Adhesion within the stem cell niches

Extracellular matrix, integrins, and growth factors as tailors of the stem cell niche

It is widely acknowledged that integrins, the major receptors for the extracellular matrix (ECM) proteins, exert an extensive crosstalk with many growth factor and cytokine receptors. Among them, growth factor receptors, such as the EGFR, MET, PDGFR and VEGFR, and the IL-3 receptor have been shown to be physically and functionally associated to integrins. The connection between integrins and other transmembrane receptors is bidirectional, integrins being essential for receptor signalling, and receptors being involved in regulation of integrin expression or activation. Moreover, there is accumulating evidence for direct binding of specific growth factors and morphogens to the ECM proteins, suggesting that ECM might spatially integrate different types of signals in a specific microenvironment, facilitating integrin/transmembrane receptors connection. These interactions are crucial in controlling a variety of cell behaviours including proliferation, survival and differentiation. The increasing interest for cell therapy in regenerative medicine has recently emphasized the role of cell-ECM adhesion as stem cell determinant. The relevance of ECM, integrins and growth factor receptor network in the establishment of stem cell niche, in maintenance of stem cells and in their differentiation will be analyzed in the present review.

Hematopoiesis: a human perspective

Despite its complexity, blood is probably the best understood developmental system, largely due to seminal experimentation in the mouse. Clinically, hematopoietic stem cell (HSC) transplantation represents the most widely deployed regenerative therapy, but human HSCs have only been characterized relatively recently. The discovery that immune-deficient mice could be engrafted with human cells provided a powerful approach for studying HSCs. We highlight 2 decades of studies focusing on isolation and molecular regulation of human HSCs, therapeutic applications, and early lineage commitment steps, and compare mouse and humanized models to identify both conserved and species-specific mechanisms that will aid future preclinical research.

Hair Follicle Stem Cells Derived from Single Rat Vibrissa via Organ Culture Reconstitute Hair Follicles in Vivo

Hair follicle stem cells (HFSCs) are potentially useful for the treatment of skin injuries and diseases. To achieve clinical application, a prerequisite must be accomplished: harvesting enough HFSCs from limited skin biopsy. The commonly used sorting approach for isolating HFSCs, however, suffers from its intrinsic disadvantages, such as requirement of large-scale skin biopsy. Here, we report an efficient organ culture method to isolate and expand rat HFSCs from limited skin biopsy and these HFSCs could reconstitute the epidermis and the hair follicles (HFs). Seventy-three percent of cultured HFs formed hair follicle stem cell colonies from the bulge, and a single hair follicle provided all the HFSCs used in this research, demonstrating the high efficiency of this method. Quantitative RT-PCR and immunofluorescent staining results revealed that these stem cells obtained from the bulge highly expressed basal layer markers K14 and alpha-6 integrin, epithelial stem cell marker P63, and bulge stem cell marker K15. After long-term culture in vitro, GFP-labeled hair follicle stem cells formed new hair follicles, epidermis, and sebaceous glands following xenotransplantation into the back of nude mice. This study indicated that multipotent hair follicle stem cells could be efficiently harvested through organ culture from limited skin material—even a single hair follicle—and reconstitute hair follicles in vivo after long-term expansion culture, providing the basis for future clinical applications.

Stem cells: a view from the roots

In both plants and animals, regeneration requires the activation of stem cells. This is possibly related to the origin and requirements of multicellularity. Although long diverged from a common ancestry, plant and animal models such as Arabidopsis, Drosophila and mouse share considerable similarities in stem cell regulation. This includes stem cell niche organisation, epigenetic modification of DNA and histones, and the role of small RNA machinery in differentiation and pluripotency states. Dysregulation of any of these can lead to premature ageing, patterning and specification defects, as well as cancers. Moreover, emerging basal animal and plant systems are beginning to provide important clues concerning the diversity and evolutionary history of stem cell regulatory mechanisms in eukaryotes. This review provides a comparative framework, highlighting both the commonalities and differences among groups, which should promote the intelligent design of artificial stem cell systems, and thereby fuel the field of biomaterials science.

