Developmental Neurobiology Unit (Ichiro Masai)
Associate Professor Ichiro Masai
masai at oist.jp
One aim of developmental biology is to elucidate the mechanisms that control cell fate decisions and tissue pattern formation during the development of multicellular organisms. For this purpose, we are focusing on the vertebrate retina. In the vertebrate, the retinal region is initially induced in the anterior neural plate and evaginates from the forebrain to form optic cups. In this region, six major classes of retinal neurons differentiate to form a highly organized neural network that mediates visual transduction. Thus, the neural retina provides an excellent model for studying cell differentiation, neural circuit formation and neural functions in the brain. Furthermore, there are hereditary human diseases that cause a progressive degeneration of photoreceptor cells. It is important for medical sciences to understand the mechanisms controlling the maintenance of retinal neurons.
Because the fundamental structure of the neural retina is conserved among vertebrate animals from fish to humans, we use the zebrafish, which is one of the excellent animal models suitable for studies in the fields of developmental biology and medical sciences. The zebrafish has several merits for use in genetic and cell biology studies, for example, easy mating for egg collection, a short span of life cycle, and transparent tissues. Previously, we performed a large-scale mutagenesis study using zebrafish and identified more than three hundred mutants showing defects in retinal development, including those in neuronal differentiation, neural circuit formation, and the maintenance of photoreceptor neurons. To elucidate the mechanism that regulates neuronal development, we will examine the retinal phenotypes of these mutants and identify their mutated genes.
Ongoing research projects
1) Mechanism regulating spatial and temporal pattern of retinal neurogenesis The neural retina is initially composed of multipotent mitotic progenitor cells. At the stage in which retinal neurogenesis occurs, retinal progenitor cells exit from the cell cycle and differentiate into postmitotic cells. This is the initial step of neurogenesis; however, the mechanism that determines whether retinal progenitor cells continue to proliferate or exit from the cell cycle remains unknown. Previously, we found that retinal neurogenesis is initiated at one location in the neural retina and progresses throughout the entire neural retina in a wavelike manner (Masai et al., 2000), suggesting that retinal neurogenesis is spatially and temporally regulated during development. Our previous studies revealed that at least four signaling molecules, namely Hedgehog (Hh), Wnt, Notch and Histone deacetylase 1 (HDAC1), regulate the ratio and timing of retinal neurogenesis in the zebrafish (Masai et al., 2005; Yamaguchi et al., 2005). Here, we will elucidate the molecular network that regulates the switch from proliferation to differentiation in the zebrafish retina by identifying factors that interact with the signaling molecules.
2) Mechanism regulating later steps of retinal development Previous cell lineage studies using vertebrate retina revealed that cell fate determination is independent cell lineage and retinal progenitor cells can give rise to all retinal cell-types. Current model is that signals from environment changes the competence of retinal progenitor cells to generate different types of retinal cell, but the mechanism underlying the generation of different retinal cell-types still remains to be elucidated. In our screening for mutations, we identified many zebrafish retinal mutants in which cell differentiation and neural circuit formation are severely disrupted. They include mutants showing apoptosis of retinal neurons, disorganization of retinal layers, abnormal cell differentiation and photoreceptor degeneration. To elucidate an entire signaling network that regulates developmental processes from retinal progenitor cells to mature retinal neurons, we will characterize these mutants and identify their mutated genes.
Contribution of our research to medical sciences
In fish, retinal stem cells are located in the peripheral retina and continue to produce neurons throughout the life. In contrast of fish, such retinal stem cells are exhausted in mammals including human. However, it is essentially unclarified why such difference exits between fish and mammals. Recently it has been reported that adult mammal iris cell can produce retinal photoreceptors by artificially introducing a regulatory factors that directs photoreceptor differentiation, suggesting that iris pigment cells have retinal stem cell properties. However, it is still difficult to prepare retinal stem cells that can be applicable to medical treatment. Here we will elucidate the mechanism underling the switch from proliferation to differentiation in the retina. Such knowledge will led to the development of a method of preparing retinal stem cells safely and the advancement of therapeutic strategies of eye diseases in the future.
