Antibodies to the period gene product of Drosophila reveal diverse tissue distribution and rhythmic changes in the visual system

Isolation of timeless by PER protein interaction: defective interaction between timeless protein and long-period mutant PERL

The period (per) gene likely encodes a component of the Drosophila circadian clock. Circadian oscillations in the abundance of per messenger RNA and per protein (PER) are thought to arise from negative feedback control of per gene transcription by PER. A recently identified second clock locus, timeless (tim), apparently regulates entry of PER into the nucleus. Reported here are the cloning of complementary DNAs derived from the tim gene in a two-hybrid screen for PER-interacting proteins and the demonstration of a physical interaction between the tim protein (TIM) and PER in vitro. A restricted segment of TIM binds directly to a part of the PER dimerization domain PAS. PERL, a mutation that causes a temperature-sensitive lengthening of circadian period and a temperature-sensitive delay in PER nuclear entry, exhibits a temperature-sensitive defect in binding to TIM. These results suggest that the interaction between TIM and PER determines the timing of PER nuclear entry and therefore the duration of part of the circadian cycle.

Molecular biological approach to the circadian clock mechanism

Many circadian phenomena have been described in a diverse range of species, from single cellular organisms to higher species of plants and animals. From several lines of evidence from Drosophila and Neurospora, the oscillation of the circadian clock seems to involve cycling gene expression. Although a great deal of information concerning the anatomy, neurophysiology and neurochemistry of circadian pacemakers has been obtained over the last decade, molecular and cellular approaches to this problem have only just begun. I will summarize recent progress of the molecular biological approach to the circadian clock mechanism. Finally, the importance of transcription factors to envision the common mechanism of circadian clock in the diverged species will be discussed considering with the existence of a hypothetical 'Time Box'.

Molecular control of circadian rhythms

Circadian rhythms are virtually ubiquitous in eukaryotes and have been shown to exist even in some prokaryotes. The generally accepted view is that these rhythms are generated by an endogenous clock. Recent progress, especially in the Drosophila, Neurospora and mouse systems, has revealed new clock components and mechanisms. These include the mouse clock gene, the Drosophila timeless gene, and the role of light in Neurospora.

The circadian clock: from molecules to behaviour

Circadian rhythms are a cardinal feature of living organisms. The stereotypical organization of homeostatic, endocrine and behavioural variables around the 24-hour cycle constitutes one of the most conserved attributes among species. It is now well established that circadian rhythmicity is not a learned behaviour, but is genetically transmitted and therefore subject to genetic manipulations. Recent advances in the circadian field have demonstrated that circadian oscillations are cell autonomous, that the circadian mechanism operates through a negative feedback loop and that a growing number of genes is under circadian control. Furthermore, single-gene mutations have been isolated in mammals that have profound effects on circadian behaviour. The production and mapping of one of these mutations in the mouse, an organism about which there exists a wealth of genetic information, should accelerate the elucidation of the molecular events involved in the generation of circadian rhythms in mammals.

Circadian rhythms in Drosophila can be driven by period expression in a restricted group of central brain cells

Neural tissues controlling circadian rhythmicity have been identified in a variety of organisms and are often closely associated with the visual system. In Drosophila, the clock gene period (per), which is required for circadian rhythms, is expressed in many neurons and glia throughout the eye and brain. We asked whether biological rhythms could be generated if per expression were restricted to a subset of these cells that is involved in photoreception. Here we demonstrate that expression of per under the control of the glass promoter confers both behavioral and molecular rhythmicity. glass is required for development of Drosophila photoreceptors, and this promoter is active in eyes, ocelli, and certain cells of the central brain. When we genetically removed all external photoreceptor cells, rhythms persisted in these transgenic animals. This suggests that a few central brain cells producing glass and per are capable of generating biological rhythms.

Period protein from the giant silkmoth Antheraea pernyi functions as a circadian clock element in Drosophila melanogaster

Homologs of the Drosophila clock gene per have recently been cloned in Lepidopteran and Blattarian insect species. To assess the extent to which clock mechanisms are conserved among phylogenetically distant species, we determined whether PER protein from the silkmoth Antheraea pernyi can function in the Drosophila circadian timing system. When expressed in transgenic Drosophila, the silkmoth PER protein is detected in the expected neural cell types, with diurnal changes in abundance that are similar to those observed in wild-type fruitflies. Behavioral analysis demonstrates that the silkmoth protein can serve as a molecular element of the Drosophila clock system; expression of the protein shortens circadian period in a dose-dependent manner and restores pacemaker functions to arrhythmic per0 mutants. This comparative study also suggests that the involvement of PER in different aspects of circadian timing, such as period determination, strength of rhythmicity, and clock out-put, requires distinct molecular interactions.

Are competing intermolecular and intramolecular interactions of PERIOD protein important for the regulation of circadian rhythms in Drosophila?

