BIOLOGICAL OVERVIEW

daughterless is so named because of its role in sex determination. It is required for the maturation of follicle cells during egg chamber morphogenesis. The dimerization partner of daughterless in the maturation of follicle cells is unknown (Gonzalez-Crespo, 1993 and Cummings, 1994). In other roles daughterless interacts with Dorsal to bring about the induction of twist and snail, genes required for gastrulation. daughterless is a cofactor for their activation.

However, it is the involvement of daughterless in neural differentiation that is considered of primary developmental importance. Although daughterless is not required for the formation and delamination of "nascent" neuronal precursors from the epidermal layer, it is required for expression of neuron specific genes. Mutation of da blocks transformation of presumptive precursors into true precursors. Since AS-C genes are required for cells to become neuronal precursors, this requirement is fulfilled in the absence of daughterless (Vassin, 1994). This result is paradoxical because it is presumed that DA is the dimerization partner of AS-C proteins. How can Achaete and Scute carry out their proneural function without DA?

The list of genes activated by Daughterless as a cofactor with achaete-scute complex genes will continue to grow. Known targets include prospero, cyclin A and calmodulin (Vaessin, 1994 and Kovalick, 1992).

Proneural gene products like Daughterless and Lethal of scute can bind to promoters of Enhancer of split and achaete genes, and by so doing, activate their transcription. Two proteins of the E(spl)-C (HLH-M5 and Enhancer of split) attenuate the transcriptional activation mediated by the proneural genes. This observation begins to untangle the complicated role of E(spl)-C genes in neurogenesis. Once neuroblasts have segregated, products of proneural genes become restricted to the neuroblasts. Products of the E(spl)-C genes are restricted to cells remaining in the epithelium. Therefore it appears that E(spl)-C functionally antagonizes the proneural proteins and thus silences expression of genes that are activated by the proneural genes (Oellers, 1994).

Daughterless couples the control of differentiation and cell cycle programs in in the developing sensory organ precursor (SOP). Although Daughterless is required for the proper expression of neuronal precursor genes and lineage identity genes in the peripheral nervous system (PNS) of Drosophila embryos, this requirement does not explain the failure of the nascent PNS precursors to undergo a normal cell cycle and divide in da mutants. Four genes whose products are required for various stages of the cell cycle are misexpressed in the PNS of da mutant embryos. Cyclin A, barren, disc proliferation abnormal and Histone H1 transcripts are significantly reduced or undetectable in the precursors of the PNS at stages 11 and 12. Precursors are still present at these stages in da mutants. This suggests that all aspects of PNS precursor differentiation examined so far are under the transcriptional control of da. Sensory organ precursors lacking Da may fail to express and/or accumulate other factors, such as critical differentiation genes, required for SOP entry into the cell cycle. It should be pointed out that these factors are unlikely to be the thus-far described neuronal precursor genes, as mutations in these genes do not result in any obvious cell cycle defects. Thus daughterless controls the expression of cell cycle genes in the PNS sensory organ precursors but nowhere else (Hassan, 1997).

The basic helix-loop-helix transcription factor Twist regulates a series of distinct cell fate decisions within the Drosophila mesodermal lineage. These twist functions are reflected in its dynamic pattern of expression, which is characterized by initial uniform expression during mesoderm induction, followed by modulated expression at high and low levels in each mesodermal segment, and finally restricted expression in adult muscle progenitors. Two distinct partner-dependent functions for Twist were found that are crucial for cell fate choice. Twist can form homodimers and heterodimers in vitro with the Drosophila E protein homolog, Daughterless. Using tethered dimers to assess directly the function of these two particular dimers in vivo, it has been shown that Twist homodimers specify mesoderm and the subsequent allocation of mesodermal cells to the somatic muscle fate. Misexpression of Twist-tethered homodimers in the ectoderm or mesoderm leads to ectopic somatic muscle formation overriding other developmental cell fates. In addition, expression of tethered Twist homodimers in embryos null for twist can rescue mesoderm induction as well as somatic muscle development. Loss of function analyses, misexpression and dosage experiments, and biochemical studies indicate that heterodimers of Twist and Daughterless repress genes required for somatic myogenesis. It is proposed that these two opposing roles explain how modulated Twist levels promote the allocation of cells to the somatic muscle fate during the subdivision of the mesoderm. Moreover, this work provides a paradigm for understanding how the same protein controls a sequence of events within a single lineage (Castanon, 2001).

At stage 10, in response to transcriptional regulators such as Sloppy paired and Even skipped, as well as signals from the overlying ectoderm such as Wingless, the uniform expression of Twist modulates into regions of high and low expression within each segment. Da is expressed uniformly in the mesoderm at this time. The region that maintains high Twist levels subsequently gives rise to somatic muscles whereas the region that has lower Twist levels gives rise to tissues such as visceral muscle, fat body, gonadal mesoderm and some glia cells. The heart is derived from the region that initially expresses high levels of Twist; however these cells lose Twist expression, an event necessary for the execution of heart fate. Expressing high Twist levels in cells destined to become visceral muscle, for example, blocks visceral muscle differentiation and promotes somatic muscle. Reduction of Twist levels in cells normally expressing high Twist levels blocks somatic myogenesis (Castanon, 2001 and references therein).

Several possible mechanisms are provided to explain these observations and illustrate the in vivo roles for the two opposing activities of Twist homodimers and Twist/Da heterodimers. Regions that normally express lower Twist levels do not form somatic muscles owing to higher concentrations of Twist/Da heterodimers as compared to Twist homodimers. These heterodimers repress transcription of pro-muscle genes, such as lísc as well as founder cell genes such as Kr, thereby prohibiting somatic muscle development. Other differentiation programs for visceral muscle or fat body development can proceed unaffected. No evidence is found that Twist/Da heterodimers promote visceral mesoderm or fat body fate through the direct activation of targets such as Fas III. Regions that normally express higher Twist levels do form somatic muscle owing to higher concentrations of Twist homodimers as compared to Twist/Da heterodimers. Dimer competition, then, restricts the developmental potential of mesodermal cells, by not allowing Twist homodimers to convert all mesodermal cells into somatic muscle (Castanon, 2001).

These conclusions are consistent with the observations that increasing Twist/Da levels, either by overexpression of Da or the tethered Twist-Da heterodimer, repress the earliest steps in somatic myogenesis. These are the same steps that are activated by Twist homodimers. For example, Lísc expression, which marks clusters of equipotential cells that segregate the muscle founder cells, is drastically reduced or absent upon an increase of Twist/Da heterodimers. This indicates an early failure in the somatic muscle program. Likewise failure in subsequent steps is seen; for example, few founder cells as well as few identifiable muscles are detected. These failures in muscle development are interpreted as an outcome of the initial block in the differentiation pathway. The possibility that overexpression of Da or of Twist-Da could directly repress these subsequent steps is not eliminated. Gal4 lines that drive expression at later stages of muscle development or in particular subsets of muscle cells (i.e., the S59-expressing founder cells) could provide insight into this alternative (Castanon, 2001).

GENE STRUCTURE

Bases in 5' UTR - 229

Exons - two

Bases in 3' UTR - 992

PROTEIN STRUCTURE

Amino Acids - 710

Structural Domains

One-third of the way into the protein there is a tandem repeat of a histidine-rich region. This is followed by a PEST sequence, a "myc similarity region," (a basic HLH domain) and a C terminal lysine repeat region (Caudy, 1988 and Cronmiller, 1988).

date revised: 15 November 2001  

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