basket/JNK: Evolutionary Homologs | Regulation | Protein Interactions | Developmental Biology | Effects of Mutation | References

Gene name - basket

Synonyms - Jun amino terminal kinase (DJNK)

Cytological map position - 31B-C

Function - MAP kinase - Jun N-terminal kinase

Keywords - dorsal closure, immune response, JNK pathway

Symbol - bsk

FlyBase ID:FBgn0000229

Genetic map position - 2-32

Classification - MAP kinase homolog

Cellular location - cytoplasmic and nuclear



NCBI links: Precomputed BLAST | Entrez Gene | UniGene |

Recent literature
Huang, J. and Xue, L. (2015). Loss of flfl triggers JNK-dependent cell death in Drosophila. Biomed Res Int 2015: 623573. PubMed ID: 26583122
Summary:
falafel (flfl) encodes a Drosophila homolog of human SMEK whose in vivo functions remain elusive. Gain-of-function and loss-of-function analysis was performed in in Drosophila and flfl was identified as a negative regulator of JNK pathway-mediated cell death. While ectopic expression of flfl suppresses TNF-triggered JNK-dependent cell death, loss of flfl promotes JNK activation and cell death in the developing eye and wing. These data report for the first time an essential physiological function of flfl in maintaining tissue homeostasis and organ development. As the JNK signaling pathway has been evolutionary conserved from fly to human, a similar role of PP4R3 in JNK-mediated physiological process is speculated.

Bornstein, B., et al. (2015). Developmental axon pruning requires destabilization of cell adhesion by JNK signaling. Neuron 88: 926-940. PubMed ID: 26586184
Summary:
Developmental axon pruning is essential for normal brain wiring in vertebrates and invertebrates. How axon pruning occurs in vivo is not well understood. In a mosaic loss-of-function screen, this study found that Bsk, the Drosophila JNK, is required for axon pruning of mushroom body γ neurons, but not their dendrites. By combining in vivo genetics, biochemistry, and high-resolution microscopy, this study demonstrated that the mechanism by which Bsk is required for pruning is through reducing the membrane levels of the adhesion molecule Fasciclin II (FasII), the NCAM ortholog. Conversely, overexpression of FasII is sufficient to inhibit axon pruning. Finally, this study showed that overexpressing other cell adhesion molecules, together with weak attenuation of JNK signaling, strongly inhibits pruning. Taken together, this study uncovered a novel and unexpected interaction between the JNK pathway and cell adhesion and found that destabilization of cell adhesion is necessary for efficient pruning.

Khoshnood, B., Dacklin, I. and Grabbe, C. (2015). Urm1: an essential regulator of JNK signaling and oxidative stress in Drosophila melanogaster. Cell Mol Life Sci [Epub ahead of print]. PubMed ID: 26715182
Summary:
Ubiquitin-related modifier 1 (Urm1) is a ubiquitin-like molecule (UBL) with the dual capacity to act both as a sulphur carrier and posttranslational protein modifier. This study characterizes the Drosophila homologues of Urm1 (CG33276) and its E1 activating enzyme Uba4 (CG13090) and shows that they function together to induce protein urmylation in vivo. Urm1 conjugation to target proteins in general, and to the evolutionary conserved substrate Peroxiredoxin 5 (Prx5) specifically, is dependent on Uba4. A complete loss of Urm1 is lethal in flies, although a small number of adult zygotic Urm1 n123 mutant escapers can be recovered. These escapers display a decreased general fitness and shortened lifespan, but in contrast to their S. cerevisiae counterparts, they are resistant to oxidative stress. Providing a molecular explanation, this study demonstrates that cytoprotective JNK signaling is increased in Urm1 deficient animals. In agreement, molecular and genetic evidence suggest that elevated activity of the JNK downstream target genes Jafrac1 (thioredoxin peroxidase 1) and gstD1 (Glutathione S transferase D1) strongly contributes to the tolerance against oxidative stress displayed by Urm1 n123 null mutants. In conclusion, Urm1 is a UBL that is involved in the regulation of JNK signaling and the response against oxidative stress in the fruit fly.

