Photic regulation of pineal function. Analogies between retinal and pineal photoreception

Light absorbed by a photopigment in a photoreceptor cell causes a photochemical reaction converting the 11-cis retinal chromophore into the all-trans configuration. These changes lead to a series of events that causes cGMP hydrolysis, a following decrease of cGMP in the cytoplasm of the photoreceptor outer segment and a closure of cGMP-gated cationic channels. As a consequence of these processes the membrane hyperpolarizes. In pineal photoreceptor cells of lower vertebrates these processes are only partly investigated. Molecules involved in the phototransduction process and the desensitization, like opsin, vitamin A, α-transducin and arrestin, have been immunocytochemically localized in pineal photoreceptors and also electrophysiological studies have shown that phototransduction mechanisms in pineal photoreceptors might be very similar to those found in retinal photoreceptors. This review will summarize some of the current knowledge on pineal photoreception and compare it with retinal processes.     

DNA repair in photoreceptor survival

Light triggers a sequence of events that damage photoreceptor cells within the superior central portion of the retina, resulting in apoptotic cell death. This damage is mediated by energy absorbed by rhodopsin and the intermediates of the rhodopsin-bleaching process. Furthermore, inhibition of the visual cycle and the re-isomerization of all-trans retinol preserve photoreceptors. We have recently shown light-induced DNA fragmentation to occur only within photoreceptors, and, in time-courses following light treatment, these cells exhibit two peaks of damage, approx 24 h apart. This was also observed by quantification of nucleosome-length DNA fragments and their multimers (DNA ladders) as well as by highly repetitive short interspersed nuclear element (SINE) analysis. This bimodal pattern of photoreceptor DNA fragmentation suggests two populations of cells, and each of these were affected by light at a different rate or time. However, the rat retina is composed of 500 nm-sensitive rods, and approx 2% cones, suggesting that a two-cell-type hypothesis is incorrect. Thus, there is a possibility that light-induced DNA fragmentation is triggered and that some photoreceptors are able to initiate a repair mechanism, resulting in a temporary decrease in DNA damage followed by another wave of fragmentation that ultimately leads to cell death. Subsequently, we observed that the repair enzyme DNA polymerase beta was upregulated following light treatment, again suggesting the presence of a repair mechanism. Our results suggest that a DNA-repair mechanism exists within photoreceptors, and indicate that manipulation of this process may provide additional protection and/or recovery from events that trigger DNA fragmentation and apoptotic cell death in photoreceptors.     

Cryptochromes in the field: how blue light influences crop development

Light is a pivotal environmental element capable of influencing multiple physiological processes across the entire plant life cycle. Over the course of their evolution, plants have developed several families of photoreceptors such as phytochromes, phototropins, ultraviolet (UV) resistance locus 8 and cryptochromes (crys), in order to sense light stimuli and respond to their changes. Numerous genetic studies have demonstrated that functional alterations to these photoreceptors cause a change in important agronomical traits. In particular, crys, which absorb UVA/blue light, can influence seed germination, flowering induction, plant architecture, fruit metabolic content and resistance to biotic and abiotic stresses. In the years to come, the rising temperatures and alterations to precipitation patterns generated by climate change will present a dramatic challenge for our agricultural system, with its few varieties characterized by a narrow genetic pool derived from artificial selection. Here, we review the main roles of crys in determining important agronomic traits in crops, we discuss the opportunities of using these photoreceptors as genetic targets for tuning plant physiological responses to environmental change, and the molecular strategies used so far to manipulate this family of photoreceptors.     

Optogenetic control of cell function using engineered photoreceptors

Over the past decades, there has been growing recognition that light can provide a powerful stimulus for biological interrogation. Light‐actuated tools allow manipulation of molecular events with ultra‐fine spatial and fast temporal resolution, as light can be rapidly delivered and focused with sub‐micrometre precision within cells. While light‐actuated chemicals such as photolabile ‘caged’ compounds have been in existence for decades, the use of genetically encoded natural photoreceptors for optical control of biological processes has recently emerged as a powerful new approach with several advantages over traditional methods. Here, we review recent advances using light to control basic cellular functions and discuss the engineering challenges that lie ahead for improving and expanding the ever‐growing optogenetic toolkit.     

