Apoptosis Regulates ipRGC Spacing Necessary for Rods and Cones to Drive Circadian Photoentrainment

SummaryThe retina consists of ordered arrays of individual types of neurons for processing vision. Here, we show that such order is necessary for intrinsically photosensitive retinal ganglion cells (ipRGCs) to function as irradiance detectors. We found that during development, ipRGCs undergo proximity-dependent Bax-mediated apoptosis. Bax mutant mice exhibit disrupted ipRGC spacing and dendritic stratification with an increase in abnormally localized synapses. ipRGCs are the sole conduit for light input to circadian photoentrainment, and either their melanopsin-based photosensitivity or ability to relay rod/cone input is sufficient for circadian photoentrainment. Remarkably, the disrupted ipRGC spacing does not affect melanopsin-based circadian photoentrainment but severely impairs rod/cone-driven photoentrainment. We demonstrate reduced rod/cone-driven cFos activation and electrophysiological responses in ipRGCs, suggesting that impaired synaptic input to ipRGCs underlies the photoentrainment deficits. Thus, for irradiance detection, developmental apoptosis is necessary for the spacing and connectivity of ipRGCs that underlie their functioning within a neural network

Pupillary response to moving stimuli of different speeds

Purpose: To investigate the pupillary response to moving stimuli of different speeds and the influence of different luminance environments. Methods: Twenty-eight participants with normal or corrected-to-normal vision were included. The participants were required to track moving optotypes horizontally, and their pupils were videoed with an infrared camera. Stimuli of different speeds were presented in different luminance environments. Results: Experiment 1 demonstrated that the motion stimuli induced pupil dilation in a speed-dependent pattern. The pupil dilation increased as the speed increased, and the pupil dilation gradually increased, then reached saturation. Experiment 2 showed that a stimulus targeting the rod- or cone-mediated pathway could induce pupil dilation in a similar speed-dependent pattern. The absolute but not relative pupil dilation in the cone paradigm was significantly larger than that in the rod paradigm. As the speed increased, the pupil dilation in the cone paradigm reached saturation at speed slower than the rod paradigm. Conclusions: Motion stimuli induced pupil dilation in a speed-dependent pattern, and as the motion speed increased, the pupil dilation gradually increased and reached saturation. And the speed required to reach saturation in the cone paradigm was slower than in the rod paradigm

Melanopsin Sensitivity in the Human Visual System

The human retina contains long [L]-wavelength, medium [M]-wavelength, and short [S]-wavelength cones, rods, and intrinsically photosensitive retinal ganglion cells expressing the blue-sensitive (λmax = ~480 nm) photopigment melanopsin. Previous animal studies have pointed to a role of melanopsin in advancing circadian phase, melatonin suppression, the pupillary light reflex (PLR), light avoidance, and brightness discrimination, often relying on genetic tools to study melanopsin in isolation in animal models. This work addresses the question of human melanopsin sensitivity and function in vivo using a spectrally tunable light source and the method of silent substitution, allowing for the selective stimulation of melanopsin in the human retina, in combination of pupillometry, psychophysics, and BOLD functional neuroimaging (fMRI). In three studies, we find (1) that the temporal transfer function of melanopsin in controlling the pupil in humans is low-pass, peaking at slow temporal frequencies (0.01 Hz), with a sharp drop off at higher frequencies (1-2 Hz); (2) that signals originating from S cones get combined in an antagonistic fashion with melanopsin signals and signals from L and M cones cones, demonstrating spectral opponency in the control of the human PLR; (3) that nominally cone-silent melanopsin-directed spectral modulations stimulate cones in the partial shadow of the retinal blood vessels (termed penumbral cones), leading to the entoptic percept of the subjective retinal vasculature; and (4) that there is no measurable signal due to melanopsin stimulation in human visual cortical areas (V1, V2/V3, MT, LOC; measured with BOLD fMRI) at temporal frequencies most relevant to spatial vision (0.5–64 Hz) while modulations directed at L+M, L–M and S photoreceptor combinations yield characteristic temporal transfer functions in these areas. This work advances to our understanding of the functional significance of melanopsin function in the human visual system, contributing to the study of human health in relation to light and color

Passive frame theory: A new synthesis.

