SFRP1 modulates retina cell differentiation through a beta-catenin-independent mechanism

SFRP1 upregulation causes hippocampal synaptic dysfunction and memory impairment

Impaired neuronal and synaptic function are hallmarks of early Alzheimer's disease (AD), preceding other neuropathological traits and cognitive decline. We previously showed that SFRP1, a glial-derived protein elevated in AD brains from preclinical stages, contributes to disease progression, implicating glial factors in early pathogenesis. Here, we generate and analyze transgenic mice overexpressing astrocytic SFRP1. SFRP1 accumulation causes early dendritic and synaptic defects in adult mice, followed by impaired synaptic long-term potentiation and cognitive decline, evident only when the animals age, thereby mimicking AD's structural-functional temporal distinction. This phenotype correlates with proteomic changes, including increased structural synaptic proteins like neurexin, which localizes in close proximity with SFRP1 in cultured hippocampal neurons. We conclude that excessive SFRP1 hinders synaptic protein turnover, reducing synaptic plasticity-a mechanism that may underlie the synaptopathy observed in the brains of prodromal AD patients.

Transcriptomic Profiling of Human Pluripotent Stem Cell-derived Retinal Pigment Epithelium over Time

Human pluripotent stem cell (hPSC)-derived progenies are immature versions of cells, presenting a potential limitation to the accurate modelling of diseases associated with maturity or age. Hence, it is important to characterise how closely cells used in culture resemble their native counterparts. In order to select appropriate time points of retinal pigment epithelium (RPE) cultures that reflect native counterparts, we characterised the transcriptomic profiles of the hPSC-derived RPE cells from 1- and 12-month cultures. We differentiated the human embryonic stem cell line H9 into RPE cells, performed single-cell RNA-sequencing of a total of 16,576 cells to assess the molecular changes of the RPE cells across these two culture time points. Our results indicate the stability of the RPE transcriptomic signature, with no evidence of an epithelial–mesenchymal transition, and with the maturing populations of the RPE observed with time in culture. Assessment of Gene Ontology pathways revealed that as the cultures age, RPE cells upregulate expression of genes involved in metal binding and antioxidant functions. This might reflect an increased ability to handle oxidative stress as cells mature. Comparison with native human RPE data confirms a maturing transcriptional profile of RPE cells in culture. These results suggest that long-term in vitro culture of RPE cells allows the modelling of specific phenotypes observed in native mature tissues. Our work highlights the transcriptional landscape of hPSC-derived RPE cells as they age in culture, which provides a reference for native and patient samples to be benchmarked against.

Optic Atrophy 1 Controls Human Neuronal Development by Preventing Aberrant Nuclear DNA Methylation

Optic atrophy 1 (OPA1), a GTPase at the inner mitochondrial membrane involved in regulating mitochondrial fusion, stability, and energy output, is known to be crucial for neural development: Opa1 heterozygous mice show abnormal brain development, and inactivating mutations in OPA1 are linked to human neurological disorders. Here, we used genetically modified human embryonic and patient-derived induced pluripotent stem cells and reveal that OPA1 haploinsufficiency leads to aberrant nuclear DNA methylation and significantly alters the transcriptional circuitry in neural progenitor cells (NPCs). For instance, expression of the forkhead box G1 transcription factor, which is needed for GABAergic neuronal development, is repressed in OPA1+/− NPCs. Supporting this finding, OPA1+/− NPCs cannot give rise to GABAergic interneurons, whereas formation of glutamatergic neurons is not affected. Taken together, our data reveal that OPA1 controls nuclear DNA methylation and expression of key transcription factors needed for proper neural cell specification.

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OPA1 haploinsufficiency impairs formation of DLX1/2-positive GABAergic neurons

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Reduced OPA1 levels significantly alter the transcriptional circuitry in neural cells

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Expression of the pioneer factor FOXG1 is decreased in OPA1+/− neural progenitor cells

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Impaired FOXG1 expression correlates with increased CpG methylation at its promoter

Sfrp1 deficiency makes retinal photoreceptors prone to degeneration

Millions of individuals worldwide suffer from impaired vision, a condition with multiple origins that often impinge upon the light sensing cells of the retina, the photoreceptors, affecting their integrity. The molecular components contributing to this integrity are however not yet fully understood. Here we have asked whether Secreted Frizzled Related Protein 1 (SFRP1) may be one of such factors. SFRP1 has a context-dependent function as modulator of Wnt signalling or of the proteolytic activity of A Disintegrin And Metalloproteases (ADAM) 10, a main regulator of neural cell-cell communication. We report that in Sfrp1^−/− mice, the outer limiting membrane (OLM) is discontinuous and the photoreceptors disorganized and more prone to light-induced damage. Sfrp1 loss significantly enhances the effect of the Rpe65^Leu450Leu genetic variant -present in the mouse genetic background- which confers sensitivity to light-induced stress. These alterations worsen with age, affect visual function and are associated to an increased proteolysis of Protocadherin 21 (PCDH21), localized at the photoreceptor outer segment, and N-cadherin, an OLM component. We thus propose that SFRP1 contributes to photoreceptor fitness with a mechanism that involves the maintenance of OLM integrity. These conclusions are discussed in view of the broader implication of SFRP1 in neurodegeneration and aging.

Olfactory ensheathing cells abutting the embryonic olfactory bulb express Frzb, whose deletion disrupts olfactory axon targeting

We and others previously showed that in mouse embryos lacking the transcription factor Sox10, olfactory ensheathing cell (OEC) differentiation is disrupted, resulting in defective olfactory axon targeting and fewer gonadotropin‐releasing hormone (GnRH) neurons entering the embryonic forebrain. The underlying mechanisms are unclear. Here, we report that OECs in the olfactory nerve layer express Frzb—encoding a secreted Wnt inhibitor with roles in axon targeting and basement membrane breakdown—from embryonic day (E)12.5, when GnRH neurons first enter the forebrain, until E16.5, the latest stage examined. The highest levels of Frzb expression are seen in OECs in the inner olfactory nerve layer, abutting the embryonic olfactory bulb. We find that Sox10 is required for Frzb expression in OECs, suggesting that loss of Frzb could explain the olfactory axon targeting and/or GnRH neuron migration defects seen in Sox10‐null mice. At E16.5, Frzb‐null embryos show significant reductions in both the volume of the olfactory nerve layer expressing the maturation marker Omp and the number of Omp‐positive olfactory receptor neurons in the olfactory epithelium. As Omp upregulation correlates with synapse formation, this suggests that Frzb deletion indeed disrupts olfactory axon targeting. In contrast, GnRH neuron entry into the forebrain is not significantly affected. Hence, loss of Frzb may contribute to the olfactory axon targeting phenotype, but not the GnRH neuron phenotype, of Sox10‐null mice. Overall, our results suggest that Frzb secreted from OECs in the olfactory nerve layer is important for olfactory axon targeting.

