Gene name - rolled
Synonyms - MAP kinase
Cytological map position - h38--h41
Function - serine/threonine protein kinase
Symbol - rl
Genetic map position - 2-55.1
Classification - Map kinase - ERK
Cellular location - cytoplasmic and nuclear
|Recent literature||Lim, B., Dsilva, C.J., Levario, T.J., Lu, H., Schüpbach, T., Kevrekidis, I.G. and Shvartsman, S.Y. (2015). Dynamics of inductive ERK signaling in the Drosophila embryo. Curr Biol 25(13):1784-90. PubMed ID: 26096970
Transient activation of the highly conserved extracellular-signal-regulated kinase (ERK) establishes precise patterns of cell fates in developing tissues. Quantitative parameters of these transients are essentially unknown. This study provides a detailed quantitative picture of an ERK-dependent inductive signaling event in the early Drosophila embryo, an experimental system that offers unique opportunities for high-throughput studies of developmental signaling. Data analysis reveals a spatiotemporal pulse of ERK activation that is consistent with a model in which transient production of a short-ranged ligand feeds into a simple signal interpretation system. The pulse of ERK signaling acts as a switch in controlling the expression of the ERK target gene. The quantitative approach that leads to this model, based on the integration of data from fixed embryos and live imaging, can be extended to other developmental systems patterned by transient inductive signals.
|Auer, J. S., Nagel, A. C., Schulz, A., Wahl, V. and Preiss, A. (2015). Local overexpression of Su(H)-MAPK variants affects Notch target gene expression and adult phenotypes in Drosophila. Data Brief 5: 852-863. PubMed ID: 26702412
In Drosophila, Notch and EGFR signalling pathways are closely intertwined. Their relationship is mostly antagonistic, and may in part be based on the phosphorylation of the Notch signal transducer Suppressor of Hairless [Su(H)] by MAPK. Su(H) is a transcription factor that together with several cofactors regulates the expression of Notch target genes. This study addresses the consequences of a local induction of three Su(H) variants on Notch target gene expression. To this end, wild-type Su(H), a phospho-deficient Su(H)MAPK-ko and a phospho-mimetic Su(H)MAPK-ac isoform were overexpressed in the central domain of the wing anlagen. The expression of the Notch target genes cut, wingless, E(spl)m8-HLH and vestigial, was monitored. For the latter two, reporter genes were used (E(spl)m8-lacZ and vgBE-lacZ). In general, Su(H)MAPK-ko induced a stronger response than wild-type Su(H), whereas the response to Su(H)MAPK-ac was very weak. Notch target genes cut, wingless and vgBE-lacZ were ectopically activated, whereas E(spl)m8-lacZ was repressed by overexpression of Su(H) proteins. In addition, in epistasis experiments an activated form of the EGF-receptor (DERact) or the MAPK (rlSEM) and individual Su(H) variants were co-overexpressed locally, to compare the resultant phenotypes in adult flies (thorax, wings and eyes) as well as to assay the response of the Notch target gene cut in cell clones (Auer, 2015).
|Wong, Z. S., Brownlie, J. C. and Johnson, K. N. (2016). Impact of ERK activation on fly survival and Wolbachia-mediated protection during virus infection. J Gen Virol [Epub ahead of print]. PubMed ID: 26977591
Elevated levels of reactive oxygen species (ROS) provide protection against virus-induced mortality in Drosophila. In addition to contributing to oxidative stress, ROS are known to activate a number of signaling pathways including the extracellular signal-regulated kinases (ERK) signaling cascade. It was recently shown that ERK signaling is important for resistance against viral replication and invasion in cultured Drosophila cells and the gut epithelium of adult flies.bUsing a Drosophila loss-of-function ERK (rolled) mutant this study demonstrated that ERK is important for fly survival during virus infection. ERK mutant flies subjected to Drosophila C virus (DCV) oral and systemic infection were more susceptible to virus-induced mortality as compared to wild type flies. It was demonstrated experimentally that ERK activation is important for fly survival during oral and systemic virus infection. Given that elevated ROS correlates with Wolbachia-mediated antiviral protection, the involvement of ERK in antiviral protection was also investigated in flies infected by Wolbachia. The results indicate that ERK activation is increased in the presence of Wolbachia but this does not appear to influence Wolbachia-mediated antiviral protection, at least during systemic infection.
|Ashton-Beaucage, D., Lemieux, C., Udell, C. M., Sahmi, M., Rochette, S. and Therrien, M. (2016). The deubiquitinase USP47 stabilizes MAPK by counteracting the function of the N-end rule ligase POE/UBR4 in Drosophila. PLoS Biol 14: e1002539. PubMed ID: 27552662
RAS-induced MAPK signaling is a central driver of the cell proliferation apparatus. Disruption of this pathway is widely observed in cancer and other pathologies. Consequently, considerable effort has been devoted to understanding the mechanistic aspects of RAS-MAPK signal transmission and regulation. While much information has been garnered on the steps leading up to the activation and inactivation of core pathway components, comparatively little is known on the mechanisms controlling their expression and turnover. Several factors have been identified that dictate Drosophila MAPK levels. This study describes the function of one of these, the deubiquitinase (DUB) USP47. USP47 was shown to act post-translationally to counteract a proteasome-mediated event that reduces MAPK half-life and thereby dampens signaling output. Using an RNAi-based genetic interaction screening strategy, UBC6, POE/UBR4, and UFD4 (CG5604) were identified, respectively, as E2 and E3 enzymes that oppose USP47 activity. Further characterization of POE-associated factors uncovered KCMF1 (CG11984) as another key component modulating MAPK levels. Together, these results identify a novel protein degradation module that governs MAPK levels. Given the role of UBR4 as an N-recognin ubiquitin ligase, these findings suggest that RAS-MAPK signaling in Drosophila is controlled by the N-end rule pathway and that USP47 counteracts its activity.
