The Recent Cloning Of A Sheep Is Relevant to Ageing Research

Recently, the first successful cloning of a sheep was report; and, as warranted, it has received a great deal of public attention. The reference for the full report is cited below.

Among the many issues (scientific, economic, and ethical) which this event entails, perhaps the most significant one is its relevance to basic ageing research; and this aspect was not a part of the public or even scientific discussions.

There is only a handful of theories of ageing which are currently governing the research directions into the basic mechanisms. Of these, the theory which has guided much (probably most) of the research for the last 30 years is the idea that cells have a limited capacity for division. Because of that, it is postulated that when the capacity for division is exhausted, cells accumulated irreversible wear and tear and eventually die; and, so it is said, that phenomenon is the cause of ageing. This hypothesis was originally proposed mainly by Leonard Hayflick and was based on his work on how cells behave in culture (Hayflick L and Moorhead PS, 1961). Going all the way back to 1912 and the original work of Alexis Carrel, there is a long and involved history around this work, and many scientists never gave it much credence because it is mostly based on how cells behave in culture, which is substantially different than how they behave in a body. Still, a great deal of money is currently being spent on "telomeres" which are said to be the genetic segment which limits the number of cell doublings; and recent articles have been published in Time magazine (Kluger J, 1996) and in the New Yorker (Gladwell M, 1996), both of which feature telomeres and limited cell capacity for cell division as the major mechanism of ageing. However, the recent cloing experiment clearly demonstrates that one can take the genetic material from one cell which, itself, has divided many times during the development of the animal, and obtain a complete, biological entity from those genes. In otherwords, all of the information is still in the genome and there is no loss of information - particularly, the capacity for division and expression. This clearly refutes the "Hayflick" paradigm; and should redirect the research away from the genome to the cytoplasm or, as Strohman suggests:

Background

For those who want further background on this issue, I have provided a brief review.

Since the origin of the idea that the cell is the essential living unit, one of the important questions in biology has been whether the cell has, inherently, a definite life-span or an indefinite life-span. (Clearly, the reproductive cell line can go on forever, so the questions centers around somatic cells.) The key issue centers around the term "inherently". One experimental model for addressing this question is to study cells in vitro - i.e., let them live in culture, independently of interaction with or dependence on other cells in a body.

Alexis Carrel, a famous scientist in the history of physiology, initiated a study using fibroblast cells from the heart of a chicken embryo; and he reported in 1912 that they lived indefinitely (Carrel A, 1921). In his experiment, these cells were raised on culture plates in an incubator, and they were sustained nutritionally by a crude extract from other chicken embryos. His cultured cells would divide and multiply; he would cull down the number; let them re-grow; re-cull them; let them re-grow, etc. This same batch of cells was sustained throughout thousands of cycles until 1946, when, two years after Carrell's death, the experiment was abandoned. From that time until the 1960's, the reigning dogma in biology was that cells had, intrinsically, a potential for infinite survival and that the reason for the observed, limited life-span in living organisms must be due to some factor in the normal cellular milieu. There seemed to be two possibilities: 1) cells, once liberated from the body, were no longer constrained by some inhibitory factor that existed in the whole organism; or 2) some essential life-sustaining, growth factor was contained in the embryonic extract which was being fed to the cells, and this factor was lost in mature adults.

Carrell's experiment oriented gerontology and general physiology down a certain direction. On the one hand, if the life-sustaining, growth factor could be isolated, then it could be synthesized and delivered as a therapy to cure ageing. This notion was probably the origin of what is called "cell therapy" that was devised around that time by Paul Niehans, a Swiss surgeon. Neihans took fresh cells from the organs of sheep fetuses and injected this preparation into patients. This procedures is still used today in a clinic in Vevey, Switzerland and in the Bahamas. Claims of efficacy are made; but gerontologists do not take them seriously; and Niehans, himself, aged and died at an unexceptional life-span of 89.

If, on the other hand, the immortality of cells was being inhibited by some factor in the body's milieu, then that might be isolated and chemical methods found for intervention. This is probably the underlying notion for what came be know hypothetically as the "death hormone", which was espoused by Denckla in the 1970's (Denckla WD, 1974) but, subsequently, appears to have been abandoned.

