Roles of mesenchymal stem cells and exosomes in interstitial cystitis/bladder pain syndrome

Abstract Interstitial cystitis/bladder pain syndrome (IC/BPS) is characterized by several symptoms of higher sensitivity of the lower urinary tract, such as bladder pain/discomfort, urgency, urinary frequency, pelvic pain and nocturia. Although the pathophysiology of IC/BPS is not fully understood, the hypothesis suggests that mast cell activation, glycosaminoglycan (GAG) layer defects, urothelium permeability disruption, inflammation, autoimmune disorder and infection are potential mechanisms. Mesenchymal stem cells (MSCs) have been proven to protect against tissue injury in IC/BPS by migrating into bladders, differentiating into key bladder cells, inhibiting mast cell accumulation and cellular apoptosis, inhibiting inflammation and oxidative stress, alleviating collagen fibre accumulation and enhancing tissue regeneration in bladder tissues. In addition, MSCs can protect against tissue injury in IC/BPS by secreting various soluble factors, including exosomes and other soluble factors, with antiapoptotic, anti‐inflammatory, angiogenic and immunomodulatory properties in a cell‐to‐cell independent manner. In this review, we comprehensively summarized the current potential pathophysiological mechanisms and standard treatments of IC/BPS, and we discussed the potential mechanisms and therapeutic effects of MSCs and MSC‐derived exosomes in alleviating tissue injury in IC/BPS models.


| INTRODUC TI ON
Interstitial cystitis/bladder pain syndrome (IC/BPS) is an umbrella term of chronic debilitating conditions characterized by symptoms of lower urinary tract hypersensitivity that lead to poor quality of life in IC/BPS patients. Although it is not easy to evaluate the true prevalence of IC/BPS according to the heterogenic definitions and nomenclature, approximately 3%-7% prevalence is reported according to current studies. 1 These patients always complain of chronic clinical symptoms such as bladder pain/discomfort, pelvic pain, urinary frequency, urgency and nocturia, but there is no bacterial infection in the urinary system after laboratory examinations. 2 Cameron et al. demonstrated that some women with IC/BPS also had several complaints of bladder outlet obstruction (BOO) with sensation of incomplete emptying, slow stream, straining to void and dribbling.
They postulated that painful voiding was at the root of the problem with pain causing reflexive poor pelvic floor relaxation. 3 In addition, specific individuals have accompanying symptoms, including chronic stress, sleep dysfunction, anxiety and sexual dysfunction. 4 The clinical diagnosis of IC/BPS is based on these typical symptoms after exclusion of other alternative diseases with a similar presentation, including urinary tract infection, neoplasia and bladder calculi. In more recent years, IC/BPS has been reported to be closely related to other systemic pain syndromes, including chronic fatigue syndrome, fibromyalgia and irritable bowel syndrome. 5 Stem cell-based therapy has emerged as a valuable treatment to protect against tissue injury in different diseases. Stem cells can be classified into various cell types, such as mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) and hematopoietic stem cells. 6 In comparison with ESCs and iPSCs, MSCs have less tumorigenicity in vivo, which enables them to be safer in clinical treatments. MSCs can be isolated from different adult tissues such as umbilical cord blood, bone marrow, adipose tissue, placenta, amniotic fluid, urine and other tissues. [7][8][9][10][11] They express low levels of major histocompatibility complex (MHC) class I molecules and MHC class II molecules, which enable them to have low immunogenicity. 12 In vitro, MSCs can differentiate into a range of cell types, including adipocytes, osteocytes, hepatocytes, chondrocytes, muscle, vascular smooth muscle cells and other connective tissues. [13][14][15] In response to injury signals, MSCs migrate into damaged target tissues 16,17 and differentiate into local progenitor cells to replace ageing cells or apoptotic cells in vivo. 18,19 In addition, the cytokines and growth factors secreted by MSCs yield beneficial outcomes in injured tissues or organs by stimulating angiogenesis and proliferation of endogenous progenitor cells but alleviating inflammatory reactions and tissue fibrosis. These paracrine effects are considered the primary therapeutic mode of action for these cells in some respects. 20 All of these mechanisms enable MSCs and their exosomes to be potential therapeutic agents for IC/BPS according to current studies.
In this review, we comprehensively summarized the current potential pathophysiological mechanisms and standard treatments of IC/BPS, and we discussed the potential mechanisms and therapeutic effects of MSCs and MSC-derived exosomes in alleviating tissue injury in IC/BPS models.

