Gene name - snail
Cytological map position - 35 D1-2
Function - transcription factor
Symbol - sna
Genetic map position - 2-51
Classification - zinc finger
Cellular location - nuclear
|Recent literature||Tseng, C. Y., Kao, S. H. and Hsu, H. J. (2016). Snail controls proliferation of Drosophila ovarian epithelial follicle stem cells, independently of E-cadherin. Dev Biol [Epub ahead of print]. PubMed ID: 27141871
Epithelial-mesenchymal transition (EMT), which is primarily mediated by Snail via the suppression of E-cadherin, is able to generate cells with stem cell properties. However, the role of Snail in epithelial stem cells remains unclear. This study reports that Snail directly controls proliferation of follicle stem cells (FSCs) in Drosophila females. Disruption of Snail expression in FSCs compromises their proliferation, but not their maintenance. Conversely, FSCs with excessive Snail expression display increased proliferation and lifespan, which is accompanied by a moderate decrease in the expression of E-cadherin (required for adhesion of FSCs to their niche) at the junction between their adjacent cells, indicating a conserved role of Snail in E-cadherin inhibition, which promote epithelial cell proliferation. Interestingly, a decrease in E-cadherin in snail-knock down FSCs does not restore the decreased proliferation of snail-knock down FSCs, suggesting that adhesion strength of FSCs to their niche is dispensable for Snail-mediated FSC division. These results demonstrate that Snail controls epithelial stem cell division independently of its known role in the EMT, which contributes to induction of cancer stem cells.
|Esposito, E., Lim, B., Guessous, G., Falahati, H. and Levine, M. (2016). Mitosis-associated repression in development. Genes Dev 30: 1503-1508. PubMed ID: 27401553
Transcriptional repression is a pervasive feature of animal development. This study employed live-imaging methods to visualize the Snail repressor, which establishes the boundary between the presumptive mesoderm and neurogenic ectoderm of early Drosophila embryos. Snail target enhancers were attached to an MS2 reporter gene, permitting detection of nascent transcripts in living embryos. The transgenes exhibit initially broad patterns of transcription but are refined by repression in the mesoderm following mitosis. These observations reveal a correlation between mitotic silencing and Snail repression. The study proposes that mitosis and other inherent discontinuities in transcription boost the activities of sequence-specific repressors, such as Snail.
Gastrulation is a process of tissue involution that creates three germ layers from one. In Drosophila, mesoderm and endoderm are created by different invagination events. Mesoderm is created as a result of ventral furrow formation, one of the three invagination events taking place during gastrulation [Images]. Invagination is initiated by two regulatory genes, twist (twi) and snail (sna). sna is sufficient to initiate the invagination of the ventral-most embryonic cells in the absence of twist gene activity, but without twist the invaginated cells fail to express mesodermal genes under the control of twist, such as tinman and bagpipe. Low levels of SNA that fail to repress neuroectoderm determinants in the presumptive mesoderm are nonetheless able to promote invagination (Ip, 1994a).
snail is a transcriptional repressor. It acts to restrict neuroectoderm and neural fate in the invaginating mesoderm. snail mutants display a massive derepression of mesectodermal genes such as single-minded and rhomboid (Kasai, 1992 and Ip, 1992a).
The opposing actions of twist and snail can best be observed by examining the extent of altered mitotic domains in mutant embryos. The first 13 cleavage divisions of a wild type embryo are synchronous. The cells divide in uninterruped waves, like continuously cheering fans in a stadium. The cell membranes that will later separate individual cells have not yet formed, so each cell does exactly what its neighbors do.
Following cellularization, groups of cells enter mitosis 14 in an established non-synchronous spatial and temporal sequence. In twist mutants, the position of dividing cells in the mesectoderm is shifted closer to the ventral midline, indicating a smaller presumptive mesodermal domain. In snail mutants, groups of dividing cells that have been delayed in the first non-synchronous division are seen in the mid-ventral region, suggesting that they represent an expansion of the neuroectoderm. What had been presumptive mesoderm has been transformed into neuroectoderm (Aurora, 1992). This demonstrates that snail, in its capacity as a transcriptional repressor, assures the extent and integrity of the mesodermal domain.
