Focusing on regenerative growth in the mammalian optic tract, the goal of the project is to understand age-dependent changes in the ability of brain connections to regenerate after injury, and to sprout new connections. Explant co-cultures are used to view axonal regeneration from retina to midbrain tectum in vitro. Using this model, it has been shown that here are two distinct major declines in regenerative potential in development. the first occurs suddenly on postnatal day 4, there is a programmed failure of regrowth capacity in an estimated 80% of the retinotectal axons, due to a critical change in the retina. The second is a slower decline in regrowth by the remaining 20%, attributable to changes in the brain substrate; at maturity, no regeneration is seen. Thus, control of substrate factors should be able to prevent the second decline in regenerative potential.
An aim of he project is to find conditions for maximizing the regeneration of this subpopulation, using growth factors and blocking of inhibitory factors. An important cause of inhibition is the tissue reaction at the site of injury, resulting in scar formation. The tissue culture model will be used to gain new information on how the growth-inhibiting nature of the lesion scar changes during development, and how blockage by such scars can be prevented. The first, and larger, decline in regenerative capacity results from a change in the retinal cells of origin of the optic nerve and tract, and is correlated with the onset of naturally occurring axonal pruning and loss, with death of many of the retinal ganglion cells. Genes coding for proteins which can inhibit programmed cell death in the retina has been found in several labs, and transenic mice which over-express one of these proteins have been created. Such mutants will be tested for regenerative potential of retinotectal axons at various ages, both in vitro and in vivo, to discover whether the first major decline in regrowth potential is decreased. In addition, new tests of return of vision after partial regeneration of a severed optic tract will be carried out, with attention to the nature of the functional capacity of a tract containing a greatly reduced number of axons, a relatively neglected issue in regeneration studies.
