From the Section on Neuroendocrinology, Laboratory of
Developmental Neurobiology, NICHD, National Institutes of Health,
Bethesda, Maryland 20892-4480 and the § Laboratoire de
Neurobiologie Cellulaire, Département des Neurosciences, CNRS-UMR
6558, F-86022 Poitiers Cedex, France
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ABSTRACT |
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Serotonin N-acetyltransferase
(arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is
the first enzyme in the conversion of serotonin to melatonin. Large
changes in AANAT activity play an important role in the daily rhythms
in melatonin production. Although a single AANAT gene has been found in
mammals and the chicken, we have now identified two AANAT genes in
fish. These genes are designated AANAT-1 and
AANAT-2; all known AANATs belong to the AANAT-1
subfamily. Pike AANAT-1 is nearly exclusively
expressed in the retina and AANAT-2 in the pineal gland.
The abundance of each mRNA changes on a circadian basis, with
retinal AANAT-1 mRNA peaking in late afternoon and
pineal AANAT-2 mRNA peaking 6 h later. The pike
AANAT-1 and AANAT-2 enzymes (66% identical
amino acids) exhibit marked differences in their affinity for
serotonin, relative affinity for indoleethylamines versus
phenylethylamines and temperature-activity relationships. Two AANAT
genes also exist in another fish, the trout. The evolution of two
AANATs may represent a strategy to optimally meet tissue-related
requirements for synthesis of melatonin: pineal melatonin serves an
endocrine role and retinal melatonin plays a paracrine role.
Melatonin is synthesized in two vertebrate tissues, the pineal
gland and the retina (1-5). Production of melatonin is almost always
under control of a circadian clock, with the highest levels occurring
at night (6-10). The pineal gland produces circulating melatonin,
which serves as an endocrine signal of the night period. This signal of
night length is important in circadian and seasonal physiology (11,
12). In contrast, retinal melatonin functions as a paracrine signal
within the retina to enhance retinal functioning under low light
conditions. This is done by shifting the positions of the rods and
cones and regulating the turnover rates of the photoreceptive
apparatuses of the rods and cones and surrounding pigment epithelium
(5, 13, 14).
The enzyme which controls the circadian pattern of melatonin synthesis
(serotonin It has been generally thought that a single AANAT gene is expressed in
both the pineal gland and retina. AANATs from various species are
characterized by three highly conserved peptide sequences (C/c-1,
D/c-1, and D/c-2; see Fig. 1) that have been proposed to be involved in
arylalkylamine binding (15, 17).
The single AANAT gene hypothesis is consistent with the results from
studies in mammals and the chicken, but not with those from studies in
teleosts (bony fishes). Biochemical investigations of the Northern pike
indicate that pineal and retinal AANAT have distinctly different
affinities toward arylalkylamine substrates and different
temperature-activity relationships (18). Furthermore, mRNA analyses
in both pike and trout indicate that a trout pineal-derived AANAT probe
does not detect retinal AANAT mRNA (19). These observations are
reconciled by the results presented here demonstrating the existence of
a second AANAT gene in teleosts.
cDNA Synthesis, Reverse Transcriptase PCR, and Genomic
PCR--
RNA was extracted from pike, trout, or sturgeon pineal glands
and retinae as described below. cDNA was synthesized on oligo-dT magnetic beads (Dynal) as described previously (20). PCR was conducted
in a 50-µl volume containing 10 mM Tris (pH 8.3), 50 mM KCl, 1.5 mM MgCl2, 200 µM dNTPs, 500 nM primers, and 0.5 units of
ExTaq DNA polymerase (TaKaRa). The cycling regime was as follows: 94 °C for 5 min; 30-35 cycles of annealing at 55-60 °C
(55 °C was used for primers from a species different from the
cDNA; 60 °C was used when primers and DNA were from the same
species) and extension at 72 °C for 1 min; and final extension at
72 °C for 9 min. Conditions for amplification of genomic DNA were
the same as for cDNA, except the annealing temperature was 58 °C
and 50 ng of DNA was used as template. PCR products were cloned into pGEM-T-Easy (Promega) for sequencing.
