Reduced PaxillinB localization to cell-substrate adhesions promotes cell migration in Dictyostelium

Many cells adhere to extracellular matrix for efficient cell migration. This adhesion is mediated by focal adhesions, a protein complex linking the extracellular matrix to the intracellular cytoskeleton. Focal adhesions have been studied extensively in mesenchymal cells, but recent research in physiological contexts and amoeboid cells suggest focal adhesion regulation differs from the mesenchymal focal adhesion paradigm. We used Dictyostelium discoideum to uncover new mechanisms of focal adhesion regulation, as Dictyostelium are amoeboid cells that form focal adhesion-like structures for migration. We show that PaxillinB, the Dictyostelium homologue of Paxillin, localizes to dynamic focal adhesion-like structures during Dictyostelium migration. Unexpectedly, reduced PaxillinB recruitment to these structures increases Dictyostelium cell migration. Quantitative analysis of focal adhesion size and dynamics show that lack of PaxillinB recruitment to focal adhesions does not alter focal adhesion size, but rather increases focal adhesion turnover. These findings are in direct contrast to Paxillin function at focal adhesions during mesenchymal migration, challenging the established focal adhesion model.

Transferred mitochondria accumulate reactive oxygen species, promoting proliferation

Recent studies reveal that lateral mitochondrial transfer, the movement of mitochondria from one cell to another, can affect cellular and tissue homeostasis. Most of what we know about mitochondrial transfer stems from bulk cell studies and have led to the paradigm that functional transferred mitochondria restore bioenergetics and revitalize cellular functions to recipient cells with damaged or non-functional mitochondrial networks. However, we show that mitochondrial transfer also occurs between cells with functioning endogenous mitochondrial networks, but the mechanisms underlying how transferred mitochondria can promote such sustained behavioral reprogramming remain unclear. We report that unexpectedly, transferred macrophage mitochondria are dysfunctional and accumulate reactive oxygen species in recipient cancer cells. We further discovered that reactive oxygen species accumulation activates ERK signaling, promoting cancer cell proliferation. Pro-tumorigenic macrophages exhibit fragmented mitochondrial networks, leading to higher rates of mitochondrial transfer to cancer cells. Finally, we observe that macrophage mitochondrial transfer promotes tumor cell proliferation in vivo. Collectively these results indicate that transferred macrophage mitochondria activate downstream signaling pathways in a ROS-dependent manner in cancer cells, and provide a model of how sustained behavioral reprogramming can be mediated by a relatively small amount of transferred mitochondria in vitro and in vivo.

Lack of Paxillin phosphorylation promotes single-cell migration in vivo

Focal adhesions are structures that physically link the cell to the extracellular matrix for cell migration. Although cell culture studies have provided a wealth of information regarding focal adhesion biology, it is critical to understand how focal adhesions are dynamically regulated in their native environment. We developed a zebrafish system to visualize focal adhesion structures during single-cell migration in vivo. We find that a key site of phosphoregulation (Y118) on Paxillin exhibits reduced phosphorylation in migrating cells in vivo compared to in vitro. Furthermore, expression of a non-phosphorylatable version of Y118-Paxillin increases focal adhesion disassembly and promotes cell migration in vivo, despite inhibiting cell migration in vitro. Using a mouse model, we further find that the upstream kinase, focal adhesion kinase, is downregulated in cells in vivo, and cells expressing non-phosphorylatable Y118-Paxillin exhibit increased activation of the CRKII-DOCK180/RacGEF pathway. Our findings provide significant new insight into the intrinsic regulation of focal adhesions in cells migrating in their native environment.

