Pineal gene expression: dawn in a dark matter

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

SPACRCAN in the interphotoreceptor matrix of the mouse retina: molecular, developmental and promoter analysis

SPACRCAN is a novel proteoglycan present in the interphotoreceptor matrix (IPM) of the rat and human retina that resists aqueous extraction through its binding to hyaluronan. The purpose of this study was: to clone mouse Spacrcan; to characterize the promoter elements; to define the deduced amino acid sequence; to establish the time of Spacrcan expression during retinal development; and to determine the time of appearance and distribution of SPACRCAN protein. Spacrcan cDNA clone was obtained through PCR amplification of a mouse retina cDNA library, and RT-PCR amplification and 5'RACE of mouse retina RNA. The deduced polypeptide sequence of mouse SPACRCAN contains a signal peptide at the N-terminal, seven N-link glycosylation sites, numerous potential O-linked glycosylation sites in a central mucin-like domain, two glycosaminoglycan attachment sites, five potential hyaluronan-binding motifs, two epidermal growth factor-like domains, and a hydrophobic stretch of 23 amino acids near the C-terminal. Comparison of the genomic structure of mouse and human SPACRCAN showed significant structure conservation. Analysis of the promoter region revealed several important putative regulatory elements including a Ret-1/PCE-1 element, an 11 base motif for Crx binding, six copies of PIRE, a Ret-4 element, three copies of AP-1, a CRE element, and five copies of GATA3. Northern blot analysis and immunohistochemistry were used to determine the tissue specificity of Spacrcan mRNA and to localize SPACRCAN in developing retina. Spacrcan mRNA is expressed in both retina and pineal gland and was detectable as early as embryonic day 15. The protein is first detectable in the IPM at postnatal day 8 where it increases in concert with the extension of photoreceptor inner and outer segments from the outer retinal surface. The presence of several unique regulatory elements in the promoter region and characteristic molecular features shared with the orthologue in human and rat suggest an important functional role of SPACRCAN in the IPM. The time of appearance of the SPACRCAN protein during retinal development suggests that this matrix protein may establish the extracellular microenvironment into which photoreceptor outer segments are elaborated.

Rhythmic gene expression in pituitary depends on heterologous sensitization by the neurohormone melatonin

In mammals, many daily cycles are driven by a central circadian clock, which is based on the cell-autonomous rhythmic expression of clock genes. It is not clear, however, how peripheral cells are able to interpret the rhythmic signals disseminated from this central oscillator. Here we show that cycling expression of the clock gene Period1 in rodent pituitary cells depends on the heterologous sensitization of the adenosine A2b receptor, which occurs through the nocturnal activation of melatonin mt1 receptors. Eliminating the impact of the neurohormone melatonin simultaneously suppresses the expression of Period1 and evokes an increase in the release of pituitary prolactin. Our findings expose a mechanism by which two convergent signals interact within a temporal dimension to establish high-amplitude, precise and robust cycles of gene expression.

Of rodents and ungulates and melatonin: creating a uniform code for darkness by different signaling mechanisms

Melatonin synthesis in the mammalian pineal gland is one of the best investigated output pathways of the circadian clock because it can be readily measured and is tightly regulated by a clearly defined input, the neurotransmitter norepinephrine. In this system, a regulatory scenario was deciphered that is centered around the cyclic AMP pathway but shows peculiar species-specific differences. In rodents, the cyclic AMP-mediated, temporally sequential up-regulation of two transcription factors, the activator CREB (cyclic AMP-responsive element-binding protein) and the inhibitor ICER (inducible cyclic AMP-dependent early repressor), is the core mechanism to determine rhythmic accumulation of the mRNA encoding for the rate-limiting enzyme in melatonin synthesis, the arylalkylamine N-acetyltransferase (AA-NAT). Thus, in rodents, the regulation of melatonin synthesis bears an essential transcriptional component, which, however, is flanked by posttranscriptional mechanisms. In contrast, in ungulates, and possibly also in primates, AA-NAT appears to be regulated exclusively on the posttranscriptional level. Here, increasing cyclic AMP levels inhibit the breakdown of constitutively synthesized AA-NAT protein by proteasomal proteolysis, leading to an elevated enzyme activity. Thus, self-restriction of cellular responses, as a reaction to external cues, is accomplished by different mechanisms in pinealocytes of different mammalian species. In such a temporally gated cellular adaptation, transcriptionally active products of clock genes may play a supplementary role. Their recent detection in the endogenously oscillating nonmammalian pineal organ and, notably, also in the slave oscillator of the mammalian pineal gland underlines that the mammalian pineal gland will continue to serve as an excellent model system to understand mechanisms of biological timing.

