Transcriptional stimulation of the retina-specific QR1 gene upon growth arrest involves a Maf-related protein

MafB is a downstream target of the IL-10/STAT3 signaling pathway, involved in the regulation of macrophage de-activation

In spite of the numerous reports implicating MafB transcription factor in the molecular control of monocyte-macrophage differentiation, the precise genetic program underlying this activity has been, to date, poorly understood. To clarify this issue, we planned a number of experiments that were mainly conducted on human primary macrophages. In this regard, a preliminary gene function study, based on MafB inactivation and over-expression, indicated MMP9 and IL-7R genes as possible targets of the investigated transcription factor. Bioinformatics analysis of their promoter regions disclosed the presence of several putative MARE elements and a combined approach of EMSA and luciferase assay subsequently demonstrated that expression of both genes is indeed activated by MafB through a direct transcription mechanism. Additional investigation, performed with similar procedures to elucidate the biological relevance of our observation, revealed that MafB is a downstream target of the IL-10/STAT3 signaling pathway, normally inducing the macrophage de-activation process. Taken together our data support the existence of a signaling cascade by which stimulation of macrophages with the IL-10 cytokine determines a sequential activation of STAT3 and MafB transcription factors, in turn leading to an up-regulated expression of MMP9 and IL-7R genes.

Down regulation of pRb in cultures of avian neuroretina cells promotes proliferation of reactive Müller-like cells and emergence of retinal stem/progenitors

The aim of this work was to define the role of pRb depletion in the proliferation and differentiation of avian retinoblasts in vitro. For this purpose vectors expressing pRb short hairpin RNA were used to deplete pRb in cultures of avian neuroretinal cells. Down regulation of pRb was observed by Western blot and quantification of nuclear pRb. Cell proliferation and differentiation were studied following BrdU labeling and immunostaining. Transfection significantly down-regulated pRb in neuroretinal cells. Long-term effect of pRb depletion mainly induced proliferation of epithelial-like cells that expressed markers of reactive Müller glial cells. A minority of these cells that survived passaging could be maintained as neurosphere-like aggregates with low pRb, not observed in control cultures. BrdU labeling followed by a two week chase showed the presence of cells still remained labelled, indicating low cell cycling. Under appropriate conditions, these aggregates differentiate in precursors of amacrine interneurons shown by the expression of AP2, in absence of the photoreceptors marker visinin and the late neuronal marker MAP2. Taken together these data show that decrease pRb level in cultures of avian neuroretinal cells promotes the emergence and proliferation of stem cell/progenitors from reactive-like Muller cells.

L-Maf regulates p27kip1 expression during chick lens fiber differentiation

Organ formation requires spatio-temporal proliferation and differentiation of precursor cells. During lens development, placodal cells in the posterior lens vesicle exit from the cell cycle and enter into the process of differentiation. Cyclin-dependent kinase inhibitors play critical roles in cell cycle exit and promote differentiation in several tissues. We have found that p27kip1 is expressed in the posterior lens cells that undergo differentiation to form the differentiated fiber cells. The transcription factor L-Maf is expressed in these cells earlier than p27kip1. From in ovo gain- or loss-of-function experiments, we have found that L-Maf can, respectively, induce or inhibit the expression of p27kip1 in lens cells. Promoter assays using the 5' upstream sequences of the human p27kip1 gene indicate that L-Maf can activate p27kip1 transcription through the basal regulatory region. We suggest that L-Maf regulates cell cycle exit of the posterior lens cells by activating p27kip1, and thus directs fiber cell differentiation during lens formation in chick.

