Advertisement
RESEARCH ARTICLE
Prenatal ethanol exposure in mice phenocopies Cdon mutation by impeding Shh function in the etiology of optic nerve hypoplasia
Benjamin M. Kahn, Tanya S. Corman, Korah Lovelace, Mingi Hong, Robert S. Krauss, Douglas J. Epstein
Disease Models & Mechanisms 2017 10: 29-37; doi: 10.1242/dmm.026195

ABSTRACT

Septo-optic dysplasia (SOD) is a congenital disorder characterized by optic nerve, pituitary and midline brain malformations. The clinical presentation of SOD is highly variable with a poorly understood etiology. The majority of SOD cases are sporadic, but in rare instances inherited mutations have been identified in a small number of transcription factors, some of which regulate the expression of Sonic hedgehog (Shh) during mouse forebrain development. SOD is also associated with young maternal age, suggesting that environmental factors, including alcohol consumption at early stages of pregnancy, might increase the risk of developing this condition. Here, we address the hypothesis that SOD is a multifactorial disorder stemming from interactions between mutations in Shh pathway genes and prenatal ethanol exposure. Mouse embryos with mutations in the Shh co-receptor, Cdon, were treated in utero with ethanol or saline at embryonic day 8 (E8.0) and evaluated for optic nerve hypoplasia (ONH), a prominent feature of SOD. We show that both Cdon−/− mutation and prenatal ethanol exposure independently cause ONH through a similar pathogenic mechanism that involves selective inhibition of Shh signaling in retinal progenitor cells, resulting in their premature cell-cycle arrest, precocious differentiation and failure to properly extend axons to the optic nerve. The ONH phenotype was not exacerbated in Cdon−/− embryos treated with ethanol, suggesting that an intact Shh signaling pathway is required for ethanol to exert its teratogenic effects. These results support a model whereby mutations in Cdon and prenatal ethanol exposure increase SOD risk through spatiotemporal perturbations in Shh signaling activity.

INTRODUCTION

Septo-optic dysplasia (SOD) is a clinically heterogeneous disorder that is diagnosed on the presence of at least two of the following conditions: optic nerve hypoplasia (ONH), hypopituitarism and absence of the septum pellucidum (Webb and Dattani, 2010). The severity of these features varies widely in SOD, which has an incidence of 1 in 10,000 live births (Patel et al., 2006). ONH is the most common finding in SOD, and manifests as a thinning of the optic nerve as it exits the eye, resulting in insufficient photo-transduction to the brain and, in many instances, blindness (Morishima and Aranoff, 1986; Cemeroglu et al., 2015). Variable pituitary dysfunction, including isolated growth hormone deficiency, central hypothyroidism, and panhypopituitarism is also observed in individuals with SOD, with decreased levels of one or more pituitary hormones being diagnosed by two years of age (Cemeroglu et al., 2015). Cognitive delay and seizure disorders are also frequently seen in SOD.

The cause of SOD is poorly understood. Most cases are idiopathic, but in rare instances (<1%) inherited mutations have been described in a small number of transcription factors (SOX2, SOX3, HESX1, OTX2, TCF7L1) expressed during embryonic brain development (McCabe et al., 2011; Gaston-Massuet et al., 2016). The high phenotypic variability, coupled with its sporadic nature, suggest that SOD might be influenced by a combination of environmental and genetic factors.

Insight into the pleiotropic nature of the SOD phenotype was recently realized from the study of a conditional mouse mutant lacking Shh in the developing hypothalamus (ShhΔhyp). ShhΔhyp mutants display optic nerve and pituitary defects with similarities to SOD in humans (Zhao et al., 2012). The eye and pituitary develop in close proximity to the source of SHH in the anterior hypothalamus and depend on this signal for formation of the optic disc, from where the optic nerve exits the eye, and for coordinating pituitary morphogenesis. These findings raise the possibility that reduced SHH expression and or signaling activity from the hypothalamus might underlie the pathogenesis of SOD in humans. In support of this hypothesis, SOX2 and SOX3 – two SOD-associated genes – were shown to be dose-dependent regulators of SHH transcription that directly bind and activate a long-range SHH forebrain enhancer (Zhao et al., 2012).

Nonetheless, loss-of-function mutations in SHH are not associated with SOD (Paulo et al., 2015; Gregory et al., 2015), but instead are known to cause another brain malformation, holoprosencephaly (HPE), with partially overlapping features to SOD (Roessler et al., 1996). HPE results from imperfect separation of the cerebral hemispheres and craniofacial structures as a result of reduction in SHH signaling from the prechordal plate, a transient embryonic tissue required for early aspects of forebrain development, including the specification of the hypothalamic territory (Chiang et al., 1996). Therefore, HPE and SOD could be distinguished by the timing and location of SHH signal disruption, with an early loss of SHH from the prechordal plate giving rise to HPE, and a slightly later absence of SHH from the presumptive hypothalamus resulting in SOD.

The SHH pathway has many roles during eye development. Early functions include separation of the eye fields and patterning of the optic cup (Chiang et al., 1996). At later stages, Shh secreted from retinal ganglion cells (RGCs) controls the proliferation of multipotent retinal progenitor cells (RPCs), the timing of their differentiation, as well as the guidance of RGC axons out of the eye (Wang et al., 2005; Kolpak et al., 2005; Sanchez-Camacho and Bovolenta, 2008; Stacher Hörndli and Chien, 2012). Mice lacking Shh in RGCs display ONH resulting from a failure in optic disc formation (Dakubo et al., 2003). Thus, ONH can arise by interfering with SHH signaling from two independent sources, anterior hypothalamus and RGCs, at distinct stages of eye development.

Epidemiological studies indicate that SOD associates with young maternal age and primiparity (Haddad and Eugster, 2005; Murray et al., 2005; Garcia-Filion and Borchert, 2013; Cemeroglu et al., 2015). How these risk factors contribute to the etiology of SOD is unknown, but they might be linked to adverse maternal behavior during early stages of pregnancy (Garcia-Filion and Borchert, 2013). For instance, several clinical features of fetal alcohol syndrome overlap with HPE and SOD, suggesting that prenatal ethanol exposure might increase the risk of both conditions, depending on the timing of the insult (Sulik et al., 1981; Strömland, 1987; Coulter et al., 1993; Ashwell and Zhang, 1994; Blader and Strähle, 1998; Hellström, 1999; Ribeiro et al., 2007; Aoto et al., 2008; Loucks and Ahlgren, 2009; Lipinski et al., 2010, 2012; Zhang et al., 2011).

The SHH signaling pathway is a key target of prenatal ethanol exposure and its perturbation explains much of the HPE-like phenotype observed in animal models of this condition (Ahlgren et al., 2002; Li et al., 2007; Higashiyama et al., 2007; Aoto et al., 2008). Interestingly, mouse embryos with mutations in Shh pathway genes that have no, or minimal, phenotypic consequence on their own, show a profound increase in the penetrance and severity of HPE when exposed to sub-teratogenic doses of ethanol (Hong and Krauss, 2012; Kietzman et al., 2014). The synergy between these genetic and environmental risk factors for HPE is dependent on the timing of ethanol administration during pregnancy, with a strong interaction observed at embryonic day 7 (E7.0), coinciding with a disruption in Shh signaling from the prechordal plate (Hong and Krauss, 2012).

On the basis of these studies, we postulate that SOD is a multifactorial condition that results from interactions between genetic and environmental risk factors acting at slightly later stages of forebrain development than those that cause HPE. To test this hypothesis and better define the relationship between ethanol intake, Shh signaling and SOD, we examined eye development in mouse embryos with mutations in the Shh co-receptor, Cdon, that were exposed in utero to either ethanol or saline at E8.0. Wild­-type embryos treated with ethanol phenocopied Cdon−/− mutants treated with saline in the manifestation of ONH by selectively impeding Shh signaling activity in RPCs. The combination of Cdon mutation and ethanol exposure did not worsen the ONH phenotype, indicating that this gene–environment interaction is not additive or synergistic. These results support a model whereby mutations in Cdon and prenatal ethanol exposure are risk factors for SOD and HPE through temporally and spatially distinct perturbations in Shh signaling activity.

