Support by the Skłodowska-Curie Action allowed performing the following work:
- 15 months old mice showed deficits in learning and memory in the novel object location task (Fig. 1B, black group) and those deficits were unaffected by the colonization with GM from their own group (Fig. 1B, red group) whereas it was rescued after colonization with the GM from young adult (4 months old) mice (Fig. 1B, blue group). At the contrary, learning and memory abilities of young mice was, normal before the GM transfer (Fig. 1A, black group), unaffected by a transfer with young GM (Fig. 1A, blue group) but affected by the transfer of an aged GM (Fig. 1A, red group). Memory deficits were also associated with a reactive astrogliosis (Fig. 1C) and a decrease in the expression level of synaptic markers (Fig. 1E) in hippocampal CA1, and a decrease in the number of newly generated DCX-positive neurons in the hippocampal dentate gyrus (Fig. 1D).
- the detrimental effect of the aged GM on cognition was shown to depend on the vagus nerve (VN) as aged GM led to a decrease of vagal tone seen in the brain region that receives vagal inputs (Fig. 2A) while counteracting this effect with chemogenetic activation of the VN (Fig. 2B, picture left) in young mice colonized with an aged microbiota rescued the memory deficits (Fig. 2B, right panel), as well as the decrease in dentate gyrus DCX-positive newly generated neurons (Fig. 2C) and expression levels of a hippocampal synaptic markers in CA1 (Fig. 2D).
- specific method and protocols were implemented for the first time in our institute to perform mouse humanization with human GM. Data showed that colonization of young adult mice (4 months) with an aged (>65 years old) human GM (huAinY) led to a deficit in learning and memory in two different hippocampus-dependent tasks while mice colonized with the GM from young (<35) donors (huYinY) were unaffected (Fig. 3). The characterization of human bacteria and derived metabolites with a pro- or anti-memory/aging potential is currently underway through shotgun sequencing and statistical modelling, such as Random Forest, on available data.
- Similar work on AD will soon be undertaken as ethical and reglementary work was recently completed and AD patients GM samples from the Sainte Perine hospital geriatric department (Paris) will be available in September 2018. Mice will be colonized with AD patients GM samples in parallel with the transfer of young human GM (negative control) and aged human GM and the effect on cognition, depression-related behavioral tasks and at the molecular level on AD-related markers such as amyloid plaques deposition and neurofibrillary tangles will be characterized.
Collectively, our data showed that the aged GM is necessary and sufficient to induce age-related deficits in hippocampal learning and memory through a diminution of vagus nerve activity. Artificially reinstating VN activity through chemogenetic activation counteracted this effect and restored normal memory and associated markers. Human aged GM was also shown to be sufficient for induction of age-related memory deficits and characterization of the effect of human AD GM as well as the bacteria and derived metabolites responsible for the maintenance of loss of memory abilities will be sought.
A manuscript on the role of the mouse aged GM part and including data from mice humanized with the aged human GM is currently in writing. Metagenomics/metabolic data on potential bacteria/metabolites with an anti-aging effect as well as our method to deplete the preexisting GM will be patented. In term of promotion of science toward the general public, our work was disseminated through elearning with the creation of a MOOC and several presentations to the general public.
