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Significance What defines the number and dynamics of the stem cells that generate and renew biological tissues? Although several molecular markers have been described to predict stem cell potential, we propose a complementary approach that mathematically describes “stemness” as an emergent property arising from a stochastic competition for space. We predict from that competition the robust emergence of a region made of functional stem cells, as well as give simple predictions on lineage-survival probability. We test our results with data obtained from intravital live-imaging experiments in mammary gland development, existing data from kidney development, and from the self-renewal of the crypt to show that our framework can predict the number of functional stem cells and lineage-survival probability., Understanding to what extent stem cell potential is a cell-intrinsic property or an emergent behavior coming from global tissue dynamics and geometry is a key outstanding question of systems and stem cell biology. Here, we propose a theory of stem cell dynamics as a stochastic competition for access to a spatially localized niche, giving rise to a stochastic conveyor-belt model. Cell divisions produce a steady cellular stream which advects cells away from the niche, while random rearrangements enable cells away from the niche to be favorably repositioned. Importantly, even when assuming that all cells in a tissue are molecularly equivalent, we predict a common (“universal”) functional dependence of the long-term clonal survival probability on distance from the niche, as well as the emergence of a well-defined number of functional stem cells, dependent only on the rate of random movements vs. mitosis-driven advection. We test the predictions of this theory on datasets of pubertal mammary gland tips and embryonic kidney tips, as well as homeostatic intestinal crypts. Importantly, we find good agreement for the predicted functional dependency of the competition as a function of position, and thus functional stem cell number in each organ. This argues for a key role of positional fluctuations in dictating stem cell number and dynamics, and we discuss the applicability of this theory to other settings.

The intestinal epithelium is a repetitive sheet of crypt and villus units with stem cells at the bottom of the crypts. During postnatal development, crypts multiply via fission, generating 2 daughter crypts from 1 parental crypt. In the adult intestine, crypt fission is observed at a low frequency. Using intravital microscopy in Lgr5EGFP-Ires-CreERT2 mice, we monitored individual crypt dynamics over multiple days with single-cell resolution. We discovered the existence of crypt fusion, an almost exact reverse phenomenon of crypt fission, in which 2 crypts fuse into 1 daughter crypt. Examining 819 crypts in 4 mice, we found that 3.5% ± 0.6% of all crypts were in the process of fission, whereas 4.1 ± 0.9% of all crypts were undergoing crypt fusion. As counteracting processes, crypt fission and fusion could regulate crypt numbers during the lifetime of a mouse. Identifying the mechanisms that regulate rates of crypt fission and fusion could provide insights into intestinal adaptation to altered environmental conditions and disease pathogenesis.

High-resolution intravital microscopy through imaging windows has become an indispensable technique for the long-term visualization of dynamic processes in living animals. Easily accessible sites such as the skin, the breast and the skull can be imaged using various different imaging windows; however, long-term imaging studies on cellular processes in abdominal organs are more challenging. These processes include colonization of the liver by metastatic tumor cells and the development of an immune response in the spleen. We have recently developed an abdominal imaging window (AIW) that allows long-term imaging of the liver, the pancreas, the intestine, the kidney and the spleen. Here we describe the detailed protocol for the optimal surgical implantation of the AIW, which takes approximately 1 h, and subsequent multiphoton imaging, which takes up to 1 month.

Leucine-rich repeat-containing G-protein coupled receptor 5-positive (Lgr5(+)) stem cells reside at crypt bottoms of the small and large intestine. Small intestinal Paneth cells supply Wnt3, EGF, and Notch signals to neighboring Lgr5(+) stem cells. Whereas the colon lacks Paneth cells, deep crypt secretory (DCS) cells are intermingled with Lgr5(+) stem cells at crypt bottoms. Here, we report regenerating islet-derived family member 4 (Reg4) as a marker of DCS cells. To investigate a niche function, we eliminated DCS cells by using the diphtheria-toxin receptor gene knocked into the murine Reg4 locus. Ablation of DCS cells results in loss of stem cells from colonic crypts and disrupts gut homeostasis and colon organoid growth. In agreement, sorted Reg4(+) DCS cells promote organoid formation of single Lgr5(+) colon stem cells. DCS cells can be massively produced from Lgr5(+) colon stem cells in vitro by combined Notch inhibition and Wnt activation. We conclude that Reg4(+) DCS cells serve as Paneth cell equivalents in the colon crypt niche.

