Pain due to osteoarthritis (OA) is one of the most frequent causes of chronic pain. However, the mechanisms of OA pain are poorly understood. This review addresses the mechanisms which are thought to be involved in OA pain, derived from studies on pain mechanisms in humans and in experimental models of OA. Three areas will be considered, namely local processes in the joint associated with OA pain, neuronal mechanisms involved in OA pain, and general factors which influence OA pain. Except the cartilage all structures of the joints are innervated by nociceptors. Although the hallmark of OA is the degradation of the cartilage, OA joints show multiple structural alterations of cartilage, bone and synovial tissue. In particular synovitis and bone marrow lesions have been proposed to determine OA pain whereas the contribution of the other pathologies to pain generation has been studied less. Concerning the peripheral neuronal mechanisms of OA pain, peripheral nociceptive sensitization was shown, and neuropathic mechanisms may be involved at some stages. Structural changes of joint innervation such as local loss and/or sprouting of nerve fibers were shown. In addition, central sensitization, reduction of descending inhibition, descending excitation and cortical atrophies were observed in OA. The combination of different neuronal mechanisms may define the particular pain phenotype in an OA patient. Among mediators involved in OA pain, nerve growth factor (NGF) is in the focus because antibodies against NGF significantly reduce OA pain. Several studies show that neutralization of interleukin-1β and TNF may reduce OA pain. Many patients with OA exhibit comorbidities such as obesity, low grade systemic inflammation and diabetes mellitus. These comorbidities can significantly influence the course of OA, and pain research just began to study the significance of such factors in pain generation. In addition, psychologic and socioeconomic factors may aggravate OA pain, and in some cases genetic factors influencing OA pain were found. Considering the local factors in the joint, the neuronal processes and the comorbidities, a better definition of OA pain phenotypes may become possible. Studies are under way in order to improve OA and OA pain monitoring.

PB1-F2 is a pro-apoptotic polypeptide of many influenza A virus (FLUAV) isolates encoded by an alternative ORF of segment 2. A comprehensive GenBank search was conducted to analyse its prevalence. This search yielded 2226 entries of 80 FLUAV subtypes. Of these sequences, 87 % encode a PB1-F2 polypeptide greater than 78 aa. However, classic swine influenza viruses and human H1N1 isolates collected since 1950 harbour a truncated PB1-F2 sequence. While PB1-F2 of human H1N1 viruses terminates after 57 aa, classic swine H1N1 sequences have in-frame stop codons after 11, 25 and 34 codons. Of the avian sequences, 96 % encode a full-length PB1-F2. One genetic lineage of segment 2 sequences which is avian-like and different from the classic swine FLUAV comprises PB1-F2 sequences of porcine FLUAVs isolated in Europe (H1N1, H1N2, H3N2). Of these PB1-F2 sequences, 42 % also exhibit stop codons after 11, 25 and 34 codons. These amino acid positions are highly conserved among all FLUAV isolates irrespective of their origin. Molecular genetic analyses reveal that PB1-F2 is under constraint of the PB1 gene. The PB1-F2 polypeptide of FLUAVs isolated from European pigs is expressed in host cells as demonstrated by immunohistochemistry. Using different PB1-F2 versions fused to an enhanced GFP, mitochondrial localization is demonstrated for those PB1-F2 polypeptides which are greater than 78 aa while a truncated version (57 aa) shows a diffuse cytoplasmic distribution. This indicates similar properties and function of porcine and human FLUAV PB1-F2.

