staufen


DEVELOPMENTAL BIOLOGY

Staufen mRNA is expressed very early in oogenesis in the germline cells of stage 3-4 follicles. By stage 6, the RNA is most concentrated in a single posterior cell in the follicle, the presumptive ooctye, but is also visible in the 15 more anterior nurse cells. As the oocyte grows, this localization of the Staufen transcript diminishes, and by stage 10 almost all of the RNA is found in the nurse cells. Later in oogenesis, the nurse cells contract and transfer their cytoplasm to the oocyte. Staufen mRNA is presumably also transferred to the oocyte at this point. Freshly laid egg contains high levels of uniformly distributed Staufen transcripts. At cellularization, this RNA is incorporated into the basal regions of the blastoderm and persists until after gastrulation (St Johnston, 1991).

During early stages of oogenesis, Staufen protein is uniformly distributed within the ooyte cytoplasm. The protein begins to localize during stage 8, accumulating at the anterior margins of the oocyte, as well as at the posterior pole. By stage 10b there is almost no unlocalized protein. The protein is highly concentrated in the pole plasm at the posterior end of the freshly laid egg. A much lower level of staining can also be seen throughout the egg cytoplasm. The protein is also enriched at the anterior pole of the egg. Despite the early localization of Staufen protein to pole plasm, the protein is not specifically incorporated into pole cells, but instead gradually diffuses away from the posterior pole during the syncytial blastoderm stages. By mid-cellularization Staufen staining has completely disappeared from the posterior pole and the pole cells. Late in germband extension, Staufen protein reappears in the region of the developing nervous system, including the brain. This staining is strongest after germband shortening. This expression depends upon the zygotic genotype of the embryos (St Johnston, 1991).

The posterior group gene staufen is required both for the localization of maternal determinants to the posterior pole of the Drosophila egg and for Bicoid RNA to localize correctly to the anterior pole. staufen has been cloned and sequenced. Staufen protein is one of the first molecules to localize to the posterior pole of the oocyte, perhaps in association with Oskar RNA. Once localized, Staufen is found in the polar granules and is required to hold other polar granule components at the posterior pole. By the time the egg is laid, Staufen protein is also concentrated at the anterior pole, in the same region as Bicoid mRNA (St Johnston, 1991).

The staufen/pumilio pathway is involved in Drosophila long-term memory

Memory formation after olfactory learning in Drosophila displays behavioral and molecular properties similar to those of other species. Particularly, long-term memory requires CREB-dependent transcription, suggesting the regulation of 'downstream' genes. At the cellular level, long-lasting synaptic plasticity in many species also appears to depend on CREB-mediated gene transcription and subsequent structural and functional modification of relevant synapses. To date, little is known about the molecular-genetic mechanisms that contribute to this process during memory formation. Two complementary strategies were used to identify these genes. From DNA microarrays, 42 candidate memory genes were identified that appear to be transcriptionally regulated in normal flies during memory formation. Via mutagenesis, 60 mutants with defective long-term memory have been independently identified and molecular lesions have been identified for 58 of these. The pumilio translational repressor was found from both approaches, along with six additional genes with established roles in local control of mRNA translation. In vivo disruptions of four genes, staufen, pumilio, oskar, and eIF-5C, yield defective memory. It is concluded that convergent findings from the behavioral screen for memory mutants and DNA microarray analysis of transcriptional responses during memory formation in normal animals suggest the involvement of the pumilio/staufen pathway in memory. Behavioral experiments confirm a role for this pathway and suggest a molecular mechanism for synapse-specific modification (Daubnau, 2003).

The 60 memory mutants were generated with enhancer-trap transposons, which often drive reporter genes (lacZ or GFP) in patterns of expression similar to those of the endogenous genes they disrupt. Thus, reporter gene expression patterns were examined for milord-1 and -2 (pum), norka (oskar), and krasavietz (eIF-5C) in the adult CNS. Each of these enhancer-trap memory mutants shows common reporter gene expression in the mushroom body, an anatomical focus with a demonstrated role for olfactory memory. The norka and krasavietz strains carry a PGAL4 transposon that can drive expression of GFP in neuronal somata and processes. These data clearly reveal a common site of expression in a subset of intrinsic mushroom body neurons (Kenyon cells) that comprise the α and β lobes. The milord-1 and milord-2 strains, in contrast, carry a PlacZ transposon that expresses β galactosidase only in somata and, thus, only around the calyx region of mushroom bodies (Daubnau, 2003).