Determination of the Sequence of AANAT cDNAs--
The
initial step leading to identification of two pike AANATs (pAANATs)
involved reverse transcriptase PCR generation of fragments of AANAT
from sturgeon and trout pineal cDNA. A 179-bp sturgeon fragment
(clone S12) was obtained using the sheep-based primers 562 and 563 (Table I). A 138-bp trout fragment (clone
T14/9) was then obtained using primer 562 and primer S02R, which was based on the sequence of the sturgeon clone S12. A longer trout fragment (clone T3; 184 bp) was obtained using primers T01R (based on
trout clone T14/9) and D07L (a degenerate primer based on mammalian AANATs). Trout clone T3 overlapped with the sturgeon clone S12 by 78 bp
and was 63% identical within the overlap region. Probes derived from
these clones generated different patterns when used in Northern blot
analysis: the sturgeon pineal-based probe (S12) detected multiple bands
of mRNA in the trout and pike pineal gland and only a single band
in the retina from these species, whereas the trout pineal-based probe
(T3) only detected mRNA in the pike and trout pineal gland. Based
on this, we hypothesized that two AANAT genes are expressed in trout
and pike, with the one corresponding to S12 expressed preferentially in
the teleost retina and the second corresponding to T3 expressed
preferentially in the teleost pineal gland.
Sturgeon AANAT-derived primers S03L and S02R were used with pike
retinal cDNA to obtain clone 11/15; primers derived from clone
11/15 were used to obtain a full-length sequence from pike retinal
cDNA by 3'-RACE and 5'-RACE (see below). Trout AANAT-derived primers T04L and T05R were used with pike pineal cDNA to obtain clone 2/6; primers derived from clone 2/6 were used to obtain a
full-length sequence from pike pineal mRNA by 3'-RACE and
5'-RACE.
3'-RACE was performed using pike AANAT-specific nested primers in
conjunction with oligo-dT primers (GAATTCT20N). The initial round of PCR consisted of 10 cycles with the first specific primer only
at 60 °C annealing temperature, followed by the addition of oligo-dT
primers and continuing the PCR at 50 °C annealing temperature for 5 cycles then at 60 °C for 20 cycles. An aliquot of this first
reaction was used as a template for nested PCR employing 30 cycles at
60 °C. To obtain sequence 3' of clone 11/15, immobilized pike
retinal cDNA was amplified using nested primers (P05L and P06L)
based on clone 11/15 sequence. To obtain sequence 3' of clone 2/6, pike
pineal cDNA was amplified using nested primers (P01L and P02L)
based on clone 2/6 sequence.
5'-RACE was performed by a modification of a previously described
method using nested primers from the initial pike AANAT fragments and a
primer corresponding to the amplimer ligated to the end of the cDNA
as described (20). To obtain sequence 5' of clone 11/15, anchor
oligonucleotide B was ligated onto immobilized retinal cDNA, and
this cDNA was amplified using anchor primer AMPB and nested primers
P32R and P30R, which were based on clone 11/15 sequence. To obtain
sequence 5' of clone 2/6, anchor oligonucleotide A was ligated onto
immobilized pineal cDNA, and this cDNA was amplified using
anchor primer AMPA and clone 2/6-based primer P04R. A second round of
5'-RACE was done to further extend clone 2/6 using AMPA with nested
primers P10R and P40R, which were based on the sequence of the product
of the previous round.
The full-length sequence of AANAT expressed in the trout pineal gland
was determined using a strategy similar to that used to determine the
sequence of the AANAT mRNA expressed in the pike pineal gland.
3'-RACE used nested primers T03L and T04L. Two rounds of 5'-RACE were
employed; the first used nested primers T01R and T05R while the second
used T01R along with primer T10R, which was based on the product of the
first round.
Multiple clones of all 3'- and 5'-RACE products were sequenced, and
consensus sequences were generated. The contiguity of 3'- and
5'-flanking regions and the locations of intron/exon junctions were
determined by PCR of genomic DNA. Clones containing the full ORFs were
constructed as described below. Sequence analysis and data base
searching were conducted using the GCG package from the University of
Wisconsin (21).
