cactus: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - cactus

Synonyms -

Cytological map position - 35E6-36A9

Function - regulates nuclear entry of Dorsal

Keywords - Dorsal group

Symbol - cact

FlyBase ID:FBgn0000250

Genetic map position - 2-52

Classification - I kappa B homolog

Cellular location - cytoplasmic



NCBI links: Precomputed BLAST | Entrez Gene
Recent literature
O'Connell, M.D. and Reeves G.T. (2015). The presence of nuclear Cactus in the early Drosophila embryo may extend the dynamic range of the dorsal gradient. PLoS Comput Biol 11: e1004159. PubMed ID: 25879657
Summary:
In a developing embryo, the spatial distribution of a signaling molecule, or a morphogen gradient, has been hypothesized to carry positional information to pattern tissues. Recent measurements of morphogen distribution have allowed this hypothesis to be subjected to rigorous physical testing. In the early Drosophila embryo, measurements of the morphogen Dorsal, which is a transcription factor responsible for initiating the earliest zygotic patterns along the dorsal-ventral axis, have revealed a gradient that is too narrow to pattern the entire axis. This study uses a mathematical model of Dorsal dynamics, fit to experimental data, to determine the ability of the Dorsal gradient to regulate gene expression across the entire dorsal-ventral axis. Two assumptions were required for the model to match experimental data in both Dorsal distribution and gene expression patterns. First, Cactus, an inhibitor that binds to Dorsal and prevents it from entering the nuclei, must itself be present in the nuclei. And second, fluorescence measurements of Dorsal reflect both free Dorsal and Cactus-bound Dorsal. The model explains the dynamic behavior of the Dorsal gradient at lateral and dorsal positions of the embryo, the ability of Dorsal to regulate gene expression across the entire dorsal-ventral axis, and the robustness of gene expression to stochastic effects. These results have a general implication for interpreting fluorescence-based measurements of signaling molecules.

Liu, B., Zheng, Y., Yin, F., Yu, J., Silverman, N. and Pan, D. (2016). Toll receptor-mediated Hippo signaling controls innate immunity in Drosophila. Cell 164: 406-419. PubMed ID: 26824654
Summary:
The Hippo signaling pathway functions through Yorkie to control tissue growth and homeostasis. How this pathway regulates non-developmental processes remains largely unexplored. This study reports an essential role for Hippo signaling in innate immunity whereby Yorkie directly regulates the transcription of the Drosophila IκB homolog, Cactus, in Toll receptor-mediated antimicrobial response. Loss of Hippo pathway tumor suppressors or activation of Yorkie in fat bodies, the Drosophila immune organ, leads to elevated cactus mRNA levels, decreased expression of antimicrobial peptides, and vulnerability to infection by Gram-positive bacteria. Furthermore, Gram-positive bacteria acutely activate Hippo-Yorkie signaling in fat bodies via the Toll-Myd88-Pelle cascade through Pelle-mediated phosphorylation and degradation of the Cka subunit of the Hippo-inhibitory STRIPAK PP2A complex. These results elucidate a Toll-mediated Hippo signaling pathway in antimicrobial response, highlight the importance of regulating IκB/Cactus transcription in innate immunity, and identify Gram-positive bacteria as extracellular stimuli of Hippo signaling under physiological settings. 

