Dll1 and Dll4 function sequentially in the retina and pV2 domain of the spinal cord to regulate neurogenesis and create cell diversity

Notch pathway mutants do not equivalently perturb mouse embryonic retinal development

In the vertebrate eye, Notch ligands, receptors, and ternary complex components determine the destiny of retinal progenitor cells in part by regulating Hes effector gene activity. There are multiple paralogues for nearly every node in this pathway, which results in numerous instances of redundancy and compensation during development. To dissect such complexity at the earliest stages of eye development, we used seven germline or conditional mutant mice and two spatiotemporally distinct Cre drivers. We perturbed the Notch ternary complex and multiple Hes genes to understand if Notch regulates optic stalk/nerve head development; and to test intracellular pathway components for their Notch-dependent versus -independent roles during retinal ganglion cell and cone photoreceptor competence and fate acquisition. We confirmed that disrupting Notch signaling universally blocks progenitor cell growth, but delineated specific pathway components that can act independently, such as sustained Hes1 expression in the optic stalk/nerve head. In retinal progenitor cells, we found that among the genes tested, they do not uniformly suppress retinal ganglion cell or cone differentiation; which is not due differences in developmental timing. We discovered that shifts in the earliest cell fates correlate with expression changes for the early photoreceptor factor Otx2, but not with Atoh7, a factor required for retinal ganglion cell formation. During photoreceptor genesis we also better defined multiple and simultaneous activities for Rbpj and Hes1 and identify redundant activities that occur downstream of Notch. Given its unique roles at the retina-optic stalk boundary and cone photoreceptor genesis, our data suggest Hes1 as a hub where Notch-dependent and -independent inputs converge.

Not all Notch pathway mutations are equal in the embryonic mouse retina

In the vertebrate retina, combinations of Notch ligands, receptors, and ternary complex components determine the destiny of retinal progenitor cells by regulating Hes effector gene activity. Owing to reiterated Notch signaling in numerous tissues throughout development, there are multiple vertebrate paralogues for nearly every node in this pathway. These Notch signaling components can act redundantly or in a compensatory fashion during development. To dissect the complexity of this pathway during retinal development, we used seven germline or conditional mutant mice and two spatiotemporally distinct Cre drivers. We perturbed the Notch ternary complex and multiple Hes genes with two overt goals in mind. First, we wished to determine if Notch signaling is required in the optic stalk/nerve head for Hes1 sustained expression and activity. Second, we aimed to test if Hes1, 3 and 5 genes are functionally redundant during early retinal histogenesis. With our allelic series, we found that disrupting Notch signaling consistently blocked mitotic growth and overproduced ganglion cells, but we also identified two significant branchpoints for this pathway. In the optic stalk/nerve head, sustained Hes1 is regulated independent of Notch signaling, whereas during photoreceptor genesis both Notch-dependent and -independent roles for Rbpj and Hes1 impact photoreceptor genesis in opposing manners.

Paralogous Genes Involved in Embryonic Development: Lessons from the Eye and Other Tissues

During evolution, gene duplications lead to a naturally increased gene dosage. Duplicated genes can be further retained or eliminated over time by purifying selection pressure. The retention probability is increased by functional diversification and by the acquisition of novel functions. Interestingly, functionally diverged paralogous genes can maintain a certain level of functional redundancy and at least a partial ability to replace each other. In such cases, diversification probably occurred at the level of transcriptional regulation. Nevertheless, some duplicated genes can maintain functional redundancy after duplication and the ability to functionally compensate for the loss of each other. Many of them are involved in proper embryonic development. The development of particular tissues/organs and developmental processes can be more or less sensitive to the overall gene dosage. Alterations in the gene dosage or a decrease below a threshold level may have dramatic phenotypic consequences or even lead to embryonic lethality. The number of functional alleles of particular paralogous genes and their mutual cooperation and interactions influence the gene dosage, and therefore, these factors play a crucial role in development. This review will discuss individual interactions between paralogous genes and gene dosage sensitivity during development. The eye was used as a model system, but other tissues are also included.

The Roles of Long-Term Hyperhomocysteinemia and Micronutrient Supplementation in the AppNL–G–F Model of Alzheimer’s Disease

A causal contribution of hyperhomocysteinemia to cognitive decline and Alzheimer’s disease (AD), as well as potential prevention or mitigation of the pathology by dietary intervention, have frequently been subjects of controversy. In the present in vivo study, we attempted to further elucidate the impact of elevated homocysteine (HCys) and homocysteic acid (HCA) levels, induced by dietary B-vitamin deficiency, and micronutrient supplementation on AD-like pathology, which was simulated using the amyloid-based App^NL–G–F knock-in mouse model. For this purpose, cognitive assessment was complemented by analyses of ex vivo parameters in whole blood, serum, CSF, and brain tissues from the mice. Furthermore, neurotoxicity of HCys and HCA was assessed in a separate in vitro assay. In confirmation of our previous study, older App^NL–G–F mice also exhibited subtle phenotypic impairment and extensive cerebral amyloidosis, whereas dietary manipulations did not result in significant effects. As revealed by proximity extension assay-based proteome analysis, the App^NL–G–F genotype led to an upregulation of AD-characteristic neuronal markers. Hyperhomocysteinemia, in contrast, indicated mainly vascular effects. Overall, since there was an absence of a distinct phenotype despite both a significant amyloid-β burden and serum HCys elevation, the results in this study did not corroborate the pathological role of amyloid-β according to the “amyloid hypothesis,” nor of hyperhomocysteinemia on cognitive performance. Nevertheless, this study aided in further characterizing the App^NL–G–F model and in elucidating the role of HCys in diverse biological processes. The idea of AD prevention with the investigated micronutrients, however, was not supported, at least in this mouse model of the disease.

Concomitant genetic defects potentiate the adverse impact of prenatal alcohol exposure on cardiac outflow tract maturation

BACKGROUND

Prenatal alcohol exposure (PAE) is associated with an increased incidence of congenital heart defects (CHD), in particular outflow tract (OFT) defects. However, the variability in the incidence of CHD following PAE has not been fully explored. We hypothesize that a concomitant, relevant genetic defect would potentiate the adverse effect of PAE and partially explain the variability of PAE-induced CHD incidence.

METHODS

The OFT is formed by the second heart field (SHF). Our PAE model consisted of two intraperitoneal injections (3 g/kg, separated by 6 hr) of 30% ethanol on E6.5 during SHF specification. The impact of genetic defects was studied by SHF-specific loss of Delta-like ligand 4 (Dll4), fibroblast growth factor 8 (Fgf8) and Islet1.

RESULTS

Acute PAE alone significantly increased CHD incidence (4% vs. 26%, p = .015) with a particular increase in OFT alignment defects, viz., double outlet right ventricle (0 vs. 9%, p = .02). In embryos with a SHF genetic defect, acute PAE significantly increased CHD incidence (14 vs. 63%, p < .001), including double outlet right ventricle (6 vs. 50%, p < .001) compared to controls. PAE (p = .01) and heterozygous loss of Dll4 (p = .04) were found to independently contribute to CHD incidence, while neither Islet1 nor Fgf8 defects were found to be significant.

CONCLUSIONS

Our model recapitulates the increased incidence of OFT alignment defects seen in the clinic due to PAE. The presence of a concomitant SHF genetic mutation increases the incidence of PAE-related OFT defects. An apparent synergistic interaction between PAE and the loss of DLL4-mediated Notch signaling in OFT alignment requires further analysis.

Terminal 6q deletions cause brain malformations, a phenotype mimicking heterozygous DLL1 pathogenic variants: A multicenter retrospective case series

OBJECTIVE

Terminal 6q deletion is a rare genetic condition associated with a neurodevelopmental disorder characterized by intellectual disability and structural brain anomalies. Interestingly, a similar phenotype is observed in patients harboring pathogenic variants in the DLL1 gene. Our study aimed to further characterize the prenatal phenotype of this syndrome as well as to attempt to establish phenotype-genotype correlations.