An existing mouse polyclonal antibody was to determine the expression pattern of Pum protein in the adult CNS. Consistent with the pattern of enhancer-trap expression for milord-1 and -2, Pum immunoreactivity is detected broadly in the CNS but appears to be expressed at high levels in mushroom body neurons. Strong immunoreactivity appears to be perinuclear in Kenyon cells, whereas weaker, punctate expression is detected in mushroom body neuropil (calyx). This antibody shows appreciable specificity on Western blots of embryonic extracts. It was not possible to use pumilio null mutants to establish antibody specificity for adult brain tissue, however, because the null mutants are not viable as adults. Coexpression in mushroom bodies of the reporter genes for oskar, pum, and eIF-5C and anti-Pum immunostaining are consistent with the notion that these genes function together in the CNS during long-term memory formation (Daubnau, 2003).

STAU already has been implicated in mRNA localization in mammalian CNS. In cultured hippocampal neurons, for instance, STAU has a punctate, somato-dendritic distribution and is a component of large RNP-containing neural granules, which themselves are associated with microtubules. These neural granules seem to play an analogous role in targeting mRNA translation to subcellular (synaptic) compartments in neurons, as do STAU-containing RNP particles (polar granules) in Drosophila oocytes. In cultured hippocampal neurons, neural granules are located near dendritic spines, appear to dissociate in response to local synaptic activity, and thereby release translationally repressed mRNAs. This process has been proposed as a mechanism for synapse-specific modification via local protein synthesis in response to neural activity in vitro. The data indicate that this staufen-dependent pathway underlies memory formation per se. Moreover, the further identification of oskar as a memory mutant and of moesin and orb as confirmed candidate memory genes suggests that additional genetic components of the machinery used for mRNA translocation and translation in oocytes also may function in neurons (Daubnau, 2003).

Combined with these observations from the literature, the data suggest a molecular mechanism for synapse-specific delivery of gene products during long-term memory formation. (1) Behavioral training results in the activation of CREB-mediated transcription, and nascent mRNAs are packaged into an RNP complex, a neural granule. In addition to staufen, oskar, and moesin, these granules may well include other components of polar granules such as mago and faf. (2) These neural granules then are transported into dendritic shafts along an organized microtubule network, as proposed above for vertebrate neurons. These activity-induced transcripts may be delivered nonselectively throughout the neuron or selectively to sites of recent synaptic activity. In either case, packaged mRNAs probably are translationally quiescent while in transport, thereby preventing ubiquitous expression of protein products. The data implicate pumilio as part of this translational repression complex. Finally, synapse-specific modification may result from the depolarization-dependent release of neural granule-associated mRNAs and a concomittant translational derepression (Daubnau, 2003 and references therein).

Local derepression of translation, in part, may involve phosphorylation of CPEB (orb) by aurora kinase, resulting in cytoplasmic polyadenylation and the dissociation of MASKIN from eIF-4E, which then allows interaction between eIF-4E and eIF-4G. Release of eIF-4E via phosphorylation of other 4E binding proteins also may promote assembly of the rest of the translation initiation complex. The presence during synaptic or behavioral plasticity of several other persistently active kinases also might contribute to such phosphorylation. CPEB-mediated translational activation in Xenopus oocytes, for instance, is associated with phosphorylation of ORB by CDC2 kinase (which is a dimer of CycB and CDC2) and ubiquitin-mediated degradation of Orb, perhaps modulated by faf or another ubiquitin hydrolase. Here again, DNA chip and memory mutant experiments have identified several of these additional components (Daubnau, 2003 and references therein).