In 1961, Leonard Hayflick and Paul Moorehead overturned the "Carrellian" dogma of the natural immortality of cells (Hayflick L and Moorhead P, 1961). They meticulously attempted to repeat Carrell's experiment but found that normal, cultured, embryonic fibroblasts will only divide approximately 50 times before they cease replication, deteriorate, and die. In other words, cells are not intrinsically immortal. Apparently, Carrell had accidently been injecting new embryonic cells in his feeding medium and thereby refreshing the colony intermittently. Thus, it appeared as though the original colony was being sustaining itself; but, in fact, it was not. Because of his strong reputation in science and probably also his considerable influence in the politics of science, no one challenged the veracity of Carrell's work until Hayflick and Moorehead in the 1960's.

In addition to demonstrating that normal cells in culture cannot divide indefinitely, Hayflick and Moorehead proposed that the intrinsic limitation on the number of times which a cell can divide might be one of the basic mechanisms of ageing; and this line of inquiry is still very much at the center of gerontological research to this day. But the story is not that simple.

There are some strange twists to this phenomenon. Normal cells are not intrinsically immortal - that seems fairly well established. However, transformed, or cancer, cells are immortal. Cancer cells are normal cells which have dedifferentiated and reverted back, so to speak, to a more independent, rapidly dividing state. As long as a cancer cell can feed, it will keep multiplying and live forever. For example, the famous HeLa cells are cancer cells which were taken from the cervix of one particular woman in 1952 by George O. Gey at Johns Hopkins University School of Medicine. These cells have been propagated and sent all over the world for study and are still going to this day.

Thus, normal differentiated cells have the capacity to divide and regenerate themselves only a finite number of times (50 +/- 10 times); but the same cells, if genetically reactivated somewhat, have the ability to divide and regenerate themselves forever.

The "Hayflick phenomenon", as it has come to be called, has always been subject to question as to its relevance to normal ageing. The basis of the criticism is quite simple: cells living in culture are in a completely different situation than cells living in a body; and therefore, their behaviour in culture may not be a proper model to study any normal, biological processes, including ageing. The objections to this model have been review by Witkowski (Witkowski, J.A. 1987). Further, Charles Daniel has performed experiments involving the serial transplantation of mouse mammary glands which demonstrates that the same tissue can survival for more than 4 full life-spans. Thus, if there is a Hayflick limit in living systems, then it is probably not a limiting factor within the normal life-span of a whole organism (Daniel, C.W. and Young, L.J.T. 1971). However, Daniel does consider the limitation of cell replication to be a fundamental mechanisms in ageing and Witkowski holds the discovery of cell ageing in culture to be a "scientific revolution". Irrespective of the precise applicability of the Hayflick model to aging, the fact remains that cell division is at the heart of the matter.

Rejuvenation and Cell Proliferation

While caloric restriction definitely slows ageing and will increase your healthy life-span by a good 10-20 years, it does not stop ageing; nor, most importantly, it does not reverse it. To reverse ageing will require, as proposed by Everone, the controlled induction of eumitosis (Everone, C.A. 1981). In other words, full biological regeneration does occur naturally when a cell divides properly. Therefore, instead of slowing the cell's rate of operation to preserve it longer (as is probably happening in caloric restriction), or instead of attempting to buffer degradative processes (as is the focus of antioxidants or proteolygic enzyme inhibitors), or instead of supplementing hormones which have declined (as is the aim of the administration of growth hormone or DHEA), or instead of the myriad of ways in which people might conceive of to reconstruct the damaged aspects of the cell which occur from normal wear and tear or disease processes, the more direct approach would be to figure out how to signal the cell to redivide properly and to do so when it is needed. This is something which the cell already knows how to do. All that we have to figure out is how to initiate the process properly; and thereafter, the genetic and cytoplasmic systems will do the massive amount of detail work.

