Gene name - Cyclin D
Synonyms - Cdi3: Cyclin-dependent kinase interactor 3
Cytological map position -
Function - Regulatory subunit of cyclin dependent kinase
Keywords - cell cycle
Symbol - CycD
Genetic map position -
Classification - G1 cyclin
Cellular location - nuclear
The morphogenetic furrow of the developing Drosophila eye provides a model system for studying the role of cell cycle genes and their control of cell proliferation and differentiation. A depression in the apical surface of the eye imaginal disc epithelium of the third instar larva, the morphogenetic furrow moves from the posterior region of the disc to the anterior. The passing of the furrow results in cell differentiation and in the induction of synchronous cell proliferation marked by the orderly expression of cell cycle genes.
Prior to the passing of the furrow there are many S phase and mitotic cells expressing Cyclin A and Cyclin B. Initially, a domain of string expression occurs, prior to the passing of the furrow, found in a narrow band of cell roughly 5- to 6-cell diameters wide. Cyclin D expression reaches high levels immediately after the induction of string. The string domain represents a transition point: S phase ceases at its anterior boundary, expression of Cyclin A and Cyclin B terminates, and an increase in cells in mitosis is seen within the string domain (Thomas 1994). Significantly, the band of Cyclin D expression occurs before cells reenter into the cell cycle. Cyclin E is expressed posterior to the band of Cyclin D expression, in a region partially overlapping the synchronous band of S-phase cells (Thomas 1994).
What is the function of string and Cyclin D expression ahead of the developing furrow? The expression of string is accompanied by an increase in the number of cells in mitosis as would be expected if the function of string were to drive cells in G2 into a round of mitosis. This increase in density of mitotic activity represents a two fold increase over the number seen in the unpatterned region anterior to the furrow (Thomas, 1994).
Cells in G1 are inhibited from entering S phase. Thus some aspect of cell cycle regulation inhibits passage into S for a certain period of time. The occasional S phase cells seen overlapping the anterior edge of STG mRNA suggests that cells already in S phase complete it before progressing into G2. One gene involved in regulation of the G1 phase is roughex. Mutations in roughex cause cells to circumvent G1, and all cells enter S phase, including cells that would normally differentiate. This leads to defects in early steps of pattern formation and cell fate determination.
The G1 period is accompanied by cell shape changes that define the furrow and marks the beginning of cluster differentiation. In the midst of the furrow, cellular interactions play a role in restricting the number of R8 cells that form within the morphogenetic furrow to one per cluster. At the posterior edge of the morphogenetic furrow, cells are formed into regularly spaced preclusters that contain postmitotic precursors serving as the first five photoreceptor cells to differentiate, R8 and R2-5. Following an extended G1 period, marked by Cyclin D and Cyclin E expression and the passing of the furrow, the remaining cells pass synchronously into S phase (following the passage of the furrow) and then into G2 and M, accompanied by the expression and degradation of Cyclins A and B. The expression of Cyclin D and E prior to the passing of the furrow set the stage for the cycle of synchronous division in the cells that do not immediately differentiate. Paradoxically, although all cells passing through the furrow express Cyclin D and Cyclin E, only a fraction of those cells undergo the synchronized division that follows the passing of the furrow (Thomas, 1994 and Finley, 1996).
The three mammalian D-type cyclins are thought to promote progression throughthe G1 phase of the cell cycle as regulatory subunits ofcyclin-dependent kinase 4 and 6. In addition, they have been proposed to controlthe activity of various transcription factors without a partner kinase. This study describes phenotypic consequences of null mutations in Cyclin D, thesingle D-type cyclin gene in Drosophila. As previously observed with nullmutations in the single Drosophila Cdk4 gene, these mutations do notprimarily affect progression through the G1 phase. Moreover, theapparently indistinguishable phenotypes of double (CycD and Cdk4)and single mutants (CycD or Cdk4) argue against major independentfunctions of Cyclin D and Cdk4. The reduced cellular and organismal growth ratesobserved in both mutants indicate that Cyclin D-Cdk4 acts as a growth driver (Emmerich, 2004).
