p38b


DEVELOPMENTAL BIOLOGY

Embryonic

The two Drosophila p38 genes exhibit different embryonic expression patterns. Mpk2 (p38a) mRNA expression occurs predominantly at the preblastoderm stage, indicating a high level of maternal deposition. Zygotic expression is below the detectable level during most of embryonic development. However, Northern analysis demonstrates the presence of low mRNA levels throughout development. At stage 16, there is a low level of staining in the posterior region, which may correspond to the developing hindgut. The preblastoderm staining indicates that Mpk2 may participate in early embryonic development. The p38b gene is expressed throughout embryonic development. There is a high level of maternal deposition, and at later stages, zygotic expression is present in most of the tissues. At midembryogenesis, higher levels of mRNA are detected in the developing anterior and posterior midguts. The expression pattern of p38b is very similar to that of the two MKKs, which also have high maternal deposition in early embryos and zygotic expression in the midguts. One noteworthy difference between the two MKKs is that MKK4 expression is sustained in the ventral nerve cord, while MKK3 (Licorne) expression is less detectable in this tissue but is more prominent in the midgut. Since each of these kinases is deposited maternally, they may all be involved in early development. At later stages, p38b may be the primary mediator of the p38 MAPK pathway, particularly in the developing midgut (Z. Han, 1998).

The MAP kinase p38 is part of Drosophila melanogaster's circadian clock

All organisms have to adapt to acute as well as to regularly occurring changes in the environment. To deal with these major challenges organisms evolved two fundamental mechanisms: the p38 mitogen-activated protein kinase (MAPK) pathway, a major stress pathway for signaling stressful events, and circadian clocks to prepare for the daily environmental changes. Both systems respond sensitively to light. Recent studies in vertebrates and fungi indicate that p38 is involved in light-signaling to the circadian clock providing an interesting link between stress-induced and regularly rhythmic adaptations of animals to the environment, but the molecular and cellular mechanisms remained largely unknown. This study demonstrates by immunocytochemical means that p38 is expressed in Drosophila melanogaster's clock neurons and that it is activated in a clock-dependent manner. Surprisingly, it was found that p38 is most active under darkness and, besides its circadian activation, additionally gets inactivated by light. Moreover, locomotor activity recordings revealed that p38 is essential for a wild-type timing of evening activity and for maintaining approximately 24 h behavioral rhythms under constant darkness: flies with reduced p38 activity in clock neurons, delayed evening activity and lengthened the period of their free-running rhythms. Furthermore, nuclear translocation of the clock protein Period was significantly delayed on the expression of a dominant-negative form of p38b in Drosophila's most important clock neurons. Western Blots revealed that p38 affects the phosphorylation degree of Period, what is likely the reason for its effects on nuclear entry of Period. In vitro kinase assays confirmed the Western Blot results and point to p38 as a potential 'clock kinase' phosphorylating Period. Taken together, these findings indicate that the p38 MAP Kinase is an integral component of the core circadian clock of Drosophila in addition to playing a role in stress-input pathways (Dusik, 2014 - Open access: 25144774).

Effects of Mutation

Misshapen acts in the Frizzled (Fz) mediated epithelial planar polarity (EPP) signaling pathway in eyes and wings. Both msn loss- and gain-of-function result in defective ommatidial polarity and wing hair formation. Genetic and biochemical analyses indicate that msn acts downstream of fz and dishevelled (dsh) in the planar polarity pathway, and thus implicates an STE20-like kinase in Fz/Dsh-mediated signaling. This demonstrates that seven-pass transmembrane receptors can signal via members of the STE20 kinase family in higher eukaryotes. Msn acts in EPP signaling through the JNK (Jun-N-terminal kinase) module as it does in dorsal closure. Although at the level of Fz/Dsh there is no apparent redundancy in this pathway, the downstream effector JNK/MAPK (mitogen-activated protein kinase) module is redundant in planar polarity generation. To address the nature of this redundancy, evidence is provided for an involvement of the related MAP kinases of the p38 subfamily in planar polarity signaling downstream of Msn (Paricio, 1999).

