Maintenance of neuroepithelial progenitor cells by Delta-Notch signalling in the embryonic chick retina

Müller cells trophism and pathology as the next therapeutic targets for retinal diseases

Müller cells are a crucial retinal cell type involved in multiple regulatory processes and functions that are essential for retinal health and functionality. Acting as structural and functional support for retinal neurons and photoreceptors, Müller cells produce growth factors, regulate ion and fluid homeostasis, and facilitate neuronal signaling. They play a pivotal role in retinal morphogenesis and cell differentiation, significantly contributing to macular development. Due to their radial morphology and unique cytoskeletal organization, Müller cells act as optical fibers, efficiently channeling photons directly to the photoreceptors. In response to retinal damage, Müller cells undergo specific gene expression and functional changes that serve as a first line of defense for neurons, but can also lead to unwarranted cell dysfunction, contributing to cell death and neurodegeneration. In some species, Müller cells can reactivate their developmental program, promoting retinal regeneration and plasticity-a remarkable ability that holds promising therapeutic potential if harnessed in mammals. The crucial and multifaceted roles of Müller cells-that we propose to collectively call "Müller cells trophism"-highlight the necessity of maintaining their functionality. Dysfunction of Müller cells, termed "Müller cells pathology," has been associated with a plethora of retinal diseases, including age-related macular degeneration, diabetic retinopathy, vitreomacular disorders, macular telangiectasia, and inherited retinal dystrophies. In this review, we outline how even subtle disruptions in Müller cells trophism can drive the pathological cascade of Müller cells pathology, emphasizing the need for targeted therapies to preserve retinal health and prevent disease progression.

Notch Is Required for Neural Progenitor Proliferation During Embryonic Eye Regrowth

The ability of an organism to regrow tissues is regulated by various signaling pathways. One such pathway that has been studied widely both in the context of regeneration and development is the Notch signaling pathway. Notch is required for the development of the eye and regeneration of tissues in multiple organisms, but it is unknown if Notch plays a role in the regulation of Xenopus laevis embryonic eye regrowth. We found that Notch1 is required for eye regrowth and regulates retinal progenitor cell proliferation. Chemical and molecular inhibition of Notch1 significantly decreased eye regrowth by reducing retinal progenitor cell proliferation without affecting retinal differentiation. Temporal inhibition studies showed that Notch function is required during the first day of regrowth. Interestingly, Notch1 loss-of-function phenocopied the effects of the inhibition of the proton pump, vacuolar-type ATPase (V-ATPase), where retinal proliferation but not differentiation was blocked during eye regrowth. Overexpression of a form of activated Notch1, the Notch intracellular domain (NICD) rescued the loss of eye regrowth due to V-ATPase inhibition. These findings highlight the importance of the Notch signaling pathway in eye regeneration and its role in inducing retinal progenitor cell proliferation in response to injury.

Identification and Characterization of ATOH7-Regulated Target Genes and Pathways in Human Neuroretinal Development

The proneural transcription factor atonal basic helix-loop-helix transcription factor 7 (ATOH7) is expressed in early progenitors in the developing neuroretina. In vertebrates, this is crucial for the development of retinal ganglion cells (RGCs), as mutant animals show an almost complete absence of RGCs, underdeveloped optic nerves, and aberrations in retinal vessel development. Human mutations are rare and result in autosomal recessive optic nerve hypoplasia (ONH) or severe vascular changes, diagnosed as autosomal recessive persistent hyperplasia of the primary vitreous (PHPVAR). To better understand the role of ATOH7 in neuroretinal development, we created ATOH7 knockout and eGFP-expressing ATOH7 reporter human induced pluripotent stem cells (hiPSCs), which were differentiated into early-stage retinal organoids. Target loci regulated by ATOH7 were identified by Cleavage Under Targets and Release Using Nuclease with sequencing (CUT&RUN-seq) and differential expression by RNA sequencing (RNA-seq) of wildtype and mutant organoid-derived reporter cells. Additionally, single-cell RNA sequencing (scRNA-seq) was performed on whole organoids to identify cell type-specific genes. Mutant organoids displayed substantial deficiency in axon sprouting, reduction in RGCs, and an increase in other cell types. We identified 469 differentially expressed target genes, with an overrepresentation of genes belonging to axon development/guidance and Notch signaling. Taken together, we consolidate the function of human ATOH7 in guiding progenitor competence by inducing RGC-specific genes while inhibiting other cell fates. Furthermore, we highlight candidate genes responsible for ATOH7-associated optic nerve and retinovascular anomalies, which sheds light to potential future therapy targets for related disorders.

Phototactic preference and its genetic basis in the planulae of the colonial Hydrozoan Hydractinia symbiolongicarpus

Background

Marine organisms with sessile adults commonly possess motile larval stages that make settlement decisions based on integrating environmental sensory cues. Phototaxis, the movement toward or away from light, is a common behavioral characteristic of aquatic and marine metazoan larvae, and of algae, protists, and fungi. In cnidarians, behavioral genomic investigations of motile planulae larvae have been conducted in anthozoans (corals and sea anemones) and scyphozoans (true jellyfish), but such studies are presently lacking in hydrozoans. Here, we examined the behavioral genomics of phototaxis in planulae of the hydrozoan Hydractinia symbiolongicarpus.

Results

A behavioral phototaxis study of day 3 planulae indicated preferential phototaxis to green (523 nm) and blue (470 nm) wavelengths of light, but not red (625 nm) wavelengths. A developmental transcriptome study where planula larvae were collected from four developmental time points for RNA-seq revealed that many genes critical to the physiology and development of ciliary photosensory systems are dynamically expressed in planula development and correspond to the expression of phototactic behavior. Microscopical investigations using immunohistochemistry and in situ hybridization demonstrated that several transcripts with predicted function in photoreceptors, including cnidops class opsin, CNG ion channel, and CRX-like transcription factor, localize to ciliated bipolar sensory neurons of the aboral sensory neural plexus, which is associated with the direction of phototaxis and the site of settlement.

Conclusions

The phototactic preference displayed by planulae is consistent with the shallow sandy marine habitats they experience in nature. Our genomic investigations add further evidence of similarities between cnidops-mediated photoreceptors of hydrozoans and other cnidarians and ciliary photoreceptors as found in the eyes of humans and other bilaterians, suggesting aspects of their shared evolutionary history.

