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.