Clemente-Ruiz, M., Murillo-Maldonado, J.M., Benhra, N., Barrio, L., Pérez, L., Quiroga, G., Nebreda, A.R. and Milán, M. (2016). Gene dosage imbalance contributes to chromosomal instability-induced tumorigenesis. Dev Cell 36: 290-302. PubMed ID: 26859353
Summary:
Chromosomal instability (CIN) is thought to be a source of mutability in cancer. However, CIN often results in aneuploidy, which compromises cell fitness. This study used the dosage compensation mechanism (DCM) of Drosophila to demonstrate that chromosome-wide gene dosage imbalance contributes to the deleterious effects of CIN-induced aneuploidy and its pro-tumorigenic action. Resetting of the DCM counterbalances the damaging effects caused by CIN-induced changes in X chromosome number. Importantly, interfering with the DCM suffices to mimic the cellular effects of aneuploidy in terms of reactive oxygen species (ROS) production, JNK-dependent cell death, and tumorigenesis upon apoptosis inhibition. A role of ROS was found in JNK activation and a variety of cellular and tissue-wide mechanisms that buffer the deleterious effects of CIN, including DNA-damage repair, activation of the p38 pathway, and cytokine induction were found to promote compensatory proliferation. These data reveal the existence of robust compensatory mechanisms that counteract CIN-induced cell death and tumorigenesis.
Lim, B., Dsilva, C. J., Kevrekidis, I. G. and Shvartsman, S. Y. (2017). Reconstructing ERK signaling in the Drosophila embryo from fixed images. Methods Mol Biol 1487: 337-351. PubMed ID: 27924579
Summary:
The early Drosophila embryo provides unique opportunities for quantitative studies of ERK signaling. This system is characterized by simple anatomy, the ease of obtaining large numbers of staged embryos, and the availability of powerful tools for genetic manipulation of the ERK pathway. This paper describes how these experimental advantages can be combined with recently developed microfluidic devices for high throughput imaging of ERK activation dynamics. Focus was placed on the stage during the third hour of development, when ERK activation is essential for patterning of the future nerve cord. This approach starts with an ensemble of fixed embryos stained with an antibody that recognizes the active, dually phosphorylated form of ERK. Each embryo in this ensemble provides a snapshot of the spatial and temporal pattern of ERK activation during development. Then the ages of fixed embryos are quantitatively estimated using a model that links their morphology and developmental time. This model is learned based on live imaging of cellularization and gastrulation, two highly stereotyped morphogenetic processes at this stage of embryogenesis. Applying this approach, ERK signaling can be characterized at high spatial and temporal resolution. This methodology can be readily extended to studies of ERK regulation and function in multiple mutant backgrounds, providing a versatile assay for quantitative studies of developmental ERK signaling.
Nitta, Y. and Sugie, A. (2017). DISCO interacting protein 2 determines direction of axon projection under the regulation of c-Jun N-terminal kinase in the Drosophila mushroom body. Biochem Biophys Res Commun. PubMed ID: 28396149
Summary:
Precisely controlled axon guidance for complex neuronal wiring is essential for appropriate neuronal function. c-Jun N-terminal kinase (JNK) was found to play a role in axon guidance recently as well as in cell proliferation, protection and apoptosis. In spite of many genetic and molecular studies on these biological processes regulated by JNK, how JNK regulates axon guidance accurately has not been fully explained thus far. To address this question, this study used the Drosophila mushroom body (MB) as a model since the α/β axons project in two distinct directions. This study showns that DISCO interacting protein 2 (DIP2) is required for the accurate direction of axonal guidance. DIP2 expression is under the regulation of Basket (Bsk), the Drosophila homologue of JNK. The Bsk/DIP2 pathway is independent from the AP-1 transcriptional factor complex pathway, which is directly activated by Bsk. In conclusion, these findings revealed DIP2 as a novel effector downstream of Bsk modulating the direction of axon projection.
Ma, X., Lu, J. Y., Dong, Y., Li, D., Malagon, J. N. and Xu, T. (2017). PP6 disruption synergizes with oncogenic Ras to promote JNK-dependent tumor growth and invasion. Cell Rep 19(13): 2657-2664. PubMed ID: 28658615
Summary:
RAS genes are frequently mutated in cancers, yet an effective treatment has not been developed, partly because of an incomplete understanding of signaling within Ras-related tumors. To address this, a genetic screen was performed in Drosophila, aiming to find mutations that cooperate with oncogenic Ras (RasV12) to induce tumor overgrowth and invasion. fiery mountain (fmt; CG10289), a regulatory subunit of the protein phosphatase 6 (PP6) complex, was identified as a tumor suppressor that synergizes with RasV12 to drive c-Jun N-terminal kinase (JNK)-dependent tumor growth and invasiveness. Fmt was shown to negatively regulate JNK upstream of dTAK1. It was further demonstrated that disruption of PpV, the catalytic subunit of PP6, mimics fmt loss-of-function-induced tumorigenesis. Finally, Fmt synergizes with PpV to inhibit JNK-dependent tumor progression. These data here further highlight the power of Drosophila as a model system to unravel molecular mechanisms that may be relevant to human cancer biology.
BIOLOGICAL OVERVIEW

