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Calcareous sponge genomes reveal complex evolution of α-carbonic anhydrases and two key biomineralization enzymes.

Voigt O, Adamski M, Sluzek K, Adamska M - BMC Evol. Biol. (2014)

Bottom Line: We found that the CA repertoires of two calcareous sponge species are strikingly more complex than those of other sponges.The complex evolutionary history of the CA family is driven by frequent gene diversification and losses.These evolutionary patterns likely facilitated the numerous events of independent recruitment of CAs into biomineralization within Metazoa.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Calcium carbonate biominerals form often complex and beautiful skeletal elements, including coral exoskeletons and mollusc shells. Although the ability to generate these carbonate structures was apparently gained independently during animal evolution, it sometimes involves the same gene families. One of the best-studied of these gene families comprises the α- carbonic anhydrases (CAs), which catalyse the reversible transformation of CO2 to HCO3 - and fulfill many physiological functions. Among Porifera -the oldest animal phylum with the ability to produce skeletal elements- only the class of calcareous sponges can build calcitic spicules, which are the extracellular products of specialized cells, the sclerocytes. Little is known about the molecular mechanisms of their synthesis, but inhibition studies suggest an essential role of CAs. In order to gain insight into the evolution and function of CAs in biomineralization of a basal metazoan species, we determined the diversity and expression of CAs in the calcareous sponges Sycon ciliatum and Leucosolenia complicata by means of genomic screening, RNA-Seq and RNA in situ hybridization expression analysis. Active biomineralization was located with calcein-staining.

Results: We found that the CA repertoires of two calcareous sponge species are strikingly more complex than those of other sponges. By characterizing their expression patterns, we could link two CAs (one intracellular and one extracellular) to the process of calcite spicule formation in both studied species. The extracellular biomineralizing CAs seem to be of paralogous origin, a finding that advises caution against assuming functional conservation of biomineralizing genes based upon orthology assessment alone. Additionally, calcareous sponges possess acatalytic CAs related to human CAs X and XI, suggesting an ancient origin of these proteins. Phylogenetic analyses including CAs from genomes of all non-bilaterian phyla suggest multiple gene losses and duplications and presence of several CAs in the last common ancestor of metazoans.

Conclusions: We identified two key biomineralization enzymes from the CA-family in calcareous sponges and propose their possible interaction in spicule formation. The complex evolutionary history of the CA family is driven by frequent gene diversification and losses. These evolutionary patterns likely facilitated the numerous events of independent recruitment of CAs into biomineralization within Metazoa.

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Two scenarios for the potential function of scl-CA1 and scl-CA2 in spicule formation. In both, scl-CA1 catalyses the formation of mainly metabolic CO2 to HCO3− within the sclerocytes, and then transports it to the extracellular calcification site by a biacrbonate transporter. Here, scl-CA2 could also produce HCO3− from CO2 diffused into the extracellular space (left). Alternatively, scl-CA2 could catalyse the reverse reaction, in order to remove protons that were formed by the reaction of HCO3− to carbonate. The CO2 could then diffuse into the sclerocyte and again serve as substrate for sclCA1. In addtion to metabolic CO2, DIC (in form of HCO3−) might be taken up from the seawater, which could involve the activity of another CA, as had been suggested for corals [11]. The DIC transport form and mechanisms within the sponge tissue are yet unknown.
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Fig6: Two scenarios for the potential function of scl-CA1 and scl-CA2 in spicule formation. In both, scl-CA1 catalyses the formation of mainly metabolic CO2 to HCO3− within the sclerocytes, and then transports it to the extracellular calcification site by a biacrbonate transporter. Here, scl-CA2 could also produce HCO3− from CO2 diffused into the extracellular space (left). Alternatively, scl-CA2 could catalyse the reverse reaction, in order to remove protons that were formed by the reaction of HCO3− to carbonate. The CO2 could then diffuse into the sclerocyte and again serve as substrate for sclCA1. In addtion to metabolic CO2, DIC (in form of HCO3−) might be taken up from the seawater, which could involve the activity of another CA, as had been suggested for corals [11]. The DIC transport form and mechanisms within the sponge tissue are yet unknown.

