Isolation and amino acid sequence of crustacean hyperglycemic hormone precursor-related peptides

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

Insect ion transport peptides are derived from alternatively spliced genes and differentially expressed in the central and peripheral nervous system

Insect ionic and fluid homeostasis relies upon the Malpighian tubules (MT) and different hindgut compartments. Primary urine formed in MTs is finally modified by ion, solute and water reabsorptive processes primarily in the hindgut under the control of several large peptide hormones. One of these, the ion transport peptide (ITP), is a chloride transport-stimulating and acid secretion-inhibiting hormone similar to crustacean hyperglycaemic hormones (CHHs). In locusts, moths and fruit flies, ITP together with the slightly longer ITPL isoforms, inactive in hindgut bioassays, arise by alternative splicing from very similar itp genes. ITP and ITPL are differentially distributed in (1) pars lateralis/retrocerebral complex neurosecretory cells (NSCs) containing both splice forms, (2) interneurons with either one of the splice forms, (3) hindgut-innervating abdominal ITP neurons (in Drosophila only), and (4) intrinsic, putative sensory NSCs in peripheral neurohaemal perisympathetic/perivisceral organs or transverse nerves (usually containing ITPL). Both splice forms occur as hormones released into the haemolymph in response to feeding or stress stimuli. ITPL mainly released from the peripheral NSCs is discussed as a competitive inhibitor (as established in vitro) of ITP action on yet to be identified hindgut ITP receptors. Furthermore, some evidence has been provided for possible ecdysis-related functions of ITP and/or ITPL in moths. The comparative data on the highly similar gene, precursor and primary structures and similar differential distributions in insect and crustacean NSCs suggest that CHH/ITP and ITPL neuropeptide-producing cells and their gene products share common phylogenetic ancestry.

Mass spectral characterization of peptide transmitters/hormones in the nervous system and neuroendocrine organs of the American lobster Homarus americanus

The American lobster Homarus americanus is a decapod crustacean with both high economic and scientific importance. To facilitate physiological investigations of peptide transmitter/hormone function in this species, we have used matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nanoscale liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-Q-TOF MS/MS) to elucidate the peptidome present in its nervous system and neuroendocrine organs. In total, 84 peptides were identified, including 27 previously known H. americanus peptides (e.g., VYRKPPFNGSIFamide [Val(1)-SIFamide]), 23 peptides characterized previously from other decapods, but new to the American lobster (e.g., pQTFQYSRGWTNamide [Arg(7)-corazonin]), and 34 new peptides de novo sequenced/detected for the first time in this study. Of particular note are a novel B-type allatostatin (TNWNKFQGSWamide) and several novel FMRFamide-related peptides, including an unsulfated analog of sulfakinin (GGGEYDDYGHLRFamide), two myosuppressins (QDLDHVFLRFamide and pQDLDHVFLRFamide), and a collection of short neuropeptide F isoforms (e.g., DTSTPALRLRFamide and FEPSLRLRFamide). Our data also include the first detection of multiple tachykinin-related peptides in a non-brachyuran decapod, as well as the identification of potential individual-specific variants of orcokinin and orcomyotropin-related peptide. Taken collectively, our results not only expand greatly the number of known H. americanus neuropeptides, but also provide a framework for future studies on the physiological roles played by these molecules in this commercially and scientifically important species.

Midgut epithelial endocrine cells are a rich source of the neuropeptides APSGFLGMRamide (Cancer borealis tachykinin-related peptide Ia) and GYRKPPFNGSIFamide (Gly1-SIFamide) in the crabs Cancer borealis, Cancer magister and Cancer productus

