Structural and functional characterization of an egg-laying hormone signaling system in a lophotrochozoan - The pacific oyster (Crassostrea gigas)

The egg-laying hormones (ELHs) of gastropod mollusks were characterized more than forty years ago. Yet, they have remained little explored in other mollusks. To gain insights into the functionality of the ELH signaling system in a bivalve mollusk - the oyster Crassostrea gigas, this study investigates the processing of its ELH precursor (Cragi-ELH) by mass spectrometry. Some of the ELH mature peptides identified in this study were subsequently investigated by nuclear magnetic resonance and shown to adopt an extended alpha-helix structure in a micellar medium mimicking the plasma membrane. To further characterize the ELH signaling system in C. gigas, a G protein-coupled receptor phylogenetically related to ecdysozoan diuretic hormone DH44 and corticotropin-releasing hormone (CRH) receptors named Cragi-ELHR was also characterized functionally and shown to be specifically activated by the two predicted mature ELH peptides and their N-terminal fragments. Both Cragi-ELH and Cragi-ELHR encoding genes were mostly expressed in the visceral ganglia (VG). Cragi-ELH expression was significantly increased in the VG of both fully mature male and female oysters at the spawning stage. When the oysters were submitted to a nutritional or hyposaline stress, no change in the expression of the ligand or receptor genes was recorded, except for Cragi-ELHR only during a mild acclimation episode to brackish water. These results suggest a role of Cragi-ELH signaling in the regulation of reproduction but not in mediating the stress response in our experimental conditions.

The presence of abalone egg-laying hormone-like peptide in the central nervous system and ovary of the Spotted Babylon, Babylonia areolata

Recently, the neuronal classification of the ivory shell Spotted Babylon, Babylonia areolata, was readily demonstrated. Regarding its importance as marine economic molluscan species, the attempt to understand the neuroendocrine regulation is necessary. This study firstly demonstrated the neurosecretory cells as well as the existence and distribution of the egg-laying hormone (ELH)-like peptide in the central nervous system (CNS) and ovary of the B. areolata. The neurosecretory cell was characterized by the cytoplasmic purple dot-like structure as stained by the Gomori's paraldehyde fuchsin. Using the anti-abalone (a) ELH, we detected the aELH-like-peptide in neurons (Nr) and neurosecretory cells (Ns) of all ganglia including the cerebral, pleural, parietal, pedal and buccal ganglia. The aELH-like peptide was also present in the neuropil of each. It was noted that not all Ns presented the aELH-like peptide. In the ovary, the aELH-like peptide was slightly detected in early developing oocytes and strongly detected in late developing oocytes and follicular cells. This study firstly reported the evidence of ELH-like peptide in the CNS and ovary of the B. areolata. The molecular cloning as well as to investigate the function of ELH in this species is needed as it will be beneficial for future applications in aquaculture.

Reproductive neuropeptides that stimulate spawning in the Sydney Rock Oyster (Saccostrea glomerata)

The Sydney Rock Oyster, Saccostrea glomerata, is a socioeconomically important species in Australia, yet little is known about the molecular mechanism that regulates its reproduction. To address this gap, we have performed a combination of high throughput transcriptomic and peptidomic analysis, to identify genes and neuropeptides that are expressed in the key regulatory tissues of S. glomerata; the visceral ganglia and gonads. Neuropeptides are known to encompass a diverse class of peptide messengers that play functional roles in many aspects of an animal's life, including reproduction. Approximately 28 neuropeptide genes were identified, primarily within the visceral ganglia transcriptome, that encode precursor proteins containing numerous neuropeptides; some were confirmed through mass spectral peptidomics analysis of the visceral ganglia. Of those, 28 bioactive neuropeptides were synthesized, and then tested for their capacity to induce gonad development and spawning in S. glomerata. Egg laying hormone, gonadotropin-releasing hormone, APGWamide, buccalin, CCAP and LFRFamide were neuropeptides found to trigger spawning in ripe animals. Additional testing of APGWa and buccalin demonstrated their capacity to advance conditioning and gonadal maturation. In summary, our analysis of S. glomerata has identified neuropeptides that can influence the reproductive cycle of this species, specifically by accelerating gonadal maturation and triggering spawning. Other molluscan neuropeptides identified in this study will enable further research into understanding the neuroendocrinology of oysters, which may benefit their cultivation.