Nikitin, M. A. | bioRxiv Science

bioRxiv2020

Action Potentials and Na+ voltage-gated ion channels in Placozoa

Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched animal lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals. HighlightsO_LIPlacozoans are the simplest known animals without recognized neurons and muscles C_LIO_LIWith only six morphological cell types, placozoans showed complex & rapid behaviors C_LIO_LISodium-dependent action potentials have been discovered in intact animals C_LIO_LIVoltage-gated sodium channels (Nav) in Placozoa support a rapid behavioral integration C_LIO_LIPlacozoans have more Nav channels that any studied invertebrate animal so far C_LIO_LIDiversification of Nav-channels highlight the unique evolution of these nerveless animals C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=157 SRC="FIGDIR/small/243113v2_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@4d9c5forg.highwire.dtl.DTLVardef@15539bforg.highwire.dtl.DTLVardef@4246faorg.highwire.dtl.DTLVardef@141e5fe_HPS_FORMAT_FIGEXP M_FIG C_FIG

evolutionary biology↗

bioRxiv2020

Nitric oxide (NO) signaling in Trichoplax and related species: Microchemical characterization and the lineage-specific diversification

Nitric oxide (NO) is a free radical gaseous messenger with a broad distribution across the animal kingdom. However, the early evolution of nitric oxide-mediated signaling in animals is unclear due to limited information about prebilaterian metazoans such as placozoans. Here, we analyzed NO synthases (NOS) in four different species of placozoans (haplotypes H1, H2, H4, H13). In contrast to all other invertebrates studied, Hoilungia and Trichoplax have three distinct NOS genes, including PDZ domain-containing NOS. To characterize NOS activity in Trichoplax adhaerens, we used capillary electrophoresis for microchemical assays of NO-related metabolites. Specifically, we quantified nitrites (products of NO oxidation) and L-citrulline (co-product of NO synthesis from L-arginine), which were affected by NOS inhibitors confirming the presence of functional NOS. Next, using fluorescent single-molecule in situ hybridization, we showed that distinct NOSs are expressed in different subpopulations of cells, with a noticeable distribution close to the edge regions of Trichoplax. These data suggest the compartmentalized release of this messenger and a greater diversity of cell types in placozoans than anticipated. We also revealed a dramatic diversification of NO receptor machinery, including identification of both canonical and novel NIT-domain containing soluble guanylate cyclases as putative NO/nitrite/nitrate sensors. Thus, although Trichoplax is considered to be one of the morphologically simplest free-living animals, the complexity of NO-cGMP-mediated signaling is greater to those in vertebrates. This situation illuminates multiple lineage-specific diversifications of NOSs and NO/nitrite/nitrate sensors from the common ancestor of Metazoa. Short AbstractNitric oxide (NO) is a ubiquitous gaseous messenger, but we know little about its early evolution. Here, we analyzed NO synthases (NOS) in four different species of placozoans - one of the early-branching animal lineages. In contrast to other invertebrates studied, Trichoplax and Hoilungia have three distinct NOS genes, including PDZ domain-containing NOS. Using ultra-sensitive capillary electrophoresis assays, we quantified nitrites (products of NO oxidation) and L-citrulline (co-product of NO synthesis from L-arginine), which were affected by NOS inhibitors confirming the presence of functional enzymes in Trichoplax. Using fluorescent single-molecule in situ hybridization, we showed that distinct NOSs are expressed in different subpopulations of cells, with a noticeable distribution close to the edge regions of Trichoplax. These data suggest both the compartmentalized release of NO and a greater diversity of cell types in placozoans than anticipated. NO receptor machinery includes both canonical and novel NIT-domain containing soluble guanylate cyclases as putative NO/nitrite/nitrate sensors. Thus, although Trichoplax and Hoilungia exemplify the morphologically simplest free-living animals, the complexity of NO-cGMP-mediated signaling in Placozoa is greater to those in vertebrates. This situation illuminates multiple lineage-specific diversifications of NOSs and NO/nitrite/nitrate sensors from the common ancestor of Metazoa.

evolutionary biology↗