retina aberrant in pattern/fizzy-related


DEVELOPMENTAL BIOLOGY

Embryonic

Drosophila fzy expression is tightly correlated with mitotic cell proliferation during embryogenesis (Dawson, 1995). Developmental Northern blots and in situ hybridization reveal a distinct expression pattern in the case of fzr. Maternally derived fzr transcripts present at the onset of embryogenesis disappear during the syncytial embryonic cycles. Zygotic expression starts in yolk nuclei after the thirteenth embryonic mitosis (during the stages where the nuclei at the egg periphery are cellularized) and is detectable during gastrulation. In the newly formed cells at the egg periphery, no signals above background were detected during the stages of the embryonic division cycles 14 and 15. At stage 11, expression is observed in the salivary placodes and in the anterior and posterior midgut primordia at high levels, followed by expression at lower levels apparently throughout the embryo. While the fzr transcript distribution anticipates the pattern of endoreduplication in internal tissues during germband retraction, decreasing signals are observed in mesodermal and epidermal tissues. During subsequent late embryonic stages, signal intensity decreases throughout the embryo to background levels. According to these results, therefore, fzr expression is not correlated with mitotic proliferation but is observed in tissues during the stages when cells become postmitotic (Sigrist, 1997).

Induction of endocycles represses apoptosis independently of differentiation and predisposes cells to genome instability

The endocycle is a common developmental cell cycle variation wherein cells become polyploid through repeated genome duplication without mitosis. Previous studies have show that Drosophila endocycling cells repress the apoptotic cell death response to genotoxic stress. This study investigated whether it is differentiation or endocycle remodeling that promotes apoptotic repression. It was found that when nurse and follicle cells switch into endocycles during oogenesis they repress the apoptotic response to DNA damage caused by ionizing radiation; this repression has been conserved in the genus Drosophila over 40 million years of evolution. Follicle cells defective for Notch signaling fail to switch into endocycles or differentiate and remain apoptotic competent. However, genetic ablation of mitosis by knockdown of Cyclin A or overexpression of fzr/Cdh1 induces follicle cell endocycles and represses apoptosis independently of Notch signaling and differentiation. Cells recovering from these induced endocycles regained apoptotic competence, showing that repression is reversible. Recovery from fzr/Cdh1 overexpression also results in an error-prone mitosis with amplified centrosomes and high levels of chromosome loss and fragmentation. These results reveal an unanticipated link between endocycles and the repression of apoptosis, with broader implications for how endocycles may contribute to genome instability and oncogenesis (Hassel, 2014).

Specification of differentiated adult progenitors via inhibition of endocycle entry in the Drosophila trachea

A population of Drosophila adult tracheal progenitor cells arises from differentiated cells of the larval main trachea that retain the ability to reenter the cell cycle and give rise to the multiple adult tracheal cell types. These progenitors are unique to the second tracheal metamere as homologous cells from other segments, express fizzy-related (fzr), the Drosophila homolog of CDH1 protein of the APC complex, and enter endocycle and do not contribute to adult trachea. This study examined the mechanisms for their quiescence and show that they reenter the cell cycle by expression of string/cdc25 through ecdysone. Furthermore, preventing endocycle entry is both necessary and sufficient for these tracheal cells to exhibit markers of adult progenitors, thus modifying their genetic program. Finally, Hox-mediated regulation of fzr expression was shown to be responsible for progenitor identity and thus specifies a group of differentiated cells with facultative stem cell features (Djabrayan, 2014: PubMed).

Effects of mutation, deletion and ectopic expression

In the compound eye of Drosophila, cell-cell interactions are thought to play an important role in the determination of neuronal cell fate and pattern morphogenesis. Recent work on the bride of sevenless (boss) gene has demonstrated an inductive role for photoreceptor R8 in the differentiation of photoreceptor R7. These studies have shown that while R8 differentiates early in the scheme of ommatidial assembly, it continues to play an active role in subsequent patterning events. Studies on a new genetic locus, rap (retina aberrant in pattern), are described that show that rap functions are critical for normal pattern formation in the developing eye. Mutations in the rap gene perturb the early stages of pattern formation and lead to a variable number of photoreceptor cells (R cells) in each ommatidium. Experiments with a temperature-sensitive allele have shown that rap gene function is required during the period of development when pattern formation occurs. In addition, a somatic mosaic analysis of rap has shown that its function is required only in photoreceptor cell R8 for normal ommatidial patterning. These studies suggest an important role for rap in the initial events leading to pattern formation and are consistent with R8 playing a central role in directing ommatidial pattern formation (Karpilow, 1989).