Genetic analysis is revealing molecular components of circadian rhythms. The gene products of the period gene in Drosophila and the frequency gene in Neurospora oscillate with a circadian rhythm. A recent paper (1) has shown that the PERIOD protein can undergo both intermolecular and intramolecular interactions in vitro. The effects of temperature and two period mutations on these molecular interactions were compared to the effects of the mutations and temperature on the in vivo period length of circadian rhythms. The results suggest that the molecular interactions may compete to maintain a rhythm with a constant period over a wide temperature range.

Tripping along the trail to the molecular mechanisms of biological clocks

Solving the mechanism of circadian clocks has become an important goal, in part because daily rhythms are running in such a wide variety of organisms, and contribute to many aspects of their well being. Systematic genetic approaches to studying 'the clock' were initiated in fruitflies more than 20 years ago as a novel means by which neural-pacemaking mysteries might be solved. Such chronogenetic investigations gained momentum when they spread to other species, and became molecular. However, the molecular studies were misleading, that is, until some elementary neuro-anatomical observations, involving the expression of a 'clock gene' in Drosophila, gave the experiments in this molecular-neurogenetic area of chronobiology a new direction. The initially neuro-descriptive studies led to the current investigations that involve negatively acting transcription factors and other clock molecules that are presumed to interact with them. In addition, new mutants and clones have been isolated in a timely manner. These mutations and molecules should permit chronogeneticists, working on a wide variety of organisms, to unravel further details of how the clock works, how environmental information finds its way to it, and how it sends information out into the organism's physiology, biochemistry and behavior.

Circadian rhythms: peering into the molecular clockwork

Recent developments in our understanding of the molecular basis of circadian rhythms look set to provide one of the most elegant demonstrations of the relationship between the activity of individual genes and the execution of biologically relevant and complex patterns of behaviour. At the same time the lid is being opened on one of the classic 'black boxes' of biology, the circadian clock.

Analysis of conditioned courtship in dusky-Andante rhythm mutants of Drosophila

Genetic connections between learning and rhythmicity were suggested to have been established in a previous study, in part because the duskyAndante (dyAnd) mutation in Drosophila disrupted both behaviors. dyAnd, isolated as a slow-clock variant, was reported to cause an approximately fourfold decrement in courtship-suppression conditioning. These effects have been reexamined; the experiments were buttressed by testing the effects of several recently isolated mutations at the dusky locus, along with the original And Allele that had been induced there. The reexamination was also prompted by anatomical concerns, certain of which have recently focused on dy-induced decrements in cell size, but only in terms of wing morphology. Another anatomical issue involves the discovery of a neuronal pathway that seems to connect circadian pacemaker cells to a structure in the Drosophila brain that is involved in learning. In observer-blind experiments, however, it was found that neither pacemaker-slowing (Andante-like) dy mutations nor others that cause no rhythm defects produced subnormal conditioned courtship. Moreover, in the adult brain of a slow-clock dyAnd mutant, no axonal pathway defects were readily discernible and putative pacemaker neurons appeared to be normal in size.

PER protein interactions and temperature compensation of a circadian clock in Drosophila

The periods of circadian clocks are relatively temperature-insensitive. Indeed, the perL mutation in the Drosophila melanogaster period gene, a central component of the clock, affects temperature compensation as well as period length. The per protein (PER) contains a dimerization domain (PAS) within which the perL mutation is located. Amino acid substitutions at the perL position rendered PER dimerization temperature-sensitive. In addition, another region of PER interacted with PAS, and the perL mutation enhanced this putative intramolecular interaction, which may compete with PAS-PAS intermolecular interactions. Therefore, temperature compensation of circadian period in Drosophila may be due in part to temperature-independent PER activity, which is based on competition between inter- and intramolecular interactions with similar temperature coefficients.

Ritsu: a rhythm mutant from a natural population of Drosophila melanogaster

We found a new clock mutation, ritsu (rit), in an inbred strain separated from a natural population of Drosophila melanogaster in Yamaguchi, Japan. This mutation lengthens the free-running period of the wild-type locomotor activity rhythm from ca. 24 hr to ca. 27 hr by lengthening both active and rest phases equally. To determine the loci involved, we established eight lines of chromosomal substituted flies between the inbred strain and Canton-S, and found that a major gene is located on the second chromosome. Furthermore, the result of recombination analysis suggested that the locus might be near or to the left of cl (2-18.8).

The role of light in the initiation of circadian activity rhythms of adult Drosophila melanogaster

Rearing Drosophila melanogaster in constant darkness (DD) for multiple generations disrupts the circadian activity rhythm of adults. In order to determine under what conditions normal rhythms can be initiated, DD-reared Drosophila (either the wild type or the periodshort [pers] mutant) were exposed to light either as embryos, third-instar larvae, or adults. Exposing DD-reared flies to light as embryos or larvae had no effect, while exposing them as adults fully restored normal rhythms in pers and partially restored normal rhythms in the wild type. The percentage of adults with normal rhythms was not significantly different between animals given a 1-h pulse of light as adults and animals given two LD cycles as adults. LD-reared and DD-reared animals given 2 LD cycles were synchronous. In the latter, offset of activity followed the LD transition (CT 12) by 2-6 subjective hours in pers and 2-3 subjective hours in per+. Circadian rhythms did not exhibit phase coherence in the other treatments.