RESULTS

We followed a previously validated protocol for prenatal ethanol exposure (see Materials and Methods) to determine whether Cdon−/− embryos were sensitive to ethanol-induced ONH, a prominent feature of SOD. All mice described in this study were maintained on a 129S6/SvEvTac genetic background, which is largely impervious to the HPE-associated phenotypes caused by Cdon mutation or ethanol exposure observed in other mouse strains (Zhang et al., 2006; Downing et al., 2009; Hong and Krauss, 2012). Pregnant Cdon+/− females that were time-bred with Cdon+/− males received intraperitoneal injections of ethanol (3.48 g/kg) or saline at E8.0 and again four hours later. This embryonic stage was chosen because it was subsequent to the HPE-critical period at E7.0, allowing us to address the temporal specificity of gene–environment interactions in the etiology of SOD.

Cdon mutation and prenatal ethanol exposure independently cause ONH

Cdon is expressed at early stages of eye development (E9-E11.5), including progenitors of the neural retina and lens vesicle (Zhang et al., 2009). Cdon−/− and wild-type embryos were harvested at E14.5, cryo-sectioned along the coronal plane of their heads, and immunostained for neurofilament. No gross abnormalities in the size or structure of the brain were observed between wild-type and Cdon−/− embryos in either the ethanol or saline treatment groups. Moreover, none of the prominent eye defects displayed by Cdon−/− mutant embryos on the C57BL/6 genetic background, including coloboma, microphthalmia and lens dysmorphology (Zhang et al., 2009) were detected in any of the 129S6 embryos (129S6.Cdon−/−), consistent with the strain specificity of these phenotypes.

To assess the embryos for ONH, the diameter of the optic nerve was measured at the level of the optic disc. Cdon−/− embryos treated with saline showed a 39% reduction in optic nerve diameter (mean±s.d., 32.26±1.83 μm, n=9, P<0.001) compared with control littermates (52±3.47 μm, n=8) (Fig. 1A,B,E). This difference was significant after normalizing for eye size (Fig. 1F-H). Wild­-type embryos exposed to ethanol showed a similar reduction in optic nerve diameter (29.7±2.14 μm, n=9, P<0.001) compared with saline-treated controls (Fig. 1A,C). This result was unexpected given that 129S6 embryos were thought to be resistant to ethanol-mediated teratogenicity (Downing et al., 2009; Hong and Krauss, 2012), although the optic nerve was not examined in these prior studies. The combination of Cdon mutation and prenatal ethanol exposure did not exacerbate the ONH phenotype compared with embryos with either condition alone. Ethanol-treated Cdon−/− embryos showed a 37% decrease in optic nerve width (33±2.4 μm, n=8, P<0.001) compared with saline­-treated controls (Fig. 1A,D), which was a similar reduction to that seen in saline-treated Cdon−/− embryos and ethanol-treated wild-type embryos (Fig. 1A-E). These data indicate that Cdon mutation and prenatal ethanol exposure both contribute to the etiology of ONH and that additional risk factors, such as genetic background (129S6 versus C57BL/6) and timing of ethanol exposure (E7.0 versus E8.0), influence the phenotypic outcome of ONH versus HPE.

Fig. 1.
Fig. 1.

Cdon mutation and ethanol exposure independently cause optic nerve hypoplasia. (A-D) Immunostaining for neurofilament (green) on transverse sections through the eye at E14.5 labels the optic nerve (arrow). Compared with (A) saline­-treated wild-type (Cdon+/+) embryos (n=9), the diameter of the optic nerve (white line) is significantly reduced in (B) saline­-treated Cdon−/− mutants (n=8), (C) ethanol-treated wild-type embryos (n=9), and (D) ethanol-treated Cdon−/− mutants (n=9). Scale bar: 200 µm. (E-H) Quantification of (E) optic nerve diameter, (F) axial length of eye, (G) axial width of eye, and (H) optic nerve diameter (OND) normalized to axial width (AW) of the eye. Error bars represent s.d. **P<0.01, ***P<0.001 by Student's t-test.

Formation of the optic disc is not disturbed in Cdon−/− and ethanol-exposed embryos

The optic nerve exits the eye through the optic disc, which forms at the juncture of the optic stalk and cup. ONH can arise from defects in optic disc formation or from a deficit in the number of RGC axons that make up the optic nerve (Deiner et al., 1997; Dakubo et al., 2003; Zhao et al., 2012). To distinguish between these two possibilities, we evaluated the expression of Pax2 in the optic disc of Cdon−/− and wild-type embryos that were exposed in utero to either saline or ethanol at E8.0 of gestation. No significant differences were observed between the number of Pax2+ cells in embryos from the experimental and control groups (Fig. 2), thus excluding major defects in optic disc formation as a likely explanation for the ONH phenotype in either of these mouse models.

Fig. 2.
Fig. 2.

The optic disc is not compromised in Cdon−/− or ethanol-treated embryos. (A-D) Immunostaining for Pax2 on transverse sections through the eye at E14.5 marks the optic disc (arrows). No significant differences were observed in the average number of Pax2+ optic disc cells per section from (A) saline­-treated wild-type (Cdon+/+) embryos (n=6), (B) saline­-treated Cdon−/− mutants (n=8), (C) ethanol-treated wild-type embryos (n=8), and (D) ethanol-treated Cdon−/− mutants (n=8). Scale bar: 200 µm. (E) Quantification of Pax2+ optic disc cells. Error bars represent s.d. Student's t-test.

Shh-dependent proliferation of RPCs is compromised in Cdon−/− and ethanol-treated embryos

The absence of a synergistic interaction between Cdon−/− mutation and prenatal ethanol exposure in the manifestation of ONH suggested that both insults might be disrupting a common or parallel signaling pathway(s) important for eye development. Shh is the most likely pathway to be compromised in these mouse models of ONH given the established role of Cdon as a Shh co-receptor, the essential function of Shh in RPC proliferation, and the negative influence of ethanol on Shh pathway activation in a variety of developing tissues (Ahlgren et al., 2002; Wang et al., 2005; Zhang et al., 2006; Tenzen et al., 2006; Li et al., 2007; Aoto et al., 2008; McLellan et al., 2008; Allen et al., 2011; Hong and Krauss, 2013).

To evaluate the integrity of Shh signaling we assessed Gli1 expression, a reliable readout of Shh pathway activation (Marigo et al., 1996), on sections through the eye at E14.5. Wild-type embryos treated with saline showed robust expression of Gli1 in the RPC layer of the developing eye at E14.5 (Fig. 3A). In comparison, Gli1 was markedly reduced in the RPCs of wild-type and Cdon−/− embryos exposed to ethanol at E8.0, as well as Cdon−/− embryos treated with saline (Fig. 3A-D). The downregulation of Gli1 seemed to be specific to the eye as an adjacent domain of expression in the anterior hypothalamus was unaffected across genotypes and treatment groups (Fig. 3E-H). Moreover, Shh expression was not compromised in the eye or hypothalamus of any of the embryos (Fig. 3I-P), suggesting that both Cdon mutation and prenatal ethanol exposure were acting directly on some aspect of RPC development downstream of Shh.

Fig. 3.
Fig. 3.

Selective reduction of Gli1 expression in the eyes of Cdon−/− and ethanol-treated embryos. In situ hybridization for Gli1 (A-H), and Shh (I-P) on transverse sections through the eye (A-D,I-L) and hypothalamus (E-H,M-P) of E14.5 wild-type (Cdon+/+) and Cdon−/− embryos treated with saline or ethanol at E8.0. Gli1 expression is detected in retinal progenitor cells (RPCs, area marked by dotted white line) of (A) saline­-treated wild-type embryos (n=5). Gli1 expression is markedly reduced in RPCs of (B) saline­-treated Cdon−/− mutants (n=5), (C) ethanol-treated wild-type embryos (n=9), and (D) ethanol-treated Cdon−/− mutants (n=5). No differences were observed in the expression of Gli1 in the hypothalamus between genotypes or treatment groups (E-H). No differences were observed in the level of Shh expression in retinal ganglion cells (RGCs, area marked by dotted black line, I-L) or the hypothalamus (M-P) between genotypes or treatment groups. Scale bar: 200 µm.

