castor encodes a novel zinc finger protein required for the development of a subset of CNS neurons in Drosophila

CASZ1 Is Essential for Skin Epidermal Terminal Differentiation

The barrier function of skin epidermis is crucial for our bodies to interface with the environment. Because epidermis continuously turns over throughout the lifetime, this barrier must be actively maintained by regeneration. Although several transcription factors have been established as essential activators in epidermal differentiation, it is unclear whether additional factors remain to be identified. In this study, we show that CASZ1, a multi zinc-finger transcription factor previously characterized in nonepithelial cell types, shows highest expression in skin epidermis. CASZ1 expression is upregulated during epidermal terminal differentiation. In addition, CASZ1 expression is impaired in several skin disorders with impaired barrier function, such as atopic dermatitis, psoriasis, and squamous cell carcinoma. Using transcriptome profiling coupled with RNA interference, we identified 674 differentially expressed genes with CASZ1 knockdown. Downregulated genes account for 91.2% of these differentially expressed genes and were enriched for barrier function. In organotypic epidermal regeneration, CASZ1 knockdown promoted proliferation and strongly impaired multiple terminal differentiation markers. Mechanistically, we found that CASZ1 upregulation in differentiation requires the action of both the master transcription factor, p63, and the histone acetyltransferase, p300. Taken together, our findings identify CASZ1 as an essential activator of epidermal differentiation, paving the way for future studies understanding of CASZ1 roles in skin disease.

Role of the CASZ1 transcription factor in tissue development and disease

The zinc finger transcription factor gene, CASZ1/Castor (Castor zinc finger 1), initially identified in Drosophila, plays a critical role in neural, cardiac, and cardiovascular development, exerting a complex, multifaceted influence on cell fate and tissue morphogenesis. During neurogenesis, CASZ1 exhibits dynamic expression from early embryonic development to the perinatal period, constituting a key regulator in this process. Additionally, CASZ1 controls the transition between neurogenesis and gliomagenesis. During human cardiovascular system development, CASZ1 is essential for cardiomyocyte differentiation, cardiac morphogenesis, and vascular morphology homeostasis and formation. The deletion or inactivation of CASZ1 mutations can lead to human developmental diseases or tumors, including congenital heart disease, cardiovascular disease, and neuroblastoma. CASZ1 can be used as a biomarker for disease prevention and diagnosis as well as a prognostic indicator for cancer. This review explores the unique functions of CASZ1 in tissue morphogenesis and associated diseases, offering new insights for elucidating the molecular mechanisms underlying diseases and identifying potential therapeutic targets for disease prevention and treatment.

CASZ1: Current Implications in Cardiovascular Diseases and Cancers

Castor zinc finger 1 (CASZ1) is a C2H2 zinc finger family protein that has two splicing variants, CASZ1a and CASZ1b. It is involved in multiple physiological processes, such as tissue differentiation and aldosterone antagonism. Genetic and epigenetic alternations of CASZ1 have been characterized in multiple cardiovascular disorders, such as congenital heart diseases, chronic venous diseases, and hypertension. However, little is known about how CASZ1 mechanically participates in the pathogenesis of these diseases. Over the past decades, at first glance, paradoxical influences on cell behaviors and progressions of different cancer types have been discovered for CASZ1, which may be explained by a "double-agent" role for CASZ1. In this review, we discuss the physiological function of CASZ1, and focus on the association of CASZ1 aberrations with the pathogenesis of cardiovascular diseases and cancers.

Zinc finger transcription factor CASZ1b is involved in the DNA damage response in live cells

Zinc finger transcription factor CASZ1b is essential for nervous system development and suppresses neuroblastoma growth. Our previous study showed that CASZ1b interacts with DNA repair proteins, however, whether CASZ1b is involved in the DNA damage response remains unclear. In this study, we investigated the kinetic recruitment of CASZ1b to sites of DNA damage upon induction by laser microirradiation. We find that CASZ1b is transiently recruited to sites of DNA damage in multiple cell lines. Mutagenesis of either the poly-(ADP-ribose) (PAR) binding motif or NuRD complex binding region in CASZ1b significantly reduces the recruitment of CASZ1b to these sites of DNA damage (∼65% and ∼30%, respectively). In addition, treatment of cells with a poly-(ADP-ribose) polymerase (PARP) inhibitor significantly attenuates the recruitment of CASZ1b to these DNA damaged sites. Loss of CASZ1 increases cell sensitivity to DNA damage induced by gamma irradiation as shown by decreased colony formation. Our studies reveal that CASZ1b is transiently recruited to DNA damage sites mainly in a PARP-dependent way and regulates cell sensitivity to DNA damage. Our results suggest that CASZ1b has a role, although perhaps a minor one, in the DNA damage response and ultimately regulating the efficiency of DNA repair during normal development and tumorigenesis.

Specification of the Drosophila Orcokinin A neurons by combinatorial coding

The central nervous system contains a daunting number of different cell types. Understanding how each cell acquires its fate remains a major challenge for neurobiology. The developing embryonic ventral nerve cord (VNC) of Drosophila melanogaster has been a powerful model system for unraveling the basic principles of cell fate specification. This pertains specifically to neuropeptide neurons, which typically are stereotypically generated in discrete subsets, allowing for unambiguous single-cell resolution in different genetic contexts. Here, we study the specification of the OrcoA-LA neurons, characterized by the expression of the neuropeptide Orcokinin A and located laterally in the A1-A5 abdominal segments of the VNC. We identified the progenitor neuroblast (NB; NB5-3) and the temporal window (castor/grainyhead) that generate the OrcoA-LA neurons. We also describe the role of the Ubx, abd-A, and Abd-B Hox genes in the segment-specific generation of these neurons. Additionally, our results indicate that the OrcoA-LA neurons are "Notch Off" cells, and neither programmed cell death nor the BMP pathway appears to be involved in their specification. Finally, we performed a targeted genetic screen of 485 genes known to be expressed in the CNS and identified nab, vg, and tsh as crucial determinists for OrcoA-LA neurons. This work provides a new neuropeptidergic model that will allow for addressing new questions related to neuronal specification mechanisms in the future.

Single cell RNA-seq analysis reveals temporally-regulated and quiescence-regulated gene expression in Drosophila larval neuroblasts

The mechanisms that generate neural diversity during development remains largely unknown. Here, we use scRNA-seq methodology to discover new features of the Drosophila larval CNS across several key developmental timepoints. We identify multiple progenitor subtypes - both stem cell-like neuroblasts and intermediate progenitors - that change gene expression across larval development, and report on new candidate markers for each class of progenitors. We identify a pool of quiescent neuroblasts in newly hatched larvae and show that they are transcriptionally primed to respond to the insulin signaling pathway to exit from quiescence, including relevant pathway components in the adjacent glial signaling cell type. We identify candidate "temporal transcription factors" (TTFs) that are expressed at different times in progenitor lineages. Our work identifies many cell type specific genes that are candidates for functional roles, and generates new insight into the differentiation trajectory of larval neurons.

The neuroblast timer gene nubbin exhibits functional redundancy with gap genes to regulate segment identity in Tribolium

The neuroblast timer genes hunchback, Krüppel, nubbin and castor are expressed in temporal sequence in neural stem cells, and in corresponding spatial sequence along the Drosophila blastoderm. As canonical gap genes, hunchback and Krüppel play a crucial role in insect segmentation, but the roles of nubbin and castor in this process remain ambiguous. We have investigated the expression and functions of nubbin and castor during segmentation in the beetle Tribolium. We show that Tc-hunchback, Tc-Krüppel, Tc-nubbin and Tc-castor are expressed sequentially in the segment addition zone, and that Tc-nubbin regulates segment identity redundantly with two previously described gap/gap-like genes, Tc-giant and Tc-knirps. Simultaneous knockdown of Tc-nubbin, Tc-giant and Tc-knirps results in the formation of ectopic legs on abdominal segments. This homeotic transformation is caused by loss of abdominal Hox gene expression, likely due to expanded Tc-Krüppel expression. Our findings support the theory that the neuroblast timer series was co-opted for use in insect segment patterning, and contribute to our growing understanding of the evolution and function of the gap gene network outside of Drosophila.

