RNA‐binding proteins and their downstream regulatory networks in lens development and cataract

Development of the ocular lens involves dynamic control over gene expression, perturbations of which can cause congenital cataract. Thus far, several key proteins in the lens development program that are involved in signalling, transcription, and more recently, post‐transcriptional control – all distinct regulatory mechanisms that together determine the proteome – have been identified. To understand the pathology of congenital cataract, it is critical to continue to identify key regulatory genes that play an important role in the lens, and define their relationships with other regulators in the form of networks. We have characterized eye/lens defects in animal knockout models for several conserved RNA‐binding proteins (RBPs) such as Celf1, Caprin2, Rbm24 and Tdrd7 and demonstrated their important role in mediating post‐transcriptional networks in the lens. However, there are still many mechanistic events that remain to be defined in the lens. For example, while it is known that abundant levels of specific mRNAs (e.g. crystallins) need to be made in the lens, it is currently unknown whether regulators that control the ubiquitously important “transcription elongation” process, are specifically required for achieving optimal levels of such highly expressed transcripts in fibre cells. Furthermore, how these transcriptional and post‐transcriptional regulatory pathways coordinately function to achieve optimal transcriptome and proteome in lens epithelial and fibre cells is unknown. Here, we applied the bioinformatics tool iSyTE to identify the transcription elongation factor Ell2 and the RNA‐binding protein (RBP) Elavl1 in the lens. Lens‐specific conditional knockout mouse models for Elavl1 and Ell2 were generated using Pax6GFPCre . Lens tissue from Elavl1 cKO and Ell2 cKO mice was characterized by immunostaining, RNA‐sequencing (RNA‐seq) and RT‐qPCR. Elavl1 cKO and Ell2 cKO lenses exhibit morphological defects that are detected in embryonic and early postnatal stages, respectively. RNA‐seq identifies mis‐expression of many genes in both cKO lenses. Ell2 cKO lenses show significantly reduced transcript levels of a cohort of fibre‐expressed genes such as crystallins, while Elavl1 cKO lenses show abnormal mRNA levels of several novel factors in the lens, including Ell2. These data demonstrate that the new iSyTE‐predicted genes, the RBP, Elavl1, and transcription elongation factor, Ell2, mediate regulation of key genes in lens development, and provide new insights into the molecular pathology of cataract and lens defects in animals deficient in these genes. Importantly, in addition to Elavl1 cKO lenses, Ell2 is found to be mis‐expressed in mouse cKO lenses for another RBP, Celf1, suggesting a crosstalk between transcriptional and post‐transcriptional regulatory networks that governs optimal transcriptome and proteome in lens development.