TRP Channels in Vision

The transient receptor potential (TRP) field began (for reviews see Minke, 2010; Montell, 2011; Hardie, 2011) with the analysis of a spontaneously formed Drosophila mutant showing transient, rather than sustained, responses to prolonged intense illumination in electroretinogram (ERG) measurements, rendering the flies effectively blind (Cosens and Manning, 1969). Cosens and Manning (1969) isolated this mutant, designated this strain the “A-type” mutant, and attributed its phenotype to a failure of photopigment regeneration (Cosens, 1971). The isolation of this mutant, though potentially interesting, raised a number of concerns at the time. One main concern was that the results were based on a single spontaneously occurring mutant with no description of its genetic background. It was thus difficult to know what genetic alterations this strain represented. For example, the results could have been due to additive effects of alterations in several genes mapping to the same chromosome (Pak, 2010). The isolation of multiple mutated alleles from a baseline stock of known genetic background, conducted by Pak and colleagues (Pak, 2010), was important in establishing that the observed phenotype was indeed due to mutation in a single gene. Another concern at the time was that the cellular origins of ERG components were not well established. One could not be certain whether the lack of a sustained response seen in the ERG of this strain originated from the photoreceptors or from other retinal cells (Pak, 2010). This question was settled by performing intracellular recordings from the mutant photoreceptors (Minke et al., 1975). Only after determining that the defect arose from the photoreceptors, was it safe to conclude that this mutant is defective in phototransduction (Pak, 2010; Minke et al., 1975). Extensive studies of this mutant (Minke et al., 1975; Minke, 1977, 1982; Minke and Selinger, 1992a; Barash and Minke, 1994; Barash et al., 1988) provided a more descriptive name, “transient receptor potential” or trp (Minke et al., 1975) (Figure 3.1a) by Minke and colleagues, which was ultimately adopted by the scientific community to designate the entire gene family (Montell et al., 2002). These studies revealed that the mutant photopigment cycle was not altered and concluded that the defect was at an intermediate stage of the phototransduction cascade. A combination of electrophysiological, biochemical (Devary et al., 1987), and direct Ca+2 measurements in other invertebrates (Minke and Tsacopoulos, 1986) supported an hypothesis that the TRP encodes for a novel phosphoinositide-activated and Ca+2-permeable channel/transporter protein, which is defective in the trp mutant (Devary et al., 1987; Minke and Selinger, 1991; Selinger and Minke, 1988). When the trp gene was cloned, its sequence indicated a transmembrane (TM) protein with eight TM helices, a topology reminiscent of known receptor/transporter/channel proteins (Montell and Rubin, 1989; Wong et al., 1989).Immunofluorescent measurements of TRP localized the protein to the signaling compartment, the rhabdomere, further supporting its participation in phototransduction. However, due to the lack of homologous proteins to the TRP protein in available databases and results showing that in a presumably null trp alleles (Montell and Rubin, 1989; Wong et al., 1989), a sustained receptor potential persists under dim light stimulation (Minke et al., 1975; Minke, 1977), it was concluded that the trp gene does not encode for the light-sensitive channel (Montell and Rubin, 1989; Wong et al., 1989). Following later studies, based on the ability of La3+ to mimic the trp phenotype (Suss Toby et al., 1991; Hochstrate, 1989) (Figure 3.1b), it was proposed that the TRP might encode for an inositide-activated Ca2+ channel/transporter required for Ca2+ stores refilling (Minke and Selinger, 1992b). Consequently, using whole-cell voltage-clamp recordings to determine ionic selectivity, it was shown that the primary defect in the trp mutant was a drastic reduction in the Ca2+ permeability of the light-sensitive channels themselves (Hardie and Minke, 1992). This conclusion was further supported by studies using microfluorimetry (Peretz et al., 1994a) and Ca2+-selective microelectrodes (Peretz et al., 1994b) (Figure 3.2a and b). The identification of another protein similar to the trp gene product, designated TRP-like (TRPL), using a Ca2+/calmodulin binding assay, allowed for a reinterpretation of the phenotype of the trp mutation and suggested that the light response of Drosophila is mediated by channels composed from the TRP and TRPL gene products (Hardie and Minke, 1992; Phillips et al., 1992). Later, a third TRP homologue channel of Drosophila with similarity to TRP and TRPL was discovered by Montell and colleagues and was designated TRPγ (Xu et al., 2000). Using a TRPγ-antibody the authors showed that the protein is highly expressed in the retina and interacts with both the TRP and TRPL channels. Although the lack of a light response in the trpl;trp double null mutant (Scott et al., 1997) indicates that TRPγ cannot by itself form a light-sensitive channel, the authors suggested that TRPγ-TRPL heteromers may form an additional light-sensitive channel complex. In other insects, the role of TRPγ is even less certain. TRPγ expression was not detectable in the compound eyes of the moth, Spodoptera littoralis (Chouquet et al., 2009), nor in those of the cockroach, Periplaneta americana (French et al., 2015). Hence, there is no evidence so far of a functional role for trpγ in phototransduction in any species, although roles in olfaction, cardiac function, and mechanosensation have been reported in insects (Chouquet et al., 2009; Wicher et al., 2006; Akitake et al., 2015).In conclusion, the normal light-sensitive current comprises two distinct conductances: one is highly Ca2+ selective and is encoded by the trp gene, and the second is a channel responsible for the residual light-sensitive current in the trp mutants. We now know that the latter conductance is encoded by the homologous gene trpl, while the involvement of trpγ, if any, is unclear (Phillips et al., 1992; Niemeyer et al., 1996; Reuss et al., 1997).