Giovanna M. D'Abaco

Glutamate is associated with a higher risk of seizures in patients with gliomas

Objective: To investigate the relationship of glutamate and glutamate transporter expression in human gliomas and surrounding peritumoral brain to the presence of tumor-associated seizures (TAS). Methods: We studied a retrospective (group 1: 190 patients) and then a prospective (group 2: 98 patients) cohort of patients who underwent a craniotomy for a supratentorial glioma. Tumor and peritumor tissue specimens were assayed for glutamate concentration and expression of glial glutamate transporters. Differences between the seizure (TAS) and seizure-free (non-TAS) groups were compared. Results: A total of 42% of patients had TAS, with 95% of seizures first occurring preoperatively. Clinical factors independently associated with risk of TAS were younger age, temporal lobe location, and tumors with oligodendroglial components. Molecular features in tumor specimens associated with TAS were higher glutamate concentrations, reduced EAAT2 expression, and increased system Xc− expression. In group 2, these results were also replicated in the peritumor tissue. Logistic regression analysis identified raised glutamate concentrations in tumor and peritumor tissue, increased expression of peritumor system Xc−, younger age, temporal lobe location, and tumors with oligodendroglial components as independently predictive of preoperative seizures. Conclusion: Relative increased glutamate concentration in gliomas, and altered glutamate transporter expression, are associated with the presence of TAS and may play a mechanistic role in their pathogenesis.

Chondroitin sulphate-modified neuropilin 1 is expressed in human tumour cells and modulates 3D invasion in the U87MG human glioblastoma cell line through a p130Cas-mediated pathway.

Neuropilin 1 (NRP1), a non‐tyrosine kinase receptor for vascular endothelial growth factor and class 3 Semaphorins, is highly expressed in many human tumour cell lines, but its function is poorly understood. Here, we describe the expression of a new chondroitin sulphate‐modified NRP1 (NRP1‐CS) in human tumour cell lines. Expression of a non‐modifiable NRP1 mutant (S612A) in U87MG human glioma cells results in enhanced invasion in three dimensions (3D), whereas wild‐type NRP1 has no effect. Furthermore, the S612A NRP1 cells show a significant increase in p130Cas tyrosine phosphorylation compared with control and wild‐type NRP1 cells. Silencing of p130Cas in S612A NRP1 cells resulted in a loss of increased invasive phenotype. Interestingly, p130Cas silencing does not inhibit basal 3D invasion, but leads to a mesenchymal to amoeboid transition. Biopsies from both low‐ and high‐grade human gliomas show strong expression of NRP1, and little expression of NRP1‐CS. Our data establish distinct roles for NRP1 and NRP1‐CS in modulating a new NRP1‐p130Cas signalling pathway contributing to glioblastoma cell invasion in 3D.

Distinct requirements for the Sprouty domain for functional activity of Spred proteins

Sprouty and Spred {Sprouty-related EVH1 [Ena/VASP (vasodilator-stimulated phosphoprotein) homology 1] domain} proteins have been identified as antagonists of growth factor signalling pathways. We show here that Spred-1 and Spred-2 appear to have distinct mechanisms whereby they induce their effects, as the Sprouty domain of Spred-1 is not required to block MAPK (mitogen-activated protein kinase) activation, while that of Spred-2 is required. Similarly, deletion of the C-terminal Sprouty domain of Spred-1 does not affect cell-cycle progression of G(0)-synchronized cells through to S-phase following growth factor stimulation, while the Sprouty domain is required for Spred-2 function. We also demonstrate that the inhibitory function of Spred proteins is restricted to the Ras/MAPK pathway, that tyrosine phosphorylation is not required for this function, and that the Sprouty domain mediates heterodimer formation of Spred proteins. Growth-factor-mediated activation of the small GTPases, Ras and Rap1, was able to be regulated by Spred-1 and Spred-2, without affecting receptor activation. Taken together, these results highlight the potential for different functional roles of the Sprouty domain within the Spred family of proteins, suggesting that Spred proteins may use different mechanisms to induce inhibition of the MAPK pathway.

Loss of Rb overrides the requirement for ERK activity for cell proliferation.

