Interstitial cells of the bladder: the missing link?
Dr D. De Ridder, University Hospital Gasthuisberg Department of Urology Herestraat 49, 3000 Leuven, Belgium.
The innervation and function of the bladder wall is still somewhat enigmatic. Different aetiological mechanisms for bladder overactivity have been proposed. Neurogenic causes are well known. For the idiopathic forms, however, explanations remain to be established.
Previously researchers have focused on efferent and afferent innervation and on smooth muscle physiology; recent research papers show a shift of interest to the role of the urothelium and the suburothelium. The sensing function of the urothelium is being investigated as well as the role of a strange type of myofibroblast-like or interstitial cell, located in the suburothelium and in between detrusor muscle bundles.
In this paper the authors try to integrate the current data in an intriguing hypothesis. If these interstitial cells prove to have a role in integrating the information coming from the urothelium and nerves they would be able to modulate detrusor contractility; this would mean a leap forward in our understanding of bladder overactivity. Since similar cells are also present in the gut and other organs, the interaction between the mucosal lining and the underlying smooth muscle might become an interesting research area.
CLASSICAL VIEWS ON BLADDER OVERACTIVITY
Urgency and urge incontinence are very common symptoms, having a large impact on quality of life and being accountable for major healthcare expense. Underlying detrusor overactivity has a wide variety of causes, many of which are still considered to be idiopathic. The condition is also present in patients with neurological disorders like multiple sclerosis and spinal cord lesions.
The normal micturition reflex requires efferent nerve input from the spinal cord to the bladder and afferent input from the bladder to the central nervous system. The afferent nerves transmit information regarding bladder fullness and discomfort to the brain and consist of two types of nerve fibres. The myelinated and fast-conducting Aδ-fibres project sensations of fullness and wall tension to the central regulatory centres of micturition through interneurones located in the spinal cord. Normal micturition occurs through a spino-bulbo-spinal reflex arc initiated by these Aδ-fibres. Unmyelinated C-fibres connect to the motor innervation of the bladder through spinal interneurones. These C-fibres transmit noxious and painful sensations. In cats with chronic spinal cord transection these additional pathways create a spinal reflex arc and mediate automatic micturition resulting in neurogenic detrusor overactivity.1,2
Ultrastructural alterations in the detrusor smooth muscle cells, changes in myocyte function and partial denervation of the detrusor smooth muscle have been reported as another possible cause of overactive bladder disease.3
A third possible cause could lie in recently described properties of the urothelium. In response to stretch, the urothelium can release substances such as ATP and NO, which will inhibit/activate the suburothelial afferent nerves thus modifying the micturition reflex.4
The main nonsurgical treatments of overactive bladder disease are anticholinergic agents acting on the efferent parasympathetic innervation of the detrusor muscle. The success of these drugs is limited by persisting overactivity, the troublesome side-effects and the cost of these drugs.
AFFERENT NERVE INTERACTION
More recently, intravesical instillations of vanilloids such as capsaicin and its ultrapotent analogue resiniferatoxin (RTX) combine an excellent response with good clinical tolerability.5,6 The vanilloid drugs interact with a specific transmembrane receptor, the heat- and acidity-sensitive vanilloid receptor TRPV1.7 TRPV1, originally denominated VR-1, has been described in the spinal cord, in dorsal root ganglia and in different visceral organs including bladder, urethra and colon.
The vanilloid receptor is present on sensory neurones involved in nociception and neurogenic inflammation.8 Activation of this receptor by capsaicin first causes activation then desensitisation of the afferent C-fibres. Application of capsaicin to peripheral nerve endings results in depolarisation and discharge of action potentials, which provokes a burning pain sensation. This excitation is followed by a long-lasting refractory state where the neurones become unresponsive to further drug applications, referred to as desensitisation. The duration of the desensitisation depends on the dose and time of exposure to capsaicin.9
Clinical experiments in patients with multiple sclerosis or spinal cord injury who had overactive bladder, show that intravesical capsaicin significantly decreases bladder overactivity and/or the number of urinary incontinence episodes. Complete continence was achieved in 44%, satisfactory improvement occurred in 36% and treatment failed in 20%. Clinical benefit from a single instillation lasted 3–6 months.10
RTX is as an ultra-potent vanilloid, several thousand times more potent than capsaicin for desensitising contractions in the rat urinary bladder. It has the unique effect of causing desensitisation of the sensory nerve without exciting it. As a result, RTX causes less pain when instilled intravesically.11
Intravesical administration of RTX in patients with idiopathic overactive bladder significantly increases mean maximal cystometric capacity and decreases daily episodes of urinary incontinence, results lasting more than 3 months.12
This desensitisation of unmyelinated afferent C-fibres makes intravesical administration of vanilloids a valid therapeutic option to treat diseases where abnormal afferent sensory information is an aetiological factor, such as overactive bladder disease (for a review on the clinical use of vanilloid therapy in overactive bladder disease, see refs 5 and 13).