Mentions: We have two scenarios for a potential interplay of scl-CA1 and scl-CA2 (Figure 6), which differ in the role of the secreted/membrane-bound scl-CA2. In both scenarios, the intracellular scl-CA1 transforms metabolic CO2 to HCO3− within the sclerocyte, which then is secreted to the extracellular spicule formation site by a bicarbonate transporter [47]. The metabolic CO2 may not only be produced by mitochondria within the sclerocyte, but also by pinacocytes, choanocytes and other cells of the mesohyl, and diffuse into the sclerocyte. It is then possible that scl-CA2 at the calcification site transforms excess CO2 into additional HCO3−, which diffuses from the sclerocyte (Figure 6, left). In this case protons from the formation of carbonate could be actively removed, e.g. by a yet unknown Ca2+-ATPase, which in turn delivers Ca2+ to the calcification site, as has been proposed for corals [16,48]. Alternatively, the function of scl-CA2 could be to eliminate the protons by catalyzing the reaction of two HCO3−-ions and one proton to produce CO2 and H2O (Figure 6, right). The CO2 in turn could diffuse into the sclerocyte and serve as substrate for scl-CA1. This function of secreted/membrane-bound CAs was proposed for corals, if DIC in the form of HCO3− was the main carbon source for the skeleton [11,16]. This is not a prerequisite in calcareous sponges as HCO3− could also be provided by the activity of scl-CA1. However, DIC taken up by choanocytes or pinacocytes from seawater may contribute to the carbon pool, regardless of the function of scl-CA2, but neither the uptake mechanism nor the form of transportation is known. In corals, a membrane-bound CA from ectodermal cells is believed to be involved in DIC uptake by transforming HCO3− into CO2, which diffuses into the cell [11]. According to the expression profiles, the only calcarean CA that showed continuous and high expression as would be expected for an enzyme with such a critical role is SciCA3, the closest CA to scl-CA2. The expression pattern of this gene appeared ubiquitous in adult tissues but elevated in oocytes and early embryos (see Additional file 4f), consistent with involvement of this CA in DIC uptake, where expression in choanocytes and pinacocytes would be expected.Figure 6


Calcareous sponge genomes reveal complex evolution of α-carbonic anhydrases and two key biomineralization enzymes.

Voigt O, Adamski M, Sluzek K, Adamska M - BMC Evol. Biol. (2014)

Two scenarios for the potential function of scl-CA1 and scl-CA2 in spicule formation. In both, scl-CA1 catalyses the formation of mainly metabolic CO2 to HCO3− within the sclerocytes, and then transports it to the extracellular calcification site by a biacrbonate transporter. Here, scl-CA2 could also produce HCO3− from CO2 diffused into the extracellular space (left). Alternatively, scl-CA2 could catalyse the reverse reaction, in order to remove protons that were formed by the reaction of HCO3− to carbonate. The CO2 could then diffuse into the sclerocyte and again serve as substrate for sclCA1. In addtion to metabolic CO2, DIC (in form of HCO3−) might be taken up from the seawater, which could involve the activity of another CA, as had been suggested for corals [11]. The DIC transport form and mechanisms within the sponge tissue are yet unknown.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4265532&req=5