For many years immunologists have held the view that the primary goal of the immune system is discrimination between self and non-self. Because Drosophila does not have an immunoglobulin-based immune response, the fly affords a unique opportunity to examine the nature of an immune system that evolved prior to the evolution of immunoglobulin as a primary driving force in an organism's response to pathogens. An examination of Drosophila suggests that an inducible system exists and it is based on the need to detect and protect against danger. The immunologist Matzinger (1994) has argued that such a system also remains functional as a controlling influence in the vertebrate immune response, in addition to the immunoglobulin-based system. The protein JNK is one of the pivotal actors in the Drosophila immune response, and appears to be functionally conserved in the vertebrate immune response as well. What does JNK do in both Drosophila and vertebrates, and what does this teach about the nature of the immune response?

Before taking a closer look at basket/JNK, a word on the pathway in which it functions is in order. Cascades of mitogen activated protein kinases (MAPKs) transduce signals from multiple extracellular stimuli, mediating responses such as cell proliferation, differentiation, and the regulation of metabolic pathways. There are multiple MAPKs in eukaryotes. In Drosophila, two such pathways have been identified. The first is a MAPK pathway involving Ras and Rolled, two components of the Ras pathway. It is required at least three times during development: as the terminal system which mediates responses to the Torso receptor, as neurogenic and wing vein pathways mediate responses to the EGF-receptor, and for the differentiation of photoreceptors, which mediate responses to the Sevenless receptor.

The second MAPK pathway in Drosophila involves the MAPK called Jun-N-terminal kinase (DJNK), also known as Basket. This pathway involves an additional kinase, Hemipterous, which serves to phosphorylate DJNK. Both proteins are required for dorsal closure, the sealing of the dorsal region of the embryo late in embryonic development. Dorsal closure involves coordinated changes in the shape of ectodermal cells, driven by cytoskeletal changes. (For a discussion of dorsal closure see Hemipterous). The phosphorylation cascade involving HEP and DJUN terminates in the phosphorylation of DJUN. Both DJNK and DJUN are activated in the the immune response of the fly. DJNK is closely associated on chromosome 2 with dror, a Drosophila receptor tyrosine kinase homolog of vertebrate TRK kinases, which mediate responses to neurotropins. Close chromosomal linkage often, but not always, occurs as a consequence of shared developmental function.

Regulation of JNK by Src during Drosophila development

Basket (Bsk) is regulated by Src during dorsal closure. Mutants for Src42A, a Drosophila c-src protooncogene homolog, are described. Src42A functions in epidermal closure during both embryogenesis and metamorphosis. The severity of the epidermal closure defect in the Src42A mutant depends on the amount of Bsk activity, and the amount of Bsk activity depends on the amount of Src42A. Thus, activation of the Bsk pathway is required downstream of Src42A in epidermal closure. This work confirms mammalian studies that demonstrate a physiological link between Src and JNK (Tateno, 2000).

