Morphological Identification of Melanopsin-Expressing Retinal Ganglion Cell Subtypes in Mice

Flp-recombinase mouse line for genetic manipulation of ipRGCs

Light has myriad impacts on behavior, health, and physiology. These signals originate in the retina and are relayed to the brain by more than 40 types of retinal ganglion cells (RGCs). Despite a growing appreciation for the diversity of RGCs, how these diverse channels of light information are ultimately integrated by the ~50 retinorecipient brain targets to drive these light-evoked effects is a major open question. This gap in understanding primarily stems from a lack of genetic tools that specifically label, manipulate, or ablate specific RGC types. Here, we report the generation and characterization of a new mouse line (Opn4FlpO), in which FlpO is expressed from the Opn4 locus, to manipulate the melanopsin-expressing, intrinsically photosensitive retinal ganglion cells. We find that the Opn4FlpO line, when crossed to multiple reporters, drives expression that is confined to ipRGCs and primarily labels the M1-M3 subtypes. Labeled cells in this mouse line show the expected intrinsic, melanopsin-based light response and morphological features consistent with the M1-M3 subtypes. In alignment with the morphological and physiological findings, we see strong innervation of non-image forming brain targets by ipRGC axons, and weaker innervation of image forming targets in Opn4FlpO mice labeled using AAV-based and FlpO-reporter lines. Consistent with the FlpO insertion disrupting the endogenous Opn4 transcript, we find that Opn4FlpO/FlpO mice show deficits in the pupillary light reflex, demonstrating their utility for behavioral research in future experiments. Overall, the Opn4FlpO mouse line drives Flp-recombinase expression that is confined to ipRGCs and most effectively drives recombination in M1-M3 ipRGCs. This mouse line will be of broad use to those interested in manipulating ipRGCs through a Flp-based recombinase for intersectional studies or in combination with other, non-Opn4 Cre driver lines.

Melanopsin activates divergent phototransduction pathways in intrinsically photosensitive retinal ganglion cell subtypes

Melanopsin signaling within intrinsically photosensitive retinal ganglion cell (ipRGC) subtypes impacts a broad range of behaviors from circadian photoentrainment to conscious visual perception. Yet, how melanopsin phototransduction within M1-M6 ipRGC subtypes impacts cellular signaling to drive diverse behaviors is still largely unresolved. The identity of the phototransduction channels in each subtype is key to understanding this central question but has remained controversial. In this study, we resolve two opposing models of M4 phototransduction, demonstrating that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dispensable for this process and providing support for a pathway involving melanopsin-dependent potassium channel closure and canonical transient receptor potential (TRPC) channel opening. Surprisingly, we find that HCN channels are likewise dispensable for M2 phototransduction, contradicting the current model. We instead show that M2 phototransduction requires TRPC channels in conjunction with T-type voltage-gated calcium channels, identifying a novel melanopsin phototransduction target. Collectively, this work resolves key discrepancies in our understanding of ipRGC phototransduction pathways in multiple subtypes and adds to mounting evidence that ipRGC subtypes employ diverse phototransduction cascades to fine-tune cellular responses for downstream behaviors.

Alpha retinal ganglion cells in pigmented mice retina: number and distribution

Purpose: To identify and characterize numerically and topographically the population of alpha retinal ganglion cells (αRGCs) and their subtypes, the sustained-response ON-center αRGCs (ONs-αRGCs), which correspond to the type 4 intrinsically photosensitive RGCs (M4-ipRGCs), the transient-response ON-center αRGCs (ONt-αRGCs), the sustained-response OFF-center αRGCs (OFFs-αRGCs), and the transient-response OFF-center αRGCs (OFFt-αRGCs) in the adult pigmented mouse retina.

Methods: The αRGC population and its subtypes were studied in flat-mounted retinas and radial sections immunodetected against non-phosphorylated high molecular weight neurofilament subunit (SMI-32) or osteopontin (OPN), two αRGCs pan-markers; Calbindin, expressed in ONs-αRGCs, and amacrines; T-box transcription factor T-brain 2 (Tbr2), a key transcriptional regulator for ipRGC development and maintenance, expressed in ipRGCs and GABA-displaced amacrine cells; OPN4, an anti-melanopsin antibody; or Brn3a and Brn3c, markers of RGCs. The total population of RGCs was counted automatically and αRGCs and its subtypes were counted manually, and color-coded neighborhood maps were used for their topographical representation.

