Lipid raft redistribution and morphological cell polarization are separable processes providing a basis for hematopoietic stem and progenitor cell migration

Analysis of cortical cell polarity by imaging flow cytometry

Metastasis is the main cause of cancer-related death and therapies specifically targeting metastasis are highly needed. Cortical cell polarity (CCP) is a prometastatic property of circulating tumor cells affecting their ability to exit blood vessels and form new metastases that constitute a promising point of attack to prevent metastasis. However, conventional fluorescence microscopy on single cells and manual quantification of CCP are time-consuming and unsuitable for screening regulators. In this study, we developed an imaging flow cytometry-based method for high-throughput screening of factors affecting CCP in melanoma cells. The artificial intelligence-supported analysis method we developed is highly reproducible, accurate, and orders of magnitude faster than manual quantification. Additionally, this method is flexible and can be adapted to include additional cellular parameters. In a small-scale pilot experiment using polarity-, cytoskeleton-, or membrane-affecting drugs, we demonstrate that our workflow provides a straightforward and efficient approach for screening factors affecting CCP in cells in suspension and provide insights into the specific function of these drugs in this cellular system. The method and workflow presented here will facilitate large-scale studies to reveal novel cell-intrinsic as well as systemic factors controlling CCP during metastasis.

Hematopoietic progenitors polarize in contact with bone marrow stromal cells in response to SDF1

The fate of hematopoietic stem and progenitor cells (HSPCs) is regulated by their interaction with stromal cells in the bone marrow. However, the cellular mechanisms regulating HSPC interaction with these cells and their potential impact on HSPC polarity are still poorly understood. Here we evaluated the impact of cell–cell contacts with osteoblasts or endothelial cells on the polarity of HSPC. We found that an HSPC can form a discrete contact site that leads to the extensive polarization of its cytoskeleton architecture. Notably, the centrosome was located in proximity to the contact site. The capacity of HSPCs to polarize in contact with stromal cells of the bone marrow appeared to be specific, as it was not observed in primary lymphoid or myeloid cells or in HSPCs in contact with skin fibroblasts. The receptors ICAM, VCAM, and SDF1 were identified in the polarizing contact. Only SDF1 was independently capable of inducing the polarization of the centrosome–microtubule network.

Flotillins: At the Intersection of Protein S-Palmitoylation and Lipid-Mediated Signaling

Flotillin-1 and flotillin-2 are ubiquitously expressed, membrane-associated proteins involved in multifarious cellular events from cell signaling, endocytosis, and protein trafficking to gene expression. They also contribute to oncogenic signaling. Flotillins bind the cytosolic leaflet of the plasma membrane and endomembranes and, upon hetero-oligomerization, serve as scaffolds facilitating the assembly of multiprotein complexes at the membrane-cytosol interface. Additional functions unique to flotillin-1 have been discovered recently. The membrane-binding of flotillins is regulated by S-palmitoylation and N-myristoylation, hydrophobic interactions involving specific regions of the polypeptide chain and, to some extent, also by their oligomerization. All these factors endow flotillins with an ability to associate with the sphingolipid/cholesterol-rich plasma membrane domains called rafts. In this review, we focus on the critical input of lipids to the regulation of the flotillin association with rafts and thereby to their functioning. In particular, we discuss how the recent developments in the field of protein S-palmitoylation have contributed to the understanding of flotillin1/2-mediated processes, including endocytosis, and of those dependent exclusively on flotillin-1. We also emphasize that flotillins affect directly or indirectly the cellular levels of lipids involved in diverse signaling cascades, including sphingosine-1-phosphate and PI(4,5)P₂. The mutual relations between flotillins and distinct lipids are key to the regulation of their involvement in numerous cellular processes.

