E13.5 retinal progenitors induce mouse bone marrow mesenchymal stromal cells to differentiate into retinal progenitor-like cells

Background aims: Retinal progenitor cells (RPC) are an excellent resource for retinal replacement therapy but usually unavailable. We attempted to induce bone marrow mesenchymal stromal cells (BMSC) into RPC. Methods. BMSC and embryonic day 13.5 (E13.5) RPC derived from wild-type or enhanced green fluorescence protein (EGFP) transgenic (Egfp^+/+) mice were co-cultured in a transwell or re-aggregation system. Gene and protein expressions were investigated by reverse transcription-polymerase chain reaction (PCR) and immunofluorescence, respectively. Spontaneous cell fusion was evaluated by Chloromethylbenzamido derivative of 1,1′- dioctadecyl-3,3,3′,3′ - tetramethylindocarbocyanine perchlorate (CM-DiI) labeling together with EGFP tracing. Results. BMSC from both wild-type and Egfp^+/+ mice displayed similar spindle shapes. The undifferentiated BMSC already expressed immature neural markers but did not express Nfl, Gfap or the retina-related genes Pax6, Math5 and Brn3b. When co-cultured with E13.5 RPC in the transwell system, BMSC displayed transient expression of early retinal development genes, including Pax6, Math5 and Brn3b at 3 days, as well as long-term expression of Nfl (up to 21 days). No expression of the late photoreceptor gene rhodopsin could be detected at any time. In re-aggregation co-culture, E13.5 RPC induced EGFP-positive BMSC to express not only the early retinal development genes but also the late gene rhodopsin. Furthermore, a small fraction of BMSC could be induced to express the synaptophysin protein. Re-aggregation co-culture of CM-DiI-labeled BMSC and EGFP-positive E13.5 RPC displayed minimal co-localization of the two fluorescence signals. Conclusions. E13.5 RPC are capable of inducing BMSC towards an RPC fate. The differentiation is independent of cell fusion. Cytokines and cell-cell interactions exert this induction effect, but they have different functions.