ORDER-DISORDER PHENOMENA IN MYELINATED NERVE SHEATHS .6. THE EFFECTS OF QUAKING, JIMPY AND SHIVERER MUTATIONS - AN X-RAY-SCATTERING STUDY OF MOUSE SCIATIC AND OPTIC NERVES

We report the X-ray scattering study of sciatic and optic nerve myelin from shiverer, jimpy and quaking mice mutants and from the correspond ing controls. These three mutations are known to affect dramatically c entral nervous system (CNS) myelin and to induce comparatively minor a lterations in peripheral nervous system (PNS) myelin. Scattering exper iments and data reduction were carried out using the techniques and al gorithms developed in our laboratory and previously applied to several problems involving the structure of myelin. In sciatic nerve the frac tion of myelin elementary pairs of membranes (total myelin) decreases in shiverer and quaking nerves (by approximately 30%) but not in jimpy nerves; in all three mutants the fraction of myelin membrane pairs th at are not regularly stacked in the sheaths (loose myelin), the averag e number of membranes per sheath and the packing disorder are the same as in the control nerves; the repeat distance D and the membrane dist ance D-cyt across the cytoplasmic space increase in shiverer and decre ase in jimpy; in quaking, D also decreases and the decrease is smaller than in jimpy and is not specific for D-cyt; small changes are also o bserved in the electron density profiles. As for the optic nerve the m yelin content decreases dramatically in the three mutants; the very we ak signal attests to a tiny amount of pairs of membranes structurally similar to normal CNS myelin. It is surprising that the structure of C NS myelin should be almost normal in the absence of the major structur al components, namely myelin basic protein (MBP) for shiverer of prote olipid protein (PLP) for jimpy. The question arises whether the compos ition of the residual pairs of membranes, operationally identified as myelin in the X-ray scattering experiments, mirrors the composition de termined by chemical means on the fraction of nerve tissue histologica lly identified as myelin, or whether in all circumstances it remains a pproximately the same. (C) 1996 Academic Press Limited