Rho GTPase-independent regulation of mitotic progression by the RhoGEF Net1

Conditional Overexpression of Net1 Enhances the Trans-Differentiation of Lgr5+ Progenitors into Hair Cells in the Neonatal Mouse Cochlea

Sensorineural hearing loss is mainly caused by damage to hair cells (HC), which cannot be regenerated spontaneously in adult mammals once damaged. Cochlear Lgr5+ progenitors are characterised by HC regeneration capacity in neonatal mice, and we previously screened several new genes that might induce HC regeneration from Lgr5+ progenitors. Net1, a guanine nucleotide exchange factor, is one of the screened new genes and is particularly active in cancer cells and is involved in cell proliferation and differentiation. Here, to explore in vivo roles of Net1 in HC regeneration, Net1 loxp/loxp mice were constructed and crossed with Lgr5 CreER/+ mice to conditionally overexpress (cOE) Net1 in cochlear Lgr5+ progenitors. We observed a large number of ectopic HCs in Lgr5 CreER/+ Net1 loxp/loxp mouse cochlea, which showed a dose-dependent effect. Moreover, the EdU assay was unable to detect any EdU+/Sox2+ supporting cells, while lineage tracing showed significantly more regenerated tdTomato+ HCs in Lgr5 CreER/+ Net1 loxp/loxp tdTomato mice, which indicated that Net1 cOE enhanced HC regeneration by inducing the direct trans-differentiation of Lgr5+ progenitors rather than mitotic HC regeneration. Additionally, qPCR results showed that the transcription factors related to HC regeneration, including Atoh1, Gfi1 and Pou4f3, were significantly upregulated and are probably the mechanism behind the HC regeneration induced by Net1. In conclusion, our study provides new evidence for the role of Net1 in enhancing HC regeneration in the neonatal mouse cochlea.

Reduced NET1 adversely affects early embryonic development in mice

Neuroepithelial transforming gene 1 (NET1) is a RhoA subfamily-specific guanine/nucleotide-exchange factor that exhibits critical roles in diverse biological processes. However, the functions in mouse preimplantation embryonic development have not yet determined. In the present study we demonstrated that NET1 is a key factor in the outcome of early mouse embryonic development. Immunofluorescence detection showed that NET1 is principally localized to the nucleus during mouse pre-implantation embryonic development. Silencing Net1 at the zygote stage using a specific siRNA impaired the developmental competence of early mouse embryos, and Net1-knockdown (Net1-KD) induced mitotic spindle-assembly defects and chromosomal alignment abnormalities at the first embryonic cleavage. In addition, reduced NET1 exacerbated reactive oxygen species production and DNA lesions in two-cell stage embryos, further augmenting cellular apoptosis in the preimplantation blastocyst. In summary, our data display key roles for NET1 in mitotic spindle assembly, oxidative stress, and DNA damage during early mouse embryonic development.

Doublecortin regulates the mitochondrial-dependent apoptosis in glioma via Rho-A/Net-1/p38-MAPK signaling

Doublecortin (DCX) is a microtubule-associated protein known to be a key regulator of neuronal migration and differentiation during brain development. However, the role of DCX, particularly in regulating the survival and growth of glioma cells, remains unclear. In this study, we utilized CRISPR/Cas9 technology to knock down DCX in the human glioma cell line (U251). DCX depletion suppressed cell proliferation and enhanced the pro-apoptotic effects of temozolomide (TMZ) and γ-radiation treatment. DCX knockdown led to the translocation of Bax to the mitochondria and mitochondria dysfunction. Furthermore, DCX deficiency-induced apoptosis took place along with the generation of reactive oxygen species (ROS), which is crucial in triggering mitochondrial membrane depolarization, the release of cytochrome c (Cyt-c), and caspase activation. Importantly, the transcriptional inhibition of DCX downregulated Rho-A, Net-1, and activated p38-MAPK cue, critical for cell survival and proliferation. Subsequent treatment with TMZ and γ-radiation further increased p38-MAPK activity through the decreased expression of Rho-A/Net-1, resulting in a significant reduction in glioma cell migration and invasion. Additionally, intracranial xenograft tumors of DCX-modified U251 cells in nude mice demonstrated inhibited tumor growth. Tumor sections treated with TMZ and γ-radiation exhibited a higher number of TUNEL-positive cells compared to the control group, indicating increased apoptosis. Our finding suggests that DCX depletion reduces glioma cell proliferation and promotes mitochondria-dependent apoptosis by enhancing the chemo and radiotherapy response. Targeting DCX represents a potential therapeutic target for glioma treatment.