For a genetic characterization of fzr function, the gene was mapped to an X-chromosomal region that is deleted by a number of deficiencies. A transgene (Hs-fzr), allowing heat-inducible fzr expression, was introduced into the background of the smallest of these deficiencies Df(1)bi-D3. To get insights into fzr function, those phenotypic abnormalities of hemizygous deficient embryos that could be corrected by Hs-fzr expression were examined. The identification of Hs-fzr-rescuable phenotypes is important, since Df(1)bi-D3 deletes additional genes (including hindsight[hnt], which is required for amnioserosa differentiation and germband retraction). The resulting lack of hnt causes profound morphological defects in deficient embryos during the stages that follow germband retraction. In a number of control experiments, therefore, hnt mutant embryos were used in addition to wild-type embryos (Sigrist, 1997).

To evaluate a role of fzr for mitotic cyclin accumulation, deficient embryos were labelled with antibodies against cyclin A. The first abnormalities were observed at the stages where cells become postmitotic. In wild-type epidermis, cell proliferation stops after mitosis 16, when cells enter for the first time into G1 and stop mitotic cyclin accumulation (cyclins A, B, and B3) despite the presence of perduring transcripts. In contrast to wild-type and hnt mutant embryos, a rapid reaccumulation of cyclin A and cyclins B and B3 is observed in deficient embryos after mitosis 16. By in situ hybridization, no increase of the corresponding transcript levels could be detected, suggesting that cyclin A, B, and B3 reaccumulation in deficient embryos is caused by posttranscriptional regulation. This ectopic cyclin reaccumulation occurs with the same kinetics that are also observed in an exceptional restricted thoracic region where epidermal cells always progress through an additional division cycle after mitosis 16 instead of arresting in G1. The ectopic cyclin accumulation throughout the epidermis of deficient embryos is also followed by an additional division in all cells (Sigrist, 1997).

The extra division that occurs in the deficient embryos appears completely normal, also with regard to degradation of cyclins A, B, and B3. Pulse labeling with BrdU has demonstrated that the extra division is preceded by S phase. All epidermal cells in deficient embryos incorporate BrdU after mitosis 16 instead of arresting in G1 as observed during wild-type development. The spatial and temporal pattern of BrdU incorporation closely matches the pattern of the previous mitosis 16. BrdU and tubulin labeling at later stages fails to reveal further cell proliferation, indicating that the epidermal cells progress through one extra cycle only. It is emphasized that Hs-fzr expression prevents the extra S phase in fzr-deficient embryos as well as the reaccumulation of mitotic cyclins after mitosis 16 and the extra division. Moreover, this phenotypic rescue does not require Hs-fzr expression at levels above the endogenous fzr expression as revealed by parallel in situ hybridization experiments. Finally, while Hs-fzr expression prevents ectopic BrdU incorporation in the epidermis of deficient embryos, it does not generally abolish all BrdU incorporation. Incorporation is still observed in the developing nervous system in a pattern corresponding to the normal cell proliferation program of wild-type development (Sigrist, 1997).

To address a functional redundancy of fzy and fzr, the double mutant phenotype was examined. Cell cycle progression in fzy mutants is initially supported by a maternal fzy contribution that is sufficient for the completion of all of the 16 division cycles in the dorsal epidermis. However, in the ventral epidermis, where the proliferation program is delayed compared to the dorsal epidermis, the maternal contribution is insufficient. Consequently, cells in the ventral epidermis of fzy mutants frequently become arrested in metaphase. The frequency of metaphase-arrested cells appears similar in fzy and in fzy;fzr double mutants. Loss of fzr, therefore, does not enhance the fzy mutant phenotype at the stage where the phenotype normally becomes apparent. However, cells in the dorsal epidermis of double mutants enter an extra division cycle after mitosis 16, as also observed in embryos deficient for fzr. Interestingly, these dorsal epidermal cells all become arrested during metaphase of the extra division in the double mutants, while they progress normally through this extra division in embryos lacking only fzr. These observations suggested that fzy and fzr are specialized for the down-regulation of cyclin levels in either mitosis or interphase, respectively (Sigrist, 1997).

Abnormal accumulation of mitotic cyclins is not only observed in the epidermal G1 cells after mitosis 16 but also in internal tissues. The cells in these internal tissues start with endoreduplication in a defined spatial and temporal pattern already established during embryogenesis. Interestingly, in situ hybridization experiments indicate that fzr expression is especially high precisely during the transition from mitotic proliferation to endoreduplication (Sigrist, 1997).

