The zebrafish as a model system for analyzing mammalian and native α-crystallin promoter function
Authors: Posner Mason , Murray Kelly , Eighinger Hayden , Drossman Amy , Haley Zachary , Nussbaum Justin , David Larry L , Lampi Kirsten J 
Published: January 2018

Previous studies have used the zebrafish to investigate the biology of lens crystallin proteins and their roles in development and disease. However, little is known about zebrafish α-crystallin promoter function, how it compares to that of mammals, or whether mammalian α-crystallin promoter activity can be assessed using zebrafish embryos. We injected a variety of α-crystallin promoter fragments from each species combined with the coding sequence for green fluorescent protein (GFP) into zebrafish zygotes to determine the resulting spatiotemporal expression patterns in the developing embryo. We also measured mRNA levels and protein abundance for all three zebrafish α-crystallins. Our data showed that mouse and zebrafish αA-crystallin promoters generated similar GFP expression in the lens, but with earlier onset when using mouse promoters. Expression was also found in notochord and skeletal muscle in a small percentage of embryos. Mouse αB-crystallin promoter fragments drove GFP expression primarily in zebrafish skeletal muscle, with less common expression in notochord, lens, heart and in extraocular regions of the eye. A short fragment containing only a lens-specific enhancer region produced no GFP expression, suggesting that these lens responsive elements in the mouse are not used in the zebrafish. The two paralogous zebrafish αB-crystallin promoters produced subtly different expression profiles, with the αBa promoter driving expression equally in notochord and skeletal muscle while the αBb promoter resulted primarily in skeletal muscle expression. Messenger RNA for zebrafish αa, aBa and αBb were all detected by 1 day post fertilization (dpf). Parallel reaction monitoring (PRM) mass spectrometry was used to detect αA, αBa, and αBb peptides in digests of zebrafish embryos. In whole embryos, αA-crystallin was first detected by 2 dpf, peaked in abundance by 4-5 dpf, and was localized to the eye. αBa was also detected in whole embryo at nearly constant levels from 1-6 dpf, was also localized primarily to the eye, and its abundance in extraocular tissues decreased from 4-7 dpf. In contrast, due to its low abundance, no αBb protein could be detected in whole embryo, or dissected eye and extraocular tissues. Our results show that mammalian α-crystallin promoters can be efficiently screened in zebrafish embryos and that their controlling regions are well conserved, although their use in each species may reflect evolutionary changes in developmental roles for α-crystallins. An ontogenetic shift in zebrafish αBa-crystallin promoter activity provides an interesting system for examining the evolution and control of tissue specificity. Future studies that combine these promoter based approaches with the expanding ability to engineer the zebrafish genome via techniques such as CRISPR/Cas9 will allow the manipulation of protein expression to test hypotheses about lens crystallin function and its relation to lens biology and disease.