No abstract

Although placodes are ubiquitous precursors of tissue invagination, the mechanism of placode formation has not been established and the requirement of placode formation for subsequent invagination has not been tested. Earlier measurements in chicken embryos supported the view that lens placode formation occurs because the extracellular matrix (ECM) between the optic vesicle and the surface ectoderm prevents the prospective lens cells from spreading. Continued cell proliferation within this restricted area was proposed to cause cell crowding, leading to cell elongation (placode formation). This view suggested that continued cell proliferation and adhesion to the ECM between the optic vesicle and the surface ectoderm was sufficient to explain lens placode formation. To test the predictions of this “restricted expansion hypothesis,” we first confirmed that the cellular events that accompany lens placode formation in chicken embryos also occur in mouse embryos. We then showed that the failure of lens placode formation when the transcription factor, Pax6 was conditionally deleted in the surface ectoderm was associated with greatly diminished accumulation of ECM between the optic vesicle and ectoderm and reduced levels of transcripts encoding components of the ECM. In accord with the “restricted expansion hypothesis,” the Pax6-deleted ectoderm expanded, rather than being constrained to a constant area. As a further test, we disrupted the ECM by deleting Fn1, which is required for matrix assembly and cell-matrix adhesion. As in Pax6CKO embryos, the Fn1CKO lens ectoderm expanded, rather than being constrained to a fixed area and the lens placode did not form. Ectoderm cells in Fn1CKO embryos expressed markers of lens induction and reorganized their cytoskeleton as in wild type ectoderm, but did not invaginate, suggesting that placode formation establishes the minimal mechanical requirements for invagination.

The developing ocular lens provides an excellent model system with which to study the intrinsic and extrinsic cues governing cell differentiation. Although the transcription factors Pax6 and Sox2 have been shown to be essential for lens induction, their later roles during lens fiber differentiation remain largely unknown. Using Cre/loxP mutagenesis, we somatically inactivated Pax6 and Sox2 in the developing mouse lens during differentiation of the secondary lens fibers and explored the regulatory interactions of these two intrinsic factors with the canonical Wnt pathway. Analysis of the Pax6-deficient lenses revealed a requirement for Pax6 in cell cycle exit and differentiation into lens fiber cells. In addition, Pax6 disruption led to apoptosis of lens epithelial cells. We show that Pax6 regulates the Wnt antagonist Sfrp2 in the lens, and that Sox2 expression is upregulated in the Pax6-deficient lenses. However, our study demonstrates that the failure of differentiation following loss of Pax6 is independent of β-catenin signaling or Sox2 activity. This study reveals that Pax6 is pivotal for initiation of the lens fiber differentiation program in the mammalian eye.KEY WORDS: Pax6, Sox2, Lens, Crystallin, Wnt, β-catenin, Mouse Development 136, 2567Development 136, -2578Development 136, (2009 2568Chen et al., 2004;Chen et al., 2006). When the Wnt coreceptor Lrp6 was deleted in mice, aberrant LFCs appeared in the anterior pole of the lens (Stump et al., 2003). Upon inactivation of the canonical Wnt effector β-catenin, LE markers, proliferation and differentiation were disrupted (Cain et al., 2008). These findings suggest a role for Wnt signaling in LE cell fate.Owing to the severe ocular phenotype of Pax6 mutants, the later developmental roles of Pax6 could only be extrapolated from in vitro research. Herein, we introduce the first in vivo loss-of-function model of Pax6 and its presumed transcriptional partner Sox2. We show that the loss of Pax6 prevents LFC differentiation and results in cell death and in an increase in Sox2. However, conditional deletion of Sox2 reveals that it is dispensable for LFC differentiation. Furthermore, overexpression of β-catenin results in a differentiation failure that is similar to, but independent of, that observed following Pax6 loss. These findings place Pax6 upstream in the cascade of events leading to the differentiation of LE into lens fibers in mammals. MATERIALS AND METHODS Mouse linesMouse lines employed in this study were: Pax6 lox (Ashery-Padan et al., 2000), Mlr10 (Zhao et al., 2004), Catnb lox(ex3) (Harada et al., 1999) and BATlacZ (Nakaya et al., 2005) and are described in Fig. S1 in the supplementary material. The Sox2 loxP line (see Fig. 7A) contains two loxP sites inserted around the single exon of the murine Sox2 gene using conventional gene-targeting methods (Joyner, 1995). In the gene-targeting vector, loxP-frt-pMC1neopA-frt, the neo gene is flanked by frt sites. Flp recombinase activity within the B6.SJL-Tg(ACTFLPe)9205Dym/J mouse line (Rodriguez et al.,...