The extracellular matrix: a dynamic niche in cancer progression

The local microenvironment, or niche, of a cancer cell plays important roles in cancer development. A major component of the niche is the extracellular matrix (ECM), a complex network of macromolecules with distinctive physical, biochemical, and biomechanical properties. Although tightly controlled during embryonic development and organ homeostasis, the ECM is commonly deregulated and becomes disorganized in diseases such as cancer. Abnormal ECM affects cancer progression by directly promoting cellular transformation and metastasis. Importantly, however, ECM anomalies also deregulate behavior of stromal cells, facilitate tumor-associated angiogenesis and inflammation, and thus lead to generation of a tumorigenic microenvironment. Understanding how ECM composition and topography are maintained and how their deregulation influences cancer progression may help develop new therapeutic interventions by targeting the tumor niche.

Id proteins synchronize stemness and anchorage to the niche of neural stem cells

Stem-cell functions require activation of stem-cell-intrinsic transcriptional programs and extracellular interaction with a niche microenvironment. How the transcriptional machinery controls residency of stem cells in the niche is unknown. Here we show that Id proteins coordinate stem-cell activities with anchorage of neural stem cells (NSCs) to the niche. Conditional inactivation of three Id genes in NSCs triggered detachment of embryonic and postnatal NSCs from the ventricular and vascular niche, respectively. The interrogation of the gene modules directly targeted by Id deletion in NSCs revealed that Id proteins repress bHLH-mediated activation of Rap1GAP, thus serving to maintain the GTPase activity of RAP1, a key mediator of cell adhesion. Preventing the elevation of the Rap1GAP level countered the consequences of Id loss on NSC-niche interaction and stem-cell identity. Thus, by preserving anchorage of NSCs to the extracellular environment, Id activity synchronizes NSC functions to residency in the specialized niche.

Telocyte morphologies and potential roles in diseases

Telocytes (TCs) are a new type of interstitial cells, a small cellular body with the presence of 2-5 prolongations named as telopode (Tp)-very thin (less than 0.2 µm) and extremely long (10-1,000 µm), a moniliform aspect, and caveolae, containing a nucleus surrounded by a small amount of cytoplasm. The nucleus occupies about 25% of TC body volume and contains clusters of heterochromatin attached to the nuclear envelope. The perinuclear cytoplasm is rich in mitochondria and contains a small Golgi complex, rough and smooth endoplasmic reticulum and cytoskeletal elements. TCs have several immunophenotypes such as CD34, c-kit, and vimentin. TCs were found in many organs of mammals with potential biological functions, even though the exact function remains unclear. Recently, we identified and isolated TCs from the trachea for the first time and confirmed the existence of TC in lung tissues, which could have the potential significance in the pathogenesis of pulmonary diseases. Future efforts are required to clarify pathophysiological functions of TCs in the disease.

Shaping the niche: lessons from the Drosophila testis and other model systems

Stem cells are fascinating, as they supply the cells that construct our adult bodies and replenish, as we age, worn out, damaged, and diseased tissues. Stem cell regulation relies on intrinsic signals but also on inputs emanating from the neighbouring niche. The Drosophila testis provides an excellent system for studying such processes. Although recent advances have uncovered several signalling, cytoskeletal and other factors affecting niche homeostasis and testis differentiation, many aspects of niche regulation and maintenance remain unsolved. In this review, we discuss aspects of niche establishment and integrity not yet fully understood and we compare it to the current knowledge in other model systems such as vertebrates and plants. We also address specific questions on stem cell maintenance and niche regulation in the Drosophila testis under the control of Hox genes. Finally, we provide insights on the striking functional conservation of homologous genes in plants and animals and their respective stem cell niches. Elucidating conserved mechanisms of stem cell control in both lineages could reveal the importance underlying this conservation and justify the evolutionary pressure to adapt homologous molecules for performing the same task.

