Bone marrow-derived stem cells differentiate into retinal pigment epithelium-like cells in vitro but are not able to repair retinal degeneration in vivo

The bone marrow (BM) is home to different stem/progenitor populations, including tissue-committed stem cells. In this context, we have cocultured BM-derived stem cells (BMSC) in order to investigate their differentiation capacity towards the retinal pigment epithelial (RPE) lineage in vitro. Furthermore, pre-differentiated BMSC were transplanted into the pharmacologically damaged subretinal space to determine their rescue ability in vivo. BM was harvested from the tibias and femurs of adult GFP C57BL/6 mice. Differentiated hematopoietic cells were removed by lineage depletion, and CD45 BMSCs were separated by magnetic activated cell sorting (MACS). To induce differentiation, the cells were then cocultured with murine RPE for 10 days, and retinal markers were assessed using immunohistochemistry (IHC). To induce retinal degeneration, mice were treated with sodium iodate (NaIO3). Seven days later, approx. 60,000 pre-differentiated GFP BMSC, sorted by FACS, were transplanted subretinally. Optical coherence tomography (OCT) was used to follow the transplants and to quantify the retinal thickness over time. Visual acuity was measured concurrently using the optokinetic reflex (OKR). Finally, IHC was performed to investigate the expression of retina-specific markers in the transplants. CD45 BMSC adopted an RPE-like elongated morphology and showed expression of the RPE markers RPE65 and bestrophin after coculture. After transplantation of CD45 BMSC, visual acuity increased in individual animals compared to the contralateral control eye, but did not reach baseline levels. Additionally, no significant increase in retinal thickness in the transplanted eye was found. However, the cells were detectable in the subretinal space for up to 28 days and expressed the RPE markers RPE65 and bestrophin. In summary, the BMSC differentiated into RPE-like cells but were not able to restore visual function or rescue retinal morphology after subretinal transplantation.