Cerebral Mitochondrial Function and Cognitive Performance during Aging: A Longitudinal Study in NMRI Mice

Abstract

Brain aging is one of the major risk factors for the development of several neurodegenerative diseases. Therefore, mitochondrial dysfunction plays an important role in processes of both, brain aging and neurodegeneration. Aged mice including NMRI mice are established model organisms to study physiological and molecular mechanisms of brain aging. However, longitudinal data evaluated in one cohort are rare but are important to understand the aging process of the brain throughout life, especially since pathological changes early in life might pave the way to neurodegeneration in advanced age. To assess the longitudinal course of brain aging, we used a cohort of female NMRI mice and measured brain mitochondrial function, cognitive performance, and molecular markers every 6 months until mice reached the age of 24 months. Furthermore, we measured citrate synthase activity and respiration of isolated brain mitochondria. Mice at the age of three months served as young controls. At six months of age, mitochondria-related genes (complex IV, creb-1, β-AMPK, and Tfam) were significantly elevated. Brain ATP levels were significantly reduced at an age of 18 months while mitochondria respiration was already reduced in middle-aged mice which is in accordance with the monitored impairments in cognitive tests. mRNA expression of genes involved in mitochondrial biogenesis (cAMP response element-binding protein 1 (creb-1), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α), nuclear respiratory factor-1 (Nrf-1), mitochondrial transcription factor A (Tfam), growth-associated protein 43 (GAP43), and synaptophysin 1 (SYP1)) and the antioxidative defense system (catalase (Cat) and superoxide dismutase 2 (SOD2)) was measured and showed significantly decreased expression patterns in the brain starting at an age of 18 months. BDNF expression reached, a maximum after 6 months. On the basis of longitudinal data, our results demonstrate a close connection between the age-related decline of cognitive performance, energy metabolism, and mitochondrial biogenesis during the physiological brain aging process.

Figure 1

Y-Maze spontaneous alternation test of 3-, 6-, 12-, and 18-month-old mice during a five-minute period time of testing. Changes of alternation rates (% of control) (a) and changes of number of alternations (% of control) (b); n = 12, mean ± SEM, and one-way ANOVA with Tukey's post hoc test; ^∗p < 0.05. Performance of young control mice (3 months) is defined as 100%.

Figure 2

Passive avoidance test with 3-, 6-, 12-, and 18-month-old NMRI mice. On day one, mice receive a mild electric shock (0.5 mA) and time that the mouse needs to enter into the dark chamber is recorded; 24 h after the first testing period, the test is repeated and time that the mouse needs to reenter the dark chamber is recorded; n = 15, mean ± SEM, and one-way ANOVA with Tukey's post hoc test; ^∗p < 0.05 and ^∗∗p < 0.01.

Figure 3

Relative normalized mRNA expression levels of cytochrome c oxidase subunit 5A (a), cAMP response element-binding protein (creb-1) (b), AMP-activated protein kinase (β-AMPK) (c), mitochondrial transcription factor A (Tfam) (d), and brain-derived neurotrophic factor (BDNF) (e) in brain homogenate of 3-, 6-, 12-, 18-, and 24-month-old mice. mRNA expression of 3-month-old control mice is 100%. n = 9, mean ± SEM with one-way ANOVA and Tukey's post hoc test with ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, and ^∗∗∗∗p < 0.0001 against 3-month-old control animals. “A” indicates one-way ANOVA and Tukey's post hoc test against 6-month-old mice, “B” against 12-month-old mice, and “C” against 18-month-old animals. Results are normalized to the mRNA expression levels of beta 2 microglobulin (B2M) and phosphoglycerate kinase 1 (PGK1).