Intracerebral Infection with E. coli Impairs Spatial Learning and Induces Necrosis of Hippocampal Neurons in the Tg2576 Mouse Model of Alzheimer's Disease

Abstract

Background: In patients with Alzheimer's disease (AD), bacterial infections are often associated with a cognitive decline. Animal models of genuine acute infections with viable bacteria which induce deterioration of neurodegenerative diseases are missing.

Objective: We assessed the effect of an intracerebral infection with E. coli in a mouse model of AD.

Methods: 13-month-old Tg2576 +/- mice and transgene negative littermates (Tg2576 -/-) received an intracerebral injection with E. coli K1 or saline followed by treatment with ceftriaxone starting 41 h post infection (p.i.) for 5 days. For 4 weeks, mice were monitored for clinical status, weight, motor functions, and neuropsychological status using the Morris water maze. ELISAs, stainings, and immunohistochemistry in brains were performed at the end of the experiment.

Results: Mortality of the infection was approximately 20%. After 4 weeks, spatial learning of infected Tg2576 +/- mice was compromised compared to non-infected Tg2576 +/- mice (p < 0.05). E. coli infection did not influence spatial learning in Tg2576 -/- mice, or spatial memory in both Tg2576 +/- and -/- mice within 4 weeks p.i.. Necrosis of hippocampal neurons was induced in infected compared to non-infected Tg2576 +/- mice 4 weeks p.i., whereas brain concentrations of Aβ_1-40, Aβ_1-42, and phosphoTau as well as axonal damage and microglia density were not altered.

Conclusion: Here, we proved in principle that a genuine acute bacterial infection can worsen cognitive functions of AD mice. Mouse models of subacute systemic infections are needed to develop new strategies for the treatment of bacterial infections in patients with AD in order to minimize their cognitive decline.

Keywords: Alzheimer’s disease; E. coli; amyloid-β; cognitive functions; delirium; intracerebral infection; neurodegenerative disease; spatial learning; spatial memory; water maze.

Fig. 1

Acute signs of infection in Tg2576 +/- and Tg2576 -/- mice after intracerebral injection of E. coli. A) Mortality after intracerebral E. coli injection was 18.2% in Tg2576 +/- mice and 25.0% in Tg2576 -/- mice (Kaplan-Meier curve; log-rank test: p = 0.64). B) Representative HE-stainings of the right cerebral hemisphere of a deceased Tg2576 +/- mouse (left image) and a deceased Tg2576 -/- mouse (right image) showing meningeal inflammation. C) Concentrations of E. coli in cerebellum and spleen of deceased Tg2576 +/- and Tg2576 -/- mice [medians and single values; Mann-Whitney U-test: p = 0.11 (cerebellum) and p = 0.22 (spleen)].

Fig. 2

Motor function of Tg2576 +/- and Tg2576 -/- mice after surviving an intracerebral E. coli infection. A) Performance of surviving infected Tg2576 +/- mice (n = 18) in the rotarod test did not differ from that of uninfected Tg2576 +/- mice (n = 14) up to 4 weeks after intracerebral E. coli infection (comparison of AUCs, Mann-Whitney U-test, p = 0.61). B) Performance of surviving infected Tg2576 -/- mice (n = 15) in the rotarod test did not differ from that of uninfected Tg2576 -/- mice (n = 17) up to 4 weeks after intracerebral E. coli infection (comparison of AUCs, Mann-Whitney U-test, p = 0.19). Medians (25./75. percentiles) of times that mice remained on the rotarod are presented (minimum 0, maximum 300 s).

Fig. 3

Spatial memory and spatial learning of Tg2576 +/- and Tg2576 -/- mice after intracerebral E. coli infection. A-C) Water maze performances of surviving infected Tg2576 +/- mice (n = 18) and uninfected Tg2576 +/- mice (n = 14) assessed by the latencies to find the hidden platform. A) The ability to learn the location of the platform did not differ between Tg2576 +/- mice allocated to the infection-group and the non-infection-group. B) The ability to remember the location of the hidden platform, assessed once a week during four weeks after infection, did not differ significantly between infected and non-infected Tg2576 +/- mice. C) Four weeks after infection, the ability of infected Tg2576 +/- mice to learn the new location of the platform was impaired compared to non-infected Tg2576 +/-. D-F) Water maze performances of surviving infected Tg2576 -/- mice (n = 15) and uninfected Tg2576 -/- mice (n = 17) assessed by the latencies to find the hidden platform. There were no differences between Tg2576 -/- mice of the infection group and Tg2576 -/- mice of the non-infection group concerning their ability to learn the location of the platform before infection (D), to remember the location of the platform during four weeks after infection (E), and to learn the new location of the platform four weeks after infection (F). Medians (25./75. percentiles) of median times mice needed to find the platform in 3 to 6 water maze runs are presented (maximum 90 s).

Fig. 4

Aβ load and phosphoTau concentrations in brains of Tg2576 +/- mice after intracerebral infection with E. coli. A) Example of a plaque stained with thioflavine (immunofluorescence, left image), and example of thioflavin stained cortical areas automatically detected using the Definiens software (immunofluorescence, right image). B) Comparison of the plaque area (median, 25th/75th percentile) in brains of infected and non-infected Tg2576 +/- mice (p = 0.92, Mann-Whitney U-test). C) Comparison of phosphoTau concentrations (means±SD) in brains of infected and non-infected Tg2576 +/- mice as measured by ELISA (p = 0.1, Student’s t-test).

Fig. 5

Necrotic neurons in brains of Tg2576 +/- mice after intracerebral infection with E. coli. A, B) Representative HE stainings with high numbers of necrotic neurons in the hippocampus regions CA1-4 (score 3; A), and low numbers of necrotic neurons in the hippocampus regions CA1-4 (score 0; B) [34]. C) Necrotic neurons in the hippocampus regions CA1-4 (median and single values) of infected and non-infected Tg2576 +/- mice (p = 0.038, Mann-Whitney U-test). D) Necrotic neurons in the dentate gyrus (median and single values) of infected and non-infected Tg2576 +/- mice (p = 0.76, Mann-Whitney U-test).

Fig. 6

Axonal damage and microglia density in brains of Tg2576 +/- mice after intracerebral infection with E. coli. A) Representative amyloid-β protein precursor (AβPP) immunohistochemistry showing small lesions of axonal damage (pink) [35]. B) Representative Iba-1 staining showing microglial cells in the hippocampus (brown). C) Representative Iba-1 staining showing microglial cells in the neocortex (brown). D) Axonal damage quantified by a semiquantitative score (0 = no axonal damage, 3 = severe axonal damage) in brains of infected and non-infected Tg2576 +/- (medians and single values; p = 0.074, Mann-Whitney U-test). E) Numbers of microglial cells (means and single values) in the hippocampus of infected and non-infected Tg2576 +/- mice (p = 0.84, Student’s t-test). F) Numbers of microglial cells (means and single values of means of 3 areas) in the neocortex of infected and non-infected Tg2576 +/- mice (p = 0.37, Student's t-test).