescent and chromatic. This provides an incredibly rich menu of molecular entities that can be visualized and quantified in cells-giving rise to the new discipline of Molecular Pathology. We describe the evolution of two analytical techniques, cytometry and mass spectrometry, which complement histopathological visual analysis by providing automated, cellular-resolution constituent maps. For the first time, laser scanning cytometry (LSC) and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) are combined for the analysis of tissue sections. The utility of the marriage of these techniques is demonstrated by analyzing mouse brains with neuron-specific, genetically encoded, fluorescent proteins. We present a workflow that: (1) can be used with or without expensive matrix deposition methods, (2) uses LSC images to reveal the diverse landscape of neural tissue as well as the matrix, and (3) uses a tissue fixation method compatible with a DNA stain. The proposed workflow can be adapted for a variety of sample preparation and matrix deposition methods.

Diversity of the RFamide Peptide Family in Mollusks

Since the initial characterization of the cardioexcitatory peptide FMRFamide in the bivalve mollusk Macrocallista nimbosa, a great number of FMRFamide-like peptides (FLPs) have been identified in mollusks. FLPs were initially isolated and molecularly characterized in model mollusks using biochemical methods. The development of recombinant technologies and, more recently, of genomics has boosted knowledge on their diversity in various mollusk classes. Today, mollusk FLPs represent approximately 75 distinct RFamide peptides that appear to result from the expression of only five genes: the FMRFamide-related peptide gene, the LFRFamide gene, the luqin gene, the neuropeptide F gene, and the cholecystokinin/sulfakinin gene. FLPs display a complex spatiotemporal pattern of expression in the central and peripheral nervous system. Working as neurotransmitters, neuromodulators, or neurohormones, FLPs are involved in the control of a great variety of biological and physiological processes including cardiovascular regulation, osmoregulation, reproduction, digestion, and feeding behavior. From an evolutionary viewpoint, the major challenge will then logically concern the elucidation of the FLP repertoire of orphan mollusk classes and the way they are functionally related. In this respect, deciphering FLP signaling pathways by characterizing the specific receptors these peptides bind remains another exciting objective.

Global view of the evolution and diversity of metazoan neuropeptide signaling

Neuropeptides are signaling molecules that commonly act via G protein-coupled receptors (GPCRs) and are generated in neurons by proneuropeptide (pNP) cleavage. Present in both cnidarians and bilaterians, neuropeptides represent an ancient and widespread mode of neuronal communication. Due to the inherent difficulties of analyzing highly diverse and repetitive pNPs, the relationships among different families are often elusive. Using similarity-based clustering and sensitive similarity searches, I obtained a global view of metazoan pNP diversity and evolution. Clustering revealed a large and diffuse network of sequences connected by significant sequence similarity encompassing one-quarter of all families. pNPs belonging to this cluster were also identified in the early-branching neuronless animal Trichoplax adhaerens. Clustering of neuropeptide GPCRs identified several orthology groups and allowed the reconstruction of the phyletic distribution of receptor families. GPCR phyletic distribution closely paralleled that of pNPs, indicating extensive conservation and long-term coevolution of receptor-ligand pairs. Receptor orthology and intermediate sequences also revealed the homology of pNPs so far considered unrelated, including allatotropin and orexin. These findings, together with the identification of deuterostome achatin and luqin and protostome opioid pNPs, extended the neuropeptide complement of the urbilaterian. Several pNPs were also identified from the hemichordate Saccoglossus kowalevskii and the cephalochordate Branchiostoma floridae, elucidating pNP evolution in deuterostomes. Receptor-ligand conservation also allowed ligand predictions for many uncharacterized GPCRs from nonmodel species. The reconstruction of the neuropeptide-signaling repertoire at deep nodes of the animal phylogeny allowed the formulation of a testable scenario of the evolution of animal neuroendocrine systems.

Microfluidic device for the selective chemical stimulation of neurons and characterization of peptide release with mass spectrometry

Neuropeptides are synthesized in and released from neurons and are involved in a wide range of physiological processes, including temperature homeostasis, learning, memory, and disease. When working with sparse neuronal networks, the ability to collect and characterize small sample volumes is important as neurons often release only a small proportion of their mass-limited content. Microfluidic systems are well suited for the study of neuropeptides. They offer the ability to control and manipulate the extracellular environment and small sample volumes, thereby reducing the dilution of peptides following release. We present an approach for the culture and stimulation of a neuronal network within a microfluidic device, subsequent collection of the released peptides, and their detection via mass spectrometry. The system employs microvalve-controlled stimulation channels to selectively stimulate a low-density neuronal culture, allowing us to determine the temporal onset of peptide release. Released peptides from the well-characterized, peptidergic bag cell neurons of Aplysia californica were collected and their temporal pattern of release was characterized with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. We show a robust difference in the timing of release for chemical solutions containing elevated K(+) (7 ± 3 min), when compared to insulin (19 ± 7 min) (p < 0.000 01).

