Direct reprogramming of skin cells into photoreceptors in mice

The end result of many retinal dystrophies is photoreceptor cell death and the irreversible loss of vision. To prevent the progressive loss of retinal neurons, and to slow down or reverse the progression of the disease, gene therapies are currently under study to correct or replace the altered genes responsible for different retinopathies.

Another therapeutic strategy that is under intensive investigation is the replacement of damaged retinal cells, which is known as cell therapy.

Cell therapy generally uses cells obtained from induced pluripotent stem cells (or iPS cells), which have the ability to differentiate and become almost any cell type in our body. Once programmed to become photoreceptor cells, they are transplanted into the retina of a retinal degeneration animal model to study their function and ability to restore vision. This process is cumbersome and time-consuming, and can take about six months.

In a study published April 15 in Nature, researchers at the University of North Texas Health Science Center and the National Eye Institute (NEI) have shown, for the first time, that it is possible to reprogram fibroblasts (a type of skin cell) directly into photoreceptors, bypassing the requirement for pluripotent stem cells. The technique allows obtaining, in just 10 days, functional photoreceptor cells ready for transplantation. Direct reprogramming had previously been applied to generate neurons, astrocytes, and cardiomyocytes, but it had not been successful before for obtaining photoreceptors.

The researchers have identified a set of five molecules that can chemically induce transformation of fibroblasts into photoreceptor-like cells (what researchers called chemically induced photoreceptor-like cells or CiPCs). Characterization of reprogrammed rod photoreceptor cells by gene expression analysis by Anand Swaroop’s team at NEI showed that CiPCs have a similar profile to that of native rod photoreceptors. On the other hand, fibroblast-specific genes have been silenced.

To check whether CiPCs were able to activate existing neural retina circuitry and restore visual function, the researchers transplanted CiPCs into the subretinal space of retinal degeneration rd1 mice, which harbor a Pde6b gene mutation, and analyzed their pupillary reflex. In low-light conditions, the pupillary reflex depends on the function of the rods and it has been found that it is a robust method to measure photoreceptor function after cell transplantation in the retina.

One month later, six of the 14 transplanted mice showed robust pupillary constriction, which was not observed in any of the untreated rd1 mice. They also spent more time in the dark, a behavior that requires good visual function. The researchers also observed that, three months later, the transplanted cells had integrated into the retina and established connections with the inner retina neurons.

Once optimized, this protocol, more efficient and faster than previous approaches based on iPS cells, could be used to design new treatments for retinal diseases and to study the appropriate disease models. The University of North Texas has a patent pending on this technology and the start-up company CIRC Therapeutics plans to commercialize therapies based on it.