Cardoso, M. A., Fontenele, M., Lim, B., Bisch, P. M., Shvartsman, S. Y. and Araujo, H. M. (2017). A novel function for the IkappaB inhibitor Cactus in promoting Dorsal nuclear localization and activity in the Drosophila embryo. Development 144(16): 2907-2913. PubMed ID: 28705899
Summary:
The evolutionarily conserved Toll signaling pathway controls innate immunity across phyla and embryonic patterning in insects. In the Drosophila embryo, Toll is required to establish gene expression domains along the dorsal-ventral axis. Pathway activation induces degradation of the IκB inhibitor Cactus, resulting in a ventral-to-dorsal nuclear gradient of the NFκB effector Dorsal. This study investigated how cactus modulates Toll signals through its effects on the Dorsal gradient and on Dorsal target genes. Quantitative analysis using a series of loss- and gain-of-function conditions shows that the ventral and lateral aspects of the Dorsal gradient can behave differently with respect to Cactus fluctuations. In lateral and dorsal embryo domains, loss of Cactus allows more Dorsal to translocate to the nucleus. Unexpectedly, cactus loss-of-function alleles decrease Dorsal nuclear localization ventrally, where Toll signals are high. Overexpression analysis suggests that this ability of Cactus to enhance Toll stems from the mobilization of a free Cactus pool induced by the Calpain A protease. These results indicate that Cactus acts to bolster Dorsal activation, in addition to its role as a NFκB inhibitor, ensuring a correct response to Toll signals.
Cardoso, M. A., Fontenele, M., Lim, B., Bisch, P. M., Shvartsman, S. Y. and Araujo, H. M. (2017). A novel function for the IkappaB inhibitor Cactus in promoting Dorsal nuclear localization and activity in the Drosophila embryo. Development 144(16): 2907-2913. PubMed ID: 28705899
Summary:
The evolutionarily conserved Toll signaling pathway controls innate immunity across phyla and embryonic patterning in insects. In the Drosophila embryo, Toll is required to establish gene expression domains along the dorsal-ventral axis. Pathway activation induces degradation of the IkappaB inhibitor Cactus, resulting in a ventral-to-dorsal nuclear gradient of the NFkappaB effector Dorsal. This study investigated how cactus modulates Toll signals through its effects on the Dorsal gradient and on Dorsal target genes. Quantitative analysis using a series of loss- and gain-of-function conditions shows that the ventral and lateral aspects of the Dorsal gradient can behave differently with respect to Cactus fluctuations. In lateral and dorsal embryo domains, loss of Cactus allows more Dorsal to translocate to the nucleus. Unexpectedly, cactus loss-of-function alleles decrease Dorsal nuclear localization ventrally, where Toll signals are high. Overexpression analysis suggests that this ability of Cactus to enhance Toll stems from the mobilization of a free Cactus pool induced by the Calpain A protease. These results indicate that Cactus acts to bolster Dorsal activation, in addition to its role as a NFkappaB inhibitor, ensuring a correct response to Toll signals.
BIOLOGICAL OVERVIEW

The Cactus-Dorsal interaction demonstrates how the cytoplasmic protein (Cactus) can regulate the localization of a transcription factor (Dorsal) to either the cytoplasm or the nucleus. Adequate vertebrate homology exists for purposes of comparison. In vertebrates, I kappa B protein is homologous to Cactus. It regulates the nuclear entry of NF kappa B, which is the vertebrate homolog to Dorsal in Drosophila.

Cactus and Dorsal are part of an activation cascade responsible for dorsal-ventral polarity in the fly. Preceding Cactus activity in this cascade of events is Toll. The Toll receptor is first acted upon by its ligand, Spätzle. This occurs in the ventral portion of the young embryo. The Toll signal then regulates destruction of Cactus, thus allowing nuclear transport of Dorsal.

Signal dependent degradation of Cactus does not require the presence of Dorsal, indicating that Cactus degradation is a direct response to signaling, and that disruption of the Dorsal/Cactus complex is a secondary result of Cactus degradation. Neither is the PEST sequence required for the signal-dependent degradation of Cactus. However, Cactus degradation does require tube and pelle. These two genes are also necessary for the nuclear localization of Dorsal.

Signal independent degradation of Cactus occurs when Cactus is not complexed with Dorsal. This type of degradation is PEST dependent. In the absence of Dorsal, Cactus is synthesized and degrades much more rapidly. The presence of Dorsal serves to stabilize Cactus in the cytoplasm. The normal Cactus-Dorsal interaction produces a trimer consisting of two molecules of Dorsal for each molecule of Cactus. It is not known how phosphorylation regulates the Cactus-Dorsal interaction. In the mammalian system, rapid phosphorylation of I kappa B, the Cactus homolog, has been observed in response to signaling by IL-1 and TNF-alpha, but phosphorylation of Cactus in response to Toll signaling has not been observed (Belvin, 1995).