Discovery and characterization of the Crustacean hyperglycemic hormone precursor related peptides (CPRP) and orcokinin neuropeptides in the sinus glands of the blue crab Callinectes sapidus using multiple tandem mass spectrometry techniques

The crustacean sinus gland (SG) is a well-defined neuroendocrine site that produces numerous hemolymph-borne agents including the most complex class of endocrine signaling molecules-neuropeptides. Via a multifaceted mass spectrometry (MS) approach, 70 neuropeptides were identified including orcokinins, orcomyotropin, crustacean hyperglycemic hormone (CHH) precursor-related peptides (CPRPs), red pigment concentrating hormone (RPCH), pigment dispersing hormone (PDH), proctolin, RFamides, RYamides, and HL/IGSL/IYRamide. Among them, 15 novel orcokinins, 9 novel CPRPs, 1 novel orcomyotropin, 1 novel Ork/Orcomyotropin-related peptide, and 1 novel PDH were de novo sequenced via collision induced dissociation (CID) from the SG of a model organism Callinectes sapidus. Electron transfer dissociation (ETD) was used for sequencing of intact CPRPs due to their large size and higher charge state. Capillary isoelectric focusing (CIEF) was employed for separation of members of the orcokinin family, which is one of the most abundant neuropeptide families observed in the SG. Collectively, our study represents the most complete characterization of neuropeptides in the SG and provides a foundation for future investigation of the physiological function of neuropeptides in the SG of C. sapidus.

MALDI mass spectrometry imaging of neuronal cell cultures

Mass spectrometry imaging (MSI) provides the ability to detect and identify a broad range of analytes and their spatial distributions from a variety of sample types, including tissue sections. Here we describe an approach for probing neuropeptides from sparse cell cultures using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MSI--at single cell spatial resolution-in both MS and tandem MS modes. Cultures of Aplysia californica neurons are grown on an array of glass beads embedded in a stretchable layer of Parafilm M. As the membrane is stretched, the beads/neurons are separated physically and the separated beads/neurons analyzed via MALDI TOF MS. Compared with direct MS imaging of samples, the stretching procedure enhances analyte extraction and incorporation into the MALDI matrix, with negligible analyte spread between separated beads. MALDI tandem MSI using the stretched imaging approach yields localization maps of both parent and fragment ions from Aplysia pedal peptide, thereby confirming peptide identification. This methodology represents a flexible platform for MSI investigation of a variety of cell cultures, including functioning neuronal networks.

Composite modulatory feedforward loop contributes to the establishment of a network state

Feedforward loops (FFLs) are one of many network motifs identified in a variety of complex networks, but their functional role in neural networks is not well understood. We provide evidence that combinatorial actions of multiple modulators may be organized as FFLs to promote a specific network state in the Aplysia feeding motor network. The Aplysia feeding central pattern generator (CPG) receives two distinct inputs-a higher-order interneuron cerebral-buccal interneuron-2 (CBI-2) and the esophageal nerve (EN)-that promote ingestive and egestive motor programs, respectively. EN stimulation elicits a persistent egestive network state, which enables the network to temporarily express egestive programs following a switch of input from the EN to CBI-2. Previous work showed that a modulatory CPG element, B65, is specifically activated by the EN and participates in establishing the egestive state by enhancing activity of egestion-promoting B20 interneurons while suppressing activity and synaptic outputs of ingestion-promoting B40 interneurons. Here a peptidergic contribution is mediated by small cardioactive peptide (SCP). Immunostaining and mass spectrometry show that SCP is present in the EN and is released on EN stimulation. Importantly, SCP directly enhances activity and synaptic outputs of B20 and suppresses activity and synaptic outputs of B40. Moreover, SCP promotes B65 activity. Thus the direct and indirect (through B65) pathways to B20 and B40 from SCPergic neurons constitute two FFLs with one functioning to promote egestive output and the other to suppress ingestive output. This composite FFL consisting of the two combined FFLs appears to be an effective means to co-regulate activity of two competing elements that do not inhibit each other, thereby contributing to establish specific network states.

