The C. elegans cell death specification gene ces-1 encodes a snail family zinc finger protein

Intron detention tightly regulates the stemness/differentiation switch in the adult neurogenic niche

The adult mammalian brain retains some capacity to replenish neurons and glia, holding promise for brain regeneration. Thus, understanding the mechanisms controlling adult neural stem cell (NSC) differentiation is crucial. Paradoxically, adult NSCs in the subependymal zone transcribe genes associated with both multipotency maintenance and neural differentiation, but the mechanism that prevents conflicts in fate decisions due to these opposing transcriptional programmes is unknown. Here we describe intron detention as such control mechanism. In NSCs, while multiple mRNAs from stemness genes are spliced and exported to the cytoplasm, transcripts from differentiation genes remain unspliced and detained in the nucleus, and the opposite is true under neural differentiation conditions. We also show that m6A methylation is the mechanism that releases intron detention and triggers nuclear export, enabling rapid and synchronized responses. m6A RNA methylation operates as an on/off switch for transcripts with antagonistic functions, tightly controlling the timing of NSCs commitment to differentiation.

Competitive Effect of Overexpressed C-terminal of Snail-1 (CSnail) in Control of the Growth and Metastasis of Melanoma Cells

BACKGROUND

Epithelial-to-mesenchymal transition (EMT) plays a role in the invasion and metastasis of cancer cells. During this phenomenon, Snail can promote tumor progression by upregulating mesenchymal factors and downregulating the expression of pro-apoptotic proteins.

OBJECTIVE

Therefore, interventions on the expression rate of Snails may show beneficial therapeutic applications.

METHODS

In this study, the C-terminal region of Snail1, capable of binding to E-box genomic sequences, was subcloned into the pAAV-IRES-EGFP backbone to make complete AAV-CSnail viral particles. B16F10 as a metastatic melanoma cell line, with a null expression of wild type TP53 was transduced by AAV-CSnail. Moreover, the transduced cells were analyzed for in vitro expression of apoptosis, migration, and EMT-related genes, and in vivo inhibition of metastasis.

RESULTS

In more than 80% of the AAV-CSnail transduced cells, the CSnail gene expression competitively reduced the wild-type Snail functionality and consequently lowered the mRNA expression level of EMT-related genes. Furthermore, the transcription level of cell cycle inhibitory factor p21 and pro-apoptotic factors were promoted. The scratch test showed a decrease in the migration ability of AAV-CSnail transduced group compared to control. Finally, metastasis of cancer cells to lung tissue in the AAV-CSnail-treated B16F10 melanoma mouse model was significantly reduced, pointing out to prevention of EMT by the competitive inhibitory effect of CSnail on Snail1 and increased apoptosis of B16F10 cells.

CONCLUSION

The capability of this successful competition in reducing the growth, invasion, and metastasis of melanoma cells indicates that gene therapy is a promising strategy for the control of the growth and metastasis of cancer cells.

A unique cell population expressing the Epithelial-Mesenchymal Transition-transcription factor Snail moderates microglial and astrocyte injury responses

Insults to the central nervous system (CNS) elicit common glial responses including microglial activation evidenced by functional, morphological, and phenotypic changes, as well as astrocyte reactions including hypertrophy, altered process orientation, and changes in gene expression and function. However, the cellular and molecular mechanisms that initiate and modulate such glial response are less well-defined. Here we show that an adult cortical lesion generates a population of ultrastructurally unique microglial-like cells that express Epithelial-Mesenchymal Transcription factors including Snail. Knockdown of Snail with antisense oligonucleotides results in a postinjury increase in activated microglial cells, elevation in astrocyte reactivity with increased expression of C3 and phagocytosis, disruption of astrocyte junctions and neurovascular structure, increases in neuronal cell death, and reduction in cortical synapses. These changes were associated with alterations in pro-inflammatory cytokine expression. By contrast, overexpression of Snail through microglia-targeted an adeno-associated virus (AAV) improved many of the injury characteristics. Together, our results suggest that the coordination of glial responses to CNS injury is partly mediated by epithelial-mesenchymal transition-factors (EMT-Fsl).

Role of transcription factors in apoptotic cells clearance

The human body generates 10-100 billion cells every day, and the same number of cells die to maintain homeostasis. The genetically controlled, autonomously ordered cell death mainly proceeds by apoptosis. Apoptosis is an important way of programmed cell death in multicellular organisms, timely and effective elimination of apoptotic cells plays a key role in the growth and development of organisms and the maintenance of homeostasis. During the clearance of apoptotic cells, transcription factors bind to specific target promoters and act as activators or repressors to regulate multiple genes expression, how transcription factors regulate apoptosis is an important and poorly understood aspect of normal development. This paper summarizes the regulatory mechanisms of transcription factors in the clearance of apoptotic cells to date.

BLMP-1 promotes developmental cell death in C. elegans by timely repression of ced-9 transcription

Programmed cell death (PCD) is a common cell fate in metazoan development. PCD effectors are extensively studied, but how they are temporally regulated is less understood. Here, we report a mechanism controlling tail-spike cell death onset during Caenorhabditis elegans development. We show that the zinc-finger transcription factor BLMP-1, which controls larval development timing, also regulates embryonic tail-spike cell death initiation. BLMP-1 functions upstream of CED-9 and in parallel to DRE-1, another CED-9 and tail-spike cell death regulator. BLMP-1 expression is detected in the tail-spike cell shortly after the cell is born, and blmp-1 mutations promote ced-9-dependent tail-spike cell survival. BLMP-1 binds ced-9 gene regulatory sequences, and inhibits ced-9 transcription just before cell-death onset. BLMP-1 and DRE-1 function together to regulate developmental timing, and their mammalian homologs regulate B-lymphocyte fate. Our results, therefore, identify roles for developmental timing genes in cell-death initiation, and suggest conservation of these functions.

Involvement of Scratch2 in GalR1-mediated depression-like behaviors in the rat ventral periaqueductal gray

Galanin receptor1 (GalR1) transcript levels are elevated in the rat ventral periaqueductal gray (vPAG) after chronic mild stress (CMS) and are related to depression-like behavior. To explore the mechanisms underlying the elevated GalR1 expression, we carried out molecular biological experiments in vitro and in animal behavioral experiments in vivo. It was found that a restricted upstream region of the GalR1 gene, from -250 to -220, harbors an E-box and plays a negative role in the GalR1 promoter activity. The transcription factor Scratch2 bound to the E-box to down-regulate GalR1 promoter activity and lower expression levels of the GalR1 gene. The expression of Scratch2 was significantly decreased in the vPAG of CMS rats. Importantly, local knockdown of Scratch2 in the vPAG caused elevated expression of GalR1 in the same region, as well as depression-like behaviors. RNAscope analysis revealed that GalR1 mRNA is expressed together with Scratch2 in both GABA and glutamate neurons. Taking these data together, our study further supports the involvement of GalR1 in mood control and suggests a role for Scratch2 as a regulator of depression-like behavior by repressing the GalR1 gene in the vPAG.

