Neural stem cells, tumour stem cells and brain tumours: dangerous relationships?

Neural stem cells (NSC) have been implicated not only in brain development and neurogenesis but also in tumourigenesis. Brain tumour stem cells (BTSC) have been isolated from several paediatric or adult human brain tumours, however their origin is still disputed. This review discusses the normal role of NSC in the adult mammalian brain and their anatomical location. It compares the molecular characteristics and the biological behaviour of NSC/BTSC, and describes the molecular pathways involved in controlling self-renewal and maintenance of adult NSC/BTSC and brain tumour development. It also assesses the current hypotheses about the origin of BTSC and the clinical consequences.     

CD133: holy of grail of neuro‐oncology or promiscuous red‐herring?

The CD133 glycoprotein is a controversial cancer stem cell marker in the field of neuro‐oncology, based largely on the now considerable experimental evidence for the existence of both CD133+ve and CD133?ve populations as tumour‐initiating cells. It is thought that decreasing oxygen tension enhances the complex regulation and phenotype of CD133 in glioma. In light of these ideologies, establishing the precise functional role of CD133 is becoming increasingly critical. In this article, we review the complex regulation of CD133 and its extracellular epitope AC133, and associated alterations, to tumour cell behaviour by hypoxia. Furthermore, its role in functional modulation of tumours, rather than determination of a specific stem cell type is therefore alluded to, while evidence for and against its ability as a cancer stem cell marker in primary brain tumours, is critically evaluated. Thus, the suggestion that CD133 may be a central ‘holy grail’ in identifying core cells for propagation of malignant glial neoplasms seems increasingly less convincing. It remains to be seen, however, whether CD133 is randomly expressed on such brain tumour cell populations or whether it is of major significance to brain biological behaviour.     

Glycosylation diseases: Quo vadis?

About 250 to 500 glycogenes (genes that are directly involved in glycan assembly) are in the human genome representing about 1–2% of the total genome. Over 40 human congenital diseases associated with glycogene mutations have been described to date. It is almost certain that the causative glycogene mutations for many more congenital diseases remain to be discovered. Some glycogenes are involved in the synthesis of only a specific protein and/or a specific class of glycan whereas others play a role in the biosynthesis of more than one glycan class. Mutations in the latter type of glycogene result in complex clinical phenotypes that present difficult diagnostic problems to the clinician. In order to understand in biochemical terms the clinical signs and symptoms of a patient with a glycogene mutation, one must understand how the glycogene works. That requires, first of all, determination of the target protein or proteins of the glycogene followed by an understanding of the role, if any, of the glycogene-dependent glycan in the functions of the protein. Many glycogenes act on thousands of glycoproteins. There are unfortunately no general methods to identify all the potentially large number of glycogene target proteins and which of these proteins are responsible for the mutant phenotypes. Whereas biochemical methods have been highly successful in the discovery of glycogenes responsible for many congenital diseases, it has more recently been necessary to use other methods such as homozygosity mapping. Accurate diagnosis of many recently discovered diseases has become difficult and new diagnostic procedures must be developed. Last but not least is the lack of effective treatment for most of these children and of animal models that can be used to test new therapies.     

Chrysanthemum Biotechnology: Quo vadis?

Chrysanthemum is globally the second most important ornamental in terms of socioeconomic importance. Even though the vast range of flower colors, shapes and forms were initially created using conventional and mutation breeding, transgenic strategies are now more frequently used with Agrobacterium-mediated transformation being the most popular form of introducing foreign genes into chrysanthemums. Even so, transformation efficiency remains dependent on cultivar and regeneration procedure. Transgenic molecular breeding has seen the introduction of important traits such as novel flower color and form and plant architecture, prolonged cut-flower vase-life, resistance to biotic stresses such as viruses/viroids, pathogens and insects. However, chimerism and transgene silencing continue to be limiting factors. Transgenic strategies, despite opening up new avenues for creating new cultivars with improved agronomic and horticultural traits, may be limited due to the risk of transgenic pollen escaping into the wild.     

Quo vadis porphyrin chemistry?

This review summarizes recent developments in the area of porphyrin chemistry in the direction of biological applications. Novel synthetic methodologies are reviewed for porphyrin synthesis, porphyrin analog synthesis, stable porphyrinogens -- calixpyrroles, expanded porphyrins. Unique biological properties of those compounds are desribed with focus on photodynamic therapy (PDT) and molecular recognition properties. Special attentions given to metalloporphyrins with potential to affect heme degradation and CO formation.     

Dermis‐derived stem cells: a source of epidermal melanocytes and melanoma?