CpG stimulation of chronic lymphocytic leukemia cells induces a polarized cell shape and promotes migration in vitro and in vivo

In order to accomplish their physiological functions leukocytes have the capability to migrate. As a prerequisite they need to adopt a polarized cell shape, forming a leading edge at the front and a uropod at rear pole. In this study we explored the capability of chronic lymphocytic leukaemia (CLL) cells to adopt this leukocyte-specific migration phenotype. Furthermore, we studied the impact of the Toll-like receptor 9 (TLR9) agonists CpGs type A, B and C and the antagonist oligodesoxynucleotide (ODN) INH-18 on the cell polarization and migration process of primary human CLL cells. Upon cultivation, a portion of purified CLL cells adopted polarized cell shapes spontaneously (range 10–38%). Stimulation with CpG ODNs type B (ODN 2006) and CpGs type C (ODN 2395) significantly increased the frequency of morphologically polarized CLL cells, while ODN INH-18 was hardly able to act antagonistically. Like in human hematopoietic stem and progenitor cells, in morphologically polarized CLL cells CXCR4 was redistributed to the leading edge and CD50 to the uropod. Coupled to the increased frequencies of morphologically polarized cells, CpGs type B and C stimulated CLL cells showed higher migration activities in vitro and following intravenous injection higher homing frequencies to the bone marrow of immunocompromised NOD.Cg-Prkdc^scidIl2rg^tm1Wjl/SzJ (NSG) mice. Thus, presumably independent of TLR-9 signaling, CpGs type B and C promote the cellular polarization process of CLL cells and their ability to migrate in vitro and in vivo.

CXCL12/CXCR4 activation by cancer-associated fibroblasts promotes integrin β1 clustering and invasiveness in gastric cancer

Cancer-associated fibroblasts (CAFs) are reportedly involved in invasion and metastasis in several types of cancer, including gastric cancer (GC), through the stimulation of CXCL12/CXCR4 signaling. However, the mechanisms underlying these tumor-promoting effects are not well understood, which limits the potential to develop therapeutic targets against CAF-mediated CXCL12/CXCR4 signaling. CXCL12 expression was analyzed in resected GC tissues from 110 patients by immunohistochemistry (IHC). We established primary cultures of normal fibroblasts (NFs) and CAFs from the GC tissues and examined the functional differences between these primary fibroblasts using co-culture assays with GC cell lines. We evaluated the efficacy of a CXCR4 antagonist (AMD3100) and a FAK inhibitor (PF-573,228) on the invasive ability of GC cells. High CXCL12 expression levels were significantly associated with larger tumor size, increased tumor depth, lymphatic invasion and poor prognosis in GC. CXCL12/CXCR4 activation by CAFs mediated integrin β1 clustering at the cell surface and promoted the invasive ability of GC cells. Notably, AMD3100 was more efficient than PF-573,228 at inhibiting GC cell invasion through the suppression of integrin β1/FAK signaling. These results suggest that CXCL12 derived from CAFs promotes GC cell invasion by enhancing the clustering of integrin β1 in GC cells, resulting in GC progression. Taken together, the inhibition of CXCL12/CXCR4 signaling in GC cells may be a promising therapeutic strategy against GC cell invasion.

Multipotent hematopoietic progenitors divide asymmetrically to create progenitors of the lymphomyeloid and erythromyeloid lineages

Hematopoietic stem and progenitor cells (HSPCs) can self-renew and create committed progenitors, a process supposed to involve asymmetric cell divisions (ACDs). Previously, we had linked the kinetics of CD133 expression with ACDs but failed to detect asymmetric segregation of classical CD133 epitopes on fixed, mitotic HSPCs. Now, by using a novel anti-CD133 antibody (HC7), we confirmed the occurrence of asymmetric CD133 segregation on paraformaldehyde-fixed and living HSPCs. After showing that HC7 binding does not recognizably affect biological features of human HSPCs, we studied ACDs in different HSPC subtypes and determined the developmental potential of arising daughter cells at the single-cell level. Approximately 70% of the HSPCs of the multipotent progenitor (MPP) fraction studied performed ACDs, and about 25% generated lymphoid-primed multipotent progenitor (LMPP) as wells as erythromyeloid progenitor (EMP) daughter cells. Since MPPs hardly created daughter cells maintaining MPP characteristics, our data suggest that under conventional culture conditions, ACDs are lineage instructive rather than self-renewing.