Although there is accumulating evidence that JNK-type MAPK modules are involved in planar polarity signaling, the analysis of mutant clones of either hep or bsk alleles shows no or weak phenotypes in imaginal discs. These observations suggest a high degree of redundancy at this level in the polarity signaling pathway. To address this issue further, a potential involvement of related kinases that could account for the proposed redundancy was examined. The recently described Drosophila kinases, belonging to the JNK/p38 class within the MAPK modules were examined for genetic interactions with the planar polarity phenotypes of sev-Dsh and sev-msn. These are obvious candidates to be cooperating with Hep and Bsk in polarity generation. At the level of Hep/JNKK (an MKK7 homolog), two other MKKs have been reported (DMKK3 and DMKK4). Similarly, at the level of Bsk/JNK, two p38-like kinases were isolated (Dp38a and Dp38b). Since no mutants have yet been isolated for these genes, whether deficiencies removing these kinases would show an interaction with sev-Dsh was examined. DMKK3 maps in the vicinity of hep: deficiencies removing DMKK3, Df(X)G24 and Df(X)H6, also remove hep. These deficiencies show externally a very strong suppression of sev-Dsh with a marked decrease of misrotated ommatidia as observed in tangential sections. Deficiency Df(3R)p13 removes the DMKK4 locus and also dominantly suppresses sev-Dsh. Similarly, deficiencies removing either Dp38a, Df(3L)crb87-4 and Df(3L)crbF89-4, or Dp38b, Df(2L)b80e3 and Df(2L)b87e25, are suppressors of sev-Dsh. Whether the respective deficiencies showed an interaction with sev>msn was also examined, and it was found that all of them act as dominant suppressors of this genotype as well. It is interesting to mention that the Msn-induced defects in rhabdomere morphology are also suppressed by those deficiencies. These interactions suggest that the p38 kinases are redundant with JNK in the context of planar polarity signaling (Paricio, 1999).

Although genetic evidence suggests an involvement of bsk (JNK) and hep (JNKK) in polarity signaling, phenotypic analyses suggest that the JNK module components are highly redundant in this process. It is interesting to note that all phenotypic defects of sev>Msn were dominantly suppressed by mutations in both components of the JNK and the p38 kinase module. In contrast to these interactions, tissue culture experiments in mammalian cells have shown that NIK overexpression leads to JNK phosphorylation, but no detectable p38 activation was observed. This difference can be explained by cell- and tissue-specific requirements, e.g. in Drosophila during dorsal closure, JNK activation downstream of Msn is not redundant, while redundancy and p38 interactions are observed in polarity signaling. Thus, it is tempting to speculate that both JNK and p38 kinases cooperate in polarity generation (Paricio, 1999).

The reported promiscuity of the kinases at both the MKK and the MAPK levels could account for the redundancy. The Drosophila MKKs and JNK/p38 MAPKs also appear to act (at least partially) on overlapping downstream targets. Whereas DMKK3 appears rather specific for p38 activation (although it activates both p38s), DMKK4 and Hep (the MKK7 cognate) both activate Bsk/JNK. Similarly, Bsk/JNK and both Dp38s can phosphorylate the downstream targets dJun and ATF2. Thus, a potential downstream target can still be phosphorylated when one of the upstream kinases is removed, and likewise for their upstream activators. An even more complicated picture may emerge when all relevant kinases are identified. Other examples of redundancy are described in yeast MAP kinases. Although KSS1 and FUS3 normally have specific roles in different pathways, it has been shown that they are redundant in the process of mating and in this case KSS1 replaces Fus3 when the latter is not present. The isolation and analysis of all the respective kinases and their mutants will be necessary to understand fully the contribution of each single kinase in planar polarity signaling (Paricio, 1999).

MK2-dependent p38b signalling protects Drosophila hindgut enterocytes against JNK-induced apoptosis under chronic stress

The integrity of the intestinal epithelium is crucial for the barrier function of the gut. Replenishment of the gut epithelium by intestinal stem cells contributes to gut homeostasis, but how the differentiated enterocytes are protected against stressors is less well understood. This study used the Drosophila larval hindgut as a model system in which damaged enterocytes are not replaced by stem cell descendants. By performing a thorough genetic analysis, it was demonstrates that a signalling complex consisting of p38b and MK2 forms a branch of SAPK signalling that is required in the larval hindgut to prevent stress-dependent damage to the enterocytes. Impaired p38b/MK2 signalling leads to apoptosis of the enterocytes and a subsequent loss of hindgut epithelial integrity, as manifested by the deterioration of the overlaying muscle layer. Damaged hindguts show increased JNK activity, and removing upstream activators of JNK suppresses the loss of hindgut homeostasis. Thus, the p38/MK2 complex ensures homeostasis of the hindgut epithelium by counteracting JNK-mediated apoptosis of the enterocytes upon chronic stress (Seisenbacher, 2011).

