InteractiveFly: GeneBrief

shuttle craft : Biological Overview | Developmental Biology | Effects of Mutation | Evolutionary Homologs | References

BIOLOGICAL OVERVIEW

shuttle craft (stc) was identified in a cDNA expression cloning screen for single-stranded nucleic acid binding proteins expressed during Drosophila oogenesis. stc is a dual personality gene: stc maternal and zygotic transcripts control two distinct traits. Maternal mutants exhibit defects in segmentation, while zygotic mutants show defects in axon guidance. Possessing a novel double stranded DNA-binding domain, and a separate single-stranded nucleic acid binding domain that binds nucleic acids non-specifically, Stc, along with its mammalian homolog NF-X1, define a new class of transcription factors (Stroumbakis, 1996).

Embryos lacking the stc maternal function display defects in segment polarity gene expression (these segmentation defects are detected by examination of cuticle preparations), and die during early embryogenesis. Although wild-type embryos exhibit a dynamic cell-cycle-dependent STC nuclear distribution pattern during cellularization (see stc Developmental Biology ), stc maternal mutants show no defects in pre-cellularization mitosis. Examination of cuticle preparations from late embryos derived from stc maternal mutants shows poor cuticle development and variable segmentation defects consisting of either the loss or the fusion of abdominal denticle belts. In extreme cases, embryos are missing as many as five segments. In addition, most of the maternally mutant embryos appear to arrest development by stage 16 of embryogenesis (Stroumbakis, 1998).

In wild type flies, during germband extension, Engrailed protein is expressed as a position-specific series of three oral, three thoracic, and nine abdominal stripes. During this same developmental period, virtually all embryos derived from maternally mutant oocytes fertilized with wild-type sperm display severe segmentation defects evident in distinct regions of the embryo. The defects are most obvious in thoracic segments T1-T3 as well as in abdominal segments. The T1-T3 regions contain a deletion of one or two segments as judged by the reduction in the number of Engrailed stripes. A more variable phenotype is associated in A4-A8 where Engrailed stripes appear disrupted, fused or deleted. Compared with wild-type embryos, three segments are missing (Stroumbakis. 1998).

shuttle craft zygotic mutants show defects in neural projections that are independent of the segmentation defects observed in maternal mutations. Wild-type flies project nerves (the segmental nerve [SNB] and the intersegmental nerve [ISN]) that follow routes from specific motoneurons and initiate their journey laterally from the ventral nerve cord. These nerves begin their migration at the end of germband retraction, which, coincidentally, is the same period when Stc protein is first detected in the CNS. As they exit the CNS and join together, the SNB and the ISN normally project in a characteristically direct path along specific guideposts, initially provided by the segmental branches of the trachea and later by axons derived from the dorsal sensory neurons. However, this is not the case in stc mutant embryos: in these mutants, the nerves appear to either navigate in a misguided manner or fail to maintain their precise positions after migration. Unlike previously described mutations in genes that appear to block migration of growth cones at specific locations (e.g., short stop, stranded, and beaten path), the projection of these nerves in the stc mutant embryo is not blocked. Instead, these nerves appear disarrayed, possibly because they are either partially misrouted during their migration or their exact position is not maintained after migration, as they continue to extend to their ultimate muscle field targets. Toward the end of embryogenesis, stc mutant embryos also display a slight failure in their ability to completely condense their CNS (Stroumbakis, 1996).

Maternal stc mutants exhibit their own axon guidance phenotypes. An additional phenotype distinct from either the early maternal effect segmentation or the late zygotic loss-of-function defects affects the migrating ISN and SNB nerves, which often fail to join when exiting the CNS and continue traveling in a misguided manner. These motoneuronal axon guidance defects are fully penetrant and occur in neuromeric segments that do not otherwise display severe maternal effect segmentation abnormalities (Stroumbakis, 1998).

It is concluded that the maternal and zygotic sources of Stc protein are functionally independent and play different roles during embryonic development. The presence of a maternally associated axon guidance phenotype, in addition to the segmentation defect, implies that there are at least two distinct independently-acting activities associated with maternally derived Stc protein in the embryo. The stc maternal effect phenotype is similar to that associated with disruptions in hopscotch and marelle, which encode two important components of the Drosophila embryonic JAK/STAT signaling pathway. It is also interesting to note that the human Stc homolog, NF-X1, is a transcription factor activated by the cytokine IFN-gamma (Song, 1994), one of many ligands that are know to function in JAK/STAT signaling. These observations raise the possibility that the stc maternal function may participate in or may be a target of the JAK/STAT signaling pathway (Stroumbakis, 1998).

