Dynamics of biosynthesis and release of crustacean hyperglycemic hormone isoforms in the X-organ-sinus gland complex of the crayfish Orconectes limosus

The Crustacean Hyperglycemic Hormone Superfamily: Progress Made in the Past Decade

Early studies recognizing the importance of the decapod eyestalk in the endocrine regulation of crustacean physiology-molting, metabolism, reproduction, osmotic balance, etc.-helped found the field of crustacean endocrinology. Characterization of putative factors in the eyestalk using distinct functional bioassays ultimately led to the discovery of a group of structurally related and functionally diverse neuropeptides, crustacean hyperglycemic hormone (CHH), molt-inhibiting hormone (MIH), gonad-inhibiting hormone (GIH) or vitellogenesis-inhibiting hormone (VIH), and mandibular organ-inhibiting hormone (MOIH). These peptides, along with the first insect member (ion transport peptide, ITP), constitute the original arthropod members of the crustacean hyperglycemic hormone (CHH) superfamily. The presence of genes encoding the CHH-superfamily peptides across representative ecdysozoan taxa has been established. The objective of this review is to, aside from providing a general framework, highlight the progress made during the past decade or so. The progress includes the widespread identification of the CHH-superfamily peptides, in particular in non-crustaceans, which has reshaped the phylogenetic profile of the superfamily. Novel functions have been attributed to some of the newly identified members, providing exceptional opportunities for understanding the structure-function relationships of these peptides. Functional studies are challenging, especially for the peptides of crustacean and insect species, where they are widely expressed in various tissues and usually pleiotropic. Progress has been made in deciphering the roles of CHH, ITP, and their alternatively spliced counterparts (CHH-L, ITP-L) in the regulation of metabolism and ionic/osmotic hemostasis under (eco)physiological, developmental, or pathological contexts, and of MIH in the stimulation of ovarian maturation, which implicates it as a regulator for coordinating growth (molt) and reproduction. In addition, experimental elucidation of the steric structure and structure-function relationships have given better understanding of the structural basis of the functional diversification and overlapping among these peptides. Finally, an important finding was the first-ever identification of the receptors for this superfamily of peptides, specifically the receptors for ITPs of the silkworm, which will surely give great impetus to the functional study of these peptides for years to come. Studies regarding recent progress are presented and synthesized, and prospective developments remarked upon.

Crustacean hyperglycemic hormone is synthesized in the eyestalk and brain of the crayfish Procambarus clarkii

Crustacean hyperglycemic hormone (CHH) is a neuropeptide that is synthesized, stored, and released by brain and eyestalk structures in decapods. CHH participates in the regulation of several mechanisms, including increasing the level of glucose in hemolymph. Although CHH mRNA levels have been quantified and the CHH protein has been localized in various structures of the crayfish P. clarkii, CHH synthesis has only been reported in the X-organ-sinus gland (XO-SG). Therefore, the aim of this study was to use in situ hybridization to determine whether CHH mRNA is located in other structures, including the putative pacemaker, eyestalk and brain, of crayfish P. clarkii at two times of day. CHH mRNA was observed in both the eyestalk and the brain of P. clarkii, indicating that CHH is synthesized in several structures in common with other crustaceans, possibly to provide metabolic support for these regions by increasing glucose levels.

Different transcription regulation routes are exerted by L- and D-amino acid enantiomers of peptide hormones

Conversion of one or more amino acids in eukaryotic peptides to the D-configuration is catalyzed by specific L/D peptide isomerases and it is a poorly investigated post-translational modification. No common modified amino acid and no specific modified position have been recognized and mechanisms underlying changes in the peptide function provided by this conversion were not sufficiently studied. The 72 amino acid crustacean hyperglycemic hormone (CHH) of Astacidea crustaceans exhibits a co-existence of two peptide enantiomers alternately having D- or L-phenylalanine in their third position. It is a pleiotropic hormone regulating several physiological processes in different target tissues and along different time scales. CHH enantiomers differently affect time courses and intensities of examined processes. The short-term effects of the two isomers on gene expression are presented here, examined in the hepatopancreas, gills, hemocytes and muscles of the astacid Pontastacus leptodactylus. Muscles and hemocytes were poorly affected by both isomers. Two CHH modes of action were elucidated in the hepatopancreas and the gills: specific gene induction by D-CHH only, elucidated in both organs and mutual targeted attenuation affected by both enantiomers elucidated in the gills. Consequently a two-receptor system is hypothesized for conveying the effect of the two CHH isomers.

