NEMO/NLK phosphorylates PERIOD to initiate a time-delay phosphorylation circuit that sets circadian clock speed.

The speed of circadian clocks in animals is tightly linked to complex phosphorylation programs that drive daily cycles in the levels of PERIOD (PER) proteins. Using Drosophila, we identify a time-delay circuit based on hierarchical phosphorylation that controls the daily downswing in PER abundance. Phosphorylation by the NEMO/NLK kinase at the "per-short" domain on PER stimulates phosphorylation by DOUBLETIME (DBT/CK1δ/ɛ) at several nearby sites. This multisite phosphorylation operates in a spatially oriented and graded manner to delay progressive phosphorylation by DBT at other more distal sites on PER, including those required for recognition by the F box protein SLIMB/ß-TrCP and proteasomal degradation. Highly phosphorylated PER has a more open structure, suggesting that progressive increases in global phosphorylation contribute to the timing mechanism by slowly increasing PER susceptibility to degradation. Our findings identify NEMO as a clock kinase and demonstrate that long-range interactions between functionally distinct phospho-clusters collaborate to set clock speed.

A novel indole derivative, 2-{3-[1-(benzylsulfonyl)piperidin-4-yl]-2-methyl-1H-indol-1-yl}-1-(pyrrolidin-1-yl)ethenone, suppresses hedgehog signaling and drug-resistant tumor growth.

The Hedgehog (Hh) signaling pathway plays important roles in various physiological functions. Several malignancies, such as basal cell carcinoma (BCC) and medulloblastoma (MB), have been linked to the aberrant activation of Hh signaling. Although therapeutic drugs have been developed to inhibit Hh pathway-dependent cancer growth, drug resistance remains a major obstacle in cancer treatment. Here, we show that the newly identified, 2-{3-[1-(benzylsulfonyl)-1,2,3,6-tetrahydropyridin-4-yl]-2-methyl-1H-indol-1-yl}-1-(pyrrolidin-1-yl)ethenone analog (LKD1214) exhibits comparable potency to vismodegib in suppressing the Hh pathway activation. LKD1214 represses Smoothened (SMO) activity by blocking its ciliary translocation. Interestingly, we also identified that it has a distinctive binding interface with SMO compared with other SMO-regulating chemicals. Notably, it maintains an inhibitory activity against the SmoD477H mutant, as observed in a patient with vismodegib-resistant BCC. Furthermore, LKD1214 inhibits tumor growth in the mouse model of MB. Collectively, these findings suggest that LKD1214 has the therapeutic potential to overcome drug-resistance in Hh-dependent cancers.

KLC3 Regulates Ciliary Trafficking and Cyst Progression in CILK1 Deficiency-Related Polycystic Kidney Disease.

BACKGROUND: Ciliogenesis-associated kinase 1 (CILK1) is a ciliary gene that localizes in primary cilia and regulates ciliary transport. Mutations in CILK1 cause various ciliopathies. However, the pathogenesis of CILK1-deficient kidney disease is unknown. METHODS: To examine whether CILK1 deficiency causes PKD accompanied by abnormal cilia, we generated mice with deletion of Cilk1 in cells of the renal collecting duct. A yeast two-hybrid system and coimmunoprecipitation (co-IP) were used to identify a novel regulator, kinesin light chain-3 (KLC3), of ciliary trafficking and cyst progression in the Cilk1-deficient model. Immunocytochemistry and co-IP were used to examine the effect of KLC3 on ciliary trafficking of the IFT-B complex and EGFR. We evaluated the effects of these genes on ciliary trafficking and cyst progression by modulating CILK1 and KLC3 expression levels. RESULTS: CILK1 deficiency leads to PKD accompanied by abnormal ciliary trafficking. KLC3 interacts with CILK1 at cilia bases and is increased in cyst-lining cells of CILK1-deficient mice. KLC3 overexpression promotes ciliary recruitment of IFT-B and EGFR in the CILK1 deficiency condition, which contributes to the ciliary defect in cystogenesis. Reduction in KLC3 rescued the ciliary defects and inhibited cyst progression caused by CILK1 deficiency. CONCLUSIONS: Our findings suggest that CILK1 deficiency in renal collecting ducts leads to PKD and promotes ciliary trafficking via increased KLC3.

