Vertebrate Vision: TRP Channels in the Spotlight

Abstract

The recent discovery of the critical role of TRPM1 in retinal function in vertebrates has come as a surprise and has already provided important insights into a common cause of blindness

A fundamental feature of visual processing in vertebrates is the segregation of visual signals into ON and OFF channels that detect increases and decreases in light intensity, respectively [1]. This step originates at the first retinal synapse between photoreceptors and bipolar cells. Photoreceptors hyperpolarize in response to light and reduce the rate of glutamate release, which in turn causes depolarization of ON bipolar cells and hyperpolarization of OFF bipolar cells. The polarity of the light responses of bipolar cells is determined by the subtype of glutamate receptor expressed on their dendrites: ionotropic a-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate (KA) receptor channels on OFF bipolar cells and metabotropic glutamate receptor 6 (mGluR6) on ON bipolar cells [1,2] (Figure 1). The basic components of the signaling cascades in ON and OFF bipolar cells have been known for some time [1,2], but the molecular identity of the mGluR6-gated cation channel that produces the downstream depolarization in ON bipolar cells has been the subject of debate for about 20 years [2]. Most of the early claims were that this channel was a cyclic guanosine monophosphate (cGMP)-gated channel, similar to the one that generated the light response in photoreceptors [1,2]. This possibility was attractive because it suggested analogous transduction mechanisms for light in photoreceptors and glutamate in ON bipolar cells. However, it gradually became clear that cGMP had only a modulatory role in the mGluR6 transduction cascade and did not directly gate the cation channel [2], leaving the molecular nature of this channel an open question. Several independent studies now provide strong evidence that this channel is the type 1 melastatin-related transient receptor potential (TRP) channel TRPM1 [3–8]. This major breakthrough highlights the importance of TRP channels in vertebrate vision and makes an interesting story in the light of the serendipitous manner in which their role came to be discovered.

Figure 1.

Segregation of ON and OFF channels at the photoreceptor-bipolar cell synapse. Glutamate, which is released by photoreceptor terminals in the dark, opens AMPA/KA receptor channels on OFF bipolar cells (left) and activates mGluR6 on ON bipolar cells (right). Because mGluR6 is negatively coupled to a cation-selective transduction channel, binding of glutamate to mGluR6 keeps the cation channel closed and the ON bipolar cells in a hyperpolarized state. The decrease in glutamate release in response to light triggers the closure of the AMPA/KA receptor channel and hyperpolarization of OFF bipolar cells and the opening of the mGluR6-gated cation channel and the depolarization of ON bipolar cells (see [2] for details). Despite intensive research, the molecular nature of the mGluR6-gated channel has remained elusive. Recent evidence indicates that transient receptor potential M1 (TRPM1) is the mGluR6-gated channel.

The first hint at the molecular identity of the channel came unexpectedly from studies on the genetic basis of coat color in the Appaloosa horse [9,10]. The Appaloosa coat spotting pattern in horses is caused by a single incomplete dominant gene (LP-for Leopard complex) [9]. Interestingly, Appaloosa horses homozygous for LP (LP/LP) have congenital stationary night blindness (CSNB) [9,10], a non-progressive scotopic, i.e. dark-adapted, visual deficit also found in humans [11]. CSNB is diagnosed by an abnormal scotopic electroretinogram (ERG): the a-wave (initial negative deflection), which reflects the light-induced reduction in dark current in photoreceptors, is normal but the b-wave (positive deflection), which mostly reflects electrical responses of ON bipolar cells, is absent, indicative of normal photoreceptor function but impaired ON bipolar cell function [9–11]. Genomic mapping of the LP locus identified a small region on chromosome ECA1 composed of five genes, of which one — TRPM1 — was expressed at much lower levels in CSNB-affected (LP/LP) horses than in unaffected (LP/lp) horses [10].

TRP channels are non-selective cation channels [12] and their involvement in the visual system has been well established in invertebrates, where they play key roles in phototransduction signaling [12,13]. However, their presence in the vertebrate visual system was thought to be vestigial and restricted to the transduction cascade associated with the photopigment melanopsin in the subset of intrinsically photosensitive ganglion cells (ipRGCs) that are involved in non-image-forming visual tasks [13,14], although TRP channel expression has been reported in other retinal cell types as well [15,16]. Yet, observations in the Appaloosa horse clearly associated TRPM1 with CSNB [10]. At about the same time, TRPM1 expression was reported in bipolar cells in the mouse retina [17]. It then did not take long before the functional significance of TRP channels in ON bipolar cell signal transduction was tested.

Three independent experimental studies provided molecular, immunohistochemical, and electrophysiological evidence that TRPM1 is required for the depolarizing light response in ON bipolar cells [3–5]. TRPM1 expression was found to be restricted to rod and cone ON bipolar cells, and absent from OFF bipolar cells, in the mouse and primate retinas [4,5]. Whole-cell patch-clamp recording from ON bipolar cells in mouse retinal slices demonstrated that light- or chemically-evoked responses in ON bipolar cells were dramatically impaired in TRPM1-deficient mice [4,5]. Unfortunately, the pharmacological approach was limited by the few available TRP channel agonists and antagonists, which primarily target channels of the vanilloid-related TRP (TRPV) family [12]. Type 1 TRPV (TRPV1) agonists evoked a transduction-like current, and TRPV1 antagonists inhibited a mGluR6 antagonist-induced current [3]. Although this pharmacological approach suggested a role for TRPV1, this theory was ruled out because TRPV1-deficient mice had a normal ERG, and TRPV1 agonists continued to activate the transduction cascade in ON bipolar cells in TRPV1-deficient mice [3]. Thus, the pharmacological data questioned the specificity of the TRPV1 ligands and called for studies to determine the pharmacological profile of TRPM1, which is still largely unknown [12]. However, in strong support for a role for TRPM1, TRPM1-deficient mice lack the ERG b-wave [3–5], a phenotype similar to CSNB-affected horses and humans [9–11]. Collectively, the data support the view that TRPM1 is essential for normal function of ON bipolar cells. Whether TRPM1 is the sole TRP channel involved in this process remains to be clarified, as small-amplitude transient currents induced by a mGluR6 antagonist or a TRPV1 agonist persist in some ON bipolar cells in TRPM1-deficient mice [4].

Additional support for TRPM1 involvement in ON bipolar cell function came from genetic studies in humans. Complete CSNB (type 1 CSNB) had previously been linked to mutations in the mGluR6 and nyctalopin (whose function is still unknown) genes [11]. Three independent studies now show that mutations in TRPM1 are also a common cause of type 1 CSNB [6–8]. These observations confirm a crucial role for TRPM1 in the ON bipolar cell transduction cascade and provide new insights into the etiology of CSNB.

The identification of TRPM1 as the channel gated by the mGluR6 signaling cascade in ON bipolar cells reveals a major role for TRP channels in vertebrate vision. Dogma has long held that TRP channels are characteristic attributes of the invertebrate visual system, lost during the evolution of vertebrate vision to be replaced by cGMP-gated channels [13]. This view was first challenged a few years ago by the discovery of a functional role for TRP channels in ipRGCs [14]. The recent discovery that TRPM1 is essential for ON bipolar cell function clearly demonstrates that TRP channels are key players in vertebrate image-forming vision. Given that classical TRP (TRPC) channel expression has been reported in most classes of cells in the vertebrate retina [15,16] and that chemically-evoked responses persist in some ON bipolar cells in TRPM1-deficient mice [4], it is likely that additional physiological roles will be found for TRP channels in the vertebrate retina. Clarifying these roles will undoubtedly shed more light onto retinal disorders.