Staufen- and FMRP-containing neuronal RNPs are structurally and functionally related to somatic P bodies

Local control of mRNA translation modulates neuronal development, synaptic plasticity, and memory formation. A poorly understood aspect of this control is the role and composition of ribonucleoprotein (RNP) particles that mediate transport and translation of neuronal RNAs. This study shows that staufen- and FMRP-containing RNPs in Drosophila neurons contain proteins also present in somatic 'P bodies,' including the RNA-degradative enzymes Decapping protein 1 (Dcp1p) and Xrn1p/Pacman and crucial components of miRNA (Argonaute), NMD (Upf1p), and general translational repression (Dhh1p/Me31B) pathways. Drosophila Me31B, a DEAD-box helicases, is shown to participate (1) with an FMRP-associated, P body protein (Scd6p/Trailer hitch) in FMRP-driven, Argonaute-dependent translational repression in developing eye imaginal discs; (2) in dendritic elaboration of larval sensory neurons; and (3) in bantam miRNA-mediated translational repression in wing imaginal discs. These results argue for a conserved mechanism of translational control critical to neuronal function and open up new experimental avenues for understanding the regulation of mRNA function within neurons (Barbee, 2006).

Several observations now indicate that P bodies, maternal granules, and a major subclass of neuronal RNP are similar in underlying composition and represent a conserved system for the regulation of cytoplasmic mRNAs. Known RNA transport and translational repressors shared between maternal and neuronal staufen granules now include, Stau, Btz, dFMR1, Pum, Nos, Yps, Me31B, Tral, Cup, eIF4E, Ago-2, and Imp. Strikingly, in human cells, the Me31B homolog RCK/p54, the Tral homolog RAP55, the four human argonaute proteins, eIF4E, and a eIF4E-binding protein analogous to Cup, 4E-T, are all found in P bodies. In yeast, homologs of Me31B (Dhh1p) and Tral (Scd6p) are also known to be in P bodies, and Dhh1p in particular plays a role in recruiting RNA-decapping proteins and exonucleases to these RNPs. Consistent with the above observations in yeast, the enzymes involved in mRNA hydrolysis including the 5′ to 3′ RNA exonuclease Xrn1p/Pcm and the RNA-decapping enzyme DCP1 are present on Drosophila neuronal staufen RNPs and maternal RNA granules. These data unequivocally demonstrate tight spatial proximity of components mediating various RNA regulatory processes in Drosophila neurons (Barbee, 2006).

The large collection of proteins and processes common to P bodies, staufen granules, and likely maternal RNA granules suggests that they share an underlying core biochemical composition and function, which would then be elaborated in different biological contexts. For example, one anticipates that proteins involved in mRNA transport will be more prevalent in maternal and neuronal RNPs, which need to be transported for their biological function (Barbee, 2006).

An interesting aspect of neuronal staufen RNPs described in this study is the diversity of translational repression systems that are present within them. (1) In Me31B, these RNPs contain a protein that works in general translation repression of a wide variety of mRNAs and can also affect miRNA-based repression. (2) In Ago-2, they contain a component specific to miRNA/RNAi-dependent repression. (3) Neuronal staufen granules also contain UPF1, which was originally thought to be solely involved in mRNA degradation. However, because UPF1 can act as a translation repressor and physically interacts with Stau, a reasonable hypothesis is that UPF1 might work in neuronal granules, in conjunction with Stau, to repress the translation of a subset of mRNAs. The presence of multiple mechanisms for translation repression colocalizing in granules in Drosophila neurons may allow for differential translation control of subclasses of mRNA in response to different stimuli (Barbee, 2006).

Evidence accumulating in the literature suggests that there is a potential diversity of RNA granule types in neurons. Observations in Drosophila neurons are most consistent with a model in which a major subclass of neuronal RNP, in which various translational repressor and mRNA turnover proteins colocalize, is related to other compositionally distinct, diverse RNPs. A major subclass of staufen-containing RNP is indicated by data showing substantial colocalization among various proteins analyzed. Diversity is indicated by the lack of 100% colocalization: for instance, 55% of staufen-positive particles in wild-type neurons do not contain detectable dFMR1 (Barbee, 2006).