The major burden of knee joint osteoarthritis (OA) is pain. Since in elder patients diabetes mellitus is an important comorbidity of OA, we explored whether the presence of diabetes mellitus has a significant influence on pain intensity at the end stage of knee OA, and we aimed to identify factors possibly related to changes of pain intensity in diabetic patients. In 23 diabetic and 47 nondiabetic patients with OA undergoing total knee arthroplasty, we assessed the pain intensity before the operation using the "Knee Injury and Osteoarthritis Outcome Score". Furthermore, synovial tissue, synovial fluid (SF), cartilage, and blood were obtained. We determined the synovitis score, the concentrations of prostaglandin E2 and interleukin-6 (IL-6) in the SF and serum, and of C-reactive protein and HbA1c and other metabolic parameters in the serum. We performed multivariate regression analyses to study the association of pain with several parameters. Diabetic patients had on average a higher Knee Injury and Osteoarthritis Outcome Score pain score than nondiabetic patients (P < 0.001). Knee joints from diabetic patients exhibited on average higher synovitis scores (P = 0.024) and higher concentrations of IL-6 in the SF (P = 0.003) than knee joints from nondiabetic patients. Multivariate regression analysis showed that patients with higher synovitis scores had more intense pain independent of all investigated confounders, and that the positive association between pain intensities and IL-6 levels was dependent on diabetes mellitus and/or synovitis. These data suggest that diabetes mellitus significantly increases pain intensity of knee OA, and that in diabetic patients higher pain intensities were determined by stronger synovitis.

Almost 10 years ago, an eleventh protein of influenza A viruses was discovered in a search for CD8+ T-cell epitopes. This protein was named PB1-F2 since it is encoded in the +1 reading frame of the PB1 gene segment. Various studies have shown that PB1-F2 has a pleiotropic effect: (1) The protein can induce apoptosis in a cell type-dependent manner, (2) PB1-F2 is able to promote inflammation, and (3) finally it up-regulates viral polymerase activity by its interaction with the PB1 subunit. These properties could contribute to an enhanced pathogenicity. However, the underlying mechanism is not fully understood yet. New data suggest that some effects of PB1-F2 are strain-specific and host-specific.

Inflammatory changes in the synovium of OA joints are associated with a massive destruction of the capillary and neuronal network which is present in normal synovium. Due to the disappearance of the sensory fibres it is unlikely that OA pain is initiated directly in the synovium. The loss of normally innervated vascularisation may have multiple consequences for the physiological functions of the synovium.

The pain mediator prostaglandin E 2 (PGE 2 ) sensitizes nociceptive pathways through EP2 and EP4 receptors, which are coupled to G s proteins and increase cAMP. However, PGE 2 also activates EP3 receptors, and the major signaling pathway of the EP3 receptor splice variants uses inhibition of cAMP synthesis via G i proteins. This opposite effect raises the intriguing question of whether the G i -protein-coupled EP3 receptor may counteract the EP2 and EP4 receptor-mediated pronociceptive effects of PGE 2 . We found extensive localization of the EP3 receptor in primary sensory neurons and the spinal cord. The selective activation of the EP3 receptor at these sites did not sensitize nociceptive neurons in healthy animals. In contrast, it produced profound analgesia and reduced responses of peripheral and spinal nociceptive neurons to noxious stimuli but only when the joint was inflamed. In isolated dorsal root ganglion neurons, EP3 receptor activation counteracted the sensitizing effect of PGE 2 , and stimulation of excitatory EP receptors promoted the expression of membrane-associated inhibitory EP3 receptor. We propose, therefore, that the EP3 receptor provides endogenous pain control and that selective activation of EP3 receptors may be a unique approach to reverse inflammatory pain. Importantly, we identified the EP3 receptor in the joint nerves of patients with painful osteoarthritis. mechanical hyperalgesia | sodium currents P rostaglandins regulate immune responses (1), and they are key mediators of pain and other sickness symptoms such as fever, sleepiness, and anorexia (2). In particular, prostaglandin E 2 (PGE 2 ) is a key mediator of pain because it sensitizes peripheral and spinal nociceptive pathways (3-7). Hence the most common pain treatment is the inhibition of prostaglandin synthesis by cyclooxygenase inhibitors. PGE 2 activates neuronal EP1-4 receptors (8). In this context, it is noteworthy that different EP receptors are coupled to different, partly even opposing intracellular signaling pathways. EP2 and EP4 receptors, which sensitize neurons (9-11), are coupled to G s proteins and increase cAMP (12, 13). In contrast, the major signaling pathway of the EP3 receptor splice variants uses inhibition of cAMP synthesis via G i proteins (12, 13). The functional significance of such opposite effects raises the intriguing question of whether the G i -protein-coupled EP3 receptor may counteract the EP2 and EP4 receptor-mediated pronociceptive effects of PGE 2 (9). Thus, the role of PGE 2 may not be just pronociceptive as usually assumed but it may be rather more diverse and depend on the biological context, as during inflammation (1). In the present experiments, we addressed the hypothesis that EP3 receptor activation is rather antinociceptive than pronociceptive. We found that the EP3 receptor is heavily expressed in rat sensory dorsal root ganglia (DRG) and spinal cord as well as in peripheral nerves including nerve fibers of osteoarthritic knees of humans. Selective activation of the EP3 receptor did no...

