InteractiveFly: GeneBrief

Taranis: Biological Overview | References

BIOLOGICAL OVERVIEW

Sleep is an essential and conserved behavior whose regulation at the molecular and anatomical level remains to be elucidated. This study identifies Taranis (Tara), a Drosophila homolog of the Trip-Br (SERTAD) family of transcriptional coregulators, as a molecule that is required for normal sleep patterns. Through a forward-genetic screen, tara was isolated as a novel sleep gene associated with a marked reduction in sleep amount. Targeted knockdown of tara suggests that it functions in cholinergic neurons to promote sleep. tara encodes a conserved cell-cycle protein that contains a Cyclin A (CycA)-binding homology domain. Tara regulates CycA protein levels and genetically and physically interacts with CycA to promote sleep. Furthermore, decreased levels of Cyclin-dependent kinase 1 (Cdk1), a kinase partner of CycA, rescue the short-sleeping phenotype of tara and CycA mutants, while increased Cdk1 activity mimics the tara and CycA phenotypes, suggesting that Cdk1 mediates the role of Tare and CycA in sleep regulation. Finally, a novel wake-promoting role was described for a cluster of ∼14 CycA-expressing neurons in the pars lateralis (PL), previously proposed to be analogous to the mammalian hypothalamus. The study proposes that Taranis controls sleep amount by regulating CycA protein levels and inhibiting Cdk1 activity in a novel arousal center (Alfonso, 2015).

Most animals sleep, and evidence for the essential nature of this behavior is accumulating. However, how sleep is controlled at a molecular and neural level is far from understood. The fruit fly, Drosophila, has emerged as a powerful model system for understanding complex behaviors such as sleep. Mutations in several Drosophila genes have been identified that cause significant alterations in sleep. Some of these genes were selected as candidates because they were implicated in mammalian sleep. However, others (such as Shaker and CREB) whose role in sleep was first discovered in Drosophila have later been shown to be involved in mammalian sleep, validating the use of Drosophila as a model system for sleep research. Since the strength of the Drosophila model system is the relative efficiency of large-scale screens, unbiased forward-genetic screens have been conducted to identify novel genes involved in sleep regulation. Previous genetic screens for short-sleeping fly mutants have identified genes that affect neuronal excitability, protein degradation, and cell-cycle progression. However, major gaps remain in understanding of the molecular and anatomical basis of sleep regulation by these and other genes (Afonso, 2015).

Identifying the underlying neural circuits would facilitate the investigation of sleep regulation. The relative simplicity of the Drosophila brain provides an opportunity to dissect these sleep circuits at a level of resolution that would be difficult to achieve in the more complex mammalian brain. Several brain regions, including the mushroom bodies, pars intercerebralis, dorsal fan-shaped body, clock neurons, and subsets of octopaminergic and dopaminergic neurons, have been shown to regulate sleep. However, the recent discovery that Cyclin A (CycA) has a sleep-promoting role and is expressed in a small number of neurons distinct from brain regions suggests the existence of additional neural clusters involved in sleep regulation (Afonso, 2015).

From an unbiased forward-genetic screen, this study discovered taranis (tara), a mutant that exhibits markedly reduced sleep amount. tara encodes a Drosophila homolog of the Trip-Br (SERTAD) family of mammalian transcriptional coregulators that are known primarily for their role in cell-cycle progression. TARA and Trip-Br proteins contain a conserved domain found in several CycA-binding proteins. This research shows that tara regulates CycA levels and genetically interacts with CycA and its kinase partner Cyclin-dependent kinase 1 (Cdk1) to regulate sleep. Furthermore, a cluster of CycA-expressing neurons in the dorsal brain was shown to lie in the pars lateralis (PL), a neurosecretory cluster previously proposed to be analogous to the mammalian hypothalamus, a major sleep center. Knockdown of tara and increased Cdk1 activity in CycA-expressing PL neurons, as well as activation of these cells, reduces sleep. Collectively, these data suggest that TARA promotes sleep through its interaction with CycA and Cdk1 in a novel arousal center (Afonso, 2015).

The data demonstrate that TARA interacts with CycA to regulate its levels and promote sleep. Cdk1 was also identified as a wake-promoting molecule that interacts antagonistically with TARA. Given the fact that TARA regulates CycA levels, the interaction between TARA and Cdk1 may be mediated by CycA. The finding that Cdk1 and CycA also exhibit an antagonistic interaction supports this view. The previous discovery that CycE sequesters its binding partner Cdk5 to repress its kinase activity in the adult mouse brain points to a potential mechanism, namely that TARA regulates CycA levels, which in turn sequesters and inhibits Cdk1 activity. TARA and its mammalian homologs (the Trip-Br family of proteins) are known for their role in cell-cycle progression. However, recent data have shown that Trip-Br2 is involved in lipid and oxidative metabolism in adult mice, demonstrating a role beyond cell-cycle control. Other cell-cycle proteins have also been implicated in processes unrelated to the cell cycle. For example, CycE functions in the adult mouse brain to regulate learning and memory. Based on the finding that CycA and its regulator Rca1 control sleep, it was hypothesized that a network of cell-cycle genes was appropriated for sleep regulation. The current data showing that two additional cell-cycle proteins, TARA and Cdk1, control sleep and wakefulness provide support for that hypothesis. Moreover, the fact that TARA and CycA, factors identified in two independent unbiased genetic screens, interact with each other highlights the importance of a network of cell-cycle genes in sleep regulation (Afonso, 2015).