Two types of observations suggest that the apparent subclasses of particles containing Stau or dFMR1, but not both, are related to the particles in which they colocalize: (1) these two types of RNPs are clearly compositionally related to particles that contain both proteins; (2) this is supported by the observation that colocalization can be substantially increased under some conditions. Overexpression of either dFMR1 or Stau:GFP increases colocalization between Stau and dFMR1 from 45% in wild-type neurons to more than 80%. Concurrent with increased frequency of colocalization, Stau:GFP or dFMR1 induction increases apparent particle size (or brightness) and reduces the total number of particles. The increase in colocalization and brightness, as well as reduction in particle number, is most easily explained by growth and/or fusion of related RNPs. Significantly, similar effects on mammalian neuronal granule size and number have been reported following overexpression of Stau or another granule protein, RNG105. Thus, the underlying regulatory processes appear conserved between Drosophila and mammalian neurons (Barbee, 2006).

While it remains unclear how FMRP, Stau, or RNG105 enhance granule growth or fusion, it is conceivable that individual mRNAs first form small RNPs whose compositions reflect specific requirements for translational repression of the mRNAs they contain. These small RNPs exist in dynamic equilibrium with larger RNPs in which multiple, diverse translational repression complexes are sequestered. Induction of factors that promote granule assembly could push the equilibrium toward mRNP sequestration within large granules. A requirement of this dynamic model, which postulates interactions among different types of RNP, is that the RNPs themselves can change in composition during transport to synaptic domains. This is supported by FRAP analyses showing rapid exchange of Stau:GFP between cytosol and granule (Barbee, 2006).

Additional types of RNPs have also been described in neurons. For example, polysomes apparently arrested in translation have been observed near dendritic spines, and these RNPs show no obvious similarity to large, ribosome-containing particles, termed neuronal RNA granules. In addition, a potentially distinct RNP containing Stau, kinesin, and translationally repressed RNAs, but not ribosomes, has been purified from the mammalian brain. More recently, it has been shown that RNPs containing stress-granule markers TIA-1 and TIA-R as well as pumilio2 are induced by arsenate-treatment of mammalian cultured neurons. Interestingly, as previously shown for somatic cells, these large stress granules appear tightly apposed to domains containing DCP1 and Lsm1, markers of P bodies. Determining the temporal and compositional relatedness of such varied RNPs, their pathways of assembly as well as their functions, is a broad area of future research not only in neuroscience but also in cell biology (Barbee, 2006).

These diverse types of biochemical compartments for individual mRNAs suggest that neural activity or other developmental signaling events would influence translation in two steps: first, by desequestering mRNPs held within large granules and, then, by derepressing quiescent mRNAs in individual mRNPs. Thus, RNPs described here could have a complex precursor-product relationship with other RNPs, including polysomes discovered by now-classical studies at dendritic spines (Barbee, 2006).

Despite the complexity revealed by the diversity of neuronal RNPs, the importance and significance of the observed colocalization of Me31B, Tral, argonaute, and dFMR1 in staufen-positive neuronal RNPs is most clearly demonstrated by functional analyses revealing biological pathways in which these proteins function together (Barbee, 2006).

Several independent lines of evidence are consistent with a function for Me31B in neuronal translational repression as part of a biochemical complex that includes dFMR1. (1) Subcellular localization studies indicate that Me31B and Tral localize to dFMR1-containing RNPs especially prominent at neurite branch points in cultured Drosophila neurons. (2) Me31B, Tral, and dFMR1 coimmunoprecipitate from Drosophila head extract, thus confirming the physical association of three proteins. (3) Loss-of-function alleles of either Me31B or Tral suppress the rough eye phenotype seen when dFMR1 is overexpressed in the sev-positive photoreceptors. (4) Overexpression of Me31B in sensory neurons leads to altered branching of terminal dendrites, a phenotype also seen with overexpression analyses of Nos, Pum, and dFMR1. (5) Reduction of Me31B expression in sensory neurons by RNAi results in abnormal dendrite morphogenesis and tiling defects, phenotypes similar to that observed following loss of nanos, pum, or dFmr1 function. Significantly, the effect of Me31B on dendritic growth is correlated with its ability to function in translational repression. These five independent lines of evidence provide considerable support for Me31B (and Tral) function in neuronal translation control processes. While the site of functional interaction between dFMR1, Me31B, and Tral (soma or neuronal processes) is not identified here, the importance of the physical interactions is clearly demonstrated (Barbee, 2006).

