Recent work has demonstrated that apo E secretion and accumulation increase in the regenerating peripheral nerve. The fact that apoE, in conjunction with apoA-I and LDL receptors, participates in a well-established lipid transfer system raised the possibility that apoE is also involved in lipid transport in the injured nerve. In the present study of the crushed rat sciatic nerve, a combination of techniques was used to trace the cellular associations of apoE, apoA-I, and the LDL receptor during nerve repair and to determine the distribution of lipid at each stage. After a crush injury, as axons died and Schwann cells reabsorbed myelin, resident and monocyte-derived macrophages produced large quantities of apoE distal to the injury site. As axons regenerated in the first week, their tips contained a high concentration of LDL receptors. After axon regeneration, apoE and apoA-I began to accumulate distal to the injury site and macrophages became increasingly cholesterol-loaded. As remyelination began in the second and third weeks after injury, Schwann cells exhausted their cholesterol stores, then displayed increased LDL receptors. Depletion of macrophage cholesterol stores followed over the next several weeks. During this stage of regeneration, apoE and apoA-I were present in the extracellular matrix as components of cholesterol-rich lipoproteins. Our results demonstrate that the regenerating peripheral nerve possesses the components of a cholesterol transfer mechanism, and the sequence of events suggests that this mechanism supplies the cholesterol required for rapid membrane biogenesis during axon regeneration and remyelination.

A 37-kDa glycoprotein has been described recently, whose synthesis is dramatically increased after injury of the rat sciatic and optic nerves. Cells in the nerve sheath, distal to the site of injury, produce and secrete large amounts of this protein, so that by 3 weeks after injury, it represents 2-5% of the total soluble extracellular protein in the regenerating sciatic nerve sheath, although it fails to accumulate in damaged optic nerve. Results presented here reveal extensive homology between the 37-kDa nerve injury-induced protein and a well-studied serum protein, apolipoprotein E (apoE), that is involved in lipid and cholesterol metabolism and that has been shown recently to be present in adult and developing rat astroglia. Both proteins have identical isoelectric focusing points and similar molecular masses. Antibodies raised against the 37-kDa protein recognize apoE and anti-apoE serum crossreacts with the 37-kDa protein. Sequence data for two 14 amino acid stretches of the 37-kDa protein match identical regions of apoE. These data suggest that the 37-kDa protein is identical to serum apoE and that it could have similar functions to the latter. In the nervous system, for example, it may be involved in the mobilization and reutilization of lipid in the repair, growth, and maintenance of myelin and axonal membranes, both during development and after injury.

Macrophages that rapidly enter injured peripheral nerve synthesize and secrete large quantities of apolipoprotein E. This protein may be involved in the redistribution of lipid, including cholesterol released during degeneration, to the regenerating axons. To test this postulate, apolipoprotein E-associated lipid particles released from segments of injured rat sciatic nerve and apolipoprotein E-containing lipoproteins from plasma were used to determine whether sprouting neurites, specifically their growth cones, possessed lipoprotein receptors. Pheochromocytoma (PC12) cells, which can be stimulated to produce neurites in vitro, were used as a model system. Apolipoprotein E-containing lipid particles and lipoproteins, which had been labeled with fluorescent dye, were internalized by the neurites and their growth cones; the unmetabolized dye appeared to be localized to the lysosomes. The rapid rate of accumulation in the growth cones precludes the possibility of orthograde transport of the fluorescent particles from the PC12 cell bodies. Thus, receptor-mediated lipoprotein uptake is performed by the apolipoprotein B,E(LDL) (low density lipoprotein) receptors, and in the regenerating peripheral nerve apolipoprotein E may deliver lipids to the neurites and their growth cones for membrane biosynthesis.

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