As FD-OCT is noninvasive, it should be possible to visualize and monitor, in vivo, the time course of disease progression in individual animals. To assess the utility of FD-OCT for imaging progressive retinal degeneration in a given tadpole, we used a drug-inducible
X. laevis model of retinal degeneration that allows rapid induction of rod cell death by direct activation of apoptosis (programmed cell death).
13 These transgenic
X. laevis express a drug-activatable form of caspase-9 (iCasp-9) in the rod photoreceptors. Caspase-9 is an initiator caspase that, once activated, triggers the caspase cascade, resulting in apoptosis. Administration of the drug AP20187 to these transgenic tadpoles induces rapid death of the rod photoreceptors, with ablation of most of the rods by day 5 afterward.
13 We used FD-OCT to monitor the retina of stage 47/48 tadpoles before administering the drug and at various time points over 4 days after drug administration. At each time point, a subset of tadpoles were euthanatized for histology.
Figure 4 shows representative images of iCasp-9 tadpole retinas at three time points: immediately before, 2 days after, and 4 days after administration of the drug. Before exposure to the drug, the INL, ONL, OS and RPE can be observed from FD-OCT, indicative of a healthy retina. At the intermediate time point (2 days) after drug exposure, histology showed that the majority of rods were dysmorphic and rod OS bodies were seen within the RPE, indicating rod cell death and clearing of dead rods by the RPE. We observed an analogous disruption in the OS layer in the FD-OCT images and a corresponding disruption in the RPE layer as well (
Fig. 4, lower panels). The disruption in the RPE layer in OCT images indicates a change in reflectivity and backscatter in the RPE, consistent with the accumulation of phagosomes in the RPE. At day 4 after drug administration, histology indicated that most of the dead rods were cleared, and relatively few phagosomes remained in the RPE. Similarly, the OS layer could not be identified in FD-OCT retinal scans of 4 day postdrug tadpoles, and the RPE appeared as a featureless single layer. These data demonstrate that FD-OCT is able to image intermediate rod degeneration as well as phagosomes in the RPE in vivo.