The p38 SAPK belongs to the MAPK family and is conserved from yeast to humans. In higher eukaryotes, p38 associates with its major target, the MAPK-activated protein kinase MK2. This complex resides in the nucleus in the resting state. Upon stress, p38 is activated by MKK3/MKK6 and phosphorylates MK2, which results in an exposure of the nuclear export signal of MK2 and a subsequent nuclear export of the complex (ter Haar, 2008). Another consequence of the p38/MK2 complex formation is the stabilisation of p38 protein. Interestingly, the kinase activity of MK2 is required neither for the nucleo-cytoplasmic shuttling nor for the p38 protein stabilisation (see ter Haar, 2008). Conversely, MK2 kinase activity is crucial to phosphorylate small heat-shock proteins, transcription factors (e.g., SRF and HSF-1), and Tristetraprolin (TTP) is the only trans-acting factor shown to be capable of regulating AU-rich element-dependent mRNA turnover at the level of the intact animal. The inhibitory phosphorylation of TTP by the p38/MK2 complex has been shown to increase the translation of AU-rich elements (ARE)-containing mRNAs including the mRNA encoding the proinflammatory cytokine TNFα. Furthermore, p38 and MK2 have been shown to act as cytoplasmic checkpoint kinases in parallel to CHK1. Due to the plethora of p38/MK2 functions, targeting the p38 SAPK branch with inhibitors might lead to harmful side effects. Thus, it is important to understand the roles of p38 signalling in a tissue-specific context. The availability of mouse models has helped to decipher some in vivo roles of p38 SAPK signalling but the complex nature of the intestinal system has hampered a detailed analysis (Seisenbacher, 2011 and references therein).

Gut homeostasis, under normal and stress conditions, is ensured by complex interactions between the intestinal epithelium, the immune system and the gut flora. Drosophila has been used as a simple model organism to address different aspects of intestinal homeostasis. Replenishment of the gut epithelium by intestinal stem cells (ISCs) clearly contributes to epithelial homeostasis but how the differentiated enterocytes (ECs) are protected against stressors has remained largely unknown. This study used the larval hindgut of Drosophila as a simple intestinal model organ to address how stress signalling in the hindgut ECs ensures intestinal homeostasis in the absence of proliferative cells. This analysis identifies the p38b/MK2 signalling module as a critical component in the protection of hindgut ECs against salt stress (Seisenbacher, 2011).

A model is proposed that puts a p38b/MK2 complex in the centre of stress-protection of the hindgut ECs . In the absence of this signalling module, cells are undergoing JNK-dependent apoptosis upon stress. The lesion in the EC monolayer results in the damage of the overlying hindgut musculature. This regional loss of the barrier function leads to systemic defects in the larvae, further weakening the larvae and impairing growth under stress conditions. As a consequence, pathogens and toxic substances might enter the body cavity, eventually resulting in the melanisation of pericardial cells and the induction of cecropin in the midgut (Seisenbacher, 2011).

Interestingly, JNK activation in MK2 mutant hindguts precedes the melanisation, and it consistently occurs in patches. Within these areas, some cells acquire highest amounts of JNK activity and eventually undergo apoptosis. The surrounding cells maintain JNK activity, forming a rim around the scar in the tissue. The number of affected ECs remains roughly constant for a given stress. Presently it is not known what determines the patches with high JNK activity within the tissue. Although the dorsal domain (hd, positive for engrailed expression) ECs of the hindgut form a homogeneous epithelium and are facing the same stressor, JNK signalling is only induced in clusters of a certain size but not in surrounding cells. Increased JNK activation was also observed in the p38α deleted intestinal epithelium of a mouse model for inflammatory bowel diseases (IBDs). Furthermore, ulcerations occur in similar patchy patterns in IBDs. Thus, the MK2 mutant phenotype may be useful to decipher how a group of cells within a tissue of genetically identical cells transforms into the weakest link in the chain upon stressful conditions (Seisenbacher, 2011).

Several lines of evidence support the notion that the p38b/MK2 signalling complex is key to EC protection against chronic salt stress. (1) p38b and MK2 mutant larvae both develop BDs upon stress conditions. (2) The severity of the p38b; MK2 double mutant phenotype upon stress is similar to the p38b single mutant phenotype, suggesting that they act in the same signalling pathway. (3) p38b but not p38a physically associates with MK2 via its C-terminal CD domain (DPTD motif). (4) The binding of p38b to MK2 stabilises MK2. (5) Upon co-expression, p38b but not p38a redirects MK2 to the cytoplasm. (6) Both the activation and the catalytic activity of p38b are required to efficiently relocalise MK2. (7) The binding of MK2 to p38 and the catalytic activities of both kinases are essential to protect the ECs of the larval hindgut upon salt stress. Taken together, stabilisation, localisation and activation of MK2 by p38b are required for a proper stress response (Seisenbacher, 2011).