Stable expression of intracellular Notch suppresses v-Src-induced transformation in avian neural cells

Understanding how disruption of differentiation contributes to the cancer cell phenotype is required to identify alterations essential for malignant transformation and provide experimental basis for their correction. We investigated whether primary quail neuroretina cells, transformed by a conditional v-Src mutant (QNR/v-src(ts)), could revert to a normal phenotype, in response to the stable expression of constitutively active Notch1 intracellular domain (ICN). This model system was chosen because Notch signaling plays an instructive role in cell fate determination during NR development, and because the intrinsic capacity of QNR cultures to differentiate is blocked by v-Src. We report that stable ICN expression results in suppression of QNR/v-src(ts) cell transformation in the presence of an active oncoprotein. This phenotypic reversion coincides with a major switch in cell identity, as these undifferentiated cells acquire glial differentiation traits. Both changes appear to be mediated by CBF, a transcription factor that binds to ICN and activates target genes. Cells restored to a normal and differentiated phenotype have undergone changes in the functioning of signaling effectors, essentially regulating cell morphology and cytoskeleton organization. This dominant interference may be partially mediated by an autocrine/paracrine mechanism, as revertant cells secrete a factor(s), which inhibits transformation properties of QNR/v-src(ts) cells.

Cell context reveals a dual role for Maf in oncogenesis

Maf b-Zip transcription factors are involved in both terminal differentiation and oncogenesis. To investigate this apparent contradiction, we used two different primary cell types and performed an extensive analysis of transformation parameters induced by Maf proteins. We show that MafA and c-Maf are potent oncogenes in chicken embryo fibroblasts, while MafB appears weaker. We also provide the first evidence that MafA can confer growth factor independence and promote cell division at low density. Moreover, using MafA as a model, we identified several parameters that are critical for Maf transforming activities. Indeed, MafA ability to induce anchorage-independent cell growth was sensitive to culture conditions. In addition, the transforming activity of MafA was dependent on its phosphorylation state, since mutation on Ser65 impaired its ability to induce growth at low density and anchorage-independent growth. We next examined transforming activity of large Maf proteins in embryonic neuroretina cells, where they are known to induce differentiation. Unlike v-Jun, MafA, MafB and c-Maf did not show oncogenic activity in these cells. Moreover, they counteracted transformation induced by constitutive activation of the Ras/Raf/MEK pathway. Taken together, our results show that Maf proteins could display antagonistic functions in oncogenesis depending on the cellular context, and support a dual role for Maf as both oncogenes and tumor suppressor-like proteins.

Regulation of the MAFF transcription factor by proinflammatory cytokines in myometrial cells

The MAF (proto-)oncogene family of basic-leucine zipper transcription factors plays crucial roles in the control of mammalian gene expression and development. Here we analyzed the regulation of the human MAFF gene, coding for a small MAF transcription factor, in uterine smooth muscle cells. We found that MAFF transcript levels are induced by proinflammatory cytokines in PHM1-31 myometrial cells. We observed an important induction by interleukin 1 beta (IL1B) and a weaker upregulation by tumor necrosis factor (TNF), whereas interleukin 6 (IL6) treatment had no effect. Time course experiments revealed a rapid induction of MAFF transcripts within 30 min following IL1B treatment. The presence of actinomycin D inhibited the upregulation, suggesting that regulation of MAFF mRNA levels occurs at the transcriptional level. We generated a MAFF-specific antiserum and determined that MAFF protein was also induced by TNF and IL1B in PHM1-31 cells. In contrast, it was particularly interesting that the transcript and protein levels of the highly homologous MAFG and MAFK genes are not modulated by these cytokines. Our results suggest a possible specific role for MAFF in proinflammatory cytokine-mediated control of myometrial gene expression and provide the first link between a small MAF transcription factor and the inflammatory response.

MafA transcription factor is phosphorylated by p38 MAP kinase

Basic-leucine zipper transcription factors of the Maf family are key regulators of various developmental and differentiation processes. We previously reported that the phosphorylation status of MafA is a critical determinant of its biological functions. Using Western blot and mass spectrometry analysis, we now show that MafA is phosphorylated by p38 MAP kinase and identify three phosphoacceptor sites: threonine 113 and threonine 57, evolutionarily conserved residues located in the transcription activating domain, and serine 272. Mutation of these residues severely impaired MafA biological activity. Furthermore, we show that p38 also phosphorylates MafB and c-Maf. Together, these findings suggest that the p38 MAP kinase pathway is a novel regulator of large Maf transcription factors.

