All Things Stem Cell

“STEM CELL REVOLUTIONS” is an informative and engaging documentary recently distributed by the Scottish Documentary Institute. It’s a very useful film to see if you want to learn more about the history of stem cells, and where the clinical, cutting-edge technology is at currently. The documentary gives an overview of international stem cell history, starting with the discovery of stem cells and ending with the newest members of the ever-growing stem cell family. To summarize such a wealth of research, research that has been going on for over half a century, the film tells the story of a few key stem cell discoveries and applications. Each story is described through interviews with stem cell researchers who were directly involved or appeared on the scene later but can knowledgably discuss the event’s impact. The first group of stories is related to adult stem cells (although this is not explicitly stated or explained): the discovery of stem cells during WWII, the amazing rescue of two boys in the early 1980s using stem cell-based skin grafts, and the present-day treatment of blind patients in a stem cell clinic in India. The final group of stories is related to pluripotent stem cells: the discovery of embryonic stem cells (ESCs) in mice in 1981 by Martin Evans (it was a treat to see Evans, who won the Nobel Prize in 2007 for the research he discusses in the film!) and of human ESCs (hESCs) in 1998 by Jamie Thomson, present-day use of hESCs to treat patients with retinal disorders in London (although I shuddered a little when Pete Coffee handled a flask of cells without gloves on!), and the creation of induced pluripotent stem cells (iPSCs) by Shinya Yamanaka in 2006.

The science presented in the film is well-explained and even though the focus of the film is on medical breakthroughs accomplished using stem cells, the scientists interviewed do not try to over-hype current stem cell applications. Most helpful in making the technical information accessible are several short, accurate, and intriguing animations (made by Cameron Duguid). During a segment on Yamanaka’s research, one of these animations is particularly useful in explaining how chromatin regulation of gene expression is different in different types of tissues. However, it is repeatedly jarring when the interviews with down-to-earth stem cell scientists, who mostly do not over-hype their research, are bookended by interviews with Margaret Atwood (a writer who is confusingly repeatedly interviewed in a laboratory setting). She makes repeated references to The Fountain of Youth – at odds with the scientists’ messages. Similarly, repeatedly interspersed videos of a topless man doing what looked to be the Brazilian martial art of Capoeira seemed out of place.

Perhaps the only shortcoming of the film, if a bit minor, is that it shies away from getting into some of the nitty-gritty of why iPSCs may be better than hESCs or vice versa, but instead falls back upon the standard argument that hESCs are surrounded by ethical concerns. For a 71-minute-long film, it only makes sense that some issues be simplified, but additional details may have helped viewers better understand this important and hotly-debated topic. Specifically, a lot of the ethical arguments against hESCs are outdated or ill-founded. Probably most importantly, in 2006, Irina Klimanskaya and colleagues found how to isolate hESCs while leaving the donor embryo intact and potentially able to develop normally, weakening the argument against the generation of hESC lines on the grounds that they require the destruction of a potential embryo. Additionally, many researchers use blastocysts that would have been discarded by the in vitro fertilization clinic because the embryos were damaged in some way and would never develop properly. However, a significant strike against using hESCs in treatments, which the film does not touch upon, is the potential for immune rejection. Human iPSCs, on the other hand, are very appealing because they potentially may not have immune rejection problems in treatments, as mentioned in the film. However, human iPSCs are much newer to the stem cell scene and have similarities with cancer cells that researchers should probably better understand before iPSCs are widely used clinically. It is also a little surprising that Jamie Thomson is not mentioned in the human iPSC segment, as his group independently created human iPSCs at the same time as Yamanaka’s group.

The researchers interviewed in the film emphasize the importance of striking a balance between regulation and progress, but then the film seems to not take its own advice and gets bogged down in the regulation of stem cells in the very last segment of the film, when it may have been more useful to focus on the near-future applications of these cells. There’s a surprising focus on the hypothetical ethical arguments that would arise should human iPSCs be made into function eggs and sperm (which has not been done yet, and may not even be possible). However, it may be more useful to first focus on whether human iPSCs can even be successfully used in the clinic before diverting attention to this hypothetical ethical argument, which is much further down the road. It would also have been nice to see a mention of direct reprogramming, the latest stem cell technology that may one day make even iPSCs obsolete.

While there are amazing advances being made with stem cell technology, the film rightly cautions viewers about the dangers of going to a stem cell clinic abroad. A great resource for those considering stem cell treatments abroad is A Closer Look at Stem Cell Treatments, a website made by the reputable International Society for Stem Cell Research.

