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STUDIES on the CONTROL of MYELINOGENESIS IV. Neuronal 0270-6474/82/0209-1252$02.00/O The Journal of Neuroscience Copyright 0 Society for Neuroscience Vol. 2, No. 9, pp. 1252-1266 Printed in U.S.A. September 1982 STUDIES ON THE CONTROL OF MYELINOGENESIS IV. Neuronal Induction of Schwann Cell Myelin-specific Protein Synthesis during Nerve Fiber Regeneration’ M. J. POLITIS,’ N. STERNBERGER,* KATHY EDERLE, AND PETER S. SPENCER Departments of Neuroscience and Pathology, Institute of Neurotoxicology, Albert Einstein College of Medicine, Bronx, New York 10461 and *Center for Brain Research, University of Rochester Medical Center, Rochester, New York 14627 Received June 9,1981; Revised April 2, 1982; Accepted April 5, 1982 Abstract The temporal sequence of axon-Schwann cell interaction during regeneration is examined in cat tibia1 nerves surgically denervated for 8 weeks and, subsequently, coapted to a freshly severed peroneal nerve for 3 weeks. Prior to association with regenerating axons, Schwann cells resident in denervated and reinnervated stumps failed to synthesize proteins co-migrant with PO, PI, and Pz myelin proteins in contrast to normal nerves. Axonal association with Schwann cells stimulated synthesis of amino acid-labeled proteins co-migrating with myelin-specific proteins prior to elabo- ration of myelin lamellae. Radioactivity from these peaks was precipitated by antibodies raised against myelin-specific proteins. The synthesis of P1 and PZ proteins was evident before PO synthesis in reinnervated stumps. Immunocytochemical staining with antibody to PO, PI, Pz, and myelin- associated glycoprotein (MAG) appeared only after myelin lamellae had been formed. These data suggest that Schwann cells: (a) synthesize proteins co-migrant with PI, Pz, PO, and MAG in normal cat nerves; (b) cease detectable manufacture of these proteins after axonal loss; (c) regain their capacity to synthesize these proteins upon re-establishment of axonal association; and (d) during regeneration, express the synthesis of P1 and Pa before that of PO. Earlier studies in this series of papers and from other changes that usually accompany this process (Politis and laboratories established that axons dictate the myelinat- Spencer, 1981). ing behavior of Schwann cells during nerve fiber regen- The model is established by a two-stage surgical ma- eration (reviewed by Spencer, 1979). The mechanism of nipulation of tibia1 and peroneal nerves in the hindlimb regulation and the sequence of biochemical events lead- of the cat. Initially, the tibial nerve is transected to ing to myelinogenesis are not defined, previous papers stimulate the loss of axons and myelin from the distal describing peripheral nerve fiber development and regen- stump and the proliferation of Schwann cells (Spencer et eration having focused on morphological analysis of the al., 1979,198l). After several weeks, viable Schwann cells, isolation, envelopment, and elaboration of myelin lamel- which do not elaborate myelin, populate the nerve stump. lae by the Schwann cell (e.g., Webster et al., 1973; These Schwann cells then are challenged with regener- Schriider, 1975). Some of the many unresolved biochem- ating axons by coapting the tibia1 nerve distal stump to ical questions can now be addressed with the aid of a new the proximal stump of the neighboring peroneal nerve. in vivo model of nerve regeneration which spatially sep- After 3 weeks of axonal regeneration, the temporal se- arates the temporal sequence of axon-Schwann cell in- quence of cellular events associated with axon-Schwann teraction and avoids the reactive and degenerative cell contact and myelinogenesis in the tibia1 nerve stump is separated reproducibly in a proximal-distal gradient divisible into three well defined regions: (a) a proximal- ’ We wish to thank Dr. Steven Cohen for donating antiserum to most, myelinated zone (M; between 0 and 1.5 to 2 cm myelin basic protein and Monica Bischoff for preparation of the figures from the position of peroneal-tibia1 coaptation) contain- and legends. These studies were supported by Grant NS 13106 from the National Institutes of Health. M. J. P. was supported by National ing Schwann cells beginning to elaborate myelin lamellae Institutes of Health Training Grant NS 07098. around some regenerating axons; (b) an intermediate, *To whom correspondence should be addressed at Institute of relatively immature, contact zone (C; between 1.5 to 2 Neurotoxicology, Albert Einstein College of Medicine, 1300 Morris and 6 cm from the site of coaptation) exhibiting Schwann Park Avenue, Bronx, NY 10461. cells enveloping axons prior to the formation of myelin 1252 The Journal of Neuroscience Axonal Regulation of Myelin-specific Protein Synthesis 1253 lamellae; and (c) a distal, noncontact zone (NC), where (proximal myelinated, intermediate contact, and distal axons have yet to advance and Schwann cells appear noncontact) of Schwann cell differentiation (Figs. 1, dormant. A previous detailed description of this model right, and 2 to 4). (a) The first (proximalmost) 2.5-cm demonstrated that the position of separation between segment (M) comprised