Plasticity of TRPM1 expression and localization in the wild type and degenerating mouse retina

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

Functional Availability of ON-Bipolar Cells in the Degenerated Retina: Timing and Longevity of an Optogenetic Gene Therapy

Degenerative diseases of the retina are responsible for the death of photoreceptors and subsequent loss of vision in patients. Nevertheless, the inner retinal layers remain intact over an extended period of time, enabling the restoration of light sensitivity in blind retinas via the expression of optogenetic tools in the remaining retinal cells. The chimeric Opto-mGluR6 protein represents such a tool. With exclusive ON-bipolar cell expression, it combines the light-sensitive domains of melanopsin and the intracellular domains of the metabotropic glutamate receptor 6 (mGluR6), which naturally mediates light responses in these cells. Albeit vision restoration in blind mice by Opto-mGluR6 delivery was previously shown, much is left to be explored in regard to the effects of the timing of the treatment in the degenerated retina. We performed a functional evaluation of Opto-mGluR6-treated murine blind retinas using multi-electrode arrays (MEAs) and observed long-term functional preservation in the treated retinas, as well as successful therapeutical intervention in later stages of degeneration. Moreover, the treatment decreased the inherent retinal hyperactivity of the degenerated retinas to levels undistinguishable from healthy controls. Finally, we observed for the first time micro electroretinograms (mERGs) in optogenetically treated animals, corroborating the origin of Opto-mGluR6 signalling at the level of mGluR6 of ON-bipolar cells.

Alterations in Kainate Receptor and TRPM1 Localization in Bipolar Cells after Retinal Photoreceptor Degeneration

Photoreceptor degeneration differentially impacts glutamatergic signaling in downstream On and Off bipolar cells. In rodent models, photoreceptor degeneration leads to loss of glutamatergic signaling in On bipolar cells, whereas Off bipolar cells appear to retain glutamate sensitivity, even after extensive photoreceptor loss. The localization and identity of the receptors that mediate these residual glutamate responses in Off bipolar cells have not been determined. Recent studies show that macaque and mouse Off bipolar cells receive glutamatergic input primarily through kainate-type glutamate receptors. Here, we studied the impact of photoreceptor degeneration on glutamate receptor and their associated proteins in Off and On bipolar cells. We show that the kainate receptor subunit, GluK1, persists in remodeled Off bipolar cell dendrites of the rd10 mouse retina. However, the pattern of expression is altered and the intensity of staining is reduced compared to wild-type retina. The kainate receptor auxiliary subunit, Neto1, also remains in Off bipolar cell dendrites after extensive photoreceptor degeneration. Similar preservation of kainate receptor subunits was evident in human retina in which photoreceptors had degenerated due to serous retinal detachment. In contrast, photoreceptor degeneration leads to loss of synaptic expression of TRPM1 in mouse and human On bipolar cells, but strong somatic expression remains. These findings demonstrate that Off bipolar cells retain dendritic glutamate receptors during retinal degeneration and could thus serve as a conduit for signal transmission from transplanted or optogenetically restored photoreceptors.

Restoration of synaptic function in sight for degenerative retinal disease

Synaptic disorganization is a prominent feature of many neurological diseases of the CNS, including Parkinson's disease, intellectual development disorders, and autism. Although synaptic plasticity is critical for learning and memory, it is unclear whether this innate property helps restore synaptic function in disease once the primary cause of disease is abrogated. An answer to this question may come from a recent investigation in X-linked retinoschisis, a currently untreatable retinopathy. In this issue of the JCI, Ou, Vijayasarathy, and colleagues showed progressive disorganization of key functional elements of the synapse between photoreceptors and ON-bipolar cells in a retinoschisin-deficient mouse model. Moreover, they demonstrated that adeno-associated virus-mediated (AAV-mediated) delivery of the retinoschisin gene restores structure and function to the photoreceptor to ON-bipolar cell synapse in mouse models, even in adults at advanced stages of the disease. The results of this study hold promise that AAV-based supplemental gene therapy will benefit patients with X-linked retinoschisis in a forthcoming clinical trial.

