The spaces between membrane bilayers within PNS myelin as characterized by X-ray diffraction

Area of Science:

• Neuroscience
• Biophysics
• Structural Biology

Background:

• Myelin sheaths insulate nerve fibers in both the peripheral nervous system (PNS) and central nervous system (CNS).
• The structural organization of myelin, particularly the spaces between bilayers, is crucial for its function.

Purpose of the Study:

• To characterize the structural spaces within PNS and CNS myelin using X-ray diffraction.
• To investigate the variability of these spaces across different animal species.
• To propose a molecular mechanism for myelin bilayer organization in the PNS.

Main Methods:

• X-ray diffraction analysis of myelin from a wide range of animal species.
• Detailed structural profiling of swollen rat sciatic nerve myelin.

Main Results:

• Two distinct spaces, 'extracellular' and 'cytoplasmic', were identified in both PNS and CNS myelin.
• The extracellular space exhibited remarkable constancy across species, while the cytoplasmic space showed significant variability.
• Swollen rat sciatic myelin revealed projections from the bilayer into the extracellular space.

Conclusions:

• The structural organization of myelin differs between the PNS and CNS, particularly concerning cytoplasmic space variability.
• Interdigitating Protein Zero (Po) molecules are proposed to maintain the separation of adjacent myelin bilayers within the PNS extracellular space.