Paranodal reorganization results in the depletion of transverse bands in the aged central nervous system.
Paranodal axo-glial junctional complexes anchor the myelin sheath to the axon and breakdown of these complexes presumably facilitates demyelination. Myelin deterioration is also prominent in the aging central nervous system (CNS); however, the stability of the paranodal complexes in the aged CNS has not been examined. Here, we show that transverse bands, prominent components of paranodal junctions, are significantly reduced in the aged CNS; however, the number of paired clusters of both myelin and axonal paranodal proteins is not altered. Ultrastructural analyses also reveal that thicker myelin sheaths display a "piling" of paranodal loops, the cytoplasm-containing sacs that demarcate the paranode. Loops involved in piling are observed throughout the paranode and are not limited to loops positioned in either the nodal- or juxtanodal-most regions. Here, we propose that as myelination continues, previously anchored loops lose their transverse bands and recede away from the axolemma. Newly juxtaposed loops then lose their transverse bands, move laterally to fill in the gap left by the receded loops and finally reform their transverse bands. This paranodal reorganization results in conservation of paranodal length, which may be important in maintaining ion channel spacing and axonal function. Furthermore, we propose that transverse band reformation is less efficient in the aged CNS, resulting in the significant reduction of these junctional components. Although demyelination was not observed, we propose that loss of transverse bands facilitates myelin degeneration and may predispose the aged CNS to a poorer prognosis following a secondary insult.
Shepherd
Mark N
MN
Department of Anatomy and Neurobiology, Virginia Commonwealth University, Richmond, VA 23298, USA.
Pomicter
Anthony D
AD
Velazco
Cristine S
CS
Henderson
Scott C
SC
Dupree
Jeffrey L
JL
D000375:Aging / Q000473:pathology*; D000818:Animals; D001369:Axons / Q000473:pathology; D015815:Cell Adhesion Molecules / Q000378:metabolism; D015816:Cell Adhesion Molecules, Neuronal / Q000378:metabolism; D002490:Central Nervous System / Q000378:metabolism / Q000473:pathology* / Q000648:ultrastructure; D058966:Contactins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008854:Microscopy, Electron; D009186:Myelin Sheath / Q000378:metabolism / Q000473:pathology / Q000648:ultrastructure; D009410:Nerve Degeneration / Q000378:metabolism / Q000473:pathology; D009414:Nerve Growth Factors / Q000378:metabolism; D009457:Neuroglia / Q000473:pathology; D024642:Potassium Channels, Voltage-Gated / Q000378:metabolism; D015222:Sodium Channels / Q000378:metabolism
D015815:Cell Adhesion Molecules; D015816:Cell Adhesion Molecules, Neuronal; C491322:Cntnap1 protein, mouse; D058966:Contactins; D009414:Nerve Growth Factors; C504593:Nfasc protein, mouse; D024642:Potassium Channels, Voltage-Gated; D015222:Sodium Channels