DEFECTIVE EMBRYO AND MERISTEMS genes are required for cell division and gamete viability in Arabidopsis.

The DEFECTIVE EMBRYO AND MERISTEMS 1 (DEM1) gene encodes a protein of unknown biochemical function required for meristem formation and seedling development in tomato, but it was unclear whether DEM1's primary role was in cell division or alternatively, in defining the identity of meristematic cells. Genome sequence analysis indicates that flowering plants possess at least two DEM genes. Arabidopsis has two DEM genes, DEM1 and DEM2, which we show are expressed in developing embryos and meristems in a punctate pattern that is typical of genes involved in cell division. Homozygous dem1 dem2 double mutants were not recovered, and plants carrying a single functional DEM1 allele and no functional copies of DEM2, i.e. DEM1/dem1 dem2/dem2 plants, exhibit normal development through to the time of flowering but during male reproductive development, chromosomes fail to align on the metaphase plate at meiosis II and result in abnormal numbers of daughter cells following meiosis. Additionally, these plants show defects in both pollen and embryo sac development, and produce defective male and female gametes. In contrast, dem1/dem1 DEM2/dem2 plants showed normal levels of fertility, indicating that DEM2 plays a more important role than DEM1 in gamete viability. The increased importance of DEM2 in gamete viability correlated with higher mRNA levels of DEM2 compared to DEM1 in most tissues examined and particularly in the vegetative shoot apex, developing siliques, pollen and sperm. We also demonstrate that gamete viability depends not only on the number of functional DEM alleles inherited following meiosis, but also on the number of functional DEM alleles in the parent plant that undergoes meiosis. Furthermore, DEM1 interacts with RAS-RELATED NUCLEAR PROTEIN 1 (RAN1) in yeast two-hybrid and pull-down binding assays, and we show that fluorescent proteins fused to DEM1 and RAN1 co-localize transiently during male meiosis and pollen development. In eukaryotes, RAN is a highly conserved GTPase that plays key roles in cell cycle progression, spindle assembly during cell division, reformation of the nuclear envelope following cell division, and nucleocytoplasmic transport. Our results demonstrate that DEM proteins play an essential role in cell division in plants, most likely through an interaction with RAN1.

Cytoplasmic Ca2+ changes dynamically during the interaction of the pollen tube with synergid cells.

The directional growth of the pollen tube from the stigma to the embryo sac in the ovules is regulated by pollen-pistil interactions based on intercellular communication. Although pollen tube growth is regulated by the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)), it is not known whether [Ca(2+)](cyt) is involved in pollen tube guidance and reception. Using Arabidopsis expressing the GFP-based Ca(2+)-sensor yellow cameleon 3.60 (YC3.60) in pollen tubes and synergid cells, we monitored Ca(2+) dynamics in these cells during pollen tube guidance and reception under semi-in vivo fertilization conditions. In the pollen tube growing towards the micropyle, pollen tubes initiated turning within 150 µm of the micropylar opening; the [Ca(2+)](cyt) in these pollen tube tips was higher than in those not growing towards an ovule in assays with myb98 mutant ovules, in which pollen tube guidance is disrupted. These results suggest that attractants secreted from the ovules affect Ca(2+) dynamics in the pollen tube. [Ca(2+)](cyt) in synergid cells did not change when the pollen tube grew towards the micropyle or entered the ovule. Upon pollen tube arrival at the synergid cell, however, [Ca(2+)](cyt) oscillation began at the micropylar pole of the synergid, spreading towards the chalazal pole. Finally, [Ca(2+)](cyt) in the synergid cell reached a maximum at pollen tube rupture. These results suggest that signals from the pollen tube induce Ca(2+) oscillations in synergid cells, and that this Ca(2+) oscillation is involved in the interaction between the pollen tube and synergid cell.

Molecular characterization of the glauce mutant: a central cell-specific function is required for double fertilization in Arabidopsis.

Double fertilization of the egg cell and the central cell by two sperm cells, resulting in the formation of the embryo and the endosperm, respectively, is a defining characteristic of flowering plants. The Arabidopsis thaliana female gametophytic mutant glauce (glc) can exhibit embryo development without any endosperm. Here, we show that in glc mutant embryo sacs one sperm cell successfully fuses with the egg cell but the second sperm cell fails to fuse with the central cell, resulting in single fertilization. Complementation studies using genes from the glc deletion interval identified an unusual genomic locus having homology to BAHD (for BEAT, AHCT, HCBT, and DAT) acyl-transferases with dual transcription units and alternative splicing that could rescue the sterility defect of glc. Expression of these transcripts appears restricted to the central cell, and expression within the central cell is sufficient to restore fertility. We conclude that the central cell actively promotes its own fertilization by the sperm cell through a signaling mechanism involving products of At1g65450. Successful fertilization of the egg cell is not blocked in the glc mutant, suggesting that evolution of double fertilization in flowering plants involved acquisition of specific functions by the central cell to enable its role as a second female gamete.