The Rax homeoprotein in Müller glial cells is required for homeostasis maintenance of the postnatal mouse retina

Müller glial cells, which are the most predominant glial subtype in the retina, play multiple important roles, including the maintenance of structural integrity, homeostasis, and physiological functions of the retina. We have previously found that the Rax homeoprotein is expressed in postnatal and mature Müller glial cells in the mouse retina. However, the function of Rax in postnatal and mature Müller glial cells remains to be elucidated. In the current study, we first investigated Rax function in retinal development using retroviral lineage analysis and found that Rax controls the specification of late-born retinal cell types, including Müller glial cells in the postnatal retina. We next generated Rax tamoxifen-induced conditional KO (Rax iCKO) mice, where Rax can be depleted in mTFP-labeled Müller glial cells upon tamoxifen treatment, by crossing Raxflox/flox mice with Rlbp1-CreERT2 mice, which we have produced. Immunohistochemical analysis showed a characteristic of reactive gliosis and enhanced gliosis of Müller glial cells in Rax iCKO retinas under normal and stress conditions, respectively. We performed RNA-seq analysis on mTFP-positive cells purified from the Rax iCKO retina and found significantly reduced expression of suppressor of cytokinesignaling-3 (Socs3). Reporter gene assays showed that Rax directly transactivates the Socs3 promoter. We observed decreased expression of Socs3 in Müller glial cells of Rax iCKO retinas by immunostaining. Taken together, the present results suggest that Rax suppresses inflammation in Müller glial cells by transactivating Socs3. This study sheds light on the transcriptional regulatory mechanisms underlying retinal Müller glial cell homeostasis.

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.

Key transcription factors influence the epigenetic landscape to regulate retinal cell differentiation

How the diverse neural cell types emerge from multipotent neural progenitor cells during central nervous system development remains poorly understood. Recent scRNA-seq studies have delineated the developmental trajectories of individual neural cell types in many neural systems including the neural retina. Further understanding of the formation of neural cell diversity requires knowledge about how the epigenetic landscape shifts along individual cell lineages and how key transcription factors regulate these changes. In this study, we dissect the changes in the epigenetic landscape during early retinal cell differentiation by scATAC-seq and identify globally the enhancers, enriched motifs, and potential interacting transcription factors underlying the cell state/type specific gene expression in individual lineages. Using CUT&Tag, we further identify the enhancers bound directly by four key transcription factors, Otx2, Atoh7, Pou4f2 and Isl1, including those dependent on Atoh7, and uncover the sequential and combinatorial interactions of these factors with the epigenetic landscape to control gene expression along individual retinal cell lineages such as retinal ganglion cells (RGCs). Our results reveal a general paradigm in which transcription factors collaborate and compete to regulate the emergence of distinct retinal cell types such as RGCs from multipotent retinal progenitor cells (RPCs).

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.

The neurogenic fate of the hindbrain boundaries relies on Notch3-dependent asymmetric cell divisions

Elucidating the cellular and molecular mechanisms that regulate the balance between progenitor cell proliferation and neuronal differentiation in the construction of the embryonic brain demands the combination of cell lineage and functional approaches. Here, we generate the comprehensive lineage of hindbrain boundary cells by using a CRISPR-based knockin zebrafish transgenic line that specifically labels the boundaries. We unveil that boundary cells asynchronously engage in neurogenesis undergoing a functional transition from neuroepithelial progenitors to radial glia cells, coinciding with the onset of Notch3 signaling that triggers their asymmetrical cell division. Upon notch3 loss of function, boundary cells lose radial glia properties and symmetrically divide undergoing neuronal differentiation. Finally, we show that the fate of boundary cells is to become neurons, the subtype of which relies on their axial position, suggesting that boundary cells contribute to refine the number and proportion of the distinct neuronal populations.

Müller Glia in Retinal Development: From Specification to Circuit Integration

Müller glia of the retina share many features with astroglia located throughout the brain including maintenance of homeostasis, modulation of neurotransmitter spillover, and robust response to injury. Here we present the molecular factors and signaling events that govern Müller glial specification, patterning, and differentiation. Next, we discuss the various roles of Müller glia in retinal development, which include maintaining retinal organization and integrity as well as promoting neuronal survival, synaptogenesis, and phagocytosis of debris. Finally, we review the mechanisms by which Müller glia integrate into retinal circuits and actively participate in neuronal signaling during development.

TMEM8C-mediated fusion is regionalized and regulated by NOTCH signalling during foetal myogenesis

The location and regulation of fusion events within skeletal muscles during development remain unknown. Using the fusion marker myomaker (Mymk), named TMEM8C in chicken, as a readout of fusion, we identified a co-segregation of TMEM8C-positive cells and MYOG-positive cells in single-cell RNA-sequencing datasets of limbs from chicken embryos. We found that TMEM8C transcripts, MYOG transcripts and the fusion-competent MYOG-positive cells were preferentially regionalized in central regions of foetal muscles. We also identified a similar regionalization for the gene encoding the NOTCH ligand JAG2 along with an absence of NOTCH activity in TMEM8C+ fusion-competent myocytes. NOTCH function in myoblast fusion had not been addressed so far. We analysed the consequences of NOTCH inhibition for TMEM8C expression and myoblast fusion during foetal myogenesis in chicken embryos. NOTCH inhibition increased myoblast fusion and TMEM8C expression and released the transcriptional repressor HEYL from the TMEM8C regulatory regions. These results identify a regionalization of TMEM8C-dependent fusion and a molecular mechanism underlying the fusion-inhibiting effect of NOTCH in foetal myogenesis. The modulation of NOTCH activity in the fusion zone could regulate the flux of fusion events.

Loss of Elp1 perturbs histone H2A.Z and the Notch signaling pathway

Elongator dysfunction is increasingly recognized as a contributor to multiple neurodevelopmental and neurodegenerative disorders including familial dysautonomia, intellectual disability, amyotrophic lateral sclerosis, and autism spectrum disorder. Although numerous cellular processes are perturbed in the context of Elongator loss, converging evidence from multiple studies has resolved Elongator's primary function in the cell to the modification of tRNA wobble uridines and the translational regulation of codon-biased genes. Here we characterize H2a.z, encoding the variant H2a histone H2A.Z, as an indirect Elongator target. We further show that canonical Notch signaling, a pathway directed by H2A.Z, is perturbed as a consequence of Elp1 loss. Finally, we demonstrate that hyperacetylation of H2A.Z and other histones via exposure to the histone deacetylase inhibitor Trichostatin A during neurogenesis corrects the expression of Notch3 and rescues the development of sensory neurons in embryos lacking the Elp1 Elongator subunit.