Putative pacemakers in the eyestalk and brain of the crayfish Procambarus clarkii show circadian oscillations in levels of mRNA for crustacean hyperglycemic hormone

Crustacean hyperglycemic hormone (CHH) synthesizing cells in the optic lobe, one of the pacemakers of the circadian system, have been shown to be present in crayfish. However, the presence of CHH in the central brain, another putative pacemaker of the multi-oscillatory circadian system, of this decapod and its circadian transcription in the optic lobe and brain have yet to be explored. Therefore, using qualitative and quantitative PCR, we isolated and cloned a CHH mRNA fragment from two putative pacemakers of the multi-oscillatory circadian system of Procambarus clarkii, the optic lobe and the central brain. This CHH transcript synchronized to daily light-dark cycles and oscillated under dark, constant conditions demonstrating statistically significant daily and circadian rhythms in both structures. Furthermore, to investigate the presence of the peptide in the central brain of this decapod, we used immunohistochemical methods. Confocal microscopy revealed the presence of CHH-IR in fibers and cells of the protocerebral and tritocerebal clusters and neuropiles, particularly in some neurons located in clusters 6, 14, 15 and 17. The presence of CHH positive neurons in structures of P. clarkii where clock proteins have been reported suggests a relationship between the circadian clockwork and CHH. This work provides new insights into the circadian regulation of CHH, a pleiotropic hormone that regulates many physiological processes such as glucose metabolism and osmoregulatory responses to stress.

Application of D-Crustacean Hyperglycemic Hormone Induces Peptidases Transcription and Suppresses Glycolysis-Related Transcripts in the Hepatopancreas of the Crayfish Pontastacus leptodactylus - Results of a Transcriptomic Study

The crustacean Hyperglycemic Hormone (cHH) is a neuropeptide present in many decapods. Two different chiral isomers are simultaneously present in Astacid crayfish and their specific biological functions are still poorly understood. The present study is aimed at better understanding the potentially different effect of each of the isomers on the hepatopancreatic gene expression profile in the crayfish Pontastacus leptodactylus, in the context of short term hyperglycemia. Hence, two different chemically synthesized cHH enantiomers, containing either L- or D-Phe(3), were injected to the circulation of intermolt females following removal of their X organ-Sinus gland complex. The effects triggered by the injection of the two alternate isomers were detected after one hour through measurement of circulating glucose levels. Triggered changes of the transcriptome expression profile in the hepatopancreas were analyzed by RNA-seq. A whole transcriptome shotgun sequence assembly provided the assumedly complete transcriptome of P. leptodactylus hepatopancreas, followed by RNA-seq analysis of changes in the expression level of many genes caused by the application of each of the hormone isomers. Circulating glucose levels were much higher in response to the D-isoform than to the L-isoform injection, one hour from injection. Similarly, the RNA-seq analysis confirmed a stronger effect on gene expression following the administration of D-cHH, while just limited alterations were caused by the L-isomer. These findings demonstrated a more prominent short term effect of the D-cHH on the transcription profile and shed light on the effect of the D-isomer on specific functional gene groups. Another contribution of the study is the construction of a de novo assembly of the hepatopancreas transcriptome, consisting of 39,935 contigs, that dramatically increases the molecular information available for this species and for crustaceans in general, providing an efficient tool for studying gene expression patterns in this organ.

Novel protocol for the chemical synthesis of crustacean hyperglycemic hormone analogues--an efficient experimental tool for studying their functions

The crustacean Hyperglycemic Hormone (cHH) is present in many decapods in different isoforms, whose specific biological functions are still poorly understood. Here we report on the first chemical synthesis of three distinct isoforms of the cHH of Astacus leptodactylus carried out by solid phase peptide synthesis coupled to native chemical ligation. The synthetic 72 amino acid long peptide amides, containing L- or D-Phe³ and (Glp¹, D-Phe³) were tested for their biological activity by means of homologous in vivo bioassays. The hyperglycemic activity of the D-isoforms was significantly higher than that of the L-isoform, while the presence of the N-terminal Glp residue had no influence on the peptide activity. The results show that the presence of D-Phe³ modifies the cHH functionality, contributing to the diversification of the hormone pool.

