Biomedical subjects

Mark W Hankins

Publications and source records attributed to Mark W Hankins.

12 recordsLinked to original sources

PubMed2006

Melanopsin regulates visual processing in the mouse retina.

The discovery of melanopsin-dependent inner retinal photoreceptors in mammals has precipitated a fundamental reassessment of such non-image forming (NIF) light responses as circadian photoentrainment and the pupil light reflex. By contrast, it remains unclear whether these new photoreceptors also play a role in classical image-forming vision. The retinal ganglion cells that subserve inner retinal photoreception (ipRGCs) project overwhelmingly to brain areas involved in NIF responses, indicating that, in terms of central signaling, their predominant function is non-image forming. However, ipRGCs also exhibit intraretinal communication via gap junction coupling, which could allow them to modulate classical visual pathways within this tissue. Here, we explore this second possibility by using melanopsin knockout (Opn4-/-) mice to examine the role of inner retinal photoreceptors in diurnal regulation of retinal function. By using electroretinography in wild-type mice, we describe diurnal rhythms in both the amplitude and speed of the retinal cone pathway that are a function of both prior light exposure and circadian phase. Unexpectedly, loss of the melanopsin gene abolishes circadian control of these parameters, causing significant attenuation of the diurnal variation in cone vision. Our results demonstrate for the first time a melanopsin-dependent regulation of visual processing within the retina, revealing an important function for inner retinal photoreceptors in optimizing classical visual pathways according to time of day.

Animals↗

PubMed2006

Light-evoked FOS induction within the suprachiasmatic nuclei (SCN) of melanopsin knockout (Opn4-/-) mice: a developmental study.

The aims of this study were to address three related questions: (1) Do the photosensitive ganglion cells of the mouse convey light information to the suprachiasmatic nuclei (SCN) at P0? (2) Do the differentiating rods and cones contribute to light-evoked FOS induction within the murine SCN at P4? (3) How does light-evoked FOS induction within the SCN of melanopsin knockout (Opn4-/-) mice differ at P4 and P14? Our approaches took advantage of the published descriptions of murine ocular development, melanopsin knockout (Opn4-/-) mouse, and light-induced expression of FOS (the phosphoprotein product of immediate early gene c-fos) within the SCN as a marker of retinohypothalamic tract competence. Collectively, our results show that photosensitive melanopsin-dependent retinal ganglion cells provide light information to the murine SCN on the day of birth, and possibly beforehand, and that developing rods and cones fail to provide light information to the SCN during early postnatal life. On the basis of previous publications and data presented here, we suggest that at ages around P14 the rods and cones might be capable of fully compensating for the loss of melanopsin-photosensitive ganglion cells if exposure to light is of sufficiently long duration. These results are related to the broader context of recent findings and the potential role(s) of a neonatal photoreceptor.

Animals↗

PubMed2006

Evolution of melanopsin photoreceptors: discovery and characterization of a new melanopsin in nonmammalian vertebrates.

In mammals, the melanopsin gene (Opn4) encodes a sensory photopigment that underpins newly discovered inner retinal photoreceptors. Since its first discovery in Xenopus laevis and subsequent description in humans and mice, melanopsin genes have been described in all vertebrate classes. Until now, all of these sequences have been considered representatives of a single orthologous gene (albeit with duplications in the teleost fish). Here, we describe the discovery and functional characterisation of a new melanopsin gene in fish, bird, and amphibian genomes, demonstrating that, in fact, the vertebrates have evolved two quite separate melanopsins. On the basis of sequence similarity, chromosomal localisation, and phylogeny, we identify our new melanopsins as the true orthologs of the melanopsin gene previously described in mammals and term this grouping Opn4m. By contrast, the previously published melanopsin genes in nonmammalian vertebrates represent a separate branch of the melanopsin family which we term Opn4x. RT-PCR analysis in chicken, zebrafish, and Xenopus identifies expression of both Opn4m and Opn4x genes in tissues known to be photosensitive (eye, brain, and skin). In the day-14 chicken eye, Opn4m mRNA is found in a subset of cells in the outer nuclear, inner nuclear, and ganglion cell layers, the vast majority of which also express Opn4x. Importantly, we show that a representative of the new melanopsins (chicken Opn4m) encodes a photosensory pigment capable of activating G protein signalling cascades in a light- and retinaldehyde-dependent manner under heterologous expression in Neuro-2a cells. A comprehensive in silico analysis of vertebrate genomes indicates that while most vertebrate species have both Opn4m and Opn4x genes, the latter is absent from eutherian and, possibly, marsupial mammals, lost in the course of their evolution as a result of chromosomal reorganisation. Thus, our findings show for the first time that nonmammalian vertebrates retain two quite separate melanopsin genes, while mammals have just one. These data raise important questions regarding the functional differences between Opn4x and Opn4m pigments, the associated adaptive advantages for most vertebrate species in retaining both melanopsins, and the implications for mammalian biology of lacking Opn4x.