There are two main regulatory mechanisms for sleep: the circadian mechanism that controls the timing of sleep and the homeostatic mechanism that controls the sleep amount. This study has shown that TARA has a profound effect on total sleep time. TARA also affects rhythmic locomotor behavior. Since TARA is expressed in clock cells, whereas CycA is not, it is possible that TARA plays a non-CycA dependent role in clock cells to control rhythm strength. The finding that tara mutants exhibit severely reduced sleep in constant light suggests that the effect of TARA on sleep amount is not linked to its effect on rhythmicity. Instead, TARA may have a role in the sleep homeostatic machinery, which will be examined in an ongoing investigation (Afonso, 2015).

To fully elucidate how sleep is regulated, it is important to identify the underlying neural circuits. This study has shown that activation of the CycA-expressing neurons in the PL suppresses sleep while blocking their activity increases sleep, which establishes them as a novel wake-promoting center. Importantly, knockdown of tara and increased Cdk1 activity specifically in the PL neurons leads to decreased sleep. A simple hypothesis, consistent with the finding that both activation of PL neurons and increased Cdk1 activity in these neurons suppress sleep is that Cdk1 affects neuronal excitability and synaptic transmission. Interestingly, large-scale screens for short-sleeping mutants in fruit flies and zebrafish have identified several channel proteins such as SHAKER, REDEYE, and ETHER-A-GO-GO and channel modulators such as SLEEPLESS and WIDE AWAKE. Thus, it is plausible that Cdk1 regulates sleep by phosphorylating substrates that modulate the function of synaptic ion channels or proteins involved in synaptic vesicle fusion, as has previously been demonstrated for Cdk5 at mammalian synapses (Afonso, 2015).

Whereas the data mapped some of TARA's role in sleep regulation to a small neuronal cluster, the fact that pan-neuronal tara knockdown results in a stronger effect on sleep than specific knockdown in PL neurons suggests that TARA may act in multiple neuronal clusters. PL-specific restoration of TARA expression did not rescue the tara sleep phenotype, further implying that the PL cluster may not be the sole anatomical locus for TARA function. Given that CycA is expressed in a few additional clusters, TARA may act in all CycA-expressing neurons including those not covered by PL-Gal4. TARA may also act in non-CycA-expressing neurons. The data demonstrate that tara knockdown using Cha-Gal4 produces as strong an effect on sleep as pan-neuronal knockdown. This finding suggests that TARA acts in cholinergic neurons, although the possibility that the Cha-Gal4 expression pattern includes some non-cholinergic cells cannot be ruled out. Taken together, these data suggest that TARA acts in PL neurons as well as unidentified clusters of cholinergic neurons to regulate sleep (Afonso, 2015).

Based on genetic interaction studies, tara has been classified as a member of the trithorax group genes, which typically act as transcriptional coactivators. However, TARA and Trip-Br1 have been shown to up- or downregulate the activity of E2F1 transcription factor depending on the cellular context, raising the possibility that they also function as transcriptional corepressors. Interestingly, TARA physically interacts with CycA and affects CycA protein levels but not its mRNA expression. These findings suggest a novel non-transcriptional role for TARA, although an indirect transcriptional mechanism cannot be ruled out. The hypothesis that TARA plays a non-transcriptional role in regulating CycA levels and Cdk1 activity at the synapse may provide an exciting new avenue for future research (Afonso, 2015).

Taranis protects regenerating tissue from fate changes induced by the wound response in Drosophila

Regenerating tissue must replace lost structures with cells of the proper identity and function. How regenerating tissue establishes or maintains correct cell fates during regrowth is an open question. This study has identified a gene, taranis, that is essential for maintaining proper cell fate in damaged and regenerating Drosophila wing imaginal discs but that is dispensable for these fates in normal wing development. In regenerating tissue with reduced levels of Taranis, expression of the posterior selector gene engrailed is silenced through an autoregulatory silencing mechanism that requires the PRC1 component polyhomeotic, resulting in a transformation of posterior tissue into anterior tissue late in regeneration. An essential component of the wound response, JNK signaling, induces this misregulation of engrailed expression. Taranis can suppress these JNK-induced cell fate changes without interfering with JNK signaling activity. Thus, taranis protects regenerating tissue from deleterious side effects of wound healing and regeneration (Schuster, 2015).