Experimental strategies for the analysis of D-amino acid containing peptides in crustaceans: a review

Detection of D-amino acids in natural peptides has been, and remains a challenging task, as peptidyl isomerization is a peculiar and subtle posttranslational modification that does not induce any change in primary sequence or in physicochemical properties of the molecule such as molecular mass or pI. Therefore, the presence of a D-amino acid residue in a peptide chain is generally transparent to classical methods of peptide analysis (electrophoresis, chromatography, mass spectrometry, molecular biology). In this article, we will review the various experimental strategies and analytical techniques, which have been used to characterize and to study D-amino acid containing peptides in crustaceans.

Molecular cloning of a putative crustacean hyperglycemic hormone (CHH) isoform from extra-eyestalk tissue of the blue crab (Callinectes sapidus), and determination of temporal and spatial patterns of CHH gene expression

Crustacean hyperglycemic hormone (CHH) is a polypeptide neurohormone involved in regulation of multiple physiological processes. We report here the cloning from thoracic ganglia of the blue crab (Callinectes sapidus) a cDNA (CsCHH-2) encoding a putative CHH isoform (CsCHH-2). CsCHH-2 is structurally similar to a putative preproCHH (CsCHH-1) previously cloned from eyestalk ganglia of C. sapidus. The two preprohormones possess an identical signal peptide and CHH precursor related peptide, but differ in the mature CHH polypeptide. An analysis by RT-PCR of the tissue distribution of CsCHH-1 and CsCHH-2 revealed the former is restricted to eyestalk neural ganglia, while the latter is widely distributed among tissues. The type of CHH transcript present in eyestalk and thoracic ganglia did not vary as a function of the molt cycle. An assessment of transcript abundance in tissues of intermolt crabs showed the abundance of the CsCHH-1 transcript in eyestalk ganglia far exceeds the abundance of the CsCHH-2 transcript in extra-eyestalk tissue. An assessment of transcript abundance during a molt cycle showed CsCHH-1 transcript abundance in eyestalk ganglia was low during intermolt, rose during premolt, reaching a peak in D(3), then fell prior to molting, and remained low during postmolt. By contrast, CsCHH-2 transcript abundance in thoracic ganglia was low during intermolt, rose sharply during D(2), then dropped in D(3) and remained low during postmolt. The results are consistent with the hypothesis that CsCHH-1 and CsCHH-2 differ with respect to physiological function.

Molecular and cellular specificity of post-translational aminoacyl isomerization in the crustacean hyperglycaemic hormone family

D-aminoacyl residues have been detected in various animal peptides from several taxa, especially vertebrates and arthropods. This unusual polymorphism was shown to occur in isoforms of the crustacean hyperglycaemic hormone (CHH) of the American lobster because a D-phenylalanyl residue was found in position 3 of the sequence (CHH and D-Phe3 CHH). In the present study, we report the detailed strategy used to characterize, in the lobster neuroendocrine system, isomers of another member of the CHH family, vitellogenesis inhibiting hormone (VIH). We have demonstrated that the fourth residue is either an L- or a D- tryptophanyl residue (VIH and D-Trp4 VIH). Furthermore, use of antisera specifically recognizing the epimers of CHH and VIH reveals that aminoacyl isomerization occurs in specialized cells of the X organ-sinus gland neurosecretory system and that the D-forms of the two neuropeptides are not only present in the same cells, but, importantly, also are co-packaged within the same secretory vesicles.

