Molecular and cellular aspects of hydroxyindole-O-methyltransferase expression in the developing chick pineal gland

Influence of Different Light Spectra on Melatonin Synthesis by the Pineal Gland and Influence on the Immune System in Chickens

It is well known that the pineal gland in birds influences behavioural and physiological functions, including those of the immune system. The purpose of this research is to examine the endocrine-immune correlations between melatonin and immune system activity. Through a description of the immune-pineal axis, we formulated the objective to determine and describe: the development of the pineal gland; how light influences secretory activity; and how melatonin influences the activity of primary and secondary lymphoid organs. The pineal gland has the ability to turn light information into an endocrine signal suitable for the immune system via the membrane receptors Mel1a, Mel1b, and Mel1c, as well as the nuclear receptors RORα, RORβ, and RORγ. We can state the following findings: green monochromatic light (560 nm) increased serum melatonin levels and promoted a stronger humoral and cellular immune response by proliferating B and T lymphocytes; the combination of green and blue monochromatic light (560-480 nm) ameliorated the inflammatory response and protected lymphoid organs from oxidative stress; and red monochromatic light (660 nm) maintained the inflammatory response and promoted the growth of pathogenic bacteria. Melatonin can be considered a potent antioxidant and immunomodulator and is a critical element in the coordination between external light stimulation and the body's internal response.

Embryonic Ontogeny of 5-Hydroxyindoles and 5-Methoxyindoles Synthesis Pathways in the Goose Pineal Organ

The aim of this study was to characterize the embryonic ontogeny of 5-hydroxyindoles and 5-methoxyindoles synthesis pathways in the goose pineal organ. The study was performed on embryos aged 14–28 days, which have been incubated under a 12L:12D cycle. The pineal organs were collected for measurements of indole content by HPLC every 6 h on embryonic day (ED) 14, ED 16, ED 18 and ED 22 or every 2 h on ED 24, ED 26 and ED 28. The level of tryptophan showed no significant changes during development and no day-night variations. The content of 5-hydroxytryptophan increased between ED 14 and ED 26. It was significantly higher during scotophase than during photophase starting from ED 14. The serotonin content was low during the early stages of development (ED 14–ED 18) and prominently increased from ED 20. The serotonin levels also showed day-night differences; however, they were less conspicuous than those of 5-hydroxytryptophan. The changes in the level of 5-hydroxyindole acetic acid were similar to those of serotonin. 5-Hydroxytryptophol was measurable from ED 18. Levels of N-acetylserotonin, which were detectable for the first time on ED 16, prominently increased between ED 22 and ED 28 and showed significant day–night differences from ED 20. Melatonin was detectable from ED 18. Like N-acetylserotonin, its content increased rapidly between ED 22 and ED 28, and from ED 20 showed diurnal variations. 5-Methoxyindole acetic acid and 5-methoxytryptophol occurred at measurable levels from ED 18 and ED 26, respectively. The obtained results showed that embryonic development of indole metabolism in the goose pineal organ starts with the beginning of serotonin synthesis. The processes of serotonin acetylation and 5-hydroxyindoles methylation were turned on later. Diurnal rhythmicity develops very early in the embryonic pineal organ of the goose when the eggs are incubated under a 12 h light: 12 h dark schedule. Two processes are responsible for generation of the diurnal rhythms of 5-hydroxyindoles and 5-methoxyindoles: (i) hydroxylation of tryptophan and (ii) acetylation of serotonin.

Circadian phosphorylation of ATF-2, a potential activator of Period2 gene transcription in the chick pineal gland

Stimulus-induced transcription of the Period gene is a critical step for phase-shift of vertebrate circadian systems. The promoter region of chicken Period2 contains a canonical calcium/cAMP-responsive element, but its functional relevance is not known. The present study shows that cAMP-responsive element-binding protein (CREB) and activating transcription factor-2 (ATF-2) bind to the promoter region of the Period2 gene in the chick pineal gland. In transient transfection assays, a reporter construct containing 0.7-kbp upstream region of chicken Period2 was transactivated by ATF-2, but it was poorly responsive to CREB. In the chick pineal gland, phosphorylation of CREB protein at the kinase-inducible domain was negatively regulated by light. On the other hand, phosphorylation of ATF-2 at the amino-terminal transactivation domain exhibited a circadian rhythm with a daytime peak, suggesting a role for ATF-2 in circadian rhythmicity in the chick pineal gland.