The replacement of lost or damaged tissues and appendages through regeneration is a fascinating phenomenon that occurs to varying extents among metazoans. The rebuilding of a structure after loss or damage depends on proliferation accompanied by proper cell fate specification and patterning. Recent work in several model organisms has begun to elucidate the genes and signaling pathways that initiate regeneration and promote regenerative growth. Some of these signals occur in response to wounding, such as the release of reactive oxygen species, activation of Jun N-terminal kinase (JNK) signaling, and the production of mitogens such as Fgf20 and other growth-promoting signals such as nAG (Schuster, 2015 and references therein).

While progress has been made identifying early regeneration genes, little is known about the genes that regulate repatterning and adoption or maintenance of appropriate cell fates late in regeneration. Whereas the mechanisms that establish these cell fates during regeneration are often thought to recapitulate development and regenerative medicine seeks to replicate development, deviation from developmental patterning and reprogramming of positional identity can occur in regenerating tissue. Furthermore, changes in cell lineage can occur when necessitated by depletion of the preferred progenitor pool. Moreover, while regeneration can be induced in adult Xenopus limbs by grafting progenitor cells onto amputation stumps, application of developmental signaling molecules to provide pattern instruction and positional information did not generate limbs with complete patterning and structure, indicating that additional factors are needed to ensure the proper regenerated form. Thus, very important open questions remain regarding patterning and cell fate during regeneration. What are the genes and signals that control patterning and cell fate during the later steps of regeneration? Are these genes different from those that control patterning and cell fate in the same tissue during normal development? If so, why is the normal developmental program insufficient during regeneration? Identification of these unknown factors that enable regenerating structures to attain proper cell fates and form will be key to employing regenerative mechanisms in wounded tissue (Schuster, 2015 and references therein).

This study describes the identification of taranis (tara), a homolog of the vertebrate TRIP-Br (Transcriptional Regulators Interacting with plant homeodomain [PHD] zinc fingers and/or Bromodomains) family of proteins, as a regeneration-specific patterning gene in Drosophila. Vertebrate TRIP-Br proteins can regulate transcription through Dp/E2F and p53 and can regulate the cell cycle through direct binding of CyclinD/Cdk4 and by regulating expression of CyclinE. Drosophila Tara genetically interacts with E2F/Dp (Manansala, 2013) and with Polycomb Group and Trithorax Group genes (Calgaro, 2002 and Fauvarque, 2001) but otherwise remains uncharacterized at the molecular and functional levels (Schuster, 2015).

This study shows that regenerating tissue with reduced levels of Tara undergoes posterior-to-anterior fate transformations late in regeneration. These fate changes occur because expression of the posterior selector gene engrailed (en) becomes deregulated, leading to autoregulatory silencing of the engrailed locus, which requires the Polycomb Repressor Complex 1 (PRC1) subunit polyhomeotic (ph). The misregulation and subsequent silencing of en are induced by JNK signaling, which is essential for wound closure and regenerative growth. Tara is able to suppress these JNK-dependent fate changes without reducing JNK signaling activity. Thus, Tara stabilizes engrailed expression downstream of JNK signaling to maintain proper cell fate during regeneration (Schuster, 2015).

This study has shown that the endogenous wound response, orchestrated in part by JNK signaling, can induce inappropriate cell fate changes in regenerating tissue through misregulation of en. Although this finding was unexpected, it is not surprising that such strong signaling at the wound and in regenerating tissue, which can include reactive oxygen species (ROS) and Ca2+ release, as well as JNK, FGF, EGF, and WNT signals could affect the regenerating tissue in many deleterious ways. Indeed, the presence of this signaling is a primary difference between regenerating tissue and developing tissue and may account for many of the ways in which regeneration is distinct from development (Schuster, 2015).

This study also identified Taranis as a regeneration factor that protects regenerating tissue from the adverse side effects of JNK signaling. The molecular function of Tara is not known, although genetic interactions with E2F/Dp (Manansala, 2013) and with Polycomb Group and Trithorax Group genes (Calgaro, 2002 and Fauvarque, 2001) have been reported. Vertebrate TRIP-Br proteins can bind to and regulate transcription through E2F/Dp and can interact with the CREB-binding protein to activate p53 (Hayashi, 2006, Hsu, 2001 and Watanabe-Fukunaga, 2005). Given these reports, Tara may act by regulating transcription factors directly or by recruiting chromatin modifiers to influence transcription by altering the chromatin landscape. While this study has shown that Tara does not regulate en expression indirectly through modifying JNK signaling, Tara may regulate en directly or indirectly through a different intermediary. In addition, the upstream signals that activate tara expression during regeneration are unknown. Clarifying the function of the Tara protein will be important to understanding how cells protect their identity from perturbation by the signaling that orchestrates the wound response and regeneration (Schuster, 2015).