Basic leucine zipper transcription factors C/EBP and MafL in the hydrozoan jellyfish Podocoryne carnea

Members of the CCAAT/enhancer binding protein (C/EBP) and the Maf protein subfamilies have been characterized in a variety of bilaterian organisms. This is the first report of C/EBP and MafL genes in a basal organism, the hydrozoan jellyfish Podocoryne carnea. Transcripts of both genes are present in all life cycle stages: egg, embryo, larva, polyp, and medusa. During early development, both factors appear to regulate metamorphosis of the larva to the primary polyp. Both genes are also expressed in the striated muscle of the developing and adult medusa. During in vitro transdifferentiation of striated muscle cells to smooth muscle and nerve cells, C/EBP is continuously expressed, whereas MafL expression is turned off during transdifferentiation and reactivated when nerve cells differentiate. Thus, both factors may be involved in muscle and nerve cell differentiation. In the mature medusa both genes are also implicated in gametogenesis. Developmental and evolutionary aspects of the gene structures and expression patterns are discussed.

Control of roof plate development and signaling by Lmx1b in the caudal vertebrate CNS

Numerous studies have identified the roof plate as an important signaling center controlling dorsal interneuron specification and differentiation in the developing spinal cord. Currently, the molecular pathways of roof plate formation and function are poorly understood. We determined that the LIM-homeodomain transcription factor Lmx1b is sufficient to induce functional roof plate in the early chick developing spinal cord. In the chick, Lmx1b acts upstream of Lmx1a in the roof plate developmental program. Once the roof plate forms, we show that Bmp and Wnt signaling are the major components of Lmx1a/b-dependent roof plate dorsal patterning activity. The roof plate function of Lmx1b is not conserved across vertebrates because Lmx1b is not expressed in mouse roof plate progenitors. Instead, mouse caudal CNS roof plate formation relies entirely on Lmx1a. Lmx1b can, however, partially rescue roof plate development in dreher (Lmx1a-/-) mice, indicating that Lmx1b has some functional redundancy to Lmx1a. Furthermore, we demonstrate that the roof plate-inducing activity of Lmx1b can be suppressed by Mash1 (Cash1), which is normally expressed in intermediate neural tube in both chick and mouse. Our data identify Lmx1b as a key regulator of spinal cord roof plate induction and function.

Control of roof plate formation by Lmx1a in the developing spinal cord

Numerous studies have identified the roof plate as an embryonic signaling center critical for dorsal central nervous system patterning, but little is known about mechanisms that control its formation and its separation from clonally related neural crest cells and dI1 sensory interneurons. We demonstrate that the LIM homeodomain transcription factor, Lmx1a,mutated in the dreher mouse, acts to withdraw dorsal spinal cord progenitors from the cell cycle and simultaneously direct their differentiation into functional roof plate cells. Lmx1a cell-autonomously represses the dI1 progenitor fate, distinguishing the roof plate and dI1 interneuron programs, two major developmental programs of the dorsal neural tube. Lmx1a is not directly involved in neural crest development. We establish that Bmp signaling from epidermal ectoderm is necessary and sufficient for inducing Lmx1a and other co-factors that also regulate the extent of roof plate induction. We conclude that Lmx1a controls multiple aspects of dorsal midline patterning and is a major mediator of early Bmp signaling in the developing spinal cord.

