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D. C. Mitchell, L. Greeley, M. J. Richards, S. J. Fliesler; Rod Outer Segment (ROS) Membrane Fluidity Is Decreased Relative to Controls in an Animal Model of Smith-Lemli-Opitz Syndrome. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2417.
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Smith-Lemli-Opitz syndrome (SLOS) is a hereditary disease caused by defective enzymatic conversion of 7-dehydrocholesterol (7DHC) to cholesterol (Chol). Treatment of rats with AY9944, an inhibitor of the enzyme that is genetically altered in SLOS, provides an animal model of the human disease. We previously reported lipidomic changes in this model, notably the marked reduction of docosahexaenoic acid (DHA) levels in retinas and ROS membranes, relative to controls, as well as substantial replacement of Chol with 7DHC. Here we examined how these changes affect ROS membrane fluidity.
ROS membranes were purified from 3-mo old control and AY9944-treated rats and DPH (1,6-diphenyl-1,3,5-hexatriene) was added to ROS membranes at a ratio of 1 DPH per 300 phospholipids. Fluorescence lifetime and dynamic depolarization of DPH were measured at 37 oC to assess phospholipid acyl chain packing and dynamics.
Intensity-weighted fluorescence lifetime of DPH in ROS membranes from AY9944-treated rats was slightly lower than in controls (9.17 vs. 9.36 ns; P = 0.017), indicating somewhat increased water permeability. DPH fluorescence anisotropy decays were analyzed empirically and in terms of the Brownian rotational diffusion model; both yielded similar results in terms of DPH orientational order and rotational motion. Rotational motion of DPH was significantly decreased and its orientational order was greater in ROS from AY9944-treated animals, relative to controls.
The small reduction in DPH fluorescence lifetime in AY9944-treated membranes indicates slightly looser headgroup packing with respect to water penetration. In the membrane hydrophobic core DPH reported greater acyl chain order and slower motion in ROS membranes from AY9944-treated rats, relative to age-matched controls. The differential effects of AY9944 treatment at different depths in the membrane suggest that SLOS produces a unique set of changes in ROS membrane physical properties. Decreased fluidity in the acyl chain region correlates well with the observed retinal lipidome changes, and predicts lower phototransduction efficiency in the SLOS rat model compared to controls. This is consistent with the observed retinal electrophysiological defects found both in this rat model as well as in SLOS patients.
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