Summary:nubbin and the gap genes knirps and giant redundantly repress Krüppel expression during segmentation. Simultaneous knockdown of all three genes causes ectopic Krüppel expression and loss of abdominal segment identity.

(Epi)Genetic Mechanisms Underlying the Evolutionary Success of Eusocial Insects

Eusocial insects, such as bees, ants, and wasps of the Hymenoptera and termites of the Blattodea, are able to generate remarkable diversity in morphology and behavior despite being genetically uniform within a colony. Most eusocial insect species display caste structures in which reproductive ability is possessed by a single or a few queens while all other colony members act as workers. However, in some species, caste structure is somewhat plastic, and individuals may switch from one caste or behavioral phenotype to another in response to certain environmental cues. As different castes normally share a common genetic background, it is believed that much of this observed within-colony diversity results from transcriptional differences between individuals. This suggests that epigenetic mechanisms, featured by modified gene expression without changing genes themselves, may play an important role in eusocial insects. Indeed, epigenetic mechanisms such as DNA methylation, histone modifications and non-coding RNAs, have been shown to influence eusocial insects in multiple aspects, along with typical genetic regulation. This review summarizes the most recent findings regarding such mechanisms and their diverse roles in eusocial insects.

A dynamic and mosaic basement membrane controls cell intercalation in Drosophila ovaries

Basement membranes (BM) are extracellular matrices assembled into complex and highly organized networks essential for organ morphogenesis and function. However, little is known about the tissue origin of BM components and their dynamics in vivo Here, we unravel the assembly and role of the BM main component, Collagen type IV (ColIV), in Drosophila ovarian stalk morphogenesis. Stalks are short strings of cells assembled through cell intercalation that link adjacent follicles and maintain ovarian integrity. We show that stalk ColIV has multiple origins and is assembled following a regulated pattern leading to a unique BM organisation. Absence of ColIV leads to follicle fusion, as observed upon ablation of stalk cells. ColIV and integrins are both required to trigger cell intercalation and maintain mechanically strong cell-cell attachment within the stalk. These results show how the dynamic assembly of a mosaic BM controls complex tissue morphogenesis and integrity.

Circular RNA circANKRD36 regulates Casz1 by targeting miR-599 to prevent osteoarthritis chondrocyte apoptosis and inflammation

Osteoarthritis (OA) is an ageing‐related disease characterized by articular cartilage degradation and joint inflammation. circRNA has been known to involve in the regulation of multiple inflammatory diseases including OA. However, the mechanism underlying how circRNA regulates OA remains to be elucidated. Here, we report circANKRD36 prevents OA chondrocyte apoptosis and inflammation by targeting miR‐599, which specifically degrades Casz1. We performed circRNA sequencing in normal and OA tissues and found the expression of circANKRD36 is decreased in OA tissues. circANKRD36 is also reduced in IL‐1β–treated human chondrocytes. FACS analysis and Western blot showed that the knockdown of circANKRD36 promotes the apoptosis and inflammation of chondrocytes in IL‐1β stress. We then found miR‐599 to be the target of circANKRD36 and correlate well with circANKRD36 both in vitro and in vivo. By database analysis and luciferase assay, Casz1 was found to be the direct target of miR‐599. Casz1 helps to prevent apoptosis and inflammation of chondrocytes in response to IL‐1β. In conclusion, our results proved circANKRD36 sponge miR‐599 to up‐regulate the expression of Casz1 and thus prevent apoptosis and inflammation in OA.

Drosophila Voltage-Gated Sodium Channels Are Only Expressed in Active Neurons and Are Localized to Distal Axonal Initial Segment-like Domains

In multipolar vertebrate neurons, action potentials (APs) initiate close to the soma, at the axonal initial segment. Invertebrate neurons are typically unipolar with dendrites integrating directly into the axon. Where APs are initiated in the axons of invertebrate neurons is unclear. Voltage-gated sodium (NaV) channels are a functional hallmark of the axonal initial segment in vertebrates. We used an intronic Minos-Mediated Integration Cassette to determine the endogenous gene expression and subcellular localization of the sole NaV channel in both male and female Drosophila, para Despite being the only NaV channel in the fly, we show that only 23 ± 1% of neurons in the embryonic and larval CNS express para, while in the adult CNS para is broadly expressed. We generated a single-cell transcriptomic atlas of the whole third instar larval brain to identify para expressing neurons and show that it positively correlates with markers of differentiated, actively firing neurons. Therefore, only 23 ± 1% of larval neurons may be capable of firing NaV-dependent APs. We then show that Para is enriched in an axonal segment, distal to the site of dendritic integration into the axon, which we named the distal axonal segment (DAS). The DAS is present in multiple neuron classes in both the third instar larval and adult CNS. Whole cell patch clamp electrophysiological recordings of adult CNS fly neurons are consistent with the interpretation that Nav-dependent APs originate in the DAS. Identification of the distal NaV localization in fly neurons will enable more accurate interpretation of electrophysiological recordings in invertebrates.SIGNIFICANCE STATEMENT The site of action potential (AP) initiation in invertebrates is unknown. We tagged the sole voltage-gated sodium (NaV) channel in the fly, para, and identified that Para is enriched at a distal axonal segment. The distal axonal segment is located distal to where dendrites impinge on axons and is the likely site of AP initiation. Understanding where APs are initiated improves our ability to model neuronal activity and our interpretation of electrophysiological data. Additionally, para is only expressed in 23 ± 1% of third instar larval neurons but is broadly expressed in adults. Single-cell RNA sequencing of the third instar larval brain shows that para expression correlates with the expression of active, differentiated neuronal markers. Therefore, only 23 ± 1% of third instar larval neurons may be able to actively fire NaV-dependent APs.

Temporal groups of lineage-related neurons have different neuropeptidergic fates and related functions in the Drosophila melanogaster CNS

The central nervous system (CNS) of Drosophila is comprised of the brain and the ventral nerve cord (VNC), which are the homologous structures of the vertebrate brain and the spinal cord, respectively. Neurons of the CNS arise from neural stem cells called neuroblasts (NBs). Each neuroblast gives rise to a specific repertory of cell types whose fate is unknown in most lineages. A combination of spatial and temporal genetic cues defines the fate of each neuron. We studied the origin and specification of a group of peptidergic neurons present in several abdominal segments of the larval VNC that are characterized by the expression of the neuropeptide GPB5, the GPB5-expressing neurons (GPB5-ENs). Our data reveal that the progenitor NB that generates the GPB5-ENs also generates the abdominal leucokinergic neurons (ABLKs) in two different temporal windows. We also show that these two set of neurons share the same axonal projections in larvae and in adults and, as previously suggested, may both function in hydrosaline regulation. Our genetic analysis of potential specification determinants reveals that Klumpfuss (klu) and huckebein (hkb) are involved in the specification of the GPB5 cell fate. Additionally, we show that GPB5-ENs have a role in starvation resistance and longevity; however, their role in desiccation and ionic stress resistance is not as clear. We hypothesize that the neurons arising from the same neuroblast lineage are both architecturally similar and functionally related.