| THE PATHOPHYS IOLOG IC AL MECHANIS MS OF IC/B PS
Although the pathophysiological mechanisms are not identifiable and not fully understood, a hypothesis suggests that mast cell activation, glycosaminoglycan (GAG) layer defects, urothelium permeability disruption, inflammation, autoimmune disorder and infection are potential mechanisms (Figure 1). The urothelium in bladder tissue is covered by mucopolysaccharides and consists of a basal cell layer, an intermediate layer and a superficial layer. The urothelium provides a barrier to defend the underlying tissues against urine and create a selected and specific movement of molecules towards the bladder wall. 21 The umbrella cells were denudated to develop defects in integrity and microplicae and severe pleomorphism in patients with IC/BPS. 22 Injury factors or allergic reactions induce urothelium dysfunction and result in secreting different inflammatory cytokines (vasoactive intestinal peptide [VIP] and tumour necrosis factor [TNF]), which subsequently activate mast cells. 23 Moreover, the specifically activated mast cells secrete a group of inflammatory and nociceptive factors and neurotransmitters (acetylcholine and substance P), which further promote the stimulation of other mast cells. 24 The bladder surface mucus secretes a hydrophilic GAG layer that traps water at the outer layer to form a key protective barrier at the important space between the urine and the bladder. 25 GAG molecules consist of a family of polysaccharide molecules and play critical roles in the regulation of enzymes, growth factors, protease inhibitors and cytokines. GAGs, fibronectin, elastin, collagen and laminin are known to form the basic framework of the extracellular matrix. 21 However, disruption of the GAG layer by inflammation, infection, damage, lesions and other factors results in upregulation of urothelial permeability and imbalance in urine storage. 26 Tamm-Horsfall protein (THP) is the most abundant protein that is protective in human urine and terminates in sialic acid, 27,28 while the THP of IC/ BPS patients is less protective against urinary cations that can injure the mucus of the urothelium. 29 Potassium sensitivity is also a key mechanism in the progression of IC/BPS, since protamine sulfate administration can induce potassium absorption and injury to the bladder surface mucus. 30 The upregulation of influx of potassium ions and urothelial permeability leads to the activation of sensory afferent nerves in a vicious cycle, which contributes to the painful symptoms of IC/BPS. 31,32 Neurogenic inflammation contributes to bladder afferent hypersensitivity and stromal fibrotic alterations via nerve-mast cell interactions. 33 Moreover, IC/BPS patients were found to have increased angiogenesis, glomerulation formation and urothelial deficiency. 22 Intriguingly, a relationship between IC/BPS and other autoimmune diseases has been reported. A large number of T cells, monocytes, mast cells, B cells and plasma cells infiltrate the urinary bladders of IC/BPS patients, 34,35 and the ratio of inflammatory cells and T helper cells to suppressor cells is upregulated in the peripheral blood of these patients. 36 An anti-C-X-C motif chemokine ligand (CXCL)-10 antibody effectively alleviated the severity of chronic IC/BPS by locking chemokine (C-X-C motif) receptor-3 ligands and reducing the release of proinflammatory cytokines and infiltrated immune cells. 37 Upon exposure to bacterial infection, the symptoms were exacerbated, and the expression of E-cadherin was downregulated in the urothelium. 38 Removal of mucus with an acidic solution or detergents results in a significant increase in bacterial adherence or bacterial infection. 39

