Hedgehog regulates Norrie disease protein to drive neural progenitor self-renewal

Importin-alpha transports Norrin to the nucleus to promote proliferation and Notch signaling in glioblastoma stem cells

Norrin, a secreted protein encoded by NDP gene, is recognized for its established role as a paracrine canonical Frizzled-4/Wnt ligand that mediates angiogenesis and barrier function in the brain. However, emerging evidence suggests that Norrin possesses Frizzled-4-independent functions, notably impacting Notch activation and proliferation of cancer stem cells. We conducted a BioID protein-proximity screen to identify Norrin-interacting proteins. Surprisingly, a significant proportion of the proteins we identified were nuclear. Through comprehensive tagging and proximity ligation assays, we demonstrate that Norrin is transported to the nucleus through KPNA2 (member of the Importin-alpha family). Subsequently, we demonstrate that KPNA2 loss of function in patient-derived primary glioblastoma stem cells results in a nuclear to cytoplasmic shift of Norrin distribution, and a complete abrogation of its function in stimulating Notch signaling and cellular proliferation. These results indicate that Norrin is actively transported into the nucleus to regulate vital signaling pathways and cellular functions.

Identification of a Subpopulation of Astrocyte Progenitor Cells in the Neonatal Subventricular Zone: Evidence that Migration is Regulated by Glutamate Signaling

In mice engineered to express enhanced green fluorescent protein (eGFP) under the control of the entire glutamate transporter 1 (GLT1) gene, eGFP is found in all 'adult' cortical astrocytes. However, when 8.3 kilobases of the human GLT1/EAAT2 promoter is used to control expression of tdTomato (tdT), tdT is only found in a subpopulation of these eGFP-expressing astrocytes. The eGFP mice have been used to define mechanisms of transcriptional regulation using astrocytes cultured from cortex of 1-3 day old mice. Using the same cultures, we were never able to induce tdT+ expression. We hypothesized that these cells might not have migrated into the cortex by this age. In this study, we characterized the ontogeny of tdT+ cells, performed single-cell RNA sequencing (scRNA-seq), and tracked their migration in organotypic slice cultures. At postnatal day (PND) 1, tdT+ cells were observed in the subventricular zone and striatum but not in the cortex, and they did not express eGFP. At PND7, tdT+ cells begin to appear in the cortex with their numbers increasing with age. At PND1, scRNA-seq demonstrates that the tdT+ cells are molecularly heterogeneous, with a subpopulation expressing astrocytic markers, subsequently validated with immunofluorescence. In organotypic slices, tdT+ cells migrate into the cortex, and after 7 days they express GLT1, NF1A, and eGFP. An ionotropic glutamate receptor (iGluR) antagonist reduced by 50% the distance tdT+ cells migrate from the subventricular zone into the cortex. The pan-glutamate transport inhibitor, TFB-TBOA, increased, by sixfold, the number of tdT+ cells in the cortex. In conclusion, although tdT is expressed by non-glial cells at PND1, it is also expressed by glial progenitors that migrate into the cortex postnatally. Using this fluorescent labeling, we provide novel evidence that glutamate signaling contributes to the control of glial precursor migration.

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.

