An isoform of retinoid-related orphan receptor β directs differentiation of retinal amacrine and horizontal interneurons

Cone photoreceptor differentiation regulated by thyroid hormone transporter MCT8 in the retinal pigment epithelium

The key role of a thyroid hormone receptor in determining the maturation and diversity of cone photoreceptors reflects a profound influence of endocrine signaling on the cells that mediate color vision. However, the route by which hormone reaches cones remains enigmatic as cones reside in the retinal photoreceptor layer, shielded by the blood-retina barrier. Using genetic approaches, we report that cone differentiation is regulated by a membrane transporter for thyroid hormone, MCT8 (SLC16A2), in the retinal pigment epithelium (RPE), which forms the outer blood-retina barrier. Mct8-deficient mice display hypothyroid-like cone gene expression and compromised electroretinogram responses. Mammalian color vision is typically facilitated by cone types that detect medium-long (M) and short (S) wavelengths of light but Mct8-deficient mice have a partial shift of M to S cone identity, resembling the phenotype of thyroid hormone receptor deficiency. RPE-specific ablation of Mct8 results in similar shifts in cone identity and hypothyroid-like gene expression whereas reexpression of MCT8 in the RPE in Mct8-deficient mice partly restores M cone identity, consistent with paracrine-like control of thyroid hormone signaling by the RPE. Our findings suggest that in addition to transport of essential solutes and homeostatic support for photoreceptors, the RPE regulates the thyroid hormone signal that promotes cone-mediated vision.

A multiplexed, single-cell sequencing screen identifies compounds that increase neurogenic reprogramming of murine Muller glia

Retinal degeneration in mammals causes permanent loss of vision, due to an inability to regenerate naturally. Some non-mammalian vertebrates show robust regeneration, via Muller glia (MG). We have recently made significant progress in stimulating adult mouse MG to regenerate functional neurons by transgenic expression of the proneural transcription factor Ascl1. While these results showed that MG can serve as an endogenous source of neuronal replacement, the efficacy of this process is limited. With the goal of improving this in mammals, we designed a small molecule screen using sci-Plex, a method to multiplex up to thousands of single nucleus RNA-seq conditions into a single experiment. We used this technology to screen a library of 92 compounds, identified, and validated two that promote neurogenesis in vivo. Our results demonstrate that high-throughput single-cell molecular profiling can substantially improve the discovery process for molecules and pathways that can stimulate neural regeneration and further demonstrate the potential for this approach to restore vision in patients with retinal disease.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

An allelic series of spontaneous Rorb mutant mice exhibit a gait phenotype, changes in retina morphology and behavior, and gene expression signatures associated with the unfolded protein response

The Retinoid-related orphan receptor beta (RORβ) gene encodes a developmental transcription factor and has 2 predominant isoforms created through alternative first exon usage; one specific to the retina and another present more broadly in the central nervous system, particularly regions involved in sensory processing. RORβ belongs to the nuclear receptor family and plays important roles in cell fate specification in the retina and cortical layer formation. In mice, loss of RORβ causes disorganized retina layers, postnatal degeneration, and production of immature cone photoreceptors. Hyperflexion or "high-stepping" of rear limbs caused by reduced presynaptic inhibition by Rorb-expressing inhibitory interneurons of the spinal cord is evident in RORβ-deficient mice. RORβ variants in patients are associated with susceptibility to various neurodevelopmental conditions, primarily generalized epilepsies, but including intellectual disability, bipolar, and autism spectrum disorders. The mechanisms by which RORβ variants confer susceptibility to these neurodevelopmental disorders are unknown but may involve aberrant neural circuit formation and hyperexcitability during development. Here we report an allelic series in 5 strains of spontaneous Rorb mutant mice with a high-stepping gait phenotype. We show retinal abnormalities in a subset of these mutants and demonstrate significant differences in various behavioral phenotypes related to cognition. Gene expression analyses in all 5 mutants reveal a shared over-representation of the unfolded protein response and pathways related to endoplasmic reticulum stress, suggesting a possible mechanism of susceptibility relevant to patients.

Retinoic Acid Receptor-Related Orphan Receptors (RORs) in Eye Development and Disease

The retinoic acid receptor-related orphan receptors (RORs) are ligand-mediated transcription factors with important biological roles in regulating circadian rhythms, metabolism, immunity, angiogenesis, inflammation, and development. They belong to the superfamily of nuclear receptors and include three family members: RORα, RORβ, and RORγ. Currently identified ROR ligands include cholesterol and cholesterol derivatives for RORα and RORγ, and stearic acid and all-trans retinoic acid for RORβ. Aberrant signaling of the RORs is involved in the pathogenesis of several human diseases including autoimmune diseases, metabolic disorders, and certain cancers. In the eye, RORs regulate normal development of the lens and the retina, and also contribute to potentially blinding eye diseases, especially retinal vascular diseases. Here, we review the role of RORs in eye development and disease to highlight their potential as druggable targets for therapeutic development in retinal vascular and degenerative diseases.

Cellular and Molecular Determinants of Retinal Cell Fate

The vertebrate retina is regarded as a simple part of the central nervous system (CNS) and thus amenable to investigations of the determinants of cell fate. Its five neuronal cell classes and one glial cell class all derive from a common pool of progenitors. Here we review how each cell class is generated. Retinal progenitors progress through different competence states, in each of which they generate only a small repertoire of cell classes. The intrinsic state of the progenitor is determined by the complement of transcription factors it expresses. Thus, although progenitors are multipotent, there is a bias in the types of fates they generate during any particular time window. Overlying these competence states are stochastic mechanisms that influence fate decisions. These mechanisms are determined by a weighted set of probabilities based on the abundance of a cell class in the retina. Deterministic mechanisms also operate, especially late in development, when preprogrammed progenitors solely generate specific fates.

Regulation of retinal amacrine cell generation by miR-216b and Foxn3

The mammalian retina contains a complex mixture of different types of neurons. We find that microRNA miR-216b is preferentially expressed in postmitotic retinal amacrine cells in the mouse retina, and expression of miR-216a/b and miR-217 in retina depend in part on Ptf1a, a transcription factor required for amacrine cell differentiation. Surprisingly, ectopic expression of miR-216b directed the formation of additional amacrine cells and reduced bipolar neurons in the developing retina. We identify the Foxn3 mRNA as a retinal target of miR-216b by Argonaute PAR-CLIP and reporter analysis. Inhibition of Foxn3, a transcription factor, in the postnatal developing retina by RNAi increased the formation of amacrine cells and reduced bipolar cell formation. Foxn3 disruption by CRISPR in embryonic retinal explants also increased amacrine cell formation, whereas Foxn3 overexpression inhibited amacrine cell formation prior to Ptf1a expression. Co-expression of Foxn3 partially reversed the effects of ectopic miR-216b on retinal cell formation. Our results identify Foxn3 as a novel regulator of interneuron formation in the developing retina and suggest that miR-216b likely regulates Foxn3 and other genes in amacrine cells.

Human photoreceptor cells from different macular subregions have distinct transcriptional profiles

The human neural retina is a light sensitive tissue with remarkable spatial and cellular organization. Compared with the periphery, the central retina contains more densely packed cone photoreceptor cells with unique morphologies and synaptic wiring. Some regions of the central retina exhibit selective degeneration or preservation in response to retinal disease and the basis for this variation is unknown. In this study, we used both bulk and single-cell RNA sequencing to compare gene expression within concentric regions of the central retina. We identified unique gene expression patterns of foveal cone photoreceptor cells, including many foveal-enriched transcription factors. In addition, we found that the genes RORB1, PPFIA1 and KCNAB2 are differentially spliced in the foveal, parafoveal and macular regions. These results provide a highly detailed spatial characterization of the retinal transcriptome and highlight unique molecular features of different retinal regions.