Downregulation of mafB expression in T-helper cells during early differentiation in vitro

We have studied the expression of a human homologue of mafB (maf-1), a member of the family of large maf transcription factors. In support of the suggested key role that mafB expression plays in differentiating macrophages, we found mafB to be expressed at a very high level in monocytic U937 and THP-1 cell lines. However, we show here that mafB transcription is not restricted to myeloid cells but can also be detected in lymphoid cells, indicating transcriptional plasticity during haematopoiesis. In conclusion, strong proliferative signals mediated by T-cell activation and interleukins (IL-4 and IL-12) downregulate the mafB messenger RNA transcript level when resting naïve CD4+ T-helper cells enter the differentiation pathway in vitro.

Phosphorylation of MafA is essential for its transcriptional and biological properties

We previously described the identification of quail MafA, a novel transcription factor of the Maf bZIP (basic region leucine zipper) family, expressed in the differentiating neuroretina (NR). In the present study, we provide the first evidence that MafA is phosphorylated and that its biological properties strongly rely upon phosphorylation of serines 14 and 65, two residues located in the transcriptional activating domain within a consensus for phosphorylation by mitogen-activated protein kinases and which are conserved among Maf proteins. These residues are phosphorylated by ERK2 but not by p38, JNK, and ERK5 in vitro. However, the contribution of the MEK/ERK pathway to MafA phosphorylation in vivo appears to be moderate, implicating another kinase. The integrity of serine 14 and serine 65 residues is required for transcriptional activity, since their mutation into alanine severely impairs MafA capacity to activate transcription. Furthermore, we show that the MafA S14A/S65A mutant displays reduced capacity to induce expression of QR1, an NR-specific target of Maf proteins. Likewise, the integrity of serines 14 and 65 is essential for the MafA ability to stimulate expression of crystallin genes in NR cells and to induce NR-to-lens transdifferentiation. Thus, the MafA capacity to induce differentiation programs is dependent on its phosphorylation.

Induction of postmitotic neuroretina cell proliferation by distinct Ras downstream signaling pathways

Ras-induced cell transformation is mediated through distinct downstream signaling pathways, including Raf, Ral-GEFs-, and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathways. In some cell types, strong activation of the Ras-Raf-MEK-extracellular signal-regulated kinase (ERK) cascade leads to cell cycle arrest rather than cell division. We previously reported that constitutive activation of this pathway induces sustained proliferation of primary cultures of postmitotic chicken neuroretina (NR) cells. We used this model system to investigate the respective contributions of Ras downstream signaling pathways in Ras-induced cell proliferation. Three RasV12 mutants (S35, G37, and C40) which differ by their ability to bind to Ras effectors (Raf, Ral-GEFs, and the p110 subunit of PI 3-kinase, respectively) were able to induce sustained NR cell proliferation, although none of these mutants was reported to transform NIH 3T3 cells. Furthermore, they all repressed the promoter of QR1, a neuroretina growth arrest-specific gene. Overexpression of B-Raf or activated versions of Ras effectors Rlf-CAAX and p110-CAAX also induced NR cell division. The mitogenic effect of the RasC40-PI 3-kinase pathway appears to involve Rac and RhoA GTPases but not the antiapoptotic Akt (protein kinase B) signaling. Division induced by RasG37-Rlf appears to be independent of Ral GTPase activation and presumably requires an unidentified mechanism. Activation of either Ras downstream pathway resulted in ERK activation, and coexpression of a dominant negative MEK mutant or mKsr-1 kinase domain strongly inhibited proliferation induced by the three Ras mutants or by their effectors. Similar effects were observed with dominant negative mutants of Rac and Rho. Thus, both the Raf-MEK-ERK and Rac-Rho pathways are absolutely required for Ras-induced NR cell division. Activation of these two pathways by the three distinct Ras downstream effectors possibly relies on an autocrine or paracrine loop, implicating endogenous Ras, since the mitogenic effect of each Ras effector mutant was inhibited by RasN17.

