Deciphering the retina's wiring diagram

Integrated transcriptomic and metabolomic analysis reveals adaptive changes of hibernating retinas

Hibernation is a seasonally adaptive strategy that allows hibernators to live through extremely cold conditions. Despite the profound reduction of blood flow to the retinas, hibernation causes no lasting retinal injury. Instead, hibernators show an increased tolerance to ischemic insults during the hibernation period. To understand the molecular changes of the retinas in response to hibernation, we applied an integrative transcriptome and metabolome analysis to explore changes in gene expression and metabolites of 13-lined ground squirrel retinas during hibernation. Metabolomic analysis showed a global decrease of ATP synthesis in hibernating retinas. Decreased glucose and galactose, increased beta-oxidation of carnitine and decreased storage of some amino acids in hibernating retinas indicated a shift of fuel use from carbohydrates to lipids and alternative usage of amino acids. Transcriptomic analysis revealed that the down-regulated genes were enriched in DNA-templated transcription and immune-related functions, while the up-regulated genes were enriched in mitochondrial inner membrane and DNA packaging-related functions. We further showed that a subset of genes underwent active alternative splicing events in response to hibernation. Finally, integrative analysis of the transcriptome and metabolome confirmed the shift of fuel use in the hibernating retina by the regulation of catabolism of amino acids and lipids. Through transcriptomic and metabolomic data, our analysis revealed the altered state of mitochondrial oxidative phosphorylation and the shift of energy source in the hibernating retina, advancing our understanding of the molecular mechanisms employed by hibernators. The data will also serve as a useful resource for the ocular and hibernation research communities.

Autoimmunity in visual loss

There are a number of autoimmune disorders which can affect visual function. There are a very large number of mechanisms in the visual pathway which could potentially be the targets of autoimmune attack. In practice it is the retina and the anterior visual pathway (optic nerve and chiasm) that are recognised as being affected in autoimmune disorders. Multiple Sclerosis is one of the commonest causes of visual loss in young adults because of the frequency of attacks of optic neuritis in that condition, however the basis of the inflammation in Multiple Sclerosis and the confirmation of autoimmunity is lacking. The immune process is known to be highly unusual in that it is not systemic and confined to the CNS compartment. Previously an enigmatic partner to Multiple Sclerosis, Neuromyelitis Optica is now established to be autoimmune and two antibodies - to Aquaporin4 and to Myelin Oligodendrocyte Glycoprotein - have been implicated in the pathogenesis. The term Chronic Relapsing Inflammatory Optic Neuropathy is applied to those cases of optic neuritis which require long term immunosuppression and hence are presumed to be autoimmune but where no autoimmune pathogenesis has been confirmed. Optic neuritis occurring post-infection and post vaccination and conditions such as Systemic Lupus Erythematosus and various vasculitides may cause direct autoimmune attack to visual structures or indirect damage through occlusive vasculopathy. Chronic granulomatous disorders such as Sarcoidosis affect vision commonly by a variety of mechanisms, whether and how these are placed in the autoimmune panoply is unknown. As far as the retina is concerned Cancer Associated Retinopathy and Melanoma Associated Retinopathy are well characterised clinically but a candidate autoantibody (recoverin) is only described in the former disorder. Other, usually monophasic, focal retinal inflammatory disorders (Idiopathic Big Blind Spot Syndrome, Acute Zonal Occult Outer Retinopathy and Acute Macular Neuroretinitis) are of obscure pathogenesis but an autoimmune disorder of the post-infectious type is plausible. Visual loss in autoimmunity is an expanding field: the most significant advances in research have resulted from taking a well characterised phenotype and making educated guesses at the possible molecular targets of autoimmune attack.

Overlapping structures in sensory-motor mappings

This paper examines a biologically-inspired representation technique designed for the support of sensory-motor learning in developmental robotics. An interesting feature of the many topographic neural sheets in the brain is that closely packed receptive fields must overlap in order to fully cover a spatial region. This raises interesting scientific questions with engineering implications: e.g. is overlap detrimental? does it have any benefits? This paper examines the effects and properties of overlap between elements arranged in arrays or maps. In particular we investigate how overlap affects the representation and transmission of spatial location information on and between topographic maps. Through a series of experiments we determine the conditions under which overlap offers advantages and identify useful ranges of overlap for building mappings in cognitive robotic systems. Our motivation is to understand the phenomena of overlap in order to provide guidance for application in sensory-motor learning robots.

Mesopic rod and S-cone interactions revealed by modulation thresholds

We analyzed mesopic rod and S-cone interactions in terms of their contributions to the blue-yellow opponent pathway. Stimuli were generated using a four-primary colorimeter. Mixed rod and S-cone modulation thresholds (constant L-, M-cone excitation) were measured as a function of their phase difference. Modulation amplitude was equated using threshold units and contrast ratios. This study identified three interaction types: (1) a linear and antagonistic rod:S-cone interaction, (2) probability summation, and (3) a previously unidentified mutual nonlinear reinforcement. Linear rod:S-cone interactions occur within the blue-yellow opponent pathway. Probability summation involves signaling by different postreceptoral pathways. The origin of the nonlinear reinforcement is possibly at the photoreceptors.

Imaging visual function with the multifocal m-sequence technique

The multifocal m-sequence technique is a versatile set of tools for visual electrophysiology designed to provide access to the complex dynamic interplay of converging signals in the central nervous system. Here, a number of uses for the technique are demonstrated, with examples from human electroretinography. A simple relationship between the binary kernels extracted from a single experiment permits us to distinguish local from lateral interactions in the retina. Transformation of the series of binary kernels into response sequences provides new insight into unexpected fast dynamic properties of retinal responses and facilitates future modeling of the signals as well as identification of the signal sources.

Seeing with S cones

The S cone is highly conserved across mammalian species, sampling the retinal image with less spatial frequency than other cone photoreceptors. In human and monkey retina, the S cone represents typically 5-10% of the cone mosaic and distributes in a quasi-regular fashion over most of the retina. In the fovea, the S cone mosaic recedes from a central "S-free" zone whose size depends on the optics of the eye for a particular primate species: the smaller the eye, the less extreme the blurring of short wavelengths, and the smaller the zone. In the human retina, the density of the S mosaic predicts well the spatial acuity for S-isolating targets across the retina. This acuity is likely supported by a bistratified retinal ganglion cell whose spatial density is about that of the S cone. The dendrites of this cell collect a depolarizing signal from S cones that opposes a summed signal from M and L cones. The source of this depolarizing signal is a specialized circuit that begins with expression of the L-AP4 or mGluR6 glutamate receptor at the S cone-->bipolar cell synapse. The pre-synaptic circuitry of this bistratified ganglion cell is consistent with its S-ON/(M+L)-OFF physiological receptive field and with a role for the ganglion cell in blue/yellow color discrimination. The S cone also provides synapses to other types of retinal circuit that may underlie a contribution to the cortical areas involved with motion discrimination.