The Snail transcription factor CES-1 regulates glutamatergic behavior in C. elegans

Regulation of AMPA-type glutamate receptor (AMPAR) expression and function alters synaptic strength and is a major mechanism underlying synaptic plasticity. Although transcription is required for some forms of synaptic plasticity, the transcription factors that regulate AMPA receptor expression and signaling are incompletely understood. Here, we identify the Snail family transcription factor ces-1 in an RNAi screen for conserved transcription factors that regulate glutamatergic behavior in C. elegans. ces-1 was originally discovered as a selective cell death regulator of neuro-secretory motor neuron (NSM) and I2 interneuron sister cells in C. elegans, and has almost exclusively been studied in the NSM cell lineage. We found that ces-1 loss-of-function mutants have defects in two glutamatergic behaviors dependent on the C. elegans AMPA receptor GLR-1, the mechanosensory nose-touch response and spontaneous locomotion reversals. In contrast, ces-1 gain-of-function mutants exhibit increased spontaneous reversals, and these are dependent on glr-1 consistent with these genes acting in the same pathway. ces-1 mutants have wild type cholinergic neuromuscular junction function, suggesting that they do not have a general defect in synaptic transmission or muscle function. The effect of ces-1 mutation on glutamatergic behaviors is not due to ectopic cell death of ASH sensory neurons or GLR-1-expressing neurons that mediate one or both of these behaviors, nor due to an indirect effect on NSM sister cell deaths. Rescue experiments suggest that ces-1 may act, in part, in GLR-1-expressing neurons to regulate glutamatergic behaviors. Interestingly, ces-1 mutants suppress the increased reversal frequencies stimulated by a constitutively-active form of GLR-1. However, expression of glr-1 mRNA or GFP-tagged GLR-1 was not decreased in ces-1 mutants suggesting that ces-1 likely promotes GLR-1 function. This study identifies a novel role for ces-1 in regulating glutamatergic behavior that appears to be independent of its canonical role in regulating cell death in the NSM cell lineage.

PIG-1 MELK-dependent phosphorylation of nonmuscle myosin II promotes apoptosis through CES-1 Snail partitioning

The mechanism(s) through which mammalian kinase MELK promotes tumorigenesis is not understood. We find that the C. elegans orthologue of MELK, PIG-1, promotes apoptosis by partitioning an anti-apoptotic factor. The C. elegans NSM neuroblast divides to produce a larger cell that differentiates into a neuron and a smaller cell that dies. We find that in this context, PIG-1 MELK is required for partitioning of CES-1 Snail, a transcriptional repressor of the pro-apoptotic gene egl-1 BH3-only. pig-1 MELK is controlled by both a ces-1 Snail- and par-4 LKB1-dependent pathway, and may act through phosphorylation and cortical enrichment of nonmuscle myosin II prior to neuroblast division. We propose that pig-1 MELK-induced local contractility of the actomyosin network plays a conserved role in the acquisition of the apoptotic fate. Our work also uncovers an auto-regulatory loop through which ces-1 Snail controls its own activity through the formation of a gradient of CES-1 Snail protein.

Apoptosis is critical for the elimination of ‘unwanted’ cells. What distinguishes wanted from unwanted cells in developing animals is poorly understood. We report that in the C. elegans NSM neuroblast lineage, the level of CES-1, a Snail-family member and transcriptional repressor of the pro-apoptotic gene egl-1, contributes to this process. In addition, we demonstrate that C. elegans PIG-1, the orthologue of mammalian proto-oncoprotein MELK, plays a critical role in controlling CES-1^Snail levels. Specifically, during NSM neuroblast division, PIG-1^MELK controls partitioning of CES-1^Snail into one but not the other daughter cell thereby promoting the making of one wanted and one unwanted cell. Furthermore, we present evidence that PIG-1^MELK acts prior to NSM neuroblast division by locally activating the actomyosin network.

Evolution of Snail-mediated regulation of neural crest and placodes from an ancient role in bilaterian neurogenesis

A major challenge in vertebrate evolution is to identify the gene regulatory mechanisms that facilitated the origin of neural crest cells and placodes from ancestral precursors in invertebrates. Here, we show in lamprey, a primitively jawless vertebrate, that the transcription factor Snail is expressed simultaneously throughout the neural plate, neural plate border, and pre-placodal ectoderm in the early embryo and is then upregulated in the CNS throughout neurogenesis. Using CRISPR/Cas9-mediated genome editing, we demonstrate that Snail plays functional roles in all of these embryonic domains or their derivatives. We first show that Snail patterns the neural plate border by repressing lateral expansion of Pax3/7 and activating nMyc and ZicA. We also present evidence that Snail is essential for DlxB-mediated establishment of the pre-placodal ectoderm but is not required for SoxB1a expression during formation of the neural plate proper. At later stages, Snail regulates formation of neural crest-derived and placode-derived PNS neurons and controls CNS neural differentiation in part by promoting cell survival. Taken together with established functions of invertebrate Snail genes, we identify a pan-bilaterian mechanism that extends to jawless vertebrates for regulating neurogenesis that is dependent on Snail transcription factors. We propose that ancestral vertebrates deployed an evolutionarily conserved Snail expression domain in the CNS and PNS for neurogenesis and then acquired derived functions in neural crest and placode development by recruitment of regulatory genes downstream of neuroectodermal Snail activity. Our results suggest that Snail regulatory mechanisms in vertebrate novelties such as the neural crest and placodes may have emerged from neurogenic roles that originated early in bilaterian evolution.

A molecular signature for anastasis, recovery from the brink of apoptotic cell death

Cells can survive executioner caspase activation following transient apoptotic stimuli, a process called anastasis. Using whole-transcriptome RNA sequencing, Sun et al. show that anastasis is an active, two-stage program and characterize the cell behaviors and molecular signature involved in the process.

During apoptosis, executioner caspase activity has been considered a point of no return. However, recent studies show that cells can survive caspase activation following transient apoptotic stimuli, a process called anastasis. To identify a molecular signature, we performed whole-transcriptome RNA sequencing of untreated, apoptotic, and recovering HeLa cells. We found that anastasis is an active, two-stage program. During the early stage, cells transition from growth-arrested to growing. In the late stage, HeLa cells change from proliferating to migratory. Recovering cells also exhibited prolonged elevation of proangiogenic factors. Strikingly, some early-recovery mRNAs, including Snail, were elevated first during apoptosis, implying that dying cells poise to recover, even while under apoptotic stress. Snail was also required for recovery. This study reveals similarities in the anastasis genes, pathways, and cell behaviors to those activated in wound healing and identifies a repertoire of potential targets for therapeutic manipulation.

