Interactive Fly, Drosophila

Fos-related antigen


Fos and uterine development in C. elegans

The Notch pathway is the key signal for many cell fate decisions in the nematode Caenorhabditis elegans including the uterine pi cell fate, crucial for a proper uterine-vulval connection and egg laying. Expression of the egl-13 SOX domain transcription factor is specifically upregulated upon induction of the pi lineage and not in response to other LIN-12/Notch-mediated decisions. Dual regulation by LIN-12 and FOS-1 is required for egl-13 expression at specification and for complete rescue of egl-13 mutants. fos-1 mutants exhibit uterine defects and fail to express pi markers. FOS-1 is expressed at pi cell specification was demonstrated and that FOS-1 can bind in vitro to egl-13 upstream regulatory sequence (URS) as a heterodimer with C. elegans Jun (Oommen, 2007).

Multiple isoforms of Fos in mammals

Structure-function analysis as well as studies with knock-out and transgenic mice have assigned distinct functions to c-Fos and Fra-1, two components of the transcription factor AP-1 (activator protein-1). To test whether Fra-1 could substitute for c-Fos, knock-in mice that express Fra-1 in place of c-Fos were generated. Fra-1 rescues c-Fos-dependent functions such as bone development and light-induced photoreceptor apoptosis. Importantly, rescue of bone cell differentiation, but not photoreceptor apoptosis, is gene-dosage dependent. Moreover, Fra-1 fails to substitute for c-Fos in inducing expression of target genes in fibroblasts. These results show that c-Fos and Fra-1 have maintained functional equivalence during vertebrate evolution (Fleischmann, 2000).

Fos binding to DNA and phosphorylation of Fos

The use of a DNA minicircle competition binding assay, together with DNA cyclization kinetics and gel-phasing methods, was used to show that the DNA-binding domains (dbd) of the heterodimeric leucine zipper protein Fos-Jun do not bend the AP-1 target site. The DNA constructs contain an AP-1 site phased by 1-4 helical turns against an A-tract-directed bend. Competition binding experiments reveal that (dbd)Fos-Jun has a slight preference for binding to linear over circular AP-1 DNAs, independent of whether the site faces in or out on the circle. This result suggests that (dbd)Fos-Jun slightly stiffens rather than bends its DNA target site. A single A-tract bend replacing the AP-1 site is readily detected by its effect on cyclization kinetics, in contrast to the observations for Fos-Jun bound at the AP-1 locus. In contrast, comparative electrophoresis reveals that Fos-Jun-DNA complexes, in which the A-tract bend is positioned close (1-2 helical turns) to the AP-1 site, show phase-dependent variations in gel mobilities that are comparable with those observed when a single A-tract bend replaces the AP-1 site. Whereas gel mobility variations of Fos-Jun-DNA complexes decrease linearly with increasing Mg2+ contained in the gel, the solution binding preference of (dbd)Fos-Jun for linear over circular DNAs is independent of Mg2+ concentration. Hence, gel mobility variations of Fos-Jun-DNA complexes are not indicative of (dbd)Fos-Jun-induced DNA bending (upper limit 5 degrees ) in the low salt conditions of gel electrophoresis. Instead, it is proposed that the gel anomalies depend on the steric relationship of the leucine zipper region with respect to a DNA bend (Sitlani, 1998).

The proto-oncogenes jun and fos are members of the AP-1 family of transcription factors, which activate transcription of target genes via the tetradecanoyl phorbol acetate response element (TRE). Both jun and fos contain activation domains, but their relative contributions to transcriptional activation of different TREs remain unclear. It is not apparent whether the cellular availability of specific AP-1 members is the major determinant for regulation of TREs or whether other factors including the TRE sequence itself contribute to selectivity. A novel AP-1 site has been identified in the promoter of the rat atrial natriuretic factor (ANF), which is unresponsive to jun homodimers and is inducible only in the presence of c-fos. This activation is potentiated by mitogen-activated protein (MAP) kinase. The jun proteins appear to be required solely to tether c-fos to the promoter; c-fos mutants lacking putative activation domains abrogate transactivation. Unexpectedly, the oncogenic form of c-fos that diverges most significantly in the carboxy-terminal 50 amino acids is unable to mediate transactivation at this specialized AP-1 site. Mutations within the C terminus of c-fos at serine residues that are phosphorylation targets for growth factors and MAP kinase completely abrogate transactivation and block potentiation by MAP kinase. Using GAL4 fusions, it has been shown that the 90-amino-acid C terminus of c-fos contains autonomous activation domains and that the serine residues are essential for full activity. These results suggest that phosphorylation of the C terminus of c-fos affects its transactivation properties and provide evidence for novel regulatory mechanisms that may contribute to biologic specificities of the AP-1 transcription complex (McBride, 1998).

