Unexpected Participation of Intercalated Cells in Renal Inflammation and Acute Kidney Injury

Epithelial cells constitute the 1st line of defense against pathogens, and their participation in innate immunity is rapidly emerging. In this mini-review, we discuss the noncanonical role of renal intercalated cells (ICs) in pathogen defense and in the initiation of sterile inflammation. This last function has strong implications in the onset of acute kidney injury (AKI), a potentially fatal medical complication that is seen in hospitalized patients. AKI is associated with inflammation, and it is often diagnosed only after the kidneys have suffered significant and often irreversible damage. While examining the regulation of proton secretion by type A ICs (A-ICs), we unexpectedly found high expression of the pro-inflammatory purinergic receptor P2Y14 in these cells. This receptor is located on the apical surface of A-ICs and binds UDP-glucose (UDP-Glc), a danger-associated molecular pattern molecule released from injured cells that is filtered by the glomeruli and is concentrated in the collecting duct lumen. UDP-Glc activates P2Y14 in A-ICs and triggers the production of chemokines that attract pro-inflammatory immune cells into the kidney stroma and aggravate ischemia-induced proximal tubule injury. Inhibition of P2Y14 or deletion of its gene specifically in ICs in a murine model of ischemia-reperfusion injury attenuated these effects. Thus, together with their previously recognized role in pathogen defense, A-ICs are now recognized as sensors and mediators of renal sterile inflammation that participate in the onset of AKI. Blocking the UDP-Glc/P2Y14 pathway in A-ICs provides new insights into the development of novel AKI therapeutics.

Induction of autotetraploidy and microbiome associations mediate differential responses to pathogens

It has become increasingly clear that the microbiome plays a critical role in shaping the host organism’s response to disease. There also exists mounting evidence that an organism’s ploidy level is important in their response to pathogens and parasites. However, no study has determined if or how these two factors influence one another. We investigate the effect of whole-genome duplication in Arabidopsis thaliana on their above-ground (phyllosphere) microbiome, and determine the interacting impacts of ploidy and the microbiome on disease outcome. Using seven independently derived synthetic auto-tetraploid Arabidopsis accessions, a synthetic leaf-associated bacterial community, and the model pathogen Pseudomonas syringae pv. Tomato DC3000, we confirm that polyploids are generally more resistant to pathogens, but illustrate that this resistance may be in part due to a decrease in the reliance on beneficial bacteria. Polyploids fare better against the pathogen than diploids regardless of microbial inoculation, while we observed that diploids harboring an intact microbiome have lower pathogen densities than those without. We then use RNA sequencing to show that diploids have many more differentially expressed defense-related genes in the presence of their phyllosphere microbiota, while polyploids exhibit constitutively activated defenses regardless of exposure to the synthetic community. These results imply that whole-genome duplication can disrupt historical host-microbiome associations, and suggest that a potential cause or consequence of disruption is a heightened capacity for pathogen defense that is less impacted by the microbiome.

The Flowering Time Regulator FLK Controls Pathogen Defense in Arabidopsis thaliana

Plant disease resistance is a complex process that is maintained in an intricate balance with development. Increasing evidence indicates the importance of post-transcriptional regulation of plant defense by RNA binding proteins. The K homology (KH) repeat is an ancient RNA binding motif found in proteins from diverse organisms. The role of KH domain proteins in pathogen resistance is not well known. From a genetic screen aimed to uncover novel defense genes in Arabidopsis, we identified a new allele of the canonical flowering regulatory gene, FLOWERING LOCUS KH Domain (FLK), encoding a putative triple KH-repeat protein. In addition to late flowering, the flk mutants exhibited decreased resistance to the bacterial pathogen Pseudomonas syringae and increased resistance to the necrotrophic fungal pathogen Botrytis cinerea. We found that the flk mutations compromised basal defense and defense signaling mediated by salicylic acid and led to increased reactive oxygen species (ROS) scavenging, likely through FLK’s regulation of the ROS scavenging enzyme catalases. RNA-seq data revealed that major defense signaling genes are regulated by FLK, providing a molecular basis for FLK’s contribution to pathogen defense. Together our data support that FLK is a multifunctional protein regulating pathogen defense and development of plants.

UGT76B1, a promiscuous hub of small molecule-based immune signaling, glucosylates N-hydroxypipecolic acid and controls basal pathogen defense

ABSTRACTGlucosylation modulates the biological activity of small molecules and frequently leads to their inactivation. The Arabidopsis thaliana glucosyltransferase UGT76B1 is involved in conjugating the stress hormone salicylic acid (SA) as well as isoleucic acid (ILA). Here, we show that UGT76B1 also glucosylates N-hydroxypipecolic acid (NHP), which is synthesized by FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) and activates systemic acquired resistance (SAR). Upon pathogen attack, Arabidopsis leaves accumulate two distinct NHP hexose conjugates, NHP-O-β-glucoside and NHP glucose ester, which are oppositely regulated by SA. ugt76b1 mutants specifically fail to generate the NHP-O-β-glucoside, and recombinant UGT76B1 synthesizes NHP-O-β-glucoside in vitro in competition with SA and ILA. The loss of UGT76B1 elevates the endogenous levels of NHP in addition to SA and ILA and establishes a SAR-like, primed immune status without pathogen infestation. The introgression of the fmo1 background lacking NHP biosynthesis into ugt76b1 abolishes the SAR-like resistance phenotype indicating an important function of UGT76B1-mediated NHP glucosylation in balancing the defense status. Our results further indicate that ILA promotes and SA finally executes the NHP-triggered immunity via the glucosyltransferase UGT76B1 as the common metabolic hub. Thus, UGT76B1 controls the levels of active NHP, SA, and ILA in concert to modulate plant immune signaling.

β-Defensin Strengthens Antimicrobial Peritoneal Mast Cell Response

Mast cells (MCs) are engaged in the processes of host defense, primarily via the presence of receptors responsible for the detection of pathogen-associated molecular patterns (PAMPs). Since BDs are exclusively host defense molecules, and MCs can elicit the antimicrobial response, this study is aimed at determining whether BDs might be involved in MC pathogen defense. We found that defensin BD-2 significantly augments the mRNA and protein expression of Toll-like receptors (TLRs) and retinoic acid-inducible gene-I-like receptor (RLR) essential for the detection of viral molecules, i.e., TLR3, TLR7, TLR9, and RIG-I in mature tissue rat peritoneal MCs (PMCs). We established that BD-2 might stimulate PMCs to release proinflammatory and immunoregulatory mediators and to induce a migratory response. Presented data on IgE-coated PMC upon BD-2 treatment suggest that in the case of allergies, there is an enhanced MC immune response and cell influx to the site of the ongoing infection. In conclusion, our data highlight that BD-2 might strongly influence MC features and activity, mainly by strengthening their role in the inflammatory mechanisms and controlling the activity of cells participating in antimicrobial processes.