Hwa A. Lim
D’Trends, Inc., California, USA
The human body may look like a fairly permanent structure. Most of it is actually in a state of constant flux as old cells are discarded and new ones generated in its place. Each kind of tissue has its own turnover time, partially dependent on the workload endured by its cells,.
The epidermis (surface layer of the skin) is recycled every two weeks or so. The reason for the relatively quick replacement is that the skin surface is the body’s “saran wrap”, and it can be easily damaged by scratching, solvents, and wear and tear. The red blood cells bruised and battered after traversing nearly 1,000 miles through the maze of the body’s circulatory system, last only 120 days or so on average. They are then dispatched to the graveyard in the spleen. The liver is the detoxifier of all natural plant poisons and drugs that pass a person’s lips. Its life on the chemical warfare front is quite short, with a turnover time of between 300 - 500 days.
Other tissues have lifetimes measured in years, but they are still far from permanent. For a structure that looks as permanent as the skeleton, the body’s twin construction crews of bone-dissolving and bone-rebuilding cells combine to remodel it constantly. The entire human skeleton is replaced every 10 years or so in adults.
The same may not be said of the neurons of the cerebral cortex, the inner lens cells of the eye and perhaps muscle cells of the heart. On present evidence, they seem to be the only pieces of the body that last a lifetime. The inner lens cells, for example, form during embryonic development and then lapse into inertness for the rest of the owner’s lifetime. In fact, they are so inert that they dispense altogether with their nucleus and other cellular organelles. Once these cells are established, they function until they die or the organism of which they are a part dies.
Development does not cease after birth. Some cells in the body cannot reproduce. Other cells, including those in the liver, blood vessels, pancreas and body tissues, however, multiply by simply replicating. This is evident in liver cells — when liver cells die, other liver cells divide to make up for the loss.
There is yet another category of cells which originate from undifferentiated cells known as stem cells. When a stem cell divides, each of its two daughter cells has two choices: it can remain a stem cell, or it can differentiate into another cell type. In other words, stem cells remain uncommitted until they receive signals from the body to develop into specialized cells. For example, blood cells — white and red blood cells — originate from a single type of stem cell. Stem cells are thus the body’s primordial master cells that have the capacity to grow into other specialized cells, including those that replicate, and those that do not regenerate.
Now comes the burning question: If the body remains so perpetually youthful and vigorous, and so eminently capable of renewing its tissues, why does not regeneration continue forever? A theory is that the DNA accumulates mutations and its information is gradually degraded. A second theory blames the DNA of the mitochondria, which lacks the repair mechanisms available for the chromosomes. A third theory is that stem cells that are the source of new cells in each tissue eventually grow feeble with age.
But this is only part of life’s journey. In the course of life, we sometimes get injured in our quests or in self-defense. Modern life has changed so much that everyone — young and old, rich and poor, employed and unemployed, married and unmarried, well and sick — experiences stresses of his or her own: Norms are now becoming abnorms; anachronistic views are being displaced; keeping up with the Joneses; social issues, familial problems — both intra- and inter-family — are all adding to the challenges of modern life. Stresses of life arising from these fears and other “uninvited guests” can adversely affect health.
Our immune system may come under attack (in infectious diseases such as flu), or our immune system may run amok (in immune system disorders such as asthma, type 1 diabetes, and leukemia). Degenerative diseases (including cancer, diabetes, heart disease, Parkinson’s disease and Alzheimer’s disease) lead to the function or structure of the affected tissues or organs to increasingly deteriorate over time, whether due to normal bodily wear or lifestyle choices such as eating habits...
All of these — injuries, illnesses, and stresses — contribute to compromise the regenerative capacity of tissues and organs.
Currently the only way to compensate for diseased or injured tissues or organs is through bionic implants and organ transplants.
