Induction of Metamorphosis in the Marine GastropodIlyanassa obsoleta: 5HT, NO and Programmed Cell Death

Abstract

Abstract:The central nervous system (CNS) of a metamorphically competent larva of the caenogastropod Ilyanassa obsoleta contains a medial, unpaired apical ganglion (AG) of approximately 25 neurons that lies above the commissure connecting the paired cerebral ganglia. The AG, also known as the cephalic or apical sensory organ (ASO), contains numerous sensory neurons and innervates the ciliated velar lobes, the larval swimming and feeding structures. Before metamorphosis, the AG contains 5 serotonergic neurons and exogenous serotonin can induce metamorphosis in competent larvae. The AG appears to be a purely larval structure as it disappears within 3 days of metamorphic induction. In competent larvae, most neurons of the AG display nitric oxide synthase (NOS)-like immunoreactivity and inhibition of NOS activity can induce larval metamorphose. Because nitric oxide (NO) can prevent cells from undergoing apoptosis, a form of programmed cell death (PCD), we hypothesize that inhibition of NOS activity triggers the loss of the AG at the beginning of the metamorphic process. Within 24 hours of metamorphic induction, cellular changes that are typical of the early stages of PCD are visible in histological sections and results of a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay in metamorphosing larvae show AG nuclei containing fragmented DNA, supporting our hypothesis. Keywords: Apoptosis -caenogastropod -mollusc -nitric oxide -serotonin Article: INTRODUCTION Larval Ilyanassa obsoleta, like most marine molluscs, undergo a series of physiological and anatomical transformations at metamorphosis which allow them to begin their juvenile life history phase. The most obvious external change is the loss of the ciliated larval feeding organ or velum [39], which, in the laboratory, occurs with a delay of some 12-36 hours after exposure to an inducing substance. In addition to loss of the velum, within 48 hours of metamorphic induction, internal transformations include rearrangements within the digestive tract and nervous system [13,14,27]. However, until recently, few changes in larval physiology or morphology during the 12-36 hour delay period in I. obsoleta had been described. In this species, by about 83% of larval development [27,42], the CNS contains rudiments of all of the adult ganglia along with a medial, unpaired apical ganglion (AG). Typically, the larval AG innervates the muscular and ciliary components of the velar lobes [20, 29-31, 34, 35]. The AG is an outgrowth of the trochophore apical tuft and has long been postulated to have a sensory function [6,34,35,38]. Recent experiments on a nudibranch provide evidence that the AG can detect a metamorphic cue [17], but a review of literature about molluscan AGs suggests that they are sensorimotor, coordinating the functions of the velar lobes [34,35] and sensing inductive and other stimuli [34]. Currently, the AG appears to be the only part of the larval CNS that is lost at metamorphosis. Some evidence from I. obsoleta and other species suggests t...

The regulatory role of the NO/cGMP signal transduction cascade during larval attachment and metamorphosis of the barnacleBalanus (=Amphibalanus) amphitrite

“…For instance, Robertson and colleagues reported that NO regulates tactile learning (Robertson et al, 1994) and visual learning (Robertson et al, 1996) processes in Octopus vulgaris. It was also found that NO is involved in regulating neural transmissions to intestinal muscles of the common snail Helix lucorum (Röszer et al, 2004) and in the fine olfactory discrimination of the terrestrial slug Limax valentianus (Sakura et al, 2004), and, importantly, it was found that NO plays a crucial role in regulating metamorphosis of the marine snail Ilyanssa obsolete (Leise et al, 2004). In the present study, the involvement of NO and the associated cGMP signaling pathway during larval settlement of B. amphitrite were investigated.…”

Nitric oxide signaling differentially affects habitat choice by two larval morphs of the sea slugAlderia willowi: mechanistic insight into evolutionary transitions in dispersal strategies.

“…The neurotransmitters dopamine and serotonin typically have opposing effects on swimming and metamorphosis of molluscan larvae (Leise et al, 2004;Braubach et al, 2006;Yamamoto et al, 1999;Zega et al, 2005). Larvae were exposed to the dopamine precursor L-DOPA and to serotonin by diluting stock solutions to final concentrations of 10 ; assays were performed in the dark to prevent photochemical degradation of L-DOPA.…”

Diatom cultures exhibit differential effects on larval metamorphosis in the marine gastropod Ilyanassa obsoleta (Say)

“…Because exogenous 5-HT can elicit metamorphosis, presumably by mimicking the action of serotonergic neurons (Couper and Leise, 1996;Leise et al, 2001), we anticipate that 5-HT will potentiate larval responses to natural inducers. Conversely, because nitric oxide inhibits metamorphosis in competent larvae (Froggett and Leise, 1999;Leise et al, 2004;Hens et al, 2006), we expect this diatom inducer to antagonize this activity.…”

“…When larval metamorphosis is initiated by the reception of an appropriate pharmacological stimulus in the laboratory, the ensuing morphological and physiological transformations include a decrease in the expression of the gene coding for nitric oxide synthase, the enzyme that produces NO (Froggett and Leise, 1999;Leise et al, 2001Leise et al, , 2004Gifondorwa and Leise, 2006;Hens et al, 2006). Leise and colleagues also confirmed Levantine and Bonar's (1986) discovery that the classical neurotransmitter serotonin (5-HT) can induce metamorphosis in competent larvae, and they provided pharmacological data supporting the idea that exogenously applied 5-HT mimics the action of serotonergic neurons in the larval nervous system (Couper and Leise, 1996;Leise et al, 2001Leise et al, , 2004. Our studies of the cellular and physiological mechanisms that bring about metamorphosis in this species continue, but to understand the regulation of this process from an environmentally relevant viewpoint, we need specific knowledge of how I. obsoleta reacts to a natural inducer.…”