The specific binding sites of eyestalk- and pericardial organ-crustacean hyperglycaemic hormones (CHHs) in multiple tissues of the blue crab, Callinectes sapidus

Crustacean hyperglycaemic hormone from the pericardial organ (PO-CHH) is a CHH-related neuropeptide but its function and target tissues are not known in crustaceans. To investigate this issue, we employed radiolabelled ligand binding and cGMP assays, using eyestalk-CHH (ES-CHH) as a reference neuropeptide. The membranes were prepared from various tissues of Callinectes sapidus: hepatopancreas, hindgut, midgut, gills, heart, abdominal muscles and scaphognathites. Like ES-CHH, recombinant PO-CHH (rPO-CHH) specifically bound to the membranes of scaphognathites=abdominal muscles>midgut>gills> heart>hindgut and hepatopancreas (list order corresponds to the number of binding sites). The specific binding sites of (125)I-ES-CHH in hepatopancreas and gills were saturable and displaceable. The abdominal muscle membrane binding sites were specific and saturable to both CHHs. These binding sites were displaced by homologous neuropeptides, but poorly displaced by the heterologous counterpart. As for the second messenger, the expected increment (3- to >20-fold) in the amount of cGMP produced by ES-CHH was noted in most tissues tested except midgut. Recombinant PO-CHH increased cGMP production 1.5- to 4-fold in scaphognathites, heart, midgut, hindgut and abdominal muscles. The results obtained from the binding study suggest that PO-CHH also has multiple target tissues of which abdominal muscles and scaphognathites are the primary ones. The differences in the primary amino acid sequences of PO-CHH and ES-CHH, particularly in the C-terminal region and in the amidation at C-terminus, may contribute to the truncated responses of hyperglycaemia, cGMP stimulation and binding affinity.

Molecular cloning and differential expression pattern of two structural variants of the crustacean hyperglycemic hormone family from the mud crab Scylla olivacea

Two full-length cDNA sequences encoding a crustacean hyperglycemic hormone (CHH) precursor were cloned from tissues of the mud crab Scylla olivacea. Sco-CHH (S. olivacea CHH) was cloned from eyestalk ganglia, whereas Sco-CHH-L (S. olivacea CHH-like peptide) was cloned from extra-eyestalk tissues (pericardial organ and thoracic ganglia). Each conceptually translated precursor is expected to be processed into a signal peptide, a CHH precursor-related peptide (CPRP), and a mature CHH or CHH-like peptide. The two precursors are identical in amino acid sequence through the 40th residue of the mature peptide, but different from each other substantially in the C-terminus. Both CHH variants contain the six highly conserved cysteine residues characteristic of the CHH family peptides, and share higher sequence identities with other brachyuran CHH sequences than with those of other taxonomic groups. As determined by reverse transcription-polymerase chain reaction (RT-PCR), the transcripts of Sco-CHH and Sco-CHH-L were present in eyestalk ganglia and several extra-eyestalk tissues (the thoracic ganglia, pericardial organ, brain, circumesophageal connectives, and gut). Sco-CHH was the predominant form in eyestalk ganglia, while Sco-CHH-L was the predominant form in several extra-eyestalk tissues. Neither transcript was expressed in the muscle, hepatopancreas, ovary, testis, heart, or gill. Antisera were raised against synthetic peptides corresponding to a stretch of sequence-specific to the C-terminus of Sco-CHH or Sco-CHH-L. Western blot analyses of tissues expressing Sco-CHH and Sco-CHH-L detected a Sco-CHH immunoreactive protein in the sinus gland, and a Sco-CHH-L immunoreactive protein in the pericardial organ. Immunohistochemical analyses of the eyestalk ganglia localized both Sco-CHH and Sco-CHH-L immunoreactivity to the sinus gland, and only Sco-CHH immunoreactivity to the X-organ somata; analyses of the pericardial organs also localized both Sco-CHH and Sco-CHH-L immunoreactivity to the anterior and posterior bars, as well as to longitudinal trunks joining the two bars. The combined data provided supporting evidence that Sco-CHH and Sco-CHH-L are co-localized in the same tissue.