Fibroblast growth factor receptor influences primary cilium length through an interaction with intestinal cell kinase.

Vertebrate primary cilium is a Hedgehog signaling center but the extent of its involvement in other signaling systems is less well understood. This report delineates a mechanism by which fibroblast growth factor (FGF) controls primary cilia. Employing proteomic approaches to characterize proteins associated with the FGF-receptor, FGFR3, we identified the serine/threonine kinase intestinal cell kinase (ICK) as an FGFR interactor. ICK is involved in ciliogenesis and participates in control of ciliary length. FGF signaling partially abolished ICK's kinase activity, through FGFR-mediated ICK phosphorylation at conserved residue Tyr15, which interfered with optimal ATP binding. Activation of the FGF signaling pathway affected both primary cilia length and function in a manner consistent with cilia effects caused by inhibition of ICK activity. Moreover, knockdown and knockout of ICK rescued the FGF-mediated effect on cilia. We provide conclusive evidence that FGF signaling controls cilia via interaction with ICK.

Upregulated Guanine Deaminase Is Involved in Hyperpigmentation of Seborrheic Keratosis via Uric Acid Release.

Seborrheic keratosis, which is a benign tumor composed of epidermal keratinocytes, develops common in the elderly. Uric acid generated by upregulated guanine deaminase (GDA) has been identified to cause UV-induced keratinocyte senescence in seborrheic keratosis. Seborrheic keratosis is also frequently pigmented. Growing evidences indicate that hyperuricemia is a risk factor of acanthosis nigricans, an acquired skin hyperpigmentation. The objective of this study was to investigate role of GDA and its metabolic end product, uric acid, in hyperpigmentation of patients with seborrheic keratosis using their lesional and non-lesional skin specimen sets and cultured primary human epidermal keratinocytes with or without GDA overexpression or uric acid treatment. GDA-overexpressing keratinocytes or their conditioned media containing uric acid increased expression levels of MITF and tyrosinase in melanocytes. Uric acid released from keratinocytes was facilitated by ABCG2 transporter with the help of PDZK1 interaction. Released uric acid was taken by URAT1 transporter in melanocytes, stimulating melanogenesis through p38 MAPK activation. Overall, GDA upregulation in seborrheic keratosis plays a role in melanogenesis via its metabolic end product uric acid, suggesting that seborrheic keratosis as an example of hyperpigmentation associated with photoaging.

Cell Cycle-Related Kinase (CCRK) regulates ciliogenesis and Hedgehog signaling in mice.

The Hedgehog (Hh) signaling pathway plays a key role in cell fate specification, proliferation, and survival during mammalian development. Cells require a small organelle, the primary cilium, to respond properly to Hh signals and the key regulators of Hh signal transduction exhibit dynamic localization to this organelle when the pathway is activated. Here, we investigate the role of Cell Cycle Related kinase (CCRK) in regulation of cilium-dependent Hh signaling in the mouse. Mice mutant for Ccrk exhibit a variety of developmental defects indicative of inappropriate regulation of this pathway. Cell biological, biochemical and genetic analyses indicate that CCRK is required to control the Hedgehog pathway at the level or downstream of Smoothened and upstream of the Gli transcription factors, Gli2 and Gli3. In vitro experiments indicate that Ccrk mutant cells show a greater deficit in response to signaling over long time periods than over short ones. Similar to Chlamydomonas mutants lacking the CCRK homolog, LF2, mouse Ccrk mutant cells show defective regulation of ciliary length and morphology. Ccrk mutant cells exhibit defects in intraflagellar transport (the transport mechanism used to assemble cilia), as well as slowed kinetics of ciliary enrichment of key Hh pathway regulators. Collectively, the data suggest that CCRK positively regulates the kinetics by which ciliary proteins such as Smoothened and Gli2 are imported into the cilium, and that the efficiency of ciliary recruitment allows for potent responses to Hedgehog signaling over long time periods.

