TRP channels: Emerging targets for respiratory disease

Abstract

The mammalian transient receptor potential (TRP) superfamily of cation channels is divided into six subfamilies based on sequence homology TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPP (polycystin) and TRPML (mucolipin). The expression of these channels is especially abundant in sensory nerves, and there is increasing evidence demonstrating their existence in a broad range of cell types which are thought to play a key role in respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD). These ion channels can be activated by a diverse range of chemical and physical stimuli. Physical stimuli include temperature, membrane potential changes and osmotic stress, and some of the more well known chemical stimuli include capsaicin (TRPV1), menthol (TRPM8) and acrolein (TRPA1). There is increasing evidence in this rapidly moving field to suggest that selective blockers of these channels may represent attractive novel strategies to treat characteristic features of respiratory diseases such as asthma and COPD. This review focuses on summarising the evidence that modulation of selected TRP channels may have beneficial effects at targeting key features of these respiratory diseases including airways inflammation, airways hyper-reactivity, mucus secretion and cough.

Introduction

Asthma and chronic obstructive pulmonary disease (COPD) (a group of diseases including chronic bronchitis and emphysema) are now very common chronic diseases and there is evidence to suggest that the prevalence of both diseases is increasing worldwide (Manino and Buist, 2007, Pearce et al., 2007). COPD, in particular, is a major cause of mortality and morbidity globally, and currently, unlike asthma, existing drug treatments do not prevent progression of the disease. There remains a pressing need for new and effective therapies to treat both COPD and severe asthma (Barnes and Hansel, 2004, Barnes, 2006).

Airways obstruction is a shared characteristic feature of both asthma and COPD. However, whereas in asthma the airflow obstruction is variable and normally reversible, in COPD it is progressive and largely irreversible. Airways hyperreactivity, a defining feature of asthma, is not a characteristic of COPD, although enhanced responsiveness to some agonists may be seen. Chronic inflammation of the respiratory tract is a common feature of both diseases as are acute episodes of worsening or exacerbations (reviewed by Kim & Rhee, 2010). These exacerbations, most frequently caused by viral and/or bacterial infections, are a major cause of increased patient suffering and are associated with intensified airways inflammation (Wark and Gibson, 2006, Celli and Barnes, 2007). However, the type of inflammation and the range of inflammatory cell types and mediators involved in the two diseases are distinctly different (Jeffery, 1998, Barnes, 2008) (simplified version of the inflammation is depicted in Fig. 1). In addition, there is a high unmet medical need to treat cough; a common symptom of both asthma and COPD (Higenbottam, 2002).

In COPD patients, lung inflammation is characterised by increased numbers of macrophages (5–10-fold increase above normal levels in bronchoalveolar lavage fluid), neutrophils and CD8⁺ T-lymphocytes in the airways and lung parenchyma (Keatings et al., 1996, Saetta et al., 1998). The parenchyma of emphysema patients has been reported to show a 25-fold increase in the number of macrophages both in the tissue and alveoli in comparison with normal smokers and there is increasing evidence that macrophages play a crucial role in orchestrating the inflammatory response in COPD (Retamales et al., 2001, Barnes, 2004). In response to cigarette smoke (the single most important cause of COPD) or other irritant stimuli in the airways, macrophages and epithelial cells release chemotactic factors including the CXCR1/2 chemokine receptor ligands CXCL1 (Groα), CXCL8 (IL-8), the CCR2 ligand CCL2 (MCP-1), the CXCR3 ligands CXCL9 (MIG), CXCL10 (IP-10), CXCL11 (ITAC) and leukotriene B₄ (LTB₄) which attract neutrophils, monocytes and CD8⁺ T-lymphocytes into the lung (Saetta et al., 2002, Grumelli et al., 2004, Traves et al., 2004, Costa et al., 2008). Upon activation these cells can release proteolyic enzymes such as the matrix metalloproteinases (MMPs)-1, MMP-2, MMP-9 and MMP-12 as well as the cathepsins and elastase, which are capable of causing destruction of the lung matrix leading to emphysema (Shapiro, 1994, Barnes, 2004). Certain proteases such as neutrophil elastase are also potent mucus secretagogues stimulating mucus hypersecretion (Nadel & Takeyama, 1999) — a hallmark feature of chronic bronchitis.

Bronchial biopsies from COPD patients have shown an increase in CD8⁺ T-lymphocytes, in addition to macrophages, in the subepithelium and this increase in CD8⁺ T cells correlates with the severity of airflow limitation (O'Shaughnessy et al., 1997, Saetta et al., 1998).

In contrast to COPD patients the airways inflammation in asthmatic patients is characterised by increased numbers of eosinophils and CD4⁺ T-lymphocytes (Azzawi et al., 1990, Kon and Kay, 1999). The presence of mast cells in the airway smooth muscle of asthmatic patients but not non-asthmatic subjects is also a feature of asthma and has been linked to airway hyper-responsiveness (Brightling et al., 2002). T-lymphocytes are now recognised to play a central role in driving and perpetuating the chronic inflammation that results in airway tissue damage in asthma and aberrant T cell-mediated immune responses to a range of inhaled allergens are believed to be responsible for these inflammatory changes (Heijink et al., 2008). In asthma, the CD4⁺ T helper 2 [T_H2] lymphocyte subset plays a dominant role in mediating the allergic response (Meyer et al., 2008) through the secretion of interleukin (IL)-4 and IL-13 which induce B cells to produce IgE; IL-5, which drives eosinophil differentiation, and IL-9 which leads to the recruitment and differentiation of mast cells (Hamid et al., 1991, Humbert et al., 1996, Kay, 2006).