An evolutionary and physiological perspective on cell-substrate adhesion machinery for cell migration

Cell-substrate adhesion is a critical aspect of many forms of cell migration. Cell adhesion to an extracellular matrix (ECM) generates traction forces necessary for efficient migration. One of the most well-studied structures cells use to adhere to the ECM is focal adhesions, which are composed of a multilayered protein complex physically linking the ECM to the intracellular actin cytoskeleton. Much of our understanding of focal adhesions, however, is primarily derived from in vitro studies in Metazoan systems. Though these studies provide a valuable foundation to the cell-substrate adhesion field, the evolution of cell-substrate adhesion machinery across evolutionary space and the role of focal adhesions in vivo are largely understudied within the field. Furthering investigation in these areas is necessary to bolster our understanding of the role cell-substrate adhesion machinery across Eukaryotes plays during cell migration in physiological contexts such as cancer and pathogenesis. In this review, we review studies of cell-substrate adhesion machinery in organisms evolutionary distant from Metazoa and cover the current understanding and ongoing work on how focal adhesions function in single and collective cell migration in an in vivo environment, with an emphasis on work that directly visualizes cell-substrate adhesions. Finally, we discuss nuances that ought to be considered moving forward and the importance of future investigation in these emerging fields for application in other fields pertinent to adhesion-based processes.

Genetic Modification of Primary Human Myeloid Cells to Study Cell Migration, Activation, and Organelle Dynamics

Myeloid dendritic cells (DCs) and macrophages are mononuclear phagocytes with key roles in the immune system. As antigen-presenting cells, they link innate detection of microbes with programming adaptive immune responses. Myeloid DCs and macrophages also play critical roles in development, promote tissue homeostasis, and direct repair in response to injury and inflammation. As cellular migration and organelle dynamics are intimately connected with these processes, it is necessary to develop tools to track myeloid cell behavior and function. Here, we build on previously established protocols to isolate primary human myeloid cells from peripheral blood and report an optimized method for their genetic modification with lentiviral vectors to study processes related to cell migration, activation, and organelle dynamics. Specifically, we provide a protocol for delivering genetically encoded fluorescent markers into primary monocyte-derived DCs (MDDCs) and monocyte-derived macrophages (MDMs) to label mitochondria, peroxisomes, and whole cells. We describe the isolation of primary CD14+ monocytes from peripheral blood using positive selection with magnetic beads and, alternatively, isolation based on plastic adherence. Isolated CD14+ cells can be transduced with lentiviral vectors and subsequently cultured in the presence of cytokines to derive MDDCs or MDMs. This protocol is highly adaptable for cotransduction with vectors to knock down or overexpress genes of interest. These tools enable mechanistic studies of genetically modified myeloid cells through flow cytometry, fluorescence microscopy, and other downstream assays. © 2022 Wiley Periodicals LLC. Basic Protocol: Transduction of MDDCs and MDMs with lentiviral vectors encoding fluorescent markers Alternate Protocol 1: Isolation of monocytes by plastic adhesion Alternate Protocol 2: Transduction of MDDCs and MDMs with lentiviral vectors to knock down or overexpress genes of interest Support Protocol 1: Production and purification of lentiviral vectors for transduction into primary human myeloid cells Support Protocol 2: Flow cytometry of MDDCs and MDMs Support Protocol 3: Fixed and live-cell imaging of fluorescent markers in MDMs and MDDCs.

The biochemical basis of mitochondrial dysfunction in Zellweger Spectrum Disorder

Peroxisomal biogenesis disorders (PBDs) are genetic disorders of peroxisome biogenesis and metabolism that are characterized by profound developmental and neurological phenotypes. The most severe class of PBDs-Zellweger spectrum disorder (ZSD)-is caused by mutations in peroxin genes that result in both non-functional peroxisomes and mitochondrial dysfunction. It is unclear, however, how defective peroxisomes contribute to mitochondrial impairment. In order to understand the molecular basis of this inter-organellar relationship, we investigated the fate of peroxisomal mRNAs and proteins in ZSD model systems. We found that peroxins were still expressed and a subset of them accumulated on the mitochondrial membrane, which resulted in gross mitochondrial abnormalities and impaired mitochondrial metabolic function. We showed that overexpression of ATAD1, a mitochondrial quality control factor, was sufficient to rescue several aspects of mitochondrial function in human ZSD fibroblasts. Together, these data suggest that aberrant peroxisomal protein localization is necessary and sufficient for the devastating mitochondrial morphological and metabolic phenotypes in ZSDs.

