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Myelin-associated glycoprotein: electron microscopic immunocytochemical localization in compact developing and adult
Abstract:
Light microscopic immunocytochemical studies have shown that myelin-associated glycoprotein (MAG) is localized in myelin of the developing CNS; but in the adult, MAG appears to be restricted to periaxonal regions of myelinated fibers. To extend these observations, we embedded optic nerves of 15-day-old rats, adult rats, and an adult human in epon after aldehyde and osmium tetroxide fixation. After 5% H2O2 pretreatment, thin sections were immunostained with 1:250-1:5,000 rabbit antiserum to rat CNS MAG according to the avidin-biotin-peroxidase complex (ABC) method. Dense deposits of reaction product covered compact myelin in both developing and adult optic nerves. When we used 1:500, 1:1,000, and 1:2,000 anti-MAG, less intense immunostaining of myelin was found. We also obtained the same localization in compact myelin with the peroxidase-antiperoxidase (PAP) method. With 1:250 anti-MAG, dense deposits of reaction product were not observed on axolemmal membranes or on oligodendroglial membranes located periaxonally and paranodally. In thin sections of adult human optic nerve, anti-MAG also stained compact myelin intensely. When thin sections of rat and human optic nerves were treated with preimmune or absorbed serum, no immunostaining was observed. Immunoblot tests showed that our MAG antisera did not react with any non-MAG myelin proteins. In contrast with earlier light microscopic data, this study shows that MAG localization does not change during CNS development; both developing and adult compact myelin sheaths contain MAG. As many biochemical studies also show that MAG is present in compact myelin, we suggest that this 100,000 dalton glycoprotein now be called myelin glycoprotein (MGP) instead of MAG.
Insights
Myelin-associated glycoprotein (MAG) is present in compact myelin throughout central nervous system development in both rats and humans. This finding challenges previous research and suggests renaming MAG to myelin glycoprotein (MGP).
Area of Science:
- Neuroscience
- Cell Biology
- Immunocytochemistry
Background:
- Previous light microscopy suggested myelin-associated glycoprotein (MAG) localization shifts from myelin in developing CNS to periaxonal regions in adults.
- This shift implies developmental changes in MAG's role and location within myelinated fibers.
Purpose of the Study:
- To clarify the precise localization of MAG in the central nervous system (CNS) during development and in adulthood.
- To determine if MAG's location changes from developing to adult stages using electron microscopy and immunocytochemistry.
Main Methods:
- Optic nerves from developing and adult rats, and adult humans were fixed and embedded.
- Thin sections underwent immunocytochemical staining using antibodies against MAG (avidin-biotin-peroxidase complex and peroxidase-antiperoxidase methods).
- Immunoblot tests confirmed antibody specificity for MAG.
Main Results:
- Dense reaction product indicated MAG presence in compact myelin of both developing and adult rat optic nerves.
- Intense staining of compact myelin was also observed in adult human optic nerves.
- No significant MAG staining was detected on axolemmal or oligodendroglial membranes.
Conclusions:
- MAG is consistently localized within compact myelin sheaths throughout CNS development in both species studied.
- This contradicts earlier light microscopic findings suggesting a developmental shift in MAG localization.
- The study proposes renaming myelin-associated glycoprotein (MAG) to myelin glycoprotein (MGP) due to its consistent presence in compact myelin.