The Ameliorative Effects of Choline on Bilirubin-induced Inhibition of L1 Cell Adhesion Molecule

Hyperbilirubinemia in preterm babies occurs with alarming frequency, yet the mechanism of neurotoxicity is still unclear. The L1 cell adhesion molecule (L1) is a lipid raft dependent protein and is critical for the development of the central nervous system. L1 is phosphorylated in lipid rafts before it is trafficked out and subsequently dephosphorylated in the neural outgrowth pathway. Altering the phosphorylation status of L1 would cause a change in the lipodome of affected lipid rafts, and thus impede the development of the affected brain regions. We hypothesize bilirubin will alter the phosphorylation status of L1, as well as the location of L1 in lipid rafts of cerebellar granule neurons (CGN), and that choline will act to reduce the impact of these effects. The %L1 located in lipid rafts was measured as well as the amount of phosphorylated L1 and dephosphorylated L1 in the CGN of six groups. Three groups of rat pups lacked glucuronyl transferase (jj) and therefore are unable to excrete bilirubin, and three groups of rat pups had a 50% reduction in glucuronyl transferase (Nj). Each group was given daily saline (S) or choline (C) treatments. On P5, two Nj groups and two jj groups were treated with sulfodimethoxine (SD) to mimic acute hyperbilirubinemia seen in preterm neonates, and the other groups were treated with saline (S). The %L1, dephosphorylated L1, and phosphorylated L1 for SD-treated jj mice without choline pretreatment was significantly different from all other groups tested. The ex-vivo model of hyperbilirubinemia shows that elevated bilirubin levels significantly affects the relative amount of phosphorylated L1 and dephosphorylated L1 in CGN, as well as the %L1 found in lipid rafts. This model also shows that choline pretreatment ameliorates the effect of bilirubin on L1 signaling.