Results: The total mean number of αRGCs per retina is 2,252 ± 306 SMI32+αRGCs and 2,315 ± 175 OPN+αRGCs (n = 10), representing 5.08% and 5.22% of the total number of RGCs traced from the optic nerve, respectively. αRGCs are distributed throughout the retina, showing a higher density in the temporal hemiretina. ONs-αRGCs represent ≈36% [841 ± 110 cells (n = 10)] of all αRGCs and are located throughout the retina, with the highest density in the temporal region. ONt-αRGCs represent ≈34% [797 ± 146 cells (n = 10)] of all αRGCs and are mainly located in the central retinal region. OFF-αRGCs represent the remaining 32% of total αRGCs and are divided equally between OFFs-αRGCs and OFFt-αRGCs [363 ± 50 cells (n = 10) and 376 ± 36 cells (n = 10), respectively]. OFFs-αRGCs are mainly located in the supero-temporal peripheral region of the retina and OFFt-αRGCs in the mid-peripheral region of the retina, especially in the infero-temporal region.

Conclusions: The combination of specific antibodies is a useful tool to identify and study αRGCs and their subtypes. αRGCs are distributed throughout the retina presenting higher density in the temporal area. The sustained ON and OFF response subtypes are mainly located in the periphery while the transient ON and OFF response subtypes are found in the central regions of the retina.

Differential susceptibility of retinal ganglion cell subtypes against neurodegenerative diseases

Retinal ganglion cells (RGCs) are essential to propagate external visual information from the retina to the brain. Death of RGCs is speculated to be closely correlated with blinding retinal diseases, such as glaucoma and traumatic optic neuropathy (TON). Emerging innovative technologies have helped refine and standardize the classification of RGCs; at present, they are classified into more than 40 subpopulations in mammals. These RGC subtypes are identified by a combination of anatomical morphologies, electrophysiological functions, and genetic profiles. Increasing evidence suggests that neurodegenerative diseases do not collectively affect the RGCs. In fact, which RGC subtype exhibits the strongest or weakest susceptibility is hotly debated. Although a consensus has not yet been reached, it is certain that assorted RGCs display differential susceptibility against irreversible degeneration. Interestingly, a single RGC subtype can exhibit various vulnerabilities to optic nerve damage in diverse injury models. Thus, elucidating how susceptible RGC subtypes are to various injuries can protect vulnerable RGCs from damage and improve the possibility of preventing and treating visual impairment caused by neurodegenerative diseases. In this review, we summarize in detail the progress and status quo of research on the type-specific susceptibility of RGCs and point out current limitations and the possible directions for future research in this field.

Characterization of Tbr2-expressing retinal ganglion cells

The mammalian retina contains more than 40 retinal ganglion cell (RGC) subtypes based on their unique morphologies, functions, and molecular profiles. Among them, intrinsically photosensitive RGCs (ipRGCs) are the first specified RGC type emerging from a common retinal progenitor pool during development. Previous work has shown that T-box transcription factor T-brain 2 (Tbr2) is essential for the formation and maintenance of ipRGCs, and that Tbr2-expressing RGCs activate Opn4 expression upon native ipRGC ablation, suggesting that Tbr2⁺ RGCs contain a reservoir for ipRGCs. However, the identity of Tbr2⁺ RGCs has not been fully vetted. Here, using genetic sparse labeling and single cell recording, we showed that Tbr2-expressing retinal neurons include RGCs and a subset of GABAergic displaced amacrine cells (dACs). Most Tbr2⁺ RGCs are intrinsically photosensitive and morphologically resemble native ipRGCs with identical retinofugal projections. Tbr2⁺ RGCs also include a unique and rare Pou4f1-expressing OFF RGC subtype. Using a loss-of-function strategy, we have further demonstrated that Tbr2 is essential for the survival of these RGCs and dACs, as well as maintaining the expression of Opn4. These data set a strong foundation to study how Tbr2 regulates ipRGC development and survival, as well as the expression of molecular machinery regulating intrinsic photosensitivity.

