All in the family: using inherited cancer syndromes to understand de-regulated cell signaling in brain tumors

The cell signaling pathways that are tightly regulated during development are often co-opted by cancer cells to allow them to escape from the constraints that normally limit cell growth and cell movement. In this regard, de-regulated signaling in cancer cells confers a number of key tumor-associated properties, including increased cell proliferation, decreased cell death, and increased cell motility. The identification of some of these critical signaling pathways in the nervous system has come from studies of inherited cancer syndromes in which affected individuals develop brain tumors. The study of brain tumors arising in patients with neurofibromatosis 1 (NF1), neurofibromatosis 2 (NF2), and tuberous sclerosis complex (TSC) has already uncovered several key intracellular signaling pathways important for modulating brain tumor growth. An in-depth analysis of these intracellular signaling pathways will not only lead to an improved understanding of the process of brain tumorigenesis, but may also provide important molecular targets for future therapeutic drug design.     

Akt-dependent cell size regulation by the adhesion molecule on glia occurs independently of phosphatidylinositol 3-kinase and Rheb signaling

The role of cell adhesion molecules in mediating interactions with neighboring cells and the extracellular matrix has long been appreciated. More recently, these molecules have been shown to modulate intracellular signal transduction cascades critical for cell growth and proliferation. Expression of adhesion molecule on glia (AMOG) is downregulated in human and mouse gliomas, suggesting that AMOG may be important for growth regulation in the brain. In this report, we examined the role of AMOG expression on cell growth and intracellular signal transduction. We show that AMOG does not negatively regulate cell growth in vitro or in vivo. Instead, expression of AMOG in AMOG-deficient cells results in a dramatic increase in cell size associated with protein kinase B/Akt hyperactivation, which occurs independent of phosphatidylinositol 3-kinase activation. AMOG-mediated Akt phosphorylation specifically activates the mTOR/p70S6 kinase pathway previously implicated in cell size regulation, but it does not depend on tuberous sclerosis complex/Ras homolog enriched in brain (Rheb) signaling. These data support a novel role for a glial adhesion molecule in cell size regulation through selective activation of the Akt/mTOR/S6K signal transduction pathway.     

Inactivation of NF1 in CNS causes increased glial progenitor proliferation and optic glioma formation

The gene responsible for neurofibromatosis type 1 (NF1) encodes a tumor suppressor that functions as a negative regulator of the Ras proto-oncogene. Individuals with germline mutations in NF1 are predisposed to the development of benign and malignant tumors of the peripheral and central nervous system (CNS). Children with this disease suffer a high incidence of optic gliomas, a benign but potentially debilitating tumor of the optic nerve; and an increased incidence of malignant astrocytoma, reactive astrogliosis and intellectual deficits. In the present study, we have sought insight into the molecular and cellular basis of NF1-associated CNS pathologies. We show that mice genetically engineered to lack NF1 in CNS exhibit a variety of defects in glial cells. Primary among these is a developmental defect resulting in global reactive astrogliosis in the adult brain and increased proliferation of glial progenitor cells leading to enlarged optic nerves. As a consequence, all of the mutant optic nerves develop hyperplastic lesions, some of which progress to optic pathway gliomas. These data point to hyperproliferative glial progenitors as the source of the optic tumors and provide a genetic model for NF1-associated astrogliosis and optic glioma.     

Role of class A penicillin-binding proteins in PBP5-mediated beta-lactam resistance in Enterococcus faecalis

Peptidoglycan polymerization complexes contain multimodular penicillin-binding proteins (PBP) of classes A and B that associate a conserved C-terminal transpeptidase module to an N-terminal glycosyltransferase or morphogenesis module, respectively. In Enterococcus faecalis, class B PBP5 mediates intrinsic resistance to the cephalosporin class of beta-lactam antibiotics, such as ceftriaxone. To identify the glycosyltransferase partner(s) of PBP5, combinations of deletions were introduced in all three class A PBP genes of E. faecalis JH2-2 (ponA, pbpF, and pbpZ). Among mutants with single or double deletions, only JH2-2 DeltaponA DeltapbpF was susceptible to ceftriaxone. Ceftriaxone resistance was restored by heterologous expression of pbpF from Enterococcus faecium but not by mgt encoding the monofunctional glycosyltransferase of Staphylococcus aureus. Thus, PBP5 partners essential for peptidoglycan polymerization in the presence of beta-lactams formed a subset of the class A PBPs of E. faecalis, and heterospecific complementation was observed with an ortholog from E. faecium. Site-directed mutagenesis of pbpF confirmed that the catalytic serine residue of the transpeptidase module was not required for resistance. None of the three class A PBP genes was essential for viability, although deletion of the three genes led to an increase in the generation time and to a decrease in peptidoglycan cross-linking. As the E. faecalis chromosome does not contain any additional glycosyltransferase-related genes, these observations indicate that glycan chain polymerization in the triple mutant is performed by a novel type of glycosyltransferase. The latter enzyme was not inhibited by moenomycin, since deletion of the three class A PBP genes led to high-level resistance to this glycosyltransferase inhibitor.     

