All Things Stem Cell

International Stem Cell Awareness Day is October 3, 2012, so on this day please help spread the word about the importance of stem cell research! Stem cell researchers across the world are investigating how stem cells can be used to improve our lives, from repairing and regenerating damaged or lost tissues, to developing cures for numerous devastating diseases and conditions, such as cancer, Alzheimer’s, macular degeneration, Parkinson’s, and paralyzing spinal cord injuries, and various other useful applications in between: They’re being used to help us learn more about the entire developmental process (giving us a better understanding of how to fix problems that can arise during development), the efficacies of different drugs are studied and characterized using stem cells, and their unique biological roles make them ideal for use in better understanding aging.

So please be sure to get out the word on stem cells this October 3! For more information on International Stem Cell Awareness Day (and free wallpapers and downloadable stem cell images!), visit StemCellsOfferHope.com, which is affiliated with the Sue & Bill Gross Stem Cell Research Center at the University of California, Irvine. Read on for a summary of stem cell history and recent research breakthroughs and highlights.

With all of the breaking news stories that come out on cutting-edge stem cell findings all the time, it can be easy to lose sight of the bigger picture. Yes, the stem cell family, which includes all of the varieties of stem cells that have been discovered so far, is very large, and growing larger with new children, cousins, uncles, and aunts being discovered or created all the time. But a key feature they all share is their potential to improve our lives.

Our understanding of these cells and their incredible potential for treating diseases, fight cancers, heal wounds, and, in essence, saving lives, has grown hugely since we first unknowingly used them in World War II. However, the more we learn about them the more we realize we have yet to understand. This blog has strived to explore the different stem cell types in detail, including their biology, history, potential, clinical applications, and numerous remaining questions. However, the ways in which the different types of stem cells came to be accepted into the stem cell family is itself an interesting story, and one that can help paint a useful bigger picture, and that is why this story will be the focus for this blog post to celebrate International Stem Cell Awareness Day.

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A recently published paper showed that mice with colon cancer can be “vaccinated” with human embryonic stem cells and have a significant immune response against the cancer (Li et al., 2009). This study relates to a big hurdle that needs to be overcome in order to better fight cancer: immune tolerance. The immune system usually fails to detect and attack cancerous tumors, and consequently many cancer treatments are currently being developed that stimulate the immune system to fight back (e.g. the growing field of cancer vaccines).

Cancerous tumors and embryonic tissues have been found to share many of the same antigens, which are detected by the immune system through antibodies. This group of antigens is called oncofetal antigens. Consequently, animals can be vaccinated with embryonic tissues/cells (most recently done with human embryonic stem cells) and develop an immune response against cancer.

Interestingly, this state of immune tolerance is similar to what happens during pregnancy, and, more specifically, it’s been found that the body’s response to a tumor is very similar to its response to embryonic tissues. While much recent research has not been published in this area, there is actually a long history of studies that show: (1) there is a significant number of antigens shared between tumors and embryonic tissues (called “oncofetal antigens”) and, consequently, antibodies made against tumors can also recognize embryonic tissues, and vice versa; (2) pregnancy confers some immunity against cancer (accompanied by antibody production against oncofetal antigens), not only against its occurrence but also against its growth; (3) similar to pregnancy, an immune response against cancer can be generated by vaccinating animals with embryonic tissues. These studies and the recent re-visitation will be explored below (for a more detailed review, see Brewer et al., 2009).

The first published suggestion that tumors may have an embryonic nature came in the early 1800s (Muller, 1838). Tumors were suspected to be tissues that had been triggered to become embryonic-like again, and it is now generally accepted that tumors are indeed more “embryonic” than the tissues they are derived from, due to the re-expression of embryonic-related genes. By the late 1800s, researchers understood cancer enough to realize that they must better understand normal development in order to better combat cancerous tumors and their embryonic-like cells (Brewer et al., 2009). In the 1880s, these studies shifted focus; the field of immunology was born (from research conducted by Louis Pasteur, at the University of Strasbourg, and Robert Koch, as a medical officer in Poland) and many researchers focused on creating vaccines to cure diseases. Cancer was no exception.
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A goal of regenerative medicine has been to be able to take any cell from a person’s body and turn it in to any other cell type that may be desired (such as insulin-producing beta-cells for treating diabetes, or creating neurons to treat a neurodegenerative disease). This would eliminate several donor-compatibility problems, and potentially eliminate the need for a donor (who isn’t the patient) altogether. In 2007, human induced pluripotent stem cells (iPSCs) were created and this goal seemed a bit closer (Yu et al., 2007; Takahashi et al., 2007). iPSCs are cells that can be take from adult tissue and “reprogrammed” into embryonic stem cell (ESC)-like cells. Because iPSCs are pluripotent, these cells can then differentiate into (or become) any cell type (for more information, see the All Things Stem Cell article on “Induced Pluripotent Stem Cells: A New Stem Cell Line with a Long History”).