An inactivating mutation in intestinal cell kinase, ICK, impairs hedgehog signalling and causes short rib-polydactyly syndrome.

The short rib polydactyly syndromes (SRPS) are a group of recessively inherited, perinatal-lethal skeletal disorders primarily characterized by short ribs, shortened long bones, varying types of polydactyly and concomitant visceral abnormalities. Mutations in several genes affecting cilia function cause SRPS, revealing a role for cilia function in skeletal development. To identify additional SRPS genes and discover novel ciliary molecules required for normal skeletogenesis, we performed exome sequencing in a cohort of patients and identified homozygosity for a missense mutation, p.E80K, in Intestinal Cell Kinase, ICK, in one SRPS family. The p.E80K mutation abolished serine/threonine kinase activity, resulting in altered ICK subcellular and ciliary localization, increased cilia length, aberrant cartilage growth plate structure, defective Hedgehog and altered ERK signalling. These data identify ICK as an SRPS-associated gene and reveal that abnormalities in signalling pathways contribute to defective skeletogenesis.

PEGylated hyaluronic acid-coated liposome for enhanced in vivo efficacy of sorafenib via active tumor cell targeting and prolonged systemic exposure.

This study aimed to design an effective formulation for enhancing the tumor-targeted delivery of sorafenib. Three sorafenib-loaded liposomal formulations including uncoated liposome (SF-Lip), hyaluronic acid-coated liposome (HA-SF-Lip), and PEGylated hyaluronic acid-coated liposome (PEG-HA-SF-Lip) were developed with narrow size distribution and high encapsulation efficiency. The cellular uptake and cytotoxicity of HA-SF-Lip and PEG-HA-SF-Lip were greater than those of SF-Lip in MDA-MB-231 cells overexpressing CD44, whereas there were no significant differences in MCF-7 cells with low CD44 expression, indicating the CD44-mediated cellular uptake of coated liposomes. In comparison with sorafenib solution, PEG-HA-SF-Lip increased the systemic exposure and plasma half-life in rats by 3-fold and 2-fold, respectively. Consistently, PEG-HA-SF-Lip was the most effective for tumor growth inhibition through CD44 targeting in the MDA-MB-231 tumor xenograft mouse model. Taken together, the present study suggests that PEG-HA-SF-Lip might be effective for the tumor-targeted delivery of sorafenib with enhanced systemic exposure and longer blood circulation.

Vaccinia-related kinase 3 (VRK3) sets the circadian period and amplitude by affecting the subcellular localization of clock proteins in mammalian cells.

In the eukaryotic circadian clock machinery, negative feedback repression of CLOCK (CLK) and BMAL1 transcriptional activity by PERIOD (PER) and CRYPTOCHROME (CRY) underlies the basis for 24 h rhythmic gene expression. Thus, precise regulation of the time-dependent nuclear entry of circadian repressors is crucial to generating normal circadian rhythms. Here, we sought to identify novel kinase(s) that regulate nuclear entry of mammalian CRY1 (mCRY1) with an unbiased screening using red fluorescent protein (RFP)-tagged human kinome expression plasmids in mammalian cells. Transient expression of human vaccinia-related kinase 3 (hVRK3) reduced the nuclear presence of mCRY1. hVRK3 expression also induced alterations in the subcellular localization of other core clock proteins, including mCRY2, mPER2, and BMAL1. In contrast, the subcellular localization of mCLK was not changed. Given that singly expressed mCLK mostly resides in the cytoplasm and that nuclear localization sequence (NLS) mutation of hVRK3 attenuated the effect of hVRK3 co-expression on subcellular localization, ectopically expressed hVRK3 presumably reduces the retention of proteins in the nucleus. Finally, downregulation of hvrk3 using siRNA reduced the amplitude and lengthened the period of the cellular bioluminescence rhythm. Taken together, these data suggest that VRK3 plays a role in setting the amplitude and period length of circadian rhythms in mammalian cells.