METHOD

We collected ultrasound findings from 22 fetuses diagnosed with a pure 6qter deletion. We reviewed the literature and compared our 22 cases with 14 fetuses previously reported as well as with patients with heterozygous DLL1 pathogenic variants.

RESULTS

Brain structural alterations were observed in all fetuses. The most common findings (>70%) were cerebellar hypoplasia, ventriculomegaly, and corpus callosum abnormalities. Gyration abnormalities were observed in 46% of cases. Occasional findings included cerebral heterotopia, aqueductal stenosis, vertebral malformations, dysmorphic features, and kidney abnormalities.

CONCLUSION

This is the first series of fetuses diagnosed with pure terminal 6q deletion. Based on our findings, we emphasize the prenatal sonographic anomalies, which may suggest the syndrome. Furthermore, this study highlights the importance of chromosomal microarray analysis to search for submicroscopic deletions of the 6q27 region involving the DLL1 gene in fetuses with these malformations.

Notch signaling represses cone photoreceptor formation through the regulation of retinal progenitor cell states

Notch signaling is required to repress the formation of vertebrate cone photoreceptors and to maintain the proliferative potential of multipotent retinal progenitor cells. However, the mechanism by which Notch signaling controls these processes is unknown. Recently, restricted retinal progenitor cells with limited proliferation capacity and that preferentially generate cone photoreceptors have been identified. Thus, there are several potential steps during cone genesis that Notch signaling could act. Here we use cell type specific cis-regulatory elements to localize the primary role of Notch signaling in cone genesis to the formation of restricted retinal progenitor cells from multipotent retinal progenitor cells. Localized inhibition of Notch signaling in restricted progenitor cells does not alter the number of cones derived from these cells. Cell cycle promotion is not a primary effect of Notch signaling but an indirect effect on progenitor cell state transitions that leads to depletion of the multipotent progenitor cell population. Taken together, this suggests that the role of Notch signaling in cone photoreceptor formation and proliferation are both mediated by a localized function of Notch in multipotent retinal progenitor cells to repress the formation of restricted progenitor cells.

Murine Model of Cardiac Defects Observed in Adams-Oliver Syndrome Driven by Delta-Like Ligand-4 Haploinsufficiency

Heterozygous loss-of-function mutation in Delta-like ligand-4 (Dll4) is an important cause of Adams-Oliver syndrome (AOS). Cardiac defects, in particular outflow tract (OFT) alignment defects, are observed in about one-fourth of patients with this syndrome. The mechanism underlying this genotype-phenotype correlation has not yet been established. Dll4-mediated Notch signaling is known to play a crucial role in second heart field (SHF) progenitor cell proliferation. We hypothesized that the depletion of the SHF progenitor pool of cells due to partial loss of Dll4 is responsible for the OFT alignment defects seen in AOS. To demonstrate this, we studied Dll4 expression by murine SHF progenitor cells around E9.5, a crucial time-point in SHF biology. We used SHF-specific (Islet1-Cre) conditional knockout of Dll4 to bypass the early embryonic lethality seen in global Dll4 heterozygotes. Dll4-mediated Notch signaling is critically required for SHF proliferation such that Dll4 knockout results in a 33% reduction in proliferation and a fourfold increase in apoptosis in SHF cells, leading to a 56% decline in the size of the SHF progenitor pool. A reduction in SHF cells available for incorporation into the developing heart leads to underdevelopment of the SHF-derived right ventricle and OFT. Similar to the clinical syndrome, 32% of SHF-specific Dll4 heterozygotes demonstrate foreshortened and misaligned OFT, resulting in a double outlet right ventricle. Our murine model provides a molecular mechanism to explain the cardiac defects observed in AOS and establishes a novel clinical role for Dll4-mediated Notch signaling in SHF progenitor biology.

Effects of pharmacological calcimimetics on colorectal cancer cells over-expressing the human calcium-sensing receptor

The calcium-sensing receptor (CaSR) is a ubiquitously expressed multifunctional G protein-coupled receptor. Several studies reported that the CaSR plays an anti-inflammatory and anti-tumorigenic role in the intestine, and that it is down-regulated during colorectal carcinogenesis. We hypothesized that positive allosteric CaSR modulators (type II calcimimetics) selectively targeting the intestinal cells could be used for the treatment of intestinal pathologies. Therefore, the aim of this study was to determine the effect of pharmacological stimulation of CaSR on gene expression in vitro and on tumor growth in vivo. We stably transduced two colon cancer cell lines (HT29 and Caco2) with lentiviral vectors containing either the CaSR fused to GFP or GFP only. Using RNA sequencing, RT-qPCR experiments and ELISA, we determined that CaSR over-expression itself had generally little effect on gene expression in these cells. However, treatment with 1 μM of the calcimimetic NPS R-568 increased the expression of pro-inflammatory factors such as IL-23α and IL-8 and reduced the transcription of various differentiation markers in the cells over-expressing the CaSR. In vivo, neither the presence of the CaSR nor p.o. treatment of the animals with the calcimimetic cinacalcet affected tumor growth, tumor cell proliferation or tumor vascularization of murine HT29 xenografts. In summary, CaSR stimulation in CaSR over-expressing cells enhanced the expression of inflammatory markers in vitro, but was not able to repress colorectal cancer tumorigenicity in vivo. These findings suggest potential pro-inflammatory effects of the CaSR and type II calcimimetics in the intestine.

Delta-like ligand 4-mediated Notch signaling controls proliferation of second heart field progenitor cells by regulating Fgf8 expression

The role played by the Notch pathway in cardiac progenitor cell biology remains to be elucidated. Delta-like ligand 4 (Dll4), the arterial-specific Notch ligand, is expressed by second heart field (SHF) progenitors at time-points that are crucial in SHF biology. Dll4-mediated Notch signaling is required for maintaining an adequate pool of SHF progenitors, such that Dll4 knockout results in a reduction in proliferation and an increase in apoptosis. A reduced SHF progenitor pool leads to an underdeveloped right ventricle (RV) and outflow tract (OFT). In its most severe form, there is severe RV hypoplasia and poorly developed OFT resulting in early embryonic lethality. In its milder form, the OFT is foreshortened and misaligned, resulting in a double outlet right ventricle. Dll4-mediated Notch signaling maintains Fgf8 expression by transcriptional regulation at the promoter level. Combined heterozygous knockout of Dll4 and Fgf8 demonstrates genetic synergy in OFT alignment. Exogenous supplemental Fgf8 rescues proliferation in Dll4 mutants in ex-vivo culture. Our results establish a novel role for Dll4-mediated Notch signaling in SHF biology. More broadly, our model provides a platform for understanding oligogenic inheritance that results in clinically relevant OFT malformations.

Neurogenesis and Specification of Retinal Ganglion Cells

Across all species, retinal ganglion cells (RGCs) are the first retinal neurons generated during development, followed by the other retinal cell types. How are retinal progenitor cells (RPCs) able to produce these cell types in a specific and timely order? Here, we will review the different models of retinal neurogenesis proposed over the last decades as well as the extrinsic and intrinsic factors controlling it. We will then focus on the molecular mechanisms, especially the cascade of transcription factors that regulate, more specifically, RGC fate. We will also comment on the recent discovery that the ciliary marginal zone is a new stem cell niche in mice contributing to retinal neurogenesis, especially to the generation of ipsilateral RGCs. Furthermore, RGCs are composed of many different subtypes that are anatomically, physiologically, functionally, and molecularly defined. We will summarize the different classifications of RGC subtypes and will recapitulate the specification of some of them and describe how a genetic disease such as albinism affects neurogenesis, resulting in profound visual deficits.

Deterministic fate assignment of Müller glia cells in the zebrafish retina suggests a clonal backbone during development

The optic cup houses multipotent retinal progenitor cells that proliferate and differentiate to form the mature retina, containing five main types of neurons and a single glial cell type, the Müller cell. Progenitors of the zebrafish optic cup generate clones that vary regarding the number and types of neurons, a process we previously showed could be described by stochastic models. Here, we present data indicating that each retinal progenitor cell, in the 24 hrs post‐fertilization optic cup, is predestined to form a single Müller cell. This striking fate assignment of Müller cells reveals a dual nature of retinal lineages where stochastic mechanisms produce variable numbers of neurons while there is a strong deterministic component governing the formation of glia cells. A possible mechanism for this stereotypic fate assignment could be the maintenance of a clonal backbone during retina development, which would be similar to invertebrate and rodent cortical neurogenesis.