"The proliferation theory of rejuvenation" is a term that was invented by David Danner of the Laboratory of Molecular Genetics, at the National Institute of Aging. In his paper, he describes this perspective as follows:

"This theory assumes that aging is due to the accumulation of multiple forms of molecular damage and that rejuvenation is due to repair. ... it is proposed that cell proliferation is required for full rejuvenation." (Danner, D.B. 1992)

By cell "proliferation", he means the induction of eumitosis or proper division. The area of cell proliferation is currently an exploding area of science. Within the last 3 years, 19,386 original citations on this specific topic have been deposited in the Medline databank, which is a very large number of reports. The intensity of focus can be attributed to progress in cell culture techniques and molecular probes, which now make it possible to study the details of the cell cycle. Cell proliferation is of interest not only to ageing research, but is essential to cancer and all aspects of medicine. And here is the profound convergence that represents the next, major leap forward in medicine.

As Pasteur recognized in the laster part of the 1800's, the fundamental cause of disease is not external pathogens but rather host vitality. Over the last 100 years, medicine, public sanitation, personal hygiene, and industrialization have whittled away at all of the major external pathogens. That approach to health has hit a cul de sac, and the amount of resources required to get only a small benefit has almost reached the point of a negative return. In order to sustain progress, the only place for medicine to go is to a deeper level of biological control and that can only be the manipulation of cells to reconstruct themselves properly - in other words, regeneration. Thus, the future of medicine and the pursuit of life-extension and control of ageing go hand in hand.

This area of cell senescence and regeneration is so new and the data so disparate, at this point in time, that an attempt to provide a synthesis here would be voluminous and entail a maze of technicalities, most of which would turn out to be not substantiated within a very short period of time. A brief and superficial synthesis will suffice for your purposes here.

Cell senescence occurs for two reasons: 1) the random accumulation of cellular damage that is not repaired, errors in synthesis, and DNA damage and 2) a genetic program that is associated with causing the cell to become differentiated. The loss of the cell's ability to proliferative is an active process. And this process is a deliberate blocking of the reinitiation of DNA synthesis which causes mitosis or cell division. The reinitiation of controlled cell division depends on signals from stimulatory and inhibitor growth factors and on the genes or gene products that interact with these factors. Reproductive failure (i.e., the inability of controlled eumitosis and cell proliferation), is the fundamental characteristic of cellular senescence.

For more detailed information, reference is made to the report from a workshop in which more than 50 scientists convened to "evaluate the current status of research on the molecular basis of cell senescence and aging." (Warner, H.R., et al. 1992)

Carrel A, 1921. On the permanent life of tissues outside of the organism. Journal of Experimental Medicine, 1921 (15) pp. 516-528.

Daniel CW and Young LJT, 1971. Life span of mouse mammary epithelium during serial propagation in vivo: influence of cell division on an aging process. Experimental Cell Research, 1971 (65) pp. 27-32.

Danner DB, 1992. The proliferation theory of rejuvenation. Mechanisms of Ageing and Development, 1992 (65) 85-107.

Denckla WD, 1974. Role of the pituitary and thyroid glands in the decline of minimal O2 consumption with age. Journal of Clinical Investigations, 1974 (53) 572-581.

Everone CA, 1981 et seq. Life-Extension & Control of Ageing Program - Manual of Principles and Procedures. Foundation for Infinite Survival, Inc., Berkeley, California.

Hayflick L and Moorhead PS, 1961. The serial cultivation of human diploid cell strains. Experimental Cell Research, vol.25, p.585-621, 1962.

Gladwell M, 1996. The New Age of Man. e New Yorker, Sept. 30, 1996, p.56-67.

Kluger J, 1996. Can We Stay Young?, Time, Nov.25, 1996, p.90-98.

Strohman R, 1997. Personal correspondence.

Warner HR, et al. 1992. Control of cell proliferation in senescent cells. Journal of Gerontology: Biological Sciences, 1992 (vol. 47 #6) B185-189.

Witkowski JA, 1987. Cell aging in vitro - a historical perspective. Experimental Gerontology 1987 (22) pp. 231 (575 references).