Frzb is expressed by olfactory ensheathing cells abutting the embryonic mouse olfactory bulb.

Frzb expression requires Sox10. Deletion of Frzb disrupts olfactory receptor neuron maturation, likely reflecting a defect in olfactory axon targeting.

Induction of CXC chemokines in human mesenchymal stem cells by stimulation with secreted frizzled-related proteins through non-canonical Wnt signaling

AIM: To investigate the effect of secreted frizzled-related proteins (sFRPs) on CXC chemokine expression in human mesenchymal stem cells (hMSCs).

METHODS: CXC chemokines such as CXCL5 and CXCL8 are induced in hMSCs during differentiation with osteogenic differentiation medium (OGM) and may be involved in angiogenic stimulation during bone repair. hMSCs were treated with conditioned medium (CM) from L-cells expressing non-canonical Wnt5a protein, or with control CM from wild type L-cells, or directly with sFRPs for up to 10 d in culture. mRNA expression levels of both CXCL5 and CXCL8 were quantitated by real-time reverse transcriptase-polymerase chain reaction and secreted protein levels of these proteins determined by ELISA. Dose- (0-500 ng/mL) and time-response curves were generated for treatment with sFRP1. Signal transduction pathways were explored by western blot analysis with pan- or phosphorylation-specific antibodies, through use of specific pathway inhibitors, and through use of siRNAs targeting specific frizzled receptors (Fzd)-2 and 5 or the receptor tyrosine kinase-like orphan receptor-2 (RoR2) prior to treatment with sFRPs.

RESULTS: CM from L-cells expressing Wnt5a, a non-canonical Wnt, stimulated an increase in CXCL5 mRNA expression and protein secretion in comparison to control L-cell CM. sFRP1, which should inhibit both canonical and non-canonical Wnt signaling, surprisingly enhanced the expression of CXCL5 at 7 and 10 d. Dickkopf1, an inhibitor of canonical Wnt signaling prevented the sFRP-stimulated induction of CXCL5 and actually inhibited basal levels of CXCL5 expression at 7 but not at 10 d post treatment. In addition, all four sFRPs isoforms induced CXCL8 expression in a dose- and time-dependent manner with maximum expression at 7 d with treatment at 150 ng/mL. The largest increases in CXCL5 expression were seen from stimulation with sFRP1 or sFRP2. Analysis of mitogen-activated protein kinase signaling pathways in the presence of OGM showed sFRP1-induced phosphorylation of extracellular signal-regulated kinase (ERK) (p44/42) maximally at 5 min after sFRP1 addition, earlier than that found in OGM alone. Addition of a phospholipase C (PLC) inhibitor also prevented sFRP-stimulated increases in CXCL8 mRNA. siRNA technology targeting the Fzd-2 and 5 and the non-canonical Fzd co-receptor RoR2 also significantly decreased sFRP1/2-stimulated CXCL8 mRNA levels.

CONCLUSION: CXC chemokine expression in hMSCs is controlled in part by sFRPs signaling through non-canonical Wnt involving Fzd2/5 and the ERK and PLC pathways.

Secreted frizzled related proteins modulate pathfinding and fasciculation of mouse retina ganglion cell axons by direct and indirect mechanisms

Retina ganglion cell (RGC) axons grow along a stereotyped pathway undergoing coordinated rounds of fasciculation and defasciculation, which are critical to establishing proper eye-brain connections. How this coordination is achieved is poorly understood, but shedding of guidance cues by metalloproteinases is emerging as a relevant mechanism. Secreted Frizzled Related Proteins (Sfrps) are multifunctional proteins, which, among others, reorient RGC growth cones by regulating intracellular second messengers, and interact with Tolloid and ADAM metalloproteinases, thereby repressing their activity. Here, we show that the combination of these two functions well explain the axon guidance phenotype observed in Sfrp1 and Sfrp2 single and compound mouse mutant embryos, in which RGC axons make subtle but significant mistakes during their intraretinal growth and inappropriately defasciculate along their pathway. The distribution of Sfrp1 and Sfrp2 in the eye is consistent with the idea that Sfrp1/2 normally constrain axon growth into the fiber layer and the optic disc. Disheveled axon growth instead seems linked to Sfrp-mediated modulation of metalloproteinase activity. Indeed, retinal explants from embryos with different Sfrp-null alleles or explants overexpressing ADAM10 extend axons with a disheveled appearance, which is reverted by the addition of Sfrp1 or an ADAM10-specific inhibitor. This mode of growth is associated with an abnormal proteolytic processing of L1 and N-cadherin, two ADAM10 substrates previously implicated in axon guidance. We thus propose that Sfrps contribute to coordinate visual axon growth with a dual mechanism: by directly signaling at the growth cone and by regulating the processing of other relevant cues.

The CRB1 and adherens junction complex proteins in retinal development and maintenance

The early developing retinal neuroepithelium is composed of multipotent retinal progenitor cells that differentiate in a time specific manner, giving rise to six major types of neuronal and one type of glial cells. These cells migrate and organize in three distinct nuclear layers divided by two plexiform layers. Apical and adherens junction complexes have a crucial role in this process by the establishment of polarity and adhesion. Changes in these complexes disturb the spatiotemporal aspects of retinogenesis, leading to retinal degeneration resulting in mild or severe impairment of retinal function and vision. In this review, we summarize the mouse models for the different members of the apical and adherens junction protein complexes and describe the main features of their retinal phenotypes. The knowledge acquired from the different mutant animals for these proteins corroborate their importance in retina development and maintenance of normal retinal structure and function. More recently, several studies have tried to unravel the connection between the apical proteins, important cellular signaling pathways and their relation in retina development. Still, the mechanisms by which these proteins function remain largely unknown. Here, we hypothesize how the mammalian apical CRB1 complex might control retinogenesis and prevents onset of Leber congenital amaurosis or retinitis pigmentosa.