The Armadillo repeat gene ZAK IXIK promotes Arabidopsis early embryo and endosperm development through a distinctive gametophytic maternal effect.

The proper balance of parental genomic contributions to the fertilized embryo and endosperm is essential for their normal growth and development. The characterization of many gametophytic maternal effect (GME) mutants affecting seed development indicates that there are certain classes of genes with a predominant maternal contribution. We present a detailed analysis of the GME mutant zak ixik (zix), which displays delayed and arrested growth at the earliest stages of embryo and endosperm development. ZIX encodes an Armadillo repeat (Arm) protein highly conserved across eukaryotes. Expression studies revealed that ZIX manifests a GME through preferential maternal expression in the early embryo and endosperm. This parent-of-origin-dependent expression is regulated by neither the histone and DNA methylation nor the DNA demethylation pathways known to regulate some other GME mutants. The ZIX protein is localized in the cytoplasm and nucleus of cells in reproductive tissues and actively dividing root zones. The maternal ZIX allele is required for the maternal expression of miniseed3. Collectively, our results reveal a reproductive function of plant Arm proteins in promoting early seed growth, which is achieved through a distinct GME of ZIX that involves mechanisms for maternal allele-specific expression that are independent of the well-established pathways.

The Chromatin Remodeler CHD4 Sustains Ewing Sarcoma Cell Survival by Controlling Global Chromatin Architecture.

Ewing sarcoma is an aggressive cancer with a defective response to DNA damage leading to an enhanced sensitivity to genotoxic agents. Mechanistically, Ewing sarcoma is driven by the fusion transcription factor EWS-FLI1, which reprograms the tumor cell epigenome. The nucleosome remodeling and deacetylase (NuRD) complex is an important regulator of chromatin function, controlling both gene expression and DNA damage repair, and has been associated with EWS-FLI1 activity. Here, a NuRD-focused CRISPR/Cas9 inactivation screen identified the helicase CHD4 as essential for Ewing sarcoma cell proliferation. CHD4 silencing induced tumor cell death by apoptosis and abolished colony formation. Although CHD4 and NuRD colocalized with EWS-FLI1 at enhancers and super-enhancers, CHD4 promoted Ewing sarcoma cell survival not by modulating EWS-FLI1 activity and its oncogenic gene expression program but by regulating chromatin structure. CHD4 depletion led to a global increase in DNA accessibility and induction of spontaneous DNA damage, resulting in an increased susceptibility to DNA-damaging agents. CHD4 loss delayed tumor growth in vivo, increased overall survival, and combination with PARP inhibition by olaparib treatment further suppressed tumor growth. Collectively, these findings highlight the NuRD subunit CHD4 as a therapeutic target in Ewing sarcoma that can potentiate the antitumor activity of genotoxic agents. SIGNIFICANCE: CRISPR/Cas9 screening in Ewing sarcoma identifies a dependency on CHD4, which is crucial for the maintenance of chromatin architecture to suppress DNA damage and a promising therapeutic target for DNA damage repair-deficient malignancies.

The differentially regulated genes TvQR1 and TvPirin of the parasitic plant Triphysaria exhibit distinctive natural allelic diversity.

