Journal of Neuroscience Research 31:231-244 (1992) z Structure and Expression of Proteolipid Protein in the Peripheral Nervous System J. Kamholz, M. Sessa, S. Scherer, H. Vogelbacker, K. Mokuno, P. Baron, L. Wrabetz, M. Shy, and D. Pleasure Department of Neurology, University of Pennslyvania (J.K., M.Se., S.S., H.V., K.M., P.B., L. W., D.P.), Neurology Research, Children’s Hospital of Philadelphia (K.M., P.B., D.P.), and Department of Neurology, Thomas Jefferson University (M.Sh.), Philadelphia Proteolipid protein (PLP), the major myelin protein in the central nervous system (CNS), is also made by Schwann cells (SC) in the peripheral nervous system (PNS) but is not incorporated into the SC myelin sheath. We analyzed several PLP cDNA clones iso- lated from a rat sciatic nerve cDNA library and found that their coding sequences were identical to PLP cDNAs previously isolated from the CNS. In addi- tion, we have discovered an unusual form of PLP message, present in both brain and sciatic nerve RNA, that is likely formed by alternative splicing within the 3’ untranslated region of the primary PLP transcript. The absence of PLP from the SC myelin sheath thus cannot be explained by an alteration in its amino acid sequence. Steady-state levels of PLP mRNA in SC cultures treated with the cAMP ana- logue dibutyryl cAMP (dBcAMP) were not increased, whereas dBcAMP increased steady-state levels of mRNA encoding the major myelin protein, PO. We have also shown that expression of PLP, unlike that of PO, is regulated in SC in vitro at a posttranscrip- tional level. Finally, the steady-state levels of PO mRNA are much more dramatically reduced than those of PLP mRNA during Wallerian degeneration of the peripheral nerve. Thus PLP expression in the PNS is probably controlled by different molecular mechanisms from PO, and may not be part of the coordinate program of myelin gene expression. In contrast to its expression in the PNS, transcription of PLP in the CNS is coordinately regulated along with the other myelin protein genes, suggesting there may be differences in the cis-acting elements and trans- acting factors involved in the regulation of PLP tran- scription in SC and oligodendrocytes (OC). Consis- tent with this notion, we have found that most PLP transcripts are initiated at the more proximal of two start sites in the PNS, while in the CNS proportion- ally more PLP transcripts are initiated from the distal start site. We propose that the proximal site, utilized predominantly in SC, is responsible for maintenance expression of PLP and is not inducible, while the di- stal site is responsible for the rapid, inducible in- crease of PLP message during brain development. Key words: PLP, Schwann cells, PNS INTRODUCTION Myelin is a multilamellar structure that ensheaths axons in the central and peripheral nervous systems (CNS and PNS). In the CNS, myelin is synthesized by oligodendrocytes (OC) (Wood and Bunge, 1984), while, in the PNS, it is synthesized by Schwann cells (SC) (Asbury, 1975). The myelin sheath of these two cell types is morphologically similar, although each contains a unique subset of myelin structural proteins (Braun, 1984). In CNS myelin, myelin basic protein (MBP) and proteolipid protein (PLP) make up the bulk of the struc- tural protein; CNS myelin also contains cyclic nucleotide phosphohydrolase (CNP) and myelin associated glyco- protein (MAG). In PNS myelin, PO, a transmembrane glycoprotein, is the major structural protein; PNS myelin also contains MBP, MAG, and CNP, as well as P2, a basic protein like MBP, and P170, a large-molecular- weight glycoprotein (Shuman et al., 1986). CNS and PNS myelin thus share a number of structural proteins, including MBP and MAG. PO protein, however, is found only in PNS myelin, while PLP is found only in CNS myelin. In contrast to its role in CNS myelin, PLP in the PNS is not incorporated into the myelin sheath (Puckett et al., 1987). PLP immunoreactivity can be localized to SC but has not been detected in the SC plasma membrane (On0 et al., 1990). To extend these findings, we identified and se- Received April 19, 1991; revised July 8, 1991; accepted July 17, 1991. Address reprint requests to John Kamholz MD, PhD, Department of Neurology, Hospital of the University of Pennslyvania, Philadelphia, PA 19104. zyxwvu 0 1992 Wiley-Liss, Inc.