The lipid bilayer represents the fundamental permeability barrier to the nonspecific passage of polar molecules into and out of a cell due to its high hydrophobicity. The incorporation of saturating amounts of cholesterol into lens lipid membranes ensures the rectangular shape of the hydrophobicity profile across the PCD (phospholipid cholesterol domain) with the hydrophobicity in the membrane center comparable to that in hexane and dipropylamine (ε from 2 to 3) (). This greatly increases the activation energy required for polar and small ionic molecules to pass through the membrane. Thus, the rate-limiting step for the permeability of small polar molecules is likely to be the process of crossing the hydrophobic barrier at the membrane center.
The incorporation of saturating amounts of cholesterol also creates resistance to the permeation of small hydrophobic molecules (like oxygen) across the membrane. To better illustrate the transport of oxygen and other small hydrophobic molecules within the lens lipid membrane saturated and oversaturated with cholesterol, we constructed , in which the inverse of the oxygen transport parameter (which is a measure of resistance to oxygen permeation (Widomska et al., 2007b
)) is plotted as a function of the position across the lens lipid cortical and nuclear membrane of a two-year-old cow. It can be seen that in the PCD of cortical and nuclear membrane, a rather high permeability barrier to oxygen transport is located in the polar headgroup region, and in the hydrophobic region, to the depth of the ninth carbon, which is approximately where the rigid steroid-ring structure of cholesterol reaches into the membrane. Resistance to oxygen permeation in this region is much higher than resistance in the water phase. However, resistance to oxygen permeation in the membrane center is much less than in the water phase and is comparable to resistance in pure phospholipid membranes (see profile for pure POPC bilayer included in ). Thus, the rate-limiting step for permeation of small, nonpolar molecules across the membrane (including molecular oxygen) is likely to be the process of crossing the rigidity barrier located near the membrane surface (see Refs. (Raguz et al., 2008
; Raguz et al., 2009
; Widomska et al., 2007b
) for further discussion). Similar conclusions can be made for the PCDs of other lens lipid membranes.
Fig. 9 Profiles of the resistance to oxygen permeation (the inverse of the oxygen transport parameter) across the PCD of lens lipid membranes made of lipids extracted from the cortex and nucleus of a two-year-old cow at 35°C are plotted to show the oxygen (more ...)
These results indicate that the locations of permeation barriers are different for polar and nonpolar molecules. For polar molecules, the major resistance to permeation is the hydrophobic barrier in the central part of the membrane. For nonpolar molecules, the major resistance to permeation is the rigidity barrier near the membrane surface. We can conclude that cholesterol has some functions specific to lens fiber cell membranes, which are saturated or oversaturated with cholesterol. Since the layers of fiber cells separate the lens interior from the external environment, the membrane barrier must be very high to block nonspecific permeation of small molecules across the membrane into the lens interior. Incorporation of cholesterol into the membrane serves this purpose well because cholesterol simultaneously raises the hydrophobic barrier for polar molecules and increases the rigidity barrier for nonpolar molecules.
also includes the profile of resistance to oxygen permeation across the CBD (cholesterol bilayer domain), coexisting with the PCD, in the nuclear membrane. The resistance to oxygen permeation in this domain is higher than in water and in the surrounding PCD at all depths. In the membrane center, the difference between resistance in the CBD and the PCD is as great as seven times.
