Systematic spatio-temporal mapping reveals divergent cell death pathways in three mouse models of hereditary retinal degeneration

Calcium (Ca2+) dysregulation has been linked to neuronal cell death, including in hereditary retinal degeneration. Ca2+ dysregulation is thought to cause rod and cone photoreceptor cell death. Validation of this hypothesis has been hampered so far by spatial and temporal heterogeneities in retinal disease models. Here, we examined the role of Ca2+ in photoreceptor degeneration by assessing the activation pattern of Ca2+-dependent calpain proteases. We generated spatio-temporal maps of the entire retina in the cpfl1 mouse model for primary cone degeneration, as well as for primary rod degeneration in rd1 and rd10 mutant mice. In the latter two models, the spatio-temporal pattern of increased calpain activity matched the progression of primary rod degeneration. High calpain activity coincided with activation of the calpain-2 isoform but not with calpain-1, suggesting differential roles for the two calpain isoforms. Furthermore, primary rod loss was associated with an upregulation of apoptosis-inducing factor (AIF), even though only a minute fraction of cells showed activity of the apoptotic marker caspase-3. However, after the conclusion of primary rod degeneration, caspase-3 activation appeared in cones, suggesting apoptosis as the dominant mechanism for secondary cone loss. To distinguish the temporal sequences of degenerative molecular processes from other sources of variability, we used Gaussian process models, which highlighted calpain activity as a key event during primary rod photoreceptor cell death. Taken together, our data suggests a causal link between Ca2+ dysregulation and primary, non-apoptotic degeneration of photoreceptors. On the other hand, it also suggests a role for apoptosis in secondary degeneration of cones. This systematic investigation of retinal degeneration highlights the importance of the spatial and temporal location of key molecular events, and may guide the design of future studies, including the evaluation of new therapies.