Chimaerin and Rac regulate cell number, adherens junctions, and ERK MAP kinase signaling in the Drosophila eye

FRL and DAAM are required for lateral adhesion of interommatidial cells and patterning of the retinal floor

Optical insulation of the unit eyes (ommatidia) is an important prerequisite of precise sight with compound eyes. Separation of the ommatidia is ensured by pigment cells that organize into a hexagonal lattice in the Drosophila eye, forming thin walls between the facets. Cell adhesion, mediated by apically and latero-basally located junctional complexes, is crucial for stable attachment of these cells to each other and the basal lamina. Whereas former studies have focused on the formation and remodelling of the cellular connections at the apical region, here, we report a specific alteration of the lateral adhesion of the lattice cells, leaving the apical junctions largely unaffected. We found that DAAM and FRL, two formin-type cytoskeleton regulatory proteins, play redundant roles in lateral adhesion of the interommatidial cells and patterning of the retinal floor. We show that formin-dependent cortical actin assembly is crucial for latero-basal sealing of the ommatidial lattice. We expect that the investigation of these previously unreported eye phenotypes will pave the way toward a better understanding of the three-dimensional aspects of compound eye development.

CHN1 promotes epithelial-mesenchymal transition via the Akt/GSK-3β/Snail pathway in cervical carcinoma

Background

Metastasis and invasion are crucial in determining the mortality of cervical carcinoma (CC) patients. The epithelial–mesenchymal transition (EMT) is now a universal explanation for the mechanisms of tumor metastasis. Α-chimeric protein (α-chimaerin, CHN1) plays an important role in the regulation of signal transduction and development. However, the molecular regulatory relationships between CHN1 and CC progression in relation to EMT have not yet been identified.

Methods

The expression of CHN1 in CC tissues, adjacent tissues, and lymph node metastases from CC patients was detected by immunohistochemistry. Upregulation and knockdown of CHN1 were achieved by transfection of CC cells. The effect of CHN1 on cell proliferation was determined by CCK-8 and plate clone formation assays. Changes in migration and invasion capabilities were evaluated using scratch migration and transwell invasion assays. The effect of CHN1 overexpression and interference on xenograft tumor growth was determined by tumor weight and pathological analyses. The expression of EMT-related mRNAs was measured by qRT-PCR in transfected CC cells. EMT-related proteins and Akt/GSK-3β/Snail signaling pathway-related proteins were also evaluated by western blotting.

Results

CHN1 was overexpressed in CC tissues and was associated with lymph node metastasis and low survival in CC patients. Overexpression of CHN1 promoted cell proliferation, migration, and invasion in CC cells. In contrast, silencing of CHN1 inhibited these phenomena. Overexpression of CHN1 promoted tumor formation in an in vivo xenograft tumor mouse model, with increased tumor volumes and weights. In addition, CHN1 induced the expression of EMT-related transcription factors, accompanied by the decreased expression of epithelial markers and increased expression of mesenchymal markers. The Akt/GSK-3β/Snail signaling pathway was activated by overexpression of CHN1 in vitro, and activation of this pathway was inhibited by the signaling pathway inhibitor LY294002.

Conclusion

These results suggest that CHN1 promotes the development and progression of cervical carcinoma via the Akt/GSK-3β/Snail pathway by inducing EMT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-02963-7.

The WAVE Regulatory Complex and Branched F-Actin Counterbalance Contractile Force to Control Cell Shape and Packing in the Drosophila Eye

Contractile forces eliminate cell contacts in many morphogenetic processes. However, mechanisms that balance contractile forces to promote subtler remodeling remain unknown. To address this gap, we investigated remodeling of Drosophila eye lattice cells (LCs), which preserve cell contacts as they narrow to form the edges of a multicellular hexagonal lattice. We found that during narrowing, LC-LC contacts dynamically constrict and expand. Similar to other systems, actomyosin-based contractile forces promote pulses of constriction. Conversely, we found that WAVE-dependent branched F-actin accumulates at LC-LC contacts during expansion and functions to expand the cell apical area, promote shape changes, and prevent elimination of LC-LC contacts. Finally, we found that small Rho GTPases regulate the balance of contractile and protrusive dynamics. These data suggest a mechanism by which WAVE regulatory complex-based F-actin dynamics antagonize contractile forces to regulate cell shape and tissue topology during remodeling and thus contribute to the robustness and precision of the process.

A new role of the Rac-GAP β2-chimaerin in cell adhesion reveals opposite functions in breast cancer initiation and tumor progression

β2-chimaerin is a Rac1-specific negative regulator and a candidate tumor suppressor in breast cancer but its precise function in mammary tumorigenesis in vivo is unknown. Here, we study for the first time the role of β2-chimaerin in breast cancer using a mouse model and describe an unforeseen role for this protein in epithelial cell-cell adhesion. We demonstrate that expression of β2-chimaerin in breast cancer epithelial cells reduces E-cadherin protein levels, thus loosening cell-cell contacts. In vivo, genetic ablation of β2-chimaerin in the MMTV-Neu/ErbB2 mice accelerates tumor onset, but delays tumor progression. Finally, analysis of clinical databases revealed an inverse correlation between β2-chimaerin and E-cadherin gene expressions in Her2+ breast tumors. Furthermore, breast cancer patients with low β2-chimaerin expression have reduced relapse free survival but develop metastasis at similar times. Overall, our data redefine the role of β2-chimaerin as tumor suppressor and provide the first in vivo evidence of a dual function in breast cancer, suppressing tumor initiation but favoring tumor progression.

