Cardiac responses to hypoxia and reoxygenation in Drosophila.

An adequate supply of oxygen is important for the survival of all tissues, but it is especially critical for tissues with high-energy demands, such as the heart. Insufficient tissue oxygenation occurs under a variety of conditions, including high altitude, embryonic and fetal development, inflammation, and thrombotic diseases, often affecting multiple organ systems. Responses and adaptations of the heart to hypoxia are of particular relevance in human cardiovascular and pulmonary diseases, in which the effects of hypoxic exposure can range in severity from transient to long-lasting. This study uses the genetic model system Drosophila to investigate cardiac responses to acute (30 min), sustained (18 h), and chronic (3 wk) hypoxia with reoxygenation. Whereas hearts from wild-type flies recovered quickly after acute hypoxia, exposure to sustained or chronic hypoxia significantly compromised heart function upon reoxygenation. Hearts from flies with mutations in sima, the Drosophila homolog of the hypoxia-inducible factor alpha subunit (HIF-α), exhibited exaggerated reductions in cardiac output in response to hypoxia. Heart function in hypoxia-selected flies, selected over many generations for survival in a low-oxygen environment, revealed reduced cardiac output in terms of decreased heart rate and fractional shortening compared with their normoxia controls. Hypoxia-selected flies also had smaller hearts, myofibrillar disorganization, and increased extracellular collagen deposition, consistent with the observed reductions in contractility. This study indicates that longer-duration hypoxic insults exert deleterious effects on heart function that are mediated, in part, by sima and advances Drosophila models for the genetic analysis of cardiac-specific responses to hypoxia and reoxygenation.

Cardiac deficiency of single cytochrome oxidase assembly factor scox induces p53-dependent apoptosis in a Drosophila cardiomyopathy model.

The heart is a muscle with high energy demands. Hence, most patients with mitochondrial disease produced by defects in the oxidative phosphorylation (OXPHOS) system are susceptible to cardiac involvement. The presentation of mitochondrial cardiomyopathy includes hypertrophic, dilated and left ventricular noncompaction, but the molecular mechanisms involved in cardiac impairment are unknown. One of the most frequent OXPHOS defects in humans frequently associated with cardiomyopathy is cytochrome c oxidase (COX) deficiency caused by mutations in COX assembly factors such as Sco1 and Sco2. To investigate the molecular mechanisms that underlie the cardiomyopathy associated with Sco deficiency, we have heart specifically interfered scox expression, the single Drosophila Sco orthologue. Cardiac-specific knockdown of scox reduces fly lifespan, and it severely compromises heart function and structure, producing dilated cardiomyopathy. Cardiomyocytes with low levels of scox have a significant reduction in COX activity and they undergo a metabolic switch from OXPHOS to glycolysis, mimicking the clinical features found in patients harbouring Sco mutations. The major cardiac defects observed are produced by a significant increase in apoptosis, which is dp53-dependent. Genetic and molecular evidence strongly suggest that dp53 is directly involved in the development of the cardiomyopathy induced by scox deficiency. Remarkably, apoptosis is enhanced in the muscle and liver of Sco2 knock-out mice, clearly suggesting that cell death is a key feature of the COX deficiencies produced by mutations in Sco genes in humans.

Wnt signaling interacts with bmp and edn1 to regulate dorsal-ventral patterning and growth of the craniofacial skeleton.

Craniofacial development requires signals from epithelia to pattern skeletogenic neural crest (NC) cells, such as the subdivision of each pharyngeal arch into distinct dorsal (D) and ventral (V) elements. Wnt signaling has been implicated in many aspects of NC and craniofacial development, but its roles in D-V arch patterning remain unclear. To address this we blocked Wnt signaling in zebrafish embryos in a temporally-controlled manner, using transgenics to overexpress a dominant negative Tcf3, (dntcf3), (Tg(hsp70I:tcf3-GFP), or the canonical Wnt inhibitor dickkopf1 (dkk1), (Tg(hsp70i:dkk1-GFP) after NC migration. In dntcf3 transgenics, NC cells in the ventral arches of heat-shocked embryos show reduced proliferation, expression of ventral patterning genes (hand2, dlx3b, dlx5a, msxe), and ventral cartilage differentiation (e.g. lower jaws). These D-V patterning defects resemble the phenotypes of zebrafish embryos lacking Bmp or Edn1 signaling, and overexpression of dntcf3 dramatically reduces expression of a subset of Bmp receptors in the arches. Addition of ectopic BMP (or EDN1) protein partially rescues ventral development and expression of dlx3b, dlx5a, and msxe in Wnt signaling-deficient embryos, but surprisingly does not rescue hand2 expression. Thus Wnt signaling provides ventralizing patterning cues to arch NC cells, in part through regulation of Bmp and Edn1 signaling, but independently regulates hand2. Similarly, heat-shocked dkk1+ embryos exhibit ventral arch reductions, but also have mandibular clefts at the ventral midline not seen in dntcf3+ embryos. Dkk1 is expressed in pharyngeal endoderm, and cell transplantation experiments reveal that dntcf3 must be overexpressed in pharyngeal endoderm to disrupt D-V arch patterning, suggesting that distinct endodermal roles for Wnts and Wnt antagonists pattern the developing skeleton.

Ring Finger Protein 14 is a new regulator of TCF/ß-catenin-mediated transcription and colon cancer cell survival.

T-cell factor/lymphoid enhancer factor (TCF/LEF) proteins regulate transcription by recruiting ß-catenin and its associated co-regulators. Whether TCF/LEFs also recruit more factors through independent, direct interactions is not well understood. Here we discover Ring Finger Protein 14 (RNF14) as a new binding partner for all TCF/LEF transcription factors. We show that RNF14 positively regulates Wnt signalling in human cancer cells and in an in vivo zebrafish model by binding to target promoters with TCF and stabilizing ß-catenin recruitment. RNF14 depletion experiments demonstrate that it is crucial for colon cancer cell survival. Therefore, we have identified a key interacting factor of TCF/ß-catenin complexes to regulate Wnt gene transcription.

Ovo1 links Wnt signaling with N-cadherin localization during neural crest migration.

A fundamental issue in cell biology is how migratory cell behaviors are controlled by dynamically regulated cell adhesion. Vertebrate neural crest (NC) cells rapidly alter cadherin expression and localization at the cell surface during migration. Secreted Wnts induce some of these changes in NC adhesion and also promote specification of NC-derived pigment cells. Here, we show that the zebrafish transcription factor Ovo1 is a Wnt target gene that controls migration of pigment precursors by regulating the intracellular movements of N-cadherin (Ncad). Ovo1 genetically interacts with Ncad and its depletion causes Ncad to accumulate inside cells. Ovo1-deficient embryos strongly upregulate factors involved in intracellular trafficking, including several rab GTPases, known to modulate cellular localization of cadherins. Surprisingly, NC cells express high levels of many of these rab genes in the early embryo, chemical inhibitors of Rab functions rescue NC development in Ovo1-deficient embryos and overexpression of a Rab-interacting protein leads to similar defects in NC migration. These results suggest that Ovo proteins link Wnt signaling to intracellular trafficking pathways that localize Ncad in NC cells and allow them to migrate. Similar processes probably occur in other cell types in which Wnt signaling promotes migration.