DEFINITION
Merlin(protein) (also called Neurofibromin 2 or schwannomin) is a cytoskeletal protein that in humans is a tumor suppressor protein involved in Neurofibromatosis type II. Sequence data reveal its similarity to the ERM protein family. The name "merlin" is an acronym for "Moesin-Ezrin-Radixin-Like Protein". Merlin is a 70-kDa protein. There are 10 known isoforms of human merlin molecule (the full molecule being 595amino acids in length).
THE GENE
Merlin gene is located in Chr 22q12.2. This gene is expressed at high levels during embryonic development; in adults, significant expression is found in Schwann cells, meningeal cells, lens and nerve. Mutations in this gene are associated with neurofibromatosis type II which is characterized by nervous system and skin tumors and ocular abnormalities.
CHEMICAL STRUCTURE AND IMAGES
Protein Aminoacids Percentage
SYNTHESIS AND TURNOVER
Merlin gene encodes a protein that is similar to some members of the ERM (Ezrin, Radixin, Moesin) family of proteins that are thought to link cytoskeletal components with proteins in the cell membrane. This gene product has been shown to interact with cell-surface proteins, proteins involved in cytoskeletal dynamics and proteins involved in regulating ion transport.
Two predominant isoforms and a number of minor isoforms are produced by alternatively spliced
transcripts.
Post translational modification: Phosphorylation of Ser-518 inhibits nuclear localization by disrupting the intramolecular association of the FERM domain with the C-terminal tail. Ubiquitinated by the CUL4A-RBX1- DDB1-DCAF1/VprBP E3 ubiquitin-protein ligase complex for ubiquitination and subsequent proteasome- dependent degradation.
CELLULAR FUNCTIONS
Merlin gene encodes a protein that is similar to some members of the ERM (Ezrin, Radixin, Moesin) family of proteins that are thought to link cytoskeletal components with proteins in the cell membrane. This gene product has been shown to interact with cell-surface proteins, proteins involved in cytoskeletal dynamics and proteins involved in regulating ion transport.
Merlin interacts with other ERM proteins and with components of cell-cell adherens junctions (AJs). Merlin stabilizes the links of AJs to the actin cytoskeleton. Paradoxically, the "closed" conformation of merlin- 1, where its N-terminal Four-point-one, Ezrin, Radixin, Moesin (FERM) domain binds to its C-terminal tail domain, directs its tumor suppressor functions.
DIAGNOSTIC USE
Merlin (Moesin-Ezrin-Radixin Like Protein) is the product of neurofibromatosis type 2 (
NF2) gene. It serves as a linker between transmembrane proteins and the actin-cytoskeleton. Mutations and deletions of merlin cause
Neurofibromatosis type 2 (NF2) familial cancer syndrome, a dominantly inherited autosomal disease characterized by the development of
NF2-associated tumors including schwannomas, meningiomas and ependymomas in the central and peripheral nervous system. Mutations of the
NF2 gene have also been found in other cancers, suggesting that merlin regulates a variety of cancer types. The
NF2 gene is located on human chromosome 22q12 and alterations of the gene have been detected in the germline of
NF2 patients and in sporadic
NF2-associated tumors. It has been well established that mutations and deletions of the
NF2 gene lead to development of
NF2-associated tumors and that loss of heterozygosity (LOH) of the gene is associated with sporadic schwannomas, ependymomas, and meningiomas. The
NF2 gene mutations have also been found in
thyroid cancer, mesotheliomas, and
melanoma albeit less frequently. Genetic analysis of
NF2 demonstrated that loss of merlin is embryonic lethal both in mouse and fly, which implies broad roles of merlin during key stages of embryonic development. Furthermore, the heterozygous merlin knockout mice (NF2+/-) develop metastatic osteosarcomas, fibrosarcomas, and hepatocellular carcinomas. Nearly all of these tumors have lost their wild type
NF2 allele, suggesting that merlin may serve as a tumor suppressor in a wider spectrum of cells and that loss of merlin function may play an important role in tumor growth and progression
Merlin, a magic linker between extracellular cues and intracellular signaling pathways that regulate cell motility,proliferation, and survival, 2010
(Figure 1