DEVELOPMENTAL BIOLOGY

Embryonic

During cycles of syncytial nuclear division in early embryogenesis, a maternally derived source of Stc protein exhibits a dynamic cell cycle-dependent nuclear distribution pattern, which is eventually lost once the synchronous division cycles end at blastoderm cellularization. After fertilization, Stc protein derived from maternal transcripts are evenly distributed throughout the embryo. By the ninth syncytial nuclear cleavage cycle, Stc protein specifically localizes within the nuclei. At the completion of syncytial division cycles, at blastoderm cellulariztion, Stc protein is no longer localized in interphase nuclei, and its intracellular distribution is not localized. Zygotic expression of Stc protein is detected in stage 13 to 17 embryos where it is prominent in the nuclei of subsets of cells in the CNS. Expression is restricted to the nuclei of repeated clusters of cells located in each neuromere along the length of the ventral cord and to distinct groups of cells located within the brain lobes. Hence, its limited zygotic distribution suggests a possible role for Stc protein in the development of the CNS (Stroumbakis, 1996).

Adult

STC transcripts are expressed throughout oogenesis in all follicle cells and nurse cells and are transferred by the latter into the oocyte at stage 10b, where they are maintained after fertilization within the embryo. Stc protein is first detected in the nuclei of follicle cells present in the posterior half of the germarium. Throughout oogenesis, STC protein is abundantly present in the nuclei of anterior and posterior terminal follicle cells. This nuclear expression first covers a number of cells within these spatial domains but is eventually refined by stage 10 of oogenesis to two cells at the anterior (i.e. a subset of migrating border cells) and two at the posterior end of the oocyte. By stage 12 of oogenesis, all follicle cells accumulate Stc protein in their nuclei. Stc protein is also observed in the cytoplasm of nurse cells and the oocyte. In addition, nurse cell nuclei contain Stc protein concentrated in two or three distinct spherical structures of unknown origin. Though Stc protein is abundant in ovaries, studies of germ-line mutants shows that it is not required for the completion of oogenesis (Stroumbakis, 1998).

EFFECTS OF MUTATION

shuttle craft mutants die at the end of embryogenesis, when they appear to be incapable of coordinating the typical peristaltic contraction waves normally required for embryos to hatch into feeding first instar larvae. Preliminary evidence indicates that the resulting lethality of this behavioral defect is accompanied by subtle morphological abnormalities in the central nervous system, where in wild-type embryos, Stc protein is normally localized in the nuclei of repeated cell clusters within each neuromere and brain lobe (Stroumbakis, 1996). shuttle craft is expressed zygotically in the embryonic central nervous system (CNS) where it is required to maintain the proper morphology of motoneuronal axon nerve routes following their migration from the ventral cord. A prominent maternal source of Stc protein is also present throughout both oogenesis and embryogenesis. To determine whether this maternal component is required in the ovary and/or embryo, the Drosophila autosomal dominant female sterile technique was employed to generate germ-line clones that lack the stc maternal function. A maternally derived source of STC protein is required during embryogenesis but not oogenesis. In contrast to the zygotic phenotype, the primary defect in embryos derived from stc germ-line clones affects segmentation by causing disruptions and deletions in distinct thoracic (T1-T3) and abdominal (A4-A8) segments. These localized defects are responsible for additional phenotypes observed later in development that include gaps in the ventral nerve cord and deletions of denticle belts in the cuticle. An additional phenotype occurring in all other neuromeric segments consists of the misguided migration of motoneuronal axons as they project out of the ventral nerve cord. Thus, the stc zygotic function is required later in development and cannot correct the segmentation and subsequent CNS abnormalities associated with loss of its earlier acting maternally derived activity (Tolias, 1998).

Variation in longevity in natural populations is attributable to the segregation of multiple interacting loci, whose effects are sensitive to the environment. Although there has been considerable recent progress towards understanding the environmental factors and genetic pathways that regulate lifespan, little is known about the genes causing naturally occurring variation in longevity. Deficiency complementation mapping was used to map two closely linked quantitative trait loci (QTL) causing female-specific variation in longevity between the Oregon (Ore) and 2b strains of Drosophila melanogaster to 35B9-C3 and 35C3 on the second chromosome. The 35B9-C3 QTL encompasses a 50-kb region including four genes, for one of which, shuttle craft (stc), mutations have been generated. The 35C3 QTL localizes to a 200-kb interval with 15 genes, including three genes for which mutations exist [reduced (rd), guftagu (gft) and ms(2)35Ci]. Quantitative complementation tests to mutations at these four positional candidate genes were performed; ms(2)35Ci and stc are novel candidate quantitative trait genes affecting variation in Drosophila longevity. Complementation tests with stc alleles reveal sex- and allele-specific failure to complement, and complementation effects are dependent on the genetic background, indicating considerable epistasis for lifespan. In addition, a homozygous viable stc allele has a sex-specific effect on lifespan. stc encodes an RNA polymerase II transcription factor, and is an attractive candidate gene for the regulation of longevity and variation in longevity, because it is required for motoneuron development and is expressed throughout development. Quantitative genetic analysis of naturally occurring variants with subtle effects on lifespan can identify novel candidate genes and pathways important in the regulation of longevity (Pasyukova, 2004).

