Tim2 is expressed in mouse fetal hepatocytes and regulates their differentiation.
Liver development is regulated by various extracellular molecules such as cytokines and cell surface proteins. Although several such regulators have been identified, additional molecules are likely to be involved in liver development. To identify such molecules, we employed the signal sequence trap (SST) method to screen cDNAs encoding a secreted or membrane protein from fetal liver and obtained a number of clones. Among them, we found that T cell immunoglobulin and mucin domain 2 (Tim2) was expressed specifically on immature hepatocytes in the fetal liver. Tim2 has been shown to regulate immune responses, but its role in liver development had not been studied. We have examined the possible role of Tim2 in hepatocyte differentiation. At first, we prepared a soluble Tim2 fusion protein consisting of its extracellular domain and the Fc domain of human IgG (Tim2-hFc) and found that it bound to fetal and adult hepatocytes, suggesting that there are Tim2-binding molecules on hepatocytes. Second, Tim2-hFc inhibited the differentiation of hepatocytes in fetal liver primary culture, i.e., the expression of mature hepatic enzymes and accumulation of glycogen were severely reduced. Third, Tim2-hFc also inhibited proliferation of fetal hepatocytes. Fourth, down-regulation of Tim2 expression by small interfering RNA (siRNA) enhanced the expression of liver differentiation marker genes.


CONCLUSIONS
It is strongly suggested that Tim2 is involved in the differentiation of fetal hepatocytes.
Watanabe
Natsumi
N
Institute of Molecular and Cellular Biosciences, the University of Tokyo, Tokyo, Japan.
Tanaka
Minoru
M
Suzuki
Kaori
K
Kumanogoh
Atsushi
A
Kikutani
Hitoshi
H
Miyajima
Atsushi
A
D000818:Animals; D002454:Cell Differentiation / Q000502:physiology*; D015536:Down-Regulation; D005434:Flow Cytometry; D018507:Gene Expression Regulation, Developmental; D022781:Hepatocytes / Q000166:cytology / Q000378:metabolism*; D008099:Liver / Q000196:embryology; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000502:physiology; D051379:Mice; D021382:Protein Sorting Signals
D008565:Membrane Proteins; D021382:Protein Sorting Signals; C442242:Timd2 protein, mouse