Cell type and gene regulatory network approaches in the evolution of spiralian biomineralisation

Biomineralisation is the process by which living organisms produce hard structures such as shells and bone. There are multiple independent origins of biomineralised skeletons across the tree of life. This review gives a glimpse into the diversity of spiralian biominerals and what they can teach us about the evolution of novelty. It discusses different levels of biological organisation that may be informative to understand the evolution of biomineralisation and considers the relationship between skeletal and non-skeletal biominerals. More specifically, this review explores if cell type and gene regulatory network approaches could enhance our understanding of the evolutionary origins of biomineralisation.

A bone morphogenetic protein regulates the shell formation of Crassostrea gigas under ocean acidification

Bone morphogenetic proteins (BMPs) are key factors controlling osteoblast differentiation, which have been proved to be involved in the hard tissue formation of marine mollusks. In the present study, a member of BMPs gene (CgBMP7) was identified from Pacific oyster Crassostrea gigas (C. gigas) with the aim to understand its possible role in the regulation of shell formation under ocean acidification (OA) conditions. The open reading frame (ORF) of CgBMP7 was of 1254 bp encoding a polypeptide of 417 amino acids. The deduced amino acid sequence of CgBMP7 was comprised of one signal peptide, one prodomain and one TGF-β domain, which shared 21.69%-61.10% identities with those from other species. The mRNA transcript of CgBMP7 was ubiquitously expressed in all the tested tissues of adult oysters with a higher expression level in mantle, notably highest in the middle fold (MF) of the three folds of mantle. The expression level of bone morphogenetic protein type I receptor (CgBMPR1B) mRNA was also highest in the MF and up-regulated dramatically post recombinant BMP7 protein (rCgBMP7) stimulation. After the blockage of BMPR1B with inhibitor LDN193189 (LDN), the mRNA expression level and phosphorylation level of CgSmad1/5/8 in mantle were decreased, and the mRNA expression levels of CgCaM and Cgengrailed-1 were down-regulated significantly. During the oysters were exposed to acidified seawater for weeks, the expression levels of CgBMP7, CgBMPR1B and CgSmad1/5/8 in the MF decreased significantly (p < 0.01) at the 4th week, and CgCaM and Cgengrailed-1 also exhibited the same variable expression patterns as CgBMP7. In addition, the growth of shell in the treatment group (pH 7.8) was slower than that in the control group (pH 8.1). These results collectively indicated that BMP7 was able to trigger the BMPR-Smad signaling pathway and involved in controlling the formation of oyster calcified shell under OA conditions.

The Increased Expression of an Engrailed to Sustain Shell Formation in Response to Ocean Acidification

Engrailed is a transcription factor required in numerous species for important developmental steps such as neurogenesis, segment formation, preblastoderm organization, and compartment formation. Recent study has proved that engrailed is also a key gene related to shell formation in marine bivalves. In the present study, the expression pattern of an engrailed gene (Cgengrailed-1) in Pacific oyster Crassostrea gigas under CO₂-driven acidification was investigated to understand its possible role in the regulation of shell formation and adaptation to ocean acidification (OA). The open reading frame (ORF) of Cgengrailed-1 was obtained, which was of 690 bp encoding a polypeptide of 229 amino acids with a HOX domain. Phylogenetic analysis indicated that the deduced amino acid sequence of Cgengrailed-1 shared high homology with other engraileds from Drosophila melanogaster, Mizuhopecten yessoensi, and Crassostrea virginica. The mRNA transcripts of Cgengrailed-1 were constitutively expressed in various tissues with the highest expression levels detected in labial palp and mantle, which were 86.83-fold (p < 0.05) and 75.87-fold (p < 0.05) higher than that in hepatopancreas. The mRNA expression of Cgengrailed-1 in mantle decreased dramatically after moderate (pH 7.8) and severe (pH 7.4) acidification treatment (0.75- and 0.15-fold of that in control group, p < 0.05). The results of immunofluorescence assay demonstrated that the expression level of Cgengrailed-1 in the middle fold of mantle increased significantly upon moderate and severe acidification treatment. Moreover, after the oyster larvae received acidification treatment at trochophore stage, the mRNA expression levels of Cgengrailed-1 increased significantly in D-shape larvae stages, which was 3.11- (pH 7.8) and 4.39-fold (pH 7.4) of that in control group (p < 0.05). The whole-mount immunofluorescence assay showed that Cgengrailed-1 was mainly expressed on the margin of shell gland, and the periostracum in trochophore, early D-shape larvae and D-shape larvae in both control and acidification treatment groups, and the intensity of positive signals in early D-shape larvae and D-shape larvae increased dramatically under acidification treatment. These results collectively suggested that the expression of Cgengrailed-1 could be triggered by CO₂-driven acidification treatment, which might contribute to induce the initial shell formation in oyster larvae and the formation of periostracum in adult oyster to adapt to the acidifying marine environment.