The genetic analysis also revealed that p38 SAPK signalling is contributing to stress protection in different ways in addition to the pivotal role of the p38b/MK2 complex. First, p38b impacts on hindgut homeostasis in an MK2-independent manner. This is apparent from the p38b mutant larvae that, in contrast to MK2 mutant larvae, develop BDs even at normal conditions. Consistently, a p38b protein version that no longer binds MK2 partially rescues the p38b mutant phenotype. Second, p38a is also required for full stress protection. The strong phenotype of MK2; p38a double mutants underscores the importance of the p38a SAPK pathway upon severe salt stress. However, the double mutants do not display an increase in BD formation but rather a decrease in viability. Thus, p38a may be involved in general stress protection that is not specific to the hindgut tissue. Third, a common p38 SAPK branch, encompassing p38a, p38b and potentially also p38c, is essential as the p38a; p38b double mutants die. Since the p38a1 allele affects the coding sequences of p38a and p38c, the p38a1; p38bd27 double mutants may even represent p38a; p38b; p38c triple mutants. However, it is unclear to date whether p38c is a pseudogene. Recent studies have suggested an involvement of p38c function in immune gene regulation, early larval survival, and fertility. Thus, further studies will be required to clarify whether p38c contributes to p38 signalling subbranches. Finally, the slight increase in BDs seen in MK2; p38b double mutants under normal conditions suggests that MK2 also performs a p38b-independent function in the hindgut. p38b mutants always impact on MK2 signalling since the MK2 protein is not stable and probably not correctly localised if not bound to p38b. Negative feedback regulation acting from MK2 on the activation of p38b further complicates the SAPK signalling network (Seisenbacher, 2011).

What are the upstream components regulating the p38b/MK2 complex? Surprisingly, MKK3/Lic does not appear to play a role in the hindgut function of the p38b/MK2 branch. MKK3 but not MKK4 can activate p38 proteins in cell culture. On the other hand, siRNA mediated knockdown of both MKK4 and MKK3 is required to fully block the activation of p38 under certain stresses in S2 cells. Both p38b and MKK3/lic mutants show a strong reduction in p38 activation but no BDs are observed in MKK3/lic mutants. In contrast, mutants for MEKK1, which acts upstream of MKK3, do develop BDs similar to p38b and MK2 mutants. In mammalian cells, it has been shown that p38 can be activated independently of a MAP2 kinase. However, no activation of p38 occurred in fibroblasts of MKK3 MKK6 double mutant mice. Since MKK4 is a suppressor of the BD phenotype and MKK7 most likely does not activate p38 in Drosophila, a scenario of MAP2K-independent p38 activation could also apply for the p38b/MK2 signalling branch in the larval hindgut (Seisenbacher, 2011).

Interestingly, overexpression of the kinase-dead or of the non-activatable p38b protein version results in an even stronger phenotype than the deletion of p38b, probably by titrating upstream partners that would have additional functions besides activating p38b. A common p38a/p38b activator would be a strong candidate for such an upstream component. Saturating this common p38 activator with p38bKR or p38bAGF would essentially result in a p38a/p38b double mutant situation and therefore would explain the strong phenotype, especially at high salt conditions (Seisenbacher, 2011).

This analysis of the p38b/MK2 signalling module in hindgut ECs reveals how deletion of SAPK members results in increased sensitivity towards a particular stressor from the molecular level to the level of the whole organism. These findings provide a new model of how hindgut homeostasis is maintained and how different SAPK branches act together in vivo to ensure cellular survival upon stress. The p38 SAPK pathway efficiently protects hindgut ECs over a wide range of stress conditions and for an astonishingly long time period of at least five days without cell replenishment. In this light, a comparative analysis of the larval and the adult hindgut (which has to be maintained for a time period of up to forty days) will be of great interest (Seisenbacher, 2011).

How does p38 interfere with JNK activation in the hindgut epithelium? In mammalian cells, p38α binds to and phosphorylates TAB1 (and potentially TAB2). As a consequence, the activities of TAK1 and thereby JNK are reduced. TAB1 is not conserved in Drosophila but TAB2 might be involved in a similar negative feedback loop. TAB2 has been implicated in JNK activation in response to peptidoglycans and lipopolysaccharides. However, in the absence of TAB2, no change in JNK activation in response to Sorbitol or NaCl has been observed. Alternatively, p38 might induce a JNK phosphatase. p38α has been shown to impact on JNK activation by inducing DUSP1/MKP-1 in mammalian cells. Sustained JNK activation due to a loss of DUSP1/MKP-1 resulted in increased cell death in response to UV stress. Interestingly, p38 activation was also increased in DUSP1/MKP-1 mutant cells but p38 activity was not linked to cell death induction. Similarly, MK2 mutant larvae bearing BDs display an overactivation of both p38 and JNK, and blocking JNK only is sufficient to prevent the BD phenotype (Seisenbacher, 2011).

