A new member of the runt domain family from Pacific oyster Crassostrea gigas (CgRunx) potentially involved in immune response and larvae hematopoiesis

The Runx family is a kind of heteromeric transcription factors, which is defined by the presence of a runt domain. As transcriptional regulator during development and cell fate specification, Runx is best known for its critical roles in hematopoiesis. In the present study, a Runx transcription factor (designed as CgRunx) was identified and characterized from the oyster Crassostrea gigas. The complete coding sequence of CgRunx was of 1638 bp encoding a predicted polypeptide of 545 amino acids with one conserved runt domain, which shared high similarity with other reported Runx proteins. CgRunx was highly expressed in hemocytes, gill and mantle both at the protein and nucleic acid levels. CgRunx protein was localized specifically in the cell nuclei of hemocytes, and distributed at the tubule lumen of gill filament. During the larval developmental stages, the mRNA transcripts of CgRunx gradually increased after fertilization, reached to a relative high level at the 8 cell embryos and the blastula stage of 2-4 hpf (hours post fertilization) (about 40-fold), and peaked at early trochophore larvae (10 hpf) (about 60-fold). Whole-mount immunofluorescence assay further revealed that the abundant immunofluorescence signals of CgRunx distributed through the whole embryo at blastula stage (5 hpf), and progressively reduced with the development to a ring structure around the dorsal region in trochophore larvae (10 hpf). Scattered positive immunoreactivity signals finally appeared in the velum region of D-veliger larvae. After LPS and Vibrio splendidus stimulations, the expression levels of CgRunx mRNA in hemocytes were upregulated significantly compared with that in the control (0 h), which were 2.98- and 2.46-fold (p < 0.05), 2.67-and 1.5-fold (p < 0.05), 2.36- and 1.38-fold (p < 0.05) at 3 h, 6 h and 12 h, respectively. These results collectively suggested that CgRunx involved in immune response and might participate in larvae hematopoiesis in oyster.