Functional studies of crustacean hyperglycemic hormones (CHHs) of the blue crab, Callinectes sapidus - the expression and release of CHH in eyestalk and pericardial organ in response to environmental stress

The rapid increase in the number of putative cDNA sequences encoding crustacean hyperglycemic hormone (CHH) family in various tissues [either from the eyestalk (ES) or elsewhere] underscores a need to identify the corresponding neuropeptides in relevant tissues. Moreover, the presence of provided structural CHH implies the level of the complexity of physiological regulation in crustaceans. Much less is known of the functions of non-ES CHH than of those of its counterpart present in ESs. In the blue crab, Callinectes sapidus, we know little of CHH involvement in response to the stressful conditions that naturally occur in Chesapeake Bay. We have identified two isoforms of CHH neuropeptide in the sinus gland of the ES and isolated a full-length cDNA encoding CHH from the pericardial organ (PO). The functions of ES-CHH and PO-CHH in this species were studied with regard to expression and release in response to stressful episodes: hypoxia, emersion, and temperatures. Animals exposed to hypoxic conditions responded with concomitant release of both CHHs. In contrast, the mRNA transcripts encoding two CHHs were differentially regulated: PO-CHH increased, whereas ES-CHH decreased. This result suggests a possible differential regulation of transcription of these CHHs.

Crustacean hyperglycemic hormone from the tropical land crab, Gecarcinus lateralis: cloning, isoforms, and tissue expression

Crustacean hyperglycemic hormone (CHH) regulates carbohydrate metabolism, molting, and ion and water transport. cDNAs encoding four CHH isoforms (designated EG-CHH-A, -B, -C, and -D) were cloned from eyestalk ganglia (EG) from land crab, Gecarcinus lateralis. The isoforms differed in the 3' region of the open reading frame and/or the length of the 3' untranslated region. All encoded essentially identical preprohormones containing a 28-amino acid (aa) signal peptide, a 42-aa precursor related peptide and a 72-aa mature CHH. All deduced aa sequences had the six cysteines, two arginines, one aspartate, one phenylalanine, and one arginine originally identified as characteristic of this neuropeptide family. There was a single aa difference between the EG-CHH-D mature hormone and the other three isoforms. The EG-CHH isoforms were expressed in EG, hindgut, and thoracic ganglion. A fifth CHH isoform, designated pericardial organ (PO)-CHH, was similar to the PO-CHH isoform described in green crab, Carcinus maenas. It was expressed in hindgut and testis, but not in eyestalk ganglia; its expression in PO was not determined. The deduced aa sequence of the PO-CHH was identical to that of the EG-CHH isoforms through aa #40 of the mature peptide. The divergent aa sequence between positions #41 and #73 was encoded by an insertion of a 111-bp sequence absent in EG-CHH cDNAs. The data suggest that EG-CHH and PO-CHH isoforms are generated by alternative splicing of at least two CHH genes.

Crustacean hyperglycemic and vitellogenesis-inhibiting hormones in the lobster Homarus gammarus

Crustacean hyperglycemic hormone (CHH) and vitellogenesis-inhibiting hormone (VIH), produced by the X organ-sinus gland neurosecretory complex, belong to a peptide group referred to as the CHH family, which is widely distributed in arthropods. In this study, genetic variants and post-translationally modified isoforms of CHH and VIH were characterized in the European lobster Homarus gammarus. With the use of RP-HPLC and ELISA with specific antibodies that discriminate between stereoisomers of CHH and VIH, two groups of CHH-immunoreactive peaks were characterized from HPLC fractions of sinus gland extract (CHH A and CHH B); each group contained two variants (CHH and D-Phe3CHH). In the same way, two VIH-immunoreactive peaks (VIH and D-Trp4VIH) were demonstrated in HPLC fractions from sinus gland extract. The masses of these different neuropeptides were determined by FT-ICR MS: CHH A and CHH B spectra exhibited monoisotopic ions at 8557.05 Da and 8527.04 Da, respectively, and both VIH isomers displayed an m/z value of 9129.19 Da. Two full-length cDNAs encoding preprohomones of CHH A and CHH B and only one cDNA for VIH precursor were cloned and sequenced from X organ RNA. Comparison of CHH sequences between European lobster and other Astacoidea suggests that the most hydrophobic form appeared first during crustacean evolution.

