No abstract

In vertebrates, many cytokines and growth factors have been identified as activators of the JAK/STAT signaling pathway. In Drosophila, JAK and STAT molecules have been isolated, but no ligands or receptors capable of activating the pathway have been described. We have characterized the unpaired (upd) gene, which displays the same distinctive embryonic mutant defects as mutations in the Drosophila JAK (hopscotch) and STAT (stat92E) genes. Upd is a secreted protein, associated with the extracellular matrix, that activates the JAK pathway. We propose that Upd is a ligand that relies on JAK signaling to stimulate transcription of pair-rule genes in a segmentally restricted manner in the early Drosophila embryo.

In mammals, many cytokines and growth factors stimulate members of the Janus kinase (JAK) family to transduce signals for the proliferation and differentiation of various cell types, particularly in hematopoietic lineages. Mutations in the Drosophila hopscotch (hop) gene, which encodes a JAK, also cause proliferative defects. Loss‐of‐function alleles result in lethality and underproliferation of diploid tissues of the larva. A dominant gain‐of‐function allele, Tumorous‐lethal (hopTum‐l), leads to formation of melanotic tumors and hypertrophy of the larval lymph glands, the hematopoietic organs. We show that a single amino acid change in Hop is associated with the hopTum‐l mutation. Overexpression of either wild‐type hop or hopTum‐l in the larval lymph glands causes melanotic tumors and lymph gland hypertrophy indistinguishable from the original hopTum‐l mutation. In addition, overexpression of Hop in other tissues of the larva leads to pattern defects in the adult or to lethality. Finally, overexpression of either hop or hopTum‐l in Drosophila cell culture results in tyrosine phosphorylation of Hop protein. However, overexpression of hopTum‐l results in greater phosphorylation than overexpression of the wild‐type. We conclude that hopTum‐l encodes a hyperactive Hop kinase and that overactivity of Hop in lymph glands causes malignant neoplasia of Drosophila blood cells.

Laminin-1 is essential for early embryonic basement membrane assembly and differentiation. Several steps can be distinguished, i.e., the expression of laminin and companion matrix components, their accumulation on the cell surface and assembly into basement membrane between endoderm and inner cell mass, and the ensuing differentiation of epiblast. In this study, we used differentiating embryoid bodies derived from mouse embryonic stem cells null for γ1-laminin, β1-integrin and α/β-dystroglycan to dissect the contributions of laminin domains and interacting receptors to this process. We found that (a) laminin enables β1-integrin–null embryoid bodies to assemble basement membrane and achieve epiblast with β1-integrin enabling expression of the laminin α1 subunit; (b) basement membrane assembly and differentiation require laminin polymerization in conjunction with cell anchorage, the latter critically dependent upon a heparin-binding locus within LG module-4; (c) dystroglycan is not uniquely required for basement membrane assembly or initial differentiation; (d) dystroglycan and integrin cooperate to sustain survival of the epiblast and regulate laminin expression; and (e) laminin, acting via β1-integrin through LG1–3 and requiring polymerization, can regulate dystroglycan expression.

The Drosophila egg develops through closely coordinated activities of associated germline and somatic cells. An essential aspect of egg development is the differentiation of the somatic follicle cells into several distinct subpopulations with specific functions. Here we demonstrate that the graded activity of the Janus kinase (JAK) pathway, stimulated by the Unpaired ligand, patterns the anterior-posterior axis of the follicular epithelium. Different levels of JAK activity instruct adoption of distinct anterior cell fates. Further, the coordinated activities of the JAK/STAT and epidermal growth factor receptor (EGFR) pathways are required to specify the posterior terminal cell fate. We propose that Upd secreted from the polar cells may act as a morphogen to stimulate A/P-derived follicular fates through JAK pathway activation.