Cellular and molecular regulation of serotonin N-acetyltransferase activity in chicken retinal photoreceptors

Constant light in early life induces fear-related behavior in chickens with suppressed melatonin secretion and disrupted hippocampal expression of clock- and BDNF-associated genes

Background

Light management plays an important role in the growth and behavior of broiler chickens. Constant light in early post hatch stage has been a common practice in broiler industry for improving growth performance, while whether and how constant light in early life affects the behavior of broiler chickens is rarely reported.

Results

In this study, newly hatched chicks were kept in either constant (24 L:0 D, LL) or (12 L:12 D, LD) photoperiod for 7 d and then maintained in 12 L:12 D thereafter until 21 days of age. Constant light increased the average daily feed intake but not the body weight, which led to higher feed conversion ratio. Chickens in LL group exhibited fear-related behaviors, which was associated with higher corticosterone, lower melatonin and 5-HT levels. Concurrently, constant light exposure increased the mRNA expression of clock-related genes and suppressed the expression of antioxidative genes in the hippocampus. Moreover, brain derived neurotrophic factor/extracellular signal-regulated kinase (BDNF/ERK) pathway was suppressed in the hippocampus of chickens exposed to constant light in the first week post hatching.

Conclusions

These findings indicate that constant light exposure in early life suppress melatonin secretion and disrupts hippocampal expression of genes involved in circadian clock and BDNF/ERK pathway, thereby contributing to fear-related behaviors in the chicken.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00720-4.

Retinal Circadian Clocks and Control of Retinal Physiology

Retinas of all classes of vertebrates contain endogenous circadian clocks that control many aspects of retinal physiology, including retinal sensitivity to light, neurohormone synthesis, and cellular events such as rod disk shedding, intracellular signaling pathways, and gene expression. The vertebrate retina is an example of a “peripheral” oscillator that is particularly amenable to study because this tissue is well characterized, the relationships between the various cell types are extensively studied, and many local clock-controlled rhythms are known. Although the existence of a photoreceptor clock is well established in several species, emerging data are consistent with multiple or dual oscillators within the retina that interact to control local physiology. Aprominent example is the antiphasic regulation of melaton in and dopamine in photoreceptors and inner retina, respectively. This review focuses on the similarities and differences in the molecular mechanisms of the retinal versus the SCN oscillators, as well as on the expression of core components of the circadian clockwork in retina. Finally, the interactions between the retinal clock(s) and the master clock in the SCN are examined.

Avian photoreceptors and their role in the regulation of daily and seasonal physiology

Birds time their activities in synchronization with daily and seasonal periodicities in the environment, which is mainly provided by changes in day length (=photoperiod). Photoperiod appears to act at different levels than simply entraining the hypothalamic clock via eyes in birds. Photoreceptor cells that transmit light information to an avian brain are localized in three independent structures, the retina of eyes, pineal gland and hypothalamus, particularly in the paraventricular organ and lateral septal area. These hypothalamic photoreceptors are commonly referred to as encephalic or deep brain photoreceptors, DBPs. Eyes and pineal are known to contribute to the circadian regulation of behavior and physiology via rhythmic melatonin secretion in several birds. DBPs have been implicated in the regulation of seasonal physiology, particularly in photoperiod induced gonadal growth and development. Here, we briefly review limited evidence that is available on the roles of these photoreceptors in the regulation of circadian and seasonal physiology, with particular emphasis placed on the DBPs.

Early appearance of nonvisual and circadian markers in the developing inner retinal cells of chicken

The retina is a key component of the vertebrate circadian system; it is responsible for detecting and transmitting the environmental illumination conditions (day/night cycles) to the brain that synchronize the circadian clock located in the suprachiasmatic nucleus (SCN). For this, retinal ganglion cells (RGCs) project to the SCN and other nonvisual areas. In the chicken, intrinsically photosensitive RGCs (ipRGCs) expressing the photopigment melanopsin (Opn4) transmit photic information and regulate diverse nonvisual tasks. In nonmammalian vertebrates, two genes encode Opn4: the Xenopus (Opn4x) and the mammalian (Opn4m) orthologs. RGCs express both Opn4 genes but are not the only inner retinal cells expressing Opn4x: horizontal cells (HCs) also do so. Here, we further characterize primary cultures of both populations of inner retinal cells (RGCs and HCs) expressing Opn4x. The expression of this nonvisual photopigment, as well as that for different circadian markers such as the clock genes Bmal1, Clock, Per2, and Cry1, and the key melatonin synthesizing enzyme, arylalkylamine N-acetyltransferase (AA-NAT), appears very early in development in both cell populations. The results clearly suggest that nonvisual Opn4 photoreceptors and endogenous clocks converge all together in these inner retinal cells at early developmental stages.