Summary: The maldevelopment of sensory neurons in Elongator knockout embryos is associated with elevated H2A.Z and perturbed Notch signaling that can be rescued by Trichostatin A.

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.

Potential role of NGF, BDNF, and their receptors in oligodendrocytes differentiation from neural stem cell: An in vitro study

Neural stem cells (NSCs) or neuronal progenitor cells are cells capable of differentiating into oligodendrocytes, myelin-forming cells that have the potential of remyelination. Brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are two neurotrophic factors that have been studied to stimulate NSC differentiation thus playing a role in multiple sclerosis pathogenesis and several other demyelinating disorders. While several studies have demonstrated the proliferative and protective capabilities of these neurotrophic factors, their cellular and molecular functions are still not well understood. Thus, in the present study, we focus on understanding the role of these neurotrophins (BDNF and NGF) in oligodendrogenesis from NSCs. Both neurotrophic factors have been shown to promote NSC proliferation and NSC differentiation particularly into oligodendroglial lineage in a dose-dependent fashion. Further, to establish the role of these neurotrophins in NSC differentiation, we have employed pharmacological inhibitors for TrkA and TrkB receptors in NSCs. The use of these inhibitors suppressed NSC differentiation into oligodendrocytes along with the downregulation of phosphorylated ERK suggesting active involvement of ERK in the functioning of these neurotrophins. The morphometric analysis also revealed the important role of both neurotrophins in oligodendrocytes development. These findings highlight the importance of neurotrophic factors in stimulating NSC differentiation and may pave a role for future studies to develop neurotrophic factor replacement therapies to achieve remyelination.

Asymmetric neurogenic commitment of retinal progenitors involves Notch through the endocytic pathway

During brain development, progenitor cells need to balanceproliferation and differentiation in order to generate different neurons in the correct numbers and proportions. Currently, the patterns of multipotent progenitor divisions that lead to neurogenic entry and the factors that regulate them are not fully understood. We here use the zebrafish retina to address this gap, exploiting its suitability for quantitative live-imaging. We show that early neurogenic progenitors arise from asymmetric divisions. Notch regulates this asymmetry, as when inhibited, symmetric divisions producing two neurogenic progenitors occur. Surprisingly however, Notch does not act through an apicobasal activity gradient as previously suggested, but through asymmetric inheritance of Sara-positive endosomes. Further, the resulting neurogenic progenitors show cell biological features different from multipotent progenitors, raising the possibility that an intermediate progenitor state exists in the retina. Our study thus reveals new insights into the regulation of proliferative and differentiative events during central nervous system development.

Notch3 and DeltaB maintain Müller glia quiescence and act as negative regulators of regeneration in the light-damaged zebrafish retina

Damage to the zebrafish retina stimulates resident Müller glia to reprogram, reenter the cell cycle, divide asymmetrically, and produce neuronal progenitor cells that amplify and differentiate into the lost neurons. The transition from quiescent to proliferative Müller glia involves both positive and negative regulators. We previously demonstrated that the Notch signaling pathway represses retinal regeneration by maintaining Müller glia quiescence in zebrafish. Here we examine which Notch receptor is necessary to maintain quiescence. Quantitative RT-PCR and RNA-Seq analyses reveal that notch3 is expressed in the undamaged retina and is downregulated in response to light damage. Additionally, Notch3 protein is expressed in quiescent Müller glia of the undamaged retina, is downregulated as Müller glia proliferate, and is reestablished in the Müller glia. Knockdown of Notch3 is sufficient to induce Müller glia proliferation in undamaged retinas and enhances proliferation during light damage. Alternatively, knockdown of Notch1a, Notch1b, or Notch2 decreases the number of proliferating cells during light damage, suggesting that Notch signaling is also required for proliferation during retinal regeneration. We also knockdown the zebrafish Delta and Delta-like proteins, ligands for the Notch receptors, and find that the deltaB morphant possesses an increased number of proliferating cells in the light-damaged retina. As with Notch3, knockdown of DeltaB is sufficient to induce Müller glia proliferation in the absence of light damage. Taken together, the negative regulation of Müller glia proliferation in zebrafish retinal regeneration is mediated by Notch3 and DeltaB.

Glycosylation of Specific Notch EGF Repeats by O-Fut1 and Fringe Regulates Notch Signaling in Drosophila

Fringe glycosyltransferases differentially modulate the binding of Notch receptors to Delta/DLL versus Serrate/Jagged ligands by adding GlcNAc to O-linked fucose on Notch epidermal growth factor-like (EGF) repeats. Although Notch has 22 O-fucosylation sites, the biologically relevant sites affecting Notch activity during animal development in vivo in the presence or absence of Fringe are not known. Using a variety of assays, we find important roles in Drosophila Notch signaling for GlcNAc-fucose-O glycans on three sites: EGF8, EGF9, and EGF12. O-Fucose monosaccharide on EGF12 (in the absence of Fringe) is essential for Delta-mediated lateral inhibition in embryos. However, wing vein development depends on the addition of GlcNAc to EGF8 and EGF12 by Fringe, with a minor contribution from EGF9. Fringe modifications of EGF8 and EGF12 together prevent Notch from cis-inhibiting Serrate, thereby promoting normal wing margin formation. Our work shows the combinatorial and context-dependent roles of GlcNAc-fucose-O glycans on these sites in Drosophila Notch-ligand interactions.

POFUT1/O-Fut1 and Fringe glycosyltransferases regulate Notch signaling by adding fucose and GlcNAc, respectively, to Notch EGF repeats. Using in vitro and in vivo experiments, Pandey et al. define the critical target sites of these enzymes on Drosophila Notch and determine the distinct roles of each sugar in Notch-dependent processes.