Spectrum of Mutations in NDP Resulting in Ocular Disease; a Systematic Review

Aims and Rationale: The inner retina is supplied by three intraretinal capillary plexi whereas the outer retina is supplied by the choroidal circulation: NDP is essential for normal intraretinal vascularisation. Pathogenic variants in NDP (Xp11.3) may result in either a severe retinal phenotype associated with hearing loss (Norrie Disease) or a moderate retinal phenotype (Familial Exudative Vitreoretinopathy, FEVR). However, little is known about whether the nature or location of the NDP variant is predictive of severity. In this systematic review we summarise all reported NDP variants and draw conclusions about whether the nature of the NDP variant is predictive of the severity of the resulting ocular pathology and associated hearing loss and intellectual disability. Findings: 201 different variants in the NDP gene have been reported as disease-causing. The pathological phenotype that may result from a disease-causing NDP variant is quite diverse but generally comprises a consistent cluster of features (retinal hypovascularisation, exudation, persistent foetal vasculature, tractional/exudative retinal detachment, intellectual disability and hearing loss) that vary predictably with severity. Previous reviews have found no clear pattern in the nature of NDP mutations that cause either FEVR or Norrie disease, with the exception that mutations affecting cysteine residues have been associated with Norrie Disease and that visual loss amongst patients with Norrie disease tends to be more severe if the NDP mutation results in an early termination of translation as opposed to a missense related amino acid change. A key limitation of previous reviews has been variability in the case definition of Norrie disease and FEVR amongst authors. We thus reclassified patients into two groups based only on the severity of their retinal disease. Of the reported pathogenic variants that have been described in more than one patient, we found that any given variant caused an equivalent severity of retinopathy each time it was reported with very few exceptions. We therefore conclude that specific NDP mutations generally result in a consistent retinal phenotype each time they arise. Reports by different authors of the same variant causing either FEVR or Norrie disease conflict primarily due to variability in the authors' respective case definitions rather than true differences in disease severity.

Endolymphatic Hydrop Phenotype in Familial Norrie Disease Caused by Large Fragment Deletion of NDP

Norrie disease (ND; OMIM 310600), a rare X-linked recessive genetic disorder, is characterized by congenital blindness and occasionally, sensorineural hearing loss, and developmental delay. The congenital blindness of ND patients is almost untreatable; thus, hearing is particularly important for them. However, the mechanism of hearing loss of ND patients is unclear, and no good treatment is available except wearing hearing-aid. Therefore, revealing the mechanism of hearing loss in ND patients and exploring effective treatment methods are greatly important. In addition, as a serious monogenic genetic disease, convenient gene identification method is important for ND patients and their family members, as well as prenatal diagnosis and preimplantation genetic diagnosis to block intergenerational transmission of pathogenic genes. In this study, a Norrie family with two male patients was reported. This pedigree was ND caused by large fragment deletion of NDP (norrin cystine knot growth factor NDP) gene. In addition to typical severe ophthalmologic and audiologic defects, the patients showed new pathological features of endolymphatic hydrops (EH), and they also showed acoustic nerves abnormal as described in a very recent report. PCR methods were developed to analyze and diagnose the variation of the family members. This study expands the understanding of the clinical manifestation and pathogenesis of ND and provides a new idea for the treatment of patients in this family and a convenient method for the genetic screen for this ND family.

Norrin mediates tumor-promoting and -suppressive effects in glioblastoma via Notch and Wnt

Glioblastoma multiforme (GBM) contains a subpopulation of cells, GBM stem cells (GSCs), that maintain the bulk tumor and represent a key therapeutic target. Norrin is a Wnt ligand that binds Frizzled class receptor 4 (FZD4) to activate canonical Wnt signaling. Although Norrin, encoded by NDP, has a well-described role in vascular development, its function in human tumorigenesis is largely unexplored. Here, we show that NDP expression is enriched in neurological cancers, including GBM, and its levels positively correlated with survival in a GBM subtype defined by low expression of ASCL1, a proneural factor. We investigated the function of Norrin and FZD4 in GSCs and found that it mediated opposing tumor-suppressive and -promoting effects on ASCL1lo and ASCL1hi GSCs. Consistent with a potential tumor-suppressive effect of Norrin suggested by the tumor outcome data, we found that Norrin signaling through FZD4 inhibited growth in ASCL1lo GSCs. In contrast, in ASCL1hi GSCs Norrin promoted Notch signaling, independently of WNT, to promote tumor progression. Forced ASCL1 expression reversed the tumor-suppressive effects of Norrin in ASCL1lo GSCs. Our results identify Norrin as a modulator of human brain cancer progression and reveal an unanticipated Notch-mediated function of Norrin in regulating cancer stem cell biology. This study identifies an unanticipated role of Norrin in human brain cancer progression. In addition, we provide preclinical evidence suggesting Norrin and canonical Wnt signaling as potential therapeutic targets for GBM subtype-restricted cancer stem cells.

