Sonic hedgehog is required during an early phase of oligodendrocyte development in mammalian brain.

Oligodendrocyte precursor development in the embryonic spinal cord is thought to be regulated by the secreted signal, Sonic hedgehog (Shh). Such precursors can be identified by the expression of Olig genes, encoding basic helix-loop-helix factors, in the spinal cord and brain. However, the signaling pathways that govern oligodendrocyte precursor (OLP) development in the rostral central nervous system are poorly understood. Here, we show that Shh is required for oligodendrocyte development in the mouse forebrain and spinal cord, and that Shh proteins are both necessary and sufficient for OLP production in cortical neuroepithelial cultures. Moreover, adenovirus-mediated Olig1 ectopic expression can promote OLP formation independent of Shh activity. Our results demonstrate essential functions for Shh during early phases of oligodendrocyte development in the mammalian central nervous system. They further suggest that a key role of Shh signaling is activation of Olig genes.

Generation and utilization of phosphorylation state-specific antibodies to investigate signaling pathways.

Phosphorylation state-specific antibodies can be of great use, for example, in studying individual steps within a given signal transduction pathway. This unit presents a general approach to the generation and purification of phosphorylation state-specific antibodies. In addition to their ability to detect phosphorylation at a particular key site, these antibodies are often more sensitive for biochemical studies. Besides their application in immunoblotting procedures, activation state-specific antibodies can be used as immunohistochemical reagents. Thus, critical changes in phosphorylation can be monitored as described on an individual cell basis or in fixed tissue sections. Such antibodies can be used to address fundamental questions about signal transduction pathways during physiologic events that cannot be resolved by more conventional methodologies.

Intracranial inhibition of platelet-derived growth factor-mediated glioblastoma cell growth by an orally active kinase inhibitor of the 2-phenylaminopyrimidine class.

Glioblastoma multiforme is the most common primary human brain tumor, and it is, for all practical purposes, incurable in adult patients. The high mortality rates reflect the fact that glioblastomas are resistant to adjuvant therapies (radiation and chemicals), the mode of action of which is cytotoxic. We show here that an p.o.-active small molecule kinase inhibitor of the 2-phenylaminopyrimidine class may have therapeutic potential for glioblastomas. STI571 inhibits the growth of U343 and U87 human glioblastoma cells that have been injected into the brains of nude mice, but it does not inhibit intracranial growth of ras-transformed cells. Studies on a broad panel of genetically validated human and animal cell lines show that STI571 acts by disruption of the ligand:receptor autocrine loops for platelet-derived growth factor that are a pervasive feature of malignant astrocytoma. The cellular response of glioblastoma cells to STI571 does not appear to involve an apoptotic mechanism.

Sonic hedgehog--regulated oligodendrocyte lineage genes encoding bHLH proteins in the mammalian central nervous system.

During development, basic helix-loop-helix (bHLH) proteins regulate formation of neurons from multipotent progenitor cells. However, bHLH factors linked to gliogenesis have not been described. We have isolated a pair of oligodendrocyte lineage genes (Olg-1 and Olg-2) that encode bHLH proteins and are tightly associated with development of oligodendrocytes in the vertebrate central nervous system (CNS). Ectopic expression of Olg-1 in rat cortical progenitor cell cultures promotes formation of oligodendrocyte precursors. In developing mouse embryos, Olg gene expression overlaps but precedes the earliest known markers of the oligodendrocyte lineage. Olg genes are expressed at the telencephalon-diencephalon border and adjacent to the floor plate, a source of the secreted signaling molecule Sonic hedgehog (Shh). Gain- and loss-of-function analyses in transgenic mice demonstrate that Shh is both necessary and sufficient for Olg gene expression in vivo.

Bipotent cortical progenitor cells process conflicting cues for neurons and glia in a hierarchical manner.

