Rod photoreceptors protect from cone degeneration-induced retinal remodeling and restore visual responses in zebrafish.

Humans are largely dependent upon cone-mediated vision. However, death or dysfunction of rods, the predominant photoreceptor subtype, results in secondary loss of cones, remodeling of retinal circuitry, and blindness. The changes in circuitry may contribute to the vision deficit and undermine attempts at restoring sight. We exploit zebrafish larvae as a genetic model to specifically characterize changes associated with photoreceptor degenerations in a cone-dominated retina. Photoreceptors form synapses with two types of second-order neurons, bipolar cells, and horizontal cells. Using cell-specific reporter gene expression and immunolabeling for postsynaptic glutamate receptors, significant remodeling is observed following cone degeneration in the pde6c(w59) larval retina but not rod degeneration in the Xops:mCFP(q13) line. In adults, rods and cones are present in approximately equal numbers, and in pde6c(w59) mutants glutamate receptor expression and synaptic structures in the outer plexiform layer are preserved, and visual responses are gained in these once blind fish. We propose that the abundance of rods in the adult protects the retina from cone degeneration-induced remodeling. We test this hypothesis by genetically manipulating the number of rods in larvae. We show that an increased number and uniform distribution of rods in lor/tbx2b(p25bbtl) or six7 morpholino-injected larvae protect from pde6c(w59)-induced secondary changes. The observations that remodeling is a common consequence of photoreceptor death across species, and that in zebrafish a small number of surviving photoreceptors afford protection from degeneration-induced changes, provides a model for systematic analysis of factors that slow or even prevent the secondary deteriorations associated with neural degenerative disease.