Metamorphosis inHydractinia: Studies with activators and inhibitors aiming at protein kinase C and potassium channels

Signalling molecules inducing metamorphosis in marine organisms

Covering: findings from early 1980s until early 2022Microbial-derived cues of marine biofilms induce settlement and metamorphosis of marine organisms, a process responsible for the emergence of diverse flora and fauna in marine habitats. Although this phenomenon is known for more than 80 years, the research field has only recently gained much momentum. Here, we summarize the currently existing biochemical and microbial knowledge about microbial signalling molecules, con-specific signals, and synthetic compounds that induce or prevent recruitment, settlement, and metamorphosis in invertebrate larvae. We discuss the possible modes of action and conclude with perspectives for future research directions in the field of marine chemical ecology.

The Influence of Bacteria on Animal Metamorphosis

The swimming larvae of many marine animals identify a location on the seafloor to settle and undergo metamorphosis based on the presence of specific surface-bound bacteria. While bacteria-stimulated metamorphosis underpins processes such as the fouling of ship hulls, animal development in aquaculture, and the recruitment of new animals to coral reef ecosystems, little is known about the mechanisms governing this microbe-animal interaction. Here we review what is known and what we hope to learn about how bacteria and the factors they produce stimulate animal metamorphosis. With a few emerging model systems, including the tubeworm Hydroides elegans, corals, and the hydrozoan Hydractinia, we have begun to identify bacterial cues that stimulate animal metamorphosis and test hypotheses addressing their mechanisms of action. By understanding the mechanisms by which bacteria promote animal metamorphosis, we begin to illustrate how, and explore why, the developmental decision of metamorphosis relies on cues from environmental bacteria.

Warm temperature alters the chemical cue preference of Acropora tenuis and Heliopora coerulea larvae

Larvae released into the water column rely on chemical cues from the benthos for successful settlement. However, larval preference for substrates may be affected by rising seawater temperature brought about by global climate change. In this study, we examined the effect of elevated temperature on chemical cue preference by larvae of the scleractinian coral, Acropora tenuis, and the octocoral, Heliopora coerulea, collected from northwestern Philippines. At ambient temperature (28 °C), both H. coerulea and A. tenuis larvae showed preference for substrates containing either crustose coralline algae or crude ethanolic extracts from conspecific or congeneric corals. In contrast, at higher temperature (30 °C), greater preference was shown for substrates containing the crude extract from conspecific or congeneric corals. These results demonstrate that elevated temperature can change larval substrate preference, which will have downstream impacts on crucial biological processes, such as larval settlement and recruitment.

Molecular characterization of larval development from fertilization to metamorphosis in a reef-building coral

BACKGROUND

Molecular mechanisms underlying coral larval competence, the ability of larvae to respond to settlement cues, determine their dispersal potential and are potential targets of natural selection. Here, we profiled competence, fluorescence and genome-wide gene expression in embryos and larvae of the reef-building coral Acropora millepora daily throughout 12 days post-fertilization.

RESULTS

Gene expression associated with competence was positively correlated with transcriptomic response to the natural settlement cue, confirming that mature coral larvae are "primed" for settlement. Rise of competence through development was accompanied by up-regulation of sensory and signal transduction genes such as ion channels, genes involved in neuropeptide signaling, and G-protein coupled receptor (GPCRs). A drug screen targeting components of GPCR signaling pathways confirmed a role in larval settlement behavior and metamorphosis.

CONCLUSIONS

These results gives insight into the molecular complexity underlying these transitions and reveals receptors and pathways that, if altered by changing environments, could affect dispersal capabilities of reef-building corals. In addition, this dataset provides a toolkit for asking broad questions about sensory capacity in multicellular animals and the evolution of development.

Cnidarian chemical neurotransmission, an updated overview

The ultrastructural, histochemical, immunocytochemical, biochemical, molecular, behavioral and physiological evidence for non-peptidergic and peptidergic chemical neurotransmission in the Anthozoa, Hydrozoa, Scyphozoa and Cubozoa is surveyed. With the possible exception of data for the catecholamines and peptides in some animals, the set of cumulative data - the evidence from all methodologies - is incomplete. Taken together, the evidence from all experimental approaches suggests that both classical fast (acetylcholine, glutamate, GABA, glycine) and slow (catecholamines and serotonin) transmitters, as well as neuropeptides, are involved in cnidarian neurotransmission. Ultrastructural evidence for peptidergic, serotonergic, and catecholaminergic synaptic localization is available, but the presence of clear and dense-cored synaptic vesicles also suggests both fast and slow classical transmission. Immunocytochemical studies, in general, reveal a continuous, non-localized distribution of neuropeptides, suggesting a neuromodulatory role for them. Immunocytochemical and biochemical studies indicate the presence of glutamate, GABA, serotonin, catecholamines (and/or their receptors), RFamides, nitric oxide and eicosanoids in cnidarian neurons and tissues. Gene sequences for peptidergic preprohormones have been reported; putative gene homologies to receptor proteins for vertebrate transmitters have been found in Hydra. Behavioral and physiological studies implicate classical transmitters, neuropeptides, eicosanoids and nitric oxide in the coordination of the neuroeffector systems.