Delta-Notch Signaling: The Long and The Short of a Neuron's Influence on Progenitor Fates

Maintenance of the neural progenitor pool during embryonic development is essential to promote growth of the central nervous system (CNS). The CNS is initially formed by tightly compacted proliferative neuroepithelial cells that later acquire radial glial characteristics and continue to divide at the ventricular (apical) and pial (basal) surface of the neuroepithelium to generate neurons. While neural progenitors such as neuroepithelial cells and apical radial glia form strong connections with their neighbours at the apical and basal surfaces of the neuroepithelium, neurons usually form the mantle layer at the basal surface. This review will discuss the existing evidence that supports a role for neurons, from early stages of differentiation, in promoting progenitor cell fates in the vertebrates CNS, maintaining tissue homeostasis and regulating spatiotemporal patterning of neuronal differentiation through Delta-Notch signalling.

Basal Protrusions Mediate Spatiotemporal Patterns of Spinal Neuron Differentiation

During early spinal cord development, neurons of particular subtypes differentiate with a sparse periodic pattern while later neurons differentiate in the intervening space to eventually produce continuous columns of similar neurons. The mechanisms that regulate this spatiotemporal pattern are unknown. In vivo imaging in zebrafish reveals that differentiating spinal neurons transiently extend two long protrusions along the basal surface of the spinal cord before axon initiation. These protrusions express Delta protein, consistent with the hypothesis they influence Notch signaling at a distance of several cell diameters. Experimental reduction of Laminin expression leads to smaller protrusions and shorter distances between differentiating neurons. The experimental data and a theoretical model support the proposal that neuronal differentiation pattern is regulated by transient basal protrusions that deliver temporally controlled lateral inhibition mediated at a distance. This work uncovers a stereotyped protrusive activity of newborn neurons that organize long-distance spatiotemporal patterning of differentiation.

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.

Ligand-Induced Cis-Inhibition of Notch Signaling: The Role of an Extracellular Region of Serrate

Cellular development can be controlled by communication between adjacent cells mediated by the highly conserved Notch signaling system. A cell expressing the Notch receptor on one cell can be activated in trans by ligands on an adjacent cell leading to alteration of transcription and cellular fate. Ligands also have the ability to inhibit Notch signaling, and this can be accomplished when both receptor and ligands are coexpressed in cis on the same cell. The manner in which cis-inhibition is accomplished is not entirely clear but it is known to involve several different protein domains of the ligands and the receptor. Some of the protein domains involved in trans-activation are also used for cis-inhibition, but some are used uniquely for each process. In this work, the involvement of various ligand regions and the receptor are discussed in relation to their contributions to Notch signaling.

Wiring the Binocular Visual Pathways

Retinal ganglion cells (RGCs) extend axons out of the retina to transmit visual information to the brain. These connections are established during development through the navigation of RGC axons along a relatively long, stereotypical pathway. RGC axons exit the eye at the optic disc and extend along the optic nerves to the ventral midline of the brain, where the two nerves meet to form the optic chiasm. In animals with binocular vision, the axons face a choice at the optic chiasm—to cross the midline and project to targets on the contralateral side of the brain, or avoid crossing the midline and project to ipsilateral brain targets. Ipsilaterally and contralaterally projecting RGCs originate in disparate regions of the retina that relate to the extent of binocular overlap in the visual field. In humans virtually all RGC axons originating in temporal retina project ipsilaterally, whereas in mice, ipsilaterally projecting RGCs are confined to the peripheral ventrotemporal retina. This review will discuss recent advances in our understanding of the mechanisms regulating specification of ipsilateral versus contralateral RGCs, and the differential guidance of their axons at the optic chiasm. Recent insights into the establishment of congruent topographic maps in both brain hemispheres also will be discussed.

Role of Notch Signaling Pathway in Glioblastoma Pathogenesis

Notch signaling is an evolutionarily conserved pathway that regulates important biological processes, such as cell proliferation, apoptosis, migration, self-renewal, and differentiation. In mammals, Notch signaling is composed of four receptors (Notch1–4) and five ligands (Dll1-3–4, Jagged1–2) that mainly contribute to the development and maintenance of the central nervous system (CNS). Neural stem cells (NSCs) are the starting point for neurogenesis and other neurological functions, representing an essential aspect for the homeostasis of the CNS. Therefore, genetic and functional alterations to NSCs can lead to the development of brain tumors, including glioblastoma. Glioblastoma remains an incurable disease, and the reason for the failure of current therapies and tumor relapse is the presence of a small subpopulation of tumor cells known as glioma stem cells (GSCs), characterized by their stem cell-like properties and aggressive phenotype. Growing evidence reveals that Notch signaling is highly active in GSCs, where it suppresses differentiation and maintains stem-like properties, contributing to Glioblastoma tumorigenesis and conventional-treatment resistance. In this review, we try to give a comprehensive view of the contribution of Notch signaling to Glioblastoma and its possible implication as a target for new therapeutic approaches.

Cancer Stem Cells: Emergent Nature of Tumor Emergency

A functional analysis of 167 genes overexpressed in Krebs-2 tumor initiating cells was performed. In the first part of the study, the genes were analyzed for their belonging to one or more of the three groups, which represent the three major phenotypic manifestation of malignancy of cancer cells, namely (1) proliferative self-sufficiency, (2) invasive growth and metastasis, and (3) multiple drug resistance. 96 genes out of 167 were identified as possible contributors to at least one of these fundamental properties. It was also found that substantial part of these genes are also known as genes responsible for formation and/or maintenance of the stemness of normal pluri-/multipotent stem cells. These results suggest that the malignancy is simply the ability to maintain the stem cell specific genes expression profile, and, as a consequence, the stemness itself regardless of the controlling effect of stem niches. In the second part of the study, three stress factors combined into the single concept of "generalized cellular stress," which are assumed to activate the expression of these genes, were defined. In addition, possible mechanisms for such activation were identified. The data obtained suggest the existence of a mechanism for the de novo formation of a pluripotent/stem phenotype in the subpopulation of "committed" tumor cells.