ROR: Nuclear Receptor for Melatonin or Not?

Whether the retinoic acid-related orphan receptor (ROR) is a nuclear receptor of melatonin remains controversial. ROR is inextricably linked to melatonin in terms of its expression, function, and mechanism of action. Additionally, studies have illustrated that melatonin functions analogous to ROR ligands, thereby modulating the transcriptional activity of ROR. However, studies supporting these interactions have since been withdrawn. Furthermore, recent crystallographic evidence does not support the view that ROR is a nuclear receptor of melatonin. Some other studies have proposed that melatonin indirectly regulates ROR activity rather than directly binding to ROR. This review aims to delve into the complex relationship of the ROR receptor with melatonin in terms of its structure, expression, function, and mechanism. Thus, we provide the latest evidence and views on direct binding as well as indirect regulation of ROR by melatonin, dissecting both viewpoints in-depth to provide a more comprehensive perspective on this issue.

Early cis-regulatory events in the formation of retinal horizontal cells

During retinal development, multipotent and restricted progenitor cells generate all of the neuronal cells of the retina. Among these are horizontal cells, which are interneurons that modulate the light-induced signal from photoreceptors. This study utilizes the identification of novel cis-regulatory elements as a method to examine the gene regulatory networks that direct the development of horizontal cells. Here we describe a screen for cis-regulatory elements, or enhancers, for the horizontal cell-associated genes PTF1A, ONECUT1 (OC1), TFAP2A (AP2A), and LHX1. The OC1ECR22 and Tfap2aACR5 elements were shown to be potential enhancers for OC1 and TFAP2A, respectively, and to be specifically active in developing horizontal cells. The OC1ECR22 element is activated by PTF1A and RBPJ, which translates to regulation of OC1 expression and suggests that PTF1A is a direct activator of OC1 expression in developing horizontal cells. The region within the Tfap2aACR5 element that is responsible for its activation was determined to be a 100 bp sequence named Motif 4. Both OC1ECR22 and Tfap2aACR5 are negatively regulated by the nuclear receptors THRB and RXRG, as is the expression of OC1 and AP2A, suggesting that nuclear receptors may have a role in the negative regulation of horizontal cell development.

A loss-of-function mutation in RORB disrupts saltatorial locomotion in rabbits

Saltatorial locomotion is a type of hopping gait that in mammals can be found in rabbits, hares, kangaroos, and some species of rodents. The molecular mechanisms that control and fine-tune the formation of this type of gait are unknown. Here, we take advantage of one strain of domesticated rabbits, the sauteur d'Alfort, that exhibits an abnormal locomotion behavior defined by the loss of the typical jumping that characterizes wild-type rabbits. Strikingly, individuals from this strain frequently adopt a bipedal gait using their front legs. Using a combination of experimental crosses and whole genome sequencing, we show that a single locus containing the RAR related orphan receptor B gene (RORB) explains the atypical gait of these rabbits. We found that a splice-site mutation in an evolutionary conserved site of RORB results in several aberrant transcript isoforms incorporating intronic sequence. This mutation leads to a drastic reduction of RORB-positive neurons in the spinal cord, as well as defects in differentiation of populations of spinal cord interneurons. Our results show that RORB function is required for the performance of saltatorial locomotion in rabbits.

Jmjd3 Plays Pivotal Roles in the Proper Development of Early-Born Retinal Lineages: Amacrine, Horizontal, and Retinal Ganglion Cells

Purpose

Trimethylation of histone H3 at lysine 27 (H3K27me3) is a critical mediator of transcriptional gene repression, and Jmjd3 and Utx are the demethylases specific to H3K27me3. Using an in vitro retinal explant culture system, we previously revealed the role of Jmjd3 in the development of rod bipolar cells; however, the roles of Jmjd3 in the development of early-born retinal cells are unknown due to limitations concerning the use of retinal explant culture systems. In this study, we investigated the roles of Jmjd3 in the development of early-born retinal cells.

Methods

We examined retina-specific conditional Jmjd3 knockout (Jmjd3-cKO) mice using immunohistochemistry and quantitative reverse transcription PCR and JMJD3 binding to a target locus by chromatin immunoprecipitation analysis.

Results

We observed reductions in amacrine cells (ACs) and horizontal cells (HCs), as well as lowered expression levels of several transcription factors involved in the development of ACs and HCs in the Jmjd3-cKO mouse retina. JMJD3 bound the promoter regions of these transcription factors. Notably, an elevated number of retinal ganglion cells (RGCs) was observed at embryonic stages, whereas RGCs were moderately reduced at later postnatal stages in the Jmjd3-cKO retina. We also observed reduced expression of Eomes, which is required for the maintenance of RGCs, as well as lower H3K27me3 level and lower JMJD3 binding in the promoter region of Eomes in RGC-enriched cells.

Conclusions

The results indicated that Jmjd3 has critical roles in the development of early-born retinal subtypes, and suggested biphasic roles of Jmjd3 in RGC production and maintenance.

Cortical RORβ is required for layer 4 transcriptional identity and barrel integrity

Retinoic acid-related orphan receptor beta (RORβ) is a transcription factor (TF) and marker of layer 4 (L4) neurons, which are distinctive both in transcriptional identity and the ability to form aggregates such as barrels in rodent somatosensory cortex. However, the relationship between transcriptional identity and L4 cytoarchitecture is largely unknown. We find RORβ is required in the cortex for L4 aggregation into barrels and thalamocortical afferent (TCA) segregation. Interestingly, barrel organization also degrades with age in wildtype mice. Loss of RORβ delays excitatory input and disrupts gene expression and chromatin accessibility, with down-regulation of L4 and up-regulation of L5 genes, suggesting a disruption in cellular specification. Expression and binding site accessibility change for many other TFs, including closure of neurodevelopmental TF binding sites and increased expression and binding capacity of activity-regulated TFs. Lastly, a putative target of RORβ, Thsd7a, is down-regulated without RORβ, and Thsd7a knock-out alone disrupts TCA organization in adult barrels.

Distinct functions and prognostic values of RORs in gastric cancer

Retinoic acid receptor-related orphan receptors (RORs) are frequently abnormally expressed in several human malignancies, including gastric cancer (GC). RORs are involved in the development and progression of GC through Wnt signaling pathway receptors and other common receptors. However, the prognostic roles of individual RORs in patients with GC remain elusive. We accessed the prognostic roles of three RORs (RORα, RORβ, and RORγ) through "The Kaplan-Meier plotter" (KM plotter) database in patients with GC. For all patients with GC who were followed for 20 years, the low mRNA expression of all three RORs showed a significant correlation with better outcomes. We further accessed the prognostic value of individual RORs in different clinical pathological features including Lauren classification, clinical stages, pathological grades, HER2 status, and different treatments methods. The RORs demonstrated critical prognostic roles in GC. Expressions of RORs were higher in GC tissues when compared with normal gastric tissues. Moreover, knockdown of RORs significantly inhibited cell proliferation and migration, suggesting an oncogenic role of RORs in human GC. These findings suggest potential roles of RORs as biomarkers for GC prognosis and as oncogenes in GC.