Thus the generation of dorso-ventral polarity in Drosophila relies on the formation of a nuclear gradient of the rel/nuclear factor kappa B transcription factor Dorsal in the pre-cellular syncytial embryo by a process of differential nuclear localization. It is thought that the gradient is formed by activation of Toll's membrane receptor at ventral positions. This in turn causes the local dissociation of Dorsal from the cytoplasmic anchor protein Cactus.

Cactus and Dorsal play a critical role in a hemocyte-dependent function in Drosophila. In invertebrates such as Drosophila, hematopoietic stem cells are located in the lymph glands. They give rise to progenitors of at least two lineages, plasmatocytes and crystal cells. Plasmatocytes are the predominant form of hemocytes in the wild-type larval hemolymph and, like mammalian macrophages or neutrophils, they perform phagocytic functions. Plasmatocytes are small, spherical and non-adhesive, and engulf bacteria and cell debris. Plasmatocytes also secrete extracellular matrix components. When a larva experiences an immune challenge, plasmatocytes become stimulated, increase in number and, depending on the nature of infection, engage in phagocytosis or differentiate into discoidal and adhesive lamellocytes. Lamellocytes do not show any capacity for phagocytosis. Instead, they form multilayered capsules around foreign invaders or objects that are too large for phagocytosis. These capsules get melanized by the activities of crystal cells. Crystal cells house the substrates and enzymes for melanization reactions. In the absence of an immune challenge, plasmatocytes of a normal larva differentiate into lamellocytes at the onset of pupariation. However, in certain Drosophila mutants, lamellocytes form melanotic capsules around self tissue, even in the absence of an immune challenge. The mechanisms that control the production, differentiation and functions of these cells in wild-type and mutant Drosophila are poorly understood but they appear to represent constitutive activation of the immune system (Qiu, 1998).

Cactus and Dorsal play a critical role in a hemocyte-dependent function in Drosophila. The zygotic lethal phenotype of cact was examined. The absence of Cactus results in a highly penetrant overproliferation of hemocytes. To identify the lethal phase of cact mutants, the null (cact E8/cact D13 ) and hypomorphic (cact E8/cact IIIG ) larvae were examined through different larval and pupal stages. These larvae are viable at 24 and 48 hours after fertilization. By 72 hours, there is some larval lethality and delay in the rate of development of the null animals when compared to their heterozygous siblings. A clear difference between null and hypomorphic animals in their viability and melanotic capsule phenotype becomes evident by 120 hours. Whereas only 40% of the expected cact null animals survive to 120 hours, more than 80% of the expected hypomorphic animals are alive. With time this difference is even more pronounced: only 4 out of 100 cact null larvae make white prepupae; the remaining animals persist as larvae and eventually die. In contrast, more than 80% of the hypomorphs progress into pupal stages. However, less than 1% of these animals eclose, while the majority die as pupae (Qiu, 1998).

To determine if the incidence of encapsulation in cact animals correlates with zygotic lethality and if capsules are present in null larvae, larvae of different allele classes were examined for the presence of dark spots. Neither larvae nor adults of the V4, V3 or gain-of-function cact classes have melanotic capsules. In contrast, about 40% of the hypomorphic larvae and almost all (over 90%) of the null larvae bear melanotic capsules. The high penetrance of capsule formation in cact null animals and the strong correlation of encapsulation with lethality suggest that encapsulation of self tissue is at least one of the primary zygotic phenotypes of cact. To identify the tissues or organs where cact function may be required, capsules and accompanying tissues were examined from mutant larvae. Aggregates of hemocytes are often found in association with the larval fat body. Melanization in the cact capsules appears to initiate at discrete spots. As larvae grow, the size of capsules and the area of the melanized foci become larger. Cells of the cact fat body show variable loss of intercellular adhesion. In addition to these defects, salivary glands appear atrophied and sometimes show melanization that is not accompanied by hemocytic encapsulation. These phenotypic defects are strikingly similar to those observed when Dorsal is overexpressed, suggesting that they represent a hyperactive immune system. To determine if the high incidence of melanotic capsules in cact mutants is caused by an increase in hemocyte concentration, by increased hemocyte differentiation, or by both these processes, the hemocyte concentration and differentiation were compared in mutants and heterozygotes. The number of hemocytes/ml in cact - hemolymph is more than ten-fold higher than in the hemolymph of heterozygous siblings or wild-type larvae (Qiu, 1998).