Transcriptional regulation of L-type calcium channel expression in cardiac myocytes

The L-type calcium channel is a heteromultimeric protein complex, which is expressed in the cardiac sarcolemma. Although post-translational regulation of its subunits by protein kinase A (PKA) has been widely reported, little is known about molecular processes that regulate expression of calcium channel subunits (alpha(1C), alpha(2)- delta, and beta(2A)subunits). Previous studies from our group demonstrate that the steady-state mRNA level of the alpha(1C)unit is increased by treatment of myocytes with beta -adrenergic agonists. The current study is designed to determine whether the mRNA levels for all subunits of the L-type calcium channel are coordinately controlled by a beta -adrenergic agonist, and whether this occurs predominantly through control of rate of transcription. Nuclear run-on assays were used to determine the transcription initiation rate of these genes in cultured neonatal rat cardiac myocytes. In isoproterenol (10(-7)m)-treated myocytes, transcription of genes encoding the alpha(1C), alpha(2)- delta, and beta(2A)subunits was enhanced. The increases in transcription initiation rate for alpha(1C), alpha(2)- delta, and beta(2A)subunits genes were 404%, 367%, and 240% of control, respectively. Pretreatment with the beta -adrenergic antagonist propranolol (10(-5)m) or PKA inhibitor H-89 (10(-6)m) blocked the effects of isoproterenol, while either drug alone did not affect the gene transcription rate significantly. Steady state mRNA levels of the subunits increased following isoproterenol treatment. These results suggest that beta -adrenergic stimulation and the PKA signaling pathway play an important role in transcriptional regulation of the L-type calcium channel in myocyte. The expression of all the subunits of this ion channel is under coordinate transcriptional control.

Melatonin limits transcriptional impact of phosphoCREB in the mouse SCN via the Mel1a receptor

In the mouse, activity phase-shifts of the endogenous clock in the suprachiasmatic nucleus (SCN) are associated with phosphorylation of the transcription factor Ca2+/cAMP responsive element binding protein (CREB). CREB phosphorylation is induced by the retino-hypothalamic transmitter pituitary adenylate cyclase-activating polypeptide (PACAP). As detected by immunohistochemistry in SCN slices from wild-type mice, melatonin completely blocked PACAP-stimulated CREB phosphorylation at low concentrations (1 nM). In Mel1a melatonin receptor-deficient mice, the PACAP-induced CREB phosphorylation was inhibited only at melatonin concentrations of 100 nM. This inhibition was, however, blunted by blocking the Mel1b melatonin receptor. Thus, melatonin modulates PACAP-mediated retinal stimuli for clock entrainment primarily via the Mel1a melatonin receptor through molecular interaction within the cAMP-signalling pathway.

Signal transduction in the rodent pineal organ. From the membrane to the nucleus

The rodent pineal organ transduces a photoneural input into a hormonal output. This photoneuroendocrine transduction leads to highly elevated levels of the hormone melatonin at night-time which serves as a message for darkness. The melatonin rhythm depends on transcriptional, translational and posttranslational regulation of the arylalkylamine-N-acetyltransferase, the key enzyme of melatonin biosynthesis. These regulatory mechanisms are fundamentally linked to two second messenger systems, namely the cAMP- and the Ca(2+)-signal transduction pathways. Our data gained by molecular biology, immunohistochemistry and single-cell imaging demonstrate a time- and substance-specific activation of these signaling pathways and provide a framework for the understanding of the complex signal transduction cascades in the rodent pineal gland which in concert not only regulate the basic profile but also fine-tune the circadian rhythm in melatonin synthesis.

Transcription factor dynamics and neuroendocrine signalling in the mouse pineal gland: a comparative analysis of melatonin-deficient C57BL mice and melatonin-proficient C3H mice

In rodents, the nocturnal rise and fall of arylalkylamine N-acetyltransferase (AANAT) activity controls the rhythmic synthesis of melatonin, the hormone of the pineal gland. This rhythm involves the transcriptional regulation of the AANAT by two norepinephrine (NE)-inducible transcription factors, e.g. the activator pCREB (phosphorylated Ca2+/cAMP-response element binding protein) and the inhibitor ICER (inducible cAMP early repressor). Most inbred mouse strains do not produce melatonin under standard laboratory light/dark conditions. As melatonin-deficient mice are often the founders for transgenic animals used for chronobiological experimentations, molecular components of neuroendocrine signalling in the pineal gland as an integral part of clock entrainment mechanisms have to be deciphered. We therefore compared calcium signalling, transcriptional events and melatonin synthesis in the melatonin-deficient C57BL mouse and the melatonin-proficient C3H mouse. Pineal glands and primary pinealocytes were cultured and stimulated with NE or were collected at various times of the light/dark (LD) cycle. Changes in intracellular calcium concentrations, the phosphorylation of CREB, and ICER protein levels follow similar dynamics in the pineal glands of both mouse strains. pCREB levels are high during the early night and ICER protein shows elevated levels during the late night. In the C57BL pineal gland, a low but significant increase in melatonin synthesis could be observed upon NE stimulation, and, notably, also when animals were exposed to long nights. We conclude that the commonly used C57BL mouse is not completely melatonin-deficient and that this melatonin-deficiency does not affect molecular details involved in regulating transcriptional events of melatonin synthesis.

