In Drosophila, localized RNAs program the antero-posterior and dorso-ventral axis of the oocyte and embryo. Soon after, an RNA localized to the anterior pole of the Drosophila oocyte and early embryo was discovered (6).
PATTERNS OF RNA LOCALIZATION
One such RNA is oskar, which is present only at the posterior pole of the oocyte by the end of stage 9 (24, 25). At this stage, several newly localized RNAs can be seen at the rear of the oocyte.
After the first phase of ooplasmic segregation, shortly after fertilization, YC transcripts are localized to the vegetal cap of the myoplasm ( 101 ). There is one example of a maternally synthesized mRNA localized to the echinoderm ( Strongylocentrotus purpuratus ) oocyte and early embryo ( 103 ).
MECHANISMS OF RNA LOCALIZATION
Egalitarian and Bicaudal-D proteins are produced in nurse cells and transported to the rear of the oocyte, presumably along minus-directed microtubules (20). Thus, the role of the cytoskeleton in RNA localization is highly conserved across evolution (see below). Anchoring of RNAs at the posterior pole of the Drosophila oocyte also requires the integrity of the polar granules.
Many posteriorly localized RNAs are either components of the polar granules or are associated with the granules (reviewed in 142). Developmental functions of the posterior polar granules and their associated localized RNAs are considered below. There is therefore rarely any doubt about the in vivo significance of the results obtained.
In the case of bicoid RNA, distinct elements within the 3' UTR have been defined that confer different aspects of the RNA localization pattern. BLE1 interacts with Exl protein, which could function in localization to the leading edge of the oocyte (177) (see below). For example, Oskar RNA is not translated until it is localized to the posterior pole of the stage 9 oocyte.
TRANS-ACTING FACTORS INVOLVED IN RNA LOCALIZATION AND TRANSLATIONAL
In staufen mutants, oskar RNA is kept at the front of the oocyte until stage 10, when it is delocalized (25) (in wild-type oocytes, oskar RNA is transported from the anterior pole to the posterior by stage 9). In the early embryo, Staufen protein colocalizes with oskar RNA at the posterior pole and with bicoid RNA at the anterior pole (179). Endogenous oskar mRNA is translated only after the transcript is localized to the posterior part of the oocyte at stages 8–9 (184).
Exuperantia protein is highly concentrated in the anterior cortex of the oocyte between stages 8 and. In stage 10 egg chambers mutant for exuperantia, bicoid RNA delocalizes from the anterior of the oocyte (7, 150). However, in orb mutants, gurken transcripts are present throughout the anterior of the oocyte.
In the wild type, oskar mRNA is localized to the posterior pole of the oocyte. Since cucumber is transcribed in the oocyte nucleus (R Cohen, personal communication) and becomes restricted to the oocyte cytoplasm dorso-anterior to the oocyte nucleus, K10 protein may function specifically in vectorial nucleo-cytoplasmic transport of cucumber transcripts (R Cohen, personal communication). ). The bicaudal-D protein includes a region of homology to the coiled-coil domains of several cytoskeletal proteins and is required for maintaining oskar RNA localization at the posterior pole of the oocyte (160).
DEVELOPMENTAL FUNCTIONS OF RNA LOCALIZATION
Thus the antero-posterior axis is primary and the creation of the dorso-ventral axis is secondary (35). First, mislocalization of bicoid RNA to the posterior pole can result in developmental defects in the posterior part of the embryo through cells that mistakenly adopt anterior fates (224). Thus, a subsequent function of anterior bicoid transcript localization is to prevent bicoid protein synthesis in the posterior part of the embryo.
Nano mRNA is localized to the posterior pole of the late oocyte and early embryo, although some nonlocalized RNA is present throughout the embryo (56, 231). The protein Nanos in the posterior part of the embryo prevents this by translationally suppressing humpback RNA. Translation of oskar RNA at this site results in the formation of posterior polar granules and polar plasm.
In the latter two situations the ectopic pole cells form at or near the anterior part of the embryo. The three embryonic layers of the early embryo (ectoderm, mesoderm, and endoderm) are located along the animal-vegetal axis (72, 73). The orientation of the dorso-ventral axis of the early embryo is not determined before fertilization.
EVOLUTIONARY CONSIDERATIONS
In this case, during cell division, the ASHJ mRNA is localized to the site of the bud and then into the daughter cell that forms there (128, 129). The ASHJ protein acts as a repressor of the HO endonuclease, which is responsible for mating type switching (128, 129). Localization of ASHJ rnRNA and its asymmetric segregation in the daughter cell thus ensures that the daughter cell cannot switch mating type, while the mother cell (which does not inherit ASHJ mRNA) can switch.