|Johnson, H. E., Goyal, Y., Pannucci, N. L., Schupbach, T., Shvartsman, S. Y. and Toettcher, J. E. (2017). The Spatiotemporal Limits of Developmental Erk Signaling. Dev Cell 40(2): 185-192. PubMed ID: 28118601
Animal development is characterized by signaling events that occur at precise locations and times within the embryo, but determining when and where such precision is needed for proper embryogenesis has been a long-standing challenge. This study addressed this question for extracellular signal regulated kinase (Erk) signaling, a key developmental patterning cue. An optogenetic system is described for activating Erk with high spatiotemporal precision in vivo. Implementing this system in Drosophila, it was found that embryogenesis is remarkably robust to ectopic Erk signaling, except from 1 to 4 hr post-fertilization, when perturbing the spatial extent of Erk pathway activation leads to dramatic disruptions of patterning and morphogenesis. Later in development, the effects of ectopic signaling are buffered, at least in part, by combinatorial mechanisms. This approach can be used to systematically probe the differential contributions of the Ras/Erk pathway and concurrent signals, leading to a more quantitative understanding of developmental signaling.
|]Park, S.M., Park, H.R. and Lee, J.H. (2017).
MAPK3 at the autism-linked human 16p11.2
locus influences precise synaptic target selection at Drosophila
larval neuromuscular junctions. Mol Cells [Epub ahead of print].
PubMed ID: 28196412
Proper synaptic function in neural circuits requires precise pairings between correct pre- and post-synaptic partners. Errors in this process may underlie development of neuropsychiatric disorders, such as autism spectrum disorder (ASD). Development of ASD can be influenced by genetic factors, including copy number variations (CNVs). This study focused on a CNV occurring at the 16p11.2 locus in the human genome and investigated potential defects in synaptic connectivity caused by reduced activities of genes located in this region at Drosophila larval neuromuscular junctions, a well-established model synapse with stereotypic synaptic structures. A mutation of rolled, the Drosophila homolog of human mitogen-activated protein kinase 3 (MAPK3) at the 16p11.2 locus, causes ectopic innervation of axonal branches and their abnormal defasciculation. The specificity of these phenotypes was confirmed by expression of wild-type rolled in the mutant background. Albeit to a lesser extent, ectopic innervation patterns were also observed in mutants defective in Cdk2, Gaq, and Gp93, all of which are expected to interact with Rolled MAPK3. Further genetic analysis in double heterozygous combinations reveals a synergistic interaction between rolled and Gp93. In addition, results from RT-qPCR analyses indicate consistently reduced rolled mRNA levels in Cdk2, Gaq, and Gp93 mutants. Taken together, these data suggest a central role of MAPK3 in regulating the precise targeting of presynaptic axons to proper postsynaptic targets, a critical step that may be altered significantly in ASD.
|Tomer, D., Chippalkatti, R., Mitra, K. and Rikhy, R. (2018). ERK regulates mitochondrial membrane potential in fission deficient Drosophila follicle cells during differentiation. Dev Biol 434(1):48-62. PubMed ID: 29157562
Mitochondrial morphology regulatory proteins interact with signaling pathways involved in differentiation. In Drosophila oogenesis, EGFR signaling regulates mitochondrial fragmentation in posterior follicle cells (PFCs). EGFR driven oocyte patterning and Notch signaling mediated differentiation are abrogated when PFCs are deficient for the mitochondrial fission protein Drp1. It is not known whether fused mitochondrial morphology in drp1 mutant PFCs exerts its effects on these signaling pathways through a change in mitochondrial electron transport chain (ETC) activity. This study shows that aggregated mitochondria in drp1 mutant PFCs have increased mitochondrial membrane potential. Experiments were performed to assess the signaling pathway regulating mitochondrial membrane potential and how this impacts follicle cell differentiation. drp1 mutant PFCs were found to show an increase in phosphorylated ERK (dpERK) formed downstream of EGFR signaling. ERK regulates high mitochondrial membrane potential in drp1 mutant PFCs. PFCs depleted of ERK and drp1 are able to undergo Notch mediated differentiation. Notably mitochondrial membrane potential decrease via ETC inhibition activates Notch signaling at an earlier stage in wild type and suppresses the Notch signaling defect in drp1 mutant PFCs. Thus, this study shows that the EGFR pathway maintains mitochondrial morphology and mitochondrial membrane potential in follicle cells for its functioning and decrease in mitochondrial membrane potential is needed for Notch mediated differentiation.
|Molnar, C., Estrada, B. and de Celis, J. F. (2018). Tay bridge and ERK activity are required for motoneuron function in the Drosophila neural system. Genes Brain Behav. PubMed ID: 29524312
Extracellular regulated kinase (Erk) activity is required during neural development for the specification of cell fates in neuroblasts and neuronal lineages, and also regulates several aspects of the activity and survival of mature neurons. The activation of Erk is regulated at multiple levels by kinases and phosphatases that alter its phosphorylation state and by other proteins that regulate its subcellular localization. This study finds that tay bridge (tay), a negative regulator of Erk in Drosophila imaginal discs, is required in the motoneurons to regulate the number and size of neuromuscular synapses in these cells. The expression of Tay is maximal in motoneurons with low levels of activated ERK, suggesting that Tay modulates the activity of Erk in these cells. Loss of tay expression and increased Erk activity specifically in the motoneurons cause a reversible decrease in walking speed. Impaired motoneurons activity may be caused by alterations in the functionality and number of synaptic boutons developing at the neuromuscular junction in tay mutants.