Animal Culture--
Pike (Esox lucius; 300-1,500 g;
male and female) were collected (mid-October to mid-March) from ponds
of the Poitou-Charentes (France) and held in a commercial hatchery in
small ponds supplied with river water under natural conditions. Pike
were adapted for at least 24 h to the experimental photoperiod
which reflected natural conditions (except dim red light was
substituted for the dark period). Fish were killed by decapitation at
selected times during 24-h cycles of white light/red light, constant
white light, or constant red light conditions. Each lighting regime was
set up and sampled during three sequential weeks. The entire experiment was conducted once in October and repeated in February; the natural lighting regime during both periods was approximately 11 h
light/13 h dark. Tissues were obtained by rapid dissection and were
frozen in liquid nitrogen for subsequent analysis.
Northern Blot Analysis--
RNA extraction and Northern blot
analysis were performed as described previously (19, 22). Briefly,
total RNA was extracted using a guanidine HCl-phenol method (TRIzol;
Life Technologies, Inc.), fractionated on an agarose-formaldehyde gel,
and transferred to a nylon membrane (Hybond-N, Amersham Pharmacia
Biotech). Analytical probes for Northern blots were generated by PCR.
The probe for AANAT-1 corresponds to bp 1009-1628 in the
3'-untranslated region of pAANAT-1; the probe for AANAT-2
corresponds to bp 961-1590 in the 3'-untranslated region of
pAANAT-2. Probes were labeled with 32P by random priming.
All blots were probed for both AANAT-1 and AANAT-2. For normalization, blots were stripped and probed
with a random primed human Expression of Glutathione S-Transferase-AANAT Fusion
Proteins--
The full-length ORF of each pike AANAT cDNA was
subcloned into the procaryotic expression vector pGEX-4T-3 (Amersham
Pharmacia Biotech). The resulting expression clones were pRX50 for the
form dominating in the retina and pPX36 for the pineal form. Both
clones were expressed as glutathione S-transferase fusion
proteins in BL21(DE3)pLysS bacteria. Culture sonicates (2×
phosphate-buffered saline + protease inhibitors + 10 mM
dithiothreitol) were diluted (0.5 M NaCl + 50 mM sodium citrate + 1 mM dithiothreitol) and assayed.
Enzyme Activity Analysis (23)--
Enzyme activity was
measured by the formation of 3H-acetylated product by
incubating preparations with arylalkylamine substrates at the indicated
concentrations and 0.5 mM [3H]acetyl-coenzyme
A (4 Ci/mol) in 40-µl final volume of 100 mM sodium
phosphate buffer (pH 6.8) at 37 °C for 20 min, unless otherwise indicated. Radiolabeled products were separated from [ 3H]acetyl-coenzyme A and quantitated by one of two
methods. To isolate and measure
N-[3H]acetyltryptamine,
N-[3H]acetyl-5-methoxytryptamine, or
N-[3H]acetylphenylethylamine, the assay
mixtures were extracted into chloroform; the chloroform was taken to
dryness; and the residue was redissolved in scintillation fluid. To
isolate and measure N-[3H]acetylserotonin or
N-[3H]acetyltyramine, a sample of the assay
was mixed with ethanol containing the appropriate authentic carrier,
the mixture was subjected to thin layer chromatography, products were
located, corresponding gel was removed and product was extracted into
scintillation fluid. Radioactivity was determined by routine methods.
Sequence Analysis
The deduced amino acid sequences of the two pAANAT clones are 66%
identical and 76% similar (Fig.
1A). Both sequences are ~65-75% identical to known mammalian and avian AANAT clones (Fig. 1B). Both contain known conserved features of AANATs
including two putative protein kinase A phosphorylation sites, a
conserved putative ubiquitination site (K9), a histidine-rich stretch
of residues proposed to be important in catalysis (residues 113-128) and two regions, described as motifs A and B, which characterize members of a large acetyltransferase superfamily referred to as the A/B
or GNAT superfamily (15, 17, 22).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
N-acetylserotonin
melatonin) is serotonin N-acetyltransferase (arylalkylamine
N-acetyltransferase;
AANAT1; E.C. 2.3.1.87) (15).
