# Direct Conversion of Mouse Fibroblasts into Photoreceptor-like Cells

## TL;DR

Scientists converted mouse fibroblasts into photoreceptor-like cells using three transcription factors, which could help treat blinding diseases.

## Contribution

A three-transcription-factor combination (CNO) enables direct conversion of fibroblasts into functional photoreceptor-like cells.

## Key findings

- The CNO combination (Crx, Nrl, Otx2) is sufficient to convert fibroblasts into photoreceptor-like cells.
- Induced cells express photoreceptor markers and integrate into the mouse retina's outer nuclear layer.
- This method bypasses pluripotent stem cells, offering a safer approach for cell replacement therapies.

## Abstract

What are the main findings?
The study identified a minimal combination of just three transcription factors—Crx, Nrl, and Otx2 (CNO)—that is sufficient to directly convert MEFs into photoreceptor-like cells.The induced photoreceptor-like cells expressed a comprehensive panel of photoreceptor-specific markers. Notably, upon transplantation into adult mouse retinas, these induced photoreceptor-like cells migrated and integrated into the outer nuclear layer and continued to express photoreceptor markers in vivo.

The study identified a minimal combination of just three transcription factors—Crx, Nrl, and Otx2 (CNO)—that is sufficient to directly convert MEFs into photoreceptor-like cells.

The induced photoreceptor-like cells expressed a comprehensive panel of photoreceptor-specific markers. Notably, upon transplantation into adult mouse retinas, these induced photoreceptor-like cells migrated and integrated into the outer nuclear layer and continued to express photoreceptor markers in vivo.

What are the implications of the main findings?
This work explores a feasible method to generate photoreceptor-like cells from an abundant and easily accessible somatic cell source (fibroblasts). The direct conversion approach bypasses the pluripotent stem cell stage, making the process faster and eliminating the associated risk of teratoma formation, thereby presenting a potentially safer and more practical strategy for future cell replacement therapies for blinding diseases like AMD and RP.The finding that the CNO combination is the core driver of reprogramming provides preliminary experimental evidence for the synergistic role of these factors in defining photoreceptor identity and cell-based therapies for the photoreceptor repair.

This work explores a feasible method to generate photoreceptor-like cells from an abundant and easily accessible somatic cell source (fibroblasts). The direct conversion approach bypasses the pluripotent stem cell stage, making the process faster and eliminating the associated risk of teratoma formation, thereby presenting a potentially safer and more practical strategy for future cell replacement therapies for blinding diseases like AMD and RP.

The finding that the CNO combination is the core driver of reprogramming provides preliminary experimental evidence for the synergistic role of these factors in defining photoreceptor identity and cell-based therapies for the photoreceptor repair.

The purpose of our study is to explore the potential of a transcription factor-based strategy for directly converting mouse fibroblasts into photoreceptor-like cells. The mouse cDNAs of Ascl, Crx, Ngn1, Nrl, and Otx2 were cloned into a modified commercial adenoviral vector. Mouse embryonic fibroblasts (MEFs) were isolated from E13.5 embryos, and mouse postnatal fibroblasts (MPFs) were isolated from three-day-old mice. A pool of adenoviruses containing five genes was prepared to infect MEFs or MPFs once daily for two days. The MEFs or MPFs were incubated in a specific medium supplemented with forskolin and were changed every two days. After 7 or 14 days, the photoreceptor-like cells were assayed via immunofluorescence or polymerase chain reaction with reverse transcription (RT–PCR). The photoreceptor-like cells were then transplanted into adult C57BL/6 mouse retinas and were assessed by immunofluorescence 14 days following transplantation. Screening from a pool of five candidate genes, we reported that a combination of only three factors—Crx, Nrl, and Otx2—was sufficient to convert mouse embryonic and postnatal fibroblasts into photoreceptor-like cells. The induced photoreceptor-like cells expressed photoreceptor-specific proteins such as Recoverin, Rhodopsin, and Opsin and integrated into the outer nuclear layer of the retina following transplantation. This exploratory study provides preliminary evidence that fibroblasts can be directly converted into photoreceptor-like cells, suggesting a cellular model and potential source for future transplantation strategies aimed at retinal repair.