Transcriptome dataset of light-dependent expression in the early onset retinal degeneration model, Mcoln1-/- mouse.

Retinal degenerative diseases (RDDs) are a diverse group of retinal disorders that cause visual impairment. While RDD prevalence is high, little is known about the molecular mechanisms underlying the pathogenesis within many of these disorders. Here we use transcriptome analysis to elucidate the molecular mechanisms that drive early onset photoreceptor neuron function loss in the mouse model of the RDD Mucolipidosis type IV (MLIV). MLIV is a lysosomal storage disorder resulting from loss of function mutations in the MCOLN1 gene. MCOLN1 encodes a lysosomal cation channel, the transient receptor potential channel mucolipin 1 (Trpml1). To identify changes in gene expression during onset in MLIV we used a genetic mouse model (Mcoln1-/-) which recapitulates clinical attributes of the human disease. We conducted transcriptome analysis in 6-week old control and Mcoln1-/- mice under normal 12:12 light cycle as well as low and high light stress conditions. These data will be valuable to the vision research community for identifying differentially expressed in early onset MLIV potentially leading to new insights into the pathophysiology of this RDD. Raw FASTQ files and processed counts files for the RNA-seq libraries are deposited in the NCBI Sequence Read Archive (SRA) and have been assigned BioProject accession PRJNA1002601 [1].

Trpm1: Novel function at the intersection of light and pain response in the iris.

In mammals, the retina is the photosensitive tissue that is responsible for the capture of light and the transduction of the light-initiated signals to the brain. These visual signals help to drive image and non-image forming behaviors. The pupillary light reflex (PLR) is an involuntary non-image forming behavior which involves the constriction of the iris muscle tissue in response to ambient light intensity. A subset of photosensitive retinal ganglion cells provides the principal pathway for all light input to the olivary pretectal nucleus which directs the neuronal input to drive iris constriction. Transient receptor potential melastatin 1 (Trpm1) knockout mice have a severe defect in PLR, but it remains unclear how the Trpm1 channel contributes to this behavior. We have demonstrated that the reduced PLR in Trpm1-/- mice at scotopic and photopic intensities is due to a functional loss of Trpm1 in the retina as well as the iris sphincter muscle. We have also tested constriction in isolated eyes and have shown that light-driven constriction independent of signaling from the brain also requires Trpm1 expression. In both the in vivo PLR and the iris photomechanical response, melanopsin is required for the light-dependent activation. Finally, pharmacological experiments using capsaicin to activate pain afferents in the eye demonstrate that Trpm1 expression is required for all sensory driven iris constriction. Our results demonstrate for the first time that Trpm1 has a novel and necessary role in iridial cells and is required for all sensory-driven constriction in the iris.

Suppression of the motor deficit in a mucolipidosis type IV mouse model by bone marrow transplantation.

Mucolipidosis IV (MLIV) is a severe lysosomal storage disorder, which results from loss of the TRPML1 channel. MLIV causes multiple impairments in young children, including severe motor deficits. Currently, there is no effective treatment. Using a Drosophila MLIV model, we showed previously that introduction of trpml+ in phagocytic glia rescued the locomotor deficit by removing early dying neurons, thereby preventing amplification of neuronal death from cytotoxicity. Because microglia, which are phagocytic cells in the mammalian brain, are bone marrow derived, and cross the blood-brain barrier, we used a mouse MLIV model to test the efficacy of bone marrow transplantation (BMT). We found that BMT suppressed the reduced myelination and the increased caspase-3 activity due to loss of TRPML1. Using a rotarod test, we demonstrated that early BMT greatly delayed the motor impairment in the mutant mice. These data offer the possibility that BMT might provide the first therapy for MLIV.

RdgB2 is required for dim-light input into intrinsically photosensitive retinal ganglion cells.

A subset of retinal ganglion cells is intrinsically photosensitive (ipRGCs) and contributes directly to the pupillary light reflex and circadian photoentrainment under bright-light conditions. ipRGCs are also indirectly activated by light through cellular circuits initiated in rods and cones. A mammalian homologue (RdgB2) of a phosphoinositide transfer/exchange protein that functions in Drosophila phototransduction is expressed in the retinal ganglion cell layer. This raised the possibility that RdgB2 might function in the intrinsic light response in ipRGCs, which depends on a cascade reminiscent of Drosophila phototransduction. Here we found that under high light intensities, RdgB2(-/-) mutant mice showed normal pupillary light responses and circadian photoentrainment. Consistent with this behavioral phenotype, the intrinsic light responses of ipRGCs in RdgB2(-/-) were indistinguishable from wild-type. In contrast, under low-light conditions, RdgB2(-/-) mutants displayed defects in both circadian photoentrainment and the pupillary light response. The RdgB2 protein was not expressed in ipRGCs but was in GABAergic amacrine cells, which provided inhibitory feedback onto bipolar cells. We propose that RdgB2 is required in a cellular circuit that transduces light input from rods to bipolar cells that are coupled to GABAergic amacrine cells and ultimately to ipRGCs, thereby enabling ipRGCs to respond to dim light.

Photochemistry of retinal chromophore in mouse melanopsin.

In mammals, melanopsin is exclusively expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs), which play an important role in circadian photoentrainment and other nonimage-forming functions. These ipRGCs reside in the inner retina, far removed from the pigment epithelium, which synthesizes the 11-cis retinal chromophore used by rod and cone photoreceptors to regenerate opsin for light detection. There has been considerable interest in the identification of the melanopsin chromophore and in understanding the process of photopigment regeneration in photoreceptors that are not in proximity to the classical visual cycle. We have devised an immuno-magnetic purification protocol that allows melanopsin-expressing retinal ganglion cells to be isolated and collected from multiple mouse retinas. Using this technique, we have demonstrated that native melanopsin in vivo exclusively binds 11-cis retinal in the dark and that illumination causes isomerization to the all-trans isoform. Furthermore, spectral analysis of the melanopsin photoproduct shows the formation of a protonated metarhodopsin with a maximum absorbance between 520 and 540 nm. These results indicate that even if melanopsin functions as a bistable photopigment with photo-regenerative activity native melanopsin must also use some other light-independent retinoid regeneration mechanism to return to the dark state, where all of the retinal is observed to be in the 11-cis form.

Melanopsin forms a functional short-wavelength photopigment.

Recently, melanopsin has emerged as the leading candidate for the elusive photopigment of the mammalian circadian system. This novel opsin-like protein is expressed in retinal ganglion cells that form the retinohypothalamic tract, a neuronal connection between the retina and the suprachiasmatic nucleus. These hypothalamic structures contain the circadian pacemaker, which generates daily rhythms in physiology and behavior. In mammals, proper synchronization of these rhythms to the environmental light-dark cycle requires retinal input. Surprisingly, rod and cone photoreceptors are not required. Instead, the melanopsin-containing ganglion cells are intrinsically sensitive to light, perhaps responding via a melanopsin-based signaling pathway. To test this hypothesis, we have characterized melanopsin following heterologous expression in COS cells. We found that melanopsin absorbed maximally at 424 nm after reconstitution with 11-cis-retinal. Furthermore, melanopsin activated the photoreceptor G-protein, transducin, in a light-dependent manner. In agreement with the measured absorbance spectrum, melanopsin was most efficiently excited by blue light (420-440 nm). In contrast, published action spectra suggest that the photopigment underlying the intrinsic light sensitivity of SCN-projecting RGCs has an absorption maximum near 484 nm. In summary, our experiments constitute the first direct demonstration that melanopsin forms a photopigment capable of activating a G-protein, but its spectral properties are not consistent with the action spectrum for circadian entrainment.