Retina and melanopsin neurons

Melanopsin retinal ganglion cells (mRGCs) are the third class of retinal photoreceptors with unique anatomical, electrophysiological, and biological features. There are different mRGC subtypes with differential projections to the brain. These cells contribute to many nonimage-forming functions of the eye, the most relevant being the photoentrainment of circadian rhythms through the projections to the suprachiasmatic nucleus of the hypothalamus. Other relevant biological functions include the regulation of the pupillary light reflex, mood, alertness, and sleep, as well as a possible role in formed vision. The relevance of the mRGC-related pathways in the brain is highlighted by the role that the dysfunction and/or loss of these cells may play in affecting circadian rhythms and sleep in many neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease and in aging. Moreover, the occurrence of circadian dysfunction is a known risk factor for dementia. In this chapter, the anatomy, physiology, and functions of these cells as well as their resistance to neurodegeneration in mitochondrial optic neuropathies or their predilection to be lost in other neurodegenerative disorders will be discussed.

M1 Intrinsically Photosensitive Retinal Ganglion Cells Integrate Rod and Melanopsin Inputs to Signal in Low Light

Light influences various behaviors and physiological processes that occur outside of our conscious perception, including circadian photoentrainment, sleep, and even learning and mood. The M1, melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) relay a combination of rod/cone and melanopsin signals to drive these functions. However, little is known about how M1 ipRGCs integrate these signals in low light. We measure the dim light response of M1 ipRGCs and find that they exhibit a wide spectrum of responses to dim, scotopic light stimulation that are driven by a combination of rod pathway input and melanopsin phototransduction. The presence of rod input to M1 ipRGCs correlates with larger and more complex dendritic arbors. Collectively, these results show variability in the rod input to M1 ipRGCs and a surprising contribution of melanopsin to the light responses of M1 ipRGCs at very low light.

M1 intrinsically photosensitive retinal ganglion cells (ipRGCs) control an array of non-image-forming functions. Lee et al. report diverse light responses of M1 ipRGCs in scotopic light that are determined by the degree of rod and melanopsin inputs and find that degree of rod input correlates with dendritic complexity.

Essential Roles of Tbr1 in the Formation and Maintenance of the Orientation-Selective J-RGCs and a Group of OFF-Sustained RGCs in Mouse

In the mouse retina, more than 30 retinal ganglion cell (RGC) subtypes have been classified based on a combined metric of morphological and functional characteristics. RGCs arise from a common pool of retinal progenitor cells during embryonic stages and differentiate into mature subtypes in adult retinas. However, the cellular and molecular mechanisms controlling formation and maturation of such remarkable cellular diversity remain unknown. Here, we demonstrate that T-box transcription factor T-brain 1 (Tbr1) is expressed in two groups of morphologically and functionally distinct RGCs: the orientation-selective J-RGCs and a group of OFF-sustained RGCs with symmetrical dendritic arbors. When Tbr1 is genetically ablated during retinal development, these two RGC groups cannot develop. Ectopically expressing Tbr1 in M4 ipRGCs during development alters dendritic branching and density but not the inner plexiform layer stratification level. Our data indicate that Tbr1 plays critical roles in regulating the formation and dendritic morphogenesis of specific RGC types.

Overlapping morphological and functional properties between M4 and M5 intrinsically photosensitive retinal ganglion cells

Multiple retinal ganglion cell (RGC) types in the mouse retina mediate pattern vision by responding to specific features of the visual scene. The M4 and M5 melanopsin-expressing, intrinsically photosensitive retinal ganglion cell (ipRGC) subtypes are two RGC types that are thought to play major roles in pattern vision. The M4 ipRGCs overlap in population with ON-alpha RGCs, while M5 ipRGCs were recently reported to exhibit opponent responses to different wavelengths of light (color opponency). Despite their seemingly distinct roles in visual processing, previous reports have suggested that these two populations may exhibit overlap in their morphological and functional properties, which calls into question whether these are in fact distinct RGC types. Here, we show that M4 and M5 ipRGCs are distinct morphological classes of ipRGCs, but they cannot be exclusively differentiated based on color opponency and dendritic morphology as previously reported. Instead, we find that M4 and M5 ipRGCs can only be distinguished based on soma size and the number of dendritic branch points in combination with SMI-32 immunoreactivity. These results have important implications for clearly defining RGC types and their roles in visual behavior.