BACKGROUND: Plant parasitism represents an extraordinary interaction among flowering plants: parasitic plants use a specialized organ, the haustorium, to invade the host vascular system to deprive host plants of water and nutrients. Various compounds present in exudates of host plants trigger haustorium development. The two most effective haustorium inducing factors (HIFs) known for the parasitic plant Triphysaria versicolor (T. versicolor) are peonidin, an antioxidant flavonoid, and 2,6-dimethoxybenzoquinone (DMBQ), an oxidative stress agent. To date, two genes involved in haustorium initiation in T. versicolor have been identified: TvQR1, a quinone oxidoreductase that generates the active HIF from DMBQ, and TvPirin, a transcription co-factor that regulates several other DMBQ- responsive and -non-responsive genes. While the expression of these genes in response to DMBQ is well characterized, their expression in response to peonidin is not. In addition, the pattern of polymorphisms in these genes is unknown, even though nucleotide changes in TvQR1 and TvPirin may have contributed to the ability of T. versicolor to develop haustoria. To gain insights into these aspects, we investigated their transcriptional responses to HIFs and non-HIF and their natural nucleotide diversity. RESULTS: Here we show that TvQR1 and TvPirin are transcriptionally upregulated by both DMBQ and peonidin in T. versicolor roots. Yet, while TvQR1 also responded to juglone, a non-HIF quinone with toxicity comparable to that of DMBQ, TvPirin did not. We further demonstrate that TvPirin encodes a protein shorter than the one previously reported. In the T. versicolor natural population of Northern California, TvQR1 exhibited remarkably higher molecular diversity and more recombination events than TvPirin, with the highest non-synonymous substitution rate in the substrate recognition and catalytic domain of the TvQR1 protein. CONCLUSION: Our results suggest that TvQR1 and TvPirin have most likely evolved highly distinct roles for haustorium formation. Unlike TvPirin, TvQR1 might have been under diversifying selection to maintain a diverse collection of polymorphisms, which might be related to the recognition of an assortment of HIF and non-HIF quinones as substrates for successful haustorial establishment in a wide range of host plants.

The TvPirin gene is necessary for haustorium development in the parasitic plant Triphysaria versicolor.

The rhizosphere is teemed with organisms that coordinate their symbioses using chemical signals traversing between the host root and symbionts. Chemical signals also mediate interactions between roots of different plants, perhaps the most obvious being those between parasitic Orobanchaceae and their plant hosts. Parasitic plants use specific molecules provided by host roots to initiate the development of haustoria, invasive structures critical for plant parasitism. We took a transcriptomics approach to identify parasitic plant genes associated with host factor recognition and haustorium signaling and previously identified a gene, TvPirin, which is transcriptionally up-regulated in roots of the parasitic plant Triphysaria versicolor after being exposed to the haustorium-inducing molecule 2,6-dimethoxybenzoquinone (DMBQ). Because TvPirin shares homology with proteins associated with environmental signaling in some plants, we hypothesized that TvPirin may function in host factor recognition in parasitic plants. We tested the function of TvPirin in T. versicolor roots using hairpin-mediated RNA interference. Reducing TvPirin transcripts in T. versicolor roots resulted in significantly less haustoria development in response to DMBQ exposure. We determined the transcript levels of other root expressed transcripts and found that several had reduced basal levels of gene expression but were similarly regulated by quinone exposure. Phylogenic investigations showed that TvPirin homologs are present in most flowering plants, and we found no evidence of parasite-specific gene duplication or expansion. We propose that TvPirin is a generalized transcription factor associated with the expression of a number of genes, some of which are involved in haustorium development.

PAX3-FOXO1 uses its activation domain to recruit CBP/P300 and shape RNA Pol2 cluster distribution.

Activation of oncogenic gene expression from long-range enhancers is initiated by the assembly of DNA-binding transcription factors (TF), leading to recruitment of co-activators such as CBP/p300 to modify the local genomic context and facilitate RNA-Polymerase 2 (Pol2) binding. Yet, most TF-to-coactivator recruitment relationships remain unmapped. Here, studying the oncogenic fusion TF PAX3-FOXO1 (P3F) from alveolar rhabdomyosarcoma (aRMS), we show that a single cysteine in the activation domain (AD) of P3F is important for a small alpha helical coil that recruits CBP/p300 to chromatin. P3F driven transcription requires both this single cysteine and CBP/p300. Mutants of the cysteine reduce aRMS cell proliferation and induce cellular differentiation. Furthermore, we discover a profound dependence on CBP/p300 for clustering of Pol2 loops that connect P3F to its target genes. In the absence of CBP/p300, Pol2 long range enhancer loops collapse, Pol2 accumulates in CpG islands and fails to exit the gene body. These results reveal a potential novel axis for therapeutic interference with P3F in aRMS and clarify the molecular relationship of P3F and CBP/p300 in sustaining active Pol2 clusters essential for oncogenic transcription.

CRISPR activation screen identifies TGFß-associated PEG10 as a crucial tumor suppressor in Ewing sarcoma.