Genes that regulate cell shape changes in Drosophila are required for dorsal closure of the embryonic epidermis and thorax closure of the pupal epidermis. Mutations in genes such as hemipterous (hep) and basket (bsk, also known as DJNK) result in abnormal embryos with a dorsal hole or abnormal adults with a dorsal midline cleft. Hep and Bsk are homologous to the mammalian MKK7 (MAPK kinase 7) and JNK, and they are components of a MAPK (mitogen-activated protein kinase) cascade. Although the role of the Hep-Bsk cascade during dorsal closure has been extensively studied, the upstream trigger of this cascade is poorly understood. To identify the trigger, a screen was carried out for mutants showing the dorsal midline cleft phenotype, like a mild hep mutant. The mutant for Src42A shows this phenotype and Src42A regulates Bsk during Drosophila development (Tateno, 2000).

One line, Jp45, from the mutant collection of the P-element-inserted semilethal lines has been identified that survives to adulthood but shows various degrees of the dorsal midline cleft phenotype. Excision of the P-element eliminates the semilethality and restores the cleft phenotype. The P-element is inserted in the 5' untranslated region (UTR) of the Src42A gene, which encodes a Src-family nonreceptor tyrosine kinase. Ethyl methanesulfonate (EMS) mutant screening was used to isolate two strong alleles of Src42A, Src42AE1 and Src42Amyristylation (myri). In Src42AE1, a stop codon at codon 483 eliminates the COOH-terminal part of the kinase domain of Src42A. Src42Amyri has a point mutation in codon 2, which causes an amino acid substitution from Gly2 to Asp. Gly2 is conserved in all members of the Src family and must be myristylated for localization of Src to the cellular membrane in mammals. About 50% of the Src42Amyri homozygotes die before they hatch, and most of the remainder die during the first-instar larval stage. Therefore, Gly2 is required for development (Tateno, 2000).

Because the adult Src42AJp45 phenotype resembles that of hep, it was suspected that Src42A is involved in Hep and Bsk function. A mutation in hep or bsk dominantly enhances the lethality and the phenotypic severity of Src42AJp45 homozygotes. Conversely, reducing the gene dosage of puckered (puc), a gene encoding a phosphatase that inactivates Bsk, suppresses the lethality and the severity of the cleft phenotype of Src42AJp45. Thus, Src42A may function in the Bsk pathway during metamorphosis (Tateno, 2000).

Dorsal closure is the process by which a pair of epidermal layers elongate dorsally and fuse at the dorsal midline of the embryo. This process is not completed in hep and bsk mutants, yielding a dorsal open phenotype. Strong Src42A mutants do not show the dorsal open phenotype but display malformed mouth parts. This defect is similar to the defect in the embryo of the Tec29 mutant. Tec29 is a Src-related nonreceptor tyrosine kinase and is regulated by SRC64, another Drosophila Src homolog, during oogenesis. Thus, Tec29 may be involved in the function of Src42A. A mutation in Tec29 dominantly enhances the lethality of Src42AJp45 (Tateno, 2000).

Furthermore, the Tec29 Src42A double mutant shows complete embryonic lethality, and a certain fraction of the dead embryos show the dorsal open phenotype. Activated DJun, a transcription factor downstream of Bsk, partially rescues the dorsal open phenotype in the Tec29 Src42A double mutant. Thus, Src42A appears to regulate Bsk in the fusion of epithelial sheets during embryogenesis and metamorphosis, and Tec29 is involved in this regulation. The double mutant for Src64 and Src42A manifests a mild but clear dorsal open phenotype, which suggests a functional redundancy between Src64 and Src42A (Tateno, 2000).