Neurons and glia of the cerebral cortex are thought to arise from a common, multipotent progenitor cell that is instructed toward alternate fates by extracellular cues. How do these cells behave when confronted with conflicting cues? We show here that nestin-positive neuroepithelial (NE) cells from embryonic day 14 rat cortex coexpress surface receptor proteins for ciliary neurotrophic factor (CNTF) and platelet-derived growth factor (PDGF). Both sets of these receptor proteins are functional in NE cells, as shown by ligand-dependent activation of downstream signal-generating proteins. Transient (30') exposure to CNTF instructs NE cells toward an astrocyte fate. Brief exposure to PDGF initiates neuronal differentiation. However, when challenged with conflicting cues, PDGF is dominant to CNTF. Moreover, CNTF-treated NE cells can be "redirected" by a subsequent exposure to PDGF to form neurons instead of astrocytes, whereas the converse is not true. The asymmetric relationship between CNTF and PDGF indicates that these two growth factors act on a common progenitor cell that has, at a minimum, two fates available to it rather than separate populations of precommitted neuroblasts and astroblasts. This bipotent progenitor cell processes conflicting cues for neurons and glia in a hierarchical manner.

Platelet-derived growth factor stimulation of monocyte chemoattractant protein-1 gene expression is mediated by transient activation of the phosphoinositide 3-kinase signal transduction pathway.

Platelet-derived growth factor (PDGF) stimulates transcription of an immediate-early gene set in Balb/c 3T3 cells. One cohort of these genes, typified by c-fos, is induced within minutes following activation of PDGF receptors. A second cohort responds to PDGF only after a significant time delay, although induction is still a primary response to receptor activation as shown by "superinduction" in the presence of the protein synthesis inhibitor cycloheximide. PDGF-receptor activated signaling pathways for the "slow" immediate-early genes are poorly resolved. Using gain-of-function mutations together with small molecule inhibitors of kinase activity, we show that activation of PI 3-kinase is both necessary and sufficient for the induction of the prototype slow immediate-early gene, monocyte chemoattractant-1 (MCP-1). Following activation of PDGF receptors, MCP-1 mRNA does not begin to accumulate for at least 90 min. However, only a brief (10 min) interval of PI 3-kinase activity is required to trigger this delayed response. The serine/threonine protein kinase, Akt/PKB, likely functions as a downstream affector of PI 3-kinase for this induction.

Activation of ErbB2 during wallerian degeneration of sciatic nerve.

We used anti-phosphopeptide-immunodetecting antibodies as immunohistochemical reagents to define the location and activity state of p185(erbB2) during Wallerian degeneration. Nerve damage induces a phosphorylation event at Y1248, a site that couples p185(erbB2) to the Ras-Raf-MAP kinase signal transduction pathway. Phosphorylation of p185(erbB2) occurs within Schwann cells and coincides in time and space with Schwann cell mitotic activity, as measured by bromodeoxyuridine uptake. These visual images of receptor autophosphorylation link activation of p185(erbB2) to the Schwann cell proliferation that accompanies nerve regeneration.

Detection of activated platelet-derived growth factor receptors in human meningioma.

The beta receptor subunit of platelet-derived growth factor (PDGF) and its corresponding ligand (PDGF-BB) are coordinately expressed in fresh surgical isolates of human meningioma. These observations imply that PDGF autocrine loops are engaged in human meningioma and suggest that activated PDGF-beta receptors might contribute to the pathology of this common brain neoplasm. The study of PDGF autocrine loops and human meningioma has been slowed by the scarcity of meningioma cell culture model systems. Furthermore, in meningioma tumor tissue, the activation state of PDGF receptors is difficult to assess with conventional reagents, because the tumor is intermixed with normal stroma. In fact, there is no evidence that PDGF receptors within the tumor are activated by ligand. We used a synthetic tyrosine phosphopeptide to raise an antibody that reports the phosphorylation state of tyrosine 751 in the human PDGF-beta receptor. Phosphorylated tyrosine 751 is a recognition site for phosphatidylinositol 3'-kinase, a cytoplasmic effector of PDGF-induced mitogenesis, chemotaxis, and membrane ruffling. Immunoblotting and immunostaining analyses with this antibody show that the PDGF-beta receptor is constitutively phosphorylated at tyrosine 751 within multiple fresh surgical isolates of human meningioma. These findings are consistent with a role for activated PDGF receptors in the proliferation of human meningiomas.