As the second most common pediatric bone and soft tissue tumor, Ewing sarcoma (ES) is an aggressive disease with a pathognomonic chromosomal translocation t(11;22) resulting in expression of EWS-FLI1, an "undruggable" fusion protein acting as transcriptional modulator. EWS-FLI1 rewires the protein expression in cancer cells by activating and repressing a multitude of genes. The role and contribution of most repressed genes remains unknown to date. To address this, we established a CRISPR activation system in clonal SKNMC cell lines and interrogated a custom focused library covering 871 genes repressed by EWS-FLI1. Among the hits several members of the TGFß pathway were identified, where PEG10 emerged as prime candidate due to its strong antiproliferative effect. Mechanistic investigations revealed that PEG10 overexpression caused cellular dropout via induction of cell death. Furthermore, non-canonical TGFß pathways such as RAF/MEK/ERK, MKK/JNK, MKK/P38, known to lead to apoptosis or autophagy, were highly activated upon PEG10 overexpression. Our study sheds new light onto the contribution of TGFß signalling pathway repression to ES tumorigenesis and suggest that its re-activation might constitute a novel therapeutic strategy.

Inhibition of HDACs reduces Ewing sarcoma tumor growth through EWS-FLI1 protein destabilization.

Oncogenic transcription factors lacking enzymatic activity or targetable binding pockets are typically considered "undruggable". An example is provided by the EWS-FLI1 oncoprotein, whose continuous expression and activity as transcription factor are critically required for Ewing sarcoma tumor formation, maintenance, and proliferation. Because neither upstream nor downstream targets have so far disabled its oncogenic potential, we performed a high-throughput drug screen (HTS), enriched for FDA-approved drugs, coupled to a Global Protein Stability (GPS) approach to identify novel compounds capable to destabilize EWS-FLI1 protein by enhancing its degradation through the ubiquitin-proteasome system. The protein stability screen revealed the dual histone deacetylase (HDAC) and phosphatidylinositol-3-kinase (PI3K) inhibitor called fimepinostat (CUDC-907) as top candidate to modulate EWS-FLI1 stability. Fimepinostat strongly reduced EWS-FLI1 protein abundance, reduced viability of several Ewing sarcoma cell lines and PDX-derived primary cells and delayed tumor growth in a xenograft mouse model, whereas it did not significantly affect healthy cells. Mechanistically, we demonstrated that EWS-FLI1 protein levels were mainly regulated by fimepinostat's HDAC activity. Our study demonstrates that HTS combined to GPS is a reliable approach to identify drug candidates able to modulate stability of EWS-FLI1 and lays new ground for the development of novel therapeutic strategies aimed to reduce Ewing sarcoma tumor progression.

NuRD subunit CHD4 regulates super-enhancer accessibility in rhabdomyosarcoma and represents a general tumor dependency.

The NuRD complex subunit CHD4 is essential for fusion-positive rhabdomyosarcoma (FP-RMS) survival, but the mechanisms underlying this dependency are not understood. Here, a NuRD-specific CRISPR screen demonstrates that FP-RMS is particularly sensitive to CHD4 amongst the NuRD members. Mechanistically, NuRD complex containing CHD4 localizes to super-enhancers where CHD4 generates a chromatin architecture permissive for the binding of the tumor driver and fusion protein PAX3-FOXO1, allowing downstream transcription of its oncogenic program. Moreover, CHD4 depletion removes HDAC2 from the chromatin, leading to an increase and spread of histone acetylation, and prevents the positioning of RNA Polymerase 2 at promoters impeding transcription initiation. Strikingly, analysis of genome-wide cancer dependency databases identifies CHD4 as a general cancer vulnerability. Our findings describe CHD4, a classically defined repressor, as positive regulator of transcription and super-enhancer accessibility as well as establish this remodeler as an unexpected broad tumor susceptibility and promising drug target for cancer therapy.

Phenotypic profiling with a living biobank of primary rhabdomyosarcoma unravels disease heterogeneity and AKT sensitivity.

Cancer therapy is currently shifting from broadly used cytotoxic drugs to patient-specific precision therapies. Druggable driver oncogenes, identified by molecular analyses, are present in only a subset of patients. Functional profiling of primary tumor cells could circumvent these limitations, but suitable platforms are unavailable for most cancer entities. Here, we describe an in vitro drug profiling platform for rhabdomyosarcoma (RMS), using a living biobank composed of twenty RMS patient-derived xenografts (PDX) for high-throughput drug testing. Optimized in vitro conditions preserve phenotypic and molecular characteristics of primary PDX cells and are compatible with propagation of cells directly isolated from patient tumors. Besides a heterogeneous spectrum of responses of largely patient-specific vulnerabilities, profiling with a large drug library reveals a strong sensitivity towards AKT inhibitors in a subgroup of RMS. Overall, our study highlights the feasibility of in vitro drug profiling of primary RMS for patient-specific treatment selection in a co-clinical setting.