Sharing Is Caring

It is well known that melanoma lipid metabolism is regulated by intrinsic genetic and epigenetic changes in the cancer cells. However, recent work reveals that lipid metabolism is also controlled by the tumor microenvironment through the direct transfer of lipids from adipocytes to melanoma cells.

Cell fusion potentiates tumor heterogeneity and reveals circulating hybrid cells that correlate with stage and survival

High lethality rates associated with metastatic cancer highlight an urgent medical need for improved understanding of biologic mechanisms driving metastatic spread and identification of biomarkers predicting late-stage progression. Numerous neoplastic cell intrinsic and extrinsic mechanisms fuel tumor progression; however, mechanisms driving heterogeneity of neoplastic cells in solid tumors remain obscure. Increased mutational rates of neoplastic cells in stressed environments are implicated but cannot explain all aspects of tumor heterogeneity. We present evidence that fusion of neoplastic cells with leukocytes (for example, macrophages) contributes to tumor heterogeneity, resulting in cells exhibiting increased metastatic behavior. Fusion hybrids (cells harboring hematopoietic and epithelial properties) are readily detectible in cell culture and tumor-bearing mice. Further, hybrids enumerated in peripheral blood of human cancer patients correlate with disease stage and predict overall survival. This unique population of neoplastic cells provides a novel biomarker for tumor staging, as well as a potential therapeutic target for intervention.

Competition between TIAM1 and Membranes Balances Endophilin A3 Activity in Cancer Metastasis

Normal cells acquire aggressive behavior by modifying signaling pathways. For instance, alteration of endocytosis profoundly impacts both proliferation and migration during tumorigenesis. Here we investigate the mechanisms that enable the endocytic machinery to coordinate these processes. We show that a membrane curvature-sensing protein, endophilin A3, promotes growth and migration of colon cancer cells through two competing mechanisms: an endocytosis pathway that is required for proliferation and a GTPase regulatory pathway that controls cell motility. EndoA3 stimulates cell migration by binding the Rac GEF TIAM1 leading to activation of small GTPases. Competing interactions of EndoA3 with membrane versus TIAM1 modulate hyperproliferative and metastatic phenotypes. Disruption of EndoA3-membrane interactions stimulates TIAM1 and small GTPases in vitro, and further promotes pro-metastatic phenotypes in vivo. Together, these results uncover a coupling mechanism, by which EndoA3 promotes growth and migration of colon cancers, by linking membrane dynamics to GTPase regulation.

Macrophage-Dependent Cytoplasmic Transfer during Melanoma Invasion In Vivo

Interactions between tumor cells and tumor-associated macrophages play critical roles in the initiation of tumor cell motility. To capture the cellular interactions of the tumor microenvironment with high-resolution imaging, we directly visualized tumor cells and their interactions with macrophages in zebrafish. Live imaging in zebrafish revealed that macrophages are dynamic, yet maintain sustained contact with tumor cells. In addition, the recruitment of macrophages to tumor cells promotes tumor cell dissemination. Using a Cre/LoxP strategy, we found that macrophages transfer cytoplasm to tumor cells in zebrafish and mouse models. Remarkably, macrophage cytoplasmic transfer correlated with melanoma cell dissemination. We further found that macrophages transfer cytoplasm to tumor cells upon cell contact in vitro. Thus, we present a model in which macrophage/tumor cell contact allows for the transfer of cytoplasmic molecules from macrophages to tumor cells corresponding to increased tumor cell motility and dissemination.