Fos post-transcriptional regulation

mRNA turnover mediated by the major protein-coding-region determinant of instability (mCRD) of the c-fos proto-oncogene transcript illustrates a functional interplay between mRNA turnover and translation. The function of mCRD depends on its distance from the poly(A) tail. Five mCRD-associated proteins were identified: Unr, a purine-rich RNA binding protein; PABP, a poly(A) binding protein; PAIP-1, a poly(A) binding protein interacting protein; hnRNP D, an AU-rich element binding protein, and NSAP1, an hnRNP R-like protein. These proteins form a multiprotein complex. Overexpression of these proteins stabilizes mCRD-containing mRNA by impeding deadenylation. It is proposed that a bridging complex forms between the poly(A) tail and the mCRD and ribosome transit disrupts or reorganizes the complex, leading to rapid RNA deadenylation and decay (Grosset, 2000).

Messenger RNA decay mediated by the c-fos major protein coding-region determinant of instability (mCRD) is a useful system for studying translationally coupled mRNA turnover. Among the five mCRD-associated proteins identified previously, UNR was found to be an mCRD-binding protein and also a PABP-interacting protein. Interaction between UNR and PABP is necessary for the full destabilization function of the mCRD. By testing different classes of mammalian poly(A) nucleases, CCR4 was identified as a poly(A) nuclease involved in the mCRD-mediated rapid deadenylation in vivo and CCR4 also associates with UNR. Blocking either translation initiation or elongation greatly impedes poly(A) shortening and mRNA decay mediated by the mCRD, demonstrating that the deadenylation step is coupled to ongoing translation of the message. These findings suggest a model in which the mCRD/UNR complex serves as a 'landing/assembly' platform for formation of a deadenylation/decay mRNA-protein complex on an mCRD-containing transcript. The complex is dormant prior to translation. Accelerated deadenylation and decay of the transcript follows ribosome transit through the mCRD. This study provides new insights into a mechanism by which interplay between mRNA turnover and translation determines the lifespan of an mCRD-containing mRNA in the cytoplasm (Chang, 2004).

Signaling pathways activating Fos

The baculovirus/Sf9 cell system can be used to dissect signaling pathways involved in transmitting activating signals from the cell surface to the nucleus. Different combinations of the critical signaling proteins pp60v-src, p21v-ras, Raf-1 and ERK-1 were coexpressed. The effects of resulting signaling cascades on the modifications of coexpressed transcription factors c-jun or c-fos were assayed. Activation of ERK-1 (Drosophila homolog: Rolled) via Raf-1 and p21ras (see Ras) dependent signals can result in the hyperphosphorylation of c-jun. In contrast, c-fos appears to be the target of two Raf-1 activated modifying signals: one independent of ERK-1 and the other dependent on ERK-1 stimulation. Thus, coexpression of c-fos with pp60v-src, p21v-ras or constitutively active forms of Raf-1 results in a dramatic reduction of c-fos's electrophoretic mobility in the absence of coexpressed ERK-1. Activation of this ERK-1-independent pathway, together with the ERK-1 dependent pathway that modifies c-jun, results in additional modification of c-fos. The observation of a Raf-1 activated, ERK-independent signaling pathway is consistent with previous reports that constitutively active Raf-1, in some cell types, can result in transformation or differentiation without activation of ERKs. These data indicate the presence of multiple Raf-1 activated pathways that lead to modification of transcription factors (Agarwal, 1995).