The period clock gene is expressed in central nervous system neurons which also produce a neuropeptide that reveals the projections of circadian pacemaker cells within the brain of Drosophila melanogaster

The period protein (PER) is a essential component of the circadian clock in Drosophila melanogaster. Although PER-containing pacemaker cells have been previously identified in the brain, the neuronal network that comprises the circadian clock remained unknown. Here it is shown that some PER neurons are also immunostained with an antiserum against the crustacean pigment-dispersing hormone (PDH). This antiserum reveals the entire arborization pattern of these pacemaker cells. The arborizations of these neurons are appropriate for modulation of the activity of many neurons and they might interact with PER-containing glial cells. A putative physiological role of PDH in the circadian system is discussed.

An ultrashort clock mutation at the period locus of Drosophila melanogaster that reveals some new features of the fly's circadian system

A rhythm mutant of Drosophila melanogaster was induced by chemical mutagenesis and isolated by testing for locomotor activity rhythms, which in the new variant had periods of approximately 16 hr. The sex-linked mutation responsible for this ultrashort period causes 20-hr rhythms when heterozygous with a normal X. This semidominance notwithstanding, the new mutation was revealed to be an allele of the period (per) gene by noncomplementation with per-null variants, in the sense that females heterozygous for perT (as the ultrafast-clock allele is called) and per- exhibited periods that were much shorter than in the case of perT/+. These tests also revealed in a clearer manner than in previous cases that two "doses" of a fast-clock per mutation lead to appreciably shorter periods than those exhibited by one-dose females whose other per allele is a loss-of-function variant. In light-dark cycles (LD 12:12), flies carrying perT in a genotypic condition leading to free-running periods that are 8 hr faster than normal nevertheless entrained, by phase-shifting that large number of hours each day; the evening peak of locomotor activity was, however, many hours earlier than normal. The use of a newly developed device for monitoring Drosophila eclosion automatically showed that perT exhibits a very marginal emergence rhythm at 25 degrees C, but periodicity of ca. 17-18 hr at 19 degrees. Staining of the per-encoded protein (PER) in sections of perT versus normal pharate adults revealed for the first time that the immunohistochemically detected signal cycles in its intensity in wild-type, in a manner that is similar to the PER rhythm previously demonstrated in adults. The staining cycle in pharate adults expressing perT differed from that of wild-type. Temperature compensation of the adult activity rhythm of perT was found to be faulty, in that periods became appreciably shorter as the flies were heated. However, the mutant exhibited a normal degree of period lengthening when its locomotor activity was monitored in the presence of heavy water. The perT mutation interacted with the long-period Andante allele of the dusky locus in a manner that was anomalous (in comparison to dyAnd interactions with per+ or another short-period per mutation). This and other unique features of perT are discussed from the standpoint of the new mutation's heuristic value, including that which may stimulate a deeper understanding of the period gene's action at the molecular level.

Analysis of period mRNA cycling in Drosophila head and body tissues indicates that body oscillators behave differently from head oscillators

The period (per) gene is thought to be part of the Drosophila circadian pacemaker. The circadian fluctuations in per RNA and protein that constitute the per feedback loop appear to be required for pacemaker function, and have been measured in head neuronal tissues that are necessary for locomotor activity and eclosion rhythms. The per gene is also expressed in a number of neuronal and nonneuronal body tissues for which no known circadian phenomena have been described. To determine whether per might affect some circadian function in these body tissues, per RNA cycling was examined. These studies show that per RNA cycles in the same phase and amplitude in head and body tissues during light-dark cycles. One exception to this is the lack of per RNA cycling in the ovary, which also appears to be the only tissue in which PER protein is primarily cytoplasmic. In constant darkness, however, the amplitude of per RNA cycling dampens much more quickly in bodies than in heads. Taken together, these results indicate that circadian oscillators are present in head and body tissues in which PER protein is nuclear and that these oscillators behave differently.

Cloning of a structural and functional homolog of the circadian clock gene period from the giant silkmoth Antheraea pernyi

The period (per) gene of Drosophila plays an important role in circadian clock function. Interestingly, homologs of per have not been cloned outside of dipteran species. Using a PCR strategy, we now report the cloning of the cDNA of a per homolog from the silkmoth Antheraea pernyi. The cDNA encodes a protein of 849 amino acids, which shows highest identity (39%) with the per protein of Drosophila virilis. Stretches of high identity between moth and fly proteins are in the amino terminus, the PAS region, and the region surrounding the site of the per mutation in Drosophila. Moth per homolog mRNA levels exhibit a prominent circadian variation in adult heads, and per protein antibodies show a pronounced variation of per antigen staining in photoreceptor nuclei. With sequence information derived from moth and flies, per-like cDNA fragments were readily cloned by PCR from other moth species and a third insect order.