Role of nuclear-cytoplasmic protein localization during Drosophila neuroblast development

Nuclear-cytoplasmic localization is an efficient way to regulate transcription factors and chromatin remodelers. Altering the location of existing protein pools also facilitates a more rapid response to changes in cell activity or extracellular signals. There are several examples of proteins that are regulated by nucleo-cytoplasmic shuttling, which are required for Drosophila neuroblast development. Disruption of the localization of homologs of these proteins has also been linked to several neurodegenerative disorders in humans. Drosophila has been used extensively to model the neurodegenerative disorders caused by aberrant nucleo-cytoplasmic localization. Here, we focus on the role of alternative nucleo-cytoplasmic protein localization in regulating proliferation and cell fate decisions in the Drosophila neuroblast and in neurodegenerative disorders. We also explore the analogous role of RNA binding proteins and mRNA localization in the context of regulation of nucleo-cytoplasmic localization during neural development and a role in neurodegenerative disorders.

A repressor-decay timer for robust temporal patterning in embryonic Drosophila neuroblast lineages

Biological timers synchronize patterning processes during embryonic development. In the Drosophila embryo, neural progenitors (neuroblasts; NBs) produce a sequence of unique neurons whose identities depend on the sequential expression of temporal transcription factors (TTFs). The stereotypy and precision of NB lineages indicate reproducible TTF timer progression. We combine theory and experiments to define the timer mechanism. The TTF timer is commonly described as a relay of activators, but its regulatory circuit is also consistent with a repressor-decay timer, where TTF expression begins when its repressor decays. Theory shows that repressor-decay timers are more robust to parameter variations than activator-relay timers. This motivated us to experimentally compare the relative importance of the relay and decay interactions in vivo. Comparing WT and mutant NBs at high temporal resolution, we show that the TTF sequence progresses primarily by repressor-decay. We suggest that need for robust performance shapes the evolutionary-selected designs of biological circuits.

Conserved Genes Underlie Phenotypic Plasticity in an Incipiently Social Bee

Despite a strong history of theoretical work on the mechanisms of social evolution, relatively little is known of the molecular genetic changes that accompany transitions from solitary to eusocial forms. Here, we provide the first genome of an incipiently social bee that shows both solitary and social colony organization in sympatry, the Australian carpenter bee Ceratina australensis. Through comparative analysis, we provide support for the role of conserved genes and cis-regulation of gene expression in the phenotypic plasticity observed in nest-sharing, a rudimentary form of sociality. Additionally, we find that these conserved genes are associated with caste differences in advanced eusocial species, suggesting these types of mechanisms could pave the molecular pathway from solitary to eusocial living. Genes associated with social nesting in this species show signatures of being deeply conserved, in contrast to previous studies in other bees showing novel and faster-evolving genes are associated with derived sociality. Our data provide support for the idea that the earliest social transitions are driven by changes in gene regulation of deeply conserved genes.

Casz1 controls higher-order nuclear organization in rod photoreceptors

Eukaryotic cells depend on precise genome organization within the nucleus to maintain an appropriate gene-expression profile. Critical to this process is the packaging of functional domains of open and closed chromatin to specific regions of the nucleus, but how this is regulated remains unclear. In this study, we show that the zinc finger protein Casz1 regulates higher-order nuclear organization of rod photoreceptors in the mouse retina by repressing nuclear lamina function, which leads to central localization of heterochromatin. Loss of Casz1 in rods leads to an abnormal transcriptional profile followed by degeneration. These results identify Casz1 as a regulator of higher-order genome organization.

Genome organization plays a fundamental role in the gene-expression programs of numerous cell types, but determinants of higher-order genome organization are poorly understood. In the developing mouse retina, rod photoreceptors represent a good model to study this question. They undergo a process called “chromatin inversion” during differentiation, in which, as opposed to classic nuclear organization, heterochromatin becomes localized to the center of the nucleus and euchromatin is restricted to the periphery. While previous studies showed that the lamin B receptor participates in this process, the molecular mechanisms regulating lamina function during differentiation remain elusive. Here, using conditional genetics, we show that the zinc finger transcription factor Casz1 is required to establish and maintain the inverted chromatin organization of rod photoreceptors and to safeguard their gene-expression profile and long-term survival. At the mechanistic level, we show that Casz1 interacts with the polycomb repressor complex in a splice variant-specific manner and that both are required to suppress the expression of the nuclear envelope intermediate filament lamin A/C in rods. Lamin A is in turn sufficient to regulate heterochromatin organization and nuclear position. Furthermore, we show that Casz1 is sufficient to expand and centralize the heterochromatin of fibroblasts, suggesting a general role for Casz1 in nuclear organization. Together, these data support a model in which Casz1 cooperates with polycomb to control rod genome organization, in part by silencing lamin A/C.

Downregulation of castor zinc finger 1 predicts poor prognosis and facilitates hepatocellular carcinoma progression via MAPK/ERK signaling

Background

Castor zinc finger 1 (CASZ1) plays critical roles in various biological processes and pathologic conditions, including cancer. However, the prognostic importance and biologic functions of CASZ1 in hepatocellular carcinoma (HCC) are still unclear.

Methods

qRT-PCR, western blot and immunohistochemistry analyses were used to determine CASZ1 expression in HCC samples and cell lines. The clinical significance of CASZ1 was assessed in two independent study cohorts containing 232 patients with HCC. A series of in vitro and in vivo experiments were performed to explore the role and molecular mechanism of CASZ1 in HCC progression.

Results

Here we report that CASZ1 expression was downregulated in HCC tissues and cell lines. Low CASZ1 expression was closely correlated with aggressive clinicopathological features, poor clinical outcomes and early recurrence of HCC patients. Moreover, overexpression of CASZ1 in HCCLM3 cells significantly inhibited cell proliferation, migration, invasion in vitro and tumor growth and metastasis in vivo, whereas silencing CASZ1 significantly enhanced the above abilities of PLC/PRF/5 cells. Further mechanism study indicated that these phenotypic changes were mediated by MAPK/ERK signaling pathway and involved altered expression of MMP2, MMP9 and cyclinD1. Finally, we proved that CASZ1 exerted its tumor-suppressive effect by directly interacting with RAF1 and reducing the protein stability of RAF1.

Conclusions

Our study for the first time demonstrated that CASZ1 is a tumor suppressor in HCC, which may serve as a novel prognostic predictor and therapeutic target for HCC patients.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0720-8) contains supplementary material, which is available to authorized users.

Identification of Casz1 as a Regulatory Protein Controlling T Helper Cell Differentiation, Inflammation, and Immunity

While T helper (Th) cells play a crucial role in host defense, an imbalance in Th effector subsets due to dysregulation in their differentiation and expansion contribute to inflammatory disorders. Here, we show that Casz1, whose function is previously unknown in CD4+ T cells, coordinates Th differentiation in vitro and in vivo. Casz1 deficiency in CD4+ T cells lowers susceptibility to experimental autoimmune encephalomyelitis, consistent with the reduced frequency of Th17 cells, despite an increase in Th1 cells in mice. Loss of Casz1 in the context of mucosal Candida infection severely impairs Th17 and Treg responses, and lowers the ability of the mice to clear the secondary infection. Importantly, in both the models, absence of Casz1 causes a significant diminution in IFN-γ+IL-17A+ double-positive inflammatory Th17 cells (Th1* cells) in tissues in vivo. Transcriptome analyses of CD4+ T cells lacking Casz1 show a signature consistent with defective Th17 differentiation. With regards to Th17 differentiation, Casz1 limits repressive histone marks and enables acquisition of permissive histone marks at Rorc, Il17a, Ahr, and Runx1 loci. Taken together, these data identify Casz1 as a new Th plasticity regulator having important clinical implications for autoimmune inflammation and mucosal immunity.