The transcription factor AP-1, composed of Fos-Jun dimers, mediates some aspects of the cellular response to growth factors. Transcriptional activation and neoplastic transformation by FosB, a member of the Fos family of proteins, require the presence of a potent C-terminal activation domain. The FosB C-terminal domain has a proline-based motif that is essential for both of these functions. Phosphopeptide mapping experiments show that the C terminus of FosB is phosphorylated within a cluster of functionally redundant serine residues, which are adjacent to this proline-based motif. Mutation of these serine residues to alanine severely reduces the ability of this region to function as an activation domain and inhibits the ability of FosB protein to function as a transforming protein. Several observations suggest that the kinase responsible for phosphorylation of these sites is distinct from the mitogen-activation protein kinases and stress-activated protein kinases (Skinner, 1997).

Hypoxia is a pathophysiological condition that occurs during injury, ischemia, and stroke. It is characterized by a decrease of reactive oxygen intermediates and a change of the intracellular redox level. In tumors, hypoxia is regarded as a trigger for enhanced growth and metastasis. In HeLa cells, hypoxic conditions induce the transcriptional activation of c-fos transcription via the serum response element. Mutations in both the binding site for the ternary complex factor Elk-1 (Drosophila homolog: Pointed) and the serum response factor abolish this induction, indicating that a ternary complex at the serum response element is necessary for the induction of the c-fos gene under hypoxia. The transcription factor Elk-1 is covalently modified by phosphorylation in response to hypoxia. This hyperphosphorylation of Elk-1, as well as the activation of mitogen-activated protein kinase (MAPK), and the induction of c-fos transcripts are all blocked by a specific inhibitor of mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase kinase 1. An in vitro kinase assay with Elk-1 as substrate shows that MAPK is activated under hypoxia. The activation of MAPK corresponds temporally with the phosphorylation and activation of Elk-1. Thus, a decrease of the intracellular reactive oxygen intermediate level by hypoxia induces c-fos via the MAPK pathway. These results suggest that the intracellular redox levels may be directly coupled to tumor growth, invasion, and metastasis via Elk-1-dependent induction of c-Fos controlled genes (Muller, 1997).

Growth factors induce c-fos transcription by stimulating phosphorylation of transcription factor TCF/Elk-1, which binds to the serum response element (SRE). Under such conditions Elk-1 can be phosphorylated by the mitogen-activated protein kinases (MAPKs) ERK1 and ERK2. However, c-fos transcription and SRE activity are also induced by stimuli (such as UV irradiation and activation of the protein kinase MEKK1) that cause only an insignificant increase in ERK1/2 activity. However, both of these stimuli strongly activate two other MAPKs, JNK1 and JNK2, and stimulate Elk-1 transcriptional activity and phosphorylation. The JNKs are the predominant Elk-1 activation domain kinases in extracts of UV-irradiated cells; immunopurified JNK1/2 phosphorylates Elk-1 on the same major sites recognized by ERK1/2, thus potentiating Fos's transcriptional activity. UV irradiation, but not serum or phorbol esters, stimulates translocation of JNK1 to the nucleus. As Elk-1 is most likely phosphorylated while bound to the c-fos promoter, these results suggest that UV irradiation and MEKK1 activation stimulate TCF/Elk-1 activity through JNK activation, while growth factors induce c-fos through ERK activation (Cavigelli, 1995).