Over a quarter of a century ago, Mykles described the presence of putative endocrine cells in the midgut epithelium of the crab Cancer magister(Mykles, 1979). In the years that have followed, these cells have been largely ignored and nothing is known about their hormone content or the functions they play in this species. Here,we used a combination of immunohistochemistry and mass spectrometric techniques to investigate these questions. Using immunohistochemistry, we identified both SIFamide- and tachykinin-related peptide (TRP)-like immunopositive cells in the midgut epithelium of C. magister, as well as in that of Cancer borealis and Cancer productus. In each species, the SIFamide-like labeling was restricted to the anterior portion of the midgut, including the paired anterior midgut caeca, whereas the TRP-like immunoreactivity predominated in the posterior midgut and the posterior midgut caecum. Regardless of location, label or species, the morphology of the immunopositive cells matched that of the putative endocrine cells characterized ultrastructurally by Mykles(Mykles, 1979). Matrix-assisted laser desorption/ionization-Fourier transform mass spectrometry identified the peptides responsible for the immunoreactivities as GYRKPPFNGSIFamide (Gly1-SIFamide) and APSGFLGMRamide [Cancer borealis tachykinin-related peptide Ia (CabTRP Ia)], respectively, both of which are known neuropeptides of Cancer species. Although the function of these midgut-derived peptides remains unknown, we found that both Gly1-SIFamide and CabTRP Ia were released when the midgut was exposed to high-potassium saline. In addition, CabTRP Ia was detectable in the hemolymph of crabs that had been held without food for several days, but not in that of fed animals, paralleling results that were attributed to TRP release from midgut endocrine cells in insects. Thus, one function that midgut-derived CabTRP Ia may play in Cancer species is paracrine/hormonal control of feeding-related behavior, as has been postulated for TRPs released from homologous cells in insects.

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.

Identification of Neuropeptides from the Sinus Gland of the Crayfish Orconectes limosus Using Nanoscale On-line Liquid Chromatography Tandem Mass Spectrometry

In this article, a novel and sensitive analytical strategy for direct characterization of neuropeptides from the X-Organ-sinus gland neurosecretory system of the crayfish Orconectes limosus is presented. A desalted extract corresponding to 0.5 sinus gland equivalents was analyzed in a nanoflow liquid chromatography system coupled to quadrupole time-of-flight tandem mass spectrometry (nanoLC-QTOF MS/MS). The existence and structural identity of four crustacean hyperglycemic hormone precursor-related peptide variants and two new genetic variants of the pigment-dispersing hormone, not detected by conventional chromatographic systems, molecular cloning, or immunochemical methods before, was revealed. The here-presented approach of the combined LC-QTOF MS/MS technique is a powerful tool to discover new peptide hormones in biological systems, due to its sensitivity, accuracy, and speed.

Crustacean hyperglycaemic hormone (CHH)-like peptides and CHH-precursor-related peptides from pericardial organ neurosecretory cells in the shore crab, Carcinus maenas, are putatively spliced and modified products of multiple genes

About 24 intrinsic neurosecretory neurons within the pericardial organs (POs) of the crab Carcinus maenas produce a novel crustacean hyperglycaemic hormone (CHH)-like peptide (PO-CHH) and two CHH-precursor-related peptides (PO-CPRP I and II) as identified immunochemically and by peptide chemistry. Edman sequencing and MS revealed PO-CHH as a 73 amino acid peptide (8630 Da) with a free C-terminus. PO-CHH and sinus gland CHH (SG-CHH) share an identical N-terminal sequence, positions 1-40, but the remaining sequence, positions 41-73 or 41-72, differs considerably. PO-CHH may have different precursors, as cDNA cloning of PO-derived mRNAs has revealed several similar forms, one exactly encoding the peptide. All PO-CHH cDNAs contain a nucleotide stretch coding for the SG-CHH(41-76) sequence in the 3'-untranslated region (UTR). Cloning of crab testis genomic DNA revealed at least four CHH genes, the structure of which suggest that PO-CHH and SG-CHH arise by alternative splicing of precursors and possibly post-transcriptional modification of PO-CHH. The genes encode four exons, separated by three variable introns, encoding part of a signal peptide (exon I), the remaining signal peptide residues, a CPRP, the PO-CHH(1-40)/SG-CHH(1-40) sequences (exon II), the remaining PO-CHH residues (exon III) and the remaining SG-CHH residues and a 3'-UTR (exon IV). Precursor and gene structures are more closely related to those encoding related insect ion-transport peptides than to penaeid shrimp CHH genes. PO-CHH neither exhibits hyperglycaemic activity in vivo, nor does it inhibit Y-organ ecdysteroid synthesis in vitro. From the morphology of the neurons it seems likely that novel functions remain to be discovered.