Purification and immunohistochemical analysis of calcium-binding proteins expressed in the chick pineal gland

The pineal gland is a site of melatonin production, of which intracellular calcium ions (Ca2+) are likely involved in various aspects. To investigate the identity of molecules responsible for the Ca2+-dependent processes in the pineal cells, we prepared a cellular extract from 2000 chick pineal glands and isolated a series of Ca2+-binding proteins by taking advantage of their Ca2+-dependent hydrophobic interaction with phenyl-Sepharose beads. The proteins identified by micro-sequencing analysis included calmodulin, neurocalcin, sorcin, annexin II and annexin V. Immunohistochemical analysis of the chick pineal sections revealed that both calmodulin and sorcin are expressed in the follicular and parafollicular pinealocytes. On the other hand, neurocalcin was expressed in a few neuron-like cells located predominantly in the parafollicular layer of the pineal follicle. These results suggest that calmodulin and sorcin may contribute to cellular functions in the chick pinealocytes.

Transcriptional regulation of the chicken hydroxyindole-O-methyltransferase gene by the cone-rod homeobox-containing protein

Functional differentiation of photoreceptor cells involves the expression of two sets of genes: those encoding the proteins of the phototransduction cascade and those encoding the enzymes of melatonin synthesis. The transcription factor Crx (cone-rod homeobox) plays a major role in the differentiation and maintenance of the photoreceptor phenotype. Previous studies have shown that this effect of Crx is correlated with its ability to transactivate several genes of the phototransduction cascade. Here, we show that Crx can also act on the gene encoding the melatonin-synthesizing enzyme, hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4). Three of the six putative Crx-binding sites found in the chicken HIOMT promoter interact directly with recombinant Crx and bound a pineal/retina-specific protein showing DNA-binding characteristics similar to those of Crx. In transient transfection experiments, Crx transactivated transcription of a reporter gene from the chicken HIOMT promoter. Transactivation was observed even with a portion of promoter carrying only one Crx-binding site and it was abolished by a mutation in this cis-regulatory element. These data indicate that Crx may participate in photoreceptor cell differentiation, not only by acting on the genes of the phototransduction cascade, but also by controlling the expression of the genes involved in melatonin synthesis.

Immunocytochemical localization of hydroxyindole-o-methyltransferase (HIOMT) in the brain of Myoisophagos lacteus (Nemertea: Heteronemertea: Lineidae)

In an attempt to identify the brain structures that synthesize melatonin and that probably mediate the photoperiodic response of the heteronemertean Myoisophagos lacteus, we utilized immunocytochemical techniques and employed immunoglobulins directed against hydroxyindole-O-methyltransferase (HIOMT, EC 2.1.1.4). This enzyme catalyzes the last step of melatonin biosynthesis. In immunocytochemically treated head sections of Myoisophagos lacteus, antibodies labelled a few cells in the dorsal region of the dorsal cerebral ganglia. Previous studies have shown that melatonin is present both in the brain and eyes of this nemertean species and that melatonin is involved in control of the worm reproduction. Other studies have demonstrated the presence of photoreceptor-like cells in the same region of the worm brain that showed HIOMT immunostaining. Therefore, anatomical findings of the present study, coupled with results of previous works, provide strong evidence that this region of the worm brain houses a photoperiodic receptor involved in melatonin biosynthesis. J. Exp. Zool. 290:156-162, 2001.

Cellular circadian clocks in the pineal

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Immunocytochemical identification of pinopsin in pineal glands of chicken and pigeon

Pinopsin is a blue-sensitive photoreceptive molecule possibly involved in photic entrainment of the circadian pacemaker in the chicken pineal gland. To characterize pinopsin as a circadian photoreceptor, antibodies were raised against the C-terminal portion of pinopsin. As expected from the divergence of the amino acid sequence of this region, the resultant antibody cross-reacted with neither chicken rhodopsin nor red-sensitive cone pigment (chicken red). In Western blot analysis, the antibody stained a single band of 42-kDa protein in a detergent-extract of chicken pineal membranes, suggesting that pinopsin (calculated molecular weight, 38187) might be glycosylated and/or palmitoylated. Immunocytochemical examination of pineal sections of the chicken and the pigeon with this antibody revealed strong positive images for most of the membrane structures in the lumen of the follicles. This antibody also stained string- and bulb-shaped structures of the chicken parafollicular cells, the morphology of which resembles those of retinal photoreceptor cells. In contrast to the predominant distribution of pinopsin, a monoclonal antibody specific for chicken red stained a smaller number of membrane structures in the lumen of chicken pineal follicles. These results strongly suggest that the chicken pineal gland contains at least two types of photoreceptive molecules, pinopsin (major) and chicken red (minor). We show that the former molecule is localized in parafollicular pinealocytes and in the outer segments of pinealocytes that make contact with the follicular lumen.

