Swine PAX6-, ISL1+, RCVRN+ Cone Progenitors and NRL+ Rod progenitors Arise Sequentially From Distinct Retinal Stem Cell Populations and Display Transient, Nonoverlapping Regenerative Potential After Retinal Transplantation

Abstract

Purpose: To identify and characterize rod and cone photoreceptor progenitors in the developing swine retina, and assessed their capacity and time course for differentiation into cones and rods after retinal transplantation.

Methods: Adult swine retinas and embryonic retinas were harvest and processing for histology and immunostaining. Embryonic retinal cells from GFP-expressing transgenic swine were transplanted into the subretinal space of adult pigs following retinal damage with iodoacetic acid to assess the time course and capacity of retinal progenitors for differentiation into rods and cones.

Results: Cones and rods are completely segregated in the swine retina with cones comprising the outer two rows of the outer nuclear layer (ONL). This segregation of rods and cones allowed us to follow these two lineages in situ. Cone progenitors are in place in the outer retina at midgestation, and marked by loss of PAX6, transient expression of the Islet1 (ISL1) and early Recoverin (RCVRN) expression. The NRL is expressed and rod progenitors are derived from underlying stem cells, distinct from the preformed cone progenitors. We then transplanted different stage of retinal progenitors into adult retina. At embryonic day (E)50 where PAX6-, ISL1+, Recoverin+ cone progenitors are present but NRL+ rod progenitors are not yet evident, transplanted cells only gave rise to cones. At E65 where NRL expression is evident, transplanted stem cells only gave rise to rods, indicating a transient potential for cone progenitors that prevents their functional overlap with rod progenitors. By E105, progenitors had differentiated to form the retinal layers, and transplantation of retinal cells led to few integrated differentiated photoreceptors, demonstrating that only progenitor cells effectively integrate into the retinal following transplantation.

Conclusions: Distinct populations of rod and cone progenitors are present in the embryonic swine retina. Consistent with our identification of distinct sequentially appearing populations of cone and rod progenitors based on unique marker expression and spatial position, the transplantation assays link the appearance of these progenitors to sequential cone and rod differentiation potential.

Keywords: 648 photoreceptors • 497 development • 741 transplantation