Axon-oligodendrocyte interactions are vital for the propagation of fast saltatory nerve impulses in the central nervous system (CNS). Axonal signals are involved in the survival and maturation of oligodendrocytes during development but are poorly characterised. Early in Multiple Sclerosis (MS) remyelination of demyelinated axons is
common. However, the hallmark of MS pathology is the demyelinated plaque. Oligodendrocyte precursor cells (OPCs) are present in the adult CNS and can
remyelinate axons. OPCs have been found in chronically demyelinated lesions and
appear to contact axons but not to engage in myelin formation, suggesting that they may not be receiving the correct signals.
Molecules that are expressed on the axon and interact with molecules expressed on the
surface of the pre-myelinating oligodendrocyte are candidates to mediate contactdependent signalling, which may control the late stages of oligodendrocyte differentiation and myelination. These molecules could also have a role in MS pathology. We have investigated the role that neural cell adhesion molecule (NCAM) might play in such axon-oligodendrocyte interactions using an in vitro system. NCAM
has been shown to have a role in neurite outgrowth and neuronal survival. OPCs were isolated from the neonatal rat brain and differentiated in vitro to the O4+ preoligodendrocyte stage. Western blotting and immunofluorescence confirmed the
expression of NCAM. To mimic the effects of axonal NCAM binding to OPCs we used
a small peptide from the FnIII domain of NCAM (FRM) and a full length soluble protein (NCAM-Fc). FRM peptide and NCAM-Fc increased OPC survival, as detected
by propidium iodide staining after four days in culture, to a level comparable to that obtained with the OPC survival factor insulin. NCAM has previously been shown to
increase neuronal survival and process outgrowth in vitro via the FGF receptor. The
FGF receptor inhibitor PD173074 specifically blocked the survival effects of FRM
peptide and NCAM-Fc, leading to a return to control cell survival rates. Selective inhibitors of MAP kinase, Fyn and PI3 kinase demonstrated the involvement of MAP kinase and Fyn in NCAM-mediated OPC survival. This was supported with Western blotting using OPC cell lysates stimulated with FRM petide or NCAM-Fc, which
detected phosphoERK1/2 but not phosphoAkt. In addition, co-culture of OPCs on NCAM-expressing fibroblasts increased process outgrowth after one day in vitro, and this was dependent on signalling via Fyn. However, FRM peptide and NCAM-Fc were not able to replicate this effect, suggesting a requirement for NCAM to be presented in a cellular substratum to induce outgrowth.
We have shown that NCAM mimetics increase OPC survival and that this is dependent on activation of the FGF receptor and downstream signalling. These results suggest that the binding of axonal NCAM may promote OPC survival and radial process outgrowth, enabling initiation of myelination.