The principal mechanism for transduction of salty taste involves the passage of cations through specific ion channels within the apical membrane of receptor cells [1]. Pure salty taste is defined as the taste elicited by sodium chloride. In humans, while many non-sodium salts such as potassium have aspects of salty taste, their saltiness is accompanied by additional qualitative attributes most frequently described as bitterness [2]. In mammals, suggested components of the salty taste pathway include an amiloride-sensitive, cation selective (Na+ and Li+) epithelial sodium channel (ENaC) and an amiloride-insensitive, cation generalist transient receptor potential cation channel subfamily V family 1(TrpV1) [3, 4]. Moreover, salt is unique in that increasing concentration can induce a powerful aversion from the innate appetitive stimulus. Studies suggest that ENaC may be required for low-concentration salt taste; and the high-concentration salt taste pathway differentially recruits aversive taste pathways [3, 5, 6]. Some evidence suggests TrpV1 may also be involved in the high salt pathway [4]. Much remains unknown regarding the specificity of salt responses and the innate attractive and aversive properties of salts. Further understanding here can offer insights into how hedonics can contribute to the body’s mechanisms of balancing ions.
Analogous to humans, naive C. elegans display chemoattraction toward low concentrations of salt while avoiding high concentrations [7]. The chemoattraction to salts is thought to be mediated by four pairs of amphid sensory neurons ADF, ASE, ASG, and ASI [8]. Ablation of the ASE neurons results in a greatly reduced attraction response to sodium, with a residual response that is likely mediated by ADF, ASG, and ASI neurons. In contrast, avoidance of sodium was found to be mostly mediated by the ASH sensory neurons, which are primarily responsible for sensing chemical repellants and avoiding noxious stimuli such as high osmolarity and nose touch [9, 10]. In the absence of ASH, there is evidence that sensory neuron ADL can modulate sodium avoidance [10].
Recently, transmembrane channel-like 1 (tmc-1) in C. elegans has been suggested to encode a protein expressed in ASH that may be sodium-sensitive and/or alkali-activated with reports linking it to sodium and alkaline chemosensation [11, 12]. Transmembrane channel-like (TMC) genes encode a family of channel-like proteins that are evolutionarily conserved in humans and nematodes [13, 14]. In humans and mice, mutation of transmembrane channel-like 1, TMC-1 is known to cause dominant and recessive forms of deafness [14,15,16]. While previous studies suggest that TMC-1 is a mechano-electrical transducer channel, there is evidence that it could function independently as an ion channel [16, 17]. Recent data suggests that TMC-1 in nematodes may mediate Na+-leak currents to stabilize membrane potential in excitable cells, including amphid sensory cells [18].
Utilizing transgenic lines expressing a fluorescent reporter under the tmc-1 promoter, Chatzigeorgiou et al. observed that tmc-1 is expressed primarily in aforementioned sensory neurons ASH, ADF, ASE, and ADL, in addition to PHA. By applying an escape “drop test” assay [9], Chatzigeorgiou et al. concluded that tmc-1 mutants were strongly defective in the avoidance of high sodium concentrations above 100 mM, as mediated by activity in the ASH neuron. These authors additionally found that heterologous expression of TMC-1 in non-salt responsive ASK amphid neurons was sufficient to confer sodium sensitivity to these cells and finally showed that TMC-1 is sufficient to generate a sodium receptor in vitro. Together their data suggests that TMC-1 in the ASH neuron may function directly as a sodium receptor, leading to the aversive effects of high sodium.
However more recently, Wang et al. were unable to confirm a role for TMC-1 in sensing high concentrations of sodium or sodium chemosensory aversion in the nematode, even with similar methodologies. In contrast, they found that sodium sensation by the ASH neuron is dependent on the G protein ODR-3, opening the door for the possibility that high sodium aversion is mediated by a GPRC. Additionally, these authors concluded that TMC-1 is necessary for alkalinity sensation in the ASH neuron [12]. Ultimately, the channel’s role in sodium chemosensation remains in question.
Additional studies in C. elegans provide evidence that the nematode’s behavior can be altered by previous experiences. Most notably, in a phenomenon known as gustatory plasticity, prolonged pre-exposure to a variety of salts has been shown to induce aversion to innately appetitive low salt concentrations [19]. Pre-exposure to salts can abolish chemoattraction to compounds in a partly salt specific yet reversible manner. To measure the specificity of the conditioning, Jansen et al., utilized a cross-adaptation assay in which nematodes pre-exposed to NH4Cl were found to significantly reduce chemotaxis to NaOAc. However, the conditioned avoidance intensity was less than when nematodes were pre-exposed to NaOAc suggesting the presence of both a salt-specific and an aspecific response [19]. In a series of follow up studies it was hypothesized that gustatory plasticity following pre-exposure to NaCl leads to an ASE induced signal that sensitizes ADF, ADL, ASI, and ASH neurons ultimately resulting in aversion. G-protein signaling, serotonin, and glutamate have all been implicated in gustatory plasticity [20, 21]. A related phenomenon called salt chemotaxis learning has been shown to be dependent on insulin signaling and the calcium/calmodulin-dependent kinase, CMK-1 [22,23,24,25]. Here nematodes will avoid sodium following a period of starvation in the presence of NaCl. Much remains unknown regarding the mechanism of other signaling pathways and genes that may contribute to gustatory plasticity.
Here we sought to clarify the role of TMC-1 in the C. elegans salty taste pathway. In our study, we further characterized TMC-1 using a chemotaxis behavior assay developed by Wicks et al. to assess TMC-1’s contribution to chloride salts taste pathways for sodium, lithium, potassium, and magnesium [26]. Our findings suggest that while TMC-1 is required for NaCl and LiCl induced attraction behaviors, the channel likely has no significance in NaCl induced avoidance behaviors or the MgCl2 and KCl induced attraction behaviors. Additionally, we show that pre-exposure to NaCl not only disrupts NaCl induced attraction, but also LiCl. Further, conditioning to sodium is dependent on TMC-1. The findings of our project clarify TMC-1’s role in sodium chemotaxis in the nematode.