Rhythms in AANAT activity in most vertebrates are driven by circadian
clocks; light acts to reset and entrain these clocks. Light also acts
independently via a downstream mechanism to turn off AANAT activity by
initiating AANAT proteolysis (16).
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
PCR primers used for cloning AANAT cDNAs
-actin cDNA
(CLONTECH).
RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
Sequence analysis of pike AANAT-1
and AANAT-2. A, alignment of pAANAT-1
and pAANAT-2 deduced amino acid sequences. It is not clear which of the
two methionines within the first four residues of pAANAT-2 is the
preferred start of translation. Residues linked by vertical
lines are identical; vertical dots indicate similarity.
B, alignment of pAANATs and published AANATs
(GenBankTM accession numbers: human, U40347; rat, U38306;
sheep, U29663; cow, AD000742; chicken, U46502; quail, AF007068; trout
AANAT-1, AB007294). The two motifs that identify proteins as members of
the motif A/B superfamily of acetyltransferases and the three highly
conserved regions (C/c-1, D/c-1, D/c-2) which distinguish members of
the AANAT subfamilies are indicated. pka, putative cyclic
nucleotide-dependent kinase site. The consensus sequences
for AANAT-1 and AANAT-2 match all known
sequences. Capitalized residues in individual AANAT-1
and AANAT-2 sequences match the consensus. The
consensus for AANAT-1/-2 indicates residues identical
between all AANAT-1 and AANAT-2 sequences. The
last 35 amino acids of trout AANAT-1 have been omitted for
clarity. C, the organization of pAANAT-1 and pAANAT-2 genes,
compared with the human AANAT-1 (hAANAT-1) gene (25).
Dark boxes indicate the ORF.
Clone pRX50, from the pike retina, contains the three highly conserved peptides (C/c-1, D/c-1, and D/c-2) found in all previously published AANATs (17); this clone and the known AANATs will be designated members of a single AANAT-1 subfamily (15, 22-28). The corresponding peptides in clone pPX36, from the pike pineal gland, differ at 6 of 36 amino acids in these sequences. This clone, along with the trout pineal AANAT cloned here, will be considered to represent a second AANAT subfamily, designated AANAT-2. This segregation is also consistent with the results of computer-assisted phylogenetic analyses (ClustalW and Phylip) which cluster all AANAT-1 s on a separate branch from pAANAT-2.2
Analysis of genomic sequences indicates that the locations of the intron/exon junctions in the ORF of both pAANAT genes are identical to that of human, sheep, and chicken AANAT-1 (Fig. 1C, and Ref. 15). Genomic Southern blot analysis confirmed that each gene was present as a single copy and did not co-migrate following digestion with five different restriction enzymes.3 This and the above results indicate that it is highly likely that AANAT-1 and AANAT-2 evolved from a common ancestor rather than resulting from convergent evolution or alternative splicing.
Tissue Distribution
Northern blot analysis reveals that pAANAT-1 and pAANAT-2 exhibit strikingly different expression patterns. pAANAT-1 is expressed at high levels only in the retina and pAANAT-2 only in the pineal gland. A trace amount of a 1.6-kilobase pAANAT-2-positive transcript is also found in the ovary. Northern blot analysis did not detect expression of either gene in other tissues. Day/night differences in expression also occur in retina and pineal gland (discussed below).
pAANAT-2 is present in the pineal gland as 2.1- and 2.8-kilobase transcripts; a single 2.2-kilobase pAANAT-1 transcript is found in the retina. The relative amounts of the two pAANAT-2 transcripts varied between samples for unexplained reasons not linked to experimental history or sample preparation, as judged from the actin mRNA signal. Although Northern blot analysis failed to reveal expression of pAANAT-1 in the pineal gland or of pAANAT-2 in the retina, reverse transcriptase PCR analysis detected trace levels of pAANAT-2 mRNA in the retina and of pAANAT-1 mRNA in the pineal gland.3
Kinetic Characterization of Recombinant AANAT-1 and AANAT-2 Proteins
Temperature Dependence-- Temperature-activity relationships of the recombinant enzymes were different. Maximal activity with 10 mM tryptamine for pAANAT-1 occurred at 37 °C and that for pAANAT-2 at 18 °C. The temperature profile of pAANAT-1 corresponds to that reported for pike retinal homogenates and that of pAANAT-2 to pike pineal homogenates (Ref. 18; Fig. 3A, inset), consistent with the strong expression of pAANAT-1 in the retina and of pAANAT-2 in the pineal gland (Fig. 2). It is interesting that recombinant pAANAT-1 and pAANAT-2 enzymes exhibit significant levels of activity at 0 to 4 °C, as is also true of the native enzymes (18).