Synaptic pathology and therapeutic repair in adult retinoschisis mouse by AAV-RS1 transfer

Strategies aimed at invoking synaptic plasticity have therapeutic potential for several neurological conditions. The human retinal synaptic disease X-linked retinoschisis (XLRS) is characterized by impaired visual signal transmission through the retina and progressive visual acuity loss, and mice lacking retinoschisin (RS1) recapitulate human disease. Here, we demonstrate that restoration of RS1 via retina-specific delivery of adeno-associated virus type 8-RS1 (AAV8-RS1) vector rescues molecular pathology at the photoreceptor-depolarizing bipolar cell (photoreceptor-DBC) synapse and restores function in adult Rs1-KO animals. Initial development of the photoreceptor-DBC synapse was normal in the Rs1-KO retina; however, the metabotropic glutamate receptor 6/transient receptor potential melastatin subfamily M member 1-signaling (mGluR6/TRPM1-signaling) cascade was not properly maintained. Specifically, the TRPM1 channel and G proteins Gαo, Gβ5, and RGS11 were progressively lost from postsynaptic DBC dendritic tips, whereas the mGluR6 receptor and RGS7 maintained proper synaptic position. This postsynaptic disruption differed from other murine night-blindness models with an electronegative electroretinogram response, which is also characteristic of murine and human XLRS disease. Upon AAV8-RS1 gene transfer to the retina of adult XLRS mice, TRPM1 and the signaling molecules returned to their proper dendritic tip location, and the DBC resting membrane potential was restored. These findings provide insight into the molecular plasticity of a critical synapse in the visual system and demonstrate potential therapeutic avenues for some diseases involving synaptic pathology.

Properties and functions of TRPM1 channels in the dendritic tips of retinal ON-bipolar cells

An increase in light intensity induces a depolarization in retinal ON-bipolar cells via a reduced glutamate release from presynaptic photoreceptor cells. The underlying transduction cascade in the dendritic tips of ON-bipolar cells involves mGluR6 glutamate receptors signaling to TRPM1 proteins that are an indispensable part of the transduction channel. Several other proteins are recognized to participate in the transduction machinery. Deficiency in many of these leads to congenital stationary night blindness, because rod bipolar cells, a subgroup of ON-bipolar cells, constitute the main route for sensory information under scotopic conditions. Here, we review the current knowledge about TRPM1 ion channels and how their activity is regulated within the postsynaptic compartment of ON-bipolar cells. The functional properties of TRPM1 channels in the dendritic compartment are not well understood as they differ substantially from those of recombinant TRPM1 channels. Critical evaluation of possible explanations of these discrepancies indicates that some key components of this transduction pathway might still not be known. The continued exploration of this pathway will yield further clinically useful insights.

Synaptic remodeling generates synchronous oscillations in the degenerated outer mouse retina

During neuronal degenerative diseases, neuronal microcircuits undergo severe structural alterations, leading to remodeling of synaptic connectivity. The functional consequences of such remodeling are mostly unknown. For instance, in mutant rd1 mouse retina, a common model for Retinitis Pigmentosa, rod bipolar cells (RBCs) establish contacts with remnant cone photoreceptors (cones) as a consequence of rod photoreceptor cell death and the resulting lack of presynaptic input. To assess the functional connectivity in the remodeled, light-insensitive outer rd1 retina, we recorded spontaneous population activity in retinal wholemounts using Ca(2+) imaging and identified the participating cell types. Focusing on cones, RBCs and horizontal cells (HCs), we found that these cell types display spontaneous oscillatory activity and form synchronously active clusters. Overall activity was modulated by GABAergic inhibition from interneurons such as HCs and/or possibly interplexiform cells. Many of the activity clusters comprised both cones and RBCs. Opposite to what is expected from the intact (wild-type) cone-ON bipolar cell pathway, cone and RBC activity was positively correlated and, at least partially, mediated by glutamate transporters expressed on RBCs. Deletion of gap junctional coupling between cones reduced the number of clusters, indicating that electrical cone coupling plays a crucial role for generating the observed synchronized oscillations. In conclusion, degeneration-induced synaptic remodeling of the rd1 retina results in a complex self-sustained outer retinal oscillatory network, that complements (and potentially modulates) the recently described inner retinal oscillatory network consisting of amacrine, bipolar and ganglion cells.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Pharmacological analysis of intrinsic neuronal oscillations in rd10 retina

