Bird Lens Research Articles

Nicotinamide adenine dinucleotide reduced (NADH) is a natural UV filter of certain bird lens

In this work, we for the first time report the identification of UV filters in the bird eye lens. We found that lenses of some raptors (black kite, common buzzard) and waterfowl (birds from Podicipedidae family) contain unusually high levels of reduced nicotinamide adenine dinucleotide (NADH)—a compound with high absorption in the UV-A range with a maximum at 340 nm. The lens metabolome of these birds also features an extremely low [NAD +]/[NADH] ratio. Chemometric analysis demonstrates that the differences between the metabolomic compositions of lenses with low and high NADH abundances should be attributed to the taxonomic features of bird species rather to the influence of the low [NAD +]/[NADH] ratio. We attributed this observation to the low metabolic activity in lens fiber cells, which make up the bulk of the lens tissue. Photochemical measurements show that properties of NADH as a UV filter are as good as that of UV filters in the human lens, including strong absorption in the UV-A spectral region, high photostability under both aerobic and anaerobic conditions, low yields of triplet state, fluorescence, and radicals under irradiation. Lenticular UV filters protect the retina and the lens from photo-induced damages and improve the visual acuity by reducing chromatic aberrations; therefore, the results obtained contribute to our understanding of the extremely high acuity of the raptor vision.

γ-Crystallins of the chicken lens: remnants of an ancient vertebrate gene family in birds.

γ-Crystallins, abundant proteins of vertebrate lenses, were thought to be absent from birds. However, bird genomes contain well-conserved genes for γS- and γN-crystallins. Although expressed sequence tag analysis of chicken eye found no transcripts for these genes, RT-PCR detected spliced transcripts for both genes in chicken lens, with lower levels in cornea and retina/retinal pigment epithelium. The level of mRNA for γS in chicken lens was relatively very low even though the chicken crygs gene promoter had lens-preferred activity similar to that of mouse. Chicken γS was detected by a peptide antibody in lens, but not in other ocular tissues. Low levels of γS and γN proteins were detected in chicken lens by shotgun mass spectroscopy. Water-soluble and water-insoluble lens fractions were analyzed and 1934 proteins (< 1% false discovery rate) were detected, increasing the known chicken lens proteome 30-fold. Although chicken γS is well conserved in protein sequence, it has one notable difference in leucine 16, replacing a surface glutamine conserved in other γ-crystallins, possibly affecting solubility. However, L16 and engineered Q16 versions were both highly soluble and had indistinguishable circular dichroism, tryptophan fluorescence and heat stability (melting temperature Tm ~ 65 °C) profiles. L16 has been present in birds for over 100 million years and may have been adopted for a specific protein interaction in the bird lens. However, evolution has clearly reduced or eliminated expression of ancestral γ-crystallins in bird lenses. The conservation of genes for γS- and γN-crystallins suggests they may have been preserved for reasons unrelated to the bulk properties of the lens.

Anomalous behavior of βB1-crystallin subunits from avian lenses

In soluble extracts of bird lenses, crystallin subunits of apparent sizes between 30kDa and 40kDa show considerable variability in both abundance and mobility in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). This is only partly due to the taxon-specific distribution of lactate dehydrogenase-B (LDH-B)/epsilon-crystallin. Here we show that, in spite of high conservation in primary sequence, beta B1-crystallin subunits of avian lenses have a markedly slower migration in SDS PAGE than those of mammals and that the apparent subunit size of beta B1-crystallin varies among birds. This may be at least partly due to unusual post-translational modifications, since beta B1-crystallins of adult birds react strongly in a glycan detection procedure. No other crystallins in birds or mammals share this strong reaction. Modification of beta B1-crystallin could have interesting consequences for the formation of high molecular weight beta-crystallin aggregates in bird lenses.

Lens fiber organization in four avian species: A scanning electron microscopic study

