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Posts Tagged ‘adult’

Cancer Stem Cells: A Possible Path to a Cure

July 5th, 2009

Cancer stem cells (CSCs), as their name implies, are stem cells that have been discovered to reside within cancerous tumors. Tumors are made up of a heterogeneous mixture of cells. Consequently, if the growth comes from a common origin it must be a cell, or cells, capable of becoming many different types of cells. This makes stem cells a very likely suspect as they, by definition, are able to give rise to a variety of cells. CSCs have been broadly defined as cells within a tumor that are able to self-renew, regenerating a population of multipotent CSCs, as well as differentiate into other cells, which can create the heterogeneity seen in tumors (Vermeulen et al., 2008).

Although the theory of cancer stem cells has been around since the 1970s (Hamburger and Salmon, 1977), recently it has gained a spotlight in the scientific community. The first functional identification of CSCs was in 1997 in acute myeloid leukemia (Bonnet and Dick, 1997). Researchers found that although there are many different populations of cells within a tumor, only one population has the ability to generate the tumor. This was determined by separating the populations from each other and engrafting them into an immuno-compromised (NOD/SCID) mouse; the population identified as CSCs was able to recreate the original tumor, including morphology and the specific differentiated cell types observed within the tumor (Vermeulen et al., 2008).

The different populations within a tumor can be separated and identified according to the proteins expressed (or produced) on the surface of a particular cell; cells expressing the same set of proteins are grouped into one population. Because such proteins are commonly used to identify and categorize cells, they are called cell markers. CSCs from the same tumor type usually have the same set of markers expressed, although the markers expressed can vary much more between CSCs from different tissues (Vermeulen et al., 2008). For example, breast cancer CSCs have been found to express a marker called CD44, but are distinct for also not expressing the marker CD24 (making this CSC population be labeled CD44+/CD24) (Al-Hajj et al., 2002). In comparison, pancreatic cancer CSCs express CD44, but also express CD24 (Li et al., 2007). Although there are differences like this in marker expression between CSCs from different tumor types, some markers are present in CSCs from many different types of tumors, such as CD44. CSCs from ovarian tumors (Zhang et al., 2008) and head and neck squamous cell carcinomas (Prince et al., 2006) have also been found to express CD44. Another major marker protein expressed in CSCs across tissue types is CD133; it is expressed by CSCs found in brain (Singh et al., 2003), prostate (Lang et al., 2008), colon (O’Brien et al., 2007), lung (Eramo et al., 2007), and hepatic (Suetsugu et al., 2006) tumors. For a more detailed summary of marker expression of CSCs from the different tumors they have been discovered in, see Table 1.

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Table 1. Cancer Stem Cell Populations Detected in Different Cancerous Tumors (CSC Markers and Percent of the Total Tumor)

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Induced Pluripotent Stem Cells: A New Stem Cell Line with a Long History

June 7th, 2009

Virtually identical to human embryonic stem cells (hESCs) except for their origin of isolation, the recently created induced pluripotent stem cells (iPSCs) (Yu et al., 2007; Takahashi et al., 2007) hold much potential for use in regenerative therapies. iPSCs are cells that were originally from adult tissues, but have been forced to produce proteins that are thought to be essential for the pluripotency of human embryonic stem cells. By making cells express these embryonic stem cell proteins, adult cells can be created that look and act nearly identical to hESCs.

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Applying Somatic Cell Nuclear Transfer in the Creation of Dolly the Cloned Sheep. Dolly the sheep was cloned through somatic cell nuclear transfer (SCNT). An adult cell from the mammary gland of a Finn-Dorset ewe acted as the nuclear donor; it was fused with an enucleated egg from a Scottish Blackface ewe, which acted as the cytoplasmic (or egg) donor. An electrical pulse acted to fuse the cells and activate the oocyte after injection into the surrogate mother ewe. A successfully implanted oocyte developed into the lamb Dolly, a clone of the nuclear donor, the Finn-Dorset ewe.