Molecular and functional analysis of PmC1qDC in nacre formation of Pinctada fucata martensii

The C1q-domain-containing (C1qDC) proteins are a family of proteins characterized by a globular C1q (gC1q) domain in their C-terminus which hold the potential function in the shell formation as shell matrix proteins. In this study, a C1qDC protein was identified and characterized in pearl oyster (Pinctada fucata martensii) (PmC1qDC) to explore its function in nacre formation. The PmC1qDC-deduced protein sequence carried a typical globular C1q (gC1q) domain that possessed the typical 10-stranded β-sandwich fold with a jelly-roll topology common to all C1qDC family members and shared high homology with other gC1q domains. Homologous analysis of PmC1qDC presented it contained conserved secondary structure and Phe135, Phe155, Tyr166, Phe173, Tyr181, Phe183, and Phe256 amino acid residues. Expression pattern analysis showed that PmC1qDC expressed in all the detected tissues and exhibited a significantly higher expression level in nacre formation-associated tissues. After the shell notching, the expression level of PmC1qDC showed significantly up-regulation after 12 h in the central zone of mantle (MC). PmC1qDC expression significantly decreased in the MC after RNA interference (RNAi). Furthermore, disordered crystals with evident rough surface and irregular crystal tablets were observed in the nacre after RNAi. Results suggested that PmC1qDC affects the shell nacre formation, which is significant to improve the pearl production of pearl oyster.

PfSMAD1/5 Can Interact with PfSMAD4 to Inhibit PfMSX to Regulate Shell Biomineralization in Pinctada fucata martensii

The BMP2 signal transduced by SMAD1/5 plays an important role in osteoblast differentiation and bone formation. Shell formation of Pinctada fucata martensii is a typical biomineralization process that is similar to that of teeth/bone formation. However, whether the Pinctada fucata BMP2 (PfBMP2) signal transduced by PfSMAD1/5 occurs in P. f. martensii, how the PfBMP2 signal is transduced by PfSMAD1/5, and how PfSMAD1/5 regulates the biomineralization process in this species and other shellfish are poorly understood. Therefore, injection experiments of recombinant PfBMP2 and inhibitor dorsomorphin revealed that PfSMAD1/5 can transduce PfBMP2 signals. Subcellular localization and bimolecular fluorescence complementation assays indicated that PfSMAD1/5 phosphorylated by PfBMPR1b interacts with PfSMAD4 in the cytoplasm to form a complex, which translocates to the nucleus to transduce PfBMP2 signals. Co-immunoprecipitation and luciferase assays revealed that PfSMAD1/5 may interact with PfMSX to dislodge it from its binding element, resulting in initiation of mantle gene transcription. The in vivo functional assay showed that knockdown of PfMSAD1/5 decreased expression of shell matrix genes and disordered the nacreous layer, and the correlation assay of shell regeneration showed the concomitant expression pattern of PfSMAD1/5 and shell matrix genes. Together, these data showed that PfSMAD1/5 can transduce PfBMP2 signals to regulate shell biomineralization in P. f. martensii, which illustrated conservation of the BMP2-SMAD signal pathway among invertebrates. Particularly, the results suggest that there is only one PfMSX gene, which functions like the Hox gene in vertebrates, that interacts with PfSMAD1/5 in a protein-protein action form and plays the role of transcription repressor.