Cystitis is associated with TRPV1b-downregulation in rat dorsal root ganglia

Human podocytes express functional thermosensitive TRPV channels

BACKGROUND AND PURPOSE

Heat-sensitive transient receptor potential vanilloid (TRPV) channels are expressed in various epithelial tissues regulating, among else, barrier functions. Their expression is well established in the distal nephron; however, we have no data about their presence in podocytes. As podocytes are indispensable in the formation of the glomerular filtration barrier, we investigated the presence and function of Ca²⁺ -permeable TRPV1-4 channels in human podocyte cultures.

EXPERIMENTAL APPROACH

Expression of TRPV1-4 channels was investigated at protein (immunocytochemistry, Western blot) and mRNA (Q-PCR) level in a conditionally immortalized human podocyte cell line. Channel function was assessed by measuring intracellular Ca²⁺ concentration using Flou-4 Ca²⁺ -indicator dye and patch clamp electrophysiology upon applying various activators and inhibitors.

KEY RESULTS

Thermosensitive TRP channels were expressed in podocytes. The TRPV1-specific agonists capsaicin and resiniferatoxin did not affect the intracellular Ca²⁺ concentration. Cannabidiol, an activator of TRPV2 and TRPV4 channels, induced moderate Ca²⁺ -influxes, inhibited by both tranilast and HC067047, blockers of TRPV2 and TRPV4 channels respectively. The TRPV4-specific agonists GSK1016790A and 4α-phorbol 12,13-didecanoate induced robust Ca²⁺ -signals which were abolished by HC067047. Non-specific agonists of TRPV3 channels induced marked Ca²⁺ transients. However, TRPV3 channel blockers, ruthenium red and isopentenyl diphosphate only partly inhibited the responses and TRPV3 silencing was ineffective suggesting remarkable off-target effects of the compounds.

CONCLUSION AND IMPLICATIONS

Our results indicate the functional presence of TRPV4 and other thermosensitive TRPV channels in human podocytes and raise the possibility of their involvement in the regulation of glomerular filtration barrier.

Prolonged exposure to bradykinin and prostaglandin E2 increases TRPV1 mRNA but does not alter TRPV1 and TRPV1b protein expression in cultured rat primary sensory neurons

Sensitisation of the capsaicin receptor, transient receptor potential vanilloid type 1 (TRPV1) ion channel in nociceptive primary sensory neurons (PSN) underlies the development of inflammatory heat hyperalgesia. Removal of the negative-dominant splice variant of the TRPV1 molecule, TRPV1b from TRPV1/TRPV1b heterotetrameric channels, which should be associated with changes in the expression of TRPV1 and TRPV1b transcripts and proteins, has been suggested to contribute to that sensitisation. Respective reverse-transcriptase polymerase chain reaction (RT-PCR) and Western-blotting revealed that both TRPV1 and TRPV1b mRNA, and their encoded proteins are expressed in rat cultured PSN. Sequencing of the RT-PCR products showed that TRPV1b mRNA lacks the entire exon 7. Further, growing PSN for 2 days in the presence of 10μM bradykinin (BK) and 10μM prostaglandin E2 (PGE2) significantly increases TRPV1 responsiveness and TRPV1 mRNA expression, without producing any changes in TRPV1b mRNA, and TRPV1 and TRPV1b protein expression. These data challenge the hypothesis that alterations in the composition of the TRPV1 ion channel contributes to the sensitisation.

Transient receptor potential channels in bladder function

The transient receptor potential (TRP) superfamily of cationic ion channels includes proteins involved in the transduction of several physical and chemical stimuli to finely tune physiological functions. In the urinary bladder, they are highly expressed in, but not restricted to, primary afferent neurons. The urothelium and some interstitial cells also express several TRP channels. In this review, we describe the expression and the known roles of some members of TRP subfamilies, namely TRPV, TRPM and TRPA, in the urinary bladder. The therapeutic interest of modulating the activity of TRP channels to treat bladder dysfunctions is also discussed.

Acute colonic inflammation triggers detrusor instability via activation of TRPV1 receptors in a rat model of pelvic organ cross-sensitization