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Substrate Preferences-- The affinities of pAANAT-1 and of pAANAT-2 for tryptamine differ by more than 2 orders of magnitude (pAANAT-1, apparent Km ~ 0.02 mM; pAANAT-2, apparent Km ~ 7 mM) (Fig. 3B). An almost identical pattern exists between AANATs from pike retina and pineal gland homogenates (Ref. 18; Fig. 3B, inset). The apparent Km values for serotonin are also very different between pAANAT-1 (~0.02 mM) and pAANAT-2 (~3 mM); however, whereas pAANAT-1 acetylates tryptamine and serotonin with very similar affinities, pAANAT-2 acetylates serotonin about twice as well as tryptamine (Fig. 3C).
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pAANAT-1 and pAANAT-2 also have different relative affinities for indoleethylamines versus phenylethylamines. pAANAT-1 does not appear to differentiate between indoleethylamines (serotonin, methoxytryptamine, and tryptamine) and phenylethylamines (tyramine and phenylethylamine). In contrast, pAANAT-2 has almost no ability to acetylate phenylethylamines (Fig. 3C).
The substrate-product formation relationships of pAANAT-1 and pAANAT-2 also differ in that only pAANAT-1 generates a pattern where acetylation is inhibited at substrate concentrations above 0.3 mM with indoleethylamine substrates; this same pattern is observed at 4 °C, as well as 37 °C. Neither enzyme exhibits such a pattern with phenylethylamines. We failed to obtain evidence for product inhibition of pAANAT-1 with up to 1 mM N-acetylserotonin or melatonin.3 This suggests that substrate inhibition is likely to explain the observed pattern.
Circadian Pattern of Abundance of Retinal pAANAT-1 and Pineal pAANAT-2
The abundance of AANAT mRNA transcripts changes on a 24-h basis in both the retina and pineal gland in animals maintained in a lighting environment providing 12 h of light and 12 h of darkness (Fig. 4). The amplitude of the retinal AANAT-1 mRNA rhythm is >20-fold and that of pineal pAANAT-2 mRNA is >8-fold. The phasing of the rhythms differs, with the retinal AANAT-1 mRNA rhythm peaking at dusk, 6 h prior to the middark period peak in pineal AANAT-2 mRNA.
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Daily rhythms in the abundance of retinal pAANAT-1 mRNA and pineal
pAANAT-2 mRNA persist under constant lighting conditions of dim red
light or constant light, indicating they are not dependent upon
light/dark transitions but are controlled by an endogenous circadian clock.
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DISCUSSION |
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Two AANAT Genes Explains Paradoxical Results in Pike-- The existence of a second AANAT gene in pike resolves two paradoxical observations regarding AANAT in pike retina and pineal gland (18, 19). First, the report that AANAT mRNA was undetectable in the pike retina is explained by the finding that the trout pineal-derived probe used in those studies was AANAT-2-selective and did not detect pAANAT-1 under the conditions used, in part reflecting only a 68% bp match. As reported here, pike retina preferentially expresses AANAT-1; AANAT-2 is not detectable in the retina by Northern blot analysis.