Neural Lineage Progression Controlled by a Temporal Proliferation Program

Great progress has been made in identifying transcriptional programs that establish stem cell identity. In contrast, we have limited insight into how these programs are down-graded in a timely manner to halt proliferation and allow for cellular differentiation. Drosophila embryonic neuroblasts undergo such a temporal progression, initially dividing to bud off daughters that divide once (type I), then switching to generating non-dividing daughters (type 0), and finally exiting the cell cycle. We identify six early transcription factors that drive neuroblast and type I daughter proliferation. Early factors are gradually replaced by three late factors, acting to trigger the type I→0 daughter proliferation switch and eventually to stop neuroblasts. Early and late factors regulate each other and four key cell-cycle genes, providing a logical genetic pathway for these transitions. The identification of this extensive driver-stopper temporal program controlling neuroblast lineage progression may have implications for studies in many other systems.

Stem Cell-Intrinsic, Seven-up-Triggered Temporal Factor Gradients Diversify Intermediate Neural Progenitors

Building a sizable, complex brain requires both cellular expansion and diversification. One mechanism to achieve these goals is production of multiple transiently amplifying intermediate neural progenitors (INPs) from a single neural stem cell. Like mammalian neural stem cells, Drosophila type II neuroblasts utilize INPs to produce neurons and glia. Within a given lineage, the consecutively born INPs produce morphologically distinct progeny, presumably due to differential inheritance of temporal factors. To uncover the underlying temporal fating mechanisms, we profiled type II neuroblasts' transcriptome across time. Our results reveal opposing temporal gradients of Imp and Syp RNA-binding proteins (descending and ascending, respectively). Maintaining high Imp throughout serial INP production expands the number of neurons and glia with early temporal fate at the expense of cells with late fate. Conversely, precocious upregulation of Syp reduces the number of cells with early fate. Furthermore, we reveal that the transcription factor Seven-up initiates progression of the Imp/Syp gradients. Interestingly, neuroblasts that maintain initial Imp/Syp levels can still yield progeny with a small range of early fates. We therefore propose that the Seven-up-initiated Imp/Syp gradients create coarse temporal windows within type II neuroblasts to pattern INPs, which subsequently undergo fine-tuned subtemporal patterning.

Neuronal cell fate specification by the molecular convergence of different spatio-temporal cues on a common initiator terminal selector gene

The extensive genetic regulatory flows underlying specification of different neuronal subtypes are not well understood at the molecular level. The Nplp1 neuropeptide neurons in the developing Drosophila nerve cord belong to two sub-classes; Tv1 and dAp neurons, generated by two distinct progenitors. Nplp1 neurons are specified by spatial cues; the Hox homeotic network and GATA factor grn, and temporal cues; the hb -> Kr -> Pdm -> cas -> grh temporal cascade. These spatio-temporal cues combine into two distinct codes; one for Tv1 and one for dAp neurons that activate a common terminal selector feedforward cascade of col -> ap/eya -> dimm -> Nplp1. Here, we molecularly decode the specification of Nplp1 neurons, and find that the cis-regulatory organization of col functions as an integratory node for the different spatio-temporal combinatorial codes. These findings may provide a logical framework for addressing spatio-temporal control of neuronal sub-type specification in other systems.

Integration of temporal and spatial patterning generates neural diversity

In the Drosophila optic lobes, 800 retinotopically organized columns in the medulla act as functional units for processing visual information. The medulla contains over 80 types of neuron, which belong to two classes: uni-columnar neurons have a stoichiometry of one per column, while multi-columnar neurons contact multiple columns. Here we show that combinatorial inputs from temporal and spatial axes generate this neuronal diversity: all neuroblasts switch fates over time to produce different neurons; the neuroepithelium that generates neuroblasts is also subdivided into six compartments by the expression of specific factors. Uni-columnar neurons are produced in all spatial compartments independently of spatial input; they innervate the neuropil where they are generated. Multi-columnar neurons are generated in smaller numbers in restricted compartments and require spatial input; the majority of their cell bodies subsequently move to cover the entire medulla. The selective integration of spatial inputs by a fixed temporal neuroblast cascade thus acts as a powerful mechanism for generating neural diversity, regulating stoichiometry and the formation of retinotopy.

Neuronal cell fate diversification controlled by sub-temporal action of Kruppel

During Drosophila embryonic nervous system development, neuroblasts express a programmed cascade of five temporal transcription factors that govern the identity of cells generated at different time-points. However, these five temporal genes fall short of accounting for the many distinct cell types generated in large lineages. Here, we find that the late temporal gene castor sub-divides its large window in neuroblast 5–6 by simultaneously activating two cell fate determination cascades and a sub-temporal regulatory program. The sub-temporal program acts both upon itself and upon the determination cascades to diversify the castor window. Surprisingly, the early temporal gene Kruppel acts as one of the sub-temporal genes within the late castor window. Intriguingly, while the temporal gene castor activates the two determination cascades and the sub-temporal program, spatial cues controlling cell fate in the latter part of the 5–6 lineage exclusively act upon the determination cascades.

DOI:http://dx.doi.org/10.7554/eLife.19311.001

As a nervous system develops, stem cells generate different types of nerve cells at different times. This series of events follows a fixed schedule in developing embryos, and even a single stem cell that is removed and then grown outside the body will follow the same schedule. This strongly suggests that stem cells have a built-in clock that controls their development.

Studies of the developing nervous system of fruit flies reveal that this clock works by switching genes on in specific sequences, which defines which nerve cells are produced at different stages of development. However, a clock built from the genes that are currently known to be involved in the process is simply not fine-tuned enough to explain how so many different types of nerve cell develop at such precise times. This implies that scientists do not yet know all of the genes that are involved.

Using genetic experiments in stem cells from fruit flies, Stratmann, Gabilondo et al. now identify additional clock genes that act to divide broad windows of time during development into smaller, more precise ones. Genes that define broad windows of time switch on the “small window” genes at specific times – a bit like large cogs turning small cogs in a clock. One small window gene, called Kruppel, works at different stages of development and it is possible that other small window genes multi-task in similar ways in other developmental clocks, such as those found in more complex organisms like humans.

It is clear that many genes work in sequence in the developing nervous system to ensure that developmental stages happen at precise times. Stratmann, Gabilondo et al. will next investigate the molecular details of this timing, specifically how genes in sequential time windows connect together like cogs in the developmental clock.

DOI:http://dx.doi.org/10.7554/eLife.19311.002

Zinc finger transcription factor CASZ1 interacts with histones, DNA repair proteins and recruits NuRD complex to regulate gene transcription

The zinc finger transcription factor CASZ1 has been found to control neural fate-determination in flies, regulate murine and frog cardiac development, control murine retinal cell progenitor expansion and function as a tumor suppressor gene in humans. However, the molecular mechanism by which CASZ1 regulates gene transcription to exert these diverse biological functions has not been described. Here we identify co-factors that are recruited by CASZ1b to regulate gene transcription using co-immunoprecipitation (co-IP) and mass spectrometry assays. We find that CASZ1b binds to the nucleosome remodeling and histone deacetylase (NuRD) complex, histones and DNA repair proteins. Mutagenesis of the CASZ1b protein assay demonstrates that the N-terminus of CASZ1b is required for NuRD binding, and a poly(ADP-ribose) binding motif in the CASZ1b protein is required for histone H3 and DNA repair proteins binding. The N-terminus of CASZ1b fused to an artificial DNA-binding domain (GAL4DBD) causes a significant repression of transcription (5xUAS-luciferase assay), which could be blocked by treatment with an HDAC inhibitor. Realtime PCR results show that the transcriptional activity of CASZ1b mutants that abrogate NuRD or histone H3/DNA binding is significantly decreased. This indicates a model in which CASZ1b binds to chromatin and recruits NuRD complexes to orchestrate epigenetic-mediated transcriptional programs.

