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Neutron scattering from myelin revisited: bilayer asymmetry and water-exchange kinetics.

Denninger AR, Demé B, Cristiglio V, LeDuc G, Feller WB, Kirschner DA - Acta Crystallogr. D Biol. Crystallogr. (2014)

Bottom Line: Understanding the processes that govern myelin biogenesis, maintenance and destabilization requires knowledge of myelin structure; however, the tight packing of internodal myelin and the complexity of its junctional specializations make myelin a challenging target for comprehensive structural analysis.This investigation revealed the dimensions of the bilayers and aqueous spaces of myelin, asymmetry between the cytoplasmic and extracellular leaflets of the membrane, and the distribution of water and exchangeable hydrogen in internodal multilamellar myelin.It also uncovered differences between CNS and PNS myelin in their water-exchange kinetics.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biology Department, Boston College, Chestnut Hill, MA 02467, USA.

ABSTRACT
Rapid nerve conduction in the central and peripheral nervous systems (CNS and PNS, respectively) of higher vertebrates is brought about by the ensheathment of axons with myelin, a lipid-rich, multilamellar assembly of membranes. The ability of myelin to electrically insulate depends on the regular stacking of these plasma membranes and on the presence of a number of specialized membrane-protein assemblies in the sheath, including the radial component, Schmidt-Lanterman incisures and the axo-glial junctions of the paranodal loops. The disruption of this fine-structure is the basis for many demyelinating neuropathies in the CNS and PNS. Understanding the processes that govern myelin biogenesis, maintenance and destabilization requires knowledge of myelin structure; however, the tight packing of internodal myelin and the complexity of its junctional specializations make myelin a challenging target for comprehensive structural analysis. This paper describes an examination of myelin from the CNS and PNS using neutron diffraction. This investigation revealed the dimensions of the bilayers and aqueous spaces of myelin, asymmetry between the cytoplasmic and extracellular leaflets of the membrane, and the distribution of water and exchangeable hydrogen in internodal multilamellar myelin. It also uncovered differences between CNS and PNS myelin in their water-exchange kinetics.

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Neutron scattering length density profiles from (a) rat sciatic nerves, (b) rat optic nerves, (c) mouse sciatic nerves and (d) mouse spinal cords in 0–100% D2O-saline. Scattering length density is plotted against radial distance r, with the centre of the cytoplasmic apposition at r = 0. For clarity in the bilayer regions, uncertainty (grey borders) was included only for the profiles calculated for myelin in 0 and 100% D2O-saline. The arrow indicates the higher level of neutron scattering density in the extracellular half of the bilayer, which is proposed to relate to an asymmetric distribution of cholesterol. For each panel, the upper x axis indicates the positions of 0.25d, 0.5d, 0.75d and d.
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fig5: Neutron scattering length density profiles from (a) rat sciatic nerves, (b) rat optic nerves, (c) mouse sciatic nerves and (d) mouse spinal cords in 0–100% D2O-saline. Scattering length density is plotted against radial distance r, with the centre of the cytoplasmic apposition at r = 0. For clarity in the bilayer regions, uncertainty (grey borders) was included only for the profiles calculated for myelin in 0 and 100% D2O-saline. The arrow indicates the higher level of neutron scattering density in the extracellular half of the bilayer, which is proposed to relate to an asymmetric distribution of cholesterol. For each panel, the upper x axis indicates the positions of 0.25d, 0.5d, 0.75d and d.

Mentions: Neutron scattering density profiles were calculated for myelin from rat sciatic and optic nerves and mouse sciatic nerves and spinal cords (Fig. 5 ▶). For both PNS (Figs. 5 ▶a and 5 ▶c) and CNS myelin (Figs. 5 ▶b and 5 ▶d) at high %D2O, two regions of high neutron scattering density characterized the profiles, centred at r = 0 and r = 0.5d and corresponding to the two distinct aqueous spaces within myelin: the cytoplasmic and extracellular compartments, respectively. As D2O was replaced with H2O, these spaces displayed dramatic changes in scattering density owing to the high proportion of exchangeable hydrogen in water and in other constituents. Between this pair of aqueous compartments were regions of relative constancy that were largely unaffected by alterations in H/D content. These stable regions, near r = 0.25d and r = 0.75d, correspond to the hydrocarbon layers in the membrane, which exclude water and are rich in nonexchangeable hydrogen. At 0% D2O, the scattering density from the aqueous layers decreased sufficiently to reveal four distinct peaks (in the membrane pair, from 0 to d) corresponding to the lipid polar groups, which are relatively water-poor and hydrogen-poor and rich in more strongly scattering phosphorus, carbon and oxygen. Across all samples, and especially in rat PNS myelin, a shoulder was observed in the extracellular leaflet of the bilayer, proximal to the lipid polar group region. This asymmetry in neutron scattering density within the bilayer is consistent with the postulated enrichment of cholesterol in the extracellular leaflet. The steroid nucleus of cholesterol has a higher neutron scattering density (0.07 × 1011 cm−2) than stiff-chain hydrocarbon (−0.01 × 1011 cm−2) (Kirschner, 1974 ▶), which could account for its detection here.