Shh/Boc signaling is required for sustained generation of ipsilateral projecting ganglion cells in the mouse retina

Sonic Hedgehog (Shh) signaling is an important determinant of vertebrate retinal ganglion cell (RGC) development. In mice, there are two major RGC populations: (1) the Islet2-expressing contralateral projecting (c)RGCs, which both produce and respond to Shh; and (2) the Zic2-expressing ipsilateral projecting RGCs (iRGCs), which lack Shh expression. In contrast to cRGCs, iRGCs, which are generated in the ventrotemporal crescent (VTC) of the retina, specifically express Boc, a cell adhesion molecule that acts as a high-affinity receptor for Shh. In Boc(-/-) mutant mice, the ipsilateral projection is significantly decreased. Here, we demonstrate that this phenotype results, at least in part, from the misspecification of a proportion of iRGCs. In Boc(-/-) VTC, the number of Zic2-positive RGCs is reduced, whereas more Islet2/Shh-positive RGCs are observed, a phenotype also detected in Zic2 and Foxd1 null embryos. Consistent with this observation, organization of retinal projections at the dorsal lateral geniculate nucleus is altered in Boc(-/-) mice. Analyses of the molecular and cellular consequences of introducing Shh into the developing VTC and Zic2 and Boc into the central retina indicate that Boc expression alone is insufficient to fully activate the ipsilateral program and that Zic2 regulates Shh expression. Taking these data together, we propose that expression of Boc in cells from the VTC is required to sustain Zic2 expression, likely by regulating the levels of Shh signaling from the nearby cRGCs. Zic2, in turn, directly or indirectly, counteracts Shh and Islet2 expression in the VTC and activates the ipsilateral program.

Genome-wide analysis points to roles for extracellular matrix remodeling, the visual cycle, and neuronal development in myopia

Myopia, or nearsightedness, is the most common eye disorder, resulting primarily from excess elongation of the eye. The etiology of myopia, although known to be complex, is poorly understood. Here we report the largest ever genome-wide association study (45,771 participants) on myopia in Europeans. We performed a survival analysis on age of myopia onset and identified 22 significant associations (), two of which are replications of earlier associations with refractive error. Ten of the 20 novel associations identified replicate in a separate cohort of 8,323 participants who reported if they had developed myopia before age 10. These 22 associations in total explain 2.9% of the variance in myopia age of onset and point toward a number of different mechanisms behind the development of myopia. One association is in the gene PRSS56, which has previously been linked to abnormally small eyes; one is in a gene that forms part of the extracellular matrix (LAMA2); two are in or near genes involved in the regeneration of 11-cis-retinal (RGR and RDH5); two are near genes known to be involved in the growth and guidance of retinal ganglion cells (ZIC2, SFRP1); and five are in or near genes involved in neuronal signaling or development. These novel findings point toward multiple genetic factors involved in the development of myopia and suggest that complex interactions between extracellular matrix remodeling, neuronal development, and visual signals from the retina may underlie the development of myopia in humans.

The genetic basis of myopia, or nearsightedness, is believed to be complex and affected by multiple genes. Two genetic association studies have each identified a single genetic region associated with myopia in European populations. Here we report the results of the largest ever genetic association study on myopia in over 45,000 people of European ancestry. We identified 22 genetic regions significantly associated with myopia age of onset. Two are replications of the previously identified associations, and 20 are novel. Ten of the novel associations replicate in a small separate cohort. Sixteen of the novel associations are in or near genes implicated in eye development, neuronal development and signaling, the visual cycle of the retina, and general morphology: BMP3, BMP4, DLG2, DLX1, KCNMA1, KCNQ5, LAMA2, LRRC4C, PRSS56, RBFOX1, RDH5, RGR, SFRP1, TJP2, ZBTB38, and ZIC2. These findings point to numerous biological pathways involved in the development of myopia and, in particular, suggest that early eye and neuronal development may lead to the eventual development of myopia in humans.

Wnt signaling in eye organogenesis

The vertebrate eye consists of multiple tissues with distinct embryonic origins. To ensure formation of the eye as a functional organ, development of ocular tissues must be precisely coordinated. Besides intrinsic regulators, several extracellular pathways have been shown to participate in controlling critical steps during eye development. Many components of Wnt/Frizzled signaling pathways are expressed in developing ocular tissues, and substantial progress has been made in the past few years in understanding their function during vertebrate eye development. Here, I summarize recent work using functional experiments to elucidate the roles of Wnt/Frizzled pathways during development of ocular tissues in different vertebrates.

Antagonistic cross-regulation between Wnt and Hedgehog signalling pathways controls post-embryonic retinal proliferation

Continuous neurogenesis in the adult nervous system requires a delicate balance between proliferation and differentiation. Although Wnt/β-catenin and Hedgehog signalling pathways are thought to share a mitogenic function in adult neural stem/progenitor cells, it remains unclear how they interact in this process. Adult amphibians produce retinal neurons from a pool of neural stem cells localised in the ciliary marginal zone (CMZ). Surprisingly, we found that perturbations of the Wnt and Hedgehog pathways result in opposite proliferative outcomes of neural stem/progenitor cells in the CMZ. Additionally, our study revealed that Wnt and Hedgehog morphogens are produced in mutually exclusive territories of the post-embryonic retina. Using genetic and pharmacological tools, we found that the Wnt and Hedgehog pathways exhibit reciprocal inhibition. Our data suggest that Sfrp-1 and Gli3 contribute to this negative cross-regulation. Altogether, our results reveal an unexpected antagonistic interplay of Wnt and Hedgehog signals that may tightly regulate the extent of neural stem/progenitor cell proliferation in the Xenopus retina.

Rediscovering the chick embryo as a model to study retinal development

The embryonic chick occupies a privileged place among animal models used in developmental studies. Its rapid development and accessibility for visualization and experimental manipulation are just some of the characteristics that have made it a vertebrate model of choice for more than two millennia. Until a few years ago, the inability to perform genetic manipulations constituted a major drawback of this system. However, the completion of the chicken genome project and the development of techniques to manipulate gene expression have allowed this classic animal model to enter the molecular age. Such techniques, combined with the embryological manipulations that this system is well known for, provide a unique toolkit to study the genetic basis of neural development. A major advantage of these approaches is that they permit targeted gene misexpression with extremely high spatiotemporal resolution and over a large range of developmental stages, allowing functional analysis at a level, speed and ease that is difficult to achieve in other systems. This article provides a general overview of the chick as a developmental model focusing more specifically on its application to the study of eye development. Special emphasis is given to the state of the art of the techniques that have made gene gain- and loss-of-function studies in this model a reality. In addition, we discuss some methodological considerations derived from our own experience that we believe will be beneficial to researchers working with this system.