Identification of CASZ1 NES reveals potential mechanisms for loss of CASZ1 tumor suppressor activity in neuroblastoma

As a transcription factor, localization to the nucleus and the recruitment of cofactors to regulate gene transcription is essential. Nuclear localization and nucleosome remodeling and histone deacetylase (NuRD) complex binding are required for the zinc-finger transcription factor CASZ1 to function as a neuroblastoma (NB) tumor suppressor. However, the critical amino acids (AAs) that are required for CASZ1 interaction with NuRD complex and the regulation of CASZ1 subcellular localization have not been characterized. Through alanine scanning, immunofluorescence cell staining and co-immunoprecipitation, we define a critical region at the CASZ1 N terminus (AAs 23-40) that mediates the CASZ1b nuclear localization and NuRD interaction. Furthermore, we identified a nuclear export signal (NES) at the N terminus (AAs 176-192) that contributes to CASZ1 nuclear-cytoplasmic shuttling in a chromosomal maintenance 1-dependent manner. An analysis of CASZ1 protein expression in a primary NB tissue microarray shows that high nuclear CASZ1 staining is detected in tumor samples from NB patients with good prognosis. In contrast, cytoplasmic-restricted CASZ1 staining or low nuclear CASZ1 staining is found in tumor samples from patients with poor prognosis. These findings provide insight into mechanisms by which CASZ1 regulates transcription, and suggests that regulation of CASZ1 subcellular localization may impact its function in normal development and pathologic conditions such as NB tumorigenesis.

Neuronal Cell Fate Specification by the Convergence of Different Spatiotemporal Cues on a Common Terminal Selector Cascade

Specification of the myriad of unique neuronal subtypes found in the nervous system depends upon spatiotemporal cues and terminal selector gene cascades, often acting in sequential combinatorial codes to determine final cell fate. However, a specific neuronal cell subtype can often be generated in different parts of the nervous system and at different stages, indicating that different spatiotemporal cues can converge on the same terminal selectors to thereby generate a similar cell fate. However, the regulatory mechanisms underlying such convergence are poorly understood. The Nplp1 neuropeptide neurons in the Drosophila ventral nerve cord can be subdivided into the thoracic-ventral Tv1 neurons and the dorsal-medial dAp neurons. The activation of Nplp1 in Tv1 and dAp neurons depends upon the same terminal selector cascade: col>ap/eya>dimm>Nplp1. However, Tv1 and dAp neurons are generated by different neural progenitors (neuroblasts) with different spatiotemporal appearance. Here, we find that the same terminal selector cascade is triggered by Kr/pdm>grn in dAp neurons, but by Antp/hth/exd/lbe/cas in Tv1 neurons. Hence, two different spatiotemporal combinations can funnel into a common downstream terminal selector cascade to determine a highly related cell fate.

A study of neuropeptide neurons in the Drosophila nervous system reveals that two different combinations of spatiotemporal cues—active in different progenitors—converge on a common terminal selector gene to trigger a similar neuronal subtype identity.

A fundamental challenge in developmental neurobiology is to understand how the great diversity of neuronal subtypes is generated during nervous system development. Neuronal subtype cell fate is established in a stepwise manner, starting with spatial and temporal cues that confer distinct identities to neural progenitors and trigger expression of terminal selector genes in the early-born neurons. Terminal selectors are those that determine the final neuronal subtype cell fate. Intriguingly, similar neuronal subtypes can be generated by different progenitors and under the control of different spatiotemporal cues; thus, we wondered how such convergence is achieved. To address this issue, we have decoded the specification of two highly related neuropeptide neurons, which are generated at different locations and time-points in the Drosophila nervous system. We find that two different combinations of spatiotemporal cues, in two different neural progenitors, funnel onto the same terminal selector gene, which in turn activates a shared regulatory cascade, ultimately resulting in the specification of a similar neuronal cell subtype identity.

Zinc finger transcription factor Casz1 expression is regulated by homeodomain transcription factor Prrxl1 in embryonic spinal dorsal horn late-born excitatory interneurons

The transcription factor Casz1 is required for proper assembly of vertebrate vasculature and heart morphogenesis as well as for temporal control of Drosophila neuroblasts and mouse retina progenitors in the generation of different cell types. Although Casz1 function in the mammalian nervous system remains largely unexplored, Casz1 is expressed in several regions of this system. Here we provide a detailed spatiotemporal characterization of Casz1 expression along mouse dorsal root ganglion (DRG) and dorsal spinal cord development by immunochemistry. In the DRG, Casz1 is broadly expressed in sensory neurons since they are born until perinatal age. In the dorsal spinal cord, Casz1 displays a more dynamic pattern being first expressed in dorsal interneuron 1 (dI1) progenitors and their derived neurons and then in a large subset of embryonic dorsal late-born excitatory (dILB) neurons that narrows gradually to become restricted perinatally to the inner portion. Strikingly, expression analyses using Prrxl1-knockout mice revealed that Prrxl1, a key transcription factor in the differentiation of dILB neurons, is a positive regulator of Casz1 expression in the embryonic dorsal spinal cord but not in the DRG. By performing chromatin immunoprecipitation in the dorsal spinal cord, we identified two Prrxl1-bound regions within Casz1 introns, suggesting that Prrxl1 directly regulates Casz1 transcription. Our work reveals that Casz1 lies downstream of Prrxl1 in the differentiation pathway of a large subset of dILB neurons and provides a framework for further studies of Casz1 in assembly of the DRG-spinal circuit.

Sequence-specific interaction between ABD-B homeodomain and castor gene in Drosophila

We have examined the effect of bithorax complex genes on the expression of castor gene. During the embryonic stages 12-15, both Ultrabithorax and abdominal-A regulated the castor gene expression negatively, whereas Abdominal-B showed a positive correlation with the castor gene expression according to real-time PCR. To investigate whether ABD-B protein directly interacts with the castor gene, electrophoretic mobility shift assays were performed using the recombinant ABD-B homeodomain and oligonucleotides, which are located within the region 10 kb upstream of the castor gene. The results show that ABD-B protein directly binds to the castor gene specifically. ABD-B binds more strongly to oligonucleotides containing two 5'-TTAT-3' canonical core motifs than the probe containing the 5'-TTAC-3' motif. In addition, the sequences flanking the core motif are also involved in the protein-DNA interaction. The results demonstrate the importance of HD for direct binding to target sequences to regulate the expression level of the target genes. [BMB Reports 2014; 47(2): 92-97]

Essential role of the zinc finger transcription factor Casz1 for mammalian cardiac morphogenesis and development

Chromosome 1p36 deletion syndrome is one of the most common terminal deletions observed in humans and is related to congenital heart disease (CHD). However, the 1p36 genes that contribute to heart disease have not been clearly delineated. Human CASZ1 gene localizes to 1p36 and encodes a zinc finger transcription factor. Casz1 is required for Xenopus heart ventral midline progenitor cell differentiation. Whether Casz1 plays a role during mammalian heart development is unknown. Our aim is to determine 1p36 gene CASZ1 function at regulating heart development in mammals. We generated a Casz1 knock-out mouse using Casz1-trapped embryonic stem cells. Casz1 deletion in mice resulted in abnormal heart development including hypoplasia of myocardium, ventricular septal defect, and disorganized morphology. Hypoplasia of myocardium was caused by decreased cardiomyocyte proliferation. Comparative genome-wide RNA transcriptome analysis of Casz1 depleted embryonic hearts identifies abnormal expression of genes that are critical for muscular system development and function, such as muscle contraction genes TNNI2, TNNT1, and CKM; contractile fiber gene ACTA1; and cardiac arrhythmia associated ion channel coding genes ABCC9 and CACNA1D. The transcriptional regulation of some of these genes by Casz1 was also found in cellular models. Our results showed that loss of Casz1 during mouse development led to heart defect including cardiac noncompaction and ventricular septal defect, which phenocopies 1p36 deletion syndrome related CHD. This suggests that CASZ1 is a novel 1p36 CHD gene and that the abnormal expression of cardiac morphogenesis and contraction genes induced by loss of Casz1 contributes to the heart defect.