The Ras GTPase is a critical transducer of mitogenic signals ultimately leading to inactivation of the retinoblastoma (Rb) protein, but the molecular basis underlying Ras-dependent control of cell cycle kinetics remains to a great extent unknown. In an effort to further elucidate the role of Ras activation in cell cycle control, we have studied the role of the downstream Mek-ERK pathway in facilitating exit from the quiescent G0 state and passage through the G1/S transition. We have adopted a genetic approach in combination with U0126, an inhibitor of Mek activation to study the role of Mek in cell cycle progression. Here we report that whereas wild-type (Wt) mouse embryo fibroblasts (MEFs) depend on ERK activation to enter the cell cycle, Rb-deficient (Rb-/-) MEFs have a reduced requirement for ERK signalling. Indeed in the presence of U0126 we found that Rb-null MEFs can exit G0, make the G1/S transition and proliferate. Analysis of Rb-deficient tumour cell lines also revealed a reduced requirement for ERK signalling in asynchronous growth. We discuss the molecular mechanism that may underlie this escape from MAP kinase signalling.

Selective CREB-dependent cyclin expression mediated by the PI3K and MAPK pathways supports glioma cell proliferation

The cyclic-AMP response element binding (CREB) protein has been shown to have a pivotal role in cell survival and cell proliferation. Transgenic rodent models have revealed a role for CREB in higher-order brain functions, such as memory and drug addiction behaviors. CREB overexpression in transgenic animals imparts oncogenic properties on cells in various tissues, and aberrant CREB expression is associated with tumours. It is the central position of CREB, downstream from key developmental and growth signalling pathways, which gives CREB this ability to influence a spectrum of cellular activities, such as cell survival, growth and differentiation, in both normal and cancer cells. We show that CREB is highly expressed and constitutively activated in patient glioma tissue and that this activation closely correlates with tumour grade. The mechanism by which CREB regulates glioblastoma (GBM) tumour cell proliferation involves activities downstream from both the mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K) pathways that then modulate the expression of three key cell cycle factors, cyclin B, D and proliferating cell nuclear antigen (PCNA). Cyclin D1 is highly CREB-dependent, whereas cyclin B1 and PCNA are co-regulated by both CREB-dependent and -independent mechanisms. The precise regulatory network involved appears to differ depending on the tumour-suppressor phosphatase and tensin homolog status of the GBM cells, which in turn allows CREB to regulate the activity of the PI3K itself. Given that CREB sits at the hub of key cancer cell signalling pathways, understanding the role of glioma-specific CREB function may lead to improved novel combinatorial anti-tumour therapies, which can complement existing PI3K-specific drugs undergoing early phase clinical trials.

ADAM22, Expressed in Normal Brain but not in High-Grade Gliomas, Inhibits Cellular Proliferation via the Disintegrin Domain

OBJECTIVE:To study the expression and function of the brain-specific proteinase deficient disintegrins, ADAM11 and ADAM22 (a disintegrin and metalloproteinase). METHODS:Specimens of low- and high-grade gliomas and normal brain were analyzed for ADAM11 and ADAM22 expression using Western blotting. The effects of overexpression of ADAM11 and ADAM22 in glioma cells on growth were analyzed using bromodeoxyuridine incorporation linked to immunocytochemistry. Similarly analyzed were the effects on cell proliferation of bacterially expressed glutathione S-transferase fusion proteins with the disintegrin domain of ADAM11 and ADAM22. RESULTS:ADAM22 is expressed in normal brain and some low-grade gliomas, but not in high-grade gliomas, whereas ADAM11 is expressed in all low- and high-grade gliomas. In vitro, ADAM22 inhibits cellular proliferation of glioma derived astrocytes. The growth inhibition appears to be mediated by interactions between the disintegrin domain of ADAM22 and specific integrins expressed on the cell surface. This growth inhibition can be avoided by over-expression of integrin linked kinase. CONCLUSION:ADAM22, a brain-specific cell surface protein, mediates growth inhibition using an integrin dependent pathway. It is expressed in normal brain but not in high-grade gliomas. A related protein, ADAM11, has only a minor effect on cell growth, and its expression is unchanged in low- and high-grade gliomas.

IDH1 mutation is associated with seizures and protoplasmic subtype in patients with low‐grade gliomas

The isocitrate dehydrogenase 1 (IDH1) R132H mutation is the most common mutation in World Health Organization (WHO) grade II gliomas, reported to be expressed in 70–80%, but only 5–10% of high grade gliomas. Low grade tumors, especially the protoplasmic subtype, have the highest incidence of tumor associated epilepsy (TAE). The IDH1 mutation leads to the accumulation of 2‐hydroxyglutarate (2HG), a metabolite that bears a close structural similarity to glutamate, an excitatory neurotransmitter that has been implicated in the pathogenesis of TAE. We hypothesized that expression of mutated IDH1 may play a role in the pathogenesis of TAE in low grade gliomas.