Macrophage-dependent tumor cell transendothelial migration is mediated by Notch1/MenaINV-initiated invadopodium formation

The process of intravasation involving transendothelial migration is a key step in metastatic spread. How the triple cell complex composed of a macrophage, Mena over-expressing tumor cell and endothelial cell, called the tumor microenvironment of metastasis (TMEM), facilitates tumor cell transendothelial migration is not completely understood. Previous work has shown that the physical contact between a macrophage and tumor cell results in the formation of invadopodia, actin-rich matrix degrading protrusions, important for tumor cell invasion and transendothelial migration and tumor cell dissemination. Herein, we show that the macrophage-induced invadopodium is formed through a Notch1/Mena^INV signaling pathway in the tumor cell upon macrophage contact. This heterotypic tumor cell – macrophage interaction results in the upregulation of Mena^INV through the activation of MENA transcription. Notch1 and Mena^INV expression are required for tumor cell transendothelial migration, a necessary step during intravasation. Inhibition of the Notch signaling pathway blocked macrophage-induced invadopodium formation in vitro and the dissemination of tumor cells from the primary tumor in vivo. Our findings indicate a novel role for Notch1 signaling in the regulation of Mena^INV expression and transendothelial migration and provide mechanistic information essential to the use of therapeutic inhibitors of metastasis.

Abstract 4034: Macrophage-dependent activation of a non-canonical NOTCH-RhoA signaling pathway regulates tumor cell intravasation

Intravital imaging of rodent mammary tumors has shown that direct contacts between a Mena expressing tumor cell, a perivascular macrophage and an endothelial cell forms a microanatomical structure named TMEM (Tumor Microenvironment of Metastasis). Clinical studies have shown that the number of TMEM is correlated with an increased risk of developing distant metastasis in breast cancer patients. While TMEM is an excellent prognostic marker for predicting metastasis the mechanisms of TMEM assembly and function are not understood. Recently, we showed that heterotypic cell contact between tumor cells and macrophages induces the formation of invadopodia in tumor cells, invasive structures necessary for matrix degradation and required for tumor cell intravasation. Using high resolution FRET imaging, we further found that macrophage-induced invadopodium formation is dependent on RhoA activation in the tumor cell. However, what remained to be determined was the signaling pathway that regulated this heterotypic cell contact-mediated phenomenon. We explored the role of NOTCH, a known receptor involved in heterotypic cell contact signaling, in stimulating macrophage-dependent tumor cell invadopodium formation and transendothelial migration. We found that, in the absence of Notch signaling, macrophage-induced invadopodium formation and tumor cell intravasation are abolished. Moreover, RhoA is no longer activated in tumor cells upon macrophage contact when Notch signaling is perturbed. These results show that Notch signaling regulates heterotypic cell contact mediated invadopodium formation through RhoA activation, and reveals a novel non-canonical NOTCH/RhoA pathway as a molecular target to prevent TMEM function and therefore metastasis.

Citation Format: Jose Javier Bravo Cordero, Minna Roh-Johnson, Jeanine Pignatelli, Maja Oktay, Louis Hodgson, John Condeelis. Macrophage-dependent activation of a non-canonical NOTCH-RhoA signaling pathway regulates tumor cell intravasation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4034. doi:10.1158/1538-7445.AM2015-4034

Abstract 3593: Examining the role of tumor cell secreted factors in intravasation and enhancing paracrine loop invasion

The interaction of breast cancer cells with other cells in the tumor microenvironment plays an important role in metastasis. Invasion and intravasation, two critical steps in the metastatic process, are thought to be influenced by these interactions in addition to their intrinsic invasive properties. Macrophages are of particular interest when it comes to studying tumor cell invasiveness. Previous studies have shown that there is paracrine loop signaling between breast cancer cells and macrophages in which colony stimulating factor 1 (CSF-1) produced by tumor cells stimulates epidermal growth factor (EGF) production by macrophages. This EGF, in turn, stimulates invasion of the tumor cells. In addition to the classically described paracrine loop, it has been seen that tumor cell expression of ErbB3, a member of the EGFR family of receptor tyrosine kinases, may play a role in facilitating tumor cell intravasation. Overexpression of ErbB3 in breast cancer patients correlates with decreased survival. We show that using a blocking antibody against ErbB3 results in a significant decrease in macrophage-induced transendothelial migration of breast cancer cells. Tumor cell secretion of Neuregulin 1 (NRG1), a ligand of the ErbB3 receptor, may also play a role in the tumor cell- macrophage paracrine interaction. We show that stimulation of macrophages with NRG1 upregulates mRNA expression of JAG1, a ligand of the Notch receptor. Activation of the Notch receptor pathway has been shown to be involved in the process of tumor cell invasion. Overall, our studies look to examine this interaction between tumor cells and macrophages and these observations indicate that ErbB3, NRG1, and JAG1 could serve as novel targets in metastasis and the tumor microenvironment.

