The ageing systemic milieu negatively regulates neurogenesis and cognitive function

Abstract

In the central nervous system, ageing results in a precipitous decline in adult neural stem/progenitor cells and neurogenesis, with concomitant impairments in cognitive functions. Interestingly, such impairments can be ameliorated through systemic perturbations such as exercise. Here, using heterochronic parabiosis we show that blood-borne factors present in the systemic milieu can inhibit or promote adult neurogenesis in an age-dependent fashion in mice. Accordingly, exposing a young mouse to an old systemic environment or to plasma from old mice decreased synaptic plasticity, and impaired contextual fear conditioning and spatial learning and memory. We identify chemokines--including CCL11 (also known as eotaxin)--the plasma levels of which correlate with reduced neurogenesis in heterochronic parabionts and aged mice, and the levels of which are increased in the plasma and cerebrospinal fluid of healthy ageing humans. Lastly, increasing peripheral CCL11 chemokine levels in vivo in young mice decreased adult neurogenesis and impaired learning and memory. Together our data indicate that the decline in neurogenesis and cognitive impairments observed during ageing can be in part attributed to changes in blood-borne factors.

Figure 1. Heterochronic parabiosis alters neurogenesis in an age-dependent fashion

a, Schematic showing parabiotic pairings. b,e, Representative fields of Doublecortin (b) and BrdU (e) immunostaining of young (3–4 months; yellow) and old (18–20 months; gray) isochronic and heterochronic parabionts five weeks after parabiosis (arrowheads point to individual cells, scale bar: 100μm). c–f Quantification of neurogenesis (c,d) and proliferating cells (e,f) in the young (c,e; top) and old (d,f; bottom) DG after parabiosis. Data from 12 young isochronic, 10 young heterochronic, 6 old isochronic and 12 old heterochronic parabionts. g,h, Population spike amplitude (PSA) was recorded from DG of young parabionts. Representative electrophysiological profiles (g) and LTP levels (h) are shown for young heterochronic and isochronic parabionts. Data from 4–5 mice per group. All data represented as Mean + SEM; *P<0.05; **P<0.01 t-test.

Figure 2. Factors from an old systemic environment decrease neurogenesis and impair learning and memory

a, Schematic of young (3–4 months) or old (18–22 months) plasma extraction and intravenous injection into young (3 months) adult mice. b, Representative field of Doublecortin immunostaining of young adult mice after plasma injection treatment four times over ten days (scale bar: 100μm). c, Quantification of neurogenesis in the young DG after plasma injection. Data from 8 young plasma and 7 old plasma injected mice. d,e Hippocampal learning and memory assessed by contextual fear conditioning (d) and RAWM (e) paradigms in young adult mice after young or old plasma injections nine times over 24 days. d, Percent freezing time 24 hours after training. Data from 8 mice per group. e, Number of entry arm errors prior to finding platform. Data from 12 mice per group. All data represented as Mean ± SEM; *P< 0.05; **P< 0.01; t-test (c,d), repeated measures ANOVA, Bonferroni post-hoc test (e).

Figure 3. Systemic chemokine levels increase during aging and heterochronic parabiosis and correlate with decreased neurogenesis

a, Venn diagram of results from aging and parabiosis proteomic screens. Seventeen age-related plasma factors correlated strongest with decreased neurogenesis in gray, fifteen plasma factors increased between young isochronic and young heterochronic parabionts in red, and six factors elevated in both screens in brown intersection. Data from 5–6 animals per age group. b,c Changes in plasma concentrations of CCL11 with age (b) and young heterochronic parabionts pre- and post- parabiotic pairing (c). d,e Changes in plasma (d) and CSF (e) concentrations of CCL11 with age in healthy human subjects. All data represented as dot plots with mean; *P< 0.05; **P< 0.01; ***P< 0.001 t-test (c,e), ANOVA, Tukey’s post-hoc test (a,b), and Mann-Whitney U Test (d).

Figure 4. Systemic exposure to CCL11 inhibits neurogenesis and impairs learning and memory

a, Schematic of young (3–4 months) mice injected intraperitoneally with CCL11 or vehicle, and in combination with anti-CCL11 neutralizing or isotype control antibody. b, Representative field of Dcx-positive cells for each treatment group (n = 6–10 mice) treated four times over ten days. (scale bar: 100μm). c, Quantification of neurogenesis in the DG after treatment. d, Schematic of young adult mice given unilateral stereotaxic injections of anti-CCL11 neutralizing or isotype control antibody followed by systemic injections with either recombinant CCL11 or PBS (vehicle). e, Representative field of Dcx-positive cells in adjacent sides of the DG for each treatment group (n = 3–11 mice) f, Quantification of neurogenesis in the DG after systemic and stereotaxic treatment. g,h, Learning and memory assessed by contextual fear conditioning (g) and RAWM (h) paradigms in young adult mice injected with CCL11 or vehicle every three days for five weeks (n = 12–16 mice per group). All data represented as Mean ± SEM; *P< 0.05; **P<0.01; ANOVA, Dunnet’s or Tukey’s post-hoc test (c,f); repeated measures ANOVA, Bonferroni post-hoc test (k).