Wnt2b inhibits differentiation of retinal progenitor cells in the absence of Notch activity by downregulating the expression of proneural genes

Abstract

During the development of the central nervous system, cell proliferation and differentiation are precisely regulated. In the vertebrate eye, progenitor cells located in the marginal-most region of the neural retina continue to proliferate for a much longer period compared to the ones in the central retina, thus showing stem-cell-like properties. Wnt2b is expressed in the anterior rim of the optic vesicles, and has been shown to control differentiation of the progenitor cells in the marginal retina. In this paper,we show that stable overexpression of Wnt2b in retinal explants inhibited cellular differentiation and induced continuous growth of the tissue. Notably,Wnt2b maintained the undifferentiated progenitor cells in the explants even under the conditions where Notch signaling was blocked. Wnt2b downregulated the expression of multiple proneural bHLH genes as well as Notch. In addition,expression of Cath5 under the control of an exogenous promoter suppressed the negative effect of Wnt2b on neuronal differentiation. Importantly, Wnt2b inhibited neuronal differentiation independently of cell cycle progression. We propose that Wnt2b maintains the naive state of marginal progenitor cells by attenuating the expression of both proneural and neurogenic genes, thus preventing those cells from launching out into the differentiation cascade regulated by proneural genes and Notch.

Loss of adenomatous polyposis coli (apc) results in an expanded ciliary marginal zone in the zebrafish eye

“…The Wnt/b-catenin pathway is not only important for the specification of peripheral fates but it also maintains stem cells in the CMZ of chick (Kubo et al, 2003(Kubo et al, , 2005Kubo and Nakagawa, 2009). In the presence of activated Wnt/b-catenin, signaling neuronal differentiation is inhibited, CMZ markers are expanded toward the central retina, and cells remain proliferative.…”

“…We observed that, in zebrafish apc mutant eyes, cells that express Wnt/ b-catenin pathway members do not co-express retinal identity and proneural genes, such as sox2 and atoh7 suggesting that Wnt/b-catenin signaling inhibits these genes (Kubo et al, 2003(Kubo et al, , 2005Cho and Cepko, 2006;Liu et al, 2007). The neural competence factor Sox2 plays a crucial role in retina development as it is essential for neural specification maintenance of neural progenitor cells (Mizuseki et al, 1998;Bylund et al, 2003;Pevny and Placzek, 2005;Van Raay et al, 2005;Taranova et al, 2006;Agathocleous et al, 2009).…”

“…However, overactivation of this pathway causes loss of differentiated cells (Kubo et al, 2003(Kubo et al, , 2005Cho and Cepko, 2006;Agathocleous et al, 2009). It was reported that retinal ganglion cells differentiate in apc mutant zebrafish, even though they are disorganized (Nadauld et al, 2006;Paridaen et al, 2009).…”

“…However, Wnt/bcatenin signaling influences proliferation independently from its inhibitory role on cell differentiation, e.g., by means of the induction of the differentiation inhibitor Sox2 (Kubo et al, 2005;Agathocleous et al, 2009). Wnt/ b-catenin target genes that are essential for proliferation are cyclinD1 and myca that promote transition into Sphase (Schreiber-Agus et al, 1993;He et al, 1998;Tetsu and McCormick, 1999;Megason and McMahon, 2002;Pelengaris and Khan, 2003;Levine and Green, 2004).…”

“…The Wnt/bcatenin pathway regulates stem cells, proliferation and differentiation in many other systems and also plays an important role in the regulation of the CMZ in vertebrates (Van Raay and Vetter, 2004;Van Raay et al, 2005). Studies in mice and chick have emphasized that activation of the Wnt pathway is required for the development of peripheral structures, such as the ciliary body and the iris, whereas other studies propose that the Wnt pathway also serves an important function in the maintenance of stem cells (Kubo et al, 2003(Kubo et al, , 2005Liu et al, 2007;Denayer et al, 2008;Kubo and Nakagawa, 2009). Here we propose that the zebrafish adenomatous polyposis coli (apc mcr ) mutation, which leads to constitutive up-regulation of the Wnt/b-catenin pathway, is an excellent model to decipher the roles of signaling pathways in the developing eye.…”