CREB phosphorylation and melatonin biosynthesis in the rat pineal gland: involvement of cyclic AMP dependent protein kinase type II

Phosphorylation of cyclic AMP response element binding protein (CREB) at amino acid serine 133 appears as an important link between the norepinephrine (NE)-induced activation of second messenger systems and the stimulation of melatonin biosynthesis. Here we investigated in the rat pineal gland: 1) the type of protein kinase that mediates CREB phosphorylation: and 2) its impact on melatonin biosynthesis. Immunochemical or immunocytochemical demonstration of serine133-phosphorylated cyclic AMP regulated element binding protein (pCREB) and radioimmunological detection of melatonin revealed that only cyclic AMP-dependent protein kinase (PKA) inhibitors suppressed NE-induced CREB phosphorylation and stimulation of melatonin biosynthesis, whereas inhibitors of cyclic GMP-dependent protein kinase (PKG), mitogen-activated protein kinase kinase, protein kinase C, or calcium-calmodulin-dependent protein kinase (CaMK) were ineffective. Investigations with cyclic AMP-agonist pairs that selectively activate either PKA type I or II link NE-induced CREB phosphorylation and stimulation of melatonin biosynthesis to the activation of PKA type II. Our data suggest that PKA type II plays an important role in the transcriptional control of melatonin biosynthesis in the rat pineal organ.

The onset of increased melatonin secretion after the onset of darkness in sheep depends on the photoperiod

In sheep, melatonin secretion occurs rapidly after the onset of darkness, but the interval fluctuates according to different authors. The aim of this study was to determine this interval in sheep subjected to a long or a short photoperiod. Blood samples were taken at the right jugular vein every 100 s for 1 hr after the onset of darkness. The experiment was repeated on three consecutive days either in long (LD 16:8) or in short photoperiod (LD 8:16) on the same animals. Melatonin secretion was found to begin within 11 min under long photoperiod and 20 min under short period. It can be concluded that the onset of melatonin secretion depends on the duration of the photoperiod.

Inducible cyclic AMP early repressor protein in rat pinealocytes: a highly sensitive natural reporter for regulated gene transcription

Rhythmic activity of arylalkylamine N-acetyltransferase (AANAT) determines melatonin synthesis in rat pineal gland. The transcriptional regulation of AANAT involves the activating and inhibiting transcription factors of the cyclic AMP (cAMP)-signaling pathway, cAMP response element-binding protein and inducible cAMP early repressor (ICER), respectively. Activation of this pathway is centered around norepinephrine, stimulating beta(1)-adrenergic receptors, but various other transmitters can modulate melatonin biosynthesis. To compare the transcriptional impact of norepinephrine with that of other neurotransmitters on melatonin synthesis, we determined ICER protein levels in pinealocytes and, in parallel, hormone secretion. The dose-dependent inductions of ICER protein by norepinephrine, the beta(1)-adrenergic receptor agonist isoproterenol, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, and adenosine are correlated to regulatory dynamics in melatonin production. Importantly, ICER protein induction required lower ligand concentrations than the induction of melatonin biosynthesis. Although neuropeptide Y, glutamate, and vasopressin altered norepinephrine-stimulated hormone production without affecting ICER levels, the activation of voltage-gated cation channels increased ICER without affecting hormone synthesis. Sensitivity and versatility of ICER induction in pinealocytes make these neuroendocrine cells a valuable model system in which to study molecular interactions determining a regulated gene expression.