In Rat-1 fibroblasts, nonmitogenic doses of lysophosphatidic acid (LPA) stimulate a transient activation of mitogen-activated protein kinase (MAPK), whereas mitogenic doses elicit a sustained response. This sustained phase of MAPK activation regulates cell fate decisions such as proliferation or differentiation, presumably by inducing a program of gene expression that is not observed in response to transient MAPK activation. The expression of members of the AP-1 transcription factor complex has been examined in response to stimulation with different doses of LPA. c-Fos, c-Jun, and JunB are induced rapidly in response to LPA stimulation, whereas Fra-1 and Fra-2 are induced after a significant lag. The expression of c-Fos is transient, whereas the expression of c-Jun, JunB, Fra-1, and Fra-2 is sustained. The early expression of c-Fos can be reconstituted with nonmitogenic doses of LPA, but the response is transient when compared to that observed with mitogenic doses. In contrast, expression of Fra-1, Fra-2, and JunB and optimal expression of c-Jun are observed only with doses of LPA, which induce sustained MAPK activation and DNA synthesis. LPA-stimulated expression of c-Fos, Fra-1, Fra-2, c-Jun, and JunB is inhibited by the MEK1 inhibitor PD098059, indicating that the Raf-MEK-MAPK cascade is required for their expression. In cells expressing a conditionally active form of Raf-1 (DeltaRaf-1:ER), selective, sustained activation of Raf-MEK-MAPK is sufficient to induce expression of Fra-1, Fra-2, and JunB but, interestingly, such activation induces little or no c-Fos or c-Jun. The induction of c-Fos observed in response to LPA is strongly inhibited by buffering the intracellular [Ca2+]. Moreover, although Raf activation or calcium ionophores induce little c-Fos expression, a synergistic induction in response to the combination of DeltaRaf-1:ER and ionomycin is observed. These results suggest that kinetically distinct phases of MAPK activation serve to regulate the expression of distinct AP-1 components, such that sustained MAPK activation is required for the induced expression of Fra-1, Fra-2, c-Jun, and JunB. However, in contrast to the case for Fra-1, Fra-2, and JunB, activation of the MAPK cascade alone is not sufficient to induce c-Fos expression, which rather requires cooperation with other signals such as Ca2+ mobilization. Finally, the identification of the Fra-1, Fra-2, c-Jun, and JunB genes as those that are selectively regulated by sustained MAPK activation, or in response to activated Raf, suggests that these genes are candidates to mediate certain effects of Ras proteins in oncogenic transformation (Cook, 1999).

In cell culture systems, the TCF Elk-1 represents a convergence point for extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) subclasses of mitogen-activated protein kinase (MAPK) cascades. Its phosphorylation strongly potentiates its ability to activate transcription of the c-fos promoter through a ternary complex assembled on the c-fos serum response element. In rat brain postmitotic neurons, Elk-1 is strongly expressed. However, its physiological role in these postmitotic neurons remains to be established. To investigate biochemically the signaling pathways targeting Elk-1 and c-fos in mature neurons, a semi-in vivo system was used, composed of brain slices stimulated with the excitatory neurotransmitter glutamate. Glutamate treatment leads to a robust, progressive activation of the ERK and JNK/SAPK MAPK cascades. This corresponds kinetically to a significant increase in Ser383-phosphorylated Elk-1 and the appearance of c-fos mRNA. Glutamate also causes increased levels of Ser133-phosphorylated cyclic AMP-responsive element-binding protein (CREB) but only transiently relative to Elk-1 and c-fos. ERK and Elk-1 phosphorylation are blocked by the MAPK kinase inhibitor PD98059, indicating the primary role of the ERK cascade in mediating glutamate signaling to Elk-1 in the rat striatum in vivo. Glutamate-mediated CREB phosphorylation is also inhibited by PD98059 treatment. Interestingly, KN62, which interferes with calcium-calmodulin kinase (CaM-K) activity, leads to a reduction of glutamate-induced ERK activation and of CREB phosphorylation. These data indicate that ERK functions as a common component in two signaling pathways (ERK/Elk-1 and ERK/?/CREB) converging on the c-fos promoter in postmitotic neuronal cells and that CaM-Ks act as positive regulators of these pathways (Vanhoutte, 1999)