Regulatory mechanisms of EGFR signalling during Drosophila eye development

EGFR signalling is a well-conserved signalling pathway playing major roles during development and cancers. This review explores what studying the EGFR pathway during Drosophila eye development has taught us in terms of the diversity of its regulatory mechanisms. This model system has allowed the identification of numerous positive and negative regulators acting at specific time and place, thus participating to the tight control of signalling. EGFR signalling regulation is achieved by a variety of mechanisms, including the control of ligand processing, the availability of the receptor itself and the transduction of the cascade in the cytoplasm. Ultimately, the transcriptional responses contribute to the establishment of positive and negative feedback loops. The combination of these multiple mechanisms employed to regulate the EGFR pathway leads to specific cellular outcomes involved in functions as diverse as the acquisition of cell fate, proliferation, survival, adherens junction remodelling and morphogenesis.

BIGH3 modulates adhesion and migration of hematopoietic stem and progenitor cells

Cell adhesion and migration are important determinants of homing and development of hematopoietic stem and progenitor cells (HSPCs) in bone marrow (BM) niches. The extracellular matrix protein transforming growth factor-β (TGF-β) inducible gene H3 (BIGH3) is involved in adhesion and migration, although the effect of BIGH3 is highly cell type-dependent. BIGH3 is abundantly expressed by mesenchymal stromal cells, while its expression in HSPCs is relatively low unless induced by certain BM stressors. Here, we set out to determine how BIGH3 modulates HSPC adhesion and migration. We show that primary HSPCs adhere to BIGH3-coated substrates, which is, in part, integrin-dependent. Overexpression of BIGH3 in HSPCs and HL60 cells reduced the adhesion to the substrate fibronectin in adhesion assays, which was even more profound in electrical cell-substrate impedance sensing (ECIS) assays. Accordingly, the CXCL12 induced migration over fibronectin-coated surface was reduced in BIGH3-expressing HSPCs. The integrin expression profile of HSPCs was not altered upon BIGH3 expression. Although expression of BIGH3 did not alter actin polymerization in response to CXCL12, it inhibited the PMA-induced activation of the small GTPase RAC1 as well as the phosphorylation and activation of extracellular-regulated kinases (ERKs). Reduced activation of ERK and RAC1 may be responsible for the inhibition of cell adhesion and migration by BIGH3 in HSPCs. Induced BIGH3 expression upon BM stress may contribute to the regulation of BM homeostasis.

Systematic expression and loss-of-function analysis defines spatially restricted requirements for Drosophila RhoGEFs and RhoGAPs in leg morphogenesis

The Drosophila leg imaginal disc consists of a peripheral region that contributes to adult body wall, and a central region that forms the leg proper. While the patterning signals and transcription factors that determine the identity of adult structures have been identified, the mechanisms that determine the shape of these structures remain largely unknown. The family of Rho GTPases, which consists of seven members in flies, modulates cell adhesion, actomyosin contractility, protrusive membrane activity, and cell-matrix adhesion to generate mechanical forces that shape adult structures. The Rho GTPases are ubiquitously expressed and it remains unclear how they orchestrate morphogenetic events. The Rho guanine nucleotide exchange factors (RhoGEFs) and Rho GTPase activating proteins (RhoGAPs), which respectively activate and deactivate corresponding Rho GTPases, have been proposed to regulate the activity of Rho signaling cascades in specific spatiotemporal patterns to orchestrate morphogenetic events. Here we identify restricted expression of 12 of the 20 RhoGEFs and 10 of the 22 Rho RhoGAPs encoded in Drosophila during metamorphosis. Expression of a subset of each family of RhoGTPase regulators was restricted to motile cell populations including tendon, muscle, trachea, and peripodial stalk cells. A second subset was restricted either to all presumptive joints or only to presumptive tarsal joints. Depletion of individual RhoGEFs and RhoGAPs in the epithelium of the disc proper identified several joint-specific genes, which act downstream of segmental patterning signals to control epithelial morphogenesis. Our studies provide a framework with which to understand how Rho signaling cascades orchestrate complex morphogenetic events in multi-cellular organisms, and evidence that patterning signals regulate these cascades to control apical constriction and epithelial invagination at presumptive joints.