Phosphorylation-directed antibodies in high-flux screens for compounds that modulate signal transduction.

Synthetic tyrosine phosphopeptides were used to generate antibodies that recognize phosphotyrosine in the context of a defined sequence of flanking amino acids. Using phosphopeptide immunogens derived from regulatory or signal-generating motifs, "phosphorylation-directed antibodies" can be targeted to specific growth factor receptors or signal-generating proteins. In his paper, we show how phosphorylation-directed antibodies can be used in a colorimetric, high-throughput screen for drugs that modulate the function of specific growth factor receptors or signal-generating proteins.

A PDGF-regulated immediate early gene response initiates neuronal differentiation in ventricular zone progenitor cells.

When exposed to platelet-derived growth factor (PDGF), uncommitted neuroepithelial cells from the developing cortex of embryonic day 14 (E14) rats develop into neurons. Outward signs of the neuronal phenotype are not observed for 4 days following exposure to PDGF. However, only a brief (2-3 hr) period of PDGF receptor activation is required to initiate neuronal development. During the window of receptor activation, RNA synthesis is essential, but protein synthesis is not. These observations indicate that specification of neuronal fate is mediated by an immediate early gene response.

Differential utilization of Trk autophosphorylation sites.

Tyrosine autophosphorylation controls the catalytic and signaling activities of the neurotrophin receptors, the Trks. To analyze the regulation of distinct tyrosine sites, we generated a panel of antibodies that report the phosphorylation state of individual tyrosines within the Trk cytoplasmic domain. Using pheochromocytoma-derived cell lines, we show that individual tyrosines within the nerve growth factor receptor TrkA are phosphorylated in a non-coordinate fashion following receptor activation. The non-coordinate response of these tyrosines reflects their separate functions in regulating the catalytic and signaling activities of Trk receptors. The differential utilization of distinct sites on Trk receptor tyrosine kinases suggests that the receptor can specify both the timing and the nature of neurotrophin-stimulated signal transduction pathways. Moreover, we show that these Trk autophosphorylation sites, which have hitherto been mapped and characterized only in non-neuronal cell lines, are activated in normal neurons in response to ligand stimulation.

Platelet-derived growth factor induction of the immediate-early gene MCP-1 is mediated by NF-kappaB and a 90-kDa phosphoprotein coactivator.

A broad panel of agents including serum, interleukin-1, double-stranded RNA, and platelet-derived growth factor (PDGF) stimulate transcription of the "slow" immediate-early gene MCP-1. These disparate inducers act through a tight cluster of regulatory elements in the distal 5'-flanking sequences of the MCP-1 gene. We describe a 22-base element in this cluster which, in single copy, confers PDGF-inducibility to a tagged MCP-1 reporter gene. In mobility shift assays, the element binds a PDGF-activated form of NF-kappaB, and a 90-kDa protein (p90) which binds constitutively. Antibody supershift and UV cross-linking experiments indicate that the PDGF-activated NF-kappaB species is a Rel A homodimer. The DNA binding form of p90 is a nuclear-restricted serine/threonine phosphoprotein. Mutagenesis of the 22-base element shows that the NF-kappaB and p90 binding sites overlap, but binding of the two species is mutually independent. Both sites, however, are required for optimum PDGF induction of MCP-1. Therefore, p90 appears to be a coactivator with NF-kappaB in PDGF-mediated induction of MCP-1.

Applications of electroporation of adherent cells in situ, on a partly conductive slide.

