- Email: [email protected]
Hox Service Warranty Extends to Adult Bone Repair
Developmental Cell
Previews Hox Service Warranty Extends to Adult Bone Repair Andrew J. Saurin,1 Marie-Claire Delfini,1 and Yacine Graba1,* 1Aix-Marseille Universite ´ , CNRS, IBDM, UMR7288, case 907, 13288 Marseille Cedex 09, France *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.12.007
Hox genes are key developmental regulators. In this issue of Developmental Cell, Rux et al. (2016) uncover an adult role for Hox11 genes in regionalized bone repair. This function relies on Hox activity in bone marrow multipotent mesenchymal stem progenitor cells, which promotes skeletal cell differentiation. Hox genes were initially recognized as key developmental regulators, providing antero-posterior positional information and promoting diversified morphogenesis (Bobola and Merabet, 2016; Rezsohazy et al., 2015). They also similarly provide proximo-distal positional information along the developing fore- and hindlimbs, with the Hox9/10, Hox11, and Hox13 paralogs expressed in the proximal (stylopod), median (zeugopod), and distal (autopod) limb parts, respectively (Figure 1, top; Pineault and Wellik, 2014). Consequently, each limb compartment will acquire bones with distinct identities: humerus, radius/ulna, and carpals/metacarpals in the forelimb; and femur, tibia/ fibula, and tarsal/metatarsal in the hindlimb. Besides these well-studied developmental functions, it was also early recognized that Hox gene expression is maintained in postnatal and adult stages, in patterns that recapitulate embryonic characteristics, suggesting that adult Hox gene function may be in continuity with embryonic activity (Morgan, 2006). However, with the exception of hematopoiesis and the female reproductive tract (Alharbi et al., 2013; Du and Taylor, 2015), which to some extent are seen as developmental processes occurring in the adult body, roles for regionalized adult Hox gene expression have not been determined. In this issue of Developmental Cell, Rux et al. (2016) report on an adult function of Hox11 paralog group genes in limb bone repair. Previous work established that continued Hox expression in the periosteum allows the promotion of bone regeneration, through repair processes that are distinct from those occurring in Hox-free regions (Leucht et al., 2008). Using a Hoxa11 reporter line to follow expression in the limb, Rux et al. (2016) found that adult expres-
sion not only is maintained in a pattern reminiscent of embryonic expression within the periosteum, but also appears in the zeugopod bone marrow (Figure 1, bottom). Using relevant markers, the authors provide compelling evidence that the Hoxa11-expressing cells are bone marrow multipotent mesenchymal stem/progenitor cells (BM-MSCs), a low abundant subpopulation of nonendothelial stromal cells (Zhou et al., 2014). The identity of Hox11-expressing bone marrow cells was confirmed by the demonstration that Hoxa11-expressing cells display the functional characteristics of BM-MSCs in ex vivo differentiation (capacity to generate chondrocytes, osteoblasts, and adipocytes) and self-renewing assays. The origin of adult Hoxa11 expression in the bone marrow and how it relates to embryonic expression is still unclear. It is at present unknown whether zeugopod BM-MSCs originate from cells that were earlier expressing Hoxa11 or whether they correspond to a fully de novo expression pattern, without any lineage dependence toward Hoxa11-expressing cells. Solving this question should not only clarify the embryonic origin of these cells but also put within reach the mechanisms underlying transitioning from embryonic to adult Hox expression patterns. Given the role of BM-MSCs in bone repair, the authors next elegantly combined classical mouse genetics to ex vivo analyses after FACS to probe the contribution of Hox11 genes to this process. They found that upon bone injury, Hoxa11 BM-MSC-positive cells expand, and transplant experiments showed that the progeny of Hoxa11 BM-MSCs participate in fracture repair. Most convincingly, the authors found that in Hoxa11 compound mutants, frac-