D-type cyclin-cdk complexes are of crucial importance in human tumorigenesis.Since these complexes have been conserved in evolution, it is readily possibleto use model organisms like D. melanogaster for functionalcharacterizations. This study extends previous characterization of DrosophilaCdk4 mutants by phenotypic comparisons with CycD mutants. Asobserved for Cdk4, Cyclin D is not required for progression throughthe G1 phase of the cell cycle. Some escapers develop to the adultstage even when both maternal and zygotic Cdk4+ orCycD+ function is abolished. Moreover, FACS analysesdemonstrate that the cell-cycle profile of wing-imaginal disc cells homozygousfor null mutations in Cdk4 or CycD is essentially indistinguishable from that of wildtype. The evidence therefore is not consistent with the prevailing idea thatD-type cyclin-cdk complexes primarily regulate progression through theG1 phase. In cultured mammalian cells, where the most support forthis suggestion has accumulated, D-type cyclin-cdk complexes have been shown toact in part by titrating CIP/KIP inhibitors away from CycE/Cdk2 complexes, whichare thus freed to stimulate cell-cycle progression.In contrast, binding of Dacapo, the single known DrosophilaCIP/KIP family member, to Drosophila Cyclin D-Cdk4 has not been detectable. This providesa potential explanation for the apparent discrepancy. It should be noted,however, that the strong genetic interactions reported among CycD,CycE, and Cdk2, previous interactiontests performed with Rbf indicate that DrosophilaCyclinD-Cdk4 complexes do play a significant, if redundant, activating role inthe E2F/RBF network, just as described in mammals. In the CycD andCdk4 mutants, CycE/Cdk2 complexes are presumably sufficient to perform this function (Emmerich, 2004).
While not revealing a specific role during G1, the Drosophila mutantphenotypes provide compelling evidence that Cyclin D-Cdk4 promotes cellulargrowth and thereby accelerates progression through all the cell-cycle phasesproportionally. CycD and Cdk4 mutants develop into small butnormally proportioned flies with an average weight of ~20% less than thatof wild-type siblings. Conversely, overexpression of Cyclin D and Cdk4 hasthe opposite effect, causingincreased growth in organs such as the eye, wing, and salivary glands. Moreover,growth regulation by Cyclin D and Cdk4 is also clearly apparent at the cellularlevel. Clones of wing-imaginal disc cells either lacking one of the CyclinD-Cdk4 complex partners or overexpressing the complex grow slower or faster,respectively, than wild-type clones (Emmerich, 2004).
In Drosophila, the growth-promoting function of Cyclin D-Cdk4 can beinterrogated using genetic approaches. Initial results have so far argued thatCyclin D-Cdk4 is not part of one of the other pathways (insulin/TOR, ras, myc,bantam), which are known to control cellular and organismal growth rates.However, the Hif-1 prolyl hydroxylase has been identified asa key growth stimulator downstream of Cyclin D-Cdk4. This raises thepossibility that Cyclin D-Cdk4 is interconnected with metabolic pathwayssensitive to oxygen levels. Future analysesmight therefore reveal whether an involvement in oxygen-related metabolismrepresents the evolutionary conserved role of Cyclin D-Cdk4 in multicellulareukaryotes and throw a new light on its significance in human tumors, whereoxygen limitation is a known and crucial challenge. Cyclin D-Cdk4 has alsorecently been implicated in the JAK-STAT pathway by an independent geneticapproach in Drosophila (Emmerich, 2004 and references therein).
This comparison of CycD and Cdk4 mutant phenotypes is also ofinterest with regard to functions provided by these proteins independently. Inparticular, D-type cyclins have been proposed to regulate a number oftranscription factors without a partner kinase. Moreover,overexpression of UAS-CycD alone or UAS-Cdk4 alone does often havephenotypic consequences that vary in extent with different GAL4 driver lines.For instance, ey-GAL4-driven UAS-CycD expression suppresses theinhibitory effects of simultaneous UAS-RBF1 expression dramatically. Similarly,da-GAL4-driven UAS-CycD expression during development of otherwisewild-type flies results in an increased adult fly weight. However, in these experiments in Drosophila, the overexpressed Cyclin Dmight execute its effect in combination with excess Cdk4 expressed from theendogenous Cdk4 gene, as suggested by the finding thatda-GAL4-driven UAS-CycD expression in Cdk4 mutantsincreases adult fly weight at most marginally. The findingsthat loss of CycD+ or Cdk4+ function, aswell as simultaneous loss of both CycD+ andCdk4+ function, results in essentially indistinguishablephenotypes and does not necessarily prevent development to the adult stage,demonstrate that neither Cyclin D nor Cdk4 provides essential functions inDrosophila independently of each other (Emmerich, 2004).
Drosophila Cyclin D shows strongest similarity to cyclins in a 135 amino acid region, extending from residue 157 to residue 292. This region contains matches to the 20 most conserved residues of most cyclins. The first 97 amino acids contain PEST sequences involved in rapid protein turnover. Drosophila Cyclin D is 39% identical to human Cyclin D2 but is only 18 to 24% identical to the other classes of cyclins identified either in humans or Drosophila. The consensus Cyclin D sequence consists of 65 residues, of which 24 are unique to D cyclins (Finley, 1996).
date revised: 6 Jan 96
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