Characterization of a novel quiescence responsive element downregulated by v-Src in the promoter of the neuroretina specific QR1 gene

The neuroretina is a functional unit of the central nervous system which arises through successive steps of division, growth arrest and differentiation of neuroectodermal precursors. Postmitotic quail neuroretina (QNR) cells are conditionally induced to divide upon infection with temperature sensitive mutants of Rous sarcoma virus (RSV), since QNR cell division can be arrested by either inactivating p60v-Src at the nonpermissive temperature (41 degrees C) or by serum deprivation at 37 degrees C. We are studying the transcriptional control of QR1, a neuroretina specific gene, whose expression is down-regulated in proliferating cells at 37 degrees C and is fully restored when these cells are made quiescent. We previously showed that this quiescence specific upregulation implicates a promoter region named A box, which binds Maf transcription factors. We report the identification of the C box, a second promoter sequence that activates QR1 transcription in non dividing cells. This sequence is able to form two DNA-protein complexes, one of which (C4) is predominantly detected in growth arrested NR cells. We identified the DNA binding site for C4 and described mutations that abolish both C4 binding and promoter activity in quiescent cells. Moreover, we show that a multimerized C box is able to stimulate a heterologous promoter in non dividing cells and constitutes, therefore, a novel quiescence responsive enhancer. Finally, we report that QR1 transcriptional response to cell quiescence requires cooperation between the C box and A box.

Human KRML (MAFB): cDNA cloning, genomic structure, and evaluation as a candidate tumor suppressor gene in myeloid leukemias

Members of the MAF family of basic region/leucine zipper transcription factors can affect transcription in either a positive or a negative fashion, depending on their partner protein(s) and the context of the target promoter. The KRML (MAFB) transcriptional regulator plays a pivotal role in regulating lineage-specific hematopoiesis by repressing ETS1-mediated transcription of erythroid-specific genes in myeloid cells. In previous studies, we mapped the human KRML gene within a genomic contig on human chromosome 20, bands q11.2-q13.1. We have isolated the human cDNA containing the full-length predicted open reading frame (ORF). Multiple KRML transcripts of approximately 1.8 and approximately 3 kb, which differ in the length of the 3' untranslated region, are ubiquitously expressed in hematopoietic tissues and encode a protein with 323 amino acids (MW 35,832). The protein has 84% identity and 92% similarity to the murine protein. The ORF of the human KRML gene contains no introns, and the gene spans approximately 3 kb. KRML maps within the smallest commonly deleted segment in malignant myeloid disorders characterized by a deletion of 20q; however, we detected no mutations of KRML in leukemia cells with loss of 20q. Thus, KRML is unlikely to be involved in the pathogenesis of malignant myeloid disorders characterized by abnormalities of chromosome 20.

C/EBPbeta (NF-M) is essential for activation of the p20K lipocalin gene in growth-arrested chicken embryo fibroblasts

The p20K gene is induced in conditions of reversible growth arrest in chicken embryo fibroblasts (CEF). This expression is dependent on transcriptional activation and on a region of the promoter designated the quiescence-responsive unit (QRU). In this report, we describe the regulatory elements of the QRU responsible for activation in resting cells and characterize the trans-acting proteins interacting with these elements. We show that the QRU consists of functionally distinct domains including quiescence-specific and weak proliferation-responsive elements. The quiescence responsiveness of the QRU was mapped to two C/EBP binding sites, and the activity of the p20K promoter and its QRU was inhibited by the expression of a dominant negative mutant of C/EBPbeta in nondividing cells. The activation of QRU in response to serum starvation and contact inhibition correlated with the presence of a growth arrest-specific complex in electrophoretic mobility shift assays. This complex was supershifted by antibody for C/EBPbeta. C/EBPbeta accumulated in conditions of contact inhibition as a result of transcriptional activation. Therefore, C/EBPbeta was itself regulated as a growth arrest-specific gene in CEF. Finally, we show that the expression of p20K is regulated by linoleic acid, an essential fatty acid binding to p20K. The addition of linoleic acid to contact-inhibited CEF markedly repressed the synthesis of p20K without inducing mitogenesis. The activity of the QRU was inhibited by linoleic acid or the peroxisome proliferator-activated receptor PPARgamma2 in transient expression assays. Therefore, we have identified C/EBPbeta as a key activator of a growth arrest-specific gene in CEF and implicated an essential fatty acid, linoleic acid, in regulation of the QRU and the p20K lipocalin gene.