The Leukemogenic TCF3-HLF Complex Rewires Enhancers Driving Cellular Identity and Self-Renewal Conferring EP300 Vulnerability.

The chimeric transcription factor TCF3-HLF defines an incurable acute lymphoblastic leukemia subtype. Here we decipher the regulome of endogenous TCF3-HLF and dissect its essential transcriptional components and targets by functional genomics. We demonstrate that TCF3-HLF recruits HLF binding sites at hematopoietic stem cell/myeloid lineage associated (super-) enhancers to drive lineage identity and self-renewal. Among direct targets, hijacking an HLF binding site in a MYC enhancer cluster by TCF3-HLF activates a conserved MYC-driven transformation program crucial for leukemia propagation in vivo. TCF3-HLF pioneers the cooperation with ERG and recruits histone acetyltransferase p300 (EP300), conferring susceptibility to EP300 inhibition. Our study provides a framework for targeting driving transcriptional dependencies in this fatal leukemia.

A calcium dialog mediated by the FERONIA signal transduction pathway controls plant sperm delivery.

Sperm delivery for double fertilization of flowering plants relies on interactions between the pollen tube (PT) and two synergids, leading to programmed cell death (PCD) of the PT and one synergid. The mechanisms underlying the communication among these cells during PT reception is unknown. We discovered that the synergids control this process by coordinating their distinct calcium signatures in response to the calcium dynamics and growth behavior of the PT. Induced and spontaneous aberrant calcium responses in the synergids abolish the two coordinated PCD events. Components of the FERONIA (FER) signaling pathway are required for initiating and modulating these calcium responses and for coupling the PCD events. Intriguingly, the calcium signatures are interchangeable between the two synergids, implying that their fates of death and survival are determined by reversible interactions with the PT. Thus, complex intercellular interactions involving a receptor kinase pathway and calcium-mediated signaling control sperm delivery in plants.

Arabidopsis GLAUCE promotes fertilization-independent endosperm development and expression of paternally inherited alleles.

Early seed development of sexually reproducing plants requires both maternal and paternal genomes but is prominently maternally influenced. A novel gametophytic maternal-effect mutant defective in early embryo and endosperm development, glauce (glc), has been isolated from a population of Arabidopsis Ds transposon insertion lines. The glc mutation results from a deletion at the Ds insertion site, and the molecular identity of GLC is not known. glc embryos can develop up to the globular stage in the absence of endosperm and glc central cells appear to be unfertilized. glc suppresses autonomous endosperm development observed in the fertilization-independent seed (fis) class mutants. glc is also epistatic to mea, one of the fis class mutants, in fertilized seeds, and is essential for the biparental embryonic expression of PHE1, a repressed downstream target of MEA. In addition, maternal GLC function is required for the paternal embryonic expression of the ribosome protein gene RPS5a and the AMP deaminase gene FAC1, both of which are essential for early embryo and endosperm development. These results indicate that factors derived from the female gametophyte activate a subset of the paternal genome of fertilized seeds.

The TORMOZ gene encodes a nucleolar protein required for regulated division planes and embryo development in Arabidopsis.

Embryogenesis in Arabidopsis thaliana is marked by a predictable sequence of oriented cell divisions, which precede cell fate determination. We show that mutation of the TORMOZ (TOZ) gene yields embryos with aberrant cell division planes and arrested embryos that appear not to have established normal patterning. The defects in toz mutants differ from previously described mutations that affect embryonic cell division patterns. Longitudinal division planes of the proembryo are frequently replaced by transverse divisions and less frequently by oblique divisions, while divisions of the suspensor cells, which divide only transversely, appear generally unaffected. Expression patterns of selected embryo patterning genes are altered in the mutant embryos, implying that the positional cues required for their proper expression are perturbed by the misoriented divisions. The TOZ gene encodes a nucleolar protein containing WD repeats. Putative TOZ orthologs exist in other eukaryotes including Saccharomyces cerevisiae, where the protein is predicted to function in 18S rRNA biogenesis. We find that disruption of the Sp TOZ gene results in cell division defects in Schizosaccharomyces pombe. Previous studies in yeast and animal cells have identified nucleolar proteins that regulate the exit from M phase and cytokinesis, including factors involved in pre-rRNA processing. Our study suggests that in plant cells, nucleolar functions might interact with the processes of regulated cell divisions and influence the selection of longitudinal division planes during embryogenesis.