While the regulation of en and preservation of P identity could be specific to Drosophila wing disc regeneration, it is possible that Tara and vertebrate TRIP-Br proteins regulate expression of relevant genes at other wound sites. Indeed, Tara is also upregulated after pathogen-induced damage in the adult Drosophila gut (Chakrabarti, 2012). Furthermore, transcriptional profiling of regenerating tissue reveals the presence of TRIP-Br family members in the Xenopus tropicalis tadpole tail blastema and upregulation of TRIP-Br family members in regenerating zebrafish spinal cord and the axolotl limb blastema (Schuster, 2015 and references therein).

It is unlikely that Tara is the only protective factor required for regeneration. Future studies in experimental regeneration systems such as Drosophila will likely identify additional genes required for patterning and cell fate after regeneration. Current efforts to engineer regeneration for medical purposes often seek to replicate development. However, it is now clear that they must account for the unwanted side effects of regenerative signaling, whether endogenous to the wound or applied as therapy, and seek to deploy such protective factors to aid in regeneration (Schuster, 2015).

The Hippo pathway coactivator Yorkie can reprogram cell fates and create compartment-boundary-like interactions at clone margins

During development, tissue-specific patterns of gene expression are established by transcription factors and then stably maintained via epigenetic mechanisms. Cancer cells often express genes that are inappropriate for that tissue or developmental stage. This study shows that high activity levels of Yki, the Hippo pathway coactivator that causes overgrowth in Drosophila imaginal discs, can also disrupt cell fates by altering expression of selector genes like engrailed (en) and Ultrabithorax (Ubx). Posterior clones expressing activated Yki can down-regulate en and express an anterior selector gene, cubitus interruptus (ci). The microRNA bantam and the chromatin regulator Taranis both function downstream of Yki in promoting ci expression. The boundary between Yki-expressing posterior clones and surrounding wild-type cells acquires properties reminiscent of the anteroposterior compartment boundary; Hedgehog signaling pathway activation results in production of Dpp. Thus, at least in principle, heterotypic interactions between Yki-expressing cells and their neighbors could activate boundary-specific signaling mechanisms (Bairzin, 2020).

Human cancers are characterized by multiple genetic lesions, a subset of which are driver mutations that are thought to be responsible for their tumorous characteristics. It is estimated that most cancers have two to eight driver mutations. This makes it difficult to evaluate the contribution of each mutation to any particular characteristic of the tumor. This study has taken advantage of the ability of single-gene manipulations to cause overgrowth in Drosophila imaginal discs to assess the ability of three different oncogenes to destabilize established patterns of selector gene expression; yki, the Drosophila ortholog of Yap and Taz, is especially potent in doing so. The patterns of expression of En, Ci, and Ubx are established relatively early in embryogenesis and maintained stably in imaginal discs during the larval stages of development. These patterns of expression can be disrupted in clones expressing an activated form of Yki. Expression of a wild-type form of Yki is capable of disrupting these expression patterns in combination with other genetic manipulations such as overexpression of ban or tara. This latter scenario is more likely to apply to human cancers; increased Yap or Taz activity has been described in multiple human cancers, which often also have other genetic lesions (Bairzin, 2020).

These studies show that sd, ban, and tara make important contributions to the pathway by which Yki^CA destabilizes gene expression; reducing the expression of any of these in clones expressing Yki^CA greatly reduces ectopic Ci expression, and increasing expression of both genes can cause ectopic Ci expression. It is likely that other mechanisms function in parallel to destabilize selector gene expression since combined overexpression of ban and tara increased ectopic Ci expression but did not reduce En expression (Bairzin, 2020).

Changing selector gene expression within an overgrowing clone can create interactions at the clone margin that are reminiscent of compartment boundaries and result in the production of morphogens. A recent study showed that forced expression of En in lgl clones can elicit similar phenomena in anterior clones. In addition, Yki^CA clones are often extruded, consistent with previous observations that heterotypic interactions caused by overexpressing patterning genes also promotes extrusion. Previous work found that ci RNA levels were increased in wts mutant tissue, this study did not see ectopic Ci protein expression in wts mutant clones or wild-type Yki-overexpressing clones. This work shows therefore that sustained expression of very high Yki levels is necessary to destabilize expression of selector genes. However, even under these conditions, the effect on ectopic Ci expression is Sd dependent. Moreover, this study shows that even wild-type Yki can, in combination with increased expression of ban or tara, induce ectopic Ci expression. While these changes in gene expression are most obvious with above-physiological levels of Yki, they nevertheless reflect a previously unknown ability of this pathway to alter patterning gene expression and furthermore to change the growth characteristics of neighboring wild-type cells. Differences in selector gene expression between human cancers or precancerous lesions and their wild-type neighbors have received relatively little attention, and the current results call attention to tumor margins as sites where heterotypic interactions could create signaling centers that affect the behavior of tumor cells (Bairzin, 2020).

