Joiirnul zyxwvutsrqponm of zyxwvutsrqponmlkji Neurochemislry zyxwvutsrqponm Raven Press, Ltd., New zyxwvutsrqpon York (0 I993 International Society for Neurochemistry Rapid Communication Nerve Regeneration Occurs in the Absence of Apolipoprotein E in Mice *?$Brian Popko, *§Jeffry F. Goodrum, *§Thomas W. Bouldin, §Sunny H. Zhang, and $§Nobuyo Maeda *Brain and Development Research Center, Departments of ?Biochemistry and Biophysics and of §Pathology, and $Program in Molecular Biology and Biotechnology, University zyxwvut cf North Carolina at Chapel Hill, Chapel Hill, North Carolina, U.S.A. Abstract: The concentration of apolipoprotein E (apoE), a high-af- finity ligand for the low-density lipoprotein receptor, increases dra- matically in peripheral nerve following injury. This endoneurial apoE is thought to play an important role in the redistribution of lipids from the degenerating axonal and myelin membranes to the regenerating axons and myelin sheaths. The importance ofapoE in nerve repair was examined using mutant mice that lack apoE. We show that at 2 and zyxwvutsrqp 4 weeks following sciatic nerve crush, regenerat- ing nerves in apoE-deficient mice were morphologically similar to regenerating nerves in control animals, indicating that apoE is not essential for peripheral nerve repair. Moreover, cholesterol synthe- sis was reduced in regenerating nerves of apoE-deficient mice as much as in regenerating nerves of control animals. These results suggest that the intraneuralconservation and reutilization ofcholes- terol following nerve injury do not require apoE. Key Words: Apo- lipoprotein E-deficient mice-Gene targeting-3-Hydroxy-3- methylglutaryl-CoA reductase-Wallerian degeneration-Choles- terol. Popko B. et al. Nerve regeneration occurs in the absence of apolipoprotein E in mice. J. Neurochem. 60, 1 155-1 158 (1993). Axons of the PNS are distinguished from their counter- parts in the CNS by their much greater ability to regenerate and reinnervate successfully their targets after axonal in- terruption (Fawcett and Keynes, 1990). Following nerve in- jury and the consequent degeneration of the axon and its Schwann cell-derived myelin sheath distal to the site ofdam- age (Wallerian degeneration), the axon proximal to the in- jury site reacts by elongating and growing back into the denervated distal nerve. The regenerating axon is myelin- ated by proliferated Schwann cells in the distal nerve. The environment in the nerve distal to the injury site is believed to be critical for successful nerve regeneration. Hematogenously derived macrophages infiltrate the nerve during Wallerian degeneration and are thought to be responsible for clearing the axonal and myelin debris (Beuche and Friede, 1984; Perry et al., 1987). The lipids (especially cholesterol) present in the nerve before injury are reutilized in the regenerating axonal and myelin mem- branes (Rawlins et al., 1970, 1972; Goodrum, 199 1). Apoli- poprotein E (apoE), a 37-kDa glycoprotein that is synthe- sized and secreted by the infiltrating macrophages, is be- lieved to play a key role in this local reutilization of lipids following nerve injury (Ignatius et al., 1987a; Mahley, 1988). ApoE is a structural component of most lipoprotein particles and serves as a high-affinity ligand for the low-den- sity lipoprotein (LDL) receptor. During Wallerian degener- ation, the concentration of apoE increases -250-fold (Skene and Shooter, 1983; Muller et al., 1985, 1986; Snipes et al., 1986; Ignatius et al., 1986; Boyles et al., 1990). Ele- vated concentrations of apoE are also found in experimen- tal models of demyelinating neuropathy and in both axonal and demyelinating human neuropathies (Gelman et al., 1991). Current evidence suggests that this apoE binds the cholesterol derived from the catabolism of degenerating axons and myelin and that these cholesterol-apoE com- plexes are stored in endoneurial macrophages after being taken up by LDL receptors on the surface of these cells (Boyles et al., 1989; Goodrum, 199 1). During nerve regen- eration, the growth cones of the regenerating axons and the Schwann cells myelinating the regenerating axons express elevated levels of the LDL receptor (Ignatius et al., 19876; Boyles et al., 1989; Rothe and Muller, 1991). These recep- tors are thought to mediate the uptake of apoE-lipid com- plexes donated by the endoneurial macrophages. Recently, mutant mice have been generated, using the gene targeting approach in embryonic stem cells, that are specifically and completely deficient in the expression of apoE (Piedrahita et al., 1992; Zhang et al., 1992). These mice have approximately fivefold higher than normal levels of serum cholesterol and generally survive well into adult- hood. The availability of apoE-deficient mice allows us to Resubmitted manuscript received November 24, 1992;accepted December 3, 1992. Address correspondence and reprint requests to Dr. B. Popko at Brain and Development Research Center, CB 7250, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599, USA. Abbreviations used. apoE, apolipoprotein E; LDL, low-density lipoprotein; HMG-CoA, zyxw 3-hydroxy-3-methylglutaryl-CoA. zyx 1155