Gastrulation and neuroectodermal gene expression are subject to differential regulation by Snail. In snail mutants, the ventral cell invagination during gastrulation is blocked and no mesoderm-derived tissue is formed. One of the functions of Snail is to repress neuroectodermal genes and restrict their expressions to the lateral regions. The derepression of the neuroectodermal genes into the ventral region in snail mutants is a possible cause of defects in gastrulation and in mesoderm differentiation. To investigate such a possibility, a series of snail mutant alleles was analyzed. Different neuroectodermal genes respond differently in various snail mutant backgrounds. Due to the differential response of target genes, one of the mutant alleles, V2, which manifests reduced Snail function, also shows an intermediate phenotype. In V2 embryos, neuroectodermal genes, such as single-minded and rhomboid, are derepressed while ventral invagination proceeds normally. However, the differentiation of these invaginated cells into mesodermal lineage is disrupted. The results suggest that the establishment of mesodermal cell fate requires the proper restriction of neuroectodermal genes, while the ventral cell movement is independent of the expression patterns of these genes. The expression of some ventral genes disappear in snail mutants. Snail function is required for activation of genes such as dGATAb (serpent) and zfh-1. It is proposed that Snail may repress or activate another set of target genes, including folded gastrulation, that are required specifically for gastrulation (Hemavathy, 1997).
snail has a second function later, as a regulator of neurogenesis. Its role here is independent of daughterless/achaete-scute, and it acts in both the central and peripheral nervous systems. The nature of snail's neural actions are uncharacterized. It may act to repress non neural fates. If so, then its role in the nervous system is the reverse of its role in the mesoderm (Ip, 1994b).
snail is also involved is the specification of the wing disc. In this role, snail is expressed in the ectoderm. escargot and snail are required for the maintenance of vestigial expression in the wing disc (Fuse, 1996).
By the time of neuroblast delamination, Sna is present in most of the neuroblasts that have segregated from the ectoderm. Despite the extensive expression in the neuroblasts, prior to this study, Sna had no known function in the developing nervous system. The neuroblast pattern of sna resembles that of a group of genes called pan-neural genes. One of these genes, scratch (scrt), encodes a protein that has sequence similarity to Sna in the zinc-finger domain. Mutations of scrt have no obvious phenotype except that viable escapers have morphological defects in the eyes. Furthermore, no nervous system defect can be seen in sna scrt double mutants. However, the scrt dpn double mutants exhibit some defects in nervous system development. deadpan (dpn) is another pan-neural gene that encodes a basic helix-loop-helix protein. Therefore, scrt does have a function in the central nervous system (CNS), but the function of sna, if any, in the nervous system does not overlap with that of scrt (Ashraf, 1999 and references therein).
Escargot (Esg) is another protein that contains five zinc fingers with sequences highly homologous to those of Sna. The expression of esg is rather dynamic during embryonic development. The gene is expressed in the epidermis, neuroectoderm and imaginal precursor cells. The Esg protein probably acts through the cdc2 kinase to maintain the proper cell cycle in larval imaginal disc cells; in esg mutant larvae the imaginal disc cells lose their diploidy as they re-enter the S phase without going through mitosis. Moreover, esg and sna are both expressed in the embryonic wing imaginal disc primodia and the two genes have redundant functions in this tissue; the vestigial marker gene expression in the disc is lost in esg sna double mutants. Despite a clear demonstration of the redundant requirements of sna and esg in the wing disc, the double mutant has been reported to have no significant embryonic CNS phenotype. Thus, the function of sna in nervous system development has remained a mystery (Ashraf, 1999 and references therein).