Nontraumatic, simple, and reproducible procedures for the introduction of nonpermeant molecules into adherent mammalian cells by in situ electroporation are described. Cells are grown on a glass slide, half of which is coated with electrically conductive, optically transparent, indium-tin oxide. An electric pulse is applied in the presence of the molecules to be introduced, and their effect on the cellular phenotype can be observed. The cells growing on the nonconductive side of the slide do not receive any pulse and serve as controls. Careful adjustment of electric field strength can achieve the introduction of the molecules into essentially 100% of the cells, and this treatment causes no detectable disruption to cellular metabolism. This is applied in the presence of the fluorescent dye, Lucifer yellow, causing its penetration into the cells growing on the conductive half of the slide. The migration of the dye to the nonelectroporated cells growing on the nonconductive area is microscopically observed under fluorescence illumination.

Electroporation of peptides into adherent cells in situ.

To study the effect of protein interactions in vivo upon cellular functions,such complexes may be disrupted through the introduction of peptides corresponding to the proteins' points of contact. In this communication a simple, rapid and reproducible procedure for peptide introduction into adherent cells by electroporation is described. Cells are grown on electrically conductive, optically transparent indium-tin oxide at the time of pulse delivery. Several electrode and slide configurations, necessary to obtain non-electroporated cells adjacent to the electroporated ones as a control, are outlined. Careful control of electric field strength achieved the introduction of the peptide into essentially 100% of the cells while this treatment caused no detectable disruption of their division cycle.

A new platelet-derived growth factor-regulated genomic element which binds a serine/threonine phosphoprotein mediates induction of the slow immediate-early gene MCP-1.

The MCP-1 chemokine gene belongs to a cohort of immediate-early genes that are induced with slower kinetics than c-fos. In this study, we identified a cluster of four platelet-derived growth factor (PDGF)-responsive elements within a 240-bp enhancer found in the distal 5' flanking MCP-1 sequences. Two of the elements bind one or more forms of the transcription factor NF-kappa B. We focused on the other two elements which are hitherto unreported, PDGF-regulated genomic motifs. One of these novel elements, detected as a 28-mer by DNase I footprinting, restores PDGF inducibility when added in two copies to a 5' truncated MCP-1 gene. A single copy of the second novel element, a 27-mer, restores PDGF inducibility to a 5' truncated MCP-1 gene. The 27-base element interacts with a PDGF-activated serine/threonine phosphoprotein that is detected only within the nucleus of PDGF-treated 3T3 cells. DNA binding of this phosphoprotein is activated by PDGF treatment with slow kinetics that match the time course of MCP-1 gene expression, and activation is not inhibited by cycloheximide. PDGF-activated binding to the 27-mer is shown to involve a single 30-kDa protein by UV-cross-linking analysis.

Multiple, functional DBP sites on the promoter of the cholesterol 7 alpha-hydroxylase P450 gene, CYP7. Proposed role in diurnal regulation of liver gene expression.

