Isolation and characterization of a MAPKK gene from Penaeus monodon in response to bacterial infection and low-salinity challenge

Mitogen-activated protein kinase kinase (MAPKK), a component of the MAPK signal transduction pathway, plays a crucial role in stress resistance and immunity. Herein, a MAPKK cDNA from Penaeus monodon (black tiger shrimp) was cloned (PmMAPKK), and its effect on bacterial infection and low-salinity stress was investigated. The full-length PmMAPKK cDNA was 3665 bp long, with a 1224-bp open reading frame (ORF), encoding a 408-amino acid (aa) protein. Results revealed that PmMAPKK was expressed in all examined tissues of P. monodon, including hepatopancreas, gills, and lymphoid organ. The PmMAPKK expression level was found to be highest in ovary (approximately 8.7-fold higher than in hepatopancreas), and lowest in testis (approximately 0.54 times that in hepatopancreas). In early developmental stages of P. monodon, PmMAPKK expression was highest during the zygote stage. From the zoea1 stage onward, a steady decrease in expression was registered. Meanwhile, in the bacterial infection experiment, the hepatopancreas was found to be more sensitive than gills after bacterial infection. Following infection with Staphylococcus aureus and Vibrio anguillarum, PmMAPKK expression was appeared significantly higher than that in the control group (P < 0.05). However, infection with Vibrio harveyi caused a significant decrease in the expression level compared to the control group (P < 0.05). Furthermore, low-salinity environment significantly altered PmMAPKK expression in hepatopancreas and gills (P < 0.05). Silencing of PmMAPKK expression caused the mortality of P. monodon to increase under low-salinity stress conditions. Furthermore, the ERK, JNK, and p38 signaling pathways were activated to varying degrees concomitantly, reflecting the complex regulatory mechanisms in the MAPK signaling pathway. Hence, the results of the present study indicate that PmMAPKK is involved in the innate immune mechanism of P. monodon as well as its adaptation to low-salinity environments.