The receptor tyrosine phosphatase Lar regulates adhesion between Drosophila male germline stem cells and the niche

The stem cell niche provides a supportive microenvironment to maintain adult stem cells in their undifferentiated state. Adhesion between adult stem cells and niche cells or the local basement membrane ensures retention of stem cells in the niche environment. Drosophila male germline stem cells (GSCs) attach to somatic hub cells, a component of their niche, through E-cadherin-mediated adherens junctions, and orient their centrosomes toward these localized junctional complexes to carry out asymmetric divisions. Here we show that the transmembrane receptor tyrosine phosphatase Leukocyte-antigen-related-like (Lar), which is best known for its function in axonal migration and synapse morphogenesis in the nervous system, helps maintain GSCs at the hub by promoting E-cadherin-based adhesion between hub cells and GSCs. Lar is expressed in GSCs and early spermatogonial cells and localizes to the hub-GSC interface. Loss of Lar function resulted in a reduced number of GSCs at the hub. Lar function was required cell-autonomously in germ cells for proper localization of Adenomatous polyposis coli 2 and E-cadherin at the hub-GSC interface and for the proper orientation of centrosomes in GSCs. Ultrastructural analysis revealed that in Lar mutants the adherens junctions between hub cells and GSCs lack the characteristic dense staining seen in wild-type controls. Thus, the Lar receptor tyrosine phosphatase appears to polarize and retain GSCs through maintenance of localized E-cadherin-based adherens junctions.

Extracellular matrix degradation and remodeling in development and disease

The extracellular matrix (ECM) serves diverse functions and is a major component of the cellular microenvironment. The ECM is a highly dynamic structure, constantly undergoing a remodeling process where ECM components are deposited, degraded, or otherwise modified. ECM dynamics are indispensible during restructuring of tissue architecture. ECM remodeling is an important mechanism whereby cell differentiation can be regulated, including processes such as the establishment and maintenance of stem cell niches, branching morphogenesis, angiogenesis, bone remodeling, and wound repair. In contrast, abnormal ECM dynamics lead to deregulated cell proliferation and invasion, failure of cell death, and loss of cell differentiation, resulting in congenital defects and pathological processes including tissue fibrosis and cancer. Understanding the mechanisms of ECM remodeling and its regulation, therefore, is essential for developing new therapeutic interventions for diseases and novel strategies for tissue engineering and regenerative medicine.

Organ aging and susceptibility to cancer may be related to the geometry of the stem cell niche

Telomere loss at each cell replication limits the proliferative capacity of normal cells, including adult stem cells. Entering replicative senescence protects dividing cells from neoplastic transformation, but also contributes to aging of the tissue. Recent experiments have shown that intestinal mouse stem cells divide symmetrically, at random make decisions to remain stem cells or to differentiate, and gradually lose telomeric DNA. A cell's decision whether to differentiate or to remain a stem cell depends on the local cellular and chemical environment and thus tissue architecture is expected to play role in cell proliferation dynamics. To take into account the structure of the stem cell niche in determining its proliferative potential and susceptibility to cancer, a theoretical model is introduced and the niche proliferative potential is quantified for different architectures. The niche proliferative potential is quantitatively related to the proliferative potential of the individual stem cells for different structural classes of the stem cell niche. Stem cells at the periphery of a niche are under pressure to divide and to differentiate, as well as to maintain the stem cell niche boundary, and thus the geometry of the stem cell niche is expected to play a role in determining the stem cell division sequence and differentiation. Smaller surface-to-volume ratio is associated with higher susceptibility to cancer, higher tissue renewal capacity, and decreased aging rate. Several testable experimental predictions are discussed, as well the presence of stochastic effects.