cAMP-regulated dynamics of the mammalian circadian clock

Previous molecular description of the mammalian timekeeping mechanism was based mainly on transcriptional/translational feedback loops (TTFLs). However, a recent experimental report challenges such a molecular architecture, showing that the cAMP signaling is an indispensable component of the mammalian circadian clock. In this paper, we develop a reduced mathematical model that characterizes the mammalian circadian network. The model with 8-state differential equations incorporates both TTFLs and cAMP-mediated feedback loop. In agreement with experimental observations, our results show that: (1) the model simulates sustained circadian (23.4-h periodic) oscillations in constant darkness and entrained circadian dynamics by light-dark cycles; (2) circadian rhythmicity is lost without cAMP signaling; (3) the system is resilient to large fluctuations in transcriptional rates; (4) it successfully simulates the phenotypes of Per1(-/-)/Per2(-/-) double-mutant mice and Bmal1(-/-) mutant mice. Our study implies that to understand the circadian pacemaking in suprachiasmatic nucleus neurons, the TTFLs should not be isolated from intracellular cAMP-dependent signaling.

Circadian regulation of ion channels and their functions

Ion channels are the gatekeepers to neuronal excitability. Retinal neurons of vertebrates and invertebrates, neurons of the suprachiasmatic nucleus (SCN) of vertebrates, and pinealocytes of non-mammalian vertebrates display daily rhythms in their activities. The interlocking transcription-translation feedback loops with specific post-translational modulations within individual cells form the molecular clock, the basic mechanism that maintains the autonomic approximately 24-h rhythm. The molecular clock regulates downstream output signaling pathways that further modulate activities of various ion channels. Ultimately, it is the circadian regulation of ion channel properties that govern excitability and behavior output of these neurons. In this review, we focus on the recent development of research in circadian neurobiology mainly from 1980 forward. We will emphasize the circadian regulation of various ion channels, including cGMP-gated cation channels, various voltage-gated calcium and potassium channels, Na(+)/K(+)-ATPase, and a long-opening cation channel. The cellular mechanisms underlying the circadian regulation of these ion channels and their functions in various tissues and organisms will also be discussed. Despite the magnitude of chronobiological studies in recent years, the circadian regulation of ion channels still remains largely unexplored. Through more investigation and understanding of the circadian regulation of ion channels, the future development of therapeutic strategies for the treatment of sleep disorders, cardiovascular diseases, and other illnesses linked to circadian misalignment will benefit.

Night/day changes in pineal expression of >600 genes: central role of adrenergic/cAMP signaling

The pineal gland plays an essential role in vertebrate chronobiology by converting time into a hormonal signal, melatonin, which is always elevated at night. Here we have analyzed the rodent pineal transcriptome using Affymetrix GeneChip(R) technology to obtain a more complete description of pineal cell biology. The effort revealed that 604 genes (1,268 probe sets) with Entrez Gene identifiers are differentially expressed greater than 2-fold between midnight and mid-day (false discovery rate <0.20). Expression is greater at night in approximately 70%. These findings were supported by the results of radiochemical in situ hybridization histology and quantitative real time-PCR studies. We also found that the regulatory mechanism controlling the night/day changes in the expression of most genes involves norepinephrine-cyclic AMP signaling. Comparison of the pineal gene expression profile with that in other tissues identified 334 genes (496 probe sets) that are expressed greater than 8-fold higher in the pineal gland relative to other tissues. Of these genes, 17% are expressed at similar levels in the retina, consistent with a common evolutionary origin of these tissues. Functional categorization of the highly expressed and/or night/day differentially expressed genes identified clusters that are markers of specialized functions, including the immune/inflammation response, melatonin synthesis, photodetection, thyroid hormone signaling, and diverse aspects of cellular signaling and cell biology. These studies produce a paradigm shift in our understanding of the 24-h dynamics of the pineal gland from one focused on melatonin synthesis to one including many cellular processes.