Genetic and molecular analysis of light-regulated plant development

Light regulates many physiological and developmental events in plants through the action of multiple sensory pigment systems. Although our understanding of the regulatory photoreceptors, including phytochromes (that principally absorb red and far-red energy) and blue light receptors, has advanced considerably in the recent past, the mechanisms of light signal transduction in higher plants are poorly understood. To unravel the molecular events associated with light-regulated plant development, a large number of photomorphogenic mutants have been isolated in several different plant species, including Arabidopsis, cucumber, tomato, pea, Brassica and Sorghum, which are either impaired in normal perception of light signal (photoreceptor mutants) or are affected in some specific or a sub-set of phenotypic traits (signal transduction mutants). Their physiological and molecular analysis is proving to be valuable in (1) assigning specific function to discrete phytochrome species, (2) elucidation of elements that constitute the transduction pathway downstream of signal perception, and (3) determining how different photosensory systems regulate many diverse responses. The progress made in the analysis of photomorphogenic mutants, as reviewed in this article, clearly indicates that multiple photoreceptors, either of the same or different class, interact through an intricate network of signal transduction pathways to finally determine the light-dependent phenotype of both monocots and dicots.     

Light signals in leaf and chloroplast development: photoreceptors and downstream responses in search of a transduction pathway

Light affects both the development and the metabolism of plants. In addition to the role of light in providing energy for photosynthesis, light signals cause profound changes in the morphology of the developing young seedling, including cotyledon expansion, leaf development, inhibition of stem growth, and production of chlorophyll in the photosynthetically competent chloroplast. The light-dependent development of plants (photomorphogenesis) is a complex process resulting from the combined action of several photoreceptors. This review summarizes what is known of the red- and blue-light photoreceptors that regulate dicotyledonous seedling development and the complexity of the downstream responses. Special emphasis is placed on the recent progress made toward genetic and biochemical dissection of the signal transduction pathways.     

Distinct lobes of Limulus ventral photoreceptors. I. Functional and anatomical properties of lobes revealed by removal of glial cells

Removing the glial cells that encase Limulus ventral photoreceptors allows direct observation of the cell surface. Light microscopy of denuded photoreceptors reveals a subdivision of the cell body into lobes. Often one lobe, but sometimes several, is relatively clear and translucent (the R lobes). The lobe adjacent to the axon (the A lobe) has a textured appearance. Scanning electron microscopy shows that microvilli cover the surface of R lobes and are absent from the surface of A lobes. When a dim spot of light is incident on the R lobe, the probability of evoking a single photon response is two to three orders of magnitude higher than when the same spot is incident on the A lobe. We conclude that the sensitivity of the cell to light is principally a function of the R lobe.     

LOVely enzymes – towards engineering light‐controllable biocatalysts

Light control over enzyme function represents a novel and exciting field of biocatalysis research. Blue-light photoreceptors of the L ight, O xygen, V oltage (LOV) family have recently been investigated for their applicability as photoactive switches. We discuss here the primary photochemical events leading to light activation of LOV domains as well as the proposed signal propagation mechanism to the respective effector domain. Furthermore, we describe the construction of LOV fusions to different effector domains, namely a dihydrofolate reductase from Escherichia coli and a lipase from Bacillus subtilis. Both fusion partners retained functionality, and alteration of enzyme activity by light was also demonstrated. Hence, it appears that fusion of LOV photoreceptors to functional enzyme target sites via appropriate linker structures may represent a straightforward strategy to design light controllable biocatalysts.     

Nucleo-cytoplasmic partitioning of the plant photoreceptors phytochromes

Phytochromes in harmony with blue light photoreceptors play a major role in controlling plant growth and development from germination to seed maturation. Light absorption by phytochromes triggers a signaling cascade, phototransduction, which culminates in regulated gene expression. A major regulatory step at the cellular level, which affects specificities of light-induced physiological responses, seems to be the light-quality and light-quantity dependent nuclear import of the phytochromes themselves. The correlations found between the nuclear import of phytochromes (phyA and phyB) and various physiological responses regulated by these photoreceptors provides strong support for this hypothesis.     