Neoblast specialization in regeneration of the planarian Schmidtea mediterranea

Planarians can regenerate any missing body part in a process requiring dividing cells called neoblasts. Historically, neoblasts have largely been considered a homogeneous stem cell population. Most studies, however, analyzed neoblasts at the population rather than the single-cell level, leaving the degree of heterogeneity in this population unresolved. We combined RNA sequencing of neoblasts from wounded planarians with expression screening and identified 33 transcription factors transcribed in specific differentiated cells and in small fractions of neoblasts during regeneration. Many neoblast subsets expressing distinct tissue-associated transcription factors were present, suggesting candidate specification into many lineages. Consistent with this possibility, klf, pax3/7, and FoxA were required for the differentiation of cintillo-expressing sensory neurons, dopamine-β-hydroxylase-expressing neurons, and the pharynx, respectively. Together, these results suggest that specification of cell fate for most-to-all regenerative lineages occurs within neoblasts, with regenerative cells of blastemas being generated from a highly heterogeneous collection of lineage-specified neoblasts.

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Forty-one transcription factors are expressed in subsets of planarian neoblasts

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Specific combinations of transcription factors mark different neoblast subsets

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Specific cell-type regeneration failures follow transcription factor RNAi

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The neoblast population contains many specified progenitors after wounding

Congenital heart disease protein 5 associates with CASZ1 to maintain myocardial tissue integrity

The identification and characterization of the cellular and molecular pathways involved in the differentiation and morphogenesis of specific cell types of the developing heart are crucial to understanding the process of cardiac development and the pathology associated with human congenital heart disease. Here, we show that the cardiac transcription factor CASTOR (CASZ1) directly interacts with congenital heart disease 5 protein (CHD5), which is also known as tryptophan-rich basic protein (WRB), a gene located on chromosome 21 in the proposed region responsible for congenital heart disease in individuals with Down's syndrome. We demonstrate that loss of CHD5 in Xenopus leads to compromised myocardial integrity, improper deposition of basement membrane, and a resultant failure of hearts to undergo cell movements associated with cardiac formation. We further report that CHD5 is essential for CASZ1 function and that the CHD5-CASZ1 interaction is necessary for cardiac morphogenesis. Collectively, these results establish a role for CHD5 and CASZ1 in the early stages of vertebrate cardiac development.

CASZ1 inhibits cell cycle progression in neuroblastoma by restoring pRb activity

Dysregulation of cell cycle genes such as Cyclin D1 and Chk1 contributes to the undifferentiated phenotype of neuroblastoma (NB). CASZ1 functions as a tumor suppressor in NB; here we sought to determine how loss of CASZ1 contributes to cell cycle dysregulation in NB. CASZ1 restoration in NB cells delays NB cell cycle progression. The earliest changes occur within 8 h of CASZ1 restoration in SY5Y cells with a 2.8-fold increase in the level of p21, an inhibitor of Cdk2/4. By 16 h, there is a 40% decrease in the steady-state levels of Cdk6. Restoration of CASZ1 decreases Cdk2-dependent cyclins A and E protein levels and Cdk4/6-dependent Cyclin D1 protein levels. The restoration of CASZ1 resulted in a decrease in pRb phosphorylation and a significant reduction of E2F transcriptional activity. Subsequent to the changes in the G 1/S transition, induction of CASZ1 results in a decrease in Cyclin B levels and Cdc25c phosphatase levels, an upstream activator of the G 2/M regulator CyclinB:Cdk1. In addition, induction of CASZ1 results in a decrease in the levels of phospho-Chk1, a key M-phase regulatory kinase. Similar results were found in a NB cell line with MYCN amplification. Taken together, this study indicates that restoration of CASZ1 activates pRb in G 1 and inhibits the G 2/M regulators Cyclin B1 and Chk1, leading to a lengthening of NB cell cycle progression and a subsequent decrease in cell proliferation.

The CASZ1/Egfl7 transcriptional pathway is required for RhoA expression in vascular endothelial cells

Vertebrate development depends on the formation of a closed circulatory loop consisting of intricate networks of veins, arteries, and lymphatic vessels. The coordinated participation of multiple molecules including growth factors, transcription factors, extracellular matrix proteins, and regulators of signaling such as small GTPases is essential for eliciting the desired cellular behaviors associated with vascular assembly and morphogenesis. We have recently demonstrated that a novel transcriptional pathway involving activation of the Epidermal Growth Factor-like Domain 7 (Egfl7) gene by the transcription factor CASTOR (CASZ1) is required for blood vessel assembly and lumen morphogenesis. Furthermore, this transcriptional network promotes RhoA expression and subsequent GTPase activity linking transcriptional regulation of endothelial gene expression to direct physiological outputs associated with Rho GTPase signaling, i.e., cell adhesion and cytoskeletal dynamics. Here we will discuss our studies with respect to our current understanding of the mechanisms underlying regulation of RhoA transcription, protein synthesis, and activity.

Klumpfuss controls FMRFamide expression by enabling BMP signaling within the NB5-6 lineage

A number of transcription factors that are expressed within most, if not all, embryonic neuroblast (NB) lineages participate in neural subtype specification. Some have been extensively studied in several NB lineages (e.g. components of the temporal gene cascade) whereas others only within specific NB lineages. To what extent they function in other lineages remains unknown. Klumpfuss (Klu), the Drosophila ortholog of the mammalian Wilms tumor 1 (WT1) protein, is one such transcription factor. Studies in the NB4-2 lineage have suggested that Klu functions to ensure that the two ganglion mother cells (GMCs) in this embryonic NB lineage acquire different fates. Owing to limited lineage marker availability, these observations were made only for the NB4-2 lineage. Recent findings reveal that Klu is necessary for larval neuroblast growth and self-renewal. We have extended the study of Klu to the well-known embryonic NB5-6T lineage and describe a novel role for Klu in the Drosophila embryonic CNS. Our results demonstrate that Klu is expressed specifically in the postmitotic Ap4/FMRFa neuron, promoting its differentiation through the initiation of BMP signaling. Our findings indicate a pleiotropic function of Klu in Ap cluster specification in general and particularly in Ap4 neuron differentiation, indicating that Klu is a multitasking transcription factor. Finally, our studies indicate that a transitory downregulation of klu is crucial for the specification of the Ap4/FMRFa neuron. Similar to WT1, klu seems to have either self-renewal or differentiation-promoting functions, depending on the developmental context.

Castor is required for Hedgehog-dependent cell-fate specification and follicle stem cell maintenance in Drosophila oogenesis

Asymmetric division of stem cells results in both self-renewal and differentiation of daughters. Understanding the molecules and mechanisms that govern differentiation of specific cell types from adult tissue stem cells is a major challenge in developmental biology and regenerative medicine. Drosophila follicle stem cells (FSCs) represent an excellent model system to study adult stem cell behavior; however, the earliest stages of follicle cell differentiation remain largely mysterious. Here we identify Castor (Cas) as a nuclear protein that is expressed in FSCs and early follicle cell precursors and then is restricted to differentiated polar and stalk cells once egg chambers form. Cas is required for FSC maintenance and polar and stalk cell fate specification. Eyes absent (Eya) is excluded from polar and stalk cells and represses their fate by inhibiting Cas expression. Hedgehog signaling is essential to repress Eya to allow Cas expression in polar and stalk cells. Finally, we show that the complementary patterns of Cas and Eya reveal the gradual differentiation of polar and stalk precursor cells at the earliest stages of their development. Our studies provide a marker for cell fates in this model and insight into the molecular and cellular mechanisms by which FSC progeny diverge into distinct fates.

Hedgehog signaling acts with the temporal cascade to promote neuroblast cell cycle exit

During the development of the Drosophila nervous system, the developmentally regulated Hedgehog pathway, together with a series of temporal transcription factors, schedules the end of neurogenesis.

