There are several ideas about how the GI tract transduces chemical and nutrient stimuli. One idea is that stimulants cross the epithelium and then interact with specialised receptors on afferent nerve terminals. Some stimulants may be ferried across by transport proteins while other may cross by simple diffusion. Liu et al. (1999) have hypothesised the former mechanism for glucose, suggesting that it is ferried across the epithelium by specific transport proteins (Figure 2A). The latter mechanism is likely to be used by capsaicin which can diffuse through the epithelium where it then has access to TRPV1 receptors (transient receptor potential ion channels) located on afferent nerve terminals. Alternatively, receptors and transduction machinery may exist on the luminal aspect of specialised epithelial cells within the mucosal epithelium (Figure 2B). The EE cell is one class of epithelial cell which much research has focused on in particular. Many EE cells have luminal microvilli and granules full of a variety of neuroactive transmitters and hormones. It is known that many of these transmitters can act on receptors on the afferent nerve terminals and it is generally believed that this is the sensory transduction mechanism through which many stimulants act.

Work in the last 15 years has focused on the idea that the GI tract uses much of the same transduction machinery as do other senses. Models of how the GI tract senses mechanical or chemical stimuli have been based on hearing transduction (Gershon, 1995), on glucose sensing by the β-cells of the pancreas (Raybould et al., 2004) and on the internal chemosenses (e.g., carotid bodies). However, what is exciting is the recent evidence that shows that machinery for taste (e.g., Sutherland et al., 2007) and smell (e.g., Braun et al., 2007) are localised to the EE cells in the GI tract. Since these discoveries there has been an explosion of interest in these transduction molecules with studies using selective antibodies and animals with genetic deficiencies to provide evidence for the presence, if not the function, of this transduction machinery in the GI tract. Some of these many new findings are highlighted below.

Taste is perhaps the best used analogy for explaining GI sensory transduction, even before the discovery of many of the taste molecules in the GI tract (e.g., Rozengurt and Sternini, 2007). It is natural to extend lingual taste down into the GI tract. Anatomically, the EE cells and other specialised epithelial cells resemble the taste receptor cells in the taste buds. Both have apical microvilli that are exposed to the ingested contents, and both contain granules with transmitters that can be released. Further, it has been assumed that similar sorts of ingested chemical stimuli would activate both taste receptors and EE cells (e.g., sugars). It is now clear that sub-sets of EE cells have many of the elements that make up the taste transduction machinery (Figure 3). Some of the more important components of the taste transduction machinery are the taste receptors, T1Rs which respond to sweet tastant, and T2Rs which responds to bitter tastants (for review, see Sternini, 2007). These receptors couple to the taste G-protein (gustducin) and to taste specific second messenger systems (phospholipase C; PLCβ2). There are also specialised ion channel receptors such as TRPM5 which is required for the transduction of bitter, sweet and umami tastes (Liman, 2007).

Concrete interest in components of the taste transduction machinery in the GI tract began in the late 1990’s when Hofer et al. (1996, 1998) demonstrated α-gustducin mRNA and protein in a sub-set of epithelial cells. Interestingly, this first study identified cells that were not EE cells, but a poorly characterised cell called a brush cell (also called a tufted cell). One major issue with accepting that these cells participated in sensory transduction was the lack of evidence for granules or other evidence of transmitter storage. Luckily, Hofer et al. had recently shown that nitric oxide synthase was also expressed by the brush cells (Kugler et al., 1994), suggesting at least a plausible sensory transduction pathway via the generation of nitric oxide.

Some of the first work to look beyond gustducin was by Dyer et al. (2005), who showed that T1R sweet taste receptor mRNA and protein (as well as α-gustducin) were expressed in the STC-1 enteroendocrine cell line. These data were recently confirmed by studies showing α-gustducin in brush cells from mouse stomach (Hass et al., 2007) and in brush cells and some EE cells from mouse small intestine (Sutherland et al., 2007). For example, Sutherland et al. (2007) used immunohistochemical techniques to localise α-gustducin and found over half of the cells were brush cells while the remainder were EE cells (of these, 25% were 5-HT containing enterochromaffin (EC) cells and about 15% were GLP containing L-cells).

The presence of components of the taste transduction machinery has also been examined in human tissues. When the mucosal epithelium from biopsies were analysed using PCR methods, taste receptors, α-gustducin, PLCβ2 and TRPM5 were found to be expressed along the GI tract (Bezençon et al., 2007). Bezençon et al. also used the promoter for the Trpm5 gene to express eGFP in a transgenic mouse and then used antibodies to colocalise TRPM5 protein with some morphologically identified EE cells (containing PLCβ2 and TRPM5) and brush cells (containing TRPM5, α-gustducin, T1Rs). Unfortunately, there was no colocalisation of Trpm5-eGFP expression with EE cell transmitters such as ghrelin, orexin, peptide YY (PYY), glucagon-like peptide (GLP-1), or cholecystokinin (CCK) but recently Kokrashvili et al. (2009) have shown the colocalisation of Trpm5-eGFP with some mouse duodenal EE cells that contain enkephalin and uroguanylin. On the other hand, Jang et al. (2007) in human and mouse demonstrated immunohistochemical localisation of α-gustducin in GLP-1 containing L-cells. Using a cell line, they also showed that sugars caused the release of GLP-1 which was blocked by inhibition of taste receptors or α-gustducin (Jang et al., 2007).

