A signaling-focused session chaired by Vijaya Ramesh (Harvard Medical School/Massachusetts General Hospital) and Frank McCormick (University of California, San Francisco) spanned talks on NF2/merlin functions, Schwann cell biology and NF1/Ras signaling. The session opened with Brendan Manning (Harvard School of Public Health) giving an overview of the complexity of mTOR signaling. There is significant convergence of many tumor suppressors on the mTORC1 pathway including NF1, NF2, PTEN, LKB1, TSC1 and TSC2. However, the downstream consequences of aberrant mTORC1 activation and therapeutic response of these different tumor suppressor syndromes remain poorly understood. The Manning laboratory focuses on the cellular and molecular effects of uncontrolled mTORC1 activity in TSC, via a combination of TSC gene disruption and rapamycin treatment to activate and inhibit mTORC1 respectively. From this system, his group has identified and characterized a set of transcripts that are strictly regulated by mTORC1 signaling. They have also shown that aberrant mTORC1 signaling can trigger adaptive stress response pathways that dampen the deleterious effects of metabolic changes and uncontrolled protein synthesis. The approaches taken by Dr. Manning’s laboratory offer novel points of therapeutic intervention and may provide potential targets of cytotoxic agents to selectively kill tumor cells exhibiting elevated mTORC1 activity, a molecular defect common to NF1, NF2 and TSC.
Chunling Yi (The Wistar Institute), a postdoctoral fellow in the group of Joseph Kissil, has purified a novel protein complex composed of merlin and a number of tight junction associated proteins. A direct interaction between merlin and the tight junction proteins as well as co-localization of the proteins to both tight junctions and adherens junctions were presented. One of the interacting proteins was shown to function downstream of merlin as a positive regulator of Rac-Pak and MAPK signaling. This study elegantly demonstrated a novel merlin-mediated signaling route at the site of cell:cell junctions, which may contribute to the tumor suppressive function of merlin.
Marlan Hansen (University of Iowa) presented data suggesting that ErbB2 and c-Jun N-terminal kinase (JNK) signaling contribute to vestibular schwannoma growth and radiosensitivity. ErbB2, a receptor tyrosine kinase essential for Schwann cell development and proliferation resides in lipid rafts and is active in vestibular schwannoma (VS) cells. In addition, MEK/ERK, PI3K/AKT and JNK signaling are active in VS cells, which is independent of ErbB2 signaling. Inhibition of JNK resulted in VS cell apoptosis and an increase in radiosensitivity.
Betty Chow (Fox Chase Cancer Center), a postdoctoral fellow in the group of Jonathan Chernoff, showed that a peptide inhibitor of Pak1, Pak2 and Pak3 restored normal morphology and slowed the growth of cells transformed with a dominant mutant allele of merlin. Xenografts of cells expressing the peptide inhibitor showed markedly reduced growth, when compared with cells expressing an inactive mutant form of this inhibitor. The growth of the xenograft tumors did not correlate with the level of ERK activation, indicating that the effects of Pak on cell proliferation and tumor growth are not mediated by ERK.
Pablo Hollstein (Harvard Medical School) discussed the molecular mechanisms by which neurofibromin is degraded during normal cell signaling. He reported identification of a E3 ubiquitin ligase that is specifically required for neurofibromin degradation. Depletion of this ubiquitin ligase specifically attenuates Ras signaling, resulting in impaired cellular proliferation. Dr Hollstein suggested that understanding the mechanisms that regulate neurofibromin degradation may provide the opportunity for designing or applying effective therapies aimed at increasing neurofibromin protein stability, a strategy that may be most useful in manifestations related to haploinsufficiency at the NF1 locus, which has been shown to contribute to some symptoms of the disease. Regulation of neurofibromin degradation might also be a useful strategy in other conditions driven by hyperactive Ras and cell proliferation.
Frank McCormick discussed therapeutic approaches to the Ras pathway, with special emphasis of the issue of “oncogene addiction”. According to this concept, cancer cells often become increasingly dependent on driver oncogenes for their survival, and therefore respond dramatically when these drivers are blocked through therapeutic intervention. In the case of NF1, Shannon, Lauchle and colleagues (submitted for publication) showed that early stages of myeloid disease associated with NF1 are not sensitive to intervention, using MEK inhibitors, but later stages that progress to AML, become sensitive. This suggests that drugs blocking hyperactive Ras signaling may have little effect on normal tissue in NF1 patients, but could selectively inhibit advanced disease. Dr McCormick also discussed intrinsic cellular pathways, involving ephrin signaling and sprouty-like proteins that regulate normal Ras and could, potentially, be a source of new drug targets to treat this disease.
Karen Cichowski (Harvard Medical School/Brigham and Women’s Hospital) discussed the use of mouse models to optimize treatment protocols for treating NF1. While mTOR inhibitors are currently being tested in the clinic, it is likely that sequential or combination therapies may ultimately be more effective. The Cichowski group has been delineating mechanisms of NF1 pathogenesis and investigating the biology of therapeutic response in these models. Responses in vitro have, so far, not been predictive of efficacy in vivo, stressing the importance of advanced mouse models to properly mimic disease and therefore predict response. In glioblastoma, loss of neurofibromin occurs through deletion of the gene and through increased turnover and degradation, a process initiated by PKC. These insights may be useful in identifying new therapeutic approaches to NF1.
Jan Manent (INSERM, France) presented data evaluating the transcriptome of Nf2-deficient primary mouse Schwann cells (SCs) and tumors from mice and patients. In mouse SCs, loss of Nf2 led to the activation of a molecular program reminiscent of myelination via the PI3K/Akt pathway. Mouse schwannoma transcription profiles share striking similarity to developing immature SCs and comparison of mouse and human schwannoma gene expression revealed common transcriptional signature. The data presented suggested that NF2 protein plays a role in axon-SC crosstalk and that loss of this specific function may be relevant to schwannoma formation.
Steven Scherer (University of Pennsylvania) provided an overview on junctional specialization of Schwann cells. Myelinating Schwann cells have gap, tight and adherens junctions that are composed of connexin32, claudin-19 and E-cadherin respectively. In addition, they have specialized contacts with the axons at the nodes of Ranvier and the flanking paranodal region mediated by a different set of cell adhesion molecules. Cultured normal Schwann cells are joined by adherens junctions comprised of N-cadherin. Beta-catenin is associated with the adherens junctions of myelinating, non-myelinating and cultured normal Schwann cells. In contrast, NF2-deficient Schwannoma cells revealed poorly formed adherens junctions and an irregular distribution of N-cadherin and Beta-catenin, which has been implicated in tumorigenesis.
Andrea McClatchey (Harvard Medical School/Massachusetts General Hospital) presented on behalf of her predoctoral fellow Zachary Morris and showed how merlin controls EGFR distribution and signaling. Employing single particle tracking of individual EGFR molecules, it was shown that merlin immobilizes EGFR at the cell surface in confluent mouse-derived cells. Furthermore, merlin appears to govern the mobility and distribution of EGFR within the plasma membrane in a contact-independent manner. The efficacy of several EGFR inhibitors was tested in preclinical in vitro studies and pilot study results indicated that the compounds were effective in blocking proliferation and signaling in Nf2−/− mouse cells.
Ueli Suter (ETH Zürich, Switzerland) closed the session with a focus on Rho-GTPases in Schwann cells. The role of integrin-linked kinase (ILK) in Schwann cell migration, proliferation and morphological changes associated with the sorting, ensheathment and myelination of axons was reviewed as well as the roles of RhoGTPase and AKT signaling in signal transduction in Schwann cell development.