In Drosophila postembryonic neuroblasts, transition in gene expression programs of a cascade of transcription factors (also known as the temporal series) acts together with the asymmetric division machinery to generate diverse neurons with distinct identities and regulate the end of neuroblast proliferation. However, the underlying mechanism of how this “temporal series” acts during development remains unclear. Here, we show that Hh signaling in the postembryonic brain is temporally regulated; excess (earlier onset of) Hh signaling causes premature neuroblast cell cycle exit and under-proliferation, whereas loss of Hh signaling causes delayed cell cycle exit and excess proliferation. Moreover, the Hh pathway functions downstream of Castor but upstream of Grainyhead, two components of the temporal series, to schedule neuroblast cell cycle exit. Interestingly, hh is likely a target of Castor. Hence, Hh signaling provides a link between the temporal series and the asymmetric division machinery in scheduling the end of neurogenesis.

In almost all metazoans, neurons are produced by a group of neural stem cells/progenitors in a precise temporal manner, which is important for generating a functional nervous system. In Drosophila, this “timing” mechanism is mainly governed by the sequential switching of transcription factors in neural stem cells called neuroblasts, such that neuronal fate is associated with its birth order. These temporal factors also coordinate the termination of neuroblast division towards the end of neurogenesis. In this study, we show that Hedgehog (Hh) signaling also regulates the division rate of neuroblasts during their proliferative phase at larval stage, as well as the cessation of proliferation at early pupal stage. Excessive Hh signaling causes premature neuroblast cell cycle exit and early termination of neurogenesis, while loss of Hh signaling results in prolonged proliferation of neuroblasts beyond its physiological window. We also find that Hh signaling acts in concert with the temporal transcription factors, and is itself regulated by these factors. We hypothesize that this mode of interaction (temporal transcription factors with developmentally regulated signals like Hh) during neurogenesis could be widely conserved in other organisms.

Temporal patterning of neural progenitors in Drosophila

Drosophila has recently become a powerful model system to understand the mechanisms of temporal patterning of neural progenitors called neuroblasts (NBs). Two different temporal sequences of transcription factors (TFs) have been found to be sequentially expressed in NBs of two different systems: the Hunchback, Krüppel, Pdm1/Pdm2, Castor, and Grainyhead sequence in the Drosophila ventral nerve cord; and the Homothorax, Klumpfuss, Eyeless, Sloppy-paired, Dichaete, and Tailless sequence that patterns medulla NBs. In addition, the intermediate neural progenitors of type II NB lineages are patterned by a different sequence: Dichaete, Grainyhead, and Eyeless. These three examples suggest that temporal patterning of neural precursors by sequences of TFs is a common theme to generate neural diversity. Cross-regulations, including negative feedback regulation and positive feedforward regulation among the temporal factors, can facilitate the progression of the sequence. However, there are many remaining questions to understand the mechanism of temporal transitions. The temporal sequence progression is intimately linked to the progressive restriction of NB competence, and eventually determines the end of neurogenesis. Temporal identity has to be integrated with spatial identity information, as well as with the Notch-dependent binary fate choices, in order to generate specific neuron fates.

The cis-regulatory dynamics of the Drosophila CNS determinant castor are controlled by multiple sub-pattern enhancers

In the developing CNS, unique functional identities among neurons and glia are, in part, established as a result of successive transitions in gene expression programs within neural precursor cells. One of the temporal-identity windows within Drosophila CNS neural precursor cells or neuroblasts (NBs) is marked by the expression of a zinc-finger transcription factor (TF) gene, castor (cas). Our analysis of cis-regulatory DNA within a cas loss-of-function rescue fragment has identified seven enhancers that independently activate reporter transgene expression in specific sub-patterns of the wild-type embryonic cas gene expression domain. Most of these enhancers also regulate different aspects of cas expression within the larval and adult CNS. Phylogenetic footprinting reveals that each enhancer is made up of clusters of highly conserved DNA sequence blocks that are flanked by less-conserved inter-cluster spacer sequences. Comparative analysis of the conserved DNA also reveals that cas enhancers share different combinations of sequence elements and many of these shared elements contain core DNA-binding recognition motifs for characterized temporal-identity TFs. Intra-species alignments show that two of the sub-pattern enhancers originated from an inverted duplication and that this repeat is unique to the cas locus in all sequenced Drosophila species. Finally we show that three of the enhancers differentially require cas function for their wild-type regulatory behavior. Cas limits the expression of one enhancer while two others require cas function for full expression. These studies represent a starting point for the further analysis of cas gene expression and the TFs that regulate it.

Loss of gene function as a consequence of human papillomavirus DNA integration

Integration of the human papillomavirus (HPV) genome into the host chromatin is a characteristic step in cervical carcinogenesis. Integration ensures constitutive expression of the viral oncogenes E6 and E7 which drive carcinogenesis. However, integration has also an impact on host DNA. There is increasing evidence that integration not only occurs in fragile sites and translocation breakpoints but also in transcriptionally active regions. Indeed, a substantial number of integration sites actually disrupt host genes and may thereby affect gene expression. No doubt, even subtle changes in gene expression may influence the cell phenotype but small fold changes are difficult to quantify reliably in biopsy material. We have, therefore, addressed the question whether a complete loss of gene function that is insertional mutagenesis in combination with deletion or epigenetic modification of the second allele is also a phenomenon pertinent to cervical cancer. Out of the ten preselected squamous cell carcinomas analyzed, all viral integration sites were located within the intron sequences of known genes, giving rise to viral-cellular fusion transcripts of sense orientation. Moreover, for two tumors, we provide evidence for complete functional loss of the gene affected by HPV integration. Of particular note is that one of the genes involved is the recently described novel tumor suppressor gene castor zinc finger 1. Although our study provides no functional proof that any of the genes affected by HPV integration are causally involved in the transformation process, an exhaustive systematic look at the role of insertional mutagenesis in cervical cancer appears to be warranted.

Characterization of critical domains within the tumor suppressor CASZ1 required for transcriptional regulation and growth suppression

CASZ1 is a zinc finger (ZF) transcription factor that is critical for controlling the normal differentiation of subtypes of neural and cardiac muscle cells. In neuroblastoma tumors, loss of CASZ1 is associated with poor prognosis and restoration of CASZ1 function suppresses neuroblastoma tumorigenicity. However, the key domains by which CASZ1 transcription controls developmental processes and neuroblastoma tumorigenicity have yet to be elucidated. In this study, we show that loss of any one of ZF1 to ZF4 resulted in a 58 to 79% loss in transcriptional activity, as measured by induction of tyrosine hydroxylase promoter-luciferase activity, compared to that of wild-type (WT) CASZ1b. Mutation of ZF5 or deletion of the C-terminal sequence of amino acids (aa) 728 to 1166 (a truncation of 38% of the protein) does not significantly alter transcriptional function. A series of N-terminal truncations reveals a critical transcriptional activation domain at aa 31 to 185 and a nuclear localization signal at aa 23 to 29. Soft agar colony formation assays and xenograft studies show that WT CASZ1b is more active in suppressing neuroblastoma growth than CASZ1b with a ZF4 mutation or a deletion of aa 31 to 185. This study identifies key domains needed for CASZ1b to regulate gene transcription. Furthermore, we establish a link between loss of CASZ1b transcriptional activity and attenuation of CASZ1b-mediated inhibition of neuroblastoma growth and tumorigenicity.