Hydroxyindole-O-methyltransferase gene expression in the pineal gland of chicken embryo: development of messenger RNA levels and regulation by serum

Hydroxyindole-O-methyltransferase (HIOMT), the enzyme which catalyzes the final step of melatonin biosynthesis, constitutes a marker of the functional differentiation of pineal cells. In addition, a day/night rhythm of HIOMT mRNA concentration, previously described in the chicken pineal gland [6], would suggest that HIOMT gene transcription is one output of the circadian system that controls pineal function. The study sought to monitor the developmental expression of HIOMT mRNA in the chick pineal gland and to investigate a possible role of instructive signals in this differentiation process. RT-PCR analysis indicated that HIOMT mRNA is expressed at embryonic day 8 (E8). At E12, HIOMT mRNA became detectable on northern blots and traces of HIOMT activity could be measured. HIOMT mRNA concentration increased 100-fold between E14 and day 10 post-hatch, then levelled off. A day/night rhythm of HIOMT mRNA concentration was readily observed in the pineal gland of 2-day-old chicks. Pineal glands isolated on minimum culture medium at E11 stopped developing HIOMT gene expression. However, the addition of serum to the culture medium restored HIOMT mRNA concentration to the levels observed in vivo. The data suggest that the functional differentiation of melatoninergic cells observed during the second week of embryonic life may be controlled [correction of controled] by serum factors.

Regulation of melatonin secretion by light in the isolated pineal organ of the white sucker (Catostomus commersoni)

The effects of different lighting conditions and physical parameters of light were investigated in the isolated pineal organ of the white sucker kept under static or superfusion culture. The secretion of pineal melatonin is directly controlled by the photoperiod and completely suppressed under constant illumination. When pineal organs are exposed to unexpected light at night, the secretion of melatonin is significantly reduced within 15 min and reaches basal value after 30-35 min of light treatment. The inhibition of melatonin secretion by unexpected light at night depends on the irradiance, duration, timing of the treatment and the lighting history of the pineal organ.

Calbindin-D28k, calretinin, and recoverin immunoreactivities in developing chick pineal gland

Calbindin-D28k, calretinin, and recoverin, three intracellular calcium-binding proteins belonging to the troponin C/calmodulin superfamily, were immunohistochemically localized in chick pineal during development [from embryonic day 16 (E16) to postnatal day 14 (P14)]. At E18, only calretinin immunoreactivity could be detected in nuclei from follicular pinealocytes. With development, calretinin immunoreactivity expanded from nucleus to cytoplasm, and calretinin immuno-positive cell number increased. At P14 almost al pinealocytes were calretinin positive. Calbindin-D28k immunoreactivity was not detected before E20. During development, many follicular and parafollicular pinealocytes became strongly calbindin-D28k positive, reaching a peak both in intensity and in number at P7; thereafter their number decreased. In addition to pinealocytes, neuron-like cells appeared calbindin-D28k positive at E20 and calretinin positive at P7. Recoverin, a myristoylated protein isolated from vertebrate photoreceptor and which might participate in the inactivation of the phototransduction cascade, was transiently expressed in follicular and parafollicular pinealocytes from P1 to P14 with a maximal expression at P7. This transitory expression may coincide with a transitory light sensitivity period in chick pinealocytes, before complete maturity of the pineal gland.

Clonidine in vivo mimics the acute suppressive but not the phase-shifting effects of light on circadian rhythm of serotonin N-acetyltransferase activity in chick pineal gland

Comparative in vivo studies on effects of pulses of light and clonidine, a selective agonist of alpha 2-adrenergic receptors, on the circadian rhythm of serotonin N-acetyltransferase (NAT) activity in chick pineal gland were performed. Six-hour pulses of white light caused an acute suppression of the nocturnal NAT activity and phase-dependent phase shifts of the circadian rhythm of the enzyme activity relative to controls. Systemic administration of clonidine acutely suppressed NAT activity of chick pineal gland, but did not affect the phase of subsequent cycles in constant darkness. These results give further support to the concept based on in vitro studies that alpha 2-adrenergic receptors are involved in regulation of melatonin biosynthesis in chick pineal gland by a mechanism distal to the pacemaker that generates the circadian melatonin rhythm.