Analysis of period mRNA cycling in Drosophila head and body tissues indicates that body oscillators behave differently from head oscillators

The period (per) gene is thought to be part of the Drosophila circadian pacemaker. The circadian fluctuations in per RNA and protein that constitute the per feedback loop appear to be required for pacemaker function, and have been measured in head neuronal tissues that are necessary for locomotor activity and eclosion rhythms. The per gene is also expressed in a number of neuronal and nonneuronal body tissues for which no known circadian phenomena have been described. To determine whether per might affect some circadian function in these body tissues, per RNA cycling was examined. These studies show that per RNA cycles in the same phase and amplitude in head and body tissues during light-dark cycles. One exception to this is the lack of per RNA cycling in the ovary, which also appears to be the only tissue in which PER protein is primarily cytoplasmic. In constant darkness, however, the amplitude of per RNA cycling dampens much more quickly in bodies than in heads. Taken together, these results indicate that circadian oscillators are present in head and body tissues in which PER protein is nuclear and that these oscillators behave differently.

A period (per)-like protein exhibits daily rhythmicity in the suprachiasmatic nuclei of the rat

The period (per) gene of Drosophila melanogaster is considered an important biological clock gene, since it regulates multiple behavioral rhythms. Per mRNA and protein exhibit circadian rhythms in the fruitfly brain and these rhythms appear to influence each other through a feedback loop. More recently, using the same antibody as was used in the Drosophila studies, PER-like proteins were detected in the suprachiasmatic nuclei (SCN) of male rats. This region of the brain is considered to be a major neural circadian pacemaker in mammals. The purpose of this study was to confirm that PER-like proteins are detectable in the SCN of female rats and to determine whether PER-like proteins exhibit a circadian rhythm. Female rats were killed at several times of day under both light/dark and constant conditions. Using the same anti-PER antibody in Western blots with Enhanced Chemiluminescence (Western-ECL) detection, the levels of the PER-like proteins were quantified in the SCN and cerebral cortex. The antibody identified a doublet band of approximately 170-160 kDa and a single band at 115 kDa. Of the three PER-like proteins only the largest exhibited a daily rhythm in the SCN, which peaked in the middle of the dark and attained its nadir around lights off; levels during the light were intermediate with a tendency towards a second drop around lights on. This rhythm did not persist under constant dim red light.(ABSTRACT TRUNCATED AT 250 WORDS)

Pigment-dispersing hormone-immunoreactive neurons in the cockroach Leucophaea maderae share properties with circadian pacemaker neurons

Neurons immunoreactive with antisera against the crustacean peptide beta-pigment dispersing hormone fulfill several anatomical criteria proposed for circadian pacemakers in the brain of the cockroach Leucophaea maderae. These include position of somata, projections to the lamina and midbrain and possible coupling pathways between the two pacemakers through commissural fibers. In behavioral experiments combined with lesion studies and immunocytochemical investigations we examined whether the presence of pigment-dispersing hormone-immunoreactive arborizations in the midbrain of the cockroach correlates with the presence of circadian locomotor activity. No rhythm was detected after severing both optic stalks in any animal for at least 12 days. Within the same time pigment-dispersing hormone-immunoreactive fibers in the midbrain disappeared. Two to seven weeks after the operation some of the cockroaches regained circadian locomotor activity, while others remained arrhythmic. In all cockroaches which regained rhythmic behavior pigment-dispersing hormone-immunoreactive fibers had regenerated and had largely found their original targets within the brain. In all arrhythmic cockroaches either none or very little regeneration had occurred. The period of the regained circadian activity inversely correlated with the number of regenerated immunoreactive commissural fibers. These data provide further evidence for the involvement of pigment-dispersing hormone-immunoreactive neurons in circadian clocks of orthopteroid insects.

Block in nuclear localization of period protein by a second clock mutation, timeless

In wild-type Drosophila, the period protein (PER) is found in nuclei of the eyes and brain, and PER immunoreactivity oscillates with a circadian rhythm. The studies described here indicate that the nuclear localization of PER is blocked by timeless (tim), a second chromosome mutation that, like per null mutations, abolishes circadian rhythms. PER fusion proteins without a conserved domain (PAS) and some flanking sequences are nuclear in tim mutants. This suggests that a segment of PER inhibits nuclear localization in tim mutants. The tim gene may have a role in establishing rhythms of PER abundance and nuclear localization in wild-type flies.