Chronic pelvic pain of unknown etiology is a common clinical condition and may develop as a result of cross-sensitization in the pelvis when pathological changes in one of the pelvic organs result in functional alterations in an adjacent structure. The aim of the current study was to compare transient receptor potential vanilloid 1 (TRPV1) activated pathways on detrusor contractility in vivo and in vitro using a rat model of pelvic organ cross-sensitization. Four groups of male Sprague-Dawley rats (N = 56) were included in the study. Animals received intracolonic saline (control), resiniferatoxin (RTX, TRPV1 agonist, 10(-7) M), 2,4,6-trinitrobenzene sulfonic acid (TNBS, colonic irritant), or double treatment (RTX followed by TNBS). Detrusor muscle contractility was assessed under in vitro and in vivo conditions. Intracolonic RTX increased the contractility of the isolated detrusor in response to electric field stimulation (EFS) by twofold (P ≤ 0.001) and enhanced the contractile response of the bladder smooth muscle to carbachol (CCh). Acute colonic inflammation reduced detrusor contractility upon application of CCh in vitro, decreased bladder capacity by 28.1% (P ≤ 0.001), and reduced micturition volume by 60% (P ≤ 0.001). These changes were accompanied by an increased number of nonmicturition contractions from 3.7 ± 0.7 to 15 ± 2.7 (N = 6 in both groups, P ≤ 0.001 vs. control). Desensitization of intracolonic TRPV1 receptors before the induction of acute colitis restored the response of isolated detrusor strips to CCh but not to EFS stimulation. Cystometric parameters were significantly improved in animals with double treatment and approximated the control values. Our data suggest that acute colonic inflammation triggers the occurrence of detrusor instability via activation of TRPV1-related pathways. Comparison of the results obtained under in vitro vs. in vivo conditions provides evidence that intact neural pathways are critical for the development of an overactive bladder resulting from pelvic organ cross talk.

Modulation of urinary bladder innervation: TRPV1 and botulinum toxin A

The persisting interest around neurotoxins such as vanilloids and botulinum toxin (BoNT) derives from their marked effect on detrusor overactivity refractory to conventional antimuscarinic treatments. In addition, both are administered by intravesical route. This offers three potential advantages. First, intravesical therapy is an easy way to provide high concentrations of pharmacological agents in the bladder tissue without causing unsuitable levels in other organs. Second, drugs effective on the bladder, but inappropriate for systemic administration, can be safely used as it is the case of vanilloids and BoNT. Third, the effects of one single treatment might be extremely longlasting, contributing to render these therapies highly attractive to patients despite the fact that the reasons to the prolonged effect are still incompletely understood. Attractive as it may be, intravesical pharmacological therapy should still be considered as a second-line treatment in patients refractory to conventional oral antimuscarinic therapy or who do not tolerate its systemic side effects. However, the increasing off-label use of these neurotoxins justifies a reappraisal of their pharmacological properties.

Afferent mechanism in the urinary tract

Much of the current research on lower urinary tract dysfunction is focused on afferent mechanisms. The main goals are to define and modulate the signaling pathways by which afferent information is generated and conveyed to the central nervous system. Alterations in bladder afferent mechanisms are a potential source of voiding dysfunction and an emerging source of drug targets. Even some established drug therapies such as muscarinic receptor antagonists, as well as emerging therapies such as botulinum toxin type-A, may act partly through afferent mechanisms. This review presents up-to-date findings on the localization of afferent fiber types within the bladder wall, afferent receptors and transmitters, and how these may communicate with the urothelium, interstitial cells, and detrusor smooth muscle to regulate micturition in normal and pathological bladders. Peripheral and central mechanisms of afferent sensitization and myogenic mechanisms that lead to detrusor overactivity, overactive bladder symptoms, and urgency sensations are also covered as well as new therapeutic approaches and new and established methods of measuring afferent activity.

TRPV1 splice variants: structure and function

The capsaicin receptor (TRPV1) is a non-selective cation channel predominantly expressed in specialized sensory neurons that detect painful stimuli. Although its many functional roles continue to be revealed, it has been confirmed to play a critical role in the perception of peripheral inflammatory hyperalgesia and pain. TRPV1 not only is sensitized and/or activated under a wide range of conditions including inflammation and nerve injury but also undergoes changes in expressed levels in response to these same pathologic conditions. Just as our understanding of the structural requirements of TRPV1 activation has grown, there is evidence that TRPV1 forms heteromeric channel complexes. This review is focused on the structural and functional consequence of TRPV1 splice variants: VR.5'sv, TRPV1b/beta and TRPV1var. Through their co-expression and formation of heteromeric complexes with TRPV1, they have been shown to modulate TRPV1 activation. Moreover, TRPV1 splice variant subunits may also contribute unique properties of activation such as the detection of hypertonic conditions.

Sequestration of brain derived nerve factor by intravenous delivery of TrkB-Ig2 reduces bladder overactivity and noxious input in animals with chronic cystitis