It is widely debated whether all tumor cells in mammary tumors have the same potential to propagate and maintain tumor growth or whether there is a hierarchical organization. Evidence for the latter theory is mainly based on the ability or failure of transplanted tumor cells to produce detectable tumors in mice with compromised immune systems; however, this assay has lately been disputed to accurately reflect cell behavior in unperturbed tumors. Lineage tracing experiments have recently shown the existence of a small population of cells, referred to as cancer stem cells (CSCs), that maintains and provides growth of squamous skin tumors and intestinal adenomas. However, the lineage tracing techniques used in these studies provide static images and lack the ability to study whether stem cell properties can be obtained or lost, a process referred to as stem cell plasticity. Here, by intravital lineage tracing, we report for the first time the existence of CSCs in unperturbed mammary tumors and demonstrate CSC plasticity. Our data indicate that existing CSCs disappear and new CSCs form during mammary tumor growth, illustrating the dynamic nature of these cells.

Polarized cell migration is a crucial process in the development and repair of tissues, as well as in pathological conditions, including cancer. Recent studies have elucidated important roles for Rho GTPases in the establishment and maintenance of polarity prior to and during cell migration. Here, we show that Tiam1, a specific activator of the small GTPase Rac, is required for the polarized outgrowth of protrusions in primary astrocytes during the initial phase of cell polarization after scratch-wounding monolayers of cells. Tiam1 deficiency delays closure of wounds in confluent monolayers. Lack of Tiam1 impairs adoption of an asymmetrical cell shape as well as microtubule organization within protrusions. Positioning of the centrosome and Golgi apparatus, however, are independent of Tiam1-Rac signaling. We speculate that the function of Tiam1 in polarized outgrowth of astrocyte protrusions involves regulation of microtubule organization, possibly by stabilizing the microtubule cytoskeleton. Our results add Tiam1 as a player to the growing list of proteins involved in polarized outgrowth of protrusions and further elucidate the signaling pathways leading to cell polarization.

Migration toward chemoattractants is a hallmark of T-cell trafficking and is essential to produce an efficient immune response. Here, we have analyzed the function of the Rac activator Tiam1 in the control of T-cell trafficking and transendothelial migration. We found that Tiam1 is required for chemokine- and S1P-induced Rac activation and subsequent cell migration. As a result, Tiam1-deficient T cells show reduced chemotaxis in vitro, and impaired homing, egress, and contact hypersensitivity in vivo. Analysis of the T-cell transendothelial migration cascade revealed that PKCζ/Tiam1/Rac signaling is dispensable for T-cell arrest but is essential for the stabilization of polarization and efficient crawling of T cells on endothelial cells. T cells that lack Tiam1 predominantly transmigrate through individual endothelial cells (transcellular migration) rather than at endothelial junctions (paracellular migration), suggesting that T cells are able to change their route of transendothelial migration according to their polarization status and crawling capacity.

Background: The establishment and maintenance of cell polarity is crucial for many biological functions and is regulated by conserved protein complexes. The Par polarity complex consisting of Par3, Par6, and PKCζ, in conjunction with Tiam1-mediated Rac signaling, controls apical-basal cell polarity in contacting epithelial cells. Here we tested the hypothesis that the Par complex, in conjunction with Tiam1, controls "front-rear" polarity during the persistent migration of freely migrating keratinocytes. Results: Wild-type (WT) epidermal keratinocytes lacking cell-cell contacts are stably front-rear polarized and migrate persistently. In contrast, Tiam1-deficient (Tiam1 KO) and (si)Par3-depleted keratinocytes are generally unpolarized and migrate randomly because front-rear polarity is short lived. Immunoprecipitation experiments show that in migrating keratinocytes, Tiam1 associates with Par3 and PKCζ. Moreover, Par3, PKCζ, and Tiam1 proteins are enriched at the leading edges of polarized keratinocytes. Tiam1 KO keratinocytes are impaired in chemotactic migration toward growth factors, whereaes haptotactic migration is similar to WT. Par3 depletion or the blocking of PKCζ signaling in WT keratinocytes impairs chemotaxis but has no additional effect on Tiam1 KO cells. The migratory and morphological defects in keratinocytes with impaired Par-Tiam1 function closely resemble cells with pharmacologically destabilized microtubules (MTs). Indeed, MTs in Tiam1 KO keratinocytes and WT cells treated with a PKCζ inhibitor are unstable, thereby negatively influencing directional but not random migration. Conclusions: We conclude that the Par-Tiam1 complex stabilizes front-rear polarization of noncontacting migratory cells, thereby stimulating persistent and chemotactic migration, whereas in contacting keratinocytes, the same complex controls the establishment of long-lasting apical-basal polarity. These findings underscore a remarkable flexibility of the Par polarity complex that, depending on the biological context, controls distinct forms of cellular polarity.

