Casein kinase Iepsilon modulates the signaling specificities of dishevelled

Wnt signaling is critical to many aspects of development, and aberrant activation of the Wnt signaling pathway can cause cancer. Dishevelled (Dvl) protein plays a central role in this pathway by transducing the signal from the Wnt receptor complex to the beta-catenin destruction complex. Dvl also plays a pivotal role in the planar cell polarity pathway that involves the c-Jun N-terminal kinase (JNK). How functions of Dvl are regulated in these two distinct pathways is not clear. We show that deleting the C-terminal two-thirds of Dvl, which includes the PDZ and DEP domains and is essential for Dvl-induced JNK activation, rendered the molecule a much more potent activator of the beta-catenin pathway. We also found that casein kinase Iepsilon (CKIepsilon), a previously identified positive regulator of Wnt signaling, stimulated Dvl activity in the Wnt pathway, but dramatically inhibited Dvl activity in the JNK pathway. Consistent with this, overexpression of CKIepsilon in Drosophila melanogaster stimulated Wnt signaling and disrupted planar cell polarity. We also observed a correlation between the localization and the signaling activity of Dvl in the beta-catenin pathway and the JNK pathway. Furthermore, by using RNA interference, we demonstrate that the Drosophila CKIepsilon homologue Double time positively regulates the beta-catenin pathway through Dvl and negatively regulates the Dvl-induced JNK pathway. We suggest that CKIepsilon functions as a molecular switch to direct Dvl from the JNK pathway to the beta-catenin pathway, possibly by altering the conformation of the C terminus of Dvl.

The Doubletime and CKII Kinases Collaborate to Potentiate Drosophila PER Transcriptional Repressor Activity

In all genetically studied model organisms, a negative feedback loop of gene expression contributes to the circadian rhythm mechanism. In the Drosophila system, it has been proposed that the delay between the synthesis and function of clock proteins is due to phosphorylation-regulated nuclear entry. To test this hypothesis, we assayed the relationship between PER phosphorylation, nuclear localization, and transcriptional repression activity in cultured S2 cells. The results indicate that the two putative PER kinases DBT and CKII work together to phosphorylate PER and increase repression activity. Experiments combining kinase inhibition with inhibition of PER nuclear export suggest that phosphorylation directly affects PER repression activity and that PER nuclear localization is an indirect consequence of the association of active PER with DNA or chromatin. This interpretation suggests further that the circadian regulation of PER nuclear localization in flies reflects changes in PER transcriptional activity rather than in PER nuclear import or export activity.

Role for Slimb in the degradation of Drosophila Period protein phosphorylated by Doubletime

Protein phosphorylation has a key role in modulating the stabilities of circadian clock proteins in a manner specific to the time of day. A conserved feature of animal clocks is that Period (Per) proteins undergo daily rhythms in phosphorylation and levels, events that are crucial for normal clock progression. Casein kinase Iepsilon (CKIepsilon) has a prominent role in regulating the phosphorylation and abundance of Per proteins in animals. This was first shown in Drosophila with the characterization of Doubletime (Dbt), a homologue of vertebrate casein kinase Iepsilon. However, it is not clear how Dbt regulates the levels of Per. Here we show, using a cell culture system, that Dbt promotes the progressive phosphorylation of Per, leading to the rapid degradation of hyperphosphorylated isoforms by the ubiquitin-proteasome pathway. Slimb, an F-box/WD40-repeat protein functioning in the ubiquitin-proteasome pathway interacts preferentially with phosphorylated Per and stimulates its degradation. Overexpression of slimb or expression in clock cells of a dominant-negative version of slimb disrupts normal rhythmic activity in flies. Our findings suggest that hyperphosphorylated Per is targeted to the proteasome by interactions with Slimb.

