Expression of neurofibromin, the neurofibromatosis 1 gene product: studies in human neuroblastoma cells and rat brain

Genetics of Neurofibromatosis 1-Associated Peripheral Nerve Sheath Tumors

Neurofibromatosis 1, an inherited disorder that affects 1/3500 individuals worldwide, predisposes to the development of benign and malignant peripheral nerve sheath tumors. The disorder results from inactivation of one of the NFI genes. The second NFI gene is typically inactivated in Schwann cells during tumor formation. This article reviews the different types of genetic alterations in NFI in both constitutional and tumor tissues and genetic alterations of other genes that may affect tumorigenesis. These studies have provided insight into the genetic basis of both the variable expression of the disorder and of benign and malignant peripheral nerve sheath tumorigenesis.

Neurofibromin in the brain

Neurofibromatosis 1 is one of the most common autosomal dominant disorders affecting the nervous system. Individuals with neurofibromatosis 1 present with abnormalities of both astrocytes and neurons that result from reduced or absent expression of the NF1 gene product neurofibromin. Impaired neurofibromin function in these nervous system cells contributes to the development of astrocytomas, learning disabilities, and radiographic abnormalities of the brain. With the identification of NF1, significant advances have begun to unlock some of the mysteries that surround the molecular pathogenesis of neurofibromatosis 1-associated brain abnormalities. With continued advances in our basic understanding of NF1 function, future targeted therapies for neurofibromatosis 1-associated central nervous system abnormalities can be developed.

Haploinsufficiency for the neurofibromatosis 1 (NF1) tumor suppressor results in increased astrocyte proliferation

Individuals affected with neurofibromatosis 1 (NF1) harbor increased numbers of GFAP-immunoreactive cerebral astrocytes and develop astrocytomas that can lead to blindness and death. Mice heterozygous for a targeted Nf1 mutation (Nf1+/-) were employed as a model for the human disease to evaluate the hypothesis that reduced NF1 protein (neurofibromin) expression may confer a growth advantage for astrocytes, such that inactivation of only one NF1 allele is sufficient for abnormal astrocyte proliferation. Here, we report that Nf17+/- mice have increased numbers of cerebral astrocytes and increased astrocyte proliferation compared to wild-type littermates. Intriguingly, primary Nf1+/- astrocyte cultures failed to demonstrate a cell-autonomous growth advantage unless they were cocultured with C17 neuronal cells. This C17 neuronal cell-induced Nf1+/- increase in proliferation was blocked by MEK inhibition (PD98059), suggesting a p21-ras-dependent effect. Furthermore, mice heterozygous for a targeted mutation in another GAP molecule, p120-GAP, demonstrated no increases in cerebral astrocyte number. These findings suggest that reduced NF1 expression results in a cell context-dependent increase in astrocyte proliferation that may be sufficient for the development of astrocytic growth abnormalities in patients with NF1.

Region-specific astrogliosis in brains of mice heterozygous for mutations in the neurofibromatosis type 1 (Nf1) tumor suppressor

Brains from human neurofibromatosis type 1 (NF1) patients show increased expression of glial fibrillary acidic protein (GFAP), consistent with activation of astrocytes (M.L. Nordlund, T.A. Rizvi, C.I. Brannan, N. Ratner, Neurofibromin expression and astrogliosis in neurofibromatosis (type 1) brains, J. Neuropathol. Exp. Neurology 54 (1995) 588-600). We analyzed brains from transgenic mice in which the Nf1 gene was targeted by homologous recombination. We show here that, in all heterozygous mice analyzed, there are increased numbers of astrocytes expressing high levels of GFAP in medial regions of the periaqueductal gray and in the nucleus accumbens. More subtle, but significant, changes in the number of GFAP positive astrocytes were observed in the hippocampus in 60% of mutant mice analyzed. Astrocytes with elevated GFAP were present at 1 month, 2 months, 6 months and 12 months after birth. Most brain regions, including the cerebellum, basal ganglia, cerebral cortex, hypothalamus, thalamus, cortical amygdaloid area, and white matter tracts did not show any gliotic changes. No evidence of degenerating neurons was found using de Olmos' cupric silver stain. We conclude that Nf1/nf1 mice provide a model to study astrogliosis associated with neurofibromatosis type 1.