The idea of reprogramming a cell from adult tissue into an embryonic-like, pluripotent cell existed long before the creation of iPSCs. In 1938, Hans Spemann showed that a nucleus from a fertilized salamander egg that had already undergone cell division several times could be implanted into a cell from a newly fertilized salamander egg that is enucleated (has had its nucleus removed) and create an entire adult salamander (Spemann, 1938). Consequently, Spemann’s work suggests that an embryonic nucleus remains totipotent, or is able to develop into any cell type of the adult body, even after several cell divisions. Due to technical difficulties, it was several years before researchers could repeat these experiments using older nuclei to see how long the nucleus retains its pluripotency. In the early 1950s, Robert Briggs and Thomas King repeated Spemann’s experiments using a species of leopard frog, Rana pipiens, first with a nucleus from young embryos (Briggs and King, 1952) then from older embryos (King and Briggs, 1954); both the younger and older implanted nuclei could still be reprogrammed by the enucleated host cell. However, they also observed that the older the donor nucleus was, the more difficult it was to reprogram it to a totipotent state. For years it was unclear whether the nucleus from a fully differentiated, adult cell could be completely reprogrammed, as conflicting results were published by different groups (Briggs and King, 1957; Fishberg et al., 1958; Gurdon and Byrne, 2003).

Although the studies done by Spemann, Briggs, and King used nuclei from embryos, their results are the basis for somatic cell nuclear transfer (SCNT). SCNT is a technique wherein the nucleus from a somatic cell (an adult cell that is not a sperm or egg, i.e. not the gametes) is implanted into an enucleated egg cell which can then be implanted into, and develop in, a surrogate mother, and potentially become an adult organism. The resultant organism is a clone of the animal that donated the nucleus. The first widely-accepted successful use of SCNT came with the creation of the sheep Dolly in 1997, the first cloned animal from an adult cell and the first cloned mammal (Wilmut et al., 1997). Since then, several other animals have been successfully cloned, though many problems still remain and there are low success rates (Wilmut et al., 1997; Wakayama et al., 1998; Solter, 1998; McKinnell and Di Bernardino, 1999; Gurdon and Byrne, 2003).

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Stem Cells Discovered in Menstrual Blood: Endometrial Regenerative Stem Cells

March 27th, 2009

Often the feasibility of using stem cells for regenerative therapies is limited by two factors: obtaining a significant number of cells and doing so in a relatively noninvasive manner. Because our bodies freely shed a limited and select number of cells, many stem cell types must be obtained using a rather invasive procedure. However, around the beginning of last year two laboratories independently reported the discovery of a new type of stem cell that may overcome both obstacles; stem cells were found to reside in menstrual blood (Meng et al., 2007; Patel et al., 2008). These stem cells, termed endometrial regenerative cells (ERCs), are not only harvested in a noninvasive manner and relatively readily available in large quantities, but they potentially overcome the problem of immune rejection in many female patients as well.

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The uterus is lined by a layer of cells called the endometrium. During the menstrual cycle, the endometrium cycles between thickening and being broken down if fertilization does not occur. The break down and expulsion of the endometrium is called menstruation, or menstrual bleeding, and is the source of endometrial regenerative cells (ERCs).

Researchers suspected stem cells to be present in menstrual blood because stem cells were previously found to be present in the lining of the uterus. The wall of the uterus is lined by a layer of cells called the endometrium (see figure). To create ideal conditions for the uterus to accept and nurture an embryo, the endometrium lining becomes thicker and increases the number of blood vessels and glands within it. However, if implantation does not occur, the endometrium lining is broken down and shed. Overall, the endometrium is quite a hyperproliferative tissue, continuously being broken down and rebuilt; it is an ideal tissue to investigate for the presence of stem cells. In the menstrual cycle, the shedding is known as menstruation, or menstrual bleeding; the excreted menstrual blood is made up of blood as well as cells from the endometrium layer. Researchers previously reported the presence of stem cells in the intact endometrium lining of the uterus (Cho et al., 2004; Schwab et al., 2005; Du and Taylor, 2007). Because stem cells were found in the endometrium, researchers thought it likely that stem cells could also be found in the shed endometrium in the form of menstrual blood, which can be obtained in relatively large quantities in a much less invasive manner. However, the stem cells discovered in menstrual blood, ERCs, appear to be rather different from stem cells derived from the intact endometrium.

While stem cells from the intact endometrium appear to be mesenchymal stem cells (MSCs, as discussed earlier), ERCs do not; they are distinctly different not only in their undifferentiated state, but in the cells they can differentiate into as well. Researchers categorize stem cells into certain groups based off of, among other factors, their cell morphology and the proteins they express. An established stem cell group usually expresses a distinct set of proteins. ERCs, though morphologically appearing mesenchymal, were found to express only some, but not all, proteins characteristic of MSCs. Additionally, ERCs were reported to be able to differentiate into, or become, cells from the three different germ layers (see the previous post on MSCs for more details): mesoderm (muscle, bone, fat, cartilage, and endothelial cells), ectoderm (neurons), and endoderm (liver, pancreas, and lung cells) (Meng et al., 2007; Patel et al., 2008). However, the mesenchymal stem cells from the intact endometrium cannot generate cells from all three germ layers. Overall, ERCs were determined to be functionally distinct from endometrium MSCs (Meng et al., 2007; Hida et al., 2008).