But is it possible to get rid of the iPSC-middle man? Is it possible to take any cell in the adult body and directly reprogram it, skipping the iPSC state, into the final desired cell type? There have been several studies over the last few decades that show this is quite possible, though it still has a ways to go before it can be regularly used in the clinic.

Reprogramming of cells to a different cell type is usually done by either somatic cell nuclear transfer (SCNT) or by using transcription factors. This post will focus on work done with transcription factors (for more information on using SCNT, see the “Induced Pluripotent Stem Cells…” post). Transcription factors are expressed (or made) at different levels in different cell types, and control what genes are expressed in every cell, making sure, for example, that a liver cell remains a liver cell and does not become a neuron. A famous example of how transcription factor expression can be used to alter a cell’s identity is the creation of iPSCs, where adult cells were forced to express transcription factors normally expressed in ESCs, which made the adult cells express genes specific to ESCs, and consequently become nearly identical to ESCs.

There are many degrees of direct reprogramming that have been reported over the last few decades. Several progenitor cells, cells that appear to be committed to their fate but not yet fully differentiated, have been shown to be capable of dedifferentiating into a different cell type; this process is called transdetermination. However, in a few cases it has been shown that a fully differentiated cell can actually become a different cell type; this process is called transdifferentiation (Graf and Enver, 2009). Over the last few decades, much progress has been made in direct reprogramming with muscle, blood, the pancreas, and neurons.

Muscle

In the 1980s, the first reprogramming experiments using transcription factors took place. In 1987, a group reported using MyoD to make fibroblasts become muscle cells (Davis et al., 1987). Fibroblasts are cells important for wound healing (they secrete essential extracellular matrix proteins) and are common in connective tissues. The specific fibroblasts used were embryonic mouse fibroblasts. Because they were embryonic, this process is called transdetermination; the embryonic fibroblasts could probably differentiate more easily than adult fibroblasts (Graf and Enver, 2009). To convert the fibroblasts into muscle cells, the researchers transfected the fibroblasts with the cDNA of MyoD, forcing the cells to express MyoD (Davis et al., 1987). MyoD is normally only expressed in skeletal muscle, and it was later found to be a transcription factor involved in the differentiation of muscle cells and also a very early marker of muscle cell fate commitment.

Because of its success with the fibroblasts, MyoD was subsequently used in many other reprogramming studies to see what other cells it could make into muscle. It was found that while MyoD could indeed convert many different cell types into muscle, including fibroblasts in the dermal layer of skin, immature chondrocytes (cells in cartilage), smooth muscle, and retinal cells (Choi et al., 1990), MyoD could not turn any cell type into muscle; it was found incapable of making muscle out of hepatocytes (cells in the liver) (Schäfer et al., 1990).

Blood

In the 1990s, another key direct reprogramming factor was discovered, specifically involved in hematopoiesis. Hematopoiesis is the process by which the different types of blood cells are generated in the body (the term literally means “to make blood”). (For information on hematopoietic stem cells, see the All Things Stem Cell article “Hematopoietic Stem Cells: A Long History in Brief”). The central hematopoiesis-regulating factor discovered was the transcription factor GATA-1.

In 1995, a group reported that when GATA-1 was added to or removed from avian monocyte precursors, it could turn them into erythrocytes, megakaryocytes, and eosinophils (Kulessa et al., 1995). To understand the significance of these findings an inspection of hematopoiesis is required (see Figure). During hematopoiesis, hematopoietic stem cells (HSCs) (also called hemocytoblasts) give rise to all the different types of blood cells. Specifically, HSCs can first differentiate into either a common myeloid progenitor cell or a common lymphoid progenitor cell; either progenitor then further differentiates into specific blood cell types.

Direct Reprogramming in the Hematopoietic System. Several different transcription factors have been found that can directly reprogram one type of blood cell into another. Changing the expression levels of GATA-1 in monocytes (red) can make them differentiate into eosinophils, erythrocytes, or megakaryocytes. Making B-cells (B lymphocytes) express C/EBP transcription factors (blue) can cause them to differentiate into macrophages. Lastly, C/EBPs can also inhibit the function of the transcription factor Pax5; when Pax5 is deleted in B-cells they differentiate into T-cells (T lymphocytes), though they first dedifferentiate into a common lymphoid progenitor.

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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.

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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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