Calpain-1 C2L domain peptide protects retinal photoreceptor cells in rhodopsin P347L transgenic rabbits

Retinitis pigmentosa (RP) leads to visual impairment by causing the death of photoreceptor cells. Mitochondrial calpain-1, an intracellular enzyme dependent on Ca²⁺ signaling, induces cell death in RP by cleaving and releasing apoptosis-inducing factors from the mitochondria. Previously, we developed Tat-μCL, a mitochondrial calpain-1 inhibitor that protects the retina from degeneration in a rat model of RP. Herein, we investigated its effect on a rabbit model of RP, which is more human-like than the rat model. We used a transgenic (Tg) rabbit harboring a P347L rhodopsin mutation and applied saline or Tat-μCL to each eye of the Tg rabbit using eye drops. Before and after 8 and 16 weeks of saline or Tat-μCL instillation, we performed electroretinography (ERG) and optical coherence tomography (OCT). After measurement in week 16, each eye was collected and histologically analyzed using hematoxylin and eosin (HE) staining and immunostaining for glial fibrillary acidic protein, ionized calcium-binding adapter molecule 1, apoptosis-inducing factor, and Tat-μCL. Tat-μCL substantially inhibited retinal thinning as determined via OCT and tended to improve the amplitude of the a- and b-waves in ERG measurement compared to saline. HE staining showed that Tat-μCL preserved the number of nuclear layers in the outer nuclear layer of the retina. Mitochondrial calpain-1 inhibition using Tat-μCL as eye drops prevented retinal degeneration in Tg rabbits, similar to our previous results from the rat model. These results validate our earlier findings that Tat-μCL preserves photoreceptor cells and delays disease progression in RP. Our findings need further validation in clinical settings.