Modulation of kinase activity and oncogenic properties by alternative splicing reveals a novel regulatory mechanism for B-Raf

Members of the raf oncogene family encode serine/threonine protein kinases, which activate the mitogen-activated protein kinase kinase MEKs (MAPK or ERK kinases) through direct interaction and phosphorylation. Several recent studies have revealed interesting differences between two members of this family, Raf-1 and B-Raf, regarding their activation, regulation, and kinase activity. In particular, B-Raf was shown to display higher MEK kinase activity than Raf-1. By using both two-hybrid analysis and coimmunoprecipitation experiments, we demonstrate here that B-Raf also markedly differs from Raf-1 by a higher affinity for MEK. We previously reported that the B-raf gene encodes multiple protein isoforms resulting from complex alternative splicing of two exons (exons 8b and 10) located upstream of B-Raf kinase domain. In the present study, we show that these naturally occurring modifications within the protein sequence markedly modulate both the biochemical and oncogenic properties of B-Raf. The presence of exon 10 sequences enhances the affinity for MEK, the basal kinase activity, as well as the mitogenic and transforming properties of full-length B-Raf, whereas the presence of exon 8b sequences seems to have opposite effects. Therefore, alternative splicing represents a novel regulatory mechanism for a protein of the Raf family.

Interaction of the CNC-bZIP factor TCF11/LCR-F1/Nrf1 with MafG: binding-site selection and regulation of transcription

We have previously shown that the widely expressed human transcription factor TCF11/LCR-F1/Nrf1 interacts with small Maf proteins and binds to a subclass of AP1-sites. Such sites are required for beta-globin 5' DNase I hypersensitive site 2 enhancer activity, erythroid porphobilinogen deaminase inducibility, hemin responsiveness by heme-oxygenase 1 and expression of the gene NAD(P)H:quinone oxidoreductase1. Here we report the optimal DNA-binding sequences for TCF11/LCR-F1/Nrf1 alone and as a heterodimer with MafG, identified by using binding-site selection. The heterodimer recognises a 5'-TGCTgaGTCAT-3' binding-site that is identical to the established NF-E2-site, the antioxidant response element and the heme-responsive element while the binding specificity of the homomer is less stringent. To investigate the activity of TCF11 through this selected site, both alone and in the presence of MafG, we have used a transient transfection assay. TCF11 alone activates transcription while MafG alone acts as a repressor. When co-expressed, MafG interferes with TCF11 transactivation in a dose dependent manner. This indicates that MafG protein, which heterodimerises efficiently with TCF11 in vitro (the heterodimer having a higher affinity for DNA than TCF11 alone), does not co-operate with TCF11 in transactivating transcription. We propose that since both these factors are widely expressed, they may act together to contribute to the negative regulation of this specific target site. Efficient positive regulation by TCF11 may require alternative partners with perhaps more restricted expression patterns.

Murine Ksr interacts with MEK and inhibits Ras-induced transformation

BACKGROUND

Ksr (kinase supressor of Ras) was identified as a regulator of the Ras-MAP kinase (mitogen-activated protein kinase) pathway by genetic screens in Drosophila and Caenorhabditis elegans. Ksr is a kinase with similarities to the three conserved regions of Raf kinases, especially within the kinase domain. To investigate whether these structural similarities correlated with common functional properties, we examined the ability of mKsr-1, the murine homolog of Ksr, to interact with components of the vertebrate MAP kinase pathway.