Dominant mutations in Toll or constitutive expression of dorsal can induce lamellocyte differentiation and cause the formation of melanotic capsules. The hemocyte density of mutant Toll, tube or pelle hemolymph is significantly lower than that of the wild type. Lethality of mutant cactus animals can be rescued either by the selective expression of wild-type Cactus protein in the larval lymph gland or by the introduction of mutations in Toll, tube or pelle. Cactus, Toll, Tube and Pelle proteins are expressed in the nascent hemocytes of the larval lymph gland. These results suggest that the Toll/Cactus signal transduction pathway plays a significant role in regulating hemocyte proliferation and hemocyte density in the Drosophila larva (Qui, 1996).

Since effects of the Toll/Cactus pathway mutants appear to be confined to the regulation of hemocyte density in the larva, other basal and regulatory signals must contribute more directly to lineage specification, hemocyte turnover and differentiation. Phenotypes of other hematopoietic mutants in Drosophila range from the absence of lymph glands to the presence of massively overgrown lymph glands. Analysis of some of these mutants suggests that these genes play specific roles in hemocyte specification, differentiation or turnover. For example, recent studies on the constitutive JAK mutant hop Tum-l (which results in lymph gland overgrowth and hematopoietic neoplasm) suggest that, like the Toll/Cactus pathway, the JAK/STAT signaling pathway also regulates hematopoiesis and hemocyte density in the Drosophila larva. It is possible that multiple regulatory inputs, such as the Toll/Cactus and JAK/STAT signals, are integrated with, or superimposed on one another, to ensure that hematopoietic precursors survive and divide, and progress normally through their developmental program to give rise to mature hemocytes in a consistently controlled manner (Qiu, 1998 and references).

NF-kappaB, IkappaB, and IRAK control glutamate receptor density at the Drosophila NMJ

NF-κB signaling has been implicated in neurodegenerative disease, epilepsy, and neuronal plasticity. However, the cellular and molecular activity of NF-κB signaling within the nervous system remains to be clearly defined. This study shows that the NF-κB and IκB homologs Dorsal and Cactus surround postsynaptic glutamate receptor (GluR) clusters at the Drosophila NMJ. Mutations in dorsal, cactus, and IRAK/pelle kinase specifically impair GluR levels, assayed immunohistochemically and electrophysiologically, without affecting NMJ growth, the size of the postsynaptic density, or homeostatic plasticity. Additional genetic experiments support the conclusion that cactus functions in concert with, rather than in opposition to, dorsal and pelle in this process. Finally, evidence is provided that Dorsal and Cactus act posttranscriptionally, outside the nucleus, to control GluR density. Based upon these data it is speculated that Dorsal, Cactus, and Pelle function together, locally at the postsynaptic density, to specify GluR levels (Heckscher, 2007).

NF-κB signaling has been implicated in the mechanisms of neural plasticity, learning, epilepsy, neurodegeneration, and the adaptive response to neuronal injury. The data presented in this study advance the understanding of neuronal NF-κB signaling in two ways. First, multiple lines of evidence are presented that NF-κB/Dorsal signaling is required for the control of GluR density at the NMJ. These data provide a synaptic function for NF-κB signaling that may be directly relevant to the diverse activities ascribed to NF-κB in the nervous system. Second, molecular and genetic evidence is provided that Dorsal, Cactus, and Pelle may function posttranscriptionally, at the postsynaptic side of the NMJ, to specify GluR density during postembryonic development (Heckscher, 2007).