ture repair is severely impaired. The basis for failure of fracture repair results from the requirement of Hox11 genes for proper differentiation of skeletal cell from BM-MSCs: in vitro, Hox11 BMMSC mutant cells fail to differentiate into osteogenic and chondrogenic cells but instead produce adipose cells, whereas in vivo, cartilage that serves as an anlage for the formation of new bones poorly develops at sites of injury when compared to wild-type mice. Whether fracture repair exclusively relies on cells initially expressing Hoxa11, or also involves the conversion of cells toward a Hoxa11-positive state, as may be the case for periosteal cells (Leucht et al., 2008), will need to be addressed to apprehend more extensively the cellular processes underlying bone marrowbased Hox-mediated repair. The work of Rux et al. (2016) also investigates more broadly the expression of other Hox genes in the adult bone marrow. Like Hoxa11 in the zeugopod, Hoxa9 and Hoxa10 are expressed only in the non-endothelial stromal cells that comprise stylopod BM-MSCs. A similar restriction of expression was also found outside the limbs in the sternum for Hoxa5, Hoxb6, and Hoxc6, suggesting that adult Hox gene expression in BM-MSCs may be a general feature of Hox adult expression patterns. This sets the context for a general role for Hox genes in the biology of BMMSCs. The extension of the findings from Hoxa11 to additional Hox genes may be one of the most exciting prospects of the work, as it suggests that bone repair in different animal body parts is likely controlled through different Hox proteins. Suggested by the expression patterns, this is also supported by the finding that the Hox11 compound mutation delays
Developmental Cell 39, December 19, 2016 ª 2016 Elsevier Inc. 627
Developmental Cell
Previews
Figure 1. From Skeletal Patterning to Bone Repair In the developing embryonic limb, differential posterior Hox gene expression along the proximo-distal axis provides positional information allowing for the establishment of distinct bone identities. In the adult, in addition to continued expression in the periosteum (PO), Hox genes are expressed in BM-MSCs in the bone marrow (BM), in-frame with embryonic expression territories: Paralog Group (PG) 5–6 genes (orange) in the sternum (not depicted), PG9–10 (red) genes in the stylopod, PG11 (green) in the zeugopod, and PG12–13 (blue) predictively in the autopod. In the zeugopod, PG11-expressing BM-MSCs (dark green) are mobilized at the site of fracture and favor fracture repair by promoting the differentiation into skeletal cells (light green). PG9–10 and PG12–13 may similarly control bone repair in the stylopod (pink) and autopod (light blue). The extent of conservation/divergence of the cellular and molecular events underlying Hox-mediated bone repair in the different limb parts remains to be established.
bone repair only in the zeugopod and not the neighboring stylopod. This is a somewhat unexpected prediction as the process does not have, or need to have, any apparent regional specificity, a usual attribute of Hox paralog proteins. A similar conclusion also recently emerged from the study of Drosophila Hox proteins in the fat body, where most Hox proteins are repressing autophagy, irrespective of their paralog identity (Banreti et al., 2014). This collectively suggests that Hox proteins, besides their specific functions, may also have shared generic functions, which may be the result of their phylogenetic origin. Demonstrating the
dependence of bone repair upon the activity of Hox9 and Hox10 genes in BMMSC stylopod cells would clarify whether bone repair represents a case of shared generic Hox activity. If so, it will also provide a framework to explore whether the molecular control by Hox genes in the different limb parts relies on the activation of a same/similar genetic program or on region-specific distinct repair programs instead. Transplant experiments and/or Hox gene swap experiments, aimed at probing the importance of the initial origin of BM-MSCs and of the Hox expression status for efficient repair, is also a clear prospect for further investi-
628 Developmental Cell 39, December 19, 2016
gations. Solving these questions not only is of fundamental relevance to the biology of Hox proteins, but also is crucial when considering the potential use of BM-MSCs for stem cell-based regenerative medicine, as it may impose spatial constraints on the origin of BMMSCs to be used.