Foxn4 is a temporal identity factor conferring mid/late-early retinal competence and involved in retinal synaptogenesis

During development, neural progenitors change their competence states over time to sequentially generate different types of neurons and glia. Several cascades of temporal transcription factors (tTFs) have been discovered in Drosophila to control the temporal identity of neuroblasts, but the temporal regulation mechanism is poorly understood in vertebrates. Mammalian retinal progenitor cells (RPCs) give rise to several types of neuronal and glial cells following a sequential yet overlapping temporal order. Here, by temporal cluster analysis, RNA-sequencing analysis, and loss-of-function and gain-of-function studies, we show that the Fox domain TF Foxn4 functions as a tTF during retinogenesis to confer RPCs with the competence to generate the mid/late-early cell types: amacrine, horizontal, cone, and rod cells, while suppressing the competence of generating the immediate-early cell type: retinal ganglion cells (RGCs). In early embryonic retinas, Foxn4 inactivation causes down-regulation of photoreceptor marker genes and decreased photoreceptor generation but increased RGC production, whereas its overexpression has the opposite effect. Just as in Drosophila, Foxn4 appears to positively regulate its downstream tTF Casz1 while negatively regulating its upstream tTF Ikzf1. Moreover, retina-specific ablation of Foxn4 reveals that it may be indirectly involved in the synaptogenesis, establishment of laminar structure, visual signal transmission, and long-term maintenance of the retina. Together, our data provide evidence that Foxn4 acts as a tTF to bias RPCs toward the mid/late-early cell fates and identify a missing member of the tTF cascade that controls RPC temporal identities to ensure the generation of proper neuronal diversity in the retina.

A Non-canonical Feedback Circuit for Rapid Interactions between Somatosensory Cortices

Sensory perception depends on interactions among cortical areas. These interactions are mediated by canonical patterns of connectivity in which higher areas send feedback projections to lower areas via neurons in superficial and deep layers. Here, we probed the circuit basis of interactions among two areas critical for touch perception in mice, whisker primary (wS1) and secondary (wS2) somatosensory cortices. Neurons in layer 4 of wS2 (S2_L4) formed a major feedback pathway to wS1. Feedback from wS2 to wS1 was organized somatotopically. Spikes evoked by whisker deflections occurred nearly as rapidly in wS2 as in wS1, including among putative S2_L4 → S1 feedback neurons. Axons from S2_L4 → S1 neurons sent stimulus orientation-specific activity to wS1. Optogenetic excitation of S2_L4 neurons modulated activity across both wS2 and wS1, while inhibition of S2_L4 reduced orientation tuning among wS1 neurons. Thus, a non-canonical feedback circuit, originating in layer 4 of S2, rapidly modulates early tactile processing.

Notch Signaling-Induced Oscillatory Gene Expression May Drive Neurogenesis in the Developing Retina

After integrating classic and cutting-edge research, we proposed a unified model that attempts to explain the key steps of mammalian retinal neurogenesis. We proposed that the Notch signaling-induced lateral inhibition mechanism promotes oscillatory expression of Hes1. Oscillating Hes1 inhibitory activity as a result leads to oscillatory expression of Notch signaling inhibitors, activators/inhibitors of retinal neuronal phenotypes, and cell cycle-promoting genes all within a retinal progenitor cell (RPC). We provided a mechanism explaining not only how oscillatory expression prevents the progenitor-to-precursor transition, but also how this transition happens. Our proposal of the mechanism posits that the levels of the above factors not only oscillate but also rise (with the exception of Hes1) as the factors accumulate within a progenitor. Depending on which factors accumulate fastest and reach the required supra-threshold levels (cell cycle activators or Notch signaling inhibitors), the progenitor either proliferates or begins to differentiate without any further proliferation when Notch signaling ceases. Thus, oscillatory gene expression may regulate an RPC's decision to proliferate or differentiate. Meanwhile, a post-mitotic precursor's selection of one retinal neuronal phenotype over many others depends on the expression level of key transcription factors (activators) required for each of these retinal neuronal phenotypes. Because the events described above are stochastic due to oscillatory gene expression and gene product inheritance from a mother RPC after its division, an RPC or precursor's decision requires the assignment of probabilities to specific outcomes in the selection process. While low and sustained (non-oscillatory) Notch signaling activity is required to promote the transition of retinal progenitors into various retinal neuronal phenotypes, we propose that the lateral inhibition mechanism, combined with high expression of the BMP signaling-induced Inhibitor of Differentiation (ID) protein family, promotes high and sustained (non-oscillatory) Hes1 and Hes5 expression. These events facilitate the transition of an RPC into the Müller glia (MG) phenotype at the late stage of retinal development.

Rorβ regulates selective axon-target innervation in the mammalian midbrain

Developmental control of long-range neuronal connections in the mammalian midbrain remains unclear. We explored the mechanisms regulating target selection of the developing superior colliculus (SC). The SC is a midbrain center that directs orienting behaviors and defense responses. We discovered that a transcription factor, Rorβ, controls establishment of axonal projections from the SC to two thalamic nuclei: the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior nucleus (LP). A genetic strategy used to visualize SC circuits revealed that in control animals Rorβ⁺ neurons abundantly innervate the dLGN but barely innervate the LP. The opposite phenotype was observed in global and conditional Rorb mutants: projections to the dLGN were strongly decreased, and projections to the LP were increased. Furthermore, overexpression of Rorb in the wild type showed increased projections to the dLGN and decreased projections to the LP. In summary, we identified Rorβ as a key developmental mediator of colliculo-thalamic innervation. Such regulation could represent a general mechanism orchestrating long-range neuronal connections in the mammalian brain.

Transcription factor Ptf1a in development, diseases and reprogramming

The transcription factor Ptf1a is a crucial helix-loop-helix (bHLH) protein selectively expressed in the pancreas, retina, spinal cord, brain, and enteric nervous system. Ptf1a is preferably assembled into a transcription trimeric complex PTF1 with an E protein and Rbpj (or Rbpjl). In pancreatic development, Ptf1a is indispensable in controlling the expansion of multipotent progenitor cells as well as the specification and maintenance of the acinar cells. In neural tissues, Ptf1a is transiently expressed in the post-mitotic cells and specifies the inhibitory neuronal cell fates, mostly mediated by downstream genes such as Tfap2a/b and Prdm13. Mutations in the coding and non-coding regulatory sequences resulting in Ptf1a gain- or loss-of-function are associated with genetic diseases such as pancreatic and cerebellar agenesis in the rodent and human. Surprisingly, Ptf1a alone is sufficient to reprogram mouse or human fibroblasts into tripotential neural stem cells. Its pleiotropic functions in many biological processes remain to be deciphered in the future.

Late neuroprogenitors contribute to normal retinal vascular development in a Hif2a-dependent manner

In the adult central nervous system, endothelial and neuronal cells engage in tight cross-talk as key components of the so-called neurovascular unit. Impairment of this important relationship adversely affects tissue homeostasis, as observed in neurodegenerative conditions including Alzheimer's and Parkinson's disease. In development, the influence of neuroprogenitor cells on angiogenesis is poorly understood. Here, we show in mouse that these cells interact intimately with the growing retinal vascular network, and we identify a novel regulatory mechanism of vasculature development mediated by hypoxia-inducible factor 2a (Hif2a). By Cre-lox gene excision, we show that Hif2a in retinal neuroprogenitor cells upregulates the expression of the pro-angiogenic mediators vascular endothelial growth factor and erythropoietin, whereas it locally downregulates the angiogenesis inhibitor endostatin. Importantly, absence of Hif2a in retinal neuroprogenitor cells causes a marked reduction of proliferating endothelial cells at the angiogenic front. This results in delayed retinal vascular development, fewer major retinal vessels and reduced density of the peripheral deep retinal vascular plexus. Our findings demonstrate that retinal neuroprogenitor cells are a crucial component of the developing neurovascular unit.