Shh signaling maintains RPCs in a mitotically active state until they are poised to differentiate into RGCs (Zhang and Yang, 2001; Wang et al., 2005). Therefore, we next determined whether the downregulation in Shh signaling observed in Cdon−/− and ethanol-treated embryos compromised the growth and differentiation properties of RPCs. The proliferation marker Ki67 labeled 670 RPCs per section (n=3) in saline­-treated wild-type embryos at E14.5 (Fig. 4A,I). By contrast, a drastic reduction in the number of Ki67-positive RPCs was observed in Cdon−/− embryos treated with ethanol (88 RPCs/section, n=3, P<0.001) or saline (117 RPCs/section, n=3, P<0.001), as well as wild-type embryos exposed to ethanol (113 RPCs/section, n=3, P<0.001) (Fig. 4A-D,I). Despite the significant reduction in Ki67 staining, trace amounts were still detected in Cdon−/− and ethanol-treated embryos upon increased exposure times. Reduced proliferation was also noted in the lens epithelium of Cdon−/− and ethanol-treated embryos (Fig. 4J), as described previously (Zhang et al., 2009). The proliferation defects seem to be specific to the eye as no significant differences were detected in the number of Ki67-positive neural progenitors in adjacent brain regions from either genotype or treatment group (Fig. 4E-H,K). These results suggest that the failure of RPCs to respond to Shh signaling in both Cdon−/− and ethanol-treated embryos at E8.0 compromises their ability to replicate, in agreement with other studies of Shh signaling in the eye (Wang et al., 2005).

Fig. 4.
Fig. 4.

Reduced proliferation of RPCs in Cdon−/− and ethanol-treated embryos. (A-H) Immunostaining for Ki67 on transverse sections through the eye (A-D) and hypothalamus (E-H) of E14.5 embryos labels proliferating progenitors. (A) The majority of retinal progenitor cells (RPCs, area marked by dotted white line) in saline­-treated wild-type (Cdon+/+) embryos (n=3), are marked by Ki67. The number of Ki67+ RPCs is significantly reduced in (B) saline­-treated Cdon−/− embryos (n=3), (C) ethanol-treated wild-type embryos (n=3), and (D) ethanol-treated Cdon−/− embryos (n=3). No differences in the number of Ki67+ cells in the ventricular layer of the ventral hypothalamus (boxed area) were observed between genotypes or treatment groups (E-H). Scale bar: 200 µm. (I-K) Quantification of Ki67+ cells in (I) retina, (J) lens and (K) hypothalamus sections across all experimental groups. Error bars represent s.d. *P<0.05, **P<0.01, ***P<0.001 by Student's t-test.

Precocious differentiation of RGCs in Cdon−/− and ethanol-treated embryos

To determine if the differentiation of RPCs was affected by their premature cell-cycle exit we assessed expression of Math5 (also known as Atoh7), a bHLH transcription factor required at the onset of RGC differentiation (Wang et al., 2001). In control embryos, Math5 expression was confined to postmitotic progenitors in the ventricular zone. By contrast, in Cdon−/− and ethanol-treated embryos, Math5 expression extended from the ventricular zone into the ganglion cell layer (Fig. 5A-D). This observation is similar to the previous report of expanded Math5 expression in mouse mutants that lack Shh signaling in the eye (Sakagami et al., 2009), and suggests that loss of Shh-dependent RPC proliferation might be associated with precocious differentiation of RPCs.

Fig. 5.
Fig. 5.

Precocious differentiation of RGCs in Cdon−/− and ethanol-treated embryos. (A-D) In situ hybridization for Math5 on transverse sections through the eye at E14.5 labels postmitotic progenitors in the ventricular zone (vz) of control embryos (n=3) (A). Math5 expression expands into the ganglion cell layer (gcl) of Cdon−/− and ethanol-treated embryos (n=3 for each experimental group) (B-D). Dashed line marks the vz–gcl boundary. (E-H) Immunostaining for Isl1/2 on transverse sections through the eye of E14.5 embryos primarily labels differentiating retinal ganglion cells (RGCs, arrows). Compared with (E) saline­-treated wild-type (Cdon+/+) embryos (n=3), the number of Isl1/2+ RGCs is significantly increased in (F) saline­-treated Cdon−/− mutants (n=3), (G) ethanol-treated wild-type embryos (n=3), and (H) ethanol-treated Cdon−/− mutants (n=3). (I-L) Immunostaining for AP-2α in amacrine cells. No significant differences were observed between the average number of AP-2α+ amacrine cells per section from (I) saline­-treated wild-type (Cdon+/+) embryos (n=4), (J) saline­-treated Cdon−/− mutants (n=3), (K) ethanol-treated wild-type embryos (n=4), and (L) ethanol-treated Cdon−/− mutants (n=3). Scale bar: 50 µm. (M,N) Quantification of cells expressing Isl1/2 (M) and AP-2α (N). Error bars represent s.d. *P<0.05, **P<0.01 by Student's t-test.

RGCs are the earliest born retinal cell type originating from a subset of RPCs expressing Math5 (Feng et al., 2010; Brzezinski et al., 2012). The LIM homeobox transcription factor Isl1 functions downstream of Math5, and in conjunction with the POU domain protein Pou4F2 promotes RGC differentiation (Mu et al., 2008; Pan et al., 2008; Prasov and Glaser, 2012; Wu et al., 2015). We evaluated the status of RGC differentiation in Cdon−/− and ethanol-treated embryos by immunostaining for Isl1. At E14.5, RGCs are still early in their differentiation as evidenced by the sparse labeling of Isl1 in saline­-treated wild-type embryos (225 cells/section, n=3) (Fig. 5E). However, the number of Isl1-positive cells was increased by 32% in wild-type (329 cells/section, n=3, P<0.05) and Cdon−/− (338 cells/section, n=3, P<0.01) embryos exposed to ethanol at E8.0, as well as in saline­-treated Cdon−/− mutants (349 cells/section, n=3, P<0.05) (Fig. 5E-H,M). Although Isl1 is not exclusively expressed by RGCs, we did not observe significant differences in the number of other early-born retinal progenitors, such as amacrine cells expressing AP-2α (also known as Tfap2a), between controls and treatment groups, suggesting that the precocious differentiation was limited to RGCs (Fig. 5I-L,N).

These data suggest that the loss of Shh signaling in Cdon−/− and ethanol-treated embryos results in the precocious differentiation of RGCs, which would likely deplete the pool of non-proliferating RPCs over time (Wang et al., 2005). The significant thinning of the optic nerve in experimental embryos likely results from the failure of these prematurely differentiating RGCs to properly extend axons to the optic disc, a premise that is supported by a previously characterized role for Shh in regulating the guidance of RGC axons (Sanchez-Camacho and Bovolenta, 2008). Taken together, our results demonstrate that prenatal ethanol exposure at E8.0 phenocopies 129S6.Cdon−/− mutant embryos in the manifestation of ONH by selective interference with Shh-dependent expansion and differentiation of RPCs in the eye.

DISCUSSION

Ethanol and Cdon mutation impede Shh signaling in RPCs to cause ONH

The association of SOD with young maternal age led to the hypothesis that adverse behavior, including prenatal alcohol exposure, is a predisposing factor in its etiology (Haddad and Eugster, 2005; Murray et al., 2005; Garcia-Filion and Borchert, 2013; Cemeroglu et al., 2015). Fetal exposure to alcohol causes a spectrum of developmental disorders; however, direct evidence linking ethanol to SOD has been lacking. Here, we used a mouse model to demonstrate that in utero exposure to ethanol at E8.0 causes ONH, the most prevalent SOD-associated phenotype. We show that ethanol causes ONH through a similar mechanism to that observed in Cdon−/− embryos, involving the inhibition of Shh signaling activity in retinal progenitor cells, which leads to their premature cell-cycle arrest, precocious differentiation, and failure to properly extend axons to the optic nerve (Fig. 6).