REFERENCES Alharbi, R.A., Pettengell, R., Pandha, H.S., and Morgan, R. (2013). Leukemia 27, 1000–1008. Banreti, A., Hudry, B., Sass, M., Saurin, A.J., and Graba, Y. (2014). Dev. Cell 28, 56–69.
Developmental Cell
Previews Bobola, N., and Merabet, S. (2016). Curr. Opin. Genet. Dev. 43, 1–8. Du, H., and Taylor, H.S. (2015). Cold Spring Harb. Perspect. Med. 6, a023002. Leucht, P., Kim, J.B., Amasha, R., James, A.W., Girod, S., and Helms, J.A. (2008). Development 135, 2845–2854.
Morgan, R. (2006). Trends Genet. 22, 67–69. Pineault, K.M., and Wellik, D.M. (2014). Curr. Osteoporos. Rep. 12, 420–427.
Rux, D.R., Song, J.Y., Swinehart, I.T., Pineault, K.M., Schlientz, A.J., Trulik, K.J., Goldstein, S.A., Kozloff, K.M., Lucas, D., and Wellik, D.M. (2016). Dev. Cell 39, this issue, 653–666.
Rezsohazy, R., Saurin, A.J., Maurel-Zaffran, C., and Graba, Y. (2015). Development 142, 1212– 1227.
Zhou, B.O., Yue, R., Murphy, M.M., Peyer, J.G., and Morrison, S.J. (2014). Cell Stem Cell 15, 154–168.
Robo-Enabled Tumor Cell Extrusion Helena E. Richardson1,* and Marta Portela2 1Department
of Biochemistry and Genetics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, VIC 3086, Australia of Molecular, Cellular, and Developmental Neurobiology, Cajal Institute (CSIC), Avenida Doctor Arce, 37, Madrid 28002, Spain *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.12.008 2Department
How aberrant cells are removed from a tissue to prevent tumor formation is a key question in cancer biology. Reporting in this issue of Developmental Cell, Vaughen and Igaki (2016) show that a pathway with an important role in neural guidance also directs extrusion of tumor cells from epithelial tissues. In all multicellular organisms, damaged or mutant cells need to be recognized and removed to maintain tissue function and prevent tumor formation. A surveillance mechanism known as cell competition is important for the detection and elimination of aberrant cells in order to maintain tissue homeostasis (reviewed by Merino et al., 2016). In cell competition, cellular ‘‘fitness,’’ a reflection of translation rates, cellular growth, mitogenic signaling, and cell polarity, is monitored within a tissue, and cells with lower fitness (losers) due to genetic or physical damage are recognized and actively eliminated from the tissue. Cell competition was originally discovered in the vinegar fly model organism, Drosophila melanogaster, but is now also known to occur in mammalian systems (reviewed by Merino et al., 2016). Studies in Drosophila have revealed that the mechanism by which loser cells, with reduced translation, cellular growth, or mitogenic signaling, are recognized depends upon cell-surface receptors and innate immune system signaling, which result in the induction of caspase-dependent cell death and extrusion from the epithelium (reviewed by Merino et al., 2016). In 95% of cases, extrusion occurs basally and the dying cells are engulfed by macrophage-like cells (hemocytes), while in 5% of cases the loser cells are en-
gulfed by their epithelial neighbors in the apical region (Casas-Tinto´ et al., 2015). Cells with aberrant cell polarity or morphology within an epithelium appear to require different mechanisms to elicit their removal and engulfment (Ohsawa et al., 2011; reviewed by Pastor-Pareja and Xu, 2013). Disruption of cell polarity, as occurs with loss-of-function mutations in the apicobasal cell polarity regulator Scribbled (Scrib) leads to elevation of the Jun kinase (JNK) signaling pathway, which in a heterogenic epithelium triggers caspase-mediated cell death of the scrib mutant cells (reviewed by Pastor-Pareja and Xu, 2013). However, elevated JNK signaling in polarity-impaired cells also regulates the expression of differentiation and signaling pathway genes (Bunker et al., 2015), which might impact cell competition, extrusion, and elimination processes. In this issue of Developmental Cell, Vaughen and Igaki (2016) have discovered targets of JNK signaling that are important in the extrusion mechanism of scrib mutant cells from a heterogenic epithelium in Drosophila. Vaughen and Igaki identified Enabled/ VASP (Ena), a regulator of actin nucleation, in a genetic screen for genes required for scrib mutant cell elimination in the Drosophila eye epithelium. Ena is known to be regulated by the Slit ligand and