Amino Acid Sequence↗

PubMed2005

Numerical study of short-term afterimages and associate properties in foveal vision.

A cellular model of the primate retina has been developed. Unlike existing models, it incorporates spatial non-uniformities, such as the random arrangement of L and M cones, and the radial dilation with eccentricity. Based on a population of ganglion cell activities, colour-image representation is modelled with the luminance and the R-G opponent channels. The developed model reproduces experimentally known properties in temporal and spatial vision. Furthermore, spatio-temporally coupled properties such as transition from positive to negative phases in an afterimage, are recapped. In colour vision, the model can explain the insensitivity in our colour perception to the L/M cone ratio.

Afterimage↗

PubMed2005

Mammalian photoentrainment: results, methods, and approaches.

Research on circadian biology over the past decade has paid increasing attention to the photoreceptor mechanisms that align the molecular clock to the 24-h light/dark cycle, and some of the results to emerge are surprising. For example, the rods and cones within the mammalian eye are not required for entrainment. A population of directly light-sensitive ganglion cells exists within the retina and acts as brightness detectors. This article provides a brief history of the discovery of these novel ocular photoreceptors and then describes the methods that have been used to study the photopigments mediating these responses to light. Photopigment characterization has traditionally been based on a number of complementary approaches, but one of the most useful techniques has been action spectroscopy. A photopigment has a discrete absorbance spectrum, which describes the probability of photons being absorbed as a function of wavelength, and the magnitude of any light-dependent response depends on the number of photons absorbed by the photopigment. Thus, a description of the spectral sensitivity profile (action spectrum) of any light-dependent response must, by necessity, match absorbance spectra of the photopigment mediating the response. We provide a step-by-step approach to conducting action spectra, including the construction of irradiance response curves, the calculation of relative spectral sensitivities, and photopigment template fitting, and discuss the underlying assumptions behind this approach. We then illustrate action spectrum methodologies by an in-depth analysis of action spectra obtained from rodless/coneless (rd/rd cl) mice and discuss, for the first time, the full implications of these findings.

Animals↗

PubMed2005

Long-term constant light induces constitutive elevated expression of mPER2 protein in the murine SCN: a molecular basis for Aschoff's rule?

Circadian rhythms in behavior, metabolism, and physiology are based upon transcriptional/translational feedback loops involving a core set of clock genes that interact to regulate their own expression. In mammals, the SCN is the site of a master biological clock regulating circadian locomotor rhythms. The products of the clock genes mPer1, mPer2, mCry1, and mCry2 form essential components of both negative and positive elements within the SCN oscillator. The primary aims of this study were to examine clock gene abundance under longterm LL in an attempt to provide molecular correlates of the lengthened tau and daily phase delays described by Aschoff's rule. Wheel-running behavior was recorded from mice maintained in either DD or LL for 50 days. The abundance of the clock genes mPer1, mPer2, mCry1, and mCry2 and their protein products was then examined (every approximately 4 h) within the SCN using in situ hybridization and immunocytochemistry. Under LL conditions, mPer1, mPer2, mCry1, and mCry2 messages remained rhythmic, although the waveform of mCry2 was altered compared to DD. In LL, mPER1, mCRY1, and mCRY2 protein levels were also rhythmic and comparable to the patterns observed in DD. However, mPER2 is elevated and constitutively expressed under LL. Thus, rhythmic expression of these clock genes is not dependent on the rhythmic production of mPER2, and the acute up-regulation of mPer1 and mPer2 described for short (nonparametric) light pulses is not sustained under LL conditions. These findings suggest that mPER2 is important for the generation of phase delays in the molecular clockwork, providing a possible molecular explanation for Aschoff's rule: LL lengthens the circadian period by inhibiting the degeneration of mPER2, and constitutively elevated levels of mPER2 enhance the phase-delaying limb of the molecular oscillator.