Osteoprotection Through the Deletion of the Transcription Factor Rorβ in Mice

There is a clinical need to identify new molecular targets for the treatment of osteoporosis, particularly those that simultaneously inhibit bone resorption while stimulating bone formation. We have previously shown in overexpression studies that retinoic acid receptor-related orphan receptor β (Rorβ) suppresses in vitro osteoblast differentiation. In addition, the expression of Rorβ is markedly increased in bone marrow-derived mesenchymal stromal cells with aging in both mice and humans. Here we establish a critical role for Rorβ in regulating bone metabolism using a combination of in vitro and in vivo studies. We used Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 gene editing to demonstrate that loss of Rorβ in osteoblasts enhances Wnt signaling, specifically through increased recruitment of β-catenin to T-cell factor/lymphoid enhancer factor (Tcf/Lef) DNA binding sites in the promoters of the Wnt target genes Tcf7 and Opg. This resulted in increased osteogenic gene expression and suppressed osteoclast formation through increased osteoprotegerin (OPG) secretion in Rorβ-deficient cells. Consistent with our in vitro data, genetic deletion of Rorβ in both female and male mice resulted in preserved bone mass and microarchitecture with advancing age due to increased bone formation with a concomitant decrease in resorption. The improved skeletal phenotype in the Rorβ-/- mice was also associated with increased bone protein levels of TCF7 and OPG. These data demonstrate that loss of Rorβ has beneficial skeletal effects by increasing bone formation and decreasing bone resorption, at least in part through β-catenin-dependent activation of the Wnt pathway. Thus, inhibition of Rorβ represents a novel approach to potentially prevent or reverse osteoporosis. © 2017 American Society for Bone and Mineral Research.

CuCr2O4@rGO Nanocomposites as High-Performance Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries

Rechargeable lithium-oxygen batteries have been considered as a promising energy storage technology because of their ultra-high theoretical energy densities which are comparable to gasoline. In order to improve the electrochemical properties of lithium-oxygen batteries (LOBs), especially the cycling performance, a high-efficiency cathode catalyst is the most important component. Hence, we aim to demonstrate that CuCr₂O₄@rGO (CCO@rGO) nanocomposites, which are synthesized using a facile hydrothermal method and followed by a series of calcination processes, are an effective cathode catalyst. The obtained CCO@rGO nanocomposites which served as the cathode catalyst of the LOBs exhibited an outstanding cycling performance for over 100 cycles with a fixed capacity of 1000 mAh g^-1 at a current density of 200 mA g^-1. The enhanced properties were attributed to the synergistic effect between the high catalytic efficiency of the spinel-structured CCO nanoparticles, the high specific surface area, and high conductivity of the rGO.

Prdm13 is required for Ebf3+ amacrine cell formation in the retina

Amacrine interneurons play a critical role in the processing of visual signals within the retina. They are highly diverse, representing 30 or more distinct subtypes. Little is known about how amacrine subtypes acquire their unique gene expression and morphological features. We characterized the gene expression pattern of the zinc-finger transcription factor Prdm13 in the mouse. Consistent with a developmental role, Prdm13 was expressed by Ptf1a+ amacrine and horizontal precursors. Over time, Prdm13 expression diverged from the transiently expressed Ptf1a and marked just a subset of amacrine cells in the adult retina. While heterogeneous, we show that most of these Prdm13+ amacrine cells express the transcription factor Ebf3 and the calcium binding protein calretinin. Loss of Prdm13 did not affect the number of amacrine cells formed during development. However, we observed a modest loss of amacrine cells and increased apoptosis that correlated with the onset timing of Ebf3 expression. Adult Prdm13 loss-of-function mice had 25% fewer amacrine cells, altered calretinin expression, and a lack of Ebf3+ amacrines. Forcing Prdm13 expression in retinal progenitor cells did not significantly increase amacrine cell formation, Ebf3 or calretinin expression, and appeared detrimental to the survival of photoreceptors. Our data show that Prdm13 is not required for amacrine fate as a class, but is essential for the formation of Ebf3+ amacrine cell subtypes. Rather than driving subtype identity, Prdm13 may act by restricting competing fate programs to maintain identity and survival.

Pax6 is essential for the generation of late-born retinal neurons and for inhibition of photoreceptor-fate during late stages of retinogenesis

In the developing retina, as in other regions of the CNS, neural progenitors give rise to individual cell types during discrete temporal windows. Pax6 is expressed in retinal progenitor cells (RPCs) throughout the course of retinogenesis, and has been shown to be required during early retinogenesis for generation of most early-born cell types. In this study, we examined the function of Pax6 in postnatal mouse retinal development. We found that Pax6 is essential for the generation of late-born interneurons, while inhibiting photoreceptor differentiation. Generation of bipolar interneurons requires Pax6 expression in RPCs, while Pax6 is required for the generation of glycinergic, but not for GABAergic or non-GABAergic-non-glycinergic (nGnG) amacrine cell subtypes. In contrast, overexpression of either full-length Pax6 or its 5a isoform in RPCs induces formation of cells with nGnG amacrine features, and suppresses generation of other inner retinal cell types. Moreover, overexpression of both Pax6 variants prevents photoreceptor differentiation, most likely by inhibiting Crx expression. Taken together, these data show that Pax6 acts in RPCs to control differentiation of multiple late-born neuronal cell types.

Combinatorial regulation of a Blimp1 (Prdm1) enhancer in the mouse retina

The mouse retina comprises seven major cell types that exist in differing proportions. They are generated from multipotent progenitors in a stochastic manner, such that the relative frequency of any given type generated changes over time. The mechanisms determining the proportions of each cell type are only partially understood. Photoreceptors and bipolar interneurons are derived from cells that express Otx2. Within this population, Blimp1 (Prdm1) helps set the balance between photoreceptors and bipolar cells by suppressing bipolar identity in most of the cells. How only a subset of these Otx2+ cells decides to upregulate Blimp1 and adopt photoreceptor fate is unknown. To understand this, we investigated how Blimp1 transcription is regulated. We identified several potential Blimp1 retinal enhancer elements using DNase hypersensitivity sequencing. Only one of the elements recapitulated Blimp1 spatial and temporal expression in cultured explant assays and within the retinas of transgenic mice. Mutagenesis of this retinal Blimp1 enhancer element revealed four discrete sequences that were each required for its activity. These included highly conserved Otx2 and ROR (retinoic acid receptor related orphan receptor) binding sites. The other required sequences do not appear to be controlled by Otx2 or ROR factors, increasing the complexity of the Blimp1 gene regulatory network. Our results show that the intersection of three or more transcription factors is required to correctly regulate the spatial and temporal features of Blimp1 enhancer expression. This explains how Blimp1 expression can diverge from Otx2 and set the balance between photoreceptor and bipolar fates.