Neurofibromin regulation of ERK signaling modulates GABA release and learning

We uncovered a role for ERK signaling in GABA release, long-term potentiation (LTP), and learning, and show that disruption of this mechanism accounts for the learning deficits in a mouse model for learning disabilities in neurofibromatosis type I (NF1). Our results demonstrate that neurofibromin modulates ERK/synapsin I-dependent GABA release, which in turn modulates hippocampal LTP and learning. An Nf1 heterozygous null mutation, which results in enhanced ERK and synapsin I phosphorylation, increased GABA release in the hippocampus, and this was reversed by pharmacological downregulation of ERK signaling. Importantly, the learning deficits associated with the Nf1 mutation were rescued by a subthreshold dose of a GABA(A) antagonist. Accordingly, Cre deletions of Nf1 showed that only those deletions involving inhibitory neurons caused hippocampal inhibition, LTP, and learning abnormalities. Importantly, our results also revealed lasting increases in GABA release triggered by learning, indicating that the mechanisms uncovered here are of general importance for learning.     

Astrocyte-specific neurofibromatosis 1 gene deletion is insufficient for astrocytoma formation in mice

To gain insight into the role of the NF1 (Neurofibromatosis type 1) gene during neural development and in tumorigenesis, we have utilized the bacteriophage P1, Cre/loxP system to generate a conditional allele at the NF1 locus (NF1 flox) that permits temporal and spatial ablation of function through Cre-mediated recombination. We have been using these mice to assess the scope of NF1 requirement in distinct cell types. At the center of this approach is to identify the cells that give origin to the tumors most frequently found in NF1 patients: neurofibromas, neurofibrosarcomas, and astrocytomas. We have hypothesized that specific stem cells must lose NF1 by LOH to begin this process. I will discuss the consequences of NF1 loss in neurons, Schwann cells, and neural precursors. Distinct tumor phenotypes appear in each case. In malignant tumors, our mouse models indicate that the p53 pathway must also become mutated to cooperate with loss of NF1. Additionally, we have genetic evidence that the haploin-sufficient state is essential for tumor appearance. These data suggest that profilactic therapies preceding tumor appearance should be considered for NF1. Acknowledgements:  Funded by NINDS, NNFF, and DOD.     

Konstruktion und Selektion: Argumente gegen einen morphologisch verkürzten Selektionismus

Zusammenfassung Es wird aufgezeigt, da\ der aufDarwin selbst zurückgehende eingeengte Gebrauch des Selektionskonzeptes auf die Beziehung zwischen Organismus und Umwelt die biologische Konstruktion der vorphylogenetischen idealistischen Morphologie überlassen hat. Ein zutreffendes VerstÄndnis des Evolutionsmechanismus begreift aber die biologische Apparatur ein, entzieht der Morphologie, soweit sie sich als Vorbedingung phylogenetischen Arbeitens versteht, den Boden.

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Summary It is argued that the narrow concept of selection used since the time ofDarwin is only concerned with the relation between organism and its environment (niche). It did not include the organism itself, which was the object of the pre-phylogenetic idealistic morphology. An adequate understanding of the mechanism of evolution dispenses with the concept of morphology as the basis of phylogeny.

     

Tumor suppressor Tsc1 is a new Hsp90 co‐chaperone that facilitates folding of kinase and non‐kinase clients

The tumor suppressors Tsc1 and Tsc2 form the tuberous sclerosis complex (TSC), a regulator of mTOR activity. Tsc1 stabilizes Tsc2; however, the precise mechanism involved remains elusive. The molecular chaperone heat‐shock protein 90 (Hsp90) is an essential component of the cellular homeostatic machinery in eukaryotes. Here, we show that Tsc1 is a new co‐chaperone for Hsp90 that inhibits its ATPase activity. The C‐terminal domain of Tsc1 (998–1,164 aa) forms a homodimer and binds to both protomers of the Hsp90 middle domain. This ensures inhibition of both subunits of the Hsp90 dimer and prevents the activating co‐chaperone Aha1 from binding the middle domain of Hsp90. Conversely, phosphorylation of Aha1‐Y223 increases its affinity for Hsp90 and displaces Tsc1, thereby providing a mechanism for equilibrium between binding of these two co‐chaperones to Hsp90. Our findings establish an active role for Tsc1 as a facilitator of Hsp90‐mediated folding of kinase and non‐kinase clients—including Tsc2—thereby preventing their ubiquitination and proteasomal degradation.