Donor and host photoreceptors engage in material transfer following transplantation of post-mitotic photoreceptor precursors

Abstract

Photoreceptor replacement by transplantation is proposed as a treatment for blindness. Transplantation of healthy photoreceptor precursor cells into diseased murine eyes leads to the presence of functional photoreceptors within host retinae that express an array of donorspecific proteins. The resulting improvement in visual function was understood to be due to donor cells integrating within host retinae. Here, however, we show that while integration occurs the majority of donor-reporter-labelled cells in the host arises as a result of material transfer between donor and host photoreceptors. Material transfer does not involve permanent donor-host nuclear or cell-cell fusion, or the uptake of free protein or nucleic acid from the extracellular environment. Instead, RNA and/or protein are exchanged between donor and host cells in vivo. These data require a re-evaluation of the mechanisms underlying rescue by photoreceptor transplantation and raise the possibility of material transfer as a strategy for the treatment of retinal disorders.

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.…”

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.…”

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.…”