Merlin: the wizard requires protein stability to function as a tumor suppressor. Biochim Biophys Acta. 2012)
Figure 1
MERLIN STRUCTURE AND FUNCTION
Increasing amount of evidence indicated that merlin regulates the functions and activities of cell surface receptor tyrosine kinases (RTKs) and adhesion/extracellular matrix (ECM) receptors and serves as a key regulator of several important signaling pathways that regulate cell motility, proliferation, and survival in various cancer metabolisms. Phosphorylation of merlin at its COOH terminus especially at Ser518 abolishes the head-to-tail self-association and leads to an “open” conformation and loss of tumor-suppressor activity. Several kinases including p21-activated kinase 1 and 2 (PAK1/2) and cAMP-dependent protein kinase A (PKA) phosphorylate merlin at Ser518, which leads to the open inactive conformation. Merlin inhibits PAK1 activity through a feedback loop. Loss of merlin results in increased PAK1 activity. In addition to PAK1, AKT phosphorylates merlin at Thr230 and Ser315, which promotes merlin degradation by proteasome. Conversely, myosin phosphatase MYPT1–PP1 dephosphorylates merlin at Ser518, which results in merlin activation.
Studies have shown that merlin binds to numerous transmembrane and intracellular proteins including the hyaluronic acid (HA) receptor, CD44. These merlin binding partners are likely play important roles in exerting the effects of merlin on the signaling pathways mediated by RTKs, adhesion and extracellular matrix (ECM) receptors, PI3K/AKT/mTOR, and small GTPases. Merlin serves as a linker between the plasma membrane and the actin cytoskeleton and regulates cytoskeleton remodeling, cell motility, and cell proliferation in response to the extracellular signals. Tumors developed in the heterozygous NF2 knockout mice (NF2+/-) are highly spreading and display metastatic potential. As a linker between transmembrane proteins and the actin-cytoskeleton, merlin is uniquely positioned to regulate cell proliferation in response to the signals derived from their microenvironment. Studies have demonstrated that merlin is dephosphorylated and activated in confluent cells and that merlin accumulates and stabilizes at the adherens junctions in keratinocytes and fibroblasts. Furthermore, merlin mediates contact inhibition of cell growth through its interaction with CD44 or by blocking recruitment of Rac to the plasma membrane. Accordingly, loss of merlin destabilizes the cadherin-containing cell-cell junctions, leading to loss of the contact inhibition and increased Rac activity, lamellipodia formation, and increased cell motility. The inhibition of the CD44-hyaluronan interaction by merlin contributes to the tumor suppressor activity of merlin and CD44 functions upstream of merlin (Figure 2).
Merlin, a magic linker between extracellular cues and intracellular signaling pathways that regulate cell motility,proliferation, and survival, 2010
Figure 2
MERLIN AND THE HIPPO SIGNALLING PATHWAY
The Hippo signaling pathway plays an essential role in regulating organ size, development and differentiation, and tissue regeneration by restricting cell growth, regulating cell division, and promoting apoptosis and recent results placed merlin upstream of the Hippo signaling pathway. Other than merlin the molecules included in this pathway are MST1/2, salvador homolog 1 (SAV1), MOB1 and LATS1/2. These four proteins are often referred to as the core components of the Hippo pathway. After receiving upstream signaling, MST1/2 kinase, which makes a complex with a scaffold protein SAV1, phosphorylates and activates LATS1/2. LATS1/2, which is activated by another scaffold protein MOB1, phosphorylates and inactivates YAP, a transcriptional coactivator. YAP itself does not bind DNA, but activates transcription factors to which it binds, including p53, RUNX, and TEAD family members. Overexpression of YAP leads to loss of contact inhibition and anchorage-independent growth to EMT. As a transcriptional co-activator, YAP promotes proliferation and inhibits apoptosis by up-regulating expression of cyclin E and cIAP1/2, respectively (Figure 3). Inactivation of Merlin in malignant mesothelioma cells and the Hippo signaling cascade dysregulation, 2011
Figure 3
MERLIN AND RECEPTOR TYROSINE KINASES