The Drosophila SERTAD protein Taranis determines lineage-specific neural progenitor proliferation patterns

Neural progenitors of the Drosophila larval brain, called neuroblasts, can be divided into distinct populations based on patterns of proliferation and differentiation. Type I neuroblasts produce ganglion mother cells (GMCs) that divide once to produce differentiated progeny, while type II neuroblasts produce self-renewing intermediate neural progenitors (INPs) and thus generate lineages containing many more progeny. This study identified Taranis (Tara) as an important determinant of type I lineage-specific neural progenitor proliferation patterns. Tara is an ortholog of mammalian SERTAD proteins that are known to regulate cell cycle progression. Tara is differentially-expressed in neural progenitors, with high levels of expression in proliferating type I neuroblasts but no detectable expression in type II lineage INPs. Tara is necessary for cell cycle reactivation in quiescent neuroblasts and for cell cycle progression in type I lineages. Cell cycle defects in tara mutant neuroblasts are due to decreased activation of the E2F1/Dp transcription factor complex and delayed progression through S-phase. Mis-expression of tara in type II lineages delays INP cell cycle progression and induces premature differentiation of INPs into GMCs. Premature INP differentiation can also be induced by loss of E2F1/Dp function and elevated E2F1/Dp expression suppresses Tara-induced INP differentiation. These results show that lineage-specific Tara expression is necessary for proper brain development and suggest that distinct cell cycle regulatory mechanisms exist in type I versus type II neural progenitors (Manansala, 2013).

Brain development requires precise control of neural progenitor proliferation and differentiation. Attempts were made to identify regulators of these processes starting with a TU-tagging approach (Miller, 2009) to identify mRNAs that are enriched in neural progenitors. From the TU-tagging data, tara as a candidate regulator of neurogenesis. Analysis of tara-lacZ expression in larval brains revealed interesting temporal and spatial expression patterns. tara is not expressed in quiescent neuroblasts of newly hatched larvae but is expressed in the persistently proliferating mushroom body neuroblasts. Similarly, tara expression ceases at the end of larval neurogenesis when neuroblasts exit the cell cycle but tara expression continues through pupal stages in the proliferating mushroom body neuroblasts. These temporal patterns suggest that tara expression is regulated by signals that control cell cycle entry and exit. In developing larvae, nutrient status controls secretion of insulin/IGF-like peptides from glial cells and this signaling system is necessary for neuroblast reactivation. Importantly, tara-lacZ expression is only detected after quiescent neuroblasts receive these reactivation signals. Tara is therefore not likely to function in the transduction of these signals but is a likely downstream target of reactivation pathways. This is similar to serum-induced expression of the mammalian Tara orthologs SERTAD1 and SERTAD2 (Sim, 2006). tara-lacZ expression is also detected in post-mitotic neurons of late-stage larval brains, particularly in early-born neurons that surround the central neuropil. This expression pattern suggests that Tara has an additional role in neural function, separate from the cell cycle roles described in this study (Manansala, 2013).

In addition to the temporal patterns of tara-lacZ expression, spatial patterns of tara-lacZ expression were identified that correspond to distinct progenitor populations. tara-lacZ is not expressed in optic lobe neuroepithelia but is expressed in neuroblasts derived from these progenitors. tara is also differentially-expressed in central brain neuroblasts, with high levels of tara-lacZ expression in type I neuroblasts, low or undetectable levels in type II neuroblasts (following reactivation), and no detectable expression in type II lineage INPs. Compared to previously described lineage-specific proteins (Ase, Erm, PntP1), Tara is unique in that it is only expressed in type I neuroblast lineages following reactivation. Importantly, the tara-lacZ expression patterns in type I versus type II lineages correlate with tara loss-of-function and mis-expression phenotypes. Type I lineage tara loss-of-function clones had dramatic proliferation defects while type II lineage loss-of-function clones were not significantly different from wildtype, as predicted based on the absence of tara-lacZ expression in type II lineages. Type II lineages were more dramatically affected by tara mis-expression, compared to the effect of tara over-expression in type I lineages, providing evidence that the absence of Tara is important for normal type II lineage development (Manansala, 2013).

