Wheel running attenuates microglia proliferation and increases expression of a proneurogenic phenotype in the hippocampus of aged mice

Abstract

Aging is associated with low-grade neuroinflammation including primed microglia that may contribute to deficits in neural plasticity and cognitive function. The current study evaluated whether exercise modulates division and/or activation state of microglia in the dentate gyrus of the hippocampus, as activated microglia can express a classic inflammatory or an alternative neuroprotective phenotype. We also assessed hippocampal neurogenesis to determine whether changes in microglia were associated with new neuron survival. Adult (3.5 months) and aged (18 months) male BALB/c mice were individually housed with or without running wheels for 8 weeks. Mice received bromodeoxyuridine injections during the first or last 10 days of the experiment to label dividing cells. Immunofluorescence was conducted to measure microglia division, co-expression of the neuroprotective indicator insulin-like growth factor (IGF-1), and new neuron survival. The proportion of new microglia was increased in aged mice, and decreased from wheel running. Running increased the proportion of microglia expressing IGF-1 suggesting exercise shifts microglia phenotype towards neuroprotection. Additionally, running enhanced survival of new neurons in both age groups. Findings suggest that wheel running may attenuate microglia division and promote a proneurogenic phenotype in aged mice.

Figure 1

Average distance (km) run per day by aged and adult mice over eight weeks of running wheel access. Means ± standard error of the mean (SEM).

Figure 2

(A) Representative sections triple labeled with antibodies against Iba-1 (macrophage/microglia; blue), NeuN (mature neuron; green), and BrdU (new cell; red) from adult and aged mice injected with BrdU during the first ten days (BrdU Early). (B) Number of new neurons in the granular cell layer of the hippocampus of adult and aged mice housed with or without running wheel access. The number of new neurons was assessed in cells labeled with BrdU during the first (BrdU Early) or last (BrdU Late) ten days of the study. * indicates a significant difference from age-matched sedentary mice. + indicates a significant difference between adult and aged mice within an exercise condition. Means ± SEM.

Figure 3

(A) Representative hippocampal sections triple labeled with antibodies against Iba-1 (macrophage/microglia; blue), NeuN (mature neuron; green), and BrdU (new cell; red) from adult and aged mice. (B) Average percentage of microglia that underwent division in the dentate gyrus of adult and aged mice. Percentages were calculated by dividing the number of Iba-1 positive cells that expressed BrdU by the number of Iba-1 positive cells and multiplying by 100. * indicates a significant difference from age-matched sedentary mice. + indicates a significant difference between adult and aged mice within an exercise condition. Means ± SEM.

Figure 4

(A) Representative image of a cell co-labeled with antibodies against Iba-1 (macrophage/microglia; blue) and IGF-1 (growth factor; red) from an aged mouse. (B) Average percentage of Iba-1 positive cells that expressed IGF-1 in the dentate gyrus of adult and aged mice. Data are collapsed across BrdU timing. (C) Representative image of a triple-labeled cell with antibodies against Iba-1 (macrophage/microglia; blue), IGF-1 (growth factor; red), and BrdU (new cell; green) from an aged mouse. (D) Average percentage of Iba-1 positive cells that are new (BrdU+) and express IGF-1. * indicates a significant difference from age-matched sedentary mice. + indicates a significant difference between adult and aged mice within an exercise condition. Means ± SEM.