Fig. 6.
Fig. 6.

Model depicting the influence of Cdon mutation and ethanol exposure on Shh signaling activity in the developing eye. (A) In wild-type embryos (E14.5), the binding of Shh to Cdon and Ptch1 releases the inhibition on smoothened (Smo), facilitating the transcription by Gli activator (GliA) of target genes involved in RPC proliferation. (B) In the eyes of 129S6.Cdon−/− embryos, there is persistent inhibition of Smo by Ptch1, even in the presence of Shh, causing Gli repressor (GliR) to block transcription of genes involved in RPC proliferation, resulting in precocious RPC differentiation. (C) Ethanol exposure at E8.0 interferes with Shh signaling in the eye through a variety of proposed mechanisms. The lengthy delay between ethanol exposure (E8.0) and its negative effects on Shh signaling activity (E14.5), suggests that the epigenetic landscape of Shh target genes might be modified to suppress RPC proliferation. (D) Cdon mutation or ethanol exposure at E8.0 impedes Shh signaling activity in RPCs resulting in optic nerve hypoplasia.

These data are consistent with previous studies showing that Shh secreted from RGCs is required to maintain RPCs in a proliferative state, thus preventing their differentiation (Zhang and Yang, 2001; Wang et al., 2005; Sakagami et al., 2009). RGCs also remain dependent on Shh during their maturation, as evidenced by the axonal outgrowth defects that occur upon further inhibition of Shh (Kolpak et al., 2005; Sanchez-Camacho and Bovolenta, 2008). Taken together, our findings implicate the disruption of RGC-derived Shh signaling as the pathogenic mechanism by which Cdon mutation and prenatal ethanol exposure cause ONH (Fig. 6D).

Interestingly, Cdon has also been reported to antagonize hedgehog (Hh) signaling in the optic vesicle of zebrafish and chick embryos (Cardozo et al., 2014). However, we did not observe any of the gain of Hh function phenotypes described in Cdon morphants, including expansion of Pax2-expressing cells in the ventral retina, or increased Hh signaling in the hypothalamic territory adjacent to the eye. Moreover, the HPE phenotype displayed by Cdon−/− mouse embryos exposed to ethanol at E7.0 was rescued by increasing Shh signaling activity (Hong and Krauss, 2013), in contrast to the decrease in Hh that restored eye patterning in Cdon morphants (Cardozo et al., 2014). The differences between our findings and those of Cardozo et al. (2014) might be related to the species in which the experiments were performed, or possibly the nature of the genetic manipulations – germline mutation versus morpholino knockdown – that in some cases might result in phenotypic differences resulting from distinct modes of genetic compensation (Rossi et al., 2015).

Strain-dependent modifiers and timing of prenatal ethanol exposure influence Shh-related phenotypes

A particularly striking feature of our mouse model is the influence that genetic background and timing of prenatal ethanol exposure have on the variable phenotypic severity, in keeping with other studies of ethanol-induced teratogenesis (Downing et al., 2009; Lipinski et al., 2012). When bred on the 129S6/SvEvTac strain, both Cdon−/− mutants and wild-type embryos exposed to ethanol at E8.0 presented with ONH. By contrast, when raised on a C57BL/6 (C57BL/6NTac or C57BL/6J) genetic background, both Cdon−/− embryos, and wild-type embryos exposed to ethanol one day earlier at E7.0, exhibited HPE (Zhang et al., 2006; Higashiyama et al., 2007; Aoto et al., 2008; Godin et al., 2010). Thus, strain-dependent modifiers of the Cdon−/− mutation and timing of prenatal ethanol exposure affect the spatiotemporal dynamics of Shh pathway disruption in the eye and prechordal plate, which influences the likelihood of developing ONH versus HPE, respectively.

It is intriguing that we did not detect any interaction between Cdon−/− mutation and ethanol in the manifestation of ONH, or other SOD-related phenotypes, whereas synergy between the two insults was observed for HPE (Hong and Krauss, 2012). This finding suggests that the eye is especially vulnerable to genetic and environmental perturbations in Shh signaling, at least on the more resistant 129S6 background. Pituitary hypoplasia is another prominent feature of SOD that arises from Shh pathway disruption (Treier et al., 2001; Wang et al., 2010; Zhao et al., 2012). However, Shh expression in the anterior hypothalamus, which is required for pituitary morphogenesis, was not affected in the embryos analyzed in our study. Hence, more impactful perturbations in Shh signaling might be needed to compromise pituitary development, as described in other mouse models of SOD (Zhao et al., 2012; Gaston-Massuet et al., 2016).

Effects of ethanol on Shh signaling

Another interpretation for the inability of ethanol to worsen the ONH phenotype in Cdon−/− mutants is that an intact Shh signaling pathway is required for ethanol to exert its teratogenic effect. Ethanol treatment reduces Shh signaling through diverse mechanisms, including the activation of Shh pathway antagonists (PKA), repression of Shh pathway modulators (cholesterol), and indirect consequences that decrease the survival of Shh-expressing and/or responsive cells, possibly owing to increased oxidative stress (Ahlgren et al., 2002; Li et al., 2007; Aoto et al., 2008; Zhang et al., 2011). In each of these examples the acute effect of ethanol on Shh signaling is short-lived, occurring close to the developmental stage when Shh function is required. However, in our study Shh is not expressed in the eye until several days after ethanol administration, suggesting that ethanol-induced alterations persist beyond the time of exposure.

One potential mechanism by which ethanol might invoke long-lasting changes in gene expression is through epigenetic modifications of DNA and chromatin structure (Kleiber et al., 2014). Acetyl-CoA is an end product of ethanol metabolism and, among its many cellular functions, serves as a substrate for histone acetylation. Stable alterations in the acetylation and methylation of histone tails at several loci were detected in the cerebral cortex of E17 mouse embryos after in utero ethanol exposure at E7.0 (Veazey et al., 2015). Whether these ethanol-induced changes in histone modifications alter gene expression programs that are responsible for specific developmental defects requires further experimentation. Nonetheless, these observations suggest an intriguing model in which prenatal ethanol exposure at E8.0 perturbs the epigenetic landscape leading to alterations in Shh-dependent gene expression in the eye at E14.5 (Fig. 6C).

SOD is a multifactorial disorder

The idiopathic nature of most SOD cases suggests a multifactorial etiology to this debilitating condition, including sporadic mutations and environmental teratogens that impinge on Shh-dependent mechanisms of eye and pituitary development. Exome and whole-genome sequencing of SOD cases should assist in the identification of as-yet undiscovered genetic variants that increase disease risk. Although our study demonstrated the adverse effects of prenatal ethanol exposure on Shh signaling during eye development, other drugs, including cannabinoids and their more potent synthetic derivatives, might also contribute to disease pathogenesis by interfering with Shh signal transduction at key stages of embryonic development (Khaliullina et al., 2015; Gilbert et al., 2015). The use of drugs and alcohol at early stages of pregnancy is particularly harmful to the embryo because it coincides with a sensitive period of brain development during the first month when young mothers are often unaware of their pregnancy. A better understanding of the gene–environment interactions underlying SOD risk might improve treatment options, time to diagnosis, and public awareness of the importance for early prenatal care, even when pregnancy is inadvertent.

MATERIALS AND METHODS

Mice

All animal work was approved by the Institutional Animal Care and Use Committee (IACUC) at the Icahn School of Medicine at Mount Sinai and the Perelman School of Medicine, University of Pennsylvania. The animal facilities at both institutions are accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC). Detailed methods for all mouse breeding experiments, in utero ethanol administration, measurements of maternal blood alcohol concentration and embryo harvest are described in Hong and Krauss (2012). Briefly, Cdon+/− mice on a 129S6/SvEvTac (129S6) background were mated for one hour in the dark and checked for the presence of a vaginal plug. The time of plug detection was designated as embryonic day 0 (E0). Pregnant female mice were injected intraperitoneally with 15 μl/g body weight of a solution of 30% ethanol in saline (3.48 g/kg) at E8.0, and again 4 h later. Saline injections were used as a control. Generation of mice with a targeted Cdon null allele was described previously (Cole and Krauss, 2003).