RESULTS

In the yeast two-hybrid interaction assay, mKsr-1 did not bind to either Ras, B-Raf or Raf-1, but interacted strongly with both MEK-1 and MEK-2, activators of MAP kinase. The Ksr-MEK interaction was confirmed by co-immunoprecipitation experiments. Ectopically expressed mKsr-1 co-precipitated with endogenous MEK-1 in COS-1 cells, and endogenous Ksr and MEK co-precipitated from PC12 cells. Phosphorylation of MEK by mKsr-1 was not detected, however. In contrast, the MEK subpopulation complexed with mKsr-1 in COS-1 cells or PC12 cells did not display kinase activity. This ability of Ksr to block MEK in an inactive form correlated with a biological response: mKsr-1 did not transform NIH3T3 cells, and, furthermore, mKsr-1 reduced Ras-induced transformation. Similarly, mKsr-1 inhibited the proliferation of embryonic neuroretina cells induced by Ras and B-Raf but not that induced by MEK.

CONCLUSIONS

Our results suggest a novel mechanism for Ksr in regulating the MAP kinase pathway, at least in part through an ability to interact with MEK.

The Maf transcription factors: regulators of differentiation

Since the identification of the v-maf oncogene in an avian tumor virus, the Maf protein family has grown rapidly, forming a unique subclass of basic-leucine zipper transcription (bZIP) factors. Maf family members appear to play important roles in the regulation of differentiation.

Human bZIP transcription factor gene NRL: structure, genomic sequence, and fine linkage mapping at 14q11.2 and negative mutation analysis in patients with retinal degeneration

The NRL gene encodes an evolutionarily conserved basic motif-leucine zipper transcription factor that is implicated in regulating the expression of the photoreceptor-specific gene rhodopsin. NRL is expressed in postmitotic neuronal cells and in lens during embryonic development, but exhibits a retina-specific pattern of expression in the adult. To understand regulation of NRL expression and to investigate its possible involvement in retinopathies, we have determined the complete sequence of the human NRL gene, identified a polymorphic (CA)n repeat (identical to D14S64) within the NRL-containing cosmid, and refined its location by linkage analysis. Since a locus for autosomal recessive retinitis pigmentosa (arRP) has been linked to markers at 14q11 and since mutations in rhodopsin can lead to RP, we sequenced genomic PCR products of the NRL gene and of the rhodopsin-Nrl response element from a panel of patients representing independent families with inherited retinal degeneration. The analysis did not reveal any causative mutations in this group of patients. These investigations provide the basis for delineating the DNA sequence elements that regulate NRL expression in distinct neuronal cell types and should assist in the analysis of NRL as a candidate gene for inherited diseases/syndromes affecting visual function.

A role for the roof plate and its resident TGFbeta-related proteins in neuronal patterning in the dorsal spinal cord

Distinct neuronal cell types are generated at characteristic times and positions in the dorsal horn of the spinal cord. We provide evidence that the identity and pattern of generation of dorsal neurons depend initially on BMP-mediated signals that derive from the epidermal ectoderm and induce dorsal midline cells of the roof plate. Roof plate cells provide a secondary source of TGFbeta-related signals that are required for the generation of distinct classes of dorsal interneurons. These inductive interactions involve both qualitative and quantitative differences in signaling by TGFbeta-related factors and temporal changes in the response of neural progenitor cells.

