Xenopus gamma-crystallin gene expression: evidence that the gamma-crystallin gene family is transcribed in lens and nonlens tissues.

Abstract

Crystallins, the major gene products of the lens, accumulate to high levels during the differentiation of the vertebrate lens. Although crystallins were traditionally thought to be lens specific, it has recently been shown that some are also expressed at very low levels in nonlens tissues. We have examined the embryonic expression pattern of gamma-crystallins, the most abundant crystallins of the embryonic lens in Xenopus laevis. The expression profile of five Xenopus gamma-crystallin genes mirrors the pattern of lens differentiation in X. laevis, exhibiting on average a 100-fold increase between tailbud and tadpole stages. Four of these genes are also ubiquitously expressed outside the lens at a very low level, the first demonstration of nonlens expression of any gamma-crystallin gene; expression of the remaining gene was not detected outside the head region, thus suggesting that there may be two classes of gamma-crystallin genes in X. laevis. Predictions regarding control mechanisms responsible for this dual mode of expression are discussed. This study raises the question of whether any crystallin, on stringent examination, will be found exclusively in the lens.

Crystallins, genes and cataract

“…Recently, low levels of y-crystallin transcripts have been detected in developing Xenopus laevis tadpoles [44,45]. The yScrystallin (a monomeric protein of the ~/y family, which has a blocked N-terminus like ~-crystallins) [248] has been shown by RT-PCR to be expressed in the adult bovine eye cornea and retina.…”

A crystallin gene network in the mouse retina

“…Initially, the ubiquitous crystallins were recognized as the primary component of the vertebrate lens, lending the lens its optical properties of transparency and refractive focusing of light. Although initially they were thought of as being solely lens proteins, it is now known that the crystallins are expressed in other tissues such as the brain, heart, kidney, lungs, and retina (Clayton et al, 1986; Head et al, 1991; Smolich et al, 1994). The α-crystallin proteins are the most studied, and have a high degree of homology with small heat shock proteins (HSP).…”

Extralenticular Expression of the Rodent βB2-Crystallin Gene

“…Recently, β-and γ-crystallin mRNA was found to be present in early embryonic stages of Xenopus laevis (Smolich et al, 1994 ;Brunekreef et al, 1997) and the members of the β\γ-crystallin superfamily could play some unknown role during early development. Whether βB2-crystallin plays a vital role in the mouse or man is questionable, given the fact that the only visible phenotypical effect of a mutation in the βB2-crystallin gene is the development of cataract (Chambers and Russell, 1991 ;Litt et al, 1997).…”

“…There is accumulating evidence that the β-and γ-crystallin genes are also expressed at low levels outside the lens. Expression of β-crystallins has been detected in the mouse, cat and chicken retina (Head, Peter and Clayton, 1991 ;Head, Sedowofia and Clayton, 1995) and extralenticular expression of β-and γ-crystallin mRNA was detected in early embryonic stages of Xenopus development (Smolich et al, 1994 ;Brunekreef et al, 1997).…”