Micro- and nanoengineering for stem cell biology: the promise with a caution

Current techniques used in stem cell research only crudely mimic the physiological complexity of the stem cell niches. Recent advances in the field of micro- and nanoengineering have brought an array of in vitro cell culture models that have enabled development of novel, highly precise and standardized tools that capture physiological details in a single platform, with greater control, consistency, and throughput. In this review, we describe the micro- and nanotechnology-driven modern toolkit for stem cell biologists to design novel experiments in more physiological microenvironments with increased precision and standardization, and caution them against potential challenges that the modern technologies might present.

After the bomb drops: a new look at radiation-induced multiple organ dysfunction syndrome (MODS)

PURPOSE

There is increasing concern that, since the Cold War era, there has been little progress regarding the availability of medical countermeasures in the event of either a radiological or nuclear incident. Fortunately, since much is known about the acute consequences that are likely to be experienced by an exposed population, the probability of survival from the immediate hematological crises after total body irradiation (TBI) has improved in recent years. Therefore focus has begun to shift towards later down-stream effects, seen in such organs as the gastrointestinal tract (GI), skin, and lung. However, the mechanisms underlying therapy-related normal tissue late effects, resulting from localised irradiation, have remained somewhat elusive and even less is known about the development of the delayed syndrome seen in the context of whole body exposures, when it is likely that systemic perturbations may alter tissue microenvironments and homeostasis.

CONCLUSIONS

The sequence of organ failures observed after near-lethal TBI doses are similar in many ways to that of multiple organ dysfunction syndrome (MODS), leading to multiple organ failure (MOF). In this review, we compare the mechanistic pathways that underlie both MODS and delayed normal tissue effects since these may impact on strategies to identify radiation countermeasures.

Microfluidic devices for studying heterotypic cell-cell interactions and tissue specimen cultures under controlled microenvironments

Microfluidic devices allow for precise control of the cellular and noncellular microenvironment at physiologically relevant length- and time-scales. These devices have been shown to mimic the complex in vivo microenvironment better than conventional in vitro assays, and allow real-time monitoring of homotypic or heterotypic cellular interactions. Microfluidic culture platforms enable new assay designs for culturing multiple different cell populations and∕or tissue specimens under controlled user-defined conditions. Applications include fundamental studies of cell population behaviors, high-throughput drug screening, and tissue engineering. In this review, we summarize recent developments in this field along with studies of heterotypic cell-cell interactions and tissue specimen culture in microfluidic devices from our own laboratory.

Hair follicle stem cells provide a functional niche for melanocyte stem cells

In most stem cell systems, the organization of the stem cell niche and the anchoring matrix required for stem cell maintenance are largely unknown. We report here that collagen XVII (COL17A1/BP180/BPAG2), a hemidesmosomal transmembrane collagen, is highly expressed in hair follicle stem cells (HFSCs) and is required for the maintenance not only of HFSCs but also of melanocyte stem cells (MSCs), which do not express Col17a1 but directly adhere to HFSCs. Mice lacking Col17a1 show premature hair graying and hair loss. Analysis of Col17a1-null mice revealed that COL17A1 is critical for the self-renewal of HFSCs through maintaining their quiescence and immaturity, potentially explaining the mechanism underlying hair loss in human COL17A1 deficiency. Moreover, forced expression of COL17A1 in basal keratinocytes, including HFSCs, in Col17a1-null mice rescues MSCs from premature differentiation and restores TGF-β signaling, demonstrating that HFSCs function as a critical regulatory component of the MSC niche.

Adhesion molecules in the stem cell niche--more than just staying in shape?

The expression of adhesion molecules by stem cells within their niches is well described, but what is their function? A conventional view is that these adhesion molecules simply retain stem cells in the niche and thereby maintain its architecture and shape. Here, we review recent literature showing that this is but one of their roles, and that they have essential functions in all aspects of the stem cell-niche interaction--retention, division and exit. We also highlight from this literature evidence supporting a simple model whereby the regulation of centrosome positioning and spindle angle is regulated by both cadherins and integrins, and the differential activity of these two adhesion molecules enables the fundamental stem cell property of switching between asymmetrical and symmetrical divisions.