Brain derived nerve factor (BDNF) is a trophic factor belonging to the neurotrophin family. It is upregulated in various inflammatory conditions, where it may contribute to altered pain states. In cystitis, little is known about the relevance of BDNF in bladder-generated noxious input and bladder overactivity, a matter we investigated in the present study. Female rats were intraperitoneally (i.p.) injected with cyclophosphamide (CYP; 200 mg/kg). They received saline or TrkB-Ig(2) via intravenously (i.v.) or intravesical administration. Three days after CYP-injection, animals were anaesthetized and cystometries performed. All animals were perfusion-fixed and the spinal cord segments L6 collected, post-fixed and processed for c-Fos and phosphoERK immunoreactivity. BDNF expression in the bladder, as well as bladder histology, was also assessed. Intravesical TrkB-Ig(2) did not change bladder reflex activity of CYP-injected rats. In CYP-animals treated with i.v. TrkB-Ig(2) a decrease in the frequency of bladder reflex contractions, in comparison with saline-treated animals, was observed. In spinal sections from the latter group of animals, the number of phosphoERK and c-Fos immunoreactive neurons was lower than in sections from saline-treated CYP-animals. BDNF immunoreactivity was higher during cystitis but was not changed by TrkB-Ig(2) i.v. treatment. Evaluation of the bladder histology showed similar inflammatory signs in the bladders of inflamed animals, irrespective of the treatment. Data show that i.v. but not intravesical administration of TrkB-Ig(2) reduced bladder hyperactivity in animals with cystitis to levels comparable to those observed in unirritated rats. Since i.v. TrkB-Ig(2) also reduced spinal extracellular signal-regulated kinase (ERK) activation, it is possible that BDNF contribution to inflammation-induced bladder hyperactivity is via spinal activation of the ERK pathway. Finally, the reduction in c-Fos expression indicates that TrkB-Ig(2) also reduced bladder-generated noxious input. Our results show that sequestration of BDNF may be considered a new therapeutic strategy to treat chronic cystitis.

Acute bladder inflammation differentially affects rat spinal visceral nociceptive neurons

The present investigation examined the effect of inflammation produced by intravesical zymosan on spinal dorsal horn neuronal responses to urinary bladder distension (UBD). Extracellular single-unit recordings of neurons excited by UBD were obtained in spinalized female Sprague-Dawley rats. Neurons were classified as Type I-inhibited by heterotopic noxious conditioning stimuli (HNCS) or as Type II-not inhibited by a HNCS. In Experiment 1-following neuronal characterization, 1% zymosan was infused into the bladder and after 2h spinal units were recharacterized. Control rats received intravesical saline or subcutaneous zymosan. In Experiment 2-rats were pretreated with intravesical zymosan 24h prior to surgical preparation. Control rats received anesthesia only. 137 spinal dorsal horn neurons excited by UBD were characterized. In comparison with controls, Type II neurons demonstrated increased spontaneous and UBD-evoked activity following intravesical zymosan treatment (both Experiments 1 and 2) whereas Type I neurons demonstrated either no change (Experiment 1) or decreased activity (Experiment 2) following bladder inflammation. No significant changes were noted in neuronal activity in control experiments. Inflammation differentially affects subpopulations of spinal dorsal horn neurons excited by UBD that can be differentiated according to the effect of HNCS. This results in an altered pattern of spinal sensory transmission that may serve as the mechanism for the generation of visceral nociception.

Ion channel and receptor mechanisms of bladder afferent nerve sensitivity

Sensory nerves of the urinary bladder consist of small diameter A(delta) and C fibers running in the hypogastic and pelvic nerves. Neuroanatomical studies have revealed a complex neuronal network within the bladder wall. Electrophysiological recordings in vitro and in vivo have revealed several distinct classes of afferent fibers that may signal a wide range of bladder stimulations including physiological bladder filling, noxious distension, cold, chemical irritation and inflammation. The exact mechanisms that underline mechanosensory transduction in bladder afferent terminals remain ambiguous; however, a wide range of ion channels (e.g., TTX-resistant Na(+) channels, Kv channels and hyperpolarization-activated cyclic nucleotide-gated cation channels) and receptors (e.g., TRPV1, TRPM8, TRPA1, P2X(2/3), etc) have been identified at bladder afferent terminals and implicated in the generation and modulation of afferent signals. Experimental investigations have revealed that expression and/or function of these ion channels and receptors may be altered in animal models and patients with overactive and painful bladder disorders. Some of these ion channels and receptors may be potential therapeutic targets for bladder diseases.

Where is TRPV1 expressed in the bladder, do we see the real channel?

Transient receptor potential channel-vanilloid subfamily member 1 (TRPV1) is an important target in the treatment of bladder overactivity. This receptor is suggested to function as a mechanosensor in the normal bladder and to mediate the development of bladder overactivity during cystitis. Our aim was to determine the cellular distribution of TRPV1 in mouse and rat bladder tissue. We used three different commercial TRPV1 antibodies to perform immunohistochemistry on bladder tissue from rats and wild-type and TRPV1(-/-) mice, using trigeminal ganglia as a control tissue for TRPV1 expression. Although two of the antibodies seemed to react specifically in trigeminal ganglion tissue, all the antibodies produced a similar staining pattern in the urothelium of wild-type and TRPV1(-/-) mice. These data show that TRPV1 antibodies can cause an aspecific immunostaining in bladder tissue, urging for additional research to confirm the exact distribution of TRPV1 in bladder. In conclusion, we think that the use of negative controls on knockout mice, whenever available, is mandatory when conducting immunohistochemical localization studies.