Several observations also argue that Me31B acts, at least in part, within neurons to promote translation repression and/or mRNA degradation in response to miRNAs. This possibility was first suggested by the physical and genetic interactions of Me31B with dFMR1, a protein that has been implicated in the miRNA-mediated repression. Using direct assays for miRNA-mediated function in vivo, this study shows that Me31B is required for efficient repression by the bantam miRNA in developing wing imaginal discs. This identifies Me31B as a protein required for efficient miRNA-based repression (Barbee, 2006).

Recently, miRNA-based regulation has been shown to be important for the control of spine growth in hippocampal neurons and to be a target of protein-degradative pathways involved in long-term memory formation in Drosophila. Thus, the data predict that Me31B will be important in modulating miRNA function pertinent to development of functional neuronal plasticity. More generally, because Me31B homologs in yeast and mammals have been shown to function in P body formation in somatic cells, the requirement for Me31B in miRNA function provides evidence to support a model in which formation of P bodies is required for efficient miRNA-based repression in varied cell types and biological contexts (Barbee, 2006).

The conclusion that staufen- and dFMR1-containing neuronal RNPs are similar in organization and function to P bodies has several implications for neuronal translational control. (1) The presence of diverse translational repression systems on these RNPs suggests that, like in P bodies, different classes of mRNAs will be repressed by different mechanisms. This may allow specific RNA classes to be released for new translation in response to different stimuli. Such diversity of control may allow synapses to remodel themselves differently, depending on the frequency and strength of stimulation (e.g., LTD or LTP). (2) FRAP experiments indicate that both P bodies and staufen granules are dynamic structures. This argues that, like P bodies, staufen granules are in a state of dynamic flux, perhaps in activity-regulated equilibrium with the surrounding translational pool. (3) The presence of mRNA-degradative enzymes on staufen granules suggests regulation of mRNA turnover may play an important role in local synaptic events. For example, if synaptic signaling were to induce turnover of specific mRNAs at a synapse, then stimulated synapses could acquire properties different from unstimulated ones that retain a 'naive' pool of stored synaptic mRNAs. Finally, these observations imply that the proteins known to function in translation repression within P bodies will play important roles in modulating translation in neurons. Thus, it is anticipated that proteins of mammalian or yeast P bodies such as Edc3p, Pat1p, the Lsm1-7p complex, GW182, and FAST will be present on and influence assembly and function of neuronal granules (Barbee, 2006).

Effects of Mutation or Deletion

A group of maternal genes, the posterior group, is required for the development of the abdominal region in the Drosophila embryo. Genetic as well as cytoplasmic transfer experiments were used to order seven of the posterior group genes (nanos, pumilio, oskar, valois, vasa, staufen and tudor) into a functional pathway. An activity present in the posterior pole plasm of wild-type embryos can restore normal abdominal development in posterior group mutants. This activity is synthesized during oogenesis: the gene nanos most likely encodes this activity. The other posterior group genes have distinct accessory functions: pumilio acts downstream of nanos and is required for the distribution or stability of the nanos-dependent activity in the embryo. staufen, oskar, vasa, valois and tudor act upstream of nanos. Embryos from females mutant for these genes lack the specialized posterior pole plasm and consequently fail to form germ-cell precursors. Results suggest that the products of these genes provide the physical structure necessary for the localization of nanos-dependent activity and of germ line determinants (Lehmann, 1991).

Hsp83 is the Drosophila homolog of the mammalian Hsp90 family of regulatory molecular chaperones. Maternally synthesized Hsp83 transcripts are localized to the posterior pole of the early Drosophila embryo by a novel mechanism involving a combination of generalized RNA degradation and local protection at the posterior. This protection of Hsp83 RNA occurs in wild-type embryos and embryos produced by females carrying the maternal effect mutations nanos and pumilio, which eliminate components of the posterior polar plasm without disrupting polar granule integrity. In contrast, Hsp83 RNA is not protected at the posterior pole of embryos produced by females carrying maternal mutations that disrupt the posterior polar plasm and the polar granules--cappuccino, oskar, spire, staufen, tudor, valois, and vasa. Mislocalization of Oskar RNA to the anterior pole, which has been shown to result in induction of germ cells at the anterior, leads to anterior protection of maternal Hsp83 RNA. These results suggest that Hsp83 RNA is a component of the posterior polar plasm that might be associated with polar granules. In addition, it is shown that zygotic expression of Hsp83 commences in the anterior third of the embryo at the syncytial blastoderm stage and is regulated by the anterior morphogen, bicoid (Ding, 1993a).