Furthermore, there are hereditary retinal diseases in humans, for example, retinitis pigmentosa, in which photoreceptors undergo a progressive degeneration. To date, about 160 genetic loci associated with retinal diseases have been mapped to human chromosomes. The recent completed human genome project accelerated the cloning of these mutated genes, and more than one hundred genes have been identified. These genes function in signaling pathways involved in various biological processes of photoreceptors: phototransduction, retinoid metabolism, and intracellular protein transport. However, there are still many molecules whose function remains unclarified. In our screening of mutations, we found several zebrafish mutants that show a progressive degeneration of photoreceptors. The analysis on these mutants will reveal the mechanism underlying retinal pathogenesis in human.
Mochizuki, T., Suzuki, S., and Masai, I. (2014) Spatial pattern of cell geometry and cell-division orientation in zebrafish lens epithelium. Biol. Open 3, 982–994.
Imai, F., Yoshizawa, A., Matsuzaki, A., Oguri, E., Araragi, M., Nishiwaki, Y. & Masai, I. (2014) Stem-loop binding protein is required for retinal cell proliferation, neurogenesis, and intraretinal axon pathfinding in zebrafish. Dev. Biol. 391, 94-109.
Mochizuki, T. & Masai, I. (2014) The lens equator: a platform for molecular machinery that regulates the switch from cell proliferation to differentiation in the vertebrate lens. Dev. Growth Differ. 56, 387-401.
Nishiwaki, Y., Yoshizawa, A., Kojima, Y., Oguri, E., Nakamura, S., Suzuki, S., Yuasa-Kawada, J., Kinoshita-Kawada, M., Mochizuki, T., and Masai, I. (2013) The BH3-only SNARE BNip1 mediates photoreceptor apoptosis in response to vesicular fusion defects. Dev. Cell 25, 374 – 387.
Imai, F., Yoshizama, A., Fujimori-Tonou, N., Masai, I. (2010) The ubiquitin proteasome system is required for cell-cycle progression of the lens epithelium and differentiation of lens fiber cells in zebrafish. Development 137, 3257-3268.
Yamaguchi M, Imai F, Tonou-Fujimori N, Masai I. (2010) Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina. Mech. Dev. 127, 247-264.
Yamaguchi M, Fujimori-Tonou N, Yoshimura Y, Kishi T, Okamoto H, Masai I. (2008) Mutation of DNA primase causes extensive apoptosis of retinal neurons through the activation of DNA damage checkpoint and tumor suppressor p53. Development 135, 1247-1257.
Nishiwaki Y, Komori A, Sagara H, Suzuki E, Manabe T, Hosoya T, Nojima Y, Wada H, Tanaka H, Okamoto H, Masai I. (2008) Mutation of cGMP phosphodiesterase 6alpha'-subunit gene causes progressive degeneration of cone photoreceptors in zebrafish. Mech. Dev. 125, 932-946.
Yamaguchi M, Tonou-Fujimori N, Komori A, Maeda R, Nojima Y, Li H, Okamoto H, Masai I. (2005). Histone deacetylase 1 regulates retinal neurogenesis in zebrafish by suppressing Wnt and Notch signaling pathways. Development 132, 3027-3043.
Masai I, Yamaguchi M, Tonou-Fujimori N, Komori A, Okamoto H. (2005) The hedgehog-PKA pathway regulates two distinct steps of the differentiation of retinal ganglion cells: the cell-cycle exit of retinoblasts and their neuronal maturation. Development 132, 1539-1553.
Masai I, Lele Z, Yamaguchi M, Komori A, Nakata A, Nishiwaki Y, Wada H, Tanaka H, Nojima Y, Hammerschmidt M, Wilson SW, Okamoto H. (2003) N-cadherin mediates retinal lamination, maintenance of forebrain compartments and patterning of retinal neurites. Development 130, 2479-2494.
Masai I, Stemple DL, Okamoto H, Wilson SW. (2000) Midline signals regulate retinal neurogenesis in zebrafish. Neuron 27, 251-263.