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A Kinase Gets Caspases into Shape

2006; Cell Press; Volume: 126; Issue: 3 Linguagem: Inglês

10.1016/j.cell.2006.07.017

1097-4172

Denise J. Montell,

Signaling Pathways in Disease

Two new papers (Kuranaga et al., 2006Kuranaga E. Kanuka H. Tonoki A. Takemoto K. Tomioka T. Kobayashi M. Hayashi S. Miura M. Cell. 2006; (this issue)PubMed Google Scholar, Oshima et al., 2006Oshima K. Takeda M. Kuranaga E. Ueda R. Aigaki T. Miura M. Hayashi S. Curr. Biol. 2006; (in press Published online August 3, 2006)https://doi.org/10.1016/j.cub.2006.06.032Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar) describe a previously uncharacterized Drosophila kinase (DmIKKɛ) that regulates the abundance of DIAP1, a protein best known for its ability to inhibit apoptosis. However, DmIKKɛ-mediated degradation of DIAP1 does not regulate apoptosis as might be predicted but instead regulates actin dynamics, cell morphology, and the differentiation of sensory organ precursor cells. Two new papers (Kuranaga et al., 2006Kuranaga E. Kanuka H. Tonoki A. Takemoto K. Tomioka T. Kobayashi M. Hayashi S. Miura M. Cell. 2006; (this issue)PubMed Google Scholar, Oshima et al., 2006Oshima K. Takeda M. Kuranaga E. Ueda R. Aigaki T. Miura M. Hayashi S. Curr. Biol. 2006; (in press Published online August 3, 2006)https://doi.org/10.1016/j.cub.2006.06.032Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar) describe a previously uncharacterized Drosophila kinase (DmIKKɛ) that regulates the abundance of DIAP1, a protein best known for its ability to inhibit apoptosis. However, DmIKKɛ-mediated degradation of DIAP1 does not regulate apoptosis as might be predicted but instead regulates actin dynamics, cell morphology, and the differentiation of sensory organ precursor cells. Life is fragile. Within multicellular organisms, cells are bombarded with signals from within and from without in a constant struggle between life and death. When the balance tips toward death, cells undergo a stereotypical series of morphological changes in the process known as apoptosis. They shrink, break contacts with neighboring cells, bleb; their microvilli disappear, their nuclei condense, and their chromosomes fragment. Cells undergo apoptosis for a variety of reasons, sometimes to the benefit of the organism, sometimes to its detriment. So, understanding the mechanisms that regulate the balance between life and death is of obvious importance. In addition, many components of the apoptosis molecular machinery participate in cellular activities that are non-life-threatening (Figure 1). Work described in two new papers (Kuranaga et al., 2006Kuranaga E. Kanuka H. Tonoki A. Takemoto K. Tomioka T. Kobayashi M. Hayashi S. Miura M. Cell. 2006; (this issue)PubMed Google Scholar, Oshima et al., 2006Oshima K. Takeda M. Kuranaga E. Ueda R. Aigaki T. Miura M. Hayashi S. Curr. Biol. 2006; (in press Published online August 3, 2006)https://doi.org/10.1016/j.cub.2006.06.032Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar) now identifies a new kinase that regulates actin dynamics, cell morphology, and cell differentiation via the nonapoptotic functions of the cell death pathway. In this issue of Cell, Kuranaga et al., 2006Kuranaga E. Kanuka H. Tonoki A. Takemoto K. Tomioka T. Kobayashi M. Hayashi S. Miura M. Cell. 2006; (this issue)PubMed Google Scholar demonstrate that DmIKKɛ is a new type of antagonist of the DIAP1 (Drosophila inhibitor of apoptosis 1) protein. Independently of other known antagonists, DmIKKɛ phosphorylates DIAP1, leading to its degradation (Figure 1). The authors first carried out a genetic screen in the fruit fly Drosophila for mutations that modify the eye-loss phenotype caused by overexpression of the proapoptotic gene reaper, which is a known antagonist of DIAP1. This led them to a previously uncharacterized gene that encodes a protein kinase, which they named DmIKKɛ. Overexpression of DmIKKɛ led to increased caspase activity and apoptotic cell death in imaginal discs. Furthermore, they found that DIAP1 protein levels were reduced in cells overexpressing DmIKKɛ, both in vivo and in vitro. In cultured S2 cells, DmIKKɛ associated with and phosphorylated DIAP1. In addition, expression of NAK (NF-κB-activating kinase)—the human ortholog of DmIKKɛ—in S2 cells caused depletion of DIAP1 and cell death. NAK also phosphorylated XIAP, one of the mammalian IAPs, suggesting that this relationship has been conserved during evolution. These observations are important because they identify DmIKKɛ as a new determinant of IAP protein level, which can mean the difference between life and death. IAPs bind to caspases and inhibit their activities, and caspases are primary molecular arbiters of apoptosis. Proteins such as Reaper bind to DIAP1 and cause it to self-destruct, thereby derepressing caspase activity and promoting apoptosis (Figure 1). If the level of DIAP1 exceeds that of its antagonists, caspases will be shut down and the cell will live; otherwise, it will die. But that is not the whole story because DIAP1 and caspases do not exist solely to inhibit cell death. DIAP1 can also promote cell migration, and caspases have been shown to control a variety of nonapoptotic functions including cell proliferation, differentiation, morphogenesis, and migration. Nonapoptotic functions have been discovered in both mammals (Launay et al., 2005Launay S. Hermine O. Fontenay M. Kroemer G. Solary E. Garrido C. Oncogene. 