Expression of puc is known to be induced by the Bsk signal. In the wing disc of the wild-type third-instar larva, puc is expressed in the dorsal midline of the adult notum. In the wing disc of the Src42AJp45 mutant, puc expression is reduced. In contrast, larvae with a constitutively activated form of Src42A (Src42ACA) shows ectopic expression of puc. Further, introduction of a hep null mutation reduces the amount of ectopic puc expression. It is known that Bsk induces expression of puc and decapentaplegic (dpp) during embryonic dorsal closure. The embryos of the Tec29 Src42A double mutant do not show any puc or dpp expression in the leading edge cells. These results indicate that Src42A, Tec29, Hep, and Bsk regulate dpp and puc expression during embryonic dorsal closure (Tateno, 2000).

To investigate the ability of Src42ACA to activate Bsk, the amount of phosphorylated Bsk was directly assessed by immunoblot analysis. Forced expression of Src42ACA does not affect the quantity of total Bsk protein but induces more phosphorylated Bsk than the controls. Thus, Src42A appears to regulate the phosphorylation level of Bsk (Tateno, 2000).

During embryonic dorsal closure, the Hep-Bsk signal is required for elongation of the leading edge cells. In the absence of the Bsk signal, these cells do not fully elongate. The accumulation of F-actin and phosphotyrosine (P-Tyr) in leading edge cells is associated with the elongation of these cells. Accumulation of these substances is disturbed in the DJun and the puc mutants. In the double mutant for Tec29 and Src42A, the leading edge cells contain reduced quantities of F-actin and P-Tyr, and these cells are only partially elongated. Thus, the defect in embryonic dorsal closure in the Tec29 Src42A double mutant is caused by this failure in cell shape change, as is the case in the DJun mutant (Tateno, 2000).

A model is proposed in which Src42A, upon receiving an unidentified signal, activates the Hep-Bsk pathway to regulate cell shape change and epidermal layer movement. This is consistent with the observation in mammals that c-Src regulates the cell morphogenetic and migratory processes and is known to activate JNK. As in Drosophila, c-Src definitely affects F-actin organization and P-Tyr localization during cell morphogenesis. Therefore, Src regulation of JNK activity toward a change in cell shape may be conserved (Tateno, 2000).

It can be also interpreted that Src42A acts upstream of DFos, a dimerization partner of DJun. Although the Src42A, Tec29, and Src64 single mutants do not show a dorsal open phenotype, the DFos mutant clearly exhibits it. This relationship is also analogous to that in mammals. Both c-src and c-fos knockout mice have a similar defect, osteopetrosis caused by reduced osteoclast function. But the phenotypic severity is milder in c-src than in c-fos knockouts; this can be explained by the functional overlap in multiple Src-family tyrosine kinases. Accordingly, in both Drosophila and mammals, multiple nonreceptor tyrosine kinases may cooperate to regulate the function of the Jun/Fos complex (Tateno, 2000).

Cell competition modifies adult stem cell and tissue population dynamics in a JAK-STAT-dependent manner

Throughout their lifetime, cells may suffer insults that reduce their fitness and disrupt their function, and it is unclear how these potentially harmful cells are managed in adult tissues. This question was addressed using the adult Drosophila posterior midgut as a model of homeostatic tissue and ribosomal Minute mutations to reduce fitness in groups of cells. A quantitative approach was taken, combining lineage tracing and biophysical modeling, and how cell competition affects stem cell and tissue population dynamics was addressed. Healthy cells were shown to induce clonal extinction in weak tissues, targeting both stem and differentiated cells for elimination. It was also found that competition induces stem cell proliferation and self-renewal in healthy tissue, promoting selective advantage and tissue colonization. Finally, winner cell proliferation was shown to be fueled by the JAK-STAT ligand Unpaired-3, produced by Minute-/+ cells in response to chronic JNK stress signaling (Kolahgar, 2015).