MANCR lncRNA Modulates Cell-Cycle Progression and Metastasis by Cis-Regulation of Nuclear Rho-GEF

A significant number of the genetic alterations observed in cancer patients lie within nonprotein-coding segments of the genome, including regions coding for long noncoding RNAs (lncRNAs). LncRNAs display aberrant expression in breast cancer (BrCa), but the functional implications of this altered expression remain to be elucidated. By performing transcriptome screen in a triple negative BrCa (TNBC) isogenic 2D and 3D spheroid model, we observed aberrant expression of >1000 lncRNAs during BrCa progression. The chromatin-associated lncRNA MANCR shows elevated expression in metastatic TNBC. MANCR is upregulated in response to cellular stress and modulates DNA repair and cell proliferation. MANCR promotes metastasis as MANCR-depleted cells show reduced cell migration, invasion, and wound healing in vitro, and reduced metastatic lung colonization in xenograft experiments in vivo. Transcriptome analyses reveal that MANCR modulates expression and pre-mRNA splicing of genes, controlling DNA repair and checkpoint response. MANCR promotes the transcription of NET1A, a Rho-GEF that regulates DNA damage checkpoint and metastatic processes in cis, by differential promoter usage. Experiments suggest that MANCR regulates the expression of cancer-associated genes by modulating the association of various transcription factors and RNA-binding proteins. Our results identified the metastasis-promoting activities of MANCR in TNBC by cis-regulation of gene expression.

Role of Small GTPase RhoA in DNA Damage Response

Accumulating evidence has suggested a role of the small GTPase Ras homolog gene family member A (RhoA) in DNA damage response (DDR) in addition to its traditional function of regulating cell morphology. In DDR, 2 key components of DNA repair, ataxia telangiectasia-mutated (ATM) and flap structure-specific endonuclease 1 (FEN1), along with intracellular reactive oxygen species (ROS) have been shown to regulate RhoA activation. In addition, Rho-specific guanine exchange factors (GEFs), neuroepithelial transforming gene 1 (Net1) and epithelial cell transforming sequence 2 (Ect2), have specific functions in DDR, and they also participate in Ras-related C3 botulinum toxin substrate 1 (Rac1)/RhoA interaction, a process which is largely unappreciated yet possibly of significance in DDR. Downstream of RhoA, current evidence has highlighted its role in mediating cell cycle arrest, which is an important step in DNA repair. Unraveling the mechanism by which RhoA modulates DDR may provide more insight into DDR itself and may aid in the future development of cancer therapies.

Cdk1 phosphorylation negatively regulates the activity of Net1 towards RhoA during mitosis

The Neuroepithelial transforming gene 1 (Net1) is a RhoA subfamily guanine nucleotide exchange factor that is overexpressed in a number of cancers and contributes to cancer cell motility and proliferation. Net1 also plays a Rho GTPase independent role in mitotic progression, where it promotes centrosomal activation of Aurora A and Pak2, and aids in chromosome alignment during prometaphase. To understand regulatory mechanisms controlling the mitotic function of Net1, we examined whether it was phosphorylated by the mitotic kinase Cdk1. We observed that Cdk1 phosphorylated Net1 on multiple sites in its N-terminal regulatory domain and C-terminus in vitro. By raising phospho-specific antibodies to two of these sites, we also demonstrated that both endogenous and transfected Net1 were phosphorylated by Cdk1 in cells. Substitution of the major Cdk1 phosphorylation sites with aliphatic or acidic residues inhibited the interaction of Net1 with RhoA, and treatment of metaphase cells with a Cdk1 inhibitor increased Net1 activity. Cdk1 inhibition also increased Net1 localization to the plasma membrane and stimulated cortical F-actin accumulation. Moreover, Net1 overexpression caused spindle polarity defects that were reduced in frequency by acidic substitution of the major Cdk1 phosphorylation sites. These data indicate that Cdk1 phosphorylates Net1 during mitosis and suggest that this negatively regulates its ability to signal to RhoA and alter actin cytoskeletal organization.

Neuroepithelial Cell Transforming Gene 1 Acts as an Oncogene and Is Mediated by miR-22 in Human Non-Small-Cell Lung Cancer

Abnormal expression of neuroepithelial cell transforming gene 1 (NET1) has been authenticated in many human cancers, including lung cancer. We have previously reported that NET1 functioned as an oncogene and promoted human non-small-cell lung cancer (NSCLC) growth and migration. However, the correlation between NET1 and its upstream miRNAs needed further illustration. Our present work demonstrated that miR-22 had a relatively low expression, and NET1 had a relatively high expression in both NSCLC samples and lung adenocarcinoma cell lines compared with corresponding normal controls. Moreover, miR-22 directly regulated NET1 and was verified to weaken cancer cell proliferation and migration, as well as enhance cell apoptosis by suppressing NET1. Furthermore, the inhibitory effect of miR-22 can be reversed via overexpressing NET1 using an ectopic expression vector in NSCLC cells. Our findings showed that miR-22/NET-1 axis may contribute to the inhibition of NSCLC growth and migration and represents a promising therapeutic target for NSCLC.