Caenorhabditis elegans CES-1 Snail Represses pig-1 MELK Expression To Control Asymmetric Cell Division

Snail-like transcription factors affect stem cell function through mechanisms that are incompletely understood. In the Caenorhabditis elegans neurosecretory motor neuron (NSM) neuroblast lineage, CES-1 Snail coordinates cell cycle progression and cell polarity to ensure the asymmetric division of the NSM neuroblast and the generation of two daughter cells of different sizes and fates. We have previously shown that CES-1 Snail controls cell cycle progression by repressing the expression of cdc-25.2 CDC25. However, the mechanism through which CES-1 Snail affects cell polarity has been elusive. Here, we systematically searched for direct targets of CES-1 Snail by genome-wide profiling of CES-1 Snail binding sites and identified >3000 potential CES-1 Snail target genes, including pig-1, the ortholog of the oncogene maternal embryonic leucine zipper kinase (MELK). Furthermore, we show that CES-1 Snail represses pig-1 MELK transcription in the NSM neuroblast lineage and that pig-1 MELK acts downstream of ces-1 Snail to cause the NSM neuroblast to divide asymmetrically by size and along the correct cell division axis. Based on our results we propose that by regulating the expression of the MELK gene, Snail-like transcription factors affect the ability of stem cells to divide asymmetrically and, hence, to self-renew. Furthermore, we speculate that the deregulation of MELK contributes to tumorigenesis by causing cells that normally divide asymmetrically to divide symmetrically instead.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

miRNAs cooperate in apoptosis regulation during C. elegans development

Sherrard et al. demonstrate that, during embryogenesis, miR-35 and miR-58 bantam family miRNAs cooperate to prevent the precocious death of mothers of cells programmed to die by repressing the gene egl-1, which encodes a proapoptotic BH3-only protein.

Programmed cell death occurs in a highly reproducible manner during Caenorhabditis elegans development. We demonstrate that, during embryogenesis, miR-35 and miR-58 bantam family microRNAs (miRNAs) cooperate to prevent the precocious death of mothers of cells programmed to die by repressing the gene egl-1, which encodes a proapoptotic BH3-only protein. In addition, we present evidence that repression of egl-1 is dependent on binding sites for miR-35 and miR-58 family miRNAs within the egl-1 3′ untranslated region (UTR), which affect both mRNA copy number and translation. Furthermore, using single-molecule RNA fluorescent in situ hybridization (smRNA FISH), we show that egl-1 is transcribed in the mother of a cell programmed to die and that miR-35 and miR-58 family miRNAs prevent this mother from dying by keeping the copy number of egl-1 mRNA below a critical threshold. Finally, miR-35 and miR-58 family miRNAs can also dampen the transcriptional boost of egl-1 that occurs specifically in a daughter cell that is programmed to die. We propose that miRNAs compensate for lineage-specific differences in egl-1 transcriptional activation, thus ensuring that EGL-1 activity reaches the threshold necessary to trigger death only in daughter cells that are programmed to die.

Size Matters: How C. elegans Asymmetric Divisions Regulate Apoptosis

Apoptosis is a form of programmed cell death used by metazoans to eliminate abnormal cells, control cell number, and shape the development of organs. The use of the nematode Caenorhabditis elegans as a model for the study of apoptosis has led to important insights into how cells die and how their corpses are removed. Eighty percent of these apoptotic cell deaths occur during nervous system development and in daughters of neuroblasts that divide asymmetrically. Pioneering work defined a conserved apoptosis pathway that is initiated in C. elegans by the BH3-only protein EGL-1 and that leads to the activation of the caspase CED-3. While the execution of the apoptotic fate is well understood, much less is known about the mechanisms that specify the apoptotic fate of particular cells. In some cells fated to die, this regulation occurs at the level of the egl-1 gene transcription, and investigators have identified several lineage-specific transcription factors that both positively and negatively regulate egl-1. In this review, we focus on a second set of molecules that appear to influence apoptosis by controlling the position of the cleavage plane in divisions that produce apoptotic cells.

Programmed cell death and clearance of cell corpses in Caenorhabditis elegans

Programmed cell death is critical to the development of diverse animal species from C. elegans to humans. In C. elegans, the cell death program has three genetically distinguishable phases. During the cell suicide phase, the core cell death machinery is activated through a protein interaction cascade. This activates the caspase CED-3, which promotes numerous pro-apoptotic activities including DNA degradation and exposure of the phosphatidylserine "eat me" signal on the cell corpse surface. Specification of the cell death fate involves transcriptional activation of the cell death initiator EGL-1 or the caspase CED-3 by coordinated actions of specific transcription factors in distinct cell types. In the cell corpse clearance stage, recognition of cell corpses by phagocytes triggers several signaling pathways to induce phagocytosis of apoptotic cell corpses. Cell corpse-enclosing phagosomes ultimately fuse with lysosomes for digestion of phagosomal contents. This article summarizes our current knowledge about programmed cell death and clearance of cell corpses in C. elegans.

Characterization of innate immunity genes in the parasitic nematode Brugia malayi

The filarial nematode Brugia malayi is one of the causative agents of lymphatic filariasis, a neglected tropical disease that affects 120 million people worldwide. The limited effectiveness of available anthelmintics and the absence of a vaccine have prompted extensive research on the interaction between Brugia and its obligate bacterial endosymbiont, Wolbachia. Recent studies suggest that Wolbachia is able to manipulate its nematode host immunity but relatively little is known about the immune system of filarial nematodes. Therefore, elucidation of the mechanisms underlying the immune system of B. malayi may be useful for understanding how the symbiotic relationship is maintained and help in the identification of new drug targets. In order to characterize the main genetic pathways involved in B. malayi immunity, we exposed adult female worms to two bacterial lysates (Escherichia coli and Bacillus amyloliquefaciens), dsRNA and dsDNA. We performed transcriptome sequencing of worms exposed to each immune elicitor at two different timepoints. Gene expression analysis of untreated and immune-challenged worms was performed to characterize gene expression patterns associated with each type of immune stimulation. Our results indicate that different immune elicitors produced distinct expression patterns in B. malayi, with changes in the expression of orthologs of well-characterized C. elegans immune pathways such as insulin, TGF-β, and p38 MAPK pathways, as well as C-type lectins and several stress-response genes.

Engulfment pathways promote programmed cell death by enhancing the unequal segregation of apoptotic potential

Components of the conserved engulfment pathways promote programmed cell death in Caenorhabditis elegans (C. elegans) through an unknown mechanism. Here we report that the phagocytic receptor CED-1 mEGF10 is required for the formation of a dorsal–ventral gradient of CED-3 caspase activity within the mother of a cell programmed to die and an increase in the level of CED-3 protein within its dying daughter. Furthermore, CED-1 becomes enriched on plasma membrane regions of neighbouring cells that appose the dorsal side of the mother, which later forms the dying daughter. Therefore, we propose that components of the engulfment pathways promote programmed cell death by enhancing the polar localization of apoptotic factors in mothers of cells programmed to die and the unequal segregation of apoptotic potential into dying and surviving daughters. Our findings reveal a novel function of the engulfment pathways and provide a better understanding of how apoptosis is initiated during C. elegans development.

Programed cell death occurs in a stereotypic fashion during C. elegans development, and it is thought that engulfment promotes programmed cell death. Here the authors present evidence that a signaling function of the conserved engulfment pathways, not the process of engulfment itself, promotes apoptotic cell death.

