Phospholipid Synthesis in the Squid Giant Axon: Enzymes of Phosphatidylinositol Metabolism

Abstract

We examined the properties of several enzymes of phospholipid metabolism in axoplasm extruded from squid giant axons. The following synthetic enzymes, CDP-diglyceride: inositol transferase (EC 2.7.8.11), ATP:diglyceride phosphotransferase, diglyceride kinase (EC 2.7.2.-), and phosphatidylinositol kinase (EC 2.7.1.67), were all present in axoplasm. Phospholipid exchange proteins, which catalyzed the transfer of phosphatidylinositol and phosphatidylcholine between membrane preparations and unilamellar lipid vesicles, were also found. However, we did not find conditions under which the synthesis of CDP-diglyceride, phosphatidylserine, and phosphatidylinositol-4,5-diphosphate could be measured. Subcellular fractionation by differential centrifugation showed that the axoplasmic inositol transferase and phosphatidylinositol kinase activities were largely "microsomal," while the diglyceride kinase and exchange protein activities were primarily "cytosolic."

Effect of Electrical Stimulation on Phosphoinositide Metabolism in Rat Sciatic Nerve In Vivo

“…Whether this result means that the axons and Schwann cells both have a small portion of the labeled lipid that is responsive to stimulation, or that the active labeled axonal pool is still present and responsive after the 20-h prelabeling period is not known. Of possible relevance is the recent finding that PI kinase is an active enzyme in axoplasm extruded from the squid giant axon (Gould et al, 1983b).…”

Axonal synthesis of phosphatidylcholine is required for normal axonal growth in rat sympathetic neurons.

“…For example, the activities of some enzymes of glycolipid biosynthesis were detected in rat axolemma (De Vries et al, 1983), and the synthesis of small amounts of several phospholipids was reported in extruded squid axoplasm (Tanaka et al, 1987) and in mouse sciatic nerves (Gould, 1976;Gould et al, 1987;Kumari-Siri and Gould, 1980;Padilla and Pope, 1991). Moreover, phosphatidylinositol synthesis was observed in mouse sciatic nerves (Gould, 1976) and squid axons (Gould et al, 1983). In addition, enzymes catalyzing the final steps in the biosynthesis of phosphatidylcholine (PtdCho)~ and phosphatidylethanolamine (PtdEtn) were detected in lysed rat brain synaptosomes (Strosznajder et al, 1979).…”

Evidence that the Major Membrane Lipids, Except Cholesterol, Are Made in Axons of Cultured Rat Sympathetic Neurons

“…The biosynthesis of small amounts of several phospholipids has been reported previously in extruded squid axoplasm (Tanaka et al, 1987) and in mouse sciatic nerves . However, with the exception of the synthesis of phosphatidylinositol (PtdIns) in mouse sciatic nerves (Kumari-Siri and Gould, 1980) and in the giant squid axon (Gould et al, 1983), little evidence exists for the synthesis of phospholipids in pure axons (Ledeen, 1985). Strosznajder et al (1979) have reported that lysed, but not intact, synaptosomes of rat brain contain the activities of the enzymes catalyzing the final steps in the biosynthesis of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn), i.e., the cholineand ethanolaminephosphotrailsferases.…”

“…Highly purified preparations of myelin from rat brain have been shown to possess several phospholipid biosynthetic enzyme activities, including those of ethanolaminephosphotransferase (Wu and Ledeen, 1980), cholinephosphotransferase, and CTP:phosphoethanolamine cytidylyltransferase (Kunishita and Ledeen, 1984). Moreover, the transport of phospholipids from myelin to axons has been demonstrated (Ledeen et al, 1985(Ledeen et al, , 1992Gould et al, 1987;Boiron et al, 1993). Retrograde axonal transport of lipids synthesized in axons also apparently occurs.…”