Cellular Reprogramming: The Extraordinary Potential of Transplanting Stem Cells Produced by Oocyte Stem Cell Factories Whose Own Nuclei Have Been Removed and Replaced by Specialized Adult Cellular Nuclei

According to the journal Science, (see Alberts,  and Gurdon et al. below) the most important story in all 2008 science was progress made in the understanding of the ability of some cells to be reprogrammed from being one kind of functional cell into another kind doing a very different job.  

The story includes a kind of time machine effect, whereby the nuclei of functionally specialized adult cells could, through the cooperation of young cells otherwise intended to become something else (namely eggs)  be turned instead into factories for making  young stem cells with great curative potential for jobs other than being a female  gamete.

Cellular reprogramming holds out great promise for a number of reasons, not the least of which is their  ability somehow  to grow corrected (as opposed to inherently faulty) replacement tissues for malfunctioning hearts, pancreases, and perhaps even brains.

This process does not work equally well using all types of cells as starting material.

In fact, the most direct adult-for-adult cellular replacement approach, often works the least well.

Attempts to go about replacing sick adult heart cells with other healthier adult heart cells, for example, has rarely succeeded thus far, for reasons of both “foreign” tissue rejection, and the fact that adult stage tissue when placed among other adult stage tissue already in place within the organism,  do not often go on to yield highly viable, self-reproducing healthy tissues.

A statistic recently reported is that only about one in 10,000 such transplants survives.

This held true even in some cases when it was the patient’s own “corrected” adult cells that had been cultured and reintroduced. Having essentially identical DNA did not seem reliably to guarantee success or even always to confer protection from immunological attack.  

For years, it was thought that the closeness of genetic match, for example, using human cells ( as opposed to say, mouse cells)  for human cellular replacement, would be  the only or at least key feature making  cellular transplants more successful.

But that genetic relatedness notion told only half the story. The environment in which the very same genes function   can yield very different results in terms of what tissues develop or get reprogrammed.

Stem cells -----pre-adult-cells that have wide range of potential capabilities, even if they ultimately turn into a particular type of adult cellular functional specialist --------- work much better, even if they are not as fully trained for the particular specialty job which they will eventually assume upon transplant.

First, they have an advantage in that they appear to generate much less immunological attack. (Being very young appears to render them somewhat harmless appearing to many immune systems).

And, stem cells are like very bright college students   before they have definitively   decided on a college major. They are amenable to being trained one way or another, even if they all have the same DNA at the start.

Take identical twins, who are somehow assigned different training tracks in their freshman year in terms of what group of mentors with which they come into contact.

Through exposing one of   them to a working group in chemistry, and the other to  one in economics,  an academic dean has a higher rate of turning them separately into chemists or economists, even though they both had an essentially identical genetic repertories.  

The trick is first, to develop a production system that will make stem cells abundantly; second, it is to influence the resulting stem cell to take on the role you want.

The best, but no longer only,   cellular factory for creating new stem cells is the oocyte. Ooctytes are essentially egg cells about one step from becoming independently viable mature eggs capable of fertilization, after which they would turn into embryos.

Today it has been recognized that timing with ooocytes, if not quite everything, is really important. Using too mature an egg cell does not work as well as using these young oocytes.

 It appears that oocytes present a window of opportunity for  having their own nuclei extracted without having too much of their own original  genetic propensities remain residually in a manner that would inhibit,  or work at cross purposes with the purpose of the DNA introduced by a transplanted nucleus.

In other words, an oocyte which has its own DNA removed, and is then injected with the nucleus of another mature cell, seems to offer fewer “objections” to making stem cells that are more influenced by the injected nucleus.

How then are resulting stem cells made to orient themselves in a manner to get the job required done?  In other words, how does one recruit or transform the best starting material once it has begun being made in some quantity?

While there are several approaches possible, three  appear  more practical at this time.

·        The first is to find the  best available subpopulation of cells within the original source of the more specialized cells, even when dealing with very sick individuals, and to culture them to obtain enough healthy nucleic for transplant into oocytes.

To continue our college analogy:  Even among some very badly performing high schools, are likely to be some heroically performing, over-achieving  individual students who ought  to be selected.  They ought to be taken out of their otherwise hopeless underperforming environment, and be bolstered by association with other successful “despite-the-odds”  students. As we have repeatedly seen, throwing high school overachievers directly into the adult world through direct injection  rarely works (the 1 in 10,000 statistic). They need instead, to get a fresh start where allowance for growth in ability and some shelter from immunological attack can be had by first   attaining stem cell status.

These  overachievers are to be excised from their sick  tissue, multiplied in the lab, and then instead of being directly injected back into the patient, have their nuclei injected into oocytes whose own nuclei have been removed immediately prior. These oocytes then go on in a maternal way to reproduce the next generation of what would otherwise have been their own egg cells as instead, stem cells having a propensity for achievement.

Being exposed to the best and the brightest tends to multiply natural propensities, in a sense “ the collective genetic and epigenetic environment” steers the resulting stem cells successfully,   so that eventually the resulting cell culture is not seen as a clone of a failing system but as the stem cell of a particularly promising “campus culture” that tends to have a great propensity for functioning as chemists or economists, so to speak.

·        The second approach is to encourage the nucleus-transferred oocytes to develop into early stage embryos with multiple types of harvestable stem cells in different areas of the proto-fetal body.

 

In other words, let the process of embryonic development   build an assortment of easily harvested of quasi-specialized  tissues with some degree of potential already: nervous system-oriented  stem cells could be harvested for brain and neurological transplants,  proto-blood cells (hemoblasts)  could be used for blood or marrow transplants, etc.

·        The third approach is to use either method one or  method two, and find a chemical agent, or a benign virus bearing some added genes,  that amplifies the effectiveness or favors the production of stem cells in a manner that yield a desired disproportion of more vibrant cells for transplant.  This is the method that probably has the most promise at this point.

Ultimately, the new thinking is that introducing a fresh supply of many healthier, more viable stem cells that have a chance for reproducing is likely to work better than attempts to someone do single gene replacement or single gene silencing therapy. Healthy tissue that reproduces well in the patient after transplantation, is likely to become a more viable option than hoping that a newly  altered gene will not be shut down or revert to “wild” or “carcinogenic” or otherwise dysfunctional status.

Tony Stankus tstankus@uark.edu Life Sciences Librarian & Professor

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