Cell Death in C. elegans Development

Cell death is a common and important feature of animal development, and cell death defects underlie many human disease states. The nematode Caenorhabditis elegans has proven fertile ground for uncovering molecular and cellular processes controlling programmed cell death. A core pathway consisting of the conserved proteins EGL-1/BH3-only, CED-9/BCL2, CED-4/APAF1, and CED-3/caspase promotes most cell death in the nematode, and a conserved set of proteins ensures the engulfment and degradation of dying cells. Multiple regulatory pathways control cell death onset in C. elegans, and many reveal similarities with tumor formation pathways in mammals, supporting the idea that cell death plays key roles in malignant progression. Nonetheless, a number of observations suggest that our understanding of developmental cell death in C. elegans is incomplete. The interaction between dying and engulfing cells seems to be more complex than originally appreciated, and it appears that key aspects of cell death initiation are not fully understood. It has also become apparent that the conserved apoptotic pathway is dispensable for the demise of the C. elegans linker cell, leading to the discovery of a previously unexplored gene program promoting cell death. Here, we review studies that formed the foundation of cell death research in C. elegans and describe new observations that expand, and in some cases remodel, this edifice. We raise the possibility that, in some cells, more than one death program may be needed to ensure cell death fidelity.

Screening of suppressors of bax-induced cell death identifies glycerophosphate oxidase-1 as a mediator of debcl-induced apoptosis in Drosophila

Members of the Bcl-2 family are key elements of the apoptotic machinery. In mammals, this multigenic family contains about twenty members, which either promote or inhibit apoptosis. We have previously shown that the mammalian pro-apoptotic Bcl-2 family member Bax is very efficient in inducing apoptosis in Drosophila, allowing the study of bax-induced cell death in a genetic animal model. We report here the results of the screening of a P[UAS]-element insertion library performed to identify gene products that modify the phenotypes induced by the expression of bax in Drosophila melanogaster. We isolated 17 putative modifiers involved in various function or process: the ubiquitin/proteasome pathway; cell growth, proliferation and death; pathfinding and cell adhesion; secretion and extracellular signaling; metabolism and oxidative stress. Most of these suppressors also inhibit debcl-induced phenotypes, suggesting that the activities of both proteins can be modulated in part by common signaling or metabolic pathways. Among these suppressors, Glycerophosphate oxidase-1 is found to participate in debcl-induced apoptosis by increasing mitochondrial reactive oxygen species accumulation.

FBXO11 promotes ubiquitination of the Snail family of transcription factors in cancer progression and epidermal development

The Snail family of transcription factors are core inducers of epithelial-to-mesenchymal transition (EMT). Here we show that the F-box protein FBXO11 recognizes and promotes ubiquitin-mediated degradation of multiple Snail family members including Scratch. The association between FBXO11 and Snai1 in vitro is independent of Snai1 phosphorylation. Overexpression of FBXO11 in mesenchymal cells reduces Snail protein abundance and cellular invasiveness. Conversely, depletion of endogenous FBXO11 in epithelial cancer cells causes Snail protein accumulation, EMT, and tumor invasion, as well as loss of estrogen receptor expression in breast cancer cells. Expression of FBXO11 is downregulated by EMT-inducing signals TGFβ and nickel. In human cancer, high FBXO11 levels correlate with expression of epithelial markers and favorable prognosis. The results suggest that FBXO11 sustains the epithelial state and inhibits cancer progression. Inactivation of FBXO11 in mice leads to neonatal lethality, epidermal thickening, and increased Snail protein levels in epidermis, validating that FBXO11 is a physiological ubiquitin ligase of Snail. Moreover, in C. elegans, the FBXO11 mutant phenotype is attributed to the Snail factors as it is suppressed by inactivation/depletion of Snail homologs. Collectively, these findings suggest that the FBXO11-Snail regulatory axis is evolutionarily conserved and critically governs carcinoma progression and mammalian epidermal development.

Coordination of cell proliferation and cell fate determination by CES-1 snail

The coordination of cell proliferation and cell fate determination is critical during development but the mechanisms through which this is accomplished are unclear. We present evidence that the Snail-related transcription factor CES-1 of Caenorhabditis elegans coordinates these processes in a specific cell lineage. CES-1 can cause loss of cell polarity in the NSM neuroblast. By repressing the transcription of the BH3-only gene egl-1, CES-1 can also suppress apoptosis in the daughters of the NSM neuroblasts. We now demonstrate that CES-1 also affects cell cycle progression in this lineage. Specifically, we found that CES-1 can repress the transcription of the cdc-25.2 gene, which encodes a Cdc25-like phosphatase, thereby enhancing the block in NSM neuroblast division caused by the partial loss of cya-1, which encodes Cyclin A. Our results indicate that CDC-25.2 and CYA-1 control specific cell divisions and that the over-expression of the ces-1 gene leads to incorrect regulation of this functional ‘module’. Finally, we provide evidence that dnj-11 MIDA1 not only regulate CES-1 activity in the context of cell polarity and apoptosis but also in the context of cell cycle progression. In mammals, the over-expression of Snail-related genes has been implicated in tumorigenesis. Our findings support the notion that the oncogenic potential of Snail-related transcription factors lies in their capability to, simultaneously, affect cell cycle progression, cell polarity and apoptosis and, hence, the coordination of cell proliferation and cell fate determination.

Animal development is a complex process and requires the coordination in space and time of various processes. These processes include the controlled production of cells, also referred to as ‘cell proliferation’, and the adoption by cells of specific fates, also referred to as ‘cell fate determination’. The observation that uncontrolled cell proliferation and cell fate determination contribute to conditions such as cancer, demonstrates that a precise coordination of these processes is not only important for development but for the prevention of disease throughout life. Snail-related transcription factors have previously been shown to be involved in the regulation of cell proliferation and cell fate determination. For example, the Caenorhabditis elegans Snail-related protein CES-1 affects cell fate determination in a specific cell lineage, the NSM (neurosecretory motorneuron) lineage. We now present evidence that CES-1 also controls cell proliferation in this lineage. Within a short period of time, CES-1 therefore coordinates cell proliferation and cell fate determination in one and the same lineage. Based on this finding, we propose that CES-1 is an important coordinator that is involved in the precise control - in space (NSM lineage) and time (<150 min) - of processes that are critical for animal development.

Deletion of Snai2 and Snai3 results in impaired physical development compounded by lymphocyte deficiency

The Snail family of transcriptional regulators consists of three highly conserved members. These proteins regulate (repress) transcription via the recruitment of histone deacetylases to target gene promoters that possess the appropriate E-box binding sequences. Murine Snai1 is required for mouse development while Snai2 deficient animals survive with some anomalies. Less is known about the third member of the family, Snai3. To investigate the function of Snai3, we generated a conditional knockin mouse. Utilizing Cre-mediated deletion to facilitate the ablation of Snai3 in T cells or the entire animal, we found little to no effect of the loss of Snai3 in the entire animal or in T cell lineages. This finding provided the hypothesis that absence of Snai3 was mitigated, in part, by the presence of Snai2. To test this hypothesis we created Snai2/Snai3 double deficient mice. The developmental consequences of lacking both of these proteins was manifested in stunted growth, a paucity of offspring including a dramatic deficiency of female mice, and impaired immune cell development within the lymphoid lineages.