Retinal myeloid cells regulate tip cell selection and vascular branching morphogenesis via Notch ligand Delta-like 1

During angiogenesis, single endothelial cells (EC) specialize into tip cells that guide vessel sprouting towards growth factor gradients and instruct the adjacent vessel stalk. The balance between tip and stalk cells is regulated by endothelial Notch signalling through the expression of Notch ligand Delta-like 4 (Dll4) in tip cells, which suppresses a tip cell fate in adjacent stalk cells. Here we show, using genetic reporter and conditional deletion strategies, that myeloid cells regulate tip cell numbers and Dll4 expression via the Notch ligand Dll1 during vascular development in the retina. Dll1 is selectively expressed by a subpopulation of retinal myeloid cells, which progressively localizes to the sprouting vascular network. Conditional, myeloid-specific deletion of Dll1 impairs endothelial Dll4 tip-stalk gradient resulting in an increase of endothelial tip cells and EC filopodia, accompanied by an increase in vascular density and branching. In vitro, co-culture of human EC with monocyte-derived macrophages induced Dll1 upregulation in macrophages and Dll4 upregulation and an endothelial tip cell signature in EC. Furthermore, culturing human EC on recombinant DLL1 induced endothelial Dll4 expression and a tip cell program, indicating that changes are Dll1-dependent. Thus, myeloid cells regulate tip cell fate and angiogenesis through expression of Notch ligand Dll1.

Derivation of Specific Neural Populations From Pluripotent Cells for Understanding and Treatment of Spinal Cord Injury

Due to the nature of the biological response to traumatic spinal cord injury, there are very limited therapeutic options available to patients. Recent advances in cell transplantation have demonstrated the therapeutic potential of transplanting supportive cell types following spinal cord injury. In particular, pluripotent stem cell derived neural cells are of interest for future investigation. Use of pluripotent stem cells as the source allows many cell types to be produced from a population that can be expanded in vitro. In this review, we will discuss the signaling pathways that have been used to differentiate spinal neural phenotypes from pluripotent stem cells. Additionally, we will highlight methods that have been developed to direct the differentiation of pluripotent stem cells to specific neural fates. Further refinement and elaboration of these techniques might aid in elucidating the multitude of neuronal subtypes endogenous to the spinal cord, as well as produce further therapeutic options for spinal cord injury recovery. Developmental Dynamics 248:78-87, 2019. © 2018 Wiley Periodicals, Inc.

Mib1 prevents Notch Cis-inhibition to defer differentiation and preserve neuroepithelial integrity during neural delamination

The vertebrate neuroepithelium is composed of elongated progenitors whose reciprocal attachments ensure the continuity of the ventricular wall. As progenitors commit to differentiation, they translocate their nucleus basally and eventually withdraw their apical endfoot from the ventricular surface. However, the mechanisms allowing this delamination process to take place while preserving the integrity of the neuroepithelial tissue are still unclear. Here, we show that Notch signaling, which is classically associated with an undifferentiated state, remains active in prospective neurons until they delaminate. During this transition period, prospective neurons rapidly reduce their apical surface and only later down-regulate N-Cadherin levels. Upon Notch blockade, nascent neurons disassemble their junctions but fail to reduce their apical surface. This disrupted sequence weakens the junctional network and eventually leads to breaches in the ventricular wall. We also provide evidence that the Notch ligand Delta-like 1 (Dll1) promotes differentiation by reducing Notch signaling through a Cis-inhibition mechanism. However, during the delamination process, the ubiquitin ligase Mindbomb1 (Mib1) transiently blocks this Cis-inhibition and sustains Notch activity to defer differentiation. We propose that the fine-tuned balance between Notch Trans-activation and Cis-inhibition allows neuroepithelial cells to seamlessly delaminate from the ventricular wall as they commit to differentiation.

Establishment of 2 control and 2 hPSC cell lines constitutively expressing the Notch ligand DLL4

The Notch ligand DLL4 has key roles during embryonic development of different tissues, but most of the data comes from animal models. Here we describe the generation and characterization of 2 human Pluripotent Stem Cell (hPSC) lines that overexpress DLL4, as well as the two corresponding control hPSC lines. DLL4 expression can be detected at the mRNA and protein level, and does not affect the pluripotency of the cells. These hPSC lines can be used to study the role of DLL4 during human embryonic development.

The Regulation of Notch Signaling in Retinal Development and Regeneration

Purpose of review

Notch signaling is an important component of retinal progenitor cell maintenance and MG specification during development, and its manipulation may be critical for allowing MG to re-enter the cell cycle and regenerate neurons in adults. In mammals, MG respond to retinal injury by undergoing a gliotic response rather than a regenerative one. Understanding the complexities of Notch signaling may allow for strategies that enhance regeneration over gliosis.

Recent findings

Notch signaling is regulated at multiple levels, and is interdependent with various other signaling pathways in both the receptor and ligand expressing cells. The precise spatial and temporal patterning of Notch components is necessary for proper retinal development. Regenerative species undergo a dynamic regulation of Notch signaling in MG upon injury, whereas non-regenerative species fail to productively regulate Notch.

Summary

Notch signaling is malleable, such that the altered composition of growth and transcription factors in the developing and mature retinas result in different Notch mediated responses. Successful regeneration will require the manipulation of the retinal environment to foster a dynamic rather than static regulation of Notch signaling in concert with other reprogramming and differentiation factors.

Separate and coincident expression of Hes1 and Hes5 in the developing mouse eye

BACKGROUND

Notch signaling is broadly required during embryogenesis, frequently activating the transcription of two basic helix-loop-helix transcription factors, Hes1 and Hes5. But, it remains unresolved when and where Hes1 and Hes5 act alone or together during development. Here, we analyzed a Hes5-green fluorescent protein (GFP) bacterial artificial chromosome (BAC) transgenic mouse, as a proxy for endogenous Hes5. We directly compared transgenic GFP expression with Hes1, and particular markers of embryonic lens and retina development.

RESULTS

Hes5-GFP is dynamic within subsets of retinal and lens progenitor cells, and differentiating retinal ganglion neurons, in contrast to Hes1 found in all progenitor cells. In the adult retina, only Müller glia express Hes5-GFP. Finally, Hes5-GFP is up-regulated in Hes1 germline mutants, consistent with previous demonstration that Hes1 suppresses Hes5 transcription.

CONCLUSIONS

Hes5-GFP BAC transgenic mice are useful for identifying Hes5-expressing cells. Although Hes5-GFP and Hes1 are coexpressed in particular developmental contexts, we also noted cohorts of lens or retinal cells expressing just one factor. The dynamic Hes5-GFP expression pattern, coupled with its derepressed expression in Hes1 mutants, suggests that this transgene contains the relevant cis-regulatory elements that regulate endogenous Hes5 in the mouse lens and retina. Developmental Dynamics 247:212-221, 2018. © 2017 Wiley Periodicals, Inc.

Photoreceptor Fate Determination in the Vertebrate Retina

Photoreceptors are highly specialized primary sensory neurons that sense light and initiate vision. This critical role is well demonstrated by the fact that visual impairment accompanies photoreceptor loss or dysfunction in many human diseases. With the remarkable advances in stem cell research, one therapeutic approach is to use stem cells to generate photoreceptors and then engraft them into diseased eyes. Knowledge of the molecular mechanisms that control photoreceptor genesis during normal development can greatly aid in the production of photoreceptor cells for this approach. This article will discuss advances in our understanding of the molecular mechanisms that regulate photoreceptor fate determination during development. Recent lineage studies have shown that there are distinct retinal progenitor cells (RPCs) that produce specific combinations of daughter cell types, including photoreceptors and other types of retinal cells. Gene regulatory networks, in which transcription factors interact via cis-regulatory DNA elements, have been discovered that operate within distinct RPCs, and/or newly postmitotic cells, to direct the choice of photoreceptor fate.