Injury-stimulated Sonic hedgehog expression in microglia contributes to neuroinflammatory response in the MPTP model of Parkinson's disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder in which dopamine (DA) neurons in the substantia nigra pars compacta (SNpc) region are selectively destroyed. Sonic hedgehog (Shh) has been well known to play a key role in a variety of processes such as embryogenesis, cell proliferation and protection, and tissue repair during inflammation. However, the evidences for the innate role of Shh in adult brain injury are presently lacking and studies have been needed to unveil the importance of Shh in the process of neurodegeneration. Here, we investigated the role of Shh in the pathologic progress of Parkinson's disease in MPTP-induced animal model system. Interestingly, we observed that Shh expression was gradually increased in MPTP affected SNpc region. Activated microglia exclusively expressed SHH in vivo and we could recapitulate Shh induction in activated cultured primary microglia cells. Using the SHH responsive Cre-loxP binary genetic reporter transgenic mouse system, we also found that most of the cell types except for oligodendrocyte in the SNpc region reacted to the SHH by MPTP injection. Taken together, activated microglia induced Shh expression and most neural cells except oligodendrocyte responded to microglia-derived SHH in MPTP-treated SN. These results suggest that SHH in activated microglia by MPTP-injection might be involved in the innate processes of recovery from neurotoxin induced injury in the PD animal model system.

Intestinal cell kinase, a protein associated with endocrine-cerebro-osteodysplasia syndrome, is a key regulator of cilia length and Hedgehog signaling.

Endocrine-cerebro-osteodysplasia (ECO) syndrome is a recessive genetic disorder associated with multiple congenital defects in endocrine, cerebral, and skeletal systems that is caused by a missense mutation in the mitogen-activated protein kinase-like intestinal cell kinase (ICK) gene. In algae and invertebrates, ICK homologs are involved in flagellar formation and ciliogenesis, respectively. However, it is not clear whether this role of ICK is conserved in mammals and how a lack of functional ICK results in the characteristic phenotypes of human ECO syndrome. Here, we generated Ick knockout mice to elucidate the precise role of ICK in mammalian development and to examine the pathological mechanisms of ECO syndrome. Ick null mouse embryos displayed cleft palate, hydrocephalus, polydactyly, and delayed skeletal development, closely resembling ECO syndrome phenotypes. In cultured cells, down-regulation of Ick or overexpression of kinase-dead or ECO syndrome mutant ICK resulted in an elongation of primary cilia and abnormal Sonic hedgehog (Shh) signaling. Wild-type ICK proteins were generally localized in the proximal region of cilia near the basal bodies, whereas kinase-dead ICK mutant proteins accumulated in the distal part of bulged ciliary tips. Consistent with these observations in cultured cells, Ick knockout mouse embryos displayed elongated cilia and reduced Shh signaling during limb digit patterning. Taken together, these results indicate that ICK plays a crucial role in controlling ciliary length and that ciliary defects caused by a lack of functional ICK leads to abnormal Shh signaling, resulting in congenital disorders such as ECO syndrome.

Ciliary smoothened-mediated noncanonical hedgehog signaling promotes tubulin acetylation.

