Force-distance measurements between supported lipid bilayers mimicking the cytoplasmic surface of myelin at various surface coverages of myelin basic protein (MBP) indicate that maximum adhesion and minimum cytoplasmic spacing occur when each negative lipid in the membrane can bind to a positive arginine or lysine group on MBP. At the optimal lipid/protein ratio, additional attractive forces are provided by hydrophobic, van der Waals, and weak dipolar interactions between zwitterionic groups on the lipids and MBP. When MBP is depleted, the adhesion decreases and the cytoplasmic space swells; when MBP is in excess, the bilayers swell even more. Excess MBP forms a weak gel between the surfaces, which collapses on compression. The organization and proper functioning of myelin can be understood in terms of physical noncovalent forces that are optimized at a particular combination of both the amounts of and ratio between the charged lipids and MBP. Thus loss of adhesion, possibly contributing to demyelination, can be brought about by either an excess or deficit of MBP or anionic lipids.biomembrane adhesion ͉ lipid-protein interactions ͉ multiple sclerosis ͉ myelin membrane structure ͉ experimental allergic encephalomyelitis T he myelin sheath is a multilamellar membrane surrounding the axons of neurons in both the central nervous system (CNS) and peripheral nervous system (PNS) (1) as shown in 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). Dehydrated myelin is unusual in that it is composed of 75-80% lipid and 20-25% protein by weight, compared with Ϸ50% of most other cell membranes (3) (Fig. 1 C and D). Multiple lipids make up the myelin sheath (Table 1), and each sheath, with its own distinct physical properties, contributes to the structure, adhesive stability, and possibly the pathogenesis of the myelin membrane. The asymmetric distribution of lipid composition on the cytoplasmic and extracellular faces likely also plays an important role (4). Myelin basic protein (MBP) constitutes 20-30% of total protein by weight and is located only between the 2 cytoplasmic faces, where it acts as an intermembrane adhesion protein.The myelin sheath acts as an electrical insulator, forming a capacitor surrounding the axon, which allows for faster and more efficient conduction of nerve impulses than unmyelinated nerves (5). According to cable theory, the time to transmit a signal over a distance x is ϭ 1 ⁄2RC myelin x 2 , where R is the resistance per unit length, and C myelin is the capacitance between the axon and its surroundings, which is given by (5) C myelin ϭ 2 0 myelin log͑R O /R I ͒ per unit length,where R O and R I are the outer and inner radii (Fig. 1B), 0 is the permittivity of free space, and the mean dielectric constant of the myelin sheath iswhere (Fig. 1C). Low capacitance necessitates a low value of myelin , which is promoted by the much l...

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The goals of the current study were threefold: first, to confirm previous single volume proton (1H) magnetic resonance spectroscopy results of reduced N-acetyl aspartate (NAA, a putative marker of neurons) in multiple sclerosis (MS) white matter lesions using multiple volume 1H magnetic resonance spectroscopic imaging (MRSI); second, to measure the phospholipid metabolites phosphomonoesters and phosphodiesters in such lesions using phosphorus (31P) MRSI; and third, to test the hypothesis that biochemical changes occur in the normal-appearing (on spin echo T2-weighted magnetic resonance images) white matter in patients with MS. Thirteen subjects with clinically definite MS were studied with both 1H and 31P MRSI, and 19 controls were studied with either 1H MRSI, 31P MRSI, or both. MS lesion, MS normal-appearing white matter, and region-matched control spectra from the centrum semiovale were analyzed. The major findings of this study were that in both white matter lesions and normal-appearing white matter in patients with MS, the metabolite ratio NAA/creatine and the total 31P peak integrals were significantly reduced compared with controls. In addition, in MS lesions NAA/choline and phosphodiesters/total 31P were significantly reduced compared with controls, and in MS normal-appearing white matter there was a trend for NAA/choline to be reduced compared with controls. In normal-appearing white matter in patients with MS, total creatine and phosphocreatine were significantly increased compared to controls, as detected with both 1H (total creatine peak integrals) and 31P (phosphocreatine/total 31P) MRSI techniques. These results suggest reduced neuronal density and altered phospholipid metabolites in white matter lesions in patients with MS.(ABSTRACT TRUNCATED AT 250 WORDS)

This report describes force measurements and atomic force microscope imaging of lipid-protein interactions that determine the structure of a model membrane system that closely mimics the myelin sheath. Our results suggest that noncovalent, mainly electrostatic and hydrophobic, interactions are responsible for the multilamellar structure and stability of myelin. We find that myelin basic protein acts as a lipid coupler between two apposed bilayers and as a lipid ''hole-filler,'' effectively preventing defect holes from developing. From our protein-mediated-adhesion and forcedistance measurements, we develop a simple quantitative model that gives a reasonably accurate picture of the molecular mechanism and adhesion of bilayer-bridging proteins by means of noncovalent interactions. The results and model indicate that optimum myelin adhesion and stability depend on the difference between, rather than the product of, the opposite charges on the lipid bilayers and myelin basic protein, as well as on the repulsive forces associated with membrane fluidity, and that small changes in any of these parameters away from the synergistically optimum values can lead to large changes in the adhesion or even its total elimination. Our results also show that the often-asked question of which membrane species, the lipids or the proteins, are the ''important ones'' may be misplaced. Both components work synergistically to provide the adhesion and overall structure. A better appreciation of the mechanism of this synergy may allow for a better understanding of stacked and especially myelin membrane structures and may lead to better treatments for demyelinating diseases such as multiple sclerosis.lipid-protein interactions ͉ myelin membrane structure ͉ membrane adhesion ͉ membrane regeneration͞healing ͉ demyelinating diseases T his communication addresses the general question of the molecular interactions that determine the structure and stability of membranes that are stabilized by bridging proteins, with the myelin sheath of the central nervous system (CNS) being taken as a prime example of this type of structure. The myelin sheath is formed by extensions of oligodendrocyte cell membranes that wrap around the axon to form a cylindrical scroll a few tens of micrometers in diameter (1, 2). The sheath consists of repeat units of ''double'' bilayers separated by 3-to 4-nm-thick aqueous layers that alternate between the cytoplasmic and extracellular spaces (2). Seventy to 80% of the dry weight of myelin consists of lipids, a proportion that is significantly higher than in most other cell membranes. There are two major proteins: myelin basic protein (MBP), which spans the aqueous cytoplasmic spaces, and proteolipid protein, which spans the bilayers (2). Myelin acts as a transmitter of electric signals known as action potentials, its efficiency being attributed to the low dielectric constant associated with the high lipid content of its closely apposed membranes. Electrical impulses are transmitted along myelinated axons orders of magnitude...