|Zhang, X., Li, Q., Wang, L., Liu, Z. J. and Zhong, Y. (2018). Active Protection: Learning-Activated Raf/MAPK Activity Protects Labile Memory from Rac1-Independent Forgetting. Neuron 98(1):142-155. PubMed ID: 29551489
Active forgetting explains the intrinsic instability of a labile memory lasting for hours. However, how such memory maintains stability against unwanted disruption is not completely understood. This study reports a learning-activated active protection mechanism that enables labile memory to resist disruptive sensory experiences in Drosophila. Aversive olfactory conditioning activates mitogen-activated protein kinase (MAPK) transiently in the mushroom-body gamma lobe, where labile-aversive memory is stored. This increased MAPK activity significantly prolongs labile memory retention and enhances its resistance to disruption induced by heat shock, electric shock, or odor reactivation. Such experience-induced forgetting cannot be prevented by inhibition of Rac1 activity. Instead, protection of Rac1-independent forgetting correlates with non-muscle myosin II activity and persistence of learning-induced presynaptic structural changes. Increased Raf/MAPK activity, together with suppressed Rac1 activity, completely blocks labile memory decay. Thus, learning not only leads to memory formation, but also activates active protection and active forgetting to regulate the formed memory.
|Gupta, S., Varshney, B., Chatterjee, S. and Ray, K. (2018). Somatic ERK activation during transit amplification is essential for maintaining the synchrony of germline divisions in Drosophila testis. Open Biol 8(7). PubMed ID: 30045884
Transit amplification (TA) of progenitor cells maintains tissue homeostasis by balancing proliferation and differentiation. In Drosophila testis, the germline proliferation is tightly regulated by factors present in both the germline and the neighbouring somatic cyst cells (SCCs). Although the exact mechanism is unclear, the epidermal growth factor receptor (EGFR) activation in SCCs has been reported to control spermatogonial divisions within a cyst, through downstream activations of Rac1-dependent pathways. This study reports that somatic activation of the mitogen-activated protein kinase (Rolled/ERK) downstream of EGFR is required to synchronize the mitotic divisions and regulate the transition to meiosis. The process operates independently of the Bag-of-marble activity in the germline. Also, the integrity of the somatic cyst enclosure is inessential for this purpose. Together, these results suggest that synchronization of germ-cell divisions through somatic activation of distinct ERK-downstream targets independently regulates TA and subsequent differentiation of neighbouring germline cells.
|Ogura, Y., Sami, M. M., Wada, H. and Hayashi, S. (2019). Automated FRET quantification reveals distinct subcellular ERK activation kinetics in response to graded EGFR signaling in Drosophila. Genes Cells. PubMed ID: 30851218
Threshold responses to an activity gradient allow a single signaling pathway to yield multiple outcomes. Extracellular signal-regulated kinase (ERK) is one such signal, which couples receptor tyrosine kinase signaling with multiple cellular responses in various developmental processes. Recent advances in the development of fluorescent biosensors for live imaging have enabled the signaling activities accompanying embryonic development to be monitored in real time. This study used an automated computational program to quantify the signals of a fluorescence resonance energy transfer (FRET) reporter for activated ERK, and this system was used to monitor the spatio-temporal dynamics of ERK during neuroectoderm patterning in Drosophila embryos. The cytoplasmic and nuclear ERK activity gradients were found to exhibit distinct kinetics in response to epidermal growth factor (EGF) receptor activation. The ERK activation patterns implied that the cytoplasmic ERK activity is modulated into a threshold response in the nucleus.
|Johnson, H. E. and Toettcher, J. E. (2019). Signaling dynamics control cell fate in the early Drosophila embryo. Dev Cell 48(3): 361-370.e363. PubMed ID: 30753836
The Erk mitogen-activated protein kinase plays diverse roles in animal development. Its widespread reuse raises a conundrum: when a single kinase like Erk is activated, how does a developing cell know which fate to adopt? This study combined optogenetic control with genetic perturbations to dissect Erk-dependent fates in the early Drosophila embryo. Erk activity was found to be sufficient to "posteriorize" 88% of the embryo, inducing gut endoderm-like gene expression and morphogenetic movements in all cells within this region. Gut endoderm fate adoption requires at least 1 h of signaling, whereas a 30-min Erk pulse specifies a distinct ectodermal cell type, intermediate neuroblasts. The endoderm-ectoderm cell fate switch is controlled by the cumulative load of Erk activity, not the duration of a single pulse. The fly embryo thus harbors a classic example of dynamic control, where the temporal profile of Erk signaling selects between distinct physiological outcomes.
rolled/MAP kinase is essential to the proper functioning of the Ras signaling pathway. Mammalian homologs are known as ERKs (extracellular signal-regulated kinases) because of their roles in transducing signals from outside the cell into the nucleus. Targets of Rolled in Drosophila include Pointed, Anterior open (commonly referred to as Yan), Seven in absentia (Sina) and Jun related antigen (Jun or DJun). For example, although induction of Sina is not regulated by Rolled, its biological activity is regulated by Ras pathway phosphorylation (Dickson, 1992). The same holds true for Anterior open. Anterior open activity is targeted by the Ras pathway (Gabay, 1996). Likewise, the Ras pathway activates Pointed protein (Brunner, 1994b) and Jun related antigen, commonly known as Jun (Peverali,1996).
The development of the Drosophila eye is controlled in part, by the Ras pathway. This provides a good illustration of rolled/Mapk functions. The Drosophila compound eye is made up of a cluster of single simplified eye elements, collectively termed ommatidia. Each ommatidium contains eight photoreceptors (R1-R8), differentiated during larval development in a process of discrete steps from the larval eye imaginal disc: in order of determination, they are R8, followed by the pairs R2/R5, R3/R4, and R1/R6, and finally R7. Induction of R7 involves signals from the R8 photoreceptor. The R8 photoreceptor presents on its surface a ligand, Bride of Sevenless, that binds and activates Sevenless receptor tyrosine kinase in the R7 precursor. Autophosphorylated Sevenless initiates a Ras1-mediated cascade, which eventually activates transcription factors in the nucleus via Raf1 and MAP kinases, resulting in R7 development (Yamamoto, 1994 and references).