Citation Format: Ramon M. Cabrera, Zhen N. Zhou, Minna Roh-Johnson, John Condeelis, Dianne Cox, Jeffrey E. Segall. Examining the role of tumor cell secreted factors in intravasation and enhancing paracrine loop invasion. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3593. doi:10.1158/1538-7445.AM2014-3593

Macrophage contact induces RhoA GTPase signaling to trigger tumor cell intravasation

Most cancer patients die as a result of metastasis, thus it is important to understand the molecular mechanisms of dissemination, including intra- and extravasation. Although the mechanisms of extravasation have been vastly studied in vitro and in vivo, the process of intravasation is still unclear. Furthermore, how cells in the tumor microenvironment facilitate tumor cell intravasation is still unknown. Using high-resolution imaging, we found that macrophages enhance tumor cell intravasation upon physical contact. Macrophage and tumor cell contact induce RhoA activity in tumor cells, triggering the formation of actin-rich degradative protrusions called invadopodia, enabling tumor cells to degrade and break through matrix barriers during tumor cell transendothelial migration. Interestingly, we show that macrophage-induced invadopodium formation and tumor cell intravasation also occur in patient-derived tumor cells and in vivo models, revealing a conserved mechanism of tumor cell intravasation. Our results illustrate a novel heterotypic cell contact mediated signaling role for RhoA, as well as yield mechanistic insight into the ability of cells within the tumor microenvironment to facilitate steps of the metastatic cascade.

Autocrine HBEGF expression promotes breast cancer intravasation, metastasis and macrophage-independent invasion in vivo

Increased expression of HBEGF in ER negative breast tumors is correlated with enhanced metastasis to distant organ sites and more rapid disease recurrence upon removal of the primary tumor. Our previous work has demonstrated a paracrine loop between breast cancer cells and macrophages in which the tumor cells are capable of stimulating macrophages through the secretion of CSF-1 while the tumor associated macrophages (TAMs) in turn aid in tumor cell invasion by secreting EGF. To determine how the autocrine expression of EGFR ligands by carcinoma cells would affect this paracrine loop mechanism, and in particular whether tumor cell invasion depends on spatial ligand gradients generated by TAMs, we generated cell lines with increased HBEGF expression. We find that autocrine HBEGF expression enhanced in vivo intravasation and metastasis, and resulted in a novel phenomenon in which macrophages were no longer required for in vivo invasion of breast cancer cells. In vitro studies revealed that expression of HBEGF enhanced invadopodium formation, thus providing a mechanism for cell autonomous invasion. The increased invadopodium formation was directly dependent on EGFR signaling, as demonstrated by a rapid decrease in invadopodia upon inhibition of autocrine HBEGF/EGFR signaling as well as inhibition of signaling downstream of EGFR activation. HBEGF expression also resulted in enhanced invadopodium function via upregulation of MMP2 and MMP9 expression. We conclude that high levels of HBEGF expression can short-circuit the tumor cell/macrophage paracrine invasion loop, resulting in enhanced tumor invasion that is independent of macrophage signaling.

Spatial regulation of RhoC activity defines protrusion formation in migrating cells

Protrusion formation is the first step that precedes cell movement of motile cells. Spatial control of actin polymerization is necessary to achieve directional protrusion during cell migration. Here we show that the spatial coordinators p190RhoGEF and p190RhoGAP regulate actin polymerization during leading edge protrusions by regulating the actin barbed end distribution and amplitude. The distribution of RhoC activity and proper balance of cofilin activation achieved by p190RhoGEF and p190RhoGAP determines the direction of final protrusive activity. These findings provide a new insight into the dynamic plasticity in the amplitude and distribution of barbed ends, which can be modulated by fine-tuning RhoC activity by upstream GEFs and GAPs for directed cell motility.