The expression levels for components of the taste transduction machinery are not static but seem to respond dynamically to nutrient conditions. For example, Young et al. (2009) have shown in human and mouse that mRNA expression for sweet taste receptors (T1R2 and T1R3 subunits), TRPM5 and α-gustducin were inversely related to blood glucose levels. Similarly, the Na⁺/glucose co-transporter 1 (SGLT-1) has been shown to be upregulated via a taste receptor mediated pathway. Margolskee et al. (2007) have shown that sweet taste receptors (T1R3 subunit) and α-gustducin are coupled to the levels of SGLT mRNA expression in mouse small intestine and an endocrine cell line (GLUTag). Similarly, ingested sugars have been found to increase expression of glucose transporter-2 (GLUT2) in rat small intestine with the taste receptors (T1R2 and T1R3 subunits) and α-gustducin being implicated in this process (Mace et al., 2009). Finally, molecules involved in the control of fat metabolism have been shown to upregulate T2R bitter receptors on STC-1 cells and in EE cells from mouse intestine (Jeon et al., 2008).

Components of the taste transduction machinery are not the only sensory transduction elements which have been localised to the GI tract. Molecules involved in vision and smell transduction have also been found. Hofer et al. (1999) showed that the vision G-protein, transducin, was present in GI epithelium. More recently, Mace et al. (2007, 2009) have provided support for this study by showing that transducin could couple through the taste machinery present in rat jejunum. Smell transduction machinery is also present in the GI tract with a recent study by Braun et al. (2007) showing odorants, such as those from roses and raspberries, evoked 5-HT release from an EC cell model (BON cells). In particular, a sub-set of olfactory receptors was detected, a finding supported by Kidd et al. (2008) who showed that activation of class II olfactory receptors stimulated 5-HT release from an EC cell model (KRJ-I cells).

Glucose sensing has been well reviewed recently; please see Dyer et al. (2007). In brief, sugar sensing has been linked to taste machinery (Mace et al., 2009) and to mechanisms of glucose sensing used by the pancreatic β-cells (Raybould, 2002; Raybould et al., 2004). Pancreatic β-cells use the ATP activated potassium channel (K_ATP) to monitor glucose concentrations indirectly. In the GI tract glucose evokes enteropancreatic reflexes which are reduced by a K_ATP blocker (Kirchgessner et al., 1996). The GI tract uses the Na⁺/glucose co-transporter 1 (SGLT) to transport simple sugars into the enterocyte, and these have also been found on enteric neurons (Liu et al., 1999). Perhaps more importantly, Kidd et al. (2008) have shown mRNA for GLUT1/3 and SGLT1 transporters are enriched in human EC cells and have demonstrated release of 5-HT evoked by sugars. Similarly, sweet taste receptors (T1R3 subunit) and α-gustducin have been shown to be coupled to SGLT expression in mouse and the GLUTag cell line (Margolskee et al., 2007) and GLUT2 (Mace et al., 2007).

It is clear that lipids affect the motility of the GI tract predominately through the release of CCK from the type of EE cell called the ‘I’ cell. Fatty acids can induce segmentation in the small intestine (Gwynne et al., 2004) and propulsion in the colon (Grider and Piland, 2007). They can activate enteric sensory neurons (Bertrand et al., 1997) as well as the extrinsic sensory nerves (Eastwood et al., 1998). Fat sensing has been linked to fatty acid receptors which include the short chain fatty acid receptors, FFAR2 (free fatty acid receptor 2; also called GPR43: G-protein coupled receptor 43) and FFAR3 (GPR41) as well as two long chain fatty acid receptors FFAR1 (GPR40) and GPR120 (for reviews see Brown et al., 2005; Ichimura et al., 2009; Wellendorph et al., 2009). FFAR2 and FFAR3 are both known to be expressed within the intestinal mucosa while FFAR1 and FFAR3 have been identified in the cytoplasm of enterocytes and in L-cells but not EC cells (Karaki et al., 2006; Tazoe et al., 2008, 2009). GPR120 has been shown to release CCK from STC-1 cells (Tanaka et al., 2008). There is also evidence that fatty acid transport proteins regulate the uptake of fatty acids (Stahl, 2004) suggesting that they may be capable of regulating the chemosensory transduction of fats.

Many of these actions appear to be via the release of CCK from EE cells, but other mechanisms exist. For example, molecules involved in the control of fat metabolism have been shown to upregulate T2R bitter receptors on STC-1 cells and in EE cells from mouse intestine causing release of CCK and GLP-1 (Jeon et al., 2008). Similarly, short or long chain fatty acids can release 5-HT from the EC cells (Bertrand and Bertrand, 2009) and mice deficient in enzymes that re-synthesize triglyceride may directly regulate the release of GLP-1 and PYY from L-cells (Okawa et al., 2009).

