Abstract
Purpose: :
Development in marsupials takes place predominantly ex utero with the young attached to a nipple in the mother’s pouch. Substantial changes in the milk composition during pouch life lead to changes in the digestive and metabolic functions of the developing animal. This study was undertaken to examine if there may also be developmental changes in the production of lens proteins during pouch life in young kangaroos.
Methods: :
Fresh lenses were obtained at official culls from kangaroos ranging in age from 3 months to 10 years. Ages were determined using both molar progression and total lens protein content. Wet weights were recorded and then the lenses were divided into concentric layers by controlled dissolution using PBSA. Samples were taken for determination of protein contents, which were then used to determine the age of the layer removed. Protein size distributions were determined by fractionation of the extracts using GPC-HPLC and polypeptide contents by SDS-PAGE.
Results: :
Lens growth and protein accumulation from birth to weaning were continuous and extended smoothly into post pouch life. Three major crystallin classes, α-, β- and γ-, comprising approximately 30, 40 and 30%, respectively -were identified in the very young PY animals. With increasing time of development, there was a gradual increase in the β-crystallin population and a decrease in the γ-crystallins. SDS-PAGE revealed that the polypeptide composition of α-crystallin changes with developmental age, from1:3, A:B early in development to 10:1 A:B after weaning. At around the time that the young first released the nipple, the synthesis of a 37.5kDa polypeptide, identified as µ-crystallin, started to increase from around 0.5% of the total proteins and reached around 14% of the total by the time the animal was fully weaned. Purification of µ-crystallin was achieved by affinity chromatography on Blue-Sepharose. The native protein was found to be a tetramer of a 37.5 kDa polypeptides.
Conclusions: :
µ-crystallin synthesis is stimulated around the time that the young kangaroo first releases the nipple and ventures out of the pouch. Around the same time, the source of lens energy changes from galactose to glucose. Recent identification of µ-crystallin as a thyroid binding protein, suggests it may play a role in regulation of carbohydrate metabolism in the periphery of the adult kangaroo lens.
Keywords: development • crystallins • metabolism