Abstract LB-274: MenaINV expression is associated with discohesion, intravasation and possibly metastasis in human invasive ductal carcinoma of the breast

Metastasis is a multistep process that leads to cancer cell dissemination and patient demise. Current technologies for identification of patients at risk for metastatic disease and the treatments for patients with distant metastases are unsatisfactory. Since metastasis is in part a motility disease, development of cell motility assays will complement current cell proliferation-based gene expression profiling assays and may improve breast cancer prognostication. The Mena family of actin binding proteins functions at the convergence of actin polymerization, directional cell movement towards EGF, and invadopodium formation pathways and plays a pivotal role in regulating cell motility. In particular, increased expression of Mena isoform, MenaINV, combined with the decreased expression of the Mena11a isoform decreases breast cancer cell cohesion and enhances cancer cell invasion and dissemination in rodent models. Tumor cells with this Mena expression pattern participate with macrophages in mouse mammary carcinoma invasion and intravasation. Intravasation sites, consisting of Mena expressing tumor cells in association with perivascular macrophages, called Tumor Microenvironment of Metastasis (TMEM), have also been found in human breast cancer and their number positively correlates with metastatic outcome in patients.

In a cohort of 100 cases of human invasive ductal carcinoma of the breast, we demonstrated that MenaINV expression levels in breast cancer cells obtained by fine needle aspiration (FNA) biopsy positively correlated with the number of TMEMs in a pre-specified field in the matching tumor tissue, while Mena11a levels showed a weak negative correlation. This suggests that MenaINV is the isoform associated with breast cancer cell invasion, intravasation and possibly metastasis in humans. The MenaINV/TMEM correlation is particularly strong in the ERPR+/Her2- cancers, implying that the MenaINV/ macrophage dependent mechanism of intravasation is particularly important in this clinical subtype. In addition, only in the ERPR+/Her2- cases, high MenaINV expression levels and TMEM scores are associated with lymph node metastasis. Furthermore, using the in vitro intravasation assay, we showed that intravasation-competent breast cancer cells obtained from patients by FNA preferentially express the MenaINV isoform. We also showed that human invasive ductal carcinomas of the breast with high MenaINV expression levels and high TMEM score express less E-cadherin, which may render these cells less cohesive and thus more motile and prone to invasion, intravasation, and metastasis.

Our data indicate that MenaINV is the isoform of Mena associated with cell discohesion, intravasation competency and potentially metastatic outcome in human breast carcinoma. Thus, MenaINV has the potential to become both a prognostic marker and a target for therapy.

Citation Format: Maja Oktay, Sumanta Goswami, Minna Roh-Johnson, Sara Maleki, Shannon Hughes-Alford, Thomas Rohan, Frank Gertler, Joan Jones, John Condeelis. MenaINV expression is associated with discohesion, intravasation and possibly metastasis in human invasive ductal carcinoma of the breast. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-274. doi:10.1158/1538-7445.AM2013-LB-274

Triggering a cell shape change by exploiting preexisting actomyosin contractions

Apical constriction changes cell shapes, driving critical morphogenetic events, including gastrulation in diverse organisms and neural tube closure in vertebrates. Apical constriction is thought to be triggered by contraction of apical actomyosin networks. We found that apical actomyosin contractions began before cell shape changes in both Caenorhabitis elegans and Drosophila. In C. elegans, actomyosin networks were initially dynamic, contracting and generating cortical tension without substantial shrinking of apical surfaces. Apical cell-cell contact zones and actomyosin only later moved increasingly in concert, with no detectable change in actomyosin dynamics or cortical tension. Thus, apical constriction appears to be triggered not by a change in cortical tension, but by dynamic linking of apical cell-cell contact zones to an already contractile apical cortex.

