Renal concentration capacity in adult patients with urinary tract infections

Regulation of renal aquaporin water channels in acute pyelonephritis

Acute pyelonephritis (APN) is most frequently caused by uropathogenic Escherichia coli (UPEC), which ascends from the bladder to the kidneys during a urinary tract infection. Patients with APN have been reported to have reduced renal concentration capacity under challenged conditions, polyuria, and increased aquaporin-2 (AQP2) excretion in the urine. We have recently shown increased AQP2 accumulation in the plasma membrane in cell cultures exposed to E. coli lysates and in the apical plasma membrane of inner medullary collecting ducts in a 5-day APN mouse model. This study aimed to investigate if AQP2 expression in host cells increases UPEC infection efficiency and to identify specific bacterial components that mediate AQP2 plasma membrane insertion. As the transepithelial water permeability in the collecting duct is codetermined by AQP3 and AQP4, we also investigated whether AQP3 and AQP4 localization is altered in the APN mouse model. We show that AQP2 expression does not increase UPEC infection efficiency and that AQP2 was targeted to the plasma membrane in AQP2-expressing cells in response to the two pathogen-associated molecular patterns (PAMPs), lipopolysaccharide and peptidoglycan. In contrast to AQP2, the subcellular localizations of AQP1, AQP3, and AQP4 were unaffected both in lysate-incubated cell cultures and in the APN mouse model. Our finding demonstrated that cellular exposure to lipopolysaccharide and peptidoglycan can trigger the insertion of AQP2 in the plasma membrane revealing a new regulatory pathway for AQP2 plasma membrane translocation, which may potentially be exploited in intervention strategies.NEW & NOTEWORTHY Acute pyelonephritis (APN) is associated with reduced renal concentration capacity and increased aquaporin-2 (AQP2) excretion. Uropathogenic Escherichia coli (UPEC) mediates changes in the subcellular localization of AQP2 and we show that in vitro, these changes could be elicited by two pathogen-associated molecular patterns (PAMPs), namely, lipopolysaccharide and peptidoglycan. UPEC infection was unaltered by AQP2 expression and the other renal AQPs (AQP1, AQP3, and AQP4) were unaltered in APN.

Urinary Tract Infections: Renal Intercalated Cells Protect against Pathogens

Urinary tract infections affect more than 1 in 2 women during their lifetime. Among these, more than 10% of patients carry antibiotic-resistant bacterial strains, highlighting the urgent need to identify alternative treatments. While innate defense mechanisms are well-characterized in the lower urinary tract, it is becoming evident that the collecting duct (CD), the first renal segment encountered by invading uropathogenic bacteria, also contributes to bacterial clearance. However, the role of this segment is beginning to be understood. This review summarizes the current knowledge on CD intercalated cells in urinary tract bacterial clearance. Understanding the innate protective role of the uroepithelium and of the CD offers new opportunities for alternative therapeutic strategies.

Urine specific gravity, pyuria, and neutrophil gelatinase-associated lipocalin for identifying urinary tract infection in young children

BACKGROUND

To determine whether urine neutrophil gelatinase-associated lipocalin (uNGAL) might be superior to pyuria for detecting urinary tract infection (UTI) regardless of urine specific gravity (SG) in young children.

METHODS

We conducted a retrospective analysis of children aged < 3 years who were evaluated for UTI with urinalysis, urine culture, and uNGAL measurements during a 5-year period. Sensitivity, specificity, likelihood ratios (LRs), predictive values (PVs), area under the curves (AUCs) of uNGAL cut-off levels, and various microscopic pyuria thresholds for detecting UTI were calculated for dilute (SG < 1.015) and concentrated urine (SG ≥ 1.015).

RESULTS

Of 456 children included, 218 had UTI. The diagnostic value of urine white blood cell (WBC) concentration to define UTI changed with urine SG. For detecting UTI, uNGAL cut-off of 68.4 ng/mL had higher AUC values than pyuria ≥ 5 WBCs/high power field (HPF) for dilute and concentrated urine samples (both P < 0.05). Positive LR and PV and specificity of uNGAL were all greater than those of pyuria ≥ 5 WBCs/HPF regardless of urine SG, although the sensitivity of pyuria ≥ 5 WBCs/HPF was higher than that of uNGAL cut-off for dilute urine (93.8% vs. 83.5%) (P < 0.05). At uNGAL ≥ 68.4 ng/mL and ≥ 5 WBCs/HPF, posttest probabilities of UTI were 68.8% and 57.5% for dilute urine and 73.4% and 57.3% for concentrated urine, respectively.