The three-dimensional organization of the eye lenses of the chicken, the canary, the song-thrush and the kestrel was studied using light and scanning electon microscopy. The lenses of birds are characterized by the presence of two distinct compartments: the annular pad and the main lens body, separated by a cavum lenticuli. The annular pad fibers had a hexagonal circumference all contained a round nucleus and except for the canary were smooth-surfaced and lacking anchoring devices. In the canary, however, the annular pad fibers were studded with edge protrusions and ball-and-socket junctions. The semicircular main lens body fibers of all four species were studded with ball-and-socket junctions and edge protrusions. In contrast with mammals these anchoring devices were present throughout the lens up to the embryonal nucleus. Superficially the main lens body fibers were extremely flat. Additionally membrane elevations and depressions and globular elements were found on these central fibers in three species, the kestrel being the exception. At the transition between annular pad and main lens body the fibers turned their course and the nuclei became oval and disappeared in the deeper aspect of the main lens body. The cavum lenticuli was filled with globules tied off from the annular pad fibers. It seems attractive to assume that the presence of a separated annular pad, a cavum lenticuli filled with globular elements, the extreme flatness of the superficial central fibers and the studding of these central fibers with anchoring devices up to the embryonal nucleus are morphological expressions of the mouldability of the bird's eye lenses and consequently would explain their efficient accommodative mechanism including formation of a lenticonus. The presence of nuclei in the annular pad fibers and their typical change at the transitional zone between annular pad and main lens body are suggestive for a two-phased differentiation in bird's lens fibers: differentiation of the germinative epithelial cells to annular pad fibers which migrate to the main lens body after which they differentiate further to main lens body fibers.

On the existence of γ-crystallin in the bird lens

Sauropsidans until recently were acknowledged to possess the lens-specific proteins, α and β-crystallins, while lacking γ-crystallins. The γ-crystallins have been employed as specific indicators of normal and regenerating lens fiber cell differentiation in several other organisms. It has been reported, however, that indeed, birds do possess γ-crystallins, and that furthermore this protein is not lens-fiber specific, since it could be found in annular pad and epithelium as well as fibers. Several physical characteristics of this protein cited by those suggesting its existence and at odds with those known for γ-crystallins, led the authors to further investigate the putative γ-crystallin. Pigeon “γ”-crystallin was prepared exactly according to previously published methods, and subjected to amino acid and N-terminal analyses, and peptide mapping. The fraction isolated satisfied none of the commonly-accepted criteria for γ-crystallins. Thus the pigeon “γ”-crystallin did not exhibit low values for alanine and lysine; a free N-terminal amino acid could not be detected; and peptide mapping did not reveal the distinctive peptide of γ-crystallins, the fastest moving orange spot in the electrophoresis direction. The bird lens, then, does not contain γ-crystallin; the γ-crystallins, when present, remain as reliable indicators of lens fiber differentiation.

Raman spectra of bird and reptile lens proteins

Lenses of birds and several reptiles have been examined by Raman spectroscopy of intact lenses and some chick lens protein fractions to elucidate the relationship between chemical structure and various unusual properties of these lenses. The δ-crystallin, which is the major protein by far of young bird and reptile lenses, and which is unique to these classes, exists in situ and after chromatography chiefly in the α-helical conformation, in contrast to the lens proteins of all other species examined. This determination by Raman spectroscopy for several species of these two classes has been confirmed by circular dichroism studies of chick lens δ-crystallin. Raman spectroscopy also reveals the glycogen in pigeon lens nucleus and its absence in chicken lens and in pigeon annular pad. A comparison of the spectrum of heat-coagulated δ-crystallin as a powder with the spectrum of native δ-crystallin reveals a considerable conversion of the α-helical form to the β-pleated sheet. This is the first case in which a major conformational change has been shown to be associated with the formation of lens opacification.

Prevalence of α-helical form in avian lens proteins

Laser Raman spectra of the intact capsulated living lenses from chick and pigeon are reported. Evidence is presented that the First Important Soluble Crystallin (FISC) protein exists chiefly in the α-helical conformation, in contrast with the lens proteins from bovine and dogfish (mixtures of β-pleated and unordered structures). The occurrence of this low thiol protein in the α-helical form may explain the failure of the bird lens to develop cold cataract or a firm nucleus. The circular dichroism (CD) curve of chick lens proteins also indicate the presence of α-helical form. The overall “helix-forming potential” as calculated by the method of Chou and Fasman is high, supporting our Raman spectroscopic conclusion.