Pαx6 expression in postmitotic neurons mediates the growth of axons in response to SFRP1

During development, the mechanisms that specify neuronal subclasses are coupled to those that determine their axonal response to guidance cues. Pax6 is a homedomain transcription factor required for the specification of a variety of neural precursors. After cell cycle exit, Pax6 expression is often shut down in the precursor progeny and most postmitotic neurons no longer express detectable levels of the protein. There are however exceptions and high Pax6 protein levels are found, for example, in postmitotic retinal ganglion cells (RGCs), dopaminergic neurons of the olfactory bulb and the limbic system in the telencephalon. The function of Pax6 in these differentiating neurons remains mostly elusive. Here, we demonstrate that Pax6 mediates the response of growing axons to SFRP1, a secreted molecule expressed in several Pax6-positive forebrain territories. Forced expression of Pax6 in cultured postmitotic cortical neurons, which do not normally express Pax6, was sufficient to increment axonal length. Growth was blocked by the addition of anti-SFRP1 antibodies, whereas exogenously added SFRP1 increased axonal growth of Pax6-transfected neurons but not that of control or untransfected cortical neurons. In the reverse scenario, shRNA-mediated knock-down of Pax6 in mouse retinal explants specifically abolished RGCs axonal growth induced by SFRP1, but had no effect on RGCs differentiation and it did not modify the effect of Shh or Netrin on axon growth. Taken together these results demonstrate that expression of Pax6 is necessary and sufficient to render postmitotic neurons competent to respond to SFRP1. These results reveal a novel and unexpected function of Pax6 in postmitotic neurons and situate Pax6 and SFRP1 as pair regulators of axonal connectivity.

Secreted frizzled-related proteins are required for Wnt/β-catenin signalling activation in the vertebrate optic cup

Secreted frizzled-related proteins (Sfrps) are considered Wnt signalling antagonists but recent studies have shown that specific family members enhance Wnt diffusion and thus positively modulate Wnt signalling. Whether this is a general and physiological property of all Sfrps remains unexplored. It is equally unclear whether disruption of Sfrp expression interferes with developmental events mediated by Wnt signalling activation. Here, we have addressed these questions by investigating the functional consequences of Sfrp disruption in the canonical Wnt signalling-dependent specification of the mouse optic cup periphery. We show that compound genetic inactivation of Sfrp1 and Sfrp2 prevents Wnt/β-catenin signalling activation in this structure, which fails to be specified and acquires neural retina characteristics. Consistent with a positive role of Sfrps in signalling activation, Wnt spreading is impaired in the retina of Sfrp1(-/-);Sfrp2(-/-) mice. Conversely, forced expression of Sfrp1 in the wing imaginal disc of Drosophila, the only species in which the endogenous Wnt distribution can be detected, flattens the Wg gradient, suppresses the expression of high-Wg target genes but expands those typically activated by low Wg concentrations. Collectively, these data demonstrate that, in vivo, the levels of Wnt signalling activation strongly depend on the tissue distribution of Sfrps, which should be viewed as multifunctional regulators of Wnt signalling.

Down-regulation of the canonical Wnt β-catenin pathway in the airway epithelium of healthy smokers and smokers with COPD

BACKGROUND

The Wnt pathway mediates differentiation of epithelial tissues; depending on the tissue types, Wnt can either drive or inhibit the differentiation process. We hypothesized that key genes in the Wnt pathway are suppressed in the human airway epithelium under the stress of cigarette smoking, a stress associated with dysregulation of the epithelial differentiated state.

METHODOLOGY/PRINCIPAL FINDINGS

Microarrays were used to assess the expression of Wnt-related genes in the small airway epithelium (SAE) obtained via bronchoscopy and brushing of healthy nonsmokers, healthy smokers, and smokers with COPD. Thirty-three of 56 known Wnt-related genes were expressed in the SAE. Wnt pathway downstream mediators β-catenin and the transcription factor 7-like 1 were down-regulated in healthy smokers and smokers with COPD, as were many Wnt target genes. Among the extracellular regulators that suppress the Wnt pathway, secreted frizzled-related protein 2 (SFRP2), was up-regulated 4.3-fold in healthy smokers and 4.9-fold in COPD smokers, an observation confirmed by TaqMan Real-time PCR, Western analysis and immunohistochemistry. Finally, cigarette smoke extract mediated up-regulation of SFRP2 and down-regulation of Wnt target genes in airway epithelial cells in vitro.

CONCLUSIONS/SIGNIFICANCE

Smoking down-regulates the Wnt pathway in the human airway epithelium. In the context that Wnt pathway plays an important role in differentiation of epithelial tissues, the down-regulation of Wnt pathway may contribute to the dysregulation of airway epithelium differentiation observed in smoking-related airway disorders.

Sonic hedgehog guides post-crossing commissural axons both directly and indirectly by regulating Wnt activity

After midline crossing, axons of dorsolateral commissural neurons turn rostrally into the longitudinal axis of the spinal cord. In mouse, the graded distribution of Wnt4 attracts post-crossing axons rostrally. In contrast, in the chicken embryo, the graded distribution of Sonic hedgehog (Shh) guides post-crossing axons by a repulsive mechanism mediated by hedgehog-interacting protein. Based on these observations, we tested for a possible cooperation between the two types of morphogens. Indeed, we found that Wnts also act as axon guidance cues in the chicken spinal cord. However, in contrast to the mouse, Wnt transcription did not differ along the anteroposterior axis of the spinal cord. Rather, Wnt function was regulated by a gradient of the Wnt antagonist Sfrp1 (Secreted frizzled-related protein 1) that in turn was shaped by the Shh gradient. Thus, Shh affects post-crossing axon guidance both directly and indirectly by regulating Wnt function.