Regenerative medicine is a new innovative technique to replace or repair defective or diseased tissues or organs by in vivo (in the living body) means or in vitro (in the laboratory) design for in situ (in the natural environment) usage to restore the structure and function of those damaged tissues or organs.4
This new technique — for therapeutic use in the treatment of incurable, debilitating, and chronic disease — encompasses many novel approaches to treatment of disease and restoration of biological function through the following methods:
- Using therapies that prompt the body to autonomously regenerate damaged tissues or organs (in vivo means),
- Using tissue engineered implants to prompt regeneration (in vitro design and in vivo stimulation),
- Direct transplantation of healthy tissues or organs into damaged environments (in vitro design and in situ usage).
Collectively, these new treatments of regenerative medicine allow for at least two substantial advances over the current state of medicine. The first advance is the potential to in vivo (in the living body) regenerate currently irreparable damaged tissues or organs so that they return to full functionality. The second advance is to be able to produce tissues or organs in vitro (in the laboratory) to be used for transplantation purposes when regeneration is not possible.
The failure of a tissue to regenerate can be due to various factors. The two most obvious are a lack of regenerative-competent cells, the lack of an environment favorable to regeneration, or a combination of both. This suggests that virtually all adult cells (with the exception of B and T cells) have the potential to engage in regeneration. This idea is compatible with the fact that present in all cells is a complete genome. The genome can, in principle, be reprogrammed to regenerate tissues and appendages, or even complete organisms.
Stem Cell — The Magic Cell
Stem cells are almost magical in their power. Imagine a single cell that has the ability to turn into almost anything — a heart cell, a pancreatic insulin-producing cell, or a brain cell. Imagine the good that could be accomplished with these cells: to repair a heart damaged by heart attack, or cure diabetes, or repair a brain damaged by stroke. The possibilities for regeneration and repair of diseased or damaged organs are almost endless. Stem cells could truly usher in a new age of medicine, in particular, regenerative medicine.
At present there are a limited number of effective stem cell therapies available for people. The most common, and most proven, is the blood-stem-cell transplant. This can be accomplished through a bone-marrow transplant or by using umbilical-cord cells, which also have blood stem cells. This sort of transplant can restore the ability of a person to make both red cells, which carry oxygen, and the blood cells of the immune system, which are needed to fight infection.
Stem-cell therapies have also been used in people to treat diabetes, heart disease, and kidney disease, with promising results. Another use of stem cells is to correct corneas of the eyes that have been scarred by infection or trauma. But we are really in just the beginning phases or learning how to use stem cells to treat human diseases.
In mouse models researchers have used stem cells to treat spinal injuries, Parkinson’s and other devastating conditions, with encouraging outcomes. Over the past few years, legions of scientists have worked out conditions for converting mouse embryonic stem cells into many different types of cells that could be used to help patients. The stem cell work on mice has also been extended to humans. In the future, these cells could provide a powerful elixir for the treatment of many diseases, helping us extend lifespan dramatically, and perhaps even making it possible for paraplegics to walk again.
Master Switch in Adult Cells
Could it be possible to find a gene or genes that could take an adult, differentiated cell, and turn back the clock, reverse its development, and transform or reprogram it back into a (embryonic) stem cell? At first glance this would seem an impossible fantasy, but incredibly, however, it has now been done!
As you might expect, this was not an easy task. Scientists had studied embryonic stem cells carefully and knew all of the genes they expressed. The problem is they express a lot of genes, over ten thousand. Which of these genes might be the secret answer to making adult stem cells revert back to stem cells? The detective work begins with some clues. We knew that one class of genes, those that encode transcription factors, would provide the best candidates; these are the genes that regulate other genes and are capable of initiating genetic programs.
By limiting the candidate genes to these encoding transcription factors we were still left with a list of over a thousand. To trim the list further, we can compare the genes expressed in the embryonic stem cells to those expressed by other cell types. These genes are rarely expressed anywhere else, suggesting that they might be particularly good candidates for driving the stem-cell genetic program, and for reverting adult differentiated cells into stem cells. After applying this filter there were a few dozen candidate genes left. And when each of these genes was tried individually, nothing happened.