Passive frame theory attempts to illuminate what consciousness is, in mechanistic and functional terms; it does not address the “implementation” level of analysis (how neurons instantiate conscious states), an enigma for various disciplines. However, in response to the commentaries, we discuss how our framework provides clues regarding this enigma. In the framework, consciousness is passive albeit essential. Without consciousness, there would not be adaptive skeletomotor action

The functional neuroanatomy of emotion processing in frontotemporal dementias

This is a pre-copyedited, author-produced version of an article accepted for publication in Brain following peer review. The version of record: Charles R Marshall, Christopher J D Hardy, Lucy L Russell et al., The functional neuroanatomy of emotion processing in frontotemporal dementias, Brain, awz204, is available online at: https://doi.org/10.1093/brain/awz204Brain Research TrustAlzheimer’s SocietyLeonard Wolfson Experimental Neurology CentreMedical Research Council UKNIHR UCLH Biomedical Research Centr

Photoreception in ipRGCs and Their Developmental Roles in Neural Circuit Formation and Refinement

During development, neurons must form a precise network in order to generate a variety of complex behaviors. It is critical for many neurons to accurately achieve both local and long-distance connections. Retinal ganglion cells (RGCs) must both be integrated into the network of retinal neurons and form precise long-distance connections with a variety of different brain nuclei. Thus, these cells provide an excellent system to investigate the mechanisms underlying key processes of neural development. My thesis work has focused on intrinsically photosensitive retinal ganglion cells (ipRGCs) because these cells are one of the few RGC subtypes that has highly specific molecular marker, which allows for the production of many genetic tools. Additionally, ipRGCs have defined projections that are distinct from other RGC subtypes, and ipRGCs have a well-established role in several testable behaviors, which provide a reliable output of their function. While ipRGC have a well-established role in mediating non-image light responses such as circadian photoentrainment, these cells are functional at early postnatal stages. Using genetic tools, labeling methods, and a variety of light cycles and conditions, I investigated how ipRGCs become integrated into retinal circuitry and whether they can influence the development of non-image or image forming visual systems. I found that ipRGCs undergo proximity-dependent Bax-mediated apoptosis to become evenly distributed across the retina and that disrupting this retinal mosaic impairs integration of ipRGCs into retinal circuitry. Additionally, I found ipRGCs are necessary during development to set the period of the circadian clock, a process that I further showed to be light-dependent. ipRGCs also have a light-independent developmental role in regulating the spatiotemporal properties of spontaneous activity in the retina, and without this regulation, retinotopic circuitry in the brain abnormal, which in turn results in reduced visual acuity. Lastly, since ipRGCs have a critical light-dependent role in circadian regulation, I investigated the phototransduction cascade of ipRGCs, and revealed that it is more complicated than previously appreciated. My graduate work has extended our understanding of how ipRGCs develop and revealed their critical role in the development of both the circadian and image forming visual systems

Molecular dissection of the retinal projectome

The retina transforms visual sensation into perception. Extracted visual features are encoded by retinal ganglion cells (RGCs), the output neurons of the eye, and sent to the brain in parallel processing channels. Morphologically, RGCs fall into more than fifty diverse types, which innervate distinct brain areas. Such visual pathways differentially regulate various behaviors. However, the genetic determinants of RGC type diversity are unknown and thus we lack genetic access to study visual pathways. A generation of a more comprehensive RGC type atlas integrating molecular, morphological and functional properties is essential to dissect the functional architecture of the visual system. In a collaborative effort, I used single cell transcriptomics to molecularly classify RGCs during larval and adult stages. RGC types segregate into many discrete transcriptional clusters each with a unique molecular composition. Relatedness of clusters revealed a molecular taxonomy, in which RGC types are arranged into major RGC groups that comprise subclasses and diversify into individual types. This organization of RGC type diversification underlies a code of gene expression patterns, composed primarily of transcription factors. Differential gene expression analysis identified dozens of novel cluster-specific genetic markers for RGC types. Comparison of transcriptional signatures revealed that larval RGCs exhibit higher molecular diversity, which facilitates segregation of similar types, while adult RGCs maintain a core molecular identity suggesting a tight correspondence between larval and adult RGC types. Next, I mapped transcriptional clusters to RGC morphotypes. Select candidate markers were exploited as genetic entry points in a CRISPR-Cas9 transgenesis approach. To restrict labeling specifically to cluster-specific RGC types, I established a genetic intersection with a broad RGC marker. This intersectional transgenic approach allowed to correspond various clusters to distinct morphologically classified RGC types. I generated two transgenic lines using RGC subclass markers, one of which is based on the transcription factor eomesa expressed by RGC types routing to visual areas in hypothalamus, pretectum and tectum. Based on homologies to RGC types characterized in other species, I hypothesized that eomesa+ RGCs constitute intrinsically photosensitive RGCs and have non-image forming functions. I tested this hypothesis by characterizing their response profiles to a battery of visual stimuli and found that they are not tuned to canonical pattern stimuli. Rather eomesa+ RGCs encode ambient luminance levels corroborating my hypothesis. I further tested their necessity in non-image forming behavior, specifically visual background adaptation, which by initial investigation appears to not be affected by chemogenetic ablation of eomesa+ RGCs. In conclusion, this thesis presents a strong foundation for a RGC type atlas and reconciles molecular, morphological and functional features of discrete cell types. This comprehensive molecular classification of RGC types, together with the identified markers and newly established transgenic tools, provides a rich resource towards a better understanding of visual pathway function

