For most, the idea of loosing one's eyesight is unbearable, and despite many technical breakthroughs, blindness still remains largely untreatable. Douglas Blackiston from Tufts University, USA, explains why: ‘Implanting an artificial [retinal implant] or biological replacement eye would require connecting it to the nervous system in some way. For those with damaged optic nerves, or those missing the eye completely, retinal implants are not possible treatment options.’ If connection through the optic nerve is not possible, could replacement eyes connect elsewhere? And just how hardwired are our nervous systems to expect data from specific organs in pre-determined locations? This question has intrigued Blackiston's supervisor, Michael Levin, for many years. He realised that they could use blind Xenopus tadpoles to investigate how adaptable the brain and central nervous system is to receiving information from abnormally located eyes (p. 1031).
To begin, Blackiston induced blindness in the tadpoles by surgically removing their eyes, with some of the blinded amphibian patients also receiving donor eyes, which were transplanted onto unusual positions along their torsos and tails. Whilst grafting over 200 tiny donor eyes was painstaking work, it was the next step that proved most challenging, recalls Levin. The investigating duo needed to develop a test to determine whether these transplanted ectopic eyes allowed blind tadpoles to see. ‘While physiology can show that an eye sends action potentials [electrical signals] in response to light, a behavioural regime is necessary to show that the brain is receiving such data and processing the information in a meaningful way’, Levin points out.
After nearly a year of hard work the pair came up with a suitable eye test for blind tadpoles. They placed their amphibious subjects in a well where half of the dish was illuminated with red light and the other half with blue light, which they inverted at regular intervals. During training sessions, whenever the tadpoles ventured into areas bathed in red light they received a little warning zap of electricity. After a break the tadpoles were tested to see whether they had learnt to associate the red light with electrical punishment and whether they would stick to the blue side of the dish. While blind tadpoles never showed a preference for blue light, six tadpoles with donor eyes behaved like their full-sighted relatives and showed a learnt desire to remain in the safe, blue-illuminated areas. These fortunate six tadpoles obviously were able to see through their new ectopic eyes.
However, the team had tested 134 tadpoles endowed with transplanted eyes, so what made these six tadpoles different? The answer lies in the nerve patterns extended by the donor eye after transplantation. Levin and Blackiston had cleverly used tadpoles expressing a red fluorescent protein as donors, and so they were able to see the pattern of red neurons extending from the new ectopic eye using a microscope. Half of the transplant patients showed no innervation. Of the remaining half, 26% showed neurons projecting from the donor eye towards the gut and 24% had neurons extending towards the spine. It was within this latter group that the six lucky tadpoles with colour vision fell.
‘The [tadpole's] ability to see when ectopic eyes are connected to spinal cord and not directly to the brain was stunning’, remembers Levin. ‘We believe that future biomedical treatments for sensory or motor disorders may not need to target the original brain locations to restore function’, he adds. It is clear that his finding could radically change our future approach to regenerative medicine for a wide range of disorders.