The Retinal Pigment Epithelium Is a Notch Signaling Niche in the Mouse Retina

Notch signaling in neural progenitor cell is triggered by ligands expressed in adjacent cells. To identify the sources of active Notch ligands in the mouse retina, we negatively regulated Notch ligand activity in various neighbors of retinal progenitor cells (RPCs) by eliminating mindbomb E3 ubiquitin protein ligase 1 (Mib1). Mib1-deficient retinal cells failed to induce Notch activation in intra-lineage RPCs, which prematurely differentiated into neurons; however, Mib1 in post-mitotic retinal ganglion cells was not important. Interestingly, Mib1 in the retinal pigment epithelium (RPE) also contributed to Notch activation in adjacent RPCs by supporting the localization of active Notch ligands at RPE-RPC contacts. Combining this RPE-driven Notch signaling and intra-retinal Notch signaling, we propose a model in which one RPC daughter receives extra Notch signals from the RPE to become an RPC, whereas its sister cell receives only a subthreshold level of intra-retinal Notch signal and differentiates into a neuron.

Iterative Role of Notch Signaling in Spinal Motor Neuron Diversification

The motor neuron progenitor domain in the ventral spinal cord gives rise to multiple subtypes of motor neurons and glial cells. Here, we examine whether progenitors found in this domain are multipotent and which signals contribute to their cell-type-specific differentiation. Using an in vitro neural differentiation model, we demonstrate that motor neuron progenitor differentiation is iteratively controlled by Notch signaling. First, Notch controls the timing of motor neuron genesis by repressing Neurogenin 2 (Ngn2) and maintaining Olig2-positive progenitors in a proliferative state. Second, in an Ngn2-independent manner, Notch contributes to the specification of median versus hypaxial motor column identity and lateral versus medial divisional identity of limb-innervating motor neurons. Thus, motor neuron progenitors are multipotent, and their diversification is controlled by Notch signaling that iteratively increases cellular diversity arising from a single neural progenitor domain.

PlexinA2 and Sema6A are required for retinal progenitor cell migration

In the vertebrate retina six types of neurons and one glial cell type are generated from multipotent retinal progenitor cells (RPCs) whose proliferation and differentiation are regulated by intrinsic and extrinsic factors. RPCs proliferate undergoing interkinetic nuclear migration within the neuroblastic layer, with their nuclei moving up and down along the apico-basal axis. Moreover, they only differentiate and therefore exit the cell cycle at the apical side of the neuroblastic layer. Sema6A and its receptors PlexinA4 and PlexinA2 control lamina stratification of the inner plexiform layer in the mouse retina. Nevertheless, their function in earlier developmental stages is still unknown. Here, we analyzed the embryonic retina of PlexinA2 and Sema6A knockout mice. Using time-lapse videomicroscopy we provide evidence that Sema6A/PlexinA2 signaling participates to interkinetic nuclear migration of RPCs around birth. When disrupted, RPCs migration is blocked at the apical side of the neuroblastic layer. This is the first evidence supporting a role for transmembrane molecules in the regulation of interkinetic nuclear migration in the mouse retina.

A Disintegrin and Metalloproteinase10 (ADAM10) Regulates NOTCH Signaling during Early Retinal Development

ADAM10 and ADAM17 are two closely related members of the ADAM (a disintegrin and metalloprotease) family of membrane-bound sheddases, which proteolytically cleave surface membrane proteins. Both ADAM10 and ADAM17 have been implicated in the proteolytic cleavage of NOTCH receptors and as such regulators of NOTCH signaling. During retinal development, NOTCH signaling facilitates retinal neurogenesis by maintaining progenitor cells in a proliferative state and by mediating retinal cell fates. However, the roles of ADAM10 and ADAM17 in the retina are not well defined. In this study, we set out to clarify the roles of ADAM10 and ADAM17 during early retinal development. The retinal phenotype of conditionally abated Adam17 retinae (Adam17 CKO) did not differ from the controls whereas conditionally ablated Adam10 retinae (Adam10 CKO) exhibited abnormal morphogenesis characterized by the formation of rosettes and a loss of retinal laminae phenotypically similar to morphological abnormalities identified in mice with retinal NOTCH signaling deficiency. Additionally, Adam10 CKO retinae exhibited abnormal neurogenesis characterized by fewer proliferating progenitor cells and greater differentiation of early photoreceptors and retinal ganglion cells. Moreover, constitutive activation of the NOTCH1-intracellular domain (N1-ICD) rescued Adam10 CKO abnormal neurogenesis, as well as abnormal retinal morphology by maintaining retinal cells in the progenitor state. Collectively these findings provide in vivo genetic evidence that ADAM10, and not ADAM17, is indispensable for proper retinal development as a regulator of NOTCH signaling.

Oncogenic role of the Notch pathway in primary liver cancer

Primary liver cancer, which includes hepatocellular carcinoma (HCC), intrahepatic cholangiocarcinoma (ICC) and fibrolamellar HCC, is one of the most common malignancies and the third leading cause of cancer-associated mortality, worldwide. Despite the development of novel therapies, the prognosis of liver cancer patients remains extremely poor. Thus, investigation of the genetic background and molecular mechanisms underlying the development and progression of this disease has gained significant attention. The Notch signaling pathway is a crucial determinant of cell fate during development and disease in several organs. In the liver, Notch signaling is involved in biliary tree development and tubulogenesis, and is also significant in the development of HCC and ICC. These findings suggest that the modulation of Notch pathway activity may have therapeutic relevance. The present review summarizes Notch signaling during HCC and ICC development and discusses the findings of recent studies regarding Notch expression, which reveal novel insights into its function in liver cancer progression.

Transitional Progenitors during Vertebrate Retinogenesis

The retina is a delicate neural tissue responsible for light signal capturing, modulating, and passing to mid-brain. The brain then translated the signals into three-dimensional vision. The mature retina is composed of more than 50 subtypes of cells, all of which are developed from a pool of early multipotent retinal progenitors, which pass through sequential statuses of oligopotent, bipotent, and unipotent progenitors, and finally become terminally differentiated retinal cells. A transitional progenitor model is proposed here to describe how intrinsic developmental programs, along with environmental cues, control the step-by-step differentiation during retinogenesis. The model could elegantly explain many current findings as well as predict roles of intrinsic factors during retinal development.

Cell autonomous and nonautonomous requirements for Delltalike1 during early mouse retinal neurogenesis

BACKGROUND

In the vertebrate retina, six neuronal and one glial cell class are produced from a common progenitor pool. During neurogenesis, adjacent retinal cells use Notch signaling to maintain a pool of progenitors by blocking particular cells from differentiating prematurely. In mice there are multiple Notch pathway ligands and receptors, but the role(s) of each paralogue during retinal histogenesis remains only partially defined.

RESULTS

Here we analyzed the cell autonomous and nonautonomous requirements for the Deltalike1(Dll1) ligand during prenatal retinogenesis. We used the α-Cre driver to simultaneously delete a Dll1 conditional allele and activate the Z/EG reporter, then quantified Dll1 mutant phenotypes within and outside of this α-Cre GFP-marked lineage. We found that Dll1 activity is required for Hes1 expression, both autonomously and nonautonomously, but were surprised that retinal ganglion cell differentiation is only blocked cell autonomously. Moreover, Dll1 does not act during cone photoreceptor neurogenesis. Finally, Dll1 mutant adult retinas contained small retinal rosettes and RGC patterning defects but were otherwise normal.

CONCLUSIONS

Although Dll1 participates in bidirectional (cis + trans) Notch signaling to regulate Hes1 expression, it only acts cell autonomously (in cis) to interpret inhibitory signals from other cells that block RGC neurogenesis. Developmental Dynamics 245:631-640, 2016. © 2016 Wiley Periodicals, Inc.