Hedgehog (Hh) signaling plays key roles in animal development and tissue homeostasis. Binding of the secreted ligand to its Ptch1 receptor triggers Hh signaling through distinct canonical or noncanonical signaling pathways. Canonical Hh signaling leads to the activation of Gli transcription factors to induce Hh target-gene expression. In contrast, noncanonical Hh signaling regulates cytoskeleton rearrangement and apoptosis. Recently, it has been shown that primary cilia are important for canonical Hh signaling, but the ciliary role for signaling through the noncanonical pathway remains unresolved. Here, we examine the role of primary cilia in noncanonical Hh signaling in cultured mammalian cells. We found that Hh pathway activation in mouse embryonic fibroblast cells (MEFs) increases microtubule acetylation via smoothened (Smo), and suppression of Hh signaling by a Smo antagonist abrogates the microtubule acetylation. Using genetically engineered MEFs, we revealed that the increase in microtubule acetylation by Hh is dependent on Smo, but not on Sufu or Gli. In Kif3a-/- MEFs, which cannot form primary cilia, we observed that primary cilia were required for transducing noncanonical Hh signaling. Furthermore, we revealed that an increase in intracellular calcium is important for Hh-dependent tubulin acetylation at the downstream of Smo. Collectively, these findings suggest that Smo and primary cilia-dependent noncanonical Hh signaling leads to post-translational regulation of microtubules and may be important for modulating cell behaviors.

TRPV1 on astrocytes rescues nigral dopamine neurons in Parkinson's disease via CNTF.

Currently there is no neuroprotective or neurorestorative therapy for Parkinson's disease. Here we report that transient receptor potential vanilloid 1 (TRPV1) on astrocytes mediates endogenous production of ciliary neurotrophic factor (CNTF), which prevents the active degeneration of dopamine neurons and leads to behavioural recovery through CNTF receptor alpha (CNTFRα) on nigral dopamine neurons in both the MPP(+)-lesioned or adeno-associated virus α-synuclein rat models of Parkinson's disease. Western blot and immunohistochemical analysis of human post-mortem substantia nigra from Parkinson's disease suggests that this endogenous neuroprotective system (TRPV1 and CNTF on astrocytes, and CNTFRα on dopamine neurons) might have relevance to human Parkinson's disease. Our results suggest that activation of astrocytic TRPV1 activates endogenous neuroprotective machinery in vivo and that it is a novel therapeutic target for the treatment of Parkinson's disease.

Drosophila peptidyl-prolyl isomerase Pin1 modulates circadian rhythms via regulating levels of PERIOD.

In animal circadian clock machinery, the phosphorylation program of PERIOD (PER) leads to the spatio-temporal regulation of diverse PER functions, which are crucial for the maintenance of ~24-hr circadian rhythmicity. The peptidyl-prolyl isomerase PIN1 modulates the diverse functions of its substrates by inducing conformational changes upon recognizing specific phosphorylated residues. Here, we show that overexpression of Drosophila pin1, dodo (dod), lengthens the locomotor behavioral period. Using Drosophila S2 cells, we demonstrate that Dod associates preferentially with phosphorylated species of PER, which delays the phosphorylation-dependent degradation of PER. Consistent with this, PER protein levels are higher in flies overexpressing dod. Taken together, we suggest that Dod plays a role in the maintenance of circadian period by regulating PER metabolism.

Pax3 function is required specifically for inner ear structures with melanogenic fates.

Pax3 mutations result in malformed inner ears in Splotch mutant mice and hearing loss in humans with Waardenburg's syndrome type I. In the inner ear, Pax3 is thought to be involved mainly in the development of neural crest. However, recent studies have shown that Pax3-expressing cells contribute extensively to multiple inner ear structures, some of which were considered to be derived from the otic epithelium. To examine the specific functions of Pax3 during inner ear development, fate mapping of Pax3 lineage was performed in the presence or absence of functional Pax3 proteins using Pax3(Cre) knock-in mice bred to Rosa26 reporter (R26R) line. ß-gal-positive cells were widely distributed in Pax3(Cre/+); R26R inner ears at embryonic day (E) 15.5, including the endolymphatic duct, common crus, cristae, maculae, cochleovestibular ganglion, and stria vascularis. In the absence of Pax3 in Pax3(Cre/Cre); R26R inner ears, ß-gal-positive cells disappeared from regions with melanocytes such as the stria vascularis of the cochlea and dark cells in the vestibule. Consistently, the expression of Dct, a melanoblast marker, was also absent in the mutant inner ears. However, when examined at E11.5, ß-gal positive cells were present in Pax3(Cre/Cre) mutant otocysts, whereas Dct expression was absent, suggesting that Pax3 lineage with a melanogenic fate migrated to the inner ear, yet failed to differentiate and survive without Pax3 function. Gross inner ear morphology was generally normal in Pax3(Cre/Cre) mutants, unless neural tube defects extended to the cranial region. Taken together, these results suggest that despite the extensive contribution of Pax3-expressing cells to multiple inner ear tissues, Pax3 function is required specifically for inner ear components with melanogenic fates.