Hepatic cytochrome P450 cholesterol 7 alpha-hydroxylase, CYP7, is regulated in vivo at the protein and the mRNA level in response to multiple physiological factors, including liver cholesterol synthesis, bile acid feedback inhibition, and diurnal rhythm. In the present study we investigated whether the liver transcription factor DBP (albumin promoter D-site binding protein), which undergoes a striking diurnal rhythm in rat liver (DBP levels during evening/morning approximately 100:1), contributes to the diurnal regulation of CYP7 gene expression. DNase I footprinting analysis using bacterially expressed DBP and a cloned 5'-flanking DNA segment of the rat CYP7 gene revealed five distinct DBP-binding sites, designated A-E, distributed between nucleotides (nts) -41 and -295 relative to the CYP7 transcription start site. CYP7-directed gene transcription in HepG2 cells transfected with a 5'-CYP7 promoter-chloramphenicol acetyl-transferase reporter was activated up to 12-fold upon cotransfection of a DBP expression vector, whereas an HNF-1 alpha expression vector did not stimulate CYP7 gene activity. 5'-Deletion analyses and site-specific mutagenesis revealed that this stimulating effect of DBP can in part be ascribed to its functional interaction with DBP binding sites B (nts -115/-125), C (nts -172/-195), and D (nts -214/-230). C/EBP beta (LAP), another liver-enriched basic-leucine zipper transcription factor, bound to these same sites but effected a more modest increase in CYP7-directed gene transcription (up to 3-4-fold) when expressed in HepG2 cells. Competition for CYP7 promoter-binding sites between C/EBP, which undergoes < or = 2-fold diurnal change in rat liver, and the diurnally regulated DBP is proposed to determine the relative rates of basal versus diurnally regulated CYP7 gene transcription and thus may be a primary mechanism for setting the 3-6-fold amplitude that characterizes the circadian rhythm of liver CYP7 expression. Moreover, since DBP is first expressed in rat liver 3-4 weeks after birth, these findings may account for both the enhanced expression and the onset of the diurnal pattern of CYP7 enzyme levels at this stage of development.

Identification of an activity that interacts with the 3'-untranslated region of c-myc mRNA and the role of its target sequence in mediating rapid mRNA degradation.

Single or multiple copies of the pentameric motif, AUUUA, are found in the 3'-untranslated domain of most immediate early mRNAs. A broad body of data links this 5-base element to the selective degradation of these mRNAs. However, AUUUA motifs are neither always necessary nor always sufficient to tag an mRNA for rapid degradation. Here, we describe a cytoplasmic, protease-sensitive factor in Balb/c3T3 cells that interacts with the 3'-untranslated region of c-myc mRNA. The factor recognizes a 39-base uridine-rich domain adjacent to, but distinct from, the c-myc AUUUA motif. In chimeric mRNA constructs, the 39-base binding element cooperates with adjacent sequence material to destabilize beta-globin mRNA.

Sex-specific, growth hormone-regulated transcription of the cytochrome P450 2C11 and 2C12 genes.

Growth hormone (GH) differentially regulates the expression of several male-specific and female-specific liver cytochrome P450 mRNAs as a function of its sex-dependent ultradian secretory pattern. Pulsatile GH release stimulates expression of the male-specific P450 2C11, while a continuous GH secretion pattern suppresses expression of 2C11 and stimulates the expression of the female-specific P450 2C12. To help define the level at which GH regulates the expression of 2C11 and 2C12 mRNA, liver nuclear RNA samples isolated from rats differing in GH status were analyzed for 2C11 and 2C12 hnRNAs by hybridization to 2C11 and 2C12 gene-specific exonic oligonucleotide probes, as well as exon/intron junction probes. The 2C11 and 2C12 hnRNAs were found to be responsive to circulating GH profiles in a manner indistinguishable from the corresponding mature, cytoplasmic mRNAs, with no 2C12 mRNA precursors found in untreated male or hypophysectomized female liver nuclei, and no 2C11 mRNA precursors in untreated female or hypophysectomized male liver nuclei. Thus, transport of 2C11 and 2C12 RNA to the cytoplasm and cytoplasmic mRNA stability are unlikely to be important GH-regulated control points for sex-specific P450 RNA expression. Run-on transcription analysis further established that GH regulates the sex-specific expression of the 2C11 and 2C12 genes at the level of transcript initiation. Transcription was also shown to be the major step for regulation of the male-specific P450 2A2 RNA, whose expression, unlike 2C11, is not obligatorily dependent on pulsatile GH release. In vitro footprinting analysis of 2C11 and 2C12 promoter fragments incubated with liver nuclear proteins isolated from rats differing in GH status revealed several sex- and GH-dependent differences in DNase cleavage patterns ("hypersensitivity sites"), demonstrating that GH can regulate specific protein-DNA interactions in the 5'-flanking sequences of these two genes. In vitro transcription assays driven by 2C11 and 2C12 5'-flanking DNA sequences fused to TATAA box-G-less cassette template constructs did not, however, faithfully mimic the sex-specific transcription of the 2C11 and 2C12 genes, indicating that additional cis-elements or trans-acting factors may be required to achieve the transcriptional regulation of these genes that occurs in vivo.