The presence of glycogen in lenses of different species

The presence of glycogen in lenses has occasionally been the subject of studies, mostly by histochemical procedures. Quantitative values for the glycogen content are, however, rare. Results obtained by both methods corresponded in so far as the glycogen content of lenses proved to be very low and it is doubtful whether it may be considered at all as a reserve carbohydrate for lens metabolism. After histochemical investigations with acridine-free parosaniline-(PAS)-staining and fluorescence identification with salicylic acid-hydrazide had confirmed the presence of glycogen in the lenses of pigeon, guinea pig, rat, rabbit, calf and bovine, we tried to get more information about such compounds participating in the glycogen cycle. Besides determination of their concentration in total lenses, lens cortex and lens nucleus, the data for their specific activities after incorporation of labelled inorganic phosphate into the organophosphorous-compounds give an indication of the metabolic rates of these reactions. All present findings show that the glycogen present in the lens is located in the nucleus to a considerable degree, which was most obvious in the case of the pigeon lens, being in good agreement with the numerous results Rabaey (1963) obtained with bird lenses. In Rabaey's opinion the importance of glycogen in the nucleus of the bird lens, regarding its special accommodation mechanism, lies in the increase of the refractive index, substituting for the protein of low molecular weight lacking in this region. We however tend to assume that in calf and bovine lenses there is an active participation of glycogen in the carbohydrate metabolism. This may be true also for the lenses of rabbit and rat as well as those of humans.

Evaluation of the Eye Lens Technique for Aging Columbian Black-Tailed Deer

In order to evaluate the eye lens technique for aging Columbian black-tailed deer (Odocoileus hemionus columbianus), lenses from 83 known-age animals were fixed, desiccated, and weighed in a standardized manner. Several potential sources of error in the preparation and weighing of lenses were explored, including length of desiccating time, length of cooling period before weighing, removal of lenses from eyeballs before fixing, and freezing prior to fixing. The effects of freezing and removal of the lens before fixing were investigated on a series of domestic sheep (Ovis aries) eyes. Freezing had no significant effect, but it was determined that fresh lenses were easily damaged with consequent loss of weight. For statistical comparisons of the influence of sex, nutrition, body weight, and geographical variation, a mathematical expression of the lens weight-age relationship was necessary, and the equation Y = a + b log X was found to give a satisfactory representation of the data. A comparison was made with weights of lenses from 20 known-age Rocky Mountain mule deer (Odocoileus hemionus hemionus) from Utah, and significant differences were found. Lenses from male and female deer differed significantly in weight. The effects of nutrition on lens weight are not clear cut, and even though there is a good general correlation with body weight, trends vary with increasing age. Using the regression, a test was made of the usefulness of the lens technique by estimating the ages of the knownage deer. The margin of error was found to be directly proportional to age, but did not bear a constant relationship and increased rapidly in deer older than 5 years. Accuracy was improved by comparing males and females with their appropriate regressions. In general, this technique is considered to be satisfactory for aging deer at least through 5 years, especially if only year classes are desired. Since Lord (1959) described the use of the dry weight of the eye lens for aging cottontail rabbits (Sylvilagus floridanus), this technique has been applied to a number of other mammals and birds with varying degrees of success. Lord (1961) has also investigated aging of the gray fox (Urocyon cinereoargenteus) by this means, and Sanderson (1961) reported on a study of the raccoon (Procyon lotor). Other applications of this technique have been published by Beale (1962) on fox squirrels (Sciurus niger); Martinson et al. (1961) on swamp rabbits (Sylvilagus aquaticus); Wight and Conaway (1962) on cottontails; and Kolenosky and Miller (1962) for pronghorn antelope (Antilocapra americana). Lord (1962) also applied this method to white-tailed deer (Odocoileus virginianus) and described the relationship between lens weight and nutritional status of the deer. In this paper he also mentioned unpublished studies on the deer mouse (Peromyscus maniculatus) and the Norway rat (Rattus norvegicus). Montgomery (1963) has studied the effects of freezing and decomposition on the weights of raccoon lenses and attributed some of the variability in Sanderson's data (1961) to these causes. In Australia, Dudzinski and Mykytowycz (1961) have also used this method for aging wild rabbits (Oryctolagus cuniculus). Various investigators have attempted to use the lenses of birds for aging purposes. Payne (1961) reported this method to be unsatisfactory for house sparrows (Passer domesticus) and cited unsuccessful studies with other birds. Campbell and Tomlinson (1962) attempted to age chukar partridges (Alectoris graeca) with similarly discouraging results. Generally birds appear to have too much individual variation and overlap in weights between age-classes for successful use of this technique. The principal objective of the present

The Effect of X-Radiation on the Lens of the Embryo and the Adult Hen

X irradiation of tbe chick embryo or of the eye of the young hen does not lead to lens opacity. The immediate radiation damage to the bird lens appears from histological studies to be the same as it is in the mammalian lens, but it is suggested that the damaged cells are lysed from the bird lens whereas they remain in perpetuity in the lens of the mammal. (auth)