The late and dual origin of cerebrospinal fluid-contacting neurons in the mouse spinal cord

Considerable progress has been made in understanding the mechanisms that control the production of specialized neuronal types. However, how the timing of differentiation contributes to neuronal diversity in the developing spinal cord is still a pending question. In this study, we show that cerebrospinal fluid-contacting neurons (CSF-cNs), an anatomically discrete cell type of the ependymal area, originate from surprisingly late neurogenic events in the ventral spinal cord. CSF-cNs are identified by the expression of the transcription factors Gata2 and Gata3, and the ionic channels Pkd2l1 and Pkd1l2. Contrasting with Gata2/3(+) V2b interneurons, differentiation of CSF-cNs is independent of Foxn4 and takes place during advanced developmental stages previously assumed to be exclusively gliogenic. CSF-cNs are produced from two distinct dorsoventral regions of the mouse spinal cord. Most CSF-cNs derive from progenitors circumscribed to the late-p2 and the oligodendrogenic (pOL) domains, whereas a second subset of CSF-cNs arises from cells bordering the floor plate. The development of these two subgroups of CSF-cNs is differentially controlled by Pax6, they adopt separate locations around the postnatal central canal and they display electrophysiological differences. Our results highlight that spatiotemporal mechanisms are instrumental in creating neural cell diversity in the ventral spinal cord to produce distinct classes of interneurons, motoneurons, CSF-cNs, glial cells and ependymal cells.

Context-Dependent Sensitivity to Mutations Disrupting the Structural Integrity of Individual EGF Repeats in the Mouse Notch Ligand DLL1

The highly conserved Notch-signaling pathway mediates cell-to-cell communication and is pivotal for multiple developmental processes and tissue homeostasis in adult organisms. Notch receptors and their ligands are transmembrane proteins with multiple epidermal-growth-factor-like (EGF) repeats in their extracellular domains. In vitro the EGF repeats of mammalian ligands that are essential for Notch activation have been defined. However, in vivo the significance of the structural integrity of each EGF repeat in the ligand ectodomain for ligand function is still unclear. Here, we analyzed the mouse Notch ligand DLL1. We expressed DLL1 proteins with mutations disrupting disulfide bridges in each individual EGF repeat from single-copy transgenes in the HPRT locus of embryonic stem cells. In Notch transactivation assays all mutations impinged on DLL1 function and affected both NOTCH1 and NOTCH2 receptors similarly. An allelic series in mice that carried the same point mutations in endogenous Dll1, generated using a mini-gene strategy, showed that early developmental processes depending on DLL1-mediated NOTCH activation were differently sensitive to mutation of individual EGF repeats in DLL1. Notably, some mutations affected only somite patterning and resulted in vertebral column defects resembling spondylocostal dysostosis. In conclusion, the structural integrity of each individual EGF repeat in the extracellular domain of DLL1 is necessary for full DLL1 activity, and certain mutations in Dll1 might contribute to spondylocostal dysostosis in humans.

The Determination of Rod and Cone Photoreceptor Fate

Photoreceptors have been the most intensively studied retinal cell type. Early lineage studies showed that photoreceptors are produced by retinal progenitor cells (RPCs) that produce only photoreceptor cells and by RPCs that produce both photoreceptor cells and other retinal cell types. More recent lineage studies have shown that there are intrinsic, molecular differences among these RPCs and that these molecular differences operate in gene regulatory networks (GRNs) that lead to the choice of the rod versus the cone fate. In addition, there are GRNs that lead to the choice of a photoreceptor fate and that of another retinal cell type. An example of such a GRN is one that drives the binary fate choice between a rod photoreceptor and bipolar cell. This GRN has many elements, including both feedforward and feedback regulatory loops, highlighting the complexity of such networks. This and other examples of retinal cell fate determination are reviewed here, focusing on the events that direct the choice of rod and cone photoreceptor fate.

Context-Dependent Functional Divergence of the Notch Ligands DLL1 and DLL4 In Vivo

Notch signalling is a fundamental pathway that shapes the developing embryo and sustains adult tissues by direct communication between ligand and receptor molecules on adjacent cells. Among the ligands are two Delta paralogues, DLL1 and DLL4, that are conserved in mammals and share a similar structure and sequence. They activate the Notch receptor partly in overlapping expression domains where they fulfil redundant functions in some processes (e.g. maintenance of the crypt cell progenitor pool). In other processes, however, they appear to act differently (e.g. maintenance of foetal arterial identity) raising the questions of how similar DLL1 and DLL4 really are and which mechanism causes the apparent context-dependent divergence. By analysing mice that conditionally overexpress DLL1 or DLL4 from the same genomic locus (Hprt) and mice that express DLL4 instead of DLL1 from the endogenous Dll1 locus (Dll1Dll4ki), we found functional differences that are tissue-specific: while DLL1 and DLL4 act redundantly during the maintenance of retinal progenitors, their function varies in the presomitic mesoderm (PSM) where somites form in a Notch-dependent process. In the anterior PSM, every cell expresses both Notch receptors and ligands, and DLL1 is the only activator of Notch while DLL4 is not endogenously expressed. Transgenic DLL4 cannot replace DLL1 during somitogenesis and in heterozygous Dll1Dll4ki/+ mice, the Dll1Dll4ki allele causes a dominant segmentation phenotype. Testing several aspects of the complex Notch signalling system in vitro, we found that both ligands have a similar trans-activation potential but that only DLL4 is an efficient cis-inhibitor of Notch signalling, causing a reduced net activation of Notch. These differential cis-inhibitory properties are likely to contribute to the functional divergence of DLL1 and DLL4.

From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate

During vertebrate development, the central (CNS) and peripheral nervous systems (PNS) arise from the neural plate. Cells at the margin of the neural plate give rise to neural crest cells, which migrate extensively throughout the embryo, contributing to the majority of neurons and all of the glia of the PNS. The rest of the neural plate invaginates to form the neural tube, which expands to form the brain and spinal cord. The emergence of molecular cloning techniques and identification of fluorophores like Green Fluorescent Protein (GFP), together with transgenic and electroporation technologies, have made it possible to easily visualize the cellular and molecular events in play during nervous system formation. These lineage-tracing techniques have precisely demonstrated the migratory pathways followed by neural crest cells and increased knowledge about their differentiation into PNS derivatives. Similarly, in the spinal cord, lineage-tracing techniques have led to a greater understanding of the regional organization of multiple classes of neural progenitor and post-mitotic neurons along the different axes of the spinal cord and how these distinct classes of neurons assemble into the specific neural circuits required to realize their various functions. Here, we review how both classical and modern lineage and marker analyses have expanded our knowledge of early peripheral nervous system and spinal cord development.

Notch signaling differentially regulates Atoh7 and Neurog2 in the distal mouse retina

Notch signaling regulates basic helix-loop-helix (bHLH) factors as an evolutionarily conserved module, but the tissue-specific mechanisms are incompletely elucidated. In the mouse retina, bHLH genes Atoh7 and Neurog2 have distinct functions, with Atoh7 regulating retinal competence and Neurog2 required for progression of neurogenesis. These transcription factors are extensively co-expressed, suggesting similar regulation. We directly compared Atoh7 and Neurog2 regulation at the earliest stages of retinal neurogenesis in a broad spectrum of Notch pathway mutants. Notch1 and Rbpj normally block Atoh7 and Neurog2 expression. However, the combined activities of Notch1, Notch3 and Rbpj regulate Neurog2 patterning in the distal retina. Downstream of the Notch complex, we found the Hes1 repressor mediates Atoh7 suppression, but Hes1, Hes3 and Hes5 do not regulate Neurog2 expression. We also tested Notch-mediated regulation of Jag1 and Pax6 in the distal retina, to establish the appropriate context for Neurog2 patterning. We found that Notch1;Notch3 and Rbpj block co-expression of Jag1 and Neurog2, while specifically stimulating Pax6 within an adjacent domain. Our data suggest that Notch signaling controls the overall tempo of retinogenesis, by integrating cell fate specification, the wave of neurogenesis and the developmental status of cells ahead of this wave.