•

The HC7 anti-CD133 antibody allows analyses of ACDs on fixed human HSPCs

•

HC7 and AC133 anti-CD133 antibodies allow tracking of CD133 in living HSPCs

•

Cells of the MPP fraction divide asymmetrically to create LMPP- and EMP-like cells

•

ACDs of MPPs are lineage instructive rather than self-renewing

Endocytic trafficking of membrane-bound cargo: a flotillin point of view

The ubiquitous and highly conserved flotillin proteins, flotillin-1 and flotillin-2, have been shown to be involved in various cellular processes such as cell adhesion, signal transduction through receptor tyrosine kinases as well as in cellular trafficking pathways. Due to the fact that flotillins are acylated and form hetero-oligomers, they constitutively associate with cholesterol-enriched lipid microdomains. In recent years, such microdomains have been appreciated as platforms that participate in endocytosis and other cellular trafficking steps. This review summarizes the current findings on the role of flotillins in membrane-bound cargo endocytosis and endosomal trafficking events. We will discuss the proposed function of flotillins in endocytosis in the light of recent findings that point towards a role for flotillins in a step that precedes the actual endocytic uptake of cargo molecules. Recent findings have also revealed that flotillins may be important for endosomal sorting and recycling of specific cargo molecules. In addition to these aspects, the cellular trafficking pathway of flotillins themselves as potential cargo in the context of growth factor signaling will be discussed.

New relationships of human hematopoietic lineages facilitate detection of multipotent hematopoietic stem and progenitor cells

Three important goals of hematopoietic stem cell research are to understand of how hematopoietic stem cells (HSCs) self-renew, how lineage commitment takes place, and how HSCs can be expanded ex vivo. Research in this area requires a reliable model of hematopoiesis. Performing detailed functional analyses of human hematopoietic progenitor subsets, we recently gained evidence for new hematopoietic lineage relationships. (1) According to our data, neutrophils belong to the same branch of the hematopoietic tree as lymphocytes. In contrast, eosinophils and basophils derive from another branch, the erythro-myeloid branch. Here, after introducing the newly proposed hematopoietic model, we discuss its consequences for the identification and expansion of human multipotent progenitors and suggest a fast and reliable method to screen for multipotent hematopoietic cells in vitro.

Lipid raft regulates the initial spreading of melanoma A375 cells by modulating β1 integrin clustering

Cell adhesion and spreading require integrins-mediated cell-extracellular matrix interaction. Integrins function through binding to extracellular matrix and subsequent clustering to initiate focal adhesion formation and actin cytoskeleton rearrangement. Lipid raft, a liquid ordered plasma membrane microdomain, has been reported to play major roles in membrane motility by regulating cell surface receptor function. Here, we identified that lipid raft integrity was required for β1 integrin-mediated initial spreading of melanoma A375 cells on fibronectin. We found that lipid raft disruption with methyl-β-cyclodextrin led to the inability of focal adhesion formation and actin cytoskeleton rearrangement by preventing β1 integrin clustering. Furthermore, we explored the possible mechanism by which lipid raft regulates β1 integrin clustering and demonstrated that intact lipid raft could recruit and modify some adaptor proteins, such as talin, α-actinin, vinculin, paxillin and FAK. Lipid raft could regulate the location of these proteins in lipid raft fractions and facilitate their binding to β1 integrin, which may be crucial for β1 integrin clustering. We also showed that lipid raft disruption impaired A375 cell migration in both transwell and wound healing models. Together, these findings provide a new insight for the relationship between lipid raft and the regulation of integrins.