The presence of functional R7 photoreceptor cells can be detected by a simple behavioral test. Given a choice between an ultraviolet (UV) and a visible light source, wild-type flies will move towards the UV light. Flies lacking R7 cells, however, will move towards the visible light. This behavior has been used to identify mutations in genes that prevent the development of R7 photoreceptor cells and has led to the identification of sevenless and bride of sevenless genes. This behavioral screen can be used to identify dominant mutations that result in the activation of the sevenless signal transduction pathway, even in the absence of its inducing signal, the Boss protein (Brunner, 1994a and references).
Sevenmaker, a gain of function rolled mutation, specifies R7 photoreceptor cells independently of boss and sev functions. In boss mutant flies, the R7 cell is missing, whereas in Sev gain of function mutants many ommatidia contain multiple R7-like cells (Brunner, 1994a). Rolled acts downstream of Ras1 and Raf in specifying the R7 fate. In addition, rolled plays a key role in the Torso pathway. Gain of function Sev results in the ectopic acivation of the Torso signaling pathway in a manner similar to its effect on the sevenless pathway in the developing eye. Loss-of-function mutations in rolled can suppress gain-of-function torso mutations. Thus Rolled acts in the terminal pathway responsible for specification of terminal structures immediately after fertilization. Likewise rolled mutations affect signaling of the Drosophila Epidermal growth factor receptor. Gain-of-function Sevenmaker mutations produce extra wing veins close to the wing margin. In addition, rolled mutations modify EGF-R signaling in the establishment of the dorsoventral polarity of the egg shell and the embryo (Brunner, 1994a).
Rolled activity is targeted during heat shock induced stress in cultured Drosophila cells. Heat shock activates a MAP kinase phosphatase, resulting in a reduction of Rolled activity over the course of a 15 minute heat shock regime (raising the temperature from 25 to 37 degrees) (Cornelius, 1995).
Trophic mechanisms in which neighboring cells mutually control their survival by secreting extracellular factors play an important role in determining cell number. However, how trophic signaling suppresses cell death is still
poorly understood. The survival of a subset of midline glia cells in Drosophila depends upon direct suppression of the proapoptotic protein Hid via the Egf receptor/RAS/MAPK pathway. The TGF alpha-like ligand Spitz is activated in the neurons, and glial cells compete for limited amounts of secreted Spitz to survive. In midline glia that fail to activate the Egfr pathway, Hid induces apoptosis by
blocking a caspase inhibitor, Diap1. Therefore, a direct pathway linking a specific extracellular survival factor with a caspase-based death program has been established (Bergmann, 2002).
The reduction in midline glia (MG) cell number due to apoptosis, as well as the requirement of the RAS/MAPK pathway for MG survival, has been documented using various MG-specific enhancer trap lines and reporter fusion constructs. The MG are visualized using a reporter fusion construct for the slit gene (sli-lacZ) in which a 1 kb fragment of the slit promoter confers expression specifically to the MG. Using a ß-gal antibody to monitor the developmental profile of the MG, about ten cells per segment expressing sli-lacZ are detectable at midembryogenesis (stage 13). By the end of embryogenesis at stage 17, the number of sli-lacZ-positive cells is reduced to approximately three per segment. Since the sli-lacZ expression is specific for the MG, sli-lacZ-expressing cells are referred to as MG (Bergmann, 2002).
Prominent activation of MAPK has been identified in the MG cells, but its functional role has not been determined. The fate of the MG in mapk-deficient embryos has been analyzed. Compared to wild-type embryos, the initial generation of the MG appears to be normal. However, by stage 17 (the end of embryogenesis), none of the MG in mapk-deficient embryos survive. This finding suggests that MAPK is required for MG survival in wild-type embryos (Bergmann, 2002).
The genetic requirement of mapk for MG survival and of hid for MG apoptosis prompted the assumption that MAPK promotes survival of the MG by inhibition of HID activity. According to this model, the MG would be unprotected from HID-induced apoptosis in mapk-deficient embryos, and die. Consistent with this idea, HID protein is detectable in the MG of late stage wild-type embryos. To test this further, embryos that were mutant for both mapk and hid were examined. In early stage mapk;hid double mutant embryos, the initial generation of the MG appears to be normal. However, in contrast to mapk mutants alone, the MG is rescued in mapk;hid double mutant embryos although the survival function of MAPK is missing in these embryos. Dissection revealed that the MG are located directly at the cuticle of the embryos. Because segmental fusions occur in these embryos, some of the MG cluster in groups of up to 20 cells. In individual segments, five to six MG are visible. This number is larger compared to wild-type (three MG per segment), and is remarkably similar to the number of surviving MG in hid mutant embryos alone, indicating that MAPK promotes MG survival largely through inhibition of HID (Bergmann, 2002).
The mutant analysis revealed that MAPK is required to suppress the activity of HID for MG survival. If hid is mutant in mapk-deficient embryos (i.e., in mapk; hid double mutants), MAPK is no longer needed for the survival of the MG. Thus, MAPK-mediated survival of the MG functions through inhibition of HID (Bergmann, 2002).
In hid mutant embryos there is a 2-fold increase of the MG compared to wild-type. Approximately six MG per segment survive in hid embryos compared to the 2.8 MG per segment in wild-type, indicating that hid is genetically required for MG cell death. MAPK activity is required for MG survival. Does the level of MAPK activity determine the final number of surviving MG cells? Mutational activation of MAPK, using a dominant allele of MAPK termed Sevenmaker or mapkSem, promotes survival of extra MG. About 6.0 MG per segment survive in stage 17 mapkSem embryos, providing additional evidence that MAPK is required for MG survival. Remarkably, the number of surviving MG in mapkSem embryos is very similar to the number of surviving MG in hid mutant embryos. In both cases, approximately six MG survive per segment. Therefore, it was determined whether the six surviving MG in mapkSem embryos correspond to the same MG that survive in hid mutant embryos by double mutant analysis. Stage 17 mapkSem; hid double mutant embryos contain on average 6.6 MG, or slightly more than the single mutants alone. This result strongly suggests that hid expression and MAPK activation occur in largely the same set of MG, that is, in a group of about six MG. If MAPK activation and hid expression would occur in different MG independently of each other, then the mapkSem;hid double mutant would be expected to be the composite of the individual mutants and a total of about ten to twelve MG would survive in the double mutant, similar to what has been observed in H99 mutant embryos. It is inferred from the double mutant analysis that the survival of approximately six MG is regulated by MAPK-dependent inhibition of HID. As long as MAPK is activated, these MG survive (as seen in the activated mapkSem mutant). However, MG in this group that does not maintain activated MAPK are eliminated by HID-induced apoptosis. Thus, the coordinated expression of HID and activation of MAPK regulate the final MG cell number (Bergmann, 2002).