The second paradox explained by these findings is that the kinetics of AANAT in the pike pineal gland and retina are different. This is explained by the dominant expression of pAANAT-1 in the retina and pAANAT-2 in the pineal gland, and the inherent differences in the proteins encoded by pAANAT-1 and pAANAT-2. This is supported by the remarkable similarity of the published kinetic characteristics of pike retinal AANAT and recombinant pAANAT-1 and that of pineal AANAT and recombinant pAANAT-2. It is noteworthy that retinal AANAT activity and recombinant pAANAT-1 appear to have nearly identical temperature dependence and substrate specificity characteristics, as did pineal AANAT activity and recombinant pAANAT-2, despite the fact that native enzymes were studied in crude pineal or retinal homogenates and fusion proteins were studied in bacterial extracts. The remarkable similarities observed between recombinant pAANAT-1 and retinal AANAT versus recombinant pAANAT-2 and pineal AANAT provide strong indications that the differences seen are valid reflections of the AANAT proteins and are not caused by other proteins or factors in the assay. Further investigations with pure preparations of the expressed proteins are required to fully characterize and provide a molecular explanation for these differences.
Functional Significance of Two AANAT Genes-- It is not unreasonable to consider that the retinal/pineal expression pattern of pAANAT-1 and pAANAT-2 reflects a fine-tuning of AANAT's regulatory role in melatonin synthesis in each tissue. As indicated above, rhythmic changes in melatonin synthesis are regulated by changes in AANAT activity. Recent studies have established that AANAT activity is modulated through control of enzyme protein levels by regulated proteolysis (16). With this type of regulation, changes in enzyme protein will be most effectively converted into changes in product formation if the Km value approximates the substrate concentration. This is consistent with our finding that pAANAT-1 is expressed in the retina, which typically has a relatively low concentration of serotonin and that pAANAT-2 is expressed in the pineal gland, which contains one of the highest concentrations of serotonin (29). The low Km of pAANAT-1 appears to best support melatonin synthesis if very low levels of serotonin are available, as is the case in the retina. This level of synthesis appears to be sufficient for local demands, and the higher Km of pAANAT-2 in the pineal gland is consistent with the higher concentration of serotonin there and the role of the pineal gland as a factory for circulating melatonin.
There may also be functional significance to the differences in substrate specificity between the proteins encoded by pAANAT-1 and pAANAT-2. The former acetylates indoleethylamines and phenylethylamines equally well, whereas the latter exhibits strong preference for indoleethylamines. This weak affinity of pAANAT-2 for phenylethylamines eliminates or minimizes acetylation as a factor influencing phenylethylamine metabolism in the pineal gland. Likewise, the strong affinity of pAANAT-1 for phenylethylamines suggests the enzyme might function in the retina to control phenylethylamine levels. This is of interest because 3,4-dihydroxyphenylethylamine (dopamine) is an important retinal neurotransmitter (5, 9).
The remarkable activity-temperature dependence difference between these two enzymes raises two questions: one being the molecular basis of the difference, and the second being the physiological significance. It has been suggested that molecular adaptation of closely related enzymes in different fish species to different physiological temperatures can be achieved through modification of flexibility and mobility within the molecule (30). The recent publication of the crystal structure of ovine AANAT-1 may help determine whether sequence differences between pAANAT-1 and pAANAT-2 could result in similar effects (31). The physiological significance is difficult to explain but may relate to the fact that the Northern pike is a cold water fish; the steep rise in pAANAT-2 activity within the range of seasonal temperatures may be important in integrating environmental parameters.
Two AANAT Genes-Exception or the Rule?-- The retinal/pineal expression pattern of AANAT-1 and AANAT-2 seen in pike is also present in trout because trout AANAT-1 was previously cloned from retina (28), and AANAT-2 was cloned from the trout pineal gland in this study. However, the strict pattern of expression of AANAT-1 in the retina and of AANAT-2 in the pineal gland is not conserved within or across classes. For example, AANAT-1 is expressed at high levels in both the retina and pineal gland of the chicken and some mammals (22, 25-27), and AANAT-2 is expressed at high levels in both the pineal gland and retina of zebrafish (19). Accordingly, species-to-species variations in tissue expression of AANAT-1 or AANAT-2 might reflect unique species- and tissue-dependent requirements for acetylation of serotonin and perhaps other arylalkylamines. Such biochemical and genetic variation is consistent with the anatomical and molecular diversity of these tissues (15, 32-34) and the important phototransduction roles the pineal gland and retina play in survival.