The world according to Maf

Maf family proteins are so named because of their structural similarity to the founding member, the oncoprotein v-Maf. The small Maf proteins (MafF, MafG and MafK), as do all family members, include a characteristic basic region linked to a leucine zipper (b-Zip) domain which mediate DNA binding and subunit dimerization respectively. The small Maf proteins form homodimers or heterodimers with other b-Zip proteins present in the cell and bind to Maf recognition elements (MARE) in DNA. Since they lack known transcriptional activation domains, the small Maf proteins function either as obligatory heterodimeric partner molecules with numerous large subunits, discussed below, or alternatively as homo- or heterodimeric transcriptional repressors. The three small Maf proteins are expressed in a number of overlapping tissues, but their expression profiles nonetheless appear to be under meticulous tissue- and developmental stage-specific control. The MARE bears a striking resemblance to the NF-E2 binding sequence. NF-E2 binding sites in the human beta-globin locus control region have been directly implicated as integral components in the circuitry required for eliciting changes in chromatin structure that precede globin gene activation. While the NF-E2 DNA sequence has been shown to be important for erythroid-specific gene regulation, a growing list of other genes may also be regulated through the same, or very similar, cis elements in non-erythroid cells. Taken together, these observations argue that comprehensive analysis of the activities of the small Maf proteins may provide a unique perspective for expanding our understanding of transcriptional regulation that can be elicited through interacting transcription factor networks.

Human MafG Is a Functional Partner for p45 NF-E2 in Activating Globin Gene Expression

Mammalian globin gene expression is activated through NF-E2 elements recognized by basic-leucine zipper proteins of the AP-1 superfamily. The specificity of NF-E2 DNA binding is determined by several nucleotides adjacent to a core AP-1 motif, comprising a recognition site for transcription factors of the Maf subfamily. Earlier work proposed that p18(MafK) forms a heterodimer with hematopoietic-specific protein p45 NF-E2 to activate transcription through NF-E2 sites. However, there was no direct evidence that p18(MafK) serves this function in vivo; in fact, mice lacking p18(MafK) have no phenotype. Here we describe a novel cDNA clone that encodes the human homolog of chicken MafG. Human MafG heterodimerizes with p45 NF-E2 and binds DNA with specificity identical to that of purified NF-E2 DNA binding activity. A tethered heterodimer of p45 and MafG is fully functional in supporting expression of α- and β-globin, and in promoting erythroid differentiation in CB3, a p45-deficient mouse erythroleukemia cell line. These results indicate that human MafG can serve as a functional partner for p45 NF-E2, and suggest that the p45/MafG heterodimer plays a role in the regulation of erythropoiesis.

QRI, a retina-specific gene, encodes an extracellular matrix protein exclusively expressed during neural retina differentiation

Neural retina development results from growth arrest of neuroectodermal precursors and differentiation of postmitotic cells. The QRI gene is specifically expressed in Müller retinal glial cells. Its expression coincides with the stage of withdrawal from the cell cycle and establishment of differentiation and is repressed upon induction of retinal cell proliferation by the v-src gene product. In this report, we show that the QR1 gene encodes several glycosylated proteins that are secreted and can either associate with the extracellular matrix or remain diffusible in the medium. By using pulse-chase experiments, the 100-103 kDa forms seem to appear first and are specifically incorporated into the extracellular matrix, whereas the 108 and 60 kDa polypeptides appear later and are detected as soluble forms in the culture medium. We also report that expression of the QR1 gene is developmentally regulated in the chicken. Its mRNA is first detectable at embryonic day 10, reaches a maximal level at embryonic day 15 and is no longer detected at embryonic day 18. Immunolocalization of the QR1 protein in chicken retina sections during development shows that expression of the protein parallels the differentiation pattern of post-miotic cells (in particular Müller cells and rods), corresponding to the two differentiation gradients in the retina: from the ganglion cell layer to the inner nuclear layer and outer nuclear layer, and from the optic nerve to the iris. At embryonic day 10, expression of the QR1 protein(s) is restricted to the optic nerve region and the inner nuclear layer, colocalizing with Müller cell bodies. As development proceeds, QR1 protein localization spreads towards the iris and towards the outer nuclear layer, following Müller cell elongations towards the photoreceptors. Between embryonic days 16 and 18, the QR1 protein is no longer detectable in the optic nerve region and is concentrated around the basal segment of the photoreceptors in the peripheral retina. Our results suggest a role for the QR1 gene product in the process of growth arrest and establishment of photoreceptor differentiation.