This suggested that perhaps we just would not be able to reverse development and turn adult cells back into embryonic stem cells, or that perhaps it would be possible, but would require a combination of genes instead of a single gene. So many different combinations were tried.
Amazingly, it worked! Shinya Yamanaka and his colleagues at Kyoto University found that when a magic mix of four active genes (including c-myc, sox2, klf4, and oct4, which are now called Yamanaka factors) were introduced into an adult mouse cell it was possible to transform (or reprogram) the adult cell into a stem cell. This new stem cell was essentially indistinguishable from one derived from an early embryo: It was possible to induce it to form many distinct types of adult differentiated cells, including those from the pancreas, heart, and brain. Furthermore, this adult-derived stem cell passed the ultimate test: it was possible to turn this stem cell into a complete individual, a mouse, with all cell types present and normal. In the lexicon of developmental biology, this cell is totipotent — it is able to give rise to all of the different kinds of cells found in an adult.
Since then, further progress has been made, and the list of factor genes has been cut down to two, then to one factor with substituting chemicals (small molecules), and then to all substituting chemicals.
The discovery of a method to turn (or reprogram) differentiated adult cells into stem cells completely changed the landscape of therapeutic medicine, in particular, regenerative medicine. It is now possible to think of taking a patient, removing some skin cells (for example), turning these into stem cells, and then using these stem cells for a host of different therapeutic applications. These cells, called induced pluripotent stem cells (iPSCs), eliminate the ethical controversies surrounding stem cells derived from embryos, and because they are actually derived from the patient’s adult cells, they are a perfect genetic match and would not be rejected by the patient’s immune system when reintroduced into the patient’s body. The power of these adult-derived embryonic stem cells is unmistakable and they offer enormous promise in saving lives.
There are still many, mysteries associated with these cells, but progress has been steadily made. For example, the magic cocktail of four genes only worked in a fraction of cells. When the procedure was first described it was very inefficient: of ten thousand skin cells from an adult treated with these genes, only a few, may be ten at most, would actually become stem cells. More recent developments have dramatically improved the efficiency, with success rates as high as one in ten reported.
We still do not understand why some adult cells are reversible while others fail to do so (at least at present time). Nevertheless, the efficiency is still high enough to be of use because a single stem cell can be grown in culture under the right conditions to give millions, or billions, of stem cells. Stem cells (and iPSCs) continue to divide for a very long time. So it will indeed be possible to turn skin cells into stem cells, albeit with low efficiency, and then grow the stem cells in culture to expand their numbers, and to use them for disease therapy.
Pathbreaking to Nobel
Shinya Yamanaka first created iPSCs in 2006, using mouse adult cells. Unfortunately, two of the four factor genes used — c-myc and klf4 — are oncogenic.11
A year later in 2007, at least two groups, including that of S. Yamanaka and of James Thomson, successfully extended the technique used in mouse on human cells (fibroblasts).15—19 In later studies, Yamanaka showed that iPSCs could be created even without c-myc. The process would take longer and was not as efficient, but the resulting tissues, organs or organisms did not develop cancer.20—22 Since, induced pluripotent stem cells (iPSCs) have been made from adult stomach, liver, blood cells and other tissues.
Now, hundreds of scientists around the world are employing the Yamanaka factors and related techniques to search for solutions to a host of relentless illnesses. Expectedly, for his pathbreaking work on reversing adult cells into induced pluripotent stem cells, Yamanaka (with John B. Gurdon for his work done in 1962, the year Yamanaka was born) was awarded the 2012 Nobel Prize in Physiology or Medicine, only six years after his discovery. This is rather unusual for the Nobel committees typically reward research done more than a decade before; to make sure it has stood the test of time.