AF6/s-afadin is a dual residency protein and localizes to a novel subnuclear compartment

The AF6/afadin protein is a component of cell membranes at specialized sites of cell–cell contact. Two main splice variants exist, known as l‐ and s‐afadin, respectively. L‐afadin is widely expressed in cells of epithelial origin, whilst s‐afadin expression is restricted to the brain. Here we demonstrate that the short form of AF6/s‐afadin is a dual residency protein able to localize to the plasma membrane or nucleus whilst the long form of AF6, l‐afadin is unable to localize to the nucleus. AF6/s‐afadin clusters in a distinctive speckled pattern in the nucleus, but is unable to do so when cell cycle progression is inhibited at the G1/S or G2/M checkpoints. The formation of AF6/s‐afadin nuclear bodies is also sensitive to the transcriptional activity of the cell with inhibition of RNA polymerase activity abolishing AF6/s‐afadin nuclear clustering. AF6/s‐afadin nuclear bodies localize to a novel subnuclear compartment, failing to colocalize with other known nuclear bodies. Formation of the AF6/s‐afadin nuclear foci can be regulated by specific growth factor receptor mediated signaling events and by cytoplasmic tyrosine kinases, but does not correlate with tyrosine phosphorylation of AF6/s‐afadin. AF6/s‐afadin is a candidate for mediating control of cellular growth processes by regulated translocation to the nucleus. J. Cell. Physiol. 210: 212–223, 2007.

Efficient ADAM22 surface expression is mediated by phosphorylation-dependent interaction with 14-3-3 protein family members

ADAM22 is one of three catalytically inactive ADAM family members highly expressed in the brain. ADAM22 has numerous splice variants, all with considerable cytoplasmic tails of up to 148 amino acids. ADAM22 can act to inhibit cell proliferation, however, it has been suggested that it also acts as an adhesion protein. We identified three 14-3-3 protein members by a yeast two-hybrid screen and show by co-immunoprecipitation that the cytoplasmic domain of ADAM22 can interact with all six 14-3-3 proteins expressed in the brain. In addition, we show that 14-3-3 proteins interact preferentially with the serine phosphorylated precursor form of ADAM22. ADAM22 has two 14-3-3 protein binding consensus motifs; the first binding site, spanning residues 831-834, was shown to be the most crucial for 14-3-3 binding to occur. The interaction between ADAM22 and 14-3-3 proteins is dependent on phosphorylation of ADAM22, but not of 14-3-3 proteins. ADAM22 point mutants lacking functional 14-3-3 protein binding motifs could no longer accumulate efficiently at the cell surface. Deletion of both 14-3-3 binding sites and newly identified ER retention motifs restored localization of ADAM22 at the cell surface. These results reveal a role for 14-3-3 proteins in targeting ADAM22 to the membrane by masking ER retention signals.

Stargazin and AMPA receptor membrane expression is increased in the somatosensory cortex of Genetic Absence Epilepsy Rats from Strasbourg.

Absence-like seizures in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS) model are believed to arise in hyperexcitable somatosensory cortical neurons, however the cellular basis of this increased excitability remains unknown. We have previously shown that expression of the Transmembrane AMPA receptor Regulatory Protein (TARP), stargazin, is elevated in the somatosensory cortex of GAERS. TARPs are critical regulators of the trafficking and function of AMPA receptors. Here we examine the developmental expression of stargazin and the impact this may have on AMPA receptor trafficking in the GAERS model. We show that elevated stargazin in GAERS is associated with an increase in AMPA receptor proteins, GluA1 and GluA2 in the somatosensory cortex plasma membrane of adult epileptic GAERS. Elevated stargazin expression is not seen in the epileptic WAG/Rij rat, which is a genetically distinct but phenotypically similar rat model also manifesting absence seizures, indicating that the changes seen in GAERS are unlikely to be a secondary consequence of the seizures. In juvenile (6 week old) GAERS, at the age when seizures are just starting to be expressed, there is elevated stargazin mRNA, but not protein expression for stargazin or the AMPA receptor subunits. In neonatal (7 day old) pre-epileptic GAERS there was no alteration in stargazin mRNA expression in any brain region examined. These data demonstrate that stargazin and AMPA receptor membrane targeting is altered in GAERS, potentially contributing to hyperexcitability in somatosensory cortex, with a developmental time course that would suggest a pathophysiological role in the epilepsy phenotype.