Localized mRNAs are critical early in embryogenesis to establish axes, and have been identified via molecular screening in frogs and genetic studies in Drosophila.
|Anterior||Anterior class genes control the head and thorax. bicoid mRNA is the localized cytoplasmic determinant of the anterior class. Translation of the anterior and posterior class mRNA leads to generation of a morphogen gradient within the large, syncytical embryo.|
|Posterior||Posterior class genes control the abdomen. nanos mRNA is the localized cytoplasmic determinant of the posterior class. Translation of the anterior and posterior class mRNA leads to generation of a morphogen gradient within the large, syncytical embryo.|
|Terminal||Terminal class genes control the termini -- most of the anterior and most of the posterior. Translation of the terminal and dorsal-ventral class mRNA generates a morphogen gradient of small molecules which diffuse between cells. Activation of a receptor by a locally activated ligand leads to generation of a spatially localized transcription factor.|
|Dorsal-Ventral||Dorsal-ventral class genes control the dorsal-ventral axis. Translation of the terminal and dorsal-ventral class mRNA generates a morphogen gradient of small molecules which diffuse between cells. Activation of a receptor by a locally activated ligand leads to generation of a spatially localized transcription factor.|
|Bicoid||Bicoid, encoded by bcd, is a homoedomain (certain kind of DNA-binding) transcription factor necessary for formation of thoracic (anterior) segments. bcd mRNA is localized to one end of the Drosophila egg (evidenced by in situ hybridization), and is only translated after fertilization. Bicoid protein diffuses freely (evidenced by antibody staining) to form a morphogen gradient within the syncitial blastoderm embryo. Bicoid and hunchback gradients together divide the embryo into broad stripes of expresion of gap genes.|
- High bicoid concentration activates transcription of head gap genes.
- Intermediate bicoid concentration activates hb transcription and represses kr transcription.
- Low bicoid concentration activates kr transcription.
|Nanos||Nanos, encoded by nos is a translation factor that is only translated after fertilization and which is necessary for formation of abdominal (posterior) segments. Nanos mRNA is localized opposite bicoid mRNA, leading to an reversed morphogen gradient. Nanos represses hb translation, contributing to a gradient of hunchback even while hb mRNA is spread evenly throughout the cell.|
|Vg1||The Vg1 protein is a TGF-β molecule (a type of growth factor) that
plays an important role in establishing the dorsal side of the amphibian embryo. We will discuss
its function in more detail later.|
|Localization||The localization signal for bcd, nos and Vg1 mRNA is located in the 3' UTR; thus, bcd mRNA with a 3' UTR from nanos will be localized to the posterior. Also, localization of these three genes is dependent on microtubules, as determined by microtubule inhibitor colchicine.|
|Hunchback||Encoded by hb, hunchback is a peptide transcription factor that represses kni and gt transcription and whose mRNA is evenly distributed (with bicoid-dependent translation that is repressed by nos). Bicoid and hunchback gradients together divide the embryo into broad stripes of expresion of gap genes.|
- Low hunchback concentration allows kni transcription.
- Even lower hunchback concentration (at posterior) allow gt transcription.
|Tailless||tailless patterns the posterior gut primordium|
|Dorsal & Toll||dorsal encodes Dorsal, a ligand required to establish the ventral region and also establishes and intranucleic TGF-β gradient (encoded by dpp); dorsal- mutants are completely dorsalized. Prior to activation, Dorsal is bound to Cactus, its own inhibitor. The pathway begins with another ligand, encoded in an inactive form by maternal mRNA into the perivitelline space (between embryo and vitelline membrane). A protein (secreted by follicle cells during oogenesis) in the ventral region of the perivitelline space activates a protease cascade which eventually activates the ligand. This activated ligand is localized to the ventral region, binding the uniformly distributed Toll receptor (encoded by toll) to into dorsalactive. Dorsalactive can bind the toll-encoded receptor on the ventral side of the embryo, which transduces a signal into the cytoplasm. This signal activates a kinase which phosphorylates Cactus, thus releasing (thereby activating) Dorsal so that Dorsal can enter the nucleus from the ventral side of the embryo. This intra-nucleus morphogen gradient of Dorsal activates genes for ventral determination (including sog, twist and snail) and repressing dorsal determination genes (including dpp, and a protease that increases Dpp activity -- thus, these are only expressed o the dorsal side of the embryo).|
|Short Gastrulation||Short Gastrulation, encoded by sog, antagonizes Dpp activity by preventing Dpp from binding its receptor. This contributes to the slope of the Dpp activity gradient.|
|Twist & Snail||Activated by Dorsal, twist and snail genes encode transcription factors that are required for formation of the ventral mesoderm and therefore for formation of the ventral furrow and gastrulation.|
|Decapentaplegic||Decapentaplegic (aka Dpp, a TGF-β homologous to mammalian BMP-4) is encoded by dpp and is transcriptionally repressed by Dorsal. In absence of Dorsal from the embryo nucleus, dpp is expressed in the dorsal-most 40% of the embryo. Different levels of Dpp activity dictate different cell fates in the dorsal half of the embryo, from the amnioserosa in the most dorsal position, to the dorsal cuticle more laterally, and finally to the lateral-most positions of the ventral denticle belts. These different levels of dpp expression are thus considered to constitute a morphogen gradient. Note that this morphogen gradient is based on the diffusion of a small protein growth factor (Dpp) between cells.|
|Fushi Tarazu||Ftz, which encodes fushi tarazu, is a pairrule gene affected T2, A1, A3, A5 and A7; ftz- mutants lack those segments.|
The localization signal for bcd, nos and Vg1 mRNA is located in the 3' UTR; thus, bcd mRNA with a 3' UTR from nanos will be localized to the posterior. Also, localization of these three genes is dependent on microtubules, as determined by microtubule inhibitor colchicine.By screening duplicate copies of a cDNA library with labeled cDNA probe made from mRNA from either the animal or vegetal pole of the Xenopus egg, the gene encoding the Vg1 protein was identified. The Vg1 protein is a TGF-β molecule (a type of growth factor) that plays an important role in establishing the dorsal side of the amphibian embryo. We will discuss its function in more detail later.
|Twist||Twist transcription activated by Dorsal. twist mutants fail to gastrulate |
High levels of bcd activate hunchback. High levels of hunchback represses Kruppel.
Low levels of bcd activates Kruppel. Low levels of hunchback turns on Kruppel.
If bcd gradient is decreased overall:smaller zygotic domain of high [hb], and Kr expands toward anterior. Conclusion: Kr is downstream of hb, bcd.
If there is no Bcd: no Hb expression, no Kr expression. Kr- and Hb- have no effect on Bcd gradient. Thus, Kr is downstream of Hb, Bcd.