Results presented in this review can help us to better understand the molecular nature of the internal barrier to diffusion of small molecules that is formed in the human lens during middle age and is hypothesized to be a key event in the development of age-related nuclear cataract (Moffat et al., 1999
; Sweeney & Truscott, 1998
). The binding of denatured proteins to the fiber-cell membrane as the mechanism responsible for the barrier (Friedrich & Truscott, 2009
) (possibly by occluding membrane pores and channels) is more probable than a recent hypothesis which states that changes in membrane lipids with age may be responsible (Deeley et al., 2010
). The latter mechanism was proposed based on results that revealed that sphingomyelin levels increased with age in the barrier region, until reaching a plateau at approximately 40 years of age. Deeley et al. states that such changes in lipid composition will have a significant impact on the physical properties of fiber-cell membranes. This contrasts our conclusion that states that membrane properties, including barrier properties, are independent of phospholipid composition until the membrane is saturated with cholesterol. However, when the cholesterol concentration is low (~30 mol%), these membranes (especially those composed of saturated sphingomyelin) can be very rigid (Borchman et al., 1996
; Kusumi et al., 1986
; Wisniewska & Subczynski, 2008
The striking similarity between profiles of the oxygen transport parameter ( and ) and profiles of hydrophobicity ( and ) in lens lipid membranes and model membranes saturated with cholesterol suggests a possibility for lateral transport of molecular oxygen and other small, nonpolar molecules along the inner core of the membrane (parallel to the membrane surface), which is referred to as “hydrophobic channeling.” As shown in , the resistance to the transport of oxygen and other small, hydrophobic molecules in the membrane center is much lower than in the water phase and the solubility of small, hydrophobic molecules is much higher (as indicated by high hydrophobicity in this region [ and ]). To better illustrate the phenomenon of hydrophobic channeling in lens lipid membranes, we constructed , in which we displayed the temperature dependence of the permeability coefficient for oxygen across the membrane region, where the major resistance to oxygen permeation is located (from the membrane surface to the depth of the ninth carbon), and for the membrane center, where oxygen transport is enhanced (between the tenth carbons in each leaflet). To compare the permeability properties of certain membrane regions with those of water, we display these data as a ratio of oxygen permeability across the appropriate membrane region to oxygen permeability across a water layer of the same thickness. We observed that centers of membranes saturated with cholesterol (both lens lipid and model) can serve as channels for oxygen transport, as they have a much higher oxygen permeability than water. To escape from these channels, oxygen has to cross high barriers with low oxygen permeability existing on both sides of the membrane. Additionally, activation energy for oxygen translational diffusion in the lens lipid membrane is significantly greater in the region where the rigidity barrier is located than in the membrane center (Raguz et al., 2009
). This supports our hypothesis that in the lens lipid membrane a high cholesterol content is responsible for creating hydrophobic channels for oxygen transport parallel to the membrane surface, and at the same time, a high cholesterol content is responsible for creating the rigidity barrier to oxygen transport across the membrane.
Fig. 10 The permeability coefficient for oxygen across a specific membrane region (P’M(EPR)) relative to that across a water layer of the same thickness as the membrane region (P’W(EPR); i.e., P’M(EPR)/P’W(EPR)) for cortical (□,■) (more ...)
Finally, the question is raised: Is high cholesterol content in the fiber-cell plasma membrane beneficial or harmful to the lens? We addressed this “conflict” of membrane properties in the paper entitled “Membranes: Barriers or Pathways for Oxygen Transport” (Subczynski & Hyde, 1998
). This conflict is evident in the eye lens where the barrier to oxygen transport created by fiber-cell plasma membranes can be beneficial, helping to maintain low oxygen partial pressure in the lens nucleus. On the other hand, the hydrophobic channel makes it possible for the system of fiber-cell membranes to form conduits for oxygen molecules from the anterior and posterior surfaces into the deeper regions of the lens. This phenomenon could be harmful to the lens, especially when the low oxygen level around the lens is disturbed. Such an event can occur after vitrectomy, when the partial pressure of oxygen at the posterior of the human lens increases to ~13 mmHg (Siegfried et al., 2010
). This increase in oxygen partial pressure is associated with rapid (less than 2 years) opacification of the lens nucleus. Also, in older individuals, when the structure of the vitreous body breaks down (Harocopos et al., 2004
), decreasing the rate of ascorbate-dependent oxygen consumption within the vitreous fluid (Shui et al., 2009
), the posterior of the lens is exposed to the increased oxygen partial pressure (Beebe et al., 2011
). In these conditions, pathways for oxygen transport provided by the fiber-cell membranous system supply more oxygen to the lens center, disturbing the delicate balance between oxygen consumption and oxygen delivery, and increasing oxygen partial pressure in the lens nucleus, which, as a consequence, leads to cataract development.