Grass carp (Ctenopharyngodon idellus, C.idellus) is one of the most important globally cultured freshwater fish species, and its growth trait can be improved by dietary supplementation of oligopeptides which is transported through the intestinal oligopeptide transporter PepT1. However, many aspects of the molecular regulation mechanism of C.idellus PepT1 still remains elusive. In this study, the regulation mechanism of C.idellus PepT1 by CDX2 was extensively studied. Firstly, the interaction of the C.idellus CDX2 and SP1 was investigated by mammalian two hybrid assays and co-localization; The promoter sequence of PepT1 was cloned and its activity was directly up-regualted by CDX2. In the further study, the effect of CDX2 changes at both mRNA and protein levels on the expression of PepT1 was examined in vivo and in vitro. The mRNA expression of PepT1 was significantly decreased when CDX2 was repressed and its expression change profile was almost consistent with that of CDX2. To understand the regulation of CDX2 on the expression of PepT1 at protein level, the stability regulation mechanism of CDX2 was initially studied. The results showed that CDX2 protein was degraded via the ubiquitin-proteasome pathways, and increasing phosphorylation level of CDX2 protein declined its turnover and elevated its accumulation level in cells. The expression of PepT1 was significantly elevated with increasing CDX2 protein stability, and expressions of both CDX2 and PepT1 were stimulated by the oligopeptide. In conclusion, these observations may shed new light on the molecular regulation mechanism of PepT1 by CDX2 in C.idellus and provide new perceptions and strategies for the efficient transportation of intestinal oligopeptides in teleost fish.

草鱼（Ctenopharyngodon idellus，C.idellus）是全球最重要的淡水养殖淡水鱼种之一，其生长性状可以通过膳食补充通过肠道寡肽转运蛋白PepT1转运的寡肽来改善。然而， C.idellus PepT1的分子调控机制的许多方面仍然难以捉摸。本研究广泛研究了CDX2对C.idellus PepT1的调控机制。首先，C.idellus的相互作用CDX2 和 SP1 通过哺乳动物两种杂交试验和共定位进行了研究；克隆了PepT1的启动子序列，其活性被CDX2直接上调。在进一步的研究中，在体内和体外检测了 CDX2 在 mRNA 和蛋白质水平上的变化对 PepT1 表达的影响。当 CDX2 被抑制时，PepT1 的 mRNA 表达显着降低，其表达变化谱与 CDX2 几乎一致。为了解CDX2在蛋白水平上对PepT1表达的调控作用，对CDX2的稳定性调控机制进行了初步研究。结果表明，CDX2蛋白通过泛素-蛋白酶体途径降解，增加CDX2蛋白的磷酸化水平降低了其周转率，提高了其在细胞中的积累水平。PepT1 的表达随着 CDX2 蛋白稳定性的增加而显着升高，并且 CDX2 和 PepT1 的表达都受到寡肽的刺激。总之，这些观察结果可能为 CDX2 对 PepT1 的分子调控机制提供新的启示。C.idellus并为硬骨鱼肠道寡肽的有效运输提供新的认识和策略。