The demonstration that the 3'-UTR of yeast ASH I mRNA carries information for intracellular targeting implies that the position of cis-acting localization elements may be conserved in mRNAs from yeast to mammals. Future studies focusing on the identification and analysis of trans-acting factors that target RNA for localization are likely to uncover additional conserved components of the cytoplasmic RNA localization machinery. The concerted action of two RNA degradation pathways controls the timing of maternal transcript secretion at the mid-blastula transition.
Joint action of two RNA degradation pathways controls the timing of maternal transcript
Hsp83 transcripts are particularly abundant, accounting for 1% of polyadenylated transcripts in the early embryo (Zimmerman et al., 1983). By MBT (2.5-3.0 hours post-fertilization), >96% of maternal Hsp83 transcripts loaded into the embryo had disappeared (Fig. IK). Degradation of maternal Hsp83 transcripts, arrays, and nano begins at fertilization and >95% of transcripts are removed by the MBT.
In unfertilized eggs, >99% of nanos transcripts, 99% of Hsp83 transcripts, and 90% of string transcripts are degraded (string mother, upper band in J). Addition of Hsp83 HDE restores degradation to the otherwise stable nos[l'lTCE] transcript (Figure 4E and F), while addition of nanos TCE similarly targets the stable Hsp83-lacZ[I'iliDE] transcript for degradation (Figure 4G and H) ). Translation of nanos in the head region of the embryo causes suppression of bicoid translation and thus head skeletal defects (Dahanukar and Wharton, 1996; Smibert et al., 1996).
Furthermore, the severity of the skull defects was greater in the case of nos[l'lTCE+HDE] than nos[l'lTCE]. E and F) Replacement of the IIWIOS TCE with the Hsp83 HOW (Hsp83 3'-UTR nucleotides 253-349) restores transcriptional degradation. The fourth class (eg, Hsp83, tou and nanos) is degraded by the combined action of the maternal and zygotic machinery.
Mammalian NUMB is an evolutionarily conserved signaling adapter protein that specifies cell fate
A portion of the dNUMB protein exhibits strong homology to the SHC PTB domain [ 18 ], suggesting a functional link to tyrosine kinase signal transduction. The sequence of the encoded protein is 67% similar to that of dNUMB (Fig. 1b), with the highest sequence homology in the putative PTB. A comparison of SHC and NUMB PTB domains shows that amino acids critical for the binding of the SHC PTB domain to tyrosine-phosphorylated substrates are conserved in the mNUMB protein [24–26] (Fig. lc).
To study the distribution of the mNUMB protein, a peptide corresponding to the carboxy-terminal 15 amino acids of mNUMB (Nb-C peptide) was used to generate a rabbit polyclonal antiserum (anti-Nb-C). Previous analyzes of the effects of overexpression of the Drosophila numb gene by Rhyu et al. During the first division, SOP dNUMB determines the fate of lib (neuron and sheath precursor) versus Ila (hair and socket precursor).
Misexpression of mNUMB during Drosophila notum and wing development resulted in the same cell fate. We expect that the Drosophila and mammalian NUMB PTB domain targets are also evolutionarily conserved. Specificity of the PTB domain of SHC for beta-formations of amino-terminal pentapeptide motifs on phophotyrosine.
Fringe boundaries coincide with Notch-dependent
Analysis of mouse Delta-like gene (DU1, Notch-ligand) expression in the mesoderm undergoing somitogenesis in ES-5 embryos reveals that Dill is highly expressed in the presomitic mesoderm and further induced in the nascent somite (Fig. 3a) 10• Despite this due to the extremely high level of Dill expression in the nascent somite, Notchl activation is probably restricted to the tissue between the somite9 _ Lunatic margin is expressed at this stage in two bands surrounding the nascent somite (Fig. 3b)-A. mouse. Similarly, the epithelium of the tongue is continuously generated by the division of basally located stem cells that express the lunatic margin (Fig. 2h)_ As the cells differentiate, they move apically, turning off the nori res and generating both manic and radical fringe genes (Fig._ 2i, J)- As a result, in neural tissue and epidermis the boundaries between progenitor cells expressing lunatic edge and their committed progeny expressing manic and radical edge genes.
In Drosophila, 0-fringes are expressed in the dorsal but not ventral compartment of the developing wing disc. In the wing imaginal disc, patched is expressed in a strip of cells on the anterior side of the AP compartment boundary1•1•16•. 96Iine expresses GAL4 at the dorso-ventral space boundary in the wing disc, which represents the future wing margin !7.
-l is expressed at extremely high levels in the forming somite, where Notch I would not be expected to be activated111•. In the developing neural tube, a crazy fringe expression boundary coincides with a Dll-1//agged1 expression boundary, suggesting that the crazy fringe might control development. ligands and crazy edges during somitogenesis and neural tube patterning. The lunatic edge may therefore function together with Notch I to control lineage commitment in the thymus.