Biochemical and functional aspects of crustacean hyperglycemic hormone in decapod crustaceans: review and update

In crustaceans, neuroendocrine centers are located in different structures of the nervous system. One of these structures, the X-organ-sinus gland complex of the eyestalk, produces several neuropeptides that belong to the two main functionally different families: firstly, the chromatophorotropins, and secondly, a large family comprising various closely related peptides, commonly named CHH/MIH/GIH family. This review updates some aspects of the structural, biochemical and functional properties of the main hyperglycemic neuropeptide of this family, the crustacean hyperglycemic hormone (CHH). The first part of this work is a survey of the neuroendocrine system that produces the neurohormones of the CHH/MIH/GIH family, focusing on recent reports that propose new possible neuroendocrine loci of CHH production, secondly we revise general aspects of the CHH biochemical, and structural characteristics and thirdly, we present a review of the role of CHH in the regulation of several physiological processes of crustaceans as well as new reports on the ontogenetic aspects of CHH. The review is centered only on one group of malacostracan crustaceans, the Decapoda.

Dynamics of in vivo release of molt-inhibiting hormone and crustacean hyperglycemic hormone in the shore crab, Carcinus maenas

Very little is known regarding the release patterns or circulating titers of neuropeptides in crustaceans, in particular those concerned with regulation of molting hormone (ecdysteroid) synthesis, molt-inhibiting hormone (MIH), and crustacean hyperglycemic hormone (CHH), which is also an adaptive hormone, centrally important in carbohydrate metabolism. Furthermore, the currently accepted model of molt control is founded on an untested hypothesis suggesting that molting can proceed only after decline in MIH titer. Accordingly, we measured simultaneous circulating neuropeptide profiles for both MIH and CHH by RIA of purified hemolymph during the molt cycle at fine temporal scale during day/night cycles and seasonally. For CHH we additionally determined release patterns after physiologically relevant stress. Results show that both hormones are released exclusively and episodically, rather than continuously, with notably short half-lives in circulation, suggesting dynamic and short-lived variations in levels of both hormones. During the molt cycle, there are no overt changes in MIH titer, except a massive and unprecedented increase in MIH during late premolt, just before ecdysis. The function of this hormone surge is unknown. Treatment with various stressors (hypoxia, temperature shock) showed that CHH release occurs extremely rapidly, within minutes of stress. Release of CHH after stressful episodes during premolt (when gut endocrine cells synthesize large quantities of CHH) is exclusively from the sinus gland: CHH from the gut is never involved in the stress response. The results show a hitherto unsuspected dynamism in release of MIH and CHH and suggest that currently accepted models of molt control must be reconsidered.

Tissue distribution, subcellular localization and endocrine disruption patterns induced by Cr and Mn in the crab Ucides cordatus

The essential trace elements Cr and Mn are toxic at high concentrations and information about low concentration is insufficient in the literature. In polluted mangroves, the crab Ucides cordatus can represent a useful tool to assess information on the potential impact of trace elements like Cr and Mn on the environment, since this species is comestible and thus, commercially negotiated. Therefore, U. cordatus crabs were exposed in vivo to different concentrations of Cr and Mn solved in seawater and had their tissue distribution and subcellular deposits evaluated. The gill, hepatopancreas and muscle concentrations were determined by atomic absorption spectroscopy and the results showed that Cr and Mn presented the highest values in the gills rather than in the hepatopancreas and muscular tissue. Electron microscopy and analytical X-ray microanalysis revealed Cr precipitates on the gill surface, co-localized with epiphyte bacteria. In addition, since Cr and Mn did not equally accumulate in most of the tissues studied, glycemic rate of animals, which received injections of extracts of eyestalks of the contaminated crabs, were measured in order to evaluate whether the studied concentrations of Cr and Mn could produce any metabolic alteration. The results indicated that extracts of the eyestalks of crabs submitted to Cr and Mn salts and injected into normal crabs markedly influenced crustacean hyperglycemic hormone synthesis and/or release. The results are discussed with respect to sensitivity of the employed methods and the possible significance of the concentrations of Cr and Mn in the organisms.