Calcium and melatonin production in dissociated trout pineal photoreceptor cells in culture

Trout pineal cells maintained in primary culture produce melatonin in high amounts during night time and low amounts during daytime. The dark-induced increase in melatonin production was enhanced, in a dose-dependent manner, by elevating extracellular calcium concentration. Low external calcium concentration reduced nocturnal and diurnal melatonin production. Bay K 8644 increased, in a dose-dependent manner, the dark-induced rise in melatonin output, and this effect was antagonized by nifedipine and verapamil. This suggests a role for the dihydropyridine calcium channels in the regulation of the melatonin output. To confirm this, patch-clamp recordings (whole-cell perforated) were run in a 20 mmol/l barium medium at different holding potentials from -80 mV. A voltage-dependent inward current was activated from -30 mV to +40 mV with a maximal amplitude being observed at 0 mV. This current was drastically increased in the presence of Bay K 8644. Nifedipine inhibited the current both in the absence or in the presence of Bay K 8644. Our results are consistent with the idea that extracellular calcium participates in the control of melatonin secretion by photoreceptor cells. It is suggested that activation of the voltage-dependent L-type channel may modulate this secretion.

Immunocytochemical localisation of hydroxyindole-O-methyltransferase in pineal photoreceptor cells of several fish species

Melatonin is an internal "Zeitgeber," involved in the timing and control of a number of rhythmic functions and behaviours. Its synthesising cells remain to be identified in the fish pineal. The last step in the melatonin biosynthetic pathway is catalysed by the enzyme hydroxyindole-O-methyltransferase. An affinity-purified antibody, directed against chicken pineal hydroxyindole-O-methyltransferase, was used in the present study to identify the melatonin synthesising cells in four fish species: a primitive chondrostean (sturgeon), a saltwater teleost (dorado), and two freshwater teleosts (pike, trout). Western blot immunolabeling of pike and trout pineal proteins revealed a single band at 38 KDa, which corresponds to the known molecular weight of the enzyme in bovine, rat, and chicken pineal. Regardless of the species, a specific immunocytochemical labeling, visualised by means of the peroxidase-antiperoxidase method, was exclusively associated with the photoreceptor cells. These results provide evidence that photoreceptors of the fish pineal are responsible for the biosynthesis of 5-methoxyindoles, including melatonin. In the pike, reactions were less intense in the distal portion of the pineal vesicle than in the other regions of the organ. It is questioned whether this might be related to the existence of a germinative zone, generating new photoreceptor cells in this distal portion. Hydroxyindole-O-methyltransferase has been previously demonstrated in mammalian pinealocytes, and modified photoreceptors of the avian pineal. It is now demonstrated in pineal photoreceptors of a primitive fish and of more evolved saltwater and freshwater fish. The results strengthen the view that these cells are related through phylogeny and that their well conserved melatoninergic function appears early in the course of evolution.

Multiple circadian oscillators in the photosensitive pike pineal gland: a study using organ and cell culture

The fish pineal organ contains typical and, in some species, modified photoreceptor cells involved in the photoperiodic control of melatonin production. In the majority of species studied, the rhythm in melatonin production is driven by an intra-pineal circadian oscillator synchronized by the light:dark cycle. In the present study, it is shown that the endogenous rhythm in melatonin release of superfused pike pineals maintained under constant darkness is expressed at temperatures of 19 degrees C, 20 degrees C, 25 degrees C, and 30 degrees C (period > 24 hr), but not at temperatures of 10 degrees C and 15 degrees C. Under constant darkness, pineal fractions containing either typical photoreceptors, modified photoreceptors, or both behaved like total organs. Dissociated pike pineal cells, cultured statically at 20 degrees C, expressed a high amplitude rhythm in melatonin secretion under a light:dark cycle. Under constant darkness, circadian oscillations, which appeared better sustained than in organ culture, were also observed. This study provides the first evidence that the rhythmic production of melatonin, by a fish pineal, is driven by a population of circadian oscillators or clocks. It is hypothesized that each typical and modified photoreceptor might be the locus of a circadian clock. Damping of the overall rhythm under constant darkness might reflect the desynchronization (uncoupling) between these clocks and/or damping of individual oscillators.