Selective neuronal lineages derived from Dll4-expressing progenitors/precursors in the retina and spinal cord

BACKGROUND

During retinal and spinal cord neurogenesis, Notch signaling plays crucial roles in regulating proliferation and differentiation of progenitor cells. One of the Notch ligands, Delta-like 4 (Dll4), has been shown to be expressed in subsets of retinal and spinal cord progenitors/precursors and involved in neuronal subtype specification. However, it remains to be determined whether Dll4 expression has any progenitor/precursor-specificity contributing to its functional specificity during neural development.

RESULTS

We generated a Dll4-Cre BAC transgenic mouse line that drives Cre recombinase expression mimicking that of the endogenous Dll4 in the developing retina and spinal cord. By fate-mapping analysis, we found that Dll4-expressing progenitors/precursors give rise to essentially all cone, amacrine and horizontal cells, a large portion of rod and ganglion cells, but only few bipolar and Müller cells. In the spinal cord, Dll4-expressing progenitors/precursors generate almost all V2a and V2c cells while producing only a fraction of the cells for other interneuron and motor neuron subtypes along the dorsoventral axis.

CONCLUSIONS

Our data suggest that selective expression of Dll4 in progenitors/precursors contributes to its functional specificity in neuronal specification and that the Dll4-Cre line is a valuable tool for gene manipulation to study Notch signaling.

Distinct expression patterns of Notch ligands, Dll1 and Dll4, in normal and inflamed mice intestine

Reports have suggested that the two Notch ligands, Dll1 and Dll4, are indispensable to maintain the homeostasis of the intestinal epithelium. However, within the intestinal epithelium, the precise distribution of the cells that express those ligands at the protein level remains largely unknown. Here, we show a series of immunohistochemical analysis through which we successfully identified mice intestinal epithelial cells (IECs) that endogenously express Dll1 or Dll4. Results showed that Dll1-positive (Dll1+ve) IECs reside exclusively within the crypt, whereas Dll4-positive (Dll4+ve) IECs can locate both in the crypt and in the villus of the small intestine. Also in the colon, Dll1+ve IECs resided at the lower part of the crypt, whereas Dll4+ve IECs resided at both upper and lower part of the crypt, including the surface epithelium. Both Dll1+ve and Dll4+ve IECs were ATOH1-positive, but Hes1-negative cells, and located adjacent to Hes1-positive cells within the crypts. A sub-population of both Dll1+ve and Dll4+ve IECs appeared to co-express Muc2, but rarely co-expressed other secretory lineage markers. However, as compared to Dll1+ve IECs, Dll4+ve IECs included larger number of Muc2-postive IECs, suggesting that Dll4 is more preferentially expressed by goblet cells. Also, we identified that Dll4 is expressed in the Paneth cells of the small intestine, whereas Dll1 and Dll4 is expressed in the c-kit-positive IECs of the colon, indicating that Dll1+ve and Dll4+ve IECs may contribute to constitute the intestinal stem cell niche. Compared to the normal colon, analysis of DSS-colitis showed that number of Dll1+ve IECs significantly decrease in the elongated crypts of the inflamed colonic mucosa. In sharp contrast, number of Dll4+ve IECs showed a significant increase in those crypts, which was accompanied by the increase in number of Hes1-positive IECs. Those Dll4+ve IECs were mostly found adjacent to the Hes1-positive IECs, suggesting that Dll4 may act as a major Notch ligand in the crypts of the inflamed colonic mucosa. Our results illustrate distinct expression patterns of Dll1 and Dll4 within the intestinal epithelium, and suggest that these two ligands may have different roles in normal and inflamed mucosa.

Different combinations of Notch ligands and receptors regulate V2 interneuron progenitor proliferation and V2a/V2b cell fate determination

The broad diversity of neurons is vital to neuronal functions. During vertebrate development, the spinal cord is a site of sensory and motor tasks coordinated by interneurons and the ongoing neurogenesis. In the spinal cord, V2-interneuron (V2-IN) progenitors (p2) develop into excitatory V2a-INs and inhibitory V2b-INs. The balance of these two types of interneurons requires precise control in the number and timing of their production. Here, using zebrafish embryos with altered Notch signaling, we show that different combinations of Notch ligands and receptors regulate two functions: the maintenance of p2 progenitor cells and the V2a/V2b cell fate decision in V2-IN development. Two ligands, DeltaA and DeltaD, and three receptors, Notch1a, Notch1b, and Notch3 redundantly contribute to p2 progenitor maintenance. On the other hand, DeltaA, DeltaC, and Notch1a mainly contribute to the V2a/V2b cell fate determination. A ubiquitin ligase Mib, which activates Notch ligands, acts in both functions through its activation of DeltaA, DeltaC, and DeltaD. Moreover, p2 progenitor maintenance and V2a/V2b fate determination are not distinct temporal processes, but occur within the same time frame during development. In conclusion, V2-IN cell progenitor proliferation and V2a/V2b cell fate determination involve signaling through different sets of Notch ligand-receptor combinations that occur concurrently during development in zebrafish.

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.

S/T phosphorylation of DLL1 is required for full ligand activity in vitro but dispensable for DLL1 function in vivo during embryonic patterning and marginal zone B cell development

Interaction of Notch receptors with Delta- and Serrate-type ligands is an evolutionarily conserved mechanism that mediates direct communication between adjacent cells and thereby regulates multiple developmental processes. Posttranslational modifications of both receptors and ligands are pivotal for normal Notch pathway function. We have identified by mass spectrometric analysis two serine and one threonine phosphorylation sites in the intracellular domain of the mouse Notch ligand DLL1. Phosphorylation requires cell membrane association of DLL1 and occurs sequentially at the two serine residues. Phosphorylation of one serine residue most likely by protein kinase B primes phosphorylation of the other serine. A DLL1 variant, in which all three identified phosphorylated serine/threonine residues are mutated to alanine and valine, was more stable than wild-type DLL1 but had reduced relative levels on the cell surface and was more effectively cleaved in the extracellular domain. In addition, the mutant variant activated Notch1 significantly less efficient than wild-type DLL1 in a coculture assay in vitro. Mice, however, whose endogenous DLL1 was replaced with the phosphorylation-deficient triple mutant developed normally, suggesting compensatory mechanisms under physiological conditions in vivo.

Normal development in mice over-expressing the intracellular domain of DLL1 argues against reverse signaling by DLL1 in vivo

The Notch signaling pathway mediates the direct communication between adjacent cells and regulates multiple developmental processes. Interaction of the Notch receptor with its ligands induces the liberation of the intracellular portion of Notch (NICD) referred to as regulated intramembraneous proteolysis (RIP). NICD translocates to the nucleus, and by complexing with the DNA binding protein RBPjκ and other cofactors activates transcription of bHLH genes. RIP-like processing of various mammalian Notch ligands (DLL1, JAG1 and JAG2) and the translocation of their intracellular domains (ICDs) to the nucleus has also been observed. These observations together with effects of over-expressed ligand ICDs in cultured cells on cell proliferation, differentiation, and Notch activity and target gene expression have led to the idea that the intracellular domains of Notch ligands have signaling functions. To test this hypothesis in vivo we have generated ES cells and transgenic mice that constitutively express various versions of the intracellular domain of mouse DLL1. In contrast to other cell lines, expression of DICDs in ES cells did not block proliferation or stimulate neuronal differentiation. Embryos with ubiquitous DICD expression developed to term without any apparent phenotype and grew up to viable and fertile adults. Early Notch-dependent processes or expression of selected Notch target genes were unaltered in transgenic embryos. In addition, we show that mouse DICD enters the nucleus inefficiently. Collectively, our results argue against a signaling activity of the intracellular domain of DLL1 in mouse embryos in vivo.