Contributions of the RhoA guanine nucleotide exchange factor Net1 to polyoma middle T antigen-mediated mammary gland tumorigenesis and metastasis

Background

The RhoA activating protein Net1 contributes to breast cancer cell proliferation, motility, and invasion in vitro, yet little is known about its roles in mammary gland tumorigenesis and metastasis.

Methods

Net1 knockout (KO) mice were bred to mice with mammary gland specific expression of the polyoma middle T antigen (PyMT) oncogene. Mammary gland tumorigenesis and lung metastasis were monitored. Individual tumors were assessed for proliferation, apoptosis, angiogenesis, RhoA activation, and activation of PyMT-dependent signaling pathways. Primary tumor cells from wild-type and Net1 KO mice were transplanted into the mammary glands of wild-type, nontumor-bearing mice, and tumor growth and metastasis were assessed. Gene expression in wild-type and Net1 KO tumors was analyzed by gene ontology enrichment and for relative activation of gene expression signatures indicative of signaling pathways important for breast cancer initiation and progression. A gene expression signature indicative of Net1 function was identified. Human breast cancer gene expression profiles were screened for the presence of a Net1 gene expression signature.

Results

We show that Net1 makes fundamental contributions to mammary gland tumorigenesis and metastasis. Net1 deletion delays tumorigenesis and strongly suppresses metastasis in PyMT-expressing mice. Moreover, we observe that loss of Net1 reduces cancer cell proliferation, inhibits tumor angiogenesis, and promotes tumor cell apoptosis. Net1 is required for maximal RhoA activation within tumors and for primary tumor cell motility. Furthermore, the ability of PyMT to initiate oncogenic signaling to ERK1/2 and PI3K/Akt1 is inhibited by Net1 deletion. Primary tumor cell transplantation indicates that the reduction in tumor angiogenesis and lung metastasis observed upon Net1 deletion are tumor cell autonomous effects. Using a gene expression signature indicative of Net1 activity, we show that Net1 signaling is activated in 10% of human breast cancers, and that this correlates with elevated proliferation and PI3K pathway activity. We also demonstrate that human breast cancer patients with a high Net1 gene expression signature experience shorter distant metastasis-free survival.

Conclusions

These data indicate that Net1 is required for tumor progression in the PyMT mouse model and suggest that Net1 may contribute to breast cancer progression in humans.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-0966-2) contains supplementary material, which is available to authorized users.

mDia2 and CXCL12/CXCR4 chemokine signaling intersect to drive tumor cell amoeboid morphological transitions

Morphological plasticity in response to environmental cues in migrating cancer cells requires F-actin cytoskeletal rearrangements. Conserved formin family proteins play critical roles in cell shape, tumor cell motility, invasion and metastasis, in part, through assembly of non-branched actin filaments. Diaphanous-related formin-2 (mDia2/Diaph3/Drf3/Dia) regulates mesenchymal-to-amoeboid morphological conversions and non-apoptotic blebbing in tumor cells by interacting with its inhibitor diaphanous-interacting protein (DIP), and disrupting cortical F-actin assembly and bundling. F-actin disruption is initiated by a CXCL12-dependent mechanism. Downstream CXCL12 signaling partners inducing mDia2-dependent amoeboid conversions remain enigmatic. We found in MDA-MB-231 tumor cells CXCL12 induces DIP and mDia2 interaction in blebs, and engages its receptor CXCR4 to induce RhoA-dependent blebbing. mDia2 and CXCR4 associate in blebs upon CXCL12 stimulation. Both CXCR4 and RhoA are required for CXCL12-induced blebbing. Neither CXCR7 nor other Rho GTPases that activate mDia2 are required for CXCL12-induced blebbing. The Rho Guanine Nucleotide Exchange Factor (GEF) Net1 is required for CXCL12-driven RhoA activation and subsequent blebbing. These results reveal CXCL12 signaling, through CXCR4, directs a Net1/RhoA/mDia-dependent signaling hub to drive cytoskeleton rearrangements to regulate morphological plasticity in tumor cells. These signaling hubs may be conserved during normal and cancer cells responding to chemotactic cues.