Genotype-phenotype correlation in eight new patients with a deletion encompassing 2q31.1

Microdeletions of the 2q31.1 region are rare. We present the clinical and molecular findings of eight previously unreported patients with overlapping deletions in 2q31.1. The patients have a variable clinical phenotype and present with developmental delay (7/8), growth retardation (5/8), seizures (2/8) and a craniofacial dysmorphism consisting of microcephaly (4/8), short palpebral fissures (7/8), broad eyebrows with lateral flare (7/8), low-set ears with thickened helices and lobules (5/8), and micrognathia (6/8). Additional congenital anomalies were noted, including limb abnormalities (8/8), heart defects (3/8), genital anomalies (3/8), and craniosynostosis (1/8). Six of these microdeletions, ranging in size from 1.24 to 8.35 Mb, were identified by array CGH, one larger deletion (19.7 Mb) was detected by conventional karyotyping and further characterized by array CGH analysis. The smallest region of overlap in all eight patients spans at most 88 kb and includes only the WIPF1 gene. This gene codes for the WAS/WASL interacting protein family member 1. The patients described here do not present with clinical signs of the Wiskott-Aldrich syndrome and the deletion of this single gene does not allow explaining the phenotype in our patients. It is likely that the deletion of different but overlapping sets of genes from 2q31 is responsible for the clinical variability in these patients. To further dissect the complex phenotype associated with deletions in 2q31, additional patients with overlapping phenotypes should be examined with array CGH. This should help to link particular phenotypes to specific genes, and add to our understanding of the underlying developmental processes.

A novel cross-talk in diacylglycerol signaling: the Rac-GAP beta2-chimaerin is negatively regulated by protein kinase Cdelta-mediated phosphorylation

Although the family of chimaerin Rac-GAPs has recently gained significant attention for their involvement in development, cancer, and neuritogenesis, little is known about their molecular regulation. Chimaerins are activated by the lipid second messenger diacylglycerol via their C1 domain upon activation of tyrosine kinase receptors, thereby restricting the magnitude of Rac signaling in a receptor-regulated manner. Here we identified a novel regulatory mechanism for beta2-chimaerin via phosphorylation. Epidermal growth factor or the phorbol ester phorbol 12-myristate 13-acetate caused rapid phosphorylation of beta2-chimaerin on Ser(169) located in the SH2-C1 domain linker region via protein kinase Cdelta, which retained beta2-chimaerin in the cytosol and prevented its C1 domain-mediated translocation to membranes. Furthermore, despite the fact that Ser(169) phosphorylation did not alter intrinsic Rac-GAP activity in vitro, a non-phosphorylatable beta2-chimaerin mutant was highly sensitive to translocation, and displayed enhanced association with activated Rac, enhanced Rac-GAP activity, and anti-migratory properties when expressed in cells. Our results not only revealed a novel regulatory mechanism that facilitates Rac activation, but also identified a novel mechanism of cross-talk between diacylglycerol receptors that restricts beta2-chimaerin relocalization and activation.

Cdc42 antagonizes Rho1 activity at adherens junctions to limit epithelial cell apical tension

Rho promotes actomyosin contractility during epithelial cell remodeling, but Cdc42 keeps the epithelium in shape by limiting RhoA activity.

In epithelia, cells are arranged in an orderly pattern with a defined orientation and shape. Cadherin containing apical adherens junctions (AJs) and the associated actomyosin cytoskeleton likely contribute to epithelial cell shape by providing apical tension. The Rho guanosine triphosphatases are well known regulators of cell junction formation, maintenance, and function. Specifically, Rho promotes actomyosin activity and cell contractility; however, what controls and localizes this Rho activity as epithelia remodel is unresolved. Using mosaic clonal analysis in the Drosophila melanogaster pupal eye, we find that Cdc42 is critical for limiting apical cell tension by antagonizing Rho activity at AJs. Cdc42 localizes Par6–atypical protein kinase C (aPKC) to AJs, where this complex limits Rho1 activity and thus actomyosin contractility, independent of its effects on Wiskott-Aldrich syndrome protein and p21-activated kinase. Thus, in addition to its role in the establishment and maintenance of apical–basal polarity in forming epithelia, the Cdc42–Par6–aPKC polarity complex is required to limit Rho activity at AJs and thus modulate apical tension so as to shape the final epithelium.

Rac1 activation controls nuclear localization of beta-catenin during canonical Wnt signaling

Canonical Wnt signaling critically regulates cell fate and proliferation in development and disease. Nuclear localization of beta-catenin is indispensable for canonical Wnt signaling; however, the mechanisms governing beta-catenin nuclear localization are not well understood. Here we demonstrate that nuclear accumulation of beta-catenin in response to Wnt requires Rac1 activation. The role of Rac1 depends on phosphorylation of beta-catenin at Ser191 and Ser605, which is mediated by JNK2 kinase. Mutations of these residues significantly affect Wnt-induced beta-catenin nuclear accumulation. Genetic ablation of Rac1 in the mouse embryonic limb bud ectoderm disrupts canonical Wnt signaling and phenocopies deletion of beta-catenin in causing severe truncations of the limb. Finally, Rac1 interacts genetically with beta-catenin and Dkk1 in controlling limb outgrowth. Together these results uncover Rac1 activation and subsequent beta-catenin phosphorylation as a hitherto uncharacterized mechanism controlling canonical Wnt signaling and may provide additional targets for therapeutic intervention of this important pathway.