Overall, “STEM CELL REVOLUTIONS” is a great film for anyone wanting to learn more about the history of stem cells, hear legendary researchers talk about their ground-breaking work and patients talk about how stem cell therapies have changed their lives, and still get a down-to-earth idea of what is realistically being accomplished with these cells.

admin Bioengineering, Embryonic Stem Cells, Hematopoietic Stem Cells, Reprogramming, Review adult, clinical trials, embryonic, history, regenerative medicine, review

It’s known that stem cells, the key players in regenerative processes in the body, play a key role in continually making new hair. This role created interest in studying hair follicle stem cells to better understand androgenetic alopecia (AGA), or male pattern baldness, the most frequent type of hair loss among men. Naturally, the hair follicle stem cells were the prime suspects for causing AGA. However, earlier this month a study by George Cotsarelis at the University of Pennsylvania School of Medicine and colleagues published in The Journal of Clinical Investigation (Garza et al., 2011) revealed that patients with AGA actually had had a normal amount of follicle stem cells in their scalps. Surprisingly, it was found that different progenitor cell populations, suspected to be derived from the hair follicle stem cells, were in fact the ones playing the key roles in causing AGA. (Progenitor cells are like stem cells in that they can differentiate into different cell types, but progenitors’ fates are more limited and they can replicate only a restricted number of times.) By better understanding the exact cell types involved, it may help researchers devise better therapies for treating AGA.

A Hair’s Life-Cycle: In order to understand AGA and the newly discovered key role of these progenitor cells in it, it’s helpful to first review the normal life of a hair. In the skin, every hair sits inside a hair follicle, a little cavity that goes down through the dermis layer and has connected sebaceous glands (which lubricate the hair by secreting an oily substance) an arrector pili (a small bundle of muscles that can make the hair stand on end) (see Hair Follicle figure). Each hair carries out its own life-cycle. The first lifecycle phase is called anagen, a growing period that about 85 percent of the hairs on a person’s head are in at any given time. During anagen, which can last two to six years for one hair, the hair grows at the rate of about five inches a year. After anagen, the hair enters catagen, a transitional one- to two week-long stage when the hair follicle and root both shrink. The hair then enters the last stage, telogen, which is a resting phase that lasts about five to six weeks, during which time the old hair does not grow. At the end of telogen the hair follicle re-enters anagen, the growth phase, and often a new hair will push the old one out, starting the growth cycle over again (Furdon & Clark, 2003; Garza et al., 2011).

Every hair sits inside a hair follicle, which goes down through the epidermis and dermis of the skin. Connected to the follicle are sebaceous glands, which release oils onto the hair, and arrector pili muscles, which can cause hairs to stand on end. The bulge is where the majority of the hair follicle stem cells reside, and these can give rise to multipotent progenitor cells.

Androgenetic Alopecia: Normally, the new hair will grow similar to how the last one did. However, with AGA this isn’t the case. In AGA, hair follicles get smaller over time, and consequently make smaller and smaller, eventually microscopic, hairs. How is this caused? It’s not that well understood; it’s known that testosterone is necessary for this miniaturization (as inhibiting testosterone conversion to its active form can delay AGA progression), but not much else is known about what causes AGA (Garza et al., 2011).

But even if it’s not known what happens to cause AGA, researchers have done a lot of work to figure out what stem cells are normally active in the hair follicle. Within a hair follicle, there are stem cells that reside in an area called the hair follicle “bulge,” which is a small compartment located where the outer root sheath meets the arrector pili muscle (see Hair Follicle figure). The stem cells in the bulge are multipotent epithelial stem cells, and can become, or differentiate into, all the epithelial cell types in the follicle (including hair follicles, epidermis, and sebaceous glands) (Oshima et al., 2001). They’re intimately involved in the hair follicle lifecycle. Given this, it shouldn’t come as a surprise that if these stem cells are destroyed, so is the hair follicle (Ohyama et al., 2006; Ohyama 2007).
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admin Hair Stem Cells adult, regenerative medicine

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.

admin Embryonic Stem Cells, Induced Pluripotent Stem Cells, Reprogramming adult, cancer, developmental, embryonic, hematopoietic, history, news, potency, regenerative medicine

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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admin Bioengineering, Mesenchymal Stem Cells adult, clinical trials, mesenchymal, regenerative medicine

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