Transcriptional regulation of gene expression in C. elegans

Protein coding gene sequences are converted to mRNA by the highly regulated process of transcription. The precise temporal and spatial control of transcription for many genes is an essential part of development in metazoans. Thus, understanding the molecular mechanisms underlying transcriptional control is essential to understanding cell fate determination during embryogenesis, post-embryonic development, many environmental interactions, and disease-related processes. Studies of transcriptional regulation in C. elegans exploit its genomic simplicity and physical characteristics to define regulatory events with single-cell and minute-time-scale resolution. When combined with the genetics of the system, C. elegans offers a unique and powerful vantage point from which to study how chromatin-associated proteins and their modifications interact with transcription factors and their binding sites to yield precise control of gene expression through transcriptional regulation.

Scratch2 modulates neurogenesis and cell migration through antagonism of bHLH proteins in the developing neocortex

Scratch genes (Scrt) are neural-specific zinc-finger transcription factors (TFs) with an unknown function in the developing brain. Here, we show that, in addition to the reported expression of mammalian Scrt2 in postmitotic differentiating and mature neurons in the developing and early postnatal brain, Scrt2 is also localized in subsets of mitotic and neurogenic radial glial (RGP) and intermediate (IP) progenitors, as well as in their descendants-postmitotic IPs and differentiating neurons at the border subventricular/intermediate zone. Conditional activation of transgenic Scrt2 in cortical progenitors in mice promotes neuronal differentiation by favoring the direct mode of neurogenesis of RGPs at the onset of neurogenesis, at the expense of IP generation. Neuronal amplification via indirect IP neurogenesis is thereby extenuated, leading to a mild postnatal reduction of cortical thickness. Forced in vivo overexpression of Scrt2 suppressed the generation of IPs from RGPs and caused a delay in the radial migration of upper layer neurons toward the cortical plate. Mechanistically, our results indicate that Scrt2 negatively regulates the transcriptional activation of the basic helix loop helix TFs Ngn2/NeuroD1 on E-box containing common target genes, including Rnd2, a well-known major effector for migrational defects in developing cortex. Altogether, these findings reveal a modulatory role of Scrt2 protein in cortical neurogenesis and neuronal migration.

Neuron-specific expression of scratch genes during early zebrafish development

Scratch (scrt) genes are neural-specific in mammals, but their homologues have not been well studied in non-mammalian vertebrates. In this report, we isolated three zebrafish scrt genes, scratch1a (scrt1a), scratch1b (scrt1b), and scratch2 (scrt2), which belong to the Snail superfamily of zinc finger transcription factors. Spatiotemporal expression analysis revealed that scrt1a and scrt2 were initially detected in the central nervous system (CNS) during early somitogenesis while scrt1b was first detectable in neuronal clusters in the brain during late somitogenesis. Interestingly, scrt-expressing cells largely overlapped with huC-positive differentiating neurons and partially with neurogenin1-positive neuronal precursor cells. In addition, scrt-expressing cells were dramatically increased in mind bomb, a neurogenic mutant. Taken together, these results suggest that each zebrafish scrt gene is specifically expressed in neuronal cells and may be involved in differentiation of distinct neuronal populations in the vertebrate nervous system.

Repression of Puma by scratch2 is required for neuronal survival during embryonic development

Although Snail factors promote cell survival in development and cancer, the tumor-suppressor p53 promotes apoptosis in response to stress. p53 and Snail2 act antagonistically to regulate p53 upregulated modulator of apoptosis (Puma) and cell death in hematopoietic progenitors following DNA damage. Here, we show that this relationship is conserved in the developing nervous system in which Snail genes are excluded from vertebrate neurons and they are substituted by Scratch, a related but independent neural-specific factor. The transcription of scratch2 is induced directly by p53 after DNA damage to repress puma, thereby antagonizing p53-mediated apoptosis. In addition, we show that scratch2 is required for newly differentiated neurons to survive by maintaining Puma levels low during normal embryonic development in the absence of damage. scratch2 knockdown in zebrafish embryos leads to neuronal death through the activation of the intrinsic and extrinsic apoptotic pathways. To compensate for neuronal loss, the proliferation of neuronal precursors increases in scratch2-deficient embryos, reminiscent of the activation of progenitor/stem cell proliferation after damage-induced apoptosis. Our data indicate that the regulatory loop linking p53/Puma with Scratch is active in the vertebrate nervous system, not only controlling cell death in response to damage but also during normal embryonic development.

SPK-1, an SR protein kinase, inhibits programmed cell death in Caenorhabditis elegans

To identify genes involved in protecting cells from programmed cell death in Caenorhabditis elegans, we performed a genetic screen to isolate mutations that cause an increase in the number of programmed cell deaths. We screened for suppressors of the cell-death defect caused by a partial loss-of-function mutation in ced-4, which encodes an Apaf-1 homolog that promotes programmed cell death by activating the caspase CED-3. We identified one extragenic ced-4 suppressor, which has a mutation in the gene spk-1. The spk-1 gene encodes a protein homologous to serine-arginine-rich (SR) protein kinases, which are thought to regulate splicing. Previous work suggests that ced-4 can be alternatively spliced and that the splice variants function oppositely, with the longer transcript (ced-4L) inhibiting programmed cell death. spk-1 might promote cell survival by increasing the amount of the protective ced-4L splice variant. We conclude that programmed cell death in C. elegans is regulated by an alternative splicing event controlled by the SR protein kinase SPK-1.

Cell lineage and cell death: Caenorhabditis elegans and cancer research

Cancer is a complex disease in which cells have circumvented normal restraints on tissue growth and have acquired complex abnormalities in their genomes, posing a considerable challenge to identifying the pathways and mechanisms that drive fundamental aspects of the malignant phenotype. Genetic analyses of the normal development of the nematode Caenorhabditis elegans have revealed evolutionarily conserved mechanisms through which individual cells establish their fates, and how they make and execute the decision to survive or undergo programmed cell death. The pathways identified through these studies have mammalian counterparts that are co-opted by malignant cells. Effective cancer drugs now target some of these pathways, and more are likely to be discovered.

Six and Eya promote apoptosis through direct transcriptional activation of the proapoptotic BH3-only gene egl-1 in Caenorhabditis elegans

The decision of a cell to undergo programmed cell death is tightly regulated during animal development and tissue homeostasis. Here, we show that the Caenorhabditis elegans Six family homeodomain protein C. elegans homeobox (CEH-34) and the Eyes absent ortholog EYA-1 promote the programmed cell death of a specific pharyngeal neuron, the sister of the M4 motor neuron. Loss of either ceh-34 or eya-1 function causes survival of the M4 sister cell, which normally undergoes programmed cell death. CEH-34 physically interacts with the conserved EYA domain of EYA-1 in vitro. We identify an egl-1 5' cis-regulatory element that controls the programmed cell death of the M4 sister cell and show that CEH-34 binds directly to this site. Expression of the proapoptotic gene egl-1 in the M4 sister cell requires ceh-34 and eya-1 function. We conclude that an evolutionarily conserved complex that includes CEH-34 and EYA-1 directly activates egl-1 expression through a 5' cis-regulatory element to promote the programmed cell death of the M4 sister cell. We suggest that the regulation of apoptosis by Six and Eya family members is conserved in mammals and involved in human diseases caused by mutations in Six and Eya.

