Nociceptors regulate osteoimmune transcriptomic response to infection

Abstract

Osteoimmune diseases, such as apical periodontitis, are prevalent, often painful, inflammatory conditions resulting in bone loss and reduced quality of life. There is growing evidence that the nociceptive fibers densely innervating affected tissues regulate disease progression; therefore, we hypothesized that nociceptors regulate the transcriptomic profile of the periapical osteolytic lesion in a mouse model of apical periodontitis. Male control and nociceptor-ablated mice underwent pulp exposures, and after 0, 7, or 14 days, total RNA from periapical tissues was submitted for sequencing and bioinformatic analysis. Pulp exposure triggers the differential expression of hundreds of genes over the course of infection. At 14 days post pulp exposure, 422 genes, including Tnf, Il1a, and Il1b, were differentially expressed between nociceptor-ablated and control mice with greater enrichment of biological processes related to inflammation in nociceptor-ablated mice. Nociceptor ablation regulates the transcriptomic profile of periapical lesions in a mouse model of apical periodontitis, shifting the gene expression profile to a greater enrichment of inflammatory genes, suggesting nociceptors play a role in the kinetics of the immune response. This newly uncovered neuro-immune axis and its mechanisms in apical periodontitis can be an important therapeutic target for the treatment of this prevalent disease.

Figure 1

Pulp exposure results in the differential expression of genes within the periapical lesion over the course of 14 days in nociceptor-ablated and control mice. Volcano plots of differentially expressed genes (DEGs) in periapical lesions compared between 0- and 7-days (A), 0- and 14-days (B), and 7- and 14-days (C) following pulp exposure in cre-control and nociceptor-ablated mice, where log₂(fold change [FC]) is plotted against –log(pval) (n = 3–4 mice/strain/time point). The R package ‘DESeq’ was used to normalize data and find group-pairwise differential gene expression.

Figure 2

Nociceptor-ablated mice exhibit unique differentially expressed genes throughout the progression of apical periodontitis compared to cre-control. Venn diagrams of number of up- or down-regulated DEGs and percent of total up- or down-regulated DEGs for nociceptor-ablated and cre-control mice between 0 and 7, 0 and 14, and 7 and 14 (A). DEGs between cre-control and nociceptor-ablated at Days 0, 7, and 14 are shown (B). DEGs were defined as genes where p < 0.05, and FC > 1.5 (n = 3–4 mice/strain/time point). The R package ‘DESeq’ was used to normalize data and find group-pairwise differential gene expression.

Figure 3

Nociceptor-ablation results in a greater enrichment of biological processes related to inflammation, specifically immune cell chemotaxis and migration, compared to cre-control after 14 days of infection. Enriched gene ontology (GO) biological processes of DEGs between cre-control and nociceptor-ablated lesions at 14 days of infection are plotted as Fold Enrichment for each biological process, where color correlates to -log(pValue) and size corresponds to the number of genes enriched in each process.

Figure 4

After 14 days of infection, nociceptor-ablated lesions display unique patterns of expression of genes related to bone, immune, and inflammatory processes. Heatmap of select inflammatory DEGs expressed in the periapical lesions from mice on Day 14 post- pulp exposure related to bone (A) and immune and inflammatory (B) biological processes. Color corresponds to z score of Reads Per Kilobase of transcript per Million mapped (RPKM) values of each gene (n = 3 mice/strain).