Shortly after fertilization in Drosophila embryos, the G-protein alpha subunit, Gi alpha, undergoes a dramatic redistribution. Initially granules containing Gi alpha are present throughout the embryonic cortex but during nuclear cleavage they become concentrated at the posterior pole, and by the blastoderm stage they are lost. Mutations that eliminate anterior structures (bicoid, swallow, and exuperantia) do not prevent the posterior accumulation of Gi alpha. Likewise, embryos from mothers with dominant gain of function mutations in the Bicaudal D gene show normal polarization of Gi alpha granules. By contrast, members of a a subset of mutations that eliminate posterior structures (cappuccino, spire, staufen, mago nashi, valois, and oskar) prevent the posterior accumulation of Gi alpha. It is important to note that mutations in posterior genes that are lower in the putative hierarchy (vasa, tudor, nanos, and pumilio) do not affect Gi alpha redistribution. From these results it is concluded that Gi alpha redistribution to the posterior pole depends on maternal factors involved in the localization of the posterior morphogen Nanos (Wolfgang, 1995).

The proteins encoded by polar-localized mRNAs play an important role in cell fate specification along the anteroposterior axis of the Drosophila embryo. The only maternally synthesized mRNA known previously to be localized to the anterior cortex of both the oocyte and the early embryo is the Bicoid mRNA whose localization is required to generate a homeodomain protein gradient that specifies position along the anteroposterior embryonic axis. A second mRNA is localized to the anterior pole of the oocyte and early embryo. This mRNA encodes a Drosophila homolog of mammalian adducin, a membrane-cytoskeleton-associated protein that promotes the assembly of the spectrin-actin network. A comparison of the spatial distribution of Bicoid and Adducin-like transcripts in the maternal-effect RNA-localization mutants exuperantia, swallow, and staufen indicates different genetic requirements for proper localization of these two mRNAs to the anterior pole of the oocyte and early embryo. While exu and swallow are required for Adducin mRNA localization, there is no requirement for staufen (Ding, 1993b).

Pole plasm formation depends on the stepwise recruitment of a number of posterior group gene products to the posterior pole. The posterior localization of Stauffen and Oskar mRNA leads to the translational activation of the latter to produce Osk protein, which then anchors the complex and recruits Vasa protein. Par-1 family members share a conserved function in the generation of cell polarity. Drosophila par-1 mutants show a novel polarity phenotype in which Bicoid mRNA accumulates normally at the anterior, but Oskar mRNA is redirected to the center of the oocyte, resulting in embryonic patterning defects. These phenotypes arise from a disorganization of the oocyte microtubule cytoskeleton. To determine where PAR-1 lies in this hierarchy, its localization was examined in various posterior group mutants. Par-1 localization at the oocyte posterior is unaffected in vasPD egg chambers, and in homozygotes for osk missense mutations, in which OSK mRNA is localized and anchored at the posterior, but fail to recruit Vasa. In contrast, the strong osk nonsense allele, osk54, completely abolishes the posterior localization of Par-1 and null mutations in stau have a similar effect. Thus, the recruitment of PAR-1 to the posterior is upstream and independent of vasa, but requires osk and stau (Shulman, 2000).

Asymmetric cell divisions and segregation of fate determinants are crucial events in the generation of cell diversity. Fly neuroblasts, the precursors that self-reproduce and generate neurons, represent a clear example of asymmetrically dividing cells. Less is known about how neurons and glial cells are generated by multipotent precursors. Flies provide the ideal model system to study this process. Indeed, neuroglioblasts (NGBs) can be specifically identified and have been shown to require the gcm fate determinant to produce glial cells, which otherwise would become neurons. The division of a specific NGB (NGB6-4T), which produces a neuroblast (NB) and a glioblast (GB), has been followed. To generate the glioblast, gcm RNA becomes progressively unequally distributed during NGB division and preferentially segregates. Subsequently, a GB-specific factor is required to maintain gcm expression. Both processes are necessary for gliogenesis, showing that the glial vs. neuronal fate choice is a two-step process. This feature, together with gcm subcellular RNA distribution and the behavior of the NGB mitotic apparatus identify a novel type of division generating cell diversity (Ragone, 2001).