To analyze the role of fzr during the transition to endocycles, a concentration was placed on the prospective salivary gland cells. These cells stop to divide after mitosis 15. This final division is followed by an immediate entry into S phase 16, as also observed in all other cells of the embryo. In contrast to the epidermis, however, S phase 16 is no longer followed by a division in the salivary gland but by the first endoreduplication S phase. Mitotic cyclins are no longer detectable in wild-type salivary glands before entry into the first endoreduplication S phase. In the deficient embryos, however, these cyclins are clearly present and salivary glands fail to incorporate BrdU at the stage where the first endoreduplication occurs in wild type. Endoreduplication fails also in the other internal tissues of deficient embryos. Hs-fzr expression restors BrdU incorporation in salivary glands of deficient embryos at the stage of the first endoreduplication and in other internal tissues as well, even when expressed at levels below the endogenous fzr expression. Moreover, Hs-fzr expression also suppresses the ectopic accumulation of mitotic cyclins in internal tissues. It is concluded, therefore, that fzr is required for suppression of cyclin A, B, and B3 accumulation and for entry into endoreduplication in the salivary gland (Sigrist, 1997).

Loss of fzr results in ectopic accumulation of cyclins A, B, and B3 in both epidermis and internal tissues. Additional support for the idea that fzr acts as a negative regulator of cyclin levels was provided by the analysis of deficient embryos that were also mutant in Cyclin A (CycA). In CycA mutants, the maternally derived cyclin A is no longer sufficient for progression through mitosis 16 in the epidermis. In double mutants, however, lacking both zygotic fzr and CycA expression, a normal progression through mitosis 16 is observed followed by an ectopic S phase. It was of interest to test whether premature expression of fzr already during the embryonic division cycles is sufficient to bring about a premature down-regulation of mitotic cyclins. For these experiments, prd-Gal4 and UAS-fzr transgenes were used. The simultaneous presence of these two transgenes results in premature fzr overexpression in alternating epidermal segments. prd-Gal4-directed expression starts during embryonic cycle 15 and continues during cycle 16. Until G2 of cycle 16, differences between UAS-fzr-expressing and nonexpressing stripes could not be detected. However, in G2 of cycle 16, mitotic cyclins are severely decreased in the fzr-expressing stripes according to immunolabeling. Moreover, mitosis 16 is blocked, and therefore, cell density is found to be reduced in the UAS-fzr-expressing stripes at a stage where the epidermal cell proliferation is completed. It is concluded that premature fzr overexpression down-regulates mitotic cyclins in epidermal cells and inhibits mitosis. Hs-fzr expression in wild-type embryos during G2 of cycle 16 also results in a complete inhibition of mitosis 16, but prd-Gal4-directed expression of UAS-fzy has no effect (Sigrist, 1997).

Mitotic divisions are also inhibited during the imaginal disc cell proliferation, when UAS-fzr is expressed with the help of an en-Gal4 transgene in the cells of the posterior compartment of imaginal discs. However, DNA labeling of imaginal discs at the end of larval development indicates that these UAS-fzr-expressing cells in the posterior compartment do not arrest in G2 but continue with DNA overreplication. While the cells in the anterior compartment of en-Gal4, UAS-fzr discs are indistinguishable from control discs as expected, fewer but very big and intensely labeled cell nuclei are found in the UAS-fzr-expressing posterior compartments (Sigrist, 1997).

Completion of mitosis requires neither fzr/rap nor fzr2, a male germline-specific Drosophila Cdh1 homolog

Proteolysis of mitotic regulators like securins and cyclins requires Fizzy(Fzy)/Cdc20 and Fizzy-related(Fzr)/Hct1/Cdh1 proteins. Budding yeast Cdh1 acts not only during G1, but is also required for B-type cyclin degradation during exit from mitosis when Cdh1 is a target of the mitotic exit network controlling progression through late mitosis and cytokinesis. In contrast, observations in frog and Drosophila embryos have suggested that the orthologous Fzr is not involved during exit from mitosis. However, the potential involvement of minor amounts of maternally derived Fzr was not excluded in these studies. Similarly, the reported absence of severe mitotic defects in chicken Cdh1-/- cells might be explained by the recent identification of multiple Cdh1 genes. This study carefully analyzed the Fzr requirement during exit from mitosis in Drosophila, which, apart from fzr, has only one additional homolog. This fzr2 gene, although expressed in the male germline, is not expressed during mitotic divisions. Moreover, by characterizing fzr alleles, it has been demonstrated that completion of mitosis including Cyclin B degradation does not require Fzr. However, fzr is an essential gene corresponding to the rap locus, and Fzr, which accumulates predominantly in the cytoplasm, is clearly required during G1 (Jacobs, 2002).

Apart from the previously characterized genes fzy, fzr, and cortex, the Drosophila genome contains one additional member of the fzy/fzr family (CG16783). This fizzy-related 2 (fzr2) gene is intronless and nested within the last intron of another uncharacterized gene. The embedding gene, CG2970, encodes a stomatin-like protein that therefore presumably functions independently of fzr2. The putative Fzr2 amino acid sequence is more similar to metazoan Fzr than to Fzy homologs. Similarity is most extensive within the C-terminal region, which includes the characteristic seven WD40 repeats. Within the N-terminal regions, similarity among fzy/fzr family members is generally very low, except for a small region recently named C-box, which is required for binding to the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). Although recognizable, the C-box of Fzr2 is slightly diverged (Jacobs, 2002).