Recent studies have shown that cell competition can also take place in adult tissues. This work has taken this notion forward and delineated quantitatively how adult stem cells and tissue population dynamics are affected by cell competition. In the subfit population, differentiated cells are killed by apoptosis followed by cell delamination; stem cells are also eliminated, possibly via induction of differentiation, as dying stem cells have not been detected. In parallel, as this study has shown, the healthy tissue expands due to an increase in stem cell proliferation and self-renewal. Indeed, biophysical modeling shows that changes in these parameters of a magnitude comparable to what observed experimentally is sufficient to recapitulate the stem cell dynamics of wild-type tissue undergoing Minute cell competition. Interestingly, accelerated proliferation of fitter stem cells has been seen in mouse embryonic stem cells using in vitro models of cell competition. However, in those studies, increased stem cell self-renewal has not been observed, probably because stemness in vitro is artificially maintained by exogenous factors in the culture medium (Kolahgar, 2015).

In many adult homeostatic tissues, stem cells stochastically differentiate or self-renew, and this leads to clonal extinction balanced by clonal expansion. This is known as neutral drift competition, because through this process, stem cell compartments stochastically tend toward monoclonality. It has also been shown that stem cell competition can be nonneutral (i.e., biased) when stem cells acquire a cell-autonomous advantage. In these cases, the bias derives from intrinsic differences (e.g., faster proliferation) and does not rely on cell interactions. This study shows instead that in adult homeostatically maintained tissues, competitive cell interactions can act as extrinsic cues that actively modify stem cell behavior, and that this confers on winners an advantage (e.g., as this study observed, increased proliferation rate and self-renewal) and on losers a disadvantage (e.g., as observed induced cell death), influencing tissue colonization. It is important to note that clones of wild-type cells that have lost proliferative capability because they are devoid of ISCs are equally able to induce death in neighboring M/+cells. This rules out the possibility that physical displacement due to a faster clonal expansion is the cause of cell competition in this case. This process instead, like the recent reports of cell competition in the mouse heart and fly nervous system, likely corresponds to the adult equivalent of the cellular competition observed in developing tissues (Kolahgar, 2015).

This work shows that M/+ midguts suffer from a chronic inflammatory response, which through JNK signaling activation and the ensuing production of the JAK-STAT ligand Upd-3 promotes wild-type tissue overgrowth. Thus, in this tissue, the overproliferation of winner cells stems from the increased availability of proliferative signals in the M/+ environment. The results suggest that wild-type cells respond more efficiently than M/+ cells to this proliferation stimulus, and that this difference results in their preferential overgrowth, contributing to cell competition. It has long been suggested that cell competition may result from the limiting availability of growth factors, which would compromise the viability of loser cells. Here it was instead found that excess production of a growth factor (Upd-3) can boost cell competition by promoting preferential proliferation of fitter cells. Given that JNK and JAK-STAT are frequently activated in response to stress or deleterious mutations, it would be interesting to test whether this is a general mechanism used by loser cells to promote the overgrowth of fitter neighbors. Notably, differences in JAK-STAT signaling are sufficient to trigger cell competition and, consistent with this, reducing JAK-STAT signaling in wild-type cells compromises their ability to eliminate scribble/losers. Thus, increased JAK-STAT signaling may in addition provide wild-type cells with a heightened fitness state and help promote the elimination of M/losers (Kolahgar, 2015).

Ribosomal mutations are linked with many adult disorders, not just in Drosophila but more importantly in humans, where they are associated with a number of severe pathologies, collectively known as ribosomopathies. Given that 79 proteins make up the eukaryotic ribosome (and several more are involved in ribosomal production) and that many Minute mutations are dominant, the sporadic insurgence of M/+ in adult tissues is likely to be one of the most common spontaneous generations of somatic mutant cells in our bodies. The elimination of these cells via cell competition is likely to play an unappreciated role in maintaining healthy adult tissues (Kolahgar, 2015).

