All Things Stem Cell

While it is widely accepted that embryonic stem cells (ESCs) have the ability to become any type of cell, the molecular causes for this characteristic are still under much investigation, although one suspected player is chromatin. Recently, more evidence has been reported to support the important role of chromatin structure in maintaining an undifferentiated state in ESCs; the specific protein involved is called Chd1 (Gaspar-Maia et al., 2009).

DNA is condensed on histones, creating a structure called chromatin. (Left) A single DNA strand (formed by a sugar-phosphate backbone and nucleotide base-pairs). (Right) Chromatin is the complex formed by histones (green) and DNA (blue); the DNA can be tightly wrapped around the histones. (DNA bound to histones may be inaccessible to the transcription machinery, preventing the transcription of these genes, while unbound DNA allows space for the machinery and the genes may be transcribed.) Chd1 may function in ESCs to maintain chromatin in an open (euchromatin) state and potentially promote pluripotency in this way.

Chromatin structure plays an important role in regulating what genes are created, or expressed, in a given cell. In eukaryote organisms (almost all large organisms, such as animals, plants, and fungi, but not bacteria), DNA forms a complex with proteins that are called histones. This complex of DNA and histones is called chromatin (see figure). Histones act as spools for the DNA to be spun around, binding to DNA and packaging it into tightly coiled units (without histones, the long DNA strands would take up a very large amount of space). Whether the histones bind to the DNA or not can be regulated through chemical modification of the histones (they can be methylated or acetylated). When histones are bound to the DNA, the chromatin is in a condensed state (called heterochromatin) and the genes are not expressed because they cannot be accessed by the gene transcription machinery. However, when the histones are not bound to the DNA, the chromatin is extended (called euchromatin), and the DNA can be accessed and these genes can be expressed.

It was previously believed that embryonic stem cells had lots of open chromatin (euchromatin), but this was not a proven theory. A study on stem cells and gene expression (Efroni et al., 2008) reported that, globally but at low-levels, more genes in ESCs are actively turned into protein than are in differentiated cells. Additionally, proteins involved in changing chromatin structure and transcribing genes were expressed at relatively high levels in ESCs too. When the function of some proteins involved in chromatin-remodeling was changed, normal ESC proliferation and differentiation was also affected. Overall, Efroni et al. suggested that the differentiation of ESCs may correlate with a loss of active transcription of the cell genome.

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admin Embryonic Stem Cells, Induced Pluripotent Stem Cells embryonic, history, potency © 2012. Teisha Rowland. All rights reserved.

While embryonic stem cells are widely studied, a lesser known, but still significant, population of stem cells also resides within the early developing embryo: trophoblast stem cells (TSCs).

In brief, in most mammals the trophoblast is the part of the early embryo that later significantly contributes to the placenta of the fetus. The embryo and mother work together to create the placenta; while the trophoblast of the embryo becomes the chorion part of the placenta, the maternal uterine cells and surrounding blood vessels form the maternal placental components (Gilbert, 2003).

The placenta is the organ in mammals that connects the uterine wall to the developing fetus, bringing the two blood systems close together. The placenta allows the fetus to safely receive essential gases, such as oxygen, and nutrients from the mother. At the same time, it also lets the fetus expel waste through the mother’s kidneys. Additionally, the placenta releases essential pregnancy-related hormones and growth factors that, for example, let the uterus hold the fetus. Lastly, the placenta secretes immune response regulators to give the fetus immune protection against the mother (so that the fetus is not rejected by the mother’s immune system, as a tissue graft or organ transplant would be) (Rossant and Cross, 2001; Gilbert, 2003). Overall, the placenta plays a key role in early development; even small abnormalities in the placenta can lead to death of the fetus (Rossant and Cross, 2001).

Figure 1: The blastocyst is a hollow sphere made of approximately 150 cells and contains three distinct areas: the trophoblast, which is the surrounding outer layer that contains the trophoblast stem cells and later becomes the placenta, the blastocoel, which is a fluid-filled cavity within the blastocyst, and the inner cell mass, also known as the embryoblast, which can become the embryo proper, or fetus, and is where human embryonic stem cells are isolated from. When the late blastocyst is implanted in the uterine wall, at day 7 or 8 in human development, the trophoblast stem cells (in the trophoblast) quickly differentiate to form cells required for a firm implantation and, later, for the placenta.

While TSCs give rise to the placenta, these stem cells establish their identity long before the placenta develops; their fate is determined during the early embryo. Soon after the egg and sperm join during fertilization, the resultant zygote (fertilized egg cell) starts undergoing cell division. The resulting cells continue to undergo synchronous cell division. When the embryo is at the 16-cell stage (called a morula), it is a solid sphere of cells and already the precursors of the trophoblast cells are defined; the external, relatively larger cells mostly become the trophoblast cells. By the 64-cell stage, these cells’ fates are set; while the trophoblast will become the placenta, the other cells in the embryo can become the fetus. In mammalian development, this is the first differentiation event (Rossant and Cross, 2001; Gilbert, 2003).