Immunohistochemistry and in situ hybridization

Embryos were harvested at E14.5, fixed overnight in 4% paraformaldehyde at 4°C, washed in PBS, dehydrated through a graded ethanol series, and stored in 100% ethanol at −20°C. Embryos were rehydrated in PBS, cryoprotected in 30% sucrose overnight at 4°C, embedded in Tissue-Tek OCT Compound (Sakura Finetek USA, Inc., Torrance, CA), quick-frozen on dry ice, and cryosectioned at 16 µm. Primary antibodies used for immunohistochemistry and their dilutions are as follows: mouse anti-neurofilament (1:250, 2H3), mouse anti-islet1/2 (1:100, 39.4D5), mouse anti AP-2alpha (1:100, 5E4) were obtained from Developmental Studies Hybridoma Bank (University of Iowa, Iowa City, IA); rabbit anti-Pax2 (1:250, 71-6000, Invitrogen); mouse anti-Ki67 (1:1000, ACK02, Leica Biosystems). Detection of primary antibodies was achieved using secondary antibodies conjugated to Cy3 (1:100, 115-106-006, Jackson ImmunoResearch Laboratories) or Alexa 488 (1:100, A32723, Molecular Probes). Section in situ hybridization was performed with digoxygenin-UTP-labeled riboprobes essentially as described (Nissim et al., 2007). At least three to five embryos in the experimental and control groups were evaluated for each antibody or in situ probe.

Quantification and statistical analysis

All cell counts were performed using the cell counter function in ImageJ (NIH) on tissue sections from at least three embryos of each experimental and control group. The width of the optic nerve was determined at its mid-point using image software in the Leica Application Suite (Leica Microsystems). The axial width and length of each eye was also determined. Eye measurements were taken from at least eight embryos of each experimental and control group that were blind to the observer. Statistical analysis was performed in GraphPad Prism using the Student's t-test.

Acknowledgements

We thank members of the Epstein Lab for helpful discussions and comments on the manuscript. We also thank Jeremy Horrell for advice on Ki67 immunostaining. The support of Danielle Marino and the Franklin Institute STEM Scholars Program is greatly appreciated.

Footnotes

  • Competing interests

    The authors declare no competing or financial interests.

  • Author contributions

    B.M.K., T.S.C. and K.L. performed the experiments. M.H. and R.S.K. generated the Cdon+/− mouse mutant and provided embryos. D.J.E. conceived the project and wrote the manuscript with B.M.K.

  • Funding

    This work was supported by the National Institutes of Health [R01 NS039421 to D.J.E.]; and National Institute on Alcohol Abuse and Alcoholism [R01 AA018825 to R.S.K.].

  • Received May 11, 2016.
  • Accepted November 16, 2016.
http://creativecommons.org/licenses/by/3.0