Animals↗

PubMed2003

Neuropsin (Opn5): a novel opsin identified in mammalian neural tissue.

We have cloned and characterised the expression of a new opsin gene, neuropsin (Opn5), in mice and humans. Neuropsin comprises seven exons on mouse chromosome 17. Its deduced protein sequence suggests a polypeptide of 377 amino acids in mice (354 in humans), with many structural features common to all opsins, including a lysine in the seventh transmembrane domain required to form a Schiff base link with retinaldehyde. Neuropsin shares 25-30% amino acid identity with all known opsins, making it the founding member of a new opsin family. It is expressed in the eye, brain, testis and spinal cord.

Amino Acid Sequence↗

PubMed2003

VA opsin, melanopsin, and an inherent light response within retinal interneurons.

BACKGROUND: Although photoreception is best understood in rods and cones, it is increasingly clear that these are not the only photoreceptive cells of the vertebrate retina. While considerable attention has been paid to the role of melanopsin in the generation of intrinsic light sensitivity in the retinal ganglion cells of mammals, nothing is known about the photoreceptive capacity of the horizontal cells of the fish retina in which both VA opsin and melanopsin are expressed. As yet, there has been little more than speculation as to the physiological function of these opsins within local retinal circuit neurons. RESULTS: VA opsin and melanopsin have been isolated and localized within the well-characterized cyprinid retina of the roach (Rutilus rutilus). Parallel electrophysiological studies identified a novel subtype of horizontal cell (HC-RSD) characterized by a depolarizing response that fits an opsin photopigment with a lambda(max) of 477 nm. The HC-RSD cells mediate responses to light that are characterized by long integration times, well beyond those observed for rods and cones. Significantly, HC-RSD responses persist when the conventional photoreceptor inputs are saturated by background light. CONCLUSIONS: The syncytium of coupled horizontal cells has long been considered to provide a signal of overall retinal irradiance. Our data suggest that this light information is, at least in part, derived from a population of intrinsically photosensitive VA opsin and/or melanopsin horizontal cells.

Animals↗

PubMed2003

Calcium imaging reveals a network of intrinsically light-sensitive inner-retinal neurons.

BACKGROUND: Mice lacking rod and cone photoreceptors (rd/rd cl) are still able to regulate a range of responses to light, including circadian photoentrainment, the pupillary light reflex, and suppression of pineal melatonin by light. These data are consistent with the presence of a novel inner-retinal photoreceptor mediating non-image-forming irradiance detection. RESULTS: We have examined the nature and extent of intrinsic light sensitivity in rd/rd cl retinae by monitoring the effect of light stimulation (470 nm) on intracellular Ca(2+) via FURA-2 imaging. Using this approach, which does not rely on pharmacological or surgical isolation of ganglion cells from the rod and cone photoreceptors, we identified a population of light-sensitive neurons in the ganglion cell layer (GCL). Retinal illumination induced an increase of intracellular Ca(2+) in approximately 2.7% of the neurons. The light-evoked Ca(2+) fluxes were dependent on the intensity and duration of the light stimulus. The light-responsive units formed an extensive network that could be uncoupled by application of the gap junction blocker carbenoxolone. Three types of light-evoked Ca(2+) influx were observed: sustained, transient, and repetitive, which are suggestive of distinct functional classes of GCL photoreceptors. CONCLUSIONS: Collectively, our data reveal a heterogeneous syncytium of intrinsically photosensitive neurons in the GCL coupled to a secondary population of light-driven cells, in the absence of rod and cone inputs.

Animals↗

PubMed2003

Expression of opsin genes early in ocular development of humans and mice.

