ERK signaling pathway regulates embryonic survival and eye development in goldfish, Carassius auratus

The extracellular signal-regulated kinase (ERK) is one of the three major types of mitogen-activated protein kinases. Previous studies showed that ERKs mediate various signaling pathways for cell proliferation, differentiation, survival and transformation in mammals. In the present study, we use goldfish as a model system and demonstrate that ERK kinases play important roles in promoting embryonic survival and regulate development of eye and trunk in vertebrates. ERKs are highly expressed in multiple tissues including lens epithelial cells, lens fiber cells, retina, brain, muscle and heart of adult goldfish. Injection of the dominant negative ERK mutant (DNM-ERK) into the fertilized eggs of goldfish significantly inhibited ERK activity at blastula stage, and completely blocked ERK activity at gastrula and later stages. As a result, the blastula cells were induced into apoptosis, and majority of the injected embryos were lethal at embryonic stages. At the molecular level, inhibition of ERK activity by DNM-ERKs suppressed phosphorylation of Bad at Ser-112 to promote apoptosis. Similar results were observed when MEK activity was inhibited by U0126 treatment. The survived embryos display significant abnormality in the phenotypes of both eye and trunk. Associated with the abnormality in the eye development, phosphorylation in Pax-6 and expression of HSF4 were significantly decreased and expression of the β-crystallin gene was also downregulated. These results provide novel information regarding the roles of ERKs in regulating vertebrate development.