Pten Regulates Retinal Amacrine Cell Number by Modulating Akt, Tgfβ, and Erk Signaling

All tissues are genetically programmed to acquire an optimal size that is defined by total cell number and individual cellular dimensions. The retina contains stereotyped proportions of one glial and six neuronal cell types that are generated in overlapping waves. How multipotent retinal progenitors know when to switch from making one cell type to the next so that appropriate numbers of each cell type are generated is poorly understood. Pten is a phosphatase that controls progenitor cell proliferation and differentiation in several lineages. Here, using a conditional loss-of-function strategy, we found that Pten regulates retinal cell division and is required to produce the full complement of rod photoreceptors and amacrine cells in mouse. We focused on amacrine cell number control, identifying three downstream Pten effector pathways. First, phosphoinositide 3-kinase/Akt signaling is hyperactivated in Pten conditional knock-out (cKO) retinas, and misexpression of constitutively active Akt (Akt-CA) in retinal explants phenocopies the reduction in amacrine cell production observed in Pten cKOs. Second, Akt-CA activates Tgfβ signaling in retinal explants, which is a negative feedback pathway for amacrine cell production. Accordingly, Tgfβ signaling is elevated in Pten cKO retinas, and epistatic analyses placed Pten downstream of TgfβRII in amacrine cell number control. Finally, Pten regulates Raf/Mek/Erk signaling levels to promote the differentiation of all amacrine cell subtypes, which are each reduced in number in Pten cKOs. Pten is thus a positive regulator of amacrine cell production, acting via multiple downstream pathways, highlighting its diverse actions as a mediator of cell number control.

SIGNIFICANCE STATEMENT

Despite the importance of size for optimal organ function, how individual cell types are generated in correct proportions is poorly understood. There are several ways to control cell number, including readouts of organ function (e.g., secreted hormones reach functional levels when enough cells are made) or counting of cell divisions or cell number. The latter applies to the retina, where cell number is regulated by negative feedback signals, which arrest differentiation of particular cell types at threshold levels. Herein, we show that Pten is a critical regulator of amacrine cell number in the retina, acting via multiple downstream pathways. Our studies provide molecular insights into how PTEN loss in humans may lead to uncontrolled cell division in several pathological conditions.

Identification and prediction of alternative transcription start sites that generate rod photoreceptor-specific transcripts from ubiquitously expressed genes

Transcriptome complexity is substantially increased by the use of multiple transcription start sites for a given gene. By utilizing a rod photoreceptor-specific chromatin signature, and the RefSeq database of established transcription start sites, we have identified essentially all known rod photoreceptor genes as well as a group of novel genes that have a high probability of being expressed in rod photoreceptors. Approximately half of these novel rod genes are transcribed into multiple mRNA and/or protein isoforms through alternative transcriptional start sites (ATSS), only one of which has a rod-specific epigenetic signature and gives rise to a rod transcript. This suggests that, during retina development, some genes use ATSS to regulate cell type and temporal specificity, effectively generating a rod transcript from otherwise ubiquitously expressed genes. Biological confirmation of the relationship between epigenetic signatures and gene expression, as well as comparison of our genome-wide chromatin signature maps with available data sets for retina, namely a ChIP-on-Chip study of Polymerase-II (Pol-II) binding sites, ChIP-Seq studies for NRL- and CRX- binding sites and DHS (University of Washington data, available on UCSC mouse Genome Browser as a part of ENCODE project) fully support our hypothesis and together accurately identify and predict an array of new rod transcripts. The same approach was used to identify a number of TSS that are not currently in RefSeq. Biological conformation of the use of some of these TSS suggests that this method will be valuable for exploring the range of transcriptional complexity in many tissues. Comparison of mouse and human genome-wide data indicates that most of these alternate TSS appear to be present in both species, indicating that our approach can be useful for identification of regulatory regions that might play a role in human retinal disease.

Prenatal thalamic waves regulate cortical area size prior to sensory processing

The cerebral cortex is organized into specialized sensory areas, whose initial territory is determined by intracortical molecular determinants. Yet, sensory cortical area size appears to be fine tuned during development to respond to functional adaptations. Here we demonstrate the existence of a prenatal sub-cortical mechanism that regulates the cortical areas size in mice. This mechanism is mediated by spontaneous thalamic calcium waves that propagate among sensory-modality thalamic nuclei up to the cortex and that provide a means of communication among sensory systems. Wave pattern alterations in one nucleus lead to changes in the pattern of the remaining ones, triggering changes in thalamic gene expression and cortical area size. Thus, silencing calcium waves in the auditory thalamus induces Rorβ upregulation in a neighbouring somatosensory nucleus preluding the enlargement of the barrel-field. These findings reveal that embryonic thalamic calcium waves coordinate cortical sensory area patterning and plasticity prior to sensory information processing.

How sensory maps are formed in the brain is only partially understood. Here the authors describe spontaneous calcium waves that propagate across different sensory nuclei in the embryonic thalamus; disrupting the wave pattern triggers thalamic gene expression changes and eventually alters the size of cortical areas.

Up-regulated NRIP2 in colorectal cancer initiating cells modulates the Wnt pathway by targeting RORβ

BACKGROUND

Colorectal cancer remains one of the most common malignant tumors worldwide. Colorectal cancer initiating cells (CCICs) are a small subpopulation responsible for malignant behaviors of colorectal cancer. Aberrant activation of the Wnt pathways regulates the self-renewal of CCIC. However, the underlying mechanism(s) remain poorly understood.

METHODS

Via retroviral library screening, we identified Nuclear Receptor-Interacting Protein 2 (NRIP2) as a novel interactor of the Wnt pathway from enriched colorectal cancer colosphere cells. The expression levels of NRIP2 and retinoic acid-related orphan receptor β (RORβ) were further examined by FISH, qRT-PCR, IHC and Western blot. NRIP2 overexpressed and knockdown colorectal cancer cells were produced to study the role of NRIP2 in Wnt pathway. We also verified the binding between NRIP2 and RORβ and investigated the effect of RORβ on CCICs both in vitro and in vivo. Genechip-scanning speculated downstream target HBP1. Western blot, ChIP and luciferase reporter were carried to investigate the interaction between NRIP2, RORβ, and HBP1.

RESULTS

NRIP2 was significantly up-regulated in CCICs from both cell lines and primary colorectal cancer tissues. Reinforced expression of NRIP2 increased Wnt activity, while silencing of NRIP2 attenuated Wnt activity. The transcription factor RORβ was a key target through which NRIP2 regulated Wnt pathway activity. RORβ was a transcriptional enhancer of inhibitor HBP1 of the Wnt pathway. NRIP2 prevented RORβ to bind with downstream HBP1 promoter regions and reduced the transcription of HBP1. This, in turn, attenuated the HBP1-dependent inhibition of TCF4-mediated transcription.

CONCLUSIONS

NRIP2 is a novel interactor of the Wnt pathway in colorectal cancer initiating cells. interactions between NRIP2, RORβ, and HBP1 mediate a new mechanism for CCIC self-renewal via the Wnt activity.