Distinct mechanisms produce functionally complementary actions of neuropeptides that are structurally related but derived from different precursors

Many bioactive neuropeptides containing RFamide at their C terminus have been described in both invertebrates and vertebrates. To obtain insight into the functional logic of RFamide signaling, we investigate it here in the feeding system of Aplysia. We focus on the expression, localization, and actions of two families of RFamide peptides, the FRFamides and FMRFamide, in the central neuronal circuitry and the peripheral musculature that generate the feeding movements. We describe the cloning of the FRFamide precursor protein and show that the FRFamides and FMRFamide are derived from different precursors. We map the expression of the FRFamide and FMRFamide precursors in the feeding circuitry using in situ hybridization and immunostaining and confirm proteolytic processing of the FRFamide precursor by mass spectrometry. We show that the two precursors are expressed in different populations of sensory neurons in the feeding system. In a representative feeding muscle, we demonstrate the presence of both FRFamides and FMRFamide and their release, probably from the processes of the sensory neurons in the muscle. Both centrally and in the periphery, the FRFamides and FMRFamide act in distinct ways, apparently through distinct mechanisms, and nevertheless, from an overall functional perspective, their actions are complementary. Together, the FRFamides and FMRFamide convert feeding motor programs from ingestive to egestive and depress feeding muscle contractions. We conclude that these structurally related peptides, although derived from different precursors, expressed in different neurons, and acting through different mechanisms, remain related to each other in the functional roles that they play in the system.

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.

Aplysia Bag Cells Function as a Distributed Neurosecretory Network

The anatomical organization of many neuroendocrine systems implies multiple sites of hormone release in areas mediating multiple aspects of physiology and behavior, yet this neurosecretory complexity has not often been verified. Here we probe the well-characterized hormonal model, the reproductive bag cell neuroendocrine system of the sea slug Aplysia californica. The bag cell neurons of Aplysia mediate egg-laying behavior through the coordinated secretion of a suite of peptides derived from a single gene product, the egg-laying prohormone (proELH). Although the majority of bag cell neurons are located within two major abdominal bag cell clusters, smaller groups of egg-laying hormone-expressing cells have been observed in specific pleural and cerebral ganglia regions, some of which have been reported to be electrically connected to the abdominal bag cell clusters. Releasates are sampled from discrete locations within the Aplysia CNS before and during stimulation of afterdischarge activity and subsequently analyzed with matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Site-specific release profiles are observed at bag cell cluster, pleural, and genital ganglion sites after site-specific electrophysiological activation of bag cell afterdischarges. These data demonstrate that the bag cell network has multiple neurohemal release sites, exhibits descending activation that travels from the cerebral and pleural ganglia down to the abdominal bag cell clusters, and releases spatially distinct profiles of proELH-derived peptides within the Aplysia nervous system. Such distributed neurosecretory organization may be a common feature of neuroendocrine systems across higher order organisms linking multiple behavioral aspects to a single neuronal network.

Characterizing peptides in individual mammalian cells using mass spectrometry

Cell-to-cell chemical signaling plays multiple roles in coordinating the activity of the functional elements of an organism, with these elements ranging from a three-neuron reflex circuit to the entire animal. In recent years, single-cell mass spectrometry (MS) has enabled the discovery of cell-to-cell signaling molecules from the nervous system of a number of invertebrates. We describe a protocol for analyzing individual cells from rat pituitary using matrix-assisted laser desorption/ionization MS. Each step in the sample preparation process, including cell stabilization, isolation, sample preparation, signal acquisition and data interpretation, is detailed here. Although we employ this method to investigate peptides in individual pituitary cells, it can be adapted to other cell types and even subcellular sections from a range of animals. This protocol allows one to obtain 20-30 individual cell samples and acquire mass spectra from them in a single day.

Identification and characterization of homologues of vertebrate beta-thymosin in the marine mollusk Aplysia californica

The beta-thymosins have been known as actin-sequestering proteins, but now are recognized as molecules with multiple and diverse intracellular and extracellular functions. Two closely related proteins, beta-thymosin(His) and beta-thymosin(Gln), have been de novo sequenced by top-down mass spectrometry in the common neurobiology model, Aplysia californica. As determined by nanoelectrospray quadrupole-enhanced Fourier-Transform mass spectrometry with collisionally activated and electron-capture dissociations, both of these Aplysia beta-thymosins are acetylated and differ by a single residue in the central actin-binding domain. Profiling of individual cells and tissue by matrix-assisted laser desorption/ionization mass spectrometry reveals that these proteins are widely expressed in the Aplysia central nervous system, including in individual identified neurons, neuronal clusters, nerves and connective tissues. Newly identified beta-thymosin(His) and beta-thymosin(Gln) are also detected by mass spectrometry in hemolymph, and in releasates collected from whole ganglia. When applied exogenously, beta-thymosin proteins, purified from nerve cell extract, support the anchoring of neurons, and increase neurite sprouting and total neurite outgrowth in culture. These positive effects on neurite regeneration in cell culture suggest that the beta-thymosin proteins have an extracellular function in the central nervous system of Aplysia californica.