Negative feedback defining a circadian clock: autoregulation of the clock gene frequency

The frequency (frq) locus of Neurospora crassa was originally identified in searches for loci encoding components of the circadian clock. The frq gene is now shown to encode a central component in a molecular feedback loop in which the product of frq negatively regulated its own transcript, which resulted in a daily oscillation in the amount of frq transcript. Rhythmic messenger RNA expression was essential for overt rhythmicity in the organism and no amount of constitutive expression rescued normal rhythmicity in frq loss-of-function mutants. Step reductions in the amount of FRQ-encoding transcript set the clock to a specific and predicted phase. These results establish frq as encoding a central component in a circadian oscillator.

Loss of circadian behavioral rhythms and per RNA oscillations in the Drosophila mutant timeless

Eclosion, or emergence of adult flies from the pupa, and locomotor activity of adults occur rhythmically in Drosophila melanogaster, with a circadian period of about 24 hours. Here, a clock mutation, timeless (tim), is described that produces arrhythmia for both behaviors. The effects of tim on behavioral rhythms are likely to involve products of the X chromosome-linked clock gene period (per), because tim alters circadian oscillations of per RNA. Genetic mapping places tim on the left arm of the second chromosome between dumpy (dp) and decapentaplegic (dpp).

Temporal phosphorylation of the Drosophila period protein

The period gene (per) is required for Drosophila melanogaster to manifest circadian (congruent to 24 hr) rhythms. We report here that per protein (PER) undergoes daily oscillations in apparent molecular mass as well as abundance. The mobility changes are largely or exclusively due to multiple phosphorylation events. The temporal profile of the classic short-period form of PER (PERS) is altered in a manner consistent with the mutant strain's behavioral phenotype. As changes in abundance and phosphorylation persist under constant environmental conditions, they reflect or contribute to a free-running rhythm. We suggest that the phosphorylation status of PER is an important determinant in the Drosophila clock's time-keeping mechanism.

A promoterless period gene mediates behavioral rhythmicity and cyclical per expression in a restricted subset of the Drosophila nervous system

Transgenic flies carrying a 7.2 kb piece of DNA from the period (per) gene were analyzed for the presence of circadian locomotor activity rhythms and fluctuations of per-encoded mRNA and protein. The 5' end of this genomic fragment is within the first intron, which precedes the coding region. This promotorless fragment could rescue circadian behavioral rhythms and mediate spatial expression of PER in a subset of wild-type per cells within the CNS and PNS. In one behaviorally rhythmic line, PER protein was found in only "per lateral neurons." In the rhythmic transgenics, per mRNA and protein levels undergo circadian cycling, as previously described for wild type. Cycling of PER in brain cells of flies carrying the same 7.2 kb piece of per DNA under the control of a heat shock promoter corroborated the hypothesis that per's molecular cyclings and behavioral rhythmicity are causally related.

Phase shifting of the circadian clock by induction of the Drosophila period protein

Virtually all organisms manifest circadian (24-hour) rhythms, governed by an ill-defined endogenous pacemaker or clock. Several lines of evidence suggest that the Drosophila melanogaster period gene product PER is a clock component. If PER were central to the time-keeping mechanism, a transient increase in its concentration would cause a stable shift in the phase of the clock. Therefore, transgenic flies bearing a heat-inducible copy of PER were subjected to temperature pulses. This treatment caused long-lasting phase shifts in the locomotor activity circadian rhythm, a result that supports the contention that PER is a bona fide clock component.

Pigment-dispersing hormone-immunoreactive neurons in the nervous system of wild-type Drosophila melanogaster and of several mutants with altered circadian rhythmicity

Antisera against the crustacean pigment-dispersing hormone (beta-PDH) were used in immunocytochemical preparations to investigate the anatomy of PDH-immunoreactive neurons in the nervous system of wild-type Drosophila melanogaster and in that of several brain mutants of this species, some of which express altered circadian rhythmicity. In the wild-type and in all rhythmic mutants (small optic lobes, sine oculis, small optic lobes; sine oculis), eight cell bodies at the anterior base of the medulla (PDFMe neurons) exhibit intense PDH-like immunoreactivity. Four of the eight somata are large and four are smaller. The four large PDFMe neurons have wide tangential arborizations in the medulla and send axons via the posterior optic tract to the contralateral medulla. Fibers from the four small PDFMe neurons ramify in the median protocerebrum dorsal to the calyces of the mushroom bodies. Their terminals are adjacent to other PDH-immunoreactive somata (PDFCa neurons) which send axons via the median bundle into the tritocerebrum. The results suggest a possible involvement of the PDFMe neurons in the circadian pacemaking system of Drosophila. The location and size of the PDFMe neurons are identical with those of neurons containing the period protein which is essential for circadian rhythmicity. Changes in the arborizations of the PDFMe neurons in small optic lobes; sine oculis mutants are suited to explain the splitting in the locomotor rhythm of these flies. In the arrhythmic mutant, disconnected, the PDFMe neurons are absent. The arrhythmic mutant per0, however, shows normal PDH immunoreactivity and therefore, does not prevent the expression of PDH-like peptides in these neurons.