The timing of tara expression during neuroblast reactivation suggested that Tara is necessary for cell cycle re-entry. This role was confirmed in tara¹ mutants, in which quiescent neuroblasts fail to reactivate. A neuroblast-autonomous role for Tara in reactivation was demonstrated using the wor-GAL4 driver to express a tara transgene in the tara¹ mutant background. Wor-GAL4 driven tara expression was sufficient to rescue quiescent neuroblast reactivation in tara¹ mutants, although there were fewer M phase neuroblasts relative to wildtype. Wor-GAL4 driven E2F1/Dp expression in tara¹ mutants also rescued quiescent neuroblast reactivation, placing E2F1/Dp downstream of Tara in the reactivation pathway. Given the requirement for Tara during neuroblast reactivation, precocious expression of tara in quiescent neuroblasts might be expected to cause early cell cycle re-entry. Expression of tara in quiescent neuroblasts was sufficient to activate CycE expression but was not sufficient for progression to S phase. Similarly, previous work has shown that precocious expression of CycE fails to induce premature S phase in quiescent neuroblasts. These results are interpreted as evidence that Tara regulates the initial transition from G0 to G1, but temporally-regulated growth signals are required for transition past the G1/S checkpoint. It is worth noting that ectopic expression of tara or E2F1/Dp in neuroblasts using wor-GAL4 did not rescue the lethality of tara¹ mutants. This suggests that endogenous regulation of Tara in the nervous system is essential or that Tara is required in other tissues such as mesoderm or muscles (Manansala, 2013).

MARCM analysis showed that tara loss-of-function in type I lineages caused a delay in neuroblast and GMC cell cycle progression, primarily during S-phase. In Drosophila embryos, decreased E2F1/DP activity has been shown to inhibit the G1→S transition (which requires the E2F1/DP target gene CycE) and to delay S phase progression (which requires the E2F1/DP target gene PCNA). Following reactivation, the delayed S phase progression observed in tara¹ clones may be due to decreased transcription of E2F1/DP targets, particularly PCNA. tara¹ neuroblasts express low levels of PCNA and this would be expected to limit the processivity of DNA polymerase delta and prolong S phase, as previously described (Manansala, 2013).

The absence of tara expression in INPs led to a hypothesis that tara mis-expression might interfere with type II lineage development. tara mis-expression in type II lineages was found to delay INP cell cycle progression and reduced the number of INPs in type II neuroblast MARCM clones. The decreased numbers of INPs appeared to be due to premature INP differentiation, particularly based on the analysis of INP MARCM clones. If tara mis-expression only decreased the rate of INP self-renewing divisions, the frequency of clones that still contain an INP would not differ between wildtype and tara mis-expressing clones. However, there is a significant decrease in the number of clones containing an INP when tara is mis-expressed, indicating premature INP differentiation into a GMC. Delayed progression through G1 and S-phases in INPs could be sufficient to cause premature differentiation, as has been demonstrated in mammalian cortical progenitors. However, experiments using Dap to delay the G1/S transition in INPs demonstrate that this delay is not sufficient to cause differentiation. These results suggest that there is something unique about the cell cycle delay induced by Tara or that Tara causes INP differentiation independent of its effects on cell cycle timing (Manansala, 2013).

Mammalian SERTAD proteins regulate cell cycle entry and progression via interaction with Cdk4 and link cell cycle progression with transcription via interactions with E2F/Dp. SERTAD1 regulates Cdk4 kinase activity in a dose-dependent manner, stimulating Cdk4 at low concentrations and inhibiting Cdk4 at high concentrations (Li, 2004). It was hypothesized that Tara might inhibit Cdk4 in INPs but found that Cdk4 is dispensable for type II lineage development. Similarly, elevated Cdk4 expression does not suppress Tara-induced INP differentiation. These results agree with previous work showing that Cdk4 does not significantly affect cell proliferation rates in Drosophila and instead regulates cell growth in certain tissues. In contrast to Cdk4, loss of E2F1/Dp-dependent transcription causes type II lineage defects that are identical to those observed in tara mis-expressing INPs. Similarly, E2F1/Dp over-expression suppresses Tara-induced INP differentiation. These results support a model in which Tara represses transcription of E2F1/Dp target genes that are necessary for cell cycle progression and self-renewal in INPs. In contrast to mammals, the Drosophila genome encodes a single Dp protein, and only two E2F proteins, E2F1 and E2F2. E2F1/Dp activates transcription of genes necessary for cell cycle progression and E2F2/Dp can repress transcription of these same genes in addition to repressing transcription of a distinct set of genes involved in differentiation. This study did not investigate the potential contribution of E2F2/Dp-Tara complexes in INP differentiation, but the fact that E2F1i2 mutants mimic tara mis-expression phenotypes and elevated E2F1/Dp suppresses tara-mis-expression phenotypes suggests that E2F1 target genes (as opposed to E2F2-specific target genes) are relevant to INP differentiation (Manansala, 2013).