Pituitary regulation of the male-specific steroid 6 beta-hydroxylase P-450 2a (gene product IIIA2) in adult rat liver. Suppressive influence of growth hormone and thyroxine acting at a pretranslational leve;.

Oligonucleotide probes that distinguish between two closely related mRNAs encoding steroid 6 beta-hydroxylases of rat P-450 gene family CYP3A were used to individually assess their responsiveness to pituitary hormone regulation. Northern blot analysis revealed that the elevation of immunoreactive P-450 IIIA2 in livers of hypophysectomized rats reflects an elevation of the constitutive, male-specific P-450 IIIA2 (P-450 2a) and not an induction of the drug-inducible P-450 IIIA1 (P-450p). P-450 IIIA2 mRNA levels in intact adult male rats were found to be markedly reduced by GH administered as a continuous infusion at levels as low as 1 mU/h, indicating that GH acts at a pretranslational step to suppress expression of this P-450 enzyme. In hypophysectomized male rats, however, this same hormone treatment was only partially effective at suppressing P-450 IIIA2 mRNA and protein, suggesting that other pituitary-dependent factors contribute to the suppression observed in the intact rats. Further analysis revealed that T4, but not ACTH or human CG, can act in concert with GH to effect a more complete suppression of hepatic P-450 IIIA2 mRNA and protein in hypophysectomized rats. T4 also suppressed the expression of another GH-regulated, male-specific hepatic enzyme, designated P-450 IIA2 (P-450 RLM2), particularly in hypophysectomized female rats. In contrast, the GH-responsive P-450 IIA1 (P-450 3) was much less affected by T4 treatment. Thus, while T4 can modulate P-450 IIIA2 expression, it does not serve as a universal regulator for hepatic expression of GH-responsive P-450s.

Spectroscopic characterization of secondary amine mono-oxygenase. Comparison to cytochrome P-450 and myoglobin.

Secondary amine mono-oxygenase from Pseudomonas aminovorans catalyzes the NAD(P)H- and dioxygen-dependent N-dealkylation of secondary amines to yield a primary amine and an aldehyde. Heme iron, flavin, and non-heme iron prosthetic groups are known to be present in the oligomeric enzyme. The N-dealkylation reaction is also catalyzed by the only other heme-containing mono-oxygenase, cytochrome P-450. In order to identify the heme iron axial ligands of secondary amine mono-oxygenase so as to better define the structural requirements for oxygen activation by heme enzymes, we have investigated the spectroscopic properties of the enzyme. The application of three different spectroscopic techniques, UV-visible absorption, magnetic circular dichroism and electron paramagnetic resonance, to study eight separate enzyme derivatives has provided extensive and convincing evidence for the presence of a proximal histidine ligand. This conclusion is based primarily on comparisons of the spectral properties of the enzyme with those of parallel derivatives of myoglobin (histidine proximal ligand) and P-450 (cysteinate proximal ligand). Spectral studies of ferric secondary amine mono-oxygenase as a function of pH have led to the proposal that the distal ligand is water. Deprotonation of the distal water ligand occurs upon either raising the pH to 9.0 or substrate (dimethylamine) binding. In contrast, the deoxyferrous enzyme appears to have a weakly bound nitrogen donor distal ligand. Initial spectroscopic studies of the iron-sulfur units in the enzyme are interpreted in terms of a pair of Fe2S2 clusters. Secondary amine mono-oxygenase is unique in its ability to function as cytochrome P-450 in activating molecular oxygen but to do so with a myoglobin-like active site. As such, it provides an important system with which to probe structure-function relations in heme-containing oxygenases.