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

References

  1. ↵
    1. Ahlgren, S. C.,
    2. Thakur, V. and
    3. Bronner-Fraser, M.
    (2002). Sonic hedgehog rescues cranial neural crest from cell death induced by ethanol exposure. Proc. Natl. Acad. Sci. USA 99, 10476-10481. doi:10.1073/pnas.162356199
    OpenUrlAbstract/FREE Full Text
  2. ↵
    1. Allen, B. L.,
    2. Song, J. Y.,
    3. Izzi, L.,
    4. Althaus, I. W.,
    5. Kang, J.-S.,
    6. Charron, F.,
    7. Krauss, R. S. and
    8. McMahon, A. P.
    (2011). Overlapping roles and collective requirement for the coreceptors GAS1, CDO, and BOC in SHH pathway function. Dev. Cell 20, 775-787. doi:10.1016/j.devcel.2011.04.018
    OpenUrlCrossRefPubMedWeb of Science
  3. ↵
    1. Aoto, K.,
    2. Shikata, Y.,
    3. Higashiyama, D.,
    4. Shiota, K. and
    5. Motoyama, J.
    (2008). Fetal ethanol exposure activates protein kinase A and impairs Shh expression in prechordal mesendoderm cells in the pathogenesis of holoprosencephaly. Birth Defects Res. A Clin. Mol. Teratol. 82, 224-231. doi:10.1002/bdra.20447
    OpenUrlCrossRefPubMed
  4. ↵
    1. Ashwell, K. W. S. and
    2. Zhang, L.-L.
    (1994). Optic nerve hypoplasia in an acute exposure model of the fetal alcohol syndrome. Neurotoxicol. Teratol. 16, 161-167. doi:10.1016/0892-0362(94)90113-9
    OpenUrlCrossRefPubMedWeb of Science
  5. ↵
    1. Blader, P. and
    2. Strähle, U.
    (1998). Ethanol impairs migration of the prechordal plate in the zebrafish embryo. Dev. Biol. 201, 185-201. doi:10.1006/dbio.1998.8995
    OpenUrlCrossRefPubMedWeb of Science
  6. ↵
    1. Brzezinski, J. A. IV.,
    2. Prasov, L. and
    3. Glaser, T.
    (2012). Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle. Dev. Biol. 365, 395-413. doi:10.1016/j.ydbio.2012.03.006
    OpenUrlCrossRefPubMed
  7. ↵
    1. Cardozo, M. J.,
    2. Sánchez-Arrones, L.,
    3. Sandonis, A.,
    4. Sánchez-Camacho, C.,
    5. Gestri, G.,
    6. Wilson, S. W.,
    7. Guerrero, I. and
    8. Bovolenta, P.
    (2014). Cdon acts as a Hedgehog decoy receptor during proximal-distal patterning of the optic vesicle. Nat. Commun. 5, 4272. doi:10.1038/ncomms5272
    OpenUrlCrossRefPubMed
  8. ↵
    1. Cemeroglu, A. P.,
    2. Coulas, T. and
    3. Kleis, L.
    (2015). Spectrum of clinical presentations and endocrinological findings of patients with septo-optic dysplasia: a retrospective study. J. Pediatr. Endocrinol. Metab. 28, 1057-1063. doi:10.1515/jpem-2015-0008
    OpenUrlCrossRef
  9. ↵
    1. Chiang, C.,
    2. Litingtung, Y.,
    3. Lee, E.,
    4. Young, K. E.,
    5. Corden, J. L.,
    6. Westphal, H. and
    7. Beachy, P. A.
    (1996). Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function. Nature 383, 407-413. doi:10.1038/383407a0
    OpenUrlCrossRefPubMedWeb of Science
  10. ↵
    1. Cole, F. and
    2. Krauss, R. S.
    (2003). Microform holoprosencephaly in mice that lack the Ig superfamily member Cdon. Curr. Biol. 13, 411-415. doi:10.1016/S0960-9822(03)00088-5
    OpenUrlCrossRefPubMedWeb of Science
  11. ↵
    1. Coulter, C. L.,
    2. Leech, R. W.,
    3. Schaefer, G. B.,
    4. Scheithauer, B. W. and
    5. Brumback, R. A.
    (1993). Midline cerebral dysgenesis, dysfunction of the hypothalamic-pituitary axis, and fetal alcohol effects. Arch. Neurol. 50, 771-775. doi:10.1001/archneur.1993.00540070083022
    OpenUrlCrossRefPubMedWeb of Science
  12. ↵
    1. Dakubo, G. D.,
    2. Wang, Y. P.,
    3. Mazerolle, C.,
    4. Campsall, K.,
    5. McMahon, A. P. and
    6. Wallace, V. A.
    (2003). Retinal ganglion cell-derived sonic hedgehog signaling is required for optic disc and stalk neuroepithelial cell development. Development 130, 2967-2980. doi:10.1242/dev.00515
    OpenUrlAbstract/FREE Full Text
  13. ↵
    1. Deiner, M. S.,
    2. Kennedy, T. E.,
    3. Fazeli, A.,
    4. Serafini, T.,
    5. Tessier-Lavigne, M. and
    6. Sretavan, D. W.
    (1997). Netrin-1 and DCC mediate axon guidance locally at the optic disc: loss of function leads to optic nerve hypoplasia. Neuron 19, 575-589. doi:10.1016/S0896-6273(00)80373-6
    OpenUrlCrossRefPubMedWeb of Science
  14. ↵
    1. Downing, C.,
    2. Balderrama-Durbin, C.,
    3. Broncucia, H.,
    4. Gilliam, D. and
    5. Johnson, T. E.
    (2009). Ethanol teratogenesis in five inbred strains of mice. Alcohol. Clin. Exp. Res. 33, 1238-1245. doi:10.1111/j.1530-0277.2009.00949.x
    OpenUrlCrossRefPubMed
  15. ↵
    1. Feng, L.,
    2. Xie, Z. H.,
    3. Ding, Q.,
    4. Xie, X.,
    5. Libby, R. T. and
    6. Gan, L.
    (2010). MATH5 controls the acquisition of multiple retinal cell fates. Mol. Brain 3, 36. doi:10.1186/1756-6606-3-36
    OpenUrlCrossRefPubMed
  16. ↵
    1. Garcia-Filion, P. and
    2. Borchert, M.
    (2013). Prenatal determinants of optic nerve hypoplasia: review of suggested correlates and future focus. Surv. Ophthalmol. 58, 610-619. doi:10.1016/j.survophthal.2013.02.004
    OpenUrlCrossRefPubMed
  17. ↵
    1. Gaston-Massuet, C.,
    2. McCabe, M. J.,
    3. Scagliotti, V.,
    4. Young, R. M.,
    5. Carreno, G.,
    6. Gregory, L. C.,
    7. Jayakody, S. A.,
    8. Pozzi, S.,
    9. Gualtieri, A.,
    10. Basu, B. et al.
    (2016). Transcription factor 7-like 1 is involved in hypothalamo–pituitary axis development in mice and humans. Proc. Natl. Acad. Sci. USA 113, E548-E557. doi:10.1073/pnas.1503346113
    OpenUrlAbstract/FREE Full Text
  18. ↵
    1. Gilbert, M. T.,
    2. Sulik, K. K.,
    3. Fish, E. W.,
    4. Baker, L. K.,
    5. Dehart, D. B. and
    6. Parnell, S. E.
    (2015). Dose-dependent teratogenicity of the synthetic cannabinoid CP-55,940 in mice. Neurotoxicol. Teratol. [Epub ahread of press], doi:10.1016/j.ntt.2015.12.004doi:10.1016/j.ntt.2015.12.004.
    OpenUrlCrossRef
  19. ↵
    1. Godin, E. A.,
    2. O'Leary-Moore, S. K.,
    3. Khan, A. A.,
    4. Parnell, S. E.,
    5. Ament, J. J.,
    6. Dehart, D. B.,
    7. Johnson, B. W.,
    8. Allan Johnson, G.,
    9. Styner, M. A. and
    10. Sulik, K. K.
    (2010). Magnetic resonance microscopy defines ethanol-induced brain abnormalities in prenatal mice: effects of acute insult on gestational day 7. Alcohol. Clin. Exp. Res. 34, 98-111. doi:10.1111/j.1530-0277.2009.01071.x
    OpenUrlCrossRefPubMedWeb of Science
  20. ↵
    1. Gregory, L. C.,
    2. Gaston-Massuet, C.,
    3. Andoniadou, C. L.,
    4. Carreno, G.,
    5. Webb, E. A.,
    6. Kelberman, D.,
    7. McCabe, M. J.,
    8. Panagiotakopoulos, L.,
    9. Saldanha, J. W.,
    10. Spoudeas, H. A. et al.
    (2015). The role of the sonic hedgehog signalling pathway in patients with midline defects and congenital hypopituitarism. Clin. Endocrinol. (Oxf.) 82, 728-738. doi:10.1111/cen.12637
    OpenUrlCrossRefPubMed
  21. ↵
    1. Haddad, N. G. and
    2. Eugster, E. A.
    (2005). Hypopituitarism and neurodevelopmental abnormalities in relation to central nervous system structural defects in children with optic nerve hypoplasia. J. Pediatr. Endocrinol. Metab. 18, 853-858. doi:10.1515/JPEM.2005.18.9.853
    OpenUrlCrossRefPubMed
  22. ↵
    1. Hellström, A.
    (1999). Optic nerve morphology may reveal adverse events during prenatal and perinatal life—digital image analysis. Surv. Ophthalmol. 44 Suppl. 1, S63-S73. doi:10.1016/S0039-6257(99)00067-3
    OpenUrlCrossRefPubMed
  23. ↵
    1. Higashiyama, D.,
    2. Saitsu, H.,
    3. Komada, M.,
    4. Takigawa, T.,
    5. Ishibashi, M. and
    6. Shiota, K.
    (2007). Sequential developmental changes in holoprosencephalic mouse embryos exposed to ethanol during the gastrulation period. Birth Defects Res. A Clin. Mol. Teratol. 79, 513-523. doi:10.1002/bdra.20367
    OpenUrlCrossRefPubMedWeb of Science
  24. ↵
    1. Hong, M. and
    2. Krauss, R. S.
    (2012). Cdon mutation and fetal ethanol exposure synergize to produce midline signaling defects and holoprosencephaly spectrum disorders in mice. PLoS Genet. 8, e1002999. doi:10.1371/journal.pgen.1002999
    OpenUrlCrossRefPubMed
  25. ↵
    1. Hong, M. and
    2. Krauss, R. S.
    (2013). Rescue of holoprosencephaly in fetal alcohol-exposed Cdon mutant mice by reduced gene dosage of Ptch1. PLoS ONE 8, e79269. doi:10.1371/journal.pone.0079269
    OpenUrlCrossRefPubMed
  26. ↵
    1. Khaliullina, H.,
    2. Bilgin, M.,
    3. Sampaio, J. L.,
    4. Shevchenko, A. and
    5. Eaton, S.
    (2015). Endocannabinoids are conserved inhibitors of the Hedgehog pathway. Proc. Natl. Acad. Sci. USA 112, 3415-3420. doi:10.1073/pnas.1416463112
    OpenUrlAbstract/FREE Full Text
  27. ↵
    1. Kietzman, H. W.,
    2. Everson, J. L.,
    3. Sulik, K. K. and
    4. Lipinski, R. J.
    (2014). The teratogenic effects of prenatal ethanol exposure are exacerbated by Sonic Hedgehog or GLI2 haploinsufficiency in the mouse. PLoS ONE 9, e89448. doi:10.1371/journal.pone.0089448
    OpenUrlCrossRefPubMed
  28. ↵
    1. Kleiber, M. L.,
    2. Diehl, E. J.,
    3. Laufer, B. I.,
    4. Mantha, K.,
    5. Chokroborty-Hoque, A.,
    6. Alberry, B. and
    7. Singh, S. M.
    (2014). Long-term genomic and epigenomic dysregulation as a consequence of prenatal alcohol exposure: a model for fetal alcohol spectrum disorders. Front. Genet. 5, 161. doi:10.3389/fgene.2014.00161
    OpenUrlCrossRefPubMed
  29. ↵
    1. Kolpak, A.,
    2. Zhang, J. and
    3. Bao, Z.-Z.
    (2005). Sonic hedgehog has a dual effect on the growth of retinal ganglion axons depending on its concentration. J. Neurosci. 25, 3432-3441. doi:10.1523/JNEUROSCI.4938-04.2005
    OpenUrlAbstract/FREE Full Text
  30. ↵
    1. Li, Y.-X.,
    2. Yang, H.-T.,
    3. Zdanowicz, M.,
    4. Sicklick, J. K.,
    5. Qi, Y.,
    6. Camp, T. J. and
    7. Diehl, A. M.
    (2007). Fetal alcohol exposure impairs Hedgehog cholesterol modification and signaling. Lab. Invest. 87, 231-240. doi:10.1038/labinvest.3700516
    OpenUrlCrossRefPubMedWeb of Science
  31. ↵
    1. Lipinski, R. J.,
    2. Godin, E. A.,
    3. O'leary-Moore, S. K.,
    4. Parnell, S. E. and
    5. Sulik, K. K.
    (2010). Genesis of teratogen-induced holoprosencephaly in mice. Am. J. Med. Genet. C Semin. Med. Genet. 154C, 29-42. doi:10.1002/ajmg.c.30239
    OpenUrlCrossRef
  32. ↵
    1. Lipinski, R. J.,
    2. Hammond, P.,
    3. O'Leary-Moore, S. K.,
    4. Ament, J. J.,