Several independent lines of experimentation suggest that Cactus, Dorsal, and Pelle function together at the PSD to specify GluR density. Evidence is provided that Cactus and Dorsal localize to a similar postsynaptic domain. In addition, overexpression of a GFP-tagged Pelle protein that is sufficient to rescue a pelle mutation, can traffic to the PSD where Cactus and Dorsal reside. Next, genetic evidence is presented that cactus, dorsal, and pelle function together, in the same genetic pathway, to control GluR density. It is particularly surprising that mutations in cactus behave similarly to dorsal and pelle. In other systems (embryonic patterning and immunity), Cactus inhibits Dorsal-mediated transcription by binding and sequestering cytoplasmic Dorsal protein. As a result, in these other systems, cactus mutations cause phenotypes that are opposite to those observed in dorsal mutations. This study used the same cactus and dorsal mutations that previously have been observed to generate the predicted opposing phenotypes during embryonic patterning, and yet it was observed that cactus phenocopies the dorsal mutations. In addition, genetic epistasis experiments indicate that these genes function together to facilitate GluR density. Thus, at the NMJ, Cactus functions in concert with, rather than in opposition to, Dorsal (Heckscher, 2007).

One explanation for this observation could be that Dorsal does not function as a nuclear transcription factor during the control of GluR levels. In support of this idea it has been demonstrated that (1) Dorsal protein is not detected in the nucleus, (2) reporters of Dorsal-dependent transcription fail to show activity in muscle nuclei, and (3) mutation of the Dorsal transactivation domain, dlU5 does not impair GluR abundance even though this same mutation has been shown to impair transcription-dependent patterning during embryogenesis. An alternative explanation for the observation that dorsal and cactus have similar phenotypes at the NMJ could be that Cactus and Dorsal act synergistically to control the transcription of GluRs at the NMJ. Indeed, there is evidence in other systems that IκB can shuttle with NF-κB to the nucleus. A previous study shows Cactus accumulation in Drosophila larval muscle nuclei in a dorsal mutant background (Cantera, 1999). However, this result could not be repeated despite examination of Cactus localization in five allelic combinations of dorsal. Furthermore, the data from vertebrate systems suggest that IκB should shuttle into the nucleus with NF-κB, not in its absence. Thus, a model is favored in which Dorsal and Cactus function together at the postsynaptic membrane to facilitate GluR abundance during development (Heckscher, 2007).

If this model is correct, then it is predicted that NF-κB does not control GluR density through transcriptional regulation. This prediction is supported by two experimental observations: (1) GluR transcript levels (assessed by QT PCR) are not statistically different from wild-type in dorsal and cactus mutations that cause an ~50% decrease in GluR abundance; (2) it was demonstrated that overexpression of a myc-tagged GluRIIA cDNA using a heterologous, muscle-specific promoter is not able to restore synaptic GluRIIA levels in either the cactus or dorsal mutant backgrounds. These data are consistent with Dorsal and Cactus acting posttranscriptionally to control GluR density at the NMJ. There are two general mechanisms by which GluR levels could be controlled posttranscriptionally: (1) altered receptor delivery to the NMJ or (2) altered receptor internalization/degradation. If receptor internalization/degradation were enhanced in the cactus, dorsal, or pelle mutant backgrounds, one might expect GluRIIA-myc overexpression to overcome this change and restore normal receptor levels. In addition, less myc-tagged protein might be seen in the mutants in comparison to wild-type. This is not what was observed. Therefore, the hypothesis is favored that Cactus, Dorsal, and Pelle function together to promote the delivery of glutamate receptors to the NMJ during development (Heckscher, 2007).

