The αA‐crystallin gene expression in differentiating lens fiber cells, FGF signaling, and transcriptional factories

Abstract

PurposeEmbryonic lens development requires intricate temporal and spatial control of gene expression that is executed through specific gene regulatory networks regulated by FGF signaling. The αA‐crystallin is the most abundant mammalian lens protein; transcriptional studies of this gene provide detailed insights into the molecular mechanisms governing lens fiber cell differentiation.MethodsDynamic changes in chromatin structure and gene expression during lens developments were evaluated by a combination of RNA‐seq and ATAC‐seq. Expression of αA‐crystallin (ch1) and other crystallins (Cryga, Crybb1, and Crybb3, localized on chromosomes 1 and 5, respectively) in individual nuclei of the developing mouse lens were analyzed by RNA‐FISH. Transgenic mice with c‐Maf promoter were generated and expression of EGFP was evaluated in mouse embryos. Localization of c‐Jun, c‐Maf, Etv5/ERM, and Pax6 was determined by ChIP using the c‐Maf and Cryaa loci and mouse lens chromatin. Co‐transfection experiments with c‐Jun and Etv5 were conducted using c‐Maf and αA‐crystallin reporters.ResultsTranscriptional factories, a subset of topologically associating domains (TADs), are formed between different crystallin loci in lens cell nuclei. These specific TADs are enriched by RNA polymerase II and other proteins. Novel FGF2‐responsive region in the c‐Maf promoter was found. Both c‐Maf and Cryaa regulatory regions contain arrays of AP‐1 and Ets‐binding sites determined by ChIP assays.ConclusionsCollectively, these studies show that the lens fiber cell nuclei spatially organize crystallin loci into TADs, and that FGF signaling up‐regulates expression of αA‐crystallin both directly and indirectly via up‐regulation of c‐Maf. These molecular mechanisms are applicable for other crystallins and genes highly expressed in terminally differentiating lens fibers.