Neutron scattering from myelin revisited: bilayer asymmetry and water-exchange kinetics.

Denninger AR, Demé B, Cristiglio V, LeDuc G, Feller WB, Kirschner DA - Acta Crystallogr. D Biol. Crystallogr. (2014)

Neutron scattering length density profiles from (a) rat sciatic nerves, (b) rat optic nerves, (c) mouse sciatic nerves and (d) mouse spinal cords in 0–100% D2O-saline. Scattering length density is plotted against radial distance r, with the centre of the cytoplasmic apposition at r = 0. For clarity in the bilayer regions, uncertainty (grey borders) was included only for the profiles calculated for myelin in 0 and 100% D2O-saline. The arrow indicates the higher level of neutron scattering density in the extracellular half of the bilayer, which is proposed to relate to an asymmetric distribution of cholesterol. For each panel, the upper x axis indicates the positions of 0.25d, 0.5d, 0.75d and d.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4257619&req=5

fig5: Neutron scattering length density profiles from (a) rat sciatic nerves, (b) rat optic nerves, (c) mouse sciatic nerves and (d) mouse spinal cords in 0–100% D2O-saline. Scattering length density is plotted against radial distance r, with the centre of the cytoplasmic apposition at r = 0. For clarity in the bilayer regions, uncertainty (grey borders) was included only for the profiles calculated for myelin in 0 and 100% D2O-saline. The arrow indicates the higher level of neutron scattering density in the extracellular half of the bilayer, which is proposed to relate to an asymmetric distribution of cholesterol. For each panel, the upper x axis indicates the positions of 0.25d, 0.5d, 0.75d and d.
Mentions: Neutron scattering density profiles were calculated for myelin from rat sciatic and optic nerves and mouse sciatic nerves and spinal cords (Fig. 5 ▶). For both PNS (Figs. 5 ▶a and 5 ▶c) and CNS myelin (Figs. 5 ▶b and 5 ▶d) at high %D2O, two regions of high neutron scattering density characterized the profiles, centred at r = 0 and r = 0.5d and corresponding to the two distinct aqueous spaces within myelin: the cytoplasmic and extracellular compartments, respectively. As D2O was replaced with H2O, these spaces displayed dramatic changes in scattering density owing to the high proportion of exchangeable hydrogen in water and in other constituents. Between this pair of aqueous compartments were regions of relative constancy that were largely unaffected by alterations in H/D content. These stable regions, near r = 0.25d and r = 0.75d, correspond to the hydrocarbon layers in the membrane, which exclude water and are rich in nonexchangeable hydrogen. At 0% D2O, the scattering density from the aqueous layers decreased sufficiently to reveal four distinct peaks (in the membrane pair, from 0 to d) corresponding to the lipid polar groups, which are relatively water-poor and hydrogen-poor and rich in more strongly scattering phosphorus, carbon and oxygen. Across all samples, and especially in rat PNS myelin, a shoulder was observed in the extracellular leaflet of the bilayer, proximal to the lipid polar group region. This asymmetry in neutron scattering density within the bilayer is consistent with the postulated enrichment of cholesterol in the extracellular leaflet. The steroid nucleus of cholesterol has a higher neutron scattering density (0.07 × 1011 cm−2) than stiff-chain hydrocarbon (−0.01 × 1011 cm−2) (Kirschner, 1974 ▶), which could account for its detection here.

Bottom Line: Understanding the processes that govern myelin biogenesis, maintenance and destabilization requires knowledge of myelin structure; however, the tight packing of internodal myelin and the complexity of its junctional specializations make myelin a challenging target for comprehensive structural analysis.This investigation revealed the dimensions of the bilayers and aqueous spaces of myelin, asymmetry between the cytoplasmic and extracellular leaflets of the membrane, and the distribution of water and exchangeable hydrogen in internodal multilamellar myelin.It also uncovered differences between CNS and PNS myelin in their water-exchange kinetics.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biology Department, Boston College, Chestnut Hill, MA 02467, USA.

ABSTRACT
Rapid nerve conduction in the central and peripheral nervous systems (CNS and PNS, respectively) of higher vertebrates is brought about by the ensheathment of axons with myelin, a lipid-rich, multilamellar assembly of membranes. The ability of myelin to electrically insulate depends on the regular stacking of these plasma membranes and on the presence of a number of specialized membrane-protein assemblies in the sheath, including the radial component, Schmidt-Lanterman incisures and the axo-glial junctions of the paranodal loops. The disruption of this fine-structure is the basis for many demyelinating neuropathies in the CNS and PNS. Understanding the processes that govern myelin biogenesis, maintenance and destabilization requires knowledge of myelin structure; however, the tight packing of internodal myelin and the complexity of its junctional specializations make myelin a challenging target for comprehensive structural analysis. This paper describes an examination of myelin from the CNS and PNS using neutron diffraction. This investigation revealed the dimensions of the bilayers and aqueous spaces of myelin, asymmetry between the cytoplasmic and extracellular leaflets of the membrane, and the distribution of water and exchangeable hydrogen in internodal multilamellar myelin. It also uncovered differences between CNS and PNS myelin in their water-exchange kinetics.

Show MeSH
Related in: MedlinePlus