The Cellular and Synaptic Architecture of the Mechanosensory Dorsal Horn

The deep dorsal horn is a poorly characterized spinal cord region implicated in processing low-threshold mechanoreceptor (LTMR) information. We report an array of mouse genetic tools for defining neuronal components and functions of the dorsal horn LTMR-recipient zone (LTMR-RZ), a role for LTMR-RZ processing in tactile perception, and the basic logic of LTMR-RZ organization. We found an unexpectedly high degree of neuronal diversity in the LTMR-RZ: seven excitatory and four inhibitory subtypes of interneurons exhibiting unique morphological, physiological, and synaptic properties. Remarkably, LTMRs form synapses on between four and 11 LTMR-RZ interneuron subtypes, while each LTMR-RZ interneuron subtype samples inputs from at least one to three LTMR classes, as well as spinal cord interneurons and corticospinal neurons. Thus, the LTMR-RZ is a somatosensory processing region endowed with a neuronal complexity that rivals the retina and functions to pattern the activity of ascending touch pathways that underlie tactile perception.

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Convergent LTMR and cortical inputs define the mechanosensory dorsal horn

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A dorsal horn molecular-genetic toolbox defines 11 interneuron subtypes

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Dorsal horn interneurons receive specific patterns of cortical and LTMR inputs

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Dorsal horn interneurons modulate output pathways and tactile perception

Retinoid-Related Orphan Receptor β and Transcriptional Control of Neuronal Differentiation

The ability to generate neuronal diversity is central to the function of the nervous system. Here we discuss the key neurodevelopmental roles of retinoid-related orphan receptor β (RORβ) encoded by the Rorb (Nr1f2) gene. Recent studies have reported loss of function of the human RORB gene in cases of familial epilepsy and intellectual disability. Principal sites of expression of the Rorb gene in model species include sensory organs, the spinal cord, and brain regions that process sensory and circadian information. Genetic analyses in mice have indicated functions in circadian behavior, vision, and, at the cellular level, the differentiation of specific neuronal cell types. Studies in the retina and sensory areas of the cerebral cortex suggest that this orphan nuclear receptor acts at decisive steps in transcriptional hierarchies that determine neuronal diversity.

Loss of function of the retinoid-related nuclear receptor (RORB) gene and epilepsy

Genetic generalized epilepsy (GGE), formerly known as idiopathic generalized epilepsy, is the most common form of epilepsy and is thought to have predominant genetic etiology. GGE are clinically characterized by absence, myoclonic, or generalized tonic-clonic seizures with electroencephalographic pattern of bilateral, synchronous, and symmetrical spike-and-wave discharges. Despite their strong heritability, the genetic basis of generalized epilepsies remains largely elusive. Nevertheless, recent advances in genetic technology have led to the identification of numerous genes and genomic defects in various types of epilepsies in the past few years. In the present study, we performed whole-exome sequencing in a family with GGE consistent with the diagnosis of eyelid myoclonia with absences. We found a nonsense variant (c.196C>T/p.(Arg66*)) in RORB, which encodes the beta retinoid-related orphan nuclear receptor (RORβ), in four affected family members. In addition, two de novo variants (c.218T>C/p.(Leu73Pro); c.1249_1251delACG/p.(Thr417del)) were identified in sporadic patients by trio-based exome sequencing. We also found two de novo deletions in patients with behavioral and cognitive impairment and epilepsy: a 52-kb microdeletion involving exons 5-10 of RORB and a larger 9q21-microdeletion. Furthermore, we identified a patient with intellectual disability and a balanced translocation where one breakpoint truncates RORB and refined the phenotype of a recently reported patient with RORB deletion. Our data support the role of RORB gene variants/CNVs in neurodevelopmental disorders including epilepsy, and especially in generalized epilepsies with predominant absence seizures.

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.

Loss of Maspardin Attenuates the Growth and Maturation of Mouse Cortical Neurons

BACKGROUND

Mast syndrome, an autosomal recessive, progressive form of hereditary spastic paraplegia, is associated with mutations in SPG21 loci that encode maspardin protein. Although SPG21-/- mice exhibit lower limb dysfunction, the role of maspardin loss in mast syndrome is unclear.

OBJECTIVE

To test the hypothesis that loss of maspardin attenuates the growth and maturation of cortical neurons in SPG21-/- mice.

METHODS AND RESULTS

In a randomized experimental design SPG21-/- mice demonstrated significantly less agility and coordination compared to wild-type mice in beam walk, ledge, and hind limb clasp tests for assessing neuronal dysfunction (p ≤ 0.05). The SPG21-/- mice exhibited symptoms of mast syndrome at 6 months which worsened in 12-month-old cohort, suggesting progressive dysfunction of motor neurons. Ex vivo, wild-type cortical neurons formed synapses, ganglia and aggregates at 96 h, whereas SPG21-/- neurons exhibited attenuated growth with markedly less axonal branches. Additionally, epidermal growth factor markedly promoted the growth and maturation of SPG21+/+ cortical neurons but not SPG21-/- neurons. Consequently, quantitative RT-PCR identified a significant reduction in the expression of a subset of EGF-EGFR signaling targets.

CONCLUSIONS

Our current study uncovered a direct role for maspardin in normal and EGF-induced growth and maturation of primary cortical neurons. The loss of maspardin resulted in attenuated growth, axonal branching, and attenuation of EGF signaling. Reinstating the functions of maspardin may reverse hind limb impairment associated with neuronal dysfunction in mast syndrome patients.

Photoreceptor cell fate specification in vertebrates

Photoreceptors--the light-sensitive cells in the vertebrate retina--have been extremely well-characterized with regards to their biochemistry, cell biology and physiology. They therefore provide an excellent model for exploring the factors and mechanisms that drive neural progenitors into a differentiated cell fate in the nervous system. As a result, great progress in understanding the transcriptional network that controls photoreceptor specification and differentiation has been made over the last 20 years. This progress has also enabled the production of photoreceptors from pluripotent stem cells, thereby aiding the development of regenerative medical approaches to eye disease. In this Review, we outline the signaling and transcription factors that drive vertebrate photoreceptor development and discuss how these function together in gene regulatory networks to control photoreceptor cell fate specification.

Molecular features distinguish ten neuronal types in the mouse superficial superior colliculus

The superior colliculus (SC) is a midbrain center involved in controlling head and eye movements in response to inputs from multiple sensory modalities. Visual inputs arise from both the retina and visual cortex and converge onto the superficial layer of the SC (sSC). Neurons in the sSC send information to deeper layers of the SC and to thalamic nuclei that modulate visually guided behaviors. Presently, our understanding of sSC neurons is impeded by a lack of molecular markers that define specific cell types. To better understand the identity and organization of sSC neurons, we took a systematic approach to investigate gene expression within four molecular families: transcription factors, cell adhesion molecules, neuropeptides, and calcium binding proteins. Our analysis revealed 12 molecules with distinct expression patterns in mouse sSC: cadherin 7, contactin 3, netrin G2, cadherin 6, protocadherin 20, retinoid-related orphan receptor β, brain-specific homeobox/POU domain protein 3b, Ets variant gene 1, substance P, somatostatin, vasoactive intestinal polypeptide, and parvalbumin. Double labeling experiments, by either in situ hybridization or immunostaining, demonstrated that the 12 molecular markers collectively define 10 different sSC neuronal types. The characteristic positions of these cell types divide the sSC into four distinct layers. The 12 markers identified here will serve as valuable tools to examine molecular mechanisms that regulate development of sSC neuronal types. These markers could also be used to examine the connections between specific cell types that form retinocollicular, corticocollicular, or colliculothalamic pathways. J. Comp. Neurol. 524:2300-2321, 2016. © 2016 Wiley Periodicals, Inc.

