Autoradiographic evidence that transport of newly synthesized neuropeptides is directed to release sites in the X-organ--sinus gland of Cardisoma carnifex

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.

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.

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 (CHH) is the major neuropeptide produced by the X-organ-sinus gland neurosecretory system of the crayfish, Orconectes limosus. This hormone is synthesized by two different cell types, as two isomers (CHH and D-Phe3-CHH) which display different activities The aim of this report is to analyze and compare the synthetic and secretory activities of these specialized cells. In vitro pulse-chase incubations and time-course experiments were conducted on isolated X-organ-sinus gland (XO-SG) complexes, followed by analysis of the labeled peptides. The different steps of the post-translational processing of the CHH precursor, including proteolytic cleavage of the propeptide, C-terminal amidation and N-terminal pyroglutamylation were characterized and the kinetics of CHHs maturation were estimated in the different parts of the neuroendocrine complex. Furthermore, synthesis of CHHs in XO-SG complexes and release in incubation media were investigated using combined HPLC/immunoassay. Under basal conditions, i.e. without stimulation, similar dynamics for both isomers were found and results indicate that newly synthesized CHHs are preferentially released.

Fine structure of the neurohemal sinus gland of the shore crab, Carcinus maenas, and immuno-electron-microscopic identification of neurosecretory endings according to their neuropeptide contents

The sinus gland of the shore crab, Carcinus maenas, is a compact assembly of interdigitating neurosecretory axon endings abutting upon the thin basal lamina of a central hemolymph lacuna. Four types of axon endings are distinguishable by the size distribution, shape, electron density and core structure of their neurosecretory granules. One additional type of axon ending is characterized by electron-lucent vacuoles and vesicles. The axon profiles are surrounded by astrocyte-like glial cells. Various fixations followed by epoxy- or Lowicryl-embedding were compared in order to optimize the preservation of the fine structure of the granule types and the antigenicity of their peptide hormone contents. By use of specific rabbit antisera, the crustacean hyperglycemic, molt-inhibiting, pigment-dispersing, and red-pigment-concentrating hormones were assigned to the four distinct granule types which showed no overlap of immunostaining. Epi-polarization microscopy and ultrathin section analysis of immunogold-stained Lowicryl-embedded specimens revealed that immunoreactivity to Leu-enkephalin and proctolin is co-localized with molt-inhibiting hormone immunoreactivity in the same type of granule. The size and core structure of the immunocytochemically identified granule types vary little with the different pretreatments but, in some cases, to a statistically significant extent. The present results are compared with those from earlier studies of sinus glands in different crustaceans. The methods of granule identification used in this study supplement the classical approach in granule typing; they are easier to perform and more reliable for the analysis of release phenomena in identified secretory neurons supplying the neurohemal sinus gland.