Human multipotent dermal stem cells (DSCs) have been isolated and propagated from the dermal region of neonatal foreskin. DSCs can self-renew, express the neural crest stem cell markers NGFRp75 and nestin, and are capable of differentiating into a wide variety of cell types including mesenchymal and neuronal lineages and melanocytes, indicative of their neural crest origin. When placed in the context of reconstructed skin, DSCs migrate to the basement membrane zone and differentiate into melanocytes. These findings, combined with the identification of NGFRp75-positive cells in the dermis of human foreskin, which are devoid of hair, suggest that DSCs may be a self-renewing source of extrafollicular epidermal melanocytes. In this review, we discuss the properties of DSCs, the pathways required for melanocyte differentiation, and the value of 3D reconstructed skin to assess the behavior and contribution of DSCs in the naturalized environment of human skin. Potentially, DSCs provide a link to malignant melanoma by being a target of UVA-induced transformation.     

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is ‘hormone profiling’, i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.     

Non‐neural adult stem cells: tools for brain repair?

Stem cells isolated from adult mammalian tissues may provide new approaches for the autologous treatment of disease and tissue repair. Although the potential of adult stem cells has received much attention, it has also recently been brought into question. This article reviews the recent work describing the ability of non-hematopoietic stem cells derived from adult bone marrow to form neural derivatives and their potential for brain repair. Earlier transplantation experiments imply that grafted adult stem cells can differentiate into neural derivatives. Recent reports suggest, however, that such findings may be misleading and grafted cells acquiring different identities may merely be explained by their fusion with host cells and not the result of radical changes to their program of cellular differentiation. Nonetheless, in vitro studies have shown that neural development by bone-marrow-derived stem cells also appears possible. Understanding the molecular mechanisms that specify the neural lineage will lead to the development of tools for the targeted production of neural cell types in vitro that may ultimately provide a source of material to treat specific neurological deficits.     

A polarized anchor for hematopoietic stem cells: Synapse between stem cells and their niche?

Multipotent hematopoietic stem cells are maintained by the bone marrow niche, but how niche-derived membrane-bound stem cell factor (mSCF) regulates HSCs remains unclear. In this issue, Hao et al. (2021. J. Cell Biol. https://doi.org/10.1083/jcb.202010118) describe that mSCF, synergistically with VCAM-1, induces large, polarized protrusions that serve as anchors for HSCs to their niche.

