Biomedical subjects

Yuji Yokouchi

Publications and source records attributed to Yuji Yokouchi.

4 recordsLinked to original sources

PubMed2007

Dan is required for normal morphogenesis and patterning in the developing chick inner ear.

During vertebrate inner ear development, compartmentalization of the auditory and vestibular apparatuses along two axes depends on the patterning of transcription factors expressed in a region-specific manner. Although most of the patterning is regulated by extrinsic signals, it is not known how Nkx5.1 and Msx1 are patterned. We focus on Dan, the founding member of the Cerberus/Dan gene family that encodes BMP antagonists, and describe its function in morphogenesis and patterning. First, we confirmed that Dan is expressed in the dorso-medial region of the otic vesicle that corresponds to the presumptive endolymphatic duct and sac (ed/es). Second, we used siRNA knockdown to demonstrate that depletion of Dan induced both a severe reduction in the size of the ed/es and moderate deformities of the semicircular canals and cochlear duct. Depletion of Dan also caused suppression of Nkx5.1 in the dorso-lateral region, suppression of Msx1 in the dorso-medial region, and ectopic induction of Nkx5.1 and Msx1 in the ventro-medial region. Most of these phenotypes also appeared following misexpression of the constitutively active form of BMP receptor type Ib. Thus, Dan is required for the normal morphogenesis of the inner ear and, by inhibiting BMP signaling, for the patterning of the transcription factors Nkx5.1 and Msx1.

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PubMed2006

Id3 is important for proliferation and differentiation of the hepatoblasts during the chick liver development.

The specified hepatic endoderm (hepatoblasts), the bipotential progenitor for hepatocytes and bile duct epithelial cells, proliferates during the primordial stages of liver development. Despite extensive studies, the mechanism that regulates proliferation of bipotential hepatoblasts is not fully understood. Here we show that Id3, a negative regulator of helix-loop-helix transcription factors, is an important regulator of hepatoblast proliferation in the developing chick liver. Id3 was expressed in hepatoblasts at early developmental stages (stages 12-29) but not in hepatocytes at later developmental stages (stage 34 onwards). Depletion of Id3 in hepatoblasts by siRNA results in failure of cell proliferation, but is not associated with either cell death or failure of expression of Hhex and Fibrinogen, the earliest hepatoblast markers. These observations suggest that at early developmental stages, Id3 functions as a positive regulator of hepatoblast proliferation, independent of cell death or maintenance of the non-terminally differentiated state. Interestingly at later developmental stages, the expression pattern of Id3 is complementary to that of Albumin, a marker of mature hepatocytes. Overexpression of Id3 in liver explants delayed the initiation of Albumin expression. Taken together, our observations show that Id3 is not only a positive regulator of hepatoblast proliferation, but also an inhibitor of their differentiation into hepatocytes in the developing chick liver.

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PubMed2005

Analysis of gene expression patterns in the developing chick liver.

The chick embryo has been used widely for studying liver development. However, in the past 30 years, the usage has decreased markedly due to lack of appropriate marker genes for differentiation in the developing chick liver. To use the chick embryo for analyzing the molecular mechanism of liver development, we surveyed marker genes in the developing chick liver by examining the expression pattern of genes that are well-characterized in the developing mammalian liver. By whole-mount in situ hybridization, Fibrinogen-gamma (FIB) expression was first detected at stage 12, specifically in the anterior intestinal portal, and its liver-specific expression persisted in the later stages. Albumin (ALB) expression was first detected at stage 30, when the liver starts maturing. Cytokeratin 19 (CK19) was first detected at stage 37 in the ductal plate of the liver, and its expression continued in the intrahepatic bile ducts derived from the ductal plate. Hex, a transcription factor, is an additional marker of bile duct differentiation. Hence, FIB, ALB, and CK19 expression can be used to trace hepatic induction, maturation, and bile duct differentiation, respectively.

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PubMed2005

Establishment of a chick embryo model for analyzing liver development and a search for candidate genes.

The liver plays a crucial role in metabolism. There is considerable interest in how the liver develops, as such knowledge could prove of importance in regenerative medicine. However, our understanding of liver development remains somewhat limited. We have developed a model system using the chick embryo that is cost effective and is easy to manipulate experimentally. We performed four fundamental studies: (i) construction of an atlas of the developing chick liver; (ii) identification of differentiation marker genes in the developing chick embryo; (iii) development of germ-layer specific electroporation; and (iv) establishment of organ culture from the developing chick liver. Using this system, we have been able to demonstrate the functions of candidate genes within a shorter period and in a more cost-effective manner. In parallel with the establishment of this system, we examined the expression patterns of genes known to be required for organ development in the developing chick embryo in order to identify genes also involved in liver development. To date, we have found sixteen genes that are expressed in the developing chick liver (GELD, genes expressed in liver development). This knowledge will be fundamental to the establishment of the basic technology for engineering liver tissue in the future.

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