Evidence is provided that CNS expression of Snail is required for nervous system development. The neural function of snail is masked by two closely linked genes, escargot and worniu. worniu (pronounced war-niu, Chinese for 'snail') encodes a protein with a zinc-finger domain highly homologous to those of Sna and Esg; it has been identified from the Berkeley Drosophila Genome Project database. RNA in situ hybridization reveals extensive expression of worniu in the developing nervous system. wor is located between esg and sna, ~100 kb apart, in the 35D region of the second chromosome. Although not affecting expression of early neuroblast markers, the deletion of the region containing all three genes correlates with loss of expression of CNS determinants including fushi tarazu, pdm-2 and even-skipped. Transgenic expression of each of the three Snail family proteins efficiently rescues the fushi tarazu defects, and partially rescues the pdm-2 and even-skipped CNS patterns. These results demonstrate that the Snail family proteins have essential functions during embryonic CNS development, around the time of ganglion mother cell formation (Ashraf, 1999).
The putative Wor protein sequence contains a C-terminal domain with six zinc fingers that are very similar to those of Sna and Esg, even though those proteins contain only five fingers. The N-terminal halves of these proteins have rather divergent sequences, except that they all contain a conserved basic motif very close to the N-termini. The function of this motif is not known. Moreover, the proteins contain two P-DLS-K motifs. The P-DLS-K domains in Sna have been shown to interact with the Drosophila C-terminal binding protein (dCtBP) and to play important roles in transcriptional repression. Since all three Sna family proteins contain highly homologous corepressor-interacting and DNA-binding domains, and can bind to similar DNA sequences, it is possible that they bind to promoters of overlapping sets of target genes and repress transcription (Ashraf, 1999 and references therein).
While there is no maternal RNA deposition of wor, zygotic expression can be detected first at the onset of neurogenesis. At a late stage 8, WOR transcript can be observed in two small patches of cells in the dorsal head region anterior to the cephalic furrow, representing precursor cells of the developing brain. At stage 9 wor expresses in the first wave of delaminating neuroblasts along either side of the midline, as well as in cells in the head region. Later in the germ band-extended embryo, most of the neuroblasts contain WOR mRNA. This pattern greatly resembles that of sna at this stage of development, except that sna expression in some of the centrally located neuroblasts in each hemisegment is at lower levels. In later stages, wor continues to express in the brain and part of the ventral nerve cord. No expression of wor is detected in any other embryonic tissue (Ashraf, 1999).
There is no extensive expression of esg in the neuroblasts similar to that shown for wor or sna. However, it has been demonstrated that esg RNA is expressed in the ventral neuroectoderm. Careful examination of the expression reveals that esg transcript is probably present in the CNS, albeit at variable levels. Based on the expression analyses, it is hypothesized that the newly identified wor might serve a redundant function with that of sna or esg during neural development. This would explain why neither single nor double mutants of sna and esg show severe defects in the nervous system (Ashraf, 1999).
To test the hypothesis that the Sna family proteins function redundantly in the developing nervous system, the neural phenotype associated with a deletion that uncovers all three genes was examined. wor is located between esg and sna, ~100 kb apart, in the 35D region of the second chromosome. Advantage of the close proximity of these genes and the phenotypes of a deficiency mutant are examined. Since high levels of Sna and Wor are present in the neuroblasts, the expression of the proneural gene achaete, which marks a subset of early delaminating neuroblasts was examined. This expression is not affected in the osp29 deficiency mutants. The expression patterns of additional neuroblast markers including hunchback, dpn, scrt and lethal of scute also are similar in wild-type and mutant embryos. Therefore, the early waves of neuroblast delamination are normal in the absence of the Snail family proteins (Ashraf, 1999).
The CNS patterns of GMC markers ftz, pdm-2 and eve were examined. ftz is expressed in a number of midline precursor cells and extensively in GMC. In contrast to the neuroblast markers, the ftz expression is almost abolished in the mutant embryo. The pdm-2 gene is also expressed in some neuroblasts and GMC. The early neuroblast expression of pdm-2 in the mutant is nearly normal, while the expression in later staged embryos is highly defective. eve gene products are present in a number of GMC and postmitotic neurons during normal development. All the eve CNS expression is absent in homozygous osp29 deletion mutant embryos. Taken together, the deletion mutant that uncovers the three sna family genes shows severe defects in CNS development (Ashraf, 1999).