Protein kinase C (PKC) is a multigene family of enzymes consisting of at least 11 isoforms. It has been implicated in the induction of c-fos and other immediate response genes by various mitogens. The serum response element (SRE) in the c-fos promoter is necessary and sufficient for induction of transcription of c-fos by serum, growth factors, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). It forms a complex with the ternary complex factor (TCF) and with a dimer of the serum response factor (SRF). TCF is the target of several signal transduction pathways and SRF is the target of the rhoA pathway. Dominant-negative and constitutively active mutants of PKC-alpha, PKC-delta, PKC-epsilon, and PKC-zeta have been generated to determine the roles of individual isoforms of PKC in the activation of the SRE. Transient-transfection assays with NIH 3T3 cells, using an SRE-driven luciferase reporter plasmid, indicate that PKC-alpha and PKC-epsilon, but not PKC-delta or PKC-zeta, mediate SRE activation. TPA-induced activation of the SRE is partially inhibited by dominant negative c-Raf, ERK1, or ERK2, and constitutively active mutants of PKC-alpha and PKC-epsilon activate the transactivation domain of Elk-1. TPA-induced activation of the SRE was also partially inhibited by a dominant-negative MEKK1. Furthermore, TPA treatment of serum-starved NIH 3T3 cells leads to phosphorylation of SEK1, and constitutively active mutants of PKC-alpha and PKC-epsilon activate the transactivation domain of c-Jun, a major substrate of JNK. Constitutively active mutants of PKC-alpha and PKC-epsilon can also induce a mutant c-fos promoter that lacks the TCF binding site, and they also induce transactivation activity of the SRF. Furthermore, rhoA-mediated SRE activation is blocked by dominant negative mutants of PKC-alpha or PKC-epsilon. Taken together, these findings indicate that PKC-alpha and PKC-epsilon can enhance the activities of at least three signaling pathways that converge on the SRE: c-Raf > MEK1 > ERK > TCF; MEKK1 > SEK1 > JNK > TCF, and rhoA > SRF. Thus, specific isoforms of PKC may play a role in integrating networks of signal transduction pathways that control gene expression (Soh, 1999).

Heat shock factor 1 (see Drosophila HSF) inhibits the expression of c-fos, an immediate early gene that controls responses to extracellular stimuli for growth and differentiation. Heat shock factor 1 inhibits the transcription of the c-fos gene and antagonizes the activating effects of the signal transducing protein Ras on the c-fos promoter and on the promoter of another Ras responsive gene, uPA. This property is specific for heat shock factor 1; c-fos repression is not seen with the structurally related protein heat shock factor 2. Repression involves different molecular mechanisms when compared with those involved in transcriptional activation by heat shock factor 1, and specifically does not require binding to the c-fos promoter. Thus, in addition to its known role as a transcriptional activator of the cellular heat shock response, heat shock factor 1 also antagonizes the expression of Fos, a key component of the ubiquitous AP-1 transcription factor complex; as such, it could influence multiple aspects of cell regulation (Chen, 1997).

The serum response element (SRE), which is pivotal for transcriptional up-regulation of the c-fos protooncogene, is constitutively occupied by a protein complex comprising the serum response factor and a ternary complex factor (TCF). Phosphorylation of the TCFs Elk-1 and Sap-1a by the ERK and JNK subclasses of MAP kinases triggers c-fos transcription. Elk-1 is barely activated by a third subclass of MAP kinases (p38), most likely because the critical residues Ser383 and Ser389 are poorly phosphorylated by p38 MAP kinase. In contrast, the TCF Sap-1a is efficiently phosphorylated by p38 MAP kinase in vitro and in vivo on the homologous residues Ser381 and Ser387. Mutation of these sites to alanine severely reduces c-fos SRE-dependent transcription mediated by Sap-1a and p38 MAP kinase. Thus, Sap-1a may be an important target for mitogens, stress and apoptotic signals to elicit a nuclear response. However, signaling from p38 MAP kinase to Sap-1a or from Sap-1a to the basal transcription machinery does not occur in all cell types nor at promoters other than the c-fos SRE, which may ensure the specificity of signaling (Janknecht, 1997).

