Department of Biological Sciences

Currently our laboratory has two major overlapping research interests, directed at understanding the molecular mechanism involved in the maintenance of pluripotency of human embryonic stem (ES) cells, and how organs are formed using the vertebrate model organism, zebrafish. We are particularly interested in how zebrafish thyroid receptors (TRs) and fushi-tarazu factor-1 (Ff1) nuclear receptors regulate organ formation during embryonic and early larval phases. In addition, in collaboration with Professor Ariff Bongso (Department of Obstetics and Gynaecology, NUS) we are learning how human ES cells maintain their pluripotency and self-renewal capability through functional genomic approaches, and the molecular players that are involved in theri subsequent differentiation into specific cell lineages.

Current Projects
Developmental Roles of Nuclear Receptors During Vertebrate Development

The nuclear receptor gene superfamily comprises a large number of ligand-dependent and 'orphan' transcription factors. They have many crucial roles in vertebrate development. Of these, we are interested in how the Ff1 orphan nuclear receptors regulate gonadogenesis, a process that directs an immature, bipotential gonad into forming either the ovary or testis, and the specification and formation of the interrenal gland, which is the teleost homolog of the mammalian adrenal cortex.

We have recently established the pivotal role of ff1b, the zebrafish homologue of the mammalian SF-1 nuclear receptor, in the initial specification of the interrenal primordium and its subsequent acquisition of steroidogenic capacity. In addition, we have shown that Prox1, a divergent homeodomain protein, is capable of interacting with Ff1b and thereby represses its transcriptional ability. Intriguingly, the formation of the Prox1/Ff1b repressor complex appears to be necessary for the final maturation of the interrenal gland. We are now trying to understand if Prox1 has similar roles during embryonic development in other organs (eg. liver, brain, intestine) where they are co-expressed with Ff1a and Ff1b. We are also interested to understand if Prox1 might also act as a coregulator for other nuclear receptors like TRs. Lineage tracing experiments to further understand the developmental fate of Ff1b- and Prox1-expressing cells are currently on-going. The interaction with transcription factors and co-regulators will be a key focus since they are likely to be important for the actions of Ff1 isoforms in the differentiation of the bipotential gonad, interrenal gland and liver.

We are also actively involved in understanding the process of metamorphosis, which transforms a larva into an adult through extensive tissue modification and re-organisation. Although freshwater fish species do not undergo obvious metamorphosis, two important transitory phases have been identified. They are the embryonic-to-larval and larval-to-adult transitory phases. In zebrafish, thyroid hormones appear to be obligatory for the completion of these two phases. Efforts to understand the downstream target genes that are activated by TRs during metamorphosis are now underway.

Developmental Potential of Human Embryonic Stem Cells

Our main interest is to develop the necessary tools and reagents to unravel the molecular events leading to the terminal differentiation of mesodermal and endodermal lineages in humans and zebrafish. The major focus will be the identification of potential master regulator molecules in this differentiation cascade. Once identified, they will be transfected into human ES cells to produce stable cell lines. These cell lines will find tremendous application in the treatment of human diseases by transplantation therapy. Finally, we would like to document the global transcription events that are activated during the differentiation process using genomics tools.

Research or Studentships vacancies are currently available for both research areas of zebrafish developmental biology and stem cell biology. Please contact Associate Professor Chan Woon Khiong via email (dbscwk@nus.edu.sg) for the list of projects that are available.

Selected Publications (Transgenic Fish, Zebrafish & Nuclear Receptors):

Chai C and Chan WK (2000). Developmental expression of a novel Ftz-F1 homologue, ff1b (NR5A4), in the zebrafish Danio rerio. Mechanisms of Development 91: 423-428.
Liu YW and Chan WK (2002). Thyroid hormones are important for embryonic to larval transition in zebrafish. Differentiation 70:36-45
Chai C and Chan WK (2003). ff1b is required for the development of steroidogenic component of the zebrafish interrenal organ. Developmental Biology 260:226-244.
Liu YW, Gao W, Teh WL, Tan JH, Chai C and Chan WK (2003). Prox1 is a novel corepressor of Ff1b and acts downstream of ff1b in the development of interrenal tissue. Molecular and Cellular Biology 23:7243-7255.
Kuo MW, Postlethwait J, Lee WC, Lou SW, Chan WK and Chung BC (2005). Gene duplication, gene loss and evolution of expression domains in the vertebrate nuclear receptor NR5A (Ftz-F1) family. The Biochemical Journal 389:19-26.

Human Embryonic Stem Cells

Richards M, Fong CY, Chan WK, Wong PC and Bongso A (2002). Human feeders support prolonged undifferentiated growth of human inner cell masses and embryonic stem cells. Nature Biotechnology 20:933-936.
Richards M, Tan S, Fong CY, Biswas A, Chan WK and Bongso A (2003). Comparative evaluation of various human feeders for prolonged undifferentiated growth of human embryonic stem cells. Stem Cells 21: 546-556.
Richards M, Tan SP, Tan JH, Chan WK* and Bongso A* (2004). The transcriptome profile of human embryonic stem cells as defined by SAGE. Stem Cells 22:51-64. (*Joint Corresponding authors)
Richards M, Tan SP, Chan WK* and Bongso* (2006). Reverse SAGE identification of orphan SAGE tags from human embryonic stem cells identifies the presence of novel transcripts and antisense transcription of key pluripotency genes. Stem Cells 24:1162-1173 (*Joint Corresponding authors).
Kueh J, Mark R, Ng SW, Chan WK and Bongso TA (2006). The search for factors in human feeders that support the derivation and propagation of human embryonic stem cells: Preliminary studies using transcriptome profiling by serial analysis of gene expression. Fertility and Sterility 85:1843-1846.