Degraded myelin‐associated glycoprotein (dMAG) formation from pure human brain myelin‐associated glycoprotein (MAG) is not mediated by calpain or cathepsin L‐like activities

• Published in: Journal of neurochemistry 2003-02, Vol.84 (3), p.533-545
• Main Authors: Päiväläinen, Satu, Suokas, Marko, Lahti, Outi, Heape, Anthony M.
• Format: Article
• Language: English
• Subjects: Aged; Aged, 80 and over; Biological and medical sciences; Brain Chemistry; Buffers; Calcium - chemistry; Calpain - chemistry; Calpain - metabolism; Cathepsin L; Cathepsins - chemistry; Cathepsins - metabolism; cell adhesion molecule; Chelating Agents - chemistry; CNS; Cysteine Endopeptidases; cysteine proteases; Electrophoresis, Polyacrylamide Gel; Fundamental and applied biological sciences. Psychology; glial cells; Humans; Hydrogen-Ion Concentration; Isolated neuron and nerve. Neuroglia; Middle Aged; myelin; Myelin-Associated Glycoprotein - chemistry; Myelin-Associated Glycoprotein - metabolism; Protease Inhibitors - chemistry; proteolysis; Temperature; Vertebrates: nervous system and sense organs
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1046/j.1471-4159.2003.01539.x

The myelin‐associated glycoprotein (MAG) is a transmembrane cell adhesion molecule participating in myelin formation and maintenance. Calcium‐activated/‐dependent proteolysis of myelin‐associated glycoprotein by calpain and cathepsin L‐like activities has already been detected in purified myelin fractions, producing a soluble fragment, called degraded (d)MAG, characterized by the loss of the transmembrane and cytoplasmic domains. Here, we demonstrate and analyze dMAG formation from pure human brain myelin‐associated glycoprotein. The activity never exhibited the high rate previously reported in human myelin fractions. Degradation is time‐, temperature‐, buffer‐ and structure‐dependent, is inhibited at 4°C and by denaturation of the sample, and is mediated by a trans‐acting factor. There is no strict pH dependency of the proteolysis. Degradation was inhibited by excess aprotinin, but not by 1–10 µg/mL aprotinin and was not eliminated by the use of an aprotinin‐sepharose matrix during the purification. dMAG formation was not enhanced by calcium, nor inhibited by a wide variety of protease inhibitors, including specific calpain and cathepsin L inhibitors. Therefore, while cysteine proteases may be present in human myelin membrane fractions, they are not involved in dMAG formation from highly purified human brain myelin‐associated glycoprotein preparations.