In a recent study published in Science Immunology, a group of researchers investigated how sensory nerve sprouting and mast cell activity, driven by nerve growth factor (NGF), contribute to persistent bladder pain and dysfunction in recurrent urinary tract infection (rUTI) cases.
Study: Recurrent infections drive persistent bladder dysfunction and pain via sensory nerve sprouting and mast cell activity. Image Credit: Rabizo Anatolii/Shutterstock.com
Background
Urinary tract infections (UTIs), primarily caused by uropathogenic Escherichia coli (UPEC), lead to numerous clinical visits annually due to their high recurrence rate, especially in women.
Recurrences are often attributed to UPEC persisting within the urinary tract's epithelial cells, causing rUTIs even in individuals with intact immune systems.
Standard UTI treatment involves a short course of antibiotics, with longer prophylactic use in cases of frequent recurrences.
rUTI patients frequently experience symptoms like pelvic pain and increased urination frequency, often treated with antibiotics despite negative urine cultures.
Further research is needed to develop targeted therapies that address the underlying neuro-immunological mechanisms contributing to chronic pelvic pain and bladder dysfunction in rUTI patients.
About the study
In the present study, researchers employed a rUTI mouse model to mimic symptoms and phenotypes observed in human clinical settings.
They evaluated voiding behavior, pain-like responses, and nerve anatomy through various techniques such as cystometry, histology, and three-dimensional (3D) model reconstruction.
Additionally, enzyme-linked immunosorbent assay (ELISA) and flow cytometry were utilized to identify proteins and cell types relevant to rUTI pathology.
To validate their findings, the team implemented multiple targeting strategies, including receptor antagonists and neutralizing antibodies against NGF, as well as genetically modified mice lacking specific immune cell receptors or cells.
To further establish the connection between their observations in mice and human patients, the researchers administered NGF, histamine, and bradykinin to naive mice.
This replicated the primary observations from the rUTI model, suggesting a possible translational significance of their findings.
Human bladder biopsies and urine samples were collected from control individuals and rUTI patients experiencing pain to confirm the animal model data with human clinical observations.
These human samples underwent similar analysis protocols to those applied in animal studies, ensuring that the insights gained from the research were grounded in both preclinical and clinical realities.
The study explored nerve growth in rUTI through various methods. Researchers collected dorsal root ganglia (DRGs) from mice, treated them with enzymes for dissociation, and then cultured the cells on coated coverslips. Adding NGF or its antagonist revealed NGF's role in neurite outgrowth.
This was paralleled by in vivo experiments where NGF was instilled into mouse bladders to induce nerve sprouting, contrasting with treatments that neutralized NGF or blocked its receptor, which prevented such growth.
Additionally, monocytes and mast cells were cultured with neurons to demonstrate further NGF's influence on nerve development.
Various techniques were employed to assess pelvic sensitivity and bladder innervation effects, underscoring NGF's central role in sensory nerve sprouting associated with rUTI symptoms.
Study results
Patients experiencing rUTI often report persistent symptoms such as pelvic sensitivity and urinary frequency, even after bacterial cultures show no signs of infection. This observation led researchers to investigate the role of bladder sensory nerves and their connection to rUTI symptoms.
Upon examining bladder biopsies from rUTI patients, an increase in Substance P (SP+), a marker for nociceptive sensory nerves, was observed, suggesting alterations in nociceptive activity.
A mouse model of rUTI was employed to explore these clinical findings further, revealing similar symptoms of increased pelvic sensitivity and urinary frequency alongside significant sensory nerve sprouting in the lamina propria.
The investigation into the underlying mechanisms of these observations pointed towards the role of NGF, a key mediator in nerve sprouting and sensitization. Elevated levels of NGF, but not brain-derived neurotrophic factor (BDNF), were identified in the bladders of rUTI mice.
Targeting NGF signaling through the administration of native NGF to naive mice or utilizing NGF-neutralizing strategies replicated the rUTI phenotype or alleviated symptoms, respectively, confirming NGF's pivotal role.
The cellular source of NGF was traced back to recruited monocytes and bladder-resident mast cells, which were identified as primary contributors to the elevated NGF levels during rUTI.
These findings were further confirmed by in vitro experiments showing that monocytes and mast cells induced nerve growth. This process could be blocked by Tropomyosin receptor kinase A (TrkA) antagonism, indicating the specificity of NGF from these immune cells in inducing sensory nerve sprouting.
Exploring the link between sensory nerve sprouting, sensitization, and the observed rUTI symptoms, researchers discovered that the sustained release of NGF-sensitized nociceptors leads to pain-like responses and urinary frequency.
This process involved the activation of Transient Receptor Potential Vanilloid 1 Positive (TRPV1+) nerves, with mast cell products such as histamine and bradykinin playing significant roles in mediating these symptoms.
Interestingly, the administration of histamine or bradykinin directly into the bladder of naive mice induced similar symptoms to those observed in rUTI, suggesting a role for these mast cell products in the pathological process.
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