To evaluate whether fzr2 is a functional fzy/fzr family member, an analysis was performed to see whether its expression prevents fzy or fzr mutant phenotypes during Drosophila embryogenesis. A UAS-fzr2 transgene in combination with prd-GAL4 results in expression within alternating epidermal segments. While UAS-fzr2 expression does not prevent the characteristic metaphase arrest observed in fzy mutants, it clearly suppresses the fzr mutant phenotype. A lack of zygotic fzr function results in a failure to degrade the mitotic Cyclins A, B, and B3 during the first G1 phase occurring after mitosis 16 in the epidermis. After mitosis 16, therefore, Cyclin B is readily detectable by immunofluorescence in all epidermal cells of fzr mutant embryos, while, in fzr+ sibling embryos, Cyclin B is only observed in the nervous system, but not in the epidermis. Within the UAS-fzr2-expressing regions of fzr mutant embryos, ectopic reaccumulation of Cyclin B after mitosis 16 does not occur, demonstrating that Fzr2 can provide Fzr function. However, Fzr2 might be less active than Fzr, because prd-GAL4-mediated UAS-fzr expression already induces premature degradation of mitotic cyclins before mitosis 16, while analogous UAS-fzr2 expression had little effect before mitosis 16 (Jacobs, 2002).

The finding that only targeted UAS-fzr2 expression, but not the endogenous fzr2 gene, prevents Cyclin B reaccumulation after mitosis 16 in fzr mutant epidermis suggested that fzr2 is not normally expressed when epidermal cells become postmitotic. Indeed, in situ hybridization failed to reveal fzr2 expression during embryogenesis. Similarly, RT-PCR experiments failed to detect fzr2 transcripts during embryogenesis and larval development. However, low transcript levels are observed in pupae, and higher levels are observed in adult males, but not adult females. Additional RT-PCR experiments have demonstrated that fzr2 is expressed in testes, and in situ hybridization confirmed this finding. fzr2 transcripts are detected primarily in premeiotic spermatocytes. Only background signals were observed in the apical testis region containing the somatic and germline stem cells as well as the immature spermatocyte clusters during the mitotic gonial divisions. Interestingly, cortex, the most distant member of the Drosophila fzy/fzr family, is exclusively expressed during oogenesis. fzr2 and cortex, therefore, might have similar roles during male or female meiosis, respectively (Jacobs, 2002).

fzr transcripts were also detected in testes, although with a slightly different distribution. fzr signals are reproducibly of maximal intensity within the subapical region containing primary spermatocyte clusters that have just completed the last mitotic division. Interestingly, apart from in the testes, fzr is also expressed in other adult tissues that lack proliferating cells. Expression in postmitotic cells has also been demonstrated for the mammalian fzr homolog Cdh1 (Jacobs, 2002).

Since fzr2 is not expressed during embryogenesis, it cannot explain the normal progression through mitosis in fzr-deficient embryos. However, since there are maternal fzr transcripts in early embryos, whether maternally derived Fzr might mask mitotic defects in embryos lacking zygotic fzr expression was addressed. Maternal fzr transcripts are detected during the first few syncytial division cycles. Although these transcripts are no longer detected during cell division cycles 14–16 following cellularization, the Fzr protein translated early on from the maternal transcripts might be more stable and perdure throughout the epidermal cell proliferation period (Jacobs, 2002).

To estimate the levels of perduring maternally derived Fzr protein, immunoblotting experiments were performed. fzr+ and fzr- sibling embryos during the stage at which the epidermal cells enter the first G1 phase during wild-type development were sorted manually before preparation of extracts and immunoblotting with anti-Fzr antibodies. During this stage, zygotic fzr expression is upregulated in almost all embryonic tissues. The level of maternally derived Fzr protein present in fzr- embryos was found to be at most 1% of the amount present in fzr+ embryos (Jacobs, 2002).

The antibodies against Fzr also allowed an analysis of the intracellular Fzr localization. Interestingly, specific anti-Fzr signals are observed in the cytoplasm. In contrast, the Drosophila emi1 homolog RCA1, which is thought to bind and inhibit Fzr, has been reported to accumulate in the cell nucleus after overexpression. This apparent difference in intracellular localization of Fzr and RCA1 might represent an artifact of rca1 overexpression or indicate regulated binding of these proteins (Jacobs, 2002).