For a more in depth overview of the clinical and pathological features of asthma and COPD and the key cell types and inflammatory mediators involved please refer to the reviews of Jeffery (1998), Barnes (2008) and Holgate (2008).

A rise in the cytosolic free calcium concentration [Ca²⁺]_i is a powerful stimulus to cell activation and plays a pivotal role in regulating the activity of the cell types found in the lung, whether resident cells such as macrophages, mast cells, epithelial cells and airways smooth muscle cells or cell types such as eosinophils, neutrophils, monocytes and T lymphocytes which migrate to the lung in response to injury and infection. [Ca²⁺]_i plays a crucial role in determining cellular responsiveness by regulating a wide range of functional responses including, degranulation, reactive oxygen species (ROS) generation, lipid mediator release, cell proliferation and airways smooth muscle contraction (Li et al., 2002).

Members of the transient receptor potential (TRP) family of cation channels are good candidates for receptor-operated calcium channels. The mammalian TRP family consists of 28 known members and can be subdivided into six subfamilies based on sequence homology (Clapham, 2003). Presently, the following mammalian TRP proteins have been identified: TRPC (canonical subfamily, 7 members), TRPV (vanilloid, 6), TRPM (melastatin, 8), TRPA (ankyrin, 1), TRPP (polycystin, 3) and TRPML (mucolipin, 3) (reviewed in Pedersen et al., 2005, Ramsey et al., 2006, Venkatachalam and Montell, 2007).

TRP proteins are transmembrane proteins with six putative transmembrane domains and a pore-forming loop between the fifth and sixth segments (Ramsey et al., 2006). The C- and N-termini are located intracellularly and are involved in regulation of channel function and channel assembly. The subfamilies differ in the presence of ankyrin repeats, coiled-coil regions, a TRP signature motif and other domains. TRP channels are thought to be composed of four pore-forming TRP protein subunits. Both homo- and heterotetramers have been described (Hoenderop et al., 2003, Kobori et al., 2009) and recent data has shown that depletion of calcium stores stimulates translocation of TRPV4-C1 heteromeric channels into the plasma membrane resulting in an augmented calcium influx in response to flow in the human embryonic kidney cell overexpression system and native endothelial cells (Ma et al., 2010).

The expression of TRP channels is well described in sensory nerve cells, but more recently there is evidence to suggest that they are present in other cell types which are thought to play a role in respiratory diseases (Jia and Lee, 2007, Colsoul et al., 2009). TRP channels influence cell function by mediating the flux of cations across the plasma membrane into the cytoplasm. Most TRP channels are relatively nonselective for cations. The inward current generated by this cationic influx depolarizes the membrane triggering the opening of voltage gated ion channels. In this way, TRP channels can induce action potentials in excitable cells and contribute to a range of sensory processes including thermosensation, phototransduction, chemosensation, and nociception. In all cell types, the [Ca²⁺]_i is an important mediator of signal transduction and influences various downstream events, such as transcription, translation, contraction, and migration (Jia and Lee, 2007, Colsoul et al., 2009).

Ion channels in the TRP family can be opened by a wide range of chemical or physical stimuli. Known physical activators are noxious temperature (heat or cold), changes in membrane potential, and mechanical or osmotic stress. Moreover, various chemical agonists can activate TRP channels. These agonists include specific exogenous molecules, such as capsaicin (TRPV1), menthol (TRPM8), and acrolein (TRPA1), as well as endogenous signal transduction molecules, such as phospatidylinositol phosphates, and arachidonic acid metabolites (Caterina et al., 1997, McKemy et al., 2002, Bautista et al., 2006, Jia and Lee, 2007, Colsoul et al., 2009).

Some of these activators can also enhance the responsiveness of a TRP channel to other stimuli, which represents a mechanism to integrate such diverse inputs. For example, the threshold for activation of TRPV1 by temperature is strongly dependent on the transmembrane voltage (Voets et al., 2004) and the pH (Tominaga et al., 1998). Similarly, inflammatory mediators can lower the concentration of capsaicin needed for TRPV1 activation (Chuang et al., 2001). Moreover, TRP sensitivity is also regulated by intracellular signalling cascades (Colsoul et al., 2009; Jia & Lee, 2007). In general, phosphorylation by protein kinase A (PKA) or C (PKC) causes sensitization, whereas dephosphorylation by protein phosphatases promotes desensitisation. G-protein-coupled receptors influence TRP channel function, e.g. by activation of PKA, PKC or phospholipase C (PLC), leading to either phosphorylation of TRP channels or generation of endogenous regulators, such as diacylglycerol (DAG) or phosphatidylinositol(4,5)phosphate (PI(4,5)P₂). For some TRP channels however phosphoinositide regulation seems more complex with activating and inhibitory effects being reported (reviewed by Rohacs, 2009). In addition, for several stimuli it is not entirely clear whether they are direct openers or sensitizers of the channel or whether they act via secondary mediators. Selected TRP channels and their potential relevance for respiratory diseases will be discussed below.