Ageing-associated gut dysbiosis deteriorates mouse cognition

Abstract

Ageing is an independent factor for cognitive dysfunction. Ageing-associated alterations in the gut microbiota also affect cognition. The present study is designed to investigate changes in the gut microbiota and their participation in ageing-associated cognitive impairment. Both 10-week-old and 18-month-old mice are used. Mouse cognition is examined by novel object recognition and T-maze tests. Mouse feces are collected for sequencing and transplantation. Protein expression in the mouse intestine and hippocampus is studied using immunohistochemistry and immunofluorescence staining. Senescent neurons are induced by hydrogen peroxide in vitro. The cell lysates are used for western blot analysis and adenosine triphosphate (ATP) measurement. Our results show that 18-month-old mice exhibit cognitive dysfunction compared with young mice. In aged mice, transplanting the microbiota of young mice increases the protein presence of synaptophysin in the hippocampus and partially restores cognition. The protein expressions of mucin-2 and E-cadherin in the intestine are reduced in aged mice but are increased by transplantation. Gut microbiota analyses reveal that the reduced abundance of the microbe Bacilli-Lactobacillales-Lactobacillaceae-Lactobacillus in aged mice is restored by transplantation. Fecal microbiota transplantation in young mice increases the serum level of acetic acid in aged mice. Hydrogen peroxide stimulation induces senescence and reduces the protein expression levels of synaptophysin and acetyl-coenzyme A synthetase member 2 (ACSS2) in primary neurons. Incubation with acetic acid upregulates the protein expressions of ACSS2 and synaptophysin and further increases ATP production in senescent neurons. In summary, gut microbiota transplantation increases the abundance of Lactobacillales, elevates serum acetic acid level, and improves cognitive function in aged mice. Gut microbiota transplantation has therapeutic importance for ageing-associated cognitive decline.

1. Introduction

Medical advances and hygiene improvements have significantly extended lifespan in modern society. However, the incidence of ageing-associated diseases, including neurological disorders, cardiovascular disease and malignant cancers, has also increased. It has been reported that more than 50 million senile people worldwide suffer from cognitive impairments, which will reach 130 million by 2050 [1]. Ageing-induced neuron impairment and microglial activation in the central nervous system are critical mechanisms underlying cognitive decline, shown as impaired learning capacity, memory loss, confusion, anxiety, and personality changes.

In addition to the well-studied mechanisms in the central nervous system, the gut microbiota plays a role in neurological disorders through the gut-brain axis [2], probably through the release of metabolites and neurotransmitters [3]. Notably, patients suffering from cognitive decline have altered gut microbiota communities, as shown by decreases in the beneficial bacteria Akkermansia and Bifidobacterium and increases in the detrimental bacteria Ruminococcus and Eggerthella [4–7]. In a rodent model of Alzheimer’s disease, germ-free mice presented reduced cerebral amyloid-β plaques and blunted neurofibrillary tangles compared with their counterparts under specific pathogen-free conditions [8]. Transplanting the gut microbiota from Alzheimer’s disease model mice impaired memory in C57BL/6 mice [9]. Short-chain fatty acids (SCFAs), including acetic acid, pentanoic acid, hexanoic acid, isovaleric acid, and butyric acid, are the primary metabolites of bacteria. SCFAs are involved in regulating neurotransmitters [10], neurotrophic factors [11], myelination [12], and microglial homeostasis [13]. The administration of acetate increases the production of glutamate-glutamine, a regulatory neuropeptide, and suppresses appetite in mice [10]. Sodium butyrate increases mitochondrial biogenesis in astrocytes and ameliorates cognitive impairments in Alzheimer’s disease model mice [14]. Therefore, fecal microbiota transplantation (FMT) has been developed as an alternative treatment for cognitive impairment. Treatment with Bacillus subtilis probiotics inhibits α-synuclein aggregation in C. elegans [15]. In aged mice with cognitive dysfunction, probiotic treatment improved memory by restoring the anatomic structure of the gut and the blood-brain barrier [16].

Thus, the present study was designed to investigate the effects of FMT on ageing-associated cognitive decline and the underlying mechanisms involving gut microbiota and metabolites.