Stem Cells

Cancer stem cells are rare immortal cells within a tumour that can both self-renew by dividing and give rise to many cell types that constitute the tumour, and can therefore form tumours. Such cells have been found in various types of human tumours and might be attractive targets for cancer treatment. Cancer stem cells (CSCs) are self-renewable cell types that are identified in most types of liquid and solid cancers and contributed to tumor onset, expansion, resistance, recurrence, and metastasis after therapy. CSCs are identified from the expression of cell surface markers, which is tumor-type dependent. The transition between CSCs with cancer cells and other non-CSCs occurs in cancers, which is possibly under the control of signals from CSCs and tumor microenvironment (TME), including CSC niche. Cancer-associated fibroblasts are among the most influential cells for promoting both differentiation of CSCs and dedifferentiation of non-CSCs toward attaining a CSC-like phenotype. WNT/β-catenin, transforming growth factor-β, Hedgehog, and Notch are important signals for maintaining self-renewal in CSCs. An effective therapeutic strategy relies on targeting both CSCs and non-CSCs to remove a possible chance of tumor relapse. There are multiple ways to target CSCs, including immunotherapy, hormone therapy, (mi)siRNA delivery, and gene knockout. Such approaches can be designed for suppressing CSC stemness, tumorigenic cues from TME, CSC extrinsic and/or intrinsic signaling, hypoxia or for promoting differentiation in the cells. Because of sharing a range of characteristics to normal stem/progenitor cells, CSCs must be targeted based on their unique markers and their preferential expression of antigens.

Embryonic stem (ES) cells are pluripotent stem cells derived from the inner cell mass of preimplantation embryos. Induced pluripotent stem (iPS) cells can be generated by somatic cell reprogramming following the exogenous expression of specific transcription factors (Oct-3/4, KLF4, SOX2, and c-Myc). Stem cell pluripotency can be defined and experimentally evaluated by assessing the ability of starting cell populations to differentiate into somatic cell types that arise from the three vertebrate germ layers: ectoderm, mesoderm, and endoderm. In addition, embryonic and induced pluripotent stem cells are characterized by the ability to undergo long-term self-renewal in vitro when cultured with growth factors provided by feeder cells or defined culture media. Pluripotent stem cell phenotypes can be modulated by chemical compounds and small bioactive molecules that enhance the proliferation or direct the differentiation of pluripotent stem cells.

Hematopoietic stem cells that provide long-term reconstitution can be separated from more mature hematopoietic cells that provide radioprotection and short-term repopulation. All differentiated blood cells from the lymphoid and myeloid lineages arise from HSCs. HSCs can be found in adult bone marrow, peripheral blood, and umbilical cord blood. Classic studies in mice describe two populations of Hematopoietic Stem Cells, Long Term and Short Term. Long term HSCs are capable of self renewal, while short term HSCs do not have this capacity. Short term HSCs, also called progenitor or precursor cells, can differentiate into all types of blood cells, which can be characterized by specific markers. For example, cells that differentiate into lymphocytes and granulocytes can be identified by detecting the expression of certain SLAM family receptors and Integrin alpha M/CD11b, respectively.

Mesenchymal stem cells, or MSCs, are multipotent adult stem cells that are present in multiple tissues, including umbilical cord, bone marrow and fat tissue. Mesenchymal stem cells can self-renew by dividing and can differentiate into multiple tissues including bone, cartilage, muscle and fat cells, and connective tissue. MSCs are rare in bone marrow, representing ~1 in 10,000 nucleated cells. Although not immortal, they have the ability to expand manyfold in culture while retaining their growth and multilineage potential. MSCs are identified by the expression of many molecules including CD105 (SH2) and CD73 (SH3/4) and are negative for the hematopoietic markers CD34, CD45, and CD14. The properties of MSCs make these cells potentially ideal candidates for tissue engineering. It has been shown that MSCs, when transplanted systemically, are able to migrate to sites of injury in animals, suggesting that MSCs possess migratory capacity. However, the mechanisms underlying the migration of these cells remain unclear. Chemokine receptors and their ligands and adhesion molecules play an important role in tissue-specific homing of leukocytes and have also been implicated in trafficking of hematopoietic precursors into and through tissue. Several studies have reported the functional expression of various chemokine receptors and adhesion molecules on human MSCs. Harnessing the migratory potential of MSCs by modulating their chemokine-chemokine receptor interactions may be a powerful way to increase their ability to correct inherited disorders of mesenchymal tissues or facilitate tissue repair in vivo.

Neural Stem Cells (NSCs) are the most primordial and uncommitted cells of the nervous system, and are believed to give rise to the vast array of more specialized cells of the CNS and peripheral nervous system (PNS). To be considered a “neural stem cell,” in contrast to a “progenitor” cell (i.e., cells that have already become lineage committed to give rise to only one category of neural component, e.g., glial cells versus neurons), that cell must be capable of (1) generating all neural lineages (neurons, astrocytes, and oligodendrocytes) throughout the nervous system, (2) having some capacity for self-renewal, and (3) being able to give rise to cell types in addition to themselves through asymmetric cell division

Stem cells reside within distinct microenvironments known as stem cell niches. Within the niche, stem cells may be in contact with their daughter cells, the extracellular matrix (ECM), and stromal cells. Adhesion molecules, such as Cadherins and Integrins, mediate stem cell contacts with other cells and ECM proteins, respectively. E-Cadherin-based cell adhesion is important for embryonic stem cell self-renewal and maintenance of pluripotency. In the developing mammalian embryonic brain, the loss of Integrin molecules and subsequent neural stem cell detachment from the ECM results in apoptosis. Integrins are also thought to regulate hematopoietic stem cell homing and retention in the bone marrow niche. In addition, adhesion molecules may influence asymmetric stem cell division by interacting with microtubules of the mitotic spindle and directing the plane of cell division. The daughter cell that maintains contact with the niche receives self-renewal signals, while the sister cell inherits differentiation factors and lacks potency signals from the niche. Pathologically, the loss of cell-cell adhesion molecules in cancer stem cells is thought to contribute to an epithelial to mesenchymal transition and an invasive, migratory phenotype.

The individual properties of hematopoietic stem cells, including self-renewal, maintenance of pluripotency, and asymmetric cell division, must depend at some level on the functions of specific transcription factors. Cellular programs regarding proliferation, potency, and cell fate determination can be mediated by signal transduction events that modulate transcription factor (TF) expression and/or activation. TFs promote the diferentiation of pluripotent embryonic stem (ES) cells derived from the inner cell mass of the blastocyst into stem or progenitor cells of all three vertebrate germ layers: ectoderm, mesoderm, and endoderm. In addition, the expression of specifc TFs is sufcient to reprogram somatic cells back to a pluripotent state. Induced pluripotent stem (iPS) cells, like ES cells, give rise to adult stem cells such as neural and epithelial stem cells. Adult stem cells function to replenish cells of the tissue in which they are found.

Detecting the expression of cell type-specific markers is a critical tool for defining starting stem cell populations and confirming differentiation status. Using techniques such as immunocytochemistry (ICC) and flow cytometry, these important methodological steps can reduce experimental variability, improve data consistency, and provide valuable insight that may prevent weeks of wasted effort and reagents.