| S TANDARD TRE ATMENTS FOR I C/B PS
Targeted treatments have mixed outcomes, since the mechanisms are complex and the complicated symptoms are not easy to control.
Specific treatments that target the related mechanisms are recommended. IC/BPS patients have a relatively low quality of life because of unmanageable symptoms, and the main target is to improve the quality of life of IC/BPS patients. 41 Behavioural, physical, pharmacological, endoscopic and surgical therapies must be simultaneously considered to synergistically improve the quality of life of patients.
The first step in the management of patients with IC/BPS includes conservative and behavioural guidance, including timed voiding and bladder training, which serve as the foundation for long-term treatment because they are risk-free and relatively inexpensive for all patients. 42 Physiotherapy includes soft tissue massage of the pelvic floor, and myofascial trigger point release is also a good option for patients with pelvic floor dysfunction and trigger point or myofascial tenderness. 43 Pharmacotherapy includes a wide range of drugs and is currently applied in the clinic via the oral or intravesical route, while a few of them are recommended because of heterogeneity.
Although amitriptyline is not licensed for application in IC/BPS, F I G U R E 1 Potential pathophysiological mechanisms of IC/BPS according to current studies this tricyclic antidepressant is used to effectively alleviate neuropathic pain when patients can tolerate a dose of at least 50 mg. 44 Pentosan polysulfate is a licensed drug for the treatment of IC/ BPS. Van et al. 45 demonstrated that pentosan polysulfate effectively alleviated discomfort symptoms such as pain, urgency and frequency compared with the placebo, and pentosan polysulfate could replace the GAG layer and inhibit mast cell degranulation. 46 Antihistamines benefit from preventing histamine release from mast cells and are potential therapeutic agents in IC/BPS patients, but evidence of efficacy is conflicting and based on low-quality studies. 47 Cyclosporin A targets T-cell activation and cytokine release and has emerged as a treatment option for IC/BPS patients.
The application should be cautious because it may exert severe side effects. It is preferable for patients with refractory BPS who are not sensitive to other oral or intravesical agents. 48 Intravesical dimethylsulfoxide (DMSO) demonstrated high efficacy in symptom improvements and urodynamic and voiding diary changes with minor side effects. 49 In addition, DMSO remains the only approved drug by the Food and Drug Administration (FDA) to treat IC/BPS, and it is hypothesized to inhibit inflammatory mast cells and promote the relaxation of bladder muscles. 50 Intravesical pentosan polysulfate was reported to more significantly increase the efficacy of pentosan polysulfate in alleviating IC/BPS symptoms than oral pentosan polysulfate. 51 Intravesical lidocaine is another optional treatment with 30%-50% symptom improvements in IC/ BPS. 52  Bladder distention has served as both treatment and diagnostic tool for IC/BPS patients for many years, while the technique has an unverified outcome because of the heterogeneity in patient selection and hydrodistension techniques. 56 Meanwhile, the implanted electrode and conducting lead in sacral neuromodulation stimulate the afferent sacral nerves when they exit the sacral foramina, which obviously alleviates symptoms of pelvic pain, frequency, nocturia and urgency in IC/BPS patients. 57 However, some patients with implantation of the device found that the symptoms were aggravated, which indicates that revision or explanation of the device is necessary.
Radical surgery is the last resort option for severe and disabling IC/BPS patients who are not sensitive to other treatments. Most recently, in 2021, a systematic review concluded that 77% of 448 IC/ BPS patients had symptomatic improvement after they underwent radical surgery, and total cystectomy with orthotopic neobladder formation had the highest clinical response compared to subtotal cystectomy with cystoplasty and urinary diversion alone. 58 Although there are various treatment tools for IC/BPS patients, there are still no definite treatments in terms of recurrence, side effects or lack of response. IC/BPS is a chronic condition that may be controlled but not cured. It is necessary to investigate new and safer agents to inhibit pathophysiological processes.

| MSC s AND I C/B PS
Although there have been no definitive treatments to cure this heterogeneous disease until now, MSCs have emerged as a new therapeutic agent in IC/BPS. Bladder dysfunction is an excellent candidate for stem cell therapy due to its chronic nature and high prevalence. 59 Compared to radical surgery, MSC transplantation is easier to manipulate. In comparison with pharmacological agents, they are pleiotropic and have no severe side effects. MSCs can be isolated from autologous tissue without rejection reaction. Moreover, cell-free therapy based on MSC-derived exosomes is an emerging treatment that benefits from abundant anti-inflammatory and anti-immune cytokines.

| MSCs effectively alleviated tissue injury of IC/BPS via different mechanisms
Mesenchymal stem cells have been proven to protect against tissue injury in IC/BPS via different mechanisms (Table 1)     transplantation. 70 We have found that the anti-inflammatory and immunoregulatory capacities of MSCs are more important and widely studied after we take a panoramic view of the situation from the current studies.

| MSCs effectively alleviated tissue injury of partial BOO
Partial BOO is a common pathological condition induced by sev-

ACK N OWLED G EM ENTS
This work was supported by the National Natural Science Foundation of China (no. 81700553) and Zhejiang basic public welfare research programme (no. LGF20H030008).

CO N FLI C T S O F I NTE R E S T
The authors declare that they have no conflict of interest.