NMR analysis of Caenorhabditis elegans FLP-18 neuropeptides: implications for NPR-1 activation

Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides (FLPs) are the largest neuropeptide family in animals, particularly invertebrates. FLPs are characterized by a C-N-terminal gradient of decreasing amino acid conservation. Neuropeptide receptor 1 (NPR-1) is a G-protein coupled receptor (GPCR), which has been shown to be a strong regulator of foraging behavior and aggregation responses in Caenorhabditis elegans. Recently, ligands for NPR-1 were identified as neuropeptides coded by the precursor genes flp-18 and flp-21 in C. elegans. The flp-18 gene encodes eight FLPs including DFDGAMPGVLRF-NH2 and EMPGVLRF-NH2. These peptides exhibit considerably different activities on NPR-1, with the longer one showing a lower potency. We have used nuclear magnetic resonance and biological activity to investigate structural features that may explain these activity differences. Our data demonstrate that long-range electrostatic interactions exist between N-terminal aspartates and the C-terminal penultimate arginine as well as N-terminal hydrogen-bonding interactions that form transient loops within DFDGAMPGVLRF-NH2. We hypothesize that these loops, along with peptide charge, diminish the activity of this peptide on NPR-1 relative to that of EMPGVLRF-NH2. These results provide some insight into the large amino acid diversity in FLPs.

Identification of a new neuropeptide precursor reveals a novel source of extrinsic modulation in the feeding system of Aplysia

The Aplysia feeding system is advantageous for investigating the role of neuropeptides in behavioral plasticity. One family of Aplysia neuropeptides is the myomodulins (MMs), originally purified from one of the feeding muscles, the accessory radula closer (ARC). However, two MMs, MMc and MMe, are not encoded on the only known MM gene. Here, we identify MM gene 2 (MMG2), which encodes MMc and MMe and four new neuropeptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to verify that these novel MMG2-derived peptides (MMG2-DPs), as well as MMc and MMe, are synthesized from the precursor. Using antibodies against the MMG2-DPs, we demonstrate that neuronal processes that stain for MMG2-DPs are found in the buccal ganglion, which contains the feeding network, and in the buccal musculature including the ARC muscle. Surprisingly, however, no immunostaining is observed in buccal neurons including the ARC motoneurons. In situ hybridization reveals only few MMG2-expressing neurons that are mostly located in the pedal ganglion. Using immunohistochemical and electrophysiological techniques, we demonstrate that some of these pedal neurons project to the buccal ganglion and are the likely source of the MMG2-DP innervation of the feeding network and musculature. We show that the MMG2-DPs are bioactive both centrally and peripherally: they bias egestive feeding programs toward ingestive ones, and they modulate ARC muscle contractions. The multiple actions of the MMG2-DPs suggest that these peptides play a broad role in behavioral plasticity and that the pedal-buccal projection neurons that express them are a novel source of extrinsic modulation of the feeding system of Aplysia.

Discovering new invertebrate neuropeptides using mass spectrometry

Neuropeptides are a complex set of messenger molecules controlling a wide array of regulatory functions and behaviors within an organism. These neuromodulators are cleaved from longer protein molecules and often experience numerous post-translational modifications to achieve their bioactive form. As a result of this complexity, sensitive and versatile analysis schemes are needed to characterize neuropeptides. Mass spectrometry (MS) through a variety of approaches has fueled the discovery of hundreds of neuropeptides in invertebrate species in the last decade. Particularly successful are direct tissue and single neuron analyses by matrix-assisted laser desorption/ionization (MALDI) MS, which has been used to elucidate approximately 440 neuropeptides, and examination of neuronal homogenates by electrospray ionization techniques (ESI), also leading to the characterization of over 450 peptides. Additional MS methods with great promise for the discovery of neuropeptides are MS imaging and large-scale peptidomics studies in combination with a sequenced genome.

Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry quantitation via in cell combination

Herein we describe a novel method for quantitation using a Fourier transform mass spectrometer (FTMS) equipped with a MALDI ion source. The unique instrumental configuration of FTMS and its ion trapping and storing capabilities enable ion packets originating from two physically distinct samples to be combined in the ion cyclotron resonance (ICR) cell prior to detection. These features are exploited to combine analyte ions from two differentially labeled samples spotted separately and then combined in the ICR cell to generate a single mass spectrum containing isotopically paired peaks for quantitative comparison of relative ion abundances. The utility of this new quantitation via in cell combination (QUICC) approach is explored using peptide standards, a bovine serum albumin tryptic digest, and a crude neuronal tissue extract. We show that spectra acquired using the QUICC scheme are comparable to those obtained from premixing the isotopically labeled samples in solution. In addition, we show direct tissue in situ isotopic formaldehyde labeling of a crustacean neuroendocrine organ, thus demonstrating the potential application of the QUICC methodology for direct tissue quantitative analysis.

