The fate of newly synthesized hormone from neuroendocrine cells of Aplysia

Gonadotropin-releasing hormone in invertebrates: structure, function, and evolution

Gonadotropin-releasing hormone (GnRH) is central to the initiation and maintenance of reproduction in vertebrates. GnRH is found in all major groups of Phylum Chordata, including the protochordates. Studies on functional and structural evolution of GnRH have, in the past, focused exclusively on chordates. However, the recent structural elucidation of an octopus GnRH-like molecule and increasing evidence that GnRH-like substances are present in multiple invertebrate phyla suggest GnRH is an ancient peptide that arose prior to the divergence of protostomes and deuterostomes. The extraordinary conservation of GnRH structure and function raises interesting questions regarding the functional role assumed by GnRH over the course of evolution. This review will focus on the current understanding of GnRH structure and function in non-chordate invertebrates. Special emphasis will be placed upon the possible and speculated functions of GnRH in mollusks.

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.

Regional differences in processing of locally translated prohormone in peptidergic neurons of Aplysia californica

Earlier work showed that cell bodies and neurites of the peptidergic bag cell neurons of Aplysia californica contain mRNA for egg-laying hormone. The purpose of the present study was to determine if egg-laying hormone synthesis and prohormone processing is similar in the pleurovisceral connective nerves (containing neurites of bag cell neurons) and the bag cell neuron clusters (containing both cell bodies and neurites of bag cell neurons). Initial experiments confirmed by RT-PCR and sequencing that egg-laying hormone mRNA was present in the pleurovisceral connective nerves. To investigate possible regional differences in translation of mRNA and prohormone processing, clusters were separated from connective nerves and newly synthesized egg-laying hormone-immunoreactive proteins were analyzed. Results showed that synthesis and processing of prohormone occurred in both the clusters and isolated connective nerves; however, the relative abundance of prohormone, processing intermediates, and egg-laying hormone was different. Pulse-chase experiments showed that prohormone was processed more slowly in the connective nerves than in the clusters. These results show that mRNA in isolated neural processes of neuroendocrine cells can be translated, and that the cellular machinery for protein synthesis is present, but processing of the ELH prohormone is significantly compromised.