Molecular evolution of proteins involved in vertebrate phototransduction

Vision is one of the most important senses for vertebrates. As a result, vertebrates have evolved a highly organized system of retinal photoreceptors. Light triggers an enzymatic cascade, called the phototransduction cascade, that leads to the hyperpolarization of photoreceptors. It is expected that a systematic comparison of phototransduction cascades of various vertebrates can provide insights into the diversity of vertebrate photoreceptors and into the evolution of vertebrate vision. However, only a few attempts have been made to compare each phototransduction protein participating in this cascade. Here, we determine phylogenetic trees of the vertebrate phototransduction proteins and compare them. It is demonstrated that vertebrate opsin sequences fall into five fundamental subfamilies. It is speculated that this is crucial for the diversity of the spectral sensitivity observed in vertebrate photoreceptors and provides the vertebrates with the molecular tools to discriminate the color of incident light. Other phototransduction proteins can be classified into only a few subfamilies. Cones generally share isoforms of phototransduction proteins that are different from those found in rods. The difference in sensitivity to light between rods and cones is likely due to the difference in the molecular properties of these isoforms. The phototransduction proteins seem to have co-evolved as a system. Switching the expression of these isoforms may characterize individual vertebrate photoreceptors.     

Tuning in to the signals controlling photoregulated gene expression in plants.

Plants have developed flexible mechanisms to respond appropriately to environmental signals. These stimuli are transduced by largely unknown signalling pathways that are likely to be modulated by endogenous developmental signals to produce an integrated response that coordinately regulates gene expression. Light is a critical environmental signal that controls many aspects of plant development via a series of photoreceptors that are able to respond to different light wavelengths. Light is also the principal energy source for photosynthesis. The photosynthetic products are carbohydrates which are translocated in the form of sucrose from the photosynthetic (source) to non-photosynthetic (sink) organs. Consequently, the control of photoregulated genes must integrate developmental inputs with signals derived from the photoreceptors, from the photosynthetic apparatus and from metabolites such as sucrose.     

蛹虫草光受体研究进展

光调控真菌的生长发育和代谢产物合成,是大多数食用菌子实体分化必不可少的环境因子。蛹虫草是一种具有重要药用价值的食用真菌,光因素对蛹虫草的分生孢子数量、生长速率、子实体发育、昼夜节律以及次级代谢产物虫草素和类胡萝卜素的合成均有较大的影响。目前,基于蛹虫草全基因组的解析和遗传操作系统的建立,已鉴定出7种光受体,white collar-1（WC-1）、WC-2、Drosophila-Arabidopsis- Synechocystis-human type cryptochromes（CRY-DASH）、CRY-2、环丁烷嘧啶二聚体光裂合酶（cyclobutane pyrimidine dimer,CPD）、VIVID（VVD）和光敏色素（phytochrome,PHY）。本文较系统地总结了上述光受体的蛋白结构特征,全面阐述了光受体介导光信号调控蛹虫草菌丝生长、子实体发育及次级代谢产物合成等方面的分子机制,并对后续研究进行了展望,为深入研究蛹虫草光受体的具体功能和相关机制提供参考。     

Blue news update: BODIPY-GTP binds to the blue-light receptor YtvA while GTP does not

Light is an important environmental factor for almost all organisms. It is mainly used as an energy source but it is also a key factor for the regulation of multiple cellular functions. Light as the extracellular stimulus is thereby converted into an intracellular signal by photoreceptors that act as signal transducers. The blue-light receptor YtvA, a bacterial counterpart of plant phototropins, is involved in the stress response of Bacillus subtilis. The mechanism behind its activation, however, remains unknown. It was suggested based on fluorescence spectroscopic studies that YtvA function involves GTP binding and that this interaction is altered by absorption of light. We have investigated this interaction by several biophysical methods and show here using fluorescence spectroscopy, ITC titrations, and three NMR spectroscopic assays that while YtvA interacts with BODIPY-GTP as a fluorescent GTP analogue originally used for the detection of GTP binding, it does not bind GTP.     

