Recombinant peripheral myelin protein Po confers both adhesion and neurite outgrowth‐promoting properties

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Abstract
To probe into the functional properties of the major peripheral myelin cell surface glycoprotein Po, its ability to confer adhesion and neurite outgrowth‐promoting properties was studied in cell culture. To this aim, Po was expressed as integral membrane glyco protein at the surface of CV‐1 cells with the help of a recombinant vaccinia virus expression system. Furthermore, the immunoglobulin‐like extracellular domain of Po (Po‐ED) was expressed as soluble protein in a bacterial expression system and used as substrate coated to plastic dishes or as competitor in cell adhesion and neurite outgrowth‐promoting assays. The adhesion of Po‐expressing CV‐1 cells to Po‐ED substrate was specifically inhibitable by polyclonal Po antibodies (54% ± 6%). In addition, the specific interaction between Po molecules could be reduced (49% ± 8%) by adding soluble Po‐ED to the culture medium, demonstrating that the homophilic interaction between recombinant Po molecules can be mediated, at least on one partner of interacting molecules, by the unglycosylated Ig‐like domain. Substrate‐coated Po‐ED also conferred adhesion and neurite outgrowth ability to dorsal root ganglion neurons with neurites of a mean length of about 150 μm. This neurite outgrowth was specifically inhibitable by soluble Po (74% ± 14%) and Po antibodies (65% ± 9%). These observations indicate that Po is capable of displaying two different types of functional roles in the myelination process of peripheral nerves: The heterophilic interaction with neurons may be responsible for the recognition between axon and myelinating Schwann cell at the onset of myelination, whereas the homophilic interaction may indicate its role in the self‐recognition of the apposing loops of Schwann cell surface membranes during the myelination process and in the mature compact myelin sheath.

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