Overexpression of GCAP2 in transgenic mice leads to shortening of ribbon length and to an increase in the fraction of club-shape and spherical morphologies representing disassembling ribbons.

<p>A. Electron micrographs of rod synaptic terminals of dark-reared C57Bl, GCAP2+ or GCAP2+/+ mice at p40 that were processed in darkness or immediately after 1–5 h of light exposure, showing transversal sections of synaptic ribbons. Notice the difference in length in C57Bl [left], GCAP2+ [middle] and GCAP2+/+ [right panel] ribbons. In addition to synaptic vesicles, vesicle-like particles that are smaller in diameter than synaptic vesicles were seen forming clusters in the cytosol [GCAP2+/+ panel]. These clusters found in the vicinity of the ribbons were more extensive in GCAP2+/+ samples than in C57Bl samples. B. Club-shape ribbons were more abundant in GCAP2+ and GCAP2+/+ than in C57Bl samples. Two examples of the density of club-shape and spherical-ribbons are shown in 8,000× visual fields of GCAP2+ and GCAP2+/+ retinal sections. Club-shape and spherical ribbons pointed by arrows are shown at higher magnification in the right panels. C. Statistical analysis of ribbon length in C57Bl, GCAP2+ and GCAP2+/+ mice that were either raised in constant darkness (D); or raised in constant darkness and subsequently exposed to 1–5 h light (L). A minimum of forty synaptic ribbons were measured from at least two mice per phenotype. Plotted in the histogram are mean values ± standard error. *** denotes P<0.0001 in Student’s t-test. ** denotes P≤0,001 in Student’s t-test. *denotes PP≤0,01 in Student’s t-test. D. Statistical analysis of ribbon length in GCAP2+ and WT littermate control mice raised in standard cyclic light and processed at p60. GCAP2-expressing mice showed a 10% reduction in ribbon length compared to WT littermate controls. Notice the difference in the Y-axis scale. *** denotes P≤0,0001 in Student’s t-test. E. Histogram comparing the percentage of club-shape and spherical ribbons [of total synaptic ribbons] in C57Bl, GCAP2+ and GCAP2+/+ at p40 processed in darkness [D] or immediately after 1 h or 5 h of light exposure.