A new type of putative non-visual photoreceptors in the optic lobe of beetles

A putative photoreceptor organ is described in the carabid beetle, Pachymorpha sexguttata. The elongated structure, about 20-40 microns wide and more than 300 microns long, is situated within the optic lobe at the fronto-dorsal rim of the lamina. It lies, deep in the head capsule, in front of the compound eyes and beneath window-like thinnings of the cuticle. The organ is composed of two types of cells: (1) clear sheath cells and (2) well-organized inner receptor cells that appear in a horseshoe-like or circular array in cross-section. Common histological features of all inner cells include a distal trunk ending in microvilli that form a rhabdom-like structure, an axon at the proximal end of the cell, lamellar and multivesicular bodies within the trunk, and clusters of small mitochondria. The organ has no shielding pigment. It is connected by thin axons to a circumscribed neuropil that parallels the organ, and thence via a fiber tract to the medulla accessoria, a possible site of the circadian pacemaker in insects. Immunoreactivity to anti-per(s), an antibody recognizing the Drosophila period (per) protein that plays a central role in the function of the circadian pacemaker in fruit flies, is demonstratable in thin efferent terminals within the organ, in the associated neuropil and in its fiber connection to the medulla. A second receptor organ displaying the same fine structure lies near the second optic chiasm. This set of putative photoreceptors also occurs in the tenebrionid beetle, Zophobas morio, and its pupa. The possible function of these receptor organs is discussed with respect to former chronobiological data and some recently described types of extraretinal photoreceptors in arthropods.

The major-histocompatibility-complex-encoded beta-type proteasome subunits LMP2 and LMP7. Evidence that LMP2 and LMP7 are synthesized as proproteins and that cellular levels of both mRNA and LMP-containing 20S proteasomes are differentially regulated

The proteasome (high-molecular-mass multicatalytic proteinase complex) is composed of a large number of non-identical protein subunits of the alpha and beta types. The mouse beta-type subunits LMP2 and LMP7 (LMP, low-molecular-mass protein) are encoded within the mouse major histocompatibility complex (MHC II) region, and are thought to connect the proteasome to the MHC class-I antigen-processing pathway. In the present communication, we have analysed the two proteasome subunits with regard to their identity within the proteasome complex, their protein levels, their amounts of mRNA in different mouse tissues and cell lines, and have investigated the intracellular localization of LMP2 and LMP7 subunits in thymus and liver by immunocytology. Our experiments indicate that LMP2 and LMP7 subunits are synthesized as precursor proteins of 24 kDa and 30 kDa, respectively, and that only the processed 21-kDa and 23-kDa subunits are part of the 20S proteasome complex. The proportion of LMP2-subunit-containing and LMP7-subunit-containing proteasome complexes, as well as LMP2 and LMP7 mRNA levels, vary strongly and are shown to be dependent on the tissues or cell lines analysed. Furthermore, high LMP2 and LMP7 mRNA levels do not always correlate with high protein levels, suggesting a specific translational mechanism which controls proteasome subunit synthesis. Generally, mRNA levels appear to be particularly high in those tissues which are known to be involved in MHC class-I antigen presentation. Immunocytological analysis shows a strong nuclear localization of the subunits in cells of the thymus, while in the liver they appear to be evenly distributed between the two cellular compartments. Our data support the idea that both LMP2 and LMP7 proteins are non-essential proteasome subunits which are probably involved in the regulation of proteasome activities. The function of the two subunits, however, may not be restricted to the proposed role of proteasomes in antigen presentation.

Antiserum to an eye-specific protein identifies photoreceptor and circadian pacemaker neuron projections in Aplysia

The marine gastropod Aplysia has a circadian clock in each eye that generates a circadian rhythm of optic nerve activity. The axons of pacemaker neurons carry the rhythmic activity to the brain where it can be recorded from various ganglionic connectives as it is distributed throughout the CNS. We had previously identified an eye-specific 48-kD protein using an antiserum, anti-S, that recognizes the period gene product of Drosophila. We have now obtained two partial amino acid sequences of the 48-kD protein and raised a polyclonal antiserum using a synthetic peptide with the amino acid sequence of one of them. The antiserum recognizes a family of spots of M(r) 47-48 kD and Pi 5.9-6.0 on 2D immunoblots of eye proteins. The immunoblot staining intensity does not exhibit a circadian rhythm. Used in immunocytochemistry, the antiserum recognizes fibers in the optic nerve and retinal neuropil, pacemaker neurons, certain photoreceptors, and the photoreceptor rhabdom layer. It stains the optic nerve fibers and optic fiber terminals in the cerebral optic ganglion and recognizes the cerebral optic tracts, putative synaptic exchange areas, and optic tract projections from the cerebral ganglion into various head nerves and interganglionic connectives. The function of the 48-kD protein is not known but it could be involved in the maintenance or regulation of the retinal afferent pathways, including the pacemaker neuron axons, known from previous axonal transport and electrical recording studies to be the circadian output pathway.