MAPK suppresses hid activity in two ways: via downregulation of its transcription and via phosphorylation of HID protein. However, hid mRNA and protein are readily detectable in the surviving MG of wild-type embryos. Therefore, transcriptional downregulation of hid does not account for MG survival. This prompted a test to see whether inhibitory phosphorylation of HID by MAPK might be critical for MG survival. For this purpose, advantage was taken of an observation that overexpression of HID in the MG using the MG-specific sli-GAL4 driver and UAS-hid transgenes is not sufficient to induce MG apoptosis. Even two copies of the UAS-hid transgenes are not able to ablate the MG. This is contrary to findings in other tissues in which expression of hid induces cell death very well. However, since MAPK is activated in the MG and required for MG survival, it was hypothesized that even overexpressed HID might be inactivated via MAPK phosphorylation (Bergmann, 2002).
To examine this further, UAS-hid transgenes were generated that alter the five phosphoacceptor residues of the MAPK phosphorylation sites to nonphosphorylatable Ala residues (UAS-hidAla5). The UAS-hidAla5 transgenes driven by sli-GAL4 induce apoptosis in the MG very efficiently. One copy of a UAS-hidAla5 transgene is sufficient for the ablation of the MG. Occasionally, some embryos are recovered in which the ablation of the MG is incomplete. However, nerve cord preparations reveal that in these embryos, only a small fraction of the MG survives compared to wild-type. Some segments completely lack MG cells, while others just contain one remaining MG. The MG is required for separation of the commissural axon tracts of the CNS. Consistently, expression of the UAS-hidAla5 transgenes and consequently ablation of the MG causes a fused commissure phenotype. In summary, this analysis demonstrates that MG survival requires suppression of HID activity by MAPK. The MAPK phosphorylation sites in HID are critical for this response, providing an important mechanism for the regulation of MG number (Bergmann, 2002).
Activation of MAPK usually requires activation of RAS, which in turn is activated by receptor tyrosine kinase (RTK) signaling: this demonstrates that MG survival depends on RAS, which is consistent with the model. Within the embryonic CNS, the Drosophila homolog of the Epidermal growth factor receptor (Egfr) is specifically expressed and required for MG differentiation. The requirement of Egfr signaling for MG survival was examined (Bergmann, 2002).
Due to severe developmental defects in egfr mutants, only a few MG start forming at stage 11, and none of them survive. Thus, it is difficult to directly study the requirement of the Egfr for MG survival. To overcome this problem, a dominant-negative mutant of the Egfr (UAS-EgfrDN) was expressed in the MG using the sli-GAL4 driver in otherwise wild-type embryos. In this way, Egfr activity is specifically diminished in the MG after their generation. As expected, the MG form normally in these embryos. However, most of the MG die during subsequent developmental stages and only a few survive to the end of embryogenesis, indicating a direct requirement of the Egfr for MG survival. To determine whether the MG death in this experimental condition is due to failure to inhibit HID, EgfrDN was expressed in the MG of hid mutants. In this genetic background, on average 6.1 MG cells survive, demonstrating that MG survival requires functional Egfr signaling to suppress HID activity (Bergmann, 2002).
The spi gene is required for MG survival and encodes a candidate trophic factor for MG survival. To prove that spi function is required to suppress hid activity, the fate of the MG in spi;hid double mutant embryos was determined. The MG survive in spi embryos if hid is removed as well, and it is concluded that the survival function of spi is mediated through suppression of hid-induced apoptosis (Bergmann, 2002).
The question arises as to which cells process mSPI and provide a source of sSPI for MG survival. Since spi is ubiquitously expressed, it is difficult to determine histochemically where sSPI, the active ligand, is generated. Therefore, a genetic approach was used; whether the loss of MG in spi mutant embryos can be rescued by expression of the membrane bound inactive precursor (UAS-mSPI) either in the MG (using the sim-GAL4 driver) or in neuronal axons (using the elav-GAL4 driver) was examined. It was reasoned that the MG would be rescued in spi mutant embryos only if mSPI is presented in the location where it is normally processed for MG survival in wild-type embryos. Presentation of mSPI by the MG itself does not result in rescue of the MG in spi mutant embryos, ruling out an autocrine mechanism. In contrast, expression of mSPI in neuronal axons appears to be sufficient for MG survival in spi embryos. This argues in favor of a paracrine mechanism. In control experiments, sSPI was examined using these two Gal4 drivers in wild-type embryos. With both GAL4 drivers an increase in the number of MG cells is detected, indicating that they are expressed at the right time and that the MG does not fail to secrete Spi once it has been processed (Bergmann, 2002).
A key regulator of Spi activation is rhomboid, a gene encoding a cell surface, seven-pass transmembrane protein that appears to function as a serine protease directly cleaving mSPI. rhomboid has been implicated in suppression of MG apoptosis. Ectopic expression of Rho in neurons (elav-Gal4/UAS-Rhomboid) promotes an excess of MG, suggesting that neurons have the capacity to process endogenous mSPI. Another essential protein for Spi processing is Star. Star mutants display an MG phenotype similar to spi. STAR regulates intracellular trafficking of mSPI. Expression of Star from the neurons but not from the MG rescues the Star phenotype in the MG. Thus, this analysis clearly demonstrates that the sSPI signal for MG survival is generated and secreted by neurons (Bergmann, 2002).