Proper differentiation of photoreceptors and amacrine cells depends on a regulatory loop between NeuroD and Six6

Timely generation of distinct neural cell types in appropriate numbers is fundamental for the generation of a functional retina. In vertebrates, the transcription factor Six6 is initially expressed in multipotent retina progenitors and then becomes restricted to differentiated retinal ganglion and amacrine cells. How Six6 expression in the retina is controlled and what are its precise functions are still unclear. To address this issue, we used bioinformatic searches and transgenic approaches in medaka fish (Oryzias latipes) to characterise highly conserved regulatory enhancers responsible for Six6 expression. One of the enhancers drove gene expression in the differentiating and adult retina. A search for transcription factor binding sites, together with luciferase, ChIP assays and gain-of-function studies, indicated that NeuroD, a bHLH transcription factor, directly binds an 'E-box' sequence present in this enhancer and specifically regulates Six6 expression in the retina. NeuroD-induced Six6 overexpression in medaka embryos promoted unorganized retinal progenitor proliferation and, most notably, impaired photoreceptor differentiation, with no apparent changes in other retinal cell types. Conversely, Six6 gain- and loss-of-function changed NeuroD expression levels and altered the expression of the photoreceptor differentiation marker Rhodopsin. In addition, knockdown of Six6 interfered with amacrine cell generation. Together, these results indicate that Six6 and NeuroD control the expression of each other and their functions coordinate amacrine cell generation and photoreceptor terminal differentiation.

Growth and differentiation of the retina and the optic tectum in the medaka fish requires olSfrp5

Secreted Frizzled-Related Proteins (SFRPs) are extracellular modulators of Wnt and Bmp signaling. Previous studies in birds and fishes have shown that Sfrp1, a member of this family, is strongly expressed throughout the development of the eye contributing to the specification of the eye field, retina neurogenesis and providing guidance information to retina ganglion cell axons. Here, we report that in medaka fish (Oryzias latipes) the expression of olSfrp5, which is closely related to olSfrp1, largely overlaps with that of olSfrp1 in the eye, but is additionally expressed in the developing midbrain and gut primordium. Morpholino-based interference with olSfrp5 expression causes microphthalmia and reduction of the tectum size associated with an increase in apoptotic cell death in these structures. Furthermore, interference with the levels of olSfrp5 expression impairs the patterning of the ventral portion of the optic cup, leading in some cases to a fissure coloboma. These early defects are followed by an abnormal retinal and tectal neurogenesis. In particular, only reduced numbers of photoreceptor and RGC were generated in olSfrp5 morphants retinas. The results point to an important role of olSfrp5 in visual system formation and indicate that olSfrp1 and olSfrp5, despite their overlapping expression, have only partially redundant function during eye development.

A global survey identifies novel upstream components of the Ath5 neurogenic network

Regulators of vertebrate Ath5 expression were identified by high-throughput screening; extending the current gene regulatory model network controlling retinal neurogenesis.

Background

Investigating the architecture of gene regulatory networks (GRNs) is essential to decipher the logic of developmental programs during embryogenesis. In this study we present an upstream survey approach, termed trans-regulation screen, to comprehensively identify the regulatory input converging on endogenous regulatory sequences.

Results

Our dual luciferase-based screen queries transcriptome-scale collections of cDNAs. Using this approach we study the regulation of Ath5, the central node in the GRN controlling retinal ganglion cell (RGC) specification in vertebrates. The Ath5 promoter integrates the input of upstream regulators to enable the transient activation of the gene, which is an essential step for RGC differentiation. We efficiently identified potential Ath5 regulators that were further filtered for true positives by an in situ hybridization screen. Their regulatory activity was validated in vivo by functional assays in medakafish embryos.

Conclusions

Our analysis establishes functional groups of genes controlling different regulatory phases, including the onset of Ath5 expression at cell-cycle exit and its down-regulation prior to terminal RGC differentiation. These results extent the current model of the GRN controlling retinal neurogenesis in vertebrates.

The Netrin-related domain of Sfrp1 interacts with Wnt ligands and antagonizes their activity in the anterior neural plate

BACKGROUND

Secreted frizzled related proteins (SFRPs) are multifunctional modulators of Wnt and BMP (Bone Morphogenetic Protein) signalling necessary for the development of most organs and the homeostasis of different adult tissues. SFRPs fold in two independent domains: the cysteine rich domain (SfrpCRD) related to the extracellular portion of Frizzled (Fz, Wnt receptors) and the Netrin module (SfrpNTR) defined by homologies with molecules such as Netrin-1, inhibitors of metalloproteinases and complement proteins. Due to its structural relationship with Fz, it is believed that SfrpCRD interferes with Wnt signalling by binding and sequestering the ligand. In contrast, the functional relevance of the SfrpNTR has been barely addressed.

RESULTS

Here, we combine biochemical studies, mutational analysis and functional assays in cell culture and medaka-fish embryos to show that the Sfrp1NTR mimics the function of the entire molecule, binds to Wnt8 and antagonizes Wnt canonical signalling. This activity requires intact tertiary structure and is shared by the distantly related Netrin-1NTR. In contrast, the Sfrp1CRD cannot mirror the function of the entire molecule in vivo but interacts with Fz receptors and antagonizes Wnt8-mediated beta-catenin transcriptional activity.

CONCLUSION

On the basis of these results, we propose that SFRP modulation of Wnt signalling may involve multiple and differential interactions among Wnt, Fz and SFRPs.

Beyond Wnt inhibition: new functions of secreted Frizzled-related proteins in development and disease

The secreted Frizzled-related proteins (SFRPs) are a family of soluble proteins that are structurally related to Frizzled (Fz) proteins, the serpentine receptors that mediate the extensively used cell-cell communication pathway involving Wnt signalling. Because of their homology with the Wnt-binding domain on the Fz receptors, SFRPs were immediately characterised as antagonists that bind to Wnt proteins to prevent signal activation. Since these initial studies, interest in the family of SFRPs has grown progressively, offering new perspectives on their function and mechanism of action in both development and disease. These studies indicate that SFRPs are not merely Wnt-binding proteins, but can also antagonise one another's activity, bind to Fz receptors and influence axon guidance, interfere with BMP signalling by acting as proteinase inhibitors, and interact with other receptors or matrix molecules. Furthermore, their expression is altered in different types of cancers, bone pathologies, retinal degeneration and hypophosphatemic diseases, indicating that their activity is fundamental for tissue homeostasis. Here we review some of the debated aspects of SFRP-Wnt interactions and discuss the new and emerging roles of SFRPs.