The cAMP/cAMP-dependent protein kinase (A-kinase) and Ca2+/calmodulin-dependent protein kinase (Cam-kinase) signal transduction pathways are well known to regulate gene transcription, but this has not been demonstrated directly for the cGMP/cGMP-dependent protein kinase (G-kinase) signal transduction pathway. Transfection of G-kinase into G-kinase-deficient cells causes activation of the human c-fos promoter in a strictly cGMP-dependent manner. The effect of G-kinase appears to be mediated by several sequence elements, most notably the serum response element (SRE), the AP-1 binding site (FAP), and the cAMP response element (CRE). The magnitude of G-kinase transactivation of the fos promoter is similar to that of A-kinase, but there are significant differences between G-kinase and A-kinase activation of single enhancer elements and of a chimeric Gal4-CREB transcription factor. These results indicate that G-kinase transduces signals to the nucleus independently of A-kinase or Ca2+, although it may target some of the same transcription factors as A-kinase and Cam-kinase (Gudi, 1996).

The small GTPase Rho (see Drosophila Rho1) is implicated in cytoskeletal rearrangements, including the formation of stress fibers and focal adhesion, and in the transcriptional activation of c-fos serum response element. In vitro, Rho-kinase, which is activated by Rho, phosphorylates not only myosin light chain (MLC) (thereby activating myosin ATPase) but also myosin phosphatase, thus inactivating myosin phosphatase. Rho-kinase is involved in the formation of stress fibers and focal adhesions in fibroblasts. The expression of constitutively active Rho-kinase increases the level of MLC phosphorylation. The activity of Rho-kinase is necessary for maintaining the vinculin-containing focal adhesions, whereas organized actin stress fibers are not necessary for this. The microinjection of constitutively active Rho-kinase into fibroblasts induces the formation of focal adhesions to some extent under the conditions where organized actin stress fibers are disrupted. The expression of constitutively active Rho-kinase also stimulates the transcriptional activity of c-fos serum response element. These results suggest that Rho-kinase has distinct roles in divergent pathways downstream of Rho, including MLC phosphorylation leading to stress fiber formation, focal adhesion formation, and gene expression (Chihara, 1997).

RhoA and two other Rho-family proteins, Cdc42 and Rac1, regulate Serum Response Factor (SRF) activation of the c-fos serum response element. This pathway acts independently of known MAPK pathways and is regulated by agents such as serum and LPA, acting via heterotrimeric G protein-coupled receptors. Constitutively active forms of either of the small GTPases -- RhoA (RhoA.V14) or Cdc42 (Cdc42.V12) -- induces expression of extrachromosomal SRF reporter genes in microinjection experiments, but only Cdc42.V12 can efficiently activate a chromosomal template. Both SAPK/JNK-dependent or -independent signals can cooperate with RhoA.V14 to activate chromosomal SRF reporters; it is SAPK/JNK activation by Cdc42.V12 that allows SAPK/JNK to activate chromosomal templates. Cooperating signals can be bypassed by deacetylase inhibitors. Three findings show that histone H4 hyperacetylation is one target for cooperating signals, although it alone is not sufficient: (1) Cdc42.V12, but not RhoA.V14, induces H4 hyperacetylation; (2) cooperating signals use the same SAPK/JNK-dependent or -independent pathways to induce H4 hyperacetylation, and (3) growth factor and stress stimuli induce substantial H4 hyperacetylation, detectable in reporter gene chromatin. These data establish a link between signal-regulated acetylation events and gene transcription. Thus, in isolation, the SRF-controlled extrachromosomal reporter gene is a target for only a subset of signals that can activate the chromsomal c-fos promoter. This is thought to reflect differences in chromatin structure associated with the two types of templates (Alberts, 1998).