Recent studies have demonstrated the important contribution of merlin in regulating the distribution, aggregation, and availability of several cell surface receptors, especially receptor tyrosine kinases (RTKs), in the plasma membrane. This leads to accumulation of these receptors on the cell surface and corresponding activation of downstream signaling such as Ras-ERK, JNK, Rac, Pak, and FAK that leads to increased cell proliferation. Similar results have been obtained in mammalian cells with respect to EGFR. However, merlin seems to block the internalization of ligand-bound EGFR, an event that is necessary for the EGF-EGFR-mediated signaling, and sequester EGFR into a non-signaling membrane compartment, which results in reduced cell proliferation. Recent studies also demonstrated that merlin regulates glial cell growth in ErbB2- and Src dependent manner and that merlin inhibits schwannoma cell proliferation by promoting platelet-derived growth factor receptor (PDGFR) degradation. Together, these results suggest an important role of merlin in regulating the activity and availability of several RTKs that play essential roles in tumor initiation and progression. Merlin, a magic linker between extracellular cues and intracellular signaling pathways that regulate cell motility,proliferation, and survival, 2010
Furthermore merlin has many intracellular interacting partners, which can be classified into several major signaling pathways: Hippo, PI3K/AKT/mTOR, RTKs, and small GTPases. Pharmacological inhibitors of some of these signaling pathways are currently available, and it will be interesting to investigate their efficacy in the mouse models and patients bearing tumors with dysregulated merlin signaling pathways (Figure 4). Inactivation of Merlin in malignant mesothelioma cells and the Hippo signaling cascade dysregulation, 2011
Figure 4
MERLIN IN MALIGNANT PLEURAL MESOTHELIOMA
Malignant pleural mesothelioma (MPM) is an aggressive neoplasm that arises primarily from the surface serosal cells of the pleural cavities. In most patients with MPM, a clinically overt tumor is diagnosed in the 30–40 years after exposure to asbestos, indicating a long latency for tumor development.
Pathologically, the epithelial type accounts for 60%, sarcomatous type for 20%, and the biphasic type with both components ranges around 20%. Since patients with MPM are usually diagnosed at advanced stages and MPM is largely unresponsive to conventional therapy, the prognosis for patients with MPM is very poor. The median survival of patients is 9 to 12 months after diagnosis, regardless of the advancement in chemotherapeutical modalities combining cisplatin and antifolates such as pemetrexed or raltitrexed. Since MPM is a relatively rare malignancy and early preneoplastic lesions are difficult to identify clinically, understanding of molecular pathogenesis including sequential accumulation of genetic/epigenetic alterations for MPM development has lagged behind other common malignancies.
A clear link has been established between asbestos exposure and MPM development. There are two subgroups of asbestos: (i) the amphiboles, a group of rod-like fibers including amosite (brown asbestos), crocidolite (blue asbestos), anthophyllite, actinolite, and tremolite; and (ii) the serpentine group, consisting of chrysotile (white asbestos). The association between amphibole asbestos exposure and MPM development is well accepted. In particular, crocidolite is considered to be the most oncogenic type of asbestos. Chrysotile is the most common type of asbestos, accounting for 90% of the world’s asbestos production. It is still controversial whether chrysotile causes MPM. It remains unclear whether asbestos fibers act directly on mesothelial cells or indirectly cause mesothelioma. There are several possible mechanisms as to how asbestos fibers cause MPM. First, long and thin asbestos fibers can be inhaled deeply into the lungs and can penetrate them. Asbestos fibers repeatedly scratch the mesothelial surface and cause prolonged cycles of damage, repair, and local inflammation, thus inducing pleural irritation. Second, asbestos fibers can also physically interfere with the mitotic process of the cell cycle via disrupting the mitotic spindle, which may result in chromosomal structural abnormalities and aneuploidy of mesothelial cells. Third, asbestos fibers generate highly reactive oxygen species (ROS) and reactive nitrogen species (RNS), which lead to DNA damage and strand breaks. Finally, asbestos can induce cytokines and growth factors in exposed mesothelial cells and nearby macrophages. In addition, crocidolite asbestos was shown to induce autophosphorylation of EGFR in rat mesothelial cells. Genomic abnormalities and signal transduction dysregulation in malignant mesothelioma cells. 2010