The current findings support a model in which Tara stimulates E2F1/Dp activity in type I lineages but inhibits E2F1/Dp activity when mis-expressed in type II lineages. A likely explanation for these opposing lineage-specific roles of Tara is provided by the fact that mammalian SERTAD1 can interact with transcription factors that either stimulate or inhibit E2F1/Dp1 activity. This interaction occurs via PHD and bromodomain binding motifs that are conserved in Tara (Calgaro, 2002). It is proposed that distinct E2F/Dp-Tara complexes form in type I versus type II lineages and that the absence of Tara from INPs is necessary to avoid formation of complexes that inhibit E2F1/Dp activity and favor differentiation. Elevated Pros expression in INPs has been shown to induce premature differentiation and this study observed an increased number of Proshigh cells in tara mis-expressing clones. Elevated Pros may be the cause of differentiation in tara mis-expressing INPs. Tara is not required for induction of Pros expression in type I lineages, since tara loss-of-function does not result in any detectable changes in Pros levels, but tara mis-expression may increase Pros transcription in INPs. The current findings suggest that Tara establishes lineage-specific E2F1/Dp-dependent transcription programs, and identification of relevant target genes and Tara-interacting transcription factors will be important areas of future investigation. The discovery that a Drosophila SERTAD protein differentially regulates the development of neural lineages also raises the possibility that mammalian SERTAD proteins control cell cycle and cell fate decisions in a lineage-specific or tissue-specific manner (Manansala, 2013).

Dicer partner proteins tune the length of mature miRNAs in flies and mammals

Drosophila Dicer-1 produces microRNAs (miRNAs) from pre-miRNA, whereas Dicer-2 generates small interfering RNAs (siRNAs) from long dsRNA. Alternative splicing of the loquacious (loqs) mRNA generates three distinct Dicer partner proteins. To understand the function of each, flies were constructed expressing Loqs-PA, Loqs-PB, or Loqs-PD. Loqs-PD promotes both endo- and exo-siRNA production by Dicer-2. Loqs-PA or Loqs-PB is required for viability, but the proteins are not fully redundant: a specific subset of miRNAs requires Loqs-PB. Surprisingly, Loqs-PB tunes where Dicer-1 cleaves pre-miR-307a, generating a longer miRNA isoform with a distinct seed sequence and target specificity. The longer form of miR-307a represses glycerol kinase and taranis mRNA expression. The mammalian Dicer-partner TRBP, a Loqs-PB homolog, similarly tunes where Dicer cleaves pre-miR-132. Thus, Dicer-binding partner proteins change the choice of cleavage site by Dicer, producing miRNAs with target specificities different from those made by Dicer alone or Dicer bound to alternative protein partners (Fukunaga, 2012).

The Drosophila gene taranis encodes a novel trithorax group member potentially linked to the cell cycle regulatory apparatus

Genes of the Drosophila Polycomb and trithorax groups (PcG and trxG, respectively) influence gene expression by modulating chromatin structure. Segmental expression of homeotic loci (HOM) initiated in early embryogenesis is maintained by a balance of antagonistic PcG (repressor) and trxG (activator) activities. This study identified a novel trxG family member, taranis (tara), on the basis of the following criteria: (1) tara loss-of-function mutations act as genetic antagonists of the PcG genes Polycomb and polyhomeotic and (2) they enhance the phenotypic effects of mutations in the trxG genes trithorax (trx), brahma (brm), and osa. In addition, reduced tara activity can mimic homeotic loss-of-function phenotypes, as is often the case for trxG genes. tara encodes two closely related 96-kD protein isoforms (TARA-α/-β) derived from broadly expressed alternative promoters. Genetic and phenotypic rescue experiments indicate that the TARA-α/-β proteins are functionally redundant. The TARA proteins share evolutionarily conserved motifs with several recently characterized mammalian nuclear proteins, including the cyclin-dependent kinase regulator TRIP-Br1/p34(SEI-1), the related protein TRIP-Br2/Y127, and RBT1, a partner of replication protein A. These data raise the possibility that TARA-α/-β play a role in integrating chromatin structure with cell cycle regulation (Calgaro, 2002).

