Organization of lipids in fiber-cell plasma membranes of the eye lens

Abstract

The plasma membrane together with the cytoskeleton forms the only supramolecular structure of the matured fiber cell which accounts for mostly all fiber cell lipids. The purpose of this review is to inform researchers about the importance of the lipid bilayer portion of the lens fiber cell plasma membranes in the maintaining lens homeostasis, and thus protecting against cataract development.

Keywords: Cholesterol; Cholesterol bilayer domain; Cholesterol crystals; Membrane domains; Oxygen permeation; Spin labeling.

Fig.1

Schematic drawing illustrating, induced by the increased cholesterol content, changes in the organization of lipids in the lipid bilayer membranes. As indicated in the text, structures (B, C, and D may be relevant to the eye lens lipid membranes).

Fig. 2

Schematic drawing of the human lens intact membranes indicating the tightly adherent cell-cell junctions formed by membrane proteins (AQP0, Cx46, and Cx50). These proteins permit also communication between tightly packed layers of fiber-cells. The lipid bilayer portion of the intact fiber cell membrane provides high hydrophobic barrier which protect against uncontrolled leaking of small polar molecules. Purported lipid domains induced by the high Chol content and the presence of integral membrane proteins are indicated. The intact membranes were found to contain three distinct lipid environments termed the bulk lipid domain, boundary lipid domain, and trapped lipid domain [18, 96]. However, the cholesterol bilayer domain (CBD), which was detected in cortical and nuclear lens lipid membranes [43, 55], was not yet detected in intact membranes. Note that Chol is excluded from boundary lipids [102, 103].

Fig. 3

(A) Diagram of the human eye lens section showing the location of the lens cortex (blue) and nucleus (white). Differentiating fibers (dark blue) near the lens surface contain a normal complement of organelles, including mitochondria. Maturate fibers located in the deeper central region of the lens (light blue in cortex and white in nucleus) do not contain mitochondria. Values of the oxygen partial pressure at the surface of the anterior and posterior cortex of the lens in the healthy eye are taken from [27]. Arrow heads indicate the purported changes of the oxygen partial pressure toward the lens center with the thickness proportional to the partial pressure value (note that 90% of oxygen flux is consumed by mitochondria located in the differentiating zone [93]). (B) Oxygen partial pressure difference across the fiber cell plasma membrane formed by oxygen consumption by fiber cells at deeper locations in the lens. Note that oxygen is crossing the outermost plasma membrane of the fiber cell from extracellular space to the cytoplasm, going through cytoplasm, and crossing the innermost membrane of the same fiber cell from the cytoplasm to the extracellular space (indicated in parenthesis).