In the widely used mouse model of retinal degeneration, rd1, the loss of photoreceptors leads to rhythmic electrical activity of around 10-16 Hz in the remaining retinal network. Recent studies suggest that this oscillation is formed within the electrically coupled network of AII amacrine cells and ON-bipolar cells. A second mouse model, rd10, displays a delayed onset and slower progression of degeneration, making this mouse strain a better model for human retinitis pigmentosa. In rd10, oscillations occur at a frequency of 3-7 Hz, raising the question whether oscillations have the same origin in the two mouse models. As rd10 is increasingly being used as a model to develop experimental therapies, it is important to understand the mechanisms underlying the spontaneous rhythmic activity. To study the properties of oscillations in rd10 retina we combined multi electrode recordings with pharmacological manipulation of the retinal network. Oscillations were abolished by blockers for ionotropic glutamate receptors and gap junctions. Frequency and amplitude of oscillations were modulated strongly by blockers of inhibitory receptors and to a lesser extent by blockers of HCN channels. In summary, although we found certain differences in the pharmacological modulation of rhythmic activity in rd10 compared to rd1, the overall pattern looked similar. This suggests that the generation of rhythmic activity may underlie similar mechanisms in rd1 and rd10 retina.

TRPM channels: same ballpark, different players, and different rules in immunogenetics

Transient receptor potential (TRP) channels belong to a large family of cation channels and are the "border guards" predominantly localized to the plasma membrane. Research over the years has considerably and highly developed the knowledge of expression and functional aspects of the TRPM channels. A closer look at the channel dynamics has dismantled undeniable substantiation for multifaceted roles for TRPM channel-mediated extracellular Ca(2+) influx in several physiological and pathophysiological functions. Given the wealth of literature unfolding the multiple roles of TRP channels in physiology in a very extensive range of different mammalian tissues, this review confines itself to the literature describing the multiple roles of TRPM channels in diabetes, smooth muscle cell regulation, immunological responses, and emerging aspects of cancer. We also focus on differential activities of TRPM channels after post-transcriptional and post-translational processing and their exquisite roles at various cellular and molecular levels.

Missing optomotor head-turning reflex in the DBA/2J mouse

PURPOSE

The optomotor reflex of DBA/2J (D2), DBA/2J-Gpnmb+ (D2-Gpnmb+), and C57BL/6J (B6) mouse strains was assayed, and the retinal ganglion cell (RGC) firing patterns, direction selectivity, vestibulomotor function and central vision was compared between the D2 and B6 mouse lines.

METHODS

Intraocular pressure (IOP) measurements, real-time PCR, and immunohistochemical analysis were used to assess the time course of glaucomatous changes in D2 retinas. Behavioral analyses of optomotor head-turning reflex, visible platform Morris water maze and Rotarod measurements were conducted to test vision and vestibulomotor function. Electroretinogram (ERG) measurements were used to assay outer retinal function. The multielectrode array (MEA) technique was used to characterize RGC spiking and direction selectivity in D2 and B6 retinas.

RESULTS

Progressive increase in IOP and loss of Brn3a signals in D2 animals were consistent with glaucoma progression starting after 6 months of age. D2 mice showed no response to visual stimulation that evoked robust optomotor responses in B6 mice at any age after eye opening. Spatial frequency threshold was also not measurable in the D2-Gpnmb+ strain control. ERG a- and b-waves, central vision, vestibulomotor function, the spiking properties of ON, OFF, ON-OFF, and direction-selective RGCs were normal in young D2 mice.

CONCLUSIONS

The D2 strain is characterized by a lack of optomotor reflex before IOP elevation and RGC degeneration are observed. This behavioral deficit is D2 strain-specific, but is independent of retinal function and glaucoma. Caution is advised when using the optomotor reflex to follow glaucoma progression in D2 mice.