Mass spectrometric characterization of crustacean hyperglycemic hormone precursor-related peptides (CPRPs) from the sinus gland of the crab, Cancer productus

Crustacean hyperglycemic hormone (CHH) precursor-related peptides (CPRPs) are produced during the proteolytic processing of CHH preprohormones. Currently, the physiological roles played by CPRPs are unknown. Due to their large size, direct mass spectrometric sequencing of intact CPRPs is difficult. Here, we describe a novel strategy for sequencing Cancer productus CPRPs directly from a tissue extract using nanoflow liquid chromatography coupled to quadrupole time-of-flight tandem mass spectrometry. Four novel CPRPs were characterized with the aid of MS/MS de novo sequencing of 27 truncated CPRP peptides. Extensive modifications (methionine oxidation and carboxy-terminal methylation) were identified in both the full-length and truncated peptides. To investigate the origin of the modifications and truncations, a full-length CPRP was synthesized and subjected to the same storage and extraction protocols used for the characterization of the native peptides. Here, some methionine oxidation was seen, however, no methylation or truncation was evident suggesting much of the chemical complexity seen in the native CPRPs is unlikely due to a sample preparation artifact. Collectively, our study represents the most complete characterization of CPRPs to date and provides a foundation for future investigation of CPRP function in C. productus.

Mass spectrometric investigation of the neuropeptide complement and release in the pericardial organs of the crab, Cancer borealis

The crustacean stomatogastric ganglion (STG) is modulated by both locally released neuroactive compounds and circulating hormones. This study presents mass spectrometric characterization of the complement of peptide hormones present in one of the major neurosecretory structures, the pericardial organs (POs), and the detection of neurohormones released from the POs. Direct peptide profiling of Cancer borealis PO tissues using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) revealed many previously identified peptides, including proctolin, red pigment concentrating hormone (RPCH), crustacean cardioactive peptide (CCAP), several orcokinins, and SDRNFLRFamide. This technique also detected corazonin, a well-known insect hormone, in the POs for the first time. However, most mass spectral peaks did not correspond to previously known peptides. To characterize and identify these novel peptides, we performed MALDI postsource decay (PSD) and electrospray ionization (ESI) MS/MS de novo sequencing of peptides fractionated from PO extracts. We characterized a truncated form of previously identified TNRNFLRFamide, NRNFLRFamide. In addition, we sequenced five other novel peptides sharing a common C-terminus of RYamide from the PO tissue extracts. High K+ depolarization of isolated POs released many peptides present in this tissue, including several of the novel peptides sequenced in the current study.

Endogenous neurotrophic factors enhance neurite growth by bag cell neurons of Aplysia

Mechanisms that regulate neurite outgrowth are phylogenetically conserved, including the signaling molecules involved. Here, we describe neurotrophic effects on isolated bag cell neurons (BCNs) of substrate-bound growth factors endogenous to the sea slug Aplysia californica. Sheath cells dissociated from the pleural-visceral connectives of the Aplysia CNS and arterial cells dissociated from the anterior aorta enhance neurite outgrowth when compared to controls, i.e., BCNs grown in defined medium alone. In addition, the substrate remaining after sheath cells or arterial cells are killed significantly enhances growth, relative to all other conditions tested. For instance, primary neurites are more numerous and greater in length for BCNs cultured on substrate produced by arterial cells. These results suggest that sheath and arterial cells produce growth-promoting factors, some of which are found in the substrates produced by these cell types. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), we found that Aplysia collagen-like peptides are produced by dissociated arterial cells, and therefore likely contribute to the observed growth effects. Collagen-like peptides and other factors produced by sheath and arterial cells likely influence neurite growth in the Aplysia CNS during development, learning and memory, and regeneration after injury.

Spatial profiling invertebrate ganglia using MALDI MS

The ability of MALDI TOF MS to spatially map peptides and proteins directly from a tissue is an exciting advance to imaging mass spectrometry. Recent advances in instrumentation for MS have resulted in instruments capable of achieving several micron spatial resolution while acquiring high-resolution mass spectra. Currently, the ability to obtain high quality mass spectrometric images depends on sample preparation protocols that often result in limited spatial resolution. A number of sample preparation and matrix deposition protocols are evaluated for spatial profiling of Aplysia californica exocrine gland and neuronal tissues. Such samples are different from mammalian tissues, but make good targets for method optimization because of the wealth of biochemical information available on neuropeptide processing and distribution. Electrospray matrix deposition and a variety of freezing methods have been found to be optimum for these invertebrate tissues, with the exact protocols being tissue dependent.