Interphase adhesion geometry is transmitted to an internal regulator for spindle orientation via caveolin-1

Despite theoretical and physical studies implying that cell-extracellular matrix adhesion geometry governs the orientation of the cell division axis, the molecular mechanisms that translate interphase adhesion geometry to the mitotic spindle orientation remain elusive. Here, we show that the cellular edge retraction during mitotic cell rounding correlates with the spindle axis. At the onset of mitotic cell rounding, caveolin-1 is targeted to the retracting cortical region at the proximal end of retraction fibres, where ganglioside GM1-enriched membrane domains with clusters of caveola-like structures are formed in an integrin and RhoA-dependent manner. Furthermore, Gαi1–LGN–NuMA, a well-known regulatory complex of spindle orientation, is targeted to the caveolin-1-enriched cortical region to guide the spindle axis towards the cellular edge retraction. We propose that retraction-induced cortical heterogeneity of caveolin-1 during mitotic cell rounding sets the spindle orientation in the context of adhesion geometry.

Studies imply that cell adhesion geometry during interphase dictates the orientation of the cell division axis. Here the authors show that accumulation of caveolin-1 to rapidly retracting regions during cell rounding sets the spindle orientation by recruiting Gαi1-LGN-NuMA to the cortex.

The RhoGEF Net1 is required for normal mammary gland development

Neuroepithelial transforming gene 1 (Net1) is a RhoA subfamily-specific guanine nucleotide exchange factor that is overexpressed in human breast cancer and is required for breast cancer cell migration and invasion. However, the role of Net1 in normal mammary gland development or function has never been assessed. To understand the role of Net1 in the mammary gland, we have created a conditional Net1 knockout mouse model. Whole-body deletion of Net1 results in delayed mammary gland development during puberty characterized by slowed of ductal extension and reduced ductal branching. Epithelial cells within the developing ducts show reduced proliferation that is accompanied by diminished estrogen receptor-α expression and activity. Net1-deficient mammary glands also exhibit reduced phosphorylation of regulatory subunits of myosin light chain and myosin light-chain phosphatase, indicating that RhoA-dependent actomyosin contraction is compromised. Net1 deficiency also leads to disorganization of myoepithelial and ductal epithelial cells and increased periductal collagen deposition. Mammary epithelial cell transplantation experiments indicate that reduced ductal branching and disorganization are cell autonomous. These data identify for the first time a role for NET1 in vivo and indicate that NET1 expression is essential for the proliferation and differentiation of mammary epithelial cells in the developing mammary gland.

Rho GTPase independent regulation of ATM activation and cell survival by the RhoGEF Net1A

ATM activation following DNA damage is a critical event which is required for efficient DNA repair and cell survival, yet signaling mechanisms controlling its activation are incompletely understood. The RhoGEF Net1 has previously been reported to control Rho GTPase activation and downstream cell survival outcomes following double strand DNA damage. However the role of Net1 isoforms in controlling ATM-dependent cell signaling has not been assessed. In the present work we show that expression of the Net1A isoform is specifically required for efficient activation of ATM but not the related kinase DNA-PK after ionizing radiation. Surprisingly Net1A overexpression also potently suppresses ATM activation and phosphorylation of its substrate H2AX. This effect does not require catalytic activity towards RhoA or RhoB, and neither Rho GTPase affects ATM activation, on its own. Consistent with a role in controlling ATM activation, Net1A knockdown also impairs DNA repair and cell survival. Taken together these data indicate that Net1A plays a plays a previously unrecognized, Rho GTPase-independent role in controlling ATM activity and downstream signaling after DNA damage to impact cell survival.

Controlling the switches: Rho GTPase regulation during animal cell mitosis

Animal cell division is a fundamental process that requires complex changes in cytoskeletal organization and function. Aberrant cell division often has disastrous consequences for the cell and can lead to cell senescence, neoplastic transformation or death. As important regulators of the actin cytoskeleton, Rho GTPases play major roles in regulating many aspects of mitosis and cytokinesis. These include centrosome duplication and separation, generation of cortical rigidity, microtubule-kinetochore stabilization, cleavage furrow formation, contractile ring formation and constriction, and abscission. The ability of Rho proteins to function as regulators of cell division depends on their ability to cycle between their active, GTP-bound and inactive, GDP-bound states. However, Rho proteins are inherently inefficient at fulfilling this cycle and require the actions of regulatory proteins that enhance GTP binding (RhoGEFs), stimulate GTPase activity (RhoGAPs), and sequester inactive Rho proteins in the cytosol (RhoGDIs). The roles of these regulatory proteins in controlling cell division are an area of active investigation. In this review we will delineate the current state of knowledge of how specific RhoGEFs, RhoGAPs and RhoGDIs control mitosis and cytokinesis, and highlight the mechanisms by which their functions are controlled.