Membrane association of the PTEN tumor suppressor: molecular details of the protein-membrane complex from SPR binding studies and neutron reflection.

The structure and function of the PTEN phosphatase is investigated by studying its membrane affinity and localization on in-plane fluid, thermally disordered synthetic membrane models. The membrane association of the protein depends strongly on membrane composition, where phosphatidylserine (PS) and phosphatidylinositol diphosphate (PI(4,5)P(2)) act pronouncedly synergistic in pulling the enzyme to the membrane surface. The equilibrium dissociation constants for the binding of wild type (wt) PTEN to PS and PI(4,5)P(2) were determined to be K(d)∼12 µM and 0.4 µM, respectively, and K(d)∼50 nM if both lipids are present. Membrane affinities depend critically on membrane fluidity, which suggests multiple binding sites on the protein for PI(4,5)P(2). The PTEN mutations C124S and H93R show binding affinities that deviate strongly from those measured for the wt protein. Both mutants bind PS more strongly than wt PTEN. While C124S PTEN has at least the same affinity to PI(4,5)P(2) and an increased apparent affinity to PI(3,4,5)P(3), due to its lack of catalytic activity, H93R PTEN shows a decreased affinity to PI(4,5)P(2) and no synergy in its binding with PS and PI(4,5)P(2). Neutron reflection measurements show that the PTEN phosphatase "scoots" along the membrane surface (penetration <5 Å) but binds the membrane tightly with its two major domains, the C2 and phosphatase domains, as suggested by the crystal structure. The regulatory C-terminal tail is most likely displaced from the membrane and organized on the far side of the protein, ∼60 Å away from the bilayer surface, in a rather compact structure. The combination of binding studies and neutron reflection allows us to distinguish between PTEN mutant proteins and ultimately may identify the structural features required for membrane binding and activation of PTEN.

Gamma-secretase inhibitors enhance temozolomide treatment of human gliomas by inhibiting neurosphere repopulation and xenograft recurrence.

Malignant gliomas are treated with a combination of surgery, radiation, and temozolomide (TMZ), but these therapies ultimately fail due to tumor recurrence. In glioma cultures, TMZ treatment significantly decreases neurosphere formation; however, a small percentage of cells survive and repopulate the culture. A promising target for glioma therapy is the Notch signaling pathway. Notch activity is upregulated in many gliomas and can be suppressed using gamma-secretase inhibitors (GSI). Using a neurosphere recovery assay and xenograft experiments, we analyzed if the addition of GSIs with TMZ treatment could inhibit repopulation and tumor recurrence. We show that TMZ + GSI treatment decreased neurosphere formation and inhibited neurosphere recovery. This enhancement of TMZ treatment occurred through inhibition of the Notch pathway and depended on the sequence of drug administration. In addition, ex vivo TMZ + GSI treatment of glioma xenografts in immunocompromised mice extended tumor latency and survival, and in vivo TMZ + GSI treatment blocked tumor progression in 50% of mice with preexisting tumors. These data show the importance of the Notch pathway in chemoprotection and repopulation of TMZ-treated gliomas. The addition of GSIs to current treatments is a promising approach to decrease brain tumor recurrence.

A mutant form of PTEN linked to autism.

The tumor suppressor, phosphatase, and tensin homologue deleted on chromosome 10 (PTEN), is a phosphoinositide (PI) phosphatase specific for the 3-position of the inositol ring. PTEN has been implicated in autism for a subset of patients with macrocephaly. Various studies identified patients in this subclass with one normal and one mutated PTEN gene. We characterize the binding, structural properties, activity, and subcellular localization of one of these autism-related mutants, H93R PTEN. Even though this mutation is located at the phosphatase active site, we find that it affects the functions of neighboring domains. H93R PTEN binding to phosphatidylserine-bearing model membranes is 5.6-fold enhanced in comparison to wild-type PTEN. In contrast, we find that binding to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)) model membranes is 2.5-fold decreased for the mutant PTEN in comparison to wild-type PTEN. The structural change previously found for wild-type PTEN upon interaction with PI(4,5)P(2), is absent for H93R PTEN. Consistent with the increased binding to phosphatidylserine, we find enhanced plasma membrane association of PTEN-GFP in U87MG cells. However, this enhanced plasma membrane association does not translate into increased PI(3,4,5)P(3) turnover, since in vivo studies show a reduced activity of the H93R PTEN-GFP mutant. Because the interaction of PI(4,5)P(2) with PTEN's N-terminal domain is diminished by this mutation, we hypothesize that the interaction of PTEN's N-terminal domain with the phosphatase domain is impacted by the H93R mutation, preventing PI(4,5)P(2) from inducing the conformational change that activates phosphatase activity.

Clinically relevant doses of chemotherapy agents reversibly block formation of glioblastoma neurospheres.