We have compared the onsets of expression of the classical visual opsins with those of the non-rod, non-cone opsins in foetal and post-natal eye tissue from mice and humans. Mouse Rgr-opsin, peropsin, encephalopsin and melanopsin are all expressed in foetal development by E11.5, unlike the murine rod and cone opsins that exhibit post-natal expression, e.g. P1 for ultraviolet cone opsin and P5 for rod opsin. Human non-rod, non-cone opsins are also all expressed early, by 8.6 weeks post-conception. The implications of these observations are discussed with regard to the possible functions of these opsins at early stages of ocular development.

Animals↗

PubMed2002

Non-rod, non-cone photoreception in the vertebrates.

When reflected from a surface, light can provide a representation of the spatial environment, whilst gross changes in environment light can signal the time of day. The differing sensory demands of using light to detect environmental space and time appear to have provided the selection pressures for the evolution of different photoreceptor systems in the vertebrates, and probably all animals. This point has been well recognised in the non-mammals, which possess multiple opsin/vitamin A-based photoreceptor populations in a variety of sites distributed both within and outside the CNS. By contrast, eye loss in mammals abolishes all responses to light, and as a result, all photoreception was attributed to the rods and cones of the retina. However, studies over the past decade have provided overwhelming evidence that the mammalian eye contains a novel photoreceptor system that does not depend upon the input from the rods and cones. Mice with eyes but lacking rod and cone photoreceptors can still detect light to regulate their circadian rhythms, suppress pineal melatonin, modify locomotor activity, and modulate pupil size. Furthermore, action spectra for some of these responses in rodents and humans have characterised at least one novel opsin/vitamin A-based photopigment, and molecular studies have identified a number of candidate genes for this photopigment. Parallel studies in fish showing that VA opsin photopigment is expressed within sub-sets of inner retina neurones, demonstrates that mammals are not alone in having inner retinal photoreceptors. It therefore seems likely that inner retinal photoreception will be a feature of all vertebrates. Current studies are directed towards an understanding of their mechanisms, determining the extent to which they contribute to physiology and behaviour in general, and establishing how they may interact with other photoreceptors, including the rods and cones. Progress on each of these topics is moving very rapidly. As a result, we hope this review will serve as an introduction to the cascade of papers that will emerge on these topics in the next few years. We also hope to convince the more casual reader that there is much more to vertebrate photoreceptors than the study of retinal rods and cones.

Animals↗

PubMedDate not supplied

Residual photosensitivity in mice lacking both rod opsin and cone photoreceptor cyclic nucleotide gated channel 3 alpha subunit.

The mammalian retina contains three classes of photoreceptor. In addition to the rods and cones, a subset of retinal ganglion cells that express the putative sensory photopigment melanopsin are intrinsically photosensitive. Functional and anatomical studies suggest that these inner retinal photoreceptors provide light information for a number of non-image-forming light responses including photoentrainment of the circadian clock and the pupil light reflex. Here, we employ a newly developed mouse model bearing lesions of both rod and cone phototransduction cascades (Rho(-/-) Cnga3(-/-)) to further examine the function of these non-rod non-cone photoreceptors. Calcium imaging confirms the presence of inner retinal photoreceptors in Rho(-/-) Cnga3(-/-) mice. Moreover, these animals retain a pupil light reflex, photoentrainment, and light induction of the immediate early gene c-fos in the suprachiasmatic nuclei, consistent with previous findings that pupillary and circadian responses can employ inner retinal photoreceptors. Rho(-/-) Cnga3(-/-) mice also show a light-dependent increase in the number of FOS-positive cells in both the ganglion cell and (particularly) inner nuclear layers of the retina. The average number of cells affected is several times greater than the number of melanopsin-positive cells in the mouse retina, suggesting functional intercellular connections from these inner retinal photoreceptors within the retina. Finally, however, while we show that wild types exhibit an increase in heart rate upon light exposure, this response is absent in Rho(-/-) Cnga3(-/-) mice. Thus, it seems that non-rod non-cone photoreceptors can drive many, but not all, non-image-forming light responses.

Animals↗