Effects of temperature on reproductive neuroendocrine function in Aplysia californica

Characterization and spatiotemporal expression of gonadotropin-releasing hormone in the Pacific abalone, Haliotis discus hannai

Gonadotropin-releasing hormone (GnRH) is a key neuropeptide regulating reproduction in humans and other vertebrates. Recently, GnRH-like cDNAs and peptides were reported in marine mollusks, implying that GnRH-mediated reproduction is an ancient neuroendocrine system that arose prior to the divergence of protostomes and deuterostomes. Here, we evaluated the reproductive control system mediated by GnRH in the Pacific abalone Haliotis discus hannai. We cloned a prepro-GnRH cDNA (Hdh-GnRH) from the pleural-pedal ganglion (PPG) in H. discus hannai, and analyzed its spatiotemporal gene expression pattern. The open reading frame of Hdh-GnRH encodes a protein of 101 amino acids, consisting of a signal peptide, a GnRH dodecapeptide, a cleavage site, and a GnRH-associated peptide. This structure and sequence are highly similar to GnRH-like peptides reported for mollusks and other invertebrates. Quantitative polymerase chain reaction demonstrated that Hdh-GnRH mRNA was more strongly expressed in the ganglions (PPG and cerebral ganglion [CG]) than in other tissues (gonads, gills, intestine, hemocytes, muscle, and mantle) in both sexes. In females, the expression levels of Hdh-GnRH mRNA in the PPG and branchial ganglion (BG) were significantly higher at the ripe and partial spent stages than at the early and late active stages. In males, Hdh-GnRH mRNA levels in the BG showed a significant increase in the partial spent stage. Unexpectedly, Hdh-GnRH levels in the CG were not significantly different among the examined stages in both sexes. These results suggest that Hdh-GnRH mRNA expression profiles in the BG and possibly the PPG are tightly correlated with abalone reproductive activities.

Localization and functional characterization of a novel adipokinetic hormone in the mollusk, Aplysia californica

Increasing evidence suggests that gonadotropin-releasing hormone (GnRH), corazonin, adipokinetic hormone (AKH), and red pigment-concentrating hormone all share common ancestry to form a GnRH superfamily. Despite the wide presence of these peptides in protostomes, their biological effects remain poorly characterized in many taxa. This study had three goals. First, we cloned the full-length sequence of a novel AKH, termed Aplysia-AKH, and examined its distribution in an opisthobranch mollusk, Aplysia californica. Second, we investigated in vivo biological effects of Aplysia-AKH. Lastly, we compared the effects of Aplysia-AKH to a related A. californica peptide, Aplysia-GnRH. Results suggest that Aplysia-AKH mRNA and peptide are localized exclusively in central tissues, with abdominal, cerebral, and pleural ganglia being the primary sites of Aplysia-AKH production. However, Aplysia-AKH-positive fibers were found in all central ganglia, suggesting diverse neuromodulatory roles. Injections of A. californica with Aplysia-AKH significantly inhibited feeding, reduced body mass, increased excretion of feces, and reduced gonadal mass and oocyte diameter. The in vivo effects of Aplysia-AKH differed substantially from Aplysia-GnRH. Overall, the distribution and biological effects of Aplysia-AKH suggest it has diverged functionally from Aplysia-GnRH over the course of evolution. Further, that both Aplysia-AKH and Aplysia-GnRH failed to activate reproduction suggest the critical role of GnRH as a reproductive activator may be a phenomenon unique to vertebrates.