Evidence against alpha2-adrenoceptor involvement in the regulation of rat melatonin synthesis by ambient lighting

This study was carried out to clarify the role of alpha2-adrenoceptors in the regulation of pineal melatonin synthesis. Medetomidine, a selective alpha2-adrenoceptor agonist, was previously found to be a potent suppressor of nocturnal melatonin levels in rats. Medetomidine and alpha2-adrenoceptor antagonists atipamezole and yohimbine were injected into rats in different conditions, and their pineal melatonin contents were measured by radioimmunoassay. Experiment 1: Blocking the alpha2-adrenoceptors and possible non-adrenergic binding sites with atipamezole did not counteract the light-induced suppression of nocturnal melatonin. These receptors are, thus, not essential for the suppression of melatonin by light. Experiment 2: Blocking the alpha2-adrenoceptors with atipamezole or yohimbine did not sensitize the pineal melatonin synthesis to daytime darkness in the light/dark-entrained rats. The binding sites are not involved in keeping the daytime melatonin levels low, even in darkness. Experiment 3: The rats were sensitized to daytime darkness by keeping them for seven days in constant light. The dark-elicited melatonin rise was suppressed by a lower dose of medetomidine than the normal nocturnal rise in light/dark-entrained rats, while atipamezole had no effect. The results showed that alpha2-adrenoceptor insufficiency is not involved in the constant light-induced pineal supersensitivity. In summary, the experiments indicated that the physiological regulation of melatonin synthesis by ambient lighting in rats does not depend on alpha2-adrenergic mechanisms.

Synaptic-like microvesicles in mammalian pinealocytes

The recent deciphering of the protein composition of the synaptic vesicle membrane has led to the unexpected identification of a compartment of electron-lucent microvesicles in neuroendocrine cells which resemble neuronal synaptic vesicles in terms of molecular structure and function. These vesicles are generally referred to as synaptic-like microvesicles (SLMVs) and have been most intensively studied in pancreatic beta-cells, chromaffin cells of the adrenal medulla, and pinealocytes of the pineal gland. This chapter focuses on the present knowledge of SLMVs as now well-established constituents of mammalian pinealocytes. I review the results of morphological, immunocytochemical, and biochemical studies that were important for the characterization of this novel population of secretory vesicles in the pineal organ. The emerging concept that SLMVs serve as a device for intercellular communication within the pineal gland is outlined, and unanswered questions such as those pertaining to the physiological function and regulation of pineal SLMVs are discussed.

Transcription factors in neuroendocrine regulation: rhythmic changes in pCREB and ICER levels frame melatonin synthesis

Neurotransmitter-driven activation of transcription factors is important for control of neuronal and neuroendocrine functions. We show with an in vivo approach that the norepinephrine cAMP-dependent rhythmic hormone production in rat pineal gland is accompanied by a temporally regulated switch in the ratio of a transcriptional activator, phosphorylated cAMP-responsive element-binding protein (pCREB), and a transcriptional inhibitor, inducible cAMP early repressor (ICER). pCREB accumulates endogenously at the beginning of the dark period and declines during the second half of the night. Concomitant with this decline, the amount of ICER rises. The changing ratio between pCREB and ICER shapes the in vivo dynamics in mRNA and, thus, protein levels of arylalkylamine-N-acetyltransferase, the rate-limiting enzyme of melatonin synthesis. Consequently, a silenced ICER expression in pinealocytes leads to a disinhibited arylalkylamine-N-acetyltransferase transcription and a primarily enhanced melatonin synthesis.

A semiquantitative image-analytical method for the recording of dose-response curves in immunocytochemical preparations

Knowledge about intracellular signal transduction cascades is largely based on investigations of cultured cells whose responses to different stimuli are typically quantified via RIA, ELISA, or immunoblots. These techniques, which require relatively large amounts of biological material, are performed with homogenized cells and therefore do not allow localization of the molecules under investigation. We describe a protocol for recording dose-response curves directly from immunocytochemical preparations using rat pinealocytes as a model system. The cells were exposed to beta-adrenergic stimuli inducing the phosphorylation of the transcription factor CREB (mediated by PKA), an increase in ICER protein levels, and synthesis and release of melatonin. Melatonin concentrations were determined by ELISA. cPKA, phosphorylated CREB, and ICER were demonstrated by immunocytochemistry and immunoblots. Dose-response curves were recorded by measuring the integrated density of the immunoreactive sites with an image analysis program. Dose-response curves from immunoblots and immunocytochemical preparations showed almost identical dynamics, validating the immunocytochemical approach, which minimizes the amount of biological material needed for such studies, allows combined quantification and localization of biomolecules, and may even be more sensitive than immunoblotting.