Molecular dissection of the retinal projectome

The retina transforms visual sensation into perception. Extracted visual features are encoded by retinal ganglion cells (RGCs), the output neurons of the eye, and sent to the brain in parallel processing channels. Morphologically, RGCs fall into more than fifty diverse types, which innervate distinct brain areas. Such visual pathways differentially regulate various behaviors. However, the genetic determinants of RGC type diversity are unknown and thus we lack genetic access to study visual pathways. A generation of a more comprehensive RGC type atlas integrating molecular, morphological and functional properties is essential to dissect the functional architecture of the visual system. In a collaborative effort, I used single cell transcriptomics to molecularly classify RGCs during larval and adult stages. RGC types segregate into many discrete transcriptional clusters each with a unique molecular composition. Relatedness of clusters revealed a molecular taxonomy, in which RGC types are arranged into major RGC groups that comprise subclasses and diversify into individual types. This organization of RGC type diversification underlies a code of gene expression patterns, composed primarily of transcription factors. Differential gene expression analysis identified dozens of novel cluster-specific genetic markers for RGC types. Comparison of transcriptional signatures revealed that larval RGCs exhibit higher molecular diversity, which facilitates segregation of similar types, while adult RGCs maintain a core molecular identity suggesting a tight correspondence between larval and adult RGC types. Next, I mapped transcriptional clusters to RGC morphotypes. Select candidate markers were exploited as genetic entry points in a CRISPR-Cas9 transgenesis approach. To restrict labeling specifically to cluster-specific RGC types, I established a genetic intersection with a broad RGC marker. This intersectional transgenic approach allowed to correspond various clusters to distinct morphologically classified RGC types. I generated two transgenic lines using RGC subclass markers, one of which is based on the transcription factor eomesa expressed by RGC types routing to visual areas in hypothalamus, pretectum and tectum. Based on homologies to RGC types characterized in other species, I hypothesized that eomesa+ RGCs constitute intrinsically photosensitive RGCs and have non-image forming functions. I tested this hypothesis by characterizing their response profiles to a battery of visual stimuli and found that they are not tuned to canonical pattern stimuli. Rather eomesa+ RGCs encode ambient luminance levels corroborating my hypothesis. I further tested their necessity in non-image forming behavior, specifically visual background adaptation, which by initial investigation appears to not be affected by chemogenetic ablation of eomesa+ RGCs. In conclusion, this thesis presents a strong foundation for a RGC type atlas and reconciles molecular, morphological and functional features of discrete cell types. This comprehensive molecular classification of RGC types, together with the identified markers and newly established transgenic tools, provides a rich resource towards a better understanding of visual pathway function

Social and Affective Neuroscience of Everyday Human Interaction

This Open Access book presents the current state of the art knowledge on social and affective neuroscience based on empirical findings. This volume is divided into several sections first guiding the reader through important theoretical topics within affective neuroscience, social neuroscience and moral emotions, and clinical neuroscience. Each chapter addresses everyday social interactions and various aspects of social interactions from a different angle taking the reader on a diverse journey. The last section of the book is of methodological nature. Basic information is presented for the reader to learn about common methodologies used in neuroscience alongside advanced input to deepen the understanding and usability of these methods in social and affective neuroscience for more experienced readers