On the synergy between myelin proteins P0, MBP, and P2 in peripheral nerve major dense line formation

The proper formation and function of the myelin sheath, a proteolipid membrane multilayer, relies on the coordinated action of several key myelin proteins. We studied how proteins from the peripheral myelin cytoplasmic apposition-myelin basic protein (MBP), the cytoplasmic tail of myelin protein zero (P0ct), and peripheral myelin protein 2 (P2)-interact with each other and with myelin-like membranes using various techniques, such as small-angle X-ray diffraction, differential scanning calorimetry (DSC), surface plasmon resonance (SPR), and electron and live epifluorescence microscopy. DSC revealed changes in lipid interactions depending on the protein combination, with altered membrane fluidity and stability. These results were supported by SPR, which indicated that the myelin proteins may compete for membrane surface binding. Analysis of the Bragg peaks induced by the myelin proteins in lipidic environments showed both lamellar and nonlamellar phases in protein-lipid complexes, indicating the formation of nanoscale structures that may be relevant for myelin assembly. Microscopy experiments showed the formation of new membrane structures with each of the proteins separately and together. Our data indicate both synergy and competition between the three main proteins residing in the peripheral nervous system myelin major dense line. The observed direct effects of myelin proteins on lipid membrane structure and properties may be relevant to their function in myelinating cells as well as their role in myelin disorders.