Objective. In arthritis, macrophages invade the affected joint. Experimental arthritis models have shown that macrophages also invade the dorsal root ganglia (DRGs) of the inflamed segments in which the perikarya of sensory neurons are located. It is unclear whether this macrophage invasion contributes to arthritis pain and/or furthers neuronal damage. The present study was undertaken to investigate how differently activated macrophages affect DRG neurons.Methods. We determined the phenotype of macrophages in the DRGs of rats with antigen-induced arthritis (AIA). In a DRG neuron-macrophage coculture system, we investigated whether differently activated macrophages (stimulated with either lipopolysaccharide [LPS]/interferon-g [IFNg], tumor necrosis factor [TNF], or interleukin-4) damage DRG neurons and/or stimulate them to release the mediator calcitonin generelated peptide (CGRP), which promotes pain and neurogenic inflammation.Results. Macrophages in the DRGs of rats with AIA showed the phenotype of TNF-stimulated macrophages but did not express inducible nitric oxide synthase, which was found in cultured macrophages only after LPS/ IFNg activation. In neuron-macrophage cocultures, activation of macrophages stimulated DRG neurons to release CGRP within 1 hour, indicating neuronal activation by macrophages. Only 48-hour activation of macrophages with LPS/IFNg increased the neuronal cell death rate in culture, provided that the macrophages were in direct contact with DRG neurons. This effect was dependent on nitric oxide.Conclusion. Macrophages have the potential to stimulate sensory neurons in the DRGs, and this may contribute to arthritis pain. If they are classically activated, such as after LPS/IFNg stimulation, this may also further neuronal cell death. This is not the case in AIA but may occur in models involving damage of sensory neurons.

The expression of proteinase-activated receptor (PAR)(2) in human hepatocellular carcinoma (HCC) was established by reverse transcription-polymerase chain reaction, confocal immunofluorescence and electron microscopy in permanent cell lines, primary HCC cell cultures and HCC tumor tissue. Stimulation of HCC cells with trypsin and the PAR(2)-selective activating peptide, 2-furoyl-LIGRLO-NH(2), increased cell invasion across Matrigel. Both effects were blocked by a PAR(2)-selective pepducin antagonist peptide (pal-PAR(2)) and by PAR(2) silencing with specific small interfering RNA (siRNA). PAR(2)-initiated HCC cell invasion was also blocked by inhibiting the hepatocyte growth factor receptor (Met receptor tyrosine kinase) with the receptor-targeted kinase inhibitors, SU 11274 and PHA 665752, or by downregulation of Met with specific siRNA. The involvement of Met in PAR(2)-mediated HCC invasive signaling was further supported by the finding that treatment of HCC cells with trypsin or the PAR(2)-selective agonist peptide, 2-furoyl-LIGRLO-NH(2), stimulated Met activation-phosphorylation. In addition, Met-dependent stimulation of p42/p44 mitogen-activated protein Kinases was found to be critical for the PAR(2)-Met receptor tyrosine kinase-invasive signaling axis in HCC cells. Our study establishes an important link between the PAR(2) and Met receptor tyrosine kinase signaling in promoting HCC cell invasion.