Differential synthesis of rat lens proteins during development

Age-dependent changes in resistivity and electrolytes related to lens development and growth in the rat

Alterations in resistivity measurements of lens homogenates, lens percent water and cation concentrations of sodium and potassium show a complex pattern in the Sprague-Dawley rat during lens development and maturation. During the neonatal period, the data provide evidence for three distinct periods: days 5-12, a pre-critical maturation period (pre-CMP), a steep decline in cation concentration and a minimal change in percent water were accompanied by an expected sharp rise in resistivity; days 12-16, critical maturation period (CMP), a further decrease in ion concentration and water was concomitant with the unexpected observation of no significant change in resistivity; and days 16-30, post-CMP, no significant changes were observed for cation concentrations, percent water, or resistivity. From 30 to 100 days, an adult nuclear maturation period (NMP), a decrease in cation concentration and percent water was reflected in a rise in resistivity. A comparison of 100 and 500 day lenses revealed that the concentrations of Na and K, and percent water are essentially unchanged. The K/Na ratio, which had decreased from an initial value of 5.9 at 5 days, stabilized at approximately 4 by day 100. A comparison of the resistivity measured in both lens homogenates and KCl solutions at identical molar strengths revealed that, for the ages studied, ion concentration and ion mobility play important roles in determining this parameter. The underlying cause for age related variations in lens susceptibility to cataractogenic insult is most likely related to complex changes in composition and resistivity which undoubtedly reflect adjustments in molecular organization. The proposition that the lens is a 'free flowing' syncytium of cells appears unwarranted.

Differential synthesis of crystallins in the developing rat eye lens

The patterns of protein synthesis in rat lenses ranging in age from newborn to 4 months were compared. After incubation of lenses in [35S]methionine-containing medium it was possible to identify the de novo synthesized crystallins by two-dimensional gel electrophoresis and fluorography, in combination with peptide mapping and immunoblotting. It was found that the relative synthesis of alpha A and beta A3 stays fairly constant in rat lenses of all investigated ages. The relative synthesis of beta B2 and gamma s shows a pronounced increase with age in these post-natal lenses. A differential decrease can be observed in the relative synthesis of the other six gamma-crystallins (gamma A-gamma F). There appears to be a good correlation between the changes in relative synthesis of the various crystallins and previously reported alterations in mRNA levels, although certain mRNAs exhibit marked differences in translational efficiency.

Immunological studies on gamma crystallins from Xenopus: localization, tissue specificity and developmental expression of proteins

In an effort to understand the spatio-temporal regulation of crystallins and their genes during lens development, the gamma crystallins from the frog lens have been isolated, purified and characterized. Using an immunological approach, they were found to be localized exclusively in the lens fiber cells and were not detected in any other lens cells or non-lens tissues including mature oocytes. During embryogenesis, the antigens were first detected in stage 25 embryos (but not in stage 20 embryos). Their level first decreased and then increased during subsequent stages of development. A different member of the family was also found to be expressed during later stages of embryogenesis.

Crystallin gene expression during rat lens development

The analysis of the developmental pattern of the alpha A-, alpha B-, beta B1-, beta B2-, beta B3-, beta A3/A1-, and beta s-crystallin genes during fetal and postnatal development of the rat shows that the differential regulation of crystallin synthesis relies on differential gene shutdown rather than differential gene activation; that is, all crystallin genes are active during early development but turn off at different stages. The only two exceptions to this rule are the alpha B- and beta s-crystallin genes. The alpha B-crystallin gene transcript becomes first detectable at 18 days of fetal development, while the beta s-crystallin gene appears to be active only in the postnatal period. We also determined the absolute numbers of the alpha A-, alpha B-, beta B1-, beta B2-, beta B3-, beta A3/A1-, beta s-, and gamma-crystallin gene transcripts present in the lens at various times after birth. Comparison of these RNA data with the published protein data shows that the alpha B- and beta B2-crystallin RNAs are relatively overrepresented, suggesting the possibility that these two RNA species are not used as efficiently as other crystallin mRNAs. Examination of the known (hamster) alpha B-crystallin sequence and elucidation of the (rat) beta B2-crystallin sequence yielded no evidence for aberrant codon usage. These two RNAs have one sequence motif in common: they are the only crystallin mRNAs in which the translation initiation codon is preceded by CCACC.