Even though Fzr levels in fzr- embryos are strongly reduced compared to fzr+ embryos, residual maternally derived Fzr might still be sufficient to satisfy a mitotic requirement. In principle, fzr- progeny derived from fzr- mutant germline clones can be expected to lack Fzr protein completely, and their analysis should reveal the functional significance of maternally derived protein. However, initial phenotypic characterizations were accomplished with a chromosomal deficiency, Df(1)bi-D3, which deletes other genes in addition to fzr. Thus, Df(1)bi-D3 is not amenable to germline clonal analyses, and fzr null alleles had to be generated first. Recessive lethal P element insertions within either the 5' untranslated leader region or the first intron of the fzr gene were found to cause a weaker phenotype than previously observed in Df(1)bi-D3 embryos (Jacobs, 2002).

The P element insertions allowed the isolation of imprecise excisions that were associated with a cell cycle phenotype indistinguishable from Df(1)bi-D3 embryos. As in Df(1)bi-D3 embryos, all epidermal cells reaccumulate mitotic cyclins after mitosis 16 in embryos hemizygous for the imprecise excision fzrie28. Moreover, in addition to mitotic cyclin reaccumulation, the epidermal cells progress through a complete additional division cycle, as previously described in Df(1)bi-D3 embryos. Based on labeling with antibodies against Cyclin B, tubulin, and a DNA stain, this additional mitosis 17 appeared to be completely normal. Exit from mitosis is accompanied by chromosome segregation to the poles, mitotic cyclin degradation, and midbody formation. Molecular analyses by PCR indicate that the genes flanking fzr are both present in fzrie28, while part of the fzr promoter and coding regions are deleted, indicating that fzrie28 is a null allele. fzrie28 is a recessive lethal mutation, demonstrating that fzr is an essential gene. Unfortunately, females carrying germline clones failed to lay eggs. fzr, therefore, is presumably required in the germline for oogenesis (Jacobs, 2002).

The hypomorphic phenotype resulting from a partial loss of fzr function, however, suggests an alternative strategy to address the functional significance of the maternally derived Fzr protein during the mitotic divisions in fzr- embryos. Interestingly, in individual epidermal cells within embryos hemizygous for hypomorphic fzr alleles (fzrG0418 and fzrG0326), mitotic cyclins appear to be either absent after mitosis 16, as in wild-type, or present at the same high level as in fzr null mutant embryos. The fraction of epidermal cells displaying the abnormal cyclin reaccumulation after mitosis 16 was correlated with the allelic strength of the mutations. Similarily, Fzr levels determined by quantitative immunoblotting experiments were correlated with allelic strength. These findings reveal a switch-like character of the regulation of the Fzr-APC/C degradation pathway, which tends to be either fully active or completely inactive. Decreasing levels of fzr function bias the degradation system toward the off state. They do not simply reduce the efficiency of mitotic cyclin degradation gradually in epidermal cells, since a uniform reaccumulation of mitotic cyclins throughout the epidermis at a rate slower than in fzr null mutant embryos was not observed with the hypomorphic mutants (Jacobs, 2002).

The correlation of allelic strength with the fraction of epidermal cells displaying unscheduled reaccumulation of mitotic cyclins after mitosis 16 was further corroborated by the analysis of rap alleles, which were found to represent hypomorphic fzr mutations. Analysis of embryos hemizygous for some of the rap alleles revealed that the fraction of the epidermal cells that reaccumulate Cyclin B after mitosis 16 is even lower than in embryos hemizygous for fzrG0326 and fzrG0418 (Jacobs, 2002).

The conclusion that the fraction of epidermal cells reaccumulating Cyclin B after mitosis 16 is sensitive to the extent of fzr function was further confirmed by RNA interference experiments. Injection of fzr dsRNA into early syncytial embryos at high concentration results in Cyclin B reaccumulation throughout the entire epidermis in half of the injected embryos after the terminal mitosis 16 and phenocopies fzr null mutants. A decrease in the injected dsRNA concentration is accompanied by an increase in the fraction of embryos displaying only a partial, graded fzr phenocopy. Epidermal regions in which all cells were Cyclin B-positive were always observed near the posterior injection site in these embryos. In contrast, far from the injection site, i.e., in the anterior epidermis, Cyclin B-positive cells were absent. Transition zones between anterior and posterior regions, usually 1–3 segments wide, were characterized by a mosaic of Cyclin B-negative and -positive cells (Jacobs, 2002).