Yamanaka’s work has modified the trajectory of embryo stem cell research, circumventing ethical concerns, into a path that is acceptable to all. Some scientists even joke that Yamanaka deserves not only a Nobel Prize for medicine, but also a Nobel Prize for ethics!
The idea of reprogramming cells has also been put to work in basic research on disease, through an approach sometimes called “disease in a dish” (or in vitro). The reprogramming allows scientists to create particular kinds of tissue they want to study, like lung tissues for studying cystic fibrosis, or brain tissue for Huntington’s disease. By reprogramming cells from patients with a particular disease, they can create new tissue with the same genetic background, and study in the lab (in vitro). That can give new insights into the roots of the disease.
In November 2012, within a month after the Nobel Prize announcement, researchers from Austria, Hong Kong and China presented a protocol for generating human iPSCs from exfoliated renal epithelial cells present in urine. This method of acquiring donor cells is comparatively less invasive and simple. These urinary iPSCs (UiPSCs) were found to show good differentiation potential, and thus represent an alternative choice for producing pluripotent cells from normal individuals or patients with genetic diseases, including those affecting the kidney.
Stem cell research is progressing, rapidly and steadily, in various fronts. The current areas of research include:
An international collaborative effort, StemBANCC, formed in 2012 to build a collection of iPS cell lines for drug screening for a variety of hard-to-treat diseases (including peripheral nervous disorders, pain, dementia, migraine, autism, schizophrenia, bipolar disorder and diabetes), is being managed by Oxford University, UK. As of date, the effort pools funds and resources from 10 pharmaceutical companies and 23 universities. The goal is to generate a library of 1,500 iPS cell lines to be used in early drug testing.
Organs for transplantation
A proof-of-concept effort to use iPSCs to generate human organs for transplantation has been carried out in Japan. Human “liver buds” (iPSC-LBs) were grown from a mixture of three different kinds of stem cells: hepatocytes coaxed from iPSCs, endothelial stem cell (to form lining of blood vessel) from umbilical cord blood, and mesenchymal stem cells (to form connective tissue). This new approach allows different cell types to self-organize into complex organ, mimicking the process of fetal development. After growing in vitro for a few days, the liver buds were transplanted into mice (in situ) where the “liver” quickly connected with the host blood vessels and continue to grow. Most importantly, it performed regular liver functions, including metabolizing drugs and producing liver-specific proteins.
The first human clinical trial using autologous (from one part of the body to another in the same individual) iPSCs has been approved by the Japan Ministry of Health and would be conducted in 2014 by the Riken Center for Developmental Biology and the Institute of Biomedical Research and Innovation Hospital in Kobe. iPSCs derived from skin cells of six patients suffering from age-related macular degeneration (AMD) — AMD is the leading cause of blindess in the industrialized world and accounts for about 9% of all blindness globally — would be reprogrammed to differentiate into retina pigment epithelial (RPE) cells. The cell sheet would be transplanted into the affected retina where the degenerated RPE tissue has been excised. The benefits of using autologous iPSCs are that there is theoretically no risk of rejection and it sidesteps ethical quandaries of using embryonic stem cells.
The eyes themselves are particularly well suited to transplant therapies because they are inherently protected from normal inflammatory immune responses that counter diseases. This protected status is referred to as “immune privilege”, displayed only by the eye and the brain.
While the stem cell and regenerative medicine communities are working hard to have further breakthroughs, other opportunists have been tapping into stem cell and stem cell research breakthroughs, sometimes prematurely.
Cosmeceutical skincare products, for instance, have been claiming that they are using stem cell technology to aid rejuvenation. In actual fact, as far as we can tell, at least at the point of this writing, these products contain not a strain of stem cell, contrary to their misleading claims. The active ingredients in these products are Mitostime (an algae extract) and Phyko AI-PF (phyco: seaweed), and Derm SRC/Seractin. The claim should properly be phrased as follows: “These ingredients support a healthy environment for stem cells”.