During development, tissue-specific transcription factors regulate both protein-coding and non-coding genes to control differentiation. Recent studies have established a dual role for the transcription factor Pax6 as both an activator and repressor of gene expression in the eye, central nervous system, and pancreas. However, the molecular mechanism underlying the inhibitory activity of Pax6 is not fully understood. Here, we reveal that Trpm3 and the intronic microRNA gene miR-204 are co-regulated by Pax6 during eye development. miR-204 is probably the best known microRNA to function as a negative modulator of gene expression during eye development in vertebrates. Analysis of genes altered in mouse Pax6 mutants during lens development revealed significant over-representation of miR-204 targets among the genes up-regulated in the Pax6 mutant lens. A number of new targets of miR-204 were revealed, among them Sox11, a member of the SoxC family of pro-neuronal transcription factors, and an important regulator of eye development. Expression of Trpm/miR-204 and a few of its targets are also Pax6-dependent in medaka fish eyes. Collectively, this study identifies a novel evolutionarily conserved mechanism by which Pax6 controls the down-regulation of multiple genes through direct up-regulation of miR-204.

Repeated alcohol intake leads to mesostriatal neuroadaptations, resulting in drinking escalation and addiction phenotypes. Fibroblast growth factor 2 (FGF2) has been shown to interact with the mesostriatal dopaminergic system, and has been implicated in the actions of psychostimulants in the brain, and in several psychiatric disorders. Here, we report on a positive regulatory feedback loop of alcohol and FGF2 in rodent models. Specifically, we found that acute alcohol exposure (2.5 g/kg, i.p.) increased the mRNA expression of in the dorsal hippocampus, nucleus accumbens, and dorsal striatum. Longer alcohol exposure (7 d × 2.5 g/kg, i.p.) restricted these increases to the dorsal striatum, and the latter effect was blocked by the dopamine D2-like receptor antagonist haloperidol. Voluntary prolonged and excessive alcohol consumption in a 2-bottle choice procedure increased expression selectively in dorsomedial striatum (DMS) of both mice and rats. Importantly, we found that systemic administration of recombinant FGF2 (rFGF2) in mice, or rFGF2 infusion into the dorsal striatum or DMS of rats, increased alcohol consumption and preference, with no similar effects on saccharin or sucrose consumption. Finally, we found that inhibition of the endogenous FGF2 function in the DMS, by an anti-FGF2 neutralizing antibody, suppressed alcohol consumption and preference. Together, our results suggest that alcohol consumption increases the expression of in the DMS, and that striatal FGF2 promotes alcohol consumption, suggesting that FGF2 in the DMS is a positive regulator of alcohol drinking. Long-term alcohol intake may lead to neuroadaptations in the mesostriatal reward system, resulting in addiction phenotypes. Fibroblast growth factor 2 (FGF2) is crucial for the development and maintenance of the mesostriatal dopaminergic system. Here, we provide evidence for the involvement of FGF2 in alcohol-drinking behaviors. We show that alcohol increases expression in the dorsal striatum, an effect mediated via dopamine D2-like receptors. Importantly, we show that infusion of recombinant FGF2 into the dorsomedial striatum increases alcohol consumption, whereas inhibiting the endogenous FGF2 function suppresses consumption. Thus, FGF2 is an alcohol-responsive gene constituting a positive regulatory feedback loop with alcohol. This loop leads to facilitation of alcohol consumption, marking FGF2 as a potential new therapeutic target for alcohol addiction.