Role of CYP1B1 in Glaucoma

Glaucoma is a leading cause of blindness, estimated to affect 60 million people by 2010, and represents a heterogeneous group of neurodegenerative disease. The two major types of glaucoma include primary open-angle glaucoma (POAG) and primary congenital glaucoma (PCG). A genetically heterogeneous group of developmental disorders known as anterior segment dysgenesis (ASD) have been reported to be associated with increased intraocular pressure (IOP) and glaucoma. These include Peters' anomaly, Rieger's anomaly, aniridia, iris hypoplasia, and iridogoniodysgenesis. Genetic linkage analysis and mutation studies have identified CYP1B1 as a causative gene in PCG, as a modifier gene in POAG, and, on rare occasions, as causative gene in POAG as well as in several ASD disorders. CYP1B1-deficient mice exhibit abnormalities in their ocular drainage structure and trabecular meshwork that are similar to those reported in human PCG patients. Accordingly, it is speculated that diminished or absent metabolism of key endogenous CYP1B1 substrates adversely affects the development of the trabecular meshwork. CYP1B1 protein is involved in the metabolism of steroids, retinol and retinal, arachidonate, and melatonin. The conserved expression of CYP1B1 in both murine and human eyes, its higher expression in fetal than adult eyes, and its biochemical properties are consistent with this hypothesis. The exact role of CYP1B1 in the pathogenesis of glaucoma and other ASD disorders remains to be elucidated.

Expression of circadian clock genes in retinal dopaminergic cells

The mammalian neural retina contains single or multiple intrinsic circadian oscillators that can be directly entrained by light cycles. Dopaminergic amacrine (DA) cells represent an especially interesting candidate as a site of the retinal oscillator because of the crucial role of dopamine in light adaptation, and the widespread distribution of dopamine receptors in the retina. We hereby show by single-cell, end-point RT-PCR that retinal DA cells contain the transcripts for six core components of the circadian clock: Bmal1, Clock, Cry1, Cry2, Per1, and Per2. Rod photoreceptors represented a negative control, because they did not appear to contain clock transcripts. We finally confirmed that DA cells contain the protein encoded by the Bmal1 gene by comparing immunostaining of the nuclei of DA cells in the retinas of wildtype and Bmal1-/- mice. It is therefore likely that DA cells contain a circadian clock that anticipates predictable variations in retinal illumination.

Arylalkylamine N-acetyltransferase: "the Timezyme"

Arylalkylamine N-acetyltransferase controls daily changes in melatonin production by the pineal gland and thereby plays a unique role in biological timing in vertebrates. Arylalkylamine N-acetyltransferase is also expressed in the retina, where it may play other roles in addition to signaling, including neurotransmission and detoxification. Large changes in activity reflect cyclic 3',5'-adenosine monophosphate-dependent phosphorylation of arylalkylamine N-acetyltransferase, leading to formation of a regulatory complex with 14-3-3 proteins. This activates the enzyme and prevents proteosomal proteolysis. The conserved features of regulatory systems that control arylalkylamine N-acetyltransferase are a circadian clock and environmental lighting.

Molecular cloning, mRNA expression, and immunocytochemical localization of a putative blue-light photoreceptor CRY4 in the chicken pineal gland

In non-mammalian vertebrates, the pineal gland contains an endogenous circadian oscillator and serves as a photosensitive neuroendocrinal organ. To better understand the pineal phototransduction mechanism, we focused on the chicken putative blue-light photoreceptive molecule, Cryptochrome4 (cCRY4). Here we report the molecular cloning of pineal cCry4 cDNA, the in vivo expression of cCry4 mRNA, and the detection of cCRY4 protein. cCry4 is transcribed in a wide variety of chick tissues out of which the pineal gland and retina contain high levels of cCry4 mRNA. In the pineal gland, under 12 h light : 12 h dark cycles, the levels of both cCry4 mRNA and cCRY4 protein showed diurnal changes, and in cultured chick pineal cells, the cCry4 mRNA level was not only up-regulated by light but also controlled by circadian signals. Immunoblot analysis with a cCRY4-specific antibody detected cCRY4 in a soluble fraction of the pineal lysate. Immunocytochemistry revealed that cCRY4 was expressed in many parenchymal cells and a limited number of stromal cells. These cCRY4 features strikingly contrast with those of the chick pineal photoreceptor pinopsin, suggesting a possible temporal and/or spatial duplicity of the pineal photoreceptive system, the opsin- and CRY-based mechanisms.