That is, these products are designed to stimulate the skin’s own stem cells, which are layered between the epidermis and dermis. These cells are constantly dividing, with newer cells slowly moving to the surface and older cells being shed from the top layer. As people age, this turnover process slows, causing the loss of elasticity, uniform color and other characteristics that give skin a youthful appearance. The goal of many skincare manufacturers is to find substances that provoke adult skin stem cells to behave like younger cells, speeding up the skin-turnover cycle.
On another but related front, in the quest to show how single genes can initiate extensive programs of development, it has been discovered that there are genes that are capable of dramatically altering the developmental state of individual cell types. One of the first discovered extraordinary examples of this is the MyoD gene, which can change many different cell types into muscle.
In a culture that fetishizes slimness, this was quite a striking finding. For example, the addition of an active MyoD gene to an adipoblast, a fat cell, would convert it into a muscle cell. Instead of driving the formation of an entire structure or organ, which involves many different cell types, the MyoD gene is converting one type of cell into another. This is still a considerable accomplishment, requiring the MyoD gene to start within the cell a genetic cascade that involves large numbers of genes. Someday we might be able to rub a MyoD cocktail on our fat tummies and develop six-pack abs, without the exercise.
It is also conceivable that we can turn induced stem cells into eggs and sperms by using the right set of culture conditions. This could have enormous implications in terms of offering much higher numbers of embryos to screen for the presence of desired gene combinations.
As is well known, men produce huge numbers of sperm, but women produce relatively few eggs at a time. Even with multiple rounds of hormone treatments it is possible to retrieve at best a few dozen eggs from a woman for in vitro fertilization, for example. This means there can only be a few dozen embryos to choose among. The use of adult-derived stem cells (iPSCs) offers the potential to produce unlimited numbers of embryos for screening. For example, a tiny amount of skin (fibroblasts) is taken from a woman, and a few of those skin cells are turned into induced pluripotent stem cells (iPSCs). These few cells are amplified by growing in cell culture to essentially unlimited numbers of stem cells, which would then be turned into eggs by changing the culture conditions.
The challenges now is we have not yet worked out all of the details of how to turn stem cells into functional eggs, but much progress have been made, allowing us to turn stem cells into the precursors of eggs. It is not unlikely that in the future we will figure out how to turn these precursors into eggs, which would then be fertilized by sperm from the father, using in vitro fertilization techniques. This would give, in principle, an unlimited numbers of embryos, which would then be genetically assayed by removing single cells and performing whole genome DNA sequencing. The DNA sequences would reveal the predicted character combinations of the potential progeny, such as health, longevity, appearance, intelligence, and so on. The parents could choose the embryo with the best or most desirable combination of genes, according to their criteria, and this embryo would be implanted into the uterus of the mother and be born nine months later, ushering in a new age of designer-gene baby.
About the Author
Dr. Hwa A. Lim. is an internationally respected authority on bioinformatics and biotechnology, active in both the academic and the private sectors. He has been senior executive, and is on board of a few companies; Program Director and tenured state-line faculty member at a university. His career started with a brief stint at the Strong Memorial Hospital, Rochester, New York.
Besides these appointments, Dr. Lim is an articulate and well sought-after speaker at international meetings. Well published, with more than 15 books and 150 papers in peer-reviewed journals, Dr. Lim travels extensively to do business, to lecture, and to mingle with locals to experience and learn firsthand—something that he enjoys as an author writing on diverse topics.
Dr. Lim gained his Ph.D. (science), M.A. (science), and MBA (strategy and business laws) from the United States, his B.Sc. (Honours) and ARCS from Imperial College of Sc. Tech. & Medicine, the University of London, United Kingdom.
He currently resides in Santa Clara, “The Heart of Silicon Valley” SM, California, USA, which, besides the high concentration of high-tech companies, has the largest cluster of life science companies.
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