The development and function of the Drosophila CNS midline cells

1. The midline cells of the Drosophila embryonic CNS comprise a discrete neuroanatomical structure consisting of a small subset of neurons and glia. 2. Developmental commitment of the CNS midline cells requires the action of dorsal/ventral patterning genes. 3. The single-minded gene encodes a basic-helix-loop-helix transcription factor and acts as a master regulator for the CNS midline lineage. 4. A number of different transcription factors and proteins involved in cell-cell interactions are necessary for the differentiation of midline neurons and glia. 5. CNS midline cells have important functions in the formation of the ventral epidermis and axon commissures.

Development and tissue-specific distribution of mouse small heat shock protein hsp25

We have investigated the developmental and tissue-specific distribution of the mouse small hsp25 by immunohistology using an antibody that specifically identifies hsp25. Our analysis shows that the relative amount of hsp25 increases during embryogenesis. Through days 13-20 of embryogenesis, hsp25 accumulation is predominant in the various muscle tissues, including the heart, the bladder, and the back muscles. hsp25 is detectable also in neurons of the spinal cord and the purkinje cells. Furthermore analysis of the closely related alpha, B-crystallin shows that in several tissues, including the bladder, the notochordal sheath and the eye lens both proteins are coexpressed. Our studies demonstrate that mammalian hsp25 accumulation is developmentally regulated during mouse embryogenesis and support the view of an important functional role of small heat shock proteins in normal cell metabolism.

Circadian oscillations in period gene mRNA levels are transcriptionally regulated

The period (per) gene is involved in regulating circadian rhythms in Drosophila melanogaster. The per gene is expressed in a circadian manner, where fluctuations in per mRNA abundance are influenced by its own translation product, which also cycles in abundance. Since per gene expression is necessary for circadian rhythmicity, we sought to determine how certain features of this feedback loop operate. The results of this study reveal that fluctuations in per mRNA are primarily controlled by fluctuations in per gene transcription, that per mRNA has a relatively short half-life, and that sequences sufficient to drive per mRNA cycling are present in 1.3 kilobases of 5' flanking sequences. These and other results indicate that the per feedback loop has all of the basic properties necessary to be a component of a circadian oscillator.

Stopping the circadian pacemaker with inhibitors of protein synthesis

The requirement for protein synthesis in the mechanism of a circadian pacemaker was investigated by using inhibitors of protein synthesis. Continuous treatment of the ocular circadian pacemaker of the mollusc Bulla gouldiana with anisomycin or cycloheximide substantially lengthened (up to 39 and 52 hr, respectively) the free-running period of the rhythm. To determine whether high concentrations of inhibitor could stop the pacemaker, long pulse treatments of various durations (up to 44 hr) were applied and the subsequent phase of the rhythm was assayed. The observed phases of the rhythm after the treatments were a function of the time of the end of the treatment pulse, but only for treatments which spanned subjective dawn. The results provide evidence that protein synthesis is required in a phase-dependent manner for motion of the circadian pacemaker to continue.

Protein differences in tau mutant hamsters: candidate clock proteins

In the tau mutant hamster, the period of the circadian rhythm is shortened from about 24 h to about 22 h in heterozygotes and to about 20 h in homozygotes. Understanding the biochemical basis of the period changes in the tau mutant may elucidate the regulation of the vertebrate pacemaker. Using two-dimensional gel electrophoresis, we have found two sets of proteins that differ between the different genotypes. P33tau (about 33 kDa; pI 6.5) was found in all gels from wild type and heterozygous animals, but was absent in gels from all except one of the homozygous mutant animals. P32tau (about 32 kDa; pI 4.8) was a chain of spots, which showed a striking difference in pattern between gels from wild type animals and from mutant animals. P33tau was greatly enriched in soluble cellular fractions, whereas P32tau was found only in insoluble fractions. These differences between P33tau and P32tau were apparent in gels from both SCN and cortical tissue, suggesting that both proteins are distributed throughout the brain. These proteins should be useful as new tools to explore the biochemistry of circadian pacemakers.

New short period mutations of the Drosophila clock gene per

Earlier work has indicated that the period length of Drosophila circadian behavioral rhythms is dependent on the abundance of the period (per) gene product. Increased expression of this gene has been associated with period shortening for both the circadian eclosion (pupal hatching) rhythm and circadian locomotor activity rhythms of adult Drosophila. In this study it is shown that a wide variety of missense mutations, affecting a region of the per protein consisting of approximately 20 aa, predominantly generate short period phenotypes. The prevalence of such mutations suggests that short period phenotypes may result from loss or depression of function in this domain of the per protein. Possibly mutations in the region eliminate a regulatory function provided by this segment, or substantially increase stability of the mutant protein.

