A thymosin beta-4 is involved in production of hemocytes and immune defense of Hong Kong oyster, Crassostrea hongkongensis

Thymosin beta-4 (T beta 4) is a ubiquitous protein with multiple and diverse intracellular and extracellular functions in vertebrates. In this study, the full-length cDNA of T beta 4 was cloned and identified in Crassostrea hongkongensis, designated as ChT beta 4. The full-length cDNA of ChT beta 4 consists of 530 bp with an open reading frame of 126 bp encoding a 41 amino acid polypeptide. SMART analysis indicated that there is one thymosin domain and a highly conserved actin-binding motif (18LKKTET23) in ChT beta 4. In vivo injection of recombinant ChT beta 4 protein could significantly increase total hemocytes count in oysters, and knockdown of the expression of ChT beta 4 resulted in a significant decrease in the circulating hemocytes. Tissue distribution analysis revealed a ubiquitous presence of ChT beta 4, with the highest expression in hemocytes. The upregulated transcripts of ChT beta 4 in response to bacterial challenge and tissue injury suggest that ChT beta 4 is involved in both innate immunity against pathogen infection and wound healing. Moreover, bacteria-clearance experiment showed ChT beta 4 could facilitate the clearance of injected bacteria in oysters. In vivo injection with ChT beta 4 resulted in reduction of the intracellular ROS in hemocytes, which was associated with increased expression of antioxidant enzymes Cu/Zn superoxide dismutase (SOD), Catalase, and Glutathione Peroxidase (GPX) by pre-treatment with ChT beta 4. These results suggest that ChT beta 4 is a thymosin beta-4 homolog and plays a vital role in the immune defense of C. hongkongensis. (C) 2015 Elsevier Ltd. All rights reserved