Calmodulin immunoreactivity in the chicken pineal gland: comparison with calbindin-D28k, calretinin, and S100

Calmodulin distribution in the chicken pineal organ was investigated by immunohistochemistry. Calmodulin immunoreactivity was detected in ependymocytes in the follicular zone and in interstitial cells in the parafollicular zone. No calmodulin immunoreactivity was detected in pinealocytes. Lack of calmodulin immunoreactivity in pinealocytes raises questions about its proposed function in melatonin synthesis as suggested by pharmacological studies using calmodulin antagonists. The calmodulin distribution was comparable to that of S100, a glial cell marker. Two other markers, calbindin-D28k and calretinin, which in neuroanatomical studies give excellent cytoarchitectonic staining, in the chick pineal permitted the detection of two subclasses of pinealocytes. One was darkly stained by calbindin-D28k and rare. The other was very abundant and calretinin positive. In the parafollicular zone, calbindin-D28k and/or calretinin antibodies allowed us to visualize cells presenting a neuron-like morphology. Calretinin immunoreactivity was detected in nearly all pinealocytes in which hydroxy-indol-O-methyl transferase was also located. Comparison between the lack of calmodulin and the presence of calretinin, belonging to the same calcium-binding protein family, in chick pinealocytes raises the hypothesis about a possible role of calretinin in melatonin synthesis.

Phototransduction-related circadian changes in indoleamine metabolism in the chick pineal gland in vivo

The purpose of this study was to examine the day/night levels of pineal melatonin and its rate limiting enzyme N-acetyltransferase (NAT) in relationship to the ratio of 11-cis- to all-trans-retinal. Three-week-old chicks were placed in 12:12 light:dark (LD 12:12) cycle for one week, pineals were collected during the light phase at 1500 (i.e., after 10 hr light), during the dark phase at 1900 (i.e., 2 hr after dark), at 2100 (i.e., 4 hr after dark), and at 2300 (i.e., 6 hr after dark) and after light extension to 1900. The results show that light-sensitive 11-cis-retinal in the chick pineal has the same diurnal rhythm as NAT and melatonin; all constituents increased within 2 hr of darkness onset (at 1900) and reached their peak after 4 hr of dark. All values were lowest during the light phase at 1500. Low values for 11-cis-retinal, NAT, and melatonin were also seen in the group of chicks which experienced light extension to 1900. The data indicate that in vivo light plays a major role in triggering rhodopsin-bound 11-cis-retinal production within 2-4 hr after darkness onset; this change likely serves as the signal for the subsequent formation of the hormonal product of the pineal gland, melatonin.

Localization of mRNA encoding the indolamine synthesizing enzyme, hydroxyindole-O-methyltransferase, in chicken pineal gland and retina by in situ hybridization

Hydroxyindole-O-methyltransferase (HIOMT) is the enzyme that catalyzes the final step in the synthesis of the hormone melatonin. We have examined the localization of expression of the mRNA encoding HIOMT by in situ hybridization in the 3-day-old and adult chicken retina and pineal gland. The riboprobe utilized for this study was transcribed from the complete coding region of HIOMT cDNA synthesized from retina RNA and amplified by polymerase chain reaction (PCR). High levels of HIOMT mRNA were present in the pinealocytes of the pineal gland. In the retina, the hybridization signal was localized to the photoreceptors. The retinal photoreceptors of the 3-day-old chick displayed a much lower level of hybridization than did the photoreceptors of the adult chicken. This study strongly suggests that the photoreceptors are the sites of melatonin synthesis in the retina.

Long-term effects of constant light or darkness on chicken pineal hydroxyindole-O-methyltransferase expression: biochemical and cellular aspects

1. Chickens kept in constant light, as opposed to constant darkness, display a twofold increase in the activity of pineal hydroxyindole-O-methyltransferase (HIOMT), the last acting enzyme in the melatonin pathway. 2. Using an immunological approach, we presently show that this regulation of HIOMT activity reflects changes in the concentration of a single molecular form of the enzyme protein (a 38 kDa polypeptide). Immunohistofluorescence indicates that these concentration changes concurrently affect modified photoreceptors and pinealocyte-like cells in the chicken pineal organ. 3. Together, the present data support the hypothesis that environmental lighting might regulate the expression of the HIOMT gene.