Embryonic expression of shuttle craft, a Drosophila gene involved in neuron development, is associated with adult lifespan

Despite the progress in aging research that highlights the role of the nervous system in longevity, whether genes that control development and consequently structure of the nervous system affect lifespan is unclear. This study demonstrates that a mutation in shuttle craft, a gene involved in the nervous system development, increased the lifespan of unmated females and decreased the lifespan of mated females, without affecting males. Precise reversions of the mutation lead to the restoration of the lifespan specific to control females. In mutant unmated females, increased lifespan was associated with elevated locomotion at older ages, indicating slowed aging. In mutant mated females, reproduction was decreased compared to controls, indicating a lack of tradeoff between this trait and lifespan. No differences in shuttle craft transcription were observed between whole bodies, ovaries, and brains of mutant and control females of different ages, either unmated or mated. The amount of shuttle craft transcript appeared to be substantially decreased in mutant embryos. These results demonstrated that a gene that regulates development of the nervous system might also influence longevity, and thus expanded the spectrum of genes involved in lifespan control. It is hypothesized that this 'carry-over' effect might be the result of transcription regulation in embryos (Roshina, 2014: PubMed).

EVOLUTIONARY HOMOLOGS

The class II major histocompatibility complex (MHC) molecules function in the presentation of processed peptides to helper T cells. As most mammalian cells can endocytose and process foreign antigen, the critical determinant of an antigen-presenting cell is its ability to express class II MHC molecules. Expression of these molecules is usually restricted to cells of the immune system and dysregulated expression is hypothesized to contribute to the pathogenesis of a severe combined immunodeficiency syndrome and certain autoimmune diseases. Human complementary DNA clones encoding a newly identified, cysteine-rich transcription factor, NF-X1, which binds to the conserved X-box motif of class II MHC genes, have been obtained, and the primary amino acid sequence deduced. The major open reading frame encodes a polypeptide of 1,104 amino acids with a symmetrical organization. A central cysteine-rich portion encodes the DNA-binding domain, and is subdivided into seven repeated motifs. This motif is similar to but distinct from the LIM domain and the RING finger family, and is reminiscent of known metal-binding regions. The unique arrangement of cysteines indicates that the consensus sequence CX3CXL-XCGX1-5HXCX3CHXGXC represents a novel cysteine-rich motif. Two lines of evidence indicate that the polypeptide encodes a potent and biologically relevant repressor of HLA-DRA transcription: (1) overexpression of NF-X1 from a retroviral construct strongly decreases transcription from the HLA-DRA promoter, and (2) the NF-X1 transcript is markedly induced late after induction with interferon gamma (IFN-gamma), coinciding with postinduction attenuation of HLA-DRA transcription. The NF-X1 protein may therefore play an important role in regulating the duration of an inflammatory response by limiting the period in which class II MHC molecules are induced by IFN-gamma (Song, 1994).

REFERENCES

Search PubMed for articles about Drosophila shuttle craft

Pasyukova, E. G., Roshina, N. V. and Mackay, T. F. (2004). Shuttle craft: a candidate quantitative trait gene for Drosophila lifespan. Aging Cell. 3(5): 297-307. 15379853

Roshina, N. V., Symonenko, A. V., Krementsova, A. V., Trostnikov, M. V. and Pasyukova, E. G. (2014). Embryonic expression of shuttle craft, a Drosophila gene involved in neuron development, is associated with adult lifespan. Aging (Albany NY) 6(12): 1076-93. PubMed ID: 25567608

Song, Z., et al. (1994). A novel cysteine-rich sequence-specific DNA-binding protein interacts with the conserved X-box motif of the human major patibility complex class II genes via a repeated Cys-His domain and functions as a transcriptional repressor. J. Exp. Med. 180(5): 1763-74. PubMed ID: 7964459

Stroumbakis, N. D., Li, Z. and Tolias, P. P. (1996). A homolog of human transcription factor NF-X1 encoded by the Drosophila shuttle craft gene is required in the embryonic central nervous system. Mol. Cell. Biol. 16(1): 192-201. PubMed ID: 8524296

Tolias, P. P. and Stroumbakis, N. D. (1998). The Drosophila zygotic lethal gene shuttle craft is required maternally for proper embryonic development. Dev. Genes Evol. 208(5): 274-82. PubMed ID: 9683743