The surviving MG in late stage embryos are in close contact to commissural axons. In embryos lacking the commissureless (comm) gene, the commissural axons are absent. In comm embryos the MG die prematurely, and some survivors become misplaced laterally along the longitudinal axon tracts. The location of the MG along the longitudinal axons in comm mutant embryos as well as their close contact to commissural axons in wild-type embryos has prompted the suggestion that axon contact is required for MG survival. Axon contact appears to permit the MG to respond to trophic signaling, which is necessary for its survival. Consistent with this notion trophic signaling provided by sSPI/Egfr is present only in MG associated with longitudinal axons in comm;hid double mutants. Thus, it was asked whether axon contact-mediated Egfr signaling in the MG is required to activate MAPK, which in turn suppresses the cell death-inducing ability of HID (Bergmann, 2002).
To address this question, the fate of the MG was examined in comm mutant embryos which are at the same time mutant for hid (comm;hid double mutants) or carry the dominant active mapk allele, mapkSem (mapkSem;comm double mutants). Strikingly, a substantial number of the MG survive even in the absence of axonal contact if hid function is removed or if MAPK is activated. This strongly suggests that axon contact is necessary to suppress HID via MAPK. Only MG in proximity to neurons undergo Egfr signaling. The additional MG that survive along the midline in comm;hid mutants do not express spry, that is, do not receive an Egfr signal, and survive only because hid is absent in this experimental condition (Bergmann, 2002).
It is noted that active MAPK is capable of rescuing a total of six MG based on analysis of mapkSem embryos. Presumably, this MAPK activation in mapkSem embryos is inherited from the differentiation period of the MG. However, only three MG survive by stage 17 in wild-type embryos. It is proposed that of the group of six MG that require MAPK for survival, only the three surviving cells make adequate axon contacts necessary to receive sufficient quantities of the survival factor sSPI. According to this model, the remaining three MG die because they lose the competition for axon contact and do not receive levels of sSPI that are high enough to inactivate HID via phosphorylation by MAPK. If additional sSPI is provided in the midline, additional MG can be rescued. The limited availability of axon-derived sSPI would serve to match the number of MG to the length of commissural axons requiring ensheathment. Thus, the regulation of MG number and survival represents a genetically defined example of the classical trophic theory of cell survival (Bergmann, 2002).
The regulation of MG apoptosis in Drosophila bears striking overall similarity to the regulation of glial cell death in the rat optic nerve. There is an early dependence of the oligodendroglia in the rat optic nerve on growth factors for differentiation followed by a dependence on axon contact for survival. However, it is not clear how the oligodendroglia in the rat optic nerve survive upon axon contact. Since mammalian homologs for many of the components in the apoptotic pathway both upstream and downstream of Drosophila HID are known, it will be interesting to analyze whether similar molecules regulate apoptosis and cell number in the mammalian nervous system. Therefore, molecular genetic studies in Drosophila promise considerable insight for advancing an understanding of the basic control mechanisms involved in the regulation of apoptosis in the context of a developing organism in vivo (Bergmann, 2002).
The rolled locus is found in a heterochromatic region of chromosome 2 that is considered to remain condensed (and for the most part transcriptionally inactive) throughout all or most of the cell cycle. rolled lies in what is considered to be alpha heterochromatin, a chromosome region that makes up the chromocenter of polytene salivary gland chromosomes. The chromocenter is not thought to be polytenized, that is, it is not thought to undergo repeated rounds of DNA replication resulting in multiple copies of active genes. The chromocenter is thought to be made up of DNA and protein in a tightly knit dense structure of which is transcriptionally inactive. Such heterochomatic regions, which make up 30% of the Drosophila genome, have a much lower density of genes as compared to euchromatin.
rolled gene activity is unusual in that it requires the surrounding heterochromatin for gene function. rolled gene activity is severly impaired by bringing rolled close to any euchromatic position; however, these position effects can be reversed by chromosomal rearrangements that bring the rolled gene closer to any block of autosomal or X chromosome heterochromatin (Eberl, 1993). Heterochromatic rolled is extensively polytenized and transcriptionally active in salivary gland chromosomes. rolled undergoes polytenization in salivary gland chromosomes to a degree comparable to that of euchromatic genes, despite its deep heterochromatic location. Sequences on either side of rolled are severly underrepresented in polytene chromosomes and are considered to appear in alpha-heterochromatin. It is suggested that multiple domains of functionally active, complex DNA sequences, composed of single-copy genes and middle-repetitive elements, are present within the proximal heterochromatin of chromosome 2. It may well be that heterochromatin is not the transcriptional desert that it was once thought to be (Berghella, 1996).
Patterning of the terminal regions of the Drosophila embryo relies on the gradient of phosphorylated ERK/MAPK (dpERK), which is controlled by the localized activation of the Torso receptor tyrosine kinase. This model is supported by a large amount of data, but the gradient itself has never been quantified. This study presents the first measurements of the dpERK gradient and establishes a new intracellular layer of its regulation. Based on the quantitative analysis of the spatial pattern of dpERK in mutants with different levels of Torso as well as the dynamics of the wild-type dpERK pattern, it is proposed that the terminal-patterning gradient is controlled by a cascade of diffusion-trapping modules. A ligand-trapping mechanism establishes a sharply localized pattern of the Torso receptor occupancy on the surface of the embryo. Inside the syncytial embryo, nuclei play the role of traps that localize diffusible dpERK. It is argued that the length scale of the terminal-patterning gradient is determined mainly by the intracellular module (Coppey, 2008).