Identification of Phox2b-regulated genes by expression profiling of cranial motoneuron precursors

Background

Branchiomotor neurons comprise an important class of cranial motor neurons that innervate the branchial-arch-derived muscles of the face, jaw and neck. They arise in the ventralmost progenitor domain of the rhombencephalon characterized by expression of the homeodomain transcription factors Nkx2.2 and Phox2b. Phox2b in particular plays a key role in the specification of branchiomotor neurons. In its absence, generic neuronal differentiation is defective in the progenitor domain and no branchiomotor neurons are produced. Conversely, ectopic expression of Phox2b in spinal regions of the neural tube promotes cell cycle exit and neuronal differentiation and, at the same time, induces genes and an axonal phenotype characteristic for branchiomotor neurons. How Phox2b exerts its pleiotropic functions, both as a proneural gene and a neuronal subtype determinant, has remained unknown.

Results

To gain further insights into the genetic program downstream of Phox2b, we searched for novel Phox2b-regulated genes by cDNA microarray analysis of facial branchiomotor neuron precursors from heterozygous and homozygous Phox2b mutant embryos. We selected for functional studies the genes encoding the axonal growth promoter Gap43, the Wnt antagonist Sfrp1 and the transcriptional regulator Sox13, which were not previously suspected to play roles downstream of Phox2b and whose expression was affected by Phox2b misexpression in the spinal cord. While Gap43 did not produce an obvious phenotype when overexpressed in the neural tube, Sfrp1 induced the interneuron marker Lhx1,5 and Sox13 inhibited neuronal differentiation. We then tested whether Sfrp1 and Sox13, which are down-regulated by Phox2b in the facial neuron precursors, would antagonize some aspects of Phox2b activity. Co-expression of Sfrp1 prevented Phox2b from repressing Lhx1,5 and alleviated the commissural axonal phenotype. When expressed together with Sox13, Phox2b was still able to promote cell cycle exit and neuronal differentiation, but the cells failed to relocate to the mantle layer and to extinguish the neural stem cell marker Sox2.

Conclusion

Our results suggest novel roles for Sfrp1 and Sox13 in neuronal subtype specification and generic neuronal differentiation, respectively, and indicate that down-regulation of Sfrp1 and Sox13 are essential aspects of the genetic program controlled by Phox2b in cranial motoneurons.

Secreted frizzled related protein 1 is a paracrine modulator of epithelial branching morphogenesis, proliferation, and secretory gene expression in the prostate

Previous in vitro studies identified secreted frizzled related protein 1 (SFRP1) as a candidate pro-proliferative signal during prostatic development and cancer progression. This study determined the in vivo roles of SFRP1 in the prostate using expression studies in mice and by creating loss- and gain-of-function mouse genetic models. Expression studies using an Sfrp1(lacZ) knock-in allele showed that Sfrp1 is expressed in the developing mesenchyme/stroma of the prostate. Nevertheless, Sfrp1 null prostates exhibited multiple prostatic developmental defects in the epithelium including reduced branching morphogenesis, delayed proliferation, and increased expression of genes encoding prostate-specific secretory proteins. Interestingly, over-expression of SFRP1 in the adult prostates of transgenic mice yielded opposite effects including prolonged epithelial proliferation and decreased expression of genes encoding secretory proteins. These data demonstrated a previously unrecognized role for Sfrp1 as a stromal-to-epithelial paracrine modulator of epithelial growth, branching morphogenesis, and epithelial gene expression. To clarify the mechanism of SFRP1 action in the prostate, the response of WNT signaling pathways to SFRP1 was examined. Forced expression of SFRP1 in prostatic epithelial cells did not alter canonical WNT/beta-catenin signaling or the activation of CamKII. However, forced expression of SFRP1 led to sustained activation of JNK, and inhibition of JNK activity blocked the SFRP1-induced proliferation of prostatic epithelial cells, suggesting that SFRP1 acts through the non-canonical WNT/JNK pathway in the prostate.

Regulation of endothelial cell cytoskeletal reorganization by a secreted frizzled-related protein-1 and frizzled 4- and frizzled 7-dependent pathway: role in neovessel formation

Consistent with findings of Wnt pathway members involved in vascular cells, a role for Wnt/Frizzled signaling has recently emerged in vascular cell development. Among the few Wnt family members implicated in vessel formation in adult, Wnt7b and Frizzled 4 have been shown as involved in vessel formation in the lung and in the retina, respectively. Our previous work has shown a role for secreted Frizzled-related protein-1 (sFRP-1), a proposed Wnt signaling inhibitor, in neovascularization after an ischemic event and demonstrated its role as a potent angiogenic factor. However the mechanisms involved have not been investigated. Here, we show that sFRP-1 treatment increases endothelial cell spreading on extracellular matrix as revealed by actin stress fiber reorganization in an integrin-dependent manner. We demonstrate that sFRP-1 can interact with Wnt receptors Frizzled 4 and 7 on endothelial cells to transduce downstream to cellular machineries requiring Rac-1 activity in cooperation with GSK-3beta. sFRP-1 overexpression in endothelium specifically reversed the inactivation of GSK-3 beta and increased neovascularization in ischemia-induced angiogenesis in mouse hindlimb. This study illustrates a regulated pathway by sFRP-1 involving GSK-3beta and Rac-1 in endothelial cell cytoskeletal reorganization and in neovessel formation.

Sox2 and Pou2f1 interact to control lens and olfactory placode development

Sox2, which encodes an SRY-like HMG box transcription factor, is critical for vertebrate development. Sox2 mediates its transcriptional effects through the formation of complexes with specific co-factors, many of which are unknown. In this report, we identify Oct-1, encoded by the Pou2f1 gene, as a co-factor for Sox2 in the context of mouse lens and nasal placode induction. Oct-1, Sox2, and Pax6 are co-expressed during lens and nasal placode induction and during subsequent developmental stages. Genetic combination of Sox2 and Pou2f1 mutant alleles results in impaired induction of the lens placode, an ocular phenotype that includes anophthalmia, and a complete failure of nasal placode induction. These ocular and nasal phenotypes closely resemble those observed in Pax6 null embryos. Moreover, we identify DNA-binding sites that support the cooperative formation of a complex between Sox2 and Oct-1 and mediate Sox2/Oct-1-dependent transactivation of the Pax6 lens ectoderm enhancer in vitro. We demonstrate that the same Sox- and Octamer-binding sites are essential for Pax6 enhancer activity in the lens placode and its derivatives in transgenic mouse embryos. Collectively, these results indicate that Pou2f1, Sox2 and Pax6 are interdependent components of a molecular pathway utilized in both lens and nasal placode induction.

