The eye is a critical sensory organ, and malformations of the eye, due to defective development, commonly account for serious visual impairment in newborns. My laboratory uses the zebrafish (Danio rerio) as a model for eye development. Because the process is conserved among all vertebrates, understanding the mechanisms underlying zebrafish eye formation is likely to improve our understanding of the processes underlying human eye development. Although genetic studies have identified many genes involved in eye development, we still have surprisingly poor understanding of its morphogenesis, the process by which the organ achieves proper cellular organization and shape. Our research, using live imaging and zebrafish molecular genetics and cell biology, aims to understand how a simple mass of cells becomes organized into the properly formed and patterned optic cup.
My lab studies the cellular and molecular mechanisms underlying tissue morphogenesis: the process by which a group of cells achieves its proper cellular organization and shape. Using the vertebrate eye as a model, we want to understand how the cells that comprise the vertebrate optic cup – neural retina, retinal pigmented epithelium, and lens – form the stereotyped structure that is critical for visual function. Developmental defects in eye morphogenesis represent a common cause of serious visual impairment in newborns.
Using confocal microscopy and a custom-built software suite for tracking cell behaviors in four dimensions, we have previously generated a map encompassing all cellular movements and divisions during zebrafish optic cup morphogenesis. We found that a complex set of cell movements, coordinated between different tissues, is responsible for shaping the eye. Using this unprecedented dataset as a reference point, our current studies focus on the molecular mechanisms underlying eye morphogenesis. Our studies make use of zebrafish molecular genetics, cell biology, and live imaging, and include the following projects:
Cell-matrix adhesion and cell polarity
We found that the extracellular matrix component laminin is required for eye morphogenesis: in bashfulUW1 (laminin-1) mutants, optic stalk constriction and optic vesicle invagination are impaired (Figure 2). In addition, establishment of apicobasal polarity is impaired. However, we still do not understand how disrupted polarity leads to this phenotype, how polarity is initially established, or the signaling pathway downstream of laminin.
Developmental signaling pathways
While signaling pathways such as Shh and Wnt are known to play a role in patterning the optic vesicle, their role in morphogenesis itself is unknown. By performing 4-dimensional cell tracking on zebrafish mutants and morphants, we will quantitatively determine the contributions of different developmental signaling pathways to eye morphogenesis.
During eye morphogenesis, we find that retinal progenitors have unexpectedly motile behaviors despite their epithelial polarization. Such behaviors are frequently regulated by Rho-family small GTPases, and we will investigate the involvement of these signaling molecules in coordinating the motility of single cells and the morphogenesis of entire sheets if tissue.
New genes involved in eye morphogenesis
Our 4D analysis suggests that coordination of cell movements between tissues is critical for eye morphogenesis. In addition, in a forward genetic screen, I isolated a mutant, O15, in which extremely tight adhesion between the optic vesicle and the overlying ectoderm impairs invagination of both the optic vesicle and lens. We are isolating the underlying mutation and will determine the nature of the tissue-tissue interactions regulated by O15.
References to Publications:
K.M. Kwan*, H. Otsuna*, H. Kidokoro, K.R. Carney, Y. Saijoh, and C.-B. Chien. (2012) A Complex Choreography of Cell Movements Shapes the Vertebrate Eye. Development 139: 359-372.
K.M. Kwan. (2010) 25 years on, Developmental Biology remains dynamic, competent, and instructive (book review). Developmental Dynamics 239: 3506-7
K.M. Kwan, E. Fujimoto, C. Grabher, B.D. Mangum, M.E. Hardy, D.S. Campbell, J.M. Parant, H.J. Yost, J.P. Kanki and C.-B. Chien. (2007) The Tol2kit: a multisite-gateway based construction kit for Tol2 transposon transgenesis constructs. Developmental Dynamics 236: 3088-99
C.J. Lowe, M. Terasaki, M. Wu, R.M. Freeman Jr., L. Runft, K. Kwan, S. Haigo, J. Aronowicz, E. Lander, C. Gruber, M. Smith, M.W. Kirschner and J. Gerhart. (2006) Dorsoventral patterning in hemichordates: insights into early chordate evolution. PLoS Biology 4(9): e291
K.M. Kwan and M.W. Kirschner. (2005) A microtubule-binding Rho-GEF controls cell morphology during Xenopus convergent extension. Development 132: 4599-610
I. Ivanovska, E. Lee, K.M. Kwan, D.D. Fenger and T.L. Orr-Weaver. (2004) The Drosophila MOS ortholog is not essential for meiosis. Current Biology 14: 75-80
L.C. Murtaugh, B.Z. Stanger, K.M. Kwan and D.A. Melton. (2003) Notch signaling controls multiple steps of pancreatic differentiation. Proceedings of the National Academy of Sciences USA 100: 14920-5
K.M. Kwan and M.W. Kirschner. (2003) Xbra functions as a switch between cell migration and convergent extension in the Xenopus gastrula. Development 130: 1961–72
Kristen Kwan, Ph.D.
Department of Human Genetics
University of Utah
15 N 2030 E rm. 7160A
Salt Lake City, Utah 84112-5330