MERLIN AND HIPPO SIGNALING IN MPM
Nearly half of MPM tumors harbor a NF2 inactivating mutation as an acquired somatic event. Among genetic alterations in MPM, the NF2 gene the second most frequently mutated, which indicates that the inactivation of merlin is a key event that could lead major susceptibility to asbestos exposure thus activating MPM development and progression. However, it has long been an enigma why not all MPM have the NF2 mutation. In this regard, an insufficient level of a tumor-suppressing form of merlin due to suppression of expression, alternative splicing, or phosphorylation has been proposed as a reasonable alternative mechanism to inactivate tumor-suppressing merlin in MPM cells. Since the functions of Merlin have not been determined clearly, the surrogate molecules for NF2 inactivation in MPM cells, if any, remain unclear. However, recent studies demonstrate that several components of the Hippo signaling cascade, such as LATS2, are also genetically deleted or mutated in MPM cells, indicating that the Hippo signaling cascade is indeed one of the most important downstream cascades of merlin to be inactivated for the development or progression of MPM cells. Furthermore, the dysregulation of Merlin-Hippo signaling enhanced YAP phosphorylation and translocation from the nucleus to the cytoplasm in MPM cells, thus inactivating the transcriptional coactivator activity of YAP, indicating that merlin is an upstream regulator of YAP in MPM cells. Finally, one MPM cell line was also shown to have a homozygous deletion at SAV1. Since a homozygous deletion was also reported in renal cell cancer, the Hippo pathway can be inactivated with a SAV1 alteration, even though it seems to be a rare event in human malignancies.
Genomic abnormalities and signal transduction dysregulation in malignant mesothelioma cells. 2010
MERLIN AND mTOR SIGNALING IN MPM
The mTOR signaling pathway plays a major role in the regulation of protein translation, cell growth and metabolism. Two multiprotein mTOR complexes (mTORC), mTORC1 and mTORC2, exist and mTORC1 regulates eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and S6 kinase 1 (S6K1) Regarding mesothelioma, rapamycin, an mTORC1 inhibitor, showed enhanced cell death with cisplatin on MPM cell lines. In patients with malignant peritoneal mesothelioma, the activation of both phosphatidylinositol 3-kinase (PI3K) and mTOR signaling pathways was shown to be associated with a shortened survival. Merlin has also been shown to be a novel negative regulator of mTORC1. The NF2 gene is inactivated by nonsense mutations or missense mutations as well as homozygous deletion in 40–50% of MPM. Merlin-negative, but not Merlin-positive, MPM cells were shown to display unregulated mTORC1 signaling, including phosphorylation of 4E-BP1 and S6K1. As expected, Merlin negative MPM cells showed the much enhanced growth inhibition effect of rapamycin compared to Merlin-positive cells. Thus, mTORC1 inhibitors seemed to be more effective for MM cells with the NF2 mutation. Inactivation of Merlin in malignant mesothelioma cells and the Hippo signaling cascade dysregulation, 2011
CONCLUSIONS
Merlin is a multifunctional protein that is involved in integrating and regulating extracellular cues and intracellular signaling pathways that control cell fate, shape, motility, proliferation, and survival. MPM is an aggressive malignancy caused by multiple genetic and epigenetic changes. Molecular biological studies have been determining the underlying key events responsible for the development of MPM, some of which may be directly caused by asbestos fibers. Nearly half of MM tumors harbor a NF2 inactivating mutation as an acquired somatic event. Thus, new diagnostic and therapeutic tools against this highly aggressive malignancy could be developed based on more extensive and detailed studies on the Merlin-Hippo signaling cascade in future. Genomic abnormalities and signal transduction dysregulation in malignant mesothelioma cells. 2010