This study identifies the nuclear trapping of dpERK as a mechanism responsible for the intracellular spatial processing of the terminal signal. This conclusion is based on the analysis of the dynamics of the wild-type dpERK gradient. Between nuclear cycles 10 and 14, the dpERK levels are amplified at the termini and attenuated in the subterminal regions of the embryo. The observed dynamics of the dpERK gradient is consistent with a model where dpERK is a diffusible molecule, which is trapped and dephosphorylated by the nuclei. A uniform increase in the nuclear density would increase the trapping of the dpERK molecules at the poles and prevent their diffusion to the middle of the embryo. This is consistent with biochemical and imaging data showing that dpERK rapidly translocates to the nucleus, which can also serve as a compartment of dpERK dephosphorylation. In addition, the model makes a testable prediction about the dynamics of the nucleocytoplasmic (N/C) ratio of phosphorylated MAPK (Coppey, 2008).
The nuclear and cytoplasmic levels of dpERK were quantified in cycle 13 and 14 embryos. Plotting the nuclear and cytoplasmic profiles against each other gives a clear linear relationship, as predicted by a simple formula. Furthermore, the nucleocytoplasmic ratio clearly increases between these two nuclear cycles: the N/C ratio is ~1.4 and ~2 at nuclear cycles 13 and 14, respectively. These measurements show that the nuclear trapping rate is indeed an increasing function of the nuclear density, as predicted by the model (Coppey, 2008).
The observed N/C ratios show that a significant fraction of total dpERK nuclear. As a consequence, defects in the nuclear density should generate clear defects in the gradient. This can be tested in mutants with 'holes' in the nuclear density in blastoderm embryos. For example, in shakleton (shkl) embryos, the migration of nuclei to the poles is delayed and a number of embryos exhibit major disruptions in nuclear density. As predicted by the model, shkl embryos show striking disruptions in the dpERK gradient. The quantified posterior gradient of this particular mutant embryo shows a clear local correlation with the nuclear distribution, emphasizing the role of the nuclei at this stage. In early embryos, the gradient is more extended, presumably reflecting the lack of the nuclei at the poles. Similar defects were found in the giant nuclei (gnu) mutant embryos, which show a different type of defect in nuclear organization. These results support the model in which the syncytial nuclei play an important role in shaping the dpERK gradient (Coppey, 2008).
To summarize, it is proposed that the dpERK gradient is controlled by a cascade of at least two diffusion-trapping modules. In the extracellular compartment, a ligand-trapping mechanism, identified in previous studies, establishes a sharp gradient of Torso receptor occupancy. A similar mechanism regulates the dpERK gradient inside the embryo, where syncytial nuclei act as traps that localize diffusible dpERK. At this time, it cannot be ruled out that the observed sharpening of the dpERK gradient can be modulated also by changes in the spatial distribution of the Torso ligand, but currently there are no data in support of this mode of regulation (Coppey, 2008).
The dynamics of the dpERK gradient is qualitatively different from that of the Bicoid gradient, which remains stable during the last five nuclear divisions. It has been proposed that a stable gradient of Bicoid can be established in the absence of Bicoid degradation, because of the reversible trapping of Bicoid by an exponentially increasing number of nuclei. The differences between the dynamics of the Bicoid and dpERK gradients are attributed to two effects. The first effect is due to the differences in the 'chemistries' of the two systems: the morphogen in the terminal system is degraded (MAPK is dephosphorylated), whereas the anterior morphogen is stable (it is proposed that Bicoid is not degraded on time scale of the gradient formation). The second effect is due to the differences in the initial conditions: by the 10th nuclear division, which is the starting point of the activation of the terminal system, Bicoid gradient is essentially fully established. Thus, a common biophysical framework can describe the Bicoid and dpERK gradients. It remains to be determined whether the nuclear export affects the length scale of the Dorsal gradient, which patterns the dorsoventral axis of the embryo (Coppey, 2008)
Given the relationship between sleep and plasticity, this study examined the role of Extracellular signal-regulated kinase (ERK, Rolled in Drosophila) in regulating baseline sleep, and modulating the response to waking experience. Both sleep deprivation and social enrichment increase ERK phosphorylation in wild-type flies. The effects of both sleep deprivation and social enrichment on structural plasticity in the within the Pigment Dispersing Factor (PDF)-expressing ventral lateral neurons (LNvs) can be recapitulated by expressing an active version of ERK (UAS-ERKSEM) pan-neuronally in the adult fly using GeneSwitch (Gsw) Gsw-elav-GAL4. Conversely, disrupting ERK reduces sleep and prevents both the behavioral and structural plasticity normally induced by social enrichment. Finally, using transgenic flies carrying a cAMP response Element (CRE)-luciferase reporter it was shown that activating ERK enhances CRE-Luc activity while disrupting ERK reduces it. These data suggest that ERK phosphorylation is an important mediator in transducing waking experience into sleep (Vanderheyden, 2013).
ERK plays a key role in regulating not only cell differentiation and proliferation during development, but is also critical for regulating long-term potentiation and plasticity related events in the fully developed adult. Recent studies have highlighted the important relationship between plasticity induced by waking-experience and sleep need. With that in mind, it was hypothesized that ERK may provide a molecular link between plasticity and sleep. Since, ERK phosphorylation has been previously correlated with sleep time following rhomboid mediated activation of EGFR, this study over-expressed an active version of ERK pan-neuronally in the adult fly and found a significant increase in sleep. ERK activation increased sleep during the day, was rapidly reversible, and was associated with increased activity during waking. In contrast, disrupting ERK signaling by feeding adults the MEK inhibitor SL327 decreased daytime sleep and lowered waking activity. Together these data indicate ERK activation plays a role in sleep regulation (Vanderheyden, 2013).
As mentioned, inducing EGFR signaling resulted in an increase in sleep which seemed to correlate nicely with the increase in ppERK. Although these data strongly suggested that the increase in sleep was due to ppERK activation, ppERK was not directly manipulated. Thus these studies confirm and extend data demonstrating that directly activating ppERK can increase sleep. While the largest effects of sleep were obtained using a pan neuronal activation of EGFR signaling, it has also been reported that the EGFR induced increase in sleep could be mapped to the PI. Surprisingly, this study did not see any changes in sleep when ppERK was expressed in the PI using the same GAL4 drivers as a previous study. These data suggest that in the PI, EGFR activation may recruit additional factors along with ppERK to alter sleep. Such regulation may be particularly important for allowing ppERK to carry out multiple functions in various circuits as needed (Vanderheyden, 2013).