Foxn4: a multi-faceted transcriptional regulator of cell fates in vertebrate development

Vertebrate development culminates in the generation of proper proportions of a large variety of different cell types and subtypes essential for tissue, organ and system functions in the right place at the right time. Foxn4, a member of the forkhead box/winged-helix transcription factor superfamily, is expressed in mitotic progenitors and/or postmitotic precursors in both neural (e.g., retina and spinal cord) and non-neural tissues (e.g., atrioventricular canal and proximal airway). During development of the central nervous system, Foxn4 is required to specify the amacrine and horizontal cell fates from multipotent retinal progenitors while suppressing the alternative photoreceptor cell fates through activating Dll4-Notch signaling. Moreover, it activates Dll4-Notch signaling to drive commitment of p2 progenitors to the V2b and V2c interneuron fates during spinal cord neurogenesis. In development of non-neural tissues, Foxn4 plays an essential role in the specification of the atrioventricular canal and is indirectly required for patterning the distal airway during lung development. In this review, we highlight current understanding of the structure, expression and developmental functions of Foxn4 with an emphasis on its cell-autonomous and non-cell-autonomous roles in different tissues and animal model systems.

Heterochronic misexpression of Ascl1 in the Atoh7 retinal cell lineage blocks cell cycle exit

Retinal neurons and glia arise from a common progenitor pool in a temporal order, with retinal ganglion cells (RGCs) appearing first, and Müller glia last. The transcription factors Atoh7/Math5 and Ascl1/Mash1 represent divergent bHLH clades, and exhibit distinct spatial and temporal retinal expression patterns, with little overlap during early development. Here, we tested the ability of Ascl1 to change the fate of cells in the Atoh7 lineage when misexpressed from the Atoh7 locus, using an Ascl1-IRES-DsRed2 knock-in allele. In Atoh7(Ascl1KI/+) and Atoh7(Ascl1KI/Ascl1KI) embryos, ectopic Ascl1 delayed cell cycle exit and differentiation, even in cells coexpressing Atoh7. The heterozygous retinas recovered, and eventually produced a normal complement of RGCs, while homozygous substitution of Ascl1 for Atoh7 did not promote postnatal retinal fates precociously, nor rescue Atoh7 mutant phenotypes. However, our analyses revealed two unexpected findings. First, ectopic Ascl1 disrupted cell cycle progression within the marked Atoh7 lineage, but also nonautonomously in other retinal cells. Second, the size of the Atoh7 retinal lineage was unaffected, supporting the idea of a compensatory shift of the non-proliferative cohort to maintain lineage size. Overall, we conclude that Ascl1 acts dominantly to block cell cycle exit, but is incapable of redirecting the fates of early RPCs.

Gene networks: dissecting pathways in retinal development and disease

During retinal neurogenesis, diverse cellular subtypes originate from multipotent neural progenitors in a spatiotemporal order leading to a highly specialized laminar structure combined with a distinct mosaic architecture. This is driven by the combinatorial action of transcription factors and signaling molecules which specify cell fate and differentiation. The emerging approach of gene network analysis has allowed a better understanding of the functional relationships between genes expressed in the developing retina. For instance, these gene networks have identified transcriptional hubs that have revealed potential targets and pathways for the development of therapeutic options for retinal diseases. Much of the current knowledge has been informed by targeted gene deletion experiments and gain-of-functional analysis. In this review we will provide an update on retinal development gene networks and address the wider implications for future disease therapeutics.

Regulation of neuronal differentiation at the neurogenic wavefront

Signaling mediated by the Delta/Notch system controls the process of lateral inhibition, known to regulate neurogenesis in metazoans. Lateral inhibition takes place in equivalence groups formed by cells having equal capacity to differentiate, and it results in the singling out of precursors, which subsequently become neurons. During normal development, areas of active neurogenesis spread through non-neurogenic regions in response to specific morphogens, giving rise to neurogenic wavefronts. Close contact of these wavefronts with non-neurogenic cells is expected to affect lateral inhibition. Therefore, a mechanism should exist in these regions to prevent disturbances of the lateral inhibitory process. Focusing on the developing chick retina, we show that Dll1 is widely expressed by non-neurogenic precursors located at the periphery of this tissue, a region lacking Notch1, lFng, and differentiation-related gene expression. We investigated the role of this Dll1 expression through mathematical modeling. Our analysis predicts that the absence of Dll1 ahead of the neurogenic wavefront results in reduced robustness of the lateral inhibition process, often linked to enhanced neurogenesis and the presence of morphological alterations of the wavefront itself. These predictions are consistent with previous observations in the retina of mice in which Dll1 is conditionally mutated. The predictive capacity of our mathematical model was confirmed further by mimicking published results on the perturbation of morphogenetic furrow progression in the eye imaginal disc of Drosophila. Altogether, we propose that Notch-independent Delta expression ahead of the neurogenic wavefront is required to avoid perturbations in lateral inhibition and wavefront progression, thus optimizing the neurogenic process.

Attenuation of Notch and Hedgehog signaling is required for fate specification in the spinal cord

During the development of the spinal cord, proliferative neural progenitors differentiate into postmitotic neurons with distinct fates. How cells switch from progenitor states to differentiated fates is poorly understood. To address this question, we studied the differentiation of progenitors in the zebrafish spinal cord, focusing on the differentiation of Kolmer-Agduhr″ (KA″) interneurons from lateral floor plate (LFP) progenitors. In vivo cell tracking demonstrates that KA″ cells are generated from LFP progenitors by both symmetric and asymmetric cell divisions. A photoconvertible reporter of signaling history (PHRESH) reveals distinct temporal profiles of Hh response: LFP progenitors continuously respond to Hh, while KA″ cells lose Hh response upon differentiation. Hh signaling is required in LFP progenitors for KA″ fate specification, but prolonged Hh signaling interferes with KA″ differentiation. Notch signaling acts permissively to maintain LFP progenitor cells: activation of Notch signaling prevents differentiation, whereas inhibition of Notch signaling results in differentiation of ectopic KA″ cells. These results indicate that neural progenitors depend on Notch signaling to maintain Hh responsiveness and rely on Hh signaling to induce fate identity, whereas proper differentiation depends on the attenuation of both Notch and Hh signaling.

During tissue formation, progenitor cells generate both differentiated cells and progenitor cells. It is poorly understood how this balance between self-renewal and differentiation generates the correct number of different cell types. Here, we use zebrafish spinal cord development as a model system to investigate how neural progenitor cells switch from progenitor states to differentiated fates. Combining genetic manipulation and a novel method to study cell signaling in live embryos, our data show that this process requires the dynamic regulation of two signaling pathways: the Notch signaling pathway and the Hedgehog (Hh) signaling pathway. In neural progenitors, Notch signaling maintains the competence of neural progenitors to respond to Hh signaling. In parallel, Hedgehog signaling functions to induce cell fate identity. As cells switch from progenitor states to differentiated states, both Notch and Hh signaling become attenuated. Thus, the dynamic deployment of Notch and Hh signaling controls the renewal and differentiation of progenitor cells.

Forkhead box N4 (Foxn4) activates Dll4-Notch signaling to suppress photoreceptor cell fates of early retinal progenitors

The generation of diverse neuronal types and subtypes from multipotent progenitors during development is crucial for assembling functional neural circuits in the adult central nervous system. During mouse retinogenesis, early retinal progenitors give rise to several cell types, including ganglion, amacrine, horizontal, cone, and rod cells. It is unknown at present how each of these fates is selected from the multiple neuronal fates available to the early progenitor. By using a combination of bioinformatic, genetic, and biochemical approaches, we investigated the mechanism by which Foxn4 selects the amacrine and horizontal cell fates from multipotential retinal progenitors. These studies indicate that Foxn4 has an intrinsic activity to suppress the alternative photoreceptor cell fates of early retinal progenitors by selectively activating Dll4-Notch signaling. Gene expression and conditional ablation analyses reveal that Dll4 is directly activated by Foxn4 via phylogenetically conserved enhancers and that Dll4 can partly mediate the Foxn4 function by serving as a major Notch ligand to expand the progenitor pool and limit photoreceptor production. Our data together define a Foxn4-mediated molecular and signaling pathway that underlies the suppression of alternative cell fates of early retinal progenitors.