Activity-Dependent Regulation of Neurohormone Synthesis and Its Impact on Reproductive Behavior in Aplysia1

The bag cell neurons (BCNs) of the mollusk Aplysia californica provide a simple model system for investigating cellular and molecular events regulating synthesis and secretion of a reproductive neuropeptide and their impact on physiology and behavior. The BCNs secrete a large amount of egg-laying hormone (ELH) in response to an electrical afterdischarge. The afterdischarge also triggers cellular and molecular events leading to upregulation of ELH biosynthesis to replenish the supply of releasable hormone that was lost because of secretion. In the present review, we discuss signal-transduction events that link membrane excitability to ELH biosynthesis. We present evidence that the afterdischarge stimulates ELH synthesis by upregulating translation of ELH mRNA rather than by activating ELH gene transcription. This increase in ELH synthesis is accompanied by a decrease in total protein synthesis, suggesting that the synthetic machinery is being funneled selectively toward ELH. We also discuss work showing that afterdischarge-induced ELH synthesis uses a novel mechanism of translation initiation, one involving a switch from cap-dependent to cap-independent translation initiation that activates an internal ribosome entry site (IRES) located in the 5'-untranslated region of ELH mRNA. The IRES-regulated translation provides a unique cellular mechanism to selectively upregulate synthesis of a critical reproductive hormone at the expense of nonessential proteins.

Regulation of seasonal reproduction in mollusks

Understanding the physiological basis of environmental regulation of reproduction at the cellular level has been difficult or unfeasible in vertebrate species because of the highly complex and diffuse nature of vertebrate neuroendocrine systems. This is not the case with the simple nervous system of mollusks in which reproductive neuroendocrine cells are often readily identifiable in living tissue. Given that there are mollusks that are seasonal breeders, that the neuroendocrine cells controlling reproduction have been identified in several molluskan species, that these neurons are conducive to cell physiological analysis, and that basic features of cell biology have been highly conserved between mammals and mollusks, it seems that the mollusk would provide an excellent model system to investigate cell-physiological events that mediate effects of environmental signals on reproduction. The purpose of this review is to explore this idea in three species in which the topic of the neural basis of seasonal reproduction has been studied: the giant garden slug Limax maximus, the freshwater pond snail Lymnaea stagnalis, and the marine snail Aplysia californica.

Proteolytic processing of the Aplysia egg-laying hormone prohormone

By using matrix-assisted laser desorption/ionization time-of-flight MS, individual peptidergic neurons from Aplysia are assayed. A semiquantitative method is developed for comparing single-cell profiles by using spectral normalization, and peptides are localized to specific cells by mass spectrometric cell mapping. In addition to all previously identified products of the egg-laying hormone (ELH) gene, other peptides are formed from proteolytic hydrolysis of Leu-Leu residues within ELH and acidic peptide (AP). AP exhibits further processing to yield AP1-20 and AP9-27. These peptides appear to be colocalized in vesicles with ELH, transported to specific neuronal targets, and released in a Ca2+-dependent manner. A differential peptide distribution is observed at a specific target cell, and a low-frequency variation of AP, [Thr21]AP, is detected in a single animal.

The fate of newly synthesized hormone from neuroendocrine cells of Aplysia

The neuroendocrine bag cells from Aplysia control reproductive functions through the regulated release of egg-laying hormone (ELH). The purposes of this study were to investigate (1) if synthesis of ELH in bag cells compensates for the release and degradation of the hormone, (2) the ratio of released to stored ELH, (3) the fate of newly synthesized ELH and its relative contribution to the overall pattern of ELH secretion, and (4) the consequences of blocking protein synthesis in bag cells on the pattern of ELH secretion and subsequent reproductive function. Excised bag cells were incubated with [3H]leucine to radiolabel newly synthesized ELH and were then electrically stimulated to trigger secretion of the hormone on 4 successive days, mimicking bag cell activities during the breeding season. Samples of bag cell secretion were collected; total (new and old) and radiolabeled (new) ELH levels were determined. The results showed that ELH synthesis could not keep up with the daily loss of ELH through release and degradation, but the ratio of released to stored ELH remained constant over the 4-day study. In addition, releasable ELH was secreted preferentially within 24 hr after it was synthesized. Blocking protein synthesis resulted in a 50% decrease in total ELH released, suggesting that newly synthesized ELH contributes to half of the total ELH secreted. Furthermore, this lowered level of total ELH, i.e., older ELH alone, was not sufficient to stimulate egg laying in Aplysia, indicating that the contribution of newly synthesized ELH was physiologically relevant.