The role of proteases in pain

Alpha-1 antitrypsin in autoimmune diseases: Roles and therapeutic prospects

Alpha-1 antitrypsin (A1AT) is a protease inhibitor in the serum. Its primary function is to inhibit the activity of a series of proteases, including proteinase 3, neutrophil elastase, metalloproteases, and cysteine-aspartate proteases. In addition, A1AT also has anti-inflammatory, anti-apoptotic, anti-oxidative stress, anti-viral, and anti-bacterial activities and plays essential roles in the regulation of tissue repair and lymphocyte differentiation and activation. The overactivation of the immune system characterizes the pathogenesis of autoimmune diseases. A1AT treatment shows beneficial effects on patients and animal models with autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus. This review summarizes the functions and therapeutic prospects of A1AT in autoimmune diseases.

Role of cathepsin K in the expression of mechanical hypersensitivity following intra-plantar inflammation

Persistent/chronic inflammatory pain involves multiple pathophysiological mechanisms and is far more complex than acute/momentary pain. Current therapeutics for chronic inflammatory pain are often not effective because the etiology responsible for the pain is not addressed by traditional pharmacological treatments. Cathepsin K is a cysteine protease that has mostly been studied in the context of bone and joint disorders. Previous work by others has shown that inhibition of cathepsin K activity reduces osteoarthritis-associated nociception in joints. However, the role of cathepsin K in cutaneous inflammation is understudied. We assessed the effectiveness of genetic deletion or pharmacological inhibition of cathepsin K in male mice on the expression of nocifensive behaviors after formalin injection or mechanical and thermal hypersensitivity after injection of complete Freund’s adjuvant (CFA) into the mouse hind paw. Our data demonstrate that cathepsin K knockout mice (Ctsk^−/−) have a reduction in nocifensive behaviors in the formalin test. In addition, Ctsk^−/− do not develop mechanical hypersensitivity after CFA injection for up to 7 days. Moreover, we found that inhibition of cathepsin K reduced mechanical hypersensitivity after CFA injection and mRNA levels, protein levels, and cathepsin K activity levels were elevated after CFA injection. Based upon our data, cathepsin K is indicated to play a role in the expression of chemically-induced cutaneous hypersensitivity, as Ctsk^−/− mice do not develop mechanical hypersensitivity and show a reduction in nocifensive behaviors. Further research is needed to determine whether attenuating cathepsin K activity may generate a clinically relevant therapeutic.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Intracellular versus extracellular inhibition of calpain I causes differential effects on pain in a rat model of joint inflammation

Calpain I is a calcium-dependent cysteine protease which has dual effects on tissue inflammation depending on its cellular location. Intracellularly, calpain I has pro-inflammatory properties but becomes anti-inflammatory when exteriorised into the extracellular space. In this study, the effect of calpain I on joint pain was investigated using the kaolin/carrageenan model of acute synovitis. Evoked pain behaviour was determined by von Frey hair algesiometry and non-evoked pain was measured using dynamic hindlimb weight bearing. Local administration of calpain I reduced secondary allodynia in the acute inflammation model and this effect was blocked by the cell impermeable calpain inhibitor E-64c. Calpain I also blocked the algesic effect of the protease activated receptor-2 (PAR-2) cleaving enzyme mast cell tryptase. The cell permeable calpain blocker E-64d also produced analgesia in arthritic joints. These data suggest that calpain I produces disparate effects on joint pain viz. analgesia when present extracellularly by disarming PAR-2, and pro-algesic when the enzyme is inside the cell.

Subchondral bone microenvironment in osteoarthritis and pain

Osteoarthritis comprises several joint disorders characterized by articular cartilage degeneration and persistent pain, causing disability and economic burden. The incidence of osteoarthritis is rapidly increasing worldwide due to aging and obesity trends. Basic and clinical research on osteoarthritis has been carried out for decades, but many questions remain unanswered. The exact role of subchondral bone during the initiation and progression osteoarthritis remains unclear. Accumulating evidence shows that subchondral bone lesions, including bone marrow edema and angiogenesis, develop earlier than cartilage degeneration. Clinical interventions targeting subchondral bone have shown therapeutic potential, while others targeting cartilage have yielded disappointing results. Abnormal subchondral bone remodeling, angiogenesis and sensory nerve innervation contribute directly or indirectly to cartilage destruction and pain. This review is about bone-cartilage crosstalk, the subchondral microenvironment and the critical role of both in osteoarthritis progression. It also provides an update on the pathogenesis of and interventions for osteoarthritis and future research targeting subchondral bone.