Beyond epithelial to mesenchymal transition: a novel role for the transcription factor Snail in inflammation and wound healing

INTRODUCTION

Snail, a transcription factor linked to epithelial to mesenchymal transition (EMT) during embryonic development and tumor progression, is associated with migration of cells. During inflammation and tissue injury, cell movement is also observed to provide the first line of defense against bacteria and to promote wound healing. Therefore, we studied the function of Snail in activated macrophages in a variety of inflammatory processes.

MATERIALS AND METHODS

In this study, we examined the expression and localization of Snail during inflammation and tissue injury in rats and human tissue specimens, by immunohistochemistry, Western blot, and real-time PCR. We investigated Snail expression after stimulation of macrophages with TGF-beta1, LPS, Interleukin-8, and MMP-3 in vitro. To further understand the role of Snail in activated macrophages, we used Stealth siRNA against Snail, transfected the human macrophage cell line THP-1, and measured migration of cells in an in vitro invasion assay.

RESULTS AND DISCUSSION

We found a strong, transient, and time-dependent activation of Snail in migrating macrophages at the sites of injury in vivo and in vitro, as well as in patients with inflammatory bowel disease. Furthermore, we showed that induction of Snail in macrophages is dependent on TGF-beta1 signaling pathway. Downregulation of Snail by Stealth siRNA led to impaired migration of THP-1 cells in an invasion assay after stimulation with TGF-beta1.

CONCLUSION

We conclude that TGF-beta1 induced migration of activated macrophages during inflammation and wound healing is mediated by snail. These results give insights in a novel EMT-like mechanism present in immune cell movement during tissue injury.

Up-regulation of survivin by the E2A-HLF chimera is indispensable for the survival of t(17;19)-positive leukemia cells

The E2A-HLF fusion transcription factor generated by t(17;19)(q22;p13) translocation is found in a small subset of pro-B cell acute lymphoblastic leukemias (ALLs) and promotes leukemogenesis by substituting for the antiapoptotic function of cytokines. Here we show that t(17;19)+ ALL cells express Survivin at high levels and that a dominant negative mutant of E2A-HLF suppresses Survivin expression. Forced expression of E2A-HLF in t(17;19)(-) leukemia cells up-regulated Survivin expression, suggesting that Survivin is a downstream target of E2A-HLF. Analysis using a counterflow centrifugal elutriator revealed that t(17;19)+ ALL cells express Survivin throughout the cell cycle. Reporter assays revealed that E2A-HLF induces survivin expression at the transcriptional level likely through indirect down-regulation of a cell cycle-dependent cis element in the promoter region. Down-regulation of Survivin function by a dominant negative mutant of Survivin or reduction of Survivin expression induced massive apoptosis throughout the cell cycle in t(17;19)+ cells mainly through caspase-independent pathways involving translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus. AIF knockdown conferred resistance to apoptosis caused by down-regulation of Survivin function. These data indicated that reversal of AIF translocation by Survivin, which is induced by E2A-HLF throughout the cell cycle, is one of the key mechanisms in the protection of t(17;19)+ leukemia cells from apoptosis.

The C. elegans Snail homolog CES-1 can activate gene expression in vivo and share targets with bHLH transcription factors

Snail-type transcription factors (TFs) are found in numerous metazoan organisms and function in a plethora of cellular and developmental processes including mesoderm and neuronal development, apoptosis and cancer. So far, Snail-type TFs are exclusively known as transcriptional repressors. They repress gene expression by recruiting transcriptional co-repressors and/or by preventing DNA binding of activators from the basic helix-loop-helix (bHLH) family of TFs to CAGGTG E-box sequences. Here we report that the Caenorhabditis elegans Snail-type TF CES-1 can activate transcription in vivo. Moreover, we provide results that suggest that CES-1 can share its binding site with bHLH TFs, in different tissues, rather than only occluding bHLH DNA binding. Together, our data indicate that there are at least two types of CES-1 target genes and, therefore, that the molecular function of Snail-type TFs is more plastic than previously appreciated.

Trithorax, Hox, and TALE-class homeodomain proteins ensure cell survival through repression of the BH3-only gene egl-1

Mutations that aberrantly activate trithorax-group proteins, Hox transcription factors and TALE-class Hox cofactors promote leukemogenesis, but their target genes critical for leukemogenesis remain largely unknown. Through genetic analyses in C. elegans, we find that the trithorax-group gene lin-59 and the TALE-class Hox cofactor unc-62 are required for survival of the VC motor neurons. With the goal of providing a model for how aberrantly active Hox complexes might promote leukemia, we elucidate the mechanism through which these new inhibitors of programmed cell death act: lin-59 maintains transcription of the Hox gene lin-39, while unc-62 promotes nuclear localization of the TALE-class Hox cofactor ceh-20. A LIN-39/CEH-20 complex binds the promoter of the pro-apoptotic BH3-only gene egl-1, repressing its transcription and ensuring survival of the VC neurons. In the absence of this regulatory mechanism, egl-1 is transcribed and the VC neurons die. Furthermore, ectopic expression of the Hox gene lin-39, as occurs for human Hox genes in leukemia, is sufficient to block death of some cells. This work identifies BH3-only pro-apoptotic genes as targets of Hox-mediated repression and suggests that aberrant activation of Hox networks may promote leukemia in part by inhibiting apoptosis.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

Asymmetric divisions, aggresomes and apoptosis

Asymmetric cell division (ACD) is a fundamental process used to generate cell diversity during metazoan development that occurs when a cell divides to generate daughter cells adopting distinct fates. Stem cell divisions, for example, are a type of ACD and provide a source of new cells during development and in adult animals. Some ACDs produce a daughter cell that dies. In many cases, the reason why a cell divides to generate a dying daughter remains elusive. It was shown recently that denatured proteins are segregated asymmetrically during cell division. Here, we review data that provide interesting insights into how apoptosis is regulated during ACD and speculate on potential connections between ACD-induced cell death and partitioning of denatured proteins.

Cell death specification in C. elegans

Years of research have identified a highly conserved mechanism required for apoptotic cell killing. How certain cells are specified to die is not well understood. With a rich history in programmed cell death research, the nematode C. elegans offers an excellent animal model with which to study cell death specification events. Developing hermaphrodites have 131 invariant cell death events that can be studied with single cell resolution. Recent genetic studies have begun to identify diverse sets of factors required for the proper specification of individual cell death events. The limited findings thus far suggest that cell death specification is controlled through transcriptional regulation of at least two members of the core cell death pathway, egl-1 and ced-3. However, it remains unclear if additional modes of cell death specification exist. Here we briefly summarize current findings in the field of C. elegans cell death specification and consider those questions that remain to be answered.