A striking feature emerging from the results is that, in response to cell competition, normal cells can efficiently repopulate adult tissues, thus effectively replacing potentially diseased cells. This bears striking resemblance to the phenomenon of mosaic revertants, observed in a number of human skin and blood diseases. Spontaneous sporadic reversion of genetically inherited, disease-bearing mutations leads to the generation of revertant cells, which effectively repopulate tissues, at times ameliorating the condition. In some instances, the revertants' expansion is so efficient that selective advantage has been. Intriguingly, ichthyosis with confetti, a skin disease characterized by confetti-like appearance of revertant skin spots, is associated with a mutation in Keratin 10, which, due to its nucleolar mislocalization, could affect ribosome production similar to M-/+mutants. Thus, based on these findings, it is tentative to speculate that selective advantage in mosaic revertants could in some cases be driven by cell competition (Kolahgar, 2015).

The work shows that M/+ midguts suffer from a chronic inflammatory response, which through JNK signaling activation and the ensuing production of the JAK-STAT ligand Upd-3 promotes wild-type tissue overgrowth. Thus, in this tissue, the overproliferation of winner cells stems from the increased availability of proliferative signals in the M/+ environment. The results suggest that wild-type cells respond more efficiently than M/+ cells to this proliferation stimulus, and that this difference results in their preferential overgrowth, contributing to cell competition. It has long been suggested that cell competition may result from the limiting availability of growth factors, which would compromise the viability of loser cells. This study found instead that excess production of a growth factor (Upd-3) can boost cell competition by promoting preferential proliferation of fitter cells. Given that JNK and JAK-STAT are frequently activated in response to stress or deleterious mutations, it would be interesting to test whether this is a general mechanism used by 'loser' cells to promote the overgrowth of fitter neighbors. Notably, differences in JAK-STAT signaling are sufficient to trigger cell competition and, consistent with this, reducing JAK-STAT signaling in wild-type cells compromises their ability to eliminate scribble/ losers. Thus, increased JAK-STAT signaling may in addition provide wild-type cells with a heightened fitness state and help promote the elimination of M/+ losers (Kolahgar, 2015).

Ribosomal mutations are linked with many adult disorders, not just in but more importantly in humans, where they are associated with a number of severe pathologies, collectively known as ribosomopathies. Given that 79 proteins make up the eukaryotic ribosome (and several more are involved in ribosomal production) and that many Minute mutations are dominant, the sporadic insurgence of M/+ cells in adult tissues is likely to be one of the most common spontaneous generations of somatic mutant cells in our bodies. The elimination of these cells via cell competition is likely to play an unappreciated role in maintaining healthy adult tissues (Kolahgar, 2015).

A striking feature emerging from the current results is that, in response to cell competition, normal cells can efficiently repopulate adult tissues, thus effectively replacing potentially diseased cells. This bears striking resemblance to the phenomenon of mosaic revertants, observed in a number of human skin and blood diseases. Spontaneous sporadic reversion of genetically inherited, disease-bearing mutations leads to the generation of revertant cells, which effectively repopulate tissues, at times ameliorating the condition. In some instances, the revertants' expansion is so efficient that selective advantage has been proposed. Intriguingly, ichthyosis with confetti, a skin disease characterized by confetti-like appearance of revertant skin spots, is associated with a mutation in Keratin 10, which, due to its nucleolar mislocalization, could affect ribosome production similar to M/+ mutants. Thus, based on the current findings, it is tentative to speculate that selective advantage in mosaic revertants could in some cases be driven by cell competition (Kolahgar, 2015).

Bacterial infection causes insect immune tissues to synthesize a spectrum of antibacterial peptides, including Cecropin and Diptericin, which lyse the invading microorganisms (Hultmark, 1993 and Peterson, 1995). Exposure of Drosophila to environmental stress (such as a pathogenic onslaught) activates two groups of transcription factors: AP-1 (coded for by Drosophila jun and fos) and two related proteins; Dorsal and Dorsal related factor (Dif).

Dorsal and Dif are homologs of NFkappaB (also known as Rel), an essential transcription factor involved in the vertebrate immune response (Ip, 1993). Both Dif and Dorsal act in the insect immune defense against bacterial infection (see Dorsal for more information).

