Françoise Z. Huang

Stochastic WNT signaling between nonequivalent cells regulates adhesion but not fate in the two-cell leech embryo.

BACKGROUND In the leech Helobdella robusta, an annelid worm, the early pattern of cell divisions is stereotyped. The unequal first cleavage yields cells AB and CD, which differ in size, cytoplasmic inheritance, normal fate, and developmental potential. RESULTS Here we report a dynamic and transcription-independent pattern of WNT signaling in the two-cell stage of H. robusta. Surprisingly, HRO-WNT-A is first expressed in a stochastic manner, such that either AB or CD secretes the protein in each embryo. This stochastic phase is followed by a deterministic phase during which first AB, then CD expresses HRO-WNT-A. When contact between the cells is reduced or eliminated, both AB and CD express HRO-WNT-A simultaneously. Finally, bathing embryos in anti-HRO-WNT-A antibody during first cleavage reduces the adhesion between cells AB and CD. CONCLUSIONS Our findings show that the stochastic phase of HRO-WNT-A signaling in the two-cell stage of Helobdella is negatively regulated by cell-cell contact and that this early signaling affects cell adhesion without affecting cell fate. We speculate that the primordial function of wnt class genes may have been to regulate cell-cell adhesion and that the nuclear signaling components of the wnt pathway arose later in association with the evolution of diverse cell types.

A hedgehog homolog regulates gut formation in leech(Helobdella)

Signaling by the hedgehog (hh)-class gene pathway is essential for embryogenesis in organisms ranging from Drosophila to human. We have isolated a hh homolog (Hro-hh) from a lophotrochozoan species, the glossiphoniid leech, Helobdella robusta, and examined its expression by reverse transcription polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization. The peak of Hro-hh expression occurs during organogenesis (stages 10-11). No patterned expression was detected within the segmented portion of the germinal plate during the early stages of segmentation. In stage 10-11 embryos, Hro-hh is expressed in body wall, foregut, anterior and posterior midgut, reproductive organs and in a subset of ganglionic neurons. Evidence that Hro-hh regulates gut formation was obtained using the steroidal alkaloid cyclopamine, which specifically blocks HH signaling. Cyclopamine induced malformation of both foregut and anterior midgut in Helobdella embryos, and no morphologically recognizable gonads were seen. In contrast, no gross abnormalities were observed in the posterior midgut. Segmental ectoderm developed normally, as did body wall musculature and some other mesodermal derivatives, but the mesenchymal cells that normally come to fill most of the coelomic cavities failed to develop. Taken with data from Drosophila and vertebrates, our data suggest that the role of hh-class genes in gut formation and/or neural differentiation is ancestral to the bilaterians, whereas their role in segmentation evolved secondarily within the Ecdysozoa.

Contribution of the geneextramacrochaetae to the precise positioning of bristles inDrosophila

We examine the effect of mutations in theextramacrochaetae (emc) gene on the positioning of macrochaetes on the notum ofDrosophila. We show that, inemc mutants, most of the precursor cells appear earlier than in wild-type individuals, consistent with an antagonizing effect ofemc on the action of the proneural genesachaete andscute. We also show that reducingemc function affects the position of three bristles and/or of their precursors, but has no marked effect on the positioning of the other bristles.

4 The Other Side of the Embryo: An Appreciation of the Non-D Quadrants in Leech Embryos

Publisher Summary This chapter summarizes findings that reveal a phenomenologically rich developmental cell biology in the A, B, and C quadrants in embryos of glossiphoniid leeches, such as Helobdella robusta (class Hirudinea; phylum Annelida). In glossiphoniid leeches, these non-D quadrants are precursors of definitive endoderm among other tissues. To appreciate the roles of these cells in embryogenesis, however, it is necessary first to summarize glossiphoniid leech development in terms of the contributions of the D quadrant.

Cell interactions that affect axonogenesis in the leech Theromyzon rude

The leech nervous system comprises a relatively simple network of longitudinal (connective) and transverse (segmental) nerves. We have followed the normal pattern of axon development in the glossiphoniid leech Theromyzon rude by immunostaining embryonic preparations with antibody to acetylated α-tubulin. The dependence of the normal pattern of axon growth on cells in the mesodermal (M) and ectodermal (N, O, P and Q) lineages was examined by selectively ablating subsets of these lineages in developing embryos. We found that ablating mesoderm severely disrupted overall axonogenesis, while various ectodermal ablations induced a range of more specific phenotypes. In particular, formation of the posterior segmental nerve (PP) was abnormal in embryos deficient in primary neuroectoderm (N lineage). More specific ablations demonstrated that a subset of N-derived cells were required for establishing the PP nerve root. Previous studies have shown that the PP nerve root is normally pioneered by an O lineage-derived neuron (PD). Our results suggest that the role of the N lineage-derived cells is to induce the migration of neuron PD to its normal position in the posterior compartment of the hemiganglion.