Age and Sex Influence the Hippocampal Response and Recovery Following Sepsis

Abstract

Although in-hospital mortality rates for sepsis have decreased, survivors often experience lasting physical and cognitive deficits. Moreover, older adults are more vulnerable to long-term complications associated with sepsis. We employed a murine model to examine the influence of age and sex on the brain's response and recovery following sepsis. Young (~ 4 months) and old (~ 20 months) mice (C57BL/6) of both sexes underwent cecal ligation and puncture (CLP) with restraint stress. The hippocampal transcriptome was examined in age- and sex-matched controls at 1 and 4 days post-CLP. In general, immune- and stress-related genes increased, while neuronal, synaptic, and glial genes decreased 1 day after CLP-induced sepsis. However, specific age and sex differences were observed for the initial responsiveness to sepsis as well as the rate of recovery examined on day 4. Young females exhibited a muted transcriptional response relative to young males and old females. Old females exhibited a robust shift in gene transcription on day 1, and while most genes recovered, genes linked to neurogenesis and myelination continued to be downregulated by day 4. In contrast, old males exhibited a more delayed or prolonged response to sepsis, such that neuronal and synaptic genes continued to decrease while immune response genes continued to increase on day 4. These results suggest that aging is associated with delayed recovery from sepsis, which is particularly evident in males.

Keywords: Aging; Cecal ligation and puncture; Hippocampus; Mice; Sepsis; Transcriptome.

Fig. 1

Diagram summarizing the initial analysis performed on the transcriptome. a For each age and sex group, gene changes following sepsis were examined by generating lists of differentially expressed genes. Gene lists were generated by statistical filtering (p < 0.01), comparing the transcriptome on day 1 or day 4 following sepsis relative to control. Moreover, we examined the changes in expression from day 1 to day 4. Upregulated and downregulated genes were separately analyzed for enrichment of biological processes or cellular components using DAVID. b The gene lists generated from the above analysis were then used to examine the prolonged effects or recovery after sepsis. Analysis for the prolonged effect of sepsis was determined by genes that continued to be modified in the same direction on day 1 and day 4 relative to control. For recovery, genes were selected if they were differentially expressed on day 1 (e.g., increasing on day 1) and expression was in the opposite direction on day 4 compared to day 1 (e.g., decreased on day 4 relative to day 1). For prolonged and recovery effects, gene lists were separately analyzed for enrichment of biological processes or cellular components using DAVID

Fig. 2

The number of genes differentially expressed for each age and sex group. Summary of the total number of genes that increased (black) or decreased (white) the expression in the hippocampus 1 or 4 days after sepsis relative to age-matched controls

Fig. 3

Selected gene ontology (GO) terms for genes that were differentially expressed following sepsis in young (a) and old (b) males. These clusters were separately generated for genes that were increased or decreased 1 or 4 days after sepsis compared to controls. GO terms were loosely grouped into three main categories: gene/protein expression (blue), immune/stress response (green), and neuronal/synaptic function (gray), and the bar represents the –log(p value)

Fig. 4

Prolonged dysregulation or recovery of genes that were altered after sepsis in young (a) and old (b) males. Pie charts illustrate the proportion of genes that either continued to increase (yellow) or decrease (teal) expression across both days compared to controls, genes that recovered expression (pink), and genes that did not exhibit continued dysregulation or recovery across conditions (white; NS = not significant). Recovery was grouped by the direction of change at 1 and 4 days after sepsis. Also, significant clusters associated with each category are listed to the right

Fig. 5

Selected GO terms for genes that were differentially expressed following sepsis in young (a) and old females (b). Genes that were differentially expressed 1 or 4 days after sepsis were separated into increasing and decreasing. GO terms were loosely grouped into three main categories: gene/protein expression (blue), immune/stress response (green), and neuronal/synaptic function (gray), and the bar represents the –log(p value)

Fig. 6

Continued dysregulation or recovery of genes that were altered after sepsis in young (a) and old (b) females. Pie charts illustrate the proportion of genes that either continued to increase (yellow) or decrease (teal) expression across both days compared to controls, genes that recovered expression (pink), and genes that did not exhibit continued dysregulation or recovery across conditions (white; NS = not significant). Recovery was grouped by the direction of change at 1 and 4 days after sepsis. Also, significant clusters associated with each category are listed to the right

Fig. 7

Age-related changes in baseline gene expression and the response to sepsis. a The number of DEGs between young and aged control animals separated based on the direction of change and sex. b List of selected GO terms that were upregulated or downregulated with age separated based on sex (blue = male; pink = female). The bar represents the –log(p value). PCA analysis of the genes that increased (c) and decreased (d) with age in males (top) and females (bottom)

Fig. 8

A heatmap illustrating the effect of sepsis on gene expression levels within each age and sex group. Expression levels for each gene were converted into a z-score and averaged for each independent group. Genes were grouped into the GO terms immune response, synaptic function, gene expression, and energy metabolism. The color represents the standard deviation, either increasing (yellow) or decreasing (green), relative to the mean (black)

Fig. 9

The effect of sepsis, age, and sex on plasma cytokine levels. Bar graphs of the mean cytokine concentration (± SEM) split by group. Cytokines are split into three graphs based on if the cytokine was significant for an interaction or factor a sepsis × age × sex interaction, b sepsis × sex, and c sepsis. *p < 0.05