    5. Pecevich, S. J.,
    6. Jiang, Y.,
    7. Budin, F.,
    8. Parnell, S. E.,
    9. Suttie, M.,
    10. Godin, E. A. et al.
    (2012). Ethanol-induced face-brain dysmorphology patterns are correlative and exposure-stage dependent. PLoS ONE 7, e43067. doi:10.1371/journal.pone.0043067
    OpenUrlCrossRefPubMed
  33. ↵
    1. Loucks, E. J. and
    2. Ahlgren, S. C.
    (2009). Deciphering the role of Shh signaling in axial defects produced by ethanol exposure. Birth Defects Res. A Clin. Mol. Teratol. 85, 556-567. doi:10.1002/bdra.20564
    OpenUrlCrossRefPubMed
  34. ↵
    1. Marigo, V.,
    2. Johnson, R. L.,
    3. Vortkamp, A. and
    4. Tabin, C. J.
    (1996). Sonic hedgehog differentially regulates expression of GLI and GLI3 during limb development. Dev. Biol. 180, 273-283. doi:10.1006/dbio.1996.0300
    OpenUrlCrossRefPubMedWeb of Science
  35. ↵
    1. McCabe, M. J.,
    2. Alatzoglou, K. S. and
    3. Dattani, M. T.
    (2011). Septo-optic dysplasia and other midline defects: the role of transcription factors: HESX1 and beyond. Best Pract. Res. Clin. Endocrinol. Metab. 25, 115-124. doi:10.1016/j.beem.2010.06.008
    OpenUrlCrossRefPubMed
  36. ↵
    1. McLellan, J. S.,
    2. Zheng, X.,
    3. Hauk, G.,
    4. Ghirlando, R.,
    5. Beachy, P. A. and
    6. Leahy, D. J.
    (2008). The mode of Hedgehog binding to Ihog homologues is not conserved across different phyla. Nature 455, 979-983. doi:10.1038/nature07358
    OpenUrlCrossRefPubMedWeb of Science
  37. ↵
    1. Morishima, A. and
    2. Aranoff, G. S.
    (1986). Syndrome of septo-optic-pituitary dysplasia: the clinical spectrum. Brain Dev. 8, 233-239. doi:10.1016/S0387-7604(86)80075-4
    OpenUrlCrossRefPubMedWeb of Science
  38. ↵
    1. Mu, X.,
    2. Fu, X.,
    3. Beremand, P. D.,
    4. Thomas, T. L. and
    5. Klein, W. H.
    (2008). Gene regulation logic in retinal ganglion cell development: Isl1 defines a critical branch distinct from but overlapping with Pou4f2. Proc. Natl. Acad. Sci USA 105, 6942-7. doi:10.1073/pnas.0802627105
    OpenUrlAbstract/FREE Full Text
  39. ↵
    1. Murray, P. G.,
    2. Paterson, W. F. and
    3. Donaldson, M. D. C.
    (2005). Maternal age in patients with septo-optic dysplasia. J. Pediatr. Endocrinol. Metab. 18, 471-476. doi:10.1515/JPEM.2005.18.5.471
    OpenUrlCrossRefPubMed
  40. ↵
    1. Nissim, S.,
    2. Allard, P.,
    3. Bandyopadhyay, A.,
    4. Harfe, B. D. and
    5. Tabin, C. J.
    (2007). Characterization of a novel ectodermal signaling center regulating Tbx2 and Shh in the vertebrate limb. Dev. Biol. 304, 9-21. doi:10.1016/j.ydbio.2006.12.010
    OpenUrlCrossRefPubMedWeb of Science
  41. ↵
    1. Pan, L.,
    2. Deng, M.,
    3. Xie, X. and
    4. Gan, L.
    (2008). ISL1 and BRN3B co-regulate the differentiation of murine retinal ganglion cells. Development 135, 1981-1990. doi:10.1242/dev.010751
    OpenUrlAbstract/FREE Full Text
  42. ↵
    1. Patel, L.,
    2. McNally, R. J. Q.,
    3. Harrison, E.,
    4. Lloyd, I. C. and
    5. Clayton, P. E.
    (2006). Geographical distribution of optic nerve hypoplasia and septo-optic dysplasia in Northwest England. J. Pediatr. 148, 85-88. doi:10.1016/j.jpeds.2005.07.031
    OpenUrlCrossRefPubMedWeb of Science
  43. ↵
    1. Paulo, S. S.,
    2. Fernandes-Rosa, F. L.,
    3. Turatti, W.,
    4. Coeli-Lacchini, F. B.,
    5. Martinelli, C. E., Jr..,
    6. Nakiri, G. S.,
    7. Moreira, A. C.,
    8. Santos, A. C.,
    9. de Castro, M. and
    10. Antonini, S. R.
    (2015). Sonic Hedgehog mutations are not a common cause of congenital hypopituitarism in the absence of complex midline cerebral defects. Clin. Endocrinol. (Oxf). 82, 562-569. doi:10.1111/cen.12565
    OpenUrlCrossRefPubMed
  44. ↵
    1. Prasov, L. and
    2. Glaser, T.
    (2012). Dynamic expression of ganglion cell markers in retinal progenitors during the terminal cell cycle. Mol. Cell. Neurosci. 50, 160-168. doi:10.1016/j.mcn.2012.05.002
    OpenUrlCrossRefPubMed
  45. ↵
    1. Ribeiro, I. M.,
    2. Vale, P. J.,
    3. Tenedorio, P. A.,
    4. Rodrigues, P. A.,
    5. Bilhoto, M. A. and
    6. Pereira, H. C.
    (2007). Ocular manifestations in fetal alcohol syndrome. Eur. J. Ophthalmol. 17, 104-109.
    OpenUrlPubMedWeb of Science
  46. ↵
    1. Roessler, E.,
    2. Belloni, E.,
    3. Gaudenz, K.,
    4. Jay, P.,
    5. Berta, P.,
    6. Scherer, S. W.,
    7. Tsui, L.-C. and
    8. Muenke, M.
    (1996). Mutations in the human Sonic Hedgehog gene cause holoprosencephaly. Nat. Genet. 14, 357-360. doi:10.1038/ng1196-357
    OpenUrlCrossRefPubMedWeb of Science
  47. ↵
    1. Rossi, A.,
    2. Kontarakis, Z.,
    3. Gerri, C.,
    4. Nolte, H.,
    5. Hölper, S.,
    6. Krüger, M. and
    7. Stainier, D. Y. R.
    (2015). Genetic compensation induced by deleterious mutations but not gene knockdowns. Nature 524, 230-233. doi:10.1038/nature14580
    OpenUrlCrossRefPubMed
  48. ↵
    1. Sakagami, K.,
    2. Gan, L. and
    3. Yang, X.-J.
    (2009). Distinct effects of Hedgehog signaling on neuronal fate specification and cell cycle progression in the embryonic mouse retina. J. Neurosci. 29, 6932-6944. doi:10.1523/JNEUROSCI.0289-09.2009
    OpenUrlAbstract/FREE Full Text
  49. ↵
    1. Sanchez-Camacho, C. and
    2. Bovolenta, P.
    (2008). Autonomous and non-autonomous Shh signalling mediate the in vivo growth and guidance of mouse retinal ganglion cell axons. Development 135, 3531-3541. doi:10.1242/dev.023663
    OpenUrlAbstract/FREE Full Text
  50. ↵
    1. Stacher Hörndli, C. and
    2. Chien, C.-B.
    (2012). Sonic hedgehog is indirectly required for intraretinal axon pathfinding by regulating chemokine expression in the optic stalk. Development 139, 2604-2613. doi:10.1242/dev.077594
    OpenUrlAbstract/FREE Full Text
  51. ↵
    1. Strömland, K.
    (1987). Ocular involvement in the fetal alcohol syndrome. Surv. Ophthalmol. 31, 277-284. doi:10.1016/0039-6257(87)90028-2
    OpenUrlCrossRefPubMedWeb of Science
  52. ↵
    1. Sulik, K. K.,
    2. Johnston, M. C. and
    3. Webb, M. A.
    (1981). Fetal alcohol syndrome: embryogenesis in a mouse model. Science 214, 936-938. doi:10.1126/science.6795717
    OpenUrlAbstract/FREE Full Text
  53. ↵
    1. Tenzen, T.,
    2. Allen, B. L.,
    3. Cole, F.,
    4. Kang, J.-S.,
    5. Krauss, R. S. and
    6. McMahon, A. P.
    (2006). The cell surface membrane proteins Cdo and Boc are components and targets of the Hedgehog signaling pathway and feedback network in mice. Dev. Cell 10, 647-656. doi:10.1016/j.devcel.2006.04.004
    OpenUrlCrossRefPubMedWeb of Science
  54. ↵
    1. Treier, M.,
    2. O'Connell, S.,
    3. Gleiberman, A.,
    4. Price, J.,
    5. Szeto, D. P.,
    6. Burgess, R.,
    7. Chuang, P. T.,
    8. McMahon, A. P. and
    9. Rosenfeld, M. G.
    (2001). Hedgehog signaling is required for pituitary gland development. Development 128, 377-386.
    OpenUrlAbstract
  55. ↵
    1. Veazey, K. J.,
    2. Parnell, S. E.,
    3. Miranda, R. C. and
    4. Golding, M. C.
    (2015). Dose-dependent alcohol-induced alterations in chromatin structure persist beyond the window of exposure and correlate with fetal alcohol syndrome birth defects. Epigenetics Chromatin 8, 39. doi:10.1186/s13072-015-0031-7
    OpenUrlCrossRefPubMed
  56. ↵
    1. Wang, S. W.,
    2. Kim, B. S.,
    3. Ding, K.,
    4. Wang, H.,
    5. Sun, D.,
    6. Johnson, R. L.,
    7. Klein, W. H. and
    8. Gan, L.
    (2001). Requirement for math5 in the development of retinal ganglion cells. Genes Dev. 15, 24-29. doi:10.1101/gad.855301
    OpenUrlAbstract/FREE Full Text
  57. ↵
    1. Wang, Y.,
    2. Dakubo, G. D.,
    3. Thurig, S.,
    4. Mazerolle, C. J. and
    5. Wallace, V. A.
    (2005). Retinal ganglion cell-derived sonic hedgehog locally controls proliferation and the timing of RGC development in the embryonic mouse retina. Development 132, 5103-5113. doi:10.1242/dev.02096
    OpenUrlAbstract/FREE Full Text
  58. ↵
    1. Wang, Y.,
    2. Martin, J. F. and
    3. Bai, C. B.
    (2010). Direct and indirect requirements of Shh/Gli signaling in early pituitary development. Dev. Biol. 348, 199-209. doi:10.1016/j.ydbio.2010.09.024
    OpenUrlCrossRefPubMed
  59. ↵
    1. Webb, E. A. and
    2. Dattani, M. T.
    (2010). Septo-optic dysplasia. Eur. J. Hum. Genet. 18, 393-397. doi:10.1038/ejhg.2009.125
    OpenUrlCrossRefPubMed
  60. ↵
    1. Wu, F.,
    2. Kaczynski, T. J.,
    3. Sethuramanujam, S.,
    4. Li, R.,
    5. Jain, V.,
    6. Slaughter, M. and
    7. Mu, X.
    (2015). Two transcription factors, Pou4f2 and Isl1, are sufficient to specify the retinal ganglion cell fate. Proc. Natl. Acad. Sci. USA 112, E1559-E1568. doi:10.1073/pnas.1421535112
    OpenUrlAbstract/FREE Full Text
  61. ↵
    1. Zhang, X. M. and
    2. Yang, X. J.
    (2001). Regulation of retinal ganglion cell production by Sonic hedgehog. Development 128, 943-957.
    OpenUrlAbstract
  62. ↵
    1. Zhang, W.,
    2. Kang, J.-S.,
    3. Cole, F.,
    4. Yi, M.-J. and
    5. Krauss, R. S.
    (2006). Cdo functions at multiple points in the Sonic Hedgehog pathway, and Cdo-deficient mice accurately model human holoprosencephaly. Dev. Cell 10, 657-665. doi:10.1016/j.devcel.2006.04.005
    OpenUrlCrossRefPubMedWeb of Science
  63. ↵
    1. Zhang, W.,
    2. Mulieri, P. J.,
    3. Gaio, U.,
    4. Bae, G.-U.,
    5. Krauss, R. S. and
    6. Kang, J.-S.
    (2009). Ocular abnormalities in mice lacking the immunoglobulin superfamily member Cdo. FEBS J. 276, 5998-6010. doi:10.1111/j.1742-4658.2009.07310.x
    OpenUrlCrossRefPubMed
  64. ↵
    1. Zhang, C.,
    2. Turton, Q. M.,
    3. Mackinnon, S.,
    4. Sulik, K. K. and
    5. Cole, G. J.
    (2011). Agrin function associated with ocular development is a target of ethanol exposure in embryonic zebrafish. Birth Defects Res. A Clin. Mol. Teratol. 91, 129-141. doi:10.1002/bdra.20766
    OpenUrlCrossRefPubMed
  65. ↵
    1. Zhao, L.,
    2. Zevallos, S. E.,
    3. Rizzoti, K.,
    4. Jeong, Y.,
    5. Lovell-Badge, R. and
    6. Epstein, D. J.
    (2012). Disruption of SoxB1-dependent Sonic hedgehog expression in the hypothalamus causes septo-optic dysplasia. Dev. Cell 22, 585-596. doi:10.1016/j.devcel.2011.12.023
    OpenUrlCrossRefPubMedWeb of Science
Back to top

