Runx1 expression defines a subpopulation of displaced amacrine cells in the developing mouse retina

AML1/Runx1 (Runx1) is a mammalian transcription factor that plays critical roles in regulating the differentiation of a number of different cell types. In the present study, we have utilized mice expressing beta-galactosidase (beta-gal) under the control of the Runx1 promoter to characterize the spatiotemporal expression pattern of Runx1 during retinogenesis. Expression of beta-gal was first detected at embryonic day 13.5 in post-mitotic cells located in the inner retina and overlapped with expression of the early amacrine and ganglion cell marker protein Islet1. During subsequent developmental stages, the number of beta-gal-positive cells increased in a central-to-peripheral gradient until late embryogenesis but then decreased in the early post-natal retina. beta-gal-positive cells were located primarily in the ganglion cell layer by late embryonic/early post-natal stages and were identified as a subpopulation of displaced amacrine cells by the continued expression of Islet1, as well as Pax6, and the coexpression of the amacrine cell subtype-specific markers choline acetyltransferase, calretinin and the 65-kDa isoform of glutamic acid decarboxylase. These findings identify Runx1 as a novel marker for a restricted amacrine cell subtype and suggest a role for this gene in regulating the post-mitotic development of these cells.