Growth and transparency in the lens, an epithelial tissue, stimulated by pulses of PDGF

The rat lens undergoes dramatic growth during early postnatal development. Lens weight increased by a factor of 23 in 26 days. Growth rate per day oscillated between 0 and 87 percent. A new culture system was designed to study the oscillations in growth during development. Lens growth and transparency in vitro required pulsatile delivery of platelet-derived growth factor (PDGF) in HL-1 serum-free medium. Continuous delivery of HL-1 medium with PDGF or pulsatile delivery of HL-1 medium without PDGF resulted in lens opacity and no growth. These results provide direct evidence that PDGF stimulates an epithelial tissue and that oscillations in growth occur during normal development of the rat lens.

Rat lens gamma-crystallins. Characterization of the six gene products and their spatial and temporal distribution resulting from differential synthesis

We have isolated, purified and characterized six individual gamma-crystallin polypeptides present in the rat lens. Comparison of their amino acid compositions with the known structure of the six gamma-crystallin genes permits a one-to-one correspondence to be made between each protein synthesized and the encoding gene. This demonstrates that each of the six genes is actually expressed in vivo. Two classes of three gamma-crystallins each, which we have designated classes gamma ABC and gamma DEF, are known to exist, on the basis of internal sequence homology. We have measured the temperature-dependent phase-separation characteristics of solutions of the six purified gamma-crystallins, and find that the three members of the gamma DEF class (gamma 2-2, gamma 3-1 and gamma 4-1) are all cryo-proteins with relatively high phase-separation temperatures, whereas the three gamma ABC crystallins (gamma 1-1, gamma 1-2 and gamma 2-1) do not show phase separation above -7 degrees C. We have measured the spatial distribution in rat lens of each of the alpha-, beta- and gamma-crystallins as a function of age from 1 to 420 days, using size-exclusion and ion-exchange high-pressure liquid chromatography (HPLC). Our findings in the cortical layer permit us to establish the differential synthesis of each of the crystallins during lens development. Particular attention has been devoted to the spatial and temporal distribution of the six individual gamma-crystallins. Up to birth, synthesis of the three components of the gamma DEF class predominates, and in particular that of gamma 2-2. In subsequent development the three components of the gamma ABC class assume a greater proportion of monomeric crystallins synthesized, while beta s-crystallin synthesis predominates in late development. Our analysis of different layers within single lenses provides novel information on spatial gradients of the water-soluble and water-insoluble protein fractions as a function of age. We consider the consequences of these findings for lens transparency and opacity in both rat and mouse lens. We show that the high concentrations of gamma DEF-crystallins appear to be responsible for the opacity known to occur in young rat lenses. We conclude from these observations that close control of the differential synthesis of gamma-crystallins plays an important role in maintaining lens transparency during development.

Age-related changes in fibre differentiation of rat lens epithelial cells in vitro

Epithelial cells in explants from rat lenses differentiate into fibre cells when cultured with retina conditioned medium (RCM). This differentiation is characterised by cell elongation, the accumulation of alpha-crystallin and the onset of synthesis of the fibre specific beta- and gamma-crystallins. To determine the effect of donor age on the synthesis of beta- and gamma-crystallins in response to culture with RCM, we explanted lens epithelia from 3-, 10- and 21-day-old rats. Explants were trimmed to obtain a population of central epithelial cells and at the end of the culture period crystallin content was measured by ELISA methods and expressed per unit DNA. Explants from 3- and 10-day-old rats showed significant increases in the levels of beta-crystallin/DNA within four days after the addition of RCM. The synthesis of beta-crystallin in explants from 21-day-old rats was delayed until after day 4 of culture, but rose quickly to reach the same levels at 10 days as the explants from younger lenses. The ability of lens epithelia to synthesise gamma-crystallin in response to RCM showed a dramatic decline as the age of the donor rat increased. After 10 days of culture, explants from 10-day-old rats contained only 12% of the amount of gamma-crystallin accumulated by explants from 3-day-old rats. Explants from 21-day-old rats accumulated no significant gamma-crystallin until day 10 of culture, and at this time the amount was significantly less than that accumulated by explants from younger rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein distribution patterns in concentric layers from single bovine lenses: changes with development and ageing