The multisystemic functions of FOXD1 in development and disease

Transcription factors (TFs) participate in a wide range of cellular processes due to their inherent function as essential regulatory proteins. Their dysfunction has been linked to numerous human diseases. The forkhead box (FOX) family of TFs belongs to the "winged helix" superfamily, consisting of proteins sharing a related winged helix-turn-helix DNA-binding motif. FOX genes have been extensively present during vertebrates and invertebrates' evolution, participating in numerous molecular cascades and biological functions, such as embryonic development and organogenesis, cell cycle regulation, metabolism control, stem cell niche maintenance, signal transduction, and many others. FOXD1, a forkhead TF, has been related to different key biological processes such as kidney and retina development and embryo implantation. FOXD1 dysfunction has been linked to different pathologies, thereby constituting a diagnostic biomarker and a promising target for future therapies. This paper aims to present, for the first time, a comprehensive review of FOXD1's role in mouse development and human disease. Molecular, structural, and functional aspects of FOXD1 are presented in light of physiological and pathogenic conditions, including its role in human disease aetiology, such as cancer and recurrent pregnancy loss. Taken together, the information given here should enable a better understanding of FOXD1 function for basic science researchers and clinicians.

Signal Propagation in Sensing and Reciprocating Cellular Systems with Spatial and Structural Heterogeneity

Sensing and reciprocating cellular systems (SARs) are important for the operation of many biological systems. Production in interferon (IFN) SARs is achieved through activation of the Jak-Stat pathway, and downstream upregulation of IFN regulatory factor (IRF)-7 and IFN transcription, but the role that high- and low-affinity IFNs play in this process remains unclear. We present a comparative between a minimal spatio-temporal partial differential equation model and a novel spatio-structural-temporal (SST) model for the consideration of receptor, binding, and metabolic aspects of SAR behaviour. Using the SST framework, we simulate single- and multi-cluster paradigms of IFN communication. Simulations reveal a cyclic process between the binding of IFN to the receptor, and the consequent increase in metabolism, decreasing the propensity for binding due to the internal feedback mechanism. One observes the effect of heterogeneity between cellular clusters, allowing them to individualise and increase local production, and within clusters, where we observe 'subpopular quiescence'; a process whereby intra-cluster subpopulations reduce their binding and metabolism such that other such subpopulations may augment their production. Finally, we observe the ability for low-affinity IFN to communicate a long range signal, where high affinity cannot, and the breakdown of this relationship through the introduction of cell motility. Biological systems may utilise cell motility where environments are unrestrictive and may use fixed system, with low-affinity communication, where a localised response is desirable.

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.

Notch Signaling Activation Enhances Human Adipose-Derived Stem Cell Retinal Differentiation

Retinal disease treatment by stem cell-based replacement relies on stem cell differentiation into retinal cells. We previously demonstrated that human periodontal ligament-derived stem cells can be directed into retinal lineage upon induction. Here, we report the transdifferentiation potential of human adipose-derived stem cells (ASCs) into retinal lineage and its enhancement by Notch signaling modulation. Human ASCs, isolated from abdominal fat, expressed mesenchymal but not hematopoietic stem cell markers, and they can differentiate into adipocytes, chondrocytes, and osteoblasts in vitro. Upon noggin/Dkk-1/IGF-1 induction, the treated ASCs showed elevated expression of retinal progenitor, retinal ganglion, and photoreceptor cell markers as well as the glutamate-evoked calcium response, which was not observed in the noninduced cells. Compared to the regular induction treatment, Notch signaling activation by JAG1 enhanced the expression of retinal progenitor and precursor markers without affecting the glutamate-evoked calcium response. In contrast, Notch signaling inhibition by DAPT showed more retinal ganglion cells, but delayed the response to glutamate stimulation. In summary, our results revealed that human ASCs possess a retinal transdifferentiation potential upon noggin/Dkk-1/IGF-1 induction, which can further be enhanced by Notch signaling activation.

Spatial and Temporal Control of Senescence

Cellular senescence is an autonomous tumor suppressor mechanism leading to stable cell cycle arrest. Senescent cells are highly secretory, driving a range of different functions through the senescence-associated secretory phenotype (SASP). Recent findings have suggested that the composition of the SASP is dynamically and spatially regulated and that the changing composition of the SASP can determine the beneficial and detrimental aspects of the senescence program, tipping the balance to either an immunosuppressive/profibrotic environment or proinflammatory/fibrolytic state. Here, we discuss the current understanding of the temporal and spatial regulation of the SASP and the novel finding of NOTCH signaling as a regulator of SASP composition.

Notch1 deficiency in postnatal neural progenitor cells in the dentate gyrus leads to emotional and cognitive impairment

It is well known that Notch1 signaling plays a crucial role in embryonic neural development and adult neurogenesis. The latest evidence shows that Notch1 also plays a critical role in synaptic plasticity in mature hippocampal neurons. So far, deeper insights into the function of Notch1 signaling during the different steps of adult neurogenesis are still lacking, and the mechanisms by which Notch1 dysfunction is associated with brain disorders are also poorly understood. In the current study, we found that Notch1 was highly expressed in the adult-born immature neurons in the hippocampal dentate gyrus. Using a genetic approach to selectively ablate Notch1 signaling in late immature precursors in the postnatal hippocampus by cross-breeding doublecortin (DCX)⁺ neuron-specific proopiomelanocortin (POMC)-α Cre mice with floxed Notch1 mice, we demonstrated a previously unreported pivotal role of Notch1 signaling in survival and function of adult newborn neurons in the dentate gyrus. Moreover, behavioral and functional studies demonstrated that POMC-Notch1^-/- mutant mice showed anxiety and depressive-like behavior with impaired synaptic transmission properties in the dentate gyrus. Finally, our mechanistic study showed significantly compromised phosphorylation of cAMP response element-binding protein (CREB) in Notch1 mutants, suggesting that the dysfunction of Notch1 mutants is associated with the disrupted pCREB signaling in postnatally generated immature neurons in the dentate gyrus.-Feng, S., Shi, T., Qiu, J., Yang, H., Wu, Y., Zhou, W., Wang, W., Wu, H. Notch1 deficiency in postnatal neural progenitor cells in the dentate gyrus leads to emotional and cognitive impairment.

