Axonal transport of phosphatidylcholine and two synthetic analogs

Sonicated emulsions of egg phosphatidylcholine containing either [(14)C]-dipalmitoyl phosphatidylcholine (diester-PC) or two metabolically inert analogs. [ (14)C]-1- octadecyl -2- hexadecyl -sn- glycero -3- phosphocholine (diether-PC) and [(3)H]-2-tetradecyloctadecano-(1)-phosphocholine (dialkyl-PC) were injected into the vitreous of the eye of adult rabbits. After 1-40 days, radioactivities were measured in the stations of the optical pathway, and the identities of the labelled lipids arrived at the superior colliculus were ascertained by thin-layer chromatography and treatment with phospholipase A(2), with the following results: (1) phosphatidylcholine and its analogs were taken up from the vitreous by the retina at similar rates: (2) all three lipids were transported in the optic nerve axons at similar rates ('fast'). They reached maximal concentration in the superior colliculus some 20 days after injection: (3) phosphatidylcholine travelled from vitreous to superior colliculus as the intact molecule: (4) maximal accumulation of the two analogs in the superior colliculus reached only about 1 per cent of that of phosphatidylcholine. The results suggest that the vehicles of fast axonal transport can pick up intact phospholipid molecules originating in the ganglionic cell plasma membrane (and, likely, from other cellular compartments). The packaging process is promoted by the presence of carboxyl ester groups in the phospholipid; this fact suggests the involvement of ganglionic phospholipid transfer protein with specificity for these groups.