Dynamic localization of C. elegans TPR-GoLoco proteins mediates mitotic spindle orientation by extrinsic signaling

Cell divisions are sometimes oriented by extrinsic signals, by mechanisms that are poorly understood. Proteins containing TPR and GoLoco-domains (C. elegans GPR-1/2, Drosophila Pins, vertebrate LGN and AGS3) are candidates for mediating mitotic spindle orientation by extrinsic signals, but the mechanisms by which TPR-GoLoco proteins may localize in response to extrinsic cues are not well defined. The C. elegans TPR-GoLoco protein pair GPR-1/2 is enriched at a site of contact between two cells - the endomesodermal precursor EMS and the germline precursor P(2) - and both cells align their divisions toward this shared cell-cell contact. To determine whether GPR-1/2 is enriched at this site within both cells, we generated mosaic embryos with GPR-1/2 bearing a different fluorescent tag in different cells. We were surprised to find that GPR-1/2 distribution is symmetric in EMS, where GPR-1/2 had been proposed to function as an asymmetric cue for spindle orientation. Instead, GPR-1/2 is asymmetrically distributed only in P(2). We demonstrate a role for normal GPR-1/2 localization in P(2) division orientation. We show that MES-1/Src signaling plays an instructive role in P(2) for asymmetric GPR-1/2 localization and normal spindle orientation. We ruled out a model in which signaling localizes GPR-1/2 by locally inhibiting LET-99, a GPR-1/2 antagonist. Instead, asymmetric GPR-1/2 distribution is established by destabilization at one cell contact, diffusion, and trapping at another cell contact. Once the mitotic spindle of P(2) is oriented normally, microtubule-dependent removal of GPR-1/2 prevented excess accumulation, in an apparent negative-feedback loop. These results highlight the role of dynamic TPR-GoLoco protein localization as a key mediator of mitotic spindle alignment in response to instructive, external cues.

In vivo roles for Arp2/3 in cortical actin organization during C. elegans gastrulation

The Arp2/3 complex is important for morphogenesis in various developmental systems, but specific in vivo roles for this complex in cells that move during morphogenesis are not well understood. We have examined cellular roles for Arp2/3 in the Caenorhabditis elegans embryo. In C. elegans, the first morphogenetic movement, gastrulation, is initiated by the internalization of two endodermal precursor cells. These cells undergo a myosin-dependent apical constriction, pulling a ring of six neighboring cells into a gap left behind on the ventral surface of the embryo. In agreement with a previous report, we found that in Arp2/3-depleted C. elegans embryos, membrane blebs form and the endodermal precursor cells fail to fully internalize. We show that these cells are normal with respect to several key requirements for gastrulation: cell cycle timing, cell fate, apicobasal cell polarity and apical accumulation and activation of myosin-II. To further understand the function of Arp2/3 in gastrulation, we examined F-actin dynamics in wild-type embryos. We found that three of the six neighboring cells extend short, dynamic F-actin-rich processes at their apical borders with the internalizing cells. These processes failed to form in embryos that were depleted of Arp2/3 or the apical protein PAR-3. Our results identify an in vivo role for Arp2/3 in the formation of subcellular structures during morphogenesis. The results also suggest a new layer to the model of C. elegans gastrulation: in addition to apical constriction, internalization of the endoderm might involve dynamic Arp2/3-dependent F-actin-rich extensions on one side of a ring of cells.

Apical constriction: a cell shape change that can drive morphogenesis

Biologists have long recognized that dramatic bending of a cell sheet may be driven by even modest shrinking of the apical sides of cells. Cell shape changes and tissue movements like these are at the core of many of the morphogenetic movements that shape animal form during development, driving processes such as gastrulation, tube formation, and neurulation. The mechanisms of such cell shape changes must integrate developmental patterning information in order to spatially and temporally control force production-issues that touch on fundamental aspects of both cell and developmental biology and on birth defects research. How does developmental patterning regulate force-producing mechanisms, and what roles do such mechanisms play in development? Work on apical constriction from multiple systems including Drosophila, Caenorhabditis elegans, sea urchin, Xenopus, chick, and mouse has begun to illuminate these issues. Here, we review this effort to explore the diversity of mechanisms of apical constriction, the diversity of roles that apical constriction plays in development, and the common themes that emerge from comparing systems.