To confirm the function of these three proteins in neural development, transgenic rescue plasmids were constructed in which individual genes (esg, wor or sna) were placed under the control of a sna promoter, containing an enhancer element that directs expression in the neuroblasts. The transgenic flies obtained were then crossed with the osp29 strain and analyzed for CNS development. In the presence of any one of the three constructs the ftz expression is restored significantly. Analysis of the rescued pattern under higher magnification reveals that part of the ftz staining is clearly absent. However, more detailed analysis is required to pinpoint the exact cell lineages that are missing. Nevertheless, the results demonstrate that each of the three sna family genes can perform essential functions in the CNS in the absence of the other two. The rescue by the transgenes of the expression of pdm-2 and eve, both of which are defective in the osp29 mutant, was also examined. While all three sna family genes clearly can rescue the expression of pdm-2 , the effect is not as extensive compared with that of ftz. For eve RNA, the transgenes rescue the expression in a significant number of cells when compared with the total loss of expression in the parental osp29 mutant. The rescue of eve, again, is not as extensive as that of ftz. Later stage CNS morphology in the rescued embryos was also monitored by BP102 staining. The embryos carrying the transgenes have slightly better overall CNS axonal morphology, but they are still highly abnormal when compared with the wild type (Ashraf, 1999).
Pairwise recombination of the transgenes were constructed and a test was performed to see whether they could achieve better rescue. By staining embryos obtained from stable lines that are homozygous for two transgenes, the constructs were found to give slightly direct the expression of ftz slightly better. Meanwhile, the eve and BP102 antigen expression in the presence of two transgenes reveals only minor improvement of the axonal morphology. These results suggest that the three proteins may have some collaborative function. It is also possible that the promoter used has some limitation in driving the rescue transgenes or that there are additional genes involved for the severe CNS phenotype (Ashraf, 1999).
Increasing numbers of sna-related genes have been identified in diverse species. These proteins have been assigned to the Sna family based mostly on the similarity of the sequences in the zinc-finger domains. The expression patterns and some functional studies of the vertebrate proteins suggest a role in regulating cell movement. However, gene knock-out experiments have demonstrated that mutating a mouse Slug homolog does not lead to a detectable cell movement defect. Such a result suggests a possible redundant function provided by other genes, similar to this report. If the vertebrate homologs do have a function in controlling cell movement, it would be reminiscent of the control of cell movement during gastrulation by Drosophila Sna. However, the expression of vertebrate Sna proteins in developing CNS has not been demonstrated. A careful examination of the expression and function in the CNS is needed to reveal the importance of Sna expression. The analysis of the functions of Sna, Esg and Wor in Drosophila CNS development will certainly provide a foundation for similar analysis in other species (Ashraf, 1999 and references therein).
Although Snail is essential for disassembly of adherens junctions during epithelial-mesenchymal transitions (EMTs), loss of adherens junctions in Drosophila melanogaster gastrula is delayed until mesoderm is internalized, despite the early expression of Snail in that primordium. By combining live imaging and quantitative image analysis, the behavior of E-cadherin-rich junction clusters were tracked, demonstrating that in the early stages of gastrulation most subapical clusters in mesoderm not only persist, but move apically and enhance in density and total intensity. All three phenomena depend on myosin II and are temporally correlated with the pulses of actomyosin accumulation that drive initial cell shape changes during gastrulation. When contractile myosin is absent, the normal Snail expression in mesoderm, or ectopic Snail expression in ectoderm, is sufficient to drive early disassembly of junctions. In both cases, junctional disassembly can be blocked by simultaneous induction of myosin contractility. These findings provide in vivo evidence for mechanosensitivity of cell-cell junctions and imply that myosin-mediated tension can prevent Snail-driven EMT (Weng, 2016).