Fig6: Two scenarios for the potential function of scl-CA1 and scl-CA2 in spicule formation. In both, scl-CA1 catalyses the formation of mainly metabolic CO2 to HCO3− within the sclerocytes, and then transports it to the extracellular calcification site by a biacrbonate transporter. Here, scl-CA2 could also produce HCO3− from CO2 diffused into the extracellular space (left). Alternatively, scl-CA2 could catalyse the reverse reaction, in order to remove protons that were formed by the reaction of HCO3− to carbonate. The CO2 could then diffuse into the sclerocyte and again serve as substrate for sclCA1. In addtion to metabolic CO2, DIC (in form of HCO3−) might be taken up from the seawater, which could involve the activity of another CA, as had been suggested for corals [11]. The DIC transport form and mechanisms within the sponge tissue are yet unknown.
Mentions: We have two scenarios for a potential interplay of scl-CA1 and scl-CA2 (Figure 6), which differ in the role of the secreted/membrane-bound scl-CA2. In both scenarios, the intracellular scl-CA1 transforms metabolic CO2 to HCO3− within the sclerocyte, which then is secreted to the extracellular spicule formation site by a bicarbonate transporter [47]. The metabolic CO2 may not only be produced by mitochondria within the sclerocyte, but also by pinacocytes, choanocytes and other cells of the mesohyl, and diffuse into the sclerocyte. It is then possible that scl-CA2 at the calcification site transforms excess CO2 into additional HCO3−, which diffuses from the sclerocyte (Figure 6, left). In this case protons from the formation of carbonate could be actively removed, e.g. by a yet unknown Ca2+-ATPase, which in turn delivers Ca2+ to the calcification site, as has been proposed for corals [16,48]. Alternatively, the function of scl-CA2 could be to eliminate the protons by catalyzing the reaction of two HCO3−-ions and one proton to produce CO2 and H2O (Figure 6, right). The CO2 in turn could diffuse into the sclerocyte and serve as substrate for scl-CA1. This function of secreted/membrane-bound CAs was proposed for corals, if DIC in the form of HCO3− was the main carbon source for the skeleton [11,16]. This is not a prerequisite in calcareous sponges as HCO3− could also be provided by the activity of scl-CA1. However, DIC taken up by choanocytes or pinacocytes from seawater may contribute to the carbon pool, regardless of the function of scl-CA2, but neither the uptake mechanism nor the form of transportation is known. In corals, a membrane-bound CA from ectodermal cells is believed to be involved in DIC uptake by transforming HCO3− into CO2, which diffuses into the cell [11]. According to the expression profiles, the only calcarean CA that showed continuous and high expression as would be expected for an enzyme with such a critical role is SciCA3, the closest CA to scl-CA2. The expression pattern of this gene appeared ubiquitous in adult tissues but elevated in oocytes and early embryos (see Additional file 4f), consistent with involvement of this CA in DIC uptake, where expression in choanocytes and pinacocytes would be expected.Figure 6

Bottom Line: We found that the CA repertoires of two calcareous sponge species are strikingly more complex than those of other sponges.The complex evolutionary history of the CA family is driven by frequent gene diversification and losses.These evolutionary patterns likely facilitated the numerous events of independent recruitment of CAs into biomineralization within Metazoa.

View Article: PubMed Central - PubMed

ABSTRACT

Background: Calcium carbonate biominerals form often complex and beautiful skeletal elements, including coral exoskeletons and mollusc shells. Although the ability to generate these carbonate structures was apparently gained independently during animal evolution, it sometimes involves the same gene families. One of the best-studied of these gene families comprises the α- carbonic anhydrases (CAs), which catalyse the reversible transformation of CO2 to HCO3 - and fulfill many physiological functions. Among Porifera -the oldest animal phylum with the ability to produce skeletal elements- only the class of calcareous sponges can build calcitic spicules, which are the extracellular products of specialized cells, the sclerocytes. Little is known about the molecular mechanisms of their synthesis, but inhibition studies suggest an essential role of CAs. In order to gain insight into the evolution and function of CAs in biomineralization of a basal metazoan species, we determined the diversity and expression of CAs in the calcareous sponges Sycon ciliatum and Leucosolenia complicata by means of genomic screening, RNA-Seq and RNA in situ hybridization expression analysis. Active biomineralization was located with calcein-staining.

Results: We found that the CA repertoires of two calcareous sponge species are strikingly more complex than those of other sponges. By characterizing their expression patterns, we could link two CAs (one intracellular and one extracellular) to the process of calcite spicule formation in both studied species. The extracellular biomineralizing CAs seem to be of paralogous origin, a finding that advises caution against assuming functional conservation of biomineralizing genes based upon orthology assessment alone. Additionally, calcareous sponges possess acatalytic CAs related to human CAs X and XI, suggesting an ancient origin of these proteins. Phylogenetic analyses including CAs from genomes of all non-bilaterian phyla suggest multiple gene losses and duplications and presence of several CAs in the last common ancestor of metazoans.

Conclusions: We identified two key biomineralization enzymes from the CA-family in calcareous sponges and propose their possible interaction in spicule formation. The complex evolutionary history of the CA family is driven by frequent gene diversification and losses. These evolutionary patterns likely facilitated the numerous events of independent recruitment of CAs into biomineralization within Metazoa.

Show MeSH
Related in: MedlinePlus