Mapping Sub-Second Structure in Mouse Behavior

Complex animal behaviors are likely built from simpler modules, but their systematic identification in mammals remains a significant challenge. Here we use depth imaging to show that 3D mouse pose dynamics are structured at the sub-second timescale. Computational modeling of these fast dynamics effectively describes mouse behavior as a series of reused and stereotyped modules with defined transition probabilities. We demonstrate this combined 3D imaging and machine learning method can be used to unmask potential strategies employed by the brain to adapt to the environment, to capture both predicted and previously hidden phenotypes caused by genetic or neural manipulations, and to systematically expose the global structure of behavior within an experiment. This work reveals that mouse body language is built from identifiable components and is organized in a predictable fashion; deciphering this language establishes an objective framework for characterizing the influence of environmental cues, genes and neural activity on behavior.

Retinoic Acid-Related Orphan Receptors (RORs): Regulatory Functions in Immunity, Development, Circadian Rhythm, and Metabolism

In this overview, we provide an update on recent progress made in understanding the mechanisms of action, physiological functions, and roles in disease of retinoic acid related orphan receptors (RORs). We are particularly focusing on their roles in the regulation of adaptive and innate immunity, brain function, retinal development, cancer, glucose and lipid metabolism, circadian rhythm, metabolic and inflammatory diseases and neuropsychiatric disorders. We also summarize the current status of ROR agonists and inverse agonists, including their regulation of ROR activity and their therapeutic potential for management of various diseases in which RORs have been implicated.

Prdm13 regulates subtype specification of retinal amacrine interneurons and modulates visual sensitivity

Amacrine interneurons, which are highly diversified in morphological, neurochemical, and physiological features, play crucial roles in visual information processing in the retina. However, the specification mechanisms and functions in vision for each amacrine subtype are not well understood. We found that the Prdm13 transcriptional regulator is specifically expressed in developing and mature amacrine cells in the mouse retina. Most Prdm13-positive amacrine cells are Calbindin- and Calretinin-positive GABAergic or glycinergic neurons. Absence of Prdm13 significantly reduces GABAergic and glycinergic amacrines, resulting in a specific defect of the S2/S3 border neurite bundle in the inner plexiform layer. Forced expression of Prdm13 distinctively induces GABAergic and glycinergic amacrine cells but not cholinergic amacrine cells, whereas Ptf1a, an upstream transcriptional regulator of Prdm13, induces all of these subtypes. Moreover, Prdm13-deficient mice showed abnormally elevated spatial, temporal, and contrast sensitivities in vision. Together, these results show that Prdm13 regulates development of a subset of amacrine cells, which newly defines an amacrine subtype to negatively modulate visual sensitivities. Our current study provides new insights into mechanisms of the diversification of amacrine cells and their function in vision.

Tfap2a and 2b act downstream of Ptf1a to promote amacrine cell differentiation during retinogenesis

Retinogenesis is a precisely controlled developmental process during which different types of neurons and glial cells are generated under the influence of intrinsic and extrinsic factors. Three transcription factors, Foxn4, RORβ1 and their downstream effector Ptf1a, have been shown to be indispensable intrinsic regulators for the differentiation of amacrine and horizontal cells. At present, however, it is unclear how Ptf1a specifies these two cell fates from competent retinal precursors. Here, through combined bioinformatic, molecular and genetic approaches in mouse retinas, we identify the Tfap2a and Tfap2b transcription factors as two major downstream effectors of Ptf1a. RNA-seq and immunolabeling analyses show that the expression of Tfap2a and 2b transcripts and proteins is dramatically downregulated in the Ptf1a null mutant retina. Their overexpression is capable of promoting the differentiation of glycinergic and GABAergic amacrine cells at the expense of photoreceptors much as misexpressed Ptf1a is, whereas their simultaneous knockdown has the opposite effect. Given the demonstrated requirement for Tfap2a and 2b in horizontal cell differentiation, our study thus defines a Foxn4/RORβ1-Ptf1a-Tfap2a/2b transcriptional regulatory cascade that underlies the competence, specification and differentiation of amacrine and horizontal cells during retinal development.

Retinoic acid-related orphan receptor RORβ, circadian rhythm abnormalities and tumorigenesis (Review)

Nuclear receptors are a superfamily of transcription factors including the steroid hormone receptors, non-steroid hormone receptors and the orphan nuclear receptor family. Retinoic acid-related orphan receptor (ROR)β, as a member of the orphan nuclear receptor family, plays an important regulatory role in the maintenance of a variety of physiological and pathological processes. RORβ has been determined to act as an osteogenic repressor in regulating bone formation, and is involved in regulating circadian rhythm. The findings of recent studies concerning the association between tumorigenesis and circadian rhythm have shown that an aberrant circadian rhythm may promote tumorigenesis and tumor progression. The mechanisms discussed in this review demonstrate how aberrant RORβ-induced circadian rhythm may become a new direction for future studies on tumorigenesis and strategy design for cancer prevention.

Onecut1 and Onecut2 transcription factors operate downstream of Pax6 to regulate horizontal cell development

Genetic studies of the last decades strongly indicated that generation of particular retinal cell types is governed by gene regulatory networks of transcription factors and their target genes. The paired and homeodomain transcription factor Pax6 plays a pivotal role in retinal development as its inactivation in the retinal progenitor cell population leads to abolished differentiation of all retinal cell types. However, until now, only a few transcription factors operating downstream of Pax6 responsible for generation of individual retinal cell types have been identified. In this study, we identified two transcription factors of the Onecut family, Onecut1 and Onecut2, as Pax6 downstream-acting factors. Onecut1 and Onecut2 were previously shown to be expressed in developing horizontal cells, retinal ganglion cells and cone photoreceptors; however, their role in differentiation of these cell types is poorly understood. In this study, we show that the horizontal cell genesis is severely disturbed in Onecut-deficient retinae. In single Onecut1 and Onecut2 mutants, the number of horizontal cells is dramatically reduced while horizontal cells are completely missing in the Onecut1/Onecut2 compound mutant. Analysis of genes involved in the horizontal cell genesis such as Foxn4, Ptf1a, Prox1 and Lim1 showed that although horizontal cells are initially formed, they are not maintained in Onecut-deficient retinae. Taken together, this study suggests the model in which Pax6 regulates the maintenance of horizontal cells through the activation of Onecut1 and Onecut2 transcription factors.

Alternative splicing of the LIM-homeodomain transcription factor Isl1 in the mouse retina

Islet-1 (Isl1) is a LIM-homeodomain (LIM-HD) transcription factor that functions in a combinatorial manner with other LIM-HD proteins to direct the differentiation of distinct cell types within the central nervous system and many other tissues. A study of pancreatic cell lines showed that Isl1 is alternatively spliced generating a second isoform, Isl1β, which is missing 23 amino acids within the C-terminal region. This study examines the expression of the canonical and alternative Isl1 transcripts across other tissues, in particular, within the retina, where Isl1 is required for the differentiation of multiple neuronal cell types. The alternative splicing of Isl1 is shown to occur in multiple tissues, but the relative abundance of Isl1α and Isl1β expression varies greatly across them. In most tissues, Isl1α is the more abundant transcript, but in others the transcripts are expressed equally, or the alternative splice variant is dominant. Within the retina, differential expression of the two Isl1 transcripts increases as a function of development, with dynamic changes in expression peaking at E16.5 and again at P10. At the cellular level, individual retinal ganglion cells vary in their expression, with a subset of small-to-medium sized cells expressing only the alternative isoform. The functional significance of the difference in protein sequence between the two Isl1 isoforms was also assessed using a luciferase assay, demonstrating that the alternative isoform forms a less effective transcriptional complex for activating gene expression. These results demonstrate the differential presence of the canonical and alternative isoforms of Isl1 amongst retinal ganglion cell classes. As Isl1 participates in the differentiation of multiple cell types within the CNS, the present results support a role for alternative splicing in the establishment of cellular diversity in the developing nervous system.