Two distinct light regulated G-proteins in octopus photoreceptors

Two distinct light-regulated G-proteins were found in octopus photoreceptors. Gip, a 41 kDa protein from washed microvilli, was ADP ribosylated by pertussis toxin in the presence of GDP in the dark. Light and GTP analogues were inhibitory as with transducin (Gt; G-protein in vertebrate photoreceptors). G34, a 34 kDa protein from fresh octopus retina, was ADP ribosylated by both cholera and pertussis toxin in the dark. Light inhibited labeling of the 34 kDa protein by both toxins. Unlike Gip, G34 is soluble and is very labile to heat, freezing and thawing. Prolonged incubation of octopus retina with cholera toxin and labeled NAD produced an additional radioactive band at 46 kDa. Labeling of the 46 kDa protein, Gsp, was greatly enhanced by GTP analogues, but inhibited by a GDP analogue as with Gs in hormone-sensitive adenylate cyclase. In contrast to Gip and G34, labeling of the 46 kDa protein (Gsp) was not influenced by light. The two distinct light-regulated G-proteins, Gip and G34, found in octopus photoreceptors might be involved in either phototransduction or photoadaptation. The function of Gsp is not known.     

The role of dermal photoreceptors during the sea lamprey (<Emphasis Type="Italic">Petromyzon marinus</Emphasis>) spawning migration

Light avoidance in larval lampreys is mediated by dermal photoreceptors located in the tail. These photoreceptors continue to function in adults, but they seem redundant because post-metamorphic lampreys possess well-developed eyes. This study examined the role of dermal photoreceptors in adult sea lampreys by testing whether temperature-induced changes in refuge-seeking behavior are mediated by a reduction in dermal photosensitivity. In a lighted arena containing a single shaded refuge platform, lampreys at 22°C displayed five times less search activity and were less likely to attach beneath the refuge platform than lampreys at either 7 or 15°C. A behavioral assay for tail photosensitivity (locomotor response to tail illumination) revealed a corresponding reduction in dermal photosensitivity at 22°C. Moreover, the responses to head illumination (eyes and pineal) did not correspond with the observed light avoidance behaviors. The head was less responsive to light than the tail and was not influenced by temperature. These results provide strong evidence that the dermal photoreceptors continue to mediate light avoidance in adult lampreys, even though adults possess fully functional eyes. The fact that the eyes apparently do not take on this role suggests that there is functional specialization between these two light sensing systems.     

LOV domain-containing F-box proteins: light-dependent protein degradation modules in Arabidopsis

Plants constantly survey the surrounding environment using several sets of photoreceptors. They can sense changes in the quantity (=intensity) and quality (=wavelength) of light and use this information to adjust their physiological responses, growth, and developmental patterns. In addition to the classical photoreceptors, such as phytochromes, cryptochromes, and phototropins, ZEITLUPE (ZTL), FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1), and LOV KELCH PROTEIN 2 (LKP2) proteins have been recently identified as blue-light photoreceptors that are important for regulation of the circadian clock and photoperiodic flowering. The ZTL/FKF1/LKP2 protein family possesses a unique combination of domains: a blue-light-absorbing LOV (Light, Oxygen, or Voltage) domain along with domains involved in protein degradation. Here, we summarize recent advances in our understanding of the function of the Arabidopsis ZTL/FKF1/LKP2 proteins. We summarize the distinct photochemical properties of their LOV domains and discuss the molecular mechanisms by which the ZTL/FKF1/LKP2 proteins regulate the circadian clock and photoperiodic flowering by controlling blue-light-dependent protein degradation.     

How plants tell the time

The components of the circadian system that have recently been discovered in plants share some characteristics with those from cyanobacterial, fungal and animal circadian clocks. Light input signals to the clock are contributed by multiple photoreceptors: some of these have now been shown to function specifically in response to light of defined wavelength and fluence rate. New reports of clock-controlled processes and genes are highlighting the importance of time management for plant development.     

Phototransduction: shedding light on translocation

Light induces the migration of arrestin to the photosensitive membrane in both vertebrate and invertebrate photoreceptors. New work has identified a phosphoinositide lipid binding domain in Drosophila arrestin and implicates PIP(3) in control of arrestin translocation.