Because of the correlation between the level of fzr function and the fraction of Cyclin B-positive epidermal cells after mitosis 16, a reduction of the maternal fzr contribution would be expected to increase this Cyclin B-positive fraction in hypomorphic fzr mutant embryos, if functionally relevant amounts of maternally derived Fzr protein perdure until after mitosis 16. In contrast, if the maternally derived Fzr protein does not perdure long enough to assist in the degradation of mitotic cyclins after mitosis 16, the maternal status of fzr function should be irrelevant for the severity of hypomorphic mutant phenotypes. By counting the number of Cyclin B-positive cells after mitosis 16 in embryos hemizygous for hypomorphic fzr alleles (rape2, rape4, or rape6) derived from mothers with either only a mutant fzr copy (rape2, rape4, or rape6) or with a fzr+ copy in addition to the mutant fzr copy, no effect of the maternal fzr genotype was observed. This result strongly argues that functionally effective levels of maternally derived Fzr protein do not perdure until after mitosis 16. The additional mitosis 17 that occurs in embryos hemizygous for fzr null alleles therefore cannot be supported by maternally derived Fzr protein. Because this additional mitosis 17 is completed normally, it is concluded that exit from mitosis, including degradation of B-type cyclins, is not dependent on Fzr function (Jacobs, 2002).

Analysis of mutant embryos lacking zygotic expression of both fzy and fzr has demonstrated that Fzy is required for exit from the additional mitosis 17 including mitotic cyclin degradation. Since no other fzy/fzr family member is expressed during the additional mitosis 17 in fzr mutants, it is concluded that Fzy alone is sufficient for the completion of mitosis. Sequential APC/C activation first by Fzy, promoting securin degradation and thereby the metaphase-to-anaphase transition, followed by Fzr, allowing B-type cyclin degradation, telophase, and cytokinesis, as originally proposed in budding yeast, therefore, is not obligatory for higher eukaryote mitosis. In addition, these results confirm that Fzr is essential during the G1 phase for preventing unscheduled accumulation of mitotic cyclins (Jacobs, 2002).

Degradation of origin recognition complex large subunit by the anaphase-promoting complex in Drosophila

The kinetics of Origin recognition complex subunit 1 ORC1 expression in cells is consistent with the idea that ORC1 accumulates at the G1/S boundary immediately after a pulse of E2F-dependent transcription and persists until its catastrophic destruction at the M/G1 boundary. In this scenario, cell cycle-modulated transcription and proteolysis both contribute to setting the level of ORC1. This idea was tested directly by uncoupling the expression of orc1 mRNA from its normal transcriptional signals. To this end, constitutive expression of ORC1-GFP was driven using the GMR promoter, which is turned on in all cells posterior to the morphogenetic furrow in the eye disc, and the distribution of green fluorescence was visualized both in situ and in FACS experiments (Araki, 2003).

In cultured cells, human ORC1 is degraded during S phase by Skp2-dependent SCF activity (2002). In contrast, the timing of ORC1 degradation in Drosophila strongly implies degradation by the APC, which degrades mitotic cyclins and securin to promote passage through and exit from mitosis. The APC is generally thought to be activated in succession, first by Fizzy (Fzy)/Cdc20, which promotes passage out of metaphase, and subsequently by Fizzy-related (Fzr)/Cdh1, which promotes exit from mitosis and suppresses CycB accumulation into G1. The ultimate consequence of APC activity is proteasome-dependent degradation of targeted substrates. The degradation of ectopically expressed ORC1 in G1 suggests the involvement of Fzr (Araki, 2003).

To examine the role of Fzr, the behavior of ORC1 was examined in mutant animals lacking Fzr activity. fzr mutants die in late embryogenesis, long before the imaginal discs can be studied; it also seems unlikely that fzr mutant somatic clones would proliferate and survive, precluding analysis in mosaic imaginal discs. Therefore, epithelial cells in stage 12-13 embryos were examined as they exited from M phase of division cycle 16 into G1 of cycle 17 (Araki, 2003). Whether ORC1 and ORC1-GFP behave in a similar manner in these embryonic cells as in proliferating imaginal disc cells was examined. In wild-type stage 12-13 embryos, most of the dorsal epithelial cells have entered a prolonged G1 arrest, with only a few cells still proliferating. These laggards have both detectable ORC1 and CycB, and thus are in S or G2 of cycle 16, whereas most of their neighbours have entered the prolonged G1 of cycle 17 and have neither protein. When expressed under the transcriptional control of the strong, constitutive actin5C promoter, ORC1-GFP accumulates to high levels only in the few epithelial cells still in S or G2, whereas the protein is destabilized in the remaining (G1) cells. The selective degradation of ORC1-GFP in G1 is mediated by N-terminal signals, as is the case in imaginal disc cells. Analysis of Western and Northern blots of embryonic samples is consistent with the idea that regulation of ORC1 accumulation is post-transcriptional. Thus, the stability of both the endogenous ORC1 and ectopically expressed ORC1-GFP is regulated in essentially the same fashion in the embryo and the imaginal disc (Araki, 2003).

