A Role for Nyctalopin, a Small Leucine-Rich Repeat Protein, in Localizing the TRP Melastatin 1 Channel to Retinal Depolarizing Bipolar Cell Dendrites

Department of Biochemistry and Molecular Biology, University of Louisville, Louisville, Kentucky 40202, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 07/2011; 31(27):10060-6. DOI: 10.1523/JNEUROSCI.1014-11.2011
Source: PubMed


Expression of channels to specific neuronal sites can critically impact their function and regulation. Currently, the molecular mechanisms underlying this targeting and intracellular trafficking of transient receptor potential (TRP) channels remain poorly understood, and identifying proteins involved in these processes will provide insight into underlying mechanisms. Vision is dependent on the normal function of retinal depolarizing bipolar cells (DBCs), which couple a metabotropic glutamate receptor 6 to the TRP melastatin 1 (TRPM1) channel to transmit signals from photoreceptors. We report that the extracellular membrane-attached protein nyctalopin is required for the normal expression of TRPM1 on the dendrites of DBCs in mus musculus. Biochemical and genetic data indicate that nyctalopin and TRPM1 interact directly, suggesting that nyctalopin is acting as an accessory TRP channel subunit critical for proper channel localization to the synapse.

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Available from: Scott Nawy, Jan 18, 2015
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    • "Transient receptor potential cation channel, subfamily M, member 1 (also known as melastatin) (TRPMI), G protein coupled receptor 179 (GPR179), and metabotropic glutamate receptor 6 (mGluR6) (GRM6), and NYX all play a key role in ON bipolar cell depolarization that leads to the rise of b-wave after light stimulation. Loss of function of these genes results in impairment of signal mediation between photoreceptors and ON bipolar cells, presenting as a loss of the scotopic b-wave in ERG and night vision disturbance [15,16,19-25,49,50]. "
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    ABSTRACT: Congenital stationary night blindness (CSNB) is a non-progressive retinal disorder that shows genetic and clinical heterogeneity. CSNB is inherited as an autosomal recessive, autosomal dominant, or X-linked recessive trait and shows a good genotype-phenotype correlation. Clinically, CSNB is classified as the Riggs type and the Schubert-Bornschein type. The latter form is further sub-classified into complete and incomplete forms based on specific waveforms on the electroretinogram (ERG). There are no molecular genetic data for CSNB in the Indian population. Therefore, we present for the first time molecular profiling of eight families with complete CSNB (cCSNB). The index patients and their other affected family members were comprehensively evaluated for the phenotype, including complete ophthalmic evaluation, ERG, fundus autofluorescence, optical coherence tomography, and color vision test. The known gene defects for cCSNB, LRIT3, TRPM1, GRM6, GPR179, and NYX, were screened by PCR direct sequencing. Bioinformatic analyses were performed using SIFT and PolyPhen for the identified missense mutations. All eight affected index patients and affected family members were identified as having cCSNB based on their ERG waveforms. Mutations in the TRPM1 gene were identified in six index patients. The two remaining index patients each carried a GPR179 and GRM6 mutation. Seven of the patients revealed homozygous mutations, while one patient showed a compound heterozygous mutation. Six of the eight mutations identified are novel. This is the first report on molecular profiling of candidate genes in CSNB in an Indian cohort. As shown for other cohorts, TRPM1 seems to be a major gene defect in patients with cCSNB in India.
    Full-text · Article · Mar 2014 · Molecular vision
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    • "When the photoreceptors are stimulated by light, the deactivation of the G-protein in the ON-bipolar cells is responsible for the opening of TRPM1, resulting in the formation of the ERG b-wave [15], [25], [32]. GRM6 and NYX interact with TRPM1 and are essential for its localization at the dendritic tips of ON-bipolar cells [23], [24]. In addition GPR179 is essential for the action of the G-protein downstream of mGluR6 via the correct localization of Regulator of G protein Signaling proteins (RGS) [20], GPR179 also interacts with both mGluR6 and TRPM1 and its correct localization is mediated through mGluR6 [40]. "
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    ABSTRACT: Mutations in LRIT3, coding for a Leucine-Rich Repeat, immunoglobulin-like and transmembrane domains 3 protein lead to autosomal recessive complete congenital stationary night blindness (cCSNB). The role of the corresponding protein in the ON-bipolar cell signaling cascade remains to be elucidated. Here we genetically and functionally characterize a commercially available Lrit3 knock-out mouse, a model to study the function and the pathogenic mechanism of LRIT3. We confirm that the insertion of a Bgeo/Puro cassette in the knock-out allele introduces a premature stop codon, which presumably codes for a non-functional protein. The mouse line does not harbor other mutations present in common laboratory mouse strains or in other known cCSNB genes. Lrit3 mutant mice exhibit a so-called no b-wave (nob) phenotype with lacking or severely reduced b-wave amplitudes in the scotopic and photopic electroretinogram (ERG), respectively. Optomotor tests reveal strongly decreased optomotor responses in scotopic conditions. No obvious fundus auto-fluorescence or histological retinal structure abnormalities are observed. However, spectral domain optical coherence tomography (SD-OCT) reveals thinned inner nuclear layer and part of the retina containing inner plexiform layer, ganglion cell layer and nerve fiber layer in these mice. To our knowledge, this is the first time that SD-OCT technology is used to characterize an animal model for CSNB. This phenotype is noted at 6 weeks and at 6 months. The stationary nob phenotype of mice lacking Lrit3, which we named nob6, confirms the findings previously reported in patients carrying LRIT3 mutations and is similar to other cCSNB mouse models. This novel mouse model will be useful for investigating the pathogenic mechanism(s) associated with LRIT3 mutations and clarifying the role of LRIT3 in the ON-bipolar cell signaling cascade.
    Full-text · Article · Mar 2014 · PLoS ONE
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    • "That this domain is critical to function in nyctalopin is highlighted by the fact that mutations in the LRR domain of nyctalopin in humans cause CSNB1 [1], [2]. These data, combined with our observation that nyctalopin is required for the localization of TRPM1 to the dendritic tips of DBCs [52], suggest several possible mechanisms of action. Functional TRP channels are homo or hetero tetramers (for reviews see [57], [58]). "
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    ABSTRACT: Nyctalopin is a small leucine rich repeat proteoglycan (SLRP) whose function is critical for normal vision. The absence of nyctalopin results in the complete form of congenital stationary night blindness. Normally, glutamate released by photoreceptors binds to the metabotropic glutamate receptor type 6 (GRM6), which through a G-protein cascade closes the non-specific cation channel, TRPM1, on the dendritic tips of depolarizing bipolar cells (DBCs) in the retina. Nyctalopin has been shown to interact with TRPM1 and expression of TRPM1 on the dendritic tips of the DBCs is dependent on nyctalopin expression. In the current study, we used yeast two hybrid and biochemical approaches to investigate whether murine nyctalopin was membrane bound, and if so by what mechanism, and also whether the functional form was as a homodimer. Our results show that murine nyctalopin is anchored to the plasma membrane by a single transmembrane domain, such that the LRR domain is located in the extracellular space.
    Full-text · Article · Apr 2012 · PLoS ONE
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