Different findings, however, have cast doubt on the exclusive neuronal localisation of the vanilloid receptor TRPV1 in the urinary bladder.
Birder et al.14 reported expression of TRPV1 mRNA in isolated urothelial and smooth muscle cells. Immunostaining revealed TRPV1 immunoreactivity in both basal and superficial urothelial cells. These urothelial cells react to vanilloid application by releasing chemical mediators such as NO and ATP. These findings suggest specialised sensory and signalling properties for the urothelium, at least partially depending on the presence of the TRPV1 receptor, which allows it to engage in reciprocal communication with nerves in the bladder wall.14
Vanilloid receptor immunoreactivity also has been described on the interstitial cells of the bladder (ICB), both on suburothelial ICB and on interstitial cells located in between the detrusor smooth muscle bundles.15 The discovery that cells other than afferent nerves are immunoreactive for the human vanilloid receptor suggests that the effect of vanilloids administration in the treatment of bladder overactivity is achieved in part through an effect on these cells, making them possible targets of pharmacotherapy.
CANNABINOIDS AND BLADDER
The role of cannabinoids in treating overactive bladder disease is also worth investigating. Patients with multiple sclerosis and symptoms of overactive bladder report an improvement of urinary complaints after inhalation of marihuana.16 The main active component of smoked cannabinoids, Δ9-THC, acts on the cannaboinoid receptors CB1 and CB2. While the CB2 receptor is present on immunological cells, the CB1 receptor is present on central and peripheral nerves and seems to be responsible for the perceived effect of smoked cannabis on urinary complaints.
CB1 is a G-protein coupled receptor, either negatively or positively associated with selective ion channels. It is expressed strongly in the basal ganglia, cerebellum, hippocampus and dorsal primary afferent regions of the spinal cord.17
Several endogenous ligands, known as endocannabinoids, have been found. Anandamide and 2-arachidonylglycerol are the most studied endocannabinoids, but anandamide is also a weak vanilloid receptor agonist.18 The main function of the endocannabinoid system is to regulate synaptic neurotransmission of excitatory and inhibitory circuits.19
Prejunctional CB1 receptors are present in the mouse urinary bladder, which could mediate inhibition of electrically evoked contractions by reducing contractile transmitter release.20 Ost et al. demonstrated the presence of CB1 in human urinary bladder.21 Administration of WIN 55,212–2, a CB1 agonist, in bladder inflammation significantly increased the micturition threshold in comparison with administration in normal bladders.22 This suggests an upregulation of CB1 receptors in bladder inflammation to explain this increased effect of cannabinoid agonists, similar to the upregulation of CB1 in mouse intestine after intestinal inflammation.23
The intraperitoneal administration of a CB1 agonist in spinal cord injured rats significantly reduces detrusor pressure compared with an intraperitoneal CB1 antagonist.24 Anandamide has been described as an agonist of both CB1 and TRPV1, suggesting a link between the cannabinoid and the vanilloid system.25
Own observations on human prostate26 and human urinary bladder demonstrate a spatial relationship between the two systems: both prostatic and bladder interstitial cells are positive for the TRPV1 and are surrounded by patches of CB1 immunoreactivity. These findings might suggest that the interstitial cells are a source of endocannabinoid release in the bladder and prostate.