The possibility that Cactus, Dorsal, and Pelle act posttranscriptionally to control GluR density raises many questions. For example, do Dorsal and Cactus exist as a protein complex at the PSD? If so, is this complex regulated and how might such a complex influence GluR density? Since pelle kinase-dead mutants impair GluR density, it is possible that Dorsal and Cactus recruit Pelle to the PSD. If so, what are the targets of Pelle kinase that are relevant to establishing or maintaining the proper density of glutamate receptors at the PSD? Finally, the demonstration that cytoplasmic NF-κB/Dorsal can influence GluR density does not rule out the possibility that NF-κB/Dorsal may also translocate to the muscle nucleus at the Drosophila NMJ under certain stimulus conditions. Indeed, in both the vertebrate central and peripheral nervous systems NF-κB is found within neuronal and muscle nuclei, and nuclear translocation can be stimulated by neuronal activity, glutamate, injury, and disease. For nuclear entry of Dorsal, two events must occur: (1) Cactus must be degraded and (2) Dorsal must be phosphorylated. It remains possible that one or both of these criteria are not met during the normal development of the Drosophila NMJ but could be met under as-yet-to-be-identified stimulus conditions. The possibility that NF-κB acts both locally at the synapse and globally via the nucleus is not unique to this signaling pathway. A similar organization has been documented for wingless/wnt signaling where noncanonical cytoplasmic signaling can impact cytoskeletal organization while canonical signaling involves the nuclear translocation of downstream beta-catenin and TCF-dependent gene transcription (Heckscher, 2007).

It remains unknown how NF-κB signaling is activated at the Drosophila NMJ. In Drosophila embryonic patterning and innate immunity, NF-κB signaling is initiated through activation of Toll or Toll-like receptors. There are nine Toll and Toll-like receptors encoded in the Drosophila genome. However, none of these receptors appear to be present in Drosophila larval muscle. The Toll receptor is expressed in a subset of embryonic muscle fibers, but is absent from larval muscle. None of the Toll-like receptors are expressed in Drosophila embryonic muscle and none appear to be expressed in larval muscle. An alternative possibility is that TNF-α receptors activate NF-κB in Drosophila muscle as has been observed in vertebrate skeletal muscle. Indeed, a TNF-α receptor homolog (Wengen) has been identified, and it is expressed in Drosophila skeletal muscle. The possibility that TNF-α signaling is mediated via NF-κB is intriguing given the recent demonstration that TNF-α regulates GluR abundance in the vertebrate central nervous system. In both cultured neurons and hippocampal slices glial-derived TNF-α signaling is required for the increase in postsynaptic AMPA receptor abundance observed following chronic activity blockade. Thus, the current data in combination with work in the vertebrate CNS raise the possibility that a conserved TNFα/NF-κB signaling system controls GluR abundance at both neuromuscular and central synapses during development and in response to chronic activity blockade (Heckscher, 2007).


GENE STRUCTURE

Two Cactus transcripts have been detected. The 2.2 kb transcript (maternal/zygotic) is more abundant and is present throughout development. A 2.6 kb transcript (zygotic) is present from 4 hours of development onward. Both these transcripts initiate from the same start site but differ at their 3' ends (Kidd, 1992).
Exons - seven


PROTEIN STRUCTURE

Amino Acids - 482 (zygotic); 500 (maternal/zygotic)

Structural Domains

Among the maternally active genes of Drosophila, cactus is the only one whose loss of function mutations specifically produce ventralized embryos. Its product inhibits nuclear translocation of the Dorsal morphogen in the dorsal region of the embryo. cactus encodes an acidic, cytoplasmic protein with six ankyrin repeats. The sequence has similarity to the I kappa B proteins that inhibit the vertebrate transcription factor NF-kappa B. By analogy to results obtained with I kappa B and NF-kappa B, bacterially expressed Cactus protein can inhibit DNA binding of Dorsal protein in vitro (Geisler, 1992). There is an N-terminal acidic domain, six ankyrin repeats in the C-terminal (involved in the interaction with Dorsal), followed by a PEST sequence conferring rapid degradation (Kidd, 1992).


cactus: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 6 July 98 

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