Investigating the use of mycophenolic acid for the treatment of Retinitis Pigmentosa

Abstract

Purpose: Retinitis pigmentosa is a degenerative genetic disorder in which pho- toreceptor cell death can be connected to high cGMP levels. This is exemplified by the rd1 mouse model where a mutation in the Pde6b gene leads to decreased cGMP hydrolysis in photoreceptors. While the enzyme guanylyl cyclase (GC) synthesizes cGMP in photoreceptors, inosine monophosphate dehydrogenase-1 (IMPDH1) catalyzes the rate-limiting step in the biosynthesis leading up to cGMP. Hence, inhibiting IMPDH1 may be a strategy for the reduction of photoreceptor cGMP levels and cell death. I explored the capacity of the registered immunosup- pressive drug mycophenolic acid (MPA) to reduce photoreceptor cGMP levels and cell death in rd1 retinal explant cultures. Methods: The retinal expression patterns of IMPDH1 and GC were assessed in wild-type and rd1 mouse retina using immunofluorescence. Organotypic retinal explant cultures derived from post-natal day (P) 5 rd1 mice were treated with MPA in six different concentrations ranging from 1 to 1000 μM. Parallel control cultures received vehicle or medium only. The retinal explants were cultured from P5 to P11 with medium changes every two days. At P11, the explants were fixed in paraformaldehyde, cryosectioned, and stained for cell death using the TUNEL- assay. TUNEL-positive cells were counted and compared with controls. To de- termine treatment effects on cone photoreceptors, these were quantified using PNA labelling. Results: IMPDH1 was expressed in photoreceptor inner segments, outer plexi- form layer, neurites in the outer nuclear layer, and cell bodies in the inner nuclear layer. GC staining labelled the outer segments of photoreceptors. In rd1 ex vivo explant cultures treated with MPA, cell death decreased in a concentration-de- pendent manner. Between 40 and 250 μM cell death was significantly reduced, while at 1000 μM cell death was strongly increased, and retinal structure was lost. No significant differences in cone cell numbers were found by PNA staining. Conclusion: The localization of IMPDH1 expression to photoreceptor inner seg- ments makes it a potentially druggable target for the treatment of retinitis pigmen- tosa. Importantly, the treatment with MPA revealed a neuroprotective effect in a concentration range achievable in a clinical setting.

Dissertation ist gesperrt bis 31. März 2026 !