Occurrence of L- and D-crustacean hyperglycemic hormone isoforms in the eyestalk X-organ/sinus gland complex during the ontogeny of the crayfish Astacus leptodactylus

We studied the ontogeny of the eyestalk structure and of the L-CHH and d-Phe3-CHH synthesis in the X-organ/sinus gland (XO/SG) complex by light microscopy and immunocytochemistry in the freshwater crustacean Astacus leptodactylus. The optic ganglia start to differentiate in embryos at EI 190 microm (EI: eye index; close to 410 microm at hatching). At EI 270 microm, the three medullae (externa, interna, and terminalis) and the lamina ganglionaris are present and are organized as in the adult eyestalk. The L-CHH was localized in perikarya of neuroendocrine cells, in their tracts, and in SG from the metanauplius stage to the adult. The d-Phe3-CHH was visualized in XO perikarya, in their tracts and in SG of embryos from EI 350 microm and in all later studied stages. Co-localization of both CHH stereoisomers always occurred in the d-Phe3-CHH-producing cells. These results show that the synthesis of CHH enantiomers starts during the embryonic life in A. leptodactylus, and that the d-isomer is synthesized later than its L-counterpart. We discuss the post-translational isomerization as a way to generate hormonal diversity and the putative relation between d-Phe3-CHH synthesis and the ability to osmoregulate, occurring late during the embryonic life of Astacus leptodactylus.

Posttranslational isomerization of a neuropeptide in crustacean neurosecretory cells studied by ultrastructural immunocytochemistry

Isomerization of the third amino acid residue (a phenylalanine) of crustacean hyperglycemic hormone (CHH) has been previously reported to occur as a late step of hormone precursor maturation in a few neurosecretory cells in the X-organ-sinus gland complex of the crayfish Orconectes limosus. In the present report, using conformation-specific antisera combined with immunogold labeling, we have studied, at the ultrastructural level, the distribution of L- and D-CHH immunoreactivity in CHH-secreting cells of the crayfish Astacus leptodactylus. Two CHH-secreting cell populations were observed, the first one (L-cells), the most numerous, exhibited only labeling for L-CHH. In the second one (D-cells), four secretory granule populations were distinguished according to their labeling: unlabeled, either L- or D- exclusively or both L- and D-granules. Labeling quantification by image analysis in D-cells showed a marked increase in D-labeling from the cell body to the axon terminal. However some L- and mixed granules remain in axon terminals. Our results demonstrate that Phe3 isomerization of CHH occurs within the secretory granules of specialized neurosecretory cells and progresses as the granules migrate along the axonal tract. The observation that not all the CHH synthesized is isomerized, and the great variability in the proportion of L- and D-immunoreactivity in granules in every cell region may suggest an heterogeneous distribution of the putative enzyme involved in Phe3 isomerization, a peptide isomerase, within the secretory pathway.

Two genetic variants of the crustacean hyperglycemic hormone (CHH) from the Australian crayfish, Cherax destructor: detection of chiral isoforms due to posttranslational modification

From sinus glands of the Australian crayfish Cherax destructor, two genetic variants of the crustacean hyperglycemic hormone (CHH) were isolated by HPLC and fully characterized by mass spectrometry and Edman sequencing. Both CHH A (8350.38 Da) and CHH B (8370.34 Da) consist of 72 amino acid residues, with pyroGlu as N-terminus and an amidated (Val-NH2) C-terminus. They differ in 14 residues (81% identity). Both sequences are significantly different from those of the hitherto known three CHHs of Astacoidea species (Northern hemisphere crayfish), which among themselves are extremely conserved. This may reflect the long, separate evolution of the Astacoidea lineage and the Parastacoidea (Southern hemisphere crayfish) lineage, to which Cherax belongs. CHH A and CHH B genes are expressed at comparable levels, as indicated by the similar amounts of mature peptides in the sinus gland. In addition to each of the major peptides, which share the identical N-terminal tripeptide pyroGlu-Val-L-Phe, one chiral isoform containing pyroGlu-Val-D-Phe was identified. Compared to the main peptides, the amounts of the D-isoforms are lower, but significant, amounting to 30-40% of L-isoforms. These results demonstrate that two genes can give rise to a total of four different peptides in the secretory terminals of the sinus gland. All peptides gave a highly significant hyperglycemic in vivo response in C. destructor.