Pros transcription factor is necessary to maintain gcm expression and thereby activate the glial program in the glioblast. Indeed, in the absence of Pros, gcm RNA progressively disappears from the GB. gcm displays several differences with respect to pros with regard to RNA localization. (1) asymmetric distribution is not evident before metaphase; (2) asymmetry occurs progressively during cell division rather than being sharply apical at interphase and basal at metaphase; (3) gcm transcripts are present at the cortex and in the cytoplasm. These differences suggest the existence of different RNA localization pathways in asymmetrically dividing cells. That Stau and Mira may participate to the process is suggested by the mislocalization of gcm RNA in stau and mira mutants. In addition, the gcm 3'UTR displays a stem-loop secondary structure, a conformation that is necessary for the interaction of Staufen with Bicoid 3'UTR. However, this mechanism is not sufficient to ensure a correct fate choice. Moreover, mira and stau are not fully penetrant with respect to gcm RNA distribution. Finally, and more importantly, the cytoplasmic localization of some gcm transcripts as well as the kinetics of asymmetry calls for a cortical microfilament independent mechanism. Thus, the same RNA may be the target of two localization pathways: this complements the observation that the same RNA binding protein may localize transcripts using pathways with different cytoskeletal requirements (Ragone, 2001).

During embryonic development, orderly patterns of gene expression eventually assign each cell in the embryo its particular fate. For the anteroposterior axis of the Drosophila embryo, the first step in this process depends on a spatial gradient of the maternal morphogen Bicoid (Bcd). Positional information of this gradient is transmitted to downstream gap genes, each occupying a well defined spatial domain. The precision of the initial process has been determined by comparing expression domains in different embryos. The Bcd gradient displays a high embryo-to-embryo variability, but this noise in the positional information is strongly decreased ('filtered') at the level of hunchback (hb) gene expression. In contrast to the Bcd gradient, the hb expression pattern already includes the information about the scale of the embryo. Genes known to interact directly with Hb are not responsible for its spatial precision, but the maternal gene staufen may be crucial (Houchmandzadeh, 2002).

Among all the mutations studied, the only ones that affect Hb boundary precision are certain alleles of staufen. In embryos from mothers homozygous for either stauHL or staur9, the Hb boundary position shows a variability of 6%, comparable to the observed Bcd variability. Surprisingly, this variability is largely reduced (to 2%) in another strong allele of stau, D3. Mutations in stau disrupt bcd and osk mRNAs and decrease Bcd protein level about twofold. Whether the effect of stau on Hb is simply an indirect effect of its variable effect on bicoid was tested. From the pool of embryos in stauHL background that were double stained for Bcd and Hb, two populations were selected: one that displayed an anterior Hb boundary shift, and one that displayed a posterior shift. The corresponding average Bcd profiles for these two populations are very similar, both in the Bcd level and in its spatial distribution. Thus, the observed variability in the Hb boundary position may reflect an activity of staufen independent of bcd. The disruption of Hb precision in stauHL is transmitted to downstream genes, and is not corrected before gastrulation. For instance, double staining for Hb and Kr shows that the variability of the Kr boundary in the stauHL background is similar to that of the Hb boundary. Moreover, the positions of these two boundaries remain tightly correlated, as in the wild type (Houchmandzadeh, 2002).

By quantitatively analyzing the protein profiles of maternal morphogens and zygotic gap genes in numerous wild-type and mutant embryos, two phenomena that take place in the early Drosophila development have been demonstrated: (1) at a very early stage, noise associated with the maternal gradient of Bcd is filtered out, and (2) at the same time, the genetic network, which includes the Hb gap gene, establishes spatial proportions (scaling) in the embryo. It is potentially significant that staufen, the one gene affecting the process, makes a product that localizes to both poles of the egg. More work is needed to establish the mechanisms that control the spatial scaling and precision. It would then be interesting to investigate whether similar phenomena are present in other developmental processes in Drosophila and other organisms (Houchmandzadeh, 2002).


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staufen: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 15 April 2014
 

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