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Mesenchymal Stem Cells: A Diverse Family, Large and Still Growing

March 15th, 2009

Perhaps containing more different cell types than any other stem cell category, mesenchymal stem cells (MSCs) can be isolated from a wide variety of tissues in the human body. These cells have been grouped and labeled as “mesenchymal” because they are thought to have a common progenitor in the mesenchyme, an embryonic tissue (Caplan, 2005). In the developing vertebrate embryo, there are three distinct “germ layers,” or layers of cells: the endoderm, the mesoderm, and the ectoderm. Together with the germ cells, these three layers pattern out the entire body (see figure). The mesenchyme is a collection of cells mostly derived from the mesoderm that later becomes supportive structures throughout the body, including bone, cartilage, connective tissue, smooth muscle, adipose tissue, as well as the lymphatic and hematopoietic systems. Most MSCs are thought to contain progenitors in the mesenchyme (Gilbert, 2003; Conrad et al., 2009; Caplan, 2005).

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The endoderm layer later becomes skin (epidermis) and the nervous system, the ectoderm becomes the digestive tract and respiratory system, and the mesoderm becomes bone, blood, muscles, connective tissue, and several organs (heart, kidney, and gonads).

However, calling MSCs “mesenchymal” can be misleading. Because this term refers to a precursor of the large MSC family, it is referring to an embryonic tissue, though the descendant MSCs can be found in both fetal and adult tissues. MSCs have been isolated from adult muscle, bone marrow, adipose tissue, cartilage, bone, potentially teeth (Caplan, 2005) as well as some fetal tissues (fetal liver, lung, amniotic fluid, and umbilical cord) (Phinney and Prockop, 2007). The MSCs isolated from any one of these tissues are multipotent and are usually shown to be MSCs by being able to differentiate into at least three different, standard mesenchymal cell types: osteocytes (bone), chondrocytes (cartilage), and adipocytes (fat) (Baksh et al., 2004). There is much evidence, though somewhat inconsistent, showing that MSCs can also differentiate into neuronal cells, which may be from mesenchyme derived from the endoderm instead of the mesoderm (Gilbert, 2003; Phinney and Prockop, 2007). Overall, MSC differentiation potentials can vary depending on what mesenchyme-derived tissue the MSCs were harvested from (Phinney and Prockop, 2007). However, MSCs cannot become hematopoietic cells (which are derived from hematopoietic stem cells), even though these cells are derived from the mesenchyme, making the label “mesenchymal” more deceptive (Gilbert, 2003; Caplan, 2005).

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Potential of Stem Cells to Cure HIV

March 1st, 2009

Recently, a patient with leukemia and human immunodeficiency virus (HIV) had apparent remission of both after stem cell transplants (Hütter et al., 2009). As discussed earlier, hematopoietic stem cells have been used in transplants to rescue patients with leukemia, but this method has not previously been as successful for treating HIV, the virus that causes acquired immunodeficiency syndrome (AIDS).

Once in the body, HIV primarily attacks the immune system, such as T cells, though some individuals have T cells that are naturally resistant to HIV infection. Over a decade ago, this resistance was found to be due to a mutation in a receptor that is normally on the cell surface of T cells, called chemokine receptor 5 (CCR5) (Liu et al., 1996). CCR5 is a chemokine receptor, meaning it normally binds and receives signals from chemokines, which are molecules cells can release and receive to cause an immune system response. CCR5 is thought to normally be involved in causing a response to infection, though its exact function is not fully understood. HIV normally interacts with CCR5 to gain entry into the target T cell, but some individuals have a mutation in the CCR5 gene, specifically a 32 base-pair deletion, that renders the resultant receptor completely nonfunctional and consequently prevents HIV from being taken into these cells (Liu et al., 1996).

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The T cell membrane (shown as the purple, semicircle double line) allows entry of HIV (in pink) into the cell through multiple cell receptors anchored on the membrane, including CCR5.

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