The sign-inverting synapse between photoreceptors and ON-BC neurons provides a conversion of the light-induced hyperpolarizing response of photoreceptors into the excitatory depolarization of the downstream ON-BC. The main players, as well as the key steps of this process at the postsynaptic site have now been established (Vardi et al., 2002
; Okawa and Sampath, 2007
; Morgans et al., 2010
). In the hypothetical sequence of events, decrease in the glutamate release by the photoreceptors during light response leads to a reduction in mGluR6 activation, subsequent Go deactivation and opening of the TRPM1 channels, that are kept closed in the dark by tonic mGluR6-Gαo signaling (Vardi et al., 2002
; Snellman et al., 2008
). This makes TRPM1 the central effector channel ultimately responsible for providing ON-BC responses to light (Morgans et al., 2010
Currently, the mechanisms governing gating and cellular regulation of TRPM1 in ON-BC neurons are completely unknown. What is known, however, is the remarkable speed with which ON-BC mount the response. The TRPM1 channels fully open in less than 100 ms from the moment mGluR6 receptors detect changes in glutamate concentration (Field and Rieke, 2002
). In comparison, corresponding step in G protein cascade of rod photoreceptors, deactivation of rhodopsin-transducin signaling and opening of the cGMP-gated channels, takes approximately 5 times longer (Pugh, 2006
; Fain et al., 2010
). These considerations suggest that postsynaptic G protein cascade of ON-BC is organized in a way that allows streamlining signal transmission from mGluR6 to TRPM1 channel. Indeed, mGluR6 cascade displays a remarkable compartmentalization with all key components being enriched at the tips of the ON-BC dendrites in immediate apposition from the photoreceptor axonal terminals.
One of the main results of our study is a demonstration that mGluR6 is required for targeting of TRPM1 channel to the dendritic tips, where it functions. In the absence of mGluR6, transmembrane channel TRPM1 is still able to reach plasma membrane but completely looses its accumulation at postsynaptic sites. This observation parallels our earlier report showing that targeting of RGS proteins to the dendritic tips is also mediated by their association with mGluR6. Together, these findings indicate an important role that mGluR6 receptor plays in subcellular targeting of pathway components in addition to its role in signaling.
The second key finding of this study is the demonstration of the macromolecular complex formation between principle components of the ON-BC signaling cascade: TRPM1 and mGluR6 which both physically associate with synaptic protein nyctalopin. We propose that assembly of this complex plays an essential role in enabling fast transmission of signal from mGluR6 to TRPM1, ensuring millisecond timing kinetics of ON-BC responses to light. Because neither mGluR6 (Beqollari et al., 2009
; Masuho et al., 2010
) nor TRPM1 (Oancea et al., 2009
; Koike et al., 2010
; Lambert et al., 2011
) appear to require nyctalopin for their functional activity in cells per se, we envision that the role of nyctalopin is to coordinate the assembly of mGluR6-TRPM1 complex in the optimal configuration for signaling, which ultimately makes the fast signal transmission possible. Loss of nyctalopin in this scheme would disrupt the coupling of mGluR6 receptor to TRPM1 to result in inefficient response that would not reach the threshold level necessary for the generation of the light response. This situation is reminiscent of invertebrate phototransduction cascade where the coupling of the G protein coupled receptor to the TRP effector channel requires participation of auxiliary scaffolding protein (INAD) that brings the components in the complex together and plays a critical role in visual signaling (Tsunoda and Zuker, 1999
; Wang and Montell, 2007
). Interestingly, several G protein coupled receptors carry extracellular nyctalopin-like leucine-rich repeat (LRR) domains (Hsu et al., 2000
), and there is also a precedent for the involvement of LRR-containing proteins in allosteric modulation of the ion channel properties (Yan and Aldrich, 2010
). These observations suggest a possible general mechanism of synaptic signaling where numerous LRR proteins (Ko and Kim, 2007
) could be involved in modulating signal transmission events from G protein coupled receptors to TRP channels.