The Drosophila double-timeS mutation delays the nuclear accumulation of period protein and affects the feedback regulation of period mRNA

The Drosophila double-time (dbt) gene, which encodes a protein similar to vertebrate epsilon and delta isoforms of casein kinase I, is essential for circadian rhythmicity because it regulates the phosphorylation and stability of period (per) protein. Here, the circadian phenotype of a short-period dbt mutant allele (dbt(S)) was examined. The circadian period of the dbt(S) locomotor activity rhythm varied little when tested at constant temperatures ranging from 20 to 29 degrees C. However, per(L);dbt(S) flies exhibited a lack of temperature compensation like that of the long-period mutant (per(L)) flies. Light-pulse phase-response curves were obtained for wild-type, the short-period (per(S)), and dbt(S) genotypes. For the per(S) and dbt(S) genotypes, phase changes were larger than those for wild-type flies, the transition period from delays to advances was shorter, and the light-insensitive period was shorter. Immunohistochemical analysis of per protein levels demonstrated that per protein accumulates in photoreceptor nuclei later in dbt(S) than in wild-type and per(S) flies, and that it declines to lower levels in nuclei of dbt(S) flies than in nuclei of wild-type flies. Immunoblot analysis of per protein levels demonstrated that total per protein accumulation in dbt(S) heads is neither delayed nor reduced, whereas RNase protection analysis demonstrated that per mRNA accumulates later and declines sooner in dbt(S) heads than in wild-type heads. These results suggest that dbt can regulate the feedback of per protein on its mRNA by delaying the time at which it is translocated to nuclei and altering the level of nuclear PER during the declining phase of the cycle.

Phosphorylation of period is influenced by cycling physical associations of double-time, period, and timeless in the Drosophila clock

The clock gene double-time (dbt) encodes an ortholog of casein kinase Iepsilon that promotes phosphorylation and turnover of the PERIOD protein. Whereas the period (per), timeless (tim), and dClock (dClk) genes of Drosophila each contribute cycling mRNA and protein to a circadian clock, dbt RNA and DBT protein are constitutively expressed. Robust circadian changes in DBT subcellular localization are nevertheless observed in clock-containing cells of the fly head. These localization rhythms accompany formation of protein complexes that include PER, TIM, and DBT, and reflect periodic redistribution between the nucleus and the cytoplasm. Nuclear phosphorylation of PER is strongly enhanced when TIM is removed from PER/TIM/DBT complexes. The varying associations of PER, DBT and TIM appear to determine the onset and duration of nuclear PER function within the Drosophila clock.

Casein kinase I: another cog in the circadian clockworks

Multiple components of the circadian central clock are phosphoproteins, and it has become increasingly clear that posttranslational modification is an important regulator of circadian rhythm in diverse organisms, from dinoflagellates to humans. Genetic studies in Drosophila have identified double-time (dbt), a serine/threonine protein kinase that is highly homologous to human casein kinase I epsilon (CKIepsilon), as the first kinase linked to behavioral rhythms. Identification of a missense mutation in CKIepsilon as the tau mutation in the Syrian hamster places CKIepsilon within the core clock machinery in mammals. Most recently, identification of a phosphorylation site mutant of hPER2 in a family with an inherited circadian rhythm abnormality strongly suggests that PER2 is a physiologically relevant substrate of CKI. Phosphorylation may regulate multiple properties of clock proteins, including stability and intracellular localization.

Short-period mutations of per affect a double-time-dependent step in the Drosophila circadian clock

Circadian (24 hour) PERIOD (PER) protein oscillation is dependent on the double-time (dbt) gene, a casein kinase Ivarepsilon homolog [1-3]. Without dbt activity, hypophosphorylated PER proteins over-accumulate, indicating that dbt is required for PER phosphorylation and turnover [3,4]. There is evidence of a similar role for casein kinase Ivarepsilon in the mammalian circadian clock [5,6]. We have isolated a new dbt allele, dbt(ar), which causes arrhythmic locomotor activity in homozygous viable adults, as well as molecular arrhythmicity, with constitutively high levels of PER proteins, and low levels of TIMELESS (TIM) proteins. Short-period mutations of per, but not of tim, restore rhythmicity to dbt(ar) flies. This suppression is accompanied by a restoration of PER protein oscillations. Our results suggest that short-period per mutations, and mutations of dbt, affect the same molecular step that controls nuclear PER turnover. We conclude that, in wild-type flies, the previously defined PER'short domain' [7,8] may regulate the activity of DBT on PER.

