Bilirubin inhibits lipid raft dependent functions of L1 cell adhesion molecule in rat pup cerebellar granule neurons

BACKGROUND: The mechanism of bilirubin neurotoxicity is poorly understood. We hypothesize that bilirubin inhibits the function of lipid rafts (LR), microdomains of the plasma membrane critical for signal transduction. To test this hypothesis, we measured the effect of free bilirubin (Bf) between 7.6 – 122.5 nM on LR dependent functions of L1 cell adhesion molecule (L1). METHODS: Cerebellar granule neurons (CGN) were plated on poly L-lysine overnight, and neurite length was determined after 1 h treatment with L1 alone or L1 and bilirubin. L1 activation of ERK1/2 was measured in CGN in the presence or absence of bilirubin. The effect of bilirubin on L1 distribution in LR was quantitated, and the localization of bilirubin to LR was determined. RESULTS: The addition of bilirubin to CGN treated with L1 significantly decreased neurite length compared to L1 alone. L1 activation of ERK1/2 was inhibited by bilirubin. Bilirubin redistributed L1 into LR. Bilirubin was associated only with LR containing fractions of a sucrose density gradient. CONCLUSION: Bf significantly inhibits LR-dependent functions of L1 and are found only associated with LR suggesting one mechanism by which bilirubin may exert neurotoxicity is through the dysfunction of protein-LR interactions.