Roundabout (Robo) receptor, which play a repulsive role in neural pathfinding. Consistent with the conservation of this regulatory mechanism, Vaughen and Igaki also found that Slit and one of the three Drosophila Robo receptors, Robo2, were required for the elimination of scrib mutant cells. Also in line with the role of Slit-Robo2Ena in cell-cell repulsion, Vaughen and Igaki (2016) observed that this pathway was involved in the extrusion of scrib mutant cells from the epithelium, predominantly basally, where the cells rapidly underwent apoptosis. When Slit-Robo2-Ena signaling was disrupted, scrib mutant cells remained in the epithelial layer and overproliferated to form tumors. Moreover, they showed that slit, robo2, and ena are targets of the JNK signaling pathway and identified binding sites for the JNK-regulated transcription factor AP1 (Jun/Fos), in the intronic regions of these genes. Vaughen and Igaki (2016) also found that JNK signaling was important for the basal extrusion of scrib mutant cells and that Slit-Robo2-Ena signaling was required downstream of JNK for scrib mutant cell extrusion. Interestingly, they also found that Slit-Robo2-Ena signaling resulted in elevated F-actin polymerization and JNK activation, triggering a positive feedback loop enhancing slit, robo2, and ena gene expression. Altogether, their results
Developmental Cell 39, December 19, 2016 ª 2016 Elsevier Inc. 629
Previews Hox Service Warranty Extends to Adult Bone Repair Andrew J. Saurin,1 Marie-Claire Delfini,1 and Yacine Graba1,* 1Aix-Marseille Universite ´ , CNRS, IBDM, UMR7288, case 907, 13288 Marseille Cedex 09, France *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.12.007
Hox genes are key developmental regulators. In this issue of Developmental Cell, Rux et al. (2016) uncover an adult role for Hox11 genes in regionalized bone repair. This function relies on Hox activity in bone marrow multipotent mesenchymal stem progenitor cells, which promotes skeletal cell differentiation. Hox genes were initially recognized as key developmental regulators, providing antero-posterior positional information and promoting diversified morphogenesis (Bobola and Merabet, 2016; Rezsohazy et al., 2015). They also similarly provide proximo-distal positional information along the developing fore- and hindlimbs, with the Hox9/10, Hox11, and Hox13 paralogs expressed in the proximal (stylopod), median (zeugopod), and distal (autopod) limb parts, respectively (Figure 1, top; Pineault and Wellik, 2014). Consequently, each limb compartment will acquire bones with distinct identities: humerus, radius/ulna, and carpals/metacarpals in the forelimb; and femur, tibia/ fibula, and tarsal/metatarsal in the hindlimb. Besides these well-studied developmental functions, it was also early recognized that Hox gene expression is maintained in postnatal and adult stages, in patterns that recapitulate embryonic characteristics, suggesting that adult Hox gene function may be in continuity with embryonic activity (Morgan, 2006). However, with the exception of hematopoiesis and the female reproductive tract (Alharbi et al., 2013; Du and Taylor, 2015), which to some extent are seen as developmental processes occurring in the adult body, roles for regionalized adult Hox gene expression have not been determined. In this issue of Developmental Cell, Rux et al. (2016) report on an adult function of Hox11 paralog group genes in limb bone repair. Previous work established that continued Hox expression in the periosteum allows the promotion of bone regeneration, through repair processes that are distinct from those occurring in Hox-free regions (Leucht et al., 2008). Using a Hoxa11 reporter line to follow expression in the limb, Rux et al. (2016) found that adult expres-
