Membrane interactions in nerve myelin. I. Determination of surface charge from effects of pH and ionic strength on period

Abstract

We have used x-ray diffraction to study the interactions between myelin membranes in the sciatic nerve (PNS) and optic nerve (CNS) as a function of pH (2-10) and ionic strength (0-0.18). The period of myelin was found to change in a systematic manner with pH and ionic strength. PNS periods ranged from 165 to 250 A or more, while CNS periods ranged from 150 to 230 A. The native periods were observed only near physiological ionic strength at neutral or alkaline pH. The smallest periods were observed in the pH range 2.5-4 for PNS myelin and pH 2.5-5 for CNS myelin. The minimum period was also observed for PNS myelin after prolonged incubation in distilled water. At pH 4, within these acidic pH ranges, myelin period increased slightly with ionic strength; however, above these ranges, the period increased with pH and decreased with ionic strength. Electron density profiles calculated at different pH and ionic strength showed that the major structural alteration underlying the changes in period was in the width of the aqueous space at the extracellular apposition of membranes; the width of the cytoplasmic space was virtually constant. Assuming that the equilibrium myelin periods are determined by a balance of nonspecific forces/i.e., the electrostatic repulsion force and the van der Walls attractive force, as well as the short-range repulsion force (hydration force, or steric stabilization), then values in the period-dependency curve can be used to define the isoelectric pH and exclusion length of the membrane. The exclusion length, which is related to the minimum period at isoelectric pH, was used to calculate the electrostatic repulsion force given the other forces. The electrostatic repulsion was then used to calculate the surface potential, which in turn was used to calculate the surface charge density (at different pH and ionic strength). We found the negative surface charge increases with pH at constant ionic strength and with ionic strength at constant pH. We suggest that the former is due to deprotonation of the ionizable groups on the surface while the latter is due to ion binding. Interpretation of our data in terms of the chemical composition of myelin is given in the accompanying paper (Inouye and Kirschner, 1988). We also calculated the total potential energy functions for the different equilibrium periods and found that the energy minima became shallower and broader with increasing membrane separation. Finally, it was difficult to account directly for certain structural transitions from a balance of nonspecific forces. Such transitions included the abrupt appearance of the native period at alkaline pH and physiological ionic strength and the discontinuous compaction after prolonged treatment in distilled water. Possibly, in PNS myelin conformational modification of PO glycoprotein occurs under these conditions. The invariance of the cytoplasmic space suggests the presence of specific short-range interactions between surfaces at this apposition.

Interaction forces and adhesion of supported myelin lipid bilayers modulated by myelin basic protein

“…2 and 4, the cytoplasmic and extracellular faces of the myelin bilayer have very different compositions. We used the distributions of the lipids determined by Inouye and Kirschner (2,4) to calculate the composition of the cytoplasmic and extracellular monolayers of the control and EAE membranes (Table 1), given the overall lipid compositions of normal and EAE myelin measured in our previous work (10). The asymmetry in lipid composition between the extracellular and cytoplasmic monolayers was believed to be the same for both control and EAE (4,15).…”

“…The estimate gives the excess number of positive charges (arginine and lysine AAs) per 18.5-kDa MBP molecule to be Ϸ20 (15). Thus, for full-charge neutralization each protein molecule should cover an area of 20 ϫ 5.4 Ϸ 108 nm 2 , which corresponds to a surface coverage of ⌫ MBP Ϸ [18,500/(6.02 ϫ 10 23 ϫ 108)] g/nm 2 ϭ 0.28 mg/m 2 per surface. § This value, when compared with the measured coverage of ⌫ MBP Ϸ 2 mg/m 2 obtained at maximum adhesion (Fig.…”

“…1 A and B. The myelin sheath consists of repeating units of double bilayers separated by 3-to 4-nm-thick aqueous layers that alternate between the cytoplasmic and extracellular faces of cell membranes (2) (Fig. 1C).…”

Lipid domains control myelin basic protein adsorption and membrane interactions between model myelin lipid bilayers

“…We reconstructed and used two types of supported model monolayers with a lipid composition characteristic of "normal" or "healthy" and "disease-like" EAE myelin deposited on a dipalmitoylphosphatidylethanolamine (DPPE) monolayer to examine the effect of lipid composition, domains, and fluidity on the interaction forces, film viscosity, and MBP adsorption mechanism between these model myelin bilayers. The lipid composition used is based on data from Inouye and Kirschner (1,24) and Ohler et al (6) (Table 1). For convenience, these reconstituted bilayers are referred to as "model normal bilayers," "model EAE bilayers," and "model myelin bilayers" throughout this article.…”

“…lipid raft | biomembrane adhesion | myelin structure | multiple sclerosis | intrinsically unstructured proteins M yelin is an asymmetric multilamellar membrane wrapped around the axons of the central nervous system (CNS) and consists of alternating extracellular and cytoplasmic leaflets (1)(2)(3). The bilayer-associated proteins, mainly myelin basic protein (MBP) and proteolipid protein, play an essential role in stabilizing and maintaining the myelin structure.…”