Functional studies on the role of Notch signaling in Hydractinia development

The function of Notch signaling was previously studied in two cnidarians, Hydra and Nematostella, representing the lineages Hydrozoa and Anthozoa, respectively. Using pharmacological inhibition in Hydra and a combination of pharmacological and genetic approaches in Nematostella, it was shown in both animals that Notch is required for tentacle morphogenesis and for late stages of stinging cell maturation. Surprisingly, a role for Notch in neural development, which is well documented in bilaterians, was evident in embryonic Nematostella but not in adult Hydra. Adult neurogenesis in the latter seemed to be unaffected by DAPT, a drug that inhibits Notch signaling. To address this apparent discrepancy, we studied the role of Notch in Hydractinia echinata, an additional hydrozoan, in all life stages. Using CRISPR-Cas9 mediated mutagenesis, transgenesis, and pharmacological interference we show that Notch is dispensable for Hydractinia normal neurogenesis in all life stages but is required for the maturation of stinging cells and for tentacle morphogenesis. Our results are consistent with a conserved role for Notch in morphogenesis and nematogenesis across Cnidaria, and a lineage-specific loss of Notch dependence in neurogenesis in hydrozoans.

Effects of the Notch signalling pathway on hyperoxia-induced immature brain damage in newborn mice

Hyperoxia exposure can cause dramatic release of proinflammatory cytokines, leading to neuronal apoptosis and inducing white matter damage in newborn mouse brains. Some studies indicated that the Notch activation was provoked during inflammation and might regulate adaptive and innate immune responses. Moreover, the pathway also regulated oligodendrocyte maturation which was disrupted in neonatal mice after hyperoxia exposure. This study sought to investigate whether the Notch signalling activation contributed to immature brain damage after hyperoxia exposure. Cellular changes in the white matter (WM) of neonatal wild-type mice exposed to 80% oxygen from postnatal day 3 (P3) to day 5 (P5) were determined. Moreover, in order to further confirm the relationship between the Notch signalling pathway and hyperoxia-induced periventricular white matter injury, mice were pre-treated with a γ-secretase inhibitor (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester, DAPT), which inhibits activation of the Notch pathway before exposure to hyperoxia. The results suggested that expression of myelin basic protein (MBP) increased in P12 mice subjected to hyperoxia after DAPT pretreatment. Moreover, hyperoxia could cause mature oligodendrocytes (MBP+) counts decreased with an obvious inverse increase in OPCs (NG2+) after hyperoxia on P12, DAPT pretreatment significantly ameliorated disruption of oligodendrocytes maturation induced by hyperoxia. Our results also demonstrated that DAPT could reduce memory impairment induced by hyperoxia exposure. Taken together, these results suggest that hyperoxia exposure induces both brain damage in the developing brain and behavioural abnormalities through the Notch signalling activation. And modulation of γ-secretase, selectively interfering with the Notch signalling pathway, could improve adverse outcomes induced by hyperoxia.

Self-organized Notch dynamics generate stereotyped sensory organ patterns in Drosophila

The emergence of spatial patterns in developing multicellular organisms relies on positional cues and cell-cell communication. Drosophila sensory organs have informed a paradigm in which these operate in two distinct steps: Prepattern factors drive localized proneural activity, then Notch-mediated lateral inhibition singles out neural precursors. Here we show that self-organization through Notch signaling also establishes the proneural stripes that resolve into rows of sensory bristles on the fly thorax. Patterning, initiated by a gradient of Delta ligand expression, progresses through inhibitory signaling between and within stripes. Thus, Notch signaling can support self-organized tissue patterning as a prepattern is transduced by cell-cell interactions into a refined arrangement of cellular fates.

Signaling and Transcription Factors during Inner Ear Development: The Generation of Hair Cells and Otic Neurons

Integration between cell signals and bHLH transcription factors plays a prominent role during the development of hair cells of the inner ear. Hair cells are the sensory receptors of the inner ear, responsible for the mechano-transduction of sound waves into electrical signals. They derive from multipotent progenitors that reside in the otic placode. Progenitor commitment is the result of cell signaling from the surrounding tissues that result in the restricted expression of SoxB1 transcription factors, Sox2 and Sox3. In turn, they induce the expression of Neurog1 and Atoh1, two bHLH factors that specify neuronal and hair cell fates, respectively. Neuronal and hair cell development, however, do not occur simultaneously. Hair cell development is prevented during neurogenesis and prosensory stages, resulting in the delay of hair cell development with respect to neuron production. Negative interactions between Neurog1 and Atoh1, and of Atoh1 with other bHLH factors driven by Notch signaling, like Hey1 and Hes5, account for this delay. In summary, the regulation of Atoh1 and hair cell development relies on interactions between cell signaling and bHLH transcription factors that dictate cell fate and timing decisions during development. Interestingly, these mechanisms operate as well during hair cell regeneration after damage and during stem cell directed differentiation, making developmental studies instrumental for improving therapies for hearing impairment.

SoxC Transcription Factors Promote Contralateral Retinal Ganglion Cell Differentiation and Axon Guidance in the Mouse Visual System

Transcription factors control cell identity by regulating diverse developmental steps such as differentiation and axon guidance. The mammalian binocular visual circuit is comprised of projections of retinal ganglion cells (RGCs) to ipsilateral and contralateral targets in the brain. A transcriptional code for ipsilateral RGC identity has been identified, but less is known about the transcriptional regulation of contralateral RGC development. Here we demonstrate that SoxC genes (Sox4, 11, and 12) act on the progenitor-to-postmitotic transition to implement contralateral, but not ipsilateral, RGC differentiation, by binding to Hes5 and thus repressing Notch signaling. When SoxC genes are deleted in postmitotic RGCs, contralateral RGC axons grow poorly on chiasm cells in vitro and project ipsilaterally at the chiasm midline in vivo, and Plexin-A1 and Nr-CAM expression in RGCs is downregulated. These data implicate SoxC transcription factors in the regulation of contralateral RGC differentiation and axon guidance.

Muscle contraction is required to maintain the pool of muscle progenitors via YAP and NOTCH during fetal myogenesis

The importance of mechanical activity in the regulation of muscle progenitors during chick development has not been investigated. We show that immobilization decreases NOTCH activity and mimics a NOTCH loss-of-function phenotype, a reduction in the number of muscle progenitors and increased differentiation. Ligand-induced NOTCH activation prevents the reduction of muscle progenitors and the increase of differentiation upon immobilization. Inhibition of NOTCH ligand activity in muscle fibers suffices to reduce the progenitor pool. Furthermore, immobilization reduces the activity of the transcriptional co-activator YAP and the expression of the NOTCH ligand JAG2 in muscle fibers. YAP forced-activity in muscle fibers prevents the decrease of JAG2 expression and the number of PAX7+ cells in immobilization conditions. Our results identify a novel mechanism acting downstream of muscle contraction, where YAP activates JAG2 expression in muscle fibers, which in turn regulates the pool of fetal muscle progenitors via NOTCH in a non-cell-autonomous manner.