Metamorphosis in the Cnidaria

The free-living stages of sedentary organisms are an adaptation that enables immobile species to exploit scattered or transient ecological niches. In the Cnidaria the task of prospecting for and identifying a congenial habitat is consigned to tiny planula larvae or larva-like buds, stages that actually transform into the sessile polyp. However, the sensory equipment of these larvae does not qualify them to locate an appropriate habitat from a distance. They therefore depend on a hierarchy of key stimuli indicative of an environment that is congenial to them; this is exemplified by genera of the Anthozoa (Nematostella, Acropora), Scyphozoa (Cassiopea), and Hydrozoa (Coryne, Proboscidactyla, Hydractinia). In many instances the final stimulus that triggers settlement and metamorphosis derives from substrate-borne bacteria or other biogenic cues which can be explored by mechanochemical sensory cells. Upon stimulation, the sensory cells release, or cause the release of, internal signals such as neuropeptides that can spread throughout the body, triggering decomposition of the larval tissue and acquisition of an adult cellular inventory. Progenitor cells may be preprogrammed to adopt their new tasks quickly. Gregarious settlement favours the exchange of alleles, but also can be a cause of civil war. A rare and spatially restricted substrate must be defended. Cnidarians are able to discriminate between isogeneic and allogeneic members of a community, and may use particular nematocysts to eliminate allogeneic competitors. Paradigms for most of the issues addressed are provided by the hydroid genus Hydractinia.

Taurine in the marine hydrozoan Hydractinia echinata: stabilizer of the larval state?

Taurine (beta-aminoethane sulfonic acid) is present in high concentrations in tissue of planula larvae of the marine hydrozoan Hydractinia echinata. It has been proposed to function as a stabilizer of the larval state mainly because of the previous findings that larvae induced to undergo metamorphosis appeared to lose most of their taurine, and taurine added to the medium antagonizes metamorphosis. Release of taurine was assumed to be a necessary prerequisite for the onset of metamorphosis. The primary aim of the present study was to confirm this by determination of taurine release accompanying metamorphosis induction by inducers other than CsCl. However, a decrease of the larval tissue taurine content was not found, irrespective of schedule of treatment and the inducer applied. The cause for this difference from the preceding study could not be clarified. Taurine in the medium, even at low concentration, causes elevated tissue concentrations high enough to cause general adverse effects on cell physiology. In order to ascribe an alternative function to taurine in H. echinata variations of the free amino acid pool under osmotic stress were examined. The tissue concentration of beta-alanine strongly correlates with the salinity of the medium. Large amounts of gamma-aminobutyric acid (GABA) are present in animals adapted to high salinity. Taurine content appears not to depend on osmolarity of the medium. Nevertheless, taurine may constitute the foundation of the cellular organic osmolyte system of the H. echinata larva.

Serotonin plays an early role in the metamorphosis of the hydrozoan Phialidium gregarium

Hydrozoan larvae normally metamorphose in response to an obligate external environmental cue. Application of certain artificial chemical stimuli will also induce metamorphosis. These chemicals and their inhibitors have been used to define and order some of the signal transduction events involved in this process. Results from this study show that exogenous application of serotonin (5-HT) will induce metamorphosis and that 5-HT immunoreactive cells are present in larvae when they are competent to metamorphose. The 5-HT inhibitors ketanserin, clozapine, and 5,7-DHT prevent metamorphosis from occurring as a response to a natural inducing stimulus. Additionally, 5-HT signaling occurs prior to both an influx of external Ca2+ from seawater and activation of protein kinase C, two other steps in the metamorphic signal transduction pathway. The neuropeptide LWamide, previously shown to induce metamorphosis in a related hydrozoan, Hydractinia echinata, also induced metamorphosis in Phialidium. When larvae were cotreated with LWamide and the 5-HT antagonist ketanserin, settlement occurred but was not followed by polyp morphogenesis. These results are used to present a model for the action of 5-HT during metamorphosis in Phialidium gregarium.