Drosophila JNK is activated by endotoxic lipopolysaccharide (LPS). LPS is a component of bacterial cell walls, and is known to be a stimulant for the immune response in both insects and mammals. The response of Drosophila to bacterial infection can be studied by examining the effect of LPS on cultured cell lines. Addition of LPS to cultured cell lines, including mbn-2 hemocytes and Schneider S2 embryonic cells, causes marked induction of cecropin and diptercin genes. basket is activated within 5 minutes of LPS addition. However, basket activation is transient and returns to basal levels after 1 hour. The activation of DJun by DJNK in LPS-treated cells may lead to increased AP-1 (a heterodimer of DFos and DJun) transcriptional activity. Targets of Drosophila AP-1 may include the DJun promoter (Sluss, 1996).

Although the pathway in Drosophila for pathogen activation of the DJNK-DJun system and the immune system targets of DJNK and DJun are not yet known, the cloning of DJNK should help clarify the roles of these proteins in the fly's immune response.

Mammalian JNK has been implicated in a variety of immune-related signaling pathways, including the response to pro-inflammatory cytokines, the respone to LPS, and T-cell activation (Su, 1994, Westwick, 1994 and 1995 and Raingeaud, 1995). The role of JNK and of Rel proteins in the vertebrate immune response harkens back to the roles of these proteins in Drosophila: both proteins are involved in systems that have developed to detect and protect against danger. When viewed from the perspective of signal transduction cascades and transcription factors it is clear that at its core, the vertebrate immune response system is one which detects and protects against danger. This conclusion is made clearer by looking at the roles of JNK and Rel in vertebrate responses to danger and in the related immune responses (see the basket/JNK Evolutionary homologs section).

The "simple" immune response of Drosophila, involving two signal transduction pathways and two groups of transcription factors, serves as a model for the inducible response to pathogens that is found in both invertebrates and vertebrates. This system functions quite adequately without the added level of antigen specific immunoglobulins.

Where then does the vertebrate immunoglobulin system come from? Invertebrates have a dozen or more cell surface proteins possessing domains resembling those found in vertebrate immunoglobulins. Examples in Drosophila include, Dlar, Fasciclin II, Fibroblast growth factor receptor 1, Frazzled, Hikaru genki, Neuroglian, and Semaphorin 2. The Ig domain, as it is found in invertebrates, is involved in cell-cell interactions, and many of the proteins have roles in the complex process whereby axons find their way to target tissues. It is likely that the immunglobulin system, both T and B cell based, evolved from invertebrate Ig domain proteins. What particular aspect of the Ig domain makes it ideal for functions as diverse as homophilic cell adhesion and antigen recognition? The folded structure of the Ig domain allows for a rigid central core that acts to stabilize exposed polypeptide loops. These exposed loops can freely vary in sequence without seriously disrupting the stable structure of the protein. This special structure of the Ig domain has permitted the multitude of functions for which it serves.


GENE STRUCTURE

Genomic size - 4186 bp

cDNA clone length - 1831 bases

Bases in 5' UTR - 535

Exons - 6

Bases in 3' UTR - 141


PROTEIN STRUCTURE

Amino Acids - 372

Structural Domains

Drosophila Basket/JNK contains kinase subdomains I-XI. The dual phosphorylation motif Thr-Xaa-Tyr required for MAP kinase activation is located in kinase subdomain VIII (Thr-Pro-Tyr). Sequence comparison with other MAP kinases demonstrates that DJNK is a member of the JNK group of protein kinases. The amino acid sequence of DJNK is 65% identical to human JNK1. The amino acid sequence between kinase subdomains IX and X varies between the JNKs, and determines the efficiency of binding to c-Jun. In this region, DJNK is most homologous to JNK2. DJNK is related more distantly to the Drosophila rolled gene product ERKA (Sluss, 1996 and Riesgo-Escovar, 1996).


basket/JNK: | Evolutionary Homologs | Regulation | Protein Interactions | Developmental Biology | Effects of Mutation | References
date revised: 16 Dec 96  
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