Two novel doubletime mutants alter circadian properties and eliminate the delay between RNA and protein in Drosophila

Phosphorylation is an important feature of pacemaker organization in Drosophila. Genetic and biochemical evidence suggests involvement of the casein kinase I homolog doubletime (dbt) in the Drosophila circadian pacemaker. We have characterized two novel dbt mutants. Both cause a lengthening of behavioral period and profoundly alter period (per) and timeless (tim) transcript and protein profiles. The PER profile shows a major difference from the wild-type program only during the morning hours, consistent with a prominent role for DBT during the PER monomer degradation phase. The transcript profiles are delayed, but there is little effect on the protein accumulation profiles, resulting in the elimination of the characteristic lag between the mRNA and protein profiles. These results and others indicate that light and post-transcriptional regulation play major roles in defining the temporal properties of the protein curves and suggest that this lag is unnecessary for the feedback regulation of per and tim protein on per and tim transcription.

double-time is identical to discs overgrown, which is required for cell survival, proliferation and growth arrest in Drosophila imaginal discs

We have isolated the discs overgrown gene of Drosophila and shown that it encodes a homolog of the Casein kinase I(delta)/(epsilon) subfamily and is identical to the double-time gene. However, in contrast to the weak double-time alleles, which appear to affect only the circadian rhythm, discs overgrown alleles, including bona fide null alleles, show strong effects on cell survival and growth control in imaginal discs. Analysis of their phenotypes and molecular lesions suggests that the Discs overgrown protein is a crucial component in the mechanism that links cell survival during proliferation to growth arrest in imaginal discs. This work provides the first analysis in a multicellular organism of Casein kinase I(delta)/(epsilon) functions necessary for survival. Since the amino acid sequences and three-dimensional structures of Casein kinase I(delta)/(epsilon) enzymes are highly conserved, the results suggest that these proteins may also function in controlling cell growth and survival in other organisms.

Requirement of circadian genes for cocaine sensitization in Drosophila

The circadian clock consists of a feedback loop in which clock genes are rhythmically expressed, giving rise to cycling levels of RNA and proteins. Four of the five circadian genes identified to date influence responsiveness to freebase cocaine in the fruit fly, Drosophila melanogaster. Sensitization to repeated cocaine exposures, a phenomenon also seen in humans and animal models and associated with enhanced drug craving, is eliminated in flies mutant for period, clock, cycle, and doubletime, but not in flies lacking the gene timeless. Flies that do not sensitize owing to lack of these genes do not show the induction of tyrosine decarboxylase normally seen after cocaine exposure. These findings indicate unexpected roles for these genes in regulating cocaine sensitization and indicate that they function as regulators of tyrosine decarboxylase.

The molecular control of circadian behavioral rhythms and their entrainment in Drosophila

Molecular and genetic characterizations of circadian rhythms in Drosophila indicate that function of an intracellular pacemaker requires the activities of proteins encoded by three genes: period (per), timeless (tim), and doubletime (dbt). RNA from two of these genes, per and tim, is expressed with a circadian rhythm. Heterodimerization of PER and TIM proteins allows nuclear localization and suppression of further RNA synthesis by a PER/TIM complex. These protein interactions promote cyclical gene expression because heterodimers are observed only at high concentrations of per and tim RNA, separating intervals of RNA accumulation from times of PER/TIM complex activity. Light resets these molecular cycles by eliminating TIM. The product of dbt also regulates accumulation of per and tim RNA, and it may influence action of the PER/TIM complex. The recent discovery of PER homologues in mice and humans suggests that a related mechanism controls mammalian circadian behavioral rhythms.

The Drosophila clock gene double-time encodes a protein closely related to human casein kinase Iepsilon

The cloning of double-time (dbt) is reported. DOUBLETIME protein (DBT) is most closely related to human casein kinase Iepsilon. dbtS and dbtL mutations, which alter period length of Drosophila circadian rhythms, produce single amino acid changes in conserved regions of the predicted kinase. dbtP mutants, which eliminate rhythms of per and tim expression and constitutively overproduce hypophosphorylated PER proteins, abolish most dbt expression. dbt mRNA appears to be expressed in the same cell types as are per and tim and shows no evident oscillation in wild-type heads. DBT is capable of binding to PER in vitro and in Drosophila cells, suggesting that a physical association of PER and DBT regulates PER phosphorylation and accumulation in vivo.