Pain is initiated when specific pain-sensitive nerve fibres (nociceptors) are activated by painful stimuli such as heat, strong mechanical stimuli and chemical stimuli. Examples of chemical stimuli include acid (e.g. from lemon juice) or capsaicin, the active ingredient of chilli peppers. In the last 10 years much progress has been made in understanding the cellular basis of pain, and in particular in isolating and characterising the membrane ion channels which open in response to these stimuli and thus initiate action potentials in primary pain-sensitive afferent nerve fibres. Amongst these ion channels are the heat and capsaicin-sensitive ion channel, TRPV1, and other related members of the TRP family, and the acid-sensitive family of ion channels, the ASICs. Our overall aim is to use a range of state-of-the-art cellular and molecular techniques to unravel an age-old problem which lies at the root of much human suffering.
Pain is unique amongst sensations in that it increases with prolonged exposure, a process known as sensitization, while in other sensory systems adaptation to a prolonged stimulus is observed. Sensitization has obvious protective value for the organism, but it is also responsible for much suffering in chronic pain states. Sensitization is caused by release of inflammatory mediators, such as bradykinin, prostaglandins and nerve growth factor (NGF), which bind to receptors on the neuronal cell membrane and activate cellular signalling pathways, which in turn modify the activity of downstream targets such as TRPV1.
Our group has been interested in heat-activated ion channels for many years, and we first characterised a heat-gated ion channel in nociceptive neurones (Cesare & McNaughton, 1996; Cesare & McNaughton, 1997; Cesare et al., 1999). We found that the heat-activated channel was sensitised by bradykinin, and we showed that this process depended on phosphorylation by the epsilon isoform of protein kinase C (PKC-epsilon - see Cesare et al, 1999). Since the cloning of TRPV1, we have shown that the heat and capsaicin responses in neurones and in TRPV1-transfected cells were comparable, with both being sensitised by PKC-epsilon phosphorylation (Vellani et al., 2001).
We currently focus on obtaining a greater understanding of the mechanisms of activation and modulation of TRPV1. Work in our lab has identified the pathways involved in TRPV1 modulation by NGF (Bonnington & McNaughton, 2003), and we have recently identified the residues on both the NGF receptor, TrkA, and TRPV1 itself that are responsible for NGF mediated sensitisation (Zhang et al., 2005). TRPV1 is sensitised by two distinct processes: gating of ion channels already in the membrane can be enhanced, or additional ion channels can be inserted into the membrane. Some agents (e.g bradykinin) act mainly to enhance gating, while others (e.g. NGF) have their main effect by inserting new channels. Phosphorylation of the channel at different sites controls both processes (see diagram below).
- Schematic diagram of the signaling pathways important in sensitization of TRPV1 by TrkA. Functionally most significant pathway is shown at left (yellow, solid arrows). A smaller component of sensitization following exposure to NGF is mediated by phosphorylation of TRPV1 at residues S502 and S801, probably by the PLC-gamma/PKC-epsilon pathway (green, dashed arrows). PKC-epsilon is a crucial intermediate in sensitization of TRPV1 by bradykinin (pathway shown at lower right of diagram). See Zhang et al, 2005.
A second important contributor to sensitisation is expression of receptors for inflammatory mediators. We have been interested in the factors controlling expression of B1 and B2 receptors for bradykinin. B2 receptors are upregulated by NGF (Lee et al., 2002), while B1 receptors are not expressed normally but appear following injury, a process which is mediated by a different growth factor, GDNF (Vellani et al., 2004).
Recent work has shown TRPV4 to be activated by cell swelling. What are the mechanisms of activation and modulation of TRPV4 when a cell swells? Understanding in more detail how TRPV4 is modulated could have potential therapeutic benefit in helping to treat patients suffering from syndromes such as SIADS (syndrome of inappropriate secretion of anti-diuretic hormone).
ASICs are expressed in the peripheral nervous system suggesting a role in nociception, which we have confirmed in recent work our group conducted in collaboration with Prof. Steve McMahon's group at King's College London (Jones et al., 2004). Currently there is little known about how ASICs might be sensitised by inflammation and work in our laboratory involves investigating the effects of different inflammatory mediators upon ASICs. The mechanism(s) by which ASICs are activated by low pH is not fully understood and another aim of our current research is to understand the activation of ASICs by protons.
During chronic pain nociceptive neurones can display spontaneous activity, in the form of repetitive action potentials. One membrane ionic current that may contribute to such behavious is the Ih current, a slow developing non-inactivating, non-selective inward current activated by membrane hyperpolarisation. The family of ion channels responsible for carrying the Ih current are termed the hyperpolarisation-activated cyclic nucleotide gated channels (HCN). Our lab is interested in identifying which HCN channels are present in which neuronal phenotypes, and how these may be modified by cyclic AMP and other cell signalling pathways.
Many animals, both invertebrates and vertebrates, have been shown to be sensitive to magnetic fields. A growing number of observations support the theory that they may use this 'sixth sense' to navigate over long distances, especially in environments lacking visual cues (sky, deep ocean), but the cellular basis of magnetoreception has remained mysterious. The discovery of chains of magnetite (Fe3O4) in cells of the trout olfactory organ (Walker et al, Nature 390, 371-6, 1997) suggests that these cells may be the elusive magnetoreceptors. Our aim is to study the cellular mechanisms underlying the activation of these receptors, in collaboration with Professor Michael Walker at the University of Auckland.
- Cell culture (neuronal and transfected cell lines)
- Electrophysiology (whole-cell and single channel)
- Confocal microscopy and calcium imaging
- Molecular biology (site-directed mutagenesis, immunoprecipitation, western blots, immunohistochemistry, biotinylation to label membrane expressed proteins and PCR)