Glioblastoma patients have a poor prognosis, even after surgery, radiotherapy, and chemotherapy with temozolomide or 1,3-bis(2-chloroethy)-1-nitrosourea. We developed an in vitro recovery model using neurosphere cultures to analyze the efficacy of chemotherapy treatments, and tested whether glioblastoma neurosphere-initiating cells are resistant. Concentrations of chemotherapy drugs that inhibit neurosphere formation are similar to clinically relevant doses. Some lines underwent a transient cell cycle arrest and a robust recovery of neurosphere formation. These results indicate that glioblastoma neurospheres can regrow after treatment with chemotherapy drugs. This neurosphere recovery assay will facilitate studies of chemo-resistant subpopulations and methods to enhance glioblastoma therapy.

Doublecortin is preferentially expressed in invasive human brain tumors.

Doublecortin (DCX) is required for neuroblastic migration during the development of the cerebral cortex. DCX is a microtubule-associated protein that plays a role in cellular motility. These facts led us to hypothesize that DCX is increased in invasive brain tumors. DCX expression was assessed in 69 paraffin-embedded brain tumors of neuroepithelial origin. In addition, mouse brain sections of the subventricular zone and dentate gyrus were used as positive controls for immunostaining, and specificity of antibody staining was demonstrated by peptide neutralization. DCX was highly expressed in both high-grade invasive tumors (glioblastoma, n=11; anaplastic astrocytoma/oligoastrocytoma, n=7; and medulloblastoma/PNET, n=6) and low-grade invasive tumors (oligodendroglioma, n=3; and astrocytoma/oligoastrocytoma, n=5). However, DCX was less intensely expressed in the circumscribed group of tumors (pilocytic astrocytoma, n=6; ependymoma/subependymoma, n=7; dysembryoplastic neuroepithelial tumor, n=4; ganglioglioma, n=2; meningioma, n=9; and schwannoma, n=9). By the Cochran-Mantel-Haenszel statistical test, the circumscribed group was significantly different from both the high-grade invasive group (P<0.0001) and the low-grade invasive group (P<0.0001). We conclude that DCX is preferentially expressed in invasive brain tumors. In addition, DCX immunostaining was stronger at the margin of the tumor than at the center. For a subset of these tumors, we also detected DCX mRNA and protein by Northern and Western blotting. DCX mRNA and protein was detected in glioma cell lines by Northern blotting, immunofluorescence microscopy and Western blotting. Collectively, the immunohistochemistry, Western blots and Northern blots conclusively demonstrate expression of DCX by human brain tumors.

Nitric oxide-mediated inhibition of Hdm2-p53 binding.

It has become increasingly evident that nitric oxide exerts its effects, in part, by S-nitrosylation of cysteine residues. We tested in vitro whether nitric oxide may indirectly control p53 by S-nitrosylation and inactivation of the p53 negative regulator, Hdm2. Treatment of Hdm2 with a nitric oxide donor inhibits Hdm2-p53 binding, a critical step in Hdm2 regulation of p53. The presence of excess amounts of cysteine or dithiothreitol blocks this inhibition of binding. Moreover, nitric oxide inhibition of Hdm2-p53 binding was found to be reversible. Sulfhydryl sensitivity and reversibility are consistent with nitrosylation. Finally, we have identified a critical cysteine residue that nitric oxide modifies to disrupt Hdm2-p53 binding. This cysteine is proximal to the Hdm2-p53 binding interface and is conserved across species from zebrafish to humans. Mutation of this residue from a cysteine to an alanine does not interfere with binding but rather eliminates the sensitivity of Hdm2 to nitric oxide inactivation.

Novel functional interactions between Trk kinase and p75 neurotrophin receptor in neuroblastoma cells.

To understand the functional interactions between the TrkA and p75 nerve growth factor (NGF) receptors, we stably transfected LAN5 neuroblastoma cells with an expression vector for ET-R, a chimeric receptor with the extracellular domain of the epidermal growth factor receptor (EGFR), and the TrkA transmembrane and intracellular domains. EGF activated the ET-R kinase and induced partial differentiation. NGF, which can bind to endogenous p75, did not induce differentiation but enhanced the EGF-induced response, leading to differentiation of almost all cells. A mutated NGF, 3T-NGF, that binds to TrkA but not to p75 did not synergize with EGF. Enhancement of EGF-induced differentiation required at least nanomolar concentrations of NGF, consistent with the low-affinity p75 binding site. EGF may induce a limited number of neuronal cells because it also enhanced apoptosis. Both NGF and a caspase inhibitor reduced apoptosis and, thereby, enhanced differentiation. NGF seems to enhance survival through the phosphatidylinositol-3 kinase (PI3K) pathway. Consistent with this hypothesis, Akt, a downstream effector of the PI3K pathway, was hyperphosphorylated in the presence of EGF+NGF. These results demonstrate that TrkA kinase initiates differentiation, and p75 enhances differentiation by rescuing differentiating cells from apoptosis via the PI3K pathway. Even though both EGF and NGF are required for differentiation of LAN5/ET-R cells, only NGF is required for survival of the differentiated cells. In the absence of NGF, the cells die by an apoptotic mechanism, involving caspase-3. An anti-p75 antibody blocked the survival effect of NGF. Brain-derived neurotrophic factor also enhanced cell survival, indicating that in differentiated cells, NGF acts through the p75 receptor to prevent apoptosis.