Adrenergic signalling in osteoarthritis

Adrenoceptors (ARs) mediate the effects of the sympathetic neurotransmitters norepinephrine (NE) and epinephrine (E) in the human body and play a central role in physiologic and pathologic processes. Therefore, ARs have long been recognized as targets for therapeutic agents, especially in the field of cardiovascular medicine. During the past decades, the contribution of the sympathetic nervous system (SNS) and particularly of its major peripheral catecholamine NE to the pathogenesis of osteoarthritis (OA) attracted growing interest. OA is the most common degenerative joint disorder worldwide and a disease of the whole joint. It is characterized by progressive degradation of articular cartilage, synovial inflammation, osteophyte formation, and subchondral bone sclerosis mostly resulting in chronic pain. The subchondral bone marrow, the periosteum, the synovium, the vascular meniscus and numerous tendons and ligaments are innervated by tyrosine hydroxylase-positive (TH+) sympathetic nerve fibers that release NE into the synovial fluid and cells of all abovementioned joint tissues express at least one out of nine AR subtypes. During the past decades, several in vitro studies explored the AR-mediated effects of NE on different cell types in the joint. So far, only a few studies used animal OA models to investigate the contribution of distinct AR subtypes to OA pathogenesis in vivo. This narrative review shortly summarizes the current background knowledge about ARs and their signalling pathways at first. In the second part, we focus on recent findings in the field of NE-induced AR-mediated signalling in different joint tissues during OA pathogenesis and at the end, we will delineate the potential of targeting the adrenergic signalling for OA prevention or treatment. We used the PubMed bibliographic database to search for keywords such as 'joint' or 'cartilage' or 'synovium' or 'bone' and 'osteoarthritis' and/or 'trauma' and 'sympathetic nerve fibers' and/or 'norepinephrine' and 'adrenergic receptors / adrenoceptors' as well as 'adrenergic therapy'.

Cytotoxic Immunity in Peripheral Nerve Injury and Pain

Cytotoxicity and consequent cell death pathways are a critical component of the immune response to infection, disease or injury. While numerous examples of inflammation causing neuronal sensitization and pain have been described, there is a growing appreciation of the role of cytotoxic immunity in response to painful nerve injury. In this review we highlight the functions of cytotoxic immune effector cells, focusing in particular on natural killer (NK) cells, and describe the consequent action of these cells in the injured nerve as well as other chronic pain conditions and peripheral neuropathies. We describe how targeted delivery of cytotoxic factors via the immune synapse operates alongside Wallerian degeneration to allow local axon degeneration in the absence of cell death and is well-placed to support the restoration of homeostasis within the nerve. We also summarize the evidence for the expression of endogenous ligands and receptors on injured nerve targets and infiltrating immune cells that facilitate direct neuro-immune interactions, as well as modulation of the surrounding immune milieu. A number of chronic pain and peripheral neuropathies appear comorbid with a loss of function of cellular cytotoxicity suggesting such mechanisms may actually help to resolve neuropathic pain. Thus while the immune response to peripheral nerve injury is a major driver of maladaptive pain, it is simultaneously capable of directing resolution of injury in part through the pathways of cellular cytotoxicity. Our growing knowledge in tuning immune function away from inflammation toward recovery from nerve injury therefore holds promise for interventions aimed at preventing the transition from acute to chronic pain.

