A functional association between merlin and HEI10, a cell cycle regulator

Abstract

Merlin and ezrin are homologous proteins with opposite effects on neoplastic growth. Merlin is a tumor suppressor inactivated in the neurofibromatosis 2 disease, whereas upregulated ezrin expression is associated with increased malignancy. Merlin's tumor suppressor mechanism is not known, although participation in cell cycle regulation has been suggested. To characterize merlin's biological activities, we screened for molecules that would interact with merlin but not ezrin. We identified the cyclin Bbinding protein and cell cycle regulator HEI10 as a novel merlin-binding partner. The interaction is mediated by the alpha-helical domain in merlin and the coiled-coil domain in HEI10 and requires conformational opening of merlin. The two proteins show partial subcellular colocalization, which depends on cell cycle stage and cell adhesion. Comparison of Schwann cells and schwannoma cultures demonstrated that the distribution of HEI10 depends on merlin expression. In transfected cells, a constitutively open merlin construct affected HEI10 protein integrity. These results link merlin to the cell cycle control machinery and may help to understand its tumor suppressor function.

HEI10 negatively regulates cell invasion by inhibiting cyclin B/Cdk1 and other promotility proteins

“…However, HEI10 depletion did not affect steadystate levels of Merlin or FAK (Figure 4a and b). This finding was consistent with the prediction of Gronholm et al (2006), which placed HEI10 downstream of Merlin in cell signaling cascades. In contrast, the phosphorylation of FAK on Y 397 , associated with kinase activation (Ruest et al, 2000) and increased cellular migration, was consistently elevated by 4-to 6-fold following HEI10 depletion ( Figure 4b).…”

“…HEI10 directly interacts with Merlin (NF2) (Gronholm et al, 2006). In cells induced to express Merlin, HEI10 moved from the nucleus to the cytoplasm in NF2 À/À cells, HEI10 concentrated in the nucleus.…”

“…This last identification is particularly intriguing, as Merlin, encoded by the neurofibromatosis 2 (NF2) gene, regulates both cell proliferation and migration (Evans et al, 2000;Xiao et al, 2005). Further, the expression of HEI10 and Merlin are interdependent, and the proteins colocalize in some phases of cell cycle, with a subpopulation of HEI10 located at cortical actin at the cell periphery (Gronholm et al, 2006). Together, these studies caused us to hypothesize that HEI10 not only regulates cell cycle, but also influences cell migration and invasion.…”

“…A study of mRNA transcripts elevated in melanoma suggested that HEI10 is highly upregulated, specifically in melanoma metastases (Smith et al, 2004). Gronholm et al (2006) have recently identified HEI10 as a physical interactor with the Merlin tumor-suppressor protein. This last identification is particularly intriguing, as Merlin, encoded by the neurofibromatosis 2 (NF2) gene, regulates both cell proliferation and migration (Evans et al, 2000;Xiao et al, 2005).…”

Neuropathology for the Neuroradiologist: Antoni A and Antoni B Tissue Patterns

“…Although the mechanism by which merlin influences the cell cycle is not fully understood, recent studies describe merlin's interaction with and inhibition of several proteins involved in the cell cycle and mitogenic signal intensity transduction. These proteins include the following: MLK3 (mixed lineage kinase 3), which stabilizes the B-Raf/Raf-1 complex within the mitogenactivated protein kinase pathway and is required for schwannoma tumor cell proliferation 38 ; HEI10, which controls the accumulation of cyclin B, a protein critical to cell cycle control 39 ; and GTPase PIKE-L (phosphatidylinositol 3-kinase [PI3Kinase] enhancer), which promotes cell survival by activating PI3Kinase. 40 Neurofibromas also demonstrate loss of a tumor suppressor gene NF1, located on chromosome 17q12.…”