Crucial Role of the Small GTPase ARF6 in Hepatic Cord Formation during Liver Development

Abstract

The mammalian small GTPase ADP-ribosylation factor 6 (ARF6) plays important roles in a wide variety of cellular events, including endocytosis, actin cytoskeletal reorganization, and phosphoinositide metabolism. However, physiological functions for ARF6 have not previously been examined. Here, we described the consequence of ARF6 ablation in mice, which manifests most obviously in the context of liver development. Livers from ARF6^−/− embryos are smaller and exhibit hypocellularity, due to the onset of midgestational liver cell apoptosis. Preceding the apoptosis, however, defective hepatic cord formation is observed; the liver cells migrate abnormally upon exiting the primordial hepatic epithelial sheet and clump rather than becoming dispersed. Consistent with this observation, the ability of hepatocyte growth factor/scatter factor (HGF) to induce hepatic cord-like structures from ARF6^−/− fetal hepatocytes cultured in vitro in collagen gel matrix is impaired. Finally, we show that endogenous ARF6 in wild-type fetal hepatocytes is activated in response to HGF stimulation. These results provide evidence that ARF6 is an essential component in the signaling pathway coupling HGF signaling to hepatic cord formation.

This work was supported by research grants from the Ministry of Education, Science, Sports and Culture, Japan to Y. K. and from the Mitsubishi research foundation to Y. K.

We thank H. Nishina and K. Miyazawa for valuable comments and advice. We are also greatly appreciative to J. Penninger, J. G. Donaldson, and K. Nakayama for their generous gifts of E14K ES cells and a mouse 129/Ola genomic library, the anti-ARF6 antiserum, and ARF6 cDNA, respectively.