Protease-Mediated Suppression of DRG Neuron Excitability by Commensal Bacteria

Peripheral pain signaling reflects a balance of pronociceptive and antinociceptive influences; the contribution by the gastrointestinal microbiota to this balance has received little attention. Disorders, such as inflammatory bowel disease and irritable bowel syndrome, are associated with exaggerated visceral nociceptive actions that may involve altered microbial signaling, particularly given the evidence for bacterial dysbiosis. Thus, we tested whether a community of commensal gastrointestinal bacteria derived from a healthy human donor (microbial ecosystem therapeutics; MET-1) can affect the excitability of male mouse DRG neurons. MET-1 reduced the excitability of DRG neurons by significantly increasing rheobase, decreasing responses to capsaicin (2 μm) and reducing action potential discharge from colonic afferent nerves. The increase in rheobase was accompanied by an increase in the amplitude of voltage-gated K⁺ currents. A mixture of bacterial protease inhibitors abrogated the effect of MET-1 effects on DRG neuron rheobase. A serine protease inhibitor but not inhibitors of cysteine proteases, acid proteases, metalloproteases, or aminopeptidases abolished the effects of MET-1. The serine protease cathepsin G recapitulated the effects of MET-1 on DRG neurons. Inhibition of protease-activated receptor-4 (PAR-4), but not PAR-2, blocked the effects of MET-1. Furthermore, Faecalibacterium prausnitzii recapitulated the effects of MET-1 on excitability of DRG neurons. We conclude that serine proteases derived from commensal bacteria can directly impact the excitability of DRG neurons, through PAR-4 activation. The ability of microbiota-neuronal interactions to modulate afferent signaling suggests that therapies that induce or correct microbial dysbiosis may impact visceral pain.SIGNIFICANCE STATEMENT Commercially available probiotics have the potential to modify visceral pain. Here we show that secretory products from gastrointestinal microbiota derived from a human donor signal to DRG neurons. Their secretory products contain serine proteases that suppress excitability via activation of protease-activated receptor-4. Moreover, from this community of commensal microbes, Faecalibacterium prausnitzii strain 16-6-I 40 fastidious anaerobe agar had the greatest effect. Our study suggests that therapies that induce or correct microbial dysbiosis may affect the excitability of primary afferent neurons, many of which are nociceptive. Furthermore, identification of the bacterial strains capable of suppressing sensory neuron excitability, and their mechanisms of action, may allow therapeutic relief for patients with gastrointestinal diseases associated with pain.

Emerging Targets for the Management of Osteoarthritis Pain

Worldwide, osteoarthritis (OA) is one of the leading causes of chronic pain, for which adequate relief is not available. Ongoing peripheral input from the affected joint is a major factor in OA-associated pain. Therefore, this review focuses predominantly on peripheral targets emerging in the preclinical and clinical arena. Nerve growth factor is the most advanced of these targets, and its blockade has shown tremendous promise in clinical trials in knee OA. A number of different types of ion channels, including voltage-gated sodium channels and calcium channels, transient receptor potential channels, and acid-sensing ion channels, are important for neuronal excitability and play a role in pain genesis. Few channel blockers have been tested in preclinical models of OA, with varying results. Finally, we discuss some examples of G-protein coupled receptors, which may offer attractive therapeutic strategies for OA pain, including receptors for bradykinin, calcitonin gene-related peptide, and chemokines. Since many of the pathways described above can be selectively and potently targeted, they offer an exciting opportunity for pain management in OA, either systemically or locally.

Hypersensitivity Induced by Activation of Spinal Cord PAR2 Receptors Is Partially Mediated by TRPV1 Receptors

Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) receptors in the peripheral nerve endings are implicated in the development of increased sensitivity to mechanical and thermal stimuli, especially during inflammatory states. Both PAR2 and TRPV1 receptors are co-expressed in nociceptive dorsal root ganglion (DRG) neurons on their peripheral endings and also on presynaptic endings in the spinal cord dorsal horn. However, the modulation of nociceptive synaptic transmission in the superficial dorsal horn after activation of PAR2 and their functional coupling with TRPV1 is not clear. To investigate the role of spinal PAR2 activation on nociceptive modulation, intrathecal drug application was used in behavioural experiments and patch-clamp recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs, eEPSCs) were performed on superficial dorsal horn neurons in acute rat spinal cord slices. Intrathecal application of PAR2 activating peptide SLIGKV-NH₂ induced thermal hyperalgesia, which was prevented by pretreatment with TRPV1 antagonist SB 366791 and was reduced by protein kinases inhibitor staurosporine. Patch-clamp experiments revealed robust decrease of mEPSC frequency (62.8 ± 4.9%), increase of sEPSC frequency (127.0 ± 5.9%) and eEPSC amplitude (126.9 ± 12.0%) in dorsal horn neurons after acute SLIGKV-NH₂ application. All these EPSC changes, induced by PAR2 activation, were prevented by SB 366791 and staurosporine pretreatment. Our results demonstrate an important role of spinal PAR2 receptors in modulation of nociceptive transmission in the spinal cord dorsal horn at least partially mediated by activation of presynaptic TRPV1 receptors. The functional coupling between the PAR2 and TRPV1 receptors on the central branches of DRG neurons may be important especially during different pathological states when it may enhance pain perception.