DOI:http://dx.doi.org/10.7554/eLife.15593.001

Skeletal muscle is attached to the skeleton and allows the body to move. Making a new muscle, or repairing an existing one, relies on stem cells that are present inside muscles. A major goal of skeletal muscle research is to understand the signals that regulate the abilities of muscle stem cells to divide and give rise to more stem cells or to become muscle cells. Molecular signals are known to regulate the numbers of stem cells in the muscle. Skeletal muscles become larger if they are exercised, but it is not clear if mechanical forces generated by muscle contractions directly affect the number of muscle stem cells.

The NOTCH signaling pathway contributes to maintaining the population of stem cells in muscles by forcing the stem cells to divide and preventing them from becoming muscle cells. Here, Esteves de Lima et al. investigated whether muscle contraction regulates NOTCH signaling during muscle formation in chick fetuses.

The experiments show that muscle contraction stimulates the activity of a protein called YAP in muscle cells, which in turn, activates a gene in the NOTCH signaling pathway known as JAG2. This increases NOTCH signaling activity in the neighboring stem cells and maintains the number of stem cells in the muscle. The next step following this work will be to establish if this mechanism also operates during muscle formation and regeneration in other animals such as mice and zebrafish.

DOI:http://dx.doi.org/10.7554/eLife.15593.002

Eye development and photoreceptor differentiation in the cephalopod Doryteuthis pealeii

Photoreception is a ubiquitous sensory ability found across the Metazoa, and photoreceptive organs are intricate and diverse in their structure. Although the morphology of the compound eye in Drosophila and the single-chambered eye in vertebrates have elaborated independently, the amount of conservation within the 'eye' gene regulatory network remains controversial, with few taxa studied. To better understand the evolution of photoreceptive organs, we established the cephalopod Doryteuthis pealeii as a lophotrochozoan model for eye development. Utilizing histological, transcriptomic and molecular assays, we characterize eye formation in Doryteuthis pealeii Through lineage tracing and gene expression analyses, we demonstrate that cells expressing Pax and Six genes incorporate into the lens, cornea and iris, and the eye placode is the sole source of retinal tissue. Functional assays demonstrate that Notch signaling is required for photoreceptor cell differentiation and retinal organization. This comparative approach places the canon of eye research in traditional models into perspective, highlighting complexity as a result of both conserved and convergent mechanisms.

Lhx2 Is an Essential Factor for Retinal Gliogenesis and Notch Signaling

Müller glia (MG) are the only glial cell type produced by the neuroepithelial progenitor cells that generate the vertebrate retina. MG are required to maintain retinal homeostasis and support the survival of retinal neurons. Furthermore, in certain vertebrate classes, MG function as adult stem cells, mediating retinal regeneration in response to injury. However, the mechanisms that regulate MG development are poorly understood because there is considerable overlap in gene expression between retinal progenitor cells and differentiated MG. We show that the LIM homeodomain transcription factor Lhx2 is required for the development of MG in the mouse retina. Temporally controlled knock-out studies reveal a requirement for Lhx2 during all stages of MG development, ranging from the proliferation of gliocompetent retinal progenitors, activation of Müller-specific gene expression, and terminal differentiation of MG morphological features. We show that Lhx2 regulates gliogenesis in part by regulating directly the expression of Notch pathway genes including Notch1, Dll1, and Dll3 and gliogenic transcription factors such as Hes1, Hes5, Sox8, and Rax. Conditional knock-out of Lhx2 resulted in a rapid downregulation of Notch pathway genes and loss of Notch signaling. We further demonstrate that Müller gliogenesis induced by misexpression of the potently gliogenic Notch pathway transcriptional effector Hes5 requires Lhx2 expression. These results indicate that Lhx2 not only directly regulates expression of Notch signaling pathway components, but also acts together with the gliogenic Notch pathway to drive MG specification and differentiation.

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.

A Notch-Gli2 axis sustains Hedgehog responsiveness of neural progenitors and Müller glia

Neurogenesis is regulated by the dynamic and coordinated activity of several extracellular signalling pathways, but the basis for crosstalk between these pathways remains poorly understood. Here we investigated regulatory interactions between two pathways that are each required for neural progenitor cell maintenance in the postnatal retina; Hedgehog (Hh) and Notch signalling. Both pathways are activated in progenitor cells in the postnatal retina based on the co-expression of fluorescent pathway reporter transgenes at the single cell level. Disrupting Notch signalling, genetically or pharmacologically, induces a rapid downregulation of all three Gli proteins and inhibits Hh-induced proliferation. Ectopic Notch activation, while not sufficient to promote Hh signalling or proliferation, increases Gli2 protein. We show that Notch regulation of Gli2 in Müller glia renders these cells competent to proliferate in response to Hh. These data suggest that Notch signalling converges on Gli2 to prime postnatal retinal progenitor cells and Müller glia to proliferate in response to Hh.

The bHLH Transcription Factor NeuroD Governs Photoreceptor Genesis and Regeneration Through Delta-Notch Signaling

PURPOSE

Photoreceptor genesis in the retina requires precise regulation of progenitor cell competence, cell cycle exit, and differentiation, although information around the mechanisms that govern these events currently is lacking. In zebrafish, the basic helix-loop-helix (bHLH) transcription factor NeuroD governs photoreceptor genesis, but the signaling pathways through which NeuroD functions are unknown. The purpose of this study was to identify these pathways, and during photoreceptor genesis, Notch signaling was investigated as the putative mediator of NeuroD function.

METHODS

In embryos, genetic mosaic analysis was used to determine if NeuroD functions is cell- or non-cell-autonomous. Morpholino-induced NeuroD knockdown, CRISPR/Cas9 mutation, and pharmacologic and transgenic approaches were used, followed by in situ hybridization, immunocytochemistry, and quantitative RT-PCR (qRT-PCR), to identify mechanisms through which NeuroD functions. In adults, following photoreceptor ablation and NeuroD knockdown, similar methods as above were used to identify NeuroD function during photoreceptor regeneration.