Parallels between tuberous sclerosis complex and neurofibromatosis 1: common threads in the same tapestry

Neurofibromatosis type 1 (NF1) and tuberous sclerosis complex (TSC) represent two neurocutaneous disorders in which affected individuals develop tumors at an increased frequency. Although the clinical manifestations of these disorders are distinctive, the identification of the genes responsible for these disorders has demonstrated remarkable similarities on a molecular level between the NF1 and TSC tumor suppressor gene products. The NF1 and TSC2 gene products are hypothesized to function as growth regulators by modulating the activities of small GTPase molecules. The overlap between the functions of these tumor suppressor genes has yielded important insights into the molecular pathogenesis underlying each of these disorders and suggested possible pharmacological therapies specifically targeted for affected individuals.

GapIII, a new brain-enriched member of the GTPase-activating protein family

Ras GTPase-activating proteins (GAPs) are negative regulators of ras, which controls proliferation and differentiation in many cells. Ras GAPs have been found in a variety of species from yeast to mammals. We describe here a newly identified mammalian GAP, GapIII, which was obtained by differential screening of a rat oligodendrocyte cDNA library. GapIII putatively encodes a 834 amino acid protein with a predicted molecular weight of 96 kDa, which contains a consensus GAP-related domain (GRD). The protein encoded by this cDNA has high homology with Gap1m, which was recently identified as a putative mammalian homolog of Drosophila Gap1. These proteins contain three structural domains, an N-terminal calcium-dependent phospholipid binding domain, GRD, and a C-terminal PH/Btk domain. Because of the sequence homology and the structural similarities of this protein with Gap1m, we hypothesize that GapIII and Gap1m may be members of a mammalian GAP gene family, separate from p120GAP, neurofibromin (NF1), and IQGAP. To confirm the GapIII protein activity, constructs containing different GapIII-GRD domains were transformed into iral mutant yeast to determine their relative ability to replace IRA1 functionally. Constructs that contained essentially the full-length protein (all three domains), the GRD alone, or the GRD plus PH/Btk domain suppressed heat shock sensitivity of ira1, whereas constructs that contained the GRD with part of the PH/Btk domain had only a weak ability to suppress heat shock sensitivity. These results suggest that the GapIII GRD itself is sufficient to down-regulate ras proteins in yeast. Expression of GapIII mRNA (4.2 kb) was examined by Northern analysis and in situ hybridization. This mRNA was expressed at highest levels in the brain, where its expression increased with development. Lower levels of the mRNA were expressed in the spleen and lung. Among neural cells, GapIII mRNA was expressed in neurons and oligodendrocytes, but not in astrocytes. Interestingly, the expression pattern in brain is reminiscent of type 1 NF1 expression reported by Gutmann et al. (Cell Growth Differ in press, 1995). We propose that in addition to p120GAP and neurofibromin, the GapIII/Gap1m family may be important for modulating ras activity in neurons and oligodendrocytes during normal brain development and in particular in the adult brain.

Regulated expression of neurofibromin in migrating neural crest cells of avian embryos