2005; 24: 5137-5148Crossref PubMed Scopus (197) Google Scholar, Schwerk and Schulze-Osthoff, 2003Schwerk C. Schulze-Osthoff K. Biochem. Pharmacol. 2003; 66: 1453-1458Crossref PubMed Scopus (177) Google Scholar) and Drosophila (Cashio et al., 2005Cashio P. Lee T.V. Bergmann A. Semin. Cell Dev. Biol. 2005; 16: 225-235Crossref PubMed Scopus (58) Google Scholar) . The new work by Kuranaga et al., 2006Kuranaga E. Kanuka H. Tonoki A. Takemoto K. Tomioka T. Kobayashi M. Hayashi S. Miura M. Cell. 2006; (this issue)PubMed Google Scholar combines in vivo genetic experiments with biochemical analysis of proteins expressed in cultured cells to define the function of DmIKKɛ. Although high levels of DmIKKɛ expression in vivo cause apoptosis, and although reducing DmIKKɛ inhibited artificially induced cell death, the normal function of this kinase does not appear to be regulation of apoptosis. Reduction of DmIKKɛ expression does not lead to increased apoptosis in embryos or imaginal discs, even though it does reduce caspase activity. So what is the function of DmIKKɛ-mediated caspase activation, if not to control apoptosis? In earlier work, the same lab showed that caspase activity can be detected in imaginal disc cells that are not undergoing apoptosis and that sensory organ precursor (SOP) development depends on caspase activity but not apoptosis (Kanuka et al., 2005Kanuka H. Kuranaga E. Takemoto K. Hiratou T. Okano H. Miura M. EMBO J. 2005; 24: 3793-3806Crossref PubMed Scopus (84) Google Scholar). They also showed that the apical caspase DRONC can cleave and thereby activate one particular isoform of the kinase Shaggy, which participates in Wingless signaling. The new work shows that the number of SOP cells is also affected by DmIKKɛ through this previously defined nonapoptotic function of DRONC. In a collaborative and complementary study recently published in Current Biology, Oshima et al., 2006Oshima K. Takeda M. Kuranaga E. Ueda R. Aigaki T. Miura M. Hayashi S. Curr. Biol. 2006; (in press Published online August 3, 2006)https://doi.org/10.1016/j.cub.2006.06.032Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar came across the same DmIKKɛ gene in a different genetic screen. These investigators screened for genes that, when overexpressed, caused defects in the organization of the Drosophila tracheal system, an interconnected network of tubes that constitutes the breathing system of the fly. Oshima et al. found that overexpression of DmIKKɛ caused loss of apical-basal polarity in tracheal epithelial cells and, as a consequence, disorganization of the tracheal network. Suspecting that the actin cytoskeleton might be involved, they directly examined the effects of inhibiting or enhancing the activity of DMIKKɛ on actin dynamics in cultured S2 cells. Expression of a dominant-negative kinase caused morphological changes in the cells and enhanced the stability of the actin cytoskeleton. Shrinkage of the dynamic domain of peripheral actin was also observed in ruffling cells, and, together, these observations suggested that DmIKKɛ might be a negative regulator of F-actin polymerization. Consistent with this notion, overexpression of the kinase destabilized the actin cytoskeleton and caused excess membrane ruffling. These effects were apparently independent of apoptosis. The investigators went on to examine the effect of manipulating DmIKKɛ function in specific cells in vivo, either reducing or increasing its activity. They found a variety of effects on the morphology of actin-based structures such as sensory bristles, tracheal cell protrusions, and lateral branches of the arista (thin sensory protrusions that extend off of the fly antenna). For example, DmIKKɛ depletion caused ectopic branching of each of these structures, a phenotype that is also observed following overexpression of well-characterized regulators of actin polymerization such as profilin. Based on genetic interactions, ectopic branching observed following depletion of DmIKKɛ appears to result from excess DIAP1 protein. So how does excess DIAP1 lead to increased branching of actin-rich protrusions? A previous study showed that DIAP1 overexpression suppressed defects in border cell migration that were caused by dominant-negative Rac, another well-characterized regulator of cell morphology and actin protrusions. Dominant-negative DRONC also rescued the phenotype (Geisbrecht and Montell, 2004Geisbrecht E.R. Montell D.J. Cell. 2004; 118: 111-125Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar). Consistent with these observations, Oshima et al., 2006Oshima K. Takeda M. Kuranaga E. Ueda R. Aigaki T. Miura M. Hayashi S. Curr. Biol. 2006; (in press Published online August 3, 