Fabrication of liraglutide-encapsulated triple layer hyaluronic acid microneedles (TLMs) for the treatment of obesity.

Obesity is a chronic metabolic disease that is prevalent worldwide, causing complications that affect the quality of life and longevity of humans. Currently, the low bioavailability upon subcutaneous injection of an appetite suppressant, liraglutide, and health problems in the locally injected region remain to be overcome. In this study, we developed a novel hyaluronic acid-based liraglutide-encapsulated triple-layer microneedle (TLM) as a painless and patient-friendly long-term drug delivery system. In contrast to previous anti-obesity microneedle approaches, this TLM is composed of three layers for complete skin insertion, protecting the encapsulated liraglutide from environmental stresses. Daily topical application of the liraglutide-loaded TLM significantly reduced body weight and improved body composition in a mouse model of high-fat diet-induced obesity. Additionally, it ameliorated diet-induced hepatic steatosis in obese mice. This novel TLM could promote a glucagon-like peptide-1 drug release system for long-term daily administration with relatively higher patient compliance compared to subcutaneous injection.

Egg Microneedle for Transdermal Delivery of Active Liraglutide.

Liraglutide, a human glucagon-like peptide-1 (GLP-1) analog, is promising for safely treating type 2 diabetes mellitus (T2DM), compared to insulin, by significantly reducing the risk of glucose-dependent hypoglycemia. Concerns related to injection prevent T2DM patients from taking liraglutide regularly, even though once-a-day subcutaneous (SC) injections. Dissolving microneedles (DMNs) are promising substitutes for SC injection and for improving patient convenience. However, there are two fundamental limitations: the low drug delivery due to incomplete insertion and loss of drug activity during DMN fabrication. Here, it is shown that an egg microneedle (EMN) designed with three functional layered structures can maintain the maximum activity of the loaded compound during DMN fabrication and deliver it completely into the skin, with the base layer allowing the complete delivery of liraglutide, and the shell layer maintaining the drug activity by mimicking the role of albumin in eggs. In a diabetic mouse model, liraglutide administration via EMN exhibited similar effect when compared to that of injection. Therefore, EMN-mediated liraglutide administration is a good potential option for replacing liraglutide injections in T2DM treatment.

Tissue-specific requirement of sodium channel and clathrin linker 1 (Sclt1) for ciliogenesis during limb development.

Primary cilia have essential roles as signaling centers during development and adult homeostasis. Disruption of ciliary structure or function causes congenital human disorders called ciliopathies. Centriolar distal appendage (DAP) proteins are important for anchoring cilia to the membrane. However, the exact functions of DAP during in vivo ciliogenesis and animal development remain poorly understood. Here, we showed that the DAP component sodium channel and clathrin linker 1 (Sclt1) mutant mice had abnormal craniofacial and limb development with postnatal lethality. In mutant embryos, most of the affected tissues had defects in DAP recruitment to the basal body and docking to the membrane that resulted in reduced ciliogenesis and disrupted hedgehog (Hh) signaling in limb bud mesenchymal cells. However, limb digit formation and ciliogenesis in Sclt1 mutant mice were differentially affected between the fore- and hindlimb buds. The forelimbs developed normally in Sclt1 mutants, but the hindlimbs had preaxial polydactyly. Heterozygous loss of Cep83, another core DAP component, in Sclt1 mutant mice, caused forelimb and hindlimb polydactyly. These findings revealed the tissue-specific differential requirement of DAPs. Taken together, these results indicated that during limb development the ciliary base components, DAPs, play an essential role in ciliogenesis and Hh signaling in vivo in a position-dependent manner.

