Selective cell uptake of modified Tat peptide-fluorophore conjugates in rat retina ex vivo and in vivo models

PURPOSE. To determine the pattern of retinal uptake of modified Tat peptide-fluorophore conjugates in the rat after ex vivo application and intravitreal injection. METHODS. Modified Tat peptide (RKKRRORRRGC) was conjugated at the C terminus to Alexa Fluor 594 to enable visualization of uptake. In the ex vivo model, posterior segments were incubated for up to 120 minutes in peptide solution. In the in vivo model, intravitreal injections of 5 mu L peptide solution were performed in anesthetized rats, which were then euthanatized from I hour to 7 days after injection. Retinal and optic nerve paraffin sections were examined for fluorescent labeling. Immunohistochemistry for retinal cell markers was performed to identify cell types exhibiting uptake. RESULTS. The pattern of labeling seen in retinal sections was highly similar for the ex vivo and in vivo experiments, with specific uptake by retinal ganglion cells (RGCs) and by a subset of inner nuclear layer cells. The pattern of labeling remained specific even at the later time points. In the in vivo model, fluorescence was also noted in the nerve fiber layer and anterior optic nerve, extending posteriorly along the optic nerve at later time points. CONCLUSIONS. A specific pattern of uptake for modified Tat peptides was consistently seen in the rodent retina. Given the preferential uptake of these peptides by RGCs and the potential to conjugate diverse moieties, modified Tat peptides may be useful for delivery of therapeutic agents or molecular imaging probes to RGCs.