2006)https://doi.org/10.1016/j.cub.2006.06.032Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar found that overexpression of DIAP1 or dominant-negative DRONC enhanced the excessive branching caused by low DmIKKɛ. Taken together, the new data in both papers show that DmIKKɛ functions as a negative regulator of DIAP1 protein levels and is thereby a positive regulator of caspase activity. The primary function of DmIKKɛ in vivo seems to be to promote the nonapoptotic functions of the caspase DRONC in a variety of cell types. These functions include specification of SOPs on the one hand and regulation of actin polymerization and actin-based cell morphology on the other. This intriguing new work adds to the existing literature implicating caspases and their regulators in nonapoptotic functions and raises a number of compelling questions. For example, what activates DmIKKɛ? One study in cultured HEK293 cells showed that the platelet-derived growth factor (PDGF) stimulates NAK activity via protein kinase C, providing one possible mechanism of activation of the mammalian homolog of DmIKKɛ (Tojima et al., 2000Tojima Y. Fujimoto A. Delhase M. Chen Y. Hatakeyama S. Nakayama K. Kaneko Y. Nimura Y. Motoyama N. Ikeda K. et al.Nature. 2000; 404: 778-782Crossref PubMed Scopus (314) Google Scholar). However, it is unclear whether this is the only activation pathway or whether there might be different inputs in different biological contexts. In keeping with this idea, NAK was originally identified as regulating the NF-κB pathway, which is not related in any obvious way to the new findings. A second question concerns the identity of the critical targets of DRONC in its nonapoptotic functions. With respect to changes in cell morphology and actin organization, the small GTPase Rac, actin, and other proteins that affect actin are known caspase targets. Which of these, if any, mediate the reported biological effects remains to be determined. What is the unifying theme, if any, between the different biological processes that employ caspase activity? Some of the differentiation events controlled by caspases, such as lens differentiation and sperm individualization, resemble incomplete apoptosis. For example, the nucleus is extruded from differentiated lens cells, whereas in sperm individualization, the cytoplasmic volume is reduced while the nucleus is spared. In these cases, it is not difficult to imagine how the apoptotic machinery came to be involved. But what about the others? Apoptosis was originally described as a series of morphological changes. One possibility is that a subset of the morphological changes associated with apoptosis are related to normal changes that cells undergo during development. For example, apoptotic cells round up, detach from their neighbors, and undergo reorganization of the actin cytoskeleton, behaviors that also characterize motile cells leaving an epithelium. Perhaps certain cells have harnessed the activities of caspases and their regulators in order to accomplish a subset of the features of apoptosis and put them to use. But how do they survive in the presence of activated caspase? Is caspase activity physically compartmentalized so that access to substrates is very limited? Or is it simply a question of the level of caspase activity that determines how a cell responds? Perhaps the presence (or absence) of other factors limits the effects of an active caspase? Might the answers be different for different cell types? These are some of the questions that promise to keep investigators interested in life and death busy for the foreseeable future. Drosophila IKK-Related Kinase Regulates Nonapoptotic Function of Caspases via Degradation of IAPsKuranaga et al.CellAugust 11, 2006In BriefCaspase activation has been extensively studied in the context of apoptosis. However, caspases also control other cellular functions, although the mechanisms regulating caspases in nonapoptotic contexts remain obscure. Drosophila IAP1 (DIAP1) is an endogenous caspase inhibitor that is crucial for regulating cell death during development. Here we describe Drosophila IKK-related kinase (DmIKKɛ) as a regulator of caspase activation in a nonapoptotic context. We show that DmIKKɛ promotes degradation of DIAP1 through direct phosphorylation. Full-Text PDF Open ArchiveIKKɛ Regulates F Actin Assembly and Interacts with Drosophila IAP1 in Cellular MorphogenesisOshima et al.Current BiologyAugust 08, 2006In BriefDifferentiated cells assume complex shapes through polarized cell migration and growth. These processes require the restricted organization of the actin cytoskeleton at limited subcellular regions. IKKɛ is a member of the IκB kinase family, and its developmental role has not been clear. Drosophila IKKɛ was localized to the ruffling membrane of cultured cells and was required for F actin turnover at the cell margin. In IKKɛ mutants, tracheal terminal cells, bristles, and arista laterals, which require accurate F actin assembly for their polarized elongation, all exhibited aberrantly branched morphology. Full-Text PDF Open Archive