A circadian clock of Drosophila: effects of deuterium oxide and mutations at the period locus

Mutations at the period (per) locus (1:1.3; 3B1-2) in Drosophila melanogaster lengthen (perL), shorten (pers), or abolish (per0) overt circadian rhythmicity. Deuterium oxide lengthens the free-running circadian period. We tested the effects of deuterium on three mutants of the per gene (pers, perL, and per0) and wild-type Drosophila melanogaster (per+) to assess interactions. With increasing concentrations of deuterium, the free-running circadian period of locomotor activity rhythms increased. The dose-response was linear in all genotypes tested. With increasing dosages of deuterium, circadian rhythms became weaker as evidenced by the signal-to-noise ratio (SNR). Genotype and deuterium changed circadian period length independently and additively, showing no interaction. SNRs for all genotypes converged on a low level as deuterium concentration increased. Deuterium increased life span, except at high concentrations (40 and 50%).

Visual receptor cycle in normal and period mutant Drosophila: microspectrophotometry, electrophysiology, and ultrastructural morphometry

Visual pigment, sensitivity, and rhabdomere size were measured throughout a 12-h light/12-h dark cycle in Drosophila. Visual pigment and sensitivity were measured during subsequent constant darkness [dark/dark (D/D)]. MSP (microspectrophotometry) and the ERG (electroretinogram) revealed a cycling of visual pigment and sensitivity, respectively. A visual pigment decrease of 40% was noted at 4 h after light onset that recovered 2-4 h later in white-eyed (otherwise wild-type, w per+) flies. The ERG sensitivity [in w per+ flies in light/dark (L/D)] decreased by 75% at 4 h after light onset, more than expected if mediated by visual pigment (MSP) changes alone. ERG sensitivity begins decreasing 8 h before light onset while decreases in visual pigment begin 2 h after light onset. These cycles continue in constant darkness (D/D), suggesting a circadian rhythm. White-eyed period (per) mutants show similar cycles of visual pigment level and sensitivity in L/D; per's alterations, if any on the D/D cycles were subtle. The cross-sectional areas of rhabdomeres in w per+ were measured using electron micrographic (EM) morphometry. Area changed little through the L/D cycle.

The Drosophila per gene homologs are expressed in mammalian suprachiasmatic nucleus and heart as well as in molluscan eyes

This study presents evidence for the conservation of Drosophila per gene homologs in mammalian DNA and for their expression in a number of tissues which are involved in various aspects of circadian timekeeping. Distinct 5 kb sequences, which hybridized to a non repetitive fragment of the Drosophila per gene under stringent conditions, were detected by Southern blotting. Sequences homologous to per gene of Drosophila were also amplified from rat and mouse brain cDNA libraries and from a mouse anterior hypothalamus and human hypothalamus libraries. Degenerate PCR primer design was based on conserved segments of the per protein. The per homologs were shown directly (by RT-PCR) to be expressed in hamster and mouse SCN, in hamster heart and in Aplysia and Bulla eyes.

Drosophila single-minded gene and the molecular genetics of CNS midline development

Our goal is to understand the molecular mechanisms that govern the formation of the central nervous system. In particular, we have focused on the development of a small group of neurons and glia that lie along the midline of the Drosophila CNS. These midline cells possess a number of unique attributes which make them particularly amenable to molecular, cellular, and genetic examinations of nervous system formation and function. In addition, the midline cells exhibit distinctive ontogeny, morphology, anatomical position, and patterns of gene expression which suggest that they may provide unique functions to the developing CNS. The single-minded gene encodes a nuclear protein which is specifically expressed in the midline cells and has been shown to play a crucial role in midline cell development and CNS formation. Genetic experiments reveal that sim is required for the expression of many CNS midline genes which are thought to be involved in the proper differentiation of these cells. In order to identify additional genes which are expressed in some or all of the midline cells at different developmental stages, a technique known as enhancer trap screening was employed. This screen led to the identification of a large number of potential genes which exhibit various midline expression patterns and may be involved in discrete aspects of midline cell development. Further molecular, genetic, and biochemical analyses of sim and several of the enhancer trap lines are being pursued. This should permit elucidation of the genetic hierarchy which acts in the specification, differentiation, and function of these CNS midline cells.

Ontogeny of a biological clock in Drosophila melanogaster

Drosophila melanogaster born and reared in constant darkness exhibit circadian locomotor activity rhythms as adults. However, the rhythms of the individual flies composing these populations are not synchronized with one another. This lack of synchrony is evident in populations of flies commencing development at the same time, indicating that a biological clock controlling circadian rhythmicity in Drosophila begins to function without a requirement for light and without a developmentally imparted phase. It is possible to synchronize the phases of rhythms produced by dark-reared flies with light treatments ending as early as the developmental transition from embryo to first-instar larva: Light treatments occurring at developmental times preceding hatching of the first-instar larva fail to synchronize adult locomotor activity rhythms, while treatments ending at completion of larval hatching entrain these rhythms. The synchronized rhythmic behavior of adult flies receiving such light treatments suggests that a clock controlling circadian rhythms may function continuously from the time of larval hatching to adulthood.