Role of capsaicin-sensitive nerves and tachykinins in mast cell tryptase-induced inflammation of murine knees

OBJECTIVE, DESIGN

Mast cell tryptase (MCT) is elevated in arthritic joints, but its direct effects are not known. Here, we investigated MCT-evoked acute inflammatory and nociceptive mechanisms with behavioural, in vivo imaging and immunological techniques.

MATERIAL AND SUBJECTS

Neurogenic inflammation involving capsaicin-sensitive afferents, transient receptor potential vanilloid 1 receptor (TRPV1), substance P (SP), neurokinin A (NKA) and their NK1 tachykinin receptor were studied using gene-deleted mice compared to C57Bl/6 wildtypes (n = 5-8/group).

TREATMENT

MCT was administered intraarticularly or topically (20 μl, 12 μg/ml). Capsaicin-sensitive afferents were defunctionalized with the TRPV1 agonist resiniferatoxin (RTX; 30-70-100 μg/kg s.c. pretreatment).

METHODS

Knee diameter was measured with a caliper, synovial perfusion with laser Doppler imaging, mechanonociception with aesthesiometry and weight distribution with incapacitance tester over 6 h. Cytokines and neuropeptides were determined with immunoassays.

RESULTS

MCT induced synovial vasodilatation, oedema, impaired weight distribution and mechanical hyperalgesia, but cytokine or neuropeptide levels were not altered at the 6-h timepoint. Hyperaemia was reduced in RTX-treated and TRPV1-deleted animals, and oedema was absent in NK1-deficient mice. Hyperalgesia was decreased in SP/NKA- and NK1-deficient mice, weight bearing impairment in RTX-pretreated, TRPV1- and NK1-deficient animals.

CONCLUSIONS

MCT evokes synovial hyperaemia, oedema, hyperalgesia and spontaneous pain. Capsaicin-sensitive afferents and TRPV1 receptors are essential for vasodilatation, while tachykinins mediate oedema and pain.

Emerging concepts of pain therapy based on neuronal mechanisms

Current pain treatment is successful in many patients, but nevertheless numerous problems have to be solved because still about 20% of the people in the population suffer from chronic pain. A major aim of pain research is, therefore, to clarify the neuronal mechanisms which are involved in the generation and maintenance of different pain states and to identify the mechanisms which can be targeted for pain treatment. This volume on pain control addresses neuronal pain mechanisms at the peripheral, spinal, and supraspinal level which are thought to significantly contribute to pain and which may be the basis for the development of new treatment principles. This introductory chapter addresses the types of pain which are currently defined based on the etiopathologic considerations, namely physiologic nociceptive pain, pathophysiologic nociceptive pain, and neuropathic pain. It briefly describes the structures and neurons of the nociceptive system, and it addresses molecular mechanisms of nociception which may become targets for pharmaceutical intervention. It will provide a frame for the chapters which address a number of important topics. Such topics are the concept of hyperalgesic priming, the role of voltage-gated sodium channels and nerve growth factor (NGF) in different inflammatory and neuropathic pain states, the hyperalgesic effects of NGF in different tissues, the contribution of proteinase-activated receptors (PARs) to the development of pain in several chronic pain conditions, the role of spinal NO and of glial cell activation in the generation and maintenance of inflammatory and neuropathic pain, the potential role of spinal inhibitory interneurons, the endogenous endocannabinoid system, and the importance of nonneuronal immune mechanisms in opioid signaling in the control of pain, the influence of spinal mechanisms on the expression of peripheral inflammation, the role of the amygdala and their connections to the medial prefrontal cortex in pain states, the experimental methods to test central sensitization of the nociceptive system in humans, and differences and similarities of the neuronal systems of pain and itch. Finally it will be discussed that both the concentration on single key molecules of nociception and the interference with disease-related mediators may provide novel approaches of pain treatment.