Next, the dependence of ORC1 and CycB degradation on Fzr was examined, comparing the accumulation of each protein in sibling wild-type and fzr mutant embryos. Essentially every epithelial cell in fzr mutant embryos has appreciable levels of both ORC1 and CycB. This observation is consistent with the idea that ORC1 degradation is dependent on Fzr. However, loss of Fzr activity perturbs the cell cycle, promoting epithelial cells into an extra division. The accumulation of ORC1 might simply correlate with the progression of these cells into S and G2, where ORC1 has been shown to be stabilized (Araki, 2003).

To better test the role of Fzr in regulating ORC1 stability in vivo, it was desirable to perturb its activity without causing attendant dramatic changes in the cell cycle profile. To this end, either ORC1-GFP or the stable ORC1C-NLS-GFP derivative (as a control) were co-expressed in the eye imaginal disc with either Fzr or Rca1, a specific inhibitor of Fzr. Overexpression of Fzr causes a dramatic destabilization of ORC1-GFP, but not ORC1C-NLS-GFP. Conversely, co-expression of Rca1 modestly stabilizes ORC1-GFP but has no effect on ORC1C-NLS-GFP. Importantly, the effect of ectopic Fzr and Rca1 cannot readily be explained by primary effects on cell cycle progression with subordinate effects on ORC1 stability. Misexpression of neither Fzr nor Rca1 promotes significant S phase entry in the eye disc cells. Misexpression of Rca1 has no apparent effect on cell cycle progression analyzed by FACS, and misexpression of Fzr actually leads to accumulation of additional cells in G2 (where ORC1 is normally stable). In similar experiments, it was found that co-expression of Fzy has no apparent effect on ORC1-GFP stability, suggesting that ORC1 is preferentially targeted by Fzr. The simplest interpretation of these findings is that ORC1 is normally degraded upon exit from mitosis by Fzr-dependent APC activity (Araki, 2003).

To test the idea that Fzr acts directly to target ORC1 for degradation, it was asked whether Drosophila ORC1 is an APC substrate in vitro using a heterologous purified system. Ubiquitylation of a positive control, human polo-like kinase (Plk1), is stimulated by addition of APC activators from humans and flies, including Drosophila Fzr. Ubiquitinylation of ORC1N is also stimulated by Drosophila Fzr (and the human homolog, Cdh1, whereas none of the APC activators tested significantly stimulates ubiquitylation of ORC1C. These observations strongly support the idea that Drosophila ORC1 is targeted for degradation by Fzr by a mechanism that is conserved between vertebrates and flies (Araki, 2003).

Regulation of glia number in Drosophila by Rap/Fzr, an activator of the anaphase-promoting complex, and Loco, an RGS protein

Glia mediate a vast array of cellular processes and are critical for nervous system development and function. Despite their importance in neurobiology, glia remain understudied and the molecular mechanisms that direct their differentiation are poorly understood. Rap/Fzr is the Drosophila homolog of the mammalian Cdh1, a regulatory subunit of the anaphase-promoting complex/cyclosome (APC/C). APC/C is an E3 ubiquitin ligase complex well characterized for its role in cell cycle progression. This study uncovered a novel cellular role for Rap/Fzr. Loss of rap/fzr function leads to a marked increase in the number of glia in the nervous system of third instar larvae. Conversely, ectopic expression of UAS-rap/fzr, driven by repo-GAL4, results in the drastic reduction of glia. Data from clonal analyses using the MARCM technique show that Rap/Fzr regulates the differentiation of surface glia in the developing larval nervous system. Genetic and biochemical data further indicate that Rap/Fzr regulates glial differentiation through its interaction with Loco, a regulator of G-protein signaling (RGS) protein and a known effector of glia specification. It is proposed that Rap/Fzr targets Loco for ubiquitination, thereby regulating glial differentiation in the developing nervous system (Kaplow, 2008).

The APC/C is a multi-subunit ubiquitination complex that has been well characterized for its role in regulating mitotic exit. Rap/Fzr/Cdh1 is an activator of APC/C and plays a key role in the regulation of mitosis by targeting cell cycle regulators, such as cyclins and cyclin-dependent kinases, for ubiquitination. This study uncover a novel role for Rap/Fzr in the regulation of glia differentiation. Loss-of-function rap/fzr mutants display an increase in glia number and a corresponding decrease in neuronal number. Conversely, targeted overexpression of Rap/Fzr in glia leads to a severe reduction in glia number with a corresponding increase in neuronal number. This change in glia and neuron number occurs without significantly altering the mitotic index. Similarly, Pereanu (2005) reported a change in glial cell number in the larval brain without a significant change in mitotic index and suggested that the additional glial cells arise from the differentiation of secondary neuro-glioblasts located in the surface of the brain. Clonal analysis data derived from MARCM experiments suggest that Rap/Fzr specifically regulates differentiation of a subset of glia, the surface glia. Several lines of evidence presented here support the model that Rap/Fzr regulates gliogenesis by targeting the RGS protein, Loco, for ubiquitination. First, genetic interaction studies show that a single copy of the loco mutation is a dominant suppressor of both the rap/fzr rough-eye phenotype and the glial phenotype in the larval brain. Second, biochemical data show an interaction between Rap/Fzr and Loco in larval brain tissue and that Loco is ubiquitinated in larval extracts. Third, results from immunolocalization experiments show that Rap/Fzr and Loco colocalize within surface glia in the postembryonic larval brain. It is concluded that Loco is targeted for ubiquitination by Rap/Fzr through its D-box and/or KEN box, two signature ubiquitination-targeting motifs recognized by the APC/C (Kaplow, 2008).