Crosstalk: The diversity of melanopsin ganglion cell types has begun to challenge the canonical divide between image-forming and non-image-forming vision

Melanopsin ganglion cells have defied convention since their discovery almost 20 years ago. In the years following, many types of these intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged. In the mouse retina, there are currently six known types (M1-M6) of melanopsin ganglion cells, each with unique morphology, mosaics, connections, physiology, projections, and functions. While melanopsin-expressing cells are usually associated with behaviors like circadian photoentrainment and the pupillary light reflex, the characterization of multiple types has demonstrated a reach that may extend far beyond non-image-forming vision. In fact, studies have shown that individual types of melanopsin ganglion cells have the potential to impact image-forming functions like contrast sensitivity and color opponency. Thus, the goal of this review is to summarize the morphological and functional aspects of the six known types of melanopsin ganglion cells in the mouse retina and to highlight their respective roles in non-image-forming and image-forming vision. Although many melanopsin ganglion cell types do project to image-forming brain targets, it is important to note that this is only the first step in determining their influence on image-forming vision. Even so, the visual system has canonically been divided into these two functional realms and melanopsin ganglion cells have begun to challenge the boundary between them, providing an overlap of visual information that is complementary rather than redundant. Further studies on these ganglion cell photoreceptors will no doubt continue to illustrate an ever-expanding role for melanopsin ganglion cells in image-forming vision.

Potential mechanisms of retinal ganglion cell type-specific vulnerability in glaucoma

Glaucoma is a neurodegenerative disease characterised by progressive damage to the retinal ganglion cells (RGCs), the output neurons of the retina. RGCs are a heterogenous class of retinal neurons which can be classified into multiple types based on morphological, functional and genetic characteristics. This review examines the body of evidence supporting type-specific vulnerability of RGCs in glaucoma and explores potential mechanisms by which this might come about. Studies of donor tissue from glaucoma patients have generally noted greater vulnerability of larger RGC types. Models of glaucoma induced in primates, cats and mice also show selective effects on RGC types - particularly OFF RGCs. Several mechanisms may contribute to type-specific vulnerability, including differences in the expression of calcium-permeable receptors (for example pannexin-1, P2X7, AMPA and transient receptor potential vanilloid receptors), the relative proximity of RGCs and their dendrites to blood supply in the inner plexiform layer, as well as differing metabolic requirements of RGC types. Such differences may make certain RGCs more sensitive to intraocular pressure elevation and its associated biomechanical and vascular stress. A greater understanding of selective RGC vulnerability and its underlying causes will likely reveal a rich area of investigation for potential treatment targets.

μ-Opioid Receptor Activation Directly Modulates Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode light intensity and trigger reflexive responses to changes in environmental illumination. In addition to functioning as photoreceptors, ipRGCs are post-synaptic neurons in the inner retina, and there is increasing evidence that their output can be influenced by retinal neuromodulators. Here we show that opioids can modulate light-evoked ipRGC signaling, and we demonstrate that the M1, M2 and M3 types of ipRGCs are immunoreactive for μ-opioid receptors (MORs) in both mouse and rat. In the rat retina, application of the MOR-selective agonist DAMGO attenuated light-evoked firing ipRGCs in a dose-dependent manner (IC₅₀ < 40 nM), and this effect was reversed or prevented by co-application of the MOR-selective antagonists CTOP or CTAP. Recordings from solitary ipRGCs, enzymatically dissociated from retinas obtained from melanopsin-driven fluorescent reporter mice, confirmed that DAMGO exerts its effect directly through MORs expressed by ipRGCs. Reduced ipRGC excitability occurred via modulation of voltage-gated potassium and calcium currents. These findings suggest a potential new role for endogenous opioids in the mammalian retina and identify a novel site of action-MORs on ipRGCs-through which opioids might exert effects on reflexive responses to environmental light.