Cellular component transfer between photoreceptor cells of the retina

Photoreceptor transplantation is a potential therapeutic strategy for degenerative retinal diseases. Studies on mechanisms contributing to retinal regeneration and vision repair identified cellular components transfer (CCT) as playing a role, in addition to somatic augmentation (referred to as "cell replacement" in this paper). In CCT, donor photoreceptors shuttle proteins, RNA, and mitochondria to host photoreceptors through intercellular connections. The discovery of CCT in the transplantation context triggered a re-interpretation of prior transplantation studies that generally did not include specific CCT assays and thereby broadly emphasized the cell replacement model, reflecting the prevailing understanding of retinal transplantation biology at that time. In addition to clarifying our understanding of photoreceptor biology, CCT has raised the possibility of developing treatments to replenish molecular deficiencies in diseased photoreceptor cells. As the CCT field evolves, investigators have used diverse terminology, and implemented different CCT assays following transplantation in animal models. The non-standardized terminology of CCT and absent minimal assay standards for detection can hinder communication between investigators and comparison between studies. In this review, we discuss the current understanding of CCT, provide an overview of transplantation and regeneration studies in small and large animals, and propose terminology and a minimal assay standard for CCT. Further research on CCT may eventually provide new avenues to treat a range of hereditary and acquired retinopathies while illuminating mechanisms of cell-cell interaction in the retina.