Loco has been previously shown to be a positive effector of glia development during Drosophila embryogenesis. Recently, Loco has also been reported to have a role during the asymmetric cell division of embryonic neuroblasts. The current results suggest a new role for Loco in postembryonic development of Drosophila CNS and, specifically, in glial differentiation. It is proposed that the cellular level of Loco in the postembryonic GMC is a key positive effector in the binary switch model of glia-neuron differentiation. In this model, Rap/Fzr negatively regulates glia number by targeting Loco for ubiquitination and eventual proteosomal degradation. The model further predicts that alteration in the rap/fzr gene dosage would change cellular levels of Loco, with resulting effects on glia number. In larval neuroblasts, compartmentalization of Loco within GMCs may be critical in promoting a glial cell lineage. The results showed that, in the larval brain, Loco is colocalized with Miranda and Rap/Fzr in the basal axis, whereas during asymmetric division of embryonic neuroblasts, Loco is expressed in the apical axis. Although Miranda is a known mediator of asymmetric division of embryonic neuroblasts and a specific marker for larval neuroblasts, its function in postembryonic development has not been completely elucidated. Colocalization of Loco with Miranda and Rap/Fzr suggests a possible functional role for these molecules during postembryonic neuroblast division (Kaplow, 2008).

Collectively, these data suggest that Rap/Fzr regulates glia differentiation during two phases of development: (1) initially, Rap/Fzr controls the proliferation and self-renewal of dividing neuroblasts, and (2) subsequently, Rap/Fzr regulates the differentiation of GMCs. This model is consistent with evidence from other studies showing that proliferation of larval neuroblasts is controlled by other components of the APC/C, such as ida (a subunit of the APC/C), and Aurora-A kinase, a known target of APC/C-mediated ubiquitination during mitotic progression. Since work by Slack (2006) has shown a possible role for ida and, in turn, for the APC/C during neuroblast division, it would be interesting to determine if additional components of the APC/C have roles during later phases of development. Preliminary analysis of glia number in morula/APC2 (a catalytic subunit of APC/C) mutants showed a significant increase in glia number similar to rap/fzr loss-of-function mutants. However, the precise roles of additional components of the APC/C, a complex of 11 subunits, during glial differentiation have yet to be elucidated. While the results suggest that Rap/Fzr regulates neuroblast number by targeting Loco for degradation, Rap/Fzr may also regulate neuroblast self-renewal through its interactions with other proteins such as Aurora-A kinase. In Drosophila larval neuroblasts, Aurora-A kinase is an important regulator of neuroblast self-renewal and is known to be a substrate for APC/C in vertebrates (Kaplow, 2008).

The data presented in this article support a model in which components of the ubiquitin ligase complex, APC/C, mediate a post-translational regulatory mechanism critical to the glial differentiation program. During the past 2 years, other studies have also reported novel roles for the APC/C and its components during nervous system development, independent of its function during cell cycle regulation. Studies have demonstrated a role for Cdh1, the mammalian homolog of Rap/Fzr, in axon growth through its interaction with the transcriptional corepressor SnoN. Furthermore, in vitro cell culture studies using neuroblastoma cell lines and silencing of Cdh1 in postmitotic cerebellar granule neurons demonstrate that the DNA-binding protein inhibitor of differentiation 2 (Id2) is a target for Cdh1-mediated ubiquitination. The current results show that Rap/Fzr is involved in glia differentiation and are consistent with other data that demonstrate that Cdh1 targets transcriptional regulators involved in the differentiation program of the developing nervous system. Thus, in addition to its role in the regulation of cell cycle progression, Rap/Fzr/Cdh1 promotes neuron formation and inhibits gliogenesis. These studies here lend further support to the idea that ubiquitination functions as a key regulatory mechanism during nervous system development (Kaplow, 2008).


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retina aberrant in pattern/fizzy-related: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 15 April 2014

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