Using the photopigment melanopsin, intrinsically photosensitive retinal ganglion cells (ipRGCs) respond directly to light to drive circadian clock resetting and pupillary constriction. We now report that ipRGCs are more abundant and diverse than previously appreciated, project more widely within the brain, and can support spatial visual perception. A Cre-based melanopsin reporter mouse line revealed at least five subtypes of ipRGCs with distinct morphological and physiological characteristics. Collectively, these cells project beyond the known brain targets of ipRGCs to heavily innervate the superior colliculus and dorsal lateral geniculate nucleus, retinotopically-organized nuclei mediating object localization and discrimination. Mice lacking classical rod-cone photoreception, and thus entirely dependent on melanopsin for light detection, were able to discriminate grating stimuli from equiluminant gray, and had measurable visual acuity. Thus, non-classical retinal photoreception occurs within diverse cell types, and influences circuits and functions encompassing luminance as well as spatial information.

Rod and cone photoreceptors detect light and relay this information through a multisynaptic pathway to the brain via retinal ganglion cells (RGCs) 1 . These retinal outputs support not only pattern vision, but also non-image forming (NIF) functions, which include circadian photoentrainment and pupillary light reflex (PLR). In mammals, NIF functions are mediated by rods, cones and the melanopsincontaining intrinsically photosensitive retinal ganglion cells (ipRGCs) 2, 3 . Rod/cone photoreceptors and ipRGCs are complementary in signalling light intensity for NIF functions 4-12 . The ipRGCs, in addition to being directly photosensitive, also receive synaptic input from rod/cone networks 13, 14 . To determine how the ipRGCs relay rod/cone light information for both image and non-image forming functions, we genetically ablated ipRGCs in mice. Here we show that animals lacking ipRGCs retain pattern vision, but have deficits in both PLR and circadian photoentrainment that are more extensive than those observed in melanopsin knockouts 8,10,11 . The defects in PLR and photoentrainment resemble those observed in animals that lack phototransduction in all three †To whom correspondence should be addressed.

SUMMARY Transcriptional regulatory networks that control the morphologic and functional diversity of mammalian neurons are still largely undefined. Here we dissect the roles of the highly homologous POU-domain transcription factors Brn3a and Brn3b in retinal ganglion cell (RGC) development and function using conditional Brn3a and Brn3b alleles that permit the visualization of individual wild type or mutant cells. We show that Brn3a- and Brn3b-expressing RGCs exhibit overlapping but distinct dendritic stratifications and central projections. Deletion of Brn3a alters dendritic stratification and the ratio of monostratified: bistratified RGCs, with little or no change in central projections. In contrast, deletion of Brn3b leads to RGC transdifferentiation and loss, axon defects in the eye and brain, and defects in central projections that differentially compromise a variety of visually-driven behaviors. These findings reveal distinct roles for Brn3a and Brn3b in programming RGC diversity, and they illustrate the broad utility of germ-line methods for genetically manipulating and visualizing individual identified mammalian neurons.

BackgroundThe response to hypoxia in tissues is regulated by the heterodimeric transcription factor Hypoxia Inducible Factor-1 (HIF-1).Methodology/Principal FindingsWe have created a strain of mice with inducible cardiomyocyte-specific expression of a mutated, oxygen-stable, form of HIF-1α. Cardiac function steadily decreased with transgene expression, but recovered after the transgene was turned off. Using long-oligo microarrays, we identified 162 transcripts more than 3-fold dysregulated in these hearts after transgene expression. Among the down-regulated genes the transcript for SERCA was reduced 46% and the protein 92%. This led us to an evaluation of calcium flux that showed diminished reuptake of cytoplasmic calcium in myocytes from these hearts, suggesting a mechanism for cardiac dysfunction.Conclusions/SignificanceThese results provide a deeper understanding of transcriptional activity of HIF in the heart, and show that enhanced HIF-1 activity is sufficient to cause contractile dysfunction in the adult heart. HIF is stabilized in the myocardium of patients with ischemic cardiomyopathy, and our results suggest that HIF could be contributing directly to the contractile dysfunction in this disease.

BackgroundMost retinal ganglion cells (RGCs) convey contrast and motion information to visual brain centers. Approximately 2% of RGCs are intrinsically photosensitive (ipRGCs), express melanopsin and are necessary for light to modulate specific physiological processes in mice. The ipRGCs directly target the suprachiasmatic nucleus (SCN) to photoentrain circadian rhythms, and the olivary pretectal nucleus (OPN) to mediate the pupillary light response. How and when this ipRGC circuitry develops is unknown.ResultsHere, we show that some ipRGCs follow a delayed developmental time course relative to other image-forming RGCs. Specifically, ipRGC neurogenesis extends beyond that of other RGCs, and ipRGCs begin innervating the SCN at postnatal ages, unlike most RGCs, which innervate their image-forming targets embryonically. Moreover, the appearance of ipRGC axons in the OPN coincides precisely with the onset of the pupillary light response.ConclusionsSome ipRGCs differ not only functionally but also developmentally from RGCs that mediate pattern-forming vision.

The visual system consists of two major subsystems, image-forming circuits that drive conscious vision and non-image-forming circuits for behaviors such as circadian photoentrainment. While historically considered non-overlapping, recent evidence has uncovered crosstalk between these subsystems. Here, we investigated shared developmental mechanisms. We revealed an unprecedented role for light in the maturation of the circadian clock and discovered that intrinsically photosensitive retinal ganglion cells (ipRGCs) are critical for this refinement process. In addition, ipRGCs regulate retinal waves independent of light, and developmental ablation of a subset of ipRGCs disrupts eye-specific segregation of retinogeniculate projections. Specifically, a subset of ipRGCs, comprising ~200 cells and which project intraretinally and to circadian centers in the brain, are sufficient to mediate both of these developmental processes. Thus, this subset of ipRGCs constitute a shared node in the neural networks that mediate light-dependent maturation of the circadian clock and light-independent refinement of retinogeniculate projections.DOI: http://dx.doi.org/10.7554/eLife.22861.001

SUMMARY The 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.

BackgroundNeuropeptides are critical integrative elements within the central circadian clock in the suprachiasmatic nucleus (SCN), where they mediate both cell-to-cell synchronization and phase adjustments that cause light entrainment. Forward peptidomics identified little SAAS, derived from the proSAAS prohormone, among novel SCN peptides, but its role in the SCN is poorly understood.Methodology/Principal FindingsLittle SAAS localization and co-expression with established SCN neuropeptides were evaluated by immunohistochemistry using highly specific antisera and stereological analysis. Functional context was assessed relative to c-FOS induction in light-stimulated animals and on neuronal circadian rhythms in glutamate-stimulated brain slices. We found that little SAAS-expressing neurons comprise the third most abundant neuropeptidergic class (16.4%) with unusual functional circuit contexts. Little SAAS is localized within the densely retinorecipient central SCN of both rat and mouse, but not the retinohypothalamic tract (RHT). Some little SAAS colocalizes with vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP), known mediators of light signals, but not arginine vasopressin (AVP). Nearly 50% of little SAAS neurons express c-FOS in response to light exposure in early night. Blockade of signals that relay light information, via NMDA receptors or VIP- and GRP-cognate receptors, has no effect on phase delays of circadian rhythms induced by little SAAS.Conclusions/SignificanceLittle SAAS relays signals downstream of light/glutamatergic signaling from eye to SCN, and independent of VIP and GRP action. These findings suggest that little SAAS forms a third SCN neuropeptidergic system, processing light information and activating phase-shifts within novel circuits of the central circadian clock.