Dominant modifiers of the polyhomeotic extra-sex-combs phenotype induced by marked P element insertional mutagenesis in Drosophila

Members of the Polycomb group (Pc-G) and trithorax group (trx-G) of genes, as well as the enhancers of trx-G and Pc-G (ETP), function together to maintain segment identity during Drosophila development. In order to obtain new marked P mutations in these genes, a screen was carried out for dominant modifiers of the extra-sex-combs phenotype displayed by males mutant for the polyhomeotic (ph) gene, a member of the Pc-G group. Five P(lacW) insertions in four different genes were found to stably suppress ph: two are allelic to trithorax, one is the first allele specific to the Minute(2)21C gene, and the remaining two define new trx-G genes, toutatis (tou) in 48A and taranis (tara) in 89B10-13. tou is predicted to encode a 3109 amino acid sequence protein (TOU), which contains a TAM DNA-binding domain, a WAKZ motif, two PHD zinc fingers and a C-terminal bromodomain, and as such is likely to be involved in regulation of chromatin structure as a subunit of a novel chromatin remodelling complex. A previous study found that insertion of a P(ph) transposable element containing ph regulatory sequences creates a high frequency of mutations modifying ph homeotic phenotypes. One such insertion enhanced the ph phenotype and was shown that to be new allele of UbcD1/eff, a gene encoding a ubiquitin-conjugating enzyme that is involved in telomere association and potentially in chromatin remodelling (Fauvarque, 2001).

Search PubMed for articles about Drosophila Taranis

Afonso, D.J., Liu, D., Machado, D.R., Pan, H., Jepson, J.E., Rogulja, D. and Koh, K. (2015). TARANIS functions with Cyclin A and Cdk1 in a novel arousal center to control sleep in Drosophila. Curr Biol 25(13):1717-26. PubMed ID: 26096977

Bairzin, J. C. D., Emmons-Bell, M. and Hariharan, I. K. (2020). The Hippo pathway coactivator Yorkie can reprogram cell fates and create compartment-boundary-like interactions at clone margins. Sci Adv 6(50). PubMed ID: 33298454

Calgaro, S., Boube, M., Cribbs, D. L. and Bourbon, H. M. (2002). The Drosophila gene taranis encodes a novel trithorax group member potentially linked to the cell cycle regulatory apparatus. Genetics 160: 547-560. PubMed ID: 11861561

Chakrabarti, S., Liehl, P., Buchon, N. and Lemaitre, B. (2012). Infection-induced host translational blockage inhibits immune responses and epithelial renewal in the Drosophila gut. Cell Host Microbe 12: 60-70. PubMed ID: 22817988

Fauvarque, M. O., Laurenti, P., Boivin, A., Bloyer, S., Griffin-Shea, R., Bourbon, H. M. and Dura, J. M. (2001). Dominant modifiers of the polyhomeotic extra-sex-combs phenotype induced by marked P element insertional mutagenesis in Drosophila. Genet Res 78: 137-148. PubMed ID: 11732091

Fukunaga, R., Han, B. W., Hung, J. H., Xu, J., Weng, Z. and Zamore, P. D. (2012). Dicer partner proteins tune the length of mature miRNAs in flies and mammals. Cell 151: 533-546. PubMed ID: 23063653

Hayashi, R., Goto, Y., Ikeda, R., Yokoyama, K. K. and Yoshida, K. (2006). CDCA4 is an E2F transcription factor family-induced nuclear factor that regulates E2F-dependent transcriptional activation and cell proliferation. J Biol Chem 281: 35633-35648. PubMed ID: 16984923

Hsu, S. I., Yang, C. M., Sim, K. G., Hentschel, D. M., O'Leary, E. and Bonventre, J. V. (2001). TRIP-Br: a novel family of PHD zinc finger- and bromodomain-interacting proteins that regulate the transcriptional activity of E2F-1/DP-1. EMBO J 20: 2273-2285. PubMed ID: 11331592

Li, J., Melvin, W. S., Tsai, M. D. and Muscarella, P. (2004). The nuclear protein p34SEI-1 regulates the kinase activity of cyclin-dependent kinase 4 in a concentration-dependent manner. Biochemistry 43: 4394-4399. PubMed ID: 15065884

Manansala, M. C., Min, S. and Cleary, M. D. (2013). The Drosophila SERTAD protein Taranis determines lineage-specific neural progenitor proliferation patterns. Dev Biol 376: 150-162. PubMed ID: 23376107

Miller, M. R., Robinson, K. J., Cleary, M. D. and Doe, C. Q. (2009). TU-tagging: cell type-specific RNA isolation from intact complex tissues. Nat Methods 6: 439-441. PubMed ID: 19430475

Schuster, K. J. and Smith-Bolton, R. K. (2015). Taranis protects regenerating tissue from fate changes induced by the wound response in Drosophila. Dev Cell 34: 119-128. PubMed ID: 26096735

Sim, K. G., Cheong, J. K. and Hsu, S. I. (2006). The TRIP-Br family of transcriptional regulators is essential for the execution of cyclin E-mediated cell cycle progression. Cell Cycle 5: 1111-1115. PubMed ID: 16721052

Watanabe-Fukunaga, R., Iida, S., Shimizu, Y., Nagata, S. and Fukunaga, R. (2005). SEI family of nuclear factors regulates p53-dependent transcriptional activation. Genes Cells 10: 851-860. PubMed ID: 16098148

Biological Overview

date revised: 12 April 2021

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