Retinal transplantation of photoreceptors results in donor–host cytoplasmic exchange

Abstract

Pre-clinical studies provided evidence for successful photoreceptor cell replacement therapy. Migration and integration of donor photoreceptors into the retina has been proposed as the underlying mechanism for restored visual function. Here we reveal that donor photoreceptors do not structurally integrate into the retinal tissue but instead reside between the photoreceptor layer and the retinal pigment epithelium, the so-called sub-retinal space, and exchange intracellular material with host photoreceptors. By combining single-cell analysis, Cre/lox technology and independent labelling of the cytoplasm and nucleus, we reliably track allogeneic transplants demonstrating cellular content transfer between graft and host photoreceptors without nuclear translocation. Our results contradict the common view that transplanted photoreceptors migrate and integrate into the photoreceptor layer of recipients and therefore imply a re-interpretation of previous photoreceptor transplantation studies. Furthermore, the observed interaction of donor with host photoreceptors may represent an unexpected mechanism for the treatment of blinding diseases in future cell therapy approaches.

Transfer of pathogenic and nonpathogenic cytosolic proteins between spinal cord motor neurons in vivo in chimeric mice

“…Interestingly, cytosolic transfer of nonpathogenic proteins that seems to resemble the transfer observed here has recently been reported to occur between retinal photoreceptor cells in the context of transplantation experiments injecting fluorescently labeled donor photoreceptor cells into the subretinal space of adult wild-type mice (26)(27)(28). Transfer of cytosolic proteins-GFP, cre recombinase, and α-transducin-was observed between donor photoreceptor cells and the neighboring host photoreceptors, and transfer appeared to occur bidirectionally, taking place in the absence of nuclear transfer/ fusion.…”

Cone fusion confusion in photoreceptor transplantation

“…With discordant genetic fluorescent markers in host and donor cells, they showed not only double labelling, but also radial alignment of labelled retinal cells in the ONL with the same colour cell residing in the subretinal space (12). The mechanism of cytoplasmic fusion was subsequently verified and published in three seminal papers in Nature Communications at the end of 2016 (13)(14)(15). This observation has now overturned many of the key papers that had set the field of photoreceptor transplantation up to that point.…”

“…In the first instance they were unable to identify any cells with double nuclei. Polyploidy was previously identified as the mechanism of haemopoetic cell fusion and transfer of donor cell markers (16), but photoreceptor transplantation is different, because the host and donor cells make contact via the cytoplasm and exchange proteins whilst each maintains a separate nucleus (12)(13)(14)(15). In the second instance, they used antibodies to a human stem cell marker STEM121 (SC121) to confirm that the green fluorescent ONL photoreceptor cells were human derived.…”

Rebuilding the Missing Part—A Review on Photoreceptor Transplantation

“…Until very recently donor photoreceptors were understood to physically migrate and integrate into the recipient outer nuclear layer (ONL) as identified by fluorescent donor proteins being present within the host tissue. However, three recent studies provided evidence that structural integration represents a minor mechanism for this observation and that exchange of cell material between donor and host photoreceptors account for the majority of reporter labeled cells seen in the host retina (Pearson et al, 2016; Santos-Ferreira et al, 2016a; Singh et al, 2016). These novel results are discussed in detail in section “Paradigm Shift in Photoreceptor Replacement Therapy”, but for simplicity purposes and taking in account the historical context, we will describe all reporter labeled cells found within the host retina of previous studies as “integrated” or refer generally to improved/reduced transplantation outcomes.…”