u-0126 and Neurofibromatosis-1

Patients with neurofibromatosis type 1 (NF1), resulting from neurofibromin gene mutations, frequently suffer from deficits in learning and spatial memory. Mice heterozygous for functional deletion of the NF1 gene (NF1(+/-) mice) also exhibit compromised spatial learning, and deficits in early-stage hippocampal long-term potentiation (LTP). Neurofibromin is a multifunctional protein which acts in part as an inhibitory constraint on Ras signalling, and the deficits in early-stage LTP and spatial learning have been linked to Ras hyperactivation. However, the downstream targets of Ras hyperactivation that lead to cognitive disruption are unknown. The levels of activity of signalling molecules potentially downstream of Ras were therefore studied in NF1(+/-) mice. Elevated phospho-ERK (pERK) levels were observed in the hippocampi from NF1(+/-) mice, while phospho-Akt/PKB (pAkt) and phospho-eIF4E (peIF4E) levels were unchanged relative to wild-type mice. Hippocampal levels of phospho-CREB (pCREB) were also increased, suggesting potential changes in late-phase LTP in NF1(+/-) mice. Indeed, LTP was found to be impaired for at least 4 h following induction in NF1(+/-) mice, linking neurofibromin function with the long-term maintenance of LTP. Remarkably, U0126, an inhibitor of ERK activation, at doses which reduced the hyperactive pERK levels in NF1(+/-) mice to the levels observed in control mice, caused a reduction in the deficits in early-phase LTP and completely rescued the long-term LTP deficits. In contrast to the abundant evidence that reductions in ERK activity lead to impaired plasticity, these data indicate that ERK hyperactivation in a partial model of type 1 neurofibromatosis leads to deficits in long-lasting hippocampal plasticity.

Topics: Animals; Butadienes; Cyclic AMP Response Element-Binding Protein; Disease Models, Animal; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Electric Stimulation; Enzyme Inhibitors; Excitatory Postsynaptic Potentials; Extracellular Signal-Regulated MAP Kinases; Gene Expression Regulation; Hippocampus; In Vitro Techniques; Mice; Mice, Inbred C57BL; Mice, Knockout; Neurofibromatosis 1; Neuronal Plasticity; Nitriles; Phosphorylation; Recovery of Function

The neurofibromatosis Type 1 (NF1) gene functions as a tumor suppressor gene. One known function of neurofibromin, the NF1 protein product, is to accelerate the slow intrinsic GTPase activity of Ras to increase the production of inactive rasGDP, with wide-ranging effects on p21ras pathways. Loss of neurofibromin in the autosomal dominant disorder NF1 is associated with tumors of the peripheral nervous system, particularly neurofibromas, benign lesions in which the major affected cell type is the Schwann cell (SC). NF1 is the most common cancer predisposition syndrome affecting the nervous system. We have developed an in vitro system for differentiating mouse embryonic stem cells (mESC) that are NF1 wild type (+/+), heterozygous (+/-), or null (-/-) into SC-like cells to study the role of NF1 in SC development and tumor formation. These mES-generated SC-like cells, regardless of their NF1 status, express SC markers correlated with their stage of maturation, including myelin proteins. They also support and preferentially direct neurite outgrowth from primary neurons. NF1 null and heterozygous SC-like cells proliferate at an accelerated rate compared to NF1 wild type; this growth advantage can be reverted to wild type levels using an inhibitor of MAP kinase kinase (Mek). The mESC of all NF1 types can also be differentiated into neuron-like cells. This novel model system provides an ideal paradigm for studies of the role of NF1 in cell growth and differentiation of the different cell types affected by NF1 in cells with differing levels of neurofibromin that are neither transformed nor malignant.

Topics: Animals; Antibodies; Butadienes; Cell Differentiation; Cell Line; Cell Proliferation; Chick Embryo; Culture Media; DNA Primers; Embryonic Stem Cells; Enzyme Inhibitors; Ganglia; Genes, Neurofibromatosis 1; Immunohistochemistry; Indicators and Reagents; MAP Kinase Kinase Kinases; Mice; Mice, Knockout; Neurites; Neurofibromatosis 1; Neurons; Nitriles; Reverse Transcriptase Polymerase Chain Reaction; Schwann Cells

Type 1 neurofibromatosis (NF1) is a common autosomal dominant disorder that results in neuroectodermal tumors. The NF1 tumor-suppressor gene encodes neurofibromin, which includes a GTPase-activating domain for Ras inactivation. Affinity purification showed N-Ras to be the predominant activated isoform of Ras in two independent neurofibrosarcoma cell lines from NF1 patients (lines ST88-14 and NF90-8). These NF1 cells also demonstrated increased constitutive activity of the extracellular signal-regulated kinases 1 and 2 (ERK1,2) mitogen-activated protein (MAP) kinases compared with a sporadic malignant schwannoma cell line that maintains neurofibromin expression (STS-26T). Thus, MAP kinase kinase (MEK) inhibitors may be a rational approach to NF1 therapy. The MEK inhibitors PD98059 [2'-amino-3'-methoxyflavone], PD184352 (also called CI-1040) [2-(2-chloro-4-iodo-phenylamino)-N-cyclopropylmethoxy-3,4-difluoro-benzamide], and U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene] all produced concentration-dependent suppression of the proliferation of the three cell lines. Individual MEK inhibitors had similar effects in all three cell lines. However, only the antiproliferative effects of PD184352 correlated closely with the elimination of ERK1,2 MAP kinase activities. PD98059 was primarily cytostatic, whereas U0126 and PD184352 were cytotoxic. Only PD184352 induced apoptosis in all three lines, as indicated by morphology, activation of DEVDase, procaspase-3 cleavage, and the appearance of populations having sub-G(0)/G(1) DNA contents. The differential effects of the MEK inhibitors on cell survival were not dependent on p53 status or effects on the ERK5 pathway. PD184352 was also proapoptotic to primary rat Schwann cells. Hence, although PD184352 effectively killed neurofibrosarcoma cells, its effects on normal Schwann cells may limit its usefulness in the clinic.

Topics: Apoptosis; Benzamides; Blotting, Western; Butadienes; Cell Line, Tumor; Cell Proliferation; Enzyme Inhibitors; Flavonoids; Flow Cytometry; Gene Expression Regulation, Neoplastic; Genes, p53; Genes, ras; Humans; Mitogen-Activated Protein Kinase 7; Mitogen-Activated Protein Kinase Kinases; Neurilemmoma; Neurofibromatosis 1; Nitriles; Phosphorylation; Reverse Transcriptase Polymerase Chain Reaction; Transfection