Jagged2 controls the generation of motor neuron and oligodendrocyte progenitors in the ventral spinal cord

In the developing spinal cord, motor neurons (MNs) and oligodendrocytes arise sequentially from a common pool of progenitors. However, the genetic network responsible for this neurogenesis to gliogenesis switch is largely unknown. A transcriptome analysis identified the Notch ligand Jagged2 (JAG2) as a Sonic hedgehog-regulated factor transiently expressed in MN progenitors (pMNs). In vivo loss- and gain-of-function experiments show that JAG2 schedules the differentiation of the pMN progenitors. At early developmental stages, Olig2 expressing pMN progenitors that enter the differentiation pathway exclusively generate MNs. At these times, the activation of the Notch pathway by JAG2 maintains selected pMN progenitors in an undifferentiated state by two mechanisms; first it inhibits MN generation by reducing Olig2 proteins levels, and second it directly inhibits the premature generation of oligodendrocyte progenitors (OLPs) by maintaining high levels of Hes5. Later, extinction of JAG2 from the pMN results in the loss of Hes5 expression, heralding the gliogenic phase of pMN progenitors. Strikingly, downregulation of JAG2 in pMN progenitors is sufficient to promote the precocious generation of OLPs. Together these data provide evidence that JAG2 is a key regulator of the timely and ordered generation of two of the defining cell types in the spinal cord, MNs and OLPs.

Regulation of spinal interneuron development by the Olig-related protein Bhlhb5 and Notch signaling

The neural circuits that control motor activities depend on the spatially and temporally ordered generation of distinct classes of spinal interneurons. Despite the importance of these interneurons, the mechanisms underlying their genesis are poorly understood. Here, we demonstrate that the Olig-related transcription factor Bhlhb5 (recently renamed Bhlhe22) plays two central roles in this process. Our findings suggest that Bhlhb5 repressor activity acts downstream of retinoid signaling and homeodomain proteins to promote the formation of dI6, V1 and V2 interneuron progenitors and their differentiated progeny. In addition, Bhlhb5 is required to organize the spatially restricted expression of the Notch ligands and Fringe proteins that both elicit the formation of the interneuron populations that arise adjacent to Bhlhb5(+) cells and influence the global pattern of neuronal differentiation. Through these actions, Bhlhb5 helps transform the spatial information established by morphogen signaling into local cell-cell interactions associated with Notch signaling that control the progression of neurogenesis and extend neuronal diversity within the developing spinal cord.

Trisomy 1q42-qter associated with monosomy 6q27-qter: a case report

PURPOSE

Partial trisomy 1q42-qter is a rare chromosomal aberration. Most cases arise from de novo unbalanced translocations or from unbalanced inheritance of parental balanced rearrangements.

METHODS

Descriptive case report.

RESULTS

A 4-year-old boy had shown an increased neck translucency at the fetal ultrasound examination performed at the 11th week of gestation. Amniocentesis, performed at the 18th week of gestation, did not demonstrate any genetic abnormality. A second fetal ultrasound examination, carried out at the 35th week of gestation, showed congenital clubfeet and hydrocephalus. At birth, clinical examination revealed congenital bilateral ventriculomegaly, bilateral congenital equinovarus clubfeet, low-set ears, plagiocephaly, micrognathia, hypertelorism, prominent forehead, broad nasal bridge, hypertonic syndrome, and inguinal hernia. Ophthalmologic consultation showed the presence of optic pit in his left eye. Genetic counseling was performed. Chromosome analysis demonstrated a partial trisomy 1q42.2-qter associated with a partial monosomy 6q27-qter. Moreover, deletions of the distal region on the long arm of chromosome 6 are frequently associated with both ocular abnormalities and several solid tumor types. Moderate mental and psychomotor retardation has occurred.

CONCLUSIONS

This case emphasizes the importance of scheduling a screening test for eye diseases and tumor in these patients.

The Notch signaling pathway in retinal dysplasia and retina vascular homeostasis

The retina is one of the most essential elements of vision pathway in vertebrate. The dysplasia of retina cause congenital blindness or vision disability in individuals, and the misbalance in adult retinal vascular homeostasis leads to neovascularization-associated diseases in adults, such as diabetic retinopathy or age-related macular degeneration. Many developmental signaling pathways are involved in the process of retinal development and vascular homeostasis. Among them, Notch signaling pathway has long been studied, and Notch signaling-interfered mouse models show both neural retina dysplasia and vascular abnormality. In this review, we discuss the roles of Notch signaling in the maintenance of retinal progenitor cells, specification of retinal neurons and glial cells, and the sustaining of retina vascular homeostasis, especially from the aspects of conditional knockout mouse models. The potential of Notch signal manipulation may provide a powerful cell fate- and neovascularization-controlling tool that could have important applications in treatment of retinal diseases.

Two Notch ligands, Dll1 and Jag1, are differently restricted in their range of action to control neurogenesis in the mammalian spinal cord

Background

Notch signalling regulates neuronal differentiation in the vertebrate nervous system. In addition to a widespread function in maintaining neural progenitors, Notch signalling has also been involved in specific neuronal fate decisions. These functions are likely mediated by distinct Notch ligands, which show restricted expression patterns in the developing nervous system. Two ligands, in particular, are expressed in non-overlapping complementary domains of the embryonic spinal cord, with Jag1 being restricted to the V1 and dI6 progenitor domains, while Dll1 is expressed in the remaining domains. However, the specific contribution of different ligands to regulate neurogenesis in vertebrate embryos is still poorly understood.

Methodology/Principal Findings

In this work, we investigated the role of Jag1 and Dll1 during spinal cord neurogenesis, using conditional knockout mice where the two genes are deleted in the neuroepithelium, singly or in combination. Our analysis showed that Jag1 deletion leads to a modest increase in V1 interneurons, while dI6 neurogenesis was unaltered. This mild Jag1 phenotype contrasts with the strong neurogenic phenotype detected in Dll1 mutants and led us to hypothesize that neighbouring Dll1-expressing cells signal to V1 and dI6 progenitors and restore neurogenesis in the absence of Jag1. Analysis of double Dll1;Jag1 mutant embryos revealed a stronger increase in V1-derived interneurons and overproduction of dI6 interneurons. In the presence of a functional Dll1 allele, V1 neurogenesis is restored to the levels detected in single Jag1 mutants, while dI6 neurogenesis returns to normal, thereby confirming that Dll1-mediated signalling compensates for Jag1 deletion in V1 and dI6 domains.

Conclusions/Significance

Our results reveal that Dll1 and Jag1 are functionally equivalent in controlling the rate of neurogenesis within their expression domains. However, Jag1 can only activate Notch signalling within the V1 and dI6 domains, whereas Dll1 can signal to neural progenitors both inside and outside its domains of expression.

Domain-specific control of neurogenesis achieved through patterned regulation of Notch ligand expression

Homeodomain (HD) transcription factors and components of the Notch pathway [Delta1 (Dll1), Jagged1 (Jag1) and the Fringe (Fng) proteins] are expressed in distinct progenitor domains along the dorsoventral (DV) axis of the developing spinal cord. However, the internal relationship between these two regulatory pathways has not been established. In this report we show that HD proteins act upstream of Notch signalling. Thus, HD proteins control the spatial distribution of Notch ligands and Fng proteins, whereas perturbation of the Notch pathway does not affect the regional expression of HD proteins. Loss of Dll1 or Jag1 leads to a domain-specific increase of neuronal differentiation but does not affect the establishment of progenitor domain boundaries. Moreover, gain-of-function experiments indicate that the ability of Dll1 and Jag1 to activate Notch is limited to progenitors endogenously expressing the respective ligand. Fng proteins enhance Dll1-activated Notch signalling and block Notch activation mediated by Jag1. This finding, combined with the overlapping expression of Fng with Dll1 but not with Jag1, is likely to explain the domain-specific activity of the Notch ligands. This outcome is opposite to the local regulation of Notch activity in most other systems, including the Drosophila wing, where Fng co-localizes with Jagged/Serrate rather than Dll/Delta, which facilitates Notch signalling at regional boundaries instead of within domains. The regulation of Notch activation in the spinal cord therefore appears to endow specific progenitor populations with a domain-wide autonomy in the control of neurogenesis and prevents any inadequate activation of Notch across progenitor domain boundaries.