Despite the more complex nature of mammalian guts, the strong conservation of stress-signalling pathways and the similar demands of ECs make it likely that the current results will also be important in the context of various diseases of the intestinal system. A variety of different signalling pathways have been implicated in IBDs, underscoring the complex nature of these diseases. p38 and MK2 are critical regulators of TNFα production and are thereby associated with IBDs but the role of p38 SAPK in IBDs has remained controversial. This study identifies a crucial role of p38b/MK2 signalling in the first line of defence against a particular stressor in a model system devoid of an adaptive immune system. The consequences of lacking this immune system-independent protective function of a SAPK branch might parallel early steps of IBD development in intestinal epithelial cells. Indeed, a recent study has revealed tissue-specific effects of p38α in a DSS-induced mouse model for IBDs. Deleting p38α in the myeloid lineage had beneficial effects, consistent with the inflammatory nature of IBDs. In contrast, p38α deletion in the intestinal epithelium resulted in a loss of gut homeostasis, marked by increased proliferation and by a reduction in goblet cells. Thus, these studies on Drosophila p38 signalling and its role in the larval hindgut provide a basis to specifically address the role of ECs in the maintenance of an intestinal epithelium in the absence of proliferation and immune response (Seisenbacher, 2011).

In vivo interaction proteomics reveal a novel p38 mitogen-activated protein kinase/Rack1 pathway regulating proteostasis in Drosophila muscle

Several recent studies suggest that systemic aging in metazoans is differentially affected by functional decline in specific tissues, such as skeletal muscle. In Drosophila, longevity appears to be tightly linked to myoproteostasis, and the formation of misfolded protein aggregates is a hallmark of senescence in aging muscle. Similarly, defective myoproteostasis is described as an important contributor to the pathology of several age-related degenerative muscle diseases in humans, e.g., inclusion body myositis. p38 mitogen-activated protein kinase (MAPK) plays a central role in a conserved signaling pathway activated by a variety of stressful stimuli. Aging p38 MAPK mutant flies display accelerated motor function decline, concomitant with an enhanced accumulation of detergent-insoluble protein aggregates in thoracic muscles. Chemical genetic experiments suggest that p38-mediated regulation of myoproteostasis is not limited to the control of reactive oxygen species production or the protein degradation pathways but also involves upstream turnover pathways, e.g., translation. Using affinity purification and mass spectrometry, this study identified Rack1 as a novel substrate of p38 MAPK in aging muscle and showed that the genetic interaction between p38b and Rack1 controls muscle aggregate formation, locomotor function, and longevity. Biochemical analyses of Rack1 in aging and stressed muscle suggest a model whereby p38 MAPK signaling causes a redistribution of Rack1 between a ribosome-bound pool and a putative translational repressor complex (Belozerov, 2014).

p38 MAP Kinase regulates circadian rhythms in Drosophila

The large repertoire of circadian rhythms in diverse organisms depends on oscillating central clock genes, input pathways for entrainment, and output pathways for controlling rhythmic behaviors. Stress-activated p38 MAP Kinases (p38K), although sparsely investigated in this context, show circadian rhythmicity in mammalian brains and are considered part of the circadian output machinery in Neurospora. This study found that Drosophila p38Kb is expressed in clock neurons, and mutants in p38Kb either are arrhythmic or have a longer free-running periodicity, especially as they age. Paradoxically, similar phenotypes are observed through either transgenic inhibition or activation of p38Kb in clock neurons, suggesting a requirement for optimal p38Kb function for normal free-running circadian rhythms. This study also found that p38Kb genetically interacts with multiple downstream targets to regulate circadian locomotor rhythms. More specifically, p38Kb interacts with the period gene to regulate period length and the strength of rhythmicity. In addition, p38Kb was shown to suppress the arrhythmic behavior associated with inhibition of a second p38Kb target, the transcription factor Mef2. Finally, manipulating p38K signaling in free-running conditions was found to alter the expression of another downstream target, MNK/Lk6, which has been shown to cycle with the clock and to play a role in regulating circadian rhythms. These data suggest that p38Kb may affect circadian locomotor rhythms through the regulation of multiple downstream pathways (Vrailas-Mortimer, 2014).


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p38b: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology

date revised: 15 December 2014

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