Single-minded 2 is required for left-right asymmetric stomach morphogenesis.

The morphogenesis of left-right (LR) asymmetry is a crucial phase of organogenesis. In the digestive tract, the development of anatomical asymmetry is first evident in the leftward curvature of the stomach. To elucidate the molecular events that shape this archetypal laterality, we performed transcriptome analyses of the left versus right sides of the developing stomach in frog embryos. Besides the known LR gene pitx2, the only gene found to be expressed asymmetrically throughout all stages of curvature was single-minded 2 (sim2), a Down Syndrome-related transcription factor and homolog of a Drosophila gene (sim) required for LR asymmetric looping of the fly gut. We demonstrate that sim2 functions downstream of LR patterning cues to regulate key cellular properties and behaviors in the left stomach epithelium that drive asymmetric curvature. Our results reveal unexpected convergent cooption of single-minded genes during the evolution of LR asymmetric morphogenesis, and have implications for dose-dependent roles of laterality factors in non-laterality-related birth defects.

Acetylcholinesterase plays a non-neuronal, non-esterase role in organogenesis.

Acetylcholinesterase (AChE) is crucial for degrading acetylcholine at cholinergic synapses. In vitro studies suggest that, in addition to its role in nervous system signaling, AChE can also modulate non-neuronal cell properties, although it remains controversial whether AChE functions in this capacity in vivo Here, we show that AChE plays an essential non-classical role in vertebrate gut morphogenesis. Exposure of Xenopus embryos to AChE-inhibiting chemicals results in severe defects in intestinal development. Tissue-targeted loss-of-function assays (via microinjection of antisense morpholino or CRISPR-Cas9) confirm that AChE is specifically required in the gut endoderm tissue, a non-neuronal cell population, where it mediates adhesion to fibronectin and regulates cell rearrangement events that drive gut lengthening and digestive epithelial morphogenesis. Notably, the classical esterase activity of AChE is dispensable for this activity. As AChE is deeply conserved, widely expressed outside of the nervous system, and the target of many environmental chemicals, these results have wide-reaching implications for development and toxicology.

Vangl2 coordinates cell rearrangements during gut elongation.

BACKGROUND: The embryonic gut tube undergoes extensive lengthening to generate the surface area required for nutrient absorption across the digestive epithelium. In Xenopus, narrowing and elongation of the tube is driven by radial rearrangements of its core of endoderm cells, a process that concomitantly opens the gut lumen and facilitates epithelial morphogenesis. How endoderm rearrangements are properly oriented and coordinated to achieve this complex morphogenetic outcome is unknown. RESULTS: We find that, prior to gut elongation, the core Wnt/PCP component Vangl2 becomes enriched at both the anterior and apical aspects of individual endoderm cells. In Vangl2-depleted guts, the cells remain unpolarized, down-regulate cell-cell adhesion proteins, and, consequently, fail to rearrange, leading to a short gut with an occluded lumen and undifferentiated epithelium. In contrast, endoderm cells with ectopic Vangl2 protein acquire abnormal polarity and adhesive contacts. As a result, endoderm cells also fail to rearrange properly and undergo ectopic differentiation, resulting in guts with multiple torturous lumens, irregular epithelial architecture, and variable intestinal topologies. CONCLUSIONS: Asymmetrical enrichment of Vangl2 in individual gut endoderm cells orients polarity and adhesion during radial rearrangements, coordinating digestive epithelial morphogenesis and lumen formation with gut tube elongation.

Jun N-terminal kinase maintains tissue integrity during cell rearrangement in the gut.

Tissue elongation is a fundamental morphogenetic process that generates the proper anatomical topology of the body plan and vital organs. In many elongating embryonic structures, tissue lengthening is driven by Rho family GTPase-mediated cell rearrangement. During this dynamic process, the mechanisms that modulate intercellular adhesion to allow individual cells to change position without compromising structural integrity are not well understood. In vertebrates, Jun N-terminal kinase (JNK) is also required for tissue elongation, but the precise cellular role of JNK in this context has remained elusive. Here, we show that JNK activity is indispensable for the rearrangement of endoderm cells that underlies the elongation of the Xenopus gut tube. Whereas Rho kinase is necessary to induce cell intercalation and remodel adhesive contacts, we have found that JNK is required to maintain cell-cell adhesion and establish parallel microtubule arrays; without JNK activity, the reorganizing endoderm dissociates. Depleting polymerized microtubules phenocopies this effect of JNK inhibition on endoderm morphogenesis, consistent with a model in which JNK regulates microtubule architecture to preserve adhesive contacts between rearranging gut cells. Thus, in contrast to Rho kinase, which generates actomyosin-based tension and cell movement, JNK signaling is required to establish microtubule stability and maintain tissue cohesion; both factors are required to achieve proper cell rearrangement and gut extension. This model of gut elongation has implications not only for the etiology of digestive tract defects, but sheds new light on the means by which intra- and intercellular forces are balanced to promote topological change, while preserving structural integrity, in numerous morphogenetic contexts.

Rare variants in CAPN2 increase risk for isolated hypoplastic left heart syndrome.