RSSRSS

Keywords

 Download PDF

Email
RESEARCH ARTICLE
Prenatal ethanol exposure in mice phenocopies Cdon mutation by impeding Shh function in the etiology of optic nerve hypoplasia
Benjamin M. Kahn, Tanya S. Corman, Korah Lovelace, Mingi Hong, Robert S. Krauss, Douglas J. Epstein
Disease Models & Mechanisms 2017 10: 29-37; doi: 10.1242/dmm.026195
del.icio.us logo Digg logo Reddit logo Twitter logo CiteULike logo Facebook logo Google logo Mendeley logo
Citation Tools
Alerts

Article navigation

Other journals from The Company of Biologists

Advertisement

DMM and COVID-19

We are aware that the COVID-19 pandemic is having an unprecedented impact on researchers worldwide. The Editors of all The Company of Biologists’ journals have been considering ways in which we can alleviate concerns that members of our community may have around publishing activities during this time. Read about the actions we are taking at this time.

Please don’t hesitate to contact the Editorial Office if you have any questions or concerns.

Monica Justice bids farewell to DMM

In her farewell Editorial, outgoing Editor-in-Chief Monica Justice reminds us of the past half-decade of growth and of DMM's commitment to support the disease modelling community, concluding, “The knowledge and experience I gained during my time as Senior Editor and EiC at DMM is invaluable: working within a not-for-profit community publishing environment is a joy.”

3D imaging of beta cell mass in diabetic mouse models

In their inducible mouse model of diabetes, Roostalu et al. demonstrate how quantitative light-sheet imaging can capture changes in individual islets to help pharmacological research in diabetes.

Visit our YouTube channel to watch more videos from DMM, our sister journals and the Company.

Modelling Joubert syndrome patient-derived mutations in C. elegans

In this issue’s Editor’s choice, Karen Lange and colleagues used C. elegans to model and characterise two patient-derived mutations that cause the ciliopathy Joubert syndrome.

Interview – Karen Lange

First author of our current Editor’s choice, Karen Lange takes us behind the scenes of the paper, and shares her thoughts on how the lack of both time and job security will impact her research.