Multiple levels of regulation specify the polarity of an asymmetric cell division in C. elegans

Wnt signaling systems play important roles in the generation of cell and tissue polarity during development. We describe a Wnt signaling system that acts in a new way to orient the polarity of an epidermal cell division in C. elegans. In this system, the EGL-20/Wnt signal acts in a permissive fashion to polarize the asymmetric division of a cell called V5. EGL-20 regulates this polarization by counteracting lateral signals from neighboring cells that would otherwise reverse the polarity of the V5 cell division. Our findings indicate that this lateral signaling pathway also involves Wnt pathway components. Overexpression of EGL-20 disrupts both the asymmetry and polarity of lateral epidermal cell divisions all along the anteroposterior (A/P) body axis. Together our findings suggest that multiple, inter-related Wnt signaling systems may act together to polarize asymmetric cell divisions in this tissue.     

Environmental RNA interference

The discovery of RNA interference (RNAi), the process of sequence-specific gene silencing initiated by double-stranded RNA (dsRNA), has broadened our understanding of gene regulation and has revolutionized methods for genetic analysis. A remarkable property of RNAi in the nematode Caenorhabditis elegans and in some other multicellular organisms is its systemic nature: silencing signals can cross cellular boundaries and spread between cells and tissues. Furthermore, C. elegans and some other organisms can also perform environmental RNAi: sequence-specific gene silencing in response to environmentally encountered dsRNA. This phenomenon has facilitated significant technological advances in diverse fields including functional genomics and agricultural pest control. Here, we describe the characterization and current understanding of environmental RNAi and discuss its potential applications.     

Noninvasive Identification of Viable Cell Populations in Docetaxel-Treated Breast Tumors Using Ferritin-Based Magnetic Resonance Imaging

Background

Cancer stem cells (CSCs) are highly tumorigenic and are responsible for tumor progression and chemoresistance. Noninvasive imaging methods for the visualization of CSC populations within tumors in vivo will have a considerable impact on the development of new CSC-targeting therapeutics.

Methodology/Principal Findings

In this study, human breast cancer stem cells (BCSCs) transduced with dual reporter genes (human ferritin heavy chain [FTH] and enhanced green fluorescence protein [EGFP]) were transplanted into NOD/SCID mice to allow noninvasive tracking of BCSC-derived populations. No changes in the properties of the BCSCs were observed due to ferritin overexpression. Magnetic resonance imaging (MRI) revealed significantly different signal intensities (R₂* values) between BCSCs and FTH-BCSCs in vitro and in vivo. In addition, distinct populations of pixels with high R₂* values were detected in docetaxel-treated FTH-BCSC tumors compared with control tumors, even before the tumor sizes changed. Histological analysis revealed that areas showing high R₂* values in docetaxel-treated FTH-BCSC tumors by MRI contained EGFP+/FTH+ viable cell populations with high percentages of CD44+/CD24− cells.

Conclusions/Significance

These findings suggest that ferritin-based MRI, which provides high spatial resolution and tissue contrast, can be used as a reliable method to identify viable cell populations derived from BCSCs after chemotherapy and may serve as a new tool to monitor the efficacy of CSC-targeting therapies in vivo.     

A Wnt signaling system that specifies two patterns of cell migration in C. elegans

In C. elegans, a bilateral pair of neuroblasts, QL and QR, give rise to cells that migrate in opposite directions along the anteroposterior (A/P) body axis. QL and its descendants migrate posteriorly whereas QR and its descendants migrate anteriorly. We find that a Wnt family member, EGL-20, acts in a dose-dependent manner to specify these opposite migratory behaviors. High levels of EGL-20 promote posterior migration by activating a canonical Wnt signal transduction pathway, whereas low levels promote anterior migration by activating a separate, undefined pathway. We find that the two Q cells respond differently to EGL-20 because they have different response thresholds. Thus, in this system two distinct dose-dependent responses are specified not by graded levels of the Wnt signal, but instead by left-right asymmetrical differences in the cellular responsiveness to Wnt signaling.     

Identification of genes that regulate a left-right asymmetric neuronal migration in Caenorhabditis elegans

In C. elegans, cells of the QL and QR neuroblast lineages migrate with left-right asymmetry; QL and its descendants migrate posteriorly whereas QR and its descendants migrate anteriorly. One key step in generating this asymmetry is the expression of the Hox gene mab-5 in the QL descendants but not in the QR descendants. This asymmetry appears to be coupled to the asymmetric polarizations and movements of QL and QR as they migrate and relies on an asymmetric response to an EGL-20/Wnt signal. To identify genes involved in these complex layers of regulation and to isolate targets of mab-5 that direct posterior migrations, we screened visually for mutants with cell migration defects in the QL and QR lineages. Here, we describe a set of new mutants (qid-5, qid-6, qid-7, and qid-8) that primarily disrupt the migrations of the QL descendants. Most of these mutants were defective in mab-5 expression in the QL lineage and might identify genes that interact directly or indirectly with the EGL-20/Wnt signaling pathway.