Revisiting the proteolytic processing of cell adhesion molecule L1

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Cell adhesion molecule L1 (L1) is crucial for nervous system function.
• L1 proteolytic fragments are generated by various enzymes, with proposed cleavage sites in fibronectin type III (FNIII) domains.
• Previous studies suggested specific fragments (L1-80, L1-140) arise from cleavage in the third FNIII domain.

Purpose of the Study:

• To precisely identify the L1 cleavage sites responsible for generating specific L1 fragments in the mouse nervous system.
• To clarify the roles of the first and third FNIII domains in L1 proteolysis.
• To reconcile conflicting previous findings regarding L1 fragment generation.

Main Methods:

• Generation of gene-edited mice with mutated L1 cleavage sites in the first or third FNIII domains.
• Immunoprecipitation and immunoblot analyses of brain homogenates from wild-type and mutant mice.
• Mass spectrometry to confirm fragment identification.
• Protease treatment (trypsin, plasmin) of cultured cerebellar neurons followed by immunoblot analysis.

Main Results:

• L1-70 and L1-135 fragments were detected in wild-type mice but significantly reduced or absent in mutant mice lacking functional cleavage sites.
• L1-80 and L1-140 fragments were not detected in either wild-type or mutant mice.
• In vitro experiments showed trypsin and plasmin generated L1-70 and L1-135, not L1-80 and L1-140.

Conclusions:

• L1-70 and L1-135 are the predominant L1 fragments in the mouse nervous system.
• The third FNIII domain is critical for the generation of these predominant L1 fragments.
• Previous discrepancies in L1 fragment identification may stem from differing experimental conditions or interpretations.