Calcium is an important intracellular signaling molecule that requires tight regulation of the intracellular concentrations for optimal triggering of signaling cascades and cell survival.
31 In the mouse photoreceptor, normal calcium levels range between approximately 250 nM (in complete darkness) and approximately 60 nM (in the light).
32 In the
rd1 mouse photoreceptors, however, calcium levels increased up to approximately 190% over the wild-type mice levels.
11 Consequently, messages for calcium-binding proteins and specific calcium sensors, such as calpains, may be upregulated in the
rd1 retina. Calpains are cysteine proteases activated by calcium during the apoptotic processes.
33 Two ubiquitously expressed calpains are the isozymes calpain I (μ-calpain) and calpain II (
m-calpain), which are activated in vitro by micromolar and millimolar calcium concentrations, respectively.
34 Calpain I and calpain II are expressed in the retina, and they were activated in the
rd1 mouse.
35 36 37 Calpains do not directly cause chromatin condensation, but they are proteases that activate apoptotic factors. Several proteins are known targets of calpain protease activity, such as caspase-12. Caspase-12 is localized on the cytoplasmic side of the ER, which enables direct sensing of ER perturbations.
38 Caspase-12 is activated when the ER undergoes stress, but not by membrane- or mitochondrial-targeted apoptotic signals. Mice deficient for caspase-12 are resistant to inducers of ER stress, suggesting that caspase-12 is significant in ER stress-induced apoptosis.
39 In the present study, accompanying photoreceptor apoptosis in the
rd1 mouse, the expression of procaspase-12 and cleaved caspase-12 were markedly upregulated. The expression of procaspase-12 was downregulated at P10, but this might have been because some of the procaspase-12 was cleaved. The upregulation of caspase-12 coincided with the onset and peak of photoreceptor apoptosis, suggesting that caspase-12 is involved in the photoreceptor degeneration in the
rd1 mouse. The immunofluorescence study demonstrated that at the peak of its expression, caspase-12 was primarily located at the inner segments of photoreceptor cells, with some positive staining also showing in the ONL. This is consistent with the theory that caspase-12 is localized in the ER, so that the protein is most likely localized in the inner segments of photoreceptor cells containing mitochondria and the ER. The cleaved active form of caspase-12 participates in the apoptotic event by translocation to the nucleus or possibly through other caspases.
40 41 42