Rhythmic variation in beta1-adrenergic receptor mRNA levels in the rat pineal gland: circadian and developmental regulation

In the rat pineal gland noradrenaline is released in large quantities from sympathetic nerve endings at the onset of darkness, thereby driving rhythmic melatonin synthesis with elevated levels at night-time. Upon release, noradrenaline interacts with postsynaptic beta1-adrenergic receptors to activate the cyclic AMP signalling pathway. Well characterized third messengers of this signalling cascade affect cyclic AMP-inducible genes that are crucially involved in initiation, maintenance and termination of hormone production. Among these third messengers are CREB (cyclic AMP responsive element binding protein) as an activating and ICER (inducible cyclic AMP early repressor) as an inhibitory transcription factor. Because a cyclic AMP-inducible promoter element is present on the beta1-adrenergic receptor gene, the expression of the receptor itself may be under control of the cyclic AMP-signalling pathway. By in situ hybridization, Northern blot analysis and RT-PCR we demonstrate a day/night rhythm in beta1-adrenergic receptor mRNA in the rat pineal gland with elevated levels during the dark period. As this rhythm persists, under constant darkness but is abolished upon removal of the sympathetic innervation, it is truly circadian. A marked day/night difference in the levels of beta1-adrenergic receptor mRNA becomes evident only after postnatal day 10, coinciding with the appearance of a functional cyclic AMP signalling pathway in the rat pineal gland. Furthermore, targeting ICER expression by transfection of pinealocytes with an antisense ICER construct, clearly indicates that the levels of the beta1-adrenergic receptor mRNA are regulated by the cyclic AMP-signalling pathway in a feedback mechanism.

Ontogeny of a diurnal rhythm in arylalkylamine-N-acetyltransferase mRNA in rat pineal gland

Melatonin synthesis in the pineal gland of adult rats is linked to cAMP-dependent transcriptional and post-transcriptional regulatory mechanisms affecting its rate-limiting enzyme, the arylalkylamine-N-acetyltransferase (AA-NAT). During development of the pineal gland, neuronal control gains access to the earlier matured cAMP-signaling pathway to shape the day-night rhythm in AA-NAT enzymatic activity. By semiquantitative in situ hybridization we analyzed if the developmental onset of a rhythmic AA-NAT activity is correlated to a temporally parallel onset in AA-NAT transcription. We found that AA-NAT mRNA levels in rat pineal gland become rhythmic at postnatal day 5. Thus, AA-NAT gene transcription in rat pineal gland starts to show day-night differences shortly prior to the appearance of a rhythmic AA-NAT activity.

The validity of melatonin as an oncostatic agent

The validity of melatonin as a prominent, naturally occurring oncostatic agent is examined in terms of its putative oncostatic mechanism of action, the correlation between melatonin levels and neoplastic activity, and the outcome of therapeutically administered melatonin in clinical trials. Melatonin's mechanism of action is summarized in a brief analysis of its actions at the cellular level, its antioxidative functions, and its indirect immunostimulatory effects. The difficulties of interpreting melatonin levels as a diagnostic or prognostic aid in cancer is illustrated by referral to breast cancer, the most frequently studied neoplasm in trials regarding melatonin. Trials in which melatonin was used therapeutically are reviewed, i.e., early studies using melatonin alone, trials of melatonin in combination with interleukin-2, and controlled studies comparing routine therapy to therapy in combination with melatonin. A table compiling the studies in which melatonin was used in the treatment of cancer in humans is presented according to the type of neoplasm. Melatonin's suitability in combination chemotherapy, where it augments the anticancer effect of other chemotherapeutic drugs while decreasing some of the toxic side effects, is described. Based on the evidence derived from melatonin's antiproliferative, antioxidative, and immunostimulatory mechanisms of action, from its abnormal levels in cancer patients and from clinical trials in which melatonin was administered, it is concluded that melatonin could indeed be considered a physiological anticancer substance. Further well-controlled trials should, however, be performed in order to find the link between its observed effects and the underlying mechanisms of action and to define its significance as a therapeutic oncostatic agent.

Signal transduction molecules in the rat pineal organ: Ca2+, pCREB, and ICER

The mammalian pineal organ transduces light-dependent neural inputs into a hormonal output. This photoneuroendocrine transduction results in a largely elevated concentration of the pineal hormone melatonin at night. The rhythm in melatonin production and secretion depends on activation and inactivation of transcriptional, translational, and posttranslational mechanisms fundamentally linked to two second messenger systems, the cAMP- and the Ca(2+)-signal transduction pathways. Here we review molecular biological, immunocytochemical, and single-cell imaging studies, which demonstrate a time- and substance-specific activation of these signaling pathways in rat pinealocytes. The data provide a framework for understanding the complex interactions between second messengers (cAMP, Ca2+), transcription factors (CREB, ICER), and their role in regulation of melatonin synthesis. The data have proven the rat pinealocyte to be an interesting model to study transmembrane signaling pathways which may be common to both neuroendocrine and neuronal cells.