sion not only is maintained in a pattern reminiscent of embryonic expression within the periosteum, but also appears in the zeugopod bone marrow (Figure 1, bottom). Using relevant markers, the authors provide compelling evidence that the Hoxa11-expressing cells are bone marrow multipotent mesenchymal stem/progenitor cells (BM-MSCs), a low abundant subpopulation of nonendothelial stromal cells (Zhou et al., 2014). The identity of Hox11-expressing bone marrow cells was confirmed by the demonstration that Hoxa11-expressing cells display the functional characteristics of BM-MSCs in ex vivo differentiation (capacity to generate chondrocytes, osteoblasts, and adipocytes) and self-renewing assays. The origin of adult Hoxa11 expression in the bone marrow and how it relates to embryonic expression is still unclear. It is at present unknown whether zeugopod BM-MSCs originate from cells that were earlier expressing Hoxa11 or whether they correspond to a fully de novo expression pattern, without any lineage dependence toward Hoxa11-expressing cells. Solving this question should not only clarify the embryonic origin of these cells but also put within reach the mechanisms underlying transitioning from embryonic to adult Hox expression patterns. Given the role of BM-MSCs in bone repair, the authors next elegantly combined classical mouse genetics to ex vivo analyses after FACS to probe the contribution of Hox11 genes to this process. They found that upon bone injury, Hoxa11 BM-MSC-positive cells expand, and transplant experiments showed that the progeny of Hoxa11 BM-MSCs participate in fracture repair. Most convincingly, the authors found that in Hoxa11 compound mutants, frac-
ture repair is severely impaired. The basis for failure of fracture repair results from the requirement of Hox11 genes for proper differentiation of skeletal cell from BM-MSCs: in vitro, Hox11 BMMSC mutant cells fail to differentiate into osteogenic and chondrogenic cells but instead produce adipose cells, whereas in vivo, cartilage that serves as an anlage for the formation of new bones poorly develops at sites of injury when compared to wild-type mice. Whether fracture repair exclusively relies on cells initially expressing Hoxa11, or also involves the conversion of cells toward a Hoxa11-positive state, as may be the case for periosteal cells (Leucht et al., 2008), will need to be addressed to apprehend more extensively the cellular processes underlying bone marrowbased Hox-mediated repair. The work of Rux et al. (2016) also investigates more broadly the expression of other Hox genes in the adult bone marrow. Like Hoxa11 in the zeugopod, Hoxa9 and Hoxa10 are expressed only in the non-endothelial stromal cells that comprise stylopod BM-MSCs. A similar restriction of expression was also found outside the limbs in the sternum for Hoxa5, Hoxb6, and Hoxc6, suggesting that adult Hox gene expression in BM-MSCs may be a general feature of Hox adult expression patterns. This sets the context for a general role for Hox genes in the biology of BMMSCs. The extension of the findings from Hoxa11 to additional Hox genes may be one of the most exciting prospects of the work, as it suggests that bone repair in different animal body parts is likely controlled through different Hox proteins. Suggested by the expression patterns, this is also supported by the finding that the Hox11 compound mutation delays
Developmental Cell 39, December 19, 2016 ª 2016 Elsevier Inc. 627
Developmental Cell
Previews
Figure 1. From Skeletal Patterning to Bone Repair In the developing embryonic limb, differential posterior Hox gene expression along the proximo-distal axis provides positional information allowing for the establishment of distinct bone identities. In the adult, in addition to continued expression in the periosteum (PO), Hox genes are expressed in BM-MSCs in the bone marrow (BM), in-frame with embryonic expression territories: Paralog Group (PG) 5–6 genes (orange) in the sternum (not depicted), PG9–10 (red) genes in the stylopod, PG11 (green) in the zeugopod, and PG12–13 (blue) predictively in the autopod. In the zeugopod, PG11-expressing BM-MSCs (dark green) are mobilized at the site of fracture and favor fracture repair by promoting the differentiation into skeletal cells (light green). PG9–10 and PG12–13 may similarly control bone repair in the stylopod (pink) and autopod (light blue). The extent of conservation/divergence of the cellular and molecular events underlying Hox-mediated bone repair in the different limb parts remains to be established.