Protein distribution patterns were determined in concentric layers removed from 24 bovine lenses ranging in age from about 6 months before birth to 180 months post-natal. It was possible to distinguish alterations in protein synthesis patterns during development and changes due to ageing, i.e., prolonged existence of the proteins. It was found that alpha-crystallin represents a constant 50% of the proteins synthesized by the fibre cells throughout life. However, the protein becomes progressively less soluble with increasing age. Beta-crystallin synthesis increases from 30% of the total proteins during prenatal development to around 40% in post-natal fibre cells. This increase is due to increased production of the beta-crystallin. In old tissues, beta H-crystallin is converted to a high molecular weight from (HMW beta) gamma-crystallins account for 22% of the proteins synthesized in the earliest prenatal fibre cells. This level decreases rapidly through prenatal development until they represent about 4% of the total at birth. Beta S-crystallin synthesis commences around this time and in the post-natal fibre cells is essentially the only low molecular weight protein. The possible significance of some of these changes is discussed with regard to the functional requirements of the lens.

Defects in lens fiber differentiation are linked to c-mos overexpression in transgenic mice

We describe three strains of transgenic mice derived by embryo microinjection of DNA consisting of a long terminal repeat (LTR) of Moloney murine sarcoma virus (Mo-MSV) linked to the murine c-mos coding sequences. Southern analysis of the genomic DNA of these strains suggested that in each case the transgene had integrated at a different chromosomal location. The strains were characterized by dominant changes in secondary lens fiber differentiation. Shortly after birth, insufficient elongation of differentiating lens fibers and lack of basement membrane secretion resulted in breakdown of the posterior lens capsule. This, in turn led to posterior protrusion and swelling of lens tissue. In the course of the first 3 weeks after birth, globular lens cells began to fill the entire anterior and posterior chambers of the eye. Concomitantly, there was massive overexpression of c-mos RNA in the lens. Whereas this construct has high transforming activity when transfected into NIH-3T3 cells, no hyperplasia or neoplasia have been observed in the affected lenses. Increased expression of c-mos RNA was not confined to the lens of the eye but has been detected in any of several tissues tested.

Human lens gamma-crystallins: isolation, identification, and characterization of the expressed gene products

We have isolated the individual gamma-crystallins expressed in young human lenses and identified with which of the six known human gamma-crystallin genes they each correspond. We find that at least 90% of the gamma-crystallins synthesized in the young human lens are the products of genes gamma G3 and gamma G4. We demonstrate that gamma G4-crystallin undergoes a temperature-dependent phase separation, and we have measured the low-concentration branch of its coexistence curve (phase separation temperature vs. concentration) up to about 40 mg/ml. By comparison, we found no evidence of gamma G3-crystallin phase separating, even at lower temperatures and higher concentrations. This is consistent with predictions based on sequence homology between human and rat gamma-crystallins. The implications of these findings for human inherited and senile cataracts are considered.

A critical maturation period in neonatal-rat-lens development

The transition of the immature lens to the mature lens was investigated in the Sprague-Dawley rat during the neonatal period which extends for 21 days after birth. The lens of the newborn rat consists of uniformly hydrated cortical and primary fiber cells while the mature lens is characterized by the presence of a nucleus which has a lower water content relative to the surrounding cortex. Our studies reveal that a critical maturation period occurs between days 12-16. In this investigation, the period is defined by the following physiological and anatomical alterations: A significant decline in the rate of lens wet weight growth occurs between days 12-16. However, the growth in dry weight is linear throughout the neonatal period. There is a significant decline in lens percentage water during this 12-16-day period. The loss in lens water is primarily from the intracellular compartment since the lens extracellular space is constant throughout the neonatal period. Lens anterior-posterior and equatorial dimensions exhibit a zero growth rate between days 14-16 and 12-14, respectively. The permeability of lens extracellular space declines significantly during the critical maturation period as determined by Procion Yellow, an extracellular marker. The significant decline in lens percentage water and the decline in the depth of penetration of Procion Yellow to the lens interior during the critical maturation period is related to the transition of an immature to a mature lens.