A few cell divisions later, the trophoblast contributes to significant cellular rearrangements in the embryo which make it enter the blastocyst stage (see Figure 1). The blastocyst, which contains approximately 150 cells, is made up of three main parts: the blastocoel (an internal, fluid-filled cavity), the inner cell mass (ICM), and the trophoblast. When the embryo was a morula, the surrounding trophoblast precursors caused fluid to be secreted into the morula (utilizing sodium pumps in the trophoblast cell membranes); this secretion created the blastocoel cavity. The ICM is a cluster of cells inside the blastocyst that will later become the adult organism; human embryonic stem cells can be derived from the ICM, as was previously discussed. Lastly, the trophoblast is a monolayer of cells, specifically polarized epithelial cells, which surround the blastocoel and ICM, similar to their future role of surrounding the fetus as its placenta (Rossant and Cross, 2001; Gilbert, 2003).

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admin Trophoblast Stem Cells developmental, embryonic, regenerative medicine © 2012. Teisha Rowland. All rights reserved.

Teisha J. Rowland, the author of All Things Stem Cell, recently started a general biology column with The Santa Barbara Independent. This new column, titled “Biology Bytes,” will have weekly stories posted on a wide variety of biology topics, so far ranging from snails, marsupials, and parrots, to stem cells.

The most recent article, “Likely Suspects in Cancer Growth,” is on cancer stem cells — it is a modified version of the “All Things Stem Cell” post “Cancer Stem Cells: A Possible Path to a Cure” to fit a more lay public audience.

Tune in to “Biology Bytes” for weekly stories on a wide array of fascinating biology topics, including more accessible explanations of stem cell biology.

admin Cancer Stem Cells, Resource Biology Bytes, cancer © 2012. Teisha Rowland. All rights reserved.

While there is great potential for using stem cells in regenerative therapies, there is still a ways to go before it can be considered a proven practice, although recent breakthroughs, and one specific trial in particular, makes it seem much closer. Recently, the first human tissue-engineered organ using stem cells was created and transplanted successfully into a patient. Other tissue regeneration efforts with stem cells have also recently made many breakthroughs, emphasizing the potential of using stem cells in future tissue transplants.

In the first reported instance of using stem cells to bioengineer a functional human organ, Paolo Macchiarini and his research group used a patient’s own stem cells to generate an airway, specifically a bronchus, and successfully grafted it into the patient to replace her damaged bronchus (See Figure 1). Macchiarini’s group bypassed the problem of immune rejection by using the patient’s own stem cells. Additionally, by combining a variety of bioengineering efforts, no synthetic parts were involved in the creation of the organ; it was made entirely of cadaveric and patient-derived tissues (Macchiarini et al., 2008; Hollander et al., 2009).

Figure 1. In order to create a patient-compatible replacement bronchus, Macchiarini’s group removed and decellularized a trachea from a cadaveric donor, grew cells removed from the patient on the trachea in a bioreactor, and then transplanted the bioengineered airway into the patient, successfully replacing their defective bronchus (Macchiarini et al., 2008).

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Recently, many papers have come out that highlight connections between cancer and induced pluripotent stem cells (iPSCs), the latter of which was discussed previously. These papers hold many implications for not only iPSCs, but for our understanding of cancer as well. Additionally, these papers should not at all be thought of as invalidating the importance of iPSCs for studying and treating future therapies, but they should help us better understand what iPSCs are and how to use them appropriately.

The most recent and most publicized link between iPSCs and cancer is p53. p53, also known as protein 53 (53 referring to its molecular mass), is a well-studied protein whose normal function is important in preventing cancer. Though p53 has many different roles, they are quite related. In essence, the job of p53 is to make sure the cell does not accumulate DNA damage, or DNA mutations, which could eventually make the cell cancerous. When a cell has its DNA damaged, often from external stresses, p53 stops the normal cell cycle to fix the DNA damage. If the damage is too great to repair, p53 can prevent the cell from dividing, which would create more damaged cells; p53 initiates programmed cell death, or apoptosis. The potential tumor cell dies. Overall, p53 functions as a “tumor suppressor” to prevent abnormal cells from occurring and multiplying into a cancer (Vazquez et al., 2008). Consequently, it has been found that p53 is mutated in approximately 50% of all human tumors, and other tumors may have mutations in the pathway regulating p53 activity (Vazquez et al., 2008). p53 is therefore well-studied as an oncogene, or a gene that when not functioning normally can contribute to a normal cell becoming cancerous.

So what does p53 have to do with iPSCs? One recently discovered connection is with the generation of iPSCs. Recently, many research groups discovered that when p53 is deleted from, or damaged in, their cells, they could more easily become iPSCs (Hong et al., 2009; Kawamura et al., 2009; Utikal et al., 2009; Li et al., 2009; Zhao et al., 2008). As posted earlier, iPSCs are cells that were originally from adult tissues, but have been “reprogrammed” to be pluripotent stem cells, or stem cells able to become all the adult cells of the body, looking and functioning nearly identical to human embryonic stem cells (hESCs) (Takahashi et al., 2007; Yu et al., 2007).

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admin Cancer Stem Cells, Embryonic Stem Cells, Induced Pluripotent Stem Cells adult, cancer, embryonic, news, regenerative medicine © 2012. Teisha Rowland. All rights reserved.