Concepto de Valor

Yeast two-hybrid, biochemical and co-localisation experiments show that Willin is able to bind neurofascin 155. Previous work in this laboratory has shown that Ezrin and neurofascin 155 interact (Gunn-Moore et al., 2006), and work is

ongoing to determine whether Merlin interacts with neurofascin 155. In addition, it is known that Merlin and Ezrin interact (Nguyen et al., 2001), and theDrosophila

knowledge that all of these proteins are found in Schwann cells, with all but (thus far) Willin confirmed to be at the paranode (Scherer et al., 2001; Tait et al., 2000), leads to the tantalising idea of a regulatory signalling complex guiding Schwann cell

differentiation, growth and myelination activity. This possibility is bolstered by what is known of Drosophila homologues of these proteins and the parallel nature of the invertebrate septate junction (SJ) and the paranodal septate-like junction (PSJ).

As discussed in sections 4.3 and 4.4, Willin has been observed to slow growth and cause cell death when overexpressed in cell lines. This correlates with the

function of its proposedDrosophilahomologue, expanded, which regulates growth and apoptosis in disc development (Boedigheimer and Laughon, 1993; Blaumueller and Mlodzik, 2000). The L1 family also has aDrosophilahomologue, neuroglian (Hortsch, 1996), and the ERM proteins are represented only by Moesin (McCartney and Fehon, 1996). All are present at epithelial membranes, but from there the situation becomes murkier.

The literature is unclear about the exact locations of these proteins. Luque and Milán (2007) refer to neuroglian as being a component of adherens junctions, while others place it at the septate junctions (Genova and Fehon, 2003). Expanded, meanwhile, is said to be ‘at or very near’ adherens junctions (Boedigheimer et al., 1997); interestingly, a later paper states simply that expanded is found at adherens junctions (Blaumueller and Mlodzik, 2000), apparently based on this reference. Hamaratoglu et al. (2006) mention Merlin and expanded together as being at adherens junctions in Drosophila epithelium, and three references are cited for this. However, the first paper was concerned with mammalian cells, not Drosophila (Lallemand et al., 2003); the second (McCartney et al., 2000) refers to an older publication that stated ‘at leastpart[emphasis added] of the detected DMerlin … protein was associated

with the adherens junction’, but the data was not shown (McCartney and Fehon, 1996); and the third, refering to expanded, stated ‘Ex protein is close to or in adherens junctions’ (Boedigheimer et al., 1997). Other papers are more cautious and state that Merlin and/or expanded are adjacent to adherens junctions without stating whether they are apical or basal to them (Edgar, 2006) or avoid specificity altogether, referring only to ‘apical junctions’ as the localisation of Merlin and expanded (Maitra et al., 2006). Another FERM containing protein, coracle, is found at septate junctions, and appears to be required for SJ integrity as well as embryonic and larval development (Ward et al., 2001).

It is apparent that much clarification is required in this field. However, some information can be gleaned from the chaos. In Drosophila, neuroglian, moesin, merlin, expanded and another FERM protein, coracle, are all found at signalling junctions of the epithelium, potentially at structures that are homologous to

mammalian paranodes, where neurofascin 155, Ezrin, and Merlin are found. Willin has also been seen in Schwann cells (Figure 4.6), and thus may also turn out to

localise at the paranode. It is not yet known what role the FERM proteins may play at the paranode, but neurofascin is required for its formation and proper recruitment of many of its proteins (Sherman et al., 2005). It is also known that merlin defects can lead to abnormal myelination (Giovannini et al., 2000) and to dedifferentiation of Schwann cells from a myelinating stage to a growth stage (Hung et al., 2002). Of course, merlin defects are also the chief cause of Schwann cell tumours (Giovannini et al., 2000). Clearly, proliferative and developmental processes are at work, and the nature of these proteins makes it likely that they are being put together in a molecular architecture that allows for the complex signalling required of such a tightly-regulated system as myelination. One model for explaining the potential interactions and

physiological functions of this complex arrangement is shown in Figure 6.1 and is as follows: in development of the peripheral nervous system, neurofascin 155 and Ezrin interact to promote growth and axoglial contact. Merlin and Willin then join the complex, perhaps by one binding to Ezrin to oppose its action, while the other binds to neurofascin 155 to stabilise the paranode. A loss of Merlin leads to a loss of compact myelin and, possibly by the continued action of Ezrin, Schwann cells develop into schwannomas. The action of Willin alone may be enough to slow, but not prevent, proliferation, leading to the known slow growth phenotype of these tumours (Propp et al., 2006). It is certain that this is far too simplistic an explanation; it does not explain how Caspr, a known interactant of merlin (Denisenko-Nehrbass et al., 2003) and neurofascin (Charles et al., 2002), nor how the various potential

signalling pathways known to affect and be affected by the ERMs, merlin and neurofascin are involved. However, it does provide a basic set of interactions for future study, able to be done in eitherDrosophilaor mammalian systems, which may lead to greater insights into this complex field.

Figure 6.1. A proposed mechanism for FERM protein action in paranode development. During Schwann cell growth and proliferation, Ezrin is bound to neurofascin to promote growth. During formation and stabilisation of the paranode, Merlin binds to Ezrin to stop this action and willin binds neurofascin to stabilise the paranode. When Merlin is lost, Willin can still bind neurofascin and control

proliferation to some extent, but Ezrin stays bound, leading to misformation of the paranode, abnormal myelination and slow but unceasing growth.