Extracellular matrix proteoglycan plays a pivotal role in sensitization by low pH of mechanosensitive currents in nociceptive sensory neurones

Key points Tissue acidosis is caused in many pathological and physiological conditions (e.g. ischaemia, inflammation and exercise) and induces pain and mechanical hyperalgesia. An augmented mechanical response of thin-fibre afferents by low pH has been reported, but the sensitizing mechanism has not been determined. In this study we examined whether mechanically activated (MA) currents recorded from the soma of cultured sensory neurones were augmented by low pH application. We showed that low pH mainly sensitized MA currents of IB4-positive neurones expressing an extracellular matrix proteoglycan, versican, and this sensitization was attenuated by manipulating the extracellular matrix proteoglycan, but not by blocking intracellular signalling pathways. These results show us a novel sensitizing mechanism involving extracellular matrix proteoglycans, which is different from currently popular sensitizing mechanisms involving intracellular signalling pathways. Abstract Ischaemia, inflammation, and exercise lead to tissue acidosis, which induces pain and mechanical hyperalgesia. Corresponding to this, enhanced thin-fibre afferent responses to mechanical stimulation have been recorded in vitro at low pH. However, knowledge about how this sensitization by low pH occurs is lacking. In this study, we found that all three types (rapidly adapting (RA), intermediately adapting and slowly adapting) of mechanically activated currents recorded with the whole cell patch-clamp method were sensitized by low pH in rat cultured dorsal root ganglion neurones. This sensitization was mainly observed in neurones positively labelled with isolectin B4 (IB4), which binds to versican, a chondroitin sulfate proteoglycan. Inhibitors of acid-sensitive channels (amiloride and capsazepine) did not block sensitization by low pH except in RA neurones, and extracellular calcium was not involved even in the sensitization of this type of neurone. A broad spectrum kinase inhibitor and a phospholipase C inhibitor (staurosporine and U73122) failed to block pH-induced sensitization in IB4-positive neurones, suggesting that these intracellular signalling pathways are not involved. Notably, both excess chondroitin sulfate in the extracellular solution and pretreatment of the neurone culture with chondroitinase ABC attenuated this low pH-induced sensitization in IB4-positive neurones. These findings suggest that a change in interaction between mechanosensitive channels and/or their auxiliary molecules and the side chain of versican on the cell surface causes this sensitization, at least in IB4-positive neurones. This report proposes a novel mechanism for sensitization that involves extracellular proteoglycans (versican).