Wnt2b/β-catenin-mediated canonical Wnt signaling determines the peripheral fates of the chick eye

Wnt signaling orchestrates multiple aspects of central nervous system development, including cell proliferation and cell fate choices. In this study, we used gene transfer to activate or inhibit canonical Wnt signaling in vivo in the developing eye. We found that the expression of Wnt2b or constitutively active (CA) β-catenin inhibited retinal progenitor gene(RPG) expression and the differentiation of retinal neurons. In addition, Wnt signal activation in the central retina was sufficient to induce the expression of markers of the ciliary body and iris, two tissues derived from the peripheral optic cup (OC). The expression of a dominant-negative (DN)allele of Lef1, or of a Lef1-engrailed fusion protein, led to the inhibition of expression of peripheral genes and iris hypoplasia, suggesting that canonical Wnt signaling is required for peripheral eye development. We propose that canonical Wnt signaling in the developing optic vesicle (OV) and OC plays a crucial role in determining the identity of the ciliary body and iris. Because wingless (wg) plays a similar role in the induction of peripheral eye tissues of Drosophila, these findings indicate a possible conservation of the process that patterns the photoreceptive and support structures of the eye.

Beta-catenin is essential for lamination but not neurogenesis in mouse retinal development

During vertebrate retinal development, the seven retinal cell types differentiate sequentially from a single population of retinal progenitor cells (RPCs) and organize themselves into a distinct laminar structure. The purpose of this study was to determine whether beta-catenin, which functions both as a nuclear effector for the canonical Wnt signaling pathway and as a regulator of cell adhesion, is required for retinal neurogenesis or lamination. We used the Cre-loxP system to either eliminate beta-catenin or to express a constitutively active form during retinal neurogenesis. Eliminating beta-catenin did not affect cell differentiation, but did result in the loss of the radial arrangement of RPCs and caused abnormal migration of differentiated neurons. As a result, the laminar structure was massively disrupted in beta-catenin-null retinas, although all retinal cell types still formed. In contrast to other neural tissues, eliminating beta-catenin did not significantly reduce the proliferation rate of RPCs; likewise, activating beta-catenin ectopically in RPCs did not result in overproliferation, but loss of neural retinal identity. These results indicate that beta-catenin is essential during retinal neurogenesis as a regulator of cell adhesion but not as a nuclear effector of the canonical Wnt signaling pathway. The results further imply that retinal lamination and retinal cell differentiation are genetically separable processes.

Duplicate sfrp1 genes in zebrafish: sfrp1a is dynamically expressed in the developing central nervous system, gut and lateral line

The secreted frizzled-related proteins (Sfrp) are a family of soluble proteins with diverse biological functions having the capacity to bind Wnt ligands, to modulate Wnt signalling, and to signal directly via the Wnt receptor, Frizzled. In an enhancer trap screen for embryonic expression in zebrafish we identified an sfrp1 gene. Previous studies suggest an important role for sfrp1 in eye development, however, no data have been reported using the zebrafish model. In this paper, we describe duplicate sfrp1 genes in zebrafish and present a detailed analysis of the expression profile of both genes. Whole mount in situ hybridisation analyses of sfrp1a during embryonic and larval development revealed a dynamic expression profile, including: the central nervous system, where sfrp1a was regionally expressed throughout the brain and developing eye; the posterior gut, from the time of endodermal cell condensation; the lateral line, where sfrp1a was expressed in the migrating primordia and interneuromast cells that give rise to the sensory organs. Other sites included the blastoderm, segmenting mesoderm, olfactory placode, developing ear, pronephros and fin-bud. We have also analysed sfrp1b expression during embryonic development. Surprisingly this gene exhibited a divergent expression profile being limited to the yolk syncytium under the elongating tail-bud, which later covered the distal yolk extension, and transiently in the tail-bud mesenchyme. Overall, our studies provide a basis for future analyses of these developmentally important factors using the zebrafish model.

SFRP1 regulates the growth of retinal ganglion cell axons through the Fz2 receptor

Axon growth is governed by the ability of growth cones to interpret attractive and repulsive guidance cues. Recent studies have shown that secreted signaling molecules known as morphogens can also act as axon guidance cues. Of the large family of Wnt signaling components, only Wnt4 and Wnt5 seem to participate directly in axon guidance. Here we show that secreted Frizzled-related protein 1 (SFRP1), a proposed Wnt signaling inhibitor, can directly modify and reorient the growth of chick and Xenopus laevis retinal ganglion cell axons. This activity does not require Wnt inhibition and is modulated by extracellular matrix molecules. Intracellularly, SFRP1 function requires G(alpha) protein activation, protein synthesis and degradation, and it is modulated by cyclic nucleotide levels. Because SFRP1 interacts with Frizzled-2 (Fz2) and interference with Fz2 expression abolishes growth cone responses to SFRP1, we propose a previously unknown function for this molecule: the ability to guide growth cone movement via the Fz2 receptor.

Involvement of FrzA/sFRP-1 and the Wnt/frizzled pathway in ischemic preconditioning

Phosphorylation and subsequent inactivation of glycogen synthase kinase (GSK)-3beta via the Akt/PI3-Kinase pathway during ischemic preconditioning (PC) has been shown to be cardioprotective. As FrzA/sFRP-1, a secreted antagonist of the Wnt/Frizzled pathway, is expressed in the heart and is able to decrease the phosphorylation of GSK-3beta in vitro on vascular cells, we examined its effect during PC using transgenic mouse overexpressing FrzA in cardiomyocytes (alpha-MHC promoter) under a conditional transgene expression approach (tet-off system). Overexpression of FrzA inhibited the increase in GSK-3beta phosphorylation as well as protein kinase C (PKC) epsilon activation in transgenic mice after PC as compared with littermates. Phospho-Akt (P-Akt), phospho-JNK, or the cytoplasmic beta-catenin levels were not modified, phospho-p38 (P-p38) was slightly increased in transgenic mice after PC as compared with littermates. FrzA transgenic mice displayed a larger infarct size and a greater worsening of cardiac function compared with littermates. All these differences were reversed by the addition of doxycycline. This study demonstrates for the first time that disruption of a beta-catenin independent Wnt/Frizzled pathway induces the activation of GSK-3beta and reverses the benefit of preconditioning.