This study shows that during Drosophila gastrulation, subapical junctions are repositioned toward the apical surface and are strengthened as the cortical tension increases. Both these phenomena follow apical myosin activation and thus may reflect a mechanosensitive response of junctional complexes to the tension generated by this activation of myosin. The junctional responses occur on the time scale of individual myosin pulses and are temporally correlated with those pulses. Such junctional changes depend on myosin activity but do not require Sna, given that ectopic myosin activation recapitulates similar junctional responses in Sna-negative tissues. This phenomenon may not be restricted to Drosophila embryos. The increased contractile actomyosin on the apical cortex of human cell lines deficient for the cortex actin regulator Merlin is associated with a condensation of adherens junctions toward the apical surface, suggesting that the response of adherens junctions to cortical tension can be of general significance (Weng, 2016).
The changes in junction mass and density suggest that, rather than being simple passive anchors for contractile actomyosin filaments, adherens junctions respond to the contractile actomyosin by restructuring and repositioning themselves, potentially involving aggregation and rearrangement of E-Cad molecules within the plasma membrane or vesicle-based redistribution of E-Cad. Indeed, actomyosin organization has been shown to be critical in the lateral clustering of E-Cad molecules. The change in E-Cad clustering is considered an active mechanosensitive mechanism to strengthen the adhesion. Alternatively, the adhesion can also be remodeled through the vesicle-based mechanisms, and endocytosis of E-Cad has been shown to be up-regulated when junctions are under actomyosin-generated stress. The repositioning could also arise through restructuring rather than passive dragging, if for example recycling and turnover rates in the basal regions of the junctions differ from apical regions. Overall, regardless of the underlying mechanism, this mechanosensitivity may be advantageous, providing a direct self-corrective mechanism that allows junctions to adjust their localization and intensity to match the mechanical force they experience (Weng, 2016).
Although the molecular mechanism for the junction strengthening requires further investigation, the data suggest that it is resistant to the posttranscriptional disassembly of adherens junctions downstream of Sna. The phenotype of myosin knockdown in this study resembles that previously described for cta; T48 double mutants, in which apical actomyosin cannot be activated and junctions are lost only in the ventral mesodermal cells. In all scenarios in which Sna expression is associated with junction loss (ventral cells in cta; T48 mutants, ventral cells in myosin knockdown mutants, and ectodermal cells with ectopic Sna expression), Sna is expressed in cells in the absence of myosin contractility. Maintenance of adherens junctions ultimately relies on the balance between assembly and disassembly rates of junctional components. Thus mechanical force likely modulates the assembly/disassembly balance and therefore remains in a homeostatic relationship with the junctions bearing the force (Weng, 2016).
In the early stages of embryogenesis analyzed in this study, E-Cad is maternally provided and thus not subject to direct transcriptional repression. The disassembly of junctions in the absence of myosin contraction must therefore reflect a posttranscriptional regulation on junctions, likely performed by one or several of Sna’s transcriptional targets. Much effort has been invested in identifying transcription targets of Sna, but it is not known which, if any, of its known targets might play such a role. One mesodermally expressed gene, Traf4, is required for fine-tuning junction morphology, but its expression appears to depend on the other mesodermal determinant, Twist, rather than Sna. One gene repressed by Sna in Drosophila mesoderm, bearded, is required for the subapical positioning of adherens junctions in cells not expressing Snail. It is not clear, however, whether Bearded plays a direct role in junction disassembly or a more general role in apical polarity or the apical myosin contractility that drives repositioning. The posttranscriptional regulation of adherens junction disassembly may allow more rapid and effective EMT than a disassembly relying on transcriptional down-regulation of junctional components such as E-Cad. Identifying and characterizing the relevant Sna targets in Drosophila may provide insights into the underlying mechanism for this disassembly, especially with respect to its apparent sensitivity to externally exerted tension. The force-dependent resistance to this Sna function may help in dissecting the underlying molecular functions. Further exploration of Sna’s posttranscriptional effect on junctions and how myosin contraction antagonizes Sna will shed light on understanding of EMT processes (Weng, 2016).
Bases in 5' UTR - 163
Bases in 3' UTR - 340
There are five C terminal zinc fingers (Boulay, 1987).
date revised: 20 December 99
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