Receptors coupled to heterotrimeric G proteins can effectively stimulate growth promoting pathways in a large variety of cell types, and if persistently activated, these receptors can also behave as dominant-acting oncoproteins. Consistently, activating mutations for G proteins of the Galphas (see Drosophila G protein salpha 60A) and Galphai2 families have been found in human tumors; members of the Galphaq and Galpha12 families are fully transforming when expressed in murine fibroblasts. In an effort aimed to elucidate the molecular events involved in proliferative signaling through heterotrimeric G proteins, this study has focused on gene expression regulation. Using NIH 3T3 fibroblasts expressing m1 muscarinic acetylcholine receptors as a model system, it was observed that activation of these transforming G protein-coupled receptors induces the rapid expression of a variety of early responsive genes, including the c-fos protooncogene. One of the c-fos promoter elements, the serum response element (SRE), plays a central regulatory role; activation of SRE-dependent transcription has been found to be regulated by several proteins, including the serum response factor and the ternary complex factor. Stimulation of m1 muscarinic acetylcholine receptors potently induces SRE-driven reporter gene activity in NIH 3T3 cells. In these cells, only the Galpha12 family of heterotrimeric G protein alpha subunits strongly induces the SRE, while Gbeta1gamma2 dimers activate SRE to a more limited extent. M1, Galpha12 and the small GTP-binding protein RhoA are components of a novel signal transduction pathway that leads to the ternary complex factor-independent transcriptional activation of the SRE and to cellular transformation (Fromm, 1997).

Exposure of Syrian hamsters to light 1 h after lights-off rapidly (10 min) induces nuclear immunoreactivity (-ir) to the phospho-Ser133 form of the Ca2+/cAMP response element (CRE) binding protein (pCREB) in the retinorecipient zone of the suprachiasmatic nuclei (SCN). Light also induces nuclear Fos-ir in the same region of the SCN after 1 h. The glutamatergic N-methyl-D-aspartate (NMDA) receptor blocker MK801 attenuates the photic induction of both factors. To investigate glutamatergic regulation of pCREB and Fos further, tissue blocks and primary cultures of neonatal hamster SCN were examined by Western blotting and immunocytochemistry in vitro. The pCREB-ir signal at 45 kDa is enhanced by glutamate or a mixture of glutamatergic agonists [NMDA, amino-methyl proprionic acid (AMPA), and Kainate (KA)], whereas total CREB does not change. Glutamate or the mixture of agonists also induces a 56 kDa band identified as Fos protein in SCN tissue. In dissociated cultures of SCN, glutamate causes a rapid (15 min) induction of nuclear pCREB-ir and Fos-ir (after 60 min) exclusively in neurons, both GABA-ir and others. Treatment with NMDA alone has no effect on pCREB-ir. AMPA alone causes a slight increase in pCREB-ir. However, kainate alone or in combination with NMDA and AMPA induces nuclear pCREB-ir equal to that induced by glutamate. The effects of glutamate on pCREB-ir and Fos-ir are blocked by antagonists of both NMDA (MK801) and AMPA/KA (NBQX) receptors. In the absence of extracellular Mg2+, MK801 blocks glutamatergic induction of Fos-ir. However, the AMPA/KA receptor antagonist is no longer effective at blocking glutamatergic induction of either Fos-ir or pCREB-ir, consistent with the model that glutamate regulates gene expression in the SCN by a co-ordinate action through both NMDA and AMPA/KA receptors. Glutamatergic induction of nuclear pCREB-ir in GABA-ir neurons is blocked by an inhibitor of Ca2+/Calmodulin (CaM)-dependent kinases, implicating Ca2+-dependent signaling pathways in the glutamatergic regulation of gene expression in the SCN (Schurov, 1999).

The canonical Wnt-β-catenin signaling pathway is important for a variety of developmental phenomena as well as for carcinogenesis. In hippocampal neurons, NMDA-receptor-dependent activation of calpain induces the cleavage of β-catenin at the N terminus, generating stable, truncated forms. These β-catenin fragments accumulate in the nucleus and induced Tcf/Lef-dependent gene transcription. Fosl1, one of the immediate-early genes, was identified as a target of this signaling pathway. In addition, exploratory behavior by mice resulted in a similar cleavage of β-catenin, as well as activation of the Tcf signaling pathway, in hippocampal neurons. Both β-catenin cleavage and Tcf-dependent gene transcription are suppressed by calpain inhibitors. These findings reveal another pathway for β-catenin-dependent signaling, in addition to the canonical Wnt-β-catenin pathway, and suggest that this other pathway could play an important role in activity-dependent gene expression (Abe, 2007).

Transcription factors targeting the Fos promoter

Continued: see Fos-related antigen Evolutionary Homologs part 2/4  | part 3/4 | part 4/4  |

Fos-related antigen: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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