Disrupted Neurogenesis in Germ-Free Mice: Effects of Age and Sex

Abstract

The gut microbiome has profound effects on development and function of the nervous system. Recent evidence indicates that disruption of the gut microbiome leads to altered hippocampal neurogenesis. Here, we examined whether the effects of gut microbiome disruption on neurogenesis are age-dependent, given that both neurogenesis and the microbiome show age-related changes. Additionally, we examined memory induced functional connectivity of hippocampal networks. Control and germ-free mice at three different ages (4, 8, and 12 weeks) were trained in contextual fear-conditioning, then subsequently tested the following day. Hippocampal neurogenesis, quantified via BrdU and doublecortin, exhibited age-dependent changes relative to controls, with the established age-dependent decrease in neurogenesis being delayed in germ-free mice. Moreover, we found sex-dependent effects of germ-free status on neurogenesis, with 4 week old male germ-free mice having decreased neurogenesis and 8 week old female germ-free mice having increased neurogenesis. To assess systems-level consequences of disrupted neurogenesis, we assessed functional connectivity of hippocampal networks by inducing c-Fos expression with contextual memory retrieval and applying a previously described network analysis. Our results indicate impaired connectivity of the dentate gyrus in germ-free mice in a pattern highly correlated with adult neurogenesis. In control but not germ-free mice, functional connectivity became more refined with age, indicating that age dependent network refinement is disrupted in germ-free mice. Overall, the results show that disruption of the gut microbiome affects hippocampal neurogenesis in an age-and sex-dependent manner and that these changes are also related to changes in the dentate gyrus functional network.

Tryptophan-metabolizing gut microbes regulate adult neurogenesis via the aryl hydrocarbon receptor

“…Previous studies aimed at assessing the impact of microbes on hippocampal neurogenesis have revealed somewhat differing results, possibly due to different experimental conditions and timelines for monitoring neurogenesis. For instance, in one study, a different mouse line (Swiss Webster) was used (66), whereas in another, hippocampal neurogenesis was assessed earlier in life (58). A third study reported a decrease in adult neurogenesis of adult male C57BL/6J mice after antibiotic treatment (67), which is consistent with our results.…”

The Potential Role of Inflammation in Modulating Endogenous Hippocampal Neurogenesis After Spinal Cord Injury

“…Therefore, reducing systemic inflammation through modulation of the microbiome might be a strategy to increase neurogenesis. A recent study showed that alteration of the gut microbiome alters hippocampal neurogenesis in both an age-and sex-dependent manner, with young male germfree mice showing decreased hippocampal neurogenesis and increased neuron apoptosis (Scott et al, 2020). Gut microbiome from 2-year-old mice transplanted into 6-week-old germ-free mice increased the number of newborn hippocampal neurons and local BDNF expression in the short-term while the transplantation of microbiome from 6-week-old mice had no effect (Kundu et al, 2019).…”

Brain iron deficiency and affected contextual fear memory in mice with conditional Ferroportin1 ablation in the brain

“…BrdU + and DCX + cells were counted following the instructions from the previous literature. 63 Quantification of BrdU and DCX is shown in Figures D and E, Figure 3A and Figure 3B show the mean fluorescence intensities of FPN1, which are displayed as the fold change normalized to the Fpn1 flox/flox group. Values are presented as the mean ± SD of three mice.…”