In flies, sleep homeostasis is primarily observed during the subjective day. Similarly, social enrichment also produces increases in daytime sleep. Thus, the data indicate that modulating ERK activity in the adult produces a change in sleep during the portion of the circadian day during which sleep deprivation and social enrichment modify sleep time. Interestingly, ERK activation not only increases daytime sleep, but it also results in the proliferation of terminals in the wake-promoting LNvs. The ability of ERK activation to increase synaptic terminals is reminiscent of the change in synaptic markers and structural morphology that are independently observed following sleep deprivation and social enrichment. Interestingly, arouser mutants show both enhanced ethanol sensitivity and an increase in terminals from the LNvs through its activation by EGFR/ERK. Given that a well characterized function of ERK is to regulate synaptic morphology, the current results suggest that ERK activation may be a common mechanism linking waking experience, plasticity and sleep (Vanderheyden, 2013).
As mentioned, ERK activation has been correlated with sleep time following rhomboid mediated activation of EGFR. Interestingly, in that study, ppERK was not detectable in cell bodies following rho mediated increases in sleep suggesting that, during rho activation, ppERK might be modifying sleep at the level of translation initiation. The current data extend these observations and provide genetic evidence that ERK activation may also play a role in regulating sleep and plasticity by activating gene transcription. That is, pan-neuronal expression of RSK, which retains ERK in the cytoplasm and prevents its nuclear translocation, results in a decrease in daytime sleep similar to that observed in wild type flies fed SL327. Although previous studies have established a link between CREB and sleep this study evaluated CRE-Luc solely as a reporter of transcriptional activation. The data indicates that the expression of UAS-ERKSEM increased CRE-Luc activity. In contrast, transgenic CRE-Luc reporter flies show reduced bioluminescence when crossed into a rl10a mutant background. Finally, flies fed SL327 also showed a reduction of CRE-Luc activity. These data are consistent with a recent report demonstrating that activating MEK increases bioluminescence in flies carrying a CRE-Luc reporter. Together these data suggest that ERK activation may alter plasticity and sleep, in part, by activating gene transcription (Vanderheyden, 2013).
Interestingly, expressing UAS-ERKSEM in PDF neurons does not change the number of PDF-terminals and does not alter sleep time. This is in contrast to the effects of expressing UAS-ERKSEM pan-neuronally which increases both the number of PDF-terminals and increases sleep. These data suggest that ERK activation can either influence PDF neurons in a non-cell autonomous fashion or that ERK activation is required in multiple circuits to modulate plasticity. Indeed, it has been recently shown that increasing sleep by activating the dorsal Fan Shaped Body significantly reduces the number of PDF-terminals. Thus, PDF terminal number provides an accessible read-out of brain plasticity that can be used to elucidate molecular mechanisms linking sleep and plasticity at the circuit level (Vanderheyden, 2013).
It is important to note that in flies there is a critical window of adult development that can influence sleep and learning. For example, 0-3 day old rut2080 mutants are able to respond to social enrichment with an increase in sleep but their older siblings (>3days) cannot. In other words, rut2080 mutants can exhibit higher or lower amounts of sleep as adults depending upon environmental context, not levels of rutabaga per se. Indeed, rutabaga mutants have been reported to have significant variations in sleep (both longer and shorter) compared to controls. Given that the environment can stably modify sleep during adult development, even in the absence of memory related genes, care must be taken when classifying a mutant as either long or short sleepers. It should be emphasize that the current experiments were designed to avoid making manipulations during this critical time window to avoid such confounds. However, it remains possible that ERK may modify sleep by activating additional downstream targets and/or by regulating translation initiation at the synapse (Vanderheyden, 2013).
Recent studies have shown that waking experience, including both prolonged wakefulness and exposure to enriched environments, independently produce dramatic increases in both synaptic markers and structural morphology throughout the fly brain and that these changes are reversed during sleep. To date, most studies have evaluated mutations that disrupt synaptic plasticity to identify the molecular mechanisms linking sleep with plasticity. Given that ERK is a key molecule for the regulation of synaptic plasticity and long-term potentiation, this study evaluated its ability to alter both sleep and structural plasticity. The data indicate that both sleep deprivation and social enrichment independently increase ERK phosphorylation in wild-type flies. It is also reported that expressing an active version of ERK (UAS-ERKSEM) in the adult fly results in an increase in sleep and an increase in structural plasticity in the LNvs. These data suggest that ERK phosphorylation is an important mediator in transducing waking experience into sleep (Vanderheyden, 2013).
There are several extended regions of amino acid identity with rat ERK1 and -2 and yeast FUS3 and KSS1. The sequence is most similar to rat ERK1 and -2 over its entire length (80% identity) (Biggs, 1992).
Rolled protein contains 11 conserved kinase subdomains characteristic of kinase proteins. The gain of function Sevenmaker mutation (see above: Biological overview ) is found in the C-terminal end of kinase domain XI, the putative adenosine triphospate-binding site is located in conserved domain II (Brunner, 1994a and references).
The structure of the active form of the MAP kinase ERK2 has been solved, phosphorylated on a threonine and a tyrosine residue within the phosphorylation lip. The lip is refolded, bringing the phosphothreonine and phosphotyrosine into alignment with surface arginine-rich binding sites. Conformational changes occur in the lip and neighboring structures, including the P+1 site, the MAP kinase insertion, the C-terminal extension, and helix C. Domain rotation and remodeling of the proline-directed P+1 specificity pocket account for the activation. The conformation of the P+1 pocket is similar to a second proline-directed kinase, CDK2-CyclinA, thus permitting the origin of this specificity to be defined. Conformational changes outside the lip provide loci at which the state of phosphorylation can be felt by other cellular components (Canagarajah, 1997).
date revised: 30 March 2002Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
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