Gene sharing in lens and cornea: facts and implications

Abstract

The major water-soluble proteins (crystallins) responsible for the optical properties of the cellular lenses of vertebrates and invertebrates are surprisingly diverse and often differ among species (i.e., are taxon-specific). Many crystallins are encoded by the identical gene specifying a stress protein or a metabolic enzyme which has non-refractive functions in numerous tissues. This double use of a distinct protein has been called gene sharing. Abundant expression of various metabolic enzymes also occurs in a taxon-specific manner in corneal epithelial cells, suggesting that gene sharing extends to this transparent tissue. It has been proposed that one of the most abundant corneal enzymes (aldehyde dehydrogenase class 3) may protect the eye by directly absorbing ultraviolet light, as well as by providing an enzymatic function. It also seems possible that the high expression of corneal enzymes (5–40% of the water-soluble proteins) may reduce scattering in the corneal epithelium by minimizing spatial fluctuations in refractive index as they do in the lens. Thus, gene sharing may be a widespread phenomenon encompassing the lens, cornea and probably other systems. Lens-preferred expression of crystallin genes is integrated in a complex developmental program utilizing in many cases Pax-6. The differential expression of αB-crystallin (a small heat shock protein) in different tissues involves the combinatorial use of both shared and lens-specific cis-control elements. Corneal-preferred gene expression appears to depend in part on induction by environmental influences. Among the implications of gene sharing are that gene duplication is not required for the evolution of a new protein phenotype, a change in gene regulation is sufficient, that proteins may be under more than one selective constraint, affecting their evolutionary clock, and that it would be prudent to consider the possibility that any given gene may have important, unrecognized roles when planning to implement gene therapy in the future.

Introduction

Certain rules for the specialization and evolution of proteins have been steadfastly believed. First, it has been generally accepted that genes encode proteins with distinct functions, even if those particular functions affect multiple biological processes, as might be expected for a metabolic enzyme. Second, no one would argue that the evolution of a new function for a protein occurs by a process of molecular tinkering resulting in small adaptive changes over time (Jacob, 1977). Finally, the prevalence of gene families implies that the freedom for a protein to adopt a new role depends on gene duplication (Kimura and Ohta, 1974). It has also been recognized that the expression of a gene is subject to change, and that this can have profound effects on developmental processes (Britten and Davidson, 1971; Zuckerkandl, 1994) as well as on evolution (King and Wilson, 1975; Wilson et al., 1977, Wilson et al., 1987). Recent studies on lens crystallins have provided a compelling case that changes in gene regulation, either without or before gene duplication, can also lead to new protein functions without loss of the original function by a process, described below, called “gene sharing” (Piatigorsky et al., 1988). It is ironic that the crystallins, generally considered lens-specific, structural proteins specialized for the transparent and refractive properties of the lens, have become a model of functional diversity associated with evolutionary changes in gene regulation.

The transparent lens and cornea of the vertebrate eye are responsible for transmitting incident light into the eye and casting an image onto the photoreceptors of the retina (Land, 1988; Land and Fernald, 1992). In terrestrial vertebrates, the cornea refracts about two-thirds and the lens one-third of the incident light, while in aquatic animals the refraction is accomplished entirely by the lens owing to the similarity of the refractive index of the cornea and the surrounding water. The refractive power of the cellular lens is due to a gradient of protein concentration from the center to the edge of the tissue. Surrounded by a collagenous capsule, the lens is neither innervated nor vascularized, and has an anterior layer of cuboidal epithelial cells and a posterior array of fiber cells (Kuzak and Brown, 1994). Lens differentiation involves the cessation of cell division, extensive cell elongation, loss of organelles, including the nuclei in the central fiber cells, formation of cell junctions, and extreme accumulation of crystallins, which represent 80–90% of the water-soluble proteins of the lens (Piatigorsky, 1981). The gradient of protein concentration responsible for lens refraction is established by the differential accumulation of crystallins in the concentric layers of the fiber cells. Lens transparency is due to short range order interactions among the crystallins and reduced spatial density fluctuations in the cytoplasm (Benedek, 1971; Benedek, 1983; Bettelheim and Siew, 1983; Delaye and Tardieu, 1983; Clark, 1994).

The abundant lens crystallins have been generally viewed as static proteins serving a strictly structural role. Even their name—crystallins—reflecting their accumulation in the crystal-clear lens, implies that these soluble, globular proteins have a crystalline structure and inert role within the cells. Thus, although crystallins have been of general interest as markers of lens cell differentiation (Piatigorsky, 1981) and as evolutionary paradigms (de Jong, 1981, de Jong, 1982; Lubsen et al., 1988), their physicochemical properties have been studied largely in terms of their structural roles in transparency and cataract (Harding and Crabbe, 1984; Bettelheim, 1985; Spector, 1991; Clark, 1994).

Recently, comparative and sequence studies have revealed that the lens crystallins are much more diverse than previously recognized and that many are related or identical to metabolic enzymes and stress proteins found in numerous tissues. These findings have been reviewed extensively (Wistow and Piatigorsky, 1988; Piatigorsky and Wistow, 1989; de Jong et al., 1989; Bloemendal and de Jong, 1991; de Jong et al., 1993, de Jong et al., 1994; Groenen et al., 1994; Wistow, 1993, Wistow, 1995; Slingsby et al., 1997). The fact that crystallins are multifunctional proteins has raised new questions with respect to crystallin evolution, the nature of crystallin gene regulation, and the roles that crystallins may be playing within the lens as well as in other tissues, which are examined in the present article.

A second goal of the present review is to consider the possibility that the use of enzymes and other specialized proteins for multiple purposes by a gene-sharing strategy is not limited to lens crystallins. The cornea, in particular, is examined in greater detail. The current literature indicates that, like the lens, the corneal epithelial cells accumulate different enzymes at concentrations approaching those of crystallins, suggesting that they have structural, as well as metabolic roles. I thus propose that gene sharing is a widespread phenomenon exploiting the different potentials of proteins and is used by the lens, cornea and probably other systems.