The question of whether AANAT-2 is expressed in non-teleosts remains open at this time. Efforts to clone chicken and human AANAT-2 s have been unsuccessful so far.3 Based on the degree of difference between pAANAT-1 and AANAT-2, it is not unreasonable that pAANAT-1 and pAANAT-2 existed prior to the emergence of teleosts; both might have been present in a vertebrate ancestor common to both teleosts and higher vertebrates. However, it is also possible that AANAT-2 appeared only after fish emerged from the vertebrate line that led to higher vertebrates.
Circadian Patterns of Expression of pAANAT-1 and pAANAT-2-- The circadian expression of pAANAT-1 and pAANAT-2 is consistent with previous reports indicating that there are rhythms in AANAT mRNA in the chicken retina and in the pineal glands of the rat, pike, and chicken. This appears to reflect the role of endogenous clocks which determines that melatonin synthesis can be significantly elevated only during the night and not during the day. This improves the reliability of melatonin as an indicator of the night period. The clock is reset and modulated by light. However, as indicated in the Introduction, the clock is not the only mechanism controlling AANAT activity. An additional control mechanism involves light acting downstream of the clock to suppress AANAT activity; this apparently involves light-initiated proteasomal proteolysis of AANAT protein (16). This provides a mechanism through which the system can respond immediately to changes in photoperiod length that are not anticipated by the endogenous clock. This is of special significance in considering the importance of the different phasing of the rhythms of retinal AANAT-1 and pineal AANAT-2 mRNAs.
We suspect that the earlier peak in retinal AANAT-1 mRNA may reflect a requirement for maximal melatonin production early in the night period, perhaps to optimally promote dark adaptation. As a result, as soon as light levels fall below a threshold and retinal AANAT-1 proteolysis ceases, retinal AANAT activity and melatonin production will increase immediately.
The later peak in pineal melatonin is consistent with the observation that in most species the circulating melatonin signal is broad and peaks in the middle of the dark period; it provides a hormonal signal that is proportional in length to the duration of the night period. Light participates by tailoring the duration of this signal. It is of interest to note that limited data suggests that a similar phase difference may exist between the rhythms in AANAT-1 mRNA in the rat pineal gland and retina (35), both of which are generated by endogenous oscillators.
The molecular basis of the different phasing of the retinal pAANAT-1 and pineal pAANAT-2 is not clear. It might reflect tissue-related differences in clock function, in the links between the clock and the AANAT genes, or in regulatory elements in the AANAT-1 and AANAT-2 promoters. Accordingly, continued analysis of this system will provide valuable insights into molecular aspects of clock function and the relationship of clocks to clock-driven genes.
Final Comment--
The presence of a second AANAT gene in the pike
provides evidence of intricate and divergent strategies for regulating
melatonin production in the pike retina and pineal gland, and also
provides unexpected opportunities for further investigation. For
example, determination of the three-dimensional differences between
pAANAT-1 and pAANAT-2 will help understand the molecular basis of
kinetic differences, as will side-by-side determination of effects of site-directed mutagenesis on three-dimensional structure and kinetic activity. In addition, analysis of promoter elements in pAANAT-1 and
pAANAT-2 will enhance our understanding of the molecular basis of
tissue specificity and circadian-timed gene expression.
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FOOTNOTES |
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* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF034081, AF034082, AF034084, AF034083, and AF106006.
¶ To whom correspondence should be addressed: Bldg. 49, Rm. 6A-82, National Institutes of Health, Bethesda, MD 20892-4480; Tel.: 301-496-6915; Fax: 301-480-3526; E-mail: klein{at}helix.nih.gov.
2 E. Koonin (National Institutes of Health), personal communication.
3 S. L. Coon and D. C. Klein, unpublished results.
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ABBREVIATIONS |
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The abbreviations used are: AANAT, arylalkylamine N-acetyltransferase (serotonin N-acetyltransferase); ORF, open reading frame; RACE, rapid amplification of cDNA ends; PCR, polymerase chain reaction; bp, base pair(s); pAANAT, pike AANAT.
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