A hierarchical phosphorylation cascade that regulates the timing of PERIOD nuclear entry reveals novel roles for proline-directed kinases and GSK-3beta/SGG in circadian clocks.

The daily timing of when PERIOD (PER) proteins translocate from the cytoplasm to the nucleus is a critical step in clock mechanisms underpinning circadian rhythms in animals. Numerous lines of evidence indicate that phosphorylation plays a prominent role in regulating various aspects of PER function and metabolism, including changes in its daily stability and subcellular distribution. In this report, we show that phosphorylation of serine 661 (Ser661) by a proline-directed kinase(s) is a key phospho-signal on the Drosophila PER protein (dPER) that regulates the timing of its nuclear accumulation. Mutations that block phosphorylation at Ser661 do not affect dPER stability but delay its nuclear entry in key pacemaker neurons, yielding longer behavioral rhythms. Intriguingly, abolishing phosphorylation at Ser661 also attenuates the extent of dPER hyperphosphorylation in vivo, suggesting the phosphorylated state of Ser661 regulates phosphorylation at other sites on dPER. Indeed, we identify Ser657 as a site that is phosphorylated by the glycogen synthase kinase GSK-3ß (SHAGGY; SGG) in a manner dependent on priming at Ser661. Although not as dramatic as mutating Ser661, mutations that abolish phosphorylation at Ser657 also lead to longer behavioral periods, suggesting that a multi-kinase hierarchical phosphorylation module regulates the timing of dPER nuclear entry. Together with evidence in mammalian systems, our findings implicate proline-directed kinases in clock mechanisms and suggest that PER proteins are key downstream targets of lithium therapy, a potent inhibitor of GSK-3ß used to treat manic depression, a disorder associated with clock malfunction in humans.

Two distinct modes of PERIOD recruitment onto dCLOCK reveal a novel role for TIMELESS in circadian transcription.

Negative transcriptional feedback loops are a core feature of eukaryotic circadian clocks and are based on rhythmic interactions between clock-specific repressors and transcription factors. In Drosophila, the repression of dCLOCK (dCLK)-CYCLE (CYC) transcriptional activity by dPERIOD (dPER) is critical for driving circadian gene expression. Although growing lines of evidence indicate that circadian repressors such as dPER function, at least partly, as molecular bridges that facilitate timely interactions between other regulatory factors and core clock transcription factors, how dPER interacts with dCLK-CYC to promote repression is not known. Here, we identified a small conserved region on dPER required for binding to dCLK, termed CBD (for dCLK binding domain). In the absence of the CBD, dPER is unable to stably associate with dCLK and inhibit the transcriptional activity of dCLK-CYC in a simplified cell culture system. CBD is situated in close proximity to a region that interacts with other regulatory factors such as the DOUBLETIME kinase, suggesting that complex architectural constraints need to be met to assemble repressor complexes. Surprisingly, when dPER missing the CBD (dPER(ΔCBD)) was evaluated in flies the clock mechanism was operational, albeit with longer periods. Intriguingly, the interaction between dPER(ΔCBD) and dCLK is TIM-dependent and modulated by light, revealing a novel and unanticipated in vivo role for TIM in circadian transcription. Finally, dPER(ΔCBD) does not provoke the daily hyperphosphorylation of dCLK, indicating that direct interactions between dPER and dCLK are necessary for the dCLK phosphorylation program but are not required for other aspects of dCLK regulation.