Norrin mediates opposing effects on tumor progression of glioblastoma stem cells

Glioblastoma is the most common human brain cancer entity and is maintained by a glioblastoma stem cell (GSC) subpopulation. In this issue of the JCI, El-Sehemy and colleagues explored the effects that Norrin, a well-characterized activator of Wnt/β-catenin signaling, had on tumor growth. Norrin inhibited cell growth via β-catenin signaling in GSCs that had low expression levels of the transcription factor ASCL1. However, Norrin had the opposite effect in GSCs with high ASCL1 expression levels. The modulation of Norrin expression, with respect to high or low ASCL1 levels in GSCs, significantly reduced tumor growth in vivo, and subsequently increased the survival rate of mice. Notably, Norrin mediates enhanced tumor growth of glioblastomas by activating the Notch pathway. This study clarifies the opposing effects of Norrin on glioblastoma tumor growth and provides potential therapeutic targets for glioblastoma treatment.

The neuroprotective role of Wnt signaling in the retina

The canonical Wnt/β-catenin signaling pathway has been shown to play a major role during embryonic development and maturation of the central nervous system including the retina. It has a significant impact on retinal vessel formation and maturation, as well as on the establishment of synaptic structures and neuronal function in the central nervous system. Mutations in components of the Wnt/β-catenin signaling cascade may lead to severe retinal diseases, while dysregulation of Wnt signaling can contribute to disease progression. Apart from the angiogenic role of Wnt/β-catenin signaling, research in the last decades leads to the theory of a protective effect of Wnt/β-catenin signaling on damaged neurons. In this review, we focus on the neuroprotective properties of the Wnt/β-catenin pathway as well as its downstream signaling in the retina.

Endothelial Cell-Specific Inactivation of TSPAN12 (Tetraspanin 12) Reveals Pathological Consequences of Barrier Defects in an Otherwise Intact Vasculature

Supplemental Digital Content is available in the text.

Objective—

Blood-CNS (central nervous system) barrier defects are implicated in retinopathies, neurodegenerative diseases, stroke, and epilepsy, yet, the pathological mechanisms downstream of barrier defects remain incompletely understood. Blood-retina barrier (BRB) formation and retinal angiogenesis require β-catenin signaling induced by the ligand norrin (NDP [Norrie disease protein]), the receptor FZD4 (frizzled 4), coreceptor LRP5 (low-density lipoprotein receptor-like protein 5), and the tetraspanin TSPAN12 (tetraspanin 12). Impaired NDP/FZD4 signaling causes familial exudative vitreoretinopathy, which may lead to blindness. This study seeked to define cell type-specific functions of TSPAN12 in the retina.

Approach and Results—

A loxP-flanked Tspan12 allele was generated and recombined in endothelial cells using a tamoxifen-inducible Cdh5-CreERT2 driver. Resulting phenotypes were documented using confocal microscopy. RNA-Seq, histopathologic analysis, and electroretinogram were performed on retinas of aged mice. We show that TSPAN12 functions in endothelial cells to promote vascular morphogenesis and BRB formation in developing mice and BRB maintenance in adult mice. Early loss of TSPAN12 in endothelial cells causes lack of intraretinal capillaries and increased VE-cadherin (CDH5 [cadherin5 aka VE-cadherin]) expression, consistent with premature vascular quiescence. Late loss of TSPAN12 strongly impairs BRB maintenance without affecting vascular morphogenesis, pericyte coverage, or perfusion. Long-term BRB defects are associated with immunoglobulin extravasation, complement deposition, cystoid edema, and impaired b-wave in electroretinograms. RNA-sequencing reveals transcriptional responses to the perturbation of the BRB, including genes involved in vascular basement membrane alterations in diabetic retinopathy.

Conclusions—

This study establishes mice with late endothelial cell–specific loss of Tspan12 as a model to study pathological consequences of BRB impairment in an otherwise intact vasculature.