Identification and characterization of the feeding circuit-activating peptides, a novel neuropeptide family of aplysia

We use a multidisciplinary approach to identify, map, and characterize the bioactivity of modulatory neuropeptides in the circuitry that generates feeding behavior in Aplysia. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the cerebral-buccal connective (CBC), a nerve containing axons of many interneurons that control feeding behavior of Aplysia, was used to identify neuropeptides that may participate in generation and shaping of feeding motor programs. Using this functionally oriented search, we identified a novel family of peptides that we call the feeding circuit-activating peptides (FCAPs). Two peptides with masses identical to those observed in the CBCs (molecular weight 1387 and 1433) were purified from buccal ganglia and partially sequenced using mass spectrometry. The amino acid sequence was then used to clone the FCAP precursor, which encodes multiple copies of eight different FCAPs. The two FCAPs present in highest copy number correspond to those observed in the CBC. The distribution of FCAP expression was mapped using Northern analysis, whole-mount in situ hybridization, and immunocytochemistry. Consistent with our initial findings, FCAP-immunopositive axons were observed in the CBC. Furthermore, we found that FCAP was present in some cerebral-buccal and buccal-cerebral interneurons. As their name suggests, FCAPs are capable of initiating rhythmic feeding motor programs and are the first neuropeptides with such activity in this circuit. The actions of FCAPs suggest that these peptides may contribute to the induction and maintenance of food-induced arousal. FCAPs were also localized to several other neuronal systems, suggesting that FCAPs may play a role in the regulation of multiple behaviors.

Potent antagonistic action of synthetic analogues of APGWGNamide, an antagonist of molluscan neuropeptide APGWamide

Fifty-five kinds of analogues of APGWGNamide (Ala-Pro-Gly-Trp-Gly-Asn-NH2), which is an antagonist of molluscan neuropeptide APGWamide, were synthesized and their antagonistic activities were examined on two molluscan smooth muscles. Among all the analogues tested, on spontaneous contraction of the crop of the land snail, Euhadra congenita, APGWG(L-biphenylalanine, Bip)amide showed the most potent antagonistic activity and its potency was 50-100 times higher than that of APGWGNamide. Likewise, on phasic contraction of the anterior byssus retractor muscle (ABRM) of the sea mussel, Mytilus edulis, the effect of APGWG(D-homophenylalanine, dHfe) was the most potent and showed 5-10 times stronger activity than that of APGWGNamide. In the tolerance test to known exo- and endopeptidases or the crop tissue homogenate, APGWGNamide was not only easily degraded by a proline-specific endopeptidase but also by the homogenate. Two kinds of potent antagonists were thus developed: APGWG(Bip)amide and APGWG(dHfe)amide, which will be useful tools for investigation of the function of APGWamide in the snail and the mussel, respectively.

Direct assay of Aplysia tissues and cells with laser desorption/ionization mass spectrometry on porous silicon

Desorption/ionization on porous silicon (DIOS) is a form of laser desorption mass spectrometry that allows for the direct mass analysis of a variety of analytes without the addition of organic matrix. Protocols are described for the direct analysis of exocrine tissue and single neurons using DIOS-MS. The atrial gland of Aplysia californica was blotted on to porous silicon and analyzed with DIOS-MS in the range m/z 1000-4000. The ability to culture invertebrate neurons directly on porous silicon is also presented. Isolated bag cells regenerated neuronal processes in culture on porous silicon. DIOS-MS allowed the direct detection of the peptides contained in individual cultured neurons indicating that with appropriate protocols, DIOS can be used with biological samples with considerable thickness.

Cloning, expression and processing of the CP2 neuropeptide precursor of Aplysia

The cDNA sequence encoding the CP2 neuropeptide precursor is identified and encodes a single copy of the neuropeptide that is flanked by appropriate processing sites. The distribution of the CP2 precursor mRNA is described and matches the CP2-like immunoreactivity described previously. Single cell RT-PCR independently confirms the presence of CP2 precursor mRNA in selected neurons. MALDI-TOF MS is used to identify additional peptides derived from the CP2 precursor in neuronal somata and nerves, suggesting that the CP2 precursor may give rise to additional bioactive neuropeptides.

The enterins: a novel family of neuropeptides isolated from the enteric nervous system and CNS of Aplysia

To identify neuropeptides that have a broad spectrum of actions on the feeding system of Aplysia, we searched for bioactive peptides that are present in both the gut and the CNS. We identified a family of structurally related nonapeptides and decapeptides (enterins) that are present in the gut and CNS of Aplysia, and most of which share the HSFVamide sequence at the C terminus. The structure of the enterin precursor deduced from cDNA cloning predicts 35 copies of 20 different enterins. Northern analysis, in situ hybridization, and immunocytochemistry show that the enterins are abundantly present in the CNS and the gut of Aplysia. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry we characterized the enterin-precursor processing, demonstrated that all of the precursor-predicted enterins are present, and determined post-translational modifications of various enterins. Enterin-positive neuronal somata and processes were found in the gut, and enterins inhibited contractions of the gut. In the CNS, the cerebral and buccal ganglia, which control feeding, contained the enterins. Enterin was also present in the nerve that connects these two ganglia. Enterins reduced the firing of interneurons B4/5 during feeding motor programs. Such enterin-induced reduction of firing also occurred when excitability of B4/5 was tested directly. Because reduction of B4/5 activity corresponds to a switch from egestive to ingestive behaviors, enterin may contribute to such program switching. Furthermore, because enterins are present throughout the nervous system, they may also play a regulatory role in nonfeeding behaviors of Aplysia.