all of the myelinated zone plus zones C and NC is defined precisely by the distal limit of a small portion of the contact zone. Sampling this length movement of a radioactive label incorporated into the of nerve insured the absence of myelin lamellae in seg- fast axonal transport system of the regenerating axons ment Cl. (b) The second segment (Cl) contained and that the division between zones M and C is dictated Schwann cells at an advanced stage of axonal association by the border between regions with myelin lamellae and prior to the formation of myelin lamellae. (c) The third those without (Politis and Spencer, 1981). segment (C,) included Schwann cells at an early stage of The biochemical properties of the three clearly defined axonal association and, distally, a portion of the proximal zones can be examined and compared by sampling ap- noncontact zone. Sampling this length of nerve insured propriate portions of the reinnervated nerve stump. For the absence of axons in segment NC. (d and e) The example, incubation of tissue from the three zones in fourth and fifth segments (NC and NCZ) contained tritiated thymidine demonstrated a significant and repro- Schwann cells free of regenerating axons (Fig. 2). The ducible increase of thymidine incorporation in the mye- absence of axons from these two segments was demon- linated and contact zones relative to that in the noncon- strated previously by electron microscopy and by the tact zone (Pellegrino et al., 1980). These find.ings dem- disappearance of an axonally transported radiolabel at onstrated that mitotic activity is induced when regener- the interface between zones Ca and NC1 (Politis and ating axons reach Schwann cells. Spencer, 1981). The present study uses this new model of peripheral Control material consisted of 2.5-cm lengths of intact nerve regeneration to characterize Schwann cell protein sciatic or brachial nerves and tibia1 nerve distal stumps synthesis as a function of axon contact and the formation denervated for 10 weeks. of myelin lamellae. Four principal questions are ad- Assessment of protein synthesis. Eleven reinnervated dressed: (a) do Schwann cells which have lost axonal tibia1 nerves were used. Nerve segments a to e from each apposition synthesize myelin proteins? (b) Do regener- of four nerves were incubated in [3H]leucine (300 pC!i, 40 ating axons initiate changes in Schwann cell protein to 60 Ci/mmol; Amersham Corp.), four were incubated synthesis? (c) Can myelin protein synthesis be detected in [3H]fucose (300 PCi, 20 Ci/mmol; Amersham Corp.), before the elaboration of myelin lamellae? (d) In what and three were incubated in [3H]tryptophan (150 pC!i, 27 sequence are specific myelin proteins synthesized? Ci/mmol; Amersham Corp.). The latter amino acid was A preliminary report of this work has appeared else- used because it is not incorporated into histones which where (Politis et al., 1980). migrate in the vicinity of myelin basic proteins (H. Agra- wal, personal communication). To facilitate incorpora- Materials and Methods tion and subsequent analysis, endoneurial tissue was Surgical procedure. A total of 11 cats, each weighing plucked rapidly from the unwanted nerve sheath (peri- 2.5 to 3.5 kg, was used for these experiments. For surgery, neurium plus epineurium) with the aid of watchmaker’s the animals were anesthetized deeply with sodium bar- forceps and a stereoscopic dissecting microscope. Ap- bitone, and bilateral procedures were carried out under proximately 90% of the cells in denervated distal stumps aseptic conditions. are Schwann cells (Spencer et al., 1979). These tissue Complete denervation of the tibia1 nerve was achieved samples were incubated in a shaking water bath at 37°C by transecting the nerve 2 cm below the sciatic notch, for 2 hr in 3 ml of oxygenated (95% 02, 5% COZ) Krebs’ ligating its cut ends with 4/O suture thread, and securing buffer containing 2 mg/ml of glucose and 0.1 mg/ml of both nerve stumps 1 cm apart onto underlying muscle chloramphenicol. The latter was added to inhibit any (Fig. 1, step 1). Morphological examination of the distal mitochondrial protein synthesis in axons or Schwann stumps 7 to 8 weeks post-transection revealed axon- and cells. After incubation was complete, further amino acid myelin-free Schwann cells aligned longitudinally into incorporation was stopped by several washes of cold columns (bands of Biingner), each delimited by a conduit buffer. of basal lamina. Incubated nerve segments were prepared for slab gel Regenerating axons were introduced into tibia1 nerve electrophoresis by homogenization and partial delipida- distal stumps 7 weeks after nerve transection. To pro- tion of the homogenate with 3:2 ether:ethanol. Sodium mote axon penetration, the ligated proximal l- to 1.5-cm dodecyl sulfate-polyacrylamide gel electrophoresis (SDS- portion of the distal nerve stump was removed and PAGE) was performed using a 3% stacking gel and a 12% discarded. The peroneal nerve then was severed and its running gel by a modification of the methods of Laemmli proximal stump was ligated to the tibia1 nerve distal (1970), Greenfield et al.
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