K21 Improves Mitochondrial Function in Macrophages, Extending Life in Nematode Worms
The antimicrobial drug K21 acts via disruption of bacterial membranes, but is also known to produce improved wound healing, which suggests the involvement of mechanisms other than antimicrobial activity. Here, K21 is shown to favorably modulate macrophage activity, in that exposure improves the operation of the mitochondrial quality control processes of mitophagy in macrophages, which in turn improves mitochondrial function. Delivery of K21 to nematode worms resulted in longer life spans, as one might expect from any intervention that improves mitochondrial function without meaningful side effects.
Here, we sought to use single-cell RNA-seq to explore the diversity of the innate immune response and to understand its potential to be reprogrammed by antimicrobial therapy. Antimicrobials primarily target specific aspects of the pathogen's lifecycle to inhibit its spread, yet few drugs can effectively control viruses, bacteria, and other pathogens, including fungi, while improving cellular health. The only possible way to achieve this is to alter the host immune repertoire to effectively inhibit pathogenic survival. We chose to study the antimicrobial compound K21, a quaternary ammonium silane compound that not only exhibits broad-spectrum effects against viruses, bacteria, and fungi but also demonstrates potent wound healing activity.
As macrophages and macrophage polarization orchestrate the phases of wound-healing, shifting from pro-inflammatory signaling that combats infection to pro-regenerative signaling that recruits fibroblasts and keratinocytes to repair the epithelial tissue, we reasoned that K21 might operate by modulating macrophage properties, classes, and/or dynamics. We chose to examine the effects of K21 on monocyte-derived macrophages from human peripheral blood monocytes (PBMCs) because of their technical accessibility, their ability to be isolated in a relatively naïve state and co-cultured with other cell types, and their controllable M1/M2 polarization.
We found that supplementing cytokine and cell-surface marker profiling with scRNA-seq transcriptomic analysis provides unparalleled resolution of macrophage diversity and plasticity, allowing us to demonstrate how treatment with K21 remodels the macrophage repertoire both inside and outside the cell. We show that a key aspect of K21 treatment is the induction of mitochondrial fission and autophagy (mitophagy). We turned to the distantly related model system C. elegans to examine whether the mechanism used by K21 is used systemically in vivo and evolutionarily conserved. K21 induced DRP-1-mediated mitochondrial fission and mitophagy in vivo without impairing viability, development, or fertility; indeed, it reprogrammed metabolic gene expression and extended lifespan in these nematodes, consistent with its effects observed in mammalian cells.