Aplysia attractin: biophysical characterization and modeling of a water-borne pheromone

Attractin, a 58-residue protein secreted by the mollusk Aplysia californica, stimulates sexually mature animals to approach egg cordons. Attractin from five different Aplysia species are approximately 40% identical in sequence. Recombinant attractin, expressed in insect cells and purified by reverse-phase high-performance liquid chromatography (RP-HPLC), is active in a bioassay using A. brasiliana; its circular dichroism (CD) spectrum indicates a predominantly alpha-helical structure. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) characterization of proteolytic fragments identified disulfide bonds between the six conserved cysteines (I-VI, II-V, III-IV, where the Roman numeral indicates the order of occurrence in the primary sequence). Attractin has no significant similarity to any other sequence in the database. The protozoan Euplotes pheromones were selected by fold recognition as possible templates. These diverse proteins have three alpha-helices, with six cysteine residues disulfide-bonded in a different pattern from attractin. Model structures with good stereochemical parameters were prepared using the EXDIS/DIAMOD/FANTOM program suite and constraints based on sequence alignments with the Euplotes templates and the attractin disulfide bonds. A potential receptor-binding site is suggested based on these data. Future structural characterization of attractin will be needed to confirm these models.

Cerebrin prohormone processing, distribution and action in Aplysia californica

The isolation, characterization, and bioactivity in the feeding circuitry of a novel neuropeptide in the Aplysia californica central nervous system are reported. The 17-residue amidated peptide, NGGTADALYNLPDLEKIamide, has been termed cerebrin due to its primary location in the cerebral ganglion. Liquid chromatographic purification guided by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry allowed the isolation of the peptide with purity adequate for Edman sequencing. The cerebrin cDNA has been characterized and encodes an 86 amino acid prohormone that predicts cerebrin and one additional peptide. Mapping using in situ hybridization and immunocytochemistry showed that cerebrin containing neuronal somata are localized almost exclusively in the cerebral ganglion, mostly in the F- and C-clusters. Both immunostaining and mass spectrometry demonstrated the presence of cerebrin in the neurohemal region of the upper labial nerve. In addition, immunoreactive processes were detected in the neuropil of all of the ganglia, including the buccal ganglia, and in some interganglionic connectives, including the cerebral-buccal connective. This suggests that cerebrin may also function as a local signaling molecule. Cerebrin has a profound effect on the feeding motor pattern elicited by the command-like neuron CBI-2, dramatically shortening the duration of the radula protraction in a concentration-dependent manner, mimicking the motor-pattern alterations observed in food induced arousal states. These findings suggest that cerebrin may contribute to food-induced arousal in the animal. Cerebrin-like immunoreactivity is also present in Lymnaea stagnalis suggesting that cerebrin-like peptides may be widespread throughout gastropoda.

Mass spectrometric survey of peptides in cephalopods with an emphasis on the FMRFamide-related peptides

A matrix-assisted laser desorption/ionization (MALDI) mass spectrometric (MS) survey of the major peptides in the stellar, fin and pallial nerves and the posterior chromatophore lobe of the cephalopods Sepia officinalis, Loligo opalescens and Dosidicus gigas has been performed. Although a large number of putative peptides are distinct among the three species, several molecular masses are conserved. In addition to peptides, characterization of the lipid content of the nerves is reported, and these lipid peaks account for many of the lower molecular masses observed. One conserved set of peaks corresponds to the FMRFamide-related peptides (FRPs). The Loligo opalescens FMRFa gene has been sequenced. It encodes a 331 amino acid residue prohormone that is processed into 14 FRPs, which are both predicted by the nucleotide sequence and confirmed by MALDI MS. The FRPs predicted by this gene (FMRFa, FLRFa/FIRFa and ALSGDAFLRFa) are observed in all three species, indicating that members of this peptide family are highly conserved across cephalopods.

Single-cell MALDI: a new tool for direct peptide profiling

Matrix-assisted laser desorption-ionization (MALDI) mass spectrometry (MS) is a rapid and sensitive analytical approach that is well suited for obtaining molecular weights of peptides and proteins from complex samples. MALDI-MS can profile the peptides and proteins from single-cell and small tissue samples without the need for extensive sample preparation, except for the cell isolation and matrix application. Strategies for peptide identification and characterization of post-translational modifications are presented. Furthermore, several recent enhancements in MALDI-MS technology, including in situ peptide sequencing as well as the direct spatial mapping of peptides in cells and tissues are discussed.