CONCLUSIONS

Urine SG can affect the diagnostic performance of pyuria for detecting UTI and uNGAL might be helpful for identifying UTI regardless of urine SG in young children. A higher resolution version of the Graphical abstract is available as Supplementary information.

Acute pyelonephritis: Increased plasma membrane targeting of renal aquaporin-2

AIM

Aquaporin-2 (AQP2) shuttling between intracellular vesicles and the apical plasma membrane is pivotal in arginine vasopressin-mediated urine concentration and is dysregulated in multiple diseases associated with water balance disorders. Children and adults with acute pyelonephritis have a urinary concentration defect and studies in children revealed increased AQP2 excretion in the urine. This study aimed to analyse AQP2 trafficking in response to acute pyelonephritis.

METHODS

Immunofluorescence analysis was used to evaluate subcellular localization of AQP2 and AQP2-S256A (mimicking non-phosphorylated AQP2 on serine 256) in cells stimulated with bacterial lysates and of AQP2 and pS256-AQP2 in a mouse model at day 5 of acute pyelonephritis. Western blotting was used to evaluate AQP2 levels and AQP2 phosphorylation on S256 upon incubation with bacterial lysates. Time-lapse imaging was used to measure intracellular cAMP levels in response to incubation with bacterial lysates.

RESULTS

In cell cultures, lysates from both uropathogenic and nonpathogenic bacteria-mediated AQP2 plasma membrane targeting and increased AQP2 phosphorylation on serine 256 (pS256) without increasing cAMP levels. Both bacterial lysates induced plasma membrane targeting of AQP2-S256A. Immunofluorescence analysis of renal sections from mice after 5 days of acute pyelonephritis revealed apical plasma membrane targeting of AQP2 and pS256-AQP2 in inner medullary collecting ducts.

CONCLUSION

Uropathogenic bacteria induce AQP2 plasma membrane targeting in vitro and in vivo. cAMP levels were not elevated by the bacterial lysates and AQP2 plasma membrane targeting could occur without S256 phosphorylation. This may explain increased AQP2 excretion in the urine during acute pyelonephritis.

The bacteria and the host: a story of purinergic signaling in urinary tract infections

The local environment forces a selection of bacteria that might invade the urinary tract, allowing only the most virulent to access the kidney. Quite similar to the diet in setting the stage for the gut microbiome, renal function determines the conditions for bacteria-host interaction in the urinary tract. In the kidney, the term local environment or microenvironment is completely justified because the environment literally changes within a few micrometers. The precise composition of the urine is a function of the epithelium lining the microdomain, and the microenvironment in the kidney shows more variation in the content of nutrients, ion composition, osmolality, and pH than any other site of bacteria-host interaction. This review will cover some of the aspects of bacterial-host interaction in this unique setting and how uropathogenic bacteria can alter the condition for bacteria-host interaction. There will be a particular focus on the recent findings regarding how bacteria specifically trigger host paracrine signaling, via release of extracellular ATP and activation of P2 purinergic receptors. These finding will be discussed from the perspective of severe urinary tract infections, including pyelonephritis and urosepsis.

Renal concentrating capacity in female outpatients with symptomatic urinary tract infection

OBJECTIVE

Renal concentrating capacity (RCC) has been used as a tool in the diagnosis of the site of urinary tract infection (UTI). In this study, RCC was measured in women with symptoms of UTI in relation to the clinical picture and the bacterial species isolated.

MATERIAL AND METHODS

In 633 outpatients aged > or =18 years with symptoms of UTI, a renal concentration test with desmopressin was performed at baseline. The test was repeated after antibiotic treatment in a subset of patients with osmolality values below the reference levels at entry. The bacterial species were identified and patients were classified as having either pyelonephritis or cystitis.

RESULTS

RCC was reduced in patients infected with Escherichia coli or Staphylococcus saprophyticus compared to those who had negative urine cultures and increased after antibiotic treatment. Patients with symptoms of both pyelonephritis and cystitis had reduced RCC: 44/68 (65%) and 205/457 (45%), respectively.

CONCLUSIONS

These findings demonstrate a considerable overlap in RCC between patients with pyelonephritis and cystitis, making the osmolality test unreliable for the purpose of diagnosis of the site of UTI.