Control of apoptosis by asymmetric cell division

Asymmetric cell division and apoptosis (programmed cell death) are two fundamental processes that are important for the development and function of multicellular organisms. We have found that the processes of asymmetric cell division and apoptosis can be functionally linked. Specifically, we show that asymmetric cell division in the nematode Caenorhabditis elegans is mediated by a pathway involving three genes, dnj-11 MIDA1, ces-2 HLF, and ces-1 Snail, that directly control the enzymatic machinery responsible for apoptosis. Interestingly, the MIDA1-like protein GlsA of the alga Volvox carteri, as well as the Snail-related proteins Snail, Escargot, and Worniu of Drosophila melanogaster, have previously been implicated in asymmetric cell division. Therefore, C. elegans dnj-11 MIDA1, ces-2 HLF, and ces-1 Snail may be components of a pathway involved in asymmetric cell division that is conserved throughout the plant and animal kingdoms. Furthermore, based on our results, we propose that this pathway directly controls the apoptotic fate in C. elegans, and possibly other animals as well.

Asymmetric cell division and apoptosis (programmed cell death) are two fundamental processes that are important for the development and function of multicellular organisms. Asymmetric cell division creates daughter cells of different fates, and this is critical for the generation of cellular diversity. Apoptosis eliminates superfluous cells from the organism, which is critical for cellular homeostasis. We found that the processes of asymmetric cell division and apoptosis can be functionally linked. Specifically, we show that asymmetric cell division in the nematode Caenorhabditis elegans is mediated by a pathway involving three genes, dnj-11 MIDA1, ces-2 HLF, and ces-1 Snail, that directly control the enzymatic machinery responsible for apoptosis. Interestingly, the role of this pathway in asymmetric cell division and the control of apoptosis might be evolutionarily conserved. Furthermore, it might have an unexpected role in stem cell biology: the process of asymmetric cell division plays an essential role in the ability of stem cells to self-renew, and the mammalian counterparts of two components of the dnj-11 MIDA1, ces-2 HLF, ces-1 Snail pathway have recently been implicated in stem cell function. For this reason, we speculate that a dnj-11 MIDA1, ces-2 HLF, ces-1 Snail–like pathway might function in stem cells to coordinate self-renewal and apoptosis and, hence, the number of stem cells.

A pathway involved in asymmetric cell division in the nematode Caenorhabditis elegans, the dnj-11 MIDA1, ces-2 HLF, ces-1 Snail pathway, directly controls the enzymatic machinery responsible for apoptosis.

Noncanonical cell death programs in the nematode Caenorhabditis elegans

Genetic studies of the nematode Caenorhabditis elegans have uncovered four genes, egl-1 (BH3 only), ced-9 (Bcl-2 related), ced-4 (apoptosis protease activating factor-1), and ced-3 (caspase), which function in a linear pathway to promote developmental cell death in this organism. While this core pathway functions in many cells, recent studies suggest that additional regulators, acting on or in lieu of these core genes, can promote or inhibit the onset of cell death. Here, we discuss the evidence for these noncanonical mechanisms of C. elegans cell death control. We consider novel modes for regulating the core apoptosis genes, and describe a newly identified cell death pathway independent of all known C. elegans cell death genes. The existence of these noncanonical cell death programs suggests that organisms have evolved multiple ways to ensure appropriate cellular demise during development.

The C. elegans protein CEH-30 protects male-specific neurons from apoptosis independently of the Bcl-2 homolog CED-9

The developmental control of apoptosis is fundamental and important. We report that the Caenorhabditis elegans Bar homeodomain transcription factor CEH-30 is required for the sexually dimorphic survival of the male-specific CEM (cephalic male) sensory neurons; the homologous cells of hermaphrodites undergo programmed cell death. We propose that the cell-type-specific anti-apoptotic gene ceh-30 is transcriptionally repressed by the TRA-1 transcription factor, the terminal regulator of sexual identity in C. elegans, to cause hermaphrodite-specific CEM death. The established mechanism for the regulation of specific programmed cell deaths in C. elegans is the transcriptional control of the BH3-only gene egl-1, which inhibits the Bcl-2 homolog ced-9; similarly, most regulation of vertebrate apoptosis involves the Bcl-2 superfamily. In contrast, ceh-30 acts within the CEM neurons to promote their survival independently of both egl-1 and ced-9. Mammalian ceh-30 homologs can substitute for ceh-30 in C. elegans. Mice lacking the ceh-30 homolog Barhl1 show a progressive loss of sensory neurons and increased sensory-neuron cell death. Based on these observations, we suggest that the function of Bar homeodomain proteins as cell-type-specific inhibitors of apoptosis is evolutionarily conserved.

Mechanisms of transcription factor deregulation in lymphoid cell transformation

The most frequent targets of genetic alterations in human lymphoid leukemias are transcription factor genes with essential functions in blood cell development. TAL1, LYL1, HOX11 and other transcription factors essential for normal hematopoiesis are often misexpressed in the thymus in T-cell acute lymphoblastic leukemia (T-ALL), leading to differentiation arrest and cell transformation. Recent advances in the ability to assess DNA copy number have led to the discovery that the MYB transcription factor oncogene is tandemly duplicated in T-ALL. The NOTCH1 gene, which is essential for key embryonic cell-fate decisions in multicellular organisms, was found to be activated by mutation in a large percentage of T-ALL patients. The gene encoding the FBW7 protein ubiquitin ligase, which regulates the turnover of the intracellular form of NOTCH (ICN), is also mutated in T-ALL, resulting in stabilization of the ICN and activation of the NOTCH signaling pathway. In mature B-lineage ALL and Burkitt lymphoma, the MYC transcription factor oncogene is overexpressed due to translocation into the IG locus. PAX5, a transcription factor essential for B-lineage commitment, is inactivated in 32% of cases of B-progenitor ALL. Translocations resulting in oncogenic fusion transcription factors also occur frequently in this form of ALL. The most frequent transcription factor chimeric fusion, TEL-AML1, is an initiating event in B-progenitor ALL that acts by repressing transcription. Therefore, deregulated transcription and its consequent effects on key developmental pathways play a major role in the molecular pathogenesis of lymphoid malignancy. Once the full complement of cooperating mutations in transformed B- and T-progenitor cells is known, and the deregulated downstream pathways have been elucidated, it will be possible to identify vulnerable components and to target them with small-molecule inhibitors.

Snail, Zeb and bHLH factors in tumour progression: an alliance against the epithelial phenotype?

The molecular mechanisms that underlie tumour progression are still poorly understood, but recently our knowledge of particular aspects of some of these processes has increased. Specifically, the identification of Snail, ZEB and some basic helix-loop-helix (bHLH) factors as inducers of epithelial-mesenchymal transition (EMT) and potent repressors of E-cadherin expression has opened new avenues of research with potential clinical implications.

A morphologically conserved nonapoptotic program promotes linker cell death in Caenorhabditis elegans

Apoptosis, cell death characterized by stereotypical morphological features, requires caspase proteases. Nonapoptotic, caspase-independent cell death pathways have been postulated; however, little is known about their molecular constituents or in vivo functions. Here, we show that death of the Caenorhabditis elegans linker cell during development is independent of the ced-3 caspase and all known cell death genes. The linker cell employs a cell-autonomous death program, and a previously undescribed engulfment program is required for its clearance. Dying linker cells display nonapoptotic features, including nuclear crenellation, absence of chromatin condensation, organelle swelling, and accumulation of cytoplasmic membrane-bound structures. Similar features are seen during developmental death of neurons in the vertebrate spinal cord and ciliary ganglia. Linker cell death is controlled by the microRNA let-7 and Zn-finger protein LIN-29, components of the C. elegans developmental timing pathway. We propose that the program executing linker cell death is conserved and used during vertebrate development.