Measuring the peptides in individual organelles with mass spectrometry

New sampling protocols combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) allow the assay of single dense core vesicles. Understanding the packaging of vesicles is important as vesicles are the quanta of information for intercellular communication. Using vesicles from the exocrine atrial gland of Aplysia californica as the model, a wide range of bioactive peptides are detected within each vesicle. Although the expression of the egg-laying hormone gene family of type 1 atrial gland cells has been previously examined, chemical characterization of individual 1-2 microm diameter vesicles demonstrates that products from several genes are colocalized. The mass sensitivity of MALDI MS can be further improved to enable the analysis of even smaller subcellular organelles.

The Aplysia mytilus inhibitory peptide-related peptides: identification, cloning, processing, distribution, and action

Neuropeptides are a ubiquitous class of signaling molecules. In our attempt to understand the generation of feeding behavior in Aplysia, we have sought to identify and fully characterize the neuropeptides operating in this system. Preliminary evidence indicated that Mytilus inhibitory peptide (MIP)-like peptides are present and operating in the circuitry that generates feeding in Aplysia. MIPs were originally isolated from the bivalve mollusc Mytilus edulis, and related peptides have been identified in other invertebrate species, but no precursor has been identified. In this study, we describe the isolation and characterization of novel Aplysia MIP-related peptides (AMRPs) and their precursor. Several AMRPs appear to have some structural and functional features similar to vertebrate opioid peptides. We use matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to confirm that all 14 AMRPs predicted by the precursor are processed in isolated neurons. Northern analysis, whole-mount in situ hybridization, and immunohistochemistry are used to map the abundant expression of these peptides in the CNS and peripheral tissues such as the digestive tract, vasculature, and the reproductive organs. Physiological studies demonstrate that the rank order of the inhibitory actions of these peptides is different for three target muscles. These results underscore the importance of using a multidisciplinary approach to identifying and characterizing the actions of neuropeptides in an effort to gain understanding of their role in systems of interest. The widespread distribution of the AMRPs indicates that they may be operating in many different systems of Aplysia.

Egg-laying hormone peptides in the aplysiidae family

The neuropeptidergic bag cells of the marine mollusc Aplysia californica are involved in the egg-laying behavior of the animal. These neurosecretory cells synthesize an egg-laying hormone (ELH) precursor protein, yielding multiple bioactive peptides, including ELH, several bag cell peptides (BCP) and acidic peptide (AP). While immunohistochemical studies have involved a number of species, homologous peptides have been biochemically characterized in relatively few Aplysiidae species. In this study, a combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MS) and electrospray ionization Fourier transform ion cyclotron resonance MS is used to characterize and compare the ELH peptides from related opisthobranch molluscs including Aplysia vaccaria and Phyllaplysia taylori. The peptide profiles of bag cells from these two Aplysiidae species are similar to that of A. californica bag cells. In an effort to characterize further several of these peptides, peptides from multiple groups of cells of each species were extracted, and microbore liquid chromatography was used to separate and isolate them. Several MS-based sequencing approaches are applied to obtain the primary structures of bag cell peptides and ELH. Our studies reveal that (α)-BCPs are 100 % conserved across all species studied. In addition, the complete sequences of (ε)-BCP and ELH of A. vaccaria were determined. They show a high degree of homology to their counterparts in A. californica, with only a few amino acid residue substitutions.

Insulin prohormone processing, distribution, and relation to metabolism in Aplysia californica

The first Aplysia californica insulin gene is characterized and its proteolytic processing from prohormone to final peptides elucidated using a combination of biochemical and mass spectrometric methods. Aplysia insulin (AI) is one of the largest insulins found, with a molecular weight of 9146 Da, and an extended A chain compared with other invertebrate and vertebrate insulins. The AI prohormone produces a series of C peptides and also a unique N-terminally acetylated D peptide. AI-producing cells are restricted to the central region of the cerebral ganglia mostly within the F and C clusters, and AI is transported to neurohemal release sites located on the upper labial and anterior tentacular nerves. The expression of AI mRNA decreases when the animal is deprived of food, and injections of AI reduce hemolymph glucose levels, suggesting that the function of insulin-regulating metabolism has been conserved.

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.

Characterization of peptides from Aplysia using microbore liquid chromatography with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry guided purification

Liquid chromatography (LC) has been used extensively for the separation and isolation of peptides due to its high selectivity and peak capacity. An approach combining microbore LC with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS) detection is described to identify peptides in cells and guide the purification of peptides from the marine mollusc Aplysia californica. Direct MALDI-MS of neurons and processes provides molecular mass information for unknown peptides with almost no sample preparation, and LC-MALDI-MS allows the isolation and purification of these peptides from pooled samples, thus enabling new putative neuropeptides to be isolated from complex cellular samples. Both direct MALDI-MS and LC-MALDI-MS are compared in terms of detecting peptides from neuronal samples. Using both approaches, two peaks from Aplysia californica connectives having molecular masses of 5013 and 5021 have been isolated, partially sequenced and identified as novel collagen-like peptides.