bone repair only in the zeugopod and not the neighboring stylopod. This is a somewhat unexpected prediction as the process does not have, or need to have, any apparent regional specificity, a usual attribute of Hox paralog proteins. A similar conclusion also recently emerged from the study of Drosophila Hox proteins in the fat body, where most Hox proteins are repressing autophagy, irrespective of their paralog identity (Banreti et al., 2014). This collectively suggests that Hox proteins, besides their specific functions, may also have shared generic functions, which may be the result of their phylogenetic origin. Demonstrating the
dependence of bone repair upon the activity of Hox9 and Hox10 genes in BMMSC stylopod cells would clarify whether bone repair represents a case of shared generic Hox activity. If so, it will also provide a framework to explore whether the molecular control by Hox genes in the different limb parts relies on the activation of a same/similar genetic program or on region-specific distinct repair programs instead. Transplant experiments and/or Hox gene swap experiments, aimed at probing the importance of the initial origin of BM-MSCs and of the Hox expression status for efficient repair, is also a clear prospect for further investi-
628 Developmental Cell 39, December 19, 2016
gations. Solving these questions not only is of fundamental relevance to the biology of Hox proteins, but also is crucial when considering the potential use of BM-MSCs for stem cell-based regenerative medicine, as it may impose spatial constraints on the origin of BMMSCs to be used.
REFERENCES Alharbi, R.A., Pettengell, R., Pandha, H.S., and Morgan, R. (2013). Leukemia 27, 1000–1008. Banreti, A., Hudry, B., Sass, M., Saurin, A.J., and Graba, Y. (2014). Dev. Cell 28, 56–69.
Developmental Cell
Previews Bobola, N., and Merabet, S. (2016). Curr. Opin. Genet. Dev. 43, 1–8. Du, H., and Taylor, H.S. (2015). Cold Spring Harb. Perspect. Med. 6, a023002. Leucht, P., Kim, J.B., Amasha, R., James, A.W., Girod, S., and Helms, J.A. (2008). Development 135, 2845–2854.
Morgan, R. (2006). Trends Genet. 22, 67–69. Pineault, K.M., and Wellik, D.M. (2014). Curr. Osteoporos. Rep. 12, 420–427.
Rux, D.R., Song, J.Y., Swinehart, I.T., Pineault, K.M., Schlientz, A.J., Trulik, K.J., Goldstein, S.A., Kozloff, K.M., Lucas, D., and Wellik, D.M. (2016). Dev. Cell 39, this issue, 653–666.
Rezsohazy, R., Saurin, A.J., Maurel-Zaffran, C., and Graba, Y. (2015). Development 142, 1212– 1227.
Zhou, B.O., Yue, R., Murphy, M.M., Peyer, J.G., and Morrison, S.J. (2014). Cell Stem Cell 15, 154–168.