Sumoylation is an essential posttranslational modification in eukaryotes that has emerged as an important pathway in oncogenic processes. Most human cancers display hyperactivated sumoylation and many cancer cells are remarkably sensitive to its inhibition, thus supporting application of chemical sumoylation inhibitors in cancer treatment. Here we show, first, that transformed embryonic fibroblasts derived from mice haploinsufficient for Ubc9, the essential and unique gene encoding the SUMO E2 conjugating enzyme, exhibit enhanced proliferation and transformed phenotypes in vitro and as xenografts ex vivo. To then evaluate the possible impact of loss of one Ubc9 allele in vivo, we used a mouse model of intestinal tumorigenesis. We crossed Ubc9+/− mice with mice harboring a conditional ablation of Apc either all along the crypt–villus axis or only in Lgr5+ crypt-based columnar (CBC) cells, the cell compartment that includes the intestinal stem cells proposed as cells-of-origin of intestinal cancer. While Ubc9+/− mice display no overt phenotypes and no globally visible hyposumoylation in cells of the small intestine, we found, strikingly, that, upon loss of Apc in both models, Ubc9+/− mice develop more (>2-fold) intestinal adenomas and show significantly shortened survival. This is accompanied by reduced global sumoylation levels in the polyps, indicating that Ubc9 levels become critical upon oncogenic stress. Moreover, we found that, in normal conditions, Ubc9+/− mice show a moderate but robust (15%) increase in the number of Lgr5+ CBC cells when compared to their wild-type littermates, and further, that these cells display higher degree of stemness and cancer-related and inflammatory gene expression signatures that, altogether, may contribute to enhanced intestinal tumorigenesis. The phenotypes of Ubc9 haploinsufficiency discovered here indicate an unanticipated tumor-suppressive role of sumoylation, one that may have important implications for optimal use of sumoylation inhibitors in the clinic.

Summary Most cancer cells release heterogeneous populations of extracellular vesicles (EVs) containing proteins, lipids, and nucleic acids. In vitro experiments showed that EV uptake can lead to transfer of functional mRNA and altered cellular behavior. However, similar in vivo experiments remain challenging because cells that take up EVs cannot be discriminated from non-EV-receiving cells. Here, we used the Cre-LoxP system to directly identify tumor cells that take up EVs in vivo. We show that EVs released by malignant tumor cells are taken up by less malignant tumor cells located within the same and within distant tumors and that these EVs carry mRNAs involved in migration and metastasis. By intravital imaging, we show that the less malignant tumor cells that take up EVs display enhanced migratory behavior and metastatic capacity. We postulate that tumor cells locally and systemically share molecules carried by EVs in vivo and that this affects cellular behavior., Graphical Abstract, Highlights • Visualization of extracellular vesicle exchange between tumor cells in living mice • Direct in vivo evidence of local and systemic mRNA exchange between tumor cells • The exchange of biomolecules is mediated through extracellular vesicles • Metastatic behavior can be phenocopied through extracellular vesicle exchange, Intravital imaging experiments reveal the extracellular vesicle-mediated exchange of molecules important for metastasis between tumor cells in living mice. This exchange results in metastatic properties being conferred to the receiving cell.