Activin receptor type IIB (ActRIIB) is a transmembrane serine/threonine kinase receptor which plays a pivotal role in regulating the reproduction in vertebrates including teleost. Earlier studies have documented its importance in governing gonadal maturation in higher vertebrates. However, reports on the regulation of fish reproductive system by ActRIIB gene are still limited. Here, we report the identification and characterization of ActRIIB cDNA of Labeo rohita, a commercially important fish species of the Indian subcontinent. The full-length gene encoding rohu ActRIIB was cloned and found to be of 1674 bp in length. Functional similarities were evident from evolutionary analysis across vertebrates. Real-time PCR to measure the expression of ActRIIB transcript in rohu revealed significant mRNA levels in gonads followed by non-reproductive tissues, including the brain, pituitary and muscle. With respect to different gonadal maturation stages, predominant expression of ActRIIB mRNA was observed during the pre-spawning phase of both sexes. To further delineate its role in rohu reproduction, a recombinant protein of the extracellular domain of ActRIIB (rECD-ActRIIB) was produced, and polyclonal antibody is raised against the protein for its immuno-localization studies during different gonadal maturation stages. Strong immunoreactivity was noticed in the pre-vitellogenic oocytes which decreased dramatically in the fully mature oocytes. Similarly, the strong and intense immunoreactivity was found in the spermatids and spermatocytes of the immature testis, and eventually the intensity reduced with the progression of the maturation stage. These results provide the first evidence of the presence of ActRIIB in rohu gonadal tissues. Taken together, our observations lay the groundwork for further understanding and investigating on the potential role of ActRIIB in fish reproduction system in the event of gonadal maturation.

IIB 型激活素受体 (ActRIIB) 是一种跨膜丝氨酸/苏氨酸激酶受体，在调节包括硬骨鱼在内的脊椎动物的繁殖中起关键作用。早期的研究已经记录了它在控制高等脊椎动物性腺成熟方面的重要性。但ActRIIB基因调控鱼类生殖系统的报道仍然有限。在这里，我们报告了印度次大陆重要的商业鱼类Labeo rohita的ActRIIB cDNA的鉴定和表征。克隆编码rohu ActRIIB的全长基因，发现其长度为1674 bp。从脊椎动物的进化分析中可以明显看出功能相似性。实时荧光定量 PCR 测量rohu 中的ActRIIB转录本显示性腺中有显着的 mRNA 水平，其次是非生殖组织，包括大脑、垂体和肌肉。对于不同的性腺成熟阶段，在两性的产卵前阶段均观察到ActRIIB mRNA 的主要表达。为了进一步描述其在罗胡繁殖中的作用， ActRIIB细胞外结构域的重组蛋白(rECD-ActRIIB) 已产生，并针对该蛋白质产生多克隆抗体，用于在不同性腺成熟阶段进行免疫定位研究。在卵黄前卵母细胞中观察到强烈的免疫反应性，在完全成熟的卵母细胞中显着降低。同样，在未成熟睾丸的精子细胞和精母细胞中发现了强烈而强烈的免疫反应，最终强度随着成熟阶段的进展而降低。这些结果提供了在罗胡性腺组织中存在ActRIIB的第一个证据。总之，我们的观察结果为进一步了解和研究ActRIIB在性腺成熟时在鱼类繁殖系统中的潜在作用奠定了基础。