Y. Tony Ip

Signal transduction by the c-Jun N-terminal kinase (JNK) — from inflammation to development

The c-Jun amino-terminal kinase (JNK) group of MAP kinases has been identified in mammals and insects. JNK is activated by exposure of cells to cytokines or environmental stress, indicating that this signaling pathway may contribute to inflammatory responses. Genetic and biochemical studies demonstrate that this signaling pathway also regulates cellular proliferation, apoptosis, and tissue morphogenesis. A functional role for JNK is therefore established in both the cellular response to stress and in many normal physiological processes.

Dif, a dorsal-related gene that mediates an immune response in Drosophila

There are striking parallels between the regulation of gene expression along the dorsoventral (DV) axis of Drosophila embryos and lymphoid-restricted expression in the mammalian immune system. Both depend on regulatory factors containing rel domains (dorsal and NF-kappa B) that are controlled at the level of nuclear transport. A novel Rel-containing gene in Drosophila, Dif (dorsal-related immunity factor), provides a potential link between these seemingly disparate processes. Although Dif maps close to dorsal, it does not appear to participate in DV patterning, but instead mediates an immune response in Drosophila larvae. Dif is normally localized in the cytoplasm of the larval fat body, but quickly accumulates in the nucleus upon bacterial infection or injury. Evidence is presented that once in the nucleus, Dif binds to kappa B-like sequence motifs present in promoter regions of immunity genes. These results suggest that mammalian and insect immunity share a common evolutionary origin.

Tissue damage-induced intestinal stem cell division in Drosophila

Stem cell division is essential for tissue integrity during growth, aging, and pathogenic assaults. Adult gastrointestinal tract encounters numerous stimulations, and impaired tissue regeneration may lead to inflammatory diseases and cancer. Intestinal stem cells in adult Drosophila have recently been identified and shown to replenish the various cell types within the midgut. However, it is not known whether these intestinal stem cells can respond to environmental challenges. By feeding dextran sulfate sodium and bleomycin to flies and by expressing apoptotic proteins, we show that Drosophila intestinal stem cells can increase the rate of division in response to tissue damage. Moreover, if tissue damage results in epithelial cell loss, the newly formed enteroblasts can differentiate into mature epithelial cells. By using this newly established system of intestinal stem cell proliferation and tissue regeneration, we find that the insulin receptor signaling pathway is required for intestinal stem cell division.

Snail/slug family of repressors: slowly going into the fast lane of development and cancer.

The existence of homologous genes in diverse species is intriguing. A detailed comparison of the structure and function of gene families may provide important insights into gene regulation and evolution. An unproven assumption is that homologous genes have a common ancestor. During evolution, the original function of the ancestral gene might be retained in the different species which evolved along separate courses. In addition, new functions could have developed as the sequence began to diverge. This may also explain partly the presence of multipurpose genes, which have multiple functions at different stages of development and in different tissues. The Drosophila gene snail is a multipurpose gene; it has been demonstrated that snail is critical for mesoderm formation, for CNS development, and for wing cell fate determination. The related vertebrate Snail and Slug genes have also been proposed to participate in mesoderm formation, neural crest cell migration, carcinogenesis, and apoptosis. In this review, we will discuss the Snail/Slug family of regulators in species ranging from insect to human. We will present the protein structures, expression patterns, and functions based on molecular genetic analyses. We will also include the studies that helped to elucidate the molecular mechanisms of repression and the relationship between the conserved and divergent functions of these genes. Moreover, the studies may enable us to trace the evolution of this gene family.

Hippo signaling regulates Drosophila intestine stem cell proliferation through multiple pathways

Intestinal stem cells (ISCs) in the Drosophila adult midgut are essential for maintaining tissue homeostasis and replenishing lost cells in response to tissue damage. Here we demonstrate that the Hippo (Hpo) signaling pathway, an evolutionarily conserved pathway implicated in organ size control and tumorigenesis, plays an essential role in regulating ISC proliferation. Loss of Hpo signaling in either midgut precursor cells or epithelial cells stimulates ISC proliferation. We provide evidence that loss of Hpo signaling in epithelial cells increases the production of cytokines of the Upd family and multiple EGFR ligands that activate JAK-STAT and EGFR signaling pathways in ISCs to stimulate their proliferation, thus revealing a unique non–cell-autonomous role of Hpo signaling in blocking ISC proliferation. Finally, we show that the Hpo pathway mediator Yorkie (Yki) is also required in precursor cells for injury-induced ISC proliferation in response to tissue-damaging reagent DSS.

Toll and IMD Pathways Synergistically Activate an Innate Immune Response in Drosophila melanogaster

ABSTRACT The inducible expression of antimicrobial peptide genes in Drosophila melanogaster is regulated by the conserved Toll and peptidoglycan recognition protein LC/immune deficiency (PGRP-LC/IMD) signaling pathways. It has been proposed that the two pathways have independent functions and mediate the specificity of innate immune responses towards different microorganisms. Scattered evidence also suggests that some antimicrobial target genes can be activated by both Toll and IMD, albeit to different extents. This dual activation can be mediated by independent stimulation or by cross-regulation of the two pathways. We show in this report that the Toll and IMD pathways can interact synergistically, demonstrating that cross-regulation occurs. The presence of Spätzle (the Toll ligand) and gram-negative peptidoglycan (the PGRP-LC ligand) together caused synergistic activation of representative target genes of the two pathways, including Drosomycin, Diptericin, and AttacinA. Constitutive activation of Toll and PGRP-LC/IMD could mimic the synergistic stimulation. RNA interference assays and promoter analyses demonstrate that cooperation of different NF-κB-related transcription factors mediates the synergy. These results illustrate how specific ligand binding by separate upstream pattern recognition receptors can be translated into a broad-spectrum host response, a hallmark of innate immunity.