Retinoic Acid-Related Orphan Receptors (RORs): Regulatory Functions in Immunity, Development, Circadian Rhythm, and Metabolism

In this overview, we provide an update on recent progress made in understanding the mechanisms of action, physiological functions, and roles in disease of retinoic acid related orphan receptors (RORs). We are particularly focusing on their roles in the regulation of adaptive and innate immunity, brain function, retinal development, cancer, glucose and lipid metabolism, circadian rhythm, metabolic and inflammatory diseases and neuropsychiatric disorders. We also summarize the current status of ROR agonists and inverse agonists, including their regulation of ROR activity and their therapeutic potential for management of various diseases in which RORs have been implicated.

Feedback induction of a photoreceptor-specific isoform of retinoid-related orphan nuclear receptor β by the rod transcription factor NRL

Vision requires the generation of cone and rod photoreceptors that function in daylight and dim light, respectively. The neural retina leucine zipper factor (NRL) transcription factor critically controls photoreceptor fates as it stimulates rod differentiation and suppresses cone differentiation. However, the controls over NRL induction that balance rod and cone fates remain unclear. We have reported previously that the retinoid-related orphan receptor β gene (Rorb) is required for Nrl expression and other retinal functions. We show that Rorb differentially expresses two isoforms: RORβ2 in photoreceptors and RORβ1 in photoreceptors, progenitor cells, and other cell types. Deletion of RORβ2 or RORβ1 increased the cone:rod ratio ∼2-fold, whereas deletion of both isoforms in Rorb(-/-) mice produced almost exclusively cone-like cells at the expense of rods, suggesting that both isoforms induce Nrl. Electroporation of either RORβ isoform into retinal explants from Rorb(-/-) neonates reactivated Nrl and rod genes but, in Nrl(-/-) explants, failed to reactivate rod genes, indicating that NRL is the effector for both RORβ isoforms in rod differentiation. Unexpectedly, RORβ2 expression was lost in Nrl(-/-) mice. Moreover, NRL activated the RORβ2-specific promoter of Rorb, indicating that NRL activates Rorb, its own inducer gene. We suggest that feedback activation between Nrl and Rorb genes reinforces the commitment to rod differentiation.

Identification of Rorβ targets in cultured osteoblasts and in human bone

Control of osteoblastic bone formation involves the cumulative action of numerous transcription factors, including both activating and repressive functions that are important during specific stages of differentiation. The nuclear receptor retinoic acid receptor-related orphan receptor β (Rorβ) has been recently shown to suppress the osteogenic phenotype in cultured osteoblasts, and is highly upregulated in bone marrow-derived osteogenic precursors isolated from aged osteoporotic mice, suggesting Rorβ is an important regulator of osteoblast function. However the specific gene expression patterns elicited by Rorβ are unknown. Using microarray analysis, we identified 281 genes regulated by Rorβ in an MC3T3-E1 mouse osteoblast cell model (MC3T3-Rorβ-GFP). Pathway analysis revealed alterations in genes involved in MAPK signaling, genes involved in extracellular matrix (ECM) regulation, and cytokine-receptor interactions. Whereas the identified Rorβ-regulated ECM genes normally decline during osteoblastic differentiation, they were highly upregulated in this non-mineralizing MC3T3-Rorβ-GFP model system, suggesting that Rorβ may exert its anti-osteogenic effects through ECM disruption. Consistent with these in vitro findings, the expression of both RORβ and a subset of RORβ-regulated genes were increased in bone biopsies from postmenopausal women (73±7 years old) compared to premenopausal women (30±5 years old), suggesting a role for RORβ in human age-related bone loss. Collectively, these data demonstrate that Rorβ regulates known osteogenic pathways, and may represent a novel therapeutic target for age-associated bone loss.

Meis1 regulates Foxn4 expression during retinal progenitor cell differentiation

The transcription factor forkhead box N4 (Foxn4) is a key regulator in a variety of biological processes during development. In particular, Foxn4 plays an essential role in the genesis of horizontal and amacrine neurons from neural progenitors in the vertebrate retina. Although the functions of Foxn4 have been well established, the transcriptional regulation of Foxn4 expression during progenitor cell differentiation remains unclear. Here, we report that an evolutionarily conserved 129 bp noncoding DNA fragment (Foxn4CR4.2 or CR4.2), located ∼26 kb upstream of Foxn4 transcription start site, functions as a cis-element for Foxn4 regulation. CR4.2 directs gene expression in Foxn4-positive cells, primarily in progenitors, differentiating horizontal and amacrine cells. We further determined that the gene regulatory activity of CR4.2 is modulated by Meis1 binding motif, which is bound and activated by Meis1 transcription factor. Deletion of the Meis1 binding motif or knockdown of Meis1 expression abolishes the gene regulatory activity of CR4.2. In addition, knockdown of Meis1 expression diminishes the endogenous Foxn4 expression and affects cell lineage development. Together, we demonstrate that CR4.2 and its interacting Meis1 transcription factor play important roles in regulating Foxn4 expression during early retinogenesis. These findings provide new insights into molecular mechanisms that govern gene regulation in retinal progenitors and specific cell lineage development.

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.

Thioredoxin Interacting Protein (TXNIP) and Pathogenesis of Diabetic Retinopathy

Chronic hyperglycemia (HG)-associated reactive oxygen/nitrogen species (ROS/RNS) stress and low grade inflammation are considered to play critical roles in the development of diabetic retinopathy (DR). Excess glucose metabolic flux through the aldose reductase/polyol pathway, advanced glycation end product (AGE) formation, elevated hexosamine biosynthesis pathway (HBP), diacyl glycerol/PKC activation, and mitochondrial ROS generation are all implicated in DR. In addition, endoplasmic reticulum stress/unfolded protein response (er-UPR) and deregulation of mitochondrial quality control by autophagy/mitophagy are observed causing cellular bioenergetic deficiency and injury. Recently, a pro-oxidant and pro-apoptotic thioredoxin interacting protein (TXNIP) was shown to be highly upregulated in DR and by HG in retinal cells in culture. TXNIP binds to thioredoxin (Trx) inhibiting its oxidant scavenging and thiolreducing capacity. Hence, prolonged overexpression of TXNIP causes ROS/RNS stress, mitochondrial dysfunction, inflammation and premature cell death in DR. Initially, DR was considered as microvascular complications of endothelial dysfunction and pericyte loss characterized by capillary basement membrane thickening, pericyte ghost, blood retinal barrier leakage, acellular capillary and neovascularization. However, it is currently acknowledged that neuro-glia are also affected by HG in diabetes and that neuronal injury, glial activation, innate immunity/sterile inflammation, and ganglion apoptosis occur early in DR. In addition, retinal pigment epithelium (RPE) becomes dysfunctional in DR. Since TXNIP is induced by HG in most cells, its effects are not restricted to a particular cell type in DR. However, depending on the metabolic activity and anti-oxidant capacity, some cells may be affected earlier by TXNIP than others. Identification of TXNIP sensitive cells and elucidating the underlying mechanism(s) will be critical for preventing pre-mature cell death and progression of DR.