Tumor-initiating cells are not enriched in cisplatin-surviving BRCA1;p53-deficient mammary tumor cells in vivo

Although many breast cancers respond to chemotherapy or hormonal therapy, lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. Using the K14cre;Brca1(F5-13/F5-13);p53(F2-10/F2-10) mouse model for hereditary breast cancer, we have previously studied responses of mammary tumors to clinically relevant anti-cancer drugs, including cisplatin. The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to cisplatin and never become resistant to this agent due to the large, irreversible deletion in Brca1. We show here that even dose-dense treatment with a maximum tolerated dose of cisplatin does not result in complete tumor eradication. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified tumor-initiating cells in BRCA1- and p53-deficient tumors. In addition to the Lin⁻/CD24(+)/CD49f(+) subpopulation, we show that a larger population of Lin⁻/CD24(+)/CD49f-cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants. We conclude that in this model the tolerance of the cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs.

E-cadherin is required for centrosome and spindle orientation in Drosophila male germline stem cells

Many adult stem cells reside in a special microenvironment known as the niche, where they receive essential signals that specify stem cell identity. Cell-cell adhesion mediated by cadherin and integrin plays a crucial role in maintaining stem cells within the niche. In Drosophila melanogaster, male germline stem cells (GSCs) are attached to niche component cells (i.e., the hub) via adherens junctions. The GSC centrosomes and spindle are oriented toward the hub-GSC junction, where E-cadherin-based adherens junctions are highly concentrated. For this reason, adherens junctions are thought to provide a polarity cue for GSCs to enable proper orientation of centrosomes and spindles, a critical step toward asymmetric stem cell division. However, understanding the role of E-cadherin in GSC polarity has been challenging, since GSCs carrying E-cadherin mutations are not maintained in the niche. Here, we tested whether E-cadherin is required for GSC polarity by expressing a dominant-negative form of E-cadherin. We found that E-cadherin is indeed required for polarizing GSCs toward the hub cells, an effect that may be mediated by Apc2. We also demonstrated that E-cadherin is required for the GSC centrosome orientation checkpoint, which prevents mitosis when centrosomes are not correctly oriented. We propose that E-cadherin orchestrates multiple aspects of stem cell behavior, including polarization of stem cells toward the stem cell-niche interface and adhesion of stem cells to the niche supporting cells.

Cell adhesion in regulation of asymmetric stem cell division

Adult stem cells inevitably communicate with their cellular neighbors within the tissues they sustain. Indeed, such communication, particularly with components of the stem cell niche, is essential for many aspects of stem cell behavior, including the maintenance of stem cell identity and asymmetric cell division. Cell adhesion mediates this communication by placing stem cells in proximity to the signaling source and by providing a polarity cue that orients stem cells. Here, I review the recent discovery that cell adhesion molecules govern the behavior of stem cells.

Desmosomes in vivo

The structure, function, and regulation of desmosomal adhesion in vivo are discussed. Most desmosomes in tissues exhibit calcium-independent adhesion, which is strongly adhesive or “hyperadhesive”. This is fundamental to tissue strength. Almost all studies in culture are done on weakly adhesive, calcium-dependent desmosomes, although hyperadhesion can be readily obtained in confluent cell culture. Calcium dependence is a default condition in vivo, found in wounds and embryonic development. Hyperadhesion appears to be associated with an ordered arrangement of the extracellular domains of the desmosomal cadherins, which gives rise to the intercellular midline identified in ultrastructural studies. This in turn probably depends on molecular order in the desmosomal plaque. Protein kinase C downregulates hyperadhesion and there is preliminary evidence that it may also be regulated by tyrosine kinases. Downregulation of desmosomes in vivo may occur by internalisation of whole desmosomes rather than disassembly. Hyperadhesion has implications for diseases such as pemphigus.