Introgression of regulatory alleles and a missense coding mutation drive plumage pattern diversity in the rock pigeon

Birds and other vertebrates display stunning variation in pigmentation patterning, yet the genes controlling this diversity remain largely unknown. Rock pigeons (Columba livia) are fundamentally one of four color pattern phenotypes, in decreasing order of melanism: T-check, checker, bar (ancestral), or barless. Using whole-genome scans, we identified NDP as a candidate gene for this variation. Allele-specific expression differences in NDP indicate cis-regulatory divergence between ancestral and melanistic alleles. Sequence comparisons suggest that derived alleles originated in the speckled pigeon (Columba guinea), providing a striking example of introgression. In contrast, barless rock pigeons have an increased incidence of vision defects and, like human families with hereditary blindness, carry start-codon mutations in NDP. In summary, we find that both coding and regulatory variation in the same gene drives wing pattern diversity, and post-domestication introgression supplied potentially advantageous melanistic alleles to feral populations of this ubiquitous urban bird.

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.

Epilepsy phenotypes in siblings with Norrie disease

Norrie disease is an X-linked recessive disorder that is characterized by congenital blindness. Although epileptic seizures are observed in some patients with Norrie disease, little is known about this phenomenon. Here, we report the manifestation of epilepsy in siblings with Norrie disease to increase our knowledge of epilepsy in this condition. Three brothers with congenital blindness were diagnosed with Norrie disease after genetic analyses indicated the deletion of exon 2 of the NDP gene. The eldest brother had suffered from epileptic seizures since the age of 11years, and his seizures were resistant to antiepileptic drugs. Although the second brother had no epileptic seizures, the youngest sibling had experiences epileptic seizures since the age of 8years. His seizures were controlled using lamotrigine and levetiracetam. An electroencephalography (EEG) revealed epileptiform discharges in the occipital areas in all three brothers. A study of these patients will increase our knowledge of epilepsy in patients with Norrie disease.

CEND1 and NEUROGENIN2 Reprogram Mouse Astrocytes and Embryonic Fibroblasts to Induced Neural Precursors and Differentiated Neurons

Recent studies demonstrate that astroglia from non-neurogenic brain regions can be reprogrammed into functional neurons through forced expression of neurogenic factors. Here we explored the effect of CEND1 and NEUROG2 on reprogramming of mouse cortical astrocytes and embryonic fibroblasts. Forced expression of CEND1, NEUROG2, or both resulted in acquisition of induced neuronal cells expressing subtype-specific markers, while long-term live-cell imaging highlighted the existence of two different modes of neuronal trans-differentiation. Of note, a subpopulation of CEND1 and NEUROG2 double-transduced astrocytes formed spheres exhibiting neural stem cell properties. mRNA and protein expression studies revealed a reciprocal feedback loop existing between the two molecules, while knockdown of endogenous CEND1 demonstrated that it is a key mediator of NEUROG2-driven neuronal reprogramming. Our data suggest that common reprogramming mechanisms exist driving the conversion of lineage-distant somatic cell types to neurons and reveal a critical role for CEND1 in NEUROG2-driven astrocytic reprogramming.

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CEND1 reprograms astrocytes and fibroblasts to GABAergic neurons

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Neurospheres are formed from CEND1⁺ and NEUROG2⁺ cells through the β-catenin pathway

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CEND1 and NEUROG2 participate in a reciprocal feedback loop leading to neurogenesis

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CEND1 is a key mediator of NEUROG2 reprogramming function

Autocrine signaling based selection of combinatorial antibodies that transdifferentiate human stem cells

We report here the generation of antibody agonists from intracellular combinatorial libraries that transdifferentiate human stem cells. Antibodies that are agonists for the granulocyte colony stimulating factor receptor were selected from intracellular libraries on the basis of their ability to activate signaling pathways in reporter cells. We used a specialized "near neighbor" approach in which the entire antibody library and its target receptor are cointegrated into the plasma membranes of a population of reporter cells. This format favors unusual interactions between receptors and their protein ligands and ensures that the antibody acts in an autocrine manner on the cells that produce it. Unlike the natural granulocyte-colony stimulating factor that activates cells to differentiate along a predetermined pathway, the isolated agonist antibodies transdifferentiated human myeloid lineage CD34+ bone marrow cells into neural progenitors. This transdifferentiation by agonist antibodies is different from more commonly used methods because initiation is agenetic. Antibodies that act at the plasma membrane may have therapeutic potential as agents that transdifferentiate autologous cells.