Analysis of Biological Development (K. Kalthoff)

Updates to Topic 22A: Maternal Genes Affecting the Anteroposterior Body Pattern

New Review Articles

van Eeden F. and St Johnston D. (1999) The pol;arization of the anterior-posterior and dorsal-ventral axes during Drosophila oogenesis. Cuur. Opin. Genet. Devel. 9: 396-404

New Research Articles

Casali A. and Casanova J. (2001) The spatial control of Torso RTK activation: a C-terminal fragment of the Trunk protein acts as a signal for Torso receptor in the Drosophila embryo. Development 128: 1709-1715

The investigators analyzed how the Torso (Tor) receptor, which is present on the entire surface of the Drosophila embryo, is activated only in the pole regions by a locally produced ligand. By rescuing trunk mutants with transgenes bearing various deletions, they found that a carboxy-terminal fragment of the Trunk protein retains the ability to activate Tor. This fragment also bypasses the requirements for the other genes involved in the activation of the Tor. The results confirm the hypothesis that Tor is activated locally by the spatially restricted proteolysis of the Trunk protein.

Cha B.-J., Koppetsch B.S. and Theurkauf W.E. (2001) In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway. Cell 106:35-46

The investigators traced the localization of fluorescently labeled bicoid (bcd) mRNA in cultured Drosophila egg chambers by time-lapse confocal microscopy. Upon injection into nurse cell cytoplasm, bcd mRNAwas assembled into particles that were transported via the cytoplasmic bridges to the oocyte, where they became localized in the anterior cortex. This localization process depended on microtubules, functional exuperantia+ (exu+) gene product, and a localization element in the 3' UTR of bcd mRNA identified previously by others. Curiously, bcd mRNA injected directly into the oocyte showed microtubule-dependent movement to the closest cortical surfaces, not just the anterior one. In contrast, bcd mRNA injected into nurse cells, then withdrawn and injected into the oocyte of another egg chamber was localized specifically to the anterior cortex. Based on their observations, the authors propose that bcd mRNA particles in the nurse cell cytoplasm combine in a microtubule-dependent way with Exu protein and with at least one other component that occurs in nurse-cells but not in oocytes. These combined mRNP particles are transported along microtubules and via the cytoplasmic bridges into the oocyte. Here, they move along a subset of polarized microtubules that end in the anterior cortex. In contrast, bcd mRNP particles that have not passed through the nurse cells and therefore lack a nurse cell-specific component are transported on all oocyte microtubules to produce a non-polar cortical distribution.

Chang J.S., Tan L., Wolf M.R. and Schedl P. (2001) Functioning of the Drosophila orb gene in gurken mRNA localization and translation. Development 128: 3169-3177

Tan L., Chang J.S., Costa A. and Schedl P. (2001) An autoregulatory feedback loop directs the localized expression of the Drosophila CPEB protein Orb in the developing oocyte. Development 128: 1159-1169

One of the maternally expressed genes required for the establishment of both the anteroposterior and the dorsoventral polarity in the Drosophila oocyte is the oo 18 RNA binding+ (orb+) gene. The orb protein is a member of the cytoplasmic polyadenylation element binding (CPEB) family of translational regulators (see Topic 18 updates). Mutant females with weak orb loss-of-function alleles have offspring with phenotypes that are best interpreted by failure to localize and translate oskar and gurken mRNAs. In each case, orb protein is already localized at the appropriate site within the oocyte before the gurken and oskar mRNAs arrive. The localization of orb protein seems to involve a positive feedback loop of the protein on the translation of its own orb mRNA. However, the mechanisms that initiate this feedback loop in a localized fashion remain to be elucidated.

Edwards T.A., Pyle S.E., Wharton R.P., and Aggarwal A.K. (2001) Structure of Pumilio reveals similarity between RNA and peptide binding motifs. Cell 105: 281-289.

Sonoda J. and Wharton R.P. (1999) Recruitment of Nanos to hunchback mRNA by Pumilio. Genes Dev. 13: 2704-2712.

Sonoda J. and Wharton R.P. (2001) Drosophila Brain Tumor is a translational repressor. Genes Dev. 15: 762-773.

Translational regulation of hunchback (hb) mRNA is essential for posterior patterning of the Drosophila embryo. This regulation is mediated by sequences in the 3'-untranslated region of hb mRNA (the Nanos response elements or NREs), as well as three trans-acting factors: Pumilio, Nanos, and Brain Tumor. The critical region of Pumilio, the Puf domain, organizes a multivalent repression complex on the 3' untranslated region of hunchback mRNA.

Niessling D., Blanke S. and Jaeckle H. (2003) Bicoid associates with the 5'-cap-bound complex of caudal mRNA and represses translation. Genes & Dev. 16: 2576-2582

Bicoid protein not only acts as a transcription factor but also causes specific translational repression of the ubiquitous caudal mRNA in the anterior part of the embryo.

Schaeffer V., Killian D., Desplan C. and Wimmer E.A. (2000) High Bicoid levels render the terminal system dispensable for Drosophila head development. Development 127: 3993-3999

Wimmer E.A., Carleton A., Harjes P., Turner T. and Desplan C. (2000) bicoid-independent formation of thoracic segments in Drosophila. Science 287: 2476-2479

Wimmer et al. address the questions how the bicoid+ (bcd+) gene could have evolved late and be limited to the cyclorraphan flies, to which Drosophila belongs. They tried to reconstruct genetic conditions in Drosophila under which the Bicoid-independent enhancers of hb are sufficient to form the normal segment pattern. To this end, they engineered a hunchback (hb) transgene lacking the Bicoid-dependent enhancer elements and the adjacent promoter P2. Using this hbP1only transgene they obtained hb-/hb-, hbP1only offspring from females expressing various numbers of hb copies during oogenesis. Such offspring from females with one expressed hb gene lack the labial and all three thoracic (T1 through T3) segments. In contrast, offspring from females with two expressed hb genes lack only T2 and T3. With four maternal copies of hb, and the embryonic dosage of knirps+ reduced to one, the development of a normal body pattern is restored. Schaeffer et al. show that increasing the number of bcd+ from two to six can rescue much of the phenotype caused by the lack of Torso-mediated signaling. Together, these studies show that anterior patterning in the Drosophila embryo relies on several overlapping mechanisms.

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