The EE cells contain and release secretory granules into the sub-epithelial space – near where the sensory nerve terminals are. The cells are generated in the crypts and migrate to the tip of the villus; as a result they cannot form lasting appositions with these nerve terminals (Berthoud and Patterson, 1996) so transmission is likely to be paracrine. Most EE cells are of the open type with microvilli accessing the lumen, as has been described above. However, some are of the closed type, having no clear access to lumenal contents. This lack of lumenal contact has been confirmed by serial section (Kusumoto et al., 1988). As these EE cells make contact with the lumen they may have a different role in chemosensory transduction. For instance, EC cells of the closed type may only participate in mechanically evoked responses. Alternatively, closed EE cell types may only respond to release of other mediators, for example, ATP released from epithelial cells (e.g., Cooke et al., 2003).

As Table 1 suggests, a variety of transmitters are found in different types of EE cells. For example, CCK is found only in a type of EE cell called an ‘I’ cell and is widely believed to participate in signalling the presence of lipids to the extrinsic sensory nerves (e.g., Grundy et al., 1998) and to the intrinsic innervation (e.g., Gwynne et al., 2004). In fact, different types of EE cells contain a host of putative GI hormones\transmitters such as neurotensin, somatostatin (SOM) and GABA. Some transmitters are co-stored within a single type of EE cell, while others like 5-HT (EC cells) and CCK (I cells) represent distinct populations of EE cells. Gut hormones such as orexin and ghrelin are also important for nutrient signalling and are contained within G cells and P/D1 cells (also known as X cells), respectively.

A fair question to ask is why there are so many types of EE cell. The simplest explanation is that each type has a specific function and perhaps communicates with a specific sub-population of sensory nerves. There is some evidence for this as mentioned for lipid and CCK containing I cells, and for sugars and GLP containing L-cells. On the other hand, the 5-HT containing EC cells seem to respond to a wide variety of stimulants. It is beyond the scope of this review to examine this issue in detail, but it is likely that different types of EE cell do have a specific function, but that they function together to control GI function. Nonetheless, despite this variety of EE cell types, it is the 5-HT containing EC cell that has received the most experimental attention.

The EC cells are one of the most studied type of EE cell and for good reason. 5-HT is stored in dense core granules located in the basal and apical parts of the EC cell and EC cells are responsible for the production and storage of the largest pool of 5-HT in the body (Erspamer, 1954). Many pharmacology and physiology studies, past and present, suggest that the release of 5-HT from the EC cells is a critical step in the transduction of chemical and mechanical information to the afferent neurons (for reviews see Bertrand, 2003; Grundy, 2006; Bertrand and Bertrand, 2009).

Studies over the past 50 years have shown that the EC cells release 5-HT during physiologically meaningful stimuli (e.g., Bülbring and Crema, 1959; Kirchgessner et al., 1992; Foxx-Orenstein et al., 1996; Grider et al., 1996; Linden et al., 2003; O'Hara et al., 2004). Similarly, nutrients – such as short or long chain fatty acids, peptides, glucose – or chemical stimuli (e.g., acid, base) can release 5-HT from the EC cells (Bertrand and Bertrand, 2009). Once in the lamina propria, high concentrations of released 5-HT are thought to activate sensory nerve terminals via 5-HT₃ receptors as does exogenously applied 5-HT (Bertrand et al., 2000). Many in vitro studies have shown that 5-HT₃ receptors are important for initiation or propagation of motor reflexes (Kadowaki et al., 1996; Tuladhar et al., 1997; Grider et al., 1998; Jin et al., 1999) or secretory reflexes (Sidhu and Cooke, 1995; Cooke et al., 1997a,b). Despite this keen interest in the EC cell, studies have yet to firmly nail down the transduction machinery that the EC cell uses to detect luminal stimuli or to couple this to release of 5-HT.

We are well on the way to understanding the role of 5-HT containing EC cells thanks to work which has lead to their enrichment and a subsequent global survey of EC cell genes (e.g., Kidd et al., 2006). The selective real time recording of 5-HT during reflexes (e.g., Bertrand, 2006a) is also helping with this understanding. Other EE cell types are not as well characterised but new work to generate transgenic mice with specific expression of a fluorescent protein in GLP containing L-cells (Reimann et al., 2008), CCK containing I cells (Samuel et al., 2008) and TRPM5 expressing EE cells (Bezençon et al., 2007) will undoubtedly help. Another hot topic will be to define the control of EE cell proliferation as has been done for the EC cells (e.g., O'Hara and Sharkey, 2007; Yadav et al., 2008). Finally, the physiological functions in the GI tract of these neuroactive transmitters after they are released, especially when released together, has not yet been defined.

Control of the GI tract is at once complex yet simple as there are many stimuli, but few outcomes. The challenge is to determine how these many signals act in concert to produce coordinated activity. There is also a need to re-examine stimulants that cause EE cells to release transmitters. Genetic clues have provided potential stimulants to be tested while the enrichment of some sub-types of EE cells has provided a standard upon which to test. Perhaps a more important goal will be to translate these insights into useful information about the physiology or pathophysiology of the EE cell. In the end, there are many types of EE cell, each with an important role in paracrine signalling which function together, rather than alone.