Smet and Jonavicius were the first to describe the interstitial cells of the bladder, both in guinea pig and in human detrusor.27 They now have been encountered in the entire urinary tract of different species. Interstitial cells have been described in the pelvi-calyceal junction and the renal pelvis of the guinea pig28 and in the mouse ureter.29 Interstitial cells have also been identified in the rabbit urethra30 and in the human26 and guinea pig prostate.31
Interstitial cells (or ‘myofibroblasts’) have been described in two networks in the bladder, a suburothelial network directly beneath the urothelium and a network in between the detrusor smooth muscle cells, both in human and in guinea pig bladders.15,27,32–35
An exact role for these interstitial cells of the bladder (ICB) in bladder function has not yet been described, but can be hypothesised from analogous cells located in the gut.
The interstitial cells of Cajal (ICC) are located in most parts of the digestive system and they have a specific role in gastro-intestinal motility. Electrophysiological experiments on isolated ICC demonstrate that they act as an electrical pacemaker generating rhythmic electric activity36,37; and that they transmit this signal to the smooth muscle.38 Networks of ICC interconnect through gap junctions and function as preferential conductors of electrical activity. Both roles have already been attributed to ‘ICC-like’ cells in the upper28 and lower30 urinary tract. The ICC of the gut have also been placed in a central position in the nerve-to-muscle signal transmission, receiving direct neuronal input and transferring it to the smooth muscle cells.39,40
Morphologically, ICC have both myoïd and fibroblast features. Typically, numerous mitochondria, abundant intermediate filaments and the presence of gap junctions characterise ICC.41
ICC contain the tyrosine kinase receptor kit, which is a selective marker for these cells.39 The kit-receptor is located on the plasma membrane and is a receptor for stem cell factor, a growth factor.42 Thus it is possible to identify the ICC by immunohistochemistry, using antibodies directed against the kit-receptor.43 ICC also possess abundant vimentin filaments44 and are interconnected though gap junctions built of Connexin 43 (Cx43).45
Human and rat ICB are also immunoreactive for kit, vimentin and Cx43 antibodies.33,34 Double-labelling confocal microscopy experiments revealed that the ICB were positioned in proximity to nerves in the guinea pig bladder.33 Using electron microscopy in human bladders, Sui et al.34 identified a suburothelial network of interstitial cells interconnected by gap junctions. Wiseman et al.35 found human suburothelial cells with the typical ultrastructural characteristics of myofibroblasts. Close contacts between these cells and unmyelinated axonal varicosities were identified. Our own unpublished electron microscopy observations show close contacts between suburothelial ICB and nerve fibres.
Since the interstitial cells of the bladder are comparable to the interstitial cells of the gut from an immunohistochemical and morphological point of view, their functions could also be similar. Using Calcium imaging techniques, McCloskey et al.33 described a spontaneous activation of interstitial cells obtained from guinea pig detrusor. This suggests that the ICB may have a pacemaker function, as previously demonstrated for interstitial cells of the urethra30 and of the prostate.31
Hashitani et al.46 reported that spontaneous calcium waves originated along the boundary of guinea pig detrusor smooth muscle cells and propagated through the muscle via gap junctions. The interstitial cells of the detrusor could be these pacemaker cells as they are located in the fibromuscular stroma adjacent to the detrusor and are forming a functional syncitium interconnected with gap junctions.
Recent reports have shown that the ICB respond to application of neurotransmitters, firing calcium waves when stimulated by carbachol33 or ATP.47 Combining this with the knowledge that the suburothelial ICB are in close contact with the suburothelial nerves, it may be that the ICB are the target of neurotransmitters released by the nerve endings.
We hypothesise that the suburothelial network of interstitial cells of the bladder, functions as a sensing network, receiving information from the urothelium and from the efferent nerve and modulating afferent bladder innervation.
The detrusor network of interstitial cells might play a role in electrical pacemaking, in electrical coupling and in neuromodulation. The vanilloid-cannabinoid system could regulate the tone of the detrusor.
Overactive bladder disease could result from an imbalance in the vanilloid-cannabinoid system, from an aberrant pacemaker signal generated by the interstitial cells, from an improper electrical coupling between the interstitial cells or from an aberrant modulation of neurotransmission by the interstitial cells of the bladder. Further functional research will clarify the role of these cells in the pathophysiology of bladder overactivity.