Seven up acts as a temporal factor during two different stages of neuroblast 5-6 development

Drosophila embryonic neuroblasts generate different cell types at different time points. This is controlled by a temporal cascade of Hb→Kr→Pdm→Cas→Grh, which acts to dictate distinct competence windows sequentially. In addition, Seven up (Svp), a member of the nuclear hormone receptor family, acts early in the temporal cascade, to ensure the transition from Hb to Kr, and has been referred to as a ‘switching factor’. However, Svp is also expressed in a second wave within the developing CNS, but here, the possible role of Svp has not been previously addressed. In a genetic screen for mutants affecting the last-born cell in the embryonic NB5-6T lineage, the Ap4/FMRFamide neuron, we have isolated a novel allele of svp. Expression analysis shows that Svp is expressed in two distinct pulses in NB5-6T, and mutant analysis reveals that svp plays two distinct roles. In the first pulse, svp acts to ensure proper downregulation of Hb. In the second pulse, which occurs in a Cas/Grh double-positive window, svp acts to ensure proper sub-division of this window. These studies show that a temporal factor may play dual roles, acting at two different stages during the development of one neural lineage.

EZH2 Mediates epigenetic silencing of neuroblastoma suppressor genes CASZ1, CLU, RUNX3, and NGFR

Neuroblastoma (NB) is the most common extracranial pediatric solid tumor with an undifferentiated status and generally poor prognosis, but the basis for these characteristics remains unknown. In this study, we show that upregulation of the Polycomb protein histone methyltransferase EZH2, which limits differentiation in many tissues, is critical to maintain the undifferentiated state and poor prognostic status of NB by epigenetic repression of multiple tumor suppressor genes. We identified this role for EZH2 by examining the regulation of CASZ1, a recently identified NB tumor suppressor gene whose ectopic restoration inhibits NB cell growth and induces differentiation. Reducing EZH2 expression by RNA interference-mediated knockdown or pharmacologic inhibiton with 3-deazaneplanocin A increased CASZ1 expression, inhibited NB cell growth, and induced neurite extension. Similarly, EZH2(-/-) mouse embryonic fibroblasts (MEF) displayed 3-fold higher levels of CASZ1 mRNA compared with EZH2(+/+) MEFs. In cells with increased expression of CASZ1, treatment with histone deacetylase (HDAC) inhibitor decreased expression of EZH2 and the Polycomb Repressor complex component SUZ12. Under steady-state conditions, H3K27me3 and PRC2 components bound to the CASZ1 gene were enriched, but this enrichment was decreased after HDAC inhibitor treatment. We determined that the tumor suppressors CLU, NGFR, and RUNX3 were also directly repressed by EZH2 like CASZ1 in NB cells. Together, our findings establish that aberrant upregulation of EZH2 in NB cells silences several tumor suppressors, which contribute to the genesis and maintenance of the undifferentiated phenotype of NB tumors.

Use of a Drosophila genome-wide conserved sequence database to identify functionally related cis-regulatory enhancers

Background: Phylogenetic footprinting has revealed that cis-regulatory enhancers consist of conserved DNA sequence clusters (CSCs). Currently, there is no systematic approach for enhancer discovery and analysis that takes full-advantage of the sequence information within enhancer CSCs. Results: We have generated a Drosophila genome-wide database of conserved DNA consisting of >100,000 CSCs derived from EvoPrints spanning over 90% of the genome. cis-Decoder database search and alignment algorithms enable the discovery of functionally related enhancers. The program first identifies conserved repeat elements within an input enhancer and then searches the database for CSCs that score highly against the input CSC. Scoring is based on shared repeats as well as uniquely shared matches, and includes measures of the balance of shared elements, a diagnostic that has proven to be useful in predicting cis-regulatory function. To demonstrate the utility of these tools, a temporally-restricted CNS neuroblast enhancer was used to identify other functionally related enhancers and analyze their structural organization. Conclusions:cis-Decoder reveals that co-regulating enhancers consist of combinations of overlapping shared sequence elements, providing insights into the mode of integration of multiple regulating transcription factors. The database and accompanying algorithms should prove useful in the discovery and analysis of enhancers involved in any developmental process. Developmental Dynamics 241:169–189, 2012. © 2011 Wiley Periodicals, Inc.

CASZ1b, the short isoform of CASZ1 gene, coexpresses with CASZ1a during neurogenesis and suppresses neuroblastoma cell growth

In Drosophila, the CASZ1 (castor) gene encodes a zinc finger transcription factor and is a neural fate-determination gene. In mammals, the CASZ1 gene encodes two major isoforms, CASZ1a with 11 zinc fingers and CASZ1b with 5 zinc fingers. CASZ1b is more evolutionally conserved since it is the only homologue found in drosophila and Xenopus. Our previous study showed that full length CASZ1 (CASZ1a) functions to suppress growth in neuroblastoma tumor. However, the function of CASZ1b isoform in mammals is unknown. In this study, realtime PCR analyses indicate that mouse CASZ1b (mCASZ1b) is dynamically expressed during neurogenesis. CASZ1b and CASZ1a co-exist in all the neuronal tissues but exhibit distinct expression patterns spatially and temporally during brain development. CASZ1b and CASZ1a expression is coordinately upregulated by the differentiation agent Retinoic Acid, as well as agents that modify the epigenome in neural crest derived neuroblastoma cell lines. In contrast CASZ1b is down regulated while CASZ1a is upregulated by agents that raise intracellular cAMP levels. CASZ1b and CASZ1a have no synergistic or antagonistic activities on the regulation of their target NGFR gene transcription. Specific restoration of CASZ1b in NB cells suppresses tumor growth in vitro and in vivo. Consistent with its function role, we find that low CASZ1b expression is significantly associated with decreased survival probability of neuroblastoma patients (p<0.02). This study indicates that although their mechanisms of regulation may be distinct, both CASZ1b and CASZ1a have largely redundant but critical roles in suppressing tumor cell growth.

CASZ1, a candidate tumor-suppressor gene, suppresses neuroblastoma tumor growth through reprogramming gene expression

Neuroblastoma (NB) is a common childhood malignant tumor of the neural crest-derived sympathetic nervous system. In NB the frequent loss of heterozygosity (LOH) on chromosome 1p raises the possibility that this region contains tumor-suppressor genes whose inactivation contributes to tumorigenesis. The human homolog of the Drosophila neural fate determination gene CASZ1, a zinc-finger transcription factor, maps to chromosome 1p36.22, a region implicated in NB tumorigenesis. Quantitative real-time PCR analysis showed that low-CASZ1 expression is significantly correlated with increased age (≥18 months), Children's Oncology Group high-risk classification, 1p LOH and MYCN amplification (all P<0.0002) and decreased survival probability (P=0.0009). CASZ1 was more highly expressed in NB with a differentiated histopathology (P<0.0001). Retinoids and epigenetic modification agents associated with regulation of differentiation induced CASZ1 expression. Expression profiling analysis revealed that CASZ1 regulates the expression of genes involved in regulation of cell growth and developmental processes. Specific restoration of CASZ1 in NB cells induced cell differentiation, enhanced cell adhesion, inhibited migration and suppressed tumorigenicity. These data are consistent with CASZ1 being a critical modulator of neural cell development, and that somatically acquired disruption of normal CASZ1 expression contributes to the malignant phenotype of human NB.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

Coincidence of P-insertion sites and breakpoints of deletions induced by activating P elements in Drosophila

We isolated a set of seven deletions in the 67B region by activating a nearby P-element insertion. The structures of the deletions were characterized by cloning and sequencing. The results showed that the P-induced deletions occurred nonrandomly in the genomic sites. One breakpoint of the deletions was located precisely at the end of the starting element, i.e., at the end of the inverted terminal repeats. The other breakpoint was nearby the retained starting element and coincided with preferential P-element insertion sites that harbor transcription initiation activities. It is known that P elements induce male recombination near the starting elements, giving rise to deletions with one breakpoint precisely located at an inverted terminal repeat of the retained starting element. Database analyses further revealed that deletions generated in P-induced male recombination also contained the other breakpoint in genomic regions that coincided with preferential P-insertion sites. The results suggest that nonrandom distribution of the deletion breakpoints is characteristic of the mechanism by which P elements induce deletions near the starting elements.