Sufficient Activity of the Ubiquitin Proteasome System in Aged Mice and During Retinal Degeneration Supports DHFR-Based Conditional Control of Protein Abundance in the Retina

Abstract

The Escherichia coli dihydrofolate reductase (DHFR) destabilizing domain (DD) serves as a promising approach to conditionally regulate protein abundance in a variety of tissues. In the absence of TMP, a DHFR stabilizer, the DD is degraded by the ubiquitin proteasome system (UPS). To test whether this approach could be effectively applied to a wide variety of aged and disease-related ocular mouse models, which may have a compromised UPS, we evaluated the DHFR DD system in aged mice (up to 24 mo), a light-induced retinal degeneration (LIRD) model, and two genetic models of retinal degeneration (rd2 and Abca4-/- mice). Aged, LIRD, and Abca4-/- mice all had similar proteasomal activities and high-molecular weight ubiquitin levels compared to control mice. However, rd2 mice displayed compromised chymotrypsin activity compared to control mice. Nonetheless, the DHFR DD was effectively degraded in all model systems, including rd2 mice. Moreover, TMP increased DHFR DD-dependent retinal bioluminescence in all mouse models, however the fold induction was slightly, albeit significantly, lower in Abca4-/- mice. Thus, the destabilized DHFR DD-based approach allows for efficient control of protein abundance in aged mice and retinal degeneration mouse models, laying the foundation to use this strategy in a wide variety of mice for the conditional control of gene therapies to potentially treat multiple eye diseases.