Signal transduction in barnacle settlement: Calcium re-visited

The search for marine natural product antifoulants is being hampered by problems associated with conventional settlement assays. Yet it has been recognised that the study of how chemical cues are perceived by fouling organisms may offer clues to settlement inhibitors and may identify novel biochemical assays for antifoulants based on signal transduction pathways. Here the role of calcium in barnacle settlement is re-examined. A requirement for calcium in settlement of the cypris larva of Balanus amphitrite has been confirmed; settlement was inibited in low calcium, and calcium-free, seawater. Although 10 mM (19.27 mM) excess calcium had no effect and higher concentrations were inhibitory, a 5 mM excess stimulated settlement. Stimulation is proposed to be effected by an increase in intracellular calcium. The release of calcium from intracellular pools with thapsigargin (but not cyclopiazonic acid) induced settlement and an antagonist of intracellular calcium, TMB-8, generally inhibited settlement. Nevertheless, the calcium ionophore A23187 did not induce settlement at the concentrations tested. Consequently, the relative importance of external calcium and intracellular pools to increased intracellular calcium has yet to be determined. Pharmacological manipulations of calcium channels with organic and inorganic channel blockers strongly indicate calcium channel involvement in barnacle settlement. The data are summarised in an hypothetical scheme for signal transduction at settlement and are compared to those obtained for other marine invertebrate larvae.

Possible involvement of arachidonic acid and eicosanoids in metamorphic events in Hydractinia echinata (Coelenterata; Hydrozoa)

Upon induction of metamorphosis, larvae of the marine hydroid Hydractinia echinata release [14C]-arachidonic acid from previously labeled endogenous sources. The lipoxygenase inhibitors nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid inhibited metamorphosis induced by Cs+ and 1,2-sn-dioctanoylglycerol, whereas the inhibitors of cyclooxygenase, indomethacin, and acetylsalicylic acid were ineffective, suggesting a role for lipoxygenase metabolites of arachidonic acid in induction of metamorphosis. Lipoxygenase products in Hydractinia echinata were isolated and identified by gas chromatography/mass spectrometry. 8- and 12-HETE were the most abundant metabolites. In cytosolic fractions from larvae activity of an arachidonic acid metabolizing enzyme, presumably a lipoxygenase, was found. The metabolic product was identified by 1H-NMR and chiral phase HPLC as 8(R)-HETE. Its production was strongly inhibited by NDGA, but not by indomethacin.

Protein kinase C in hydrozoans: involvement in metamorphosis of Hydractinia and in pattern formation of Hydra

A wealth of information has suggested the involvement of protein kinase C (PKC) in metamorphosis of Hydractinia echinata and in pattern formation of Hydra magnipapillata. We have identified a Ca²⁺- and phospholipid-dependent kinase activity in extracts of both species. The enzyme was characterized as being similar to mammalian PKC by ion exchange chromatography. Gel filtration experiments revealed a molecular weight of about 70 kD. In phosphorylation assays of endogenous Hydractinia proteins, a protein with a molecular weight of 22.5 kD was found to be phoshorylated upon addition of phosphatidylserine. Bacterial induction of metamorphosis of Hydractinia echinata caused an increase in endogenous diacylglycerol, the physiological activator of PKC, suggesting that the bacterial inducer acts by activating receptor-regulated phospholipid metabolism. Exogenous diacylglycerol leads to membrane translocation of PKC, indicative of an activation. On the basis of our results and those of Freeman and Ridgway (1990) a model for the biochemical events during metamorphosis is presented.

Calcium control of metamorphosis in polychaete larvae

The importance of Ca2+ in the control of metamorphosis of a marine invertebrate larva was investigated. An excess of [Ca2+] in the external medium induced metamorphosis of Phragmatopoma californica (polychaete) larvae in a concentration-dependent manner. This effect is specific for calcium, and not simply the result of osmotic changes, as an excess of Mg2+ did not induce metamorphosis. Consistent with this finding, the calcium ionophore, A23187, also induced metamorphosis in a concentration-dependent manner. Paradoxically, however, the aromatic compounds diltiazem, verapamil, D600, and nifedipine, known to block Ca2+ channels in other systems, also induced metamorphosis. When exposed to diltiazem for only 20 h and subsequently washed free of this compound, 95% of the larvae metamorphosed and developed normally. Previous studies have demonstrated that the induction of metamorphosis in Phragmatopoma californica is controlled by chemosensory recognition of an exogenous morphogen and mediated by an excitatory pathway that involves adenyl cyclase and cyclic AMP. Because cellular excitation and cyclic AMP-dependent signal transduction generally involve the participation of calcium ion, the most parsimonious explanation for the results reported here include (1) direct control of the morphogenetic pathway by calcium ion, and (2) complexities of the calcium regulation of this process, or a functional similarity between the structurally related aromatic effectors tested and the natural inducer of metamorphosis.