Moult cycle-related changes in biological activity of moult-inhibiting hormone (MIH) and crustacean hyperglycaemic hormone (CHH) in the crab, Carcinus maenas. From target to transcript

The currently accepted model of moult control in crustaceans relies entirely on the hypothesis that moult-inhibiting hormone (MIH) and crustacean hyperglycaemic hormone (CHH) repress ecdysteroid synthesis of the target tissue (Y-organ) only during intermoult, and that changes in synthesis and/or release of these neurohormones are central to moult control. To further refine this model, we investigated the biological activities of these neuropeptides in the crab Carcinus maenas, at the target tissue, receptor and cellular level by bioassay (inhibition of ecdysteroid synthesis), radioligand (receptor) binding assays, and second messenger (cGMP) assays, at defined stages of the moult cycle. To investigate possible moult cycle-related changes in neuropeptide biosynthesis, steady-state transcript levels of both neuropeptide mRNAs were measured by quantitative RT-PCR, and stored neuropeptide levels in the sinus gland were quantified during intermoult and premoult. The results show that the most important level of moult control lies within the signalling machinery of the target tissue, that expression and biosynthesis of both neuropeptides is constant during the moult cycle, and are not central to the currently accepted model of moult control.

Enkephalinergic control of the secretory activity of neurons producing stereoisomers of crustacean hyperglycemic hormone in the eyestalk of the crayfish Orconectes limosus

A subgroup of neurons in the classical X organ sinus gland neuroendocrine system of the crayfish (Orconectes limosus) eyestalk produces two chiral forms of the crustacean hyperglycemic hormone (CHH) in two different types of neurons: CHH in 22 cells and D Phe(3) CHH in eight cells. Previous reports have demonstrated that release of CHH from the sinus gland is inhibited by enkephalins. Here, we have addressed the questions of 1) whether this inhibition affects one or both types of CHH neurons, 2) where the site of enkephalinergic control of CHH and/or D Phe(3) CHH is, and 3) whether the inhibitory effect is due to direct or indirect interactions of enkephalinergic neurons with CHH cells. In vitro incubations of neurosecretory complexes followed by immunoassays of CHH isoforms indicated that both methionine and leucine enkephalins inhibit release of the two CHH isoforms from crayfish eyestalks, by a receptor mediated process. Whole mount double or triple immunofluorescence labelings combined with confocal microscopy revealed enkephalin immunostaining in all neuropils of the eyestalk, except in the sinus gland. Virtual thin confocal sections showed many close appositions between terminals of enkephalinergic neurons and dendritic arborizations of specific CHH immunoreactive cells in the medulla terminalis neuropil. This provides the first evidence for direct inputs from enkephalinergic neurons into dendrites of both CHH cell types, which suggests that enkephalins inhibit release of both CHH isoforms via synaptic contacts.

Structure and phylogeny of the crustacean hyperglycemic hormone and its precursor from a hydrothermal vent crustacean: the crab Bythograea thermydron

The structure of a well-known neurohormone involved in homeostasis regulation and stress response, the crustacean hyperglycemic hormone, was investigated in the deep-sea hydrothermal vent crab Bythograea thermydron. The neuropeptide was isolated from neurohemal organs (sinus glands) and its biological activity checked using an homologous bioassay. Partial amino acid sequence was established by a combination of Edman chemistry and mass spectrometry. Then, the sequence of the cDNA encoding the hormone precursor was determined. The preprohormone is composed of a 29 amino acid signal peptide, followed by a 41 amino acid associated peptide flanking the 72 amino acid hyperglycemic hormone. Comparison of these data with other known crab hyperglycemic hormone and prohormone sequences was performed using phylogenetic analysis methods.

L to D amino acid isomerization in a peptide hormone is a late post-translational event occurring in specialized neurosecretory cells

Modification of the chirality of a single amino acid residue within a peptide chain appears to be novel additional mechanism leading to structural and functional diversification of eukaryotic bioactive peptides. This phenomenon has been studied at the cellular level in a neuroendocrine organ which elaborates a mixture of diastereoisomers of a 72-residue neuropeptide, crustacean hyperglycemic hormone. For the first time, amino acid isomerization has been shown to occur in the perikarya of fully specialized neurosecretory cells, as a late step of the maturation of the hyperglycemic hormone precursor and after propeptide cleavage. The specificity and efficiency of this phenomenon indicates the existence of a new enzyme family involved in the biogenesis of peptide hormones.