A remarkable, precisely timed release of hyperglycemic hormone from endocrine cells in the gut is associated with ecdysis in the crab Carcinus maenas

Molting or ecdysis is the most fundamentally important process in arthropod life history, because shedding of the exoskeleton is an absolute prerequisite for growth and metamorphosis. Although the hormonal mechanisms driving ecdysis in insects have been studied extensively, nothing is known about these processes in crustaceans. During late premolt and during ecdysis in the crab Carcinus maenas, we observed a precise and reproducible surge in hemolymph hyperglycemic hormone (CHH) levels, which was over 100-fold greater than levels seen in intermolt animals. The source of this hormone surge was not from the eyestalk neurosecretory tissues but from previously undescribed endocrine cells (paraneurons), in defined areas of the foregut and hindgut. During premolt (the only time when CHH is expressed by these tissues), the gut is the largest endocrine tissue in the crab. The CHH surge, which is a result of an unusual, almost complete discharge of the contents of the gut endocrine cell, regulates water and ion uptake during molting, thus allowing the swelling necessary for successful ecdysis and the subsequent increase in size during postmolt. This study defines an endocrine brain/gut axis in the arthropods. We propose that the ionoregulatory process controlled by CHH may be common to arthropods, in that, for insects, a similar mechanism seems to be involved in antidiuresis. It also seems likely that a cascade of very precisely coordinated release of (neuro) hormones controls ecdysis.

Amino acid sequences of both isoforms of crustacean hyperglycemic hormone (CHH) and corresponding precursor-related peptide in Cancer pagurus

Both isoforms of the crustacean hyperglycemic hormone (CHH) and corresponding crustacean hyperglycemic hormone precursor-related peptide (CPRP) derived from HPLC-purified sinus gland extracts from the edible crab Cancer pagurus were fully characterised by microsequencing and mass spectrometry. The amino acid sequences of the CHH isoforms were almost identical except that the N-terminus of the minor isoform (CHH-I), was glutamine rather than pyroglutamate in the major isoform (CHH-II). Both CHH isoforms were of similar biological activity, as tested by in vivo hyperglycemia bioassays and in vitro repression of ecdysteroid synthesis. Comparison with other published CHH and CPRP sequences show that for crabs, these peptides form a distinct group, that the presence of CHH isoforms with free and blocked N-termini seems unique to crabs. It is argued that this phenomenon reflects a slow post-translational modification in sinus gland neurosecretory terminals. This study appears to complete the entire sinus gland inventory of functionally and structurally characterised CHH-related peptides in a crab.

Molecular cloning of a cDNA encoding molt-inhibiting hormone of the crab, Cancer magister

A neuropeptide molt-inhibiting hormone (MIH) negatively regulates crustacean molting glands (Y-organs). We report here the molecular cloning of a cDNA encoding putative MIH of the Dungeness crab, Cancer magister. A cDNA library was commercially prepared using poly (A+) RNA isolated from C. magister eyestalk neural ganglia. The library was screened using as probe a previously cloned portion of a cDNA encoding MIH of the blue crab, Callinectes sapidus. DNA sequence analysis of one positive clone revealed a 339 base pair open reading frame encoding a 78 amino acid putative MIH and a 35 amino acid signal peptide. The deduced amino acid sequence of C. magister MIH shows high sequence identity (80-98%) with MIH of three other brachyuran crabs, but lower identity (26-45%) with MIH and MIH-like peptides from astacurans and shrimp. Studies using reverse transcription-polymerase chain reaction (RT-PCR) indicate the MIH gene is expressed in eyestalk but not control (muscle, gill, gonad, hepatopancreas) tissue.

cDNA cloning of a mandibular organ inhibiting hormone from the spider crab Libinia emarginata

Mandibular organs (MO) produce a crustacean juvenile hormone, methyl farnesoate (MF). MO activity is negatively regulated by factors, called mandibular organ inhibiting hormones (MOIHs), from the crustacean sinus gland X-organ complex in the eyestalks. Three MOIHs have been isolated previously from the spider crab Libinia emarginata and are characterized as members of the crustacean hyperglycemic hormone (CHH) neuropeptide family. In the research reported here, a full length cDNA sequence of 972 bp of a MOIH was isolated by screening a cDNA library constructed from the eyestalks of Libinia emarginata. This cDNA sequence encodes a preprohormone peptide with 137 amino acid residues, including a 26-amino acid long signal peptide, a 34-amino acid long precursor peptide, a dibasic peptide, the full length of 72-amino acid long MOIH, and a tri-peptide Gly-Lys-Lys which designates the potential amidation site at the C-terminus of the mature peptide.