Analysis of an inductive interaction between lens and neural retina in rats of different ages

Lens epithelial cells from neonatal rats cultured with neural retinas or neural retina-conditioned medium (RCM), undergo fibre differentiation. This is characterized by cell elongation, increased alpha-crystallin synthesis and the initiation of beta and gamma-crystallin synthesis. To determine if this tissue interaction continues in later life we developed an ELISA method to analyse patterns of alpha and beta-crystallin accumulation in epithelia from 3-day-, 10-day- and 21-day-old rats. Culture of lens epithelia with RCM resulted in the formation of multilayers of elongated fibres and the accumulation of alpha and beta-crystallins. The patterns of crystallin accumulation were essentially similar whether expressed as microgram crystallin per explant, or crystallin per DNA (ng per ng). alpha- and beta-Crystallins accumulated rapidly in explants after 2 days of culture in RCM, whereas explants grown in control medium showed no change in the crystallin levels from day 0 to day 10. Patterns of alpha- and beta-crystallin accumulation showed that there were no significant differences between the ability of lens epithelia from 3-day-, 10-day or 21-day-old rats to undergo fibre differentiation in response to RCM. Therefore we conclude that the inductive interaction between lens and neural retina is not restricted to embryonic or neonatal stages, but continues on throughout life maintaining normal patterns of fibre differentiation in the lens.

Immunocytochemical localization of the 27K beta-crystallin polypeptide in the mouse lens during development using a specific monoclonal antibody: implications for cataract formation in the Philly mouse

The Philly mouse develops a hereditary cataract about 5 weeks after birth. Although the causative agent is not known, data suggest that there is a correlation between cataract formation and the selective absence of a 27 kilodalton (27K) beta-crystallin lens polypeptide. The ontogeny of the 27K beta-crystallin polypeptide was examined in normal mice in order to evaluate its role in normal development and determine what impact its absence may have on the Philly mouse lens. A monoclonal antibody was used with the PAP method to immunocytochemically localize the 27K polypeptide in lenses of normal mice during development. beta-Crystallins detected with polyclonal antisera were found in differentiated fiber cells throughout the lens. In contrast, the 27K beta-crystallin polypeptide detected with a specific monoclonal antibody was not found in the fiber cells of the inner part of the lens (nucleus), but was specifically localized in the fiber cells of the outer part of the lens called the cortex. The polypeptide was found only in elongating and differentiated fiber cells and not in mitotically active epithelial cells. Although a minor component of the 2-day-old lens, the 27K polypeptide comprised a large portion of the 16-day-old lens including the anterior and posterior poles. These data show that the 27K polypeptide is a minor component of the embryonic lens, but becomes a major contributor to the postnatal lens. The 27K beta-crystallin lens polypeptide is abundant in the fiber cells of the normal postnatal mouse lens. The absence of the 27K polypeptide in the Philly mouse may contribute to the observed failure of fiber cells to differentiate in the Philly mouse after birth or may be deleterious in some other manner to normal lens development. The selective absence of the 27K beta-crystallin polypeptide, a defect which precedes cataract formation in the Philly mouse, is intriguing since it suggests a relationship between this major lens polypeptide and lens clarity.

Comparison of the lens crystallin proteins from normal, rd, and rds mutant mice utilizing specific monoclonal antibodies

The lens proteins from three lines of congenic mice which are homozygous for the gene retinal degeneration (rd) or retinal degeneration slow (rds) or carrying the normal alleles (normal) were analyzed by SDS-PAGE and by immunological reactivity to specific monoclonal antibodies. The lens proteins of normal, rd, and rds mice showed a similar developmental pattern between postnatal day 0 and postnatal day 30. The expression of the 25000 molecular weight (MW) beta-crystallin polypeptide which appears postnatally in the normal lens was not affected by retinal abnormalities in the mutant mice. It is concluded that the regulation of the 25000 MW beta-crystallin polypeptide is not dependent upon differentiation or maintenance of the photoreceptor outer segments or continued presence of the photoreceptor cells.