The extracellular matrix component WIF-1 is expressed during, and can modulate, retinal development

We have shown previously that components of the extracellular matrix (ECM) modulate neuronal development. Here, we searched for additional ECM elements that might play roles in retinal histogenesis and identified a secreted glycoprotein that is heavily expressed in the retina. This molecule, named by others Wnt Inhibitory Factor-1 (WIF-1), is expressed during and after the period of rod photoreceptor morphogenesis in the mouse. We show that a potential WIF-1 ligand, Wnt4, as well as a potential Wnt4 receptor, fzd4, and a potential Wnt4 coreceptor, LRP6, are expressed in the region of, and at the time of, rod photoreceptor genesis. WIF-1 and Wnt4 are coexpressed during retinal development and bind to each other; therefore, they are likely to interact during rod production. WIF-1 protein inhibits rod production, and anti-WIF-1 antibodies increase rod production; in contrast, Wnt4 promotes rod production. Together, these data suggest that WIF-1 and Wnt4, both components of the ECM, regulate mammalian photoreceptor development.

Transcriptional program of bone morphogenetic protein-2-induced epithelial and smooth muscle differentiation of pluripotent human embryonal carcinoma cells

Pluripotent human embryonal carcinoma NTera2/cloneD1 (NT2/D1) cells respond to multiple vertebrate patterning factors and offer a unique model system to investigate the signaling events associated with lineage determination and cell differentiation. Here, we define the temporal changes in global gene expression patterns in NT2/D1 cells upon treatment with bone morphogenetic protein-2 (BMP-2). Exposure to BMP-2 rapidly induced the expression of several transcription factors involved in establishing non-neural ectodermal fate followed by the appearance of epithelial-specific markers. Subsequent loss of stem cell markers was coupled to gene expression changes associated with decreased proliferative activity. Temporal clustering of gene expression patterns revealed a concurrent down-regulation of multiple transcripts involved in neurogenesis, neurite outgrowth, and axonal guidance, suggesting that the BMP-mediated differentiation process involves pro-epithelial as well as anti-neurogenic mechanisms. In addition, increased expression of smooth muscle markers both by gene expression and immunohistochemistry was detected. Several neural crest markers were induced preceding such a differentiation, compatible with a neural crest origin of NT2/D1-derived smooth muscle cells. Comparison of changes in transcript expression between BMP-2-induced epithelial versus all-trans-retinoic acid (ATRA)-induced neural differentiation revealed potential candidates for regulation of BMP-2 signaling and suppression of neural fate by BMP-2. This study suggests that BMP-2-induced differentiation of NT2/D1 cells provides a powerful assay to study early human epithelial and smooth muscle development.

Fluid shear stress induces beta-catenin signaling in osteoblasts

beta-Catenin plays a dual role in cells: one at cell-cell junctions and one regulating gene transcription together with TCF (T-cell Factor) in the nucleus. Recently, a role for beta-catenin in osteoblast differentiation and gene expression has begun to be elucidated. Herein we investigated the effects of fluid shear stress (FSS) on beta-catenin signaling. FSS is a well-characterized anabolic stimulus for osteoblasts; however, the molecular mechanisms for the effects of this stimulation remain largely unknown. We found that 1 hour of laminar FSS (10 dynes/cm(2)) induced translocation of beta-catenin to the nucleus and activated a TCF-reporter gene. Analysis of upstream signals that may regulate beta-catenin signaling activity revealed two potential mechanisms for increased beta-catenin signaling. First, FSS induced a transient, but significant, increase in the phosphorylation of both glycogen synthase kinase 3beta (GSK-3beta) and Akt. Second, FSS reduced the levels of beta-catenin associated with N-cadherin, suggesting that less sequestration of beta-catenin by cadherins occurs in osteoblasts subjected to FSS. Functional analysts of potential genes regulated by beta-catenin signaling in osteoblasts revealed two novel observations. First, endogenous, nuclear beta-catenin purified from osteoblasts formed a complex with a TCF -binding element in the cyclooxygenase-2 promoter, and, second, overexpression of either a constitutively active beta-catenin molecule or inhibition of GSK-3beta activity increased basal cyclooxygenase-2 levels. Together, these data demonstrate for the first time that FSS modulates the activity of both GSK-3beta and beta-catenin and that these signaling molecules regulate cyclooxygenase-2 expression in osteoblasts.

SFRP1 is required for the proper establishment of the eye field in the medaka fish

Secreted Frizzled Related Proteins (SFRPs) are a family of soluble molecules structurally related to the Wnt receptors. Functional analysis in different vertebrate species suggests that these molecules are multifunctional modulators of Wnt and possibly other signalling pathways. Sfrp1 a member of this family, is strongly expressed throughout embryonic development in different vertebrate species. Its function is, however, poorly understood. To address the role of this protein at early stages of embryonic development, we have used the medaka fish (Oryzias latipes) as a model system. Here, we describe the characterisation and the expression analysis of olSfrp1. We also show that morpholino-based interference with olSfrp1 expression results in embryos with a reduced eye field, a phenotype that, in the most affected embryos, is associated with a shortening and widening of the A-P axis. Because the expression of posterior diencephalic markers is unchanged but that of rostral telencephalic ones is expanded, we propose that olSfrp1 is needed for a proper establishment of the eye field within the forebrain. In addition, olSfrp1 may contribute to the control of mesodermal convergence extension movements that take place during gastrulation.

An analysis of the gene expression program of mammalian neural progenitor cells

A diverse range of neural cell types is generated from a pool of dividing stem and progenitor cells in an orderly manner during development. Little is known of the molecular and cellular biology underpinning the intrinsic control of this process. We have used a nonbiased method to purify populations of neural progenitor cells from the murine CNS to characterize the gene expression program of mammalian retinal progenitor cells. Analysis of these data led to the identification of a core set of >800 transcripts enriched in retinal progenitor cells compared to both their immediate postmitotic progeny and to differentiated neurons. This core set was found to be shared by progenitors in other regions of the developing CNS, with important regional differences in key functional families. In addition to providing an expression fingerprint of this cell type, this set highlights several key aspects of progenitor biology.