An accidental discovery could pave the way to one day coaxing stem cells to develop into human eyes in the lab

A team of scientists at the University of Warwick in England studying the development of motility in frogs found that a certain ectoenzyme (a cell-surface protein) injected into a tadpole embryo triggered the development of tissues that eventually form eyes.

Further experimentation led the researchers to conclude that the surface protein is, in fact, an early player in the cellular cascade that leads to eye formation. Researchers say the finding could be harnessed in the future to make an "eye in a dish," a tool that would be invaluable in coaxing stem cells to develop into ocular tissues.

"Our study provides clear discovery of upstream signals controlling a previously known pathway controlling eye development and therefore provides a step closer to being able to manipulate eye development," explains Elizabeth A. Jones, a professor in the university's biological sciences department and a co-author of the study published in Nature.

Ectonucleoside triphosphate diphosphohydrolase 2 (E-NTPDase2) is an ectoenzyme that, along with family members E-NTPDase1 and E-NTPDase3, is known to degrade the chemical compound ATP (adenosine triphosphate) into ADP (adenosine diphosphate) for the purpose of sending messages to cells to change the fleet of proteins they are producing. Primarily, ATP functions as the energy currency of cells but, in some varieties, a tiny amount is secreted into the space between cells, where it latches onto a neighbor to induce particular responses and modulations. Both ATP and ADP, known as purines, can transmit signals to cells that change their developmental activity. The research team found that when it increased levels of E-NTPDase2 in tadpole embryos that consisted of only eight cells, they could cause parts of the eye to form not only on the heads of the amphibians, but also in tissues in other parts of their bodies, including their tails. Minute pulses of ATP are released into extracellular areas mostly by cells in the head where the eyes are supposed to develop. Jones notes that at temporally distinct moments, other cells in the body may expel small packets of ATP, which in the presence of E-NTPDase2 can cause eye tissue to form.

Through many rounds of analysis, both by amplifying and decreasing the levels of certain chemicals as well as knocking out the function of certain genes that code for proteins that regulate eye development (called eye field transcription factors), the scientists determined that E-NTPDase2 (although not E-NTPDase 1 or 3) was the only ectoenzyme that could drive eye development. Further, they determined that it must act early in the pathway that leads to the formation of the eye. After it converts ATP to ADP, the level of the latter accumulates outside the cell and the purine can bind to a purine receptor called P2Y1.

"It is the activation of this receptor that either directly or indirectly turns on the expression of the eye-field transcription factors," Jones says. "We don?t quite know the mechanisms involved between going from the receptor and turning on the genes, and this is an area for future investigation."

Jones and her colleagues believe that most of the eye development pathway is conserved between frogs and humans. Damage to human chromosome 9 (of the cell's 24 pairs) where the gene that codes for E-NTPDase2 resides is known to cause eye and brain defects, such as microphthalmia—literally, small eyes. This means that down the road, researchers might be able to create an "eye in a dish."

"This work may have interesting implications for the stem cell field," says Richard Lang, a professor of developmental biology at the Cincinnati Children's Hospital Research Foundation. "The activity of purine signaling in inducing eye field precursors," he says, "might be a very useful tool for the culture dish–generation of progenitor cells for a variety of eye cell types."