RESULTS

In embryos, NeuroD function is non-cell-autonomous, NeuroD knockdown increases Notch pathway gene expression, Notch inhibition rescues the NeuroD knockdown-induced deficiency in cell cycle exit but not photoreceptor maturation, and Notch activation and CRISPR/Cas9 mutation of neurod recapitulate NeuroD knockdown. In adults, NeuroD knockdown prevents cell cycle exit and photoreceptor regeneration and increases Notch pathway gene expression, and Notch inhibition rescues this phenotype.

CONCLUSIONS

These data demonstrate that during embryonic development, NeuroD governs photoreceptor genesis via non-cell-autonomous mechanisms and that, during photoreceptor development and regeneration, Notch signaling is a mechanistic link between NeuroD and cell cycle exit. In contrast, during embryonic development, NeuroD governs photoreceptor maturation via mechanisms that are independent of Notch signaling.

Jagged1 intracellular domain-mediated inhibition of Notch1 signalling regulates cardiac homeostasis in the postnatal heart

Aims

Notch1 signalling in the heart is mainly activated via expression of Jagged1 on the surface of cardiomyocytes. Notch controls cardiomyocyte proliferation and differentiation in the developing heart and regulates cardiac remodelling in the stressed adult heart. Besides canonical Notch receptor activation in signal-receiving cells, Notch ligands can also activate Notch receptor-independent responses in signal-sending cells via release of their intracellular domain. We evaluated therefore the importance of Jagged1 (J1) intracellular domain (ICD)-mediated pathways in the postnatal heart.

Methods and results

In cardiomyocytes, Jagged1 releases J1ICD, which then translocates into the nucleus and down-regulates Notch transcriptional activity. To study the importance of J1ICD in cardiac homeostasis, we generated transgenic mice expressing a tamoxifen-inducible form of J1ICD, specifically in cardiomyocytes. Using this model, we demonstrate that J1ICD-mediated Notch inhibition diminishes proliferation in the neonatal cardiomyocyte population and promotes maturation. In the neonatal heart, a response via Wnt and Akt pathway activation is elicited as an attempt to compensate for the deficit in cardiomyocyte number resulting from J1ICD activation. In the stressed adult heart, J1ICD activation results in a dramatic reduction of the number of Notch signalling cardiomyocytes, blunts the hypertrophic response, and reduces the number of apoptotic cardiomyocytes. Consistently, this occurs concomitantly with a significant down-regulation of the phosphorylation of the Akt effectors ribosomal S6 protein (S6) and eukaryotic initiation factor 4E binding protein1 (4EBP1) controlling protein synthesis.

Conclusions

Altogether, these data demonstrate the importance of J1ICD in the modulation of physiological and pathological hypertrophy, and reveal the existence of a novel pathway regulating cardiac homeostasis.

Otic placode cell specification and proliferation are regulated by Notch signaling in avian development

BACKGROUND

The entire inner ear including the cochlear-vestibular ganglion arises from a simple epithelium, the otic placode. Precursors for the placode originate from a pool of progenitors located in ectoderm next to the future hindbrain, the pre-otic field, where they are intermingled with future epibranchial and epidermal cells. While the importance of secreted proteins, such as FGFs and Wnts, in imparting otic identity has been well studied, how precursors for these different fates segregate locally is less well understood.

RESULTS

(1) The Notch ligand Delta1 and the Notch target Hes5-2 are expressed in a part of pre-otic field before otic commitment, indicative of active Notch signaling, and this is confirmed using a Notch reporter. (2) Loss and gain-of-function approaches reveal that Notch signaling regulates both proliferation and specification of pre-otic progenitors.

CONCLUSIONS

Our results identify a novel function of Notch signaling in cell fate determination in the pre-otic field of avian embryos.

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.

Notch1 is pan-endothelial at the onset of flow and regulated by flow

Arteriovenous differentiation is a key event during vascular development and hemodynamic forces play an important role. Arteriovenous gene expression is present before the onset of flow, however it remains plastic and flow can alter arteriovenous identity. Notch signaling is especially important in the genetic determination of arteriovenous identity. Nevertheless, the effect of the onset of circulation on Notch expression and signaling has not been studied. The aim of this study is therefore to investigate the interaction of Notch1 signaling and hemodynamic forces during early vascular development. We find that the onset of Notch1 expression coincides with the onset of flow, and that expression is pan-endothelial at the onset of circulation in mouse embryos and only becomes arterial-specific after remodeling has occurred. When we ablate flow in the early embryo, endothelial cells fail to express Notch1. We show that low and disturbed flow patterns upregulate Notch1 expression in endothelial cells in vitro, but that higher shear stress levels do not (≥10 dynes/cm²). Using siRNA, we knocked down Notch1 to investigate the role of Notch1 in mechanotransduction. When we applied shear stress levels similar to those found in embryonic arteries, we found an upregulation of Klf2, Dll1, Dll4, Jag1, Hey1, Nrp1 and CoupTFII but that only Dll4, Hey1, Nrp1 and EphB4 required Notch1 for flow-induced expression. Our results therefore indicate that Notch1 can modulate mechanotransduction but is not a critical mediator of the process since many genes mechanotransduce normally in the absence of Notch1, including genes involved in arteriovenous differentiation.

Application of stem cells in targeted therapy of breast cancer: a systematic review

BACKGROUND

The aim of this systematic review was to investigate whether stem cells could be effectively applied in targeted therapy of breast cancer.

MATERIAL AND METHOD

A systematic literature search was performed for original articles published from January 2007 until May 2012.

RESULTS

Nine studies met the inclusion criteria for phase I or II clinical trials, of which three used stem cells as vehicles, two trials used autologous hematopoetic stem cells and in four trials cancer stem cells were targeted. Mesenchymal stem cells (MSCs) were applied as cellular vehicles to transfer therapeutic agents. Cell therapy with MSC can successfully target resistant cancers. Cancer stem cells were selectively targeted via a proteasome-dependent suicide gene leading to tumor regression. Wnt/β-catenin signaling pathway has been also evidenced to be an attractive CSC-target.

CONCLUSIONS

This systematic review focused on two different concepts of stem cells and breast cancer marking a turning point in the trials that applied stem cells as cellular vehicles for targeted delivery therapy as well as CSC-targeted therapies. Applying stem cells as targeted therapy could be an effective therapeutic approach for treatment of breast cancer in the clinic and in therapeutic marketing; however this needs to be confirmed with further clinical investigations.