Cloning and expression of two mRNAs encoding structurally different crustacean hyperglycemic hormone precursors in the lobster Homarus americanus

The crustacean hyperglycemic hormone (CHH) of the X-organ sinus gland complex is a multifunctional neurohormone primarily involved in the regulation of blood sugar levels. HPLC analysis of lobster sinus glands revealed two CHH-immunoreactive groups, each consisting of two isoforms with identical amino acid sequences and molecular weights. In order to obtain more information concerning the number and sequences of preproCHHs, and to study their expression, we isolated two full-length cDNAs encoding two different CHH preprohormones. Both preprohormone structures consist of a signal peptide, a CHH-precursor-related peptide and a highly-conserved CHH peptide. Expression studies revealed that the X-organ is not the only source of CHH mRNA because the ventral nerve system also expresses this mRNA. Based on these findings and earlier studies on the effect of eyestalk ablation, implantation of thoracic/abdominal ganglia as well as the multifunctionality of CHH, we postulate that CHH, present in the ventral nerve system is a good candidate for a supplementary role in the control of reproduction and molting.

Cloning and expression of mRNA encoding prepro-gonad-inhibiting hormone (GIH) in the lobster Homarus americanus

The gonad-inhibiting hormone (GIH) is produced in the eyestalk X-organ sinus gland complex of male and female lobsters, and plays a prominent role in the regulation of reproduction, e.g. inhibition of vitellogenesis in female animals. To study this neurohormone at the mRNA level, we cloned and sequenced a cDNA which encodes GIH in the lobster Homarus americanus. The structure of preproGIH consists of a signal peptide and the GIH peptide itself. A comparative analysis revealed that lobster GIH, together with crab molt-inhibiting hormone, belongs to a separate group of the crustacean hyperglycemic hormone (CHH) peptide family which seems to be unique for crustaceans. Expression studies showed that GIH mRNA is expressed in the eyestalk, indicating that the neuroendocrine center in this optic structure is the only source of GIH. As this center modulates the other (neuro)endocrine organs in crustaceans, it is postulated that GIH regulates production and release of hormones involved in reproduction/molting processes.

Cloning and expression of two crustacean hyperglycemic-hormone mRNAs in the eyestalk of the crayfish Orconectes limosus

Crustacean hyperglycemic hormone (CHH) is a multifunctional neurohormone produced in the eyestalk of crustaceans and is primarily involved in the regulation of carbohydrate metabolism. In several crustacean species, CHH isoforms with identical amino acid sequences and molecular masses, but with different chromatographic elution patterns, are synthesized. To obtain sequence information on the CHH preprohormone in the crayfish Orconectes limosus we isolated two full-length cDNAs encoding two structurally different preproCHH species. The sequences of these precursors differ slightly in the signal peptide, the CHH-precursor-related peptide(CPRP)-coding sequences and in the non-coding regions, but are identical in the CHH peptide-coding sequence. Determination of the levels of preproCHH mRNAs and the amount of CHH peptide in the eyestalks of individual animals revealed that the ratio between the two preproCHH mRNAs varies for different individuals while the ratio between the two CHH peptide isoforms does not differ among animals. Our results suggest that the existence of two CHH isoforms in the crayfish O. limosus is due to a post-translational modification event. Northern-blot analysis showed only one band in eyestalk tissue with a size of approximately 2.4 kb, similar to the sizes of the cDNA sequences. Southern-blot analysis revealed the presence of at least two preproCHH genes in the crayfish suggesting a gene duplication event. Slight modifications in the duplicated genes could be responsible for the existence of the two preproCHH-encoding mRNAs.

Molecular cloning of crustacean putative molt-inhibiting hormone (MIH) precursor

A cDNA encoding the complete precursor of the putative molt-inhibiting hormone (MIH) of the shore crab, Carcinus maenas, was isolated and sequenced. The precursor consists of a putative 35 amino acid signal peptide and the 78 amino acid mature MIH. The deduced MIH amino acid sequence is in complete agreement with the sequence previously determined by Edman degradation. In situ hybridization revealed MIH-expression in a subpopulation of large neurosecretory perikarya of the medulla terminalis X-organ in the eyestalk.