Hematopoietic stem cells (HSCs) generate all blood and immune cells throughout life via self-renewal and multilineage differentiation within the bone marrow niche. HSCs are the basis for bone marrow transplantation, saving thousands of lives yearly. The bone marrow niche often serves as a paradigm for studying stem cell biology. In addition, elucidating the underlying mechanism in the niche helps devise strategies to expand functional HSCs for clinical use. Within the niche, leptin receptor–positive perisinusoidal stromal cells and endothelial cells are the major source of essential cytokines for HSC maintenance, including vascular cell adhesion molecule 1 (VCAM-1) and stem cell factor (SCF; 1, 2). Locally produced soluble and membrane-bound cytokines preserve the unique localization and anchorage of HSCs to stromal cells within their niche. Consistent with this notion, mouse genetic data have shown that membrane-bound SCF (mSCF) is important for HSC maintenance in vivo (3). However, given that both soluble and membrane-bound forms of SCF can engage with the cognate cKIT receptors, the mechanisms by which mSCF sustains HSCs function in vivo remain elusive. Likewise, it is unclear why the expansion and maintenance of HSCs ex vivo by adding SCF to culture as an either soluble or immobilized form has only been achieved with limited success.In this issue, Hao et al. addressed this question by using a supported lipid bilayer (SLB) system to model the interaction between HSCs and membrane-bound cytokines, including SCF (4). SLBs present an advantage over conventional immobilization methods; they allow the lateral mobility of membrane-bound proteins and clustering of receptors and signaling complexes, thus resembling the lipid bilayer of plasma membrane in vivo. Focusing on HSC cytokines that may be presented as membrane-bound forms in the bone marrow niche, the authors performed an imaging screen in vitro using SLBs and found that mSCF but not soluble SCF (sSCF) induced mSCF/cKIT clustering and the formation of membrane protrusions on HSCs. While mSCF alone was sufficient to promote cell protrusions, HSCs required both mSCF and VCAM-1 for large, polarized protrusions. They followed HSCs at different time points after exposure to VCAM-1 and mSCF by scanning electron microscopy and observed that HSCs first formed diffuse mSCF clusters and multifocal thin protrusions and then proceeded to a polarized, clustered morphology with larger and thicker protrusions. Using a controlled sheer stress device, Hao et al. showed that these polarized protrusions had a functional consequence on the adhesion strength of HSCs. mSCF and VCAM-1 dramatically increased the adhesion of HSCs to SLB compared with VCAM-1 or mSCF alone. Interestingly, the effect was more prominent in HSCs compared with their immediate downstream progenies, multipotent progenitors. This phenotype was also specific to ligands presented on SLB because the effect was canceled when the cytokines were directly immobilized onto the glass surface. Then, they had a close look into the cytoskeletal organization of HSCs in the presence of both mSCF and VCAM-1 on SLB. They found that F-actin and myosin IIa concentrated at the protrusion, which led them to speculate that the cytoskeleton remodeling mediates the formation of the polarized morphology. Indeed, chemical inhibitors blocking myosin contraction, actin polymerization, or Rho-associated protein kinase disrupted the formation of the large and polarized protrusion. The authors noted that phosphatidylinositol 3-kinase (PI3K) also localized with mSCF/cKIT clusters, so they further assessed the contribution of the PI3K/Akt pathway to the polarized morphology of HSCs by using total internal reflection fluorescence microscopy and PI3K and Akt chemical inhibitors. PI3K/Akt activation contributed downstream of the mSCF–VCAM-1 synergy to regulating HSC cell adhesion and polarized mSCF/cKIT distribution. In addition, PI3K signaling enhanced the nuclear retention of FOXO3a, a crucial factor for HSC self-renewal; this enhancement was induced by mSCF but lessened by sSCF. Intriguingly, sSCF also competed with mSCF and abrogated the effect of the mSCF–VCAM-1 synergy on polarized protrusion formation. However, whether and how PI3K transmits the mSCF–VCAM-1 synergy into proliferation or quiescence cues in HSCs requires further investigation. Taken together, these data suggest that mSCF and VCAM-1 synergize to induce polarized protrusions on HSCs, which regulates their adhesion to the niche (Fig. 1). These protrusions share many features with the immunological synapse (5), which points toward the existence of a similar model for stem cells, “stem cell synapse,” where HSCs interact with and receive a variety of signals from their niche cells.Open in a separate windowFigure 1.VCAM-1 and mSCF synergistically promote the formation of polarized protrusions (stem cell synapse) on HSCs. (A and B) VCAM-1 or mSCF alone does not induce apparent polarized morphology on HSCs. The signaling and adhesion of HSCs to the niche is not at its full potential. (C) VCAM-1 and mSCF together induce robust receptor clustering on HSCs, optimal signaling, and strong adhesion. (D) sSCF can competitively disrupt the polarized protrusions on HSCs. The figure was created with BioRender.com.While the study by Hao et al. sheds light on how niche signals, particularly mSCF, regulate HSCs, several outstanding questions remain. First, even though many hematopoietic cells express cKIT (some of them even express higher levels than HSCs), HSCs respond to mSCF + VCAM-1 the strongest by recruiting the most mSCF to clusters. What is the specific mechanism in HSCs underlying this specificity? Second, SCF is produced both as mSCF and sSCF in vivo, through alternative splicing and proteolytic cleavage; if mSCF is mainly responsible for anchoring HSCs in the niche, what is the function of sSCF in vivo? Does sSCF modulate the available pool of mSCF? Third, robust maintenance of HSCs in culture has been challenging. HSCs can be maintained in a system composed of sSCF, thromopoietin (TPO), fibronectin, and polyvinyl alcohol (6). Tethering cytokines to SLB elicits more physiological response from HSCs compared with soluble cytokines or direct immobilization. Does SLB improve maintenance of HSCs in in vitro culture? Fourth, some cytokines, such as TPO, act on HSCs in a long-range manner (7). How do these systemic cytokines induce robust signaling in HSCs? Do they participate in the stem cell synapse even if they are not the initiators? Finally, do stem cells and their niche interact by forming similar synapses in other stem cell systems? Answering these questions will deepen our understanding of the stem cell niche and help integrate the niche component into potential, more successful applications in regenerative medicine.     

Chaperoning stem cells: a role for heat shock proteins in the modulation of stem cell self‐renewal and differentiation?

Self‐renewal and differentiation of stem cells are tightly regulated processes subject to intrinsic and extrinsic signals. Molecular chaperones and co‐chaperones, especially heat shock proteins (Hsp), are ubiquitous molecules involved in the modulation of protein conformational and complexation states. The function of Hsp, which are typically associated with stress response and tolerance, is well characterized in differentiated cells, while their role in stem cells remains unclear. It appears that embryonic stem cells exhibit increased stress tolerance and concomitant high levels of chaperone expression. This review critically evaluates stem cell research from a molecular chaperone perspective. Furthermore, we propose a model of chaperone‐modulated self‐renewal in mouse embryonic stem cells.