P0-glycoprotein of myelin: Orientation of its extracellular domain in homophilic adhesion

153 P0-GLYCOPROTEIN OF MYELIN: ORIENTATION OF ITS EXTRACELLULAR DOMAIN IN HOMOPHILIC ADHESION D.A. Kirschner, R. A. Saavedra and H. Inouye Children's Hospital end Harvard Medical School, Boston, MA 02115 U.S,A. The major protein of peripheral nervous sys'.em(PNS) myelin is P0, a trmmnemlxan~*glycoprotein, which accounts for over 50% of the myelin protein. Suuctural and biochemical studies of normal end shiverer mutant mice led to the hypothesis that PO is responsible for membr~meadhesion and compaction of the myelin lamellae. The extracellular domain of P0 (P0-ED) is similar to the variable immunoglobulin-like domain, and carries one accepter sequence for N-linked glyensylation. Recent experiments on PO-trausfectedcells support the notion that PO is involved in membrme-membrane adhesion. Current schematics that depict PO-POinteraction ignore the width of the inter-mernbr~nespace that must accommodate the extracellulm"domains of PO molecules. In this work. we consider the constraints imposed by X-ray diff~ractiondata from lamellar myelin on the conformatiun of PO-ED. and we propose how this immunoglobulin-like domain could be accommodated in the extracallular space between myelin membranes. The modeling makes use of our previous finding that I~-slrandpl'ediotions for PO-ED are virtually superimpusable with those o the VH domain of the phosphorylclmline-binding immunoglobulin M603 of mouse, which has a similar nmnher of residues as PO-EDand whose strucun'e has been solved crystallographically. Thus. the dimensions of PO-ED are about 36 x 30 x 30 A. X-ray diffzaction analysis of PNS myelin has demonstrated reversible ~ransitio.-~sthat depend on pH ,ee~lionic s~rength, and result in thre distinct swactures characterized by pe,io~ of 165 A. 180 A (native) and 220 A. The widths of the inter-membrane spaces at the ext, scellular apposition in these structures are: 36 A, 50 A and 90 A. respectively. These spaces would most readily accommodate P0-ED fxom spposed membranes ff the molecules were oricnted so that the 13-strandswere approxhnately perpendicular to the membran~ surface. Appose~ P0-EDs would be fully \1~1~1 overlapping at closest apposition of membranes (panelA , . below) partially ove~lappingin the native state (pane/ B), ~ ~ ~ e L / ~ ~ and end-to-end in the incompletely swollen state (panel -u-~-~ff--~J~,~,,3-- - ~ ef,~t~ C). Otis"model explaL'lShow the single imraunoglobulin- P O - ~ ~ ~i~t~ ~s-'~ llke domain of PO can account for distinct structural statcs "C,~_ -C~-(~" ~-~" ~ ~l~/ of myelin membrane p~klng by homol~ili¢ interactions. / ~ / ~ F~ /~ ~ (Supported by NIH-NINDS 20824 end National Multiple A "~ ~it '~ ~ Sclerosis Society) c

N-CAM IN CSF AND SERUM OF MULTIPLE SCLEROSIS (MS) PATIENTS: A CANDIDATE MARKER OF REMYELINATION A.R.Massaro, G.Carbone, R.P.Ciofli, P.Tonali. Iustimte of Neurology of the "Universit~ del Sacro Cuore". 00168 Roma. Italy; Fax: 6-3385411. Bath and Silberberg showed that oligodend~ocyteadhesion molecule is related to N-CAM of molecular weight of 120000 and detected a role for it in phenomena of myelination and remyelinatlon. Further, they demonslrated that in dysmyelinating mouse mv~nt~ different molecular forms of N-CAM are reduced: N-CAM-120 in Jimpy, N-CAM-180 in quaking, beth -120 and -180 in Shiverer mousas. Previously we showed that CSF levels of N-CAM increase in patients with MS following an scute exacerbation, and parallel the cfinical improvement of the patients. This prompted us to study in depth CSF and blood levels of N-CAM in order to define the origin of the increasing levels we detected, i.e., ¢arebr~*v.~.hemati¢, and whether a temporal relationship exists between N-CAM levels and clinical parameters we monitored. We followed longitudinally 16 patients shortly after an -',cuteepisode of exacerbation, monitoring in the 30 days of follow-up the clinical coudifions, serum N-CAM levels every other day. and CSF N-CAM levels every 8 days. All patients, submitted to steroid ¢eatment, showed marked clinical improvement. The time cot~se of N-CAM levels resembled closely the incrae~e we had t~viously shown. Such inerease, starting with CSF levels not different from normal conurols,reached in the followig two weeks significantly higher values. On the contrary, patients in the stationary phase, similarly ueated and monitored, did not show any significant clinical improvement nor CSF N-CAM increase Serum N-CAM levels, which are usually higher than CSF levels, showed an appare,-itlyrandom time course and were no~ correlated tc the respective CSF values. The increase of CSF levels was apparently correlated to the beginning of the ~linieal improvement rather than to the beginning of the acute exacerbation. These dam suggests that what we see in CSF reflect patho-biological events typical of the CNS. These events are Frobably linked to th~ healing process of the recently affected area; we are inclined to think that phenomena of myelin repair and remyelinatlon may be at the base of this marked increase.