Time keeping by the quail's eye: circadian regulation of melatonin production

Previous studies have shown that eye removal disrupts the circadian body temperature and activity rhythms of Japanese quail supporting the hypothesis that the eyes act as pacemakers within the quail circadian system. Furthermore, the putative ocular pacemakers are coupled to the rest of the circadian system via neural and hormonal outputs. Although the neural pathway has yet to be identified, experiments suggest that the daily rhythm of ocular melatonin synthesis and release is the hormonal output. We sought to strengthen the hypothesis that the eyes are the loci of circadian pacemakers, and that melatonin output is involved, by examining melatonin secretion in cultured quail retinas. Using an in vitro flow-through system we demonstrated that (1) isolated retinal tissue could exhibit a rhythm of melatonin release, (2) the rhythm of melatonin synthesis is directly entrainable by 24-h light-dark cycles, and (3) supplementation of the culture medium with serotonin is necessary for robust, rhythmic production of melatonin in constant darkness. These results show definitively that the eyes are the loci of a biological clock and, in light of previous studies showing the disruptive effects of blinding on the circadian system, strengthen the hypothesis that the ocular clock is a circadian pacemaker that can affect the rest of the circadian system via the cyclic synthesis and release of melatonin. The quail retina is proving to be a valuable in vitro model for investigating properties of circadian pacemakers.

Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina

Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.

Transcriptional Profiling of Circadian Patterns of mRNA Expression in the Chick Retina

Previous transcriptome analyses have identified candidate molecular components of the avian pineal clock, and herein we employ high density cDNA microarrays of pineal gland transcripts to determine oscillating transcripts in the chick retina under daily and constant darkness conditions. Subsequent comparative transcriptome analysis of the pineal and retinal oscillators distinguished several transcriptional similarities between the two as well as significant differences. Rhythmic retinal transcripts were classified according to functional categories including phototransductive elements, transcription/translation factors, carrier proteins, cell signaling molecules, and stress response genes. Candidate retinal clock transcripts were also organized relative to time of day mRNA abundance, revealing groups accumulating peak mRNA levels across the circadian day but primarily reaching peak values at subjective dawn or subjective dusk. Comparison of the chick retina transcriptome to the pineal transcriptome under constant conditions yields an interesting group of conserved genes. This group includes putative clock elements cry1 and per3 in addition to several previously unidentified and uninvestigated genes exhibiting profiles of mRNA abundance that varied markedly under daily and constant conditions. In contrast, many transcripts were differentially regulated, including those believed to be involved in both melatonin biosynthesis and circadian clock mechanisms. Our results indicate an intimate transcriptional relationship between the avian pineal and retina in addition to providing previously uncharacterized molecular elements that we hypothesize to be involved in circadian rhythm generation.

Gating of the cAMP signaling cascade and melatonin synthesis by the circadian clock in mammalian retina

Melatonin is synthesized in retinal photoreceptor cells and acts as a neuromodulator imparting photoperiodic information to the retina. The synthesis of melatonin is controlled by an ocular circadian clock and by light in a finely tuned mechanism that ensures that melatonin is synthesized and acts only at night in darkness. Here we report that the circadian clock gates melatonin synthesis in part by regulating the expression of the type 1 adenylyl cyclase (AC1) and the synthesis of cAMP in photoreceptor cells. This gating is effected through E-box-mediated transcriptional activation of the AC1 gene, which undergoes robust daily fluctuations that persist in constant illumination. The circadian control of the cAMP signaling cascade indicates that the clock has a more general and profound impact on retinal functions than previously thought. In addition, rhythmic control of AC1 expression was observed in other parts of the central circadian axis, the suprachiasmatic nucleus and pineal gland, but not in other brain areas examined. Thus, clock control of the cAMP signaling cascade may play a central role in the integration of circadian signals that control physiology and behavior.

Comparative anatomy of nitrergic intrinsic choroidal neurons (ICN) in various avian species