The dorsal morphogen is a sequence-specific DNA-binding protein that interacts with a long-range repression element in drosophila

A gradient of the maternal morphogen dorsal (dl) establishes dorsal-ventral (D-V) polarity in the early Drosophila embryo. The dl concentration gradient is initiated by regulated nuclear transport, and only protein that enters nuclei is active in the D-V patterning process. Here we show that dl is a DNA-binding protein that specifically interacts with distal sequences of the zerknüllt (zen) promoter, one of the genetic targets of the morphogen. These zen sequences have the properties of a silencer element and can act over long distances to repress the expression of a heterologous promoter. The dl protein recognizes a sequence motif similar to that of the mammalian transcriptional activator NF-kappa B, which was shown to contain extensive homology with dl and the oncoprotein rel. We present evidence that the DNA-binding activity of the dl protein is mediated by the region of homology (the rel domain) conserved in the rel and NF-kappa B proteins.

Human Slug Is a Repressor That Localizes to Sites of Active Transcription

ABSTRACT Snail/Slug family proteins have been identified in diverse species of both vertebrates and invertebrates. The proteins contain four to six zinc fingers and function as DNA-binding transcriptional regulators. Various members of the family have been demonstrated to regulate cell movement, neural cell fate, left-right asymmetry, cell cycle, and apoptosis. However, the molecular mechanisms of how these regulators function and the target genes involved are largely unknown. In this report, we demonstrate that human Slug (hSlug) is a repressor and modulates both activator-dependent and basal transcription. The repression depends on the C-terminal DNA-binding zinc fingers and on a separable repression domain located in the N terminus. This domain may recruit histone deacetylases to modify the chromatin and effect repression. Protein localization study demonstrates that hSlug is present in discrete foci in the nucleus. This subnuclear pattern does not colocalize with the PML foci or the coiled bodies. Instead, the hSlug foci overlap extensively with areas of the SC-35 staining, some of which have been suggested to be sites of active splicing or transcription. These results lead us to postulate that hSlug localizes to target promoters, where activation occurs, to repress basal and activator-mediated transcription.

The Drosophila Toll-9 activates a constitutive antimicrobial defense.

The Toll family of transmembrane proteins participates in signaling infection during the innate immune response. We analyzed the nine Drosophila Toll proteins and found that wild‐type Toll‐9 behaves similar to gain‐of‐function Toll‐1. Toll‐9 activates strongly the expression of drosomycin, and utilizes similar signaling components to Toll‐1 in activating the antifungal gene. The predicted protein sequence of Toll‐9 contains a tyrosine residue in place of a conserved cysteine, and this residue switch is critical for the high activity of Toll‐9. The Toll‐9 gene is expressed in adult and larval stages prior to microbial challenge, and the expression correlates with the high constitutive level of drosomycin mRNA in the animals. The results suggest that Toll‐9 is a constitutively active protein, and implies its novel function in protecting the host by maintaining a substantial level of antimicrobial gene products to ward off the continuous challenge of microorganisms.

RACE: A DROSOPHILA HOMOLOGUE OF THE ANGIOTENSIN CONVERTING ENZYME

We report the isolation and characterization of a putative angiotensin converting enzyme (ACE) in Drosophila, called Race. General interest in mammalian ACE stems from its association with high blood pressure; ACE has also been implicated in a variety of other physiological processes including the processing of neuropeptides and gut peristalsis. Mammalian ACE is a membrane associated zinc binding protease that converts angiotensin I (A I) into angiotensin II (A II). A II functions as a potent vasoconstrictor by triggering a G-coupled receptor system in the smooth muscles that line blood vessels. Drosophila Race is composed of 615 amino acid residues, and shares extensive sequence identity with mammalian ACE over its entire length (over 42% overall identity and greater than 60% similarity). Evidence is presented that Race might correspond to a target of the homeobox regulatory gene, zerknullt (zen). Soon after zen expression is restricted to the dorsal-most regions of the embryonic ectoderm, Race is activated in a coincident pattern and becomes associated with the amnioserosa during germ band elongation, shortening and heart morphogenesis. After germ band elongation, Race is also expressed in both the anterior and posterior midgut, where it persists throughout embryogenesis. Race expression is lost from the dorsal ectoderm in either zen- or dpp- mutants, although gut expression is unaffected. P-transformation assays and genetic complementation tests suggest that Race corresponds to a previously characterized lethal complementation group, 1(2)34Eb. Mutants die during larval/pupal development, and transheterozygotes for two different lethal alleles exhibit male sterility. We propose that Race might play a role in the contractions of the heart, gut, or testes and also suggest that Hox genes might be important for coordinating both developmental and physiological processes.