Robo-Enabled Tumor Cell Extrusion Helena E. Richardson1,* and Marta Portela2 1Department
of Biochemistry and Genetics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, VIC 3086, Australia of Molecular, Cellular, and Developmental Neurobiology, Cajal Institute (CSIC), Avenida Doctor Arce, 37, Madrid 28002, Spain *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.12.008 2Department
How aberrant cells are removed from a tissue to prevent tumor formation is a key question in cancer biology. Reporting in this issue of Developmental Cell, Vaughen and Igaki (2016) show that a pathway with an important role in neural guidance also directs extrusion of tumor cells from epithelial tissues. In all multicellular organisms, damaged or mutant cells need to be recognized and removed to maintain tissue function and prevent tumor formation. A surveillance mechanism known as cell competition is important for the detection and elimination of aberrant cells in order to maintain tissue homeostasis (reviewed by Merino et al., 2016). In cell competition, cellular ‘‘fitness,’’ a reflection of translation rates, cellular growth, mitogenic signaling, and cell polarity, is monitored within a tissue, and cells with lower fitness (losers) due to genetic or physical damage are recognized and actively eliminated from the tissue. Cell competition was originally discovered in the vinegar fly model organism, Drosophila melanogaster, but is now also known to occur in mammalian systems (reviewed by Merino et al., 2016). Studies in Drosophila have revealed that the mechanism by which loser cells, with reduced translation, cellular growth, or mitogenic signaling, are recognized depends upon cell-surface receptors and innate immune system signaling, which result in the induction of caspase-dependent cell death and extrusion from the epithelium (reviewed by Merino et al., 2016). In 95% of cases, extrusion occurs basally and the dying cells are engulfed by macrophage-like cells (hemocytes), while in 5% of cases the loser cells are en-
gulfed by their epithelial neighbors in the apical region (Casas-Tinto´ et al., 2015). Cells with aberrant cell polarity or morphology within an epithelium appear to require different mechanisms to elicit their removal and engulfment (Ohsawa et al., 2011; reviewed by Pastor-Pareja and Xu, 2013). Disruption of cell polarity, as occurs with loss-of-function mutations in the apicobasal cell polarity regulator Scribbled (Scrib) leads to elevation of the Jun kinase (JNK) signaling pathway, which in a heterogenic epithelium triggers caspase-mediated cell death of the scrib mutant cells (reviewed by Pastor-Pareja and Xu, 2013). However, elevated JNK signaling in polarity-impaired cells also regulates the expression of differentiation and signaling pathway genes (Bunker et al., 2015), which might impact cell competition, extrusion, and elimination processes. In this issue of Developmental Cell, Vaughen and Igaki (2016) have discovered targets of JNK signaling that are important in the extrusion mechanism of scrib mutant cells from a heterogenic epithelium in Drosophila. Vaughen and Igaki identified Enabled/ VASP (Ena), a regulator of actin nucleation, in a genetic screen for genes required for scrib mutant cell elimination in the Drosophila eye epithelium. Ena is known to be regulated by the Slit ligand and
Roundabout (Robo) receptor, which play a repulsive role in neural pathfinding. Consistent with the conservation of this regulatory mechanism, Vaughen and Igaki also found that Slit and one of the three Drosophila Robo receptors, Robo2, were required for the elimination of scrib mutant cells. Also in line with the role of Slit-Robo2Ena in cell-cell repulsion, Vaughen and Igaki (2016) observed that this pathway was involved in the extrusion of scrib mutant cells from the epithelium, predominantly basally, where the cells rapidly underwent apoptosis. When Slit-Robo2-Ena signaling was disrupted, scrib mutant cells remained in the epithelial layer and overproliferated to form tumors. Moreover, they showed that slit, robo2, and ena are targets of the JNK signaling pathway and identified binding sites for the JNK-regulated transcription factor AP1 (Jun/Fos), in the intronic regions of these genes. Vaughen and Igaki (2016) also found that JNK signaling was important for the basal extrusion of scrib mutant cells and that Slit-Robo2-Ena signaling was required downstream of JNK for scrib mutant cell extrusion. Interestingly, they also found that Slit-Robo2-Ena signaling resulted in elevated F-actin polymerization and JNK activation, triggering a positive feedback loop enhancing slit, robo2, and ena gene expression. Altogether, their results
Developmental Cell 39, December 19, 2016 ª 2016 Elsevier Inc. 629