Intrinsic choroidal neurons (ICN) represent a peculiar feature of eyes in higher primates and birds. They account for up to 2000 in human and duck eyes but are virtually absent or rare in all other mammalian species investigated so far. It has been suggested that ICN are involved in regulation of ocular blood supply, hence influencing intraocular pressure, and changes in choroidal thickness, thus influencing accommodation. The present study was undertaken in order to compare differences in various avian species with respect to ICN as well as to provide data on some avian species relevant for experimental ophthalmic research, i.e. chicken and quail. Choroids from 12 avian species were processed for NADPH-diaphorase histochemistry or, in some cases, neuronal nitric oxide synthase immunocytochemistry. ICN were quantified and normalized to mean choroidal area. Three choroids of each galliformes (i.e. chicken, quail, turkey) and anseriformes (i.e. Muscovy duck, Mallard duck, goose) were rastered in squares of 1 mm2 and x/y coordinates were transferred into a 3D-diagram with the amount of ICN represented in the z-axis. ICN were detected in all species investigated. They were predominantly small cells with soma diameters of 20-30 microm. In turkey, and to a lesser amount in chicken, a subpopulation of ICN with somal diameters of up to 70 microm was observed. Highest mean cell counts were found in goose (6195.4; turkey 3558.4; chicken 1681.4; Muscovy duck 785.4; Mallard duck 640.8; quail 440.2). Normalized to choroidal area, highest mean cell counts were (per mm2): 12.62 in goose, 4.42 in both chicken and turkey, 2.86 in quail, 2.66 in Mallard duck and 1.89 in Muscovy duck. In galliformes, ICN were found to be accumulated temporo-cranial, while in anseriformes they were arranged in a more belt-like fashion, passing from cranio-nasal to temporo-caudal. Our results show that besides Muscovy duck, other avian species appear as suitable models for further functional experiments on ICN. The temporo-cranial accumulation of ICN in galliformes and the belt-like arrangement in anseriformes may reflect special functional requirements in regions of high visual acuity.

Circadian rhythm and photic control of cAMP level in chick retinal cell cultures: a mechanism for coupling the circadian oscillator to the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, in photoreceptor cells

Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.

Chick pineal melatonin synthesis: light and cyclic AMP control abundance of serotonin N-acetyltransferase protein

Melatonin production in the pineal gland is high at night and low during the day. This rhythm reflects circadian changes in the activity of serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1.87], the penultimate enzyme in melatonin synthesis. The rhythm is generated by an endogenous circadian clock. In the chick, a clock is located in the pinealocyte, which also contains two phototransduction systems. One controls melatonin production by adjusting the clock and the other acts distal to the clock, via cyclic AMP mechanisms, to switch melatonin synthesis on and off. Unlike the clock in these cells, cyclic AMP does not appear to regulate activity by altering AA-NAT mRNA levels. The major changes in AA-NAT mRNA levels induced by the clock seemed likely (but not certain) to generate comparable changes in AA-NAT protein levels and AA-NAT activity. Cyclic AMP might also regulate AA-NAT activity via changes in protein levels, or it might act via other mechanisms, including posttranslational changes affecting activity. We measured AA-NAT protein levels and enzyme activity in cultured chick pineal cells and found that they correlated well under all conditions. They rose and fell spontaneously with a circadian rhythm. They also rose in response to agents that increase cyclic AMP. They were raised by agents that increase cyclic AMP, such as forskolin, and lowered by agents that decrease cyclic AMP, such as light and norepinephrine. Thus, both the clock and cyclic AMP can control AA-NAT activity by altering the total amount of AA-NAT protein. Effects of proteosomal proteolysis inhibitors suggest that changes in AA-NAT protein levels, in turn, reflect changes in the rate at which the protein is destroyed by proteosomal proteolysis. It is likely that cyclic AMP-induced changes in AA-NAT protein levels mediate rapid changes in chick pineal AA-NAT activity. Our results indicate that light can rapidly regulate the abundance of a specific protein (AA-NAT) within a photoreceptive cell.

Regulation of tryptophan hydroxylase activity in Xenopus laevis photoreceptor cells by cyclic AMP

The aim of this study was to investigate the role of cyclic AMP in the regulation of tryptophan hydroxylase activity localized in retinal photoreceptor cells of Xenopus laevis, where the enzyme plays a key role in circadian melatonin biosynthesis. In photoreceptor-enriched retinas that lack serotonergic neurons, tryptophan hydroxylase activity is markedly stimulated by treatments that increase intracellular levels of cyclic AMP or activate cyclic AMP-dependent protein kinase, including forskolin, phosphodiesterase inhibitors, and cyclic AMP analogues. In contrast, cyclic AMP has no effect on tryptophan hydroxylase mRNA abundance. Experiments using cycloheximide and actinomycin D demonstrate that cyclic AMP exerts its regulatory effect via posttranslational mechanisms mediated by cyclic AMP-dependent protein kinase. The effect of cyclic AMP is independent of the phase of the photoperiod, suggesting that the nucleotide is not a mediator of the circadian rhythm of tryptophan hydroxylase. Cyclic AMP accumulation is higher in darkness than in light, as is tryptophan hydroxylase activity. Furthermore, the stimulatory effect of forskolin and that of darkness are inhibited by H89, an inhibitor of cyclic AMP-dependent protein kinase. In conclusion, cyclic AMP may mediate the acute effects of light and darkness on tryptophan hydroxylase activity of retinal photoreceptor cells.