Neurofibromatosis type 1 (NF1) is a common human genetic disease involving various neural crest (NC)-derived cell types, in particular, Schwann cells and melanocytes. The gene responsible for NF1 encodes the protein neurofibromin, which contains a domain with amino acid sequence homology to the ras-guanosine triphosphatase activating protein, suggesting that neurofibromin may play a role in intracellular signaling pathways regulating cellular proliferation or differentiation, or both. To determine whether neurofibromin plays a role in NC cell development, we used antibodies raised against human neurofibromin fusion proteins in western blot and immunocytochemical studies of early avian embryos. These antibodies specifically recognized the 235 kD chicken neurofibromin protein, which was expressed in migrating trunk and cranial NC cells of early embryos (E1.5 to E2), as well as in endothelial and smooth muscle cells of blood vessels and in a subpopulation of non-NC-derived cells in the dermamyotome. At slightly later stages (E3 to E5), neurofibromin immunostaining was observed in various NC derivatives, including dorsal root ganglia and peripheral nerves, as well as non-NC-derived cell types, including heart, skeletal muscle, and kidney. At still later stages (E7 to E9), neurofibromin immunoreactivity was found in almost all tissues in vivo. To determine whether the levels of neurofibromin changed during melanocyte and Schwann cell development, tissue culture experiments were performed. Cultured NC cells were found to express neurofibromin at early time points in culture, but the levels of immunoreactivity decreased as the cells underwent pigmentation. Schwann cells, on the other hand, continued to express neurofibromin in culture. These data suggest, therefore, that neurofibromin may play a role in the development of both NC cells and a variety of non-NC-derived tissues.

Expression of two new protein isoforms of the neurofibromatosis type 1 gene product, neurofibromin, in muscle tissues

The neurofibromatosis type 1 (NF1) gene encodes a tumor suppressor protein, termed neurofibromin, which is expressed predominantly in neurons, Schwann cells, oligodendrocytes, and leukocytes. There are at least three isoforms of neurofibromin produced by the alternative use of exons 23a and 48a. Previously we described the identification of an NF1 mRNA isoform containing an additional 54 nucleotides from exon 48a (type 3 NF1) in human skeletal, cardiac and smooth muscle tissues by reverse-transcribed (RT)-PCR. To extend our initial observations, we have produced high titer chicken IgY antibodies which specifically recognize this muscle-specific neurofibromin isoform. An NF1 cDNA was generated containing human exon 48a sequences and expressed as a fusion protein in bacteria. The muscle-specific neurofibromin antibodies detected this exon 48a fusion protein by Western immunoblotting. Immunoprecipitation using these type 3 neurofibromin antibodies also specifically detected a 250 kDa protein in human and rat muscle tissues. Type 3 neurofibromin was found in rat heart and muscle, but not in liver brain, kidney or spleen with levels of expression declining after postnatal day 7. Expression of total NF1 RNA during rat embryonic development was detected at high levels in E15 heart, tongue, and limb bud. In addition, using type 2 neurofibromin-specific antibodies, the existence of a fourth isoform of neurofibromin (type 4 neurofibromin) containing both exon 23a and 48a sequences was demonstrated in rat heart muscle tissues. The identification of two muscle-specific isoforms of neurofibromin expands our definition of this important tumor suppressor protein and suggests additional roles for neurofibromin in muscle development and differentiation.

Regulated expression of the neurofibromin type I transcript in the developing chicken brain

Neurofibromatosis type 1 (NF-1) is among the most common inherited diseases affecting cells of the central and peripheral nervous systems. A region of the NF-1 gene is similar in sequence to the ras-GTPase activator protein (ras-GAP), and investigations have confirmed that the NF-1 gene product (now known as neurofibromin) stimulates ras-GTPase activity in vitro and in vivo. Neurofibromin modulates the ability of ras proteins to regulate cellular proliferation and/or differentiation, suggesting a possible role in normal development. An alternative form of the neurofibromin transcript with an additional 63-bp exon inserted in the GAP-related domain (GRD) has been described recently. To determine whether differential expression of the two forms of neurofibromin GRD mRNA plays a role in embryonic development, we have isolated and characterized the corresponding chicken cDNA. The predicted amino acid sequence for the inserted exon is identical between chick and human, as are the exon-intron boundaries. RNase protection and RNA-polymerase chain reaction analyses demonstrate that most tissues express predominantly type II mRNA (which contains the insert) throughout embryonic development. In contrast, whereas type II is the major form in the brain early in development, expression of the type I transcript (without the insert) in this tissue increases dramatically at later times. Analysis of primary cultures derived from chick embryo brain indicates that the type I mRNA is enriched in neurons.