DPL-1 DP, LIN-35 Rb and EFL-1 E2F act with the MCD-1 zinc-finger protein to promote programmed cell death in Caenorhabditis elegans

The genes egl-1, ced-9, ced-4, and ced-3 play major roles in programmed cell death in Caenorhabditis elegans. To identify genes that have more subtle activities, we sought mutations that confer strong cell-death defects in a genetically sensitized mutant background. Specifically, we screened for mutations that enhance the cell-death defects caused by a partial loss-of-function allele of the ced-3 caspase gene. We identified mutations in two genes not previously known to affect cell death, dpl-1 and mcd-1 (modifier of cell death). dpl-1 encodes the C. elegans homolog of DP, the human E2F-heterodimerization partner. By testing genes known to interact with dpl-1, we identified roles in cell death for four additional genes: efl-1 E2F, lin-35 Rb, lin-37 Mip40, and lin-52 dLin52. mcd-1 encodes a novel protein that contains one zinc finger and that is synthetically required with lin-35 Rb for animal viability. dpl-1 and mcd-1 act with efl-1 E2F and lin-35 Rb to promote programmed cell death and do so by regulating the killing process rather than by affecting the decision between survival and death. We propose that the DPL-1 DP, MCD-1 zinc finger, EFL-1 E2F, LIN-35 Rb, LIN-37 Mip40, and LIN-52 dLin52 proteins act together in transcriptional regulation to promote programmed cell death.

An NF-kappaB and slug regulatory loop active in early vertebrate mesoderm

Background

In both Drosophila and the mouse, the zinc finger transcription factor Snail is required for mesoderm formation; its vertebrate paralog Slug (Snai2) appears to be required for neural crest formation in the chick and the clawed frog Xenopus laevis. Both Slug and Snail act to induce epithelial to mesenchymal transition (EMT) and to suppress apoptosis.

Methodology & Principle Findings

Morpholino-based loss of function studies indicate that Slug is required for the normal expression of both mesodermal and neural crest markers in X. laevis. Both phenotypes are rescued by injection of RNA encoding the anti-apoptotic protein Bcl-xL; Bcl-xL's effects are dependent upon IκB kinase-mediated activation of the bipartite transcription factor NF-κB. NF-κB, in turn, directly up-regulates levels of Slug and Snail RNAs. Slug indirectly up-regulates levels of RNAs encoding the NF-κB subunit proteins RelA, Rel2, and Rel3, and directly down-regulates levels of the pro-apopotic Caspase-9 RNA.

Conclusions/Significance

These studies reveal a Slug/Snail–NF-κB regulatory circuit, analogous to that present in the early Drosophila embryo, active during mesodermal formation in Xenopus. This is a regulatory interaction of significance both in development and in the course of inflammatory and metastatic disease.

The expression of Scratch genes in the developing and adult brain

The Scratch genes belong to the Snail superfamily of zinc-finger transcription factors present in the metazoa, represented in mammals by the Scratch1 and Scratch2 genes. We have analyzed the expression of these genes in the brain of mice at developmental stages between 9.5 days-post-coitum to adulthood. Both genes are expressed in the mantle layer of the neuroepithelium at mid-gestational stages in all regions except for the region corresponding to the V2 interneuron column, which lacked Scratch2 transcripts. From perinatal to adult stages, the expression patterns of the two genes differ. Scratch1 remains strongly expressed in almost all brain regions, although it is not found in some ventral structures such as motor nuclei and hypothalamic regions. In contrast, Scratch2 expression progressively diminishes and virtually no expression can be detected in the adult brain. Nevertheless, strong expression of Scratch2 is retained in the postnatal cortical subventricular zone, in the inner part of the cerebellar external granular layer, and in the glial cells of the adult vomeronasal nerve.

A novel role for proline- and acid-rich basic region leucine zipper (PAR bZIP) proteins in the transcriptional regulation of a BH3-only proapoptotic gene

Proline- and acid-rich (PAR) basic region leucine zipper (bZIP) proteins thyrotroph embryonic factor (TEF), D-site-binding protein (DBP), and hepatic leukemia factor have been involved in neurotransmitter homeostasis and amino acid metabolism. Here we demonstrate a novel role for these proteins in the transcriptional control of a BH3-only gene. PAR bZIP proteins are able to transactivate the promoter of bcl-gS. This promoter is particularly responsive to TEF activation and is silenced by NFIL3, a repressor that shares the consensus binding site with PAR bZIP proteins. Consistently, transfection of TEF induces the expression of endogenous bcl-gS in cancer cells, and this induction is independent of p53. A naturally occurring variant of DBP (tDBP), lacking the transactivation domain, has been identified and shown to impede the formation of active TEF dimers in a competitive manner and to reduce the TEF-dependent induction of bcl-gS. Of note, treatment of cancer cells with etoposide induces TEF activation and promotes the expression of bcl-gS. Furthermore, blockade of bcl-gS or TEF expression by a small interfering RNA strategy or transfection with tDBP significantly reduces the etoposide-mediated apoptotic cell death. These findings represent the first described role for PAR bZIP proteins in the regulation of a gene involved in the execution of apoptosis.

The nongenotoxic carcinogens naphthalene and para-dichlorobenzene suppress apoptosis in Caenorhabditis elegans

Naphthalene (1) and para-dichlorobenzene (PDCB, 2), which are widely used as moth repellents and air fresheners, cause cancer in rodents and are potential human carcinogens. However, their mechanisms of action remain unclear. Here we describe a novel method for delivering and screening hydrophobic chemicals in C. elegans and apply this technique to investigate the ways in which naphthalene and PDCB may promote tumorigenesis in mammals. We show that naphthalene and PDCB inhibit apoptosis in C. elegans, a result that suggests a cellular mechanism by which these chemicals may promote the survival and proliferation of latent tumor cells. In addition, we find that a naphthalene metabolite directly inactivates caspases by oxidizing the active site cysteine residue; this suggests a molecular mechanism by which these chemicals suppress apoptosis. Naphthalene and PDCB are the first small-molecule apoptosis inhibitors identified in C. elegans. The power of C. elegans molecular genetics, in combination with the possibility of carrying out large-scale chemical screens in this organism, makes C. elegans an attractive and economic animal model for both toxicological studies and drug screens.

Developmental apoptosis in C. elegans: a complex CEDnario

Apoptosis, an evolutionarily conserved programme of cellular self-destruction, is essential for the development and survival of most multicellular animals. It is required to ensure functional organ architecture and to maintain tissue homeostasis. During development of the simple nematode Caenorhabditis elegans, apoptosis claims over 10% of the somatic cells that are generated - these cells were healthy but unnecessary. Exciting insights into the regulation and execution of apoptosis in C. elegans have recently been made. These new findings will undoubtedly influence our perception of developmental apoptosis in more complex species, including humans.