Hypoplastic left heart syndrome (HLHS) is a severe congenital heart defect (CHD) characterized by hypoplasia of the left ventricle and aorta along with stenosis or atresia of the aortic and mitral valves. HLHS represents only ∼4%-8% of all CHDs but accounts for ∼25% of deaths. HLHS is an isolated defect (i.e., iHLHS) in 70% of families, the vast majority of which are simplex. Despite intense investigation, the genetic basis of iHLHS remains largely unknown. We performed exome sequencing on 331 families with iHLHS aggregated from four independent cohorts. A Mendelian-model-based analysis demonstrated that iHLHS was not due to single, large-effect alleles in genes previously reported to underlie iHLHS or CHD in >90% of families in this cohort. Gene-based association testing identified increased risk for iHLHS associated with variation in CAPN2 (p = 1.8 × 10-5), encoding a protein involved in functional adhesion. Functional validation studies in a vertebrate animal model (Xenopus laevis) confirmed CAPN2 is essential for cardiac ventricle morphogenesis and that in vivo loss of calpain function causes hypoplastic ventricle phenotypes and suggest that human CAPN2707C>T and CAPN21112C>T variants, each found in multiple individuals with iHLHS, are hypomorphic alleles. Collectively, our findings show that iHLHS is typically not a Mendelian condition, demonstrate that CAPN2 variants increase risk of iHLHS, and identify a novel pathway involved in HLHS pathogenesis.

Regulation of survivin by ErbB2 signaling: therapeutic implications for ErbB2-overexpressing breast cancers.

In breast cancer, overexpression of ErbB2 or aberrant regulation of survivin, a member of the inhibitor of apoptosis family, is associated with resistance to chemo/hormone therapy and predicts for a poor clinical outcome. A functional link between the two predictive factors has not been previously shown. Here, using genetic and pharmacologic approaches to block ErbB2 signaling, we show that ErbB2 regulates survivin protein expression in ErbB2-overexpressing breast cancer cells. Selective knockdown of ErbB2 using small interfering RNA markedly reduced survivin protein, resulting in apoptosis of ErbB2-overexpressing breast cancer cell lines such as BT474. Alternatively, inhibition of ErbB2 signaling using lapatinib (GW572016), a reversible small-molecule inhibitor of ErbB1/ErbB2 tyrosine kinases, at pharmacologically relevant concentrations, leads to marked inhibition of survivin protein with subsequent apoptosis. The effect of lapatinib on survivin seems to be predominantly posttranslational, mediated by ubiquitin-proteosome degradation as lactacystin, a proteosome inhibitor, reverses these effects. Furthermore, lapatinib down-regulated the expression of His-tagged survivin, which was under the transcriptional control of a heterologous promoter, providing additional evidence supporting a posttranslational mechanism of regulation. In contrast, trastuzumab and gefitinib failed to down-regulate survivin in ErbB2-overexpressing breast cancer cells. Importantly, the clinical relevance of these findings was illustrated in patients with ErbB2-overexpressing breast cancer whose clinical response to lapatinib was associated with marked inhibition of survivin in their tumors. These findings shed new light on the mechanism by which ErbB2 overexpression protects against apoptotic stimuli in breast cancer and identifies therapeutic interventions to improve clinical outcomes in these aggressive tumors.

Heterotaxin: a TGF-ß signaling inhibitor identified in a multi-phenotype profiling screen in Xenopus embryos.

Disruptions of anatomical left-right asymmetry result in life-threatening heterotaxic birth defects in vital organs. We performed a small molecule screen for left-right asymmetry phenotypes in Xenopus embryos and discovered a pyridine analog, heterotaxin, which disrupts both cardiovascular and digestive organ laterality and inhibits TGF-ß-dependent left-right asymmetric gene expression. Heterotaxin analogs also perturb vascular development, melanogenesis, cell migration, and adhesion, and indirectly inhibit the phosphorylation of an intracellular mediator of TGF-ß signaling. This combined phenotypic profile identifies these compounds as a class of TGF-ß signaling inhibitors. Notably, heterotaxin analogs also possess highly desirable antitumor properties, inhibiting epithelial-mesenchymal transition, angiogenesis, and tumor cell proliferation in mammalian systems. Our results suggest that assessing multiple organ, tissue, cellular, and molecular parameters in a whole organism context is a valuable strategy for identifying the mechanism of action of bioactive compounds.

Functional analysis of a phosphatidic acid binding domain in human Raf-1 kinase: mutations in the phosphatidate binding domain lead to tail and trunk abnormalities in developing zebrafish embryos.

Previously, we and others identified a 35-amino acid segment within human Raf-1 kinase that preferentially binds phosphatidic acid. The presence of phosphatidic acid was found to be necessary for the translocation of Raf-1 to the plasma membrane. We have now employed a combination of alanine-scanning and deletion mutagenesis to identify the critical amino acid residues in Raf-1 necessary for interaction with phosphatidic acid. Progressive mutations within a tetrapeptide motif (residues 398-401 of human Raf-1) reduced and finally eliminated binding of Raf-1 to phosphatidic acid. We then injected zebrafish embryos with RNA encoding wild-type Raf-1 kinase or a mutant version with triple alanine mutations in the tetrapeptide motif and followed the morphological fate of embryonic development. Embryos with mutant but not wild-type Raf-1 exhibited defects in posterior axis formation exemplified by bent trunk and tail structures. Molecular evidence for lack of signaling through mutated Raf-1 was obtained by aberrant in situ hybridization of the ntl (no tail) gene, which functions downstream of Raf-1. Our results demonstrate that a functional phosphatidate binding site is necessary for Raf-1 function in embryonic development.