ΔmtDNA load and mitochondrial respiration in brain tissues. (A) Accumulation of ΔmtDNA. Because ΔmtDNA is distributed nearly uniformly throughout all the tissues of a mito-mouse at age 4 weeks, we estimated the increased proportion of ΔmtDNA in brain and skeletal muscle tissues after the experiment (at age 8.5 months), based on ∆mtDNA proportions in tail samples at age 4 weeks. (B) Relationship between mitochondrial respiratory function and phenotypic expression of impaired spatial remote memory. Sections of visual cortex from B6, low group, and high group (Mice 52, 64, and 76) were stained with anti-COI antibody and DAPI. Mice 52, 64, and 76 carried 52%, 64%, and 76% ΔmtDNA, respectively, in their tails at age 4 weeks. Numbers under the sections for Mice 52, 64, and 76 indicate scores in the spatial remote memory experiment shown in Figure 3C. (C) Graphic representation of Figures 4A and 4B. In the left graph, bars indicate values obtained from deduction of ΔmtDNA proportion in the tail at age 4 weeks from that in brain (black bar) or skeletal muscle (white bar) tissues at age 8.5 months. In the right graph, quantification of COI positive signals in visual cortex of B6 (white bar), low group (black bar), and high group (gray bar) mice were shown. (P=0.5942 in B6 vs low group, P < 0.0001 in B6 vs high group, and P < 0.0001 in low vs high groups). (D) Biochemical analysis of COX activity in brain tissues. Whole brain samples from B6 (white bar), low group (black bar), and high group (gray bar) were used for the analysis (P=0.3469 in B6 vs low group, P < 0.0005 in B6 vs high group, and P < 0.002 low vs high groups). All values are means ± SE. All asterisks indicate significant differences. n.d., not determined.